Method and system for monitoring and protection of single-phase earth fault of three-phase low voltage line

Abstract

The application discloses a three-phase low-voltage line single-phase grounding fault monitoring and protection method and system in the field of power system relay protection and electrical monitoring, and the application takes the bus zero sequence current amplitude over-limit as the starting criterion of the system, obtains the transient current and voltage at the bus and at the outlet of each line, selects the fault line by the Daubechies wavelet modulus extreme value comparison method of the transient current traveling wave, further calculates the Hermitian wavelet energy of each phase current of the fault line to select the fault phase, triggers the bypass grounding switch to act by the microprocessor, and makes the fault current discharge to the ground through the current-limiting resistor without affecting the operation of the power grid line, thereby protecting the operation stability of the system. The protection method provided by the application has the advantages of fast detection speed, low cost, simple device, higher reliability and better safety compared with the existing single-phase grounding protection device.

Description

Technical Field

[0001] This invention belongs to the field of power system relay protection and electrical monitoring, specifically relating to a method and system for monitoring and protecting single-phase grounding faults in three-phase low-voltage lines. Background Technology

[0002] Single-phase grounding faults are among the most common faults in power systems. For 6kV and below 1:N type low-current grounding systems, single-phase grounding faults, such as those occurring in underground coal mine distribution networks, directly impact human life and property safety, potentially leading to electric shock accidents. Timely detection and rapid isolation of grounding faults are crucial for the stable operation of low-voltage distribution networks. Therefore, research is urgently needed on monitoring and protection methods and systems for single-phase grounding faults in three-phase low-voltage lines.

[0003] In my country, low-voltage distribution networks of 6kV and below mostly adopt a neutral point grounded via an arc suppression coil. When a single-phase ground fault occurs, the large current flowing through the grounding point is approximately the sum of the ground capacitance current of the entire system. The voltage of the faulted phase approaches zero, while the voltage of the non-faulted phases rises to... The fault current flowing through the faulty line after a single-phase ground fault is related to the transition resistance. For ground faults with a transition resistance of less than or equal to 1000Ω, in order to ensure system stability, the total time required for fault detection and protection actions must not exceed 1 / 4 of a power frequency signal cycle (i.e., 5ms). Existing single-phase ground fault detection and protection methods are mainly divided into steady-state method, transient method, and injection signal method. They are slow in detection speed, have low reliability, and the device settings are easily affected by the distributed parameters of the line. Moreover, the protection methods mostly adopt the means of directly disconnecting the faulty line or the leakage detection device built by physical circuit, which cannot achieve safe isolation of the fault. In 2018, the State Grid Ningxia Electric Power Research Institute proposed a method for preventing electric shock in neutral-point ungrounded distribution networks (application publication number: CN107979068A). This method uses zero-sequence voltage and instantaneous three-phase voltage values ​​to determine the occurrence of an accident and triggers a circuit breaker to disconnect the faulty line. However, this fault-clearing method allows the induced current generated by the back electromotive force of inductive loads (energy storage elements) in the system to continue flowing through the fault point, potentially causing secondary injuries. In 2021, the Hangzhou Power Supply Company of State Grid Zhejiang Electric Power Co., Ltd. proposed a method for selecting the fault location in distribution networks based on the phase difference of phase currents (application publication number: CN114047402A). This method uses the phase current amplitude and phase comparison method to determine the faulty line. However, in reality, due to the distribution of line capacitance and the presence of arc suppression coils, the amplitude difference of the currents in each phase is not significant after a single-phase ground fault occurs, making the reliability of the fault location difficult to guarantee. Summary of the Invention

[0004] The purpose of this invention is to overcome the above-mentioned shortcomings and provide a method and system for monitoring and protecting single-phase grounding faults in three-phase low-voltage lines. This method and system can achieve rapid fault detection, diagnosis of the faulty phase line, and triggering of the corresponding bypass grounding switch to discharge the capacitor current. It is a low-cost and easy-to-implement detection method and protection system.

[0005] To achieve the above objectives, the methods for monitoring and protecting against single-phase grounding faults in three-phase low-voltage lines include:

[0006] The transient current and transient voltage of each branch in the busbar are obtained, the zero-sequence current component of the busbar is calculated, and it is determined whether the zero-sequence current component exceeds the preset threshold. If it does, fault line selection is performed; otherwise, the transient current of the busbar is obtained again.

