A method for characterizing insulation safety margin of transformer oil-paper system with thermal defects

Abstract

The present invention relates to a method for characterizing the insulation safety margin of a transformer oil-paper system when there is a thermal defect, and belongs to the technical field of transformer monitoring. The method comprises the following steps: a first step: establishing a calculation model for the power frequency withstand capability index of the transformer oil-paper insulation system; a second step: obtaining the power frequency breakdown voltage values ​​of the insulation paper and the insulation oil at different temperatures; a third step: calculating the temperature distribution and electric field distribution of the transformer insulation structure under different degrees of thermal defect overheating; and a fourth step: calculating the power frequency withstand capability index value of the transformer oil-paper insulation system when there is a thermal defect. The present invention provides a calculation model for characterizing the power frequency withstand capability index of the transformer oil-paper system, which comprehensively considers the power frequency breakdown voltage variation law of the oil gap and the oil-immersed paperboard at different temperatures and the electric field and temperature distribution of the transformer under different degrees of thermal defect overheating, quantitatively analyzes the impact of thermal defects on the insulation of the oil-paper material, and provides a safety criterion for future active protection ultra-high voltage converter transformers.

Description

Technical Field

[0001] The invention belongs to the technical field of transformer monitoring and relates to a method for characterizing the insulation safety margin when a transformer oil-paper system has thermal defects. Background Art

[0002] Winding thermal defects refer to abnormal overheating spots in transformer windings, which can be caused by a variety of factors. Once a thermal defect occurs, it will cause the insulation material to degrade, further reducing the dielectric strength of the oil-paper insulation, threatening the safe operation of the equipment. In recent years, many scholars have conducted extensive research on quantitative estimation methods for the severity of transformer thermal defects. Xu Minglu et al. from State Grid Beijing Electric Power Company, based on infrared thermal imager temperature data, proposed an intelligent diagnosis method for thermal defects in transformer electrical equipment based on radial basis function neural networks, improving the accuracy of thermal defect identification. Han Zhongjie et al. from State Grid Zhejiang Electric Power Company Jiaxing Power Supply Company, by implementing the hardware and software execution environment for the diagnostic system, can effectively solve the internal overheating problem of transformer equipment, thereby providing reasonable protection for power application resources. Zhang Lei et al. from Guangxi Electric Power Research Institute inverted the internal hot spot temperature by real-time measurement of the outer casing temperature, thereby continuously sensing the thermal defects of the transformer's internal windings and providing reliable data reference for grid dispatch. Wang Jian and others from Chongqing University proposed a multi-parameter fusion method for power transformer defect identification, status classification and active safety protection by integrating multiple parameters such as the operating environment conditions, operating parameters, and thermal defect status parameters of power transformers. This method can effectively improve the effect of power transformer defect identification and enhance the objectivity and comprehensiveness of power transformer safety assessment.

[0003] Under new power system models, thermal defects significantly impact the insulation of transformer oil-paper systems, leading to thermal aging of the insulation material and, in turn, a reduction in its dielectric strength and insulation performance. However, much research has focused on the physical and chemical changes in oil-paper insulation materials caused by thermal aging, while in-depth analysis of changes in their electrical properties is relatively limited. Consequently, a method for characterizing the insulation safety margin of transformer oil-paper systems exposed to thermal defects is lacking. Summary of the Invention

[0004] In view of this, an object of the present invention is to provide a method for characterizing the insulation safety margin of a transformer oil-paper system when it is thermally defective.

[0005] In order to achieve the above object, the present invention provides the following technical solutions:

[0006] A method for characterizing insulation safety margin of a transformer oil-paper system when it is thermally defective, the method comprising the following steps:

[0007] The first step is to establish a calculation model for the power frequency withstand capability index of the transformer oil-paper insulation system; a calculation model for the power frequency withstand capability index of the oil-paper system is proposed. The input parameters of the model are the electric field strength E(x, y, z) at a certain point inside the transformer, the power frequency breakdown field strength E(x, y, z) of the oil-paper system at a certain temperature, and the power frequency withstand capability index of the transformer. B T , Maximum power frequency breakdown field strength E of oil-paper system B Max The output parameter is the power frequency withstand capability index BTI(x,y,z,T) of the oil-paper system.

