ML20199J262

Summary of 860623 Meeting W/Util in Bethesda,Md to Discuss Corrective Actions Re Electrical Sys Voltages.Licensee Will Submit Rev to Adequacy of Station Electric Distribution Sys Voltages by 860731

ML20199J262
Person / Time
Site:	Yankee Rowe
Issue date:	06/27/1986
From:	Mckenna E Office of Nuclear Reactor Regulation
To:	Office of Nuclear Reactor Regulation
References
NUDOCS 8607080260
Download: ML20199J262 (4)

Text

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JUN 2 71986 Docket No.50-029 LICENSEE: Yankee Atomic Electric Company (YAEC)

FACILITY: Yankee Nuclear Power Station

SUMMARY

OF MEETING HELD ON JUNE 23, 1986 TO DISCUSS ELECTRICAL SYSTEM V0LTAGES On June 23, 1986, members of the NRC staff met with YAEC representatives (the licensee) at NRC offices in Bethesda, MD. A list of attendees is enclosed.

The purpose of the meeting was to discuss proposed corrective actions relating to a problem with the electrical distributi.on system. In a letter dated June 19, 1986, the licensee described a situation that could occur when the 480V buses are crosstied where the minimum starting voltage requirements for the high pressure safety injection (HPSI) pumps may not be satisfied.

Also, the station service transformer could exceed its rated capacity if a safety injection actuation signal (SIAS) should occur.

SYSTEM DESCRIPTION The Yankee onsite electrical distribution system (see attachment) has three 2400V buses, three 480V buses, and three 480V emergency buses for powering the safeguards loads. Two of the three sets of buses are normally connected to the offsite grid which supplies power through the switchyard from the two incoming 115 KV lines. The third set of buses is normally powered by the main generator. When the main generator is disconnected, such as during plant startup, the third set of buses are connected to the offsite power by closing normally open bus tie breakers to the other buses. After startup when the generator is synchronized to the grid, the center buses are isolated from the other buses by manually opening the bus tie breakers. The #2 2400V bus is tied to the #1 2400V bus and 480V bus 6-3 is tied to the 480V bus 4-1 (or the alternate pairs) to split the loads.

Each of the three emergency buses can be powered by its own diesel generator if offsite power is not available to the bus. On a loss of voltage on the emergency bus, the diesel generator is automatically started and closed onto the bus.

If a safety injection actuation signal occurs, the low pressure safety injection pumps are automatically started. After a ten second time delay, the high pressure safety injection (HPSI) pumps start.

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8607080260 860627 I PDR ADOCK 05000029 P PDR

If the SIAS had occurred prior to loss of voltage on the bus, the HPSI pumps would be shed and then re-energized after with a time delay after the diesel generator start.

DISCUSSION At the meeting, the licensee described the problem that can exist when the 480V buses are crosstied. The configuration occurs only during short periods of time when coming off-line or back on line (below 30 MWE, i.e., about 15%

power). With the 480V buses crosstied (for example bus 6-3 ad 4-1), four buses are being fed from one transformer (6SST). Should a SI.; occur, the two LPSI pumps would load onto the emergency buses. Ten seconds later, the HPSI pumps would also receive a start signal, but the studies show that the bus voltage would be too low for pump start. The 480V bus breakers (BTIA, BT1B, etc.) would then trip and de-energize the two emergency buses. The diesel generators would be needed to power the two emergency buses. The third train would be unaffected.

The licensee stated that under the reduced power conditions, the requirements for emergency core cooling (i.e., minimum number of trains, actuation time) are less limiting than at full power such that a short delay (i.e., while diesel comes up to speed and loads are re-energized) is not a problem. However, the potential need for a diesel as the power source for two buses rather than offsite power is not consistent with NRC licensing criteria and, therefore, corrective actions have been proposed.

The staff noted that adequacy of station voltages was a subject that had been previously reviewed by the staff in 1981 and asked why this problem was not found previously. The licensee stated that their previous studies had assumed that the HPSI pumps were 460V motors; in actuality, they are 480V motors. Thus, the minimum required voltage is higher than assumed. Also, new loads have since been added to the 480V buses.

CORRECTIVE ACTIONS The short-term corrective action proposed by the licensee was to station a dedicated operator at the diesel generator / safety injection panel such that if a SIAS occurs, the operator could immediately open breakers such that the diesel generator could power the emergency loads for #2 emergency bus. The other two emergency buses would still be receiving offsite power. As a long-term corrective action, the licensee will install a breaker between the 2400V bus and 4SST. With this breaker, when the main generator is off-line, the crosstie at the 2400V level would be used, and the center set of 480V buses would receive power from the center 2400V bus via 4SST. This will alleviate any overloading of SSST (or 6SST) and ensure adequate voltage on the emergency bus for the HPSI motor. The new breaker would be installed at the next refueling outage (May 1987).

SUMMARY

The staff considers the above described actions for resolving the undervoltage problem acceptable. The staff requested that the licensee submit a revised report on Adequacy of Station Electric Distribution System Voltages and highlight.those aspects that have changed since the staff's July 13, 1981 evaluation on this subject was issued. The licensee agreed to provide this report by the end of July. The staff also requested that this report include a description of operator actions required for alignment of power supplies during startup/ shutdown when the new breaker has been installed.

/s/

Eileen M. McKenna, Project Manager Project Directorate #1 Division ~of PWR Licensing-A

Enclosures:

List of Attendees Onsite Electric Distribution System Diagram cc's: See Next Page l

Office: LA/ PAD #1 Pg PD/ PAD #1 Surname: PShuttle%$ EMcKenna/4g GLear Date: 06/27/86 06/q7/86 06p/86

l Distribution Copies:

$NN NRC PDR Ekeb Local PDR t PAD #1 r/f PAD #1 s/f Glear EMcKenna OELD

, EJordan BGrimes ACRS (10) 4 NRC Participants

(cc

Licensee and Plant Service List) i l

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