ML23251A030
| ML23251A030 | |
| Person / Time | |
|---|---|
| Issue date: | 08/31/2023 |
| From: | NRC/OCIO |
| To: | - No Known Affiliation |
| Shared Package | |
| ML23251A034 | List: |
| References | |
| FOIA-2023-000163 | |
| Download: ML23251A030 (1) | |
Text
ROOT CAUSE REPORT FOR THE EXIDE UPS 1A, B, C, D, G TMP EVENT OF AUGUST 13, 1991 SEPTEMBER 9, 1991 FILE CODE: NNIP77748 John Co y
oot Cause ator Tech.
Support Jo arwees Root Cause Faczlatator ZSEG Engineer Revieved-By:
es R.
pa afore Program Director, ISEG 9109ii0276 9i09i0 PDR ADQCK 050004i0 P
jl a
a f'
This report has been generated to document the analvsis of the root cause for the tripping of Uninterruptable Power Supplies (UPS) 2VBB-L.:S 1A, B, C, D and G and the failure to transfer their loads to the maintenance supply.
This analysis was performed in accordance with NDP-16.01 by reviewing plant operator and damage control team observations and actions, performance of troubleshooting activities on in-plant equipment, review of various drawings, performance of laboratory diagnostic testing, consultation with the UPS manufacturer, review of data recorded during the event, and consultation with other industry experts.
On August 13, 1991 at 5:48 AM an electrical fault on the B phase main step-up transformer occurred.
At that same time five (5) Exide Uninterruptable Power Supplies (UPS) tripped simultaneously.
Transfer of the UPS's loads to the maintenance power supplies did not occur. 'he system conditions as documented by operators that were dispatched to restore the units immediately after the incident as well as observations by the System Engineer and other damage control team members indicated that the UPS's logic had tripped their input and output breakers.
Post event review ofequipment drawings with the vendor revealed that the DC power supply which powers the system control logic normally draws its power from the maintenance power supply.
The inverter output is utilized as a backup source.
This scheme of connection allows transients on the AC power line to be transmitted to the DC logic power supply, Tests performed by the System Engineer support this conclusion.
The bypass breaker CB-4 did not close and transfer the UPS loads to the maintenance supply.
This functioned per design since perm isslves rmissives for CB-4 closure were not satisfied due to the degraded voltage conditions present on the maintenance supply.
The initial inspection of the units revealed that alarm indications on the five units were not identical. The inverter logic alarm light was not lit on UPS1G but was lit on A, B, and C.
The voltage difference alarm indication did not clear on 2 out of 5 units (Alarm should clear in 10-15 seconds after condition clears).
The over-voltage/undervoltage (OV/UV) alarm was present on 3 out of 5 units although all units should have displayed this alarm.
In addition, none of the 10 LEDs that indicate the initiating signals for a logic trip were lit on any of the UPS units.
Page 1
Qn August 13, 1991 at 5:48 AM a ground fault occurred on the B phase main transformer.
This event was detected and recorded on the Scriba Substation oscillograph.
Five Exide UPSs (2VBD-UPS 1A,B,C,D, and G) tripped during this event resulting in a loss of power to all their loads.
The results of observations by plant operators and damage control team personnel are summarized on Attachment l. All five UPS loads were initially restored to their maintenance supplies by plant operators after initiallyattempting (unsuccessfully) to restart the D unit. The damage control team was able to restart the C, D, and G units.
The A and B units were left on the maintenance supply because the damage control team was not successful in restarting those units.
As a result of these observations, it has been concluded that all five units shut down as a result of a logic initiated trip. This conclusion is based on the as found positions of breakers CB-1,2,3 on all five units and the presence of the module trip alarm on all the units except D which was reset by a plant operator while attempting to restart that unit, It is noted however, that none of the 10 LEDs on the A13A21 card which should indicate what condition caused the logic to trip were lit. In addition, two units (UPS1D, UPS1G) displayed voltage difference alarms.
This alarm indication should have cleared in 10-15 seconds after the plant operators manually restored the UPS loads to the maintenance supply.
The OV/UValarm indication was present on three units only, (UPS1C, UPS1D, UPSIG), although all units would be expected to display that alarm indication. The inverter logic alarm light was not lit on UPS1G although it was lit on the other units that were not initiallyreset (UPS1A, 1B, and 1C).
Breaker CB-4 was found open on all five units.
A review of the oscillograph recording indicates that for the duration of the transformer fault (i,e., approx.
100 msec.) the B phase voltage of the station's normal AC distribution system decreased to approximately 50% of its normal level. It has been concluded that this condition prevented the automatic transfer of the UPS's loads to their maintenance supplies.
This is due to a logic feature which prevents static switch transfer to the maintenance supply under conditions that could cause damage to the connected loads.
The following potential causes for the simultaneous tripping of the five UPSs were evaluated:
(1)
Voltage transient on the B phase of the normal AC distribution system (2)
$~egation of high frequency noise from the main transformer fault (3)
Voltage transient on the station ground system Page 2
Transmission of high frequency noise from the transformer fault through the atmosphere to the UPS units could not have caused the UPS trips.
Preoperational testing demonstrated that the units are not sensitive to radio frequency (RF) transmissions unless the panel doors are open and an RF source is in close proximity. The report provided as Attachment 2 indicates that it is unlikely that high frequency noise could have been transmitted through the station's normal AC distribution system to the UPSs due to intervening transformers that would filter such a signal.
As a result, potential cause (2) is not considered credible.
Potential Cause (3) is considered unlikely due to the fact that one of the five UPSs is located in an area substantially away from the other four units yet exhibited similar behavior.
In addition, no other station equipment (including other UPSs) appears to have been affected by a ground transient.
Initial laboratory testing indicates that a significant ground transient would have caused the destruction of numerous logic circuit components which has not been observed in the field.
Further laboratory tests are being conducted in an attempt to identify the mechanism by which inconsistent alarm light indications were received.
Potential Cause (1) was investigated as the most probable cause.
Troubleshooting performed following the event to evaluate and demonstrate the validity of potential cause (1) indicated the following:
1)
The DC logic power supply for UPSs 1A, B, C, D, G is normally fed from th B phase of the maintenance supply with the inverter output supply as a backup.
2)
The trip point of the DC logic is at 17.3 VDC for UPS1D corresponding to 84.S VAC on its input; and 16.9 VDC for UPS1C corresponding to 84.S9 VAC on its input.
New control batteries (fully charged) only provide approximately 18 VDC.
3)
Transfer to alternate power is accomplished via a K-S relay.
K-5 relay drop out voltage is 45 VAC for UPS1C and pick up voltage is 52 VAC.
K-5 relay drop out voltage is 42 VAC for UPS1D and pick up voltage is SS VAC.>>
4)
Voltage transients generated during troubleshooting on the normal AC input power line feeding UPS1C did not trip the UPS.
<<These measiasments were not repeated on the other units since the results were essentially the same for the C and D units and should not be any different for the A, B, and G units.
Page 3
x a r
5)
The internal logic batteries on all five units were in a degraded condition and were not capable of sustaining proper logic voltage when all other sources were disconnected.
There is no way to determine that the batteries are in a degraded condition with the current UPS design during normal operation.
6)
Voltage transients injected (i.e., dropping AC input voltage to near zero for l00-200 msec.)
on the maintenance power line in combination with the degraded batteries affected the DC logic such that it tripped the units without allowing the K-5 relay to change state, This was demonstrated on UPSIC and UPSlD.
7)
A sudden ggpi~tg loss of the maintenance supply voltage with both new and degraded batteries installed did
~n cause the unit to trip.
In this case, the logic power supply properly transferred to the inverter output and therefore prevented a trip.
8)
Voltage transients injected on the maintenance power line (i.e., similar to those utilized in 6) above) with good batteries installed did not produce any unit trips, although some voltage perturbations on the logic power supply were observed.
This was demonstrated on UPS1C and UPS lD.
9)
Fully charged batteries are required for successful K-5 relay transfer under some degraded voltage conditions on the maintenance line since other-wise the unit may trip on logic power supply failure (16.9 VDC (84.5 VAC) before the K-5 relay willtransfer the logic power supply to the inverter output.
Laboratory testing is being conducted to more fully evaluate the condition of critical components and to investigate why none of the 10 LEDs were lit on the AI3A21 board even though the logic was tripped. The pertinent results of this testing to date indicate the following:
1)
Significant ground voltage transients applied to certain circuit components causes their destruction.
2)
Injection of noise into the boards has not caused a trip signal to be generated.
Laboratory testing will continue to further investigate the inconsistent alarm light indications. The outcome of this work is not expected to affect this root cause determination or the functionality Of the UPSs.
Results of in-plant troubleshooting and laboratory testing to date indicate proper 5mction of the various alarms.
Page 4
l l.
A review of the UPS vendor manual resulted in the identification of the following deficiencies:
The vendor manual implies that the function of the batteries is to allow logic testing with no other input power available to the logic. This contributed to the system engineer not knowing that fully charged batteries could prevent a trip.
The following statement is from the vendor manual; "A redundant logic supply, powered by the inverter output, a separate 120 VAC bypass source, and/or internal rechargeable sealed batteries, allows logic testing with no input power applied and keeps alarms indicating for as long as any source of AC control power is available."
The section of the vendor manual which describes preventive maintenance does not mention the logic batteries, In addition, the general description section of the manual states,
"(The batteries should be replaced at 4-year intervals)"
~
The 4-year replacement frequency is not satisfactory for service over the acceptable ambient temperature range specified for the UPSs.
The description of the logic power supply in the manual (shown below) is incorrect.
"These power supplies are powered through relay A27K1, which selects inverter output (preferred) or bypass (alternate) source."
As a result of discussions with the UPS vendor it has been determined that the logic backup batteries are not designed to mitigate a degraded voltage condition.
Additionally, the UPS design does not provide a battery test feature or allow for safe replacement of the batteries without removing the entire unit from service.
Removing the unit from service would result in de-energizing the connected loads.
1)
The main transformer fault caused a voltage drop on the maintenance supply to a11 five UPS units.
2)
The degraded voltage on the maintenance supply caused the voltage on the UPS logic power supply to decrease below its trip setpoint causing the units to trip.
Page 5
l t
I 1
3)
Automatic load transfer to the maintenance supply was prevented by design due to the degraded voltage conditions on the maintenance supply.
Th~f h
i I
ippi g fth UPS i ~i~g
~~jgn.
The UPS is not designed to accomodate a degraded voltage condition.
The following design deficiencies allowed the UPS logic power supply voltage to decrease below its trip setpoint as a result of the main step up transformer fault.
The logic power supply is normally energized from the maintenance supply with the inverter output as a backup instead of visa versa.
Under degraded voltage conditions the logic power supply switching circuit does not actuate until the supply voltage has decreased to well below the level that willcause the logic to trip.
5)
Fully charged batteries probably would have prevented the tripping of the UPSs even though that is not part of their design.
1)
Modify the UPS logic power supply for units 1A,B,C,D, and G to be inverter preferred with maintenance backup prior to plant restart.
2)
Replace all UPS logic backup batteries prior to restart.
3)
Prior to restart review other plant hardware which utilizes backup batteries and verify that appropriate replacement schedules exist for those applications.
Ensure any control functions dependent on batteries are identified prior to restart.
4)
Process appropriate changes to the UPS vendor manual to address the identified deficiencies.
1)
Evaluate (post restart) further logic power supply modifications to rectify the K-5 xehy drop out characteristic problem and to provide easy access to the logic batteries for testing and replacement.
2)
Develop an appropriate replacement schedule for the logic batteries based on supplier recommendations, actual service conditions, and purpose of batteries.
Page 6
~
I t
ATTACMMVI' Page 1 of 5
/1 /
1 N
ER N TRAN IENT N A INP A.)
Operators responded to 2VBB-UPS1A, 1B, 1C, 1D, 1G and found the following:
1.)
1~ 1 A'
)
52L'UR 3 )
LEAK'
)
LZ5112'.)
b.)
c.)
d.)
e.)
f.)
s) h.)
a.)
b.)
c,)
d.)
e.)
f.)
s) h.)
a.)
b.)
c.)
d.)
e.)
f.)
s) h.)
i.)
a.)
b.)
c.)
d.)
e.)
f.)
s) h.)
i.)i) k.)
CB-1 tripped CB-2 tnpped CB-3 OPEN CB-4 OPEN AUTO restart CB-3 switch closed Module TRIP Inverter Logic Alarm CB-1 tripped CB-2 tripped CB-3 OPEN CB-4 OPEN AUTO restart CB-3 switch closed Module TRIP Inverter Logic Alarm CB-1 tripped CB-2 tripped CB-3 OPEN CBA OPEN AUTO restart CB-3 switch closed Module TRIP Inverter Logic Alarm OV/UV CB-1 tripped CB-2 tripped CB-3 OPEN CBA OPEN AUTO restart CB-3 switch closed No module TRIP No Logic TRIP OV/UV OV/UVTransfer Voltage Difference Page 7
~
I
A'ITACMKÃI' Page 2 of 5 5.)
g25lg; a.)
b.)
c.)
1.)
e.)
f.)
g.)
h.)
i.)
CB-l tripped CB-2 tripped CB-3 OPEN CB-4 OPEN AUTO restart CB-3 switch closed Module TRIP Voltage Difference OV/UV B.)
The operators did the following manipulations in attempting to restore the UPS':
l.)
LE~i a.)
b.)
c.)
d.)
Placed restart switch to MANUAL Placed the CB-3 toggle switch to OPEN position.
Reset the alarms LIFTED CB-4 MOTOR OPERATOR AND MANUALLYCLOSED CB-4. 'ee note 2.)
LlZRLB'.)
b.)
c.)
d.)
Closed CB-1 Closed CB-2 Reset the alarms LIFTED CBA MOTOR OPERATOR AND MANUALLYCLOSED CB-4.
see note 3 )
UL'BC'.)
Placed restart switch to MANUAL b.)
Placed CB-3 toggle switch to OPEN position c )
LIFI'EDC~ MOTOR OPERATOR AND MANUALLYCLOSED C&4.
~ see note 4 )
Il281D'.)
b.)
c.)
d.)
Closed CB-I Closed CB-2 Reset the alarms LIFI'EDC~ MOTOR OPERATOR AND MANUALLYCLOSED CB4.
~ see note Page 8
4
ATI'ACHMZK1 Page 3 of 5 a.)
Placed CB-3 toggle switch to OPEN position.
b.)
LIFTED CB-4 MOTOR OPERATOR AND MANUALLYCLOSED CB-4.
see note
- NOTE:
When the operators tried to restart UPSlD the procedure called out verifying that CB-4 was closed but it was open.
The operators made a decision to energize the UPS loads by manually closing CB-4 by first lifting the motor operator offof the breaker.
They restored each UPS in that same manner.
C.)
At approximately 0830 the system engineer went down with damage control team ¹3 (operators, electricians and I/C technician) to restore each UPS.
UPS1C:
Found CB-1, CB-2 tripped and CB-3 was open.
CB-4 was closed and the CBX motor operator (in the OFF position) was lifted off breaker Removed P6 plug from the CB-4 motor operator and aligned the motor operator to the ON position.
Reset all alarms.
Closed CB-1 and restarted the unit. It started up and "synced" to the maintenance supply.
Closed CB-2, restored P6 plug and reinstalled the motor operator for CB-4 back on the breaker.
Transferred the load to UPS power and put transfer switch in AUTO position.
UPS1D:
Found CB-1, CB-2 closed and CB-3 was open. CB-4 was closed and the CB-4 motor operator (in OFF position) was lifted off the breaker.
Removed P6 plug from the CB-4 motor operator and aligned the motor operator to the ON position.
Opened CB-1 and CB-2.
Closed CB-1 and restarted the unit. It started up and "synced" to the maintenance supply.
Closed CB-2, restored P6 plug and reinstalled motor operator for C~
back on breaker.
Attempted to transfer load to UPS power but CB-3 would not close.
It was found in tripped position.
CB-3 was reset, the motor operator was restored and the unit transferred to UPS power.
Put the transfer switch in AUTO position.
Page 9
I l
ATTACKHFKI' Page 4 of 5 Found CB-1 and CB-2 tripped and CB-3 was open.
CB-4 was closed and the CE4 motor operator (in OFF position) was lifted off the breaker.
Removed the P6 plug from the CB4 motor operator and aligned the motor operator to the ON position.
Clo'sed CB-1 and attempted to restart the unit.
Closing CB-1 caused an inrush to the UPS and tripped the upstream breaker, 2VBB-PNI.301, breaker ¹1.
Reset breaker in 2VBB-PNL301 and reclosed CB-1 on UPS1A.
Upstream breaker tripped again.
Wrote WR (WR ¹ 1623 19) and Deficiency tag to repair Rectifier section of UPSlA. Unit left with CB-4 closed.
Found CB-1, CB-2 closed and CB-3 open.
CB-4 was closed and the CB-4 motor operator {in OFF position) was lifted off breaker.
Removed P6 plug from the motor operator and aligned motor operator to ON position.
Opened CB-1 and CB-2.
Closed CB-1 and restarted unit. It started up and "synced" to the maintenance supply.
Closed CB-2, restored P6 plug and reinstalled motor operator for CBA back on breaker.
Attempted to transfer load to UPS power but CB-3 would not close. It was found in the tripped position.
CB-3 was reset, the motor operator was restored and attempted to transfer load to UPS power but CB-3 again would not close, CB-3 cannot be reset due to a previously identified problem.
Unit left with C&4 closed - on Maintenance supply power.
Note: WR¹ 138173 exists to replace CB-3.
Page 10
4
ATTACEBMVl' Page'5 of 5 UPS1G:
Found CB-l, CB-2 tripped and CB-3 open.
CB-4 was closed and the CB-4 motor operator {in OFF position) was lifted off breaker.
Removed P6 plug from motor operator and aligned motor operator to ON position.
Reset all alarms.
Noted 575vac input to UPS.
Closed CB-l. When CB-1 was closed it tripped its upstream breaker in 2VBB-PNL301. Breaker 0'7 in 2VBB-PNL301 was reset and CB-1 reclosed {successfully).
The unit was restarted.
It started up and "synced" to the maintenance supply.
Closed CB-2, restored P6 plug, When restoring the P6 block the CB-4 motor operator went to the OFF position.
Opened CB-2 and CB-1 and removed logic power from unit to reset all logic.
Reset motor operator on CB-4 to ON position.
Reclosed logic power, closed CB-1 and restarted UPS.
