How does the China Current Sense Transformer supplier differentiate this product

According to different purposes, current sense transformers can be roughly divided into two categories:

Current sense transformer for measurement (or measurement winding of current transformer): within the normal operating current range, it provides current information of the power grid to measurement, metering and other devices.

Protection current sense transformer (or protection winding of current sense transformer): In the state of grid fault, it provides grid fault current information to relay protection and other devices.

1. Current sense transformer for measurement

When measuring the large current of alternating current, in order to facilitate the secondary instrument measurement, it needs to be converted into a relatively uniform current (China stipulates that the secondary rating of the current transformer is 5A or 1A), and the voltage on the line is relatively high, such as direct measurement is very dangerous. The current transformer plays the role of current conversion and electrical isolation. It is a sensor for secondary equipment such as measuring instruments and relay protection in the power system to obtain electrical primary circuit current information. The current transformer converts high current into low current in proportion. The primary side of the current transformer is connected to the primary system, and the secondary side connected to measuring instruments, relay protection, etc.

During normal operation, the secondary side of the transformer is in an approximate short-circuit state, and the output voltage is very low. During operation, if the secondary winding is open or the primary winding flows abnormal current (such as lightning current, resonant overcurrent, capacitor charging current, inductive starting current, etc.), thousands of volts or even tens of thousands of volts of overvoltage will be generated on the secondary side. . This will not only cause damage to the insulation of the secondary system, but also cause the transformer to be overexcited and burn out, and even endanger the life safety of the operating personnel.

There are only 1 to several turns on the primary side, and the cross-sectional area of the wire is large, and it is connected in series with the circuit under test. The secondary side has a large number of turns and a thin wire, which forms a closed circuit with a meter with a small impedance (the current coil of the ammeter/power meter).

The operation of the current transformer is equivalent to a transformer with a short circuit on the secondary side, ignoring the excitation current, and the number of ampere turns is equal I1N1=I2N2

The current ratio of the primary winding current I1 of the current transformer to the secondary winding I2 is called the actual current ratio I1/I2=N2/N1=k.

Excitation current is a major source of error.

The accuracy grade of the current transformer for measurement is 0.2/0.5/1/3, and 1 means that the transformation ratio error does not exceed ±1%, and there are also 0.2S and 0.5S grades.

2. Current sense transformer for protection

Current transformers for protection are divided into: 1. Overload protection current transformer, 2. Differential protection current transformer, 3. Grounding protection current transformer (zero sequence current transformer)

The protective current transformer mainly cooperates with the relay device to provide a signal to the relay device to cut off the fault circuit when a fault such as short circuit or overload occurs in the line to protect the safety of the power supply system. The working conditions of the protective current transformer are completely different from those of the measuring current transformer. The protective transformer only starts to work effectively when the current is several times or tens of times larger than the normal current. Main requirements for protective transformers: 1. Reliable insulation, 2. Large enough accurate limit coefficient, 3. Sufficient thermal stability and dynamic stability.

The maximum primary current of the protective transformer that can meet the requirements of the accuracy level under the rated load is called the rated accuracy limit primary current. The accurate limit coefficient is the ratio of the rated accurate limit primary current to the rated primary current. When the primary current is large enough, the iron core will be saturated and cannot reflect the primary current, and the accurate limit coefficient represents this characteristic. The accuracy grades of protective transformers are 5P and 10P, which means that the allowable current error is 1% and 3% at the rated accurate limit primary current, and the composite error is 5% and 10% respectively.

The surge current when the line fails produces heat and electromagnetic force, and the current transformer for protection must bear it. Under the condition of the secondary winding short-circuit, the effective value of the primary current that the current transformer can withstand without damage within one second is called the rated short-time thermal current. When the secondary winding is short-circuited, the peak value of the primary current that the current transformer can withstand without damage is called the rated dynamic stable current.

The accuracy grade of the protective current transformer is 5P/10P, and the composite error of 10P is not more than 10%.

Classified by insulating medium

1. Dry-type current transformer [4]: It is insulated by ordinary insulating materials dipped in varnish.

2. Casting type current transformer: a current transformer molded by casting epoxy resin or other resin mixture materials.

3. Oil-immersed current transformer: Insulated by insulating paper and insulating oil, generally outdoor type.

4. Gas-insulated current transformer: the main insulation is composed of gas.

Classified by installation method

1. Penetrating current transformer [5]: A current transformer used to pass through screen panels or walls.

2. Pillar current transformer: a current transformer installed on a plane or a pillar and also used as a primary circuit conductor pillar.

3. Bushing current transformer: a current transformer that is directly set on an insulating bushing without a primary conductor and primary insulation.

4: Bus-type current transformer: a current transformer that has no primary conductor but has primary insulation and is directly set on the bus.

Classified by principle

1. Electromagnetic current sense transformer: a current transformer that realizes current conversion based on the principle of electromagnetic induction.

2. Electronic current sense transformer:

For example :

(1) Optical current transformer refers to the use of optical devices as the current sensor to be measured, and the optical devices are composed of optical glass, all-fiber, etc.

(2) Air-core coil current transformer. Also known as Rogowski coil current transformer. Air-core coils are often made of enameled wire 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 different from air-core coils with iron cores. A distinguishing feature of current transformers.

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

The development direction of the use of the two

Due to the problems of easy saturation, nonlinearity and narrow frequency band of electromagnetic current transformers, electronic current transformers are gradually emerging. Electronic current transformers generally have the advantages of anti-magnetic saturation, low power consumption, and wide frequency band.

Domestic representative electronic transformers include AnyWay variable frequency voltage sensor, AnyWay variable frequency current sensor and AnyWay variable frequency power sensor. Among them, AnyWay variable frequency power sensor belongs to voltage and current combined transformer.

The main features of the transformer are as follows:

1. Adopt front-end digital technology, optical fiber transmission, good electromagnetic compatibility.

2. The amplitude-frequency characteristics and phase-frequency characteristics are good, and high measurement accuracy can be obtained in a wide range of amplitude, frequency and phase.

3. It is a digital sensor, the secondary instrument will not introduce errors, and the sensor error is the system error.