Application of swarm algorithm to solving voltage unbalance problem in DC tram traction supply system

Tram traction network in Poland is supplied with DC voltage 600 V. This voltage is obtained by transformation of 6, 15 or 20 kV, 50 Hz, three-phase power network voltage. 12-pulse transforming circuit is commonly used, it is composed of 6-phase transformer and uncontrolled rectifiers. Voltage unbalance at the secondary side of the transformer, which may be due to the winding arrangement itself, leads to emergence of additional harmonics in the rectified voltage. This effect is strengthened, if transformer is supplied with distorted three-phase voltage. In order to minimize this effect, we propose to apply on-load tap changer. Setting of the tap position for different supply conditions is calculated using one of swarm optimization algorithms, namely Ant Colony Optimization. Example of THD minimization in the rectified voltage for selected supply conditions is presented in the paper.