The type current transformer

By Application
According to different purposes, the current transformer can be broadly divided into two categories:
Measuring Current Transformer (or current transformer measuring winding): In the normal operating current range, the measuring, metering and other devices to provide current information to the grid.
Protective current transformer (or current transformer winding protection): the grid fault condition, and other devices to provide power protection fault current information.
Measuring current transformer
Current transformer
In the measurement of large currents alternating current, in order to facilitate the secondary instrument measurements need to be converted to more uniform current (Chinese specified current transformer secondary is rated for 5A or 1A), another relatively high voltage on the line, such as direct measurement is very dangerous. Current transformer

Is to play the role of the current transformer and electrical isolation. It is the power system measurement instrumentation, electrical protection and other secondary devices to obtain information on a loop current sensors, current power transformers to convert high current proportionally low current, the current transformer primary side connected to a

Systems, measuring instruments connected to the secondary side, protection and so on.
Normal working hours transformer secondary side is similar to short-circuit condition, the output voltage is very low. In the operation of the secondary winding open or abnormal current flows through the primary winding (thundering current, over-current resonant capacitor charging current, the inductor current starts, etc.), will be in the secondary

Generate thousands of volts or even thousands of volts over voltage. This is not only harmful to the secondary system insulation, but also the radical and burning electric transformer , and even endanger the lives and safety of operating personnel.
Current transformer
1 side only one to a few turns, wire cross-sectional area, the string into the circuit under test. Number of turns on the secondary side, fine wire, and the smaller the impedance meter (meter / power meter current coil) constitute a closed circuit.
Equivalent to the operation of the current transformer secondary side of the transformer short circuit, ignoring the excitation current, ampere turns equal I1N1 = I2N2
Current transformer primary winding and the secondary winding current I1 I2 current ratio, called the actual current ratio I1/I2 = N2/N1 = k.
Magnetizing current is the main source of error.
Measuring current transformer accuracy class 0.2/0.5/1/3, 1 ratio error is less than ± 1%, in addition to 0.2S and 0.5S level.
Protective current transformer accuracy class 5P/10P, 10P labeled composite error does not exceed 10%.
Protective current transformer
Protective current transformer with primary and relay device, a short circuit in the line overload fault, the relay device provides a signal to cut off the fault current
Protective current transformer
Road to protect the security of the power supply system. Protective current transformer working conditions and measuring current transformer is completely different, protective current transformer is several times larger than the normal current of a few times when beginning work effectively. Main transformer protection requirements

: 1. Reliable insulation, 2. Sufficiently large accuracy limit factor, 3. Sufficient thermal stability and dynamic stability.
Protection of transformers at rated load level to meet the exact requirements of the maximum rated accuracy limit primary current is called primary current. Accuracy limit factor is rated accuracy limit primary current and the rated primary current ratio. When a current is large enough, since the core will saturate

Not reflect the role of the primary current, accuracy limit factor is expressed this feature. Precise level of protection transformer 5P, 10P, that a current at the rated accuracy limit the allowable current error of 1%, 3% of the composite error of 5%, 10%
Line failure inrush current to generate heat and electromagnetic force, protective current transformers must withstand. Secondary winding short-circuit conditions, the current transformer within one second can withstand without damage a current rms, nominal rated short-time thermal current. Secondary windings shorted

Case, a current transformer can withstand without damage a current peak, called the rated dynamic current.
Protective current transformer is divided into: 1. Overload protection current transformer, 2. Differential protection current transformer, 3 ground protection current transformer (zero sequence current transformer)

The use of current transformers

A) current transformer wiring should comply with the principles in series: the first winding in series with the circuit under test should be, while the secondary winding of the transformer load current with all instrument series
2) Press the measured current size, select the appropriate changes, otherwise the error will increase. Meanwhile, the secondary side must be grounded at one end to prevent the insulation being damaged, one side of the low pressure side of the high pressure fleeing into the secondary, resulting in personal and equipment accidents
3) Open the secondary side is absolutely not allowed, because once open, the primary current I1 all become magnetizing current, causing φm and E2 surge, causing excessive core saturation magnetization, severe fever and even burned coil; while over-saturation magnetization of the magnetic circuit after , the error increases.

