An Introduction to Completing a NERC PRC-019 Study for Traditional and DistributedGeneration SourcesMatthew Manley and Tony LimonPOWER Engineers, Inc.Abstract -- NERC PRC standards have beenimplemented as a comprehensive plan to increaseutility reliability in response to the 2003 NortheastUnited States blackout. The intent of PRC-019 isto verify regulating controls, limiters, equipmentcapabilities, and protection controls installed atgeneration facilities are appropriately coordinatedso as to not exacerbate adverse power gridconditions during a system disturbance. PRC-019provides fairly explicit guidance for what isexpected to show compliance for synchronousgenerator facilities. PRC-019 does not containexplicit guidance on how to show compliance forasynchronous or distributed generation resources.This paper describes the approach, challenges,and lessons learned from performing NERC PRC019 studies of both typical synchronous andasynchronous generators. Both the commonalitiesand variances of synchronous and asynchronousgeneration facilities are highlighted to provide theconnecting link between what is explicitly stated inPRC-019 requirements and what is commonlyexpected by compliance authorities. While theinterpretation for what is necessary todemonstrate compliance can vary betweenreliability coordinators; this paper providesexamples of proven compliance documentation fora recent PRC-019 wind farm study.I.INTRODUCTIONIn response to the 2003 Northeast United Statesblackout and subsequent governmental regulation,NERC Protective and Control Standards (PRC) werecreated. The intent of these standards is to improvethe performance and reliability of the NorthAmerican Bulk Electric Power System (BES). MostPRC standards have clearly defined requirementswith very specific examples for what analysis anddocumentation is required to demonstratecompliance. The recent NERC redefinition of whattransmission systems are included within the BES hasled to many revisions to the standards to includedistributed generation resources (DG) as applicablefacilities. Inclusion of DG facilities without explicitPRC standard criteria has created room forinterpretation in what analyses and documentationneeds to be provided to show compliance.This paper intends to provide an outline of how aPRC-019 study is completed for traditionalsynchronous generator facilities and an interpretationfor how to complete a PRC-019 study forasynchronous DG resources. This paper also providesperspective on how those two studies compare,contrast, and lessons that have been learned whilecompleting them.II. NERC PRC-019-2According to the NERC PRC-019 standard, itspurpose is to “verify coordination of generating unitFacility or synchronous condenser voltage regulatingcontrols, limit functions, equipment capabilities andprotection system settings .” The goal of PRC-019is to improve BES reliability during short timesystem transients by keeping available generation inservice to support the BES, reducing the risk of acascading blackout event.The standard establishes criteria for which facilitiesPRC-019 applies to and what coordination items atthose facilities are required to be verified. Facilitieswhich fall under PRC-019 requirements are: Generators over 20MVASynchronous condensers over 20 MVAMultiple generator facilities with anaggregate nameplate rating over 75MVABlack start generators identified inTransmission Operator’s restorationplan(s)Both Generation Owners (GO) and TransmissionOwners (TO) of synchronous condensers are requiredto demonstrate compliance for BES generatingfacilities under their control. An addition made to thesecond revision of the PRC-019 standard is section188.8.131.52. This description explicitly states dispersedpower producing resources, even if they areperforming voltage regulation at the individual unitlevel, are included in PRC-019-2 requirements.Before this revision, dispersed generating resourcessuch as a type I wind farm would have been excludedfrom the requirements.
