A Half-Bridge Modular Multilevel Converter Topology with DC Fault Tolerance Capability

Modular Multilevel Converters (MMC) are extensively used in high-voltage and high-power applications such as HVDC transmission systems due to their unique advantages, in terms of modularity and power quality in particular. In such systems, DC short-circuit fault is a common and severe fault. A Half-Bridge (HB) based MMC converter behaves similar to an uncontrolled rectifier under a DC fault condition, resulting in high fault currents. This paper proposes a pole-to-pole DC fault-tolerant half-bridge MMC converter suitable for HVDC systems. In the proposed converter, each of the arms feature the conventional HB sub-module in series with the proposed fault-tolerant sub-modules. The approach is capable of the blocking mode of operation to block the AC grid from feeding into the DC fault. Further, the work proposes a limiting mode of operation to maintain the AC grid stability while limiting the fault current during non-permanent faults thus improving the converter reliability. Together with detailed analytical results, the paper also presents simulation and experimental results.