Closed-form frequency model of 3-phase inverter drive for DC distribution system analysis

In this paper a model of a three-phase inverter drive will be presented that is suitable for inclusion in a DC distribution system analysis. It will be shown that the drive can be accurately modeled on the electrical side by a capacitor, representing the bus capacitance of the inverter, in parallel with a current source. The current source consists of a DC component, corresponding to net power flow to and from the flywheel, plus high-frequency current harmonics generated by the operation of the switch-mode inverter. Closed-form expressions for the current harmonics can be derived by analyzing the AC currents in the electric machine and the switch-mode nature of the inverter, including the "dead-time" effect, and will be presented in the paper. Comparisons between edge-based and center-based pulse-width operation suggest that center-based PWM produces less harmonic content. It is shown that "dead-time" can have a significant effect on the harmonic content. Experimental results taken on a three-phase inverter with an inductive load validate the presented model.