05000275/LER-2007-003
| Docket Number Sequential Revmonth Day Year Year Month Day Yearnumber No. | |
| Event date: | 05-28-2007 |
|---|---|
| Report date: | 07-27-2007 |
| Initial Reporting | |
| ENS 43393 | 10 CFR 50.72(b)(3)(iv)(A), System Actuation |
| 2752007003R00 - NRC Website | |
I. Plant Conditions
The Pacific Gas and Electric Company's (PG&E) Diablo Canyon Power Plant Unit 1 was in Mode 1 (Power Operation) at approximately 8 percent reactor power prior to the event.
II. Description of Problem
A. Background
The circulating water system [KE] provides a continuous ocean water cooling supply to the main condenser [COND], condensate cooler [CLR], service water cooling system [KG], and intake cooling system [KE]. Each unit has two single-stage circulating water pumps (CWPs). The CWPs are nominal 12kV, 13,000 horsepower motors and can derive their power from the 230kV Startup-Standby System [EA] or the Unit Auxiliary Power System to their respective nonvital 12kV buses. Each unit has two nonvital 12kV Buses; nonvital Bus D provides power to CWP 1-1, Reactor Coolant Pumps (RCPs) 1-2 and 1-4, and nonvital Bus E provides power to CWP 1-2, RCPs 1-1 and 1-3.
Each unit has three 4160V vital buses that can be powered from the Startup-Standby, Unit Auxiliary, or emergency diesel generators [DG] [EK]. Should a sustained low voltage condition be experienced on any of these buses, undervoltage relays will initiate a DG start signal and if undervoltage is of sufficient magnitude/duration, the bus will separate from the offsite power source and transfer to the DG for subsequent loading.
Three undervoltage relays are provided on each 4160 Class 1E vital bus for detecting sustained degraded voltage condition or a loss of bus voltage. A relay will generate a loss of power (LOP) signal (first level undervoltage relay (FLUR) setpoint) if the voltage is below 75 percent for a short time.
The second level undervoltage relay (SLUR) actuates at approximately 3785V or below, approximately 91 percent, but above the setpoints of the FLURs, the following SLUR actions occur automatically:
(1) After a less than or equal to 10 second time delay, the respective diesel generators will start.
(2) After a less than or equal to 20 second time delay, if the undervoltage condition persists, the circuit breakers for all loads to LICENSEE EVENT REPORT (LER) TEXT CONTINUATION FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6) PAGE (3) the respective vital 4160V buses, except the 4160-480V load center transformer, are opened and sequentially loaded on the DG.
B. Event Description
On May 28, 2007, at 2250 PDT, with Unit 1 at approximately 8 percent power, preparations were being made to parallel the Main Generator to the electrical transmission grid. All 12kV and 4160V electrical busses had been transferred to the Startup-Standby power supply (230kV System).
Operations started CWP 1-2, a 13,000 horsepower 12kV motor. Since the 12kV System feeds the 4kV System when aligned to Startup, the resultant voltage dip propagated throughout the Startup Standby System. The voltage transient duration of approximately 8 seconds was sufficient to time out the SLUR on the 4160V Vital Bus 'G,' causing a start of that bus's DG 1-2. The 4160V Vital Bus 'G' voltage recovered to above the SLUR voltage setpoint prior to timing out the load shed timer. Thus the bus continued to be powered by the Startup-Standby power source; loads were not transferred and DG 1-2 remained in operation unloaded.
Immediately after the component actuation, DG 1-2 was verified not to be required to respond to the voltage transient event, was shut down, and returned to its normal standby condition.
On May 29, 2007, at 0501 PDT, an eight-hour nonemergency report was made using the Event Notification System (ENS Number 43393) pursuant to 10 CFR 50.72 (b)(3)(iv)(A).
