UNITED STATES

NUCLEAR REGULATORY COMMISSION

OFFICE OF NUCLEAR REACTOR REGULATION

WASHINGTON, DC 20555-0001

March 30, 2007

NRC INFORMATION NOTICE 2007-14: LOSS OF OFFSITE POWER AND DUAL-UNIT

TRIP AT CATAWBA NUCLEAR GENERATING

STATION

ADDRESSEES

All holders of operating licenses for nuclear power reactors, except those who have

permanently ceased operations and have certified that fuel has been permanently removed

from the reactor vessel.

PURPOSE

The U.S. Nuclear Regulatory Commission (NRC) is issuing this information notice (IN) to inform

addressees of a loss-of-offsite-power (LOOP) and dual-unit trip event that occurred at the

Catawba Nuclear Generating Station (Catawba) due to circuit transformer (CT) failures and

improper switchyard bus differential relay settings. The NRC expects that addressees will

review the information for applicability to their facilities and consider actions, as appropriate, to

avoid similar problems. However, suggestions contained in this IN are not NRC requirements;

therefore, no specific action or written response is required.

DESCRIPTION OF CIRCUMSTANCES

On May 20, 2006, at 2:01 p.m., Catawba, Units 1 and 2 tripped automatically from 100 percent

power following a LOOP event. The event began when a fault occurred internal to a CT

associated with one of the 230-kV switchyard power circuit breakers (PCBs). [CTs are used in

protective circuits to step down the line current to a value suitable for use by protective relays.] 

Within a fraction of a second, another CT associated with a different switchyard PCB failed. 

The differential protective relays tripped the appropriate PCBs to clear the faults. However, due

to a low trip setting, one of the red bus differential relays inappropriately actuated and tripped

additional PCBs. This red bus differential relay should not have actuated because the CT

failures occurred outside of its zone of protection. This relay tripped most of the PCBs except

those in the middle of the "breaker-and-a-half" switchyard bus arrangement. At this point, only

two 230-kV transmission lines remained in service to carry the full power output from Catawba,

Units 1 and 2. The PCBs for these transmission lines tripped on overload, separating the units

from the grid. The emergency diesel generators (EDGs) on both units started automatically

and supplied the required essential loads.

ML070610424
IN 2007-14

Page 2 of 3

The licensee at Catawba declared a Notice of Unusual Event (NOUE) due to the loss of

alternating current electrical power from all offsite sources for more than 15 minutes. The

licensee completed restoring power to the 6.9-kV buses at Unit 2 and then Unit 1 approximately

6.6 hours after the LOOP and restored offsite power to the vital buses several hours later. The

licensee secured all four EDGs approximately 11.1 hours after the LOOP and terminated the

NOUE shortly thereafter, once it restored offsite power to vital buses on both units.

The licensee’s root cause analysis determined that certain switchyard relay tap settings, within

the “breaker-and-a-half” switchyard configuration scheme, for the red and yellow buses were

set at a value too low to handle the fault currents experienced during this transient. In 1979,

prior to the commercial operation of either Catawba unit and establishment of site system

engineering, the relay engineering department originally set the switchyard red and yellow bus

differential relays using tap settings of 100 volts. This was the appropriate setting for the

postulated fault current at the time. In 1981, the relay engineering department revised the

differential relay calculations for a new tap setting of 250 volts. The revised calculations

reflected the additional postulated fault current, as well as, the addition of transmission lines to

the switchyard. However, the revised relay settings were not implemented at the Catawba

switchyard. One relay setting card was erroneously marked as having made the change from

100 volts to 250 volts. The relay card was returned to the relay engineering department to

update the engineering records. Another relay setting card was left reflecting the 100-volt tap

setting and, thus, became the field reference for further maintenance work on the relays. 

If the actual relay settings in the switchyard had been set adequately, only certain appropriate

PCBs would have opened due to the CT failures, both units would have run back to 48 percent

main generator electrical power, a sufficient number of transmission lines would have remained

in service for this power level, and a LOOP would not have occurred. 

The NRC dispatched an augmented inspection team to review the facts surrounding the event

(NRC Inspection Report 05000413/2006-009 and 05000414/2006-009, dated June 29, 2006,

Agencywide Documents Access and Management System (ADAMS) Accession Number

ML061800329). NRC Inspection Report 05000413/2006-004 and 05000414/2006-004 dated

October 26, 2006 (ADAMS Accession No. ML062990345), also discussed the Catawba LOOP

event. 

DISCUSSION

The Catawba LOOP event occurred as a result of incorrect switchyard protective relay tap

settings. Inspection Report 05000413/2006-009 and 05000414/2006-009 concluded there were

no requirements or standards that were not met and it was not reasonable for the licensee to

have identified the lower-than-desired relay tap setting earlier. Notwithstanding, this event

illustrates the importance of determining and implementing the appropriate relay tap settings so

that no differential relay operation is obtained for faults outside the zone of protection. 
IN 2007-14

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CONTACT

This Information Notice does not contain any information collections and, therefore, is not

subject to the requirements of the Paperwork Reduction Act of 1995 (44 U.S.C. 3501 et seq.).

This IN requires no specific action or written response. Please direct any questions about this

matter to the technical contacts listed below.

/RA by TQuay for/

Michael J. Case, Director

Division of Policy and Rulemaking

Office of Nuclear Reactor Regulation

Technical Contacts: Brett A. Rini, NRR/DIRS Vijay Goel, NRR/DE

301-415-3931 301-415-3730

E-mail: bar3@nrc.gov E-mail: vkg@nrc.gov

Note: NRC generic communications may be found on the NRC public Web site,

http://www.nrc.gov under Electronic Reading Room/Document Collections.