A method, system, device and medium for calculating the radial position of a casing plate

Abstract

The present invention belongs to the technical field of insulation structure design for power equipment and discloses a method, system, device, and medium for calculating the radial position of a casing plate. A preset casing core model is established and initial conditions for the preset casing core model are set. The preset casing core model includes an epoxy core model, which includes a zero-layer plate model and a last-layer plate model. Within the epoxy core model, each layer of plate model is sequentially loaded from the zero-layer plate model toward the last-layer plate model, and the position of each layer of plate model is optimized based on the partial discharge safety margin at the upper edge and the partial discharge safety margin at the lower edge of the plate model. When the radial position of the loaded plate model exceeds the radial position of the last-layer plate model, the loaded plate model is unloaded, completing the calculation of the casing plate radial position. The present invention considers the partial discharge safety margin at the plate edge and can calculate the optimal casing plate spacing arrangement while ensuring calculation accuracy and efficiency.

Description

Technical Field

[0001] The present invention belongs to the technical field of insulation structure design of electric power equipment, and in particular relates to a method, system, equipment and medium for calculating the radial position of a bushing plate. Background Art

[0002] In high-voltage electrical equipment (such as bushings), the plate spacing directly impacts insulation strength. Properly designed plate spacing ensures proper operation of the equipment under high voltage conditions while preventing accidents such as discharge and breakdown. Plate spacing design is closely related to the insulation performance of high-voltage electrical equipment. Therefore, determining a reasonable plate spacing can improve the safety of high-voltage electrical equipment.

[0003] Regarding the design of the plate spacing within the epoxy core of a bushing, current research is based on an empirical formula for the partial discharge safety margin of the field strength within the epoxy core. This ensures that the partial discharge safety margin of all plates is consistent, thereby maximizing the partial discharge safety margin within the bushing core while maintaining the same number of plate layers. Alternatively, an empirical formula is used to balance the partial discharge safety margin with the plate spacing. However, because the design of plate spacing is influenced by multiple factors, traditional empirical formulas have limitations in complex electrical operating environments, potentially resulting in inaccurate design results and insufficient adaptability to actual operations. Summary of the Invention

[0004] In order to solve the problems existing in the prior art, the purpose of the present invention is to provide a method, system, device and medium for calculating the radial position of the casing plate. The present invention takes into account the partial discharge safety margin of the plate edge and can calculate the optimal casing plate spacing arrangement scheme while ensuring calculation accuracy and efficiency.

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

[0006] A method for calculating the radial position of a casing plate, comprising:

[0007] Establishing a preset casing core model and setting initial conditions of the preset casing core model, wherein the preset casing core model includes an epoxy core model, and the epoxy core model includes a zero-layer plate model and a last-layer plate model;

[0008] In the epoxy core model, the plate models of each layer are loaded in sequence along the direction from the zero-layer plate model to the last-layer plate model, and the position of each layer of plate model is optimized according to the partial discharge safety margin of the upper edge and the partial discharge safety margin of the lower edge of the plate model; wherein, after each layer of plate model is loaded, the position of the plate model of this layer is optimized, and after the position optimization of the plate model of this layer is completed, the next layer of plate model is loaded and the position optimization is performed, until the radial position of the loaded plate model exceeds the radial position of the last-layer plate model, the loading of the plate model is canceled, and the radial position calculation of the casing plate is completed.

[0009] Preferably, in the preset casing core model, the outer envelopes of the ends of all the plate models are predetermined curves.

[0010] Preferably, the process of sequentially loading the plate models of each layer includes:

[0011] Loading the current layer plate model within a preset radial distance from the previous layer plate model, and then optimizing the position of the current layer plate model;

[0012] The preset range is an empirical range of radial distances between adjacent plate models.

[0013] Preferably, the process of optimizing the position of the current layer plate model includes:

[0014] Load the plate model of the current layer sequentially with a preset step length within a preset range of radial distance from the plate model of the previous layer;

[0015] After loading the current layer plate model at each position loading point, the partial discharge safety margin of the upper edge and the partial discharge safety margin of the lower edge of the current layer plate model are calculated, and then the minimum value of the partial discharge safety margin of the upper edge and the partial discharge safety margin of the lower edge of the current layer plate model is obtained;

[0016] The maximum value among the minimum values ​​of the current layer plate model at all position loading points is obtained, and the position loading point of the current layer plate model corresponding to the maximum value is used as the final position loading point of the current layer plate model.

