Dynamic elimination method for residual magnetism of iron core of converter transformer

Abstract

The invention relates to a method for dynamically eliminating residual magnetism of an iron core of a converter transformer, and belongs to the field of transformer demagnetization. According to the method, a capacitance energy storage and release device is connected to the two ends of a transformer winding, capacitance energy storage causes RLC oscillation of the transformer, a magnetic field is generated through current caused by the RLC oscillation, and magnetic domains of iron core ferromagnetic materials are rearranged through the changed magnetic field, so that the demagnetization effect on residual magnetism of the transformer is achieved. The method comprises the following steps: drawing a hysteresis loop in the demagnetization process of a transformer by collecting voltage and current data in the demagnetization process, translating an end point of the hysteresis loop to an original point, and judging the residual magnetism direction and size of initial demagnetization according to the area of each quadrant of the hysteresis loop; and then transformer demagnetization is carried out again, the residual magnetism direction and size of the first demagnetization are evaluated according to the hysteresis loop of the second demagnetization, and whether the residual magnetism is smaller than a threshold value is judged. According to the method, the direction and size of residual magnetism in the demagnetization process can be evaluated in real time, the demagnetization energy is dynamically adjusted, and sufficient demagnetization of the converter transformer is ensured.

Description

Technical Field

[0001] The present invention belongs to the field of transformer degaussing, and relates to a method for dynamically eliminating the residual magnetism of a converter transformer core. Background Art

[0002] As a core device of a high-voltage direct current (HVDC) transmission system, the problem of residual magnetism in the core of a converter transformer has become a major hidden danger affecting the safe and stable operation of the power grid. After the converter transformer undergoes a circuit breaker operation or a direct current resistance test, a large amount of residual magnetism will be generated in its core. If the residual magnetism in the core cannot be eliminated in time, a large-amplitude magnetizing inrush current will be formed during closing, which will not only accelerate the aging of the winding insulation, but may also cause misoperation of the differential protection, seriously affecting the safety of the transformer body and its protection device.

[0003] Existing degaussing technologies mainly rely on the AC degaussing method and the AC-DC degaussing method, but there are significant defects in practical applications: traditional AC degaussing requires repeated adjustment of the power frequency voltage amplitude, and a single operation takes more than 30 minutes, and it cannot effectively eliminate the composite residual magnetism generated by the cross magnetic circuit; although the AC-DC degaussing can shorten the operation time to 10-15 minutes, its fixed-frequency attenuation mode is difficult to match the complex hysteresis loop characteristics of the converter transformer, resulting in insufficient degaussing completion. More seriously, the existing methods lack closed-loop monitoring of the degaussing process, and only estimate the residual magnetism by simple integration of the measured winding induced voltage, with a large error range and unable to accurately judge the degaussing end point.

[0004] With the large-scale construction of converter stations with voltage levels of ±500 kV and above, the core structure of the converter transformer presents complex characteristics such as multi-column parallel connection and segmented interleaving, further exacerbating the spatio-temporal difference of the residual magnetism distribution.

[0005] Therefore, there is an urgent need to study a precise method for eliminating the residual magnetism of the converter transformer core. Summary of the Invention

[0006] In view of this, the purpose of the present invention is to provide a method for dynamically eliminating the residual magnetism of a converter transformer core, realizing real-time evaluation of the residual magnetism direction and magnitude during the degaussing process of the converter transformer, and dynamically adjusting the degaussing energy to achieve rapid and effective degaussing of the converter transformer.

