Author: K. C. Agrawal ISBN: 81-901642-5-2

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19.1 Circuit interrupters 19/72319.1.1 Surges during a switch closure 19/72319.1.2 Surges during a switch interruption 19/723

19.2 Theory of circuit interruption with different switching mediums (theoryof deionization) 19/723

19.3 Theory of arc plasma 19/725

19.4 Circuit breaking under unfavourable operating conditions 19/726

19.5 Circuit interruption in different mediums 19/72619.5.1 Bulk Oil Circuit Breakers (BOCBs) 19/72619.5.2 Minimum Oil Circuit Breakers (MOCBs or LOCBs) 19/72719.5.3 Air Circuit Breakers (ACBs) 19/72819.5.4 Air Blast Circuit Breakers (ABCBs) 19/73219.5.5 Sulphur hexafluoride gas circuit breakers (SF6) 19/73319.5.6 Vacuum circuit interrupters 19/740

(i) Vacuum circuit breakers (VCBs) 19/740(ii) HV vacuum contactors (HVCs) 19/743(iii) LV vacuum contactors (LVCs) 19/745

19.6 Phenomenon of current chopping 19/74519.6.1 Influence of frequency on the system 19/746

19.7 Virtual current chopping 19/746

19.8 Containing the severity of switching surges 19/74719.8.1 Theory of energy balancing 19/747

19.9 Comparison of circuit breakers using different interrupting devices 19/748

19.10 Gas insulated switchgears (GIS) 19/748

19.11 Retrofitting old installations with vacuum and SF6 breakers 19/757

Relevant Standards 19/758

Further Reading 19/758

19Circuitinterrupters andtheir applications

19/721

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Circuit interrupters and their applications 19/723

19.1 Circuit interrupters

An interrupter is the electrical part and constitutes theswitching device of a breaker. The operation of this devicecauses switching surges when closing or opening a circuit.Details of such surges and causes of their generation arediscussed in chapter 17.

Surges that may appear on the LV side of a powersystem as a result of transference from the HV side of atransformer (Section 18.5.2) are different and not relatedto switching. Surges on the LV side due to switching ofstatic devices (Section 6.13) are not related to the switchingof the circuit but to the static devices themselves.

In this chapter we discuss the types of insulating andquenching mediums, their switching characteristics andmerits and demerits of their use. We also consider thebasic interrupting devices developed over the years, usingsuch mediums, keeping switching surges as the basiccriteria in mind. The theory of arc interruption is the samefor all switching devices. The following types of breakershave been developed for the purpose of switching andthey mostly relate to the HV systems, except where noted:

1 Bulk Oil Circuit Breakers (BOCBs)2 Minimum Oil or Low Oil Content Circuit Breakers

(MOCBs or LOCBs)3 Air Circuit Breakers (ACBs) – generally for an LV

system only4 Air Blast Circuit Breakers (ABCBs)5 Sulphur Hexafluoride Circuit Breakers (SF6 breakers)6 Vacuum Circuit Breakers (VCBs)7 HV and LV Vacuum Contactors (VCs). Contactors

are not breakers but covered here to comprehend theapplications of vacuum interrupters as the contactorstoo embody an interrupting device.

It is for the user to choose the most appropriate circuitbreaker to suit his requirements, application and cost.Some of the breakers in the HV range like BOCBs,MOCBs and ABCBs are now out dated in the face ofmore advanced technologies available in the form ofvacuum and SF6 breakers. Installations where old breakersare still installed, are fast getting retrofitted with the newgeneration advanced technology breakers. But to retainthe historical significance of the old breakers we havediscussed briefly these breakers also for a general referenceto the readers, to be more abreast with the evolution ofnew technologies that has taken place over the years inthe techniques of arc extinction. Here we discuss brieflythe philosophy of circuit interruption and the effect ofinsulating and quenching mediums on the arc extinctionof these breakers. We also deal briefly with theconstructional features and applications of such breakers.For more details one may refer to the manufacturers’catalogues and literature available on the subject.

To begin our discussions, let us first have a briefreview of switching surges discussed in Chapter 17.

19.1.1 Surges during a switch closure

• Switching surges may develop during a closingoperation just before the contacts are able to make. It

may occur at a stage when the gap (i.e. the dielectric)between the contacts becomes incapable ofwithstanding the impressed voltage and breaks down.When this occurs, it causes an arc between the contactsleading to such surges.

• Switching surges also develop when all the threepoles of a switching device do not make at the sameinstant. (Refer to Section 17.7.2(ii).) The surges mayhave a peak value up to 3–5 p.u. at a surge frequencyof 5–100 kHz or more, depending upon the closingcircuit constants L and C. They may exist in the systemfor a very short duration of much less than even onehalf of a cycle, i.e. up to the closure of the switchonly. Typical speeds of interrupters may fall in therange of 1–1.5 mm/ms. Extremely steep (fast-rising)transient surges, up to 3.5 p.u. have been noticed incertain switching circuits, with a front time as low as0.2 ms.

19.1.2 Surges during a switch interruption

These surges develop when interrupting a highly inductiveor capacitive circuit, such that the current phasor lags orleads the voltage phasor by so much that up to a near fullsystem voltage may appear across the parting contactson a current zero and cause re-ignition of the arc plasma.(Refer to Section 17.7.4.) These surges may also have apeak value up to 3–5 p.u. at a surge frequency of 5–100kHz or more, depending upon the interrupting circuitconstants L and C. They may exist in the system for aslightly longer duration of one half to one-and-a-halfcycles (10–30 ms for a 50 Hz system), i.e. up to circuitinterruption. Extremely steep transient surges, up to 5 p.u.in certain interrupting circuits, have been noticed with afront time as low as 0.2 ms and even less.

The phenomenon of a switching surge is related tothe performance of the switching device, i.e. its speed ofoperation and ability to quickly rebuild its dielectricstrength (deionization of the arc plasma) between theparting contacts after a current zero.

19.2 Theory of circuit interruptionwith different switchingmediums (theory ofdeionization)

When a live circuit is interrupted, an arc is invariablyformed between the parting contacts, the intensity andmagnitude of which would depend upon the quantumand the quality (p.f.) of the current being interrupted.The arc, due to its excessive heat, under high pressure orvacuum (the medium in the interrupter is maintainedthus), forms a plasma in the medium which causesdecomposition of the insulating and the quenching mediumto a few gases and vapours. The gases so formed thenionize into electrons and protons, which are chargedparticles conducting in nature, and make the arc conductingas well. How to disperse the heat of the arc plasma quicklyfor a successful interruption of the circuit is the theoryof arc extinction. The types of gases produced and their

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19/724 Electrical Power Engineering Reference & Applications Handbook

behaviour, as a consequence of ionization of the insulatingand quenching mediums are as follows:

• Oil in BOCB or MOCB This decomposes intovapourized and dissociated hydrocarbon, which in turnionizes into H2 and other gases and vapours. H2constitutes around 70% of all the gases and vapoursproduced.

• Air in ACB or ABCB This ionizes into N2, O2 andvapours; N2 constitutes most of it.

• SF6 in SF6 circuit breakers This ionizes into sulphurand fluorine.

• In a VCB This is not the vacuum but the metal ofthe parting contacts that becomes vapourized.

The main problem of circuit breaking arises out offormation of the arc and its prolonged extinction, whichmay delay the circuit interruption and lead to a restrikeof the arc plasma after a current zero. The basic conceptof a circuit breaking thus leads to the quickest extinctionof the arc plasma. It has caused many engineers andscientists to undertake extensive research and developmenton the subject over the past 50 years or so to find moresuitable mediums and to evolve better techniques toextinguish the arc plasma. The present-day hightechnology, adopted by the various manufacturers in thefield of arc quenching, is the result of these long years ofconsistent and continuous research and developmentwork.

To achieve a quicker extinction of the arc it isimperative to create one or more of the followingconditions:

1 To quench the arc plasma caused during theinterruption, quickly and continuously, is to ensurethat by the next current zero, the arc path is devoid ofany traces of arcing. In other words, the contact gapmust restore its dielectric strength before the nextcurrent zero.

2 To lengthen the arc as shown in Figures 19.11 and19.12. This is an effort to render the restriking voltage(TRV*) insufficient to re-establish an arc across theparting contacts after a current zero. The processincreases the resistance of the arc plasma that helpsto absorb a part of the TRV by causing a voltage dropacross the resistance so created, besides improvingthe p.f. of the interrupting circuit and thus dampingthe restriking voltage (TRV) to far below its peakvalue by the next current zero. Damping of TRV atimproved p.f. may be observed from curves a and bof Figure 17.11.

3 Splitting the arc into a number of series arcs (Figure19.11) so that the input power to the arc becomes lessthan the heat dissipated during the process ofdeionization. The more efficient the process of cooling,the better will be the chances of avoiding a restrikeand achieving a quicker extinction of the arc.

4 A forced interruption before a current zero, as mayoccur in an ABCB or VCB, may cause current chopping

(Section 19.6, Figure 19.27) giving rise to high TRVs,is not desirable. It is therefore important that the designof the interrupting device be such that a live circuitinterrupts only at a natural current zero, as far aspossible, to avoid generation of voltage surges. Thefollowing techniques have been developed to achievethis:• Use of high pressure at the arc plasma to drive

away the same.• Adopting forced cooling to quench the arc plasma.• Use of such constructions that can elongate arc

length and reduce the concentration of ions in thearc plasma and hence enhance the dielectric strengthbetween the parting contacts.

• Pre-inserting a resistor in the interrupter unit tocause a voltage (TRV) drop across it and to alsoimprove the p.f. of the interrupting circuit andmaking arc extinction easy. The mechanism is madesuch that a resistance commensurate with the systemparameters and switching conditions (which theuser has to stipulate to the manufacturer) is insertedinto the switching circuit. Insertion is made throughthe interrupting mechanism immediately, say, byhalf a cycle before the contacts make or open andis shortened or disconnected immediately on closingor opening of the contacts.

These techniques have been successfully implementedin interrupting devices as noted in Section 19.1.2, beingcommercially produced by various manufacturers fordifferent voltage systems and applications.

The dielectric properties of different mediums atdifferent contact gaps are illustrated in Figure 19.1. Itmay be observed, that except the medium of vacuum,which has a near constant or very little rise in dielectric

Figure 19.1 Dielectric strength of different mediums as a functionof contact gap

High vacuum

Die

lect

ric s

tren

gth

(kV

)

250

200

150

100

50

0 5 10 15 20Contact gap (mm)

Air, 1 bar

OilSF 6

, 1 b

ar

*TRV–Transient recovery or impressed voltage (Section 17.6.2)

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strength above a gap of 10 mm and pressure about 10–3

Torr or less, all other mediums, even air, have a near-linear rise in their dielectric strength with the contact gap.

The dielectric strength can also be enhanced with therise in pressure of the medium, except oil, which cannotbe compressed, and can be considered as having a near-constant dielectric properties. The characteristic of air atvery low pressures is illustrated in Figure 19.2. Thebehaviour of air at very low pressures (below 10–4 Torr)is extensively utilized in vacuum interrupters.

19.3 Theory of arc plasma

The arc plasma is caused during an interruption of thelive contacts, and also just before closing the contacts,when the contact gap falls short of the required dielectricstrength to withstand the impressed voltage. When anarc is caused the gases present in the arcing chamber,under the influence of high temperature of the arc plasmaand the high pressure or high vacuum maintained withinthe arcing chamber, become ionized. They liberate protons(positive ions, positively charged, heavier particles) andneutrons (uncharged particles) surrounded by electrons(negatively charged lighter particles (Figure 19.3)). Thetheory of arc extinction relates to the physics and behaviourof these electrically charged particles that are responsiblefor a restrike of the TRV even after a current zero. Theeffectiveness of the medium and the design of the arcchamber to diffuse these electrically charged particles toneutrons as quickly as possible determines the ability ofone type of breaker over others. In fact, the theory of arcextinction is the theory of deionization (neutralization)of the electrically charged protons and electrons. Thetheory may be briefly explained as follows.

The positive ions (protons) present in the arc plasmamove to the cathode (negative pole) and neutralize(deionize) the electrons there. Similarly, the negativeions (electrons) move to the anode (positive pole) andbecome absorbed there. Thus the process continues untilall the plasma is neutralized and contains only neutrons

to extinguish the arc. In fact, the concentration of ionsbetween the parting contacts gradually becomes dilutedby the deionization of protons and the absorption ofelectrons at the anode, and a stage is reached whenadequate dielectric strength between the parting contactsis restored to extinguish the arc. It is not necessary forall the ions present in the plasma to be deionized toextinguish the arc, rather a stage when they are not ableto hold the arc. This process also increases the resistanceof the arc plasma as a result of reduced contact pressureand arc contact area.

A proton, being 1835 times heavier than an electron,moves sluggishly compared to an electron since

m m1 12

2 22 = = constantv v

where m is the mass of an ion and v its velocity. Then,for the mass of electron as m, the velocity of the protonwill be

1835 = p2

e2◊ ◊ ◊m V m V

If Vp = velocity of the proton and Ve = velocity of the electron

then VV V

pe e =

1835 =

43, which is too slow compared to

an electron. The conductance of the arc plasma is thusthe result of the movement of electrons, rather thanprotons, which contribute only a little.

Figure 19.2 Dielectric strength of air at different pressures ata uniform contact gap of 10 mm (1 Torr � 1 mm head of Hg)

1000

0.1

1

10

100

�240

10–6 10–5 10–4 10–3 10–2 10–1 1 10 102 103 104

Die

lect

ric s

tren

gth

(kV

)

Pressure (Torr) 760 Torr �1 bar (1 atm.)

N +

A molecule of gas, underhigh temperature andpressure or vacuum

Neutron is surroundedby very lightweightelectrons

Moves towards anode and allelectrons get absorbed there.The leftout neutrons comeback to plasma uncharged

Moves towards cathode, collectsan electron and gets neutralized.The neutrons so resulted, comeback to the plasma uncharged

Proton

Return to plasma

uncharged

Ionization

Note: The mass of a nucleon (proton or neutron) is 1835 timesheavier than an electron and moves much slower than an electronsince, m v m v1 1

22 2

2= = constant. Where m is the mass and v, thevelocity of an electron or a proton. The bulk of the arc is thereforecaused by electrons rather than protons.

