Flexible DC power transmission line adaptive reclosing method based on coupling voltage analysis

Abstract

The invention discloses a flexible DC power transmission line adaptive reclosing method based on coupling voltage analysis, and the method comprises the steps: collecting a coupling voltage signal of a non-fault pole after determining that a single-pole grounding fault occurs in a flexible DC power transmission line and a fault line is removed; adopting a CEEMDAN algorithm to extract a residual term component of the signal; carrying out linear fitting on the residual term component and calculating an included angle characteristic quantity; whether the included angle characteristic quantity meets a preset included angle condition or not is judged, if yes, it is judged that the single-pole grounding fault is an instantaneous fault, and the circuit breaker is controlled to execute reclosing operation to restore power supply; and if not, determining that the single-pole grounding fault is a permanent fault, and locking and reclosing. According to the method, the dynamic trend characteristics of the coupling voltage after the fault are analyzed and the included angle criterion is constructed, so that the technical problems of insufficient quick action, poor anti-interference capability and easiness in causing secondary impact of an existing self-adaptive reclosing method in single-pole grounding fault property judgment are solved.

Description

Technical Field

[0001] This invention relates to the field of relay protection technology for flexible DC transmission systems, and specifically to an adaptive reclosing method for flexible DC transmission lines based on coupled voltage analysis. Background Technology

[0002] Flexible direct current transmission technology (MMC-HVDC) has become a key solution for large-scale, long-distance transmission of clean energy due to its renewable energy-friendly characteristics. Currently, to reduce engineering costs, onshore MMC-HVDC projects generally use overhead lines for power transmission. However, overhead lines are exposed to complex natural environments for extended periods, making them highly susceptible to single-pole grounding faults caused by lightning strikes, wind deflection, bird damage, and other factors. Statistics show that over 60% of these faults are transient, meaning the fault point can self-recover insulation after the arc is extinguished. Therefore, configuring appropriate reclosing devices can reduce power outage time and lower the probability of transient faults becoming permanent, which is crucial for improving the safe operation of the system.

[0003] Traditional solutions typically employ fixed-delay reclosing, mechanically waiting a preset time after a fault trip before issuing a closing command. This "blind reclosing" method, when facing permanent faults, can cause the circuit breaker to reclose directly at the fault point, generating a massive secondary fault current surge. This seriously threatens the safety of critical equipment such as converter valves and circuit breakers, and may expand the scope of the accident. Existing adaptive reclosing methods have significant drawbacks. The active injection method requires equipment modification and may prolong the arc extinguishing time; the line energy method relies on traveling wave detection and has weak anti-interference capabilities; and methods based on coupling voltage amplitude or integral ratio have long judgment times and insufficient speed. In summary, existing technologies either carry the risk of secondary surges due to "blind reclosing," or fail to meet the engineering requirements of rapid and reliable power restoration for flexible DC systems due to the need for equipment modification, poor anti-interference capabilities, or slow judgment speed.

[0004] Therefore, existing technologies urgently need a new technical solution to address the above problems. Summary of the Invention

[0005] To address the shortcomings of the existing technologies, this invention provides an adaptive reclosing method for flexible DC transmission lines based on coupled voltage analysis. By analyzing the dynamic trend characteristics of the coupled voltage after a fault and constructing an angle criterion, this method solves the technical problems of insufficient speed, poor anti-interference ability, and easy secondary impact in the determination of the nature of single-pole grounding faults in existing adaptive reclosing methods.

[0006] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0007] An adaptive reclosing method for flexible DC transmission lines based on coupled voltage analysis includes the following steps:

[0008] S1. After determining that a single-pole grounding fault has occurred in the flexible DC transmission line and the faulty line has been disconnected, collect the coupling voltage signal of the non-faulty pole within a preset time window.

[0009] S2. The CEEMDAN algorithm is used to adaptively decompose the coupled voltage signal and extract the residual term component that represents the linear change trend of the coupled voltage.

[0010] S3. Perform linear fitting on the residual components and calculate the included angle feature based on the fitting function obtained;

[0011] S4. Determine whether the included angle characteristic quantity meets the preset included angle condition. If it meets the condition, determine that the single-pole grounding fault is a transient fault and control the circuit breaker to perform a reclosing operation to restore power supply. If it does not meet the condition, determine that the single-pole grounding fault is a permanent fault and block the reclosing operation.

[0012] As a preferred embodiment, in step S1, a coupling voltage signal is generated between the faulty electrode and the non-faulty electrode through the equivalent inter-electrode capacitance and the capacitance to ground, and the coupling voltage signal of the non-faulty electrode is obtained based on the image method analysis.