[0008] Step 2: Obtain the power frequency breakdown voltage values ​​of the insulating paper and insulating oil at different temperatures; Based on the power frequency breakdown voltage test of the oil-paper system at different temperatures, obtain the change law of the power frequency breakdown voltage of the oil-paper system at different temperatures, and obtain the input parameter E B_Max ;

[0009] The third step is to calculate the temperature distribution and electric field distribution of the transformer insulation structure under different degrees of thermal defect overheating; taking the grid-side winding of the converter transformer as the object, based on the different degrees of thermal defect severity, the transformer temperature distribution and electric field intensity distribution are simulated, and the input parameters E(x, y, z) and E are obtained by combining the variation law of the power frequency breakdown voltage of the oil-paper system. B_T ;

[0010] Step 4: Calculate the power frequency withstand capability index value of the transformer oil-paper insulation system when it is thermally defective; substitute the input parameter values ​​obtained in steps 2 and 3 into the power frequency withstand capability index calculation model of the transformer oil-paper insulation system to obtain the output parameter BTI (x, y, z, T).

[0011] Furthermore, the first step is specifically:

[0012] The calculation model of the power frequency withstand capability index of the transformer oil-paper system is shown in formula (1);

[0013]

[0014] Among them, BTI(x,y,z,T) is the power frequency withstand capability index of the oil-paper system, E(x,y,z) is the electric field strength of a point with coordinates (x,y,z) inside the transformer, and E B_T is the power frequency breakdown field strength of insulating oil and oil-immersed insulating paperboard at this temperature, E B_Max is the maximum power frequency breakdown field strength of insulating oil and oil-immersed insulating paperboard; when the electric field strength at a certain point of the winding reaches the breakdown field strength of the insulating material, that is, E(x,y,z) and E B_T When the electric field strength E(x, y, z) at a certain point in the winding is 0, the BTI value of the point is close to 1, which is in the optimal safety state.

[0015] Furthermore, the second step is specifically as follows:

[0016] The power frequency withstand capability index of the oil-paper system under power frequency was analyzed. The power frequency breakdown voltage of the oil gap at different temperatures was tested according to GB / T 507-2002. The power frequency breakdown voltage of the insulating paper at different temperatures was tested according to GB / T 1408.1-2016. The variation pattern of the power frequency breakdown voltage of the oil-paper system at different temperatures was obtained, and then the maximum power frequency breakdown field strength E of the insulating oil and oil-immersed insulating paperboard was obtained. B_ Max.

[0017] Furthermore, the third step is specifically as follows:

[0018] (1) Calculation of temperature distribution of transformer oil-paper insulation under different degrees of thermal defect overheating

[0019] Under the condition of constant inlet oil flow rate, the temperature distribution inside the transformer under different degrees of thermal defect overheating is simulated based on the Navier-Stokes equation (2). On this basis, according to the variation law of the power frequency breakdown voltage of the oil-paper system, the power frequency breakdown field strength E of the insulating oil and oil-immersed insulating paperboard under different degrees of thermal defect overheating is obtained. B_T ;

[0020]

[0021] Among them, the horizontal axis is the r axis, the vertical axis is the z axis, and the u r and u z are the horizontal and vertical velocities of the fluid, respectively, in m / s, and ρ is the density, in kg / m 3 , F r and F z are the horizontal and vertical mass forces, in N; μ is the dynamic viscosity of the fluid, in Pa·s; c is the heat capacity, in J / (kg·K);

[0022] (2) Calculation of electric field distribution of transformer oil-paper insulation system considering the influence of temperature distribution

[0023] The rated voltage is applied to the transformer winding. Based on Maxwell's equations (3) and the temperature distribution results of the transformer under different thermal defect overheating degrees, the power frequency breakdown field strength E(x, y, z) of the insulating oil and oil-immersed insulating cardboard of the UHV converter transformer at the rated voltage is simulated.

