Information Notice No. 86-87: Loss of Offsite Power Upon an Automatic Bus Transfer
SSINS No.: 6835
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
All nuclear power reactor facilities holding an operating license or a
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.
October 10, 1986
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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
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.
October 10, 1986
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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
Frank Ashe, AEOD
Attachment: List of Recently Issued IE Information Notices
Page Last Reviewed/Updated Friday, May 22, 2015