United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 84-02: Operating a Nuclear Power Plant at Voltage Levels Lower than Analyzed

                                                           SSINS No.:  6835 
                                                           IN 84-02        

                                UNITED STATES
                        NUCLEAR REGULATORY COMMISSION
                    OFFICE OF INSPECTION AND ENFORCEMENT
                           WASHINGTON, D.C.  20555
                                     
                              January 10, 1984


IE INFORMATION  NOTICE NO. 84-02:  OPERATING A NUCLEAR POWER PLANT AT 
                                   VOLTAGE LEVELS LOWER THAN ANALYZED 

Addressees: 

All nuclear power reactor facilities holding a construction permit (CP) or 
an operating license (OL). 

Purpose: 

The purpose of this notice is to:  (1) inform licensees of an event 
resulting from operating a nuclear power plant's electrical distribution 
system at voltage levels lower than that specified in the plant's safety 
analysis, and (2) describe design deficiencies of the plant's degraded 
voltage protection system.  

Description of Circumstances: 

On August 1, 1983, at about 4 a.m., the Monticello Nuclear Generating Plant 
was being operated at rated power.  Because of a lightly loaded grid, the 
main generator was being operated at a reduced voltage(i.e., 20.9 kV rather 
than at the nominal level of 22kV) in response to a request from the load 
dispatcher in order to reduce the voltages of the Monticello switchyard 
buses.  This request was made to compensate for voltage rises that can occur
on transmission lines that are lightly loaded.  Since the main generator 
feeds both the licensee's transmission network, via a main power transformer
to the plant's switchyard, and the plant's electrical distribution system, 
via an auxiliary transformer, the voltage levels of the license's 
transmission network and of the plant's electrical distribution system are 
directly proportional to the main generator's terminal voltage. 

During normal power operation, as was the case during this event, all the 
plant's electrical loads, including the two redundant 4160 V safety-related 
buses, are served by the main generator via an auxiliary transformer.  Since
the main generator was operating at a reduced terminal voltage, the two 
safety-related buses were also being energized at a reduced voltage level 
(i.e., 3960 V rather than the nominal level of 4160 V).  During this time, a 
large safety-related pump motor served by one of these buses was started. 
The voltage drop resulting from starting this large motor, coupled with the 
low initial bus voltage, caused the voltage to dip below the degraded 
voltage protection system's trip setpoint (3885 V).  As a result, the 
degraded voltage protection system was armed, and a time delay circuit was 
activated. The purpose of this time delay circuit is to accommodate the 
voltage drops resulting from starting 


8312120104 
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                                                           IN 84-02        
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motors by blocking actuation of the degraded voltage protection system for 
10 seconds. If, during this 10-second time delay, the bus voltage recovers 
to a level above the degraded voltage protection system's reset setpoint 
(3985V), the system will be reset and no further action will ensue. 
Otherwise, the  degraded voltage protection system will be actuated. On 
August 1,1983, since the bus voltage could not recover to the reset 
setpoint, the degraded voltage protection system was actuated.  It should be 
noted that the degraded voltage protection system will always be actuated 
when the safety-related buses are operating at levels below the reset 
setpoint (as was the case on August 1, 1983) and the voltage on either bus 
dips below the trip setpoint. 

In this instance, the following sequence of events was initiated upon 
actuation of the degraded voltage protection system:  (1) the normal feeder 
breaker serving the affected 4160 V safety-related bus automatically 
tripped: (2) both emergency diesel generators (EDGs)automatically started on 
an emergency basis: (3) when the EDG serving the affected bus reached its 
design speed and voltage, the feeder breaker from the EDG to the affected 
safety-related bus automatically closed, and (4) those loads on the affected 
bus that were not automatically shed were energized by the EDG.   Since the 
redundant 4160 V safety-related bus was not affected by this event, it was 
continuously served by the main generator via the auxiliary transformer. 

Discussion: 

Our investigation of this event revealed the following particulars, some of 
which may be applicable to other nuclear power facilities: 

1.   In response to NRC's multiplant action item no. B-48, the licensee 
     submitted a worst-case analysis supporting the design of the plant's 
     electrical distribution system.  This analysis was based on a heavily 
     loaded grid, and included the assumptions that under such conditions 
     the minimum bus voltages at the Monticello switchyard would be 350.8 kV 
     and 119.7 kV on the 345 kV and 115 kV buses, respectively. Based on 
     these assumptions, the licensee calculated that the minimum 
     steady-state voltages on the 4160 V buses would be 4025 V after all ESF 
     loads were operating (i.e., greater than 4025 V prior to actuation of 
     the ESF loads, but the actual value was not stipulated).  In contrast, 
     because of the aforementioned lightly loaded grid, the 345 kV bused 
     were being operated at 344 kV and the 4160 V safety-related buses at 
     3960 V at the time of the event.  

