Individual capacitor voltage balancing in H-bridge cascaded multilevel STATCOM at zero current operating mode

Individual capacitor voltage balancing is one of the challenges in the field of multilevel converters, especially when the phase legs of the converter are connected in star configuration. This issue can be even more problematic when the converter is operating close to zero average current operating mode. This paper first shows the issue related to the capacitor voltage balancing at zero average-current mode and second proposes a novel algorithm to overcome this problem. The method proposes a modulation technique for individual balancing in H-bridge cascaded multilevel converters operated at zero average-current mode. The proposed algorithm modifies the conventional sorting algorithm based on current ripple. The individual DC-link voltage control method is applied to a 19-level star-connected cascaded converter in PSCAD and simulation results verify the ability of the proposed method in maintaining the balancing of the capacitors voltages at zero current injection mode. Several practical limitation such as forward voltage drop over semiconductor elements, noise in measured data, delay in the digital controller, switching deadtime, and grid voltage harmonics are also considered in the model.