367 / 2017-04-11 21:58:28

Ion current measurements in SF6 and Vacuum Under High Voltage DC Application

13400,13401,13402,13403

会场 > Fundamental Physics and Electrical Insulation in Switchgears

Pre-breakdown current evaluation is of importance for Gas Insulated Switchgear (GIS) under DC high voltage application. Even if very small, assuming constant polarity of the applied voltage, such currents represent a source of charges which continuously accumulate on dielectric surfaces, especially if these have high resistance. This can lead to a significant change in the distribution of the electric field, possibly compromising voltage withstand. In this work we present a set-up for pA current measurements, for the evaluation of the influence of various parameters, such as humidity, temperature, gas type and gas pressure. The system is built inside a commercial GIS vessel, not only to provide a field configuration as similar as possible to the application, but also in order to be able to capture ionization currents, which are proportional to the volume of the gas. Another advantage is that the electrodes are designed in coaxial configuration, and this facilitates the simulations and allows to predict emitted currents from closed analytical formulas. The measurements are performed under DC high voltage application (up to 120 kV), with negative and positive polarities. Particular care was taken in order to reduce residual ripple of the voltage source, which could compromise the measurement quality. Care was also taken to improve current resolution. With the adopted instrumentation it is well below 1 pA, thus allowing to resolve the resistive regime of natural ionization currents (in pressurized SF6 under low voltage) and emission currents in vacuum. Pressure values well below the Paschen minimum can be reached with the vacuum system, consisting of a fore-vacuum pump and two turbo-pumps. Several electrodes were built and tested in the realized set-up. The tested electrodes are painted, or bare and have different roughness, to evaluate the sensitivity of currents to different surface finishing. Several findings were possible with the present system. An example is that the humidity effect on ion currents has been carefully quantified. The conclusion is that in order to have a significant change in current it is necessary to increase humidity to levels which are clearly beyond the normal GIS operating level.