Interpret the role of current transformers1

A current transformer is an instrument with two currents affecting each other. So the question is, what is the function of the current transformer? Measurement? Protection? - -Introduction Current transformer, the English name is Current transformer, the symbol is TA, it is an instrument composed of a closed iron core and windings that can convert a large current into a small secondary current according to the principle of electromagnetic induction. Its structure is shown in the figure below. The primary winding has fewer turns and is connected to the circuit that needs to measure the current, while the secondary winding has more turns and is connected to the measurement surface or protection circuit. When it is in operation, the current mutual inductance The secondary circuit of the device is closed (otherwise there will be a safety hazard), and the impedance of the measurement surface or the series coil of the protection circuit is very small, making its working condition close to a short circuit.

Second, the role of the current transformer 1 - - used to measure the role of the current transformer is used for measurement, often used for billing or measuring the current size of equipment in operation. When measuring large alternating currents, in order to facilitate surface measurement and reduce the risk of directly measuring high-voltage electricity, it is often necessary to use current transformers to convert them into relatively uniform currents. to the role of current conversion and electrical isolation. The current transformer converts high current into low current proportionally as required, and is used to connect the primary side of the current transformer to the primary system and the secondary side to the measurement surface or relay protection equipment during measurement.

3. The second function of the current transformer - the second function of the current transformer is for protection, and it is often used in cooperation with the relay equipment. When the line is short-circuited or overloaded, the current transformer will send the relay to the relay. The device sends a signal to block the defective circuit, and then achieves the purpose of protecting the safety of the power supply system. The current transformer for protection is different from the current transformer for measurement. It can work effectively only when the current is several times or tens of times larger than the normal current, and it requires reliable insulation and a large and accurate limit value. coefficient, with satisfactory thermal and dynamic stability. Fourth, the role of the current transformer - - Notes 1. During the operation of the current transformer, the secondary side is not allowed to open, and the closed state must be maintained. Because once the secondary side is opened, the magnetic flux and the secondary side voltage will far exceed the normal value (up to thousands or even tens of thousands of volts), which is extremely harmful to the safety of operators and equipment. 2. The wiring method of the current transformer should follow the series connection principle, that is, the primary winding is connected in series with the circuit under test, and the secondary winding is connected in series with the measuring surface or relay equipment.