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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 .
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