Therefore the resultant area covered by fast and double-side tripping amounts to 60–80% of the entire section length (Fig. Each relay’s first zone range typically covers 80–90% of the line length (Fig. Due to the need to ensure relays’ tripping selectivity, none of their instantaneous zone ranges can be set to cover the entire protected line. The problem related to the use of distance relays for protection of two-sides powered transmission lines will be briefly discussed (Fig. However, as regards these relays’ autonomous operation at each end of the protected line, an instantaneous zone range reduction results in the inability to quickly switch off for some internal fault locations. The ability to distinguish between internal and external faults in each preset zone range and either direction seems to be the distance protection’s main advantage. Fast tripping of faults in transmission grids is also important for power system stability. This is associated with the risk of equipment damage or destruction.
For the sake of safe elimination of high-current internal faults, such lines need to be capable of fast switching-off on both sides. Power lines of the highest voltages are usually double-side supplied.
