24-Pulse active front end rectifier with low switching frequency

High-power medium voltage converters face the issues of the network interaction, reliability and efficiency. In addition, 4-quadrant operation is needed for FACTS, distributed generation and regenerative drive systems. This work presents a 24-pulse active-front-end rectifier (AFE) with low switching frequency. The system has four units, each one configured with a three-phase three-level NPC AFE rectifier (3LAFE). The 24-pulse behavior is built by the transformers´ connection with ±7.5°-degree displacement at the primary side. For achieving low switching frequency, a modulation pattern based on a selective harmonic elimination PWM technique (SHEPWM) is employed. With only 3 switching angles, the lowest injected harmonic orders are the 47^th and 49^th. Simulated and experimental results show the good performance at the mains side with unity power factor and low harmonic distortion. Further reduction of remaining 47^th and 49^th harmonic orders is discussed. This solution may be applied in high-power regenerative systems like downhill conveyors, mill drives and windmills, allowing the improvement of the overall system efficiency and the reduction of size and cost of large power harmonic filters.