Electrical World

As the name indicates, electromechanical relays use the principle of inter conversion between electrical and mechanical energies. The most common type of electromechanical relay is the induction coil. Basic operation of an electromechanical relay: The relay used should activate the circuit breaker when an over current occurs, thereby the circuit breaker disconnects the current flow and prevent the circuit components from damages. Usually a current transformer is used and current flows through its secondary only when over current flows. The current across the secondary of the transformer energizes the relay coil which in turn breaks the circuit connection in the circuit breaker. Thus further damages are prevented. In the case of induction relays, there may be one or more exciting coils wound around a magnetic core. Due to flow of current in the exciting coil, the electromagnetic torque is produced which rotates the rotor. This rotation of rotor is translated into...

Q. Mention some of the applications of Directional Over-Current relays? A.    1. Double end fed power system      2. Single end fed system of parallel feeders     3. Ring main feeder system Q. Explain differential protection? A. Differential protection is based on the fact that any fault within an electrical equipment would cause the current entering it, to be different, from that leaving it. Thus, we can compare the two currents either in magnitude or in phase or both and issue a trip output if the difference exceeds a pre- determined set value. Q. What are the different types of faults in a transformer? A. The internal faults include phase faults, ground faults, inter-turn faults, tap-changer failure, and leakage oil from tank. The external faults include system phase faults, system ground faults, over loads and over fluxing. Q. What is inrush current? A. the initial value of flux is zero when the transformer is energiz...