ML20024G682

LER 91-007-00:on 910320,480 Volt Circuit Breaker Coordination Outside Design Basis.Caused by Deficiencies in Original Sys Design.Breaker/Fuse Coordination Study to Be Completed & Problems Will Be corrected.W/910419 Ltr

ML20024G682
Person / Time
Site:	Fort Calhoun
Issue date:	04/19/1991
From:	Bannister D, Gates W OMAHA PUBLIC POWER DISTRICT
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
LER-91-007, LER-91-7, LIC-91-0015L, LIC-91-15L, NUDOCS 9104240147
Download: ML20024G682 (6)

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Omaha Public Power District 444 South 16th Street Mali Omaha, Nebraska 68102-2247 402/636-2000 April 19, 1991 LIC-91-0015L U. S. Nuclear Regulatory Commission Attn Document Control Desk Mail Station P1-137  !

Washington, DC 20555

Reference:

Docket No. 50-285 Gentlemen

Subject:

Licensee Event Report 91-07 for the Fort Calhoun Station  !

Please find attached Licensee Event Report 91-07 dated April 19, 1991. This report is being submitted pursuant to 10 CFR 50.73(a)(2)(ii)(B).

If you should have any questions, please contact me.

Sincerely, W.d M W. G. Gates Division Manager Nuclear Operations WGG/ tem Attachment c: R. D. Martin, NRC Regional Administrator W. C. Walker, NRC Project Manager R. P. Mullikin, NRC Senior Resident Inspector INPO Records Center 9104240147 93o439 FDR S ADOCK 050002Mi PDR i 45 5124 Empynent uth Equa: 0;$rrtunity n ..

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During reconstitution of the design basis for the electrical distribution overcurrent tripping scheme, the portion of a Breaker / Fuse Coordination Stud dealing with the 161 kV system down to the 480V Motor Control Centers (MCCs)ywas completed. Twenty-one 480 Volt molded case circuit breakers e re determined to have overlap)ing breaker coordination curves with the MCC fer circuit breakers. Tie lack of coordination could result in the triMng of a MCC due to a fault on one of its non coordinated loads. On March 20 19911 this condition 1 was determined to be outside the design basis of the )lant. Two of the twenty-one loads were determined to have an unacceptable pro) ability of faulting during a Design Basis Accident.

This condition was caused by deficiencies in the original system design as constructed by the plant Architect / Engineer. A contributing cause was the lack of comprehensive design basis documentation to substantiate that coordinated breaker f ault protection existed.

The two affected loads were isolated from the 480V system. Present design procedures require breaker coordination. Completion of the Breaker / Fuse Coordination Study will determine if any more coordination problems exist with the electrical distribution system. Problems identified by the Breaker Fuse Coordination Study will be corrected on a schedule commensurate with th ir safety - signi ficance.

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010 012 0F 0l5 emru . ,4 ,~ - u m .mn FCS) Unit No.1 plant electrical distribution systems The Fort Calhoun are cesigned to meetStation deman (ds for electrical energy for plant control and operation during normal, abnormal, and accident situations.

The 416,. Y system is comprised of four electrically separated buses. Buses lAl and 1A2, which supply only 4160 V loads, are normally connected to unit auxiliary transformers TIAl and TlA2. These transformers are supplied from the Main Generator 22kV/345 kV system. However if the Main Generator 22kV/345 Ky system transformers is unavailable,TlA4, TIA3 and respectively. buses lAltransformers These and 1A2 canare besupplied suppliedftom from the house service the off-site 161 kV su) ply. Buses lA3 and 1A4 (called " Safeguards" buses) are normally connected to louse service transformers TlA3 and TIA4. However, if the off-site 161 kV supp y is unavailable, buses lA3 and 1A4 can be supplied from unit auxiliary trans ormers TIAl and TIA2, respectively. In the event that both the off-site 161 kV and the Main Generator 22 kV/345 kV feeds are lost buses lA3 and 1A4 can be supplied from Diesel Generators D-1 and D-2 respectIvely for emergency power. Buses lA3 and 1A4 supp y all Engineered Safeguards (ESF and essential support systems, either direct y or through the 480 Volt distri ution system.

The 480 Volt system is comprised of nine buses, powered from six 4.16 kV to 480 Volt transformers. Three of these buses are supplied by bus lA3, three are supplied b bus lA4, and three are manually connected to either bus lA3 or lA4, but normal y not both, as this is prevented by interlocked bus-tie circuit breakers. receive power from the nine 480 Volt bus Twenty-two sections. Motor Control The MCCs whichCenters (MCCs)d throughout the plant provide are arrange power to both safety related and non-safety related equipment.

