A power distribution line tree line early fault identification method

Abstract

The application belongs to the technical field of distribution line, and discloses a kind of distribution line tree line early fault generation mechanism and identification method, including obtaining complete waveform data of fault phase voltage and fault ground current in entire fault cycle, judging waveform distortion condition according to waveform slope;Based on zero-holiday detection method, judge whether the fault waveform satisfies the three-stage form of "zero-holiday distortion-no zero-holiday distortion-zero-holiday distortion";On the basis of meeting three-stage characteristics, the change trend of current zero-holiday duration in the third stage is calculated to judge whether the fault is early fault.The distribution line tree line early fault generation mechanism and identification method provided by the application uses current intensity, distribution characteristics of zero-point distortion and change trend of zero-point distortion duration as identification parameters, extracts these fault characteristics according to interval slope, eliminates the influence of ground medium nonlinear characteristics on fault characteristic extraction, and realizes the judgment of tree line early fault.

Description

Technical Field

[0001] This invention relates to the technical field of power distribution lines, and more particularly to a method for early fault identification of power distribution line trees. Background Technology

[0002] In suburban and mountainous areas, power distribution lines are mainly overhead bare conductors. The vegetation along these lines is abundant and grows rapidly, leading to many single-phase grounding faults caused by tree-conductor contact. As a typical high-resistance grounding fault, tree-conductor faults are characterized by small grounding currents, weak electrical characteristics, and difficulty in triggering protection actions, making them one of the most difficult types of faults to detect and inspect. During the development of a tree-conductor fault, fires may occur in three ways: (1) the accompanying electric arc ignites trees or nearby combustibles; (2) molten metal fragments from the aluminum stranded wire fall to the ground and ignite dead branches and leaves; (3) the thermal effect of the fault current flowing through the tree causes the tree to heat up and catch fire. Therefore, the earlier tree-conductor faults caused by tree contact are identified, the sooner the potential for faults can be eliminated, reducing the likelihood of fire accidents.

[0003] Existing research primarily analyzes tree-line faults in two scenarios: first, faults caused by the trees themselves, such as trees falling in severe weather and coming into contact with conductors, causing tree-line grounding faults, or branches breaking in strong winds and falling over conductors, bridging multiple phase conductors and causing phase-to-phase faults; and second, tree-line grounding faults caused by overhead conductors breaking due to external factors and the broken conductor falling onto trees. However, these two types of faults are mostly permanent tree-line faults caused by relatively extreme weather conditions. In actual distribution network operation, only a few extreme scenarios such as conductor breakage and tree falling can lead to permanent tree-line grounding faults. In most cases, tree-line faults are transient grounding faults caused by branches growing near conductors making brief contact with conductors during swaying. These faults have self-clearing characteristics and are called early tree-line faults. Such faults are often considered disturbances and are ignored, leading to their intermittent and frequent occurrence over a considerable period of time. Early faults in tree-line power lines are often accompanied by arc discharge processes, which can lead to the melting and breakage of aluminum stranded wires, as well as the carbonization and burning of tree branches. This causes early faults to gradually evolve into permanent faults such as wire breaks. The resulting molten metal fragments and embers also pose a risk of fire. Researching and establishing the correspondence between the development process of early faults in tree-line power lines and the characteristics of fault signals can effectively guide the detection and identification of early faults in tree-line power lines, thereby narrowing the scope of fault investigation, improving line inspection efficiency, eliminating hidden dangers before faults develop into permanent faults or cause serious accidents such as fires, and improving the overall operation and maintenance level of power distribution lines.

[0004] The voltage and current waveform disturbances caused by early-stage tree-line faults can be detected and recorded by substation line selection devices or feeder intelligent terminals (such as FTU, DTU, TTU), and can be used as waveform data for fault identification. However, existing studies mostly classify tree-line grounding faults as high-resistance grounding faults or arcing high-resistance grounding faults, treating the grounding transition resistance as a fixed resistance, and introducing typical algebraic arc models such as the Mayr model, Cassie model, and logarithmic model as equivalents of the fault arc. However, the arc, as the main source of the nonlinear characteristics of high-resistance grounding fault waveforms, exhibits characteristics such as length, temperature, and extinguishing time that dynamically change during the development process and are closely related to factors such as the type of faulty equipment, fault conditions, and external environment. Traditional arc models can only reflect the common characteristics of arcs and fail to establish the connection between the dynamic characteristics of the arc and the fault development process, thus failing to identify the fault type. Modeling studies on treeline faults treat them as permanent faults and model and analyze the gradual change process of tree transition resistance. However, they neglect the transient characteristics of early treeline faults and the dynamic changes of fault arcs. This makes the above methods more effective in the case of permanent faults, but not suitable for the identification of early treeline faults. Summary of the Invention

[0005] The purpose of this section is to outline some aspects of embodiments of the present invention and to briefly describe some preferred embodiments. Simplifications or omissions may be made in this section, as well as in the abstract and title of this application, to avoid obscuring the purpose of these documents; however, such simplifications or omissions should not be construed as limiting the scope of the invention.

