Power system protection

The objective of a protection scheme is to keep the power system stable by isolating only the components that are under fault, whilst leaving as much of the network as possible in operation.

Some items in substations such as transformers might require additional protection based on temperature or gas pressure, among others.

If a protective relay fails to detect a fault, the resulting damage to the alternator or to the transformer might require costly equipment repairs or replacement, as well as income loss from the inability to produce and sell energy.

Instantaneous overcurrent requires that the current exceeds a predetermined level for the circuit breaker to operate.

Based on this curve, if the measured current exceeds a given level for the preset amount of time, the circuit breaker or fuse will operate.

Earth fault protection also requires current transformers and senses an imbalance in a three-phase circuit.

If one or two phases become connected to earth via a low impedance path, their magnitudes will increase dramatically, as will current imbalance.

If the ratio of voltage to current measured at the relay terminals, which equates to an impedance, lands within a predetermined level the circuit breaker will operate.

Remote back-up protection will generally remove both the affected and unaffected items of plant to clear the fault.

Protective device coordination is the process of determining the "best fit" timing of current interruption when abnormal electrical conditions occur.

Zone definitions account for generators, buses, transformers, transmission and distribution lines, and motors.

They are designed for redundancy to eliminate unprotected areas; however, overlapped regions are devised to remain as small as possible such that when a fault occurs in an overlap region and the two zones which encompass the fault are isolated, the sector of the power system which is lost from service is still small despite two zones being isolated.

Choosing the appropriate balance between security and dependability in designing the protection system requires engineering judgement and varies on a case-by-case basis.