Power Plant And Transmission System Protection .

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Power Plant and Transmission SystemProtection CoordinationGSU Phase Overcurrent (51T), GSU Ground Overcurrent(51TG), and Breaker Failure (50BF) ProtectionNERC Protection Coordination Webinar SeriesJune 9, 2010Phil TatroJon Gardell

Agenda2 Technical Reference Document Overview Objectives Description of Protection Functions Time-Current Coordination Discuss and Describe System Events that CouldCreate Conditions that Would Cause Operationof These Functions

Agenda3 Detailed Coordination Information Function 51T – GSU Phase Overcurrent Function 51TG – GSU Ground Overcurrent Function 50BF – Breaker Failure What is Important to Coordination Settings that Protect the Generator Coordination Margin Question and Answer

Disclaimer The information from this webcast is provided forinformational purposes only. An entity's adherence to theexamples contained within this presentation does notconstitute compliance with the NERC Compliance Monitoringand Enforcement Program ("CMEP") requirements, NERCReliability Standards, or any other NERC rules. While theinformation included in this material may provide some of themethodology that NERC may use to assess compliance withthe requirements of certain Reliability Standards, this materialshould not be treated as a substitute for the ReliabilityStandard or viewed as additional Reliability Standardrequirements. In all cases, the entity should rely on thelanguage contained in the Reliability Standard itself, and noton the language contained in this presentation, to determinecompliance with the NERC Reliability Standards.

Technical Reference Document Overview5 Introduction and Background – BlackoutRecommendation TR-22 SPCS’s Assignment The Need for this Technical ReferenceDocument - History and Background: August 14, 2003 Blackout Subsequent Events Specific Concerns Regarding Generator and PowerPlant Protection

Technical Reference Document Overview6 Support of PRC Standards Benefits of Coordination: To the Generator Owner To the Transmission Owner To the Planning Coordinator Reliability of the Bulk Electric System and PowerDelivery to the Customer

Objective7 Increase knowledge of recommended protectionfor GSU Phase Overcurrent, GSU GroundOvercurrent, and Breaker Failure protection. Facilitate improved coordination between powerplant and transmission system protection forthese specific protection functions.

Scope8 Focus is on the reliability of the Bulk ElectricSystem. This Technical Reference Document isapplicable to all generators, but concentrates onsynchronous generators connected at 100-kVand above. Distributed Generation (DG) facilities connectedto distribution systems are outside the scope ofthis document.

The Need for GSU Phase OvercurrentProtection – Function 51T Neither IEEE C37.91 nor IEEE C37.102 supports the use of aphase overcurrent function for backup protection for faults in boththe GSU and generator, or for system faults. IEEE C37.102 provides the following information concerning phaseovercurrent backup protection: “In general, a simple time-overcurrent relay cannot be properly set toprovide adequate backup protection. The pickup setting of this type ofrelay would normally have to be set from 1.5 to 2 times the maximumgenerator rated full-load current in order to prevent unnecessarytripping of the generator during some emergency overload condition. With this pickup setting and with time delays exceeding 0.5 s, thesimple time-overcurrent relay may never operate since the generatorfault current may have decayed below relay pickup.”IEEE C37.102-2006 – Guide for ACGenerator Protection, Section 4.6.1.29

The Need for GSU GroundOvercurrent Protection – Function 51TG The ground overcurrent function providesgenerator and GSU ground backup overcurrentprotection for uncleared system ground faults. The ground overcurrent function is connected todetect the ground current provided by the GSUtransformer when connected as a groundsource.10

The Need for Breaker FailureProtection – Function 50BF Breaker failure protection provides isolation ofthe generator in the event its breakers fail toopen subsequent to receiving a signal to trip. When a generator unit breaker fails, the breakerfailure function is required to initiate the trippingof backup breaker to isolate the failed breaker.11

Relay One-Line Showing All Generator Protectionand Identifying Function 51T, 51TG, and H2132404650/2751V78

Time-Current Coordination13Time in SecondsGSU TransformerDamage Curve These protectionsmust be coordinatedwith system faultprotection and theequipment capability Sensitivity and timingof the relaying must:Phase OC on GSU - 51GSUCT 400/1 Result in tripping ofproper system elementsTOC TAP 10ATime Dial No 1.0Curve INVERSE Permit the generator tostay on line duringsystem stressedconditionsPhase OC on Line - 51 LINECT 400/1TOC TAP 8ATime Dial No 0.5Curve INVERSEFault 11587.7AINST TAP 20ACurrent in Amperes

System Events that Could Cause UndesiredOperation of These Protection Functions Fault Conditions Miscoordination with system protection during asystem fault14

