ML19093A988

Presents Results of Analyses of a 06/03/1977 Request for Further Information on the System'S Capabilities of Handling Degraded Voltage Conditions

ML19093A988
Person / Time
Site:	Surry
Issue date:	09/26/1977
From:	Stallings C Virginia Electric & Power Co (VEPCO)
To:	Case E, Reid R Office of Nuclear Reactor Regulation
References
Download: ML19093A988 (5)

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VIRGINIA ELECTRIC AND POWER COMPANY RICHMOND,VIRGlNlA 23261 September 26, 197T

• Mr. Edson G. Case, Acting Director Serial !'lo. 234A/060377 Nuclear Reactor Regulation PO&M/ TAP : <las U. S. Nuclear Regulatory Commission Docket Nos. 50-280 Washington, D. C. 20555 50-281 License Nqs. DPR-32 Attn: Mr. Robert_W. Reid, Chief DPR-37 Operating Reactors Branch 4

Dear Mr. Case:

This letter is to present the results of our analyses of your June 3, 1977, request for further information on our system's capabilities of handling degrad-ed voltage conditions. We have reviewed our system with your staff and believe that our present system capabilities are adequate. Also, we are submitting for your study a proposed addition to our surveillance requirements.

Specifically, our responses to your Staff Positions as discussed at our meeting of September 6, 1977, follow:

1) Position 1: Second Level of Under-or-Over Voltage Protection With a Time Delay We require that a second level of voltage protection for the onsite power system be provided and that this second level of voltage protection shall satisfy the following criteria:

a) The selection of.voltage and time set points shall be determined from an analysis of the voltage requirements of the safety~related loads at all onsite system distri-bution levels.

Response

Our letter serial number 197/081276 dated November 3, 1976-thoroughly.describes the voltage protection features employed.

We will briefly summarize our* system capabilities.

The emergency buses are fed from the Reserve Station Service Transformers which each have an automatic tap changing feature.

This tap changer is activated by a change in output voltage of .:l::1.25%

for greater than*thirty seconds at which time the tap changer changes in increments of .625% per second for 16 steps. Thus, our first level of protection protects the emergency buses, plus or minus 10%, around the normal line voltage.

e e VIRGINIA ELECTRIC AND POWER COMPANY TO Mr. Edson G. Case Page No. 2 Our undervoltage detection and actuation is by coincidence logic of 2/2 relays per *bus. The relays have LV coils which.have response characteristics as shown on the attachment. Therefore, even tho.ugh the setpoint of diesel start at 84% of nominal voltage and of isola-tion from off-site power at 79.5% of nominal voltage occurs at 12.se-conds from the time of reaching the setpoint, other conditions of undervoltage will also be detected. For example, on complete loss of voltage the relays will trip in 1.2 seconds.

With these two levels of protection, the emergency bus is pro-tected at all points except for the range of 80-90% undervoltage.

However, our safety grade motors were all purchased to start at 70%

of rated voltage and thus the motors will all start and accelerate thro.ugh the range of 80-90% of nominal voltage. If the transient de-graded voltage condition does not last lo.nger than normal starting times. (10-30 seconds) the motors will suffer no harm. Any longer period than that may begin to.reduce their useful life, but they still will respond as required during the postulated problem.

The vital buses are r.egulated to compensate for unusual volt.age conditions and the margin of setpoint change under the worst cases will be minimal.

Another.level of disconnecting the emergency buses from the off-site power source occurs at 45.8% of nominal voltage as sensed on each of the transfer buses, the method of sensing is s*imilar to that on the emergency buses.

Therefore, we consider our system to be in compliance with the intent of Staff Position 1. The attached proposed (for study only)

Technical Specification change will place our .. Technical Specifications in compliance.

2) Position 2:. Interaction of*. On.site Power Sources.with* Load Shed .Feature

Response

Our system does not have a.feature to load shed the emergency buses once the onsite source is supplying power.

Therefore, we are in compliance with Staff Position.2.

3) Position 3: Onsite Power Source Testing

Response

The full operability at full load of the on-site power source is

.tested per Technical Specification requirement monthly.

Our .Technical Specification includes a.requirement for simulating

e e VIROlNIA ELECTRIC AND POWER COMPANY TO Mr. Edson G. Case Page }l'o. 3 loss of offsite power in conjunction with a safety injection actua-tion signal at each refueling. The high head and low head safety injection pumps are not started but the breakers are placed in test to preclude a RCS overpressure accident per a prior NRC commitment.

This serves the intent as the pumps are proved operable monthly and this test proves the starting sequencing during accident conditions and proper diesel loading.

• The differ.ence in diesel load is minimal as we have no load sequencer, and the other heavy loads are not start-ed immediately during a SI condition (containment spray and contain-ment recirculation *spray pumps) but would be required later.

Therefore, the monthly testing proves the operability under full load of the onsite power source; the refueling.testing proves these-quencing of diesel start and .load; and the refueling testing proves the starting of required loads.

We are in compliance with the intent of Staff Position 3 except for interrupting the load on the diesel. As during emergency condi-tions our diesel is only protected by mechanical overspeed trip, we do not feel this test is appropriate nor do we believe that further trips on the diesel are proper during emergency conditions. Justi-fication for this load interruption test is also minimal as we have

. no load sequencer or load shed.feature on our onsite power source which could cause this postulated load interruption.

Therefore, we believe that we are in compliance with the full intent of Staff Positions 1, 2 and 3 as they apply to our facility.

Very truly yours,

&.)!}/). -..dla?Cu:c_~

C.. M. Stallings Vice President-Power Supply and Production Operations Attachments cc: Mr. James_P. O'Reilly

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• ATTACHMENT Proposed (for study only) Technical Specification Change for Surry Power Station T.S. 4.6 D -

Emergency Bus Undervoltage Detection

1. Tests and Frequencies The undervoltage relays on the emergency buses shall be checked each refueling to be within the trip values stated below:

• Functional Unit Trip Value (allowable tolerance)

Loss of Power a) 4.16 KV Emergency bus 45.8% (+5.0, -5.0%)

(D,E,F) Undervoltage 12 Second delay (+/- 1.0 second)

~Loss of Voltage) b) 4.16 KV Emergency bus 80.5% (+5.0%, -5.0%)

(_11, J) Ur1deL ..vTol tagc 12 Second del.e.y (1, 0 sec0n<'I)

(Degraded Vol cage)