05000247/LER-2016-006
| 05-27-2016 | On March 9, 2016, during shutdown for a refueling outage, while performing testing of the safety injection (SI) system, the Station Auxiliary Transformer (SAT) Load Tap Changer (LTC) failed to increase per design upon actuation of an SI signal. The condition was acceptable for the current mode but is unacceptable when the offsite AC electric power distribution and SI System is required to be operable. An investigation was performed and it was discovered on March 28, 2016, that the SAT control cabinet terminal blocks W105 and W106 had their links open thereby preventing proper operation of the LTC. A review of tests and Work Orders (WO) did not identify any previous failed tests or any WO with instructions to open the links. The last successful test of LTC operation was performed on February 26, 2014. The direct cause of the event was open links for the SAT LTC SI signal. The most probable apparent cause was maintenance activities by either IPEC or supplemental personnel that left the SAT states links W105 and W106 open during the 2014 spring outage (2R21). A review was performed of 42 Work Order (WO) tasks conducted during the 2014 spring outage. None of the WO tasks required work on the SAT LTC links or required work in the specific area of the states links. Corrective actions included closure of the SAT LTC Terminal Block states links and testing. The control links for the Unit 3 SAT will be inspected to verify they are in the correct position.
All troubleshooting work orders will require concurrent verification that any equipment manipulation is restored to its original configuration. The SAT LTC Preventive Maintenance (PM) procedure will be revised to insure steps are added to require independent verification for all LTC SI logic control link manipulation. The event had no significant effect on public health and safety.. NRC FORM366AU.S. NUCLEAR REGULATORY COMMISSION FACILITY NAME (1) DOCKET (2) LER NUMBER (6) PAGE (3) Note: The Energy Industry Identification System Codes are identified within the brackets {1. ContentsDESCRIPTION OF EVENTOn March 9, 2016, during shutdown for a refueling outage, while performing testing of the safety injection (SI) system in accordance with 2-PT-R013 (Safety Injection), the Station Auxiliary Transformer (SAT) {FK} Load Tap Changer (LTC) {TTC} failed to increase per design upon actuation of an SI signal {BQ}. The condition was acceptable for the current mode but is unacceptable when the offsite AC electric power distribution and SI System is required to be operable (Mode 4 and above). An investigation was performed and it was discovered on March 28, 2016, that the SAT control cabinet {CAB}, terminal blocks {BLit} W105 and W106 in terminal box BA3 had their links open thereby preventing proper operation of the LTC. A review of tests and Work Orders (WO) did not identify any previous failed tests or any WO with instructions to open the links. The last successful test of LTC operation was performed on February 26, 2014. The event in which the SAT LTC failed to operate correctly during testing was recorded in the Indian Point Energy Center corrective action program (CAP) as CR- IP2-2016-01386. The condition where the SAT control cabinet terminal block links were discovered open was recorded in the CAP as CR-IP2-2016-02293. The SAT is part of the plant AC Electrical Power Distribution System which consists of two offsite circuits (a 138 kV circuit and a 13.8 kV circuit), each of which has a preferred and a backup feeder. The onsite AC standby power circuit consists of three Emergency Diesel Generators. The plant electrical distribution system is configured around 6.9 kV buses Nos. 1,2,3,4,5 and 6. All offsite power to the 480 volt safeguards buses enters the plant via 6.9 kV buses 5 and 6 which are normally connected to the 138 kV offsite circuit but have the ability to be connected to the 13.8 kV offsite circuit. The 138 kV offsite circuit consists of either 138"kV feeder 95332 (preferred feeder) or 138 kV feeder 95331 (backup feeder) and 138 kV/6.9 kV SAT including the automatic tap changer, circuit breakers ST5 and ST6 which supply 6.9 kV buses 5 and 6. When the plant is operating, 6.9 kV buses 1,2,3 and 4 typically receive power from the main generator via the Unit Auxiliary Transformer (UAT). However, when the main generator or UAT is not capable of supporting this configuration, 6.9 kV buses 1 and 2 receive offsite power via bus 5 and 6.9 kV buses 3 and 4 receive offsite power via 6.9 kV bus 6. Following a unit trip, 6.9 kV buses 1,2,3 and 4 will auto transfer (dead fast transfer) to 6.9 kV buses 5 and 6 in order to receive offsite power. The 6.9 kV buses Nos. 2,3,5 and 6 supply power to the 480 volts safeguards power buses (5A, 6A, 2A and 3A) using 6.9 kV/480 volt Station Service Transformers (SSTs). The SAT is a 138 kV/6.9 kV transformer with the primary function to provide electric power to 6.9 kV buses 5 and 6 during on-line operations and 6.9 kV auxiliary buses 1 through 6 during shutdown and startup modes of operations. The SAT and UAT have automatic load tap changers (LTC), which is operated under load to maintain the secondary voltage at 6.9 kV. The SAT LTC is used to mitigate the fast transfer of 6.9 kV system from buses 1 through 4 to