15 busbar protection

Introduction 15.1Busbar faults 15.2

Protection requirements 15.3Types of protection system 15.4System protection schemes 15.5

Frame-earth protection(Howard protection) 15.6

Differential protection principles 15.7

High impedancedifferential protection 15.8Low impedance biaseddifferential protection 15.9

Numerical busbar protection 15.10References 15.11

• 1 5 • B u s b a r P r o t e c t i o n

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15.1 INTRODUCTION

The protection scheme for a power system should coverthe whole system against all probable types of fault.Unrestricted forms of line protection, such as overcurrentand distance systems, meet this requirement, althoughfaults in the busbar zone are cleared only after sometime delay. But if unit protection is applied to feedersand plant, the busbars are not inherently protected.

Busbars have often been left without specific protection,for one or more of the following reasons:

a. the busbars and switchgear have a high degree ofreliability, to the point of being regarded asintrinsically safe

b. it was feared that accidental operation of busbarprotection might cause widespread dislocation ofthe power system, which, if not quickly cleared,would cause more loss than would the veryinfrequent actual bus faults

c. it was hoped that system protection or back-upprotection would provide sufficient bus protectionif needed

It is true that the risk of a fault occurring on modernmetal-clad gear is very small, but it cannot be entirelyignored. However, the damage resulting from oneuncleared fault, because of the concentration of faultMVA, may be very extensive indeed, up to the completeloss of the station by fire. Serious damage to ordestruction of the installation would probably result inwidespread and prolonged supply interruption.

Finally, system protection will frequently not provide thecover required. Such protection may be good enough forsmall distribution substations, but not for importantstations. Even if distance protection is applied to allfeeders, the busbar will lie in the second zone of all thedistance protections, so a bus fault will be clearedrelatively slowly, and the resultant duration of thevoltage dip imposed on the rest of the system may not betolerable.

With outdoor switchgear the case is less clear since,although the likelihood of a fault is higher, the risk ofwidespread damage resulting is much less. In generalthen, busbar protection is required when the systemprotection does not cover the busbars, or when, in order

• 15 • Busbar P rotect ion

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to maintain power system stability, high-speed faultclearance is necessary. Unit busbar protection providesthis, with the further advantage that if the busbars aresectionalised, one section only need be isolated to cleara fault. The case for unit busbar protection is in factstrongest when there is sectionalisation.

15.2 BUSBAR FAULTS

The majority of bus faults involve one phase and earth,but faults arise from many causes and a significantnumber are interphase clear of earth. In fact, a largeproportion of busbar faults result from human errorrather than the failure of switchgear components.

With fully phase-segregated metalclad gear, only earthfaults are possible, and a protection scheme need haveearth fault sensitivity only. In other cases, an ability torespond to phase faults clear of earth is an advantage,although the phase fault sensitivity need not be very high.

15.3 PROTECTION REQUIREMENTS

Although not basically different from other circuitprotection, the key position of the busbar intensifies theemphasis put on the essential requirements of speed andstability. The special features of busbar protection arediscussed below.

15.3.1 Speed

Busbar protection is primarily concerned with:

a. limitation of consequential damage

b. removal of busbar faults in less time than could beachieved by back-up line protection, with theobject of maintaining system stability

Some early busbar protection schemes used a lowimpedance differential system having a relatively longoperation time, of up to 0.5 seconds. The basis of mostmodern schemes is a differential system using either lowimpedance biased or high impedance unbiased relayscapable of operating in a time of the order of one cycleat a very moderate multiple of fault setting. To this mustbe added the operating time of the tripping relays, but anoverall tripping time of less than two cycles can beachieved. With high-speed circuit breakers, completefault clearance may be obtained in approximately 0.1seconds. When a frame-earth system is used, theoperating speed is comparable.

15.3.2 Stability

The stability of bus protection is of paramountimportance. Bearing in mind the low rate of fault

incidence, amounting to no more than an average of onefault per busbar in twenty years, it is clear that unlessthe stability of the protection is absolute, the degree ofdisturbance to which the power system is likely to besubjected may be increased by the installation of busprotection. The possibility of incorrect operation has, inthe past, led to hesitation in applying bus protection andhas also resulted in application of some very complexsystems. Increased understanding of the response ofdifferential systems to transient currents enables suchsystems to be applied with confidence in theirfundamental stability. The theory of differentialprotection is given later in Section 15.7.

Notwithstanding the complete stability of a correctlyapplied protection system, dangers exist in practice for anumber of reasons. These are:

a. interruption of the secondary circuit of a currenttransformer will produce an unbalance, whichmight cause tripping on load depending on therelative values of circuit load and effective setting.It would certainly do so during a through fault,producing substantial fault current in the circuit inquestion

b. a mechanical shock of sufficient severity maycause operation, although the likelihood of thisoccurring with modern numerical schemes isreduced

c. accidental interference with the relay, arising froma mistake during maintenance testing, may lead tooperation

In order to maintain the high order of integrity neededfor busbar protection, it is an almost invariable practiceto make tripping depend on two independentmeasurements of fault quantities. Moreover, if thetripping of all the breakers within a zone is derived fromcommon measuring relays, two separate elements mustbe operated at each stage to complete a trippingoperation. Although not current practice, in many casesthe relays are separated by about 2 metres so that noreasonable accidental mechanical interference to bothrelays simultaneously is possible.

The two measurements may be made by two similardifferential systems, or one differential system may bechecked by a frame-earth system, by earth fault relaysenergised by current transformers in the transformerneutral-earth conductors or by overcurrent relays.Alternatively, a frame-earth system may be checked byearth fault relays.

If two systems of the unit or other similar type are used,they should be energised by separate currenttransformers in the case of high impedance unbiaseddifferential schemes. The duplicate ring CT cores may bemounted on a common primary conductor but

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independence must be maintained throughout thesecondary circuit.

In the case of low impedance, biased differentialschemes that cater for unequal ratio CT's, the schemecan be energised from either one or two separate sets ofmain current transformers. The criteria of double featureoperation before tripping can be maintained by theprovision of two sets of ratio matching interposing CT'sper circuit. When multi-contact tripping relays are used,these are also duplicated, one being energised from eachdiscriminating relay; the contacts of the tripping relayare then series-connected in pairs to provide trippingoutputs.

Separate tripping relays, each controlling one breakeronly, are usually preferred. The importance of such relaysis then no more than that of normal circuit protection, sono duplication is required at this stage. Not least amongthe advantages of using individual tripping relays is thesimplification of trip circuit wiring, compared withtaking all trip circuits associated with a given bus sectionthrough a common multi-contact tripping relay.

In double busbar installations, a separate protectionsystem is applied to each section of each busbar; anoverall check system is provided, covering all sections ofboth busbars. The separate zones are arranged tooverlap the busbar section switches, so that a fault onthe section switch trips both the adjacent zones. Thishas sometimes been avoided in the past by giving thesection switch a time advantage; the section switch istripped first and the remaining breakers delayed by 0.5seconds.

Only the zone on the faulty side of the section switchwill remain operated and trip, the other zone resettingand retaining that section in service. This gain,applicable only to very infrequent section switch faults,is obtained at the expense of seriously delaying the busprotection for all other faults. This practice is thereforenot generally favoured. Some variations are dealt withlater under the more detailed scheme descriptions. Thereare many combinations possible, but the essentialprinciple is that no single accidental incident of asecondary nature shall be capable of causing anunnecessary trip of a bus section.

