Multi-Level Voltage Source Parallel Inverters using Coupled Inductors

Inverter legs can be connected in parallel using coupled inductors to produce multi-level PWM output voltages with PWM frequencies higher than the inverter switching frequency. Techniques are described for connecting the coupled inductor windings to parallel inverter legs that lower the effective series inductance at the system output terminals. High-quality multi-level PWM output voltages result with low fundamental voltage drops across the output impedance. The proposed PWM technique can be implemented using either multi-limb or a modular system where individual magnetic cores are connected to each inverter leg. The low output impedance means that high-frequency fundamental voltages can be generated using a relatively low carrier to fundamental ratio. Carrier/reference signal manipulation techniques are presented that improve the quality of the multi-level output voltages and that can be easily implemented in digital hardware. The inverter structure is presented for a 3-phase ac output that can be used as a motor drive system or a utility connected PWM rectifier. For a 3-phase system, 7 and 9 level line voltages can be obtained using 3 of 4 inverter legs per phase respectively. The operation of the system is demonstrated using simulations and experimental results.