Method for controlling freezing span and freezing time of oil-filled cable

Abstract

The invention relates to the technical field of high-voltage submarine cables, and discloses an oil-filled cable freezing span and freezing time control method which comprises the following steps: acquiring basic parameters such as a cable system, field working conditions and freezing conditions; for a given freezing span, acquiring key thermodynamic parameters based on the parameters, including radial thermal resistance, longitudinal thermal resistance, total heat capacity including latent heat of solidification and heat carried by oil flow; establishing a heat balance differential model for describing the temperature change of the freezing center point based on the thermodynamic parameters; and solving the model to obtain an expression that the temperature changes along with time, and deducing the freezing time corresponding to the given freezing span by setting the final temperature as an oil freezing point. According to the method, the thermodynamic model which introduces the experimental calibration correction coefficient is constructed and solved, so that accurate quantitative calculation of the freezing operation core parameters is realized, freezing failure is effectively avoided, and resource consumption is optimized.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of high-voltage submarine cables, in particular to a control method for the frozen span and frozen time of an oil-filled cable. BACKGROUND

[0002] Oil-filled cables, especially high-voltage submarine oil-filled cables, often use a frozen plugging technique when they are repaired or spliced during their life cycle. This technique uses a cryogenic medium such as liquid nitrogen to locally cool the cable, causing the internal insulating oil to solidify and form a temporary solid oil plug that can withstand a certain pressure, thereby effectively blocking the oil flow and creating an oil-free environment for subsequent operations.

[0003] In the prior art, the determination of the key construction parameters required for frozen plugging operations, namely the frozen span and the frozen time, mainly relies on the personal experience and qualitative estimation of the on-site construction personnel. This method lacks systematic theoretical basis and quantitative calculation means, making the selection of parameters highly uncertain. If the parameters are selected too conservatively, it will lead to unnecessary consumption of cryogens such as liquid nitrogen and prolong the operation cycle, directly increasing the engineering cost. Conversely, if the parameters are not sufficient, the oil plug may not form completely or its mechanical strength may not be sufficient to resist the oil pressure, leading to freezing failure, operation interruption, and even safety accidents, resulting in serious technical risks and economic losses.

[0004] Therefore, there is an urgent technical problem in the prior art: how to establish a quantitative method that can comprehensively consider the cable's own parameters, on-site working conditions, and freezing conditions to accurately guide the selection of frozen span and frozen time, thereby improving construction efficiency and controlling costs under the premise of ensuring safety and reliability. SUMMARY

[0005] To address the shortcomings of the prior art, the present application provides a control method for the frozen span and frozen time of an oil-filled cable, which solves the problem of low construction efficiency, high resource consumption, and poor safety and reliability caused by the lack of quantitative basis when determining the key parameters of the frozen plugging operation of the oil-filled cable in the prior art.

[0006] To solve the above technical problems, the present application provides a control method for the frozen span and frozen time of an oil-filled cable, which obtains accurate parameters for guiding the frozen operation by constructing a thermodynamic model and solving a thermal equilibrium model, including the following steps: S1, obtaining basic parameters; S2, for a given frozen span, obtaining key thermodynamic parameters based on the basic parameters; S3, based on the key thermodynamic parameters, a heat balance differential model for describing the temperature change of the freezing center point is established, and the heat balance differential model represents a dynamic balance relationship between the heat transferred through the radial thermal resistance and the longitudinal thermal resistance, the heat carried by the oil flow and the total heat capacity; S4, based on the heat balance differential model, a change relationship of the freezing center point temperature with time is obtained, and the freezing time corresponding to the given freezing span is obtained by configuring the freezing center point temperature in the change relationship as equal to the preset oil freezing point temperature.

[0007] Preferably, in step S1, the basic parameters include physical parameters of the cable system and materials, field working condition parameters and freezing condition parameters; the step of obtaining the physical parameters of the cable system and materials specifically includes: equivalent to the cross-sectional area of a cylinder, and the equivalent cable conductor radius is calculated; and the total cross-sectional area of the cable core is equivalent to the cross-sectional area of a cylinder, and the equivalent cable core radius is calculated.

