Type MBCH 12, 13, 16Biased Differential Protection forTransformers, Generators and

Generator Transformers

2

Type MBCH 12, 13, 16Biased Differential Protection for Transformers,Generators and Generator Transformers

Features

• Independent single phase relayssuitable for single or three phasetransformer protection schemes

• Fast operating times, typically10ms to 25ms

• Dual slope percentage biasrestraint characteristic withadjustable basic threshold settingof 10% to 50% In, selectable in10% steps

• High stability during throughfaults even under conditions of CTsaturation and with up to 20%ratio imbalance resulting from theeffects of tap changing and CTerrors

• Magnetising inrush restraint over-excitation (over-fluxing) restraint

• Up to six biased inputs

• Transformer phase group and lineCT ratio correction by means ofseparate tapped interposingtransformers where required

• Two isolated change-over trippingcontacts plus one isolatednormally open latching alarmcontact. The individual phaseelements can be interconnectedto provide six isolated change-over tripping contacts for threephase schemes

• Light emitting diode (led) faultindication.

ApplicationThe type MBCH 12, 13 and 16 arehigh-speed, single phase, biaseddifferential relays suitable for theprotection of two- or three-windingpower transformers, auto-transformers, or generator-transformer units. Up to six biasedcurrent inputs can be provided tocater for power transformers withmore than two windings and/ormore than one circuit-breaker

controlling each winding, as in‘mesh’ or ‘one and a half circuit-breaker’ busbar arrangements.Typical applications are shown inFigures 2 to 8.

As a general low impedance biaseddifferential relay, the MBCH mayalso be regarded as an alternativeto the high impedance type relay forthe protection of reactors, motorsand generators.

Models Available

Type Designation No. of Bias ApplicationCircuits

MBCH 12 2 Two-winding power transformerMBCH 13 3 Generally 3 winding power transformer,

where bias is required from each of the 3groups of CTs

MBCH 16 6 For all applications requiring 4, 5 or 6bias circuits

Figure 1:Relay Type MBCH 16withdrawn from case

3

DescriptionThe relay is extremely stable duringthrough faults and provides high-speed operation on internal faults,even when energised via linecurrent transformers of onlymoderate output. Immunity to falsetripping due to large inrush currentson energisation of the powertransformer, and also duringoverfluxing conditions, is providedby the use of a novel feature notinvolving harmonic filter circuits andtheir associated delay.

It can be beneficial to supplementthe differential protection by arestricted earth fault relay,especially where the neutral point ofthe power transformer is earthedvia a current limiting resistor. Therestricted earth fault relay (typeMCAG 14 or MFAC 14) may beconnected into the differentialcircuitry, in association with acurrent transformer in the neutralconnection of the powertransformer, as indicated in Figures3 and 6. Additional line currenttransformers are not required.

For optimum performance, thedifferential scheme should bearranged so that the relay will seerated current when full load currentflows in the protected circuit. Thismay be achieved either byappropriate choice of main linecurrent transformers, or the use ofinterposing current transformers.

When protecting a powertransformer, the differential settingshould not be less than 20% relayrated current, to give stability formoderate transient overfluxing. Themaximum spill current with throughload current should generally bekept below 20% of the relay ratedcurrent, allowing for CT mismatchand possible tap changeroperation. Where higher levels ofspill current exist, the relay settingmay need to be increased.

A tapped interposing transformer isavailable for ratio matching of themain current transformers. The tapsare spaced at intervals of 4% andbetter, allowing matching to wellwithin 2% in most cases. The sameinterposing transformers may also

be used where necessary for powertransformer phase shift correction.

For some applications, no phaseshift correction is necessary, but azero sequence current trap isrequired to prevent zero sequencecurrents, due to external earth faultsbeing seen by the differential relay.Two secondary windings areprovided on the interposingtransformer to allow the creation ofan isolated delta connection for thispurpose.

Each relay case incorporates aterminal block, for externalconnections, into which the moduleis plugged. Removal of the modulefrom the case automatically causesthe incoming line current transformerconnections to be short-circuited,followed by the open circuiting ofthe relay tripping circuit.

