ML19319C033

Responds to NRC 761001 Ltr Re Equipment Failures During Degraded Grid Voltage Condition at Millstone 2.Forwards Results of Investigation of Effects of Grid Voltage on Davis-Besse

ML19319C033
Person / Time
Site:	Millstone, Davis Besse
Issue date:	11/03/1976
From:	Roe L TOLEDO EDISON CO.
To:	Stolz J Office of Nuclear Reactor Regulation
References
NUDOCS 8001300763
Download: ML19319C033 (8)

Text

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• '"""I' NRC Dl,STRIBUTION FOn PART 50 DOCKET MATERIAL FROM: TOLEDO EDISON DATE OF DOCUMENT TO: J.F. STOLZ TOLEDO, CHIO 11-3-76 L.E. ROE oATE RECEivEo 11-10-76 O NOTO RIZ E D PROP INPUT FORM NUMBER OF COPIES RECEIVEC OLETTER (E C RIGiN AL {]UNC LASSIPIE D OCOPY 1 DEScrlPTION ENCLOSU RE LTR. RE. OUR 10-1-76 LTR.., TRANS THE FOLLOW- RESULTS OF INVESTIGATION OF THE'JEFFECTS OF GRI:I ING.... VOLTAGE W...

( 1 SIGNED CY. RECEIVED)

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TOLEDO EDISON November 3, 1976 LOWELL E. ROE v.c. m..o.at e.c,\,s.., c., mens Serial No. 144 f ' m '93 mei ass.s 4a v ss w

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Mr. John F. Stol.: a NC/lO jggy 3 Chief Light Water Reactors 9/ na C u,s.eg.;,.;7.d.

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United States Nuclear Regulatory Commission Washington, D.C. 20555  :,p

Dear Mr. Stols:

Your letter to us dated October 1,1976, described equipment failures during a degraded grid voltage condition at Millstone, Unit 2. We have analyzed per the request for information in your Enclosure 2 the effect of various grid voltage conditions on Davis-Besse Unit No. 1. A summary of this investigation is included in Attachment 1 of this letter.

Yours very truly, pc 33a/1 Attachment 1 - Results of investigation of effects of grid voltage on Davis-Besse Unit No. 1 Yj q , . - ^

THE TOLEDO EDISON COMPANY EDISON PLAZA 300 MACISON AVENUE TOLEDO. CHIO 43652

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ATTAC19EENT 1 - RESULTS OF INVESTIGATION OF EFFECTS

- 0F GRID VOLTAGE ON DAVIS-BESSE UNIT NO.1 h,h Pj N T $ ' . Y. .

Describe the plant conditions under which the plant auxiliary systems (safety related and non-safety related) will be supplied by offsite power. Include an estimate of the fraction of normal plant operating time in which this is the case.

Response: Normal occurrences for which plant auxiliary systems are supplied from offsite power include plant startup and shutdown. Presently the one month refueling opera-tion per year is the only planned occurrence.

Abnormal occurrences.for which plant auxiliary systems are cupplied from offsite power include faults associated with the auxiliary transformer. No time estimate is available for abnormal occurrences.

Item Ib The voltage used to describe the grid distribution system is usually a " nominal" value. Define the normal operating range of your grid system voltage and the corresponding voltage values at the safety re-lated buses.

Response: Tha normal operating range for the grid system voltage and the corresponding voltage values at the safety re-lated buses are given below:

Minimum Voltage Maximum Voltage 345 KV Grid 98.3% 102.2%

4.16 KV 96.6% 101.1%

480 V 94.7% 99.3%

Item Ic The transformers utilized in power systems for providing the required voltage at the various system distribution levels are normally provided with taps to allow voltage adjustment. Provide the results of an analysis of your design to determine if the voltage profiles at the safety related buses are satisfactory for the full load and no load conditions on the system and the range of grid voltage.

Response: Voltage profiles are listed below for the full load and no load conditions.

