Wye connected 3-phase to 9-phase auto-transformer with reduced winding currents

摘要

In an 18-pulse converter system a 3-phase to 9-phase auto transformer with multiple windings per phase is used to power a 9-phase AC to DC converter. The transformer kVA rating is typically about 55% of the DC load kW. Multiple delta connected coils formed around a basic wye connected structure provide a means to circulate the third harmonics of current necessary for proper operation and reduce the amplitude of load current flowing in individual stub windings. The similar current rating required in several of the multiple coils, coupled with reduced amplitude compared to other topologies, facilitates manufacturing of higher power transformers without the need to parallel conductors. The voltage ratio can be selected so as to give an average dc output from an 18-pulse converter which is generally the same as that of a conventional 3-phase bridge rectifier without transformer, however, the topology described is also suitable for step up or step down of voltage as required. Where necessary to further reduce high frequency harmonic currents, a 3-phase ac line reactor may be connected in series with the source of ac power.