United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 92-63: Cracked Insulators in ASL Dry Type Transformers Manufactured by Westinghouse Electric Corporation

                                 UNITED STATES
                         NUCLEAR REGULATORY COMMISSION
                     OFFICE OF NUCLEAR REACTOR REGULATION
                            WASHINGTON, D.C.  20555

                                August 26, 1992


NRC INFORMATION NOTICE 92-63:  CRACKED INSULATORS IN ASL DRY TYPE  
                               TRANSFORMERS MANUFACTURED BY WESTINGHOUSE 
                               ELECTRIC CORPORATION


Addressees

All holders of operating licenses or construction permits for nuclear power
reactors.

Purpose

The U.S. Nuclear Regulatory Commission (NRC) is issuing this information
notice to alert addressees to problems that could result from the cracking of
ceramic insulators on ASL Dry Type Power Center 4160/480  volt (V) 3-phase
transformers manufactured by the Westinghouse Electric Corporation
(Westinghouse).  It is expected that recipients will review the information
for applicability to their facilities and consider actions, as appropriate, to
avoid similar problems.  However, suggestions contained in this information
notice are not NRC requirements; therefore, no specific action or written
response is required. 

Description of Circumstances

On May 13, 1991, the Washington Public Power Supply System (WPPSS), the
construction permit holder for the Washington Nuclear Project, Unit 3, (WNP-3)
reported to the NRC that cracks were observed on ceramic insulators that
support the high voltage coils in certain Class 1E and Non-Class 1E 4160/480 V
transformers rated at 1000 and 1500 kVA.  

The Pacific Gas and Electric Company (PG&E), the licensee for the Diablo
Canyon Nuclear Power Plant, Units 1 and 2, found a hairline crack on an
insulator in one of the 4160/480V, 1000 KVA rated transformers in Unit 2.  In
a letter of August 30, 1991, Westinghouse, the supplier of the transformer,
recommended that PG&E replace the cracked insulator as soon as possible.  On
September 19, 1991, PG&E evaluated the operability  to justify continued
operation with the hairline crack on one of its insulators.

Discussion

The design of these transformers typically includes a magnetic iron core,
three high voltage coil assemblies and a steel frame.  The high voltage coil
assemblies are held in position and are electrically separated from the ground
plane of the transformer by a set of insulators uniformly spaced around the
top and bottom periphery of each coil assembly.  In turn, the insulators are


9208200177.

                                                            IN 92-63
                                                            August 26, 1992
                                                            Page 2 of 2


held in place by pressure rings.  A compressive force is applied to the top of
each pressure ring by four jack bolts.  The assembly is designed to maintain
the spatial relationship of the coils, insulators, and the pressure ring
during normal operation, a seismic event, or an electrical fault condition.   

The 1000 kVA transformer has six insulators at the top and bottom of each high
voltage coil assembly while the 1500 kVA transformer has eight.  The
insulators are 4 3/8 inches long and 1 inch in diameter.  The center portion
of each insulator has a series of annular indentations that form five rings or
skirts.  Typically, the cracks were found in these indentations.  

At WNP-3, both the WPPSS and NRC staff observed that some of the cracked
insulators in the affected transformers were installed off-center and were not
perpendicular to the pressure ring.   The cracks were on insulators located at
the top or bottom of the high voltage coils and at least one hairline crack
was observed along the smallest cross section in the upper half of each
affected insulator.  

On November 14, 1991, Westinghouse informed the NRC (Attachment 1) that it had
sent a customer notification letter to all utilities that may have received
Westinghouse safety-related ASL Dry Type transformers.  Westinghouse stated
that it had received information from the ABB Power Transmission and
Distribution Company, Incorporated, that the cracking of the insulators could
have a catastrophic effect on the structural integrity of the transformer.  

This information notice requires no specific action or written response.  If
you have any questions about the information in this notice, please contact
the technical contact listed below or the appropriate Office of Nuclear
Reactor Regulation (NRR) project manager.


                                  ORIGINAL SIGNED BY


                              Charles E. Rossi, Director 
                              Division of Operational Events Assessment 
                              Office of Nuclear Reactor Regulation 


Technical contact:  K. R. Naidu, NRR
                    (301) 504-2980

Attachments:
1.  Westinghouse letter ET-NRC-91-3638
2.  List of Recently Issued NRC Information Notices
.
Page Last Reviewed/Updated Tuesday, November 12, 2013