[0006] In view of the aforementioned existing problems, the present invention is proposed.

[0007] Therefore, the technical problem solved by the present invention is that the existing technology cannot identify the early fault type of the tree line.

[0008] To solve the above-mentioned technical problems, the present invention provides the following technical solution: a method for early fault identification of power distribution line trees, comprising: Obtain complete waveform data of the fault phase voltage and fault ground current throughout the entire fault cycle, and determine the waveform distortion based on the waveform slope. The method of zero-wave detection is used to determine whether the fault waveform meets the three-stage form of "with zero-wave distortion - without zero-wave distortion - with zero-wave distortion". Based on the three-stage characteristics, the changing trend of the current zero-rest duration in the third stage is calculated to determine whether the fault is an early-stage fault.

[0009] As a preferred embodiment of the early fault identification method for power distribution line trees described in this invention, wherein: the fault current recording sequence is defined as I(n), then at the nth... sat each sampling point (n) s , I(n s The interval slope k(n) s ) is represented as: Where n is the number of sampling points, n = 1, 2, 3, ..., N, To calculate the length of the interval, , N T The number of sampling points in each period.

[0010] As a preferred embodiment of the early fault identification method for power distribution lines described in this invention, the method includes: determining whether there is a zero-crossing at each zero-crossing point of the fault waveform, including: Taking the first positive / negative peak of the fault waveform as the starting point and the last positive / negative peak as the ending point, count the number of zero-crossings of the current between the starting and ending points, and define the nth zero-crossing as Z. n ; Calculate the interval slope distribution k of the waveform between the start and end points, and plot the interval slope distribution curve; In Z n Nearby, find the two nearest points with the minimum slope in the interval, and define the corresponding sampling points as N in sequence. n1 N n2 The absolute value of the corresponding current intensity is defined as |I n1 |with|I n2 |; In the interval (N) n1 , N n2 Within a range, for sampling points that satisfy |I(n)| < 0.1, the sampling point with the smallest interval slope is denoted as (N). nv k nv ).

[0011] As a preferred embodiment of the early fault identification method for power distribution lines described in this invention, the method includes: determining whether there is a zero-crossing at each zero-crossing point of the fault waveform, including: When the interval slope curve is in the interval (N) n1 , N n2 Within a certain range, there exist at least two local maxima points, and the distance from point (N) is such that... nv k nv The two most recent local maxima are (N) np1 k np1 The two local maxima (N) np2 k np2 (If zero days off are required, the following conditions must be met:) N np1 <(N n1 +Nn2 ) / 2 <N np2 N np1 <N nv <N np2 k nv <k np1 &k nv <k np2 When the state of each current zero-crossing point does not meet the conditions, the distortion of its waveform mainly originates from the nonlinear characteristics of the equivalent grounding resistance of the medium at the grounding point, which runs through the entire fault. When calculating the zero-crossing duration, it needs to be excluded.

[0012] As a preferred embodiment of the method for early fault identification of power distribution line trees according to the present invention, the method further includes: judging whether there is a zero-crossing at each zero-crossing point of the fault waveform; At the zero-crossing point where zero rest may exist, the zero-crossing point (N) nv k nv ), (N np1 k np1 ), (N np2 k np2 When zero days off exist, the following conditions must be met: k nv <0.7×k std ×(|I n1 |+|I n2 |) / 2 (k np1 + k np2 ) / 2>0.9×k std ×(|I n1 |+|I n2 |) / 2 Where, k std This represents the slope of the unit sine curve at the zero-crossing current interval under this sampling rate.

[0013] When the state of each current zero-crossing point does not meet the conditions, the distortion of its waveform mainly originates from the nonlinear characteristics of the equivalent grounding resistance of the medium at the grounding point, which runs through the entire fault. When calculating the zero-crossing duration, it needs to be excluded.

[0014] As a preferred embodiment of the method for early fault identification of power distribution line trees according to the present invention, the calculation of the zero-downtime includes: The degree of drop in the interval slope is expressed as: Dn= (k np1 -k nv ) / 2+(k np2 -knv ) / 2 The average drop level is expressed as: k ave = (D1+…+D S ) / S Where S is the number of zero crossings that do not meet the conditions.