[0008] Preferably, the key thermodynamic parameters include the radial thermal resistance of radial heat transfer, the longitudinal thermal resistance of axial heat transfer, the total heat capacity of system heat storage capacity, and the heat carried by the oil flow affected by the oil flow; the specific steps of obtaining the heat carried by the oil flow are: multiply the axial oil flow heat loss correction coefficient, the specific heat capacity of the insulating oil, the density of the insulating oil, the initial oil flow rate and the temperature difference between the initial oil temperature and the oil freezing point temperature in the freezing condition parameters to obtain a product; and multiply the product by an exponential term factor determined by the thermal resistance between the frozen oil and the flowing oil to obtain the heat carried by the oil flow.

[0009] Preferably, in step S2, the step of obtaining the total heat capacity specifically includes: respectively calculate the heat capacity of the conductor part and the insulating oil part; wherein the conductor part heat capacity is calculated according to the volume, density and specific heat capacity; the insulating oil part heat capacity is calculated according to the volume, density, specific heat capacity and freezing latent heat, and the sum of the sensible heat required for the initial oil temperature to drop to the oil freezing point and the latent heat required for the completion of freezing; and finally add the conductor part heat capacity and the insulating oil part heat capacity to obtain the total heat capacity.

[0010] Preferably, in step S2, the step of obtaining the radial thermal resistance specifically includes: the natural logarithm of the ratio of the inner radius of the steel pipe in the obtained in step S1 to the equivalent cable core radius, to obtain a geometric factor; Divide the geometric factor by the product of the thermal conductivity of the insulating oil and the given refrigeration span, and multiply the calculation result by the radial thermal resistance correction coefficient in the refrigeration condition parameter to obtain the radial thermal resistance.

[0011] Preferably, in step S2, the step of obtaining the longitudinal thermal resistance is specifically: First, based on the thermal conductivity of the insulating oil, the thermal conductivity of the conductor, and the respective cross-sectional areas determined by the equivalent cable conductor radius and the outer radius of the steel pipe obtained in step S1, the combined longitudinal thermal conductivity is calculated by weighted average; Then, the given refrigeration span is divided by the product of the combined longitudinal thermal conductivity and the cross-sectional area of the steel pipe to obtain the longitudinal thermal resistance.

[0012] Preferably, it further includes a checking step under high oil pressure difference working condition: Based on the oil pressure difference in the field working condition parameter, the minimum refrigeration span to ensure the mechanical strength of the oil plug is calculated, and it is ensured that the given refrigeration span is not less than the minimum refrigeration span.

[0013] Preferably, it further includes a refrigeration mode adjustment step under high oil flow rate working condition: When the initial oil flow rate in the field working condition parameter is higher than the preset threshold, the refrigeration mode of pre-cooling the main refrigeration cavity first and then pre-cooling the auxiliary refrigeration cavity is adopted; Wherein, the main refrigeration cavity is arranged upstream of the auxiliary refrigeration cavity, and is used for preliminary cooling and pre-cooling the oil flow with high oil flow rate; The auxiliary refrigeration cavity is used for deep cooling and solidification of the oil flow pre-cooled by the main refrigeration cavity, so as to form an effective oil plug.

[0014] Preferably, it further includes a refrigeration mode adjustment step under limited construction space working condition: When the available physical space on site is smaller than the required refrigeration span, the refrigeration mode of pre-cooling the oil flow entering the main refrigeration cavity by the auxiliary cavity is adopted to increase the equivalent refrigeration span.

[0015] Preferably, the determination method of the radial thermal resistance correction coefficient in the refrigeration condition parameter and the axial oil flow heat loss correction coefficient for calculating the heat carried by the oil flow includes: Physical experiments are carried out for a plurality of preset working condition combinations to obtain actual refrigeration time; The theoretical refrigeration time is calculated by steps S3 and S4; And the difference between the theoretical refrigeration time and the actual refrigeration time is minimized as the target, and the solution is obtained by iterative fitting algorithm.

