Information Notice 1989-83, Sustained Degraded Voltage on the Offsite Electrical Grid and Loss of Other Generating Stations As a Result of a Plant Trip

Sustained Degraded Voltage on the Offsite Electrical Grid and Loss of Other Generating Stations As a Result of a Plant Trip

ML031190117
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
Issue date:	12/11/1989
Revision:	0
From:	Rossi C E Office of Nuclear Reactor Regulation
To:
References
IN-89-083, NUDOCS 8912050250
Download: ML031190117 (7)

UNITED STATESNUCLEAR REGULATORY COMMISSIONOFFICE OF NUCLEAR REACTOR REGULATIONWASHINGTON, D.C. 20555December 11, 1989NRC INFORMATION NOTICE NO. 89-83: SUSTAINED DEGRADED VOLTAGE ON THE OFFSITEELECTRICAL GRID AND LOSS OF OTHER GENERATINGSTATIONS AS A RESULT OF A PLANT TRIP

Addressees

All holders of operating licenses or construction permits for nuclear powerreactors.

Purpose

This information notice is intended to alert addressees to potential problemsresulting from a sustained degraded grid voltage on the offsite electricalsystem. This problem could occur as a result of a plant trip if the generatorbackup relays of other generating stations on the electrical grid are impro-perly set, causing additional generating stations to be lost. It is expectedthat recipients will review the information for applicability to their facili-ties and consider actions, as appropriate, to avoid similar problems. However,suggestions contained in this Information notice do not constitute NRC require-ments; therefore, no specific action or written response is required.

Description of Circumstances

On July 11, 1989, Virgil C. Summer Nuclear Station was operating at 100-percentpower during a record power demand on the South Carolina Electric & Gas Company(SCE&G) system. During maintenance activities, the generator stator coolingwater signal was inadvertently lost, causing a turbine trip and a reactor trip.This turbine trip caused an unexpectedly large voltage disturbance on the grid;subsequently, several other electrical generating stations were lost within theSCE&G system, resulting in an estimated loss of about 1500 megawatts. Thissignificant loss of power resulted in a degraded grid voltage condition atSummer, which caused the station's second level undervoltage protection relays(degraded grid voltage protection) to Initiate load shedding and automaticstarting and loading of the diesel generators.-Previously, on July 20, 1976, as a result of a unit trip at Millstone NuclearPower Station, the grid voltage dropped from 352 kV to 333 kY. This voltagedrop, in conjunction with the voltage drops produced by the applicablestep-down transformers, reduced the control power voltage within individualmotor control centers and individual 480-volt controllers to a level thatwas insufficient to actuate the main line controller contactors. As a result,f > a,1- Ii- IN 89-83December 11, 1989 when the motors were signaled to start, the contactor control power fuses wereblown, making several motors powered from the 480-volt motor control centersinoperable.Both the Millstone trip and the recent Summer trip resulted in a degradedvoltage condition at the plant safety buses. To resolve the Millstone concern,the staff recommended in Branch Technical Position PSB-I, Adequacy of StationElectric Distribution System Voltages," that a time-delayed, degraded gridvoltage detection scheme be incorporated into the plant's safety-relatedelectrical distribution system to protect the Class 1E safety-related equip-ment. This undervoltage protection has been implemented at all operatingplants. As described in the above event at Summer, the second level of under-voltage relays separates the safety buses from the offsite power system (loadshedding) and initiates operation of the diesel generators.A licensee evaluation of the event at Summer showed that a grid instabilityafter the turbine trip and subsequent cascade tripping of other generationstations occurred because the SCE&G system could not compensate for the lossof the 440 MYARs being delivered by Summer. The 115-kY line voltage degradedto 102-kY, and the 230-kY line voltage degraded to 205-kY. This-evaluationfurther showed that the setpoints of the generator backup relays at othergenerating stations, McMeekin and Saluda, were lower than the standard set-tings. These setpoints were established many years ago and were never reverified..(Note: Generator backup relays provide protection to the generator againstinternal and external faults not properly cleared by other relays.) With thegenerators at McMeekin and Saluda in the automatic voltage control mode, thegenerators tried to increase voltage in response to the loss of power at Summer.This attempt to raise voltage resulted in a reactive generation increase thatcaused the oversensitive generator backup relays to trip the units. The as-found relay settings for those generating stations were 118 and 107 percentof generator output. SCE&G stated that its present, standard setting for thegenerator backup relays is 150 percent of the generator MVA rating. Even withthe relay settings of four Fairfield pumped storage stations set at 150 percent,the large voltage decrease that resulted from the loss of the Summer, McMeekin,and Saluda units and the automatic voltage regulation response of the Fairfieldunits also caused the Fairfield units to trip.Discussion:The offsite power system is the preferred and the most reliable source of powerfor nuclear plant safety systems. Therefore, plants remain connected to thepreferred source for as long as possible, that is, for as long as the capabilityand capacity of the offsite source permits, before switching to the emergencydiesel generators. A loss of generation capacity from a plant challenges theoffsite power supply availability; thus, during licensing, plants perform agrid stability analysis to ensure that the offsite power system will remainstable in the event of the loss of the largest single supply to the grid orthe loss of the largest load from the grid.;I E- IH 89-83December 11, 1989 The events that occurred at Summer and at Millstone were the result of theseplants being operated outside the analyzed conditions of their grid stabilityanalyses. The Summer licensee has confirmed that in the past there had beenother scrams from 100-percent power without degraded grid voltage problems.However, the maximum MVARs supplied to the grid during those plant trips was317 MVARs whereas, during the recent event, the generator was supplying 440MYARs to the grid. One short-term corrective measure taken by Summer was tolimit MVAR generation to 300 MVARs unless otherwise approved by their generalmanager of nuclear plant operations. Thus, it appears that the licensee didnot consider its maximum reactive output in its grid stability analysis.Licensees perform the grid stability analysis on the basis of system informa-tion, such as transmission, generation, and load, that is available at thetime the plant is licensed. However, licensees may wish to review their gridstability analysis whenever transmission, generation, or load system changesoccur that affect the original design basis condition to ensure that theoriginal design basis of the plant remains valid in accordance with GeneralDesign Criterion 17, UElectric Power Systems." Moreover, to avert recurrence.of conditions that occurred at Summer, licensees may also wish to periodicallycheck the setpoints of the generator backup relays.This information notice requires no specific action or written response. Ifyou have any questions about the information in this notice, please contact thetechnical contact listed below or the appropriate NRR project manager.Ciales E. Rossi, DirectorDivision of Operational Events AssessmentOffice of Nuclear Reactor Regulation

