A distribution network cable emergency current carrying capacity evaluation method and device

Abstract

The application discloses a kind of distribution network cable emergency current carrying capacity evaluation method and device, wherein, method includes: respectively collecting distribution network cable outer surface real-time temperature, environmental boundary real-time temperature and distribution network cable real-time load;Equivalent thermal circuit model and ontology transient thermal circuit model are respectively established, and the dynamic thermal evaluation model of distribution network cable;To the differential equation group of temperature node established by distribution network cable dynamic thermal evaluation model writing;Based on distribution network cable outer surface real-time temperature, environmental boundary real-time temperature and distribution network cable real-time load, the differential equation group of temperature node of distribution network cable dynamic thermal evaluation model is solved, obtains the equivalent thermal resistance parameter of distribution network cable external environment and the equivalent thermal capacity parameter of distribution network cable external environment;The differential equation group of temperature node is solved using MATLAB, obtains the corresponding emergency time of distribution network cable under different power supply load demand.The application can realize accurate thermal calculation to the conductor temperature of distribution network cable, completes the evaluation of emergency time.

Description

Technical Field

[0001] This invention belongs to the field of power cable technology, specifically relating to a method and device for assessing the emergency current-carrying capacity of distribution network cables. Background Technology

[0002] With rapid economic development, the electricity demand in Chinese cities is increasing, leading to the large-scale application of urban distribution network cables. Distribution network cables face two main operational challenges: first, the continuous growth in electricity load, coupled with the complex laying environment of distribution cables, exacerbates the problem of heavy-load operation; second, the variable load conditions of distribution network cables result in significant load fluctuations, and the integration of renewable energy into the distribution network is becoming a trend under the development of new power systems, inevitably further aggravating load fluctuations. As the lifeline of power transmission in the distribution network, the accurate assessment of the load capacity of distribution cables is crucial for the safe operation of the distribution system and provides guidance for achieving transparent operation of distribution cables and formulating control strategies for new power systems. Therefore, accurately achieving dynamic thermal assessment of distribution network cables is of significant research importance.

[0003] Accurate dynamic thermal assessment of distribution network cables relies heavily on real-time conductor temperature acquisition. Although smart distribution network cables with built-in optical fibers have emerged, many existing distribution network cables lack the capability for direct conductor temperature measurement. Therefore, for existing distribution network cables, calculating conductor temperature through thermal circuit models remains the most direct and effective method. Existing methods for dynamic thermal assessment of distribution network cables often refer to relevant standards such as IEC and national standards when selecting environmental parameter values, resulting in conservative values ​​that hinder accurate dynamic thermal assessment. When distribution network cables require a large emergency load due to load fluctuations or fault repairs, the conductor temperature will rise rapidly, potentially exceeding the long-term withstand temperature of the cable insulation material. This reduces the cable's lifespan and, in severe cases, may lead to insulation breakdown or even fires. Therefore, a method for determining the emergency current-carrying capacity of distribution network cables based on accurate dynamic thermal assessment results is needed to provide technical support for the safe operation of distribution network cables under power transfer conditions. Summary of the Invention

[0004] The technical problem to be solved by the embodiments of the present invention is to provide a method and device for assessing the emergency current carrying capacity of distribution network cables, so as to improve the accuracy of thermal assessment of distribution network cables and realize accurate thermal calculation of the conductor temperature of distribution network cables.

[0005] To address the aforementioned technical problems, this invention provides a method for assessing the emergency current-carrying capacity of distribution network cables, comprising:

[0006] Step S1: Collect the real-time temperature of the outer surface of the distribution network cable, the real-time temperature of the environmental boundary, and the real-time load of the distribution network cable, respectively.

[0007] Step S2: Establish an equivalent thermal circuit model characterizing the heat transfer process between the distribution network cable and the external environment, and a transient thermal circuit model characterizing the internal heat transfer process of the distribution network cable. Based on the transient thermal circuit model of the distribution network cable and the equivalent thermal circuit model of the external environment, establish a dynamic thermal assessment model for the distribution network cable.

[0008] Step S3: Based on thermoelectric analogy theory, write the temperature node differential equations for the established dynamic thermal evaluation model of distribution network cables.

[0009] Step S4: The real-time temperature of the outer surface of the distribution cable, the real-time temperature of the environmental boundary, and the real-time load of the distribution cable are collected as input variables of the port network. The temperature node differential equations of the dynamic thermal assessment model of the distribution cable are solved to obtain the equivalent thermal resistance parameters and equivalent heat capacity parameters of the external environment of the distribution cable.

[0010] Step S5: Based on the maximum limit value of the cable conductor temperature, use MATLAB to solve the temperature node differential equation set to obtain the emergency time corresponding to the distribution network cable under different power transfer load requirements.

