A cable type power distribution network fault classification and positioning method

Abstract

The application belongs to the technical field of power distribution network fault positioning, and provides a cable type power distribution network fault classification and positioning method, which comprises the steps of identifying three-core cable structure and electrical parameters, constructing normal and fault state models, generating fault identification signals by using power electronic switch combination of additional power supply, detecting fault types, obtaining short-circuit current data through phase-mode transformation, and calculating fault distance by using harmonic impedance method. The application uses multi-frequency harmonic injection and power electronic switch combination technology to enhance weak fault current detection capability, expands the phase-mode transformation method of Karenbauer matrix to eliminate electromagnetic coupling interference of lead core and shielding layer, uses harmonic impedance method for fault distance measurement to reduce distance measurement error, and realizes localized fault positioning by using fast Fourier transform to reduce dependence on remote monitoring system. The application realizes high-precision, strong anti-interference and fast-response power distribution network fault positioning, and provides an efficient technical solution for reliable operation of intelligent power distribution network.

Description

Technical Field

[0001] The present invention belongs to the technical field of distribution network fault location, and in particular relates to a cable type distribution network fault classification and location method. Background Art

[0002] Smart grids are an inevitable trend in the development of the energy and power industries, and are of great significance for adjusting my country's energy structure, achieving energy conservation and emission reduction, and addressing climate change. Compared to transmission networks, distribution networks have numerous devices, complex line connections, and intertwined and overlapping lines. These networks operate in a relatively complex environment, making them susceptible to various external environmental and internal issues. Compared to transmission networks, distribution networks are more prone to failures, with over 80% of power system failures occurring within them. Distribution networks both domestically and internationally have long suffered from ground faults that can cause electric shock, forest fires, and power outages. Because distribution networks extend deep into user terminals, ground faults can easily pose a safety risk to electric shock. Distribution networks are also prone to insulation faults, contact faults, and disconnection faults, which can easily trigger wildfires and cause widespread blazes.

[0003] Therefore, improving the power supply reliability of distribution networks can ensure safe power supply to power users and improve the power quality of the entire power system. For a long time, research on power system fault location has tended to focus on transmission networks, with relatively little research on fault location in distribution networks. Therefore, research on distribution network fault location is crucial for improving power system reliability. After a fault occurs in the distribution network, the protection system will activate based on the corresponding electrical quantity changes, driving circuit breakers and other devices to disconnect the faulty line. Distance measurement of the fault point on the faulty line is crucial for post-fault recovery, and rapid fault location measurement ensures timely restoration of power supply.

[0004] The widespread use of cables inevitably leads to an increase in faults. Finding underground cable faults is difficult. Existing distribution automation systems can only identify sections of permanently faulted lines and are unable to accurately measure the distance to complex underground cable faults. Furthermore, distribution automation systems are primarily based on a centralized data processing platform at the master station. The large amount of monitoring data places excessive pressure on the master station's communications, increasing fault location time and imposing high data transmission risks and poor fault location reliability. Currently, on-site inspections still rely on manual inspections or offline signal injection to precisely locate underground cable faults, which is time-consuming and labor-intensive, and does not meet the requirements for the development of intelligent distribution networks.

[0005] Fault location in distribution networks is fundamental for improving demand-side power supply reliability, a central technical step in achieving distribution network automation, and crucial for ensuring the safe and stable operation of the entire power system. Rapid and accurate fault location is the foundation for rapid self-healing in distribution networks and a key technology for implementing smart distribution networks. Detecting distribution network cable faults, identifying the faulty feeder, and ultimately locating the fault point, and performing targeted repairs on the faulty cable can prevent serious power outages in the distribution network and improve power supply reliability. Therefore, research on fault location in cable-based distribution networks is crucial for minimizing outage losses, improving fault repair efficiency, and ensuring the safe and stable operation of distribution networks. Summary of the Invention

[0006] To solve the problems in the background technology, the present invention provides a cable-type distribution network fault classification and location method, comprising the following steps:

[0007] S1. Identify the structure and electrical parameters of the three-core cable;

[0008] S2. Build a normal state model and a fault state model for the three-core cable, and simulate the operating data of the cable in normal and fault states;

[0009] S3. Generate a fault type identification signal through the power electronic switch combination of the additional power supply, traverse the cable line to detect the fault type, obtain short-circuit current data based on phase mode transformation, and calculate the fault distance using the harmonic impedance method to complete the fault location; wherein:

[0010] The power electronic switch combination of the additional power supply includes a controllable power electronic switch and a matching harmonic injection power supply. The harmonic impedance method separates the influence of fault resistance on ranging by injecting multi-frequency voltage signals into the line head end and analyzing the spectrum characteristics of the response current.

