Hexagon-Connected Transformer-Based 20-Pulse AC-DC Converter for Power Quality Improvement

This paper presents the design and analysis of a hexagon connected transformer based 20-phase ac-dc converter which supplies direct torque controlled induction motor drives (DTCIMD's) in order to have better power quality conditions at the point of common coupling. The proposed converter output voltage is accomplished via two paralleled 10-pulse ac-dc converters each of them consisting of 5-phase diode bridge rectifier. A transformer is designed to supply the rectifiers. The design procedure of magnetics is in a way such that makes it suitable for retrofit applications where a six-pulse diode bridge rectifier is being utilized. The proposed structure improves power quality criteria at ac mains and makes them consistent with the IEEE-519 standard requirements for SCR > 20. Furthermore, near unity power factor is obtained for a wide range of DTCIMD operation. A comparison is made between 6-pulse and proposed converters from view point of power quality indices. Results show that input current total harmonic distortion (THD) is less than 8% for the proposed topology at variable loads.