System Voltage 98.3% 102.2% 98.3% 102.2%

Bus Full Load No Load 4.16 KV 96.6% 101.1% 102.8% 106.9%

480 V. 94.7% 99.5% 105.0% 109.3%

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• At no load and 102.2% voltage of the grid, the calculated bus voltages

-are above the allowable voltages for some of the equipment-as listed in item ig. Therefore, when the unit is shut down, administrative pro-cedures will be taken to lower the, voltages to within acceptable limits.

Item Id Assuming the facility auxiliary loads are being carried by the station generator, provide the voltage profiles at the safety buses for grid voltage at the normal maximum value, the normal minimum value, and at the degraded conditions (high or low voltage, current, etc.) which would

, require generator trip.

Response: When Davis-Besse Unit No. I auxiliary loads are carried by the station generator, the voltage profiles at the safety buses are as follows:

, Minimum Voltage Maximum Voltage 345 KV Grid 101.7% 102.1%

4.16 KV 100.0% 97.1%

480 V 98.5% 95.3%

At degraded conditions below the minimum voltages listed above, system load shedding of 10% will occur at 59.3 Hz and 15% at 58.9 nl. If degradation continues, i.e., 58.2 Hz for 30 cycles, the generator breaker is tripped, the grid load is disconnected and the generator supplies the station auxiliaries at the generator nominal voltage.

On overload conditions, the system can operate with the generator voltage down to 95% continuously, i.e., 23.75KV. The calculated corresponding voltages on the 4.16KV and 480V safety related buses are 92.8% and 90.7%, respectively.

Item le Identify the sensor location and provide the trip setpoint for your facility's loss of Offsite Power (undervoltage trip) instrumentation.

Include the basis for your trip setpoint selection.

Response: Undervoltage relays are located at the safety related 4.16KV buses with a setting'of 59% of rated voltage.

This setting is predicated on total Loss of Offsite Power, but not on a sustained low voltage conditions.

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Item if Assuming operation on offsite power and degradation of the grid system voltage, provide the voltage values at the safety related bus,s corres-ponding to the maximum value'of grid voltage and the degraded grid voltage corresponding to the undervoltage trip setpoint.

Response: The voltage values at the safety buses corresponding to the maximum value of grid voltage are 101.1% on the 4.16KV and 99.5% on the 480V. The calculated grid voltage corresponding to the,undervoltage trip setpoint is 67%.

Question lg Utilizing the safety related bus voltage values identified in (f),

evaluate the capability of all safety related loads, including related control circuitry and instrumentation, to perform their safety functions.

Include a definition of the voltage range over which the safety related components, and non-safety components, can operate continuously in the farformance of their design function.

Response: The voltage range over which safety and non-safety related equipment can operate continuously in the performance of their design function is tabulated below:

I Equipment Rating Non-safety related motors 90% -

110%

Safety related motors

• 90% -

110%

! Non-safety related 13.8KV Switchgear 83% -

109%

Non-safety related 4.16KV Switchgear >'3% -

114%

Safety.related 4.16KV Switchgear 93% -

114%

Safety and-Non-safety 480V Switchgear maximum of 106%

120V Contactors 80% -

110%

120V Relays 95% -

110%

• Safety related motors are designed to start at 70% voltage and to provide full load torque during a momentary dip to 65% of rated motor nameplate. voltage at the motor terminals for 15 seconds.

Item lh Describe the bus voltage monitoring and abnormal voltage alarms available in the control room.

Response: Voltmeters 'qc.ated in the Control Room provide the operators with the voltage monitoring of the 345 KV, 13.8KV, 4.16KV and 480V buses.

In addition, undervoltage conditions on the above buses are detected by undervoltage relays and alarmed on the annunciator and computer. Analog voltage signals from  !

the two 345 KV buses and the two essential 4.16 KV buses l are' logged hourly. l l

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,EItem 2 The functional safety requirement of the undervoltage trip is to detect the loss of offsite (preferred) power system voltage and initiate the necessary actions required to transfer-safety related buses to the 4

onsite power system. Describe the load shedding feature of your design

. (required ' prior to transferring to the onsite (diesel generator) systems) and the capability of the onsite system to perform their function if the load shedding feature is maintained af ter the diesel generators are connected to their respective safety buses. Describe the bases (if any) for retention or reinstatement of the load shedding furetion'after the diesel generators are coanected to their respective buses.

