Current transformer usage

- Sep 27, 2019-

The secondary side of the current transformer is connected in series with the current coil of the measuring instrument to form a closed circuit. Since the impedance is small, the secondary circuit is close to the short circuit state, and the voltage is low. But if the secondary circuit is disconnected, the current transformer is actually equivalent to the step-up transformer. If it is turned on twice, the secondary circuit loses current and power. As the current balances, the magnetic flux of the core increases sharply, and the induced electromotive force also sharply increases, causing the secondary voltage to rise to several hundred volts or even thousands of volts. It is easy to cause electric shock to the human body, which may damage the insulation of the secondary wires and electrical components, which is very dangerous.

In addition, the core flow is greatly increased, the core is heated, and the transformer is burned. In addition, since the secondary current is zero, the meter is zero, so the current relay cannot work normally, and the monitoring and protection functions of the main circuit are lost.

Causes of secondary transformer grounding of voltage transformer and current transformer

1. The secondary side of the voltage transformer should have a grounding point, which is mainly used for safety considerations. When the insulation between the primary winding and the secondary winding is damaged by high voltage, the high voltage of the primary winding will jump to the secondary winding, and the grounding of the secondary winding will ensure the safety of personnel and equipment. In addition, the phase voltage can be supplied to the insulation monitoring device by grounding.

2. The second side of the current transformer should be grounded slightly. Since the main side of the high voltage current transformer is high voltage, when high voltage breakdown occurs between the two coils due to insulation damage, the high voltage will be pushed into the low voltage. When the secondary coil is grounded slightly, the high voltage is introduced into the ground to maintain the ground potential of the secondary coil to ensure the safety of personnel and equipment.