[0011] Furthermore, in step S1, the structure of the three-core cable includes: an outer sheath, a shielding layer, a filler, a main shielding layer and a conductor; the electrical parameters include conductor material, rated voltage, working capacity, impedance per unit length and operating frequency; wherein, the shielding layer is divided into a common shielding structure and an independent shielding structure, the conductors are arranged symmetrically in an equilateral triangle, and the self-impedance, mutual impedance, shielding layer self-impedance and conductor-shielding layer mutual impedance of the three-phase conductors satisfy a symmetrical relationship.

[0012] Furthermore, in step S2, the normal state model and the fault state model are:

[0013] Normal state model: The three-phase conductors are connected to the load impedance through the line running impedance, and the shielding layer is grounded through the head-end grounding resistor and the terminal grounding resistor;

[0014] Fault state model: A fault generator is added to the normal state model to simulate core-shield faults, single-phase / multi-phase grounding faults, and phase-to-phase short circuit faults. The fault generator generates different fault types by switching the fault resistance and fault point location.

[0015] Furthermore, in step S3, the power electronic switch combination of the additional power supply includes:

[0016] Multiple IGBT switches are connected between phases A, B, and C and the shield layer to form a programmable conduction path;

[0017] A harmonic injection power supply outputs a voltage pulse signal containing fundamental and harmonic components. The multi-frequency voltage signal output by the harmonic injection power supply includes the fundamental frequency and odd harmonic frequencies, where the odd harmonics have orders of 3, 5, and 7 and an amplitude of 20%-30% of the fundamental amplitude. The signal is injected into the fault line through a conductive path; the conductive combination of the IGBT switches corresponds one-to-one to the fault type, and the response current amplitude of the fault phase is greater than that of the non-fault phase.

[0018] Furthermore, in step S3, the fault type identification specifically includes:

[0019] When the response current of phase C is greater than that of phase A and phase B, it is determined to be a phase C ground fault;

[0020] When the amplitudes of the response currents of any two phases are equal and higher than those of the third phase, it is determined to be a phase-to-phase short circuit fault;

[0021] When the three-phase response currents are all lower than the threshold of 1.1 times the rated current, it is determined that there is no ground fault.

[0022] Furthermore, in step S3, the implementation of the harmonic impedance method includes:

[0023] S31. Inject multi-frequency voltage signal into the head end of the fault line V inj , and collect the response current signal I res ;

[0024] S32. Yes V inj and I res Perform fast Fourier transform to extract fundamental wave component and harmonic wave component;

[0025] S33. Calculate the harmonic impedance of the line according to formula (1) :

[0026]

[0027] in:

[0028] : No. Equivalent impedance modulus under subharmonics;

[0029] : equivalent DC resistance component;

[0030] : harmonic order;

[0031] : number of sampling points;

[0032] : time interval of a single sampling point;

[0033] : Fault circuit equivalent inductance;

[0034] The equivalent DC resistance component The response current at the head end of the fault line under zero-frequency voltage is calculated by offline measurement, specifically: ;

[0035] S34. Based on fundamental reactance X 1Calculate the fault distance S :

[0036]

[0037] in:

[0038] : Fault circuit equivalent inductance;

[0039] : Inductance per unit length.

[0040] Furthermore, the fundamental reactance X 1 is calculated by formula (3):

[0041]

[0042] in:

[0043] X 1: fundamental reactance;

[0044] Z eq1 : fundamental frequency impedance;

[0045] Z eq0 : equivalent DC resistance component;

[0046] : harmonic order;

[0047] : fundamental angular frequency;

[0048] The fundamental frequency impedance Z eq1 is the voltage injected at the fundamental frequency V inj(1) The response current collected I res(1) The ratio of .

