Information Notice 1996-23, Fires in Emergency Diesel Generator Exciters During Operation Following Undetected Fuse Blowing

Fires in Emergency Diesel Generator Exciters During Operation Following Undetected Fuse Blowing
	ML031060118
Person / Time
Site:	Beaver Valley, Millstone, Hatch, Monticello, Calvert Cliffs, Dresden, Davis Besse, Peach Bottom, Browns Ferry, Salem, Oconee, Mcguire, Nine Mile Point, Palisades, Palo Verde, Perry, Indian Point, Fermi, Kewaunee, Catawba, Harris, Wolf Creek, Saint Lucie, Point Beach, Oyster Creek, Watts Bar, Hope Creek, Grand Gulf, Cooper, Sequoyah, Byron, Pilgrim, Arkansas Nuclear, Three Mile Island, Braidwood, Susquehanna, Summer, Prairie Island, Columbia, Seabrook, Brunswick, Surry, Limerick, North Anna, Turkey Point, River Bend, Vermont Yankee, Crystal River, Haddam Neck, Ginna, Diablo Canyon, Callaway, Vogtle, Waterford, Duane Arnold, Farley, Robinson, Clinton, South Texas, San Onofre, Cook, Comanche Peak, Yankee Rowe, Maine Yankee, Quad Cities, Humboldt Bay, La Crosse, Big Rock Point, Rancho Seco, Zion, Midland, Bellefonte, Fort Calhoun, FitzPatrick, McGuire, LaSalle, Fort Saint Vrain, Shoreham, Satsop, Trojan, Atlantic Nuclear Power Plant
Issue date:	04/22/1996
From:	Crutchfield D; Office of Nuclear Reactor Regulation
To:	;
References
	IN-96-023, NUDOCS 9604170169
	Download: ML031060118 (10)
	v • d • e

UNITED STATES

NUCLEAR REGULATORY COMMISSION

OFFICE OF NUCLEAR REACTOR REGULATION

WASHINGTON, DC 20555-0001 April 22, 1996 NRC INFORMATION NOTICE 96-23: FIRES IN EMERGENCY DIESEL GENERATOR EXCITERS

DURING OPERATION FOLLOWING UNDETECTED FUSE

BLOWING

Addressees

All holders of operating licenses or construction permits for nuclear power

reactors.

Purpose

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

notice to alert addressees to the potential for damage (possibly fire) to

emergency diesel generator (EDG) exciters resulting from sustained high-power

operation after undetected blowing of a secondary fuse. 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 action or written response is required.

Description of Circumstances

On September 30, 1994, during refueling at the Wolf Creek Generating Station, the A train emergency diesel generator was undergoing post maintenance testing

and balancing. After about one hour of sustained operation above full diesel

generator power as part of a routine prolonged full and above full-power run, a fire occurred in the main power potential transformer of the static exciter- voltage regulator (exciter). The fire was extinguished quickly by deener- gizing the exciter and using a portable carbon dioxide fire extinguisher.

After the fire in the EDG-A exciter, the licensee performed electrical checks

on the EDG-B exciter and found no problems. On October 11, 1994, again after

about an hour of above full power operation of EDG-B, its exciter potential

transformer also caught fire. This fire was also quickly extinguished by

deenergizing the exciter and using a fire extinguisher. After each fire, the

licensee found that one of the 100-ampere fuses in the secondary circuits of

the respective exciter potential transformer had blown. It was later deter- mined that the fuses had not blown as a result of the fires, but that the

blown fuses were a contributing cause of the fires.

The phase B windings of the potential transformer removed from the exciter in

EDG-A were severely charred, and the primary and secondary cables were

blistered from the terminal lug back several inches. Some collateral damage

had occurred to the portions of the A- and C-phase windings closest to the

center, B-phase, windings. The condition of the damaged windings in both

cases was consistent with progressive insulation breakdown caused by sustained

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IN 96-23 April 22, 1996 current well in excess of the secondary winding ampacity. Even with lower

current in the primary, the primary and secondary windings being wound

together caused thermal damage to the primary insulation with its ultimate

electrical failure as well. The A-phase windings of the potential transformer

for EDG-B were similarly damaged.

The blown fuse in the exciter for EDG-A was a power amplifier fuse in the

phase C line of the secondary branch circuit supplying one of the power

amplifiers. The blown fuse in the exciter for EDG-B was the corresponding

fuse in its phase B line. The licensee determined that the fuses had blown in

each case as the result of manual engine shutdown without exciter shutdown.

