05000323/LER-2008-001

LER-2008-001, Reactor Trip Due to Main Electrical Transformer Failure

Docket Number
Event date:	08-16-2008
Report date:	10-15-2008
Reporting criterion:	10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications 10 CFR 50.73(a)(2)(iv)(A), System Actuation
Initial Reporting
ENS 44419	10 CFR 50.72(b)(2)(iv)(B), RPS System Actuation, 10 CFR 50.72(a)(1)(i), Emergency Class Declaration, 10 CFR 50.72(b)(3)(iv)(A), System Actuation
3232008001R00 - NRC Website

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.