ML20084C693

Ro:On 740425,generators in Both Units Tripped When Both Generator Circuit Breaker Protection Sys Relays Energized During Functional Testing of Unit 4 Startup Transformer Protection Sys Relays.No Equipment Damage or AO Resulted

ML20084C693
Person / Time
Site:	Turkey Point
Issue date:	05/31/1974
From:	Schmidt A FLORIDA POWER & LIGHT CO.
To:	Moseley N NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION II)
Shared Package
ML20084C683	List: ML20084C693
References
NUDOCS 8304080165
Download: ML20084C693 (6)

Text

. ,. -- - --- . . _ _ . _. - _ .- . --_ . . .

7. -

- c. _ .

• p.o.nox sico wAm. nemoA s2ici 0 0 c.

7=,w.q"tg h53 IDYtjY A;

//j J F t ORIDA FOi',[R & UGHT Cr '. f'AN '

' > May 31, 1974 i .

1 m (o 4 q Mr. Norman C. Moseley, Director Region II Directorate of Regulatory Operations p fg'If t/g t y, ,

gg U. S. Atomic Energy Commission -

q Wg A uri 230 Peachtree Street, N. W., Suite 818 T 'c r Atlanta, Georgia 303D3 17.- 5.#gh, .W 8 NU h:;

'\ 4,'.,'gh,#s 4 C ,

Dear Mr. Moseley:

TURKEY POINT PLANT / Q' T]"~ t '

l DOCKET NOS. 50-250 AND 50-251 UNIT TRIPS - APRIL 25, 1974 I

At the request of Mr. Austin Hardin of your staff, we are re-porting the tripping of Units 3 and 4 at the Turkey Point Plant.

SUMMARY

About 3:49 p.m., on April 25, 1974, Unit Nos. 3 and 4 ceneratun trippea wnen both generator circuit breaker protection system relays were energized during functional testing of Un'it No. 4 I

Startup Transformer protection system relays.

Trip of Unit Nos. 3 and 4 turbine-generators caused an auto- '

matic trip of both Unit Nos. 3 and 4 reactors. The reactor protection systems and engineered safeguards system equipment performed satisfactorily after the reactors tripped.

There was no equipment damage, abnormal occurrence or unusual event as a result of the unit trips.

Unit No. 3 was returned to service at 5:45 p.m., on April 25, 1974. .

Unit No. 4 was returned to service at 5:40 a.m., on April 26, 1974.

UNIT CONDITIONS BEFORE TRIPS l Unit No. 4 Startup Transformer was electrically isolated for functional testing of protection system relays and periodic maintenance on associated equipment. All of the specified 1 '

i T i-o ;c ik ,[ ] k .V

<( is i.

l 8304080165 740620 DR ADOCK 05000 COPY SENT REGION _

[ -

0400

c== u e

)

n

.Mr,.NormanC.(m .,oscicy V Page 2 May 31, 1974 _

L

~.

clearance requests, notifications, authorizations and approvals necessary to begin work on No. 4 Startup Transformer were met.

All of the conditions specified in the Technical Specifications for this work were also met.

Unit No. 4 had been operating at 723 MWe (98% R.P.) steady-state power level for several hours before the trip. Unit No. 3 had been operating at 720 MWe (97% R.P.) steady-state power level for several hours.

DESCRIPTION OF OCCURRENCE No. 4 Startup Transformer protective relays were tested and de-monstrated satisfactory operation. However, when the Startup Transformer lockout relay was " reset", current flowed through an unanticipated path (" sneak circuit") and energized Unit No. 4 generator breaker protection system relays. This opened both of the generator breakers on Unit Nos. 3 and 4. Both No. 3 and 4 reactors tripped.

automatically tripped when the turbine-generators The minimum system frequency recorded during the transient was 58.6 Hz. Underfrequency load shedding relays operated to reduce the interconnected system load by automatically opening feeder breakers. Frequency was restored to 60 Hz within six seconds interconnected system power flows were returned to normal within twenty minutes.

We estimate 99% of FPL's customers were back in service within twenty minutes, though some isolated outages lasted up to an hour. Some short term power interruptions also occurred to customers on other Florida systems as a result of the incident.

The instantaneous loss of 1443 MWe, when both Unit Nos. 3 and 4 tripped, is the largest single loss of generation experienced in FPL's system. In comparison to previous system disturbances,

' customer load was restored faster than any previous occurrence.

Refer to Table I for load shedding data.

DESIGNATION OF APPARENT CAUSE OF TRIPS Review, analysis, and evaluation of Unit Nos. 3 and 4 annunciators, records and Turkey Point Plant switchyard oscillographs concluded that both Unit Nos. 3 and 4 generators breakers. operated as a result of the actuation of the generato'r Sack-up protection relays during functional testing of Unit No. 4 Startup Transformer relays.

.-m pir. Norman C.O aseley .O

. Page'3 May 31, 1974 .

