Printed in USA

The MD32T Transformer DifferentialRelay is a member of CooperPower Systems’ Edison® line ofmicroprocessor based protectiverelays. The MD32T relay offers thefollowing functions:

■ High and low set differentialelements for each phase withmulti-slope bias characteristics.The MD32T is suitable forprotecting any two windingtransformer or a three windingtransformer with unit-directionalcurrent flow.

■ Low set differential elementresponse time of less than 2cycles, high set response timeunder 1 cycle.

■ Programmable 2nd and 5thharmonic restraint elementswhich may be reduced auto-matically during transformerenergization or at any time byan external trigger.

■ Restricted ground fault element.

■ Automatic CT ratio compen-sation for all transformerwinding and CT connectionconfigurations throughprogrammable vector groups.

■ 16 cycle oscillographic recordsof trip events. An oscillographicrecord may also be triggeredexternally.

The MD32T also shares thefollowing features common to allEdison® relays:

■ Simple five button man machineinterface (MMI) allows access toall functions, settings, andstored data without the need fora computer.

■ Bright electroluminescentdisplay easily visible even inbrightly lit environments.

■ Draw-out design permits relaytesting without disturbingconnections to case.

■ Modbus communicationprotocol and RS485 terminal onrear.

■ Modular design allows thedraw-out module to be fitted toa variety of space savingcabinet styles.

■ Three programmable Form C(SPDT) output contacts and oneForm A/B contact.

■ Pick-up (start-time) elements.

■ Programmable resetcharacteristics.

■ Dedicated power supply/relayfail output contacts.

ApplicationsThe MD32T is ideally suited for theprotection of any two windingtransformer against two or threephase internal faults, inter-turnfaults, and ground faults intransformers with low-impedanceor solidly grounded neutrals.

The MD32T may also be used toprotect multi-winding transformerswhere one winding always acts asthe source winding, and the CTs ofthe other load windings areconnected in an appropriatemanner to represent the sum of theload currents.

For autotransformer-started motors,some differential protectionschemes include theautotransformer in the zone ofprotection. In this case a rotatingmachine differential relay, such asthe MD32G, is not suitable as the2nd and 5th harmonic restraintfunctions common to transformerdifferential relays is required. Forthis application, the MD32T shouldbe used.

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Electrical Apparatus

Transformer Differential Relay

MD32T Transformer Differential Relay 150-30

August 1997 • New Issue

Figure 1.Front View of the MD32T Transformer Differential Relay

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Multi-slope BiasedDifferential ElementEach phase is provided with itsown differential element with acharacteristic as shown in Figure 2.The relay internally calculates theRMS value of the differentialcurrent compensated for any CTratio and phase angle mismatch.

The minimum differential currentrequired for operation is adjustablebetween 0.1 and 0.5 pu of the ratedcurrent. This setting is fixed up to0.5 pu of the transformers ratedcurrent. Above 0.5 pu compen-sation of the set point is requiredfor CT saturation, which may occurat higher current levels, and forvoltage ratio fluctuations caused byon-load tap changers. From 0.5 to2.5 pu of rated current, the slope ofthe differential element isadjustable between 10 and 50%.Between 2.5 pu and 18 pu of ratedcurrent, the bias slope is fixed at100%.

Low Set DifferentialElementIn addition to the multi-slope biascharacteristic, the low setdifferential element is furtherequipped with programmable 2ndand 5th harmonic restraint logic.

During transformer energization,significant 2nd harmonic current ispresent, which may result in falsetrips. This requires the relay tosuppress its response to the 2ndharmonic component of current.The MD32T allows for adjustmentof the 2nd harmonic restraintelement over the range of 0.1 to 0.3pu of the transformer’s ratedcurrent.

Similarly, over-excitation of thetransformer may result in significantlevels of 5th harmonic current,requiring similar restraintcharacteristics. The MD32T allowsfor an adjustment range of 0.2 - 0.4pu of rated current. Either or boththe 2nd and 5th harmonic restraintelements may be disabled ifdesired.

To avoid problems associated withthe harmonic restraint elementsfrom being too sensitive on actual

faults and blocking the relay, theMD32T allows the harmonicrestraint settings to be lowered toas little as one-half the normalsettings during a programmabletime period following transformerenergization. Transformer ener-gization is sensed via a digital inputconnected to an auxiliary contacton the transformer’s breaker.

