Abstract
Out of step (OOS) is considered an abnormal condition in power system. At this case, the electrical power is not equalized with the mechanical power for the generator. So, OOS leads to variation in three-phase voltage and current for the power system. The distance protection relay is depending on the voltage and current in the protection zone. So, the distance protection calculations for the load impedance may consider that the OOS condition is a three-phase fault in the protection zone for over head transmission line. This paper discusses the requirements from distance protection function in the numerical protection relay at OOS and how to overcome this case to prevent the power system blackout or separating system by the optimum design for out-of-step blocking.