This had occurred with the generators unloaded (output breakers open) at the

end of the EDG test run preceding the prolonged high-power runs during which

the exciter caught fire. The blown fuses were not detected at the time

because these fuses had no blown-fuse indication. The fires occurred in both

cases after about an hour of sustained operation above full power. Because

there was no blown-fuse indication, the normal full and above full-power runs

for routine testing were conducted subsequently without knowing that the fuses

had blown and "single phased" the potential transformers.

Discussion

Single-phasing the secondary windings of the delta-connected potential

transformers, as a result of the blown fuses, left them at about 58 percent of

rated capacity with one phase winding carrying two-thirds of the total load.

The other two windings were effectively put in series with each other;

together they were now in parallel with the first winding, but with twice the

total impedance and hence, only one-third total load. This effect is

illustrated in Figure 2 of Attachment 1 to this notice. Subsequent operation

with the undetected blown fuse for about 1 hour1.157407e-5 days <br />2.777778e-4 hours <br />1.653439e-6 weeks <br />3.805e-7 months <br /> at or above full load severely

overloaded the affected windings. It is expected that prolonged operation at

any power level significantly above the reduced capacity of the single-phased

potential transformers would eventually cause damage. However, even with the

imbalanced load, total excitation current demand from the potential

transformer, being within expected limits, was not sufficient to blow either

or both of the other two fuses. Operation continued until the overloaded

windings overheated and suffered progressive insulation breakdown, internal

short circuiting of the windings, and the resultant fire.

Although an undetected blown fuse for any reason could cause potential

transformer single-phasing and subsequent sustained high-power operation could

lead to fires, the reason for the blown fuses at Wolf Creek revealed another

apparent design deficiency. In both cases, the exciters were not shut down

automatically when the engines shut down due to mechanical causes not

accompanied by any of the normal electrical signals that would have otherwise

automatically shut down the exciters. On a normal manual shutdown from the

control panel, the exciter is turned off automatically as part of the normal

shutdown sequence of relay actuations. Similarly, in the event of one of the

standard alarm conditions that require immediate EDG shutdown, such as low

lube oil pressure, the system automatically shuts down the exciter.

IN 96-23 April 22, 1996 However, in both cases at Wolf Creek in which fuses were blown, the engines

had been shut down mechanically in a manner in which there were no attendant

electrical signals to the system to automatically shut down the exciters. In

the first instance, after it appeared that control of EDG-A may have been lost

upon a failed attempt at a normal shutdown (the unit tried to resume speed due

to an unrelated malfunction), the unit was shut down manually at the engine by

the operator shifting the engine manual control lever to the stop position

which shuts the fuel racks. In the second event, EDG-B shut itself down due

to a mechanical failure associated with the governor and the mechanical

overspeed trip device, but without an actual overspeed alarm condition

occurring. In systems equipped with underfrequency and/or voltz-per-hertz

protective features, this contingency is provided for, but the system at Wolf

Creek had no such features. Therefore, when the engines stopped in both cases

under the circumstances described, there was no electrical signal generated to

effect automatic exciter shutdown.

Engine shutdown without exciter shutdown caused a potential transformer

secondary fuse to blow in each case as follows: The exciters at Wolf Creek

(and Callaway) are type WNR manufactured by the Applied Products Division of

Westinghouse Electric Corporation (Westinghouse). Figure 1 of Attachment 1 to

this notice is a simplified functional block diagram of the affected static

exciter-voltage regulator design. The potential transformers that caught fire

(designated "PT" in Attachment 1) are Model 26616, delta-connected, 3-phase, step down 4160-Vac to 480-Vac, 45-kVA, 60-hz transformers, manufactured by the

NWL Transformer Company in Bordentown, New Jersey. The primary windings of

the PT are connected to the 4160-Vac, 3-phase generator output busses. The PT

secondary windings supply a portion of the generator DC field excitation

through power amplifiers. A major portion of the generator field excitation

with the generator under load is supplied by current transformers (designated

"CT" in Attachment 1) from the generator output through rectifiers. However, with a generator unloaded and its output breaker open, only a minimal amount

of excitation is provided by the current transformers; most of it being

provided by the potential transformers.

Expanded safety parameter display system trace printouts of EDG voltage, speed, and load for both events showed that as the generator slowed and

produced less output voltage (in both cases the generator was already unloaded

with its output circuit breaker open), the exciter, not having been turned

off, sensed this and demanded more generator field excitation current to

attempt to compensate. In both cases, the exciters attempted to maintain

voltage for about 20 seconds during engine coastdown, after which voltage

decayed rapidly. Attempting to maintain voltage with the generator slowing

caused excessive current in the potential transformer secondary (now the sole

source of excitation power with the generator breaker open) which blew one of

the fuses. With the resultant reduction in generator field excitation, coupled with the collapsing stator (armature) induced voltage as the machine

slowed, the available current was insufficient to blow either of the remaining

fuses. Although these Westinghouse exciters are used at the Standard Nuclear

Unit Power Plant System (SNUPPS) plants, Wolf Creek and Callaway, their

general features are not uncommon and other designs including Basler Electric

Type SB (series boost) exciters (without underfrequency or volts-per-hertz

IN 96-23 April 22, 1996 protection), are potentially susceptible to problems similar to those

described herein.

