Intermittent arc multi-physics field simulation method for early fault of medium-voltage three-core cable

Abstract

The invention discloses an intermittent arc multi-physical field simulation method for an early fault of a medium-voltage three-core cable, and the method comprises the following steps: building a two-dimensional model of the medium-voltage three-core cable according to a cable structure, and setting the defects of the cable; circuit setting is carried out according to the operation condition of the medium-voltage cable; performing electric field setting according to electric field characteristics of the cable model; setting a magnetic field according to the magnetic field characteristics of the cable model; fluid heat transfer setting is performed according to the heat transfer characteristics of the cable; performing multi-physical field coupling setting according to heat release of the electric arc; performing grid division on the cable model according to the structural characteristics of the cable; and setting a simulation time length and a step length of a result, and solving the model. The invention provides the multi-physical field simulation method for the intermittent arc of the early fault of the medium-voltage three-core cable, the early fault of the cable is accurately simulated by using the method, and a technical reference is provided for the research of the physical evolution process of the fault.

Description

Technical Field

[0001] The invention relates to the field of simulation of early-stage cable faults, and in particular to an intermittent arc multi-physical field simulation method for early-stage faults of a medium-voltage three-core cable. Background Art

[0002] During the manufacturing, transportation, laying and operation of cables, various types of defects may occur, such as cracks, protrusions, air gaps, etc. These defects will cause the cable to age faster and increase the possibility of insulation breakdown, thereby reducing the expected life of the cable and causing the risk of power outages and more property losses. Current research shows that the development process of cable failure can be divided into three stages. The first stage is partial discharge, which is manifested as extremely short-term pulses; the second stage is early failure, which is manifested as intermittent arc discharge in the insulation layer and may be accompanied by insulation recovery; the third stage is permanent failure, which is manifested as the arc discharge channel completely penetrating the insulation layer, causing irreversible damage to the insulation layer.

[0003] In the distribution network system, the cables are generally 10kV medium voltage cables, and the partial discharge signal is not obvious on the medium voltage cable, making it difficult to monitor. In recent years, the concept of fault warning has emerged. When the cable is in the early stage of fault, the characteristics of the intermittent arc in the insulation layer of the cable are used to evaluate the health of the cable, so as to give a warning before a permanent fault occurs, guide the operation and maintenance personnel to find the damaged insulation parts in time and replace them, curb the occurrence of permanent faults, and provide a guarantee for the safe transmission of electric energy.

[0004] Since the early faults of cables are manifested as random and intermittent arc discharges, existing studies generally use the arc black box model to simulate early faults, extract features based on the simulated voltage and current curves, and analyze the cable fault information. However, the black box model cannot display the physical field distribution of intermittent arcs, and therefore cannot reflect the physical changes of the actual cable insulation layer. In addition, there may be errors between the simulation results and the actual situation, so it is necessary to further consider multi-physical field simulation methods. Summary of the invention

[0005] The current research field of early cable faults lacks a simulation model that can accurately reproduce the physical changes of actual faults. To this end, the present invention provides an intermittent arc multi-physical field simulation method for early faults of medium-voltage three-core cables. The method is based on COMSOL finite element simulation software, and can better simulate and reproduce early cable faults. While obtaining voltage and current curves, the thermal field and electric field of the intermittent arc can also be obtained with high accuracy.

[0006] The technical solution adopted by the present invention is as follows:

[0007] A multi-physics field simulation method for intermittent arc of early fault of medium voltage three-core cable, characterized in that the simulation method comprises the following steps:

[0008] S1. A cross-sectional two-dimensional model of a medium-voltage three-core cable is established according to the structure of the medium-voltage three-core cable, which is used as a cable model, and defects of the cable are set;

[0009] S2. Setting the circuit of the cable model according to the operating conditions of the medium voltage three-core cable;

[0010] S3, setting the electric field of the cable model according to the electric field characteristics of the cable model;

[0011] S4, setting the magnetic field of the cable model according to the magnetic field characteristics of the cable model;

[0012] S5. According to the heat transfer characteristics of the medium voltage three-core cable, setting the fluid heat transfer for the cable model;

[0013] S6. Perform multi-physics field coupling setting on the cable model according to the heat release of the intermittent arc of the early fault of the medium voltage three-core cable;

[0014] S7. Mesh the cable model according to the structural characteristics of the medium voltage three-core cable;

[0015] S8. Setting the simulation time and the step size of the result, and solving the cable model.

