Station Bus Protection

11

Station-Bus Protection

Revised by: SOLVEIG WARD

1 INTRODUCTION

A bus is a critical element of a power system, as it is thepoint of convergence of many circuits, transmission,generation, or loads. The effect of a single bus fault isequivalent to many simultaneous faults and usually,due to the concentration of supply circuits, involveshigh-current magnitudes. High-speed bus protection isoften required to limit the damaging effects onequipment and system stability or to maintain serviceto as much load as possible. The bus protectiondescribed refers to protection at the bus location,independent of equipment at remote locations.

Differential protection is the most sensitive andreliable method for protecting a station bus. Thephasor summation of all the measured current enteringand leaving the bus must be 0 unless there is a faultwithin the protective zone. For a fault not in theprotective zone, the instantaneous direction of at leastone current is opposite to the others, and the sum ofthe currents in is identical to the sum out. A fault onthe bus provides a path for current flow that is notincluded in these summations. This is called thedifferential current. Detection of a difference exceedingthe predictable errors in the comparison is oneimportant basis for bus relaying.

In dealing with high-voltage power systems, therelay is dependent on the current transformers in theindividual circuits to provide information to it regard-ing the high-voltage currents. Figure 11-1 showstypical examples of the location of current transfor-mers that are used for this purpose. The arrowheadsindicate the reference direction of the currents.

1.1 Current Transformer Saturation Problemand Its Solutions on Bus Protection

Bus differential relaying is complicated by the fact thatfor an external fault on one circuit, all of the othercircuits connected to the bus contribute to that fault.The current through the circuit breaker for the faultedcircuit will be substantially higher than that for any ofthe other circuits. With this very high current flowingthrough the current transformer and its circuit breaker,there is a very high likelihood that some degree ofsaturation will occur. A saturated current transformerwill not deliver its appropriate current to the bus relay.With the lower currents in the other circuits for thisexternal fault, the degree of saturation is expected to beconsiderably lower. This may lead to a large differ-ential current that will tend to cause the relay to sensean internal fault rather than the actual external faultthat exists. The relay must accommodate this errorcurrent without misoperation.

A widely used equivalent diagram for a currenttransformer appears in Figure 11-3b. It consists of aperfect transformation from the high current side tothe low current side (e.g., 600:5). All of the significantimperfections are lumped into Rp, Rs, and Xm. The Rsrepresents the internal secondary resistance of the ct(current transformer), and the X represents a currentpath that accommodates the exciting requirements.The ct is assumed to have a uniformly distributedwinding and, therefore, to manifest no significantleakage reactance.

When the ct is subjected to excessive flux, the ct issaid to ‘‘saturate,’’ meaning that the core of the ct has

Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.

Figure 11-1 Common bus arrangements with relay input sources.


been forced to carry more flux than it can handle. Theflux then spills into the area surrounding the core,causing the magnetizing reactance to have a muchlower value than normal. It can be seen that anycurrent that flows in X subtracts from the perfectlytransformed current, producing a deficiency in thecurrent that is delivered to the devices connected to thect. The black blocks are the polarity markers. A singlepolarity marker has no significance. With two, it isacknowledged that, at the instant of time at whichcurrent is flowing into the polarity marker on the highcurrent side of the ct, current is flowing out of thepolarity marker on the low current side. Of course, thecurrent reverses every half cycle, but both the high andlow reverse together.

Direct current saturation is much more serious thanac saturation because a relatively small amount of dcfrom an asymmetrical fault wave will saturate thecurrent transformer core and appreciably reduce thesecondary output. The L/R ratio of the power-systemimpedance, which determines the decay of the dccomponent of fault current, should strongly influencethe selection of the bus protective relaying. Typically,the dc time constants for the different circuit elementscan vary from 0.01 sec for lines to 0.3 sec or more forgenerating plants. The nearer a bus location is to astrong source of generation, the greater the L/R ratioand the slower the decay of the resulting dc componentof fault current.

