Method and device for calculating electromagnetic-heat flow field under turn-to-turn short circuit fault of converter transformer

Abstract

The invention discloses a method and a device for calculating an electromagnetic-heat flow field under a turn-to-turn short circuit fault of a converter transformer. The method comprises the following steps: establishing a converter transformer multi-physical field numerical calculation theoretical model based on circuit-electromagnetic-heat flow coupling; based on the multi-physical field numerical calculation theoretical model, establishing a converter transformer electromagnetic-heat flow field model under the turn-to-turn short circuit fault; setting turn-to-turn short circuit faults at different positions in the electromagnetic-heat flow field model of the converter transformer; and performing winding current waveform, voltage waveform and electromagnetic field analysis and heat flow field analysis of the converter transformer under the set turn-to-turn short circuit fault.

Description

Technical Field

[0001] The invention relates to an electromagnetic-thermal flow field calculation method and device under a turn-to-turn short-circuit fault of a converter transformer based on field-circuit coupling, and belongs to the technical field of power systems. Background Art

[0002] The converter transformer is a key component in the ultra-high voltage and ultra-high voltage direct current transmission system. Its main function is to realize the conversion between direct current and alternating current. Together with the converter, it constitutes the hub of energy conversion in the AC / DC system. Therefore, ensuring the normal operation of the converter transformer is an indispensable part of the high voltage direct current transmission project. The converter transformer and the converter together constitute an important part of the stable operation of the power grid.

[0003] As one of the most common faults in power transformers, turn-to-turn short circuit refers to the reduction of insulation resistance or even complete short circuit caused by insulation aging or failure between two or more turns in the transformer winding coil. Under normal circumstances, the outer layer of the winding coil is covered with insulating material for isolation. During the operation of power equipment, due to the combined influence of factors such as thermal stress, mechanical stress, electrical stress, and environmental pollution, the inter-turn insulation material gradually ages, resulting in a decrease in insulation performance, thereby reducing the insulation resistance between coil turns. When the insulation performance drops to a certain extent, electric field distortion, electrical tree growth, or electrical breakdown occurs in the insulation material under the action of electrical stress, resulting in insulation failure and turn-to-turn short circuit. Turn-to-turn short circuit faults that have not been detected or handled for a long time will accelerate the aging of the transformer and shorten its service life. Frequent local overheating and insulation damage may lead to irreparable failures of the transformer, such as winding burnout or core damage. When a turn-to-turn short circuit fault with complete insulation failure occurs, the resistance between the short-circuited turns is almost zero, so a huge short-circuit current will be generated in a short time. This large current will significantly increase local losses, generate excessive heat, cause winding overheating, and further aggravate insulation degradation and faults, which may eventually lead to a larger short circuit or winding burnout. If the inter-turn short circuit causes severe winding damage or local overheating of the core, the transformer may eventually need to be overhauled or even the winding or core replaced. In severe cases, the transformer may be scrapped prematurely due to irreparable damage. For power systems, inter-turn short circuits change the turns ratio of the transformer and affect the output voltage of the transformer. Since the number of short-circuited turns reduces the effective number of turns of the winding, the output voltage on the secondary side is unstable, and voltage fluctuations, distortion, and even a sharp drop in output power may occur. Inter-turn short circuits may also cause the waveform of the current to be distorted and generate more harmonics. Harmonics not only affect the quality of power, but also interfere with other equipment in the power system and increase the loss of electrical equipment.

[0004] Since the DC transmission system has the characteristics of high voltage level, long transmission distance and large capacity, the converter transformer, as a key equipment, has many different characteristics compared with ordinary power transformers. The converter transformer needs to face the high-frequency pulses and non-sinusoidal currents generated during the commutation process, and is easily affected by harmonics and DC bias to produce high-frequency losses and additional magnetic losses caused by DC bias; compared with power transformers, the winding and core structure design of the converter transformer itself is very different from that of power transformers, and the electrical connection between the converter transformer and the converter, power supply and external load is also more complicated; in addition, the converter transformer is affected by DC voltage bias and high-frequency pulses, and the insulation material of the equipment is more likely to age and cause insulation faults such as electrical breakdown and inter-turn short circuit. Taking the above factors into consideration, the research on the fault characteristics of the converter transformer is significantly different from that of the power transformer. Therefore, in order to prevent the inter-turn short-circuit fault from causing serious damage to the converter transformer, it is necessary to simulate and analyze the different physical characteristics of the converter transformer with the inter-turn short-circuit fault, so as to fully understand the spatiotemporal evolution law of the electromagnetic-thermal flow under the inter-turn short-circuit fault of the transformer, which is conducive to early detection and resolution of the inter-turn short-circuit problem, extending the service life of the transformer, and avoiding large-scale equipment damage and power outages.

[0005] The calculation of the electromagnetic field inside the transformer is mainly divided into two calculation models: analytical model and numerical model, while the calculation of the thermal flow field can be divided into analytical model, dynamic network model and numerical model. There are many advantages to using finite element model to analyze the electromagnetic-thermal flow distribution characteristics of converter transformers. Compared with other models, finite element model can more accurately describe the complex physical phenomena inside the converter transformer, especially those involving asymmetry, local effects and multi-physical field coupling. At present, the numerical simulation of converter transformers under different working conditions is mostly focused on studying the calculation of the spatial electromagnetic field and temperature field distribution of the transformer under normal working conditions. Many current studies have provided rich references for the present invention, but there are still certain limitations, including: 1) The numerical calculation of the converter transformer under the condition of inter-turn short-circuit fault is mainly focused on the electromagnetic field distribution and the analysis of the voltage and current change waveforms, and there is a lack of research on the winding overheating and burning caused by loss changes; 2) In addition, the transformer electromagnetic-thermal flow finite element model established in previous studies is mostly based on normal operating conditions, and there is a lack of relevant modeling for the evolution law of the transformer thermal flow field under fault conditions that cause serious damage to the equipment; 3) Two-dimensional models are mostly used for winding modeling, but they cannot reflect the electromagnetic distribution and heat dissipation caused by the asymmetry of the three-dimensional structure of the transformer. It is necessary to establish a three-dimensional model to reflect the electromagnetic field distribution and thermal flow field changes in the overall space. Summary of the invention

[0006] In view of this, the present invention proposes an electromagnetic-thermal flow field calculation method and device under a turn-to-turn short-circuit fault of a converter transformer based on field-circuit coupling. By studying the electromagnetic and thermal flow field distribution of the converter transformer at different turn-to-turn short-circuit fault locations, as well as the dynamic temperature response of the winding in the early stage of the fault, the three-dimensional heat dissipation path of the converter transformer and the temperature change of the winding hot spot during the turn-to-turn short-circuit fault are accurately simulated, thereby providing rich information for the detection, identification and prevention of turn-to-turn short-circuit faults to ensure the stable operation of the converter transformer.

[0007] According to one aspect of the present invention, a method for calculating the electromagnetic-thermal flow field under a turn-to-turn short-circuit fault of a converter transformer is proposed, comprising: establishing a multi-physical field numerical calculation theoretical model of a converter transformer based on circuit-electromagnetic-thermal flow coupling; establishing an electromagnetic-thermal flow field model of a converter transformer under a turn-to-turn short-circuit fault based on the multi-physical field numerical calculation theoretical model; setting turn-to-turn short-circuit faults at different positions in the electromagnetic-thermal flow field model of the converter transformer; and performing winding current waveform, voltage waveform and electromagnetic field analysis, as well as thermal flow field analysis, of the converter transformer under the set turn-to-turn short-circuit fault.