Unit started up and "synced" to the maintenance
- supply, Closed CB-2, restored P6 plug and reinstalled the motor operator for CB-4 back on the breaker.
Transferred load to UPS power and put transfer switch in the AUTO position.
NOTE:
When a trip signal is generated within the UPS it sends a shunt trip signal to both CB-1 and CB-2. It also sends an OFF signal to CB-3 and an ON signal to CA A voltage difference alarm will inhibit a closure of CB-4.
EVE 2
- W 2
- W UPS1A Normal AC
{US3-B)
UPS1A Maint. Supply (US5)
UPS1B Maint. Supply (US6)
X X
UPS1C Normai hC (US3-B)
UPS1C Mair&Supply (US5)
UPS 1D Maint. Supply
{US6)
X UPS1G Normal AC (US3-B)
UPS1G Maint. Supply (US6)
X X
Page 1)
~
4 4
>>agara Mohawk Nine Mile Point Unit 2 Event of 13 August 1991 Report by:
Melvin L. Crensbaw ConsuIting Engineer Po~er Systems Engineeriag Department General Electric Company Schenectady, NY 8 September 1991
~
J I
'Niagara
>foham'k Aine 'dile Point Unit 2'vent of j.3 August 1.991 05:48 Qn August 13. 1991, at 5:48 A%i the Unit 2 phase B generator step-up transformer failed. Oscillographic records of the event are available from a digital data recorder at thc Scriba Substation. They show various 345 kV and 115 kV system voltages and currents.
Figure A with notations is attached.
The four cycles preceding the fault show no signs of a gradual degradation or a developing disturbance.
The oscillographic traces and station protective relay targets reported, indicate a ground fault occurred on the high voltage winding.
Depression of the 345 kU phase B bus voltage to about 39'Fo oi the prior value was observed from the oscillographic trace.
This suggests the involvement of only a portion of the entire winding. The 345 kU line currents and voltages show rapid development of the ground fault beginning at point 1 with the ground current reaching a constant value of 1,300 ampetes in 1 1/2 cycles at point 4.
The fiashover in the faulted transformer occurs just preceding a maximum in phase 2 to neutral voltage (as would have been expected) at point 2. The 345 kU linc current in an unfaulted phase increases in step function manner to 350 lo of the prefault value at point 3.
Vo high speed recordings of voltages or currents within the plant were available.
No sequence of event recordings werc available to correlate relay operation times.
Due to the large amount of magnetic energy coupling the generator rotor and stator, and known electrical parameters, thc decay of fault current contributed by the generator to the solidly connected transformer would have spanned a number of seconds as the field dccaycd.
Relay operation targets reported were:
1.
Transformer Differential Relay {Type BDD) on Transformer 2MTX-XM1B.
- 2. Transformer Neutral Current Relay (Type IAC).
9.
Overall Unit Differential Relays (Type BDD) in phases 2 and 3.
4.
Generator Phase Overcurrent Relays {Type PJC) in phases 2 and 3.
Fv Following isolation of the generator and failed transformer from the power ~rid, g
marked 5 on Figure A. only a single 345 kV phase to ground voltage record Is available.
The magnitude of this voltage on an unfaulted phase is 74<a of the pre-fault value.
Since generator neutral current is limited to less than S
- amperes, it is known that the faulted transformer appears as a line to line fault with some impedance to the generator.
By trial and error calculation, generator line currents are found to be 0, 1.9 and 1.9, multiples of the rated value'f 31,140 amperes.
The line-to-line voltages have magnitudes 74 k 74 fo, and 25 lo of the rated value of 25,000 volts. The decay of this voltage for 0.25 seconds of the recording has a measured time constant of 2.7 seconds.
The calculated value of the impedance of the faulted transformer as seen by the generator is 0.23 per unit.
Conditions prevailing during the six cycle time period following the fault, marked 2 on Figure A, cannot be determined with certainty.
The exact nature of the fault within the transformer is not known and the physical evidence will be strongly affected by the continued flow of energy from the generator due to the inherent time constant.
The flashover of only a portion of the HV winding is evident since the 345 line voltages to neutral remain at 397o, 867o and 86fo of the pre-fault values.
The presence of "residual" in the measured 345 kV line currents provides the evidence of transformer neutral to ground current.
This requires that the. fault involves a path for current to ground from the high voltage winding.
Recorded voltages and currents show a step change to new values and no dramatic change during the time period of the record, which totals somewhat less than 1/2 second.
It could be said they are "cleaner" and less distorted than commonly seen oscillograph recordings of faults.
Given these observations and since both the generator and the system were supplying fault current into the faulted transformer, generator line-to-line voltages preceding isolation would be expected to be greater than those immediately followingisolation.
It has been speculated that very high frequency energy (mHz region) tnay have caused malfunction of logic and control circuitry in the UPS equipment.
A broad range of frequencies would be expected in any arcing phenomenon such as occurred in this failure.
Nothing in the available data or design parameters of the plant equipment would suggest an extraordinary generation or propagation of higher frequency components.
The failure of a transformer and internal arcing is not a rare occurrence.
Comparison of oscillographic charts
g E
'E rom similar events in other plants shov nothing unexpected or".un'usual
";n ".us particular faiiure lt must be borne in mind that the sarripling rate ot;he recorder is listed as 5.814 kHz,and frequency components in excess of perhaps 500 Hz would not be accurately portrayed.
GE experience in testing ot typical power transformers (such as the Lnit Auxiliaries Transformers) provides an indication of the expected coupling between windings at radio frequencies in the region of 1 megahertz:
The attenuation factors range from 1.000:
1 to 10's of thousands:
1.
Direct measurements could be made in this plant to determine attenuation factors for individual transformers over a range of frequencies.
These tests would be made on non-energized transformers using an RF signal generator and a
sensitive, calibrated detector.
Attached recent articles on electro-magnetic interference, Reference 1
discusses IEC 801.4 and the characteristics of electrically fast transients.
Reference 2 discusses testing of ground connections.
V I
The possibility of elevation of the station grounding system as a result of this disturbance was postulated.
The relatively high level of ground fault current, estimated at 1,300 amperes from the available recording, would not have been conducted into the plant.
This current can only flow in from the 345 kV system for the 6 cycle period required for relay and circuit breaker operation to achieve isolation.
The generator ground current would have been limited to less than 8 amperes by the neutral grounding equipment.
Elevation or differences in ground potential within the plant would therefore not have been expected during this event.
Reference 1
discusses the problem of achieving a "super" ground and concludes that a stable ground reference for interconnected equipment is of greater significance.
Since normally circulating ground currents are not
- expected, testing with very low voltages 'and currents is recommended.
Note especially the recommendation to test with a frequency non-harmonically related to the power line &equency.
The transformers stepping the voltage down to successively lower voltage levels are connected in a manner to minimize coupling of power frequency and higher frequency.
components between the various busses.
Specific configurations are:
l.
Normal Station Service Transtormer-delta 25 kV to wye 13.8 kV with 400 ampere resistive grounding on the 13.8 kV side.
Load Center Transformers-delta 13.8 kV to wye 4.l6 kV with 4% ampere resistive grounding on the '.16 kV side.
3.
Load Center Transformers-delta 13.8 kV or 4.16 kV to wye 600 volts with neutral solidly grounded on the 600 volt side.
4.
Reserve Station Service Transformers-wye 115 kV, delta 4.16 kV, wye 13.8 kV. The 13.8 kV neutral is 430 ampere resistive grounded.
The 4.16 kV circuit is connected to a zig-zag grounding transformer with a resistor in the neutral connection, presumably for 400 amperes.
These configurations provide "effectively grounded" distribution busses as defined in IEEE Standard 142 and will serve to limit transient over voltages.
This is in accordance with design practices deemed prudent and conservative within the power industry.
The industry continues to review the effects of geomagnetic disturbances on power transformers.
While no evidence is seen of voltage distortion in the four cycles preceeding the failure, excessive duty could have occurred if these transformers had been subjected to low level direct current previously.
References 3 and 4 are attached for perusal.
~
E
~-. IJ Lll>>Ew I T~
LSssC. 2>>
K,c As =. ~ ()
I}
T Cs -/J g,4 v 'ss K
e K
S sr C4t KS>>4SSC Z>>2 CO Jg J(j JQ Qa W c'1 Q,5
>>>>KK A:
P.
I VWOz s.
3 s
~
+
r s
jpA~
~:fvV: "v
~
s D>>S>>>>s>>s>>s
} s gpss
~
s
~
~
s
~
/
I
/
jg(l - i Cs III0 4s>>st+
ps>>1$ C
~
<~sA V'J i V V 'v' V (!,.',,'
C.sva sr~a 4Q
>-a l.
~
~
I s j 7>> K~K.
/$0 PY 30O Bmoc s trsslsss4>>s s44>>ss, s>ssss>>sssss X
VVVVii ls Ls~C '}
~
s r
AV V
'v
~
>>~
~ >>
~
s Flgelk'f A
~4~4~r4ss t
~.g. Q~clshgsJ C'-'LS Sf S N>> 'C>>>>,
S QK K
~
~
Po
~
r>>
S
~
r
industrial Equipment Electronics in inctustrial Applications A Discussion of Fundamental EMC i'rincfples for Electronic Controllers ln an industrial Environment By ~gillian D. KinmeL PE Kimmei G cfire Associates. L:d Fl/C probler..s i~:h:ndustra! centrals are aggravatea by harsh en'".ronments.
mixed
- ecnnoiogies and a lack of umform E!~IC yuceures...'is aft:cie
~~ll concertnte on
- he common as;ec:s of elec'iuc controls in an:ndustriai eninronment.
which is generally muon harshc.;han the oifice environr.:cn:.
tVhat:s !he!ndustrlal environment and what can be core about:t? The ennronment includes tl;e enure gamut of '.he basic
- hreats.
power disturbances.
RFI.
and ESD. RFI and power disturbances may be locally gcneratcd or not. Mixed tcchnolo-g.es compourd:he problem. Digital circuits are used to switch linevoltages via relays.
Aralog sensors arc input devices:o digital controls.
increasingly.
!here is a
need for a
- ooperauve ffort between the designers.
.-.znu.'acturers and b:stallers to come up vrith a
rock. solid system.
A common complairt is that the inscallers or mainte-narce people won't follow the instal!ation requirements.
This may be ttue, buc it must change.
smce there arc prob!cms wiuch cannot be solved at the board leveL It is also twe that manufacturers often specify inscailaaon
. Cquiremcncs which are not pncucal to implement, and there are documcmed cases where the prescribe installauan procedures wi!I cause nther clan cure a probictsL The!ack of utuggcctc guidetinc5 has ham-pered EMC pragesa in thc industrial arena. Fortunately, the European Commu-ruty is worlang to adapc the IEC 801.x specifications.
and domestic compaaies would be wise to adopc them. even ifthere is no intcnaon to export.
The Baaic Threaca Thc three ba5ic thfeat5 to mdustrial electronics arc power distu*anccs. ndio frequency interference. and ESD.
Power Discu*ancca.
Power distur-
. EMC Test 4 Design bances are a we~J known:rdustral problem.
ln iac:. when a proaiem occurs. ile Grst ihougnt is
'.o b!arne the power company.
Often power qualitv is a problem (especially if grounding issues are:ncludcd).
but the problem is almost aiways generated by adiacer.t equ;pment.
Tradiuonal problems with power include spikes and transients.
sags and surges.
aad outages.
which threaten the e!ectrorucs via the power supply.
These prob!cms arc fairly well documented and are often solved using power conditioners or UPS, The most common power problems confronang electronics today is the sag which r/picaliy occurs dunng turn on and the spikes which tyfnca!Iyacct during turti off of heavy eductive loads.
i"he sags simply stafvc the electraiucs.
The high frequency transients barrel right through the supposedly Htered power supply=to attack the electronics inside.
Digital cirnncs are most vu!nenblc to spikes which'cause data errors or worse.
Analog nfnllt5 afc Inost vulncnblc to continuous RF riding on top of the power.
FIPS PUB 94 provides guidcbnes on e!cctrical power for commcccia! compucen.
This is good information. buc beware that factory power is much noisier than commer-cial power.
The guidehncs of IEC 801A specifies an electrically fast ~sienc [EFD that simu-lates arnng and other high speed noise.
EFTs are quite sharc nnged they diminish rapidly with distance due to induc-tance ul thc linc. But at short nngc, they arc devastating.
Unfortunately, at tention is placed on the franc end of the electfatucs, the power supply. With industrial contfcN5, the prob-lem is the controlled elements.
If tho electronics is controlling liae power, the disturbances sneak in the back end where little or no protection exists.
System ground, wtule not being specifi-cally a powe. ~stu-b~-ce -~~~'i-the car-..er )f residual e.".Cats oi -
~
disturaances.
Any ind su:al or:ommer struc:ure has sigiuficant
.'Ow "eue currents nfcu!ating:hrough
'".e;.c'ystem.
sometimes because the energ.
intenuonaHy dumped onto the ground 's.
as with an arc weider) and some because of uninteniionai coupling or.~
an inadvertent connection between re.'.
'nd ground somewhere in '..".e facility.
Radio Frequency Interference..-,
dio frequency interference affects analog and digital cin."~ts.
with an~
nrnats being generally more susceat."=
Surprising to many, the prmnple threat not the TV or FM stauon down the ro; buc nther it is thc hand held t~srn::
camcd around by faciliues personnel. A o watt ndio will result m an eiccc.".c "eld Gve volts/meter at a one meter hstanc enough to upset many electromcs system IEC 801.3 spenfies immutucy to electr GeMs of oae to ten volts per
- met, depending on the equipment.
with c."z!
volts per meter being the level for qyic equipmcnt. As can be seen!rom the aboi approximation, three volts per meter is n an excessive requirement, and even:e volts pef rnctcr is fairly modest.
Eleccfoacacfc Discharges.
Elect::
stanc discharge is an intense short dun c pu!se.
having a
riseamc of about or.
nanosecond, This is equivalent:o a bur of 300 MH5 iaterfercnce.
Static bui!dup of 15 kV are not uncommon.
Dry cfimatcs, induding northern climate WKam Eimmcfis a principal with Kimmc Gcdrc Aaaofiafca. Lfd. 77/c finn spccia:
fact in prcveaang and solving electfonug naia infcrfcrcacc and compaabQity (E.>fl EMQ problems.
Mr.
Kimmel can fcaabcd af 1$44 N Pascal.
St. Paul..'f:
551M, or telephone 612 330-3728.
I
Vcc line piotzer TRIAC I
.".i<".re.'. Ampu6er dentoauiauon.
Figu:e 2. Transient feedback path.
gut tc.".
poser
.n ante..
offer apportuiuty
.'or ESD.
Indus;nai env:ronments. with:he:r moving eq ipment. a.e loaded with potenuai ESD sources:
rubber roilers. belts. anc produc-uon ou:put sucn as plasuc and "aper rails.
a!l add ;p :o a real ESD th:eat.
and vis t.'seat is more likely to occur even m
reiauvely moist enviroi.ments. Look ta IEC 801.2.'or ESD standards.
Electronics Design Elec:ronics:s generally thc ultimate vicum of interference.
The
~ter'.ercncc ands:ts way ~Wough various paths:o the eiec'.rorucs equipmcnt itself. Let's concen-trtte on what can happen to your eiectranics rom
!he back door.
that is.
by direct radiauon into the eiectroiucs and by con-ducted ".ntcrference through:he signal and cont:oiiincs.
Sensors.
Low level sensors.
such as ther...ocouples.
pressure sensors. etc., arc charactenzed by very low bandwidths and
!ow signal!cvels. A major Great to these ser.sors is radio frequency interference, either from nearby hand hekl trinsmitters ar more distance land iaobile or Gxed transnuttcfs.
But:hese are high frequency, much above the bandpaas ofyour amphfier, right?
Wrong!
Low frequency amplifiers are plagued by two yfeaomcna:
out of band response and aero rectification.
These combine to prosit false information on leveis to the system.
All amplifiers have a normal bxndpass.
typiGcd by a 20 dB/decade roUoff or more at the high end. But resonances due to stray inductance and capacitance willgive rise to amphficr response 6vc orders of magnitude or more above thc nominal bandpass of the ampiificr. This means an audio amplifier will respond to signals in the hundreds of MHx.
The second aspect occurs when RF ercour ters a nor~earity such as a semicon-duc'.or device. All such devices give nse to a DC:eve! shift when confronted with RF.
ln a radio receiver they are called detec-tors. Nonlireanues are muunuzcd in linear devices. but:here is always enough to cause problems. The upshot is that the ampiifier demodulates the RF, generates an errone-ous signal. and passes this error on. This effect is shown m Figure 1. Output hoes are sinu!arly affected. with capacitive couphng back to the input.
The soluuon is to prevent tbe RF fram getung to the amphGcr. either by shielding or nltering. Thc most common path to thc amplificr is wa an external signai line fram thc sensor.
but if thc ckctronics is not shielded. direct radiation to the circuit board may also present a problem.
Assuming fGtering is the sdected method.
use a high ~frequency Gltc.r, designed to bhck signals up to 1 GHx or even more.
Use fcmtes and high frequency capacitors.
Do not rely on your law frequency Glter to take out RF.
At the op amp. you shauld aisa decouple your plus and minus power to ground at tbe chip. Ifyour ground is cartying RF, you can anticipate
'.he same probkm menaoncd
- above, since it wt!I cottupt the reference level.
Data Lines. Digital data lines will be upset by the RF probkrn as in aaa!og, but tbe levels necessary to upset are higher.
Instead.
digital data lines are much more susceptible to trarlicnt ghtches. Al sigaal lines should be Gitcred to pass only the frequencies ncccssary for operation. If tbc threat lies in the bandpass of tbe signaL then shielding or optical links will be needed.
Switched Power LInes. This refers specificaily to the power being controGed by the controller device. Industrial control-lers are commonly tasked to control power to heavy equipment. wiuc...s."~c..
by hcavy starung
'.oads znd irduc ve at turn off. Typically the
!ectroruc
-.".".'witch Gnc power using relays or:;.
This exposes the back end of the con":
to substanaal line transients.
wiuch "=
back to the circmt power and grounc disrupt Jx digital circultrv as show.
Figurc 2.
lt is mandatory that the transient rents be diverted or blocked.
since digital system cannot wnhstand
".e,.~
tudes likely to occur with an inoucuve
.~
unicsa special steps are taken.
Self jammmg can be ignutcd by contra.
when you switch
'.he Gnc.
usmg crossing devices. Of parur~ importa.,
is the tuni off. smcc ~Mt:s when induczzve kick occurs.
