Power distribution overhead conductor tree-touching single-phase earth fault identification method and system

Abstract

The invention discloses a power distribution overhead conductor tree-touching single-phase earth fault identification method and system, and the method comprises the following steps: constructing a tree model, and obtaining the shape parameters of a tree; in the single-phase grounding simulation process of the tree model, the tree temperature, the tree moisture content, the environment temperature, the branch side area and the phase voltage of the power distribution overhead conductor are obtained; calculating tree access resistance, current heat production, convection heat dissipation, radiation heat dissipation, moisture evaporation heat absorption and tree temperature rise heat absorption; further calculating the moisture evaporation mass to update the moisture content of the tree, and calculating the temperature rise of the tree to update the temperature of the tree; iteratively calculating tree access resistance and recording until single-phase grounding simulation is completed, and constructing a resistance-time model; and during fault identification, monitoring the resistance between the power distribution overhead conductor and the ground in real time, and comparing the resistance with the resistance-time model. The method can accurately and quickly predict the tree-touching single-phase earth fault of the power distribution overhead conductor, and is high in applicability.

Description

Technical Field

[0001] The invention belongs to the technical field of distribution network fault analysis, and in particular relates to a method and system for identifying a single-phase grounding fault of a distribution overhead conductor touching a tree. Background Art

[0002] In recent years, forest fires have occurred frequently around the world, causing huge casualties and economic losses. The issue of responsibility determination after the fire has gradually become one of the focus issues of people's attention. The total length of distribution lines in my country exceeds 5.37 million kilometers, of which the length of overhead lines is more than 4.37 million kilometers, and the length of rural overhead lines is more than 3.89 million kilometers. Single-phase grounding faults caused by overhead wires hitting trees due to tree growth, strong winds, landslides, etc. occur from time to time. The grounding current of the wire hitting the tree fault has the characteristics of a small initial value and gradually increases slowly as the fault develops.

[0003] At present, the research on the single-phase grounding fault of the distribution overhead wire touching the tree mainly focuses on the experimental simulation of the fault and the method research of extracting the electrical quantity characteristics of the specific test. For example, the current and voltage gradient and other information generated during the single-phase grounding fault of the distribution overhead wire touching the tree are measured, and the phenomenon is observed and recorded through audio and images; the characteristics of the current signal in the time domain and frequency domain are analyzed and identified by machine learning methods; the single-phase grounding fault process of the distribution overhead wire touching the tree is divided into four stages according to the changes in physical and electrical characteristics during the fault process: development temperature rise, water evaporation, tree carbonization, and flashover; and the influence of different vegetation sizes on leakage current and arc discharge characteristics is analyzed.

[0004] A Chinese invention patent application with publication number CN115113092A discloses a tree line fault identification method, simulation device, system, computer device and medium, wherein the simulation device is used to obtain simulated fault data, wherein the simulated fault data includes fault type information, separation speed information and simulated fault waveform, wherein the fault type information, the separation speed information and the simulated fault waveform information are matched one by one, and a line fault waveform is obtained, wherein the line fault waveform is a transmission waveform of an overhead bare conductor when a transmission fault occurs, and whether the line fault waveform matches the simulated fault waveform is determined, and if so, the fault type information and the separation speed information that match the simulated fault waveform are pushed.

[0005] However, the existing single-phase grounding fault model usually cannot accurately reflect the slowly changing characteristics of the transition resistance at the contact point between the conductor and the tree during the single-phase grounding fault of the distribution overhead conductor touching the tree. This slow-changing characteristic is particularly critical in the early stage of the single-phase grounding fault of the distribution overhead conductor touching the tree. The scheme ignores the dynamic changes of resistance under the influence of factors such as temperature, humidity, and tree size. Due to the lack of accurate simulation of the slow-changing characteristics, the existing models show poor adaptability in describing the development process of the single-phase grounding fault of the distribution overhead conductor touching the tree, which in turn affects the accurate assessment of the fire risk. Furthermore, some models are only valid for the parameters of the test, and when the environmental conditions such as different voltage levels, tree types, and contact positions change, the models show significant limitations. This lack of versatility makes it difficult for existing models to be widely used in complex and changeable actual environments. Summary of the invention

[0006] The present invention provides a method and system for identifying a single-phase grounding fault of a power distribution overhead conductor touching a tree, aiming to solve the problems of low adaptability and insufficient accuracy in different environments in the prior art in identifying a single-phase grounding fault.

