05000301/LER-2001-001
| Docket Number | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| Initial Reporting | |
| 3012001001R00 - NRC Website | |
Event Description:
On February 6, 2001, at 2152 (all times are CST), while operating at 100% power, the Point Beach Nuclear Plant (PBNP), Unit 2, experienced a generator [TB] lockout which caused an immediate opening of the main generator breaker [BKR], followed closely by a turbine [TA] trip, and an automatic reactor [RCT] trip. The generator lockout was caused by the 27/59N relay [64], which is one of the stator neutral ground detection devices. The 27/59N relay, hereafter referred to as the "RAGEA" relay, is an electronic device installed in Unit 2 in 1987 and Unit 1 in 1988 as part of a system wide modification to provide additional fault protection and detection for the generator. The 27 device is designed to protect the last 5% of the generator windings closest to the neutral connection from a ground fault. The other 95% of the windings is protected by the 59 device of the RAGEA relay and a separate relay (the 59N relay), both of which did not actuate during the event.
The plant operating staff responded to the reactor trip by entering the appropriate Emergency Operating Procedure, EOP 0, "Reactor Trip or Safety Injection," and transitioned to EOP 0.1, "Reactor Trip Response." As expected following a loss of load from greater than 430 MW load, the cross over steam dump system [JI], which is designed to minimize turbine overspeed, actuated. Following this actuation, the plant experienced a loss of condenser [SG] vacuum. We believe this was a result of the cross over steam dump valves failing to reseat promptly. In response to this condition, the operating staff also entered AOP-5A, "Loss of Condenser Vacuum" in parallel with EOP 0.1. AOP-5A directed the shutting of the main steam isolation valves. Reactor Coolant System [AB] temperature and pressure were then controlled using the atmospheric Steam Dump system [RV]. All other systems and equipment necessary for operation following the trip operated as expected and the unit remained stable at normal hot shutdown temperature and pressure.
A post trip review, incident investigation, and root cause evaluation were initiated. The NRC was notified of the event in accordance with 10 CFR 50.72(B)(2)(iv)(B) at 2244 (Event Notification 37722). The unit trip and the loss of condenser vacuum following the cross over steam dump actuation have been documented in the corrective action program (CR 01-0389 and CR 01-0397 respectively).
Extensive trouble shooting and investigations were completed in an attempt to determine why the RAGEA relay 27 device actuated. These efforts are discussed in detail in the Cause and Corrective Action Sections of this report.
PBNP Unit 2 was subsequently released for restart at 0100 on February 8. Unit 2 was taken critical at 1402 on the 8th and returned to service at 1426 on February 9, 2001.
Cause:
On December 17, 2000, while conducting generator voltage regulator [TL] testing, a generator lockout and turbine trip were initiated by this same RAGEA relay. At that time the reactor power was at 28% and, since the plant is designed to voltage regulator had been modified during the PBNP Unit 2 Fall 2000 refueling outage. The post modification testing required testing of the minimum excitation limiter protective settings by operating with a leading power factor, i.e.
bringing MVARS into the generator. While conducting this testing, at approximately 200 MVARS in, this same RAGEA relay actuated and caused a generator lock out and turbine trip. Our investigation of that event determined that the RAGEA relay, which uses the third harmonic (180 Hz) voltage on the generator neutral ground as an indication of a fault within the first 5% of the winding closest to neutral, had detected a decreasing third harmonic voltage and actuated correctly. Following this relay actuation, we found that the third harmonic voltage could be diminished when operating at low power levels with a leading power factor. The increase of MVARS in during the test caused the third harmonic voltage to decrease to the relay trip point. However, during the lockout on February 6, 2001, the unit was operating with a lagging power factor, 75 MVARS out, and there were no switching or other transients experienced on the transmission system. Furthermore, Point Beach Unit 1 has the same relay with the same setpoint. An actual system disturbance would, therefore, be likely to affect both units. No problems or abnormalities were observed on Unit 1.
Accordingly, we do not believe that the cause of the February 6th RAGEA relay actuation is related to the December U.S. NUCLEAR REGULATORY COMMISSION 00 2001 - 001 - Point Beach Nuclear Plant Unit 2 05000301 17th turbine generator trip.
The following actions were taken during the investigation of the cause of this trip included:
- Fault recorders in the stator ground protection relay circuit were checked and showed no evidence of an actual fault nor any abnormal third harmonic around the time of the trip. The information from the Unit 2 and Unit 1 recorders was similar.
