United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 83-37: Transformer Failure Resulting from Degraded Internal Connection Cables

                                                            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 
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                                                           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

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Page Last Reviewed/Updated Tuesday, November 12, 2013