ML13330B486
| ML13330B486 | |
| Person / Time | |
|---|---|
| Site: | San Onofre  |
| Issue date: | 03/15/1989 |
| From: | Nandy F Southern California Edison Co |
| To: | NRC/IRM |
| References | |
| NUDOCS 8903200008 | |
| Download: ML13330B486 (6) | |
Text
Southern California Edison Company P. 0. BOX 800 2244 WALNUT GROVE AVENUE.
ROSEMEAD, CALIFORNIA 91770 F. R. NANDY TELEPHONE MANAGER OF NUCLEAR LICENSING March 15, 1989 (818) 302-1896 U. S. Nuclear Regulatory Commission Attention: Document Control Desk Washington, D.C. 20555 Gentlemen:
Subject:
Docket No. 50-206 San Onofre Nuclear Generating Station Unit 1 Potential Overload of 480V Switchgear Breakers By letter dated February 21, 1989, SCE transmitted to the NRC a report discussing the potential overload of San Onofre Unit 1 480V switchgear breakers. The overload condition could occur following a safety injection actuation signal with offsite power available and assuming the worst postulated combination of non-essential and safety loads running or starting.
SCE committed to evaluate the overload condition and then to select and implement a viable solution prior to Unit I restart from the current refueling outage.
The purpose of this letter is to describe the results of our analysis of the potential overload condition and the actions we plan to complete to address this problem prior to Unit 1 restart.
In order to determine whether the breakers had sufficient design margin above the 1600 amp continuous rating to handle the worst postulated post-SIS load profile, (1896 amps for Switchgear No. 1 and 1640 amps for Switchgear No. 2),
a test was performed on an equivalent 0B-50 breaker. The test guidelines were based on ANSI C37.50, "Test Procedures for Low-Voltage AC Power Circuit Breakers Used in Enclosures."
The test results were used to predict temperatures of critical breaker components (contacts, insulation, and bus connections) for the worst postulated current profile based on graphical and analytical techniques. The predicted temperatures were plotted against time (see enclosures) and then compared to ANSI maximum recommended values. It was found that, under worst case conditions, some of those recommended maximum continuous temperatures for normal operating conditions would be exceeded but that the additional temperature rise allowed by ANSI for four-hour emergency overload conditions would not be exceeded. For the purpose of this evaluation, an ambient temperature of 40 degrees C (conservative) and a switchyard voltage of 230KV (most probable) were assumed. The test breaker was inspected following the test and neither the insulation nor contacts were degraded.
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Document Control Desk
-2 The postulated overload condition would result in peak load currents above the continuous ratings for the breakers and cables, eventually causing overheating. The overheating would be gradual over the course of several hours. However, even with no operator action to reduce loading, temperature levels would be in the range that would cause accelerated aging of the components rather than failure. The test breaker was subjected to sustained (3 hours3.472222e-5 days <br />8.333333e-4 hours <br />4.960317e-6 weeks <br />1.1415e-6 months <br />) temperatures greater than 15 degrees C above rated and peak temperatures as high as 90 degrees C above rated with no indication of impending failure. Based on these conclusions, failure of the breakers due to the postulated overload condition is not credible and the condition was determined not to be reportable under 10 CFR 50.72 or 50.73.
In addition to the main breaker issue, an analysis was performed to determine whether the remainder of the auxiliary electric system was capable of handling the worst postulated current profile. This analysis revealed that the main power cables feeding 480V Switchgear #1 are routed through a 34" fire barrier and also randomly laid in a section of covered cable tray. This condition would require that the cables be derated due to the thermal effects of the fire barrier and the random lay of the cables. The resulting ampacity of the cables is below the postulated worst case load current. However, the cables are environmentally qualified to survive high temperature LOCA environments in excess of the temperatures that would occur due to the postulated overload condition. Therefore, they would continue to perform their safety function.
Based on the results of the breaker test and analysis, no modifications are required for continued plant operation during Cycle 10. While no safety concerns exist associated with operation of the 480V breakers or cables, the following actions will be completed prior to returning Unit 1 to service from the Cycle 10 outage in order to further assure safe plant operation:
- 1. Perform preventative maintenance on installed 480V Switchgear #1 and
- 2 main breakers in order to ensure that they are in the same condition as the tested breakers.
- 2. Revise operating procedures to require operator surveillance of 480V switchgear loading within four hours after a safety injection actuation and make necessary adjustment of 480V switchgear loading to maintain acceptable continuous levels.
- 3. Rework the fire barrier to the minimum required depth of 10 inches to further assure that adequate current carrying capability exists on the main power cables for Switchgear #1.
- 4. Provide maintained spacing of Switchgear #1 power cables in the covered cable tray or modify cable tray covers to improve cable ampacity.
Doc.ument Control Desk
-3 In addition, the following operations will be investigated for possible long term corrective action for the breaker overload condition.
- 1. Rerate breakers to postulated maximum current value.
- 2. Replace breakers with breakers which are rated to continuously carry the maximum current.
- 3. Implement design modifications and/or operator and control actions to ensure that the present current rating of the existing breakers will not be exceeded.
A viable solution will be selected and implemented prior to restart from the Cycle 11 refueling outage.
If you have any questions on the 480V breaker overload or cable overheating issues, please let me know.
Very truly yours, cc: 3. B. Martin, Regional Administrator, NRC Region V F. R. Huey, NRC Senior Resident Inspector, San Onofre Units 1, 2 and 3
DB-50. MAIN CONTACT-THERMAL RE'SPONSEF tldximuW Rise @ Continuous Current 80 w
70 Ll 1900 Amps a_
Fi 40 30 202 ELAPSED TIME (HOURS)
DB-.50 INSULATION THERMAL RESPONSE 80 aximwo Rise @4 hr Overlod tkMximum Rise @8 hr Overload Id 70 MdiiiUM Rise @ Continuous Current LA 60 4:
5090 Lii 1400 Amps 040 30 1
2 0 10 0
12 3
D ELAPSEL)
TIME (HOURS)
DE3-50 BUS CONNECTION, THERMAL IRESPONSE 190 80 Maximum Rise @ 4 fir Overlod U
70-________
(A Maximfum Rise 0 hr Ovuerload I.-
I~Maximium Rise (d Conitnuous Current 50 040 Lii 4-01400 Amps 30 200 10 0...... -...
rw
-r---r-0 0
12 3
ELAPSED TIME (HOURS)