Detailedsimulationstudiesasasolutionto21st centurypowersystemchallenges

MichaelRopp,Ph.D.,P.E.NorthernPlainsPowerTechnologies

Brookings,SD57006USA

BriefintroductiontoNPPT

Keyareas:• Low-inertiasystems(microgrids,islands, remotecommunities, ≤ 30MW)

• Distributed EnergyResources (DERs),especially atthedistribution level

• “Traditional” EMT-type studies

Primaryservices:• Systemdiagnostics/forensics andeventanalysis

• Controlanddynamics• Protection• Design• Test designandresult interpretation• Systemstudies

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Trustworthypowersystemengineeringservices

movingpowerforward11/11/16

Keychallengestodiscusstoday

• Aspowersystemschange,certainkeyfunctionsdoNOTchange:• Protection• Powerquality• Dynamicperformance/stability• Reliability• Economicoptimization

• We’vebeenhandlingallofthese,successfully, foroveracentury.

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So,what’schanged?

• Powerelectronics• Muchfastertimeconstants• Little tonoinertia• Asmuchsoftware-defined asphysics-defined

• Needtostudywithhighergranularity• Keyapproximations/simplifications yieldunacceptable error

• Infinitebus• Phase-phase balance• Perfectgrounding• Fixedloadtypes,usuallywithout dynamics

• Timeconstants aremuchshorter(again)

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Modelingcanhaveveryhighvalue• Therighttypeofmodelingcanhelpusaddressthesechallenges.• Pros

üReducesrisküImprovesfundamentalunderstanding—observabilityandcontrollabilityüShortensdevelopment/commissioningtimesüAllowstestingthatcan’tbedoneexperimentallyüCansignificantlydecreaseoverallcosts

• ConsMCanbeexpensiveMNeedalotofdataMValidationiscriticalMNeedsomeonewhoknowswhatthey’redoing—easytobuildalyingmodel

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Thevalueofmodeling

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PerceivedvalueCost

0%(actualHWonactualsystem)

100%(pure simulation)

LaboratoryscaleHW

Controls andpowerHIL

Controls HIL

Understandinggained

Whatmakesagoodmodel?

• Thebestmodelisonethatincludeseverythingthat’simportant,andnothingthatisn’t.• Troubleis,youmaynotknowinadvancewhichiswhich.

• Goforoverkill/accuracy?• Goforsimplicity/speed?

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Key“gotchas”withmodeling

• Rangeofapplicability—isthemodeldesignedtosolvemyproblem?• Whatareyoutryingtostudy?• Istherightmodel beingused?

• Inputdataquality—garbagein,garbageout(GIGO)• Operatorqualifications—doestheuserunderstandthemodelandthesystem?• Validation—howdoIknowthemodelistellingthetruth?

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Mainmodeltypes

• Calculationautomators—“macros”• Modelsdesignedfordynamics—“dynamicsimulators”• Modelsdesignedfortransients—“transientsimulators”• Modelsdesignedfor“snapshots”—“steady-statesimulators”

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Simulationtoolsforpowersystems—traditionalview

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Simulationtoolsforpowersystems—evolvingview

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Simulationtoolsforpowersystems—low-inertiasystemsview

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Simulationtoolsforpowersystems—distribution-connectedDERstudies

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Calculationautomators

• Userentersinputs;programperformsapresetstandardizedcalculation• Canbethoughtofasamacro—thesearegenerallyNOTsimulators• Sufficientforperhaps75%ofthemodelingneededindistribution• Examples

• CYME• Aspen(?)• Synergi• SKM

• Thingstowatchfor• Powerfulandconvenient,butlimitedflexibility• Easytomisapply—mustbeawareoftheunderlyingassumptionsandconstraints

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Calculationautomators

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Acalculationautomator canfallanywhereonthischartaslongasthere’sastandardformula.