= lfaG power switctung used zero cross devices. the transient!eveis in the fact would bc dramaticaily reduced.
Unfor natdy, that goal is weil off in the fuu:
UatQ then, expect that high voltage poi transients willoccur. and they must be di with.
Opucal couplers and relays do riot prov
'u isolation by themseivcs.
Th high capacitance provides an excellent n:
frequency path. and if they are stacked in an arrxy, thc capacitance wtG add up
~
pass surprisingiy low frequencies.
The capacitances caa't be elinuiated. but y can design your control circuits to nunun:
couphng paths and to maximize!ow impc ance alternate paths.
Transient suppressors should be instaU at tbc had, which is the source of the spB but they can be installed at the controb as wdl.
An Interesting effect occurs when coi'ining acta crossing SCR regulators w
low kvd sensors which use line frequer.
noise cancehng techniques.
Very sensiu sensors sometimes arc
!amp:ea..'or Juil"Angus
gk P
se0 VAC High Current DC
'4co v i n PS Electronics F'8'f
- 3. 'fn.. Gn:ndustr:ai powe. supply.
."igure 4. IIulnpie groura patrs.
e..'-.
power
-,c!e:o:ancei thc ine
.'rec ency;=mponent. lft.;e sama!e occurs concu..ectly wi:h ~re power sw:rchrng on
- cif:he average:o rhe sensor wi!I be
'.pse:. and an e.-.or i4 be. ccorded.
System Design and Installation Once
>e elect:arucs is designed.
it becomes a problem of the system mtegrator ana mstaier:o er.sure:hat the electrorucs is crcvided with rre environment for which
- t was des:gned.
Most of the arne.
this work is performed by power experts and electncians.
and they are not always aware of:he inter'.erencc probkm. Oftca. on site.
the power quality is blamed tor thc equip.
ment anomalies.
But the problem can often be avoided by following a few basic prina-plcs.
The indvstrial control device is either inte~ted into a system at '.hc!actory or insta"ed separately on site.
Controlkrs hardie a varcty of devices such as motor speed controls. posiuoning devices.
weld-ers.
etc.
Interferencc presented to the electronics can be significantly reduced by appropnate measures outside of the ekc-uomcs box.
There is no way to accurate!y assess thc threat without test data. But rcgardkss of the Nor,.ation availabk.
much can bc accomplished by correct instagation.
and it doesn't cost much if dona at the start.
Retro!its become costly, espeaally if ac-comparued with factory down time, Let's consider Oe same problems from a system standpcriot. Your goal is to limit the interfcrcncc whkh must be handkd by the electronics.
Direct radiation to the ekctranics is not often a probkm in an industrial environ-ment, but it does occur. and most often with a
p!asuc enclosure.
The NEMA type enclosures pravide enough shielding for most industria needs. Ifyou don't want to use a
meta!
enclosure, be sure to gct electronics which will withstand thc RF which wrtl occur.
ERIC Test 8c De van afore oi.en
'.he prob!em is conducted.
circe. ';a power or ground. The problem occurs "e:o power and g.ound distur-bances caused by the equiprncnt. It is an all too common pract:ce
',o draw controger power I;om U.e same source as feeds the power eqrapment.
~r.is power may provide
- he necessary energy to drive the equip-ment. but it is not suitable to power the elec:romcs 'Figure 3).
Hopefully. all industrial equipment will have electronics powered fram a separate tow power !20 volt circuit. It solves several problems. First. it separates thc electron-ics power from thc probably very noisy industry grade
- power, prevenung the switching transients and startup sags fram gering to thc electronics.
Second. if it is necessary to condition the electronics power from an exter."A probkm. it is tar cheaper to condition the watts needed forelectronics power r.'ran it is to condition the kilowaus required by the system.
If power cannot bc separated, then it is necessary to provide a bulletproof power supply.
preferably including an isolation transformer, to separate the entire power supply from Jlc electrical equipment.
Ground iVoisc. Ground noise, inevita-ble in industrial environments.
must be diverted from the electronics
- module, Multiple grounds ia a system wiU often result in grovnd currents circulating through the equipment. and ground noise arculating through thc electromcs path will cause maifuncuon. Figurc 4 showa some typical gttrund loop situauons.
A comman approach is to demand a super earth ground. This is good. but it is not a cure all, and often a super ground cannot be achieved.
no matter how you try. How do you get a super ground franl thc turd floor!The real need is to get a stabk ground reference to all interconnected equipmeats.
ff this equipmcnt is closely located. then a very!ow impedance interconnect is feasi-bk.
Power conditioners arc often tasked to elm.'nate R..r pound.".cise..;.a work. but:hesc proa!ems can e
s wrh an:solat:an nns cr...er:o
=...
reutral to ground noise and wrh ""~I!".
hne alters.
So you may want ra inexpensive approacn rest.
Data I.inks. Data iinks are sr.";nr over die enure!acuvty.
exposing rhe.
two prmaple effects. ground noise an:
pickup. Ground noise wi!Icause cata e unless the electronics has been desi':t accommodate potenual differences of eral volts or more. This is accompi.
wrrhdifferential drivers and recc:vers:I must be direct coupled.
Optical inks
<<ventually take over these!inks.
The other aspect is RF pickup. Inex sive shielded cabk is suitable
!or purpose.
Ground both ends!
Do not a single point ground techniques
'.o RF.
hw frequency ground loop proalem
- threat, thea one end can be capaa" grounded:
Summary fndustrial electronics are subjected t harsh environment. Good design and in.
lation techniques willmininuae problem the 6cld.
Adherence to the Ev~p standards.
IEC 801.x is a good start. e ifyou are only markeung in the USA.
Bibliograyhy FIPS PUB 94 ~
Guideline an E!ec=
Po~er for ADP Installauons, Septemb 1983.
IEC 801-2, Ekctromagneric compaubu for industria-process measurement
'ontrol equipmcnt. Electrorrtauc dischar requirements, 1984.
IEC 801-3. Ekctramagnetic compatibi!
for industrial-process measurement; contra! equipment, Radiated ekcuom:
acdc Rckl requirements.
1984.
IEC 8014, Ekctromagrletic cornpatibi for industrial-process measurement a
coatrol equipmcnt, Electrical fast:rar sic:
burst requirements.
l984.
p ~
p
'J E
I'
industrial Equipment Equipment Grouncf Boncfing-Designing for Performance and Life A Discussion of Ground Connection Fundamentals to Control EMl By D.B.L. Durham Dytecna Ltd. L'K The problem of achieving sauslactory earth bonds or ground connecuons has plagued EMC engineers for many years. not only because the bonds are often vital for the achievement of sausfactory equipment per'-
formance but because they aifect the long term performance of eqinpment aiter it has been introduced into service.
Recommendauons on bonding have ex-isted in tfn form of nulitary specifiations, such as Mil Std 1310.
Mi1 188-124A and MiI-B.5087 (ASG) for some years and these have generally proved satisfactory for most new builds. However. these specifications have certain Iinutauons in that they gener-ally do not specify consistently low levels of bond impedance.
nor a suitabk test method.
The introduction of new EMC specidations in Europe with the EEC Direcuve on EMC and the requirements for iong tenn stability in EMC characteristics has directed the UK nnTitary to review existing specifications and introduce a new Defence Standard to tighten up perfortn-ance requirements for miStary equiptnent.
Def Stan M4 (Azt 1)/1 has been intro-duced to address 5$ 4rla as far aa ItNbllo and transportable ceasxtnications instaHa-uons are concertai, bttt the requirements shoukf have impffctfntts in industrial apph-catfons and over the whole ekctrotUcs market iflong tenn prtxfuct perfonnance is to be guaranteed.
Bond Degradatlon Earth or ground bonds are generaSy considered essential not only (or safety
- reasons, but as a means of diverting EMl currents.
"locking" circuit boards and 38 eqtuprnent to a stable ground point. achkv-ing adequate levels ofcable shiekfing and for many other reasons, Many designers un-derstand the requirement forshort, fat bond leads to mininuze ground inductance but few appreciate that a cntial aspect is the connecuon resistance with which the bond strap is attached to the equipment ground point. De basic reqturement of any bond is that it should have as Iow an impedance as possibk (unless it is a dehbente uxfuc-tive bond to iginit ground currents).
The impedance is a combination of the resistive and
~Pe inducuve components.
The resis-tive element is a functxm of the bond strap resistivity. cross sectional area and length, see Equadon 1, whSst the inductive compo.
nent is a more complex function of the bond strap characteristics as shown in Equation 2.
R~-
qf (1)
A L~
L In
+05 ~02235 j
Njt b+ cl 2.L bc 2f J (2) where R
resistance, q ~ resistivity, f~
length. A ~ area. ii, ~ penneahiSty of free space.
- l. ~
inductance, Ii, ~
relative permeability, b strap width, and c ~ strap thickness.
The frequency at which the inductive ekment dominates the impedance expres-sion when calculating the total inductaaa is, from Equadon 3. typically 1 kHx. lt wiH be seen therefore that to aS intents and purposes the bond except at DC and power frequencies, may be assumed to be an uxfuctance.
At very hgh.".equencies stray apacitance across the strap dominate.
This means that the volt ".
across a bond is generaily a iunc"'or.
inductance and frequency. Based on Ohr.
Law this volt drop is shown in Equaucr.
For transients the voltage drop is giver.
Equatxxt S.
Z Ra + (u'Lt V ~ IZ ~ jruLf Vm I, dl dt where Z ~ strap impedance.
cu ~ ad; frequency, V ~ voltage. and I ~ current.
From this. the higher the inductance ih more isolated the circuit or box become from grtxmd.
This an have signtfiar.
effects on equipment.
inchding enhance ment of noise injection onto circiuts, reduc tfon of Ster perfonnance.
Rxf loss c
coautxmicadoa range.
From a TEMPES.
standpoutt it may result in more radiauor fmm equipaxmt. lt woukf seem from tlat that the criteria for any bond is the inductatee and hence the choice of short fat David Du@tarn served for 21 years in (lie Br&fsh Anny. where he gained hi degree in efectricaf etigiaeering. Alter service in a van'ety N appointments fte retired ro join de RecabSES company as the Techmcal hfxrlger rwpoaaNe d'or the design and devwfopiinent aC ctxturxmicarion systems.
ln 158f helix'ned Dytecna as the Manager of thi Ergineerfrtg Division. and now is cunenrfy Technical Marfrea'ng Manage..
July/August !99!
t 4 ~
)
~2cii;-isis~a<-<<as<<ry
,'(ac (t w,cc
(
t5(
'.rrer.
(O Ar. as
'au A <<k5<<CD
<<as<<CR I
4 I
V s
VC s lip I
VOLTAGE 4EASI'R&lENT~
I Figure!. Bond.esisunce.
Figurc 3. Four w(re bridge method.
bend suaps.
However.
an analysis oi:he bond educunce shows that for a bond strap of 100 mm tong. 15mm i~dc and 2 mm ttuck che (mpedancc ai 1 MHz witt be 3.8 Ohms.
I:
sounds extremely sirrpte.
but work perfon",.cd in '.he USA'nd I;K shows thai if an error is made:n ke way the strap is iernunaied then a progressive increase in
- he resisunce af Ae bond strap to box juncuan can occur as thc equipment ages.
Eventually the rcs(stance wdl begin to exceed hundreds of ohms and may eventu-aily go open r cuit. This can negate the effect of chc bond strap completely as part of ihe EMI protecdon.
tVhat happens with bonds to cause this change.'ssentially a ground cannecuon is a
scnes of impcdances from the strap a@ough to the ground material.
as shown in Figurc 1. Each point of conuci contnb-utes ta the total bond pcr'.ormancc.
As a result. a change in any contact condition can result in a
change in thc total bond resistance.
As is weil appreciatel.
the contact resistance between two metal sur-faces is a fimct(on of the pressure.
The prcssure excr.cd by the tip ot a drawing pin is vasdy greater than that fram thc thumb pressing by itself. Thus the contact tram a sharp poet yves a aasch higher pressure t!un a Qat point and thgekre tower contact resistance. Measure~a have showa that sharp pa((its enable owtact resistance of a fcw microohm to be achieved whilst similar pressures on Qat surfaces result in mil-tiohms of contact resistance.
It might be felt that there is tittle or no difference between these vah(cs. but in reality there is. An essenual aspect of a good bond is that it should remain so after the equipment has cntcred usc, High pressures also have the etfect ofsqueezing out corrosive nuteri-als and insulaung 6lms. Thc former causes EMC Test & Design prog.essive "egradauon nt bonds.
whilst the tatter can reduce the ef'cicncy of the bond '-om (e mor..eni:t is insulled. It is paruculariy important in conu"..umcatians
- systems, where fittcrs are insuQcd and shielded cabie:ernunauons arc made Blat the bords are of:ow resisuncc and rcu(n their peifornance.
Bond Perfonnance and Mcaatcrcmcnc Expenencc has shown over a rnunbcr of years thac for tong tern consistent bond performance a low value af res(suacc must be achieved. This is typically 1.5 miQiahms.
In Det Stan ~ (Part 1)/1 the vah(e has been sci at a maximum of 2 mittiohms. This level is measured through thc individual bonds.
Thc logic behind this Icvd is twofold. Firstly, experience has shown that with comnnmicaions equipment in particu-lar this value af bond resistance is required ifconsistent perfonnance h to be achieved ia terms of reception et6cicncy and trans-mission characteristics.
This is particularly so for TEMPEST protcctcd equipments, The secoad point is thai if thc bond has a higher rcsisunce then there is a signi6caat likelihood that progressive degradatian wI occur aad the band resistance wig increase in value. There witt then be a progressive loss in perfonnance.
The main problem with measuring bond resistances is that it should be mcasurel ushlg a
taw voltage/cutTcilt t09taiquc.
Most techniques to date for assessing safety illvolvcs driving a large current through thc bond. This checks the bond's aMity ta carry current but does not necessaril check its EMI protection performance.
The rea-son is that many bonds may when in normal use have a high resistance due to oxide aad greasy 6lms. but when subjected to a high current the layers heat up and are vaco nscd. After the current:s
. emoved ".c "';.
can return. Thus high cur.crt tee!in(que are nat recommended
!Or:csteg bonds. Thc new Defence Sundard in '.".c bl< spcci6es a maximum probe voiiage ":
100 rmcrovolts. lYis represents typ(caJy l probe current of 50 mdliamps under shor.'irc(at
((
1 mQ) condiuans.
This:s insuf6cienc ta destroy surface 8ms.
The classic method for measuring Iow rcsisuncc has been to use a four:crminal bndgc as shawn ia Figure 2. In this case the c--.eni is drive between two poets and "le volugc across the sample is measured w(th a high rcs(stance probe. This re(caves he effects ot the probe canuct resisunce and lead rcs(stance.
This is generally cons(d-ered to bc a laboratory method as the use offour contacts caa be awkward. Ifthe!ead resistance caa be removed by a caiibrauon tech(ique then thc four terminals may be replaced with a two terminal system.
h further pouibte refincment to 'lc cd(aique is to use a frequency that is not DC or 50/60/400Hz. In this case 10.4 Hz has been chosen. Ifan active Ster is used to Ster ouc all other electrica noise. then it is possible to use the bond resistance meter on powered up systems. It is worth natmg that at thN frequency thc impedance is stQI targdy represented by rcs(stance rather thaa iad(actaace.
The two teimmal method is shown ia Figure 3, The introduction ot new EMC/EMI spcci6catioas in Europe has made it more important that once made the bands have consistent long tenn performance.
T)us means measuring on periodic inspecuon ard alter maintenance, It is an essenual aspect of insuring consistent performance.
It has been shown that within months apparently good bonds can deteriorat io iigh rcsis.
IJ
cuo!ec::o tesrrog rnd -.a.....;3:.:.-.-
33 3 tnarnrena..
e rasi.
~c hsc ~eksv1~eNr V
ci z c+ o ts>5~3NCc l
I FIXED RESISTANCE I.EAOS Figure 3. Two termrnal bndge method.
40 0
I
~ I I)nlpflallII't Qc c+, +
c pecan
+
co+
qo aeries CUSTOM25VC Tc os~% 'W o%c c'w
+b
-IION COMM@ Rf pC 0
9n goc cc~
.o>
b.~.avatars, Coaxial Term.
V c'+
c bc~ac>
connectors, s%c+ 9+~.bluest Ci 1120 12 l.incoin Avenue. FIOlbrook. NY 17741 I<16)SN-)4'AX.16-~86.H14 INFOICARO 29 LK ~filitary Experience T..ere have oeen:wc
-..3!c..-r::
caused
".v poor:o.".cs ex".e."...".. c-degrada:Ion
.n."enc:ma;.ce
~r 3oc Ironed In nus 3.-.:cie.;.".e.css cauon range.
d'or "..'ll -e.".o....ance other effects aL':onunbute:3 3 cons:c rcducuonm equipment
.':ency 3nn 1
ability. The second feet wruc.'-.:s difficultto:dentil"Is hat 3
~'o Fa ir.=-.
(NFR problems.
An analysis of re"c.
failures hom miavtary reiiaci'.ity data shown that NFF !1c:dents can oe ex:re.-.
high. parrrcurariy;n burred cbmares.
has been parnaliy confirmed by repons!:
the Cuif War when all forces.epor.ec mcrease In avariabiliry of eqtuprner.:
dner chmatc.
Many fav'rs are due:o ekctrrcai contacts m connectors. but 3.3.
number have been:denufied as excess EMI induced throvgh poor ground oon This may bc caused bv erther a loose grc strap or connector terrrariauon to:he:,
A slgnrficant unprovemcnt
.1 equ:pi..t'vailability and performance is expec:,
when morc recent stausucs are 3naryse" The introduct:on;nto the Brtusn A:.
service of Ac Dyreaa Bond Resrstan Test Set DT '09 has enabled the
'ihtary to measure bond resrstances installed equipment and renuce;he curanccs of NFF errors. The I:K m e:3.
measurement procedure uses 3 r'wo:e...
nal bridge method and an accurate miUiohm calibration standard.
This znea urernent procedure and equIpment Is 31:
in use by other NATO nauons and e's.
where by militaryand naval forces who 33c rccogrused the same pruble.
Corrcluiiorri The problems with ground bonds hav become significant with the development c sensitive and secure communicauons ecui" ment. This coupled with an increasmg ree.
to achieve higher and higher levels of EM, protectiZxr has leal to an increased emphasr.
being placed on the effectiveness ofall type of system grounds.
