The excitation current is the main cause of the current transformer error, so the reduction of the excitation current can reduce the error:
The material with high permeability core, because the core magnetic properties not only affect the ratio difference and angle difference, also affects the saturation ratio.
The increase of core section, shorten the length of magnetic path; increasing the number of turns of the coil. Changes in core section or the coil will increase and decrease the saturation ratio, in the core section of the coil is taken to increase or to improve the ratio and phase difference, must consider the impact of saturation ratio.
The impact of the two load limit. In the field, the method of increasing the effective cross section of the connecting wires is generally used, such as using larger sections of cables or multi-core parallel connection to reduce the impedance value of the two load. Two current transformers with the same type and variable ratio can be used in series, so that the load of each current transformer is half of the total load.
The increasing ratio of current transformer. In the field operation, the transformer with larger change ratio is selected.
In addition, there are two winding fractional compensation, two side capacitor shunt compensation and so on.
Calibration of 4. current transformer
Thecurrent transformer used in measurement usually does not carry out error check when it is chosen, and only the error test is carried out at the time of operation. The design and selection of the two connection wire section is selected according to the load level and the accuracy two times, while in operation only when the check limit load (such as the 10% rated current) and limit load (such as the 120% rated current) the ratio difference and phase difference, a lot of test method for difference and angle difference, such as double current meter method, communication method and compensator.
The current transformerused for relay protection is usually checked by 10% error curve (or volt ampere characteristic curve), or short circuit current multiple and two connection load check. When the power system has transient requirements for current transformers, transient error correction is still needed.
4.110% calibration of the error curve (saturation factor)
Than the difference is related to the size of two times and two times the size of the load, the load increase ratio difference curve to negative move, the same current transformer when the ratio reached -10%, two times larger than the current load, (I1/I1e) multiple small, two small load, the current ratio (I1/I1e times). If the two load is the rated value, this multiple is the saturation factor. The two different load curves are drawn and the corresponding saturation multiple is 10%, the error curve".
According to the operating experience of relay protection, the current error of thecurrent transformer used in the protection device is not allowed to exceed 10% while the angle error is not more than 7 degrees under the actual operation condition. The 10% error curve given by the manufacturer is based on the following procedure:
A given rated current limit by a factor of NE, and AWP obtained the excitation ampere turns AW0=AWP/10 and the length of the ampere turn magnetic excitation ampere turns AW0/L.
The B-H curve by AW0/L check the corresponding core, achieving the maximum magnetic flux density Bm, and according to the core and the two coil parameters, obtained two ESm corresponding induction potential.
According to ESm and two times the rated current is ISn, obtained two Sigma and ZS load impedance by two coil impedance obtained load impedance of the Zb allowed.
We are given a series of short n, can obtain the corresponding Zb value, which make the curve of n=f (Zb) is the 10% error curve.
In practice, the volt ampere characteristic is often used to measure the volt ampere characteristic curve of current transformer, and the test connection is shown in figure 2. The test will be an open circuit in the transformer coil, a secondary coil voltage, from Figure 1 (equivalent circuit of current transformer) the voltage corresponds to the voltage U2 with the current meter measured in voltage under the effect of U2 inflow of two of the secondary coil current of I0, U2=f (the relationship between current and voltage I0) is the I-V curve of current transformer
In the test, in order to make the output voltage close to the sine wave, the single-phase regulator is used to adjust the current, and the electric or electromagnetic instruments are adopted to make the 10% error curve more secure. Because the measured current contains three harmonic components, the average value is smaller than the effective value, while the electrodynamic and electromagnetic instruments reflect the effective value, and the rectifier type instrument reflects the mean value. From the equivalent circuit according to the current transformer can be obtained:
When the impedance angles of Z0, Z2 and Zfh are identical, I1, I0 and I2 are in phase, then the ratio is the maximum, and when the ratio is -10%, the formula is obtained from the formula of difference (1):
Because Z2 is very small, the pressure drop is negligible, so the U0 is U2, so the I-V curves available U0 and I0 relationship U0 = U2=f (I0). Defined by current multiples:
(Note: the current rating of the two side of the current transformeris uniform to 5 or 1.)
It is assumed that the rated change ratio Ke is equal to the ratio of turns to Kw, and formula (7) and formula (9) can be simplified as:
The volt ampere characteristic curve can also be checked to see if there is no turn to turn short circuit between the two coils. The I-V curves of current transformer of the same type is very similar, interturn short circuit, short circuit circulation will be generated in part, which is equivalent to the core installed a short turn, with voltage under the same current will increase a lot, the I-V curves significantly down, great the difference with the I-V characteristics of the interturn short-circuit.
(1) ratio difference and angle difference are the main characteristics of current transformer for measurement, while saturation coefficient is the main characteristic of relay transformer.
(2) the internal resistance and leakage reactance of the two coil, the turn number of the two coil, and the iron core
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Contact person: Elva Zhuang&Sunny Ni
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