Setting adjustment is by means offrontplate mounted switches.Indication of relay operation isprovided by an led, also mountedon the relay frontplate, and which isresettable by a push button operablewith the relay cover in position. Theoutput elements consist of auxiliaryattracted armature relays, thecontacts of which are capable ofcircuit-breaker tripping. Threeelectrically independent contacts,comprising two self-resetting change-over contacts and one hand reset,normally open contact are providedper pole, for circuit breaker trippingand alarm purposes respectively. Byinterconnecting relays as shown inFigure 9, up to six self-resettingchange-over contacts can beprovided for the three-phase trippingof up to six circuit-breakers.

Functional DescriptionThe differential transformerprotection measuring circuit is basedon the well known Merz-Pricecirculating current principle.Figure 10 shows the relay functionalblock diagram. The outputs fromeach bias restraint transformer T3 toT5, proportional to the appropriateprimary line currents, are rectifiedand summed to produce a biasrestraint voltage. Any resultingdifference current is circulatedthrough transformers T1 and T2.

The output from T1 is rectified andcombined with the bias voltage toproduce a signal which is appliedto the amplitude comparator. Thecomparator output is in the form ofpulses which vary in widthdepending on the amplitude of thecombined bias and differencevoltages. Where the measurementof the interval between these pulsesindicates less than a preset time, aninternal fault is indicated and a tripsignal, initiated after a short delay( s), level set by the bias. If, duringthe above mentioned delay, theinstantaneous value of differentialcurrent falls below the threshold andremains below for longer than afurther preset time, ( s) as it wouldduring transformer magnetisinginrush conditions, the trip timer isreset and operation of the relayblocked.

An unrestrained high-set circuit,which monitors the differentialcurrent, will override the amplitudecomparator circuit and operate therelay output element when thedifference current is above the high-set setting.

Variable percentage biasrestraint

Even under normal operatingconditions, unbalanced currents(spill current), may appear. Themagnitude of the spill currentdepends largely on the effect of tapchanging. During through faults thelevel of spill current will rise as afunction of the fault current level. Inorder to avoid unwanted operationdue to spill current and yet maintainhigh sensitivity for internal faults,when the difference current may berelatively small, the variablepercentage bias restraintcharacteristic shown in Figure 11 isused. The setting Is is defined as theminimum current, fed into one of thebias inputs and the differentialcircuit, to cause operation. This isadjustable between 10% and 50%of rated current.

The initial bias slope is 20% fromzero to rated current. This ensuressensitivity to faults whilst allowing a15% current transformer ratiomismatch when the power

1 4f

1 f

4

transformer is at the limit of its taprange, plus 5% for CT ratio error.Above rated current, extra errorsmay be gradually introduced as aresult of CT saturation. The biasslope is therefore, increased to 80%to compensate for this.

At the inception of a through faultthe bias is increased to more than100%. It then falls exponentially tothe steady state characteristic shownin Figure 11. This transient biasmatches the transient differentialcurrents that result from CTsaturation during through faults, soensuring stability.

However, during internal faults thistransient bias is suppressed toensure that no additional delay inoperation is caused.

The transient bias circuits of thethree phases are externallyinterconnected to ensure optimumstability of the protection duringthrough faults.

Magnetising inrush restraint

Particularly high inrush currents mayoccur on transformer energisationdepending on the point on wave ofswitching as well as on themagnetic state of the transformercore.

Since the inrush current flows only inthe energised winding the protectionrelay sees this current as differencecurrent. To avoid unwanted trippingof the power transformer it has beencustomary to incorporate secondharmonic restraint to block theprotection relay. Practice has shownthat this technique may result insignificantly slower operating timesfor internal faults when secondharmonics are introduced into thecurrent waveform by core saturationof line CTs.

In order to overcome the problemsassociated with second harmonic

restraint a new technique has beendeveloped to recognise magnetisinginrush current and restrain the relayduring such periods.