[0007] In order to solve the above technical problems, the present invention proposes a fault identification method, comprising the following steps:

[0008] The real morphology of the tree is mapped into a fractal tree network model to construct a tree model and obtain tree shape parameters, wherein the tree model includes a trunk and several levels of branches, and the structural dimensions between two adjacent levels are in the same proportional relationship;

[0009] In the single-phase grounding simulation process of the tree model, tree temperature, tree moisture content, ambient temperature, branch side area and phase voltage of the distribution overhead wire are obtained;

[0010] Tree path resistance was calculated based on the binary relationship between tree temperature and tree moisture content;

[0011] Calculate the heat generated by electric current, heat dissipation by convection and heat dissipation by radiation of the tree model, and then calculate the heat absorbed by water evaporation and heat absorbed by the temperature rise of the tree;

[0012] Calculate the water evaporation mass according to the water evaporation heat absorption, and use the water evaporation mass to update the water content of the tree;

[0013] Calculating the tree temperature rise based on the tree temperature rise heat absorption, and using the tree temperature rise to update the tree temperature;

[0014] Iteratively calculate and record the tree path resistance using the updated tree moisture content and tree temperature until the single-phase grounding simulation is completed, and use the recorded tree path resistance to build a resistance-time model;

[0015] During the fault identification process, the resistance between the distribution overhead wire and the ground is monitored in real time, and the resistance-time relationship is generated and compared with the resistance-time model to identify the single-phase grounding fault caused by the wire touching the tree.

[0016] Preferably, the tree moisture content is an absolute moisture content, which is obtained by experimental weighing and is calculated using the following formula:

[0017]

[0018] In the formula, m s is the fresh weight of the wood block obtained from the experimental sampling, and m0 is the absolute dry mass of the wood block.

[0019] Preferably, the calculation method of the tree path resistance is:

[0020] In the tree model, α and β are defined as the length ratio and radius ratio of the branches of the k+1th layer network and the kth layer network, expressed as follows:

[0021]

[0022] In the formula, l k , l k+1 are the branch lengths of the k-th layer network and the k+1-th layer network, r k 、r k+1 They are the branch radius of the k-th layer network and the branch radius of the k+1-th layer network respectively;

[0023] According to the known binary relationship σ between the conductivity at time t and the tree temperature T and the tree moisture content W t (W,T), calculate the tree path resistance R t :

[0024]

[0025] Where n is the level of the touch line in the tree model, l0 is the basic branch length in the tree model, that is, the trunk height, and r0 is the basic branch radius, that is, the trunk radius.

[0026] Preferably, the calculation method of the heat generated by the current is:

[0027]

[0028] In the formula, Q s is the heat generated by the current, U is the phase voltage of the overhead distribution line, R t is the tree path resistance at time t.

[0029] Preferably, the calculation method of the convective heat dissipation is:

[0030] Q c =∫kc S c (TT e )dt

[0031] In the formula, Q c is the convective heat dissipation, k c is the branch convection heat dissipation coefficient, S c is the lateral area of ​​the branch, T is the tree temperature, T e is the ambient temperature.

[0032] Preferably, the calculation method of the radiant heat dissipation is:

[0033]

[0034] In the formula, Q r is the radiation heat dissipation, k r is the branch radiation heat transfer coefficient, δ0 is the Stefan-Boltzmann constant, S c is the lateral area of ​​the branch, T is the tree temperature, T e is the ambient temperature.

[0035] Preferably, the calculation methods of the heat absorbed by water evaporation and the heat absorbed by tree temperature rise are:

[0036] Q w =(Q s -Q c -Q r )·k f

[0037] Q t =(Q s -Q c -Q r )·(1-k f )

[0038]

[0039] In the formula, Q w The heat absorbed by water evaporation, Q t The heat absorbed by the tree due to temperature rise, Q s Q is the heat generated by the current. c is the convective heat dissipation, Q r is the radiation heat dissipation, k f is the evaporation absorption proportional coefficient, and T is the tree temperature.

[0040] Preferably, the calculation method of the water evaporation mass is:

[0041]

[0042] In the formula, Δm is the mass of water evaporated, Q wThe heat absorbed by water evaporation, T w For the water temperature.

[0043] Preferably, the calculation method of the tree temperature rise is:

[0044]

[0045] Where ΔT is the temperature rise of the tree, c t is the specific heat capacity of wood, c w is the specific heat capacity of water, m s is the fresh weight of the wood block obtained from the experimental sampling, and m0 is the absolute dry mass of the wood block.

