Harmonic suppression technology of three-phase diode rectifier based on third harmonic current injection

The approaches to reduce input current´s harmonics of three-phase diode rectifier based on injection of third harmonic current is studied in this paper. Two ways, the passive and the active, of implementation of injection are introduced. The mechanism of THD reduction and the structure of star-connected capacitors are introduced. Because it tunes at frequency of third harmonics with serial resonance, the passive circuit enhances the efficiency of circulating current with limited voltage source. Optimal injection conditions and characteristics of the circuit are also given in details. While the active method, which adds an ac voltage regulation circuit to the passive and based on Pulse-Width Modulation (PWM) technology is investigated too. It acts as a controllable resistor so that the third injection current can be controlled according to the optimal condition. Results of simulation and experiments show: the passive approach has acceptable harmonic suppression performance in certain cases, while the active one is more suitable for the application of wide-range variable loads or higher performance required.