Modular multilevel converter for large-scale photovoltaic generation with reactive power flow and unbalanced active power extraction capabilities

Abstract A modular multilevel converter (MMC) topology for high-scale three-phase photovoltaic power generation and its control scheme are proposed in this work. First, a complete model of the system is derived for a generalized n-modules converter. Second, a control strategy is proposed in order to achieve independent maximum power extraction from all the modules, to regulate all dc-links, and to control the injected currents to the grid to be sinusoidal and in-phase with grid voltages if transferring only active power, or to be sinusoidal and phase-shifted in order to transfer active power and simultaneously manage reactive power. These active and reactive power transfers are achieved in a balanced way at the medium voltage level. The proposed control strategy also allows the MMC operating only with a large amount of reactive power, without having any active power production on the PV arrays, which is a valuable feature for using the MMC in smartgrids. The proposed converter is capable of extracting the maximum power of each module at its PV array, even if the powers produced by them are unbalanced due to irregular solar irradiances. Simulation tests with extreme changes are developed to validate the operation, control and feasibility of the converter approach.