In practice the magnetising inrushcurrent waveform is characterisedby a period during each cycle whenlittle or no current flows, as shown inFigure 12. By measuring thischaracteristic zero period, the relayis able to determine whether thedifference current is due tomagnetising inrush current or togenuine fault current and therebyinhibit operation only during theinrush condition. This newmeasurement technique ensures thatoperating times remain unaffectedeven during periods of significantline CT saturation.

Transformer over-excitation

When a large section of systemload is suddenly disconnected froma power transformer the voltage atthe input terminals of the transformermay rise by 10-20% of rated valuegiving rise to an appreciableincrease in transformer steady stateexciting current. The resultingexciting current may rise to a valuehigh enough to operate thedifferential protection relay, sincethis current is seen by the input linecurrent transformers only. Excitingcurrents of this order are due to theinput voltage exceeding the kneepoint voltage of the powertransformer resulting in amagnetising current wave shape asshown in Figure 13. By detectingthe periods when the currentremains close to or at zero, in asimilar manner to that used toidentify magnetising inrush current,the relay is able to detect andremain insensitive to substantialover-excitation current.

Where extremely large andpotentially damaging over-exciting

currents are possible it isrecommended that an over-fluxingrelay, responsive to V/Hz, shouldbe used. Such relays are designedto operate after a time delay ofseveral seconds

High-set

An unrestrained instantaneous high-set feature is incorporated toprovide extremely fast clearance ofheavy internal faults. Thisinstantaneous feature has an auto-ranging setting, normally low atnormal load through-put, butautomatically rising to a highervalue under heavy through faultconditions. Furthermore, immunityto magnetising current inrush isguaranteed provided the first peakof the waveform is no greater than12 times the rated rms current. Theproblem relating to choice of ahigh-set threshold which avoidstripping on magnetising currentinrush does not, therefore, occurand no user adjustment is required.

Auxiliary interposingtransformer

Auxiliary interposing transformersare available as single or triple unitassemblies as illustrated in Figure15.

A comprehensive tap adjustmentrange is available as indicated bythe details of turns per tap shown inTable 2. All line CT connectionsshould be made to the terminal stripmarked ‘S1 S2 S3 S4 P1 P2’.

Selection of appropriate primarytaps is made using the flexiblejumper leads connected to terminalsP1 and P2. See Figure 16. Thetertiary winding S3-S4 must beused in series with winding S1-S2where a delta output winding isrequired to ensure correct operationof the differential scheme.

5

A

B

C

DY1A

B

C

23 25 27

24 26 28MBCH 12 12

23 25 27

24 26 28MBCH 12 12

23 25 27

24 26 28

12

MBCH 12

Restricted E/Frelay

DY1A

B

C

23 25 27

24 26 28

MBCH 12 12Phase 'A'Relay

23 25 27

24 26 28

MBCH 12 12Phase 'B'Relay

23 25 27

24 26 28 MBCH 12

12Phase 'C'Relay

Figure 3:Typical connection diagram for MBCH 12 relays protecting a DY 1 transformerwith integral restricted earth fault relay

Figure 2:Application diagram: relay type MBCH 12 with two biased inputs

6

A

23 25 27

24 26 28

23 25 27

24 26 28

23 25 27

24 26 28

B

C

Figure 4:Typical connection diagram for MBCH relays protecting a YD 11 transformeron a phase by phase basis

Figure 5:Typical connection diagram for MBCH 12 relays protecting a series reactor

A

B

C

23 25 27

24 26 28

23 25 27

24 26 28

23 25 27

24 26 2812

12

12

7

A

B

C

21 23 25 27

12

22 24 26 28

MBCH 13

21 23 25 27

12

22 24 26 28

MBCH 13

21 23 25 27 12

22 24 26 28

MBCH 13

Y(d)YO

A

B

C

Y(d1) YO

RestrictedE/F relay

21 23 25 27

22 24 26 2812

MBCH 13

21 23 25 27

22 24 26 28 12

MBCH 13

21 23 25 27

22 24 26 28

12

MBCH 13

Figure 7:Typical connection diagram for MBCH 13 relays protecting a Y(d) YO transformer supplied from a ‘mesh’ busbar

Figure 6:Typical connection diagram for MBCH 13 relays protecting a Y(d) YO transformer with integral restricted earth fault relay

8

Figure 8:Typical connection diagram for MBCH 13 relays protecting a generator/transformer unit

Figure 9:Connection for six change-over tripping contacts for three phase tripping ofup to six circuit breakers showing correct connection of the screened lead ona three phase scheme

1

3

5

2

4

6

1

3

5

2

4

6

1

3

5

2

4

6

All output contacts shown areinstantaneously initiated for anyinternal fault condition when terminals No. 10 on each phaseunit are connected togetheras shown.

