Transformer Protection, Monitoring and Control

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    Power transformers areexpensive and importantcomponents in a powersystem. It is necessary toprovide first-classprotection to limit damagein case of severe faults andto prevent abnormalconditions which canshorten componentlifecycles and causesubsequent failures.

    Power transformersare made in a wide rangeof sizes and with a varietyof connections, from me-dium-sized distributiontransformers to hugepower station step-uptransformers and EHV sys-tem transformers. Big unitsare also made as auto-transformers with twovoltage leve Is sharing apart of the winding.Transformer windings canbe arranged for differentvector group connectionsand quite often there isalso a tap-changer with afacility for voltageregulation.A shunt reactor is insame ways sim ilar to apower transformer, butthere are also differenceswhich are significant to theprotection arrangement.An oil-immersed shuntreactor is often equippedwith protection similar tothat of a transformer.

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    Power ransformer protection should ensurefast and dependable ripping for a fallit inthe transformer or transformer switchbay. naddition, it should not causenon-selectivetripping in the event of externa! allits , burit is usually set up to offer time-graded back-up protection for externa! short-circuits andearth-faults.

    To cope with the wide spectrum ofapplication aspects or transformerprotection, a number of functions havecome into use.There are various kinds ofelectrical protection functions supplementedwith other protection and monitoringdevicesbuilt onto the transformer,such asoil and winding temperaturedevices,Buchholz gasand suddenpressure elaysandothers.

    two examples. Ii addition to theco ditions mentioned above,phaseshift,inr h current etc., the current transformerrat os usually need a balancing correction,an the effect of the tap-changerwill furtherco tribute by a varying rate of imbalance.

    The effect of an internat or externat-fault dependsvery much on the system

    ne tral-earthing at the points of powerinE;ed and other neutrals. The method ofsys em earthing varies with systemvoltagean dependsalso on the practice of the user.Sys em neutrals may be earthed directly,thr Ugh ow impedance resistors, eactors,hi impedancearc suppression eactors orhi impedance esistors,or they may be leftco nected through just a surge arrester.A igh earth-fault current should be clearedqui kly by the differential or other currentfel ying function. A low earth-fault current

    be clearedby time-graded current andne tral vol age unctions.Long experienceallied to knowledge

    of odern numerical design and simulationtec nology form the basis or the design ofthe numerical transformer protection relay.

    The most C'Jmmon types ofpower transformers are ofthe two-wirlding and three-winding type.In some cases an earthingtransformer is used tocreate a nelutral forearthing.

    The characteristicsof power trans-formers and certain phenomena n switchingand other serviceconditions are ofimportance to the designand structure ofpower transformer protection. Whenswitching-in the transformer or its paralleiunit, or in the event of a sudden return ofvoltage arter disconnection,a considerableinrush of current results,which must not beinterpreted as a fault condition.At voltage rise, the transformer may go intasaturation and draw a considerablemagneti-sing current, which is also an abnormalcondition hut not a fault requiringinstantaneous ripping.

    A transformer differential protectiondevice s a widely used orm of protectionfor instantaneousdetection of internal faultsin transformers arger than a few MYA .Therequirements concerning transformer diffe-rential funccion differ somewhat rom gene-rator or busbardifferential protection, to

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    Multifunctional protecti tnterminal RET 521Several usefulfunctionswhich have beentraditionally provided byindividual relays are herecombined inta one multi-functional protection ter-minal, RET 521.

    Transformerdifferential protection (DIF)

    The transformer differential protectiondevice s the major protection in the trans-former and shunt reactor protection termi-nal and it is designed or two and three-winding transformers. Jt is stabilised againstexternal faults, inrush cutrent and over-excitation via aselectable current biascharacteristicand harmonic restraint.The protection device ncludes aninternal current amplitude and vector groupcompensation or various transformerconnection groups, which means that nointerposing CTs areneeded.

    There is a generous ange of settingpossibilities including individual setting ofbasicsensitivity, current bias slope and highsetnon-restrained differential function forhigh cutrent faults.

    Applications such as ransformerswith on-load tap-changersor shunt reactorscan be covered.

    In the felar basicand op ional protectionand associatedunctions e available,such as:.Transformer differenti.Time-overcurrent.Restricted earth-fallIt.Earth current.Neutral voltage or ove oltage.Collection ofbinary si Dals romexternal devices or ove temperature etc..Trip logic.Logic elements or free e.Service value ndicatio.Event logging

    Exampleof functionality in one application. Current bias characteristics.

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    tnputs for externatprotection devicesTh transformer and shunt reactor

    pr tection terminal are also equipped withco ection and logic facilities for externalpr tection devices. ndication, event oggingan trip logic can thus be arranged or trans-for er oil, winding temperature devicesandBu hh?lz gas detection, to name a fewe ples.

