SYNOPSIS

J. MEINTJES* (presented by Mr Meintjes)

The operating characteristics of a modern 30 MW furnace for the production of 75 per cent ferrosilicon provided withpower-factor-correcting capacitors connected in shunt and series are presented in graphical form. The results arediscussed, and the conclusion is reached that, according to the evidence as a whole, shunt connection is to be preferred.

DISCUSSION

power, corrected grid power factor, electrode-to-hearthvoltage, and relative resistance, which is defined as theratio of the total phase resistance of the furnace system tothe inductive phase reactance of the furnace system, thelatter being assumed to be a constant for the furnace.

These quantities are plotted against electrode current asthe furnace is brought onto load on the specific trans­former tapping that will give the designed load at thedesign point. Unless a furnace has been off-line for arelatively short period, it is more usual to bring it ontoload by increases in both the electrode current and thevoltage, but the graphical comparison given will still bevalid.

The particular example chosen is that of a modem,large electric furnace for 75 per cent ferrosilicon, de­signed for optimum operation at 30 MW at a phase resist­tance of I mn, the inductive phase reactance being 1,15mn. At this load, the electrode current will be 100 kA, theelectrode-to-hearth voltage 152,4 V, and the relative re­sistance 0,870. The supply voltage of the line has beentaken as 21,5 kV, and at the design point the grid powerfactor has been corrected to 0,97.

In the calculation of the electrode-to-hearth voltage, theimpedance voltage drop of the transformer between noload and full load has been disregarded. Thus, theelectrode-to-hearth voltage is shown as constant for shuntcapacitors, whereas in practice it will slope downwards tosome extent from no load to full load, in turn affecting theother parameters. However, the effect will be similar forseries capacitors.

At the design point, active power, electrode-to-hearthvoltage, corrected grid power factor, and relative resist­ance are the same for both shunt- and series-connectedcapacitors. The following emerges from further examina­tion of the graphs.

With shunt capacitors, the electrode-to-hearth voltageremains constant at 152,4 V (if the drop in transformerimpedance voltage is neglected) as the furnace is broughtonto load, while with series capacitors this voltage risesfrom 103, I V at 0 kA to 152,4 V at lOO kA, and stillhigher values as the current is increased further. This is animportant factor when a possible breakdown of thecapacitors is considered. A failure of shunt capacitors willnot in any way affect the voltage steps obtainable from thetransformer, and furnace operation will be completelynormal except that the maximum demand will rise. Afailure with series capacitors will seriously upset furnaceoperation, since the secondary voltages obtainable from

149

COMPARISON OF CAPACITORS CONNECTEDIN SHUNT AND SERIES

The theory of capacitors employed in shunt and seriesfor power-factor correction has been adequately dealtwith by others',2. The analysis given in the Appendix hastherefore been restricted to the derivation of those equa­tions necessary for the graphical comparison of active

A Comparison of Power-factor Correction onSubmerged-arc Furnaces by Capacitors in Shuntand Series

*Rand Carbide Limited, South Africa.

INTRODUCTIONWhere a maximum kVA demand forms one of the

bases on which a consumer has to pay for the purchase ofpower to operate submerged-arc electric furnaces, it isvery advantageous economically to install power-factor­correcting capacitors. In fact, this is virtually a sine quanon with modem large furnaGes owing to the inherentlylow power factor of such furnaces. Power-factor de­creases with increase in furnace size, both because of adecrease in resistance and, to a lesser extent, because ofan increase in reactance .. Thus, for an increase in furnacesize from 15 MW to 30 MW, the power factor will usuallydecrease from more than 0,8 to less than 0,7.

The provision of capacitors, whether in shunt or inseries, cannot alter the furnace power factor and thus thepower liberated in the arc. These depend entirely upon theinherent reactance of the furnace system and the max­imum resistance that can be successfully employed for themanufacture of a specific product. The capacitors merelycorrect the incoming line power on the primary side ofthetransformer, the degree of correction employed beingdependent upon economic considerations. Owing to thelarge currents and low voltages on the secondary side ofthe transformers, it is customary to locate the capacitorson the primary side. They can theoretically be located onthe secondary side of the furnace transformer by beingconnected across the secondary of a series transformerplaced between the furnace transformer and the furnace'.In that case, the furnace transformer can be of smallercapacity, but the author knows of no practical applicationof this arrangement.