General Data Exchange Requirements –Generator Owner Data and Information The following general information must be exchanged in addition torelay settings to facilitate coordination, where applicable: Relay scheme descriptions Generator off nominal frequency operating limits CT and VT/CCVT configurations Main transformer connection configuration Main transformer tap position(s) and impedance (positive and zerosequence) and neutral grounding impedances High voltage transmission line impedances (positive and zerosequence) and mutual coupled impedances (zero sequence) Generator impedances (saturated and unsaturated reactances thatinclude direct and quadrature axis, negative and zero sequenceimpedances and their associated time constants) Documentation showing the function of all protective elements listedabove15

General Data Exchange Requirements –Transmission or Distribution Owner Data and Information The following general information must be exchanged in addition torelay settings to facilitate coordination, where applicable: Relay scheme descriptions Regional Reliability Organization’s off-nominal frequency plan CT and VT/CCVT configurations Any transformer connection configuration with transformer tapposition(s) and impedance (positive and zero sequence) and neutralgrounding impedances High voltage transmission line impedances (positive and zerosequence) and mutual coupled impedances (zero sequence) Documentation showing the function of all protective elements Results of fault study or short circuit model Results of stability study Communication-aided schemes16

Detailed Coordination Information forFunctions 51T, 51TG, and 50BF Detailed coordination information is presentedunder seven headings, as appropriate, for eachfunction in the document. The following slides present a section-by-sectionsummary for Functions 51T, 51TG, and 50BF.17

Document Format – Seven Sub-Sectionsfor Each Protection Function Purpose Coordination of Generator and Transmission System Faults Loadability Other Conditions, Where Applicable Considerations and Issues Coordination Procedure Test Procedure for Validation Setting ConsiderationsExamples Proper Coordination Improper Coordination Summary of Detailed Data Required for Coordination of the ProtectionFunction Table of Data and Information that Must be Exchanged18

Purpose – Functions 51T and 51TG19 Provide generator and GSU phase and ground backupprotection for uncleared system phase and ground 51TG67I/T50/51Gor67GI/TFigure 3.9.1 — Phase & Ground Backup Overcurrent Relays on GSU Transformer

Coordination of Generator andTransmission System – Functions 51T and 51TGCaution: Use of a GSU phase overcurrent element (51T) for backupprotection is strongly discouraged. The distance function (21) and the voltage supervised overcurrentprotection function (51V) are better suited for this purpose. The Technical Reference Document describes the use and applicationof these functions to provide the best phase backup protection that canbe coordinated between the protective relaying of a Generator Ownerand Transmission Owner. For completeness the issues required to utilize the 51T backupovercurrent protection function are presented. When used, the 51T function and associated settings need toconsider the following:20

Coordination of Generator andTransmission System – Functions 51T and 51TG Faults The 51T and 51TG must meet the following considerations forfault coordination: Pickup for the worst-case backup fault on the transmission systembased on the application. Have sufficient time delay with adequate margin to coordinate withthe worst-case clearing time of the transmission protection withbreaker failure clearing times included. Be set such that the generator has the ability to produce the faultcurrent long enough to complete the overcurrent backup functionwithout causing any misoperation.– This requires great care in determining the sensitivity (pickupvalue) and selectivity (time to operate value).21

Coordination of Generator andTransmission System – Functions 51T and 51TG Loadability The 51T function must be set to accommodate a minimumloading of 200 percent of the generator MVA rating at ratedpower factor. The above requirement allows a generator to remain online throughextreme operating system events, by allowing a generator to utilizeit full capability of field forcing. Note: Any 51 function utilized from the generator or GSU multifunctional protective relays must meet the above loadabilityrequirement. The 51TG function must accommodate the greatest systemunbalance anticipated at the GSU, with margin.22

Considerations and Issues– Function 51T and 51TG Protective functions other than 51T are available toprovide backup protection for phase faults whileproviding better coordination with the transmission andgenerator protections. The 51TG backup overcurrent provides backup and timedelayed protection for ground faults when primaryrelaying or equipment does not operate properly. Refer to IEEE C37.102 section 4.6 for recommendationson setting the 21, 51V, and 51TG relays, and refer to thereferences in IEEE C37.102 that discourage the use ofthe 51T.23

Coordination Procedure– Function 51T and 51TG Coordination of Function 51T The 51T must have a minimum pickup of twice the generatorMVA rating at rated power factor. The 51T must operate slower, with margin, than the slowesttransmission protection system that it must coordinate withbased on protection design including breaker failure time. The 51T must sense the required fault based on thetransmission protection design with the fault current availablefrom the generator in the time frame that it is set to operate. The Generator Owner must determine that the setting for the51T that coordinates with the transmission protection will alsocoordinate with the generator protection systems for the faultcurrent available from the transmission system.24