[0011] Further, in step S1, three temperature measuring points are evenly arranged radially at 120° intervals on the outer surface of the distribution cable, and the average value of the measurement results of the three surface temperature measuring points is used as the monitoring data of the real-time temperature of the outer surface of the distribution cable; a temperature measuring point is arranged in the external environmental medium at a distance of 3m below the distribution cable, and the measurement result of this temperature measuring point is used as the monitoring data of the real-time temperature of the environmental boundary; the measurement result of the CT of the distribution cable is used as the measurement data of the temperature of the distribution cable load.

[0012] Furthermore, in step S2, the heat flow sources and parallel heat capacities at the same node in the transient thermal circuit model of the distribution network cable are merged to obtain a simplified transient thermal circuit model of the distribution network cable; then, the simplified transient thermal circuit model of the distribution network cable and the equivalent thermal circuit model of the external environment are connected in series to obtain the dynamic thermal evaluation model of the distribution network cable.

[0013] Furthermore, the temperature node differential equations are shown below:

[0014]

[0015] Where θ0 is the surface temperature of the cable outer sheath, θ1 is the temperature of the cable conductor, θ2 is the temperature of the outer surface of the insulation and the metal shielding layer, θ3 is the temperature of the armor layer, T1 is the thermal resistance of the insulation layer of a single conductor, T2 is the thermal resistance of the filler layer and the inner sheath, T3 is the thermal resistance of the outer sheath, Q1 is the total heat flow into the conductor node, Q2 is the total heat flow into the metal sheath node, Q3 is the total heat flow into the armor node, C1 is the equivalent heat capacity of the conductor and the insulation layer, C2 is the equivalent heat capacity of the metal sheath and the filler layer, C3 is the equivalent heat capacity of the armor layer and the outer sheath, and t is time.

[0016] Further, step S4 specifically includes:

[0017] Step S41: Establish the heat flow balance equation for the surface temperature node of the outer sheath of the distribution network cable;

[0018]

[0019] Step S42: Set the time interval and select three adjacent time points t0, t1, and t2;

[0020] Step S43: Use the finite difference method to transform the heat flow balance equation into a set of equations related to three adjacent time points t0, t1, and t2.

[0021]

[0022]

[0023] Step S44: Based on the transformed set of equations, derive the equivalent thermal resistance T of the distribution network cable's external environment. e The equivalent heat capacity C of the external environment of the distribution network cable e The calculation formula;

[0024]

[0025] Step S45: Calculate the real-time temperature of the armor layer based on the collected real-time temperature of the outer surface of the distribution network cable and the real-time load of the distribution network cable.

[0026]

[0027]

[0028]

[0029] Step S46: Substitute the collected real-time temperature of the outer surface of the distribution cable, the real-time temperature of the environmental boundary, and the calculated real-time temperature of the armor layer into the calculation formula for the equivalent thermal parameters of the external environment of the distribution cable to calculate the equivalent thermal resistance T of the external environment of the distribution cable. e The equivalent heat capacity C of the external environment of the distribution network cablee .

[0030] Further, step S5 specifically includes: setting the maximum limit value of θ1 in the temperature node differential equation set of the dynamic thermal assessment model of the distribution network cable, changing the size of the power supply load of the distribution network cable, and using MATLAB to solve the temperature node differential equation set under the premise of considering the maximum limit value of θ1, so as to obtain the emergency time corresponding to the distribution network cable under different power supply load requirements.

[0031] The present invention also provides an emergency current-carrying capacity assessment device for distribution network cables, comprising:

[0032] The data acquisition module is used to collect the real-time temperature of the outer surface of the distribution network cable, the real-time temperature of the environmental boundary, and the real-time load of the distribution network cable, respectively.

[0033] The model building module is used to build an equivalent thermal circuit model that characterizes the heat transfer process between the distribution network cable and the external environment, and a transient thermal circuit model that characterizes the internal heat transfer process of the distribution network cable. Based on the transient thermal circuit model of the distribution network cable and the equivalent thermal circuit model of the external environment, a dynamic thermal evaluation model of the distribution network cable is built.

[0034] The first calculation module is used to write a set of temperature node differential equations for the established dynamic thermal evaluation model of distribution network cables based on thermoelectric analogy theory.

[0035] The second calculation module is used to use the real-time temperature of the outer surface of the distribution cable, the real-time temperature of the environmental boundary, and the real-time load of the distribution cable as input variables of the port network, and to solve the temperature node differential equations of the dynamic thermal assessment model of the distribution cable to obtain the equivalent thermal resistance parameters and equivalent heat capacity parameters of the external environment of the distribution cable.

[0036] The third calculation module is used to solve the temperature node differential equations using MATLAB based on the maximum limit value of the cable conductor temperature, so as to obtain the emergency time corresponding to the distribution network cable under different power transfer load requirements.

