Transformer thermal life evaluation method for metal structural member

Abstract

The invention provides a transformer thermal life evaluation method for a metal structural member. The transformer thermal life evaluation method comprises the following steps: firstly, establishing a digital twinborn model of an oil-immersed transformer; carrying out three-dimensional magnetic field calculation by taking a current source as excitation, and carrying out electromagnetic field calculation to obtain stray loss of a metal structural member in the transformer as a heat source to be coupled into a temperature field; respectively calculating convective heat transfer coefficients of the clamping piece and the pulling plate through the target Nusselt number and the hot spot position Nusselt number, and calculating heat conductivity coefficients of contact surfaces of the clamping piece and the pulling plate and the insulator through a heat conduction formula; a linear relation is obtained by simplifying the transformer heat distribution diagram, and therefore the corresponding oil temperature is calculated according to the height of the metal structural part; loading a calculation result into temperature field simulation analysis of the transformer; comparing and correcting the temperature obtained by the method with the temperature measured by the transformer optical fiber; and introducing an insulation thermal aging evaluation standard through the hot-spot temperature rise state of the metal structural member of the transformer to evaluate the thermal life of the transformer of the metal structural member.

Description

Technical Field

[0001] The invention belongs to the technical field of transformer temperature field calculation, and provides a transformer thermal life assessment method for metal structural parts. Background Art

[0002] Transformers are one of the important transmission equipment in power systems. During long-term operation, thermal life assessment of transformers has become one of the key technologies to ensure their safety and reliability. With the continuous growth of power demand, transformers usually need to work under high load and long-term operating conditions. In this case, heat accumulation inside the transformer becomes a problem that cannot be ignored. When the temperature of the structural parts is too high during the operation of the transformer, the thermal aging of the transformer insulation is accelerated, which means that the thermal life loss of the transformer during operation is aggravated. In order to ensure the safe and reliable operation of the transformer and extend its service life, it is more important to accurately obtain the temperature of the metal structural parts of the transformer. The inaccurate temperature of the metal structural parts of the transformer obtained by traditional temperature field simulation is usually due to the inaccurate value of the convective heat transfer coefficient involved in the calculation. The evaluation of the thermal life of the insulation usually only considers the influence of the hot spot temperature of the low-voltage winding on the thermal life of the insulation, and does not consider the influence of the temperature rise of the metal structural parts on the thermal life of the insulation. Therefore, it is necessary to provide a transformer thermal life assessment method for metal structural parts to evaluate the thermal life of the transformer in order to accurately evaluate the thermal life of the transformer. Summary of the invention

[0003] Purpose of the Invention

[0004] In order to solve the problems in the prior art, the present invention provides a transformer thermal life assessment method for metal structural parts, which adopts a heat transfer coefficient discrete method to more comprehensively obtain the convective heat transfer coefficient of metal structural parts, and evaluates the thermal life of the transformer through the hot spot temperature rise state of the metal structural parts.

[0005] To achieve the above object, the present invention provides the following technical solutions:

[0006] A transformer thermal life assessment method for metal structural parts comprises the following steps:

[0007] Step 1: Establish a digital twin model of the oil-immersed transformer;

[0008] Step 2: Perform electromagnetic field simulation on the digital twin model of the oil-immersed transformer to obtain the total stray loss of the metal structural parts;

[0009] Step 3: Calculate the heat dissipation coefficient of each surface of the transformer metal structure, load the calculated results into the transformer temperature fluid field simulation analysis, and obtain the temperature distribution of the transformer metal structure;

[0010] Step 4: Simplify the transformer heat distribution diagram to obtain a linear relationship, thereby calculating the relationship between the height of the metal structure and the oil temperature, and load the calculated results into the transformer temperature field simulation analysis to complete the transformer temperature field simulation calculation;

[0011] Step 5: Compare and correct the temperature of the metal structure of the transformer obtained by the heat transfer coefficient discrete method and the temperature of the metal structure of the transformer obtained by the optical fiber temperature measurement;

[0012] Step 6: Based on the transformer insulation thermal aging evaluation criteria and the temperature rise status of the transformer metal structural parts, conduct a transformer thermal life assessment for the metal structural parts.

[0013] As a further description of the above scheme, the establishment of the digital twin model of the oil-immersed transformer in step 1 is based on the physical parameters of the transformer entity, and the design parameters of the structure are obtained according to the transformer calculation sheet and drawings.

[0014] As a further description of the above scheme, the step 2 includes the following steps:

[0015] Step 2.1: Perform electromagnetic field simulation on the digital twin model of the oil-immersed transformer. First, assign electromagnetic material properties to each part of the transformer structure. According to the number of coil turns and phase current in the transformer product design data, select the winding cross-sectional area to input the corresponding number of coil turns and operating current expressions. Grid division is performed based on the structural dimension data of the digital twin transformer. The total simulation running time and step size are set. The solution algorithm is calculated based on the T-Ω method in the three-dimensional time-harmonic field.

