The increasing application of SF6 as an insulating gas has led to many studies on SF6 decomposition in gas-insulated equipment. In the presence-of an electric arc, spark or corona, SF6 decomposes to a wide variety of chemically active products which possess completely different properties from SF6. The accumulation of these decomposition products in the equipment has caused concerns regarding personnel safety and material compatibility problems. This paper reviews previous research in SF6 decomposition relating to the operation of gas-insulated switchgears, gas-insulated transmission lines, and electrostatic accelerators. Results on the qualitative and quantitative determination of the by-products and their formation ion rates in various modes of electrical discharges are summarized. The mechanisms leading to the formation of transient and stable products are described. In particular, the influence of discharge energies and impurities on the formation of SOF2 and SO2F2, the two dominant stable by-products, is discussed. The effects of the by-products on personnel safety and equipment ent dielectric integrity are presented. The application of SF6 gas analysis as a tool for diagnosing the internal condition of gas-insulated equipment is assessed. Based on the results of many phenomenological observations, future research activities are suggested to address the issues of safety, compatibility and equipment aging. More fundamental studies on electron, ion, and neutral reaction rates in an SF6 discharge are required to gain a better understanding of the decompositon mechanisms and the influence of the products on equipment operation.

SF6 Decomposition in Gas-Insulated Equipment

The breakdown field strength E/sub b/(= alpha Vb/d) of gaps made with pure copper and stainless steel depends on the effective area S/sub eff/. It can be expressed by the single experimental equation E/sub b/=K/sub 1/S/sub eff//sup -n/, where K/sub 1/ is a constant, n is about 0.24, S/sub eff/ is the electrode area subjected to more than 90% of maximum field strength, alpha is the shape factor, d is the gap length, and V/sub b/ is the breakdown voltage. The field intensification factor beta /sub l/ obtained through the measurement of prebreakdown current can be expressed by a single experimental equation beta /sub l/=K/sub 2/S/sub eff/n, where K/sub 2/ is a constant. This means that the area of effect of E/sub b/ is represented by the area effect of beta /sub l/ and that the vacuum breakdown may be initiated by field emission. >

Area effect on electric breakdown of copper and stainless steel electrodes in vacuum

In this paper vacuum breakdown phenomena are reviewed in two parts The first considers the development of thought concerning electrical breakdown in vacuum with particular regard to field emission, interaction of cathode emission at the anode, and particle effects, including impurities While Section 1 is basically historical, Section 2 considers these and other topics in the light of recent literature

Electrical Breakdown in Vacuum

The author presents a bibliography covering the field of electrical discharges in vacuum, comprising both electrical breakdown in vacuum and vacuum arcs. A brief section lists some review papers which would be helpful to the novice in this field. The bulk of the presentation consists of bibliographic listings arranged by year of publication and alphabetically by author within each year. An author index refers one to all papers authored or coauthored by a particular person. >

Electrical discharges in vacuum: 1980-90

Using three types of copper electrodes with different surface areas, experiments were performed to investigate the influence of electrode area on the conditioning effect,which is a characteristic of dielectric breakdown in vacuum gaps. The conditioning process varied with electrode area: the smaller the electrode area, the sooner conditioning ended. Breakdown voltages after completion of conditioning also depended on electrode area: the smaller the electrode area, the higher the breakdown voltage. >

Influence of electrode area on the conditioning effect in vacuum

High voltage impulses were used to investigate various possible factors influencing the prebreakdown current and breakdown in vacuum gaps of sphere-to- sphere and rod-to-plane.

Impulse Voltage Field Emission Characteristics and Breakdown Dependency Upon Field Strength in Vacuum Gaps

It is known that some insulating materials are degraded due to dissociation products of SF6 gas. Many studies related to the degradation of insulating materials have been accomplished. But no studies related to sealing materials, for instance (lubricating) grease and rubber have been made.

Degradation Process of Grease Due to SF 6 Gas Dissociation Products

Three types of cross-linked polyethyline (XLPE) wire were developed for insulation of internal pump motors (wet type) used in an advanced boiling water reactor. By applying gamma ray irradiation of 10/sup 4/ to 10/sup 6/ Gy and electric stress in hot water simultaneously or sequentially, the effect of gamma ray irradiation is examined. The effect of combined stresses in a large scale functional test is also described. It is proved that the gamma ray irradiation less than 10/sup 6/ Gy does not deteriorate the properties of the insulation of the wires. In the irradiational environment of 5*10/sup 4/ Gy where the internal pump motor is served, it is concluded that the wires have sufficient reliability. >

H. Yoshida

Papers

Impulse Voltage Field Emission Characteristics and Breakdown Dependency Upon Field Strength in Vacuum Gaps

Degradation Process of Grease Due to SF 6 Gas Dissociation Products

Voltage Deterioration of XLPE Wires in Water under Gamma Ray Irradiation