CH7 - Relay Technology

7/29/2019 CH7 - Relay Technology

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. 7 . Relay echnology7.1 INTRODUCTIONThelast thirty yearshaveseenenormouschangesin relaytechnology. The electromechanical relay in all of itsdifferent forms hasbeen replacedsuccessivelyby static,digital and numerical relays,each changebringing withit reductionsandsizeand improvementsin functionality.At the sametime, reliability levelshavebeenmaintainedor even improvedand availability significantly increaseddue to techniques not available with older relay types.This representsa tremendousachievement for aII thoseinvolved in relaydesignand manufacture.This chapter charts the course of relay technologythrough the years. As the purpose of the book is todescribemodern protection relay practice, it is naturaltherefore to concentrate on digital and numerical relaytechnology. Thevast number of electromechanical andstatic relays are still giving dependable service, butdescriptionson the technology usedmust necessarilybesomewhat brief. For those interested in the technologyof electromechanical and static technology, moredetailed descriptionscan be found in reference [7.1].'.2 ELECTROrv'E:HANICAL RELAYSTheserelayswere the earliest forms of relay usedfor theprotection of power systems,and they date back nearly100 years.They work on the principle of a mechanicalforce causingoperation of a relay contact in responsetoa stimulus. The mechanicalforce is generated throughcurrent flow in one or morewindings on a magneticcoreor cores, hence the term electromechanical relay. Theprinciple advantage of such relays is that they providegalvanic isolation between the inputs and outputs in asimple, cheap and reliable form - therefore for simpleon/off switching functions where the output contactshaveto carry substantial currents, they are still used.Electromechanical relays can be classified into severaldifferent typesasfollows:

a. attracted armatu reb. moving coilc. inductiond. thermale. motor operatedf. mechanical

However, only attracted armature types have significant


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application at this time, all other types having beensuperseded by more modern equivalents.7.2.1Attracted ArmatureRelaysThese generally consist of an iron-cored electromagnetthat attracts a hinged armature when energised. Arestoring force is provided by means of a spring orgravity so that the armature will return to its originalposition when the electromagnet is de-energised.Typicalforms of an attracted armature relay are shownin Figure7.1. Movementof the armature causes contactclosure or opening, the armature either carrying amovingcontact that engages with a fixed one,or causesa rod to move that brings two contacts together. It isvery easy to mount multiple contacts in rowsor stacks,and hence cause a single input to actuate a number ofoutputs. The contacts can be made quite robust andhence able to make, carry and break relatively largecurrents underquite onerous conditions(highlyinductivecircuits). Thisis still a significant advantage of this typeof relay that ensures its continued use.

(a) D.C. relay (c) Solenoid relay

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besimplyachievedbyaddinga permanentmagnettobasic electromagnet. Both self-reset and bi-stables can be achieved. Figure 7.2 shows the basictruction. One possibleexampleof use is to provide

operatingtimesfor a singlecontact,speedsof less1ms being possible. Figure7.3 illustrates a typicalpleof anattracted armaturerelay.STATIC RELAYS

7.4: Circuit board of static relay

term 'static' implies that the relay has no movings. Thisis not strictly the casefor a static relay,astheput contacts are still generally attracted armatureys. Ina protection relay,the term 'static' refersto theence of moving parts to create the relayoduction of static relaysbegan in the early 1960's.r design is basedon the use of analogueelectroniccesinsteadof coils and magnetsto create the relayacteristic. Earlyversionsuseddiscrete devicessuchtransistors and diodes in conjunction with resistors,citors, inductors, etc., but advancesin electronicsled the use of linear and digital integrated circuitslater versions for signal processing andementation of logic functions. While basic circuitsy be common to a number of relays,the packagings still essentially restricted to a single protectionction per case, while complex functions requirederalcasesof hardwaresuitably interconnected. Usergramming was restricted to the basic functions ofstment of relaycharacteristic curves. Theythereforebeviewed in simple terms asan analogueelectronicacement for electromechanical relays, with sometional flexibility in settings and somesaving in spaceirements. In some cases,relay burden is reduced,kingfor reducedCT/VToutput requirements.