[0016] Step 2.1.1: Use the following formula to describe the mathematical model of the T-Ω method for solving the three-dimensional eddy current field problem of the transformer. The magnetic field intensity is divided into two parts: the gradient of the scalar potential in the non-conductor area and the vector edge unit of the vector field in the conductor area, as shown in the following table:

[0017] serial number Solution Domain Transformer components <![CDATA[V1]]> Non-conductor Transformer oil <![CDATA[V2]]> conductor Metal structural parts

[0018] Assuming that the conductivity σ is a constant, it can be obtained from Introducing the vector potential T, then:

[0019]

[0020] In the formula, J S is the excitation current density, J E is the eddy current density, J is the current density, and T is the vector potential. The domain relationship between the vector potential T and the scalar magnetic potential Ω under different solution domains satisfies:

[0021]

[0022] Where H is the magnetic field intensity and Ω is the scalar magnetic potential;

[0023] In a homogeneous medium:

[0024]

[0025] Where μ is magnetic permeability, σ is electrical conductivity, and E is electric field intensity;

[0026] Step 2.1.2: The field equations under different solution domains are:

[0027]

[0028] Step 2.1.3: Use the Lorentz formula to make the solution of the above boundary value problem T,Ω unique:

[0029]

[0030] The field equations can finally be rearranged as follows:

[0031]

[0032] Step 2.2: Based on the calculation formula of the transformer electromagnetic field eddy current loss, obtain the stray loss of the transformer metal structure. The metal structure will generate eddy current inside in the magnetic field. The calculation formula of the eddy current loss of the metal structure is as follows:

[0033]

[0034] Where P e0 is the period average eddy current loss; dv is the integrated volume element;

[0035] Hysteresis loss is usually calculated by the magnetic flux density B. The calculation formula of hysteresis loss is as follows:

[0036]

[0037] Where P h0 is the hysteresis loss; V (i) is the unit volume of the i-th unit; ρ0 is the material density; is the peak value of magnetic flux density of the ith unit; is the hysteresis loss of the ith unit; N is the total number of time intervals within the period considered;

[0038] The total stray loss P of the magnetic metal structural parts is obtained as follows:

[0039] P=P e0 +P h0 ;

[0040] Where P is the total stray loss.

[0041] As a further description of the above scheme, step 3 includes the following steps:

[0042] Step 3.1: For different heat dissipation coefficients corresponding to different faces of transformer clamps and pull plates, the heat transfer coefficient discrete method is used to calculate the convection heat transfer coefficient of each metal structure surface.

[0043] The following formula is introduced for the target convective heat transfer coefficient calculation:

[0044] Nu=C(Gr·Pr) n ;

[0045] In the formula, Nu is the Nusselt number; Gr is the Grashof criterion; C and n are constants determined experimentally; Pr stands for the Prandtl number;

[0046] The calculation formula of Gr is as follows:

[0047]

[0048] In the formula, g is the acceleration of gravity; a represents the volume expansion coefficient; v1 represents the kinematic viscosity; l is the fixed size, △t is the difference between the wall temperature and the oil flow temperature;

[0049] The calculation formula of Pr is as follows:

[0050]

[0051] In the formula, c represents the specific heat capacity of the heat transfer medium, and λ represents the thermal conductivity of the heat conduction material. λ is set to 0.25 through the material properties;

[0052] The following formula is introduced for the calculation of the convective heat transfer coefficient at the hot spot position: △t in Gr is an unknown quantity, and Gr is used in the criterion correlation formula * , that is, Gr * for:

[0053]

[0054] In the formula, Gr * For the modified Grashof criterion, q is the value under the constant heat flow boundary;

[0055] The heat transfer coefficient criterion correlation formula for hot spot position is:

[0056]

[0057] In the formula, Nu x is the Nusselt number at the hotspot, is the Grashof criterion for the hotspot location;

[0058] The calculation formula for the convective heat transfer coefficient h is as follows:

[0059]

[0060] Where h is the convective heat transfer coefficient;

[0061] Convective heat transfer coefficient h at hotspot x The calculation formula is as follows:

[0062]

[0063] In the formula, h x is the convective heat transfer coefficient at the hot spot;

[0064] Step 3.2: When the average surface heat transfer coefficient of the wall is calculated by the temperature difference at 1 / 2 height of the wall, the surface heat transfer coefficient is taken as the average surface heat transfer coefficient of the whole wall, which is approximately equal to the average surface heat transfer coefficient defined by the integrated average temperature difference of the whole wall; laminar flow and turbulent flow are distinguished according to the transformer cooling oil circulation mode, the flow state of the natural oil circulation cooling mode and the forced oil circulation guided cooling mode is laminar flow, and the flow state of the forced oil circulation cooling mode is turbulent flow; the convective heat transfer coefficient is calculated by the target Nusselt number, and C and n are selected according to the cooling oil circulation mode, the shape and position of the wall and the boundary conditions as shown in the following table:

[0065]

[0066] The convective heat transfer coefficient of the clamp and the pull plate is calculated by the target Nusselt number. The convective heat transfer coefficient corresponding to each surface of the clamp is shown in the following table:

[0067]

[0068]

[0069] The corresponding convective heat transfer coefficient of each surface of the pull plate is shown in the following table:

[0070]

[0071]

[0072] Step 3.3: For the clamp surface f and the pull plate surface f, both are in contact between the metal structure and the insulation, and heat is transferred through heat conduction. The heat generated per unit time in a specific cross section during the heat transfer process is proportional to the cross-sectional area and the temperature change rate in the vertical direction. The hot surface temperature is calculated according to Fourier's law of heat conduction:

[0073]

[0074] In the formula, φ 夹件表面f is the heat transferred to the fixture surface f; Thot夹件表面f is the temperature of the clamp surface f during heat conduction; T cold夹件表面f A is the oil temperature at the surface f of the fixture during heat conduction; 夹件 is the cross-sectional area of ​​the fixture surface f in the heat conduction direction;

[0075] Temperature of the pulling plate surface f:

[0076]

[0077] In the formula, φ 拉板表面f is the heat transferred to the pull plate surface f; T hot拉板表面f is the temperature of the pull plate surface f during heat conduction;

[0078] T cold拉板表面f A is the oil temperature at the surface f of the pull plate during heat conduction; 拉板 is the cross-sectional area of ​​the pull plate surface f in the direction of heat conduction.

