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A current transformer (CT) is a type of transformer that is used to measure AC Current. It produces 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.
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.
Current transformers typically consist of a silicon steel ring core wound with many turns of copper wire as shown in the illustration to the right. The conductor carrying the primary current is passed through the ring. The CT's primary therefore consists of a single 'turn'. The primary 'winding' may be a permanent part of the current transformer, i.e. a heavy copper bar to carry current through the core. Window-type current transformers (aka zero sequence current transformers, or ZSCT) are also common, which can have circuit cables run through the middle of an opening in the core to provide a single-turn primary winding. To assist accuracy, the primary conductor should be centered in the aperture.
CTs are specified by their current ratio from primary to secondary. The rated secondary current is normally standardized at 1 or 5 amperes. For example, a 4000:5 CT secondary winding will supply an output current of 5 amperes when the primary winding current is 4000 amperes. This ratio can also be used to find the impedance or voltage on one side of the transformer, given the appropriate value at the other side. For the 4000:5 CT, the secondary impedance can be found as ZS = NZP = 800ZP, and the secondary voltage can be found as VS = NVP = 800VP. In some cases, the secondary impedance is referred to the primary side, and is found as ZS’ = N2ZP. Referring the impedance is done simply by multiplying initial secondary impedance value by the current ratio. The secondary winding of a CT can have taps to provide a range of ratios, five taps being common.
Shapes and sizes vary depending on the end user or switch gear manufacturer. Low-voltage single ratio metering current transformers are either a ring type or plastic molded case.
Split-core current transformers either have a two-part core or a core with a removable section. This allows the transformer to be placed around a conductor with minimum disturbance. Split-core current transformers are typically used in low current measuring instruments, often portable, battery-operated, and hand-held (see illustration lower right).
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