DC fault protection for modular multi-level converter-based HVDC multi-terminal systems with solid state circuit breakers

The high penetration of renewable energy requires flexible transmission of electrical energy over long distances. Onshore high-voltage DC (HVDC) interconnectors based on overhead transmission lines have already reached an advanced planning stage. These interconnectors could be extended to multi-terminal grids in the second step for higher transmission redundancy and flexibility. Fast and selective protection concepts for fault handling are required to ensure high reliability and continuous operation of these systems. Modular multi-level converters with submodules in half-bridge topology and solid-state HVDC-circuit breakers (SSCB) provide fault clearing within several microseconds to prevent converter blocking. Within this publication, a selective protection concept for SSCB and multi-terminal HVDC systems based on the overhead transmission is developed and analysed. It is based on a combination of local voltage and current signals and does not require communication between grid nodes. Additional series reactors for limiting rising fault currents are not required. Subsequent simulations for validating the protection concept and identifying its limits are carried out in power systems computer-aided design/electro-magnetic transient design and control for an exemplary HVDC system. The combination of overcurrent protection with additional excitation signals enables selective fault clearing for all types of line fault scenarios.