[0079] As a further description of the above scheme, in step 4: the thermal distribution diagram of the oil-immersed transformer is obtained by simplifying the thermal characteristics. For the oil temperature of the structural parts, it increases linearly from the bottom to the top. Then, a linear relationship y=kx+b is obtained according to the thermal distribution diagram. The oil temperature of the top layer and the bottom layer of the transformer oil tank and the corresponding height are measured by thermocouples. Finally, a linear relationship between the height of the structural parts in the oil tank and the oil temperature is obtained. The ambient temperature of each metal structural part is set according to the height corresponding to each surface of the metal structural part in the oil tank. For the vertical screen wall, the average height is used to calculate the oil temperature of the metal structural part surface.

[0080] As a further description of the above scheme, the transformer thermal life assessment for metal structural parts in step 6 includes the following steps:

[0081] Step 6.1: Calculate the thermal life loss. Due to the uneven temperature distribution, the part running at the highest temperature will generally suffer the most severe degradation. First, calculate the relative aging rate

[0082]

[0083] In the formula, θ hot is the hottest point temperature of the metal structure; V0 is the relative aging rate;

[0084] Thermal life loss is determined by the following formula:

[0085]

[0086] Where L is the thermal life loss, V n is the relative aging rate in the nth time interval; t n is the time of the nth time interval; n is the ordinal number of each time interval in the period under consideration;

[0087] Step 6.2: The transformer thermal life equation is obtained as:

[0088] O 热寿命 =SL;

[0089] In the formula, O 热寿命 is the thermal life of the transformer after running for a period of time; S is the service life of the transformer.

[0090] Advantages and effects of the present invention:

[0091] (1) In step 3 of the present invention, the heat dissipation coefficient of each surface of the transformer metal structure is accurately calculated. The convective heat transfer coefficient of each surface of the transformer metal structure is accurately calculated by the heat transfer coefficient discretization method, and the convective heat transfer coefficient of the heat dissipation surface at the hot spot position is accurately calculated. This method comprehensively considers the influence of heat transfer on each surface of the transformer internal metal structure.

[0092] (2) In step 4 of the present invention, the thermal distribution diagram of the transformer is simplified to obtain a linear relationship, thereby calculating the relationship between the height of the metal structure and the oil temperature. The ambient temperature of each metal structure can be set more accurately, thereby obtaining a more accurate temperature field temperature rise state.

[0093] (3) In step 6 of the present invention, the thermal aging of the transformer is calculated by the hot spot temperature of the metal structure. Most of the existing methods calculate the thermal life by the hot spot temperature of the winding, ignoring the impact of the remaining parts that may cause thermal life loss. Therefore, calculating the thermal life by the hot spot temperature of the metal structure can more comprehensively evaluate the thermal life of the transformer. BRIEF DESCRIPTION OF THE DRAWINGS

[0094] Figure 1 A flow chart of a method for evaluating the thermal life of a transformer of a metal structural part of the present invention;

[0095] Figure 2 is the digital twin model of the present invention;

[0096] Figure 3 is a front view of the clamp of the present invention;

[0097] Figure 4 The rear view of the clamp of the present invention Figure 1 ;

[0098] Figure 5 The rear view of the clamp of the present invention Figure 2 ;

[0099] Figure 6 It is a front view of the pull plate of the present invention;

[0100] Figure 7 It is a rear view of the pull plate of the present invention;

[0101] Figure 8 This is the thermal distribution diagram of the oil-immersed transformer of the present invention;

[0102] Fig. 9 This is a comparison diagram of the temperature measured by the optical fiber of the present invention and the temperature obtained by the heat transfer coefficient discrete method corresponding to the specific position where the optical fiber is placed.

[0103] 1-clamp surface a; 2-clamp surface b; 3-clamp surface c; 4-clamp surface d; 5-clamp surface e; 6-clamp surface f; 7-clamp surface g at the hot spot position; 8-pull plate surface a; 9-pull plate surface b; 10-pull plate surface c; 11-pull plate surface d; 12-pull plate surface e; 13-pull plate surface f; 14-pull plate surface g at the hot spot position; 15-clamp; 16-pull plate; 17-oil tank; 18-iron core; 19-winding; 20-clamp surface h; 21-clamp surface i; 22-clamp surface j; 23-pull plate surface h. DETAILED DESCRIPTION

[0104] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0105] A transformer thermal life assessment method for metal structural parts comprises the following steps:

[0106] Step 1: Establish a digital twin model of the oil-immersed transformer;

[0107] Step 2: Perform electromagnetic field simulation on the digital twin model of the oil-immersed transformer to obtain the stray losses of the metal structural parts;

[0108] Step 3: Accurately calculate the heat dissipation coefficient of each surface of the transformer metal structure, load the calculated results into the transformer temperature fluid field simulation analysis, and obtain the temperature distribution of the transformer metal structure;

[0109] Step 4: Simplify the transformer heat distribution diagram to obtain a linear relationship, thereby calculating the relationship between the height of the metal structure and the oil temperature, and load the calculated results into the transformer temperature field simulation analysis to complete the transformer temperature field simulation calculation;

[0110] Step 5: The temperature of the metal structure of the transformer obtained by the heat transfer coefficient discrete method is compared and corrected with the temperature of the metal structure of the transformer measured by the optical fiber temperature measurement. The temperature is very close to the accurate value, so the temperature field simulation of the heat transfer coefficient discrete method is successfully realized to obtain the temperature of the metal structure of the transformer.

