Advanced distance protection applications fot dynamic loading and out-of step condition

Distance relays have been successfully used for many years as the most common type of protection of transmission lines. The development of electromechanical and solid state relays with mho characteristics can be considered as an important factor in the wide spread acceptance of this type of protection at different voltage levels all over the world. Zone 1 of distance relays is used to provide primary highspeed protection of a significant portion of the transmission line. Zone 2 is used to cover the rest of the protected line and provide some backup for the remote end bus. Zone 3 is the backup protection for all the lines connected to the remote end bus. The experience from the North American blackout in August 2003, as well as the recent European disturbance in November 2006 demonstrated the importance that the behavior of many distance relays had during the dynamic loading at different stages of the disturbance and the out-of-step conditions that followed in some areas The implementation of distance relays requires understanding of its operating principles, as well as the factors that affect the performance of the device under different abnormal conditions. The paper analyzes different methods for resolving the load encroachment of the apparent impedance into the distance characteristic as a result of dynamic loading and the detection of faults at that time. Another important issue is the operation of distance relays during stable or unstable out-of-step conditions. Depending on the location of the distance relays in the system and the type of event the relays may use blocking of the selected distance elements or tripping if separation is required.