Of the several available methods for solving theunequal performance of current transformers, four arein common use:

1. Eliminating the problem by eliminating iron inthe current transformer [a linear coupler (LC)system]

2. Using a multirestraint, variable-percentage dif-ferential relay which is specifically designed tobe insensitive to dc saturation (CA-16 relaysystem)

3. Using a high impedance differential relay with aseries resonant circuit to limit sensitivity to ctsaturation (KAB relay system)

4. Using a Differential Comparator relay withmoderately high impedance to limit sensitivityto ct saturation (RED-521)

1.2 Information Required for the Preparation ofa Bus Protective Scheme

Some bus protection schemes rely on the operation ofa remote breaker. It is simple and economic, but slow

(zone-2 trip) and may interrupt unnecessarily a tappedload. When local bus protection is applied, thefollowing information is required for the schemeselection, relay selection, and setting calculations:

1. Information about the bus configuration isrequired. The common bus arrangements areas shown in Figure 11-1, such as single bus,double bus, main-and-transfer bus, ring bus,breaker and a half, bus tie-breaker, double-bus-single-breaker, etc.

2. Maximum and minimum bus fault currents(single-phase-to-ground fault and three-phasefault)

3. Current transformer information, including

Current transformer locationCurrent transformer ratiosCurrent transformer accuracy classCurrent transformer saturation curves

4. Operating speed requirement

1.3 Normal Practices on Bus Protection

The normal practices on bus protection are

1. There is one set of bus relays per bus section.2. Use a dedicated ct for bus differential protec-

tion. If possible, the connection of meters,auxiliary ct’s, and other relays in differential-type bus schemes should be avoided since thesedevices introduce an additional burden into themain circuit.

3. Lead resistance, as well as ct winding resistance,contributes to ct saturation. Therefore, thelength of secondary lead runs should be heldto a minimum.

4. Usually, the full-ct secondary winding tapshould be used. This has two advantages. Itminimizes the burden effect of the cable and,second, leads by minimizing the secondarycurrent and makes use of the full-voltagecapability of the ct.

5. Normally, there is no bus relay required for thetransfer bus on a main-and-transfer busarrangement. The transfer bus is normallydeenergized and will be included in the mainbus section when it is energized.

6. No bus relay is required for a ring bus becausethe bus section between each pair of circuitbreakers is protected as a part of the connectedcircuit.


7. Special arrangements should be considered ifthere is any other apparatus, such as stationservice transformers, capacitor banks, ground-ing transformers, or surge arresters, inside thebus differential zone.

8. There is no simple scheme available for adouble-bus-single-breaker arrangement (Fig.11-1e), because its current transformers arenormally located on the line side. Theseapplications greatly benefit from numericalschemes, such as the RED-521. (Refer to Sec.9 of this chapter for more information.)

2 BUS DIFFERENTIAL RELAYING WITHOVERCURRENT RELAYS

2.1 Overcurrent Differential Protection

This differential scheme requires that a time-over-current relay be paralleled with all of the currenttransformers for a particular phase, as shown inFigure 11-2. It is permissible to use auxiliary ct’s tomatch ratios, but it is preferred that all of the ct’s havethe same ratio on the tap chosen and that the use ofauxiliary ct’s be avoided.

In this scheme, the overcurrent relay must be set tooverride the maximum error current that results froman external fault (phase or ground). It may also benecessary to have sufficient time delay to refrain fromtripping during the time that one or more of thecurrent transformers is severely saturated by the dccomponent of the primary current. To assure this,using a simple overcurrent relay, the current transfor-mers must be chosen to have no more than 20 timesrated current flowing in their primary for the worst-case external fault, and each have a burden no morethan the rated value (relaying-accuracy-class voltage/100). The operating time of the relay must not be less

than three primary time constants, and its setting mustbe greater than the exciting current of the currenttransformer under worst-case conditions. This mayrequire a setting of 10 or more amperes and a timesetting of, say, 18 cycles. These values may beacceptable for smaller substation buses, but moresophistication and faster relaying speed are generallymandatory for more extensive and higher-voltagebuses.

In these applications a ‘‘short time’’ or ‘‘extremelyinverse’’ characteristic overcurrent relay is used in theinterests of getting faster tripping speeds at highcurrent. Operating times of 8 to 18 cycles are expected.Although the relay cost is low, the engineering costmay be high because of the usual need for considerablestudy for the application to assure correct operation.

2.2 Improved Overcurrent DifferentialProtection

The sensitivity of the overcurrent differential scheme(Fig. 11-2) can be improved by externally connecting aseries resistor with each overcurrent relay, as shown inFigure 11-3. These resistors are called stabilizingresistors. If we assume that an external fault causesthe ct on the faulted feeder to be saturated completely,the ct excitation reactance will approach 0. As shownin Figure 11-3, the error current Id that flows through

Figure 11-2 The overcurrent differential bus protection.

