ML13331A311
| ML13331A311 | |
| Person / Time | |
|---|---|
| Site: | San Onofre  |
| Issue date: | 02/21/1989 |
| From: | Nandy F SOUTHERN CALIFORNIA EDISON CO. |
| To: | NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM) |
| References | |
| NUDOCS 8903030224 | |
| Download: ML13331A311 (3) | |
Text
Southern California Edison Comp, P 0 BOx 800 2244 WALNUT GROVE AVENUE RO;E1EAO CAL0ORNiA 9 -77 February 21, 1989 U. S. Nuclear Regulatory Commission Attention:
Document Control Desk Washington, D.C. 20555 Gentlemen:
Subject:
Docket No. 50-206 Potential Overload of 480V Switchgear Breakers San Onofre Nuclear Generating Station Unit 1 While performing electrical calculations in support of San Onofre Unit I cycle 10 plant modifications, it was determined that a previously unknown. potential for overloading the main breakers for 480V switchgear Nos. I and 2 sexistediin the event of a safety injection with Offsite Power available. Ti odto was immediately reported to the NRC per regulatory requirements and work was initiated on generating an LER describing the Problem. In addition, during discussions with NRC personnel, SCE committed to supplying the NRC with a report prior to issuance of the LER.
The purpose of this letter is to transmit the attached report discussing the potential overload condition of the San Onofre.Unit 1 480V switchgear breakers.
As discussed in the report, several options are currently being evaluated to correct the potential overload problem. A viable solution will be selected and implemented prior to Unit 1 restart.
If you have any questions on the 480V breaker overload issue, please let me know.
Very truly yours, cc:
F. B. Martin, Regional Administrator, NRC Region V F. R. Huey, NRC Senior Resident Inspector, San Onofre Units 1, 2 and 3 J. H. Hickman, California Department of Health Services PAR A1ock C0i 0001
=*
POTENTIAL OVERLOAD OF 480V SWITCHGEAR MAIN BREAKERS BACKGROUND INFORMATION The 4KV and 480V AC electrical distribution systems distribute electrical power from offsite sources or the emergency diesel generators to the plant operating and safeguards equipment. The plant electrical system is provided with two distribution trains, A and B, which provide redundancy and independence of the trains.
During normal operation, the 4KV buses are powered from the main generator (Bus 1A and 1B) and offsite sources (Bus 1C and 2C).
The 480V Buses 1 and 2 are powered from their respective Station Service Transformers which are supplied from Buses IC and 2C respectively. The 480V Switchgear and the Motor Control Centers power both non-essential and safeguards loads for the plant. 480V Switchgear 3 provides power to the third of a kind loads and other plant auxiliary loads.
In the event of an occurrence requiring automatic operation of safeguards equipment, two Safeguards Load Sequencing Systems (SLSS) are provided to (1) ensure power is available, (2) isolate unnecessary and unqualified loads, and (3) energize required safeguards loads.
The SLSSs initiate the safeguards functions which are required to mitigate the consequences of the event. There are two independent SLSSs, designated SLSS-l for Train A and SLSS-2 for Train B.
In the event that injection of borated water to the reactor coolant system is required, the SLSSs initiate a Safety Injection Signal (SIS) which starts the required Safeguards components without shedding any normal plant loads (non-essential). Should there also be a Loss of Offsite Power (LOP), the SLSSs initiate a SISLOP actuation which trips the non-essential loads and then starts the required safeguards components in a timed sequence. The timed sequence serves to (1) maintain electrical starting loads within the limits of both the emergency diesel generators and the distribution system equipment, and (2) actuate the loads in the time necessary to mitigate the consequences of the event.
DESCRIPTION OF PROBLEM On January 30, 1989, with Unit I in Mode 5, it was determined as a result of revising electrical calculations that the potential for an overload condition existed on the main breakers for 480V Switchgear 1 and 2 during a SIS scenario with offsite power available.
The calculation results indicated that, following a SIS actuation signal with the worst postulated combination of non-essential and safety loads running or starting, the total loading on the 480V Switchgear 1 and 2 would exceed the rated current carrying capacity of their main breakers (52-1102 and 52-1202) before operator action could be credited (30 minutes) to shed loads. The subject breakers are rated at 1600 amps.
Load currents for e two breakers are conservative calculated to be approximately 1950 and 1650 amps (at 460V), respectively.
This condition could result in the failure of the breakers causing the loss of power to both trains of safeguards equipment on the 480V buses.
Previous load estimates did not consider all possible system combinations of non-essential and safeguard loads and as a result previous load estimates were low. During SISLOP events, the non-essential loads are automatically shed and locked out and therefore the potential overload conditions would not exist.
METHOD OF DISCOVERY While performing electrical calculations to evaluate motor starting voltages on the auxiliary power system, in support of Cycle 10 design modifications and using the latest design basis information, the overload conditions described above were identified.
CORRECTIVE ACTION The following options are being investigated for possible implementation:
- 1. Evaluate the overload capability of the main 480V breakers by testing and reviewing the breaker design basis with the vendor (Westinghouse).
- 2.
Evaluate modifications to trip and lockout the non-essential loads on 480V Switchgears 1 and 2 during a SIS event with offsite power available. This will reduce electrical loads to below breaker rating.
- 3. Replace the existing breakers with higher rated breakers.
9927F