Figure 19.3 Theory of ionization and deionization of gas atomsto extinguish the arc plasma

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19.4 Circuit breaking underunfavourable operatingconditions

Long years of experience in the field of circuit breakingwith interrupting devices have revealed that under adverseconditions of circuit parameters, interruption may not besmooth. It may result in excessive voltage surges, as aconsequence of restriking of the parting contacts. A wrongchoice of interrupting device may result in insulationfailure of the terminal equipment, such as a powertransformer, an induction motor or inter-connecting cables.This situation may arise when:

1 Interrupting small magnetizing currents, such asinterrupting an induction motor or a transformer onno-load, a situation, when the current may lag theimpressed voltage by nearly 90∞.

2 Interrupting a charged capacitor bank, when the currentwill lead the impressed voltage by nearly 90∞.

3 Interrupting an unloaded transmission or distributionline or a cable, i.e. interrupting a line charging current,which is capacitive and may lead the system voltageby nearly 90∞.

4 Interrupting an induction motor immediately after aswitch on, when the current is large and highlyinductive.

5 Interrupting fault currents that are mostly inductive(Section 13.4.1(5)) and occur at very low power factors.They are excessive in magnitude, and cause highthermal effects and electromagnetic* forces on thearc chamber, the contacts and the contact mountingsupports.

Under the above conditions, the arc, as usual, willextinguish at the first current zero but will have a tendencyto re-establish immediately again, after the current zero(Figure 17.11(c)) while the contacts are still parting. Thisis because the TRV across the parting contacts may exceedthe dielectric strength of the contact gap achieved sofar.

Restoration of the dielectric strength will depend uponthe speed of the moving contact and the insulating mediumof the arc chamber. There may be a number of arc restrikesbefore a final extinction is achieved. The frequency ofarc restrikes may be extremely high (Equation (17.1)),depending upon the L and C of the interrupting circuit,which would have the characteristics of a surge circuiton formation of an arc. In terms of actual rated frequency( f ), restoration of the dielectric strength may not takemore than one half to one and a half cycles, i.e. 10–30ms (for a 50 Hz system). The behaviour of circuit breakingthus depends upon the design and the quenching mediumof the interrupting device.

* The breaker will interrupt only during a transient state (Figure13.20) by which time the d.c. component responsible for the dynamicforces, has subsided.

Figure 19.4 Rear view of a bulk oil circuit breaker assemblyshowing single-break contacts, self aligning cluster isolatingcontacts, terminal bushing and arc control pots (Courtesy:GEC-Alsthom)

Tulipcontacts

Terminalbushings

Arc controlpots

Movingcontacts

Oil tank

19.5 Circuit interruption in differentmediums

19.5.1 Bulk Oil Circuit Breakers (BOCBs)

Refer to the general arrangement of this breaker in Figure19.4. In this device the moving contacts make and breakin an oil bath. When the arc is formed during an interruption,the oil becomes decomposed due to excessive heat, andproduces a few gases and vapours such as H2 (70%), C2H4(20%), CH4 (10%) and free carbon, say, 3 g per 10 litresof oil decomposed at a very high pressure of 100–150bars, in the shape of a bubble around the arc (Figure19.5). H2 is an extremely good medium for quenchingand does most of the cooling of the arc plasma,extinguishing it while passing through it. The gases thusproduced also cause turbulence in the oil in theneighbourhood, causing rapid replacement of the oil withcool oil from around the contacts, thus achieving a doublecooling effect. At each current zero, it almost recoversits dielectric strength and also increases its post-arcresistance as a result of cooling and arc extinction, makingthe interruption all the more easier and complete.

Simultaneously the bubble also pushes the oil awayfrom around it and reduces the cooling. Proper design,however, can ensure adequate cooling during interruption

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(arcing) by adjusting the speed of the parting contact,supplementing the cooling of oil through an additionaloil chamber, such as a side-vented explosion pot or crossjet pot, by adjusting the gap between the fixed and themoving contacts.

While breaking smaller currents, the formation of gasmay not be adequate to provide the desired cooling effect.This is, however, immaterial because of less intensivearc formation requiring much less cooling. Extinctionmay be slightly prolonged but may be achieved by thenext current zero.

The circuit tends to interrupt at a normal current zeroand causes no current chopping. A BOCB is generallydesigned with two breaks per pole that help in restoringthe dielectric strength promptly during an arc interruption.

This is the oldest version of HV circuit breakers andwas the most extensively used breaker up to 1970. Inmodern systems of power distribution, however,application of this breaker is quickly becoming outdateddue to the higher maintenance of oil that requires constantchecking. It becomes carbonized on every switching andloses its dielectric strength, leading to the possibility ofa fire hazard. Poor availability of such breakers for higherfault levels to match the complexity and a much higherfault level (Table 13.10) demand of modern transmissionand distribution networks has also rendered themunsuitable for all such applications. They are, however,still in use at many old installations that have a low faultlevel and voltages 3.6–12 kV until they too are retrofittedwith advanced technology vacuum or SF6 breakers.

Role of oil

• To insulate the live contacts from the grounded metal

tank. The dielectric strength of oil is nearly twice thatof air.

• To provide an insulating barrier between the opencontacts after the arc is extinguished.

• To produce hydrogen, a good medium for quenching,during the arcing period.

Shortcomings

1 Oil causes carbonization and sludging.2 A hydrogen–air mixture is highly explosive and fire

hazardous.3 Oil coalescing (fusion) with the tank walls may cause

an ignition and explosion. This limitation requires alarge oil tank, which becomes rather impracticable tohandle beyond a certain range of voltage and currentrating. For instance, for a 245 kV system almost20 000 litres of oil tank per phase will be essential. Thearc interruption, although highly efficient and almostautomatic as the size of the gas bubble and the gaspressure is directly related to the size of the arc plasmaor the current it is interrupting, is highly susceptible toexplosion by fusion with the tank metal or highcarbonization of oil.

4 The arc energy produced during an interruption ishigh compared to the mediums of SF6 and vacuum.Figure 19.6 makes a comparison of the arc energyproduced during interruption of a breaker in differentmediums.

The improvised version of a BOCB is achieved throughan MOCB by arranging separate and insulated arcchambers to interrupt each phase separately, thuseliminating the element of fusion with the tank wall.

19.5.2 Minimum Oil Circuit Breakers (MOCBsor LOCBs)

Refer to the general arrangement of this breaker in Figure19.7. The theory of arc extinction is the same as for

Arc

ene

rgy

(kW

h)

600

500

400

300

200

100

00 10 20 30 40

Symmetrical breakingcurrent (kA)

Oil

SF6

Vacuum

Figure 19.6 Comparison of arc energy produced duringinterruption of a breaker in different mediums

Fixed contactOil

Arc

Gas bubble

Moving contact

Oil tank

(1) Formation of gas bubble.(2) When the dimensions of the tank are inadequate, the bubbles

may communicate amongst themselves and also with thetank and cause fusion which may lead to explosion.

Figure 19.5 Interruption of circuit in oil

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Figure 19.8 Cross-sectional view of a typical pole assembly ofan 11 kV MOCB (Courtesy: NGEF Ltd.)

Oil level indicator

Vent liberated gases in the breaker

Oil filling vent

Upper pole head

Contact tube

Connecting terminals

Spring supported tulip contacts(fixed contacts)

Arc quenching device

Arc chamber

Moving contact rod

Contact roller guide

Connecting terminals

Contact rollers

Lower pole head

Oil drain plug

Level connecting operatingmechanism

Figure 19.7 Minimum oil content circuit breaker (MOCB)(Courtesy: NGEF Ltd.)

BOCBs. Here also the circuit interrupts at a normal currentzero and generally causes no current chopping. In viewof their construction which houses each arcing contactin a separate insulated cylindrical body (Figure 19.8) ithas space limitations to provide two breaks per pole. Asa result, it has a slightly delayed extinction of arc comparedto a BOCB.

However, this is an improvised version of the bulk oilcircuit breaker. Here, the oil is used only for the purposeof quenching the arc. It became the most sought-afterbreaker during the late 1960s and onwards. Their rupturingcapacity is also much higher than that of a BOCB andthey are extremely suitable for distribution systems withmoderate fault levels. MOCBs are available from 6 kVto 420 kV and have a rupturing capacity of 250–25 000MVA. The trend, however, has tilted in favour of moreadvanced technologies, now available in the form ofvacuum and SF6 breakers.

Though applications of MOCBs are rare, they maystill be preferred in the range of 72.5kV–145kV purelyon cost consideration.

19.5.3 Air Circuit Breakers (ACBs)

Refer to the general arrangement of this breaker in Figures19.9(a) and (b).

The moving contacts make and break in air as shownin Figure 19.10. During interruption, the arc is formed(Figure 19.11) producing N2 (80%) and O2 (20%) and

metallic vapours. The quenching and extinction of arcplasma is achieved through the elongation of arc, whichincreases the area of cooling, on the one hand and requiresa higher TRV to cause a restrike, on the other. To obtainthis, arc chutes are provided on the top of the interruptingcontacts, as illustrated in Figures 19.12(a) and (b). Thedesign of the arc chutes is such that it drives the arcplasma upwards and elongates it to provide the requiredcooling effect. This is achieved by constructing the arcchute housing of some insulating material, such as glass,asbestos, ceramic or Bakelite, suitable to withstand thevery high temperature of the arc plasma. Metallic arcsplitter plates (fins) made of magnetic material are fixedinside the arc chute housing so that the arc plasma producesa magnetic field through these splitters, splits into a numberof shorter arcs and rises upwards, to lose all its heatthrough convection. This renders the TRV insufficient tocause a restrike. The long arc length and subsequentcooling increases the resistance of the arc plasma andimproves the p.f. of the interrupting circuit. It thus helpsto bring the current phasor closer to the voltage, andmake interruption on a current zero less severe as a resultof low TRV. (Refer to curves a and b of Figure 17.11.)The gradual rise of arc resistance after a current zero

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Figure 19.9(b) Drawout ACB (Ir – 400–6300A, Isc – 100 kA) (Courtesy: L&T)

dampens the TRV and makes such breakers almostimmune to switching surges. For higher currents, the arcsplitter plates may be altered to have a variety of designs,such as with offset slots, serpentine splitter plates orsimilar features to effectively arrest the arc plasma withinthe arc chutes, rendering it incapable of causing a restrikeafter a current zero.

Such breakers are normally produced for use on an

LV system only. At higher voltages, while interruptingheavy currents (such as on a fault) the arc energy may beso high that a disproportionate size of arc chutes may berequired to arrest and extinguish the arc, leading todisproportionate size of ACB.

ACBs were the first to be produced commercially.They are simple to operate and cause no fire hazards.But at atmospheric pressure, they possess a low dielectric

Figure 19.9(a) Views of air circuit breakers

(Courtesy: Siemens) (Courtesy: GE Power Controls)

A Safety shutter F Secondary isolatingB Microswitch for contacts (fixed)

position indication G Grounding terminal at rearC Racking screw H Scraping ground (fixed)D Racking interlock I Telescopic railE Door interlock J Interlock support

A

B

C

D

E

F

G

H

I

J

Cradie – front view

A

B

C

D

Rear view

A Current transformer C Secondary isolatingB Scraping ground (moving) contacts (moving)

D Contact jaws

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strength and are therefore normally manufactured onlyin low voltages. Air has less contamination and thereforethese breakers require negligible maintenance, comparedto oil. They require no contact cleaning. Since there is alimit to producing these breakers for HV systems, theirnormal application is for LV systems alone, where theyare used extensively. In fact, they are the only breakersto meet the needs of an LV distribution system and are

available for current ratings 400 A-6300 A and interruptingcapacity 100 to 150 kA.

Protection releases

Modern breakers are available with various protectionconfigurations such as,

– Conventional thermo-magnetic over-current (OC) andshort-circuit (SC) releases

Figure 19.9(c) Microprocessor based and communicationcapable protection release (Courtesy: L&T)

Figure 19.9(d) Coordination of releases

Note Times shown for illustration indicatetotal tripping times

A

B

340 ms

B1 B2

240 ms 240 ms

C

C1

140 ms

C2

140 ms

F

Figure 19.10 Typical contact arrangement of an LV air circuitbreaker

Figure 19.11 Process of arc formation and quenching in anACB using splitter plates

Arc runner(Horn shaped)

Fixed & movingarcing contacts

Arc movinginto arc chute

Lengthening ofarc

Metallic arc splitters orquenching plates ofmagnetic material

Housing of insulatingmaterial (asbestos,Bakelite, FRP or DMCetc)

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– Solid state OC and SC releases– State-of-the-art microprocessor based and communica-

tion capable releases.

Microprocessor based releasesFigure 19.9(c) shows a typical release of this kind of aparticular manufacturer. This is an intelligent electronicdevice (IED) and can be incorporated with many featuressuch as,

• Display r.m.s current.• Store trip data and display trip history.

• Can be hooked up with PCs (personal computers).• Pick-up current settings can be as low as 200% Ir or

so for phase faults and short-circuits and up to 10%Ir or so for ground faults. Trip time settings can be aslow as 0.1–0.5s.

• With the application of these fast operating (£ 0.5s)protective devices it is possible to contain the faultseverity (heating µ Isc

2 . t) of a system to a low leveland enable choose power equipment and components(busbars, connecting links) with lower cross-sectionsto cut on cost.

• The electromagnetic forces ‘Fm’ may still reach theirmaximum as the protective device is fast acting notcurrent limiting and will allow the fault current ‘Isc’reach its peak. (Also see fault level of an LV system,Section 13.4.1(5))

• Being digital can use MODBUS RTU model (Section24.11.5) for serial port communication like breakercontrol through RS485 or any other port.

• Using telemetry software as noted above it is nowpossible to make a substation remotely controlledand fully automatic.