[0008] Furthermore, based on field-circuit coupling, the electromagnetic field equation and circuit equation of the converter transformer are combined to calculate the electromagnetic field distribution of the converter transformer; the electromagnetic loss equation of the converter transformer and the solution result of the electromagnetic field distribution are combined, and the electromagnetic loss is calculated using the electromagnetic field distribution; the electromagnetic loss is substituted into the fluid dynamics equation group as a heat source to realize electromagnetic-heat flow coupling, and the heat flow field distribution of the converter transformer is calculated.

[0009] Furthermore, the calculation of the electromagnetic field distribution of the converter transformer when operating at rated conditions adopts the following Maxwell equations:

[0010]

[0011] in, is the Hamilton operator, E represents the electric field intensity, B represents the magnetic flux density, H represents the magnetic field intensity, J represents the current density, D represents the electric displacement vector, and ρ represents the charge density;

[0012] Calculating electromagnetic loss using the electromagnetic field distribution includes:

[0013] Based on the magnetic flux density and current density calculated by formula (1), the electromagnetic loss in the converter transformer is calculated as shown in formula (2):

[0014]

[0015] Among them, S represents electromagnetic loss, P winding Represents the winding loss, P core represents the core loss, In Represents the normal turn winding current, R n Indicates the normal turn winding resistance, I f Indicates the fault winding current, R f represents the fault turn winding resistance, k, α, β are empirical parameters obtained by measurement, f represents the operating frequency, B m Indicates the maximum value of magnetic flux density;

[0016] The calculating of the heat flow field distribution of the converter transformer comprises:

[0017] Substitute the electromagnetic loss S calculated by formula (2) into the fluid dynamics equations (3) to solve the thermal flow field distribution of the converter transformer:

[0018]

[0019] Among them, d represents the fluid density, ω represents the fluid velocity, F represents the body force vector, p represents the fluid pressure, u represents the fluid dynamic viscosity, c represents the fluid specific heat capacity, and k represents the fluid thermal conductivity.

[0020] Furthermore, the establishment of the electromagnetic-thermal flow field model of the converter transformer under the inter-turn short-circuit fault includes: establishing a three-dimensional geometric model of the converter transformer according to the structural parameters of the converter transformer; performing three-dimensional meshing on the three-dimensional geometric model; and performing field-circuit coupling settings, material parameter settings and solver settings required for solving the model according to the three-dimensional mesh of the converter transformer.

[0021] Furthermore, the three-dimensional geometric model of the converter transformer is established according to the structural parameters of the converter transformer, including: using ANSYS Maxwell of ANSYS software to complete field-circuit coupling and electromagnetic field finite element modeling, and using ANSYS Fluent to establish and solve the fluid dynamics model; wherein, the converter transformer adopts a single-phase four-column iron core, including two main yokes and two side yokes, which are connected by upper and lower yokes; the windings are wound on the two main yokes, and from the inside to the outside are the voltage regulating winding, the grid side winding, and the valve side winding.

[0022] Furthermore, the material parameter settings, field-circuit coupling and solver settings required for solving the model are performed according to the three-dimensional grid of the converter transformer, including: ANSYS Maxwell adopts a field-circuit coupling method, the grid-side winding is powered by a rated industrial frequency AC voltage source, the load-side winding is connected to a rated load, the winding structure parameters are set, the winding current direction is calculated, and the number of turns of each winding is set; the solver is set as a transient solver, and the duration of the transient solver and the time point of the fault occurrence are set; ANSYS Fluent loads the winding and core losses obtained by electromagnetic field calculation into the thermal flow field as heat sources, sets oil flow material parameters, and sets the solver as a steady-state solver.

[0023] Furthermore, turn-to-turn short-circuit faults are set at different positions in the electromagnetic-thermal flow field model of the converter transformer, including: setting different turn-to-turn short-circuit fault positions on the voltage regulating winding; dividing the coil where the turn-to-turn short-circuit fault is located into normal turns and fault turns, and connecting a timing switch and a fault resistor to the fault turn; and simulating the occurrence of the turn-to-turn short-circuit fault by setting the timing switch to be closed at a certain moment.

[0024] Furthermore, the setting of different turn-to-turn short-circuit fault positions on the voltage regulating winding includes: evenly dividing the voltage regulating winding into 1 to 8 layers from top to bottom, and setting the turn-to-turn short-circuit fault positions on layers 1 to 8, respectively; wherein the voltage source connected to the grid-side winding, the load connected to the valve-side winding, and other parameters of the turn-to-turn short-circuit fault remain unchanged.

[0025] Furthermore, the winding current waveform, voltage waveform and electromagnetic field analysis of the converter transformer under the set turn-to-turn short-circuit fault includes: current waveform analysis: according to the change of winding current under the turn-to-turn short-circuit fault, the current surge phenomenon in the faulty winding is obtained; voltage waveform analysis: according to the voltage waveforms of the high and low voltage windings during the turn-to-turn short-circuit fault, the voltage of the grid-side winding and the valve-side winding is analyzed to be affected by the fault when the fault occurs; electromagnetic field analysis: according to the distortion and concentration of the spatial magnetic flux density caused by the surge in current, the spatial electromagnetic distribution is analyzed.

[0026] Furthermore, the thermal flow field analysis includes: analyzing the temperature distribution of each winding in the converter transformer when turn-to-turn short-circuit faults occur at different positions; analyzing the flow velocity distribution of the oil flow in the converter transformer when turn-to-turn short-circuit faults occur at different positions.

[0027] Furthermore, the analysis of the temperature distribution of each winding in the converter transformer when a turn-to-turn short-circuit fault occurs at different positions includes: based on the temperature distribution diagram obtained by setting the turn-to-turn short-circuit fault on different layers of the voltage regulating winding, it is learned that when a turn-to-turn short-circuit fault occurs, the hot spot temperature of the converter transformer is distributed on the faulty turn coil, and the temperature gradually decreases from the faulty turn to the upper and lower ends respectively.

[0028] Furthermore, the analysis of the flow velocity distribution of the oil flow in the converter transformer when turn-to-turn short-circuit faults occur at different positions includes: based on the temperature distribution diagram obtained by setting the turn-to-turn short-circuit fault on different layers of the voltage regulating winding, it is found that the oil flow distribution of the converter transformer under the fault presents an asymmetric distribution, and the overall oil flow velocity on the side where the faulty turn winding is located is higher than that on the other side.

[0029] According to another aspect of the present invention, a computer-readable storage medium is provided, on which a computer program is stored. When the computer program is executed by a processor, the steps of the electromagnetic-thermal flow field calculation method under the inter-turn short-circuit fault of the converter transformer can be implemented.

[0030] According to another aspect of the present invention, an electromagnetic-thermal flow field calculation device for a converter transformer under an inter-turn short-circuit fault is proposed, comprising: a multi-physical field numerical calculation theoretical model construction module, used to establish a multi-physical field numerical calculation theoretical model of a converter transformer based on circuit-electromagnetic-thermal flow coupling; an electromagnetic-thermal flow field model construction module, used to establish an electromagnetic-thermal flow field model of a converter transformer under an inter-turn short-circuit fault based on the multi-physical field numerical calculation theoretical model; an inter-turn short-circuit fault setting module, used to set an inter-turn short-circuit fault at different positions in the electromagnetic-thermal flow field model of the converter transformer; and an analysis module, used to perform winding current waveform, voltage waveform and electromagnetic field analysis, as well as thermal flow field analysis of the converter transformer under the set inter-turn short-circuit fault.