[0015] As a preferred embodiment of the method for early fault identification of power distribution line trees according to the present invention, the calculation of the zero-downtime includes: There exists a zero-crossing point (N) of zero rest. nz1 k nz1 ) and (N nz2 k nz2 ) Must meet: N np1 <N nz1 <N nv <N nz2 <N np2 ∀ k ni ∈(k nv k np1 ), k nz1 -(k np1 - k ave )≤ k ni -(k np1 - k ave ) ∀ k ni ∈(k nv k np2 ), k nz2 -(k np2 - k ave ) ≤ k ni -(k np2 - k ave ) At the zero-crossing point Zn, the number of sampling points for the zero-rest duration is: N nz = N nz2 - N nz1 As a preferred embodiment of the method for early fault identification of power distribution line trees according to the present invention, the calculation of the zero-downtime further includes: Arrange the durations of zero-day breaks in chronological order as N. z ={ N z1 N z2 ,…, N zn}, take N zi and N z(i+c) Form a pair of data (N) zi Nz(i+c) ), N zi and N z(i+c) ∈N z , Define Delta i =N zi -N z(i+c) Delta i The quantity >0 is represented by D. + Delta i The number of values ​​less than 0 is represented by D. - , The beneficial effects of the present invention are as follows: The method for identifying early faults in distribution lines provided by the present invention uses the distribution characteristics of current intensity, zero-point distortion and the trend of zero-point distortion duration as identification parameters. These fault features are extracted according to the interval slope, eliminating the influence of the nonlinear characteristics of the grounding medium on the extraction of fault features, and realizing the judgment of early faults in distribution lines. Attached Figure Description

[0016] To more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings used in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort. Wherein: Figure 1 This is an overall flowchart of a method for early fault identification of power distribution line tree lines according to an embodiment of the present invention; Figure 2 The slope distribution diagram of a sinusoidal discrete current sequence and a discrete current sequence with arc zero rest provided in an embodiment of the present invention for an early fault identification method for power distribution line tree lines. Figure 3 Normalized current waveform diagram of an early fault identification method for power distribution line tree lines provided in an embodiment of the present invention; Figure 4 An overall experimental circuit diagram of a method for early fault identification of power distribution line tree lines provided in one embodiment of the present invention; Figure 5 A tree-line early fault simulation diagram provided in an embodiment of the present invention for a tree-line early fault identification method for power distribution lines; Figure 6 A waveform diagram of the fault phase voltage and grounding current at the fault location in a method for early fault identification of power distribution lines provided in an embodiment of the present invention; Figure 7The normalized interval slope diagram of the current waveform at the fault point in a method for early fault identification of power distribution line tree lines provided in an embodiment of the present invention. Figure 8 The duration and variation curves of each zero point in a method for early fault identification of power distribution line tree lines provided in an embodiment of the present invention; Figure 9 This invention provides a method for early fault identification of power distribution line tree lines, showing fault current waveforms and normalized interval slope curves corresponding to different fault scenarios in an embodiment of the present invention. Figure 10 A zero-rest duration variation curve of an early fault identification method for power distribution lines provided in an embodiment of the present invention; Figure 11 The fault current waveforms and normalized interval slope curves corresponding to three different samples of an early fault identification method for power distribution line tree lines provided in an embodiment of the present invention. Figure 12 Waveform diagrams of different tree species in the non-arc contact stage of a method for early fault identification of power distribution line trees provided in an embodiment of the present invention; Figure 13 The current waveform diagram during the contact stage of an early fault identification method for power distribution lines provided in an embodiment of the present invention. Detailed Implementation

[0017] To make the above-mentioned objects, features, and advantages of the present invention more apparent and understandable, specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of them. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort should fall within the protection scope of the present invention.

[0018] Many specific details are set forth in the following description in order to provide a full understanding of the invention. However, the invention may also be practiced in other ways different from those described herein, and those skilled in the art can make similar extensions without departing from the spirit of the invention. Therefore, the invention is not limited to the specific embodiments disclosed below.

[0019] Secondly, the term "one embodiment" or "embodiment" as used herein refers to a specific feature, structure, or characteristic that may be included in at least one implementation of the present invention. The phrase "in one embodiment" appearing in different places in this specification does not necessarily refer to the same embodiment, nor is it a single or selective embodiment that is mutually exclusive with other embodiments.

[0020] This invention is described in detail with reference to the schematic diagrams. When detailing the embodiments of this invention, for ease of explanation, the cross-sectional views illustrating the device structure may be partially enlarged, not adhering to the usual scale. Furthermore, the schematic diagrams are merely examples and should not be construed as limiting the scope of protection of this invention. In actual fabrication, the three-dimensional spatial dimensions of length, width, and depth should be included.

[0021] Furthermore, in the description of this invention, it should be noted that the terms "upper," "lower," "inner," and "outer," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. These terms are used solely for the convenience of describing the invention and for simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on the invention. In addition, the terms "first," "second," or "third" are used for descriptive purposes only and should not be construed as indicating or implying relative importance.

