A unique current differential based algorithm for protection of multi-terminal lines

The paper describes a new differential relay showing the possibility of using the vector difference as a restraint quantity, while keeping the vector sum as a differential quantity. The vector difference is between the largest incremental current and the vector sum of incremental currents of the other terminals. This is as compared with the existing algorithms that use scalar sum of current terminals as a restraining quantity, which is somewhat arbitrary. This is as well as reducing the need to provide some form of synchronization to the relays at either end of the line, or to provide additional restraint when CT saturation occurred during an external fault since the algorithm considers these conditions inherently as an external fault. This paper is first of all concerned with the existing protection scheme and then with a new scheme proposed by the authors. The purpose of the paper is to establish the principle and the applicability of the algorithm in multi-terminal lines.