[0024]

[0025] Where E is the electric field intensity, unit is V / m; B is the magnetic induction intensity, unit is T; H is the magnetic field intensity, unit is A / m; D is the electric displacement vector, unit is C / m 2 ; J is the current density, unit is A / m 2; ρ is the charge density, unit is C / m 3 .

[0026] Furthermore, the fourth step is specifically as follows:

[0027] According to the variation law of power frequency breakdown voltage of oil-paper system at different temperatures, the maximum power frequency breakdown field strength E of insulating oil and oil-immersed insulating paperboard is obtained. B_Max Combined with the variation law of power frequency breakdown voltage of oil-paper system at different temperatures and the temperature distribution inside the transformer under different degrees of thermal defect overheating, the power frequency breakdown field strength E of insulating oil and oil-immersed insulating paperboard under different degrees of thermal defect overheating is obtained. B_T Combined with the temperature distribution results of the transformer under different thermal defect overheating degrees, the power frequency breakdown field strength E(x,y,z) of the insulating oil and oil-immersed insulating cardboard of the UHV converter transformer at rated voltage is simulated; E(x,y,z), E B_T 、E B_Max The calculation results are substituted into the power frequency withstand capability index calculation model to obtain the power frequency withstand capability index BTI(x,y,z,T); BTI(x,y,z,T) under different thermal defect levels provides a safety criterion for future active protection UHV converter transformers.

[0028] The beneficial effects of the present invention are as follows: the present invention provides a calculation model for the power frequency withstand capability index of the transformer oil-paper system. The model comprehensively considers the change law of the power frequency breakdown voltage of the oil gap and the oil-immersed paperboard at different temperatures and the electric field and temperature distribution of the transformer under different degrees of thermal defect overheating, quantitatively analyzes the impact of thermal defects on the insulation of the oil-paper material, and provides a safety criterion for future active protection ultra-high voltage converter transformers.

[0029] Other advantages, objects, and features of the present invention will be described in part in the following description and, in part, will be apparent to those skilled in the art upon examination of the following description or may be learned from practice of the present invention. The objects and other advantages of the present invention may be realized and obtained through the following description. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] In order to make the purpose, technical solutions and advantages of the present invention more clear, the present invention will be described in detail below with reference to the accompanying drawings, in which:

[0031] Figure 1 Flowchart of the present invention;

[0032] Figure 2 The breakdown voltage of insulating oil and insulating paper changes with temperature; (a) is insulating oil, (b) is insulating paper;

[0033] Figure 3Figure 2 shows the winding temperature distribution when the grid-side winding has different degrees of thermal defects; (a) is a normal winding, (b) is overheated at 100°C, (c) is overheated at 150°C, (d) is overheated at 225°C, (e) is overheated at 300°C, (f) is overheated at 500°C, and (g) is overheated at 700°C.

[0034] Figure 4 The electric field intensity distribution of the UHV converter transformer winding at rated voltage; (a) is the overall electric field intensity distribution of the grid-side winding and valve-side winding, (b) is the electric field intensity distribution on the outer diameter side of the upper end of the grid-side winding, and (c) is the electric field intensity distribution on the inner diameter side of the upper end of the valve-side winding;

[0035] Figure 5 Figure 2 shows the BTI distribution of the grid-side winding under different degrees of thermal defect overheating; (a) is the normal winding, (b) is overheated at 100°C, (c) is overheated at 150°C, (d) is overheated at 225°C, (e) is overheated at 300°C, (f) is overheated at 500°C, and (g) is overheated at 700°C.