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A numberof designproblemshadto besolvedwith staticrelays. In particular, the relays generally require areliable source of d.c. power and measuresto preventdamage to vulnerable electronic circuits had to bedevised.Substationenvironmentsareparticularly hostileto electronic circuits due to electrical interference ofvarious forms that are commonly found (e.g. switchingoperations andthe effect of faults). While it is possibleto arrange for the d.c. supply to be generatedfrom themeasured quantities of the relay, this has thedisadvantageof increasingthe burdenon the CT'sorVT's,and there will bea minimum primary current or voltagebelow which the relay will not operate. This directlyaffects the possiblesensitivity of the relay. Soprovisionof an independent,highly reliable and securesource ofrelay power supply was an important consideration. Topreventmaloperationor destruction of electronic devicesduring faults or switching operations,sensitive circuitryis housed in a shielded caseto exclude common modeand radiated interference. The devices may also besensitive to static charge, requiring special precautionsduring handling, as damagefrom this causemay not beimmediately apparent, but becomeapparent later in theform of prematurefailure of the relay.Therefore,radicallydifferent relay manufacturing facilities are requiredcompared to electromechanicalrelays. Calibration andrepair is no longera task performed in the field withoutspecialisedequipment. Figure7.4showsthe circuit boardfor a simplestatic relayandFigure7.5showsexamplesofsimpleandcomplexstatic relays.

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Figure 7.5: Selection of static relays


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7.4 DIGITAL REDigital protection relays introduced a step change intechnology. Microprocessors and microcontrollersreplaced analogue circuits used in static relays toimplement relay functions. Early examplesbeganto beintroduced into service around 1980, and, withimprovementsin processingcapacity,canstill be regardedas current technology for many relay applications.However,such technologywill be completely supersededwithin the next five yearsby numericalrelays.Comparedto static relays, digital relays introduce A/Dconversionof all measuredanaloguequantities and usea microprocessorto implement the protection algorithm.The microprocessor may use some kind of countingtechnique, or usethe DiscreteFourierTransform(DFT)toimplement the algorithm. However, the typicalmicroprocessorsused have limited processingcapacityand memory compared to that provided in numericalrelays. The functionality tends therefore to be limitedand restricted largely to the protection function itself.Additional functionality comparedto that providedbyanelectromechanical or static relay is usually available,typically taking the form of a wider range of settings,and greater accuracy. A communications link to aremote computer may alsobe provided.The limited power of the microprocessorsusedin digitalrelaysrestricts the numberof samplesof the waveformthat can bemeasuredpercycle. This, in turn, limits thespeedof operation of the relay in certain applications.Therefore, a digital relay for a particular protectionfunction may have a longer operation time than thestatic relay equivalent. However,the extra time is notsignificant in terms of overall tripping time and possibleeffects of power system stability. Examplesof digitalrelaysare shown in Figure7.6.

i ''F . - 0 JfC1F3Af[l F~~bJ ikJ "tJ - "~-_J ..Figure 7.6: Selection of digitol relays

7.5 NUMERICAL RELAYSThedistinction betweendigital and numerical relay restson points of fine technical detail, and is rarely found inareas other than Protection. They can be viewed asnatural developments of digital relays as a result ofadvancesin technology. Typically,they usea specialiseddigital signal processor (DSP) as the computationalhardware, together with the associatedsoftware tools.The input analoguesignals are converted into a digitalrepresentationandprocessedaccordingto the appropriatemathematicalalgorithm. Processingscarriedout usingaspecialisedmicroprocessorthat is optimised for signalprocessing applications, known as a digital signalprocessoror DSPor short. Digital processingof signalsinreal time requiresa veryhigh powermicroprocessor.In addition, the continuing reduction in the cost ofmicroprocessorsand related digital devices(memory,I/O,etc.) naturally leadsto an approachwhere a single itemof hardware is used to provide a range of functions('one-box solution' approach). By using multiplemicroprocessorsto provide the necessarycomputationalperformance, a large number of functions previouslyimplemented in separateitems of hardwarecan now beincludedwithin a single item. Table7.1providesa list oftypical functions available, while Table7.2 summarisesthe advantagesof a modern numerical relay over thestatic equivalent of only 10-15 years ago. Figure 7.7shows typical numerical relays, and a circuit board isshown in Figure7.8.Figure7.9 providesan illustration ofthe savingsin spacepossibleon a HVfeedershowing thespacerequirementfor relayswith electromechanicalandnumerical relay technology to provide the samefunctionality.