[0016] Preferably, in the medium voltage three-core cable, in the early stage fault, an air gap defect is set in the insulation layer in the model according to the development degree of the early stage fault.

[0017] Preferably, in the circuit setting, the circuits are connected in a node manner.

[0018] Preferably, in the circuit configuration, three voltage sources are used as power sources for the cable line.

[0019] Preferably, in the circuit setting, the voltage output nodes in the three dual-port nodes are connected to the three cores of the three-core cable, the cores are connected to the load resistors, and finally connected to the ground to form a loop.

[0020] Preferably, in the circuit setting, due to a defect in one of the wire cores, an intermittent arc may form an additional loop.

[0021] Preferably, in the electric field setting, the outer sheath surface of the cable is set to be electrically insulating.

[0022] Preferably, in the electric field setting, three cores of the cable are set as terminals, which are respectively connected to three external couplings of the circuit.

[0023] Preferably, in the magnetic field setting, the boundary of the cable outer sheath is set as magnetic insulation.

[0024] Preferably, the grid division sets the defective area of ​​the cable and the insulating layer connected thereto as a finer grid, and sets the other structures of the cable as a regular grid, wherein the size of the finer grid is about 0.1 mm, and the size of the regular grid is about 1 mm, so the density of the finer grid is about one hundred times that of the regular grid. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 It is the overall flow chart of this simulation method;

[0026] Figure 2 It is a schematic diagram of the two-dimensional model of a medium voltage three-core cable;

[0027] Figure 3 is a schematic diagram of the circuit setup;

[0028] Figure 4 This is a schematic diagram of the grid division of a certain type of 10kV medium voltage three-core cable;

[0029] Figure 5 It is the voltage curve intention of the feeder fault point;

[0030] Figure 6 It is a schematic diagram of the current curve at the feeder fault point;

[0031] Figure 7 It is a schematic diagram of the temperature field distribution of the intermittent arc;

[0032] Figure 8 It is a schematic diagram of the potential distribution of the internal structure of the cable. DETAILED DESCRIPTION

[0033] The following is combined with Figures 1 to 8 The scheme of the present invention is further explained. It should be noted that, in the absence of conflict, the embodiments in the present application and the technical features in the embodiments can be combined with each other. It should be pointed out that, unless otherwise specified, all technical and scientific terms used in this application have the same meanings as those generally understood by ordinary technicians in the technical field to which this application belongs. The words "including" or "comprising" and the like used in the present invention are intended to mean that the elements or objects appearing before the word cover the elements or objects listed after the word and their equivalents, without excluding other elements or objects.

[0034] The overall flow chart of this simulation method is as follows Figure 1As shown, considering that intermittent electric arcs involve the action of multiple physical fields, electric field, magnetic field, and fluid heat transfer modules are set in the model to characterize the behavior of the arc under different physical fields, and the circuit module is used as the external coupling of the electric field.

[0035] The specific steps are as follows:

[0036] 1) Physical model construction

[0037] COMSOL software provides a geometry module, in which a two-dimensional model of a medium-voltage three-core cable is established using the cable drawings or structural dimension information, and the cable defects are set. Figure 2 As shown in the figure, in the early stage of cable fault, its defects are mainly manifested as tiny air gaps in the insulation layer. Therefore, in the model, air gap defects of different sizes can be set in the insulation layer according to the development degree of the early stage fault.

[0038] 2) Circuit settings

[0039] Medium voltage cables generally use a neutral point that is not grounded or directly grounded, and the three-phase electricity is transmitted to the load end through three wire cores. In COMSOL software, the circuit module is used to set the operating conditions of the medium voltage cable. Figure 3 As shown, under normal circumstances, the power direction starts from the power supply and reaches the load end through the medium voltage three-core cable. The circuit is connected in a node manner, and three voltage sources with a phase difference of 120° are used as the power supply of the 10kV cable line. The Thevenin theorem is used to be equivalent to three two-port nodes, one of which is the ground and the other is the voltage output. The three two-port node circuits can be set as three external couplings of the circuit and connected to the terminal boundary conditions of the electric field. The voltage output nodes in the three two-port nodes are connected to the three cores of the three-core cable, and the cores are connected to the load resistors, and finally connected to the ground to form a loop. In addition, in core 1, due to the existence of defects, the intermittent arc will form an additional loop.