[0017] Preferably, the empirical range of the radial distance between adjacent plate models is 0.5-3 mm, and the preset step size is 0.05 mm.

[0018] Preferably, the initial conditions of the preset casing core model include the outer envelope of the ends of all the plate models, the position of the zero-layer plate, the position of the last-layer plate, and the operating conditions.

[0019] Preferably, the working condition includes the temperature field of the epoxy core model in the preset sleeve core model.

[0020] The present invention also provides a casing plate radial position calculation system for implementing the above casing plate radial position calculation method, comprising:

[0021] Modeling module: used to establish a preset casing core model and set the initial conditions of the preset casing core model, wherein the preset casing core model includes an epoxy core model, and the epoxy core model includes a zero-layer plate model and a last-layer plate model;

[0022] Optimization module: used to load each layer of plate model in the epoxy core model in sequence along the direction from the zero layer plate model to the last layer plate model, and optimize the position of each layer of plate model according to the partial discharge safety margin of the upper edge and the partial discharge safety margin of the lower edge of the plate model; wherein, after each layer of plate model is loaded, the position of the plate model of this layer is optimized, and after the position optimization of the plate model of this layer is completed, the next layer of plate model is loaded and the position optimization is performed, until the radial position of the loaded plate model exceeds the radial position of the last layer of plate model, the loading of the plate model is canceled, and the radial position calculation of the casing plate is completed.

[0023] The present invention also provides an electronic device, comprising:

[0024] one or more processors;

[0025] a storage device having one or more programs stored thereon;

[0026] When the one or more programs are executed by the one or more processors, the one or more processors are enabled to implement the casing pad radial position calculation method as described above in the present invention.

[0027] The present invention further provides a storage medium storing a computer program, wherein the computer program, when executed by a processor, implements the method for calculating the radial position of a casing plate as described above.

[0028] The present invention has the following beneficial effects:

[0029] The traditional plate spacing designed based on empirical formulas has limitations in certain situations. The field strength at the edge of the plate may exceed the partial discharge safety margin, and the calculation efficiency is low. In the present invention, a dynamic programming method is used. After each layer of the plate model is loaded, the position of the plate model is optimized. By sequentially loading each layer of the plate model and performing position optimization, the radial position optimization of all the plate models in the preset casing core model is finally achieved. Therefore, the present invention can reasonably arrange the order of solving sub-problems and maximize the calculation efficiency by exchanging "space" for "time". Within a relatively low time cost, the method of the present invention can obtain all the optimal solutions for the number of plate model layers within a certain range, the zero-layer plate model, and the last-layer plate model. Designers can comprehensively consider the number of plate model layers and the maximum partial discharge safety margin to select the layout plan of the plate model. The present invention can significantly change the maximum field strength distribution at the edge of the plate model, and make the distribution more uniform at the stable moment. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 1 is a schematic diagram of the overall structure after the position of the plate model in the preset casing core model is optimized in an embodiment of the present invention;

[0031] Figure 2 Schematic diagram of the relationship between the number of plate model layers and the partial discharge safety margin calculated in this embodiment of the present invention;

[0032] Figure 3 Schematic diagram of the radial position of the plate model in this embodiment of the present invention;

[0033] Figure 4 Schematic diagram of the step length of the preset casing core model in this embodiment of the present invention;

[0034] Figure 5 Schematic diagram of the maximum electric field strength at the upper end of the plate model before and after position optimization of the plate model in an embodiment of the present invention;

[0035] Figure 6 Schematic diagram of the maximum electric field strength at the lower end of the plate model before and after position optimization of the plate model in an embodiment of the present invention.

[0036] In the figure, 1 is a preset casing core model, 101 is a preset casing core model gas side, 102 is a preset casing core model oil side, 2 is an epoxy core model, 3 is an upper end portion, and 4 is a lower end portion. DETAILED DESCRIPTION

[0037] In order to enable those skilled in the art to better understand the solutions of the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the accompanying drawings in the embodiments of the present invention.

