Experiment study of surface charge accumulation and decay on a cone-type insulator in HVDC GIL

Study of insulator surface charge accumulation and its suppression is one of the key techniques in developing HVDC gas-insulated transmission line (GIL). Consequently, we established a surface charge measurement system for the cone-type insulator in a GIL unit. Based on the electrostatic probe method, the surface potentials on a cone-type insulator made of epoxy resin under DC voltage were measured. The surface charge accumulation phenomenon under different voltage polarities, voltage amplitudes, as well as polarity reversal process were studied. And the surface charge decay process was also measured. The experimental results show that surface charge distribution of insulators is closely related to the polarity of the applied voltage. In 0.5 MPa air, with the increase of the voltage amplitude (+40~+70 kV), the surface charge increases significantly (the maximum negative potential increases from -200 V to -3000 V). Moreover, in 0.5 MPa air, under polarity reversal condition (first +70 kV and then -40 kV), surface charge of some certain area can increases sharply (the maximum positive potential increases from 500 V to over 2500 V). In 0.1 MPa air, after +40 kV voltage being applied, the decay process of the surface charge within 300 minutes is similar to exponential process and the time constant of the decay rate is about 10⁴ s order of magnitude.