A method of active fault detection based on harmonic injection
By injecting harmonic signals into the low-voltage DC distribution network and combining them with curve fitting methods, the problem of traditional passive detection methods being unable to accurately identify faults in DC distribution networks has been solved, achieving high-precision fault detection and location.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Patents(China)
- Current Assignee / Owner
- STATE GRID FUJIAN ELECTRIC POWER CO LTD
- Filing Date
- 2023-12-12
- Publication Date
- 2026-06-19
AI Technical Summary
Traditional passive fault detection methods are difficult to adapt to the nonlinear and time-varying characteristics of converters in DC distribution networks, leading to the risk of false protection or failure to protect, and it is difficult to achieve accurate fault detection.
An active fault detection method based on harmonic injection is adopted. By analyzing line faults in the low-voltage DC distribution network, harmonic signals are injected, and harmonic signals with specific frequencies and amplitudes are selected. The harmonic frequency features are extracted by combining curve fitting methods to achieve fault identification and location.
It improves the accuracy and precision of fault detection, reduces the deviation in location results caused by errors, and achieves high-precision location of faults in low-voltage DC lines.
Smart Images
Figure CN117706269B_ABST
Abstract
Description
Technical Field
[0001] This invention relates to the field of low-voltage DC distribution networks, and specifically to an active fault detection method based on harmonic injection. Background Technology
[0002] Power electronic transformers are crucial equipment connecting AC / DC hybrid power grids and are essential for DC power grids. Traditional relay protection typically employs passive fault detection methods, which rely on the differential characteristics of internal and external faults based on the disturbance characteristics of the power grid. However, DC distribution networks contain numerous power electronic devices. Converters exhibit nonlinear and time-varying responses to disturbances, making their transient characteristics during faults more complex. Furthermore, the highly controllable self-protection mechanisms of converters weaken fault characteristics and shorten fault duration. These factors make them difficult to adapt to traditional detection principles and techniques, potentially leading to false protection or failure to protect. Therefore, the aforementioned passive detection methods may be difficult to implement in DC distribution networks. Summary of the Invention
[0003] The purpose of this invention is to provide an active fault detection method based on harmonic injection, which is beneficial to improving the accuracy of fault detection.
[0004] To achieve the above objectives, the technical solution adopted by this invention is: an active fault detection method based on harmonic injection, comprising the following steps:
[0005] Step 1: Perform line fault analysis on the low-voltage DC distribution network;
[0006] Step 2: Inject harmonic signals to identify fault locations in real time;
[0007] Step 3: Select the frequency of the injected harmonic signal;
[0008] Step 4: Select the amplitude of the injected harmonic signal;
[0009] Step 5: Extract harmonic frequency characteristics for calculation;
[0010] Step 6: The results obtained in Step 5 are processed using curve fitting to improve the overall location accuracy of low-voltage DC line faults and reduce the deviation in location results caused by errors.
[0011] Furthermore, the specific implementation method of step 1 is as follows:
[0012] When a fault occurs, the neutral point of the capacitor is at the same potential as ground. The DC-side capacitor, the fault line, and the transition resistor form a second-order RLC oscillating circuit, and the capacitor discharges rapidly. The oscillation pattern is related to the fault distance and the magnitude of the transition resistance. The fault loop resistance is the critical value of the oscillating circuit.
[0013]
[0014] Where R is the line resistance and grounding resistance, C is the capacitor value, and L is the equivalent inductance on the line; At this time, it is an underdamped discharge process, and the capacitor discharge circuit oscillates with reduced amplitude. At this time, it is an overdamped discharge process, and the transient energy of the faulty circuit decreases rapidly until it becomes zero;
[0015] In the case of a low-voltage positive ground short circuit, the short circuit only forms a fault loop with the faulty pole, and the negative pole does not form a short circuit with the fault point; there is only one resonant stage on the line, and the fault circuit is approximately equivalent to an RLC series resonant circuit, whose resonant frequency is expressed as:
[0016] .
