ML19309B890
| ML19309B890 | |
| Person / Time | |
|---|---|
| Site: | Pilgrim |
| Issue date: | 03/28/1980 |
| From: | Andognini G BOSTON EDISON CO. |
| To: | Ippolito T Office of Nuclear Reactor Regulation |
| References | |
| 80-49, NUDOCS 8004070304 | |
| Download: ML19309B890 (5) | |
Text
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,O BOSTON EOleON COMPANY oEuEnAL orricEm soo movLaToM sTnEET Waaron. MassacNumETTs o2199 G. cAmL ANoceNINS BUPEWINTENDENT NUCLEAR OPENATIONS DEPANTNENT y
March 28, 19,80 BEco Ltr. #80-49
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Mr. Thomas A. Ippolito, Chief Operating Reactors Branch #3 Division of Operating Reactors Office of Nuclear Reactor Regulation U.S. Nuclear Regulatory Commission Washington, D. C.
20555 License No. DPR-35 Docket No. 50-293 Conference Call Review on PNPS Electrical Power Systems
Dear Sir:
In telephone conversations on February 4, 6, 7 and March 5 of 1980 with members of your staff reviewing our correspondence on onsite emergency power systems at PNPS it was determined that the following information was needed in order to complete your review in this area.
NRC Concern BECo letter (G. C. Andognini) to NRC (T. A. Ippolito) dated September 27, 1979 stated that changes to the degraded voltage trip set points were being re-analyzed.
What are the New setpoints?
Response
Minimum # of Operable Instrument Channels Per Trip System Trip Function Trip Level Setting 2
Startup Transformer 3745V i 2% with Degraded Voltage 9.2 1 0.5 sec.
time delay NRC Concern Please provide a supplement to August 24,1977 " Proposed Electrical Power Systems O(
Technical Specifications" to include surveillance testing for the timers on the shutdown transformer (should be at same testing frequency as timers on D/G).
Response
We will provide the supplement as requested; however, due to administrative review policies for technical specification changes, it will be transmitted under a separate cover letter in the near future.
8004070 3dy
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- 30 ', TON I E!CCN COMPANY
. MriI Thomas A.'
Ippolito, Chief.
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March l28,11980-t JPage'2' i
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NRC Concern w
Ideally, degraded voltage protection should be provided for all s'afety-related eguipment under all conditions, therefore, please extend the scope of your review
-to consider installing under-voltage trip devices for the' auxiliary transformer breakers and the shutdown' transformer breakers, 'or' provide the basis for no't installing these devices.
7 i.
~ Response l'
f Shutdown Power Transformer i
- In addition to the unit, preferred and diese1~ generator, there is another " secondary AC" power
- source available in each 4160V emergency service auxiliary bus. This 1
t secondary AC power source is. connected to the 23 kV transmission line. The vol-i' tage is reduced from 23 kV.to 4160 V by a shutdown transformer.
Each shutdown power transformer breaker is automatically connected to each 4160 V emergency.
service auxiliary bus af ter a 12 second time delay, if the diesel generator breaker fails to close on the bus during a design basis event.
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' The-loss of secondary AC power source is sensed by undervoltage ' relays which are -
i located at 'the source side of the shutdown-transformer breakers 501 and. 601.
An.
undervoltage condition is also alarmed at the control room. The undervoltage relays are General Electric type and Model No IAV53B.
The trip set point has j
been. selected from a curve which insures relay trip in.1.1 second, if the voltage at the 4160 V side of the-shutdown transformer drops to zero.
It will trip in 18 seconds if the. voltage at the 4160 V side is 3094 volts.
Installation of the second level of undervoltage relays to trip the shutdown transformer. breakers is not necessary since this power source is not intended for supplying the emergency buses during startup and/or normal power plant opera-i-
tion. This : power source is designed for ' supplying power to the emergency bus if
. the diesel generator breaker fails to close on the bus during design basis events.
1 j ~
E voltage alarm at the eantrol-room are adequate for the protection of safety re-
.The present trip set points of the undervoltage relays together with the under-
[
lated equipment under a111 conditions.