Figure 11-3 The improved overcurrent differential bus

protection.


the overcurrent unit would be

Id ¼ IF2RL þRS

2RL þRS þRd

� �ð11-1Þ

where Rd is the resistance in the differential path.In order to reduce the error current Id in the

differential path for improving the sensitivity of thescheme, the most effective way is to increase the valueof Rd. The limitations of this additional resistance aredetermined by (1) the overvoltage to the ct circuit and(2) the minimum available internal fault current. Itshould be limited to

Rd ¼ VCL

46IminpickupO ð11-2Þ

Note: The multiplier 4 includes a safety factor of 2.

3 MULTIRESTRAINT DIFFERENTIAL SYSTEM

Multirestraint differential schemes use conventionalcurrent transformers, which may saturate on heavyexternal faults. For this reason, the secondary currentoutput may not represent the primary. In a differentialscheme, the current transformers and relay function asa team. When the current transformers do not performadequately, the relay can within limits make up for thedeficiency.

The multirestraint differential scheme uses the CA-16 variable-percentage differential relay, which con-sists of three induction restraint units and oneinduction operating unit per phase. Two of the unitsare placed opposite each other and operate on acommon disc. In turn, the two discs are connected to acommon shaft with the moving contacts. All four ofthe units are unidirectional; that is, current flow ineither direction through the windings generates con-tact-opening torque for the restraint units or contact-closing torque for the operating unit. Each restraintunit (called R, S, and T) also has two windings toprovide restraint proportional to the sum or difference,depending on the direction of the current flow. If thecurrents in the two paired windings are equal andopposite, the restraint is cancelled. Thus, the pairedrestraint windings have a polarity with respect to eachother. With this method six restraint windings areavailable per phase.

In addition to providing multiple restraint, thevariable-percentage characteristic helps in overcomingcurrent transformer errors. At light fault currents,current transformer performance is good, and the

percentage is small for maximum sensitivity. For heavyexternal faults, current transformer performance islikely to be poor, and the percentage is large. Thevariable-percentage characteristic is obtained by ener-gizing the operating unit through a built-in saturatingautotransformer.

The saturating autotransformer also presents a highimpedance to the false differential current, which tendsto limit the current through the operating coil and toforce more equal saturation of the current transfor-mers. On internal faults, in which a desirable highdifferential current exists, saturation reduces theimpedance. A further advantage of the saturatingautotransformer is that it provides a very effectiveshunt for the dc component, appreciably reducing thedc sensitivity of the operating units. At the minimumpickup current of 0.15+ 5% A, the restraining coilsare ineffective.

When using the CA-16 relay, the current transfor-mers should not saturate when carrying the maximumexternal symmetrical fault current; that is, the excitingcurrent should not exceed one secondary ampere rms.This requirement is met if the burden impedance doesnot exceed

½NPVCL � ðIEXT � 100Þ�RS

1:33 IEXTð11-3Þ

where

NP¼ proportion of total current transformerturns in use

VCL¼ current transformer accuracy-class voltageIEXT¼maximum external symmetrical fault current

in secondary (amperes rms) (use IEXT¼ 100if IEXT< 100)

RS¼ current transformer secondary winding resis-tance of the turns in use (in ohms); forexample, if the 400:5 tap of a 600:5 wye-connected class C200 current transformer isused, then NP¼ 400/600¼ 0.67 andVCL¼ 200

If IEXT¼ 120A and RS¼ 0.5O, then the burden ofthe ct’s secondary circuit, excluding current transfor-mer secondary winding resistance, should not exceed

0:676200� ð120� 100Þ0:51:336120

¼ 0:78O

Settings for the CA-16 relay need not be calculated.Field experience indicates that one CA-16 relay perphase is satisfactory for the vast majority of applica-tions.


External connections are as shown in Figures 11-4through 11-6. Figure 11-4 may be used if only threecircuits are involved. The term circuit refers to a sourceor feeder group.

When several circuits exist and the bus can bereduced to four circuits, then the scheme of Figure 11-5may be used. For example, assume a bus consists oftwo sources and six feeders, and that the feeders arelumped into two groups. The bus now reduces to fourcircuits.

In paralleling current transformers, each feedergroup must have less than 14 A load current (restraintcoil continuous rating).

