Working Principle and Characteristics of Current Transformers

Working Principle of Current Transformers

The working principle of current transformer is made based on the electromagnetic induction principle. A current transformer consists of a closed core and windings. Its primary winding has very few turns and is connected in series with the line where the current needs to be measured, hence the entire current of the line often flows through it. The secondary winding has more turns and is connected in series with the measuring instruments and protection circuit. When the current transformer is in operation, its secondary circuit is always closed, thus the impedance of the series coil of the measuring instruments and protection circuit is very small, and the operating state of the current transformer is close to a short circuit. The current transformer converts the high current of the primary side into a low current on the secondary side for measurement, and the secondary side should not be open-circuited.

Current Transformers are mainly used to convert large currents in AC circuits into smaller, proportionate currents(standardized at 5 amperes), for measurement and relay protection purposes. You might have seen an electrician's clamp meter, which is a device used to measure AC current; the "clamp" is a through-core current transformer.

A current transformer consists of a primary coil, a secondary coil, a core, insulating supports, and output terminals. The core of the current transformer is made of stacked silicon steel sheets, and its primary coil is connected in series with the main circuit, carrying the measured current T1, which generates an alternating magnetic flux in the core, inducing a corresponding secondary current T2 in the secondary coil. If excitation losses are ignored, then T1n1 = T2n2, where n1 and n2 are the turns of the primary and secondary coils, respectively. The transformation ratio of the current transformer K=T1/T2=n2/n1.

Since the primary coil of a current transformer is connected in the main circuit, it must use insulating materials that match the voltage level of the primary circuit for insulation to ensure the safety of the secondary circuit and personnel. The secondary circuit is composed of the secondary coil of the current transformer, instruments, and the current coils of relays connected in series. Current transformers can be roughly divided into two types: measuring current transformers and protection current transformers.

Current Transformer operates based on the principle of electromagnetic induction. Its primary winding often carries the entire current of the line. When the current transformer is in operation, its secondary circuit is always closed, thus the impedance of the series coil of the measuring instruments and protection circuit is very small, making the operating state of the current transformer close to a short circuit.

Characteristics of Current Transformers

A so-called current transformer typically comprises a closed iron core and windings, based on the principle of electromagnetic induction. The primary side of the winding has relatively few turns and is required to be connected in series with the current line, allowing all the current to pass through the primary winding. The secondary winding has more turns and, when connected in series with protection circuits and measuring instruments, forms a closed loop in the secondary circuit.

Hence, one can observe that the impedance of the series coil in measuring instruments and protection circuits is low, similar to the short-circuit operation of a transformer. Essentially, high current on the primary side is converted to low current on the secondary side using the current transformer, which allows for adequate utilization, prohibiting any open circuit on the secondary side. Typically, the secondary side of current transformers has a rated current of 5A or 1A. Since an open circuit on the secondary side can generate high voltage, directly impacting the operating personnel and equipment, the secondary circuit of an operational current transformer must not be open-circuited.

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