A method for extracting zero-sequence electromagnetic parameters of transformer under AC / DC superposition excitation

Abstract

The present invention relates to a method for extracting zero-sequence electromagnetic parameters of a transformer under AC / DC superposition excitation, and belongs to the field of electric power technology. The method establishes an electromagnetic field model of a three-phase transformer, simulates the field-circuit coupling model of the three-phase transformer under different working conditions based on the model, and simultaneously applies a time-domain field-circuit coupling method to calculate the leakage flux of the three-phase transformer under zero-sequence connection based on the energy perturbation principle to obtain a three-dimensional visual simulation diagram of the time-space distribution of the oil gap and the oil tank zero-sequence flux, thereby performing dynamic leakage inductance parameter analysis. The present invention can supplement the zero-sequence parameters that are simplified or ignored in traditional lumped models and distributed models, thereby reducing the simulation calculation error of the zero-sequence flux of the three-phase transformer and providing a basic model for large-scale full electromagnetic transient simulation of AC / DC hybrid power grids.

Description

Technical Field

[0001] The invention belongs to the technical field of electric power and relates to a method for extracting zero-sequence electromagnetic parameters of a transformer under AC / DC superposition excitation. Background Art

[0002] AC / DC hybrid power grids are a new form of grid development. Their rapid expansion has led to a surge in unbalanced DC single-pole loops or bipolar operating conditions. This has led to increasing DC intrusion into transformer neutral points, exacerbating vibration and overheating, and even causing widespread power outages. Accurately characterizing the electromagnetic properties of the oil tank and oil gap under AC / DC superposition excitation at the transformer neutral point requires a complete transformer electromagnetic transient model adapted for AC / DC hybrid power grids. This work is premised on revealing the spatiotemporal distribution of the zero-sequence magnetic flux in the oil gap and tank under AC / DC excitation, as well as analytical analysis of the zero-sequence dynamic leakage inductance parameters.

[0003] Regarding electromagnetic transient modeling of structural components such as transformer oil tanks, early traditional transformer electromagnetic transient modeling often heavily linearized the oil tank and oil gap, with its inductance parameters obtained from standard zero-sequence testing. Essentially, this model can only characterize the linear characteristics of the nonlinear inductance, but cannot characterize the saturation nonlinear characteristics of the oil tank. To enhance the ability of transformer electromagnetic transient models to characterize nonlinear zero-sequence characteristics, a lumped model based on Cauer circuits and a distributed model that takes into account structural components such as the oil tank have been proposed. The former assumes that all three-phase zero-sequence magnetic flux flows into the tank wall, simplifying the eddy currents in the tank wall to circulating currents along the perimeter of the tank wall. The entire tank wall is simulated using a 25th-order Γ-type single-value nonlinear topological circuit. The latter, based on the leakage flux distribution of the structural components and the oil gap, establishes an equivalent topological circuit model of nonlinear inductors and resistors in series and parallel, which has a certain ability to characterize the spatial distribution of leakage flux.

[0004] However, the above models all have corresponding shortcomings: 1. Under AC and DC superposition excitation, for the characterization of zero-sequence magnetic flux, the lumped model only simulates the tank wall to a high degree, simplifying the characterization of the zero-sequence magnetic flux of structural components such as the tank cover, tank bottom, oil gap, and pull plate. It does not consider the local dynamic circulating current characteristics of the tank, resulting in inaccurate zero-sequence magnetic flux characterization results. 2. Under AC and DC superposition excitation, for the characterization of zero-sequence magnetic flux, the distributed model also only considers its axial non-uniform distribution, ignoring the radial non-uniform distribution and edge effects. However, when the transformer capacity is large, the radial distribution cannot be ignored. 3. Both the lumped model and the distributed model extract model parameters based on power frequency zero-sequence test data, making it difficult to accurately characterize the time-varying local saturation and dynamic time-varying characteristics of the tank stray losses. Summary of the Invention

[0005] In view of this, the purpose of the present invention is to provide a method for extracting zero-sequence electromagnetic parameters of a transformer under AC / DC superposition excitation, extract the zero-sequence flux of the oil gap and the oil tank, supplement the zero-sequence parameters that are simplified or ignored by traditional lumped models and distributed models, thereby reducing the calculation error of the zero-sequence flux simulation of the three-phase transformer, and providing a basic model for large-scale full electromagnetic transient simulation of AC / DC hybrid power grids.