[0111] Step 6: Based on the transformer insulation thermal aging evaluation criteria and the temperature rise status of the transformer metal structural parts, conduct a transformer thermal life assessment for the metal structural parts.

[0112] The establishment of the digital twin model of the oil-immersed transformer in step 1 of the present invention is based on the physical parameters of the transformer entity, and the structural design parameters are obtained according to the transformer calculation sheet and drawings.

[0113] Step 2 of the present invention comprises the following steps:

[0114] Step 2.1: Perform electromagnetic field simulation on the digital twin model of the oil-immersed transformer. First, assign electromagnetic material properties to each part of the transformer structure. According to the number of coil turns and phase current in the transformer product design data, select the cross-sectional area of ​​winding 19 to input the corresponding number of coil turns and operating current expressions. Grid division is performed based on the structural dimension data of the digital twin transformer. The total simulation running time and step size are set. The solution algorithm is calculated based on the T-Ω method in the three-dimensional time-harmonic field.

[0115] Step 2.1.1: Use the following formula to describe the mathematical model of the T-Ω method for solving the three-dimensional eddy current field problem of the transformer. The magnetic field intensity is divided into two parts: the gradient of the scalar potential in the non-conductor area and the vector edge unit of the vector field in the conductor area, as shown in the following table:

[0116] serial number Solution Domain Transformer components <![CDATA[V1]]> Non-conductor Transformer oil <![CDATA[V2]]> conductor Metal structural parts

[0117] Assuming that the conductivity σ is a constant, it can be obtained from Introducing the vector potential T, then:

[0118]

[0119] In the formula, J S is the excitation current density, J E is the eddy current density, J is the current density, and T is the vector potential. The domain relationship between the vector potential T and the scalar magnetic potential Ω under different solution domains satisfies:

[0120]

[0121] Where H is the magnetic field intensity and Ω is the scalar magnetic potential;

[0122] In a homogeneous medium:

[0123]

[0124] Where μ is magnetic permeability, σ is electrical conductivity, and E is electric field intensity;

[0125] Step 2.1.2: The field equations under different solution domains are:

[0126]

[0127] Step 2.1.3: Use the Lorentz formula to make the solution of the above boundary value problem T,Ω unique:

[0128]

[0129] The field equations can finally be rearranged as follows:

[0130]

[0131] Step 2.2: Based on the calculation formula of the transformer electromagnetic field eddy current loss, obtain the stray loss of the transformer metal structure. The metal structure will generate eddy current inside in the magnetic field. The calculation formula of the eddy current loss of the metal structure is as follows:

[0132]

[0133] Where P e0 is the period average eddy current loss; dv is the integrated volume element;

[0134] Hysteresis loss is usually calculated by the magnetic flux density B. The calculation formula of hysteresis loss is as follows:

[0135]

[0136] Where P h0 is the hysteresis loss; V (i) is the unit volume of the i-th unit; ρ0 is the material density; is the peak value of magnetic flux density of the ith unit; is the hysteresis loss of the ith unit; N is the total number of time intervals within the period considered;

[0137] The total stray loss P of the magnetic metal structural parts is obtained as follows:

[0138] P=P e0 +P h0 ;

[0139] Where P is the total stray loss.

[0140] Step 3 of the present invention comprises the following steps:

[0141] Step 3.1: For different heat dissipation coefficients corresponding to different faces of the transformer clamp 15 and the pull plate, the heat transfer coefficient discrete method is used to calculate the convection heat transfer coefficient of each metal structural surface.

[0142] The following formula is introduced for the target convective heat transfer coefficient calculation:

[0143] Nu=C(Gr·Pr) n ;

[0144] In the formula, Nu is the Nusselt number; Gr is the Grashof criterion; C and n are constants determined experimentally; Pr stands for the Prandtl number;

[0145] The calculation formula of Gr is as follows:

[0146]

[0147] In the formula, g is the acceleration of gravity; a represents the volume expansion coefficient; v1 represents the kinematic viscosity; l is the fixed size, △t is the difference between the wall temperature and the oil flow temperature;

[0148] The calculation formula of Pr is as follows:

[0149]

[0150] In the formula, c represents the specific heat capacity of the heat transfer medium, and λ represents the thermal conductivity of the heat conduction material. λ is set to 0.25 through the material properties;

[0151] The following formula is introduced for the calculation of the convective heat transfer coefficient at the hot spot position: △t in Gr is an unknown quantity, and Gr is used in the criterion correlation formula * , that is, Gr * for:

[0152]

[0153] In the formula, Gr * To modify the Grashof criterion, q is the value under the constant heat flow boundary;

[0154] The heat transfer coefficient criterion correlation formula for hot spot position is:

[0155]

[0156] In the formula, Nu x is the Nusselt number at the hotspot, is the Grashof criterion for the hotspot location;

