05000321/FIN-2009006-08
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Finding | |
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| Title | Degraded Voltage Protection |
| Description | The team identified an Unresolved Item (URI) regarding the Hatch degraded voltage protection scheme. The existing automatic degraded voltage protection scheme employs automatic setpoints that are too low to assure operability of safety related electrical equipment in case of a sustained degraded grid condition, and instead relies on administrative controls to assure adequate voltage to safety-related equipment during an accident. In 1991, the NRC Engineering Design Safety Function Inspection determined that Hatchs calculations for the setpoints of the inverse time degraded undervoltage protection relays, then set at approximately 78.8% with a 20 second delay, were not adequate. Hatch updated the voltage calculations, and indicated in a letter dated November 22, 1993 that the setpoints would need to be raised to approximately91% of 4160V at the 4160V safety buses in order to ensure adequate voltage to safety related loads during a LOCA. Graphs attached to the letter showed that required LOCA voltages ranged from 88.46% to 90.8% for the three 4160V safety buses. During the inspection, Hatch was not able to locate calculations that supported the values (88.46%to 90.8%) given in the graph. Hatch concluded that raising the trip setpoint to 91% would result in little margin between the trip setpoint at which the buses would be separated from offsite power, and the minimum bus voltage that could occur if offsite declined to the lower end of its expected range (101.3% of 230kV). Because of the increase in risk of spurious separation of the offsite power supply that would have occurred if the trip setting of the undervoltage relay was raised, Hatch proposed a scheme where the trip setpoint of the relays providing the automatic separation feature would remain at its existing setting, and additional relays providing an alarm function would be installed, with a setpoint of approximately 92%. In addition, Hatch agreed to maintain a minimum switchyard voltage of 101.3% of 230kV, supported by a software based contingency alarm operated by the transmission system operator. This scheme was recognized as a deviation from the guidance on degraded voltage protection provided in NRC generic letter dated June 2, 1977 because it relied on an alarm followed by manual operator action, in lieu of automatic protection, but it was accepted by the NRC in an SER dated February 23, 1995. Consequently, Hatch is currently relying on measures implemented and maintained by their transmission system operator to assure adequate power to safety related equipment during an accident. This item is unresolved pending further NRC review of plant design and prior NRC inspections related to this issue. (URI 05000321/2009006-08 and05000366/2009006-08, Degraded Voltage Protection) |
| Site: | Hatch  |
|---|---|
| Report | IR 05000321/2009006 Section 1R21 |
| Date counted | Sep 30, 2009 (2009Q3) |
| Type: | URI: |
| cornerstone | Mitigating Systems |
| Identified by: | NRC identified |
| Inspection Procedure: | IP 71111.21 |
| Inspectors (proximate) | C Smith C Baron D Jones G Skinner R Lewis J Hamman B Desai R Williams J Eargle A Sengupta |
| INPO aspect | |
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Finding - Hatch - IR 05000321/2009006 | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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Finding List (Hatch) @ 2009Q3
Self-Identified List (Hatch)
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