Analysis method and device for impedance mismatch and imbalance characteristics of hybrid operation system based on XLPE cable aging and medium

Abstract

The invention provides an XLPE cable aging-based mixed operation system impedance mismatch and imbalance characteristic analysis method, equipment and a medium, and belongs to the technical field of cable aging analysis. Equivalence simplification is performed on a power grid in a simulation system, switching stations and transformer substations are reserved, and a cable structure model is constructed; cable structure parameters are obtained, three-phase unbalanced current-carrying simulation conditions are set, operation parameters are obtained, a cable finite element thermal field analysis model is constructed, cable thermal field analysis data are input, and cable burying depth and surrounding parameters are set; comparing and analyzing the temperature distribution difference, acquiring temperature data of the inner and outer surfaces of the insulating layer, and calculating an average temperature difference value of the insulating layer under the two working conditions; based on the average temperature difference value, the influence degree of the single-phase power-on working condition on the cable insulation life compared with the symmetrical operation working condition is quantified. And according to an evaluation result, a more reasonable cable maintenance plan and replacement period are formulated, faults caused by insulation aging are prevented in advance, and safe and reliable operation of a power system is guaranteed.

Description

Technical Field

[0001] The present invention belongs to the technical field of cable aging analysis, and particularly relates to an analysis method, device and medium for impedance mismatch and unbalance characteristics of a hybrid operation system based on XLPE cable aging. Background Art

[0002] With the deep promotion of the construction of the energy Internet, the cross-sea power grid connection project, as a key infrastructure for optimizing energy resource allocation and improving regional power supply reliability, has received extensive attention in terms of its technological evolution and safe operation. As one of the important equipment for offshore wind power transmission, power supply to remote islands and regional power grid interconnection, the demand scale of submarine cables has been continuously expanding. In the submarine cable project, the line often spans dozens or even hundreds of kilometers, involving huge resource and capital investments, and its construction and operation and maintenance costs will greatly affect the overall life cycle benefits of the project. Cross-linked polyethylene (XLPE) insulated cables are gradually replacing traditional oil-filled cables due to their environmental protection performance and excellent electrical characteristics and are being applied to newly built submarine power transmission projects. However, in actual projects, there is often a transitional stage where new and old technologies coexist, and the influence mechanism of the heterogeneous hybrid operation conditions of the cable system on the system stability has not been effectively analyzed.

[0003] There is an obvious phenomenon of disciplinary separation in current research: the materials science field focuses on the aging mechanism and life prediction of XLPE cables, while the power system field focuses on the impact of three-phase unbalance on system stability, and the research on the coupling effect between the two is relatively lacking. Under hybrid operation conditions, the impedance mismatch and length mismatch act together on the system unbalance characteristics, and the dynamic modulation of the system impedance parameters during the aging process further complicates the problem. At present, there is a lack of in-depth analysis of the interaction mechanism between cable aging and system unbalance under hybrid operation conditions, and it is impossible to quantify the impact of unbalanced operation conditions on the cable insulation life, resulting in the inability of operation and maintenance personnel to know the cable insulation aging state and the inability to ensure the safe and reliable operation of the power system. Summary of the Invention

[0004] The present invention provides an analysis method for impedance mismatch and unbalance characteristics of a hybrid operation system based on XLPE cable aging. According to the evaluation results, the method formulates a more reasonable cable maintenance plan and replacement cycle, and prevents in advance the faults caused by insulation aging, so as to ensure the safe and reliable operation of the power system.

[0005] The method includes: S101: Equivalently simplify the power grid in the simulation system, retain the switch stations and substations, equivalently simplify the rest of the power grid to the retained stations, and construct a structural model of a three-core cross-linked polyethylene cable and an oil-filled cable; S102: Obtain the structural parameters of the three-core cross-linked polyethylene cable and the oil-filled cable, including the nominal thickness, outer diameter and electrical characteristics of each layer of material, and form an electrical parameter table of the cable; S103: Set the simulation conditions for unbalanced three-phase current-carrying, configure the A phase with a three-core cross-linked polyethylene cable and the BC phases with oil-filled cables, set the transmission power of any one phase under full-load conditions, obtain the operating parameters when the single-phase current in the three-core cable reaches the preset current value, and compare the current differences of each phase under symmetrical conditions; S104: Construct a finite element thermal field analysis model for the cable. Based on the current parameters obtained from real-time simulation, establish a three-dimensional thermal field analysis model for the three-core cross-linked polyethylene cable, input the cable thermal field analysis data, and set the cable burial depth and the thermal conduction parameters of the surrounding seabed saturated sediment and seawater; S105: Compare and analyze the temperature distribution differences. Respectively simulate the single-phase power-on and three-phase symmetrical operation conditions in the thermal field model, obtain the temperature data on the inner and outer surfaces of the insulation layer, and calculate the average temperature difference of the insulation layer under the two conditions; S106: Based on the average temperature difference, use the Arrhenius equation to analyze the change in the aging rate of the insulation material, and quantify the influence degree of the single-phase power-on condition on the cable insulation life compared with the symmetrical operation condition.

[0006] Furthermore, it should be noted that step S101 specifically includes: constructing a hybrid operation power grid model including a 500 kV cable line, an overhead line, and a substation on the real-time digital simulation system platform. By retaining the 500 kV W switch station, A substation, X substation on the left and the 500 kV F substation, Z switch station, H substation on the right and equivalent processing the remaining power grid parts, setting the equivalent additional lines between the W switch station and the A substation, and between the Z switch station and the H substation, and at the same time inputting the conductor structure, insulation layer configuration, and sheath component parameters of the three-core cross-linked polyethylene cable and the oil-filled cable to establish a three-dimensional cable structure model.

[0007] Furthermore, it should be noted that step S102 specifically includes: obtaining the structural parameters of the three-core cross-linked polyethylene cable and the oil-filled cable, including the nominal thickness, outer diameter, and electrical characteristics of each layer of material, establishing a parameter table of geometric dimensions and material properties including the conductor shielding layer, insulation layer, sheath layer, and armor layer based on the cable structure layer data, and generating a comparison table of positive sequence, negative sequence, and zero sequence electrical parameters for different cable types.

[0008] Furthermore, it should be noted that step S103 specifically includes: Configure the rated voltage and copper core cross-sectional area parameters of the A-phase three-core cross-linked polyethylene cable and the rated voltage of the BC-phase oil-filled cable in the simulation system, and set the rated transmission power of any one phase under full-load conditions, obtain the current in the three-core cross-linked polyethylene cable during hybrid operation, and compare the difference in the current amplitudes of each phase when only using the three-core cross-linked polyethylene cable under normal power-on and symmetrical conditions.

[0009] Furthermore, it should be noted that step S103 also includes: Based on the three-phase phase voltage and current data obtained in real-time simulation, the positive sequence, negative sequence, and zero sequence components are obtained through Clarke and Fortescue sequence component transformations; Calculate the negative sequence voltage and zero sequence voltage unbalance degrees based on formula (1); (1) In the above formula, 、 are the root mean square values of the positive sequence and negative sequence voltages respectively; Calculate the negative sequence current and zero sequence current unbalance degrees based on formula (2); (2) In the above formula, 、 are the root mean square values of the positive sequence and negative sequence voltages respectively.

[0010] Furthermore, it should be noted that step S104 also includes: establishing a three-dimensional thermal field analysis model of a three-core cross-linked polyethylene cable, obtaining the temperature distribution characteristics of the insulation layer during unbalanced operation, and quantifying the temperature difference of the insulation layer compared with that during three-phase symmetrical operation; The copper conductor of the three-core cross-linked polyethylene cable is set to have a linear resistivity, and the resistivity is calculated using formula (3); (3) In the formula is the reference temperature T ref of the resistivity, is the temperature coefficient of resistivity.

