578 ABSTRACTS OF PAPERS

(/) Every air-blast circuit-breaker has critical upper and lowerlimits, which must be fully explored by short-circuit tests.

These laws are true of nearly all air-blast circuit-breakers, andin consequence a system of scale testing may be adopted, inaddition to part testing, to prove the performance at valuesbeyond the capacity of existing testing-stations. By fittingnozzles and electrodes of reduced size and testing over the avail-able current range, the breaking capacity of the full-scale circuit-breaker can be directly deduced.

RESISTANCE SWITCHING

Researches have been made into the use of resistors to modifyrestriking transients. Their application is economical forratings above 150 MVA at 11 kV and 500 MVA at 33 kV.

USE OF MULTIPLE BREAKS

From 66 kV upwards, the use of multiple breaks provides asimpler and cheaper solution than resistance switching. Theassessment of the breaking capacity of multiple-break circuit-breakers from the results of tests of a single element at theappropriate voltage is justified, provided that voltage distribution

and available pressure at the various breaks are substantiallyuniform. Fig. 4 shows that the breaking capacity of a singlenozzle at constant natural frequency increases as the voltage isreduced. Maximum breaking capacity is therefore obtained byusing as many breaks as possible if means are provided to ensureuniform voltage and pressure, and therefore uniform electricalperformance, at the various breaks.

For example, a 66-kV circuit breaker with two breaks and thesame overall air consumption as a single-break circuit-breakerwas proved to have approximately three times the breakingcapacity at the same circuit severity. Four breaks in series givestill better results.

Notwithstanding the theoretical advantages of multiple breaks,economic considerations tend to determine the most advanta-geous number. In practice, no difficulty arises in achieving abreaking capacity of 2 500 MVA at 132 kV with two breakswithout resistors, but a larger number of breaks would givegreater economy in air consumption.

The paper describes in detail the practical construction of single-break and double-break circuit-breakers. Fig. 5 shows thecross-section of a 132-kV duplex circuit-breaker, consisting oftwo exactly similar single-break units.

THE PERFORMANCE OF HIGH-VOLTAGE OIL CIRCUIT-BREAKERS INCORPORATINGRESISTANCE SWITCHING

By H. E. Cox and T. W. WILCOX, Associate Members.*(ABSTRACT of a paper which was published in August, 1947, in Part II of the Journal.)

Progress in the development of the high-voltage dead-tank oilcircuit-breaker has now made it a match for the air-blast circuit-breaker in every respect. The improvement in arc-control pots,has been accompanied by a reduction in both arc duration andtotal break time; whilst the development of compressed-air-operated closing mechanisms has now made these breakerssuitable for instantaneous automatic-reclosing cycles.

In service it has been found that the fire hazard is practicallynon-existent and that maintenance costs are reasonable.

In comparing the costs of air-blast and oil circuit-breakers itmust be remembered that a circuit breaker is invariably asso-ciated with three or six current transformers and probably oneor more voltage-indicating devices. When the unit is taken, notas the circuit breaker alone, but as the circuit breaker completewith all its associated equipment, cost considerations are over-whelmingly in favour of the dead-tank oil circuit-breaker. Thisis brought about because the primary insulation for both currenttransformers and capacitance dividers already exists in the oilcircuit-breaker terminal bushings, whereas in the air-blast circuit-breaker, separate primary insulation has to be provided.

ARC CONTROL

The heart of any oil circuit-breaker is the arc-control deviceor pot. Testing has shown that the side-vented pot having itsvents transverse to and disposed along the axis of the arc is veryeffective. Three arrangements of such pots are shown in Fig. 1.As the arc itself develops the pressure necessary to produce theblast for arc extinction, the pressure generated in the pot willincrease as the arc current increases. This increased pressuregives a more powerful blast and a greater electrical strength perunit length of arc path, with the result that the restriking voltagetransient can be withstood by shorter gaps between the moving

• British Thomson-Houston Co., Ltd.

Fig. 1.—Arrangements of side-vented pots.(fl) Pot with long initial arc "x."{b) Pot with auxiliary pressure generating arc "y."(c) Pot with compensating arc "z."

and the stationary contacts. Thus arc extinction occurs withshorter and shorter arcs as the current increases, as shown by thetypical characteristic of a side-vented pot [Fig. 2(a), curve 1].Curve 1 in Fig. 2(6) shows the probable shape of the gas-pressurecharacteristic.

Examination of these two curves shows that the limiting ratingof a pot can be reached either by the arcing time at the hump "x"becoming infinite, i.e. the pot fails to clear, or because the pressureat"y" becomes so high that the pot bursts.

For a given pot the maximum arc duration attained at "x" ismainly a function of the magnitude of the rate of rise and thepeak value of the restriking transient of voltage, whilst the

ABSTRACTS OF PAPERS 579

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maximum pressure attained at "y" is mainly a function of themagnitude of the current and is independent of the restrikingtransient of voltage over a very wide range.

The arrangement shown in Figs. 1 (a) and (6) are both designedto accumulate pressure in the pots prior to opening the orifices.Both these arrangements increase the pressure at "x" in Fig. 2(b)and thus raise the limiting-voltage rating, but they also increasethe pressure at "y" and lower the maximum breaking-currentrating.

