ML18082B285

Responds to NRC Questions Re Degraded Grid Voltage Protection at Facility.Proposed Design for Second Level of Undervoltage & Related Matl Encl

ML18082B285
Person / Time
Site:	Salem
Issue date:	03/03/1980
From:	Librizzi F Public Service Enterprise Group
To:	Schwencer A Office of Nuclear Reactor Regulation
Shared Package
ML18082B279	List: ML18082B278 ML18082B285 ML18082B289
References
NUDOCS 8010060279
Download: ML18082B285 (11)

Text

Public Service

• ATTACHMENT l Electric and Gas March 3, 1980 Company Mr. Albert Schwencer, Chief Operating Reactor Branch tl Division of Operating Reactors U.S. Nuclear Regulatory Commission Washington, D.C. 20555 Gentlemen:

DEGRADED GRID VOLTAGE PROTECTION (70-90%)

SUPPLEMENTAL INFORMATION SALEM GENERATING STATION UNITS NO. l AND 2 DOCKET NO. 50-272 The enclosures attached to this letter are submitted in response to your questions about the degraded grid voltage protection at Salem. Since the October, 1979 meeting with the NRC staff, ques-tions about the proposed design for a second level of undervoltage protection have been received and are addressed in Enclosure 1. contains related information requested by Mr. W. Ross during the October, 1979 meeting.

If you have any questions, please do not hesitate to contact us.

Very truly yours, C. i*~; ;1 :.~ ~, -*:-. *- :1 F. P. L!t.i£:::.i

.t*-4 **/, -'*

Frank P. Librizzi General Manager -

Electric Production

ENCLOSURE 1

l. The onsite distribution system for each unit at Salem is arranged so that two vital buses are connected to one station power transformer and the third is connected to the other station power transformer (FSAR figure 8.3.1).

The in-feed breakers for each ~ital bus from the two station power transformers are electrically interlocked to prevent paralleling both sources through a vital bus. The breakers also provide the means for transfer-

_ring between sources in the event of an interruption of power from one source.

Undervoltage protection for each vital bus will be pro-vided by two protective relay groups. One group is de-signed to protect the vital buses if bus supply voltage falls below 70% of its rated value. This group is al-ready installed and operableo The other group will be designed to protect the vital buses if bus supply volt-age falls below 91% of its rated value.

Each undervol tage protection- group is/will be comprised of two sets of relays:

A set for undervoltage transf~r, and A set for generating *s1ackout" signals.

The 91% group will be comprised of adjustable time delay relays. The protective relays will be connected to the electrical system in the same manner and location as the present undervoltage protection. The new design essen-tially duplicates the present protective scheme in all regards (Attachment*A) with the following exception.

The 91% relays will be equipped with an administratively controlled lockout which will allow the control room op-erator to disarm the system during the start of any re-actor coolant pump. This feature is required to fore-stall any unnecessary undervoltage signals due to the voltage transient caused by a reactor coolant pump start. The lack of this manual lockout feature would require that an extended time delay (30 seconds) be used to actuate the 91% relays. The system will be armed by the co~trol room operator upon completion of any RCP start, and will remain armed under all conditions other than a RCP start. The disarming and rearming of the system will become an integral part of the RCP starting HK19/2

procedures used at Salem and an alarm will be provided for the disarmed condition. In addition to manual re-arming, a timer will be provided to automatically rearm the system in the event the control room operator neg-lects to do so after a RCP has been started.

The time delay of the 91% undervoltage transfer relays will be 10.5 seconds when the output of the station power transformers is below 91% of its rated value (Figure 1).

The time delay of the 91% bus *blackout" relays will be 13 seconds when the voltage on the affected bus (or buses) is below 91% of rated value (Figure 1).

In the event the supply voltage to a 4 KV vital bus or buses falls below 91% of its rated voltage, the affected bus or buses will be automatically transferred to the alternate source by the action of the vital bus transfer relay (XET-230, Fig. 1) and the 91% transfer relays af-ter a 10.5 second time delay; The following conditions must be met before a bus trans-fer may be accomplished at either low-voltage condition:

a. The bus differential or overload relays have not operated.

b. Voltage on the affected bus or buses is below 35%

(this permissive prevents transfer with excessive out-of-phase residual bus voltage).

c. The in-feed breaker of the normal supply is opened.

d. The related diesel-generator circuit breaker is open.

e. The alternate source voltage is above 91%.

f. The SEC (Safeguards Equipment Control) bus under~

voltage (blackout) relays have not operated.

