CT working principle and applications

A current transformer (CT) is an instrument transformer whose primary winding carries the line current in series with the circuit and whose secondary delivers a proportionally reduced current (1 A or 5 A) to meters and relays, with a known phase relation.

Working principle

It works on the principle of electromagnetic induction, like a transformer operating with its secondary nearly short-circuited.

     I_p (line current)
  ═══════════╦═══════════  1 or few turns
          ┌──╨──┐
          │core │  flux Φ
          └──╥──┘
        N_s turns
        ┌────╨────┐
        │  A / R  │ ammeter/relay
        └─────────┘ low impedance burden

• The primary current is fixed by the load of the line, not by the CT.
• It sets up primary ampere-turns N_p·I_p. The secondary current I_s produces opposing ampere-turns; only a small difference remains to magnetise the core:

 N_p·I_p ≈ N_s·I_s   →   I_s ≈ I_p × (N_p/N_s)

• Example: a 500/5 A CT gives 5 A at 500 A primary; turns ratio 1:100.
• The flux density is low in normal operation; if the secondary is opened, the full primary ampere-turns magnetise the core, causing saturation and dangerously high secondary voltage. Hence the secondary must never be open-circuited.

Types

Wound type, bar type, window/ring type, bushing type, core-balance CT.

Application areas

1. Metering: ammeters, wattmeters, energy meters, power-factor meters on HV/LV feeders and tariff metering.
2. Protection: overcurrent and earth-fault relays, differential protection of transformers, generators and busbars, distance and directional relays.
3. Core-balance CTs: earth-leakage and sensitive earth-fault protection of cables and motors.
4. Control and monitoring: SCADA transducers, maximum demand indicators, power quality meters.
5. Isolation: keeps meters and personnel away from high voltage.