A method and system for identifying single-phase earth fault of power distribution overhead conductor

Abstract

The application discloses a kind of distribution overhead conductor tree contact single-phase ground fault identification method and system, the method includes the following steps: constructing tree model, obtaining tree shape parameter;In the single-phase ground simulation process of tree model, obtain tree temperature, tree moisture content, ambient temperature, branch side area and the phase voltage of distribution overhead conductor;Calculate tree path resistance, current heat production, convection heat dissipation, radiation heat dissipation, water evaporation heat absorption and tree temperature rise heat absorption;Further calculate water evaporation mass to update tree moisture content, calculate tree temperature rise to update tree temperature;Iterative calculation calculates tree path resistance and records, until single-phase ground simulation is completed, constructs resistance-time model;In fault identification, the resistance between distribution overhead conductor and ground is monitored in real time, compared with resistance-time model.The application can accurately and quickly predict distribution overhead conductor tree contact single-phase ground fault, and has strong applicability.

Description

Technical Field

[0001] The present 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 become frequent worldwide, causing significant casualties and economic losses. The issue of determining liability after fires has gradually become a focal point of public concern. my country's total power distribution line length exceeds 5.37 million kilometers, including over 4.37 million kilometers of overhead lines and over 3.89 million kilometers of rural overhead lines. Single-phase grounding faults caused by overhead conductors striking trees are common due to factors such as tree growth, strong winds, and landslides. The grounding current in a conductor-on-tree fault is characterized by a small initial value and a gradual increase as the fault progresses.

[0003] Currently, research on single-phase grounding faults caused by overhead distribution conductors contacting trees primarily focuses on experimental simulation of the fault and the extraction of electrical characteristics from specific tests. For example, the current and voltage gradients generated during a single-phase grounding fault caused by overhead distribution conductors contacting trees are measured, and the phenomena are observed and recorded through audio and image recording. Machine learning methods are used to analyze and identify the characteristics of the current signal in the time and frequency domains. Based on the changes in physical and electrical characteristics during the fault, the process of a single-phase grounding fault caused by overhead distribution conductors contacting trees is divided into four stages: temperature rise, water evaporation, tree carbonization, and flashover. Furthermore, the effects of different vegetation sizes on leakage current and arc discharge characteristics are analyzed.

[0004] The Chinese invention patent application with publication number CN115113092A discloses a tree line fault identification method, simulation device, system, computer equipment 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, and the fault type information, the separation speed information and the simulated fault waveform information are matched one by one to obtain a line fault waveform, wherein the line fault waveform is the 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. 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 is usually difficult to accurately reflect the slow change characteristics of the transition resistance of the contact point between the power distribution overhead conductor and the tree in the process of the single-phase grounding fault of the power distribution overhead conductor contacting the tree. This slow change characteristic is particularly important in the early stage of the single-phase grounding fault of the power distribution overhead conductor contacting the tree, and the scheme ignores the dynamic changes of the resistance under the influence of factors such as temperature, humidity and tree size. Due to the lack of accurate simulation of the slow change characteristics, the existing model shows poor adaptability in describing the fault development process of the single-phase grounding fault of the power distribution overhead conductor contacting the tree, thereby affecting the accurate assessment of the fire risk. Moreover, some models are only valid for the parameters in the test, and when the environmental conditions such as voltage level, tree type and contact position change, the model shows significant limitations. This lack of universality makes it difficult for the existing model to be widely applied to complex and variable actual environments. SUMMARY

[0006] The present application provides a method and system for identifying a single-phase grounding fault of a power distribution overhead conductor contacting 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] To solve the above technical problems, the fault identification method provided by the present application comprises the following steps:

[0008] The real shape of the tree is mapped into a fractal tree network model to construct a tree model, and the tree model includes a trunk and a plurality of levels of branches, and the structure size between adjacent two levels is in a same proportional relationship;

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

[0010] The tree path resistance is calculated according to the binary relationship between the tree temperature and the tree moisture content;

[0011] The current heat production, the convective heat dissipation and the radiative heat dissipation of the tree model are calculated, and then the water evaporation heat absorption and the tree temperature rise heat absorption are calculated;

[0012] The water evaporation mass is calculated according to the water evaporation heat absorption, and the tree moisture content is updated using the water evaporation mass;

[0013] The tree temperature rise is calculated according to the tree temperature rise heat absorption, and the tree temperature is updated using the tree temperature rise;

[0014] The updated tree moisture content and tree temperature are used to iteratively calculate and record the tree path resistance until the single-phase grounding simulation is completed, and a resistance-time model is constructed using the recorded tree path resistance;

[0015] During the fault identification process, the resistance between the distribution overhead conductor and the ground is monitored in real time, and a resistance-time relationship is generated. This relationship is then compared with the resistance-time model to identify single-phase grounding faults caused by conductors touching trees.

