NPP Voltage S rgeNPP Voltage S rgeNPP Voltage Surge NPP Voltage Surge ProtectionProtectionProtectionProtection

Presentation toPresentation toIEEE Nuclear Power Engineering Committee IEEE Nuclear Power Engineering Committee

25 July 201225 July 201225 July, 201225 July, 2012

Dr. John H. Bickel, ChairmanDr. John H. Bickel, ChairmanDr. John H. Bickel, ChairmanDr. John H. Bickel, ChairmanDIDELSYS Working GroupDIDELSYS Working Group

OECD Nuclear Energy AgencyOECD Nuclear Energy Agency

Purpose of this Talk:Purpose of this Talk:ppDescribe Forsmark and Olkiluoto NPP voltage Describe Forsmark and Olkiluoto NPP voltage surge eventssurge eventssurge eventssurge events

Describe why such events happened in Describe why such events happened in supposedlysupposedlyqualifiedqualified Class 1E Electrical SystemsClass 1E Electrical Systems

Suggest Suggest possible gapspossible gaps in current philosophy of in current philosophy of gggg p g pp g p p p yp p yVoltage Withstand Capability Voltage Withstand Capability vsvs ProtectionProtection

Suggest possible role for IEEE/IEC to standardize:Suggest possible role for IEEE/IEC to standardize:Suggest possible role for IEEE/IEC to standardize:Suggest possible role for IEEE/IEC to standardize:NPP Voltage Withstand Design Requirements NPP Voltage Withstand Design Requirements Component Voltage Withstand Component Voltage Withstand QualificationQualificationp gp g

NPPNPP Electrical SystemElectrical SystemInternational standardsInternational standards::International standardsInternational standards::IAEA NSIAEA NS--GG--1.81.8IEEE Std. 308IEEE Std. 308KTAKTA--3701 (German)3701 (German)

Common to all standards:Common to all standards:Interface Interface safety related safety related control and motive power to control and motive power to

f t AC if t AC inonnon--safety AC sources via safety AC sources via TransformersTransformersProtective Circuit BreakersProtective Circuit BreakersProtective Circuit BreakersProtective Circuit BreakersClass 1E Vital AC control Class 1E Vital AC control power generated via power generated via i t di t bi t di t bintermediate busesintermediate buses

Early Vital AC Early Vital AC Power Power Design DesignM tM t G t tG t tMotorMotor--Generator setGenerator set

Design Strength:Design Strength:

Vit l AC P iVit l AC P iVital AC Power is Vital AC Power is mechanically isolatedmechanically isolated from from switching transients, voltage switching transients, voltage surges, or dips less than surges, or dips less than Inertial time constantInertial time constant

Operations/MaintenanceOperations/Maintenance Issue:Issue:

Fire Protection concerns with Fire Protection concerns with l b i ti ill b i ti illubricating oilslubricating oilsRotating equipment missiles Rotating equipment missiles near switchboardsnear switchboardsMaintenance Maintenance of continuously of continuously running running MG setsMG sets

Modern Vital AC Modern Vital AC Power Power DesignDesignModernization Modernization efforts are efforts are replacing MG sets with replacing MG sets with Solid State UPSSolid State UPSSo d S a e U SSo d S a e U SFiltering of incoming power Filtering of incoming power fluctuations is done by fluctuations is done by capacitor bankscapacitor bankscapacitor bankscapacitor banks

DesignDesign Concerns:Concerns:

Voltage surge through Aux. Voltage surge through Aux. g g gg g gTransformer can Transformer can feedfeed surge surge into into Class 1E system Class 1E system power power faster than breakers openfaster than breakers open..ppProtection must now rely on Protection must now rely on thyristor switching controlsthyristor switching controlsCan connected equipmentCan connected equipmentCan connected equipmentCan connected equipmentwithstand voltage surge?withstand voltage surge?

2006 Forsmark Event2006 Forsmark Event

••Single Phase Single Phase Fault in Fault in SwitchyardSwitchyard

••Initial voltage dip followed by Initial voltage dip followed by 130% voltage surge130% voltage surge

••Surge Surge passed to passed to Battery Battery Charger units Charger units via via transformerstransformers

S lid t t AEG P fitS lid t t AEG P fit••Solid state AEG Profitec Solid state AEG Profitec ChargerCharger--Inverter units were Inverter units were designed for 85% designed for 85% -- 110%110%

••Overcharged Batteries caused Overcharged Batteries caused Inverter lockInverter lock--up and shutdown of up and shutdown of 2/4 Vital 2/4 Vital AC AC DivisionsDivisions

After Forsmark eventAfter Forsmark eventAEG modified Battery AEG modified Battery ChargerCharger -- Inverter DesignInverter DesignChargerCharger -- Inverter DesignInverter Design

•• SPICE simulation model of SPICE simulation model of Forsmark event identifiedForsmark event identifiedForsmark event identified Forsmark event identified insufficient thyristor gating insufficient thyristor gating controlcontrol

