Feasibility Analysis of Protection Diodes for Breakerless Dc Grids

Fault currents in medium-voltage dc grids can reach peak currents of several kiloamperes within milliseconds. In breakerless dc grids, these currents remain uninterrupted, imposing significant stress on converter freewheeling diodes. To prevent semiconductor-module damage during short-circuit faults, a protection diode is introduced that bypasses the fault current once the dc-link voltage collapses. However, the additional protection diode can exhibit problems in terms of current commutation when placed incorrectly. Therefore, this commutation is analyzed in simulation and measurements. The results demonstrate this diodes' effectiveness in both short-circuit faults and fault-current commutation for a $100 ~\text{kW}, 5 \text{kV}$ dc-dc converter system.