[0006] In order to achieve the above object, the present invention provides the following technical solutions:

[0007] A method for extracting zero-sequence electromagnetic parameters of a transformer under AC / DC superposition excitation is proposed. This method establishes an electromagnetic field model of a three-phase transformer and simulates the field-circuit coupling model of the three-phase transformer under different working conditions based on this model. At the same time, a time-domain field-circuit coupling method is applied to calculate the leakage flux of the three-phase transformer under zero-sequence connection based on the energy perturbation principle to obtain a three-dimensional visual simulation diagram of the spatiotemporal distribution of the zero-sequence flux of the oil gap and the oil tank, thereby performing analytical analysis of the dynamic leakage inductance parameters.

[0008] Preferably, an electromagnetic field model of a three-phase transformer is established using ANSYS software based on the topological structure, material properties, and dimensional parameters of a two-winding three-phase three-leg transformer. The three-phase three-leg transformer includes three-phase iron legs (A, B, and C), upper and lower iron yokes, and three-phase windings; the upper and lower iron yokes are respectively arranged on the upper and lower sides of the three-phase iron legs; the three-phase windings are arranged with the high-voltage winding outside and the low-voltage winding inside.

[0009] Preferably, the three-phase three-column transformer is arranged in an oil tank model.

[0010] Preferably, based on normal three-phase AC excitation of the three-phase transformer, a DC voltage source is connected in series with the neutral point to simulate neutral point DC intrusion, thereby obtaining a field-circuit coupling model of the three-phase transformer under different operating conditions. The operating conditions include normal three-phase AC excitation, zero-sequence connection, three-phase AC / DC superposition excitation, and three-phase AC / DC superposition excitation with zero-sequence connection.

[0011] Preferably, the method for calculating the dynamic leakage inductance of the transformer is: first, a circuit model of the three-phase transformer under DC bias is established, and a time-domain differential equation is constructed based on the model:

[0012]

[0013] Then, a T-type equivalent circuit is established, ignoring the mutual inductance between high-voltage windings of different phases and the mutual inductance between low-voltage windings of different phases. At the same time, the secondary side parameters are converted to the primary side, and the equivalent circuit matrix equation is obtained:

[0014]

[0015] Through the dynamic leakage inductance matrix [L0] and the excitation matrix [L e]Calculate the leakage flux of the three-phase transformer under zero-sequence connection.

[0016] The beneficial effects of the present invention are as follows: the present invention adopts the finite element method to establish an electromagnetic field model of a three-phase transformer in two dimensions of time and space, establishes the transmission path of the zero-sequence magnetic flux through simulation, and uses the energy balance method combined with the T-type equivalent circuit to separate the dynamic leakage inductance, thereby supplementing the zero-sequence parameters simplified or ignored by the traditional lumped model and distributed model, reducing the simulation calculation error of the zero-sequence magnetic flux of the three-phase transformer, and providing a basic model for large-scale full electromagnetic transient simulation of AC / DC hybrid power grids.

[0017] Other advantages, objects, and features of the present invention will be described in part in the following description and, in part, will be apparent to those skilled in the art upon examination of the following description or may be learned from practice of the present invention. The objects and other advantages of the present invention may be realized and obtained through the following description. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] In order to make the purpose, technical solutions and advantages of the present invention more clear, the present invention will be described in detail below with reference to the accompanying drawings, in which:

[0019] Figure 1 is the electromagnetic field model of a three-phase transformer;

[0020] Figure 2 The field-circuit coupling model of the three-phase transformer under different working conditions; (a) is the normal three-phase AC excitation working condition, (b) is the three-phase AC / DC superposition excitation working condition, (c) is the zero-sequence connection working condition, and (d) is the zero-sequence connection working condition when the three-phase AC / DC superposition excitation is applied;

[0021] Figure 3 It is the transformer circuit model under DC bias;

[0022] Figure 4 is the T-type equivalent circuit of the transformer;

[0023] Figure 5 This is the front view of the change of core magnetic flux density under normal excitation and DC intrusion;

[0024] Figure 6 This is a side view of the change in core magnetic flux density under normal excitation and DC intrusion;

[0025] Figure 7 It is a top view of the change of core magnetic flux density under normal excitation and DC intrusion;