•• Additional protective circuit Additional protective circuit added to control thyristors added to control thyristors when AC supply jumpswhen AC supply jumps

•• Active controls of thyristors Active controls of thyristors is faster than could be is faster than could be achieved by circuit breakersachieved by circuit breakersachieved by circuit breakersachieved by circuit breakers

2008 Olkiluoto Event2008 Olkiluoto Event

••Generator Generator Voltage Voltage Regulator failed high while unit at >20%Regulator failed high while unit at >20%

••Unit separated from grid and attempted runback to house load Unit separated from grid and attempted runback to house load causing large load rejectioncausing large load rejection

••Entire reactor output diverted into islanded inEntire reactor output diverted into islanded in--plant electrical plant electrical systems before turbine steam bypass systems could reactsystems before turbine steam bypass systems could reactsystems before turbine steam bypass systems could reactsystems before turbine steam bypass systems could react

••InIn--plant voltages reached >150% nominal (150% is max recorder plant voltages reached >150% nominal (150% is max recorder range)range)g )g )

••High Voltage Rectifiers on Recirculation Pumps destroyedHigh Voltage Rectifiers on Recirculation Pumps destroyed

••Identical ChargerIdentical Charger--Inverter unitsInverter units -- modified based on Forsmark eventmodified based on Forsmark eventIdentical ChargerIdentical Charger Inverter units Inverter units modified based on Forsmark event modified based on Forsmark event –– were not impactedwere not impacted

Similar events in US include:Similar events in US include:••1993 North Anna 1993 North Anna –– Generator Excitation Failure: >120%Generator Excitation Failure: >120%••1993 Sequoyah 1993 Sequoyah –– Generator Excitation Failure: >120%Generator Excitation Failure: >120%

Example Voltage Surge Example Voltage Surge TransientsTransients

Lightning strikes on transmission lines or switchyard (> 5ULightning strikes on transmission lines or switchyard (> 5Unomnom..))Protected by Lighting Surge Arrestors, GroundingProtected by Lighting Surge Arrestors, GroundingProtected by Lighting Surge Arrestors, GroundingProtected by Lighting Surge Arrestors, Grounding

Ground faults on high voltage line (~ 2UGround faults on high voltage line (~ 2Unom.nom.))Protected by Protected by Low Voltage Surge Low Voltage Surge Arrestors, Grounding Arrestors, Grounding

Large Load Rejection with proper turbine governor control response Large Load Rejection with proper turbine governor control response (~ 1.2U(~ 1.2Unomnom..))

Assumed to be protected by robustness in equipmentAssumed to be protected by robustness in equipment designdesign marginsmarginsAssumed to be protected by robustness in equipmentAssumed to be protected by robustness in equipment designdesign marginsmargins

Unit Separation from Grid with attempted Runback to House Load Unit Separation from Grid with attempted Runback to House Load (> 1.2U(> 1.2Unom.nom.))

Voltage Voltage Surge Surge Protection PhilosophyProtection Philosophy

Standards recognize: Standards recognize: OvervoltageOvervoltage vs. vs. DurationDuration spectrum and spectrum and have inherent assumptions about “have inherent assumptions about “Withstand VoltageWithstand Voltage” ” capabilitycapability

ExperienceExperience Suggests Suggests Gap in ProtectionGap in ProtectionNPP electrical systems NPP electrical systems nominally designed for nominally designed for

ti ith /ti ith / 10%10%operation with +/operation with +/--10% 10% Voltage Voltage What if Voltage isWhat if Voltage is aboveaboveWhat if Voltage is What if Voltage is aboveabove120% but 120% but belowbelow lightning lightning surge protection features?surge protection features?F k Olkil tF k Olkil t ttForsmark, Olkiluoto Forsmark, Olkiluoto events events indicate:indicate:Previously assumedPreviously assumedPreviously assumed Previously assumed “Withstand Voltage” may “Withstand Voltage” may be as low as:be as low as: ~130%~130%

ToTo Determine NPP Determine NPP Electrical System Electrical System Coping:Coping:

Identify: voltage Identify: voltage surge transients between nominal surge transients between nominal design design (110%(110%--120%) 120%) and existing lightning surge protection.and existing lightning surge protection.

Include: Include: consideration of combinations of consideration of combinations of eventsevents

Large load rejection Large load rejection attempted runback to house load attempted runback to house load ANDANDfailure of main generator excitation failure of main generator excitation system (Olkiluoto 2008)system (Olkiluoto 2008)

Review equipment to Review equipment to determine current determine current Voltage Withstand Voltage Withstand capability for capability for overover--voltage voltage transients (transients (include: include: asymmetricasymmetriccasescases))

Consider: Consider: solid state, Digital I&C equipment solid state, Digital I&C equipment that may have least that may have least Voltage WithstandVoltage Withstand capabilitycapability

Thi i l d UPS iThi i l d UPS i b hb h I&C biI&C biThis includes: UPS units, This includes: UPS units, battery chargersbattery chargers, I&C cabinet power , I&C cabinet power supplies supplies

