United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 91-29, Supplement 3: Deficiencies Identified During Electrical Distribution System Functional Inspections

WASHINGTON, D.C.  20555-0001

November 22, 1995

                                             ELECTRICAL DISTRIBUTION SYSTEM
                                             FUNCTIONAL INSPECTIONS


All holders of operating licenses or construction permits for nuclear power


The U.S. Nuclear Regulatory Commission (NRC) is issuing this information
notice supplement to provide additional information on deficiencies the staff
found during electrical distribution system functional inspections (EDSFIs) at
nuclear plants.  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.


During various multi-discipline inspections, the NRC had identified numerous
deficiencies related to the electrical distribution system.  To address these
deficiencies, the NRC developed the EDSFI to specifically evaluate the
electrical distribution system.  Attachment 1, "Related Information Notices,"
contains previously identified deficiencies from NRC inspections.  

Description of Circumstances

The EDSFIs focused on the capacity of the electrical distribution system to
perform its intended functions during all plant operating and accident
conditions and the capability and performance of engineering organizations in
providing engineering and technical support for the operation of the
electrical distribution system.  The NRC has completed inspections of all
operating plants in accordance with Temporary Instruction (TI) 2515/107,
"Electrical Distribution System Functional Inspection," and this IN supplement
addresses deficiencies identified during the EDSFIs in addition to those
discussed in the listed information notices.

The inspections findings included the following:  1) availability and
reliability of offsite power sources, 2) design review of inadequate fast bus
transfer, 3) inadequate tests for emergency diesel generator systems, and 
4) deficiencies in class 1E battery systems.  These findings are discussed in

9511130071.                                                            IN 91-29, Supp. 3
                                                            November 22, 1995
                                                            Page 2 of 2

detail in Attachment 2, "Inspection Findings for Electrical Distribution
System Functional Inspections."

This information notice requires no specific action or written response.  If
you have any questions about the information in this notice, please contact
the technical contact 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 contact:  Roy K. Mathew
                    (301) 415-2965

1.  Related Information Notices
2.  Inspection Findings for Electrical Distribution System 
      Functional Inspections
3.  List of Recently Issued NRC Information Notices

.                                                            Attachment 1
                                                            IN 91-29, Supp. 3
                                                            November 22, 1995
                                                            Page 1 of 1


(1)   IN 91-29, "Deficiencies Identified During Electrical Distribution System
      Functional Inspections," dated April 15, 1991, addressed inadequate ac
      voltages at the 480-Vac and 120-Vac distribution levels, inadequate
      procedures to test circuit breakers, and inadequate determinations and
      evaluations of setpoints.

(2)   IN 91-51, "Inadequate Fuse Control Programs," dated August 20, 1991.

(3)   IN 92-40, "Inadequate Testing of Emergency Bus Undervoltage Logic
      Circuitry," dated May 27, 1992.

(4)   IN 92-53, "Potential Failure of Emergency Diesel Generators Due to
      Excessive Rate of Loading," dated July 29, 1992.

(5)   IN 91-29, Supplement 1, "Deficiencies Identified During Electrical
      Distribution System Functional Inspections," dated September 14, 1992,
      addressed the following:  inadequate undervoltage relay setpoints for
      degraded voltage conditions, inadequate interrupting capacity of fault
      protection devices, improper coordination of fault protection devices,
      inadequate analysis of emergency diesel generator (EDG) capacity to
      power safety-related loads during postulated accidents, and deficiencies
      in EDG mechanical interface systems.

(6)   IN 92-77, "Questionable Selection and Review to Determine Suitability of
      Electropneumatic Relays For Certain Applications," dated November 17,

(7)   IN 91-29, Supplement 2, "Deficiencies Identified During Electrical
      Distribution System Functional Inspections," dated December 22, 1993,
      addressed the potential for the electrical bus transfer logic allowing
      repeated bus transfers between the two offsite sources supplying power
      to the safety-related buses.   

(8)   IN 94-19, "Emergency Diesel Generator Vulnerability to Failure From Cold
      Fuel Oil," dated March 16, 1994.
.                                                            Attachment 2
                                                            IN 91-29, Supp. 3
                                                            November 22, 1995
                                                            Page 1 of 4


Discussion of Inspection Findings

Availability and Reliability Issues Regarding Offsite Power Sources

During the EDSFI at the Millstone 1 facility (Report No. 50-245/91-81, dated
December 30, 1991), the inspection team noted that the licensee had conducted
load flow and stability studies in 1979 and had conducted additional studies
to support the startup of the Millstone 3 facility in 1982.  The team asked
the licensee to confirm the validity of those studies with respect to the
current design and the expected system load in the next few years.

