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AN ELECTRONICALLY ISOLATED 12-PULSE AUTOTRANSFORMER RECTIFICATION SCHEME TO IMPROVE INPUT POWER FACTOR AND LOWER HARMONIC DISTORTION IN VARIABLE-FREQUENCY DRIVES ABSTRACT: Diode rectifier front-ends with large dc bus capacitors used in variable-frequency drives draw discontinuous current from the power system, resulting in a poor input power factor and high input current distortion. The 12-pulse and 18-pulse techniques used to reduce input current harmonics are popular because of their simplicity but are bulky and expensive. Autotransformer techniques require additional external magnetic components that are bulky and add to the cost of the system. A new electronically isolated 12-pulse autotransformer topology that does not need bulky external magnetic components is proposed in this paper. The proposed topology employs active switches to achieve electronic isolation between the two rectifiers, resulting in continuous input current with reduced harmonics. The proposed non regenerative active scheme achieves less than 5% total harmonic distortion at a much lower cost compared with other passive or active topologies currently in the market. To reduce the influence of wiring inductance between switch–diode pairs and the main dc bus capacitance of the inverter section, a snubber capacitor is employed on the inverter module. A laminated bus structure can be used between the switches (S1∼S4) and the inverter section to reduce inductance. Switches S1-S2 and S3-S4 are operated at a high switching frequency with an equal duty ratio of 0.5 to optimize the rating of the switches. The criterion for establishing an optimal switching frequency will be discussed later in this paper. The equal duty cycle allows balanced current flow from each three phase set of the autotransformer, thereby reducing the input current THD. The voltage across switches S1∼S4 is the instantaneous difference between the voltages across either CDC1 or CDC2 and CDC as the switches that are OFF have the voltage of either CDC1 or CDC2 at one side of the switch and CDC on the other side of the switch. Hence, the voltage rating of the switches need not be that of the full dc bus voltage of the VFD.