Polyphase transformer arrangements with reduced kVA capacities for harmonic current reduction in rectifier type utility interface

In this paper, polyphase transformer arrangements with reduced kVA capacities are presented for harmonic current reduction in high power rectifier utility interface systems. Based on the concept of an autotransformer, a proposed twelve-pulse rectifier system is realized with a resultant kVA rating of 0.18P₀ (pu). In this arrangement, the rectifier diodes each conduct for 120 degrees per cycle and the fifth and seventh harmonics are absent from the utility input line current. In the second scheme, an eighteen-pulse rectifier is realized with a kVA rating of 0.16P₀ (pu), rectifier diode conduction of 120 degrees per cycle and fifth, seventh, eleventh and thirteenth harmonic cancellation in the utility line currents. Additional applications for the proposed polyphase transformer arrangements include twelve and eighteen-pulse systems feeding multiple six-pulse nonlinear loads, such as AC motor drives, with an appropriate phase shift, and this arrangement substantially reduces the utility line current harmonics. For example, it is shown that the fifth and seventh harmonics generated by two separate six-pulse nonlinear loads are subtracted and this contributes to reduced harmonic distortion. Simulation results verifying the proposed concept are presented.