[0022] Unless otherwise explicitly specified and limited, the terms "installation," "connection," and "joining" in this invention should be interpreted broadly. For example, they can refer to fixed connections, detachable connections, or integral connections; similarly, they can refer to mechanical connections, electrical connections, or direct connections, or indirect connections through an intermediate medium, or internal connections between two components. Those skilled in the art can understand the specific meaning of the above terms in this invention based on the specific circumstances.

[0023] Example 1 Reference Figure 1 —4, is an embodiment of the present invention, which provides a method for early fault identification of power distribution line trees, including: S1: Obtain complete waveform data of the fault phase voltage and fault ground current throughout the entire fault cycle, and determine the waveform distortion based on the waveform slope; Furthermore, the mechanisms underlying early treeline failures include: Trees growing along overhead power lines will not cause tree-to-line contact faults when the lines are still and windless. However, when branches sway in the wind, the distance between the branches and the conductors shortens dramatically, causing them to come into contact with the conductors and creating a tree-to-line grounding fault. As the wind weakens or disappears, the branches bounce back, and the fault is cleared by themselves.

[0024] It should be noted that in real-world scenarios, the swaying of trees in the wind is a rather complex mechanical process. The trunk of a tree vibrates at a certain natural frequency driven by the wind, while the branches vibrate along with the trunk and are further swayed by the airflow. In this study, we will ignore the overall vibration of the tree and focus on the movement of branches that come into contact with the conductor. The swaying is mainly caused by the "vortex shedding" effect formed by the airflow near the branches, so the swaying speed is closely related to the wind strength. Natural wind levels are typically 3 to 6, with wind speeds of 3.4 m / s to 13.8 m / s. For a branch with a diameter of 1 cm, at a wind speed of 10 m / s, its one-sided vortex shedding frequency is approximately 200 Hz, and the average swaying speed is approximately 0.162 m / s. Therefore, under common wind conditions, the swaying speed of the branch is approximately 0.1 m / s to 0.2 m / s.

[0025] Furthermore, such as Figure 2 As shown, the actual development process of early tree-line faults can be divided into three stages: "branch approaching the conductor - branch contacting the conductor - branch separating from the conductor". During the approaching process, the air gap between the branch and the conductor bears the system voltage. When the gap length shortens to a certain extent, the gap breaks down and forms a breakdown arc. After the conductor and branch come into contact, the arc is extinguished due to the contact of the two poles. In the separation stage, analogous to the breaking process of a circuit breaker, the fault current flowing through the contact point between the branch and the conductor is interrupted, thus forming a separation arc.

[0026] It should be noted that, as a typical high-resistance grounding fault, the early-stage fault grounding current intensity of the tree line is less than 1A.

[0027] Furthermore, if we define the fault current recording sequence as I(n), then at the nth time... s at each sampling point (n) s , I(n s The interval slope k(n) s ) is represented as:

[0028] Where n is the number of sampling points, n = 1, 2, 3, ..., N, To calculate the length of the interval, , N T The number of sampling points in each period.

[0029] It should be noted that the interval slope distribution for sinusoidal discrete current sequences and discrete current sequences with arc zero rest is as follows: Figure 3 As shown, Figure 3The upper middle section shows the normalized current waveform, with a sampling frequency of 100kHz and 2000 sampling points per power frequency cycle. The lower section shows the interval slope of each point in the normalized current sequence. Figure 3 In (a), the current sequence waveform exhibits a standard sinusoidal shape, with its interval slope reaching its maximum value when the current crosses zero. This value is directly related to the sampling rate and is denoted as kstd. For a unit sine curve at a sampling rate of 100kHz, kstd is 3.122 × 10⁻³. The interval slope reaches its minimum value of 0 when the current reaches its peak. Within half a cycle, the interval slope exhibits an inverted U-shape. Simultaneously, for Figure 3 (b) shows a current waveform with a zero-crossing slope. Its interval slope exhibits an "M-shaped" slope with two peaks and one valley within half a cycle. The interval slope corresponding to the valley is much smaller than kstd, while the interval slope corresponding to the peak increases to near or even exceeding kstd. Therefore, the drop in the interval slope of the current waveform near its zero-crossing point can be used as a criterion for determining whether a zero-crossing slope exists. In actual fault conditions, the nonlinear characteristics of the system's own harmonics and equivalent grounding resistance will cause a certain degree of distortion in the current waveform even if there is no accompanying arc during the fault, and the interval slope curve will not strictly maintain a zero-crossing slope. Figure 3 The smooth M-shaped curve shown in (a) is... Figure 4 Taking the normalized current waveform shown as an example, the following procedure is proposed to determine whether there is zero crossover at each zero-crossing point of the fault waveform.

[0030] S2: Determine whether the fault waveform meets the three-stage form of "with zero-life distortion - without zero-life distortion - with zero-life distortion" based on the zero-life detection method; It should be noted that the three-stage form of "with zero-cycle distortion - without zero-cycle distortion - with zero-cycle distortion" corresponds to the three stages of "approach → contact → separation" in the early stage of treeline faults. The duration of the first stage is usually less than 2 cycles and less than the duration of the third stage.