[0046] In another aspect of the present invention, a system for identifying a single-phase grounding fault caused by a power distribution overhead conductor touching a tree is provided. The system is used to implement the above-mentioned simulation method, and comprises:

[0047] The tree model building module is used to map the real shape of the tree into a fractal tree network model, obtain the shape parameters of the tree, including the trunk and several levels of branches, and set the structural dimensions between two adjacent levels to be in the same proportional relationship;

[0048] The parameter acquisition and setting module is used to obtain the tree temperature, ambient temperature and phase voltage of the distribution overhead wire during the single-phase grounding simulation process, and receive the parameters input manually;

[0049] A path resistance calculation module, used for calculating the path resistance of the tree according to the binary relationship between the tree temperature and the tree moisture content;

[0050] Thermodynamic calculation module, used to calculate the heat generation of electric current, convection heat dissipation, radiation heat dissipation, water evaporation heat absorption and tree temperature rise heat absorption based on the tree model;

[0051] A parameter updating module is used to calculate the water evaporation mass according to the water evaporation heat absorption, and update the tree moisture content according to the water evaporation mass; and calculate the tree temperature rise according to the tree temperature rise heat absorption, and update the tree temperature according to the tree temperature rise;

[0052] A resistance iteration module is used to iteratively calculate the tree path resistance according to the updated tree moisture content and tree temperature until the single-phase grounding simulation is completed, and record the tree path resistance to construct a resistance-time model;

[0053] The fault identification module is used to monitor the resistance between the distribution overhead wire and the earth in real time, generate the resistance-time relationship, and compare it with the resistance-time model to identify the single-phase grounding fault caused by the wire touching the tree.

[0054] Compared with the prior art, the present invention has the following technical effects:

[0055] 1. The fault identification method proposed in the present invention simplifies trees into cylinders with uniform electrical and thermal characteristics by accurately modeling the current path, combining contact resistance and arc resistance to accurately simulate the path of current through trees, and consider the change of resistance with the temperature and humidity of trees. It adopts an iterative calculation method to update the conductivity, temperature and moisture content of trees in real time in each calculation cycle, which enables the model to dynamically reflect the real-time changes in the TSF process. It realizes accurate simulation of electrical characteristics during the fault process and improves the accuracy of fault prediction.

[0056] 2. The fault identification method proposed in the present invention takes into account the slowly changing characteristics of the transition resistance at the contact point between the conductor and the tree during the single-phase grounding fault of the distribution overhead conductor touching the tree, so that the model can more accurately reflect the electrothermal characteristics of trees under different environmental conditions, avoiding errors in traditional models and improving the adaptability of the model in describing the development process of the single-phase grounding fault of the distribution overhead conductor touching the tree.

[0057] 3. The fault identification method proposed in the present invention introduces the quantitative relationship between Joule heat and water evaporation, and clarifies how the Joule heat caused by current is distributed inside the tree. Part of the Joule heat is used for the temperature rise of the tree, and the other part is used for water evaporation, which reduces the calculation deviation caused by underestimation of the temperature rise process in the traditional model and improves the accuracy of the present invention in predicting time-varying resistance.

[0058] 4. The fault identification method proposed in the present invention introduces an iterative calculation mechanism so that the model can update the temperature, humidity, conductivity and other parameters of the tree in real time, reflecting the dynamic changes during the fault process. This improvement enables the model to not only accurately predict the changing trends of current and temperature, but also provide timely fault warnings, avoiding the hysteresis problem caused by traditional static models, and effectively improving the fault warning speed.

[0059] 5. The fault identification method proposed in the present invention simplifies the geometric model of the tree into a cylinder with uniform electrothermal characteristics, thereby reducing the complexity of the model calculation, making the calculation process more efficient, reducing the system operation cost, and improving the processing speed of the model, thereby achieving the purpose of improving the early warning efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0060] Figure 1 It is a schematic diagram of the process of the identification method of the present invention;

[0061] Figure 2 is a schematic diagram of a tree model according to an embodiment of the present invention;

[0062] Figure 3 is a simplified schematic diagram of a tree model according to an embodiment of the present invention;

[0063] Figure 4 is a comparison diagram of actual values ​​and simulation values ​​described in an embodiment of the present invention;

[0064] Figure 5 is another comparison diagram of the actual value and the simulation value described in the embodiment of the present invention. DETAILED DESCRIPTION

[0065] In order to make the objectives, technical solutions and advantages of the present invention more clear, the technical solutions of the present invention will be clearly and completely described below in combination with specific embodiments of the present application and with reference to the accompanying drawings.

[0066] The purpose of the present invention is to provide a method for simulating the electrical characteristics of the entire process of a single-phase grounding fault of a distribution overhead conductor touching a tree, and to identify the fault based on the simulation results. The simulation can accurately reflect the transition resistance change characteristics during the fault process, especially the key physical processes such as temperature rise and water evaporation after the conductor contacts the tree. In view of the fact that the distribution overhead conductors of the voltage level of 10kV are more likely to touch trees in the installation environment than other conductors with higher voltage specifications, and generate more heat after touching trees than other conductors with lower voltage specifications, and are more likely to cause fires. Therefore, the embodiment of the present invention describes in detail the simulation of the electrical characteristics of the entire process of a single-phase grounding fault of a 10kV distribution overhead conductor touching a tree and the fault identification, and can also be used for the simulation of the electrical characteristics of the entire process of a single-phase grounding fault of a distribution overhead conductor touching a tree of other voltage levels and the fault identification.

