MMC-Based PV Three-Phase System with Distributed MPPT

Abstract

The use of distributed maximum power point tracking (DMPPT) algorithms is spreading because of their higher efficiency in the case of partial shading. The possibility of integrating photovoltaic (PV) modules in a modular multilevel converter (MMC) connected to the grid has been proposed in recent literature with the advantage of integrating DMPPT algorithms. In the case of partial shading, circulating currents control is necessary to extract the maximum available power and inject symmetric currents in the grid. Novel control strategies for both the ac and dc circulating current components of a three-phase MMC-based PV were proposed and analyzed in this work. Thanks to the presence of a capacitor connected to the dc-side of the MMC, the ac circulating currents could freely be controlled to extract the maximum power from all the PV modules in any irradiance condition while maintaining low power losses. Moreover, in contrast to previous works, instead of measuring the active power of legs and compensating for their imbalance using open-loop control, the power leg mismatches are compensated exploiting the dc loop currents generated through closed loop controls. The effectiveness of the proposed control strategy was proved by simulations performed using MATLAB Simulink®.

A Sizing Procedure for the DC-Side Capacitor of a Three-Phase Modular Multilevel Converter

“…The analyzed converter topologies are reported in Figure 1. They are related to the control strategy reported in [20], in the presence of the DC-side capacitor (Figure 1a), and to the control strategy reported in [16], without the DC-side capacitor (Figure 1b). Both consist of three legs, one per phase, and, in turn, each leg is made up of two arms, i.e., the upper and lower arm, connected through mutually coupled inductors.…”

“…To overcome these issues and reduce converter complexity, in [19], the authors proposed a novel control strategy for a single-phase MMC that laid the foundations for the low-voltage three-phase MMC topology proposed in [20]. In both topologies, the PV arrays were directly connected to the capacitors of MMC submodules.…”

“…In both topologies, the PV arrays were directly connected to the capacitors of MMC submodules. In the three-phase topology [20], an additional larger capacitor was connected to the DC side of the MMC. In this way, it was possible to decouple the three legs of the MMC.…”

“…In fact, in case of unbalanced irradiance conditions between the arms of only one leg, the AC circulating currents can flow through the capacitor of the DC side without interesting the other legs of the MMC; thus, the total system efficiency can increase. On the other hand, in [20], the sizing of that DC-side capacitor and its influence on system performance are missing.…”

“…In the present work, a comparison of the control strategy reported in [20] in the presence of a DC-side capacitor with respect to the one reported in [16] without a DC-side capacitor is performed, focusing on AC circulating voltage management and converter losses. Specifically, the quantification of the real benefits of the use of an additional DC-side capacitor is pointed out.…”