05000323/LER-2012-002
| Diablo Canyon Power Plant, Unit 2 | |
| Event date: | 10-11-2012 |
|---|---|
| Report date: | 06-26-2013 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 3232012002R01 - NRC Website | |
I. Plant Conditions
Just prior to the event, Unit 2 was operating in Mode 1 (Power Operation) at approximately 100 percent reactor [RCT] power with normal operating reactor coolant temperature and pressure.
II. Problem Description
A. Background
The Diablo Canyon Power Plant (DCPP) is equipped with a Class 1E alternating current electrical power distribution system [EB] that is divided into three load groups. The power sources for this system consist of two physically-independent offsite sources and multiple onsite standby power sources (three engine-driven diesel generators (DGs)[DG] for each unit). These systems have independent controls, independent protection, and separate switchyards, transmission lines, and tie-lines to the plant. In the normal alignment, the power produced at DCPP is transmitted offsite via the 500kV system [EL] and also feeds normal onsite loads via the auxiliary transformer [XFMR]. Backup power is available immediately via the 230kV system [EK] and startup transformer and in a delayed manner (for vital loads) from onsite DGs. The alternating current electrical power sources provide sufficient capacity, capability, redundancy, and reliability to ensure the availability of necessary power to engineered safety systems so that the fuel, reactor coolant system [AB], and containment design limits are not exceeded.
DCPP's 500kV system has metering hardware that provides real-time generation and consumption data to the Independent System Operator (ISO). Independent bushing current transformers (CTs) and 500kV coupling capacitor voltage transformers (CCVTs) are the sensors used for this application. The CCVTs and CTs are connected to each unit's Main Bank Transformer (MBT).
A catastrophic failure of the Unit 2 "C" phase high-voltage (HV) bushing (insulator) in 2008 led to a 2009 replacement of two of the six ceramic HV insulator bushings with a silicone polymer design that had less potential for collateral damage upon failure. All of the DCPP lightning arresters (LAs) and CCVTs have also been replaced with this same silicone polymer design.
There are three key factors to consider for an insulating system to be able to adequately withstand the applied system voltage: insulator material properties, creepage distance, and environmental conditions (contaminants, such as salt, dust, industrial pollutants, etc.). A contaminated insulator without adequate creepage margin will suffer arcing and eventually flashover.
Silicone polymer provides exceptional performance as an insulator material, especially in adverse environmental conditions. By continuously extruding a layer of silicone, it encapsulates contaminants on the surface of the insulator. This prevents the buildup of a conductive film when the insulator is exposed to moisture. The silicone layer performs the same function as the silicone grease some plants apply to porcelain insulators.
Creepage distance is the shortest path between two conductive parts. The Institute of Electrical and Electronics Engineers (IEEE) standard C57.19.100, "IEEE Guide for Application of Power Apparatus Bushings," provides recommendations for various environmental conditions. DCPP's location exposes these systems and components to environmental contaminants such as salt, dust, industrial pollutants, etc. Per IEEE C57.19.100, DCPP's environment is classified as a heavy or very heavy contamination area, which establishes the recommended minimum creepage distance as 502 or 616 inches, respectively. Pacific Gas and Electric (PG&E) corporate transmission line design standard indicates a minimum creepage distance of 400 inches, but does not refer to environmental classifications. The installed polymer CCVT bushings have a vendor stated guaranteed minimum of 400 inches of creepage distance. A DCPP generic vendor document described a porcelain CCVT creepage distance of 435 inches. A separate, model-specific vendor document detailed a porcelain CCVT creepage distance of 521 inches. DCPP later determined that the previously installed CCVTs had the longer creepage distance.
Relative to the auxiliary feedwater pump, PG&E had implemented a temporary setpoint change in March 2012 to address a deficiency related to Rosemount transmitter uncertainty error increase. This resulted in a setpoint change for each of the steam generator (SG) low-low level setpoints from 15 percent to 17 percent for the auxiliary feedwater (AFW) pump autostarts. Procedures were reviewed to identify necessary changes due to the level setpoint changes, particularly reviewing all 15 percent level values in the procedures to implement necessary changes.
The Emergency Operating Procedure EOP E-0.1, "Reactor Trip Response," Response Not Obtained, Step 6.c, checks whether the turbine-driven (TD) AFW pump should be secured by verifying SG level greater than 16 percent in at least 3 SGs. The 16 percent value is derivative of the 15 percent AFW start setpoint, but is not explained as such in the setpoint basis documents or in the text of the procedures. Without a clear link between the 15 percent and the 16 percent value provided in the procedure or setpoint bases documents, procedure writers failed to identify the need to increase the 16 percent procedure requirement.
B. Event Description
On October 11, 2012, during a light rain, plant personnel identified visible arcing on the Unit 2 "A" and B" Phase MBT CCVTs. At 1208 PDT, the "A" Phase MBT CCVT flashed over to ground, causing a single-line-to-ground fault. This caused the 500kV tie-line differential relay to actuate, resulting in a Unit 2 trip. The Unit 2 trip actuated a turbine trip and, because Unit 2 was operating above the 50 percent power permissive, the reactor protection system initiated a Unit 2 reactor trip. All plant equipment, including the auto start of the AFW system, responded as designed.
About 18 minutes after the reactor trip, plant operators manually closed the steam supply valve to the TD AFW pump to secure the pump. Operators performed this action in accordance with plant operating procedures after they verified that the indicated SG levels were greater than the procedural requirement of 16 percent narrow range span. However, because the SG low-level bistables associated with the AFW actuation circuits had not yet cleared, the emergency safeguards actuation signal drove the steam-supply valve back open, restarting the TD AFW pump. Operators increased steam generator levels to clear the bistables and successfully reclosed the steam-supply valve.