In recent years, series capacitors have replaced shuntcapacitors on many of the new furnaces installed, and it ispertinent to enquire whether this change is justified. Al­though excellent monographs',2 have been written on this·subject, it is still generally the case that the averagemanufacturer of electric-furnace products has not a veryclear picture of the difference between the two types ofpower-factor correction. It is the object of this paper toexamine the differences and to present them in a simple,easily understood graphical form.

0,3

0,4

0,1

0,2

5

Vlo

10 0,5U

COS OB20 1,0200 40

0,9

EfA0,1'>

150 30

100 20

50 10

r 1"> j

::E

UJ a..

L..__.l...-__.l...-__L.-_-L.-_-"--_.l...-_-"---"---r~-r---'------'-------'---------'~r.::---4-'0

10 20 30 40 !ill &l 70 80 ~ 9Or;: 100 110 120 130 ~lflkAI- :g ~ S

Graph,jor a 30 MW furnace, of electrode current (If) versus grid power factor (cos cPA' cos cPB),furnacepower~, (shunt), Pa (series), electrode-to-hearth voltage (EA. Em), and relative resistance (nA, nB)

the transformer will be too low for the product beingmanufactured unless transformer tappings of a suffi­ciently high value are specially provided at extra cost toguard against this eventuality. It can be argued that, withmodern capacitors, a break-down is unlikely: some typesare now fitted with internal fuses, namely, one for eachcapacitor element. They are arranged to fail before theexternal fuse, and give the advantage that a faulty elementis immediately disconnected by the element fuse withoutcausing service interruption. The unit continues to func­tion at slightly reduced output, t;md its service life remainspractically unchanged. Nevertheless, the possibility ofcapacitor failure should be given due consideration whenthe choice is made between shunt and series capacitors.

To the left of the full-load point as the furnace isbrought onto load, the relative furnace resistance is al­ways less for the same electrode current with seriescapacitors than it is with shunt capacitors, while the re­verse is true beyond the full-load point. This followsdirectly from the difference in electrode-to-hearth vol­tages for the two cases. A lower relative resistance at thesame electrode current and charge resistivity is associatedwith deeper electrode immersions. Thus, as a furnacewith series capacitors is brought onto load, electrodeimmersions tend to be deeper than in a furnace with shuntcapacitors. This is an advantage that can, however, beoffset for shunt capacitors if the load is raised on lowervoltages, as is usually done. At the full-load point, elec­trode immersions are equal, while, beyond this point,immersions will tend to be deeper with shunt than withseries capacitors, which offers some advantage, particu-

150

lady when an attempt is being made to gain immersion byan increase in electrode current.

It is further to be noted that, for the same change incurrent in the region of the full-load point, electrodeimmersions will change less with series than with shuntcapacitors. With series capacitors, therefore, the practiceof trying to gain on electrode immersions by a temporaryincrease in the electrode current will be relatively ineffec­tive. This is also apparent from the fact that, with seriescapacitors, the electrode-to-hearth voltage rises with cur­rent.

It has been claimed2 , however, that the smaller changein electrode position obtained for a determined cha~ge inelectrode current or active power at the full-load point,with series-capacitor compensation compared with shuntcompensation, offers a great advantage from both opera­ting and regulating technique. From the operating point ofview, modern furnaces are usually provided with bothcurrent and impedance regulation. If it is desired to limitelectrode movement to a minimum, the furnace withshunt capacitors can be run on constant impedance orso-called 'fixed zone' control. Thus, there need not bemore wear and tear on the regulators and electrode­hoisting equipment than for series-capacitor compensa­tion. This equipment is, in any case, robust and designedfor the life of the furnace. With series capacitors, thetransformer voltage steps can be chosen somewhat furtherapart, but the saving in cost is hardly significant.

As electrode current is increased from zero, the gridpower factor for shunt capacitors alters from leading tounity to 0,97 lagging at the full-load point, and to lower

values lagging beyond the full-load point. With seriescapacitors, the grid power factor cannot become leadingand reduces from unity at zero load to 0,97 at full load,and slightly lower values beyond full load. The avoidanceof a leading power factor is probably to be preferred,although in the author's experience a leading power factor,while bringing a furnace with shunt capacitors onto load,has not proved a disadvantage. Compaints about a leadingpower factor have never been received from theelectricity-generating authority, probably because thefurnace load constitutes only a fraction of the total load onthe grid, and because a leading power factor is generallyemployed only for a relatively short time. Moreover,shunt capacitors are usually arranged in separate banks oftwo or three, one or more of which can be switched in at atime. Thus, if a furnace has to be operated for a long timeon low load, as when a new furnace is being started up orfor any other reason, capacitor correction can be em­ployed in multiples of one-third of the total installed MVAR to suit conditions. From the electricity consumer'spoint of view, the elimination of a leading power factoroffers no advantage. With either shunt or seriescapacitors, the maximum demand will not be exceeded asthe furnace is brought onto load, even with a leadingpower factor in the shunt case, because the load is belowfull load.