[0007] Based on the transient voltage and transient current collected from each branch in the bus, the traveling wave component of the current in each branch is calculated. The forward or reverse traveling wave of the current in each branch is decomposed by Daubechies wavelet. The extreme values ​​of the modulus of the detail component at the scale that best reflects the fault characteristics are selected to determine the line where the ground fault is located as the fault line.

[0008] Based on the fault line identification results, the transient current and voltage collected from the fault line are calculated to determine the faulty phase;

[0009] Based on the fault phase identification result, the bypass grounding switch of the corresponding branch is activated to complete the fault isolation.

[0010] The method for calculating the zero-sequence current component of the bus is as follows:

[0011] The transient current of each branch of the bus is collected in real time by a fixed-step dual data window. The main window and the secondary window move synchronously at a preset signal period to obtain the zero-sequence current component of the bus.

[0012] The specific method for using Daubechies wavelet decomposition to decompose the forward or reverse traveling waves of the current in each branch is as follows:

[0013] Based on the transient voltage and transient current collected from each branch in the busbar, calculate the changes in three-phase transient voltage and current of each branch during the window period before and after the fault, and perform Clark transformation on the changes in transient voltage and current.

[0014] The traveling wave flowing from the fault point to the detection point is a positive traveling wave, and the opposite is a negative traveling wave. The positive and negative traveling wave components of the current are calculated by combining the line wave impedance.

[0015] The Daubechies wavelet is:

[0016]

[0017] Among them, b krepresents the wavelet coefficients, and 2x-k represents the support length.

[0018] The method for determining the line where a ground fault occurs is as follows:

[0019] By decomposing the forward or reverse current of each branch using Daubechies wavelet decomposition, the extreme values ​​of the detail component modulus at the scale that best reflects the fault characteristics are selected. All component modulus extreme values ​​are compared, and the line with the largest modulus extreme value and the opposite polarity to other branches is selected as the line where the ground fault occurs.

[0020] The method for determining the faulty phase is as follows:

[0021] Singular value detection is performed on the transient current of each phase using Hermitian wavelets. The time-domain convolution calculation is transformed into a frequency-domain multiplication operation. The transformed magnitude represents the instantaneous energy of the signal. The relationship between the magnitude and time is the Hermitian amplitude diagram. The energy of the amplitude diagram of each phase after Hermitian wavelet transformation at the same scale is calculated. The phase with the largest energy is identified as the fault phase.

[0022] The time-domain expression of the Hermitian wavelet without the scale factor a is:

[0023]

[0024] The frequency domain expression of the Hermitian wavelet containing the scaling factor a is:

[0025]

[0026] To calculate the energy contained in the transformed wavelet amplitude diagrams at the same scale *a*, the energy integral operation is as follows:

[0027]

[0028] In the formula, N is the number of sampling points contained in a sampling window, and the phase with the highest energy is selected as the fault phase.

[0029] A monitoring and protection system for single-phase grounding faults in three-phase low-voltage lines includes:

[0030] The zero-sequence current component acquisition module is used to acquire the transient current and transient voltage of each branch in the bus, calculate the zero-sequence current component of the bus, and determine whether to perform fault line selection.

[0031] The fault line judgment module is used to calculate the traveling wave component of the current in each branch based on the transient voltage and transient current collected in each branch of the bus. The forward or reverse traveling wave of the current in each branch is decomposed by Daubechies wavelet, and the extreme value of the modulus of the detail component at the scale that best reflects the fault characteristics is selected to determine the line where the grounding fault is located as the fault line.

[0032] The fault phase determination module is used to calculate the transient current and voltage collected from the faulty line based on the fault line identification results, and to determine the faulty phase.

[0033] The isolation module is used to activate the bypass grounding switch of the corresponding branch based on the fault phase identification result, thereby completing the fault isolation.

[0034] The isolation module includes a microprocessor, which is connected to the normally open switches and the lagging phase normally closed switches on each branch of the bus.

[0035] The grounding switch connects the fully controlled electronic device IGBT and the 24V DC power supply in series via a three-phase relay. The triggering and shutdown of each phase's fully controlled electronic device IGBT are controlled by a microprocessor.

