Component of protective system

A protection system consists of many other subsystems which contribute to the detection and removal of faults. As shown in Fig. 1.2, the main subsystems of the protection system are the transducers, relays, circuit breakers, and trip circuit constraining trip coil and battery. The transducers, i.e., the current and voltage transformers (CTs and VTs) are used to reduce currents and voltages to standard lower values and to isolate protective relays from the high voltages of the power system. They constitute a major component of the protection system. Protective relays detect and locate the fault and issue a command to the circuit breaker (CB) to disconnect the faulty element. When a fault occurs in the protected circuit (i.e., the line in this case), the relay connected to the CT and VT actuates and closes its contacts to complete the trip circuit. Current flows from the battery in the trip circuit. As the trip coil of the circuit breaker is energized, the circuit breaker operating mechanism is actuated and it operates for the opening operation to disconnect the faulty element. A circuit breaker is a mechanical switching device capable of making, carrying, and breaking currents under normal circuit conditions and also making, carrying for a specified time, and automatically breaking currents under specified abnormal circuit conditions such as those of short circuits, i.e., faults. It is used to isolate the faulty part of the power system under abnormal conditions. A protective relay detects abnormal conditions and sends a tripping signal to the circuit breaker. A circuit breaker has two contacts—a fixed contact and a moving contact. Under normal conditions, these two contacts remain in closed position. When the circuit breaker is required to isolate the faulty part, the moving contact moves to interrupt the circuit. On the separation of the contacts, the flow of current is interrupted, resulting in the formation of an arc between the contacts. The medium in which circuit interruption is performed is designated by a suitable prefix, such as oil circuit breaker, air-break circuit breaker, air blast circuit breaker, sulfur hexafluoride circuit breaker, or vacuum circuit breaker. Since the primary function of a protection system is to remove a fault, the ability to trip a circuit breaker through a relay must not be compromised during a fault, when the AC voltage available at the substation may not be of sufficient magnitude. In case of a close-in three-phase fault, the AC voltage at the substation can be zero. Therefore the tripping power, as well as the power required by the relays cannot be obtained from the AC system and is usually provided by the station battery. The battery which is permanently connected through a charger to the station AC service floats on the charger during normal steady-state conditions. The charger is of sufficient VA capacity to provide all steady-state loads powered by the battery. The battery should also be rated to maintain adequate DC power for 8-12 hours following a station blackout. For better reliability, EHV stations have duplicate batteries, each Bus bar CT CB Line VT Relay Battery Trip coil Trip circuit Fig. 1.2 Components of a protection system feeding from its charger, and connected to its own complement of relays. Since the severe transients produced by the electromechanical relays on the battery loads during their operation may cause maloperation of other sensitive relays in the substation, or may even damage them, it is common practice, as far as practicable, to separate electromechanical and static equipment by connecting them to different batteries.