Three-Phase Diode-Clamped Four-Level PWM Inverter With Offset Voltage Injection

In this paper, a three-phase diode-clamped four-level PWM inverter with offset voltage injection is proposed. The proposed inverter is based on multi-neighboring reference vector (discontinuous) pulse-width modulation (MNRV (D)PWM), where multiple adjacent reference vectors with different charging/discharging characteristics of the capacitors are selected depending on the location of the command voltage. These reference vectors are evenly utilized, and the duty ratio of the remaining reference vector as a degree of freedom is adjusted to satisfy the volt·time product. Additionally, the proposed MNRV (D)PWM can generate various (D)PWM forms by applying the offset voltage injection method. By extending the methodology applied to the four-level inverter, a general MNRV (D)PWM method applicable to general high-dimensional multilevel multiphase PWM inverters is also proposed. It is demonstrated that MNRV (D)PWM can be applied not only to dc/ac but also to dc/dc topologies. The ripple component of the dc-link voltage has been confirmed to be related to the three-phase summation of the product of the duty of the E voltage level vector and the phase current. Through various simulations and prototype experiments, the steady-state operating characteristics have been confirmed, demonstrating reduced average switching frequency and superior THD performance compared to conventional methods. Furthermore, proficient management of fluctuations in the dc-link voltage reference and sustained voltage equilibrium are exemplified.