[0067] Embodiment 1

[0068] This embodiment is a method for identifying a single-phase grounding fault of a power distribution overhead conductor touching a tree. Figure 1 As shown, the steps include steps one to eight:

[0069] Step 1: Map the real shape of the tree into a fractal tree network model to construct a tree model and obtain tree shape parameters. The tree model includes a trunk and several levels of branches, and the structural dimensions between two adjacent levels are in the same proportional relationship.

[0070] The specific implementation method of building tree models is as follows: Figure 2 , 3 As shown, in Figure 2 In (a), when the wire touches the tree, the current path from high to low is that the wire passes through one of the forked branches and finally flows into the ground through the trunk. Therefore, the equivalent circuit is as follows Figure 2 (b) shows a series connection of multiple dendrimers.

[0071] The tree is approximated as a cylinder with uniform electrical and thermal characteristics, and the transition part between branches is ignored. The model structure is simplified to facilitate calculation. The conductivity of the tree is affected by temperature and moisture content, so the main parameters in the model include temperature, moisture content and tree size. By modeling the electrical and thermal characteristics of the tree, the TSF (Tree-contact Single-phase-to-ground Faults) current and its development process can be accurately simulated.

[0072] As a typical natural fractal structure, trees have similar geometric features in their overall shape from trunks to branches to small twigs. The fractal tree network model is used to simplify and simulate the real shape of trees, such as Figure 3 As shown, the structural sizes between two adjacent levels decrease and the proportions are the same. Since the TSF pathway consists of only one tree and trunk at each level, the specific number of branches is not discussed. The tree-like branch network has countless branch layers.

[0073] In the tree model, α and β are defined as the length ratio and radius ratio of the branches of the k+1th layer network and the kth layer network, expressed as follows:

[0074]

[0075] In the formula, l k , l k+1 are the branch lengths of the k-th layer network and the k+1-th layer network, r k 、r k+1 They are the branch radius of the k-th layer network and the branch radius of the k+1-th layer network respectively.

[0076] For l k and r k ,have:

[0077] l k =l0α k

[0078] r k =r0α ｋ

[0079] Where l0 is the basic branch length in the tree model, that is, the trunk height, and r0 is the basic branch radius, that is, the trunk radius.

[0080] Step 2: During the single-phase grounding simulation of the tree model, the tree temperature, tree moisture content, ambient temperature, branch side area, and phase voltage of the distribution overhead wire are obtained.

[0081] Among them, the commonly used methods for obtaining tree temperature include:

[0082] In the experimental scenario, thermocouple sensors can be arranged at different locations of the tree (such as the trunk and main branches), or infrared thermometers can be used to measure the surface temperature of the tree;

[0083] Based on the thermal conductivity of trees and experimental data, the thermodynamic model is calibrated to estimate the internal temperature in real time.

[0084] The ambient temperature is usually regarded as the external boundary condition, which can be determined by using temperature and humidity sensors to monitor the ambient temperature at the fault site, or obtaining meteorological data of the fault area as input.

[0085] The lateral area of ​​branches is an important geometric parameter of the tree model, which is used to calculate the heat dissipation. The methods to obtain it include calculation based on the fractal structure of trees and experimental measurement. The calculation based on the fractal structure of trees assumes that branches and trunks are approximately uniform cylinders. According to the fractal geometric characteristics of trees, the cross-sectional area and total length of each level of branches are calculated, and then the lateral area is determined according to its surface geometry. Alternatively, the fractal scale factor is used to set the basic trunk area A. n and decreasing ratio r, through formula A n =A0·r n Estimate the sum of the lateral areas of all levels. Experimental measurements can use laser scanners to model the morphology of trees in three dimensions, extract geometric data, and calculate the lateral area of ​​branches.

[0086] The phase voltage of the distribution overhead conductor is used as a known input parameter for the subsequent calculation of the total heat generation, which can be obtained in the following ways:

[0087] On-site measurement, in the fault simulation device, use a voltage sensor (such as a voltage transformer or a voltage divider) to measure the phase voltage of a single-phase conductor;

[0088] Experimental setting, in the experiment or simulation, the phase voltage value of the conductor is set according to the common distribution system voltage level (such as 10kV, 35kV);

[0089] Real-time monitoring data,real-time phase voltage data is obtained from the online monitoring equipment of the distribution system,which is used as the input of the simulation model.