[0031] Furthermore, the system determines whether zero crossover occurs at each zero-crossing point of the fault waveform, including: Taking the first positive / negative peak of the fault waveform as the starting point and the last positive / negative peak as the ending point, count the number of zero-crossings of the current between the starting and ending points, and define the nth zero-crossing as Z. n ; Calculate the interval slope distribution k of the waveform between the start and end points, and plot the interval slope distribution curve; In Z n Nearby, find the two nearest points with the minimum slope in the interval, and define the corresponding sampling points as N in sequence. n1 N n2 The absolute value of the corresponding current intensity is defined as |I n1 |with|I n2 |; In the interval (N) n1 , N n2 Within a range, for sampling points that satisfy |I(n)| < 0.1, the sampling point with the smallest interval slope is denoted as (N). nv k nv ).

[0032] Furthermore, when the interval slope curve is in the interval (N) n1 , N n2 Within a certain range, there exist at least two local maxima points, and the distance from point (N) is such that... nv k nv The two most recent local maxima are (N) np1 k np1 The two local maxima (N) np2 k np2 (If zero days off are required, the following conditions must be met:) N np1 <(N n1 +N n2 ) / 2 <N np2 N np1 <N nv <N np2 k nv <k np1 &k nv <k np2 When the two local maxima do not meet the conditions, the current is judged to have hardly any distortion at the zero-crossing point, and the zero-crossing time of the current is 0.

[0033] Furthermore, determining whether a zero-crossing exists at each zero-crossing point of the fault waveform also includes: at zero-crossing points where a zero-crossing might exist, the zero-crossing point (N) nv k nv ), (N np1 k np1 ), (N np2 k np2 When zero days off exist, the following conditions must be met: k nv <0.7×k std ×(|I n1 |+|I n2 |) / 2 (k np1 + k np2 ) / 2>0.9×k std ×(|I n1 |+|I n2 |) / 2 Where, k stdThis represents the slope of the unit sine curve at the zero-crossing current interval under this sampling rate.

[0034] When the state of each current zero-crossing point does not meet the conditions, the distortion of its waveform mainly originates from the nonlinear characteristics of the equivalent grounding resistance of the medium at the grounding point, which runs through the entire fault. When calculating the zero-crossing duration, it needs to be excluded.

[0035] S3: Based on the three-stage characteristics, calculate the changing trend of the current zero-rest duration in the third stage to determine whether the fault is an early-stage fault.

[0036] It should be noted that in the third stage, the zero-rest duration of the fault current waveform gradually increases, and when the current crosses zero "from negative to positive", the zero-rest duration is shorter than when the current crosses zero "from positive to negative". In order to identify early faults in the tree line, it is necessary to analyze the fault waveform data.

[0037] Furthermore, the calculation of the duration of zero-day work week includes: The degree of drop in the interval slope is expressed as: Dn= (k np1 -k nv ) / 2+(k np2 -k nv ) / 2 The average drop level is expressed as: k ave = (D1+…+D S ) / S Where S is the number of zero crossings that do not meet the conditions.

[0038] There exists a zero-crossing point (N) of zero rest. nz1 k nz1 ) and (N nz2 k nz2 ) Must meet: N np1 <N nz1 <N nv <N nz2 <N np2 ∀ k ni ∈(k nv k np1 ), k nz1 -(k np1 - k ave )≤ k ni -(k np1 - k ave ) ∀ k ni ∈(k nv k np2 ), knz2 -(k np2 - k ave ) ≤ k ni -(k np2 - k ave ) At the zero-crossing point Zn, the number of sampling points for the zero-rest duration is: N nz = N nz2 - N nz1 Furthermore, the durations of zero-day breaks are arranged in chronological order as N. z ={ N z1 N z2 ,…,N zn}, take N zi and N z(i+c) Form a pair of data (N) zi N z(i+c) ), N zi and N z(i+c) ∈N z ,

[0039] Define Delta i =N zi -N z(i+c) Delta i The quantity >0 is represented by D. + Delta i The number of values ​​less than 0 is represented by D. - ,

[0040] Example 2 Reference Figure 5 —9 is an embodiment of the present invention, which provides a method for early fault identification of power distribution line trees. In order to verify the beneficial effects of the present invention, a simulation experiment is conducted for scientific demonstration.