[0037] Furthermore, the acquisition module is specifically used for: uniformly arranging three temperature measuring points at 120° radial intervals on the outer surface of the distribution cable, and using the average value of the measurement results of the three surface temperature measuring points as the real-time temperature monitoring data of the outer surface of the distribution cable; arranging a temperature measuring point in the external environmental medium at 3m intervals below the distribution cable, and using the measurement result of this temperature measuring point as the real-time temperature monitoring data of the environmental boundary; and using the measurement result of the CT of the distribution cable as the measurement data of the temperature of the distribution cable load.

[0038] Furthermore, the model building module is specifically used to merge the heat flow sources and parallel heat capacities at the same node in the transient thermal circuit model of the distribution network cable to obtain a simplified transient thermal circuit model of the distribution network cable; then, the simplified transient thermal circuit model of the distribution network cable and the equivalent thermal circuit model of the external environment are connected in series to obtain the dynamic thermal evaluation model of the distribution network cable.

[0039] Furthermore, the temperature node differential equations are shown below:

[0040]

[0041] Where θ0 is the surface temperature of the cable outer sheath, θ1 is the temperature of the cable conductor, θ2 is the temperature of the outer surface of the insulation and the metal shielding layer, θ3 is the temperature of the armor layer, T1 is the thermal resistance of the insulation layer of a single conductor, T2 is the thermal resistance of the filler layer and the inner sheath, T3 is the thermal resistance of the outer sheath, Q1 is the total heat flow into the conductor node, Q2 is the total heat flow into the metal sheath node, Q3 is the total heat flow into the armor node, C1 is the equivalent heat capacity of the conductor and the insulation layer, C2 is the equivalent heat capacity of the metal sheath and the filler layer, C3 is the equivalent heat capacity of the armor layer and the outer sheath, and t is time.

[0042] Furthermore, the second calculation module is specifically used for:

[0043] Establish the heat flow balance equation for the surface temperature node of the outer sheath of the distribution network cable;

[0044]

[0045] Set a time interval and select three adjacent time points t0, t1, and t2;

[0046] The heat flow balance equation is transformed into a set of equations related to three adjacent time points t0, t1, and t2 using the finite difference method.

[0047]

[0048]

[0049] Based on the transformed set of equations, the equivalent thermal resistance T of the distribution network cable's external environment is derived. e The equivalent heat capacity C of the external environment of the distribution network cable e The calculation formula;

[0050]

[0051] The real-time temperature of the armor layer is calculated based on the collected real-time temperature of the outer surface of the distribution network cable and the real-time load of the distribution network cable.

[0052]

[0053]

[0054]

[0055] The real-time temperature of the outer surface of the distribution cable, the real-time temperature of the environmental boundary, and the calculated real-time temperature of the armor layer are substituted into the calculation formula of the equivalent thermal parameters of the external environment of the distribution cable to calculate the equivalent thermal resistance T of the external environment of the distribution cable. e The equivalent heat capacity C of the external environment of the distribution network cable e .

[0056] Furthermore, the third calculation module is specifically used to: set the maximum limit value of θ1 in the temperature node differential equation set of the dynamic thermal assessment model of the distribution network cable, change the size of the power supply load of the distribution network cable, and use MATLAB to solve the temperature node differential equation set under the premise of considering the maximum limit value of θ1 to obtain the emergency time corresponding to the distribution network cable under different power supply load requirements.

[0057] Implementing this invention has the following beneficial effects: Based on real-time monitoring of the outer surface temperature and load data of distribution network cables and environmental boundary temperature data, this invention enables accurate thermal calculation of the conductor temperature of distribution network cables, thereby completing the assessment of the maximum load capacity of distribution network cables under the current operating environment and the emergency time during power transfer operation; it can be applied to the operation site of distribution network cables under different laying conditions, providing reliable guidance for load control of distribution network cable lines and determination of maintenance time under fault conditions; it can also provide data support for the construction of intelligent distribution network cable digital twin system, thereby realizing transparent operation and maintenance of distribution network cables under the new power system. Attached Figure Description

[0058] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0059] Figure 1 This is a flowchart illustrating a method for assessing the emergency current-carrying capacity of distribution network cables according to an embodiment of the present invention.

[0060] Figure 2 This is a schematic diagram of the equivalent thermal circuit model of the external environment of the distribution network cable in an embodiment of the present invention.

[0061] Figure 3 This is a schematic diagram of the transient thermal circuit model of the distribution network cable body in an embodiment of the present invention.

[0062] Figure 4This is a simplified transient thermal circuit model of the distribution network cable body in an embodiment of the present invention.