[0011] Furthermore, it should be noted that step S105 also includes: Taking the current parameters obtained from the simulation system as boundary conditions and inputting them into the established three-dimensional thermal field analysis model of the three-core cross-linked polyethylene cable, and respectively simulating the single-core energization condition and the three-phase symmetrical operation condition in the thermal field model; Obtaining the temperature data near the conductor side and outside of the insulation layer under the two conditions; Calculating the average temperature of the insulation layer under the two conditions respectively through the logarithmic mean temperature calculation method of a cylindrical insulator, and obtaining the difference in the average temperature of the insulation layer between the single-core operation condition and the three-phase symmetrical operation condition.

[0012] Furthermore, it should be noted that the logarithmic mean temperature calculation formula of a cylindrical insulator is:

[0013] Among them, is the internal temperature of the insulation layer, is the external temperature of the insulation layer; According to the Arrhenius equation:

[0014] Wherein, is the insulation life, is the activation energy, k is the Boltzmann constant, T is the absolute temperature.

[0015] According to another embodiment of the present application, there is provided an electronic device, including a memory, a processor, and a computer program stored on the memory and executable on the processor. When the processor executes the program, the steps of the analysis method for impedance mismatch and unbalance characteristics of the hybrid operation system based on XLPE cable aging are implemented.

[0016] According to still another embodiment of the present application, there is further provided a storage medium, on which a computer program is stored. When the computer program is executed by a processor, the steps of the analysis method for impedance mismatch and unbalance characteristics of the hybrid operation system based on XLPE cable aging are implemented.

[0017] From the above technical solutions, it can be seen that the present invention has the following advantages: The analysis method for impedance mismatch and unbalance characteristics of the hybrid operation system based on XLPE cable aging provided by the present application simplifies the power grid equivalent, reduces the system complexity while retaining the electrical characteristics of key nodes, reduces the consumption of simulation computing resources, and improves the modeling efficiency; constructs an actual engineering cable structure model to make the simulation close to the real scenario. Obtain the structural parameters and electrical characteristics of each layer of the cable to form a standardized electrical parameter table, providing a quantitative basis for the electrical performance analysis of the hybrid cable system and ensuring that the simulation results can reflect the electrical conduction, insulation and other characteristics of the actual cable. Set the three-phase unbalanced current-carrying condition and compare it with the symmetric condition to quantify the current difference of the cable under different operation modes and reveal the current distribution law of the hybrid cable system under unbalanced conditions. Establish a three-dimensional thermal field analysis model and input multi-physical field parameters, convert the current parameters into thermal boundary conditions, realize the coupled analysis of the electromagnetic field and the thermal field, and intuitively present the temperature distribution characteristics of the cable. By comparing the temperature differences between the single-core and symmetric conditions, quantify the impact of unbalanced operation on the temperature of the cable insulation layer, discover the risk of local overheating, and provide key indicators for cable heat dissipation design and operation monitoring. Use the Arrhenius equation to convert the temperature difference into a quantitative analysis of the insulation aging rate, establish a "temperature-aging-life" correlation model, scientifically evaluate the impact of unbalanced conditions on the cable life, form a quantitative analysis correlation relationship from current difference to temperature distribution, and then to the impact on insulation life, avoid the ambiguity of traditional qualitative analysis, and improve the scientificity and credibility of the evaluation results. Identify in advance the risks of current concentration, local overheating and accelerated insulation aging caused by unbalanced operation. Brief Description of the Drawings

[0018] In order to more clearly illustrate the technical solutions of the present invention, the drawings required in the description will be briefly introduced below. Obviously, the drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can be obtained based on these drawings.

[0019] Figure 1 It is a flowchart of an analysis method for impedance mismatch and unbalance characteristics of a hybrid operation system based on XLPE cable aging. Figure 2 It is a simplified system wiring diagram after equivalence. Figure 3 It is a structural schematic diagram of a three-core cable. Figure 4 It is a structural schematic diagram of an oil-filled cable. Figure 5 It is a schematic diagram of the unbalance degree of the hybrid operation system when not aged. Figure 6 It is a schematic diagram of the temperature distribution of hybrid operation and symmetric operation. Among them, Figure 6 a is a schematic diagram of a three-core cable in symmetric operation, and b is a schematic diagram of a three-core cable with only one core energized during hybrid operation. Figure 7 It is a schematic diagram of the change in dielectric properties after XLPE thermal aging. Among them, Figure 7 (a) is a schematic diagram of the variation law of the dielectric constant with frequency, and (b) is a schematic diagram of the variation law of the dielectric loss with frequency. Figure 8 It is a schematic diagram of the evolution trend of the negative-sequence unbalance degree at each measurement point of the system. Figure 9 It is a schematic diagram of the relationship between the growth rate characteristic of the negative-sequence unbalance degree and the distance. Figure 10 It is a schematic diagram of a three-dimensional mutual feedback relationship model of aging-impedance mismatch-unbalance degree. Figure 11 It is a schematic diagram of an electronic device. Detailed Embodiments

[0020] The analysis method of impedance mismatch and unbalance characteristics of a hybrid operation system based on XLPE cable aging in this application constructs an electric field-thermal field-aging analysis framework to explore the influence mechanism of aging on unbalance in the hybrid operation system. An accurate simulation model of the hybrid operation system is established based on actual engineering parameters, and high-precision calculations of power grid power flow and transient characteristics are realized through a real-time digital simulation system platform. Then, a cable thermal field distribution model is constructed using finite element software to analyze the temperature gradient characteristics under different working conditions. A targeted accelerated aging experiment is designed to obtain the degradation law of key parameters of XLPE materials under unbalanced electrical stress, and a mathematical model of the aging-parameter relationship is established. This application proposes an index of hybrid operation impedance mismatch coefficient, revealing the influence mechanism of XLPE cable aging on unbalance.

[0021] The following will describe in detail the specific steps of the analysis method of impedance mismatch and unbalance characteristics of a hybrid operation system based on XLPE cable aging involved in this application. For the purpose of illustration rather than limitation, specific details such as specific system structures and technologies are proposed to thoroughly understand the embodiments of this application. However, those skilled in the art should clearly understand that this application can also be implemented in other embodiments without these specific details.

[0022] It should be understood that when used in the specification of this application, the term "comprising" indicates the presence of the described features, wholes, steps, operations, elements, and / or components, but does not exclude the presence or addition of one or more other features, wholes, steps, operations, elements, components, and / or their combinations. The terms "comprising", "including", "having" and their variants all mean "including but not limited to", unless otherwise specifically emphasized in other ways.

[0023] The statements such as "in one embodiment" or "in some embodiments" described in this application mean that the specific features, structures, or characteristics described in the embodiment are included in one or more embodiments of this application. Thus, the statements such as "in one embodiment", "in some embodiments", "in other some embodiments", "in still other embodiments" and the like that appear in different parts of this application do not necessarily refer to the same embodiment, but mean "one or more but not all embodiments", unless otherwise specifically emphasized in other ways.

[0024] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the scope of protection of the present invention.

[0025] Please refer to Figure 1The following is a flowchart of an analysis method for impedance mismatch and unbalance characteristics of a hybrid operation system based on XLPE cable aging in a specific embodiment. The method includes: Step S101: Simplify the power grid equivalently in the simulation system, retain switch stations and substations, equivalent the remaining power grid to the retained stations, and construct a structural model of a three-core cross-linked polyethylene cable and an oil-filled cable.