[0041] This embodiment conducted a simulation test of early-stage tree-line faults on a 10kV distribution line in a real-world distribution network test field. The overall circuit diagram for the test is as follows: Figure 5As shown, the system power supply is provided through an 800kVA step-up transformer, which raises the 0.4kV voltage to the 10kV voltage of the station bus. The distribution network line module includes two parts: a "lumped parameter equivalent line module" and a "real line module". The lumped parameter equivalent line module is composed of two modules connected in series. One is a lumped model of a 20km overhead line, consisting of R1=5.376Ω, L1=17.985mH, and C1=0.246μF. The other is a lumped model of a 10km overhead line, consisting of R1=2.688Ω, L1=8.992mH, and C1=0.123μF. The real line model is a 400m double-circuit real overhead line erected on the site, with a load simulation device connected to its end to simulate a certain level of line load (approximately 5A).

[0042] The connection point between the real-type line module and the equivalent line module is set as the system fault point, and phase C is set as the fault phase. An early fault simulation module for the tree-line is deployed, which mainly consists of four main structures: overhead aluminum stranded wire, supporting insulators, motorized guide rails, and tree branches. The specific structure is as follows: Figure 6 As shown: The tree branch is fixed to a slider on an electric guide rail, with the guide rail perpendicular to both the ground and the conductor. A controller allows the motor to drive the guide rail belt at a certain speed, causing the slider with the tree branch fixed to it to move relative to the conductor. The guide rail has two limit switches, and the slider reciprocates between them. To simulate the tree branch's movement from "approaching the conductor → contacting the conductor → separating from the conductor," the height of slider 1 is slightly lower than the aluminum stranded wire, while the height of slider 2 ensures that the arc during the separation phase can extinguish naturally.

[0043] The specific experimental steps are as follows: ① Close KG1 to set the neutral point grounding mode of the system to ungrounded.

[0044] ② Close KL to enable the lumped parameter equivalent circuit module and the real circuit module to carry 10kV power frequency voltage.

[0045] ③ Activate the load simulation module to make each phase of the system operate under a load of about 30kVA, that is, the load current of each phase is about 5A.

[0046] ④ Fix the tree branch to the guide rail slider, start the guide rail control box, and make the slider rise to the limit switch 2.

[0047] ⑤ Close KF to make the aluminum stranded wire in the early fault simulation module at the same potential as the fault.

[0048] ⑥ Start the fault recording device to record the voltage and current waveforms at the fault point. At the same time, start the guide rail control box to make the slider move the tree branch between the two limiters, and "approach-contact-separate" with the aluminum stranded wire to realize the simulation of early faults in the tree line.

[0049] ⑦ After each branch has been in contact with the aluminum stranded wire several times, disconnect KF, stop recording the waveform, replace the branch, and repeat steps 4 to 6.

[0050] In the experiment, voltage and current waveform sampling points were set at both the conductor side and the tree branch grounding side, with a sampling frequency of 100 kHz. By changing the tree branch type and adjusting the movement speed, a total of 312 sets of data were obtained. Taking one of the recorded waveforms as an example, the fault phase voltage and grounding current waveforms at the fault location are as follows: Figure 7 As shown. Figure 7 This diagram reflects the changes in voltage and current waveforms throughout the entire process of the fault, from the tree branch approaching the conductor to contacting the conductor and then moving away from the conductor. The fault current intensity was approximately 0.228 A, lasting about 880 ms, corresponding to approximately 44 power frequency cycles. Based on the morphological characteristics of the current waveform, it can be decomposed into three stages as shown in the diagram, corresponding to the processes of "approaching → contacting → separating". During the approach phase, a faint arc appears only when the distance between the branch and the conductor is very small. From the current waveform, only one zero-crossing shoulder appears during the approach phase, indicating that this phase lasts for less than one cycle. During the contact phase, the aforementioned faint arc disappears, and the fault current waveform remains basically sinusoidal with a relatively constant current intensity. This indicates that the arc is extinguished during this phase due to direct contact between the branch and the conductor, and the resistance of the branch does not change significantly. During the separation phase, a clear "arc pulling" process occurs between the branch and the conductor. The arc gradually lengthens as the separation process progresses. This phase lasts for about 8.5 cycles, with the current crossing zero 15 times, and a relatively obvious "zero-crossing shoulder" is observed.

[0051] For the experimental results shown in the figure, after normalizing the current waveform at the fault point, the interval slope of each point is calculated, and the results are as follows. Figure 8As shown in the figure, the slope distribution of the fault current waveform is closely related to the presence or absence of arc zero-crossing. During the approach and separation phases, the slope drops significantly due to the presence of the current zero-crossing phase. In the contact phase, although the current waveform is generally sinusoidal, it still exhibits slight distortion, and the slope does not strictly follow the "M-shape" shown in the figure. Based on the zero-crossing judgment method in Section 2.1, the first zero-crossing and the 72nd-86th zero-crossings of the current waveform are considered to indicate the presence of zero-crossing. It can be seen that the fault current waveform can be clearly distinguished into three phases, basically corresponding to the "approach-contact-separation" phase. Furthermore, for the third phase with a large number of zero-crossings, the duration and variation curves of each zero-crossing can be obtained using the zero-crossing time calculation method in Section 2.1, as shown in the figure. Figure 9 As shown in the figure, the zero-rest duration of this fault gradually increased from 160 sampling points (1.6 ms) at the 72nd zero crossing to 295 sampling points (2.95 ms) at the 86th zero crossing, and the increase in the zero-rest duration showed a sawtooth pattern. The main reason for this is the difference between the zero-rest duration when the current crosses zero "from positive to negative" and when it crosses zero "from negative to positive". The curves showing the change in the zero-rest duration when the current crosses zero "from positive to negative" and "from negative to positive" are plotted separately, as shown by the dotted line in the figure. It can be seen that the zero-rest duration when the current crosses zero "from positive to negative" is significantly longer than when it crosses zero "from negative to positive". The results of this experiment are basically consistent with the aforementioned fault mechanism and characteristic analysis results, and meet the various criteria of the tree-line early fault identification process shown in the figure. Therefore, it can be correctly identified as an early tree-line fault.