The standard outlines two requirements:Requirement R1 states that “At a maximum of everyfive calendar years, each Generator Owner andTransmission Owner with applicable Facilities shallcoordinate the voltage regulating system controls,(including in-service limiters and protectionfunctions) with the applicable equipment capabilitiesand settings of the applicable Protection Systemdevicesandfunctions.Requirement R1 section 1.1 defines two coordinationitems that are to be verified, with the assumption ofnormal automatic voltage regulating control loop andsteady-state operating conditions:1.1.1 “The in-service limiters are set to operatebefore the Protection System of the applicableFacility in order to avoid disconnecting thegeneratorunnecessarily.”1.1.2 “The applicable in-service Protection Systemdevices are set to operate to isolate or deenergize equipment in order to limit the extentof damage when operating conditions exceedequipment capabilities or stability limits.”Requirement R2 requires GOs & TOs to resubmitPRC-019-2 compliance documentation within 90calendar days following applicable system equipmentor voltage regulating and protection system settingchanges.III. SYNCHRONOUS MACHINE STUDYA.Study GoalsTo show compliance with PRC-019, GOs & TOsmust provide evidence of equipment capability,protection and voltage regulating coordination in theform of P-Q, R-X, Inverse Time diagrams, or acombination of the above. In some cases availablemachine manufacturer’s data is not provided in aformat that can be displayed on one of these diagramsand in that case PRC-019-2 does make allowance forequivalent tables or other evidence.B.Study ProcessThe study process begins with acquiring informationabout the subject generator’s capabilities, in-servicelimiters and protection system. Generator propertiesinclude: impedances, MVA capability, rotor andstator current withstand, and V/Hz withstand areprovided by generator manufacturers. The data istypically provided in P-Q or Time CurrentCoordination (TCC) plot format. Excitation systemlimit settings are typically provided in the form ofexciter programming documentation or settingsreport. Generator protection settings are typicallyprovided as plots in the case of electromechanicalrelays, settings report or a settings file in the case ofmicroprocessor based relaying. An ideal data setwould include information about: Generator Capability & ImpedancesUnit Transformer (GSU) Rating andImpedanceLoss of Excitation Protection SettingsOver Excitation Limiter SettingsOver Excitation Protection SettingsGenerator Rated Power FactorUnder Excitation Limiter SettingsGSU & Generator Volts/Hertz CapabilitiesVolts/Hertz Limiter SettingsVoltage regulation system manualsOne line diagramsThree line diagramsAfter receiving data from an initial request forinformation (RFI) it must be reviewed to determine ifit is adequate to perform a PRC-019 study. The typeof data used in a PRC-019 study is not typically usedin day-to-day plant operation of a synchronousgenerator and as such it is often misplaced, forgottenabout or was never delivered. As a result it is typicalto issue one or more follow up RFIs before receivinga complete data set for a PRC-019 study, and to assistthe GO in issuing RFIs to equipment manufacturers.Once a complete data set is compiled the data isplotted on diagrams appropriate to the data received,typically P-Q, R-X and inverse time-current orinverse time-voltage plots. The PRC-019 standarddocument provides examples of these plots. Figure 1is a typical P-Q plot example shown in PRC-019-2.Microsoft Excel has proven to be a useful tool forcreating plots since capability data can be inputdirectly and characteristics such as steady statestability limit (SSSL), loss of field protection andlimiter characteristics can be calculated in the samework book that is used to produce the plot.
Figure 3 shows the typical synchronous generatorPRC-019 study process in flow chart form.Figure 1: PRC-019-2 Example P-Q PlotOnce plotting is complete, evaluating PRC-019compliance is straight forward.Non-compliantlimiters and protection characteristics are apparent byvisual inspection of the plots. Figure 2 shows anexample of a non-compliant P-Q plot. Observe inthis example that the under excitation limiter is setbelow the steady state stability limit curve, whichdoes not meet PRC-019 Requirement R1 because themachine may go out of synchronism before thelimiter takes action.Figure 2: Example Non-Compliant P-Q PlotFigure 3 – Typical PRC-019 Study Process