[0063] Figure 5 This is a schematic diagram illustrating the process of solving the equivalent thermal parameters of the external environment in an embodiment of the present invention. Detailed Implementation

[0064] The following description of the embodiments is taken with reference to the accompanying drawings, which illustrate specific embodiments in which the invention can be implemented.

[0065] Please refer to Figure 1 As shown, Embodiment 1 of the present invention provides a method for assessing the emergency current-carrying capacity of distribution network cables, comprising:

[0066] Step S1: Collect the real-time temperature of the outer surface of the distribution network cable, the real-time temperature of the environmental boundary, and the real-time load of the distribution network cable, respectively.

[0067] Step S2: Establish an equivalent thermal circuit model characterizing the heat transfer process between the distribution network cable and the external environment, and a transient thermal circuit model characterizing the internal heat transfer process of the distribution network cable. Based on the transient thermal circuit model of the distribution network cable and the equivalent thermal circuit model of the external environment, establish a dynamic thermal assessment model for the distribution network cable.

[0068] Step S3: Based on thermoelectric analogy theory, write the temperature node differential equations for the established dynamic thermal evaluation model of distribution network cables.

[0069] Step S4: The real-time temperature of the outer surface of the distribution cable, the real-time temperature of the environmental boundary, and the real-time load of the distribution cable are collected as input variables of the port network. The temperature node differential equations of the dynamic thermal assessment model of the distribution cable are solved to obtain the equivalent thermal resistance parameters and equivalent heat capacity parameters of the external environment of the distribution cable.

[0070] Step S5: Based on the maximum limit value of the cable conductor temperature, use MATLAB to solve the temperature node differential equation set to obtain the emergency time corresponding to the distribution network cable under different power transfer load requirements.

[0071] Specifically, in step S1, heavy-load distribution network cable lines are selected as the object of interest. Three temperature measuring points are evenly arranged radially at 120° intervals on the outer surface of the distribution network cable. The average value of the measurement results of the three surface temperature measuring points is used as the monitoring data of the real-time temperature of the outer surface of the distribution network cable. A temperature measuring point is arranged in the external environmental medium at a distance of 3m below the distribution network cable. The measurement result of this temperature measuring point is used as the monitoring data of the real-time temperature of the environmental boundary. The measurement result of the CT of the distribution network cable is used as the measurement data of the temperature of the distribution network cable load.

[0072] In step S2, based on heat transfer theory analysis, a first-order thermal capacity and thermal resistance equivalent thermal circuit model is used to characterize the heat transfer process between the distribution network cable and the external environment, such as... Figure 2 As shown. Figure 2 In this context, θ0 represents the surface temperature of the cable's outer sheath, and θ e For the environmental boundary temperature, T e C represents the equivalent thermal resistance parameter of the distribution network cable in the external environment. e The equivalent heat capacity parameters of the external environment of the distribution network cable are used. A transient thermal circuit model of the distribution network cable body is established, such as... Figure 3 As shown. Figure 3 In the middle, Q c The loss generated by a single conductor and inner shield of a cable; Q d λ1 represents the dielectric loss of a single conductor insulation and insulation shielding layer; λ2 represents the loss factor of a single conductor metal sheath and shielding; λ3 represents the loss factor of a single conductor armor layer; θ1 represents the cable conductor temperature; θ2 represents the temperature of the outer surface of the insulation and the metal shielding layer; θ3 represents the armor layer temperature; T1 represents the thermal resistance of a single conductor insulation layer; T2 represents the thermal resistance of the filler layer and inner sheath; T3 represents the thermal resistance of the outer sheath; C1′ represents the thermal capacitance of the conductor and inner shielding; C2′ represents the thermal capacitance of the insulation layer and insulation shielding layer; C3′ represents the thermal capacitance of the metal sheath shielding layer; C4′ represents the thermal capacitance of the filler layer and inner sheath; C5′ represents the thermal capacitance of the armor layer; C6′ represents the thermal capacitance of the outer sheath. Figure 3 In the transient thermal circuit model of the distribution network cable shown, the heat flow sources and parallel heat capacities at the same node are merged to obtain a simplified transient thermal circuit model of the distribution network cable, as follows. Figure 4 As shown, a simplified transient thermal circuit model of the distribution network cable body and a first-order thermal capacity and thermal resistance equivalent thermal circuit model of the external environment are connected in series to obtain a dynamic thermal evaluation model of the distribution network cable.

[0073] In step S3, based on thermoelectric analogy theory, a set of temperature node differential equations is written for the dynamic thermal assessment model of the distribution network cable, as shown in the following equation. Solving the set of temperature node differential equations allows for the acquisition of the dynamic conductor temperature of the distribution network cable.