[0026] In some embodiments, as Figure 2 shown, in the simulation system, based on actual data, equivalently simplify the power grid architecture. On the left side of the power grid, completely retain the 500kV W switch station, 500kV A substation, and 500kV X substation, and simplify and equivalent the remaining part of the power system to the W switch station and the A substation; on the right side of the power grid, retain the 500kV F substation, 500kV Z switch station, and 500kV H substation, equivalent the remaining 500kV power grid to the Z switch station and the H substation, and uniformly equivalent the 220kV power grid to the F substation. At the same time, construct a structural model of a three-core cross-linked polyethylene cable with a rated voltage of 500kV and a copper core cross-section parameter of 3×1000 + 2×48 (steel armor) for phase A, and construct a structural model of an oil-filled cable with a rated voltage of 500kV for phases B and C.

[0027] This embodiment can simplify the complex power grid according to the principle of equivalent circuit transformation on the premise of ensuring the unchanged electrical characteristics of key nodes. By analyzing the influence of each part of the power grid on the power, voltage, etc. of key stations, substitute the simplified part with equivalent power sources, impedances and other components, so that the complex system is transformed into a model convenient for analysis and calculation. Constructing the cable structural model is based on the physical structure and material characteristics of the cable, and transforms it into a digital model that can be recognized and processed by the simulation system.

[0028] Step S102: Obtain the structural parameters of the three-core cross-linked polyethylene cable and the oil-filled cable, including the nominal thickness, outer diameter and electrical characteristics of each layer of material, and form an electrical parameter table of the cable.

[0029] In some embodiments, for the constructed structural models of three-core cross-linked polyethylene cables and oil-filled cables, detailed structural parameters of each layer of materials are comprehensively obtained. For example, the three-core cross-linked polyethylene cable covers the combined outer diameter of the copper conductor and the water-blocking tape, the thickness and outer diameter of the semi-conductive double-sided water-blocking binding tape and the inner shield, etc.; the oil-filled cable includes the oil duct diameter, conductor size, etc. Referring to national standards, international standards, and actual engineering data, the electrical characteristics of the two types of cables are determined, such as parameters like resistance, reactance, capacitance, etc. Finally, an electrical parameter table containing cable structure, length, positive sequence, negative sequence, and zero sequence parameters is formed. It can accurately reflect the electrical behavior of the cable during actual operation, provide a reliable basis for studying the power flow distribution, voltage and current characteristics of the power grid, and improve the accuracy and credibility of the simulation results.

[0030] Step S103: Set the simulation conditions for unbalanced three-phase current-carrying. Configure phase A with a three-core cross-linked polyethylene cable and phases B and C with an oil-filled cable. Set the transmission power of any one phase under full-load conditions, obtain the operating parameters when the single-phase current in the three-core cable reaches the preset current value, and compare the current differences of each phase under symmetrical conditions.

[0031] In some embodiments, in the simulation system, according to the actual engineering topology structure and equipment parameters, the simulation conditions for unbalanced three-phase current-carrying are set. Specifically, configure phase A with a three-core cross-linked polyethylene cable and phases B and C with an oil-filled cable, and set the rated transmission power of any one phase under full-load conditions to 200 MW. After running the simulation, obtain the operating parameters when the single-phase current in the three-core cross-linked polyethylene cable reaches 858.4 A during mixed operation. At the same time, simulate the symmetrical condition of normal power-on using only the three-core cross-linked polyethylene cable, obtain the data of the current amplitude of each phase as 866.7 A, and conduct a comparative analysis of the current conditions under the two conditions. In this way, by setting different conditions and obtaining current parameters, the current characteristics of the hybrid cable system under unbalanced operating conditions can be studied, and the influence of unbalanced operation on the cable current can be clearly understood by comparing with the symmetrical condition.

[0032] Step S104: Construct a finite element thermal field analysis model of the cable. Based on the current parameters obtained from real-time simulation, establish a three-dimensional thermal field analysis model of the three-core cross-linked polyethylene cable, input the cable thermal field analysis data, and set the cable burial depth and the thermal conductivity parameters of the surrounding submarine saturated sediment and seawater.

[0033] In some embodiments, the cable thermal field analysis data includes but is not limited to conductor conductivity, insulation layer dielectric constant, thermal conductivity coefficients of various materials, and constant pressure heat capacity parameters.

[0034] Taking the current parameters obtained from real-time simulation as the boundary conditions, build a three-dimensional thermal field analysis model of the three-core cross-linked polyethylene cable on the finite element simulation platform. Sequentially input the thermal field analysis data of each part of the cable into the model, such as the conductivity of the copper conductor being 6×10 7S / m, relative permittivity of XLPE insulation is 2.3, etc.; at the same time, the burial depth of the cable is set to 1.0 m, and the thermal conductivity parameters of the surrounding environment such as the thermal conductivity of saturated sedimentary soil at the seabed of 1.0 W / (m·K) and the thermal conductivity of seawater of 0.6 W / (m·K) are set.

[0035] In this embodiment, a three-dimensional thermal field analysis model of the cable is established and the parameters are accurately set, which can simulate the temperature change of the cable in the actual operating environment, and provide an intuitive and accurate analysis method for studying the temperature characteristics of the cable insulation layer, evaluating the thermal stability and life of the cable.

[0036] Step S105: Compare and analyze the temperature distribution differences. In the thermal field model, simulate the single-phase power-on and three-phase symmetric operation conditions respectively, obtain the temperature data of the inner and outer surfaces of the insulation layer, and calculate the average temperature difference of the insulation layer under the two conditions.

[0037] In some embodiments, the current parameters output by the real-time digital simulation system are imported into the established three-dimensional thermal field analysis model of the three-core cross-linked polyethylene cable, and the single-core power-on condition and the three-phase symmetric operation condition are simulated respectively. The temperature data near the conductor side inside and outside the cable insulation layer under the two conditions are obtained. The internal temperature of the insulation layer under the three-phase symmetric operation condition is 84.9 °C, and the external temperature is 79.1 °C. The internal temperature of the insulation layer under the single-core operation condition is 87.8 °C, and the external temperature is 80.5 °C. Using the logarithmic mean temperature calculation method of the cylindrical insulator, the average temperature of the insulation layer under the two conditions is calculated respectively, and it is obtained that the average temperature of the insulation layer under the single-core operation condition is about 2.3 °C higher than that under the three-phase symmetric operation condition. By comparing and analyzing the temperature distribution differences of the cable insulation layer under different conditions, the influence of unbalanced operation on the cable temperature can be intuitively understood.

[0038] Step S106: Based on the average temperature difference, use the Arrhenius equation to analyze the change of the insulation material aging rate, and quantify the influence degree of the single-phase power-on condition on the cable insulation life compared with the symmetric operation condition.

[0039] In this embodiment, based on the average temperature difference of the insulation layer obtained in step S105, the Arrhenius equation is used to establish a quantitative relationship between temperature and the insulation material aging rate. By substituting relevant parameters, the aging rate of the cable insulation material under the single-phase power-on condition compared with the symmetric operation condition is calculated, and then the influence degree of this unbalanced operation condition on the cable insulation life is quantified.

[0040] The Arrhenius equation here describes an exponential relationship between the aging rate and temperature in the cable insulation aging, that is, the increase in temperature will accelerate the aging reaction of the insulation material. By substituting the temperature data under different conditions into the equation, the change of the aging rate is calculated, so as to evaluate the influence on the insulation life.

