Information Notice 2007-36, Emergency Diesel Generator Voltage Regulator Problems
| ML072780394 | |
| Person / Time | |
|---|---|
| Issue date: | 11/15/2007 |
| Revision: | 0 |
| From: | Case M J NRC/NRR/ADRO/DPR |
| To: | |
| References | |
| IN-07-006 | |
| Download: ML072780394 (4) | |
November 15, 2007
NRC INFORMATION NOTICE 2007-36:EMERGENCY DIESEL GENERATOR VOLTAGEREGULATOR PROBLEMS
ADDRESSEES
All holders of operating licenses for nuclear power reactors, except those who havepermanently ceased operations and have certified that fuel has been permanently removed from the reactor vessel.
PURPOSE
The U.S. Nuclear Regulatory Commission (NRC) is issuing this information notice (IN) to informaddressees of recent operating experiences involving the emergency diesel generator (EDG)
voltage regulator problem The NRC expects that recipients will review the information for applicability to their facilities and consider actions, as appropriate, to avoid similar problem Suggestions contained in this IN are not NRC requirements; therefore, no specific action or written response is required.
DESCRIPTION OF CIRCUMSTANCES
There have been recent operating experiences with EDG voltage regulator problems at thefollowing nuclear stations:Seabrook Generating StationOn August 30, 2006, during a fast start test of the "A" EDG, the EDG voltage regulator diodefailure warning light illuminate Data obtained at test points within the EDG voltage regulator indicated that there were anomalies with the gate firing circuit output pulses to the silicon controlled rectifiers (SCRs) on the rectifier assembl On September 27, 2006, the "A" EDG experienced an additional diode failure light illuminatio The subsequent troubleshooting determined that the K1 contactor was defective with one phase not making ideal contact. During surveillance testing of the "B" EDG on August 31, 2006, the EDG voltage regulatordiode failure light illuminated and the EDG output voltage failed high and could not be controlled in the automatic mod Initial troubleshooting of the "B" EDG voltage regulator failure identified degradation in one of the seven capacitors in the power chassi The licensee's root cause evaluation for this event discussed that the "B" EDG at Seabrook Station had a history of intermittent overvoltage problems, including two overvoltage events in 200 The evaluation attributed the root cause of the intermittent overvoltage problem to a higher than expectedimpedance in the flyback diode portion of the rectifier chassis circuitr The high impedance caused one of the SCRs to become latched on during the transient conditions experienced during a fast start of the ED The mock-up testing demonstrated that the rectifier chassis installed during the August 31, 2006, event exhibited an SCR lock-up when exposed to conditions that simulate an excitation system during the fast star (NRC Special Inspection Report 05000443/2006016 dated February 16, 2007, Agencywide Documents Access and Management System (ADAMS) Accession No. ML070510065)Palo Verde Generating StationOn September 22, 2006, the "A" EDG at Palo Verde Unit 3 failed to attain its required voltageduring post maintenance surveillance testin This occurred because the field shorting K1 contactor (also referred to as the K1 relay) did not fully reset, preventing field excitatio (The field flashing relay and the shut-down relay/contactor (K1) are interlocked to prevent field flashing unless the shut-down relay/contactor is reset.) The root cause was determined to be insufficient compression on the auxiliary contacts of the K1 contactor due to a bent metal actuator arm in the K1 contactor assembly.A similar prior event had occurred on July 25, 2006, at Palo Verde Unit 3 when the "A" EDGfailed to attain its required voltag At that time the licensee's investigation into that occurrence determined that an auxiliary contact of the K1 contactor had an erratic resistance reading in the closed stat The licensee determined that the most probable cause was contamination on the contact surface Investigation of the September 22, 2006, failure concluded that the K1 contactor that initially failed on July 25, 2006, and was replaced, was vulnerable to the same inadequate DC auxiliary contact compression as the September 22, 2006, failure, in addition to the debris and oxidation buildup inside the auxiliary contac (Licensee Event Report 2006-006-01 dated March 9, 2007, ADAMS Accession No. ML070820085 and NRC Special Inspection Report 05000528/2006012; 05000529/2006012; 05000530/2006012 dated December 6, 2006, ADAMS Accession No. ML063400561) Cooper Generating StationOn January 18, 2007, approximately three hours into a loaded surveillance run of EDG 2 atCooper Station, the EDG output breaker tripped on an overcurrent condition, followed immediately by an overvoltage trip of EDG The root cause was determined to be failure of the voltage regulator printed circuit board due to a failed zener diode on the printed circui The board had been installed in EDG 2 on November 11, 2006, as corrective maintenance to repair a failed potentiometer on the previously installed circuit boar (NRC Special Inspection Report 05000298/2007007 dated May 22, 2007, ADAMS Accession No. ML071430289)Part 21 NotificationOn September 21, 2007, MPR Associates Inc. issued a Part 21 report on the analog electroniccircuit card used in the automatic voltage regulator (AVR) assembly of Basler SBSR voltage regulators (ADAMS Accession No. ML072750470). The report identified that, over a period of many years, cracks can form in the solder joint connections between the electronic circuit card and the L1 magnetic amplifier module of the AV When electrical continuity is lost at the solder connections, the voltage regulator does not perform as intended, and the EDG could failto deliver emergency AC power as intended.
