United States Nuclear Regulatory Commission - Protecting People and the Environment

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

                                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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