If the bus reduces to more than four circuits, thenthe scheme of Figure 11-6 should be used. In applyingthe scheme of Figure 11-6, each primary circuit mustbe identified as either a source or feeder. As definedhere, a feeder contributes only a small portion of thetotal fault current for a bus fault. All other circuits aresources. Next, a number of feeders are lumped into afeeder group by paralleling feeder current transfor-mers. Each feeder group must have less than 14 A loadcurrent and not contribute more than 10% of the totalphase- or ground-fault current for a bus fault. Thenconnect the ‘‘source’’ and ‘‘feeder groups’’ alternatelyas shown in Figure 11-6.

Note that in Figures 11-4 through 11-6, electro-magnets R, S, and T are referred to. Each of theseelements has two windings. The polarity markings areextremely significant as related to one another on thesame electromagnet, but have no significance withrespect to one another on different electromagnets. Ifthe current into a polarity marker is equal to the

Figure 11-6 Connection of one CA-16 relay per phase to

protect a bus with six equivalent circuits. (Connections for

one phase only are shown.)


protect a bus with four equivalent circuits. (Connections for



protect a bus with three equivalent circuits. (Connections for



current out of the polarity marker on the sameelectromagnet, there will be no restraining torqueproduced by that electromagnet. The sum of all of therestraint torques is compared to that produced by theoperating coil. Current into the operating coil circuitproduces a much stronger effect than the same currentthrough a single restraint winding. For an externalfault, there is no current through the operating coil ifthe current transformers perform perfectly. There willbe substantial restraint for this same condition, eventhough in some restraint electromagnets some (or eventotal) cancellation may take place.

Consider a fault on the bus of Figure 11-5 in whichall of the high-voltage circuits contribute the samevalue of current. All of the restraint cancels because ineach of the electromagnets the current into the polaritymarker equals the current out of its paired coil. All ofthe internal fault current (in secondary terms, ofcourse) flows into the operating coil circuit and fasttripping occurs. Practical cases with widely differingfault contributions produce similar effects even thoughconsiderable restraint torque may be present.

Consider, now, an external fault on the upper circuitoff of the bus with the equal fault current contributionsthat were assumed in the previous case. Torquecancellation occurs in electromagnet T, as before.Substantial restraint torque is produced by R and S.The operating coil current cannot exceed the errorcurrent in the faulted circuit (which may well beextreme due to the effect of saturation).

This is a very sensitive bus relaying scheme, and it isvery secure against operation for external faults eventhough severe ct saturation may occur for one or morect’s. It is reasonably fast. Another advantage is that itcan accept auxiliary ct’s in the circuit, which allowsdifferent ratios of the main ct’s. Two shortcomings areits comparative inflexibility as other circuits are addedto the bus and the need to bring all circuits back fromthe switchyard to the relay location.

4 HIGH IMPEDANCE DIFFERENTIAL SYSTEM

Although the high impedance differential scheme alsouses conventional current transformers, it avoids theproblem of unequal current transformer performanceby loading them with a high impedance relay (Fig.11-7).

This arrangement tends to force the false differentialcurrents through the current transformers rather thanthe relay operating coil. Actually, the high impedancedifferential concept comes from the above ‘‘improved

overcurrent differential’’ approach. It uses a highimpedance voltage element instead of ‘‘a low impe-dance overcurrent element plus an external resistor.’’

The high impedance differential KAB relay consistsof an instantaneous overvoltage cylinder unit (V), avoltage-limiting suppressor (varistor), an adjustabletuned circuit, and an instantaneous current unit (IT).

On external faults, the voltage across the relayterminals will be low, essentially 0, unless the currenttransformers are unequally saturated. On internalfaults, the voltage across the relay terminals will behigh and will operate the overvoltage unit. Since theimpedance of the overvoltage unit is 2600O, this highvoltage may approach the open-circuit voltage of thecurrent transformer secondaries. The varistor limitsthis voltage to a safe level.

Since offset fault current or residual magnetismexists in the current transformer core, there is anappreciable dc component in the secondary current.The dc voltage that appears across the relay will befiltered out by the tuned circuit, preventing relaypickup.

The IT current unit provides faster operation onsevere internal faults and also backup to the voltageunit. The range of adjustment is 3 to 48 A.

The KAB relay has successfully performed opera-tions up to external fault currents of 200 A secondaryand down to an internal fault current of 0.27Asecondary. Its typical operating speed is 25 msec.