[0016] This invention provides a method for controlling the freezing span and freezing time of oil-filled cables. It has the following beneficial effects: 1. This invention provides a systematic method for quantitatively determining freezing span and freezing time by acquiring fundamental parameters of the cable system, on-site operating conditions, and freezing conditions, and then establishing and solving a differential thermal equilibrium model based on these key thermodynamic parameters. This method, based on a physical model, replaces the qualitative estimations relying on personal experience in existing technologies, improving the scientific rigor and accuracy of parameter determination.

[0017] 2. This invention can accurately determine the freezing time required to form an effective oil plug under a specific freezing span, thus providing precise data support for the construction plan. This avoids excessive consumption of cooling media such as liquid nitrogen and unnecessary extension of operation time due to overly conservative parameter selection, effectively reducing the direct economic cost of the project.

[0018] 3. This invention, through complete thermodynamic modeling of the oil plug formation process, ensures that the calculation results can guide the formation of an effective solid oil plug with sufficient mechanical strength, thereby avoiding the risk of freezing failure. Furthermore, by incorporating safety boundary conditions such as the risk of low-temperature brittle fracture of the steel pipe, this method can ensure the structural safety of the cable body while guaranteeing the sealing effect, comprehensively improving the reliability and safety of freezing operations. Attached Figure Description

[0019] Figure 1 This is a schematic diagram of the method flow of the present invention; Figure 2 This is a system functional block diagram of the present invention; Figure 3 This is a schematic diagram of the cross-section and equivalent model of the oil-filled cable of the present invention; Figure 4 This is a schematic diagram of the oil plug stress analysis under high oil pressure differential conditions according to the present invention; Figure 5 This is a schematic diagram of the step-by-step freezing strategy under high oil flow rate conditions according to the present invention; Figure 6 This is a schematic diagram illustrating the low-temperature brittle fracture risk of the steel pipe material according to the present invention; Figure 7 This is a flowchart of the experimental calibration process for the correction coefficient of the present invention. Detailed Implementation

[0020] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0021] See attached document Figure 1 Appendix Figure 2 and attached Figure 3 The present invention provides a method for controlling the freezing span and freezing time of an oil-filled cable. The method can be executed by a system, which may include: a basic parameter acquisition module 100, a key thermodynamic parameter module 200, a thermal balance model establishment module 300, and a freezing time solution module 400.

[0022] The method specifically includes the following steps: Step S1 is performed by the basic parameter acquisition module 100. This step involves acquiring all the input data required for subsequent calculations. The basic parameters are categorized as: cable system and material physical parameters, field operating condition parameters, and freezing condition parameters. Cable system and material physical parameters include: steel pipe inner diameter, equivalent cable conductor radius, etc. Equivalent cable core radius Outer radius of steel pipe Thermal conductivity of oil Thermal conductivity of conductors The density of oil Density of conductors Specific heat capacity of oil Specific heat capacity of conductors and the latent heat of solidification of oil .

[0023] In one specific implementation, when the object being processed is a three-phase oil-filled cable, the equivalent cable conductor radius is obtained. and equivalent cable core radius The specific steps are as follows: Equip the total cross-sectional area of ​​the cable conductor with the equivalent cross-sectional area of ​​a cylinder, and calculate the equivalent cable conductor radius. ; and the total cross-sectional area of ​​the cable core is equivalent to the cross-sectional area of ​​a cylinder, and the equivalent cable core radius is calculated. .

[0024] On-site operating parameters include: initial oil temperature Initial oil flow rate Oil pressure difference and ambient temperature .

[0025] Freezing condition parameters include: target temperature of the freezing chamber surface. Radial thermal resistance correction factor and axial oil flow heat loss correction factor .

[0026] Step S2 is performed by the critical thermodynamic parameter module 200. This step is for a given freezing span. Based on the fundamental parameters obtained in step S1, a set of key parameters describing the thermodynamic behavior of the system are calculated. These key thermodynamic parameters include: radial thermal resistance. Longitudinal thermal resistance Total heat capacity And the heat carried by the oil flow .