[0036] Compared with existing technologies, this invention uses the exceeding limit of the zero-sequence current amplitude of the busbar as the system activation criterion. After acquiring the transient current and voltage at the busbar and each line outlet, it selects the faulty line using the Daubechies wavelet modulus extremum comparison method of the transient current traveling wave. Then, it calculates the Hermitian wavelet energy of each phase current of the faulty line to select the faulty phase. The microprocessor triggers the bypass grounding switch to operate, allowing the fault current to be discharged to the ground through the current-limiting resistor without affecting the operation of the power grid, thereby protecting the system's operational stability. The protection method proposed in this invention has fast detection speed, low cost, and simple device, and offers higher reliability and better safety compared to existing single-phase grounding protection devices.

[0037] Furthermore, this invention extracts and amplifies the singular values ​​of the transient electrical parameters of the system after a fault, and uses Hermitian wavelets to transform the time-domain convolution calculation into a frequency-domain multiplication operation, which speeds up the calculation and improves the accuracy and reliability of fault location compared with existing line and phase selection methods.

[0038] The system of this invention uses a microprocessor to select the line and phase for single-phase grounding faults. Compared with traditional line and phase selection devices for distribution networks, this invention greatly improves the fault detection speed, reduces the complexity of protection devices, and saves the implementation cost of protection; it is also effective for identifying actual single-phase grounding faults. Attached Figure Description

[0039] Figure 1 This is a flowchart of the present invention;

[0040] Figure 2 This is a schematic diagram of synchronous sampling with dual data windows in this invention;

[0041] Figure 3 This is a structural diagram of a single-phase grounding fault in a 6kV multi-outgoing distribution network with the neutral point grounded by an arc suppression coil, as shown in this invention (only three branches are shown).

[0042] Figure 4 This is a diagram showing the three-phase decoupling relationship of the Clark transform in this invention;

[0043] Figure 5 This is a schematic diagram of the Daubechies wavelet decomposition principle in this invention (only three-level decomposition is shown);

[0044] Figure 6 This is a schematic diagram of the Daubechies wavelet mode extrema in this invention;

[0045] Figure 7 This is a schematic diagram of the Hermitian continuous wavelet transform principle in this invention;

[0046] Figure 8 This is a structural diagram of the system bypass grounding unit in this invention;

[0047] Figure 9 This is a circuit diagram of the grounding switch control in this invention (only the switch control of a single line is shown);

[0048] Figure 10 This is a system diagram of the present invention. Detailed Implementation

[0049] The invention will now be further described with reference to the accompanying drawings.

[0050] See Figure 1 The monitoring and protection methods for single-phase grounding faults in three-phase low-voltage lines include the following steps:

[0051] S1. Acquire the transient current and transient voltage of each branch in the busbar. Collect the transient current of each branch in the busbar in real time according to the fixed step size dual data window. The main window and the secondary window move synchronously at a preset signal period interval to obtain the zero-sequence current component of the busbar. Determine whether the zero-sequence current component exceeds the preset threshold. If yes, perform fault line selection. If no, acquire the transient current of the busbar again.

[0052] The transient current of each branch is acquired in real time using a microprocessor with a fixed-step dual data window. The zero-sequence current at the bus is calculated. The data window width is 1 / 4 of the power frequency signal cycle, and the sampling frequency is selected as 5-10kHz. The main window and the secondary window move synchronously, with an interval of 1-2 signal cycles. If the zero-sequence current exceeds the threshold, a ground fault is determined to have occurred. This threshold should be determined to be the maximum unbalanced current I of the bus during normal operation of the distribution network. u When selecting a threshold I, considering a safety margin, the threshold value is set. set For I u 1.1 to 1.3 times, of which the zero-sequence current is calculated as follows:

[0053] I0=(i A +i B +i C ) / 3

[0054] In the formula, I0 is the zero-sequence current of the line; i A i B i C These represent the instantaneous values ​​of the three-phase currents, respectively.

