10-15-2008 | On August 16, 2008, at 23:57 PDT, with Unit 2 in Mode 1 (Power Operation) at approximately 100 percent power, a main generator Unit Trip signal initiated a Reactor Trip due to the failure of the main electrical transformer "C" phase. On August 17, 2008, at 00:12 PDT, plant operators declared an Unusual Event (UE) due to an observed fire. at the "C" phase transformer. Plant operators made an Emergency Event Notification ( EN 44419) in accordance with 10 CFR 50.72(a)(1)(i) at 00:37 PDT. Plant operators stabilized Unit 2 in Mode 3 (Hot Standby) and updated the UE that the fire was out at 02:02 PDT. At 07:38 PDT, EN 44419 was updated to specify 10 CFR 50.72(b)(3)(iv)(A) and 10 CFR 50.72(b)(3)(iv)(B).
This event was due to a catastrophic failure of the main electrical transformer "C" phase high voltage bushing. The event investigation will continue, however, this event is considered a random component failure based upon onsite inspections and vendor reviews performed.
Corrective actions include the "C" phase transformer replacement, electrical tests of transformers and bushings, oil analysis of transformers and bushings, installation of a bushing monitoring system, evaluation and replacement of equipment damaged as a result of the electrical failure, and review of industry and station operating experience regarding main transformer issues. |
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Plant Conditions
Unit 2 was in Mode 1 (power operation) at approximately 100 percent power.
II.� Description of Problem
A. Background
During normal operation, the main generator's output is stepped up from 25kV to 500 kV via the main bank transformers, which consist of three separate transformers, one for each phase. The main bank transformers were placed in service during October 1999. The 4kV vital buses are powered from the auxiliary power system fed from the main electrical generator via auxiliary transformers that step the voltage down from 25kV.
The 230kV system provides an immediately available source of offsite power to the 4kV system. The 230kV system provides power to Startup Transformers (SUTs) [EA][XFMR] 1-1 and 2-1 (230kV to 12kV), which feed the SUT 1-1 and 2-2 (12 kV to 4kV), respectively. SUT 1-1 and 2-2 then supply power to the 4kV vital bus and 480V vital buses.
Each Diablo Canyon Power Plant Unit has three onsite emergency diesel generators (EDGs) [EK][DG], which supply power to the 4kV vital AC buses [EA][BU] whenever power is either unavailable, or voltage degrades below the point at which required loads could become inoperable. EDGs automatically start on a safety injection signal, degraded or loss of voltage on the associated vital bus, or undervoltage on the 230kV startup power system. This event did not result in the starting or loading of the EDGs.
The protection of the transmission system and main electrical generator (including isophase bus and connected transformers) are provided by relays, which sense line faults, and initiate signals to their respective breakers to open the breaker and isolate the fault. The main electrical generator output transformer is protected by phase differential relays that sense the difference between phases to provide equipment protection during normal operation.
Technical Specification (TS) 3.8.1, "AC Sources — Operating," Action A.1, requires verification of the operability of the independent circuits between the off-site transmission network and the on-site distribution system when in the action statement. Surveillance Test Procedure (STP) I-C, "Routine Weekly Checks Required by Licenses," Attachment 12.4, verifies, that "At least one 230kV line is energized and its respective ...{breaker} is LICENSEE EVENT REPORT (LER) TEXT CONTINUATION FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6) operable...," via the SUT. The conditional surveillance is required to be performed within, "1 hour1.157407e-5 days <br />2.777778e-4 hours <br />1.653439e-6 weeks <br />3.805e-7 months <br /> and once per 8 hours9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br /> thereafter.
B. Event Description
On August 16, 2008, at approximately 23:57 PDT a main electrical output transformer "C" phase oil filled high voltage output bushing experienced a high energy phase to ground electrical fault resulting in catastrophic failure. An automatic main generator Unit Trip signal resulted in an automatic Reactor Protection System (RPS) Reactor Trip initiate signal.
On August 17, 2008, at 00:12 PDT, licensed plant operators declared an Unusual. Event (UE) due to an observed fire at the main electrical transformer "C" phase that was not extinguished within 15 minutes.
On August 17, 2008, at 00:24 PDT plant operators initiated a required conditional surveillance in accordance with TS 3.8.1, Condition A.1, to be performed within 1 hour1.157407e-5 days <br />2.777778e-4 hours <br />1.653439e-6 weeks <br />3.805e-7 months <br /> by completing STP I-1C, Attachment 12.4.
