Characteristics and usage requirements of Electrical Current Transformer

1. Less primary turns and more secondary turns

Electrical Current Transformer used in the power system, its primary winding is usually the incoming and outgoing wires of the primary equipment, with only 1 or 2 turns; the number of secondary turns is many. For example, an Electrical Current Transformer with a transformation ratio of 1250/1 has 1 turn at the first turn and 1250 turns at the second turn.

2. The working magnetic density in the iron core is very low, and the magnetic density is large when the system fails

During normal operation, the working magnetic density in the Iron Core of the Electrical Current Transformer is very low, and its primary and secondary ampere turns are balanced. When the system fails, due to the large fault current and the high secondary voltage, the excitation current increases, and the magnetic density in the iron core rises sharply, even saturating the iron core.

3. High internal resistance, current source

Under normal conditions, the magnetic density in the iron core is very low, the excitation impedance is very large, and the number of secondary turns is large. Seen from the secondary side, its impedance is very large. Compared with the internal resistance of the Electrical Current Transformer, the load impedance can be ignored. Therefore, the change of the load impedance has little effect on the secondary current and can be called a current source.

4. The need for secondary load is small (relative to voltage transformer)

If the secondary responsibility of Electrical Current Transformer is large, its secondary voltage is very high during operation, and the excitation current will inevitably increase, which will increase the error of current conversion. Especially when the system fails, the primary current of the Electrical Current Transformer may reach dozens of times the rated current, causing the iron core to saturate and the current conversion error is large, not meeting the requirements of the relay protection, or even causing the protection to malfunction.

5. The secondary circuit must not be open

The secondary circuit of the Electrical Current Transformer must not be open. If the secondary circuit is open during operation, the secondary current disappears, and the demagnetization effect also disappears. The magnetic density in the iron core is very high; and because the number of secondary turns is extremely large, the secondary voltage will be very high, sometimes up to Thousands of volts endanger secondary equipment and personal safety.