United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 94-77: Malfunction in Main Generator Voltage Regulator Causing Overvoltage at Safety-Related Electrical Equipment

                                 UNITED STATES
                         NUCLEAR REGULATORY COMMISSION
                         WASHINGTON, D.C.  20555-0001

                               November 17, 1994

                               REGULATOR CAUSING OVERVOLTAGE AT 
                               SAFETY-RELATED ELECTRICAL EQUIPMENT


All holders of operating licenses or construction permits for nuclear power


The U.S. Nuclear Regulatory Commission (NRC) is issuing this information
notice to alert addressees that a malfunction in the main generator voltage
regulator could increase generator output voltage, which could cause an
overvoltage condition at the Class 1E buses powering safety-related electrical
equipment.  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 March 1, 1993, a steam extraction line to a feedwater heater ruptured at
Sequoyah Nuclear Plant, Unit 2.  The steam from the ruptured pipe engulfed the
nearby main generator voltage regulator cabinet.  The heat and moisture caused
the voltage regulator to malfunction, increasing the excitation to the main
generator (overexcitation).  The overexcitation caused the main generator
voltage to rise about 19 percent above the normal output voltage.  Main
generator alarms annunciated in the main control room.  After approximately 3
to 3� minutes, the operators tripped the main generator manually because they
could not control the generator output voltage.  The turbine and reactor
tripped in response to the generator trip.  Although the overvoltage transient
caused the voltage at the Class 1E buses to rise, the licensee determined
that, in this instance, the voltages were within the design limits of the
equipment involved.  

On April 16, 1993, North Anna Power Station, Unit 2, automatically tripped
while at 100-percent power in response to a main generator exciter field
breaker trip because of overexcitation (overvoltage).  The overexcitation was
caused by a malfunction in the voltage regulator circuitry.  

9411100237.                                                            IN 94-77
                                                            November 17, 1994
                                                            Page 2 of 2


The Class 1E buses that supply power to Class 1E electric equipment in a
nuclear power plant are powered either from the startup transformers or from
the main generator through the station service transformers.  Regardless of
the power source, the Class 1E buses can be exposed to overvoltage conditions
if the main generator voltage regulator fails in the high field
(overexcitation) position.  Overexcitation of the generator field can produce
overvoltage throughout the electrical system, including the switchyard.

Volts/hertz relays are usually supplied to protect generators and transformers
from overexcitation of the generator field.  When the ratio of volts to hertz
exceeds a preset value, the volts/hertz relay initiates an alarm and, after a
time delay, trips the main generator exciter field breaker.  When the exciter
field breaker opens, it interrupts excitation to the main generator field,
causing the generator to trip.  Although a volts/hertz relay was installed at
Sequoyah Unit 2, it was used only to initiate an alarm in the control room. 
In contrast, a similar relay installed at North Anna Unit 2 initiates an alarm
and also trips the main generator exciter field breaker.  In addition, the
transformers that supply power to the Class 1E buses from the grid at North
Anna are equipped with automatic load tap changers.  The load tap changers
automatically switch taps in response to changes in switchyard voltage to
maintain the proper voltage at the Class 1E buses.

After the event on March 1, 1993, the licensee for Sequoyah Unit 2 installed a
second volts/hertz relay.  This relay automatically trips the main generator
exciter field breaker to end generator overexcitation conditions that could
damage the generator, transformers, and Class 1E electrical equipment.  A
similar relay was already installed at Sequoyah Unit 1.

Main generator protective relays designed only to activate an alarm and not to
trip the field breaker in response to overexcitation may not protect the
associated Class 1E electrical equipment from voltages beyond the design
limits of the equipment.

This information notice requires no specific action or written response.  If
you have any questions about the information in this notice, please contact
one of the technical contacts listed below or the appropriate Office of
Nuclear Reactor Regulation (NRR) project manager.

                                    /S/'D BY BKGRIMES

                                    Brian K. Grimes, Director
                                    Division of Project Support
                                    Office of Nuclear Reactor Regulation

Technical contacts:  Peter J. Kang, NRR      Om P. Chopra, NRR
                     (301) 504-2779          (301) 504-3265
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