05000530/LER-2006-004
| Palo Verde Nuclear Generating Station Unit 3 | |
| Event date: | 05-06-2006 |
|---|---|
| Report date: | 10-06-2006 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation 10 CFR 50.73(a)(2)(v), Loss of Safety Function |
| Initial Reporting | |
| ENS 42559 | 10 CFR 50.72(b)(3)(iv)(A), System Actuation |
| 5302006004R01 - NRC Website | |
1. REPORTING REQUIREMENT(S):
Arizona Public Service (APS) submits LER (50-530/2006-004-01) pursuant to 10 CFR 50.73(a)(2)(iv)(A), to report an actuation of the A train emergency diesel generator in response to a valid loss of power event to,one class bus (3EPBAS03). Notification of this event was made to the NRC on ENS 42559.
2. DESCRIPTION OF STRUCTURE(S), SYSTEM(S) AND COMPONENT(S):
Class 1 E AC System Safety-related equipment is divided into two load groups. Either one of the associated load groups is capable of providing power for safely shutting down the unit. Each alternating current (AC) load group consists of one 4.16 kilo-volt (KV) bus (EIIS: EB), three 480V load centers (EIIS: ED), four 480V motor control centers (MCCs)(EIIS: ED), and two non-Class lE MCCs (EIIS: ED). The preferred power source for each load group is off-site ac power (EIIS: EK).
Standby Power� EK The standby power supply for each safety-related load group consists of one emergency diesel generator (EDG)(EIIS: EK), complete with its accessories and fuel storage and transfer systems. The standby power supply functions as a source of AC power for safe plant shutdown in the event of loss of preferred power and for post accident operation of engineered safety feature (ESF) loads.
Station Black Out and Gas Turbine Generator System The station blackout gas turbine generation system (NE) consists of non-safety related generators, 13.8 KV switchgear and associated power distribution system used to deliver AC power from the gas turbine generators to the units.
The gas turbine system (GT) consists of two gas turbine driven generators (GTGs), either of which is capable of supplying the AC power to cope with a Station Black Out (SBO) at one unit.
The GTGs are located outside the protected area, plant east of Unit 1 and south of the Water Reclamation Facility (WRF) near the WRF boundary. Power cables are run to each unit in buried conduit duct banks.
The GT and NE systems together are designated as an independent Alternate AC (AAC). The AAC power system is equipped with a completely independent start system capable of a black start. The starting system is a battery backed direct current (DC) power source that is electrically independent from the PVNGS units' power systems.
The system includes a separate diesel engine which drives a hydraulic start pump and motor to start the turbine.
The GTGs are not normally connected to the units. During a SBO, a GTG is started and then loaded manually onto a 13.8 KV bus. This requires local manual operation in the Turbine Control Room (TCR) as well as local manual closing of a circuit breaker at the 13.8 KV cubicle located at the affected unit's NANS03 bus.
3. INITIAL PLANT CONDITIONS:
Unit 3 was in Mode 5 at 50 psia and 140 F, Cold Shutdown, for a refueling outage at the time of the event. Shutdown Cooling (SDC)(EIIS: BP) train 'B' was operable and in service. Off site power (preferred) was providing power to both 'A' and 'B' train components. Operations and engineering personnel were performing a Gas Turbine Generator test on the A train class 4.16 KV bus at the time of the event.
4. EVENT DESCRIPTION:
experienced a valid loss of power (LOP) actuation on the train 'A' 4.16 kV safety bus.
The event occurred during the performance of an isochronous test for the station's GTGs.
