Multiphysikalische Simulationen und experimentelle Untersuchungen an extrudierten DC-Kabeln und Kabelgarnituren

Due to the rising share of renewable energy, which sources mostly on the countryside, it is necessary to manage the distribution and the transportation towards centres of consumption. Because of acceptance problems and public resistance, the possibility of building new overhead lines as well as cables systems is limited. Therefore, an optimal usage of existing power equipment and infrastructure is important. A possible approach to further challenges is to operate existing three-phase transmission systems with direct-current (DC) or to reinforce the existing DC grid with medium voltage direct-current transmission systems. This master thesis focuses on multiphysical simulations of DC cables and cable accessories and experimental studies about the thermal profile of cable joints and cable terminations as well as DC breakdown tests for medium voltage cables. In course of the work a literature research about the specific resistance regarding to the common insulations XLPE, SiR and EPDM/EPR was done. This research is necessary for current flow simulations because of occurring effects like the field inversion at DC stress. The field inversion depends on the specific resistance from the different insulations, more precisely on the temperature and electrical field strength coefficient.

Through various simulations it was recognised that the conductor current under DC stress can be raised between 17–36 % in comparison to the AC current with regards of reaching the same inner conductor temperature. This is possible, beside the missing of additional AC specific losses and limitations. Due to the fact, that at AC three heat sources are present, compared to two heat sources at DC. During the thesis multiphysical simulations of a cable joint and termination were done. It is very challenging to implement factors such as air, convection or the contact resistance into the model. Therefore, experiments were made to measure the temperature profile of investigated cable accessories. The simulations about the cable termination and the joint show that the insulation body of these components could operate as a resistive field control. To reach this state the insulation body must have a lower specific resistance than the insulation of the cable itself.

The last point of this thesis deals with DC breakdown tests. Down to the present day there is no consistent test method regarding the DC breakdown tests on cables. The simulation and the research about this test demonstrate that the range of the step duration must be higher than at AC testing, to reach a resistive field distribution in the equipment under test. Further, the aim was to develop a “test cable termination” for upcoming tests. This is necessary to reach the high voltages during the breakdown test and to receive results for the electric breakdown strength under DC.