ML19257A499
| ML19257A499 | |
| Person / Time | |
|---|---|
| Site: | Peach Bottom  |
| Issue date: | 12/31/1979 |
| From: | Daltroff S PECO ENERGY CO., (FORMERLY PHILADELPHIA ELECTRIC |
| To: | Ippolito T Office of Nuclear Reactor Regulation |
| References | |
| NUDOCS 8001040480 | |
| Download: ML19257A499 (17) | |
Text
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PHILAD ELPHIA ELECTRIC COM PANY 2301 MARKET STREET P.O. BOX 8699 PHILADELPHIA. PA.19101 SHIELOS L. D ALTRO FF ELtcTasc PR C ton December 31, 1979 Docket Ncs. 50-277 50-278 Mr. Thomas A.
Ippolito, Chief Operating Reactors Branch #3 Division of Operating Reactors US Nuclear Regulatory Commission Washington, DC 20555
Dear Mr. Ippolito:
This letter is in response to your letter to E.
G.
- Bauer, Jr. (PECO) dated October 30, 1979 concerning the " Adequacy of Station Electric Distribution Systems Voltages for Peach Botton Units 2 and 3".
Philadelphia Electric Company reviewed the adequacy of our of f site p ower s ources to the safety buses in accordance with the guidelines (Ecciosure 2) of your August 8, 1979 letter requesting this information.
We have determined that for all conceivable accident and transient conditions except one, the voltage at the safety loads is within the limi t s specified f or the saf ety loads.
The exception to the acceptable voltage conditions results when the plant is being supplied from a single source, that source is operating at its lowest allowable limit, and one of the Units exp erien cec a turbine trip transient.
To accommodate this scenario we propose to automatically trip the cooling tower loads whenever the plant is being supplied f rom a single of f site s ource and a turbine trip transient (or any generator upset transient) occurs.
With the tripping the voltage of the safety loads is kept within the specified limits.
The details of the analyses and the comparison to the gu i d e lin e s -- a r e-attache d_( Encl o s u r e I).
Als o attached (Enclosure II) in a description of our prop osed degraded voltage detection and isolation logic which will separate any safety bus from an of f site p ower s ource when the voltage is less than the minimum expected value.
^
/-fois s
///
%. no 1687 120 m
Mr. Thomas A.
Ippolito Page 2 Philadelphia Electric Company is proceeding with the design of the logic for the detection of degraded voltage for implementation on Peach Bottom Unit 2 during the Spring 1980 outage and Unit 3 during the Spring 1981 outage based on conceptual design approval given by the NRC at a meeting in Bethesda, MD on November 27, 1979<
Following the Plant Operating Review Committee (PORC) and Operating and Safety Review (0&SR)
Committee approval, a p rop os ed Technical Specification Amendment reflecting the changes discussed above vill be submitted to replace the p rop osed Technical Specification Amendment submitted on December 22, 1977.
Should you have any questions or require additional information, please do not hesitate to contact us.
Very truly yours, f
i./
sj c ey Attachments i687 121
ENCLOSURE I The following is a p resentation of the results of the voltage drop calculations for Peach Bottom Atomic Power Station Units 2 and 3.
The paragraph numbers correspond t o the Guidelines issued by Mr. William Gammill of the NRC (Enclosure 2) with the August 8, 1979 request f or inf ormation.
1.
Separate analyses should be perf ormed assuming the power source to saf ety buses is (a) the unit auxiliary transformer; (b ) the startup transformer; and (c) other available connections to the of f site network one by one assuming the need f or electric p ower is initiated by (1) an anticpated transient (e.g.,
unit trip) or (2) an accident, whichever p resents the largest load demand situation.
Response
The only power s ou rces for the Peach Bottom 4 kV safety buses are the two connections to the of f site network.
Although not assigned t o the respective Units, they are labeled as the #2 and #3 startup sources.
Under normal operations, both startup sources available, the 4 kV safety buses for each Unit are split between the two s ources.
The non-safety 13 kV Unit Auxiliary buses are f ed f rom their corresponding Unit Transformers, and the cooling tower load and s creen structure load is split between the two s tartup esurces.
If one of the two s tartup sources becomes unavailable (Continued operation is permissible for 7 days if Units are running; both startup sources must be available prior to Unit startup) all the 4 kV buses and all the cooling tower and screen structure load can be supplied from one startup source.
The single startup source supplying the station represents the most limiting condition as far as connected loads is concerned.
