ML20055H838
| ML20055H838 | |
| Person / Time | |
|---|---|
| Issue date: | 06/30/1990 |
| From: | Mazumdar S NRC OFFICE FOR ANALYSIS & EVALUATION OF OPERATIONAL DATA (AEOD) |
| To: | |
| Shared Package | |
| ML20055H836 | List: |
| References | |
| TASK-AE, TASK-E90-05, TASK-E90-5 AEOD-E90-05, AEOD-E90-5, NUDOCS 9007310012 | |
| Download: ML20055H838 (35) | |
Text
,
i i
l i
AE00/E90-05 L
t ENGINEERING EVALUATION REPORT i
OPERATIONAL EXPERIENCE ON BUS TRANSFER June 1990 Prepared by: Subinoy Mazumdar r
l Office for Analysis and Evaluat1on of Operation Data U.S. Nuclear Regulatory Comission l
9007310012 900725 PDR ORG 14E XD
l l
INDEX Page No.
i Summary
(
l.
Introduction 1
2.
Description of Millstone 3 Event 2
3.
Search for Similar Events and Industry Actions 3
4.
Basic Transfer Schemes 11 5.
Classification of Auxiliary Distribution Systems in Nuclear Plants 14 6.
Consequences of Bus Transfer Failures 15 7.
Findings 18
~
8.
Conclusion 19 i
9.
References 20
- 10. Diagrams 22 D
?
l l
i
--v
.-e-
P
SUMMARY
Analysis by Northeast Utilities established on November 18, 1988, that at Millstone Unit 3, under certain scenarios, the existing 4160V bus transfer practice can result in a common mode failure of Class IE loads of both trains.
On November 27, 1989, another analysis by the licensee concluded that repeated operation of the existing 4160V bus transfer could potentially damage safety-related motors.
Both analyses were reported in licensee event reports (LERs for Millstone 3.
On further study of bus transfer failures at other utilities, we have found that though there is no evidence of equipment failures that could be directly attributed to bus transfer, there have been at least 56 LERs issued between 1985 and 1989, reporting failures of bus transfer to take place. An unsuc-cessful bus transfer following a nuclear unit trip, has the potential to lead to either a full or a partial loss of offsite power to the station auxiliary electric system. A reliable bus transfer scheme is required to meet the intent of general design criteria (GDC) 17 (10 CFR 50, Appendix A) to "... minimize the probability of losing electric power from any of the rcmaining supplies as a result of, or coinciuent with, the loss of power generated by the nuclear unit, the loss of power from the transmission network, or the loss of power from the onsite power supplies."
Our detailed study on bus transfer schemes indicates that:
1)
Though the industry (ANSI), National Electrical Manufacturers Asso-standard organizations, American National Standard Institute ciation (NEMA), Institute of Electrical and Electronic Engineers (IEEE)andElectricPowerReserachInstitute(EPRI),havebeen working for over 7 years to establish a guideline on safe bus transfer, they have not come to any decision as yet, and it may be a few years before appropriate standards are issued.
2)
Most of the licensees have not updated their bus transfer schemes with the state-of-the-art development.
In most of these plants, with suitable improvements in the existing bus transfer schemes, it may be possible to improve their reliability significantly.
3)
It is desirable that relevant parts of this study and subsequent findings be communicated to the utility companies and the organi-zations trying to establish the guidelines for safe bus transfer.
1.
INTRODUCTION At a nuclear plant most electrical buses are provided with power feed from multiple sources with provision for manual or automatic power transfer from one power source to another power source to provide maximum availability of power.
i l
This study covers power transfer in nuclear plant medium voltage auxiliary distribution systems between 2kV and 15kV.
l 4
e This study was initiated by our review of two reports submitted by Northeast Utilities (Reference 1 and 2) relating to bus transfer problems at Millstone Unit 3.
Following that review, we identified similar problems at other operating nuclear plants. We noted wide variations in the medium voltage bus transfer scheme used in different plants.
This prompted us to conduct an in-depth study on the state-of-the-art of bus transfer schemes. We have also comunicatea with the various industry organizations active on this issue.
The salient features of our investigation is recorded in the report.
i 2.
DESCRIPTION OF MILLSTONE 3 EVENT The attached Figure 1 taken from Millstone 3 LER 50-422/88-026-03 is a simp-lified diagram of the electrical distribution system at Millstone 3.
The Unit 3 main generator is provided with a main generator breaker between the main generator and its two step-up main transformers connected in parallel. The high voltage side of these main transformers are connected to the 345kV switchyard in a breaker-and-half-scheme, through breakers 15G-13T-2 and 15G-14T-2. The main generator provides normal power to the 4160V plant auxi-liary buses 34A and 348 through the normal station service transformer (NSST)
A, and to the 6900Y plart auxiliary (vital) buses 34C and 340 are normal buses 35A, 35B, 350, and 35D through NSST B.
The 4160V safety related from the buses 34A and 348 respectively.
The 4160V buses 34C and 340 are provided with alternate power from the 345kV switchyard through the reserve station service transformer (RSST) A, and the 6900Y buses 35A, 358, 35C, and 35D through RSST B.
l On November 18, 1988, an engineering analysis by the licensee discovered that with the existing scheme, certain faults in the 345kV system could trip open the 345kV breakers 15G-13T-2 and 15G-14T-2, and thereby isolate the main generator from the 345kV switchyard without tripping the main generator breaker.
In this scenario, with the main generator disconnected from the grid but still connected to the NSSTs, the turbine could trip due to a pt,wer mismatch or turbine overspeed.
Following the turbine trip, the turbine-generator would coast-down and the generator voltage and frequency would continue to decrease.
A licensee computer simulation indicated that the voltage of the 4160V buses would decay to 3220 volts and the frequency to 40 hertz, approximately 86 seconds after the turbine trip.
At 3220 volts, the undervoltage relays on non-vital buses 34A and 348 would operate which in turn would trip all motor loads off buses 34A and 34B. After a time delay of 2 seconds, the supply breakers from NSST A to bus 34A and 34B would trip open. The tripping of the normal supply breakers from NSST A to buses 34A and 34B would initiate auto-matic fast transfer of buses 340 and 34D to RSST A.
This fast transfer tates about 6 cycles.
Thus, prior to the transfer,- the voltage at vital buses 34C and 34D would be below 3220V/40Hz. A fast transfer to RSST A at 4160V/60Hz would cause a transient that could damage the loads connected to vital buses 34C and 34D. This is a potential common mode failure mechanism affecting both trains of safety-related equipment (see Section 6 " Consequences of Bus Transfer Failures" on damages caused by bus transfer transients).
. Another licensee analysis, LER 50-422/89-030-00, indicates under the worst-case condition such a fast bus tranefer can produce per unit resultant voltage as i
high as 1.85 per unit volts per hertz, while the ANSI standard C50.41-1982,
" Polyphase Induction Motors for power Generating Stations" reconsnends that to avoir' damages to connected loads, the resultant voltage should be limited to 1
1.33 per unit volts per hertz.
As a solution to the issue, on June 22 1989, the licensee modified their bus transfercontrolcircuittoeliminatelastbustransferonundervoltage.. The tast bus transfer would still function as originally designed whenever the su) ply breakers from NSST to buses 34A and 348 open automatically for reasons otlerthanundervoltage(i.e.overcurrent,currentdifferential,etc.). After this modification, an undervoltage on the non-vital buses 34A and 34B will result in a slow transfer in which the vital to non-vital bus tie breakers will open and the supply breakers from RSST A to 34C and 34D will close on bus undervoltage of 27 percent.