[0055] S2. Based on the transient voltage and transient current collected from each branch of the busbar, calculate the traveling wave component of each branch current. Then, decompose the forward or reverse traveling wave of each branch current using Daubechies wavelet decomposition. Select the extreme values ​​of the detail component modulus at scale 3 or scale 4 that best represent the fault characteristics to determine the line where the ground fault occurs. The specific method for decomposing the forward or reverse traveling wave of each branch current using Daubechies wavelet decomposition is as follows:

[0056] Based on the transient voltage and transient current collected from each branch in the busbar, calculate the changes in three-phase transient voltage and current of each branch during the window period before and after the fault, and perform Clark transformation on the changes in transient voltage and current.

[0057] The traveling wave flowing from the fault point to the detection point is a positive traveling wave, and the opposite is a negative traveling wave. The positive and negative traveling wave components of the current are calculated by combining the line wave impedance.

[0058] In the microprocessor, based on the current and voltage sampled through dual data windows, the three-phase transient voltage and current changes Δu and Δi of each branch during the window period before and after the fault are calculated, and a Clark transform is performed on them.

[0059] i m =Q·Δi

[0060] u m =Q·Δu

[0061] Where Q is the Clark transformation matrix.

[0062]

[0063] Calculate the forward and reverse traveling wave components of the current. The traveling wave from the fault point to the detection point is the forward traveling wave, and vice versa; where if α if β if0 is the positive traveling wave component of the current, ir α ir β it0 is the reverse traveling wave component of the current, that is, the forward traveling wave (or reverse traveling wave) can be decomposed into α and β linear mode components and 0 mode component:

[0064]

[0065] In the formula, Z is the line surge impedance, which is determined by the positive-sequence distributed capacitance and inductance per km.

[0066]

[0067] Since the linear mode components contain a lot of fault characteristic information, Daubechies wavelet decomposition is performed on one of the linear mode components α (or β) of the transient positive traveling wave (or reverse traveling wave) of each branch. The extreme values ​​of the mode at scale 3 or scale 4 of each branch are compared, and the line with the largest extreme value of the mode and the opposite polarity to other branches is selected as the faulty line; if the polarity of the extreme values ​​of the mode corresponding to each branch is the same, it is determined that the fault occurs in the bus.

[0068] The Daubechies wavelet scale is selected based on the different line parameters. For a 1:N type distribution network system experiencing a single-phase ground fault, db4 to db6 is sufficient. The Daubechies wavelet mentioned is (excluding the scale factor).

[0069]

[0070] In the formula, b k represents the wavelet coefficients, and 2x-k represents the support length.

[0071] S3. Based on the fault line identification results, further calculations are performed on the transient currents and voltages collected from the fault line. To improve the identification speed of the fault phase, Hermitian wavelets are used to perform singular value detection on the transient currents of each phase, transforming the time-domain convolution calculation into a frequency-domain multiplication operation. The transformed modulus |WT x (a,t)| represents the instantaneous energy of the signal, and angle θ x (a,t) represents the instantaneous phase of the signal. The relationship between the magnitude and time is the Hermitian amplitude diagram. Further calculations are performed on the amplitude diagrams of each phase after Hermitian wavelet transform at the same scale a. The phase with the highest energy is identified as the fault phase.

[0072] Based on the faulty line, Hermitian wavelet transform is performed on the transient current components of each phase. The time-domain expression of the Hermitian wavelet without the scaling factor a is:

[0073]

[0074] The frequency domain expression of the Hermitian wavelet containing the scaling factor a is:

[0075]

[0076] To calculate the energy contained in the transformed wavelet amplitude diagrams at the same scale *a*, the energy integral operation is as follows:

[0077]

[0078] In the formula, N is the number of sampling points contained in a sampling window, and the phase with the highest energy is selected as the fault phase;

[0079] When using Hermitian wavelets to transform transient current components, the smaller the selected scale a, the more high-frequency components are included in the decomposition result, the more obvious the fault characteristics, and the higher the line differentiation. For the identification of single-phase ground fault phases, a scale a of less than 10 can be selected.

[0080] S4. Based on the fault phase identification result, the microprocessor sends a control command to start the bypass grounding switch of the corresponding branch. Without affecting the operation of the power grid line, the large current of the fault phase is discharged to the ground through the bypass grounding unit, thereby achieving rapid isolation of the fault.

[0081] See Figure 10 A monitoring and protection system for single-phase grounding faults in three-phase low-voltage lines, including:

[0082] The zero-sequence current component acquisition module is used to acquire the transient current and transient voltage of each branch in the bus, calculate the zero-sequence current component of the bus, and determine whether to perform fault line selection.