[0031] Furthermore, the multi-physics field numerical calculation theoretical model construction module includes: an electromagnetic field distribution calculation module, which is used to combine the electromagnetic field equations and circuit equations of the converter transformer based on field-circuit coupling to calculate the electromagnetic field distribution of the converter transformer; an electromagnetic loss calculation module, which is used to combine the electromagnetic loss equations of the converter transformer and the electromagnetic field distribution solution results, and use the electromagnetic field distribution to calculate the electromagnetic loss; a heat flow field distribution calculation module, which is used to substitute the electromagnetic loss as a heat source into the fluid dynamics equations to achieve electromagnetic-heat flow coupling and calculate the heat flow field distribution of the converter transformer.

[0032] Furthermore, the electromagnetic-thermal flow field model construction module includes: a three-dimensional geometric model construction module, which is used to establish a three-dimensional geometric model of the converter transformer according to the structural parameters of the converter transformer; a three-dimensional mesh generation module, which is used to perform three-dimensional mesh generation on the three-dimensional geometric model; and a model setting module, which is used to perform field-circuit coupling settings, material parameter settings and solver settings required for solving the model according to the three-dimensional mesh of the converter transformer.

[0033] Furthermore, the turn-to-turn short-circuit fault setting module includes: a fault position setting module, used to set different turn-to-turn short-circuit fault positions on the voltage regulating winding; a coil setting module, used to divide the coil where the turn-to-turn short-circuit fault is located into normal turns and fault turns, and the fault turn is externally connected to a timing switch and a fault resistor; a fault simulation module, used to simulate the occurrence of a turn-to-turn short-circuit fault by setting a closing timing switch at a certain moment.

[0034] Furthermore, the analysis module includes: a current waveform analysis module, which is used to obtain the current surge phenomenon in the faulty winding according to the change of the winding current under the turn-to-turn short-circuit fault; a voltage waveform analysis module, which is used to analyze the impact of the fault on the voltage of the grid-side winding and the valve-side winding when the fault occurs according to the voltage waveforms of the high and low voltage windings during the turn-to-turn short-circuit fault; an electromagnetic field analysis module, which is used to analyze the spatial electromagnetic distribution according to the distortion and concentration of the spatial magnetic flux density caused by the surge in current; a thermal flow field analysis module, which is used to analyze the temperature distribution of each winding in the converter transformer when the turn-to-turn short-circuit fault occurs at different positions, and to analyze the flow velocity distribution of the oil flow in the converter transformer when the turn-to-turn short-circuit fault occurs at different positions.

[0035] The beneficial effects of the present invention are as follows: the present invention analyzes the electromagnetic-heat flow distribution characteristics of the converter transformer under different turn-to-turn short-circuit fault positions based on electromagnetic numerical simulation method and computational fluid dynamics, proposes an electromagnetic-heat flow finite element model under turn-to-turn short-circuit fault of the converter transformer based on field-circuit coupling, analyzes the spatial electromagnetic distribution characteristics of the converter transformer and the voltage and current waveform changes of the winding when the turn-to-turn short-circuit fault occurs, and compares the heat flow field distribution characteristics of the converter transformer under different turn-to-turn short-circuit faults. The invention analyzes the significant increase of the fault turn current and the sharp rise of the winding temperature after the turn-to-turn short-circuit fault, points out the thermal effect of the fault area, especially the appearance of the hottest point, which can provide an important basis for fault location; by simulating the flow of oil flow in the transformer winding, especially when the turn-to-turn short-circuit occurs, the mutual influence of the oil flow velocity and the winding where the fault turn is located is revealed, providing a reference for the optimization of cooling efficiency; points out the relationship between the fault position and the severity of the winding fault, and quantifies the temperature rise and oil flow changes at different fault levels, especially when the fault occurs in the eighth layer, the fault degree is the most serious, which provides a new idea for the structural design and fault monitoring of the converter transformer. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 It is a wiring schematic diagram of a converter transformer according to an embodiment of the present invention.

[0037] Figure 2 It is a connection diagram of the external circuit of the converter transformer according to an embodiment of the present invention.

[0038] Figure 3 3 is the magnetization BH curve of the converter transformer core according to the embodiment of the present invention.

[0039] Figure 4 It is a three-dimensional geometric model diagram of the converter transformer according to an embodiment of the present invention.

[0040] Figure 5-1 and Figure 5-2 They are respectively the winding current waveform and the winding voltage waveform of the converter transformer according to the embodiment of the present invention under a simulated turn-to-turn short circuit fault.

[0041] Figure 6 It is a spatial magnetic flux density distribution diagram of the converter transformer of an embodiment of the present invention under a simulated turn-to-turn short circuit fault.

[0042] Figure 7 3 is a thermal flow field distribution diagram of the converter transformer at different fault locations according to an embodiment of the present invention. DETAILED DESCRIPTION

[0043] The present invention will be further described below in conjunction with the accompanying drawings and specific implementation methods and examples. It should be understood that the purpose of providing the examples is only to illustrate, rather than to limit.

[0044] The main purpose of the present invention is to propose a method for calculating the electromagnetic-thermal flow field under a short-circuit fault between turns of a converter transformer. Based on finite element analysis software and in combination with Maxwell's equations, the internal transient electromagnetic field equation of the converter transformer is established under low frequency conditions. The associated distribution matrix related to the magnetic field intensity and the transformer material is obtained by weighted discretization, and the field-circuit coupling circuit and boundary equation are constructed to build a three-dimensional finite element simulation model of the converter transformer winding. The height of the voltage-regulating winding is adjusted when the arrangement is symmetrical, and the influence of different heights of the voltage-regulating winding on the spatial magnetic field distribution distortion is compared and analyzed. A spatial magnetic field distribution cloud map is established, and a voltage-regulating winding height optimization arrangement scheme is proposed based on data analysis. The research method and results provide a theoretical basis and design reference for the internal insulation and structural optimization of the converter transformer. With the help of finite element analysis software, the windings of the ±500kV converter transformer are simulated and calculated. By analyzing and comparing the laws of spatial magnetic field distortion under different conditions of the transformer, the effectiveness and correctness of the method proposed in the present invention are proved.

[0045] The embodiment of the present invention provides a method for calculating the electromagnetic-thermal flow field under a turn-to-turn short circuit fault of a converter transformer, comprising the following steps (1) to (5):

[0046] Step (1) establishes a theoretical model for numerical calculation of multi-physics fields of converter transformers based on circuit-electromagnetic-thermal flow coupling. This step solves the electromagnetic field distribution based on Maxwell's equations through field-circuit coupling. By combining the above equations with the fluid dynamics equations, the basic formula of electromagnetic-thermal flow field can be obtained, which is applied to the calculation of solid and liquid temperature, density and fluid velocity.

[0047] Step (2), based on the multi-physics field numerical calculation theoretical model of step (1), establish the electromagnetic-thermal flow field model of the converter transformer under the turn-to-turn short-circuit fault. This step adopts the field-circuit coupling modeling method to establish the electromagnetic-thermal flow scaled model under the turn-to-turn short-circuit fault of the converter transformer. Through the field-circuit coupling, the electromagnetic loss distribution of the transformer during the turn-to-turn short-circuit can be calculated, and based on this, the thermal flow field distribution of the transformer under the turn-to-turn short-circuit can be further solved.

[0048] Step (3), setting turn-to-turn short-circuit faults at different positions in the electromagnetic-thermal flow field model of the converter transformer established in step (2). Based on the established model, the different turn-to-turn short-circuit fault positions of the embodiment of the present invention are mainly set on the voltage regulating winding. Since each voltage regulating winding has 9 taps, the voltage regulating winding is evenly divided into 1 to 8 layers from top to bottom, and the turn-to-turn short-circuit fault positions are set on 1 to 8 layers respectively. The obtained simulation results can be mutually verified with the experimental results.

