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Characteristics of corona discharges over insulating surfaces are described. Two insulating materials, PTFE and porcelain are compared and contrasted using identical insulator surface profiles and dimensions and identical impulse voltage waveforms. The insulators were in the form of right cylinders and of similar shape but with a single shed, of practical dimensions added. Properties studied include the inception conditions, spatial development, light emission and changes in corona pattern. Results are discussed in terms of the relative permittivity of the materials, the surface profile and the physical effects of the surface upon corona properties. Index Terms--Corona; insulators; PTFE, porcelain; surface discharges.

Experiments are described on the propagation of corona discharges over the surfaces of cylindrical PTFE and porcelain insulators under positive impulse voltages. A rod-plane electrode arrangement has been used to which a single value of peak voltage, with the same impulse profile, has been applied throughout. Quantities measured were inception times and voltages, corona current and light output at selected axial regions of the corona. Still photographs of the corona appearance have been taken. Comparison has been made between the surface corona and that in air alone. Interpretation is based on knowledge of the electric field profiles, as modified by the insulators and the possible effects of the surface on the discharge. These are discussed in terms of the presence or absence of charge attachment, the effects of the surface on electron production and the effect of the permittivity of the materials. Index Terms--Surface ionization, positive corona, insulating surfaces.

The uses and limitations of the electrostatic probe for the measurement of charge densities on insulating surfaces are discussed. A development of the technique is described in which two important limitations have together been overcome: (i) The effects on the probe signal of charges on all points of the surface have been taken into account by means of a matrix inversion procedure; (ii) A robotic control system has been developed which enables the probe to follow and scan a wide range of axi-symmetric insulator profiles. The degree of resolution achieved enables the probe system to display and measure charge densities in individual streamer channels of a corona discharge on a polytetrafluoroethylene (PTFE) surface. An example is given and comparison made with a dust figure of the same event. Index Terms--Electrostatic, robotic probe, scan, matrix, surface corona, PTFE, surface charge, capacitive probe, insulator.

Surface charge on an insulator's surface is studied using 3-dimensional finite element modelling. The calculations are based on measurements of surface charge deposited by a streamer on a PTFE surface. The surface electric field strengths associated with uniform surface charge density distributions over a 1[mm.sup.2] region are calculated for a range of different measured charge densities. The effect of reducing the area of the surface charge whilst keeping the total amount of charge in the region constant is observed. The surface charge is then modelled with a Gaussian distribution of charge and compared to that of a uniform distribution. By assuming a maximum allowable surface electric field strength an estimation of actual charged path diameter is made. Index Terms--Electrostatic, capacitive probe, scan, modelling, streamer, charge density, PTFE, insulator, electric field, finite element method.

A description is given of some properties of the corona discharge when propagating over a cylindrical polytetrafluoroethylene insulator surface placed along the axis of a rod-plane electrode arrangement. A scanning electrostatic probe has been used to measure the density of charge deposited on the surface; it is shown that the total net deposited charge is small compared with the total charge injected into the gap. The velocity of propagation has been measured. Effects of preceding coronas on succeeding ones and the nature of deposited charge after breakdown are described. Discussion is given in terms of electron lifetimes and attachment and photoemission processes.

Experimental results on the propagation of positive streamers along contoured, axially symmetric, polytetrafluoroethylene (PTFE) insulation surfaces are presented. A plane parallel electrode configuration provides a substantially uniform electric field for streamer propagation and a point electrode at the ground plane initiates the avalanche process. Basic streamer properties of velocity and propagation probability with field strength are measured and compared with the corresponding characteristics of air and cylindrical insulators. Several insulator profiles are investigated and the occurrence of multiple streamer paths is demonstrated, which are generally distinguishable as surface and air components with different propagation velocities. Comparative data of the breakdown fields for the various insulators is included.

Comparative measurements are reported on the breakdown, under lightning and switching impulses, of a concentric electrode arrangement in which the inter-electrode space is spanned either by an insulator surface or by air. Temperature in the system has been varied in the range 10 < T < 100 [degrees] C. The variation of positive and negative 50% flashover voltage, and the times to breakdown at 100% flashover, are presented for each temperature level tested. Flashover voltages, analyzed as a function of relative air density [Delta], show a variation of the form [[Delta].sup.m], where 0.8 < m < 1.0 for positive polarity and m > 1.0 for negative polarity at the highly stressed electrode. Variations are the same, in the presence or absence of the insulator surface, but absolute positive flashover voltages are lower across the surface, than in air, while negative flashover voltages are higher. Times to breakdown decrease with increasing temperature.

The sparkover characteristics of rod-rod and rod-plane gaps in air under dc voltages are reviewed. The linear characteristics in both configurations are associated with the prominence of streamer discharges during the prebreakdown phase. The limits of linearity are shown to be a function of humidity, gap spacing and electrode profile, all of which determine the pre-discharge mode. It is shown that the rod-plane gap offers a wider range of linearity than the rod-rod gap and that it can have a valid role as a dc voltage measurement tool.

The variation of current densities on a plane cathode in air has been investigated in a positive point-plane gap configuration. This work is concerned with the effects of gap spacing, point diameter and humidity variations on the current density profiles, according to the type of corona regime produced at the point anode during predischarge development. Cases are considered where the point electrode is energized by a positive dc voltage producing either a pure glow or streamers, according to given particular experimental conditions such as point-plane gap spacing, point diameter and ambient absolute humidity. The results obtained show that the current density variation for positive corona in air does not always follow Warburg's law but depends upon the type of corona regime generated at the HV point anode. Comparisons are made with negative point-plane gaps, studied by other workers.

In a uniform field arrangement, under direct voltage, positive streamer propagation and breakdown are investigated along cylindrical insulators with different profiles, inserted perpendicularly between two parallel plane electrodes. The basic properties of streamer propagation and breakdown, namely the electric field required for a stable propagation together with the associated velocity and the breakdown field together with time to breakdown, are measured as influenced by the pulse voltage amplitude used for the streamer initiation and by the insulator profile. It is shown that a strong relation between streamer propagation and breakdown exists, because the insulator profile exerts a similar influence on the breakdown and propagation fields. The effect of a shed on an insulating surface, forming an `obstruction' to streamer progress, is to increase the stability for propagation and breakdown fields, and to reduce the propagation velocity at all applied fields compared with those in the case of a smooth insulator Along the surface of a smooth insulator, a streamer system propagates with a `surface' and an `air' component; however, a shed on an insulating surface modifies this system, resulting in only one component reaching the cathode.

The paper describes the development of arc discharge rectifiers, employing hydrogen and mercury as working gases, to test the effects of a magnetic field on their deionization characteristics. The discharges were pulsed in the presence of (a) an established magnetic field, and (b) a pulse field switched on near the end of the discharge pulse. The arc back characteristics were studied as a function of the conditions, in which currents were varied to over 1000 amperes and magnetic fields up to 0.11 W/M peak. Deionization was studied by measurement of current drawn to the walls of the tube, together with some radial observations of light output from the discharge. The results indicated that while significant deflections of the discharge were recorded, the magnetic fields did not appreciably improve the arc-back qualities of the rectifiers. [ABSTRACT FROM AUTHOR]