C.Common Stumbling BlocksManufacturer’s machine data quantity and qualityvaries widely between manufacturers. It also tends tovary with the age of the generator. Newer generatorstend to have more data available as less time haspassed for it to be misplaced or destroyed, and withmodern numerical controls and protective relays, theO&M manuals usually include extensive listings ofsettings. Older generators tend to have less dataavailable for the reason noted above. In addition,data for older generators tends to be of poorerresolution, either in the original document or in scansand copies over time, which can be difficult tointerpret. Older generators may also make use ofanalog excitation controls that are obsolete and notcurrently supported by the OEM, requiring extensiveresearch to determine how limiters are implemented,or even if the function exists.D.ReportA PRC-019 compliance report must convey to acompliance auditor that the subject generator orsynchronous condenser meets the requirements statedin the PRC-019 standard. A secondary audience istypically engineering staff responsible for thegeneration or transmission facility the subject unitbelongs to. A tertiary audience of managers may alsoneed to be able to glean information from the report,such as the need for replacement of relays or voltageregulating equipment.The report should progress from a high leveloverview to a detailed discussion of the analysis andresults.The report must include a detaileddescription of any assumptions that were made aboutthe subject generating unit or data interpretations thatwere required. The report structure should allowmanagement personnel to be able to read the initialoverview and summary and see at a glance whereproblems were identified in a specific subsystem. Thebody of the report will include figures as described inPRC-019, and tables and text as necessary todemonstrate to a compliance auditor that limiters,protection and unit capability are coordinated per therequirements of PRC-019. The body of the reportshould also contain all of the detail a protectionengineer would need to understand the analysis thatwas performed and to reproduce any portion of thestudy if needed to confirm the results.To assist GOs in generating RSAW (ReliabilityStandard Audit Worksheet) documents in a uniformmanner, NERC has developed a standard RSAWtemplate for PRC-019 and many other compliancestandards. The PRC-019 report writer is stronglyadvised to use the RSAW template as reference toinsure the report includes all the necessaryinformation. The report itself is then used to providebackup evidence of study completion when theRSAW package is assembled and submitted forperiodic reliability audit by the GO’s complianceofficer.IV. ASYNCHRONOUS MACHINE STUDYA. Study GoalsUnlike completing a PRC-019 study for a traditionalsynchronous generator, there are no specified limiterfunctions, protective devices, or example diagrams inthe PRC-019 standard related to asynchronousgenerators. Asynchronous generators are typicallyfound in a network of multiple similar generatorsconnected into a collector substation, such as at awind farm or solar PV facility. Although wind andsolar PV generators are commonly combined at acentral facility, wind and solar generation facilitiesare generally considered distributed generation (DG)or distributed energy resources (DER) because thefacilities are an assembly of multiple individualgenerators that themselves are far smaller than the20MVA minimum described above.Completing a PRC-019 study of an asynchronousgenerator facility requires a different analysis methodthan the typical synchronous generator facility. PRC019 compliance can be completed for wind or solargenerators by understanding the goal of PRC-19 andlikening functions found in asynchronous facilities tothose at synchronous facilities. The followingdescription will focus on Type I and Type III windturbine facilities, but the same study process can beapplied to any asynchronous DER facility.B. Study DescriptionAn asynchronous generator PRC-19 study beginswith acquiring information about the individualgenerator, the collector system, and plant levelcontrols. Each of the three sections represents adifferent analysis phase, and will require informationspecific to that area. Many of the same issues withdata collection that occur in a synchronous generatorstudy are encountered while gathering data for anasynchronous generator study. Turbine manufacturer,facility designer, and current operating entity canaffect what documentation is available and whatinformation is present in each particular document.Asynchronous generator and facility data tends to bemuch less standardized than data for traditionalsynchronous generators. As responses to the initial
RFI are returned, a more specific request for theoutstanding information can be sent. Figure 4 showsa diagram for each of the four types of wind turbinegenerators (WTG) commonly used to date whichinclude: Type 1: Induction generatorType II: Induction generator with variablerotor resistanceType III: Doubly-fed induction generator(DFIG)Type IV: Asynchronous or synchronousgenerator with full converter interface. ThisWTG can make use of DC or ACgenerators, and avoid the use of a gearbox.Most existing wind farms are constructed of eitherthe older Type I induction generators with staticcapacitor compensation or newer Type III DFIGWTG. Following advancements to the newest TypeIV full converter synchronous WTG, it is possiblethat many of the same limiter and protectionfunctions found in a typical synchronous generatorcould be present. If functions matched thosespecifically listed in the PRC-019 standard, it isreasonable a more standard analysis technique couldbe performed, including the creation of a typical P-Qplot. Considering the depth of information required inregards