ML19340D868

2 Electric Distribution Sys Voltages.

ML19340D868
Person / Time
Site:	Millstone
Issue date:	12/31/1980
From:	NORTHEAST UTILITIES
To:
Shared Package
ML19340D861	List: ML19340D860 ML19340D868
References
TAC-12672, NUDOCS 8101050476
Download: ML19340D868 (21)

Text

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O Docket No. 50-336 ATTACHMENT Millstone Nuclear Power Station, Unit No. 2 Station Electric Distribution System Voltages December, 1980 Bio 105 0 4%

JTK-c27 Ref. 1: NRC letter (W. Gammill) to all Power Reactor Licensees, dated August 8, 1979.

Ref. 2: Attachment 3 of Northeast Utilities letter (W. G. Counsil) to NRC (D. L. Ziemann), dated November 15, 1979.

Ref. 3: W. G. Counsil letter to D. M. Crutchfield dated November 14, 1980.

Ref. 4: W. G. Counsil letter to R. A. Clark dated August 22, 1980.

Item 1 Guidelines 1 and 7 (Ref. 1) require that a separate analysis be performed for all available connections to the offsite network and ' hat the analysis be adequately documented for each condition analyzed. Ref. 2 does not fully meet these requirements. To confirm the acceptability of the voltage conditions on the station electric distribution system, submit adequate voltage analysis documentation for each case and condition in Ref. 2 and additional documentation, specifically:

a. Requirements of Guidelines 6 and il as well as 5 and 13 (Ref.1) must be included in each separate case analyzed. These guidelines t

refer to the use of minimum and maximum expected grid voltages, maximum loads assumed for each analyzed case and a list of assumptions made for each analyzed case.

b. Supply the calculated voltages for all low voltage AC (less than 480 volts) Class lE buses (including all available source connections) for each analyzed case. Do these buses supply instrumentation or con-trol circuits as required by GDC 13? If so, is all equipment capable of sustaining the analyzed voltages without blowing fuses, overheating, anj without affecting the equipment's ability to perform the required function?

c. Per Guidelines 3 and 9 (Ref.1), compare the effect of starting and running the largest non-Class 1E load on all Class 1E buses and loads with the required voltage range for normal operation of all Class 1E equipment (starters, contactors, motors, etc.) for each available connection of offsite power. This comparison should occur after the Class 1E buses are fully loaded.

d. Ref. 2, Page 3 Item 2 identifies a viable offsite source connection to the Class 1E buses by backfeeding from the 345 KV switchyard through the main transformer and transfonner NSST-2. A complete analysis is required or identify limiting conditions of operation.

e. From the sketches of the auxiliary buses submitted in Ref. 2, there appears a possible offsite source connection to the Class 1E buses from transformer RSST-1. An analysis is required for this source l

connection unless interlocks prevent the connection or limiting conditions of operation are identified.

, f. Submit a voltage analysis which neets Guideline 2 (Ref.1); that is, Unit #2 is experiencing an accident or anticipated transient l

with the simultaneous shutdown of Unit #1 for all available source conditions.

Response 1 l

The attached data sheets summarize the cases analyzed in the voltage studies for Millstone Unit No. 2. This data completes the information requested by the NRC in the June 25, 1980 memo from T. M. Novak to W. G. Counsil. It should be noted that the response to Question 3 of i -

2

Reference (4)isbeingrevisedduetoachangeinloading. The response in Reference (4) identified the minimum and normal loading on the 480-volt busesincases(a)and(c)respectively. In completing the response to item 1, it was observed that the minimum bus loading for bus 22F was higher than the nonnal bus loading. To correct this, the minimum bus loadings have been revised, resulting in changes to the calculated 480-volt bus voltage.

a. The data sheets include the minimum and maximum expected grid voltages, mode of operation, and the assumptions made for each case in the analysis.

The minimum expected grid voltages used in the analysis are based on the system limit of -5% except for those cases where Unit 2 is beiiig supplied by the Unit 1 Reserve Station Service Transformer.

Although the present switchyard limit for Unit 1 is 345 KV, these cases use a lower limit of 341 KV in anticipation of corrective measures to be taken by Unit 1 (this problem is discussed in the response to item 1.b. of Reference 3). The maximum expected 345 KV grid voltages are based on the system limit of +5%.