C. Status of Inoperable Structures, Systems, or Components that Contributed to the Event None.
D. Other Systems or Secondary Functions Affected None.
E. Method of Discovery The automatic actuations were immediately known to licensed plant operators in the control room by alarms and indications.
6 LICENSEE EVENT REPORT (LER) TEXT CONTINUATION FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6) PAGE (3) Diablo Canyon Unit 1 0 5 0 0 0 2 7 5 2007 0 0 3 0 0 4 OF
F. Operator Actions
Plant operators responded to alarms and indications provided in the control room, confirmed that continued DG 1-2 operation was not required, shut down the DG, and returned it to its standby status.
G. Safety System Responses
DG 1-2 automatically started when a transient undervoltage condition was sensed on the 4160V Vital Bus 'G', causing DG 1-2 to start, but did not cause it to load. Since the 4160V Vital Bus 'G' continued to be powered by the Startup-Standby power source, loads were not transferred to DG 1-2. The magnitude/duration of the voltage transient on the 4160V Vital Bus 'F' and 4160V Vital Bus 'H' was similar, but due to the relay settings being closer to 10 seconds, these relays did not time out and the DG 1-1 and DG 1-3 did not start.
Cause of the Problem
A. Immediate Cause
The duration of a 4kV undervoltage transient resulting from the start of a 13,000 horsepower CWP pump motor, while both the associated 12kV bus and the 4kV Vital Bus G were aligned to the Startup offsite power source, exceeded the setpoint of the second level undervoltage DG start time delay relay, starting DG 1-2.
B. Root Cause
Plant procedures did not clearly identify precautions necessary for the subject plant configuration to minimize voltage transient conditions that could result in an inadvertent DG start.
C. Contributing Causes Prior engineering recommendations were not captured in plant procedures.
IV.�Assessment of Safety Consequences There were no actual safety consequences involved in this event since the safety-related plant electrical systems responded as designed. All vital LICENSEE EVENT REPORT (LER) TEXT CONTINUATION FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6) PAGE (3) equipment remained aligned to the Startup-Standby power source for the duration of the event.
Final Safety Analysis Report (FSAR) Update, Section 15.2.9, "Loss of Offsite Power to the Station Auxiliaries," previously analyzed the total loss of offsite power, and bounds the transient undervoltage conditioned experienced in this event. The FSAR Update concluded that "for the loss of offsite power to station auxiliaries event, all safety criteria are met.
This condition is not considered a Safety System Functional Failure as the transient undervoltage condition is a previously analyzed condition and anticipated operational occurrence that does not create a significant condition adverse to safety. The lowest voltage experienced on 4160V Vital Bus 'G' was seen as approximately 85.6 percent; the FLUR setpoint is at 75 percent and the SLUR setpoint is at approximately 91 percent. The setpoints for the FLURs and SLURs are based on ensuring sufficient margin is available so that loads are not stripped from the bus and reloaded on the DGs. In addition, The SLUR setpoints have been selected to ensure that the technical specification allowable values are not exceeded due to normal instrument drift. The methodology used provides margin by accounting for inaccuracies, biases, and uncertainties of components including calibration equipment.
Therefore, the event is not considered risk significant and it did not adversely affect the health and safety of the public.
V.�Corrective Actions
A. Immediate Corrective Actions
Following confirmation that the DG start was due to the CWP start voltage transient, DG 1-2 was shut down and returned to its standby status.
B. Corrective Actions to Prevent Recurrence
1. PG&E revised the CWP starting procedure OP E-4:1 to add a precaution to minimize the voltage transient condition due to the large motor start.
2. PG&E will revise the RCP starting procedure OP A-6:I to add a precaution to minimize the voltage transient condition due to the large motor start.
LICENSEE EVENT REPORT (LER) TEXT CONTINUATION FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6) PAGE (3) 50Diablo Canyon Unit 1 0 VI.AAdditional Information 0 0 2 7 5 2007 300
A. Failed Components
None.
B. Previous Similar Events
None.
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