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Type Differentiation Of Current Transformers
Aug 10, 2017

Type differentiation of current transformers

Classification by usage

In accordance with different uses, the current transformer can be broadly divided into two categories, [4]:

Measuring current transformer (or current transformer measuring winding): in the normal working current range, to the measurement, measurement and other devices to provide current information of the grid.

Protection current transformer (or current transformer protection winding): in the state of power grid fault, relay protection device to provide power grid fault current information.

1 、 measuring current transformer

In the current measurement of alternating current, for two instruments need to be converted to current unified (China provisions two times rated current transformer for 5A or 1A), the voltage on the line is as high as the direct measurement is very dangerous. The current transformer plays a role of variable current and electrical isolation. It is the sensor in power system measuring instrument, relay protection, two devices to obtain information of a circuit of electrical current, current transformer high current into low current proportion, a current transformer is connected in a system, the two side connected measuring instrument, relay protection etc..

In normal operation, the two side of the transformer is in an approximate short-circuit state, and the output voltage is very low. In operation if the two winding or a winding through the abnormal current (such as lightning current, resonant current and capacitor charging current, inductance starting current, etc.) will be over voltage in the secondary side two produce thousands of volts or even tens of thousands of volts. This not only endangers the insulation of the two system, but also causes the transformer to go too far and burn out, even endangering the safety of the operators.

The 1 side only has 1 to several turns, and the wire cross section is big, and the circuit is connected into the tested circuit. The 2 side turns are many, the wire is fine, and the instrument with less impedance (Ammeter / current coil of the power meter) constitutes a closed circuit.

Operation of current transformer equivalent transformer short circuit 2 side, ignoring the excitation current, ampere turn is equal to the number of I1N1=I2N2

Current transformer primary winding current I1 and the two winding I2 current ratio, called the actual current than I1/I2=N2/N1=k.

Excitation current is the major source of error.

The accuracy level of the current transformer is 0.2/0.5/1/3,1, indicating that the ratio error is no more than + 1%, and there are also 0.2S and 0.5S levels.

2 、 protective current transformer

Protection current transformer is divided into 1. overload protection current transformer, 2. differential protection current transformer, 3. earth protection current transformer (zero sequence current transformer)

The protective current transformer mainly works with the relay device. When the circuit is short circuited or overloaded, it provides the relay with the signal to cut off the fault power

Road to protect the safety of power supply system. Working conditions of protective current transformer and current used for measurement

The transformer is completely different, and the protective transformer only works effectively when the current is several times several times higher than the normal current. The main requirements of protective transformers are: 1., reliable insulation, 2. large enough accurate limit coefficient, 3. enough thermal stability and dynamic stability.

For the protection of transformer under rated load can meet the requirements of accuracy class of one of the greatest amount of current is established that limit primary current. The accurate limit factor is the rated accurate limit, primary current and rated primary current ratio. When the current is large enough, the iron core is saturated and does not reflect the primary current. The exact limiting factor is the characteristic. The accuracy of protection transformers is 5P and 10P, which means that the allowable current error is 1% and 3% at the rated accurate limit primary current, and the compound errors are 5% and 10% respectively

When the line fails, the impact current produces thermal and electromagnetic forces, and the protective current transformer must withstand it. In the case of two winding short-circuit, the current transformer can withstand the primary current effective value in one second, which is called rated short-time thermal current. In the case of two winding short circuit, the primary current peak that current transformer can bear without damage is called rated dynamic steady current.

The accuracy level of protective current transformer is 5P/10P and 10P. The compound error is not more than 10%.

Classification by insulating medium

1 、 dry current transformer: by ordinary insulating material after dipping coating as insulation.

2 、 pouring type current transformer: the current transformer is made of epoxy resin or other resin mixed material.

3 、 oil immersed current transformer: insulated paper and insulating oil as insulation, usually outdoor type.

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

Sort by installation

1 through current transformer [5]: a current transformer used to pass through a screen board or wall.

2 、 support type current transformer: installed on the plane or pillar, as a circuit conductor pillar of the current transformer.

3) bushing type current transformer: a current transformer without one conductor and one insulation, which is directly mounted on an insulated bushing.

4: bus type current transformer: there is no one conductor, but there is a insulation, a direct current set on the bus used by a current transformer.

Classification by principle

1 、 electromagnetic current transformer: current transformer based on electromagnetic induction principle to realize current conversion.

2 、 electronic current transformer:

for example

(1) optical current transformers refer to optical devices as current sensors. Optical devices are composed of optical glass and all fiber.

(2) hollow coil current transformer. Also known as the Rogowski coil type current transformer. Hollow coil often consists of enameled wire is uniformly wound made on the ring skeleton, skeleton made of plastic, ceramic and other non ferromagnetic materials, the relative permeability and relative permeability of the air is the same, this is a hollow coil has a remarkable characteristic different from the current transformer with iron.

(3) core coil type low power current transformer (LPCT). It is a kind of development of the traditional electromagnetic current transformer. According to the high impedance design, in a very high current, saturation characteristics improved, expanding the range of reducing power consumption, high accuracy measurement without saturation up to short-circuit current over current, short circuit current offset, measurement and protection can share a core coil type low power current transformer, the output voltage signal.

Directions for the use of both

Because of the saturation, nonlinearity and narrow frequency band of the electromagnetic current transformer, the electronic current transformer is rising gradually. The electronic current transformer with anti magnetic saturation and low power consumption, wide band etc..

Electronic transformer representativedomestic AnyWay inverter voltage sensor, current sensor and AnyWay AnyWay frequency conversion power sensor, which belongs to the AnyWay variable frequency power sensor voltage and current combined transformer.

The main features of this transformer are as follows:

1 、 adopt front end digital technology, fiber transmission, good electromagnetic compatibility.

2, amplitude frequency characteristic and phase frequency characteristic are good, and can obtain higher measurement precision in wide range value, frequency and phase range.

3, belong to digital sensor, the two instrument will not introduce error, sensor error is system error.


Add: No.38 South Taoyuan Road, Yaozhuang Town,Jiashan County, Jiaxing City, Zhejiang Province, China
Contact person: Elva Zhuang&Sunny Ni
Tel:+86-573-84775555
Fax:+86-573-84776699
Phone:+86-13732570078
E-mail:sales9@great-relay.com
sunny.ni@great-relay.com