Optimized arm power reference for an MMC-based PV system operating in grid-forming mode

This paper proposes a control strategy for modular multilevel converters (MMCs) operating in grid-forming (GFOR) mode with photovoltaic (PV) panels connected to submodules (SMs). Here, the converter behaves as a controllable voltage source, establishing voltage and frequency while providing support based on available power reserve. However, potential partial shading in PV panels affects converter capability. By exploiting the converter topology, maximum available power can be extracted by regulating the SM capacitor voltages and injecting circulating currents during non-homogeneous irradiance conditions. Unlike grid-following mode, which maximizes PV power injection, GFOR mode requires maintaining power reserves, offering greater control flexibility. In this regard, circulating currents can distribute power reserve differently among the SMs. Therefore, an optimization-based reference calculation is proposed to minimize circulating currents, maximizing system efficiency and converter support capability. The optimization algorithm yields the optimal MMC arm power references, then used in an energy-based control approach. Since it targets steady-state operation, real-time execution is unnecessary. Time-domain simulations validate the proposed method, demonstrating over a 50% reduction in losses associated with circulating currents compared to an approach based on uniform power reserve distribution or one that minimizes deviations from the average power produced by the arms.