ML19256F166
| ML19256F166 | |
| Person / Time | |
|---|---|
| Site: | Haddam Neck, Millstone File:Connecticut Yankee Atomic Power Co icon.png |
| Issue date: | 11/01/1979 |
| From: | Counsil W NORTHEAST UTILITIES |
| To: | Grier B NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION I) |
| References | |
| IEB-79-23, NUDOCS 7911210565 | |
| Download: ML19256F166 (9) | |
Text
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November 1, 1979 Docket Nos. 50-213 50-245 50-336 Mr. Boyce H. Grier, Director Regiin I Office of Inspection and Enforcement U. S. Nuclear Regulatory Commission 631 Park Avenue King of Prussia, PA 19406
Reference:
(1)
B. H. Grier letter to W. G. Counsil dated September 12, 1979, transmitting I&E Bulletin No. 79-23.
Gentlemen:
Haddam Neck Plant Millstone Nuclear Power Station, Unit Nos. 1 and 2 I&E Bulletin No. 79-23, Potential Failure of Emergency Diesel Generator Exciter Transformer Reference (1) requested both Connecticut Yankee Atomic Power Company (CYAPCO) and Northeast Nuclear Energy Company (NNECO) to:
(1) Determine whether electrical connections between excitation power transformers and emergency diesel generators (EDG) exist without adequate limitations to the flow of circulating currents.
(2) Provide a schedule for completion or the results of a long duraticn, sustained full-load operation test of the EDG.
CYAPC0 and NNECO have taken exception to Item (2) of the Bulletin which requests that a twenty fcur (24) hour test be performed to demonstrate full-load carrying capability of the EDG.
^
Testing the EDG in the manner requested will not fully uncover any potential failure problems with the EDG field exciter tranr, formers, since the I&E Bulletin only addresses the requirement to maintah full output for a 24-hour period, without determining that components are operating within their design rating. CYAPC0 and NNECO have concluded that a more valid and technically defensible approach to uncover any problems with the transformers would be to determine the loading on them by measuring the transformer currents and voltages with the EDG fully loaded, subsequently comparing these readings with the transformer's nameplate rating. This mode of testing will verify whether or not the transformer in operating within its nameplate rating and also indicate any potential for 13/8
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future damage or failure of the transformer. A single twenty-four hour test for operability, as requested in the Bulletin, will not indicate probability of future failure.
Both CYAPC0 and NNECO have utilized this approach in the response to Item (2) of the Bulletin.
CYAPCO and NNECO hereby submit Attachment 1 and Attachment 2, respectively, in response to the Reference (1) Bulletin.
We trust this information satisfactorily dispositions the Reference (1) concerns and is responsive to your requests.
Very truly yours, CONNECTICUT YANKEE ATOMIC POWER COMPANY NORTHEAST NUCLEAR ENERGY COMPANY 1
,(1
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W. G. Counsil Vice President Attachments i5/8 139
DOCKET No. 50-213 ATTACINENT 1 HADDAM NECK PLAhT RESPONSE TO I&E BULLETIN No. 79-23 15/8 140 NOVEMBER, 1979
HADDAM NECK PLANT Question (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.
Response
YSe Haddam Neck Plant utilizes emergency diesel generator (EDr > excitation transformers consisting of three single phase transformers, each rated at 2400/240 V,15 KVA.
The transformers were connected in Wye :o the generator on their high side and in Delta on their low side with the high side neutral of the transformer connected to the generator grounded neutral.
In thi con-figuration, and the generator on full output, it was determined that the excitation transformers were loaded to 167% of their rating.
This configuration was altered, in 1974, by removing the grounding connection leaving the high side of the transformer in an ungrounded wye.
This type of configuration precluded zero sequence, or circulating currents such as those described in the subject bulletin. The removal of this ground connection caused their loading to decrease to 48% of their rating for the same generator output condition.
Currents, measured in the high side wye, low side delta and low side outputs of the excitation transformers conclusively proved the existence of a circulating (zero sequence) current as being the cause of the overload.
Technical considerations of the circuit connections indicated that it was not necessary to maintain the grounding connection on the wye winding, and with the engine manufacturer's concurrence it was subsequently removed.
Question (2)
Provide a schedule for the completion of a sustained fuil-load operation test of the EDGs for a duration of not less than 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />, 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 ca-rying capability for an interval of not less than 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />, of which 22 hours2.546296e-4 days <br />0.00611 hours <br />3.637566e-5 weeks <br />8.371e-6 months <br /> should be at a load equivalent to the continuous rating of the diesel generator and 2 hours2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br /> at a load equivalent to the 2 hour2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br /> rating of the diesel generator.
requirements are maintained and thatThe test should also verify that voltage and frequency limits.
the cooling system functions within design 13/8
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. Response CYAPCO performed a test on the EDG excitation transformers, utilizing the procedure described in the cover letter, subsequent to the design changes reported in the response to Item (1). Upon loading the EDG's to full-load unity power factor, it was determined that the transformers were operating at 7.2 KVA or 48%
within the nameplate rating. The transformers were recen:ly tested with the EDG's loaded to full load at approximately 0.7 power factor. At this condition, it was determined that the transformers were operating at a maximum 13.8 KVA or 92%
within their nameplate rating.