[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] Where 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 tree path resistance is calculated as follows:

[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, respectively, as follows:

[0021]

[0022] Where, l k 、l k+1 are the length of the branches of the k-th layer network and the length of the branches of the k+1-th layer network, r k 、r k+1 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] Where Q s is the heat generated by the current, U is the phase voltage of the distribution overhead wire, 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 (T-T e )dt

[0031] wherein Q c is the convective heat loss, k c is the convective heat loss coefficient of the branch, S c is the lateral area of the branch, T is the temperature of the tree, and T e is the ambient temperature.

[0032] Preferably, the calculation method of the radiative heat loss is:

[0033]

[0034] wherein Q r is the radiative heat loss, k r is the radiative heat transfer coefficient of the branch, δ0 is the Stefan-Boltzmann constant, S c is the lateral area of the branch, T is the temperature of the tree, and T e is the ambient temperature.

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

[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] wherein Q w is the water evaporation heat absorption, Q t is the temperature rise heat absorption of the tree, Q s is the heat production of the current, Q c is the convective heat loss, Q r is the radiative heat loss, k f is the evaporation absorption proportionality coefficient, and T is the temperature of the tree.

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

[0041]

[0042] wherein Δm is the water evaporation mass, Q wThe heat absorbed by water evaporation, T w is 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] Another aspect of the present invention provides a system for identifying single-phase grounding faults caused by contact with a tree of a power distribution overhead conductor. The system is used to implement the above-mentioned simulation method, comprising:

[0047] The tree model construction module is used to map the real shape of the tree into a fractal tree network model, obtain the tree's shape parameters, 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] Parameter acquisition and setting module, used to obtain tree temperature, ambient temperature and phase voltage of distribution overhead wires during single-phase grounding simulation, and receive manually input parameters;

[0049] A path resistance calculation module is used to calculate the path resistance of trees based on the binary relationship between tree temperature and 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 based on the water evaporation heat absorption, and update the tree moisture content based on the water evaporation mass; and calculate the tree temperature rise based on the tree temperature rise heat absorption, and update the tree temperature based on the tree temperature rise;

[0052] The resistance iteration module is used to iteratively calculate the tree path resistance based on 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 conductor and the ground in real time, generate a resistance-time relationship, and compare it with the resistance-time model to identify single-phase grounding faults caused by conductors touching trees.

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

[0055] 1. The fault identification method proposed in this paper accurately models the current path, simplifying the tree into a cylinder with uniform electrical and thermal properties. Combining contact resistance and arc resistance, it accurately simulates the path of current through the tree, taking into account changes in resistance with tree temperature and humidity. Using an iterative calculation approach, tree parameters such as conductivity, temperature, and moisture content are updated in real time during each calculation cycle. This enables the model to dynamically reflect real-time changes during the TSF process. This enables accurate simulation of the electrical characteristics during the fault process, improving 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 the tree under different environmental conditions, avoid the errors in the traditional model, and improve 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 this paper introduces a quantitative relationship between Joule heating and water evaporation, clarifying how the Joule heating caused by current is distributed within the tree. Part of the Joule heating is used to increase the tree's temperature, while the remaining part is used to evaporate water. This reduces the calculation error caused by underestimation of the temperature rise process in traditional models and improves the accuracy of the present invention's time-varying resistance prediction.

[0058] 4. The fault identification method proposed in this paper incorporates an iterative calculation mechanism, enabling the model to update tree parameters such as temperature, humidity, and conductivity in real time, reflecting dynamic changes during the fault process. This improvement enables the model to not only accurately predict current and temperature trends but also provide timely fault warnings, avoiding the lag associated with traditional static models and effectively improving 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 computing 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 flow chart 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 chart of actual values ​​and simulation values ​​according to 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 caused by a distribution overhead conductor touching a tree, and to perform fault identification 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, they are more likely to cause fires. Therefore, the embodiment of the present invention provides a detailed description of the electrical characteristics simulation and fault identification of the entire process of a single-phase grounding fault caused by a 10kV distribution overhead conductor touching a tree, and can also be used for the electrical characteristics simulation and fault identification of the entire process of a single-phase grounding fault caused by a distribution overhead conductor touching a tree at other voltage levels.

