05000397/LER-2007-003
| Telephone Number (Include Area Code) | |
| Event date: | 04-08-2007 |
|---|---|
| Report date: | 06-07-2007 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(A), Completion of TS Shutdown |
| Initial Reporting | |
| ENS 43291 | 10 CFR 50.72(b)(2)(i), Tech Spec Required Shutdown |
| 3972007003R00 - NRC Website | |
Plant Conditions
At the time of the event, Columbia Generating Station (Columbia) was operating in Mode 1 (Power Operation) at 95 percent power. Regulating transformer E-TR-IN/2 [EF-XFMR] was inoperable and isolated by clearance order at the time of the event. The removal of E-TR-IN/2 initiated the event leading to the shutdown of Columbia.
Event Description
On April 7, 2007 at 1327 PST, the Main Control Room received several "INVERTER IN-2A/2B TROUBLE" alarms [E-INVT]. Operators dispatched to investigate the alarms reported a fire in regulating transformer E-TR-IN/2, which provides backup power to critical instrument power panel E- PP-8AA. Operators promptly extinguished the fire.
On April 8, 2007 at approximately 0930 PST, maintenance was initiated to remove and replace E-TR IN/2. While performing this activity it was not recognized that the neutral wire for the primary power source to E-PP-8AA was routed through E-TR-IN/2 and provided the path for the remote ground reference point. Replacement of E-TR-IN/2 required lifting this neutral wire which resulted in an unanticipated loss of ground reference for E-PP-8AA. The ensuing power fluctuations resulted in E- PP-8AA being declared inoperable. At 1432 PST, Energy Northwest entered Condition A of Technical Specification 3.8.7, "Distribution Systems — Operating.
Prior to exceeding the 8 hour9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br /> completion time of Condition A, management decided to initiate a controlled reactor shutdown to address the issue. Energy Northwest entered Condition C of Technical Specification 3.8.7 at 2232 PST and notified the NRC of the shutdown via Event Notification #43291. Columbia entered Mode 3 (Hot Shutdown) at 0920 PST on April 9, 2007.
Immediate Corrective Action A Technical Issues Resolution Team was promptly initiated to troubleshoot and correct the immediate electrical conditions and restore system operability.
Cause of Event
The root cause of this event was a less than optimal design that does not accommodate maintenance and established a unique error-prone condition.
Contributing to this event was a unique grounded neutral configuration that was not well understood by plant personnel, less than adequate detail in the lead termination and splicing procedure, and unclear documentation concerning the location of the reference ground. Had drawings been clearly labeled, it is likely that the ground path vulnerability would have been recognized and the event averted.
26158 R3
Assessment of Safety Consequences
This event did not pose a threat to the health and safety of the public or plant personnel.
The act of lifting the neutral wire during removal of E-TR-IN/2 eliminated the ground reference point for power panel E-PP-8AA and created a condition where the neutral-to-ground potential was free to vary with circuit conditions. The neutral-to-ground potential was measured to be approximately 68 VAC. This increase in neutral potential in E-PP-8AA resulted in a change in the line voltage supplied from inverter IN/2 with respect to plant ground. Due to the internal impedance characteristics of the inverter, the line voltage with respect to the neutral was maintained at 120 VAC and the Line 1-to-Line 2 voltage was maintained at 240 VAC. The line-to-ground voltages changed over time as load demand changed.
Some of the loads supplied from E-PP-8AA have a neutral connection to other panels that were still at ground potential and would have seen the line-to-ground potential rather than the line-to-neutral voltage from the inverter. This resulted in blown power supply fuses in the Area Radiation Monitors (ARMs), momentary drop-out of a control relay supporting one of two inboard Main Steam Isolation Valve (MSIV) solenoids, changes in the intensity of the MSIV position indication lights, voltage fluctuations on the MSIV solenoids, and a failed surge protection power strip for the Digital Electro- Hydraulic (DEH) printer.
Once the decision was made to shut down the plant, operators were briefed and the shut down performed in a coordinated and controlled manner. The systems necessary to support a safe shutdown were operable and performed their required functions. The plant remained in safe shutdown condition until corrective actions could be performed to restore system operability.
Further Corrective Actions For components with similar vulnerabilities as E-TR-IN/2, Energy Northwest is pursuing a design change to facilitate maintenance and eliminate, as practical, vulnerabilities to loss of ground reference (e.g., move internal neutral connections to an external junction box or provide an alternate ground reference point closer to the load). Should a particular design change not be implemented, Energy Northwest will establish a justification and alternate corrective actions to prevent recurrence. Until such changes can be implemented, Energy Northwest has established an additional level of review by engineering for electrical work packages to ensure that the impact on the plant is completely understood and accounted for.
Energy Northwest will pursue other corrective actions to enhance the quality of associated drawings, procedures, and training of personnel to recognize similar conditions.
26158 R3 E-TR-IN/2 was replaced offline in 2001. During this replacement, E-PP-8AA was supplied from a temporary power source, and therefore, the neutral ground path through the transformer was not required.
There have not been any documented instances at Columbia involving the actual inadvertent removal of a neutral ground path that has resulted in an event. Nonetheless, prior events have indicated weaknesses in understanding the significance of neutral and ground references. Corrective actions associated with these prior events have been focused on correcting the condition and did not fully address the apparent/root causes or other generic implications.
EllS Information (Denoted as [XX]) Text Reference System Component E-PP-8AA EF E-TR-IN/2 EF XFMR E-IN-2A and 2B EF INVT 26158 R3