Leg capacitor energy balancing control is one of the crucial issues for stable operation of a cascaded H-bridge (CHB) converter. Because this topology inherently consists of numerous submodule cells with DC capacitors, the cell voltages and leg capacitor energy instantaneously fluctuate depending on operation sequence of the CHB converter. In general, a wye-connected CHB-converter-based static synchronous compensator (STATCOM) utilizes a zero-sequence voltage component for leg capacitor energy balancing. In this paper, to improve the dynamics of leg energy balancing control, a feedforward calculation method of the zero-sequence voltage injection is proposed. The feedforward term can be instantaneously calculated by using the information from the measured leg voltages and leg currents, and the method ensures successful regulation of the leg energy balance even under unbalanced grid and load conditions. Moreover, the verification of the proposed method is supported by the mathematical vector theorems. A 50MVA full-scale wye-connected CHB–STATCOM system simulation was performed to verify the proposed feedforward calculation method considering unbalanced grid as well as unbalanced load conditions.

In neutral point clamped (NPC) converters, the voltage ripples in each dc-link capacitor are mainly influenced by the third harmonic component in the neutral point of the three-level NPC converter. These low-order harmonic voltage ripples distort the output of the converter and compromise system stability. Moreover, as the converter's output increases, the capacitor's ripple component escalates correspondingly, necessitating capacitors with a higher capacitance to maintain the reliability of the NPC converter. In this article, a novel technique is introduced that is designed to mitigate voltage ripples in the dc-link capacitor of three-level NPC converters. By employing a zigzag transformer, the method effectively compensates for the third harmonic component of the voltage in the dc link, resulting in a significant reduction of ripple. The validity and effectiveness of the proposed method are corroborated through comprehensive simulations and experiments.