A simple way to set distance relays is to base the setting on percentages of the line impedances. However, when such simplified techniques are used, there are several factors that could affect the reliability of settings under various circumstances. In some cases these factors make it necessary to conduct additional studies to verify these settings under some critical operating conditions that could cause these relays to trip incorrectly. This paper describes an analysis made to the protection of a pair of long (221 km) parallel 220 kV transmission interconnection lines in Peru. It describes the problems that had been experienced before the settings were revised, and the basis for the new settings.

In addition to being an interesting study of complicating factors for transmission line protection, this paper also has an educational component. It demonstrates many of the effects that should taken in consideration during the setting studies such as, mutual coupling of the parallel lines, load flow effects, fault resistance effects. For example, this is a long line with power autotransformers at the remote terminal where a typical setting zone 2 reach had to be reduced from 120% to 100% in order to avoid tripping during faults at the lower voltage level.