In normal operation the current transformer, the secondary side is similar to a short circuit, it is suddenly open, the value is very small excitation force by the sudden change of the value of large value, the magnetic flux in the core presents a serious saturated flat wave, so the secondary winding of the magnetic induction through zero

A very high spire wave, its value can reach thousands or even tens of thousands of volts, endangering the safety of staff and instrument insulation.
In addition, the secondary side of the open circuit voltage of the secondary side of several hundred volts, when it comes to electric shock. Therefore, the current transformer secondary side switches are available in short, to prevent secondary side open. During use, the secondary side should immediately removed once the open-circuit load, then

, And then deal with power transformers outages. Everything handle before reuse.
4) In order to meet the measurement instrument, relay, breaker failure judgment and failure filtering device needs, generators, transformers, outlet, bus section breakers, circuit breakers, circuit breakers are located bypass 2 to 8 of the current transformer secondary winding

Makers.
5) For the installation of protective current transformer locations should try to eliminate the main protection device is not protected zone settings. For example: If two current transformers, and the location permit, should be located on both sides of the circuit breaker, the circuit breaker is in the scope of cross-protection among
6) In order to prevent pillar type current transformer bushing flashover caused by bus fault, current transformers are usually arranged in the circuit breaker or transformer side outlet
7) In order to reduce the damage to the generator internal fault, means for automatically adjusting the excitation current of transformer shall be arranged on the outlet side of the generator stator winding. In order to facilitate analysis and incorporated into the system before the discovery of the generator internal fault current transformer used for measuring instruments

Device should be installed in the generator neutral point side.

Current transformers technical parameters

First, the current transformer models:
First letter: L-current transformer
The second letter: A-through the wall; Z-pillar; M-bus type; D-type single turn throughout; V-structure inverted; J-zero sequence
Ground detection; W-anti-contamination; R-winding bare formula
The third letter: Z-epoxy mastic; C-ceramic insulation; Q-gas-insulated medium; W-specific and computer protection
The fourth letter: B-band protection level; C-differential protection; D-D level; Q-enhanced; J-reinforced ZG
Fifth figures: voltage level product serial number
Second, the main technical requirements
2.1 Rated capacity: Rated secondary current through the secondary rated load apparent power consumed. Rated capacity can be expressed by the apparent power VA, you can also use secondary rated load impedance Ω represents.
2.2 A rated current: the current transformer primary winding allows electricity load current. Current transformers for power transformers system once rated current 5 ~ 25000A, used test equipment, precision current transformer is 0.1 ~ 50000A. Current transformers in

A rated current in the long run, the load current exceeds the rated current value is called overload, overload current transformer long run, it will burn out the winding or reduced service life.
2.3 Secondary rated current: Allow a current transformer secondary winding induced current.
2.4 Rated current ratio (ratio): a rated current and the secondary rated current ratio.
2.5 Rated voltage: primary winding long on the ground to withstand the maximum voltage (in kV rms), shall not be less than the rated phase voltage connected to the line. Current transformer rated voltage divided 0.5,3,6,10,35,110,220,330,500 kV, several

Kind of voltage levels.
2.6 10% Multiples: secondary load at a specified and arbitrary power factor, the current transformer when the current error of -10%, a current rating of its multiples. 10% protection factor is related to technical indicators.
2.7 Accuracy Class: Indicates transformer itself, error (difference and angular than poor) grade. Current transformer accuracy class is divided into multiple levels of 0.001 to 1, compared with the original accuracy improved greatly. Used in power plants, substations, power distribution unit on the control panel

Electrical instrumentation generally use 0.5 or 0.2; used equipment, line protection is generally not less than 1; for energy measurement, depending on the load capacity or power consumption measured according to how many procedures required to choose.
2.8 ratio error: error includes transformer ratio error and phase error in two parts. Error ratio worse than short, generally expressed by the symbol f, which is equal to the actual secondary current and converted to the secondary side of the primary current difference, and converted to the secondary side of a current ratio to one hundred

