A power distribution network pre-reintegration fault-free identification method using low-frequency injection signals

Abstract

The application discloses a power distribution network pre-reclosing fault-free identification method using a low-frequency injection signal, comprising: constructing an active injection signal device, setting a delay, when a phase-to-phase fault three-phase reclosing trip occurs in a power distribution line, the active injection signal device applies a low-frequency voltage disturbance signal to the ABC three-phase simultaneously after the delay; collecting the response voltage and response current of the line side of the line to be detected, calculating the three-phase resistance calculation value and the inductance parameter calculation value; obtaining the three-phase resistance actual value and the inductance parameter actual value, judging the fault state, if it is a transient fault, judging whether the execution time reaches the cycle judgment time, if yes, closing the reclosing, if not, rejudging the fault type after the cycle judgment interval, if it is a permanent fault, locking the reclosing. The application has the beneficial effects that the difference between the resistance calculation value and the actual value and the inductance parameter calculation value and the actual value are used to distinguish the phase-to-phase fault state, the method is not affected by the non-periodic component, and the reliability is high and the action speed is fast.

Description

Technical Field

[0001] This application relates to the field of distribution network relay protection technology, and in particular to a method for identifying fault-free distribution network reclosing before reclosing using low-frequency injection signals. Background Technology

[0002] Most faults in medium-voltage lines of distribution networks are transient. Automatic reclosing can significantly shorten outage time, thereby improving the reliability of the distribution network. However, traditional automatic reclosing is unpredictable; if it recloses to a permanent fault, it can impact the system and exacerbate the damage. Adaptive reclosing, which identifies the nature of the fault before reclosing to avoid reclosing to the faulty line, helps reduce the harm of blind reclosing. Distribution lines differ significantly from transmission lines in voltage level and structure; existing permanent fault identification methods for high-voltage transmission lines cannot be directly applied to distribution networks.

[0003] Existing permanent fault identification methods for distribution networks are mainly divided into two categories. One category utilizes the electrical quantities of the line itself, primarily leveraging the energy storage and discharge characteristics of parallel capacitors in the distribution network. Permanent faults are identified by recognizing whether there are abrupt changes in the transition resistance. The second category mainly uses the external disturbance method, which employs controllable power electronic devices to connect an inverter power supply to the high-voltage side of the distribution transformer. The transient voltage waveform or the characteristics of line harmonic impedance changing with frequency under wavelet transform are analyzed to identify the nature of the fault. This scheme requires high communication coordination between the primary and secondary sides of the distribution transformer, making it difficult to implement in practice.

[0004] Chinese Patent No. CN107394757A, published on November 24, 2017, entitled "A Method for Identifying No Fault Before Reclosing in a Distribution Network with Inter-phase Faults," discloses a method for collecting instantaneous three-phase voltage samples uφ(k), line-start phase current iφ_l(k), and parallel capacitor phase current iφ_c(k) of the parallel compensation capacitor during the period from the fault to reclosing in a distribution network. The method calculates the phase voltage difference uφφ(k), line phase current iφ(k), phase current difference iφφ(k), and simultaneously calculates the first-order difference value diφφ(k). Where φ = A, B, C; the phase voltage difference uφφ(k) and phase current difference iφφ(k) are used to identify the fault circuit equivalent resistance Rf and fault distance p parameters using the equation uφφ(k) = Rf.Aφφ1(k) + p.Aφφ2(k). Then, the data window is sequentially shifted to obtain the sequence of identification parameters. The fault state is identified using the no-fault identification criterion before phase-to-phase fault reclosing. When the criterion is continuously valid, it is judged as a phase-to-phase transient fault and the fault arc has been extinguished, and the reclosing action is performed; if the criterion is not valid, the judgment continues until the maximum discrimination time is reached, at which point it is judged as a permanent fault, and the reclosing action is not performed. However, this scheme uses the time-domain characteristics of the voltage and current of the parallel compensation capacitor after the three-phase trip of the phase-to-phase fault in the distribution network to establish the identification equation of the fault phase circuit with respect to the arc resistance and fault location. The fault nature is identified based on the amplitude and rate of change characteristics of the calculated arc resistance, which is easily affected by harmonic components.