For the single s ource case, the loads connected to either of the startup sources will be identical so the most limiting case would be when the load is connected to the relatively weaker of the two s ources.
The existing plant procedures for operation with a single offsite source requires that a diesel generator be started to carry one 4 kV safety bus per Unit.
The analyses were
- - - ~ ~ ~ ~
'perf ormed f or both cases of diesel generator running and not running.
The case of diesel generator not running represents the more severe case, the results presented represent this case.
The results of the diesel generator running cases, although not tabulated, are discussed along with the other cases.
The load resulting f rom an accident scenerio rep resents the greatest load on startup s ou rce (s ) since the accident includes a turbine trip / transfer of 13 kV Unit Auxiliary 1687 I22
Mr. Thomas A.
Ippolito Page 2 buses to the s tartup s ource (s ) for the accident unit.
The maximum load occurs during the first few seconds of the accident.
2.
For multi-unit stations a s eparate analysis should be perf ormed f or each unit as s u min g (1) an accident in the unit being analyzed and simultaneous shutdown of all other units at that station; or (2) an anticipated transient in the unit being analyzed (e. g., unit trip) and simultaneous shutdown of all other units at that station, whichever p resents the largest load demand situation.
Response
The procadures covering the anticipated transients, accidents, and the associated alternate Unit shutdowns have been reviewed to determine at what time the various loads are manually operated.
The procedures for shutting down the non-accident Unit call for the reduction of load on the 13 kV Unit Auxiliary buses to the startup sources.
The maximum load occurs during the first few seconds of the accident, however the load during the shutdown of the non-accident Unit is within 5% of this figure.
Both of these conditions were analyzed to determine if the startup sources are adequate to support these loads.
3.
All actions the electric p ower system is designed to automatically initiate should be assumed to occur as designed (e.g.,
automatic bulk or sequential loading or automatic transfers of bulk loads from one transformer to another).
Included should be consideration of starting of large non-safety loads (e. g., condensate p ump s )..
Response
The 4 kV loads that are called up on to respond to the accident are assumed to load onto the 4 kV buses as designed.
The 13 kV Unit Auxiliary buses are assumed t o transfer automatically to the s tartup sources.
The recirculation M-G sets and two of the cooling towers connected t o these buses trip automatically during this transfer.
The transfer of the 13 kV Unit Auxiliary buses is perf ormed in synchronism s o there is essentially no additional inrush current required t o s tart the load.
4.
Manual load shedding should not be assumed.
1687 I23
Mr. Thomas A.
Ippolito Page 3 i
Response
The analyses do not assume any manual load shedding during the course of the accident or transient.
Following the accident or transient the loads are shed or added in accordance with the procedures for those events; these manual actions do not take place until at least ten minutes following the automatic actions.
5.
For each event a n a ly z e d, the maximum load necessitated by the event and the mode of operation of the plant at the time of the event should be assumed in addition t o all loads caused by expected automatic actions and manual actions permitted by ad inistrative procedures.
Response
The various modes of operations were investigated to determine which represented the most severa loads.on the startup sources.
The only voltage degradations occur when there is only one startup source available and the Unit Auxiliary load is being supplied by the startup source.
Since both startup sources must be available prior t o a Unit startup, the single source startup case is not realistic.
The two s ource startup cases do not represent a severe case; and hence, this case does not require an intense analysis.
The cases of one and.two s ou rce op e ra ti on, routine shutdown, accident with alternate Unit shutdown and transient with alternate Unit shutdown were investigated.
The results of these analyses are summariz ed below in Table 1.
6.
The voltage at the terminals of each saf ety load should be calculated based on the above listed considerations and assumptions and based on the assumption that the grid voltage is at the " minimum e xp ected value".
The " minimum exp ected value" should be selected based on the least of the following:
a.
The minimum steady-state voltage exp erienced at the connection to the offsite circuit.
b.
The minimum voltage expected at the connection to the offsite circuit due to contingency plans which may result in reduced voltage from the grid.
c.
The minimum p redicted grid voltage f rom grid s tability analysis. (e. g., load flow studies).
In the report to NRC on this matter the licensee s h ou ld state planned actions, including any p rop os ed " Limiting 1687 I24
Mr. Thoma<
a.
Ippolito Page 4 Conditions for Operation" for Technical Specifications, in response to experiencing voltage at the connection t o the offsite circuit which is less than the " minimum e xp ected value." A copy of the plant procedure in this regard should be provideds
Response
All of the analyses perf ormed assumed that the grid voltage was at 95% of its nominal value which is the minimum design value used f or contingency planning.
