Talking about Current Transformers

What is a current transformer?

A current transformer is an instrument that converts a large current on the primary side into a small current on the secondary side for measurement based on the principle of electromagnetic induction. A current transformer consists of a closed core and windings. It has a small number of turns on the primary winding and is strung in the line of the current to be measured.

What is the function of current transformer？
During the production process, the operating conditions of each primary equipment in the system are monitored to ensure the safe operation of the power system. The current transformer usually converts the large current of the primary circuit into a secondary small current proportional to it, and finally outputs the small secondary current to the secondary instrument, relay protection, and automatic device for use. Therefore, the current transformer is the AC current source of the electrical control, regulation, measurement and protection components, and it is also the hub of the electrical secondary circuit and the primary circuit.

Why should the current transformer be grounded?
It is to ensure safety, one is equipment safety, and the other is personal safety. Because there are distributed capacitances between the primary winding and the secondary winding of the current transformer and between the secondary winding and the ground. Then the voltage division of this distributed capacitance causes the secondary winding to produce a higher voltage to the ground. In addition, the insulation between the primary winding and the secondary winding is damaged due to some reasons, so the high voltage of the primary circuit will be directly added to the secondary circuit, which is very harmful to the secondary equipment and personal safety.
Therefore, the secondary side of the current transformer must be grounded, and the secondary circuit has one and only one grounding point, and multiple grounding points are not allowed.

Why can’t the secondary side of a current transformer be open circuit?
Because the current transformer secondary circuit open circuit, then the secondary current disappears, and then the demagnetization effect also disappears, will cause the core in the magnetic density is very high. Because the secondary side of the winding turns is much larger than the primary side of the winding turns, so once the secondary side of the open circuit, there will also be a good high voltage, sometimes up to thousands of volts, so not only endanger the secondary side of the equipment, but also endanger personal safety.

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