[0005] Chinese patent "Adaptive Reclosing Method for Distribution Network Based on Distributed Source Characteristic Signal Injection", publication number CN116404607A, publication date: July 7, 2023, specifically discloses a method where, after a line fault occurs, the protection device trips the power supply-side switch adjacent to the fault point, disconnecting the distributed source of the faulty line from the grid; the distributed source injects a characteristic signal; the distributed source identifies the presence of a fault based on the phase-to-phase impedance imbalance of the characteristic signal; when a fault exists, the duration of the distributed source outputting the characteristic signal is set to T1; when no fault exists, the duration of the distributed source outputting the characteristic signal is set to T2; the protection device detects the characteristic signal and, when the reclosing operation conditions are met, recloses the tripped line switch. This scheme identifies the fault nature by the change characteristics of line harmonic impedance with frequency, which requires high communication coordination between the primary and secondary sides of the distribution transformer, making it difficult to implement in practice. Summary of the Invention

[0006] This application addresses the problems in existing technologies where fault identification is easily affected by harmonic components or requires high communication coordination between the primary and secondary sides of the distribution transformer, making practical applications difficult. It provides a fault-free identification method for distribution networks before reclosing using low-frequency injected signals. This method uses line resistance and inductance as identification parameters, and uses the difference between calculated and actual resistance and inductance values ​​to determine the phase-to-phase fault state. It is unaffected by aperiodic components, has high reliability, and fast response speed. Furthermore, it is unaffected by harmonic components and the communication coordination between the primary and secondary sides of the transformer, making it simple to apply and widely applicable.

[0007] To achieve the above technical objectives, this application provides a technical solution: a method for fault-free identification before reclosing in a distribution network using low-frequency injection signals, comprising the following steps: Step 1: Constructing an active injection signal device and setting a delay. When a phase-to-phase fault occurs and the three-phase reclosing trips, the active injection signal device applies a low-frequency voltage disturbance signal to all three phases (A, B, and C) simultaneously after the delay. Step 2: Acquiring the response voltage and response current on the line side of the line to be tested, and calculating the calculated values ​​of the three-phase resistance and inductance parameters. Step 3: Obtaining the actual values ​​of the three-phase resistance and inductance parameters. Based on the difference between the calculated values ​​of the three-phase resistance and inductance parameters and the actual values ​​of the three-phase resistance and inductance parameters, the fault status is determined. If it is a transient fault, Step 4 is executed; if it is a permanent fault, reclosing is blocked. Step 4: Setting a cyclic judgment time and a cyclic judgment interval. Determining whether the execution time of Step 3 has reached the cyclic judgment time. If yes, the reclosing is closed; if not, Step 3 is re-executed after the cyclic judgment interval.

[0008] Furthermore, constructing the active injection signal device includes: building an active injection signal device on the distribution network side, the active injection signal device consisting of a three-phase AC voltage source u a u b u c It consists of control switches S1, S2, and S3.

[0009] Furthermore, the low-frequency voltage disturbance signal has an amplitude of 500V to 700V and a frequency of 10Hz to 20Hz.

[0010] Furthermore, step 2 includes: step 21: collecting response voltage and current data of the line side of the line to be tested and establishing a faultless equivalent network calculation model; step 22: constructing linear equations based on the response voltage and current data of the line side of the line to be tested, and using the faultless equivalent network calculation model and the linear equations to calculate the three-phase resistance and inductance parameters.

[0011] Furthermore, for fault-free lines, R exists: eq =R1;L eq =L l+L′ T Among them, R eq L is the calculated value of the three-phase resistance. eq Here are the calculated inductance parameters, where L1 is the line self-inductance of the distribution network, and L′ is the inductance value. T This is the equivalent inductance of a distribution transformer.