• Blocking capabilities to achieve intelligentdiscrimination and zone selective interlocking: Theycan be co-ordinated for desired discrimination betweenupstream and downstream breakers. Such as in adistribution network as shown in Figure 19.9(d), toisolate only the faulty circuit on a fault (phase orground fault) retaining the continuity of the healthycircuits upstream. Each breaker trips as per its owntrip threshold value and sends out a logic waitcommand to the next trip unit upstream to prevent itstripping. This command is automatically bypassedwhen the upstream breaker directly experiences afault current reaching its trip threshold value andtrips at its original default setting. For instance for afault in circuit C2 only breaker C2 will trip and for afault in circuit B2 only breaker B2 will trip etc. andnot the upstream breakers. In case of fault in theupstream circuits such as in circuit B2, the breakerB2 will trip quickly at a default setting which may be

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a Top terminalb Moulded–plastic baseb5 Fixed contactd Arc chuted2 Arc runnerd4 Metallic arc splittersd5 Leaf springe1 Moving contacte2 Contact carriere5 Tension springe7 Flexible connector

e9 Compression springf2 Operating shaftg1 Insulating barrierg2 Insulating barrierh4 Bimetal striph5 Intermediate shafth8 Current transformern3 Intermediate shaftn4 Magnet corep Bottom terminal

Figure 19.12(a) Operating mechanism of an LV ACB showingthe arc chute with splitter plates (Courtesy: Siemens)

Figure 19.12(b) Arc chamber with splitter plates in a powercontactor

Arc chute with arcsplitter plates

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of the order of 90–110 ms or so and not the commanddelay of 240 ms for faults in down-stream as chosenin Figure 19.9(d).

• Protective settings can be varied through PC whilethe breaker is in service.

19.5.4 Air Blast Circuit Breakers (ABCBs)

Refer to the general arrangement of this breaker in Figure19.13(a).

These are similar to ACBs, except that the process ofinterruption is accelerated by impinging a high pressureaxial air blast through the arc plasma, when the contactshave just begun to separate. (See Figure 19.13(b).) Thecompressed air has greater dielectric strength and thermalproperties than ordinary air at atmospheric pressure.

The conventional pressure of air blast is generally218 to 900 lb/in2 (1.5 to 6.2 MN/m2) up to 110 kV, up to3000 lb/in2 (20.7 MN/m2) for voltages up to 400 kV, and

still higher pressures for higher voltages. The pressurerequirement will change from one manufacturer to another,depending upon contact design, interrupting mechanismand design and value of resistors. Since most quenchingis through a predetermined force of an air blast, the forceof arc plasma quenching and extinction (deionization)remains the same for a particular size of breaker,irrespective of the amount of current the interruptingdevice may have to break. This factor inherits a tendencyto break small currents, even before their natural currentzeros, causing current chopping (Section 19.6). Currentchopping may raise the TRV up to 2.5–3 p.u. However,it is possible to design these breakers to contain the valueof a TRV to a non-striking level.

The damping of a TRV is achieved through low andhigh-resistance units provided across the contacts. Thelow unit will short at higher TRVs and the higher unit atlower TRVs. The arc interruption is fast and generally atthe first current zero due to damping. When the breakeris in the closed position, the resistors are open circuited.As soon as the main contacts begin to interrupt, thecontacts of resistor make first, before the main contactsseparate and provide the required damping. Afterextinction of the arc, the resistor circuit opens againautomatically and restores to the original position.

Until a few years ago these breakers had been quitecommon for medium voltages, up to 33 kV. Since theyrequire a powerful blast of air at high pressure and velocityinto the arcing region, they require a reliable source ofair supply. Air should be clean and dry and at the correctpressure and volume at all times. This requires an elaboratearrangement for air compression, an air storage facility,a network of feed pipes, valves and safety devices besidestheir regular maintenance and upkeep. All this is costly

Figure 19.13(a) One pole of ABCB 72.5–420 kV with verticalcompressed air tank (Courtesy: ABB)

Internal electricalconnection

Capacitor

Control valve andblast valve

Controlinsulator

Doubleinterruptingchamber

Interrupting chamberdriving mechanism

Control box

Control cubicle

Bifurcation housing

Supporting insulator

Compressed air tank

Pneumaticpressure lines

ControlwiringYR B

Figure 19.13(b) Process of arc formation and quenching in anair blast circuit breaker

Arc probeTerminal

Arc in initial andfinal positions

Low pressure

High pressure

Air flow

Terminal

Blast tube

Exhaust

Nozzle

Movingcontact

Slidingconact

Piston

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particularly when only a few breakers are installed at aparticular installation.

Moreover, compressed air is released through an orificeat the exhaust point at a high velocity and causes a soundlike thunder. This may be frightening to people in thevicinity. Substations involving a number of such breakersare a nuisance to residents nearby. The higher the kVlarger is the breaker and greater is the pressure of the airblast and its sound. Silencers, however, are provided tocontain such sound hazards. ABCBs are more appropriatefor large installations which require a large number ofbreakers to be installed in the same system to economizeon the compressed air supply arrangement. In light ofthe above impediments these breakers are now almostobsolete except for old installations or replacements.

19.5.5 Sulphur hexafluoride gas circuit breakers(SF6)

Refer to general arrangements of such breakers in differentratings as shown in Figures 19.14–19.16.

This is the latest technology in the field of arcextinction. It was introduced in the 1960s and attemptsto achieve a high dielectric strength between the contacts.At room temperature SF6 is a chemically inert, non-toxicand non-inflammable, colourless, odourless gas, havinga molecular weight of 146 and provides excellent arcquenching as a result of electronegative behaviour.

At atmospheric pressure, its dielectric strength is twoFigure 19.14 11 kV SF6 circuit breaker (Courtesy: Voltas)

(a) Exploded view of a pole (b) Breaker in a draw-out position

Figure 19.15 General arrangement of a 3–12 kV, SF6 circuit breaker in a housing (Courtesy: Voltas)

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to three times that of air, as illustrated in Figure 19.1,and its arc-quenching ability many times more than air.This gas undergoes no chemical change at hightemperatures, except small decomposition into SF2 andSF4 gases and some metallic fluorine in the form of aninsulating powder while interrupting and quenching anarc. These gases and powder, however, are readilyabsorbed by activated alumina placed in the filters in theclosed-loop circuit of the gas, as discussed later. The gascycle is such that after every interruption the consumedgas is replenished through a reservoir filled with SF6 gasat a high pressure, say, sixteen times that of the atmosphereand connected to the main interrupting chamber throughpressure valves and filters. As soon as the pressure in theinterrupting chamber falls below a preset value, the valvein the reservoir opens and builds up the lost pressure. Dueto the very high pressure in the reservoir, compared to onlyalmost three times the atmosphere in the chamber, it ispossible that the pressure inside the interrupting chambermay sometimes exceed the required value. In the interruptingchamber, therefore, are also provided high-pressure releasevalves to pump the excess gas back to the reservoir througha compressor and a filter. The total gas circuit is a closedcycle without any venting to the atmosphere.

This gas is electronegative and its molecules quicklyabsorb the free electrons in the arc path between the

Figure 19.16(b) SF6 circuit breakers 300–550 kV (Courtesy: Alstom)

1 – Breaking chamber2 – Central housing3 – Insulating column4 – Insulating rod5 – Opening spring6 – Lower housing7 – SF6 monitoring block8 – Operating mechanism9 – Closing spring

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Figure 19.16(a) An SF6 circuit breaker 123–145 kV, 31.5 kA.(Courtesy: BHEL)

1. Interrupter unit 5. Operating mechanism2. Multi-shed insulator 6. Coupler3. base tube 7. Operating rod*4. Hydraulic storage cylinder 8. Control unit

(a)

*Note: The tripping mechanism can be pneumatic, hydraulic orspring operated, depending upon the arc quenching techniqueadopted and energy required to extinguish the arc.

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contacts to form negatively charged ions. This apparenttrapping of the electrons results in a rapid buildup ofdielectric strength after a current zero. The detailedsequence of arc extinction may be summarized asfollows.

The contacts begin to compress a quantity of SF6 gasas soon as they start opening. This opening also causesarc plasma between the contacts. The temperature of thearc plasma ionizes the gas into sulphur and fluorine atomsand quickly becomes quenched through the turbulence ofthe compressed gas through a very strange process ofnegative ion formation. At higher temperatures, the S atomsbecome ionized into S+ protons and SN neutrons. The Se

electrons of the neutrons are immediately absorbed by thefluorine atoms to form fluorine ions (F –) which are heavyand are sluggish. They contribute little to maintaining theconductivity of the arc plasma. (See also Section 19.3.)This quickly immunizes the free electrons, quenches thearc plasma, extinguishes the arc and builds up the dielectricstrength after a current zero. After a current zero, theprocess quickly quenches the arc in the beginning itselfby sweeping away the arc plasma, thus improving thedielectric strength between the parting contacts andachieving successful extinction of the arc. The arc extinctionprocess may be slightly delayed when the contacts openvery close to the next current zero, and the quenchingmedium blows it out with force, before the current zero,leading to a case of current chopping. But with continuousimprovement in the techniques of arc extinction, it hasbeen possible to achieve an interruption devoid of a currentchopping or a restrike of the arc plasma.

As noted above, to quench the arc plasma successfully

it is essential to create a turbulence in the SF6 gas aroundthe arc plasma to destabilize and blow it out. This can beachieved in two ways:

1 Puffer technique (a puffer identifies the exhaling ofgas forcibly by compression)

2 Rotating arc technique

1 Puffer technique

Destabilization of the arc plasma is achieved by forcedconvection of gas created by the movement of the mainand arcing contacts through a puffer piston. This is anintegral part of the moving main and arcing contacts (bothbeing concentric).

Sequence of arc quenchingRefer to Figure 19.17. On a trip signal the main movingcontacts start separating a little ahead of the movingarcing contact and compress gas through the puffer pistoninside the tubular chamber. On separation the main fixedand moving contacts are transferred to the fixed and themoving arcing contacts as shown. As soon as the movingarcing contact starts separating, an arc is formed betweenthe fixed and the moving arcing contacts and the alreadymoderately compressed gas is compressed further. Thiscompressed gas is impinged with full force through theblast nozzle (Figures 19.18(a) and (b)) at right angles tothe arc plasma from all sides to achieve instantdestabilization of the arc.

Through radiation also, the arc plasma dissipates apart of its heat which supplements the quenching. But

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Pole in closed position. Mainand arcing contacts closed

Pole at the moment ofseparation of arching contacts

Pole duringarc quenching

Pole after arcquenching

123

– Fixed arcing contact– Tubular gas (blast) chamber– Puffer piston

456

– Main fixed contact– Main moving contact– Fixed contact assembly

78

– Arc chute– Moving contact assembly

Figure 19.17 Sequence of arc extinction through the puffer technique in an SF6 breaker through the cross-section of a pole(Courtesy: NGEF Ltd.)

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this is too meagre a contribution, as heat dissipationoccurs only through the outer surface of the arc plasma.Nevertheless, it is the major cause of gas impedimentgiving rise to the phenomenon of clogging, discussedlater, and which helps in arc extinction.

The events are so fine-tuned and the size of chamber,pressure of gas, travel, distance of the moving contactand the size of blast nozzle are so designed and minutelyadjusted that a near-strike-free interruption can be achievedfor low reactive currents (inductive or capacitive) as wellas full-load and very heavy fault currents. The advancecompression of gas through the movement of main contactplays an important role by storing a part of the gas evenbefore opening of the arcing contacts.

At high instantaneous currents the arc may occupymost of the contact area between the arcing contacts andmay impede the flow of gas through the arc plasma. Thisphenomenon is termed the clogging effect, but it assistsarc extinction in the following manner.

The SF6 gas around the arc plasma takes away a part ofits surface heat by radiation. At high temperatures, the gasloses its specific gravity, becomes light weight and diminishesin momentum (µ mu2). As a result, the gas is renderedincapable of penetrating through the arc plasma to quenchit. The flow of gas through the thick of the arc plasma isthus impeded (restrained).

As the moving contact moves away, so the arc plasmaelongates, losing its initial intensity, and as it approachesthe current zero, it loses the most of it. The gas, on theother hand, cools and regains its lost mass, while itspressure in the chamber continues to build to its optimumlevel, making it more capable of extinguishing a lesssevere arc plasma. The interrupter can thus be adjustedto blow out the arc at the first current zero, while clearingheavy to very heavy fault currents.

Similarly, at lower currents, the volume of arc plasmais too small (µ I2) and so is the clogging effect. Thepressure and volume of the quenching gas can be adjustedto interrupt the current now also at current zero. All theseadjustments are preset and sealed by the manufacturer.

Since the arc extinction technique is highly effectiveand quick and occurs when the arcing contact is stillmoving, arc length and hence contact travel, can bereduced as can the arc energy and the excessive heatingand the erosion of the arcing contacts. An extended contactlife can thus be achieved by this technique.

For MV and HV systems the normal puffer techniquehas been quite prevalent and adopted by all leadingmanufacturers. For constructional details and moreinformation on this mechanism, refer to the manufacturers’catalogues. Figure 19.18(a) illustrates a typical poleassembly of a 12 kV and Figure 19.18(b) up to 420 kV,SF6 circuit breaker.

Further developments

The operating energy requirement in the natural form ofpuffer technique was high as the tripping mechanismwas also required to supply the energy to compress thegas. Normal puffer in its natural form was thereforeincapable to interrupt the arc plasma at higher voltagessay, above 145 kV. Initially these breakers were therefore

1. Fibre glass arc chamber tube2. Lower pole head3. Cap4. Gas filling valve5. Main fixed contact6. Fixed arcing contact7. Moving contact arrangement8. Moving arcing contact9. Blast nozzle

10. Expansion chamber11. Lower contact12. Lower terminal13. Top terminal14. Spline shaft lever15. Switching lever16. Two level pressure switch

– 1st level contact for alarm– 2nd level contact for trip and lockout

17. Enclosure for alumina18. Explosionproof safety valve

Figure 19.18(a) Cross-sectional view of a pole of a 12–36 kVSF6 breaker (Courtesy: NGEF Ltd.)

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produced only up to 145 kV or so. But with continuousresearch and improvisation in tripping mechanism andarc quenching techniques it has now been possible tosubstantially reduce the earlier arc energy as discussedbelow,

As we can see there are two main factors responsible forthe arc extinction,

– Compression of SF6 gas in the arc chamber. The gasis used for the thermal blast

– Elongation and weakening of the arc plasma with themovement of the arcing contact. A thin and weak arcassists its extinction.