- These, further com-bined with a requirement to cnsurc thc long life of systems once in service.
have resulted In the assessmcnt that bonds anc tcrminations are one of the primary causes of EMI fa0urcs in systems.
The reqvire.
ment to test these is clear. however the means to do so have not always been avariable to engineers.
fu!SSAugust
'."=o:
)~
I.
~
/p
Panel Session PES Sumtner Meeting, July 12, 188$
Long Beach, Cailforah John G. Kappenman, Chairman Power System Susceptibility To Geomagnetic Disturbances:
Present And Future Concerns John G. Kappenman, Minnesota Power The effects ot Solar-Geomagnetic Olsturbances have been observed for decades.on power systems. However. the pro-tound impact ot the March 13, 1989 geomagnetic distur-bance has crested a much greeter level of concern about the phenomena in the power industry.
Several man made Iystems have suffered d)eruptions to their normal operation 4O ta the occurrence of geomagnetic phe-nomena. Most of Oe man-made systems, such aa commu-nications. have beast made less susceptible to the phenom-ena through technological evolution {microwave and fiber-optlc have replaced metall)c wire systems).
However, the bulk transmission system, If anything, is more susceptible today than ever before to geomagnetic disturbance events.
And Ifthe present trends continue, it la likely the bulk trans-mission network willbecome more susceptible In the future.
Same of the most concerning trends are: 1) The transmission systems ot today span greater distances ot eatth-surface-potentiai which result In the flow of larger gaomagneticaily-IEEE Power Eaipaeeriag Review, October 1989 induced.currents in tha system,
- 2) the interconnected,sys-tems tend to be more stressed by large region-to.region transfers, combined with GIC which willsimultaneously turn
~very transformer in the bulk system into s large reactive power consumer snd harmonic current generator and 3) in general, large EHV transformers, static vsr comoensators sna relay systems sra mora susceptible to adverse influence snd microperatlon due to GIC.
TRANSFORMER OPERhTION The primacy concern with Gaomagnatically-Induced Cur-rents Is the effect that they have upon tha operatlan of large power transformers. The throe major effects produced by GIC in transformers is 1) the Increased var consumption ot the
~ffected transformer, 2) the increased even and odd harmon-ics generated by the half~ale saturation, and 3) the possl-bllhlea of equipment damaging stray f)ux heating. As is walt documented, the presence of even ~ smaN amount ot GIC l20 amp! or less) wiII cause a large power transformer to half~cle saturate. The half~ale aaturathn distorted excit-ing current la rich In even and odd harmonics which become introduced to the pawer system. The distortion ot the excit-ing current 4ao determines the re4 and reactive power re-qu)ramenta of the transformer. The saturation of the cora steaI, under half~a saturatfan, can cause stray flux to en-ter structurai tank members or current windings which hsa tha patentIU to produce severe transformer heatlna.
I5
c l
l
a>t,t',s,rci
- a:- 'nat s t g'e casse srs'"-.c.s pat'-:vc.'e sat"fate r. ucn
-cfe easiiv arc:0 s -'-'0";rearc ccgree:ran 0
- "rec cnase ni
- s.."esc:;af slcr~e's oracluce higher mag.
nituces ct hart aries era "crsuf e:arger amounts af reac.
'..Vc ower wears fl':oafca ivt'.h '.nfce onase aesigfls.
RELhY hVD PROTECTIVE SYSTEiIS stc ta It.re ~aces 0! fctav afta protective s~stcf.s:nat can be attf:bt tca to ggcomagnenc aistur-canccs:
gec egret'0 s:ctms wt'en:ncv are.t,.c. e" eie-'.
SUNSPOT CYCLES hVD CEOifhCVETIC DISTLRBh.iCE CYCLES On the average.
solar ac::vitv. as measurea bv:"s hur cer fnantnty sunsoots.
!ollaws an I 1 year cvc:e.
te "esef sunsoot cycle 22 naa its mif,imum in Seotemoer
',986, are is exoectea to beak in i 990-1991.
Gear, agne:ic 'ela 0 s.
turbar.ce cycles ao not have tne same snaoe as '.ne surscct number cvcies. even tnough they are cyc.'ical. 0'gufe
'. sno~s t.'le r.ature ot the sunspot numoers ana georlagnettc sc:.': s Fatse Oceratror. c!.."c ratectiof: system. such as nav-
~g occurrca 'or SVC. =spacitor ana!ine;clay coera.
- .cf s where '.,"c '(ow of
., armonic currer.ts are rnisin.
- erorctca ov '.ne ~e'av as a 'suit or overload conainon.
Itis is tne mast common failure moae.
~
Failure ',o Operate wnen an operation is desirable, this
."as shown to be a oroblem for transformer aifferential "rotecttan schemes ana
.'or situations in which the output ot the current transformer is distorted.
Slower than Oesired Operation. the presence of GIC can easily build-up high levels of offset or remanent flux in a current transformer. Thc high GIC induced off-set can significantly reduce the CT time.to.saturation far offset fault currents.
Most of the relay and protective system rnisoperatians that are ettributea to GIC are directly caused by some malfunc-tion aue to the harsh harinonic environment resulting from large power transformer half.cycle saturation. Current trans-former response errors are more difficultto directly associate with the GIC event. For example in the case of CT reman-ence, the CT resoonse error may not accur unttl several days after the GIC event that produced the remanence.
Therefore, these types of failures ere more difficultto substantiate.
CONCLUSIONS As evident by the March 13th blackout in the Hydro Quebec svstem and transformer heating failures in the eastern US, the power industry is facing an immediate and serious chal-lenge. The power industry is more susceptibl ~ then ever to the influence ot geomagnetic disturbances. And the industry will continue to become mare susceptible to this phename-non unless concerted efforts are made to develop mitigation techniques.
Geomagnetic Disturbance Causes And Power System EEects WmIssf
'tysz
~ 'sac Cyct ~ iy Cycto i ~
I ttumocr of
~ Otstutttcst OcyvYear aii
(
i00 I j
50ii Ctctc ie Cycta20 Cyct ~ zl t Sunspot Humttcf
~
i 0 t...
ta00 SS ca 50 sa da ds 70 7$
do W
ost at C'stwoco Pits asst Att f 2S i20
~ 00 i
~ do r d0 I
40 I
~ 20 I 0 00 assure
- 1. Vettetktne of the Yeerty.Avcfe0ett Sunspot Number cnts Qeomegnetieally Disturbed Days from 1 932-t 98d.
cycles from 1932 to 1988 I2, 3). tVote that the geonlagnetic disturbance cyclea can have a double peak. one of which can lag the sunspot cycle pack. While geomagnetic activity in tne present cycle is expected to maximize in approximately 1993-1994, severe geomagnetic storms can occur at any time during the cycle; the K-9 storm ot March 13, 1989 was a striking example.
EhRTHSURFhCE.POTENTIhL hND GEOMhGiVETIChLLYINDUCEDWURREViTS The auroral electrojats produce transient fluctuations in the eanh's magnetic field during magnetic storms. The earth is a conducting sphere and portions of it experience this time-varying magnetic field. resulting in an induced earth. surface-potential lESPI that can have values of 1.2 to 8 vaitsttkm l2 to 10 voitslmilel during severe geomagnetic storms in re-gions af low earth conductivity I4).
Vernon D. Albertsdgz University of Minnaota SOLhR ORIGINS OF CEOMhGitfETIC STORMS The solar wind ia a rarifled plasma of protons and ~lectrons emitted from the sun. The solar wind is affected by solar flarea. coronal holes, and disappearing filaments. and the so-lar wind particles interact with the earth's magnetic field to produce auroral currents. or auroral electyojets. that follow generally circular paths around the geomagnetic poles at al-titudes of 100 kilometers or more I1). The aurora borealis ia visual evidence of the auroral
~lactrojets in the northern 16 Electri power systems become exposed to the ESP through the grounded neutrals ot wye-connected transformers at the opposite ends of long transmission lines. as shown in Figure
- 2. The ESP acts as an Ideal voltage source impressed be-tween the grounded neutrals and haa a frequency of one to a faw mllliherts. The geomagnetlcaiiy-induced currents IGIC) are then determined by dividing the ESP by the equivalent dc resistance ot the paralleled transformer windlngs and line conductors. The GIC Is e quas&lrect current, and values in excess ot 100 amperea have bean measured in transformer neutrals.
POWER SYSTEM EFFECTS OF GIC The per phase GIC in power transformer windings can be IEEE Power Engineering Review, October 1989
l I
, t
C l i, i'
~ riT>> ti,APaCE T
EART$i.si.REAGE HTEPeT'al Pigura 2. induced Earth Surface Potential (ESP) Producing Qaomag naucaity induced Curianta (GIC1 in Power Sya$ ama.
many uivies targe! than tne RMS ac magnetizing current..e.
sut:ing;n a dc oias of transtcrmer core flux. as in Figure 3.
r r
I I
I I
r '.O.p)
I
~ r>>I'4'
!~$
tsuae;e av misoperat.on 5:,
REFEBEv'CES
- 1. Akaaptu.
S. t., "The Ovnamic Aurcra. 'c;cntd c '-"e'i.
'Vtagaz:na..%lay 1989. pp, 9p 97, 2.
'Oaaiyn.
'. A.. "Raai.Time Piadieppn Cr Oidcai 340mag, 1 ~
ACuvny Solar Wind.Magnatbapnara Cou par g 141, Terra Scianutlc Pubiianing Company, orvo. '996.
3 Thompson.
R. J.. "The Ampiituda ot solar c<eie 22.
Radio ai.d Space Services Tacnnicai Report TR 87 G3.
c::
bar 1987.
4 V. O. Alber.adn and J. A. Van 8aalen. "Electric and Magna F-aids at !ne Eanh's Surtaee dua ip Aurorai Curian!$. '
anaac::ona on Power Apparatus and Svsiema. vcl. PAS
-" =
No. 2. Aprd 197P. pp. 578-584.
5.
J.
G. Kappanman.
V.
O. Atbanapn, V. Monan, "C oar Tranatdrmar and Relay Perrormanca in tne Piesanca of ".sc magnaucailv Induced Currents." IEEE TianaachCna pn Pcwe Apparatus and Syatama.
Vol. PAS-1CO.
Vo. 3. pp. 1G78-1G88, March 1981.
The Hydro-Quebec System Blackout OE March 31, 1989 ia.pl l
ietC)$rtacma Rgure 3.
OC 8taa of Tranatonnar Care Rue Oue to QIC.
The half.cycle saturatian of transformers on a power system is the source of nearly all operating and equipment problems caused by GIC's during magnetic storms. The direct conse-quences ot the halt-cycle transformer saturation are:
~
The transformer becomes a rich source of oven and add harmonica
~
A great increase in inductive vera drawn by the trans-former
~
Possible drastic stray leakage flux effects in the trans-former with resulting excessive localized heating.
There are a number of effects due to tho gonoratfon of high levels of harmonica by system power transformers, includ-
- ing,
~
Overloading of capacitor banda
~
Possible mfsoporetfon of relays
~
Sustained overvoltegoa on Iangdfne energization
~
Higher secoadery erc currents during single. polo switchfny
~
Higher cfrceN breaker recovery vottago Overtoadfnl ef hormonic fftfyrsof HVOC converter ter-minals. and distortion in ttt'e ac voltage wave shape that may result In loss of dc power transmission.
The increased Inductive vera drawn by system transfarmera during halfwycfe seturatlan are sufficien to cause intoler-able system voltage doproaafon, unusual swinge in MW and MVARflow on transmission lines. and problems with gener-ator var limits In some instances.
fn additfon to the halt-cycle saturation of power trans-formers, high levels of GIC can produce e distarted response IEEE Power Engiaeerizig Review, October I989 Daniel Soulfer, Hydro-Quebec On March 13, 1989, an exceptionally intense megneuc storm caused seven Static Var Compensators ISVC) on the 735-kV network to trip or shut down. These compensstars are es-sential for voltage control and system stability. With their toss. voltage dropped and frequency increased.
This ted to system Instability and the trfpping of all the La Granae trans-mission lines thereby depriving the HQ system af 9500 MW of generation. The remaining power system catlapsed within seconds of tho lose of tho La Grande network. The system blackout affected aN but a few substations isolated onto lo-cal generating stations.
Power was gradually roatared over a nine haurs period. De-lays in restoring power were encounterea because of dam-aged equipment on tho La Grande network and problems with cold toad pickup.
SYSTEM CONDITION PRIOR TO THE EVENTS Total system gonoratfon prior to the events was 21500 MW.
mast ot it coming from remote power-generating stations dt La Grande, Manlcouagon and ChurchiN Falte.
Exports to neighboring Systems totalled 1848 MW of which 1352 MW were on OC interconnectiona. The 735-kV transmission net-work waa iodod at 90% ot ita stability limit.
SEQUENCE OF EVENTS At 2:45 a,m. on March 13. a very intense magnetic storm ted to the consequential trfp or shut down of seven SVC's.
Contafnlng the Impact ot the event through operator inter-vention waa Impossible aff SVC'a having tripped ot ceased to functfon wfthln a one minute potfad.
A fow seconds IB-8 s.] after the lose of tho test SVC, all five 735-kV Ifneo ot the La Grande transmission network trippea duo to an out of stop condition. Those fino trips deprived the system of 9500 MWof generation and subsequently ted to a complete system collapse.
17
1
~
C1
'k
p.
ii ir i
- cc: cr ~n' re~air irg '"--.' '
sni.: =cwn av caaac::cr
.ci:agc ~rcaiancc orc:cct
> "a:vsg harmonic =cntents.
H1frnonic Orator hC Vciill3e g(.35 k1 AC Current'at l6 kV TCB Srancne
.SC Branehe
.'00 "e
~ 'j 1 Pj 3%
~ ay
~ p 5 Ri
~
3.i Idio a 38 "r 24 "o l6 ~i 5%
16' 4 4F~
Quasi OC =urrcnts gcrcrataa by:he rnagnenc disturbance, saturatirg in tnc SVC couoiing trarstormers are thaught to be ti.e cause for sucn a iarge secana narmonic component of current in the TSC branch.
ui~rurOanCeS Un rOiVer Trar~clOr~erS Rc'ocrt J. Ringjce James R. Stewart Power Technology'cs Inc.
T'l s discussion aacrcsses the effects of gear".agr.et c = st "
"ar cea on power transformers.
The prir..arv affect:s =. c:c core saturation esuiting from geomagnet callv.rcuccc
= r rents. GICs. Core saturation can imoose severe temocratu c
problems in wincirgs. cods, tanx plate ana struct" rai ~er
~
oars ot transforn ers ana alace heavy var ana harnlcriC " r ~
cans on the power svstem ana voltage sucaort Cauiarr Cr.t.
GiC's of 10 to 100 amoeres are more:hen mere nu:sarcas
'n;he operation of vower transformers,;hc manr.cr cr "cw can result in saturanon ot the ccrc ana conseauent charges in svstem var requirements.
Increases in harmomc
= r.er'.
magnituaes, increasea transformer stray ana caav iasses.
ana problems with system voltage control.
GENERhL OBSERVhTIONS ON THE SYSTEM BEHhVIOR The system blackout wes caused by loss of all SVC on La Grance Networx. Seven SVC trippea or stopped functioning.
Prior to and auring the event all the OC intarcannecnons be-haved properly. Na relay false trips or misoperation of special protection systems ware absewed.
Tetecommunicatians were not attectea. No equipment damage wes directly attrib-utable to GIC but once the system split, some equipment wes damaged aue to load rejection overvaltagas.
REMEDIhL hCTIONS ThKEN Since the event, the following actions were implemented:
~
SVC protection circuits have been readjusted on four SVC's so as to render their operation reliable during magnetic storms similar work is being performed on the four remaining SVC's.
~
Energy, Mines and Resource Canada naw provides Hy-dro Queb6c with updated forecasts on the probability of magnetic disturbances.
Those forecasts are used by the System Control Center dispatcher to position tho transmission system within secure limits.
~
A.C. voltage asymmetry is monitored at four koy lo-cations on the system IBouchorvill~. Arnsud, I.G2, Chhtgeaguoy). Upon detection of o 3% voltage asym-metry at any one location, the system control center dispatcher is alarmed snd willimmediately take action to position system transfer levels within secure limits if this hasn't alroody boon done because of forecasted magnetic activity.
OPERhTINC LIhall DURING MhCNETIC DL'i1%INLLNCES (hND hLERT SITUhTIONS)
The following operatfng limits are now being applied;
~
$ 0% safety margin shall be applied on maximum trans-fer limits.
~
Maximum transfer limits shall not take into account the availability ot static componsators deemed unreliable.
~
Adjust the loading on HVOC circuits to be within the 40% to 90%, or loss, of tho normal fullload rating.
18 GIC EFFECTS VERSUS CORE hND WINDING CONFIGURhTIONS Principal concerns in this discussion are lor EHV syster.
s with grounded Y transformer banks providing conauc::rg pa'ths for GIC aild zero sequence cuffell'ts. Core aha winaing configurations respond difterently to zero sequence open cir-cuit currents and to GICa. Note: as used here. the term "open circuit"refers ta testa performed with all delta connecnons opened or "broken." For oxampl ~. the three. phase three leg core form transformers aro loss prone to GIC induced satu-ration than threo-phase shell form transformers.
But. barn care form and shell farm sing!~ phase transtormers are sus-captibl ~ to GIC inducsa saturation.
Winding and load arrangomonts respond differentlv to GIC induced core saturation as well. For example. the current ais.
tribution within pareil ~ I winding paths and within low voltage loads depends upan the leakage flux paths and mutual cou-pling, i.asses within windings and leads may change signiti-cantly under GIC.induced saturation owing to the change in magnetic field intensity, H, and the resultant changes in the boundary conditions far tho leakage field path.
EDDY LOSSES IN STEEL MEMBERS The changes in the magnetic intensity. H. and the magnetic boundory conditions resulting from the GIC excitatian bias can increase the louse in stool plato. the losses for fields parallel ta tho ptano ot the plato increase nearly aa the square of H. Note also that the level of losses increase approxi-mately as the square root ot the frequency ot H. owing to the
~tfoct ot depth of penetration. Tho magnetic field slang yoke clamps and log plates in core form transformers and in Tee beams and tank plate in shag form transformers closely matches the magnetic gradient in the core. Areas of the tank and caro clamps are subjected to tho winding leakage field.
If tho caro seturatn, tho magnetic fiHd impressed upon tho stool members mey rise ton to one hundred times normal duo to the soturotlan and the effect of the leakage field. The lossoo in tho stool members wNI rise hundreds of times nor-rnsl. even under half~ate seturetian. On the steel surfaces.
