Information Notice 1986-87, Loss of Offsite Power Upon an Automatic Bus Transfer

Loss of Offsite Power Upon an Automatic Bus Transfer

ML031250328
Person / Time
Site:	Beaver Valley, Millstone, Hatch, Monticello, Calvert Cliffs, Dresden, Davis Besse, Peach Bottom, Browns Ferry, Salem, Oconee, Mcguire, Nine Mile Point, Palisades, Palo Verde, Perry, Indian Point, Fermi, Kewaunee, Catawba, Harris, Wolf Creek, Saint Lucie, Point Beach, Oyster Creek, Watts Bar, Hope Creek, Grand Gulf, Cooper, Sequoyah, Byron, Pilgrim, Arkansas Nuclear, Three Mile Island, Braidwood, Susquehanna, Summer, Prairie Island, Columbia, Seabrook, Brunswick, Surry, Limerick, North Anna, Turkey Point, River Bend, Vermont Yankee, Crystal River, Haddam Neck, Ginna, Diablo Canyon, Callaway, Vogtle, Waterford, Duane Arnold, Farley, Robinson, Clinton, South Texas, San Onofre, Cook, Comanche Peak, Yankee Rowe, Maine Yankee, Quad Cities, Humboldt Bay, La Crosse, Big Rock Point, Rancho Seco, Zion, Midland, Bellefonte, Fort Calhoun, FitzPatrick, McGuire, LaSalle, 05000000, Zimmer, Fort Saint Vrain, Shoreham, Satsop, Trojan, Atlantic Nuclear Power Plant, Skagit, Marble Hill
Issue date:	10/10/1986
From:	Jordan E NRC/IE
To:
References
IN-86-087, NUDOCS 8610080071
Download: ML031250328 (4)

SSINS No.: 6835 IN 86-87 UNITED STATES

NUCLEAR REGULATORY COMMISSION

OFFICE OF INSPECTION AND ENFORCEMENT

WASHINGTON, D.C. 20555 October 10, 1986 tE INFORMATION NOTICE NO.86-87P LOSS OF OFFSITE POWER UPON AN AUTOMATIC

BUS TRANSFER

Addressees

All nuclear power reactor facilities holding an operating license or a con- struction permit.

Purpose

This notice is to alert recipients of a potentially significant problem involv- ing 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. Robin- son 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 over- current 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 auto- matically, 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 hour1.157407e-5 days <br />2.777778e-4 hours <br />1.653439e-6 weeks <br />3.805e-7 months <br /> later. After investigation revealed

no transformer faults, offsite power was restored approximately 2 hours2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br /> after the

initiating trip. The dedicated shutdown diesel (EDG "A") was available throughout

the event.

8610080071

IN 86-87 October 10, 1966 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 func- tioning 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 r - meters 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.

I

IN 86-87 October 10, 1986 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.

war L. r

Divisio f Emergency Preparedness

and E ineering Response

Office of Inspection and Enforcement

Technical Contacts: James C. Stewart, IE

(301) 492-9061 Frank Ashe, AEOD

(301) 492-4442 Attachment: List of Recently Issued IE Information Notices

I

Attachment 1 IN 66-87 Octoter 10. 1986 LIST OF RECENTLY ISSUE6

E INfORP'ATION NOTICES

fnfnrretstnn Date of

Notice Nlo Subject Issue Issued to

86-86 Clarification Of Requirements 10/10/815 All registered users

For Fabrication And Export Of of NRC certified

Certain Previously Approved packages

Type S Packages

86-85 Enforce nt Actions Against 10/3/86 All NRCmedical

Medical Licensees For licensees

WilYfull Failure To Report

Nistadmristrations

86-84 Rupture Of A Nominal 9/30/86 All NRCmedical

40-Millicurie Iodine-12S Institution licensees

Brachytberapy Seed Causing

Significant Spread Of

Radioactive Contamination

86-83 Underground Pathways Into 9/19/86 All power reactor

Protected Areas. Vital Areas. facilities holding

Material Access Areas, And an OL or CP; fuel

Controlled Access Areas fabrication and

processing facilities

86-82 Failures Of Scram Discharge 9/16/86 All power reactor

Voluwe Vent And Brain Valves facilities holding

an OL or CP

86-81 Broken Imner-External Closure 9/1S/86 All power reactor

Springs On Atwrood & Morrill facilities holding

main Steam Isolation Valves an OL or CP

86-80 Unit Startup With Degraded 9/12/86 All power reactor

High Pressure Safety Injection facilities holding

System an OL or P.

86-79 Degradation Or Loss Of 9/2/86 All power reactor

Charging Systems At PWR facilities holding

Nuclear Power Plants Using an OL or CP

Swing-Pum Designs

OL

• Operating License

UNITED STATES CP* Construction Permit

NUCLEAR REGULATORY COMMISSION FIRST-CLASS MAIL

WASHINGTON, D.C. 20586 POSTAGE & FEESPAIO I

USN RC

WASH 0 C

OFFICIAL BUSINESS

PENALTY fOR PRIVATE USE. 30 PERMIT No GSL

I3%37 '-1 ;y IFPI1 Al

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T-FL 3 35 7;9