On calculation of corona and breakdown voltages of gaseous insulation with special reference to elecrro-negetive gases and their mixtures

Abstract

In the design of any insulation system, mechanical, electrical, thermal, and economic considerations are generally of prime importance, and many factors contribute to the type of insulation used in a particular system. In the transmission of electrical energy above 400 kV, conventional solid insulation becomes bulky and expensive. In such cases, gaseous insulation is preferred. Among various gases used in insulation systems, SF? is of great importance since it has a high dielectric strength-nearly 2.5 times that of air-due to its electron-attaching property. In addition, SF? has certain properties such as good thermal conductivity, arc-quenching ability, chemical inertness and stability, non-flammability, and non-toxicity compared to other gases used in insulation. However, SF? is very costly, and therefore considerable research work is being done on mixtures of electronegative gases with ordinary gases and electronegative-electronegative gases (binary and ternary mixtures) to obtain an optimum solution that will be economically and effectively favorable for use in insulation systems of transmission lines, circuit breakers, gas-insulated substations, and other power apparatus. A large amount of data is available in literature on breakdown of different gases and their mixtures, but it is difficult to use this data because it has not been formulated. For a design engineer, empirical formulae are of great use for practical application. In Chapter 2, it has been clearly brought out that there is a need to coordinate this data in the form of a simple expression for use in practical systems. Some formulae are available in literature but each has certain limitations and is applicable only under specific conditions. Hence, taking all these points into consideration, efforts are made to develop a simple formula (Chapter 3) which applies to electronegative gases and their mixtures under alternating as well as direct voltage conditions. The formula is valid under uniform as well as non-uniform field conditions. Under non-uniform field conditions, the formula is based upon calculations of a field utilization factor. The formula has been shown to be valid for 15 gases and their mixtures under different conditions of pressure, voltage, and electrode geometries. In the fourth chapter, verification of the formula is done by studying the corona and breakdown phenomena in air in non-uniform fields using point-plane and coaxial cylindrical electrodes. In these experiments, a multi-channel analyzer is used and, using computer DEC 1090, the number of pulses and associated charge is also measured. In the fifth chapter, a discussion of results, conclusion, and future work is presented. It is hoped that the formula will greatly aid design engineers working in the field of insulation and power system engineering.