[0026] Figure 8 This is a front view of the change in core magnetic flux density when zero sequence wiring and DC intrusion occurs;

[0027] Figure 9This is a side view of the change in core magnetic flux density when zero sequence wiring is connected and DC intrusion occurs;

[0028] Figure 10 It is a top view of the change of the core magnetic flux density when zero sequence wiring and DC intrusion occur;

[0029] Figure 11 This is the front view of the change of oil gap magnetic flux density under different working conditions;

[0030] Figure 12 It is the side view of the change of oil gap magnetic flux density under different working conditions;

[0031] Figure 13 It is a top view of the change of oil gap magnetic flux density under different working conditions;

[0032] Figure 14 The effect of DC intrusion on the magnetic flux leakage of the fuel tank under normal three-phase excitation;

[0033] Figure 15 The impact of DC intrusion on magnetic flux leakage in the oil tank during zero-sequence wiring;

[0034] Figure 16 The effect of DC intrusion on winding current under normal three-phase excitation;

[0035] Figure 17 This is the effect of DC intrusion on the eddy current loss of the fuel tank under normal three-phase excitation. DETAILED DESCRIPTION

[0036] The following describes the embodiments of the present invention by means of specific examples, and those skilled in the art can easily understand other advantages and effects of the present invention from the contents disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and the details in this specification can also be modified or changed in various ways based on different viewpoints and applications without departing from the spirit of the present invention. It should be noted that the illustrations provided in the following embodiments are only schematic illustrations of the basic concept of the present invention, and the following embodiments and features in the embodiments can be combined with each other without conflict.

[0037] Among them, the accompanying drawings are only for illustrative purposes and represent only schematic diagrams rather than actual pictures, and should not be understood as limiting the present invention. In order to better illustrate the embodiments of the present invention, some parts of the accompanying drawings may be omitted, enlarged or reduced, and do not represent the dimensions of actual products. For those skilled in the art, it is understandable that some well-known structures and their descriptions may be omitted in the accompanying drawings.

[0038] The same or similar numbers in the drawings of the embodiments of the present invention correspond to the same or similar parts; in the description of the present invention, it should be understood that if there are terms such as "upper", "lower", "left", "right", "front", "back", etc. indicating directions or positional relationships, they are based on the directions or positional relationships shown in the drawings. They are only for the convenience of describing the present invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific direction, be constructed and operate in a specific direction. Therefore, the terms describing the positional relationship in the drawings are only used for illustrative purposes and cannot be understood as limiting the present invention. For ordinary technicians in this field, the specific meanings of the above terms can be understood according to specific circumstances.

[0039] This paper establishes a three-phase transformer electromagnetic field model based on the finite element method and couples it with circuit models under four different operating conditions to extract a three-dimensional visualization of the spatiotemporal distribution of the zero-sequence magnetic flux in the oil gap and oil tank. This method can supplement the zero-sequence parameters that are simplified or ignored in traditional lumped and distributed models, thereby reducing the error in the simulation calculation of the zero-sequence magnetic flux of the three-phase transformer. The present invention mainly includes the following three parts:

[0040] 1. Three-phase transformer electromagnetic field model based on ANSYS

[0041] In order to study the influence of oil tank on large transformer, this paper takes the topology structure, material properties and size parameters of double-winding three-phase three-column transformer as the basis, uses ANSYS software to establish the magnetic field model of transformer, and uses finite element method to analyze and calculate the parameters of magnetic field. Figure 1 The electromagnetic field model of a three-phase transformer is shown. The three cylinders in the figure represent the transformer's three-phase windings, wound on a three-phase iron core, with the high-voltage winding on the outside and the low-voltage winding on the inside. Yokes are attached to the top and bottom of the core. The three-phase, three-leg transformer is placed inside an oil tank model. The domain for solving the magnetic field model of the three-phase, three-leg transformer is the outer surface of the mailbox model.

[0042] The upper and lower yokes and core are made of custom magnetic silicon steel sheets, the coil material is copper, and the fuel tank material is 1010 carbon steel. The excitation is the coil current calculated iteratively using a time-domain differential circuit.