When a furnace is brought onto load, active power isalways greater with shunt than with series capacitors, as isto be expected from the higher relative resistance for shuntcapacitors. This favours product output. With shuntcapacitors, if the furnace has an inherent power factor ofmore than 0,7071 at the full-load point, power will con­tinue to rise as current is increased beyond full load, butthe rate of rise will be considerably less than in a furnacewith series capacitors. On the other hand, if the full-loadpower factor is below 0,7071, active power will decreasewith further increase in current. In a furnace with seriescapacitors, the power will always increase - and increasefairly rapidly - through the full-load point. It is to benoted, however, that this is obtained only at the expenseof an increase in both the electrode-to-hearth voltage andthe relative resistance compared with the shunt case. lfthefurnace can be operated at a higher voltage, the increase involtage and power can equally well be achieved in thefurnace with shunt capacitors by the operator's simplypressing a button on the control desk to operate the on­load tap changer for a higher voltage. The rising-powercharacteristic of series-compensated furnaces is thus oflittle significance, the more so when operation is to the leftof the full-load point as often occurs in practice.

It should be noted particularly that, with series com­p~nsation, it is necessary to work exactly at the full-loadpoint for optimum operation. Below this point, powerfalls off rapidly. This is in direct contrast to a shunt­compensated furnace, where the power varies only minim­ally through the full-load point. It follows that, withseries capacitors, it is necessary to design the full-loadpoint exactly, which will in general be impossible. Withshunt capacitors, appreciably more latitude is allowable inthe design point.

If the question of shunt versus series capacitors isconsidered from a purely electrical point of view , it is saidth~t shunt capacitors are very sensitive to harmonic cur­rents, so that precautions have to be taken to prevent suchcurrents overheating and consequently damaging thecapacitors2 • On the other hand, with series capacitors,the development of ferro-resonance in the transformers can

151

give rise to the generation of excessive voltages across thecapacitors by severe current surges, particularly when anunloaded transformer is switched in or when a fullyloaded transformer is switched out. The use of arc gaps toprotect the capacitors from over-voltage is said not to becompletely satisfactory, and it has thus been proposed toreplace series static capacitors with series-connected syn­chronous capacitors, which eliminate the phenomenon offerro-resonance 3 . However, both types of static­capacitor connections are employed, so that it can betaken that difficulties of the type mentioned can be over­come, and there is from this point of view no overridingadvantage to be gained from the choice of one type ofconnection rather than the other. It should nevertheless bepointed out that, with a series-capacitor installation, if it isdesired to overload the transformer - which is not anunusual requirement - this will in general not be possiblewithout exceeding the current rating of the capacitors. Ifsuch overloading of the capacitors is to be prevented, itwill be necessary to install additional capacitors in parallelon each phase. In contrast, if capacitors are connected inshunt across the phases, the capacitor current and voltagealways remain the same irrespective of the transformerload.

To sum up, it is concluded that there are no convincingreasons for a swing-over to series capacitors, and if theevidence is taken as a whole, shunt capacitors are to bepreferred.REFERENCESI. FLAATHE, H.P. The use of series capacitors in conjunction with

electric smelting furnaces. Asea Pamphler 779IE.2. MO DAL, A. Comparison between electric smelting furnaces pro­

vided with shunt or series capacitors. Reunerr & Lenz Engineer'sDiary, 1963.

, 1963.3. BLELOCH, W. Compensation of reactance - submerged arc

furnaces. Unpublished report, 1972.