[0074]

[0075] Where θ0 is the surface temperature of the cable outer sheath, θ1 is the temperature of the cable conductor, θ2 is the temperature of the outer surface of the insulation and the metal shielding layer, θ3 is the temperature of the armor layer, T1 is the thermal resistance of the insulation layer of a single conductor, T2 is the thermal resistance of the filler layer and the inner sheath, T3 is the thermal resistance of the outer sheath, Q1 is the total heat flow into the conductor node, Q2 is the total heat flow into the metal sheath node, Q3 is the total heat flow into the armor node, C1 is the equivalent heat capacity of the conductor and the insulation layer, C2 is the equivalent heat capacity of the metal sheath and the filler layer, C3 is the equivalent heat capacity of the armor layer and the outer sheath, and t is time.

[0076] Step S4: The equivalent thermal circuit model of the cable's external environment can be considered as a port network. The internal parameters of the port network are solved by mathematical transformations of its input and output variables. Real-time monitored distribution network cable load data, distribution network cable outer surface temperature data, and environmental boundary temperature data are used as input variables for the port network. This allows for the determination of the T value in the temperature node differential equations of the dynamic thermal assessment model for the distribution network cable. e and C e The specific solution process is shown in the attached figure. Figure 4 As shown:

[0077] Step S41: Establish the heat flow balance equation for the surface temperature node of the outer sheath of the distribution network cable;

[0078]

[0079] Step S42: Set the time interval and select three adjacent time points t0, t1, and t2;

[0080] Step S43: Use the finite difference method to transform the heat flow balance equation into a set of equations related to three adjacent time points t0, t1, and t2.

[0081]

[0082]

[0083] Step S44: Based on the transformed set of equations, derive the equivalent thermal resistance T of the distribution network cable's external environment. e The equivalent heat capacity C of the external environment of the distribution network cable e The calculation formula;

[0084] Step S45: Calculate the real-time temperature of the armor layer based on the collected real-time temperature of the outer surface of the distribution network cable and the real-time load of the distribution network cable.

[0085]

[0086]

[0087]

[0088] Step S46: Substitute the collected real-time temperature of the outer surface of the distribution cable, the real-time temperature of the environmental boundary, and the calculated real-time temperature of the armor layer into the calculation formula for the equivalent thermal parameters of the external environment of the distribution cable to calculate the equivalent thermal resistance T of the external environment of the distribution cable. e The equivalent heat capacity C of the external environment of the distribution network cable e T e and C eThese are the key parameters of the differential equations for the temperature nodes of the distribution network cables in step S3. The background section also mentioned the shortcomings of existing methods for obtaining these two parameters. The emergency time is calculated by solving the differential equations for the temperature nodes of the distribution network cables, therefore, accurately calculating T... e and C e This is an important prerequisite for realizing dynamic thermal assessment of cables under current operating conditions.

[0089] In step S5, the maximum limit of θ1 in the temperature node differential equations of the dynamic thermal assessment model for distribution network cables is set to 90℃. The magnitude of the power supply load on the distribution network cables is varied. Considering the maximum limit of θ1, the temperature node differential equations are solved using MATLAB to obtain the emergency response time of the distribution network cables under different power supply load demands, thus assessing the emergency response capability of the distribution network cables under current operating conditions. It is understood that conductor temperature θ1 is the only factor affecting cable current carrying capacity, so here θ1 is limited to not exceeding 90 degrees.

[0090] Specifically, firstly, a maximum limit value for conductor temperature is set; then, the temperature node differential equation set of the dynamic thermal assessment model for distribution network cables is used to calculate the dynamic conductor temperature response under different load conditions; when the calculated conductor temperature reaches the maximum limit value, the corresponding time is the emergency time.

[0091] Corresponding to the emergency current-carrying capacity assessment method for distribution network cables provided in Embodiment 1 of the present invention, Embodiment 2 of the present invention provides an emergency current-carrying capacity assessment device for distribution network cables, comprising:

[0092] The data acquisition module is used to collect the real-time temperature of the outer surface of the distribution network cable, the real-time temperature of the environmental boundary, and the real-time load of the distribution network cable, respectively.

[0093] The model building module is used to build an equivalent thermal circuit model that characterizes the heat transfer process between the distribution network cable and the external environment, and a transient thermal circuit model that characterizes the internal heat transfer process of the distribution network cable. Based on the transient thermal circuit model of the distribution network cable and the equivalent thermal circuit model of the external environment, a dynamic thermal evaluation model of the distribution network cable is built.

[0094] The first calculation module is used to write a set of temperature node differential equations for the established dynamic thermal evaluation model of distribution network cables based on thermoelectric analogy theory.

[0095] The second calculation module is used to use the real-time temperature of the outer surface of the distribution cable, the real-time temperature of the environmental boundary, and the real-time load of the distribution cable as input variables of the port network, and to solve the temperature node differential equations of the dynamic thermal assessment model of the distribution cable to obtain the equivalent thermal resistance parameters and equivalent heat capacity parameters of the external environment of the distribution cable.

