ADDENDUM TO

BUSBAR PROTECTION

BY

G A HADLEY

ADDENDUM

Busbar Conflguration

Since the busbar is a maLn connectlngtransformer/distrlbution system, andpower lt is lloportant that it remainsconditions of major equipnent failureVarious factors govern Ehe choice ofimportant being :

a) System Security

b) Operarional Flexibility

c) SinpllciEy of protectlon

d) Ease of Maintenance

e) Ease of Extenslon

f) cost

Page I

link between the supply plant and thesinee lt wlll handle large amounts ofsecure and serviceable even underor busbar fault.

busbar configuration, the most

several switching schemes are avallable and there are many varlants of eachscheme' when selectlng a suitable scheme consideration should be glven tothe ability to take oui any clrcuit breaker or other equipment fornalntenance wlthout renovlng the corresponding circult fron service, alsothe abllity to lsolate ttre uusbar for naintenince, some schemes being moreflexlble than others in this respect.

rn addltlon to plarn singre and doubre busbar schemes, the forlowr.ng aresome of the other more popular arrangements :

l) Double Busbar with Transfer (Refer ro Flg 25A)

wlth thls double busbar variatlon, each feeder has isolators to enableswltchrng to maln or res'erve/transfer uars, -ana

also an addrtlonallsolator to enable the feeder breaker to b; bypassed. The reserve barnay then functlon also as a t.ransfer bar and the bus coupler breakertakes over the function of the feeder breaker to free lt formaintenance.

To apply discrlnlnatlve busbar prot,ectlve, sultable auxillary swLtchesare required on each lsolator to select the crs for the "orr!"a zone,and the trlp clrcuits to the appropriate relays.

Page 2

2) Triple Busbar (Refer ro Fig 258)

This s a double busbar scheme with a third, transfer busbar.under normal conditions all bus section and bus coupler breakers areclosed' During maintenance of a feeder ureatei, the transfer bus isenergised from the selected mar.n or reserve bus by the transfer breakerand Ehe feeder bypass isolator closed on the tiansfer Uar. A11 bussec.ion and bus coupler breakers renaining closed. For busbarprotection isolator auxiliary swrtch"" "rr ..q,rr."a as previously.Mesh Busbar Scheme (Refer ro Fig 26)

The nesh busbar scheme is a frequently used EHV busbar configuration.A transformer and a feeder are linked at each corner of che mesh andfour circuit breakers used to conplete the nesh interconnection thearrangement belng justlfled on the grounds of economy.

The protection shown consrsts of a fully discrininatlve scheme wlth arelay at each corner. A fault at any corner trips the trro breakersassocr-ated with Ehat corner and also initiates any intertrippingnecessary to open clrcult breakers at remote ends.

One and a Half Breaker Scheme (Refer 6 FLg 27)

This is I very popular and econonlcar scheme, three breakers and twofeeders belng arranged between the two busbais. -

under normal conditionsall breakers are- closed. During malntenance of a feeder breaker onrythat breaker would be kept op.rl

3)

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During malntenance of a busbar, allbusbar would remaln open to tsotate

the breakers connected to thatthat busbar.

when busbar Protectlon is requlred, then each busbar is consideredlndividually and a singre busbar scheme applled to each as shown,As rrlththe protectr.on for the rnesh busbar prevlously, the protection schemedoes not require isolator auxiliarles for cr zone selection or ln thetripplng circulEs, the scheme being very simpi., ""0 thrs together withthe operatlonal f1exlbl1lty of tnrJ bus6"r

"I"iiguration accounrs forits popularity.

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Meacurernents

LOW IMPEDANCE BIASED DIFFEREMIAL BUSBAR PROTECTION

I NTRODUCT ION

Busbars are divided inlo a numberor bus sect ion iso l-ators . In theare noE fixed as their boundariesse lect ion iso lators .

of zones bounded by circuit breakerscase of a nulciple busbar these zonesvary according to the position of

1

Each zone of the busbar has its own zone of Protection so thar if a

fault occurs only those circuit breakers defining the boundary of theaffected zone are tripped. For added security the whole busbar systemis treaEed as another zooe of protection. This additional zone isnon-selective and operates for a fault anywhere within the bus zone.This overall zone is used as a check feature in the trip 1ogic.

Before Ehe circrrit breakers bounding a zone can be tripped, Eheprotection for both that zone and the overall check zone rnust haveoperated. This is che dual criterion principle on which manyprotection schemes oPerate.

