Application considerations when protecting lines with tapped and in-line transformers

Transmission power lines can feed distribution networks or either serve loads or interconnect generation directly via tapped transformers. In some utilities, as many as three to five taps may be used on some lines. As a part of the bulk electric system, tapped transmission lines must be protected with instantaneous fault-clearing times and proper selectivity and sensitivity. This paper reviews various approaches to protecting tapped lines, including pilot protection schemes with distance and zero- and negative-sequence directional elements, time-overcurrent elements, line current differential schemes, and stepped distance backup. In addition, this paper discusses protection applications for in-line transformers where a transmission line terminates on a transformer and a single protection zone includes both the line and the transformer.