[0041] It can be seen that understanding the impact of unbalanced operation conditions on cable insulation life can provide a scientific basis for the operation and maintenance of power systems. Based on the evaluation results, maintenance personnel can formulate more reasonable cable maintenance plans and replacement cycles, prevent faults caused by insulation aging in advance, ensure the safe and reliable operation of power systems, and reduce operation and maintenance costs and power outage risks.

[0042] Furthermore, as a refinement and extension of the specific implementation of the above embodiments, to fully illustrate the specific implementation process in this embodiment, the analysis method for impedance mismatch and unbalance characteristics of the hybrid operation system based on XLPE cable aging further includes the following steps: Combined with Figure 2 , based on actual engineering data, a detailed model of relevant cable lines, overhead lines, and related substations in the Hainan Interconnection Project is built on the real-time digital simulation system platform. For the convenience of simulation calculation and research, the power grid in the actual project is equivalently simplified. On the left, the 500 kV W Switching Station, 500 kV A Substation, and 500 kV X Substation are retained, and the rest of the system is equivalent to the W Switching Station and A Substation; on the right, the 500 kV F Substation, 500 kV Z Switching Station, and 500 kV H Substation are retained, and the rest of the 500 kV power grid is equivalent to the Z Switching Station and H Substation, and the 220 kV power grid is equivalent to the F Substation. The lines between the W Switching Station and A Substation, and between the Z Switching Station and H Substation are all equivalent additional lines.

[0043] In the simulation system, for phase A, a three-core cross-linked polyethylene cable with a rated voltage of 500 kV and a copper core cross-sectional parameter of 3×1000 + 2×48 (steel sheath) is selected, as Figure 3 is the basic structure schematic diagram of the three-core cable, and Table 1 gives the three-core integrated structure parameters of the 500 kV AC cross-linked polyethylene cable.

[0044] Figure 3 involves that the three-core cross-linked polyethylene cable has three cores. Among them, the cable core includes a copper conductor + water-blocking tape 1, a semi-conductive double-sided water-blocking binding tape + inner shield 2, XLPE insulation 3, an outer shield 4, a semi-conductive water-blocking tape 5, an alloy lead sheath 6, and an MDPE sheath 7. The three-core cross-linked polyethylene cable internally has a filling material 8 and an optical fiber unit 12. The three-core cross-linked polyethylene cable also has an inner lining layer 9, an armor layer 10, and an outer covering layer 11.

[0045] Table 1

[0046] In the simulation system, for phases B and C, an oil-filled cable with a rated voltage of 500 kV is selected. The oil-filled cable adopts an oil-filled insulation structure and is suitable for complex marine environmental conditions of high-voltage and large-capacity AC power transmission. As Figure 4Shows a schematic diagram of the basic structure of an oil-filled cable. The oil-filled cable includes, from the inside to the outside: an oil duct 1, a conductor 2, an insulating layer 3, alloy lead 4, an inner lining layer 5, a polyethylene sheath 6, an outer lining layer 7, armor 8, and an outer sheath 9. Table 2 shows the structural parameters of a 500 kV oil-filled cable.

[0047] Table 2

[0048] After inputting the above structural parameters into the simulation model, the comparison of the electrical parameters of the two cables is shown in Table 3.

[0049] Table 3

[0050] For the modeling requirements of transformers, high reactors, and overhead lines in the engineering system, based on the topological structure and equipment parameter characteristics of the actual project, this embodiment constructs an electrical model consistent with the on-site operation scenario, and obtains that when the three-core cross-linked polyethylene cable is fully loaded during hybrid operation, that is, when any phase reaches its rated transmission power of 200 MW, the magnitude of the current in the cable is 858.4 A.

[0051] In the system, if an oil-filled cable is not used and only a three-core cross-linked polyethylene cable is normally energized, the amplitude of the current in each phase during the operation of this symmetric condition is 866.7 A.

[0052] From the three-phase phase voltage and current data obtained from the real-time simulation, through the Clarke and Fortescue sequence component transformations, the positive sequence, negative sequence, and zero sequence components can be obtained.

[0053] The unbalance degrees of the negative sequence voltage and the zero sequence voltage are respectively defined as the ratios of the negative sequence and zero sequence components divided by the positive sequence component, and are expressed in percentage form, as shown in formula (1).

[0054] (1) In formula (1), 、 are the root mean square values of the positive sequence and negative sequence voltages respectively. Calculate and analyze the voltage negative sequence and zero sequence unbalance degrees of substation X, substation F, terminal station N, and terminal station L. Similarly, the unbalance degrees of the negative sequence current and the zero sequence current are also defined as the ratios of the negative sequence and zero sequence components divided by the positive sequence component, as shown in formula (2).

[0055] (2) In the above formula, 、 are the root mean square values of the positive sequence and negative sequence voltages respectively. Calculate the unbalance degrees of the negative sequence and zero sequence of the current flowing through the cable, and plot the results together with the voltage unbalance degrees of the substation and the terminal station in Figure 5China.

[0056] For the construction method of the cable finite element model in this embodiment, the three-phase unbalanced current-carrying condition obtained from system simulation can be used as the boundary condition. Through the finite element simulation platform, a three-dimensional thermal field analysis model of the three-core XLPE cable is established to obtain the temperature distribution characteristics of the insulation layer during unbalanced operation and quantify the temperature difference of the insulation layer compared with that during three-phase symmetrical operation.

[0057] The input parameters of different parts of the cable are shown in Table 4.

[0058] Table 4

[0059] Among them, the copper conductor is set as a linear resistivity, and its resistivity is calculated using Equation (3).

[0060] (3) In the formula is the resistivity at the reference temperature T ref whose value is 1.72×10 -8 Ω·m, is the resistivity temperature coefficient, which is set to 0.0039.

[0061] According to the actual project, the buried depth of the cable is selected as 1.0 m, and its surrounding environment is mainly saturated sedimentary soil and seawater at the seabed. The thermal conductivity of the saturated sandy soil is taken as 1.0 W / (m·K), and the thermal conductivity of the seawater is taken as 0.6 W / (m·K). The seabed environment temperature is relatively stable, and the measured average seawater temperature in this area is 12°C, and the soil temperature is close to this. These parameters are used in the simulation to accurately reflect the heat conduction characteristics during cable operation and provide reliable inputs for the calculation of the temperature field.

[0062] The current parameters obtained from the real-time digital simulation system are used as boundary condition inputs to obtain the temperature distribution of the three-core cable under hybrid operating conditions, and its comparison with the temperature during normal symmetrical operation is as Figure 6 shown. Figure 6 In

[0063] From Figure 6It can be seen that when only one core of a three-core cross-linked polyethylene cable is energized and the other two cores are on no-load, the temperature of the insulation layer around the energized core is significantly higher than that under the three-phase symmetrical operation condition. This phenomenon can be attributed to two factors: the electromagnetic-thermal coupling effect and the change in the heat conduction path. At the electromagnetic-thermal coupling level, although the current passing through the conductor (858.4 A) in the single-phase energized condition is slightly smaller than the per-phase current (866.7 A) during symmetrical operation, the current density is concentrated in a single conductor, resulting in an increase in the local heat source density. More importantly, when energized symmetrically in three phases, the currents in each phase have a 120° phase difference in space, and the generated magnetic fields tend to cancel each other out in the core area of the cable, significantly reducing the induced eddy currents in the metal sheath; while in the single-phase energized condition, the lack of the magnetic field cancellation effect of other phase currents leads to a large eddy current in the cable metal sheath, and this eddy current loss is converted into an additional heat source, further increasing the temperature of the insulation layer. This additional loss can be expressed by the eddy current density formula: (5) where is the electrical conductivity of the material, is the magnetic induction intensity, is the position vector. According to the thermoelectric coupling control equation: (6) where represents the Joule heat, represents the eddy current loss, and the total heat source intensity under the single-core energized condition increases significantly, resulting in an increase in the temperature of the insulation layer.

