Non-Coherence in Transmission Line Arrangements

Non-coherence in system reliability refers to the fact that absence of a component from the system creates a more reliable system state or provides a higher system reliability level. This phenomenon may exist in transmission systems. Unfortunately, it has not been sufficiently recognized in the power industry. This paper explains the basic concept of non-coherence in transmission systems and provides a case study where the single transmission line structure with loop-in connection for supplying loads from two ends is superior to the double transmission line structure with tap-connection for supplying the same loads not only in an economic sense but also in accordance with the reliability level. This is contrary to intuition since transmission planning engineers normally make an opposite judgment. The example given in the paper is an actual application in a utility of Canada. It also has a wider significance because the double- line arrangement with tap-connection is a popular choice in transmission planning practice of utilities across the world. It is important and beneficial for utilities to recognize and identify the non-coherence feature that could exist in their transmission systems using quantified reliability evaluation. This can help improve system reliability while avoiding unnecessary capital investments in the system planning process.