Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
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
Attachment: List of Recently Issued IE Information Notices
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