Current transformer

A current transformer (CT) is a type of transformer that is used to measure AC Current. It produce an alternating current (AC) in its secondary which is proportional to the AC current in its primary. Current transformers, together with voltage transformers (VTs) or potential transformers (PTs), which are designed for measurement, are known as an Instrument transformer.

The main tasks of instrument transformers are:

− To transform currents or voltages from a usually high value to a value easy to handle for relays and instruments.

− To insulate the metering circuit from the primary high voltage system.

− To provide possibilities of standardizing the instruments and relays to a few rated currents and voltages.

When the current to be measured is too high to measure directly or the system voltage of the circuit is too high, a current transformer can be used to provide an isolated lower current in its secondary which is proportional to the current in the primary circuit. The induced secondary current is then suitable for measuring instruments or processing in electronic equipment. Current transformers have very little effect on the primary circuit.

Current transformers are the current sensing units of the power system. The output of the current transformers are used in electronic equipment and are widely used for metering and protective relays in the electrical power industry.

Like any transformer, a current transformer has a primary winding, a core and a secondary winding, although some transformers, including current transformers, use an air core. In principle, the only difference between a current transformer and a voltage transformer (normal type) is that the former is fed with a 'constant' current while the latter is fed with a 'constant' voltage, where 'constant' has the strict circuit theory meaning.

The alternating current in the primary produces an alternating magnetic field in the core, which then induces an alternating current in the secondary. The primary circuit is largely unaffected by the insertion of the CT. Accurate current transformers need close coupling between the primary and secondary to ensure that the secondary current is proportional to the primary current over a wide current range. The current in the secondary is the current in the primary (assuming a single turn primary) divided by the number of turns of the secondary. In the illustration on the right, 'I' is the current in the primary, 'B' is the magnetic field, 'N' is the number of turns on the secondary, and 'A' is an AC ammeter.

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Wikipedia