Need of instrument transformers; CT errors

Instrument transformers (current transformers and potential/voltage transformers) reduce the large currents and high voltages of the power system to standard low values (1 A or 5 A; 110 V or 110/√3 V) for meters and relays.

Need of instrument transformers

1. Isolation: meters, relays and operators are isolated from the high-voltage system, improving safety.
2. Standardisation: relays and meters of standard rating (5 A / 1 A, 110 V) can be used on any system voltage or current.
3. Economy: low-rated instruments with thin wiring are much cheaper than high-voltage, high-current instruments.
4. Remote location: relays and meters can be placed in a control room away from the switchyard.
5. Protection functions: CTs and VTs supply the current and voltage signals needed by overcurrent, differential, distance and directional relays.
6. Combination: several CT cores can serve separate metering and protection circuits.

Errors of CT

Because part of the primary ampere-turns supplies the exciting current I₀, the secondary current is not exactly I_p/K_n.

1. Ratio (current) error: (K_n·I_s − I_p)/I_p × 100 %. Caused mainly by the loss component of I₀; increases with burden.
2. Phase angle error (phase displacement): angle between I_p and reversed I_s, caused mainly by the magnetising component of I₀.
3. Composite error: r.m.s. difference between ideal and actual secondary current over one cycle including harmonics; important for protective CTs under saturation.

Errors increase with high burden, saturation (high fault current, DC offset), remanent flux, low primary current and poor core material.