[0038] The method for calculating the radial position of a casing plate of the present invention comprises the following steps:

[0039] Step 1: Establish a preset casing core model 1 and set the initial conditions of the preset casing core model 1. The preset casing core model 1 includes an epoxy core model 2, and the epoxy core model 2 includes a zero-layer plate model (which can be recorded as a zero-layer plate model). r 0) and the last plate model (which can be recorded as the last plate model r n ), where n is a positive integer; the initial conditions of the preset casing core model 1 include the outer envelope of the ends of all plate models (including the upper end 3 and the lower end 4 of the plate model), the position of the zero-layer plate, the position of the last-layer plate, and the operating conditions, and the operating conditions include the temperature field of the epoxy core model 2 in the preset casing core model 1. The technical solution of the present invention only focuses on the optimization of the positions of the plate models of each layer in the epoxy core model 2, and does not focus on the parts of the preset casing core model 1 other than the epoxy core model 2. The parts of the preset casing core model 1 other than the epoxy core model 2 can be in any form that complies with the prior art, and the present invention does not make specific limitations. The preset casing core model 1 described in the present invention is a casing core model established based on a certain model of casing core. The difference between the casing core model and the complete casing core model lies in the epoxy core model 2. In the epoxy core model 2 described in the present invention, only the zero-layer plate model is established in advance. r 0 and last plate model r n , for the zero-layer plate model r 0 and last plate model r n The plate models are not pre-established, but are instead calculated and determined layer by layer through the position optimization process described below. The outer envelope of the ends of all plate models in the epoxy core model 2 of the present invention is a predetermined curve. This outer envelope is consistent with the outer envelope of the ends of all plate models in the complete casing core model before the plate model position optimization. That is, when calculating the radial position of each plate model in this application, the shape of the outer envelope of the ends of all plates in the original complete casing core (i.e., the casing core of the specific model) is not changed.

[0040] Step 2: In the epoxy core model 2, follow the zero-layer plate model r 0-direction last plate model r ndirection, load each layer of plate model in turn, and optimize the position of each layer of plate model according to the partial discharge safety margin of the upper edge and the partial discharge safety margin of the lower edge of the plate model; wherein, after each layer of plate model is loaded, the position of the plate model of this layer is optimized, and after the position optimization of the plate model of this layer is completed, the next layer of plate model is loaded and the position optimization is performed until the radial position of the loaded plate model exceeds the radial position of the last plate model r n When the radial position of the casing plate is determined, the plate model is unloaded and the radial position calculation of the casing plate is completed.

[0041] The detailed process of step 2 is as follows:

[0042] Step 2.1, in the epoxy core model 2, along the zero layer plate model r 0-direction last plate model r n Direction, load plate model r 1. At this time, the zero-layer plate model r 0 is the plate model r 1's previous plate model, plate model r 1 is the zero-layer plate model r 0's next layer of plate model, plate model r 1 is called the current layer plate model;

[0043] Step 2.2, according to the plate model r 1. The upper edge partial discharge safety margin and the lower edge partial discharge safety margin for the plate model r 1. Perform location optimization, including:

[0044] Step 2.2.1, in the zero-layer plate model r The positions of the electrodes 0 are divided into a plurality of loading points with a preset step length (such as a step length of 0.05 mm) at a preset range (the preset range is the empirical range of the radial distance between adjacent electrode models, generally 0.5-3 mm);

[0045] Step 2.2.2, plate model r 1 Load at any loading point;

[0046] Step 2.2.3, calculate the plate model r 1. The upper edge partial discharge safety margin and the lower edge partial discharge safety margin, for the plate model r The upper edge partial discharge safety margin of 1 is compared with the lower edge partial discharge safety margin to obtain the plate model r The minimum value of the partial discharge safety margin at the upper edge and the partial discharge safety margin at the lower edge of 1;

[0047] Step 2.2.4: Repeat steps 2.2.2 to 2.2.3 to make the plate model r 1 Traverse all the position loading points to obtain the plate model r 1 is the minimum value when loading points at all positions;

[0048] Step 2.2.5, screening all the minimum values ​​obtained in step 2.2.4 to obtain the maximum value among these minimum values;

[0049] Step 2.2.6: Use the plate model corresponding to the maximum value determined in step 2.2.5 r The loading point at the location 1 is used as the plate model r 1 is the final position loading point (that is, the final position of the current layer plate model);

[0050] Step 2.3, repeat steps 2.1 to 2.2, for the plate model r 1The next layer of plate model r 2. Perform loading and position optimization;

[0051] Step 2.4, repeat step 2.3 until the radial position of the loaded plate model exceeds the radial position of the last plate model. r n When the radial position of the casing plate is determined, the plate model loading is canceled (i.e., the plate model to be loaded in step 2.4 is not loaded), and the calculation of the casing plate radial position is completed.

