BigPatents India

A bipolar converter station for conversion between alternating current and high-voltage direct current comprises a first and a second active filter (AF11, AF12), each with its own piece of control equipment (CE11, CE12). The filters are intended for reduction of harmonics in a dc link, connected to the converter station, with a first and a second pole line (PC1, PC2), and each filter generates, in dependence on a sum of harmonic currents in the respective pole line, an influencing quantity (IF11, IF12) and supplies this to the dc link. The control equipment comprises frequency-selecting means (BP, CTR) which influence the frequency content of the respective influencing quantity to lie substantially within one of a first and a second of at least two predetermined, mutually non-overlapping frequency bands (FR1, FR2). During bipolar operation of the converter station, the frequency-selecting means in the control equipment for the first active filter influence the frequency content of the influencing quantity, generated by this filter, to lie within the first frequency band and the frequency-selecting means in the control equipment for the second active filter influence the frequency content of the influencing quantity, generated by this filter, to lie within said second frequency band. (FIG.2 )