Protecting mutually coupled transmission lines: Challenges and solutions

This paper is a tutorial on the protection of mutually coupled transmission lines. It discusses how mutual coupling affects the polarizing quantities of ground directional elements, the reach of ground distance elements, and the accuracy of single-ended fault locating algorithms. The paper provides settings guidelines for instantaneous directional overcurrent and ground distance elements. It discusses in detail how transmission line mutual coupling causes overreaching or underreaching of ground distance elements. It also discusses the impact on these elements of grounding the mutually coupled line at both line ends during maintenance. The paper analyzes whether mutual coupling compensation offers any benefits to line protection. The ease and benefit of line current differential schemes are contrasted in the discussion. Lastly, the paper examines a case when a double-circuit transmission line is operated as a single circuit with jumpers placed across similar phases along the line. This situation typically arises when the utility company needs to free one of the bays to bring an additional line into the substation. The protection engineer needs to decide where to install jumpers to parallel the two circuits in order to avoid distance element underreaching. The paper provides an analysis of this problem and offers suggestions on how to address it.