In the event of total failure of supply from the Main Generator 22kV/345kV system and the off-site 161 kV system, both 4.16 kV buses lA3 and 1A4 are disconnected from their normal and alternate supp y sources. Simultaneous load shedding of motor loads connected to 4.16 kV and 80 Volt buses is initiated by undervoltage relays. Diesel generators are started, run up to operating speed and volta e, and connected automatically to the Safeguards buses, if no Design by Basis the operatoiAcc dent (DBA) dance with an Emergency Operating Procedure.

in accor If a D A exists, has reloading of the sys occurred, either a Pressurizer Pressure Low Sicnal (PPLS or Containment Hi h Pressure Signal (CPHS ection Ac uation Signal (SIA ),

which eliminates selec)ted 480 Volt MCCs and certain loads on other MCCswould cause a Safety i supplying loads not needed to mitigate the accident. PPLS or CPHS would also initiate the starting and loading of ESF equipment via automatic load sequencers.

Original design criteria as stated in the Updated Safety Analysis Report (USAR) for the electrical distribution system provide a hi h degree of reliability y using selective fault >rotection using full-rated c rcuit breakers. The fau t of a sin le load on a aus or MCC would be mitigated prior to tripping the bus /MCC' feeder circuit breaker. This is primarily due to the individual circuit breakers on the bus /MCC having lower interrupt current ratings than the bus /MCC's feeder breaker. The lack of coordination could result in the tripping of an entire MCC and loss of all equipment (both safety related and non safety related) on that MCC, due to a fault on one of its non-coordinated loads. In addition to providing reliability, breaker coordination is also credited to meet NRC Perm 384A iM9)

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EEQ for both 1 safety related and non-safety related equipment.10 CFR 50.49 Non-safety relate Environmental Qualificatio which is fed from safety related power sources and located in potential post-accident harsh environment areas is not specifically required to be environmentally qualified. This is allowed due to the fault protection equipment designed to isolate the non-safety related equipment from the safety related power source.

Design Power District Basis OPPD Documents

' Design (DBDs) Basis Reconstitution Program.have An open itembeen existed compiled as p concerning the ack f detailed documentation to substantiate the existence of coordinated relay / breaker fault )rotection. To resolve this issue a study of the entire FCS electrical distri)ution overcurrent tripping scheme was initiated.- This Breaker / Fuse Coordination Stud when completed, will encompass a ll electrical busses at FCS, including 161 kV,y,22 kV 4160 V, 480 V Switchgear,

-480 V MCCs, 120 VAC Instrument, and-125 VDC busses. EachbusorMCCwillhave i coordination curves generated containing upstream overcurrent protection devices '

and the largest load overcurrent protective device plotted. in addition to these devices, a typical fuse distribution panel and the six(or largest fuse) will be plotted for each DC inverters.

AspartofthisstudyInation" Circuit Breaker Coord Engineering Analysis (EA-FC-91-025 was conducted. "fCS 480V The analysis assumed Molded Case unacceptable probability of common mode fa lting during a DBA exists on1 in High Energy Line Break HELB or loss of Coolant Accident OCA)locations. potential harsh Consequently the analys s sco)pe was limited to safety rela ed ower sources which supply power to non-safety related equipment in potentia harsh'HELB LOCA locations. Faulting of equ pment in other locations would be of acceptabl low probabilit _

-scenarios,y during the 100 our short term and 1000 hour0.0116 days <br />0.278 hours <br />0.00165 weeks <br />3.805e-4 months <br /> long term DBA acc dent t

The following critoria were used by 0 PPD to identify potential non-coordinating breaker conditions:

(1) Only safety related power sources were reviewed.

(2) . All equipment that is tripped due to 480 V load shed resulting from SIAS-was not evaluated. SIAS 480 V Load Shed initiates in the initial stages of a LOCA or HELB such that the non safety related equipment which is load-shed is not expected to fault during the DBA.

(3) Equipment which'is normally out of service during normal plant operation was not evaluated.

