ML19317D846
| ML19317D846 | |
| Person / Time | |
|---|---|
| Site: | Oconee  |
| Issue date: | 06/16/1977 |
| From: | DUKE POWER CO. |
| To: | |
| Shared Package | |
| ML19317D834 | List: |
| References | |
| RO-269-77-17, NUDOCS 7912100574 | |
| Download: ML19317D846 (3) | |
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O DUKE POWER COMPANY OCGNEE UNIT 1 Report No.:.RO-279/77-17 Report Date: June 16, 1977 gg occurrence Date: May 17, 1977 Facility:
Oconee Unit 1, Seneca, South Carolina Identification of Occurrence: Inadvertent isolation of emergency power source Conditions Prior to Occurrence: Unit 1 in cold shutdown, Units 2 and 3 at 100% full power Description of Occurrence:
On May 17, 1977 the periodic test PT/1/A/610/lG, 4160 volt Emergency Power Switching Logic (EPSL) Load Shed Test was performed on Oconee Unit 1.
This test provides verification of the proper functional performance of the EPSL system and is performed basically as follows: The Unit 4160 volt buses are initially supplied from the main feeder buses through the startup transformer CT-1 (See FSAR Figure 8-2) and the standby buses are energized through transformer CT-5.
The Keowee hydro emergency start circuitry is inhibited for Unit 1 during the test to prevent the starting of the Keowee hydro units. To initiate the test, a startup bus undervoltage condition is simulated which causes the EPSL system to transfer the main feeder buses to the standby buses. Continuing the test, CT-5 is separated from the standby bus by opening the breakers between CT-5 and the standby buses (SL1 and SL2). This causes the EPSL system to sense a loss of standby bus voltage and the Keowee underground feeder breakers to close onto the standby buses (SKl and SK2).
Since the Keowee units are not operating, the EPSL system senses a no voltage condition and separates the main feeder buses from the standby buses and energizes the main feeder buses from the startup l
j transformer.
l The test described above was successfully comp'.eted approximately 1816 on May 17, 1977. At the completion of the test, alarms existed indicating an undervoltage condition on the standby buses.
Since these buses are normally de-energized an undervoltage condition was not an abnormal condition, however, the alarm indicating the condition was abnormal. Attempts to open Keowee standby breakers SKl and SK2 failed due to improper personnel actions.
It was believed from past experience that closing breakers SL1 When and SL2 and then tripping them would clear the undervoltage alarms.
breaker SL-2 was closed at 2003 breaker ACB-3 tripped and Keowee Unit I received an emergency lockout. This resulted from connecting transformer CT-5 to the Keowee Unit 1 generatar (not operating) through the standby bus, the Keowee standby bus breakers SKl and SK2 (these breakers were closed f rom the EPSL test and had not been returned to their normal open position),
transformer CT-4, the underground feeder and breaker ACB-3.
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In recognizing the loss of Keowee Unit 1 from the underground feeder circuit personnel immediately tried to place Keowee Unit 2 in service on the under-ground feeder by closing breaker ACB-4.
At this time, ACB-4 tripped and Keowee Unit 2 received an emergency lockout. This was the result of back-charging Keowee Unit 2 generator from the standby buses as described in the preceding paragraph.
Action was taken in accordance with Technical Specification 3.7.7 to start the Lee Combustion Turbine; however, the emergency lockout on both Keowee units was reset and the underground feeder circuit was returned to service within approximately 17 minutes thereby eliminating the need for the combustion turbine.
Designation of Apparent Cause of Occurrence:
A review has revealed several causes of this particular incident.
The normal emergency power switching logic test is performed during refueling outages. The test performed on May 17, 1977 was an abbreviated version of the normal test and had been performed only once before on Oconee 2.
A procedural inadequacy existed in that the procedure did not specify returning the Keowee standby breakers, SK1 and SK2 to their normally open position upon completion of the test.
Since personnel did not properly return breakers SKl and SK2 to their nornal positions the closing of breaker SL2 would backcharged the Keowee unit resulting in an emergency lockout of the Keowee units.
A procedural inadequacy also existed in that immediately upon the loss of the Keowee unit on the underground feeder the remaining Keowee unit was placed on the underground feeder. An investigation to determine the reason for the loss of the Keowee unit and the underground feeder should be performed prior to this operation.
The undervoltage alarm on the standby buses is a proper alarm when either the breakers SL, SL2, SKl or SK2 are closed and the standby bus is de-1 energized. The proper manner to clear these alarms after the test is to open the Keowee standby breakers SKl and SK2 and then close and trip the CT-5 standby bus feeder breakers SL1 and SL2 That this was not performed was the result of procedural inadequacy and was largely responsible for this incident.
l Analysis of Occurrence:
The Oconee Nuclear Station Technical Specification 3.7.7 makes provision for the continued operation of the reactors in the event both Keowee units are inoperable provided that a Lee Combustion Turbine is placed into opera-tion and charges the standby buses through the isolated 100 kV transmission line within 30 minutes.
In this incident, both hydro units and the under-ground feeder circuit were returned to an operable status within approximately 17 minutes. Considering the short period of time which transpired and the l
unlikely probability of the necessity for onsite emergency power, it is i
concluded that the health and safety of the public was not affected by this incident.
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l It should be pointed out that this incident occurred as a result of not i
returning the emergency power system to the correct lineup after a test.
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However, evcn in the incorrect lineup with the Keowee standby bus breakers SKl and SK2 closed the proper functioning of the emergency power system would not have been affected. Had the unit been in operation, the 4160 volt loads would have been supplied initially from the units auxiliary transformer. Upon an ES actuation and loss of the auxiliary transformer the 4160 volt loads would have transferred to the startup transf ormer CT-1 and both Keowee units would have started, the overhead transmission path to CT-1 would still be energized, and the underground path to the standby bus through the Keowee standby breakers SK1 and SK2 (assumed to have been already closed as a result of an error in the test procedure).
Should offsite power be lost the switchyard would isolate from the system grid and CT-1 would be energized from one Keowee unit through the overhead transmission path.
If this path were now to become inoperable, the main feeder bus would transfer to the standby bus and be energized from the remaining Keowee unit through the underground feeder. As long as the standby bus is energized through Keowee standby breakers SK1 and SK2, the CT-5 standby breakers SL1 and SL2 cannot be closed due to: a sync check interlock which would not be satisfied; an undervoltage interlock which prevents closing when the standby bus is energized; a redundant undervoltage interlock; auxiliary contacts from the SKl and SK2 breakers preventing closure of breakers SL1 and SL2 and the fact that SL1 and SL2 requires a manual closure action. Thus, as can be seen, if breakera SKl ani SK2 hsi been lef t. In :he improper closed position the emergency power system would have functioned properly to assure a continued source of emergency power.
Corrective Action:
In order to prevent recurrence of this incident the following corrective actions have been taken: The periodic test procedure has been revised to prcvide requirements and guidance for restoring the emergency power system to a proper lineup by opening the Keowee standby bus breakers SK1 and SK2 at.d clearing the standby bus undervoltage alarm.
The alarm response proce-dure will be revised by June 24, 1977 to provide guidance to investigate the cause of the loss of a Keowee unit and the underground feeder prior to placing the remaining Keowee unit on the underground feeder.
Operations pcrsonnel have reviewed this incident and additional formal training will be provided on the operation of the system.
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