Following an examination of the basic gas-discharge concepts, the Townsend breakdown criterion is derived for nonuniform fields and a reformulation of the streamer criterion is undertaken. A direct application of the Townsend criterion to a practical situation is hardly possible, whereas the streamer criterion leads to applicable criteria for the onset of breakdown in air as well as in strongly electronegative gases. In this approach, a knowledge of the relevant ionization coefficients is not required, because these are replaced by parameters obtained from uniform-field Paschen curve data. In addition, the criteria developed do not contain any arbitrary constants. By introducing the electrode surface mean curvature, it is shown that the calculation of breakdown data for widely different electrode shapes can be addressed through a single formula. It is emphasized that the inherent roughness of practical electrode surface demands that caution be exercised in all design criteria. >

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On the electrical breakdown of gaseous dielectrics-an engineering approach

The well known breakdown theories, Townsend's generation mechanism and the streamer mechanism, are reviewed and applied to the results of breakdown in the strongly electronegative gas, sulphur-hexafluoride. Experimental results reported in the literature on the breakdown behavior of sulphur-hexafluoride are examined in the light of these theories. The breakdown theories are used for the estimation of the breakdown voltages in pure SF6. Other factors that may affect the breakdown characteristics of SF6 have been discussed. Further areas of work have been proposed in order to obtain a better understanding of the breakdown mechanism.

Breakdown Mechanisms in Sulphur-Hexafluoride

The adoption of a field-theoretical approach to problems arising in the framework of electrical insulation is discussed with reference to six main topics, which have been addressed over the last 30 years. These include uniform field electrodes, Green's differential equation, electrode surface roughness, induced charge, electrostatic probes, and partial discharge transients, together with several follow-on aspects. Each topic is introduced and thereafter the progress achieved through the use of a field-theoretical approach is reviewed. Because the topics cover a wide spectrum of conditions, it is amply demonstrated that such an approach can lead to significant progress in many areas of electrical insulation.

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Electric fields and electrical insulation

The aim of this paper is to discuss the area left of the Paschen minimum, i.e., the area where the anomalous Paschen effect occurs. The experimental results and numerical simulation suggest a slightly different explanation of the anomalous Paschen effect. That is to say, that the anomalous Paschen effect is the result of the breakdown in a nonhomogenous electric field (edge electrode lines).

Anomalous Paschen effect

Quantitative criteria are derived for the onset of breakdown in atmospheric air and similar weakly electronegative gases. A comparable criterion is then formulated for strongly electronegative gases such as SF/sub 6/. These criteria contain no quantities other than those which can be obtained directly from breakdown voltage measurements in a uniform field, i.e. from Paschen curve data. No specific data are required for the ionizing coefficients of the gas. The advantage, from an engineering point of view, is that it is much easier to perform reliable Paschen curve measurements than it is to measure ionization growth parameters. The price paid is that criteria formulated in this way cannot be used to yield unknown Paschen curves. The criteria can be applied to any nonuniform field configuration provided the electrostatic field distribution is known. Moreover, in many cases a simple analytical approximation to the field distribution can render an extensive calculation of the electrostatic field unnecessary. The discussion deals exclusively with situations in which the onset of breakdown in the system is synonymous with the onset of a corona discharge. >

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Experiments showing anomalously low breakdown voltages in uniform field gaps in SF6 are discussed. A generalized quantitative criterion suitable for computer calculations of breakdown voltages in known fields in SF6 and other strongly electronegative gases is described, and it is shown that the anomaly in the uniform field gap breakdown voltages can be satisfactorily accounted for. Similar anomalies can occur in other field geometries and in other gases; it is shown how to avoid such breakdowns.

T. Munk Nielsen

Papers

Anomalous Breakdown in Uniform Field Gaps in SF6