Evaluation of 3-Phase to 9-Phase Transformer, 18-Pulse Converter, and Adjustable Speed Drive, Including Novel Third Harmonic Calculations

In the past 14 years, several patented 18-pulse converter methods have been accepted in the marketplace and successfully applied in various practical applications. In this paper, a widely used 18-pulse clean power converter, with "windmill" transformer topology is presented and analyzed. It provides a more economical solution to achieve harmonic mitigation in electric power systems as compared to some of the earlier methods. A complete adjustable speed drive (ASD) system model including "windmill" transformer, 9-phase diode bridge, DC link capacitors, inverter with SVPWM (space vector pulse width modulation), and induction machine, has been modeled and built for analysis. Features addressed are, input current harmonic mitigation, DC bus capacitor ripple current and lifespan, and motor ripple torque. In addition, the DC link dynamics under a motor step load change, as well as input voltage unbalance effects on the 18-pulse converter have been evaluated. Quantification of many of these issues is not found in present literature. Experimental tests with a 75 hp "windmill" 18-pulse clean power ASD system, verify the accuracy and effectiveness of the analysis.