Firing angle modulation for eliminating transformer DC currents in coupled AC-DC systems

A method of eliminating DC components of the currents in the transformer windings of a DC converter is presented. The method uses the technique of firing angle modulation. It is shown that merely eliminating the fundamental frequency component on the DC side may not remove this DC component. The impact of such control action at one converter on the other converters in the DC transmission system is also presented. It is also shown that the undesirable side effects of such a scheme include increased generation of noncharacteristic harmonics on both the AC and DC sides. The study is performed using an electromagnetic transients simulation program and theoretical calculations