Correct phase indication is maintained.

Phase 'B'

Phase 'C'

Phase 'A'

14

13

12

10

14

13

12

10

13

14

12

10

+

–

Vx

N.E.R.

Generator

UnitTransformer

22

21 23 25 27

24 26 28

12MBCH 13

21 23 25 27

24 26 28

12MBCH 13

22

21 23 25 27

24 26 28 12MBCH 1322

9

Figure 10:Block diagram: Biased differential protection relay type MBCH 13 with three biased inputs

3

2

1

0 1 2 3 4

Operate

80% Slope

Restrain

20% SlopeAllowable

20% ratio error

Effective bias (xIn) = I1 +I2 +I3 +I4 +I5 +I6

2

Differential current (xIn)

= I1 +I2 +I3 +I4 +I5 +I6

Settingrange(0.1-0.5)

A

B

C

Figure 11:Typical percentage biascharacteristic

Figure 12:Typical magnetisinginrush waveforms

Case earth

13579

111315

17

2468

10121416

18

19

21

23

20

22

24

25 26

27 28

Module terminalblock viewed

from rear

Note 1:

(a) C.T. shorting links makebefore (b) & (c) disconnect

(b) short terminals break before (c)

Note 2:Terminal 12 on each phase assembly should beinterconnected by a screened lead GJO153 001with the screen connected to terminal 14.

21

2223

T5I3

I2

2425

T4

T326

I1

Inputcircuits

IDIFF

T2

T1

27

28

Outputcircuits

RL12

Operate

Reset

RL21

1

35246

RL1-1

RL1-2Trip output

10

13

14

12

Trip other phase

+

–Vx

9

11RL2-1 Alarm

Biassee note 2

Protectivesafetyearth

10

0Figure 13:Magnetising current withtransformer overfluxed

Figure 14:Typical operating timecharacteristic

3

2

1

00.1 0.2 0.5 1 2 5 10 20 50 100

Ope

ratin

g tim

e (c

ycle

s)

Fault current (xIn)

Setting range

Figure 16:Winding arrangement ofsingle pole of auxiliaryinterposing transformer

Figure 15:Outline details of single and triple unit auxiliary interposing transformer assembly

1 2 3 4 5 6 X 7 8 9 1 2 3 4 5 6 X 7 8 9 1 2 3 4 5 6 X 7 8 9Primary taps

M4 terminal screws

119 (1Ø)83.5 (1Ø)

4 off M5clearanceholes

145310 (3Ø)

343 (3Ø)

Earth

S1 S2 S3 S4 P1 P2 S1 S2 S3 S4 P1 P2 S1 S2 S3 S4 P1 P2

117145

11,5

1 2 3 4

Outputto relay

5 6 X 7 8 9Flexiblejumperleads

Input fromline CT's

P2P1S4S3S2S1

11

Technical Data

RatingsAC Current (In) 1A or 5AFrequency 50Hz or 60Hz

Auxiliary dc supply (Vx) Voltage Rating Operating(Vdc) range (Vdc)30 - 34 24 - 4148 - 54 37.5 - 65110 - 125 87.5 - 150220 - 250 175 - 300

Minimum basic settingBiased feature 10%, 20%, 30%, 40%, 50% In

High-set feature 4In up to through bias currents of 9In,thereafter increasing gradually to 8In

at through bias currents of 14In

High-set operation Immunity to magnetising currentinrush is guaranteed provided the firstpeak of the waveform is no greaterthan 12 times the rated rms current.

Accuracy ±10%

Operating characteristic Dual slope, 20% up to In,80% above In.

