professional manufacturer of current sense transformer in China

FKS CoilS is a world-leading provider of electronic solutions for the smart society, headquartered in Xi'an, China. FKS CoilS' comprehensive, innovation-driven product portfolio includes passive components such as transformers, inductors, common mode chokes, air core coils and current transformers. FKS CoilS focuses on R&D, manufacturing and sales of high-quality magnetic components. Today we will take you to take a closer look at the working principle and main technologies of current sense transformers.

The principle of current sense transformer is based on the principle of electromagnetic induction. A current transformer consists of a closed core and windings. Its primary winding has very few turns, and it is connected in series in the line of the current to be measured, so it often has all the current of the line flowing, and the secondary winding has a large number of turns, connected in series in the measuring instrument and the protection circuit, and the current mutual inductance When the 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.

Working Principle

In the lines of power generation, power transformation, power transmission, power distribution and power consumption, the current size varies greatly, ranging from a few amps to tens of thousands of amps. In order to facilitate measurement, protection and control, it needs to be converted into a relatively uniform current. In addition, the voltage on the line is generally relatively high, so direct measurement is very dangerous. The current transformer plays the role of current conversion and electrical isolation.

For the pointer type ammeter, the secondary current of the current sense transformer is mostly ampere level (such as 5A, etc.). For digital instruments, the sampled signal is generally milliamp level (0-5V, 4-20mA, etc.). The secondary current of the miniature current transformer is milliamp level, which mainly acts as a bridge between the large transformer and the sampling.

Miniature current sense transformers are also called "instrument current transformers". ("Instrument current transformer" has a meaning of multi-current ratio precision current transformer used in the laboratory, which is generally used to expand the measuring range of the instrument.)

Similar to the transformer, the current transformer also works according to the principle of electromagnetic induction. The transformer transforms the voltage while the current transformer transforms the current. The winding of the current transformer connected to the measured current (the number of turns is N1) is called the primary winding (or primary winding, primary winding); the winding connected to the measuring instrument (the number of turns is N2) is called the secondary winding (or secondary winding) winding, secondary winding).

The current ratio of the current transformer primary winding current I1 to the secondary winding I2 is called the actual current ratio K. The current ratio of the current transformer when it works at the rated current is called the rated current ratio of the current transformer, expressed in Kn.

Kn=I1n/I2n

The function of the current transformer (CT for short) is to convert the primary current with a large value into a secondary current with a small value through a certain transformation ratio, which is used for protection, measurement and other purposes. For example, a current transformer with a transformation ratio of 400/5 can convert the actual 400A current into a 5A current.

Parameter Description

1. Current sense transformer model [3]:

First Letter: L—Current Transformer

Second letter: A—wall-through type; Z—pillar type; M—busbar type; D—single-turn through type; V—inverted structure; J—zero sequence

For ground detection; W—anti-pollution; R—winding exposed

The third letter: Z—epoxy resin pouring type; C—porcelain insulation; Q—gas insulation medium; W—special for computer protection

The fourth letter: B—with protection level; C—differential protection; D—D level; Q—enhanced type; J—enhanced type ZG

The fifth number: voltage level product serial number

2. Main technical requirements

2.1 Rated capacity: the apparent power consumed when the rated secondary current passes through the secondary rated load. The rated capacity can be expressed by the apparent power V.A, or by the secondary rated load impedance Ω.

2.2 Primary rated current: The electrical load current allowed to pass through the primary winding of the current transformer. The primary rated current of the current transformer used in the power system is 5-25000A, and the precision current transformer used in the test equipment is 0.1-50000A. The current transformer can run for a long time under the primary rated current. When the load current exceeds the rated current value, it is called overload. The long-term overload operation of the current transformer will burn out the winding or reduce the service life.

2.3 Secondary rated current: The primary induced current allowed to pass through the secondary winding of the current transformer.

2.4 Rated current ratio (transformation ratio): The ratio of the primary rated current to the secondary rated current.

2.5 Rated voltage: The maximum voltage that the primary winding can withstand to the ground for a long time (the effective value is in kV), which should not be lower than the rated phase voltage of the connected line. The rated voltage of the current transformer is divided into several voltage levels such as 0.5, 3, 6, 10, 35, 110, 220, 330, 500kV and so on.

2.6 10% multiple: Under the specified secondary load and any power factor, when the current error of the current transformer is -10%, the multiple of the primary current to its rated value. The 10% multiple is a technical index related to relay protection.

2.7 Accuracy grade: Indicates the grade of the transformer's own error (ratio difference and angle difference). The accuracy level of the current transformer is divided into various levels from 0.001 to 1, and the accuracy is greatly improved compared with the original one. The electrical instruments used on the power distribution control panels of power plants, substations, and power consumers generally adopt level 0.5 or 0.2; the relay protection used for equipment and lines is generally not lower than level 1; Measuring load capacity or power consumption is selected according to the requirements of the regulations (see the first lecture).

2.8 Ratio difference: The error of the transformer includes two parts: ratio difference and angle difference. The ratio error is referred to as the ratio difference, generally represented by the symbol f, which is equal to the difference between the actual secondary current and the primary current converted to the secondary side, and the ratio of the primary current converted to the secondary side, expressed as a percentage.

2.9 Angle difference: The phase angle error is referred to as the angle difference, generally expressed by the symbol δ, which is the phase difference between the secondary current vector and the primary current vector after rotating 180°. It is stipulated that the secondary current vector is ahead of the primary current vector δ as a positive value, otherwise it is a negative value, and the unit of calculation is minutes (').

2.10 Thermal stability and dynamic stability multiple: When the power system fails, the current transformer is subjected to the thermal effect and electrodynamic force of the huge current caused by the short-circuit current. The current transformer should have the ability to withstand without being damaged. This ability to withstand Expressed by thermal stability and dynamic stability multiples. The thermal stability multiple refers to the ratio of the current that does not cause the heat of the current transformer to exceed the allowable limit within 1s of the thermal stability current to the rated current of the current transformer. The dynamic stability multiple is the ratio of the maximum current instantaneous value that a current transformer can withstand to its rated current.