[0043] 2. Field-circuit coupling model of three-phase transformer under different working conditions

[0044] The primary side of the three-phase transformer is connected in star-type grounding mode, and the secondary side is also star-type and short-circuited. Based on the traditional zero-sequence experiment and normal three-phase excitation, the present invention connects a DC voltage source in series on the neutral point to simulate the DC intrusion of the neutral point, thereby obtaining the circuit model of the three-phase transformer under four different working conditions. The working condition types are: normal three-phase AC excitation, such as Figure 2 (a) As shown; three-phase AC and DC superposition excitation, such as Figure 2(b) As shown; zero sequence wiring, such as Figure 2 (c) As shown; zero sequence connection when three-phase AC / DC superposition excitation is applied, as shown Figure 2 (d) shown.

[0045] 3. Analysis of dynamic leakage inductance parameters of three-phase transformer under zero-sequence connection

[0046] The energy method is used to calculate the inductance parameters with high accuracy. The time domain field-circuit coupling method is used to calculate the transformer dynamic inductance based on the energy disturbance principle. The total leakage inductance can be equivalently distributed to both sides of the transformer through the T-type equivalent circuit, thereby calculating the single-side leakage inductance. The transformer circuit model under DC bias is as follows: Figure 3 As shown, taking phase A as an example, the time domain differential equation of the circuit system under DC bias is:

[0047]

[0048] Combine Figure 4 The T-type equivalent circuit shown ignores the mutual inductance between the high-voltage windings of different phases and the mutual inductance between the low-voltage windings of different phases, and converts the secondary side parameters to the primary side, and derives the equivalent circuit matrix equation:

[0049]

[0050] Through the dynamic leakage inductance matrix [L0] and the excitation matrix [L e ] can calculate the leakage flux of the three-phase transformer under zero-sequence connection, providing a basis and basis for studying the variation characteristics of oil gap leakage flux and physical partitioning.

[0051] The relevant data extracted by the method of the present invention are as follows:

[0052] 1. Spatiotemporal distribution of magnetic flux density at the transformer core interface under different working conditions

[0053] (1) Spatiotemporal distribution of magnetic flux density at different cross sections during normal three-phase AC excitation and DC intrusion

[0054] like Figure 5 As shown in the figure, when viewed from the front, the magnetic field intensity of the middle core is higher than that of the two side cores, and the magnetic flux density distribution in the iron yoke changes significantly with time. DC intrusion will lead to an increase in the magnetic flux density in the iron core, and the maximum magnetic flux density change of the iron core can reach 0.4T. Figure 6 As shown in the figure, from the side, the magnetic flux density in the middle part of the core is relatively small, and DC intrusion will increase the magnetic flux density in this part. The maximum magnetic flux density change of the core can reach 0.1T. Figure 7 As shown in the figure, the magnetic flux density of the middle core is higher than that of the cores on both sides. DC intrusion will enhance the magnetic flux density of the middle core, and the maximum magnetic flux density change of the core can reach 0.13T.

[0055] (2) Temporal and spatial distribution of magnetic flux density in different cross sections when zero-sequence connection is subjected to DC intrusion

[0056] From the positive side, DC intrusion will lead to an increase in the magnetic flux density of the core leg, such as Figure 8 As shown in the figure, compared with the three-phase excitation, the maximum magnetic flux change of the core is greater when the zero-sequence connection is used, which can reach 0.22T. Figure 9 As shown in the figure, the magnetic flux density in the middle part of the core is relatively large. DC intrusion will cause the magnetic flux density of the core column to increase. Compared with three-phase excitation, the maximum magnetic flux density of the core in zero-sequence connection changes more greatly, which can reach 0.4T. Figure 10 As shown in the figure, the three-phase core and winding flux density distributions are basically the same. DC intrusion will lead to an increase in the core flux density. Compared with three-phase excitation, the change in the maximum core flux density is greater, reaching 0.24T.

[0057] 2. Spatial and temporal distribution of magnetic flux in oil gap section under different working conditions

[0058] like Figure 11 As shown in the figure, viewed from the front, the magnetic flux density in the oil gap under zero-sequence connection is significantly higher than that under three-phase excitation. DC intrusion significantly enhances the magnetic flux density near the iron yoke in the oil gap, and the range of magnetic flux density exceeding 0.01 T expands. Under normal three-phase AC excitation, DC intrusion increases the magnetic flux density near the iron yoke in the oil gap, and the range of magnetic flux density exceeding 0.005 T expands.