[0064] In terms of the heat conduction mechanism, when a single core is energized, the no-load state of the other two cores causes a significant change in the originally balanced heat conduction path. The three-core symmetrical design is originally based on the principle of equal heat resistance sharing and follows Fourier's law of heat conduction: (7) where the heat flux density q is proportional to the temperature gradient , and the proportionality coefficient is the thermal conductivity k . When only one core is energized, the heat flux is forced to propagate along a non-optimal path, and the effective thermal resistance increases, which can be expressed as: (8) where is the heat path correction factor, and the value of increases significantly when a single core is energized. In addition, the skin effect and proximity effect lead to a more uneven current distribution under the single-core energized condition, increasing the heat source density gradient inside the conductor. When operating under normal symmetrical conditions, the temperature at the innermost part of the insulation layer is 84.9 °C, and the temperature at the outermost part is 79.1 °C. Using the logarithmic mean temperature formula for a cylindrical insulator: (9) wherein, is the temperature inside the insulation layer (near the conductor side), is the temperature outside the insulation layer. The calculated average temperature is about 81.9 °C. When only operating with a single core, the temperature inside its insulation layer is 87.8 °C and the outside temperature is 80.5 °C. According to the calculation of Equation (8), the average temperature is about 84.2 °C, which is about 2.3 °C higher than the symmetrical operating state. This temperature difference will significantly affect the aging rate of the insulation material during long-term operation. According to the Arrhenius equation: (10) where is the insulation life, is the activation energy, k is the Boltzmann constant, T is the absolute temperature.

[0065] In this embodiment, to verify the aging state of the cable, taking a specific experiment as an example below, the corresponding relationship between the actual operating temperature and the accelerated aging temperature is established. Based on the Arrhenius equation, the calculation formula for the acceleration factor (AF) can be deduced: (11) The activation energy of XLPE insulation material is generally in the range of 1.1 - 1.3 eV [20 - 21]. In this embodiment, the activation energy of 1.24 eV measured for the 500 kV XLPE cable insulation material is adopted. By transforming Equation (11), the experimental time required at different accelerated aging temperatures can be obtained: (12) At the aging temperature of 130 °C, the acceleration factor reaches 124, that is, aging for 1 hour at 130 °C in the experiment is equivalent to 124 hours under actual operating conditions. Based on this, the key aging time nodes are determined to be 360 h, 720 h, 1080 h, and 1440 h, corresponding to about 5.1 years, 10.2 years, 15.3 years, and 20.4 years of actual operation respectively, covering the first half of the key stage of the 40-year design life of the XLPE cable.

[0066] The selection of the temperature parameter 130 °C is based on dual theoretical considerations: on the one hand, this temperature is within the thermal oxidation aging active range (110 °C - 140 °C), which reasonably shortens the experimental period and avoids non-Arrhenius behavior caused by high temperature; on the other hand, Fourier transform infrared spectroscopy (FTIR) and thermogravimetric analysis (TGA) characterizations confirm that the microscopic mechanisms such as molecular chain scission and crosslinking network relaxation of the XLPE material at this temperature are consistent with the aging process under actual operating conditions.

[0067] The settings of the four time nodes are based on the key kinetic stages of cable insulation aging: 360 h corresponds to the critical point when the antioxidant is consumed, and the dielectric loss factor shows a linear increase during this stage; 720 h captures the inflection point of the change rate of the dielectric constant, corresponding to the acceleration period of molecular chain oxidation and fracture; 1080 h characterizes the stable stage of the free radical chain reaction; 1440 h reflects that the aging approaches the quasi-equilibrium state, and the microstructure adjustment of the insulating material tends to be stable. This experimental scheme meets the requirements of IEC 60216 standard for the thermal life assessment of insulating materials and covers the 50% critical point of the safe operation period of typical XLPE cable insulation.

[0068] Taking the 500 kV three-core cable product of a certain cable manufacturing enterprise as the reference object, the experimental XLPE insulation specimens are pressed from the same raw materials as the main insulation of the cable. The raw material components include cross-linking agent (dicumyl peroxide), polyethylene particles, and antioxidant. The materials are mixed by extrusion using a flat vulcanizing machine, and the insulating material is placed in a drying oven for drying treatment; secondly, the insulating material is preheated without pressure at 130 °C on a flat vulcanizing machine for 4 minutes; then it is pressurized at 130 °C for 6 minutes; again, it is pressurized at 180 °C for 15 minutes, and finally it is pressurized and cooled at room temperature for 8 minutes to produce insulating layer specimens with a thickness of about 0.3 mm.

[0069] The experimental results are analyzed below. The pressed samples are placed in a vacuum oven at 130 °C for aging, and after aging for 360 h, 720 h, 1080 h, and 1440 h, they are taken out and their dielectric properties are tested using a broadband dielectric spectrometer. The variation laws of the XLPE dielectric constant and dielectric loss at different aging time nodes are as Figure 7 shown. Figure 7 Figure (a) shows the schematic diagram of the variation law of the dielectric constant with frequency, Figure 7 and figure (b) shows the schematic diagram of the variation law of the dielectric loss with frequency.

[0070] As the aging time prolongs, the dielectric constant shows a gradually increasing trend. At 50 Hz, the dielectric constant increases from 2.283 before aging to 2.587 after 360 h, 2.739 after 720 h, 2.769 after 1080 h, and 2.816 after 1440 h in sequence, with the increase rates being 13.32%, 20.02%, 21.29%, and 23.37% respectively. This indicates that as the aging time prolongs, the polarization ability of the XLPE material is significantly enhanced. The reason is that in the thermal oxidation aging environment, the XLPE molecular chains break, generating polar groups such as carbonyl and ether groups. Under the action of an external electric field, these polar groups participate in polarization, enhancing the polarization performance of the material, and thus causing an increase in the dielectric constant. The longer the aging time, the more serious the molecular deterioration of the material, and the more the amount of polar groups generated, so the dielectric constant shows a law of increasing with the aging time.

[0071] The dielectric loss factor also increases significantly with aging time, especially in the low-frequency range. At 50 Hz, the dielectric loss factor increases from 0.0038 without aging to 0.0043 after 360 h, 0.0049 after 720 h, 0.0054 after 1080 h, and 0.0057 after 1440 h, with the increase rates being 13.16%, 28.95%, 42.11%, and 50.00% respectively. The change in the dielectric loss factor indicates that the loss characteristics of the material deteriorate continuously during the aging process. This phenomenon is mainly attributed to the combined action of high temperature and oxygen, which causes the XLPE molecular chains to break and generate a large number of polar groups. These polar groups not only increase the polarization loss but also the dielectric conduction loss, thus leading to an increase in the dielectric loss factor.

[0072] Regarding the influence mechanism of insulation aging on the unbalance characteristics of the hybrid operation system, the influencing factors of the unbalance degree under hybrid operation are analyzed below. In the hybrid operation system of cross-linked polyethylene (XLPE) cables and oil-filled cables, the mismatch of electrical parameters between the cables is the fundamental cause of system unbalance. Especially under the condition that there is a coupling relationship between the power flow distribution and the insulation aging characteristics, the unbalance degree shows a complex evolution law with the operation time. Based on the simulation results and experimental data, by analyzing the key driving factors of the unbalance characteristics of the hybrid system, the change mechanism under different working conditions is revealed.