The overvoltage unit is set by calculating themaximum possible voltage for an external fault asfollows:

VR ¼ KðRS þRLÞ IFN

ð11-4Þ

Where

VR¼ pickup setting of the V unit in volts rmsRS¼ dc resistance of current transformer secondary

winding, including internal leads to bushingterminals

RL¼ resistance of lead from junction points to themost distant current transformer (one-waylead for phase faults, two-way lead forphase-to-ground faults)

IF¼maximum external primary fault current, inamperes rms, contributed by the bus

N¼ current transformer turns ratioK¼margin factor

The maximum voltage occurs for the external faultwhen the faulted circuit current transformer is


completely saturated, and there is no saturation in thesource current transformers. The maximum voltage isequal to the resistance drop produced by the secondarycurrent through the leads and secondary winding ofthe saturated current transformer. In practice, thefaulted current transformer will never completelysaturate, and the source current transformers willtend to saturate. As a result, the actual maximumvoltage is less than the theoretical value. The marginfactor K, which modifies this voltage, varies directlywith the current transformer saturation factor SF:

1

SF¼ ðRS þRLÞIF

NVkð11-5Þ

where

Vk¼ knee voltage value of the poorest currenttransformer connected to the relay. For type

KAB relay application, the knee voltage isdefined as the intersection of the extension ofthe two straight-line portions of the saturationcurve. The ordinate and abscissa must use thesame scales.

The margin factor curve, shown in Figure 11-8, isbased on tests of the KAB relay in the high-powerlaboratory. A safety factor of 2 has been included inconstructing this curve.

The maximum number of circuits that can beconnected to the relay, or the minimum internal faultcurrent required to operate the relay, can be estimatedfrom the following equation:

Imin ¼ ðXIe þ IR þ IVÞN ð11-6Þ

where

Figure 11-7 External connection of type KAB bus differential relay.


Imin¼minimum primary fault current in amperesrms

Ie¼ secondary excitation current of the currenttransformer at a voltage equal to the settingvalue of the V unit in amperes

IR¼ current in the V unit at setting voltage VR inamperes, that is, IR¼VR/2600

IV¼ current in varistor circuit at a voltage equal tothe setting value of the V unit in amperes(generally negligible)

N¼ current transformer turns ratioX¼ number of circuits connected to the bus

In general, the following factors should be consideredwhen applying a high impedance bus differential relay.

4.1 Factors that Relate to the Relay Setting

The V-unit setting of the KAB relay is based on thecalculated result of Eq. (11–4), which is determined bythe values of K, RS, RL, and IF. In order to keep thissetting value within the available relay range of 75 to400V, it is necessary to keep the values of (RSþRL)and any additional burden in the ct secondary as lowas possible. This includes the consideration of thefollowing:

Use fully distributed winding current transformers,such as bushing ct’s or current transformers withtoroidally wound cores, such as those used inmetal-clad switchgear. These ct’s provide anegligible leakage reactance and therefore donot contribute to the internal impedance in theequivalent circuit of the ct. Only the RS resistanceis needed in series with RL in Eq. (11–4).

The use of auxiliary ct’s is discouraged, though,with proper consideration of their resistance inseries with the lead resistance (raising theeffective RL), they may be used at the sacrificeof some sensitivity of fault recognition. The samecomment applies to the introduction of otherdevices in the current transformer circuits.

The junction point for all of the ct’s in the busdifferential system should be in such a location asto equalize as much as possible the distance fromeach ct to this point. This will minimize RL, thevalue used in the setting calculation and thusallow better sensitivity to be achieved. Departurefrom this requirement is permissible in metal-cladswitchgear because of the comparatively shortdistances usually involved.

The lead resistance from the junction point to therelay terminals is not critical.

Note that with this system total saturation of thecurrent transformer on a circuit feeding an externalfault is allowed and the relay remains secure.

4.2 Factors that Relate to the High-VoltageProblem

All ct’s in the bus differential circuit should beoperated on their full-tap position. Refer toFigure 11-9; a high voltage will be induced on theunused portion of the ct circuit due to auto-transformer action.

All current transformers should have the same ratio.If taps must be used, the windings between the

Figure 11-9 High voltage induced by autotransformer

action.

Figure 11-8 Empirical margin factor for setting the V-unit

of the KAB relay.


taps must be completely distributed, and anyhigh voltage at the full-tap terminal caused byautotransformer action should be checked toavoid insulation breakdown. In general, auxiliaryct’s should not be used to match ratios.

