Overmodulation strategy for a three-phase four-leg voltage source converter

A three-phase four-leg voltage source converter (VSC) can produce three line to neutral voltages independently with one additional leg compared with three-phase three-leg VSC system. That is, the system has three-degrees of freedom in producing a d-q-o output voltage arbitrarily within a polyhedron of twelve sides, whose vertices are composed of 14 nonzero voltage switching vectors in the d-q-o reference frame. In this paper a novel overmodulation strategy for three-phase four-leg VSC is proposed. It focuses on the full utilization of the entire capability of the VSC system and simultaneously on the maximization of a d-q component voltage rather than zero sequence voltage. The proposed overmodulation strategy can be implemented by the triangular carrier-based PWM method using an offset voltage concept like a three-phase three-leg VSC system. So it is implemented by limiting four pole voltages with four saturation functions on software without any hardware burden. Finally, to show the feasibility of the proposed overmodulation strategy, simulations and experiments have been performed. The experimental results reveal that the utilization of the entire capability of the four-leg VSC is achieved by the proposed overmodulation strategy.