In addition, the control circuits of the 345kV switchyard breakers 15G-13T-2 and 15G-14T-2 have been modified to ensure that whenever both these breakers are open, the main generator output breaker and the supply breakers from the NSST to the 4160V and 6900Y auxiliary systems will trip open resulting in the fast transfer of the 4160V and 6900Y buses to the RSST.
On October 26, 1989, we visited Millstone 3 to discuss with Northeast Utility engineers the studies they have made to arrive at the adequacy of their proposed changes. As a result of this visit, the licensee has initiated an in-depth investigation of the following:
1)
Study of the Unit I and Unit 2 bus transfer schemes to ensure they do not have any similar deficiency.
2)
Study of the suitability of adding sync-check relays to prevent fast bus transfer on out-of-phase switching.
3)
Stuoy of alternative schemes in lieu of automatic bus transfer.
3.
SEARCH FOR SIMILAR EVENTS AND INDUSTRY ACTIONS In July 1989, we requested Oak Ridge National Lab (URNL) to search for LERs on failure of medium voltage bus transfer. A direct search did not identify any LERs. ORNL then tried indirect methods which identified 183 LERs.
We also obtained all LERs related to bus transfer that were 3creened by Reactor Operations Analysis Branch (ROAB) in 1989. After reviewing all these LERs, we identified 56 LERs on medium voltage bus transfer failures in the U.S. nuclear plants between 1985 and 1989. These LERs, with a brief description of the root causes for tailure of the bus transfers, are listed in Table 1.
Because of the limitations of the search procedure, this list does not cover all bus transfer failures that have occurred in U.S. nuclear plants during this period.
It can be only used as a broad guide to identify the nature of bus transfer failures and cannot be used to establish the total number of bus transfer failures.
5 l
s
[
l The search did not identify any event that directly caused equipment damage, either from failure of the bus transfer to take place, or from stresses. caused-by transients due to the bus transfer. These 56 LERs record the events in which successful bus transfer was inhibited.
About half of these failures can be attributed to the design deficiency of the bus transfer schemes, with 15 failures due to absence of a fast bus transfer scheme.
On-12 occasions, the.
bus transfer failed because of technician errors or poor maintenance. On four occcsions, defective circuits prevented bus transfer and on five occasions, the sync-check relay settings were inadequate.
In December 1989, we visited Arizona Public Service (APS), Phoenix, Arizona to study their bus transfer scheme. -Their scheme is similar to the basic scheme 2 explained in Section 4, " Basic Bus Transfer Schemes".
In this scheme, the four 13.8 kV balance of the plant buses are provided with automatic and residual voltage bus transfer supervised by static sync-check relays. The two 4.16 kV Class IE buses are fed from the 525 kV switchyard through two 57.5 kV-13.8 kV startup transformers and 13.8 kV-4.16kV ESF service transformers with provision for manual bus transfer only.
Arizona Power Services has l
. conducted detailed comp:ter simulation study to arrive at optimum setting of L
sync-check relays.
On January 28, 1986, Robinson 2 experienced a failure of bus transfer due to DC saturation of current transformers. This subject has been covered in Information Notice No. 86 87, " Loss of Offsite Power Upon An Automatic Bus Transfer", and Engineering Evaluation Report E703, " Loss of Offsite Power Due to Unneeded Actuation of Startup Transformer Prot,?ction Differential Relay".
To keep abreast of the current knowledge and effo; 3 by.the electric power industry on the issue of bus transfer, we establisned communication with IEEE,
)
EPRI, Southern Company Services, General Electric, and Beckwith Electric.
EPRI is actively studying various problems associated with bus transfer for the power industry.
In 1986 EPRI published a report.EL4286, on Bus Transfer Stuaies(Reference 8).
EPRI is currently pursuing a study (PR2626-1) to establish the safe preclosure voltage for bus transfer schemes. On May 16, 1990. EPRI made a presentation on the progress they have made thus far.
However, work under this project may be delayed because of shortage of funds and EPRI would welcome other organizations to share the cost.
GE engineers are also active on this issue.
They have conductdd an in-depth study of prevalent bus transfer practices in the U.S. under EPRI-sponsored project EL4286 and are working on report PR2626-1. They have produced a static sync-check relay, SLJ12A, which is presently being used by some utilities.
Beckwith Electric has also developed a static sync-check relay and a bus transfer system which are being used at operating stations. On December 12,-
1989, Beckwith made a presentation to the NRC staff at our White Flint office on different aspects of bus transfer schemes.
l F
,e
.-,,g,.
-.-------a
e
. g
- l. l 1
We visited Southern Company Services, Birmingham, Alabama, in November, 1989.
l They have=done substantial work on computer modeling and testing of fast bus transfer schemes, and have published their results in four technical papers.
On February 7 and 8,1990, the author attended the 1990 IEEE Power Engineering Society winter meeting to participate in the IEEE working group meeting on power plant auxiliary systems. At this meeting, the four technical papers by Southern Company Services (Reference 3 through 6) were also presented.
Reference 7 reports on the tests and computer simulation study that Consumer Power Company, Jackson, Michigan has conducted on fast bus transfer on a 400 megawatt fossil-fired plant. Their study demonstrates that in this particular plant both with normal load and shutdown load, the actual bus transfer time was well within the time permitted by the ANSI C50.41-1982 resultant voltage requirement of 1.33 per unit volts per hertz.
We have discussed the ANSI 1.33 per unit resultant voltage. issue with Dr. R. H.
Daugherty of Reliance Electric.
In 1982, Dr. Daugherty first questioned (Reference 11) the resultant voltage of 1.33 per unit volts per hertz specified in ANSI C50.41 (Reference 12). However, he indicated he had not zeroed in on any particular solution to the problem.
We have also requested nuclear agencies in West Germany France, England, Sweden, and Canada to exchange their experience with us. We have received responses from the Central Electricity Generating Board (CEGB), U.K. and the Swedish Nuclear Power Inspectorate (Reference 16, 17). We are awaiting responses from other countries.
Our in-depth review of these LERs, the single line diagr>ms of _the plants, the aus transfer schemes, and discussions with some of these licensees indicated:
1)
Wide variation in the medium voltage distribution system and the bus transfer schemes used in different plants.
A generalized description of these different schemes has been presented'in Section'5, " Classification of Auxiliary Distribution System in Nuclear Plants".
2)
While some of the licensees have done in-depth studies on the problems associated with bus transfer schemes, many of the licensees are not fully knowledgeable of the problems and the state-of-the-art know-how of bus transfer schemes.
3)
Though the industry standard ' organizations, (ANSI, NEMA, IEEE, and EPRI),
are aware of the consequences of bus transfer failures and have conducted significant studies on it, they-have not reached any final decision on the subject.
L o
1 i l
TABLE 1.
FAILURES OF MEDIUM. VOLTAGE BUS TRANSFERS BETWEEN 1985 and-1989
'IN U.-S. NUCLEAR PLANTS-PLANT SCHEME.*
LER NO.
ROOT CAUSE 1
Arkansas 1 2
87-005-00 Loads dropped out due to j
slow bus transfer.
2 Arkansas 1 2 002-00 Bus transfer failed because of system perturbation and wrong setting of sync-check relay.
3 Arkansas 1 2
89-004-00 Bus transfer inhibited by 1
wrong setting of sync-check relay.
1 4
Beaver Valley 2 2
87-036-00 Bus transfer inhibited by defective control circuit.
5 Brunswick 2 2
89-009-00
' Technician error caused loss of offsite power.
6 Callaway 1 2
85-005-00 Sync-check relay inhibited bus transfer following generator field failure.
7 Callaway 1 2
85-038-00, Sync-check relay inhibited bus transfer following generator field-ftilure.
