ML20215G278

TVA Sequoyah Electrical Medium Voltage Short Circuit Analysis Calculations, Final Technical Evaluation Rept

ML20215G278
Person / Time
Site:	Sequoyah
Issue date:	03/23/1987
From:	SCIENCE APPLICATIONS INTERNATIONAL CORP. (FORMERLY
To:	NRC
Shared Package
ML20214F629	List: ML20214F626 ML20215G278 ML20215G289
References
CON-NRC-03-82-096, CON-NRC-3-82-96 SAIC-87-3030, NUDOCS 8703310246
Download: ML20215G278 (7)

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I Technical Evaluation Report 1 of TVA Sequoyah l Electrical Medium Voltage Short Circuit '

Analysis Calculations l

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.I Technical Evaluation Report  ;

of TVA Sequoyah Electrical Medium Voltage Short Circuit Analysis Calculations 1

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March 23, 1987

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1 Prepared for:

U.S. Nuclear Regulatory Commission Washington, D.C. 20555 Contract NRC-03-82-096 I

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Technical Evaluation Report of TVA Sequoyah Electrical Medium Voltage Short Circuit Analysis Calculations NRC requested its contractor, SAIC, to conduct independent technical evaluations of selected TVA Sequoyah electrical calculations. SAIC was requested to evaluate the TVA calculational methods and to determine the reasonableness of the TVA results by performing independent analyses of portions of the TVA calculation.

NRC and SAIC selected TVA calculation SQN APS 008 "Short Circuit Study Medium Voltage System," dated December 22, 1986 (Reference 1), for this independent evaluation.

g SAIC evaluation of this calculation included working sessions with TVA personnel and use of TVA drawings and vendor material at the TVA offices in Knoxville, Tennessee on March 2 through 4, 1987.

Review of the Medium Voltaae Short Circuit Analyses I

SAIC obtained Sequoyah Station Auxiliary Power System one-line diagrams 15E500-1, Rev. 5, and 15E500-2, Rev. 5. These drawings contain all the medium voltage circuits and include 480 volt load distribution diagrams as well as the electrical sources, which include 500 KV, 161 KV switchgard g sources, and Sequoyah generators for both Units 1 and 2.

SAIC elected to evaluate postulated short circuits in the portion of the one-line diagram which I was fed from the 161 KV offsite source and Sequoyah Unit 2 generator.

The circuits selected for evaluation included Sequoyah main transformer #2, unit station transformers 2A and 28, 6.9 KV unit boards 28 and 2C, and 6.9

.l XV shutdown boards 2A-A and 28-8. Vendor data was obtained for all included equipment so that MVA ratings, impedances and other required calculational g inputs could be generated for short circuit computations. A short circuit 5 model was then generated and the model and model inputs were compared to the TVA calculations. In general, both models and most assumptions compared favorably. Specifically, the SAIC evaluation found the TVA cable impedance estimates between the 6.9 KV unit boards and the 69 KV shutdown boards are based on 90% of the installed lengths and therefore are only slightly I conservative. TVA assumption of zero impedance for the nonsegregated phase busses and isophase busses connecting the generator, main transformer, unit l station transformer and the 6.9 KV unit boards is conservative. The transformer MVA values and impedances and generator subtransient reactance 1

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values were found to be appropriately based on the vendor data sheets.

g Transformer self-cooled OA ratings were appropriately used for determining MVA values. The only area of difference was found in the estimation of the g available fault current from the 161 KV offsite power source. TVA estimated W that 12,710 MVA is the maximum short circuit contribution from the 161 KV source. TVA's basis for this estimate is a December 2, 1986, letter from l the TVA Transmission and Planning Branch to the TVA Sequoyah Chief Electrical Engineer. Since SAIC could not easily estimate this value, SAIC I conservatively assumed that the 161 KV fault contribution could be as large as the installed 161 KV circuit breakers. The breaker rating is 25,000 MVA.

This assumption produced slightly larger short circuit results in the medium l voltage short circuit analysis. However, the increase is less than 1% and therefore does not significantly change TVA results or conclusions. TVA selected 4 faults and analyzed each. They chose a fault on the 6.9 KV unit board 2C bus, a fault on the 6.9 KV unit board 2B bus, a fault on the 6.9 KV shutdown board 2A-A bus, and a fault on the 6.9 KV shutdown board 28-B bus.

l The choice of four faults is a reasonable selection as this allows the analyst to evaluate the operation of the six highest level protective devices in the medium voltage system.

