Bulletin 79-23: Potential Failure of Emergency Diesel Generator Field Exciter Transformer
Accession No. 7908220104 SSINS No.: 6820 UNITED STATES NUCLEAR REGULATORY COMMISSION OFFICE OF INSPECTION AND ENFORCEMENT WASHINGTON, D.C. 20555 September 12, 1979 IE Bulletin No. 79-23 POTENTIAL FAILURE OF EMERGENCY DIESEL GENERATOR FIELD EXCITER TRANSFORMER Description of Circumstances: Florida Power and Light and Light Company recently reported a problem encountered during a 24-hour full load test of the emergency diesel generators (EDG) at their Turkey Point facility. Approximately 10 hours into the test, the A-EDG tripped due to a differential-relay lockout on B and C phases; the B-EDG was manually stopped, thus interrupting the test at that point in time. Subsequent investigation and testing by the licensee revealed a design error on both the A and B EDGs which resulted in overheating of the Exciter Power Transformers (EPTs) at sustained high load operation. The following nameplate data applies to the equipment installed at Turkey Point: Emergency Diesel Generator General Motors (Electro-Motive Division)* Model EMD-999-20 Engine-turbocharged, 2 cycle, EMD design 20-64SE4 Generator-EMD-design Model A-20 Exciter Power Transformer GE-single phase Model - 9T24Y1004 Serial-MD Cycles-60 KVA 15 Insulation-4160 V The manufacturer's findings and recommendations regarding the above problem are described below: "A potential problem can exist if the neutral of the generator and the neutral of the primary windings of the excitation power transformer (EPT) (sometimes referred to as the control power transformer (CPT)) are connected. A direct connection . IE Bulletin No. 79-23 September 12, 1979 Page 2 of 3 between the neutrals, or a connection through common grounding of both neutrals, are equally undesirable conditions. Whenever either of these undesirable conditions exist, high circulating currents can be induced by harmonics. These currents may exceed transformer ratings and result in transformer damage or failure. The connection between the neutrals, either direct or through common grounding, may have been designated in the original wiring design, or may have been subsequently added by a contractor or by power plant personnel to balance excitation transformer voltages relative to the generator. No significant benefit is obtained by balancing the primary voltage of the excitation transformer by means of these connections. According to the manufacturer, "to avoid this potential transformer damage or failure, the circuitry of each installation should be examined to determine if a circuit exists between the neutral of the primary windings of the EPT or CPT and the generator neutral. If the condition exists, the neutral circuit should be disconnected, and the transformer primary neutral allowed to float." Licensees may utilize Regulatory Guide 1.108, Revision 1, as a reference for the periodic testing of diesel generator units used as on-site electric power systems at nuclear power plants. Action to be Taken by Licensees: For all power reactor facilities with an operating license or a construction permit: 1. Determine whether or not connections have been made between low KVA rated transformers and high KVA rated EDGs without adequate limitations on the flow of circulating currents. If applicable, provide a description of the corrective action being taken to address this problem. 2. Provide a schedule for the completion of a sustained full-load operation test of the EDGs for a duration of not less than 24 hours, or provide the results of the similar long duration, full-load test which has already been completed on the EDGs installed at your facility. The test should demonstrate full-load carrying capability for an interval of not less than 24 hours, of which 22 hours should be at a load equivalent to the continuous rating of the diesel generator and 2 hours at a load equivalent to the 2 hour rating of the diesel generator. The test should also verify that voltage and frequency requirements are maintained and that the cooling system functions within design limits. 3. Provide a written report of the above actions within 45 days of the receipt of this Bulletin. Reports should be submitted to the Director of the appropriate NRC Regional Office. A copy of your report should be sent to the U. S. Nuclear Regulatory Commission, Office of Inspection and Enforcement, Division of Reactor Operations Inspection, Washington, D.C 20555. . IE Bulletin No. 79- 23 September 12, 1979 Page 3 of 3 Approved by GAO, B180225 (ROO72); clearance expires 7/31/80. Approval was given under a blanket clearance specifically for identified generic problems.
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