8 Callaway 1 2
88-015-00=
Operator error caused.
failure of power supply to Class 1E bus.
9 Catawba 1 3
89-001-02 Train A blackout due to defective relay installation.
10 Cooper 2
87-013-00 Cause-of failure unknown.
11 Crystal River 3 2
89-013 Unplanned load growth caused excessive voltage drop after bus transfer.
See Section 5, " Classification of Auxiliary Distribution Systems in Nuclear Plants," for description of the schemes.
1 i
i
--,r
..n.
.,--,--,,e-
-a
i e
7-12 Crystal River 3 2
89-023-00 Technician error and defective relay caused L
failure of offsite power
- supply, i
l 13 Davis-Besse 1 2
87-011 Bus transfer failed due' to improper breaker trip q
lever setting.
L 14 Diablo Canyon 2 2
88-008-00 Bus transfer failed due to poor maintenance and cable-layout.
15 Dresden 2 2
85-034-00 Cus transfer inhibited by defective control circuit.-
16 Dresden 3 2
89-001-01 Bus transfer. inhibited by bad breaker auxiliary contacts.
17 Duane Arnold 2
85-010-00 Bus : transfer occured.
I 30 seconds after reactor trip causing dropout /of RCPs, CWPs and a CP.-
18 Farley 2 2
85-010 Loads dropped out because of slow bus transfer.
19-Hatch 1 2
J-018-00 Loss of offsite power after successful bus transfer due to' defective relay.
20 LaSalle 2 2
89-007-00 Loads dropped out because of-slow bus transfer.
21 Maine Yankee 1 2
88-006-00 A fault on main transformer caused sufficient dip in grid voltage to inhibit bus transter.-
22 Millstone 2 2 011-01 Bus transfer inhibited by ground fault on vital 4160V bus. caused by personnel error.
23 Millstone 3 3
88-026-03 Study indicates possibility of loss of redundant. trains of safety related equipment on bus transfer.
\\l L
{
. 24 -
Millstone 3 3-89-030-00 Engineering evaluation established repeated bus transfer operation could I
damage safety related loads.
25 Nine Mile Point 2 2 88-012-00 Fast bus transfer inhibited by defective 1
relay circuits.
26 Oconee 1 2
89-010-01 Bus transfer considered inadequate because of low HV grid voltage.
27 Oconee 2 2
88-002-00 Susceptibility of bus transfer failure due to HV breaker ferroresonance.
28 Palisades 2
87-024-00 Inadvertent operation of
. deluge system caused fault on startup transformer which inhibited bus transfer.
29 Palisades 2
89-015-00 Bus transfer eliminated by connecting ~ Class 1E buses l
to startup transformer during normal operation to avoid failure of bus
. transfer.from stuck breaker.
I 30 Palo Verde 1 2 004-00 Bus transfer' inhibited by stuck breakers.
l 31 Palo Verde 2 2
89-001-01 Simultaneous failure of both transformers caused I
loss of-offsite power.
l 32 Peach Bottom 2 2
86-010-00 Loads dropped'out'during bus transfer.:
33 Peach Bottom 2 2
87-012-00 Loads. dropped out during bus' transfer..
U 34 Point Beach 2 2
85-005-00 Sync-check relay inhibited bus transfer.
35 Point Beach 2 2
87-O0'2-00 Non-Class 1E bus failed to transfer due to transient caused by lightning strike.
- l 4
-g-36 Point Beach 2 2
89-002-00 Loss of offsite power due 1
to low bus voltage after bus transfer, 37 Qitad Cities 2 2
87-009-00
~ Slow bus transfer caused load drop-off.
38 Raiicho Seco
.2 88-015-00 Loads dropped out during bus transfer.
39 River Bend 2
88-018-04 Relay logic inhibited bus transfer.
40 H. B. Robinson 2 1 86-005-00 Bus transfer inhibited by CT DC saturation.
41 Salem 2 2
86-007-00 Bus transfer inhibited by-low bus voltage.
42 Seabrook 3
89-004 Loads dropped out during t
bus transfer.'
43 Seabrook' 3-89-014 Bus transfer inhibited by low DC battery voltage.:
44 Shoreham 4
87-003-00 Differential relays-for both normal and reserve i
station service transformers tripped because' maintenance-personnel lef_t jumper across these relay cts.
45 Shoreham 4
87-026-Loads dropped out during slow bus transfer.
46 South Texas ~2 3
89-009 A generator trip caused m
momentary loss of offsite i
power.
- 47 South Texas 2 3
89-014-00 Trip of the 345.-kV breaker; feeding the main transformer caused: loss of=
offsite' power.
48 Susquehanna 1 2
85-035-Loads-dropped out during-bus transfer 49 Susquehanna 1 2
87-007-00 Loads dropped out'during.
bus transfer.
I
, i 50 Susquehanna 1 2
87-015-00 Loads ~ dropped.out because of slow bus transfer.
1 51 Susquehanna;1-2 87-020-00 Loads dropped out during bus transfer.
52 Susquehanna 1 2-88-014-00 Loads dropped out during j
bus transfer.
53 Turkey Point 4 2
85-004-00 A flash over on the 240 kV system caused failure of.
both sources.of. power-supply.-
54 Vogtle 2 2
89-023 Improper CT connection inhibited bus' transfer.
- 1 55 WNP 2' 2-85-007-00
' Relay failure inhibited'
' bus transfer.
t 56 Yankee Rowe. -88-008-01 Fast bus transfer failed because of defective' circuit.
1 J
f e
I
I.
3 !
4 BASIC BUS TRANSFER SCHEMES i'
i Bus transfer primarily involves transfer of power supply to a bus from one power. source to another, normally during nuclear plant startup or shutdown.
i The intent of bus transfer schemes'is to provide power to a bus and its con-nected loads, with minimum or no interruption.
In a majority of the nuclear j
plants, during a unit start up, the unit auxiliary power is manually trans-ferred from the station offsite power, through station service transformer (SST), to the unit main generator, through the unit auxiliary transformer (UAT).
In this transfer, the two sources are first brought to syreronism (i.e.
equal [themagnitudeandphaseangleofthetwosourcevoltagesarenearly andthenconnectedinparallelforashortduration(fewseconds)before disconnecting the offsite power source. Because of this, the auxiliary j
equipment connected to the bus is subjected to minimum electrical and mechanical transients during this transfer.
However,- as the two power sources are connected in parallel, the connected equipment are susceptible to excessive fault current interruption beyond the equipment rating if a fault were to occur during the bus transter, j
The bus transfer during the unit _ shutdown can be either in the normal mode or in the emergency mode.
In the normal shutdown bus transfer operation, the operator manually transfers the auxiltary power from the UAT to the SST usually in the-reverse order of the-startup bus transfer.
The emergency mode bus transfer is usually an automatic bus transfer initiated by a loss of the power supply from the VAT. The UAT-power loss normally occurs after a generator trip, a turbine trip, a reactor' trip, or a main / unit aux-iliary transformer trip. The automatic bus transfer can be in the residual mode, the in-phase mode, the fast bus transfer mode, or a combination of the
- three, in the residual mode, also called dead bus transfer, the bus transfer takes place only af ter the bus voltage decays below 25 percent of the rated bus voltage. This is achieved either by. sensing bus undervoltage or by using an adeouate time delay in the bus transfer circuit. Tha main draw-back of this scheme is that it is the slowest of the three automatic bus transfer schemes.-
Residual transfers are susceptible to load dropoff and may. require sequential start of the loads to avoid block start which can cause excessive voltage drop or excessive stress on the station service-transformer'(Reference.3).