[0073] In the power flow distribution model of the hybrid operation system established based on simulation, the impedance difference between the XLPE cable and the oil-filled cable directly leads to the asymmetry of the three-phase current and voltage, thus causing the continuous accumulation of negative-sequence current. The equivalent impedance per unit length of the XLPE cable is significantly higher than that of the oil-filled cable. By simulating and evaluating the operation parameters of the actual system, it can be obtained that under normal working conditions, the impedance of the oil-filled cable is while the impedance of the XLPE cable, due to its material characteristics and structural differences, is . The power carried by the XLPE cable with a larger impedance is significantly lower than that of the oil-filled cable, resulting in an inclination of the overall power distribution and ultimately causing system unbalance. To more accurately quantify this phenomenon, the "hybrid operation impedance mismatch coefficient " is defined to measure the impedance difference between the XLPE cable and the oil-filled cable: (12) The calculation results of the impedance mismatch coefficient show that although the impedance per unit length of the XLPE cable is higher than that of the oil-filled cable, the difference is relatively low under normal working conditions. However, in actual operation, due to the difference in cable lengths, this impedance mismatch effect will accumulate with the length, further exacerbating the impact on power distribution. To quantify the amplification effect of the length difference on system unbalance, the "hybrid operation length mismatch coefficient " is defined: (13) According to the calculation results, the hybrid operation length mismatch coefficient increases as the difference in the lengths of XLPE cables and oil-filled cables gradually increases. The coefficient value indicates that the impedance mismatch between the two types of cables in the hybrid system is more obvious when the length difference is large. When the length mismatch coefficient is low, the system imbalance is relatively slight, and the current distribution remains relatively balanced; when the length mismatch coefficient exceeds a certain threshold, the negative sequence current increases significantly, and the system imbalance shows a non-linear growth trend. This result shows that length mismatch has an amplifying effect on the imbalance of the hybrid operation system. In actual operation, the impedance characteristics per unit length of XLPE cables determine the sensitivity of the system to length differences. As the mismatch coefficient increases, the oil-filled cable side will carry a smaller actual current, while the burden on XLPE cables increases accordingly. This uneven distribution will directly affect the overall stability of the hybrid operation system. By calculating the length mismatch coefficient, it is possible to provide a necessary theoretical basis for the design and operation of the system, guide the reasonable matching of the cable operation lengths, and thus reduce the imbalance effect.

[0074] In a hybrid operation environment, the aging process of XLPE cables will have complex effects on the imbalance characteristics of the system. This embodiment reveals how insulation aging modulates the imbalance behavior of the hybrid operation system and establishes a mathematical relationship between cable aging parameters and system imbalance.

[0075] The imbalance at all measurement points increases with the extension of aging time. The imbalance at the L terminal station increases from the initial 0.38% to 0.56% after 1440 h, with a total increase of 47.4%; the imbalance at the X substation increases from 0.28% to 0.36%, with an increase of 28.6%; the imbalance at the Z switching station and the F substation increases from 0.22% and 0.18% to 0.26% and 0.21% respectively, with increases of 18.2% and 16.7%. The change in the curve slope reflects the material kinetic transition during the aging process. The growth rate in the initial stage (0 - 720 h) is significantly higher than that in the later stage (720 - 1440 h). This stage characteristic is consistent with the aging mechanism of XLPE materials - in the initial stage, antioxidant consumption and oxidative chain scission reactions are dominant, and in the later stage, it enters a relatively stable structural reorganization stage.

[0076] Figure 8 and Figure 9 Analysis from the spatial dimension presents the distribution characteristics of the negative sequence imbalance at each measurement point in the system and its variation law with the evolution of aging. By comparing the negative sequence imbalances at the four measurement points in five aging stages through bar charts, the spatial distribution characteristics of "high at the proximal end and low at the distal end" are quantified. Figure 8 It is a schematic diagram of the evolution trend of the negative sequence imbalance at each measurement point in the system. Figure 9Schematic diagram of the relationship between the negative sequence unbalance rate growth characteristic and distance; the growth rate changes at each measurement point in different aging stages are quantitatively analyzed through a stacked bar chart, revealing the stage-by-stage cumulative effect of the growth rate, and the spatial decay law is quantified through an exponential decay curve. The relationship between the unbalance rate growth and the measurement point position is accurately described by an exponential decay curve: (14) where x represents the electrical distance (km) between the measurement point and the XLPE cable section. This curve quantifies the spatial decay law of the unbalance growth and has a high fitting accuracy. The physical meaning corresponds to a 9.5% reduction in the unbalance rate growth for every 10 km increase in electrical distance.

[0077] The unbalance growth characteristic caused by aging can be explained by the coupled analysis of the evolution of material parameters and the change of electrical characteristics. Experimental tests show that after 1440 hours of aging, the dielectric constant of the XLPE material increases from 2.283 to 2.816, with an increase of 23.37%; the dielectric loss factor increases from 0.0038 to 0.0057, with an increase of 50.00%. The changes in these key parameters directly lead to the adjustment of the cable impedance parameters, thereby causing the increase of the system unbalance rate. Through the correlation analysis of the unbalance rate increment at the L terminal station and the change of material parameters, it is found that the two show a non-linear relationship, which can be characterized by a modified exponential function: (15) where is the saturation amplification coefficient, is the time scale parameter, n is the non-linear exponent. The experimental data fitting gives n = 0.68, indicating that the unbalance growth caused by aging shows a sub-linear characteristic, which is consistent with the diffusion control mechanism of insulation material aging.

[0078] From a theoretical perspective, the growth of the negative sequence unbalance rate mainly stems from the change of impedance parameters caused by the aging of XLPE insulation materials. In a three-phase system, the relationship between the negative sequence voltage and current can be expressed as: (16) where is the negative sequence self-impedance, is the mutual impedance between the negative sequence and the zero sequence. The polar groups formed during the thermo-oxidative aging process of XLPE insulation materials change the dielectric polarization characteristics, resulting in drift, thereby affecting the negative sequence current distribution.

[0079] Electromagnetic field analysis shows that after 1440 hours of aging, the maximum field strength of the cable insulation layer increases by about 9.1%, the field strength enhancement coefficient rises from 1.18 to 1.29, the non-uniformity of the field strength distribution increases, and the accumulation of space charge is accelerated. The time evolution of the unbalance degree shows significant piecewise characteristics - the growth rate in the early stage (0 - 720h) is generally higher than that in the later stage (720 - 1440h), and in the later stage, there is a rebound in the growth rate from 1080 - 1440h. This non-monotonic change trend reflects the complex non-linear characteristics of the material aging process, which is related to the acceleration of the free radical chain reaction formed in the XLPE insulation in the later stage of aging.

[0080] The unbalanced growth caused by aging is closely related to the defined mismatch coefficient of the hybrid operation length Experimental data analysis shows that There is a linear relationship between and the growth rate of the unbalance degree: (17) The goodness of fit R² = 0.962. This relationship reveals the role of length mismatch in the unbalanced growth caused by aging and provides a quantitative basis for the optimal design of the hybrid operation system. Figure 10 It shows the process of the negative sequence unbalance degree climbing non-linearly with the extension of aging time and the increase of the impedance mismatch coefficient. In the initial stage (t = 0h), when the impedance mismatch coefficient is 9.4%, the corresponding unbalance degree is 0.28%; as the aging process progresses, at t = 1440h it rises to 15.6%, and the unbalance degree increases to 0.36%, with an increase of 28.6%. Figure 9 The non-uniformity of the surface gradient distribution in reveals the critical characteristics of the system behavior - when

[0081] exceeds the critical threshold of 15%, the growth rate of the unbalance degree increases significantly, indicating that the system enters the unstable interval.