[0096] The third calculation module is used to solve the temperature node differential equations using MATLAB based on the maximum limit value of the cable conductor temperature, so as to obtain the emergency time corresponding to the distribution network cable under different power transfer load requirements.

[0097] Furthermore, the acquisition module is specifically used for: uniformly arranging three temperature measuring points at 120° radial intervals on the outer surface of the distribution cable, and using the average value of the measurement results of the three surface temperature measuring points as the real-time temperature monitoring data of the outer surface of the distribution cable; arranging a temperature measuring point in the external environmental medium at 3m intervals below the distribution cable, and using the measurement result of this temperature measuring point as the real-time temperature monitoring data of the environmental boundary; and using the measurement result of the CT of the distribution cable as the measurement data of the temperature of the distribution cable load.

[0098] Furthermore, the model building module is specifically used to merge the heat flow sources and parallel heat capacities at the same node in the transient thermal circuit model of the distribution network cable to obtain a simplified transient thermal circuit model of the distribution network cable; then, the simplified transient thermal circuit model of the distribution network cable and the equivalent thermal circuit model of the external environment are connected in series to obtain the dynamic thermal evaluation model of the distribution network cable.

[0099] Furthermore, the temperature node differential equations are shown below:

[0100]

[0101] Where θ0 is the surface temperature of the cable outer sheath, θ1 is the temperature of the cable conductor, θ2 is the temperature of the outer surface of the insulation and the metal shielding layer, θ3 is the temperature of the armor layer, T1 is the thermal resistance of the insulation layer of a single conductor, T2 is the thermal resistance of the filler layer and the inner sheath, T3 is the thermal resistance of the outer sheath, Q1 is the total heat flow into the conductor node, Q2 is the total heat flow into the metal sheath node, Q3 is the total heat flow into the armor node, C1 is the equivalent heat capacity of the conductor and the insulation layer, C2 is the equivalent heat capacity of the metal sheath and the filler layer, C3 is the equivalent heat capacity of the armor layer and the outer sheath, and t is time.

[0102] Furthermore, the second calculation module is specifically used for:

[0103] Establish the heat flow balance equation for the surface temperature node of the outer sheath of the distribution network cable;

[0104]

[0105] Set a time interval and select three adjacent time points t0, t1, and t2;

[0106] The heat flow balance equation is transformed into a set of equations related to three adjacent time points t0, t1, and t2 using the finite difference method.

[0107]

[0108]

[0109] Based on the transformed set of equations, the equivalent thermal resistance T of the distribution network cable's external environment is derived. e The equivalent heat capacity C of the external environment of the distribution network cable e The calculation formula;

[0110]

[0111] The real-time temperature of the armor layer is calculated based on the collected real-time temperature of the outer surface of the distribution network cable and the real-time load of the distribution network cable.

[0112]

[0113]

[0114]

[0115] The real-time temperature of the outer surface of the distribution cable, the real-time temperature of the environmental boundary, and the calculated real-time temperature of the armor layer are substituted into the calculation formula of the equivalent thermal parameters of the external environment of the distribution cable to calculate the equivalent thermal resistance T of the external environment of the distribution cable. e The equivalent heat capacity C of the external environment of the distribution network cable e .

[0116] Furthermore, the third calculation module is specifically used to: set the maximum limit value of θ1 in the temperature node differential equation set of the dynamic thermal assessment model of the distribution network cable, change the size of the power supply load of the distribution network cable, and use MATLAB to solve the temperature node differential equation set under the premise of considering the maximum limit value of θ1 to obtain the emergency time corresponding to the distribution network cable under different power supply load requirements.

[0117] For the working principle and process of this embodiment, please refer to the description of the aforementioned Embodiment 1 of the present invention, which will not be repeated here.

[0118] As can be seen from the above description, compared with the prior art, the beneficial effects of the present invention are as follows: Based on real-time monitoring of the outer surface temperature and load data of distribution network cables and environmental boundary temperature data, the present invention realizes accurate thermal calculation of the conductor temperature of distribution network cables, thereby completing the assessment of the maximum load capacity of distribution network cables under the current operating environment and the emergency time during power transfer operation; it can be applied to the operation site of distribution network cables under different laying conditions, providing reliable guidance for load control of distribution network cable lines and determination of maintenance time under fault conditions; it can also provide data support for the construction of intelligent distribution network cable digital twin system, thereby realizing transparent operation and maintenance of distribution network cables under the new power system.

[0119] The above description is merely a preferred embodiment of the present invention and should not be construed as limiting the scope of the invention. Therefore, any equivalent variations made in accordance with the claims of the present invention are still within the scope of the present invention.

