Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555
December 11, 1989
Information Notice No. 89-83: SUSTAINED DEGRADED VOLTAGE ON THE OFFSITE
ELECTRICAL GRID AND LOSS OF OTHER
GENERATING STATIONS AS A RESULT OF A
PLANT TRIP
Addressees:
All holders of operating licenses or construction permits for nuclear power
reactors.
Purpose:
This information notice is intended to alert addressees to potential
problems resulting from a sustained degraded grid voltage on the offsite
electrical system. This problem could occur as a result of a plant trip if
the generator backup relays of other generating stations on the electrical
grid are improperly set, causing additional generating stations to be lost.
It is expected that recipients will review the information for applicability
to their facilities and consider actions, as appropriate, to avoid similar
problems. 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 July 11, 1989, Virgil C. Summer Nuclear Station was operating at
100-percent power during a record power demand on the South Carolina
Electric & Gas Company (SCE&G) system. During maintenance activities, the
generator stator cooling water signal was inadvertently lost, causing a
turbine trip and a reactor trip. This turbine trip caused an unexpectedly
large voltage disturbance on the grid; subsequently, several other
electrical generating stations were lost within the SCE&G system, resulting
in an estimated loss of about 1500 megawatts. This significant loss of
power resulted in a degraded grid voltage condition at Summer, which caused
the station's second level undervoltage protection relays (degraded grid
voltage protection) to initiate load shedding and automatic starting and
loading of the diesel generators.
Previously, on July 20, 1976, as a result of a unit trip at Millstone
Nuclear Power Station, the grid voltage dropped from 352 kV to 333 kV. This
voltage drop, in conjunction with the voltage drops produced by the
applicable step-down transformers, reduced the control power voltage within
individual motor control centers and individual 480-volt controllers to a
level that was insufficient to actuate the main line controller contactors.
As a result,
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IN 89-83
December 11, 1989
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when the motors were signaled to start, the contactor control power fuses
were blown, making several motors powered from the 480-volt motor control
centers inoperable.
Both the Millstone trip and the recent Summer trip resulted in a degraded
voltage condition at the plant safety buses. To resolve the Millstone
concern, the staff recommended in Branch Technical Position PSB-1, "Adequacy
of Station Electric Distribution System Voltages," that a time-delayed,
degraded grid voltage detection scheme be incorporated into the plant's
safety-related electrical distribution system to protect the Class 1E
safety-related equipment. This undervoltage protection has been implemented
at all operating plants. As described in the above event at Summer, the
second level of undervoltage relays separates the safety buses from the
offsite power system (load shedding) and initiates operation of the diesel
generators.
A licensee evaluation of the event at Summer showed that a grid instability
after the turbine trip and subsequent cascade tripping of other generation
stations occurred because the SCE&G system could not compensate for the loss
of the 440 MVARs being delivered by Summer. The 115-kV line voltage
degraded to 102-kV, and the 230-kV line voltage degraded to 205-kV. This
evaluation further showed that the setpoints of the generator backup relays
at other generating stations, McMeekin and Saluda, were lower than the
standard settings. These setpoints were established many years ago and were
never reverified. (Note: Generator backup relays provide protection to the
generator against internal and external faults not properly cleared by other
relays.) With the generators at McMeekin and Saluda in the automatic
voltage control mode, the generators tried to increase voltage in response
to the loss of power at Summer. This attempt to raise voltage resulted in a
reactive generation increase that caused the oversensitive generator backup
relays to trip the units. The as-found relay settings for those generating
stations were 118 and 107 percent of generator output. SCE&G stated that
its present, standard setting for the generator backup relays is 150 percent
of the generator MVA rating. Even with the relay settings of four Fairfield
pumped storage stations set at 150 percent, the large voltage decrease that
resulted from the loss of the Summer, McMeekin, and Saluda units and the
automatic voltage regulation response of the Fairfield units also caused the
Fairfield units to trip.
Discussion:
The offsite power system is the preferred and the most reliable source of
power for nuclear plant safety systems. Therefore, plants remain connected
to the preferred source for as long as possible, that is, for as long as the
capability and capacity of the offsite source permits, before switching to
the emergency diesel generators. A loss of generation capacity from a plant
challenges the offsite power supply availability; thus, during licensing,
plants perform a grid stability analysis to ensure that the offsite power
system will remain stable in the event of the loss of the largest single
supply to the grid or the loss of the largest load from the grid.
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IN 89-83
December 11, 1989
Page 3 of 3
The events that occurred at Summer and at Millstone were the result of these
plants being operated outside the analyzed conditions of their grid
stability analyses. The Summer licensee has confirmed that in the past
there had been other scrams from 100-percent power without degraded grid
voltage problems. However, the maximum MVARs supplied to the grid during
those plant trips was 317 MVARs whereas, during the recent event, the
generator was supplying 440 MVARs to the grid. One short-term corrective
measure taken by Summer was to limit MVAR generation to 300 MVARs unless
otherwise approved by their general manager of nuclear plant operations.
Thus, it appears that the licensee did not consider its maximum reactive
output in its grid stability analysis.
Licensees perform the grid stability analysis on the basis of system
information, such as transmission, generation, and load, that is available
at the time the plant is licensed. However, licensees may wish to review
their grid stability analysis whenever transmission, generation, or load
system changes occur that affect the original design basis condition to
ensure that the original design basis of the plant remains valid in
accordance with General Design Criterion 17, "Electric Power Systems."
Moreover, to avert recurrence of conditions that occurred at Summer,
licensees may also wish to periodically check the setpoints of the generator
backup relays.
This information notice requires no specific action or written response. If
you have any questions about the information in this notice, please contact
the technical contact listed below or the appropriate NRR project manager.
Charles E. Rossi, Director
Division of Operational Events Assessment
Office of Nuclear Reactor Regulation
Technical Contact: Om Chopra, NRR
(301) 492-0835
Attachment: List of Recently Issued NRC Information Notices
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