[0081] During the aging process of XLPE cables, the evolution of the sequence impedance parameters shows significant differential characteristics. Experimental measurements show that after 1440 hours of aging, the positive, negative, and zero sequence impedances increase by 7.3%, 9.1%, and 13.6% respectively, while for the oil-filled submarine cable, due to the continuous replenishment and self-healing characteristics of the insulating oil, its impedance parameters basically remain stable. This differential evolution of the impedance parameters directly leads to in the hybrid operation system rising from the initial 9.4% to 15.6%, which is consistent with the growth trajectory of the unbalance degree in Figure 10 .

[0082] Zero-sequence unbalance shows higher sensitivity to the aging state than negative sequence. The zero-sequence current unbalance of the cable increases from 2.54% to 3.27% within the same aging period, with an increase rate of 28.7%, and the growth rate accelerates in the later stage of aging (1080h - 1440h). This sequence-selective response stems from the structural characteristics of different sequence circuits. The sensitivity coefficient of zero-sequence impedance to the change of insulation parameters: (17) Calculation shows SZ 0 >SZ 2 , which verifies the high-sensitivity characteristic of zero-sequence impedance to the change of dielectric parameters. The zero-sequence circuit involves the grounding loop and the metal shielding layer, and the relationship between the loop impedance and the insulation parameters is more complex: (18) Dielectric constant and loss factor affect and to change the zero-sequence impedance. After being amplified by non-linear mapping, this effect leads to higher sensitivity of zero-sequence unbalance to aging.

[0083] Analyzed from the perspective of energy conversion, the increase in unbalance results in a 11.6% increase in the AC loss of the cable, and the actual measured temperature rise is 2.3℃ higher than that under symmetric operation conditions. The relationship between the dielectric loss power density and the electric field strength and loss factor is: (19) This additional loss accelerates the insulation thermal aging process, forming a typical positive feedback mechanism. The eigenvalue analysis shows that the damping ratio of the most unstable mode of the system decreases with the deepening of the aging degree, and it decreases by 8.3% after 1440 hours of aging, verifying the progressive decrease of the system stability margin.

[0084] Based on the close relationship between aging and unbalance, a comprehensive evaluation index is constructed: (21) where α 2 and α 0 are weight coefficients, reflecting the relative importance of negative-sequence and zero-sequence unbalances in system evaluation. Based on experimental data, take α 2 =0.6, α 0 =0.4. This index can be used for the comprehensive evaluation of the aging state of the cable system in mixed operation.

[0085] The unbalance and the aging degree show an exponential correlation, forming a state evaluation function: (22) In the formula, is the system imbalance state index, is the initial value, λ is the sensitivity coefficient, is the time-varying sequence impedance mismatch degree. This function reflects the cumulative effect of the aging process on the imbalance characteristics and provides a theoretical basis for the state assessment of the hybrid operation cable system.

[0086] Through comprehensive analysis, it can be obtained that the influence of XLPE cable aging on the system imbalance characteristics presents three characteristics: in the time dimension, it shows stage growth, reflecting the kinetic characteristics of the evolution of the material microstructure; in the parameter dimension, there is a critical threshold effect, revealing the non-linear transformation of the system stability; in the sequence component dimension, it shows the differential sensitivity characteristic that the zero sequence is higher than the negative sequence.

[0087] In this embodiment, for the XLPE / oil-filled cable hybrid operation system, an electric field-thermal field-aging multi-physical field coupling analysis framework is constructed. Through theoretical analysis and experimental verification, the influence mechanism of cross-linked polyethylene cable aging on the system imbalance characteristics is obtained. Specifically: 1) The negative sequence and zero sequence unbalance degrees caused by XLPE cable aging show differential non-linear growth characteristics. Experiments have confirmed that after the cable is aged for 1440 hours, the increase rate of the system negative sequence unbalance degree reaches 24.4%, and the increase rate of the zero sequence unbalance degree is 28.7%. The sensitivity of the zero sequence unbalance degree to material aging is higher than that of the negative sequence unbalance degree, and it shows obvious segmented growth characteristics, and the growth rate is significantly accelerated in the deep aging stage (aging time > 1080h). During the aging process, the hybrid operation impedance mismatch coefficient gradually increases from 9.4% in the initial state to 15.3%, resulting in the rapid accumulation of unbalanced components.

[0088] 2) Under the hybrid operation conditions, the attenuation of the negative sequence unbalance degree along the electrical distance conforms to the exponential function distribution law, and the attenuation parameter is significantly affected by the aging degree of the XLPE cable. The measurement results show that the unbalance degree detected at the measuring point closer to the XLPE cable in the electrical distance is significantly higher than that at the far-end measuring point, and the unbalance degree attenuation coefficient is about 0.008 / km. With the progress of aging, the distribution of the system unbalance degree shows a gradient characteristic of "high at the proximal end and low at the distal end", and this gradient becomes more significant with the deepening of aging. After aging for 1440 hours, the maximum field strength in the cable insulation layer increases by about 9.1%, and the non-uniformity of the field strength distribution is enhanced.

[0089] 3) There is a critical impedance mismatch threshold in the hybrid operation system. When the hybrid operation impedance mismatch coefficient or the length mismatch coefficient When exceeding the critical value, the growth of the system unbalance degree changes from linear to non-linear acceleration trend. The thermal effect and vibration effect of negative sequence current are superimposed. Along with the aging process of XLPE cables, the dielectric constant increases by 23.37% and the dielectric loss factor increases by 50.34%. The unbalance characteristics of the system continue to deteriorate. The eigenvalue analysis shows that the damping ratio of the most unstable mode of the system decreases with the deepening of the aging degree, and it decreases by about 8.3% after 1440 hours of aging.

[0090] As Figure 11 shown, the present application also provides an electronic device, including a display module 103, a memory 102, a processor 101, and a computer program stored on the memory and executable on the processor 101. When the processor 101 executes the program, it realizes the steps of an analysis method for impedance mismatch and unbalance characteristics of a hybrid operation system based on XLPE cable aging.

[0091] In the embodiments of the present invention, the electronic device includes, but is not limited to, a laptop computer, a desktop computer, a workbench, a personal digital assistant, a server, a blade server, a mainframe computer, and other suitable computers. The electronic device can also represent various forms of mobile devices, such as personal digital processing, cellular phones, smart phones, wearable devices, and other similar computing devices. The components, their connections and relationships, and their functions shown in this embodiment are only examples and are not intended to limit the implementation of the embodiments of the present application described and / or required in this application.

[0092] In the embodiments of the present application, the processor 101 can be implemented by using at least one of an application specific integrated circuit (ASIC), a programmable logic device (PLD), a field programmable gate array (FPGA), a processor, a controller, a microcontroller, a microprocessor, and an electronic unit designed to execute the functions described herein. In some cases, such an implementation can be implemented in a controller. For a software implementation, an implementation of a process or function can be implemented with a separate software module that allows the execution of at least one function or operation. The software code can be implemented by a software application (or program) written in any suitable programming language. The software code can be stored in the memory and executed by the controller.

[0093] The display module 103 is used to display the information input by the user or the information provided to the user. The display module 103 may include a display panel, and the display panel may be configured in the form of a liquid crystal display (LCD), an organic light-emitting diode (OLED), etc.

[0094] The memory 102 can be used to store software programs and various data. The memory 102 may include high-speed random access memory, and may also include non-volatile memory, such as at least one magnetic disk storage device, a flash memory device, or other volatile solid-state storage devices.

[0095] This application also provides a storage medium, on which a computer program is stored. When the computer program is executed by a processor, the steps of the analysis method for impedance mismatch and imbalance characteristics of the hybrid operation system based on XLPE cable aging are implemented.