Claims

1. A method for evaluating emergency current carrying capacity of a distribution cable, characterized in that, The application relates to a method for evaluating the dynamic heat of a power distribution network cable. The method comprises the following steps: S1, collecting the real-time temperature of the outer surface of the power distribution network cable, the real-time temperature of the environmental boundary and the real-time load of the power distribution network cable respectively; S2, establishing an equivalent thermal circuit model for representing the heat transfer process between the power distribution network cable and the external environment and an intrinsic transient thermal circuit model for representing the internal heat transfer process of the power distribution network cable, and establishing a dynamic heat evaluation model of the power distribution network cable according to the intrinsic transient thermal circuit model of the power distribution network cable and the equivalent thermal circuit model of the external environment; S3, based on the thermoelectric analogy theory, writing the temperature node differential equation set of the established dynamic heat evaluation model of the power distribution network cable; S4, taking the collected real-time temperature of the outer surface of the power distribution network cable, the real-time temperature of the environmental boundary and the real-time load of the power distribution network cable as the port network input variables, solving the temperature node differential equation set of the dynamic heat evaluation model of the power distribution network cable, and obtaining the equivalent thermal resistance parameters of the external environment of the power distribution network cable and the equivalent thermal capacity parameters of the external environment of the power distribution network cable; S5, based on the maximum limit value of the cable conductor temperature, solving the temperature node differential equation set by using MATLAB, and obtaining the emergency time corresponding to different power supply load demands of the power distribution network cable. The temperature node differential equation set is shown in the following formula: Wherein, theta0 is the cable outer sheath surface temperature, theta1 is the cable conductor temperature; theta2 is the insulation outer surface and metal shielding layer temperature; theta3 is the armored layer temperature; T1 is the single core insulation layer thermal resistance; T2 is the filling layer and inner sheath thermal resistance; T3 is the outer sheath thermal resistance; Q1 is the total heat flow flowing into the conductor node; Q2 is the total heat flow flowing into the metal sheath node; Q3 is the total heat flow flowing into the armored node; C1 is the equivalent thermal capacity of the conductor and the insulation layer; C2 is the equivalent thermal capacity of the metal sheath and the filling layer; C3 is the equivalent thermal capacity of the armored layer and the outer sheath; t is time; The step S4 specifically comprises: S41, establishing the heat flow balance equation of the temperature node of the outer sheath surface of the power distribution network cable; S42, setting a time interval, and selecting three adjacent time points t0, t1 and t2; Step S44, based on the transformed equation set, deduce the calculation formula of the equivalent thermal resistance T of the external environment of the power distribution cable e and the equivalent heat capacity C of the external environment of the power distribution cable e . S43, using the difference method to convert the heat flow balance equation into an equation set related to the three adjacent time points t0, t1 and t2; Step S46, the collected real-time temperature of the outer surface of the distribution network cable, the real-time temperature of the environmental boundary and the calculated real-time temperature of the armored layer are substituted into the calculation formula of the equivalent thermal parameter of the external environment of the distribution network cable, and the equivalent thermal resistance T of the external environment of the distribution network cable is calculated e and the equivalent heat capacity C of the external environment of the distribution network cable e .

2. The method of claim 1, wherein, S45, based on the collected real-time temperature of the outer surface of the power distribution network cable and the real-time load of the power distribution network cable, calculating the real-time temperature of the armored layer; In the step S1, three temperature measuring points are uniformly arranged at an interval of 120 degrees along the radial direction of the outer surface of the power distribution network cable, and the average value of the measurement results of the three surface temperature measuring points is taken as the monitoring data of the real-time temperature of the outer surface of the power distribution network cable; one temperature measuring point is arranged in the external environment medium below the power distribution network cable at an interval of 3m, and the measurement result of the temperature measuring point is taken as the monitoring data of the real-time temperature of the environmental boundary; 3. The method of claim 1, wherein, The measurement result of the power distribution network cable CT is taken as the measurement data of the temperature of the load of the power distribution network cable. In the step S2, the heat flow source and the parallel heat capacity on the same node in the intrinsic transient thermal circuit model of the power distribution network cable are combined to obtain a simplified intrinsic transient thermal circuit model of the power distribution network cable; Then, the simplified intrinsic transient thermal circuit model of the power distribution network cable and the equivalent thermal circuit model of the external environment are connected in series to obtain the dynamic heat evaluation model of the power distribution network cable.

4. The method of claim 1, wherein, The step S5 specifically comprises: setting a maximum limit value of θ1 in a temperature node differential equation set of the dynamic thermal evaluation model of the power distribution network cable, changing the size of the power supply load of the power distribution network cable, and solving the temperature node differential equation set by using MATLAB under the premise of considering the maximum limit value of θ1 to obtain the emergency time of the power distribution network cable under different power supply load demands.

