H∞ control of the neutral point in 3-phase 4-wire DC-AC converters

This paper proposes a new circuit topology and the H^∞ control design for the neutral point in a 3-phase 4-wire DC-AC power converter, which can be connected to 3-phase 4-wire loads or the power grid. It offers a significant advantage: the neutral leg and the three-phase converter can be controlled independently. The disadvantages of the conventional neutral leg and the split DC link (e.g. the complicated control design, the very large capacitance and the drifting neutral point) have been overcome. The control system (including the PWM waveform generating scheme) is considerably simplified. The neutral point can be steadily regulated by a very simple controller (e.g. a proportional controller) even when the three-phase converter system (mostly, the load) is extremely unbalanced. A voltage-current controller is designed using H^∞ techniques. This topology together with the H^∞ control also offers a solution to the DC link balancing problem inherently existing in three-level converters. The design is verified by simulation results.