Protection challenges for transmission lines with long taps

Abstract

Tapped transmission lines are quite common as they provide a low cost solution to connect remote loads without incurring the prohibitive cost of building a substation and the associated protective equipment. However, adding a tap in the line complicates the protection scheme and introduces unique challenges for the protection engineer. Protecting transmission lines with long taps is even more challenging. The effects of infeed can result in a fault on the tap having a larger apparent impedance than a fault at the remote end of the line. Setting relay elements to provide adequate coverage of long taps can cause coordination issues with remote lines. This paper will illustrate, through some real world examples, the issue of protecting transmission lines with long taps and discuss some options for protecting long taps. Three examples will be used to explore different scenarios with tapped transmission lines: •The first example will look at how a redundant POTT scheme is used to protect a line with a 15 mile tap in the middle of a 20 mile 69 kV sub-transmission line. •The second example will highlight the effect of the location of the tap on the line by examining a line with a tap located at 95% of the line length from one end. •The third example will explore how the relative strength of the system will impact the protection of lines with a long tap. In this example, one terminal is much stronger source than the other. The system strength on the line under study will be determined by calculation of the source impedance ratio. A short circuit program with automated scripts will be used to illustrate these examples and run different contingency scenarios. Sensitivity and coordination scripts will be run to check the validity of the proposed settings. The last part of the paper will discuss the issues related to fault location on transmission lines with long taps.