The application of a variable inductor to the design of a Direct Current (DC) circuit breaker (CB) designed to break the fault current during short circuit or over-current event in medium-voltage and high voltage DC grids. This includes an ultra-fast CB consisting of a fast-mechanical switch branch and a solid-state branch. The solid-state branch consists of a fault current commutation converter and a solid-state circuit breaker in serial connection and adds a variable inductor into the switch branch for a "safe-to-open" signal.
