Suppression of Circulating Current in parallel operation of three-level converters

The Zero Sequence Circulating Current (ZSCC) flows inevitably in parallel converters sharing common DC and AC sources. The currents flowing commonly in all converters increase losses and decrease the overall capacity of parallel converters. This paper proposes a simple and effective ZSCC suppression method based on the Space Vector PWM (SVPWM) method with the ZSCC controller. The zero sequence voltage for the proposed SVPWM is calculated based not on the phase voltage references, but on the grid voltage. The limit of linear modulation index of the converters with the proposed method is analyzed compared to other methods, and it is proven that the limit of linear modulation index can be maximized with the proposed method. The effectiveness of the proposed method has been verified through the experimental set-up consisted of the four parallel three-level converters. It has been confirmed that the ZSCC is well suppressed and larger linear modulation index is achieved at the same time with the proposed method. Moreover, the proposed control method does not require any communication between converters to suppress the ZSCC unlike the conventional methods.