Stable operation of grid connected Cascaded H-Bridge inverter under unbalanced insolation conditions

This paper presents a single-phase Cascaded H-Bridge (CHB) inverter for photovoltaic applications. The proposed control strategy permits the optimum control of each string of PV panels in different insolation conditions. Based on the presented mathematical analysis, an analytic constraint is derived to check the stable operation of the inverter. A modified MPPT control strategy is also proposed based on this new condition. This strategy enhances the stability margins of the system by changing the operating point of some PV arrays. In this approach, the extracted power from high power cell is reduced to prevent instability issue and to avoid injection of highly distorted current into the grid, keeping the total harmonic distortion (THD) below 5%. The performance and effectiveness of the proposed method is verified by simulations in PSCAD/EMTDC environment, under severe imbalance insolation conditions.