4.3 Setting Example for the KAB Bus Protection

Assume a six-circuit bus for which the maximumexternal three-phase fault current is 60,000A rms,symmetrical; the maximum external phase-to-groundfault current is 45,000A, and the minimum internalfault current is 10,000A. The current transformerratios are 2000:5, ANSI class C400, Vk is 375V. Thesecondary winding resistance RS is 0.93, and one-waylead resistance to junction point RL is 1.07O.

4.3.1 Settings for the V Voltage Unit

For the three-phase fault condition [using Eq. (11-5)],

1

SF¼ ð0:93þ 1:07Þ60,000

4006375¼ 0:8

From Figure 11-8, 1.2>K� 0.82 (use the lower valueof 0.82 for sensitivity); therefore, using Eq. (11-4), weget

VR � 0:82ð0:93þ 1:07Þ 60,000400

¼ 246V

For the phase-to-ground fault condition,

1

SF¼ ð0:93þ 261:07Þ645,000

4006375¼ 0:92

And from Figure 11-8, 1.1>K� 0.77; therefore, usingEq. (11-4) yields

VR � 0:77ð0:93þ 261:07Þ 45,000400

¼ 266V

The minimum setting of the V unit in the KAB relay,therefore, should be 266V, the larger value for eitherthe three-phase or phase-to-ground conditions, ascalculated.

4.3.2 Setting for the IT Current Unit

The IT setting is determined from Figure 11-10. Thehigher value is used as the ordinate as determined fromthe three-phase and phase-to-ground fault. Thus, for

the example, the ordinate value is

Three-phase fault ¼ ð0:93þ 1:07Þ60,000400

¼ 300

Phase-to-ground fault

¼ ð0:93þ 261:07Þ45,000400

¼ 345

From these numbers, it is obvious from Figure 11-10that the IT unit is incapable of operating for anexternal fault. The lowest available setting of 3A willusually be adequate because of the high conductionlevel of present-day varistors. The principal trippingfunction is accomplished at high speed by the voltageunit, and only in extreme circumstances will the IT unitoperate for an internal fault.

5 DIFFERENTIAL COMPARATOR RELAYS

These relays use the fundamental principle described inFigure 11-11. The RADSS is a solid-state version, theREB-103 is similar to this, but the logic is accomplishedwith a microprocessor, while the RED-521 is entirely anumerical relay. All are very high-speed relays (9- to 16-msec tripping) and are very secure against misoperationfor external faults; all reliably and sensitively detectinternal faults and are quite flexible in accommodatingadditional circuits. They may also be used for generatorstator protection and for shunt reactor protectionthough their prime application area is for bus protection.

The RADSS and the REB-103 relays use externalauxiliary current transformers which allow substan-

Figure 11-10 Setting of KAB instantaneous unit.


tially different main circuit current transformers to beaccommodated and also reduce current to a suitablelevel for the relay. The RED-521, being a micropro-cessor relay, is able to accept widely varying inputsfrom the main current transformers and to provide,internally, the appropriate scaling factors. The RED-521 is therefore very suitable for double-bus-single-breaker arrangements as no external ct switching takesplace. The ct is connected to the appropriate protectionzone numerically inside the relay.

Taking advantage of Kirchoff’s law, the schemecompares the sum of all of the currents entering thebus with the sum of all of the currents leaving the bus.These are instantaneous currents (as opposed to rms oraverage currents.)

In the circuit of Figure 11-11, the currents aredelivered to the relay through the diodes. The sum ofthe currents through the lower group of diodes isrepresentative of the instantaneous sum of the incom-ing currents to the bus, and the current flowing to theupper group of diodes is representative of theinstantaneous sum of the currents leaving the bus.These two sum currents are always in perfect balanceprovided the current transformers perform their jobfaithfully and there is no fault on the bus (or to state itmore correctly, provided there are no current paths offof the bus that are unaccounted for).

If an internal fault (phase or ground) were to occur,the currents in and out would no longer match. Theywould differ by the amount of the fault current. Thisdifference current appears as I DIFF in the relay.

To accommodate the inherent errors in the currenttransformers for an external fault, particularly in the ct

associated with the circuit on which the external faultoccurred, restraint is developed across the resistor Rs.