the generator characteristics, it would seemlikely for a study of that type, the generatormanufacturer would need to provide a majority of theinformation.C. Phase 1 - Individual Generator AnalysisDepending on the manufacturer the followingfunctions can be found in Type I machines: Normal condition shutdown limiter(s)Emergency condition shutdown limiter(s)Overcurrent or overpower relaysMachine thermal overload curveTurbine main overcurrent relay (MCCB1)Step-up transformer specificationsStep-up transformer protection (fuse orrelay)If the information above is available, individualanalysis is a fairly simple task of plotting allfunctions on a TCC plot. Figure 4 shows an exampleTCC for a 1MW Type I WTG.Figure 4 –Example Type I WTG TCCConsidering a simplified wording of the twocoordination items 1.1.1 and 1.1.2 listed in R1 of thePRC-019-2 standard, limiters should operate beforeprotective devices, and protective devices shouldoperate to limit equipment damage. It is reasonable toconsider that Figure 4 TCC verifies both requiredcoordination items. If adequate wind conditions arepresent, all WTG is able to operate at full poweroutput before a hard-stop limiter function operates byadjusting prop pitch to reduce power output. Thenormal and emergency limiters are properlycoordinated to operate before either protective devicewould trip the generator offline thus demonstratingcompliance for Requirement R1 1.1.1. All protectivedevices are properly coordinated to operate beforedamage would occur to either the WTG or the stepup transformer demonstrating compliance forRequirement R1 1.1.2. The decision to consider thestep-up transformer protection and damage curve aspart of the individual analysis is up to interpretation.While damage to the transformer would only occurfor a near bus fault, it seems consistent to includethose protection elements and damage curves in theindividual analyses considering a damagedtransformer will result in the WTG also being offlineuntil equipment can be repaired or replaced.
Individual analysis of a type III WTG is very similarto the process described above for a type I WTG, butwith one main difference: power output comes fromboth the stator and rotor converter of a type III WTG.Along with a majority of the same informationrequired for a type I turbine analysis, the outputcapabilities of both the stator and converter areneeded for type III turbine analysis. Figure 5 showsan example TCC for a 2.08 MVA type III WTG.protective device that would trip the generatoroffline, thus demonstrating compliance forRequirement R1 1.1.1. All protective devices (relays)are properly coordinated to operate before extensivedamage would occur to either the WTG or the stepup transformer demonstrating Requirement R1 1.1.2.There is an argument to be made that this level ofanalysis is satisfactory to verify NERC PRC-019-2compliance, but some protective elements as part ofthe collective system can behave in the manor of alimiter which requires additional analysis.D. Combined System AnalysisFigure 5 –Example Type III WTG TCCOf a maximum 2080KVA combined power output,the stator produces 1887KVA and converter produces193KVA of the total output. These output limits aretypically not present in documentation available atthe generation site, but can be normally be found inthe manufacture’s turbine specific O&M manual.Because of output from both the converter and stator,an individual breaker is usually installed on each.Much like the individual machine analysis for a typeI WTG, analysis of a type III WTG simply requiresplotting all limit and protection functions on a TimeCurrent Coordination plot. Due to the nature of a typeIII WTG by design there is very little coordinationmargin between the output limit function and theassociated protective device. As shown in Figure 5,Both the stator and converter output limits areproperly coordinated to operate before eitherCombined system analysis begins with the sameprocess for individual generator analyses. Thegrouping of generators can typically be seen in twolevels. The first level is a feeder string consisting ofbetween 5-15 WTG, and the second is a collector orfeeder bus made up of multiple strings with a total of30-150 WTG. Depending on the overall facilityconfiguration the collector bus connects to one ormore generator step-up (GSU) transformers andvoltage regulating equipment. Voltage regulatingequipment can include switchable shunt capacitorbanks, reactor banks, or control systems withcapacitive capabilities such as a DVAR system.Protective relaying is typically present at each arealisted above: feeder, collector bus, GSU transformer,and voltage regulating equipment. Feeder limits arethe individual WTG power output limits multipliedby the number of interconnected WTG. Collector buslimit is the individual WTG power output limitsmultiplied by the total number of interconnectedWTG plus the full VAR output of the voltageregulating equipment. Any protective device must beset above this these output limits to be compliant withPRC-019 Requirement R1. Figure 6 shows acombined TCC considering the heaviest loadedfeeder, collector, and control system limits andprotection functions.