[0007] To achieve the above purpose, the present invention provides the following technical solutions:

[0008] A dynamic method for eliminating residual magnetism in the core of a converter transformer. First, charge the capacitor energy storage and release device. Then, connect this device across the transformer windings. An alternating current is generated in the transformer windings, and the alternating current induces an alternating magnetic field, causing the magnetic domains in the transformer core material to deflect and disrupting the original directional arrangement of the magnetic domains to eliminate the residual magnetism in the transformer core. At the same time, record the data of the current in the grid-side winding and the terminal voltage of the winding of the converter transformer during demagnetization, and plot the hysteresis loop during the demagnetization process of the transformer. Then, translate the end point of the hysteresis loop to the origin, and judge the direction and magnitude of the residual magnetism of the initial demagnetization based on the areas of the quadrants of the hysteresis loop.

[0009] Subsequently, perform transformer demagnetization again. Evaluate the direction and magnitude of the residual magnetism of the first demagnetization based on the hysteresis loop of the second demagnetization, and judge whether the residual magnetism is less than the threshold. If it is less than the threshold, stop demagnetization; if it is greater than the threshold, continue to adjust the energy of the capacitor energy storage and release device, perform demagnetization on the transformer and evaluate the residual magnetism until the value of the residual magnetism is less than the threshold.

[0010] Further, the capacitor energy storage and release device includes an electrolytic capacitor, an AC voltage source, a phase-controlled switch, and a current-limiting resistor.

[0011] The working mode of the capacitor energy storage and release device is as follows: Charge the electrolytic capacitor using the AC voltage source, and adjust the energy storage of the capacitor by selecting the opening angle of the phase-controlled switch after a period of time. The charging circuit consists of a current-limiting resistor, an AC voltage source, a phase-controlled switch, and an electrolytic capacitor. The energy release circuit includes a switch, an electrolytic capacitor, and the grid-side winding of the converter transformer (including the DC resistance of the grid-side winding of the converter transformer and the equivalent inductance of the grid-side winding of the converter transformer). After the electrolytic capacitor is fully charged, release the capacitor energy through the energy release circuit, generate an RLC oscillation in the energy release circuit, and the alternating current in the transformer winding generates an alternating magnetic field, which disrupts the arrangement of the magnetic domains in the transformer core material, thereby achieving demagnetization of the transformer core.

[0012] Further, the steps for plotting the hysteresis loop during the demagnetization process of the transformer specifically include the following steps:

[0013] S101: Collect the voltage and current data of the converter transformer during the demagnetization process when the capacitor energy storage and release device is connected to the grid-side winding of the converter transformer, and record the time t corresponding to each row of voltage u(t) and current i(t).

[0014] S102: Plot the curve of time-voltage data, with the moment of the first voltage peak as the starting point, denoted as t start , and the moment when the voltage drops to 0 as the end point, denoted as t end ; Integrate the voltage u(t) to calculate the magnetic flux ψ(t), and the calculation formula is as follows:

[0015]

[0016] After obtaining the numerical value of the magnetic flux ψ corresponding to each moment, using the current i(t) as the X-axis data and the magnetic flux ψ(t) as the Y-axis data, the curve of the current i(t) and the magnetic flux ψ(t) plotted is the hysteresis loop during the demagnetization process.

[0017] Furthermore, the translation of the end point of the hysteresis loop to the origin is specifically to calculate the change in magnetic flux Δψ caused by the externally applied energy during the transformer demagnetization process, and translate the hysteresis loop by Δψ so that its end point falls at the origin (0,0). The specific steps are as follows:

[0018] S201: Based on the curve of the current i(t) and the magnetic flux ψ(t), analyze the current i(t) at the end point of the curve end and the magnetic flux numerical value ψ end , since the initial value of the magnetic flux integral is 0, the change in magnetic flux Δψ caused by the externally applied energy during the transformer demagnetization process is ψ end , that is, Δψ = ψ end ;

[0019] S202: Subtract the numerical value of Δψ from all the data in the column of the magnetic flux ψ(t), and keep the current numerical value unchanged, so that the end point of the curve of the current i(t) and the magnetic flux ψ(t) falls at the origin (0,0).