[0012] Furthermore, when the active injection signal device simultaneously applies a low-frequency voltage disturbance signal to the three phases (A, B, and C) after a delay, the response voltage, response current, and derivative of the response current exhibit a linear relationship, with the proportionality coefficient being equal to the equivalent resistance R. eq Equivalent inductance L eq : Among them, u φ (t)(φ=a,b,c) is the response voltage, i φ (t)(φ=a,b,c) is the response current.

[0013] Furthermore, the fault-free equivalent network calculation model is as follows:

[0014]

[0015] Furthermore, based on the response voltage and current data of the line side of the line under test, a linear equation is constructed as follows: The least squares method is used to calculate the collected response voltage u. φ (t)(φ=a,b,c), response current i φ (t)(φ=a,b,c) is fitted to a linear equation: Where x = R eq ,y=L eq ;u φ (k) represents the response voltage at the k-th sampling point, i φ (k) represents the response current at the kth sampling point, and N represents the number of sampling points.

[0016] Furthermore, judging the fault status based on the calculated values ​​of three-phase resistance and inductance parameters, and the differences between the actual values ​​of three-phase resistance and inductance parameters, includes: constructing a fault-free criterion before overlap based on the calculated values ​​of three-phase resistance, inductance parameters, and the actual values ​​of three-phase resistance and inductance parameters.

[0017] Where n is the length of the calculated result sequence; R eq For least squares calculation of the resistance sequence, R real This represents the actual value of the line resistance; L eq For least squares calculation of the inductance sequence; L real K represents the true sum of the line inductance and the distribution transformer inductance; rel_1 K rel_2 This is the margin coefficient.

[0018] Furthermore, judging the fault status based on the calculated values ​​of the three-phase resistance and inductance parameters and the differences between the actual values ​​of the three-phase resistance and the actual values ​​of the inductance parameters also includes: judging whether the differences between the calculated values ​​of the three-phase resistance and inductance parameters and the actual values ​​of the three-phase resistance and inductance parameters meet the criteria of no fault before re-closing. If so, it is judged as a transient fault; if not, it is judged as a permanent fault.

[0019] The beneficial effects of this application are as follows: After a three-phase trip due to a phase-to-phase fault in a distribution network, a low-frequency voltage disturbance signal is simultaneously injected into all three phases. Using an equivalent model of a transient fault as the parameter identification benchmark, and with line resistance and inductance as identification parameters, the phase-to-phase fault state is determined by the difference between the calculated and actual values ​​of resistance and inductance. This method is simple in principle, has minimal impact from harmonic components, does not require sophisticated filter design, and is well-applicable to various phase-to-phase faults. Attached Figure Description

[0020] Figure 1 This is a flowchart illustrating a method for identifying fault-free distribution network reclosing using low-frequency injection signals, as proposed in this application.

[0021] Figure 2 This application is as follows Figure 1 A schematic diagram of the active injection signal device in the embodiment shown.

[0022] Figure 3 This application is as follows Figure 1 A schematic diagram comparing the calculated and actual resistance parameters of the illustrated embodiment.

[0023] Figure 4 This application is as follows Figure 1 A schematic diagram showing the comparison between calculated and actual inductance parameters in the illustrated embodiment.

[0024] Figure 5 This application is as follows Figure 1 The fault-free equivalent network diagram of the embodiment shown.

[0025] Figure 6 This application is as follows Figure 1 The single-phase equivalent circuit diagram of the embodiment shown is illustrated.