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DistanceProtection- severalschemesincludinguserdefinable)OvercurrentProtection(directional/non-directional)SeveralSetting Groupsfor protection values

Switch-on-to-Fault ProtectionPowerSwingBlocking

VoltageTransformerSupervisionNegativeSequenceCurrentProtection

UndervoltageProtectionOvervoltageProtectionCBFail Protection

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FaultLocationCTSupervision

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IVTSupervision

CheckSynchronisation IAutoreclose ICBCondition Monitoring ICBStateMonitoring IUser-DefinableLogic IBrokenConductorDetection

Ieasurementof PowerSystemQuantities(Current,Voltage,etc.)Fault/Event/Disturbanceecorder ~

Table7.1: Numericaldistance relayfeatures Im /. 102 .

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understood and suitable precautions taken at the designstage. Software problemsare minimisedby rigoroususeof software design techniques, extensive prototypetesting (see Chapter 21) and the ability to downloadamended software into memory(possiblyusinga remotetelephone link for download). Practical experienceindicates that numerical relays are at least as reliableand have at least as good a record of availability asrelaysof earlier technologies.As the technology of numerical relays has only becomeavailable in recent years, a presentation of the conceptsbehind a numerical relay is presented in the followingsections.

Hardware ArchltectL.eThe typical architecture of a numerical relay is shownin Figure 7.10. It consists of one or more DSPmicroprocessors, some memory, digital and analogueinput/output (I/O). and a power supply. Wheremultiple processors are provided, it is usual for one ofthem to be dedicated to executing the protection relayalgorithms, while the remainder implements anyassociated logic and handles the Human MachineInterface (HMI)interfaces. Byorganising the I/Oon aset of plug-in printed circuit boards (PCB's).additionalI/O up to the limits of the hardware/software can beeasily added. The internal communications bus linksthe hardware and therefore is critical component in

Figure 7.9: Spacerequirementsof differentrelay technologies for same functionality

Figure 7.8: Circuit board for numerical relay

the design. It must work at high speed, use lowvoltage levels and yet be immune to conducted andradiated interference from the electrically noisysubstation environment. Excellent shielding of therelevant areas is therefore required. Digital inputs areoptically isolated to prevent transients beingtransmitted to the internal circuitry. Analogue inputsare isolated using precision transformers to maintainmeasurement accuracy while removing harmful

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7.10: Relaymodules and information flow

Additionally, the input signals must belitude limited to avoid them exceeding the powerly voltages, as otherwise the waveform will appear

orted, as shown in Figure 7.11.alogue signals are converted to digital form using an

converter. The cheapest method is to use a singleconverter, preceded by a multiplexer to connectch of the input signals in turn to the converter. Thenals may be initially input to a number ofultaneous sample-and-hold circuits prior toltiplexing, or the time relationship betweencessive samples must be known if the phase

ationship between signals is important. Thernative is to provide each input with a dedicatedA/Dverter, and logic to ensure that all convertersorm the measurementsimultaneously.

quencyof sampling must be carefully considered,Nyquist criterion applies:is? 2 Xfh

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Legend:SRAM- Static ReadOnlyMemoryCPU- Central ProcesingUnitIRIG-8 TimeSynchronisationignalFPGAFieldProgrammableogicArrayADC- Analogto Digital ConverterE2PROM- ElectricallyErasableProgrammableReadOnlyMemoryEPROM ElectricallyProgrammableReadOnlyMemoryLCD- LiquidCrystalDisplay

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~is = sampling frequencyfh = highest frequency of interest

If too Iowa samplingfrequency is chosen,aliasing of theinput signal can occur (Figure 7.12), resulting in highfrequencies appearingas part of signal in the frequencyrange of interest. Incorrect resultswill then beobtained.The solution is to apply an anti-aliasing filter, coupledwith an appropriate choiceof samplingfrequency,to theanalogue signal, so those frequency components thatcould cause aliasing are filtered out. Digital sine andcosinefilters are used(Figure7.13).with a frequencyresponseshownin Figure7.14,to extract the real andimaginary componentsof the signal. Frequencytrackingof the input signals is applied to adjust the samplingfrequency so that the desirednumber of samples/cycleisalways obtained. A modern numerical relay may sampleeach analogue input quantity at between 16 and 24samplesper cycle.

