05000390/FIN-2013004-01
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Finding | |
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| Title | Contribution of Potential Current Transformer Imbalance to Reactor Trip |
| Description | The inspectors monitored the licensee performing a trouble shooting plan following the reactor trip. The licensee attributed the cause of the reactor trip to a loose phase A connection on a digital fault recorder. The inspectors continued to monitor the root cause development following reactor startup to determine the validity of the cause and review the associated LER for closure. On June 28, 2013, an A-phase high impedance ground fault occurred on the Roane 500kV transmission line approximately 22 miles from Watts Bar Nuclear Unit 1. Concurrently, the licensee experienced a reactor trip due to the actuation of the 1A Main Bank Transformer Feeder Differential Relay 187TF. The 500kV transmission line fault was caused by a tree that fell onto the A phase of the transmission line. The tree was cut by a local land owner. Operations personnel stabilized the plant using the Auxiliary Feedwater (AFW) System and the main steam dump valves. The secondaryside steam generator (SG) atmospheric relief valves, SG power operated relief valves (PORVs) and SG safety valves were not challenged during the transient. The Reactor Coolant System (RCS) responded to the initial plant transient as expected without actuating Pressurizer PORVs or initiating Safety Injection signals. Per design, a differential relay, such as the 187TF relay, should not trip due to an event occurring outside of the relays zone of protection because the input amperage subtracts from the output amperage equaling zero so no amperage is available to trip the relay. Specifically, the 187TF relay zone of protection covers the bus network between the two main generator output breakers and the 1A main bank transformer, which is within the plants switch yard, whereas the fault was 22 miles from the plant site. Initially, the licensee tried to verify that the differential circuits current transformers (CTs) were electrically balanced over their range of operation by injecting increasing levels of test amperage into the circuit. The CTs measure the amperage of the 500KV power and feed that measurement to the 187TF relay so verifying that the input CT amperage properly subtracts from the output CT amperage would validate the CTs characteristics. Because the technicians injecting the amperage did not disable the 86 relay, all of the remaining circuit breakers connected to the X bus opened. The 86 relay detects if any breaker on the bus fails to open when called upon. The 86 relay operation had no significant effects on the shutdown of the plant, but because the technicians did not secure the circuit properly, TVA management decided to stop the verification of the CTs. Alternatively, the technicians tried to verify circuit connections by physically moving the wiring. While handling an A phase wire connected to one of the Digital Fault Recorders, a technician stated that he noticed about 1/8 inch of movement and heard a click. The click is the locking mechanism that ensures the connection remains secure; however the design of the connector ensures electrical connection over one inch of movement. Before the 86 relay tripped the remaining bus relays, the amperage injection had passed approximately 2 amps through this portion of the circuit, which would have detected a loose connection. Inspectors observed that TVA stopped assessing other electrically significant reasons that could have tripped the 187TF relay such as CT imbalances. The root cause team determined that the click heard by the technician was the cause of the relay trip even though subsequent bench testing could not support it. In response to inspector questions, the licensee hired a 3rd party consultant, which also discounted the connector as an obvious cause of the 187TF relay trip. Both the inspectors and the 3rd party consultant believe that neither of the licensees troubleshooting techniques nor their root cause analysis has adequately addressed the cause of the 187TF relay trip, which can continue to challenge the reactor protection system on subsequent high impedance ground faults outside of the plant. The licensee plans to disable the 187TF relay during the next shutdown for refueling, in the spring of 2014, in order to measure the current sensed by the relay as the main generator load is decreased for shut down. This item is identified as unresolved item (URI) 050000390/2013004-01, Contribution of Potential Current Transformer Imbalance to Reactor Trip. |
| Site: | Watts Bar  |
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| Report | IR 05000390/2013004 Section 4OA3 |
| Date counted | Sep 30, 2013 (2013Q3) |
| Type: | URI: |
| cornerstone | Mitigating Systems |
| Identified by: | NRC identified |
| Inspection Procedure: | IP 71153 |
| Inspectors (proximate) | S Shaeffer A Nielsen K Miller R Hamilton R Kellner R Monkc Dykesc Kontz E Patterson J Bartley J Rivera Ortiz K Miller M Magyar N Karlovich P Cooper R Monk T Steadham W Loo |
| INPO aspect | |
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Finding - Watts Bar - IR 05000390/2013004 | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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Finding List (Watts Bar) @ 2013Q3
Self-Identified List (Watts Bar)
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