Subsequent analysis by the licensee determined that the offsite power supply
could become unstable after an occurrence of disturbances in the vicinity of
Millstone station caused by over 5,000 MW of power that had been added to the
New England Power Pool by cogeneration and independent power producers.  The
grid power flow conditions had changed as a result of the addition of these
generating units not considered in the earlier studies.  The licensee imposed
restrictions on the power level of the Millstone units to prevent instability.

To account for fluctuations in power flow conditions and to ensure stability,
licensees generally perform periodic analyses for the following conditions in
order to maintain current licensing bases:  (1) the largest generating
capacity being supplied to the grid is lost; (2) the largest load is lost from
the grid; (3) the most critical transmission line is lost; or (4) the unit
itself is lost.

At the Arkansas Nuclear One (ANO) facility (Report No. 50-313/92-16, dated
July 2, 1992), the EDSFI team observed that under certain loading conditions,
sufficient voltage would not be available for one of the offsite electrical
power supplies.  The two required sources of offsite power are supplied to the
ANO site through the 500-kV switchyard and the 161-kV switchyard.  The 500-kV
switchyard normally supplies power through an onsite autotransformer to the
161-kV switchyard.

The licensee engineering evaluation showed that the voltage at the 161-kV
switchyard would not be adequate under all operating conditions if the 500-kV
system was not available.  The degradation in the available voltage occurred
because of increased offsite loading of the 161-kV transmission system.  The
degradation had not been detected because the 500-kV transmission system had
maintained the 161-kV transmission system at acceptable voltage levels.  After
becoming aware of the deficient condition, the licensee implemented immediate
procedural changes to ensure that two acceptable sources of offsite electrical
power were available and that long-term modifications were made to the 161-kV
transmission system before peak loading.  The procedural changes and the
modifications ensured that adequate voltage levels would be available under
all operating conditions.
.                                                            Attachment 2
                                                            IN 91-29, Supp. 3
                                                            November 22, 1995
                                                            Page 2 of 4

Design Review of Inadequate Fast Bus Transfer

At the Palo Verde site (Report No. 50-528/90-42, dated December 27, 1990), the
EDSFI team noted that non-Class 1E loads from the unit auxiliary transformer
were transferred to the startup transformer by an automatic bus transfer
scheme upon a reactor trip, a turbine trip, or a loss-of-coolant accident. 
The effects of these additional loads on the ratings of startup transformer
windings and available voltages to engineered safety features components had
not been considered.  As a result, the availability of the offsite power
source could have been affected.

At the Millstone 1 facility (Report No. 50-245/91-81, dated
December 30, 1991), the EDSFI team noted that the licensee was unable to
verify that the fast bus transfer was analyzed or tested to demonstrate the
operation of fast bus transfer logic.  Further analysis and tests by the
licensee to verify the operation of the fast bus transfer scheme showed that
the scheme would not function as designed.  Specifically, the completion time
for sending the signal and tripping the unit generator output breakers was
longer than assumed; hence, the breaker could have closed with an unacceptable
phase angle between the unit and the offsite power source.  In such a case, it
would be possible to damage running safety-related motors.  

The licensee review also identified that if a three phase bolted fault in the
main stepup transformer occurs, with the switchyard generator output breakers
taking more time than assumed to trip, it could result in the loss of the 345-
kV grid.

Inadequate Tests for Emergency Diesel Generator Systems 

At Vermont Yankee (Report No. 50-271/92-81, dated September 30, 1992) and
other facilities, the EDSFI teams determined that the check valves between the
non-Class 1E air compressors and the diesel generator air start receivers were
not tested for any backleakage.  Under the inservice test program, these
valves were not adequately tested to demonstrate that the disk traveled to the
seat promptly on cessation or reversal of flow.  Therefore, there was no
assurance that an adequate supply of air would be available to start the
diesel generators if the nonsafety-related compressor was lost or a failure
occurred in the nonsafety-related portion of the air start system.

