Near Unity Input Displacement Factor for Current Source PWM Drives

In highpower/medium voltage(2300V-6900V) ac drives a PWM CSI is often used to control the speed of the motor, and the front-end rectifier is a 6 or 12 pulse thyristor rectifier (pictured on this page). Interfacing with ac mains requires low input current harmonics (to comply with recommendations such as IEEE-519), high input power factor, and good voltage/current regulation. Therefore, the thyristor rectifier must be either combined with passive/ active filters or replaced with other suitable topologies. One such topology is the PWM Current Source Rectifier (PWM CSR), Fig. 1(a). Since, in high power applications, the switching frequency is limited, the best PWM strategy is to eliminate specific harmonics while minimizing the switching frequency. Therefore, off-line Selective Harmonic Elimination can be used [1], [2], The regulation of the dc link current is achieved by either phase shifting the gating pattern or varying the modulation index [3]· Phase shifting strategy is simple but has the inherent disadvantage of obtaining a low and varying Input Displacement Factor (IDF). If modulation index control is opted for, the IDF at the rectifier terminal is unity, but as the modulation index decreases (for light loads) and because of the existence of the input filter capacitors, a low and leading power factor is obtained at the ac source. In order to achieve unity, IDF at source, i.e., including the effect of the input capacitors, closed loop control of the input line currents are proposed in the literature [4]-[6]. These schemes control and waveshape the input line currents but require on-line pattern generation and a high switching frequency, which is not feasible with GTO switches used in medium voltage applications. Feedforward techniques for compensation of the IDF are also proposed in the literature [7], [8]. These techniques are also implemented with high switching frequency [7], and some use on-line pattern generators [7], [8]. A feedforward control scheme suitable for low switching frequency circuits, which requires input voltage and current sensors to extract information about displacement angle, is proposed in [9].