Propagation of unbalance in electric power systems

Power quality has become an important issue for electric power engineering. Nowadays, the distribution electric networks have unbalanced operating regimes, mainly produced by the great number of single-phase loads. Unbalanced line currents produce unbalanced voltage drops on the three phases of the supply system. Consequently, the voltage system within the supply network will become unbalanced. Voltage unbalance influences different components of the electric networks: the effects on the motors are the growths of losses, supplementary heating, and finally, the motors life is shorted. In the distribution and transmission electric networks the main effect of the unbalanced currents is the existence of additional power losses. Taking into account the above mentioned aspects, it is necessary to study the propagation of unbalance through the electric power system, upstream from the LV distribution level. By using the symmetrical components theory, transformers with different types of connections were studied, and their influence on the unbalance propagation was analyzed. A numerical example is presented for the propagation of unbalance from the secondary side of a transformer with Yz_n connection to its primary side.