It has been known for many years that the grounding resistance of a concentrated electrode drops when it is subjected to a high current charge. This helps reduce the ground potential rise. The degree of the resistance depends on the magnitude of the ionization gradient of the soil E/sub o/. Based on both a theoretical analysis and a critical review of the large number of available measurements, this paper recommends that E/sub o/ be taken equal to 300 kV/m for typical soils. This is significantly less than the 1000 kV/m value used by some authors. Graphs are also given describing the behaviour of the rod electrodes which are used in many field installations. >

The soil ionization gradient associated with discharge of high currents into concentrated electrodes

Various factors controlling the flashover of solid insulators in pressurized SF6, are reviewed and their influences in gas insulated systems are discussed from a practical point of view. Flashover voltage of clean insulator surface is under the influence of the insulator-metal contact as well as the macroscopic electric field distortion due to the high dielectric permittivity of solid insulator. Conducting particles or even fine metal powder can reduce the flashover voltage. Their effects are strongly dependent on the position they are located, the size of the insulator and gas pressure. Humidity of SF, gas should be strictly governed in SF, gas insulated apparatuses, since the condensation of water can decrease flashover voltage considerably. Decomposition products of SF6 due to the arcing in switchgears are deleterious to epoxy insulators particularly when silica is used as their filler. The decomposition products decrease the leakage resistance on the insulation surface. The field strength near positive electrode is enhanced by the electrolytic effect in the surface conduction layer. In some extreme condition, it initiates tracking on the insulator surface.

Factors Controlling Surface Flashover in SF6 Gas Insulated Systems

Electrode surface roughness can cause a large reduction of the threshold for break down in SF 6 insulated apparatus. This effect is discussed by applying the streamer theory of breakdown to the microscopic field regions associated with surface roughness. A quantitative relation is derived between the practical breakdown threshold and gas pressure times maximum surface roughness.

The effect of surface roughness on breakdown in SF 6

The long term behaviour of epoxy resin subjected to high alternating electrical stress was studied using optical magnifications of up to 1500, a scanning electron microscope, and a high sensitivity discharge detector. The formation of voids in initially void-free specimens was investigated when voltages of duration between ten seconds and several thousand hours were applied using a pin-plane configuration. Defects in epoxy resins, mechanisms of their electrical aging, and their long term dielectric strength are discussed in relation to the phenomenon of void growth.

Void formation and electrical breakdown in epoxy resin

This paper presents experimental results on the statistical properties of ac and dc breakdown field strength of compressed SF6. The experiment is performed on four gaps of different electrode area, ranging from 0.2 cm2 to 3000 cm2. Patterns of breakdown characteristics are classified into four categories depending on the type of distribution and the characteristics of conditioning effect. The criterion of the transition of these categories are discussed quantitatively.

Area Effect of Electrical Breakdown in Compressed SF6

The electrical discharge characteristics of SF6 are discussed theoretically in relation to the field dependence of the ionization coefficient a and the electron attachment coefficient The results are compared with the characteristics of air. A simple theoretical formulation of breakdown or corona inception voltages of gaps in SF6 is derived. The formulation has been examined by experiments on several electrode configurations. At low pressures of less than 4 atm, the agreement of the theoretical and the experimental results is fairly good. The breakdown voltage decreases from the theoretical estimation at higher pressure.

Y. Shibuya

Papers

Electrical Breakdown of Long Gaps in Sulfur Hexafluoride