United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 86-87: Loss of Offsite Power Upon an Automatic Bus Transfer

                                                            SSINS No.:  6835
                                                            IN 86-87 

                                UNITED STATES
                        NUCLEAR REGULATORY COMMISSION
                    OFFICE OF INSPECTION AND ENFORCEMENT
                           WASHINGTON, D.C. 20555

                              October 10, 1986

Information Notice No. 86-87:   LOSS OF OFFSITE POWER UPON AN AUTOMATIC 
                                   BUS TRANSFER 

Addressees: 

All nuclear power reactor facilities holding an operating license or a 
construction permit. 

Purpose: 

This notice is to alert recipients of a potentially significant problem 
involving the loss of offsite power following an automatic bus transfer. It 
is expected that recipients will review this information for applicability 
to their facilities and consider actions, if appropriate, to preclude a 
similar problem occurring at their facilities. However, suggestions 
contained in this notice do not constitute NRC requirements; therefore, no 
specific action or written response is required. 

Description of Circumstances: 

On January 28, 1986 the Carolina Power and Light (CP&L) Company's H. B. 
Robinson Unit 2 nuclear power plant experienced a reactor trip from 80% 
power followed by a loss of offsite power. The event was initiated when the 
emergency diesel generator (EDG) "B" output breaker was removed to install a 
solid-state overcurrent trip device. The breaker had just been racked out 
when emergency bus "E-2" tripped because of a blown potential transformer 
fuse. The subsequent loss of an instrumentation channel followed by turbine 
rollback led to the reactor trip on high pressurizer pressure. 

One minute after the reactor trip, the plant auxiliaries were transferred 
automatically, as designed, from the auxiliary transformer to the startup 
transformer. However, the startup transformer differential protective relay 
subsequently operated, which opened the source and load supply breakers and 
isolated the plant from all offsite power. EDG "A" started and loaded 
Emergency Bus "E-1." EDG "B" was restored to service approximately 1 hour 
later. After investigation revealed no transformer faults, offsite power was 
restored approximately 2 hours after the initiating trip. The dedicated 
shutdown diesel (EDG "A") was available throughout the event. 



8610080071 
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                                                       IN 86-87 
                                                       October 10, 1986 
                                                       Page 2 of 3 

Discussion: 

The licensee investigation concluded that the two primary events (loss of E-
2 bus and the loss of offsite power) were separate events. The loss of the 
E-2 bus was attributed to a loose fuse holder which caused the fuse to blow 
when the EDG "B" breaker was racked out. In addition to replacement of the 
loose fuse holders the licensee is investigating long-term changes to 
increase the reliability of the potential transformer circuitry. 

The investigation into the loss of offsite power revealed no faults in the 
transformer, no faults in the auxiliary loading, and no problems in the 
functioning of the differential relay. The licensee determined that the 
current transformers (CTs) had saturated as a result of the dc component of 
the in-rush current when the startup transformer was loaded. The saturated 
CT provided an erroneous indication to the differential relay. The 
differential relay, by design, then isolated the startup transformer. Since 
this condition had never occurred in the approximately 15 years of 
commercial operation, the system was reviewed and four conditions that 
apparently contributed to an in-rush current higher than previously 
experienced were noted. 

1.   The CP&L system voltage was recently increased so voltage was higher 
     than had been the case in most prior transfers. The adjacent fossil 
     unit was also at full load so the system was at its lowest impedance 
     which would tend to maximize in-rush current. 

2.   Auxiliary loading has slowly increased over the years due to various 
     modifications and additions. 

3.   The cold weather on the day of the event required the freeze protection
     circuits to run near maximum, which added further to the auxiliary 
     loading. 

4.   The transfer of auxiliary power uses a typical break-before-make scheme
     with a normal dead bus time of approximately six cycles. The additional
     resistive heating load on that day may have slowed the induction motors
     down more than normal during the dead bus time and would have increased
     the dc component and magnitude of the in-rush current. 

After obtaining all of the operating parameters the licensee's calculations 
verified that the startup transformer current transformers were susceptible 
to dc saturation during loading. Similar calculations or verification of 
existing calculations could assist in preventing other licensees from 
experiencing a transformer isolation. The H. B. Robinson plant has made some 
CT modifications (increased turn ratios) and has planned others to eliminate 
this potential for loss of offsite power. 

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                                                       IN 86-87       
                                                       October 10, 1986 
                                                       Page 3 of 3 

No specific action or written response is required by this information 
notice. If you have any questions about this matter, please contact the 
Regional Administrator of the appropriate regional office, or this office. 


                                   Edward L. Jordan, Director 
                                   Division of Emergency Preparedness 
                                     and Engineering Response 
                                   Office of Inspection and Enforcement 

Technical Contacts:  James C. Stewart, IE 
                     (301) 492-9061 

                     Frank Ashe, AEOD 
                     (301) 492-4442 

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