[0027] Obtain total heat capacity The specific process involves calculating the heat capacities of the conductor and insulating oil components separately. The heat capacity of the conductor component is calculated based on its volume, density, and specific heat capacity, determining its sensible heat. The heat capacity of the insulating oil component is calculated based on its volume, density, specific heat capacity, and latent heat of solidification, determining its sensible heat from the initial oil temperature. Drop to the oil's freezing point The sum of the sensible heat required and the latent heat required for solidification. Finally, the heat capacity of the conductor portion and the heat capacity of the insulating oil portion are added together to obtain the total heat capacity. The formula for its calculation is: ; in, For heat mechanical equivalent, The outer radius of the steel pipe, For the equivalent cable conductor radius, For oil density, The specific heat capacity of oil, The latent heat of oil solidification, Initial oil temperature This is the solidification point temperature of the oil. For conductor density, For the specific heat capacity of a conductor, For a given freezing span, This indicates the cross-sectional area of ​​the insulating oil filling. This indicates the equivalent specific heat capacity of the insulating oil. This represents the cross-sectional area of ​​the conductor portion.

[0028] Obtaining radial thermal resistance The specific process is as follows: take the inner radius of the steel pipe and the equivalent cable core radius. The geometric factor is obtained by taking the natural logarithm of the ratio; then the geometric factor is divided by the thermal conductivity of the oil. With a given freezing span The product of these factors is then multiplied by the radial thermal resistance correction factor. The radial thermal resistance is obtained. .

[0029] The formula is: ; in, This is the radial thermal resistance correction factor. In this calculation formula, the radius represents the outer boundary radius of the heat transfer path. This is the equivalent cable core radius. The thermal conductivity of oil is... For a given freezing span, It is the natural logarithm of the radius ratio.

[0030] Obtaining longitudinal thermal resistance The process is as follows: First, based on the thermal conductivity of oil... Thermal conductivity of conductors And the equivalent cable conductor radius and the outer radius of the steel pipe Given the determined cross-sectional areas, the combined longitudinal thermal conductivity is calculated by weighted averaging. : ; in, Let be the cross-sectional area of ​​the annular region.

[0031] Then, the given freezing span Divide by the combined longitudinal thermal conductivity The longitudinal thermal resistance is obtained by multiplying the thermal resistance by the cross-sectional area of ​​the steel pipe. The formula for its calculation is: ; in, For a given freezing span, The outer radius of the steel pipe, The combined longitudinal thermal conductivity.

[0032] Obtain the heat carried by the oil flow The specific process involves: calculating the axial oil flow heat loss correction factor. Specific heat capacity of oil The density of oil Initial oil flow rate and initial oil temperature With oil pour point The product of these five terms—temperature difference, etc.—is then multiplied by an exponential factor determined by the thermal resistance between the frozen and flowing oil to obtain the heat carried by the oil flow. .

[0033] Step S3 is executed by the thermal balance model establishment module 300. This step involves establishing a thermal balance differential model based on the key thermodynamic parameters obtained in step S2 to describe the temperature change of the freezing center point over time. This model follows the law of conservation of energy, which characterizes that at any given time, the rate of change of the total heat capacity of the freezing region is equal to the difference between the net heat transferred through radial and longitudinal thermal resistances and the heat carried by the oil flow.

[0034] Step S4 is executed by the freezing time solution module 400. This step involves solving the thermal equilibrium differential model established in step S3. First, by solving this differential model, an analytical expression for the change of the freezing center point temperature over time is obtained. Then, in this expression, the freezing center point temperature is set to be equal to a preset oil freezing point temperature. This allows us to inversely solve for the time variable, which is the time span relative to the initially given freezing period. Corresponding freezing time By varying the given freezing span... By repeating steps S2 to S4 above, a set of correspondences between freezing span and freezing time can be obtained to guide actual freezing operations.

[0035] See attached document Figure 4 When applied to specific engineering projects, the method of the present invention may also include a series of adaptive strategies for specific working conditions to ensure the effectiveness of the results and the safety of construction.

[0036] Under conditions of high oil pressure differential, such as static pressure caused by a large drop, the refrigeration system needs to be recalibrated. This calibration step ensures that the final solid oil plug has sufficient mechanical strength to withstand the oil pressure differential. The thrust. Therefore, based on the oil pressure difference in the field operating parameters. A minimum freezing span that can ensure the mechanical stability of the oil plug can be calculated in advance. When performing the aforementioned calculation processes S2 to S4, the selected given freezing span... It must be no less than this minimum freezing span This incorporates construction safety into the calculation process.

