Method for determining relevance between maximum value of main insulation electric field intensity of oil-immersed current transformer and size of ellipsoid metal particles

Abstract

The invention relates to a method for determining relevance between main insulation electric field distortion and oval metal particle size of an oil-immersed current transformer. The method comprises the following steps: firstly, drawing a full-size finite element simulation model according to the actual size of the oil-immersed current transformer by utilizing Solidworks software, and importing the full-size finite element simulation model into COMSOLMultiphysics software; then, acquiring the maximum electric field intensity corresponding to the elliptical particles with different sizes on the basis of three-dimensional simulation, and extracting data between the volume size of the elliptical particles and the maximum electric field intensity; preferably, non-linear fitting is carried out on the simulated data through Matlab, a corresponding correlation formula is obtained, and technical guarantee is provided for fault diagnosis of the current transformer and stable operation of power equipment.

Description

Technical Field

[0001] The present invention belongs to the field of insulation fault diagnosis of electrical equipment, and more specifically, relates to a method for determining the correlation between the maximum value of the main insulation electric field strength of an oil-immersed current transformer and the size of ellipsoidal metallic particles. Background Art

[0002] Oil-immersed current transformers (OCTs) are core measurement and protection equipment in high-voltage power grids. The reliability of their primary insulation structure is directly related to the safe and stable operation of the power system. The primary insulation, typically composed of an oil-paper composite dielectric, is susceptible to mechanical stress, thermal aging, and the intrusion of external contaminants over long-term operation, leading to insulation degradation. Metallic particle contamination is a significant cause of insulation failure. During equipment manufacturing, transportation, or operation, metal particles can enter the insulating oil due to mechanical wear or process defects, forming localized conductive foreign matter. These particles, when exposed to electric fields, can easily cause electric field distortion, leading to partial discharge and even insulation breakdown, seriously threatening the life of the equipment.

[0003] Currently, methods for detecting and evaluating metallic particles in the main insulation of oil-immersed current transformers primarily focus on offline testing and simplified model analysis. For example, the degree of particle contamination can be indirectly assessed through oil sample chromatography, moisture detection, or partial discharge measurements. However, these methods struggle to quantify the correlation between particle size and shape and electric field distortion. Existing research often assumes spherical particles, ignoring the ellipsoidal nature of actual particles, resulting in deviations between assessment results and actual operating conditions. Therefore, an efficient method is urgently needed to quantify the correlation between the maximum electric field strength of the main insulation and the size of ellipsoidal metallic particles to guide equipment condition assessment and defect warning. Summary of the Invention

[0004] The purpose of this method is to propose a method for determining the correlation between the maximum value of the main insulation electric field intensity of an oil-immersed current transformer and the size of ellipsoidal metallic particles, providing a simulation strategy for fault state assessment of oil-immersed current transformers.

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

[0006] S1: Taking a 220kV oil-immersed inverted current transformer as the object, a full-scale finite element simulation model was drawn in the modeling software Solidworks based on the actual dimensions of the current transformer's main insulation. The 3D model was then imported into the current field module of the finite element simulation software Comsol. The 3D model includes the shielding cover, high-voltage screen, end screen, end screen, oil-impregnated paper, and insulating oil.

[0007] S2: Set an ellipsoidal particle defect in the model and set the material properties of the simulation model in Comsol. The main insulation material uses the frequency domain dielectric parameters of oil-impregnated paper and insulating oil. The conductivity of the ellipsoidal metallic particles is set as conductor properties, and the major and minor axis size ranges of the particles are defined.

[0008] S3: Apply high-voltage excitation to the simulation model and set ground boundary conditions. Change the volume of the ellipsoid by changing its semi-axes a, b, and c. A total of 30 different ellipsoid volumes with different semi-axes a, b, and c were set. The electric field was simulated to obtain the maximum electric field strength in the main insulation of the current transformer.

[0009] S4: Extract the volume of different elliptical particles and their corresponding maximum electric field intensity data, and establish a correlation data set between particle volume and maximum electric field intensity;

[0010] S5: The dataset was imported into Matlab, and a nonlinear fitting algorithm was used to construct a correlation formula between the volume of elliptical particles and the maximum electric field strength. The electric field distortion risk of the main insulation of the current transformer was evaluated based on this formula.

[0011] Preferably, the three-dimensional model of the main insulation of the oil-immersed current transformer described in step S1 simplifies the design of the insulating bushing part and ignores the outer porcelain sleeve; the current transformer secondary junction box, expander, and expansion cover can also be ignored during calculation because they have little effect on the main insulation electric field.

