ML20206J618

Submits Addl Info Re Auxiliary Power Sys Voltage Study for Facility,Per NRC 861224 Request

ML20206J618
Person / Time
Site:	Yankee Rowe
Issue date:	04/10/1987
From:	Papanic G YANKEE ATOMIC ELECTRIC CO.
To:	Mckenna E NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM), Office of Nuclear Reactor Regulation
References
FYR-87-037, FYR-87-37, NUDOCS 8704160136
Download: ML20206J618 (6)

Text

Teuphone (t I7) 872-81CD TWX 710380 7619 YANKEE ATOMIC ELECTRIC COMPANY

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1671 Worcester Road. Framingham, Massachusetts 01701

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April 10, 1987 FYR 87-037 United States Nuclear Regulatory Commission Document Control Desk Washington, DC 20555 Attention: Eileen M. McKenna PWR Project Directorate No.1 Division of PWR Licensing - A

References:

(a) License No. DPR-3 (Docket No. 50-29)

(b) USNRC Letter to YAEC, NYR 86-286, dated December 24, 1986 (c) YAEC Letter to USNRC, FYR 86-059, dated June 19, 1986 (d) USNRC Letter to YAEC, dated June 27, 1986 (e) USNRC Letter to YAEC, dated July 13, 1986

Subject:

Request for Additional Information on Auxiliary Power System Voltage Study

Dear Ms. McKenna:

As requested in Reference (b), we are providing additional information regarding the Auxiliary Power System Voltage Study for the Yankee Nuclear Power Station.

The four items below correspond to the questions of Reference (b).

1. Question The previous Auxiliary Power System Voltage Study, dated February 27, 1980, demonstrated the adequacy of the Station Electric Distribution System by using an analytical model and computer program. To obtain further confidence, the results of the study were verified by an actual test to assure that the computer analysis was valid. Our review of the new study performed to resolve concerns related to the adequacy of the Plant Distribution System concludes that those issues should have been addressed and resolved in the previous study. The licensee should provide the basis that the new study assurer the adequacy of the Plant Distribution System voltage this time.

8704160136 670410 PDR P

ADOCK 05000029 PDR ([>\

T' I I O k

Unitsd States Nucisgr Rigulstory Commisaicn April 10, 1987 Attention: Eileen M. McKenna Page 2 FYR 87-037 4

Response

4 At.the direction of.the Nuclear Regulatory Commission (NRC), the 1980. voltage study addressed the' ability of the off-site source to provide adequate voltage for safeguard loads under certain. >

conditions of operation. This voltage. analysis,' based on 1980 plant -

i loading assumptions, revealed that when the generator was off-line,.

the plant in a bus tie configuration, and the grid voltage-at the minimum expected value, the off-site system might'not be capable of starting all the. safeguard loads if a safety Injection Actuation -

Signal (SIAS) was received. Based on the voltage study results, ,

voltage regulators were installed on the secondary side of Station

- Service Transformers (SST) 2 and 3 and resolved the voltage concerns.

j The 1986 Electrical System Study was performed independent of the i 1980 Voltage Study and at our own initiative. This study was r performed using current plant. loading which differ from the loading assumptions of 1980. In addition to voltage profile analysis, a complete set of electrical studies including load flow,'short

circuit, and coordination were conducted. The 1986 Yankee Atomic Electric Report (YAEC 1561) updates the 1980 report on voltage analysis and identifies potential problems as noted below.

When the generator is off line and the center buses are tied to the -

outer buses, the Auxiliary Power System voltage levels might not be of sufficient capacity to start and operate all safeguard loads if a SIAS was received when the buses are in a heavily loaded condition.

l The loading study also indicated that the 2400/480 volt station

service transformer which feeds the two tied 480 volt

buses could have been tripped on overload had an SIAS occurred.

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In order to resolve the above concerns,. Yankee committed in Reference (c) to install during the 1987 refueling a 2400 V circuit breaker to feed Station Service Transformer (SST) No. 4 from 2400 V i Bus No. 1. In Reference (d), the NRC approved Yankee's actions

! toward resolving the transformer overloading and safeguard load i starting and operating voltage concerns.