The licensee has installed blown fuse indication on the EDG exciter cabinets.

Prior to the installation of the blown fuse indication, the licensee

established procedures and trained operators to verify the condition of the

potential transformer secondary fuses following all EDG shutdowns, particularly those in which there are indications (such as no exciter shutdown

light) that the exciter was not turned off automatically. Procedures and

training also covered the conditions under which the exciter shutdown

pushbutton is to be actuated and under which the alternate power amplifier is

to be selected to maintain operability, e.g., in case an EDG demand signal is

received during the fuse verification, or in case a fuse should blow during

operation with or without an attendant fault or overload condition.

In other designs, such as newer Basler equipment, underfrequency protection is

often available that will independently shut down the exciter upon loss of the

prime mover. The licensee has installed volts-per-hertz protection to avoid

the conditions in question. However, EDG exciter systems of other designs

that remain on, either through a system design flaw or malfunction, after

engine mechanical shutdown may behave in a similar manner.

No specific action or written response is required by this information notice.

If you have any questions regarding this matter, please contact one of the

technical contacts listed below or the appropriate Office of Nuclear Reactor

Regulation (NRR) project manager.

4 tet- Dennis M. Crutche irector

Division of Reactor Program Management

Office of Nuclear Reactor Regulation

Technical contacts: Stephen D. Alexander, NRR

(301) 415-2995 Internet:sda@nrc.gov

John Whittemore, RIV

(817) 860-8294 Internet:jcw@nrc.gov

Attachments:

1. Exciter Block Diagram and Blown Fuse Effect Diagram

2. List of Recently Issued NRC Information Notices

d44o fi-tX

Attachment 1 IN 96-23 April 22, 1996 Figure 1: Emergency aiesel Genertr Static Eaei-Voltage Regulator

SfrnpIlIled Functional Block Diagram

F1 Phnao A

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With F2 blown, whi rigs Xl and X3 xi Sare X2 put in sedes an*dtogether, put In

o ~wbal <parallel with Xi. Thia capacity of the

Transformer PT Isthen reduced to about 58%.

6ftandsry t Powvr

PhaseD Amplfier

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The erips Impadence of X2 and XS Istwice that of Xi. so

tht Xi must cauy twios the curent.

.s

Phase C

U-J

Foire 2: Effect on Potential Transfromr Sewondary Windings of Single-Pasing

Due to Blown Fum

Attachment 2 IN 96-23 April 22, 1996 LIST OF RECENTLY ISSUED

NRC INFORMATION NOTICES

Information Date of

Notice No. Subject Issuance Issued to

96-22 Improper Equipment Set- 04/11/96 All holders of OLs or CPs

tings Due to the Use of for nuclear power reactors

Nontemperature-Compensated

Test Equipment

96-21 Safety Concerns Related 04/10/96 All U.S. NRC Medical to the

to the Design of the Door Licensees authorized to use

Interlock Circuit on brachytherapy sources in

Nucletron High-Dose Rate high- and pulsed-dose-rate

and Pulsed Dose Rate remote afterloaders

Remote Afterloading

Brachytherapy Devices

96-20 Demonstration of Associ- 04/04/96 All industrial radiography

ated Equipment Compliance licensees and radiography

with 10 CFR 34.20 equipment manufacturers

96-19 Failure of Tone Alert 04/02/96 All holders of OLs or CPs

Radios to Activate When for nuclear power reactors

Receiving a Shortened

Activation Signal

96-18 Compliance with 10 CFR 03/25/96 All material licensees

Part 20 for Airborne authorized to possess and

Thorium use thorium in unsealed

form

95-03 Loss of Reactor Coolant 03/25/96 All holders of OLs or CPs

Supp. 1 Inventory and Potential for PWR power plants

Loss of Emergency Mitiga- tion Functions While in a

Shutdown Condition

96-17 Reactor Operation Incon- 03/18/96 All holders of OLs or CPs

sistent with the Updated for nuclear power reactors

Final Safety Analysis

Report

96-16 BWR Operation with 03/14/96 All holders of OLs or CPs

Indicated Flow Less Than for boiling-water reactors

Natural Circulation

OL - Operating License

CP - Construction Permit