Effect of Phase-Shift Angle on a Delta-Connected Autotransformer Applied to a 12-Pulse Rectifier

The delta-connected autotransformer with π/6 phase-shift angle is widely used in the multipulse rectifier when the output voltage of the autotransformer is approximately equal to its input voltage. If its output voltage is slightly greater than its input voltage, the equivalent kilovoltampere (kVA) rating of the autotransformer increases rapidly, and each core limb of the autotransformer has five windings. The increase in winding number will increase the asymmetry and manufacture difficulty of the autotransformer. Therefore, the autotransformer with π/6 phase-shift angle is not suitable in the boost voltage application. This paper analyzes the effect of phase-shift angle of the autotransformer with three windings in each core limb on the input line current, output voltage, and equivalent kVA rating and proposes a new phase-shift angle. The equivalent kVA rating of the delta-connected autotransformer with the new phase-shift angle is slightly greater than that of the autotransformer with π/6 phase-shift angle under the same turn ratio, but the autotransformer with the new phase-shift angle has three windings in each core limb and is propitious to design and manufacture. Some simulation and experiment are carried out to verify the theoretical analysis.