The in-phase bus transfer scheme uses special relays that ensure that tho transfer will take place when the incoming source is in phase with the residual voltage of the bus. This scheme has the same merits as the manual parallel transfer during normal shutdown. However, this transfer scheme is usually slower than the fast transfer scheme (30 cycles as opposed to 6 cycles) and it may not have any practical advantage over the residual transfer scheme.
I
-- - - - - - - ~ - - - - - - '
,l
. The fast bus transfer scheme tries to keep the transfer time to a minimum, normally within 6 cycles.
The two main versions of this scheme are the
" sequential" and " simultaneous" type.
In the " sequential" type, also called
" break-before-make-scheme", the incoming source breaker closes af ter the
}
opening of the outgoing source breaker and is achieved by using a "b" or an -
early "b" contact of the' outgoing breaker.
In the simultaneous type, the operation of both the incoming and outgoing breakers are initiated at the same time, (i.e., the outgoing source breaker is signalled to open and the incoming source breaker is signalled to close, simultaneously). Normally, the' simul-taneous type has a shorter dead band (the time period during which the aux-iliary bus is disconnected from both sources), usually between one or two-cycles compared to five to ten cycles in sequential transfer.
However, if the-outgoing breaker is slow in opening, or fails to open, then the two sources would be connected in parallel. This parallel operation is avoid a in i
sequential operation.
In the U.S. nuclear plants, the-sequential type of fast bus transfer is the most popular scheme.
Washington Nuclear Plant,(Unit 2, Donald C. Coo'k, Units 1 and 2, and all' nuclear plants in Sweden Reference 16), use the simultaneous bus transfer scheme. Thus far, none of these units have reported any problems attributable to the transfer scheme.
In the Swedish design, transfer is permitted only if the two sources are nearly equal (V)705, delta f <0.05, and delta phase angle <
12-24 degrees), and if a breaker failure occurs such that both sources are paralleled for more than 0.1 second, then both closed breakers will be signalled to open.
There are two concerns when the two sources are connected in parallel. The-first is that such an operation can cause instability'in the system and i
stresses in the connected components.
In most cases, both the main generator-step-up transformers and the station reserve transformers are supplied from the same switchyard or from switchyards that are synchronized to each other.
Further, the prerequisite condition for successful bus transfer, u:ually monitored by sync-check relays, ensures that the two sources are reasonably in phase and equal in magnitude.- (The two sources can be in. parallel only when the outgoing breaker is slow in opening or fails to open). Thus, the system disturbance and equipment stresses experienced during this transfer is minimal.
The second concern is the occurrence of electrical faults beyond the designed fault duty of the equipment and buses when the two sources are in parallel.
To address this concern, the Swedish design has allowed only a 0.1-second-l period in the transfer scheme during which both breakers are permitted to remain closed.
The EPRI Project Report El.-4286 (Reference 8) on bus transfer studies indicates, that for a typical nuclear plant, the critical parameters for comparison between the merits of simultaneous and sequential schemes are as follows:
l
, TABLE II. COMPARISON OF CRITICAL PARAMETERS BETWEEN SIMULTANE0US AND SEQUENTIAL FAST BUS TRANSFER SCHEMES.
t PARAMETER SIMULTANEOUS SEQUENTIAL SCHEME SCHEME-(MaximumValue)
(2 Cycle Deadband)
(6CycleDeadband) 1 i
Angle in electrical degrees 32.9-155 between motor internal voltage and system Resultant volts per hertz between
.575 1.828 motor internal voltage and system
-Motor transient torque per unit 3.46 9.36 on motor basis Motor transient current per unit 3.22' 9.67 on motor basis e
This table shows that in sequential fast bus transfer, the connected electrical equipment is subjected to substantial electro-mechanical stresses and can exceed the 1.33 volts per hertz requirement of ANSI C50.41-1982. On the other hand, these stresses are lower in-the simultaneous fast bus transfer scheme (about one-third of sequential transfer) and there is less possibility of any equipment damage.
The worst case resultant volts per hertz between the motor internal voltage and the system is only 43 percent of the ANSI C50.41-1982 s
requirement.
t The fast bus transfer schemes can be either supervised-type using' a sync-check i
device or unsupervised type.
In the past, the sync-check relays were of the induction disc type which are much slower than the static' relays developed by Beckwith Electric and GE in the 1980s.
Before mid-1950, most fossil-fired plants used a' version of the: dead bus transfer in which during a loss of the normal power source, all loads were first tripped and finally the auxiliary bus was connected to the incoming source and then the auxiliary loads were re-energized in= sequence..In late 1950, with-the introduction of stored energy breakers, fast transfer schemes 1
with about six-cycle dead band gained popularity.
In 1977, ANSI C50.41,
" Polyphase Induction Motors for Power Generating Stations," first introduced 3
the requirement for limiting " pre-closure voltage"' to 1.33 per unit volts per hertz, as it was realized that with the six-cycle or longer dead band bus transfer,.the connected equipment could be damaged from switching transients.
This issue is discussed further in Section 6 and references 3 and 8.
f 5.
CLASSIFICATION OF AUXILIARY DISTRIBUTION SYSTEMS-IN NUCLEAR PLANTS Electric power systems in US nuclear power plants are designed and operated to meet the requirements of 10 CFR 50, Appendix A, GDC 17.
This criterion in part states:
" Electric power from the transmission network to the onsite.
electric distribution system shall be supplied by two physically independent circuits.... Each of these circuits shall be designed to be available in sufficient time following a loss of all onsite alternating current power supplies and the other offsite power circuit, to assure that specified accep-table fuel design limits and design conditions of the reactor coolant pressure 1
- boundary are not exceeded. One of these circuits shall be designed to be available within a few seconds following a loss-of-coolant accident to assure that core cooling, containment integrity, and other vital safety functions are maintained. Provisions shall be included to minimize the probability of losing electric power from any of the remaining supplies as a result of, or coincident with, the loss of power ' generated by the nuclear power unit, the loss of power from the transmission network, or the loss of power from the onsite electric power supplies." To meet the intent of the criterion on availability within a few seconds and' minimizing the probability of losing electric power on loss of the generating unit, power system designs use the bus transfer scheme.
However, there is wide variation in the medium voltage (between 2kV and 15kV) auxiliary power distribution systems used in nuclear plants. Broadly speaking, these different schemes can be categorized into four basic schemes from-the bus transfer analysis point of view.
L The first scheme shown in Figure 2 uses the automatic bus transfer scheme to meet the intent of GDC 17. Plants 1that use this scheme normally supply all plant auxiliaries from the main generator through the unit auxiliary trans-3 former. Upon loss of the nuclear power unit (i.e., the main generator),
electric power is supplied to the auxiliaries from the offsite preferred power source b/ the start-up transformer.
This transfer of power from the unit auxiliary to the start up transformer is accomplished by the automatic fast bus tranifer scheme.
For plants with this scheme, a failure of the bus transfer to take place leads to loss of the preferred offsite power to.the station asxiliaries, l
The second~ scheme shown.in Figure 3 (reproduced from Figure 2.3 of reference 8) is the most popular scheme. The requirements'of a minimum of two offsite power j
sources are provided.
In some of the plants using.this scheme, each Class 1E j
bus is normally fed from one of the two offsite sources with provision for manual transfer to the alternate offsite source.
In other plants,-all Class'1E loads are fed from one'of the two offsite power sources, with provision for automatic transfer to the second offsite' source.
The non-Class IE (balance ~of the' plant)busesareusuallyfedfromtheunitgeneratorthroughtheunit~
auxiliary transformer with provision for automatic fast bus transfer to offsite power source.
In some others, all loads are fed by the unit auxiliary trans-former during normal operation, with provision for automatic transfer to one of the offsite sources upon a unit trip.