[0017] Furthermore, the specific implementation method of step 2 is as follows:
[0018] A sinusoidal harmonic signal is superimposed on the bridge arm modulation signal, as shown in the following formula:
[0019]
[0020] Among them, u j Here, A is the harmonic signal amplitude, ω is the harmonic signal angular frequency, and φ is the initial phase angle of the harmonic signal.
[0021] After superimposing harmonic signals, the bridge arm voltage modulation signal of SPWM modulation is:
[0022]
[0023] Among them, u abc_ref For the initial bridge arm modulation signal, u abc This is the bridge arm modulation signal after superimposing harmonic signals.
[0024] Furthermore, in step 3, the harmonic signal frequency is selected near the resonant frequency to induce resonance in the fault circuit, generating a larger characteristic current and making the fault response characteristics more obvious. For a ground fault circuit that is equivalent to an RLC series resonant circuit, the input circuit parameters are used to obtain the harmonic signal frequency f. s as follows:
[0025] .
[0026] Furthermore, in step 4, the range of amplitude selection for the injected harmonic signal is as follows:
[0027]
[0028] Among them, udc This is the rated DC voltage.
[0029] Furthermore, an amplitude of 4% of the rated DC voltage is selected, i.e. .
[0030] Furthermore, in step 5, when a single-pole ground fault occurs on the line, the voltage and current signals of the upper half of the output capacitor of the voltage source converter VSC are acquired, and harmonic frequency characteristics are extracted for calculation; the low-voltage DC distribution network system adopts the grid impedance model, and the system impedance is expressed as RL+jXL; for voltage and current signals of the same frequency, the impedance Z on the line is... line It can be obtained from the following formula:
[0031]
[0032] in, and These are the port voltage and loop current, respectively. Given the system's internal grounding resistance and the impedance of known components, ω = 2πf s f s This refers to the frequency of the harmonic signal.
[0033] Furthermore, considering the steady-state distortion and background measurement noise of the low-voltage DC distribution network system, there is an error between the frequency domain reactance estimate and the actual reactance value directly calculated by FFT. Considering that the error distribution is relatively random and fluctuates within a certain range, a curve fitting method based on least squares is used to process the results obtained in step 5 to improve the overall location accuracy of low-voltage DC line faults and reduce the location result deviation caused by errors.
[0034] Furthermore, step 6 specifically includes the following steps:
[0035] 601) The results obtained in step 5 are processed using a curve fitting method based on the least squares method. The expression for the quadratic fitting curve is:
[0036]
[0037] Where a0, a1, and a2 are the coefficients of the fitted curve, and x is the estimated reactance value obtained in the frequency domain at a characteristic frequency; the mean square error of the estimated inductance value is defined as:
[0038]
[0039] Among them, y i Let n be the actual reactance value of the system, and n be the number of estimated reactance values required for the statistics.
[0040] 602) To minimize the mean square error of the inductance value at each point, by the extremum theorem for multivariable functions, R(a0, a1, a2) should satisfy the following equation:
[0041]
[0042]
[0043]
[0044] 603) Combining the above three formulas, we obtain the values of a0, a1, and a2:
[0045]
[0046] Substituting the calculated values of a0, a1, and a2 into the quadratic curve fitting expression, we obtain f(x) = L e The correction curve of (x) is calculated by sampling fault points at different distances; based on the measured distance and the actual distance, the fitting curve of the estimated reactance deviation at the corresponding frequency is obtained.
[0047] Compared with the prior art, the present invention has the following beneficial effects: The present invention provides an active fault detection method based on harmonic injection. This method is based on the idea of active detection and protection. By analyzing the characteristics of single-pole grounding faults in low-voltage DC distribution networks and combining the high controllability of VSC converters, it actively injects harmonic signals of characteristic frequencies. By sampling the voltage and current values of the harmonics, calculating the impedance characteristics, and using quadratic curve fitting for distance measurement, the fault identification and location are achieved with high accuracy. Attached Figure Description
[0048] Figure 1 This is a flowchart illustrating the method implementation of an embodiment of the present invention;
[0049] Figure 2 This is a block diagram illustrating the control principle of identifying fault locations by injecting harmonic signals in an embodiment of the present invention. Detailed Implementation
[0050] The present invention will be further described below with reference to the accompanying drawings and embodiments.
