05000247/LER-2011-001
| Indian Point 2 | |
| Event date: | 3-1-2011 |
|---|---|
| Report date: | 2-21-2012 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(A), Completion of TS Shutdown 10 CFR 50.73(a)(2)(iv)(B), System Actuation 10 CFR 50.73(a)(2)(iv)(A), System Actuation 10 CFR 50.73(a)(2)(v), Loss of Safety Function |
| 2472011001R01 - NRC Website | |
Note: The Energy Industry Identification System Codes are identified within the brackets {}.
DESCRIPTION OF EVENT
On March 1, 2011, at approximately 11:00 hours, while at 100% steady state reactor power, all three Emergency Diesel Generators (EDGs) {EK} 21, 22 and 23 automatically actuated as a result of an undervoltage on 480 Volt safeguards buses {ED} 5A and 6A due to a loss of 138 kV offsite power to 6.9 kV buses 5 and 6. The 480 volt safeguard buses 2A and 3A remained energized as 6.9 kV buses 2, and 3 as well as 1 and 4 were energized from the Unit Auxiliary Transformer (UAT) {XFMR} which is connected to the Main Generator {TB}. The event was recorded in the Indian Point Energy Center corrective action program (CAP) as CR-IP2-2011-01108.
All EDGs operated as designed. EDGs 21 and 23 were manually loaded and energized buses 5A and 6A. EDG-22 started but was not loaded onto its assigned bus (2A/3A) as power remained available via the UAT. The plant responded normally and all parameters remained within operating limits except two battery chargers (BCs) {EJ} were declared inoperable and the circulating water (CW) pumps 22 and 25 tripped and associated condenser vacuum was partially lost. The loss of the BCs is an expected condition for a loss of offsite power (LOOP). At approximately 11:00 hours, TS 3.8.1 (AC Sources- Operating) was entered for one offsite circuit inoperable, TS 3.8.9 (Distribution System-Operating) was entered for loss of power to 480 V bus 5A and 6A, and TS 3.8.4 was entered for 21 and 24 battery charges inoperable due to loss of buses 5A and 6A.
As TS 3.8.4 does not allow for separate entries and does not have a condition for two inoperable BCs, TS 3.0.3 was entered. TS 3.8.9 was exited at 11:03 hours after EDG 21 and 23 were powering their assigned bus. At 11:12 hours, the availability of 13.8 kV offsite power via feeder 13W92 and that it was energized and carrying load was verified. At 11:22 hours, TS 3.0.3 was exited when BC 21 was returned to service. At 11:41 hours, TS 3.8.4 was exited when BC 24 was returned to service. No power reduction was commenced for TS 3.0.3. At 12:44 hours, bus 6A was energized from its normal power supply. At 13:45 hours, bus 5A was energized from its normal supply breaker. The 21, 22 and 23 EDGs were returned to standby. At 12:17 hours the 25 CW pump was restarted and at 12:19 hours the 22 CW pump was restarted.
Prior to the event Con Edison personnel were performing troubleshooting on the metering circuit {FJ} associated with 138 kV feeder 95332 in the Buchanan switchyard (FK). The troubleshooting was being performed due to the metering circuit not communicating with the New York Independent System Operator (NYISO). As part of the troubleshooting, the test switch associated with the metering circuit was required to be opened. Upon opening the test switch for one of the current transformers (CT) {XCT} inputs to the metering circuit, feeder 95332 tripped on primary pilot wire protection. The CTs associated with the metering circuit in the Buchanan switchyard are also used in the feeders primary pilot wire protection. The pilot wire protection compares the current on both ends of the feeder (Buchanan and Indian Point) to each other. This comparison is used to detect fault conditions on the feeder. When a fault is detected both ends of the feeder are cleared (circuit breakers tripped open), thereby de-energizing the feeder and clearing the fault. For feeder 95332, circuit breakers {BKR} F2, BT3-4, BT4-5, 52/ST-5 and 52/ST-6 opened to clear the fault.
The plant turbine generator is the main source of 6.9 kV auxiliary electric power during on-line operation. Power to the auxiliaries on 6.9 kV buses 1-4 is supplied by the UAT which is connected to the main generator. Power to auxiliaries on 6.9 kV buses 5 and 6 during on-line operation is supplied by the SAT which is connected to 138 kV offsite power sources. Alternatively, buses 5 and 6 can be powered from a 13.8 kV offsite power source. The onsite AC power distribution system includes 480 Volt AC buses 5A, 6A, 2A and 3A which are divided into three safeguards power trains.
Cause of Event
The direct cause of the transient was loss of power to 480 volt safeguards buses 5A and 6A due to isolation of 138 kV feeder 95332 to the Station Auxiliary Transformer (SAT).
