Decoupling Modulation Strategy for Three-Level Four-Leg Inverters Based on Independent Control of Fourth Leg

Three-level four-Leg inverters can be used to supply high-power balanced, unbalanced and/or nonlinear loads. However, the addition of a fourth leg extends the voltage vectors from two- to three-dimensional space and increases the complexity of the neutral-point (NP) potential control. To solve the above problems, a decoupling modulation strategy, which separates the control of the fourth leg from that of the former three legs, is presented to simplify the selection of the modulation vectors, output symmetrical three-phase voltages, and get balanced NP potential. The switching states of the former three legs are established by applying the three-leg nearest-three-virtual-vector (NTV2) modulation method to the four-leg topology. And the switching state of the fourth leg is selected to meet the requirements of power quality and capacitor voltage. The behaviors of the proposed strategy are verified by the simulation results.