[0096] The storage medium can adopt any combination of one or more readable media. The readable medium can be a readable signal medium or a readable storage medium. The readable storage medium can be, for example, but not limited to, an electrical, magnetic, optical, electromagnetic, infrared, or semiconductor system, apparatus, or device, or any combination of the above. More specific examples (non-exhaustive list) of the readable storage medium include: an electrical connection with one or more wires, a portable disk, a hard disk, a random access memory (RAM), a read-only memory (ROM), an erasable programmable read-only memory (EPROM or flash memory), an optical fiber, a portable compact disc read-only memory (CD-ROM), an optical storage device, a magnetic storage device, or any suitable combination of the above.

[0097] The above description of the disclosed embodiments enables those skilled in the art to implement or use the present invention. Various modifications to these embodiments will be obvious to those skilled in the art, and the general principles defined in this embodiment can be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention will not be limited to these embodiments shown in this embodiment, but will conform to the widest scope consistent with the principles and novel features disclosed in this embodiment.

Claims

1. An analysis method for impedance mismatch and unbalance characteristics of a hybrid operation system based on XLPE cable aging, characterized in that, The method includes: S101: Simplify the power grid equivalently in the simulation system, retain switchyards and substations, equivalent the rest of the power grid to the retained sites, and construct a structural model of three-core cross-linked polyethylene cables and oil-filled cables; S102: Obtain the structural parameters of three-core cross-linked polyethylene cables and oil-filled cables, including the nominal thickness, outer diameter and electrical characteristics of each layer of material, and form an electrical parameter table of the cables; S103: Set the simulation conditions for unbalanced three-phase current carrying, configure the A phase with three-core cross-linked polyethylene cables and the BC phases with oil-filled cables, set the transmission power of any one phase under full-load conditions, obtain the operating parameters when the single-phase current in the three-core cable reaches the preset current value, and compare the current differences of each phase under symmetrical conditions; S104: Construct a finite element thermal field analysis model of the cable, establish a three-dimensional thermal field analysis model of the three-core cross-linked polyethylene cable according to the current parameters obtained from real-time simulation, input the cable thermal field analysis data, and set the buried depth of the cable and the heat conduction parameters of the surrounding seabed saturated sediment and seawater; S105: Compare and analyze the temperature distribution differences, simulate the single-phase energization and three-phase symmetrical operation conditions in the thermal field model respectively, obtain the temperature data on the inner and outer surfaces of the insulation layer, and calculate the average temperature difference of the insulation layer under the two conditions; S106: Based on the average temperature difference, analyze the change of the insulation material aging rate using the Arrhenius equation, and quantify the influence degree of the single-phase energization condition on the cable insulation life compared with the symmetrical operation condition.

2. The analysis method for impedance mismatch and unbalance characteristics of the hybrid operation system based on XLPE cable aging according to claim 1, characterized in that, Step S101 specifically includes: Construct a hybrid operation power grid model including 500 kV cable lines, overhead lines and substations on the real-time digital simulation system platform. By retaining the 500 kV W switchyard, A substation, X substation on the left and the 500 kV F substation, Z switchyard, H substation on the right and equivalently processing the rest of the power grid part, set the equivalent additional lines between the W switchyard and the A substation, and between the Z switchyard and the H substation. At the same time, input the conductor structure, insulation layer configuration and sheath component parameters of three-core cross-linked polyethylene cables and oil-filled cables to establish a three-dimensional cable structure model.

3. The analysis method for impedance mismatch and unbalance characteristics of the hybrid operation system based on XLPE cable aging according to claim 1, wherein Step S102 specifically includes: Obtain the structural parameters of three-core cross-linked polyethylene cables and oil-filled cables, including the nominal thickness, outer diameter and electrical characteristics of each layer of material. Based on the hierarchical data of the cable structure, establish a parameter table of the geometric dimensions and material properties of the conductor shielding layer, insulation layer, sheath layer and armor layer, and generate a comparison table of positive sequence, negative sequence and zero sequence electrical parameters of different cable types.

4. The analysis method for impedance mismatch and unbalance characteristics of the hybrid operation system based on XLPE cable aging according to claim 1, characterized in that Step S103 specifically includes: Configure the rated voltage and copper core cross-section parameters of the three-core cross-linked polyethylene cable in the A phase in the simulation system, and the rated voltage of the oil-filled cable in the BC phases, and set the rated transmission power of any one phase under full-load conditions. Obtain the current in the three-core cross-linked polyethylene cable during hybrid operation, and compare the differences in the current amplitudes of each phase when only using three-core cross-linked polyethylene cables and being under normal energization and symmetrical conditions.

5. The analysis method for impedance mismatch and unbalance characteristics of the hybrid operation system based on XLPE cable aging according to claim 1, characterized in that, Step S103 also includes: Based on the three-phase phase voltage and current data obtained from real-time simulation, through Clarke and Fortescue sequence component transformation, obtain their positive sequence, negative sequence and zero sequence components; Calculate the negative-sequence voltage and zero-sequence voltage unbalance degree based on formula (1); （1） In the above formula, and are the root mean square values of the positive-sequence and negative-sequence voltages respectively; Calculate the negative-sequence current and zero-sequence current unbalance degree based on formula (2); （2） In the above formula, , are the root mean square values of the positive sequence and negative sequence voltages respectively.

6. The analysis method for impedance mismatch and unbalance characteristics of the hybrid operation system based on XLPE cable aging according to claim 1, wherein Step S104 further includes: establishing a three-dimensional thermal field analysis model of a three-core cross-linked polyethylene cable, obtaining the temperature distribution characteristics of the insulating layer during unbalanced operation, and quantifying the temperature difference of the insulating layer compared with that during three-phase symmetrical operation; The copper conductor of the three-core cross-linked polyethylene cable is set with a linear resistivity, and the resistivity is calculated using formula (3); （3） where is the reference temperature T ref is the resistivity at is the temperature coefficient of resistivity 7. The analysis method for impedance mismatch and unbalance characteristics of the hybrid operation system based on XLPE cable aging according to claim 1, wherein Step S105 further includes: Taking the current parameters obtained by the simulation system as boundary conditions and inputting them into the established three-dimensional thermal field analysis model of the three-core cross-linked polyethylene cable, and respectively simulating the single-core energization condition and the three-phase symmetrical operation condition in the thermal field model; Obtain the temperature data inside the insulating layer near the conductor side and outside under the two conditions; Through the logarithmic mean temperature calculation method of the cylindrical insulator, calculate the average temperature of the insulating layer under the two conditions respectively, and obtain the difference in the average temperature of the insulating layer between the single-core operation condition and the three-phase symmetrical operation condition.

8. The analysis method for impedance mismatch and unbalance characteristics of the hybrid operation system based on XLPE cable aging according to claim 7, wherein The logarithmic mean temperature calculation formula of the cylindrical insulator is: Among them, is the internal temperature of the insulating layer, is the external temperature of the insulating layer; According to the Arrhenius equation: Among them, is the insulation life, is the activation energy, k is the Boltzmann constant, T is the absolute temperature.

9. An electronic device, comprising a memory, a processor, and a computer program stored on the memory and executable on the processor, characterized in that, When the processor executes the program, it implements the steps of the analysis method for impedance mismatch and unbalance characteristics of the hybrid operation system based on XLPE cable aging as described in any one of claims 1 to 7.

10. A storage medium having a computer program stored thereon, characterized in that, When the computer program is executed by the processor, it implements the steps of the analysis method for impedance mismatch and unbalance characteristics of the hybrid operation system based on XLPE cable aging as described in any one of claims 1 to 7.

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