Current and voltage transformers

Current or voltage instrument transformers are necessary to isolate the protection, control and measurement equipment from the high voltages of a power system, and for supplying the equipment with the appropriate values of current and voltage generally these are 1 or 5 A for the current coils, and 120 V for the voltage coils. The behaviour of current and voltage transformers (VTs) during and after the occurrence of a fault is critical in electrical protection because errors in the signal from a transformer can cause maloperation of the relays. In addition, factors such as the transient period and saturation must be taken into account when selecting the appropriate transformer. When only voltage or current magnitudes are required to operate a relay, the relative direction of the current flow in the transformer windings is not important. However, the polarity must be kept in mind when the relays compare the sum or difference of the currents.