[0049] Step (4), perform winding current waveform, voltage waveform and electromagnetic field analysis, and thermal flow field analysis of the converter transformer under the set inter-turn short-circuit fault. The inter-turn short-circuit fault simulation method proposed in the embodiment of the present invention is based on the solution of the field-circuit coupled electromagnetic-heat flow equation. The current change of the fault turn is the main cause of transformer failure and even burning, so it is necessary to analyze the voltage and current of the fault turn and compare it with the winding without fault. The thermal flow field analysis of this step mainly includes the analysis of temperature field distribution and oil flow velocity field distribution. The thermal flow field analysis is used to understand the influence of different fault positions on the thermal flow field distribution results.

[0050] The above steps of the method for calculating the electromagnetic-thermal flow field under the inter-turn short-circuit fault of the converter transformer according to the embodiment of the present invention are described in detail below.

[0051] Step (1) establishes a multi-physics field numerical calculation theoretical model of the converter transformer based on circuit-electromagnetic-thermal flow coupling, which specifically includes the following steps (1.1) to (1.3):

[0052] Step (1.1), based on field-circuit coupling, combine the electromagnetic field equation and circuit equation of the converter transformer to calculate the electromagnetic field distribution of the converter transformer when operating under rated conditions:

[0053] When operating at rated conditions, the electromagnetic field distribution of the converter transformer can be calculated using ANSYS Maxwell software. The calculation principle is based on the Maxwell equation shown in formula (1):

[0054]

[0055] in, is the Hamilton operator, E represents the electric field intensity, B represents the magnetic flux density, H represents the magnetic field intensity, J represents the current density, D represents the electric displacement vector, and ρ represents the charge density;

[0056] Step (1.2), combine the electromagnetic loss equation of the converter transformer and the electromagnetic field distribution solution, and use the electromagnetic field distribution to calculate the electromagnetic loss:

[0057] In three-dimensional space, the relationship between the various electromagnetic field quantities of the converter transformer is usually expressed by the magnetic vector potential equation (2):

[0058]

[0059] Among them, A represents the magnetic vector potential, which is used to describe the magnetic field distribution; σ represents the conductivity of the medium; μ represents the magnetic permeability of the medium;

[0060] For the windings of the converter transformer, in addition to the field equations, circuit equations are also needed to describe the electrical characteristics of the windings. In this case, the relationship between the winding current and voltage can be expressed by Kirchhoff's law. For each winding, the circuit equation can be written as follows:

[0061]

[0062] Among them, v i Indicates the voltage of the winding; R i Represents the resistance of the winding; i i Represents the current of the winding; λ i Represents the flux linkage of the winding, which is equal to the magnetic flux φ i Multiply by the number of turns N of the winding i ;

[0063] Since the magnetic flux λ i It is directly related to the magnetic vector potential A in the magnetic field, so the circuit equations and field equations can be linked. The flux linkage in the circuit equations can be calculated from the magnetic vector potential A in the field equations:

[0064]

[0065] Where S is the area through which the magnetic flux density passes, N i Represents the number of winding turns.

[0066] Thus, the winding voltage v i It has a direct relationship with the magnetic vector potential A, thus completing the coupling of the circuit and the electromagnetic field equation.

[0067] Based on the magnetic flux density and current density distribution calculated by formula (1), the current in the converter transformer can be calculated, thereby obtaining the total electromagnetic loss, as shown in formula (5):

[0068]

[0069] Among them, S represents electromagnetic loss, P winding Represents the winding loss, P core represents the core loss, I n Represents the normal turn winding current, R n Indicates the normal turn winding resistance, I f Indicates the fault winding current, R frepresents the fault turn winding resistance, k, α, β are empirical parameters obtained by measurement, f represents the operating frequency, B m Indicates the maximum value of magnetic flux density;

[0070] Step (1.3), substitute the electromagnetic loss calculated above as the heat source into the fluid dynamics equations to realize electromagnetic-heat flow coupling and calculate the heat flow field distribution of the converter transformer:

[0071] Substitute the electromagnetic loss S calculated by formula (5) into the fluid dynamics equations (6) to solve the thermal flow field distribution of the converter transformer:

[0072]

[0073] Among them, d represents the fluid density, ω represents the fluid velocity, F represents the body force vector, p represents the fluid pressure, u represents the fluid dynamic viscosity, c represents the fluid specific heat capacity, and k represents the fluid thermal conductivity.

[0074] Step (2) is based on the multi-physics field numerical calculation theoretical model to establish the electromagnetic-thermal flow field model of the converter transformer under the inter-turn short circuit fault. The specific process is as follows:

[0075] First, the external wiring diagram of the converter transformer is given to determine the type of voltage to be loaded, and then the method of using the external circuit to apply the rated voltage and simulate the inter-turn short-circuit fault is further introduced (that is, the fault is simulated by setting a timed closed switch in the external circuit). Among them, the wiring diagram of the converter transformer is shown in Figure 1 As shown, A is the first end connection of the grid-side winding for external voltage. The grid-side winding is connected in series with the voltage regulating winding through the K and g connections, and then grounded through the B connection. a and b are the first and last connections of the valve-side winding, respectively. The load is connected in parallel to the valve-side winding through a and b. The external circuit is connected as shown in Figure 2 As shown. In practice, the voltage regulating winding is connected in series with the grid-side winding, and the voltage level of the grid-side output is controlled by adjusting the tap of the voltage source connection. Therefore, the two windings are simplified into the same winding in the external circuit. The grid-side windings are connected in parallel to the voltage source for excitation, and the valve-side windings are connected in parallel to the rated load. The coil where the inter-turn short-circuit fault is located is divided into normal turns and fault turns, and the fault turn is connected to a timing switch and a fault resistor. The occurrence of an inter-turn short-circuit fault is simulated by setting the timing switch to close at a certain time.

[0076] Based on the aforementioned wiring, a scaled-down modeling of the electromagnetic-thermal flow field of the converter transformer under a turn-to-turn short-circuit fault is performed. The word "scaled" is mentioned because a converter transformer scaled down to 20:1 is used as a demonstration case in the embodiment of the present invention. However, this ratio is only exemplary, and other ratios can also be used in actual implementation, and the present invention does not limit this. The specific modeling process includes: establishing a three-dimensional geometric model of the converter transformer according to the structural parameters of the converter transformer; performing three-dimensional meshing on the three-dimensional geometric model; and performing field-circuit coupling settings, material parameter settings, and solver settings required for solving the model according to the three-dimensional mesh of the converter transformer.

[0077] Specifically, the embodiment of the present invention uses ANSYS software to establish a three-dimensional geometric model of the converter transformer. The converter transformer of the embodiment of the present invention adopts a single-phase four-column iron core, which is composed of two main yokes and two side yokes, and is connected by upper and lower yokes. The windings are wound on two main yokes, from the inside to the outside, respectively, the voltage regulating winding, the grid side winding, and the valve side winding. Among them, ANSYSMaxwell is used to complete the field-circuit coupling and electromagnetic field finite element modeling, and ANSYS Fluent is used to establish and solve the computational fluid dynamics model. The mesh generation adopts the adaptive mesh generation method of ANSYS Maxwell and ANSYS Fluent respectively. The parameters of the converter transformer are shown in Table 1, and the winding structure is shown in Table 2.

[0078] Table 1 Technical performance parameters of converter transformer

[0079]

[0080]

[0081] Table 2 Winding structure parameters of converter transformer three-dimensional geometric model

[0082]

[0083] The basic structure of the converter transformer core is the same as that of the AC transformer. The core material is B30P120, and 12-grade silicon steel sheets are stacked and rolled. The size of the body (length * width * height / mm) is: 2731*942*1277, the iron yoke width is 316mm, and the core BH curve is as follows Figure 3 shown.