[0083] The fault line judgment module is used to calculate the traveling wave component of the current in each branch based on the transient voltage and transient current collected in each branch of the bus. The forward or reverse traveling wave of the current in each branch is decomposed by Daubechies wavelet, and the extreme value of the modulus of the detail component at the scale that best reflects the fault characteristics is selected to determine the line where the grounding fault is located as the fault line.

[0084] The fault phase determination module is used to calculate the transient current and voltage collected from the faulty line based on the fault line identification results, and to determine the faulty phase.

[0085] The isolation module is used to activate the bypass grounding switch of the corresponding branch based on the fault phase identification result, thereby completing the fault isolation.

[0086] The isolation module includes a microprocessor, which is connected to the normally open switches and the lagging phase normally closed switches on each branch of the bus.

[0087] The grounding switch connects the fully controlled electronic device IGBT and the 24V DC power supply in series via a three-phase relay. The triggering and shutdown of each phase's fully controlled electronic device IGBT are controlled by a microprocessor.

[0088] The fault protection strategy involves using a bypass grounding unit installed at the outlet of each line in the distribution network to discharge the current flowing through the fault point to the ground. This isolates the fault without affecting line operation and provides a path for the current caused by the back electromotive force generated by the sudden power failure of inductive loads (energy storage elements) in the system. Each phase of the grounding device consists of a normally open switch for the current phase, a normally closed switch for the lagging phase, and a current-limiting resistor connected in series. The two switches are controlled by two-phase relays respectively. After a fault occurs, the normally open switch for the current phase closes, and the normally closed switch for the corresponding lagging phase in the healthy phase opens, preventing a phase-to-phase short circuit caused by the switch operation when two-phase ground faults occur simultaneously. The current-limiting resistor is selected to protect the switch from burning out, and its resistance value is selected from 300 to 1000 Ω. The higher the voltage level, the larger the value of the current-limiting resistor should be. The grounding switch control circuit consists of three-phase relays connected in series with fully controlled electronic IGBTs to a 24V DC power supply. Each IGBT is controlled by a microprocessor to trigger and turn off. When a phase is identified as a faulty phase, the microprocessor sends a trigger signal to the corresponding IGBT to turn it on, energizes the relay, and controls the corresponding switch to close and open.

[0089] Example:

[0090] To illustrate the implementation method of this invention in detail, this invention takes a 1:6 type distribution network system with a 6kV neutral point grounded by an arc suppression coil as an example. The line adopts a π-type circuit model, with an inductive load at the end and a load capacity of (4+j3)MW. The neutral point arc suppression coil is set to 3H (theoretical value is 2.976H) with 10% overcompensation. The lengths of the six lines are 20km, 25km, 15km, 44km, 30km, and 12km, respectively. The principles and implementation methods of this invention are further explained and discussed in conjunction with the accompanying drawings.

[0091] The present invention proposes a method and system for monitoring and protecting against single-phase grounding faults in three-phase low-voltage lines, including fault line and phase selection and a bypass grounding unit. Figure 1 The logic flow for locating and protecting against single-phase grounding faults in the system. Figure 2 This is a schematic diagram of the dual-data-window dynamic sampling principle. The transient current and voltage used are all derived from... Figure 3 The voltage and current sensors installed at the line port are shown. The system sampling frequency is set to 5-10kHz, the width of the dual data window is 1 / 4 cycle (i.e., 5000 sampling points), and the main window leads the secondary window by 1-2 signal cycles.

[0092] The detection method is activated when the zero-sequence current at the busbar exceeds the threshold I. setWhen the system is operating normally, there is no zero-sequence current at the bus. Considering the slight differences between the lines in reality and the existence of incomplete line asymmetry, the zero-sequence current fluctuates around 0. Therefore, a zero-sequence current threshold is set to improve the reliability of the method. This threshold should be selected to avoid the maximum unbalanced current of the bus when the 6kV power grid line is operating normally. In this embodiment, I is selected. set 1A is sufficient;

[0093] Three-phase transient electrical parameters include transient voltage and transient current. Since there is a coupling relationship between the three phases, the sampled parameters in three-dimensional space are projected onto a two-dimensional coordinate system and a Clark decoupling transformation is performed:

[0094] i m =Q·Δi

[0095] u m =Q·Δu

[0096] Where Δi and Δu are the current and voltage changes sampled by the dual data windows, and Q is the Clark transformation matrix.

