View source for BECO-91-102, Submits Justification for Continued Operation to Address NRC Concerns Identified w/long-term Plan & During NRC Review While Preparing for Electrical Distribution Sys Functional Insp

{{Adams
| number = ML20082G258
| issue date = 08/09/1991
| title = Submits Justification for Continued Operation to Address NRC Concerns Identified w/long-term Plan & During NRC Review While Preparing for Electrical Distribution Sys Functional Insp
| author name = Davis G
| author affiliation = BOSTON EDISON CO.
| addressee name = Martin T
| addressee affiliation = NRC OFFICE OF INSPECTION &amp; ENFORCEMENT (IE REGION I)
| docket = 05000293
| license number = 
| contact person = 
| document report number = BECO-91-102, NUDOCS 9108160275
| document type = CORRESPONDENCE-LETTERS, INCOMING CORRESPONDENCE
| page count = 5
}}

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4 BOSTON EDISON Pilgrim Nucicer Power Station Rocky HiH Flood Plymouth, Massachusetts o2300 George W. Davis                                                        August 09, 1991 Senor Vice President - Ndes'                                            BECo 91-102 Mr. Thomas T. Martin Regional Administrative, Region i U. S. Nuclear Regulatory Commission 475 Allendale Road King of Prussia, PA 19406 License DPR-35 Docket 50-293 Degraded Grid Voltage Concerns and Justification for Continued Operation

==Dear Sir:==

This-letter discusses concerns identified during degraded voltage analysis being conducted in accordance with our Long Term Plan and during reviews while
                        -preparing for an Electrical Distribution System Functional Inspection.               The concerns and our assessment and corrective measures are discussed below.
                                                                                ~
Statement of Concerns                                                                         j
                        'A Boston Edison Company (BECo) Nuclear Engineering Department design review of                i the degraded grid voltage calculations indicated that under certain statinn                   [
operating conditions sufficient voltage may not be-available to selected loads                ;
at the 480V;and 120V levels given the existing degraded voltage relay setpoints-         .    ,
i Similarly, when swing Bus-86 is being fed from-the alternate power supply Bus                 ,
                        -B2 (back-up alignment) and the grid voltage is between 340KV and 349KV-, the                  !
potential exists that the-~ voltage at 480V HCCs B14, B18 and S20 and 120V                    '
distribution panels Y4 and Y41 would be below the manufact'Jrer's minimum voltage specification required-to ensure proper operation mf some associated                  j safety-related loads'.
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Plan' of Action                                                                              .j We have evaluated system conditions and concluded the existing system with
: established administrative controls provides assurance the_ electrical system                (
will perform its safety function. Our intent is to operate in this                            e configuration until the next planned outage of sufficient duration to implement corrective action but not later than Refueling Outage 9. During this outage,                  .
                        -the equipment will be upgraded.                                                               !
A     K             b                                                            l
                                                                                                                       +