    Three-phase timeovercurrent function (OC)

    The three-phase ime overcurrent unctionhastwo current levelswith a wide settingrange:

    .LeveIl: A choice of inverseanddefinite time-overcurrentcharacteristics

    .Level 2: Instantaneousor definitetime setting

    Overcurrent protection can be individuallyapplied to the HY side and to the LV side. Indications and event recordingRE 521 has a facility for indications and

    eve trecording. It includes spontaneousfro tal MMI indication of operations,rea ing of servicevalues such asphasecu ents and differential currents, plus time-tag ed logging of protection operations ando events.

    Technicalparticulars.

    Restrictedearth-fault function (REF)

    The restricted earth-fault function offersinstantaneousprotection for internal earth-faults in low impedanceearthed systems.It usescurrent measurementn the phaseconductors and in the connection fromneutral to earth. It can be made moresensitive han the transformer differentialand still be stable n the event of externalfaults and inrush current.

    Earth-fault current protectionFor low impedance or directly earthedsystems option). The earth currentprotection function has he same eaturesasfor the three-phaseOC protection, but it is asingie-phase unction and can be set forgreatersensitivity.

    Single-phase or neutralovervoltage function (NOV)

    The neutral or singie-phase vervoltagefunction has one voltage evel and twodefinite time steps.

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    The Binary In/Out mol:lulesare optional. The numberof input transformers andA/D conversion modulesdepends on actual demandfor the variant.Data communication withSubstation Controi andSubstation Monitoring isoptional.

    The RET 521 transform r proteccion felarconforms to the 500 Seri s design, with ahousing made of sheetst el, height 6U.

    The 500 ~;eries uses1hesame hardware andsoftware platform forprotection and controiapplications. This resultsin a user-friendly productwhich is programmablethrough the built-in MMI,the PC connection at thefront, the StationMonitori/1lg System orSubstatic,nControi Sys-tem. The compact, saf,eand user-friendly 500Series allows you toconfigure your own intelli-gent terminal to ensureoptimum performanceThis can lreduce both in-stallation and mainten.Jncecosts and also provide amore effiGient "power :;ys-tem management",reducing operational anddisturbance costs and per-mitting ~!tter utilisation ofhigh-voltage equipme/1lt.

    Each protection terminal is always equipped with a built-in local MMI, but a PC can also be connected to the frontas a complement, for maintenance and fastcommissioning.

    Electrical connections are made by screwterminals at the rear.The felar can bemounted in a 19" rack, or it can be flush orsemi-flush mounted in.a panel.

    A Combitest test switch for injectiontesting is available on request.Behind the front there is an imerconnectionboard inta which circuit boards are pluggedfrom the rear. There are modules with inputtransformers,AlD conversion,modules forprocessing,power supply and binary inputand output signals.

    The basic unit has four binary inputcircuits and flve output relays.Additional1/0 modules with binary inputs and outputcontacts are availableas an option.Processings carried out in a centralmicroprocessorand digital signal processors.All binary inputs and felar outputs areconflgurable for different functions.

    The relays n the 500 Seriescan beequipped with two communication ponsoperating independenclyof each other, per-mitting simultaneouscommunication withtwo different masters.This gives he user hepossibility of having both a monitoring anda contro" system n the samestation.

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    The intelligent path to reliablecommunicatibn

    The 500 Series s a part of the PYRAMID@concept for SubstationAutomation and thePanoramaconcept for PowerNetwork Ma-nagiment: the intelligent terminal in the in-telligent substation for the intelligent powernetwork, from generation o consumption.

    In the intelligent substation, hecommunication philosophy s of the utmostimportance in order to provide the right in-formation to the right person at the righttime, and in a reliable war. For this reason,the numerical protection terminals in the500 Seriescan be equipped with two remotecommunication ports operating ndepen-dendy of eachother, permittingsimultaneouscommunication with theStation ControI and Station MonitoringSystem.

    Communication gives possibilities of intelligent sharing of information.

    Application flexibility isachieved by selection ofbasic and optionalfunctions and I/O-modules.Through the MMI, func-tians and free logicalelements can be inter-connected and configuredto inputs and outputs toadapt to a particular need.External auxiliary relaysand test switch may supp-lement the unit.

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    Panoramas the ABB solution for efficientand reliable managementof powenetworks.Panorama tands or an op n viewin all directions, utilizing innovat einformation technology.

    Panorama nables he user o alwaysbe in perfect controI of the power process,from generation o consumption.Panoramas the complete conceptfor today and for the future, fromABB Network Partner.

    ABB Network Partner AB8-72171 Vsters8wedenTel +4621321300Fax +4621 146918

    ABB Network Partner