Grid topology and technology influences on selective protection concepts for multi-terminal medium voltage DC grids

The integration of distributed renewable energy resources in combination with advancements in power electronics make the application of multi-terminal medium voltage DC grids a promising and flexible solution. Fast and selective protection concepts in combination with DC (disconnecting) switches for fault handling are required to ensure high reliability and safe grid operation of these systems. Medium voltage DC grids may be realised based on several converter technologies with different fault clearing capability. Depending on the converter technology, switches for isolating faulted branches or DC circuit breakers (CBs) for fault clearance become necessary. DC medium voltage CB concepts have already been proposed. However, the switching times of these CBs vary significantly depending on the presence or absence of mechanical components resulting in different requirements for fault detection. This contribution evaluates the relations between different converter and DC CB technologies, DC grid topologies and fault detection methods according to selective fault clearing. Transient simulation studies are carried out for the development of protection concepts. Based on the results, reasonable technological combinations of the above-mentioned technologies are identified. Special consideration is given to the dimensioning of series reactors as fault current rise limiting devices.