Measurement of air breakdown voltage and electric field using standad sphere gap method

Abstract

Rapid growth in power sector of nation has given the opportunity to power engineers to protect the power equipment for reliable operation during their operating life. It has been seen from the studies conducted by power engineers that one of the main problem in high voltage (HV) power equipment is the degradation of insulation i.e., quality of insulation of power equipment. In electrical power system, HV power equipments are mainly subjected with spark over voltage. These over voltage which may causes by the lighting strokes, switching action, determine the safe clearance required for proper insulation level. Normally, the standard sphere gaps are widely used for protective device in such electrical power equipments. The sphere gaps are commonly used for measurements of peak values of high voltages and have been adopted by IEC and IEEE as a calibration device. The sphere gaps are filled up with insulating medium such as liquid insulation (transformer oil), solid insulation (polyester, paper) and gas insulation (SF6, N2, CO2, CCl2F2 etc). Normally air medium is widely use as an insulating medium in different electrical power equipments as its breakdown strength is 30 kV/cm. Therefore electrical breakdown characteristic of small air gap under the different applied voltage has its great significance for the design consideration of various air insulated HV equipment.
In this work to simulate the air breakdown voltage experimentally in high voltage laboratory, standard diameter of 25 Cm spheres are used for measurement of air breakdown voltages and electric field of the high voltage equipments. The above experiment is conducted at the normal temperature and pressure. Finite element method is also used for finding the electric field between standard sphere electrodes. The relative air density factor and maximum electric field are measured in MATLAB environment for different temperature and pressure. The electric field distribution for sphere gap arrangements is also calculated with the help of COMSOL. In addition the influence of the humidity on air breakdown test has been also considered in this study. Humidity correction factor also considered in this work for maintain constant air breakdown voltage. Finally, the experimental result has been compared with theoretical, and simulation results.