ML19325F337

LER 89-033-00:on 890908,second Level Undervoltage Relay Sys Setpoint for Engineered Safeguards Buses Not Conservative & Led to Operation Outside Plant Design Basis.Caused by Personnel Error.Setpoint changed.W/891110 Ltr

ML19325F337
Person / Time
Site:	Crystal River
Issue date:	11/10/1989
From:	Moffatt L, Ken Wilson FLORIDA POWER CORP.
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
3F1189-10, LER-89-033, LER-89-33, NUDOCS 8911200157
Download: ML19325F337 (5)

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CORPOR At lON 4

November 10, 1989 371189-10 U. S. Nuclear Regulatory Comuniasion Attentient Document control Desk Washingtm, D. C. 20555

Subject:

Crystal River Unit 3 Docket No 50-302 Operating License No. DRt-72 Licensee Event Report No.89-033 Dear Sirt i EnclW is Licensee Event Report (IIR)89-033 which is submitted I

in acocedance with 10 CFR 50.73.

should there be any questions, please contact this office, very truly yours,

(.dk L '

Fanneth R. Wilson l

Manager, Nuclear Licensing l WIRimag 1

Enclosure l xct Regional Administrator, Region II

j. Senior Resident Inspector 1

l' 57 8911200bCKb91110 PDR. AD 5000302 S PDC

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Crystal River Unit 3 was in PODE 5 (CDID SIUIDEN) replacing a faulted Enginenred Safeguards (ES) transformer. On September 8, 1989, during the design review of the replacement 480 Volt transformer, design engineers discovered that the Second IEvel Urdezvoltage Relay (SIUR) system setpoint for the Engineered Safeguards buses was not conservative. 'Ihe setpoint had boon ,

calculated without considerirg the voltage drop between the Engineered Safeguards Motor Control Centers and their associated loads. 'Ihe cetpoint calculated to correct this problem was initially set too high. An ES actuation while supplying normal plant loads would have caused a spurious separation from i the offsite power supply. 'Ihese errors were causcd by personrol error.

'Ihe SIUR setpoint was recalculated considering the voltage drops ard the spurious separation issue ard changed on October 9,1989. Additionally, Florida Ptwer will be revalidatirg the rajor safety relatcd ES bus calculations.

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As a result of a transformer fault on August 28, 1989, Crystal River Unit 3 (G-3) was replacing the A 480 Volt Engineered Safeguartis (ES) transfomer l [ED,XINR) . The new replacement transfomer had a slightly higher inpedance.

l The higher i=whnoe of the replacement transfomer decreased the secondary i cide voltage for a given prinary side voltage. This affected the calculations l used to detemine the Second level Undervoltage Relay (EB,27) (SWR) setpoint.

Therefore, the architecturcl engineering flru, Gilbert Ocamonwealth, Inc. ,

(GCI), performed a review to a-a the effect of the fiTwhnoe charge. On I Atqust 30, 1989, GCI informed Florida Power (FPC) that the SWR setpoint was  !

not high enough to positively assure sufficient terminal voltage at the end i devices. During the installation of the replacement transfomer on August 31,  !

, 1989, the primary voltage tap was changed frtxn 4160 volts to 4055 volts l cffectively increasing the voltage on the secondary side while calculations to  ;

detemine a conservative setpoint were performed.

At 1600 on September 8, 1989, after utility ergineering revi w of the engineering ard licensirg files zulated to SWR, FTC concluded the SWR setpoint for both ES buses was inadequate. G-3 was in MODE 5 (ODID SHUPD0hH) ct the time ard imediate actions were not r e s w y. A four hour i notification was made to the IRC per 10CFR50.72(b)(2)(1). j l

On October 1, 1989, FPC ocmpleted a conservative calculation of the voltage '

drops and established a new SWR setpoint of 3941 volts. During further review, the design engineer detemined the new setpoint was too high. The 3941 volt setpoint would have caused spurious separation frm the offsite pwer supply if an ES actuation occurred khile power sources were also supplyirg the normal plant loads. Voltage drops were recalculated with more realistic assumptions ard the SWR setpoint was lowered to 3864 volts on October 9,1989.

This cordition is considered to be a cordition outside the plant design basis ard is being reported per 10CFR50.73(a)(2)(ii)(B).

CAUSE:

1he original lw SWR setpoint was caused by a cognitive personnel error. The utility design engineer believod the voltage drop between the Engineered Safeguartis Motor Control Centers (ES MCC's) [ED,MCC) ard their associated loads could be disregartled. ,

The second error was caused by personnel error. The engineering effort was focused on making sure the ord devices would not see a low voltage. Raising the setpoint was a conservative decision, for the lw voltage concem. The ergineers failed to consider that an operatirg event would result in the 4160V buses voltage beirg reduced ard activation of SWRs unrwwwrily. This was discovered while performing voltage drop calculations to support another project.