[0084] ANSYS Maxwell uses the field-circuit coupling method. The grid-side winding is powered by a rated power frequency AC voltage source, the load-side winding is connected to a rated load, the winding material is set to copper, the winding current direction is calculated, and the number of turns of each winding is set. Figure 4The transient solver is used for the solution, and the duration is set to 100ms, of which the fault occurs at 30ms.

[0085] ANSYS Fluent loads the winding and core losses calculated by the electromagnetic field into the thermal flow field as heat sources. The solid materials of the core and winding are set to copper and iron respectively, and the transformer oil flow material is set to fluid. The nonlinear material parameters shown in Table 3 are used. The solution adopts steady-state solution.

[0086] Table 3 Transformer oil flow material parameters

[0087]

[0088]

[0089] Step (3) sets turn-to-turn short-circuit faults at different positions in the electromagnetic-thermal flow field model of the converter transformer, which specifically includes the following contents: setting different turn-to-turn short-circuit fault positions on the voltage regulating winding; dividing the coil where the turn-to-turn short-circuit fault is located into normal turns and fault turns, and connecting the fault turn to a timing switch and a fault resistor; simulating the occurrence of the turn-to-turn short-circuit fault by setting the timing switch to close at a certain time. This step mainly introduces the setting method of the turn-to-turn short-circuit fault of the converter transformer, and the influence of the change of different fault positions on the thermal flow field. Since each voltage regulating winding has 9 taps, the embodiment of the present invention evenly divides the voltage regulating winding into 1 to 8 layers from top to bottom, and the turn-to-turn short-circuit fault can be set on 1 to 8 layers respectively, and the obtained simulation results can be mutually verified with the experimental results. In order to avoid too much repetitive work, only the temperature and flow field distribution of 4 representative faults are selected in the embodiment of the present invention. Among them, the voltage source connected to the grid-side winding, the load connected to the valve-side winding, and other parameters of the turn-to-turn short-circuit fault remain unchanged to explore the influence of different fault positions on the electromagnetic-thermal flow field.

[0090] Step (4) performs winding current waveform, voltage waveform and electromagnetic field analysis, as well as thermal flow field analysis of the converter transformer under the set turn-to-turn short-circuit fault, specifically including the following contents:

[0091] (4.1) The change of winding current under turn-to-turn short-circuit fault directly reflects the current surge phenomenon in the faulty winding;

[0092] (4.2) The distortion and concentration of the spatial magnetic flux density caused by the surge in current form an explanation for the spatial electromagnetic distribution;

[0093] (4.3) provides additional explanations for small changes in winding voltage and a comprehensive analysis of the winding's electrical characteristics (voltage, current).

[0094] The current waveform of the winding during a turn-to-turn short circuit fault is as follows: Figure 5-1As shown, since the grid side, voltage regulating and valve side windings are all double windings in parallel, the parallel windings are numbered 1 and 2 in the figure, the fault side is 1 and the other side is 2. When a turn-to-turn short circuit fault occurs, the current of the faulty turn winding surges from the rated 61.9A to 3.5kA, indicating that the turn-to-turn short circuit fault will cause the faulty turn current to increase sharply by dozens of times in a short period of time, while the current of other normal turn windings does not change much, the grid side winding remains at 62A, and the valve side winding remains at 195A. This shows that when a turn-to-turn short circuit fault occurs, the non-fault coil current is minimally affected by the fault and can be ignored. The spatial magnetic flux density distribution during the fault is shown as follows: Figure 6 As shown in the figure, it can be observed that the magnetic flux density near the fault winding is distorted and concentrated, and the maximum value of the magnetic flux density near the fault winding is about 0.7T. The voltage waveforms of the high and low voltage windings during the fault are as follows: Figure 5-2 As shown in Figure 1, when a fault occurs, the voltage of the grid-side winding and the valve-side winding is less affected by the fault and can be ignored. The peak voltage of the grid-side winding is 22 kV, and the peak voltage of the valve-side winding is 6.5 kV.

[0095] (4.4) Thermal flow field analysis mainly analyzes the temperature distribution of each winding in the converter transformer and the flow velocity distribution of the oil flow when a turn-to-turn short circuit fault occurs at different locations. The specific contents are as follows:

[0096] The inter-turn short-circuit fault is set on different layers of the voltage regulating winding, and the obtained temperature distribution and oil flow velocity distribution of the transformer cross section are as follows: Figure 7As shown. From the temperature distribution diagram, it can be seen that when a turn-to-turn short-circuit fault occurs, the hot spot temperature of the transformer is distributed on the fault turn coil, and the temperature gradually decreases from the fault turn to the upper and lower ends. Compared with the second, fourth, and sixth fault layers, the fault turn temperature is the highest when the eighth layer fails, reaching 575K. In addition, when the fault occurs in the second and eighth layers, the temperature difference on the voltage regulating winding is the largest, and when the fault occurs in the fourth and sixth layers, the temperature distribution on the voltage regulating winding is more uniform, although the winding temperature at this time is still maintained above 370K. This is because when the position of the turn-to-turn short-circuit fault is different, the reverse potential induced by the short-circuit turn at different positions is also different, thus affecting the winding current in each winding that affects the resistance heat. The magnitude of the induced potential is related to the speed of change of the magnetic flux density passing through the coil. When the fault position is different, the magnetic flux density passing through the fault coil is affected by the change of the leakage flux at different heights, resulting in a change in the total magnetic flux passing through the fault turn. The leakage flux is large near the upper and lower ends of the main yoke, and the induced potential increases accordingly; the leakage flux near the middle of the main yoke is small, and the induced potential decreases accordingly. The oil flow distribution of the transformer under fault is asymmetric, and the overall speed of the oil flow on the side where the fault turn is located is significantly higher than that on the other side. Taking the fault on the sixth layer as an example, the oil flow speed near and above the fault turn is high, reaching 0.012m / s. The main oil flow flows upward along the voltage regulating winding from the vicinity of the voltage regulating winding, and disperses to the surroundings after contacting the lower surface of the iron core. In addition, the oil flow on both sides of the oil tank forms eddy currents. The transformer oil in the eddy current area on the side where the fault turn is located reaches the maximum speed, about 0.0177m / s, while the oil flow speed in the eddy current area on the other side is small, reaching only 0.008m / s. When the fault occurs at the eighth layer, the oil flow rate is significantly faster, with the maximum speed reaching 0.043m / s. This is because the faulty winding has a greater heating power, resulting in a higher winding temperature. After the transformer oil absorbs heat, its density decreases, and it flows upward under the action of buoyancy. The more heat it absorbs, the greater the degree of density reduction, and the higher the speed of upward flow.

[0097] The calculation method for the evolution of the inter-turn short-circuit fault of the converter transformer proposed in the embodiment of the present invention compares and analyzes the spatial electromagnetic and thermal flow field distribution of the normal winding and the faulty winding under the condition of inter-turn short-circuit fault, and finds that the electromagnetic field near the faulty turn winding is distorted and the leakage flux is concentrated, the loss of the faulty turn winding increases sharply, and the temperature rises significantly. The simulation of different fault locations shows that the abnormal heating of the transformer is the most serious when the fault occurs in the first layer, and the hot spot temperature of the transformer exceeds the upper limit of the temperature allowed for the A-class insulation transformer within 5s after the fault occurs.

