ML19256F602

Responds to NRC 790918 Request for Addl Info Re Loss of Offsite Sys on 760222.Four Lines Lost Feeding 345 Kv Switchyard Ring Bus Connected to Startup Transformer.Cause, Fault on 345 Kv Relay & False Relaying Resulting from Storm

ML19256F602
Person / Time
Site:	Cooper
Issue date:	12/14/1979
From:	Pilant J NEBRASKA PUBLIC POWER DISTRICT
To:	Ippolito T Office of Nuclear Reactor Regulation
References
NUDOCS 7912190549
Download: ML19256F602 (3)

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GENERAL OFFICE P. O. 80X 499, COLUMBUS. NEBRASKA 68601

'1 Nebraska Public Power District retEmoNE <4ou s.. 8..i

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December 14, 1979 Mr. Thomas A. Ippolito, Chief Operating Reactors Branch No. 3 Division of Operating Reactors U.S. Nuclear Regulatory Commission Washington, DC 20555

Subject:

Review of Loss of Offsite Power Events

Reference:

Ippolito to Pilant Letter dated September 18, 1979

Dear Mr. Ippolito:

This letter is written in response to your letter requesting additional information concerning the Cooper Nuclear Power Station offsite systems.

The offsite power sources at CNS are a startup transformer which con-nects to the CNS 345 kV switchyard and a separate emergency transformer energized by a 69 kV line. The 345 kV switchyard has four (4) lines which terminate in switchyards near Des Moines, Iowa; Lincoln, Nebraska; St. Joseph, Missouri and South Omaha, Nebraska. The emergency trans-former is fed by a 69 kV line which is part of a distribution grid of another utility. If the normal transformer (powered by the main gen-erator) is lost, the startup transformer, which is normally energized, will automatically energize all plant buses, including the emergency buses. If the startup transformer fails to energize the emergency buses, the emergency transformer, which is normally energized, will automatically energize both emergency buses. If the emergency trans-former were also to fail, the emergency diesels would automatically energize their respective buses.

Since commercial operation, one partial loss of offsite power was expe-rienced on February 21, 1976. The questions pertinent to this event are answered below.

A. For losses of offsite power where less than all offsite power was lost:

1. How many circuits to the offsite network are normally avail-able and how many were lost during the event?

Normally there are four lines feeding the 345 kV switchyard ring bus which is connected to the startup transformer. All four lines were lost during the event in question. No rmally one independent 69 kV line feeds the emergency transformer, /h0 / Jf' it was not lost during this event.

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( 7912100 549

Mr. Thomas A. Ippolito December 14, 1979 Page 2

2. What was the cause of the event?

Prior to the event on February 21, 1976, a storm was moving through the area. A fault on one 345 kV line caused relaying to open the ring bus to that line and false relaying opened breakers on two other lines. This resulted in all generator power being transmitted via one transmission line. This situation, coupled with the main generator operating with a manually controlled portable exciter operating near its max-imum rating, caused the remaining line to trip on what appeared to be an out-of-step condition. The unit then tripped on load rejection as all four lines to the CNS switchyard vere lost.

The fault was caused by the 345 kV line coming close enough to the shield wire on another distribution line so that it arced across.

3. Why did the other lines not fail when some did fail?

The 69 kV line to the emergency transformer did not fail because it is physically separate from the 345 kV distribution system.

4. Was any voltage increase or decrease experienced just prior to or during the outage? If so, please give details, voltages reached, affects, etc.

No noticeable voltage changes were noted just prior to the outage.

5. Was any frequency decay experienced just prior to or during the outage? If so, please give details, lowest frequency reached, decay rate, affects on equipment operation, etc.

No frequency fluctuations were experienced just prior to the outage.

6. How long was power unavailable from the circuit?

Offsite power was lost to the startup transformer for approx-imately 29 minutes. At no time was power not available to the emergency buses.

7. Date of Event.

February 21, 1976.

B. For losses of all offsite power:

We have had no complete loss of offsite power. g)fj h-1624 26

~

Mr. Thomas A. Ippolito December 14, 1979 Page 2 C. Were there any other loss of offsite power events other than we have listed? If so, please give details of each event.

There have not been other losses of offsite power.

Since the loss of offsite power event on February 21, 1976, several changes have been made which increase the reliability of offsite power from the 345 kV grid. The main generator's pemanent exciter was returned to service in May 1976. The voltage control system of this exciter can maintain the system stability with only one 345 kV line in service. The South Omaha line, originally 64 miles in length, now teminates at a fossil powered station at Nebraska City, Nebraska. This line length is now approximately 28 miles. The Des Moines line, originally 145 miles long, now teminates at a switchyard near Booneville, Iowa. This line length is approximately 121 miles. The reduced distances reduce the possibility for fault relaying occurrences. The St. Joseph line relaying was changed to eliminate the direct transfer trip, a feature that does not exist on any other lines in the NPPD system. The feature was intended to completely deenergize the line if the breakers on either end remained open for any reason for a specified time. Elimination of this feature increases the reliability of the CNS ring bus. The line clearance between the 345 kV line and the shield wire has been increased to preclude the interference which caused the fault on February 21, 1976.

NPPD is now constructing a 161 kV switchyard at CNS which will have a ring bus fed from the CNS 345 kV switchyard via a 345/161 kV transfomer and a 161 kV grid separate from the NPPD 345 kV grid. This arrangement will increase the reliability of the offsite power supply to the station.

Should you have any further questions concerning this response, please contact me.

Sincerely, rM M Pilant Director of Licensing and Quality Assurance JMP:PJB:cmk 1624 M