[0090] The water content of the tree is the absolute water content, which is obtained by experimental weighing and the calculation formula is:

[0091]

[0092] In the formula, m s is the fresh weight of the wood block obtained from the experimental sampling, and m0 is the absolute dry mass of the wood block.

[0093] In some other embodiments of the present invention, the moisture content can be indirectly calculated by measuring parameters such as resistance and capacitance of tree branches. The moisture content of trees can also be estimated using methods such as nuclear magnetic resonance (NMR), infrared spectroscopy, microwave measurement, and ultrasonic method.

[0094] Step three, calculate the tree path resistance based on the binary relationship between tree temperature and tree moisture content.

[0095] The calculation method of the tree path resistance is:

[0096] Assume that the conductivity at time t has a binary relationship σ with respect to the tree temperature T and the tree moisture content W t (W, T) is known, based on the characteristics of the tree branch network, the conductivity R of a branch in the kth level can be obtained k for:

[0097]

[0098] Then the tree path resistance R in the tree model is t It is expressed as follows:

[0099]

[0100] Where n is the level of the touch line in the tree model, and the comprehensive result is:

[0101]

[0102] Step 4: Calculate the heat generated by the electric current, the heat dissipated by convection and the heat dissipated by radiation of the tree model, and then calculate the heat absorbed by water evaporation and the heat absorbed by the temperature rise of the tree.

[0103] The calculation method of the heat generated by the current is:

[0104]

[0105] In the formula, Q s is the heat generated by the current, U is the phase voltage of the overhead distribution line, R t is the tree path resistance at time t.

[0106] It should be noted that the heat generated by the current calculated in the above formula is Q s The heat generated by a branch in one level of the tree model. When considering the current heat generation of the entire tree model, it is necessary to calculate the heat generated by a branch in each level separately, and then calculate the sum as the current heat generation of the entire tree model. The subsequent calculations of convective heat dissipation, radiation heat dissipation, water evaporation heat absorption and tree temperature rise heat absorption are all single-level heat in the tree model, and each level of heat dissipation or heat absorption must be summarized as the heat dissipation or heat absorption of the entire tree model.

[0107] When electric current passes through trees, the Joule heat (i.e. the heat generated by electric current) generated is consumed inside the trees in three ways: part of the heat is used to increase the temperature of the wet wood itself, part of it is transferred to the external environment through convection and radiation on the surface of the wood, and the rest is absorbed by water and becomes latent heat of evaporation of water, which is used for the phase change of water from liquid to gas.

[0108] The heat transferred to the external environment through convection and radiation from the wood surface can be calculated based on known conditions.

[0109] The calculation method of the convective heat dissipation per unit time is:

[0110] Q c =∫k c S ｃ (TT e )dt

[0111] In the formula, Q c is the convective heat dissipation, k c is the branch convection heat dissipation coefficient, S c is the lateral area of ​​the branch, T is the tree temperature, T e is the ambient temperature.

[0112] The Stefan-Boltzmann law states that an object at any temperature, if it is above absolute zero, will radiate energy to the outside world, and this radiation power is proportional to the fourth power of the surface temperature of the object. Therefore, the calculation method of the radiation heat dissipation per unit time is:

[0113]

[0114] In the formula, Q r is the radiation heat dissipation, k r is the branch radiation heat transfer coefficient, δ0 is the Stefan-Boltzmann constant, S c is the lateral area of ​​the branch, T is the tree temperature, T e is the ambient temperature.

[0115] When electric current passes through a tree, the Joule heat generated is distributed in different ways depending on the temperature of the tree. Apart from heat dissipation, when the temperature of the tree is low, most of the Joule heat is used to overcome the specific heat capacity of the wood, which is mainly manifested as an increase in the temperature of the wood. However, as the temperature gradually increases, the absorption of heat by water evaporation becomes the main way to dissipate heat, thereby inhibiting the further increase in temperature and converting heat into the energy required for the phase change of water. Therefore, the evaporation absorption proportional coefficient k in the Joule heat is defined as f for:

[0116] k f =(1+e 98-T ) -1

[0117] Where T is the tree temperature.

[0118] The calculation methods of the heat absorbed by water evaporation and the heat absorbed by tree temperature rise are:

[0119] Q w =(Q s -Q ｃ -Q r )·k f

[0120] Q t =(Q s -Q c -Q r )·(1-k f )

[0121] In the formula, Q w The heat absorbed by water evaporation, Q t The heat absorbed by the tree due to temperature rise, Q s Q is the heat generated by the current. c is the convective heat dissipation, Q r is the radiation heat dissipation, k f is the evaporation absorption proportional coefficient.

