Application of Pulse Multiplication in T-Connected Autotransformer Based 28-Pulse AC-DC Converter for Power Quality Improvement

This paper presents a pulse doubling technique in a T-Connected autotransformer based 28-pulse ac-dc converter which supplies direct torque controlled motor drives (DTCIMDs) in order to have better power quality conditions at the point of common coupling. The proposed technique increases the number of rectification pulses without significant changes in the installations and yields in harmonic reduction in both ac and dc sides. The 28-pulse rectified output voltage is accomplished via two paralleled 14-pulse acdc converters each of them consisting of seven-phase diode bridge rectifier. The Tconnected autotransformer makes use of only two single-phase transformers, resulting in savings in space, volume, weight, and, finally, the cost of the drive. The design procedure of magnetics is in a way such that makes it suitable for retrofit applications where a sixpulse diode bridge rectifier is being utilized. Independent operation of paralleled diodebridge rectifiers, i.e. dc-ripple re-injection methodology, requires a Zero Sequence Blocking Transformer (ZSBT). Finally, a tapped interphase reactor is connected at the output of ZSBT to double the pulse numbers of output voltage up to 56 pulses. The aforementioned 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, 28-pulse, and proposed converters from view point of power quality indices. Results show that input current total harmonic distortion (THD) is less than 3% for the proposed topology at variable loads.

AC–DC Converter, T-Connected Autotransformer, Power Quality, 56 pulse, Pulse Doubling, Direct Torque Controlled Motor Drives (DTCIMDs)