.i
. The third scheme shown in Figure 4 (reproduced from Figure 2.4 of reference 8) uses a generator circuit breaker or load break switch.
Browns Ferry 1, 2 & 3, Catawba 1 & 2, McGuire 1 & 2. Millstone 3, North Anna 1, Seabrook, South Texas 1 & 2 and Summer have generator breakers.
In the event of a generator fault or unit trip, the generator breaker trips open and.the auxiliary loads are fed without interruption from the offsite source through the main transformer and the unit auxiliary transformer.
In the event of problems with the main or unit auxiliary transformer, the non-Class 1E buses and, and in some designs, the Class 1E buses are usually connected to the offsite source through the station service or start-up transformer using automatic bus transfer scheme. The Swedish nuclear. plants also use such a scheme (Reference 16).
The basic advantages of this scheme are:
1)
Elimination of a second station service transformer with its associated switchgear, and 2)
Elimination of bus transfer operation on unit trip i
The fourth scheme does not use any unit auxiliary transformer and relies on two station service transformers for supplying power to auxiliary loads at all times, except for emergency operation of Class 1E sources from emergency diesel generators when both or one offsite source fails. This scheme is used at Calvert Cliff 1 & 2, Fermi 2, Grand Gulf, Hope Creek, Shoreham, and TMI 1.
In this scheme, bus transfer is not required for any unit trip or fault in the unit generator and the main transformer.
Our evaluation of the 56 events reported in Table 1, indicates that Scheme 3 nad five failures, Scheme 4 had two failures and Scheme 2 had the rest of the tailures.
Some licensees (e.g., Palisades, Davis-Besse, Millstone 3) have eliminated or plan to eliminate the fast bus transfer of safety-related buses because of the unreliability of the transfer scheme.
However, such eliminations should consider the design bases of the orig 1nal design, especially as they-pertain to the intent of GDC 17 on availability and reliability'of available sources of power to-safety-related loads.
The Standard Review Plan, NUREG 0800, Section 8.2, deals with the review of offsite power system and the interface between the offsite.nd onsite power.
systems. The medium voltage bus transfer scheme used by r.uclear plants in the interface between offsite.and onsite power systems, would rightly fall into this section.
However, the section contains little e no guidance for the
- review of the various' types of bus. transfer schemes used by nuclear plant
-designs.
6.
CONSEQUENCES OF BUS TRANSFER FAILURES Bus transfer failures can be divided into two classes:
1)
Failures-that inhibit bus transfers.
2)
Failures that can damage the loads connected to the auxiliary buses.
.j
. The first failure category covers cases where the transfer does not take place, resulting in loss of power to the bus that is being transferred from one source to the other.
In several nuclear plants, such a failure leads to the loss of a preferred power source to plant auxiliaries, which in some cases,. include the Class 1E loads. Hence, with-this type of bus transfer failure, the Class IE loads will have to rely on the emergency diesel generators, and the reactor coolant system on natural circulation. Thus such failures defeat the main purpose of providing the bus transfer facility - that of meeting the intent of l
GDC 17 on availability and reliability of the preferred ~ source.
Although nuclear plants are designed for. safe shutdown on failure of offsite power, it is desirable that such failures are kept to a minimum to reduce stress on the nuclear units and to improve the availability of offsite power sources.
Every improvement in availability of offsite power source is an improvement on station blackout.
Several factors can inhibit' a bus transfer. Our review of the 56 LERs docu '
menting the failures of bus transfers has identified defective system design, slow sync-check relay speed (generally the slow electro-mechanical relays),
improper relay settings, slow operation of the outgoing breaker, bad auxiliary contacts, system undervoltage, and human error as_ the main causes for these kind of failures.
The second class of failures-are caused by the excessive voltage difference (in magnitude and phase angle) between the auxiliary load bus and the incoming power source. When the normal power source to an auxiliary bus is interrupted, the trapped magnetic flux and the-induced voltage of the motors start to decay and-the motor starts to slow down causing voltage magnitude and phase angle decay. Thus, when the auxiliary loads are connected to the incomin0 source, there can be substantial voltage difference between the auxiliary bus and the incoming source.
This excessive resultant voltage will cause transient current flows in the system which can damage the transformers, the buses,~and the connected loads due to stresses from electromagnetic forces, overheating, and transient torques. Thus far, there has not been any reported equipment failure in nuclear plants that can be directly attributed to.this cause, although potentially, some equipment is stressed in this process.
It is to be noted that such stresses experienced by connected equipment are cumulative in nature and unless specifically monitored for,' can remain undetected until failure
- occurs, l'
l ANSI C50.41 " Polyphase Induction Motors for Power Generating Stations" first addressed this issue in 1977. ANSI C50.41'-1982, states that a motor is inherently capable of developing transient torque (and current) considerably in excess of rated torque when exposed to an out-of-phase bus transfer or momentary voltage interruption. The magnitude of this transient torque can range from approximately 2 to 20 times rated torque and'is a function of the machine, operating conditions, switching times, system inertia, etc.
i -
1 To limit the possibility of damaging the motor or driven equipment, or both, it is recommended that the power supply system be designed so that the resultant vectorial volts per hertz between the motor residual volts per hertz and the incoming source volts per hertz at the instant the transfer or reclosing is completed does not exceed 1.33'per unit. volts per hertz on the motor rated voltage and frequency _ bases (see Fig. -5 reproduced from Figure 1 of Reference 12).
NEMA accepted this as a sate criteria 1' NEMA MG-1-1978.
In 1982, R.
H.-
n Daugherty challenged this criterion (Reference 11) and A. S. C. Htsui further substantiatedthechallengein1985.and1986(Reference 9and10). They established that ANSI C50.41-1982 criterion on resultant voltage limit of 1.33 per unit volts per hertz is unrelated to the motor shaft torque. As a result of these concerns, NEMA MG-1, 1987 has rejected the ANSI C50.41-1982 criterion.
Recently EPRI has initiated a study on the whole issue (RP2626-1). GE is conducting this study for EPRI, and the results are expected to be published by the middle of 1991.
At present, at most of the nuclear plants, the bus transfer schemes generally meet the criterion of resultant voltage of 1.33 per unit volts per hertz.
With no equipment failure reported, this appears to be a safe figure that can be used in bus transfer design and test verification.
However, there is no-strong technical basis for selecting this l.33 value (see Reference 11,.
3 discussionsection).
An increase in the value of the resultant voltage will permit more successful bus transfers, but increases the susceptibility of connected equipment to damage, while a decrease of this-voltage may unduly increase failure of successful bus transfer.
In this respect it is desirable for the NRC to keep close liaison with the EPRI and IEEE working groups active on these issues.
From the review of the LERs and referenced documents, the following findings relate to bus transfer criteria:
1)
A malfunction of the generator excitation system can cause excessive voltage deviation between the generator terminal and the offsite power.
2)
As established in the Millstone LERs 88-026-03 and 89-030-00 under special operating condition, the generator voltage and frequency can be substan-4 tially different from the offsite source.
3)
In certain plants there may be significant generator voltage and offsite power source, phase shift between the especially when they are not 4
from the same switchyard.
4)
A fault or lightning can cause transient system. disturbance in'which event, a supervised scheme is the best way to ensure optimum safe transfer.
i
~ 5)
Loads with high moment of inertia are suitable for fast bus transfer and-loads with low moment of inertia are more suitable for residual transfer (seeReference3and15).
q These findings lead to the conclusion that a way to ensure safe transfer under all operational conditions is to use the sync-check relay.
One way to ensure adequate setting of the sync-check relay is to carry out field tests under worst-case. loading conditions.
