A Discontinuous PWM Scheme for Lowering the Switching Frequency and Losses in a 3-phase 6-switch 3/5-Level PWM VSI using a 3-limb Inductor

Standard interleaved 3-level pwm schemes used in multi-level 3-phase inverters such as the NPC are shown not to be suitable for multi-level inverters using a 3-limb coupled inductor. This paper illustrates how a standard discontinuous 3-level pwm scheme can be modified to provide an appropriate excitation for a 3/5 level 6-switch coupled inductor inverter (CII). The pwm reference signals and switching patterns are chosen to excite the inductor phase windings so that only 3 high permeability flux paths in the inductor core are used. As a result, the high frequency current ripple in the inductor windings are reduced together with the high frequency flux produced in the core. The resultant inverter operation can be operated at much lower switching frequencies (2-8 kHz), and levels that are more suitable for standard industrial applications. Experimental results illustrate that the new pwm scheme improves the efficiency of the inverter and lowers the power losses in the coupled inductor. The lower inductor losses also means that the physical size of the coupled inductor can be reduced. Experimental and simulated results are used to demonstrate the operation of the new pwm scheme using a variable speed inverter drive.