ANALYSIS OF OCCURRENCE UNIT NO. 3 When No. 3 reactor tripped, the reactor protection system and engineered safeguard system equipment operated properly to place the unit in a safe condition. There was no equipment damage, abnormal occurrence or unusual event as a result of this trip.

After the cause of the trip was determined and conditions ev-aluated as satisfactory, the reactor was returned to service.

No. 3 unit conditions after the unit trip were more conservative than similar incidents analyzed and presented in Section 14 of Turkey Point Unit Nos. 3 and 4 FSAR. Therefore, Unit No. 3 trip did not present any danger to the health and safety of the public.

UNIT NO. 4 When No. 4 reactor. tripped, all reactor protection system and engineered safeguards system equipment operated properly to place the unit in a safe condition even though No. 4 Startup Transformer was electrically isolated and normal auxiliary power supply was unavailable. Emergency power supply was~ supplied by automatic start of both A and B Emergency Diesel-generators.

The emergency Diesel-generators continued to operate until No. 4 Gia: Lup Transformer was returned to service.

About five minutes after No. 4 reactor tripped, the 1085 psig safety valve on No. 4A Steam Generator main steam line header lifted and did not rescat. Immediate operator actions included additional boration of the reactor coolant system to ensure that adequate reactor shutdown margin was maintained. Feedwater flow-to No. 4A Steam Generator was controlled to maintain adequate water level while efforts were made to close the safety valve. When the manual lif ting device was removed from the safety valve, the safety valve rescated. (Refer to Figure 1 for details)

Review of Unit No. 4 recorder charts, indicators and annunciators show that No. 4A loop cold Icg temperature was reduced to a minimum of 517 F (compared to normal hot shutdown temperature of 547 F) and maintained this temperature for about. thirty minutes.

No. 4A loop cold leg temperature returned to 547 F within an' additional thirty minutes. This rate of temperature change is well within allowable cooldown rates. Nos. 4B and 4C loop cold leg temperatures increased 10 to 15 F to about 565 F af ter the unit trip, then returned to 547 F within about thirty minutes.

l

Mr.NormanC.o u scley .O

'Page 4 May 31, 1974 There was no equipment damage, abnormal occurence, or unusual event as a result of this trip. After the cause of the unit trip was determined, No. 4 Startup Transformer returned to servicq and auxiliary power supply restored, the reactor was returned to service.

Comparison of No. 4 reactor conditions after the trip with similar incidents analyzed and presented in Section 14 of Turkey Point Unit Nos. 3 and 4 Final Safety Analysis Report concludes that actual conditions were equal to or more conservative than the conditions assumptions in the Safety Analysis. Therefore, the Unit No. 4 trip did not present any danger to the health and safety of the public.

CORRECTIVE ACTION Florida Power 4 Light Company Relay Engineers have recommended that both Unit Nos. 3 and 4 Startup Transformer protective system be modified by installing additional isolation switches and a diode to provide for complete isolation of the primary and the back-up protection system relays during relay tests. This recom-mended change is being reviewed to obtain approval and authorization necessary to make this change.

If you have further questions about this occurrence, please do not hccitate to call me.

Very truly yours, Ic-

. D.' Schmidt Director of Power Resources VTC/kaw cc: Mr. John F. O' Leary Jack R. Newman, Esquire m

9

FLORIDA POWER 6 LIGilT COMPANY TURKEY POINT UNIT NOS. 364 UNIT TRIPS - APRIL 25, 1974 TABL'E I The underfrequency relay trips shed the interconnected system load as follows:

UTILITY LOAD - MW Florida Power 6 Light Company 850 Florida Power Corporation 330 Tampa Electric Company 130 Jacksonville Electric Authority 100 Orlando Utilities Commission 34 City of Lakeland 18 TOTAL 1 462

)

J

, FIGURE 1 a ,.

Jimat/d&/de

p. 4L'.._-g -

au j l

h{f7 .

e=s N

legwe '

pj: W/s, Ec2 ES

c A

1.

! bh h CISC p HOLDEM j p.  ;

umn su -

j>jr. ,i p - =

. . - us:

q r&"% **

.l

• n(MWlQ hnrcu~

. mu l Y '

ecor I

[ .e veur eacx p tI r

?I Puti n. ..on.

'1 -

1; 7EM)' Powr ,%r sk erag r Il c5krfr?'t4t.Iw i '

The failure of the safety valve to rescat after opening was found to be caused by the loss of a cotter key in the safety valve topworks.

This cotter key locks the manual 11f t collar in proper position on the safety valve spindle. When.the cotter key vibrated out, the manual i lift collar vibrated down and made contact with the manual lifting device.

When the safety valve operated, the manual lift collar rotated further dowrl the spindle. This physically prevented the safety valve from rescating as the steam pressure decreased.

The safety vdive was rescated by removirig the manual i i f ting arm.

The safety valve was repaired by returned the manual. lift collar to j

its original position, installing a new cotter key, and replacing the manual lifting arm. .The safety valve.is now operable.

,