[0012] Preferably, in step S2, an ellipsoidal particle defect is set, with semi-axis a being the longest semi-axis and semi-axes b and c being relatively short. The ellipsoidal particle material is set to have conductive properties. Furthermore, the main insulation material uses the frequency-domain dielectric parameters of oil-impregnated paper and insulating oil.

[0013] Preferably, in step S3, the current field of the COMSOL Multiphysics AC / DC module is selected for solution, a high-voltage screen voltage excitation is given, the end screen is set to ground, each end screen is set to a suspended potential, and electrical insulation is added to the remaining sections of the simulation model except the high-voltage screen. The volume of the ellipsoid is changed by changing the a, b, and c semi-axes of the ellipsoid. A total of 30 groups of ellipsoids with different volumes are set, and the electric field is simulated to obtain the maximum electric field strength in the main insulation of the current transformer. Among them, changing the volume of the ellipsoid by changing the a, b, and c semi-axes of the ellipsoid is described by formula (1):

[0014]

[0015] Preferably, in step S4, a correlation data set between the particle volume and the maximum electric field intensity is extracted.

[0016] Preferably, in step S5, the above data set is imported into Matlab, and a nonlinear fitting algorithm is used to construct a discriminant formula between the volume of elliptical particles and the maximum electric field intensity, which is described by formula (2):

[0017]

[0018] Among them, E max is the maximum electric field strength, in V / m; V sys is the system voltage, which is 220kV; L eff is the characteristic length, in m, which can be described by formula (3):

[0019]

[0020] C is the dimensionless correction factor; V is the volume of the ellipsoid; V0 is the reference volume, in mm 3 , and V0=1mm 3 ; m is the volume index, dimensionless; n is the aspect ratio index, dimensionless; λ is the aspect ratio, which can be described by formula (4):

[0021]

[0022] Where a, b, and c are the lengths of the semi-axes a, b, and c of the ellipsoid, respectively.

[0023] Compared with the prior art, the present invention has the following beneficial effects:

[0024] This paper combines a Comsol 3D simulation model with Matlab's nonlinear fitting tool to quantify the correlation between the maximum electric field strength of the main insulation of an oil-immersed current transformer and the size of ellipsoidal metal particles. Based on the corresponding discriminant formula, this method provides technical support for current transformer fault diagnosis and the stable operation of power equipment. The established full-scale finite element simulation 3D model improves the accuracy of electric field strength assessment of oil-immersed current transformers using the finite element method. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 This is a flow chart of the method of the present invention

[0026] Figure 2 This is the 3-D simulation model diagram of the transformer main insulation provided by the present invention

[0027] Figure 3 This is a diagram of the defect model of ellipsoidal particles embedded in the main insulation provided by the present invention. DETAILED DESCRIPTION

[0028] The following is a clear and complete description of the technical solutions of the present invention in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the embodiments described are only part of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making any creative work are within the scope of protection of the present invention. The technical solution details of the present invention are further described here.

[0029] Example 1

[0030] The method for determining the correlation between the maximum value of the main insulation electric field strength of an oil-immersed current transformer and the size of ellipsoidal metallic particles described in the present invention comprises the following steps:

[0031] S1, taking the 220kV oil-immersed inverted current transformer as the object, based on the actual size of the current transformer main insulation, a full-scale finite element simulation model is drawn in the modeling software Solidworks, and the 3D model is imported into the finite element simulation software Comsol current field module. The 3D model includes the shielding cover, high-voltage screen, end screen, end screen, oil-immersed paper, and insulating oil. The main insulation 3D simulation model is as follows: Figure 2 As shown;

[0032] S2, set an ellipsoidal particle defect in the model, set the material properties of the simulation model in Comsol, where the main insulation material uses the frequency domain dielectric parameters of oil-impregnated paper and insulating oil, the conductivity of the elliptical metallic particles is set to conductor characteristics, and the major axis and minor axis size ranges of the particles are defined. Figure 3 As shown;

[0033] In step S3, high-voltage excitation is applied to the simulation model and ground boundary conditions are set. The volume of the ellipsoid is changed by changing the a, b, and c semi-axes of the ellipsoid. A total of 30 different ellipsoid volumes are set, with different a, b, and c semi-axes. The electric field is simulated to obtain the maximum electric field strength in the main insulation of the current transformer. The change in the volume of the ellipsoid by changing the a, b, and c semi-axes of the ellipsoid is described by formula (1):

[0034]

[0035] S4, extracting the volume of different elliptical particles and their corresponding maximum electric field intensity data, and establishing a correlation data set between particle volume and maximum electric field intensity;

[0036] S5, the data set was imported into Matlab, and a nonlinear fitting algorithm was used to construct the correlation formula between the volume of elliptical particles and the maximum electric field intensity, which is described by formula (2):

[0037]

