Function and Basic Operation of Current Transformers
– Current transformers are used to reduce or multiply alternating current (AC).
– They produce a current in the secondary that is proportional to the current in the primary.
– Current transformers are instrument transformers that scale voltage or current to standardized values for measuring instruments and protective relays.
– They isolate measurement or protection circuits from high voltage.
– Current transformers provide a secondary current accurately proportional to the primary current.
– Current transformers have a primary winding, a core, and a secondary winding.
– The primary circuit is largely unaffected by the insertion of the current transformer.
– Close coupling between the primary and secondary ensures accurate current measurement.
– The secondary current is the primary current divided by the number of turns on the secondary.
– Current transformers typically consist of a silicon steel ring core wound with copper wire.
Specifications and Use of Current Transformers
– Current transformers are specified by their current ratio from primary to secondary.
– The rated secondary current is usually standardized at 1 or 5 amperes.
– CTs can have taps to provide a range of ratios.
– The secondary impedance and voltage can be calculated based on the current ratio.
– Different shapes and sizes of current transformers are available for different applications.
– Current transformers are used in digital clamp meters for measuring AC.
– They are extensively used for measuring current and monitoring the power grid.
– CTs are mounted on porcelain or polymer insulators for high-voltage applications.
– They can be mounted on low voltage or high voltage leads of power transformers.
– Multiple CTs can be installed as a stack for various uses, such as protection and metering.
Safety and Accuracy of Current Transformers
– Current transformers should not be disconnected from their burden while current is in the primary, as it may generate high voltages.
– Technical standards and design practices ensure the safety of installations using current transformers.
– The burden impedance should not exceed the specified maximum value to avoid exceeding the secondary voltage limits.
– The primary current rating should not be exceeded to prevent core saturation and accuracy compromise.
– Current transformers play a role in safety devices like ground fault circuit interrupters (GFCIs) to protect against electrical hazards.
– Factors affecting the accuracy of a CT include burden, burden class/saturation class, rating factor, load, and external electromagnetic fields.
– Accuracy classes for various types of measurement and standard loads are defined in IEC 61869-1 as classes 0.1, 0.2s, 0.2, 0.5, 0.5s, 1, and 3.
– The ratio error of a Class1 CT is 1% at rated current, while the ratio error of a Class0.5 CT is 0.5% or less.
– CTs used for protective relaying have accuracy requirements at overload currents to ensure accurate performance during system faults.
Construction and Special Types of Current Transformers
– Bar-type CTs have terminals for source and load connections of the primary circuit and provide insulation between the primary circuit and ground.
– Ring-type CTs are installed over a bus bar or insulated cable and have low insulation on the secondary coil.
– Split ring-type CTs can be slipped over a cable without disconnecting it.
– Split-core CTs have a laminated iron core and a hinged section for installation over the cable.
– High-voltage CTs may have multiple taps on their secondary windings for different ratios and protective circuits.
– Wideband CTs are used to measure high-frequency or pulsed currents within pulsed power systems.
– The Rogowski coil uses the inductive effect and requires an external integrator to provide a voltage output proportional to the measured current.
– Specially constructed wideband CTs are used with an oscilloscope to measure waveforms.
High Voltage Types and Applications of Current Transformers
– Current transformers are used for protection, measurement, and control in high-voltage electrical substations and the electrical grid.
– Current transformers may be installed inside switchgear or in apparatus bushings.
– Free-standing outdoor current transformers are often used in switchyards.
– Live tank current transformers have a substantial part of their enclosure energized at the line voltage and must be mounted on insulators.
– Dead tank current transformers isolate the measured circuit from the enclosure.
– Current transformers are used for metering circuits, control, or protection purposes.
– Neutral current transformers are used for earth fault protection to measure fault current flowing through the neutral line.
– High-voltage current transformers may contain several cores, each with a secondary winding, for different purposes.
– Live tank current transformers have a short primary conductor, providing better stability and a higher short-circuit current rating.
– The primary winding can be evenly distributed around the magnetic core, improving performance for overloads and transients.
– The major insulation of a live-tank current transformer is not exposed to the heat of the primary conductors, improving insulation life and thermal stability.
– Live tank CTs are mounted on insulators due to a substantial part of their enclosure being energized at the line voltage. Source: https://en.wikipedia.org/wiki/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.
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.
