05000352/LER-2005-002

LER-2005-002, Offsite Source Trip Due To Water Intrusion Into Transformer Winding Temperature Switch

Docket Number
Event date:	04-06-2005
Report date:	06-02-2005
Reporting criterion:	10 CFR 50.73(a)(2)(iv)(A), System Actuation
Initial Reporting
ENS 41573	10 CFR 50.72(b)(3)(iv)(A), System Actuation
3522005002R00 - NRC Website

Unit Conditions Prior to the Event Unit 1 was in Operational Condition (OPCON) 1 (Power Operation) at approximately 100% power. Unit 2 was in Operational Condition (OPCON) 1 (Power Operation) at approximately 100% power. There were no structures, systems or components out of service that contributed to this event.

Description of the Event

On Wednesday April 6, 2005, at 16:32 hours the main control room (MCR) received alarms (EIIS:ALM) "4A/4B Bus Tie & 20 Reg Trouble" and "500KV Sub Trouble". An Equipment Operator (EO) was dispatched to the 500 kV substation (EIIS:FK). Concurrently the Power System Director (PSD) called the MCR and notified the operator of a high temperature alarm on 4A/4B transformer (EIIS:XFMR). At 16:54 the 4B Transformer "General Trouble" alarm was acknowledged and reset in the 500 kV substation and the MCR alarms were reset. The EO investigation that was conducted using the annunciator response card (ARC) did not identify any alarm condition present on 4A/4B transformer.

At 16:55 hours, six circuit breakers (115, 315, 525, 625, 205, 15) tripped in the 500 kV and 220 D23) 4kV safeguard busses automatically transferring to the energized 10 Bus off-site source. In automatically started and ran unloaded as designed. The 4kV transfer and EDG starts were as designed.

The investigation identified a flag on the 4B Aux Differential lockout relay (EIIS:87). This relay operation caused all of the breakers connected to 4B transformer to trip. The cause of the lockout relay actuation was a false actuation of the 4B transformer B phase high winding temperature switch (EIIS:TIS) caused by water intrusion. A solid DC ground was identified.

The 4B Transformer has relay protection for "Hot spot temperatures and total loss of coolers".

When any of three phases exceeds 140 degrees Centigrade with no coolers operating the protective lock-out-relay is actuated. This relay trips all of the breakers connected to the transformer. This includes 500 kV breakers 115 and 315, 220 kV breakers 525 and 625, and 13 kV breakers 205 and 15. Breaker 205 is the 13 kV off-site source feed from the tertiary winding in 4A/4B transformer.

4A and 4B transformers are parallel connected 500 kV to 220 kV transformers located in the 500 kV substation on the tie line between the 500 kV substation and the 220 kV substation.

Each transformer has a 13 kV tertiary winding that is used to provide one of two off-site sources required by Technical Specifications.

Operations addressed several equipment challenges during the event (all expected for this transient). The most significant was an entry into procedure OT-116, Loss of Condenser Vacuum, due to the automatic closure of the air valves on the Unit 2 steam jet air ejector (SJAE). This required lowering Unit 2 power to 95% to provide operating margin.

An 8-hour NRC ENS notification was required by 10CFR50.72(b)(3)(iv)(A) for a valid actuation of the emergency AC power system. The ENS notification (#41573) was completed on Wednesday April 6, 2005 at 22:56 EST. Technical Specification 3.8.1.1, A.C. Sources - Operating, 72-hour action was entered on both units while the off-site source was inoperable (for approximately 10.5 hours5.787037e-5 days <br />0.00139 hours <br />8.267196e-6 weeks <br />1.9025e-6 months <br />).

This event involved a valid automatic actuation of the emergency AC electrical power system and the emergency service water system. Therefore, this LER is being submitted pursuant to the requirements of 10CFR50.73(a)(2)(iv)(A).

Analysis of the Event

There were no actual safety consequences associated with this event. The potential safety consequences of this event were minimal. All emergency AC electrical power systems functioned as designed to re-energize the affected 4kV safeguard busses. The D22 EDG was unavailable due to scheduled maintenance during the event but the power supply to D22 4 kV safeguard bus was not interrupted.

The examination of the degraded temperature gauge revealed evidence of water intrusion. In addition the gauge wires showed signs of arcing. The failed gauge was a General Electric, Qualitherm Winding Temperature Gauge, Type AWR.102, GEK-43770.

The 4A and 4B transformer protective relay scheme includes a loss of cooling concurrent with high winding temperature trip function that falsely actuated due to the water intrusion.

Maintenance on the failed switch was last performed in October 2004 when a DC ground was repaired. The switch was inspected, cleaned and repaired. Screw holes were weather proofed and the gasket was turned. Routine inspections following the maintenance did not identify any signs of water intrusion on the gauge faceplate.

Cause of the Event

The false tripping of the 4A and 4B transformers was caused by water intrusion into the 4B transformer B phase winding hot spot temperature gauge.

Corrective Action Completed The degraded 4B transformer B phase winding hot spot temperature gauge and the 4B transformer liquid temperature gauge were replaced and the other switches were inspected but no evidence of water intrusion was identified.

Previous Similar Occurrences There were no previous similar occurrences of false transformer protective relay actuations due to water intrusion.

Component data:

Cause:� B�(Design, Manufacturing, Construction / Installation) System: FK (Switchyard System) Component:�TIS (Switch, Indicating, Temperature) Manufacturer: G080 (General Electric) Type:� AWR.102 Model: GEK-43770 Reportable to EPIX: Yes