05000302/LER-1989-013, Corrected LER 89-013-01:on 890409,when Time Delay Satisfied & Voltage Had Not Recovered,Emergency Diesel Generator Start Signal Was Initiated.Caused by Degraded Voltage on Engineered Safeguard 4,160 Volt Buses

Corrected LER 89-013-01:on 890409,when Time Delay Satisfied & Voltage Had Not Recovered,Emergency Diesel Generator Start Signal Was Initiated.Caused by Degraded Voltage on Engineered Safeguard 4,160 Volt Buses

ML20246D767
Person / Time
Site:	Crystal River
Issue date:	07/05/1989
From:	Moffatt L, Widell R FLORIDA POWER CORP.
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
3F0789-03, 3F789-3, LER-89-013-01, LER-89-13-1, NUDOCS 8907110422
Download: ML20246D767 (5)

LER-2089-013, Corrected LER 89-013-01:on 890409,when Time Delay Satisfied & Voltage Had Not Recovered,Emergency Diesel Generator Start Signal Was Initiated.Caused by Degraded Voltage on Engineered Safeguard 4,160 Volt Buses

Event date:
Report date:
3022089013R00 - NRC Website

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3 Power COR POR ATION July 5, 1989 3F0789-03 U. S. Nuclear Regulatory Comission Attention: Mment Control Desk Washington, D. C. 20555

Subject:

Crystal River Unit 3 Docket No. 50-302 Operating License No. DPR-72 Licensee Event Report No. 89-13-01

Dear Sir:

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Florida Power Corporation's letter 3F0689-20 dated June 30, 1989 to transmit supplement one to IER 89-13 was sent without the revision lines arxi contained several other errors.

. Please disregard the June 30, 1989 supplement to IER 89-13 and replace it with the attached IER 89-13-01 supplement.

We are sorry for any inconvenience this may have caused. Should there be any questions, please contact this office.

Yours very truly, Rolf C. Widell Director, Nuclear Operations Site Support WIR: mag Enclosuru xc: Regional Administrator, Region II ,

Senior Resident Inspector pAA

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On April 9,1989 Crystal River Unit 3 was in OPERATIONAL MXE 5 (COID SERJIDOWN) for repair of Reactor Coolant Pumps. Maintenance was beirg performed on the Unit 3 Start-Up Transformer. The 4160V Engineered Safeguards (ES) Buses were being supplied through an underground connection by the Unit 1 and 2 (Coal Fired Plants) Start-Up Transformer. 'Ihe voltage of the ES buses degraded to i the setpoint for actuation of the Second Level Undervoltage Relays and these '

time delay relays actuated. When the time delay was satisfied and the voltage had not recovered, an Emergency Diesel Generatcr (EGDG) start signal was )

initiated. 'Ihis is an actuation of an Engineered Safety feature ard is l therefore reportable. The voltage remained degraded long enough for the EGDG I to come up to full geed. However, the voltage did not stay degraded long l enough to require the diesel to pick up the ES Buses. Appropriate Operator l action was acocxnplished to verify automatic start of both EGDGs and to verify recovery of the ES Bus voltage to the required value. 'Ihe EGDGs were returned 1 to standby status. 'Ibe cause of the degraded voltage was an overload condition l on the Unit 1 & 2 Start-Up Transformer caused by large base loads and by loads added as starting currents. Proper administrative controls governing the  !

loading of the transformer were not originally established teame> the need for '

them was not recognized by the original designer. Administrative control of the loading and alarm indication of non-availability of the transformer has been established. l N

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On April 9,1989 Crystal River Unit 3 was in OPERATIQaL PODE: 5 (ODID SEUIDOWN) for repair of Reactor Coolant PLmps [AB,P]. Maintenance was being performed on the Unit 3 Start-Up Transformer [EA,XFMR]. 'Ihe 4160V Engineered Safeguards (ES) Buses [EB,BU] were being supplied through an uMerground connection by the Unit 1 (Coal Fired Plant) 4160V buses. At 2349 the voltage of the ES buses degraded to the setpoint for actuation of the Second IcVel Undervoltage Relays

