Information Notice 2002-01, Metal Clad Switchgear Failures and Consequent Losses of Offsite Power

Metal Clad Switchgear Failures and Consequent Losses of Offsite Power

ML013540193
Person / Time
Site:	San Onofre
Issue date:	01/08/2002
From:	Beckner W Division of Regulatory Improvement Programs
To:
References
TAC MB1793 IN-02-001
Download: ML013540193 (7)

UNITED STATES

NUCLEAR REGULATORY COMMISSION

OFFICE OF NUCLEAR REACTOR REGULATION

WASHINGTON, D. C. 20555-0001 January 8, 2002 NRC INFORMATION NOTICE 2002-01: METALCLAD SWITCHGEAR FAILURES AND

CONSEQUENT LOSSES OF OFFSITE POWER

Addressees

All holders of licenses for nuclear power reactors.

Purpose

The U.S. Nuclear Regulatory Commission (NRC) is issuing this information notice to inform

addressees of electrical equipment failure modes and design vulnerabilities identified following

recent transients at the San Onofre Nuclear Generating Station and at a foreign nuclear power

station. The most interesting aspect in both events was propagation of damage from an

electrical fault in one breaker cubicle to other breakers and buswork in the same enclosure. It

is expected that recipients will review the information for applicability to their facilities and

consider actions, as appropriate, to avoid similar problems. However, suggestions contained in

this information notice are not NRC requirements; therefore, no specific actions or written

response is required.

Description of Circumstances

San Onofre Nuclear Generating Station Event

On February 3, 2001, the reactor at the San Onofre Nuclear Generating Station, Unit 3 (SONGS-3), was at 39 percent power following a refueling outage. While shifting loads to the

unit auxiliary transformers, a fault on a 4.16 kV supply circuit breaker from the unit auxiliary

transformer caused a fire and loss of offsite power.

The fault was caused by failure of the 4.16 kV breakers C phase main contacts to close fully.

This resulted in arcing and a production of a thick, dark ionized smoke. The breaker was a

Brown Bovari Type HK three pole, medium-voltage ac power circuit breaker rated for 3000

amps (continuous) and 350 MVA (interrupting). The breaker was approximately 25 years old

and had its last preventive maintenance performed in 1997. Due to the extensive fire damage, the cause of the breakers failure to close could not be definitively established.

Offsite power was lost when ionized smoke (which is conductive) diffused through holes

(through which wires passed) and conduits between adjacent cubicles. This shorted the

energized incoming terminals of the offsite power supply from the reserve auxiliary transformer.

The fault blew open the cubicle door of the offsite supply circuit breaker and blew off an

insulating boot that covered the A phase bus bar. The high voltage supply breakers upstream

of the reserve auxiliary transformer opened to clear the fault. This interrupted non-vital offsite

power to the unit.

As a consequence of the loss of offsite power, neither turbine ac lube oil pump started. Failure

of the ac pumps to start should have initiated an automatic start of the backup emergency dc

turbine lube oil pump; however, the backup dc pump failed to start. This failure to start was

caused by a defect in the trip setting device of the dc lube oil pumps power supply breaker.

The breakers trip setting device (a mechanical device that maintains spring tension and thus

the breakers trip setting) was found damaged and incapable of maintaining spring tension and

thus the trip setting. The dc pump supply breaker is a 250 volt dc General Electric molded case

circuit breaker, Type TBC 400. The dc pump failure led to extensive damage to the unit main

turbine, generator and exciter resulting from inadequate lubrication.

Foreign Nuclear Power Station Event

On March 18, 2001, at a foreign PWR, a fault in a 4.16 kV load center caused a fire and loss of

offsite power while the reactor was shutdown but with significant decay heat. This, combined

with a subsequent independent failure in the onsite standby power supply, resulted in a station

blackout (i.e., a loss of ac power to both redundant safety systems). Recovery from the event

was further complicated by smoke and dependence on ac powered emergency lighting and

ventilation.

This foreign plant is a PWR in commercial operation since the mid 1980s. Offsite power is

provided by two independent circuits from the transmission network (see Attachment 1). The

circuit from the 345 kV transmission network is the normal source of offsite power to each of

the two redundant 4.16 kV safety-related trains. The circuit from 161 kV transmission network

provides a backup offsite source to each train.

For several days prior to the event, foggy, misty weather had caused salt deposition on the

insulators on the 345 kV transmission system and consequent power fluctuations and

interruptions. On March 17, the day prior to the failure, the 345 kV transmission system had

been interrupted, resulting in an automatic reactor shutdown and transfer to the backup 161 kV

offsite source. On March 18, with the reactor shutdown and with off-site power being supplied

to the unit from the 161 kV backup source, the 345 kV source was recovered and the circuit into

the plant re-energized.

After the switchyard 345 kV circuit breaker was closed (energizing the 345/4.16 kV transformer

and the 4.16 kV circuits into the vital load centers while the 4.16 kV supply breakers were still

open), a fault occurred in the A train 4.16 kV load center. The fault was caused by insulator

failure on one phase of the A train 4.16 kV safety-related switchgear on the supply side of the

supply breaker from the 345/4.16 kV transformer. The cause of the insulator failure is

unknown. The resulting fault de-energized the 345 kV offsite source to both the A and B

4.16 kV safety-related switchgear. The fault also produced voluminous smoke and ionized gas.

Migration of the smoke through the A train 4.16 kV switchgear enclosure resulted in multiple

arcing faults. One of these faults, apparently on the supply side of supply breaker from the

161 kV source caused loss of the 161 kV supply to both the A and B trains. Both trains were affected because they share a common feeder cable downstream of the step-down transformer

from the 161 kV system. Another fault in the internal bus-work of the switchgear caused the A

train EDG supply breaker to trip after the EDG started and came to rated voltage.