APPENDIX

LIST OF SYMBOLS USEDSubscript A denotes magnitudes in the case of shunt

capacitor compensation.Subscript B denotes magnitudes in the case of series

capacitor compensation.

a = Transformer ratio in the case of shunt compen­sation

transf. primary phase voltagetransf. secondary phase voltage

b = Transformer ratio in the case of series compen­sationtransf. primary phase voltagetransf. secondary phase voltage

E = Grid phase voltage

Ee = Voltage drop across series capacitor

EJ = Furnace electrode-to-hearth voltage

i = Grid phase current

le = Capacitor current in the case of shunt compen-sation

I J = Electrode current

I L = Inductive component of electrode current

I R = Resistive component of electrode current

j = The operator J=I ,

i.e., n =j(I:XJ2 -1 (4)*

2 _(~)2or n + 1 - I f XL'

Y2 = admittance of the capacitor branch,

Y= 1 = R-jX L

1 a 2 (R +j XL) a2 (R 2 + X /)

1 jY2 = -' X =y

J C C

Also Ef = I fJR 2 + XL 2 = If XLfiT+l, (2)*

i.e., _1__ If XL- E , (3)

fiT+l f

with shunt

TransformerS~ondary

Transforme-rPrimary

Figure J

Simplified circuit diagram for a furnacecapacitors

· x Xk = The degree of compensatIOn = -2_c_ or b2 XC

a XL L

R hI' h '.n = X' t ere atIve p ase resistanceL

P = Active power (P A for shunt or P B for seriescompensation)

r = Bus-bar and transformer secondary to. phaseresistance converted to equivalent Y phaseresistance

R = r + R f = total phase resistance of the furnacesystem

Rf = Variable phase resistance of the furnace

Xc = Phase reactance of the capacitors.

XL = Inductive phase reactance of the furnace system,assumed to be a constant for the furnace

cjJ = Phase angle of the grid

cjJ f = Phase angle of the furnace system

Ifa

SHUNT CAPACITORS

nk---- (6)*

yn2+(1-k)2

l+n 1_1__ 1'IJ cos2 cjJi.e., k = (7)*

n2

1- ----=-,..,-----;:--,-(n2 + 1)cos2 cjJ

From (3)

k is obtained by solving (5) for k, whence

k = 1+ n tan cjJn2

1- ----;:------;:-(n 2 + 1)cos2 cjJ

n2 + 1-kBut from (1), tan cjJ = n k and, from the manner

in which Figure 3 has been drawn, it is obvious that,for the phase-grid current to be lagging the phase-gridvoltage, tan cjJ must be negative, i.e., k > n2 + 1. Forthis to be the case, it can readily be shown that the +sign must be chosen in the equation for k, so that

k = 1+n tan cjJn2

1- ----;:------;:-(n 2 + 1)cos2 cjJ

From (I), coscjJ = nk/Jn2 k2+(n2+ 1- k)2

nk------ (5)~. yn 2+(1-k)2

152

Refer to Figures 1, 2, and 3.

The impedance of the furnace secondary circuitreferred to the primary side of the transformer isa 2 (R +j XL)'

If Y1 = admittance of furnace secondary circuitreferred to the primary side, and

...

Figure 3

Current vector diagram for a furnace with shuntcapacitors

I l '---------------.,.

Figure 2

Equivalent phase diagram (shunt capacitors)

!J.J-------~K..'"a

Ifi a

Efl_:'--,'

i.e.,

i.e.,

= 1- nJco:2 cP - 1 . . . . . . . . . . . . . . . . . .. (15)*

Also PB = J I / R = 3 1/ XL n (16)*

Ef = I f JR2+X L2

= If X LP+1'···· .. (17)*

EI X = (f L

P+1

b= ER+}Ef Jn2+(1-k)2

E cos cP *E

fcos cP

f, (18)

. A. n A. RSInce cos 'P = and cos 'Pf = -~::====::;:

Jn2+(1-k)2 JR2+X/

SO!Ving(ll)fOrn,n=J( E )2_(1_k)2 ... (12)*b If XL

and. .by substitution of this value in (11)

k is obtained by solving (13) for k, whence

k = I ±n tan cP.I-k

But from (10) tan cP = -- and, from the manner inn

which Figure 6 has been drawn, it is obvious that, forthe phase-grid current to be lagging the phase-gridvoltage, tan cP must in this case be positive, i.e., k < 1.

Hence, the negative sign must be chosen in theequation for k

and k = I - n tan cP

nFrom (10), cos cP = (13)

Jn2+(1- k)2

_ b I ( XL n (14)*- E from (11) .

n

PRACTICAL EXAMPLEFor a graphical comparison of shunt versus series

capacitor compensation as applied to a particular furnace,relative resistance, grid power factor, electrode-to-hearthvoltage, and active power from the above equations canbe plotted against electrode current, the condition beingchosen that, for a particular transformer tapping, there isequal power in the two cases at the design point of relativeresistance.

