What is an electronic current transformer?

The principle of the current transformer is based on the principle of electromagnetic induction. A current transformer is made up of a closed core and windings. Its primary side winding turns are very small, stringed in the line of the current that needs to be measured, so it often has all the current of the line flowing through, the number of turns of the secondary side winding is more, connected in series in the measuring instrument and the protection loop, when the current transformer is working, its secondary side loop is always closed, so the impedance of the series coil of the measuring instrument and the protection loop is very small, and the working state of the current transformer is close to short circuit.

The current transformer is used to convert the large current on the primary side into a small current on the secondary side, and the secondary side cannot be opened.

Electronic current transformers can be divided into the following categories:

1) Optical current transformer. It refers to the use of optical devices as the measured current sensor, and the optical devices are composed of optical glass, all-optical fiber, etc. The optical fiber used in the transmission system is proportional to the size of the measured current.

The physical characteristics of the light wave modulated by the measured current can be divided into intensity modulation, wavelength modulation, phase modulation and polarization modulation.

2) Air-core coil current transformer. Also known as Rogowski coil current transformer. The hollow coil is often made of enameled wire evenly wound on the annular skeleton, and the skeleton is made of plastic, ceramic and other non-ferromagnetic materials, and its relative permeability is the same as the relative permeability of air, which is a significant feature of the hollow coil that is different from the current transformer with an iron core.

3) Core coil type low power current transformer (LPCT). It is a development of the traditional electromagnetic current transformer. It is designed according to the high impedance resistor, under the very high primary current, the saturation characteristics are improved, the measurement range is expanded, the power consumption is reduced, and the overcurrent and total offset short-circuit current can be measured with high accuracy without saturation, and the measurement and protection can share a core coil type low-power current transformer, and its output is a voltage signal.

The electronic sensor outputs an analog quantity. But it can be converted into a digital quantity. Zhihao Electric DOM-200 digital output module is to convert the small signal of the electronic sensor (PT CT) (e.g

Converted to the Manchester encoding format and compliant with the FT3 protocol in accordance with IEC 60870-5-1, and output to the feeder terminal FTU in accordance with the EIA RS-485 differential interface specification, the conversion module is built into the switch for use with electronic sensors. It solves the problems that the small signal transmission of traditional electronic sensors is easy to be interfered with and affects the stability and accuracy.

The IEC60044-8 Electronic Current Transformer Standard defines an electronic transformer as follows: a device consisting of one or more voltage or current sensors connected to a transmission system and a secondary converter to transmit a quantity proportional to the quantity to be measured, and to supply measuring instruments, meters, and relay protection or control devices. In the case of a digital interface, a group of electronic transformers share a single combined unit, which mainly consists of a voltage sensor and a current sensor.

Electronic transformers that are used alone to measure current are called electronic current transformers.

A current transformer is a device used to convert current from a higher value to a current proportional to a lower value. It converts high-voltage current into low-voltage current, allowing large currents to flow through the transmission line and is safely monitored by an ammeter.

Current transformers are used with AC instruments, meters, or control devices where the current to be measured is so large that it is not convenient to give the meter or meter coil sufficient current-carrying capacity. The current transformer is shown in the figure below.

The core of the current transformer is composed of silicon steel laminations. For high precision, permalloy or mumetal is used to manufacture cores. The primary windings of the current transformer carry the current to be measured and are connected to the main circuit.

The secondary windings of the transformer carry a current proportional to the current being measured and are connected to the current winding of the meter or instrument.

The primary and secondary windings are insulated from the core and insulated from each other. The primary winding is a single-turn winding (also known as a rod primary) and carries a full-load current. The secondary windings of a transformer have a large number of turns.

The working principle of a current transformer is slightly different from that of a power transformer. In a current transformer, the impedance or secondary load of the load is slightly different from that of the power transformer. As a result, the current transformer operates under secondary circuit conditions.

The current transformer is very similar to the transformer, the transformer is mainly used to change the voltage of the line, and the current transformer is mainly used to change the current of the line, so the current transformer was also called a converter in the past. Later, the instruments and equipment that turn direct current into alternating current are generally called converters, and the electrical appliances that change the current on the line are called current transformers according to the working principle of mutual inductance.

The line needs to change the current principle:

This is because according to the different situations of power generation and consumption, the current on the line is different, and the difference is huge, some are only a few amperes, and some are as large as tens of thousands of amperes. In order to directly measure these large and small currents, it is necessary to make many ammeters and other electrical meters that vary from a few amperes to tens of thousands of amperes, depending on the size of the line current.

This will make it extremely difficult to manufacture a liter of the instrument. In addition, some lines are high-voltage, such as 220,000 volts or 10,000 volts and other high-voltage transmission and power supply lines, to directly use electrical instruments to measure the current on the high-voltage line, which is extremely dangerous and absolutely not allowed.

If the transformer is connected to the road with electric defeat, then the large and small currents on the line can be unified into currents of similar size according to different proportions. As long as an electrical instrument with a current specification, such as a general electrical meter with a current of 5A, you can measure the current on the line as small as a few amperes and as large as tens of thousands of amperes through the current transformer.

At the same time, the basic structure of the current transformer is very similar to the transformer, it also has two windings, one is called the primary winding or the primary winding; One is called the secondary winding or secondary winding. There is insulation between the two windings, so that there is electrical isolation between the two windings. When the current transformer is running, the primary winding W1 is connected to the first road, and the secondary winding W2 is connected to the electrical instrument, so when measuring the current on the high-voltage line, although the primary side voltage is very high, the secondary side voltage is very low, and the operator and the instrument are very safe.