These faults resulted in unavailability of power to A train safety systems from either the A train

onsite emergency diesel generator or from either the 345 kV or the 161 kV offsite sources. The

faults inside the A train load center on the supply side of each of the offsite source circuits

tripped and locked out the high voltage supply breakers for both the 345 and 161 kV circuits

and prevented the connection of offsite power to the B train safety systems as well.

The B train onsite emergency diesel generator failed independently. The diesel generator failed

to achieve normal voltage due to a failure in its excitation circuit. Further local starting attempts

were prevented by heavy smoke from the burning insulation in the A load center. The heavy

smoke could not be cleared from the corridors because of lack of ac power to operate either

normal ventilation or emergency blowers; also only very limited emergency lighting was

available in that area.

The remaining ac power supply (a station blackout diesel generator capable of being connected

to either of the two trains) was started but tripped on low lube oil pressure. It was later

determined that the blackout diesel had tripped because the loss of offsite power interrupted

power to the engines keep-warm system.

All primary injection systems were unavailable due to the loss of ac power. The steam-driven

auxiliary feed pump provided sufficient cooling for the primary system. The blackout diesel was

eventually restarted and the B train was energized about two hours after the event and the plant

was stabilized.

Discussion

These electrical events provide insight into possible collateral damage and cascading failures

resulting from a single electrical failure and consequent challenges to plant operation. At

SONGS-3, the event originated with the failure of a single non-safety-related 4.16 kV

switchgear supply breaker. Smoke from the event propagated causing loss of offsite power to

other non-safety-related equipment and partial loss of offsite power to safety-related equipment.

The loss of offsite power resulted in the need for dc backup power to operate the non-safety- related turbine lube oil pump. The failure of the dc supply breaker for the lube oil pump resulted

in significant damage to the main turbine.

At the foreign plant, the event was caused by a single failure of an insulator on the offsite

supply cable to one train of 4.16 kV safety-related switchgear. Smoke from the event spread, causing multiple failures in the switchgear, loss of both sources of offsite power to both trains, and total loss of power to one safety train. An unrelated failure of the other source of onsite

power led to a total loss of power to both trains. In addition, the loss of offsite power disabled

the backup station blackout power supply. An electrical fault occurring in medium voltage switchgear enclosure can cascade, causing

collateral damage within the switchgear and consequent loss of multiple circuits. When both

safety trains are supplied from the same offsite supply with a common upstream circuit breaker

(see Attachment 1), one fault in the supply circuit can disable offsite power to both redundant

trains. Operators ought to be aware of such a vulnerability and to have considered means to

isolate the faulted feeder if both offsite power sources could be affected by a fault in the feeder

circuit.

This information notice requires no specific action or written response. If you have any

questions about the information in this notice, please contact one of the technical contacts

listed below or the appropriate Office of Nuclear Reactor Regulation (NRR) project manager.

/RA/

William D. Beckner, Program Director

Operating Reactor Improvements Program

Division of Regulatory Improvement Programs

Office of Nuclear Reactor Regulation

Technical contacts: Thomas Koshy, NRR James Lazevnick, NRR

(301) 415-1176 (301) 415-2782 E-mail: txk@nrc.gov E-mail: jjl@nrc.gov

Attachments:

1. AC Power Distribution System Figure

2. List of Recently Issued NRC Information Notices

ML013540193 Template No.: NRR-052 To receive a copy of this document, indicate in the box C=Copy w/o attachment/enclosure E=Copy with attachment/enclosure N = No copy

OFFICE REXB Tech Editor* C:EEIB RORP C RORP/DRIP

NAME EGoodwin* JCalvo* TKoshy /TK/ WBeckner /WDB/

DATE 01/02/02 7 / 31 /01 01/04/02 01/04/02 01/07/02

Attachment 1 AC Power

Distribution System

with Y Connection

to Safety Buses

VVVVVV 161 KV

VV VV Startup

VVVVVV

VV VV 345 KV Transformer

Startup

Transformer

Swing D/G

17 15 12 25 03 05 08 25

4.16 KV Safety Bus A EDG A 4.16 KV Safety Bus B EDG B

Attachment 2 LIST OF RECENTLY ISSUED

NRC INFORMATION NOTICES

_____________________________________________________________________________________

Information Date of

Notice No. Subject Issuance Issued to

_____________________________________________________________________________________

2001-19 Improper Maintenance and 12/17/2001 All holders of operating licenses

Reassembly of Automatic Oil for nuclear power reactors, Bubblers except those who have

permanently ceased operations

and have certified that fuel has

been permanently removed from

the reactor vessel.

2001-18 Degraded or Failed Automated 12/14/2001 All uranium fuel conversion, Electronic Monitoring, Control, enrichment, and fabrication

Alarming, Response, and licensees and certificate holders

Communications Needed for authorized to receive safeguards

Safety and/or Safeguards information. Information notice is

not available to the public

because it contains safeguards

information.

2001-17 Degraded and Failed 12/14/2001 All uranium fuel conversion, Performance of Essential enrichment, and fabrication

Utilities Needed for Safety and licensees and certificate holders

Safeguards authorized to receive safeguards

information. Information notice is

not available to the public

because it contains safeguards

information.

2001-08, Update on Radiation Therapy 11/20/2001 All medical licensees.

Sup. 2 Overexposures in Panama

2001-16 Recent Foreign and Domestic 10/31/2001 All holders of operating licenses

Experience with Degradation of for pressurized-water reactors

steam Generator Tubes and (PWR), except those who have

Internals permanently ceased operations

and have certified that fuel has

been permanently removed from

the reactor.

______________________________________________________________________________________

OL = Operating License

CP = Construction Permit