[0036] Figure 6 The distribution of the minimum power frequency withstand capability index of the grid-side winding under different overheating degrees. DETAILED DESCRIPTION

[0037] The following describes the embodiments of the present invention by means of specific examples, and those skilled in the art can easily understand other advantages and effects of the present invention from the contents disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and the details in this specification can also be modified or changed in various ways based on different viewpoints and applications without departing from the spirit of the present invention. It should be noted that the illustrations provided in the following embodiments are only schematic illustrations of the basic concept of the present invention, and the following embodiments and features in the embodiments can be combined with each other without conflict.

[0038] Among them, the accompanying drawings are only for illustrative purposes and represent only schematic diagrams rather than actual pictures, and should not be understood as limiting the present invention. In order to better illustrate the embodiments of the present invention, some parts of the accompanying drawings may be omitted, enlarged or reduced, and do not represent the dimensions of actual products. For those skilled in the art, it is understandable that some well-known structures and their descriptions may be omitted in the accompanying drawings.

[0039] The same or similar numbers in the drawings of the embodiments of the present invention correspond to the same or similar parts; in the description of the present invention, it should be understood that if there are terms such as "upper", "lower", "left", "right", "front", "back", etc. indicating directions or positional relationships, they are based on the directions or positional relationships shown in the drawings. They are only for the convenience of describing the present invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific direction, be constructed and operate in a specific direction. Therefore, the terms describing the positional relationship in the drawings are only used for illustrative purposes and cannot be understood as limiting the present invention. For ordinary technicians in this field, the specific meanings of the above terms can be understood according to specific circumstances.

[0040] like Figure 1 As shown, the present invention provides a method for characterizing the insulation safety margin of a transformer oil-paper system when it is thermally defective, which mainly includes the following four steps:

[0041] The first step is to establish a calculation model for the power frequency withstand capability index of the transformer oil-paper insulation system. The calculation model for the power frequency withstand capability index of the oil-paper system is proposed. The input parameters of the model are the electric field strength E(x, y, z) at a certain point inside the transformer and the power frequency breakdown field strength E(x, y, z) of the oil-paper system at a certain temperature. B_T , Maximum power frequency breakdown field strength E of oil-paper system B_Max The output parameter is the power frequency withstand capability index BTI (x, y, z, T) of the oil-paper system.

[0042] Step 2: Obtain the power frequency breakdown voltage values ​​of insulating paper and insulating oil at different temperatures. According to the power frequency breakdown voltage test of the oil-paper system at different temperatures, the change law of the power frequency breakdown voltage of the oil-paper system at different temperatures is obtained, and the input parameter E is obtained. B_Max .

[0043] The third step is to calculate the temperature distribution and electric field distribution of the transformer insulation structure under different degrees of thermal defect overheating. Taking the grid-side winding of the converter transformer as the object, based on the different degrees of thermal defect severity, the transformer temperature distribution and electric field intensity distribution are simulated. Combined with the variation law of the power frequency breakdown voltage of the oil-paper system, the input parameters E(x,y,z) and E B_T .

[0044] Step 4: Calculate the power frequency withstand capability index value when the transformer oil-paper insulation system is thermally defective. Substitute the input parameter values ​​obtained in steps 2 and 3 into the transformer oil-paper insulation system power frequency withstand capability index calculation model to obtain the output parameter BTI(x, y, z, T).

[0045] 1. Establish a calculation model for the power frequency withstand capability index of the transformer oil-paper insulation system

[0046] The present invention proposes a calculation model for the power frequency withstand capability index of the transformer oil-paper system, as shown in formula (1). Wherein, BTI(x, y, z, T) is the power frequency withstand capability index of the oil-paper system, E(x, y, z) is the electric field strength of a certain point with coordinates (x, y, z) inside the transformer, and E B_T is the power frequency breakdown field strength of insulating oil and oil-immersed insulating paperboard at this temperature, E B_Max is the maximum power frequency breakdown field strength of insulating oil and oil-immersed insulating paperboard. As can be seen from the formula, when the electric field strength at a certain point of the winding reaches the breakdown field strength of the insulating material, that is, E(x,y,z) and E B_T When the electric field strength E(x, y, z) at a certain point in the winding is 0, the BTI value of the point is close to 1, which is in the optimal safety state.