[0067] Example 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 following steps are included from step 1 to step 8:

[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 branches and finally flows through the trunk into the ground. 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 properties, ignoring the transitions between branches. This simplifies the model structure for ease of calculation. The electrical conductivity of trees is affected by temperature and moisture content, so the key parameters in the model are temperature, moisture content, and tree size. By modeling the electrical and thermal characteristics of trees, the current and development of TSFs (Tree-contact Single-phase-to-ground Faults) can be accurately simulated.

[0072] As a typical natural fractal structure, trees have similar geometric features in their overall shape from trunk 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 in the figure, 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 bifurcations is not discussed. The tree-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, respectively, as follows:

[0074]

[0075] Where, l k 、l k+1 are the length of branches in the k-th layer network and the length of branches in the k+1-th layer network, r k 、r k+1 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 conductor are obtained.

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

[0082] In the experimental scenario, thermocouple sensors can be placed 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 and can be determined by using temperature and humidity sensors to monitor the ambient temperature at the fault site, or by obtaining meteorological data of the fault area as input.

[0085] The lateral area of ​​branches is an important geometric parameter of the tree model and is used to calculate heat dissipation. It can be obtained by 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 geometry of trees, the cross-sectional area and total length of each level of branches are calculated, and the lateral area is determined based on their surface geometry. Alternatively, the fractal scale factor is used to set the basic trunk area A. n and decreasing ratio r, by formula A n =A0·r n Estimate the sum of the lateral areas of all layers. Experimental measurements can use a laser scanner to perform three-dimensional modeling of the tree's morphology, extract geometric data, and calculate the lateral area of ​​the branches.

[0086] The phase voltage of the distribution overhead conductor is used as a known input parameter for the subsequent calculation of the total heating value. It can be obtained in the following ways:

[0087] On-site measurement: In a fault simulation device, use a voltage sensor (such as a voltage transformer or 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 moisture content of the tree is the absolute moisture content, which is obtained by experimental weighing and the calculation formula is:

[0091]

[0092] Where 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 3: 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 t It is expressed as follows:

[0099]

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

[0101]

[0102] Step 4: Calculate the heat generated by the 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] Where 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 Q calculated in the above formula is s = is the heat generated by a branch at one level in the tree model. When considering the current heat generation of the entire tree model, the heat generation of each branch at each level needs to be calculated separately and then summed to form the current heat generation of the entire tree model. Subsequent calculations of convective heat dissipation, radiative heat dissipation, water evaporation heat absorption, and tree temperature rise heat absorption all represent the heat of a single level within the tree model. The heat dissipation or absorption at each level needs to be summed to form the heat dissipation or absorption of the entire tree model.

[0107] When electric current passes through a tree, the Joule heat (i.e., the heat generated by the electric current) generated is consumed inside the tree 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 heat dissipation on the surface of the wood, and the remaining part is absorbed by the water and becomes the latent heat of evaporation of the water, which is used for the phase change of the 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] Where 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 above absolute zero will radiate energy to the outside world, and the radiation power is proportional to the fourth power of the surface temperature. Therefore, the calculation method for the radiation heat dissipation per unit time is:

[0113]

[0114] Where 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. Excluding heat dissipation, when the tree temperature 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 wood temperature. However, as the temperature gradually rises, water evaporation heat absorption becomes the main way to dissipate heat, thereby inhibiting further temperature rise and converting heat into energy required for water phase change. Therefore, the evaporation absorption proportional coefficient k in 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 for 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] Where Q w The heat absorbed by water evaporation, Q t The heat absorbed by the tree due to temperature rise, Q s is the heat generated by the current, Q 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 based on the water evaporation heat absorption, and use the water evaporation mass to update the tree moisture content.

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

[0124]

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

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

[0127]

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

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

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

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

[0132]

[0133] In the formula, ΔT is the tree temperature rise, 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 by sampling, and m0 is the oven-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 one branch in one level in the tree model. When considering the tree temperature rise change of the entire tree model, one branch in each level needs to be calculated respectively, and each level is summarized as the tree temperature rise of the entire tree model.