Justification for Continued Opfdlon A detailed evaluation of the postulated degraded grid conditions and                                        ;
Justification for Continued Operation is provided in Attachment 1. The compensatory measures identified in the following paragraph will ensure safety-related devices will perform their safety function in the unlikely event of a degraded grid condition.
In accordance with an established agreement, the regional Power Authority (REHVEC) will notify the plant operators whenever grid voltage could drop below 340KV in the event of a PNPS unit trip. A Standing Operating Order directs the operators to refer to PNPS Procedure 2.4.144, " Degraded Voltage" and start and load the emergency diesel generators onto the emergency Buses A5 and A6 to ensure adequate voltage to the downstream safety related 480V buses and 120V                                '
distribution panels.
When the electrical distribution system is aligned with Bus 82 supplying Bus B6, the operators are directed by Standing Order to inform REHVEC that PNPS is in an abnormal electrical line-up and request PNPS be notified if switchyard voltage cannot be maintained above 349KV during this abnormal line-up. If so notified, PNPS operators are directed by Standing Order to enter PNPS Prc edure 2.4.144 and start the "B" Emergency Diesel Generator and connect it to Bus A6 to ensure adequate voltage is available for safety-related equipment in the event of a turbine generator trip.
The plant Operations Review Committee has reviewed and the Plant Manager has approved the information contained in Attachment 1.
Sianificant Hazards Consideration Boston Edison evaluated operation of the facility with the compensatory                                     >
measures described in the Justification for Continued Operation (Attachment 1) and concluded that operation under these conditions will not involve a significant increase in the probability or consequences of an accident previously evaluated and will not create the possibility of a new or different kind of accident from any accident previously evaluated. The proposed compensatory actions increase the reliability of the electrical distribution system and do not require modifications of plant equipment. Compensatory actions involve the use of previously approved procedures.
In addition, operation of the facility with the compensatory measures described in Attachment 1 does not involve a significant reduction in the margin of safety. As discussed in Attachment 1, a turbine generator trip coincide adegradedgridconditionisanextremelylowprobabilityevent(3X10gtwith           or                               ,
less). The resulting reduction in the margin of safety from this very unlikely event in terms of increased probabilistic risk is, therefore, also extremely small. Consequently, the reduction in the margin of safety is not significant.
For these reasons, Boston Edison concludes operation of the plant until the next outage of sufficient duration (but not later than RF0 9) with the compensatory measures described in Attachment 1 does not involve a significant hazards consideration.
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Conelcsion Based on the engineering evaluations of the plant design and compensatory measures outlined above and described in Attachment 1, ve believe reasonable assurance of equipment operability is provided and an acceptable basis for plant startup and operation is established.
This information is provided for your review and concurrence. Please do not hesitate to contact me if you have any questions regarding this matter.
G. . Davis cc:     U. S. Nuclear Regulatory Commission Attn: Document Control Desk Hashington, D.C. 20555 Hr. R. Eaton, Project Manager Division of Reactor Projects - I/II Office of Nuclear Reactor Regulation Mail Stop: 1401 U. S. Nuclear Regulatory Commission 1 White Flint North 11555 Rockville Pike Rockville, MD 20852 P
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e                                          Attachment 1
                               . Justification for Continued Operation Conclusion Safe operation of safety-related devices is assured during low grid voltage
       . conditions-based _on operation of the Bus A5 and A6 degraded voltage. trip and alarm relays = and the compensatory measures defined below. These measures are intended to maintain voltage on safety-related buses until stable grid voltage is restored.
Basis The. approach taken was to analyze the potential for degraded voltage conditions to develop such that operators would have to take action to protect the safety related power supplies downstream of Buses A5 and A6. -The conditions evaluated were 1) a PNPS unit trip causing degraded grid voltage, 2) multiple unit failures causing-degraded grid voltage, and 3) PNPS trip combined with an auto transfer of swing bus 86 feed from Bl to B2. These conditions were evaluated for the potential to cause a situation where voltage is not sufficient to ensure proper operation of all safety-related equipment.
An analysis of the 345KV grid reliability was conducted and it was concluded the minimum grid voltage of 340KV could be maintained without shedding of grid
       -loads. Two modes of grid degradation were analyzed; a slowly developing grid degradation and a rapid grid degradation due to nearly simultaneous (i.e. less than one-half hour) tripping of multiple generr. ting stations on the grid. The number of units.that would have to be taken s'multaneously off-line (assuming PNPS to be the last unit)-to depress the grid voltage below 340KV and improvements made to the distribution system ie the-southeastern region of Massachusetts results in a probability of slow grid degradation (on the order of 3 X 10-8). - The probability of a rapid degradation from multiple generation trips is approximately.the same.
In the event that REMVEC has determined the voltage would drop below 340KV after a PNPS unit trip,- plant operators will be notified _by REMVEC that the conditions exist. A Standing Operating Order then directs the operators to refer to PNPS Procedure 2.4.144, " Degraded Voltage'' and start and load the emergency diesel generators onto the emergency Buses A5 and A6 to ensure sufficient voltage to the downstream safety related buses.
In addition, if 480V Bus B2 (alternate supply) is feeding the Swing Bus B6 (an abnormal alignment) the calculation shows there may. not be sufficient voltage for safety-related equipment when grid voltage is less than 349KV. When the plant is aligned with Bus B2 supplying Bus B6, the operators are directed by Standing Operating Order to inform REMVEC that PNPS is in a abnormal-electrical line-up and request that PNPS be notified if switchyard voltage cannot be maintained greater than 34GV if PNPS were to trip. If so notified, PNPS operators are directed to enter PNPS Procedure 2.4.144 and start the "B" diesel generator and load it onto Bus A6 to ensure adequate voltage is available for safety-related equipment in the event of'a turbine trip. The probabilit this situation occurring has been determined to be-approximately 2x10-8.y of Page 1 of 2

  - .                                                                      Attachment 1 Justification for Continued Operation (Con't)
The purpose of starting and loading the diesel generators onto Buses AS and A6 is to increase the reliability of the electrical system during degraded voltage conditions. The emergency diesels will maintain the 4KV bus voltage at approximately 4160 volts which is well above the 4000 volts reautred to protect the 480/120 volt safety-related loads under worst caso load conditions.
Comnensatory Measures / Conditions Reagirgd In the event that REHVEC notifies PNPS operators that switchyard voltage may not be maintained greater than or equal to 340KV following a postulated trip of PHPS, the operators are directed to start the emergency diesel generators and load them to emergency Buses A5 and A6 in accordance with PNPS Procedure 2.4.144. When REHVEC notifies the station that system voltage can be maintained above 340KV following a postulated trip to PNPS, then the normal 4KV system line-up can be restored.
Any time when the plant is on line and 480V Bus B6 is powered from But B2, REHVEC will be notified that PNPS is in an abnormal electrical line-up. REHVEC is reauired to notify PHPS when system conditions change such that the switchyard voltage might drop below 349KV if PNPS were to trip off line.
Operators are then directed to start and load the "B" emergency diesel generator onto Bus A6 to ensure proper bus voltage. The bus shall be powered by the diese'. generator until:
(a) the normal supply for B6 (B1) is restored, or (b) the plant is in cold shutdown, or (c) REHVEC advises PNPS that switchyard voltage will not fall below 349KV.
Once the normal supply (B1) is restored, REMVEC will be notified that PNPS is no longer in an abnormal electrical line-up.
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