[0047]

[0048] 2. Obtain the power frequency breakdown voltage values ​​of insulating paper and insulating oil at different temperatures

[0049] The present invention analyzes the power frequency withstand capability index of the oil-paper system under power frequency, tests the power frequency breakdown voltage of the oil gap at different temperatures according to GB / T 507-2002, and tests the power frequency breakdown voltage of the insulating paper at different temperatures according to GB / T 1408.1-2016, obtains the variation law of the power frequency breakdown voltage of the oil-paper system at different temperatures, and further obtains the maximum power frequency breakdown field strength E of the insulating oil and oil-impregnated insulating paperboard. B_Max .

[0050] 3. Calculate the temperature distribution and electric field distribution of the transformer insulation structure under different degrees of thermal defect overheating

[0051] (1) Calculation of temperature distribution of transformer oil-paper insulation under different degrees of thermal defect overheating

[0052] Under the condition of constant inlet oil flow rate, the temperature distribution inside the transformer under different degrees of thermal defect overheating is simulated based on the Navier-Stokes equation (2). On this basis, according to the variation law of the power frequency breakdown voltage of the oil-paper system, the power frequency breakdown field strength E of the insulating oil and oil-immersed insulating paperboard under different degrees of thermal defect overheating is obtained. B_T .

[0053]

[0054] Among them, the horizontal axis is the r axis, the vertical axis is the z axis, and the u r and u z are the horizontal and vertical velocities of the fluid, respectively, in m / s, and ρ is the density, in kg / m 3 , F r and F zare the horizontal and vertical mass forces, in N; μ is the dynamic viscosity of the fluid, in Pa·s; c is the heat capacity, in J / (kg·K).

[0055] (2) Calculation of electric field distribution of transformer oil-paper insulation system considering the influence of temperature distribution

[0056] Rated voltage is applied to the transformer winding. Based on Maxwell's equations (3) and combined with the temperature distribution results of the transformer under different thermal defect overheating degrees, the power frequency breakdown field strength E(x, y, z) of the insulating oil and oil-immersed insulating paperboard of the UHV converter transformer at rated voltage is simulated.

[0057]

[0058] Where E is the electric field intensity, unit is V / m; B is the magnetic induction intensity, unit is T; H is the magnetic field intensity, unit is A / m; D is the electric displacement vector, unit is C / m 2 ; J is the current density, unit is A / m 2 ; ρ is the charge density, unit is C / m 3 .

[0059] 4. Calculation of the power frequency withstand capability index value when the transformer oil-paper insulation system is thermally defective

[0060] According to the variation law of power frequency breakdown voltage of oil-paper system at different temperatures, the maximum power frequency breakdown field strength E of insulating oil and oil-immersed insulating paperboard is obtained. B_Max On this basis, combined with the variation law of the power frequency breakdown voltage of the oil-paper system at different temperatures and the temperature distribution inside the transformer under different degrees of thermal defect overheating, the power frequency breakdown field strength E of the insulating oil and oil-immersed insulating paperboard under different degrees of thermal defect overheating is obtained. B_T Combined with the temperature distribution results of the transformer under different degrees of thermal defect overheating, the power frequency breakdown field strength E(x,y,z) of the insulating oil and oil-immersed insulating cardboard of the UHV converter transformer at rated voltage is simulated. B_T 、E B_Max Substituting the calculated results into the power frequency withstand capability index calculation model, we obtain the power frequency withstand capability index (BTI(x,y,z,T)). BTI(x,y,z,T) at different thermal defect levels can provide safety criteria for future active protection UHV converter transformers.