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l me Crystal Rivar Unit 3 (G-3) Second Invel Urdervoltage Relay system was installed in 1983 as a msult of NRC evaluations of irdustry degraded voltage events.  % e CR-3 S WR system is intended to prevent a minimum voltage at the l end device terminals belw the manufacturers' re----Tded minimum voltage for  !

operating equipnent for greater than 18 seconds. Since the SWR system j monitors the 4160 Volt ES bus phases and not the end device voltage, the '

setpoint must be calculated considerirg the effects of the voltage dmps across  ;

transformers and cables between the 4160 Volt bus and its ultimate loads or erd devices, mus, failure to consider the voltage dmp between the ES HCC's ard I the end devices may allw cperation belw the manufacturer's r~==nded voltage.

Se end devices include loads connected directly to the buses, the Docay lleat l Closed Cycle cooling Punps (BI,P), contml ctmplex Chillers (IN,010) l (urdervoltage limit of 414 volts), ard loads connected to the ES HOC's. Rese l loads include valves, 480V/120V transforners, and their respective loads (typically limited to 102 volts). %e loads on the vital bus inverters I (EE,DWr) would not be affected because these loads are backed by batteries (EJ,IfrRY) . A degraded voltage sensed at the inverters would not cause a degraded voltage to occur at its end devices since the batteries would pick up 4 l

the load via a high voltage auctioneer.

An effect of the incorrect setpoint is that the equipment may not have been protected frun a slightly degraded voltage. his would have occurred if the voltage had degraded belw the new setpoint but above the old setpoint. While ,

it is not Florida Itwer Corporation's intent to cperate in this condition, the '

cffect on the end devices should be minor because the actual difference in the old setpoint ard the required setpcint is small at the 480 volt and 120 volt ,

level. Se final calculation of the voltage dmps betwoon the end devices and  !

the 4160 Volt buses indicates the setpoint should be 3864 volts ocmpared to the i previous setpoint of 3780 volts. his calculation assumod worst case '

corditions and included possible setpoint errors. Using a corrected model for the voltage drops, 3780 volts coaesic-d to 402 volts at the 460 volt motor j

terminals and 97.5 volts at the 120 volt devices. While FPC can not I conclusively state the ES equipnent on the buses would operate at this slightly lower voltage, it is unlikely the Es equipent would fail. Additionally, the operators are provided with an alarm at 3857 volts. If a sustained degraded voltage belw this level occurred, then the operators would take actions to i start and load the diesel generators (EB,DG) as directed by procedures. mus, it is unlikely the previous setpoint would prevent ES equipnent frun operatirg as designed or cause any damage to the ES equipnent. I With the SWR setpoint set too high, spurious actuation of the SWR relays is more likely. We interim 3941 volt setpoint would cause spurious separation from the offsite pwer supply if an ES actuation occurred while pwer sources were also supplyirg the norml plant loads. h is situation existed for a very

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O! 3 l3 0( 0 d4 0F 0 l4 mica . < wo mmmi abort period of time between October 1st aM 9th. Daring this time, ES i actuation was not possible since }Iigh Pressum aM Im Pressure Injection l Systems were bypassed aM the plant was in EDE 4 (for SIRmXW) aM LODE 5. l While in these IODDS, less equipment in operated and loads on the buses are  ;

lower. 'Ihis condition did not degrade plant safety since it was detected and cena:ted before power cperation aM before enabling the autmatic actuation of  ;

ES systems. j CWRBCTIVE ACI'IONt Based on a preliminary calculation by GCI, G-3 temporarily lowered the primry l voltage tap frm 4160 volts to 4055 volts on the A ES transformer effectively j raising the voltage of the secondary side. Upon ocepletion of the calcalations i for the new setpoint, TPC concluded the tap chwye was not necessary. 'Ihe tap l l was then re-established at 4160 volts.

FIC has develcpod a conservative model of the voltage drops betwoon the 4160 Volt ES buses and the 480 volt and 120 volt end devices. Based on this model, G-3 determined the SWR setpoint on both 4160 Volt ES buses should be 3941 volts. 'Ihe setpoint was lowered to 3864 volts on October 9, 1989 after refining some of the conservative assunptions of the calculation.

As a result of the initial error, engineering personnel were counselled on the l need to pay attention to detail and validate assunptions khen performing analyses aM calculations.

FPC is perfoming an evaluation to detemine if the dearadod voltage event described by IIR 89-013 degraded the eM devloes on the 480 volt buses. 'Ihis IIR will be supplemented to include the results of the evaluation. FIC will clso revalidate the major safety related electrical calculations for the ES buses as described in FIC's letter dated October 20, 1989. Upon cmpletion of the Electrical Calculation Enhancenent Program, controls will be established to use this program during review of future nodifications and to maintain the program as a living ca v-nt.

PRFNICKS SIMIIAR fVENIS:

Crystal River Unit 3 has written five IIR's since 1987 describing engineering design errors. IIR 87-007 identified a deficiency in the SWR design khich violated the diesel generator block loading design which was caused by an engineering oversight. IIR 87-019 identified errors in the enginocring assurptions used to calculate diese? generator loads khich contributed to 1cading the emergency diesel generators beyond the 30 minute rating. IIR 87-020 describM hcv an incorrect assurption lead to specifying the wrong limit for the intake canal tanperature in the 'Ibchnical Specifications. IIR 89-013 described an EmenJency Diesel Generator actuation due to Unit 1 & 2 Startup Transfomer overloading. More recently, IIR 89-032 identified that incorrect engineering assumptions had caused CR-3 to exooed the station battery design load profile.

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