Security against maloperation is only achieved byincreasing the amount of equipment that is required tofunction to complete an operation; and this inevitablyincreases the statistical risk that a tripping operation dueto a fault may fail. Such a failure, leaving aside thequestion of consequential damage, may result indisruption of the power system to an extent as great, orgreater, than would be caused by an unwanted trip. Therelative risk of failure of this kind may be slight, but ithas been thought worthwhile in some instances toprovide a guard in this respect as well.

Security of both stability and operation is obtained byproviding three independent channels (say X, Y and Z)whose outputs are arranged in a ‘two-out-of three’voting arrangement, as shown in Figure 15.1.

15.4 TYPES OF PROTECTION SYSTEM

A number of busbar protection systems have beendevised:

a. system protection used to cover busbars

b. frame-earth protection

c. differential protection

d. phase comparison protection

e. directional blocking protection

Of these, (a) is suitable for small substations only, while(d) and (e) are obsolete. Detailed discussion of types (b)and (c) occupies most of this chapter.

Early forms of biased differential protection for busbars,such as versions of 'Translay' protection and also ascheme using harmonic restraint, were superseded byunbiased high impedance differential protection.

The relative simplicity of the latter, and more importantlythe relative ease with which its performance can becalculated, have ensured its success up to the presentday.

But more recently the advances in semiconductortechnology, coupled with a more pressing need to be ableto accommodate CT's of unequal ratio, have led to there-introduction of biased schemes, generally using staticrelay designs, particularly for the most extensive andonerous applications.

Frame-earth protection systems have been in use formany years, mainly associated with smaller busbarprotection schemes at distribution voltages and formetalclad busbars (e.g. SF6 insulated busbars). However,it has often been quite common for a unit protectionscheme to be used in addition, to provide two separatemeans of fault detection.

The different types of protection are described in thefollowing sections.

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Tripcircuits

X Y

Y Z

Z X

+ _

Figure 15 .1 : Two-out-of-three pr inc ip le

Chap15-232-253 17/06/02 9:46 Page 235

15.5 SYSTEM PROTECTION SCHEMES

System protection that includes overcurrent or distancesystems will inherently give protection cover to thebusbars. Overcurrent protection will only be applied torelatively simple distribution systems, or as a back-upprotection, set to give a considerable time delay.Distance protection will provide cover for busbar faultswith its second and possibly subsequent zones. In bothcases the busbar protection obtained is slow and suitableonly for limiting the consequential damage.

The only exception is the case of a mesh-connectedsubstation, in which the current transformers are locatedat the circuit breakers. Here, the busbars are included, insections, in the individual zones of the main circuitprotection, whether this is of unit type or not. In thespecial case when the current transformers are locatedon the line side of the mesh, the circuit protection willnot cover the busbars in the instantaneous zone andseparate busbar protection, known as mesh-cornerprotection, is generally used – see Section 15.7.2.1 fordetails.

15.6 FRAME-EARTH PROTECTION(HOWARD PROTECTION)

Frame leakage protection has been extensively used inthe past in many different situations. There are severalvariations of frame leakage schemes available, providingbusbar protection schemes with different capabilities.The following sections schemes have thus been retainedfor historical and general reference purposes. Aconsiderable number of schemes are still in service andframe leakage may provide an acceptable solution inparticular circumstances. However, the need to insulatethe switchboard frame and provide cable glandinsulation and the availability of alternative schemesusing numerical relays, has contributed to a decline inuse of frame leakage systems.

15.6.1 Single-Busbar Frame-Earth Protection

This is purely an earth fault system and, in principle,involves simply measuring the fault current flowing fromthe switchgear frame to earth. A current transformer ismounted on the earthing conductor and is used to energizea simple instantaneous relay as shown in Figure 15.2.

No other earth connections of any type, includingincidental connections to structural steelwork areallowed. This requirement is so that:

a. the principal earth connection and currenttransformer are not shunted, thereby raising theeffective setting. An increased effective settinggives rise to the possibility of relay maloperation.This risk is small in practice

b. earth current flowing to a fault elsewhere on thesystem cannot flow into or out of the switchgearframe via two earth connections, as this might leadto a spurious operation

The switchgear must be insulated as a whole, usually bystanding it on concrete. Care must be taken that thefoundation bolts do not touch the steel reinforcement;sufficient concrete must be cut away at each hole topermit grouting-in with no risk of touching metalwork.The insulation to earth finally achieved will not be high,a value of 10 ohms being satisfactory.

When planning the earthing arrangements of a frame-leakage scheme, the use of one common electrode forboth the switchgear frame and the power system neutralpoint is preferred, because the fault path wouldotherwise include the two earthing electrodes in series.If either or both of these are of high resistance or haveinadequate current carrying capacity, the fault currentmay be limited to such an extent that the protectionequipment becomes inoperative. In addition, if theelectrode earthing the switchgear frame is the offender,the potential of the frame may be raised to a dangerousvalue. The use of a common earthing electrode ofadequate rating and low resistance ensures sufficientcurrent for scheme operation and limits the rise in framepotential. When the system is resistance earthed, theearthing connection from the switchgear frame is madebetween the bottom of the earthing resistor and theearthing electrode.

Figure 15.3 illustrates why a lower limit of 10 ohmsinsulation resistance between frame and earth isnecessary.

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Figure 15 .2 : S ingle zoneframe-earth protect ion

I >

Trip allcircuitbreaker

Frame-earthfault relay Neutral

check relay

G KJ

+

H

I >

Chap15-232-253 17/06/02 9:46 Page 236

Under external fault conditions, the current I1 flowsthrough the frame-leakage current transformer. If theinsulation resistance is too low, sufficient current mayflow to operate the frame-leakage relay, and, as the checkfeature is unrestricted, this will also operate to completethe trip circuit. The earth resistance between the earthingelectrode and true earth is seldom greater than 1Ω, sowith 10Ω insulation resistance the current I1 is limited to10% of the total earth fault current I1 and I2. For thisreason, the recommended minimum setting for thescheme is about 30% of the minimum earth fault current.

All cable glands must be insulated, to prevent thecirculation of spurious current through the frame andearthing system by any voltages induced in the cablesheath. Preferably, the gland insulation should beprovided in two layers or stages, with an interposinglayer of metal, to facilitate the testing of the glandinsulation. A test level of 5kV from each side is suitable.

15.6.2 Frame-Earth Protection - Sectioned Busbars

Section 15.6.1 covered the basic requirements for asystem to protect switchgear as a whole. When thebusbar is divided into sections, these can be protectedseparately, provided the frame is also sub-divided, thesections mutually insulated, and each provided with aseparate earth conductor, current transformer and relay.

Ideally, the section switch should be treated as aseparate zone, as shown in Figure 15.4, and providedwith either a separate relay or two secondaries on theframe-leakage current transformer, with an arrangementto trip both adjacent zones. The individual zone relaystrip their respective zone and the section switch.

If it is inconvenient to insulate the section switch frameon one side, this switch may be included in that zone. Itis then necessary to intertrip the other zone afterapproximately 0.5 seconds if a fault persists after thezone including the section switch has been tripped. Thisis illustrated in Figure 15.5.