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Allsubsequent signal processingiscarriedout digitally insoftware, final digital outputs use relays to provideisolation or aresentvia an external communicationsbusto other devices.75.2 Re'a" ::'t' '~veThe software provided is commonly organised into aseries of tasks, operating in real time. An essentialcomponent is the RealTime Operating System (RTOS).whose function is to ensure that the other tasks areexecutedas and when required,on a priority basis.Other task software provided will naturally varyaccordingto the function of the specific relay,but can begeneralisedas follows:a. systemservicessoftware - this is akin to the BIOSof an ordinary PC,and controls the low-level I/Ofor the relay (i.e. drivers for the relay hardware,boot-up sequence,etc.)

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0 / Figure 7.11: Signal distortiondue to excessive amplitudeb. HMIinterface software - the high level softwarefor communicating with a user,via the front panelcontrols or through a data link to anothercomputer running suitable software, storage ofsetting data, etc.

c. application software - this is the software thatdefines the protection function of the relayd. auxiliaryfunctions- software to implement otherfeatures offered in the relay - often structured asa seriesof modulesto reflect the options offered toa userby the manufacturer

..., ~ 3 Anp';C;J an ~aft"'cveThe relevant software algorithm is then applied. Firstly,the values of the quantities of interest have to bedetermined from the available information contained inthe data samples. This is conveniently done by theapplication of the DiscreteFourierTransform(OFT).and

1 1 1Figure 7.12: Signal alias ing problem

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xs=1. {O+X1 +X2+X3+0- Xs -X6- X7}12 12 12 12(a) Sinefilter

x =1.{XO+XI+O- X3-XrXS+O+X7 }B 12 12 12 12

(b) CosinefilterFigure 7.13: Digital filters

result is magnitude and phase information for thectedquantity. Thiscalculation is repeatedfor all ofquantities of interest. The quantities can then bempared with the relay characteristic, and a decisionde in terms of the following:a. value abovesetting - start timers, etc.

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b. timer expired - action alarm/tripc. valuereturnedbelowsetting- reset timers, etc.d. value below setting - do nothinge.valuestill abovesetting- increment timer, etc.

Since the overall cycle time for the software is known,timers are generally implementedas counters.

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The DSP chip in a numerical relay is normally ofsufficient processing capacity that calculation of therelay protection function only occupies part of the ~processing capacity. The excesscapacity is thereforeavailable to perform other functions. Of course, care Imust be taken never to load the processor beyondcapacity, for if this happens,the protection algorithmwill not complete its calculation in the requiredtime andthe protection function will be compromised.Typicalfunctions that may befound in a numerical relaybesidesprotection functions aredescribedin this section.Note that not all functions may be found in a particularrelay. In common with earlier generations of relays,manufacturers, in accordance with their perceivedmarket segmentation, will offer different versions Ioffering a different set of functions. Functionparameterswill generally be available for display on thefront panel of the relay and also via an external

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communications port, but some by their nature may onlybe available at one output interface.-'0 ea.>uF~ a cles CI


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Programmabe Logec functions are well suited to implementation usingoprocessors.The implementation of logic in a relayt new,as functions suchas intertripping and auto-ose require a certain amount of logic. However,byiding a substantial numberof digitalljO andmakinglogic capable of being programmed using suitablene software, the functionality of such schemescannhancedand/or additional features provided. Forance, an overcurrent relay at the receiving end of aformer feeder could use the temperature inputsided to monitor transformer winding temperatureprovide alarmjtrip facilities to therator/upstream relay, eliminating the need for aarate winding temperature relay. This is anentaryexample,but other advantagesareevident torelay manufacturer - different logic schemesuired by different Utilities, etc., no longer needarate relay versions or some hard-wired logic toerrient, reducing the cost of manufacture. It is alsoerto customisea relay for a specificapplication, andinate other devices that would otherwise be

