DC Breakdown Behaviour of Liquid-Solid Interfaces Formed in Silicone Based Materials

Constantly growing amount of renewables and storage installed in the power system results in an increased interest in the power transfer under Direct Current (DC), especially in the low and medium voltage (LV and MV) networks. This is valid for both already existing as well as for newly installed cable systems. Although there is virtually no experience with MV DC networks and accessories, it is widely known that the electric stress distribution within insulation is different for AC and DC voltage. Liquid filled joints utilize an insulating liquid to fill the inner volume of the joint. A moisture sensitive, silicone based liquid can be taken as one of the examples. Beside all dielectric and thermal properties, such liquid has a property of hardening when getting in contact with moisture. By measurements of such material, it has been confirmed that the dielectric permittivity for solid and liquid state is of the same value. Thus the hardening process does not have influence on the field distribution under AC stress. However, the resistivity of the material changes when the hardening starts. This in turn, has an influence on the field distribution under DC. In order to investigate the criticality of liquid-solid interfaces, the DC breakdown testing was performed. More specifically, the testing focused on the interface being normal and tangential with respect the electric field. The literature states that the interface of two different insulating materials is an electrically weak spot. In our experiments, the contrary has been observed. The interface between liquid-solid silicone materials is at least as strong as the liquid form of the dielectric. In the current contribution, we will also discuss the implication of the mentioned findings on the feasibility of utilizing a silicone liquid filled AC MV joint under DC stress