Fraction.
2.9 angle difference: the phase angle difference angle error referred to generally by the symbol δ indicates that it is a 180 ° rotation of the secondary current vector with a phase difference between the current vector. Provides a secondary current vector ahead of the current vector δ is positive, otherwise it is negative, with the points ('

) For the unit.
2.10 thermal stability and dynamic stability factor: power system fault, the current transformer short-circuit currents caused by the huge current role of thermal and electric transformer  current transformers should be able to withstand without damage the ability of this ability to withstand heat

Stability and dynamic stability multiples. Thermal stability factor is the thermal stability current 1s-current transformer heat does not result in exceeding the allowable limit current and the rated current of the current transformer ratio. Dynamic stability factor is the current transformer can withstand the maximum current instantaneous value

Its rated current ratio.

Current transformer principle works

In power transformers generation, transformation, transmission, distribution and electricity lines in the current size of the gap, from a few to tens of thousands Ann Ann has. For ease of measurement, protection and control needs to be converted to more uniform current, the voltage on the other line higher than the general risk, such as direct measurement is

Insurance. Current transformer to play the role of current transformation and electrical isolation.
For Analog ammeter, current transformer secondary current most amp level (eg 5A, etc.). For digital instrument, the sampled signal is generally mA level (0-5V ,4-20mA, etc.). Micro current transformer secondary current of milliamps, mainly from the large

Transformer and a bridge between samples.
Micro current transformer was also known as "Instrument current transformers." ("Instrument current transformer" There is a layer of meaning is more current than laboratory use precision current transformer, generally used to expand the range of the instrument.)
Current transformers and transformer similar work is based on the principle of electromagnetic induction, the transformer transforms the voltage current transformer transforms the current Bale. electric transformer connected to the measured current winding (number of turns N1), called the primary winding (or primary winding, the primary

Winding); measuring instrument connected winding (number of turns N2) is called the secondary winding (or the secondary winding, secondary winding).
Current transformer primary winding and the secondary winding current I1 I2 current ratio, called the actual current ratio K. Current transformer at rated current work of the current ratio is called the current transformer rated current ratio, expressed by Kn.
Kn = I1n/I2n
Current Transformer (Current transformer referred CT) role is to put a value larger than the current through a certain variable values ??into smaller secondary current, used for protection, measurement and other purposes. If ratio of 400/5 current transformer, can be

To be effective for the conversion of 5A to 400A current.

Zero sequence current transformer action principle

Zero sequence current protection is generally suitable for use in TN grounding system. Because when occurs a phase to ground when the TN-S system Id loop impedance including the phase line impedance Z1, PE line impedance ZPE and contact impedance Zf, ie Zs = Z1 + ZPE + Zf; For TN-C system, Id loop impedance including the phase

Line impedance Z1, PEN line impedance ZPEN and contact resistance Zf, ie ZS = Z1 + ZPEN + Zf; For TN-CS system, Id loop impedance including the phase line impedance Z1, PEN line impedance ZPEN, PE line impedance Zf ZPE and contact resistance , the ZS = Z1 + ZPEN + ZPE + Zf, can generate a single-phase

Ground fault current Id = 220/ZS, no larger than a three-phase fault current imbalance, as long as the appropriate setting, you can detect a ground fault occurs zero sequence current in order to cut off the fault circuit. The IT systems are generally used for power supply reliability is higher

For single phase power supply Circuit breaker immediately cut unnecessary, but need to send insulation damage monitoring signal to maintain continued supply of industrial and mining enterprises for some time within the unworthiness of the neutral phase three-wire distribution lines. When single-phase ground when flowing through the fault line zero sequence

Current system-wide non-fault system capacitive currents, which can easily detect the ground fault current, it can be used zero-sequence current protection device to monitor the relative first ground fault. TT earthing system is often used in industry, agriculture, civil lighting, power supply mix

Three-phase four-wire power distribution system, often find the three-phase current imbalance greater when the event of a phase to ground, Id loop impedance including the phase line impedance Z1, PE line impedance ZPE, load side grounding resistors RA and power side grounding resistor RB, contact impedance Zf, ie