However, it is not always possible to conduct tests under worst-case loading conditions and some licensees have developed computer simulation programs to cover such conditions.
Even in computer simulation programs, information on equipment parameters are assumed and can produce erroneous results-if proper care is not used in selecting these parameters.-
~ '
7.
FINDINGS-The salient-features of this study are:
A.
There is no information on any equipment failure that can be attributed to l
ous transfer transients.
From this field experience, the present ANSI.
C50.41-1982 criterion-for safe bus transfer - 1.33 per. unit volts per hertz, appears adequate.
B.
Between 1985 and 1989, on at least 56 occasions, bus transfer failed to take place on demand.
It is possible to improve upon this bus transfer tailure rate by suitably modifying the bus transfer scheme. Thus far, the issue has not received the industry attention it deserves.
C.
There is a wide variation in bus transfer schemes'used by different licensees. While some licensees have performed in-depth studies on the problems associated with bus transfer schemes, many are not fully.
knowledgeable of the problems and state-of-the-art developments-in bus transfer schemes.
D.
Although the industry standard organizatibns (ANSI, NEMA, IEEE and EPRI) l.
are working to establish a guideline for safe bus transfer, they have not come to any decision yet, and it'may be a few years before appropriate i
l-standards are issued.
It is desirable that the NRC maintains necessary 1
communication with these organizations.
E.
Information regarding the different aspects of bus transfer, such as' types of designs and their limitations and advantages, conformance.to the.
requirements of GDC 17, safe bus transfer criteria-per ANSI C50.41 and.
the ongoing' industry ~ efforts in this area,.the use of state-of-the-art fast. acting sync-check relay, etc., need to be-fed back to licensees of operating nuclear plants.
E
.m m
- m. -,,.., -,. -,.
. q F.
The schemes that eliminate bus transfer on unit trip, for example the schemes with generator breaker (Figure 3), or-the schemes in which the auxiliary loads are normally fed from offsite source, have much lower 4
probability of failure than the conventional scheme (Figure 2), popular in the USA in which a bus transfer is initiated at every unit trip.
j G.
Though the sequential fast bus transfer scheme is popular in the USA, the i
simultaneous bus transfer scheme in conjunction with a main-generator circuit breaker is used in all_Swedish nuclear plants and appears to be a more reliable scheme, i
H.
The Standard Review Plan, NUREG 0800 Section 8.2, contains little guidanceforthereviewofmediumvoltagebustransferschemesthatare used in nuclear plant designs to meet the requirements of GDC 17, 8.
CONCLUSION Our review of operational experience and of available documentsLlike the FSARs, could not identify the design details of existing bus transfer schemes, or the modifications done by licensees of operating plants in the area of bus transfer.. We are aware that several licensees have modified their bus transfer schemes following start-up experience.
The details of the as-installed, i
as-operating bus transfer schemes, can be obtained by means of a 10 CFR 50.54(3)(f)requestfromthelicenseesofoperatingnuclear based on our review and lack of reported equipment failures, plants.
- However, it is difficult to make a cost-beneficial argument to justify such a request. Nevertheless, based on the potential consequences of failures in the-bus-transfer schemes, it would be prudent for licensees to consider-improvements to existing ~ schemes.
Though EFRI is currently active in developing a suitab1e criteria for: reliable
~
bus transfer, our survey indicates many of the licensees are not adequately informed on the state-of-the-art in bus transfer. -By suitably improving their existing bus transfer schemes, they can reduce the current rate of bus transfer failures and thereby improve the availability of offsite power significantly and reduce challenges to emergency diesel generators..
The NRC should consider coordinating with industry groups that are active in development of suitable criteria and standards for reliable bus transfers.
Revision of the Standard Review Plan should-be considered to include adequate guidance for the review of bus transfer schemes in the design of nuclear plant auxiliary electric systems.- The experiences and problems relating to medium -
voltage bus transfer schemes at operating nuclear plants should be communicated to the industry.
20 -
a
^
9.-
REFERENCES:
1.
LER 50-423/88-026-03, " Potential Damage to Safety Related Equipment Due To Design Inadequacy.", Report dated October 10, 1989.
2.
LER 50-423/89-030-00, " Potential Damage to Safety Related Motors Due to j
Fast Bus Transfer Design Inadequacy.",' Report dated December 26, 1989.
3.
T. Higgins, W. Snider, P. Young, H. Holley, " Bus Transfer Assessment and Application.", Paper 90 WM 220-4EC, IEEE/ PES Winter Meeting, 1990.
4 T. Higgins, P. Young W. Snider, H. Holley, " Computer Modeling for Bus Transfer Studies.", Paper 90 WM 221-2 EC, IEEE/ PES Winter Meeting, 1990.
5.
P. Young, W. Snider, T. Higgins, H. Holley, " Bus Tran'sfer Testing and Evaluation.", Paper 90 WM 220-0EC, IEEE/ PES Winter Meeting,1990.
6.
H. Holley, T. Higgins, P.: Young, W. Snider, "A Comparison of Induction Motor Models for Bus Transfer Studies.", Paper 90-WM 063-8 EC, IEEE/ PES Winter Meeting, 1990.-
7.
Y. E. Yeager, " Bus Transfer of Multiple Induction Motor Loads in a 400 i
Megawatt bossil Power Plant," IEEE Transaction on Energy Conversion, Volume 3, No.~3 pp 451-457, September 1988.
8.
J. C. Appiarius, E. L. Owen, R.M. McCoy, A. Murdock, " Improved Motors for Utility Applications, Volume 2:
Bus Transfer Studies." EPRI EL-4286, Volume 2, Project 1763-2, Final Report, October 1986.
9.
J. S. C. Htsui, "Nonsimultaneous Reclosing Air-Gap Transient Torque of Induction Motor: Part II, Sample Studies and Discussion on Reclosing of l
ANSI C50.41." IEEE Transactions of= Power Apparatus and. Systems, WM 214-1.
10.
J. S. C. Htsui, " Magnitude, Amplitude and Frequencies of Induction - Motor Air-Gap Transient Torque Through-Simultaneous Reclosing With;or Without Capacitors.", IEEE Transactions on Power Apparatus and Systems,. Volume PAS-104, No. 6, June 1985, 11.
R. M. Daugherty, " Analysis of Transient Electrical Torques and Shaf t Torque in Induction MotorsLas a Result of Power Supply Disturbance." IEEE Transaction on Power Apparatus and Systems, Volume PAS-108,~ No. 8, August 1982.
- 12. American National Standard for " Polyphase Induction Motors for Power Generating Stations.", C50.41-1982..
E 1
1_,
. ~.
13.
National Electrical Manufacturers Association, " Motors and Generators",
MG-1-20, 1985.
4 14 '.
L. E. Goff, J. B. Williams, J. C. Appiarius, S. L. Smith, " Application of a New Synchronism Check Relay.", Georgia Tech Relay Conference, 1979.
15.
R. D. Pettigrew, E. L. Johnson, Automated Motor Bus Transfer Theory and Application.", Paper, Thirty Seventh Annual Conference for Protective Relay Engineers, April 16, 1984, Sponsored by Texas A & M University.
3
- 16. Letter dated March 1,1990 from Swedish Nuclear Power Inspector.
17.
Letter dated April 3,1990 from the Central Electricity Generating Board, U.K.
i 3
l
l f
i 345 KV North Bus I) 15G-13T-2 l)
(Manchester T
Transmession Line
) o Haddem Neck Via Montville Central Ct)
+ (Eastem Ct)
Card Station LI
- Li"'
}
}
15G-14T-2 (Eastem Ct)
-l
+ Southington Unit 2 Unit 1 i
]}
(Central Ct) l}
l l
LI"'
h.
k 345 KV South Bus A
B RSST "B" V J.
