In power distribution cabinets, we often see square or circular components known as Current Transformers (CTs).

At first glance, many people mistake CTs for ordinary coils. However, their function is not power supply, but current sampling.

The core purpose of a CT is to convert a large primary current into a small secondary current in a fixed proportion. The common secondary output is 5 A or 1 A, which is used for sampling by ammeters, energy meters, protective devices, reactive power compensation controllers, and other equipment.

Structurally, a current transformer mainly consists of a primary conductor, iron core, secondary winding, terminals, insulating shell, and nameplate parameters. The primary conductor passes through the through-hole window; when current flows, magnetic flux is generated in the iron core, and the secondary winding induces a corresponding small current.

Therefore, the working principle of a CT is essentially electromagnetic induction.

CTs are widely used in power distribution cabinets:

In incoming cabinets: for total current sampling, metering, and protection.

In outgoing cabinets: for loop current sampling.

In capacitor cabinets: providing current signals for reactive power compensation controllers.

In metering cabinets: completing energy metering with energy meters.

In protection circuits: supplying judgment signals for relays and protective devices.

There are several critical precautions for CT operation:

The secondary side must never be open-circuited during operation, as this may produce high voltage and pose safety hazards.

The transformation ratio must match (e.g., 600/5, 400/5) with the requirements of instruments and protective devices.

Polarity must not be reversed. Incorrect connection of P1/P2 direction or S1/S2 polarity may cause inaccurate metering or maloperation of protection.

The rated capacity must be appropriate. Excessive secondary load will reduce measurement accuracy.

Grounding must comply with standards to ensure safety and anti-interference performance.