Relaying tapped substations for high-voltage faults

THE NEED for relaying a small-capacity substation tapped to a high-voltage transmission line for faults on the line is not a frequent occurrence. Most substations tapped to high-voltage transmission lines (Fig. 1) supply essentially passive or nonmotor loads and present no problem in case a fault occurs on the high-voltage transmission line. The need for special consideration of relaying at a tapped substation is encountered when a substation supplies power to a subtransmission or distribution system with considerable connected rotating machinery, and high-speed reclosing of circuit breakers is desired on the high-voltage transmission line. Typical examples of such substations are as follows: 1. A station supplying power to a low-voltage system with connected generation operating in parallel with the high-voltage transmission system, such as a municipality or a large industrial plant. 2. A station supplying power to large synchronous motors, as in an industrial or pumping plant. 3. A station supplying power to a low-voltage system where the load is predominantly small induction motors.