Coordination Procedure– Function 51T and 51TG Coordination of Function 51TG The 51TG must have a pickup with margin greaterthan the largest non-fault system unbalanceanticipated based on system design. The 51TG must operate slower with margin than theslowest transmission protection system that it mustcoordinate with based on protection design includingbreaker failure time.25

Example - Proper Coordination– Function 51T and 51TG 26Settings for Function 51T Step 1 — Rated current 1,778 A, primary (1,778A/400) 4.445 A,secondary Step 2 — Select a relay characteristic curve. [Note: Curve is typically chosento match the curve used by the Transmission Owner i.e. a Very-Inverse Curve.] Step 3 — Tap Setting of 51T 2 X I rated (4.445A) X (2) 8.89A;choose Tap 9.0A Step 4 — From short-circuit studies; obtain the 3ф through-fault current for thefault located on the generator bus shown as F1 in the diagram. I3ф 11,587-A,primary through-fault current on GSU transformer. Relay current 11,587 A,primary/400 28.96 A, secondary Step 5 — Multiple (relay current) / (Tap) 28.96A/9.0A 3.21, choose aTime Dial such that a time equal to approximate 30 cycles more than theslowest transmission overcurrent setting. Step 6 — Ensure coordination with all appropriate transmission systemprotection elements. Step 7 — The Generator Owner takes the information concerning the 51T inthe plot and determines that it will coordinate with the other generatorprotection for the available transmission system fault current for GSU andgenerator faults.

Example - Proper Coordination– Function 51T and 51TG27Time in SecondsGSU TransformerDamage CurvePhase OC on GSU - 51GSUCT 400/1TOC TAP 10ATime Dial No 1.0Curve INVERSEPhase OC on Line - 51 LINECT 400/1TOC TAP 8ATime Dial No 0.5Curve INVERSEFault 11587.7AINST TAP 20ACurrent in AmperesFigure 3.9.3 — Function 51TGSU & 51LINE (G or N) Overcurrent RelayCoordination Curves

Example - Proper Coordination– Function 51T and 51TG Setting for the 51TG Assumption: current transformer ratio (CTR) for the neutral CT on the GSU transformer is 1600/5A (CTR 120:1), multi-ratio. Step 1 — Obtain 3I0 current from short-circuit studies for fault location F2 (the primaryminimum fault current provided from the neutral of the GSU that must be detected by 51TG).F2 1930 Amperes primary. Step 2 — Select a relay characteristic curve. [Note: Curve is typically chosen to match thecurve used by the Transmission Owner, i.e. a very-inverse curve.] Step 3 — Tap Setting of 51TG [Note: Tap is typically selected based on available minimumshort-circuit current (F2) and current transformer ratio on the neutral of GSU transformer(120:1) such that two or higher times pickup is available for the fault that represents theminimum ground current that the 51TG is suppose to provide backup protection for a fault atF2, while providing for the worst case system unbalance.]. 51TG tap setting (F2)/(2.0margin *CTR) 1930 Amp/ (2.0 * 120) 8.04, choose 8.0 tap. Step 4 — From short-circuit studies; obtain the 3I0 through-fault current for the fault locatedon the generator bus shown as F1 in the diagram. 3I0 7,556 A, primary from the neutral ofGSU transformer. Relay current 7,556A/120 62.96A, secondary Step 5 — Multiple (relay current) / (Tap) 62.96/8A 7.87, choose a Time Dial equal toapproximate 30 cycles or more than the slowest transmission overcurrent setting. The timedelay setting with margin will result in a time setting in the 60 – 90 cycles range. The 30cycle margin will accommodate breaker failure clearing timers up to 20 cycles with margin.28

Example - Proper Coordination– Function 51T and 51TGTime in SecondsGSU Transformer Damage CurveGround OC on GSU - 51GGSUCT 120/1TOC TAP 8ATime Dial No 2.25Curve VERY INVERSEGround OC on Line - 51LINECT 400/1TOC TAP 2ATime Dial No 1.20Curve VERY INVERSEINST TAP 12AA Phase-to-gndFault 7557.5ACurrent in AmperesFigure 3.9.4 — Function 51TG Overcurrent Relay Characteristic Curve29

