Power transformers are

expensive and important

components in a powersystem. It is necessary to

provide first-classprotection to limit damagein case of severe faults andto prevent abnormal

conditions which canshorten componentlifecycles and cause

subsequent failures.Power transformers

are 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 a

facility for voltage

regulation.A shunt reactor is in

same ways sim ilar to a

power transformer, butthere are also differenceswhich are significant to the

protection arrangement.An oil-immersed shuntreactor is often equippedwith protection similar tothat of a transformer.

Power transformer protection should ensurefast and dependable tripping for a fallit inthe transformer or transformer switchbay. Inaddition, it should not cause non-selectivetripping in the event of externa! fallits , 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 for transformerprotection, a number of functions havecome into use. There are various kinds ofelectrical protection functions supplementedwith other protection and monitoringdevices built onto the transformer, such asoil and winding temperature devices,Buchholz gas and sudden pressure relays andothers.

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

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

ne tral- earthing at the points of powerinE; ed and other neutrals. The method ofsys em earthing varies with system voltagean depends also on the practice of the user.Sys em neutrals may be earthed directly,thr Ugh low impedance resistors, reactors,hi impedance arc suppression reactors orhi impedance resistors, 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 cleared by time-graded current andne tral vol tage functions.

Long experience allied to knowledgeof odern numerical design and simulationtec nology form the basis for 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 for

earthing.

The characteristics of power trans-formers and certain phenomena in switchingand other service conditions are ofimportance to the design and 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 considerable magneti-

sing current, which is also an abnormalcondition hut not a fault requiringinstantaneous tripping.

A transformer differential protectiondevice is a widely used form of protectionfor instantaneous detection of internal faultsin transformers larger than a few MYA .The

requirements concerning transformer diffe-rential funccion differ somewhat from gene-rator or busbar differential protection, to

Multifunctional protecti tnterminal RET 521

Several usefulfunctionswhich have been

traditionally provided byindividual relays are herecombined inta one multi-functional protection ter-minal, RET 521.

Transformer

differential protection (DIF)

The transformer differential protectiondevice is the major protection in the trans-former and shunt reactor protection termi-nal and it is designed for two and three-

winding transformers. Jt is stabilised againstexternal faults, inrush cutrent and over-excitation via aselectable current biascharacteristic and harmonic restraint.

The protection device includes aninternal current amplitude and vector group

compensation for various transformerconnection groups, which means that nointerposing CTs are needed.

There is a generous lange of setting

possibilities including individual setting ofbasic sensitivity, current bias slope and highset non-restrained differential function forhigh cutrent faults.

Applications such as transformerswith on-load tap-changers or shunt reactorscan be covered.

In the felar basicand op ional protectionand associated functions e available,such as:

.Transformer differenti

.Time-overcurrent.Restricted earth-fallIt.Earth current.Neutral voltage or ove oltage.Collection ofbinary si Dals from

external devices for ove temperature etc.

.Trip logic.Logic elements for free e.Service value indicatio.Event logging

Exampleof functionality in one application. Current bias characteristics.

tnputs for externat

protection devicesTh transformer and shunt reactorpr tection terminal are also equipped withco ection and logic facilities for external

pr tection devices. lndication, event loggingan trip logic can thus be arranged for trans-for er oil, winding temperature devices andBu hh?lz gas detection, to name a fewe ples.

Three-phase time

overcurrent function (OC)The three-phase time overcurrent functionhas 'two current levels with a wide setting

range:

.LeveIl: A choice of inverse anddefinite time-overcurrentcharacteristics

.Level 2: Instantaneous or definitetime setting

Overcurrent protection can be individuallyapplied to the HY side and to the LV side.

Indications and event recording

RE 521 has a facility for indications andeve trecording. It includes spontaneousfro tal MMI indication of operations,rea ing of service values such as phasecu ents and differential currents, plus time-tag ed logging of protection operations ando events.

Technical particulars.

Restricted

earth-fault function (REF)The restricted earth-fault function offersinstantaneous protection for internal earth-faults in low impedance earthed systems.It uses current measurement in the phaseconductors and in the connection fromneutral to earth. It can be made moresensitive than the transformer differentialand still be stable in the event of externalfaults and inrush current.

Earth-fault current protectionFor low impedance or directly earthedsystems (option). The earth currentprotection function has the same features asfor the three-phase OC protection, but it is asingie-phase function and can be set for

greater sensitivity.

Single-phase or neutralovervoltage function (NOV)

The neutral or singie-phase overvoltagefunction has one voltage level and twodefinite time steps.

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 is

optional.

The RET 521 transform r proteccion felarconforms to the 500 Seri s design, with ahousing made of sheet st 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 Station

Monitori/1lg System orSubstatic,n Controi Sys-tem. The compact, saf,eand user-friendly 500Series allows you toconfigure your own intelli-gent terminal to ensure

optimum performanceThis can lreduce both in-stallation and mainten.Jncecosts and also provide a

more effiGient "power :;ys-tem management",

reducing operational anddisturbance costs and per-mitting ~!tter utilisation of

high-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 fast

commissioning.

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 injection

testing is available on request.Behind the front there is an imerconnectionboard inta which circuit boards are pluggedfrom the rear. There are modules with input

transformers, 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 available as an option.Processing is carried out in a centralmicroprocessor and digital signal processors.All binary inputs and felar outputs areconflgurable for different functions.

The relays in the 500 Series can beequipped with two communication ponsoperating independencly of each other, per-mitting simultaneous communication withtwo different masters. This gives the user thepossibility of having both a monitoring anda contro" system in the same station.

The intelligent path to reliablecommunicatibn

The 500 Series is a part of the PYRAMID@concept for Substation Automation and thePanorama concept for Power Network Ma-nagiment: the intelligent terminal in the in-telligent substation for the intelligent powernetwork, from generation to consumption.

In the intelligent substation, thecommunication philosophy is 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 Series can be equipped with two remotecommunication ports operating indepen-dendy of each other, permittingsimultaneouscommunication with theStation ControI and Station Monitoring

System.

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 to

adapt to a particular need.External auxiliary relaysand test switch may supp-

lement the unit.

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Panorama is the ABB solution for efficientand reliable management of powenetworks.

Panorama stands for an op n viewin all directions, utilizing innovat einformation technology.

Panorama enables the user to alwaysbe in perfect controI of the power process,from generation to consumption.

Panorama is the complete conceptfor today and for the future, fromABB Network Partner.

ABB Network Partner AB8-72171 Västerås8wedenTel +4621321300Fax +4621 146918

ABB Network Partner