Consider a modem large electric furnace for the pro­duction of 75 per cent ferrosilicon designed to give op­timum operation at 30 MW, the total phase resistance atthis load being I mn, and the furnace having an inductivephase reactance of I, 15 mn.

153

Transforme-rSl?condary

TransformerPrimary

Figure 4Simplified circuit diagram for a furnace with series

capacitors

Figure 5

Equivalent phase diagram (series capacitors)

SERIES CAPACITORS

Figure 6Voltage vector diagram for a furnace with series

capacitors

Refer to Figures 4, 5, and 6.

The impedance of the furnace secondary circuitreferred to the primary side of the transformer is

b2 (R+j XL)'

Ib Ef = 1;' b2 (R +j XL) = b f f (R +j XL)

Ec=t(O-jXJ=If ( -j~)

Hence ~=bEf+Ec=!f(bR+j{bXL-~c})

= If (b R +; {h X L - ~c}),since If = If +j 0 = I r

Thus, E = If b XL {n +j (1 - k)} , . (10)

E = b If XL Jn 2 +(1- W (il)

Finally, PA = 3 If2 R = 3 If2. XL n (8)*

E *and a = - (9)Ef

S~ --bLf.J..'~_~---'lXWJ'-~ f"'~--JiIXl:\lJ'-~m--'Alternator

At this load, therefore,

Furnace power factor

Electrode-to-hearth voltage

30 x 103

RCOS~f=----

JR 2 +X L2

Electrode current

----= 0,6562

Jl +(1,15)2

3 x 100 x 0,6562

= 152,4 V (P = 3 Eflf cos ~f)

n at the design point

I = ~3 = 100 leA (P = 3 If

2R)f V~

R 1= X = 1 15 = 0,8696.

L '

Table I

Summary of equations required for a comparison of shunt and series compensation

Units: Milliohms, volts, kiloamperes, and megawatts

QUANTITY

n

cos ~

k

Transf. ratio

.Ef (volts)

P(MW)

SHUNT CAPACITORS

)(~)2_1IJ XL

nk

1l+n ---I

cos2~

n2

1------.------.---:­(n 2 + l)cos2~

EE

f

IJXLP+"I

3 I 2 X n 10- 3f L

Table 2

Values of the quantities in Table 1

SERIES CAPACITORS

)( E _\2 -(l-kfb If X]

bIrXLnE

I-n 1_1-- 1\I cos2 ~

E cos~

Ef cos ~f

I f XL n2 + 1

3 I 2 X n 10- 3f L

SHU T CAPACITORS SERIES CAPACITORS

a = 81,45 b = 120,40k A = 2,24553 k B= 0,78207

I fCkA) nA cos ~ A Ef A(V) P A (MW) nB cos ~B EfB(V) PB(MW)

° oc il 152,4 il 00 1,000 103,1 il

10 13,21 0,169 4,56 8,962 1,000 103,7 3,0920 6,550 0,333 9,04 4,477 0,999 105,5 6,1840 3,159 0,631 17,44 2,231 0,995 112,5 12,3160 1,969 0,859 24,46 1,478 0,989 123,1 18,3680 1,321 0,986 29,16 1,099 0,981 136,7 24,2787,60 1,000 0,977 142,5 26,4788,44 1,116 1,000 30,11 0,990 0,977 143,1 26,7193,71 1,000 0,994 30,30

100 0,870 0,970 30,00 0,870 0,970 152,4 30,00110 0,672 0,885 28,05 0,785 0,964 160,8 32,78120 0,469 0,716 23,28 0,715 0,957 169,6 35,50130 0,198 0,346 11,54 0,654 0,949 178,7 38,15132,52 Nil Nil If Nil 0,640 0,947 181,0 38,80

The values in Table 2 are plotted in the graph.

154

Assume that the line supply voltage = 21 500 V andthat, at the design point, it is desired to correct thegrid power factor to cos cjJ = 0,97.

The values of the quantities in Table 1 can now becalculated, and are given in Table 2.

DISCUSSION

Mr S.G. King *:I fully agree with Mr Meintjes' conclusion that shunt

capacitors seem to be better than series ones. Is thereanything to be said for a compromise between the two ­some series capacitors to enable the voltage to rise from acertain figure to the agreed full load figure, and then a setof shunt capacitors on the other side (which may knownothing about what is going on in the series side) to givethe straight-line characteristic, so that one gets the best ofboth methods?Mr Meintjes:

I have not given this combination any thought. How­ever, I still wonder if there is any advantage to be gainedby having, say, half the capacitors in series, except for nothaving a leading power factor when starting up.Mr P. van Oerst:

Before presenting this contribution, I wish to expressmy sincere thanks to Mr Meintjes, and the organizers ofINFACON 74, because series and shunt capacitor secretsexist no more.

