Capacitance minimisation & alleviation of per-phase power imbalances in cascaded PV converters

The Cascaded H-bridge topology is ideal for implementing large-scale converters for photovoltaic (PV) applications. The improved quality of output voltage waveforms, high efficiency due to transformer-less connection and ability to employ multiple Maximum Power Point Tracking (MPPT) algorithms are just some advantages. The main disadvantage is the required overrating to facilitate converter operation at times of unequal PV generation. Unequal generation occurs due to shading, temperature inhomogeneity and faulty cells. Capacitor voltage balancing under such conditions typically requires an increase in the number of series connected cells to inject zero sequence voltage. However, leakage current and safety requirements often dictate a need for isolation between PV arrays and the cascaded converter. Therefore, this paper proposes a converter topology that avoids the cost of extra series connected cells by making additional use of the DC-DC converters that provide isolation. H-bridge capacitor sizes are also shown to be reduced. Simulation results are presented that confirm correct operation of the proposed approach.