C. Status of Inoperable Structure, Systems, or Components That Contributed to the Event None.
D. Other Systems or Secondary Functions Affected
None.
E. Method of Discovery
Annunciators in the control room alerted licensed control room operators of the 500kV system problem.
F. Operator Actions
Plant operators verified appropriate plant trip response using Emergency Operating Procedure (EOP) E-0, "Reactor Trip or Safety Injection" and EOP E-0.1, "Reactor Trip Response.
G. Safety System Responses
Vital buses transferred from auxiliary power to startup power as designed.
III. Cause of the Problem
DCPP determined that the CCVT bushing failed because the insulator minimum creepage distance was not consistent with industry codes and standards for the contaminant levels in its operating environment. When the bushing was replaced in 2011, DCPP staff performing the design change referenced a design document that was not updated to reflect current industry codes and standards. When the design document provided unclear and conflicting information, DCPP staff made unvalidated assumptions, and over-relied on PG&E and industry experts. This resulted in selection of an insulator with a creepage distance that was less than the minimum creepage distance recommended by current industry codes and standards.
The cause of the AFW pump setpoint procedure issue was a human knowledge based error.
IV. Assessment of Safety Consequences
There were no safety consequences as a result of this event. The transfer of plant loads to startup occurred as designed. Equipment necessary decay heat removal was available and operated as required by plant procedures.
The unexpected restart of the AFW pump did not challenge operator restoration actions or operational limits.
Unit 1 remained at full power and all vital buses remained powered by auxiliary power. Therefore, the event is not considered risk significant and did not adversely affect the health and safety of the public.
V. Corrective Actions
A. Immediate Corrective Actions
The following immediate corrective actions relative to the CCVTs were taken following the CCVT failure:
(1) Replace the Unit 2 "A" Phase CCVT and perform Power Factor (PF) testing.
(2) Replace the Unit 2 "A" Phase LA and perform PF testing.
(3) Perform PF testing of the Unit 2 "B" and "C" Phase CCVT and LA.
(4) Clean the HV Polymer Bushings on the Unit 2 MBT "A" and "B" Phases ("C" Phase is ceramic) and clean the insulators on the Unit 2 "A," "B," and "C" Phases CCVTs and LAs.
(5) Megger the secondary circuits of the Unit 2 "A" Phase CCVT (between the CCVT and CAL-ISO metering relays).
(6) Establish monthly corona-camera monitoring and weekly infrared monitoring to identify trends.
(7) Establish visible light camera to monitor Unit 1 CCVTs with real time video.
B. Corrective Actions to Prevent Recurrence
PG&E implemented the following actions to prevent recurrence:
(1) Incorporated lessons learned in a newsletter targeted for the entire Engineering Support Personnel group to inform personnel of the event and emphasize the expectation / procedural requirements to review industry standards and codes when performing design work / evaluations.
(2) Revised Design Criteria Memorandum (DCM) S-61B to reflect:
(a) IEEE C57.19.100 and C57.19.01, "IEEE Standard Performance Characteristics and Dimensions for Outdoor Apparatus Bushings," guidance for creepage distance for all high voltage insulators including determination of contamination classifications; and (b) IEEE 693, "Nonlinear Structural Dynamics And Control Research," guidance for seismic criteria.
Additionally, PG&E will implement the following actions to prevent recurrence:
(1) Revise design procedures and process to incorporate an expectation to review current industry codes and standards for information relevant to the design work being performed and to use and understand the standard's guidance as appropriate to ensure a correct use of the guidance.
(2) Relocate the CCVTs from the MBT to the 500kV switchyard, which has lower environmental contamination levels. Unit 2 CCVTs were relocated during the Unit 2 Seventeenth Refueling Outage. Unit 1 CCVTs are scheduled to be relocated in July 2013.
C. AFW Pump Setpoint Corrective Actions (1) Update setpoint document to identify the relationship between the SG setpoint and the TD AFW pump actions.
(2) Tailboard Operations procedure writer staff on the event and the expectation to consult supporting documents when performing Emergency Operating Procedure modification scoping.
VI. Additional Information
A. Failed Components
The DCPP Unit 2 MBT "A" phase CCVT bushing failed due to an insulator flashover.
B. Previous Similar Events
On August 16, 2008, Diablo Canyon Unit 2 MBT "C" Phase experienced a catastrophic failure of the HV bushing.
A significant amount of porcelain shrapnel resulted from the failure. The debris damaged adjacent equipment and penetrated the northeast side of the administration building in multiple locations. Debris was also found in the parking lot several hundred feet away. The event occurred at 23:56 on a Saturday. The potential for injury was very high if the catastrophic failure of that HV bushing had occurred on a normal workday.
A root cause evaluation performed as a result of the MBT "C" Phase HV bushing failure found that catastrophic failure of bushings was not an uncommon occurrence. To improve personnel safety, a recommendation was made to replace the MBT porcelain bushings, the CCVTs, and the LAs with models made with a safer insulator material. Design changes were developed and components were replaced with models made of a silicone polymer insulator rather than porcelain. Bushings of Unit 2 MBT "A" and "B" phases were replaced with polymer in 2009.
The Unit 2 MBT "C" phase bushing remains of porcelain design. In Unit 2 Refueling Outage Sixteen, the CCVTs and LAs were replaced with polymer. In Unit 1 Refueling Outage 17, the CCVTs and LAs were replaced with polymer. All three Unit 1 MBTs continue to have porcelain bushings.