05000331/LER-2014-001
| Duane Arnold Energy Center | |
| Event date: | 01-17-2014 |
|---|---|
| Report date: | 04-30-2014 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 3312014001R01 - NRC Website | |
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I. Description of Event:
On January 17, 2014 at 0914, while operating at 100% power, Standby Transformer 1X4 Undervoltage Relay 127/SB2 failed to meet requirements of Surveillance Test Procedure (STP) 3.3.8.1-05B, 1A4 4KV Emergency Transformer Supply Undervoltage Calibration. Relay 127/SB2 has an intended function to actuate on loss-of-voltage to trip the Standby Transformer supply breaker (breaker 1A401) to ensure that essential bus 1A4 is de-energized to allow the "B" Emergency Diesel Generator (EDG) to carry essential loads. The relay is required to be Operable in Modes 1, 2, and 3, and when the associated "B" EDG is required to be Operable by Limiting Condition for Operation (LCO) 3.8.2, AC Sources-Shutdown to ensure that no single instrument failure can preclude the "B" EDG function.
Upon investigation, it was determined that two contacts were incorrectly configured and out of tolerance. Specifically, two contacts were configured as "Normally Open" and not as "Normally Closed" as required per the design drawing. The configuration was such that on a loss-of- voltage, one contact would close and two would open. Since the contacts are in series, the breaker trip signal would not trip breaker 1A401, and the relay could not perform its intended function. The incorrect contact configuration was corrected, and STP 3.3.8.1-05B was completed satisfactorily at 1807 on January 17, 2014.
A past operability review was performed. Relay 127/SB2 had been replaced on September 16, 2013 during pre-planned maintenance on the Standby Transformer. The transformer was placed back in service on September 19, 2013 at 0109. The relay was determined to have been inoperable for 120 days, 8 hours9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br /> and 5 minutes. Therefore, this condition is reportable to the NRC in accordance with 10CFR50.73(a)(2)(i)(B) as a condition prohibited by TS.
There were no other structures, systems or components inoperable at the start of this event that contributed to the event.
A Root Cause Evaluation was performed, two root causes were identified:
- Existing procedural guidance was inadequate to correctly configure the Standby Transformer 1X4 Undervoltage Relay 127/SB2.
- Existing procedural guidance was inadequate to develop sufficient post maintenance testing.
There were no radiological releases associated with this event. There were no other systems, structures or components inoperable at the start of this event that contributed to this event.
II. Assessment of Safety Consequences:
Duane Arnold Energy Center 05000331 A past operability review determined that Relay 127/SB2 was inoperable for approximately 120.4 days.
During that time, the Standby Transformer was supplying essential bus 1A4 for about 17 hours1.967593e-4 days <br />0.00472 hours <br />2.810847e-5 weeks <br />6.4685e-6 months <br />; during the remaining time (about 119.7 days), the Startup Transformer was supplying the bus.
Relay 127/SB2 senses Standby Transformer output voltage directly on the output of the transformer with three coils. There are six contacts associated with the relay. Three normally open contacts (one from each of the three relay coils) are in parallel in the closing circuit; the function of these contacts is to close when adequate transformer output voltage is available providing a permissive to close the Standby Transformer supply breaker to the essential bus (breaker 1A401). Three normally closed contacts (one from each of the three relay coils) are in series in the breaker trip circuit ; the function of these contacts is to close when voltage is below the trip set point (less than adequate voltage) providing a trip signal to breaker 1A401.
Breaker 1A411 ("B" EDG Supply breaker) auto transfer function requires a permissive that 1A401 Standby Transformer breaker is open. Due to the fact that two of the trip circuit contacts were configured incorrectly, breaker 1A401 would not receive a trip signal from relay 127/SB2 immediately upon loss of power on the Standby Transformer secondary winding. However, a second set of relays, the Degraded Voltage Relays 127-Al BUS1A4, 127-A2BUS1A4, 127- B1BUS1A4, and 127-B2BUS1A4, are available to trip the Standby Transformer breaker. These relays sense actual essential bus power directly from the bus. There is a time delay associated with these relays; the relays are set to trip at between 90 to 92% bus voltage with a time delay of between 7.92 to 8.5 seconds.
On a complete loss of power to the bus when the Standby transformer is supplying the bus, the degraded voltage relays would trip breaker 1A401 after 7.92 to 8.5 seconds. This delays the permissive signal to the "B" EDG auto transfer circuit, and therefore the 1A411 "B" EDG supply breaker could not close in on the emergency bus until 7.92 to 8.5 seconds from the loss of power. Technical Specifications 3.8.1.7 and 3.8.1.13 require that the "B" EDG come up to rated voltage and frequency and be able to connect to the essential bus in less than or equal to 10 seconds upon a Loss-of-Offsite-Power (LOOP) and Loss-of-Coolant-Accident (LOCA) signal.
Since the Emergency Diesel Generator start circuit was not affected by the incorrect configuration, the requirements of TS 3.8.1.7 and TS 3.8.1.13 would be met.
While the Startup Transformer was supplying the bus, there was no exposure to the 127/SB2 relay.
Upon a Loss-of-Offsite-Power (LOOP) signal, the 1A402 Startup Transformer breaker would trip. The 1A401 Standby Transformer breaker would not have closed in because the parallel 127/SB2 contacts that provide a close permissive to 1A401, which were correctly configured, would have opened upon the LOOP, thus blocking the closure of 1A401. With 1A401 and 1A402 open, the permissives would be met to allow the "B" EDG to connect to the 1A4 bus within the Technical Specification time requirements.
Based on the above, the safety significance of the event is low. This event did not result in a safety system functional failure. There were no automatic or manually initiated safety system responses.
III. Cause of Event:
A Root Cause Evaluation was performed, two root causes were identified:
1) Existing procedural guidance was inadequate to correctly configure the Standby Transformer 1X4 Undervoltage Relay 127/SB2.
2) Existing procedural guidance was inadequate to develop sufficient post maintenance testing.
IV. Corrective Actions:
On January 17, 2014, the incorrect contact configuration was corrected, and STP 3.3.8.1-05B completed satisfactorily.
A Root Cause Evaluation was performed that identified two root causes of this event. The corrective action for each root cause is listed below:
1) Root Cause - Existing procedural guidance was inadequate to correctly configure the Standby Transformer 1X4 Undervoltage Relay 127/SB2.
Corrective Action — Maintenance Procedure RELAY-G080-08 will be revised to state "When replacing a relay or contact(s) verify that contact configuration matches installed relay and applicable drawing" and address the error-likely tabulation of relay set-point criteria and rating information that appears in the procedure.
2) Root Cause - Existing procedural guidance was inadequate to develop sufficient post maintenance testing.
Corrective Action — Revise MD-024, Post-Maintenance Testing Program, to state that equipment required by the Technical Specifications, when taken out of service, requires post-maintenance testing ensuring, at a minimum, the Technical Specification functionality and operability requirements are met.
V. Additional Information:
Previous Similar Occurrences:
A review of Licensee Event Reports from the past five years did not identify a similar occurrence.
El IS System and Component Codes:
EK - Emergency Onsite Power Supply System Reporting Requirements:
This event is being reported as a condition prohibited by TS, in accordance with 10CFR50.73(a)(2)(i)(B).