[0049] Furthermore, the phase transformation uses the extended Karenbauer matrix T eq Decoupling of three-phase four-wire systems:

[0050]

[0051] The original voltage and current Convert to line mode components 、 , zero mode component and residual modulus , to eliminate the coupling effect between the three-phase conductor core and the shield layer; expand the Karenbauer matrix T eq The decoupling parameters include:

[0052] Three-phase conductor self-impedance Z s , represents the self-inductance and resistance components generated by the single-phase conductor under alternating current;

[0053] Three-phase core mutual impedance Z m , represents the mutual inductance between any two phase conductor cores;

[0054] Shield self-impedance Z ns , represents the self-inductance and resistance components generated independently by the shielding layer;

[0055] Mutual impedance between conductor and shield Z mn , represents the mutual inductance component between the single-phase conductor core and the shielding layer;

[0056] Among them, the line mode component 、 The corresponding line mode impedance satisfies:

[0057] ;

[0058] Used to characterize the balanced transmission characteristics between three-phase conductor cores;

[0059] Zero mode component Impedance:

[0060] ;

[0061] Reflects the common mode coupling effect between the three-phase conductor core and the shielding layer;

[0062] Residual modulus Impedance:

[0063] ;

[0064] Characterize the independent channel characteristics formed by the shield layer and the ground loop;

[0065] By separation and The mutual impedance term is used to eliminate the electromagnetic coupling interference between the three-phase conductor core and the shielding layer under fault conditions; where:

[0066] ;

[0067] ;

[0068] is the mutual impedance of the residual modulus to the zero mode, is the mutual impedance of the zero mode to the residual mode.

[0069] The beneficial effects achieved by the present invention are:

[0070] This invention uses five innovative technologies: multi-frequency harmonic injection, signal decoupling, impedance analysis, full-state modeling, and real-time processing. It achieves high-precision, strong anti-interference, fast response, and wide applicability in distribution network fault location. It solves the pain points of traditional methods that rely on manual labor, have low precision, and are slow, and provides an efficient technical solution for the reliable operation of smart distribution networks. This is specifically reflected in:

[0071] First, the present invention designs a combination technology of multi-frequency harmonic injection and power electronic switch. Through the coordinated operation of controllable power electronic switches and harmonic injection power supplies, a multi-frequency voltage signal containing the fundamental wave and the 3rd, 5th, and 7th harmonics is actively injected into the fault line. Combined with the switch state traversal, including turning on the fault phase and blocking the non-fault phase, the fault type can be quickly identified. The active injection signal enhances the detection capability of weak fault currents, and is particularly suitable for high-resistance grounding scenarios. It also distinguishes the fault type through harmonic spectrum characteristics, avoiding the misjudgment problem of the traditional zero-sequence current method under complex noise. In the case of a metallic short circuit, the harmonic response current amplitude increases by 5-10 times, significantly improving the signal-to-noise ratio.

[0072] Second, the present invention designs an extended Karenbauer matrix phase-mode transformation method. For three-phase, four-wire cable structures, this improved extended Karenbauer transformation matrix decouples the original electrical quantity into line-mode, zero-mode, and residual-mode components, eliminating electromagnetic coupling interference between the conductor and the shield. This decoupling significantly reduces the impedance calculation error of the line-mode component. The method supports a variety of cable types, including those with common and independent shields, without requiring algorithm adjustments.

[0073] Third, this invention designs a harmonic impedance fault distance measurement method. Based on the harmonic impedance characteristics of the multi-frequency response signal, an effective formula is designed to isolate the influence of fault resistance and combine it with the fundamental reactance to calculate the fault distance. By suppressing the resistance effect with harmonic components, ranging errors are significantly reduced, and the method remains stable in nonlinear scenarios such as arc faults and intermittent ground faults.

