Basic Requirements of Protection System:
Protection system is an extremely important part of the power system as it is provided to operate under abnormal conditions to prevent failure or isolate trouble and limit its effect. Every protection system which isolates a faulty element is required to satisfy four basic requirements: (i) reliability; (ii) selectively; (iii) sensitivity; and (iv) speed of operation. Without reliability and selectivity the protection would be rendered largely ineffective and could even become liability. The basic requirements of protection system are given below,
1. Reliability and Simplicity
Reliability is the basic requirements of protection system as it is provided to improve the reliability of the whole system. Reliability means that the protection system must be ready to function, correctly at all times and under all conditions of any fault and abnormal conditions of the power system for which it has been designed.
Reliability is a qualitative term. Reliability of protective systems is assessed from statistical data. It cannot be easily specified in terms of a mathematical expression with certainty. Statistical survey and records give idea about reliability of protective systems. With the growing size of systems, use of EHV lines, interconnections and use of large generators and transformers, the importance of reliability of protective systems has increased.
Inherent reliability is a matter of design based on long experience. This can be achieved partly by (i) simplicity and robustness in construction; (ii) high contact pressure; (iii) dust free enclosures; (iv) good contact material; (v) good workmanship, and (vi) careful maintenance.
Simplicity is closely associated with reliability. Simplicity of construction and good quality of relay, correctness of design and installation and good maintenance and supervision etc., are the main basic factors which influence the protective reliability. As a rule, the simple the protective scheme and lesser the number of relays, circuits and contacts it contains, the greater will be its reliability.
Reliability is rather closely associated with simplicity. Therefore one of the basic requirements of protection system technique is simplicity.
2. Selectivity and Discrimination
Selectively is the ability of the protective system to select correctly that part of the system in trouble and disconnect or isolate the faulty part without disturbing the rest of the system i.e., the protective system must be able to discriminate between those conditions for which prompt operation is required and those for which no operation or time-delay operation is required.
A well designed and efficient protective system should be selective i.e., it should be able to detect the point at which the fault occurs and cause the opening of the circuit breakers nearest to the fault with minimum or no damage to the system.
If the protective system is not selective and operates for faults beyond its protective zones, a larger portion of the system will get disconnected unnecessarily, causing embarrassment to supplier and consumers.
Selectivity is absolute if the protection responds only to the faults within its own zone, and relative if it is obtained by grading the settings of the protective relays of several zones all of which may respond to a given fault. Systems of protection which in principle are absolutely selective are called the unit systems. Systems in which selectivity is relative are non-unit systems. Differential protection and frame leakage protection fall in the former category while the current time graded protection and distance protection fall in the latter category.
Any protective system must be sufficiently sensitive so that it will operate reliably when required under the actual condition that produces the least operating tendency.
Sensitivity of any protective scheme refers to the smallest value of actuating quantity at which it starts operating in relation with the minimum value of fault current in the protected zone.
Sensitivity of a protective relay is a function of volt-amperes input to the relay coil necessary to cause its operation. The smaller the volt-ampere input required to cause relay operation, the more sensitive is the relay.
A protection with high degree of sensitivity is more complex and uses more equipment and circuitry and is, therefore, more expensive. Such protection is employed only in that case where simpler arrangement cannot be applied because of low degree of sensitivity.
4. Speed of Operation
The protective system should disconnect the faulty section as quick as possible. This is desirable for many reasons such as (i) reduction in the amount of damage incurred; (ii) improvement in power system stability; (iii) less annoyance to electric power consumers and decrease in total outage time for power consumers; (iv) decrease in likelihood of development of one type of fault into other more severe type and; (v) permit use of rapid reclosure of circuit breakers to restore service to consumers.
However, too fast operation may result in undesired operation during transient faults.