Multi-level Voltage Source Converters using Coupled Inductors and Parallel Connected Inverter Legs

An Enhanced PWM scheme with a flux balancing technique is described for a 3-phase voltage source converter (VSC) with 4-parallel connected inverter legs per phase. The VSC produces high-quality 5-level phase and 9-level PWM line voltages for applications such as uninterrupted power supplies, aerospace, and electric vehicles. The voltage step of the PWM line voltage is four times smaller than the dc-link voltages, and the PWM frequency is 8 times greater than the switching frequency. These PWM output voltages can easily be filtered with a small ac inductor. The flux compensation is useful to reduce the peak flux, size, and weight of interphase coupled inductors when producing high-quality multi-level PWM outputs. Modular coupled inductors with cross-coupled windings on magnetic C-cores are used to connect the outputs of the parallel-connected inverter legs. This winding arrangement makes it possible to use standard magnetic cores rather than customized multi-limb coupled inductors. The modulation scheme described can easily be implemented in reduced digital hardware like the DSP. The feasibility of the PWM control with flux compensation is verified with simulations and experimental results of a low-power laboratory prototype.