Function and Basic Structure of Current Transformer_ Parameters and Working Principle of Current Transformer

Current transformer is an instrument that converts a large current at the primary side into a small current at the secondary side according to the principle of electromagnetic induction. Current transformer is composed of closed core and winding. Its primary winding has few turns and is connected in series in the line of current to be measured.

Therefore, it often flows all the current of the line, and the number of turns of the secondary side winding is relatively large. It is connected in series in the measuring instrument and protection circuit. When the current transformer is working, its secondary side circuit is always closed. Therefore, the impedance of the series coil of the measuring instrument and protection circuit is very small, and the working state of the current transformer is close to short circuit. The current transformer converts the large current at the primary side into the small current at the secondary side for measurement, and the secondary side shall not be open circuit. The entry introduces its working principle, parameter description, classification, application introduction, etc.

Structure composition of current transformer

The current transformer is composed of primary coil, secondary coil, iron core, insulation support and outgoing terminal. The iron core of the current transformer is made of silicon steel sheets. Its primary coil is connected in series with the main circuit, and through the measured current I1, it generates alternating magnetic flux in the iron core to make the secondary coil induce the corresponding secondary current I2. If the excitation loss is ignored, i1n1 = i2n2, where N1 and N2 are the turns of primary and secondary coils respectively. Conversion ratio of current transformer k = I1 / I2 = N2 / N1.

Since the primary coil of the current transformer is connected in the main circuit, the insulation material corresponding to the primary line voltage must be adopted for the ground of the primary coil to ensure the safety of the secondary circuit and personnel. The secondary circuit is composed of the secondary coil of the current transformer, the current coil of the instrument and the relay in series. Current transformers can be roughly divided into two categories: current transformers for measurement and current transformers for protection.

Working principle of current transformer

The principle of current transformer is based on the principle of electromagnetic induction. Its primary winding often flows through all the current of the line. When the current transformer is working, its secondary circuit is always closed. Therefore, the impedance of the series coil of the measuring instrument and protection circuit is very small, and the working state of the current transformer is close to short circuit.

In an ideal current transformer, if the no-load current I0 = 0, the total magnetomotive force i0n0 = 0. According to the law of energy conservation, the magnetomotive force of the primary winding is equal to that of the secondary winding, i.e

1NI=-2N2

That is, the current of the current transformer is inversely proportional to its turns. The ratio of primary current to secondary current 1 / 2 is called the current ratio of the current transformer. When the secondary current is known, the primary current can be obtained by multiplying the current ratio. At this time, the phasor of the secondary current is 1800 different from that of the primary current.

Main parameters of current transformer

1. Rated current transformation ratio

Rated current transformation ratio refers to the ratio of primary rated current to secondary rated current (sometimes referred to as current ratio). The rated current ratio is generally expressed as an irreducible fraction. If the primary rated current i1e and secondary rated current I2E are 100 and 5A respectively, then

The so-called rated current means that under this current, the transformer can operate for a long time without being damaged by heating. When the load current exceeds the rated current, it is called overload. If the transformer operates under overload for a long time, its winding will be burned out or the service life of insulating materials will be shortened.

2. Accuracy level

Due to the certain error of current transformer, the accuracy grade of current transformer is divided according to the allowable error of current transformer. The Accuracy grades of domestic current transformers are 0.01, 0.02, 0.05, 0.1, 0.2.0.5, 1.0, 3.0, 5.0, 0.2S and 0.5s.

Current transformers above level 0.1 are mainly used for precision measurement in the laboratory, or used as a standard to test low-level transformers, or can be matched with standard instruments to test instruments, so they are also called standard current transformers. The user's electric energy metering device usually adopts 0.2 and 0.5 current transformers. For some special requirements (it is hoped that the electric energy meter can make accurate measurement under a certain current between 0.05 6a, i.e. 1% 120% of the rated current 5A), 0.2S and 0.5s current transformers can be used.

3. Rated capacity

The rated capacity of the current transformer is the apparent power S2E consumed when the rated secondary current I2E passes through the secondary rated load z2e, so

Generally, I2E = 5a, therefore, S2E = 52z2e = 25z2e, and the rated capacity can also be expressed by the rated load impedance z2e.

When the current transformer is in use, the total impedance of the secondary connecting wire and instrument current coil shall not exceed the rated capacity specified on the nameplate and shall not be less than 1 / 4 of the rated capacity, so as to ensure its accuracy. The rated secondary load calibrated on the nameplate of the manufacturer is usually expressed by the rated capacity, and its output standard values include 2.5, 5, 10, 15, 25, 30, 50, 60, 80, 100V · a, etc.

4. Rated voltage

The rated voltage of current transformer refers to the maximum voltage (effective value) that the primary winding can withstand to the ground for a long time. It only shows the insulation strength of current transformer, and has nothing to do with the rated capacity of current transformer. It is marked after the current transformer model. For example, lcw-35, where "35" refers to the rated voltage, which is in kV.

5. Polarity mark

In order to ensure the correct wiring of measurement and calibration, the terminals of primary and secondary windings of current transformer shall be marked with polarity marks.

(1) The first end of the primary winding is marked L1 and the end is marked L2. When the multi limit primary winding has a tap, the head end is marked as L1, and it is marked as L2, L3 from the first tap.

(2) The first end of the secondary winding is marked K1 and the end is marked K2. When the secondary winding has an intermediate tap, the head end is marked as K1, and the following marks are K2, K3 from the first tap.

(3) For current transformers with multiple secondary windings, numbers shall be added before the outgoing end mark "K" of each secondary winding, such as 1k1, 1k2, 1k3 ; 2K12K22K3

(4) The symbols shall be arranged so that when the primary current flows from L1 to L2, the secondary current flows out of K1 and back to K2 through the external circuit.

From the perspective of the same polarity terminals of the primary winding and secondary winding of the current transformer, the directions of currents I1 and I2 are opposite. Such a polarity relationship is called depolarization and vice versa. Current transformers are generally represented by de polarity.

Function of current transformer

1. One of the functions of current transformer is to measure, which is often used for billing or measuring the current of equipment in operation. When measuring large alternating current, in order to facilitate instrument measurement and reduce the risk of direct measurement of high-voltage power, it is often necessary to use current transformer to convert it into a more unified current. Here, current transformer plays the role of current conversion and electrical isolation. The current transformer converts the high current into low current in proportion as required. When measuring, connect the primary side of the current transformer to the primary system and the secondary side to the measuring instrument or relay protection device.

2. The second function of current transformer is for protection, which is often used in conjunction with relay devices. When there is a fault such as short circuit or overload, the current transformer sends a signal to the relay device to cut off the fault circuit, so as to achieve the purpose of protecting the safety of power supply system. The current transformer for protection is different from the current transformer for measurement. It can work effectively only when the current is several times or dozens of times larger than the normal current, and it requires reliable insulation, large enough accurate limit coefficient, sufficient thermal stability and dynamic stability.