摘要:

A study of the chemical changes produced by exposure of dielectric materials to high-energy ionising radiation has shown that the effects are similar to those involved in electrical deterioration during stress aging and when ionisation has taken place in the dielectric. The principal effects observed in hydrocarbons and polymeric materials after exposure to radiation are: the crosslinking of the molecules to form an infusible gel structure; the evolution of hydrogen and low-molecular-weight fragments; molecular degradation by main-chain scission; the formation and annihilation of unsaturated groups; and, in the presence of air, processes of oxidative degradation. The corresponding electric stress and ionisation phenomena are: wax or ‘cheese’ formation in solid-type cables; the gas evolving characteristics of paraffinic oils; the embrittlement of insulating paper; the formation of double bonds in aged cable oils, as evidenced by increased absorption in the ultraviolet region; and the oxidation effects with air resulting in an increase in power factor.While the mechanisms of radiation-induced change are not yet completely understood, they should throw light on the corresponding electrical stress and ionisation phenomena. Some of the methods used to protect organic systems against the effects of high-energy radiation are also satisfactory in reducing electrical effects, and a study of these protective mechanisms should be beneficial in the field of high-voltage dielectrics.

DOI:10.1049/piee.1965.0206

出版商:IEE    

年代:1965

数据来源: IET

摘要:

Recent experimental work on the insulating properties of high vacuum is systematically reviewed. A résumé of the prebreakdown conduction phenomena is given; in small gaps, the steady prebreakdown currents appear to be due to a modified type of field emission, while in longer gaps the contamination on the electrode surfaces gives rise to an ion-exchange mechanism, which is responsible for the microdischarges associated with these gaps. The parameters affecting the breakdown strength of a vacuum gap, and the different hypotheses postulated to explain the mechanism of breakdown, are considered in detail. No single hypothesis is able to account for all the experimental results reported, but it appears that a transition from one type of mechanism to another takes place as the interelectrode gap is increased.The paper also considers the important practical aspects of breakdown across solid insulating surfaces in vacuum.

DOI:10.1049/piee.1965.0207

出版商:IEE    

年代:1965

数据来源: IET

摘要:

Although the measurement of dielectric properties of materials at microwave frequencies by resonant-cavity techniques is well established, care must be taken when interpreting the results of measurements on pure materials with very low losses. This paper describes an H01nresonant-cavity system and analyses the influence of the degenerate E11nmode on the measurements. Some results of measurements at 9.4Gc/s on debased-alumina samples are given.

DOI:10.1049/piee.1965.0211

出版商:IEE    

年代:1965

数据来源: IET

摘要:

Both the loss tangent and the d.c. conductivity of insulating oils decrease with time after the oil is introduced into the test cell, in the absence of electric stress and at room temperature.When the cell is heated immediately after filling, the loss tangent and d.c. conductivity rise to a maximum, but decrease to a much lower value with time at constant temperature. The maximum value, the rate of rise to that value and the subsequent decrease with time are functions of the thermal capacity and shape of the cell, the technique used to clean the cell, the temperature and the oxygen content of the oil. The subsequent decrease in dielectric loss is thought to be due to catalytic oxidation of some component in the oil at the metal-electrode surface, although it is possible that ions may be merely trapped at this surface.The effects have been observed with a low-viscosity cable impregnant, both unused and severely contaminated, a transformer oil and a synthetic hydrocarbon.The implications of the effect are discussed in relation to routine testing of oils, to investigations of the effects of contamination in oils and to work on oil-impregnated paper. It is possible that the effect may be partially or wholly eliminated by the use of plastic-coated electrodes.

DOI:10.1049/piee.1965.0212

出版商:IEE    

年代:1965

数据来源: IET