- The generator phase to ground circuit was meggered and found to be satisfactory with no evidence of an actual ground. The primary and secondary neutral grounding connections were inspected with no loose connections found.
- The RAGEA relay was calibrated and tested in place and found to be functioning properly. This included verification of the four second trip time delay.
- The relay signal circuit connections were verified to be tight. The cables in the signal circuit were meggered and found to be satisfactory.
- The 2-59N-TGO1 relay which provides ground fault protection for 95% of the stator windings was calibrated and found to be functioning properly.
- The neutral grounding transformer was disconnected and had a turns ratio test performed acceptably.
- Since the RAGEA relays may be sensitive to radio frequency (RF) transmissions, the possibility of radio transmissions in proximity to the relay was investigated. Interviews with personnel using radios that evening in the vicinity of the relay cabinet established that no one was located in the area at the time of the trip. Furthermore, no one recalled hearing any radio communications just prior to the trip.
- During the trouble shooting and investigation of this fault the PBNP engineering staff was assisted by protective relay experts from Wisconsin Electric Power Company and the American Transmission Company.
Despite the extensive post trip testing and information gathering and analysis, we could not substantiate or provide a reason for the RAGEA relay actuation. Indeed, our evaluation determined that the plant was operating under steady state conditions with no abnormalities noted prior to the time of the trip. At this time, our conclusion is that either the plant experienced a spurious relay actuation or there is an intermittent ground somewhere in that circuit that is currently gone or undetectable. Based on start up monitoring and the current unit operation without relay actuation at essentially the same conditions as the trip, we consider it unlikely that the latter condition existed.
Corrective Actions:
Since a definitive reason for the relay trip was not determined, the following corrective actions have been or will be taken:
- Based on the non-conclusive findings of our incident investigation and trouble shooting and the advice of the protective relay experts we had consulted, we completed a temporary modification to change the actuation of the 27 device of the RAGEA relay from initiating a generator lockout relay trip to initiating a common alarm function in the control room. As noted above, PBNP operated until 1987 for Unit 2 and 1988 for Unit 1 without these relays.
There is minimal risk of a stator ground in the generator which would not be caught by other installed protective relaying. This action also minimizes the potential for additional plant and reactor transients which could be initiated FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6) PAGE (3) Point Beach Nuclear Plant Unit 2 05000301 by a future spurious relay actuation. Our review of industry experienced a similar trip (eventually traced to an incorrect (temporarily removing the trip function). The trip removal
- Prior to the unit restart, test equipment was connected third harmonic frequency component of the neutral to during the unit start up. During generator excitation observed by system protection experts and system with this device during the startup or subsequent operations.
- Guidance to the control room operators on how to respond
- The monitoring instrumentation and operations procedural additional evaluations of this relay and determine longer
- System engineering is evaluating the performance of system performed as expected and to determine if corrective Safety Assessment:
With the exception of the ground fault relay which initiated likely due to the cross over steam dump valves not reseating inadvertent RPS actuation was as expected. Systems transient performed as designed and maintained the plant investigation of the cause of the unit transient and the modification recurrence of the generator lock out in the event of another experienced during the subsequent unit restart and return spurious relay actuation during the subsequent Unit 2 power reactor protection system and other plant equipment necessary plant in a stable configuration, the safety significance of and the plant staff was not impacted by this event. There related safety function; therefore, this event did not involve Similar Occurrences:
A review of recent LERs (past two years) identified the following relaying related fault:
LER NUMBER � Title 7) operating experience identified another plant which had setpoint) with a RAGEA relay and took the same action in that case was subsequently made permanent.
to measure composite neutral to ground voltage and the ground voltage. � Performance of this relay was monitored and synchronization to the grid relay performance was engineers in the control room. � No problems were observed to an alarm from the RAGEA relay has been provided.
guidance will remain in effect until we can conduct term corrective actions.
the cross over steam dump system to assess whether the measures are necessary.
this event and the loss of condenser vacuum (which was promptly), the plant response during and following this and equipment necessary to mitigate the consequences of this in a stable hot shutdown condition. Following the to the relay actuation circuit to preclude a spurious relay actuation, no further problems were to full power operations. We have, likewise, experienced no operations. Other than the inadvertent challenge of the to remove shutdown decay heat and maintain the this event was minimal. The safety and welfare of the public was at no time a loss of a system, structure, or component a safety system functional failure.
event which involved a reactor trip due to a protective Phase Main Step-up Transformer Results in Reactor Scram 301/200-007-00 � Fault Associated with "C"