[0020] Furthermore, the direction and magnitude of the residual magnetism of the initial demagnetization are judged by the area of each quadrant of the hysteresis loop. Specifically, calculate the areas of the first, second, third, and fourth quadrants of the hysteresis loop respectively, and judge the direction of the original residual magnetism based on the areas of the first and third quadrants, and judge the magnitude of the original residual magnetism based on the areas of the second and fourth quadrants. The specific steps include the following:

[0021] S301: For the translated curve of the current i(t) and the magnetic flux ψ(t), calculate the areas of the curve in the first, second, third, and fourth quadrants respectively, that is, integrate the magnetic flux ψ(t) with respect to the current i(t) to calculate the area; when the curve passes through each quadrant multiple times, the areas of the corresponding quadrants are accumulated;

[0022] S302: Judge the direction of the transformer residual magnetism and the magnetic flux generated by demagnetization according to the areas of the first and third quadrants of the hysteresis loop; by default, the direction of the magnetic flux generated by the capacitor energy storage and release device is the positive direction. When the area of the first quadrant is the largest among the four quadrants of the hysteresis loop, the original residual magnetism direction of the transformer core is negative; when the area of the third quadrant is the largest among the four quadrants of the hysteresis loop, the original residual magnetism direction of the transformer core is positive;

[0023] S303: The area of the second quadrant of the hysteresis loop represents the energy of hysteresis loss during the process of demagnetizing the positive remanence until the coercive force is 0; the area of the fourth quadrant of the hysteresis loop represents the energy of hysteresis loss during the process of demagnetizing the negative remanence until the coercive force is 0; in the case of determining the original remanence direction of the converter transformer, the magnitude of the original remanence of the transformer is calculated according to the areas of the second and fourth quadrants of the hysteresis loop.

[0024] When it is determined that the original remanence is in the negative direction, the formula for calculating the remanence of the transformer based on the area of the fourth quadrant of the hysteresis loop is as follows:

[0025]

[0026] When it is determined that the original remanence is in the positive direction, the formula for calculating the remanence of the transformer based on the area of the second quadrant of the hysteresis loop is as follows:

[0027]

[0028] In the formula, S2 represents the area of the second quadrant of the hysteresis loop, S4 represents the area of the fourth quadrant of the hysteresis loop, W Br represents the core remanence energy storage, B r- , B r+ are the core remanence magnetic fluxes when the original remanence is in the negative direction and positive direction respectively, u r is the magnetic permeability corresponding to the core remanence B r , M is the weight of the transformer core, and ρ is the density of the transformer core material.

[0029] B r and u r The parameter acquisition of comes from the magnetization curve of the corresponding converter transformer silicon steel sheet model. Since B r and u r is a single function mapping relationship with respect to the magnetic field strength, the corresponding unique remanence under the area of the second or fourth quadrant of the hysteresis loop can be determined according to the magnetization curve of the converter transformer silicon steel sheet model.

[0030] Furthermore, adjusting the energy of the capacitor energy storage and release device is specifically achieved by adjusting the opening angle of the phase selection switch of the capacitor energy storage and release device to adjust the electrolytic capacitor energy storage, or by changing the voltage amplitude of the AC voltage source to adjust the electrolytic capacitor energy storage.

[0031] The calculation expression of the electrolytic capacitor energy storage is as follows:

[0032]

[0033] In the formula, W c represents the electrolytic capacitor energy storage energy, C represents the capacitance value of the electrolytic capacitor, and U is the voltage amplitude of the AC voltage source when the phase control switch is disconnected in the charging circuit.