[0037] See attached document Figure 5 Under high oil flow rate conditions, i.e., when the initial oil flow rate in the field operating parameters... When the temperature exceeds a preset threshold, the cooling power provided by a single refrigeration unit is insufficient to offset the heat brought by the high-speed oil flow, resulting in the inability to form an effective freeze.

[0038] To address this situation, the method of the present invention can incorporate a step-by-step freezing strategy. This strategy employs a main freezing chamber 210 and a secondary freezing chamber 220 arranged in series. In this way, the total cooling task is distributed to the two units, ensuring that at any cross-section, the system's cooling power is greater than the heat carried by the oil flow, thereby achieving effective freezing of high-velocity oil.

[0039] In another case where construction space is limited, i.e., when the physical length available on site for refrigeration equipment is less than the calculated required refrigeration span, the auxiliary refrigeration chamber 220 is arranged upstream of the main refrigeration chamber 210, and its function is to pre-cool the oil flow entering the main refrigeration chamber 210.

[0040] Radial thermal resistance correction coefficient in the method of the present invention and axial oil flow heat loss correction factor These are crucial for ensuring the accuracy of the computational model. The methods for determining these two coefficients include: first, conducting physical experiments for various preset operating condition combinations, which may include different initial oil temperatures, initial oil flow rates, and freezing spans. Under each preset condition, the actual freezing time required for the oil flow to completely stop is measured.

[0041] Subsequently, for each identical combination of operating conditions, steps S3 and 54 of the present invention are used to... and The theoretical freezing time was calculated using initial values. Finally, with the optimization objective of minimizing the total difference between the theoretical and actual freezing times under all operating conditions, an iterative fitting algorithm (e.g., least squares method) was used to... and The value is solved by this method. The correction coefficients calibrated using this method can effectively compensate for the deviation between the theoretical model and complex physical reality.

[0042] See attached document Figure 6 The following section uses a freezing operation of a 250mm oil-filled submarine cable as an example to illustrate the implementation of the present invention in detail. In a specific embodiment, step S1 is first performed to obtain all the basic parameters of the 250mm cable operation.

[0043] The specific freezing condition parameters obtained are: the target surface temperature of the freezing chamber determined based on the coolant used (e.g., liquid nitrogen). For example, -196°C; and a radial thermal resistance correction factor predetermined according to the calibration method of the present invention. and axial oil flow heat loss correction factor Then, step S2 is performed for a given freezing span. (For example, the construction unit selects a freezing span of 2.5m with an initial value based on the available equipment.) Based on all the specific parameter values ​​of the 250mm cable obtained in step S1, the key thermodynamic parameters are calculated.

[0044] Substitute the above parameter values ​​into the total heat capacity. Radial thermal resistance Longitudinal thermal resistance And the heat carried by the oil flow The calculation formula is used to obtain the specific values ​​of four key thermodynamic parameters under the conditions of a 250mm cable, an initial oil temperature of 20℃, a flow rate of 60L / h, and a refrigeration span of 2.5m.

[0045] Next, proceed to step S3, and use the result calculated in the previous step... The specific values ​​are substituted into the thermal equilibrium differential model used to describe the temperature change at the freezing center point. At this point, the model becomes a differential model with only two variables: temperature and time, containing specific coefficients.

[0046] Finally, step S4 is executed to solve the specific differential model, yielding a definite expression describing the temperature change of the freezing center point of the 250mm cable over time. In this expression, the temperature is set equal to the freezing point temperature of the insulating oil used in the cable. This allows us to solve for a unique time value, which is the required freezing time for a 2.5m freezing span. .

[0047] The method of this invention can also integrate safety boundary conditions to mitigate potential physical risks during the calculation process. A specific safety boundary condition is the risk of low-temperature brittle fracture of the steel pipe.

