ANALYSIS ON DIELECTRIC STRENGTH OF SF6 GAS FOR GAS INSULATED TRANSFORMER

摘要

Recently, the demand of gas insulated transformer (GIT) using SF_6 gas as insulating and cooling medium is being increased continuously in the underground substation of urban area because it has attractive advantages such as non-flammability, non-explosion, and so on. A GIT using SF_6 gas is known as relatively reliable compared with an oil-immersed transformer. Hyundai Heavy Industries Co., LTD. (HHI) has developed and commercialized a low pressure type 154kV GIT because it has many economical and reliable advantages. In this paper, dielectric characteristics of various electrode systems in SF_6 gas are tested and analyzed to improve the dielectric performance of a low pressure type 154 kV GIT using SF_6 gas. The electrode systems which simulate the internal structure of a GIT using SF_6 gas are made of stainless steel and dielectric tests according to the gap length between two electrodes and the pressure of SF_6 gas are performed. It is found that the dielectric characteristics of electrode system with non-uniform electric field are related to gap length and those of electrode system with quasi-uniform electric field are deeply dependent not only on the gap length but also on the pressure of SF_6 gas. Also, barrier effects on lightning impulse dielectric strength test in gaseous condition are experimentally verified. In this investigation, dielectric tests are performed by installing barriers made of pressboard (PB) between electrode system and breakdown voltage according to the pressure of SF_6 gas and the number of PBs is measured. Consequently, it is found that barrier effects are effective on lightning impulse dielectric strength test.