Information Notice No. 85-28: Partial Loss of AC Power and Diesel Generator Degradation
SSINS No.: 6835
NUCLEAR REGULATORY COMMISSION
OFFICE OF INSPECTION AND ENFORCEMENT
WASHINGTON, DC 20555
April 9, 1985
Information Notice No. 85-28: PARTIAL LOSS OF AC POWER AND DIESEL
All nuclear power reactor facilities holding an operating license (OL) or a
construction permit (CP).
This notice is provided to inform recipients of a potential problem with
diesel generator voltage regulation that might prevent the diesel generators
from loading on to their safety buses. It is expected that recipients will
review the information for applicability to their facilities and consider
actions, if appropriate, to preclude a similar problem occurring at their
facilities. However, suggestions contained in this information notice do not
constitute NRC requirements; therefore, no specific action or written
response is required.
Description of Circumstances:
On January 31, 1985, WNP 2 was at 100 percent power when a lockout relay,
used in the offsite power supply fast transfer logic, spuriously tripped.
This was an abnormal partial actuation that caused the 500 kV generator out-
put breaker to open and the circuit breakers from the startup transformer to
close on the plant buses even though the normal auxiliary transformers were
not disconnected from the same plant buses. The opening of the 500 kV output
breaker initiated the digital-electrohydraulic control system overspeed pro-
tection circuit which closed the turbine control valves. The turbine control
valve fast closure caused a reactor scram as designed.
As a result of this abnormal condition, the generator remained connected to
the 230 kV grid via the auxiliary and startup transformers. (See attached
simplified diagram illustrating abnormal breaker alignment during the first
seconds of this transient.) After 4 seconds, a breaker in the 230 kV line to
the startup transformer opened, leaving the plant without non-safety-related
power. Two of the buses without power, SM-1 and SM-3, ordinarily feed
safety-related buses SM-7 and SM-8. As a result of losing power to two
safety-related buses, the backup transformer, which is powered by a 115 kV
line, was automatically connected to the safety-related buses and the diesel
generators for these buses started, but were not required to assume load.
April 9, 1985
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In the control room, there were false indications such as high containment
pressure and valid indications of vessel flow level (level 2, -50 inches).
The high pressure core spray (HPCS) and reactor core isolation cooling
system (RCIC) started on the low level signal.
Eventually, the main generator's protective circuits actuated the balance of
the fast transfer logic, causing the auxiliary transformer to separate from
the plant buses.
After the event, it was found that the output voltage had been set incor-
rectly on diesel generators DG-1 and DG-2. If the backup transformer or its
supply had failed, the diesel generators would not have loaded on the safety
buses because the voltage regulators were set at their lowest voltage set
point. The safety buses have protective relaying that prevents the diesel
generators from loading on the safety-related buses if their output voltage
deviates too much from nominal. There was no control room alarm indicating
the diesel generator output voltage was too low to permit loading diesel
generators on their safety-related buses.
The condition was caused during troubleshooting of the voltage regulators
for DG-1 and DG-2. These voltage regulators have a manual "raise/lower"
handle in the control room which permitted their output voltage to be
adjusted even though the diesel was not running. If the diesels were not
running, as was the case during the troubleshooting, there was no indication
of the generator output voltage and, thus, no indication of the voltage
setting of the voltage regulator. The type of voltage regulator used on DG-1
and DG-2 allowed for inadvertent degradation without indication or alarm in
the control room. This situation did not exist on DG-3, which is dedicated
solely for the HPCS and has an automatic voltage set point reset upon start
of the diesel generator.
A number of corrective measures have been or will be taken as a result of
this event. The licensee has modified the voltage regulators for DG-1 and
DG-2 so that the output voltage of the diesel generators can be adjusted
only while the diesels are running. The licensee is pursuing an automatic
voltage set point reset feature for these diesel generators. The licensee
will evaluate the efficacy of control room annunciation for high/low diesel
generator output voltage. The licensee also has modified the offsite power
supply fast transfer logic so that if a partial spurious actuation occurs
again, then the balance of the logic will actuate. The licensee has modified
procedures and training to reflect the lessons learned from this event.
April 9, 1985
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No specific action or written response is required by this information
notice. If you have any questions about this matter, please contact the
Regional Administrator of the appropriate NRC regional office or this
Edward L. Jordan, Director
Division of Emergency Preparedness
and Engineering Response
Office of Inspection and Enforcement
Technical Contact: Eric W. Weiss, IE
1. Simplified Diagram Illustrating Abnormal Breaker Alignment
2. List of Recently Issued IE Information Notices
Page Last Reviewed/Updated Friday, May 22, 2015