[0122] Step 5: Calculate the water evaporation mass according to the water evaporation heat absorption, and use the water evaporation mass to update the water content of the tree.

[0123] The calculation method of the water evaporation mass is:

[0124]

[0125] In the formula, Δm is the mass of water evaporated, Q w The heat absorbed by water evaporation, T w For the water temperature.

[0126] The new tree moisture content becomes W-ΔW, where:

[0127]

[0128] That is, the ratio of the water evaporation mass calculated by the above formula to the absolute dry mass of the wood block obtained in the experiment is taken as the change in tree moisture content.

[0129] The water evaporation mass and moisture content change calculated in this step are also the values ​​of a branch in one level of the tree model. When considering the water evaporation mass and moisture content change of the entire tree model, it is necessary to calculate a branch at each level separately and summarize each level as the water evaporation mass and moisture content change of the entire tree model.

[0130] Step six, calculating the tree temperature rise according to the tree temperature rise heat absorption, and using the tree temperature rise to update the tree temperature.

[0131] The calculation method of the tree temperature rise is:

[0132]

[0133] Where ΔT is the temperature rise of the tree, c t is the specific heat capacity of wood, c w is the specific heat capacity of water, m s is the fresh weight of the wood block obtained from the experimental sampling, and m0 is the absolute dry mass of the wood block.

[0134] The new tree temperature becomes T+ΔT.

[0135] The tree temperature rise change calculated in this step is also the value of a branch in one level of the tree model. When considering the tree temperature rise change of the entire tree model, it is necessary to calculate a branch at each level separately and summarize each level as the tree temperature rise of the entire tree model.

[0136] Step 7: Use the updated tree moisture content and tree temperature to iteratively calculate the tree path resistance and record it until the single-phase grounding simulation is completed, and use the recorded tree path resistance to build a resistance-time model.

[0137] When the power supply voltage U, environmental variables and other constant physical parameters are known, the initial resistance of the tree can be obtained by the calculation method of step three. The initial current and the initial total Joule heat generated can be calculated, and the convective heat dissipation and radiative heat dissipation at the current moment can be calculated from the initial values ​​according to the method described in step four. The water evaporation heat absorption and tree temperature rise heat absorption at the current moment are obtained from the convective heat dissipation and radiative heat dissipation at the current moment, and the temperature and moisture content changes in the current period are calculated according to steps five and six. The tree moisture content and tree temperature at the next moment of each period can be calculated to obtain the corresponding conductivity binary relationship, through R k , R t The calculation formula is used to obtain the fault resistance and total fault resistance of each level in the path at the next moment. It can be seen that the numerical calculation can form a closed loop and the iteration can be carried out continuously. Since the electrical power dissipation and resistivity of wood are related to temperature and moisture content, the temperature of the tree continues to rise, making R t Become a time-varying resistor.

[0138] Step eight, during the fault identification process, the resistance between the overhead distribution wire and the ground is monitored in real time, a resistance-time relationship is generated, and compared with the resistance-time model to identify the wire-tree-touching single-phase grounding fault.

[0139] Specifically, the resistance between the overhead distribution wire and the earth is monitored in real time, and the data that changes over time is recorded to generate the actual resistance-time curve. According to the fractal tree network model established in the single-phase grounding simulation, the theoretical resistance-time relationship curve is calculated. The data matching algorithm is used to compare the degree of fit between the two curves (the actual measurement curve and the model output curve). For example, by calculating the square error of all sampling points, the overall difference between the two curves is evaluated; or the maximum deviation between the two curves, especially the difference between the key points, is focused on. The correlation between the two curves (such as the Pearson correlation coefficient) can also be calculated to judge the consistency of the morphology. The closer the correlation coefficient is to 1, the higher the degree of fit between the two. A threshold and a time window are set. Within the time window, when the maximum absolute error or mean square error or Pearson correlation coefficient reaches the threshold, it is determined that the actual measured resistance-time relationship is consistent with the model, and a single-phase grounding fault caused by touching the tree has occurred.

[0140] like Figure 4 As shown in FIG. 1 , a comparative experiment of this embodiment is performed. In this comparative experiment, the values ​​are as follows: the trunk height is 1.5 m, the trunk radius is 0.05 m, the initial moisture content is 0.94, the initial temperature is 32°C, the tree specific heat capacity is 1400 J / kg*K, the tree density is 700 kg / m3, and the specific heat capacity of water is 4200 J / kg*K. By comparing with the actual whole tree test data, the following is obtained: Figure 4 The blue line is the tree resistance value measured over time during the comparative experiment, and the orange line is the resistance value output by the resistance-time model constructed in this embodiment over time. It can be seen that the current and temperature change trends can be accurately predicted in the early stage of the single-phase grounding fault of the distribution overhead wire touching the tree, and timely fault warning can be provided to avoid the hysteresis problem caused by the traditional static model.

