# Current transformers - basic concepts

The operating principle of the current transformer is based on the physical principle of the electromagnetic field. The increase or decrease in the current at the output of the current transformer included in the AC circuit can increase or decrease depending on the ratio between the windings. So, for example, if the ratio of the windings is 1: 100, i.e. 100A flows through the primary winding of the current transformer, then on the secondary winding we can get a current of 1A. A current transformer is a primary and secondary winding wound on one frame and connected to each other only by an electromagnetic field.

Current transformers according to their purpose, they are divided into current transformers for measurements and current transformers for protection. In some cases, these functions are combined in a single current transformer.

Instrument current transformers are installed in circuits in which a large current flows and it is impossible to directly connect measuring instruments. They serve to transmit information to measuring instruments. Ammeters, current windings of wattmeters, meters and similar devices are connected to the secondary winding of the current transformer for measurements. Thus, the measurement current transformer provides:

•  converting an alternating current of any value into alternating current that can be measured with standard measuring instruments.
•  isolation of measuring devices from the high voltage circuit, for their safe use.

Also, an important task of the current transformer is the current control of the power grid. Thus, a transformer connected to a power relay constantly monitors the network, grounding conditions, protects and disconnects devices and equipment connected to the network in case of an emergency current value. The current flowing through the secondary winding of a current transformer is directly proportional to the current flowing in the primary winding. Thus, direct measurement of the current in the secondary winding allows you to indirectly calculate the current in the primary, i.e. high circuit current.

Protective current transformers - are intended for transferring measured information to protection and control devices. Accordingly, the protection current transformer provides:

•  conversion of alternating current of any value into alternating current suitable for supplying protection relays or automatic protection systems.
• to isolate accessed circuits and equipment from the high voltage circuit.

Current and voltage are the main parameters characterizing the mode of the electrical circuit. The measurement methods and instruments used to evaluate these quantities are different and depend on the type of current, the range of the measured quantity, the required accuracy, the permissible power consumption, the shape of the curve, the frequency, etc. First of all, it should be noted that the inclusion of an ammeter or voltmeter in the investigated circuit to measure current or voltage changes the measured value. Therefore, even with perfectly accurate measuring instruments, the result obtained differs from the value of the measured value that was in the circuit under investigation before the instrument was turned on.

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