[0034] The beneficial effects of the present invention are as follows: By connecting the transformer winding to the capacitor energy storage and release device, the RLC oscillation of the transformer is caused by the capacitor energy storage. A magnetic field is generated by the current caused by the RLC oscillation, and the magnetic domains of the ferromagnetic material of the iron core are rearranged by the changing magnetic field to achieve the effect of demagnetizing the residual magnetism of the transformer. By collecting the voltage and current data during the demagnetization process, the hysteresis loop of the demagnetization process is plotted, and the end point of the hysteresis loop is translated to the origin. The direction and magnitude of the residual magnetism of the initial demagnetization are judged by the area of the hysteresis loop. Subsequently, the transformer is demagnetized again, and the direction and magnitude of the residual magnetism of the first demagnetization are evaluated based on the hysteresis loop of the second demagnetization to determine whether the residual magnetism is less than the threshold. If it is less than the threshold, the demagnetization is stopped. If it is greater than the threshold, the energy of the capacitor energy storage and release device is continuously adjusted, and the demagnetization and residual magnetism evaluation of the converter transformer are carried out until the residual magnetism value is less than the threshold. The present invention can realize the real-time evaluation of the direction and magnitude of the residual magnetism during the demagnetization process of the converter transformer, and dynamically adjust the demagnetization energy to ensure rapid, effective and sufficient demagnetization of the converter transformer.

[0035] Other advantages, objectives and features of the present invention will be described to some extent in the subsequent specification, and to some extent, will be obvious to those skilled in the art based on the study of the following text, or can be taught from the practice of the present invention. The objectives and other advantages of the present invention can be realized and obtained through the following specification. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] In order to make the objectives, technical solutions and advantages of the present invention clearer, the present invention will be described in detail preferably with reference to the accompanying drawings, where:

[0037] Figure 1 is a flowchart of the dynamic demagnetization method for the converter transformer provided by the present invention.

[0038] Figure 2 is a schematic diagram of the capacitor energy storage and release device.

[0039] Figure 3 are the voltage curves of the capacitor energy storage and release device connected to the converter transformer during the two demagnetization processes.

[0040] Figure 4 are the current curves of the capacitor energy storage and release device connected to the converter transformer during the two demagnetization processes.

[0041] Figure 5 are the hysteresis loops plotted during the two demagnetization processes of the converter transformer.

[0042] Figure 6 are the magnetic induction intensity - magnetic field intensity and permeability - magnetic field intensity curves corresponding to the direct current magnetization of the silicon steel sheet of the converter transformer. Detailed implementation manners

[0043] The following uses specific specific examples to illustrate the implementation manners of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. The present invention can also be implemented or applied through other different specific implementation manners. Various details in this specification can also be modified or changed based on different viewpoints and applications without departing from the spirit of the present invention. It should be noted that the drawings provided in the following embodiments only illustrate the basic concept of the present invention in a schematic manner. Without conflict, the following embodiments and the features in the embodiments can be combined with each other.

[0044] Among them, the drawings are only for illustrative purposes, showing only schematic diagrams, not physical diagrams, and cannot be construed as a limitation on the present invention; in order to better illustrate the embodiments of the present invention, some components in the drawings will be omitted, enlarged or reduced, which does not represent the size of the actual product; for those skilled in the art, it is understandable that some well-known structures and their descriptions in the drawings may be omitted.

[0045] In the drawings of the embodiments of the present invention, the same or similar reference numerals correspond to the same or similar components; in the description of the present invention, it should be understood that if there are terms such as "upper", "lower", "left", "right", "front", "rear", etc. indicating the orientation or positional relationship, it is based on the orientation or positional relationship shown in the drawings. It is only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation. Therefore, the terms describing the positional relationship in the drawings are only for illustrative purposes and cannot be construed as a limitation on the present invention. For those of ordinary skill in the art, the specific meanings of the above terms can be understood according to specific circumstances.

[0046] The present invention provides a method for dynamically eliminating the residual magnetism of a converter transformer core, as Figure 1 shown, specifically including the following steps:

[0047] 1) Charge the capacitor energy storage and release device, connect the device across the transformer winding, an alternating current is generated in the transformer winding, and the alternating current induces an alternating magnetic field, causing the magnetic domains of the transformer core material to deflect and disrupting the original oriented arrangement of the magnetic domains to eliminate the residual magnetism of the transformer core.