[0157] The calculation formula for the convective heat transfer coefficient h is as follows:

[0158]

[0159] Where h is the convective heat transfer coefficient;

[0160] Convective heat transfer coefficient h at hotspot x The calculation formula is as follows:

[0161]

[0162] In the formula, h x is the convective heat transfer coefficient at the hot spot;

[0163] Step 3.2: When the average surface heat transfer coefficient of the wall is calculated by the temperature difference at 1 / 2 height of the wall, the surface heat transfer coefficient is taken as the average surface heat transfer coefficient of the whole wall, which is approximately equal to the average surface heat transfer coefficient defined by the integrated average temperature difference of the whole wall; laminar flow and turbulent flow are distinguished according to the transformer cooling oil circulation mode, the flow state of the natural oil circulation cooling mode and the forced oil circulation guided cooling mode is laminar flow, and the flow state of the forced oil circulation cooling mode is turbulent flow; the convective heat transfer coefficient is calculated by the target Nusselt number, and C and n are selected according to the cooling oil circulation mode, the shape and position of the wall and the boundary conditions as shown in the following table:

[0164]

[0165] The convective heat transfer coefficients of the clamp 15 and the pull plate 16 are calculated by the target Nusselt number. The convective heat transfer coefficients corresponding to each surface of the clamp 15 are shown in the following table:

[0166]

[0167]

[0168] The convective heat transfer coefficient corresponding to each surface of the pull plate 16 is shown in the following table:

[0169]

[0170]

[0171] Step 3.3: For the clamp surface f6 and the pull plate surface f13, both are contacts between metal structures and insulation. Heat is transferred by heat conduction. The heat generated per unit time in a specific cross section during the heat transfer process is proportional to the cross-sectional area and the temperature change rate in the vertical direction. The hot surface temperature is calculated according to Fourier's law of heat conduction:

[0172]

[0173] In the formula, φ 夹件表面f is the heat transferred to the fixture surface f; T hot夹件表面f is the temperature of the clamp surface f6 during heat conduction; T cold夹件表面f A is the oil temperature at point f6 on the clamp surface during heat conduction;夹件 is the cross-sectional area of ​​the clamp surface f6 in the heat conduction direction;

[0174] Temperature of the pulling plate surface f 13:

[0175]

[0176] In the formula, φ 拉板表面f is the heat transferred to the pull plate surface f; T hot拉板表面f is the temperature of the pull plate surface f13 during heat conduction; T cold拉板表面f A is the oil temperature at point f13 on the pull plate surface during heat conduction; 拉板 is the cross-sectional area of ​​the pull plate surface f13 in the direction of heat conduction.

[0177] In step 3 of the present invention, the heat dissipation coefficient of each surface of the transformer metal structure is accurately calculated. The convective heat transfer coefficient of each surface of the transformer metal structure is accurately calculated by the heat transfer coefficient discretization method, and the convective heat transfer coefficient of the heat dissipation surface at the hot spot position is accurately calculated. This method comprehensively considers the influence of heat transfer on each surface of the metal structure inside the transformer.

[0178] In step 4 of the present invention: the thermal distribution diagram of the oil-immersed transformer is obtained by simplifying the thermal characteristics. For the oil temperature of the structural parts, it increases linearly from the bottom to the top. Then, the linear relationship y=kx+b is obtained according to the thermal distribution diagram. The oil temperature of the top layer and the bottom layer of the transformer oil tank 17 and the corresponding height are measured by thermocouples. Finally, the linear relationship between the height of the structural parts in the oil tank 17 and the oil temperature is obtained. The ambient temperature of each metal structural part is set according to the height corresponding to each surface of the metal structural part in the oil tank 17. For the vertical screen wall, the average height is used to calculate the oil temperature of the metal structural part surface. In step 4 of the present invention, the linear relationship is obtained by simplifying the thermal distribution diagram of the transformer, thereby calculating the relationship between the height of the metal structural part and the oil temperature. The ambient temperature of each metal structural part can be set more accurately, thereby obtaining a more accurate temperature field temperature rise state.

[0179] The step 6 of the present invention includes the following steps:

[0180] Step 6.1: Calculate the thermal life loss. Due to the uneven temperature distribution, the part running at the highest temperature will generally suffer the most severe degradation. First, calculate the relative aging rate

[0181]

[0182] In the formula, θ hot is the hottest point temperature of the metal structure; V0 is the relative aging rate;

[0183] Thermal life loss is determined by the following formula:

[0184]

[0185] Where L is the thermal life loss, V n is the relative aging rate in the nth time interval; t n is the time of the nth time interval; n is the ordinal number of each time interval in the period under consideration;

[0186] Step 6.2: The transformer thermal life equation is obtained as:

[0187] O 热寿命 =SL;

[0188] In the formula, O 热寿命 is the thermal life of the transformer after running for a period of time; S is the service life of the transformer. In step 6 of the present invention, the thermal aging of the transformer is calculated by the hot spot temperature of the metal structure. Most of the existing methods calculate the thermal life by the hot spot temperature of the winding 19, ignoring the influence of the other parts that may cause the loss of thermal life. Therefore, the thermal life calculation by the hot spot temperature of the metal structure can more comprehensively evaluate the thermal life of the transformer.

[0189] Example

[0190] In order to make the advantages and technical solutions of the present invention clearer and more specific, the present invention is further described below with reference to the accompanying drawings.