[0074] Fourth, this invention uses fast Fourier transform (FFT) to perform real-time spectral analysis of the injected signal and the response current, combined with a DSP chip to achieve localized fault location without relying on master station communication. The entire process from fault occurrence to location result output can be completed within 50ms, meeting the distribution network's requirement for rapid power restoration. This reduces reliance on remote monitoring systems, conserving communication bandwidth and master station data processing resources.

[0075] Fifth, through PSCAD / EMTDC simulation verification, the present invention shows good positioning accuracy when locating various faults set in 10kV cable lines.

[0076] The present invention achieves high-precision, strong anti-interference, fast response and wide applicability of distribution network fault location through five innovative technologies: multi-frequency harmonic injection, signal decoupling, impedance analysis, full-state modeling and real-time processing. It solves the pain points of traditional methods that rely on manual labor, have low precision and slow speed, and provides an efficient technical solution for the reliable operation of smart distribution networks. BRIEF DESCRIPTION OF THE DRAWINGS

[0077] Figure 1 is an equivalent circuit diagram of a three-core cable in Example 1 of the present invention;

[0078] Figure 2 This is the equivalent circuit diagram of the three-core cable after a fault in Example 1 of the present invention;

[0079] Figure 3 This is a diagram of the three-core cable operation and fault model in Example 2 of the present invention;

[0080] Figure 4 This is a schematic diagram of the cable fault location principle in Example 3 of the present invention;

[0081] Figure 5 Is the equivalent circuit diagram of the cable fault in Example 3 of the present invention;

[0082] Figure 6 FIG. 4 is an overall architecture diagram of the fault location system of the present invention. DETAILED DESCRIPTION

[0083] The technical solutions of the present invention will be described clearly and completely below in conjunction with the drawings in the present invention. In addition, the forms of the various structures described in the following embodiments are merely examples. The present invention is not limited to the various structures described in the following embodiments. All other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention.

[0084] Reference Figures 1-6 The present invention provides a cable-type distribution network fault location method, comprising step S1, establishing a mathematical model of the cable according to the structure and parameters of the three-core cable; step S2, establishing an operation and fault model of the three-core cable based on the mathematical model of the three-core cable, and simulating the operation data of the three-core cable; step S3, establishing a power electronic device with an additional power supply for fault type identification and location according to the operation and fault model of the three-core cable, detecting different fault types, obtaining short-circuit current through phase mode transformation, and finally locating the cable fault through a harmonic impedance method.

[0085] The above steps are described in detail below through specific embodiments.

[0086] Example 1:

[0087] In this embodiment, the four metal layers of a three-core cable with a common shield structure are: phase A conductor, phase B conductor, phase C conductor, and metal shield layer S. Accordingly, the impedance or admittance matrix of this three-core cable is a 4th-order square matrix. Therefore, the impedance matrix Zcs and admittance matrix Ycs of the common shield three-core cable are expressed as follows:

[0088]

[0089]

[0090] Wherein, the impedance matrix Z CS and the admittance matrix Y CS The first to fourth rows in the matrix correspond to the common shielded three-core cable: A-phase conductor, B-phase conductor, C-phase conductor and metal shielding layer S. CS and Y CS The element z at row m and column n in mn 、y mn(1≤m, n≤4) can represent three types of impedance and admittance: the self-impedance and self-admittance of a phase conductor, the mutual impedance and mutual admittance between different phase conductors, and the mutual impedance and mutual admittance between the corresponding phase conductors and the shielding layer. In the following text, Z is used s Indicates the self-impedance of the three-phase conductor core, Z m Represents the mutual impedance between three-phase conductor cores, Z ns Indicates shield impedance and Z nm Indicates the mutual impedance between the shielding layer and the three-phase core.

[0091] According to the three-core cable structure and the above formula, it can be observed that the three-phase core conductors of the cable are symmetrically arranged in the form of an equilateral triangle. Therefore, the electrical parameters of the three-core cable are symmetrical, which can be described by the following relationship:

[0092]

[0093] Compared with high-voltage power transmission lines, distribution cables are shorter, so a lumped parameter model can be used to establish a steady-state equivalent circuit model that reflects the electrical parameters of the cable. Figure 1 The equivalent circuit of the three-core cable is established.

[0094] The equivalent circuit diagrams of cables after various types of accidents are as follows: Figure 2 As shown, it includes core-shield fault, ground fault and conductor-to-core fault.

