Balancing voltage algorithm for a medium voltage Cascaded H-Bridge STATCOM in zero-current mode

Capacitor voltage balancing in a Cascaded Η-Bridge (CHB) grid connected converter becomes critical when the converter is required to work in zero or quasi-zero current mode. When a close to zero current reference is selected, a small current ripple still flows inside the converter leading to a dangerous divergence of the capacitor voltages. In order to overcome this issue, this paper presents a new algorithm able to balance the capacitors voltage in Medium Voltage CHB STACOMs, modifying the modulating signal according to the voltage divergence among each capacitor and their average voltage on each phase. Simulation results show the effectiveness of this method, allowing a balanced dynamics within the capacitors with quasi-zero reference current values.