We would not expect to operate at this level f or a sustained period of time and many actions to alleviate the situation would be taken substantially before this de graded volt ag;e condition occurred.
We previously presented the grid strength information in our September 15, 1976 submittal, S.
L.
Daltroff (PECO) to G. Lear (NRC).
The existing Technical Specifications already address the unavailability of the startup sources.
Following the installation of the degraded voltage detection and isolation logic unavailability will then include a startup s ou rce that cannot maintain the saf ety bus voltage above the settings af the relays.
Since this action will be automatic, and will be included in the revised Technical Specifications, no procedural actions are required.
7.
The voltage analysis should include documentation f or each condition analyzed, of the voltage at the input and output of each transformer and at each intermediate bus between the connection to the offsite circuit and the terminals of each safety load.
Response
The voltages resulting f rom the various analyses are summarized on Table 1.
8.
The analysis should document the voltage setpoint and any inherent or adjustable (with nominal setting) time delay for relays which (1) initiate or execute automatic transfer of loads from one s ou rce to another; (2) initiate or execute automatic load shedding; or (3) initiate or execute automatic load sequencing.
Response
The voltage satpoints, time delays, and sequence of operations is presented in the des crip tion of the p rop osed i687 125
9 Mr. Thomas A.
Ippolito Page 5 degraded voltage detection and isolation logic for the existing sys tem and the p rop os ed logic.
9 The calculated voltages at the terminals of each safety load should be compared with tha required voltage range for normal operation and starting of that load.
Any identified inadequacies of calculated voltage require immediate remedial action and notification of NRC.
Response
The majority of the 4 kV and 460 volt safety loads were specified t o operate within a band of i 20%, however the motors for the 460 V motor operated valves were specified i 10%.
The operating setpaint for the degraded voltage logic corresponds to a figure of 90% on the 4 kV system, which corresponds to 90% on the 460 V and 115 V systems under the conditione of largest anticipats d loading.
10.
For each case evaluated voltages on each safety bus should be comparad with the voltage-time settings for the undervoltage relays on these safety buses.
Any identified inadequacies in undervoltage relay settings require immediate remedial action and notificatica of NRC.
Response
The existing installed undervoltage relays will not operate for any of the cases analyzed.
The degraded voltage logic would operate for the special case, h ow eve r, the proposed automatic tripping of the cooling towers will alleviate the voltage degradation and therefore the operation of the logic.
11.
To p rovide assurance that actions taken to assure adequate voltage le ve ls for safety loads do not result in excessive voltage, assuming the maximum expected value of voltage at the connection to tne offsite circuit, a determination should be made of the maximum voltage expected at the terminals of each safety load and its starting circuit.
If this voltage exceeds the maximum voltage rating of any item of safety equipment immediate remedial action is required and NRC shall be notified.
Response
The value of 5 % over nominal was selected as the maximum exp ected voltage at the connection to the offsite source.
Again, as explained in the discussion of Guideline item 6, 1687 126
Mr. Thomas A.
Ippolito Page 6 the system would not be op erated at this level; this level is the allowable contingency limit.
The #2 startup transf ormer has a -7 1/ 2 % t ap changer while the #3 startup trans f ormer has only a -2 1/ 2% adjustment capability.
The analysis f or the potential f or overvoltage therefore examined only the #3 startup source.
The load was assumed to be the normal plant load with both Units at p ower since this rep res ent s the smallest load on the startup sources.
Under the conditions of 105 % voltage on the offsite source the voltage on the 4 kV buses connected to the #3 startup source would be 4350 V and the no-load voltage on the 460 V motor control centers would be 502 V.
These values are within the +10 % voltage rating of the connected equipment.
Any load added to the 4 kV or 460 V buses would reduce these voltage levels closer to their respective nominal values.
Therefore any accident, transient, or Unit shutdown w ould result in voltages more p roxima te to the nominal levels.
12.
Voltage-time settings for undervoltage relays shall be selected s o as to avoid spurious separation of safety buses f rom of f site power during plant startup, normal operation and shutdown due to startup and/or operation of electric loads.
Response
As described in the attached p resentation on the degraded voltage logic the voltage-time characteristics were selected to prevent spurious operations of the logic during all modes of plant operations to include startup, normal operation, shutdown, accidents and transients.
13.
Analysis documentation should include a statement of the assumptions for each case analyzed.