Feeder 95332 isolated as a result of an arc on the feeder metering circuit current transformer (CT) switch that caused an imbalance. The CT circuit also supplies the first line pilot wire relay which tripped due to the imbalance. The pilot wire actuated due to a sensed current imbalance between the Buchanan switchyard and Indian Point. In accordance with design, the pilot wire relay tripped 138 kV breakers F2 and BT3-4 in the Buchanan switchyard and actuated protective relay 87L/138. Protective relay 87L/138 actuated lockout relay 86 STP tripping unit 2 breakers BT4-5 and 6.9 kV breakers 52/ST5 and 52/ST6 thereby isolating feeder 95332.
The apparent cause of the trip of feeder 95332 was a failure of the current transformer (CT) test switch associated with the feeder metering circuit to make-before-break. The test switch did not make-before-break due to corrosion on the contact surfaces. The switch is designed to make-before-break to prevent an energized CT from being open circuited, prevent an arc from being drawn across the contacts, and to ensure circuit continuity is maintained to other devices in the open circuit if required. The test switch is owned and operated by Con Edison in the Buchanan switchyard. Con Edison did not notify IPEC about the work in the switchyard as the work request was for work associated with RTU metering at Buchanan.
Corrective Actions
The following corrective actions have been or will be performed under the Corrective Action Program (CAP) to address the cause of this event.
- The test switch was closed and feeder 95332 restored to service.
- Con Edison placed a deficiency tag on the switch identifying it as degraded and not to use it. Subsequently, troubleshooting and testing was performed on the Buchanan switchyard feeder 95332 CT circuits and the flexi test switch was replaced in accordance with Con Edison's work process.
- The Con Edison revised their notification procedure to require notification to Indian Point for in-service work at the Buchanan switch yard 138 kV and 345 kV circuits and all connecting facilities to Indian Point.
Event Analysis
The event is reportable under 10CFR50.73(a)(2)(iv)(A). The licensee shall report any event or condition that resulted in the manual or automatic actuation of any system listed in 10CFR50.73(a)(2)(iv)(B). The systems to which the requirements of 10CFR50.73(a)(2)(iv)(A) apply include (#8) Emergency AC electrical power systems including emergency diesel generators.
The actuation and start of the EDGs due to undervoltage on buses 5A and 6A from the loss of offsite power feeder 95332 meets the reporting criteria. Because plant shutdown was not completed for TS 3.0.3 as a result of inoperable BC-21 and BC-24, the TS 3.0.3 entry is not reportable under 10CFR50.73(a)(2)(i)(A).
The loss of the offsite feeder did not result in the failure of any primary system to function properly. Therefore, there was no safety system functional failure reportable under 10CFR50.73(a)(2)(v) as the minimum required safeguards components were available.
In accordance with reporting guidance in NUREG-1022, an additional random single failure need not be assumed in that system during the condition.
Past Similar Events
A review was performed of Licensee Event Reports (LERs) for the past three years for any events reporting inadvertent EDG actuation as a result of maintenance activities.
No similar LERs were identified.
Safety Significance
This event had no significant effect on the health and safety of the public. There were no actual safety consequences for the event because there were no accidents and the emergency AC power system was available to power required safeguards buses. Power from both offsite sources and onsite emergency power were available and the actuation circuitry and EDGs performed in accordance with design and minimum safeguards power was available to power safety loads. All plant parameters responded normally for the transient and remained within operating limits.
There were no significant potential safety consequences of this event. This event was bounded by the analyzed event described in UFSAR Section 14.1.12 (Loss of all AC Power to the Station Auxiliaries). The plant is designed for a LOOP and has safety related emergency AC power by EDGs to start on a 480 VAC safety bus undervoltage and power the 480 VAC safety buses (2A/3A, 5A, 6A). Results of the analysis show that for the loss of offsite power to the station auxiliaries all safety criteria are met. The Auxiliary Feedwater capacity is sufficient to prevent water relief through the pressurizer relief and safety valves assuring the reactor coolant system (RCS) is not over pressurized.
The analysis also demonstrates that sufficient long term heat removal capability exists by the natural circulation capability of the RCS following reactor coolant pump coast down to prevent fuel or clad damage. The loss of BCs is an expected condition for a LOOP which results in the motor control centers powering the BCs from bus 5A and 6A to trip and require reset. There are four BCs and four battery installations with each battery fed from a separate charger. Each battery has been sized to carry its expected shutdown loads for a period of 2 hours2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br /> following a plant trip and a loss of all AC power. All equipment supplied by the batteries are maintained operable with minimum expected voltages at the battery terminals during the 2 hours2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br />. During this event the 21 BC was unavailable for 22 minutes and the 24 BC was unavailable for 41 minutes which is within the battery design capability of 2 hours2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br />.
In addition, there is an independent station blackout (SBO)/Appendix R diesel generator and associated switchgear necessary to achieve and maintain cold shutdown conditions independent of the normal safeguards and instrumentation power supplies and to function as the alternate ac power supply operated from outside the Control Room.