05000354/FIN-2009007-04
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Finding | |
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| Title | Degraded Voltage Protection Scheme Design |
| Description | The team identified an unresolved item with respect to the HCGS degraded voltage protection scheme. The team noted that the existing scheme was not in conformance with the guidance provided in the office of Nuclear Reactor Regulation (NRR) Branch Technical Position (PSB-1), which established a technical position for the adequacy of station electric distribution system voltages. However, it was not clear at the time of the inspection what the approved licensing bases was for HCGS with respect to the guidelines contained in the STP and postulated degraded voltage scenarios and therefore the issue was unresolved. The team noted that the existing scheme with a postulated degraded grid scenario, had the potential to automatically transfer a bus with a degraded source voltage to an alternate source that may also be degraded. Additionally, the team noted that the degraded voltage relay time delay was longer than the currently analyzed time in the accident analysis with respect to the assumption for cooling water injection to the core during a LOCA. The HCGS electrical distribution system features four 4.16kV safety related buses, each of which can be powered by its dedicated emergency diesel generator or from either of two station service transformers connected to the offsite source. The buses are normally connected to one of the two station service transformers and the bus control logic features a transfer scheme where a safety bus is transferred to its alternate station service transformer in case of the following conditions: Failure of the normal source station service transformer; or: Undervoltage on the primary source. The transfer is accomplished by opening the normal source breaker and closing the alternate source breaker for the affected 4.16 kV safety bus. Degraded voltage relays are connected on the source side of each breaker supplying a 4.16 kV safety bus from the station service transformers. These relays perform two functions in the transfer scheme; initiating the transfer on the normal source, and providing a voltage permissive on the receiving source. The relays have a nominal voltage setpoint of approximately92% of bus rated voltage, and an acceptable time delay range of 15 to 35 seconds. If a degraded voltage condition occurs where the voltage at the 4.16 kV safety buses is near or just below 92%, a transfer of one of the buses may occur. For this scenario, the team was concerned that because the loading on the receiving transformer will increase and the loading on the sending source would decrease, this would tend to decrease voltageEnclosure11on the receiving source and increase voltage on the sending source, possibly enough to reset the degraded voltage relays at the supply breakers for each bus. The receiving source (now supplying three safety buses) would experience degraded voltage and one or two buses originally being supplied by that source would transfer to the source that was degraded first, thereby causing re-degradation. Once a transfer of a particular bus occurs, it would be prevented from transferring back to its original source, but the team noted that nothing prevents the transfer of other buses to that source. Therefore, each of the four safety buses could swap sources, prior to being transferred to the EDGs. The multiple starting of loads could challenge the thermal limits and overcurrent protective devices for some equipment and safety related equipment could be challenged due to damage or tripping of overload devices. This scheme does not appear to be consistent with the guidance in Branch Technical Position PSB-1, Position B.1.b)1), which states that the degraded voltage relay should disconnect the Class 1E system from the offsite power system in case of a degraded voltage condition that exceeds the voltage and time delay setpoints. It was not clear to the team if the design has to be able to withstand a postulated scenario where degraded voltage could occur for both offsite sources such that this condition would be a concern. Additionally, the HCGS degraded voltage scheme employs only one time delay, with an allowable variation from 15 to 35 seconds instead of the two time delay scheme referenced within PSB-1. This time delay is effective whether an accident signal is present or not. The team noted that in June of 1977, the NRC had sent letters to holders of operating licenses at the time, providing guidance that the time delay for second level degraded voltage relays shall not exceed the time delay that was assumed in their FSAR accident analysis. In response to the team's inquiry, PSEG provided data that showed that even if there was no protective action function for the entire allowable 35 second time delay of the degraded voltage scheme, fuel peak cladding temperature (PCT) would remain well below the 2200F acceptance criteria. The team noted that while the PCT would remain below the criteria, the calculated PCT would increase over the current licensing bases number. The team noted that the selection of the original 15 to 35second time delay was apparently based on an attempt to coordinate bus overcurrent backup relays with the under voltage scheme and not related to accident analysis core flood requirements. Additionally, a review of the licensing record did not provide any insight regarding the rationale for omitting the second time delay referenced in BTPPSB-1 Position B.1.b)2), which described a time delay of limited duration such that permanently connected Class 1E loads would not be damaged. The team concluded that the design of the existing degraded voltage protection scheme was an issue requiring further NRC review to determine if HCGS is in compliance with their licensing bases for degraded voltage protection. |
| Site: | Hope Creek  |
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| Report | IR 05000354/2009007 Section 1R21 |
| Date counted | Dec 31, 2009 (2009Q4) |
| Type: | URI: |
| cornerstone | Mitigating Systems |
| Identified by: | NRC identified |
| Inspection Procedure: | IP 71111.21 |
| Inspectors (proximate) | G Skinner F Arner A Ziedonis D Orr L Doerflein J Richmond C Baronm Patel J Furia T Fish E Gray A Patel A Burritt L Ker |
| INPO aspect | |
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Finding - Hope Creek - IR 05000354/2009007 | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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Finding List (Hope Creek) @ 2009Q4
Self-Identified List (Hope Creek)
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