[0013] As a preferred embodiment, step S2 includes the following specific processing steps:

[0014] S201. Use the coupled voltage signal as the current input signal for the CEEMDAN algorithm;

[0015] S202. Add Gaussian white noise with preset weights to the current input signal to generate a set of parallel time series to be decomposed;

[0016] S203. Perform EMD decomposition on each time series to be decomposed to obtain the first IMF component of each time series to be decomposed; calculate the mean of the first IMF components of all time series to be decomposed, and use it as the IMF component output of the current iteration, thereby calculating the residual component of the current iteration.

[0017] S204. Determine whether the residual term component satisfies the preset convergence condition; if it does, output the residual term component that finally represents the linear change trend of the coupling voltage; if it does not satisfy the condition, use the residual term component as the new current input signal and return to step S202 for the next iteration of mode decomposition.

[0018] As a preferred embodiment, in step S203, the IMF component output by the current iteration is represented as follows:

[0019] ;

[0020] In the formula, For the first The IMF components output in the next iteration; The number of time series to be decomposed in parallel; This is the first IMF component obtained by EMD decomposition and extraction; For the first The residual components obtained after the next iteration; for The weighting coefficients of the Gaussian white noise added in the next iteration; For the first Gaussian white noise; The first one obtained by EMF decomposition One IMF component.

[0021] As a preferred embodiment, in step S203, the residual term component of the current iteration is represented as:

[0022] ;

[0023] In the formula, For the first The residual components obtained from the next iteration of decomposition.

[0024] As a preferred embodiment, in step S3, the fitting function is expressed as:

[0025] ;

[0026] In the formula, This is a linear function obtained by fitting the absolute values ​​of the residual components; The slope of a linear function; It is the constant term of a linear function; It is a time series;

[0027] The included angle characteristic quantity is then expressed as:

[0028] ;

[0029] In the formula, It is an angular characteristic quantity.

[0030] As a preferred embodiment, in step S4, the preset angle condition is: the angle feature quantity is continuously calculated using a preset sliding time window, and when the angle feature quantity calculated within several consecutive time windows is greater than the preset angle threshold, it is determined that the condition is met.

[0031] Compared with the prior art, the present invention has the following technical effects:

[0032] 1. This invention collects the coupling voltage signal of the non-faulty pole after a fault. This signal is directly derived from the electrostatic induction formed between the faulty pole and the non-faulty pole through the parasitic capacitance of the line. No external signal injection device is required, thus avoiding the disadvantages of the active injection method that may prolong the arc time and increase the equipment cost.

[0033] 2. This invention uses the CEEMDAN algorithm to effectively overcome the mode aliasing problem in traditional empirical mode decomposition by using the decomposition results under multiple noise perturbations. This separates the low-frequency residual components that characterize the overall trend of the coupled voltage signal from the high-frequency oscillation noise and interference, thus achieving robust extraction of key trend information under strong noise environment.

[0034] 3. This invention performs linear fitting on the residual term of the trend and calculates its angle with the time axis, thereby transforming the complex transient signal into a stable angle feature. Under transient faults, the amplitude of the residual term continues to increase linearly, and the fitted angle is a significant positive value. Under permanent faults, the amplitude is constrained to fluctuate at a low level, and the angle approaches zero. This eliminates the traditional method of relying on a single-moment amplitude threshold or complex integral calculation, and improves the simplicity and speed of calculation.

[0035] 4. This invention uses a continuous sliding window to determine whether the included angle is greater than a preset included angle threshold. It requires that the conditions be met for multiple consecutive time windows, thereby effectively filtering out errors caused by system noise or calculation fluctuations. This overcomes the blindness of traditional fixed-delay reclosing and improves the accuracy and speed of reclosing. Attached Figure Description

[0036] To make the objectives, technical solutions, and advantages of the invention clearer, the invention will now be described in further detail with reference to the accompanying drawings, wherein:

[0037] Figure 1 This is a flowchart of the adaptive reclosing method proposed in the embodiments of the present invention;

[0038] Figure 2 This is a schematic diagram illustrating the differences in voltage angle under different fault conditions in an embodiment of the present invention;

[0039] Figure 3 This is a timing diagram of reclosing after a fault in an embodiment of the present invention;

[0040] Figure 4 This is a topology diagram of a true bipolar MMC-HVDC system in an embodiment of the present invention;

[0041] Figure 5 This is a diagram showing the results of distinguishing between transient and permanent faults in an embodiment of the present invention;

[0042] Figure 6 The figures show the coupling voltage residual term and the fitted curve in the embodiments of the present invention. Detailed Implementation

[0043] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. The components of the embodiments of the present invention described and shown in the accompanying drawings can generally be arranged and designed in various different configurations. Therefore, the following detailed description of the embodiments of the present invention provided in the accompanying drawings is not intended to limit the scope of the claimed invention, but only to illustrate selected embodiments of the invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

[0044] The present invention will now be described in further detail with reference to the accompanying drawings.