[0051] It should be noted that the following detailed descriptions are exemplary and intended to provide further explanation of this application. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains.
[0052] It should be noted that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the exemplary embodiments according to this application. As used herein, the singular form is intended to include the plural form as well, unless the context clearly indicates otherwise. Furthermore, it should be understood that when the terms "comprising" and / or "including" are used in this specification, they indicate the presence of features, steps, operations, devices, components, and / or combinations thereof.
[0053] like Figure 1 As shown, this embodiment provides an active fault detection method based on harmonic injection, including the following steps:
[0054] Step 1: Perform line fault analysis on the low-voltage DC distribution network.
[0055] When a fault occurs, the neutral point of the capacitor is at the same potential as ground. The DC-side capacitor, the fault line, and the transition resistor form a second-order RLC oscillating circuit, and the capacitor discharges rapidly. The oscillation pattern is related to the fault distance and the magnitude of the transition resistance. The fault loop resistance is as follows, which represents the critical value for the oscillating circuit:
[0056]
[0057] Where R is the line resistance and grounding resistance, C is the capacitor value, and L is the equivalent inductance on the line; At this time, it is an underdamped discharge process, and the capacitor discharge circuit oscillates with reduced amplitude. During this time, it is an overdamped discharge process, and the transient energy of the faulty circuit decreases rapidly until it reaches zero.
[0058] Taking a low-voltage positive ground short circuit as an example, the short circuit only forms a fault loop with the faulty pole, and the negative pole does not form a short circuit with the fault point; there is only one resonant stage on the line, and the fault circuit is approximately equivalent to an RLC series resonant circuit, whose resonant frequency is expressed as:
[0059]
[0060] Step 2: Inject harmonic signals to identify the fault location in real time.
[0061] This method employs a global injection of characteristic signals to identify fault locations in real time, overcoming the problems of existing technologies. The specific control block diagram of this harmonic injection strategy is shown below. Figure 2 As shown.
[0062] The specific implementation method of step 2 is as follows:
[0063] A sinusoidal harmonic signal is superimposed on the bridge arm modulation signal, as shown in the following formula:
[0064]
[0065] Among them, u j Let A be the harmonic signal, ω be the harmonic signal amplitude, φ be the harmonic signal angular frequency, and φ be the initial phase angle of the harmonic signal.
[0066] After superimposing harmonic signals, the bridge arm voltage modulation signal of SPWM modulation is:
[0067]
[0068] Among them, u abc_ref For the initial bridge arm modulation signal, u abc This is the bridge arm modulation signal after superimposing harmonic signals.
[0069] Step 3: Select the frequency of the injected harmonic signal.
[0070] The basic principle for selecting harmonic signal frequencies is to maximize the differences in response characteristics and achieve more sensitive detection, while meeting the performance constraints of the injection equipment.
[0071] The frequency of the harmonic signal should be selected near the resonant frequency to induce resonance in the faulty circuit, generate a larger characteristic current, make the fault response characteristics more obvious, and improve the positioning accuracy.
[0072] Based on the above analysis, the ground fault circuit can be equivalently represented as an RLC series resonant circuit. For this ground fault circuit, input system parameters and obtain the harmonic signal frequency f. s as follows:
[0073]
[0074] Step 4: Select the amplitude of the injected harmonic signal.
[0075] The basic principle for selecting the amplitude of the detection signal is to minimize the impact on the power grid while meeting the detection accuracy requirements of the measuring equipment.
[0076] According to national standards, voltage deviations below 1.5kV should be between 20% and +5%. Meanwhile, voltage transformers with a measurement accuracy of 0.5 should be installed in medium- and low-voltage DC distribution networks, and their signal amplitude should be greater than 0.75% of the rated voltage. Considering these principles, the selection range for the injected harmonic signal amplitude is determined as follows:
[0077]
[0078] Among them, u dc This is the rated DC voltage.
[0079] Considering that the behavior of signal injection itself will also cause voltage fluctuations, it is preferable to choose an amplitude of 4% of the rated DC voltage, i.e. It can meet both the voltage deviation requirements and the accuracy requirements of the protection device.
