ML20211D250
| ML20211D250 | |
| Person / Time | |
|---|---|
| Site: | Vogtle  |
| Issue date: | 10/09/1986 |
| From: | Rice P GEORGIA POWER CO. |
| To: | Grace J NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION II) |
| References | |
| REF-PT21-86, REF-PT21-86-358-000 GN-1105, PT21-86-358, PT21-86-358-000, NUDOCS 8610220150 | |
| Download: ML20211D250 (4) | |
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Georgia Power Company Vogtle Project Management Post Off;ce Box 282 WJynesb0rO, Georgia 30830 Te!ephone 404 724-8114 404 554-9961 06 0CI14 P 1: 23 GeorgiaPower j
the SOUfhtYn ektfr C S(5 fern October 9, 1986 United States Nuclear Regulatory Commission Region II File:
X78G03 Suite 2900 Log:
-GN-1105 101 Marietta Street, Northwest Atlanta, Georgia 30323
Reference:
Vogtle Electric Generating Plant-Units 1 and 2; 50-424, 50-425 Tripping of Reserve Auxiliary Transformers Attention: Mr. J. Nelson Grace On May 29, 1986, a simultaneous interruption of service occurred with both Unit 1
Reserve Auxiliary Transformers, resulting in de-energization of all Unit. 1 13.8 kv ~ and 4.16 kv buses.
On June 3, 1986, both Reserve Auxiliary Transformers tripped again.
This second event was identical to the May 29, 1986, interruption.
These events were not considered to be potentially reportable pur-suant to 10 CFR 50.55(e) or 10 CFR 21 since, under normal operating conditions, both trains of the emergency diesel generators would be available to ensure the safety of the plant.
However, an evaluation was conducted to determine the cause of the events and to recommend appropriate corrective actions.
A summary of the evaluation is attached for your information. 'If you have any questions relative to this matter, feel free to contact me.
This report contains no proprietary information and may be placed in the USNRC Public Document Room.
Yours truly, P. D. Rice Vice President Vogtle Project Engineering REF/PDR/tdm Attachment (Distribution on Page Two)
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U. S. Nuclear Regulatory Commission Document Control Desk Washington, D. C.
20555 H. G. Baker R. A. Thomas L. T. Gucwa J. P. O'Reilly D. R. Altman C. W. Hayes G. F. Head P. R. Bemis G. A. McCarley R. E.'Conway J. A. Bailey R. W. McManus
-J. T. Beckham
- 0. Batum D. S. Read R. H. Pinson G. Bockhold Sr. Resident (NRC)
P.'D. Rice.
C. E. Belflower C. C. Garrett (0PC)
B. M. Guthrie J. F. D'Amico J. E. Joiner (TSLA)
D. E. Dutton E. D. Groover D. Feig (GANE)
.V0GTLE ELECTRIC GENERATING PLANT i
TRIPPING 0F RESERVE AUXILIARY TRANSFORMERS i
L
==
Introduction:==
On May 29, 1986, at approximately 8:27 a.m.,.a simultaneous interrup-tion of service occurred. on both Reserve Auxiliary Transformers INXRA and INXRB (Rat-A and B).
This service interruption de-energized both 13.8 kv buses and all five 4.16 kv buses on Unit 1.
After this event, I
a task force was formed to determine the cause of the event and to re-commend corrective action. -At 12:10 p.m.
on June 3, 1986, both RAT-A and RAT-B tripped again.
This event was identical to the May 29, 1986, event.
Hay 29, 1986 Event:
At approximately 8:27 a.m., a tripping sequence was initiated that caused a major loss of lighting and auxiliary power throughout the unit.
The substation event recorder. indicated that spurious trip current pulses were present in the trip circuits connecting the plant and switchyard for both RAT transformers at the time of the event.
Since there were no relay targets in the substation, it appeared that a 60 hertz signal had been introduced into the trip logic with sufficient magnitude and duration to operate the tripping relays. Again on June 3, 1986, a similar occurrence tripped both reserve auxiliary transformers, i
l Evaluation of Tripping Condition:
Since the only point of commonality for these circuits is in-plant 1
battery IND3AB, wiring associated with this battery.was checked for possible situations that could have. allowed AC voltage to be introduced to the DC control circuits.
A jumper wiring error was found in the boric acid blending system that could have allowed 120V AC power to be introduced to the transformer DC control circuits.
The superimposed AC voltage caused a circulatory current to flow through nctal-oxide varistors (located in the switchyard) and the trans-former trip auxiliary relay coils.
These conditions caused the ~ asso-ciated switchyard breakers to. trip.
A staged test was conducted on June 4, 1986, that re-enacted the AC-DC tie.
When the AC-DC tie.was
]
remade, a trip of both reserve auxiliary transformers occurred.
t
==
Conclusion:==
l The simultaneous tripping of Reserve Auxiliary Transformers A and l
B has been attributed to a 60 hertz voltage-to-ground connection to the battery system which supplies trip logic for both transformers.
4 l
The superimposed voltage caused a circulatory current to flow through j
the metal-oxide varistors and the transformer trip auxiliary relay coils j
which then tripped the switchyard circuit breakers.
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o Page Two Corrective Action:
1.
P.emove jumper from boric acid blender. -This eliminates the cause of this false trip.
2.
Re-verify the adequacy of cable shielding for the circuits connecting the plant and switchyard and remove the metal oxide varistors from the RAT trip circuits.
This reduces the possibility of a false
~
trip by a similar mechanism in the future.
3.
Improve reliability. by replacing targeting auxiliary trip relays with simple target-only relays.