The penetration of energy systems with renewable energy, mostly from wind turbines which are often far from the load centres, is increasing. Furthermore, shutting down of nuclear power plants in Germany, increasing the use of photovoltaic systems, and new consumer-structures, such as electric vehicles, require a redesign in the power transmission system. As an option, integration of direct current (DC) transmission systems in the existing alternating current (AC) infrastructure is proposed. The research on dielectrics under AC voltage has taken excellent progress, but analysis on DC condition is not as well developed as that under AC. In the case of the combined AC/DC voltage, the performance of insulation systems is not well known. Therefore, more research concerning the behaviour of insulation materials in combined AC/DC systems is necessary. Development of suitable models for the prediction of component behaviour in AC/DC systems, including the derivation of reliability statements is a fundamental part in this research. Obtaining new insights on the dynamics of partial discharge (PD) and breakdown of insulation systems and especially with the solid dielectrics under AC/DC combined field stress is a methodological purpose. Breakdown and PDs are now sufficiently known in AC and partly known in DC systems, but not at combined AC/DC combined field stress simultaneously. The ignorance of fundamental physical processes on insulation coordination led to oversizing, which were inefficient and often caused scale-related problems in production in the past. In this dissertation, silicone rubber, glass fibre reinforced polymer (GFRP), and epoxy resin insulation materials have been used for experimental investigations. The work covered by these target’s findings is applicable to high voltage components, e.g. cables, joints, and bushings. Regarding the mentioned targets, the existing breakdown experimental methods have been investigated. Afterwards, their advantages and disadvantages in AC and DC breakdown tests have been considered and an experimental procedure and an electrode configuration have been proposed which is useful for AC, DC, and combined AC/DC breakdown tests. PD analysis continued with experiments, and the influencing parameters on PD activities have been found and considered for the next experimental processes. These parameters have been used to develop an analytical model. This analytical model with a corresponding quivalent circuit, which can explain the internal PD in combined AC/DC systems, is a new approach in this dissertation. It explains the PD activity in combined AC/DC systems and gives a deep understanding of PD at DC systems as well. Finally, using this model, the effect of changing in the parameters of insulation material on PD activity have been investigated and analysed.weiterlesen