05000272/LER-2003-002
| Salem Generating Station Unit 1 | |
| Event date: | 01-29-2003 |
|---|---|
| Report date: | 09-24-2003 |
| 2722003002R00 - NRC Website | |
DOCKET
SC-OUENTIAL
REIASION
PLANT AND SYSTEM IDENTIFICATION
Westinghouse — Pressurized Water Reactor Reactor Protection System (JC)* Switchyard System {FK} 500KV breaker (SKR)
- Energy Industry Identification System {EIIS} codes and component function Identifier codes appear as (SS/CCC)
IDENTIFICATION OF OCCURRENCE
Event Date: July 29, 2003 Discovery Date: July 29, 2003
CONDITIONS PRIOR TO OCCURRENCE
The plant was in MODE 1 (POWER OPERATION) at the time of the event. No other structures, systems or components were inoperable at the start of this event that contributed to the event.
DESCRIPTION OF OCCURRENCE
On July 29, 2003 at 1329 Salem Unit 1 reactor tripped automatically due to Generator Protection and Turbine Trip. A short circuit in the ground fault protection circuitry on the 500KV 1-5 breaker (BKR} caused protective relaying to actuate which resulted in isolating the Unit 1 generator causing a generator and turbine trip which led to a reactor trip. The actuation of the 500KV 1-5 breaker protective relaying results in a loss of 500KV bus section 1 and isolation of the Unit 1 generator from the transmission network. The isolation of bus section 1 caused the loss of No. 2 station power transformer (SPT) resulting in the loss of Unit 1 non-vital busses 'F" and G. In addition, loss of bus section 1 caused the loss of the 14 station power transformer (SPT) which was feeding the 16 and 1C vital busses. The 1B and 1C vital busses transferred as designed to the 13 SPT, which was feeding the 1A vital bus. The loss of 14 SPT also caused three of the Circulating Water (CW) pumps to transfer to the 13 SPT as designed. As a result of the event, the 4KV vital bus voltages dropped below the second level undervoltage setting of 95.1%. Voltage on the 4KV busses did not recover to the reset setpoint before the relay timeout caused the relay logic to separate the vital busses from offsite power. The Emergency Diesel Generators (EDG) started and loaded as expected. Salem Unit 2 responded as expected and remained at power.
An Unusual Event (UE) was declared at 1401 due to a loss of offsite power for greater than 15 minutes to the vital busses. The UE was exited at 2201 after restoring normal power to the vital busses.
SAC FORM 3155A (i4)31) NRC,FORNI 366A (1-2aa1)
DOCKET
05000272 YEAR U.S. NUCLEAR REGULATORY COMMISSION LICENSEE EVENT REPORT (LER} FACILITY NAME (1) K DESCRIPTION OF OCCURRENCE (Continued) Unit 1 responded as expected except for the failure of the 13 SPT to recover voltage. Degraded voltage relays cleared the bus and loaded the vita! buses on the EDGs. The EDGs performed as designed.
ANALYSIS OF OCCURRENCE
The Initiating fault was Identified as a 500KV breaker 1-5 breaker ground relay 'B' phase target.
This fault invokes the "Breaker Failure' scheme and Isolates the 1-5 breaker from the 500kV grid. Isolating the 1-5 breaker causes the Unit 1 generator to be isolated from the 500KV grid and de-energizes bus section 1.
Troubleshooting found a current transformer (CT) control cable '0 and "C' phase conductor for breaker 1-5 shorted and grounded together. Breaker 1-5 was tested and found to operate correctly. Bus relays were checked and were in calibration.
Loss of 500kV bus section 1 results In loss of the 500KV grid to No. 2 and No. 4 station power transformers (500/13.8KV). Loss of No. 2 Station Power Transformer results in loss of °IF and °1G' Group Buses (non- vital). Loss of No. 4 station power transformer results in loss of 14 and 23 station power transformers (13.8/4.16KV). Unit 1 14 CW pump bus, 18 and IC vital buses also auto-transferred to 13 station power transformer as designed; however, 4kV secondary voltage did not recover above the degraded grid reset value of 97% of 4160. After approximately 13 seconds the 95% UV relays timed out and SEC Mode 2 actuation occurred and the Unit 1 EDGs started and picked up vital loads.
The separation of the Unit 'I vital buses from the off-site power source occurred due to the combined transfer of the 14 Circulating Water pump bus and the 1B and IC vital buses to the 13 station power transformer.
Transfer of these bulk loads caused a greater than previously analyzed voltage drop on the SPT secondary.
This in turn caused the 95% second level of under voltage protection relays to operate and time out before the transformer secondary voltage could recover above the relays' reset value of 97% or 4035 volts. Based on Plant Historian data, the bus voltages did recover to approximately W 4021 volts, °EI' 3986 volts and "C' 4011 volts prior to separation from offsite power.
The automatic transfer of the CW busses between SPTs (breaker 1CW8AD) was disabled to prevent a recurrence of separation from the preferred power source. The reduction of non-vital loads from buses (approximately 6MVA total load) during bus transfers would allow voltage recovery to higher values. The automatic transfer of Circulating Water Busses was disabled for Unit 1 and Unit 2. An operability determination was issued to address the condition pending permanent corrective action. In addition, the lower operating limit of the Salem Unit 1 and Unit 2 vital busses was raised from 4.220KV to 4.275KV, based on revised calculations, to account for non-conservative calculation bias and other non-conservatisms.
The apparent cause of the failure of the safety busses to transfer as expected was inadequate calculations that supported the switchyard modifications made in 1992 to permit CW bus transfer.
'NRC FORM 26-4AIT400l) ,NRC FORM 366A U.S. NUCLEAR REGULATORY COMMISSION (1-2001) FACIUTY NAME (1)
DOCKET
q5000272 YEAR LER NUMBER (6)
KUSER NUMER
_ PAGE (3)
CAUSE OF OCCURRENCE
The apparent cause for this event was failure of a current transformer (CT) control cable "Er and me" phase conductors that were shorted and grounded together for the 1-5 500KV breaker leading to actuation of the protective relaying for this breaker. The apparent cause of the failure of the safety busses to transfer as expected was Inadequate calculations that supported the switchyard modifications made in 1992 to permit CW bus transfer.
PREVIOUS OCCURRENCES
A review of reportable events in the last two years did not identify events of a similar nature.
SAFETY CONSEQUENCES AND IMPLICATIONS
There were no safety consequences associated with this event since all safety systems other than offsite power functioned as designed and operating procedures were adequate and property implemented.
This event constitutes a Safety System Functional Failure (SSFF) as defined In NEI 99-02.
CORRECTIVE ACTIONS
1. The fault was Isolated to the 1-5 Breaker "Breaker Failure Scheme" circuitry, the ring bus was restored and offsite power was restored to plant busses.
2. Troubleshooting of the 1-5 breaker Breaker Failure Scheme" circuitry found a shorted cable.
The "Breaker Failure Scheme" circuitry was repaired using spare conductors to restore the 1-5 breaker to service.
3. The automatic transfer of Circulating Water Busses was disabled for Unit 1 and Unit 2. An operability determination was issued to address the condition pending permanent corrective action.
4. The lower operating limit of the Salem Unit 1 and Unit 2 vital busses was raised from 4.220KV to 4.275KV.
5. The extent of condition of the "Breaker Failure Scheme" circuitry failure and inadequate design calculations will be evaluated for Salem Unit 1, Unit 2 and Hope Creek.
COMMITMENTS
The corrective actions cited in this LER do not constitute commitments.
MAC FORM 366A 0 -1,0 I )