An Optimized Reactive Power Compensation Strategy to Extend the Working Range of CHB Multilevel Grid-Tied Inverters

In the cascaded H-bridge photovoltaic (PV) grid-connected inverter system, the power of PV panels may be unbalanced due to partial shading, aging, dust accumulation, temperature difference, and other reasons. The existing control methods have problems, such as weakened PV energy harvesting ability and the requirement of expanding the working range when the imbalance is severe. This article proposes an optimized reactive power compensation strategy (ORPCS) that first extends the operating range of the system to the maximum under unity power factor. Then, when the system exceeds the upper limit of unity power factor operation under severe imbalance, not only can the maximum PV output energy be harvested with the least amount of reactive power but also the PV voltage fluctuation is small with high-quality grid-connected current when ORPCS is adopted. Last but not least, under balance or mild imbalance yet maintain the advantages of the conventional carrier phase-shift SPWM methods. Simulation and experimental results demonstrate the effectiveness of the proposed method.