For both groups (70% and 91%), the undervoltage and vital bus (XET-230) transfer relays allow the affected bus or buses to be transferred to the remaining station power transformer before the bus blackout relays are tripped.

HK19/3

-e **

If the supply voltage to the vital buses falls below 70~

of rated voltage and a transfer is not accomplished, the 70% blackout relays will provide a signal to start the diesel-generators. If the supply voltage to the vital buses falls below 91% of rated voltage and a transfer is not accomplished, the 91% bus blackout relays will pro-vide a signal to start the diesel-generators.

Undervoltage signals generated by either set of blackout relays will be combined {through the use of buffer re-lays) in a 2/3 logic matrix per bus to develop a black-out loading signal for that bus. The buffer relays will be used on each vital bus undervoltage sensor to supply independent signals to each SEC unit to maintain inde-pendence among the three buses.

If the output voltage of a station power t~ansformer supplying one vital bus falls to 70% of its rated value and the transfer mechanism fails, the 70% blackout relay for that bus will generate a signal which results in a 1/3 condition at each SEC controller. The Salem design is such that a loss of one vital bus is tolerable for all normal operating conditions; therefore, no automatic equipment actuation will take place for this condition.

This design will also apply to the new 91% protective relays. For a postulated LOCA, this criterion will not apply. A postulated LOCA concurrent with an undervolt-age condition on one vital bus is discussed in the re-sponse to question 5.

2. The proposed design will employ test switches which can be used in conjunction with any external equipment {var-iable power supply, etc.) necessary for proper calibra-tion and testing.* .

Technical specifications similar to those for the exist-ing undervoltage protection will be generated upo~ the Staff's approval of the proposed design.

3. Since this design is a duplicate of the preser.t under-voltage protection system* {except for the administrative controls), it will meet the necessary criteria for pro-t~ction anc..i control of Class lE equipment (IEEE 279-1~71).

HK19/4

4. If a LOCA concurrent with a voltage degradation which reduces the output of both station power transformers to between 90% and 70% of rated voltage is postulated, the SEC syste~will react only to the LOCA while the 91%

transfer relays are timing out. The time for the relays to actuate will be 10.5 seconds.

While the relays are timing out, the SEC system will perform the following functions:

a. Start the diesel-generator units.

b. Lockout manual control of equipment circuit breakers until the required loads are connected to the vital buses.

c. Connect all required accident loads.

The diesel-generators are started automatically so as to be available in the event they are subsequently re-quired. They are not autom~iically connected to the vi-tal buses.

The ability of the safeguards motors to start and carry their designated loads under degraded voltage conditions is described in Attachment B. The safeguards motors are capable of withstanding degraded voltage conditions for the times under consideration without suffering any thermal damage.

When the 91% transfer relays time out (10.5 seconds),

the transfer will not take place because the station power transformer potential relays will not generate a permissive. Therefore, the 91% blackout relays will be allowed to time out (in an additional 2.5 seconds), and a blackout signal will be generated. When the blackout signal is generated, the SEC will automatically shift modes from that for a LOCA (Mode I) to that for a LOCA plus blackout (Mode Ill}. The shift of nodes will re-quire less time than the recognition and action required to combat only a blackout due to the *ready* status of the diesel-generators.

HK19/5

~*--*

-s-The above mentioned sequence of operations will take place within the required time limits to successfully mitigate the consequences of a LOCA. A delay time of 15 seconds between the occurrence of the incident and the application of power to the first sequenced safeguards pumps was assumed in the original LOCA analysis

• Although regarded as extremely unlikely, it may be pos-tulated that one station power transformer may suffer a voltage degradation which reduces its output voltage to between 90% and 70% of its rated value while the output of the remaining station power transformer is reduced to just above 91% of rated voltage. Under these condi-tions, one set of 91% transfer relays will begin timing out while the other set *sees* no abnormal conditions.