[0052] Example

[0053] In this embodiment, the preset bushing core model 1 is the bushing core model corresponding to the bushing on the valve side of the ±800kV converter transformer. After the preset bushing core model 1 is subjected to the bushing plate radial position calculation method of the present invention and the plate model is optimized, the final complete bushing core model is obtained as shown below: Figure 1 The preset casing core model 1 includes an epoxy core model 2, and the epoxy core model 2 includes a zero-layer plate model. r 0 (zero layer plate model r 0 is the innermost plate model in the radial direction of the epoxy core model 2, Figure 1 Because the plate model has many layers and cannot be accurately identified, it is not marked) and the last plate model r n (Last plate model r n is the outermost plate model in the radial direction of the epoxy core model 2, Figure 1 Since the plate model has many layers, it is impossible to accurately identify them, so no identification is performed. In this embodiment, when establishing the preset casing core model 1, only the zero-layer plate model is pre-established in the epoxy core model 2.r 0 and last plate model r n The remaining plate models are then established one by one during the subsequent position optimization process. During this process, the outer envelope of the ends of all plate models is ensured to be the defined curve (this curve is the outer envelope of the ends of all plates in the valve-side bushing of the ±800kV converter transformer).

[0054] In this embodiment, the operating conditions of the ±800kV converter transformer valve-side bushing consider two voltage types and four current types, for a total of 2×4 operating conditions. The voltage type considers two conditions: initial and stable conditions. At the initial time, there is no spatial charge distribution within the ±800kV converter transformer valve-side bushing. When the transient voltage on the ±800kV converter transformer valve-side bushing reaches its maximum, partial discharge is most likely to occur at the plate edges. At stable conditions, the electric field distribution of the ±800kV converter transformer valve-side bushing is a superposition of a DC steady-state field and an AC quasi-static field. At the initial time, the maximum transient voltage of the ±800kV converter transformer valve-side bushing is the peak voltage of the operating voltage waveform, 869.3kV. At stable conditions, the amplitudes of the DC and AC components are the peak values ​​of the corresponding components in the operating voltage waveform, which are 702.2kV and 167.1kV, respectively. The current form considers four operating conditions for the UHVDC transmission line: zero load, 60% load, 80% load, and full load, corresponding to four bushing core temperature distributions. Under each operating condition, key parameters such as the bushing core temperature distribution, the radial electric field distribution of the bushing core, and the electric field non-uniformity coefficient at the plate edges (including the upper edge corresponding to upper end 3 and the lower edge corresponding to lower end 4) of the valve-side bushing of the ±800kV converter transformer are calculated.

[0055] During the optimization of the plate model position, the dynamic programming method requires the use of all possible plate model positions and the partial discharge safety margin at the edge of the plate model under the plate model spacing (partial discharge safety margin = maximum field strength / initial partial discharge field strength). The maximum field strength is the maximum field strength at the edge of the plate model, and the initial partial discharge field strength is the initial partial discharge field strength at the edge of the plate model. The initial partial discharge field strength at the edge of the plate model can be directly determined based on the temperature distribution of the actual bushing core (i.e., the bushing core of the valve-side bushing of the ±800kV converter transformer) and the outer envelope of the ends of all actual plates (i.e., the outer envelope of the ends of all plates of the valve-side bushing of the ±800kV converter transformer), resulting in a low computational cost. There are many possible scenarios for the maximum field strength at the edge of the plate model. This embodiment considers eight operating conditions, and the possible scenarios for the plate model position and plate model spacing under each operating condition are 9.24×10 4 species, and the upper end 3 and the lower end 4 of the plate model are different, so the plate model position optimization process needs to involve 1.47×10 6To quickly and accurately obtain these partial discharge safety margins, the maximum field strength at the edge of the plate model is predicted based on the empirical formula for the electric field non-uniformity coefficient at the plate model edge and the radial field strength of the preset casing core model 1 (maximum field strength at the plate model edge = electric field non-uniformity coefficient at the plate model edge × radial field strength of the preset casing core model 1).

