Fault analysis of HVDC systems with partial underground cabling

Due to recent political developments in Germany new High Voltage Direct Current (HVDC) transmission right-of-ways will have to be realised preferably as cable systems, especially near densely populated areas. Hence, overhead line (OHL) systems in combination with underground cable sections near urban areas - so-called partial underground cabling systems - become a viable option. This however leads to new challenges regarding system protection, since different converter control actions are required for OHL faults compared to cable faults. Within this paper the behaviour of HVDC systems with partial underground cabling and fullbridge Modular Multi-Level Converters (MMC) during line faults is investigated in a PSCAD|EMTDCTM model to identify possibilities to differentiate OHL and cable faults and subsequently adjust the converter operation. As the results indicate, differentiating the fault type based on local current and voltage measurements does not seem feasible, except for simple cable-OHL topologies and, if enhanced waveform analysis algorithms are applied. However, if instead cable current differential protection is used for the system's cable sections, faults can be characterised correctly within short time and the converter stations are able to alter their operating point accordingly.