Why we need for Power System Protection?
Need for power system protection are growing fast with more generators, transformers and large network in the systems. For system operation a high degree of reliability is required. In order to protect the system (lines and equipment) from damage due to undue currents and/or abnormal voltages caused by faults (such as short circuits) the need for reliable protective devices, such as relays and circuit breakers arises. As already mentioned, the most common electrical hazard against which protection is required is the short circuit. However, there are many other abnormal conditions—e.g., overloads, undervoltage and overvoltage, open phase, unbalanced phase currents, reversal of power, underfrequency and overfrequency, overtemperature, power swings, instability—for which some protection is desired.
As a rule, on the occurrence of short circuits which may lead to heavy disturbances in normal operation (damage to equipment, impermissible drop in voltage etc.), the protective scheme is designed to disconnect or isolate the faulty section from the system without any delay. The protective scheme is designed to energise an alarm or signal whenever the overloads and short circuits do not present a direct danger to the faulted circuit element and the entire installation, for example, on occurrence of a single phase fault to earth in overhead circuits operated with an insulated neutral. In such cases it is possible for the operating personnel to take the necessary measures for removal of the abnormality and prevent any interruption in power supply to consumers.
The main functions of protective relaying are to detect the presence of faults, their locations and initiate the action for quick removal from service of any element of a need for power system protection when it suffers a short circuit or when it starts to operate in any abnormal manner that might cause damage or otherwise interfere with effective operation of the rest of the system. The relaying equipment is added in this task by the circuit breakers that are capable of disconnecting the faulty element when they are called upon to do so by the relaying equipment.
Circuit breakers are generally located so that each generator, transformer, bus, transmission line etc., can be completely disconnected from the rest of the system. The circuit breakers must have sufficient capacity so that they can carry momentarily the maximum short-circuit current that can flow through them and then interrupt this current; they must also withstand closing in on such a short circuit and then interrupting it according to certain prescribed norms.
Fuses are employed where protective relays and circuit breakers are not economically justifiable.