[0052] Example 3 Reference Figure 10 —13 is an embodiment of the present invention, which provides a method for early fault identification of power distribution line trees. In order to verify the reliability and robustness of the present invention, an analysis and demonstration are carried out from the perspective of different fault scenarios based on embodiment 2.

[0053] To simulate different scenarios of early tree-line faults in actual distribution network operation, experiments were conducted using branches of different tree species and varying their relative speeds to the conductors. First, branches of the same species with roughly the same diameter and length were selected. Based on the previous experiments, the control parameters of the guide rail were changed, causing the slider to drive the branches at average speeds of 0.15 m / s and 0.2 m / s, respectively. The corresponding fault current waveforms and normalized interval slope curves are shown below. Figure 10 (a), Figure 10 As shown in (b).

[0054] When the branch's moving speed is 0.15 m / s, the current intensity of this fault is approximately 0.278 A, the fault lasts for approximately 570 ms, and the current crosses zero 56 times. According to the aforementioned criteria, there is a zero-crossing interval between the first zero-crossing and the 47th-56th zero-crossings, meaning the third stage includes 10 zero-crossings. When the branch's moving speed is 0.20 m / s, the current intensity of this fault is approximately 0.242 A, the fault lasts for approximately 490 ms, and the current crosses zero 49 times. According to the aforementioned criteria, there is a zero-crossing interval between the first zero-crossing and the 44th-49th zero-crossings, meaning the third stage includes 6 zero-crossings.

[0055] It is evident that even branches of the same tree species with similar length and diameter will exhibit varying fault current intensities due to differences in growth status, moisture content, and other factors. This means that the fault duration and the number of fault fragments are not entirely inversely proportional to the branch's movement speed. However, under otherwise approximately identical conditions, the fault duration shortens as the branch's movement speed increases. Simultaneously, during the separation phase, because the arc is stretched to a longer length within the same timeframe, its heat dissipation increases, leading to faster arc extinguishing, which is reflected in the reduced duration of the third stage in the waveform. (The following is a separate section:) Figure 11 The curve showing the change in the duration of zero rest periods indicates that the duration of zero rest periods is still on an upward trend, which is consistent with the aforementioned identification criteria.

[0056] In addition to the peach tree branches mentioned above, branches of common tall trees such as camphor and eucalyptus were also selected as test samples in the experiment. Furthermore, bamboo growing along the tree line in southern my country may also cause similar early tree line failures, so bamboo of similar length and diameter was also selected as test samples in the experiment of this paper.

[0057] The fault current waveforms and normalized interval slope curves corresponding to the three different samples are as follows: Figure 12 As shown in (a), (b), and (c). Figure 12 (a) is a camphor tree. Figure 12 (b) is a magnolia tree. Figure 12(c) For bamboo, the results of each experiment were analyzed one by one. When the test tree species was camphor tree branch, the current intensity of the fault increased from approximately 0.411A in the first cycle to 0.549A in the last cycle, lasting about 730ms with 72 zero-crossings. There was a rest period between the first and second zero-crossings and between the 69th and 72nd zero-crossings, with the rest period duration increasing from 109 to 548. When the test tree species was magnolia tree branch, the current intensity of the fault increased from approximately 0.592A in the first cycle to 0.698A in the last cycle, lasting about 620ms with 61 zero-crossings. There was a rest period between the first zero-crossing and between the 47th and 61st zero-crossings, with the rest period duration gradually increasing from 164 to 265. When the test tree species was bamboo, the current intensity of the fault remained relatively constant at 0.152A, lasting about 420ms with 41 zero-crossings. There was a rest period between the first zero-crossing and between the 37th and 41st zero-crossings, with the rest period duration gradually increasing from 156 to 336.

[0058] The experiment shows that different tree species will mainly affect the size of Rtree, resulting in differences in the intensity of the fault current. At the same time, different tree species will also lead to differences in the degree of burning of branches by the heat generated by the electric arc during the fault duration. That is, the change level of Rcontacet may be more obvious in tree species that are easily carbonized.

