As soon as the fault occurs on the power system, the protection scheme should disconnect/isolate the faulty part as quickly as possible to minimize the damage to the system and to maintain the continuity of supply.
The protective scheme may have the following qualities :
1. Sensitivity:
The ability of the protective system to operate with low values of actuating quantity is known as sensitivity.
The protective system should be enough sensitive to operate with a small quantity of fault current.
2. Speed:
As mentioned above, the protective system should disconnect the faulty part as quickly as possible for the following reasons:
1. If the fault current is allowed to flow for a longer time then there will be considerable damage to the equipment which are feeding the fault.
2. If the fault is not cleared quickly, then it is the possibility of the development of a temporary fault into a permanent fault.
3. If the faulty parts of the system are not cleared quickly then it is the possibility of spreading these faults into healthy parts of the system.
4. A fault in the system causes the reduction in the terminal voltage, causing the complete shut down of consumers' motors. Also, the generators on the system may become unstable.
3. Selectivity:
The ability of the protective system to select correctly the faulty part of the system and disconnect that part without disturbing the rest of the system is known as selectivity.
The protective system should operate the circuit breaker which is nearer to the fault otherwise, opening any circuit breaker to clear the fault will lead to disconnecting the greater portion of the power system.
Referring to Fig. if the fault occurs on the transmission line (which is located by point F), then the operating of circuit breakers 6 and 8 will cause minimum disconnection of the power system portion.
In order to provide, selectivity to the system, the entire system is divided into several protective zones. When a fault occurs in a given zone, then only the circuit breakers within that zone will be opened.
This will isolate only the faulty section and the remaining healthy is unaffected.
4. Reliability:
The ability of the protection system to operate under predetermined conditions is known as reliability.
The protection would be rendered largely ineffectively and could even become a liability without the
reliability.
5. Simplicity:
A simple protective system is easy to maintain Reliability and Simplicity are closely related.
The simpler the protection scheme, the greater will be its reliability.
6. Adequateness:
Providing a protection scheme for every abnormal condition is very costly. Therefore, protection provided for any machine should be adequate.
Adequateness of protection is judged by the following aspects :
1. Cost of the machine and importance of the machine.
2. Rating of the machine to be protected.
3. Probability of abnormal conditions due to internal and external
4. Continuity of supply as affected by the failure of the machine.
5. Location of the protected machine.
7. Economy:
Cost is the most important factor in the choice of a particular protection scheme. As a rule, the protective gear should not cost more than 5% of its total cost.
However, when the apparatus to be protected is of utmost importance, then economic considerations are often subordinated to reliability.
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