Direct Modular Multilevel Converter With Six Branches for Flexible Distribution Networks

This paper presents a complete analysis of a direct ac-to-ac modular multilevel converter (direct MMC) applied in medium-voltage distribution networks through the soft-open-point concept. The direct MMC is capable of bidirectional power flow between two feeders at any power factor, even when the feeders have different nominal voltages and operate with a phase-shift angle or unbalanced voltages. The converter has six branches, each one composed of full H-bridges cells connected in series to generate a multilevel voltage waveform, to share the blocking voltage of the power switches and to have fault-tolerant operation. This paper presents a suitable control scheme and provides a discussion about the capabilities and limitations of the converter, the capacitor voltage balance control, the efficiency, and the power-loss mitigation at various operation points. Simulation results and power-loss calculations are presented for a three-phase 11-kV 16-MVA direct MMC with 10 H-bridge cells per branch. The direct MMC is simulated in a distribution network to demonstrate the features of the converter and control under various operation conditions, including grid faults.