ZS = Z1 + ZPE + RA + RB + Zf, the earth fault current Id = 220/ZS, the RA + RB >> Z1 + ZPE + Zf, and RA + RB values ??are generally larger, it is clear ring system failure TT Road impedance of the resulting single-access fault current Id, far less than the current imbalance is difficult to check

Measure the fault current, it does not apply to TT earthing system.
For the zero-sequence current protection zero sequence CT installation process must comply with the relevant standards. For IT earthing system, as a result of single-phase ground fault, earth fault currents may not only along the cable back into the surface of the conductor, but also could not fault along the cable

The conductor surface flow back, it must be installed by the cable head to ground zero sequence CT, so as to ensure the faulted phase and non-fault phase current through the ground capacitance, which can prevent external fault protection device malfunction, but also to ensure failure device reliable action. For

IT earthing system, generally installed on the neutral line N zero sequence CT, on the low pressure side of the zero-sequence CT busbar must be installed in the neutral line N and the work of ground (or repeating ground) between the busbars. Such as zero-sequence CT installed in the N line distribution panel busbars, power distribution panel as gold

Metal enclosure generally associated directly with the ground electrode, when the bus is shorted to ground occurs, the resulting fault current Id of the metal shell along the distribution screen → → ground current electric transformer neutral point, without passing through the zero-sequence CT, reach The demands of the protection function, which is in

When the construction site is easy to neglect.

Zero sequence current transformer

Power industry used items. Zero sequence current transformer is single-turn feedthrough current transformers, and power protection devices for general use, such as: small current grounding line selection device, harmonic elimination equipment and other ancillary uses.
Outline
Zero sequence current transformer in the power transformers system to produce zero sequence ground current protection devices or signals with the use of the device element action to achieve protection or monitoring.
HS-LJK, HS-LXK series of zero-sequence current transformer is the cable type, using ABS plastic shell, resin pouring sake of sealing; using insulating oil cooling cutting process, effectively avoiding the transformer in the long-term use in the process of corrosion. Good insulation, beautiful appearance.

High sensitivity, good linearity, reliable operation, convenient installation. Its performance is superior to the general zero-sequence current transformer, using a wide range, not only for electromagnetic relay, but also apply to electronic and computer protection device. Users can according to the system

Operation mode, effectively earthed neutral or non-effectively grounded neutral point difference, the choice to adapt to zero-sequence current transformer.
Action
When the circuit breakers in an electric shock or leakage fault, the transformer secondary side output zero-sequence current, so that the connected devices on the secondary circuit protection action (cut off the power, alarms, etc.).
Zero-sequence protection principles
Principle Overview
Zero sequence current protection specific applications can be installed one each on the three-phase line current transformer (CT), or let the three phase wires together through a zero sequence CT, can also be installed on a neutral line N zero sequence CT, the use of These CT to detect the phase current vector and that the zero-sequence current

Io, IA + IB + IC = Io, when the line is connected to the three-phase load on the full balance (without ground fault, and does not consider the lines, electrical equipment leakage current), Io = 0; when the line is connected on the three phase load imbalance, then Io = IN, this time for the zero-sequence current unbalance

Current IN; when one phase ground fault occurs, inevitably produce a single-phase ground fault current Id, at this time the detected zero-sequence current IO = IN + Id, is a three-phase and single-phase unbalanced current vector sum of currents .

The difference between voltage transformer type

The most important difference is in the normal working state when operating on different:
1) during normal operation voltage transformer close to saturation flux density, magnetic flux density decreased when failure; current transformer flux density is low during normal operation, the short circuit breakers ue to the primary current becomes large, the magnetic flux density increased significantly, sometimes even as far as

Far exceeding the saturation value.
2) was used to measure voltage transformer special transformer high-voltage grid, it will be required proportion of high voltage is converted to a lower voltage, and then connect to the meter up measurements. Voltage transformer primary voltage either how many volts, while the secondary voltage are generally defined as

