A near unity input displacement factor PWM rectifier for medium voltage CSI based AC drives

The current source rectifier (CSR) operated with selective harmonic elimination (SHE) method with low switching frequencies (300-420 Hz) can meet stringent input harmonic requirements and is therefore, a suitable front end for medium voltage CSI based AC drives. Regulation of the DC link current can be achieved by either phase shifting the gating pattern or varying the modulation index. Both of these methods however, result in unacceptably low values of input displacement factor (IDF) for light loads. A control strategy based on combining modulation index and phase shift control is proposed in this paper to achieve near unity IDF for a wide range of operating points. The paper includes a description of the proposed control strategy, investigation of IDF compensation limitations, and simulation and experimental results for various modes of operation. The paper also investigates the IDF values for the above three control strategies and for three different types of industrial loads. Results show that depending on the speed/torque characteristics of the load, a power factor correction system may not always be necessary for CSI based drives with PWM rectifier front end.