Feeder & GSU Short TimeOvervoltage Trip1.25 p.u. WTG Inst TimeOvervoltage Trip1.2 p.u.Cap Bank, Feeder, & GSULong Time Overvoltage TripWTG Short TimeOvervoltage Trip1.15 p.u. WTG Medium TimeOvervoltage TripFast Voltage Control Region1.1 p.u.WTG Long TimeOvervoltage Trip1.045 p.u.Slow Voltage Control Region1.01 p.u.Deadband0.99 p.u.Slow Voltage Control Region0.955 p.u.Fast Voltage Control Region0.9 p.u.0.8 p.u.Feeder Undervoltage Trip0.5 p.u.Figure 6 – Example Collector Substation TCCE. Plant Level Control AnalysisPlant level control can vary in complexity from asingle relay switching in capacitor banks based onpower flow to a DVAR system with communicationto each individual turbine. Plant controllers typicallyoperates in either voltage or power factor controlmodes. Voltage control mode usually maintains thevoltage within a contracted range at the utility pointof interconnection (POI). Power factor control modeusually measures current and voltage at the regulatedPOI, compares these values to a contracted powerfactor range, and then calculates what VARcompensation is needed to maintain the target powerfactor. Relay switching plant level control analysis isminimal; simply review capacitor bank relays forovercurrent settings that would trip the facility offlinebefore the individual or combined WTG output limitsare met. Advanced power factor or voltage regulatingschemes require a more thorough analysis to bothcompare if control limits will curtail WTG output,but also that voltage regulating limits and protectionare properly coordinated. Figure 7 shows a voltagecoordination plot for a DVAR-STATCOM systemregulating voltage at the 138kV collector bus.Capacitor Undervoltage TripFigure 7 – Example Voltage Coordination PlotFigure 7 was created to address wording inRequirement R1 that “Facilities shall coordinate thevoltage regulating system controls, with applicableequipment capability and setting of the projectionsystem devices and functions.”  Both voltagelimits and protection set points for the WTG, feeder,collector, and plant controller are shown tocoordinated and therefore demonstrate compliance.V. CHALLENGES & LESSONS LEARNEDA. Data AvailabilityAs mentioned previously, the data used in PRC-019studies is not typically used day to day and is oftenmisplaced, of poor quality or the owner simply isn’tfamiliar with what is being requested. For olderinstallations with paper documentation, a site visit toreview all available documentation is often required.For newer installations with O&Ms in PDF format orother electronic documentation, it is typical to requestall documentation the owner has via email or FTPsite and a site visit is not required.It is not un-common to receive the exact datarequested, only to find it un-useable. Paper drawings
that have been scanned may lack the resolution topick up small text which tends to be the mostpertinent text, particularly if they have been scannedmultiple times. Nameplate photographs tend to beuseless when the nameplate has been painted overduring maintenance. Corroded nameplates for olderunits have also been an issue.MVARS axis, the general shape of the curves wouldboth drive field current limiter and loss-of-excitationsettings to very similar values. When machinecapacity plots are not available, such as for a hydroturbine generator that may be over 90 years old, it isacceptable to recreate this information using standardmethods described in engineering texts.If data is un-available it may be re-created fromequipment standards and reference texts in somecases. The P-Q Diagram in Figure 8 was calculatedusing equations from “Electric MachineryFundamentals” B. Atypical DataData sometimes comes in formats that don’t fittypical reporting diagrams. Figure 10 shows asample of a Volts-per-Hertz capability plot from agenerator manufacturer that was given simply interms of voltage and frequency per unit values, ratherthan a time based withstand curve.Figure 8 - Calculated Theoretical P-Q DiagramShortly after the P-Q Diagram in Figure 8 wasdeveloped, the manufacturer’s P-Q Diagram for thegenerator was received. Figure 9 is a plot of themanufacturer’s P-Q diagram for the generator.Figure 10 - Atypical Data FormatIn this case, protective settings were interpreted to fitthe withstand curve in the format it was provided in.Figure 11 shows the resulting plot.Figure 9 - Manufacturer's P-Q DiagramIt is apparent by observation that the theoretical plotin Figure 8 and the manufacturer’s plot in Figure 9are very similar in general shape. Although they aresignificantly different along the lower half of theFigure 11 - Atypical Voltage/Frequency Plot