[0059] In addition, by observing the waveforms of different tree species during the arc-free (contact) phase, such as... Figure 13 As shown, during the contact stage, the current waveform does not maintain a smooth sinusoidal shape, but undergoes distortion of different degrees and natures. This indicates that the equivalent resistance of trees has a certain degree of nonlinearity, and the degree and type of nonlinearity vary among different tree species. This causes the fault current to undergo other forms of distortion besides zero-degree distortion. However, these changes will not affect the aforementioned method for identifying early faults in tree lines.

[0060] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A method for early fault identification in power distribution lines, characterized in that, include: Obtain complete waveform data of the fault phase voltage and fault ground current throughout the entire fault cycle, and determine the waveform distortion based on the waveform slope. The method of zero-wave detection is used to determine whether the fault waveform meets the three-stage form of "with zero-wave distortion - without zero-wave distortion - with zero-wave distortion". Based on the three-stage characteristics, the changing trend of the current zero-rest duration in the third stage is calculated to determine whether the fault is an early-stage fault. Determine whether there is a zero-crossing at each zero-crossing point of the fault waveform, including: Taking the first positive / negative peak of the fault waveform as the starting point and the last positive / negative peak as the ending point, count the number of zero-crossings of the current between the starting and ending points, and define the nth zero-crossing as Z. n ; Calculate the interval slope distribution k of the waveform between the start and end points, and plot the interval slope distribution curve; In Z n Nearby, find the two nearest points with the minimum slope in the interval, and define the corresponding sampling points as N in sequence. n1 N n2 The absolute value of the corresponding current intensity is defined as |I n1 |with|I n2 |; In the interval (N) n1 , N n2 Within a range, for sampling points that satisfy |I(n)| < 0.1, the sampling point with the smallest interval slope is denoted as (N). nv k nv ); When the slope curve of the interval is in the interval (N) n1 , N n2 Within a certain range, there exist at least two local maxima points, and the distance from point (N) is such that... nv k nv The two most recent local maxima are (N) np1 k np1 ) and (N np2 k np2 (This may apply to zero-day workdays, which may require the following conditions to be met:) N np1 < (N n1 +N n2 ) / 2< N np2 N np1 < N nv < N np2 k nv <k np1 &k nv < k np2 When the two local maxima do not meet the conditions, the current is judged to have hardly any distortion at the zero-crossing point, and the zero-crossing time of the current is 0. At the zero-crossing point where zero rest may exist, the zero-crossing point (N) nv k nv ), (N np1 k np1 ), (N np2 k np2 When zero days off exist, the following conditions must be met: k nv <0.7×k std ×(|I n1 |+|I n2 |) / 2 (k np1 + k np2 ) / 2>0.9×k std ×(|I n1 |+|I n2 |) / 2 Where, k std This represents the slope of the unit sine curve at the zero-crossing current interval under this sampling rate.

2. The method for early fault identification of power distribution line trees as described in claim 1, characterized in that: Let the fault current waveform sequence be I(n). Then, at the nth time... s at each sampling point (n) s , I(n s The interval slope k(n) s ) is represented as: Where n is the number of sampling points, n = 1, 2, 3, ..., N, To calculate the length of the interval, , N T The number of sampling points in each period.

3. The method for early fault identification of power distribution line trees as described in claim 2, characterized in that: The calculation of the zero-rest duration includes: The degree of drop in the interval slope is expressed as: Dn= (k np1 -k nv ) / 2+(k np2 -k nv ) / 2 The average drop level is expressed as: k ave = (D1+…+D S ) / S Where S is the number of zero crossings that do not meet the conditions; There exists a zero-crossing point (N) of zero rest. nz1 k nz1 ) and (N nz2 k nz2 ) Must meet: N np1 <N nz1 < N nv < N nz2 < N np2 ∀ k ni ∈(k nv ,k np1 ), k nz1 -(k np1 - k ave )≤ k ni -(k np1 - k ave ) ∀ k ni ∈(k nv ,k np2 ), k nz2 -(k np2 - k ave ) ≤ k ni -(k np2 - k ave ) At the zero-crossing point Zn, the number of sampling points for the zero-rest duration is: N nz = N nz2 - N nz1 。 4. The method for early fault identification of power distribution line trees as described in claim 3, characterized in that: The calculation of the zero-rest duration also includes: Arrange the durations of zero-day breaks in chronological order as N. z ={ N z1 N z2 ,…, N zn }, take N zi and N z(i+c) Form a pair of data (N) zi N z(i+c) ), N zi and N z(i+c) ∈N z , Define Delta i =N zi -N z(i+c) Delta i The quantity >0 is represented by D. + Delta i The number of values ​​less than 0 is represented by D. - , 。

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