ML18136A062
| ML18136A062 | |
| Person / Time | |
|---|---|
| Site: | Surry  |
| Issue date: | 10/15/1979 |
| From: | Stallings C VIRGINIA POWER (VIRGINIA ELECTRIC & POWER CO.) |
| To: | Harold Denton Office of Nuclear Reactor Regulation |
| References | |
| 583A, NUDOCS 7910170307 | |
| Download: ML18136A062 (4) | |
Text
VrJ?GlNIA ELEC'l'RIC A.ND PowER COMPANY RICHMOND,VIRO:CN.IA 23261 October 15, 1979 Mr. Harold R. Denton, Director Office of Nuclear Reactor Regulation Attn:
Mr. A. Schwencer, Chief Operating Reactors Branch #1 Division of Operating Reactors U. S. Nuclear Regulatory Commission Washington, D. C.
20555 Serial No.
583A PO/FHT:baw Docket Nos:
50-280 50-281 License Nos:
DPR-32 DPR-37
Subject:
Surry Power Station Units 1 and 2 Degraded Grid Voltage
Dear Mr. Denton:
In accordance with your letter dated July 11, 1979, requesting additional information on the above subject and our letter dated September 4, 1979, Serial No. 583, the Virginia Electric and Power Company herewith forwards the attached response.
Our investigation has revealed that a degraded bus voltage protection system should be installed at Surry to provide undervoltage protection between 80 and 90 percent of nominal voltage.
The response we are providing is based on the addition of this system, and is responsive to the Staff's three positions relative to emergency power systems for operating reactors.
We will forward a final design for the modifications and a Technical Speci-fication change as soon as they are available.
If you have any questions or require additional information, please contact this office.
cc:
Mr. James P. O'Reilly, Director Office of Inspection and Enforcement Region II Very truly yours, uJ~t rO 1 /,/
C. M. ~~s Vice President-Power Supply and Production Operations
1.0 NRC REQUEST e
e Attachment page 1 REVIEW OF THE EMERGENCY A-C POWER SYSTEM VOLTAGE PROTECTION SYSTEM SURRY POWER STATION -
UNITS 1 & 2 VIRGINIA ELECTRIC AND POWER COMPANY Degraded voltage protection is not provided between 80-90 percent of nominal voltage; sustained voltage, even for a short period of time, within this range, may cause damage to electrical equipment.
Present an analysis that supports your design of not having protection within tbis range; or provide appropriate protection between 80-90 percent of nominal range.
RESPONSE
The existing undervoltage protection for the emergency alternating current (a-c) power systems at Surry has one level of protection designed primarily for loss-of-voltage detection.
Nuclear Regulatory Commission Staff Positions 1, 2, and 3, relative to emergency power systems for operating reactors require a second level of voltage pro-tection.
Our analysis of bus voltage levels and conditions indicates the need for a second level of voltage protection.
Modifications to Surry Units 1 & 2 emergency a-c power systems will be implemented to comply with the requirements of the NRC Staff Positions.
POSITION l Second Level of Under or Overvoltage Protection With a Time Delay The 4,160 v emergency a-c systems will be modified for Surry Units 1 & 2 as follows:
1.1 Addition of potential transformers, undervoltage relays, timers, and auxiliary relays to make the existing "loss-of-voltage11 protection system (first level) a "two-out-of-three" coincident logic scheme.
In the event of a loss of offsite power, this scheme will automatically initiate the transfer of the emergency a-c power system from the preferred to the onsite power source.
- 1. 2 Addition of a "two-out-of-three" channel, degraded (second level) voltage-protection system to include undervoltage relays, time-delay relays, auxiliary relays, and alarms.
In the event. of a sustained undervoltage condition below the safe operating level of the safety-related loads, but above the setting of the loss-of-voltage protection scheme, this will initiate transfer of the emergency a-c pow~r systems from the preferred to the onsite power source.
e e 1.3 Overvoltage protection is not required, since the automatic tap changer on the reserve station service transformers POSITION 2 can compensate for the highest postulated overvoltage con-dition.
The highest projected level of voltage on the 500 kv grid is 520 kv.
This level is well within the compensa-tion capability of the automatic load tap changer.
Interaction of Onsite Power Sources With Load Shed Feature Currently no safeguards loads are shed from the Surry*emergency buses during transfer from the offsite power source to the onsite generator.
Two nonsafeguards loads, the residual heat removal pump, and component.
cooling pump are automatically shed when transferring from the offsite to the onsite (diesel) power source.
POSTION 3 Onsite Power Source Testing An emergency bus undervoltage test scheme will be developed to demonstrate the full functional operability and independence of the_onsite power sources.
- 2. 0 "NR.C REQUEST Describe the sequence of events (such as loading, shedding, bus de-ener-gization, load sequencing, etc.) if, while on onsite power, the onsite power source is interrupted.
- Describe the reinstatement or basis for no reinstatement of load shedding.
RESPONSE
Each train of the Surry 4 kv a-c emergency buses has two parts: the main safety bus and the stub bus connected to the main bus by a.circuit breaker.
The main bus powers loads required for safe shutdown.
The stub bus powers the component cooling water pump and residual heat removal pump.
Failure of the offsite power source causes the following automatic action:
2.1 Nonna! power to the emergency buses is interrupted by tripping the normal (offsite) feeder breakers.
2.2 The stub buses are shed from the emergency buses by the automatic trip circuitry of the tie circuit breaker.
2.3 The residual heat remova~ pump circuit breakers are tripped.
2.4 The component cooling water pump circuit breakers are tripped.
l', 2.5
_The emergency diesel generator output breakers are closed to re-energize the safety buses.
If, while on onsite power (diesel), the onsite power source is interrupted, the following automatic sequence occurs:
2.1.1 Loss of voltage relays at the emergency bus function to trip the stub bus, the component cooling pump, and the residual heat removal pump.
2.1.2 The "loss-of-voltage" condition is alarmed in the control room.
The residual heat removal (RHR) pump and component cooling pump are not part of the unit safeguards system.
If the loss of offsite power is
~ot accompanied b-y a loss-of-coolant accident, the safeguards equipment
~snot required.
Under this condition the RHR pump and component cooling pump may be manually started.
If the emergency diesel generator output breaker is tripped while powering its safety bus, manual operator action is required to reset and reclose the breaker.
Depending on prior loading, manual load shedding may be required to limit the starting load connected to the bus when reclosing the breaker.
3.0 NRC REQUEST Describe how the IEEE Standard 279-1971 requirements are satisfied in the design of voltage monitors of second level protection.
RESPONSE
Currently, Surry Power Station - Units 1.& 2 do not have the "second-*
level" protection system (degraded bus).
In our response to NRC Request No. 1 of this letter, we have outlined a modification to include second-level protection at the Surry Power Station.
This system will be designed to meet IEEE Standard 279-1971.
The logic will. be a two-out-of-three coincident logic.
As required by IEEE 279-1971, precision relays with high reliability will be used for voltage monitoring.