IdentificationIdentification of events causing voltage of events causing voltage surgessurges in Germany:in Germany:surgessurges in Germany:in Germany:

Courtesy of the German Reactor Operators Association, VGB

SimulationSimulation of of surges carried surges carried out in: out in: SwedenSweden France GermanyFrance GermanySwedenSweden, France, Germany, France, Germany

Insights gained thus far:Insights gained thus far:g gg g

AEG modified design of Profitec battery charger AEG modified design of Profitec battery charger -- invertersinvertersSwedenSweden ((GothiaGothia Power forPower for VattenfallVattenfall) has performed) has performedSweden Sweden ((GothiaGothia Power for Power for VattenfallVattenfall) has performed ) has performed detailed calculations to identify detailed calculations to identify limiting limiting voltage surge voltage surge eventseventsFrance (France (EdFEdF) historically assumed 110% surge from load ) historically assumed 110% surge from load rejections. Blayaisrejections. Blayais--1 1 load rejection event: ~118load rejection event: ~118%. %. After simulations After simulations EdFEdF raised UPS trip settings to 130% for raised UPS trip settings to 130% for 50 msec (Battery charger trip is set lower)50 msec (Battery charger trip is set lower)50 msec. (Battery charger trip is set lower)50 msec. (Battery charger trip is set lower)Germany (VGB) Germany (VGB) used MATLABused MATLAB--SimulinkSimulink and and DigSILENTDigSILENT, , found found scenarios scenarios resulting in 130resulting in 130--140% in German NPPs.140% in German NPPs.There is need for standardization to assure completeness There is need for standardization to assure completeness of scenarios consideredof scenarios considered

Exchange Meetings Identified Need for Exchange Meetings Identified Need for Standardization:Standardization:Standardization:Standardization:

Studies to determine maximum Studies to determine maximum surges surges generated generated -- are are being being carried outcarried out

Slight differences in boundary Slight differences in boundary conditions and types of conditions and types of scenarios scenarios consideredconsidered

Areas of key differences in approaches taken:Areas of key differences in approaches taken:

Should Should generator voltage regulator faults generator voltage regulator faults be considered?be considered?g g gg g g

Should Should electrical fault with primary protection failure electrical fault with primary protection failure (delay) (delay)be be considered?considered?

Should Should single faults in conjunction with some offsingle faults in conjunction with some off--normalnormalboundaryboundary conditions conditions be considered?be considered?

Should Should risk assessment risk assessment be used for screening out certain be used for screening out certain combinations of combinations of faults from consideration?faults from consideration?

IEC 60071IEC 60071--11 Surge Surge QualificationQualification

Suggested IEEE, IEC Standards to Revisit:Suggested IEEE, IEC Standards to Revisit:

Recognize that actual events indicate inadequately protected Recognize that actual events indicate inadequately protected inin--plant voltage surges: “>120%” but “<surge arrestors”plant voltage surges: “>120%” but “<surge arrestors”

Existing standards Existing standards thoroughly deal thoroughly deal with seismic, EQ, shortwith seismic, EQ, short--circuit, fire, aging circuit, fire, aging butbut not so thoroughlynot so thoroughly with design margins with design margins against voltage surges Examples:against voltage surges Examples:against voltage surges. Examples:against voltage surges. Examples:

IEEE Std 323 (2003) IEEE Std 323 (2003) -- Qualifying Class 1E EquipmentQualifying Class 1E EquipmentIEEE C37.105 (1987) IEEE C37.105 (1987) –– Qualifying Class 1E Protective Relays and AuxiliariesQualifying Class 1E Protective Relays and AuxiliariesIEEE Std 650 (1990) IEEE Std 650 (1990) –– Qualification of Class 1E Static Battery Chargers, InvertersQualification of Class 1E Static Battery Chargers, InvertersStd 650 ( 990)Std 650 ( 990) Qua cat o o C ass Stat c atte y C a ge s, e te sQua cat o o C ass Stat c atte y C a ge s, e te sIEEE Std 649 (1991) IEEE Std 649 (1991) –– Qualifying Class 1E MCCsQualifying Class 1E MCCsIEEE Std 741 (1997) IEEE Std 741 (1997) –– Protection of Class 1E Power Systems and EquipmentProtection of Class 1E Power Systems and Equipment

E t bli h t f t d di dE t bli h t f t d di d D i B i V lt SD i B i V lt SEstablishment of standardized Establishment of standardized Design Basis Voltage SurgeDesign Basis Voltage Surgecharacteristicscharacteristics needs to be done in order to Qualifyneeds to be done in order to Qualify

IEEE C62.41.2 (2002) IEEE C62.41.2 (2002) –– Recommended Practice on Characterization of Surges in Recommended Practice on Characterization of Surges in L V lt AC Ci itL V lt AC Ci itLow Voltage AC CircuitsLow Voltage AC CircuitsIEEE C62.23 (1995) IEEE C62.23 (1995) –– Guideline for Surge ProtectionGuideline for Surge Protection