High Set DifferentialAn instantaneous (high set)differential element is provided toquickly remove the transformerfrom service in the event of severeinternal faults. To effectivelyeliminate the effects of dc offsetsand harmonics, the relay trips onthe sensing of sequential positiveand negative peak currents whosemagnitude exceeds the high set triplevel.

The high set trip level may be eitherset to be a fixed value, or may bebiased in the same fashion as thelow set element. Due to the oper-ational nature of the element, noharmonic programmable harmonicrestraint elements are provided.This element will operate in lessthan 1 cycle (15 msec).

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MD32T Transformer Differential Relay

Figure 2.Dual Slope Bias Characteristic of the MD32T Differential Relay

0.5 1 2 2.5 30

∆IR

∆IR

Is/In

Id=[d>] for 1F87

=[d>>] for 2F87

∆IR ∆Id

∆Id

Is = I4InIn

R%

R%100

= 100 = 100

∆Id

18

Is/In

2

1

Id/In

1

2

2

1

3

3

4

4 18

{Is= Effective relay’s operation differential currentId= Relay’s setting differential current

∆(I1 -I2)∆(I1 -I2)/2

Is =In

Is =In

IsIn

IdIn In

--+ (

+ (+

IR 0.5) •

2.5)In -IR

1002R% Id

In

▲

▲▲

▲

▲

▲

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150-30

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Ground Fault ProtectionTo provide greater coverage forground faults, the MD32T monitorsthe fundamental frequencycomponent of current fed into theground element inputs. Dependingupon the connection, this elementcan perform:

■ Restricted ground faultprotection for one Y connectedwinding (87N).

■ Residual current (3Io) protectionof one winding (51G).

■ Neutral current protection ofone winding (51N).

This ground fault element consistsof a definite time high set elementand a fixed 30 ms response timeinstantaneous element. Thiselement may be blocked duringtransformer energization.

TargetsEight bright LED targets areprovided as follows:

■ One red LED for each ofthe three phase differentialelements.

■ One red LED for the restrictedground fault element.

■ Two red LEDs to signal whenthe 2nd or 5th harmoniccurrents respectively exceed therestraint level.

For all of the above, the LEDs flashwhen the element is picked up, andconstantly illuminate upon trip. Inaddition, one yellow LED isprovided which illuminates whenthe blocking input is active. Asecond yellow LED flashes whenthe relay is in programming mode,and illuminates constantly uponrelay or power supply failure.

Blocking InputAn opto-isolated programmableblocking input is provided. Thisinput may be programmed so thatwhen activated, any combination ofthe low set differential, high setdifferential, or ground fault elementsmay be blocked.

While the blocking input is active,the pickup of any elementassociated with the blocking input isprevented. Sensing of the input

quantities and the countdown ofany timers begins only when theblocking is removed.

Reset CharacteristicThe programmable output relaysmay be programmed to reset in oneof two manners.

■ Instantaneously upon the inputor calculated quantitiesdropping below the pickupvalue.

■ Manual reset (by front panel orcomputer command) only.

Measurements andInrush ValuesThe following quantities arecontinuously monitored and areavailable for display at the relay andare accessible by software:

■ RMS values of each phase’sdifferential current

■ Fundamental frequencycomponent of the neutralcurrent in per unit of the ratedphase current.

■ RMS values of the three highside currents.

■ RMS values of the three lowside currents.

■ 2nd and 5th harmonic currentcomponents of each of thephase’s differential currents.

In addition, the maximum values ofeach of these quantities during thefirst 100 msec after transformerenergization is also recorded. Thismakes it convenient to quicklyreview the inrush currentsassociated with the most recentenergization.

Last Trip RecordThe following parameters arestored in non-volatile memory,providing details of the last five tripevents:

■ Which element was the causeof the last trip.

■ The values of all measuredcurrents at the time of trip.

In addition the relays keep acumulative total of the cause of allbreaker trips.

OscillographyThe MD32T stores twooscillographic records which aremade available for downloading toa PC for graphing. All six inputphase currents are recorded. Eachoscillographic record consists of 8pre-trigger and 8-post triggercycles, for a total of 16 cycles. Thewaveform is sampled at 12 samplesper cycle. Storage of anoscillographic record may beprogrammed to occur automaticallyevery time a protective elementtrips, or to occur only upon anexternal trigger.

Output ElementsThe following functions may beprogrammed to one or more of theoutput relays. The only limitation isthat pick-up and time delayfunctions may not be assigned tooperate the same output relay(s).