buses 5 and 6. This fast transfer typically will cause a voltage drop across the transformer. With the LTC in automatic and responding after 2 seconds, this voltage will be restored in 1 minute. In November 2002, the SAT LTC had a tap changer control circuit modification to enhance the voltage profile by raising taps on initiation of an SI signal. Upon initiation of a SI signal the new circuit takes over control of the voltage setpoint and increases the setpoint that increases voltage towards a pre-selected voltage. The SAT LTC will begin to automatically raise taps to increase secondary voltage in anticipation of the increased loading that will occur thirty seconds later when the 6.9 kV buses 1,2,3 and 4 automatically transfer (fast transfer) to the offsite power supply. The relays in this circuit are located in a box mounted on the outside of the tap changer control cubical. Cause of EventThe direct cause of the event was that SAT LTC Control Cabinet Terminal Blocks W105 and W106 had open links for the SAT LTC SI signal. An evaluation of the blocks determined they require positive force to open. An investigation determined the last documented successful test of SAT LTC SI functionality was February 26, 2014 (prior refueling outage). The most probable apparent cause was maintenance activities by either IPEC or supplemental personnel that left the SAT states links W105 and W106 open during the 2014 spring outage (2R21). A review was performed of 42 Work Order (WO) tasks conducted during the 2014 spring outage. None of the WO tasks required work on the SAT LTC links or required work in the specific area of the states links. A review was performed on the SAT I&C procedure for instrument calibrations and no documented manipulation of SAT links was identified. Prior functionality testing has been satisfactory. Additional investigations are being performed to identify the specific cause. Corrective ActionsThe following corrective actions that will be or have been performed under Entergy's Corrective Action Program to address the. cause:
Event AnalysisThe event is reportable under 10 CFR 50.73(a)(2)(i)(B). The licensee shall report any operation or condition which was prohibited by the plant's TS. This condition meets the reporting criteria because Technical Specification 3.8.1 (AC Sources-Operating) specifies the following AC electrical sources to be operable in modes 1,2,3 and 4: 1) Two qualified circuits between the offsite transmission network and the onsite AC electrical power distribution system, and 2) Three diesel generators capable of supplying the onsite power distribution subsystem. TS 3.8.1 has a Note that the automatic transfer function for the 6.9 kV buses shall be operable whenever the 138 kV offsite circuit is supplying 6.9 kV bus 5 and 6 and the UAT is supplying 6.9 kV buses 1,2,3 and 4. Qualified offsite circuits are those that are described in the UFSAR and are part of the licensing basis for the unit. Each offsite circuit must be capable of maintaining rated frequency and voltage, and accepting required loads during an accident, while connected to the Engineered Safety Features buses. UFSAR Section 7.5.2.1.12.1 discusses the SAT LTC SI signal feature. With the SAT LTC SI actuation signal disconnected, the 138 kV offsite circuit is no longer a qualified circuit therefore is inoperable. With the as-found LTC condition, an event resulting in an SI and fast bus transfer could cause the secondary voltage to drop below the degraded voltage setpoint for more than 10 seconds, resulting in a separation of the safety buses from offsite power (LOOP). Past Similar EventsA review was performed of the past three years of Licensee Event Reports (LERs) for events reporting a TS violation due to inoperable 138 kV offsite power distribution system. No LERs were identified. Safety SignificanceThis event had no effect on the health and safety of the public. There were no actual safety consequences for the event because there were no accidents or events during the degraded condition. There were no significant potential safety consequences of this event. A probabilistic risk evaluation was performed for failure of the SAT LTC to increase to the required voltage following actuation of a SI signal._The risk evaluation for the SAT LTC boosting function involved estimating the increase in core damage frequency (CDF) and large early release frequency (LERF) using a baseline case which only includes initiating events that generate an SI signal. Pending the outcome of further investigations, the time of consideration was conservatively determined to be since the SAT LTC was last verified to be operable for SI actuations on February 26, 2014. Even in this situation, the results of the risk evaluation concluded the risk impact associated with the failure of the SAT LTC to increase voltage following an SI signal is non-risk significant per NRC Regulatory Guide 1.174 (i.e., the increase in CDF is less than 1E-6/year and the increase in LERF is less than 1E-7/year). |
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| Indian Point 2 -K | |
| Event date: | 03-28-2016 |
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| Report date: | 05-27-2016 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 2472016006R00 - NRC Website | |