A Carrier-Based Modulation Method Providing Coordinated Control of Circulating Current Suppression and Neutral-Point Potential Balance in Parallel Three-Level Converters

Parallel three-level (3L) converters face two typical defects, neutral-point (NP) potential unbalance and zero-sequence circulating current (ZSCC). However, both of them are controlled by redistributing the small vector pairs in SVPWM, which means that the two different control targets share one degree of freedom. Either the existing coordinated control methods do not inherently increase the control degrees of freedom, or they are unsuitable for multi-parallel application. In this paper, firstly, the potential conflict between ZSCC and NP potential control in conventional SVPWM is discussed from the viewpoint of common-mode (CM) injection and NP current. Then, in order to control ZSCC and NP potential separately and independently, an additional NP balance control degree of freedom is introduced by constructing virtual zero levels to control the NP current, while ZSCC is controlled by a variable CM injection. The simulation results indicate the better performance of the proposed method than existing SVPWM-based coordinated methods.