SOLUTION

Requirements for Protection System

A basic requirement of a protective system is to clear the fault with sufficient speed to minimize the adverse effects of the fault. To fulfill this requirement, a protective system is required to possess the following properties.

Selectivity:- Selectivity is the ability of a protective system to isolate only a faulty section of a system from the rest of the healthy power system. Selectivity is absolute if the protective scheme responds only to faults within its own zone. Whereas it is said to be relative if it is obtained by grading the settings of protective relays of different zones, all of them respond to a given fault. The absolutely selective protective systems are known as unit systems; whereas the protection systems in which selectivity is relative are known as non-unit systems. For example, differential protection is said to be a unit protective system; whereas current time graded overcurrent protection or distance protection are said to be non-unit protective systems.

Reliability:-  Reliability is the ability of a protective system to work properly during the period for which it is in service. The term reliability includes both the security in fault clearance and the security against undesired clearances. Quantitative reliability can be expressed as an inverse of the probability of failure. Hence, it can be said that, the less the probability of failure, the better the reliability. Failure is confined not only to protective relays but also due to breaker defects, opened control circuits, or an unfaithful transformation of inputs by system transducers, etc. Hence, each and every component of a protective system must be regarded as a potential source of failure.

Speed of operation:- Speed refers to the total fault clearing time of a protective system including the protective relay tripping time as well as time taken by the breaker to open and to extinguish the arc produced between its contacts. It is obvious that the faster the speed of operation of the elements of a protective system (relay and breakers), the lesser is the damage to the power system components. This also limits the ionization at the fault, increases the chances of a successful auto-reclosing and reduces the dead time interval of a power system. Moreover, quick disconnection of the faulty section for rest of the healthy system helps in maintaining the stability of the power system.

Discrimination:- A protection system should be able to discriminate between the faulty condition and the normal loading conditions, particularly, when the minimum fault current is less than the maximum load current. As for example, while protecting a power transformer, the protective system must be able to distinguish between a fault and a magnetizing inrush.

Stability:- The term stability is often used to describe the quality of a protective system by virtue of which it remains inoperative under all conditions associated with faults outside its own tripping zone. In other words, it can be said that the protective system should remain stable against any type of disturbance present at any point of the power system except for an in-zone electrical fault.

Sensitivity:- Sensitivity is the ability of a protective relay to react correctly to the relatively low values of fault current. Sensitivity also refers to the minimum level of fault current at which a protective relay operates. As for example, high resistance ground faults are very common for EHV and UHV transmission lines; and the protective system must be able to detect such faults before they develop further serious problems. In other words, it is the protective relay setting, usually expressed in terms of the operating quantity.