[0191] The present invention proposes a transformer thermal life assessment method for metal structural parts, which mainly designs different heat dissipation coefficients corresponding to different surfaces of transformer metal structural parts, heat dissipation coefficients at hot spots, ambient temperatures corresponding to different metal structural parts, and calculation of transformer thermal life for metal structural parts. The overall process is as follows: Figure 1 shown.

[0192] First, a digital twin transformer is established based on the physical parameters of the transformer (that is, the transformer design dimensions), such as Figure 2 As shown. The structural design parameters are obtained according to the transformer calculation list and drawings. The parameters include the width and height of the core 18, the inner and outer diameters and height of the winding 19, the cross-sectional shape and thickness of the insulating spacer, the clamps, pull plates, insulation and partial structural dimensions of various metal structural parts, and the dimensions of the oil tank 17. The model is established through the physical parameters of each component and assembled based on the product drawings to form a digital twin transformer physical model. The digital twin transformer structure mainly includes the following: Figure 2 The structure shown includes the core 18, clamps, oil tank 17, pull plates, windings 19 and insulation of various metal structural parts.

[0193] When performing electromagnetic field simulation on the digital twin model of the oil-immersed transformer, the corresponding electromagnetic material properties are first assigned to each part of the transformer structure. According to the number of coil turns and phase current in the transformer product design data, the winding 19 cross-sectional area is selected to input the corresponding coil turns and operating current expressions. Based on the corresponding structural size parameters of the digital twin transformer, a suitable and as fine a grid as possible is divided. Fixed boundary conditions are set based on the physical placement of the transformer. Based on the total simulation running time and step size, simulation calculations are performed to obtain the stray losses of the transformer metal structural parts.

[0194] For the different heat dissipation coefficients corresponding to different faces of the transformer clamps and pull plates, the heat transfer coefficient discrete method is used to calculate the heat dissipation coefficient of each face of the transformer metal structure. The convective heat transfer methods for different faces are also different. The convective heat transfer coefficient is calculated by the target Nusselt number, and the convective heat transfer coefficient at the hot spot position is calculated by the Nusselt number at the hot spot position. The selection of C and n is different according to the shape, position and boundary conditions of the wall. The average surface heat transfer coefficient of the wall is calculated using the temperature difference at 1 / 2 the height of the wall as the average surface heat transfer coefficient of the entire wall, which is approximately equal to the average surface heat transfer coefficient defined by the integrated average temperature difference of the entire wall. According to the transformer cooling oil circulation method, laminar flow and turbulent flow are distinguished. The natural oil circulation cooling method and the forced oil circulation guided cooling method have laminar flow, and the forced oil circulation cooling method has turbulent flow. The convective heat transfer coefficients of the clamps and pull plates are calculated by the target Nusselt number. The corresponding positions of each face of the clamp 15 and the pull plate 16 are as follows Figure 3 , Figure 4 , Figure 5 , Figure 6 , Figure 7 As shown, the corresponding convective heat transfer coefficient of each surface of the clamp is shown in the following table:

[0195] Table 1 Calculation formula for convection heat transfer coefficient corresponding to each surface of clamp 15

[0196]

[0197]

[0198] Table 2 Calculation formula for convection heat transfer coefficient corresponding to each surface of the pull plate 16

[0199]

[0200] Step 3.3: For the clamp surface f6 and the pull plate surface f13, both are contacts between metal structures and insulation. Heat is transferred by heat conduction. The heat generated per unit time in a specific cross section during the heat transfer process is proportional to the cross-sectional area and the temperature change rate in the vertical direction. The hot surface temperature is calculated according to Fourier's law of heat conduction:

[0201]

[0202] In the formula, φ 夹件表面f is the heat transferred to the fixture surface f; T hot夹件表面f is the temperature of the clamp surface f6 during heat conduction; T cold夹件表面f A is the oil temperature at point f6 on the clamp surface during heat conduction; 夹件 is the cross-sectional area of ​​the clamp surface f6 in the heat conduction direction;

[0203] Temperature of the pulling plate surface f 13:

[0204]

[0205] In the formula, φ 拉板表面f is the heat transferred to the pull plate surface f 13; T hot拉板表面f is the temperature of the pull plate surface f13 during heat conduction; T cold拉板表面f A is the oil temperature at point f13 on the pull plate surface during heat conduction; 拉板 is the cross-sectional area of ​​the pull plate surface f13 in the direction of heat conduction.

[0206] The thermal distribution diagram of the oil-immersed transformer is obtained by simplifying the thermal characteristics, such as Figure 8 As shown, the oil temperature of the structural parts increases linearly from bottom to top regardless of the cooling method. The linear relationship y=kx+b is obtained based on the heat distribution diagram. The oil temperature of the top layer and the bottom layer of the transformer oil tank 17 are measured by thermocouples. Finally, the linear relationship between the height of the structural parts in the oil tank 17 and the oil temperature is obtained. The ambient temperature of each metal structural part is set according to the corresponding height of the metal structural part in the oil tank 17. For the vertical screen wall, the average height is used to calculate the oil temperature of the metal structural part surface. The analysis results are loaded into the transformer temperature field simulation analysis to complete the transformer temperature field simulation calculation.