[0061] This paper takes a UHV converter transformer model ZZDFPZ-405000 / 500-400 as the research object, and adopts forced oriented oil circulation air cooling (ODAF) as the cooling method. First, the power frequency breakdown voltage of a 2.5mm oil gap at different temperatures is tested according to GB / T 507-2002, and the power frequency breakdown voltage of insulating paper at different temperatures is tested according to GB / T 1408.1-2016. The variation pattern of the power frequency breakdown voltage of the oil-paper system at different temperatures is obtained, and then the maximum power frequency breakdown field strength E of the insulating oil and oil-impregnated insulating paperboard is obtained. B_Max ,like Figure 2 As shown, Figure 2 The breakdown voltage of insulating oil and insulating paper changes with temperature; (a) is insulating oil, (b) is insulating paper.

[0062] Secondly, based on the simulation of different thermal defect overheating degrees of the transformer insulation winding, the winding temperature distribution under six different thermal defect overheating degrees, such as 100℃, 150℃, 225℃, 300℃, 500℃, and 700℃, is obtained. Figure 3 shown. Figure 3 Figure 2 shows the winding temperature distribution when the grid-side winding has different degrees of thermal defects; (a) is a normal winding, (b) is overheated by 100°C, (c) is overheated by 150°C, (d) is overheated by 225°C, (e) is overheated by 300°C, (f) is overheated by 500°C, and (g) is overheated by 700°C.

[0063] Then, the grid-side winding and valve-side winding are set to rated voltages of 306 kV and 99.2 kV respectively. Based on the simulation of Maxwell's equations and the winding temperature distribution when the grid-side winding has different degrees of thermal defects, the winding electric field intensity distribution of the UHV converter transformer at rated voltage is obtained as follows: Figure 4 As shown, Figure 4 The electric field intensity distribution of the UHV converter transformer winding at rated voltage; (a) is the overall electric field intensity distribution of the grid-side winding and the valve-side winding, (b) is the electric field intensity distribution on the outer diameter side of the upper end of the grid-side winding, and (c) is the electric field intensity distribution on the inner diameter side of the upper end of the valve-side winding.

[0064] Finally, based on the temperature distribution and electric field intensity distribution results of the UHV converter transformer with different degrees of thermal defects ( Figure 3 、 Figure 4 ), and combined with the variation of power frequency breakdown voltage of oil-paper system at different temperatures ( Figure 2 ), calculate the power frequency withstand capability index of the transformer oil-paper system (Formula (1)) and obtain the BTI distribution of the grid-side winding under different thermal defect overheating degrees as follows Figure 5 , Figure 5The BTI distribution of the grid-side winding under different degrees of thermal defect overheating; (a) is a normal winding, (b) is overheated at 100°C, (c) is overheated at 150°C, (d) is overheated at 225°C, (e) is overheated at 300°C, (f) is overheated at 500°C, and (g) is overheated at 700°C. The extracted input parameters of the minimum BTI with the degree of overheating are shown in Table 1. The minimum BTI changes with the degree of overheating as shown in Figure 6 shown.

[0065] Table 1

[0066]

[0067] Based on actual transformer operating experience, mild and low-temperature overheating are common types of winding overheating. Moderate-temperature overheating often requires prompt repair, while high-temperature overheating rarely occurs in windings. The trend in BTI values ​​as a function of thermal overheating temperature is consistent with this operating experience. High-temperature overheating results in a BTI of zero, at which point the winding will experience discharge. Moderate-temperature overheating results in an extremely low BTI, posing a high risk of electrical failure and requiring prompt repair. Low-temperature overheating, on the other hand, is sensitive to changes in overheating temperature, raising the potential risk of thermal defects developing into electrical failures. Therefore, timely preventive measures are crucial at this stage. The variation in BTI values ​​at varying degrees of thermal defect severity can provide safety criteria for future UHV converter transformers with active protection.

