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SSINS No.: 6835 IN 83-37 UNITED STATES NUCLEAR REGULATORY COMMISSION OFFICE OF INSPECTION AND ENFORCEMENT WASHINGTON, D.C. 20555 June 13, 1983 Information Notice No. 83-37: TRANSFORMER FAILURE RESULTING FROM DEGRADED INTERNAL CONNECTION CABLES Addressees: All nuclear power reactor facilities holding an operating license (OL) or construction permit (CP). Purpose: This Information Notice is provided as a notification of an event that may have safety significance. It is expected that recipients will review the information for applicability to their facilities. No specific action or response is required. Description of Circumstances: On April 26, 1983, at approximately 2:30 p.m., Carolina Power and Light reported that Brunswick Unit 1 lost all offsite power. The unit was in cold shutdown for refueling at the time of the occurrence. Unit 2 was in startup from a maintenance outage and was unaffected by the loss of power to Unit 1. The loss of power resulted from the inadvertent tripping of one of the two feeds from the station auxiliary transformers (SATs) during performance testing. The other feed from the SAT was out of service for maintenance at the time. In accordance with the licensees emergency plan, an Unusual Event was declared. After loss of offsite power occurred, the appropriate automatic switching took place and the emergency diesel generators (DGs) started and loaded as designed. The 4160/480 volt transformer feeding Unit 2 reactor building emergency bus E-6 overheated, causing the feeder breaker to trip on overcurrent. Licensee investigation revealed that the E-6 transformer was damaged beyond repair by apparent arcing at its transformer winding taps. A vendor and licensee postincident review was initiated. The scenario postulated was the loss of offsite power de-energized the E-6 bus transformer, the DGs started, and a subsequent in-rush of current on the C phase of the transformer initiated major arcing in the transformer winding tap lug (4160 volt primary), causing the transformer failure. 8306090010 . IN 83-37 June 13, 1983 Page 2 of 2 The failure was attributed to improper assembly of transformer winding tap cables and long-term, undiagnosed, heat-induced degradation. It is believed that the set screw, which attaches the cable to the barrel of the lug, was overtightened during installation, which caused some of the aluminum stranded wire to break, thereby creating a high resistance joint. Arcing is thought to have started in the barrel of the lug as a result of the resistance joint. Long-term localized heating in the terminal lug over a period of time weakened and degraded the connection. It should be noted that a failure was imminent and was not created by the loss of power. Detailed transformer inspections necessary to detect these failure mechanisms prior to gross failure were not addressed in the licensee preventive maintenance procedures. Subsequent detailed inspections of the remaining 4160/480 volt safety-related transformers by visual inspections of disassembled mechanical connections, identified that 8 of the 48 connections had minor degradation at the cable-to-lug connection point. Signs of discoloration from heat and arcing were evident, and, were postulated to be early stages of degradation that resulted in the above transformer failure. Heat gun (temperature-measuring device) techniques employed in mechanical joint evaluation, identified some but not all of the terminal degradations. As corrective action, defective safety-related cables were replaced, connectors were removed, cleaned, and then reinstalled. The licensee is evaluating replacing the aluminum stranded cables and set screw lugs with copper cables and compression lugs. The transformer was manufactured by ITE in 1972 and is a dry type, 4160 volt primary (Delta) to 480/277 volt secondary (Wye). It is constructed of aluminum, including coils, bussing, and internal cables. The internal aluminum stranded cables are connected to internal bussing and transformer taps. If you have any questions regarding this matter, please contact the Regional Administrator of the appropriate NRC Regional Office, of this office. Edward L. Jordan Director Division of Emergency Preparedness and Engineering Response Office of Inspection and Enforcement Technical Contact: W. Laudan, IE (301) 492-9759 K. E. Davenport, RII (404) 221-4197 Attachment: List of Recently Issued IE Information Notices .
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