[0207] The transformer temperature field simulation obtained by the heat transfer coefficient discrete method is compared with the temperature measured by the optical fiber sensor. The comparison results are shown in Fig. 9 As shown in the figure, the horizontal axis represents measuring point 1, measuring point 2, measuring point 3 and so on. According to the comparison error between the temperature obtained by the heat transfer coefficient discrete method and the actual temperature obtained by the optical fiber sensor is less than 5%, thus verifying the accuracy of the temperature of the transformer metal structure obtained by the heat transfer coefficient discrete method temperature field simulation.

[0208] Based on the transformer insulation thermal aging evaluation criteria and the temperature rise state of the transformer metal structure, the transformer thermal life evaluation for the metal structure is carried out. Because the temperature distribution of the metal structure is uneven, the aging rate is based on the hottest temperature of the metal structure.

[0209]

[0210] In the formula, θ hot is the hottest point temperature of the metal structure, and V0 is the relative aging rate.

[0211] The thermal life loss is determined by the formula;

[0212]

[0213] Where L is the thermal life loss, V n is the relative aging rate in the nth time interval; t n is the time of the nth time interval; n is the ordinal number of each time interval in the period under consideration;

[0214] At this time, the transformer thermal life equation is:

[0215] O 热寿命 =SL;

[0216] In the formula, O 热寿命 is the thermal life of the transformer after running for a period of time; S is the service life of the transformer.

[0217] The above is only an embodiment of the present application and does not constitute any form of limitation to the present application. Although the present application is disclosed as above with a preferred embodiment, it is not intended to limit the present application. Any technician familiar with the profession, without departing from the scope of the technical solution of the present application, using the technical content disclosed above to make slight changes or modifications are equivalent to equivalent implementation cases and fall within the scope of the technical solution.

Claims

1. A transformer thermal life assessment method for metal structural parts, characterized in that: The following steps are involved: Step 1: Establish a digital twin model of the oil-immersed transformer; Step 2: Perform electromagnetic field simulation on the digital twin model of the oil-immersed transformer to obtain the total stray loss of the metal structural parts; Step 3: Calculate the heat dissipation coefficient of each surface of the transformer metal structure, load the calculated results into the transformer temperature fluid field simulation analysis, and obtain the temperature distribution of the transformer metal structure; Step 4: Simplify the transformer heat distribution diagram to obtain a linear relationship, thereby calculating the relationship between the height of the metal structure and the oil temperature, and load the calculated results into the transformer temperature field simulation analysis to complete the transformer temperature field simulation calculation; Step 5: Compare and correct the temperature of the metal structure of the transformer obtained by the heat transfer coefficient discrete method and the temperature of the metal structure of the transformer obtained by the optical fiber temperature measurement; Step 6: Based on the transformer insulation thermal aging evaluation criteria and the temperature rise status of the transformer metal structural parts, conduct a transformer thermal life assessment for the metal structural parts.

2. A transformer thermal life assessment method for metal structural parts according to claim 1, characterized in that: The establishment of the digital twin model of the oil-immersed transformer in step 1 is based on the physical parameters of the transformer entity, and the design parameters of the structure are obtained according to the transformer calculation sheet and drawings.

3. A transformer thermal life assessment method for metal structural parts according to claim 1, characterized in that: The step 2 comprises the following steps: Step 2.1: Perform electromagnetic field simulation on the digital twin model of the oil-immersed transformer. First, assign corresponding electromagnetic material properties to each part of the transformer structure. According to the coil turns and phase current in the transformer product design data, select the winding (19) cross-sectional area to input the corresponding coil turns and operating current expressions. Perform grid division based on the structural dimension data of the digital twin transformer, set the total simulation running time and step size, and perform solution algorithm calculation based on the T-Ω method in the three-dimensional time-harmonic field. Step 2.1.1: Use the following formula to describe the mathematical model of the T-Ω method for solving the three-dimensional eddy current field problem of the transformer. The magnetic field intensity is divided into two parts: the gradient of the scalar potential in the non-conductor area and the vector edge unit of the vector field in the conductor area, as shown in the following table: Assuming that the conductivity σ is a constant, it can be obtained from Introducing the vector potential T, then: In the formula, J S is the excitation current density, J E is the eddy current density, J is the current density, and T is the vector potential. The domain relationship between the vector potential T and the scalar magnetic potential Ω under different solution domains satisfies: Where H is the magnetic field intensity and Ω is the scalar magnetic potential; In a homogeneous medium: Where μ is magnetic permeability, σ is electrical conductivity, and E is electric field intensity; Step 2.1.2: The field equations under different solution domains are: Step 2.1.3: Use the Lorentz formula to make the solution of the above boundary value problem T,Ω unique: The field equations can finally be rearranged as follows: Step 2.2: Based on the calculation formula of the transformer electromagnetic field eddy current loss, obtain the stray loss of the transformer metal structure. The metal structure will generate eddy current inside in the magnetic field. The calculation formula of the eddy current loss of the metal structure is as follows: Where P e0 is the period average eddy current loss; dv is the integrated volume element; Hysteresis loss is usually calculated by the magnetic flux density B. The calculation formula of hysteresis loss is as follows: Where P h0 is the hysteresis loss; V (i) is the unit volume of the i-th unit; ρ0 is the material density; is the peak value of magnetic flux density of the ith unit; is the hysteresis loss of the ith unit; N is the total number of time intervals within the period considered; The total stray loss P of the magnetic metal structural parts is obtained as follows: P=P e0 +P h0 ; Where P is the total stray loss.

