ML19329D742

Forwards Description of Equipment Failures During Degraded Grid Voltage Condition of Millstone 2.Requests Formal Repts within 30 Days Evaluating Possible Equipment Failures

ML19329D742
Person / Time
Site:	Crystal River, Millstone
Issue date:	10/01/1976
From:	Stolz J Office of Nuclear Reactor Regulation
To:	Rodgers J FLORIDA POWER CORP.
References
NUDOCS 8003170683
Download: ML19329D742 (5)

Text

Distributit [ %1 w

Docket FiTer

~

ELD RC PDR IE (3)

OCT 1 197@ local PDR ACRS (16)

LWR 1 File J. Miller R. C. DeYoung F. J. Williams Docket Sio. 50-302 U

• fn t.

bec:

J. R. Buchanan,flSIC E. Hylton T. B. Abernathy, TIC Florida Power Corporation ATTN: Mr. J. T. Rodgers f"Knght,SS Assistant Vice President and R. Tedesco Nuclear Project Manager H* Denton P. O. Box 14042 h,h,yo$er St. Petersburg, Florida 33733 h, p

• Gentlemen:

a 11 EQUIPMENT FAILURES DURING A DEGRADED GRID VOL1 AGE C0tIDITIO'l AT llILLSTONE, UNIT 2 Provided herein as Enclosure 1 is a description of events which occurred at Millstone Unit No. 2 during July 1976 relating to plant operation and equipment failures during a degraded grid voltage condition.

As a result of our initial investigation and evaluation of the potential generic implications of the events at Millstone and our preliminary discussions with several licensees, we consider it necessary to require all plants presently in review for an operating license to conduct a thorough evaluation of the problem and to submit formal recorts. There-fore, we request that you conduct an investigation of the issue as it affects your facility using the Request for Infomation detailed in j

i as a guide, and provide the analyses and results within 30 days of your receipt of this letter or explain why you cannot meet this schedule and provide the schedule that you will meet.

The signed original and 39 copies of your response will be necessary This request for generic information was approved by GA0 under a blanket clearance number B-180225 (R0072); this clearance expires July 31, 1977.

Sincerely, Original Signed b[

' John.NNiz, Chief Light Water Reactors Branch No.1 Division of Project Management

Enclosures:

1.

Description of Events

1tlistone Unit No. 2 2.

' Request for Infomation 3./

LW LW l o,,scg.

See next_.pagt g

q

._..._L...

Nd._ JSt A 8003 1 7 0~~ >j f~.

.o

...m__.._....

.- _lell) 49LLI76 Forsa AP.C.318 (Rev. 9 53) AECM 0240 W u. s. oovsanusNT,RINTIMe 0,,tCEt 1974 024-104

. cn

• s

.a I

I 1976 Florida Power Corporation.

t 4

cc:. !4r. S. A. Brandimore Vice President and General Counsel P. O. Box 14042 St. Petersburg, Florida 33733

.1 i

i i

orrice >

sunnaus h nats >

Forma ABC 318 (Rev, p.53) AEQi 0240

• u. s; eovannuswv painvine oppics: geya.sae see

i 5

\\

~.. l

..~-

?..

ENCLOSURE NO. 1 j

' DESCRIPTION'0F' EVENTS qq m

~l

• HILLSTONE UNIT NO. 2 W6 c

On July 20,.1976, Northeast Nuclear Energy Compariy (NNECO) reported that, following a trip of Millstone Unit No. 2 on July 5,1976, several motors powered f mm 480 volt (v) motor control centers failed to start as required. The railute of the 480 y motors to start was traced to blown control power fuses on the individual motor controllers. These controllers receive control power through 480 v/120 y transformers within the controller.

HNECO's investigation disclosed that, as a result of the plant trip, the grid voltage dropped from352 kv-to 333 kv. This voltage drop, in conjunction with additional voltage drops associated with the transformers inv lved, reduced the control power and voltage within individual 480 y controllers to a voltage which was insufficient to actuate the main line controller contactors. As a result, when the motors werr ignalled to start, the control power fuses were blown.

Subsequent sting by NNEC0 showed that the contactors required at least 410 y to function properly.

HNECO concluded that under s'imilar low voltage conditions, the opera-bility of 480.v Engineered Safety Feature equipment could~not be assured.

l HNECO's immediate corrective action was to raise the setpoint of the Engineered Safeguards Actuation System (ESAS) " loss of power" under-voltage relays to assure that the plant would be separated from the

, grid and emergency power system (dual) operation would be initiated before the control voltage fell below that required for contactor operation.

A trip of the undervoltage relays causes the emergency buses to be de-energized and a load shed signal to strip the emergency b6ses, the

. diesel generators to start and power the emergency buses, ano required

~.

safety related loads to sequence start on the buses.

NNECO reported that the earlier corrective action taken

~ On July 21, 1976, L

was no longer considered appropriate because during starting of a circulating water pump, the voltage dropped below the new ESAS under-

~

~

voltage relay setting. This de-energized the emergency buses, caused load shedding to occur, started the diesel generators and began sequencing loads onto the emergency buses in accordance witli the design.

However, during sequencie of the loads onto the buses, the voltage again dropped below the undervoltage relay setting which caused the load shed signal to strip the buses. The result was energized emergency buses with no'loadssupplied.

j

..... ~ -

t

ENCLOSURE NO. 2 hh REQUEST FOR INFORMATION 1.

Evaluate the design of your facility's Class IE electrical distribution system to detemine if the operability of safety related equipment, including associated control circuitry or instrumentation, can be adversely affected by short tem or long tem degradation in the grid system voltage within the range where the offsite power is counted on to supply important equipment.

Your response should address all but not be limited to the following:

a.

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 nomal plant operating time in which this is the case.

b.

The voltage used to describe the grid distribution system is usually a " nominal" val ue. Define the nomal cperating range of your grid system voltage and the corresponding voltage values at the safety related buses.

. c.

The tiransfomers utilized in power systems for providing the required voltage at the various system distribution levels are nomally provided with taps t. 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.

.i i

d.

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 n'omal maximum value, the nomal minimum value, and at

~

the degraded conditions (high or low voltage, current, etc.) which would require generator trip. _

~

e.

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.

t f.

Assuming operation on offsite power and degradation of the grid system voltage, provide the voltage values at the safety related buses corresponding to the maximum value of grid voltage and the degraded grid voltage corresponding to the undervol.tage trip setpoint.

g.

Utilizing the safety related bu. voltag' valces identified in (f),

e evaluate the capability of all rafety related loads, including related control circuitry and instrumentation, 'to perfom their

• safety functions.

Include a definition of the voltage range over which the safety related components, and r.on-safety ccuponents, can operate continuously in the performance of their design function.

e f a I

/

A-s.

h.

Describe the bus voltage monitoring and abnomal voltage alams.

available in the control room.

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 onsite power system. Describe the load shedding feature of your design (required prior to transfering to the onsite [ diesel generator]

systenti and tne capability of the onsite systems to perform their functie i'.ie it 'd shedding feature is maintained after the diesel

~

. " cted to their respective safety buses. Describe generate:

the base m for retention or reinstatement of the load shedding function c e

• r.ne diesel generators are connected to their respective buses.

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.

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.

3 e

S g

e D

e i

l l

~

.