[0038] Among them, E max is the maximum electric field strength, in V / m;

[0039] V sys is the system voltage, which is 220kV;

[0040] L eff is the characteristic length, in m, which can be described by formula (3):

[0041]

[0042] C is the dimensionless correction coefficient, and through fitting, we can get C = 0.823;

[0043] V is the volume of the ellipsoid;

[0044] V0 is the reference volume, in mm 3 , and V0=1mm 3 ;

[0045] m is the volume index, dimensionless, and through fitting, we can get m = 0.497;

[0046] n is the aspect ratio index, dimensionless, and through fitting, we can get n = 0.315;

[0047] λ is the aspect ratio, which can be described by formula (4):

[0048]

[0049] Where a, b, and c are the lengths of the semi-axes a, b, and c of the ellipsoid, respectively.

[0050] The above content is merely an illustration of the structure of the present invention. Various modifications or additions to the specific structure described may be made by those skilled in the art. As long as they do not deviate from the structure of the invention or exceed the scope defined by the claims, they shall fall within the scope of protection of the present invention.

Claims

1. A method for determining the correlation between the maximum value of the main insulation electric field strength of an oil-immersed current transformer and the size of ellipsoidal metal particles, characterized in that: The following steps are involved: S1: Taking a 220kV oil-immersed inverted current transformer as the object, a full-scale finite element simulation model was drawn in the modeling software Solidworks based on the actual size of the current transformer's main insulation. The 3D model was then imported into the current field module of the finite element simulation software Comsol. S2: Set an ellipsoidal particle defect in the model and set the material properties of the simulation model in Comsol. The main insulation material uses the frequency domain dielectric parameters of oil-impregnated paper and insulating oil. The conductivity of the ellipsoidal metallic particles is set as conductor properties, and the major and minor axis size ranges of the particles are defined. S3: Apply high-voltage excitation to the simulation model and set ground boundary conditions. Change the volume of the ellipsoid by changing its semi-axes a, b, and c. A total of 30 different ellipsoid volumes with different semi-axes a, b, and c were set. Electric field simulation was also performed to obtain the maximum electric field strength in the main insulation of each of the 30 current transformers. S4: Extract the volume of different elliptical particles and their corresponding maximum electric field intensity data, and establish a correlation data set between particle volume and maximum electric field intensity; S5: The dataset was imported into Matlab, and a nonlinear fitting algorithm was used to construct a correlation formula between the volume of elliptical particles and the maximum electric field strength. The electric field distortion risk of the main insulation of the current transformer was evaluated based on this formula.

2. The method for determining the correlation between the maximum value of the main insulation electric field strength of an oil-immersed current transformer and the size of ellipsoidal metal particles according to claim 1, characterized in that: In step S2, the material properties of the simulation model are set as follows: the material of the oil conservator is iron, with a relative dielectric constant of 1; the material of the secondary shield is aluminum, with a relative dielectric constant of 1; the material of the insulating paper is a custom material, with a relative dielectric constant of 4; and the material of the insulating oil is transformer oil, with a relative dielectric constant of 2.

2.

3. The method for determining the correlation between the maximum value of the main insulation electric field strength of an oil-immersed current transformer and the size of ellipsoidal metal particles according to claim 1, characterized in that: In the step S4, high voltage excitation is applied to the simulation model and grounding boundary conditions are set. The high voltage panel voltage excitation is given to 220 kV, the end panel is set to grounding, and each end panel is set to a floating potential.

4. The method for determining the correlation between the maximum value of the main insulation electric field strength of an oil-immersed current transformer and the size of ellipsoidal metal particles according to claim 1, characterized in that: The step S3 changes the volume of the ellipsoid by changing the semi-axes a, b, and c of the ellipsoid, which is described by formula (1): Where V represents the volume of the ellipsoid, and a, b, and c represent the a, b, and c semi-axes of the ellipsoid, respectively.

5. The method for determining the correlation between the maximum value of the main insulation electric field strength of an oil-immersed current transformer and the size of ellipsoidal metal particles according to claim 1, characterized in that: In step S5, a nonlinear fitting algorithm is used to construct a discriminant formula between the volume of elliptical particles and the maximum electric field intensity, which is described by formula (2): Among them, E max is the maximum electric field strength, in V / m; V sys is the system voltage, which is 220kV; L eff is the characteristic length, in m, which can be described by formula (3): C is the dimensionless correction factor; V is the volume of the ellipsoid; V0 is the reference volume, in mm 3 , and V0=1mm 3 ; m is the volume index, dimensionless; n is the aspect ratio index, dimensionless; λ is the aspect ratio, which can be described by formula (4): Where a, b, and c are the lengths of the semi-axes a, b, and c of the ellipsoid, respectively.

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