Technical Contact:

Om Chopra, NRR(301) 492-0835AttachmentiList of Recently Issued NRC Information Notices JAttachentIN 81-3-ODember 11, 19SPage 1 of ILIST OF RECENTLY ISSUEDNRC IXFORATION NOTICES -*- ---- --- -!"'°&W."T a UTNotice No. Subject Issuance9-8 Recent Safety-Related t2/7/3Incidents at LargaIrradiatorsS3-5. Suppliers of Poteutially 226/inSupp. 1 nisrepresented Fasteners912 Inadequate Control of 12/6/31Teemorary Modificationsto Sty-Related Systin8940 Potential for Ieter laner 12/11/3Theal Stratificatio0n.Steam 0lnding inN119i.Pressure Coolant IajectionPiping89-79 Degraded Coatings and unl/Corrosion of SteelContainment vessels81-56. Questionable Certification 311//Supp. 1 of Naterial supplied tothe Defense Departmwotby muclear Suppliers89-78 Ii lure of Packing Nuts 11/2Di One-Inch WanimHemf loorldo Cylindervalves3.77 Debris la CoUtainment -1/21/Emrgency Sums andIncorrect screenConfigurationsIssued toAll C licenseesauthorized topossess ad vs"sealed sources atlarge Irradiators.All holders of Oisor CPs for nuticlearpoer reactors.All holders of DUor Cft for nuclearpower reactors.A13 holders of Msor Cta for nuclearpowr reactors.All holders of Olsor CPS for L1U.All holders of Duor Cps for nuclearpowr reactors.All IRC licenseesauthorized to possessand use source nter10and/or special nuclearratertal for the heating,erptying. filling, orshipping of uraniihexefluoride 1n 30-and 48-inch diametercylinders.All holders of OL0or Cps for Pwr.OL

• Operatins LicenseCP

• Construction PermitUNITED STATESNUCLEAR REGULATORY COMMISSIONWASHINGTON, D.C. 2055OFFICIAL BUSINESSPENALTY FOR PRIVATE USE. $300FIRST CLA SS MALIPOSTAGE I Ff1S PAIDnRM he 047 a -IN 89-83December 11, 1989 The events that occurred at Summer and at Millstone were the result of theseplants being operated outside the analyzed conditions of their grid stabilityanalyses. The Summer licensee has confirmed that in the past there had beenother scrams from 100-percent power without degraded grid voltage problems.However, the maximum MVARs supplied to the grid during those plant trips was317 MVARs whereas, during the recent event, the generator was supplying 440MVARs to the grid. One short-term corrective measure taken by Summer was tolimit MVAR generation to 300 MVARs unless otherwise approved by their generalmanager of nuclear plant operations. Thus, it appears that the licensee didnot consider its maximum reactive output in its grid stability analysis.Licensees perform the grid stability analysis on the basis of system informa-tion, such as transmission, generation, and load, that is available at thetime the plant is licensed. However, licensees may wish to review their gridstability analysis whenever transmission, generation, or load system changesoccur that affect the original design basis condition to ensure that theoriginal design basis of the plant remains valid in accordance with GeneralDesign Criterion 17, "Electric Power Systems." Moreover, to avert recurrenceof conditions that occurred at Summer, licensees may also wish to periodicallycheck the setpoints of the generator backup relays.This information notice requires no specific action or written response. Ifyou have any questions about the information In this notice, please contact thetechnical contact listed below or the appropriate NRR project manager.Charles E. Rossi, DirectorDivision of Operational Events AssessmentOffice of Nuclear Reactor Regulation

Technical Contact:

