Reduction of transformer rated power and line current harmonics in a primary switched converter system for traction applications

The transformer of traction vehicles fed from a 15 kV, 16² /₃ Hz AC supply has, due to the low frequency, a substantial weight. By using a primary switched converter system the transformer frequency can be increased and a reduction of the weight is possible. For a fixed frequency the transformer weight is determined mainly by its rated power. The rated power has to be about twice the transmitted power which limits the weight reduction. If the four-quadrant converter of the primary switched system is used for modulation of the line current, the transformer has to operate at a varying frequency. It is shown that the transformer rated power then can be reduced by approximately 40% compared to constant frequency. The current modulation principle requires a fairly high operating frequency of the four-quadrant converter. Current modulation has with regard to line current harmonics been compared with a principle for harmonic reduction based on a division of the primary converter into several converters connected in series