Both GTGs were started for the test. Control room operators (licensed), in conjunction with water reclamation facility (WRF) operators (non-licensed), then paralleled GTG #1 to the offsite power supplying Unit 3 'A' train (PBAS03). Once paralleled to PBAS03, loads were shifted from the normal offsite power to GTG #1. With a steady load on PBAS03, area operators (non-licensed) determined the amperage (load) on PBAS03 by recording 220 amps on the local ammeter at breaker PBASO3L (breaker for normal supply to PBAS03). Emergency Response Facility Data Acquisition Display System (ERFDADS) trends taken after the event indicate that PBAS03 amperage was 250 amps. The procedure required operators to multiply the amperage from the local meter by a factor of .3 to determine the amount of load to place on GTG #1 (220 amps x .3 = 66 amps). The Unit 3 control room operator then coordinated with the WRF operator (non-licensed) to increase GTG #1 load to achieve 66 amps locally on GTG #1 and zero (0) amps through breaker NANSO3A as read by the Unit 3 area operator at the local control room operator opened breaker NANSO3A to transfer all load on PBAS03 to GTG #1. WRF operators then commenced performance of steps of the test procedure which had four sub steps. WRF operators had completed the first step, and were proceeding to perform the second sub step when a loss of power occurred on PBAS03 at 16:35 MST due to an undervoltage condition on PBAS03. Emergency Diesel Generator (EDG) A started and loaded as expected to re-energize PBAS03.
5.A ASSESSMENT OF SAFETY CONSEQUENCES:
Unit 3 was in mode 5 with 'B' train shutdown cooling loop in service with Reactor Coolant System (RCS) loops filled and both steam generator levels greater than 25% wide range. This met technical specification Limiting Condition for Operation (LCO) 3.4:7 for RCS loops filled in mode 5. LCO 3.8.2, AC Sources Shutdown, was met with the 'B' train EDG operable and offsite power supplying the 'B' train. The plant conditions supported performance of GTG test which would parallel the GTGs to offsite power and divorce offsite power from supplying PBAS03 ('A' train) such that the GTGs would carry the full load on PBAS03. With required 'B' train AC sources available and 'A' shutdown cooling not required, no nuclear safety condition existed with the loss of power to PBAS03 during performance of the GTG test.
Power to PBAS03 was lost due to an undervoltage condition. After paralleling GTG #1 to offsite power, the undervoltage condition occurred approximately 42 seconds after offsite power was removed and GTG #1 was supplying on PBAS03. Test steps were in progress to place the GTG in isochronous mode when breaker PBASO3L opened on an undervoltage condition on PBAS03. This condition would not occur during a SBO condition because the GTG would be placed on a class bus that was deenergized in the isochronous mode. A review of emergency procedures found no condition where the GTGs are paralleled to offsite power for the purpose of placing the GTG in service on a class bus. Later on May 6, 2006, after procedure steps were resequenced, the test was performed successfully demonstrating that the GTGs can operate with steady state and transient loads in the isochronous mode. Therefore, no nuclear safety condition existed with the design function of the GTGs in a SBO event.
The event did not result in any challenges to the fission product barriers or result in the release of radioactive materials. Therefore, there were no adverse safety consequences or implications as a result of this event and the event did not adversely affect the safe operation of the plant or health and safety of the public.
The event did not result in a transient more severe than those analyzed in the updated Final Safety Evaluation Report Chapters 6 and 15. The event did not have any nuclear safety consequences or personnel safety impact.
The condition would not have prevented the fulfillment of any safety function of structures or systems as defined by 10 CFR 50.73(a)(2)(v).
6.� CAUSE OF THE EVENT:
The direct cause of the loss of power (LOP) on PBAS03 was due to an undervoltage condition that occurred while transferring offsite power to GTG #1. With GTG #1 at a set load, amps increased and voltage dropped to compensate for the higher load until breaker PBASO3L opened on undervoltage causing a LOP on PBAS03.
The root cause for the event was the test procedure was insufficient and the area operator didn't have the knowledge base to compensate for the procedure inadequacies.
7.�CORRECTIVE ACTIONS:
The investigation results identified the cause of the event was not having the GTG load adjusted properly to accept the full load from offsite power prior to opening the normal supply breaker for PBAS03.
The following corrective actions were taken or are planned, to prevent recurrence.
- Reperformed the test after test steps were resequenced (after resequencing the test steps the GTG test was successful).
o Local ammeter (3EPBASO3L) was checked for calibration and was found to be out of calibration above mid-range. It was found in tolerance for the range required for this test. Meter was replaced.
o GTG isochronous test procedure is scheduled to be rewritten.
® GTG isochronous test tailboard database will be updated with lessons learned from this event.
8.�PREVIOUS SIMILAR EVENTS:
In the past three years, there were no similar events reported for a LOP to a safety bus caused from an insufficient procedure during GTG testing.