[0095] Example 2:

[0096] In this embodiment, a power system model is provided: the power system model is located at Figure 3 On the left, it serves as the busbar for the power distribution network, providing a stable power source for the power transmission system. This model not only includes traditional ABC three-phase power transmission but also features a shield-to-ground connection, effectively suppressing electromagnetic interference and improving the system's electromagnetic compatibility.

[0097] Circuit breaker system model: The circuit breaker system model is a critical component in ensuring the safe operation of power systems. As a crucial protective device for system safety, it rapidly disconnects the circuit upon detecting an abnormality or fault signal, preventing the further spread of the fault current. This model accurately simulates the various operating states of the circuit breaker, including normal operation and fault disconnection. This model allows for simulation of circuit breaker operation, ensuring the safe operation of the power system.

[0098] Cable Fault Model: This model is used to deeply study and simulate the behavior of three-core cables under various fault conditions. The fault generator consists of a programmable resistor array and relays. By controlling the relays to switch resistor values ​​(0.1Ω-10kΩ) and grounding point locations (core-shield, core-ground, and phase-to-phase short), it simulates core-to-shield faults, single-phase ground faults, and phase-to-phase short circuits. The relays are driven by an FPGA controller, with switching times of less than 10ms. By simulating these faults, we can analyze their impact on system voltage and current distribution, supporting the development of fault diagnosis strategies and optimizing cable design. Furthermore, the model can simulate cable fault behavior under different load and environmental conditions, enhancing its practicality and accuracy.

[0099] Load Model: Designed to be located at the end of a distribution line, a load model simulates the power demands of downstream equipment or users. By introducing a load model, the power consumption characteristics of an actual distribution system can be reproduced, making the entire simulation system more realistic. The load model not only reflects the operating characteristics of different load types (such as resistive, inductive, and capacitive loads), but also allows for simulation of load variations at different times and in different scenarios by adjusting load parameters, thereby verifying the system's stability and adaptability under various operating conditions.

[0100] Example 3:

[0101] The power electronics power supply in this embodiment uses a single-phase power supply with a frequency roughly equivalent to the line power supply to power the power electronics, injecting excitation voltage into the circuit after a fault. A three-level H-bridge inverter topology is used for harmonic injection, generating a multi-frequency voltage signal containing the fundamental wave and the third, fifth, and seventh harmonics through SPWM modulation. The inverter's DC side voltage is 1000V, the switching frequency is 10kHz, and the output voltage total harmonic distortion (THD) is less than 5%. The power electronics utilize IGBTs, which are more compatible with control logic and have a higher switching frequency, as power electronic switches. Because different phase failures lead to different current paths, the corresponding fault type can be determined by cycling the conduction and shutdown cycles of the eight power electronic switches, allowing for measurement of the fault signal.

[0102] Secondly, after the signal is measured, a phase-mode transformation is performed on it to obtain relevant information about the short-circuit current.

[0103] The specific implementation principles are as follows:

[0104] Since the three-core cable model is similar to a three-phase four-wire system, this section refers to the three-phase four-wire system of the low-voltage distribution network to analyze its parameter asymmetry. By analogy with the phase-mode transformation matrix of the parameter-asymmetric three-phase four-wire system, the three-core cable and the shielding layer can be decoupled from the complex coupled system electrical quantities into four modules through phase-mode transformation, providing a theoretical basis for the calculation of three-core cable faults.

[0105] To decouple the matrix Z, we can find a similarity transformation matrix that can diagonalize the matrix Z according to the matrix calculation principle, thereby realizing the decoupling of the electrical quantities of the three-phase four-wire system. However, if this matrix is ​​to be used as a phase-mode transformation matrix, it must meet two conditions: (1) the elements in the matrix are fixed, that is, constants; (2) the transformation process corresponding to the matrix has a clear physical meaning;

[0106] According to the matrix calculation principle, if the cable parameters are completely symmetrical, the characteristic equation is written for the impedance matrix Z, and the eigenvalues ​​of the impedance matrix can be solved as follows:

[0107]