4. A transformer thermal life assessment method for metal structural parts according to claim 1, characterized in that: The step 3 comprises the following steps: Step 3.1: For different heat dissipation coefficients corresponding to different faces of the transformer clamp (15) and the pull plate (16), the heat transfer coefficient discrete method is used to calculate the convection heat transfer coefficient of each metal structural surface. The following formula is introduced for the target convective heat transfer coefficient calculation: No=C(Gr·Pr) n ; In the formula, Nu is the Nusselt number; Gr is the Grashof criterion; C and n are constants determined experimentally; Pr stands for the Prandtl number; The calculation formula of Gr is as follows: In the formula, g is the acceleration of gravity; a represents the volume expansion coefficient; v1 represents the kinematic viscosity; l is the fixed size, △t is the difference between the wall temperature and the oil flow temperature; The calculation formula of Pr is as follows: In the formula, c represents the specific heat capacity of the heat transfer medium, and λ represents the thermal conductivity of the heat conduction material. λ is set to 0.25 through the material properties; The following formula is introduced for the calculation of the convective heat transfer coefficient at the hot spot position: △t in Gr is an unknown quantity, and Gr is used in the criterion correlation formula * , that is, Gr* is: In the formula, Gr * To modify the Grashof criterion, q is the value under the constant heat flow boundary; The heat transfer coefficient criterion correlation formula for hot spot position is: In the formula, Nu x is the Nusselt number at the hotspot, is the Grashof criterion for the hotspot location; The calculation formula for the convective heat transfer coefficient h is as follows: Where h is the convective heat transfer coefficient; Convective heat transfer coefficient h at hotspot x The calculation formula is as follows: In the formula, h x is the convective heat transfer coefficient at the hot spot; Step 3.2: When the average surface heat transfer coefficient of the wall is calculated by the temperature difference at 1 / 2 height of the wall, the surface heat transfer coefficient is taken as the average surface heat transfer coefficient of the whole wall, which is approximately equal to the average surface heat transfer coefficient defined by the integrated average temperature difference of the whole wall; laminar flow and turbulent flow are distinguished according to the transformer cooling oil circulation mode, the flow state of the natural oil circulation cooling mode and the forced oil circulation guided cooling mode is laminar flow, and the flow state of the forced oil circulation cooling mode is turbulent flow; the convective heat transfer coefficient is calculated by the target Nusselt number, and C and n are selected according to the cooling oil circulation mode, the shape and position of the wall and the boundary conditions as shown in the following table: The convective heat transfer coefficients of the clamp (15) and the pull plate (16) are calculated by the target Nusselt number. The convective heat transfer coefficients corresponding to each surface of the clamp (15) are shown in the following table: The convective heat transfer coefficient corresponding to each surface of the pull plate (16) is shown in the following table: Step 3.3: For the clamp surface f(6) and the pull plate surface f(13), both are contacts between metal structures and insulation. Heat is transferred by heat conduction. The heat generated per unit time in a specific cross section during the heat transfer process is proportional to the cross-sectional area and the temperature change rate in the vertical direction. The hot surface temperature is calculated according to Fourier's law of heat conduction: In the formula, φ 夹件表面f is the heat transferred to the fixture surface f; T hot夹件表面f is the temperature of the clamp surface f(6) during heat conduction; T cold夹件表面f is the oil temperature at the clamp surface f(6) during heat conduction; A 夹件 is the cross-sectional area of ​​the clamp surface f(6) in the heat conduction direction; Temperature of the pull plate surface f(13): In the formula, φ 拉板表面f is the heat transferred to the pull plate surface f(13); T hot拉板表面f is the temperature of the pull plate surface f(13) during heat conduction; T cold拉板表面f is the oil temperature at the plate surface f(13) during heat conduction; A 拉板 is the cross-sectional area of ​​the pull plate surface f(13) in the direction of heat conduction.

5. The method for evaluating the thermal life of a transformer for a metal structural part according to claim 1, characterized in that: In the step 4, a thermal distribution diagram of the oil-immersed transformer is obtained by simplifying the thermal characteristics. The oil temperature of the structural parts increases linearly from the bottom to the top. Then, a linear relationship y=kx+b is obtained according to the thermal distribution diagram. The oil temperature of the top layer and the bottom layer of the transformer oil tank (17) and the corresponding height are measured by thermocouples. Finally, a linear relationship between the height of the structural parts in the oil tank (17) and the oil temperature is obtained. The ambient temperature of each metal structural part is set according to the height corresponding to each surface of the metal structural part in the oil tank (17). For the vertical screen wall, the average height is used to calculate the oil temperature of the metal structural part surface.

6. A transformer thermal life assessment method for metal structural parts according to claim 1, characterized in that: The transformer thermal life assessment for metal structural parts in step 6 includes the following steps: Step 6.1: Calculate the thermal life loss. Due to the uneven temperature distribution, the part running at the highest temperature will generally suffer the most severe degradation. First, calculate the relative aging rate In the formula, θ hot is the hottest point temperature of the metal structure; V0 is the relative aging rate; Thermal life loss is determined by the following formula: Where L is the thermal life loss, V n is the relative aging rate in the nth time interval; t n is the time of the nth time interval; n is the ordinal number of each time interval in the period under consideration; Step 6.2: The transformer thermal life equation is obtained as: O 热寿命 =S-L; In the formula, O 热寿命 is the thermal life of the transformer after running for a period of time; S is the service life of the transformer.

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