An Improved Coordination Control for Enhancing Photovoltaic Power Imbalance Tolerant Capability of MMC-Based Photovoltaic System

The internal reactive power compensation strategy (IRPCS) and harmonic compensation strategy (HCS) can be used in modular multilevel converter–based photovoltaic systems (MMC-PVSs) for PV power imbalance, but have requirements of extending tolerant capability and degrading internal reactive power at severe imbalance, and have the problem of overmodulation occurring on normal submodules (SMs). This article first merges IRPCS and HCS (IRPCS-HCS) to, to some extent, extend the tolerant capability and reduce the internal reactive power. Subsequently, a dual loop control and a reactive power distribution method are proposed for further optimization. The outer loop eliminates overmodulation on normal SMs via regulating the modulation index upper boundary to realize normal SM full compensation of injected harmonics of overmodulation SMs. To further expand the tolerant capability with the least amount of internal reactive power, the inner loop makes the SM maximum modulation index closely track the upper boundary to prevent overcompensation of internal reactive power, and the reactive power distribution method optimally allocates the reactive power to SMs. Finally, simulation and experimental results are illustrated to verify the feasibility of the proposed control.