[0026] Figure 7 This application is as follows Figure 1 The embodiment shown is an equivalent network diagram of a permanent fault in the distribution network AB. Detailed Implementation

[0027] To make the objectives, technical solutions, and advantages of this application clearer, the following detailed description of this application is provided in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely one preferred embodiment of this application and are only used to explain this application. They do not limit the scope of protection of this application. All other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0028] like Figure 1 As shown in the first embodiment of this application, a method for fault-free identification before reclosing in a distribution network using low-frequency injection signals includes the following steps:

[0029] Step 1: Construct an active injection signal device and set a delay. When a three-phase reclosing trip occurs in the distribution line due to a phase-to-phase fault, the active injection signal device will simultaneously apply a low-frequency voltage disturbance signal to the three phases (A, B, and C) after the delay.

[0030] Step 2: Collect the response voltage and response current on the line side of the line to be tested, and calculate the three-phase resistance and inductance parameters.

[0031] Step 3: Obtain the actual values ​​of the three-phase resistance and inductance parameters. Determine the fault status based on the difference between the calculated values ​​of the three-phase resistance and inductance parameters and the actual values ​​of the three-phase resistance and inductance parameters. If it is a transient fault, proceed to step 4. If it is a permanent fault, then reclosing is blocked.

[0032] Step 4: Set the loop judgment time and loop judgment interval, and determine whether the execution time of Step 3 has reached the loop judgment time. If yes, the reclosing is closed; if not, Step 3 is executed again after the loop judgment interval.

[0033] In this embodiment, the cyclic judgment time is set according to the maximum duration of transient fault existence based on actual conditions, and the cyclic judgment interval is set according to the minimum duration of transient fault existence based on actual conditions. Fault states include transient faults and permanent faults; a transient fault is a fault-free state, and a permanent fault is a fault-existing state. After a three-phase trip due to a phase-to-phase fault in the distribution network, low-frequency voltage disturbance signals are simultaneously injected into the three phases. Line resistance and inductance are used as identification parameters. The nature of the phase-to-phase fault is determined by the difference between the calculated and actual values ​​of resistance and inductance. The influence of harmonic components is relatively small, the requirements for filter design are not high, and it has good applicability to various phase-to-phase faults.

[0034] like Figure 2 As shown, an active injection signal device is installed on the distribution network side. The active injection signal device consists of a three-phase AC voltage source u a u b u cIt consists of control switches S1, S2, and S3. For distribution networks with distributed power sources, low-frequency disturbance signals are injected into the distribution lines by adjusting the operating mode of the distributed power source inverter, thus forming a three-phase AC voltage source for detecting disturbance injection in the lines.

[0035] After a three-phase reclosing trip occurs due to a phase-to-phase fault in a power distribution line, a delay is set to avoid the influence of residual electrical quantities in the faulty line on the judgment result. This delay allows the active injection signal device to apply a low-frequency disturbance signal. In this embodiment, the delay is 50ms to 100ms.

[0036] In this embodiment, the selection of the amplitude of the low-frequency voltage disturbance signal needs to consider the measurement accuracy of the instrument transformer and its impact on the power grid. Specifically, the voltage transformer for the 10kV distribution network should not be lower than 5%U. N The measurement accuracy, i.e., the amplitude of the applied disturbance signal, should meet the following requirements:

[0037] U x ≥5%U N ;

[0038] U N This represents the amplitude of the system phase voltage. Meanwhile, the allowable deviation of the supply voltage for 10kV and below is ±7% of the rated voltage. Considering the voltage fluctuations caused by the detection signal, the maximum amplitude of the detection signal is 7% of the rated voltage. Therefore, the amplitude of the low-frequency voltage disturbance signal is 500V~700V. While considering the measurement accuracy of the transformer, the impact of the injected signal should be minimized as much as possible.

[0039] Similarly, the frequency of the low-frequency voltage disturbance signal should, while satisfying the performance constraints of the injected signaling equipment, maximize the difference in permanent fault characteristics to improve the sensitivity of fault detection. Therefore, the frequency of the low-frequency voltage disturbance signal is 10Hz–20Hz, and the calculation step size is 10. -4 s~10 -5 This satisfies the sampling frequency requirement for differential substitution calculation of sampled values, enables phase-to-phase fault state identification through parameter identification, and reduces the impact of capacitance, communication, and detection results.