2 I

As described above, the 1986 study identified concerns regarding the capability of the Power System in addition to those identified in 1980. These concerns are the results of more conservative,,up to date assumptions regarding plant loading, and the increased scope of

our analysis. For these reasons, our 1986 electrical study is a l more realistic and in-depth electrical system analysis which

! provides assurance of the adequacy of the plant's Electrical j Distribution System.

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-United Stitas Nuclser Rigulstery Commission April 10, 1987 Attention: 'Eileen M. McKenna Page 3 l FYR 87-037 )

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2. Question '

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. Our comparison of the previous study with the new' study finds:

j a. There appears to be a significant load difference between the. _

two studies (i.e. , 5980 kVA - Table 3.1 of 1980 study versus l 4

6961 kVA - Figure 3.3.2-1 of 1986 study). Describe how the j load estimates were made for each study, and substantiate that the new study accurately represents the existing loads.

l b. From the plant configuration (Section 3.3.4) for the maximum load case, it appears that the new study reduces the loading of i the station service transformers (5 and 6) as intended.

4 However, it appears that this new configuration ~ overloads either of the upstream station service transformers (2 and 3)

i. which are rated 5000/6250 kVA each. Is this in fact true, and i if so, address why this is acceptable.

Response

j a. The loading for the Electrical Distribution System used in the j 1980 study was determined from discussions with plant-i personnel, review of plant procedures, and a review of plant ~

j drawings.

{ As stated in our response to Question 1 above, the 1986 study l was performed independent of the 1980 study. The loading used i in the 1986 maximum load case is based on both actual plant j loading and discussion with plant operations personnel, i ~

k For the 1986 study, the plant loads were reviewed to determine j operational requirements during a maximum load case which i occurs during hot standby. To be conservative, motor loads j were based on nameplate ratings instead of actual running

{ loads. The total calculated load was then compared to the 1 actual maximum load condition as recorded in the Control Room, y The total calculated load was higher than the actual measured and was used as the basis for this study. Since the higher

loading was used, the 1986 study conservatively represents

! existing loads during a maximum load condition.

i For the minimum load case, the 2400 V and'480 V bus motor loads used in the 1986 study were based on a detailed review of

operational requirements during the minimum load condition.

The motor control center loading was based on actual loading measurements taken during cold' shutdown.

i The 1980 loading estimation was adequate for the purpose of the study performed. However, the new study is more comprehensive, and thus, the 1986 loading estimate,is in much more depth and more conservatively represents the load. In addition, load l growth since 1980 was incorporated. Therefore, the 1986 study accurately represents the existing plant.

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United Statss Nuclser R gulatcry Commiccicn -April 10,'1987 4

Attention: Eileen M. McKenna Page 4 FYR 87-037-

, b. During 2400 V bus tie configurations (referred to as maximum load case in. Question 2.b), additional load will be placed on the SST which feeds the two tied 2400 V buses.

ANSI C57.92-1981 [The Guide for Loading Mineral-Oll-Immersed Power Transformers up to and Including 100 MVA with 550C or 650C Winding Rise) allows the overloading of-large oil filled i

power transformers for short time periods. Furthermore, based upon their operating experience with many transformers of this

' type, New England Electric System allows overloading up to twice the transformers forced air rating for up to eight j hours. In our specific case, the transformer would only be overloaded a maximum of 35% for 30 minutes which is within the

~

f above guidelines. This overload is based upon our conservative loading assumptions and an SIAS occurring while in a 2400 V bus l the tie configuration. With safety injection terminated within 30 minutes and the loading of the SST administratively.

controlled, the transformer-will normally be kept at or below its rated power.

The additional short time load placed on the SST'is acceptable because on an SIAS initiation, the safeguard loads will start and operate, the grid undervoltage and SST Overcurrent Protection Systems will not be activated, and the SST loading can be maintained within acceptable limits within a short

! period of time.

