Transformers for multipulse ac/dc converters

Abstract

In a 12-pulse converter system a 3-phase auto transformer with 4 windings per phase is used to power two 6-pulse converter bridges connected in parallel with a large dc filter capacitor. The transformer rating is typically about 40% of the dc kW load. The voltage ratio is typically 1:1 so that the average dc output of a multi-pulse converter is generally the same as that of a conventional 3-phase bridge rectifier without transformer, however, ac input harmonic currents are greatly reduced. A small single-phase transformer is used to block unwanted circulating currents between the two 6-pulse converters. 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. Where a smaller degree of harmonic reduction is acceptable, only 3 windings per phase are required on the transformer and the small single-phase transformer is eliminated by raising the zero-sequence impedance of the auto transformer by means of an additional magnetic path. This method provides a higher zero-sequence impedance compared to a conventional 3-limb magnetic structure used in most 3-phase transformers. The 1:1 voltage ratio feasible in this invention facilitates retrofit applications, also the concept can be applied to a greater number of parallel converters such as those giving 18-pulse operation.