     Our review of this event indicated that the licensee's analysis did not
     take into account the effects of a lightly loaded grid.  Furthermore, 
     our review revealed that the licensee did not consider the minimum 
     value assumed in the analysis as being an operational limit.  
     Consequently, at the time of the event, as well as at other times 
     during which the grid was lightly loaded, the plant was being operated 
     at voltage levels below those used to assess the adequacy of the 
     plant's safeguards distribution system and for which the degraded 
     voltage protection system had been designed. This event clearly 
     demonstrates that nuclear power facilities 
.

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     should not be operated in response to external requests unless the 
     effects of such requests are fully understood and such operation is 
     within the bounds of the plant's analysis. 

2.   It appears that the licensee had not fully evaluated the design 
     features and the operating requirements of the plant's degraded voltage 
     protection system before the August 1, 1983 event in that the operators 
     were not aware that the reset setpoint of the degraded voltage 
     protection system was higher than the actual voltages on the 4160 V 
     safety-related buses. 

     The trip setpoint of the Monticello degraded voltage protection system 
     is 3885 V, and the reset setpoint is 3985 V with a 10-second time 
     delay. 

     This means that whenever the trip setpoint is reached during an 
     electrical transient, such as the starting of a motor, the steady-state
     voltage must exceed the trip setpoint value by at least 100 V within 10
     seconds.  If these conditions are not met, the degraded voltage 
     protection system will be actuated.  Under these design conditions, if 
     the initial voltage on the 4160 V safety-related buses is only slightly
     above the reset setpoint of 3985 V and the bus voltage dips below the 
     trip setpoint of 3885 V, the degraded voltage protection system might 
     be actuated.  If the 4160 V safety related buses are being energized 
     below the reset setpoint (e.g., 3960 V as was the case on August 1, 
     1983) and the bus voltage dips below the trip setpoint of 3885 volts, 
     the degraded voltage protection system will be actuated. 

3.   The following definitions and descriptions of electrical systems used 
     in nuclear power facilities are taken directly from IEEE Std 308-1974, 
     "IEEE Standard Criteria for Class IE Power Systems for Nuclear Power 
     Generating Stations":  Preferred Power Supply (Definition ) - "That 
     power supply which is preferred to furnish electric energy under 
     accident or post-accident conditions": Preferred Power supply 
     (Description) - "The preferred power supply shall consist of one or 
     more circuits from the transmission network or equivalent source of 
     electric energy to the Class IE distribution system input terminals"; 
     Standby Power Supply (Description) - "The standby power supply shall 
     consist of all components from the stored energy to the connection to 
     the distribution system's supply circuit breaker (for example, prime 
     movers; generators and excitation equipment; control system;...)." 

     The above definitions are included to compare Monticello's degraded 
     voltage protection systems with the recommendations of IEEE Std 308-
     1974.  Actuation of Monticello's degraded voltage protection system 
     automatically transfers the affected 4160 V safety-related bus from its
     normal source of power (the main generator) to its associated EDG (a 
     standby power supply), even though a circuit from the transmission 
     network may be available.  Thus, although the design of Monticello's 
     degraded voltage protection system conforms with the requirements of 
     IEEE 308-1974, it exposes the standby power supply to unwarranted 
     challenges. 
.

                                                           IN 84-02        
                                                           January 10, 1984 
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Corrective action taken by the licensee to assure that the plant's 
electrical distribution system is operated within the bounds for which it 
was analysed included: 

1.   Reanalyzing the plant's electrical distribution system to determine the
     minimum operating voltage on the 4160 V safety-related bused: 

2.   Developing operating procedures for actions to be taken in the event 
     that the voltage on a 4160 V safety-related bus falls below 4025 volts:

3.   Evaluating the feasibility of operating the 4160 V safety-related bused
     from an offsite power source during normal plant operation; and 

4.   Developing administrative controls to assure that the voltage on the 
     345 kV and 115 kV switchyard buses remain above the minimum values 
     necessary to assure that the voltages on the 4160 V safety-related 
     buses remain above their analyzed minimum values. 

This notice highlights the need for licensees to be more fully aware of the 
bases and limits of the plant's analyses and identifies a specific 
deficiency in a degraded voltage protection system.  although no written 
response to this notice is requested, it is suggested that holders of 
licenses review the information for applicability at their facilities. 

Please refer questions regarding this notice to the Regional Administrator 
of the appropriate NRC Regional Office or a technical contact listed below. 



                                   Edward L. Jordan, Director 
                                   Division of Emergency Preparedness 
                                     and Engineering Response 
                                   Office of Inspection and Enforcement 

Technical Contacts: I. Villalva, IE
                    (301) 492-9635

                    J. T. Beard, NRR
                    (301) 492-7465

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