ENS 48660
ENS Event | |
|---|---|
05:00 Oct 27, 2012 | |
| Title | Part 21 Interim Report On The Failure Of An Emergency Diesel Generator Excitation System |
| Event Description | The following report was received from MPR Associates via facsimile:
MPR Associates (MPR) is investigating the failure of a replacement emergency diesel generator excitation system that MPR supplied to Cooper Nuclear Station. The root cause investigation is still in-process and will not be completed within 60 days of discovery as defined by 10 CFR Part 21. The 10 CFR Part 21 Interim Report [below] provides the information known at this time. An updated report will be provided once the root cause investigation is completed. IDENTIFICATION OF THE BASIC COMPONENT THAT FAILED The basic component is a Basler Electric SBSR emergency diesel generator (EDG) excitation system that was supplied as a replacement system to Cooper Nuclear Station (CNS). The replacement system included design changes relative to the original CNS excitation system, which is also a Basler Electric SBSR design. The design changes included larger magnetic components, which were intended to allow for continuous operation of the new exciter at the EDG overload rating. IDENTIFICATION OF THE SUPPLIER The excitation system was supplied by MPR Associates (headquarters in Alexandria, VA). Basler Electric (headquarters in Highland, IL) designed and fabricated the system under a commercial grade program certified to ISO 9001:2008. MPR Associates dedicated the commercial grade item for nuclear use under the MPR Nuclear QA Program, which complies with 10 CFR 50 Appendix B and ASME NQA-1. NATURE OF THE FAILURE CNS installed the replacement excitation system in the Division 1 EDG system during refueling outage RE27. Prior to declaring the EDG operable, CNS manually terminated an EDG maintenance run due to erratic EDG reactive power indication, which was followed by a sudden drop of EDG reactive load and an indication of negative reactive power. Several rounds of troubleshooting and surveillance testing were performed unsuccessfully. The surveillance testing resulted in faults to ground and overheating and failure of components in the excitation system. Some of the failed equipment included the automatic voltage regulator (AVR), manual voltage control autotransformer (T60), rectifier power diode failure indication resistors and light-emitting diodes (LEDs), insulation on the control windings of two saturable transformers, and elements of the data acquisition equipment used to record data during the testing. The root cause of the failure has not yet been determined. However, on-site troubleshooting efforts at CNS by MPR and Basler Electric identified unexpected high voltages across the direct current (DC) control winding of the saturable transformers. These voltages are likely the cause of the failures experienced in-situ at CNS. Follow-up tests at Basler Electric on a similar replacement system designed for the Hatch Plant (but not yet installed in the plant) also identified higher than expected voltages across the DC control windings of the saturable transformers. MPR and Basler Electric recommended postponement of the Hatch Plant installation until the impact of this condition (i.e., the higher than expected voltages) is evaluated. Note that the testing on the Hatch replacement system to date did not result in failure of the system or abnormal function of components external to the saturable transformers. NATURE OF THE FAILURE Testing of the replacement excitation system began on October 27, 2012. The maintenance run resulting in erratic EDG reactive power indication was performed on October 30, 2012. CNS, MPR, and Basler Electric discontinued troubleshooting efforts for the replacement excitation system on November 9, 2012. The replacement system was removed, and the original Basler Electric SBSR excitation system was re-installed. CNS declared the re-installed system operable on November 14, 2012. MPR formally documented the issue in the MPR corrective action program on November 13, 2012. NUMBER AND LOCATION OF THE AFFECTED BASIC COMPONENTS Based on the information known to date, this 10 CFR Part 21 Interim Report affects the following SBSR type excitation systems that were dedicated and supplied by MPR. Nuclear Plant Date Equipment Provided Items Supplied Cooper Nuclear 2012 1 Systems (failed during installation) Hatch 2012 5 Systems (not yet installed) CORRECTIVE ACTION PLAN MPR is performing a failure analysis and root cause investigation to determine the extent of the condition, corrective actions, and actions to prevent recurrence. The root cause investigation is scheduled for completion by March 29, 2013. ADVICE GIVEN TO PURCHASERS OR LICENSEES There are numerous Basler Electric SBSR type excitation systems in service at multiple plants throughout the nuclear industry. In addition to CNS and Hatch, MPR has dedicated and supplied SBSR excitation systems to the Beaver Valley, Davis-Besse, and Robinson plants. Each system supplied is custom designed for the generator that it is slated to control. Basler Electric SBSR excitation systems have demonstrated reliable service for many years. The replacement SBSR excitation system supplied to CNS was not identical to the original system. Specifically, there were design differences in some of the components, including larger transformers, which were intended to allow for continuous operation of the new exciter at the EDG overload rating. Although it was not foreseen (and not revealed by factory acceptance testing), it seems that the design changes in the replacement system led to its maloperation and failure when it was initially installed at CNS. This faulty operation and failure were readily observed as part of normal EDG surveillance testing. Upon re-installation, the original SBSR excitation system functioned properly. Hence, it appears that: - Differences between the replacement system and original system lead to the problem, and - The problem is readily detectable in normal surveillance testing. For these reasons, SBSR excitation systems installed at plants that have shown reliable operation during surveillance testing are in a satisfactory state and condition. MPR has no evidence that the mechanism or conditions that led to the failure at CNS will lead to failures at other installations. Therefore, plants with SBSR excitation systems installed should continue to use them and conduct normal surveillance testing.
Reporting Individual: Paul Damerell, Principal Officer, MPR Associates, Inc., 320 King Street Alexandria, VA 22314. As indicated in the interim report, the EDG excitation system supplied to Cooper Nuclear Station failed. The root cause analysis identified a manufacturing defect induced design error in the saturable transformers supplied with the excitation system. The manufacturing error allowed the transformer control winding coils to shift which weakened the dielectric strength of the transformer control winding insulation system such that it could not withstand the voltage induced in the control windings. Testing and analysis estimated induced voltages exceeded 10 kV peak at motor start. MPR Associates also determined that H.B. Robinson has 6 saturable transformers installed with 4 spares and Farley has 3 spares with potential manufacturing defects. They noted that all installed transformers that have passed installation and surveillance testing to date have no immediate concerns but could have long-term reliability issues, if the manufacturing deficiency is present. MPR and Basler are coordinating with Cooper and Hatch to replace saturable transformers supplied by MPR. Cooper has 3 and Hatch has 15 of the devices. Additionally, they will coordinate with Robinson and Farley to test their transformers to determine if the manufacturing defects exist. All affected licensees have been notified by MPR Associates. Notified R1DO (Krohn), R2DO (Seymour), R3DO (Lara), R4DO (Pick) and Part 21 Group (via email). |
| Where | |
|---|---|
| Basler Electric Alexandria, Virginia (NRC Region 1) | |
| Organization: | Mpr Associates, Inc Engineers |
| Reporting | |
| 10 CFR 21.21(a)(2), Interim Report for Comply or Defect in Component | |
| Time - Person (Reporting Time:+1788.15 h74.506 days <br />10.644 weeks <br />2.449 months <br />) | |
| Opened: | Paul Damerell 18:09 Jan 9, 2013 |
| NRC Officer: | Bill Huffman |
| Last Updated: | Mar 29, 2013 |
| 48660 - NRC Website | |
Basler Electric with 10 CFR 21.21(a)(2), Interim Report for Comply or Defect in Component | |
WEEKMONTHYEARENS 486602012-10-27T05:00:00027 October 2012 05:00:00
[Table view]10 CFR 21.21(a)(2), Interim Report for Comply or Defect in Component Part 21 Interim Report on the Failure of an Emergency Diesel Generator Excitation System 2012-10-27T05:00:00 | |