100 volts, the supply voltage meter, power transformers ,  and kilowatt hour meter relay coil voltage and voltage required.
3) could short-circuit the current transformer secondary, but not open; voltage transformer secondary can open, but not short-circuit. The large current is converted to the required proportion of low current electrical device called a current transformer. Current transformer secondary side current is generally defined as 5

Security or a security, in order to supply meter, power meter, kilowatt hour meter and relay current coil current.
4) for the second side of the load, the voltage transformer or a small internal resistance negligible, a large voltage transformer can be considered a voltage source; the current transformer has a great resistance, as well as can be infinite resistance that is a current source

Voltage transformer Precautions

Voltage transformer (PT) and current transformer (CT) is an important electrical power system equipment, which bears the high and low pressure systems and high voltage isolation between the amount of functions to convert low voltage. The wiring is correct or not, the system protection, measurement, monitoring and other equipment

Normal work is extremely important. In the new installation PT,power transformers commissioning or replacement PT, CT secondary cable, using the polarity of the test method test PT, CT wiring is correct, it is already an indispensable protection staff working procedures.
Avoid reverse polarity is to find the transformer input and output of the "dot end of the" specific method is the "point of polarity." This example shows how to point the current transformer polarity. The specific method is the multimeter connected to the output transformer secondary winding,

Multimeter hit the DC voltage profile; Then a battery of negative fixed at a current transformer output wires; reuse batteries positive to "point" of a current transformer input leads, so that the transformer primary mcb circuit breaker will generating a + (positive)

Pulse current; while observing pointer multimeter hands which direction "Offset", if the multimeter's hands shifted from left to right from 0, j namely hands "positive start" to show that you access the "current transformer input "and" Analog Multimeter positive terminal connection

A current transformer secondary output "is the same name as the end, and this connection is called a" positive polarity "or" minus polarity "; if the multimeter's hands shifted from right to left from 0, ie hands" anti-Kai "to show that you access" current transformer input "and" Pointer

Multimeter positive terminal connected to an output terminal of the current transformer secondary "is not the same name as the end, and this connection is called a" reverse polarity "or" plus polarity. "

Transformer power transmission system

In order to transmit electrical energy power systems, often using AC voltage, high current loop power was transferred to the user, the instrument can not be measured directly. Transformer, which is to the AC voltages and currents can be reduced in proportion to the value of direct measurement instrument to facilitate

Direct measurement instruments, Circuit breaker as well as protection and automatic device to provide power. Power system transformer is to network with high-voltage, high-current transmission of information to the low voltage, low current secondary metering, instrumentation and protection, a special variable automatic devices

Pressure device, is a liaison system and secondary system components, and its primary winding is connected to the grid, the secondary winding respectively measuring instruments, protective devices connected to each other. Transformers and measurement instrumentation and metering device with, a system can measure voltage, current and power

Energy; fit with relay protection and automatic devices that can pose a variety of faults on the grid electrical protection and automatic control. Transformer performance will directly affect the power system measurement,mcb circuit breaker  measurement accuracy and reliability of the relay protective devices.
Transformer voltage transformer and current transformer is divided into two categories, its main role: the primary system voltage, current, accurate information transmitted to the secondary side of the related equipment; would a system's high voltage, high current is converted into two The low voltage secondary side (standard value),

Small current (standard value), the measurement, measuring instruments and relays and other devices standardization, miniaturization, and reduces the need for secondary equipment insulation requirements; the secondary side of the device as well as the secondary system with a high-voltage equipment in the electrical aspects of the system are Good isolation, thus ensuring

The secondary equipment and personal safety

Transformer structure principle

General structure of the current transformer principle: simple structure of the current transformer, a primary winding insulated from each other, the secondary winding, core and frame, the housing, terminals and other components. Its working principle is basically the same transformer, circuit breakers the primary winding turns (N1) representing

Less directly connected in series in the power circuit, a load current () through a winding, the generated alternating magnetic flux induces a secondary current reduced proportionally (); number of turns of the secondary winding (N2) higher, and meters, relays, transmitters and other negative secondary current coil