[0040] 3) Electric field setting

[0041] Field equation settings:

[0042] The electric field of the intermittent arc can be expressed by the charge conservation equation, and the control equation is shown in equation (1):

[0043]

[0044] Where: J is the current density vector, ρ is the charge density, and t is the time.

[0045] The electric field strength can be obtained from the electric potential:

[0046]

[0047] Where: E is the electric field strength, For electric potential.

[0048] The current density vector can be calculated from the electric field strength:

[0049] J=σE (3)

[0050] Where: σ is the conductivity.

[0051] Boundary condition settings:

[0052] The outer sheath surface of the cable does not generate a current channel with the environment, so the outer sheath surface of the cable is set to be electrically insulated, that is, the outer sheath boundary of the cable should meet the following formula:

[0053] n·J=0 (4)

[0054] Where: n is the unit vector perpendicular to the outer sheath of the cable.

[0055] Since the three core conductors of the cable carry three-phase voltages with a phase difference of 120°, they are set as terminals and connected to the three external couplings of the circuit respectively, which can be expressed by the formula:

[0056] V i =U i (5) Where: V i is the voltage of the ith cable core conductor, U i is the voltage of the i-th external coupling in the circuit.

[0057] In addition, in actual operation of the cable, the metal shielding layer should be grounded, that is, the voltage of the layer is 0 at any time, so the boundary of the metal shielding layer follows the following formula:

[0058] V=0 (6)

[0059] Where: V is the metal shielding layer voltage.

[0060] Initial value setting:

[0061] In this model, the simulation starts from the moment the cable is energized. At the initial moment, the voltage of each structure inside the cable is 0, which can be expressed as:

[0062] V0=0 (7)

[0063] Where: V0 is the initial voltage value of the cable internal structure.

[0064] 4) Magnetic field setting

[0065] Field equation settings:

[0066] The magnetic field of the intermittent arc follows Ampere's law, and its control equation is shown in formula (8):

[0067]

[0068] Where: H is the magnetic field strength vector.

[0069] The magnetic flux density can be obtained from the magnetic field strength:

[0070] B=μH (9)

[0071] Where: B is the magnetic flux density vector, μ is the magnetic permeability.

[0072] The relationship between magnetic flux density and magnetic vector potential is:

[0073]

[0074] Where: A is the magnetic vector potential vector.

[0075] Boundary condition settings:

[0076] Although there is an alternating current in the internal conductor of the cable, due to the presence of the metal shielding layer, it can be considered that the magnetic field of the external environment of the cable has almost no connection with the internal conductor, that is, the boundary of the cable outer sheath is magnetically insulated:

[0077] n×A=0 (11)

[0078] Where: n is the unit vector perpendicular to the outer sheath of the cable.

[0079] Initial value setting:

[0080] There is no initial magnetic field in this model, so it is assumed that the magnetic vector potential of each structure inside the cable is 0 at the initial moment, which can be expressed as:

[0081] A0=0 (12)

[0082] Where: A0 is the initial value of the magnetic vector potential vector of the internal structure of the cable.

[0083] 5) Fluid heat transfer settings

[0084] Field Equation Settings

[0085] When analyzing heat transfer, the temperature field equation is used:

[0086]

[0087] Where: Q is the total thermal energy of the gas, ρ is the gas density, C p is the constant pressure heat capacity of the gas, T is the thermodynamic temperature of the gas, u is the velocity vector of the gas, and q is the heat flux vector of the gas.

[0088] The heat flux can be obtained from Fourier's law:

[0089]

[0090] Where: k is the thermal conductivity of the material.

[0091] Boundary settings:

[0092] There is heat exchange between the outer sheath surface of the cable and the ambient medium, which is usually not negligible. Therefore, the outer sheath boundary is set to heat flux, and the formula is as follows:

[0093] q0=h·(T ext -T o ) (15)

[0094] Where: q0 is the heat flux vector between the outer sheath and the environment, h is the heat transfer coefficient, T ext is the ambient temperature, T o is the outer sheath temperature.

[0095] Initial value setting:

[0096] At the initial moment, the entire structure of the cable is at room temperature, which can be expressed by the formula:

[0097] T0=293.15K (16)

[0098] Where: T0 is the initial temperature of each cable structure.

