Ĭurrent transformer shapes and sizes vary depending on the end user or switch gear manufacturer. The secondary winding of a CT can have taps to provide a range of ratios, five taps being common. Referring the impedance is done simply by multiplying initial secondary impedance value by the current ratio. In some cases, the secondary impedance is referred to the primary side, and is found as Z S′ = N 2Z P. For the 4000:5 CT, the secondary impedance can be found as Z S = NZ P = 800Z P, and the secondary voltage can be found as V S = NV P = 800V P. 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 example, a 4000:5 CT secondary winding will supply an output current of 5 amperes when the primary winding current is 4000 amperes. The rated secondary current is normally standardized at 1 or 5 amperes. To assist accuracy, the primary conductor should be centered in the aperture.ĬTs are specified by their current ratio from primary to secondary. Window-type current transformers 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. a heavy copper bar to carry current through the core. The primary 'winding' may be a permanent part of the current transformer, i.e. The CT's primary, therefore, consists of a single 'turn'. The conductor carrying the primary current is passed through the ring. 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.Ĭurrent 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 current in the secondary is the current in the primary (assuming a single turn primary) divided by the number of turns of the secondary. 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 primary circuit is largely unaffected by the insertion of the CT. The alternating current in the primary produces an alternating magnetic field in the core, which then induces an alternating current in the secondary. A current transformer is designed to maintain an accurate ratio between the currents in its primary and secondary circuits over a defined range. While the physical principles are the same, the details of a "current" transformer compared with a "voltage" transformer will differ owing to different requirements of the application. Ĭurrent transformers are the current-sensing units of the power system and are used at generating stations, electrical substations, and in industrial and commercial electric power distribution.Ĭurrent transformers used in metering equipment for three-phase 400-ampere electricity supplyĪ current transformer has a primary winding, a core and a secondary winding, although some transformers, including current transformers, use an air core. The current transformer presents a negligible load to the primary circuit. A current transformer provides a secondary current that is accurately proportional to the current flowing in its primary. The instrument transformers isolate measurement or protection circuits from the high voltage of the primary system. Instrument transformers scale the large values of voltage or current to small, standardized values that are easy to handle for measuring instruments and protective relays.
It produces a current in its secondary which is proportional to the current in its primary.Ĭurrent transformers, along with voltage or potential transformers, are instrument transformers. A current transformer ( CT) is a type of transformer that is used to reduce or multiply an alternating current (AC).