Dutch (Z) in series to form a closed loop, because the primary winding and the secondary winding have equal ampere turns, I1N1 = I2N2, current electric transformer rated current ratio current transformer load impedance actual operation is very small, close to the secondary winding shorted state equivalent to a short-circuit

Operation of the transformer.
Feedthrough current transformer does not have its own structure of the primary winding, the carrier (load current) wires from the L1 to L2 through the rolling silicon steel coil made of round (or other shapes) from the core role of the primary winding. Secondary winding wound directly evenly on round cores, and instrumentation

, Relays, transmitters and other secondary load current coil in series to form a closed loop, since no feedthrough current transformer primary winding, the primary winding turns ratio based on the number of turns through the transformer iron hearts determined mandrel turns The more change than the smaller; contrary, wear

Heart turns less variable ratio greater than the rated current: where I1 - Peg one turn at a time rated current; n - mandrel turns.
Multi-tap current transformers. This type of current transformer primary winding unchanged during wound secondary windings, increasing the number of taps in order to obtain a plurality of different ratios. It has a fixed core and a number of turns of the primary winding, the secondary winding is insulated copper wire

Around the suit on the insulating core barrel, the different ratio of the secondary winding tap leads, then the wiring terminal block, each tap set their own terminals, thus forming a plurality of variable ratio, such current The advantage of transformer load current is changed according to

Ratio, secondary electrical wires swapped to change the ratio, without the need to replace the current transformer, to use more convenient.
Different current transformer turns ratio. This model has the same current transformer core and primary winding, and the number of turns of the secondary winding is divided into two different, separate windings, in order to satisfy the same load current ratio different circumstances, different levels of accuracy required

, For example, under the same load conditions, in order to ensure accurate measurement of energy required becomes relatively small number (to meet the load current in a rating of 2/3), the accuracy level higher (eg 1K1.1K2 of 200/5.0. 2); and protection of electrical equipment, test

Into account the large fault current protection factor, requires relatively large number of variables, the level of accuracy can be slightly lower (eg 2K1.2K2 as 300/5.1 level).
Adjustable primary winding, the secondary winding of the current transformer and more. This current transformer turns ratio range is characterized by numerous and can be changed, more common in high-voltage current transformer. Its primary winding is divided into two sections, respectively, through the transformer core, the secondary winding is divided into two with

Taps, the different level of accuracy independent windings. Transformer with a primary winding means connected to the outer connection piece, by changing the position of connection piece, that the primary windings connected in series or parallel connection is formed, thereby changing the number of turns of the primary winding in order to obtain a different ratio

. Tapped secondary winding is divided into two different ratios, their different level of accuracy winding, with a change in position of the winding connection piece, a corresponding change in number of turns, the ratio is changed, so that the formation a multi-span ratio. Tapped

Independent secondary windings with different ratios, and different level of accuracy, can be applied separately to energy measurement, indicating instruments, transmitters, relay, etc., in order to meet their different requirements.
Combined current and voltage transformers. Combined transformer by the current transformer and voltage transformer combination, multi installed in high-pressure metering boxes, cabinets, or used as a measurement of electrical equipment, power protection devices. Combined current and voltage transformers are the two

Or three current transformer primary and secondary windings and core and voltage transformer primary and secondary windings and iron core, steel body frame fixed on the transformer oil immersed fitted cabinets, first, the secondary winding outlet all leads, then in the box outside the high and low voltage porcelain

, The formation of an insulating, closed whole. Power line connecting the primary and secondary side protection devices and metering device or connection. Depending on the needs of current and voltage transformers combined into V / V wiring and Y / Y connection both, for the measurement of three-phase load balancing or

Imbalance of power.

