United States Nuclear Regulatory Commission - Protecting People and the Environment

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 
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                                                         August 29, 1984  
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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. 

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                                                         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 

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