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
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IN 84-02
January 10, 1984
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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.
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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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