Information Notice No. 95-37: Inadequate Offsite Power System Voltages During Design-Basis Events

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

                               September 7, 1995

                               DURING DESIGN-BASIS EVENTS


All holders of operating licenses or construction permits for nuclear power


The Nuclear Regulatory Commission (NRC) is issuing this information notice to
alert addressees to circumstances that could result in inadequate offsite
power system voltages during design-basis events.  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

In response to a deficiency identified at Palo Verde Nuclear Generating
Station by an electrical distribution system functional inspection (EDSFI)
team, the licensee undertook an electrical design-basis reconstitution effort
to upgrade and reverify the voltage regulation calculations at the plant. 
Subsequently, Licensee Event Report (LER) 93-011, dated December 25, 1993,
reported on shortcomings in the plant site voltage regulation.  Specifically,
with the switchyard voltage in the lower two-thirds of its expected operating
range and the startup transformer heavily loaded, the Class 1E loads might
separate from the transformer and load onto the diesel generators; or the
Class 1E bus undervoltage relays may not actuate even though sustained,
substandard voltages might occur at the terminals of Class 1E loads.  The
heavy loading of the startup transformer would occur following a main
generator or turbine trip with successful fast bus transfer of house loads to
the transformer, or following manual transfer of house loads to the

In a recent supplement to LER 93-011, dated February 6, 1995, the Palo Verde
licensee identified a different series of events that could occur as a result
of the same problem.  If a loss of coolant accident (LOCA) should occur with
the switchyard voltage in the lower two-thirds of its operating range, the
engineered safety feature (ESF) loads would begin sequencing onto the
preferred offsite power source and the house loads would fast transfer to the
startup transformer following the main generator or turbine trip that would
accompany the LOCA.  The resulting voltage drops at the safety buses would
cause the bus degraded voltage relays to drop out during the ESF load

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sequencing and to fail to reset before timing out.  This circumstance would
result in the ESF loads separating from offsite power, load shedding of the
safety buses, closing of the diesel generator breaker, and resequencing of the
ESF loads onto the diesel generators.  The licensee is administratively
maintaining the switchyard voltage in the upper one-third of its operating
range until a long-term solution to the problem is in place.

On August 8, 1995, the licensee for Diablo Canyon reported the same voltage
vulnerability when one of the offsite power sources are unavailable.  

In LER 91-010, Supplement 1, dated March 27, 1993, the licensee for Arkansas
Nuclear One, (ANO), Units 1 and 2, reported that had a 500-kV autotransformer
been lost during summer peak conditions in the past, the 161-kV system might
not have been able to maintain adequate voltage to the safety loads of both
units unless local hydro generation was available and dispatcher actions were
taken to shed some of the 161-kV system load.  The 500-kV autotransformer is a
common link between the Unit 1 and Unit 2 startup transformers.  Upon a unit
trip the loads of each unit are automatically transferred to their respective
startup transformer.  If the 500-kV autotransformer that feeds both startup
transformers is lost or unavailable, the loads of both units are transferred
to a third common startup transformer fed from the 161-kV system.  The long-
term resolution of this problem was to install a voltage regulator from the
161-kV offsite power source.


In the case of Palo Verde and ANO, the licensees determined through their
analyses, which utilized updated computer-aided computational capabilities,
that offsite power system voltages that could occur over the course of a year
may not be adequate to support all design-basis events.  At Palo Verde the
weakness exists in the plant's electrical distribution system, and at ANO, the
problem existed in the offsite power switchyard and transmission system.

In the case of Palo Verde, the licensee determined that the normal anticipated
switchyard voltage variance is greater than the electrical system design could
accommodate and still provide acceptable onsite distribution system voltage
spread characteristics.  The licensee has indicated that the minimum and
maximum loading conditions used in previous analyses were respectively greater
than and less than those that actual plant operating experience could support,
the configuration control of transformer tap settings was not auditable, and a
fast bus transfer undervoltage blocking feature was not fully understood and
documented.  This situation occurred and went undetected because of weaknesses
in the original design and previous analyses of the plant's electrical
distribution system, and because of a less than full understanding of the
original design basis.

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At ANO, the licensee indicated that the 161-kV inadequate voltage problem
occurred because of an increase in the 161-kV grid loading with time.  The
problem went undetected because the ANO staff failed to periodically review 
the grid network voltage capability relative to the ANO voltage requirements
or to consider how many years the required minimum offsite voltage levels
could be maintained following the initial analysis.

NRC Generic Letters dated June 3, 1977 ("Millstone Nuclear Power Station
Degraded Grid Voltage"), and August 8, 1979 ("Adequacy of Station Electric
Distribution Systems Voltages"), which were subsequently replaced by Branch
Technical Position (BTP) PSB-1 in the NRC standard review plan, provide the
basis for original analyses and commitments on the degraded voltage issue.  
As part of this issue, licensees were asked to establish an anticipated range
of normal offsite grid voltages over which they were then required to
demonstrate that adequate voltages would be provided to the terminals of all
safety-related equipment for all design-basis events.  As identified by the
ANO and Palo Verde licensees, operating the plant outside a range of offsite
grid voltages that would provide adequate voltage to safety equipment, or that
would result in separation from the offsite power system because of operation
of degraded voltage protection relays may constitute a failure to meet plant
technical specifications relative to adequate capability and capacity of the
offsite power system circuits.

As demonstrated by the above described-events, failure to periodically update
the original voltage analyses as the result of changing offsite grid or plant
conditions could result in unintentional operation outside regulatory
requirements.  In addition, many licensees recently have identified a need to
increase the setpoints of their undervoltage protection relays to ensure
adequate voltages at the terminals of all safety-related equipment as a result
of NRC EDSFIs or their own in-house electrical inspection or design
reconstitution efforts.  Although this measure ensures that inadequate
voltages will not exist at the terminals of the equipment for any unacceptable
length of time, increasing the setpoints increases the potential for
separation from the offsite system during design-basis events over the range
of normally anticipated offsite system grid voltages.  An additional concern
in this area is plants with no upper limit on degraded voltage protection
relay setpoints.  If these setpoints are allowed to drift in the upward
direction, this trend could also lead to the same increased potential for
separation from the offsite power system during design-basis events.

An unanticipated voltage drop during emergency load sequencing process could
lead to sequencer lockup or circuit breaker operation that may require remote
manual reset action.  This problem was experienced at Vogtle Unit 1 and is
documented in NUREG 1410, "Loss of Vital AC Power and RHR During Mid-Loop
Operations at Vogtle Unit 1 on March 20, 1990."  Accident mitigation could be
delayed if procedures and training do not address restarting the sequencer and
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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 DMCrutchfield

                                   Dennis M. Crutchfield, Director
                                   Division of Reactor Program Management
                                   Office of Nuclear Reactor Regulation

Technical contacts:  James Lazevnick, NRR
                     (301) 415-2782
                     Thomas Koshy, NRR
                     (301) 415-1176


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