Calculation of voltage unbalance factor in power system supplying traction transformers

The article presents the models of transformers with specially connected windings for supplying catenary systems of high-speed rail. Using the symmetrical components method and equations describing the relationship between voltages and currents on the primary and secondary sides of the transformer one can derive equations describing the value of the voltage unbalance factor Eu depending on the transformer power S_T and the short-circuit power of the supplying system S_s at the place of connection of the transformer to the supplying grid. The applied procedure allows for estimating voltage unbalance for single-phase, open delta Vv, Way-Delta and Scott as well as Le Blanc, Woodbridge and Roof-Delta transformer connections. The article proves that influence caused by catenary systems of HSR depend on transformer connections, the load and the short-circuit power of the supply network The voltage unbalance factor calculated for various transformer connections can be very useful when choosing an appropriate supplying method and will help create an optimal train rímetable to minimise the unbalance in supplying power system.