A Full-Range and High-Dynamic Control Method of Neutral Point Potential for Parallel Three Level Inverters Considering Zero-Sequence Circulating Current

Parallel three-level inverters (PTLIs) are widely used in high-power applications to improve the system capacity and reliability. However, the three-level topology in PTLIs inevitably introduces the imbalance of neutral-point potential (NPP), and the NPP control (NPPC) of PTLIs must consider zero-sequence circulating current (ZSCC). The existence of ZSCC may cause the failure of NPPC methods for a single TLI to be directly applied to PTLIs. To solve this issue, a full-range and high-dynamic control method of NPP for PTLIs considering ZSCC is proposed in this article. First, the independent zero-sequence voltage injection (ZSVI) method is extended to PTLIs with the consideration of ZSCC. Meanwhile, treating PTLIs as one six-phase TLI for NPPC is proposed in this article, and the uniform ZSVI method for PTLIs is analyzed in detail. Based on the uniform ZSVI method, the modulation wave decomposition (MWD) method is implemented to balance NPP in the full modulation index range. Furthermore, to improve the dynamic performance of NPPC, an online neutral compensation current estimation (ONCCE) method combining a proportional regulator and a nonlinear disturbance observer is proposed. Combining the uniform ZSVI, MWD, and ONCCE methods organically, the full-range and high-dynamic NPPC is achieved, and the balance effect, control range, and dynamic performance are obviously improved. The experimental results obtained from an industry prototype of PTLIs verify the effectiveness of the proposed method.