ML19260C126

Corrects Util Re Max Experience Switchyard Voltage.District Changed Setpoint of Existing Overvoltage to 241 Kv During 791117 Outage.Operation at 241 Kv Acceptable

ML19260C126
Person / Time
Site:	Rancho Seco
Issue date:	12/06/1979
From:	Mattimoe J SACRAMENTO MUNICIPAL UTILITY DISTRICT
To:	Gammill W Office of Nuclear Reactor Regulation
References
TAC-10968, NUDOCS 7912180607
Download: ML19260C126 (2)

Text

..sf OtSMUD SACRAMENTO MUNICIPAL UTILITY DISTRICT O 6201 s street, Box 15a30, sacramento, California 95813; (916) 452-3211 December 6, 1979 Mr. William Gammill Acting Assisting Director for Operating Reactor Projects Division af Operating Reactors U.S. Nuclear Regulatory Commission Washington, D.C.

20555 Docket No. 50-312 Rancho Seco Nuclear Generating Station, Unit No. 1

Dear Mr. Gannill:

In Enclosure I, Response to Guideline No.11 in the District's October 17, 1979 letter, it was stated "the maximum experienced switch-yard voltage is 236.2 KV".

However, further research by the District indicated that the maximum expected switchyard voltage is 241 KV. With the existing transformation ratios and assuming a no load condition, 241 KV in the switch percent of 4160) yard will cause a maximum voltage of 4,568 volts (110 on the 4160 volt system and 515 volts (112 percent of 460) on the 460 volt system. This voltage is less than the maximum voltage rating of all safety related equipment except for the 460 volt motors and motor operated valves. Operation at this voltage is acceptable for the following reasons.

a.

The 460 volt motors will be operating at a 110 percent or less of rated voltage when equipment operation is initiated by a safety features actuation signal (SFAS) due to the voltage drop in the onsite distribution system.

b.

For the 460 volt motors, a bus voltage of 515 volts can produce a torque that is a maximum of 4 percent greater than the torque that would be produced at the allowable maximum voltage of 506 volts. However, due to voltage drop during starting and operating the motor, the maximum voltage at ths terminals of the motor will be less than 515 volts and the increase in torque will be less that 4 percent. The District's preliminary analysis indicates this marginal increase in torque is acceptable. However, the District is still evaluating operation at 515 volts and will contact the NRC immediately if this analysis indicates a less conservative result.

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Mr. William Gammill December 6,1979 c.

The 460 volt motor operated valve circuits have minimal voltage drop during starting and operation. Therefore, it is possible for the motor operated valve to produce the total increase in torque due to the overvoltage.

However, operation of the motor operated valve is tarque limited by a torque switch. Therefore, the excessive torque is not applied to the valve and operation at the overvoltage condi. tion is acceptable.

Based on the above analysis, operation with a voltage of 241 KV in the switchyard which corresponds at no load. conditions to 4,568 (110 percent of 4160) on the 4060 volt system and 515 volts (112 percent of 460) on the 460 volt system is acceptable.

To insure operation within this limit, the District is modifying its commitment in Response to Guideline #11 and is proposing to set the overvoltage relay at 4576 volts (110 percent of 4160) which corresponds to a maximum voltage of 515 volts (112 percent of 460). This setpoint is equivalent to a switchyard voltage of 241 KV. This setpoint will protect the safety related equipment at the maximum expected voltage and will not cause the safety buses to be spuriously isolated from the grid.

The District changed the setpoint of the existing overvoltage during the November 17, 1979 outage.

Sincerely, John J. Mattimoe Assistant General Manager and Chief Engineer RED:nm 1614 305