Sizing and Control of a Multilevel Converter for Photovoltaic Generation with Independent Power Extraction Capabilities per Module

This work presents the sizing and the control scheme of a modular multilevel converter for grid-connected photovoltaic generation that is capable of extracting different powers from each one of the sub-inverters, and simultaneously manage reactive power injection to the grid. With the division of the entire PV array into several equal independent subarrays and then connecting each to a module of the multilevel converter, the disadvantage of partial shading and its consequent total loss of power are removed. Conversely, the PV subarrays can deliver different powers, due to transient cloud shading, differences in ambient temperature, and even due to unequal degradation of the PV panels in the long term. The approach used to accomplish a different power extraction capability in the converter modules is based on variating the modulation indexes. The principle of operation is exposed, and then a methodology to size the dc-link voltages, number of modules, and components of the converter, based on the total power and voltage of the system, is proposed. Based on this, a control scheme is also proposed accordingly.