ML112010028

Discrepancy Regarding Spurious DVR Actuation Requirements

ML112010028
Person / Time
Site:	Diablo Canyon
Issue date:	07/05/2011
From:	Reimers G Pacific Gas & Electric Co
To:	Roy Mathew Electrical Engineering Branch
Roy Mathew, NRR/DE/EEEB 415-2965
References
Download: ML112010028 (2)

Text

From: Reimers, Gregg (DCPP) [GAR0@pge.com]

Sent: Tuesday, July 05, 2011 10:16 AM To: Mathew, Roy Cc: 'CLEFTON, Gordon'

Subject:

fyACTION: Discrepancy Regarding Spurious DVR Actuation Requirements Roy:

This e-mail is the result of the action item assigned to me during the June 28-29 Degraded Voltage Workshop. As I was unable to attend in person due to other commitments, I appreciate the opportunity provided via remote telephone participation.

The issue I am concerned about is the regulatory conflict created by requiring the DVR setpoint to preclude spurious actuation of the undervoltage protection function.

The NRC draft RIS 2011-XXX discusses spurious separation at least three times.

1. The first occurrence is an accurate restatement of the 1977 NRC position that "The voltage protection shall include coincidence logic to preclude spurious trips of the offsite power source" (See RIS Page 2, Item (b)).

2. The second occurrence is in the "Degraded Voltage Relay Design Calculations" section.

Specifically, the second sentence of the first paragraph on Page 7 reads "Voltage-time settings for DVRs should be selected so as to avoid spurious separation of safety buses from the offsite power system during unit startup, normal operation and shutdown." This introduces the DVR voltage and time setpoint interaction with the offsite power circuits as a factor in the setpoint determination. I believe a conclusion of the workshop was a common understanding that the functional requirement of the DVR protection is to prevent common mode equipment failure during a sustained degraded voltage condition. As discussed, this can best be achieved via a "bottom up" analysis without consideration of offsite power capacity and capability.

3. The third occurrence is in the "Offsite/Onsite Design Interface Calculation" Section. Page 8, Item (i) reads "For each case evaluated, the calculated voltages on each safety bus should demonstrate adequate voltage at the component level without separation from the offsite circuit due to DVR actuation."

Points #2&3 above introduce a contradiction for those stations whose current license basis is consistent with the Standard Technical Specifications. Referring to NUREG-1431, Standard Technical Specifications Westinghouse Plants (typical TS), the degraded voltage TS bases read "The Allowable Value is considered a limiting value such that a [DVR] channel is OPERABLE." Thus, at the Allowable Value lower limit, the Class 1E electrical distribution system is capable of fulfilling its ESF supporting design function. The offsite power LCO reads "Each offsite circuit must be capable of maintaining rated frequency and voltage, and accepting required loads during an accident, while connected to the ESF buses." No voltage values are defined for the offsite power TS LCO. Therefore, if the offsite power circuit can maintain the bus voltage such that the DVR lower Allowable Limit is satisfied, then the offsite power circuit would also be operable.

The DVR dropout and reset setpoints must be greater than the TS lower Allowable Value due to instrument tolerances and uncertainty. Given the DVR favors the DGs, does not mean bus voltages between the DVR

setpoint and the TS lower Allowable Value reduce the capability of the offsite power circuit. Consequently, the DVR setpoint cannot completely preclude spurious separation.

As discussed in the workshop, a voltage relay cannot predict future operating conditions. Consequently, the DVR can't distinguish between voltage transients that are expected to recover and those that are not.

Therefore, in the context of the original NRC position (i.e. Point #1), I believe the term spurious was in the context of false signals from within the DVR instrumentation and not any group of bus voltage transients.

The IEEE 308 requirement that RIS Page 8, Item (i) is trying to convey is "The preferred power supply shall be capable of starting and operating all required loads."

Licensees are correctly performing calculations to demonstrate that the DVR resets for normal and bounding electrical transients. That is the responsible thing to do relative to nuclear safety. However, difference between the DVR TS dropout lower Allowable Value (design limit) and the worst case instrument dropout/reset values (operating limit) is design margin.

Thanks you again for the workshop and your time, Gregg Reimers, P.E.

IEEE SC-4 Member Diablo Canyon Power Plant 805-545-6597