[0098] In summary, when a turn-to-turn short circuit fault occurs, the current of the faulty turn winding surges by dozens of times, causing the winding temperature to rise sharply, and the hottest point appears near the faulty turn. The overall temperature of the voltage regulating winding where the faulty turn is located rises significantly. The oil flow speed on the side where the faulty turn is located is significantly faster. The oil flow flows upward along the voltage regulating winding and disperses when it contacts the lower surface of the iron core. Eddy current zones appear on both sides of the oil tank, and the oil flow in the eddy current zone reaches the maximum speed. The severity of the winding fault is related to the location of the fault. The change in the magnetic flux density passing through the faulty turn at different fault locations is different, resulting in different induced potentials and induced currents in the winding. When the fault occurs in the eighth layer, the degree of heating is the most serious, reaching 575K, and the oil flow velocity is the fastest, reaching 0.043m / s. Therefore, when the fault occurs in the eighth layer, it is most likely to cause insulation aging, abnormal heating and burning of the transformer in a short period of time, and should be paid special attention to. When the fault occurs near the middle of the winding, the degree of fault is lower, and there is more time for transformer fault diagnosis and status detection.

[0099] Another embodiment of the present invention provides a computer-readable storage medium on which a computer program is stored, and when the computer program is executed by a processor, the steps of the electromagnetic-thermal flow field calculation method under the inter-turn short circuit fault of the converter transformer can be implemented. Based on this understanding, the aforementioned method of the present invention can be embodied in the form of a software product, which can be stored in a non-volatile storage medium (which can be a CD-ROM, a USB flash drive, a mobile hard disk, etc.), including a number of instructions for a computer device (which can be a personal computer, a server, or a network device, etc.) to execute the methods of various implementation scenarios of the present invention.

[0100] Another embodiment of the present invention further proposes an electromagnetic-thermal flow field calculation device for a converter transformer under an inter-turn short-circuit fault, comprising: a multi-physical field numerical calculation theoretical model construction module, used to establish a multi-physical field numerical calculation theoretical model of a converter transformer based on circuit-electromagnetic-thermal flow coupling; an electromagnetic-thermal flow field model construction module, used to establish an electromagnetic-thermal flow field model of a converter transformer under an inter-turn short-circuit fault based on the multi-physical field numerical calculation theoretical model; an inter-turn short-circuit fault setting module, used to set an inter-turn short-circuit fault at different positions in the electromagnetic-thermal flow field model of the converter transformer; an analysis module, used to perform winding current waveform, voltage waveform and electromagnetic field analysis, and thermal flow field analysis of the converter transformer under the set inter-turn short-circuit fault.

[0101] In some specific embodiments, the above-mentioned multi-physics field numerical calculation theoretical model construction module includes an electromagnetic field distribution calculation module, an electromagnetic loss calculation module and a heat flow field distribution calculation module; wherein: the electromagnetic field distribution calculation module is used to calculate the electromagnetic field distribution of the converter transformer when operating at rated conditions based on field-circuit coupling and the electromagnetic field equations and circuit equations of the converter transformer; the electromagnetic loss calculation module is used to calculate the electromagnetic loss based on electromagnetic-heat flow coupling and the electromagnetic loss equations and fluid dynamics equations of the converter transformer using the electromagnetic field distribution; the heat flow field distribution calculation module is used to substitute the electromagnetic loss as a heat source into the fluid dynamics equations to calculate the heat flow field distribution of the converter transformer.

[0102] Among them, the electromagnetic field distribution calculation module is based on field-circuit coupling, and the electromagnetic field equation and circuit equation of the converter transformer are combined to calculate the electromagnetic field distribution of the converter transformer when it is running at rated conditions. The Maxwell equation shown in formula (1) in the above embodiment is used, which will not be repeated here. The electromagnetic loss calculation module uses the electromagnetic field distribution obtained by the above calculation to calculate the electromagnetic loss, mainly based on the magnetic flux density and current density calculated by formula (1), and uses formula (5) of the above embodiment to calculate the electromagnetic loss in the converter transformer. The specific calculation process is the same as step (1.2) of the above embodiment, which will not be repeated here. The thermal flow field distribution calculation module can solve the thermal flow field distribution of the converter transformer by substituting the electromagnetic loss obtained by the above calculation into the fluid dynamics equation group. The specific solution process is the same as step (1.3) of the above embodiment, which will not be repeated here.

[0103] In some specific embodiments, the above-mentioned electromagnetic-thermal flow field model construction module includes a three-dimensional geometric model construction module, a three-dimensional mesh generation module and a model setting module; wherein: the three-dimensional geometric model construction module is used to establish a three-dimensional geometric model of the converter transformer according to the structural parameters of the converter transformer; the three-dimensional mesh generation module is used to perform three-dimensional mesh generation on the three-dimensional geometric model; the model setting module is used to perform field-circuit coupling settings, material parameter settings and solver settings required for solving the model according to the three-dimensional mesh of the converter transformer.

[0104] In some exemplary embodiments, the 3D geometric model building module can use ANSYS Maxwell of ANSYS software to complete field-circuit coupling and electromagnetic field finite element modeling, and use ANSYS Fluent to build and solve the fluid dynamics model. Correspondingly, the 3D mesh generation module is implemented using the adaptive mesh generation methods of ANSYS Maxwell and ANSYS Fluent respectively.

[0105] The model setting module performs material parameter setting, field-circuit coupling and solver setting required for solving the model, mainly including: ANSYS Maxwell adopts a field-circuit coupling method, the grid-side winding is powered by a rated industrial frequency AC voltage source, the load-side winding is connected to a rated load, the winding structure parameters are set as shown in Table 2 of the aforementioned embodiment, the winding current direction is calculated, and the number of turns of each winding is set, as well as the technical performance parameters of the converter transformer shown in Table 1 of the aforementioned embodiment; the solver is set as a transient solver, and the duration of the transient solver and the time point of the fault occurrence are set; ANSYS Fluent loads the winding and core losses obtained by electromagnetic field calculation into the thermal flow field as heat sources, sets the oil flow material parameters as shown in Table 3 of the aforementioned embodiment, and the solver is set as a steady-state solver.

[0106] In some specific embodiments, the above-mentioned turn-to-turn short-circuit fault setting module includes a fault position setting module, a coil setting module and a fault simulation module; wherein: the fault position setting module is used to set different turn-to-turn short-circuit fault positions on the voltage regulating winding; the coil setting module is used to divide the coil where the turn-to-turn short-circuit fault is located into normal turns and fault turns, and the fault turn is externally connected to a timing switch and a fault resistor; the fault simulation module is used to simulate the occurrence of a turn-to-turn short-circuit fault by setting a closing timing switch at a certain moment.

[0107] Among them, the main function of the fault location setting module is to set turn-to-turn short-circuit faults on different voltage regulating windings. In the aforementioned embodiment, since each voltage regulating winding has 9 taps, the voltage regulating winding can be evenly divided into 1 to 8 layers from top to bottom through the fault location setting module, and then the turn-to-turn short-circuit faults are set on 1 to 8 layers respectively, and the obtained simulation results can be mutually verified with the experimental results.

[0108] In some specific embodiments, the above-mentioned analysis modules include a current waveform analysis module, a voltage waveform analysis module, an electromagnetic field analysis module and a thermal flow field analysis module; wherein: the current waveform analysis module is used to obtain the current surge phenomenon in the faulty winding according to the change of the winding current under the turn-to-turn short-circuit fault; the voltage waveform analysis module is used to analyze the impact of the fault on the voltage of the grid-side winding and the valve-side winding when the fault occurs according to the voltage waveforms of the high and low voltage windings during the turn-to-turn short-circuit fault; the electromagnetic field analysis module is used to analyze the spatial electromagnetic distribution according to the distortion and concentration of the spatial magnetic flux density caused by the surge in current; the thermal flow field analysis module is used to analyze the temperature distribution of each winding in the converter transformer when a turn-to-turn short-circuit fault occurs at different positions, and to analyze the flow velocity distribution of the oil flow in the converter transformer when a turn-to-turn short-circuit fault occurs at different positions.

