Abstract
This paper proposes a control strategy for a modular multilevel converter (MMC)-based photovoltaic (PV) system operating under non-homogeneous irradiance and unbalanced grid voltage conditions. The considered topology integrates PV units directly into the MMC submodules, enabling distributed maximum power point tracking (DMPPT). The proposed control framework combines a generalized grid controller with dedicated AC and DC circulating current controllers to manage for vertical and horizontal power imbalances, respectively. The grid controller accommodates different operating objectives, namely balanced grid current injection, constant instantaneous real power operation, and constant instantaneous imaginary power operation, while regulating the injected total three-phase active and reactive powers to their prescribed references. The AC circulating current controller is formulated by extending a single-phase approach independently to each converter leg, thereby achieving complete decoupling among the legs in the management of vertical power mismatch under general grid unbalance conditions. Furthermore, the proposed DC circulating current controller simultaneously handles for power imbalances originating from both partial shading among the converter legs and grid voltage unbalance. The effectiveness of the proposed approach is assessed through detailed simulations under both homogeneous and non-homogeneous irradiance conditions and for different grid control objectives. The results demonstrate effective maximum power extraction, proper regulation of the internal power flows, and stable converter operation under unbalanced grid conditions, yielding an overall system efficiency greater than 97% while fulfilling the desired grid power control objectives.