[0099] 6) Multi-physics field coupling settings

[0100] During the discharge process, the intermittent arc releases extremely high heat, which can be considered as a heat source. Assuming that the arc plasma is in local thermal equilibrium, the formula for the heat source is:

[0101]

[0102] Where: Q is the heat generated by the balanced discharge heat source, k B is the Boltzmann constant, q is the charge of the electron, T is the thermodynamic temperature of the arc plasma, J is the current density vector in the arc plasma, E is the electric field intensity vector in the arc plasma, Q rad is the total volume emissivity.

[0103] In addition, the various structures of the cable will also generate electromagnetic heat due to the electromagnetic field, and the formula is:

[0104] P=J·E (18)

[0105] Where: P is the thermal power of electromagnetic heat, J is the current density vector in each section of the cable, and E is the electric field strength vector in the cable.

[0106] 7) Grid division

[0107] Since the defects of the cable are fine structures, grids of different densities are generated for different results as much as possible, and the automatic grid generation method is not used. Setting the defective area of ​​the cable and the insulation layer connected to it as a finer grid is beneficial to the accuracy of the physical field calculation; setting the other structures of the cable as a regular grid reduces the number of grids and reduces the solution time. The size of the finer grid is about 0.1mm, while the size of the regular grid is about 1mm, so the density of the finer grid is about one hundred times that of the regular grid. The grid is encrypted at the boundary of the defect to avoid non-convergence at the boundary during the solution.

[0108] 8) Calculation and solution

[0109] By setting the simulation time and the step size of the results in the solver and solving the model, you can get the corresponding physical field distribution and the voltage and current curves of the intermittent arc.

[0110] The beneficial effects of the present invention are as follows: the present invention proposes a multi-physics field simulation method for intermittent arcs of early faults in medium voltage three-core cables, and uses the method to accurately simulate early faults in cables, providing a technical reference for the study of the physical evolution process of faults. At the same time, the simulation results obtained by the method can further play a role in the fault warning algorithm or relay protection algorithm of the distribution network, and provide a basis for algorithm development as a source of fault data.

[0111] The present invention is further described below in conjunction with some embodiments:

[0112] Taking a certain type of 10kV medium voltage three-core cable as an example, its grid division is as follows: Figure 4 The cable structure is a 10kV three-core structure, which is very common in most distribution network cables in my country and has universal applicability.

[0113] The voltage curve of the feeder fault point under 1 / 4 cycle intermittent arc obtained by this method is as follows: Figure 5 shown. Figure 5 It shows that intermittent arcs in medium-voltage cables usually occur near the voltage peak. The reason is that when the voltage is at the peak, the electric field at the insulation defect is the largest, and it is most likely to break down and produce arcs. When the arc occurs, the voltage drops to a level close to 0 instantaneously, and returns to its original state after crossing the zero point. This figure reflects the voltage change characteristics of the cable in the early stage of fault, and provides data support for the distribution network fault identification algorithm based on voltage data.

[0114] The feeder fault point current curve under 1 / 4 cycle intermittent arc obtained by this method is as follows: Figure 6 shown. Figure 6It shows that when intermittent arcs occur in medium-voltage cables, the current level increases rapidly, producing a peak current with a duration of 1 / 4. This characteristic also provides data support for the distribution network fault identification algorithm based on current data.

[0115] The temperature field distribution of intermittent arc is as follows: Figure 7 As shown. The figure shows the temperature distribution characteristics of the intermittent arc in the cable insulation layer. The highest temperature is located at the upper and lower sides of the arc, and may extend to the upper and lower sides. When the cable insulation is subjected to high temperature, carbonization will occur on the surface, which may reduce the insulation level of the cable and make the intermittent arc of the cable turn into a permanent arc fault more quickly. Therefore, the temperature distribution of the intermittent arc can provide a basis for the study of cable fault evolution. At present, there has been no research on the internal temperature field of the intermittent arc. This result can further understand the temperature field distribution characteristics of the intermittent arc of the cable.