CYAPCO has de: ermined that, based on the results of tests to date, the potential for damage or failure of the EDG field exciter transformers does not exist.
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DOCKET NOS. 50-245 50-336 ATTACHMENT 2 MILLSTONE NUCLEAR POWER ST. TION, UNIT NOS.1 AND 2 RESPONSE TO I&E BULLETIN No. 79-23 1378 143 NOVEMBER, 1979
MILLSTONE UNIT NO.1 Question (1)
Determine whether or not connectiens have been made between low KVA rated transformers and high KVA rated EDGs without adequate limitacions on the flow of circulating currents.
If applicable, provide a description of the corrective action being taken to address this problem.
Response
NNECO has reviewed the connections made between both the emergency diesel generator (EDG) and emergency gas turbine generator (EGTG) and determined that the type of failure described in the Reference (1) Bulletin cannot occur in the Millstone Unit No. 1 EDG or EGTG.
Specifically, on both the EDG and EGTG, the excitation power transformers are current transformers and are connected in delta on the loa side without grounding arrangements.
This type of configuration precludes zero sequence, or circulating currents such as those described in the subject bulletin.
We have identified, however, three single phase saturable current transformers (SCT), working within the EDG exciter circuit, icith each design rated 11.6 KVA.
They are connected in Wye on their high side and in Delta on their low side, with the high side neutral of the transformer connected to the generator grounded neutral.
NNECO has performed tests on these transformers with the EDG at full load with a.8 power factcr.
During these tests, the transformers were operating at 8.3 KVA or 72% within their design rating.
Based on the results from our tests and the concurrence from the exciter manufacturer, NNECO has determined that it will be safe to operate the transformers with the high side grounded wye connected.
We have determined that any potential for damage or failure of the SCT transformers does not exist at this time.
Question (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 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />, 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 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />, of which 22 hours2.546296e-4 days <br />0.00611 hours <br />3.637566e-5 weeks <br />8.371e-6 months <br /> should be at a load equivalent to the continuous rating of the diesel 13/8 J)44
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g y generator and 2 hours2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br /> at a load equivalent to the 2 hour2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br /> 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.
Response
NNECO has performed tests on the EDG and EGTG utilizing the procedure outlined in the cover letter subsequent to the issuance of this Bulletin.
The EGTG excitation power transformers, consisting of three single phase trans-formers and each rated at 18.8 KVA, were tested with tha EGTG loaded to 9500 KW at 0.88 P.F.
At this loading (which exceeds the expected accident loading), we found that the transformers were operating at a maximum 10.2 KVA or 54% within their nameplate rating.
Based on the results from our tests, we conclude that any potential for damage or failure of the EGTG field exciter transformers does not exist at this time.
The EDG excitation power transformers, consisting of three single phase trans-formers and each rated at 5.1 KVA, were tested with the EDG loaded to' full load at approximately 0.8 P.F.
At this loading, we found that the transformers were operating at a mrximum 4.45 KVA or 87% within their design rating.
Based on the results from ou/ tests, we conclude that any potential for damage or failure of the EDG field exciter transformers does not exist at this time.
13/8 145
MILLSTONE UNIT NO. 2 Question (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.
Response
NNECO has reviewed the connections between the excitation transformer and the emergency diesel generator (EDG) and determined that the type of f ailure or potential failure described in the Reference (1) Bulletin cannot occur on the Millstone Unit No. 2 EDG excitation transformer.
Specifically, the excita-tion power transformers are current transformers and are connected in delta on the low side without grounding arrangements. This type of configuration precludes zero sequence, or circulating currents such as those described in the subject bulletin.
Question (2)
Provide a schedule for the completion of a sustained full-load operation test of the FDGs for a duration of not less than 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />, 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 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />, of which 22 hours2.546296e-4 days <br />0.00611 hours <br />3.637566e-5 weeks <br />8.371e-6 months <br /> should be at a load equivalent to the continuous rating of the diesel generator and 2 hours2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br /> at a load equivalent to the 2 hour2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br /> rating of the diesel generator.
The test should also verify that voltage and frequency requirement are maintained and that the cooling system functions within design limits.
Response
A long duration full-load test has been performed on both emergency diesel genera-tors at Millstone Unit No. 2.
Both diesels were tested for twenty-four hours at full-rated load and two hours at the two-hour rating with satisfactory results.
NNECO han performed tests on the transformers utilizing procedures described in the cover letter.
Presently, we are awaiting information from the exciter trans-former manufacturer confire'ag the design rating of the transformers, and upon receipt of this information, will supplement our response to Item (2) of the Reference (1) Bulletin.
13/8 046