Design and construction of a polygon-connected autotransformer-based 36-pulse AC-DC converter for power quality improvement in retrofit applications

This paper presents the design and analysis of a polygon connected autotransformer based 36-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 eighteen-pulse AC-DC converters each of them consisting of nine-phase diode bridge rectifier. An autotransformer is designed to supply the rectifier. 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 varying loads. 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 4% for the proposed topology at variable loads. A laboratory prototype of the proposed Polygon-Connected autotransformer-based 36-pulse AC-DC converter is developed and test results are presented to validate the developed design procedure and the simulation models of this AC-DC converter under varying loads.