05000338/LER-1980-096-01, /01T-0:on 801114,S&W Determined Significant out-of -phase Transfer Could Occur When Emergency Buses Transferred to Emergency Diesel Generators.Cause Not Determined.Time Delay Circuit Installed on Generators

/01T-0:on 801114,S&W Determined Significant out-of -phase Transfer Could Occur When Emergency Buses Transferred to Emergency Diesel Generators.Cause Not Determined.Time Delay Circuit Installed on Generators

ML19345B477
Person / Time
Site:	North Anna
Issue date:	11/21/1980
From:	Cartwright W VIRGINIA POWER (VIRGINIA ELECTRIC & POWER CO.)
To:	NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION II)
Shared Package
ML19345B467	List: 05000338/LER-1980-096, Forwards LER 80-096/01T-0 05000338/LER-1980-096-01, /01T-0:on 801114,S&W Determined Significant out-of -phase Transfer Could Occur When Emergency Buses Transferred to Emergency Diesel Generators.Cause Not Determined.Time Delay Circuit Installed on Generators
References
LER-80-096-01T, LER-80-96-1T, NUDOCS 8012010373
Download: ML19345B477 (3)

LER-1980-096, /01T-0:on 801114,S&W Determined Significant out-of -phase Transfer Could Occur When Emergency Buses Transferred to Emergency Diesel Generators.Cause Not Determined.Time Delay Circuit Installed on Generators

Event date:
Report date:
3381980096R01 - NRC Website

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U.S. NUCLEAR REGULATORY COMMISSION LICENSEE EVENT REPORT CONTROL BLOCK / /=/ / / / / (1)

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DOCKET NUMBER EVENT DATE REPORT DATE-EVENT DESCRIPTION AND PROBABLE CONSEQUENCES (10)

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On November 14, 1980, Stone and Webster Engineering Corporation determined

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that, under certain conditions, a significant out of phase transfer could occur /

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when the emergency buses are transferred to the emergency diesel generators.

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An out of phase transfer might cause vital safety equipment failures. This

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event is reportable pursuant to T.S. 6.9.1.8.i.

This event effects both

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Units 1 and 2.

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SYSTEM

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CAUSE DESCRIPTION AND CORRECTIVE ACTIONS (27)

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The cause of the design error has not been determined. A time delay circuit

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was installed on the Unit 1 emergency diesel g 7+rators to prevent significant /

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1980, prior to startup from the current outage.

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PUBLICITY ISSUED DESCRIPTION (45)

NRC USE ONLY

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- NAME OF PREPARER W. R. CARTWRIGHT PHONE (703) 894-5151 8012010373

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.s Virginia Electric and Power Company North Anna Power Station, Unit il Attachment: Page 1 of 2 Docket No. 50-338 Report No. LER 80-096/01T-0

Description of Event

On' November 12, 1980, Stone and Webster Engineering Corporation informed'Vepco that, under certain conditions, an out of phase transfer might occur when the emergency buses are transferred from the off-site power source to the emergency diesel generators after the generators have reached rated speed and voltage because there is no definitive phase relationship between the off-site source and the emergency diesel generator voltages and no time pecmissive for collapsing the residual voltage. On November 14, 1980, S'.one and Webster Engineering Corporation confirmed a significant out of phase transfer could occur under the following conditions:

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A Safety Injection signal is received and the diesels start and run on standby.

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Off-site power is lost, the emergency buses isolate from the off-site supply and are transferred to the diesel' generators.

As built, there was no time permissive to allow for collapsing of residual bus voltages; therefore, an out of phase transfer might have caused severe motor end turn forces as well as high torsional stresses that could cause motor, coupling or driven equipment shaft failures. A study of this transfer effect on the low head safety injection motors indicates that the resultant volts per Hertz may not be within the recommended allowable limit for a safe transfer per NASI C50.41-1977.

This event is reportable pursuant to T.S. 6.9.1.8.i.

Unit I and 2 are effected by this event. Whe:t the event was identified as described above, Unit I was at 90 percent power and Unit 2 was in Mode 5 (cold shutdown).

Probable 1_f2quences of Occurrence As described above, an out of phase transfer might have caused severe motor end turn forces as well as high torsional stresses that c ould cause motor, coupling or driven eqaipment shaf t failures. Since vital safet, equipment was effected, immediate corrective actions as described aelow were_taken to prevent the possibility of a significant out of phase transfer and eliminate any adverse consequences which could be caused by the event described above.

Cause of Event-The causc cf this design error has not been determined.

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Immediate Corrective Action

In order to alleviate the possiblity of a significant out of phase transfer condition, on Novermber 16, 1980 a time delay circuit was

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installed on the Unit 1 emergency diesel generator output breakers to allow a 2.0 second time delay between the opening of the off-site supply breakers and closing of the emergency diesel generator breakers. A 1.96 second time delay is sufficient for the residual voltages to dissipate to allowable levels. This modification does not create a possibility for an accident or malfunction of a different type than any evaluated in the Safety Analysis Report. This modification was made on Unit 2, on

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November 18 and 19, 1980, prior to start-up from the current outage.

Scheduled Corrective Action Stone and Webster Engineering Corporation is continuing to study the problem. When this study is complete, a final design change will be made.

Actions Taken to Prevent Recurrence No actions to prevent recurrance are required.

Generic' Implications Thie event effected North Anna Unics 1 and 2 and Surry Units 1 and 2.

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