[0048] The method of the present invention targets the surface temperature of the freezing chamber. As an adjustable input parameter, it provides a way to mitigate this risk. During calculations, instead of using the limiting temperature of the cooling medium, a relatively high safe insulation temperature can be set as the target temperature based on the characteristics of the steel pipe material. For example, -100℃. Then, steps S2 to S4 are performed to calculate the freezing span and freezing time required for effective freezing at this safe insulation temperature. Thus, the calculation result itself constitutes a construction plan that ensures the safety of the steel pipe structure while meeting freezing requirements, and the guided operation process can maintain the steel pipe temperature within its toughness range.

[0049] See attached document Figure 7 The versatility of the method of this invention lies in its parametric model. This model, through the precise definition of the cable system and material physical parameters in step S1, can adapt to oil-filled cables of different structures and specifications. For different types of oil-filled cables, such as self-contained oil-filled cables, their internal structures differ from those of steel pipe cables. The method of this invention analyzes their specific structures to extract equivalent geometric parameters (such as the equivalent radius of each layer) and material physical parameters (such as the thermal conductivity, density, and specific heat capacity of each layer).

[0050] See attached document Figure 6 The radial thermal resistance correction coefficient in the method of this invention and axial oil flow heat loss correction factor It was determined through a calibration process that combines physical experiments and numerical calculations, which ensures the consistency between the theoretical model and physical reality.

[0051] Under each defined operating condition combination, a complete cable freezing test was conducted. During the test, the actual freezing time required from the start of the cooling system to the complete cessation of oil flow within the cable or the freezing point reaching its freezing point was precisely recorded. By conducting experiments covering all preset operating condition combinations, a dataset consisting of operating condition parameters and their corresponding actual freezing times was obtained.

[0052] Next, for each working condition combination in the dataset, theoretical calculations are performed using the control method of this invention. Specifically, the parameters of the working condition combination are used as inputs to step S1, and the following settings are configured. and These are the coefficients to be determined. Then, steps S2, S3, and S4 are performed to derive a coefficient containing the undetermined coefficients. and Theoretical freezing time expression .

[0053] Finally, iterative fitting is performed to solve the problem. The theoretical freezing time under all operating conditions is used. Compared with actual freezing time The objective function is to minimize the sum of squared differences between the two values. This objective function can be expressed as: ; in, This represents the total number of experimental operating condition combinations. This is the experiment number. It is the first The difference between the actual freezing time and the theoretically predicted freezing time under various experimental conditions was analyzed. An iterative fitting algorithm, such as the nonlinear least squares method, was used to solve the objective function. The obtained results... and The value of is the final calibrated correction coefficient value.

Claims

1. A method for controlling the freezing span and freezing time of an oil-filled cable, characterized in that, Includes the following steps: S1. Obtain basic parameters; S2. For a given freezing span, obtain key thermodynamic parameters based on the aforementioned basic parameters; S3. Based on the key thermodynamic parameters, establish a thermal balance differential model to describe the temperature change at the freezing center point. The thermal balance differential model characterizes the dynamic balance relationship between the heat transferred through radial and longitudinal thermal resistances, the heat carried by the oil flow, and the total heat capacity. S4. Based on the thermal balance differential model, obtain the relationship between the freezing center point temperature and time, and by configuring the freezing center point temperature to be equal to the preset oil freezing point temperature in the relationship, obtain the freezing time corresponding to the given freezing span.

2. The method for controlling the freezing span and freezing time of an oil-filled cable according to claim 1, characterized in that, In step S1, the basic parameters include the physical parameters of the cable system and materials, the on-site working condition parameters, and the freezing condition parameters; The specific steps for obtaining the physical parameters of the cable system and materials include: The total cross-sectional area of ​​the cable conductor is equivalent to the cross-sectional area of ​​a cylinder, and the equivalent cable conductor radius is calculated. The total cross-sectional area of ​​the cable core is equivalent to the cross-sectional area of ​​a cylinder, and the equivalent cable core radius is calculated.

3. The method for controlling the freezing span and freezing time of an oil-filled cable according to claim 1, characterized in that, In step S2, the key thermodynamic parameters include radial thermal resistance for radial heat transfer, longitudinal thermal resistance for axial heat transfer, total heat capacity of the system, and heat carried by the oil flow due to the influence of the oil flow. The specific steps for obtaining the heat carried by the oil flow are as follows: The product is obtained by multiplying the following five parameters in the freezing condition parameters: axial oil flow heat loss correction coefficient, specific heat capacity of insulating oil, density of insulating oil, initial oil flow rate, and temperature difference between initial oil temperature and oil freezing point temperature. The product is then multiplied by an exponential factor determined by the thermal resistance between the frozen oil and the flowing oil to obtain the heat carried by the oil flow.

