Influence of Phase-Shifting Transformers (PSTs) on the Distance Protection of Transmission Lines and Improve the Performance of Distance Relay

With the ever-increasing growth of electric loads, the need for generating electric power grows correspondingly. By considering the limitations of power generation, utilizing novel technologies has gained persistent momentum, one of which is deploying Phase-Shifting Transformers (PSTs). Among the more important available relays for the transmission lines are Distance Relays. To this end, Distance Relays measure the voltage and current of the transmission line in its final installation location. On the other hand, the existence of Phase-Shifting Transformers on transmission lines alters the voltage and current signals at the relay location. This issue causes the impedance calculated by the relay to differ from its actual value at the fault location. As a result, the relay detects the fault location falsely, or in some cases does not recognize it at all.

The effect of phase shifting transformer on the relay performance of the distances has been investigated in this study. Furthermore, the digital distance relays are modeled in a software environment and its validity is investigated through analytical relationships. Next, the efficacy of the transformer on distance protection is analytically studied. Finally, a new method has been proposed to improve distance relay performance.

Results from analysis and modelling shows that the effect of phase shifting transformers in relay-computed impedance has two faces, the first of which is related to the internal impedance of the transformer, while the other regards the serial voltage of the transformer. The latter face is much more influential than the former one.

This fact renders the mere inner Impedance of phase-shifting transformer insufficient for using it to eliminate its effect. To this end, a method has been developed in which the voltages of both ends of the phase shifting transformer are measured by the PMUs and then sent to the facility for protecting power system after synchronization. There, this voltage is reduced from the voltage calculated by the relay, which renders the effect of the phase shifting transformer in the impedance calculated by the relay completely eliminated.