[0141] like Figure 5 As shown in FIG. 1 , it is another comparative experiment of this embodiment. In this comparative experiment, the values ​​are as follows: the trunk height is 2m, the trunk radius is 0.033m, the initial moisture content is 0.715, the initial temperature is 35°C, the tree specific heat capacity is 1500J / kg*K, the tree density is 1000kg / m3, and the specific heat capacity of water is 4200J / kg*K. Compared with the actual whole tree test data, the following is obtained: Figure 4 The blue line is the tree resistance value measured over time during the comparative experiment, and the orange line is the resistance value output by the resistance-time model constructed in this embodiment over time. It can be seen that the actual value and the simulation value can not only accurately fit in the early stage, but also maintain a good fitting effect until the time point of 1000s.

[0142] Embodiment 2

[0143] This embodiment is a system for identifying a single-phase grounding fault caused by a power distribution overhead conductor touching a tree. The system is used to implement the simulation method described in the first embodiment, including:

[0144] The tree model building module is used to map the real shape of the tree into a fractal tree network model, obtain the shape parameters of the tree, including the trunk and several levels of branches, and set the structural dimensions between two adjacent levels to be in the same proportional relationship;

[0145] The parameter acquisition and setting module is used to obtain the tree temperature, ambient temperature and phase voltage of the distribution overhead wire during the single-phase grounding simulation process, and receive the parameters input manually;

[0146] A path resistance calculation module, used for calculating the path resistance of the tree according to the binary relationship between the tree temperature and the tree moisture content;

[0147] Thermodynamic calculation module, used to calculate the heat generation of electric current, convection heat dissipation, radiation heat dissipation, water evaporation heat absorption and tree temperature rise heat absorption based on the tree model;

[0148] A parameter updating module is used to calculate the water evaporation mass according to the water evaporation heat absorption, and update the tree moisture content according to the water evaporation mass; and calculate the tree temperature rise according to the tree temperature rise heat absorption, and update the tree temperature according to the tree temperature rise;

[0149] A resistance iteration module is used to iteratively calculate the tree path resistance according to the updated tree moisture content and tree temperature until the single-phase grounding simulation is completed, and record the tree path resistance to construct a resistance-time model;

[0150] The fault identification module is used to monitor the resistance between the distribution overhead wire and the earth in real time, generate the resistance-time relationship, and compare it with the resistance-time model to identify the single-phase grounding fault caused by the wire touching the tree.

[0151] The above is only a preferred embodiment of the present invention. It should be pointed out that a person skilled in the art can make several modifications and improvements without departing from the inventive concept of the present invention, which all belong to the protection scope of the present invention.

Claims

1. A method for identifying a single-phase grounding fault caused by a power distribution overhead conductor touching a tree, characterized in that: The following steps are involved: The real morphology of the tree is mapped into a fractal tree network model to construct a tree model and obtain tree shape parameters, wherein the tree model includes a trunk and several levels of branches, and the structural dimensions between two adjacent levels are in the same proportional relationship; In the single-phase grounding simulation process of the tree model, tree temperature, tree moisture content, ambient temperature, branch side area and phase voltage of the distribution overhead wire are obtained; Tree path resistance was calculated based on the binary relationship between tree temperature and tree moisture content; Calculate the heat generated by electric current, heat dissipation by convection and heat dissipation by radiation of the tree model, and then calculate the heat absorbed by water evaporation and heat absorbed by the temperature rise of the tree; Calculate the water evaporation mass according to the water evaporation heat absorption, and use the water evaporation mass to update the water content of the tree; Calculating the tree temperature rise based on the tree temperature rise heat absorption, and using the tree temperature rise to update the tree temperature; Iteratively calculate and record the tree path resistance using the updated tree moisture content and tree temperature until the single-phase grounding simulation is completed, and use the recorded tree path resistance to build a resistance-time model; During the fault identification process, the resistance between the distribution overhead wire and the ground is monitored in real time, and the resistance-time relationship is generated and compared with the resistance-time model to identify the single-phase grounding fault caused by the wire touching the tree.

2. A method for identifying a single-phase grounding fault caused by a power distribution overhead conductor touching a tree according to claim 1, characterized in that: The water content of the tree is the absolute water content, which is obtained by experimental weighing and the calculation formula is: In the formula, m s is the fresh weight of the wood block obtained from the experimental sampling, and m0 is the absolute dry mass of the wood block.