At Nine Mile Point 1 (Report No. 50-220/91-80, dated January 10, 1992),
Millstone 1 (Report No. 50-245/91-81, dated December 30, 1991), and other
facilities, the EDSFI teams determined that the diesel generator tests were
performed to the kilowatt load requirement of the generator rather than the
kilovoltampere (kVA) and kilovars (kvar) requirements.  Specifically, the
steady-state tests were conducted at the near unity power factor rather than
at the EDG rated power factor or the actual load power factor.  This loading
condition did not meet either the licensee calculated design-basis accident
load or the rated load of the generator.  As a consequence, the current
producing/carrying components of the generators were not adequately tested to
demonstrate that the diesel generators could carry all of the safety-related
loads during a design-basis event..                                                            Attachment 2
                                                            IN 91-29, Supp. 3
                                                            November 22, 1995
                                                            Page 3 of 4

IN 91-13, dated March 4, 1991, discussed inadequate testing of emergency
diesel generators due to insufficient electrical loading as well as
insufficient compensation for the expected electrical power factor. 

Deficiencies in Class 1E Battery Systems

The EDSFI teams noted deficiencies in the Class 1E battery systems at various
sites.  The inspection reports containing various deficiencies are referenced

(1)   Several licensees were not using adequate design margins, temperature
      correction factors, or aging factors for sizing the batteries.  Normal
      industry design practices are:

      (a) Addition of a design margin of 10 to 15 percent to the cell size, as
          determined by calculations, to provide sufficient capacity margin
          for unanticipated loads, maintenance uncertainties, and poor battery
          operating conditions. 

      (b) Use of an appropriate temperature correction factor to compensate
          for the reduction in cell capacity when the anticipated minimum
          battery electrolyte temperature is below the standard value of 25 �C
          [77 �F].  The temperature correction factor ensures the available
          battery capacity during low temperature conditions.

      (c) Use of a 1.25 factor to compensate for degradation caused by aging
          to ensure that batteries have adequate capacity at the end of their
          service life.

(2)   The teams determined that the design-basis accident loads were not
      conservatively accounted for in the loading calculation.  Failure to
      determine accurate load current values for the accident loads raises
      uncertainty concerning available battery margins.

(3)   The teams determined that voltage drop studies were insufficient for
      determining if adequate voltage would be available to the Class 1E
      inverters and to the connected loads.  For voltage drop studies, the
      voltage drop for the most limiting components was not considered, nor
      were the battery discharge limits set to ensure that adequate voltage
      limits existed at the end devices. 

(4)   The teams determined that the battery service test procedures did not
      incorporate appropriate acceptance criteria for the final battery
      voltage as required by the design calculations to ensure adequate
      voltage for the safety-related components.  One team also found that the
      battery service tests did not account for all the design loads required
      by the design calculations to demonstrate the capability of the battery.

.                                                            Attachment 2
                                                            IN 91-29, Supp. 3
                                                            November 22, 1995
                                                            Page 4 of 4

(5)   The teams identified one or more of the following deficiencies in
      heating, ventilation, and air conditioning (HVAC) systems for the
      safety-related battery rooms.  Failure of the plant battery room HVAC
      systems to maintain temperature at or above the design minimum
      temperature for a design-basis event could result in decreased capacity
      and capability of the battery to meet its intended safety-related

      (a) No design provisions had been made to ensure that the temperature of
          the safety-related battery room would remain above the minimum
          required design-basis temperature.

      (b) Nonsafety-related heaters were installed to maintain an adequate
          temperature in the safety-related battery rooms, and there were no
          alarms or indications to alert the operators regarding low
          temperature conditions.

      (c) There was no evidence of seismic qualification of room heaters for
          maintaining the design-basis temperature in the battery rooms.

Inspection Report References for Class 1E Battery Systems Deficiencies

1.    ANO facility (Report No. 50-313/92-16, dated July 2, 1992)
2.    Calvert Cliffs 1 & 2 (50-317, 50-318/92-80, June 5, 1992)
3.    Catawba 1 (50-413/92-01, March 18, 1992)
4.    Crystal River 3 (50-302/93-18, September 10, 1993)
5.    Dresden 2 (50-237/91-201, September 20, 1991)
6.    Fermi 2 (50-314/91-017, October 9, 1991)
7.    Haddam Neck (50-213/91-80, May 3, 1991)
8.    Indian Point 3 (50-286/91-80, August 14, 1991)
9.    La Salle 2 (50-374/91-019, November 10, 1991)
10.   Limerick 1 & 2 (50-352, 50-353/92-81, December 4, 1992)
11.   Oyster Creek (50-219/92-80, July 7, 1992)
12.   Perry 1 (50-440/91-005, June 27, 1991)
13.   Prairie Island 1 (50-282/93-007, June 11, 1993)
14.   San Onofre 2 & 3 (50-361, 50-362/89-200, January 12, 1990)
15.   Waterford 3 (50-382/90-023, April 10, 1991)

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