4. The method for controlling the freezing span and freezing time of an oil-filled cable according to claim 1, characterized in that, In step S2, the specific steps for obtaining the total heat capacity are as follows: Calculate the heat capacity of the conductor section and the insulating oil section separately; The heat capacity of the conductor is calculated based on its volume, density, and specific heat capacity, resulting in sensible heat. The partial heat capacity of insulating oil is calculated based on its volume, density, specific heat capacity, and latent heat of solidification, which is the sum of the sensible heat required to cool the oil from its initial temperature to its freezing point and the latent heat required to complete solidification. Finally, the heat capacity of the conductor portion and the heat capacity of the insulating oil portion are added together to obtain the total heat capacity.

5. The method for controlling the freezing span and freezing time of an oil-filled cable according to claim 2, characterized in that, In step S2, the step of obtaining the radial thermal resistance specifically involves: The geometric factor is obtained by taking the natural logarithm of the ratio of the inner radius of the steel pipe to the equivalent cable core radius in the physical parameters of the cable system and materials obtained in step S1. The geometric factor is then divided by the product of the thermal conductivity of the insulating oil and the given freezing span, and the calculation result is multiplied by the radial thermal resistance correction factor in the freezing condition parameters to obtain the radial thermal resistance.

6. The method for controlling the freezing span and freezing time of an oil-filled cable according to claim 2, characterized in that, In step S2, the specific steps for obtaining the longitudinal thermal resistance are as follows: First, based on the thermal conductivity of the insulating oil and the thermal conductivity of the conductor in the physical parameters of the cable system and materials obtained in step S1, and the respective cross-sectional areas determined by the equivalent cable conductor radius and the outer radius of the steel pipe, the combined longitudinal thermal conductivity is calculated by weighted average. Then, the given freezing span is divided by the product of the combined longitudinal thermal conductivity and the cross-sectional area of ​​the steel pipe to obtain the longitudinal thermal resistance.

7. The method for controlling the freezing span and freezing time of an oil-filled cable according to claim 2, characterized in that, It also includes verification steps under high oil pressure differential conditions: Based on the oil pressure difference in the field operating parameters, the minimum freezing span to ensure the mechanical strength of the oil plug is calculated, and the given freezing span is not less than the minimum freezing span.

8. The method for controlling the freezing span and freezing time of an oil-filled cable according to claim 2, characterized in that, It also includes steps for adjusting the refrigeration method under high oil flow rate conditions: When the initial oil flow rate in the field operating parameters is higher than the preset threshold, the freezing method of first pre-cooling the main freezing chamber and then freezing the auxiliary freezing chamber is adopted. The main freezing chamber is located upstream of the secondary freezing chamber and is used to perform preliminary cooling and pre-cooling on the high oil flow rate. The secondary freezing chamber is used to deeply cool and solidify the oil flow that has been pre-cooled by the main freezing chamber, thereby forming an effective oil plug.

9. The method for controlling the freezing span and freezing time of an oil-filled cable according to claim 1, characterized in that, It also includes steps for adjusting the freezing method under conditions of limited construction space: When the available physical space on site is less than the required freezing span, a freezing method is adopted to pre-cool the oil flow entering the main freezing chamber using a secondary freezing chamber, thereby increasing the equivalent freezing span.

10. The method for controlling the freezing span and freezing time of an oil-filled cable according to claim 2, characterized in that, In step S1, the methods for determining the radial thermal resistance correction coefficient in the freezing condition parameters and the axial oil flow heat loss correction coefficient used to calculate the heat carried by the oil flow include: Physical experiments were conducted on various preset operating conditions to obtain the actual freezing time; The theoretical freezing time is calculated using steps S3 and S4. The difference between the theoretical freezing time and the actual freezing time is minimized as the objective, and is obtained by solving the problem through an iterative fitting algorithm.

Images