Basic of protection system
The system should be designed and managed to deliver this energy to the utilization points with both reliability and economy.
As these two
requirements are largely
opposed, it is
instructive to look at the relationship
between the reliability of a system and its cost and
value to the
consumer, which is shown in Figure 1.
It is important to realize that the system is viable only between the cross-over points A and B. The diagram illustrates the significance of reliability in system design, and the necessity of achieving sufficient reliability.
On the other hand, high reliability should not be pursued as an end in itself, regardless of cost, but should rather be balanced against economy, taking all factors into account.
Security of supply can be bettered by improving plant design, increasing the spare capacity margin and arranging alternative circuits to supply loads. Sub-division of the system into zones, each controlled by switchgear in association with protective gear, provides flexibility during normal operation and ensures a minimum of dislocation following a breakdown.
The greatest threat to a secure supply is the shunt fault or short circuit, which imposes a sudden and sometimes violent change on system operation.
large current which then flows, accompanied by the localized release of a
considerable quantity of energy, can cause fire at the fault
location, and mechanical damage throughout the system, particularly
to machine and transformer windings. Rapid isolation of the fault by
the nearest switch-gear will minimize the damage and disruption
caused to the system.
Revenue for the supply authority. Absolute freedom from failure of the plant and system network cannot be guaranteed.
The risk of a fault occurring, however slight for each item, is multiplied by the number of such items which are closely associated in an extensive system, as any fault produces repercussions throughout the net-work. When the system is large, the chance of a fault occurring and the disturbance that a fault would bring are both so great that without equipment to remove faults the system will become, in practical terms, inoperable.
The object of the system will be defeated if adequate provision for fault clearance is not made. Nor is the installation of switchgear alone sufficient; discriminative protective gear, designed according to the characteristics and requirements of the power system, must be provided to control the switchgear.
A system is not properly designed and managed if it is not adequately protected. This is the measure of the importance of protective systems in modern practice and of the responsibility vested in the protection engineer.