[0048] 2) Record the data of the winding current and the winding terminal voltage when the energy storage and release device is connected across the transformer winding for demagnetization, and draw the hysteresis loop during the transformer demagnetization process.

[0049] 3) Calculate the change in magnetic flux Δψ caused by the externally applied energy during the transformer demagnetization process, and translate the hysteresis loop by Δψ so that its end point falls at (0, 0).

[0050] 4) Calculate the areas of the first, second, third, and fourth quadrants of the hysteresis loop, and judge the direction of the original remanence based on the areas of the first and third quadrants, and judge the magnitude of the original remanence based on the areas of the second and fourth quadrants.

[0051] 5) Connect an energy storage and release device to the transformer winding, repeat the process of generating an alternating magnetic field and demagnetizing by the transformer current oscillation again, and calculate the areas of the first, second, third, and fourth quadrants of the hysteresis loop of the second demagnetization again. And judge the direction of the remanence after the first demagnetization based on the areas of the first and third quadrants, and judge the magnitude of the remanence after the first demagnetization based on the areas of the second and fourth quadrants.

[0052] 6) Repeat the above process repeatedly, and adjust the energy of the capacitor energy storage and release device according to the numerical value of the remanence magnitude evaluated after the previous demagnetization, until the remanence of the iron core after the (n - 1)-th demagnetization is less than the set threshold a after the n-th demagnetization, that is, stop demagnetization and consider that the transformer demagnetization is relatively sufficient.

[0053] As Figure 2 shown, the main structure and principle of the capacitor energy storage and release device are as follows:

[0054] The main structure of the capacitor energy storage and release device includes: electrolytic capacitors, AC voltage source, phase-controlled switch, current-limiting resistor.

[0055] The working mode of the capacitor energy storage and release device is: use an AC voltage source to charge the electrolytic capacitor, and adjust the energy storage size of the capacitor by selecting the opening angle of the phase-controlled switch after a period of time. The charging circuit includes a current-limiting resistor, an AC voltage source, a phase-controlled switch, and an electrolytic capacitor. The energy storage release circuit includes a switch, an electrolytic capacitor, and the grid-side winding of the commutation transformer (including the DC resistance of the grid-side winding of the commutation transformer and the equivalent inductance of the grid-side winding of the commutation transformer). After the electrolytic capacitor is fully charged, release the capacitor energy through the energy storage release circuit, generate an RLC oscillation in the energy storage release circuit, and the alternating current in the transformer winding generates an alternating magnetic field, and the alternating magnetic field disrupts the magnetic domain arrangement of the transformer core material, thereby realizing the demagnetization of the transformer core.

[0056] To illustrate the implementation method of the present invention and verify the effectiveness of the said method, use a capacitor energy storage and release device to charge and demagnetize a commutation transformer with a capacity of 237.4 MVA and grid-side and valve-side voltages of 525 kV / 161.2 kV. The total capacity of the electrolytic capacitors in the capacitor energy storage and release device is 1.5 mF. A phase-selection switch is used to disconnect at the voltage peak in the charging circuit, and the initial voltage of the AC power supply is set to 800 V. The initial energy storage of the device is calculated to be 480 J.

[0057] A capacitive energy storage and release device is connected to the grid-side winding of the converter transformer with residual magnetism. Through the energy storage release circuit in the device, the initial energy storage is released. The capacitor has an initial voltage. At the same time, the grid-side winding of the converter transformer includes inductance and resistance. Therefore, the capacitive energy storage forms an RLC circuit through the electrolytic capacitor, the DC resistance on the grid side of the converter transformer, and the inductance on the grid side of the converter transformer, which generates an oscillating and decaying voltage and current in the grid-side winding of the converter transformer. Record the data of the voltage and current, and draw the curves of the voltage and current changing with time. The curves are shown in Figure 3 and Figure 4 as shown. Draw the hysteresis loop during the demagnetization process as shown in Figure 5 as shown.