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

[0137] Under the condition that the power supply voltage U and the 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. That is, the initial current and the initial total Joule heat generated can be calculated, and the current time convective heat loss and the current time radiative heat loss can be calculated from the initial value according to the method described in step four. The current time moisture evaporation heat absorption and the current time tree temperature rise heat absorption are obtained from the current time convective heat loss and the current time radiative heat loss, and the temperature and moisture content change amount in the current period are calculated according to steps five and six. The tree moisture content and tree temperature at the next time of each period can be calculated to obtain the corresponding conductivity binary relationship, and the fault resistance of each level in the path and the total fault resistance at the next time can be obtained through the calculation formula of R k , R t . It can be seen that numerical calculation can form a closed loop, and iteration can continue. Since the electrical power dissipation and resistivity of wood are related to temperature and moisture content, the temperature of the tree is constantly rising, making R t a time-varying resistance.

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

[0139] Specifically, the resistance between the overhead power distribution conductor and the ground is monitored in real time, the data changing over time is recorded, and an actual resistance-time curve is generated. According to the fractal tree network model established in the single-phase grounding simulation, a theoretical resistance-time relationship curve is calculated. The data matching algorithm is used to compare the coincidence degree of the two curves (the actual measured curve and the model output curve). For example, by calculating the squared error of all sampling points, the overall difference between the two curves is evaluated; or the maximum deviation between the two curves is focused on, especially the difference at the key points. The correlation (such as the Pearson correlation coefficient) of the two curves can also be calculated to judge the consistency of the shape, and the closer the correlation coefficient is to 1, the higher the coincidence degree of the two curves. A threshold and a time window are set, and when the maximum absolute error or the mean square error or the Pearson correlation coefficient reaches the threshold in the time window, it is determined that the actual measured resistance-time relationship coincides with the model, and it is determined that a tree contact single-phase grounding fault occurs.

[0140] As shown in Figure 4 , it is a comparative experiment of the present embodiment, in which the numerical values are as follows: the height of the trunk is 1.5 m, the radius of the trunk is 0.05 m, the initial moisture content is 0.94, the initial temperature is 32℃, the specific heat capacity of the tree is 1400 J / kg*K, the density of the tree is 700 kg / m3, and the specific heat capacity of water is 4200 J / kg*K. Compared with the actual whole tree test data, the effect as shown in Figure 4 is obtained. Among them, the blue line is the resistance value of the tree measured over time during the comparative experiment, and the orange line is the resistance value output by the resistance-time model constructed in the present embodiment. It can be seen that the change trend of current and temperature can be accurately predicted in the early stage of the tree contact single-phase grounding fault of the overhead power distribution conductor, and timely fault warning can be provided to avoid the lag problem caused by the traditional static model.

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

[0142] Embodiment Two

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

[0144] The tree model construction module is used to map the real shape of the tree into a fractal tree network model, obtain the tree's shape parameters, 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] Parameter acquisition and setting module, used to obtain tree temperature, ambient temperature and phase voltage of distribution overhead wires during single-phase grounding simulation, and receive manually input parameters;

[0146] A path resistance calculation module is used to calculate the path resistance of trees based on the binary relationship between tree temperature and 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 based on the water evaporation heat absorption, and update the tree moisture content based on the water evaporation mass; and calculate the tree temperature rise based on the tree temperature rise heat absorption, and update the tree temperature based on the tree temperature rise;

[0149] The resistance iteration module is used to iteratively calculate the tree path resistance based on 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 conductor and the ground in real time, generate a resistance-time relationship, and compare it with the resistance-time model to identify single-phase grounding faults caused by conductors touching trees.

[0151] The above description is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this field, several variations and improvements can be made without departing from the creative concept of the present invention, which all fall within the scope of protection of the present invention.