On August 17, 2008, at 00:37 PDT plant operators made an emergency event notification (EN 44419) in accordance with 10 CFR 50.72(a)(1)(i) for the fire. They also described the resultant automatic Reactor Trip and auxiliary feedwater actuation.
On August 17, 2008, at 02:02 PDT plant operators stabilized Unit 2 in Mode 3 (Hot Standby), and updated EN 44419 to include termination of the UE.
On August 17, 2008, at 03:27 PDT plant operators logged STP I-1C, Attachment 12.4, completing TS 3.8.1, Condition A.1, as a .late conditional surveillance.
On August 17, 2008, at 07:38 PDT, plant operators updated EN 44419 to specify the system actuations in accordance with 10 CFR 50.72(b)(3)(iv)(A) and 10 CFR 50.72(b)(3)(iv)(B).
C. Status of Inoperable Structures, Systems, or Components that Contributed to the Event None.
LICENSEE EVENT REPORT (LER) TEXT CONTINUATION FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6) PAGE (3)
D. Other Systems or Secondary Functions Affected
No Unit 2 safety systems were adversely affected by this event.
Unit 1 was unaffected and remained in Mode 1 at approximately 100 percent power.
E. Method of Discovery
The event was self-revealing to licensed control room operators by annunciators indicating the Unit Trip and RPS Reactor Trip condition, and the automatic actuation of both motor-driven auxiliary feedwater pumps.
F. Operator Actions
Licensed plant operators declared an UE due to an observed fire at the "C" phase transformer and made an EN by phone via the emergency notification system (ENS). Plant operators stabilized Unit 2 in Mode 3, verified that the fire was out, and terminated the UE. Plant operators updated ENS notification EN 44419 to specify the.
10 CFR 50.72(b)(3)(iv)(A) and 10 CFR 50.72(b)(3)(iv)(B) events.
G. Safety System Responses
The Unit 2 main electrical transmission relay protection scheme automatically initiated a main electrical generator Unit Trip and RPS Reactor Trip.. The Unit Trip initiated a transfer to the preferred source of power (230kV startup power) as designed. The RPS Reactor Trip signal removed the gripper power from the reactor control rod drive system, and allowed the rods to drop into the core shutting down the reactor as designed. Two motor-driven auxiliary feedwater pumps automatically started to provide cooling water to the four steam generators to cool the reactor coolant system as designed.
III.�Cause of the Problem A.�Immediate Cause The Unit 2 main electrical transformer "C" phase high voltage bushing experienced a phase to ground high energy electrical fault resulting in catastrophic failure of the bushing. The main electrical transformer protection relays initiated a main generator Unit Trip and RPS Reactor Trip signal.
LICENSEE EVENT REPORT (LER) TEXT CONTINUATION FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6) PAGE (3) B.RRoot Cause The presumptive root cause for the failure is either an internal degraded test tap connection, or low bushing oil level in the "C" phase high voltage bushing. The failure rate of oil-filled high voltage transformer bushings is well within the assumed random single failure rate previously analyzed.
The root cause team identified that a degraded ground on the "C" phase high voltage bushing test tap may have resulted in a partial discharge condition. Due to the catastrophic failure of the bushing, arcing within the test tap and the total destruction of the test tap connection at the condenser foil, the root cause team was unable to determine if the test tap ground was degraded, or if there was a loss of oil that initiated the failure.
IV.A Assessment of Safety Consequences automatically initiated a main electrical generator Unit Trip and RPS Reactor Trip. The Unit Trip initiated an automatic transfer to the preferred source of power (230kV startup power) as designed. The RPS Reactor Trip signal allowed the reactor control rods to drop into the core shutting down the reactor as designed. Two motor driven auxiliary feedwater pumps started, and provided cooling water to the four steam generators cooling the reactor coolant system as designed. The loss of main generator electrical output is a Condition II event previously analyzed in the Final Safety Analysis Report Update, Chapter 15.2, "Condition II — Faults of Moderate Frequency.
In the unlikely event that the preferred offsite source of power (230kV startup power) were unavailable or fail, the onsite EDGs were available to provide the power to the vital buses. While the transformer failure resulted in some collateral damage to nearby equipment, no personnel or safety-related structure, system, or component was significantly damaged during this event.
Unit 1 remained at full power with vital buses powered from its auxiliary transformer.
Therefore, the event is not considered risk significant, and it did not adversely affect the health and safety of the public.
LICENSEE EVENT REPORT (LER) TEXT CONTINUATION FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6) PAGE (3) V.�Corrective Actions A.�Immediate Corrective Actions 1. Pacific Gas and Electric Company (PG&E) replaced the failed main electrical transformer "C" phase with the spare from Unit 1.