[0097]

[0098] The relationship between the Clark transform coordinate system and the three-phase coordinate system is as follows: Figure 4 According to the formulas for calculating forward and reverse traveling waves

[0099]

[0100] In the formula i m u m These represent the decoupled current and voltage, respectively; Z is the line surge impedance, determined by the positive-sequence distributed capacitance C and inductance L per km of the line. In this embodiment, the positive-sequence distributed capacitance and inductance of the line are 0.9337 mL / km and 12.74 uF / km, respectively, therefore the line surge impedance is...

[0101] The method utilizes the β component (and similarly the α component) of the transient forward current traveling wave for Daubechies wavelet decomposition. Figure 5 The diagram illustrates the principle of Daubechies wavelet decomposition (taking a three-level decomposition as an example). The original signal X is decomposed into detail components (cd) and approximation components (ca) at each scale. The approximation signal contains low-frequency information, while the detail components contain high-frequency information. This process is repeated to amplify the local features of the signal. The db6 wavelet basis function is selected, and the decomposition level is set to 6 levels. The modulus extrema of the third-level detail component (cd3) are used as the criterion for distinguishing between faulty and non-faulty lines. Figure 6The Daubechies wavelet modulus extreme value curves after a ground fault in the six outgoing lines show that the modulus extreme value amplitude of branch 4 is the largest and the polarity is opposite to that of the other lines, indicating that the fault occurred in branch 4.

[0102] Hermitian complex wavelet transform is performed using the transient phase current of the faulty line. Figure 7 Based on the principle of Hermitian complex wavelet transform, the original signal x(t) and the Hermitian wavelet basis function ψ(t) are transformed to the frequency domain using Fourier transform. Multiplication in the frequency domain avoids convolution in the time domain, yielding the wavelet transform result in the frequency domain. Performing an inverse Fourier transform on this result gives the wavelet transform WT. a (a,t). The Hermitian wavelet is a complex-valued wavelet, therefore its transformation result is a complex number. Its amplitude can represent the energy of the troughs and peaks of the original signal, and its phase can represent the time of occurrence. It corresponds one-to-one with the original signal and has no phase difference. Then, the energy integral operation is performed on the transformed wavelet amplitude at the same scale as follows.

[0103]

[0104] In the formula, N represents the number of sampling points contained in a sampling window. The fault phase A is selected based on the Hermitian wavelet energy of each transient phase current.

[0105] Bypass grounding unit such as Figure 8 As shown, each phase consists of two high-speed electronic switches connected in series with a current-limiting resistor, namely one normally open switch for the current phase and one normally closed switch for the lagging phase. The purpose of the current-limiting resistor is to limit the magnitude of the grounding current to protect the electronic switches. According to the distribution network structure and line distribution parameters of the embodiment, the resistance value is set to 300Ω. The electronic switches are controlled by the grounding switch control circuit to determine whether to open or close.

[0106] In the grounding switch control circuit, the on / off state of the relay is controlled by IGBT (or other fully controlled electronic devices). Figure 9 For the grounding switch control circuit, the microprocessor determines the phase where the fault occurs and sends a trigger pulse to the corresponding relay to activate, so that the fault current is discharged to the ground.

[0107] The relays in the grounding switch control circuit each control two switches in the circuit, with relay J... A For example, it controls the normally open switch J of this phase respectively. A2 Closed and normally closed switch J A1 Open, relay J B J C Similarly.

