AC Circuits and Power Systems in Practice

9Current Transformers

Current transformers (CTs) enable the measurement of currents in situations where either the system voltage or the magnitude of the current is too large to permit direct measurement. The current to be measured flows in the CT primary winding (usually just a single turn) and a scaled replica flows in its secondary. Current transformers are essential components in both protection and metering circuits, and while they are rugged devices they do have some limitations.

For example, like voltage transformers, CTs introduce both magnitude and phase errors into the secondary current. While these can be made arbitrarily small for metering applications, protection CTs must operate correctly during system faults, when primary currents can reach many times their normal value. Under these conditions a CT may approach saturation, and as a result its error will inevitably increase. In addition, in this region the non‐linear magnetising characteristic will introduce a significant harmonic content into the magnetising current, making phasor analysis inapplicable.

To minimise the likelihood of saturation and to reduce the size of the magnitude and phase errors, current transformers are designed to normally operate at very low flux densities. This is achieved by imposing a restriction on the size of the secondary voltage that a CT must generate, which in turn is achieved by limiting the burden impedance that can be connected across its secondary winding. As a consequence, ...