2. PG&E inspected all equipment in the vicinity of the failed transformer bushing and fire for collateral damage.
3. PG&E repaired or replaced the significantly adversely affected equipment identified as a result of the transformer failure.
B.�Corrective Actions to Prevent Recurrence PG&E will improve high voltage bushing performance monitoring by:
1. Installing on-line bushing monitoring system on the Main Bank Transformer 500kV bushings for each unit.
- 2. Establishing daily operator rounds to verify that no abnormal condition exits at the bushing condition monitors.
VI.�Additional Information
A. Failed Components
The HSP Hochspannungsgerate GmbH (formerly ELIN) 25kV to 500kV transformer output high voltage oil filled bushing.
Model TEQ-405A44D9K99 was placed in service during October 1999.
B. Previous Similar Events
Offsite 230kV Startup Power Due to an Insulator Failure and Unanticipated Relay Response at Substations," reported the loss of the offsite 230kV startup power due to a main transmission system insulator failure on May 12, 2007. The corrective actions taken for this event would not have precluded the current event as the corrective actions taken were for the offsite 230kV system. Additionally, this condition focused on the failure of an insulator of a significantly different design than the main transformer output bushing.
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05000413/LER-2008-001 | | 10 CFR 50.73(a)(2)(ii)(B), Unanalyzed Condition 10 CFR 50.73(a)(2)(v)(B), Loss of Safety Function - Remove Residual Heat 10 CFR 50.73(a)(2)(vii), Common Cause Inoperability | 05000334/LER-2008-001 | Control Room Envelope Air Intake During Normal Operation Higher Than Assumed In Design Basis Accident Dose Calculations | 10 CFR 50.73(a)(2)(v), Loss of Safety Function 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications | 05000315/LER-2008-001 | Unit 1 Manual Reactor Trip due to Main Turbine Vibrations | 10 CFR 50.73(a)(2)(iv)(A), System Actuation | 05000272/LER-2008-001 | Inadvertent Start of an Emergency Diesel Generator During Testing | 10 CFR 50.73(a)(2)(iv)(A), System Actuation | 05000251/LER-2008-001 | Manual Reactor Trip due to High Level in the 4A Steam Generator | 10 CFR 50.73(a)(2)(iv)(A), System Actuation | 05000247/LER-2008-001 | Manual Reactor Trip Due to Decreasing Steam Generator Levels Caused by Loss of Feedwater Flow as a Result of Feedwater Pump Speed Control Malfunction | 10 CFR 50.73(a)(2)(iv)(A), System Actuation 10 CFR 50.73(a)(2)(v), Loss of Safety Function | 05000530/LER-2008-001 | Manual Reactor Trip when Removing a Degraded CEDM MG Set from Service | 10 CFR 50.73(a)(2)(v), Loss of Safety Function | 05000457/LER-2008-001 | 2A Essential Service Water Train Inoperable due to Strainer Fouling from Bryozoa Deposition and Growth | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications | 05000454/LER-2008-001 | Technical Specification Non-Compliance of Containment Sump Monitor Due to Improper Installation During Oriainal construction | | 05000440/LER-2008-001 | ' Condition Prohibited by Technical Specifications Due to Unrecognized Reactor Core Isolation Cooling Inoperability | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications | 05000483/LER-2008-001 | Containment Cooler Inoperability | 10 CFR 50.73(a)(2)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident 10 CFR 50.73(a)(2)(vii), Common Cause Inoperability 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications | 05000412/LER-2008-001 | Unplanned Actuation of the Auxiliary Feedwater System During Plant Startup | 10 CFR 50.73(a)(2)(iv)(A), System Actuation | 05000374/LER-2008-001 | High Pressure Core Spray System Declared Inoperable Due to Failed Room Ventilation Supply Fan | 10 CFR 50.73(a)(2)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident | 05000323/LER-2008-001 | Reactor Trip Due to Main Electrical Transformer Failure | 10 