Delta/Fork-Connected Transformer-Based 72-Pulse AC–DC Converter for Power Quality Improvement

Abstract

ABSTRACTThis paper presents a delta/fork-connected transformer-based 72-pulse AC–DC converter feeding direct torque-controlled induction motor drives (DTCIMDs) for improving power quality. The proposed converter consists of two paralleled 18-pulse AC–DC converters along with pulse-doubling circuit at DC bus. The pulse-doubling circuit is used in achieving the pulse multiplication in a 36-pulse AC–DC converter. The Thirty-six pulse topology is obtained via two paralleled 18-pulse AC–DC converters each of them consisting of a 9-phase (9-leg) diode-bridge rectifier. The design of proposed magnetics shows the flexibility in varying the voltage ratio of the delta/fork-connected transformer for making it suitable for retrofit applications where a 6-pulse diode-bridge rectifier is being utilized. Independent operation of paralleled diode-bridge 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 72 pulses. The proposed 72-pulse AC–DC converter with the DTCIMD has been implemented and simulated using Matlab/Simulink software. Simulation results confirmed the significant improvement of the power quality indices on input AC mains and on DC bus (consistent with the IEEE-519 standard requirements) at the point of common coupling. Also, a laboratory prototype is implemented using the proposed design and the experimental results confirm the simulation results at various loads. Results show that input current total harmonic distortion (THD) is less than 3% for the proposed 72-pulse topology at variable loads. Furthermore, near unity power factor is obtained for a wide operating range of the drive.