Time dependence of spacer charging in SF6 under DC stress

As gas insulated DC transmission systems unite the advantages of space saving installation and big transmission capacity, this technology will become essential soon. The main problem in operation with respect to insulation seems to be identified today, which is charge accumulation on the surface of the insulators, so called spacers. These trapped charges can cause a reduction of dielectric strength and can lead to a breakdown, especially at polarity reversals and overvoltages. For experiments, a full encapsulated test setup was constructed. Test voltages up to 700 kV DC and a pressure up to 8 bar SF6 are possible. The main component of this setup is a 3 axis fully automated robotic handle system for moving the probe over the test samples inside the encapsulation. So the surface charge can be measured at all positions at the sample surface. The time dependence of the charging mechanism till the saturated state is of big interest. Thus the experiments presented in this paper deal with the influence of the measurement itself on the charging process.