Voltage transformer

electric transformer is a transformer with a core. It mainly consists of a secondary coil, the core and insulation components. When applying a voltage on a winding U1

when the iron core generates a magnetic flux φ, according to the law of electromagnetic induction, it is generated in the secondary winding

A secondary voltage U2. Change the primary or secondary winding turns, can produce different primary Voltage stabilizer and the secondary voltage ratio, which can be composed of

different ratios voltage transformers. Voltage transformer to convert high voltage to low voltage in proportion, ie 100V, a voltage transformer

Secondary side connected to the primary system, the secondary side connected measuring instruments, relay, etc.; mainly electromagnetic type (capacitor voltage

transformers are widely used), and another non-magnetic, such as electronics, photoelectric.
Plant and substation high voltage electrical energy measuring device, and a large user of electricity metering devices, related to the generation, transmission, supply

and multi-user interests. To ensure accurate measurement, you must follow JJG1021-2007 large-scale electronic circuits and DL/T448-2000

Design and compliance with the relevant national regulations developed. It solves the field test voltage transformers work intensity, complicated operation problems,

while the products are reliable and powerful.
Transformer calibrator, transformer tester, transformer field tester, transformer current school instrument, automatic transformer calibrator, automatic voltage

transformer tester, voltage transformer tester, transformer on-site verification meter, voltage transformer field calibrator, voltage

Transformer field tester

Transformer

Converting transformer is proportional voltage or current of the device. Its main function is to high voltage or high current proportionally converted into a standard

low voltage (100V) or standard small current (5A or 10A refer Ratings), in order to achieve measuring instruments, protective equipment and automatic control devices

Standardization of equipment, miniaturization. Meanwhile transformer can also be used to separate high-voltage systems, in order to ensure the safety of persons and

equipment.
Basic Introduction
Transformer (instrument transformer) is proportional voltage or current of a transformation. Its main function is to high voltage or high current proportionally

converted into a standard low voltage (100V) or standard small current (5A or 1A, refer to rating), in order to achieve measurement

Instrumentation, protective equipment and automatic control equipment standardization, miniaturization. Meanwhile transformer can also be used to separate high-voltage

systems, in order to ensure the safety of persons and equipment. In proportion to the voltage or current of a transformation.
Key Features
(1) a primary winding connected in series in the circuit, and rarely turns, therefore, a current in the coil depends entirely on the load current of the circuit under

test. And has nothing to do with the secondary current; (2) current transformer secondary coil connected instruments and relay current coil impedance is very small

Therefore, under normal circumstances, the current transformer at a near short-circuit state operation.
A current transformer secondary rated current ratio, known as mutual inductance Rated current transformer ratio: kn = I1n/I2n
Because the primary winding rated current I1n been standardized, unified secondary winding rated current I2n 5 (1 or 0.5) An, so the mutual inductance Rated current

transformer ratio has been standardized. kn can be approximated as a transformer primary and secondary winding turns ratio, i.e. kn ≈

kN = N1/N2 where N1.N2 as one, two coil turns.
History
Transformers first appeared in the 19th century. With the development of power transformers industry, transformer voltage levels and accurate level has greatly improved, but also

developed a lot of special transformers such as voltage, current hybrid transformer, DC current transformers, high accuracy current

Ratio and voltage ratio control, high current laser-type current transformers, electronic circuit compensation transformer, high voltage systems photoelectric

transformer, and SF6 enclosed Switchgear (GIS) in the voltage and current transformers. In the electric transformer industry, to develop what voltage

Grade and size of the power system, must develop the appropriate voltage level and accuracy of transformers for power system measurement, protection and control needs.
With many of the continuous application of new materials, transformers also appeared many new types of electromagnetic transformer has been more fully developed, in

which the core current transformer with dry, oil-filled and gas-insulated variety of structural formula adapted to the development needs of electric power construction

. However, with the power transmission capacity is growing, increasing the grid voltage level of continuous improvement and protection requirements, the general

structure of iron-core current transformers have been incompatible with gradually exposed the weaknesses inherent bulky, magnetic saturated ferromagnetic resonance

Vibration, the dynamic range is small, the use of narrow band and other weaknesses, it is difficult not meet the new generation of full power system automation, power

digital network development needs.
With the rapid development of optoelectronic technology, many technology developed has turned to the use of optical sensor technology and electronics to develop new

methods of electronic current transformer, referred to as optical current transformer. International Electrotechnical Commission has released an electronic current

transformer

Standards. Meaning electronic transformer, in addition to including the photoelectric transformer, but also a variety of other electronic test principle using the

voltage and current sensors.