[0040] In this embodiment, step 2 includes:

[0041] Step 21: Collect the response voltage and current data of the line side of the line to be tested, and establish a fault-free equivalent network calculation model;

[0042] Step 22: Construct a linear equation based on the response voltage and current data of the line side of the line to be tested, and use the fault-free equivalent network calculation model and the linear equation to calculate the three-phase resistance and inductance parameters.

[0043] Specifically, in step 21, for fault-free lines:

[0044] R eq =R1;

[0045] L eq =L l +L′ T ;

[0046] Among them, R eq L is the calculated value of the three-phase resistance. eq Here are the calculated inductance parameters, where L1 is the line self-inductance of the distribution network, and L′ is the inductance value. T For the equivalent inductance of the distribution transformer, when the active injection signal device simultaneously applies a low-frequency voltage disturbance signal to phases A, B, and C after a delay, the response voltage, response current, and derivative of the response current exhibit a linear relationship, with the proportionality coefficient being the equivalent resistance R of that phase. eq Equivalent inductance L eq :

[0047]

[0048] Among them, u φ (t)(φ=a,b,c) is the response voltage, i φ (t)(φ=a,b,c) is the response current.

[0049] Based on the sampled values ​​of response voltage and response current at different times (u φ1 i φ1 ), (u φ2 i φ2 ), ..., (u φn i φn )get:

[0050]

[0051] A fault-free equivalent network calculation model is established based on this matrix expression.

[0052] In step 22, the least squares method is used to analyze the acquired response voltage u. φ (t)(φ=a,b,c), response current i φ (t)(φ=a,b,c) is fitted to a linear equation, thus obtaining R eq With L eq The coefficients are evaluated by fitting two linear equations. The objective function is constructed as follows:

[0053]

[0054] Where x = R eq ,y=L eq ;u φ(k) represents the response voltage at the k-th sampling point, i φ (k) represents the response current at the kth sampling point, and N represents the number of sampling points.

[0055] Find the partial derivatives with respect to x and y, and then find their extrema:

[0056]

[0057] Therefore, according to x=R eq ,y=L eq get:

[0058]

[0059] The calculated value of the three-phase resistance R of the line is obtained from the above formula. eq And the calculated value of inductance parameter L eq .

[0060] like Figure 3 and Figure 4 The diagram shows the comparison between the calculated and actual values ​​of resistance and inductance parameters during the fault-prone and fault-free phases of the AB phase-to-phase fault. When the fault exists, the actual fault model is inconsistent with the identification model, and the calculated resistance and inductance values ​​differ significantly from the actual values. After the fault disappears, the actual fault model aligns with the identification model, and the calculated resistance and inductance parameters closely match the actual values.

[0061] Therefore, based on the calculated values ​​of the three-phase resistance and inductance parameters and the actual values ​​of the three-phase resistance and inductance parameters, a fault-free criterion is constructed before overlap:

[0062]

[0063] Where n is the length of the calculated result sequence; R eq For least squares calculation of the resistance sequence, R real This represents the actual value of the line resistance; L eq For least squares calculation of the inductance sequence; L real K represents the true sum of the line inductance and the distribution transformer inductance; rel_1 K rel_2 This is the margin coefficient.

[0064] A transient fault is determined to have occurred when the differences between the calculated and actual values ​​of the three-phase resistance and inductance parameters meet the fault-free criterion before resynchronization; otherwise, it is determined to be a permanent fault. Considering the influence of model errors and algorithm calculation errors, the margin coefficient K... rel_1 K rel_2 The value is set between 0.10 and 0.20 to allow for a margin and avoid misjudgment due to errors.

