CN106772152A - A kind of measuring method of transformer core remanent magnetism - Google Patents

Abstract

The invention relates to a method for measuring residual magnetism of a transformer iron core. The method comprises the following steps: 1) acquiring a magnetization curve of a transformer iron core, the magnetization curve revealing the relationship between a DC component of no-load current and the residual magnetism of the iron core; 2) 380V AC test voltage is loaded on the transformer winding, and the no-load current is obtained by measurement; 3) The DC component of the no-load current is obtained; 4) The iron core remanence is obtained according to the magnetization curve and the DC component. Compared with the prior art, the present invention has the advantages of being simple and feasible, and can quantitatively measure the residual magnetism of the main transformer iron core, guide the formulation of the degaussing scheme of the main transformer iron core residual magnetism, evaluate the degaussing effect, and test and run the main transformer. Provide technical support.

Description

A Measuring Method of Transformer Core Residual Magnetism

technical field

The invention relates to the field of transformer test and operation, in particular to a method for measuring residual magnetism of transformer iron core.

Background technique

Transformers operate by electromagnetic induction, using magnetic fields as coupling fields. Transformer cores generally use ferromagnetic materials with high magnetic permeability, such as silicon steel sheets. The magnetic permeability of ferromagnetic materials is nonlinear, and its value changes with the change of magnetic field strength. The reason why ferromagnetic materials have high magnetic permeability is that there are many small magnetic domains with definite magnetic polarity and strong magnetization inside ferromagnetic materials, as shown in Figure 1. Under the action of an external magnetic field, the magnetic domains are rearranged regularly along the direction of the external magnetic field, so that the actual magnetic field generated is much larger than that in non-ferromagnetic materials.

The polarization of the magnetic domain needs to go through a process. Therefore, during the magnetization process, the change of the magnetic induction B in the ferromagnetic material lags behind the change of the external magnetic field H, that is, hysteresis. Under the alternating external magnetic field, the variation curve of the core magnetic induction B with the external magnetic field H becomes a hysteresis loop. Different hysteresis loops can be measured with different Bm values, and all the hysteresis loops are connected at the vertices of the first quadrant, and the obtained magnetization curve is called the basic magnetization curve, as shown in Figure 2. Since the hysteresis loop of most ferromagnetic materials is very narrow, in engineering applications, the basic magnetization curve can be used instead of the hysteresis loop to solve the problem, and the error is allowed by engineering.

After the transformer cuts off the power supply, the flux linkage retained in the iron core is called residual magnetism. Residual magnetism will increase the excitation inrush current when the main transformer is switched on, increase the harmonics in the excitation current, increase the power consumption of the transformer, and may also cause relay protection malfunctions, which will adversely affect the operation of the main transformer. At the same time, the residual magnetism It may also make subsequent test results abnormal.

The main way to generate residual magnetism is the DC resistance test, and the amount of residual magnetism depends on the intensity and time of the DC current passing through the transformer winding. In the transformer direct resistance test, in order to improve the measurement accuracy and shorten the measurement time, a large current test instrument is usually used to exceed the excitation current, saturate the iron core, and cause serious residual magnetism. In addition, residual magnetism will also be generated when the no-load transformer is switched off, but at present, the iron core of the main transformer is made of soft magnetic material, and the residual magnetism is small, and the corresponding damage is relatively small.

In order to ensure the accuracy of the follow-up test and the safe operation of the main transformer, it is required to demagnetize the main transformer after the DC resistance test, but the degaussing effect cannot be checked. In the follow-up low-voltage no-load test, the test results are often inconsistent with the factory test, which cannot truly reflect the internal state of the transformer. Secondary degaussing is required on site, increasing the test workload and working hours. In addition, during the closing test of the ultra-high voltage main transformer, due to the influence of the residual magnetism of the iron core, the excitation inrush current of the main transformer was too large, which triggered the heavy gas protection action of the main transformer.

In summary, the measurement technology of the main transformer residual magnetism is of great significance to the main transformer test and safe operation. Based on the current research status at home and abroad, the current research on the remanence of the main transformer focuses on the demagnetization technology, and the measurement of the remanence is less involved.

Contents of the invention

The object of the present invention is to provide a simple and feasible method for measuring the residual magnetism of the transformer iron core in order to overcome the above-mentioned defects in the prior art.

The purpose of the present invention can be achieved through the following technical solutions:

A method for measuring residual magnetism of a transformer core, the method comprising the following steps:

1) Obtain the magnetization curve of the transformer iron core, which reveals the relationship between the DC component of the no-load current and the remanence of the iron core;

2) Load the 380V AC test voltage on the transformer winding, and measure the no-load current;

3) obtaining the DC component of the no-load current;

4) Obtain the remanent magnetization of the iron core according to the magnetization curve and the DC component.

