Effect of an additional large disturbance during power swings on the impedance seen by the out-of-step blocking function

This short article analyzes the effect of an additional large disturbance during power swings on the apparent impedances seen by the out-of-step blocking function of a distance relay. An additional large disturbance during power swings can imply a sudden change of the apparent impedance, which should be considered for the setting of out-of-step functions. The goal of this article is to describe this phenomenon, which has not been previously shown in the literature. A Venezuelan case is taken as an example, and the analyzed additional large disturbance is due to the operation of a special protective scheme for the islanding of one specific area. Maximization of the region between concentric characteristics and minimization of correspondent setting time can be useful actions to improve the expected behavior of out-of-step blocking functions. However, these actions are insufficient for the analyzed example, and two non-traditional solutions were proposed. In general, as these additional large disturbances could fortuitously occur when the apparent impedance is near of the trip region, the logical proper solution should be provided by future improvements of the algorithms from relay manufacturers.