~ddy lose density moy rise ton to thirtywatts par square inch, approschtng the thermal tlux density ot an electric range elo.
merit+
Surface temperatures rise rapidly with this thermal flux ana can result in degradation of insulation touching tho steel IREE Power Engineerin Review, October l989
I F j
Design Deficiency Ocf ir lent vendor oanual UPS has no battery test circuit Vendor aanuat naintcnance section does not nention batteries.
Design Ocficicncy gal tef les have not been replaced in 6 ycaf's Design Deficiency AC lnpUI to logic poucf SLppty is as i>>t cnanc e preferred Back LSI batteries degfadcd of dead k.s ielay
<haiactcrlst ICS preventS transfer to IIIVefICI output.
Breaker lufIclion pcr desigii Ground fault occurs on B phase of aain trans forner VOItage transient cn $1st lon AC pouer SLppty AC pouer to logic oockilc lor UPSIA.O,G cspcflenccs the transient oUIPUI voltage goes lou logic trips on povef SLSIpt y failure.
2VBB UPSIA.B, C,O,G trip BI eakei s CB 125 open; Ch C
docs not close Iowa do not aulo tranyfer lo
~Liint.
s>>HII y toss ot att
~
loads on UPSIA D.G Isuit Is clcafcd in 6 cycles; transler cocpletcd in 12 cycles Per nlvvIVIL ohlbit CB 4 Iealei floe closing CB'C ni Ivh d Io I I I>>LICI SLIIIII.SIyq>ly lo i>>ve<<II oui INII
ROOT CAUSE REPORT FOR THE EXIDE UPS 1A, B, C, D, G TRIP EVENT OF AUGUST 13, 1991 SEPTEMBER 9, 1991 FILE CODE: NKP77748 John Co y
oot Cause ator Tech.
Support Jo arwees Root Cause Face.latator ISEG Engineer Reviewed Bys es R.
pa afore Program Director, ISEG
E t
E/
P This report has been generated to document the analysis of the root cause for the tripping of Uninterruptable Power Supplies (UPS) 2VBB-I..'S lA, B, C, D and G and the failure to transfer their loads to the maintenance supply.
This analysis was performed in accordance with NDP-16.01 by reviewing plant operator and damage control team observations and actions, performance of troubleshooting activities on in-plant equipment, review of various drawings, performance of laboratory diagnostic testing, consultation with the UPS manufacturer, review of data recorded during the event, and consultation with other industry experts.
AB TRA On August 13, 1991 at 5:48 AM an electrical fault on the B phase main step-up transformer occurred.
At that same time five (5) Exide Uninterruptable Power Supplies (UPS) tripped simultaneously.
Transfer of the UPS's loads to the maintenance power supplies did not occur.
The system conditions as documented by operators that were dispatched to restore the units immediately after the incident as well as observations by the System Engineer and other damage control team members indicated that the UPS's logic had tripped their input and output breakers.
Post event review of equipment drawings with the vendor revealed that the DC power supply which powers the system control logic normally draws its power from the maintenance power supply.
The inverter output is utilized as a backup source.
This scheme of connection allows transients on the AC power line to be transmitted to the DC logic power supply.
Tests performed by the System Engineer support this conclusion.
The bypass breaker CB-4 did not close and transfer the UPS loads to the maintenance supply.
This functioned per design since permissives for CBQ closure were not satisfied due to the degraded voltage conditions present on the maintenance supply.
The initial inspection of the units revealed that alarm indications on the five units were not identical. The inverter logic alarm light was not lit on UPS1G but was lit on A, B, and C.
The voltage difference alarm indication did not clear on 2 out of 5 units (Alarm should clear in 10-15 seconds after condition clears).
The over-voltage/undervoltage (OV/UV) alarm was present on 3 out of 5 units although all units should have displayed this alarm.
In addition, none of the 10 LEDs that indicate the initiating signals for a logic trip were lit on any of the UPS units.
Page 1
I N Qn August 13, 1991 at 5:48 AM a ground fault occurred on the B phase main transformer.
This event was detected and recorded on the Scriba Substation oscillograph.
Five Exide UPSs (2VBD-UPS 1A,B,C,D, and G) tripped during this event resulting in a loss of power to all their loads.
The results of observations by plant operators and damage control team personnel are summarized on Attachment l. All five UPS loads were initially restored to their maintenance supplies by plant operators after initiallyattempting (unsuccessfully) to restart the D unit. The damage control team was able to restart the C, D, and G units.
The A and B units were left on the maintenance supply because the damage control team was not successful in restarting those units.
As a result of these observations, it has been concluded that all five units shut down as a result of a logic initiated trip. This conclusion is based on the as found positions of breakers CB-1,2,3 on all five units and the presence of the module trip alarm on all the units except D which was reset by a plant operator while attempting to restart that unit. It is noted however, that none of the 10 LEDs on the A13A21 card which should indicate what condition caused the logic to trip were lit. In addition, two units (UPS1D, UPS1G) displayed voltage difference alarms.
This alarm indication should have cleared in 10-15 seconds after the plant operator>
manually restored the UPS loads to the maintenance supply.
The OV/UValarm indication was present on three units only, (UPS1C, UPS1D, UPS1G), although all units would be expected to display that alarm indication.
The inverter logic alarm light was not lit on UPS1G although it was lit on the other units that were not initially reset (UPS1A, 1B, and 1C).
Breaker CBA was found open on all five units.
A review of the oscillograph recording indicates that for the duration of the transformer fault (i.e., approx.
100 msec.) the B phase voltage of the station's normal AC distribution system decreased to approximately 50% of its normal level. It has been concluded that this condition prevented the automatic transfer of the UPS's loads to their maintenance supplies.
This is due to a logic feature which prevents static switch transfer to the maintenance supply under conditions that could cause damage to the connected loads.
The following potential causes for the simultaneous tripping of the five UPSs were evaluated:
(1)
'Voltage transient on the B phase of the normal AC distribution system (2) 5Wpigation of high hequency noise from the main transformer fault (3)
Voltage transient on the station ground system Page 2
a J
1 V
a
Transmission of high frequency noise from the transformer fault through the atmosphere to the UPS units could not have caused the UPS trips.
Preoperational testing demonstrated that the units are not sensitive to radio frequency (RF) transmissions unless the panel doors are open and an RF source is in close proximity. The report provided as Attachment 2 indicates that it is unlikely that high frequency noise could have been transmitted through the station's normal AC distribution system to the UPSs due to intervening transformers that would filter such a signal.
As a result, potential cause (2) is not considered credible.
Potential Cause (3) is considered unlikely due to the fact that one of the five UPSs is located in an area substantially away from the other four units yet exhibited similar behavior.
In addition, no other station equipment (including other UPSs) appears to have been affected by a ground transient.
Initial laboratory testing indicates that a significant ground transient would have caused the destruction of numerous logic circuit components which has not been observed in the field.
Further laboratory tests are being conducted in an attempt to identify the mechanism by which inconsistent alarm light indications were received.
Potential Cause (1) was investigated as the most probable cause.
Troubleshooting performed following the event to evaluate and demonstrate the validity of potential cause (1) indicated the following:
1)
The DC logic power supply for UPSs 1A, B, C, D, G is normally fed from tht; B phase of the maintenance supply with the inverter output supply as a backup.
2)
The trip point of the DC logic is at 17.3 VDC for UPS1D corresponding to 84.5 VAC on its input; and 16.9 VDC for UPS1C corresponding to 84.59 VAC on its input.'ew control batteries (fully charged) only provide approximately 18 VDC.
3)
Transfer to alternate power is accomplished via a K-5 relay.
K-5 relay drop out voltage is 45 VAC for UPS1C and pick up voltage is 52 VAC.
K-5 relay drop out voltage is 42 VAC for UPS1D and pick up voltage is 55 VAC.~
4)
Voltage transients generated during troubleshooting on the normal AC input power line feeding UPS1C did not trip the UPS.
- These mea.'nmmcnts were not repeated on the other units since the results were essentially the same for the C and D units and should not be any different for the A, B, and G units.
Page 3
l I
~
~
)i I ~
5)
The internal logic batteries on all five units were in a degraded condition and were not capable of sustaining proper logic voltage when all other sources were disconnected.
There is no way to determine that the batteries are in a degraded condition with the current UPS design during normal operation.
6)
Voltage transients injected (i.e., dropping AC input voltage to near zero for 100-200 msec.)
on the maintenance power line in combination with the degraded batteries affected the DC logic such that it tripped the units without allowing the K-5 relay to change state.
This was demonstrated on UPS1C and UPSlD.
7)
A sudden g~g loss of the maintenance supply voltage with both new and degraded batteries installed did gg cause the unit to trip. In this case, the logic power supply properly transferred to the inverter output and therefore prevented a trip.
8)
Voltage transients injected on the maintenance power line (i.e., similar to those utilized in 6) above) with good batteries installed did not produce any unit trips, although some voltage perturbations on the logic power supply were observed.
This was demonstrated on UPS1C and UPS1D.
9)
Fully charged batteries are required for successful K-5 relay transfer under some degraded voltage conditions on the maintenance line since other-wise the unit may trip on logic power supply failure < 16.9 VDC (84.5 VAC) before the K-5 relay willtransfer the logic power supply to the inverter output.
Laboratory testing is being conducted to more fully evaluate the condition of critical components and to investigate why none of the 10 LEDs were lit on the A13A21 board even though the logic was tripped. The pertinent results of this testing to date indicate the following:
1)
Significant ground voltage transients applied to certain circuit components causes their destruction.
2)
Injection of noise into the boards has not caused a trip signal to be generated.
Laboratory testing will continue to further investigate the inconsistent alarm light indications.
The outcome of this work is not expected to affect this root cause determination or the functionality ofthe UPSs.
Results of in-plant troubleshooting and laboratory testing to date indicate propel'bnction of the various alarms.
Page 4
I 7
I P
J
~ >
Ci II I ~ \\'
review of the UPS vendor manual resulted in the identification of the following deficiencies:
The vendor manual implies that the function of the batteries is to allow logic testing with no other input power available to the logic. This contributed to the system engineer not knowing that fully charged batteries could prevent a trip.
The following statement is from the vendor manual:
"A redundant logic supply, powered by the inverter output, a separate 120 VAC bypass source, and/or internal rechargeable sealed batteries, allows logic testing with no input power applied and keeps alarms indicating for as long as any source of AC control power is available."
The section of the vendor manual which describes preventive maintenance does not mention the logic batteries, In addition, the general description section of the manual states,
"(The batteries should be replaced at 4-year intervals)".
The 4-year replacement frequency is not satisfactory for service over the acceptable ambient temperature range specified for the UPSs.
The description of the logic power supply in the manual (shown below) is incorrect.
"These power supplies are powered through relay A27KI, which selects inverter output (preferred) or bypass (alternate) source."
As a result of discussions with the UPS vendor it has been determined that the logic backup batteries are not designed to mitigate a degraded voltage condition.
Additionally, the UPS design does not provide a battery test feature or allow for safe replacement of the batteries without removing the entire unit from service.
Removing the unit from service would result in de-energizing the connected loads.
1)
The main transformer fault caused a voltage drop on the maintenance supply to aH five UPS units.
2)
The degraded voltage on the maintenance supply caused the voltage on the UPS logic power supply to decrease below its trip setpoint causing the units to trip.
Page 5
t I
- I
3)
Automatic load transfer to the maintenance supply was prevented by design due to the degraded voltage conditions on the maintenance supply.
Th raotautuf th i
lt t ippi g ftt UPS
'i ~idge
~Qjiin. The UPS is not designed to accomodate a degraded voltage condition.
The following design deficiencies allowed the UPS logic power supply voltage to decrease below its trip setpoint as a result of the main step up transformer fault.
The logic power supply is normally energized from the maintenance supply with the inverter output as a backup instead of visa versa.
Under degraded voltage conditions the logic power supply switching circuit does not actuate until the supply voltage has decreased to well below the level that will cause the logic to trip.
5)
Fully charged batteries probably would have prevented the tripping of the UPSs even though that is not part of their design.
A TI N 1)
Modify the UPS logic power supply for units 1A,B,C,D, and G to be inverter preferred with maintenance backup prior to plant restart.
2)
Replace all UPS logic backup batteries prior to restart.
3), Prior to restart review other plant hardware which utilizes backup batteries and verif'y that appropriate replacement schedules exist for those applications.
Ensure any control functions dependent on batteries are identified prior to restart.
4)
Process appropriate changes to the UPS vendor manual to address the identified deficiencies.
1)
'Evaluate (post restart) further logic power supply modifications to rectify the K-5 mlay drop out characteristic problem and to provide easy access to the logic batteries for testing and replacement.
2)
Develop an appropriate replacement schedule for the logic batteries based on supplier recommendations, actual service conditions, and purpose of batteries.
Page 6
P I
AITACHMENTI Page 1 of 5 FER N TRAN IENT N A INP A.)
Operators responded to 2VBB-UPS lA, 1B, 1C, 1D, 1G and found the following:
1.)
'~~1A'
)
L281K 3 )
LZ51C'
)
LIEiQ2; a.)
b.)
c.)
d.)
e.)
f.)
g) h.)
a.)
b.)
c.)
d.)
e.)
f.)
g) h.)
a.)
b.)
c.)
d.)
e.)
f.)
g) h.)
i.)
a.)
b.)
c.)
d.)
e.)
f.)
g) h.)
i.)i)
Ic.)
CB-1 tripped CB-2 tripped CB-3 OPEN CB-4 OPEN AUTO restart CB-3 switch closed Module TRIP Inverter Logic Alarm CB-1 tripped CB-2 tripped CB-3 OPEN CB-4 OPEN AUTO restart CB-3 switch closed Module TRIP Inverter Logic Alarm CB-1 tripped CB-2 tripped CB-3 OPEN CB-4 OPEN AUTO restart CB-3 switch closed Module TRIP Inverter Logic Alarm OV/UV CB-1 tripped CB-2 tripped CB-3 OPEN CB-4 OPEN AUTO restart CB-3 switch closed No module TRIP No Logic TRIP OV/UV OV/UVTransfer Voltage Difference Page 7
pl I
r
ATTACHMENT1 Page 2 of 5 S.)
mala'
)
b.)
c.)
d.)
e.)
f.)
g.)
h.)
i.)
CB-1 tripped CB-2 tripped CB-3 OPEN CB-4 OPEN AUTO restart CB-3 switch closed Module TRIP Voltage Difference OV/UV B.)
The operators did the following manipulations in attempting to restore the UPS':
1 )
LEST'.)
b.)
c.)
d.)
Placed restart switch to MANUAL Placed the CB-3 toggle switch to OPEN position.
Reset the alarms LIFTED CB-4 MOTOR OPERATOR AND MANUALLYCLOSED CB-4. 'ee note 2.)
J2PRLB'.)
b.)
c.)
d.)
Closed CB-1 Closed CB-2 Reset the alarms LIFTED CB-4 MOTOR OPERATOR AND MANUALLYCLOSED CB-4.
~ see note 3) a.)
Placed restart switch to MANUAL b.)
Placed CB-3 toggle switch to OPEN position a)
LIFI'ED CB-4 MOTOR OPERATOR AND MANUALLYCLOSED CE4.
~ see note 4 )
IJP51I2'.)
b.)
c.)
d.)
Closed CB-1 Closed CB-2 Reset the alarms LIFTED CB-4 MOTOR OPERATOR AND MANUALLYCLOSED CB-4.
~ see note Page 8
I l.
ATTACHMI'KI' Page 3 of 5 a.)
Placed CB-3 toggle switch to OPEN position.
b.)
LIFTED CB-4 MOTOR OPERATOR AND MANUALLYCLOSED CB-4.
~ see note
- NOTE:
When the operators tried to restart UPS1D the procedure called out verifying that CB-4 was closed but it was open.
The operators made a decision to energize the UPS loads by manually closing CB-4 by first lifting the motor operator offof the breaker.
They restored each UPS in that same manner.
At approximately 0830 the system engineer went down with damage control team ¹3 (operators, electricians and I/C technician) to restore each UPS.
UPSIC:
Found CB-1, CB-2 tripped and CB-3 was open.
CB-4 was closed and the CB-4 motor operator (in the OFF position) was lifted off breaker Removed P6 plug from the CB-4 motor operator and aligned the motor operator to the ON position.
Reset all alarms.
Closed CB-1 and restarted the unit. It started up and "synced" to the maintenance supply.
Closed CB-2, restored P6 plug and reinstalled the motor operator for CB-4 back on the breaker.
Transferred the load to.UPS power and put transfer switch in AUTO position.
UPS1D:
Found CB-1, CB-2 closed and CB-'3 was open. CB-4 was closed and the CB-4 motor operator (in OFF position) was lifted off the breaker.
Removed P6 plug from the CB-4 motor operator and aligned the motor operator to the ON position.
Opened CB-1 and CB-2.
Closed CB-1 and restarted the unit. It started up and "synced" to the maintenance supply.
Closed CB-2, restored P6 plug and reinstalled motor operator for CB-4 back on breaker.
Attempted to transfer load to UPS power but CB-3 would not close.
It was found in tripped position.
CB-3 was reset, the motor operator was restored and the unit transferred to UPS power.
Put the transfer switch in AUTO position.
Page 9
I r
ATI'ACHMENT1 Page 4 of 5 Found CB-1 and CB-2 tripped and CB-3 was open.
CB-4 was closed and the CB-4 motor operator (in OFF position) was lifted off the breaker.
Removed the P6 plug from the CB4 motor operator and aligned the motor operator to the ON position.
Closed CB-1 and attempted to restart the unit.
Closing CB-1 caused an inrush to the UPS and tripped the upstream breaker, 2VBB-PNL301, breaker 41.
Reset breaker in 2VBB-PNL301 and reclosed CB-1 on UPS1A.
Upstream breaker tripped again.
Wrote WR (WR 0'62319) and Deficiency tag to repair Rectifier section of UPS1A.
Unit left with CB-4 closed.
Found CB-1, CB-2 closed and CB-3 open.
CB-4 was closed and the CB-4 motor operator (in OFF position) was lifted off breaker.
Removed P6 plug from the motor operator and aligned motor operator to ON position.
Opened CB-1 and CB-2, Closed CB-1 and restarted unit. It started up-and "synced" to the maintenance supply.
Closed CB-2, restored P6 plug and reinstalled motor operator for CB-4 back on breaker.