It is clear by observation that the applicable limiter iscoordinated with the applicable protection elements.Although the plot is not in a typical format, it doesshow that the limiters and protection on thatgenerator are compliant with PRC-019.C. Manufacturer DataContacting manufacturer’s directly has been asuccessful step in receiving any outstandinginformation for generators of all types. Typicallysupplying a model and serial number for thegenerator is required to receive useful data. In someinstances the manufacturer has required the datarequest come from the generator owner directly, inothers the manufacturer has provided it to a thirdparty. In a few instances data requests have beendenied based on the claim that the requestedinformation is proprietary, an unfortunate situationwith many DER OEMs. With manufactures that havereservations about sharing certain information, manytimes holding a teleconference with the manufacture,generation owner, and the third party performing thestudy can help assure only the minimum amountinformation required to successfully complete thestudy is being asked for.D. Regulatory InterpretationPRC-019-2 was clearly written with classicsynchronous generators and condensers in mind. Toapply PRC-019 to asynchronous generators requiresmeeting the intent of the standard rather than creatinga plot explicitly given in the standard. It is importantto keep in mind that the most important audience of aPRC-019 study is the reliability coordinator for theNorth American region where the generation facilityis located. It is critically important to provideadequate information and thoroughly justify yourinterpretations of the standard to help step acompliance auditor through the study process andcome to the same conclusion. The complete studymust assure a compliance auditor that the generatoror condenser facility will operate as intended by therequirements of PRC-019.VI. CONCLUSIONSAs the bulk electric system evolves it is becomingincreasingly intricate which creates new reliabilitychallenges. Unforeseen BES events are going tooccur and NERC will respond with revised reliabilitystandards in an attempt to prevent those events fromhappening. As standards are revised, gaps insupporting documentation for what demonstratescompliance may require an interpretation of thestandard for certain facilities until more prescriptiverequirements are developed.The task of NERC PRC-019 compliance may seemcumbersome considering the amount of non-typicalinformation required and the lack of guidance in thestandard for what is expected to show compliance forDER facilities. Through those difficulties, a PRC-019study can provide an opportunity to document facilityinformation for a generating facility which wouldotherwise not be available for future use.VII. REFERENCES “NERC PRC-019-2 Coordination of GeneratingUnit or Plant Capabilities, Voltage RegulatingControls, and Protection”, National ElectricReliability Council, Version 2 May 29, 2015 “Electric Machinery Fundamentals”, StephenChapman, 2011 “NERC Considerations for Power Plant andTransmission System Protection Coordination”,National Electrical Reliability Council, July 2015 “IEEE C37.102-2006 IEEE Guide for ACGenerator Protection”VIII.BIOGRAPHIESMatthew Manley joined POWER Engineers in 2015.He is a member of the SCADA and AnalyticalServices group where he performs a variety ofelectrical system studies for transmission, substation,and generation projects. He has a background n, NERC reliability standards, and soilresistivity testing and grounding grid analysis. Mattreceived his B.S. in electrical engineering from TexasA&M University.Tony Limon joined POWER Engineers in 2015. Heis a member of the SCADA and Analytical Servicesgroup where he performs a variety of electricalsystem studies for transmission, generation, anddistribution projects. He has a background in powersystem analysis, protective relaying, and arc flashstudies. Tony holds a B.S. in electrical engineeringfrom Washington State University.
NERC Protective and Control Standards (PRC) were created. The intent of these standards is to improve the performance and reliability of the North American Bulk Electric Power System (BES). Most PRC standards have clearly defined requirements with very specific examples for what analysis