Modulated low fault-energy protection scheme for DC smart grids

DC smart grids enabled by the integration of advanced power electronic converters (PEC) can ease the integration and control of distributed renewable energy resources, electric vehicles and energy storage systems. However, these highly flexible power systems introduce many challenges when considering the design of reliable, plug-and-play protection that does not rely on dedicated communications infrastructure for device coordination. One particularly difficult challenge is the management of DC-side filter capacitor discharge during short-circuit faults where the large peak fault-current produced can permanently damage exposed semiconductor components within the converter. One solution is to ensure that the trip-time of DC protection devices is sufficiently rapid (sub-millisecond) to guarantee that fault-current is blocked prior to reaching destructive magnitudes. However, such high-speed protection devices do not offer much margin for effective selectivity with downstream devices due to the narrow time window of operation. Accordingly, this paper proposes a non-unit protection scheme for future large-scale DC smart grid applications that increases this time-window of operation to enable improved selectivity whilst retaining a lower level of energy dissipated in the fault. Reliable protection coordination is demonstrated on a DC radial network and is realized using conventional millisecond trip-time devices, and a single solid-state microsecond trip-time device.