H-bridge modular multi-level converter: control strategy for improved DC fault ride-through capability without converter blocking

This study presents a control scheme that separates control of the fundamental current associated with AC power control from the DC and harmonic components in the arms of the H-bridge modular multi-level converter (MMC). In addition, this control scheme fully exploits the subtractive and additive switch states of the H-bridge MMC to maintain capacitor voltage balance when the converter DC-link voltage collapses during DC faults. The significance of this control scheme is that it permits the H-bridge MMC to ride through DC faults without the need for converter blocking, as is presented in the literature. This study provides comprehensive discussion of MMC fundamental theory, including a logical mathematical derivation of the relationships that governed its operation and modulation. The validity of the presented control scheme is confirmed using simulations and experimentations. Besides DC fault survival, this study presents operating the H-bridge-MMC from bi-polar DC-link voltages; thus, creating the possibility of complex hybrid DC grids with reversible DC-link voltage where the conventional line commutated current-source converters can operate alongside voltage-source converters. Operation of the H-bridge-MMC with reversible DC-link voltage is validated using simulation.