[0068] Based on actual transformer operating experience, mild and low-temperature overheating are common types of winding overheating. Moderate-temperature overheating often requires prompt repair, while high-temperature overheating rarely occurs in windings. The trend in BTI values ​​as a function of thermal overheating temperature is consistent with this operating experience. High-temperature overheating results in a BTI of zero, at which point the winding will experience discharge. Moderate-temperature overheating results in an extremely low BTI, posing a high risk of electrical failure and requiring prompt repair. Low-temperature overheating, on the other hand, is sensitive to changes in overheating temperature, raising the potential risk of thermal defects developing into electrical failures. Therefore, timely preventive measures are crucial at this stage. The variation in BTI values ​​at varying degrees of thermal defect severity can provide safety criteria for future UHV converter transformers with active protection.

[0069] Winding thermal defects refer to abnormal overheating points in transformer windings. Among them, problems such as inter-turn short circuits, poor wire contact, cold solder joints, oil line blockages, and foreign matter in the oil lines can all cause thermal defects of varying degrees at different locations on the windings. Once a thermal defect occurs, it will cause the insulation material to deteriorate and further reduce the dielectric strength of the oil-paper insulation. When the insulation material deteriorates to a certain extent, it may also trigger discharges, threatening the safe operation of the equipment. Currently, there is a lack of research on the effects of thermal defects on the burning of UHV converter windings, resulting in irreparable damage to the equipment. The present invention proposes a method for characterizing the insulation safety margin of the transformer oil-paper system when thermal defects occur. When the winding thermal defect is at low temperature overheating, it is sensitive to changes in the overheat temperature, and the potential risk of the thermal defect developing into an electrical fault is high. When the winding is at medium temperature overheating, the BTI is extremely low, and the risk of an electrical fault occurring is high, and timely maintenance should be carried out. The changes in BTI values ​​under different degrees of thermal defects provide a safety criterion for future active protection UHV converter transformers.

[0070] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not limiting. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solutions of the present invention can be modified or replaced by equivalents without departing from the purpose and scope of the technical solutions, which should all be included in the scope of the claims of the present invention.

Claims

1. A method for characterizing the insulation safety margin of a transformer oil-paper system when it is thermally defective, characterized in that: The method comprises the following steps: The first step is to establish a calculation model for the power frequency withstand capability index of the transformer oil-paper insulation system; a calculation model for the power frequency withstand capability index of the oil-paper system is proposed. The input parameters of the model are the electric field strength E(x, y, z) at a certain point inside the transformer, the power frequency breakdown field strength E(x, y, z) of the oil-paper system at a certain temperature, and the power frequency withstand capability index of the transformer. B_T , Maximum power frequency breakdown field strength E of oil-paper system B_Max The output parameter is the power frequency withstand capability index BTI(x,y,z,T) of the oil-paper system. Step 2: Obtain the power frequency breakdown voltage values ​​of the insulating paper and insulating oil at different temperatures; Based on the power frequency breakdown voltage test of the oil-paper system at different temperatures, obtain the change law of the power frequency breakdown voltage of the oil-paper system at different temperatures, and obtain the input parameter E B_Max ; The third step is to calculate the temperature distribution and electric field distribution of the transformer insulation structure under different degrees of thermal defect overheating; taking the grid-side winding of the converter transformer as the object, based on the different degrees of thermal defect severity, the transformer temperature distribution and electric field intensity distribution are simulated, and the input parameters E(x, y, z) and E are obtained by combining the variation law of the power frequency breakdown voltage of the oil-paper system. B_T ; Step 4: Calculate the power frequency withstand capability index value of the transformer oil-paper insulation system when it is thermally defective; substitute the input parameter values ​​obtained in steps 2 and 3 into the power frequency withstand capability index calculation model of the transformer oil-paper insulation system to obtain the output parameter BTI (x, y, z, T).

2. The method for characterizing insulation safety margin of a transformer oil-paper system when it is thermally defective according to claim 1, characterized in that: The first step is specifically: The calculation model of the power frequency withstand capability index of the transformer oil-paper system is shown in formula (1); Among them, BTI(x,y,z,T) is the power frequency withstand capability index of the oil-paper system, E(x,y,z) is the electric field strength of a point with coordinates (x,y,z) inside the transformer, and E B_T is the power frequency breakdown field strength of insulating oil and oil-immersed insulating cardboard at a certain temperature, E B_Max is the maximum power frequency breakdown field strength of insulating oil and oil-immersed insulating paperboard; when the electric field strength at a certain point of the winding reaches the breakdown field strength of the insulating material, that is, E(x,y,z) and E B_T When the electric field strength E(x, y, z) at a certain point in the winding is 0, the BTI value of the point is close to 1, which is in the optimal safety state.

