IE Circular No. 79-02, Failure of 120 Volt Vital AC Power Supplies
CR79002
January 11, 1979
MEMORANDUM FOR: B. H. Grier, Director, Region I
J. P. O'Reilly, Director, Region II
J. G. Keppler, Director, Region III
K. V. Seyfrit, Director, Region IV
R. H. Engelken, Director, Region V
FROM: Norman C. Moseley, Director, POI, IE
SUBJECT: IE CIRCULAR 79-02, FAILURE OF 120 VOLT VITAL AC POWER
SUPPLIES
The subject document is transmitted for issuance on January 16, 1979. The
Circular should be issued to all holders of Reactor Operating Licensees and
Construction Permits.
Also enclosed is a draft copy of the transmittal letter.
Norman C. Moseley, Director
Division of Reactor Operations
Inspection
Office of Inspection and Enforcement
Enclosures:
1. IE Circular No. 79-02
2. Draft Transmittal Letter
CONTACT: V. D. Thomas, IE
49-28180
.
(Transmittal letter for Circular 79-02 to each holder of an NRC Operating
Licensees.)
IE Circular No. 79-02
Addressee:
The enclosed Circular 79-02 ia forwarded to you for information. If there
are any questions related to your understanding of suggested actions, please
contact this office.
Signature
(Regional Director)
Enclosures:
1. IE Circular No.79-02
2. List of IE Circulars
Issued in the last
12 months
.
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF INSPECTION AND ENFORCEMENT
WASHINGTON, D.C. 20555
January 16, 1979
IE Circular No.79-02
FAILURE OF 120 VOLT AC POWER
Description of Circumstances:
On September 16, 1978, Arkansas Nuclear One - Unit 2, while in hot
functional testing preceding initial criticality, suffered a degradation of
both of the independent off-site power sources. the produced an undervoltage
condition on the Engineering Safety Features (ESF) Busses and caused an
inadvertent ESF actuation. The license determine that the ESF actuation
occurred on a loss of at least two of the uninterruptable Immediate
investigation revealed that all four of the Solidstate Controls, Inc.,
(SCI) inverter static switches had automatically transferred the alternate
power supply (the 120 volt output) figure 1 (attached) shows the functional
interconnection of the 120 volt AC power supply system.
A single conclusive cause of the undesired SCI inverter static switch
transfer could not be identified. However, the following problems were
discovered during subsequent investigation.
(a) The SCI inverters have circuitry to monitor the incoming DC voltage
level which, on sensing a low voltage (nominally 104 DC volts for these
inverters) will trip both input breakers to the inverter component
after an adjustable time delay.
The setting of these time delay relays were not verified during either
preoperational testing or subsequent maintenance. The time delay is
necessary in order to accommodate transient loading conditions which
may be encountered.
(b) On one SCI inverter, a DC fuse within the inverter component was found
blown. The vendor indicated that this fuse will blow due to an
excessive DC voltage to the inverter component caused by a transient on
the 480 AC input. The licensee has subsequently reset the taps on the
480V to 120V three-phase AC input transformer to limit the DC voltage
to the inverter to less than the nameplate maximum rating in the event
of a high AC input voltage transient
1 of 3
.
IE Circular No. 79-02 January 16, 1979
(c) The SCI inverter static switch is designed to transfer to an alternate
source on inverter output overcurrent or undervoltage. Initially, these
trips were set at 125 percent overcurrent and 80 percent undervoltage.
An explanation for the transfer of two of the inverter static switches
is that during one of the transfers of site loads, the instantaneous
inductive load caused these trip set points to be exceeded. The
icensee has subsequently adjusted the transfer sensing circuitry to
higher overcurrent and lower undervoltage trip settings to assure that
the inverter will maintain load over the widest possible range of
voltage and current. (Note that in this case, one the SCI inverter
transfers to the alternated source, it will not automatically transfer
back to normal source.)
All holders of operating licenses should be aware of the potential problems
of the types noted above. It is recommended that the following items be
considered in you review of this matter:
1. Determine whether or not time delay circuitry is used in your inverter
units. If so, have they been adjusted to the appropriate set point as
required by equipment and the integrated system designs?
2. Determine if the AC input voltage and transformer tap settings are
optimized to prevent exceeding the inverter component nameplate maximum
rate DC input voltage in the event of a high AC input voltage transient
3. If an alternate 120 volt source is used in your design, determine if
the protection transfer circuitry of the inverter has been optimized
within design limits to ensure maximum possible available optimized
within design limits to ensure maximum possible availability of the
inverter system during transient loading conditions An operating
history of inexplicable transfer my be indicative of the above.
4. Determine if the administrative controls employed by your facility
ensures operability of safety systems after it subcomponents (e.g.,
time delay relays, switches, etc.) have been subjected to maintenance
or testing.
This Circular is also being forwarded to all holders of construction permits
for their information with regard to preoperational and startup testing.
2 of 3
.
IE Circular No. 79-02 January 16, 1979
No written response to this Circular is required. If you require additional
information, contact the Director of the appropriate NRC Regional Office.
Attachment:
Figure 1
3 of 3
Page Last Reviewed/Updated Tuesday, March 09, 2021
Page Last Reviewed/Updated Tuesday, March 09, 2021