Operating time Typically 10ms to 25ms

Number of current inputs MBCH 12 - 2 biased inputsMBCH 13 - 3 biased inputsMBCH 16 - 6 biased inputs

AC thermal ratingContinuous MBCH 13,16: 4In

MBCH 12: 2In

Short time, all versions 100In for 1s with amaximum of 400A

DC burdenWith output H/A not picked up

DC voltage Burden at upperrating (V) voltage rating (W)30 - 34 Approx 348 - 54 Approx 3110 - 125 Approx 4220 - 250 Approx 8

With output H/A picked upDC voltage Burden at upperrating (V) voltage rating (W)30 - 34 Approx 448 - 54 Approx 4110 - 125 Approx 7220 - 250 Approx 12

AC burdenThrough current only(per bias circuit) Approx 0.15VA (In = 1A)Ithro - 1 x In Approx 0.30VA (In = 5A)Bias and differential circuit only Approx 2.80VA (In = 1A)Idiff = 1 x In Approx 3.20VA (In = 5A)

12

Contacts Two change-over self-reset trippingcontacts per relay. (For 3 phaseschemes relays can be interconnectedas shown in Figure 9 to provideinstantaneous operation of all threesets of change-over tripping contacts.)

One normally open latching contactper relay.

Make and carry: 7500VA for 0.2swith maxima of 30A and 300V ac or dc.Carry continuously: 5A ac or dcBreak: 1250VA ac or 50W dc resistive,25W, L/R = 0.04s.Subject to maxima of 5A and 300V.

Durability

Loaded contact 10,000 operations minimumUnloaded contact 100,000 operations minimum

Operation indicator Light emitting diode (red), hand reset

Line current transformer requirementsApplication Knee Point Voltage Vk Through Fault Stability

Note: Values to be as givenbelow, with minima of 60 Infor star-connected CTs and100 for delta connected CTs

In X/R If

Transformers Vk / NM 24 In [Rct+2Rl+Rt] 40 15In

GeneratorsGeneratortransformers Vk / 24In [Rct+2Rl+Rt] 40 15InOverall generator-transformer/units Vk / 48In [Rct+2Rl+Rt] 120 15InMotorsShunt reactorsSeries Reactorsalso Transformersconnected to aMesh Corner Vk / 24In [Rct+2Rl+Rt] 40 15Inhaving two sets ofCT’s eachsupplying separate 40 40Inrelay inputs Vk / 48In [Rct+2Rl+Rt]Note: CT’s should 120 15Inbe of equal ratioand magnetisationcharacteristic

Where:

In = Rated line CT secondary current (1A or 5A).Rct = Resistance of line CT secondary winding.Rl = Resistance of a single lead from line CT to relay.Rt = Effective resistance of interposing CT where used.X/R = Maximum value of primary system reactance/resistance ratio.Ιf = Maximum value of through fault current.Table 1: Current Transformer requirements.

13

Auxiliary interposing transformersNominal secondary current rating, In 1A or 5ANominal current ratios selectable in 0.58 - 1.73/14% In steps see Table 1 2.89 - 8.66/1

2.89 - 8.66/5Note: Higher primary currents can be catered for. Refer to publication

R-6077

Rated frequency 50/60Hz

Thermal withstand 4 x In continuous30 x In for 10s100 x In for 1s with 400A maximum

Effective resistance Rt 2.0Ω for 0.58 - 1.73/1ratio0.2Ω for 2.89 - 8.66/5 ratio0.15Ω for 2.89 - 8.66/1ratio

Insulation test voltage 2kV rms 1 minute, 50Hz

Maximum terminal block wire size 84 strand, 0.3mm Ø

Dimensions Outline drawings for single and triplegroup shown in Figure 15

Weight 4kg single unit12kg triple unitNumber of turns

Primary top Transformer ratingterminals 1/1A 5/1A 5/5A

1 – 2 5 1 12 – 3 5 1 13 – 4 5 1 14 – 5 5 1 15 – 6 125 25 25X – 7 25 5 57 – 8 25 5 58 – 9 25 5 5S1 – S2 125 125 25S3 – S4 90 90 18

Table 2: Interposing CTs: number of turns per tap.