This the manufacturers have achieved by optimizing theuse of arcing heat through,– Thermal blast– Arc assisted and– Double volume or double motion techniquesThese techniques are briefly noted below.

Thermal blast and arc assisted technique

The design of the arc chamber is improvised to assist thearc-quenching capability of the arcing chamber, by furthercompressing the gas that has already expanded duringarcing and impinging this on the arc with a greater blast.The blast also helps the main moving contacts to movefarther away with a greater force, elongate the arc andreduce the energy requirement by the moving mechanismto interrupt the breaker. This makes the whole process ofarc extinction easy and smooth. This technique, instead of

equalizing the arc heat, reduces the arc energy itself,facilitating a quicker and smoother extinction of the arc.The moving mechanism that in a normal puffer is usuallyhydraulically or pneumatically operated (Figure 19.16(a))due to the higher energy requirement by the movingmechanism can now be achieved through a simple springmechanism (Figure 19.16(b)).

Double volume or double motion technique

The improvisation of the above techniques and makingthe main fixed contact also moving (double volume) havefurther enhanced the compression of gas. Elongation ofarc has reduced the arc energy requirement. All thesehave made the arc extinction easier, smoother and quicker.It has resulted in minimizing the arc energy to as low asabout 15% of the normal puffer. Most manufacturersadapt to all or some of such techniques. Figure 19.18(c)illustrates a comparison of energy requirements betweenthe various techniques developed so far.

Since all parameters as noted above like pressure andvolume of gas and elongation of arc can be fine-tunedthe latest SF6 breakers can be made restrike-free andsuitable for all applications. With these improvisationsSF6 breakers are now capable of interrupting any voltageand are produced up to 1100 kV. As SF6 breakers can bedesigned to provide a very smooth interruption of arc,devoid of current chopping or restrike of the arc plasma,they may also be termed soft break interrupters. SF6breakers are most extensively used breakers and aresuitable for practically all applications and voltage systems.The other advantage with SF6 switchgear is space saving

Figure 19.18(b) Cross-sectional view of a pole of SF6 breaker without pre-insertion resistor (up to 420 kV)(Source: GEC-Alsthom)

1. Support insulators 7. Coupling contact 13. Insulating rod2. Fixed contact 8. Moisture absorber 14. Safety diaphragm3. Moving contact 9. Density switch 15. Oil pump4. Nozzle 10. Hydraulic ram 16. Accumulators5. SF6 compression piston 11. Driving rod 17. HP pipe6. Arcing contact 12. Valve block

8 6 2 4 3 5 7 1 9 13 11 10 12 15

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up to 70–90% over the conventional type of switchgears(Table 19.2). Since these breakers are totally enclosedand sealed from the atmosphere, they are also the mostrecommended choice for all installations that are hazardousand prone to explosions.

2 Rotating arc technology

The process of arc quenching can be enhanced byincreasing the turbulence by ionizing more gas atoms, toincrease the gas pressure and therefore turbulence. Thisis possible by bringing more gas into contact with thearc plasma, and this can be achieved by rotating the arcplasma between the contacts and displacing the arc by amagnetic blowout. A general method of achieving this isby providing an electromagnetic field around the fixedcontact, which gives the required rotating motion to thearc plasma, when the moving contact moves away fromthe fixed contact, as illustrated in Figure 19.19, similarto a motoring action. Figure 19.20 illustrates one pole ofsuch a breaker, provided with a magnetic coil. The breakersbased on this principle are known as rotating arc circuitbreakers. Figure 19.21 illustrates the rotating arc formationand the direction of a magnetic field during an interruption.As the arc is made rotating over the arcing contacts, theheating and thus the erosion of the contacts is low in thesebreakers, and they have an extended contact life.

This technique, although good, is cumbersome and istherefore generally not practised now by themanufacturers.

1

3

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5

Pole duringarc quenching

1 Fixed contact assembly 4 Arc chute

2 Magnetic field coil 5 Moving arcing contact

3 Fixed arcing contact 6 Arc

Figure 19.20 Typical design of one pole of a rotating arc SF6circuit breaker

Figure 19.19 Making the arc rotate in a magnetic field

Mag

netic

flu

x

Arc current

Motion

Arc currentMagnetic flux

Magnetic flux

Motion

Rotationof arc

Contact bar

Field coil assembly(Replica of left-hand rule, Figure 1.1)

Figure 19.18(c) Comparison of energy requirement for arcextinction in an SF6 breaker, using different techniques (Courtesy:Alstom)

100%

60%

30%

20%

15%

Normal Improvised Thermal Arc Double puffer puffer blast assisted volume

(or doublemotion)

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Operating mechanism

Because of optimization of arc energy and less energyrequirement for arc extinction, it is now possible to usea low energy spring mechanism requiring littlemaintenance (in absence of air for pneumatic and oil forhydraulic mechanism) and is more reliable in operation.It can also be used in conjunction with a synchronizingrelay. Perfect timing of operation is mandatory whenoperating in association with a synchronizing relay andthat is possible with spring operated mechanism.

Pre-insertion resistorSF6 breakers, when used for switching long transmissionlines at 420 kV and above, are provided with a preinsertionresistor across each interrupting contact to limit over-voltages that may occur during a closing or openingsequence, as a result of heavy charging currents as notedin Table 24.2. The value of the resistor may be around400 W for the line parameters, considered in Table 24.2and may vary with line parameters. The resistors areconnected so that during a closing sequence they short-circuit the making contacts before closing the maincontacts for, say, 8–10 milliseconds, and open immediatelyafter the contacts are made. The same happens during anopening sequence. See also Section 17.6.2. By adjustingthe value of this resistor a restrike-free switching can beachieved while switching large capacitors or reactors atany voltage. See Figure 19.22.

Advantages of SF61 Low gas velocity and pressure minimizes the tendency

towards current chopping.2 A closed recycling of gas causes no noise or

contamination of the atmosphere.3 There is no carbonization and therefore no tracking.

(conduction of the insulating medium).4 Because of the extremely good dielectric properties

of SF6 gas, the arc gap and the contact travel andhence the arcing time are low, requiring less energyto interrupt (Figure 19.6).

Figure 19.22 Cross-sectional view of a pole of SF6 breaker with pre-insertion resistor (up to 420 kV) (Source: GEC-Alsthom)

1. Support insulators 9. Density switch 16. Accumulators2. Fixed contact 10. Hydraulic ram 17. HP pipe3. Moving contact 11. Driving rod 18. Moving contact for4. Nozzle 12. Valve block insertion resistor5. SF6 compression piston 13. Insulating rod 19. Semi-moving contact for6. Arcing contact 14. Safety diaphragm insertion resistor8. Moisture absorber 15. Oil pump 20. Insertion resistor

17 16

19 1814

8 9 1 20 6 2 4 3 5 1 13 11 10 12 15

Successive arcpositions

Contact

Cylindricalelectrode

The arrows indicate the direction of magnetic forcesMagnetic forces on the spiral arc

Figure 19.21 The spiral arc in a rotating arc SF6 circuit breaker(Source: South Wales Switchgear Ltd.)

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5 As the arcing time is very low, it causes no or only asmall amount of contact erosion.

6 It is highly suitable for hazardous locations.

19.5.6 Vacuum circuit interrupters

(i) Vacuum circuit breakers (VCBs)

Refer to the general arrangement of a loose breaker shownin Figure 19.23(a), and in housing Figure 19.23(b). Figure19.23(c) shows a 36 kV outdoor vacuum circuit breaker.The electrical breaking capacity in vacuum has beenlong known. But it was not until 1970, that it was usedin the making and breaking of currents at high voltages.It has been a greatly recognized and well accepted breaker,leaving behind all other techniques of arc breaking andextinction in its voltage range. In vacuum, a 10 mm gapat about 1/106 mm vacuum of mercury is capable ofwithstanding a peak voltage up to around 245 kV (Figure19.1). These breakers therefore, are very compact andrequire very low maintenance.

Unlike other mediums, the dielectric strength ofvacuum increases only marginally with the gap, which isthe limiting factor in producing such breakers above 36kV. These breakers are therefore used only for medium-

voltage systems (2.4–36 kV). Some manufacturers haveattempted to produce them up to 72.5 kV but they havenot shown the desired results. The application of thesebreakers therefore continues to be up to 36 kV only.

A comparison of dielectric strength of high vacuumwith the other available mediums is shown in Figure19.1. The very high dielectric strength of vacuum makesit possible to quench an arc with a very small contactgap and breakers with very compact dimensions can bedesigned.

Because of the low contact gap, low arc resistanceand fast clearance, the arc energy dissipated in vacuumfor a particular current is 1/10 that of oil and 1/4 that ofSF6, and is illustrated in Figure 19.6.

Vacuum is finally judged to be the best medium toquench the arc plasma and interrupt a circuit under themost adverse conditions. Figure 19.24 gives cross-sectional view of a vacuum interrupter and typicalconstruction of the arcing contacts. Figure 19.25 showsits assembly.

Advantages

Some advantages of vacuum circuit breakers aresummarized below:

Meteringand

relay chamberExplosion covers

Handle tomove the

trolleyR Y B

InsulatorsPlug-incontacts

Busbarchamber

C.T.

Circuitbreaker F

ront

P.T.

Cable chamber

Groundingswitch

Trolley Base frame Rollers

Figure 19.23(b) General arrangement of the breaker in a housing

7.2–36 kV vacuum circuit breaker (Courtesy: Siemens)

Figure 19.23(a) General arrangement of the breaker on a trolley

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applied voltage and the TRV approaches a full appliedvoltage (Figure 19.27) and hence, a tendency to causea restrike. A VCB is devoid of a restrike after a currentzero, as explained later.

Because of short duration of arc they have an extremelylow energy requirement to actuate the operatingmechanism and an equally short breaking time.

Disadvantages

Vacuum breakers have a few disadvantages also as notedbelow:

1 They may inherit current chopping tendencies at verylow currents of 3–5 A, varying from one manufacturerto another and depending upon the contact materialused (some manufacturers have been able to improveit to 2–3A). This is due to their extremely fast operationas a result of a high vacuum pressure of the order of10–6 Torr (1.333 ¥ 10–4 N/m2) or more (one Torr beingthe pressure equivalent to hold a column of mercury1 mm high). Thus they cause a high TRV, particularlywhen interrupting a highly inductive or capacitivecircuit (Figure 17.11(d)).

2 Very high vacuum may have a tendency to cold weldingof the making contacts. Two pure metals, when joinedhave a tendency to stick together under high vacuum.This phenomenon is termed cold welding. The contactson closing may require a lot of force to separate themwhich may prove to be detrimental in clearing a faultpromptly.

3 At no-load, opening of contacts may lead to rougheningof the surface, due to the breaking of the cold weldingthat takes place.

4 Since a very small gap in vacuum can withstand a

– Since the contacts make and break in vacuum there isno oxidation of the contacts. The contacts do notdeteriorate nor lose their dielectric properties withswitchings and provide a longer working life.

– They cause no fire hazard and are highly suitable forchemically aggravated and hazardous locations.

– They require extremely low maintenance and littledown-time.

– They make no noise during making or breaking ofcontacts as it takes place inside a hermetically sealedarc chamber.

– They do not emit any gases.– They are the only devices that are independent of the

operating system, as the breaking capacity is dependentmainly on the material and contour of the contactstructure and the quality of the vacuum.

– At lower current, say, up to 1 kA, the maximumduration of arc even at low p.fs. is of the order of justone-half to one cycle of the natural frequency of thesystem, as against nearly two cycles for an MOCB.[The low exciting currents, at low p.fs. are moredifficult to interrupt rather than large currents at highp.fs. due to an extremely adverse voltage-current phasordisposition at low p.fs.] For more clarity refer to Section17.6.2. The current now is nearly 90º lagging the

Figure 19.23(c) 36 kV outdoor vacuum circuit breaker(Courtesy: Jyoti Ltd.)

Figure 19.24 Sectional view of a 12 kV up to 2500 A, 40 kAvacuum interrupter (Courtesy: BHEL Ltd.)

Fixed contact stem

Fixed terminal pad

Glass ceramichousing

Arc chamberFixed contact

Movingcontact

Glass ceramichousing

Metallic bellowsMoving contactguide

Moving contactstem

Blown-up view of arcingcontacts. Also seeFigure 19.26.

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very high voltage, a larger gap than required will notincrease its dielectric strength. This is the limitingfactor for a VCB to exceed a certain voltage system,presently 36 kV.

5 They may cause contamination of the vacuum due togas produced by arcing.

6 They may lead to deterioration of the insulation ofthe insulating container due to condensing of the metalvapour on the inner surface of the container (more intransverse magnetic field type breakers).

7 They also have a tendency to melting and welding ofthe contacts while making or carrying large currents.However, this is overcome by suitably designing andcontouring the contacts so that the arc impinges overa large area of contacts rather than at one point onlyto prevent melting of contacts. The material of thecontact is chosen so that it will produce less gas content,have good anti-weld properties and low current-chopping tendencies (high contact resistance). Thenormal metal alloys in use are:

• Low resistance – high kA alloy (high melting point):copper – bismuth has a good resistance to coldwelding but has a higher probability of currentchopping. Refer to curve 4 of Figure 17.8.

• High resistance – low kA alloy (low melting point):copper–chromium (CuCr) which also has a goodresistance to cold welding and a lower probability

of current chopping, similar to in OCBs. Refer tocurve 2 of Figure 17.8.

Limitation of using AgW or CuW contactmaterials in VCBsThe properties of vacuum interrupters depend largely onthe material and contour of the arcing contacts.Experiments have shown (Further Reading, ref. 9) thatcontact materials AgW and CuW as used for vacuumcontactors and can limit chopping current (Ic) havelimitations in interrupting large arc energies because ofvery high melting temperature (3370∞C) of tungsten (W)compared to Ag (960∞C) or Cu (1083∞C). During an arcinterruption particularly on faults, it is only Ag or Cuthat melts and on solidification shows cracks on the contactsurface. They are thus suitable for low arc energies only.At low arc energies ( )c

2µ I AgW or CuW contacts haveshown excellent results with very low chopping currents.They are therefore ideally suited for LV and HV vacuumcontactors but not vacuum circuit breakers. On the otherhand Cu and Cr (1615∞C) jell very well with each otherat all currents and make a good contact material forVCBs. During melting they are soluble into each otherand on solidification leave a smooth contact surface.CuCr is thus ideal for all ratings of vacuum circuit breakers(VCBs) until at least a better contact material is developed.