[0109] Among them, the change of winding current under inter-turn short circuit fault directly reflects the current surge phenomenon in the fault winding. Figure 5-1The current waveform of the winding during the inter-turn short-circuit fault is shown. It can be seen that when the inter-turn short-circuit fault occurs, the fault turn winding current surges from the rated 61.9A to 3.5kA, indicating that the inter-turn short-circuit fault will cause the fault turn current to increase sharply by dozens of times in a short period of time, while the current of other normal turns does not change much, the grid side winding remains at 62A, and the valve side winding remains at 195A. This shows that when the inter-turn short-circuit fault occurs, the non-fault coil current is minimally affected by the fault and can be ignored.

[0110] The distortion and concentration of the spatial magnetic flux density caused by the surge in current form an explanation of the spatial electromagnetic distribution. The electromagnetic field analysis module is based on Figure 6 The spatial magnetic flux density distribution during the fault is shown. It can be seen that the magnetic flux density near the faulty winding is distorted and concentrated, and the maximum magnetic flux density near the faulty winding is about 0.7T.

[0111] The voltage waveform analysis module is as follows Figure 5-2 The high and low voltage winding voltage waveforms during the fault are shown. It is known that when a fault occurs, the voltage of the grid-side winding and the valve-side winding is less affected by the fault and can be ignored. The peak voltage of the grid-side winding is 22kV, and the peak voltage of the valve-side winding is 6.5kV.

[0112] When using the thermal flow field analysis module to analyze the temperature distribution of each winding in the converter transformer when a turn-to-turn short-circuit fault occurs at different locations, the temperature distribution diagram obtained by setting the turn-to-turn short-circuit fault on different layers of the voltage regulating winding shows that the hot spot temperature of the converter transformer is distributed on the faulty turn coil when a turn-to-turn short-circuit fault occurs, and the temperature gradually decreases from the faulty turn to the upper and lower ends. When using the thermal flow field analysis module to analyze the flow velocity distribution of the oil flow in the converter transformer when a turn-to-turn short-circuit fault occurs at different locations, the temperature distribution diagram obtained by setting the turn-to-turn short-circuit fault on different layers of the voltage regulating winding shows that the oil flow distribution of the converter transformer under the fault presents an asymmetric distribution, and the overall oil flow velocity on one side of the faulty turn winding is higher than that on the other side.

[0113] The electromagnetic-thermal flow field calculation device under the inter-turn short-circuit fault of the converter transformer proposed in the above embodiment of the present invention has basically the same functions as the electromagnetic-thermal flow field calculation method under the inter-turn short-circuit fault of the converter transformer in the aforementioned embodiment, can solve basically the same technical problems, and obtain the same technical effects, and will not be repeated here.

[0114] The above is a further detailed description of the present invention in combination with specific implementation methods, and it cannot be determined that the specific implementation of the present invention is limited to these descriptions. For those skilled in the art of the present invention, several equivalent substitutions or obvious variations can be made without departing from the concept of the present invention, and the performance or use is the same, which should be regarded as belonging to the protection scope of the present invention.

Claims

1. A method for calculating the electromagnetic-thermal flow field under a converter transformer turn-to-turn short-circuit fault, characterized in that: include: Establish a multi-physics numerical calculation theoretical model for converter transformers based on circuit-electromagnetic-thermal coupling; Based on the multi-physics field numerical calculation theoretical model, an electromagnetic-thermal flow field model of a converter transformer under a turn-to-turn short-circuit fault is established; Setting turn-to-turn short-circuit faults at different positions in the electromagnetic-thermal flow field model of the converter transformer; The winding current waveform, voltage waveform and electromagnetic field analysis, as well as the thermal flow field analysis of the converter transformer under the set turn-to-turn short-circuit fault are carried out.

2. The electromagnetic-thermal flow field calculation method under the inter-turn short-circuit fault of the converter transformer according to claim 1 is characterized in that: The steps of establishing a multi-physical field numerical calculation theoretical model of a converter transformer based on circuit-electromagnetic-thermal flow coupling include: Based on field-circuit coupling, the electromagnetic field equation and circuit equation of the converter transformer are combined to calculate the electromagnetic field distribution of the converter transformer; The electromagnetic loss equation of the converter transformer and the electromagnetic field distribution solution are combined, and the electromagnetic loss is calculated using the electromagnetic field distribution; The electromagnetic loss is substituted into the fluid dynamics equations as a heat source to realize electromagnetic-heat flow coupling, and the heat flow field distribution of the converter transformer is calculated.

3. The electromagnetic-thermal flow field calculation method under the inter-turn short-circuit fault of the converter transformer according to claim 2 is characterized in that: The calculation of the electromagnetic field distribution of the converter transformer under rated operating conditions adopts the following Maxwell equations: in, is the Hamilton operator, E represents the electric field intensity, B represents the magnetic flux density, H represents the magnetic field intensity, J represents the current density, D represents the electric displacement vector, and ρ represents the charge density; Calculating electromagnetic loss using the electromagnetic field distribution includes: Based on the magnetic flux density and current density calculated by formula (1), the electromagnetic loss in the converter transformer is calculated as shown in formula (2): Among them, S represents electromagnetic loss, P winding Represents the winding loss, P core represents the core loss, I n Indicates the normal turn winding current, R n Indicates the normal turn winding resistance, I f Indicates the fault winding current, R f represents the fault turn winding resistance, k, α, β are empirical parameters obtained by measurement, f represents the operating frequency, B m Indicates the maximum value of magnetic flux density; The calculating of the heat flow field distribution of the converter transformer comprises: Substitute the electromagnetic loss S calculated by formula (2) into the fluid dynamics equations (3) to solve the thermal flow field distribution of the converter transformer: Among them, d represents the fluid density, ω represents the fluid velocity, F represents the body force vector, p represents the fluid pressure, u represents the fluid dynamic viscosity, c represents the fluid specific heat capacity, and k represents the fluid thermal conductivity.

4. The electromagnetic-thermal flow field calculation method under the inter-turn short-circuit fault of the converter transformer according to claim 1 is characterized in that: The method of establishing an electromagnetic-thermal flow field model of a converter transformer under a turn-to-turn short circuit fault comprises: A three-dimensional geometric model of the converter transformer is established according to the structural parameters of the converter transformer; Performing three-dimensional meshing on the three-dimensional geometric model; According to the three-dimensional mesh of the converter transformer, the field-circuit coupling settings, material parameter settings and solver settings required for solving the model are performed.

5. The electromagnetic-thermal flow field calculation method under the inter-turn short-circuit fault of the converter transformer according to claim 4 is characterized in that: The three-dimensional geometric model of the converter transformer is established according to the structural parameters of the converter transformer, including: ANSYS Maxwell of ANSYS software is used to complete field-circuit coupling and electromagnetic field finite element modeling, and ANSYS Fluent is used to establish and solve the fluid dynamics model; Among them, the converter transformer adopts a single-phase four-column iron core, including two main yokes and two side yokes, which are connected by upper and lower yokes; the windings are wound on the two main yokes, from the inside to the outside, they are the voltage regulating winding, the grid side winding, and the valve side winding.

6. The electromagnetic-thermal flow field calculation method under the inter-turn short-circuit fault of the converter transformer according to claim 5 is characterized in that: The material parameter setting, field-circuit coupling and solver setting required for solving the model according to the three-dimensional grid of the converter transformer include: ANSYS Maxwell uses the field-circuit coupling method. The grid-side winding is powered by a rated power frequency AC voltage source, the load-side winding is connected to a rated load, the winding structure parameters are set, the winding current direction is calculated, and the number of turns of each winding is set. The solver is set to a transient solver, and the duration of the transient solver and the time point of the fault occurrence are set. ANSYS Fluent loads the winding and core losses obtained from electromagnetic field calculations into the thermal flow field as heat sources, sets the oil flow material parameters, and sets the solver to a steady-state solver.