[0045] Example 1:

[0046] As flexible DC transmission technology, centered on modular multilevel converters, becomes the mainstream choice for long-distance renewable energy consumption, its onshore engineering generally adopts overhead lines to control costs, leading to severe environmental challenges and frequent single-pole grounding faults. Against this backdrop, configuring fast and reliable reclosing is crucial to ensuring continuous power supply and preventing the escalation of transient faults. However, existing technologies all have insurmountable drawbacks: traditional fixed-delay reclosing can cause destructive secondary impacts under permanent faults; and various adaptive solutions each have their bottlenecks—active injection methods require modification of primary equipment and may interfere with the extinction of fault arcs; the line energy method relies on traveling wave signals that are susceptible to interference and have poor reliability; and methods based on coupling voltage amplitude or integration are slow and lack responsiveness. These shortcomings prevent existing methods from meeting the stringent requirements of flexible DC systems for responsiveness, reliability, and no need for modification.

[0047] To address the aforementioned technical problems, this invention proposes an adaptive reclosing method for flexible DC transmission lines based on coupled voltage analysis. This method abandons the reliance on amplitude at a single moment or simple integration, and instead analyzes the dynamic trend characteristics of the coupled voltage over time after disconnection. By employing the CEEMDAN algorithm to extract the linear trend of the coupled voltage and constructing a stable angle feature as a criterion, it can achieve fast and accurate fault identification without increasing hardware costs, ultimately driving the reclosing to perform the correct action. This improves safety while minimizing system recovery time.

[0048] Specifically, the present invention proposes an adaptive reclosing method for flexible DC transmission lines based on coupled voltage analysis, as follows: Figure 1 As shown, the specific steps include the following:

[0049] S1. After determining that a single-pole grounding fault has occurred in the flexible DC transmission line and the faulty line has been disconnected, collect the coupling voltage signal of the non-faulty pole within a preset time window.

[0050] In practice, when a single-pole grounding fault is determined, reclosing is initiated synchronously at t=0ms, and 50ms of coupled voltage signal data is taken backward. The sampling frequency of the coupled voltage signal is 10kHz.

[0051] The generation process of the coupled voltage signal is based on the image method analysis. Due to the principle of electrostatic induction, a coupled voltage is generated between the faulty and non-faulty poles through the equivalent inter-electrode capacitance and the capacitance to ground. Under transient faults, the amplitude of the coupled voltage continuously increases after the fault point disappears, while under permanent faults, the coupled voltage is constrained to a low amplitude level by the fault point. Therefore, by analyzing the electric field distribution of the positive and negative pole lines using the image method, the relationship between the voltage to ground of the positive and negative poles and the line parameters is obtained, clarifying that the voltage ratio between the faulty and non-faulty poles is only related to the positive and negative line parameters.

[0052] This embodiment collects the coupling voltage signal of the non-faulty pole after a fault. This signal is directly derived from the electrostatic induction formed between the faulty pole and the non-faulty pole through the parasitic capacitance of the line. No external signal injection device is required, thus avoiding the disadvantages of the active injection method that may prolong the arc time and increase the equipment cost.

[0053] S2. The CEEMDAN algorithm is used to adaptively decompose the coupled voltage signal and extract the residual term component that represents the linear change trend of the coupled voltage.

[0054] In practice, the initialization process begins by adding K-order Gaussian white noise to the coupled voltage signal x(t) to construct K parallel time series x to be decomposed. i (t), where i = 1, 2, ..., K; this step is expressed as:

[0055] ;

[0056] In the formula, These are the Gaussian white noise weighting coefficients; Gaussian white noise was added for the i-th processing step;

[0057] Next, the first-level mode extraction and residual calculation are performed for each time series x to be decomposed. i (t) Perform EMD decomposition on each to obtain the first modal component of each. Calculate the mean of all K first modal components to obtain the first IMF component of the CEEMDAN algorithm. It is represented as: The first residual component is calculated using the following formula. ;

[0058] ;

[0059] Then iterative mode decomposition is performed, for the j-th decomposition ( ), perform the following operations:

[0060] a) The residual signal in the (j-1)th decomposition Add Gaussian white noise to form a new set of sequences to be decomposed;

[0061] b) Perform EMD decomposition on each of the sequences in the set and take the first modal component obtained from the decomposition of each sequence;

[0062] c) Calculate the mean of these first modal components and use it as the IMF component of the j-th CEEMDAN. It is represented as:

[0063] ;

[0064] In the formula, For the first The IMF components output in the next iteration; The number of time series to be decomposed in parallel; This is the first IMF component obtained by EMD decomposition and extraction; For the first The residual components obtained after the next iteration; for The weighting coefficients of the Gaussian white noise added in the next iteration; For the first Gaussian white noise; The first one obtained by EMF decomposition One IMF component.

