ML18029A761
| ML18029A761 | |
| Person / Time | |
|---|---|
| Site: | Browns Ferry  |
| Issue date: | 07/31/1985 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML18029A760 | List: |
| References | |
| NUDOCS 8508080144 | |
| Download: ML18029A761 (3) | |
Text
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UNITEDSTATES NUCLEAR REGULATORY COMMISSION WASHINGTON, D. C. 20555 SAFFTY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION ADE UACY OF STATION ELECTRIC DISTRIBUTION SYSTEM VOLTAGES POTENTIAL OVER-VOLTAGE CONDITION - VERIFICATION TESTING TENNESSEE VALLEY AUTHORITY BROWNS FERRY NUCLEAR PLANT UNITS 1
2 AND 3
,DOCKET NOS. 50-259, 50-260 AND 50-296 References TVA Letter, L. M. Mills to H. R. Denton, April 9, 1981 TVA Memorandum, G. T. Jones to T.
G. Campbell, December 7, 1981 TVA LER, H. J.
Green to J.
P. O'Reilly, July 30, 1982 TVA Letter, D. S.
KamIer to H.
R. Denton, July 22, 1982 TVA Letter, L. M. Mills to H.
R. Denton, September 22, 1983 NRC Letter, D. B. Vassallo to H.
G. Parris, January 6,
1984 TVA Letter, L. M. Mills to H. R. Denton, March 19, 1984 TVA Letter, L. M. Mills to H.
R. Denton, September 18, 1984 In References 4 and 5, TVA provided the verification test results for NRC review.
The tests were performed in order to confirm the accuracy of the voltage analysis.
Although the review of the test results showed a close correlation between the voltage analysis and the tests, both results indicate when the safety buses are supplied via 161 kV switchyard (preferred offsite source),
an over-voltage condition exist at 480 V
safety buses.
Tennessee Valley Authority (TVA) stated in Reference 1 that, in order for an over-voltage condition to exist, the safety buses must be lightly loaded and 161 kV switchyard to be at its expected maximum voltage of 170 kV.
However, the above-mentioned test results showed that when 161 kV switchyard is at 166 kV, less than the expected maximum, the 4 kV Shutdown Board C and 480 V Shutdown Board 1B experienced voltages of 105% (4000 volt motor rating) and ill% (460 volt motor rating) respectively.
Therefore, under the maximum expected grid voltage of 170 kV, voltage at 480 V buses could reach as high as 113.5% of 460 V motors.
The 4
kV buses at Browns Ferry are equipped with over-voltage alarms which are set to actuate at 4.4 kV or 110% of 4000 volts.
However, with 4.4 kV voltage at 4 kV buses, the voltage at 480 V safety buses could reach as high as 116$ of 460 V, due to the present tap setting of the 4 kV to 480 V transformers.
In Reference 6,
we requested TVA to review the potential over-voltage condition at 480 V safety buses and implement necessary changes or procedures to alleviate the over-voltage condition.
By letter dated September 18, 1984 (Reference
- 8) TVA stated that a voltage increase of 20Ã
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beyond the motor nominal voltage rating will increase the motor torque by 44%, decrease the full load current by 11% and, therefore, decrease the full load temperature rise by 5 to 6,C.
TVA states in the above letter that since the temperature rise decreases due to an increase in voltage beyond the motor voltage rating, there would be no motor insulation damage due to temperature.
In conclusion, TVA stated, however, that various methods will be evaluated to assure proper voltage is supplied to safety equipment when these equipment are supplied via 161 kV system.
We concur with the licensee that, in general, the temperature rise on induction motors wil'l decrease due to an increase in voltage.
In a related Licensee Event Report (LER) issued on July 30, 1982 (Reference 3),
TVA had stated that the apparent cause of the overload trip associated with the Hydrogen Analyzer B pump motor was due to a high voltage condition at the 480 V supply bus.
We discussed this issue with the licensee in a recent telephone conference.
TVA stated that its investigation of the above incident revealed-that the cause of overload trip was due to a motor defect and not an over-voltage condition.
TVA further stated that since the repair of the subject pump motor by the manufacturer, no similar problem has been experienced.
Since our review of the LERs issued by TVA does not reveal any other occurrences attributed to over-voltage condition, and based on TVA's assessment of this item, we consider our concern to be satisfactorily resolved.
Principal Contributor:
J.
Emami Dated:
July 31, 1985