[EB,2] (SWRS) aM the time delay relays actuated. When the time delay was satisfied (5 sec) and the voltage had not recovered above the 95% SLUR setpoint, an Emergency Diesel Generator [EK,DG] (EGD3) engine start signal was initiated. 'Ihe voltage ruminM degraded for greater than 10 seconds which was long enough for both of tm OGDGs to come up to full speed. However, the voltage did not stay degraded long enough,18 seconds, to require stripping of the ES Buses in preparation for placing them on the EGDG. Appwsiate Operator action was accomplished to verify autctnatic start of both EDGs and to verify recovery of the ES Bus voltage to the required value. Both EGris were returned to staMby condition.

CAUSE:

'Ihe cause of the autanatic start of the EGIXss was a degraded voltage on the ES 4160v buses for a period of time sufficient for the Second Isvel Undervoltage Relays to initiate a diesel start signal. 'Ihe cause of the degraded voltage was an overload candition on the primary alternate offsite power source due to the lack of adequate administrative controls to assure reliability of the power source. 'Ihe administrative controls were not in place because the original design failed to consider starting currents in the loading scheme of the transformer. With the transformer loaded to greater than 1.5 Million Volt Amps (MVA) on the X winding, it does not have sufficient capacity to hanile the starting loads of Unit 3 Engineered Safeguards equipment.

EVENP NRIESIjS Crystal River Unit 3 uses power from the Unit 1 and 2 Start-Up Transformer .

(S/UX) as an alternate offsite source of power to the 4160v Engineered I Safeguards Buses. Units 1 & 2 are adjacent to Unit 3 and the Unit 1 & 2 S/UX is supplied from the 230 KV switchyard. 'Iho S/UX has two secoMary windings referred to as "X" and "Y" . 'Ihe alternate offsite power source for Unit 3 4160v ES Buses is the "X" wiMing of the Unit 1 & 2 S/UX. 'Ihe normal offsite power source to the ES Buses is the Unit 3 S/UX supplied directly from the 230kv switchyard. 'Ihat power source was not available because maintenance was being performed on the Unit 3 S/UX. Unit 2 was on line at full power while Urdt 1 was starting up. When the boiler feed pump at Unit 1, which is a 3500 Hp motor, was started, the Unit 1 and 2 S/UX was not able to supply all the required power without a significant voltage decrease. Voltage, as measured at the Unit 3 4160v ES buses, decreased from 4200v to 3680v instantaneously. With the approximately 500v decrease, the protective circuits on the ES buses were g ,o. m

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• YJI=U CRYSTAL RIVER UNIT 3 0151010 l0 l 3 J Ol 2 8 l9 0l1l3 011 0 l3 or O ' l4 actuated and a time delay relay was started. It should be noted that the safeguards w h ul centers starters have been specified to hold in down to 65%

- of rated voltage for 'a period of two (2) seconds. None of the safeguards equdpnent was lost in this event.

When the Unit 3 Final Safety Analysis Report (ESAR) was prepared prior to zuceipt of a license, certain conditions were aaai-d which allowed us to specify that the Unit 1 & 2 S/UX was capable of supplying the pamaaaary power -

(6.59 MVA) for Unit 3 ES loads. '1his a=== tion was stated in Qiapter 8 "gECIRICAL SYSTENS" Section 8.2.3 " SOURCES OF AUXILTARY IWER" Dages 8-17 through 8-27. 'Ihis assunption was apparently based on the steady state condition of Unit 2 operating, Unit 1 starting up, and Unit 3 in an er wfacy condition with its ES. loads already loaded on the Unit l' & 2 Start-Up Transformer. Included in the a==ntion was a stipulation that. the Unit 1 and 2 S/UX was to be carrying one fully loaded (6.65 MVA) bus, Bus 2B, frun Unit 2 and one 70% loaded (3.54 MVA) bus (Bus 1B) frcan Unit 1.

