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

A partial discharge (PD) measuring system has been deployed in order to identify and measure PD in a high voltage (HV) cable joint under impulse and superimposed voltages under laboratory conditions. The challenge is to enable the detection of PD during the impulse conditions. The method of measurement has been investigated by introducing an artificial defect in the cable joint in a controlled way to create conditions for partial discharges to occur. Next the HV cable system is subjected to AC, impulse and superimposed voltage. Two high frequency current transformers (HFCT) installed at both ends of the cable joint were used to identify PD from the cable joint and to separate PD from disturbance. Transient voltage suppressors and spark gaps are applied to protect the measuring equipment. Band pass filters with selected characteristics are applied to suppress transient disturbances and increase the chance to detect PD during the impulse. PD signals are separated from transient disturbances during data post processing and by means of pulse polarity analysis. The developed system enables the detection of so-called main and reverse discharges respectively occurring during the rise and tail time of the superimposed impulse. The measurement results obtained show the effectiveness of the presented PD measuring system for investigating the effects of voltage transients on a HV cable system in laboratory conditions.