摘要:

Using a digital computer, field factor has been calculated for spherical electrodes by the image-charge method, taking account of adjacent plane or spherical boundary surfaces. The method has been applied to a vertical asymmetric sphere gap represented by a pair of spheres above an infinite earth plane or a pair of spheres within an enclosing earthed sphere. Graphs are presented showing the percentage increase in field factor above the value applicable to isolated spheres, for different positions of the boundary surface; these graphs enable values of field factor to be deduced, for the geometries represented, with an accuracy better than 0.1%.For the standard sphere gap of BS3581and IEC52,2varying the position of the infinite earth plane over the full permitted range leads to changes of up to 0.4% (±0.2%) in field factor, for a gap spacing of one sphere radius. When the clearance from the sphere gap to additional earthed surfaces, representing the walls and ceilings of a laboratory, is varied from the minimum value permitted to about five times this value, with the earth plane as close to the sphere gap as allowed, the field factor falls by 0.5%.

DOI:10.1049/piee.1965.0256

出版商:IEE    

年代:1965

数据来源: IET

摘要:

The problem of determining conductivity distributions from measurements on magnetic loops (induction coils) is examined. Maxwell's equations are written for a cylindrically symmetric system allowing for radial variations in conductivity, and solved numerically for specific types of distribution similar to those found in plasma experiments. Methods are described by which the field impedance (E/H) at the boundary of the medium can be found and used to characterise the distribution.

DOI:10.1049/piee.1965.0257

出版商:IEE    

年代:1965

数据来源: IET

摘要:

The solution of Maxwell's equations to find the magnetic-field decay in a long rectangular iron bar of constant permeability is a classical problem in electromagnetic theory, with a well known solution. The converse problem of establishing the field when a voltage is applied to a winding on the bar is much more difficult, and has not been satisfactorily solved. Re-examination shows that the orthogonality of terms in the classical series solution leads to an exact circuit representation of the solid iron, composed of an infinite number of network loops. The explicit solution is found to be readily available by numerical methods using a digital computer, since the networks may be described by rapidly converging infinite series. Computed results are compared with answers obtained analytically by considering only a very few of the network loops. In determining time constants, accuracies of the order of within 1% are shown to be achievable, using a simple network analysis without the use of computing machines. Formulas for calculating the necessary network-component values, together with some sample computer results, are given.

DOI:10.1049/piee.1965.0258

出版商:IEE    

年代:1965

数据来源: IET

摘要:

The two theories dealing with the operation of induction meters are reviewed. Their final expressions for the torque acting on the rotor, in terms of the power, voltage, load current and speeds of revolution, are found to be the same. However, despite this coincidence, the two theories are both considered to be admissible. The existing proofs of the erroneousness of the travelling-field theory tend to lead indirectly to the conclusion that only the transformer or eddy-current theory is valid. Furthermore, the measured distributions of the magnetic flux density along the air gaps of the electromagnets in the meter are analysed for agreement with the travelling-field theory. The conclusion reached is that, although the travelling-field theory fails to give a full description of the phenomena which determine the operation of the meter, it still deserves to be retained beside the transformer, or eddy-current, theory.

DOI:10.1049/piee.1965.0259

出版商:IEE    

年代:1965

数据来源: IET

摘要:

Asbestos and glass-fibre insulation, impregnated with silicone resins, is now widely used in the construction of dry-type transformers rated as class C in BS 171: 1959, and operating at a maximum winding temperature of 200°C. Applications range from mining transformers to packaged substations, wherever oil is prohibited because of fire hazard. It is noted that the costs of class C air-cooled transformers are usually about 50% higher than for oil-cooled transformers of equivalent rating, so that these units are only selected where other factors justify this price differential. The insulation components are described, with some notes on their thermal-aging performance.Though present designs are mostly restricted to voltages of 11kV or below, it is considered that the use of fluorinated-hydrocarbon gases for cooling may lead to further developments at much higher voltages, e.g. for indoor-generator transformers.

DOI:10.1049/piee.1965.0260

出版商:IEE    

年代:1965

数据来源: IET

摘要:

Silicone rubbers are introduced in terms of their chemical composition and physical properties, with a description of different types and the methods by which they are processed. Applications described include cables, insulating sleeving and tapes, while a later Section deals with the important and versatile cold-curing rubbers, which are widely used for potting and encapsulating purposes. Recent developments are then indicated, with special mention of rubbers with improved physical properties.

DOI:10.1049/piee.1965.0261

出版商:IEE    

年代:1965

数据来源: IET

摘要:

A continuous process is described which has been used to produce thin (∽1μm) polymer films from styrene vapour subjected to an a.c. glow discharge. Dielectric losses in such films were investigated, including the effects of various gases present during polymerisation.

DOI:10.1049/piee.1965.0262

出版商:IEE    

年代:1965

数据来源: IET

摘要:

From the point of view of its electrical and mechanical properties and its ready availability in thin tape form, polythene is one of the most promising dielectrics for possible application in h.v. cable construction. To predict the behaviour of the polythene/gas dielectric system, having a lapped-tape construction, experiments were carried out on thin-walled models in nitrogen and several electronegative gases. The effects on impulse strength of a number of variables, a.c. life tests, and factors affecting the discharge-inception stress were investigated. In addition, the gas-pressurisation time was determined. Optimum electric strength, consistent with good mechanical performance, was obtained with tapes of 3 mil thickness. It was concluded that a composite polythene-tape/gas dielectric system has many advantages to recommend it for full-scale h.v. cable development.

DOI:10.1049/piee.1965.0263

出版商:IEE    

年代:1965

数据来源: IET

摘要:

Both the dust/fog and IEC methods have been used to investigate the resistance to surface tracking and erosion of a range of synthetic insulating materials under polluted conditions. The relative merits of these test methods are discussed, and reasons given for preferring the dust/fog test. Consideration is also given to methods of accelerating the dust/fog erosion test.Results of such tests have been considered from a theoretical aspect, and a relationship between chemical structure and resistance to surface tracking is proposed. Using this approach, it was deduced that the alicyclic epoxy resins would be superior to the conventional bisphenol-based resins; this has been confirmed by dust/fog tests. Tracking resistance can also be increased by use of fillers, and this is illustrated for silica and alumina trihydrate in some epoxy casting systems.The application of track- and erosion-resistant materials to service use is considered, with special reference to casting resins. It is shown that the use of fillers to improve track resistance may cause deterioration in other electrical and mechanical properties of the material.

DOI:10.1049/piee.1965.0264

出版商:IEE    

年代:1965

数据来源: IET

摘要:

High-density polyethylenes can offer an advantage of 20degC, and polypropylenes an advantage of 55 degC, in the upper temperature limit, over the currently used low-density-polyethylene insulation. None of the 16 high-density polyethylenes examined, however, combines good mechanical properties with the desirable dielectric characteristics at high temperatures. Many are relatively tough when prepared by quenching the melt but embrittle on subsequent exposure to 100° C; the majority have low impact strength and inadequate resistance to thermal cracking. The commercial polymers with the best mechanical performance have relatively poor dielectric properties.Commercial polypropylenes are all very brittle, and, if cooled slowly from melt, crystallise with visible voids between spherulites. With few exceptions their loss tangent is high at high temperatures and mains frequency.One type of modified polypropylene, made in America, shows a moderate improvement in mechanical properties coupled with excellent dielectric characteristics. Another type, made in Britain, shows outstandingly good mechanical properties, which include high impact strength and high extensibility, even after slow cooling from melt.

DOI:10.1049/piee.1965.0265

出版商:IEE    

年代:1965

数据来源: IET