[0058] According to the hysteresis loop during the demagnetization process, translate it by Δψ = -1334.76 Wb so that the end point of the hysteresis loop is translated to (0, 0). Calculate the areas of the first, second, third, and fourth quadrants of the translated hysteresis loop. The areas of each quadrant are calculated as shown in Table 1 below.

[0059] Table 1 Areas of each quadrant of the hysteresis loop after translation during the first demagnetization

[0060]

[0061] According to the calculated areas, the area of the third quadrant is greater than that of the first quadrant, and it is determined that the original residual magnetism of the converter transformer is in the positive direction. When determining that the original residual magnetism is in the positive direction, the formula for calculating the residual magnetism of the transformer based on the area of the second quadrant of the hysteresis loop is as follows:

[0062]

[0063] It is known that the core of the converter transformer weighs 153 tons, and the density of the silicon steel material is 7650 kg / m 3 . The calculated value is 10.0625. Based on the magnetic field strength - magnetic induction intensity and magnetic field strength - magnetic permeability curves of the silicon steel material, determine the unique residual magnetism corresponding to the area of the second quadrant of the hysteresis loop. The curves of magnetic induction intensity - magnetic field strength and magnetic permeability - magnetic field strength under DC magnetization of the silicon steel sheet are shown in Figure 6 as shown, and B r+ = 0.83952 T. Therefore, it is determined that the direction of the residual magnetism during the first demagnetization is positive, and the original residual magnetism of the converter transformer is 0.83952 T.

[0064] Referring to the above process, charge the capacitive energy storage and release device again and demagnetize the converter transformer. The hysteresis loop during the demagnetization process is shown in Figure 5 as shown. After translating the hysteresis loop, calculate the areas of its four quadrants. The areas of each quadrant of the hysteresis loop after processing during the second demagnetization are shown in Table 2 below.

[0065] Table 2 Areas of Each Quadrant of the Hysteresis Loop after Translation during the Second Degaussing

[0066]

[0067] The residual magnetism direction is judged to be the positive direction, and calculated according to the area of the second quadrant The value of is 0.8887. Based on the magnetic field strength - magnetic induction intensity and magnetic field strength - magnetic permeability curves of the silicon steel material, the unique residual magnetism corresponding to the area of the second quadrant of the hysteresis loop is determined. B′ r+ = 0.1857T. Therefore, the residual magnetism after the first degaussing of the transformer is 0.1857T, and the residual magnetism direction is still the positive direction. When the set threshold a is 0.2T, after the second degaussing, the degaussing stops, and it is considered that the degaussing is relatively sufficient. When the set threshold a is 0.1T, the voltage amplitude needs to be adjusted, the capacitor energy storage and release device is charged again, and the degaussing operation is carried out again. The original direction and value of the residual magnetism in the previous time are judged based on the area of the degaussing hysteresis loop, and then compared with the threshold to determine whether to stop degaussing. Combining the analysis of the no-load current test of the transformer, the no-load current under 50Hz and 380V AC voltage after the first degaussing is 3.66mA, and the no-load current of the converter transformer without residual magnetism is 3.6mA. It can be considered that the first degaussing is relatively sufficient. If it is necessary to further reduce the residual magnetism, the capacitor energy storage and release device needs to be used again for degaussing

[0068] In summary, the dynamic elimination method for the residual magnetism of the core of the converter transformer proposed by the present invention can evaluate the value and direction of the residual magnetism of the previous degaussing in real time during the degaussing process of the transformer, which can provide a reference for the rapid and effective elimination of the residual magnetism of the converter transformer

[0069] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to the preferred embodiments, those of ordinary skill in the art should understand that the technical solutions of the present invention can be modified or equivalently replaced without departing from the purpose and scope of the present technical solution, and they should all be covered by the scope of the claims of the present invention

