摘要:

The difficulty of breaking the high short-circuit powers of large modern systems has an important bearing on the security of electrical distribution. The oil switch, as usually constructed, is in some ways a fortuitous device and, as will be shown in this paper, the very act of breaking a circuit under oil, and thus bringing a high-temperature arc into contact with the oil, results in pressures being set up which are not necessarily dependent upon the actual circuit power being broken. Briefly, it can be shown that even with fixed pre-breaking conditions the formation of the arc and the oil pressure set up within the tank will vary. Moreover, such factors as the effect of phase angle, the influence of the layout of the system and, in another sense, the speed of breaking and the effect of oil head, have important effects upon the process of circuit rupture. The author has carried out an investigation in an attempt to assess the effect of these and other factors and to determine whether material improvement is possible without radical changes in design.The author's investigation has led him to conclude that the fundamental limit of breaking capacity, that is the short-circuit power which can be broken time after time by a given switch without breakdown, is for the largest examples of present designs of oil switches rather below 1 million kVA and that the present breaking-capacity rating of oil circuit-breakers is correct only if a certain factor of reliability is taken into account. For high-power breakers this factor expressed as a percentage appears to be about 96.5 per cent, and indicates that such circuit breakers can be relied upon in 96.5 operations out of 100, or, in other words, that there is a 3.5 per cent chance of breakdown. In order to eliminate this element of breakdown the breaking-capacity rating should be divided by a factor which the author's tests indicate to be 2.5. Moreover, the limitation of the short-circuit current by means of reactors may be responsible for troubles not hitherto identified with such a cause.A circuit breaker embodying entirely new principles has been evolved and tested under short-circuit conditions. Its performance is superior to that of the usual oil switch, and a unit of a given size and weight is capable of breaking many times the power broken by an oil switch of similar dimensions, while its performance appears to be wholly consistent.Throughout the paper, tests are quoted and illustrated by oscillograph records wherever possible, but it is, of course, quite impossible, while keeping the dimensions of the paper within reasonable bounds, to give anything more than a skeleton selection of the tests carried out. A few details of the tests and equipment used may, however, be of interest.The majority of the tests were carried out on a 6 000-volt, 6 000-kVA, 40-cycle alternator. Other tests were carried out at 11 000 volts with a 10 000-kVA, 25-cycle alternator, and at 6 000 volts with a 25 000-kVA alternator.The power used in the tests varied between 500 kVA and 270 000 kVA. In all, some 800 tests were carried out, in most of which such, factors as periodicity, kVA and power factor were kept reasonably constant. Factors which could not be so readily controlled were investigated by carrying out groups of rapidly repeated tests from which a selection was made in such a way that the results bore directly upon the point being investigated. This may appear to be rather a roundabout method, but it was found that groups of tests could be usefully co-ordinated and, for instance, results rejected from one group used in another. In addition to this group of 800 tests, at least an equal number was available which, although not directly forming a part of the work described in this paper, were sometimes used for comparison.The switching apparatus consisted of several model switches; 4 of these were of an experimental character, while the remainder, 5 in number, were of commercial or semi-commercial form. Compared with power-system conditions, the majority of the tests were carried out on a reduced scale, the switchgear being reduced in size so that it could be over-stressed if necessary with the power available. With a 50 000-kVA source of power, the switch models were regarded as being 1/64th of their commercial cubical content, with the object of obtaining a switch with a commercial maximum rupturing capacity of about 50 000 kVA, and comparing its performance with a full-scale switch on 1 500 000 kVA. Such a full-size switch, as the author has already suggested, would be about the largest commercially available, and thus one which would be likely to be used in practice on a short-circuit power of 1 500 000 kVA. With a larger source of testing power a correspondingly larger switch was employed. The largest used (excluding the quenched-arc switches) could break a maximum power of about 200 000 kVA.In no group of tests were fewer than 5 repeated tests carried out under similar conditions—a relatively small number, perhaps, for many conditions—but when there arose good reason to believe that closer investigation was necessary, as many as 25 repeated tests were sometimes made. The work described in this paper has been carried out—at intervals—during a period of about 8 years.

DOI:10.1049/jiee-1.1930.0121

出版商:IEE    

年代:1930

数据来源: IET

摘要:

The paper describes a study of methods of measuring dielectric losses which has been carried out with the object of determining the nature and magnitude of the various errors to which different methods are liable. The most important errors are those due to stray capacitance currents, which affect especially the phase angle of the currents in resistors commonly used in wattmeter methods. A method is developed of shielding a simple resistor of flowing water, so that the current is in phase with the voltage within about 0.0001 radian. A resistor has been developed on these lines for 150 kV.The standard of reference has been a specially designed air condenser of the parallel-plate type. The separation of the plates is variable; and at 10 cm separation, suitable for 100 kV, the capacitance is about 500 cm. The curvature of the edges and of the guard plates was decided as a result of studying the electric stress distribution by models in an electrolyte. The use of a condenser with the Schering bridge form of circuit has been found to be almost free from errors of importance which are difficult to allow for. The work has been carried out for the British Electrical and Allied Industries Research Association, more particularly with reference to the measurement of the power loss in cable dielectrics.

