472 / 2017-05-08 15:05:17

Sensitive Coefficient of Electric Field for SF6 and SF6/N2 Gas Under Equal Dielectric Strength

SF6 gas; SF6/N2 gas mixture; SCEF; non-uniform electric field; dielectric strength

SF6 is widely used in high voltage equipment due to its high dielectric strength and arc quenching ability. While SF6 has strong greenhouse effect so that it should be used very carefully. Strict recycling method still cannot avoid its leakage in normal use, which still has a large quantity. As SF6/N2 mixed gas can largely reduce the use of SF6, it is considered to replace SF6 as insulated medium in gas insulated transmission line (GIL), which needs large amounts of SF6 and needs no interruption requirement. The dielectric and physical performance of SF6/N2 has been studied for years, however, the insulated properties of SF6/N2 are rarely mentioned under equal dielectric strength with SF6, which is important for the use of SF6/N2 in practice. In this paper, the sensitive coefficient of electric field (SCEF) of insulated gas is introduced to judge the effect of non-uniform field on the gaseous dielectric strength, which obviously decides the partial discharge (PD) level to be controlled in gas insulated equipment. The SCEF equals to the ratio between breakdown voltages (BDV) of insulated gas under non-uniform electric field and normal coaxial electric field. As SF6 is verified to be much more sensitive to non-uniform electric field, the PD control is quite severe in SF6 insulated equipment. Numerous researches indicate that SF6/N2 mixture is less sensitive to non-uniform electric field than SF6 under the same pressure. However, SF6/N2 gas mixture is commonly used in higher pressure to reach required dielectric strength in the same size equipment with SF6. The SCEF of SF6 and SF6/N2 needs to be compared under equal dielectric strength, which means higher pressure for SF6/N2. As the SCEF is affected by gas pressure, the conclusion under equal dielectric strength could be contrary to that under equal gas pressure. In this paper, the SCEE of SF6 and SF6/N2 under equal dielectric strength is obtained though breakdown tests. The tests are performed under 50Hz alternating voltage and standard lightning impulse voltage, in which the point-plate electrodes and ball-plate electrodes are used to simulate non-uniform and slightly non-uniform coaxial electric fields. The dielectric strength is considered at the pressure of 0.4MPa for SF6, and 0.5MPa to 0.7MPa for SF6/N2. Experimental results indicate that the SCEF of SF6/N2 is larger than SF6 under equal gas pressure but smaller under equal dielectric strength. The conclusion means that the PD level to be controlled in high pressure SF6/N2 mixed gas insulated equipment should be more severe than SF6.