The theory of arc extinction, as related to vacuum, is

ON position OFF position

1. Upper interrupter support2. Top terminal

3,7. Glass ceramic housing4. Arc chamber5. Fixed contact6. Moving contact8. Metallic bellows9. Moving contact stem

10. Bottom terminal11. Lower interrupter support12. Lever13. Insulated coupler14. Contact pressure spring15. Release pawl

2

1

3

45

6

78

9

11 1012

1314

15

Figure 19.25 A pole assembly of a vacuum circuit breaker

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typical. No arc can take place in the absence of a gas. Themolecules of the gas alone under heat and pressure causeionization, responsible for the arc plasma and subsequentdeionization, which extinguishes it. In vacuum, the contentof gas is missing. In fact it should have been an idealcondition to interrupt a circuit without the formation of anarc and thus make the interruption devoid of high TRVsand the phenomenon of arc restrikes. But it is not so, as theheat generated at the parting contacts causes boiling of thecontact material (generally alloy of copper as mentionedabove). This boiling produces metal vapour, usually ofcopper atoms (copper, of all the other alloy metals, has thelowest melting point). Most of the metallized vapour isthus formed of copper atoms only. An electric field withinthe contacts quickly generates free electrons of this metalvapour and a constricted localized plasma is established.Beyond a certain current value, the behaviour of the arc issuddenly modified and the constricted form of the arcplasma transforms to a diffused form. The cathode spotbecomes divided into several very small spots, which thenmove very rapidly, repelling each other continually. Thisphenomenon is used in current breaking in vacuum. Inother mediums and conventional interrupters, the currentmaintains only a single arc column. These spots have anextremely high current density which can reach millionsof amperes per square centimetre. The result is that veryhigh density streams of electrons are emitted without acommensurate quantity of metal vapour. As the currentfalls to zero, at the next current zero the metal vapoursolidifies, leaving behind no medium to hold the arc andthe electrons cease to cross the contact gap. The dielectricstrength reaches its maximum. The anode, being cool isno longer able to emit more electrons, hence it is not ableto restrike after a current zero. Arc extinction in such amedium is therefore extremely quick. The arc plasmadepends largely upon the alloy being used as the contactmaterial. It is of vital importance to limit the excessiveboiling of the contacts due to the arc heat. It is possible toachieve this by suitably designing the contour of the contactsto increase their area. Depending upon the design of thecontacts’ contours, the breaker may be

• Axial magnetic field type or• Transverse magnetic field type

In axial magnetic field type the shape of the contactsmay be as shown in Figure 19.26(a). With this design ofcontacts the arc plasma will spread out axially and increasethe contact area whereas in transverse magnetic fieldtype breakers (Figure 19.26(b)) the contacts are like spiralslits in the form of petals. The design of contacts causesthe current to flow radially outward along the contact,and gives it a rotational movement under the influenceof electrodynamic forces (similar to a rotating arc SF6interrupting device, Section 19.5.5 and Figure 19.21).The rotational movement adds to the contact area andprotects the contacts from damage and a reduced life.But in this case the arc will fall perpendicularly to themagnetic field. It is possible that it may impinge on theinside insulating lining of the contact chamber and rupturethe interrupter. Axial magnetic field type contacts aretherefore generally adopted by manufacturers.

(ii) HV vacuum contactors (HVCs)

This technology (interruption in vacuum) has also beenmade use of in designing contactors. Presently vacuumcontactors are being produced by some manufacturerseven up to 24 kV and 800 A. Figure 19.26(c) shows theviews of an HVC. The use of these contactors can bemade in the switching of MV motors (Section 12.12),HV transformers, capacitors and reactors.

Magnitude of chopping current (Ic) in HVCs

With continuous researches around the world, it has beenpossible to contain it within 0.5 A to 1.2 A, even less inVCs by using AgW (silver tungsten) or CuW (coppertungsten) contacts instead of copper chromium (CuCr)contacts used in VCBs (Further Reading, ref. 9). Some

Arcing with contrate or slotted cup contact

(a) Slotted cup type contacts of a 7.2 kV, 25 kA vacuum interrupter(Axial magnetic field type) (Courtesy: Siemens)

Figure 19.26 Magnetic arc control to increase the contactarea of the arcing contacts in a VCB

(b) Arcing with spiral petal contact(Transverse magnetic field type)

CurrentArc

trajectory

Arc

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manufacturers like Toshiba have achieved it up to 0.3 A.This is against 2–3 A in a VCB noted earlier. It has alsobeen possible to contain the arc duration within one cycle(< 20 ms), closing time 3–4 cycles and interrupting time1.5–3 cycles and even less. The low Ic also helps extinguishthe arc plasma safely by the immediate current zero,point ‘a’ (low Ic makes ‘a1’ approach closer to ‘a’, Figure19.27) and reduce the TRVs making them safe for theterminal equipment. The parting contacts recoveringdielectric strength is so rapid that the arc is safely quenchedeven when the interruption occurs just before a currentzero and full recovery voltage may appear across theparting contacts (Section 19.6). VCs are therefore capableof interrupting low inductive (highly lagging) or highcapacitive (highly leading) currents with little likelihoodof an arc restrike.

Consequently, these contactors cause low arc voltages(Ic.Zs) incapable of causing a restrike of the arc plasma(Figure 19.27). Referring to Example 17.0 the arc voltagein case of VC having an Ic of 0.5 A will be only 0.5 ¥4000 or 2.0 kV for a 350 kW and 0.5 ¥ 2400 or 1.2 kVfor a 500 kW, 6.6 kV motor. These arc voltages areharmless. Also arc voltage falling in phase with TRVdoes not aggravate the situation.

NoteAt the instant of arc interruption, it is the arc voltage that may bemore severe and the terminal equipment should be capable towithstand the same. But gradually as the restrikes take place andstriking voltage builds up, it is the TRV that may become moresevere and defining surge voltage, which the terminal equipmentshould be capable to withstand or means provided to mitigate itsseverity.

AgW has proved to be the most ideal material for arcextinction in vacuum contactors except for high cost ofsilver. Varying content of Ag can vary the arccharacteristics. So is with the change in copper contentwith W for CuW contacts. Performance with Ag is foundto be far more satisfactory than with Cu. Differentmanufacturers however adopt to different mix of Ag orCu with W to meet the functional requirements.

It is noticed that Ic also varies with the surge impedance(Zs) of the interrupting circuit. Higher the Zs (as for lowerratings and higher voltage machines (Figures 17.7 a, b,c)) lower is the Ic and vice-versa, which is a redeemingfactor for the safety of the terminal equipment.

The other merits and demerits of a VC remain generallythe same as discussed for a VCB. However the followingmay be noted as additional features for vacuum contactors;

• Low arc energy helps enhance the contacts’ life andprevents contact welding.

• There being no contact bounce, which further enhancestheir contact life compared to an air-break contactor.

• Low coil consumption – Typically pick up about 300–1500 VA and run 30–50 VA or so depending uponrating of the contactor and may vary from manufacturerto manufacturer.

• Very high contact life say, over 1–3 million mechanicaland 0.5–1 million electrical operations depending uponcurrent rating of the contactor, is more than enoughfor a lifetime operation.

• High operating rate suitability, say up to 1200 ormore operations per hour (it may reduce at higherratings) as against about 150 operations in conventional

1

1

4

3

2

5

1 – Auxiliary switch2 – Closing solenoid3 – On/off indicator4 – Mechanical off-push button5 – Operation counter6 – Mechanical latching device7 – Trip coil8 – Metallic base frame

1 – Upper contact terminal2 – Epoxy cast armature assembly3 – Vacuum interrupter4 – Lower contact terminal5 – Epoxy resin moulded body

Figure 19.26c 6.6kV vacuum contactor (Courtesy: Jyoti Ltd.)

2

4

3

5

6

7

8

Front view Rear view

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air break contactors. This makes them ideal for motorswitchings and controls and suitable for applicationsrequiring frequent switchings, brakings and reversals.

• Very high making and breaking currents.• They are very compact and lightweight. The size

may be up to one-fourth that of conventional air-break contactors.

NoteValues mentioned above are just for reference. For exact valuesone must contact the manufacturer.

Applications: Specially at steel plants, power stations,fertilizers, chemicals, petrochemicals, mines and all firehazardous and corrosive locations besides normalapplications.

(iii) LV vacuum contactors (LVCs)

The advantage of vacuum interruption is now being madeuse of in LV contactors also. Some manufacturers havealready developed this in the current capacity of 160–1200 A or so and many others are in the offing to launchthis product in the market. Because of cost considerationit may have constraints of using them in still lower ratingsuntil at least it is possible to produce the LV vacuuminterrupters such as shown in Figure 19.24, in lowerratings at reasonable cost.

Vacuum contactors in LV would extend the sameadvantages as noted for HV VCs. They are speciallyuseful for applications demanding frequent switchingsand reversals. For LV motors above 100 HP they areideal interrupters.

19.6 Phenomenon of currentchopping

With advances in technology in the field of circuitinterruption, fast to extremely fast interrupting deviceshave been developed, aided by high-performing arcquenching and extinguishing mediums, as discussedabove. While such techniques have helped in theinterruption of system currents, particularly on faults (atvery low p.fs.), they have also posed some problems incertain types of circuit breaking. For instance, an airblast circuit breaker and a vacuum circuit breaker areboth extremely fast operating. When interrupting on afault, their operation is as desired but at much lowercurrents than rated, such as at no-load, they may operaterather faster than desired and interrupt the circuit beforea natural current zero. Premature interruption of a circuitsuch as this is termed current chopping and may occurjust before a natural current zero when the current issmall. In a VCB it is of the order of 3–5 A (now achievedup to 2–3 A).

When the p.f. of the interrupting circuit is low, aswhen interrupting an induction motor or a transformer,running on no-load and drawing a small but highlyinductive current, and when interrupting a highlycapacitive circuit, such as a live but unloaded cable oroverhead line carrying a high capacitive charging current,in all such cases during a circuit interruption the currentmay interrupt before a natural current zero and cause anear peak system voltage across the parting contacts.Figure 17.11(d) has been redrawn in Figure 19.27 formore clarity. Under the cumulative influence of thereflected wave (arc voltage = Ic.Zs) and the equipment’sback e.m.f., it may attain a value of high TRV, capable ofbreaking the dielectric strength across the parting contactsof the interrupting device. It may cause yet higher TRVs,until at least the immediate first natural current zero ofthe interrupting current. See TRV at current zero (Figure19.27). Thus it would endanger the terminal equipmentand the inter-connecting cable. Such surge voltages (TRVs)may rise up to 2.5–3 p.u. in normal operation. It is possiblethat the arc does not extinguish at the first current zero,point ‘a’ on the current wave. If a sufficiently highdielectric strength between the contacts is not attained,even by the next natural current zero (point ‘b’) by virtueof an extremely low contact gap or an inadequate insulationlevel in the arc chamber of the interrupting device, thearc may restrike again and cause higher TRVs. This shallfurther complicate the process of interruption andextinction of the arc. See Blower et al. (1979), Telanderet al. (1986) and IEEE transactions (1977).

Significance of chopping current (Ic)

Lower the Ic, closer will move the point ‘a1’ to current

Figure 19.27 Approximate representation of assumed voltageand current waveforms illustrating a current-chopping effect andits attenuation while interrupting a circuit having 0.3 p.f.

TRV = 2.5 pu.

* 0.95Vm Vm

Contacts break andarc extinguishes here

Voltage wave

0.02 part of one halfcycle of the current wave(enlarged for clarity)

(Ic) current choppedat this pointa1

a b

Surge current ceasesat the immediatecurrent zero ‘a ’

Current wave

* Parting contacts are subject to this voltage which they are notable to withstand and cause an arc raising the TRV up to 2.5 pu

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zero ‘a’ (Figure 19.27) during a circuit interruption. Itwill reduce the TRV and make it easier to extinguish thearc and interrupt the circuit by the immediate currentzero (point ‘a’), besides making it safer for the terminalequipment.

NoteThe surge frequency at which the TRV will restrike will be extremelyhigh. It may be of the order of 10–100 kHz, depending upon thecircuit constants L and C. To interrupt such high-frequency currentsis difficult for an ordinary breaker. But with the use of high technologies,such as adopted in SF6 and VCB interrupting devices, which makethem fast operating (for arcing time see, Table 19.1), it is possible tointerrupt, such high-frequency TRVs promptly.

19.6.1 Influence of frequency on the system

An a.c. current waveform passes through a natural zeroevery one half of a cycle. This is a highly redeemingfactor in an a.c. system. It helps to extinguish an arcpromptly, which is not so in a d.c. system. A higherpower frequency than rated would in fact support theextinction of an arc, irrespective of its other magnetizingeffects, while a lower power frequency than rated willdelay and add to the complications of extinguishing anarc. At surge frequencies the situation becomes different,as the zeros occur so frequently that the contacts aresubject to frequent restrikes of the arc and are vulnerableto damage, while the actual extinction of an arc will takeplace only at a natural current zero. To cope with suchsituations the interrupters must be fast operating. Figure19.27 illustrates the restriking phenomenon of the partingcontacts during current chopping that is assumed to occurat point ‘a1’, on the current wave. The actual current andvoltage waveforms may differ from assumptions,depending upon the speed of the breaker, rate ofdeionization, current being interrupted, its p.f. and theinstant at which the interruption initiates. In addition,the surge impedance of the circuit being interrupted. Weassume that the TRV may rise to 2.5 p.u. and is interruptedby the immediate first current zero (i.e. at point ‘a’ inFigure 19.27) in about 0.02 part of one half of a cycle ofa 50 Hz wave. During this period, if we consider thesurge frequency of the interrupting circuit to be of theorder of 13 kHz* the arc may restrike for nearly 2.6cycles or 5 times before a final interruption as determinedbelow.

