05000395/LER-2003-002
| Virgil C. Summer Nuclear Station | |
| Event date: | 05-12-2003 |
|---|---|
| Report date: | 07-02-2003 |
| Initial Reporting | |
| 3952003002R00 - NRC Website | |
PLANT IDENTIFICATION
Westinghouse - Pressurized Water Reactor
EQUIPMENT IDENTIFICATION
Main Generator Field Breaker
IDENTIFICATION OF EVENT
A reactor trip on Over Temperature Delta Temperature (OTDT) occurred from 100 percent power on May 12, 2003, at 0102 hours0.00118 days <br />0.0283 hours <br />1.686508e-4 weeks <br />3.8811e-5 months <br /> due to the main generator breaker opening.
EVENT DATE
05/12/03
REPORT DATE
07/02/03
CONDITIONS PRIOR TO EVENT
Mode 1, 100% Power
DESCRIPTION OF EVENT
On May 12, 2003, with the plant operating at 100% power, alarms for Regulator 1 and 3 were received for the Main Generator Voltage Regulator, followed shortly by a Regulator 2 alarm. The main generator breaker opened causing a turbine trip, which in turn, caused the reactor to trip. The first-out annunciator indicated that a reactor trip on Over Temperature Delta Temperature (OTDT) had occurred, at 0102 hours0.00118 days <br />0.0283 hours <br />1.686508e-4 weeks <br />3.8811e-5 months <br /> due to the main generator breaker opening at 100 percent power.
V. C. Summer Nuclear Station DESCRIPTION OF EVENT (Cont'd) Following the plant trip, all major systems functioned as expected. The plant was stabilized in Mode 3.
Condition Event Report C-03-1614 was generated to address the event to perform a root cause evaluation to understand why the event happened and to develop effective corrective actions.
CAUSE OF EVENT
The cause of this event is attributed to vibration induced wear on the main generator field breaker movable and stationary secondary disconnect contacts, and degradation of the field breaker cell switch mechanism that resulted in intermittent high resistance connections and a false field breaker open signal to the digital voltage regulator.
ANALYSIS OF EVENT
On May 12, 2003, the reactor tripped on Over Temperature Delta Temperature (OTDT) due to the main generator breaker opening at 100% power.
The trip was caused by a loss of field breaker position signal, which can be attributed to a continuity problem in the series circuit; including the breaker auxiliary switch (41MAS) contacts, the breaker cell switch (41MPS) contacts, and the breaker secondary disconnect contacts. The main generator field breaker is mounted on the breaker enclosure in the sidewall of the Alterrex Exciter enclosure, which is adjacent to the main generator. This enclosure is mounted on a common foundation pedestal with the main turbine and main generator, and as such, is subject to continuous vibration from the unit operation. Inspection of the cell switch and secondary disconnect contacts showed wear and fretting as a result of being subjected to this continuous vibration. As these contacts degraded, an intermittent high resistance condition developed (125VDC circuit). This intermittent high resistance condition was sensed by the EX2000 voltage regulator as a loss of excitation due to an open generator field breaker, when the breaker was in fact, closed. As a result, the EX2000 transferred to its backup voltage regulator.
Due to the intermittent nature of the degraded connection, the backup regulator now sensed that the main generator field breaker was still closed. However, about 65 seconds later, the degraded connection again falsely indicated that the main generator field breaker was open and the protection system opened the main generator breaker on potential loss of excitation. This was a programmed anticipatory trip function. The main generator never actually lost excitation until the trip and was fully protected.
CORRECTIVE ACTIONS
The Station is taking the following corrective actions:
- New movable and stationary secondary disconnect contacts were installed in the main generator field breaker. The auxiliary switch contacts were inspected and cleaned.
- A modification was implemented to add a redundant set of field breaker auxiliary contacts in parallel with the original contacts to reduce the vulnerability to this failure mode. The cell switch contacts for the anticipatory trip circuit were bypassed to preclude a false trip signal.
- The main generator field breaker maintenance procedure is being revised to require replacement of the movable and stationary contacts during each refueling outage (previously the contacts were inspected and replaced on an as needed basis).
PRIOR OCCURRENCES
None 05000395 YEAR