The Role of Current Transformers

A current transformer (**CT, Current Transformer**) is a core device in the power system used for **current measurement, 

protection, and control**. Its function is to convert the **large current in high-voltage or high-current lines into a standard 

small current** (such as 5A or 1A) for use by instruments, relay protection devices, and metering equipment. The following

 is a detailed analysis of it:

I. Core Functions

1. Current Transformation

- **Principle**: Through the **principle of electromagnetic induction**, the large current on the primary side (such as 1000A)

 is converted into a standard small current on the secondary side (such as 5A) according to a fixed turns ratio (such as 1000:5).

- **Function**: Make the large current easy to measure and adapt to equipment, and avoid damage to the instrument caused 

by directly measuring the large current.

2. Electrical Isolation

- There is **no direct electrical connection between the primary side and the secondary side**. It is only coupled through the iron

 core to isolate the high-voltage circuit from the low-voltage equipment, ensuring the safety of personnel and equipment.

II. Main Functions

1. Measurement and Monitoring

- Provide current signals for ammeters, power meters, energy meters, etc., and monitor the line load situation in real time (such as

 the current of the main circuit of the power distribution cabinet).

2. Relay Protection

- Provide current signals for overcurrent protection, differential protection, zero-sequence protection and other devices, and trigger 

protection actions (such as circuit breaker tripping) when the current is abnormal.

3. Electric Energy Metering

- Cooperate with the energy meter to achieve accurate metering, and it is the core device for power billing (a high-precision CT of

 class 0.2S needs to be selected).

4. Equipment Control

- Provide current feedback signals for frequency converters, soft starters, etc., to achieve automatic control (such as motor speed 

regulation).

III. Key Characteristics

1. Working Mode

- **The secondary side must be short-circuited**: In normal operation, the secondary side is connected to a low-impedance load