High Voltage withstandDielectric withstandIEC 60255-5: 1977 2kV rms for 1 minute between all

terminals and case earth.

2kV rms for 1 minute between allterminals of independent circuits, withterminals on each independent circuitconnected together.

1 kV rms for 1 minute across normallyopen contacts.

High voltage impulseIEC 60255-5: 1977 Three positive and three negative

impulses of 5kV peak, 1.2/50µs, 0.5Jbetween all terminals of the samecircuit (except output contacts),between independent circuits, andbetween all terminals connectedtogether and case earth.

14

Electrical environmentDC supply interruptionIEC 60255-11: 1979 The unit will withstand a 10ms

interruption in the auxiliary supply,under normal operating conditions,without de-energising.

AC ripple on DC supply The unit will withstand 12% ac rippleIEC 60255-11: 1979 on the dc supply

High frequency disturbanceIEC 60255-22-1: 1988 Class III 2.5kV peak between independent

circuits and case.

1.0kV peak across terminals of thesame circuit.

Fast transient disturbanceIEC 60255-22-4: 1992 Class IV 4.0kV, 2.5kHz applied directly to

auxiliary supply.

4.0kV, 2.5kHz applied directly to allinputs.

Surge immunityIEC 61000-4-5: 1995 Level 3 2.0kV peak, 1.2/50µs between all

groups and case earth.

2.0kV peak, 1.2/50µs betweenterminals of each group.

EMC compliance89\336\EEC Compliance with the European

Commission Directive on EMC isclaimed via the Technical ConstructionFile route.

EN5008-2: 1994 Generic Standards were used toEN5008-2: 1995 establish conformity.

Product safety73/23/EEC Compliance with the European

Commission Low Voltage Directive.

EN 61010-1: 1993/A2: 1995 Compliance is demonstrated byEN 60950: 1992 All: 1997 reference to generic safety standards.

Atmospheric environmentTemperatureIEC 60255-6: 1988 Storage and transit -25°C to + 70°C

Operating -25°C to +55°CIEC 60068-2-1: 1990 ColdIEC 60068-2-2: 1974 Dry heat

HumidityIEC 60068-2-3 56 days at 93% RH and 40°C

Enclosure protectionIEC 60529: 1989 IP50 (dust protected)

Mechanical environmentVibrationIEC 60255-21-1: 1988 Response Class 1

SeismicIEC 60255-21-3: 1993 Class 2

15

CasesRelays type MBCH are housed insize 4 cases as shown in Figure 17.

Figure 17:Case outline size 4

Information required with orderNote: 3 single-phase MBCH relays are required for a 3-phase scheme. Pleasestate total number of single-phase relays required.

Rated current In

Rated frequency

Auxiliary dc supply voltage Vx

Number of current bias inputs:

MBCH 12: 2 inputMBCH 13: 3 inputMBCH 16: 6 input

Auxiliary interposing transformerRated current In

Current ratio

Mounting: single or triple unit

Associated PublicationsInterposing matching CTs for use with MBCH relays R6077

MIDOS system R6001

MMLG/MMLB test block/test plug R6004

97

Push buttonprojection 10 max.

4 holes Ø4.452

23.5

159

25 min.

11

157 max.

Flush mountingAll dimensions in mm

Reset

21232

177

77

168

99Panel cut-out:Flush mounting fixing details

St Leonards Works, Stafford, ST17 4LX EnglandTel: 44 (0) 1785 223251 Fax: 44 (0) 1785 212232 Email: enquiries@pcs.alstom.co.uk Internet: www.alstom.com

©1998 ALSTOM T&D Protection & Control Ltd

Our policy is one of continuous development. Accordingly the design of our products may change at any time. Whilst every effort is made to produce up to date literature, this brochure shouldonly be regarded as a guide and is intended for information purposes only. Its contents do not constitute an offer for sale or advice on the application of any product referred to in it.

ALSTOM T&D Protection & Control Ltd cannot be held responsible for any reliance on any decisions taken on its contents without specific advice.

Publication R-6070L 060010 Printed in England.

ALSTOM T&D Protection & Control Ltd