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Figure 15 .4 : Three zone f rameearth scheme

Insulationbarriers

Triprelays

Zone HZone G

Zone Gframe leakage

relay

Zone Hframe leakage

relay

Trip K Trip L Trip M

L

L1 L2

M

M

K

I

I

>

>

K

Zone JFrame-leakagecurrenttransformer

Earthingelectroderesistance

Frameinsulationresistance to earth

Earthbar

Switchgearframe

Switchgear framebonding bar

Generator

Systemearningresistor

Outgoingfeeder

I1 I1 I2I

I1 + I2I

IFI = I1 + I2I

Figure 15 .3 : Current d ist r ibut ionfor external fault

I >

J L

K

>I

Triprelays

Trip J Trip K Trip L

Zone G

Zone G

Zone H

Zone H

Insulationbarrier

J LK1 K2K

Figure 15.5: Frame-earth scheme: bussection breaker insulated on one side only

Chap15-232-253 17/06/02 9:46 Page 237

For the above schemes to function it is necessary to havea least one infeed or earthed source of supply, and in thelatter case it is essential that this source of supply beconnected to the side of the switchboard not containingthe section switch. Further, if possible, it is preferablethat an earthed source of supply be provided on bothsides of the switchboard, in order to ensure that anyfaults that may develop between the insulating barrierand the section switch will continue to be fed with faultcurrent after the isolation of the first half of theswitchboard, and thus allow the fault to be removed. Ofthe two arrangements, the first is the one normallyrecommended, since it provides instantaneous clearanceof busbar faults on all sections of the switchboard.

15.6.3 Frame-Earth Scheme - Double Bus Substation

It is not generally feasible to separately insulate themetal enclosures of the main and auxiliary busbars.Protection is therefore generally provided as for singlebus installations, but with the additional feature thatcircuits connected to the auxiliary bus are tripped for allfaults, as shown in Figure 15.6.

15.6.4 Frame-Earth Protection - Check System

On all but the smallest equipments, a check systemshould be provided to guard against such contingencies

as operation due to mechanical shock or mistakes madeby personnel. Faults in the low voltage auxiliary wiringmust also be prevented from causing operation bypassing current to earth through the switchgear frame.A useful check is provided by a relay energised by thesystem neutral current, or residual current. If the neutralcheck cannot be provided, the frame-earth relays shouldhave a short time delay.

When a check system is used, instantaneous relays canbe used, with a setting of 30% of the minimum earthfault current and an operating time at five times settingof 15 milliseconds or less.

Figure 15.7 shows a frame-leakage scheme for ametalclad switchgear installation similar to that shownin Figure 15.4 and incorporating a neutral current checkobtained from a suitable zero sequence current source,such as that shown in Figure 15.2.

The protection relays used for the discriminating andcheck functions are of the attracted armature type, withtwo normally open self reset contacts. The trippingcircuits cannot be complete unless both thediscriminating and check relays operate; this is becausethe discriminating and check relay contacts areconnected in series. The tripping relays are of theattracted armature type.

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Figure 15 .6 : F rame-earth schemefor double busbar substat ion

II >

>II

K

Lg

g1

j1

j1M1

M2M

L1

L2

j2

j2

h1

h1

Insulationbarriers

K

_

N

G H Busbarisolatorauxiliary switchesD.C. Zone bus wires

j

M

+

jj

Zone Hrelay

Zone Grelay

Zone G

Zone J

H

Trippingrelays

KOut

_GH

M

64A-1

64B-1

CSS-H

CSS-G

L6

L4

L3

L4

L3

L5

L1

L2

74-1

74-2

64B-2

64A-2 I >

74 Alarm cancellation relayCSS Control selector switch protection in/protection outL3 Busbar protection in service lampL4 Busbar protection out of service lampL5 Tripping supply healthy lampL6 Alarm and indication supply healthy lamp

64CH-2

In

CSS-H

CSS-G

In

Out

64CH-1+ Trip relays

Figure 15.7 : Typical t r ipping and alarmcircuits for a f rame-leakage scheme

Chap15-232-253 17/06/02 9:46 Page 238

It is usual to supervise the satisfactory operation of theprotection scheme with audible and visual alarms andindications for the following:

a. busbar faults

b. busbar protection in service

c. busbar protection out of service

d. tripping supply healthy

e. alarm supply healthy

To enable the protection equipment of each zone to betaken out of service independently during maintenanceperiods, isolating switches - one switch per zone - areprovided in the trip supply circuits and an alarmcancellation relay is used.

15.7 DIFFERENTIAL PROTECTION PRINCIPLES

The Merz-Price principle is applicable to a multi-terminalzone such as a busbar. The principle is a directapplication of Kirchhoff's first law. Usually, thecirculating current arrangement is used, in which thecurrent transformers and interconnections form ananalogue of the busbar and circuit connections. A relayconnected across the CT bus wires represents a faultpath in the primary system in the analogue and hence isnot energised until a fault occurs on the busbar; it thenreceives an input that, in principle at least, representsthe fault current.

The scheme may consist of a single relay connected tothe bus wires connecting all the current transformers inparallel, one set per circuit, associated with a particularzone, as shown in Figure 15.8(a). This will give earthfault protection for the busbar. This arrangement hasoften been thought to be adequate.

If the current transformers are connected as a balancedgroup for each phase together with a three-elementrelay, as shown in Figure 15.8(b), additional protectionfor phase faults can be obtained.

The phase and earth fault settings are identical, and thisscheme is recommended for its ease of application andgood performance.

15.7.1 Differential Protectionfor Sectionalised and Duplicate Busbars

Each section of a divided bus is provided with a separatecirculating current system. The zones so formed areover-lapped across the section switches, so that a faulton the latter will trip the two adjacent zones. This isillustrated in Figure 15.9.

Tripping two zones for a section switch fault can beavoided by using the time-delayed technique of Section15.6.2. However instantaneous operation is thepreferred choice.

For double bus installation, the two busbars will betreated as separate zones. The auxiliary busbar zone willoverlap the appropriate main busbar zone at the buscoupler.

Since any circuit may be transferred from one busbar tothe other by isolator switches, these and the associatedtripping circuit must also be switched to the appropriate

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Figure 15.8: Circulating current scheme

G H J K

Differentialrelay

Differential relay

CNN

BAA

G

IdIdI I > IdIdI >

IdIdI >

H

b) Phase and earth fault circulating current scheme usingthree-element relay

f

Zone A Zone B

Zone C

B BC

BS

Typical feeder circuits

Figure 15 .9 : Zones of protect ionfor double bus stat ion

Chap15-232-253 17/06/02 9:46 Page 239

zone by 'early make' and 'late break' auxiliary contacts.This is to ensure that when the isolators are closing, theauxiliary switches make before the main contacts of theisolator, and that when the isolators are opened, theirmain contacts part before the auxiliary switches open.The result is that the secondary circuits of the two zonesconcerned are briefly paralleled while the circuit is beingtransferred; these two zones have in any case beenunited through the circuit isolators during the transferoperation.

15.7.2 Location of Current Transformers

Ideally, the separate discriminating zones should overlapeach other and also the individual circuit protections.The overlap should occur across a circuit breaker, so thatthe latter lies in both zones. For this arrangement it isnecessary to install current transformers on both sides ofthe circuit breakers, which is economically possible withmany but not all types of switchgear. With both thecircuit and the bus protection current transformers onthe same side of the circuit breakers, the zones may beoverlapped at the current transformers, but a faultbetween the CT location and the circuit breaker will notbe completely isolated. This matter is important in allswitchgear to which these conditions apply, and isparticularly important in the case of outdoor switchgearwhere separately mounted, multi-secondary currenttransformers are generally used. The conditions areshown in Figure 15.10.

Figure 15.10(a) shows the ideal arrangement in whichboth the circuit and busbar zones are overlapped leavingno region of the primary circuit unprotected.

Figure 15.10(b) shows how mounting all currenttransformers on the circuit side of the breaker results ina small region of the primary circuit unprotected. Thisunprotected region is typically referred to as the ‘shortzone’. The fault shown will cause operation of the busbarprotection, tripping the circuit breaker, but the fault willcontinue to be fed from the circuit, if a source of poweris present. It is necessary for the bus protection tointertrip the far end of the circuit protection, if the latteris of the unit type.

