ML20148J124
| ML20148J124 | |
| Person / Time | |
|---|---|
| Site: | Millstone, Yankee Rowe |
| Issue date: | 08/12/1976 |
| From: | Schwencer A Office of Nuclear Reactor Regulation |
| To: | Groce R YANKEE ATOMIC ELECTRIC CO. |
| References | |
| NUDOCS 8011210143 | |
| Download: ML20148J124 (5) | |
Text
'
(.
V r
i!!
Docket Po. 50-29 m;
,a 7:;
?l Yankee Ato Tic Electric Conpany F
ATTU: Ur. Robert H. Groce Licensino Engineer 20 Turipike Road L
Westboro, flassachusetts 01581 Gentl enen:
PE:
YA!YEE ATOMIC PO't!ER STATICH (YANKEE-p.0t!E)
Provided herein as Enclosure 1 is a description of events which occurred at hillstone Unit No. 2 during July 1976 relating to plant operation and equipnent failures during a degraded grid vol tage condition.
.\\;
On July 27, 1976, all utilities with operating reactor facilities received telephone notification froa the URC of the events at the
' ill stone Unit Ho. 2 f acility.
At that time members of your staff were as'<ed to investigate the vulnerability of your facility to similar degraded voltage conditions and to orovide a response
>>y telorbone wi thin 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />.
l.s a recul t of our initial investicction and evaluation of the ratentiel t"cric inplications of tF.e ever,ts at Millstone and nur reeli inary Ciscusc. ions viitn several licenscos, 5:e consider it v.mst,ry to require all operatinq re 'ctor licensees tn convoct a e,crei? evelvation of the probler and to suhoit for..u.1 reports.
T' oref ore, :e request that you ce,voct an investigation of the issue os it affects your facility using the Portuost for Information neti.iler in Enclosure 't as a guido, and proviric thr> ant.lysos and resul's <inin 30 ciays of ycur receipt of this letter.
39 copies of your response eill ue necessa.ry.
The sirned original ent THIS DOCUMENT CONTAINS POOR QUAUTY PAGES 1
(
C r F ::E*
g. e n. A M t +*
3 A T E t' r. A l 1, f ' '
- I f i. 9.!)) AI Od C.40 D L'. 6. G CYC h N M L NT PFtlNTING C F FIC Et IP 74 9 2 8164 8011210[ f
[
t Yankce Atolic Cicctric Company *
- : ;6 This request for aeneric infomation was approved by GA0 under a blant et clearance number B-18022b (R0072); this clearance expires July 31,1977.
ri Si ncerel y, l
C A. Schwencer, Chier Operating Reactors Branch fl Division of Operating Reactors
Enclosures:
DISTRIBIRION-1.
Description of Events Docket X ttillstone Unit I!o. 2 NRC PDR 2.
Request for Infomation Local PDR ORB #1 Reading cc: iir. Donald G. Allen, President KRGoller Yankee Ato,1c Electric Conpany TJCa rter 20 Turnnike Poad OELD ilestboro, itassachusetts 01581 OI&E(3) j ASchwencer Greenfield Public Library ABurger au2 Main Street TVerdery creenfield, !hsracnntet ts 015.11 DEisenhut SMSheppard TBAbernathy JRBuchanan ACRS(16)
VStello Dross q
- 27g3(gb ABurEM TVerde ASchw>ncer.
8/12/76 8/g/76
.8/ ABS
~
f.es. f.5 3) AICd 014 2 U *
- 8 0 V E " N " I "'I " '"I'" " ' C t i ' P 4 '
- 2 O' i 9 p
f
"..o U
W EN5LOSUREN0.1 DESCRIPTION OF' EVENTS t
' MILLSTONE UNIT NO. 2 On July 20, 1976, NortheastNuclearEnergyCompany(NNECO) reported that, following a trip of Millstone Unit No. 2 on July 5,1976, several motors powered from 480 volt (v) motor control centers failed to start as required.
The faihme of the 480 y motors to start was traced to blown control power fuses on the individual motor controllers.
These controllers receive control power thro. ugh 480 v/120 y transformers within the controller.
!7 E
HNECO's investigation disclosed that, as a result of the plant trip, E
the grid voltage dropped from352 kv to 333 kv.
This voltage drop, i:?
in conjunction with additional voltage drops associated with the E
- .3 transformers involved, reduced the control power and voltage within 5
individual 480 y controllers to a voltage which was insufficient to
. _.,,z actuate the main line controller contactors.
As a result, when the F
1*
motors were signalled to start, the control power fuses were blown.
Subsequent testing by NNECO showed that the contactors required at least 410 y to function properly.
R HNECO concluded that under similar low voltage conditions, the opera-g bility of 480 v Engineered Satety Feature equipment could not be assured.
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.
R A trip of the undervoltage relays causes the emergency buses to be de-energized and a load shed signal to strip the emergency buses, the diesel generators to start and power the emergency buses, and required j
safety related loads to sequence start on the buses.
On July 21, 1976, NNECO reported that; the earlier corrective action taken 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 witt the design.
- However, during sequencing 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. loads supplied.
~/*
9 g
~
ENCLOSURE NO. 2
~
r RE0 VEST FOR INFORMATION E
1.
Evaluate the design of your facility's -Cfass IE electrical distribution system to determine if the operability of safety related equipment, including associated control circuitry or instrumentation, can be adversely affected by short tem or long term 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
[J (safety related and non-safety related) will be supplied by offsite power.
Include an estimate of the fraction of normal plant operating time in which this is the case.
c b.
The voltage used to describe the grid distribution system is usually a " nominal" value.
Define the nomal operating range of your grid system voltage and the corresponding voltage values at the safety related buses, j
c.
The transformers utilized in power systems for providing the required voltage at the various system distribution levels are nomally provided with taps to allow voltage adjustment.
Provide i
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.
d.
Assuming the facility auxiliary loads are bcing carried by the station generator, provide the voltage profiles at the safety buses for grid voltage at the normal maximum value, the normal 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 tr'ip setpoint for your
~
I facility's Loss of Offsite Power (undervoltage trip) instrumentation.
Include the basis for your trip setpoint selection.
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 undervoltage trip setpoint.
E g.
Utilizing the safety related bus voltage values identified in (f),
evaluate the capability of all safety related loads, including related M
control circuitry and instrumentation, to perform their safety l
functions.
Include a definition of the voltage range over which the safety related components, and non-safety components, can operate continuously in the performance of their design function.
t
~/'**'
,R t -
- es
~
h.
Describe the bus voltage monitoring and abnonnal voltage alarms available in the control room.
I 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]
systems) and the capability of the onsite systems to perform their function if the load shedding feature is maintained after the diesel generators are connected to their respective safety buses.
Describe the bases (if any) for retention or reinstatement of the load shedding function after the diesel generators are connected to their respective buses.
n.
frequency and other) perating limits (real and reactive power, voltage, Define the facility o 3.
established by the grid stability analyses cited in i
the FSAR.
Describe the operating procedures or other provisions presently b
in effect for assuring that your facility is being operated within these p
V:
- limits, t.;
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.
S i..
T Y
h J
p 1
I j 1
a H
y 1
l j
l' i
i