[0065] d) The residual components of the j-th decomposition are calculated using the following formula;

[0066] ;

[0067] In the formula, For the first The residual components obtained from the next iteration of decomposition.

[0068] Finally, repeat the iterative process until the residual components satisfy the preset convergence condition, and then input the final iterative result. As the residual term output, this residual term characterizes the linear variation trend of the coupling voltage by eliminating high-frequency components and noise interference.

[0069] This embodiment uses the CEEMDAN algorithm to effectively overcome the mode aliasing problem in traditional empirical mode decomposition by using the decomposition results under multiple noise perturbations. This separates the low-frequency residual components that characterize the overall trend of the coupled voltage signal from the high-frequency oscillation noise and interference, thus achieving robust extraction of key trend information under strong noise environment.

[0070] S3. Perform linear fitting on the residual components and calculate the included angle feature based on the fitting function obtained;

[0071] In practice, the absolute value of the residual component of the coupling voltage obtained from CEEMDAN is taken, and a linear function is fitted. This fitted function is expressed as follows:

[0072] ;

[0073] In the formula, This is a linear function obtained by fitting the absolute values ​​of the residual components; The slope of a linear function; It is the constant term of a linear function; It is a time series;

[0074] The angle between the fitted function and the time axis is calculated to obtain the angle feature:

[0075] ;

[0076] In the formula, It is an angular characteristic quantity.

[0077] like Figure 2 As shown, this embodiment performs linear fitting on the trend residual term and calculates its angle with the time axis, thereby transforming the complex transient signal into a stable angle feature. Under transient faults, the residual term amplitude continues to increase linearly, and the fitted angle is a significant positive value. Under permanent faults, the amplitude is constrained to fluctuate at a low level, and the angle approaches zero. This eliminates the traditional method of relying on a single-moment amplitude threshold or complex integral calculation, improving the simplicity and speed of the calculation.

[0078] S4. Determine whether the included angle characteristic quantity meets the preset included angle condition. If it meets the condition, determine that the single-pole grounding fault is a transient fault and control the circuit breaker to perform a reclosing operation to restore power supply. If it does not meet the condition, determine that the single-pole grounding fault is a permanent fault and block the reclosing operation.

[0079] In specific implementation, the included angle feature is calculated continuously using a 5ms sliding time window. When the included angle feature calculated within three consecutive time windows... All are greater than the preset included angle threshold If the condition is met, then it is determined that the setting value is satisfied. Considering calculation errors and noise interference, the setting value should be relatively large. The final value is 10.

[0080] After determining that the fault is transient, a closing command is issued to the DC circuit breaker to directly close the fast mechanical switch of the main branch and simultaneously shut off the energy-consuming branch switch, restoring the system to normal operation. The corresponding reclosing flowchart is as follows: Figure 3 As shown.

[0081] Example 2:

[0082] To better understand the effectiveness of this embodiment, the simulation results of the embodiment of the present invention will be further described in detail below with reference to the accompanying drawings.

[0083] This embodiment takes a true bipolar flexible DC transmission system structure as the research object, and its topology is as follows: Figure 4 As shown in Table 1, the system parameters and line parameters (frequency domain correlation model) are as follows. The MMC converter consists of a three-phase, six-arm bridge, with each phase divided into an upper arm and a lower arm. Each arm is composed of N half-bridge submodules connected in series. Under normal operation, each phase simultaneously conducts N half-bridge submodules to maintain DC-side voltage stability. To limit DC-side fault current, a large inductor is connected in series at the line outlet, referencing the Zhangbei project.