With reference to Figure 15.10(b), special ‘short zone’protection can be provided to detect that the circuitbreaker has opened but that the fault current is stillflowing. Under these conditions, the protection caninitiate an intertrip to the remote end of the circuit. Thistechnique may be used, particularly when the circuitincludes a generator. In this case the intertrip proves thatthe fault is in the switchgear connections and not in thegenerator; the latter is therefore tripped electrically but notshut down on the mechanical side so as to be immediatelyready for further service if the fault can be cleared.

15.7.2.1 CT locations for mesh-connected substations

The protection of busbars in mesh connected substationsgives rise to additional considerations in respect of CTlocation. A single mesh corner is shown in Figure

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Figure 15.10: Unprotected zone withcurrent transformers mountedon one side of the circuit breaker only

protectionCircuit

-fault shown not cleared by circuit protectionb. Current transformers mounted on circuit side only of breaker

-no unprotected regiona. Current transformers mounted on both sides of breaker

Fault

Bus protection

(a) (b)

Mesh cornerprotection

Mesh corner(Note 2)

Lineprotection

Transformerprotection

(b) CT arrangements for protection - additional mesh corner protection required

Note 2: Multiple circuits may be connected to the mesh corner

Note 1: Only 1 connection to the mesh corner permitted(a) CT arrangements for protection including mesh corner

Lineprotectionrelay

Mesh corner(Note 1)

Figure 15 .11 : Mesh-corner protect ion

Chap15-232-253 17/06/02 9:48 Page 240

15.11(a). Where only one connection to the mesh ismade at a corner, CT’s located as shown will provideprotection not only to the line but the corner of the meshincluded between them. However, this arrangementcannot be used where more than one connection is madeto a mesh corner. This is because a fault on any of theconnected circuits would result in disconnection of themall, without any means of determining the faultedconnection. Protection CT’s must therefore be located oneach connection, as shown in Figure 15.11(b). This leavesthe corner of the mesh unprotected, so additional CT’sand a relay to provide mesh-corner protection are added,as also shown in Figure 15.11(b).

15.8 HIGH IMPEDANCEDIFFERENTIAL PROTECTION

This form of protection is still in common use. Theconsiderations that have to be taken into account aredetailed in the following sections.

15.8.1 Stability

The incidence of fault current with an initial unilateraltransient component causes an abnormal built-up of fluxin a current transformer, as described in Section 6.4.10.When through-fault current traverses a zone protectedby a differential system, the transient flux produced inthe current transformers is not detrimental as long as itremains within the substantially linear range of themagnetising characteristic. With fault current ofappreciable magnitude and long transient time constant,the flux density will pass into the saturated region of thecharacteristic; this will not in itself produce a spilloutput from a pair of balancing current transformersprovided that these are identical and equally burdened.A group of current transformers, though they may be ofthe same design, will not be completely identical, but amore important factor is inequality of burden. In thecase of a differential system for a busbar, an externalfault may be fed through a single circuit, the currentbeing supplied to the busbar through all other circuits.The faulted circuit is many times more heavily loadedthan the others and the corresponding currenttransformers are likely to be heavily saturated, whilethose of the other circuits are not. Severe unbalance istherefore probable, which, with a relay of normal burden,could exceed any acceptable current setting. For thisreason such systems were at one time always providedwith a time delay. This practice is, however, no longeracceptable.

It is not feasible to calculate the spill current that mayoccur, but, fortunately, this is not necessary; an alternativeapproach provides both the necessary information and thetechnique required to obtain a high performance.

An equivalent circuit, as in Figure 15.12, can represent acirculating current system.

The current transformers are replaced in the diagram byideal current transformers feeding an equivalent circuitthat represents the magnetising losses and secondarywinding resistance, and also the resistance ofthe connecting leads. These circuits can then beinterconnected as shown, with a relay connected to thejunction points to form the complete equivalent circuit.

Saturation has the effect of lowering the excitingimpedance, and is assumed to take place severely incurrent transformer H until, at the limit, the shuntimpedance becomes zero and the CT can produce nooutput. This condition is represented by a short circuit,shown in broken line, across the exciting impedance. Itshould be noted that this is not the equivalent of aphysical short circuit, since it is behind the windingresistance .

Applying the Thévenin method of solution, the voltagedeveloped across the relay will be given by:

. . .Equat ion 15 .1

The current through the relay is given by:


If RR is small, IR will approximate to IF, which isunacceptable. On the other hand, if RR is large IR isreduced. Equation 15.2 can be written, with little error,as follows:

=+( )

+ +I R R

R R Rf LH CTH

R LH CTH

IV

R R RRf

R LH CTH

=+ +

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Figure 15 .12 : Equivalent c i rcuitof c i rculat ing current system

IdIdI >ZEGZ

RR

RLHRRLGR RCTHRCTG

G H

ZEHZ

Chap15-232-253 17/06/02 9:48 Page 241

…Equat ion 15 .3or alternatively:

…Equat ion 15 .4

It is clear that, by increasing RR, the spill current IR can bereduced below any specified relay setting. RR is frequentlyincreased by the addition of a series-connected resistorwhich is known as the stabilising resistor.

It can also be seen from Equation 15.4 that it is only thevoltage drop in the relay circuit at setting current that isimportant. The relay can be designed as a voltagemeasuring device consuming negligible current; andprovided its setting voltage exceeds the value Vf ofEquation 15.4, the system will be stable. In fact, thesetting voltage need not exceed Vf, since the derivationof Equation 15.4 involves an extreme condition ofunbalance between the G and H current transformersthat is not completely realised. So a safety margin isbuilt-in if the voltage setting is made equal to Vf.

It is necessary to realise that the value of If to be insertedin Equation 15.4 is the complete function of the faultcurrent and the spill current IR through the relay, in thelimiting condition, will be of the same form. If the relayrequires more time to operate than the effective durationof the d.c. transient component, or has been designedwith special features to block the d.c. component, thenthis factor can be ignored and only the symmetricalvalue of the fault current need be entered in Equation15.4. If the relay setting voltage, Vs, is made equal to Vf,that is, If (RL + RCT), an inherent safety factor of theorder of two will exist.

In the case of a faster relay, capable of operating in onecycle and with no special features to block the d.c.component, it is the r.m.s. value of the first offset wavethat is significant. This value, for a fully offset waveformwith no d.c. decrement, is √

_3If. If settings are then

chosen in terms of the symmetrical component of thefault current, the √

_3 factor which has been ignored will

take up most of the basic safety factor, leaving only avery small margin.

Finally, if a truly instantaneous relay were used, therelevant value of If would be the maximum offset peak.In this case, the factor has become less than unity,possibly as low as 0.7. It is therefore possible to rewriteEquation 15.4 as:

…Equat ion 15 .5

where:

ISL = stability of scheme

VS = relay circuit voltage setting

IK V

R RSLS

L CT

= ×+

I R V I R RR R f f LH CTH= = +( )

IV

R

I R R

RRf

R

f LH CTH

R

= =+( ) RL + RCT = lead + CT winding resistance

K = factor depending on relay design(range 0.7 - 2.0)

It remains to be shown that the setting chosen issuitable.

The current transformers will have an excitation curvewhich has not so far been related to the relay settingvoltage, the latter being equal to the maximum nominalvoltage drop across the lead loop and the CT secondarywinding resistance, with the maximum secondary faultcurrent flowing through them. Under in-zone faultconditions it is necessary for the current transformers toproduce sufficient output to operate the relay. This willbe achieved provided the CT knee-point voltage exceedsthe relay setting. In order to cater for errors, it is usualto specify that the current transformers should have aknee-point e.m.f. of at least twice the necessary settingvoltage; a higher multiple is of advantage in ensuring ahigh speed of operation.

