Current transformer, hereinafter referred to as TA, is an instrument that converts a large current on the primary side into a small current on the secondary side based on the principle of electromagnetic induction for measurement. A current transformer consists of a closed iron core and windings. Its primary side winding has few turns and is connected in series in the line of the current that needs to be measured. Therefore, all the current of the line often flows, and the number of turns of the secondary winding is relatively large, which is connected in series in the measuring instrument and the protection circuit. When the current transformer is working, its secondary circuit is always closed, so the impedance of the series coil of the measuring instrument and the protection circuit is very small, and the working state of the current transformer is close to a short circuit. The current transformer is to convert the large current on the primary side into a small current on the secondary side for measurement, and the secondary side cannot be opened.
Similar to voltage transformers, current transformers can also be classified in different dimensions.
1. According to the application
(1) Measuring current transformer (or measuring winding of current transformer): Provide current information of power grid to measuring, metering and other devices within the normal working current range.
(2) The current transformer for protection (or the protection winding of the current transformer): in the state of power grid fault, it provides power grid fault current information to relay protection and other devices.
2. Classification by insulating medium
(1) Dry-type current transformer: It is made of ordinary insulating material and treated with varnish as insulation.
(2) Casting current transformer: a current transformer cast with epoxy resin or other resin mixed materials.
(3) Oil-immersed current transformer: It is insulated by insulating paper and insulating oil, and is generally an outdoor type.
(4) Gas-insulated current transformer: The main insulation is composed of gas.
3. Classification by installation method
(1) Through-type current transformers: current transformers used to pass through panels or walls.
(2) Pillar-type current transformer: installed on a plane or pillar, and also used as a current transformer for the conductor pillar of the primary circuit.
(3) Bushing-type current transformer: a current transformer that has no primary conductor and primary insulation, and is directly sheathed on the insulated bushing.
(4) Bus-type current transformer: a current transformer that has no primary conductor but has primary insulation and is directly fitted on the busbar.
4. Classification by principle
(1) Electromagnetic current transformer: a current transformer that realizes current transformation according to the principle of electromagnetic induction.
(2) Electronic current transformer:
1) Beixue current transformer refers to the use of Beixue devices as the current sensor under test, and the optical device is composed of optical glass, all-fiber, etc.
2) Air core coil current transformer, also known as Rogowaki coil current transformer. Air-core coils are often made of enameled wires evenly wound on a ring-shaped skeleton. The skeleton is made of non-ferromagnetic materials such as plastics and ceramics. Its relative permeability is the same as that of air. This is the difference between air-core coils and iron cores A distinctive feature of the current transformer.
3) Iron-core wire-measured low-power current transformers are a development of traditional electromagnetic current transformers. They are designed according to high-impedance resistances. Under very high primary currents, the saturation characteristics are improved and the measurement range is expanded. Power consumption is reduced, overcurrent up to short-circuit current, full-offset short-circuit current can be measured with high accuracy without saturation, measurement and protection can share an iron-core coil type low-power current transformer, and its output is a voltage signal.