3. The method for characterizing insulation safety margin of a transformer oil-paper system when it is thermally defective according to claim 2, characterized in that: The second step is specifically as follows: The power frequency withstand capability index of the oil-paper system under power frequency was analyzed. The power frequency breakdown voltage of the oil gap at different temperatures was tested according to GB / T 507-2002. The power frequency breakdown voltage of the insulating paper at different temperatures was tested according to GB / T 1408.1-2016. The variation pattern of the power frequency breakdown voltage of the oil-paper system at different temperatures was obtained, and then the maximum power frequency breakdown field strength E of the insulating oil and oil-immersed insulating paperboard was obtained. B_ Max.

4. The method for characterizing insulation safety margin of a transformer oil-paper system when it is thermally defective according to claim 3, characterized in that: The third step is specifically as follows: (1) Calculation of temperature distribution of transformer oil-paper insulation under different degrees of thermal defect overheating Under the condition of constant inlet oil flow rate, the temperature distribution inside the transformer under different degrees of thermal defect overheating is simulated based on the Navier-Stokes equation (2). On this basis, according to the variation law of the power frequency breakdown voltage of the oil-paper system, the power frequency breakdown field strength E of the insulating oil and oil-immersed insulating paperboard under different degrees of thermal defect overheating is obtained. B_T ; Among them, the horizontal axis is the r axis, the vertical axis is the z axis, and the u r and u z are the horizontal and vertical velocities of the fluid, respectively, in m / s, and ρ is the density, in kg / m 3 , F r and F z are the horizontal and vertical mass forces, in N; μ is the dynamic viscosity of the fluid, in Pa·s; c is the heat capacity, in J / (kg·K); (2) Calculation of electric field distribution of transformer oil-paper insulation system considering the influence of temperature distribution The rated voltage is applied to the transformer winding. Based on Maxwell's equations (3) and the temperature distribution results of the transformer under different thermal defect overheating degrees, the power frequency breakdown field strength E(x, y, z) of the insulating oil and oil-immersed insulating cardboard of the UHV converter transformer at the rated voltage is simulated. Where E is the electric field intensity, unit is V / m; B is the magnetic induction intensity, unit is T; H is the magnetic field intensity, unit is A / m; D is the electric displacement vector, unit is C / m 2 ; J is the current density, unit is A / m 2 ; ρ is the charge density, unit is C / m 3 .

5. The method for characterizing insulation safety margin of a transformer oil-paper system when it is thermally defective according to claim 1, characterized in that: The fourth step is specifically as follows: According to the variation law of power frequency breakdown voltage of oil-paper system at different temperatures, the maximum power frequency breakdown field strength E of insulating oil and oil-immersed insulating paperboard is obtained. B_Max Combined with the variation law of power frequency breakdown voltage of oil-paper system at different temperatures and the temperature distribution inside the transformer under different degrees of thermal defect overheating, the power frequency breakdown field strength E of insulating oil and oil-immersed insulating paperboard under different degrees of thermal defect overheating is obtained. B_T Combined with the temperature distribution results of the transformer under different thermal defect overheating degrees, the power frequency breakdown field strength E(x,y,z) of the insulating oil and oil-immersed insulating cardboard of the UHV converter transformer at rated voltage is simulated; E(x,y,z), E B_T 、E B_Max Substitute the calculated results into the power frequency withstand capability index calculation model to obtain the power frequency withstand capability index BTI(x,y,z,T); BTI(x,y,z,T) under different degrees of thermal defects provides a safety criterion for future active protection UHV converter transformers.

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