Attempted to transfer load to UPS power but CB-3 would not close. Itwas found in the tripped position.
CB-3 was reset, the motor operator was restored and attempted to transfer load to UPS power but CB-3 again would not close.
CB-3 cannot be reset due to a previously identified problem.
Unit left with CB-4 closed - on Maintenance supply power.
Note: WE4F 138173 exists to replace CB-3.
Page 10
4
(
J
ATTACHMENTI Page 5 of 5 UPS1G:
Found CB-1, CB-2 tripped and CB-3 open.
CB-4 was closed and the CB-4 motor operator (in OFF position) was lifted offbreaker.
Removed P6 plug from motor operator and aligned motor operator to ON position.
Reset all alarms.
Noted 575vac input to UPS.
Closed CB-1. When CB-1 was closed it tripped its upstream breaker in 2VBB-PNL301. Breaker f7 in 2VBB-PNL301 was reset and CB-1 reclosed (successfully).
The unit was restarted.
It started up and "synced" to the maintenance supply.
Closed CB-2, restored P6 plug.
When restoring the P6 block the CB-4 motor operator went to the OFF position.
Opened CB-2 and CB-1 and removed logic power from unit to reset all logic.
Reset motor operator on CBA to ON position.
Reclosed logic power, closed CB-1 and restarted UPS.
Unit started up and "synced" to the maintenance supply.
Closed CB-2, restored P6 plug and reinstalled the motor operator for CB-4 back on the breaker, Transferred load to UPS power and put transfer switch in the AUTO position.
NOTE:
When a trip signal is generated within the UPS it sends a shunt trip signal to both CB-1 and CB-2. It also sends an OFF signal to CB-3 and an ON signal to CA A voltage difference alarm will inhibit a closure of CB-4.
EVENT'
- W 2
- W UPS1A Normal AC (US3-B)
UPS1A Maint. Supply (US5)
X X
UPS1B Maint. Supply (US6)
X X
UPS1C Normal AC (US3-B)
UPS1C Maint..Supply (USS)
UPS1D Maint. Supply (US6)
UPS1G Normal AC (US3-B)
UPS1G Maint. Supply (US6)
X X
X X
X X
Page ll
t I
1 J
Niagara Mohawk Nine Mile Point Unit 2 Event of 13 August 1991 Report by:
Melvin L. Creasbaw Consu1ting Engineer Po~er Systems Engineering Department General Electric Company Schenectady, NY 8 September l991
I E
~'~g~~~ Mohawk Nine Mile Point Unit 2 Event of 13 August 1991 G5:48 On August 13, 1991. at 5:48 Abf the Unit 2 phase B generator step-up transformer failed. Oscillographic records of the event are available from a digital data recorder at the Scriba Substation. They show various 345 kV and 115 kU system voltages and currents.
Figure A with notations is attached.
The four cycles preceding the fault show no signs of a gradual degradation or a developing disturbance.
The oscillographic traces and station protecuve relay targets reported, indicate a ground fault occurred on the high voltage winding.
Depression of the 345 kV phase B bus voltage to about 39% of the prior value was observed from the oscillographic trace.
This suggests the involvement of only a portion of the entire winding. The 345 kV line currents and voltages show rapid development of the ground fault beginning at point 1 with the ground current reaching a constant value of 1,300 amperes in 1 1/2 cycles at point 4.
The flashover in the faulted transformer occurs just preceding a maximum in phase 2 to neutral voltage (as would have been expected) at point 2. Thc 345 kU line current in an unfaulted phase increases in step function manner to 350 lo of the prefault value at point 3.
Ao high speed recordings of voltages or currents within the plant were available.
No sequence of event recordings were available to correlate relay operation times.
Due to the large amount of magnetic energy coupling the generator rotor and stator, and known electrical parameters, the decay of fault current contributed by the generator to the solidly connected transformer would have spanned a number of seconds as the field decayed.
Relay operation targets reported were:
1.
Transformer Differential Relay (Type BDD) on Transformer 2MTX-XM1B.
2, Transformer Neutral Current Relay (Type IAC).
3.
Overall Unit Differential Relays (Type BDD) in phases 2 and 3.
4.
Generator Phase Overcurrent Relays (Type PJC) in phases 2 and 3.
I 4
P
Fv Following isolation of the generator and failed transformer from the power grid, marked 5 on Figure A, only a single 345 kV phase to ground voltage record is available.
The magnitude of this voltage on an unfaulted phase is 74% of the pre-fault value.
Since generator neutral current is limited to less than 8
- amperes, it is known that the faulted transformer appears as a line to line fault with some impedance to the generator.
By trial and error calculation, generator line currents are found to be 0, 1.9 and 1.9, multiples of the rated value of 31,140 amperes.
The line-to-line voltages have magnitudes 74% 74%, and 25%
of the rated value of 25,000 volts. The decay of this voltage for 0.25 seconds of the recording has a measured time constant of 2.7 seconds.
The calculated value of the impedance of the faulted transformer as seen by the generator is 0.23 per unit.
Conditions prevailing during the six cycle time period following the fault, marked 2 on Figure A, cannot bc determined with certainty.
The exact nature of the fault within the transformer is not known and the physical evidence will be strongly affected by the continued flow of energy from the generator due to the inherent time constant.
The flashover of only a portion of the HV winding is evident since the 345 line voltages to neutral remain at 39%, 86% and 86% of the pre-fault values.
The presence of "residual" in the measured 345 kV line currents provides the evidence of transformer neutral to ground current.
This requires that the. fault involves a path for current to ground from the high voltage winding.
Recorded voltages and currents show a step change to new values and no dramatic change during the time period of thc record, which totals somewhat less than 1/2 second.
It could be said they are "cleaner" and less distorted than commonly seen oscillograph recordings of faults.
Given these observations and since both the generator and the system were supplying fault current into the faulted transformer, generator line-to-line voltages preceding isolation would be expected to be greater than those immediately followingisolation.
It has been speculated that very high frequency energy (mHz region) may have caused malfunction of logic and control circuitry in the UPS equipment.
A broad. range of frequencies would be expected in any arcing phenomenon such as occurred in this failure.
Nothing in the available data or design parameters of thc plant equipment would suggest an extraordinary generation or propagation of higher frequency components.
The failure of a transformer and internal arcing is not a rare occurrence.
Comparison of oscillographic charts
II
from simiiar events in other plants show nothing unexpected or unusual h this particular failure.
It must be borne in mind that the sampling rate of the recorder is listed as 5.814 kHz and frequency components in excess of perhaps 500 Hz would not be accurately portrayed.
QE experience in testing of typical power transformers (such as the Lnit Auxiliaries Transformers) provides an indication of the expected coupling between windings at radio frequencies in the region of 1 megahertz:
The attenuation factors range from 1,000:
1 to 10's of thousands:
1.
Direct measurements could be made in this pLant to determine attenuation factors for individual transformers over a range of frequencies.
These tests would be made on non-energized transformers using an RF signal generator and a
sensitive, calibrated detector.
Attached recent articles on electro-magnetic interference.
Reference 1
discusses IEC 801.4 and the characteristics of electrically fast transients.
Reference 2 discusses testing of ground connections.
V 1
The possibility of elevation of thc station grounding system as a result of this disturbance was postulated.
The relatively high level of ground fault current, estimated at 1,300 amperes from the available recording, would not have been conducted into the plant. This current can only flow in from the 345 kV system for the 6 cycle period required for relay and circuit breaker operation to achieve isolation.
The generator ground current would have been limited to less than 8 amperes by the neutral grounding equipmcnt.
Elevation or differences in ground potential within the plant would therefore not have been expected during this event.
Reference 1
discusses the problem of achieving a "super" ground and concludes that a stable ground reference for interconnected equipment is of greater significance.
Since normally circulating ground currents are not
- expected, testing with very low voltages and currents is recommended.
Note especially thc recommendation to test with a frequency non-harmonically related to the power line frequency, Thc transformers stepping the voltage down to successively lower voltage levels are connected in a manner to minimize coupling of power frequency and higher frequency.
components betwccn thc various busses.
Specific configurations are:
(
J
l.
Formal Station Service Transformer-delta 25 kV to wye 13.8 kV with 400 ampere resistive grounding on the 13.8 kV side.
2.
Load Center Transformers-delta 13,8 kV to wye 4.16 kV with 400 ampere resistive pounding on the 4.16 kV side.
3.
Load Center Transformers-delta 13.8 kV or 4.16 kV to wye 600 volts with neutral solidly grounded on the 600 volt side.
4.
Reserve Station Service Transformers-wye 115 kV, delta 4,16 kV, wye 13.8 kV. The 13.8 kV neutral is 400 ampere resistive grounded.
The 4.16 kU circuit is connected to a zig-zag grounding transformer with a resistor in the neutral connection, presumably for 400 amperes.
These configurations provide "effectively grounded" distribution busses as defined in IEEE Standard 142 and will serve to limit transient over voltages.
This is in accordance with design practices deemed prudent and conservative within the power industry.
The industry continues to revie~ the effects of geomagnetic disturbances on power transformers.
While no evidence is seen of voltage distortion in the four cycles preceeding the failure, excessive duty could have occurred if these transformers had been subjected to low level direct current previously.
References 3 and 4 are attached for perusal.
t I
'I I
g/pQ
/J r
~
v W.
A 2/u 2/9 4/t} A p' D}/II}nn t C'/g
'I
~
~
L/~E w'/ 3 ~
2 LII}+2}
r,c'>} =- ~ /l
- c. 40 ~>>pc.
,~s d/ 4}/
v"VV'vv5 P} k5C rwAAh/WWVv<<'~hAVr<<rvw -~ r
'VVV IVV'dV I U',l'J(,.ug "X 5 l}HO 4>>pf. 3 C}I00 ig}l}pS C.}I}C"g$ Lave L,-c a >-o s ~} p'l I K 0 g ~ ~ I ~i J ~ ~:r ~ I r A. e I' }'l$.5'AV'}NO Jap S sr}}/}~L r~4r/S. C.]CO}I}I}} 7 /se pv ',.V v FlguRE 4
J ( I J
industriai Fquipment Electronics in indLlstrlai Applicatiens A Discussion of Fundamental ENlC Principles for Electronic Controllers in an industrial Environment By ~4'illiam D. Kimmel. PE Kimmei Gerke Associates. Ltd EMC prob!ems ~sth:ndustra! controls are aggravated by harsh en'..ronments. mixed
- ec.'".noiogies and a lack of uniform EMC guidelines.
T:"Js articie '~~il concentrate on
- he common aspects of eiectroiuc controls in an
!ndustnal eninronment. which is generaily mucn harsher;han the office environmen'.. What is the industrial environment and uhat can be dore about it.'.".e environn:ent includes the enure gamut of:he basic
- threats, power disturbances.
RFI. and ESD. RFI and power disturbances may be locally generated or not. Mixed technolo-gies compound the problem. Digital circuits are used to switch Line voltages via relays. Analog sensors are input devices:o digital controls. Increasing!y, there is a need for a
- ooperauve effort between the designers, manufacturers and installers to come up with a
rock.solid system. A common complaint is t.".at the instaUers or mainte-narce people won't follow the instaLition requirements. This may be true. but it must change. since there are problems which cannot be solved at the board levd. lt is also true that manufacturers often specify installation requirements which are not practical to implement, and there are documented cases where the prescribed installauon procedures will cause rather tlan cure a problent. The!ack of uaigacm guiddines haa ham-pered EMC ptagteas in the industria arena. Fortunately,'he European Commu-ruty is working to adopt the IEC 801.x specifications, and domestic companies would be wise to adopt them, even ifthere is no intention to export. The Basic Threats The three basic threats to industrial electronics are power disturbances. radio frequency interference. and ESD. Power Distu*ances. Power distur-EMC Test 8t Design bances are a weiJ known irdustnal problem. in lac:. when a problem occurs. Jie first thought is:o blame the power company. Often power quality is a problem (especia!iy if grounding issues are included). but the problem is almost aiways generated by adjacent equipment. Traditional problems with power include spikes and transients. sags and surges. aad outages, which threaten the e!ectromcs via the power supply. These problems are hirly well documented and are often solved using power conditioners or UPS. The most common power problems confronting electronics today is the sag which ~ically occurs during turn on and the spikes which typically occur during turn off of heavy inductive toads. ihe saga simply starve the electmnics. The high frequency transients barrel right through the supposedly FJtered power supply to attack the electronics inside. Digital circuits are most vubierab!e to spikes which cause data errors or worse. Aaahg circuits are most vu!aerab!e to continuous RF riding on top of the power. FIPS PUB 94 provides guidelines on dectrical power for commercial computers. This is good iinfonnation, but beware that hctory power is much noisier than commer-cial power. The guidehaes of IEC 801.4 specifies an electrically fast ~zansient (EFD that simu-lates arcing and other high speed noise. EFTs are quite short naged they diminish rapidly with distance due to induc-tance in the line. But at short range, they are devastating. Unfortunately. attention is placed on the front end of the electronics, the power supply. With industria controls, the pmb-iem is the controlled elements. If the electronics is controlling line power. the disturbances sneak in the back end where little or no protection exists. System ground, whi!e not being specifi-cally a power distu;barce prob'.em.:s: t the car;.'er of residual eifects nf po>> disturbances. Any 'indusinal or commer: structure has sigiuficant .'ow ~equen currents circulating:hrough the grec" system. sometimes because the nergy intentionally dumped onto the ground! s as with an arc weider) and someumi because of unintentional coupling or v'. an inadvertent connection between reu" and gmund somewhere in the!aciiity. Radio Frequency Interference. R. ~ dio frequency interference affects bc'. analog and digital circuits. with anaic circuits being generally more susceptible Surprising to many, the principle threat not the TV or FM stauon down the roac but nther it is the hand held t~snutte carried around by facilities personnel. A on watt ndio will result in an electnc fiie!d c Gve volts/meter at a one meter distance enough to upset many electronics systems IEC 801.3 specifies immunity to elec ". Gdds ofoae to ten volts per mete depending on the equiprrient, with tlire volts per meter being the level for typici equipment. As can be seen from the abov approxiraation, three volts per meter is nc an excessive requirement, and even:e: volts per meter is fairly modest. Electrostatic Diacharges. Electro static discharge is an intense short durauor puhe. having a riserime of about one nanosecond. This is equivalent:o a burst of 300 MHa iaterfereace. Static buildups of 15 kV are not uncoauaon. Dty dimates, induding'northern climates SKen Kiramefis a pn'napa) with Kimmei Gerke Associates, Lttf. The finn special-iaet in prereating aixt solving electromag-netic interference and compaab8ity fEMII EMC) proble/ns. Mr. Kiinmei can be reacherf at 1544 N pascal, St. Paul,.')fN $5108, or telephone 612 330-3728. 35
4 I l I+I
Vcc line pauer I, I I TRIAC suitct.e pouer F g.re '.. Amp"Ger dentoduianan. Figu:e 2. Transient!eedback path.
- n w;nter.
offer npportuiuty !or ESD. Induscnai environments. with the:r moving equipmenc. are loaded with potenual ESD sources: rubber rogers. belts. and produc-non ou'.put such as plasnc and paper raUs. aU add up!o a.cal ESD threat. and vis t."meat is more likely to occur even m re!anvely moist environments. Look:o IEC 80l.2.'or ESD standards. Electronics Design Electronics is generaUy the ultimate victim of interference. The biter'.erence Gnds its way through various paths:o thc electrorucs equipment itself. Let's concen-trate on what can nappen to your electronics from the back door. that is. by direct radiation into the electronics and by con-ducted ',".terference through:be signal and cont:oi tines. Sensors. Low level sensors. such as thermocouples. pressure sensors. etc.. are characterized by very low bandwidths and !ow signal!evels. A major Jireat to these sensors is radio frequency interference. either from nearby hand hekl transmitters or more distance land mobile or Gxed trails mit re f5 ~ But these are high frequency, much above thc bandpass of your amphGer, right? Wrong! Low frequency ampliGers are plagued by two,ybeaomena: out of band response and <<ztao rcctification. These combine to prerkde false information on levels to the system. AU amplUiers have a normal bandpass, typi6ed by a 20 dB/decade roUotf or more at thc high end. But resonances due to stray inductance and capacitance willgive rise to amplificr response 6vc arders of magnitude or more above the nominal bandpass of the amplifier. This means an audio ampliGer will respond to signals in the hundreds of MHz. The second aspect occurs when RF er cour ters a noriincarity such as a semicon-ductor device. AU such devices give rise to a DC level shift when confronted with RF. fn a radio receiver they are called detec-tors. Nongnearicies are mininuzcd in linear devices. but:here is always enough to cause problems. The upshot is that tbe ampliGcr demodulates the RF, generates an errone-ous signal. and passes this error on. This effect is shown in Figurc l. Output hncs are similarly affected, with capacitive couphng back to the input. The soluuon is to prevent the RF from getting to thc ampli6er. either by shielding or nltering. The most common path to thc ampli6er is via an external signa! line fram thc sensor. but if the electronics is not shielded. direct radiation to the circuit board may also present a problem. Assuming filteringis the sdectcd method, use a high z'equency 6lter, designed to bkck signals up to I GHz or even morc. Use femtes and high frequency capacitot3. Do not rely on your low frequency Glter to take out RF. At the op amp. you shouM also decouple your plus and minus power to ground at tbe chip. Ifyour ground is carzying RF, you can anticipate che same problem mentioned
- above, since it will corzupc the reference level.
Data Lines. Digital data lines will bc upset by the RF problem as in mabg, but tbe levels necessary to upset are higher.
