An analogue multi-terminal line differential protection

In many networks, ranging from distribution to subtransmission and transmission, there are several short lines. This is mostly valid for urban networks and industrial networks. Further there is an economic pressure to build lower cost and simpler to use systems. The most efficient way to decrease costs is to decrease the number of primary apparatus. A solution often used is to build multi terminal lines. For short lines, especially close to longer lines, there are some difficulties to use conventional phase overcurrent or distance protections. To assure selectivity the operation time of such protections must be delayed for most of the faults on the protected line. The long fault time will give stress to equipment, personnel danger, stability problems and others. For many years, the pilot wire current differential protection has proven to be very suitable for short lines. The analogue technique has proved to be very reliable and is very simple to use. The pilot wire line differential protection applications have been limited to two ended line applications. In this paper it is shown how a multi terminal current differential protection scheme can be applied using standard components and necessary considerations for the application are discussed. The RADHL pilot-wire relay has traditionally been used for protection of short to medium length two terminal cables or overhead lines. The relay application requires a dedicated metallic pilot wire between the line ends. The upgraded RADHL differential relay can be used for protecting lines with two or more terminals, using one relay per line end. For example, in a three terminal application, three relays are interconnected via the pilot wires. Phase-to-phase and phase-to-earth faults are detected on the line. All line ends are tripped even if the fault current is coming from one end only.