Improved control algorithm for grid-connected cascaded H-bridge photovoltaic inverters under asymmetric operating conditions

Here, a single-stage cascaded H-bridge (CHB) inverter is presented for grid-connected photovoltaic (PV) systems. The CHB inverter has separate DC links and allows individual control of PV arrays. The conversion efficiency is high and the harmonic generation is lower than conventional PV inverters. Although the CHB inverter is a good candidate for injection of solar power into grid, its control issues have not been completely solved. One of the main challenges in the CHB inverter is the harmonic generation when the connected PV arrays to the H-bridge cells have different amounts of insolation. This study deals with the asymmetrical operating conditions of PV arrays (or H-bridge cells) in the CHB inverter and presents an analytical equation for determination of cells' modulation indices based on PV arrays data. Then, a control loop is added to the tracking algorithm of conventional control systems to determine whether an H-bridge cell is in the linear modulation or not. In the case of overmodulation, the corresponding DC link voltage is increased by the controller to bring it back to the linear region. The validity of new method is confirmed by simulations and experiments on a seven-level 1.7kW CHB inverter.