[0108] Assumed eigenvalue 、 、 、 , the corresponding eigenvector is 、 、 、 , composed of 4 eigenvectors;

[0109] The matrix is , then the matrix P is the similarity transformation matrix that can diagonalize the matrix Z, that is ,in . According to the properties of matrix eigenvalues:

[0110]

[0111] Will 、 、 Substituting into the above formula, we can get:

[0112]

[0113]

[0114]

[0115] The eigenvector can be extracted from the above formula , , , the same for Substituting into the above formula, we can get:

[0116]

[0117] So we can take the eigenvector , thus we get the matrix P:

[0118]

[0119] Its inverse matrix is:

[0120]

[0121] The impedance matrix Z can be decoupled by the matrix P and its inverse matrix as:

[0122]

[0123] Therefore, for a parameter-symmetrical three-phase four-wire line impedance matrix, a matrix P can be easily obtained. P and its inverse transform the impedance matrix into a diagonal matrix, achieving system parameter decoupling. However, according to actual analysis, not all three-core cable lines meet the parameter symmetry condition. On the contrary, three-core cable lines often have asymmetric parameters. Therefore, line parameter asymmetry must still be considered in fault calculations. Therefore, a phase-mode transformation matrix suitable for matrix Z decoupling is proposed to make this phase-mode transformation method universally applicable.

[0124] To solve the above problems, we choose to expand the phase mode transformation matrix to the fourth order based on the Karenbauer transformation to obtain the final phase mode transformation matrix T:

[0125]

[0126]

[0127] The derivation of the extended Karenbauer matrix T is based on the symmetry assumption of the three-phase four-wire system. By treating the three-phase core and the shield as independent conductors, a transformation matrix that satisfies the diagonalization of the impedance matrix is ​​constructed. For cables with asymmetric parameters, a correction factor needs to be introduced into the matrix T. , so that the transformed modulus impedance matrix satisfies:

[0128]

[0129] Where Z is the original fourth-order impedance matrix, Z 120r is the diagonalized matrix after decoupling.

[0130] Based on the matrix T, the transformation relationship between the system electrical quantity and each modulus component is:

[0131]

[0132] So the voltage-current relationship equation of the three-core cable system is:

[0133]

[0134] Multiplying the change matrix on the left side of the equation yields:

[0135]

[0136] Finally, the modulus impedance equation can be obtained as:

[0137]

[0138] Right now:

[0139]

[0140] in

[0141]

[0142]

[0143]

[0144]

[0145]

[0146] Z s is the three-phase conductor core self-impedance, Z m is the three-phase conductor core mutual impedance, Z nm is the mutual impedance between the shield layer and the three-phase conductor core, Z ns is the self-impedance of the shielding layer; Z1 and Z2 are the line mode impedances, Z0 is the line zero mode impedance, Zr is the self-impedance of the residual mode, and Z 0r is the mutual impedance of the residual modulus to the zero mode, Z r0 is the mutual impedance of the zero mode to the residual mode.

[0147] According to the above formula, the transformation matrix T partially decouples the line impedance matrix Z of the three-phase four-wire system and converts the system electrical quantity into two line mode components, one zero mode component and one residual mode component.

[0148] According to the decoupled impedance matrix Z 120r It's easy to see that the two line-mode components are completely independent, with their voltage values ​​depending on the current and impedance parameters of their respective modes. However, there's a certain coupling between the 0-mode and the R-mode, with the voltages of each mode affected by the current of the other. This is due to the differences in the three-phase conductor and shield parameters.

[0149] Finally, the fault location is achieved through the harmonic detection method based on fast Fourier transform.

[0150] The principle of fast Fourier transform is as follows:

[0151] The Fourier transform is for discrete digital signals. If you want to perform spectrum analysis on a continuous analog signal through a computer, you must convert the continuous analog signal into a discrete digital signal. The discrete time domain digital signal can obtain accurate and rich frequency domain information through discrete Fourier transform.