Claims

1. A method for dynamically eliminating residual magnetism in a converter transformer core, characterized in that: First, the capacitor energy storage and release device is charged; then the device is connected to both ends of the transformer winding, an alternating current is generated in the transformer winding, and the alternating current causes an alternating magnetic field, which deflects the magnetic domains of the transformer core material and disrupts the original directional arrangement of the magnetic domains to eliminate the residual magnetism of the transformer core; at the same time, the data of the grid-side winding current and the winding terminal voltage of the converter transformer during demagnetization are recorded, and the hysteresis loop of the transformer demagnetization process is drawn; then the end point of the hysteresis loop is translated to the origin, and the direction and size of the residual magnetism of the initial demagnetization are determined by the area of ​​each quadrant of the hysteresis loop; Then the transformer is demagnetized again, and the direction and size of the residual magnetism of the first demagnetization are evaluated based on the hysteresis loop of the second demagnetization to determine whether the residual magnetism is less than the threshold. If it is less than the threshold, the demagnetization is stopped; if it is greater than the threshold, the energy of the capacitor energy storage and release device is continued to be adjusted to demagnetize the transformer and evaluate the residual magnetism until its residual magnetism value is less than the threshold.

2. The method for dynamically eliminating residual magnetism of the converter transformer core according to claim 1, characterized in that: The capacitive energy storage and release device includes an electrolytic capacitor, an AC voltage source, a phase-controlled switch and a current-limiting resistor; The working mode of the capacitor energy storage and release device is as follows: the electrolytic capacitor is charged with an AC voltage source, and after a period of time, the energy storage size of the capacitor is adjusted by selecting the opening angle of the phase-controlled switch; the charging circuit includes a current limiting resistor, an AC voltage source, a phase-controlled switch and an electrolytic capacitor; the energy storage release circuit includes a switch, an electrolytic capacitor and a grid-side winding of a converter transformer; after the electrolytic capacitor is charged, the capacitor energy is released through the energy storage release circuit, and RLC oscillation is generated in the energy storage release circuit. The alternating current in the transformer winding generates an alternating magnetic field. The alternating magnetic field disrupts the magnetic domain arrangement of the transformer core material, thereby achieving demagnetization of the transformer core.

3. The method for dynamically eliminating residual magnetism of the converter transformer core according to claim 1 or 2, characterized in that: Drawing the hysteresis loop during the demagnetization process of the transformer specifically includes the following steps: S101: collecting voltage and current data of the converter transformer during the demagnetization process of the grid-side winding of the converter transformer connected to the capacitor energy storage and release device, and recording the time t corresponding to the voltage u(t) and current i(t) of each row; S102: Draw a curve of time-voltage data, starting from the time of the first voltage peak, denoted as t start The end point is when the voltage drops to 0, denoted as t end ; Integrate the voltage u(t) to calculate the flux ψ(t), and the calculation formula is as follows: After obtaining the value of the magnetic flux ψ corresponding to each moment, the current i(t) is used as the X-axis data and the magnetic flux ψ(t) is used as the Y-axis data. The curve of current i(t) and magnetic flux ψ(t) is the hysteresis loop in the demagnetization process.

4. The method for dynamically eliminating residual magnetism of the converter transformer core according to claim 3, characterized in that: The end point of the hysteresis loop is shifted to the origin, specifically, the flux change Δψ caused by the external energy applied during the demagnetization process of the transformer is calculated, and the hysteresis loop is shifted Δψ so that its end point falls at the origin (0,0). The specific steps are as follows: S201: Based on the curve of current i(t) and flux linkage ψ(t), analyze the current i(t) at the end of the curve end and the flux value ψ end Since the initial value of the flux integral is 0, the flux change Δψ caused by the external energy during the transformer demagnetization process is ψ end , that is, Δψ=ψ end ; S202: Subtract the value of Δψ from all the data in the column of flux ψ(t), and keep the current value unchanged, so that the end point of the curve of current i(t) and flux ψ(t) falls at the origin (0,0).

