Novel voltage balancing algorithm for single-phase cascaded multilevel inverter for post-module failure operation in solar photovoltaic applications

Four cascaded quasi-impedance source inverter (qZSI) modules are required for achieving nine-level output voltage waveform. In case of one module failure, number of levels in output voltage are reduced to seven. This leads to decrease in the output voltage magnitude and increased THD (higher than conventional seven-level qZSI). This is due to the dominance of unwanted harmonic component introduced in the harmonic spectrum. To restore voltage magnitude and optimize THD performance, novel voltage balancing algorithm is proposed. To validate the control algorithm for off-grid and grid connected system, simulation results of the multilevel qZSI are discussed in two categories: (i) DC voltage source powered multilevel qZSI for RL load subjected to module failure and (ii) solar powered multilevel qZSI connected to utility grid subjected to module failure and variable solar irradiation. Pre-fault and post-fault performance of the system with the proposed control algorithm is discussed for both categories, which validates the effectiveness of the algorithm. Hardware results for proof-of-concept are discussed for DC voltage source fed cascaded qZSI connected to RL load during pre-fault and post-fault conditions. FPGA Virtex-5 is used for hardware implementation of the control algorithm. The results validate the improvement in output voltage both quantitatively and qualitatively.