Response: If loss of power is confirmed by loss ot voltage at the safety related 4.16KV bus, the following occurs:

1. All bus load breakers, except the breakers supplying power to the 480V safety related switchgear and component cooling pumps, are tripped.

2. Both source breakers are tripped and the bus is isolated.

3. The emergency diesel generator associated with the deenergized bus starts, the generator breaker closes, and the service water pump utarts after a time delay.

This operation occurs anytime an undervoltage condition exists on the bus. However, analysis has shown that the lowest voltage occurring on the bus during sequential

. loading of the diesel generator is about 78%. Therefore, since 59% voltage is required to initiate in-plant load shedding, load shedding does not interfere with the operation of the onsite system.

4 Item 3 Define the facility operating limits (real and reactive power, voltage, frequency and other) established by the grid stability analyses cited in the FSAR. Describe the operating procedures or other provisions presently in effect for assuring that your facility is being operated within these limits.

Response: The stability analysis cited in the FSAR for Davis-Besse Unit #1 determined an unstable condition resulting from a fault that left the station tied to the system via only the Ohio Edison line. This line was one of three normally available. One result of this study was the

! addition of a fifth breaker to the switchyard in such a

, way. that no single electrical fault lef t the station connected to just the Ohio Edison line.

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, Item 2 The functional safety requirement of the undervoltage trip is to detect tha loss of offsite (preferred) power system voltage and initiate the necessary actions required to transfer safety related buses to the onsite power system. Describe the load shedding feature of your design (required prior to transferring to the onsite (diesel generator) aystems) d and~ the capability of the onsite system to perform their function if the U load shedding feature is maintained af ter the diesel generators are

!! connected to their respective safety buses. Describe the bases (if any)

! for retention or reinstatement of the load shedding function after the l diesel generators are connected to their respective buses.

Lesponse: If loss of power is confirmed by loss of voltage at the safety related 4.16KV bus, the following occurs:

1. All bus load breakers, except the breakers supplying

power to the 480V safety related switchgear and component cooling pumps, are tripped.

2. Both source breakers are tripped and the bus is isolated.

3. The emergency diesel generator associated with the deenergized bus starts, the generator breaker closes, and the service water pump starts af ter a time delay.

This operation occurs ar,cime an undervoltage condition exists on the bus. Eowever, analysis has shown that the lowest voltage occurring on the bus during sequential loading of the diesel generator is about 78%. Therefore, since 597. voltage is required to initiate in-plant load shedding, load shedding does not interfere with the operation of the onsite system.

Item 3

! Define the facility operating limits (real and reactive power, voltage, frequency and other) established by the grid stability analyses cited in the FSAR. Describe the operating procedures or other provisions presently ,

in effect for assuring that your facility is being operated within these l

!! limits.

Response: The stability analysis cited in the FSAR for Davis-1 Besse Unit #1 determined an unstable condition resulting fran a fault that lef t the station tied to the system via

); only the Ohio Edison line. Ihis line was one of three normally available. One result of.this study was the addition of a fif th breaker to the switchyard in such a way that no single electrical fault left the station i- connected to just the Ohio Edison line.

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• 9 There are presently no transient stability considerations which restrict real and reactive power, voltage, and frequency of the unit beyond the limitations set by the manufacturer. Manufacturer's limitations are appro-priately identif'04 in station operating procedures.

Item 4 Provide a description of any proposed actions or' modifications to your facility based on the results of the analyses performed in response to items 1-3 above.

Response: We propose to extend the protective system to cover the conditions established by our response to items 1-3.

The feasibility of extending the protection is now under in-depth study. We will submit the final proposals no later than December 31, 1976.

pc 33a/2-6

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