Om Chopra, NRR(301) 492-0835

Attachment:

List of Recently Issued NRC Information NoticesDocument Name: IN ON GRID INSTABILITY*SEE PREVIOUS CONCURRENCES*C/OGCB:DOEA:NRRCHBerlinger12/r/89 ~ 11/28/89*OGCB:DOEA:NRR*SELB:DET:NRR *C/SELB:DET:NRR *D/DET:NRR *RPB:ARMDProchnow OChopra FRosa AThadani TechEd10/20/89 10/20/89 10/20/89 10/20/89 10/23/89r 4IN 89-XXDecember xx, 1989 The NRCt -ob -b Xhe events that occurred at Summer and atMillstone were the result of these plants being operated outside the analyzedconditions of their grid stability analyses. The Summer licensee has confirmedthat in the past there had been other scrams from 100-percent power withoutdegraded grid voltage problems. However, the maximum MYARs suppl,.d no the 4grid during those plant trips was 317 MYARs whereas, during event,the generator was supplying 440 MYARs to the grid. One short-term correctivemeasure taken by Summer was to limit MYAR generation to 300 MYARs unlessotherwise approved by their general manager of nuclear plant operations. Thus,it appears that the licensee did not consider its maximum reactive output inits grid stability analysis.Licensees perform the grid stability analysis on the basis of system informa-tion, such as transmission, generation, and load, that is available at the timethe plant is licensed. However, licensees may wish to review their gridstability analysis whenever transmission, generation, or load system changesoccur that affect the original design basis condition to ensure that theoriginal design basis of the plant remains valid in accordance with GeneralDesign Criterion 17, "Electric Power Systems." Moreover, to avert recurrenceof conditions that occurred at Summer, licensees may also wish to periodicallycheck the setpoints of the generator backup relays.This information notice requires no specific action or written response. Ifyou have any questions about the information in this notice, please contact thetechnical contact listed below or the appropriate NRR project manager.Charles E. Rossi, DirectorDivision of Operational Events AssessmentOffice of Nuclear Reactor Regulation

Technical Contact:

Om Chopra, NRR(301) 492-0835

Attachment:

List of Recently Issued NRC Information NoticesDocument Name: IN ON GRID INSTABILITY*SEE PREVIOUS CONCURRENCESD/DOEA:NRR C/OGC :CERossi CHBerlinger11/ /89 11/0J/89*OGCB:DOEA:NRR*SELB:DET:NRR *C/SELB:DET:NRR *D/DET:NRR *RPB:ARMDProchnow OChopra FRosa AThadani TechEd10/20/89 10/20/89 10/20/89 10/20/89 10/23/89

.IIN 89-XXOctober xx, 1989 The NRC staff has concluded that the events that occurred at Summer and atMillstone were the result of these plants being operated outside the analyzedconditions of their grid stability analyses. The Summer licensee has confirmedthat in the past there had been scrams from 100 percent power without degradedgrid voltage problems; however, the maximum MVARs supplied to the grid duringthese plant trips was 317 MVARs. During this recent event, the generator wassupplying 440 MVARs to the grid. One short-term corrective measure was tolimit MVAR generation at Summer station. Thus, it appears that the licenseedid not include the worst possible condition in its grid stability analysis.General Design Criterion 17, "Electric Power Systems", states that provisionsshall be included to minimize the probability of losing electric power from anyof the remaining supplies as a result of, or coincident with, the loss of powergenerated by the nuclear power unit, the loss of power from the transmissionnetwork, or the loss of power from the onsite electric power supplies. Thus,voltage degradation following a plant trip could violate GDC 17 if the offsitepower system does not have the capability to automatically start as well asoperate all required safety loads. Although protection Is provided from thedegraded grid by a second level of undervoltage protection systems, thissituation becomes a safety problem if a plant trip results in degraded voltageand unnecessarily challenges the diesel generators to start and provide powerto the safety loads.Licensees perform the grid stability analysis on the basis of system informa-tion, such as transmission, generation, and load, that is available at the timethe plant is licensed. However, licensees may wish to perform the grid stabil-ity analysis whenever system changes occur that affect the original design-basis condition to ensure that the original design-basis of the plant remainsvalid in accordance with GDC-17. Moreover, to avert recurrence of conditionsthat occurred at Summer, licensees may also wish to periodically check thesetpoints of the generator backup relays.This information notice requires no specific action or written response. Ifyou have any questions about the information in this notice, please contact thetechnical contact listed below or the appropriate NRR project manager.Charles E. Rossi, DirectorDivision of Operational Events AssessmentOffice of Nuclear Reactor Regulation

Technical Contact:

Om Chopra, NRR(301) 492-0835

Attachment:

List of Recently Issued NRC Information NoticesOGCB:DOEA:NRR SELB:DET4IRB RPB:ARM C/OGCB:DOEA:NRR D/DOEA:NRRDProchnow OChopra lf TechEd Sc- CHBerlinger CERossi1O/zo/89 10/10/89 f 10/z3/89 10/ /89 10/ /89F. g°5 A .n~x! oil10 /2 /9 rqo/20/vi