7 P'C SIGn 0 Set, 9 3vG ~Sorically, electromechanical and static relays haven provided with only one group of settings to belied to the relay. Unfortunately, power systemsnge their topology due to operational reasonson aular basis. (e.g. supply from normal/emergencyration). The different configurations may requirerent relay settings to maintain the desired level ofork protection (since, for the above example, thet levelswill be significantly different on parts of therk that remainenergisedunder both conditions).

problemcanbeovercomeby the provisionwithin theyof a numberof setting groups,only one of which iseat anyone time. Changeoverbetweengroupscan

achievedfrom a remote commandfrom the operator,siblythrough the programmablelogic system. This

y obviate the need for duplicate relays to be fittedsomeform of switching arrangementof the inputsoutputs depending on network configuration. Theator will also havethe ability to remotely programrelaywith a group of settings if required.

Conclusio'lsprovision of extra facilities in numerical relaysmayid the needfor other measurement/controldevicestofitted in a substation. A trend can therefore beerned in which protection relaysare providedwithctionality that in the past has been provided usingarate equipment. The protection relay no longer

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performsa basic protection function; but is becominganintegral and major part of a substation automationscheme. The choice of a protection relay rather thansome other device is logical, as the protection relay isprobably the only devicethat is virtually mandatory oncircuits of any significant rating. Thus, the functionspreviously carried out by separatedevices such as baycontrollers, discrete metering transducers and similardevicesare now found in a protection relay. It is nowpossibleto implement a substation automation schemeusing numerical relays as the principal or indeed onlyhardware provided at bay level. As the power ofmicroprocessors continues to grow and pressure onoperators to reduce costs continues, this trend willprobably continue, one obvious development being theprovisionof RTUfacilities in designatedrelaysthat act aslocal concentrators of information within the overallnetwork automation scheme.

-~~, "0:t ,-~ , '"' :::: u E:Theintroduction of numerical relaysreplacessomeof theissuesof previousgenerations of relayswith new ones.Someof the new issuesthat must be addressedare asfollows:

a. softwa re version controlb. relay data managementc. testing and commissioning ~b.oc-

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a. the different software versions in existenceb. the differences between each versionc. the reasons for the changed. relays fitted with each of the versions

With an effective version control system, manufacturersare able to advise users in the event of reported problemsif the problem is a known software related problem andwhat remedial action is required. With the aid ofsuitable software held by a user, it may be possible todownload the new software version instead of requiringa visit from a service engineer.

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7 7.2 Relay Data Ma'lager1er>tA numerical relay usually providesmany more featuresthan a relay using static or electromechanicaltechnology. Touse these features, the appropriate datamust be entered into the memory of the relay. Usersmust also keepa recordof all of the data, in case ofdataloss within the relay, or for use in system studies, etc.The amount of data per numerical relay may be 10-50times that of an equivalent electromechanical relay, towhichmust be added the possibilityof user-defined logicfunctions. Thetask of entering the data correctly into anumericalrelaybecomesa muchmorecomplextask thanpreviously,which adds to the possibility of a mistakebeingmade. Similarly,the amount of data that must berecorded is much larger, giving rise potentially toproblemsof storage.The problems have been addressed by the provision ofsoftware to automate the preparation and download ofrelay setting data from a portable computer connectedto a communications port of the relay. As part of theprocess,the setting data can be read backfrom the relayand compared with the desired settings to ensure thatthe download has been error-free. A copyof the settingdata (includinguser defined logic schemes where used)can also be stored on the computer, for later printoutand/or uploadto the users database facilities.More advanced software is available to perform theabove functions from an Engineering Computer in asubstation automation scheme - see Chapter 24 fordetails of such schemes).

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773 Relay Testing a"d COrY't11iSS'O'1"'gThe testing of relays based on software is of necessityradically different from earlier generations of relays. Thetopic is dealt with in detail in Chapter 21, but it can bementioned here that site commissioning is usuallyrestricted to the in-built software self-check andverification that currents and voltages measured by therelay are correct. Problems revealed by such tests requirespecialist equipment to resolve, and hence field policy isusually on a repair-by-replacement basis.

7.8 REI=E~ENCES7.1 Protective Relays Application Guide, 3rd edition.ALSTOMT&DProtectionand Control,1987.

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CH7 - Relay Technology