5. The method for dynamically eliminating residual magnetism of the converter transformer core according to claim 4, characterized in that: The method of judging the direction and size of the residual magnetism of the initial demagnetization by the area of ​​each quadrant of the hysteresis loop is specifically to calculate the areas of the first, second, third and fourth quadrants of the hysteresis loop respectively, and to judge the direction of the original residual magnetism based on the areas of the first and third quadrants, and to judge the size of the original residual magnetism based on the areas of the second and fourth quadrants, and specifically includes the following steps: S301: for the translated curve of current i(t) and flux linkage ψ(t), the areas of the curve in the first, second, third and fourth quadrants are calculated respectively, that is, the flux linkage ψ(t) is integrated with the current i(t) to calculate the area; when the curve passes through each quadrant multiple times, the areas of the corresponding quadrants are accumulated; S302: judging the direction of the flux generated by the transformer residual magnetism and demagnetization according to the areas of the first and third quadrants of the hysteresis loop; the direction of the flux generated by the capacitor energy storage and release device is assumed to be positive, and when the area of ​​the first quadrant is the largest among the four quadrants of the hysteresis loop, the original residual magnetism direction of the transformer core is negative; when the area of ​​the third quadrant is the largest among the four quadrants of the hysteresis loop, the original residual magnetism direction of the transformer core is positive; S303: The second-quadrant area of ​​the hysteresis loop represents the energy of hysteresis loss in the process of demagnetizing the transformer under positive residual magnetism until the coercive force is 0; the fourth-quadrant area of ​​the hysteresis loop represents the energy of hysteresis loss in the process of demagnetizing the transformer under negative residual magnetism until the coercive force is 0; when the original residual magnetism direction of the commutator transformer is determined, the original residual magnetism size of the transformer is calculated according to the second-quadrant and fourth-quadrant areas of the hysteresis loop.

6. The method for dynamically eliminating residual magnetism of the converter transformer core according to claim 5, characterized in that: In step S303, when it is determined that the original residual magnetism is in the negative direction, the formula for calculating the transformer residual magnetism according to the four-quadrant area of ​​the hysteresis loop is as follows: When the original residual magnetism is determined to be in the positive direction, the formula for calculating the transformer residual magnetism based on the second quadrant area of ​​the hysteresis loop is as follows: Where S2 represents the second quadrant area of ​​the hysteresis loop, S4 represents the fourth quadrant area of ​​the hysteresis loop, and W Br Characterizes the residual magnetic energy storage of the core, B r- , B r+ is the residual magnetic flux density of the core when the original residual magnetism is in the negative direction and the positive direction, u r is the core remanence B r The corresponding magnetic permeability is as follows, M is the weight of the transformer core, and ρ is the density of the transformer core material.

7. The method for dynamically eliminating residual magnetism of a converter transformer core according to claim 6, characterized in that: In step S303, B r with u r The parameters are obtained from the magnetization curve of the corresponding commutation transformer silicon steel sheet model.

8. The method for dynamically eliminating residual magnetism of the converter transformer core according to claim 6, characterized in that: The energy of the capacitor energy storage and release device is adjusted by adjusting the opening angle of the phase selection switch of the capacitor energy storage and release device to adjust the electrolytic capacitor energy storage, or by changing the voltage amplitude of the AC voltage source to adjust the electrolytic capacitor energy storage.

9. The method for dynamically eliminating residual magnetism of the converter transformer core according to claim 8, characterized in that: The calculation expression of electrolytic capacitor energy storage is as follows: Where W c It represents the energy storage capacity of the electrolytic capacitor, C represents the capacitance value of the electrolytic capacitor, and U is the voltage amplitude when the AC voltage source is disconnected by the phase-controlled switch in the charging circuit.

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