Categorized by use
According to different uses, current transformers can be roughly divided into two categories:
Current transformer for measurement (or measurement winding of current transformer): provides current information of the power grid to measurement, metering and other devices within the normal working current range.
Protection current transformer (or protection winding of current transformer): In the case of power grid fault conditions, it provides grid fault current information to relay protection and other devices.
1. Current transformer for measurement
When measuring the large current of alternating current, in order to facilitate the measurement of the secondary instrument, 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 converter and galvanic isolation. It is a sensor for secondary equipment such as measuring instruments and relay protection in the power system to obtain current information of the electrical primary circuit, the current transformer converts the high current into a low current in proportion, the current transformer is connected to the primary system on the primary side, and the secondary side is connected to the measuring instrument and relay protection.
During normal operation, the secondary side of the transformer is in an approximate short-circuit state, and the output voltage is very low. In operation, if the secondary winding is open or the primary winding flows through abnormal currents (such as lightning current, resonant overcurrent, capacitor charging current, inductor starting current, etc.), an overvoltage of thousands or even tens of thousands of volts will be generated on the secondary side. This not only causes harm to the insulation of the secondary system, but also burns the transformer overpowering, and even endangers the life safety of the operating personnel.
There are only 1 to a few turns on the primary side, and the wire cross-sectional area is large, which is connected into the circuit under test. The secondary side has a large number of turns and a thin wire, which forms a closed circuit with a small impedance meter (current coil of an ammeter/power meter).
The operation of the current transformer is equivalent to that of a transformer with a short circuit on the secondary side, ignoring the excitation current, and the number of turns is equal I1N1=I2N2
The current ratio of the primary winding current I1 to the secondary winding I2 of the current transformer is called the actual current ratio I1/I2=N2/N1=k.
The excitation current is a major source of error.
The accuracy class of the current transformer for measurement is 0.2/0.5/1/3, 1 means that the ratio error does not exceed ±1%, and there are also 0.2S and 0.5S classes.
2. Current transformer for protection
Protection current transformers are divided into: 1. Overload protection current transformer, 2. Differential protection current transformer, 3. Grounding protection current transformer (zero sequence current transformer)
The current transformer for protection is mainly cooperated with the relay device, and when the line is faulted by short circuit and overload, the relay device is provided with a signal cut-off fault circuit to protect the safety of the power supply system. The working conditions of the protection current transformer are completely different from the measurement current transformer, and the protection transformer only starts to work effectively when the current is several times and dozens of times larger than the normal current. The main requirements of the transformer for protection are: 1. Reliable insulation, 2. Accurate limit coefficient large enough, 3. Sufficient thermal stability and dynamic stability.
The maximum primary current of the transformer for protection can meet the requirements of the accuracy level under the rated load, which is called the rated accurate limit primary current. The accurate limit factor is the ratio of the rated accurate limit primary current to the rated primary current. When the primary current is large enough, the core will be saturated and will not reflect the primary current, and the accurate limit coefficient is an indication of this characteristic. The accuracy level of the transformer for protection is 5P and 10P, which means that the allowable current error is 1% and 3% when the primary current is rated at the accurate limit value, and the composite error is 5% and 10% respectively
In the event of a line fault, the inrush current generates thermal and electromagnetic forces, which must be withstood by the current transformer for protection. In the case of short circuit of the secondary winding, 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. In the case of short circuit of the secondary winding, the peak of the primary current that the current transformer can withstand without damage is called the rated dynamic and stable current.
The accuracy level of the current transformer for protection is 5P/10P, and the composite error of 10P marking is not more than 10%.
Classified by insulating medium
1. Dry-type current transformer: It is made of ordinary insulating materials and is treated with impregnated paint as insulation.
2. Pouring current transformer: current transformer cast with epoxy resin or other resin mixed materials.
3. Oil-immersed current transformer: insulating paper and insulating oil are used as insulation, generally outdoor type.
4. Gas insulated current transformer: the main insulation is composed of gas.
Classified by installation method
1. Through-type current transformer: used to pass through the screen or wall.
2. Pillar type current transformer: installed on a flat surface or pillar, and used as a current transformer for the primary circuit conductor pillar.
3. Sleeve type current transformer: a kind of current transformer directly mounted on the insulated sleeve without a primary conductor and primary insulation.
4: Busbar current transformer: There is no primary conductor but has a primary insulation, and it is directly set on the bus for use as a current transformer.
Classified by principle
1. Electromagnetic current transformer: a current transformer that realizes current conversion according to the principle of electromagnetic induction.
2. Electronic current transformer:
For example, (1) Optical current transformer 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.
(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 resistance, at 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.
The direction of the development of the use of both
Due to the problems of easy saturation, nonlinearity and narrow frequency band, electronic current transformers are gradually emerging. Electronic current transformers generally have the advantages of magnetic saturation, low power consumption, and wide frequency band.
The representative electronic transformers in China include AnyWay variable frequency voltage sensor, AnyWay variable frequency current sensor and AnyWay variable frequency power sensor, among which, AnyWay variable frequency power sensor belongs to the combined voltage and current transformer.
The main features of this 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 belongs to the digital sensor, and the secondary instrument will not introduce error, and the sensor error is the system error.
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