Bulletin 79-23: Potential Failure of Emergency Diesel Generator Field Exciter Transformer
Accession No. 7908220104
SSINS No.: 6820
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF INSPECTION AND ENFORCEMENT
WASHINGTON, D.C. 20555
September 12, 1979
IE Bulletin No. 79-23
POTENTIAL FAILURE OF EMERGENCY DIESEL GENERATOR FIELD EXCITER TRANSFORMER
Description of Circumstances:
Florida Power and Light and Light Company recently reported a problem
encountered during a 24-hour full load test of the emergency diesel
generators (EDG) at their Turkey Point facility. Approximately 10 hours into
the test, the A-EDG tripped due to a differential-relay lockout on B and C
phases; the B-EDG was manually stopped, thus interrupting the test at that
point in time.
Subsequent investigation and testing by the licensee revealed a design error
on both the A and B EDGs which resulted in overheating of the Exciter Power
Transformers (EPTs) at sustained high load operation.
The following nameplate data applies to the equipment installed at Turkey
Point:
Emergency Diesel Generator
General Motors (Electro-Motive Division)*
Model EMD-999-20
Engine-turbocharged, 2 cycle,
EMD design 20-64SE4
Generator-EMD-design Model A-20
Exciter Power Transformer
GE-single phase
Model - 9T24Y1004
Serial-MD
Cycles-60
KVA 15
Insulation-4160 V
The manufacturer's findings and recommendations regarding the above problem
are described below:
"A potential problem can exist if the neutral of the generator and the
neutral of the primary windings of the excitation power transformer (EPT)
(sometimes referred to as the control power transformer (CPT)) are
connected. A direct connection
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IE Bulletin No. 79-23 September 12, 1979
Page 2 of 3
between the neutrals, or a connection through common grounding of both
neutrals, are equally undesirable conditions. Whenever either of these
undesirable conditions exist, high circulating currents can be induced by
harmonics. These currents may exceed transformer ratings and result in
transformer damage or failure.
The connection between the neutrals, either direct or through common
grounding, may have been designated in the original wiring design, or may
have been subsequently added by a contractor or by power plant personnel to
balance excitation transformer voltages relative to the generator. No
significant benefit is obtained by balancing the primary voltage of the
excitation transformer by means of these connections.
According to the manufacturer, "to avoid this potential transformer damage
or failure, the circuitry of each installation should be examined to
determine if a circuit exists between the neutral of the primary windings of
the EPT or CPT and the generator neutral. If the condition exists, the
neutral circuit should be disconnected, and the transformer primary neutral
allowed to float."
Licensees may utilize Regulatory Guide 1.108, Revision 1, as a reference for
the periodic testing of diesel generator units used as on-site electric
power systems at nuclear power plants.
Action to be Taken by Licensees:
For all power reactor facilities with an operating license or a construction
permit:
1. Determine whether or not connections have been made between low KVA
rated transformers and high KVA rated EDGs without adequate limitations
on the flow of circulating currents. If applicable, provide a
description of the corrective action being taken to address this
problem.
2. Provide a schedule for the completion of a sustained full-load
operation test of the EDGs for a duration of not less than 24 hours, or
provide the results of the similar long duration, full-load test which
has already been completed on the EDGs installed at your facility. The
test should demonstrate full-load carrying capability for an interval
of not less than 24 hours, of which 22 hours should be at a load
equivalent to the continuous rating of the diesel generator and 2 hours
at a load equivalent to the 2 hour rating of the diesel generator. The
test should also verify that voltage and frequency requirements are
maintained and that the cooling system functions within design limits.
3. Provide a written report of the above actions within 45 days of the
receipt of this Bulletin.
Reports should be submitted to the Director of the appropriate NRC Regional
Office. A copy of your report should be sent to the U. S. Nuclear Regulatory
Commission, Office of Inspection and Enforcement, Division of Reactor
Operations Inspection, Washington, D.C 20555.
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IE Bulletin No. 79- 23 September 12, 1979
Page 3 of 3
Approved by GAO, B180225 (ROO72); clearance expires 7/31/80. Approval was
given under a blanket clearance specifically for identified generic
problems.
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