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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, 8912050250 . IN 89-83 December 11, 1989 Page 2 of 3 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. . 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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