It may be possible to reduce the output voltage of the

• normal" transformer to below 91% of its rated value af-ter the transfer from the affected transformer takes 2lace. Also, the output voltage of the initially af-fected transformer may rise above 91% of its rated value due to its partial unloading. These voltage changes will not amount to more than 3% for each transformer.

Consequently, the 91% transfer relays for the alternate transformer will begin timing out and would effect a subsequent transfer at the end of an additional 10.5 seconds. These conditions would result in a continual flip-flop condition causing intermittent power interrup-tions on the vital buses.

This action will be prevented by the installatio~ of 91%

blackout relays which have a 95% reset setting. The 91%

blackout relays began timing out at the same time as the 91% transfer relays on the initially affected trans-former. Since their time delay will be 2.5 seconds longer than that of the transfer relays, and if the transfer does not successfully raise the bus voltage above 95%, the bus relays will initiate separation of the bus from both transformers. The reset setting of 95% on the transfer relays will also ensure that the buses do not continually transfer from one source to the other.

The interlocks and permissives utilized in the transfer of buses are described in Item 1.

HK19/6

5. In the event a LOCA occurs concurrent with a voltage level on one vital bus below 90% and above 70% of rated voltage, the SEC response will be the same as that ex-plaine~ in Item 4 while the 91% transfer relays are tim-ing out. Once the relays have timed out, a transfer to the alternate source will take place.

If the transfer mechanism fails, a blackout signal will be generated *for the affected bus and the SEC will auto-matically shift from a Mode I (LOCA) to a Mode IV (LOCA plus one vital bus undervoltage) condition, whereby only the affected bus is connected to its diesel-generator.

The other two buses will remain connected to offsite power.

If a LOCA occurs concurrent with a degraded voltage con-dition on two of the three vital buses which reduces the bus voltages to between 9*0% and 70% of rated voltage, the SEC will react as explained in Item 4 while the 91%

transfer relays are timing out. Once the relays time out, the buses will be transferred to their alternate source.

If the transfer mechanism fails, a bl~ckout signal will be generated and the SEC will automatically shift from a.

Mode I (LOCA) to a Mode III (LOCA plus blackout) condi-tion, whereby all three vital buses will be shifted to diesel-generator power.

For both postulated conditions, the safeguards motors on the affected bus or buses will be subjected to degraded voltage conditions for no more than 13 seconds. Their ability to start and maintain operation (or to withstand

• a postulated voltage degradation which prevents starting) during the period prior to bus transfer, and the accepta-bility of the time delays involved are explained in I tern 4.

6. The characteristics of AC contactors and associated control fuses are described in Attachment B.

7. With regard to Staff's suggestions of utilizing only bus blackout relays in the proposed design (no attempted transfer), the following scenario is presented.

HK19/7

The existing diesel starting and sequence loading logic is located in the Safeguards Equipment Control (SEC) sys-tem associated with each bus. The plant design is predi-cated on each SEC performing the master decision making and resultant actions associated with bus loading. To make use of this existing logic with the design sug-gested, would require paralleling the existing bus under-voltage inputs and the proposed secondary bus under-voltage relays.

Assuming an initial system configuration of two buses being supplied from one station power transformer (as-sume tll) and the remaining bus powered by 112 SPT, a degraded voltage condition on ill SPT side, coupled with a single failure within the bus voltage monitoring logic, could result in the following condition:

Bus *An operating with degraded voltage due to failure in monitoring logic.

Bus *Bn operating with degraded voltage due to lack of required coincident logic in SEC (failure cascaded from failure in Bus *A* circuits).

Bus *c* operating at normal voltage supplied from 112 SPT.

The above scenario assumed that no condition exists which would generate a Safety Injection (SI) signal. If an SI signal were to exist during this occurrence, the following system configuration would result:

Bus *A* could attempt to block load all ESF loads during a degraded voltage situation. This would most likely result in a further bus voltage degradation to below 70%

and cause the diesel to start and commence sequential loading via the SEC.

• Bus *a* would follow the sequence of events described in Item 5 for a postulated LOCA concurrent with a voltage degradation on one vital bus.

• Bus *c* would *block load* the ESF loads on the normal supply from the 112 SPT.

The sequence described for the case of *No SI Signal* is unacceptable. The proposed design which includes an at-tempted transfer, precludes that sequence of events and ensures that at least two buses are capable of supplying the needed equipment.

HK19/8

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