A Quasi-Virtual DPWM Implemented With Split Modulation Waves for DC-Link Voltage Balancing in Four-Level π-Type Inverters

The balance of dc-link voltages is quite a challenge in the four level (4L) π-type inverter, which is the main factor limiting its wide application. In this article, an investigation is conducted to simultaneously balance dc-link voltages and reduce switching losses in 4L π-type inverters. First, the mechanism of the dc-link voltage balancing method based virtual vectors is analyzed, which however will inevitably increase switching losses. To reduce the switching losses, a discontinuous pulse width modulation (DPWM) implemented based on virtual vectors is presented. Then, the vector sequences are rearranged proposing a quasi-virtual DPWM for balancing dc-link voltages of the 4L π-type inverter. And a simplified carrier-based implementation method is introduced with split modulation waves. Auxiliary closed control loops are designed for controlling and balancing dc-link voltages. In addition, the advantage of low common-mode voltage in the quasi-virtual DPWM is analyzed and presented. A three-phase 4L π-type inverter experimental setup is built based on a DSP-FPGA control board. The proposed quasi-virtual DPWM is finally verified by comparative experiments, showing its controllability of dc-link voltages, high efficiency, high linear modulation index, and low common-mode voltage.