With large loads, shunt capacitors require circuit­breakers to switch them in gradually, to avoid a leadingpower factor being drawn from the supply.

It is well known that the supply authorities do not wanta leading power factor, because it gives rise to an incorrectrecording of the energy consumed.

The loss in capacitors has two implications: maximumdemand, and production.

If the preset maximum demand is not to be exceeded, aloss of production is the result, because the maximumdemand is based on the corrective kVA with shuntcapacitors.

When production is a prerequisite, the penalty is ahigher maximum demand bill, if the higher maximumdemand is allowed by Escom. This higher maximumdemand is restricted further by the permissible overload oftransformers, secondary busbars, and electrodes,whichever is the least.

The time the shunt capacitors are out of order is also avery important factor, because the operator can choosebetween the price to be paid for maximum demand andloss of production.

The following example will highlight this.If, at a normal full load without shunt capacitors, the

loss in production is x rand per hour and the cost of theincreased maximum demand on overload in order to main­tain production isy rand, then the time in hoursy/x is theperiod in which the cost of production loss is less than theincreased maximum demand costs.

This can be made attractive to the producer, if he isinsured against loss of shunt capacitors, by means of areduced premium or a bonus to the maintenance or repairdepartment of the company involved.

In general, however, it is in the interest of the producersto ensure against loss in capacitors by using shuntcapacitors, and furnace transformers and ancillary gears

*Private Consulting Engineer, South Africa.tR.M.B. Alloys, South Africa.

155

that can sustain the overload.The question arises here, of course, of why the furnace

is not normally overloaded; but this can be due to:(I) limit of the authorized maximum demand of the Elec­

tricity Supply Commission,(2) management policy, and(3) overload not being continuous.

A weak spot in the shunt-capacitor circuit is thebreaker, because of the nature of the furnace operation,which is continuous. The on-load switching demands arobust switch and continuous service. Either a standbyunit or a unit requiring a minimum of maintenance isnecessary. The shunt capacitors, because they areswitched in under load, are subject to ferro-resonance,and in this connection unwanted harmonics arise. Theseharmonics can be suppressed with a filter reactor, or withharmonic choke that can be so designed as to suppress anyseries of harmonic voltages. Of special interest here is thefifth harmonic, which is generated in an electric arc andcan disturb television receivers, so I have heard.

Series capacitors give an entirely different picture, be­cause, besides correcting the power factor, they give anincrease in the supply voltage. Subsequently, the loss inseries capacitors can result only in kiss of production. It isin the interest of the producer to insure against loss ofproduction.

The rest of my contribution to Mr Meintjes' paper mayseem like a repetition of what has already been said, but Ishall repeat it because I know the work that is involved inpreparing a paper of this kind. An added excuse for this isremarks from Mr King about the confusion in 1962, whena paper was presented by Mr C.1. Coetzee and there wassubsequent discussion about the merits of capacitors inshunt and in series by Mr Mondal.

Ferro-resonance arises also when the transformer isenergized via the series capacitors. The initial inrush ofmagnetizing current contains a series of harmonics, one ofwhich may be of the appropriate frequency to cause reso­nance, and damage can be done to transformers andcapacitors. Protection, however, is normally provided bymeans of a spark gap. This spark gap must break down atsome voltage low enough to avoid risk of damage to theseries capacitors.

A more modern method is a circuit breaker, whichcloses and short-circuits the capacitor bank.

An inspection of the spark gap will highlight furnaceoperation practice, because, if the transformer is switchedin without bringing the tap changer to the lowest voltage,the spark gap will definitely operate.

In general, shunt capacitors are used to improve thepower factor by supplying leading reactive power, andseries capacitors are used to improve voltage regulationand the power factor. Expressed differently, shuntcapacitors reduce the current by reducing the reactiveload; series capacitors reduce the current by increasing thevoltage, and, according to Mr Meintjes' paper, it does notalter the arc power, whichever system is used.

Arc power is conducted through a vapour path of such ahigh temperature as to be conductive and allow a currentto flow. Also, maximum arc power is released when thearc resistance equals the rest of the system's reactance, asis well known.

I do not know if it is possible to obtain some capaci­tance from the furnace slags at the electrode tips, but itmight be another field of investigation for somebodyinterested in this.