Principles for installation and selection of miniature current transformers

- Aug 14, 2020-

1. Primary side rated current

For the selected current transformer, the rated current of the primary side should be 1.0~1.3 times of the load current during normal operation of the line.

2. Rated voltage

The selected current transformer should have a rated voltage of 0.5 kV or 0.66 kV.

3. Accuracy class

If it is used for measurement, the current transformer of accuracy class 0.5 or 0.2 should be selected; if the load current changes greatly, or the load current is lower than 30% of the rated current of the primary side of the current transformer during normal operation, the current transformer of class 0.5 should be selected Device.

4. Number of turns and transformation ratio

When selecting a current transformer, the transformation ratio and the number of turns of the current transformer can be determined according to actual needs.

5. Model specifications

After determining the transformation ratio according to the primary load current of the power supply line, then determine the type of current transformer according to the actual installation situation.

6. Rated capacity

The secondary rated capacity of the current transformer is greater than the actual secondary load, and the actual secondary load should be 25% to 100% of the secondary rated capacity. The capacity determines the load impedance of the secondary side, which in turn affects the accuracy of measurement or control. The load impedance is mainly affected by the resistance value of the measuring instrument and the relay coil, the reactance and wiring contact resistance value, and the resistance value of the secondary connection wire.

In practical applications, if the overload protection device of the motor needs to be connected to the current transformer, the metering (control) device and the protection device should be separated to avoid affecting the reliability of protection.