Time for 0.02 part of one-half of a cycle of a normalfrequency wave of 50 Hz, during which current choppingoccurs:

= 12

150

0.02 second¥ ¥

\ Number of completed cycles of the TRV at the surgefrequency of 13 kHz

= 13 10 12

150

0.02 cycles3¥ ¥ ¥ ¥

� 2.6 cycles (approximately 5 restrikes)

*It is seen that the surge frequency during current chopping mayrarely exceed 20 kHz and which, in the context of switching surges,may be considered as low-frequency oscillations, easy to handleand interrupt.

R

Y

B

I� I� I�

iy i r i b

Arcrestrikes

(R pole opening)

i b i r i y

i b

i r

i y

C1 C2 C3

Gi g i b

i r

Gi rMotor

windings

I � = Load current i r, i y, i b = Charging or restriking currents

C1, C2, C3 = Interphase dielectric leakage lumped capacitances

Figure 19.28 One pole opening.Restriking phenomenon in phase ‘R’ causing charging currentsin phases ‘Y ’ and ‘B ’ which are still closed

By the immediate first current zero it is assumed thatthe contacts have travelled sufficiently apart to achievethe required deionization and have built up adequatedielectric strength to withstand at least 0.95 Vm. If thecircuit does not interrupt at the immediate current zeroat ‘a’, which is so near to the point of chopping ‘a1’, theinterruption will take place only by the next current zeroat point ‘b’ and result in another 260 strikes by then. Tostudy more accurate behaviour of an interrupter, withthe number of restrikes and the formation of the actualtransient voltage waveforms on current chopping,oscillograms similar to those during a short-circuit testmay be obtained (Section 14.3.6).

19.7 Virtual current chopping

This may occur during the interrupting process of aswitching device when not all the three poles will interruptsimultaneously. It is corollary to a closing phenomenon(Section 17.7.2(ii)) when not all the three poles make atthe same instant. This will also endanger the insulation ofthe other two phases. The interphase dielectric leakagecapacitances, as illustrated in Figure 19.28, are the cause.

When one of the poles, say of phase R, starts openingfirst and faces a restrike of the arc, leading to surgefrequency currents, similar (balancing) currents will beinduced in the other two phases that are still closed, inaddition to the normal current, II, that these poles willstill be carrying. The result will be that when these two

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poles also open the charging currents at a surge frequencymay virtually force a faster or premature current zero inphase B, as illustrated in Figure 19.29. This is termedvirtual current chopping and may cause an additional TRV.

The amplitude of this TRV, however, may not be largedue to a generally low surge impedance of the interruptingcircuit during an interruption. It may achieve a level ofonly 0.6–0.7 p.u. (see Telander et al., 1986), which maysometimes prove fatal for the insulation of the terminalequipment due to its steepness. Charging currents woulddevelop in phase Y also (at point ‘y’, but not shown toavoid overlapping of curves). But current chopping is notpossible in this phase because point ‘y’ will fall furtheraway from a current zero, on the one hand, and the Y-phase would carry a near-maximum current at this instant,on the other. Current chopping is a phenomenon of smallcurrents.

19.8 Containing the severity ofswitching surges

19.8.1 Theory of energy balancing

From the above we can deduce that at the instant ofcurrent chopping the arc extinguishes for a moment andre-establishes as soon as the TRV reappears. At the instantof arc extinction, it may be considered that the arc energyis transferred to the dielectric medium of the partingcontacts. This energy reappears in the form of a TRV

across them, and tends to cause a restrike of the arcplasma once again. We can generally consider the arcenergy as the magnetic energy caused by the magnetizingcurrent of the interrupting circuit and the energy receivedby the dielectric medium as the capacitive energy.

If Inl is the inductive current in amperes (no-load currentof the motor or the transformer, Figure 1.15) and L theinductance of the circuit being interrupted in henry, thenthe electromagnetic energy, J, initially contained by thearc plasma

J L I = Joules1

22n◊ ◊ �

NoteIn fact, this energy should be less by the hysteresis loss component(Section 1.6.2A.(iv)) which has been ignored in the present analysis.

If Vt is the prospective peak surge voltage (TRV) in voltsand C the dielectric capacitance of the contact gap infarad at the instant of restrike, then the capacitive energyJ, received across the contact gap is

J C V = Joules12 t

2◊ ◊

For successful interruption of the arc plasma it isessential that the energy emitted by the arc plasma is atleast equal to the capacitive energy received by thedielectric medium. At the instant of arc extinction, thisphenomenon is termed energy balancing, when

12 n

2 12 t

2 ◊ ◊ ◊ ◊L I C V� �

or V I L

Ct2

n2 � � ◊

or V I L

CI Zt s n n� �� ◊

We represent this through a circuit diagram (Figure 19.30)where

L C/ is the surge impedance, Z s, L and C the circuitconstants of the interrupting circuit, as discussed in(Section 17.6.4). C represents the dielectric capacitancebetween the parting contacts of the interrupter. Vt mustbe prevented, as far as practicable, from reachingdangerous levels with the use of surge arresters.

Inl of equipment cannot be altered once it ismanufactured. The surge voltage Vt, that will appear acrossthe contacts therefore becomes a function of theinductance, L, of the circuit being interrupted. L is theinductance of the motor or the transformer windings upto the terminals of the interrupting device, including theinductance of the inter-connecting cables or the bus system.The dielectric strength is the capacitance, C, betweenthe contacts. The C of the contacts will change with thetravel of the contacts and the deionization of the arcplasma. The influence of the inductance of the circuit L,can be reduced by introducing some capacitance orresistance into the circuit. Capacitance can be introducedby installing a few p.f. correction capacitor banks acrossthe motor or the transformer terminals and the resistancecan be introduced temporarily across the moving contactof the interrupting device through its closing mechanism.This is a practice adopted by leading manufacturers toprovide a resistance where the interrupter is required tooperate under adverse switching conditions.

Figure 19.29 An approximate illustration of a virtual currentchopping in phase ‘B ’ as a consequence of restrike of arc inphase ‘R ’

Location, r – Interruption commencesr ¢ – Restrike of arc occurs in phase ‘R ’ causing a charging

current ‘i r’y – Charging current ‘iy ’ develops in phase ‘Y ’ (Not shown

to avoid overlapping)b – Charging current ‘i b’ develops in phase ‘B ’

b ¢ – Charging current i b causes a virtual forced current zeroor virtual current chopping at this point

Virtual current chopping in phase ‘B ’

I

R Y

y

B

i b

r b ¢

r ¢b

i r

w t

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19.9 Comparison of circuit breakersusing different interruptingdevices

To assist in making an easy selection of a breaker for aparticular application, we have provided a comparisonbetween the various circuit breakers as in Table 19.1.

19.10 Gas insulated switchgears(GIS)

This is discussed only briefly to give an idea of theadvantages and applications of SF6 gas technology inHV and EHV switchgears compared to conventional airinsulated switchgears. For finer details one must consultthe manufacturer.

The advantages of SF6 gas noted in Section 19.5.5prompted its application into development of gas insulatedswitchgears even complete GIS based substations. VCBsand SF6 breakers comprising SF6 gas insulated motorizeddisconnectors and grounding switches, CTs, VTs andCVTs make a complete gas insulated substation extendingthe following advantages over conventional air insulatedswitchgears. Use of VCBs or SF6 breakers would dependupon the system voltage. Usually VCBs are used up to36 kV and SF6 breakers for 72.5 kV and above (Table19.2). A few diagrams showing VCB based GISsubstations are shown in Figures 19.31 and 19.32. Figure19.33 shows the layout of a SF6 breaker based GIS powerhouse switchyard, while Figures 19.34 and 19.35 illustratethe cross-sectional and sectional views of a SF6 breakerbased GIS with non-isolated double busbar system. Figure

19.36 illustrates sectional view of an isolated doublebusbar GIS.

Special Features

– VCBs and SF6 breakers are essentially maintenancefree. Gas insulated switchgears are therefore highlyreliable and almost maintenance free.

– The enclosure of the switchgear is at ground potentialand extends total safety to operators.

– All components are housed in hermetically sealedmetal enclosures filled with SF6 gas.

– They are environment friendly and corrosion resistantand can be installed in humid and saline (coastal)areas or industrial areas that are prone to dust andpollution and may also be fire hazardous or chemicallyaggravated.

– The plug-in or cable sealing-ends or busbar terminationsare at ground potential and safe to touch.

– It is possible to provide LEDs on the outer surface ofthe modules to ensure safe touch leakage voltage.The LEDs would glow when the leakage capacitancevoltage to ground would exceed the safe touch voltage.

– Each equipment and component is encapsulated in acompact module which occupies only a small spacecompared to if it was air insulated. Also each modulebeing at ground potential needs only small clearancesfor maintenance. GIS substations are thereforeextremely compact (see Table 19.2) and can be installedindoors or outdoors as per the site requirement.

– They extend an easy solution to space problem andare ideal for densely populated areas. VCB based GIShave extensive applications at airports, commercialbuildings, hotels, hospitals, residential colonies andpower distribution centres (substations) while SF6

j

Interrupter

Junction

Interconnectingcable

Transformer say,11 kV/6.6 kV

Interrupter


M Motor 6.6 kV

Arc duringinterruptionor a restrike

Interruptingdevice

Contact gap having adielectric capacitance‘C ’ which is subject toa TRV of Vt during arestrike

I n�

I m ¢ Im


Circuitinductance ‘L’

C

Vt

Figure 19.30(a) Figure 19.30(b) An energy-balancing phenomenon whileinterrupting a highly inductive circuit as shown in (a)

No-load magnetizing circuit of aninduction motor or a transformer

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Circuit interrupters and their applications 19/749Ta

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bove

Not

eco

nom

ical

Onl

y on

e br

eak

per

pole

,th

eref

ore

curr

ent

rati

ngs

in h

ighe

r ra

nges

are

rest

rict

ed4–

6 cy

cles

2–3

cycl

es

2 cy

cles

4–5

cycl

es

SF6a

VC

Ba

67

Sul

phur

hex

aflu

orid

eV

acuu

m

7.2–

765

kV a

nd a

bove

3.6

kV t

o 36

kV

Not

app

lica

ble

Not

app

lica

ble

Up

to 4

000

A a

nd h

ighe

rA

s re

quir

ed

4–5

cycl

es2–

3 cy

cles

12–

cyc

les

1 21–

2 cy

cles

11–

cyc

le1 2

1 2–1

cyc

le

2–4

cycl

es1

–3 c

ycle

s1 2

Em

its

ions

of

S+ (

prot

ons)

,Em

its m

etal

lic v

apou

r fro

mS

N (

neut

rons

), a

ccom

-th

e co

ntac

t mat

eria

l. Si

nce

pani

ed b

y el

ectr

ons

Se

the

cont

acts

are

ess

entia

llyan

d F

– and

met

alli

cm

ade

of c

oppe

r al

loy,

vapo

urs

copp

er h

avin

g th

e lo

wes

t

Em

its

ions

of

H2

arou

nd 7

0% a

nd r

emai

ning

as a

cety

lene

and

met

alli

c va

pour

s

(Con

td)

Auth

or: K.

C. A

graw

al

ISBN

: 81

-901

642-

5-2


12

34

56

7

3T

heor

y of

arc

quen

chin

g an

dex

tinc

tion

, th

roug

hde

ioni

zati

on o

f ar

cpl

asm

a

4N

umbe

r of

no-

load

oper

atio

ns (

depe

nds

upon

the

arc

ene

rgy:

the

low

er th

e ar

cen

ergy

, the

hig

her

will

be th

e nu

mbe

r of

ope

-ra

tions

and

vic

e ve

rsa)

.

5M

aint

enan

ce

6D

iele

ctri

c st

reng

thco

mpa

red

to a

ir

7A

rc e

nerg

y co

mpa

red

to o

il

8W

heth

er s

uita

ble

toin

terr

upt

smal

lin

duct

ive

(mag

neti

zing

) or

capa

citi

ve c

urre

nts,

wit

hout

cau

sing

curr

ent

chop

ping

Coo

ling

of

arc

plas

ma

is b

ased

on

the

bubb

le t

heor

y,H

2 co

olin

g it

by

the

turb

ulen

ce c

ause

d by

H2

bubb

le

Min

. 10

00M

in.

1000

Dei

oniz

atio

n of

N2

(O2

havi

ng a

sm

all c

onte

nt, h

asno

sig

nifi

cant

inf

luen

ce)

by l

engt

heni

ng o

f th

e ar

cpl

asm

a th

roug

h th

e ar

cch

utes

, as

a re

sult

of m

ag-

neti

c fi

eld

indu

ced

in t

hem

etal

lic s

plitt

ers

of th

e ar

cch

ute

by th

e in

duct

ive

arc

plas

ma

Min

. 10

00

boil

ing

poin

t, th

e va

pour

is l

arge

ly c

ompo

sed

ofco

pper

ion

s

Dei

oniz

atio

n o

f N

2 an

dO

2 by

a s

tron

g bl

ast

ofai

r. T

he d

eion

izat

ion

forc

e re

mai

ns t

he s

ame

for

one

size

of

brea

ker,

irre

spec

tive

of

the

curr

ent

Min

. 10

00

Dei

oniz

atio

n of

fre

esu

lphu

r el

ectr

ons

Se ,ta

kes

plac

e th

roug

h th

eir

abso

rpti

on b

y th

efl

uori

ne i

ons

F–

(ele

ctro

nega

tive

the

ory)

Min

. 50

00

Req

uire

s ve

ry l

ittl

em

aint

enan

ce,

exce

ptpe

riod

ic c

heck

s of

pres

sure

and

con

diti

onof

the

con

tact

s

2–3

Rou

ghly

1 3 (

Figu

re 1

9.6)

Dei

oniz

atio

n of

cop

per

ions

is

natu

ral

and

extr

e-m

ely

fast

Min

. 10

000

–20

000

Req

uire

s ve

ry l

ittl

em

aint

enan

ce,

exce

ptpe

riod

ic c

heck

s of

vacu

um a

nd c

ondi

tion

of

the

cont

acts

. M

axim

umco

ntac

t er

osio

n 2–

3 m

m

Nea

rly

10 t

imes

(5

tim

esof

oil

)

Rou

ghly

1 10

(Fi

gure

19.

