Analysis of Out-of-Step Blocking Functions of Distance Relays at Venezuelan 765kV System

This article describes interesting findings which were observed during the study for setting the out-of-step blocking functions of distance relays at Venezuelan 765kV system. An analysis of apparent impedances for different contingencies allowed the determination of locations where this function is necessary. For a given contingency, the apparent impedance is initially near of distance zones for some relay locations, and later for other relay locations. For a given location, the apparent impedances during power swings caused by diverse contingencies are different. That is, a unique location for the electrical center of the system does not exist. Consequently, some relays need the out-of-step blocking functions for some contingencies, other relays for other contingencies, and there are locations where this function is not necessary. The trajectories of apparent impedances tend to be almost horizontal in many cases, but there are some cases where the trajectory is almost vertical in the R-X plane.