DOI:10.1049/jiee-1.1930.0127

出版商:IEE    

年代:1930

数据来源: IET

摘要:

The B.B.C. Regional Scheme involves the construction of a number of twin wireless transmitting stations, each capable of radiating simultaneously two programmes on different wavelengths, and each at a power of 50 kW in the aerial. This paper describes the first of these twin transmitters, erected at Brookmans Park, Hertfordshire, to serve London and the Home Counties.The paper deals with the considerations underlying the choice of sites for such broadcasting stations, the type of power supply adopted, and the factors contributing to reliability in operation, as well as with the wireless transmitters themselves. Detailed considerations underlying the design of such transmitters have been avoided in the anticipation that they will form the subject of a further paper; but, nevertheless, some information in regard to the essential and major points in their design is contained herein.The paper deals further with the general station lay-out, the building and the auxiliary services, as well as with low-frequency apparatus for supplying modulation to the transmitter and apparatus for testing and monitoring purposes, etc.Of particular interest is the use of high-tension direct-current machines at this station for the production of high-tension voltage for anode supply to the main power valves. The type of generator developed especially for this work is considered in some detail.

DOI:10.1049/jiee-1.1930.0129

出版商:IEE    

年代:1930

数据来源: IET

摘要:

Formuæ are given for the distribution of radiation in different directions. The paper is divided into the following sections:—(1) Introduction.(2) Antenna elements.(3) Polarization.(4) Current distribution.(5) Effect of image in the ground.(6) A straight wire inclined at any angle.(7) A system of similar antennæ spaced in any given manner.(8) A system of antennæh uniformly distributed around the circumference of a circle.A number of special cases are deduced from these general formulæ, a curves are given to facilitate computation.

DOI:10.1049/jiee-1.1930.0132

出版商:IEE    

年代:1930

数据来源: IET

摘要:

The radiation distribution of an antenna in a vertical plane was obtained by measuring in an aeroplane the strength of a received signal from an excited antenna on the ground. The position of the aeroplane relative to the antenna was obtained by means of a theodolite on the ground, the signal strength being recorded on a cinematograph film.Although the ground site appeared very good, it was found that spurious radiations were obtained and these were probably the cause of the discrepancies found between the theoretical and the experimental results. The measurements were made on a horizontal antenna at several frequencies. The reason for this particular arrangement is that it should be possible to separate the effect of the reflection of the antenna in the earth from that of the antenna itself, and thereby obtain a double result from a single measurement.The results showed very definite maxima and minima and the position of these could be ascertained accurately within a few degrees, but the difficulties of the experiment prevented an accurate value of the field strength being obtained. Substantial agreement was obtained between theory and experiment, the discrepancies increasing with the frequency.The radiation distribution was also obtained for one of the beam stations erected by the Marconi Co. Owing to the sharpness of the beam, considerable experimental error was liable to occur. Nevertheless, substantial agreement between theory and experiment was also obtained in this case.

DOI:10.1049/jiee-1.1930.0133

出版商:IEE    

年代:1930

数据来源: IET

摘要:

The problem of the pulling into step of a synchronous motor has proved very difficult to solve, since it centres about a non-linear differential equation. One solution of the equation is not enough. All possible solutions with all combinations of the coefficients need to be made and classified into two groups, those that synchronize and those that do not. Until recently this has been practically impossible because of the amount of time required to make the solutions by graphical or point-by-point methods.The development of the integraph by Dr. Bush and his associates at the Massachusetts Institute of Technology has made it possible to solve this problem by machine integration. This integraph, which performs two successive integrations simultaneously, is particularly adapted to the solution of first- and second-order total differential equations. The solutions described in this paper were obtained by using the integraph in this way.This paper presents the complete solution of the problem for the case of a non-salient-pole synchronous motor. The results are put in a simple form convenient for practical use. By means of these results all of the necessary relationships between field current, induction-motor torque and slip, which determine whether or not a given load of a definite flywheel effect can be brought into synchronism, are determined. An example of a practical application is given.

DOI:10.1049/jiee-1.1930.0134

出版商:IEE    

年代:1930

数据来源: IET

摘要:

The paper details an attempt to measure the amount and distribution of the heat radiated from electric fires, employing the “Leeds” method already largely used for gas fires. A number of types of fire were tested, the radiant efficiency as measured lying generally within the limits of 55–70 per cent, with an average of about 60 per cent. The results are fairly reliable for comparative purposes, and the error in the radiant efficiency determination is probably well within 5 per cent.

DOI:10.1049/jiee-1.1930.0135

出版商:IEE    

年代:1930

数据来源: IET

摘要:

Two methods of measuring peak voltage have been invetigated. In the one method a condenser divider is used, and the voltage on the low-voltage arm of the divider is rectified. In the other the current through a condenser connected across the voltage to be measured is rectified, and the mean current so obtained is a measure of the peak voltage across the condenser. The accuracy of the methods is considered theoretically for different wave-froms, results arrived at by the two methods are compared, and agreement to about 2 parts in 1000 is obtained.

DOI:10.1049/jiee-1.1930.0136

出版商:IEE    

年代:1930

数据来源: IET

DOI:10.1049/jiee-1.1930.0138

出版商:IEE    

年代:1930

数据来源: IET