[0065] like Figure 5 The diagram shows the equivalent network of the distribution network when there are no faults or the transient faults have disappeared when an external disturbance is applied. Where, u φ (t)(φ=a,b,c) represents the injected voltage signal for each phase, i φ (t)(φ=a,b,c) represents the disturbance current of each phase, R l L is the line resistance of the distribution network. l For the line inductance of the distribution network, L′ T N is the equivalent inductance of the distribution transformer, and N is the virtual neutral point.

[0066] like Figure 6 The diagram shows the equivalent network circuit obtained by simplifying the equivalent network of the line. From KVL, the fault-free equivalent network equation is:

[0067]

[0068] Therefore, the response voltage u of any phase can be determined. φ (t)(φ=a,b,c) are all calculated from the equivalent line resistance, i.e., the three-phase resistance R. eq The equivalent inductance, i.e., the calculated inductance parameter L eq The linear relationship constitutes the equation.

[0069] like Figure 7 As shown, taking a two-phase short circuit between phases A and B as an example, if an external voltage is applied, the phase-to-phase fault still exists. Wherein, R... f Let Z1 be the fault transition resistance, m be the proportion of the distance from the fault point to the beginning of the line to the total length of the line, and Z1 be the equivalent impedance from the fault point to the end of the line. According to KVL, the equivalent network equation with fault is:

[0070]

[0071] Where Z1=2(1-m)R l +2jω(1-m)L l +2jωL′ T Clearly, the response voltage of any phase of the fault is related to the fault location m and the transition resistance R. f And the response voltage u of the other phase φ (t) is related and cannot be characterized as a linear relationship as shown in the fault-free equivalent network equation. For a two-phase ground fault, there exists a relationship as shown in the fault-equivalent network equation. However, since a three-phase fault is a symmetrical fault, it can also be analyzed as a single-phase fault.

[0072]

[0073] Therefore, we can know the response voltage u of each phase. φ(t)(φ=a,b,c) and the line self-resistance R from the fault point to the excitation source connection point l The line inductance L of the distribution network l The fault location (m) is relevant; the resistance and inductance parameters in the model differ significantly from those in the fault-free state. Furthermore, the equivalent network during the fault duration phase differs significantly from the fault-free equivalent network. This difference can be used to identify the state of the three-phase circuit. Using the fault-free equivalent network as the baseline model, response voltage and current are collected to calculate the equivalent resistance and inductance of the line. The fault state is identified based on the difference between the calculated and actual resistance and inductance values.

[0074] The above-described specific embodiments are preferred embodiments of a method for identifying fault-free distribution network reclosing using low-frequency injection signals, and are not intended to limit the specific scope of this application. The scope of this application includes, but is not limited to, these specific embodiments. All equivalent changes made in accordance with the shape and structure of this application are within the protection scope of this application.

Claims

1. A method for fault-free identification before reclosing in a distribution network using low-frequency injected signals, characterized in that: Includes the following steps: Step 1: Construct an active injection signal device and set a delay. When a three-phase reclosing trip occurs in the distribution line due to a phase-to-phase fault, the active injection signal device will simultaneously apply a low-frequency voltage disturbance signal to the three phases (A, B, and C) after the delay. Step 2: Collect the response voltage and response current on the line side of the line to be tested, and calculate the three-phase resistance and inductance parameters. Step 3: Obtain the actual values ​​of the three-phase resistance and inductance parameters. Determine the fault status based on the difference between the calculated values ​​of the three-phase resistance and inductance parameters and the actual values ​​of the three-phase resistance and inductance parameters. If it is a transient fault, proceed to step 4. If it is a permanent fault, then reclosing is blocked. Step 4: Set the loop judgment time and loop judgment interval, and determine whether the execution time of Step 3 has reached the loop judgment time. If yes, the reclosing is closed; if not, Step 3 is executed again after the loop judgment interval.