■ Low set differential element.

■ High set differential element.

■ Instantaneous ground faultelement.

■ Time delayed ground faultelement.

DiagnosticsComplete memory and circuitdiagnostics are run upon poweringthe relay. The revision level of thefirmware is displayed at this time.

The relay runs a comprehensive setof diagnostics every 15 minutesthat includes memory checksum,test of the A/D converters byinjection of an internally generatedreference voltage, and a check ofthe ALU.

The relay provides two manual testroutines which may be run at anytime. The first routine performs thesame 15 minute test and in additionchecks the target LEDs and thecontrol circuitry to the output relayswithout operating the output relays.The second test is identical but alsooperates the output relays.

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Figure 3.Wiring Diagram for the MD32T Transformer Differential Relay used for the 87T Protection Only

Figure 4.Wiring Diagram for the MD32T Transformer Differential Relay used for 87T and 51G Protection

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Figure 5.Wiring Diagram for the MD32T transformer Differential Relay used for 87T and 51N (Restricted Ground(Earth) Fault) Protection

Figure 6.Wiring Diagram for the MD32T Transformer Differential Relay used for 87T and 87N (Restricted Ground(Earth) Fault) Protection

150-30

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Nominal system frequency setting range.................................................50 or 60 Hz

Programmable rated primary input current of phase CTs ......................1 - 9999A in 1A steps(High and low side CT programmable separately)

Programmable rated phase-to-phase voltage of transformer ................0.20 to 380.00 kV in 0.01 kV steps......................................................................................................................(High and low side voltages programmable separately)

Transformer connection and vector group...............................................Yy0 to Yz0

Differential ElementMinimum pick-up level of low set differential element ...............................0.10 to 0.50 pu of rated CT current in 0.01 pu stepMinimum pick-up level of high set differential element..............................2.0 to 18.0 pu of rated CT current in 0.1 pu stepsBias percentage slope from 0.5 pu to 2.5 pu rated CT current.................10 - 50% in 1% steps2nd Harmonic restraint level......................................................................0.10 to 0.30 pu of the differential current in 0.01 pu steps5th Harmonic restraint level.......................................................................0.20 to 0.40 pu of the differential current in 0.01 pu stepsHarmonic restraint multiplier (reduction) ...................................................0.50 to 1.00 pu of original setting in 0.01 pu steps during energization ...................................................................................................................(2nd and 5th harmonic restraint multipliers programmable separately)Harmonic restraint reduction timer (starting at transformer energization) 0.05 to 99.00 seconds in 0.01 sec. steps

Ground Fault ElementMinimum pick-up level for ground fault element ........................................0.01 to 1.00 pu of rated CT current in 0.01 pu stepsTime delay of ground fault element ...........................................................0.02 to 9.99 seconds in 0.01 second steps as described in IEC 76

TABLE 1Functional Specifications

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MD32T Transformer Differential Relay

P.O. Box 1640, Waukesha, WI 53187http://www.cooperpower.com/

©1997 Cooper Power Systems, Inc.Edison® is a registered trademark of Cooper Industries, Inc.

Printed on Recycled Paper

Dimensional andElectrical SpecificationsSee Catalog Section 150-05 forelectrical specifications anddimensional information on allEdison® Relays.

Ordering InformationConstruct catalog number fromTable 2.

Example: MD32TL5JS is an MD32Twith low range power supply, 5A CTinputs, in a single relay case.

If ordering two or more relays to befit in a common case, the first relayordered should indicate the casestyle desired. This relay will belocated in the leftmost bay of thecase. Subsequent relays should usethe C2, C3, or C4 suffixes to denotetheir position in the case using theleftmost bay as a C1 reference.

TABLE 2Catalog Numbers

1 The power supplies are user replaceable and interchangeable. See Catalog section 150-99.2 The relay itself may be drawn out of any of the listed cases and plugged into any of the other

case styles. The catalog number specified during ordering denotes the type of cabinet in whichthe relay will be shipped.

Description Catalog Number

Base Relay MD32T

To the above add one each of thefollowing applicable suffixes

Power Supply1

24-110V AC/DC L90-220V AC/DC H

Rated CT Input1A 15A 5

Modbus Protocol J

Case Style2

Draw out relay only, no cabinet supplied DSingle relay case SDouble relay case T19" Rack mount cabinet N

Mounting PositionC2 C3C4

Denotes mounting position in either a doublecase or 19" Rack along with other relaysordered at the same time.

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