Fault-Tolerant Operation of Hybrid Modular Multilevel Converter for MVDC Applications

The utilization of unipolar half-bridge submodules (HBSMs), remains the most cost-effective option to date, but they fail to block DC-side faults. Therefore, incorporating a combination of unipolar and bipolar submodules (full-bridge) can be advantageous, as this configuration not only effectively manages DC-side faults but also enhances operational performance. In this context, a Hybrid Modular Multilevel Converter (HMMC) is considered for medium-voltage direct current (MVDC) applications. Due to more switch count in MMC, it is more prone to faults. This paper proposes a fault-tolerant algorithm specifically designed for addressing open-circuit switch faults in a HMMC. The algorithm takes into account various open switch fault scenarios, including single switch fault, multi switch fault, single SM fault, and multi SM faults. In conventional MMCs, such faults increase the stress on SM capacitors. However, the proposed fault-tolerant algorithm allows the converter to operate under fault conditions without escalating the stress on the SM capacitors. This proposed algorithm ensures that the stress on SM capacitors remains consistent during both normal operation and fault conditions. The effectiveness of the proposed technique is validated through MATLAB simulation results.