15.8.2 Effective Setting or Primary Operating Current

The minimum primary operating current is a furthercriterion of the design of a differential system. Thesecondary effective setting is the sum of the relayminimum operating current and the excitation losses inall parallel connected current transformers, whethercarrying primary current or not. This summation shouldstrictly speaking be vectorial, but is usually donearithmetically. It can be expressed as:

IR = IS +nIeS . . .Equat ion 15 .6

where:

IR = effective setting

IS = relay circuit setting current

IeS = CT excitation current at relay setting voltage

n = number of parallel - connected CT’s

Having established the relay setting voltage fromstability considerations, as shown in Section 15.8.1, andknowing the excitation characteristic of the currenttransformers, the effective setting can be computed. Thesecondary setting is converted to the primary operatingcurrent by multiplying by the turns ratio of the currenttransformers. The operating current so determinedshould be considered in terms of the conditions of theapplication.

For a phase and earth fault scheme the setting can bebased on the fault current to be expected for minimumplant and maximum system outage conditions. However,it should be remembered that:

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a. phase-phase faults give only 86% of the three-phase fault current

b. fault arc resistance and earth path resistancereduce fault currents somewhat

c. a reasonable margin should be allowed to ensurethat relays operate quickly and decisively

It is desirable that the primary effective setting should notexceed 30% of the prospective minimum fault current.

In the case of a scheme exclusively for earth faultprotection, the minimum earth fault current should beconsidered, taking into account any earthing impedancethat might be present as well. Furthermore, in the eventof a double phase to earth fault, regardless of the inter-phase currents, only 50% of the system e.m.f. is availablein the earth path, causing a further reduction in the earthfault current. The primary operating current musttherefore be not greater than 30% of the minimumsingle-phase earth fault current. In order to achievehigh-speed operation, it is desirable that settings shouldbe still lower, particularly in the case of the solidlyearthed power system. The transient component of thefault current in conjunction with unfavourable residualflux in the CT can cause a high degree of saturation andloss of output, possibly leading to a delay of several cyclesadditional to the natural operating time of the element.

This will not happen to any large degree if the faultcurrent is a larger multiple of setting; for example, if thefault current is five times the scheme primary operatingcurrent and the CT knee-point e.m.f. is three times therelay setting voltage, the additional delay is unlikely toexceed one cycle.

The primary operating current is sometimes designed toexceed the maximum expected circuit load in order toreduce the possibility of false operation under loadcurrent as a result of a broken CT lead. Desirable as thissafeguard may be, it will be seen that it is better not toincrease the effective current setting too much, as thiswill sacrifice some speed; the check feature in any case,maintains stability.

An overall earth fault scheme for a large distributionboard may be difficult to design because of the largenumber of current transformers paralleled together,which may lead to an excessive setting. It may beadvantageous in such a case to provide a three-elementphase and earth fault scheme, mainly to reduce thenumber of current transformers paralleled into one group.

Extra-high-voltage substations usually present no suchproblem. Using the voltage-calibrated relay, the currentconsumption can be very small.

A simplification can be achieved by providing one relayper circuit, all connected to the CT paralleling buswires.

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E

Zone RBus wires

Zone

c1

a1

D

Bus wiresCheck zone

same as checkZone relay Zone M1 relay

Zone M1 Zone M2

same as checkZone M2 relay

cc

GFb1

BC

ABCN

M2

c1 c2

H

same as check

95 CHX-2

IdIdI > IdIdI I

+_

Zone R

M1 First main busbarM2 Second main busbarR Reserve busbar

SupervisionRelay

High ImpedanceCirculating Current

g pCirculating Current

g p

RelayMetrosil(non-linear resistor)

Stabilising Resistor

o

Figure 15 .13 : A .C . c i rcuits for h ighimpedance c i rculat ing current scheme for dupl icate busbars

Chap15-232-253 17/06/02 9:48 Page 243

This enables the trip circuits to be confined to the leastarea and reduces the risk of accidental operation.

15.8.3 Check Feature

Schemes for earth faults only can be checked by a frame-earth system, applied to the switchboard as a whole, no

subdivision being necessary. For phase fault schemes,the check will usually be a similar type of scheme appliedto the switchboard as a single overall zone.

A set of current transformers separate from those used inthe discriminating zones should be provided. No CTswitching is required and no current transformers are

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Figure 15.14: D.C. circuits for high impedance circulating current scheme

96In Out M1

CSS-M1

87M1-1

87M2-1

CSS-M1

L1

30 Zone indicating relay74 Alarm cancellation relay80 D.C. volts supervision relay87 High impedance circulating current relay95 Bus wires supervision relay

CSS-M2

In Out

87CH-1+

D1

87R-1

CSS-R

M2 R

D296

E96a1

c1

96F2

c2

b1G

F1

96

96

H196

96H2

D.C. Buswires80T

M2

R30

30M130

87R-2

87M2-2

87M1-287CH-2

M1X95

95M1-1

M2X95

95M2-1

95RX

95R-1

95CHX

95CH-1

30M1-1

74-1

74-2

30M2-1

30R-1

95M1X-1

95M2X-1

95RX-1

95CHX-1

L2

L1

L2

CSS-R

L2

L1CSS-M2

I80

95X Zone bus wires shorting relayCSS Control selector switchL1 Indicating lamp protection in serviceL2 Indicating lamp protection out of service

74

Chap15-232-253 17/06/02 9:48 Page 244

needed for the check zone in bus-coupler and bus-section breakers.

15.8.4 Supervision of CT Secondary Circuits

Any interruption of a CT secondary circuit up to theparalleling interconnections will cause an unbalance inthe system, equivalent to the load being carried by therelevant primary circuit. Even though this degree ofspurious output is below the effective setting thecondition cannot be ignored, since it is likely to lead toinstability under any through fault condition.

Supervision can be carried out to detect such conditionsby connecting a sensitive alarm relay across the buswires of each zone. For a phase and earth fault scheme,an internal three-phase rectifier can be used to effect asummation of the bus wire voltages on to a single alarmelement; see Figures 15.13 and 15.14.

The alarm relay is set so that operation does not occurwith the protection system healthy under normal load.Subject to this proviso, the alarm relay is made assensitive as possible; the desired effective setting is 125primary amperes or 10% of the lowest circuit rating,whichever is the greater.

Since a relay of this order of sensitivity is likely tooperate during through faults, a time delay, typically ofthree seconds, is applied to avoid unnecessary alarmsignals.

15.8.5 Arrangement of CT Connections

It is shown in Equation 15.4 how the setting voltage fora given stability level is directly related to the resistanceof the CT secondary leads. This should therefore bekept to a practical minimum. Taking into account thepractical physical laying of auxiliary cables, the CT buswires are best arranged in the form of a ring around theswitchgear site.

In a double bus installation, the CT leads should be takendirectly to the isolator selection switches. The usualrouting of cables on a double bus site is as follows:

a. current transformers to marshalling kiosk

b. marshalling kiosk to bus selection isolator auxiliaryswitches

c. interconnections between marshalling kiosks toform a closed ring

The relay for each zone is connected to one point of thering bus wire. For convenience of cabling, the main zonerelays will be connected through a multicore cablebetween the relay panel and the bus section-switchmarshalling cubicle. The reserve bar zone and the checkzone relays will be connected together by a cablerunning to the bus coupler circuit breaker marshalling

cubicle. It is possible that special circumstancesinvolving onerous conditions may over-ride thisconvenience and make connection to some other part ofthe ring desirable.