[0080] Step 5: Extract harmonic frequency characteristics for calculation.
[0081] When a single-pole ground fault occurs on the line, the voltage and current signals of the upper half of the output capacitor of the voltage source converter (VSC) are acquired, and harmonic frequency characteristics are extracted for calculation. The low-voltage DC distribution network system adopts a grid impedance model, where the system impedance is expressed as RL+jXL. For voltage and current signals of the same frequency, the impedance Z on the line... line It can be obtained from the following formula:
[0082]
[0083] in, and These are the port voltage and loop current, respectively. Given the system's internal grounding resistance and the impedance of known components, ω = 2πf s f s This refers to the frequency of the harmonic signal.
[0084] Considering the steady-state distortion and background measurement noise in the low-voltage DC distribution network system, there is an error between the frequency domain reactance estimate and the actual reactance value directly calculated by FFT. Given that the error distribution is relatively random and fluctuates within a certain range, a curve fitting method based on least squares is used to process the results obtained in step 5 to improve the overall fault location accuracy of low-voltage DC lines and reduce the location result deviation caused by errors.
[0085] Step 6: The results obtained in Step 5 are processed using a curve fitting method based on the least squares method to improve the overall location accuracy of low-voltage DC line faults and reduce the deviation of the location results caused by errors.
[0086] Step 6 specifically includes the following steps:
[0087] 601) The results obtained in step 5 are processed using a curve fitting method based on the least squares method. The expression for the quadratic fitting curve is:
[0088]
[0089] Where a0, a1, and a2 are the coefficients of the fitted curve, and x is the estimated reactance value obtained in the frequency domain at a characteristic frequency. The mean square error of the estimated inductance value is defined as:
[0090]
[0091] Among them, y i Let n be the actual reactance value of the system, and n be the number of estimated reactance values required for the statistics.
[0092] 602) To minimize the mean square error of the inductance value at each point, by the extremum theorem for multivariable functions, R(a0, a1, a2) should satisfy the following equation:
[0093]
[0094]
[0095]
[0096] 603) Combining the above three formulas, we obtain the values of a0, a1, and a2:
[0097]
[0098] Substituting the calculated values of a0, a1, and a2 into the quadratic curve fitting expression, we obtain f(x) = L e The corrected curve for (x) is calculated by sampling fault points at different distances. Based on the measured distance and the actual distance, a fitted curve for the estimated reactance deviation at the corresponding frequency is obtained.
[0099] Based on the harmonic injection strategy provided by this invention, when a fault occurs in the low-voltage DC distribution network, it can be implemented according to... Figure 1 The process shown is used to identify and locate faults.
[0100] Those skilled in the art will understand that embodiments of this application can be provided as methods, systems, or computer program products. Therefore, this application can take the form of a completely hardware embodiment, a completely software embodiment, or an embodiment combining software and hardware aspects. Furthermore, this application can take the form of a computer program product embodied on one or more computer-usable storage media (including but not limited to disk storage, CD-ROM, optical storage, etc.) containing computer-usable program code.
[0101] This application is described with reference to flowchart illustrations and / or block diagrams of methods, apparatus (systems), and computer program products according to embodiments of this application. It will be understood that each block of the flowchart illustrations and / or block diagrams, and combinations of blocks in the flowchart illustrations and / or block diagrams, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, special-purpose computer, embedded processor, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, generate instructions for implementing the flowchart... Figure 1 One or more processes and / or boxes Figure 1 A device that provides the functions specified in one or more boxes.
[0102] These computer program instructions may also be stored in a computer-readable storage medium that can direct a computer or other programmable data processing device to function in a particular manner, such that the instructions stored in the computer-readable storage medium produce an article of manufacture including instruction means, which are implemented in a process Figure 1 One or more processes and / or boxes Figure 1 The function specified in one or more boxes.