CONCEPT

The type MBCZ is intended Eo saEisfy the requirements of almost anybusbar system. The design is based on a systen of standard moduleswhich can be assembled to suit a particular busbar installation, and

additional modules can be added at any time as the busbar is extended

A separate module is used for each circuit breaker and also one foreach zone of protection. In addition to these there is a coomn alarmtoodule and a number of power suppty units.

These modules are coded as folloss

MBCZ XX

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is the measured quantityCurrent

IMiscellaneous units

Bus CouplerFeeder BreakerBus Sectlon BreakerBus Se]ectlon IsolatorsBus Sect{on IsolatorZone Measur{ng UnltAlarm UnltTest Un{t

11L213I4t5I6L718

0f each of the module types listed above there are on average twelvevariations to natch the various priurary plant configurations' These

variations are achieved by links on the printed circuit boards or

adding isolator rePeat relaYs.

3.

Page 2.

CONFIGURATION OF A SCHEME

Consider the double busbar arrangement shown in figure 1. Ac the lefthand side we have a bus coupler and the module associated with thiscontains a trip output and the current input circuits.

Similarly the adjacent feeder will have the same requirements as thebus coupler except that in addition there are some isolators. Auxiliaryrelays in the feeder module are operated by auxiliary contacts on theisolators to form a repeat function in a replica circuit of the busbar.

Two measuring units are required for zones 1 and 3a.

These are followed by a second feeder module.

The bus section breaker moduleexcept that in this case'therethe reserve bar.

One extra module, not shownmulticore bus. This is thealarm circuits and the biasin through this module.

3. I Bus Wires

similar to the bus coupler modulea repeat relay for the isolator in

in this diagram, is plugged into thealarm nodule which contains the comrpnresistors. the power supplies are also fed

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Two more feeder modules, a bus coupler module and three measuring unitscomplete the set. The additional measuring unit (zCK) is for theoverall check zone.

The rnodules are then interconnectedinto the back of the modules. Thereand these are normally to be used as

l) Protection for Main busbar

via a multicore cable that plugsare five main groups of buswiresfollows : -

Protection for reserve busbar

Protection for the transfer busbar. When the reserve busbar isalso used as a transfer bar then this grouP of buswires is used.

Auxiliary connections used by the protection to combine modulesfor some of the more complex busbar configurations.

5) Protection for the check zone

3.2 Position of Current Transformers

There rnay be one or tlto sets of current transformers Per circuitbreaker and there is a standardised way of interconnecting the currenttransformers to the protection. This sinplifies the production ofapplication diagraus and ensures polarities will be correct.

2)

3)

4)

Page 3.

Figure 2 shows an example of the sLandard connections for the feederbreaker. This example shows that the current transformers may bemounted on the circuit breaker or the feeder. Their position issignificant since in Ehe former case they are bypassed when the bypassisolator is closed, whereas, in the laEer case they are not. The samefeeder module cannot, therefore, necessarily be used for bothapplications.

3.3 Power Supplies

Either two or three type MSTZO3 power supplies are used to feed theprotection, depending upon the size of the scheme. This gives ameasure of redundancy so that the protection will still function shouldone of the power supplies fail.

A separate porrer supply unit is used to feed the isolator repeatrelays. This is not duplicated because the repeat relays will remainin Lheir last set state. The protection will work correctly even ifthis power supply fails providing the busbar is not switched.

3.4 Cubicle Layout

Figure 3 shows a typical cubicle layout based on the scheme shown infigure l. The power supplies are mounted at the top to improve thermalrating. Underneath these are the measuring units and the alarm module.

The rest of the protection is then folded up so that the bus sectionmodule is under Ehe rneasuring units and the feeder modules run downeach side of the cubicle with the bus couplers at the botton.

4.

4.t

OPERATING PRINCIPLES

Bias

A11 zones of measurement are biased by the total current flowing to orfrom the busbars system via the feeders. This ensures that all zonesof measurement will have sinilar fault sensitivity under all loadcond it ions .

The bias is derived fron the check zone and the setting resistors arehoused in the alarm nodule as this is also comrrln to all zones. Thesetesistors are set to give a fixed steady state bias of 202. Thus some

ratio misnatch is tolerable.

4.2 Stability with Saturated Current Transformers

The traditional method for stabilising a differential relay is to addstabilising resistor to the differential path. I{hilsC this improvesstability it increases the burden on the current transformer forinternal faults. The technique used here overcomes this problen.

Page 4.During a through fault one of the C.T.'s may saturate. It does no!Lherefore provide a balancing current for the other C.T.'s in adifferential system. A differential current will therefore be producedby the unsaturated C.T.'s. The principle used with the new design isto detect when a C.T. is saturated and short circuit the differentialpath for the portion of the cycle for which saturation occurs. Theresultant spil1 current does not then flow through the measuringcircuit and stability is asstrred.