3. A method for identifying a single-phase grounding fault caused by a power distribution overhead conductor touching a tree according to claim 1, characterized in that: The calculation method of the tree path resistance is: In the tree model, α and β are defined as the length ratio and radius ratio of the branches of the k+1th layer network and the kth layer network, respectively, as follows: In the formula, l k , l k+1 are the branch lengths of the k-th layer network and the k+1-th layer network, r k 、r k+1 They are the branch radius of the k-th layer network and the branch radius of the k+1-th layer network respectively; According to the known binary relationship σ between the conductivity at time t and the tree temperature T and the tree moisture content W t (W,T), calculate the tree path resistance R t : Where n is the level of the touch line in the tree model, l0 is the basic branch length in the tree model, that is, the trunk height, and r0 is the basic branch radius, that is, the trunk radius.

4. A method for identifying a single-phase grounding fault caused by a power distribution overhead conductor touching a tree according to claim 1, characterized in that: The calculation method of the heat generated by the current is: In the formula, Q s is the heat generated by the current, U is the phase voltage of the overhead distribution line, R t is the tree path resistance at time t.

5. A method for identifying a single-phase grounding fault caused by a power distribution overhead conductor touching a tree according to claim 1, characterized in that: The calculation method of the convective heat dissipation is: Q c =∫k c s c (T-T e )dt In the formula, Q c is the convective heat dissipation, k c is the branch convection heat dissipation coefficient, S c is the lateral area of ​​the branch, T is the tree temperature, T e is the ambient temperature.

6. A method for identifying a single-phase grounding fault caused by a power distribution overhead conductor touching a tree according to claim 1, characterized in that: The calculation method of the radiation heat dissipation is: In the formula, Q r is the radiation heat dissipation, k r is the branch radiation heat transfer coefficient, δ0 is the Stefan-Boltzmann constant, S c is the lateral area of ​​the branch, T is the tree temperature, T e is the ambient temperature.

7. A method for identifying a single-phase grounding fault caused by a power distribution overhead conductor touching a tree according to claim 1, characterized in that: The calculation methods of the heat absorbed by water evaporation and the heat absorbed by tree temperature rise are: Q w =(Q s -Q c -Q r )·k f Q t =(Q s -Q c -Q r )·(1-k f ) k f =(1+e 98-T ) -1 In the formula, Q w The heat absorbed by water evaporation, Q t The heat absorbed by the tree due to temperature rise, Q s Q is the heat generated by the current. c is the convective heat dissipation, Q r is the radiation heat dissipation, k f is the evaporation absorption proportional coefficient, and T is the tree temperature.

8. A method for identifying a single-phase grounding fault caused by a power distribution overhead conductor touching a tree according to claim 1, characterized in that: The calculation method of the water evaporation mass is: In the formula, Δm is the mass of water evaporated, Q w The heat absorbed by water evaporation, T w For the water temperature.

9. A method for identifying a single-phase grounding fault caused by a power distribution overhead conductor touching a tree according to claim 1, characterized in that: The calculation method of the tree temperature rise is: Where ΔT is the temperature rise of the tree, c t is the specific heat capacity of wood, c w is the specific heat capacity of water, m s is the fresh weight of the wood block obtained from the experimental sampling, and m0 is the absolute dry mass of the wood block.

10. A distribution overhead conductor tree-touching single-phase grounding fault identification system, characterized in that: The system is used to implement the simulation method according to any one of claims 1 to 9, comprising: The tree model building module is used to map the real shape of the tree into a fractal tree network model, obtain the shape parameters of the tree, including the trunk and several levels of branches, and set the structural dimensions between two adjacent levels to be in the same proportional relationship; The parameter acquisition and setting module is used to obtain the tree temperature, ambient temperature and phase voltage of the distribution overhead wire during the single-phase grounding simulation process, and receive the parameters input manually; A path resistance calculation module, used for calculating the path resistance of the tree according to the binary relationship between the tree temperature and the tree moisture content; Thermodynamic calculation module, used to calculate the heat generation of electric current, convection heat dissipation, radiation heat dissipation, water evaporation heat absorption and tree temperature rise heat absorption based on the tree model; A parameter updating module is used to calculate the water evaporation mass according to the water evaporation heat absorption, and update the tree moisture content according to the water evaporation mass; and calculate the tree temperature rise according to the tree temperature rise heat absorption, and update the tree temperature according to the tree temperature rise; A resistance iteration module is used to iteratively calculate the tree path resistance according to the updated tree moisture content and tree temperature until the single-phase grounding simulation is completed, and record the tree path resistance to construct a resistance-time model; The fault identification module is used to monitor the resistance between the distribution overhead wire and the earth in real time, generate the resistance-time relationship, and compare it with the resistance-time model to identify the single-phase grounding fault caused by the wire touching the tree.

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