7. The electromagnetic-thermal flow field calculation method under the inter-turn short circuit fault of the converter transformer according to claim 1 is characterized in that: In the electromagnetic-thermal flow field model of the converter transformer, turn-to-turn short-circuit faults are set at different positions, including: Setting different inter-turn short-circuit fault positions on the voltage regulating winding; The coil where the inter-turn short-circuit fault occurs is divided into normal turns and fault turns, and the fault turn is externally connected with a timing switch and a fault resistor; The occurrence of inter-turn short-circuit fault is simulated by setting the timing switch to close at a certain moment.

8. The electromagnetic-thermal flow field calculation method under the inter-turn short-circuit fault of the converter transformer according to claim 7 is characterized in that: The method of setting different turn-to-turn short-circuit fault positions on the voltage regulating winding includes: evenly dividing the voltage regulating winding into 1 to 8 layers from top to bottom, and setting the turn-to-turn short-circuit fault positions on the 1 to 8 layers respectively; wherein the voltage source connected to the grid-side winding, the load connected to the valve-side winding, and other parameters of the turn-to-turn short-circuit fault remain unchanged.

9. The electromagnetic-thermal flow field calculation method under the inter-turn short-circuit fault of the converter transformer according to claim 1 is characterized in that: The winding current waveform, voltage waveform and electromagnetic field analysis of the converter transformer under the set turn-to-turn short-circuit fault includes: Current waveform analysis: Based on the change of winding current under turn-to-turn short-circuit fault, the current surge phenomenon in the faulty winding is obtained; Voltage waveform analysis: Based on the voltage waveforms of high and low voltage windings during turn-to-turn short circuit faults, analyze the impact of the fault on the voltages of the grid-side winding and valve-side winding when the fault occurs; Electromagnetic field analysis: Analyze the spatial electromagnetic distribution based on the distortion and concentration of the spatial magnetic flux density caused by the surge in current.

10. The electromagnetic-thermal flow field calculation method under the inter-turn short circuit fault of the converter transformer according to claim 1, characterized in that: The thermal flow field analysis includes: analyzing the temperature distribution of each winding in the converter transformer when turn-to-turn short-circuit faults occur at different positions; and analyzing the flow velocity distribution of the oil flow in the converter transformer when turn-to-turn short-circuit faults occur at different positions.

11. The electromagnetic-thermal flow field calculation method under the inter-turn short-circuit fault of the converter transformer according to claim 10, characterized in that: The analysis of the temperature distribution of each winding in the converter transformer when a turn-to-turn short-circuit fault occurs at different positions includes: according to the temperature distribution diagram obtained by setting the turn-to-turn short-circuit fault on different layers of the voltage regulating winding, it is known that when the turn-to-turn short-circuit fault occurs, the hot spot temperature of the converter transformer is distributed on the faulty turn coil, and the temperature gradually decreases from the faulty turn to the upper and lower ends respectively.

12. The electromagnetic-thermal flow field calculation method under the inter-turn short circuit fault of the converter transformer according to claim 10, characterized in that: The analysis of the flow velocity distribution of the oil flow in the converter transformer when turn-to-turn short-circuit faults occur at different positions includes: based on the temperature distribution diagram obtained by setting the turn-to-turn short-circuit fault on different layers of the voltage regulating winding, it is found that the oil flow distribution of the converter transformer under the fault presents an asymmetric distribution, and the overall oil flow velocity on the side where the faulty turn winding is located is higher than that on the other side.

13. A computer-readable storage medium having a computer program stored thereon, characterized in that: When the computer program is executed by a processor, the steps of the electromagnetic-thermal flow field calculation method under a converter transformer turn-to-turn short-circuit fault according to any one of claims 1 to 12 can be implemented.

14. An electromagnetic-thermal flow field calculation device for a converter transformer under a turn-to-turn short-circuit fault, characterized in that: include: A multi-physics field numerical calculation theoretical model building module is used to establish a multi-physics field numerical calculation theoretical model of a converter transformer based on circuit-electromagnetic-thermal flow coupling; An electromagnetic-thermal flow field model building module is used to establish an electromagnetic-thermal flow field model of a converter transformer under a turn-to-turn short-circuit fault based on the multi-physics field numerical calculation theoretical model; A turn-to-turn short-circuit fault setting module, used to set turn-to-turn short-circuit faults at different positions in the electromagnetic-thermal flow field model of the converter transformer; The analysis module is used to perform winding current waveform, voltage waveform and electromagnetic field analysis, as well as thermal flow field analysis of the converter transformer under a set turn-to-turn short-circuit fault.

15. The electromagnetic-thermal flow field calculation device for a converter transformer under a turn-to-turn short-circuit fault according to claim 14, characterized in that: The multi-physics field numerical calculation theoretical model building module includes: The electromagnetic field distribution calculation module is used to calculate the electromagnetic field distribution of the converter transformer by combining the electromagnetic field equation and the circuit equation of the converter transformer based on field-circuit coupling; An electromagnetic loss calculation module, used for combining the electromagnetic loss equation of the converter transformer with the electromagnetic field distribution solution, and calculating the electromagnetic loss using the electromagnetic field distribution; The heat flow field distribution calculation module is used to substitute the electromagnetic loss as a heat source into the fluid dynamics equations to achieve electromagnetic-heat flow coupling and calculate the heat flow field distribution of the converter transformer.

16. The electromagnetic-thermal flow field calculation device under the inter-turn short circuit fault of the converter transformer according to claim 14, characterized in that: The electromagnetic-thermal flow field model building module includes: A three-dimensional geometric model building module is used to build a three-dimensional geometric model of the converter transformer according to the structural parameters of the converter transformer; A three-dimensional mesh generation module, used for performing three-dimensional mesh generation on the three-dimensional geometric model; The model setting module is used to perform the field-circuit coupling setting, material parameter setting and solver setting required for solving the model according to the three-dimensional grid of the converter transformer.

17. The electromagnetic-thermal flow field calculation device for a converter transformer under a turn-to-turn short-circuit fault according to claim 14, characterized in that: The turn-to-turn short circuit fault setting module includes: A fault position setting module, used to set different turn-to-turn short-circuit fault positions on the voltage regulating winding; A coil setting module is used to divide the coil where the inter-turn short-circuit fault is located into a normal turn and a fault turn, and the fault turn is externally connected to a timing switch and a fault resistor; The fault simulation module is used to simulate the occurrence of an inter-turn short circuit fault by setting a closing timer switch at a certain moment.

18. The electromagnetic-thermal flow field calculation device under the inter-turn short-circuit fault of the converter transformer according to claim 14, characterized in that: The analysis module comprises: The current waveform analysis module is used to obtain the current surge phenomenon in the faulty winding based on the change of the winding current under the inter-turn short circuit fault; The voltage waveform analysis module is used to analyze the influence of the fault on the voltage of the grid-side winding and the valve-side winding according to the voltage waveforms of the high and low voltage windings when the inter-turn short circuit fault occurs; The electromagnetic field analysis module is used to analyze the spatial electromagnetic distribution based on the distortion and concentration of the spatial magnetic flux density caused by the surge in current; The thermal flow field analysis module is used to analyze the temperature distribution of each winding in the converter transformer when turn-to-turn short-circuit faults occur at different locations, and to analyze the flow velocity distribution of the oil flow in the converter transformer when turn-to-turn short-circuit faults occur at different locations.

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