05000327/FIN-2010007-02
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Finding | |
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| Title | Degraded Voltage Relay Issue |
| Description | The team indentified an Unresolved Item (URI) regarding calculations that supported the degraded voltage protection scheme. The calculations that analyzed the Class 1E 6900 VAC and 480 VAC motor loads took credit for using administrative controls for limiting the minimum 161kV offsite power supply bus voltage and credited performance of the non-safety-related automatic load tap changers on the CSSTs to limit the minimum voltage on the Class 1E 6900 VAC and 480 VAC buses. The calculations did not evaluate the Class 1E 6900 VAC and 480 VAC motor loads at the worst case possible low voltages which could drop as low as the bottom end of the acceptable tolerance band of the degraded voltage relays. Offsite power is normally provided to the Class 1E 6900 VAC buses from the 161kV offsite power system through the CSSTs. The CSSTs have automatic load tap changers which are designed to maintain approximately 6900 VAC on the Class 1E buses through a dynamic range of 161kV offsite power supply voltages. The Class 1E 480 VAC buses are then powered from fixed-tap 6900/480VAC transformers powered from the respective Class 1E 6900 VAC buses. NUREG-0800, Standard Review Plan for the Review of Safety Analysis Reports for Nuclear Power Plants, Appendix 8-A, Branch Technical Position PSB-1: Adequacy of Station Electric Distribution system Voltages, Rev. 2(07/1981) is part of the licensing basis for the Sequoyah Nuclear Plant. This document states, in part, that the selection of under-voltage and time-delay setpoints shall be determined from an analysis of the voltage requirements of the Class 1E loads at all onsite distribution levels. Calculation SQNETAPAC, AC Auxiliary Power System Analysis, Rev. 36 evaluated transient motor starting volatges at the beginning of a design basis loss of coolant accident (LOCA) and was based on the voltages where the minimum 161kv offsite power supply bus voltage was limited by taking credit for administrative controls rather than assuming a worst-case 161kV offsite power supply volatge drop which would still allow voltage recovery to the degraded voltage relay reset setpoint (minus septoint tolerance) before the expiration of the degraded voltage relay nominal 9.5 second time delay and thereby leave the Class 1E 6900 VAC buses connected to the offsite power supply. In addition, calculations for motor starting during steady-state conditions credited voltage improvement based on performance of the non-safety related CSST automatic load tap changers instead of being based on worst-case conditions. This issue is unresolved pending further inspection to determine (1) the actual worst-case voltage required to be analyzed on the Class 1E 6900 VAC and 480 VAC buses for safety-related loads in accordance with the facility licensing basis; and (2) the impact of not using the worst-case bus voltage afforded by the degraded voltage protection scheme in safety-related 6900 VAC and 480 VAC motor starting studies. (URI 05000327, 328/2010007-01, Worst Case 6900 VAC Bus Voltage in Design Calculations |
| Site: | Sequoyah  |
|---|---|
| Report | IR 05000327/2010007 Section 1R21 |
| Date counted | Jun 30, 2010 (2010Q2) |
| Type: | URI: |
| cornerstone | Mitigating Systems |
| Identified by: | NRC identified |
| Inspection Procedure: | IP 71111.21 |
| Inspectors (proximate) | R Patterson R Williams S Sandal W Deschaine W Loo C Young E Guthrie L Lake L Mahlahla M Riches M Speck P Higgins R Aiello R Baldwin R Hamiltonb Desaic Baron D Mas Penaranda G Skinner R Aiello R Berryman R Patterson |
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Finding - Sequoyah - IR 05000327/2010007 | |||||||||||||||||||||||||||||||||||||||||||||||||||||||
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Finding List (Sequoyah) @ 2010Q2
Self-Identified List (Sequoyah)
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