3. Question

, The statement in the new study (Page 8) "The analysis also assumes

, that the grid voltage is at the minimum expected value; however, voltage regulators will maintain the desired load voltage of

, 2400 volts" implies that the voltage regulator can maintain 2400 V with a 15% variation of the 115 kV sources. Thus, the voltage study was performed on the Auxiliary Power System at the 2400 V buses and below. If this is the case, we feel that the time delay associated with the voltage regulators to correct the degraded grid

voltage should be incorporated in the overall SI scenario time sequences. Otherwise, describe how the voltage regulators have been i

represented in the study during-the-transient and the steady-state cases.

Response

l The action of the voltage regulators was considered in the 1986 study as well as the.1980 study. The 1980 study assumed that the SIAS occurs subsequent to the degraded grid condition and considered that the degraded voltage condition was already corrected prior to receipt of the SIAS. The 1980 analysis, based on thic assumption was accepted by the NRC by a safety evaluation, dated

July 13, 1981 (Reference (e)).

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United Ststaa Nucisse RigulEtcry Commic2icn .

' April 10, 198F Attention: Eileen.M. McKenna' Page 5 '.

FYR 87-037 The 1986 study also assumes that any degraded voltage condition is '

corrected by the voltage regulators prior _to receipt of the SIAS.

The action of the regulators is not modeled_during the SIAS transient. The results of the study show that since voltages -

recover to within the required operating range after the start of safety loads, actuation of the regulators is not required during the l SIAS transient. The voltage regulators are modeled after the HPSI and LPSI pumps are at speed.' This simulates the intended function of the regulator which is to correct the voltage to optimum values after a change of load level.

4. Question i

With respect to the computer programs used to analyze the Auxiliary Power System, we noted that the old study used PTI program while the new study uses DAPPER (for the maximum load case) and the Stone / Webster (for the minimum load case) programs. In view of the fact that there are changes in the computer programs, system configuration, and inasmuch as the original model and method did not predict the difficulties experienced with the electrical system voltages, we are not confident that the new methodology can accurately predict the Electrical Distribution System voltages.

Therefore, we conclude that_ Yankee Rowe should perform a new verification test to verify the analytical model's ability to l accurately predict the Electrical Distribution System voltages for transient and steady-state conditions. This test should be performed according to Part B.4 of the Branch Technical Position PSB-1. Please advise when this test will be performed and

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the results submitted for our review.

Response

Yankee Atomic Electric Company has performed YAEC-1561, " Auxiliary Power System Voltage Study for the Yankee Nuclear Power Station,"

using the Distribution Analysis for Power Planning Evaluation and

, Reporting (DAPPER) program and the SWEC program, " Station Service Optimization and Analysis."

To ensure that the DAPPER results were accurate, DAPPER has been benchmarked to the SWEC program which has been fully verified and

, qualified for the Nuclear Plant Distribution System voltage analysis. The comparison of the voltages calculated by DAPPER to

the SWEC program presented in YAEC-1561 demonstrates that DAPPER is valid. To provide further assurance that the results accurately predict the Electrical Distribution System voltages at the Yankee

plant, the computer model will be verified by test during the May 1987 refueling outage, and the results submitted on

September 30, 1987. Bus voltages and loading will be monitored during a_ steady-state condition. The DAPPER program and model of I

the Auxiliary Power System will be used to predict bus voltages for the monitored condition. This steady-state test was agreed to during a conference call between YAEC and the NRC on November 11, 1986.

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United Statsa Nucisar R:guletcry C:mmisaign April 10, 1987 Attention: Eileen M. McKenna Page 6 FYR 87-037 The steady-state condition test will verify the DAPPER model of the Yankee plant's Distribution System.

4 We trust that you will find this information satisfactory, however, should you require additional information please contact us.

Very truly yours,

, G ct++w G. Pap eic Jr.

Senior Engineer, Licensing Yankee Project JP/dib l

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