This principle allows a very low inpedance circuit to be developed that,will operate successfully with smal1 C.T.'s.

The circuit shown in figure 4 has two switches connected in parallelwith the differential relay. These switches close for the portion ofthe cycle that the adjacent C.T. is saturated.

For the waveforms shown i^ is distorted due to C.T. saturation. Thedifferential current i. =zi, - i^ and as can be seen is the resultantof the two currents . tt o"l.rrt oznly whilst Ehe saturat ion is present .

The inhibit pulses are produced by a circuit that detects C.T.saturation and it is these pulses that operate the semiconductorswitches. The resultant differential current in the measuring circuitwill be a series of narro\r spikes. The area under the curve beinggreatly reduced, so aiding stability.

4.3 0peration with Saturated Current Transformers

In this case only a single input current is considered. This currenthas waveform distortion due to C.T. saturation. However, thedifferential current is now in phase with the current from thesaturated C.T. so that the resulting inhibit pulses remove aninsignificant portion of the differential current. Operation of therelay is, therefore, relatively unaffected, as shown in figure 5.

4.4 Basic Principle for CT Saturation Detectors

Figure 6 shows the circuit for the saturation detector in a sinplifiedform. The input transformers have three windings, the prinary andsecondary windings are used as a current transfer in the usual way. Ttre

tertiary is used as a voltage winding and its output is proportional tothe rate of change of flux in the core. The voltage is then rectifiedand a capacitor is charged via diode to the peak value.

A potential divider across the capacitor provides long time constantdischarge path and divides the stored voltage by 2. A comparatorcompares the unsmoothed voltage with the outPut of the potentialdivider. An output pulse is produced when the anplitude of the voltagewaveform is less than one half of the stored value. Ttre outPut pulseincreases in width when the transformer goes into saturation.

Page 5.

4.5 Master/Fo1lower Measuring Units

When two sections of busbar are connected together rria isolators(figure 7) it will result in two measuring elements being connected inparal1e1. The fault current will chen divide between t.he two ueasuringelenents in the ratio of their impedances. Thus, for two measuringelements of equal impedance Ehe effective settings will be doubled.

This problem has been overcome by using a I'IASTER/FOLLOWER arrangement.By making the impedance of one of the measuring elements very uuchhigher than the other it is possible to ensure that one of the relaysretains its original setting. Then to ensure that both the parallelconnected zones are tripped the trip circuits of the two zones areconnected in para11e1.

Any measuring unit can have the role of master or follower as it isselectable by means of a switch on the front of the module.

4,6 Criteria for Operation

For a zone to operate it is necessary for both the differentialsupervision relay and the biased differential relay to operate.

For a circuit breaker to be tripped it requires the associated nainzone to be operated and also the overall check zone.

Four separate elements must therefore operate before a trip can result.

4.7 Isolator Repeat Relays

The circuit for the isolator repeat relays is shown in figure 8. Theserelays are polarised by a permanent magnet which holds them latched intheir last energised state. A make and break auxiliary contact isrequired on each isolator to indicate the closed and open states of themain contacts.

The contacts of the auxiliary relays are used in the scheme to form areplica of the busbar, switching the current circuits to theappropriate zone, and controlling the tripping circuits.

4.8 Current Transformer Ratio Correction

The prirnary winding of each auxiliary C.T. is tapped as shown in figure9 to provide a range of racios. The tables shows the range of prinarycurrent ratings that can be used and the inEermediate steps available.

For correct ratio natching the ampere turns for each input m.rst be thesame under through fault conditions. Consider two C.T.'s, one having aratio of 2000/5A and the other 500/5A. If the same current flowsthrough the priur,ary of these two C.T.'s then the 500/5A C.T. willproduce four times the secondary current of the 2000/5A C.T.

Page 6

For correct ratio matching the 2000/5A C.T. should be connected toTl - To and the 500/5A c.T. connected to T, - TL.Ti - T6 gives four times the turns of Tt -^T4.

The fault setting is adjustable from 0.2 to 2.0 xI- but thesesettings are only achieved when the ful1 priur,ary wtnd ing is used. Forany other tap the minimuu operating current is given by Is x Tmax

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4.9 Trip Circuit Isolation

There is no provision for isolating individual trip eircuits, exceptfor what may be provided additionally in the cubicle wiring. However,there is a keyswitch on the alarm unit which isotates all the tripcircuits when turned go the'out of service'position. l.lhen set tothis position all indication and measuring circuits are fullyfunctional and can be tested withouE risking a trip.