Response
Following is a liet of assumptions made for the analyses:
a)
The offsite s ources were assumed to be op e rat ing at 95% of their nominal voltage.
Although we would not be expected to op erate at this level f or any sustained period of time it is remotely possible that we might achieve this level during system contingencies.
b)
Nominal voltages are 13.2 kV, 4160 V, 460 V and 115 V.
1687 127
Mr. Thomas A.
Ippolito Page 7 c)
The automatic transfer of the Unit Auxiliary buses and the automatic tripping of the recirculation MG sets and cooling towers were assumed to occur as designed.
This equipment is not considered safety grade a lth ou gh it is the same class of switchgear as the 4 F.V swit ch ge a r.
The automatic load tap changers were als o assumed to operate as designed.
d)
Voltage transients caused by inrush would not persist for more than ten seconds.
e)
The manual operations perf ormed t ollowing the transients were assumed to be done in accordance with the procedures and with no regard to the startup sourca loadings.
The procedures will be revised to require more awareneus of the potential f or an undervoltage condition when only one startup source is available.
f)
As mentioned in the discussion on Guideline Item 1, the loads on either of the two s tartup sources for the single source case would be identical.
The #3 startup transformer has an additional 5 % autematic tapchanger capability over the corresponding #2 startup transformer.
Therefore, as far as responding to degraded voltage conditions is concerned, the #2 startup source rep res ents the most limiting s ou rce.
The analyses, therefore, only examined the #2 startup source voltage f or both the single and two s ource cases.
The #3 startap s ou rce voltages would be capable of an additional 5 % boost for all the values presented in Table 1.
i687 128
DESCRIPTION OF CASES ANALYZED CASE I Both startup sources are available for this case.
The grid is operating at 95% of its nominal value and both Units are operating at p ow e r.
This represents the normal plant load and typical voltage levels.
CASE 2 This case represents the maximum steady state voltages that the Units would experience following (during) an accident in one Unit with the other Unit at power.
This load is greater than any of the loads associated with stabilizing the accident Unit followed by a shutdown of the non-accident Unit.
Both startup s ou rce s are available for this case and the grid is at 95% of its nominal value.
The loads experienced during startup are als o less than the load f or this case.
CASE 3 This case represents the voltages with both Units operating, one startup source available and the grid operating at 95%.
The tapchanger is at its upper limit.
CASE 4 This case represents the maximum steady state voltages that the Units would experience following (during) an accident in one Unit with the other Unit at power.
For this case only one startup source is available and the grid is operating at 95%.
As can be seen from Table 1, the voltages are outside the i 10 % limits.
As stated in the response t o Guideline 1,
the more realistic case of op erating with one diesel generator carrying one 4 kV bus per Unit would result in a 4 kV voltage of 3440 V for this same case.
Although this is a marked imp rovement over the case analyzed, it is still outside the 110% limits.
CASE 5 This case is identical to CASE 4 excep t that the cooling tower loads are tripped automatically during the c ou rs e of the accident.
This action re-establishes the voltages within acceptable limits.
If, as s tated p reviously, a diesel generator is operated to carry one 4 kV bus per Unit, the resulting voltage in this case would be 3945 V; well within the established limits.
CASE 6 This case represents the maximum s teady state voltages that the Units would experience during the shutdown of the non-accident Unit following an accident in one Unit with only one startup source available.
For this case the grid is operating at 95%.
This case represents the voltages that would be seen if the non-accident Unit was shut down in accordance with the procedure with no regard f or the load on the startup i687 i29
Mr. Thomas A.
Ippolito Page 2 source.
As can be seen f rom Table 1, the voltages are outside the +10% limits.
CASE 7 This case is identical to CASE 6 except that the loading of the startup s ou rc e is more clos ely regulated during the shutdown of the non-accident Unit.
The shutdown p rocedures will be revised to ensure that the manual t ransf erring of loads under the contingencies of the unavailability of one startup s ou rce is perf ormed in a manner conducive to the maintenance of the plan". voltage.
1687 130
TABLE 1 m
Summary of Results N
t CD C
"~
CASE (1)
(2)
(3)
(4)*
(5)
(6)*
(7)
Location 220-08 Line (kV) 218.5 218.5 218.5 218.5 218.5 218.5 218.5 i
2SU Bus (kV) 13.1 13.2 12.9 10.3 12.6 11 3 12.2 4 kV Buses (V) 4110 4090 4050 3080 3840 3475 3770 460 V Motor (V) 464 448 458 341 421 384 416 Control Centers Nominal Voltages Acceptable Limits 230 kV 241.5 - 218 5 **
13.2 kV 14.52 - 11.88 4160 V 4576 - 3740 460 V 506 - 414 The plant systems and procedures are being modified t o p reclude these cases
- These are the bounds on the operating range of the grid O
e
ENCLOSURE II Degraded Voltage Detection and Isolation Logic Following is a description of the existing undervoltage detection and isolation logic on the 4 kV safety related buses.