Any condition that produces I DIFF current will,through the transformer and the full-wave bridge,generate a voltage Vd3. For the through fault case, therestraint voltage Vs will exceed the operating voltageVd3, and the relay will refrain from operating. For theinternal fault case, I DIFF will be large, Vd3 will exceedVs, current will be passed through the diode and thereed relay DR, and tripping will occur. SR is a ‘‘start’’relay whose contact supervises tripping to add to theoverall security of the relay. It is obvious that this relayis extremely fast because the decision to trip is basedon instantaneous currents.

The RED-521 numerical relay uses this principle,but is not encumbered by need for the auxiliarymatching current transformers, the diodes, or anyother of the components inherently required in thecomparison process. The individual samples of cur-rents are collected and summed appropriately todevelop numerically the I IN and I OUT values andthe corresponding restraint quantity. This is comparedwith the difference of these individual sums, I DIFF,and a determination of the need to trip is established.

6 PROTECTING A BUS THAT INCLUDES ATRANSFORMER BANK

Ideally, when the bus includes a power transformerbank, separate protection should be provided for thebus and transformer, even though both protectionschemes must trip all breakers around the two units.

Figure 11-11 Differential comparator relay.


Such a system offers maximum continuity of service,since faults are easier to locate and isolate. Also, usinga bus differential relay for bus protection andtransformer differential relay for transformer protec-tion provides maximum sensitivity and security withminimum application engineering.

However, economics and location of current trans-formers often dictate that both units be protected inone differential zone. For these applications, either themultirestraint HU-4 or CA-26 relays should be used.

TheHU-4 relay is similar to theHUandHU-1 relays,except that it has four restraint windings. Also, therectified outputs of the restraint transformers areconnected in series, providing a higher restraint forcewhen a through fault occurs on the bus. Since the dcsaturation of current transformers will allow current topass into theHRUtransformers andpossibly pickup theIIT, the IIT unit of the HU-4 relay is set at 15 times therms tap value toprevent false tripping for external faults.

Similar to the CA-16, the CA-26 relay has astronger contact spring and higher pickup of1.25+ 5% A to help override inrush. Its variablerestraint curve is steeper than the CA-16, and itsoperating time is approximately three cycles.

Of the two types, the HU-4 relay is preferred, as it isimmune from operation on transformer magnetizinginrush. The HU-4 should always be applied for largetransformer banks or those associated with HV andEHV buses. A typical application, shown in Figure 11-12, protects a three-winding transformer bus with fourcircuits. Figure 11-13 illustrates another typical appli-cation used in EHV systems.

The CA-26 relay is applicable to relatively smalltransformers remote from generating stations, HV,and EHV buses. Here, inrush will usually be light andnot cause the CA-26 to operate. If, however, completesecurity against inrush is required, the HU-4 must beapplied.

With CA-26 relays, the four-circuit bus connectionsof Figure 11-5 are not recommended for bus protec-tion, since the relay may have too much restraint for abus fault.

The bus CA-16 relay should not be used for thetransformer differential, since it is too sensitive tooverride magnetizing inrush.

7 PROTECTING A DOUBLE-BUS SINGLE-BREAKER WITH BUS TIE ARRANGEMENT

The double-bus single-breaker with bus tie (Figure 11-1e) provides economic and operating flexibility com-parable to the double-bus double-breaker arrangement(Fig. 11-1c). However, the ct’s are normally on theline-side location, which results in increased differen-tial relaying problems. Two different approaches havebeen used in the bus protection of such arrangements:the fully switched scheme (Fig. 11-14) and theparalleling switch scheme (Fig. 11-15). They are bothcomplicated (inserting switch contacts in the ctcircuits) and/or imperfect in protection. These schemeseither require switching ct’s and/or disabling the busprotection before any switching operation. This is aperiod when the probability of a bus fault occurring ishigh and it is most desirable that the bus protection bein service. A third scheme as shown in Figure 11-16 canbe considered. It is similar to the paralleling switchedscheme except a check-zone relay is added as shown.

Figure 11-12 Typical application of HU-4 relay for

protecting a large transformer bank associated with HV

and EHV buses. (Auxiliary current transformers for ratio

matching are not shown.)

Figure 11-13 Protection of a typical transformer section

where the transformer tertiary is brought out for load or

connected to an external source.


Figure 11-14 Fully switched scheme.

Figure 11-15 Paralleled switched scheme.