In Reference 4, the response to item 3 discusses a change to the l

l no-load tap setting of NSST-2 to help eliminate overvoltages.

l However, a review of the gene.ator voltages dating from May,1978 l

to the present has revealed that the generator only rarely, and l

i l

1 for short periods of time, reaches 24.4 KY. During the review

. period the generator voltage reached this level only once (8/6/79) for a period of 8 minutes. We have therefore selected 24.4 KV as the maximum generator terminal voltage. The resulting auxiliary bus system voltages are included in the revision to the item 3 response of Reference (4).

b. All 120 VAC buses carrying Class lE equipment is supplied by inverters and regulating type transformers. Therefore, these circuits are capable of perfoming their required functions throughout the range of voltages identified in the data sheets.

c. Cases 7, 8, and 10 of the data sheets indicate the voltages which occur during the starting of the largest non-Class lE loads while the emergency buses are fully loaded. A reactor coolant pump and a service water pump were both used for this analysis (Cases 7 and

8) because they are supplied from different windings of the station service transformers. Once each of these pumps has started, the voltages return to the levels identified in Case 6 or 11 (depending on loading conditions for the motor-start case). The ten second delay in operation of the level two scheme is sufficient to allow these loads to start and for the bus voltage to be restored without inadvertent transfer to the emergency onsite power supplies.

For comparison purposes, the following voltage levels should be used as minimum bus voltages for either running or starting loads. These levels are adjusted for the cable drops listed under " assumptions" on the data sheets.

Nominal Voltage Minimum Running Voltage Minimum Starting Voltage 6900 Volt 5960 Volt 5380 Volt 4160 Volt 3620 Volt 2900 Volt 480 Volt 424 Volt 395 Volt *

• Voltages depressed due to starting of 4.16 KV loads will recover to assure adequate voltages for operation of 480-volt loads.

d. The connection from the 345 KY switchyard through the main transformer and NSST-2 to the Class lE buses is in service for two conditions:

1) When the plant is shut down and the generator links are out; and

2) When the unit .is on line with no in-plant transients which would trip the unit.

All accident scenario cases trip the unit and transfer loads to the RSST-2. The two conditions mentioned above are detailed in Cases 1 and 2 of the data sheets.

e. Transformer SDT-1 is used as the alternate offsite supply for Unit 1 and would be used only in t h'.t :rede. It does not have the capacity to serve as the Va! ' ab +rnate and carry Unit 2 loads as well. Because of the c4:pacitj : imitations on this source, the connection from SDT-1 to the Unit 2 Class lE buses is not feasible (use of such a connection is limited by procedure; no interlocks are provided).

f. The only source connection for which such an inter-related went can occur is when RSST-1 is carrying its own in-plant loads while also carrying the Unit 2 accident loads. The data for this event is contained in Cases 9,10,11, and 12. These cases use normal full load for Unit 1 rather than just shutdown loads.

Item 3 Ref. 2, Page 4 identifies three separate conditions when the +10% over-voltage capability of the motors is exceeded on the 480V, 4160V, 6.9 KV buses. Installation of overvoltage alarms will be added to initiate operator corrective action. Credit will be given for this corrective action only if the overvoltage monitors and alarms are Class 1E, and in the interim period of correction the overvoltage condition does not shorten equipment life or affect the Class lE equipment's ability to perform the required function. Provide documentation which demonstrates the equipment can meet these overvoltage conditions. Also, provide the calculated over-voltages on all Class 1E equipment for each case analyzed.

Response 3 - Revised The three cases identified which caused voltages to exceed the +10%

overvoltage limit were:

a. The unit is shut down and the NSST-2, supplied by the main generator step-up transformer is carrying minimum station auxiliary load, and system voltage is high.

. --- .~_ . . _ - -

9

b. The unit is fully loaded and the NSST-2 is carrying normal station auxiliary load, and generator voltage is high. In this case, the maximum voltage is determined by the main generator voltage rather than the system voltage.

c. The unit is shut down, RSST-2 is carrying minimum station auxiliary load, and system voltage is high. The data for these loading conditions is contained in Cases 1, 2, and 3 of the data sheets.

The maximum voltages acceptable for each bus are:

Nominal Voltage Maximum Voltage 6900 Volt 7280 Volt 4160 Volt 4420 Volt 480 Volt 516 Volt The overvoltages experienced in each case are as follows:

l

a. The 4160 and 6900-volt buses do not reach their limits. The 480-volt bus voltage exceeds the limit by 0.4%.

t l b. Using the 24.4 KV limit for generator voltage, as discussed in Item 1.a. above, no overvoltages result from this connection.

c. The 4160-volt buses do not reach their limit. The 6900-volt buses exceed their limit by 1.2%. However, no loads are connected to these buses. The 480-volt buses exceed their limit by as much as 2.6%.

It is unlikely that the plant load would ever be this low, and also unlikely that the switchyard voltage would be as high as 362 KV (especially with a unitoff-line). These effects would reduce, if not eliminate, the over-

ltages. Because the overvoltages are small and would be experienced during a small percentage of motor life, we conclude that the effect of these small overvoltages on the life of the motors is negligible.

The overvoltage monitors to be installed will be Class lE equipment which will alarm through non-Class 1E annunciators.

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