Claims

1. A method for identifying a single-phase grounding fault caused by a distribution overhead conductor touching a tree, characterized in that: The following steps are involved: Mapping the true morphology of trees 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 of two adjacent levels are in the same proportional relationship; During the single-phase grounding simulation of the tree model, tree temperature, tree moisture content, ambient temperature, branch side area, and phase voltage of the distribution overhead conductor are obtained; The tree path resistance is calculated based on the binary relationship between tree temperature and tree moisture content. The tree path resistance is calculated as follows: In the tree model, define Respectively Layer network branches and The branch length ratio and radius ratio of the layer network are expressed as follows: Where, They are The length of the branches and the The length of branches in the layer network, They are The branch radius and the The branch radius of the layer network; according to the known Temporal conductivity with respect to tree temperature and tree moisture content The binary relationship , calculate the tree path resistance : Where, is the level of the touch line in the tree model, is the basic branch length in the tree model, that is, the trunk height, is the basic branch radius, i.e. the trunk radius; Calculate the heat generated by the 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 tree's temperature rise; Calculating water evaporation mass according to the water evaporation heat absorption, and using the water evaporation mass to update the tree moisture content; calculating the tree temperature rise based on the tree temperature rise heat absorption, and updating the tree temperature using the tree temperature rise; The tree path resistance is iteratively calculated using the updated tree moisture content and tree temperature and recorded until the single-phase grounding simulation is completed. The resistance-time model is constructed using the recorded tree path resistance. During the fault identification process, the resistance between the distribution overhead conductor and the ground is monitored in real time, and a resistance-time relationship is generated. This relationship is then compared with the resistance-time model to identify single-phase grounding faults caused by conductors touching trees.

2. A method for identifying a single-phase grounding fault caused by a distribution overhead conductor touching a tree according to claim 1, characterized in that: The moisture content of the tree is the absolute moisture content, which is obtained by experimental weighing and the calculation formula is: Where, Fresh weight of wood blocks obtained for experimental sampling, It is the absolute dry mass of the wood block.

3. A method for identifying a single-phase grounding fault caused by a distribution overhead conductor touching a tree according to claim 1, characterized in that: The calculation method of the heat generated by the current is: Where, is the heat generated by the current, is the phase voltage of the distribution overhead conductor, for moment tree path resistance.

4. A method for identifying a single-phase grounding fault caused by a distribution overhead conductor touching a tree according to claim 1, characterized in that: The calculation method of the convective heat dissipation is: Where, is the convective heat dissipation, is the branch convection heat dissipation coefficient, is the lateral area of ​​the branch, is the tree temperature, is the ambient temperature.

5. The method for identifying a single-phase grounding fault caused by a power distribution overhead conductor touching a tree according to claim 1, wherein: The calculation method of the radiant heat dissipation is: Where, is the radiation heat dissipation, is the branch radiation heat transfer coefficient, is the Stefan-Boltzmann constant, is the lateral area of ​​the branch, is the tree temperature, is the ambient temperature.

6. A method for identifying a single-phase grounding fault caused by a distribution overhead conductor touching a tree according to claim 1, characterized in that: The calculation methods for the heat absorbed by water evaporation and the heat absorbed by tree temperature rise are: Where, Absorbs heat for water evaporation, As the trees absorb heat, is the heat generated by the current, is the convective heat dissipation, is the radiation heat dissipation, is the evaporation absorption proportional coefficient, is the tree temperature.

7. A method for identifying a single-phase grounding fault caused by a distribution overhead conductor touching a tree according to claim 1, characterized in that: The calculation method of the water evaporation mass is: Where, is the mass of water evaporation, Absorbs heat for water evaporation, is the water temperature.

8. The method for identifying a single-phase grounding fault caused by a distribution overhead conductor touching a tree according to claim 1, wherein: The calculation method of the tree temperature rise is: Where, For tree temperature rise, is the specific heat capacity of wood, is the specific heat capacity of water, Fresh weight of wood blocks obtained for experimental sampling, It is the absolute dry mass of the wood block.

9. A system for identifying single-phase grounding faults caused by overhead power distribution conductors touching a tree, characterized in that: The system is used to implement the simulation method according to any one of claims 1 to 8, comprising: The tree model construction module is used to map the real shape of the tree into a fractal tree network model, obtain the tree's shape parameters, including the trunk and several levels of branches, and set the structural dimensions between two adjacent levels to be in the same proportional relationship; Parameter acquisition and setting module, used to obtain tree temperature, ambient temperature and phase voltage of distribution overhead wires during single-phase grounding simulation, and receive manually input parameters; A path resistance calculation module is used to calculate the path resistance of trees based on the binary relationship between tree temperature and 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 based on the water evaporation heat absorption, and update the tree moisture content based on the water evaporation mass; and calculate the tree temperature rise based on the tree temperature rise heat absorption, and update the tree temperature based on the tree temperature rise; The resistance iteration module is used to iteratively calculate the tree path resistance based on 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 conductor and the ground in real time, generate a resistance-time relationship, and compare it with the resistance-time model to identify single-phase grounding faults caused by conductors touching trees.

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