Multi-arm MMC Analysis for Resolving Power Mismatch Issues

This paper presents a multi-arm topology of modular multilevel converters (MMC) which consists of multiple number of three-phase arms, each consisting of several halfbridge sub-modules (SM), unlike a conventional MMC that has only even number of arms (two). In this topology, there can be odd number of arms in an MMC (with half-bridge cells). The MMC does not have a central dc-link and is ideally suitable for integrating photovoltaic (PV) arrays into the SMs. Due to various factors, the PV output of each SM may vary, hence leading to various types of power mismatches in the system. This paper proposes solutions for power mismatch among the arms, or among the phases, which is essential to send balanced currents to the grid. The mismatch between arms is resolved by independent arm current control technique, wherein the currents shared by the arms are proportionate to the powers generated by them. The mismatch between the phases is resolved by introducing fundamental frequency zero sequence voltages (FFZSV). These methods of resolving power mismatch have the advantage of not involving the circulating currents among the arms.