DISCUSSION
Licensees rely on EDGs to provide emergency alternating current power in response to loss ofoffsite power event EDGs are required to be operable as specified in plant technical specification Although the first two events above are recent examples, operating experience over the last ten years shows eight additional examples where EDGs at other plants have been rendered inoperable due to K1 relays or contactor problem In addition, the voltage regulator systems of EDGs have experienced approximately fifty malfunctions of other kinds at various plants during the last ten year EDG voltage regulators, typically of 1950-1960 vintage, have recently experienced aging and obsolescence issues that have created a heightened awareness among nuclear utilities because of the threat to overall EDG performanc The industry's situation is complicated by parts shortages and limited or nonexistent manufacturer suppor The problems are of various types and are not limited to a typical single component or model of the voltage regulato In general, the performance of a voltage regulator is very sensitive to any minor defects in any component of the voltage regulation system.
CONTACT
This IN requires no specific action or written respons Please direct any questions about thismatter to the technical contacts listed below./RA by TQuay for/Michael J. Case, DirectorDivision of Policy and Rulemaking Office of Nuclear Reactor Regulation
Technical Contact:
Vijay Goel, NRR/DEJohn Thorp, NRR/DIRS 301-415-3730301-415-6584 e-mail: vkg@nrc.gov e-mail: jet3@nrc.gov Note: NRC generic communications may be found on the NRC public Web site,http://www.nrc.gov, under Electronic Reading Room/Document Collection solder connections, the voltage regulator does not perform as intended, and the EDG could fail to deliver emergency AC power as intended.
DISCUSSION
Licensees rely on EDGs to provide emergency alternating current power in response to loss ofoffsite power event EDGs are required to be operable as specified in plant technical specification Although the first two events above are recent examples, operating experience over the last ten years shows eight additional examples where EDGs at other plants have been rendered inoperable due to K1 relays or contactor problem In addition, the voltage regulator systems of EDGs have experienced approximately fifty malfunctions of other kinds at various plants during the last ten year EDG voltage regulators, typically of 1950-1960 vintage, have recently experienced aging and obsolescence issues that have created a heightened awareness among nuclear utilities because of the threat to overall EDG performanc The industry's situation is complicated by parts shortages and limited or nonexistent manufacturer suppor The problems are of various types and are not limited to a typical single component or model of the voltage regulato In general, the performance of a voltage regulator is very sensitive to any minor defects in any component of the voltage regulation system.
CONTACT
This IN requires no specific action or written respons Please direct any questions about thismatter to the technical contacts listed below./RA by TQuay for/Michael J. Case, Director Division of Policy and Rulemaking Office of Nuclear Reactor Regulation
Technical Contact:
Vijay Goel, NRR/DEJohn Thorp, NRR/DIRS 301-415-3730301-415-6584 e-mail: vkg@nrc.gov e-mail: jet3@nrc.gov Note: NRC generic communications may be found on the NRC public Web site,http://www.nrc.gov, under Electronic Reading Room/Document Collections.Distribution: IN Reading File ADAMS Accession Number: ML072780394 OFFICEEEEB:DETECH EDITORTL:IOEB:DIRSBC:IOEB:DIRSBC:EEEB:DENAMEVGoelLCulpJThorpMJRoss-LeeGWilson DATE10/11/07 10/09/07 10/18/0710/17/0710/22/07 OFFICED:DELA:PGCBPGCB:DPRBC:PGCB:DPRD:PGCB:DPR NAMEPHilandChawes CMHDBeaulieuMMurphyTQuay for MCase DATE10/22/0711/09/07 11/05/7 11/15/07 11/15/07*concurrence by e-mailOFFICIAL RECORD COPY