Electrical World

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...

Q. Explain different types of faults? A. Shunt Faults: Basically shunt faults are short circuits. When the path of the load current is cut short because of breakdown of insulation, we say that a 'short circuit' has occurred which is nothing but a shunt fault. Series Faults: Series faults are nothing but a break in the path of current. It is observed in practice that most of the open conductor faults sooner or later develop into some or the other short-circuit fault. Q. What are the various states of operation of a power system? A. Normal, Alert, Emergency, In extremis and Restorative. Q. Explain the working of a Current transformer ? A. The current transformer has two jobs to do. Firstly, it steps down the current to such a level that it can be easily handled by the relay current coil. Secondly, it isolates the relay circuitry from the high voltage of the EHV system. A CT whose primary is in series with the line in which current is to be measured. Q. Why the se...