The basic requirement on the protection equipment is that it will clear the fault with sufficient speed to limit the consequential damages in the transmission network or a plant. the fault clearance must be fast enough to avoid a total, or partial power network collapse. A number of different criteria for operation may be established, the most common criteria for power system protection are speed, sensitivity, dependability/reliability and selectivity. We shall define each criteria and more details.
1. Speed - this criteria is the most critical protective relay operating criterion. The relays have to be fast enough to allow clearing of a fault in the minimum time needed to ensure reliable and safe power system operation. In simpler term, this criteria is the ability of the protection to isolate the faults within a short possible time.
2. Sensitivity - this criteria means the capability to detect all type of fault. It is important to detect all faults even if the fault currents is smaller that the load current. Damage to equipment due to induction in low voltage equipment, or person injuries due to rise in earth potential, can occur also for low magnitude faults.
3. Reliability - it is the ability of the relay or protection scheme to operate correctly when required and to avoid unnecessary operation. Reliability consist of dependability and security of protection.
4. Selectivity - it is the capability of the protection to determine the fault location and only isolate the faulty region of the network. The consequences of a fault must be limited and the supply of electricity to the consumers secured. The protection system must therefor be capable of distinguishing between an external and an internal faults.
The protection of transmission lines varies in the principle used and the implementation approach depending on the voltage level. The main principles are associated with the implementation of transmission line protective relaying such as overcurrent protection of distribution feeders, distance and differential protection of transmission lines.
3. Reliability - it is the ability of the relay or protection scheme to operate correctly when required and to avoid unnecessary operation. Reliability consist of dependability and security of protection.
4. Selectivity - it is the capability of the protection to determine the fault location and only isolate the faulty region of the network. The consequences of a fault must be limited and the supply of electricity to the consumers secured. The protection system must therefor be capable of distinguishing between an external and an internal faults.
The protection of transmission lines varies in the principle used and the implementation approach depending on the voltage level. The main principles are associated with the implementation of transmission line protective relaying such as overcurrent protection of distribution feeders, distance and differential protection of transmission lines.
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