Twenty-one 480 Volt molded case circuit breakers on eight different MCCs were determined to have overlap)ing breaker coordination curves with the MCC feeder-

. circuit breakers On Marc 1 20, 1991,_this condition was determined to be outside the desi n basis of the lant and therefore reportable pursuant to 10 CFR 50.72(b)(1 ( i)(B) and 50.73 a)(2)(ii)(B). The plant was operating in Mode 1 at 70 percen power.

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01010l 4 or 0l5 im u . - m au.m The loads associated with nineteen of the non-coordinated breakers can be classified as one of the following types:

(1) Loads which are normally not in service.

(2) Rotating loads which are provided with thermal overload protection.

(3) Loads which feed sub-distribution panels with fused or breaker protected distribution circuits.

(4) Loads which feed static equipment.

While these loads are outside the design basis for the 480 Volt system as described in the USAR they are, however, located in DBA mild environments.

Based on engineering judgment and past plant experience, the possibility of fault on this equipment is small and not expected to occur in conjunction with a DBA. Additionally, this type of failure is bounded by worst case single failure analyses.

Based upon the assumptions made in EA FC-91-025 only two loads with uncoordinatedloadbreakerswereidentifiedaslocatedinDBAharsh environments. The two loads are welding machine outlets located in Room 81 (MCC-4A1, breaker E06) and Containment (MCC-4B1, breaker A4L).

Further analysis was conducted to determine the effect loss of MCC 4A1 or MCC-4B1 would have during either a LOCA or HELB. It was determined that safe shutdown of the reactor could still be achieved with the loss of either MCC. To eliminate this source of potential faulting, EA-FC-91-025 requested that both welding outlet breakers on the affected MCCs be danger tagged open. Plant personnel performed this on March 11, 1991 prior to the formal issuance of the analysis.

Safety Analysis for Operability (SA0) 91-03-00 was approved on March 25, 1991 which required: (1) danger tagg ng 03en or disconnecting the two non-coordinating welding recept cle if either breaker had to be re-closed then areakers the MCC on MCC's was to 4A1 and 4B1,inopera be declared and (2) ble.

In the latter case, Technical Specification Limiting Condition for Operation 2.7 (Electrical Equipment) would be applied which requires the MCC be returned to operable status within eight hours. SA0 91-03-00 also concluded that the lack of optimum breaker coordination for the other loads did not present safety considerations outside the bounds of existing accident analyses. The individual local protection of the sub-distribution panels and the installed thermal overload devices in the identified motor circuits provide an adequate level of protection against potential MCC failures resulting from this breaker coordination overlap. This satisfactorily assures continued safe operation under both normal and accident conditions.

This condition was caused by deficiencies in the original system design as constructed by the plant Architect / Engineer. A contributing cause was the lack of comprehensive design basis documentation to substantiate that coordinated breaker fault protection existed. This lack of documentation resulted in one modification, which installed inverter bypass transformer EE-4S, repeating the non-coordination error by copying what the original plant design had done.

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0l0 015 of 0l5 1ma -. w - *.cw w.mn The following corrective actions have been completed:

(1) SAO-91-03-00 was issued to justify continued operation provided the affected welding receptacle breakers in Room 81 and Containment were tagged open or disconnected.

(2) The Containment welding receptacle breaker (MCC-4Bl A4L) was danger tagged open on March 11, 1991 and has remained in that configuration.

(3) was danger The taggedRoom open81onwelding receptacle March 11, 1991. A breaker Temporary (MCC-4Al-E06)

Modifica tion (TM-91-12 was performed to disconnect power from the receptacle by removing t from the circuit. This was accomplished on March 28, 1991.

(4) Production Engineering Division modification review procedure gel-3 requires that a Breaker / Fuse coordination review be completed (per gel 9) any time new loads are added or modified.

The following corrective actions will be implemented:

(1) The Breaker / Fuse Coordination Study is scheduled to be completed by January 31, 1992. However, if any field data is required the Study completion date will be April 1, 1992. Results of the Study will determine if any more coordination problems exist with the electrical distribution system.

(2) Problems identified by the Breaker / Fuse Coordination Study will be corrected on a schedule commensurate with their safety si nificance. The schedule for resolution of these 3roblems will be provide to NRC within 90 days following completion of tie Breaker / Fuse Coordination Study.

Other Licensee Event Reports which have been submitted addressing design deficiencies are LERs 91-03, 91-04, 90 03, 90-05, 90-07, 90-09, 90-16, 90-20, 90 23, 90-25, 89 09, 89-14, 89-15, 89-24, 88-09, 88-19, 88-20, 88 32, and 88-33.

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