[0059] like Figure 12 As shown in the figure, from the side, the magnetic flux density of the upper part of the oil tank under zero-sequence connection is higher than that under three-phase excitation. DC intrusion will cause the oil gap magnetic flux density to vary more, and it is more obvious under normal three-phase AC excitation. Figure 13 As shown in Figure 2, under normal three-phase AC excitation and zero-sequence connection, DC intrusion will enhance the radial flux density in the oil gap.

[0060] 3. Impact of DC intrusion on tank magnetic leakage under different working conditions

[0061] like Figure 14 As shown in Figure 1, under normal three-phase excitation, DC intrusion will change the distribution of the transformer tank leakage flux, resulting in increased leakage flux in some areas of the tank and slight saturation. In zero-sequence connection, since the three-phase magnetic flux is in the same direction, the magnetic flux flowing into the tank increases, and some areas of the transformer tank are locally saturated, as shown in Figure 1. Figure 15 As shown in the figure, compared with the normal three-phase excitation, the saturation degree is aggravated and the area is increased.

[0062] 4. Impact of DC intrusion on other parameters under different working conditions

[0063] like Figure 16As shown in the figure, under normal three-phase excitation, DC intrusion will cause the sum of the three-phase winding currents to be non-zero, and zero-sequence current will appear at the neutral point; in zero-sequence connection, DC intrusion will cause the zero-sequence current to increase in the first half of the cycle and decrease in the second half of the cycle. Figure 17 As shown in the figure, under normal three-phase excitation, DC intrusion will cause a significant increase in eddy current in the transformer tank, resulting in a surge in eddy current losses in the tank; under zero-sequence connection, DC intrusion will cause an increase in eddy current and losses in the tank in the first half of the cycle, and a decrease in eddy current and losses in the tank in the second half of the cycle.

[0064] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not limiting. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solutions of the present invention can be modified or replaced by equivalents without departing from the purpose and scope of the technical solutions, which should all be included in the scope of the claims of the present invention.

Claims

1. A method for extracting zero-sequence electromagnetic parameters of a transformer under AC / DC superposition excitation, characterized by: This method establishes an electromagnetic field model of a three-phase transformer and simulates the field-circuit coupling model of the three-phase transformer under different working conditions based on this model. At the same time, the time-domain field-circuit coupling method is applied to calculate the leakage flux of the three-phase transformer under zero-sequence connection based on the energy perturbation principle to obtain a three-dimensional visualization simulation diagram of the time-space distribution of the zero-sequence flux of the oil gap and the oil tank, thereby performing dynamic leakage inductance parameter analysis. Among them, based on the normal three-phase AC excitation of the three-phase transformer, by connecting a DC voltage source in series on the neutral point to simulate the DC intrusion of the neutral point, the field-circuit coupling model of the three-phase transformer under different working conditions is obtained; the working conditions include normal three-phase AC excitation, zero-sequence connection, three-phase AC / DC superposition excitation, and zero-sequence connection with three-phase AC / DC superposition excitation; The method for calculating the leakage flux of a three-phase transformer under zero-sequence connection is as follows: first, a circuit model of the three-phase transformer under DC bias is established, and then a time-domain differential equation is constructed based on the model: Then, a T-type equivalent circuit is established, ignoring the mutual inductance between high-voltage windings of different phases and the mutual inductance between low-voltage windings of different phases. At the same time, the secondary side parameters are converted to the primary side, and the equivalent circuit matrix equation is obtained: Through the dynamic leakage inductance matrix [L0] and the excitation matrix [L e ]Calculate the leakage flux of the three-phase transformer under zero-sequence connection.

2. The method for extracting zero-sequence electromagnetic parameters of a transformer according to claim 1, wherein: The electromagnetic field model of the three-phase transformer is established using ANSYS software based on the topological structure, material properties and dimensional parameters of a double-winding three-phase three-column transformer. Specifically, the three-phase three-column transformer includes three-phase iron core columns A, B and C, upper and lower iron yokes, and three-phase windings; the upper and lower iron yokes are respectively arranged on the upper and lower sides of the three-phase iron core column; the three-phase windings are arranged with the high-voltage winding outside and the low-voltage winding inside.

3. The method for extracting transformer zero-sequence electromagnetic parameters according to claim 2, wherein: The three-phase three-column transformer is arranged in an oil tank model.

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