Dynamicsimulators

• Usuallybasedonphasorandsymmetricalcomponentrepresentations• Alsocalled“electromechanicalsimulators”• Designedformachinedynamicsandlonger-scaletransientsonlargesystems• Solvessystemsofdifferential/algebraicequations• Solversoptimizedforspeed(modifiedEuler,simpletrapezoidal)• Typicaltimestep:¼cycle

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Dynamicsimulators

• Examples• PSS/E• PSLF

• Thingstowatchfor• Notwellsuited forpowerelectronics (timestepstoolong)• Notespecially goodfordistribution (symmetrical component assumptions)• Built-in DERmodels aregeneric;custommodels arenotallowedbysomejurisdictions

• Solverscompromise alittle onrobustness• Usuallydon’t have“backstepping”somaymissswitchingevents

• Watchoutforinaccurate,outdated,orunverifiedmodels

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Phasorsimulators

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Therangeofapplicability ofphasorsimulators istypicallyhere.

Transientsimulators

• Baseddirectlyondifferentialequationdescriptionsofapparatusandcomponents• Designedtosimulatephysicalphenomenonat“any”timescale,aslongasyoucanrepresentitwithdifferentialequations• Maximumphysicalfidelitywithminimumassumptions,butalsomaximumdatarequirementsandcomputationalburden• Solversoptimizedforperformance,andmayhavechoicesofsolvers

• Back-stepping algorithms orvariable-step solversincluded tocatchswitchinginstants

• Typicaltimestep: 100µs(1/160th ofacycle)foraveragedmodels; 1µsforswitchingmodels; sub-microsecond forlightning orotherphysicalmodels

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Transientsimulators

• Examples• EMTPderivatives(EMTP-RV, PSCAD,ATP)• MATLAB/Simulink/SimPowerSystems• DigSilent PowerFactory• ETAP

• Thingstowatchfor• SLOWrelativetodynamicsimulators• Veryhighinputdatarequirements; GIGOcanbeabigproblem• Makesuremodeluserhasgoodphysicalknowledge(“sanitychecks”)• Controlmodels shouldbevalidated

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Transientsimulators

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Therangeofapplicability oftransient simulatorsistypicallyhere. (DependssomewhatonyourcomputerHW.)

Steady-statesimulators

• Simulatemultiplesteady-statesnapshots• Donotsimulatedynamics• Goodfor“8760”simulations• Example

• OpenDSS• CYME/Synergi/WindMil have“8760”simulatorsbuilt-in• Loadflowsolverswithbatchmodeoperation

• Thingstowatchfor• Accumulatingerror—becausedynamicsnotsimulated,changesmaynotpropagatecorrectlyfromonesteadystatetoanother

• Limitsinfundamentalmath(i.e.,Laplacetransform)

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Steady-statesimulators

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Therangeofapplicability ofsteady-state“snapshot”simulators istypicallyoverhere.

Switchedvs.averagedmodelsofinverters

• Constant-currentrepresentationsshouldbeavoided,butdoaveragedmodelswork?Yes,aslongasthecontrolsarerepresentedinsufficientdetail.ShouldalsohaveAC/DCfiltersrepresentedtocapturedynamicimpacts.• Whatyougainwithaswitchingmodel:

• MUCHfastersimulations duetoabilitytouse longertimesteps• Whatyoulosewithanaveragedmodel:

• Switchingharmonics• Saturation(“six-stepping”)• Smalldifferences inGFOVcases—zero-sequence Vslightlyunderestimated• Majoroverestimation ofLROVunless yourepresent antiparallel rectifier

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Exampleinvertermodel

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Validationresults

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6.98 6.99 7 7.01 7.02 7.03 7.04-800

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Time

Volta

ge (V

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Simulated and experimental inverter terminal voltages, no neutral, with parasitic Z

Sim ASim BSim CExp AExp BExp C

Feedermodeling

• Usually,sequenceimpedancesused• Assumes asymmetrical circuit (notusuallytrue,butnotusually important)• Losesthe identity oftheneutral (doyouneed it?)

• Sometransientsimulatorsallowtheuseoftheprimitivematrix• UseCarson’sequations toget lineparameters• Morephysicallyaccurate• Buthigher inputdataneeds(conductortypes,segment lengths, andconductorspacings required);GIGOproblem

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GFOVtestfeeder

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Aggregation

• Oftenwanttomodelmulti-inverterplantsusingasingleaggregatedinvertertospeedupsimulations• Alsooftenwantto“lump”sectionsoffeeders• Whenandhowcanyoudothis?

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Thankyou!

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Questions?

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michael.ropp@northernplainspower.com