[0116] The potential distribution of the cable internal structure is as follows Figure 8 As shown. This figure illustrates the potential distribution characteristics in the medium-voltage three-core cable at the early stage of intermittent arcing. Each line in the figure is an equipotential line of an electric potential, which means that the electric potential on the line is of the same value. When the color of the equipotential line tends to red, it indicates that the potential is positive; when it tends to blue, it indicates that the potential is negative. The greater the absolute value of the potential represented by the equipotential line, the darker the color. The voltage phases of the three cores differ by 120°, so at this moment, the two phases above and below the left are in the negative half cycle, so the equipotential lines tend to be blue, while the phase on the lower right is in the positive half cycle, so the equipotential lines tend to be red. In addition, the density of the equipotential lines reflects the magnitude of the electric field strength at that location. Where the equipotential lines are sparse, the electric field strength is small; where the equipotential lines are dense, the electric field strength is large. In the lower two-phase insulation is intact, the equipotential lines are evenly distributed, and the electric field strength distribution is good, which is a normal situation. The equipotential lines at the upper phase insulation defect are densely distributed, indicating that the electric field strength is large, which is easy to cause breakdown and arcing, so intermittent arcs are more likely to occur at insulation defects. There is currently no research on the internal electric field of intermittent arcs. This result can further understand the electric field distribution characteristics of intermittent arcs in cables. In addition, it can also serve as a reference for the detection and location of intermittent faults, and provide protection for the replacement and maintenance of damaged cables and the safety of energy transmission.

[0117] Finally, it should be noted that the described embodiments are only part of the embodiments of the present application, rather than all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without making any creative work are within the scope of protection of the present application.

[0118] The above description is only by way of illustration of certain exemplary embodiments of the present invention. It is undoubted that those skilled in the art can modify the described embodiments in various ways without departing from the spirit and scope of the present invention. Therefore, the above drawings and descriptions are illustrative in nature and should not be construed as limiting the scope of protection of the claims of the present invention.

Claims

1. A multi-physics field simulation method for intermittent arc of early fault of medium voltage three-core cable, characterized in that: The simulation method comprises the following steps: S1. A cross-sectional two-dimensional model of a medium-voltage three-core cable is established according to the structure of the medium-voltage three-core cable, which is used as a cable model, and defects of the cable are set; S2. Setting the circuit of the cable model according to the operating conditions of the medium voltage three-core cable; S3, setting the electric field of the cable model according to the electric field characteristics of the cable model; S4, setting the magnetic field of the cable model according to the magnetic field characteristics of the cable model; S5. According to the heat transfer characteristics of the medium voltage three-core cable, setting the fluid heat transfer for the cable model; S6. Perform multi-physics field coupling setting on the cable model according to the heat release of the intermittent arc of the early fault of the medium voltage three-core cable; S7. Mesh the cable model according to the structural characteristics of the medium voltage three-core cable; S8. Setting the simulation time and the step size of the result, and solving the cable model.

2. The intermittent arc multi-physics field simulation method for early fault of medium voltage three-core cable according to claim 1 is characterized in that: Preferably, in the medium voltage three-core cable, in the early stage fault, an air gap defect is set in the insulation layer in the model according to the development degree of the early stage fault.

3. The intermittent arc multi-physics field simulation method for early fault of medium voltage three-core cable according to claim 1 is characterized in that: In the circuit setting, the circuits are connected in a node manner.

4. The intermittent arc multi-physics field simulation method for early fault of medium voltage three-core cable according to claim 1 is characterized in that: In the circuit arrangement, three voltage sources are used as power sources for the cable line.

5. The intermittent arc multi-physics field simulation method for early fault of medium voltage three-core cable according to claim 1 is characterized in that: In the circuit setting, the voltage output nodes among the three dual-port nodes are connected to the three cores of the three-core cable, the cores are connected to the load resistors, and finally connected to the ground to form a loop.

6. The intermittent arc multi-physics field simulation method for early fault of medium voltage three-core cable according to claim 5 is characterized in that: In the circuit setting, due to a defect in one of the wire cores, the intermittent arc that occurs will form an additional loop.

7. The intermittent arc multi-physics field simulation method for early fault of medium voltage three-core cable according to claim 1 is characterized in that: In the electric field setting, the outer sheath surface of the cable is set to be electrically insulating.

8. The intermittent arc multi-physics field simulation method for early fault of medium voltage three-core cable according to claim 1 is characterized in that: In the electric field setting, three cores of the cable are set as terminals and are respectively connected to three external couplings of the circuit.

9. The intermittent arc multi-physics field simulation method for early fault of medium voltage three-core cable according to claim 1 is characterized in that: In the magnetic field setting, the cable outer sheath boundary is set as magnetic insulation.

10. The intermittent arc multi-physics field simulation method for early fault of medium voltage three-core cable according to claim 1, characterized in that: The grid division sets the defective area of ​​the cable and the insulating layer connected thereto as a finer grid, and sets other structures of the cable as a regular grid, wherein the size of the finer grid is about 0.1 mm, and the size of the regular grid is about 1 mm.

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