Connecting leads will usually be not less than 7/0.67mm(2.5mm2), but for large sites or in other difficultcircumstances it may be necessary to use cables of, forexample 7/1.04mm (6mm2) for the bus wire ring and theCT connections to it. The cable from the ring to the relayneed not be of the larger section.

When the reserve bar is split by bus section isolators andthe two portions are protected as separate zones, it isnecessary to common the bus wires by means of auxiliarycontacts, thereby making these two zones into one whenthe section isolators are closed.

15.8.6 Summary of Practical Details

This section provides a summary of practicalconsiderations when implementing a high-impedancebusbar protection scheme.

15.8.6.1 Designed stability level

For normal circumstances, the stability level should bedesigned to correspond to the switchgear rating; even ifthe available short-circuit power in the system is muchless than this figure, it can be expected that the systemwill be developed up to the limit of rating.

15.8.6.2 Current transformers

Current transformers must have identical turns ratios,but a turns error of one in 400 is recognised as areasonable manufacturing tolerance. Also, they shouldpreferably be of similar design; where this is not possiblethe magnetising characteristics should be reasonablymatched.

Current transformers for use with high impedanceprotection schemes should meet the requirements ofClass PX of IEC 60044-1.

15.8.6.3 Setting voltage

The setting voltage is given by the equation

Vs > If (RL + RCT)

where:

Vs = relay circuit voltage setting

If = steady-state through fault current

RL = CT lead loop resistence

RCT = CT secondary winding resistance

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15.8.6.4 Knee-point voltage of current transformers

This is given by the formula

VK ≥ 2Vs

15.8.6.5 Effective setting (secondary)

The effective setting of the relay is given by

IR = IS + nIeSIR

where:

IS = relay circuit current setting

IeS = CT excitation current at voltage setting

n = number of CT’s in parallel

For the primary fault setting multiply IR by the CT turnsratio.

15.8.6.6 Current transformer secondary rating

It is clear from Equations 15.4 and 15.6 that it isadvantageous to keep the secondary fault current low;this is done by making the CT turns ratio high. It iscommon practice to use current transformers with asecondary rating of 1A.

It can be shown that there is an optimum turns ratio forthe current transformers; this value depends on all theapplication parameters but is generally about 2000/1.Although a lower ratio, for instance 400/1, is oftenemployed, the use of the optimum ratio can result in aconsiderable reduction in the physical size of the currenttransformers.

15.8.6.7 Peak voltage developed by current transformers

Under in-zone fault conditions, a high impedance relayconstitutes an excessive burden to the currenttransformers, leading to the development of a highvoltage; the voltage waveform will be highly distortedbut the peak value may be many times the nominalsaturation voltage.

When the burden resistance is finite although high, anapproximate formula for the peak voltage is:


where:

VP = peak voltage developed

VK = knee-point voltage

VF = prospective voltage in absence of saturation

This formula does not hold for the open circuit conditionand is inaccurate for very high burden resistances thatapproximate to an open circuit, because simplifyingassumptions used in the derivation of the formula arenot valid for the extreme condition.

Another approach applicable to the open circuit

V V V VP K F K= −( )2 2

secondary condition is:


where:

If = fault current

Iek = exciting current at knee - point voltage

VK = knee - point voltage

Any burden connected across the secondary will reducethe voltage, but the value cannot be deduced from asimple combination of burden and exciting impedances.

These formulae are therefore to be regarded only as aguide to the possible peak voltage. With large currenttransformers, particularly those with a low secondarycurrent rating, the voltage may be very high, above asuitable insulation voltage. The voltage can be limitedwithout detriment to the scheme by connecting aceramic non-linear resistor in parallel with the relayhaving a characteristic given by:

V = CIβ

where C is a constant depending on dimensions and β isa constant in the range 0.2-0.25.

The current passed by the non-linear resistor at the relayvoltage setting depends on the value of C; in order tokeep the shunting effect to a minimum it isrecommended to use a non-linear resistor with a value ofC of 450 for relay voltages up to 175V and one with avalue of C of 900 for setting voltages up to 325V.

15.8.6.8 High impedance relay

Instantaneous attracted armature relays are used. Simplefast-operating relays would have a low safety factorconstant in the stability equation, Equation 15.5, asdiscussed in Section 15.8.1. The performance is improvedby series-tuning the relay coil, thereby making the circuitresistive in effect. Inductive reactance would tend toreduce stability, whereas the action of capacitance is toblock the unidirectional transient component of faultcurrent and so raise the stability constant.

An alternative technique used in some relays is to applythe limited spill voltage principle shown in Equation15.4. A tuned element is connected via a plug bridge toa chain of resistors; and the relay is calibrated in termsof voltage.

15.9 LOW IMPEDANCE BIASEDDIFFERENTIAL PROTECTION

The principles of low impedance differential protectionhave been described in Section 10.4, including theprinciple advantages to be gained by the use of a bias

VI

IVP

f

ekK= 2

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Chap15-232-253 17/06/02 9:48 Page 246

technique. Most modern busbar protection schemes usethis technique.

The principles of a check zone, zone selection, andtripping arrangements can still be applied. Currenttransformer secondary circuits are not switched directlyby isolator contacts but instead by isolator repeat relaysafter a secondary stage of current transformation. Theseswitching relays form a replica of the busbar within theprotection and provide the complete selection logic.

15.9.1 Stability

With some biased relays, the stability is not assured bythe through current bias feature alone, but is enhancedby the addition of a stabilising resistor, having a valuewhich may be calculated as follows.

The through current will increase the effective relayminimum operating current for a biased relay as follows:

IR = IS + BIF

where:

IR = effective minimum oprating current

IS = relay setting current

IF = through fault current

B = percentage restraint

As IF is generally much greater than IS, the relayeffective current, IR = BIF approximately.

From Equation 15.4, the value of stabilising resistor isgiven by:

It is interesting to note that the value of the stabilisingresistance is independent of current level, and that therewould appear to be no limit to the through faultsstability level. This has been identified [15.1] as ‘ThePrinciple of Infinite Stability’.

The stabilising resistor still constitutes a significantburden on the current transformers during internalfaults.

An alternative technique, used by the MBCZ systemdescribed in Section 15.9.6, is to block the differentialmeasurement during the portion of the cycle that acurrent transformer is saturated. If this is achieved bymomentarily short-circuiting the differential path, a verylow burden is placed on the current transformers. In thisway the differential circuit of the relay is prevented fromresponding to the spill current.

= +R RB

LH CTH

RI R R

IR

f LH CTH

R

=+( )

It must be recognised though that the use of any techniquefor inhibiting operation, to improve stability performancefor through faults, must not be allowed to diminish theability of the relay to respond to internal faults.

15.9.2 Effective Setting or Primary Operating Current

For an internal fault, and with no through fault currentflowing, the effective setting (IR) is raised above thebasic relay setting (IS) by whatever biasing effect isproduced by the sum of the CT magnetising currentsflowing through the bias circuit. With low impedancebiased differential schemes particularly where the busbarinstallation has relatively few circuits, these magnetisingcurrents may be negligible, depending on the value of IS.

The basic relay setting current was formerly defined asthe minimum current required solely in the differentialcircuit to cause operation – Figure 15.15(a). Thisapproach simplified analysis of performance, but wasconsidered to be unrealistic, as in practice any currentflowing in the differential circuit must flow in at leastone half of the relay bias circuit causing the practicalminimum operating current always to be higher than thenominal basic setting current. As a result, a laterdefinition, as shown in Figure 15.15(b) was developed.

