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

Increasing use of Direct Current (DC) in medium and high voltage electrical networks is demanding new research in the existing as well as new insulating materials. The electric field under DC is complex because it is dependent on the conductivity and temperature, which leads to formation of space and surface charges in the insulation material. The formation of space and surface charges in the insulation material can lead to premature breakdown of the insulation material. Several studies are being carried out around the world to use the existing alternating current (AC) infrastructure under DC conditions. To study the feasibility of using any equipment under DC conditions, it is therefore important to investigate the conductivity and space charge behavior of the insulation material and their dependence on the temperature and the electric field, which are not predictable yet. In this study, an attempt has been made to study the conductivity and its dependence on temperature and the electric field of liquid silicon insulation material. This liquid insulation material has a unique property of turning into solid state once exposed to moisture. In this paper, conduction Current measurements has been performed in liquid as well as solid state to investigate their conductivity values and study its dependence on the temperature and the electric field. From the results obtained, an indication of the electric field threshold value, above which space charge starts accumulating in the solid silicon insulation material is found out. These parameters then were used for performing simulations using finite element analysis software to study the behavior of insulation material under DC conditions. Due to the complex conduction phenomenon observed and the complexity in the measurement set up, it was not possible to study the electric field threshold values in liquid silicon insulation. A comparison between XLPE and solid silicon insulation under DC conditions is then studied. The electric field inversion is observed in both the insulation materials. In addition to this, it is observed that, because of the high electric field dependency on the conductivity a solid silicon insulation equalizes the inverted electric field distribution and thus reducing the magnitude of maximum electric field.