Current Transformers (CT) are the devices in which the secondary current used for calculation is proportional to the primary current produced by the electrical network. For this purpose, the primary current is tapped from the transmission line and stepped down in the secondary for measurement purposes.
In series with the current lead, the connection to the primary winding circuit is carried out. In the form of measurement instruments and various relays, the secondary winding is linked to any load.
A proportional relationship, referring to the number of turns, between the currents of both windings exists. The separation between the windings in a high voltage transformer is based on the maximum operating voltage.
As a rule, one of the ends of the secondary winding is grounded, so the winding and ground potential would be essentially the same.
Current transformers fall under the category of special accessories used in combination with various measuring devices and relays in AC circuits.
The main purpose of transformers is converting any current value to required values which is most convenient for measurements and supplying power to protective devices with low internal resistance. Due to such isolation, the maintenance electricians are protected from high voltage shock.
Instrument current transformers are designed for high voltage electrical circuits where the direct connection of the measurement instruments is not feasible. Their main function is to pass the electrical current to the measurement devices connected to the secondary winding.
An important function of transformers is to control the flow of current passing through the circuit. During connection to power relays, routine checkings of circuit networks and proper grounding are performed. When the flow of current reaches over the limit protective device is activated shutting down the transformer for avoiding further damage.
The current transformers are designed to perform two main functions: Safety and Measurement. Both functions can also be combined in certain devices.
Related: Power Transformer Working Principle, Construction and Types
The principle of operation of the current transformer is based on the law of electromagnetic induction. The transmission line voltage is supplied to the primary winding with a certain number of turns and overcomes its impedance.
This leads to the appearance of magnetic flux around the coil, which is captured by the magnetic circuit. This magnetic flux is perpendicular to the current direction.
As the secondary winding turns, located perpendicularly, are crossed, the electromotive force is activated by the magnetic flux. The loss of electrical current during the conversion would be negligible.
There is a current under the control of EMF, which is forced to overcome the impedance of the coil and the output load. At the same time, a decrease in voltage is observed at the output of the secondary winding.
All the current transformers can be categorized according to their features, method of use, and technical characteristics.
Based on the function performed,
Based on the type of construction,
In addition, there are high-precision, multi-transformation laboratory current transformers.
Invariably, the transition of energy from the primary to the secondary is followed by losses. There are no moving elements in the transformer and thus no mechanical losses.
However, there are copper losses due to the presence of electrical resistance in the windings, as well as magnetic losses in the core steel due to the resulting eddy currents and magnetization reversal.
Due to these losses, all the energy is not transferred from the primary to the secondary winding.
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