[0152] Perform N-point discrete Fourier transform on the discrete sequence x(n), and its spectrum function is:

[0153]

[0154] remember is the rotation factor, so the expression of discrete Fourier transform can be expressed as:

[0155]

[0156] Usually x(n) and They are all complex numbers. For a discrete Fourier transform of N points, it takes N to calculate all X(k). 2 The number of complex multiplications and N(N-1) complex additions increases exponentially with the increase of N, making it difficult to meet the requirements of fast and real-time signal processing by computers. So people began to look for ways to solve the problem. In 1965, Cooley and Tukey proposed a fast algorithm for discrete Fourier transform, namely the Fast Fourier Transform (FFT). The FFT algorithm uses the symmetry and reducibility of the rotation factors to transform N 2 The computational complexity of the N-point discrete Fourier transform algorithm is reduced to Nlog2N complex operations, which greatly improves the computer's Fourier analysis capability for digital signals. As a result, the fast Fourier transform has been widely used with the rapid development of computer computing power.

[0157] For a discrete digital signal sequence x(n) of length N=2m, where m is a positive integer, first decompose the time domain signal sequence x(n) into two parts, one is the even part x(2n) and the other is the odd part x(2n+1), where n=0, 1, 2, …, N / 2-1.

[0158] Obviously, the above formula is equivalent to:

[0159]

[0160] Using the reducibility of rotation factors:

[0161]

[0162] The above formula can be expressed as:

[0163]

[0164] make:

[0165]

[0166] but:

[0167]

[0168] because:

[0169]

[0170] We can get:

[0171]

[0172] After the above series of transformations, the discrete Fourier transform of N points is converted into two discrete Fourier transforms of N / 2 points. Since the transformation graph of its operation process looks like a butterfly, it is also called butterfly operation.

[0173] When locating a fault, the harmonic impedance of the distribution line at different frequencies can be obtained by calculation. Assuming the number of sampling points is N, the sampling time interval is t, and the sampling period is T, the nth harmonic impedance can be expressed as:

[0174]

[0175]

[0176] At the same time, the fundamental frequency reactance of the line for:

[0177]

[0178] Finally, the reactance is calculated based on the harmonic impedance, and the inductance of the line is further calculated based on the angular velocity information:

[0179]

[0180] The fault distance is S = L / L0, where L0 is the inductance per unit length of the line. The above steps calculate the inductance of the fault loop and determine the fault distance. Using inductance to determine the fault distance avoids the influence of fault resistance on the distance measurement results. The principle of phase-to-phase short-circuit fault distance measurement is the same as that of ground fault distance measurement. By controlling the conduction of the IGBT in the faulty phase, the excitation response signal is decomposed to obtain harmonic impedance data, which is then used to obtain inductance data for fault distance measurement.

[0181] The signal acquisition module adopts a 16-bit ADC with a sampling rate of 1 MHz; the harmonic impedance calculation module is implemented based on DSPTMS320F28335, and the FFT operation window is 10 power frequency cycles.