Conversely, it needs to be appreciated that applying thelater definition of relay setting current, which flowsthrough at least half the bias circuit, the notional mini-mum operation current in the differential circuit aloneis somewhat less, as shown in Figure 15.15(b).Using the definition presently applicable, the effectiveminimum primary operating current

where:N = CT ratio

= +[ ]∑N I B IS eS

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(a) Superseded definition (b) Current definition

IopIopI IopIopI

ISI

ISI

ISI

I'S

IBIIBI

ISI

IRI = S + BIBI IRI = I + I' I'S

IBIBI

Bias Line (B%)

Bias Line (B%)

= I' B

Figure 15 .15 : Def in i t ions of re laysett ing current for b iased re lays

Chap15-232-253 17/06/02 9:48 Page 247

Unless the minimum effective operating current of ascheme has been raised deliberately to some preferredvalue, it will usually be determined by the check zone,when present, as the latter may be expected to involvethe greatest number of current transformers in parallel.A slightly more onerous condition may arise when twodiscriminating zones are coupled, transiently orotherwise, by the closing of primary isolators.

It is generally desirable to attain an effective primaryoperating current that is just greater than the maximumload current, to prevent the busbar protection fromoperating spuriously from load current should asecondary circuit wiring fault develop. This considerationis particularly important where the check feature is eithernot used or is fed from common main CT's.

15.9.3 Check Feature

For some low impedance schemes, only one set of mainCT's is required. This seems to contradict the generalprinciple of all busbar protection systems with a checkfeature that complete duplication of all equipment isrequired, but it is claimed that the spirit of the checkingprinciple is met by making operation of the protectiondependent on two different criteria such as directionaland differential measurements.

In the MBCZ scheme, described in Section 15.9.6, theprovision of auxiliary CT's as standard for ratio matchingalso provides a ready means for introducing the checkfeature duplication at the auxiliary CT's and onwards tothe relays. This may be an attractive compromise whenonly one set of main CT's is available.

15.9.4 Supervision of CT Secondary Circuits

In low impedance schemes the integrity of the CTsecondary circuits can also be monitored. A currentoperated auxiliary relay, or element of the mainprotection equipment, may be applied to detect anyunbalanced secondary currents and give an alarm after atime delay. For optimum discrimination, the currentsetting of this supervision relay must be less than that ofthe main differential protection.

In modern busbar protection schemes, the supervision ofthe secondary circuits typically forms only a part of acomprehensive supervision facility.

15.9.5 Arrangement of CT connections

It is a common modern requirement of low impedanceschemes that none of the main CT secondary circuitsshould be switched, in the previously conventional manner,to match the switching of primary circuit isolators.

The usual solution is to route all the CT secondarycircuits back to the protection panel or cubicle toauxiliary CT's. It is then the secondary circuits of theauxiliary CT’s that are switched as necessary. Soauxiliary CT's may be included for this function evenwhen the ratio matching is not in question.

In static protection equipment it is undesirable to useisolator auxiliary contacts directly for the switchingwithout some form of insulation barrier. Positiontransducers that follow the opening and closing of theisolators may provide the latter.

Alternatively, a simpler arrangement may be provided onmultiple busbar systems where the isolators switch theauxiliary current transformer secondary circuits viaauxiliary relays within the protection. These relays forma replica of the busbar and perform the necessary logic.It is therefore necessary to route all the currenttransformer secondary circuits to the relay to enablethem to be connected into this busbar replica.

Some installations have only one set of currenttransformers available per circuit. Where the facility ofa check zone is still required, this can still be achievedwith the low impedance biased protection by connectingthe auxiliary current transformers at the input of themain and check zones in series, as shown in Figure 15.16.

15.9.6 Static Low Impedance BiasedDifferential Protection - Type MBCZ

The Type MBCZ scheme conforms in general to theprinciples outlined earlier and comprises a system ofstandard modules that can be assembled to suit aparticular busbar installation. Additional modules can beadded at any time as the busbar is extended.

A separate module is used for each circuit breaker andalso one for each zone of protection. In addition to thesethere is a common alarm module and a number of powersupply units. Ratio correction facilities are providedwithin each differential module to accommodate a widerange of CT mismatch.

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Figure 15.16: Alternative CT connections

Mainzone

Checkzone

Checkzone

Mainzone

Chap15-232-253 17/06/02 9:54 Page 248

Figure 15.17 shows the correlation between the circuitbreakers and the protection modules for a typical doublebusbar installation. In practice the modules are mountedin a multi-tier rack or cubicle.

The modules are interconnected via a multicore cablethat is plugged into the back of the modules. There arefive main groups of buswires, allocated for:

i. protection for main busbar

ii. protection for reserve busbar

iii. protection for the transfer busbar. When thereserve busbar is also used as a transfer bar thenthis group of buswires is used

iv. auxiliary connections used by the protection tocombine modules for some of the more complexbusbar configurations

v. protection for the check zone

One extra module, not shown in this diagram, is pluggedinto the multicore bus. This is the alarm module, whichcontains the common alarm circuits and the bias resistors.The power supplies are also fed in through this module.

15.9.6.1 Bias

All zones of measurement are biased by the total currentflowing to or from the busbar system via the feeders.This ensures that all zones of measurement will havesimilar fault sensitivity under all load conditions. Thebias is derived from the check zone and fixed at 20%with a characteristic generally as shown in Figure15.15(b). Thus some ratio mismatch is tolerable.

15.9.6.2 Stability with saturated current transformers

The traditional method for stabilising a differential relayis to add a resistor to the differential path. Whilst thisimproves stability it increases the burden on the currenttransformer for internal faults. The technique used inthe MBCZ scheme overcomes this problem.

The MBCZ design detects when a CT is saturated andshort-circuits the differential path for the portion of thecycle for which saturation occurs. The resultant spillcurrent does not then flow through the measuring circuitand stability is assured.

This principle allows a very low impedance differentialcircuit to be developed that will operate successfullywith relatively small CT's.

15.9.6.3 Operation for internal faults

If the CT's carrying fault current are not saturated therewill be ample current in the differential circuit to operatethe differential relay quickly for fault currents exceedingthe minimum operating level, which is adjustablebetween 20%-200% rated current.

When the only CT(s) carrying internal fault currentbecome saturated, it might be supposed that the CTsaturation detectors may completely inhibit operation byshort-circuiting the differential circuit. However, theresulting inhibit pulses remove only an insignificantportion of the differential current, so operation of therelay is therefore virtually unaffected.

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Bus coupler 1 Feeder 1 Z1 Z3a Feeder 2 Bus section Feeder 3 Z2 Z3b Feeder 4 Bus coupler 2Checkzone

Zone 1 Zone 2

Zone 3a Zone 3b

Intermodule plug-in buswire connections

Figure 15.17: Type MBCZ busbar protection showing correlationbetween circuit breakers and protection modules

Chap15-232-253 17/06/02 9:54 Page 249

15.9.6.4 Discrepancy alarm feature

As shown in Figure 15.18, each measuring modulecontains duplicated biased differential elements and alsoa pair of supervision elements, which are a part of acomprehensive supervision facility.

This arrangement provides supervision of CT secondarycircuits for both open circuit conditions and anyimpairment of the element to operate for an internalfault, without waiting for an actual system faultcondition to show this up. For a zone to operate it isnecessary for both the differential supervision elementand the biased differential element to operate. For acircuit breaker to be tripped it requires the associatedmain zone to be operated and also the overall checkzone, as shown in Figure 15.19.

