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Error And Calibration Of Current Transformer
Jul 08, 2017

Error and calibration of current transformer

overview

For large current measurement of high voltage AC circuit, usually by means of a current transformer, the transformer ratio relationship between the current into a small current, the measuring instrument not directly to the measured line, while the two loop can be connected into a wiring diagram in any way, to meet the measurement and relay protection automatic control and other requirements.

The current transformer is working as a current source, a current actually and two times for the two time is independent of the load, load conversion to a side system impedance with negligible compared to.

Error of 2. current transformer

The current transformer is mainly composed of three parts: an iron core, a primary coil and a two coil. Due to the existence of iron core reluctance, the process of current transformer in transmission current, must consume a small part of the current for excitation, the magnetic core, resulting in the EMF and two current in the two coil, current transformer error is due to the excitation current caused by the consumption of iron. Due to the existence of excitation current and iron loss, the difference between primary current and two current is a vector, and the error includes the ratio difference and the phase difference. The errors in the numerical value of primary and two currents are expressed as percentages in the form of relative error, called the ratio, and defined according to the national standard:

a primary current flowing over a I1- line;

Current in the two circuit of a I2- current transformer;

Rated current ratio of Ke- current transformer;

From the formula defined by the difference, we can see that there is a plus or minus value in the ratio. When the ratio is positive, the current reading of the measuring instrument connected to the transformer is multiplied by the ratio Ke and is greater than the actual current on the line; the reverse is negative when the ratio is negative. The same current transformer may be positive or negative at different currents and loads.

In addition, there is also a difference between the primary current and the two current. The angle between the primary current vector and the two current vector after the reversal of the 180o is called the phase difference or the angular difference. The angular difference is also positive and negative. When the current vector is reversed two times after the reversal, the angular difference is specified as positive, and the other is negative when it lags behind the primary current vector.

As thecurrent transformer used in measurement, the magnitude of the difference and the angle difference directly affect the accuracy of the measurement results. Therefore, the difference of the ratio and the angle difference are the most important characteristics of the current transformer. However, the current transformer relay protection and is not the same with the current transformer for measurement, measuring current transformer only requires accurate in normal operation, and the relay protection current transformer is accurate in short circuit condition, which requires transformer on the steady-state short-circuit current and transient short-circuit current accurate conversion.

The current transformer has the characteristics of saturated gradually, the short-circuit current,current transformer core saturation, excitation current rises sharply, the excitation current in a current account for the proportion is increasing, the difference gradually shifted to the negative and increases rapidly. When the current increases to a specific difference equal to -10%, the current to the rated current ratio (I1/I1e) is called the saturation factor. Because the current relay action in general is a few times larger than the rated current, so the current transformer relay protection should be guaranteed as to the reliability of relay action in the short circuit current several times than the rated current of the. Therefore, the main characteristics of current transformer for relay protection are not the ratio and the angle difference, but the saturation factor.


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