O J RSST "A" I
ww w
w Main Transformer-mm mm pg gg g3 p3
, NSST ~B" BOP 6.9 KV System NSST "A" W~
Nm
%J r%
T M ain em nm e
-) Gen'
,) -
--,')
,)
g)
)l
,)
,)
j l
Bkr
~; Unit]3 l
Bus 35A Bus 35B Bus 35C Bus 35D l
l Gen.
Emergency Diesel i
l
-l
'l.
4.16 KV System l
l Bus 34A Bus 34C Bus 34D Ij Bus 348 (BOP)
A l
- (Class 1E) -
. (Class 1E) y A (BOP)
Bus Tie Breaker Bus Tie Breaker
' Dqure 1. Simplified One-Line Diagram of Millstone Unit 3.
e b
Y 4
- u...
~
- 23L-i L,
c
-3 I
Switchyard and Transmission Systems I
I l
+
l l
l Generator l
j l
ww Step Up l
l mm Transformer l
I Generator _ g i-I Bus
'*^ ^'
I Auxiliary WW. Start up l
D m m Transformer, y v v i Transformer -
l 2
(Preferred Power Supply) 'l l
r7 7
l l
l.
Unit I
Generator,,,, [
L _.
)
i L_1; _1_1
%N
~~
Q m.
r1-~^A mm
.(
Standby
. Standby
)
)-
Generator Generator.
l s
LIA LIA
""~
Battery
'"~ ~
Battery LIB LIB Charger Charger.-
Battery.:::
')
)-
.::, Battery Supply
-"- Supply LIA LIA LIB LIB 4
Figure 2. Simplified One Line Diagram for Nuclear Unit with Single Offsite Power Supply.
-. - ~
. - ~ - -.. - -.. - -
L p
. Transmission Switchyard
[
Station service ww Step Up --
"" Transformer 2nd Offsite Generator
~g WW Unit Aux, W*
'l mm.
mm Transformer.
mm mm l
)
O.
O
'O l
Q O
O
.ss iE Class 1E T
T Ci i
@@l
@@ii 0
l u_____.
t_________
I Stano6y Diese!
Generators Figure 3. Simplified One Line Diagram for a Nuclear Unit with the Conventional i
Unit Connected Scheme.
Station Generator s,,y;e, ww Step Up WW Transformer mm Transformer ym
.ym s
Generator )
ww Unit Aux.
Breaker ym.
mm Transformer Generator o
gh g
O Auxiliaries Busses O
OOO M
Class IE l l
l Class IE ie i Generator Standby Diesel Generator l
Figure 4. Simplified One Line Diagram for a Nuclear Unit with Generator Circuit l
Breaker or Load Break Switch.
. d-
~
- 2$.
L Es
(
6 i
.EM En j
In this diagram, R = VE 2 + Eg E
2 - 2E Eg cos d S
S where
}
Es = System equivalent volts per hertz
- ~
= System voltage in per unit of motor rated voltage
- i t
divided by system frequency in per unit of rated frequency.
Eu = Motor residual volts per hertz:
= Motor terminal voltage in per unit of motor rated voltage divided by motor speed in per unit of synchronous speed
(
En = Resultant vectorial voltage in per unit volts per hertz-
~
on the motor rated voltage.and. frequency base' Figure 5. Determination of Resultaat Volts per Hertz on Bus Transfer or Heclosing.-
s k
C*
e.
j-DRAFT NRC INFORMATION NOTICE SSINS NO.
IN 90-XX UNITED STATES NUCLEAR REGULATORY COMMISSION OFFICE OF NUCLEAR REACTOR REGULATION WASHINGTON, D. C. 20555 J
June,-1990
)
NRC INFORMATION NOTICE NO 90-XX:
OPERATIONAL EXPERIENCE ON BUS TRANSFER Addressees:
All holders.of operating licenses (0'.s)~or construction permits (cps) for nuclearpowerreactors(NPRs),
i
Purpose:
This information notice is intended to alert addressees on our findings on failure of medium voltage (2kV to -15kV). bus transfers in nuclear plants'in the U.S. between 1985 and 1989.
It is expected that recipients will reviewithe'information for applicability to their facilities and consider actions, as appropriate, to reduce bus transfer failures..However, suggestions ~ contained in this information notice-I l
do not constitute NRC requirements; therefore, no_ specific action or written response is required.
l Description of Circumstances:
In-1988 and 1989, Northeast Utilities is'ued.two licensee event reports-(LERs f
s 50-422/88-026 and 50-422/89-030) indicating that at Millstone Uriit 3, under certain scenarios, the existing bus transfer scheme-can result in common-mode failure of Class 1E loads of both trains.and that repeated bus transfers can-damage the safety related motors.
These findings of Northeast Utilities prompted us to conduct an in-depth study of the bus transfer practices and s
operational experience at different nuclear. plants in the U.S.A.
]
2 Through the system coding and search system (SCSS) of Oak Ridge National Laboratory (0RNL) and our LER screening process, we identified 56 LERs that directly related to failure of bus transfer between 1985 and 1989.
Broadly speaking, the bus transfer failures can be divided into two classes:
- 1) Failures that can damage the loads connected to the auxiliary buses.
- 2) Failures that inhibit bus transfer.
The first class of failures are caused by the excessive voltage difference (in magnitude and phtse angle) between the auxiliary load bus and the incoming power source. Ttis excessive resultant voltage will cause transient current flows in the system which can damage the-transformers, the buses, and the connected loads due to stresses from electromagnetic forces, overheating, and transient torques.
Thus far, there have not been any recorded equipment failures in nuclear plants that can be directly attributed to this'cause, although potentially, some equipment is stressed in this process. Such stresses experienced by connected equipment are cumulative in nature and unless-specifically monitored for, can remain undetected until failure occurs.
AmericanNationalStandardInstitute(ANSI)StandardANSIC50.41," Polyphase Induction Motors for Power Generating Stations," first addressed this issue in 1977.
ANSI C50.41-1982, states that to limit the possibility of damaging the motor or driven equipment, or both, it is reccmmended that the power supply c
system be designed so that the resultant vectorial volts per he-tz between the motor residual volts per hertz and the incoming source volts per hertz at the instant the transfer or reclosing is completed does not exceed 1.33 per unit volts per hertz on the motor rated voltage and frequency bases.
TheNationalElectricalManufacturersAssociation(NEMA)acceptedthisasa safe criteria in NEMA MG-1-1978, Motors and Generators.
Subsequently, this criterion has been challenged as it is unrelated to the motor shaft torque. As a result, NEMA MG-1,1987 has rejected the ANSI C50.41-1982 criterion.
.? '
At present, in most of the U.S. nuclear plants, the bus transfer schemes generally meet the criterion of resultant voltage of 1.33 per unit volts per hertz. With no equipment failure reported, this appears to be a safe figure that can be used in bus transfer design and test verification.
Recently, the Electric Power Research Institute (EPRI) has initiated a study on the whole issue (RP2626-1).
The second f ailure category covers cases where the transfer does not take place, resulting in loss of power to the bus that is being transferred from 1
one source to the other.
In most nuclear plants, such a failure leads to the loss of a preferred power source to plant' auxiliaries, which in some cases, include the_ Class 1E loads. Hence, with this type of bus transfer failure, the Class IE loads will have to rely on the emergency diesel generators, and the reactor coolant sy. tem on natural circulation.
Thus, such failures defeat the main purpose of providing the bus transfer f acility - that of meeting the intent of General Design Criteria (GDC) 17 on availability and reliability of the preferred source.