[0103] These computer program instructions may also be loaded onto a computer or other programmable data processing equipment to cause a series of operational steps to be performed on the computer or other programmable equipment to produce a computer-implemented process, thereby providing instructions that execute on the computer or other programmable equipment for implementing the process. Figure 1 One or more processes and / or boxes Figure 1 The steps of the function specified in one or more boxes.
[0104] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention in any other way. Any person skilled in the art may make changes or modifications to the above-disclosed technical content to create equivalent embodiments. However, any simple modifications, equivalent changes, and modifications made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the protection scope of the present invention.
Claims
1. A method of active fault detection based on harmonic injection, characterized in that, Includes the following steps: Step 1: Perform line fault analysis on the low-voltage DC distribution network; The specific implementation method of step 1 is as follows: When a fault occurs, the neutral point of the capacitor is at the same potential as ground. The DC-side capacitor, the fault line, and the transition resistor form a second-order RLC oscillating circuit, and the capacitor discharges rapidly. The oscillation pattern is related to the fault distance and the magnitude of the transition resistance. The fault loop resistance is the critical value of the oscillating circuit. Where R is the line resistance and grounding resistance, C is the capacitor value, and L is the equivalent inductance of the line; At this time, it is an underdamped discharge process, and the capacitor discharge circuit oscillates with reduced amplitude. At this time, it is an overdamped discharge process, and the transient energy of the faulty circuit decreases rapidly until it becomes zero; In the case of a low-voltage positive ground short circuit, the short circuit only forms a fault loop with the faulty pole, and the negative pole does not form a short circuit with the fault point; there is only one resonant stage on the line, and the fault circuit is approximately equivalent to an RLC series resonant circuit, whose resonant frequency is expressed as: ； Step 2: Inject harmonic signals to identify fault locations in real time; The specific implementation method of step 2 is as follows: A sinusoidal harmonic signal is superimposed on the bridge arm modulation signal, as shown in the following formula: Among them, u j Here, A is the harmonic signal amplitude, ω is the harmonic signal angular frequency, and φ is the initial phase angle of the harmonic signal. After superimposing harmonic signals, the bridge arm voltage modulation signal of SPWM modulation is: Wherein, u abc_ref is the initial bridge arm modulation signal, u abc is the bridge arm modulation signal after superimposing the harmonic signal; Step 3: Select the frequency of the injected harmonic signal; In step 3, the harmonic signal frequency f s As follows: ； Step 4: Select the amplitude of the injected harmonic signal; In step 4, the range of amplitude values for the injected harmonic signal is as follows: wherein U dc is the DC voltage rating; Step 5: Extract harmonic frequency characteristics for calculation; In step 5, when a single-pole ground fault occurs on the line, the voltage and current signals of the upper half of the output capacitor of the voltage source converter VSC are acquired, and harmonic frequency characteristics are extracted for calculation; the low-voltage DC distribution network system adopts the grid impedance model; for voltage and current signals of the same frequency, the impedance Z on the line is... line It can be obtained from the following formula: in, and These are the port voltage and loop current, respectively. Given the system's internal grounding resistance and the impedance of known components, ω = 2πf s f s The frequency of the harmonic signal; Step 6: Process the results obtained in Step 5 using curve fitting; Step 6 specifically includes the following steps: 601) The results obtained in step 5 are processed using a curve fitting method based on the least squares method. The expression for the quadratic fitting curve is: Where a0, a1, and a2 are the coefficients of the fitted curve, and x is the estimated reactance value obtained in the frequency domain at a characteristic frequency; the mean square error of the estimated inductance value is defined as: where y i is the actual reactance value of the system, and n is the number of estimated reactance values required for statistics. 602) To minimize the mean square error of the inductance value at each point, by the extremum theorem for multivariable functions, R(a0, a1, a2) should satisfy the following equation: 603) Obtain the values of a0, a1, and a2: Substituting the calculated values of a0, a1, and a2 into the quadratic curve fitting expression, we obtain f(x) = L e The correction curve of (x) is calculated by sampling fault points at different distances; based on the measured distance, the fitting curve of the estimated reactance deviation at the corresponding frequency is obtained.
2. The method of claim 1, wherein, Choose an amplitude of 4% of the rated DC voltage, that is .