2. The method for fault-free identification before reclosing in a distribution network using low-frequency injection signals as described in claim 1, characterized in that: The device for constructing the active injection signal includes: An active injection signaling device is installed on the distribution network side. The active injection signaling device consists of a three-phase AC voltage source u. a u b u c It consists of control switches S1, S2, and S3.

3. The method for fault-free identification before reclosing in a distribution network using low-frequency injection signals as described in claim 1, characterized in that: The low-frequency voltage disturbance signal has an amplitude of 500V to 700V and a frequency of 10Hz to 20Hz.

4. The method for fault-free identification before reclosing in a distribution network using low-frequency injection signals as described in claim 1, characterized in that: Step 2 includes: Step 21: Collect the response voltage and current data of the line side of the line to be tested, and establish a fault-free equivalent network calculation model; Step 22: Construct a linear equation based on the response voltage and current data of the line side of the line to be tested, and use the fault-free equivalent network calculation model and the linear equation to calculate the three-phase resistance and inductance parameters.

5. The method for fault-free identification before reclosing in a distribution network using low-frequency injection signals as described in claim 4, characterized in that: For fault-free lines: R eq =R1; L eq L l +L T ′; Among them, R eq L is the calculated value of the three-phase resistance. eq Here are the calculated inductance parameters, where L1 is the line inductance of the distribution network, and L... T ′ represents the equivalent inductance of a distribution transformer.

6. The method for fault-free identification before reclosing in a distribution network using low-frequency injection signals as described in claim 5, characterized in that: When the active injection signal device applies a low-frequency voltage disturbance signal to the three phases (A, B, and C) simultaneously after a delay, the response voltage, response current, and derivative of the response current exhibit a linear relationship, with the proportionality coefficient being the equivalent resistance R. eq Equivalent inductance L eq : Among them, u φ (t)(φ=a,b,c) is the response voltage, i φ (t)(φ=a,b,c) is the response current.

7. The method for fault-free identification before reclosing in a distribution network using low-frequency injection signals as described in claim 6, characterized in that: The fault-free equivalent network calculation model is as follows:

8. The method for fault-free identification before reclosing in a distribution network using low-frequency injection signals as described in claim 4, characterized in that: Based on the response voltage and current data of the line side of the line to be tested, the linear equation is constructed as follows: The least squares method is used to analyze the acquired response voltage u. φ (t)(φ=a,b,c), response current i φ (t)(φ=a,b,c) is fitted to a linear equation: Where x = R eq ,y=L eq ;u φ (k) represents the response voltage at the k-th sampling point, i φ (k) represents the response current at the kth sampling point, and N represents the number of sampling points.

9. The method for fault-free identification of distribution network before reclosing using low-frequency injection signals as described in claim 4, characterized in that: The fault condition is determined based on the calculated values ​​of the three-phase resistance and inductance parameters, as well as the difference between the actual values ​​of the three-phase resistance and the actual values ​​of the inductance parameters. Based on the calculated values ​​of three-phase resistance and inductance parameters, and the actual values ​​of three-phase resistance and inductance parameters, a fault-free criterion is constructed before overlap: Where n is the length of the calculated result sequence; R eq For least squares calculation of the resistance sequence, R real This represents the actual value of the line resistance; L eq For least squares calculation of the inductance sequence; L real K represents the true sum of the line inductance and the distribution transformer inductance; rel_1 K rel_2 This is the margin coefficient.

10. The method for fault-free identification before reclosing in a distribution network using low-frequency injection signals as described in claim 9, characterized in that: Determining fault status based on the calculated values ​​of three-phase resistance and inductance parameters, and the difference between the actual values ​​of three-phase resistance and inductance parameters, also includes: Determine whether the difference between the calculated values ​​of the three-phase resistance and inductance parameters and the actual values ​​of the three-phase resistance and inductance parameters meets the fault-free criterion before re-synchronization. If yes, it is judged as a transient fault; if no, it is judged as a permanent fault.

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