[0056] The relationship between the number of plate layers and the partial discharge safety margin calculated in this embodiment is as follows: Figure 2 As shown in Figure 2, as the number of plate models increases to 88 layers, the partial discharge safety margin no longer increases. Therefore, this embodiment finally chooses the solution of arranging 88 layers of plate models. The specific arrangement information of the plate models is calculated according to the position equivalent method of the plate models. The results are shown in Figure 2. Figure 3 and Figure 4 As shown, Figure 3 It is a schematic diagram of the radial position of each layer of plate model; Figure 4 It is a schematic diagram of step length, where the step refers to the difference in axial position of the ends of adjacent plate models, the upper step refers to the difference in axial position of the upper end 3 of adjacent plate models, and the lower step refers to the difference in axial position of the lower end 4 of adjacent plate models.

[0057] See also Figure 5 and Figure 6 After the position of the plate model is optimized in this embodiment, the maximum electric field strength distribution at the edge of the plate model changes significantly, becoming more uniform at the stable moment. The insulation performance of the valve-side bushing of the entire ±800kV converter transformer is significantly improved, and the partial discharge safety margin of the bushing core of the valve-side bushing of the ±800kV converter transformer is increased by 40%. In addition, the total number of plates required for the valve-side bushing of the ±800kV converter transformer is relatively small.

[0058] In addition, an embodiment of the present invention further provides a system for implementing the above-mentioned method for calculating the radial position of a casing plate of the present invention, the system comprising:

[0059] Modeling module: used to establish a preset casing core model 1 and set the initial conditions of the preset casing core model 1, the preset casing core model 1 includes an epoxy core model 2, the epoxy core model 2 includes a zero layer plate model r 0 and last plate model r n ;

[0060] Optimization module: used in epoxy core model 2, along the zero layer plate model r 0-direction last plate model r ndirection, load each layer of plate model in turn, and optimize the position of each layer of plate model according to the partial discharge safety margin of the upper edge and the partial discharge safety margin of the lower edge of the plate model; wherein, after each layer of plate model is loaded, the position of the plate model of this layer is optimized, and after the position optimization of the plate model of this layer is completed, the next layer of plate model is loaded and the position optimization is performed until the radial position of the loaded plate model exceeds the radial position of the last plate model r n When the radial position of the casing plate is calculated, the loading plate model is canceled, and the radial position calculation of the casing plate is completed.

[0061] The embodiments of the present invention also provide corresponding electronic devices and computer-readable storage media for implementing the solutions provided by the embodiments of the present invention.

[0062] The device includes a memory and a processor, the memory is used to store instructions or codes, and the processor is used to execute the instructions or codes, so that the device executes the casing plate radial position calculation method described in any embodiment of the present application.

[0063] The storage medium stores a computer program, wherein when the computer program is executed by the processor, the method for calculating the radial position of the casing plate according to any embodiment of the present application is implemented.

[0064] Obviously, the embodiments described are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts should fall within the scope of protection of the present invention.

[0065] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit it. Although the present invention has been described in detail with reference to the above embodiments, ordinary technicians in the field should understand that the specific implementation methods of the present invention can still be modified or replaced by equivalents. Any modification or equivalent replacement that does not depart from the spirit and scope of the present invention should be covered by the scope of protection of the claims of the present invention.