6)

Hig

h, t

o ch

eck

the

con-

diti

on o

f oi

l, co

ntac

ts a

ndar

c qu

ench

ing

fins

and

devi

ces

> 2

1 Yes

(bu

t fo

r a

lim

ited

num

ber

of s

wit

chin

gop

erat

ions

), b

ecau

se o

fdo

uble

bre

ak p

erph

ase

that

enab

les

quic

kre

stor

atio

n of

the

diel

ectr

ic s

tren

gth

Che

ckin

g th

e co

ndit

ion

ofoi

l is

mor

e fr

eque

nt,

invi

ew o

f sm

alle

r qu

anti

tyof

oil

> 2

1 No,

bec

ause

of

gene

rall

yon

e br

eak

per

pole

, fo

rbr

eake

rs 1

2 kV

and

abov

e, u

nles

s th

ebr

eake

r is

spe

cial

lyde

sign

ed w

ith

extr

aqu

ench

ing

syst

emth

roug

h a

jet

of o

il

Ve r

y lo

w m

a int

e na n

c e e

xce p

t fo

r c h

e cki

ng t

heco

ndit

ion

of t

he a

rcin

g co

ntac

ts f

or a

ny w

ear,

tear

or

pitt

ing

1C

ompr

esse

d ai

r has

bet

ter

than

1

––

(Con

tac t

ero

sion

low

and

sui

tabl

e fo

r fr

e que

ntop

erat

ions

)

On

an L

V s

yste

m, s

uch

aph

enom

enon

has

no

rele

vanc

e, b

ecau

se o

fhi

gh d

iele

ctri

c st

reng

thbe

twee

n th

e pa

rtin

gco

ntac

ts c

ompa

red

to t

hesy

stem

vol

tage

No,

bec

ause

of

forc

e of

air

blas

t ev

en a

t sm

all

curr

ents

whi

ch m

ayde

velo

p th

e te

nden

cy t

ocu

rren

t ch

oppi

ng.

On

the

othe

r ha

nd,

it i

s po

ssib

leth

at t

he b

reak

er m

ay f

ail

to i

nter

rupt

the

ste

epfr

onte

d T

RV

s

Yes

, in

all

typ

es o

f ar

c-qu

ench

ing

tech

niqu

es:

(i)

Puf

fer t

ype:

bec

ause

the

flow

of

gas

thro

ugh

the

arc

is a

func

tion

of

mag

ni-

tude

of

curr

ent

(ii)

Rot

atin

g ar

c ty

pe:

beca

use

the

mag

neti

c fi

eld

that

Gen

eral

ly n

ot,

beca

use

of e

xtre

mel

y fa

stde

ioni

zati

on.

It h

as t

hete

nden

cy t

o cu

rren

tch

oppi

ng.

How

ever

, in

view

of

cont

inuo

usde

velo

pmen

t in

thi

sfi

eld,

it

has

now

bee

npo

ssib

le t

o ac

hiev

e a

chop

ping

cur

rent

as

low

(Con

td.)

Tab

le 1

9.1

(Con

td.)

Auth

or: K.

C. A

graw

al

ISBN

: 81

-901

642-

5-2


12

34

56

7

9A

ppli

cati

ons:

Sw

itch

ing

of∑

Indu

ctio

n m

otor

s∑

Gen

erat

ors

∑ T

rans

form

ers

∑ R

eact

ors

∑ In

duct

ive

circ

uits

∑ C

apac

itor

ban

ks∑

Dis

trib

utio

n li

nes

∑ H

ighe

r-sp

eed

auto

-re

clos

ing

(a)

Sui

tabl

e fo

r al

lap

plic

atio

ns,

not

requ

irin

g fr

eque

ntsw

itch

ing

oper

atio

ns.

(b)

Unt

il a

few

yea

rsag

o th

ey w

ere

bein

gex

tens

ivel

y us

ed f

or a

llLV

app

lica

tion

s

Ext

ensi

vely

use

d fo

r al

lLV

app

lica

tion

s an

dsu

itab

le f

or f

requ

ent

oper

atio

ns

driv

es t

he a

rc i

spr

oduc

ed b

y th

ein

terr

upti

ng c

urre

ntit

self

, an

d w

ill

vary

wit

h th

e cu

rren

t,an

d so

wil

l va

ryth

e co

olin

g ga

s in

dire

ct p

ropo

rtio

n.T

he s

mal

ler

the

curr

ent

to b

ein

terr

upte

d, t

hesm

alle

r w

ill

be t

hem

agne

tic

fiel

d an

dth

e co

olin

g fo

rce,

enab

ling

the

arc

to

inte

rrup

t at

ana

tura

l cu

rren

t ze

roon

ly.

But

thi

ste

chni

que

is n

owse

ldom

pra

ctis

ed(i

ii)

The

rma l

bla

st,

a rc

assi

sted

and

dou

ble

volu

me

type

:A

s (i

) ab

ove

Sui

tabl

e fo

r al

lap

plic

atio

ns. S

witc

hing

of

capa

cito

r ban

ks, h

owev

er,

may

pos

e pr

oble

mbe

caus

e of

hig

h T

RV

s.T

hese

may

cau

se a

rest

rike

of th

e ar

c pl

asm

a an

d gi

veri

se t

o sw

itch

ing

surg

es.

How

ever

, w

ith

the

use

of p

re-i

nser

tion

resi

stan

ce a

cros

s th

epa

rtin

g co

ntac

ts (

fixe

dan

d m

ovin

g)(F

igur

e19

.22)

the

se b

reak

ers

can

be m

ade

rest

rike

free

eve

n w

hen

swit

chin

g la

rge

capa

cito

rba

nks.

as 0

.5 t

o 1.

2A a

nd e

ven

less

. S

ecti

on 1

9.5.

6.T

here

fore

, ex

cept

inte

rrup

ting

cur

rent

slo

wer

tha

n th

is,

thes

ein

terr

upte

rs a

re s

uita

ble

to i

nter

rupt

suc

h cu

rren

tsw

itho

ut c

urre

nt c

hopp

ing

Sui

tabl

e fo

r al

l ty

pes

ofin

dust

rial

nee

ds a

ndfr

eque

nt o

pera

tion

s.T

heir

abi

lity

for

qui

ckin

terr

upti

on a

nd f

ast

buil

ding

up

of d

iele

ctri

cst

reng

th m

ake

them

suit

able

for

one

rous

duti

es a

nd h

igh

TR

Vs

(a)

Sui

tabl

e fo

r al

lap

plic

atio

ns n

otre

quir

ing

freq

uent

swit

chin

g op

erat

ions

Gen

eral

ly s

uita

ble

for

HV

dis

trib

utio

n on

ly,

and

at i

nsta

llat

ions

empl

oyin

g a

num

ber

ofsu

ch d

evic

es t

o m

ake

dry

com

pres

sed

air

syst

em h

andy

and

econ

omic

al

(Con

td.)

Tab

le 1

9.1

(Con

td.)

Auth

or: K.

C. A

graw

al

ISBN

: 81

-901

642-

5-2


12

34

56

7

10W

heth

er a

sur

gear

rest

er i

s es

sent

ial

11E

ffec

t of

lea

kage

on

perf

orm

ance

:(i

)O

pera

ting

saf

ety

(ii)

Die

lect

ric

stre

ngth

(iii

)In

terr

uptin

g ca

paci

ty

No

Gen

eral

ly n

ot

(i)

Gen

eral

ly,

the

use

ofS

F6

is s

afe,

inh

alin

g is

pois

onou

s. I

f th

ere

is a

leak

age

it w

ould

be

dang

erou

s. S

F6

itse

lf i

sno

n-to

xic,

but

dec

om-

pose

d S

F6

is m

oder

atel

yto

xic

and

mus

t be

hand

led

unde

r co

ntro

lled

cond

itio

ns. T

rain

ing

and

guid

elin

es a

re e

ssen

tial

for

pers

onne

l w

orki

ngon

suc

h eq

uipm

ent.

Sit

eda

ta c

olle

cted

fro

mva

riou

s so

urce

s,ho

wev

er,

sugg

est

ale

akag

e ra

te o

f le

ss t

han

0.1%

per

ann

um

Sti

ll h

igh

In t

he e

vent

of

ale

akag

e, t

he b

reak

er w

ill

stil

l be

in

a po

siti

on t

oin

terr

upt

the

norm

alcu

rren

ts

Bet

ter

to p

rovi

de(d

epen

ding

upo

n th

ety

pe o

f eq

uipm

ent

it i

ssw

itch

ing,

the

min

imum

curr

ent

it h

as t

o in

terr

upt

and

the

like

ly a

mpl

itud

eof

TR

V)

Not

app

lica

ble

Not

app

lica

ble

Not

app

lica

ble.

But

if

the

leak

age

is in

the

com

pres

-se

d ai

r su

pply

sys

tem

, the

brea

ker

may

not

inte

rrup

ton

fau

lt,

depe

ndin

g up

onth

e ai

r pre

ssur

e th

e sy

stem

is a

ble

to m

aint

ain

on a

leak

age

No

Not

app

lica

ble

Yes

, pa

rtic

ular

ly f

or d

ryin

sula

ted

equi

pmen

t (su

chas

an

indu

ctio

n m

otor

, a

dry

type

rea

ctor

or

a dr

yty

pe t

rans

form

er),

whi

chha

ve lo

w in

sula

tion

leve

ls.

An

oil-

imm

erse

d eq

uip-

men

t (s

uch

as a

pow

ertr

ansf

orm

er),

hav

ing

are

lati

vely

muc

h be

tter

diel

ectr

ic s

tren

gth,

may

be

swit

ched

wit

hout

an

arre

ster

Gen

eral

ly a

saf

eeq

uipm

ent.

But

in

the

even

t of

vac

uum

lea

kage

whi

ch,

alth

ough

rem

ote,

may

cau

se a

fir

e ha

zard

and

X-r

ays.

X-r

ayw

arni

ng a

nd p

rope

rsh

ield

ing

may

be

esse

ntia

l

Zer

o

In t

he e

vent

of

ale

akag

e, t

he b

reak

er w

ill

not

be i

n a

posi

tion

to

inte

rrup

t ev

en n

orm

alcu

rren

ts

No

Sin

ce th

e oi

l is

fill

ed a

t atm

osph

eric

pre

ssur

e, th

ere

isge

nera

lly

no l

eaka

ge

Not

app

lica

ble

Not

app

lica

ble

Not

app

lica

ble

Not

app

lica

ble

Not

app

lica

ble

Not

app

lica

ble

(Con

td.)

Tab

le 1

9.1

(Con

td.)

Auth

or: K.

C. A

graw

al

ISBN

: 81

-901

642-

5-2


Thi

s ha

s th

e la

test

tech

nolo

gy in

the

arc

inte

rrup

tion

and

sui

tabl

efo

r al

l app

lica

tion

s as

note

d ab

ove,

in v

iew

of

havi

ng a

chie

ved

a ve

rylo

w c

hopp

ing

curr

ent,

ofth

e or

der

of ju

st 0

.5–

1.2A

and

eve

n le

ss. I

nvi

ew o

f li

mit

ed v

olta

gera

nge,

how

ever

, it

isfa

cing

lim

itat

ions

in it

sex

tens

ive

appl

icat

ion.

The

dev

elop

men

t wor

kon

hig

her

rang

es is

sti

llun

der

way

and

not

man

ybr

eake

rs a

bove

36

kVha

ve b

een

prod

uced

so

far.

Tend

ency

to a

col

dw

eld,

dur

ing

clos

ing

and

chan

ge in

the

arc

cham

ber

pres

sure

, dur

ing

an in

terr

upti

on m

ayda

mag

e th

e m

etal

lic

bell

ows

(Fig

ure

19.2

4).

The

se a

re s

ome

impe

dim

ents

in th

eir

exte

nsiv

e us

e

Thu

nder

ous

nois

e du

ring

an in

terr

uptio

n, b

ecau

se o

fai

r bla

st. T

he in

tens

ity

can

be c

ontr

olle

d by

pro

vidi

ngsi

lenc

ers

In l

ower

ran

ges,

say

, up

to 7

2.5

kV,

this

bre

aker

is n

ow f

ast

losi

ng i

tsho

ld i

n fa

vour

of

SF

6,du

e to

cos

tco

nsid

erat

ions

and

the

cum

bers

ome

dry

air

pres

sure

sys

tem

. In

high

er r

ange

s, s

ay,

123–

765

kV,

how

ever

, th

eyar

e st

ill

bein

g us

ed b

uton

ly r

arel

y

Gen

eral

ly,

oil

is f

ire

haza

rdou

s, b

ut n

ot p

rone

to

itun

less

the

die

lect

ric

stre

ngth

rea

ches

a v

ery

low

leve

l an

d th

e m

ediu

m i

tsel

f be

com

es c

ondu

ctin

g.N

ever

thel

ess,

the

y ar

e no

t su

itab

le a

t lo

cati

ons

whi

ch a

re f

ire

haza

rdou

s. N

orm

al m

ake

or b

reak

,ev

en o

n fa

ult,

may

onl

y de

teri

orat

e th

e qu

alit

y of

oil,

but

yet

not

mak

e it

fir

e ha

zard

ous,

unl

ess

asno

ted

abov

e

Qui

et o

pera

tion

Qui

et o

pera

tion

The

se b

reak

ers

wer

e ex

tens

ivel

y us

ed u

ntil

abo

ut19

70s

but

are

now

fas

t lo

sing

the

ir h

old

in f

avou

rof

SF

6 an

d V

CB

s fo

r H

V a

nd A

CB

s fo

r LV

sys

tem

s

12

34

56

7

a The

se c

ompa

riso

ns r

elat

e on

ly t

o H

V i

nter

rupt

ers.

b For

LV s

yste

ms

only

.c R

efer

red

to a

t po

wer

fre

quen

cy (

50 o

r 60

Hz)

and

rat

ed c

urre

nt. T

hese

fig

ures

are

app

roxi

mat

e an

d fo

r a

gene

ral

refe

renc

e on

ly. T

hey

may

var

y w

ith

rati

ng, l

oadi

ng a

nd m

anuf

actu

rer.

