05000352/LER-2006-004
| Limerick Generating Station, Unit 1 | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications 10 CFR 50.73(a)(2)(v)(d) |
| 3522006004R00 - NRC Website | |
Unit Conditions Prior to the Event Unit 1 was in Operational Condition (OPCON) 1 (Power Operation) at approximately 100% power. Unit 2 was in Operational Condition (OPCON) 1 (Power Operation) at approximately 100% power. There were no structures, systems or components out of service that contributed to this event.
Description of the Event
On December 27, 1999, INPO issued SOER 99-1 Loss of Grid. On December 9, 2004 INPO issued SOER 99-1 Loss of Grid - Addendum. Recommendation 4 of the addendum stated the following:
Periodically review, confirm and update the grid reliability and stability design assumptions to ensure they remain valid following changes or modifications to the plant and to the grid. The review should include the following, as a minimum:
- Grid restoration time assumptions to restore off-site power sources to the plant
- The impact on plant voltage limits and voltage predictions following a generator trip, including whether a generator trip could result in a loss of off-site power Engineering performed a review as a result of the SOER Addendum recommendation and identified that the voltage regulation study (Calculation 6300E.20) assumptions did not match the current transmission system (EIIS:FK) operating strategy. The calculation incorrectly assumed a 1.7% grid voltage drop upon a trip of an operating unit. This value is non- conservative in that the Transmission System Operator (TSO) can, under unusual grid conditions, operate the grid to a post trip contingency voltage limit of 7%, and interfaces had not been established between the TSO and Limerick Operations for notification if the calculated limits were exceeded.
A review of the prior three years of operating history identified several occasions when calculated post trip contingency voltages could have resulted in actuation of the 4 kV safeguard bus degraded voltage relays (EIIS:RLY). Two periods, during single offsite circuit operation, were identified when a LOCA on an operating unit could have caused an actuation of the degraded voltage relays resulting in a loss of the only in-service offsite circuit. A review of actual selected grid configurations during a two-week period in the summer of 2006 identified occasions when a LOCA on an operating unit could have resulted in the loss of both offsite circuits.
These events involved a condition that could have prevented fulfillment of the safety function for the offsite power sources. In addition one or both offsite circuits may have been inoperable for periods that exceeded the Technical Specification 3.8.1.1 Action "g" allowable outage time of 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />. Therefore, this LER is being submitted pursuant to the requirements of 10CFR50.73(a)(2)(v)(d) and 10CFR50.73(a)(2)(i)(B).
Analysis of the Event
There were no actual safety consequences associated with this event. The potential safety consequences of this event were minimal. The calculation error resulted in brief periods during operation with degraded grid conditions when the 4 kV safeguard bus degraded voltage relays could have actuated following a LOCA on an operating unit. An actuation of the 4 kV safeguard bus degraded voltage relays would have tripped the offsite circuit feeder breakers to the affected 4 kV safeguard buses. The on-site emergency diesel power supply (EIIS:EK) operability was not affected.
Limerick Offsite Power System design consists of one 13 kV circuit from the 220 kV substation and one 13 kV circuit from the tertiary of a transformer (EIIS:XFMR) that connects the 220 kV substation and the 500 kV substations. The circuit from the 220 kV substation utilizes 10 station auxiliary transformer and "101 safeguard transformer to feed "101 safeguard 4 kV bus. The circuit from the tertiary utilizes "20 regulating transformer and "201 Safeguard Transformer to feed "201 safeguard 4 kV bus. Each unit has four 4kV safeguard buses. Two 4kV safeguard buses on each unit are normally energized from "101 bus and two buses on each unit are normally energized from "201 bus. Limerick has a robust onsite power supply that consists of eight emergency diesel generators (EDGs). The eight EDGs can provide power to the eight 4 kV safeguard buses.
Each 4 kV safeguard bus voltage is monitored by a set of degraded voltage relays. These relays open the source breaker supplying the bus when bus voltage remains below the setpoint for periods that exceed the time delays.
The 4 Kv safeguard bus degraded voltage relay time delays vary with bus voltage as follows: 1) 3910 volts 61 second delay, 2) 3640 volts 52 second delay, and 3) 2905 volts 1 second delay.
The 3910 volts delay is reduced to 10 seconds during LOCA conditions. The short LOCA time delay challenges the offsite circuit transformers automatic voltage control (AVC) capability to raise voltage above the relay setpoint to prevent relay actuation and subsequent trip of the offsite circuits 4 kV breakers. The long time delay for non-LOCA conditions allows the transformer AVC to increase voltage and prevent a relay actuation.
Cause of the Event
The site voltage regulation study calculation was based on a voltage drop limit of 1.7% and the TSO operational limit was 7%. Interfaces had not been established between the TSO and Limerick Operations for notification if the calculated limits were exceeded. In addition, the post trip contingency percentage voltage drop was not previously considered when determining offsite source operability.
Corrective Action Completed The site voltage regulation study calculation has been revised.
The 101 and 201 safeguard transformer load tap changer (LTC) time delays were modified to provide more operational margin and the transformer tap changers are now required to be maintained in automatic mode.
An interface agreement was established with the TSO to require a prompt notification to the station when the post trip contingency voltage drop due to a potential Limerick unit trip exceeds the calculation values.
The station grid emergency procedure (E-5) has been revised to properly address degraded grid conditions and station electrical alignments. The procedure identifies the grid conditions and electrical alignments that require the offsite circuits to be declared inoperable. Compensatory measures are also directed by the procedure to preclude offsite source inoperability.
Station procedures have also been revised to initiate compensatory measures to obtain additional post trip contingency voltage margin under certain plant configurations (ST-6-092-365-0 and ST-6-092-366-0).
Previous Similar Occurrences There was a previous occurrence of unplanned inoperability of the offsite circuits that was reported in LER 1-05-005. The implementation of a modification on the 101 and 201 transformer automatic tap changer time delays did not ensure that procedures require the offsite circuit to be declared inoperable when the tap changer was placed in manual mode.