A Research On Key Techniques Of Three-Level Three-Phase Four-Leg Inverters

A three-level three-phase four-leg inverter can meet the demands of arbitrary load: unbalanced and/or balanced load; linear and/or linear load.However, the mathematic model is much more complicated than that of either two-level three-phase four-leg inverters or three-level three-phase three-leg inverters. A dimensional reduction strategy is introduced to solve this problem, which simplifies the model into a planar one with a linear superposition, predigesting the problem greatly.What’s more, this kind of inverter shows the drawback of neutral point potential drifting. This paper proposes a novel modulation scheme based on Space Pulse Width Modulation (SVPWM) to balance the neutral point potential under arbitrary load or control method by means of optimizing the selection of synthetic vectors and their duty ratio.The three-level three-phase four-leg inverter takes on bad steady state accuracy and awful anti-interference ability under open-loop control, while bears better performance characteristics under voltage closed-loop. Theoretically, double closed-loop of voltage and current, which adds a feedback of current on the basis of voltage closed-loop, can probably get the optimal control effects:high steady state accuracy, rapid dynamic response and automatic over current protection.It has been verified by both simulation and experiment that the proposed dimensional reduction strategy and neutral point potential balancing strategy work fairly well when combined organically with all kinds of control methods.