Current Transformer

A current transformer (CT) is a type of transformer that is used to reduce or multiply an alternating current (AC). It produces a current in its secondary which is proportional to the current in its primary


Current transformers, along with voltage or potential transformers, are instrument transformers. 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. The instrument transformers isolate measurement or protection circuits from the high voltage of the primary system. A current transformer provides a secondary current that is accurately proportional to the current flowing in its primary. The current transformer presents a negligible load to the primary circuit.[1]

Current 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

A current transformer has a primary winding, a core, and a secondary winding, although some transformers, use an air core. 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. A current transformer is designed to maintain an accurate ratio between the currents in its primary and secondary circuits over a defined range.

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 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

Current transformer 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 without disconnecting it first. Split-core current transformers are typically used in low current measuring instruments, often portable, battery-operated, and hand-held (see illustration lower right).

Current transformers are used extensively for measuring current and monitoring the operation of the power grid. Along with voltage leads, revenue-grade CTs drive the electrical utility's watt-hour meter on many larger commercial and industrial supplies.

High-voltage current transformers are mounted on porcelain or polymer insulators to isolate them from ground. Some CT configurations slip around the bushing of a high-voltage transformer or circuit breaker, which automatically centers the conductor inside the CT window.

Current transformers can be mounted on the low voltage or high voltage leads of a power transformer. Sometimes a section of a bus bar can be removed to replace a current transformer.

Often, multiple CTs are installed as a "stack" for various uses. For example, protection devices and revenue metering may use separate CTs to provide isolation between metering and protection circuits and allows current transformers with different characteristics (accuracy, overload performance) to be used for the devices.

In the United States, the National Electrical Code (NEC) requires residual current devices in commercial and residential electrical systems to protect outlets installed in "wet" locations such as kitchens and bathrooms, as well as weatherproof outlets installed outdoors. Such devices, most commonly ground fault circuit interrupters (GFCIs), typically run both the 120-volt energized conductor and the neutral return conductor through a current transformer, with the secondary coil connected to a trip device.

Under normal conditions, the current in the two circuit wires will be equal and flow in opposite directions, resulting in zero net current through the CT and no current in the secondary coil. If the supply current is redirected downstream into the third (ground) circuit conductor (e.g., if the grounded metallic case of a power tool contacts a 120-volt conductor), or into earth ground (e.g., if a person contacts a 120-volt conductor), the neutral return current will be less than the supply current, resulting in a positive net current flow through the CT. This net current flow will induce current in the secondary coil, which will cause the trip device to operate and de-energize the circuit - typically within 0.2 seconds.[citation needed]

The burden (load) impedance should not exceed the specified maximum value to avoid the secondary voltage exceeding the limits for the current transformer. The primary current rating of a current transformer should not be exceeded, or the core may enter its non-linear region and ultimately saturate. This would occur near the end of the first half of each half (positive and negative) of the AC sine wave in the primary and compromise accuracy