Instrument transformers - General information. Theory. How it works

The operation of electricity transmission and distribution networks requires the metering and monitoring of the energy passing through the lines and cables.

Given the considerable installed capacity, metering for billing purposes has to be precise. An uncertainty of one per thousand in the measurement of a capacity of 1,140 MW translates into a loss of revenue of 10 GWh after one year!

In addition, all line faults, which can generate considerable short-circuit currents, must be detected and eliminated in time to prevent the destruction of equipment or the disarticulation of the network. Constant monitoring is therefore essential to ensure the protection of structures, equipment and networks.

In this treatise, the reader is referred to the article Transient regimes in power systems [D 4 410] for General information on disturbances, and to the article Lines and substations. Choosing and coordinating insulators for more details on Dielectric stresses applied to equipment.

The two essential functions (measurement and protection) can only be achieved if we have the most precise possible knowledge of two line quantities: current and voltage.