5. An emergency current carrying capacity evaluation device for distribution network cables, characterized in that, The method comprises the following steps: The acquisition module is configured to acquire real-time temperature of an outer surface of the power distribution network cable, real-time temperature of an environmental boundary, and real-time load of the power distribution network cable. The model establishment module is configured to establish an equivalent thermal circuit model representing heat transfer process between the power distribution network cable and an external environment, and an intrinsic transient thermal circuit model representing internal heat transfer process of the power distribution network cable, and establish a dynamic thermal evaluation model of the power distribution network cable according to the intrinsic transient thermal circuit model of the power distribution network cable and the equivalent thermal circuit model of the external environment. The first calculation module is configured to write a temperature node differential equation set for the established dynamic thermal evaluation model of the power distribution network cable based on a thermoelectric analogy theory. The second calculation module is configured to take the acquired real-time temperature of the outer surface of the power distribution network cable, the real-time temperature of the environmental boundary, and the real-time load of the power distribution network cable as port network input variables, solve the temperature node differential equation set of the dynamic thermal evaluation model of the power distribution network cable, and obtain equivalent thermal resistance parameters of the external environment of the power distribution network cable and equivalent thermal capacity parameters of the external environment of the power distribution network cable. The third calculation module is configured to solve the temperature node differential equation set by using MATLAB based on a maximum limit value of cable conductor temperature, and obtain the emergency time of the power distribution network cable under different power supply load demands. The temperature node differential equation set is shown in the following formula: Wherein, θ0 is a cable outer sheath surface temperature, θ1 is a cable conductor temperature, θ2 is an insulation outer surface and metal shielding layer temperature, θ3 is an armored layer temperature, T1 is a single core insulation layer thermal resistance, T2 is a filling layer and inner sheath thermal resistance, T3 is an outer sheath thermal resistance, Q1 is a total heat flow flowing into the conductor node, Q2 is a total heat flow flowing into the metal sheath node, Q3 is a total heat flow flowing into the armored node, C1 is an equivalent thermal capacity of the conductor and the insulation layer, C2 is an equivalent thermal capacity of the metal sheath and the filling layer, C3 is an equivalent thermal capacity of the armored layer and the outer sheath, and t is time. The second calculation module is specifically configured to: establish a heat flow balance equation of the temperature node of the outer sheath surface of the power distribution network cable; set a time interval, and select three adjacent time points t0, t1 and t2; convert the heat flow balance equation into an equation set related to the three adjacent time points t0, t1 and t2 by using a difference method; Based on the transformed equation set, the calculation formula of the equivalent thermal resistance T of the external environment of the distribution network cable e and the equivalent thermal capacity C of the external environment of the distribution network cable e is derived. calculate the real-time temperature of the armored layer based on the acquired real-time temperature of the outer surface of the power distribution network cable and the real-time load of the power distribution network cable. The collected real-time temperature of the outer surface of the distribution network cable, the real-time temperature of the environmental boundary and the calculated real-time temperature of the armored layer are substituted into the calculation formula of the equivalent thermal parameters of the external environment of the distribution network cable to obtain the equivalent thermal resistance T e and the equivalent thermal capacity C e of the external environment of the distribution network cable.

6. The device for evaluating emergency current carrying capacity of a power distribution cable according to claim 5, wherein The acquisition module is specifically configured to: uniformly arrange three temperature measuring points on the outer surface of the power distribution network cable at an interval of 120° along the radial direction, take an average value of measurement results of the three surface temperature measuring points as monitoring data of the real-time temperature of the outer surface of the power distribution network cable, arrange a temperature measuring point in an external environment medium 3m below the power distribution network cable, and take a measurement result of the temperature measuring point as monitoring data of the real-time temperature of the environmental boundary. Take a measurement result of the power distribution network cable CT as measurement data of the temperature of the power distribution network cable load.

7. The device for evaluating emergency current carrying capacity of a power distribution cable according to claim 5, wherein The model establishing module is specifically configured to combine the heat flow source and the parallel heat capacity on the same node in the body transient thermal circuit model of the distribution network cable, and obtain a simplified body transient thermal circuit model of the distribution network cable. The simplified body transient thermal circuit model of the distribution network cable and an equivalent thermal circuit model of the external environment are connected in series to obtain a dynamic thermal evaluation model of the distribution network cable.

8. The device for evaluating emergency current-carrying capacity of a power distribution cable according to claim 5, characterized by, The third calculation module is specifically configured to set a maximum limit value of θ1 in a temperature node differential equation set of the dynamic thermal evaluation model of the distribution network cable, change the size of the power transfer load of the distribution network cable, and solve the temperature node differential equation set by using MATLAB under the premise of considering the maximum limit value of θ1 to obtain the emergency time of the distribution network cable corresponding to different power transfer load demands.

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