Information Notice No. 84-69: Operation of Emergency Diesel Generators
SSINS No.: 6835 IN 84-69 UNITED STATES NUCLEAR REGULATORY COMMISSION OFFICE OF INSPECTION AND ENFORCEMENT WASHINGTON, D.C. 20555 August 29, 1984 Information Notice No. 84-69: OPERATION OF EMERGENCY DIESEL GENERATORS Addressees: All nuclear power reactor facilities holding an operating license (OL) or construction permit (CP). Purpose: This notice is provided to alert recipients of potentially significant safety problems that can arise when one or more emergency diesel generators (EDGs) are operated in modes (such as in parallel with the offsite power sources) other than the prescribed standby service mode. Experience has shown that such a practice can lead to a complete loss of ac power to safety buses. It is expected that recipients will review this information for applicability to their facilities and consider actions, if appropriate, to preclude similar problems occurring at their facilities. However, suggestions contained in this notice do not constitute NRC requirements; therefore, no specific action or written response is required. Description of Circumstances: On May 17, 1983, at Fort St. Vrain Unit 1, with the reactor in a shutdown condition and one of the two EDGs out of service for maintenance, the offsite power system started experiencing problems as a result of high winds and snow. As a precautionary measure, the available EDG was started and tied to the associated safety bus in parallel with the offsite power source. Approximately half an hour later, all offsite power to the plant was lost and the output breaker of the operating EDG tripped, apparently on overload. As a result, the plant was without all ac power, except for the inverter ac power off the dc power system, for approximately half an hour until the EDG was restored. The offsite power was restored after another hour. Grand Gulf Nuclear Station Unit 1 was in a startup mode on May 7, 1984, when a partial loss of offsite power occurred as a result of heavy winds and rain. This caused automatic starting and loading of the Division III EDG. While paralleling the EDG with the offsite power grid in order to restore normal power lineup, the EDG tripped on reverse power. Later, the site entered a tornado watch and all three EDGs were started and loaded on their respective buses in parallel with the offsite power grid. An hour and a half later, the Division II EDG tripped on reverse power, apparently as a result of the grid voltage fluctuations during the storm and a low reverse power trip set point. 8408027049 . IN 84-69 August 29, 1984 Page 2 of 3 Discussion The EDGs are provided as sources of standby onsite electric power in the event that offsite power is lost. Regulatory requirements have long been to minimize the probability of losing the onsite sources when power from the offsite transmission network (grid) is disturbed or lost. More specifically, the NRC Standard Review Plan prohibits the use of EDGs for purposes other than supplying standby power, when needed, and permits interconnection of the onsite and offsite sources only for short periods of time for the purpose of EDG load retesting. During such testing, only one of the redundant EDGs is to be paralleled at any one time, leaving the other EDG(s) available in standby service. Although operators may be tempted to start the EDGs when offsite power is threatened or undergoing disturbances, running the EDGs is likely to be more of a hindrance than a help. If an EDG is paralleled with the offsite power system, it is vulnerable to loss from any of the normal protective features such as overload or reverse power, especially at the moment that offsite power is interrupted. Such a practice is contrary to the intent of General Design Criterion 17. To serve as a dependable backup power source, the EDGs must be kept separate from the offsite source. One scheme suggested was to start the EDGs in anticipation of the loss of offsite power. If dummy loads are not included in the plant design, the operator is forced either to load the EDG or leave it at no-load idle. Operating experience reported some years ago in NUREG/CR-0660, "Enhancement of Onsite Diesel Generator Reliability," indicates that running an EDG at no-load or light loads may cause other EDG problems. Another scheme suggested was to run the EDGs on the safety buses but to isolate these buses from the offsite power system. While this appears, on the surface, to achieve the desired independence, licensees need to consider other aspects of the situation. In most plant designs, safety loads (needed for either an accident situation or safe shutdown without an accident) will not be automatically sequenced onto the EDG if the bus is isolated and the EDG is providing power to the bus. Applicants and licensees are expected to review this information for applicability to the onsite power system at their facilities and to initiate appropriate actions such as management directives and training to preclude similar problems from occurring at their facilities. . IN 84-69 August 29, 1984 Page 3 of 3 If you have any questions regarding this matter, please contact the Regional Administrator of the appropriate regional office or this office. Edward L. Jordan, Director Division of Emergency Preparedness and Engineering Response Office of Inspection and Enforcement Technical Contacts: J. T. Beard, NRR (301) 492-7465 R. N. Singh, IE (301) 492-0868 Attachment: List of Recently Issued IE Information Notices
Page Last Reviewed/Updated Tuesday, March 09, 2021
Page Last Reviewed/Updated Tuesday, March 09, 2021