Claims

1. A method of monitoring and protection of single-phase earth faults in a three-phase low voltage line, characterized in that, include: The transient current and transient voltage of each branch in the busbar are obtained, the zero-sequence current component of the busbar is calculated, and it is determined whether the zero-sequence current component exceeds the preset threshold. If it does, fault line selection is performed; otherwise, the transient current of the busbar is obtained again. Based on the transient voltage and transient current collected from each branch of the busbar, the traveling wave component of each branch current is calculated. The forward or reverse traveling wave of each branch current is then decomposed using Daubechies wavelet decomposition. The extreme values ​​of the modulus of the detail component at the scale that best reflects the fault characteristics are selected to determine the faulty line. The specific method for decomposing the forward or reverse traveling wave of each branch current using Daubechies wavelet decomposition is as follows: Based on the transient voltage and transient current collected from each branch in the busbar, calculate the changes in three-phase transient voltage and current of each branch during the window period before and after the fault, and perform Clark transformation on the changes in transient voltage and current. The traveling wave flowing from the fault point to the detection point is a positive traveling wave, and the opposite is a reverse traveling wave. The positive and reverse traveling wave components of the current are calculated by combining the line wave impedance. The method for determining the line where a ground fault occurs is as follows: By decomposing the forward or reverse current of each branch using Daubechies wavelet decomposition, the extreme values ​​of the detail component modulus at the scale that best reflects the fault characteristics are selected. All component modulus extreme values ​​are compared, and the line with the largest modulus extreme value and the opposite polarity to other branches is selected as the line where the ground fault is located. Based on the fault line identification results, the transient current and voltage collected from the fault line are calculated to determine the faulty phase. The specific method is as follows: Singular value detection is performed on the transient current of each phase using Hermitian wavelets. The time-domain convolution calculation is transformed into a frequency-domain multiplication operation. The transformed magnitude represents the instantaneous energy of the signal. The relationship between the magnitude and time is the Hermitian amplitude diagram. The energy of the amplitude diagram of each phase after Hermitian wavelet transformation at the same scale is calculated. The phase with the largest energy is judged as the fault phase. Based on the fault phase identification result, the bypass grounding switch of the corresponding branch is activated to complete the fault isolation.

2. The method for monitoring and protecting against single-phase grounding faults in three-phase low-voltage lines according to claim 1, characterized in that, The method for calculating the zero-sequence current component of the bus is as follows: The transient current of each branch of the bus is collected in real time by a fixed-step dual data window. The main window and the secondary window move synchronously at a preset signal period to obtain the zero-sequence current component of the bus.

3. The method for monitoring and protecting against single-phase grounding faults in three-phase low-voltage lines according to claim 1, characterized in that, The Daubechies wavelet is: in, These are wavelet coefficients. For the support length.

4. The method for monitoring and protecting against single-phase grounding faults in three-phase low-voltage lines according to claim 1, characterized in that, No scale factor The time-domain expression of the Hermitian wavelet is: Including scale factor The frequency domain expression of the Hermitian wavelet is: Solve the same scale for each phase wavelet amplitude map after transformation. The energy contained below, the energy integral calculation is as follows: In the formula, Given the number of sampling points contained in a sampling window, the phase with the highest energy is selected as the faulty phase.

5. A monitoring and protection system for single-phase grounding faults in three-phase low-voltage lines, based on the monitoring and protection method for single-phase grounding faults in three-phase low-voltage lines as described in claim 1, characterized in that, include: The zero-sequence current component acquisition module is used to acquire the transient current and transient voltage of each branch in the bus, calculate the zero-sequence current component of the bus, and determine whether to perform fault line selection. The fault line judgment module is used to calculate the traveling wave component of the current in each branch based on the transient voltage and transient current collected in each branch of the bus. The forward or reverse traveling wave of the current in each branch is decomposed by Daubechies wavelet, and the extreme value of the modulus of the detail component at the scale that best reflects the fault characteristics is selected to determine the line where the grounding fault is located as the fault line. The fault phase determination module is used to calculate the transient current and voltage collected from the faulty line based on the fault line identification results, and to determine the faulty phase. The isolation module is used to activate the bypass grounding switch of the corresponding branch based on the fault phase identification result, thereby completing the fault isolation.

6. The monitoring and protection system for single-phase grounding faults in three-phase low-voltage lines according to claim 5, characterized in that, The isolation module includes a microprocessor, which is connected to the normally open switches and the normally closed switches of the lagging phases on each branch of the bus.

7. The monitoring and protection system for single-phase grounding faults in three-phase low-voltage lines according to claim 5, characterized in that, The grounding switch connects the fully controlled electronic device IGBT and the 24V DC power supply in series via a three-phase relay. The fully controlled electronic device IGBT of each phase is controlled by a microprocessor to trigger and turn off.

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