- Instead, digital data linn arc much more susceptible to transient ghtchcs. AI signal lines should be 6!tered to pass only the frequencies necessary for operation. It the threat lies in the bandpass of the signal.
then shielding or optical links wiU be needed. Switched Power Lines. This refers specificaUY to the power being controUed by the controger device. Industrial control-lers are commonly tasked to control power to heavy equipment. which!s ->~c.e-- bY heavy starting.'oads and iaducuve at turn otf. 7ypicaUY the e!ecc anic can switch linc power using relays or;;.:. This exposes tbe back end of the conc:" to substantial line transients. which cai.;, back co tbe circuit power and ground 'isrupt tbc digital circuitly as shown Figure 2. It is mandatory that the transient nts be diverted or blocked. since digital system cannot withstand t.".e..~~, tudes likely to occur with an inducuve '.~c, unless special steps arc taken. Sdf jamming can be linuted by contra!L: when you switch !he Unc, using:e crossing devices. Of partic~ importan< is thc turn off. since ~liat is when 'nducive kick occurs. lfaU power switching used zero crossir devices. the transient levels in the facto. wouM be dramaticaily reduced. Unfor: natdy, that goal is well otf in the tucuc Until then. expect that high voltage paw transients vnlloccur, and they must be de with. Optical couplers and relays do not provi sutGcicnt isohtion by themselves. Th~ high capacitance provides an cxccgent hii frequency path, and if they arc stacked; in an array. che capacitance wiU add up pass surprisingly low frequencies. Thes ca$ 0Qtanccs c711't bc eliminated. hul Yo can design your control circuits to minimiz ceipbng paths and to maximize low impec ance alternate paths. Transient suppressors should be instaUe at the had, which is the source of the spike but they can be installed at the controge as weQ, An interesting effect occurs when corn j bining zero crossing SCR regulators wit. low icvd sensors which use line frequerc' noise cancding techniques. Verv sensiiv l sensors sometimes are samp!ed.!or z: ~ July'August '.a9,
4 ~ ~ lg
'I80 VAC HI)9h Current DC , 'ico u Rover I PS Electr onics Fig 're 3. Common industnai powe. supply. Figure 4. ~!ulnple ground paths. enure power cycle '.o;ance! the ine freq'ncy omponent. If the samp!e occurs concu-..ently with ire power svntctung on or off.:he average:o the sensor will be
- pse'!. and an e..or nil "e recorded.
System Design and Installation Once the elecuanics is designed. it becomes a problem of:he system integrator anii insta!Ier!o ersure that thc electronics is provided with tre environment for which it was designed. Most of the arne, this work is performed by power experts and electricians, and they arc not always aware of;he interference problem. Oftea, on site, the power quality is blamed for the equip-rrent anomalies. But the problem can often be avoided by fo!lowing a few basic princt. ples. The industrial control device is either inte~ted into a system at!he factory or instaLed separately on site. Controllers hardie a varety of devices such as motor speed controls. positioning devices. weld-ers. etc. Interference presented to the electronics can be signi6cantly reduced by appropriate measures outside of the elec-tronics box. There is no way to accurately assess thc threat without test data. But regardless of the Narration avaiiab!e, much can be accomplished by correct instagatian, and it doesn't cost much if done at'he start. Reuo6ts become costly, especial!y if ac-companied with factoty down time. Let's consider)twsc same problems from a system standyyiat. Your goal is to limit the interference which must be handled by the elecuotucs. Du'cct radiaaon to the clcctroiacs ls not often a problem in an industrial environ-ment. but it does occur. and most often with a plastic enclosure. The NEMA enclosures pravide enough shielding !or most indusuial needs. Ifyou don't want to use a metal enclosure, be sure '.o get electronics which will withstand the RF whch willoccur. EDDIC Test 8c De,cn .')fore ores !he problem is conducted. either aa power or grourd. The problem occurs d;e '.o power and g.ound distur-bances caused by the equipment. It is an all tao common pracr:ce to draw controller power f;om thc same source as feeds the power eqtupment. T."is power may provide '.he i.ecessary energy to drive the equip-ment. but it!s not suitable to power the electroiucs lFigure 3). Hopefully. all industrial equipment will have elecuonics powered from a separate low power 120 volt circuit. It solves several problems. First, it separates thc electron-ics power from the probably very noisy industry grade
- power, preventing the switching transients and startup sags fram getting to the electronics.
Second. if it is necessary to condiuon the electronics power from an exterrA problem, it is far cheaper to condition the watts needed forelectronics power than it is to condihon the kilowatts required by the system. It power cannot be separated. then it is neccssazy to provide a bulletproof power supply. preferably inchding an iso!ation transformer. to sc;parate thc entire power supply from the electrical equipmcnt. Ground Noise. Ground noise, inevita-ble in industrial environments. must be diverted from the electronics module. Multiple grounds in a system wGI often result in ground currents circulating through the equipment. and ground noise circtdating through the electronics path will cause iaalfuncdon. Figure 4 showa some typical ground loop situations. Acomman approach is to demand a super earth ground. This is good, but it is not a cure all, and often a super ground cannot be achieved, no matter how you try. How'o you get a super ground from thc third 4oor? The real need is to get a stable ground reference to all interconnected equipments. If this equipmcnt is clasely located. then a very low impedance interconnect is feasi-ble. Power conditioners are often tasked to egminate RF or g.-ound noise..;,at work. but:hesc prob!ems can be sc. with an!solauon uansformer to neutrd to ground noise and wth EMI pc. line 6ltcrs. So yau may wan'.;o try inexpensive approach nrst. Data I.inks. Data links are suung over the ertire!acu'ty. exposing them two principle effects. ground roise arc pickup. Ground noise weal cause data er. unless the electronics has been designe" accommodate potential differences of -:) eral volts or more. This is accompbsr with differential drivers and receivers if='. must be direct coupled. Optical gnks 'ventually take over these links. The other aspect is RF pickup. Inex pe sive shielded cable is suitable for:! purpose. Ground both ends! Do not ap~ single pomt ground techniques to RF.:: Iaw frequency ground loop problem )s threat. thea one end can be capaciuve grounded. 8uttttnctry Industrial elecuoaics arc subjected:c harsh environment. Good design and inst. lation techniques willminimize problems the 6eld. Adherence to the Europe standards. IEC 801.x is a good start, cv ifyou are only markcung in the USA. Bibliogrctphy FIPS PUB 94, Guideline on E!Cc'ac Power for ADP Installacions, Septcmbe: 1983. IEC 801-2. Electromagnetic campatibiTit for industria-process measurement an control equipmcnt, Electrostatic discharg requirements, 1984. IEC 801-3. Electromagnet!c compatibiTit tof industrial. process measuremcnt an coattaI equipment, Radiated elecuomag cetic 6eld requirements, 1984. IEC 8018, Electromagnetic compatibi!it; for industrial.process measurement ar.: control equipmcnt. Electrical fast:ransient burst requirements. l984.
c P V
industrial Equipment Equipment Ground Bonding Designing for Performance and Life A Discussion of Ground Connection Fundamentals to Control EMl By D.B.L. Durham Dytecna Ltd. UK The problem of achieving sausfactory earth bonds or ground connecuons has plagued EMC engineers for many years, not only because the bonds are often vital for the achievement of satisfactory equipment per'- formance but because they affect che long term performance of equipment after it has been inuoduced into service. Recommendations on bonding have ex-isted in the form of military spcci6cations. such as Mil Std 1310, Mil 188-124A and Mil-B.5087 (ASG) for some years and these have generally proved satisfactory for most new builds. However. these speci6cations have certam limitaaons in that they gener-al!y do not specify consistently low levels of bond impedance. nor a suitabk test method. The introduction of new'MC spcct5cations in Europe with thc EEC Directive on EMC and the requirements for iong term stabihty in EMC characteristics has directed the UK nahtary to review existing specifications and introduce a new Defence Standard to tighten up perform-ance requirements for military equipment. Def Stan N4 (Part. 1)/1 has been intro-duced to address 5W area as far as mobile and transportable ctxttmuaications installa-tions are conc~ bat thc requirements shoukl have unphcatkes in industrial apph-catioas and over the whok dectronics market iflong term product performance is to be guaranteed. Bond Degradatlon Earth or ground bonds are generally considered essential not only for safety
- reasons, but as a means of divcrthg EMl currents.
"locking" circuit boards and 38 equiprncnt to a stable ground poet. actuev. ing adequate levels ofcabk shickhng aad for many other reasons. Many designers un- . derstand the requirement for short, fat bond leads to minimize ground inductance, but fcw appreciate that a criacal aspect is the connection resistance with which the bond strap is attached to thc equipmeat gmuad point. Ae basic requirement of any bond is that it should have as Iow an impedance as poss)ble (unless it is a dehberatc induc-tive bond to limit ground currents). The impcdancc, is a combinaaoa of the resistive and the inductive components, The resis-avc element is a function of the bond strap resistivity, cross secticmal area and length, scc Equation 1, wtuist the inductive compo-nent is a more complex function ofthc bond strap characteristics as shown in Equation 2. R qt Q A
- l. ~
L In + 05 + 02235 2t b+c 2.L b c 2f (2) where R rcsistancc, q < resistivity, f> length. A ~ area, li, ~ pcrmeabiEty of free
- space, L ~
hductaacc, tt, ~ relative permeabihty, b strap width, aad c ~ strap ttuckaess. The frequency at which the inductive dement domutates the impedance expres-sion when calculating the total inductance is. from Equation 3, typically 1 kH!. lt wg! be seen therefore that to a8 intents aad purposes the bond except at DC and power frequencies, may be assumed to be an inductance. At very high ~equcncies:.- stray capacitance across the strap dommatc. This means that the volt '-; across a bond is generally a funcdon inductance and frequency. Based on Ohr.. Law'his volt drop is shown in Equadon For transients the voltage drop is given. Equation 5. Z R' cu'L' ~ IZ ~ jcuLI V ~ -L dl dt (5 where Z ~ strap impedance, e radia frequency, Y ~ voltage, and I ~ current. From this. the higher thc inductance the mote isolated the circuit or box become: from ground. This can have sitputlcan. effects on equipment, inchiding enhance. ment of noise ittjection onto carcuits, reduc. boa of Ster perfonnance. aad loss oi coataxmiation range. From a TEMPEST standpotnt it may result ia more radiation from equiparant. It would seem from this that thc 'criter for any bond is thc iaductatee aad hence the choice ofshort fat David Durhaia served for 22 years ia the Britnh Aany, where hc gained his degree in cfcctricaf eagiacen'ag. cQter service in a vs'ty of appciatiacnts hr retired to Join tht RaalSES company as the Technical Nalger respoasihlc for the design and dcveloptaeat of coauncaa'cation systems. ln 1985 hc pxned Dyrccna as the Manager of the Rtgitccring Division. and now is currcatty Technic Markea'ag Manage.. July(August !991
4 ) J
st-l 'c s>s~llv s ) Qkp yl5pcl 'ggeC4i tsar eaten: 'i:!0 >0 ArCs QJ' Als+sQ wJswcii I / scarc s>>p VOl.TAGE REMEASUREMENT Figure!. Bond resistance. Figure 2. Four iire bridge method. bond straps. However. an analysis of:he bond inductance shows chat for a bond strap of 100 mm!ong, 15 mm ~~de and 2 rnm thck the impedance at 1 MHz will be 3.8 Ohms. it sounds extremely sin:pie. but work performed in!he USA'nd L'K shows that if an error is vade in the way the strap is ternunated then a progressive increase in
- he resistance of the bond strap to box juncuon can occur as the equipment ages.
Eventually the resistance will begin to exceed hundreds of ohms and may eventu-ally go open c cuit. This can negate the effect of the bond strap completely as part of the EMI protecdon. ~Vhat happens with bonds to cause this change? Essentially a ground connection is a series of irrpedances from the strap through to the ground material, as shown in Figure l. Each point of contact contrib-utes to the total bond pcr.'otmance. As a result. a change in any contact condition can result in a change in the total bond resistance. As is weil appreciated, the contact resistance between two metal sur-faces is a function of the pressure. The pressure extr.cd by the tip of a drawmg pin is vasdy greater than that from the thumb pressing by itself. Thus the contact fram a sharp point gives a aech higher pressure t!lan a Qat point and Qgehre lower contact resistance. Measuretlmka have showa chat sharp points enable oosstacc resistance of a few microohm to be achieved whilst similar pressures on liat surfaces result in mB-liohms of contact resistance. It night be felt that there is little or no difference between these vahes. but in reality there is. An essential aspect of a good bond is that it should remain so after the equipment has entered use. High pressures also have the effect ofsqueezing out corrosive materi-als and insulaung 6lms. The (armer causes EMC Test & Design progressive degradation nf bonds. whilst the latter can reduce the ef"ciency of tht bond from the moment it is installed. It is parucularly important in communications
- systems, wi:ere Sters are insta0ed and shielded cable ternunau'ons are made Jlac the bords are of low resistance and retain their performance.
Bond Performance and Measurement Experience has shown over a number of years that for long teem consistent bond performance a low value of resistance raust be achieved. This is typically 1-5 milhohms. In Def Stan 584 (Part 1)/1 the vahie has been set at a maximum of 2 miUiohms. This level is measured through the individual bonds. The logic behind this level is twofold. Firstly. experience has shown that with communications equipment in particu-lar this value of bond resistance is required ifconsistent perfonnance is to be achieved in terms of reception ef6ciency and trans-mission characteristics. This is paructtiariy so for TEMPEST protected equipments. The secoad point is thac if the bond haa a higher resistance then there is a signi6caat likelihood that progressive dtgradauon wi1I occur aad the bond resistance wGI increase in value. There will then be a progressive loss in performance. The main problem with measuring bond resistances is that it shouM be measured usiag a low voltage/current ttchtliquc. Most techniques to date for assessing safety involves driving a large current through tht bond. This checks the bond's aMity to cany current but does aot necessarily check its EMI protection performance. The rea-son is that many bonds may when in normal use have a high resistance due to oxide aad greasy Sms. buc when subjected to a high current the layers Stat up and are vapo-rised. After the current is removed tl;e Snl can retutn. Thus high current techruques are not recommended !or:esung EIII bonds. The new Defence Standard in the UK speci6es a maximum probe voitage oi 100 microvolts. This represents typicaQy s probe current of 50 milliamps under shoe circuit (( 1 mQ) condiuons. This is insuf6cieat to destroy surface 6lms. The chssic method for measuring low resistance has been to ust a four:erminal bndge as shown ia Figure 2. In this case the current is driven between two points and ke voltage across the sample is measured with a high resistance probe. This removes Jie effects of the probe contact resistance and lead resistance. This is generally consid-ered to be a laboratory method as the use offour contacts can be awkward. Ifthe lead resistance caa be removed by a calibrauon ttclmiqut then the four terminals may be replaced with a two terminal system. h fuzcher possible re6ncment to tile ttchaique is to use a frequency that is not DC or 50/60/400Hz. h this cast 10.4 Hz haa bcca chosen. Ifan acuve 6ltcf ls Used to Ster out all other electrica noise, then it is poseble to use the bond resistance meter on powered up systems. It is wonh notaig that at this frequency the impedance ia st81 largely represented by resistance rather than inductance. The two tennmal method is shown in Figure 3. The iatroductioa of new EMC/EMI speci6catioas in Europe haa made it more important that oace made the bonds have consistent long tenn perfonnance. This means measuring on periodic inspecuon and ahtr maintenance. It is an essential aspect of iasurmg consistent performance. lt les been shown that within months apparcn Jy good bonds can deteriorate to high resis-
~ 4 J C
RiicN'i 'i.ccr oN ,0'" E EXSUR EN. ~iles ossls lilacs 0 FlXEQ RK5iSTANCE LEADS Figure 3. Two termmal bridge method. 11?0 1? t.incoln Avenue. Foibrook. 4Y 11T41 (%16) ~8S.1400 FAX (16.>85.H14 40 tNFQICARO 29 4 y, ~ a ~ jp+ o" @4~ +c ~((o~qg4 ~ y yt (o Q oc- + CA)lnptialN'0 p c~ e <( zoc' o(o p+ refills CoastiaiTerm. %~(4+ < ~c<c (4~ anne<tars, yO Cata ~ye+ q+ .quest tance. Therefore t'.i:,ces,>>'.-.-= subtec'. to tesung ~rd -.xa.-..:.-.::;cu -- ss a maintenance tas~. I:K~Iilitary Experience There haie oeen .>o ~~)cr... caused by poor bonds expe.".ended by mi4:ary equ:precut;sers.. he.'=;;:. degradation .n perfcrmacce .';ead: tioned in:his a.-.icte. The:oss i crr. cauon range. poor E~tt pe.".'o....ance other effects aL'on.'nbute io a cons:cert reduction m equipctent Ec;ercy and s ~ ability. The secord ef'.ect inich is difficultto:denufy:s hat aii~o Fault Fc" (NFR problems. An aratysis o! reuo; (ailurcs from mihtary relia".i.'ity data shown that NFF inc:dents can oe extre.-. high. particularly '.n hunud climates. has been partiallyconfirtnedby repans!-. the Gulf War when all!orces repor.e"- increase:n avaitabitity of equipn:ert 'ricr chmatc. Many fav'ts are due to = etcctricat contacts in conrec!ors, bu't a lac. number have beer. idenuficd as excess EMI induced throvgh poor ground oon-This may be caused by either a loose groi. strap or connector tenr4nauon to:he ':c A significant improvcmcnt '.n equipme: availability and perfonnance is expec: when more recent statisucs are analysed. The introducuon:nto the Bnusn Arun service of kc Dytecr~ Bond Resistanc Test Set DT I09 has enabled the L':- mihtary to rncasure bond resistances:: installed equipment and redvce;he curaaccs of NFF errors. The UK m'::a.": measurement procedure vses a two:er...: nal bridge method and an accurate miUiohm calibration standard. This meas urement procedure and equipment is als in use by other NATO nauons and e!se where by militaryand naval forces who hav recognised the same problem. Conclusions The problems with ground bonds have become sigtiificant with the devetopment o: sensitive and secure communications equip-ment. This coupled with an increasing reed to achieve higher and higher levels of EMf protection has lead to an increased emphasis being placed on thc effectiveness ofail types of system grounds.
- These, further com-bined with a requirement to ensure the tong life of systems once in service.
have resulted tn the assessment that bonds and terminations are one of the primary causes of EMI falurcs in systems. The require-ment to test these is clear. however the means to do so have not always been available to engineers. jutv.Aug.'st ".oo'-
I (
Panel Session Induced currents In the system,
- 2) the interconnected sys-tems tend to be more stressed by large region-to region transfers, combined wfth GIC which willsimultaneously turn every transformer in the bulk system inta a large reactive power consumer and harmonic current generator and 3) in general, large EHY transformers, static var compensators and relay systems are more susceptible to adverse Influence and microparation due to QIC.