CFR 50.73(a)(2)(iv)(A), System Actuation 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications | 05000305/LER-2008-001 | Pressurizer PORV and Reactor Coolant System Vent Valves Appendix R Spurious Operation Concern | | 05000271/LER-2008-001 | Crane Travel Limit Stops not Installed as Required by Technical Specifications due to an Inadequate Procedure | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications | 05000269/LER-2008-001 | Gas Void Found in High Pressure Injection System Suction Piping | | 05000263/LER-2008-001 | | | 05000261/LER-2008-001 | Appendix R Pathway Impassable due to Lock Configuration | 10 CFR 50.73(a)(2)(v)(A), Loss of Safety Function - Shutdown the Reactor | 05000361/LER-2008-001 | Valid actuation of Emergency Feedwater system following Main Feedwater pump trip | 10 CFR 50.73(a)(2)(iv)(A), System Actuation | 05000335/LER-2008-001 | Unattended Ammunition Discovered Inside Protected Area | | 05000456/LER-2008-001 | Technical Specification Non-Compliance Due to Inadequate Design of Auxiliary Feedwater (AF) Tunnel Access Covers Causing AF Valves Within the Tunnel to be Inoperable | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications | 05000423/LER-2008-001 | Postulated Fire Scenario Results in Unanalyzed Condition - Pressurizer Overfill | 10 CFR 50.73(a)(2)(ii)(B), Unanalyzed Condition | 05000369/LER-2008-001 | Potential Failure of Containment Isolation Valves (CIV) to Remain Fully Closed and Ino erable loner than allowed bCTechnical S ecification 3.6.3. | | 05000220/LER-2008-002 | Manual Reactor Scram Due to Loss of Reactor Pressure Control | 10 CFR 50.73(a)(2)(iv)(A), System Actuation | 05000261/LER-2008-002 | Manual Reactor Trip due to High Turbine Vibrations | 10 CFR 50.73(a)(2)(iv)(A), System Actuation | 05000270/LER-2008-002 | Main Steam Relief Valves Exceeded Lift Setpoint Acceptance Band 050002 | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications | 05000305/LER-2008-002 | 5 . Blocked Open Steam Exclusion Door Results in Postulated Inoperability of Safety Systems | 10 CFR 50.73(a)(2)(ii)(B), Unanalyzed Condition | 05000336/LER-2008-002 | Unplanned LCO Entry - Three Charging Pumps Aligned for Injection With the Reactor Coolant System Temperature Less than 300 Degrees F. | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications | 05000354/LER-2008-002 | BLOWN lE INVERTER MAIN FUSE WITH ONE EMERGENCY DIESEL GENERATOR INOPERABLE CAUSES LOSS OF CONTROL ROOM EMERGENCY FILTRATION LOSS OF SAFETY FUNCTION | | 05000395/LER-2008-002 | Control Room Normal and Emergency Air Handling Systems Inoperable Due to Pressure Boundary Breach | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications | 05000382/LER-2008-002 | Inoperable Steam Generator Narrow Range Level Channels | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications | 05000369/LER-2008-002 | 'Energy® BRUCE H HAMILTON Vice President McGuire Nuclear Station Duke Energy Corporation MGOIVP 112700 Hagers Ferry Road Huntersville, NC 28078 704-875-5333 704-875-4809 fax bhhamilton@duke-energy.corn August 21, 2008
U.S. Nuclear Regulatory Commission
ATTENTION: Document Control Desk
Washington, D.C. 20555
Subject: Duke Energy Carolinas, LLC
McGuire Nuclear Station, Unit 1
Docket No. 50-369
Licensee Event Report 369/2008-02, Revision 0
Problem Investigation Process No.: M-08-03862
Pursuant to 10 CFR 50.73 Sections (a)(1) and (d), attached
is Licensee Event Report (LER) 369/2008-02, Revision 0,
regarding the Unit 1 Reactor trip on June 26, 2008 due to
the 1B Reactor Coolant Pump Motor trip which was caused by
a failed surge capacitor.
This report is being submitted in accordance with 10 CFR
50.73 (a)(2)(iv)(A). This event is considered to be of no
significance with respect to the health and safety of the
public. There are no regulatory commitments contained in
this LER.
If questions arise regarding this LER, contact Lee A. Hentz
at 704-875-4187.
