United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 91-81: Switchyard Problems that Contribute to Loss of Offsite Power

                               UNITED STATES 
                       NUCLEAR REGULATORY COMMISSION 
                    OFFICE OF NUCLEAR REACTOR REGULATION 
                          WASHINGTON, D.C.  20555 

                             December 16, 1991 


NRC INFORMATION NOTICE 91-81:  SWITCHYARD PROBLEMS THAT CONTRIBUTE TO 
                               LOSS OF OFFSITE POWER 


Addressees 

All holders of operating licenses or construction permits for nuclear power 
reactors. 

Purpose 

The U.S. Nuclear Regulatory Commission (NRC) is issuing this information 
notice to alert addressees to several problems associated with plant 
switchyards.  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 are not NRC requirements; therefore, no specific action or written 
response is required. 

Description of Circumstances 

On April 23, 1991, a complete loss of offsite power occurred at the Vermont 
Yankee Nuclear Power Station as a result of maintenance activities in the 
switchyard.  Maintenance workers were installing a new battery for one of 
the two non-1E 125 VDC buses.  The two normally independent buses were cross 
tied through the swing battery charger 4A-5A after defeating a mechanical 
interlock (Figure 1).  Furthermore, the licensee was paralleling the battery 
chargers on bus DC-4A without a battery on this bus.  When the 4A-5A battery 
charger output breaker feeding bus DC-5A was opened prior to connection of 
the new battery 4A to bus DC-4A, a voltage transient propagated through the 
switchyard DC control system that caused all but one of the 345kV and 115kV 
circuit breakers to trip and lock open.  The loss of offsite power which 
occurred caused a main turbine and automatic reactor trip.  Restoration of 
full offsite power took thirteen hours. 

The NRC dispatched an augmented inspection team (AIT) to investigate this 
event, and the following generic concerns were identified by the team in its 
report (50-217/91-13):  (1) a lack of preventive maintenance on battery 
chargers, (2) presence of sensitive components in logic cards making them 
susceptible to failure, (3) switchyard protective relays that were 
susceptible to a single failure, and (4) a lack of clear authority over the 
switchyard during emergency conditions. 


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Lack of Preventive Maintenance 

Three of the four non-1E battery chargers (4A, B, and 4A-5A) in the 
switchyard DC control system were in a degraded condition at the time of the 
event.  In each of the degraded battery chargers, manufactured by Exide, the 
output filter circuit was not functional.  The problems included blown fuses 
in one filter circuit, blown and wrong-sized fuses in another, and a failed 
capacitor in the third filter circuit.  If the 4A-5A battery charger output 
filter circuit had been functional, the effect of the voltage transient may 
have been mitigated and loss of offsite power might have been prevented.  
The problems resulted directly from infrequent preventive maintenance on the 
battery chargers.  The licensee had no regular maintenance program for the 
chargers and it appeared that the last maintenance was performed in 1985.  
These battery chargers are not safety related and no surveillance testing or 
preventive maintenance is required.  However, at this facility, the 
switchyard battery system, including the switchyard batteries, their 
associated chargers, and associated distribution panels, is part of the 
auxiliary electrical power system and is required to be operable by the 
plant's Technical Specifications. 

Sensitive Components in Logic Cards 

The voltage spike in the switchyard DC control system destroyed zener diodes 
in the stuck breaker failure units (SBFUs) of two switchyard circuit 
breakers which led to a chain reaction that caused most of the breakers in 
the 345kV and 115kV switchyard to trip and lock open.  These SBFUs are 
manufactured by ASEA Brown Boveri, and previous models were made by the 
Westinghouse Electric Corporation.  The manufacturer has developed new units 
that are not as susceptible to voltage spikes because the new units do not 
contain zener diodes. 

Protective Relays Subject to a Single Failure 

All of the SBFU relays are powered from a single bus (DC-4A) even though two 
DC buses are available in the switchyard DC control system.  This design is 
subject to a single failure.  Other facilities may have similar protective 
relaying designs that could be vulnerable to a common cause failure. 

Authority Over the Switchyard Under Emergency Conditions 

The restoration of offsite power was delayed because of a lack of 
communications between plant staff and the transmission and distribution 
(T&D) personnel.  Under normal conditions, T&D is responsible for operating 
the switchyard.  The licensee's procedure specifies that, under emergency 
conditions, the responsibility is transferred to the plant staff.  However, 
the procedure does not define "emergency."  These operational problems 
suggest the need to clarify switchyard authority during a loss of offsite 
power or other emergency conditions when timely switchyard activities are 
important. 

Additional Example of Switchyard Communication Problems 

Another example of communication problems in the switchyard can be seen in 
an event that took place at the McGuire Nuclear Generating Station, Unit 1, 
on February 11, 1991.  The unit tripped from 100% power when T&D personnel 
caused 
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                                                       Page 3 of 3 


a loss of all offsite power while testing a modification to the protective 
relay circuitry in the 230 kV switchyard.  Since the majority of the 
switchyard is outside of the plant equipment boundary, the T&D personnel did 
not notify the station operations personnel that work was in progress even 
though switch-yard activities can impact the station.  Prior to this event 
there was no agreement between station operations and T&D personnel on how 
to handle switch-yard activities outside the plant equipment boundary.  
Subsequent to this event, a work control policy was established to provide 
improved communication between the station operations personnel and T&D 
personnel. 

No specific action or written response is required by this information 
notice. If you have any questions about this matter, please contact one of 
the technical contacts listed below or the appropriate Office of Nuclear 
Reactor Regulation (NRR) project manager. 




                                   Charles E. Rossi, Director 
                                   Division of Operational Events Assessment 
                                   Office of Nuclear Reactor Regulation 

Technical contacts:  Cliff Anderson, Region I 
                     (215) 337-5376 

                     Peter Kang, NRR 
                     (301) 504-0779 

                     David Skeen, NRR 
                     (301) 504-1174 

Attachments:  
1.  Figure 1:  Switchyard House DC Distribution 
      One Line Diagram 
2.  List of Recently Issued NRC Information Notices 



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