The magnetization curve of the transformer core is converted from the no-load characteristic curve of the main transformer.

When converting the no-load characteristic curve of the main transformer into the magnetization curve of the transformer core, the following assumptions are set:

a) Neglect hysteresis and eddy current losses in the iron core;

b) Neglect the winding resistance;

c) A magnetization curve is formed point by point.

In the process of forming the magnetization curve, the current value of each point in the magnetization curve is calculated by the trapezoidal integral method.

In the step 3), the DC component is obtained by performing FFT decomposition on the no-load current, and the per unit value of the DC component is obtained by the following formula:

In the formula, I _dc is the DC component of the measured no-load current, is the per unit value of the DC component, and I ₀ is the effective value of the rated current of the main transformer.

Compared with the prior art, the present invention has the following advantages:

(1) The present invention proposes a simple and feasible method for measuring the residual magnetism of the transformer iron core by studying the relationship between the no-load current and the residual magnetism of the iron core.

(2) The present invention can quantitatively measure the residual magnetism of the main transformer iron core, to guide the formulation of the degaussing scheme of the main transformer iron core residual magnetism, evaluate the degaussing effect, and provide technical support for the main transformer test and operation.

(3) The present invention can effectively measure the residual magnetism of the iron core, facilitate the inspection of the degaussing effect, thereby improving the safety of the main transformer operation.

Description of drawings

Fig. 1 is a schematic diagram of a magnetic domain in a ferromagnetic material, wherein (1a) is a schematic diagram before magnetization, and (1b) is a schematic diagram after complete magnetization;

Fig. 2 is hysteresis loop and basic magnetization curve in ferromagnetic material, wherein, (2a) is hysteresis loop, (2b) basic magnetization curve;

Figure 3 is a schematic diagram of the magnetization curve of the iron core;

Figure 4 is a schematic diagram of no-load current in the case of residual magnetism;

Figure 5 is a schematic diagram of the relationship between the magnetization curve of the iron core and the DC component of the no-load current and the residual magnetization curve;

Fig. 6 is a schematic flow chart of the present invention;

Figure 7 is a schematic diagram of the no-load current waveform of the transformer under small voltage, wherein (7a) is the waveform diagram when Br=-25 (low-voltage nonlinear region), and (7b) is the waveform when Br=-100 (low-voltage nonlinear region). Waveform diagram, (7c) is the waveform diagram when Br=-495.17 (low-voltage nonlinear region).

detailed description

The present invention will be described in detail below in conjunction with the accompanying drawings and specific embodiments. This embodiment is carried out on the premise of the technical solution of the present invention, and detailed implementation and specific operation process are given, but the protection scope of the present invention is not limited to the following embodiments.

The typical basic magnetization curve of a transformer is shown in Figure 3, which can be divided into three parts: low-voltage nonlinear region, linear region, and high-voltage nonlinear region. In the absence of residual magnetism, when the low-voltage no-load test is carried out on site, the iron core mostly works in the low-voltage nonlinear region. If there is residual magnetism in the iron core, a low-voltage test voltage is applied to the winding, and the magnetic induction in the iron core and the residual magnetic induction are superimposed, and the magnetic induction in the iron core will be biased, correspondingly, the no-load current will also A dc bias occurs, as shown in Figure 4. Therefore, the DC component in the no-load current is the direct response of the residual magnetism in the iron core. Measuring the no-load current under low voltage can indirectly measure the residual magnetism of the iron core.

In principle, the DC component in the small voltage and small no-load current and the residual magnetization of the iron core comply with the basic magnetization curve constraints of the transformer, that is, the residual magnetization of the iron core and the DC component in the no-load current are in one-to-one correspondence.

In order to verify this correspondence, take a 110kV main transformer winding as an example, apply EMTP to establish a simulation model, load a 380V sinusoidal voltage on the iron core, calculate the no-load current under different iron core remanence, and apply Fourier Analyze and calculate the DC component in the no-load current. The calculation results are shown in Table 1-2. In the table, the reference value of the current is the no-load current value (1.78A) under the rated voltage of the main transformer, and the reference value of the magnetic flux is the magnetic flux under the rated voltage (495.17Wb). When the magnetic induction in the iron core is in the low-voltage nonlinear region, the linear region and the high-voltage nonlinear region, the no-load current waveforms are shown in Figure 5 respectively.