Very truly yours,
Bruce H. Hamilton
Attachment
www. duke-energy. corn U.S. Nuclear Regulatory Commission
August 21, 2008
Page 2
cc: L. A. Reyes, Regional Administrator
U.S. Nuclear Regulatory Commission, Region II
Sam Nunn Atlanta Federal Center
61 Forsyth Street, SW, Suite 23T85
Atlanta, GA 30303
J. F. Stang, Jr. (Addressee Only)
Senior Project Manager (McGuire)
U.S. Nuclear Regulatory Commission
Mail Stop O-8G9A
Washington, DC 20555
J. B. Brady
Senior Resident Inspector
U.S. Nuclear Regulatory Commission
McGuire Nuclear Station
B. 0. Hall, Section Chief
Radiation Protection Section
1645 Mail Service Center
Raleigh, NC 27699
NRC FORM 366 U.S. NUCLEAR REGULATORY COMMISSION APPROVEDBYOMB:NO.3150-0104 EXPIRES: 08/31/2010 (9-2007) Estimated burden per response to comply with this mandatory collection request: 50 hours. Reported lessons learned are incorporated into the licensing process and fed back to industry. Send ' LICENSEE EVENT REPORT (LER) comments regarding burden estimate to the Records and FOIA/Privacy Service Branch (T-5 F52), U.S. Nuclear Regulatory Commission, Washington, DC 20555-0001, or by internet e-mail to infocollects@nrc.gov, and to the Desk Officer, Office of Information and Regulatory Affairs, NEOB-10202, (3150-0104), Office of Management and Budget, Washington, DC 20503. If a (See reverse for required number of means used to impose an information collection does not display a currently valid OMB control digits/characters for each block) number, the NRC may not conduct or sponsor, and a person is not required to respond to,' the information collection. 1, FACILITY NAME 2. DOCKET NUMBER I 3. PAGE 05000- ' 1 8McGuire Nuclear Station, Unit 1 _ 0369' OF .4, TITLE . . Unit 1 Reactor Trip due to the 1B Reactor Coolant Pump Motor Trip which was caused by a
failed Surge Capacitor | 10 CFR 50.73(a)(2)(iv)(A), System Actuation | 05000440/LER-2008-002 | Inoperable Emergency Closed Cooling System Results In Condition Prohibited By Technical Specifications | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications | 05000454/LER-2008-002 | Technical Specification Non-Compliance Due to Inadequate Design of Auxiliary Feedwater (AF) Tunnel Access Covers Causing AF Valves Within the Tunnel to be Inoperable | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications | 05000298/LER-2008-002 | Technical Specification Prohibited Condition Due to Safety Relief Valve Test Failure | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications | 05000237/LER-2008-002 | Unit 2 High Pressure Coolant Injection System Declared Inoperable | 10 CFR 50.73(a)(2)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident | 05000272/LER-2008-002 | | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications | 05000361/LER-2008-002 | Disturbance on the Pacific DC Intertie cause offsite power frequency to dip below operability limits | 10 CFR 50.73(a)(2)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident | 05000362/LER-2008-003 | Missed TS completion time results in TS Violation | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications | 05000220/LER-2008-003 | Power Supplies for Drywell Pressure Indication not Qualified for Required Post-Accident Operating Duration | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications | 05000529/LER-2008-003 | Technical Specification - Limiting Condition for Operation 3.0.3 for Greater Than 1 Hour | 10 CFR 50.73(a)(2)(v), Loss of Safety Function 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications | 05000293/LER-2008-003 | | | 05000263/LER-2008-003 | | | 05000423/LER-2008-003 | Automatic Reactor Trip During Shutdown for Refueling Outage 3R12 ., | 10 CFR 50.73(a)(2)(iv)(A), System Actuation | 05000251/LER-2008-003 | . Class 1 Weld Leak Due to Fatigue and Completion of Technical Specification Required Shutdown | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications 10 CFR 50.73(a)(2)(ii)(A), Seriously Degraded 10 CFR 50.73(a)(2)(i)(A), Completion of TS Shutdown | 05000282/LER-2008-003 | Loss of AFW Safety Function and Condition Prohibited by Technical Specifications Due to Mispositioned Isolation Valve | 10 CFR 50.73(a)(2)(v)(B), Loss of Safety Function - Remove Residual Heat 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications | 05000286/LER-2008-003 | Automatic Actuation of Emergency Diesel Generator 33 During Surveillance Testing Caused by .Inadvertent Actuation of the Undervoltage Sensing Circuit on 480 Volt AC Safeguards Bus 5A | 10 CFR 50.73(a)(2)(iv)(A), System Actuation 10 CFR 50.73(a)(2)(v), Loss of Safety Function | 05000361/LER-2008-003 | Disturbance on the Pacific DC Intertie cause offsite power frequency to dip below operability limits | 10 CFR 50.73(a)(2)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident | 05000305/LER-2008-003 | Door Bottom Seal Failure Results in Inoperability of Control Room Ventilation System | 10 CFR 50.73(a)(2)(ii)(B), Unanalyzed Condition 10 CFR 50.73(a)(2)(v)(B), Loss of Safety Function - Remove Residual Heat |
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