Claims

1. A method for calculating the radial position of a casing plate, characterized in that: include: Establishing a preset casing core model (1) and setting initial conditions of the preset casing core model (1), wherein the preset casing core model (1) includes an epoxy core model (2), and the epoxy core model (2) includes a zero-layer plate model and a last-layer plate model; In the epoxy core model (2), the plate models of each layer are loaded in sequence along the direction from the zero-layer plate model to the last-layer plate model, and the position of each plate model is optimized according to the partial discharge safety margin of the upper edge and the partial discharge safety margin of the lower edge of the plate model; wherein, after each layer of plate model is loaded, the position of the plate model of this layer is optimized, and after the position optimization of the plate model of this layer is completed, the next layer of plate model is loaded and the position optimization is performed, until the radial position of the loaded plate model exceeds the radial position of the last-layer plate model, the loaded plate model is canceled, and the radial position calculation of the casing plate is completed; The process of loading each layer of plate model in sequence includes: Loading the current layer plate model within a preset radial distance from the previous layer plate model, and then optimizing the position of the current layer plate model; The preset range is the empirical range of radial distances between adjacent plate models; The process of optimizing the position of the current layer plate model includes: Load the plate model of the current layer sequentially with a preset step length within a preset range of radial distance from the plate model of the previous layer; After loading the current layer plate model at each position loading point, the partial discharge safety margin of the upper edge and the partial discharge safety margin of the lower edge of the current layer plate model are calculated, and then the minimum value of the partial discharge safety margin of the upper edge and the partial discharge safety margin of the lower edge of the current layer plate model is obtained; The maximum value among the minimum values ​​of the current layer plate model at all position loading points is obtained, and the position loading point of the current layer plate model corresponding to the maximum value is used as the final position loading point of the current layer plate model.

2. The method for calculating the radial position of a casing plate according to claim 1, characterized in that: In the preset casing core model (1), the outer envelopes of the ends of all the plate models are determined curves.

3. The method for calculating the radial position of a casing plate according to claim 1, characterized in that: The empirical range of the radial distance between adjacent plate models is 0.5-3 mm, and the preset step size is 0.05 mm.

4. The method for calculating the radial position of a casing plate according to claim 1, characterized in that: The initial conditions of the preset casing core model (1) include the outer envelope of the ends of all plate models, the position of the zero-layer plate, the position of the last-layer plate, and the operating conditions.

5. The method for calculating the radial position of a casing plate according to claim 4, characterized in that: The working conditions include the temperature field of the epoxy core model (2) in the preset casing core model (1).

6. A casing plate radial position calculation system, characterized in that: include: Modeling module: used for establishing a preset casing core model (1) and setting initial conditions of the preset casing core model (1), wherein the preset casing core model (1) includes an epoxy core model (2), and the epoxy core model (2) includes a zero-layer plate model and a last-layer plate model; Optimization module: used for sequentially loading each layer of plate model in the epoxy core model (2) along the direction from the zero layer plate model to the last layer plate model, and optimizing the position of each layer of plate model according to the partial discharge safety margin of the upper edge and the partial discharge safety margin of the lower edge of the plate model; wherein, after each layer of plate model is loaded, the position of the plate model of this layer is optimized, and after the position optimization of the plate model of this layer is completed, the next layer of plate model is loaded and optimized, until the radial position of the loaded plate model exceeds the radial position of the last layer plate model, the loaded plate model is canceled, and the radial position calculation of the casing plate is completed; wherein, the process of sequentially loading each layer of plate model includes: Loading the current layer plate model within a preset radial distance from the previous layer plate model, and then optimizing the position of the current layer plate model; The preset range is the empirical range of radial distances between adjacent plate models; The process of optimizing the position of the current layer plate model includes: Load the plate model of the current layer sequentially with a preset step length within a preset range of radial distance from the plate model of the previous layer; After loading the current layer plate model at each position loading point, the partial discharge safety margin of the upper edge and the partial discharge safety margin of the lower edge of the current layer plate model are calculated, and then the minimum value of the partial discharge safety margin of the upper edge and the partial discharge safety margin of the lower edge of the current layer plate model is obtained; The maximum value among the minimum values ​​of the current layer plate model at all position loading points is obtained, and the position loading point of the current layer plate model corresponding to the maximum value is used as the final position loading point of the current layer plate model.

7. An electronic device, characterized in that: include: one or more processors; a storage device having one or more programs stored thereon; When the one or more programs are executed by the one or more processors, the one or more processors implement the method for calculating the radial position of a casing pad according to any one of claims 1 to 5.

8. A storage medium, characterized in that: A computer program is stored thereon, wherein when the computer program is executed by a processor, the method for calculating the radial position of the casing plate according to any one of claims 1 to 5 is implemented.

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