For

low

er v

olta

ge r

atin

gs,

they

may

be

slig

htly

hig

her.

Arc

for

mat

ion

and

exti

ncti

on t

akes

pla

ce i

nsid

e a

seal

ed c

ham

ber,

thu

s em

itti

ng n

o ga

ses

or v

apou

rsto

the

atm

osph

ere,

whi

ch m

ay b

e a

caus

e of

fir

eha

zard

. The

y ar

e th

e m

ost

appr

opri

ate

choi

ce f

orsu

ch a

reas

The

y m

ay b

e ra

ted

as m

ore

fire

haz

ardo

us t

han

aB

OC

B o

r M

OC

B i

n vi

ew o

f ar

c fo

rmat

ion

taki

ngpl

ace

in th

e op

en, a

lthou

gh u

nder

con

trol

led

cond

ition

s.T

hey

are

not

suit

able

for

ins

tall

atio

ns p

rone

to

fire

haza

rds,

unl

ess

the

sub-

stat

ion

or t

he c

ontr

ol r

oom

whe

re t

hey

are

inst

alle

d is

iso

late

d fr

om t

he a

rea

ofha

zard

s (S

ecti

on 7

.11)

. The

y ar

e ge

nera

lly

suit

able

for

all

othe

r ar

eas

12F

ire

haza

rds

13N

oise

lev

el

14T

rend

s

Exc

ept

duri

ng a

nin

terr

upti

on,

it h

as q

uiet

oper

atio

n

Onl

y br

eake

r fo

r al

l LV

appl

icat

ions

Qui

et o

pera

tion

Qui

et o

pera

tion

Thi

s ha

s be

com

e th

em

ost

pref

erre

d br

eake

rw

ith

the

wid

est

volt

age

rang

e an

d su

itab

ilit

y fo

ral

l ap

plic

atio

ns

Tab

le 1

9.1

(Con

td.)

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Figure 19.33 220 kV GIS at the Sendai thermal power station switchyard of Kyushu Electric Power Co., Inc, Japan (1973)(Source: Hitachi)

Figure 19.31 Non-withdrawable vacuum circuit breaker (VCB)switchgear with single busbars (Source: Siemens)

Figure 19.32 Rear view of Figure 19.31

AluminiumBusbars

SF6

Isometric view

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As intelligent switchgears

It is easy to make them intelligent with the use ofmicroprocessor based relays, electronic releases andmetering. See Section 13.8.1 for details.

Constructional features

– Gas tight aluminium castings, aluminium welded orstainless steel housings are used to encapsulate VCBor SF6 interrupters.

– Disconnectors and grounding switches are alsomounted inside the enclosures.

– Busbar, interrupter and switching chambers areseparated by gas tight barriers.

– There being no contact between the arc chamber of aVCB or SF6 breaker and the bus chamber. The insulatinggas of the bus chamber is not exposed to arcing chamberand causes no deterioration of the insulating SF6 gasand permits long years of maintenance-free operation.

– Operating mechanism of breakers, disconnectors andswitches and all auxiliary components like CTs orRogowski coil transducers (Section 15.11), VTs, CVTsare mounted outside the hermetically encapsulatedhousings to maintain them at ground potential andfacilitate easy maintenance without disturbing the maincomponents.

– The VCB is usually mounted in a vertical formation inthe bottom housing, each pole housed in a separateenclosure one behind the other as shown in Figure 19.31.The upper housing encapsulates the disconnector switchthat connects the busbars. SF6 breakers can be mounted

Figure 19.35 Sectional view of a 145 kV GIS bay with non-isolated double busbar system (Source: Toshiba)

breaker based GIS are used at power stations andswitchyards. They are an ideal choice for hilly areaslike for mini and large hydel power projects.

– With the global warming and consequent rising ofsea level, shrinking of available land area and risingpopulation, every effort is being made worldwide tosave on land wherever possible for the good of themankind. Use of GIS will contribute to these efforts.

– The overall compact structure of GIS also saves onI2R loss and contributes significantly towards energysaving which too is a buzzword in today’s scenario asdiscussed already.

Figure 19.34 Cross-sectional view of a GIS substation with non-isolated double busbar configuration (up to 500 kV)(Source: Hitachi)

Circuit breaker

Local control cubicle

Maintenancegroundingswitch

Grounding switchwith makingcapacity

Voltagetransformer

Removable link

Cable headMaintenancegroundingswitch

Cable isolator

Currenttransformer

Current transformerSpacer

Operatingmechanismfor circuitbreaker

Busbarisolator

B busbars

Busbarisolator

A busbars

Circuit diagram

1

1

3

7

9

22

10

11

4

5

6

8

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horizontally or vertically. Horizontal layouts at highervoltages facilitate easy erection and maintenance.

– Indication, measurement, protection, control and alarmdevices can be mounted separately on a central controlpanel.

– Busbars: They can be either single or double busbarsystems depending upon the number of supplysources.

• Each busbar system can be a conventional threephase system to economize on space and cost. Suchbusbars are used in trefoil formation to nullify theproximity effect and space field (Section 28.8) asalso enclosure currents (Section 31.4).

• They can also be single phase isolated phase bussystem to make the switchgear suitable for higherfault levels by limiting the internal faults to single

Dimensions Isolated bus (VCBs) Non isolated bus (SF6 breakers)

System Voltage kV Æ 7.2/36 72.5–145 170–245 245–300 360–420 525 765–800 1100Width-(W) m 0.6 1.0 2.8 3.1 3.6 5.5 6.5 7.5Height-(H) m 2.46 4.0 4.2 4.8 6.0 6.5 6.5 6.0Depth-(D) m 2.66 4.7* 5.7* 6.5* 8.0* 12* 20* 30*% floor space (W ¥ D) 30 14 8 6 5 4 – –required by GIScompared to AIS % Saving 70 86 92 94 95 96 – –

Notes1. These dimensions may vary by 10% or so.2. *For isolated bus system the depth will be slightly more.

Table 19.2 Typical floor dimensions for GIS (with double bus)

I(R)

I(Y)

I(B)

One set of busbars Second set of busbars

II(R) Phase

II(Y) Phase

II(B) Phase

3

7

4 3

6

26

5

8

4

3

1

Figure 19.36 Typical sectional view of an isolated double busbar GIS substation bay up to 420 kV (Source: GEC-Alsthom)

1. Circuit-breaker without closing resistor 5. Make-proof grounding switch2. Hydraulic mechanism 6. Current transformer3. Disconnector 7. Voltage transformer4. Maintenance grounding switch 8. HV cable connection

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phase faults and preventing a ground fault to developinto a phase fault.

– Each busbar system can be connected directly to atransformer.

– Gas filled in the entire enclosure forms a closed cyclesystem for easy monitoring and minimizing leakages.

– For making manufacturing simpler, cost effective andalso for ease of monitoring the gas pressure of theentire switchgear equipment is divided into separatehousings, each fitted with gas pressure monitors, fillingvalves and alarm switches. Usually an alarm is soundedwhen the pressure drops by 5% or so of the set pressure.The usual inside pressure is maintained at about 3–5kg/cm2 depending upon the system voltage.

Cable termination – Through plug-in type cabletermination or by conventional cable sealing ends.

Dimensions

Typical floor dimensions of GIS and comparativedimensions of conventional air insulated switchgears (AIS)for different interrupting devices, voltage systems andbusbar configurations are shown in Table 19.2 just forreference. For exact details the user must contact themanufacturer.

One can visualize the compact sizes of GIS andconsequently their small floor area requirement comparedto AIS. Higher the system voltage higher will be thespace saving.

Local control station

A local control panel can be provided for a complete bayof GIS and may comprise the following:

– Controls for operating ON/OFF all the switchingdevices (breakers, disconnectors and groundingswitches)

– Display of switch position– LV supply protection– Alarm display– Safety interlocks– Measuring instruments– Interface terminals for remote control– Protective relays (or the bay control unit of a substation

digital control system) as per the system requirement.– Any other requirement

Termination

The GIS can be terminated directly to cables, overheadlines, transformers or reactors through gas insulatedbusducts (GIBs). The necessary jointing kits are suppliedby the manufacturer to terminate at a bushing or aninsulator. See Section 28.2.6 for inter-connecting HVand EHV equipment.

GIS condition monitoring

Condition monitoring is sort of a preventive maintenanceand is a good service practice to enhance the reliability

of GIS that otherwise are capable to perform themselvesunattended for long years. The following monitoringdisciplines can be practised.

– SF6 breaker health monitoring: to monitor thecondition of the breaker and undertake servicing whenit is actually necessary by recording and transmittingits operating conditions such as travel curves, electricalwear and tear and other vital-parameters to a centralmonitoring station.

– Ultra high frequency (UHF) monitoring – to detectinternal partial discharges. It can be accomplishedthrough monitoring the UHF electromagnetic (EM)waves generated by partial discharges.

– SF6 gas monitoring

• Density of gas – to decide on refilling• Pressure alarm• Gas liquefaction detection• Sensor status control• Internal failure detection• Maintaining historical records• Any other vital parameter

19.11 Retrofitting old installationswith vacuum and SF6 breakers

Referring to switchgears, retrofitting aims at upgradingthe existing installations using OCBs, MOCBs or ABCBswith state-of-the-art interrupting technologies throughVCBs and SF6 breakers.

OCBs, MOCBs and ABCBs as discussed already arenow outdated at least for the new installations. But theydo exist at many old installations. In view of ever risingfault level of a system and to achieve low maintenancelevel, it is advisable to gradually changeover the oldinstallations with VCBs or SF6 breakers as per the systemvoltage and fault level. Upgradation is surely imperativefor installations undergoing renovation or expansion andall important installations like power stations, switchyardsand transformer substations, steel, automobile, chemical,petrochemical, cement and fertilizer plants. All largeindustries and other installations like airports, residentialcolonies and commercial buildings or hazardous locations.Not only to meet the fault level of the extended system,it may also be cumbersome to operate and maintainbreakers of two generations at the same installation withas many spares and maintenance experts.

Most manufacturers of switchgears and contractingcompanies are extending this service to their clients tofacilitate quick, smooth and cost-effective changeoversat most installations even without or only short shutdowns.Most switchgear structures, foundation and cabling mayremain much the same with minor modifications, savingabout half the cost of replacement by new switchgears.Replacement by new switchgear means shifting of cables,modification of foundation and other associated cost.

The normal practice is to offer new breakers in modularform (on trolley) making them matching with the oldinstallations. It is just removing the old breaker and rackingin the new one.

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Further Reading

1 Bettge, H., ‘Design and construction of vacuum interrupters’,Siemens Circuit, XVIII, No. 4 (1983).

2 Blower, R.W., Distribution Switchgear, Collins Professionaland Technical Books, 1986.

3 Blower, R.W., Cornick, K.J. and Reece, M.P. ‘Use of vacuumswitchgear for the control of motors and transformers in industrialsystems’, Electric Power Applications, 2, No. 3, June (1979).

4 Bouilliez, O., ‘Mastering switching voltage transients with SF6

switchgear’, Cahiers techniques Merlin Gerin, No. 125.5 ‘Current chopping phenomena of MV circuit breakers’, IEEE

Transactions, PAS –96, No. 1 (1977).6 Pelenc, Y. ‘Review of the main current interruption techniques’

Voltas Limited, Switchgear Division, India.

7 Ramaswamy, R., ‘Vacuum circuit breakers’, Siemens Circuit,XXIII, April (1988).

8 Telander, S.H., Wilhelm M.R. and Stump, K.B., ‘Surge limitersfor vacuum circuit breaker switchgear’, CH 2279–8/86/0000–003751.00, IEEE (1986).

9 XXth International Symposium on Discharges and ElectricalInsulation in Vacuum.

(i) X'ian Jiaotong University, X'ian, China, September 18–22,2000.

(ii) EIT- Ecole d’ Ing’enieurs de Tours, Tours, France, June 30–July 5, 2002.

10 Wong, S.M., Snider, L.A. and Lo, E.W.C., Over-voltages andReignition Behaviour of Vacuum Circuit Breaker, InternationalConference on Power System Transients – IPST-2003, NewOrleans, USA.

ANSI/IEEE-C37.010/1999ANSI/IEEE-C37.04/1999ANSI-C37.06/2000

ANSI-C37.16/2000NEMA/SG-3/1995NEMA/SG-4/2000NEMA/SG-6/2000

Application guide for a.c. HV circuit breakers.Rating structure for a.c. HV circuit breakers.A.C. HV circuit breakers rated on a symmetrical current basis – preferred ratings and related requiredcapabilities.Low voltage power circuit breakers – preferred ratings and application recommendations.Low voltage power circuit breakers.A.C. HV circuit breakers.Power switching equipment.

Related US Standards ANSI/NEMA and IEEE

Notes1 In the table of relevant Standards while the latest editions of the Standards are provided, it is possible that revised editions have become

available or some of them are even withdrawn. With the advances in technology and/or its application, the upgrading of Standards is acontinuous process by different Standards organizations. It is therefore advisable that for more authentic references, one may consult therelevant organizations for the latest version of a Standard.

2 Some of the BS or IS Standards mentioned against IEC may not be identical.3 The year noted against each Standard may also refer to the year it was last reaffirmed and not necessarily the year of publication.

IEC

60694/2001

60947-2/2003

62271-100/2003

62271-200/2003

62271-203/2003

–

Title

Common specifications for high voltage switchgear andcontrolgear standards.

Low voltage switchgear and controlgear, circuit breakers.

High voltage alternating current circuit breakers.

A.C. metal enclosed switchgear and controlgear for ratedvoltages above 1 kV and up to and including 52 kV.

Gas insulated metal-enclosed switchgear for rated voltagesof 72.5 kV and above.

Circuit breakers requirements and tests. Sec 1, voltages notexceeding 1000 V a.c. or 1200 d.c. (2 parts)

IS

12729/2000

13947-2/1998

13118/2002

12729/2000

–

13947-2/1998

BS

BS EN 60694/1997

BS EN 60947-2/2003

BS 5311/1996

BS EN 60298/1996

BS EN 60517/1997

–

–

–

–

–

–

–

Relevant Standards

Documents

Author: K. C. Agrawal ISBN: 81-901642-5-2