I TVA selected a bolted three-phase faul t as the fault type to be analyzed at each fault location.

While a three-phase bolted fault is convenient and reasonable fault to assume other

] faults such as a single phase fault to ground are also used by electrical analysts.

A single phase to ground fault is the most common fault observed 1

and this fault can produce larger asymmetrical fault currents if the ground resistances are very low. TVA, however, utilizes a nongrounded 480 V system and grounding through resistors for the 6.9 KV system which eliminates the single phase to ground fault as a likely maximum fault condition. The three-phase bolted fault should yield reasonable and conservative short circuit estimates for Sequoyah.

Results The TVA calculational methods are based on IEEE Standard 141 and ANSI C37.010 guidance, have incorporated reasonable or conservative assumptions and should yield reasonable estimates in the Sequoyah medium voltage system.

The TVA calculation, however, shows that TVA Sequoyah has installed 6.9 KV protective devices which do not meet code and industry practice for 2

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protective device sizing. For the 6.9 KV shutdown boards, TVA short circuit calculations predict symmetrical fault currents of 542 MVA for the 500 MVA class protection equipment. TVA is relying on vendor nonwarranted ultimate test results (Reference 2) which show 550 MVA ultimate interrupt limit as the justification for the continued use of these devices (see Case B of the sketch). Moreover, the protective devices located at the 6.9 KV unit boards have TVA predicted symmetrical fault currents of 673 MVA which far exceed even the ultimate nonwarranted test value for this came protective device (see sketch Case A). For Case A, an explosion accompanied by the ejection of high velocity missiles and hot conductive ionized gasses would likely I

result should the predicted fault currents ever exist. The attached sketch shows the typical location of the 6.9 KV protective devices at Sequoyah.

It is perhaps noteworthy to mention that TVA has been aware of this unsatisfactory protective device application since at least 1979. At that time TVA asked the vendor (Reference 2) to comment on the use of the circuit breaker to interrupt a fault at 7.17 KV and 44.18 KA (or 549 MVA). TVA apparently felt that a 549 MVA fault was the maximum system fault for this device. The vendor reply stated that a successful test was performed at 550 MVA but the contacts were eroded and the operation at 550 MVA creates an end-of-life condition for the breaker. Moreover, in April 1983 revised TVA fault calculations predicted fault currents of 582 MVA. TVA's review of this condition (Reference 3) led to two decisions in 1983. The first was to immediately limit the generator output voltage to 24.5 KV (from the 25.2 KV maximum output) in order to stay within the circuit breaker ultimate interrupting capacity. The second decision was to resolve this problem and j remove the low voltage limitation in five years or less. As mentioned earlier, the most recent Sequoyah calculations now predict a 673 MVA fault q for the 6.9 KV unit boards. Therefore, TVA and NRC must assume that a j

postulated fault at the 6.9 KV unit boards cannot be interrupted, will propagate back to the Unit 2 generator and to the 161 KV switchgard, and

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will likely disable both power sources. This fault should have no impact on the 500 KV source. The 500 KV source should be sufficient to deliver AC power for safe shutdown of Sequoyah Units 1 and 2. In addition, the diesel l a

generators and station oatteries should be available to supply shutdown power.

SAIC recommends that TVA either replace the undersized protective devices, or install current limiting devices.

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i 161 KV UNIT 2 GENERATOR

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CASE A 6.9 KV UNIT BOARD

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.le f'~' References u_

g, 1. "Short Circuit Study - Medium Voltage System," Calculatiot, SQN APS 008, l_ RIMS Accession Number B43 861222902, December 22, 1986.

2. Gould-Brown Boveri letter to TVA dated November 6, 1980, (response to TVA letter of December 14,1979).

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3. TVA me.mo from J.H. Boehms and 0.R. Webster to Electrical Engineering files dated April 18, 1983.

Subject:

Sequoyah Nuclear Plant Generator N Bus Voltage Limits Interrupting Capability Limits of 6900 Volt Switchgear.

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