In arrangement (c), Fig. 1, the orifices are immediately adjacentto the stationary contacts but the pot enclosure continues afterthe orifices cease, forming a "compensating chamber." Withthis arrangement high currents can be extinguished with aminimum arc-length and the current around region "x," Fig. 2(a),can continue to flow until the arc is drawn into the compensatingchamber, thus building up the pressure. This arrangement in-creases the voltage rating at "x" without decreasing the currentrating at "y," Unfortunately, with all side-vented pots it ispossible to compensate for increased pressure at "y" only up toa well-defined limit, because pots have to be made of insulatingmaterial and have to be pierced by side vents.

RESISTANCE SWITCHINGEffect of Modifying the Restriking Transient.

Shunting a pot by a resistor of suitable value reduces both therate of rise and the peak value of the restriking voltage, and thusmakes it possible to increase the voltage rating without decreasingthe current rating.

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(I) Without shunt resistor.(II) With shunt resistor.

shunting a pot rated at 57 kV, 13 kV sym., by a resistor whichcritically damped the circuit at approximately 1 200 amp andgave a logarithmic transient at all lower values of breakingcurrent. It will be seen that the hump of region "x" has disap-peared entirely.

EFFECT OF POTS IN SERIES

It is well known that with a double-break dead-tank oilcircuit-breaker the restriking voltage transient does not divideequally across each break, and also that the percentage whichappears across each break varies with the type of fault on thesystem.

When each break is shunted by a resistor, equal voltagedivision is obtained and thus the voltage rating of two pots inseries is twice that of a single pot. The paper gives a fulldescription of a double-break 132-kV oil circuit-breaker in-corporating resistance switching and rated at 2 500 MVA inaccordance with B.S. 116—1937. Fig. 4 shows a section throughone explosion pot with its associated resistor.

The breaker is of conventional and well-tried construction, anda high speed of break is obtained by the use of light alloys andhigh-tensile steels in the circuit-breaker mechanism and byrefinements in the design of the moving parts.

PERFORMANCE

The performance of this breaker is substantiated by tests basedon B.S. 116—1937, but with the 3-phase output of the test plantapplied to one pole of the breaker, the third terminal beingobtained by bringing out through the tank wall a connectionfrom the moving crossbar.

Further test data are given for the breaker with and withoutresistance switching (a) when breaking low currents on inductivecircuits, and (6) when breaking the charging current of a longcable. The effect of varying the speed of break is discussed andillustrated by test results.

CONCLUSIONS

It is shown that the application of resistance switching totank-type oil circuit-breakers offers the following advantages-j

ABSTRACTS OF PAPERS

Contact springs

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Fig. 4.—Section of side-vented explosion pot with switching resistor.

(a) It improves the operation of the pot over the critical values plants by carrying out 3-phase tests on one pole of a 3-phaseof current where the arc duration is a maximum. separate-tank breaker.

(b) It enables complete control to be exercised over the dis- id) It reduces switching over-voltages when breaking smalltribution of the restriking transient imposed on individual pots inductive or capacitive currents.when connected in series. (e) It reduces surges on transformers and other inductive

(c) It makes it possible to extend the proving capacity of test apparatus caused by switching on to the circuit.

VARIABLE SPEED REGULATION WITH A.C. PLANT*By S. R. PHELPS, Student.f

(ABSTRACT of a paper read before the CARDIFF STUDENTS' SECTION, 21th September, 1946.)

The demand for a reliable electric motor capable of operatingsteplessly throughout a range of speeds, and with a fairly closeregulation at any one speed-setting, is increasing every day.Progress in manufacturing methods sets up new requirements,and engineers are increasingly realizing that the initial cost ofthe motive-power element is often relatively unimportant inrelation to the advantages that can be obtained from a bettercontrol of the quality and speed of production.

For many applications the d.c. motor, in spite of higher costas compared with the simple squirrel-cage motor, and in spiteof the handicap of conversion losses when operated from ana.c. supply, is eminently suitable, and its merits are fullyrecognized by discerning engineers.

It is unfortunate for the full exploitation of the d.c. motor inthese fields that the problem of power generation and trans-mission has been settled, on good grounds, in favour of alter-nating current, and that the squirrel-cage motor is the simplestand most reliable power unit that has yet been evolved, and atthe same time the cheapest to buy, install and maintain. Butif it is true that this type of motor has given a great impetus tothe use of electric power, it is becoming every day more evident

• The original paper, of which this is an abstract, was awarded a Students' Premiumby the Council,

t Guest, Keen and Nettlefolds, Ltd.

that its many advantages may have acted in many instances asa brake on true progress. The squirrel-cage motor is funda-mentally a fixed-speed machine and may be described as asturdy but unintelligent entity let loose in the framework ofengineering.

The author summarizes briefly some of the possible methodsof obtaining speed regulation with a.c. drives, either existing ornew. He feels that the selection of drives or the modificationof existing drives is a problem in which all possible methodsshould receive careful consideration. These methods can beroughly classified into two groups: (a) fixed speed control, and(b) stepless speed control.

FIXED SPEED-CONTROL

Cascade combinations of two induction motors are mainlyrestricted to high-power applications such as colliery winding,haulage, fans and traction. Several disadvantages are inherentin the cascade operation of induction motors. The magnetizingcurrent of the second motor of the system is, in the first motor,superposed on its own magnetizing system, thus causing thepower factor to be low, say 0 • 5-0 • 7, as compared with 0 • 8-0 • 9for the individual machines; also it will be evident that the firstmotor will have greater losses and greater temperature rise. It