[0182] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A cable distribution network fault classification and location method, characterized in that: The following steps are involved: S1. Identify the structure and electrical parameters of the three-core cable; S2. Build a normal state model and a fault state model for the three-core cable, and simulate the operating data of the cable in normal and fault states; S3. Generate a fault type identification signal through the power electronic switch combination of the additional power supply, traverse the cable line to detect the fault type, obtain short-circuit current data based on phase mode transformation, and calculate the fault distance using the harmonic impedance method to complete the fault location; wherein: The power electronic switch combination of the additional power supply includes a controllable power electronic switch and a matching harmonic injection power supply. The harmonic impedance method injects multi-frequency voltage signals into the line headend and analyzes the spectral characteristics of the response current to isolate the influence of the fault resistance on the distance measurement. In step S3, the implementation of the harmonic impedance method includes: S31. Inject multi-frequency voltage signal into the head end of the fault line V inj , and collect the response current signal I res ; S32. Yes V inj and I res Perform fast Fourier transform to extract fundamental wave component and harmonic wave component; S33. Calculate the harmonic impedance of the line according to formula (1) : in: : No. Equivalent impedance modulus under subharmonics; : equivalent DC resistance component; : harmonic order; : number of sampling points; : time interval of a single sampling point; : Fault circuit equivalent inductance; The equivalent DC resistance component The response current at the head end of the fault line under zero-frequency voltage is calculated by offline measurement, specifically: ; S34. Based on fundamental reactance X 1Calculate the fault distance S : in: : Fault circuit equivalent inductance; : inductance per unit length; The phase mode transformation uses the extended Karenbauer matrix T eq Decoupling of three-phase four-wire systems: The original voltage and current Convert to line mode components 、 , zero mode component and residual modulus , to eliminate the coupling effect between the three-phase conductor core and the shield layer; expand the Karenbauer matrix T eq The decoupling parameters include: Three-phase conductor self-impedance Z s , represents the self-inductance and resistance components generated by the single-phase conductor under alternating current; Three-phase core mutual impedance Z m , represents the mutual inductance between any two phase conductor cores; Shield self-impedance Z ns , represents the self-inductance and resistance components generated independently by the shielding layer; Mutual impedance between conductor and shield Z nm , represents the mutual inductance component between the single-phase conductor core and the shielding layer; Among them, the line mode component 、 The corresponding line mode impedance satisfies: ； Used to characterize the balanced transmission characteristics between three-phase conductor cores; Zero mode component Impedance: ； Reflects the common mode coupling effect between the three-phase conductor core and the shielding layer; Residual modulus Impedance: ； Characterize the independent channel characteristics formed by the shield layer and the ground loop; By separation and The mutual impedance term is used to eliminate the electromagnetic coupling interference between the three-phase conductor core and the shielding layer under fault conditions; where: ； ； is the mutual impedance of the residual modulus to the zero mode, is the mutual impedance of the zero mode to the residual mode.

2. The method according to claim 1, characterized in that In step S1, the structure of the three-core cable includes: an outer sheath, a shielding layer, a filler, a main shielding layer and a conductor; the electrical parameters include conductor material, rated voltage, working capacity, impedance per unit length and operating frequency; wherein the shielding layer is divided into a common shielding structure and an independent shielding structure, the conductors are arranged symmetrically in an equilateral triangle, and the self-impedance, mutual impedance, shielding layer self-impedance and conductor-shielding layer mutual impedance of the three-phase conductors satisfy a symmetrical relationship.

3. The method according to claim 2, characterized in that In step S2, the normal state model and the fault state model are: Normal state model: The three-phase conductors are connected to the load impedance through the line running impedance, and the shielding layer is grounded through the head-end grounding resistor and the terminal grounding resistor; Fault state model: A fault generator is added to the normal state model to simulate core-shield faults, single-phase / multi-phase grounding faults, and phase-to-phase short circuit faults. The fault generator generates different fault types by switching the fault resistance and fault point location.

4. The method according to claim 1, wherein In step S3, the power electronic switch combination of the additional power supply includes: Multiple IGBT switches are connected between phases A, B, and C and the shield layer to form a programmable conduction path; A harmonic injection power supply outputs a voltage pulse signal containing fundamental and harmonic components. The multi-frequency voltage signal output by the harmonic injection power supply includes a fundamental frequency and odd harmonic frequencies, wherein the odd harmonics have orders of 3, 5, and 7 and an amplitude of 20%-30% of the fundamental amplitude. The signal is injected into the fault line through a conduction path. The conduction combination of the IGBT switches corresponds one-to-one to the fault type, and the response current amplitude of the fault phase is greater than that of the non-fault phase.

5. The method according to claim 4, characterized in that In step S3, the fault type identification specifically includes: When the response current of phase C is greater than that of phase A and phase B, it is determined to be a phase C ground fault; When the amplitudes of the response currents of any two phases are equal and higher than those of the third phase, it is determined to be a phase-to-phase short circuit fault; When the three-phase response currents are all lower than the threshold of 1.1 times the rated current, it is determined that there is no ground fault.

6. The method according to claim 1, characterized in that The fundamental reactance X 1 is calculated by formula (3): in: X 1: fundamental reactance; Z eq1 : fundamental frequency impedance; Z eq0 : equivalent DC resistance component; : harmonic order; : fundamental angular frequency; The fundamental frequency impedance Z eq1 is the voltage injected at the fundamental frequency V inj(1) The response current collected I res(1) The ratio of .

Images