摘要:

Most overhead transmission lines carry over-running earth wires, one function of which is to shield phase conductors from direct lightning strokes. Published theories of shielding are incompatible with modern knowledge of lightning. The paper emphasizes the importance of the charges bound on the earth wire and phase conductor by the leader stroke. Equality of charges is taken as a condition under which both wires are equally likely to be struck. For adequate shielding, the charge on the earth wire should exceed that on the conductor. The charges are calculated from the field due to the leader, and it is shown that equality of charges corresponds to both wires lying on the same equipotential of the leader. The limiting position for protection is determined by the slope of the equipotential, and as this depends on the distance between leader and transmission line, the limiting value of which depends on the stroke current, the protection afforded isa function of stroke current. Calculation suggests that shielding is normally adequate for conductors in a wedge of semi-vertical angle 45° of which the apex line is the earth wire.

DOI:10.1049/pi-a.1960.0070

出版商:IEE    

年代:1960

数据来源: IET

摘要:

The paper describes a new method for measuring the armature coil current in a d.c. machine as it reverses during commutation. A small search coil, fixed relative to the armature, is magnetically coupled with one of the armature coils. The e.m.f. induced in the search coil, which is proportional to the rate of change of armature coil current, is integrated to give a signal proportional to the current itself, and this is displayed on the screen of a cathode-ray tube. The method overcomes several of the difficulties associated with the use of a shunt for measuring armature coil current during commutation.A method is described for triggering the oscillograph time-base which permits examination of the current reversals as they occur in a number of consecutive revolutions of the machine.The paper is illustrated with oscillograms showing thearmature coil current during commutation in a particular aircraft generator, and the cause of poor commutation in this machine is deduced.

DOI:10.1049/pi-a.1960.0073

出版商:IEE    

年代:1960

数据来源: IET

摘要:

The paper describes an attempt to replace with solid-state devices the carbon-pile regulators and electromechanical protective circuits in a representative aircraft's power system. A complete 2-generator d.c. system has been built and found to give substantial improvements in performance over a conventional system. The only electromechanical device used was a 400 amp d.c. contactor. A new approach to generator protection, relying on a knowledge of its excitation characteristics, is also described. There seems to be no reason why similar principles should not be applied to a.c. systems.

DOI:10.1049/pi-a.1960.0075

出版商:IEE    

年代:1960

数据来源: IET

摘要:

A smoke or dust particle carried by an air stream and subjected to high-voltage corona acquires a charge approximately proportional to its external surface area. If such ionized particles are precipitated at a collector electrode, the resultant flow of charge constitutes a current proportional to the rate of deposition of total dust surface, and a significant measure of the toxicity of a chimney discharge is obtained.The paper outlines the theory of a dust monitor incorporating this principle, describes some experiments in smoke measurement and examines the operation of prototype equipment in a pulverized-fuelfired boiler installation

DOI:10.1049/pi-a.1960.0077

出版商:IEE    

年代:1960

数据来源: IET

摘要:

The performance of a 2-phase induction motor used as a servo motor depends upon the alteration of the machine from 2-phase to single-phase operation. This can be effected in three ways. Two depend upon change of applied voltage and the third depends upon alteration of the relative disposition of two stators in a specially constructed machine.The speed/torque relationship for the three conditions is deduced from the initial assumption that the flux-density distribution is sinu-soidal in space and time. The analysis reveals that single-phase motors have a maximum speed dependent upon the rotor resistance/reactance ratio as well as the frequency. It suggests a more appropriate definition of synchronous speed than that commonly accepted, namely as that speed at which the unidirectional torque becomes zero.Theoretical performance curves of the machine as a 2-phase motor, as a single-phase motor and in the intermediate condition between these two extremes are given, for various assumed resistance/reactance ratios, as a function of the parameter which alters the machine from 2-phase to single-phase operation. Comparison between theoretical and experimental curves is made.The resistance/reactance ratio of the 2-phase servo motor, required for effective speed control near zero speed, is demonstrated by the curves.

DOI:10.1049/pi-a.1960.0080

出版商:IEE    

年代:1960

数据来源: IET

摘要:

Power transformers have reached such a degree of perfection that it seems unlikely that there is room for substantial improvement in any fundamental characteristic. However, the insulation of conventional transformers is in the main dependent upon a liquid which is not only inflammable but has other undesirable characteristics. The substitution of gas for oil as the insulating medium eliminates the undesirable characteristics of the latter, and in addition, gasinsulated units have other advantages.Of the various gases having desirable properties for use in modern equipment, sulphur hexafluoride is prominent. It is produced commercially in the United States.After reviewing the advantages of gas-insulated transformers, the paper examines the fundamental characteristics of this electro-negative gas, and experimental data are presented of its electric strength relative to oil. Of considerable interest to the designer is the peculiar behaviour of gases subjected to impulse tests. It is shown that, for the same low-frequency electric strength, gas-insulated units have lower impulse strength than oil-filled transformers. This deficiency, however, can be overcome with liberal margins of safety if gas-insulated transformers are protected with modern lightning arresters.The cooling of these new transformers is by forced circulation of the gas. The thermal capacity of gas-insulated units is, of course, somewhat lower than that of oil-filled units; theoretically, it would appear that, on this account, the overload capabilities of gas-filled transformers might be much lower than those of oil-filled units. This apparent deficiency is partially nullified by the fact that the thermal ageing characteristics of conventional class-A materials used in the construction of transformers is higher in gas than in oil. Consequently, the overload capabilities of gas-insulated units compare quite favourably with those of oil-filled units cooled by forced circulation of the liquid. Two units rated at 2000kVA at 69kV are in service in New York City; a third unit rated at 10 000kVA at 69 kV is in operation at Allentown, Pennsylvania. This unit is briefly described. Several other units, some of higher apparent power and voltage are now under construction.Development is proceeding, and it appears that in the near future some very large transformers may be built which could be installed next to, or built as an integral part of, the generators, and this at considerable overall cost reduction over present practices.

DOI:10.1049/pi-a.1960.0082

出版商:IEE    

年代:1960

数据来源: IET

摘要:

The electrical resistivity of sodium or sodium-potassium is continuously measured whilst flowing at operating temperature in a pipe, to provide an indication of impurity level, particularly oxygen, and thus to enable progress of clean-up in liquid-metal loops to be followed and to give assurance that high purity is achieved and maintained.The meter is designed to be sensitive to a change of one part in 106in oxygen level, corresponding to a change in resistivity of about one part in 104. A change in liquid-metal temperature of only 0.04° C will also produce this effect, and hence temperature compensation and achieving high stability are the chief problems of the design.Two types of resistivity meter are described, both of which have been installed in the Dounreay fast reactor. Tests indicate that the design objective has been achieved and the value of the meter as an impurity monitor has been proved.

DOI:10.1049/pi-a.1960.0083

出版商:IEE    

年代:1960

数据来源: IET