A typical 4 kV safety related bus voltage scheme is shown in Figure 1.
Each of the two incoming f eeder breakers is equipped with an IAV53D relay that monitors the voltage on that feeder.
This relay has an inverse time voltage relationship, the lower the voltage the faster the drop ou t of the relay.
The relay operation begins when the voltage on the relay coil is between 65 to 70 volts.
At this voltage the relay takes approximately 1.7 seconds.
When this relay senses loss of voltage and operates, it trips its respective feeder breaker and the bus is de-energized.
Each bus has an HGA relay that initiates shedding of all connected load on the 4 kV buses and provides a permissive signal t o t rans f er to the other source, to start the diesel generator, and.to automatically close the diesel generator breaker.
Once the IAV53D relay trips its source breaker and the HGA relay drops out, a transf er to the alternate s ou rce is called f or following a time delay of.25 seconds.
If the alternate source breaker does not close in the next.25 seconds, the diesel generator is started and once proper f requency and voltage have been established, the diesel generator breaker is closed and the bus is re-energized.
Each bus als o has an SV relay that monitors bus voltage.
This relay is set to pick up above ap p r oxima t ely 105 volts.
It is this relay that permits loading of the 4 kV bus once voltage has been restored.
Once this relay is picked up it is sealed in to prevent drop out on voltage dips during loading.
The SV relay is dropped out by a contact from the HGA when bus voltage goes below the 35 volt level.
The IAV53D, the HGA and the SV relays are fed through 4200-120 volt potential transformers.
The corresponding voltages on the 4 kV bus are as follows:
Secondary (Volts)
Primary (Volts)
IAV53D 65-70 2275 - 2450 HGA 35 1225 SV 105 3675 The IAV53D setpoint was chos en at 65-70 volts to prevent spurious transfers on the starting of large motors.
Under accident conditions, two 2000 hp motors are s tart ed s imult aneou sly on the same offsite power supply.
Under these conditions, the supply 1687 i32
Mr. Thomas A.
Ippolito Page 2 voltage can momentarily dip to approximately 80% of normal value.
The setpoint was ch os en to ba near the minimum motor accelerating voltage.
The prop osed degraded voltage detection and isolation logic f or a typical 4 kV safety related bus is shown in Figure 2.
In addition to the existing IAV53D, the voltage of the incoming faeders will be monitored by a CV-1 relay and an NGV relay.
An NGV is an instantaneous relay that will be set to operate below 107 volts on the p otential trans f ormer.
The CV-1 will be essentially asymptotic t o this same voltage.
The NGV will op erat e a 60 second timer such that if the relay detects degraded voltage for more than 60 seconds it will trip its associated feeder breaker to initiate the transfer to the alternate s ou rce as described above.
The CV-1 is an inverse time voltage relay; at 96 volts it operates in 36 seconds and at 72 volts if operates in 13 seconds.
Again when the CV-1 operates and times out, it trips its associated feeder breaker to initiate the t rans f er t o the alternate source as described above.
The 107 volts at the potential transformer corresponds to 3745 volts on the 4 kV buses.
The characteristics of the NGV and CV-1 relays, along with those of the existing IAV53N relay, are shown on Figire 3.
The operation of any one of these three relays will initiate the t rans f er t o t he a.'.t ernat e s ou rce and will initiate the s tart of the diesel generator if the alternate s ource is not available.
All three of these relays provide a permissive to allow the automatic closure of their respective breakers.
Each 4 kV feeder breaker has its own individual s et of undervoltage and degraded voltage monitoring relays; there are sixteen sets of these relays for the 4 kV buses for both Units.
These relays will be installed in and considered part of the Class IE switchgear.
The installation and initial testing of these relays will require an outage of the Unit being modified.
We would exp ect to install the Unit 2 relays during the Spring 1980 refueling outage and the Unit 3 relays during the Spring 1981 refueling outage.
We would at t emp t to ins tall the Unit 3 relays prior to the refueling outage if the Unit is down f or an extended period f or another reason.
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