4-Unit Auxiliary Transformer 1
- In 1977, the. Nuclear Regulatory -Commission issued a staf f position which required all nuclear plants to institute modifications to protect redundant safety related
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electrical equipment in the -event. that the auxiliary bus voltage exceeded certain t
-pre-established limits..Such modifications.when implemented were to consist of
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automatically disconnecting the offsite power supply (unit or preferred) on sensing j:
. such abnormal conditions.
I In1 response to this NRC position, Boston Edison Company analyzed the unit auxiliary E~
system.of. Pilgrim. Station Unit-#1 and committed-to install a second level under-voltage relay protection' scheme = on the source ' side of the startup. transformer breakers in ~ order to trip these. breakers during a degraded voltage: condition at the safety.related' buses.: In addition, undervoltage. relays 'are installed on the safety 3
'related buses.to alarm at the control room to:give an early indication to the operator of a degraded voltage" condition 'at these buses. This protection scheme I
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'EOSTON E!!ZN COMPANY Mr. Thomas A. Ippolito, Chief March 28, 1980 Page 3 will protect the safety related equipment from a degraded voltage condition and also automatically disconnect the offsite power supply (startup transformer) on sensing such abnormal voltage conditions. Disconnection of the offsite power s6urce would cause a loss of power on the emergency buses which'would in turn result in automatic starting and connection of diesel generators. The diesel ginerators would now supply safety loads with a reliable onsite power.
We have analyzed the degraded voltage condition with the unit in operation and with the safety related buses energized through the unit auxiliary transformer. As stated in our previous correspondence, the load flow studies showed that the voltage ranges of the safety related buses were within the operating limits of the equipment if the grid voltage is within the normal operating range of Pilgrim Station's grid system voltage with the main generator on line. A degraded voltage condition at the safety related buses, while the unit is operating normally, will be alarmed at the control room. The operator then takes appropriate action to safeguard the nuclear facility by contacting the system dispatcher.
The operator is then able to assess and evaluate whether the voltage degradation is long-term or short-term and whether the system dispatcher has taken immediate action to correct the situation. Based on this evaluation the operator can decide whether to effect an immediate disconnection of offsite power, or whether to prepare to disconnect offsite power after a limited time period. This will preclude inadvertent loss of the generating unit from the grid every time there is a disturbance.
In an emergency situation, such as a LOCA together with a degraded voltage, the unit will be automatically disconnected from the grid, and the safety related buses will be supplied by the start-up transformer if the voltage is above an acceptable limit. When the unit is disconnected from the grid, the second level of under-voltage relays will protect the safety related systems.
During the aforementioned conference calls, members of your staff indicated that the current Commission position prefers the installation of a second level of undervol-tage relay protection scheme to trip the unit auxiliary transformer breakers. We agree with the concept of providing degraded voltage protection for safety related equipment; however, in order to provide the most positive form of protection for the safety related systems from a degraded voltage condition while the unit is in opera-tion, we plan to re-evaluate our former position to include conducting new grid studies.
Our evaluation will be based on the risks associated with those of immediate shut-down versus those associated with continued operation. Any proposed modifications as a result of our evaluation will be submitted for your review.
Also, as clarification to information given in our recent conference calls,we offer the following:
a) The bus undervoltage relays which trip the unit auxiliary transformer breakers are set at 840 V i.e., 20% of 4160 V.
b) The bus undervoltage relays which alarm at the control room are set at 3850 V (92.5%), 9.2 sec.
c) We have installed the second level of undervoltage protection scheme at the source side of startup transformer breaker but plan to connect the trip cir-cuits af ter monitoring the trip contacts during the startup of the plant.
This will provide assurance that there is no spurious separation of safety related buses from of fsite power source due to small disturbances on the buses such as the starting of large loads.
.r, BOSTON EECGN COMPANY -
- Mr. Thomas A. Ippolito, Chief
-March 28; 1980 Page 4 1
WD believe this letter adequately addresses your concerns on this subject.
O However, should you have any-further questions or concerns please do not hasitate to contact us.
Very truly yours,
~
jm An-ff, i
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