United States Nuclear Regulatory Commission - Protecting People and the Environment

Bulletin 79-23: Potential Failure of Emergency Diesel Generator Field Exciter Transformer

                                                  Accession No. 7908220104 
                                                  SSINS No.: 6820         

                                UNITED STATES
                           WASHINGTON, D.C. 20555
                             September 12, 1979

                                                      IE Bulletin No. 79-23 


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 

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
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 

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

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 

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. 

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 
Page Last Reviewed/Updated Friday, May 22, 2015