John C. Kappenman, Minnesota Power The effects of Solar-Geomagnetic Disturbances have been observed for decades.on power systems. However. the pro-tound impact of the March 13, 1989 geomagnetic distur-bance has created a much greater level of concern about the phenomena in the power industry. Several man madegratems have suffered dlsruptlans ta their normal operation 4sl to the occurrence ot geomagnetic phe-nomena. Most of~ man~e systems, such aa commu-nications, have blart made less susceptible to the phenom-ena through technotoglcU evolution (microwave and fiber-optlc have replaced metallic wire systems). However, the bulk transmissfon system, If anything, ia more susceptibf ~ today than ever befar>> ta geamagnatic disturbance events. And if the present trends continue, it ls Ifkefythe bulk trans-mission network willbecame more susceptible In the future. Same ot the moat concerning trends are: 1) Tha transmfssion systems of today span greater distances ot earth-surfaca-potential which result In the flow of larger gaomagnetically-IREE Power Etta'rteeriag Rcvtcw, October )989 TRhNSFORMER OPERhTION The primary concern with Gaomagnetfcalfy-Induced Cur-rents Ia the effect that they have upon tha operation of largo power tran!formers, The three malar effects produced by GIC in transformers ia 1) the Increased var consumption at the affected transformer, 2) tha increased oven and odd harmon-ics generated by the half.cycle saturation, and 3) the possi-bilities ot equipment damaging stray flux heating, As is well documented, the presence ot even a small amount ot GIC I20 ampa or less) wN cause a large power transformer to half-cyc/e saturate. The haffcycfe saturation distorted excit-ing current ls rich In evan and odd harmonica which become introduced to the power system. The distortion of the excit-ing current dso determines the real and reactive power re-quirements ot the transformer. The saturation of the core steel, under haft~cia saturat)an, can causa stray flux to en-ter structural tank members or current windlngs which has the potentfU to produce severe transformer heatlna. I5 PES Summer Meeting, July N, 188$ Long Beach, California John G. Kappenmaa, Chairman Power System Susceptibility To Geomagnetic Disturbances: Present And Future Concerns
C f1
- ccs, e 'ic:est esuits irai ~
- 3te '.Pat s>ngie "rase: ar 5!cr~ers naif cvcie saturate much
~are eaailV aha >O 5 -uCn 9reater aegree;han Comparable
- nree.anase units.
~ "..ese .ansformers Produce higher mag. nituces of harmonics ara corsume larger amour.ts of reac.
- ive power when cary>pared with three pnase aesigris.
RELAY AND PROTECTIVE SYSTEM(S Yi eie are:hree basic fatlire modes of relay and Protective svs:ems:nat can be attributea to ggeomagnetic aistur-cances: air' ai 'C ~ 51 ere 9eoriagretic storms when they are "'u.'r' ~ seve... SUNSPOT CYCLES AND GEOMACVETIC DISTLRSANCE CYCLES On the average, solar acnvity. as measured by tre nur ber manthiy sunspots, follows an 11 year cycle. >he presert sunspot cycle 22 bed its minimum in Seotemaer '.986, arc is expected to oeak in 1990-199'f. Geomagnetic "ela a,s. turbance cycies do not have the same shape as '.ne sunscot number cycles, even though they are cyc'ical. F gure I snows the nature ot the sunspot numbers ana geomagnetic 3ct>'v '.v ~ False Operation of:."e protection system. such as hav-ing occurred 'or SVC. capacitor and line relay ooera-tians where '.l"e.flow of harmonic currents are rnisin.
- eraretea ov the re'av as a fault or overload conditian.
This is the most common failure mode. ~ Failure to Operate when an operation is desirabfe, this has shown to be 0 Problem for transformer differential protection schemes and for snuations in which the output of the currant transformer is distorted. ~ Slower than Desired Operation, the presence of GIC can easily build.up high levels of offset or remanent flux in a current transformer. The high GIC induced off-set can significantly reduce the CT time-to.saturation for offset fault currents. Most of tha relay and protective system rnisoperatians that are attributed to GtC are directly caused by some matfunc. tian aue to the harsh harmonic environment rasuiting from large power transformer half-cycle saturation. Current trans-farmer response errors are more difficultto directly associate with the GtC event. For exampte in the casa of CT reman-ence, the CT response error may not occur until several days after the GIC event that praduced the remanence. Therefore. these types of failures are more difficultto substantiate. CONCLUSIONS As evident by the March 13th blackout in the Hydro Quebec system and transformer heating failures in the eastern US, the power industry is facing en immediate and sarfous chal-lenge. The power industry is more susceptible than ever to the influence of geomagnetic disturbances. And the industry will continue to became mare susceptibl ~ to this phenome-non unless concerted efforts are made to devetop mitigation techniques. Geomagnetic Disturbance Causes And Power System EEects uu>r oer at Cleiuroea Ceyv+eer Ap c 25 Numl>er 1532 ~ 1550 Cycie 17 Cycle 15 I i Humoer at ~ aleiuioed oeyv Year 150 I I le 19 Cycl~ 20 Cyci~ 21 I Cyc I ~ ice
- 120 Sune poi rrumoer
> 'ioo ~ 30 I ~ 50 I 40 I ~ 20 I ~ I I> >i \\i Ir I I 100 j 50!, 0 ~ ~ ~ ~ ' > ~ ~ ~ ~ i ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ > ~ ~ ~ ~ ' ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ 0 1 050 35 eo cs 50 55 50 55 70 75 50 55 So fifgure 1. Vartatlons of the Yearly-Averaged Sunspot Number end Qeama(toot(cally Dfeturbed ()aye from t932-7 985. cycles from 1932 to 1986 I2. 3). Note that the geomagnetic dtsturbance cyclea can have a double peak. one of which can Iag the sunspot cycle peak. While geomagnetic activity in the present cycle is expected ta maximize in approximately 1993-1994, severe geomagnetic storms can occur at any time during the cycle; the K-9 storm of March 13, 1989 was a striking examp(e. Eh RTH4URFhCZ.POTENTIAL AND GEOMAGNETIChLLY-INDUCEDWURREVTS The auroral electrojets produce transient fluctuations in the earth'5 magnetic field during magnattc storms. The earth is a conducting sphere and portfona of ft experience this time-varying magnetic field, raautttng in an induced earth. surface-potentlal (ESPl that can have vafuaa of 1.2 to 6 volts/km (2 to 10 voltslmlle) during severe geomagnetic storms in re-gions ot low earth conductivity (4). Electric power systems become exposed to the ESP through the grounded neutrals of wye-connected transformers at the opposite ends of long transmission Ifnes, aa shown in Figure
- 2. The ESP acta aa an ideal vottaga source impressed be-tween the grounded neutrals and haa a frequency ot ona to a few mittlhertz. The gaomagnatfcally-induced.currents IGIC) are than determined by dividing the ESP by the equivalent dc resistance of the paraftefed transformer windfngs and fine canductora. The GIC fa a quagvdfrect current. and values in excaaa at 100 amparaa have been measured in transformer neutrals.
Vernon D. Alberti)52 University of Minamata SOLAR ORIGINS OF CEOMhGiVETIC STORMS The solar wind ia a rarfffed plasma ot protons and electrons emitted from the sun. The solar wind ia affected by solar flares, coronal holes, and disappearing filaments. and the so-lar wind particles interact with the earth's magnetic field to produce auroral currents, or auraral efectrajata, that fallow generally circular paths around the geomagnetic palea at at-titudes ot 100 kilometers or more (1). The aurora borealia ia visual evidence of the auraral electrojeta in the northern t6 POWER SYSTEM EFFECTS OF GIC The per-phase GIC in power transformer windings can be IEEE Power Engineering Review, October l989
C ~a 4 'J
Y ~ 44 a i 5'rifN 9i RFal E ~ P ~\\~ I EARTH ~ SuIIFACK FOTEhfiaI. FIQui ~ 2. Induced Eanh Surfaca Patandal IESPI Producing Gaamag. noucafly Induced Curronts IGICI in Power Syafama. many times larger then the RMS ac magnetizing current, re. suiting in a dc bias ot transformer core flux, as in Figure 3. 1: Io.ai'a I l io.ai 'ma<~far.ar il '<arf:.arsiar~orq, qr "ause reiay misoperat,on I5). REFERE.'ACES I. Akaaafu, S. I., "The Oynamic Aufcra. 'c'eriufic 'me-:a Magaamo. May 1989. po. 90-97.
- 2. Jasalyn. J. A.. "Reel. Time Predicoan af Gfabai Gaamagra:
Acdvny," Solar Wind Magnatasohara Caucarg 4 3 Th 1
- 1. terra Scianuflc Publishing Company.
ax '. 986. ~ va, ~ ~ ampsan. R. J., "The Amaiituda of Solar C,cia 22. Radio and Space Services Tochnical Recaft TR.87 03. crc bar 1987. 4 V. O. Albenaan and J. A. Van 8aafafi. "Elacffic and 41agiia..:i Ffolda at Iho Eanh's Surface dua ia Auraral Cuirenfa."'EE-: Tranaactiana an Pawaf Aaoaf atua and Syatema. Vai. PAS 89 Na. 2. April 1970, pp. 578-584. 5. J. G. Kapponman, V. O. Albartaan, N. Mohan, "Cuner Tranafaimer and Relay Parfafmanca in tha Piesance af Sec magnetically Induced Currents," IEEE Transactions ail Paw a Apparatus and Syatama, Val. PAS-100, Na. 3. co. 1078-1088. March 1981. The Hydro-Quebec System Blackout Of March 31, 1989 I0IC)gfacmo Rguro 3. OC Bfaa of Trariafarmor Caro FIux Ouo ta GIC. The half.cycle saturation of transformers on a power system is the source of nearly all operatfng and equipment problems caused by GIC's during magnetic storms. The direct conse-quences of the half-cyclo transformer saturation are: ~ The transformer becomes a rich source of even and odd harmonics ~ A great increase in inductive vers drawn by the trans-foffllof ~ Possible drastic stray leakage fiux effects in the trans-former with resulting excessive localized heatfng. There are a number of effect due to tho generation of high levels of harmonics by system power transformers, incfud-ingi ~ Overloading of capacitor banda ~ Possible rnisoporation of relays ~ Sustained ovorvottagos on long.line oner gization Higher socondory arc currents during single. pole switching ~ ~ Hfgher ciraA breaker recovery voltage ~ Overtoadine et harmonic fti)pra of WVDC converter ter. mfnata. and distortion in tfio ac voltage wave shape that may result in loss of dc power transmission. The increased inductive vers drawn by system transformers during hatfwycfo saturation are sufficien to causa intolor-abto system voltage depression, unusual swings in MW and MVAAflow on transmission linea, and problems with gener-ator var limits in soma instances. In addition to tho halt-cycle saturation ot power trans-formers, high levels of GIC can produce a distorted response IEEE Power Enittoccfiag Review, October l989 Daniel Soulier, Hydro-Quebec Qn March 13, 1989, an exceptionally intense magnetic storm caused seven Static Var Comqensators (SVCl on the 735-kV network to trip or shut down. These compensetors ire es-sential for voltage control and system stability. With their loss. voltage dropped end trequency increased. This Ied to systeflt Instability and the trfpping of aN the La Grande trans-mission lines thereby depriving the HQ system of 9500 MW of generation. The remaining power system collapsed within seconds of the toss of tho La Grande network. The system blackout affected aN but a few subatations isolated onto lo-cal generating stations. Power waa gradually restored over a nine hours period. De-lays in restoring power were encountered because of dam-aged equipment on tho La Grande network and problems with cold load pickup. SYSTEM CONDITION PRIOR TO THE EVENTS Total system generation prior to the events was 21500 MW. moat ot it coming from remote power.generating stations et La Grande, Manlcouagon and ChurchlN FaNs. Exports to neighboring Systems totalled 1949 MW of which 1352 MW were on DC interconnections, The 735-kV transmission net-work waa laded at 90% of ita stability limit. SEQUENCE OF EVENTS At 2:45 a.rn, on March 13, a very intense magnetic storm ted to the consequential trip or shut down of seven SVC's. Containing the Impact of the event through operator inter-vention waa impossible atl SVC's having tripped ot ceased to function within a offo rninuto period. A fow seconds IS-9 s.) atter the loss of tho tait SVC, sll tive l35-kV lines of the La Grande transmission network tripped due to an out of stop condition. These line trips deprived the system ot 9500 MWof generation and subsequently led to a conlpleto systeITI cotlapsoa 17
$L pct C h
i'q rei el, I'ec::on rvriue rcmairir g 'cur SVC's snot cown ov capacitor ~oitage ur.caiancc protect:a" >>aivsis of vonage ana cur ~ rent asc:eograins caken at:ne Chioougamau site befare tne SVC trips snowea the 'oilowing narmonic contents. hC hC Current at l6 kV VcliaCe ai.55 ky TCB Brarche TSC Branehe Harmonic Order 100."i 1ii 3'%Ci I ~~ 3 4I' 00 &t gO Isis i a< 5~i I o 3~a IOO~e 36 "a 24 "o 'I6~i 5% l6 ~e 4~i Quasi.DC currents gei,crated by:he magnetic disturbance, saturating in thc SVC coupling transformers are thought to be tnc cause for such a large second harmonic component ot currant in the TSC branch. LIIsturoances On Popover Transformers Roocrt J. Ringjcc james R. Stewart Power Tcchnoloip'cs Inc. This discussion addresses the effects of geoinagnet c istu" oancea an power transformers. The prirnarv effect:s auc:o core saturation resulting from geomagneticaliy indvcea c r-rents. GICs. Care saturation can impose severe temacraturc problems in wi'ndings. ieads, tank plate ana structurai mer,.- bers ot transformers and place heavy var and harmoi ic ocr-dens on the power svstem and vaitage support equipment. GIC's of 10 to 100 amperes are more than mere nuisances in the operation of power transtarmers, thc manner of!Iow can result in saturation of the core and consequent changes in system var requirements, increases in harmonic current rnagmtuaes, increased transformer stray ana eady iosscs. and problems with system voltage control. CENERhL OBSERVhTIONS ON THE SYSTEM BEHAVIOR The system blackaut was caused by loss of all SVC on I.a Grande Network. Seven SVC tripped or stopped functioning. Prior to and during the event all the OC interconnections be-haved properly. No relay false trips or misoperation of special protection systems ware observed. Telecommunications were not affected. No equipment damage was directly attrib-utable to GIC but ance the system split. some equipmant wae damaged due to load rejection overvaitagas. REMEDlhL hCTIONS ThKEN Since the event, the following actions were implemented: ~ SVC protection circuits have been readjusted on four SVC's so aa to render their operation reliable during magnetic storms similar work is being performed on the four remaining SVC's. ~ Energy, Mines and Resource Canada now provides Hy-dro Queb6c with updated forecasts on the probability of magnetic disturbances. These foreceete are used by the System Control Center dispatcher to position the transmission system within secure limits, ~ A.C. voltage asymmetry ie monitored at four key lo-cations on the system IBoucherville, Arneud,
- LG2, Chetgeaguay).
Upon detection of e 3% voltage asym-metry at any one location, the eyecem control center dispatcher ie alarmed end willinlmedletely take action to position system trenefer levels within secure limits if this hasn't already been done because ot forecasted magnetic ectivity. OPERhTING LINKS DURING MhGNETIC DISTURIANCES (hND hLERT SITUhTIONS) The fallowing operating limits are now being applied: ~ 10% safety margin shell be applied on maximum trans-fer limits. ~ Maximum transfer limits shall not take into account the availability of static compeneetore deemed unrellebl ~. ~ Adjust the loading on HVOC circuits to be within the 40% to 90%, or lees, of the normal full load rating. 18 CIC EFFECTS VERSUS CORE hND Wli4DINC CONFIGURhTIONS Principal concerns in this discussion are for EHV systems with grounded Y transformer banks providing conducting paths for G(C and zero sequence currents. Core and winding configurations respond difterently to zero sequence open.cir-cuit currents and to GICe. Note: ae used here, the term "open circuit"refers to taste performed with all delta connections opened or "broken." For example. the three. phase three leg care farm tranetormere are lese prone to GIC induced satu-ration then three-phase shell form transfarmers. But. both core form and shell form single phase transtormers are sus-ceptibl ~ to GIC induced saturation. Winding and lead arrangemente respond differently to GIC induced core saturation ee well. For example, tha current dis-tribution within pareil ~ I winding paths and within low voltage leeds depends upon the leakage flux paths and mutual cou-pling. I.oeeee within windinge and leads mey change signifi-cantly under GIC.induced saturation awing to the change in magnetic fl~Id intensity, H, and the resultant changes in the boundary conditions for the leakage field path. W EDDY LOSSES IN STEEL MEMBERS The changes in che magnetic intensity. H, and the magnetic boundary conditions resulting from the GIC excitation bias can incrceee the loeeee in steel plate, the losses for fields parallel ta the plane of the plate increase nearly as the square af H, Nate alea that the level of loeeee increase approxi-mately as the square root of the frequency of H, owing to the ~tfect ot depth af penetration. The magnetic field along yoke clamps end leg plates in core form transformers and in Tee beams end tank piete In shell form transformers closely matches the magnetic gradient in the core. Areas ot the tank and core clamps are subjected to the winding leakage field. If the core saturates, the magnetic field Impressed upon the steel members mey rise ten ta one hundred time! normal due to the eeturetlan end the effecte of the leakage field. The Ioeece in Cho steel membere willrice hundreds ot times nor-mal, even under half-cycle saturation. On the steel surfaces, eddy lose deneicy mey rise ten to thirtywatts per square inch, approaching the thermal flux density of an electric range ele-itlerlco Surface cempereturee rise rapidly with this thermal flux and can result in degradation of insulation touching the steel IREE Power Engineering Review. October l989
r~ I'I
ATTACIIHfNT 3 Design Deficiency Dcf IC lent vcAUOI aanual UPS has no battery test circuit Vendor aanual aa Iht chance section does not aention batteries. Design Dcficicncy Batteries have Aot been replaced in 6 ycal'S Dcslgh Dcf Ic icAcy AC IAPUC to logtc pwer supply is as intcnance prefcrrcd Back up batteries degraded or dead k.5 relay characterist-ICS preventS tfansfcl to inverter outPUC Breaker function per design Ground fault occurs on 8 phase of aain transforaer Voltape transient on $ CaCICA AC p(wcr S~ty AC poucr to logic aodutc for UPSlA D,G experiences thc CrahslcAC '~~y'utpUl voltage goes l(w logic trips oA p(wcr suppl y failure. 2VBB.UPSCA,B> C,D,G trip Breakers CB.1,2 3 open; Ch.C dOCS hot close ups Ioads do Aot aUIO transfer to asIiht. supp 1 y loss of all loads on UPS1A'D,G fault Is cleared in 6 cycles; transfer coeplcted in 12 cycles Peraisstves rohibtt CB C rcdkcl froa clos Ing CB-4 nc(vfcd to tr.((a,fer as Iht su/Ipl y to ihveI ICI OUIPUI
4 K.i}}