05000373/FIN-2013009-01
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Finding | |
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| Title | Review of the Loss of Offsite Power Event Root Cause Evaluation and Switchyard Design Basis |
| Description | The LaSalle switchyard consists of a 345-kV section (or switchyard) with 4 offsite lines coming in and an adjacent 138-kV section (or switchyard) with 2 offsite lines coming in. The 138-kV switchyard was installed to support original plant construction and now supplies several nonsafety-related loads. On April 17, 2013, at 2:57 p.m., 138-kV line L0112 (Transmission Sub-Station (TSS) 105 Kickapoo Station (STA) 1 LaSalle) experienced a lightning-related phase-to-ground fault on a piece of equipment in the 138-kV switchyard. The fault was properly cleared at both ends in 4 cycles; however, the lightning strike and resultant fault elevated the local ground grid in the 138-kV switchyard sufficiently to affect the switchyard System 2 direct current (DC) battery system, located in the relay house in the adjacent 345-kV switchyard, with a voltage transient. Visual inspection revealed evidence of a flashover point at the Transformer 81 138-kV breakers incoming DC knife-blade terminal block point. The transient on the battery system ultimately caused all breakers on the 345-kV system to open. It should be noted that the control power for both the 138-kV and 345-kV switchyard breakers was supplied by two common redundant battery systems located in the switchyard relay house. The System 2 DC battery system was in service at the time of the event supplying control power to both the 138-kV and 345-kV switchyard breakers. Two minutes after the initial fault, 138-kV line L0112 experienced another phase-to-ground fault at the STA 1 terminal and the TSS 105 breaker tripped in 4.5 cycles and remained locked out. At STA 1, the resultant fault elevated the local ground grid in the 138-kV switchyard sufficiently to affect the System 2 battery system with a voltage transient through the Transformer 81 fuse block flash point. The fault transient caused damage to solid state/electronic devices utilizing the System 2 DC system. During a walk down of the switchyard, the licensee found damage to the C phase capacitively coupled voltage transformer west of the L0112 bushing, with collateral damage to disconnects and the oil circuit breaker L0112 B bushing. The plant effect was a dual-unit LOOP resulting in a dual-unit scram, with all five EDGs loading onto the safety busses. The foregoing description was the licensees preliminary assessment of the event that took place. At the end of this inspection, the licensee was performing its root cause evaluation. The inspectors reviewed the preliminary assessment and the design basis of the switchyard as described in the UFSAR. The inspectors noted that Section 8.2.3.2, Adequacy of Offsite Power, of the UFSAR stated, in part, that: The switchyard arrangement is such that offsite power to both units cannot be lost due to any single failure. Additionally, UFSAR Section 8.1.2.5, Unit Non-Class 1E DC System, stated, in part, that: The design of the protective relay circuits for the 345-kV oil circuit breakers and the 345-kV transmission lines is such that the loss of either battery or the loss of both batteries and associated feeder cables will not cause the loss of offsite power sources. Whether the series of events in the LaSalle switchyards that led to the dual-unit LOOP stemmed from a single failure or more than one failure, and whether the design of the switchyards meets regulatory requirements remain open questions. The issue will be treated as an Unresolved Item |
| Site: | LaSalle  |
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| Report | IR 05000373/2013009 Section 4OA3 |
| Date counted | Jun 30, 2013 (2013Q2) |
| Type: | URI: |
| cornerstone | Initiating Events |
| Identified by: | NRC identified |
| Inspection Procedure: | IP 71153 |
| Inspectors (proximate) | J Beavers M Kunowskid Szwarci Hafeez M Jeffers R Daleyb Kemkerk Carrington M Kunowski M Munir R Jickling |
| INPO aspect | |
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Finding - LaSalle - IR 05000373/2013009 | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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Finding List (LaSalle) @ 2013Q2
Self-Identified List (LaSalle)
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