Hybrid active filter for harmonically unbalanced three phase three wire railway traction loads

This paper investigates a hybrid active/passive topology and control strategy appropriate for removing harmonics caused by unbalanced nonlinear loads. Unbalanced loads such as railway traction loads may use static compensators to balance the fundamental frequency components. The harmonic currents produced by this combination are unlikely to be balanced. Purely passive structures are not sensitive to this harmonic imbalance, however some active structures are. Controllers aimed at balanced three phase loads using synchronous transformations will not remove all of the harmonic components. The controller presented in this paper uses harmonic sequence components to detect both balanced and unbalanced components and reduce both harmonic components to zero.