Though the nuclear plants are designed for safe shutdown on failure of offsite power, it is desirable that such failures are kept to a minimum to reduce stress on the nuclear units and to improve its availability.of offsite power sources. Every improvement in availability of offsite power source is an improvement on station blackout.
L l
Several factors can inhibit a bus transfer.
Our review of the 56 LERs l
documenting the failures of bus transfers has identified defective system design, slow sync-check relay speed (generally the slow electro-mechanical relays), improper relay settings, slew operation of the outgoing breakers, bad auxiliary contacts, system undervoltage, and human error as the main causes for these kind of failures.
i Discussion:
The result of our study on bus transfer is reported in our engineering report AE00/E90-05, " Operational Experience on Bus Transfer." Our study indicates that the licensees can reduce bus transfer failures in their plant by one or a combination of the following modifications:
l
a
. 4:.
a
- l 1
1)
Minimize Bus Transfer Operations:-
In most nuclear plants in the USA, the balance of.the plant loads, and in some plants, the Class IE loads are normally fed from the main generator through the unit auxiliary transformers witn no breaker between the main generator and the main step-up transformer.
When the' main generator trips, the required auxiliary loads are transferred to the offsite power source' by the bus transfer scheme.
Thus the plants with this scheme are, subjected to bus transfers every time the unit trips.
Une way to overcome this limitation is to use a generator breaker to isolate the main generator in the event of a unit trip. Thus, by avoiding bus transfer on unit-trip, the use of generator unit-breaker can sub-
~
stantially reduce bus transfer operations and subsequent bus transfer failures. This scheme is widely used overseas and at 13 plants.in the USA.
1 Another way to achieve this objective is to feed the auxiliary loads from the offsite power source all the time. This practice is followed for Class 1E loads in many nuclear plants in the USA, and for balance of 'the' l '
plant loads and Class IE loads in France, and at-seven nuclear plants in the USA.
2)
Faster bus transfer:
Fast bus transfer schemes are used in most nuclear plants to keep the transfer time to a minimum, normally within 6 cycles. The two main
"*ons of this scheme are the " sequential" and " simultaneous" type.
In
'uential" type, also called " break-before-make-scheme," the
.ig source breaker closes after the opening of the outgoing source-er and is achieved by using a "b" contact or an early "b" contact of atgoing breaker.
In the simultaneous type, the operation of both the incoming and outgoing breakers are initiated at the same time, (i.e.,'the outgoing source breaker is signaled to open and the incoming source breaker is signaled.to close, simultaneously).
Normally, the simul-taneous type has a shot cer dead band (the time period during which the
A
?
.-5 auxiliary bus is disconnected from both sources), usual.ly between one or two cycles compared to five to ten cycles in sequential transfer.
However, if'the outgoing breaker is slow in opening, or fails to open f
then the two sources would be connected in parallel.
This parallel operation is avoided in sequential operation.
In the U.S. nuclear plants,
~
the sequential type of fast bus transfer is the most popular scheme.
Washington Nuclear Plant, Unit 2, Donald C. Coo'k, Units 1 and 2, and all nuclear plants in Sweden, use the simultaneous bus transfer scheme. Thus far, none of these units have encountered any. problems attributable to this transfer scheme. -In the Swedish design, transfer is permitted only if the two sources are nearly~ equal (bus voltage is greater than 70-i percent, frequency, difference is less than 0.05, and phase angle difference is;less than 12-24 degrees), and if.a breaker failure occurs such that both sources are paralleled for more than 0.1;second, then both l
closed breakers will be signaled to open.
~
There are-two concerns when the two sources are connected in parallel.
l The first is that such an operation' can cause. instability in the system and stresses in the connected components. 'In most cases,'both the= main generator step-up transformer.and~the. station reserve transformers are supplied from the same switchyard or from switchyards that are synchro-
~
nized to each other.
Further, the prerequisite con'dition for successful tus transfer, usually monitored by sync-check relays, ensures that the two sources are reasonably in phase and equal'in magnitude.
(Thetwosources-l' can be 'n parallel only when the outgoing breaker is slow in opening or i
failstoopen). Thus, the system disturbance and equipment stresses.
l experienced during simultaneous transfer is minimal.
The second concern is the occurrence of electrical faults beyond.the designed fault duty of the equipment and buses when the two sources are in parallel. To address-this concern, the Swedish design has allowed only a 0.1 second period in the transfer scheme during-which both breakers art.
permitted to remain closed.
1 a,
l In nuclear plants in the UK, two auxiliary load buses are normally fed 3
j-from two' sources and are tie-connected'through a reactor. When one normal source fails, both buses are fed from the.available source'with the tie reactor bypassed automatically.
The EPRI Project Report EL-4286 on bus transfer studies indicates, that for a typical nuclear plant, the critical parameters for comparison between'the simultaneous and sequential schemes are as follows:
i TABLE I.
COMPARISON OF CRITICAL PARAMETERS BETWEEN SIMULTANEOUS AND SEQUENTIAL FAST-BUS TRANSFER SCHEMES PARAMETER SIMULTANEOUS SCHEME SEQUENTIAL SCHEME (Maximum Value)
(2 Cycle Deadband)
(6 Cycle Deadband)
Angle in electrical degrees 32.9 155-between motor internal voltage.
and system Resultant volts per hertz between
.575 1.828 motor internal voltage and system Motor transient torque per
'3.46 9.36 unit on motor basis Motor transient current per
-3.22 9.67 unit This table shows that in sequential fast bus transfer, the connected electrical e
equipment is subjected to substantial electro-mechanical' stresses and can l
exceed the' 1.33 per unit volts-per hertz requirement of ANSI C50.41-1982. ' On l
thE other hand, these stresses are much lower in the simultaneous fast bus transfer scheme (about one-third of sequential transfer) and there is little possibility of-any equipment damage. The worst case. resultant per unit volts per hertz between the motor internal voltage and the system is onlyE43 percent of the ANSI C50.41-1982 requirement.
n.-
w,g--r y
t v
7-=
"-9v w--v
9 o.
7 3)
Supervised bus transfer:
The fast bus transfer schemes can be either supervised or unsupervised type. Our review of the 56 LERs on bus transfer failures indicates:
a.
A malfunction of the generator excitation system can cause excessive i
voltage deviation between the generator terminal and the offsite power.
b.
As established in the Millstone LERs 88-026-03 and 89-030-00 under special operating conditions, the generator voltage and frequency can be substantially different from the offsite source.
c.
In certain plants there may be significant phase shift between the generator voltage and offsite power source, especially when they are not from the same switchyard.
d.
A fault or lightning can cause transient system disturbance in which event, a supervised scheme is the best way to ensure optimum safe
- transfer, e.
Loads with high moment of inertia are suitabic ;0r sequential fast bus transfer and loads with low moment of inertia are more suitable forresidualtransfer(forplantsthatdonotusesimultaneousfast bustransferscheme).
These findings lead to the conclusion that way to ensure safe transfer under all operational conditions is to use the sync-check relay.
In the past, the sync + heck relays were of the indication disc type which are much slower than the static relays developed by Beckwith Electric and General Electric in the 1980's.
-r----
- t o.
e' lt l a
One way to ensure adequate setting of the sync-check relay is to carry out field tests under worst-case loading conditions.
However, it is not always possible to conduct tests under worst-case loading conditions and some licensees have developed computer simulation programs to cover such conditions.
Even in computer simulation programs, information on equipment parameters are assumed and can produce erroneous results if proper care is not used in selecting these parameters.
i 1
i l
1 l
l l
t Technical
Contact:
Subinoy Mazumdar, AEOD 301-492-4308
- l j
+
+-
e n--
.