It can be seen that the correspondence between the DC component in the no-load current under small voltage and the residual magnetism of the iron core is basically consistent with the basic magnetization curve of the transformer.

In fact, the test and operation personnel can only know the no-load characteristics of the main transformer from the factory test report of the transformer. Due to the influence of the stray capacitance of the transformer, there is a certain error between it and the excitation curve of the iron core, but the error is within the allowable range of the project. . In the present invention, the no-load characteristic curve of the main transformer is used to replace the basic magnetization curve of the iron core.

As shown in Figure 6, the measuring method of transformer iron core remanence of the present invention comprises the following steps:

1) Obtain the no-load characteristic curve of the main transformer and convert it into the magnetization curve of the transformer core, which reveals the relationship between the DC component of the no-load current and the remanence of the iron core;

2) Load the 380V AC test voltage on the transformer winding, and measure the no-load current;

3) obtaining a DC component by performing FFT decomposition on the no-load current;

In this method, the DC component is recorded in the form of per unit value and obtained by the following formula:

In the formula, Idc is the _DC component of the measured no-load current of the main transformer, The per unit value of the DC component of the no-load current of the main transformer, I ₀ is the effective value of the rated current of the main transformer;

4) Obtain the remanent magnetization of the iron core according to the magnetization curve and the DC component.

The key to the measurement of residual magnetism by using no-load current under small voltage is the accuracy with which the no-load characteristic of the transformer is converted into the basic magnetization curve. During the conversion, the following assumptions are made:

1. Neglect the hysteresis and eddy current loss in the iron core;

2. Neglect the winding resistance;

3. Form the magnetization curve point by point.

The voltage in the power system can be regarded as an ideal voltage source, that is, a standard sinusoidal voltage, so the magnetic flux of the iron core is:

(In this formula, what do u _rms and ω respectively represent?)

In the formula, u _rms is the effective value of the system voltage, ω is the angular frequency of the system voltage, ω=2πf, f is the frequency of the system voltage, f=50Hz in our country.

The current conversion is more complicated. In the conversion process, the first point is selected as the reference point, and the part below the reference point is considered as the linear region.

The current value of the reference point in the magnetization curve is shown in the following formula

In the formula, i _rms,1 is the current effective value of the first point, that is, the reference point in the current conversion.

The rest of the points are calculated recursively. Assume that in is the next value to _{be found, and then assume that the sinusoidal flux linkage reaches φ n just} at the maximum value. The current curve adopts the piecewise linear method, and all known points have been determined, only the last section is uncertain because in _is unknown. Then use the integral method to _calculate the effective value of in:

Using the trapezoidal method of integration, it is possible to calculate

In the formula, a and b are the constant coefficients of the trapezoidal integral method, which can be calculated according to the magnetization curve of the main transformer.

Otherwise, I _n =i _rms,n

Then the current value in at the _nth point in the magnetization curve can be obtained, so as to form the magnetization curve point by point.

Claims (5)

1. A method for measuring transformer core remanence, characterized in that the method may further comprise the steps: 1) Obtain the magnetization curve of the transformer iron core, which reveals the relationship between the DC component of the no-load current and the remanence of the iron core; 2) Load the 380V AC test voltage on the transformer winding, and measure the no-load current; 3) obtaining the DC component of the no-load current; 4) Obtain the remanent magnetization of the iron core according to the magnetization curve and the DC component. 2. The method for measuring residual magnetism of a transformer iron core according to claim 1, wherein the magnetization curve of the transformer iron core is converted from the no-load characteristic curve of the main transformer. 3. the measuring method of transformer iron core remanence according to claim 2 is characterized in that, when main transformer no-load characteristic curve is converted into the magnetization curve of transformer iron core, set following hypothesis: a) Neglect hysteresis and eddy current losses in the iron core; b) Neglect the winding resistance; c) A magnetization curve is formed point by point. 4. The method for measuring residual magnetism of a transformer iron core according to claim 3, characterized in that, in the process of forming the magnetization curve, the current value of each point in the magnetization curve is calculated by the trapezoidal integration method. 5. the measuring method of transformer iron core remanence according to claim 1 is characterized in that, in described step 3), by carrying out FFT decomposition to described no-load current, obtains DC component, and obtains DC by following formula Component per unit value: ${\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; d</span>}\mathrm{<span\; class="notranslate">\; c</span>}}^{<span\; class="notranslate">\; *</span>}<span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; d</span>}\mathrm{<span\; class="notranslate">\; c</span>}}}{{\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}}$ In the formula, I _dc is the DC component of the measured no-load current, is the per unit value of the DC component, and I ₀ is the effective value of the rated current of the main transformer.