Summary of Protection Functions Requiredfor Coordination – Function 51T and 51TGTable 2 Excerpt — Functions 51T / 51TG Protection Coordination Data Exchange RequirementsGenerator ProtectionFunctionTransmission SystemProtection Functions5151T — Phase fault backupovercurrent6751TG — Ground fault backupovercurrent51N51G67NSystem Concerns Must have adequate margin over GSU protection andnameplate rating 51T not recommended, especially when theTransmission Owner uses distance line protectionfunctions Open phase, single-pole tripping and reclosing Generator Owners(s) needs to get Relay Data (functions51, 67, 67N, etc) and Single line diagram (including CTand PT arrangement and ratings) from TransmissionOwner(s) for function 51T coordination studies30

Protection Function Data and Information ExchangeRequired for Coordination – Functions 51T and 51TGTable 3 Excerpt — Functions 51T / 51TG Data to be Exchanged Between EntitiesGenerator OwnerFunction 51T — Phase fault backupovercurrentFunction 51TG — Ground fault backupovercurrentTransmission OwnerOne line diagram of the transmission systemup to one bus away from the generator highside busRelay timer settings.Impedances of all transmission elementsconnected to the generator high-side busTotal clearing times for the generatorbreakersRelay settings on all transmission elementsconnected to the generator high-side busTotal clearing times for all transmissionelements connected to the generator highside busTotal clearing times for breaker failure, for alltransmission elements connected to thegenerator high-side busPlanning CoordinatorNone31

Purpose — Function 50BF32 Breaker failure protection provides isolation ofthe generator in the event its breakers fail toopen subsequent to receiving a signal to trip. When a generator unit breaker fails, the breakerfailure function is required to initiate the trippingof backup breaker(s) for isolation of the failedbreaker.

Coordination of Generator andTransmission System – Function 50BF Faults Breaker failure and generator unit protection arerequired to coordinate with protective relays on thenext zone of protection including breaker failurerelaying time. Loadability There are no coordination issues related to loadabilityfor this function.33

Consideration and Issues – Function 50BF34 All upstream (next level) protection settings and systems must beconsidered when evaluating the performance of breaker failurefunctions associated with generators. Total clearing time, which includes breaker failure time, of eachbreaker in the generation station switchyard should coordinate withthe critical clearing times associated with unit stability.BREAKER FAILURE DECLARE TIMET1PROTECTIVERELAYTIME50OPERATEBREAKER INTERRUPT TIME50RESETTIMESAFETYMARGINTBKT50TMBREAKER FAILURE TIMER62BFFAULTOCCURSFAULTCLEARED86BFREMOTE BACKUP BREAKERINTERRUPT TIMETIMETRANSFER TRIP TIMETOTAL FAULT CLEARING TIMEFig u re 3.8.3 — Exa m p le o f Bre a ke r Fa ilu re Tim in g Ch a rt [1][1] This chart is excerpted from the IEEE Std. C37.119-2005 “Guide for Breaker Failure Protection of PowerCircuit Breakers.”

Consideration and Issues – Function 50BF35GSU52-G52-T52-LG50BFG52-G fail to trip or open50BF-G86T52-T Use of a 52a contactpermits operation forlow magnitude (e.g.turn-to-turn) faults andabnormal operatingconditions Use of a fault detectorpermits operation whenthe 52a contact doesnot provide an accurateindication of thebreaker statusFigure 3.8.1 — Unit Breaker Failure Logic Diagram

Coordination of Generator andTransmission System – Function 50BF The following is an example of Breaker FailureTimer Settings (62BF) of a Breaker FailureScheme for typical three-cycle. Breaker Failure Timer Breaker Interrupting Time 50 Reset Time Safety Margin 62BF TBK T50 TM 3.0 1.55 5.0 9.55cycles or 159 milliseconds36

Coordination Procedure – Function 50BF37 Transmission Owner and Generator Ownerverify: Breaker failure time is accounted for properly for eachset of relay functions requiring coordination. Appropriate backup breakers are tripped for breakerfailure operation. Total clearing time, which includes breaker failuretime, of each breaker in the generation stationswitchyard coordinate with the critical clearing timesassociated with unit stability.

Coordination Procedure – Function 50BF38 To provide proper Breaker Failure (BF)protection, the following should be considered: See C37.119 “IEEE Guide for Breaker FailureProtection of Power Circuit Breakers” for a welldesigned breaker failure scheme. Clearing time issues are addressed further inSections 4.7 and A.2.11 of C37.102-2006 — Guidefor AC Generator Protectio

9 The Need for GSU Phase Overcurrent Protection – Function 51T Neither IEEE C37.91 nor IEEE C37.102 supports the use of a phase overcurrent function for backup protection for faults in both the GSU and generator, or for system faults. IEEE C37.102 provides the following information concerning phase overcurrent backup protection:

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