Individual zones can also be switched out of service on a phase byphase basis. Zones may be selected to automatically isolate their tripcircuits when an abnormality is detected by the supervision circuits.

4. 10 Breaker Fail/Transfer Tripping

Because the realiability of circuit breakers is not 1002, there is therisk that some faults will not be cleared even though the protectionhas initiated a tripping sequence.

Serious damage could then result and even danger to life. To reducethis risk breaker fail proEectiion schemes have been developed.

These schemes are generally based on the principle that if current isstill flowing through the circuit breaker, a set time after it has beentold to trip, then it has failed to function. The next srage ofcircuit breakers back in the system are then automatically tripped.

For a bus coupler or section switch this would involve tripping all theinfeeds to the adjacent zone. This feature is therefore included inthe busbar protection scheme.

I{hen the current transformers are mounted on one side only of the buscoupler, then this feature will also ensure that a fault between theC.T. and the circuit breaker is cleared. If current transformers aremounted on both sides of the circuit breaker this is not a problemsince the protection zones can be overlapped.

The protection against failure of the feeder breakers is norurallyassociated with the feeder protection and is external to the busbarprotection. However, it requires an input to the trip logic of thebusbar protection so that the appropriate back-up breakers may be.setected and tripped. This input has been taken into account on eachof the feeder modules.

Page 7.

5. CONSTANT MONITORING

Constant monitoring is provided for all vital circuits. This isachieved by comparing signals frorn different circuits that areperforming the same function with exclusive OR gates.

5. I Alarm Module

The bias circuits are supervised by comparing the positive and negativehal-f cycles. Any faults are indicated after a short delay, whichovercomes transient drrors. The positive and negative supply rails aresupervised and separaEely alarmed.

Atrprotection operated" alarm is given if a rnain zone of protectionoperates without the check zone and vice versa.

A "protection lrippedt' alarrn is given if any main zone and the checkzone are operated together. For this condition some circuit breakersshould have tripped.

A -conmon alarm' is repeated in this module for a fault in any of theother modules in the protection.

0utputs contacts are provided for :-

* Protection Operated

* Conmon Alarm

* Unit out of Service

5.2 Circuit Breaker Modules

The outputs of the saturation detectors for the uain and check zonesare compared and alarmed if faulty.

The coils of the trip relays are supervised and open circuits alarmed.

Isolator repeat relays should normally have eiEher the set or resetcoils energised. If neither are energised or both at the same tirethen an alarm is given. By this means the repeat relay coils; externalwiring and the auxiliary contacts on the isolators are monitored.

5.3 Measuring Units

The measuring units contain the differential supervision elements andthe biased differential protection elements. These are duplicated asshown in figure 10 so that they can be compared for monitoringpurposes. The ability of the protection to trip is also improved bythe duplication

Page 8

The differential supervision elements detect differential currents thatrnay result frour open circuited current transformer circuits and henceproduce a "c.T. Faultt'if the condition persists. They also enable thebiased elements once the differential current exceeds the supervisionthreshold.

Should only one of the supervision relays operate, or only one of thebiased element.s, then a protecEion fault will be announced. Tocompensate for the small differences in the settings of the duplicateelements they are cross-coupled, so that when one element operates itcauses the setting of Ehe other to reduce by a snall percentage. Theythen both operaEe at the same leve1 to elininate any monitoringproblems.

As rnentioned previouslyswitched out of service

5.4 Pover Supplies

6.1

the measuring circuits can be automaticallyif the monitoring circuit detects a fault.

6.

Each MSTZO3 pover suppty has its own inbuilt protection and alarmcircuits.

RELIABILIlY

Figure 1l shows the trip logic which for siroplicity is drawn in rernsof contacts. This arrangement is in effect a double two out of twologic, that provides high security and availability.

Secur ity

Security is the ability to not operate when Lhe protected system is notfaulty. This is achieved by the series connection of the main andcheck zones. It can be improved by setting the minimum operating abovethe load current. Having the supervision ssitch the protection out ofcircuit also improves security.

6.2 Availability

Availability is the ability of the protection to operate shen requiredto do so. This is achieved by duplicating the measuring circuits andimproved by the coostant monitoring circuits. The latter can alarm forprotection circuit faults so that they can be repaired and theprotection restored to fu11 availability

6.3 Tine to Reparr

It will be possible in many instances to identify the faulted circuitfrom the indications displayed by the nonitoring circuits. The variouscircuits are arrranged on printed circuit boards. These plug-in and

can therefore be quickly replaced.

A HILL4 April 1985

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Dlfferenilat

TRIP LOGIC