An Improved Coordination Control for Enhancing Photovoltaic Power Imbalance Tolerant Capability of MMC-Based Photovoltaic System
Yao Pan, Xiaofeng Sun, Yao Cai, Xin Li, Wei Zhao
Abstract
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 over-modulation occurring on normal sub-modules (SMs). This paper first merges IRPCS and HCS (IRPCS-HCS) to, in 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 over-modulation on normal SMs via regulating the modulation index upper boundary to realize normal SMs full compensation of injected harmonics of over-modulation SMs. In order to further expand the tolerant capability with the least of amount of internal reactive power, the inner loop makes SM maximum modulation index closely track the upper boundary for preventing over-compensation 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.