Analysis of Inductor Current Ripple Minimization in PV-Fed Modular Multilevel DC-DC Converter With Distributed MPPT Control

Modular multilevel DC-DC converters find extensive use in power electronics due to their utilization of a single inductor, the potential for serial connection of <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N</i> number of sub-modules (SMs), and a modular design. Recent research indicates that minimizing ripple in inductor current is feasible by maintaining a fixed interleaving angle of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\frac{100}{N}$</tex-math></inline-formula> % between the SMs during equal duty ratio operating conditions. This study explores the application of these converters in photovoltaic (PV) systems incorporating individual maximum power point tracking (MPPT) control. In this context, individual MPPT control imposes varied duty ratio conditions on the SMs, arising from differences in the sources of each SM, especially in instances of partial shading condition (PSC). This variability can potentially influence the current ripple within the system. This article conducts a thorough theoretical analysis to derive the optimal interleaving angle criteria for minimizing current ripple under conditions of unequal duty ratios. Additionally, a control strategy is proposed to integrate angle optimization with MPPT control based on the derived conditions. Further, it is observed that operating the converter at this optimal angle leads to a significant reduction in ripple current, by a factor of at least <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\frac{1}{N}$</tex-math></inline-formula>. The MATLAB/PLECS simulations and experimental results verify considerable improvements in ripple reduction compared to the conventional interleaving angle method, both during steady-state and PSC.