In 1985 analyses were performed which showed that it was undesirable to allow voltage to drop to significantly less than 95% of nominal voltage at the ES 4160v Buses. For that reason, Second Isvel Undervoltage Relaying (SWRs) was installed on the Unit 3 4160v ES Naan. 'Ibe SWRs are time delayed relays.

'Ibe installation of SERs put a limit of acceptability of 95% of nominal voltage on tha ES Buses which had not been there before. 'Ibe time delay relay's function is to allow same slight and temporary voltage fluctuations without innadiately starting the EDGs. Many unr ary EDG starts are avoided by allowing a few seconds time for the voltage to recover before initiatinJ a start of an EGDG. Evidently, the expected voltage drop under the conditions a=_=d in the ISAR was not considered in the design process for the SWRs.

On the date of this event, the loads on the ES Buses totaled approximately 1670 KW ( 1.8 MVA) which, because there was no sinai f - ucy condition, is far below the FSAR a==d ES loads of 5930 Kw (6.59 MVA) . Unit 2 was operating and Unit 1 was performing a start-up which required the start of a boiler feed punp l motor (3500 Hp) . However, four (4) buses, 1A, 1B, Start-Up mm A, and B, instead of two, were being carried on the Unit 1 & 2 S/UX. 'Ihe "Y" winding was carrying buses 1B and Start 47p Bus B and any loads on those buses contributed )

to the load on the transformer. Because buses 1A and Start-Up Bus A were wisected to the "X" winding of the S/UX in parallel with Unit 3 ES Buses, they also contributed to causing the S/UX to overload when the Unit 1 boiler feed punp was started. This overloading resulted in a significant voltage drop at the 4160v ES buses at Unit 3 even without full ES loads being on the Unit 3 ES .

Buses. Consequently, when Unit 3 ms depeMwit on the power supply frun Units  ;

1 & 2 Start-Up Transformer, in was fattnd to be inadelate. >

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Fower to the ES buses was always available. There was no decrease in the -l margin of safety for the public caused by this event. None of the safety i functions of plant equipnent were cxmprmM. All in-plant equipnent functioned as designed and all plant s uc.siures were adequate for the situation. All actions by the operators were proper and timely. If this event j had occurred subsequent to an ES actuation there could have been scane inpact on j the amount of time required to supply High Pressure Injection [BQ) (HPI) water 1 to the reactor core. The delay of HPI would have been a maximum of 28 seconds beyond the ===A delay of 25 seconds for water to reach the core fcllowiJg an HFI Engineered Safeguards actuation. A reanalysis to determine the thermal-hydraulic effects has shown there would not be any significance to a total delay time of 53 seconds instead of 25 seconds for HPI water to enter the core.

umenA:nVE AcrRN:

The systems functioned as they were supposed to when a degraded voltage situation occurs. The Dnargency Diesel Generators were returned to standby status after verifying the ES buses were being properly supplied. No corrective action is required for responding systems /u.aupvimuis. Corrective actions include agreement between the Directors of the Nuclear arx1 Fossil Units as to actions required when the Unit 1 & 2 Start-Up Transformer is cuerloaded. This agreement has been issued to the operating shifts as a Memorandum of Understanding and prwides the appropriate administrative controls to assure that this transformer is not relied upon as a Technical Specification offsite power supply when its configuration will not provide sufficient capacity. An annunciator alarm has been added to the Main Control Board to provide notification to the Nuclear Operator that the Unit 1 and 2 Start-up transformer is loaded to greater than 1.5 Hva. An analysis of the ,!

impact of sequential occurrence of an ES actuation followed by a degraded voltage condition on the ES buses has been performed usiry Thermal-Hydraulic analysis.

PRFNIGJS EVINTS:

An urplanned start of an EGDG has previously occurred seven times in the 12 year operating history of the plant. 2he last event was reported in IER 87-007. This event is the first time the diesel las started due to actual degraded voltage conditions. All other events were either start circuitry problems or total loss of voltage situations. There have not been any observed occurrences of overloading the Start-Up Transformer.

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