Basic of protection system

The purpose of an electrical power generation system is to distribute energy to a multiplicity of points for diverse applications.

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.


Figure 1 Relationship between reliability of supply,
its cost and value to the consumer.

         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.

The 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.

A power system represents a very large capital investment. To maximize the return on this outlay, the system must be loaded as much as possible. For this reason it is necessary not only to provide a supply of energy which is attractive to prospective users by operating the system within the range AB (Figure 1.1), but also to keep the system in full operation as far as possible continuously, so that it may give the best service to the consumer, and earn the most.

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.