Two bus differential zones are provided, one for eachbus, with each one overlapping the bus breaker. Eachprimary circuit is normally switched to a specific bus,and relay input circuits and breaker control circuits arewired accordingly. The additional check-zone devicesupervises the trip circuits. If it becomes necessary toclear one of the buses, all the primary circuits may beswitched to the opposite bus and it is needless todisable the bus protection before any switchingoperation. However, this scheme still has two draw-backs when any one or all of the primary circuits isswitched to the opposite bus: (1) It will lose itsselectivity, and (2) it will reduce its sensitivity sincethe two relays are paralleled.

A numerical scheme, such as RED-521, overcomesthese drawbacks as there is no external ct switchinginvolved. The ct’s are connected to the appropriatezone by numerical switching in the relay.

8 OTHER BUS PROTECTIVE SCHEMES

Other methods for protecting buses are in limited use:(1) partial differential schemes, (2) directional compar-ison relaying, and (3) the fault-bus method. Except forthe latter, these schemes are most often applied aseconomic compromises for the protection of buses.

8.1 Partial Differential Relaying

This type of protection is also referred to as ‘‘busoverload’’ or ‘‘selective backup’’ protection. It is avariation of the differential principle in which currents

in one or more of the circuits are not included in thephasor summation of the current to the relay.

In this scheme, only the source circuits aredifferentially connected, as shown in Figure 11-17b,using a high-set overcurrent relay with time delay. Thect’s protecting the feeders or circuits are not in thedifferential connection.

Essentially, this arrangement combines time-delaybus protection with feeder backup protection. Thesensitivity and speed of this scheme are not as good aswith complete differential protection. This methodmay be used as a backup to a complete differentialscheme, as primary protection for a station with loadsprotected by fuses, or to provide local breaker failureprotection for load breakers.

In modern microprocessor systems, provision hasbeen included to allow communications between thefeeder breaker relaying and the source breaker relaying.The feeder breakers are each equipped with a nontrip-

Figure 11-16 Paralleled switch with check zone scheme.

Figure 11-17 Partial differential protection.


ping low-set instantaneous overcurrent function that isset somewhat above their maximum load. The sourcebreakers have an instantaneous overcurrent unit withslight time delay that is set above the maximum totalload current for the bus, and they are equipped toreceive a status input from the feeder breakers. For afault on one of the feeder circuits, the low-setinstantaneous overcurrent unit operates and applies ablock signal to the source relay. The instantaneous unitof the source breaker operates, but is unable to tripbecause of the block signal. The time-delayed andcoordinated tripping of the source breaker is notaffected so its backup function stays intact.

For a bus fault, the block signal is absent, andtripping of the source breaker occurs at high speed.

Some partial differential circuits use distance-typerelays in the scheme. The use of a distance relay for thisscheme produces both faster and more sensitiveoperation than the overcurrent scheme.

8.2 Directional Comparison Relaying

Occasionally, it is desirable to add bus protection to anolder substation where additional ct’s and control cableare too costly to install. In this instance, the existing ctcircuits used for line relaying can also be used for thedirectional comparison bus relaying protection.

As shown inFigure 11-18, the directional comparisonrelaying uses individual directional overcurrent relayson all sources and instantaneous overcurrent relays onall feeders. The directional relays close contacts whenfault power flows into the bus section. Back contacts onthe overcurrent relays open when the fault is external onthe feeder.All contacts are connected in series, andwhenthe fault occurs on the bus, the trip circuit is energizedthrough a timer. A time delay of at least four cycles willallow all the relays to decide correctly the directionof thefault and to permit contact coordination.

In this scheme, the ct’s in each circuit do not requirethe same ratio and can be used for other forms ofrelaying and metering.

The disadvantage of this scheme is the large numberof contacts and complex connections required. There isalso the remote possibility of the directional elementsnot operating on a solid three-phase bus fault as aresult of 0 voltage.

8.3 Fault Bus (Ground-Fault Protection Only)

This method requires that all the bus supportingstructure and associated equipment be interconnected

and have only one connection to ground. An over-current relay is connected in this ground path as shownin Figure 11-19. Any ground fault to the supportingstructure will cause fault current to flow through therelay circuit, tripping the bus through the multiple-contact auxiliary tripping relay. A fault detector,energized from the neutral of the grounded transfor-mer or generator, prevents accidental tripping. Thisscheme requires special construction measures and isexpensive.

Figure 11-18 Directional comparison bus protection.

Figure 11-19 Fault bus.


Documents

Station Bus Protection