15.9.6.5 Master/follower measuring units

When two sections of a busbar are connected togetherby isolators it will result in two measuring elementsbeing connected in parallel when the isolators are closed

to operate the two busbar sections as a single bar. Thefault current will then divide between the two measuringelements in the ratio of their impedances. If both of thetwo measuring elements are of low and equal impedancethe effective minimum operating current of the schemewill be doubled.

This is avoided by using a 'master/follower' arrangement.By making the impedance of one of the measuringelements very much higher than the other it is possible toensure that one of the relays retains its original minimumoperation current. Then to ensure that both the parallel-connected zones are tripped the trip circuits of the twozones are connected in parallel. Any measuring unit canhave the role of 'master' or 'follower' as it is selectable bymeans of a switch on the front of the module.

15.9.6.6 Transfer tripping for breaker failure

Serious damage may result, and even danger to life, if acircuit breaker fails to open when called upon to do so.To reduce this risk breaker fail protection schemes weredeveloped some years ago.

These schemes are generally based on the assumptionthat if current is still flowing through the circuit breakera set time after the trip command has been issued, thenit has failed to function. The circuit breakers in the nextstage back in the system are then automatically tripped.

For a bus coupler or section breaker this would involvetripping all the infeeds to the adjacent zone, a facilitythat is included in the busbar protection scheme.

• 15 •

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ar P

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Main zone

S2

S1

+ veD2

D1

Trip

S1

S2

Check zone

D2

D1

Figure 15 .19 : Busbar protect ion t r ip logic

F igure 15 .18 : B lock d iagram of measur ing unit

Enable

Diff

eren

tial

m

Bias

t

r

Enable

DifferentialBiased

DifferentialBiased

SelectionLinks

c

CurrentBuswire

Supervision

Supervision

Out of service

Trip

Protectionfault

m

t

r

TripLinksSelection

c

BuswireTrip

CT Fault

OR

r = Reserve

Alarm

t = Transfer

m = Mainc = Check

1

1

1

=1

=1

Chap15-232-253 17/06/02 9:54 Page 250

15.10 NUMERICAL BUSBAR PROTECTIONSCHEMES

The application of numerical relay technology to busbarprotection has lagged behind that of other protectionfunctions. Static technology is still usual for suchschemes, but numerical technology is now readilyavailable. The very latest developments in thetechnology are included, such as extensive use of a databus to link the various units involved, and fault toleranceagainst loss of a particular link by providing multiplecommunications paths. The development process hasbeen very rigorous, because the requirements for busbarprotection in respect of immunity to maloperation arevery high.

The philosophy adopted is one of distributed processing ofthe measured values, as shown in Figure 15.20. Feederseach have their own processing unit, which collectstogether information on the state of the feeder (currents,voltages, CB and isolator status, etc.) and communicatesit over high-speed fibre-optic data links to a central unit.For large substations, more than one central unit may beused, while in the case of small installations, all of theunits can be co-located, leading to the appearance of atraditional centralised architecture.

For simple feeders, interface units at a bay may be usedwith the data transmitted to a single centrally located peripheral unit. The central unit performs thecalculations required for the protection functions.Available protection functions are:

a. protection

b. backup overcurrent protection

c. breaker failure

d. dead zone protection

In addition, monitoring functions such as CB and isolatormonitoring, disturbance recording and transformersupervision are provided.

Because of the distributed topology used,synchronisation of the measurements taken by theperipheral units is of vital importance. A high stabilitynumerically-controlled oscillator is fitted in each of thecentral and peripheral units, with time synchronisationbetween them. In the event of loss of thesynchronisation signal, the high stability of the oscillatorin the affected feeder unit(s) enables processing of theincoming data to continue without significant errorsuntil synchronisation can be restored.

The peripheral units have responsibility for collecting therequired data, such as voltages and currents, andprocessing it into digital form for onwards transmissionto the central unit. Modelling of the CT response isincluded, to eliminate errors caused by effects such as CTsaturation. Disturbance recording for the monitoredfeeder is implemented, for later download as required.Because each peripheral unit is concerned only with anindividual feeder, the protection algorithms must residein the central unit.

The differential protection algorithm can be much moresophisticated than with earlier technology, due toimprovements in processing power. In addition tocalculating the sum of the measured currents, thealgorithm can also evaluate differences betweensuccessive current samples, since a large change above athreshold may indicate a fault – the threshold beingchosen such that normal load changes, apart from inrushconditions do not exceed the threshold. The same

• 15 •Bu

sbar

Pro

tect

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Figure 15.20: Architecture for numerical protection scheme

PersonalComputer

PU

CT

CBPU CB

CT

PU CB

CT

Feeder 1 Feeder 2

CUCentral Unit

PU

Fibre optic link

System Communication Network

PU: Peripheral UnitCU: Central Unit

CB

CT

Chap15-232-253 17/06/02 9:54 Page 251

considerations can also be applied to the phase angles ofcurrents, and incremental changes in them.

One advantage gained from the use of numericaltechnology is the ability to easily re-configure theprotection to cater for changes in configuration of thesubstation. For example, addition of an extra feederinvolves the addition of an extra peripheral unit, thefibre-optic connection to the central unit and entry viathe MMI of the new configuration into the central unit.Figure 15.21 illustrates the latest numerical technologyemployed.

15.10.1 Reliability Considerations

In considering the introduction of numerical busbarprotection schemes, users have been concerned withreliability issues such as security and availability.Conventional high impedance schemes have been one ofthe main protection schemes used for busbar protection.The basic measuring element is simple in concept andhas few components. Calculation of stability limits andother setting parameters is straightforward and schemeperformance can be predicted without the need forcostly testing. Practically, high impedance schemes haveproved to be a very reliable form of protection.

In contrast, modern numerical schemes are morecomplex with a much greater range of facilities and amuch high component count. Based on low impedancebias techniques, and with a greater range of facilities toset, setting calculations can also be more complex.

However, studies of the comparative reliability ofconventional high impedance schemes and modernnumerical schemes have shown that assessing relativereliability is not quite so simple as it might appear. Thenumerical scheme has two advantages over its oldercounterpart:

a. there is a reduction in the number of externalcomponents such as switching and other auxiliaryrelays, many of the functions of which areperformed internally within the softwarealgorithms

b. numerical schemes include sophisticatedmonitoring features which provide alarm facilitiesif the scheme is faulty. In certain cases, simulationof the scheme functions can be performed on linefrom the CT inputs through to the tripping outputsand thus scheme functions can be checked on aregular basis to ensure a full operational mode isavailable at all times

Reliability analyses using fault tree analysis methodshave examined issues of dependability (e.g. the ability tooperate when required) and security (e.g. the ability notto provide spurious/indiscriminate operation). Theseanalyses have shown that:

a. dependability of numerical schemes is better thanconventional high impedance schemes

b. security of numerical and conventional highimpedance schemes are comparable

In addition, an important feature of numerical schemesis the in-built monitoring system. This considerablyimproves the potential availability of numerical schemescompared to conventional schemes as faults within theequipment and its operational state can be detected andalarmed. With the conventional scheme, failure to re-instate the scheme correctly after maintenance may notbe detected until the scheme is required to operate. Inthis situation, its effective availability is zero until it isdetected and repaired.

15.11 REFERENCES

15.1 The Behaviour of Current Transformers subjectedto Transient Asymmetric Currents and theEffects on Associated Protective Relays. J.W.Hodgkiss. CIGRE Paper Number 329, Session15-25 June 1960.

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Figure 15.21: Busbar protection relay using thelatest numerical technology (MiCOM P740 range)

Busb

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15 busbar protection

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