[0084] surface MMC-HVDC Converter Parameter Table

[0085] parameter numerical values Rated DC voltage / kV Active power / MW Bridge arm inductance / mH Current-limiting inductance / mH Line length / km Sampling frequency / kHz Number of bridge arm submodules Submodule capacitance / mF Connecting transformer short-circuit impedance Connecting transformer group designation Connecting transformer rated voltage / kV Line length / km Line height / m Conductor radius / m Positive and negative conductor spacing / m ±500120050150300102385.110%Yn / D230 / 291300h=30r=0.0475d=10

[0086] Taking a positive ground fault occurring at point f at t=0ms as an example, fault detection is completed in 3ms, the DC circuit breaker operates in 3ms, and the fault current decays to zero within 15ms. When it is determined to be a single-pole ground fault, the reclosing mechanism proposed in this embodiment is initiated simultaneously at t=0ms. A 50ms data window is then taken, and the CEEMDAN algorithm is used to decompose the data to obtain several IMF components and residual components. The standard deviation of the Gaussian white noise added to the CEEMDAN algorithm is selected as 0.2, and 500 cycles of Gaussian white noise are continuously added. Figure 5 The calculated coupling voltage angles for transient and permanent faults are given, where transient faults are all arc faults.

[0087] from Figure 5 As can be seen, the blue line represents a transient fault. After three consecutive fulfillments of the judgment condition starting from the 11th sliding window, it is correctly judged as a transient fault, and a closing command is issued to the DC circuit breaker. The purple line represents a permanent fault. Due to the clamping of the fault point, the coupling voltage fluctuates slightly around zero, and the corresponding angle also fluctuates slightly around zero, neither exceeding the setting value, so it can be correctly judged as a permanent fault.

[0088] from Figure 5 As shown in the curve, after the fault occurs but before the fault point disappears, the fault voltage amplitude drops sharply, and the angle of the coupling voltage becomes negative. Subsequently, the coupling voltage stabilizes at around zero, and the angle of the coupling voltage also approaches zero negative value. The residual components of the coupling voltage after the fault and the fitted curve are shown below. Figure 6 As shown. After a fault, the voltage amplitude drops sharply from the rated voltage to zero potential. The drop is rapid, and the angle obtained from the fitted curve is a large negative value, which does not meet the judgment condition and will not lead to a misjudgment. However, after the fault point disappears, a coupling voltage is generated, and the voltage amplitude continues to rise along the linear curve with a positive angle.

[0089] In summary, this embodiment addresses the technical challenges of speed, reliability, and no-modification requirements in reclosing for single-pole grounding faults in flexible DC transmission lines. It proposes an adaptive reclosing method for flexible DC transmission lines based on coupled voltage analysis. After a fault trip, this method acquires the coupled voltage signal of the non-faulty pole, extracts its amplitude variation trend using the CEEMDAN algorithm, obtains the angle between the trend line and the time axis through linear fitting, and determines the fault nature (transient or permanent) based on whether this angle remains continuously greater than a threshold within a continuous sliding window. This embodiment eliminates the need for modification to primary equipment, solving the secondary impact caused by blind reclosing in traditional reclosing methods, as well as the problems of poor anti-interference, slow judgment speed, or the need for signal injection in existing adaptive methods. It achieves millisecond-level, highly reliable intelligent fault identification and recovery.

[0090] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described with reference to preferred embodiments, those skilled in the art should understand that various changes in form and detail can be made without departing from the spirit and scope of the invention as defined in the appended claims.

Claims

1. An adaptive reclosing method for flexible DC transmission lines based on coupled voltage analysis, characterized in that, Includes the following steps: S1. After determining that a single-pole grounding fault has occurred in the flexible DC transmission line and the faulty line has been disconnected, collect the coupling voltage signal of the non-faulty pole within a preset time window. S2. The CEEMDAN algorithm is used to adaptively decompose the coupled voltage signal and extract the residual term component that represents the linear change trend of the coupled voltage. S3. Perform linear fitting on the residual components, and calculate the included angle feature based on the fitting function obtained by fitting; S4. Determine whether the included angle characteristic quantity meets the preset included angle condition. If it meets the condition, determine that the single-pole grounding fault is a transient fault and control the circuit breaker to perform a reclosing operation to restore power supply. If it does not meet the condition, determine that the single-pole grounding fault is a permanent fault and block the reclosing operation.

2. The adaptive reclosing method for flexible DC transmission lines based on coupled voltage analysis according to claim 1, characterized in that, In step S1, a coupling voltage signal is generated between the faulty electrode and the non-faulty electrode through the equivalent inter-electrode capacitance and the capacitance to ground. The coupling voltage signal of the non-faulty electrode is obtained based on the image method analysis.

3. The adaptive reclosing method for flexible DC transmission lines based on coupled voltage analysis according to claim 1, characterized in that, Step S2 includes the following specific processing steps: S201. Use the coupled voltage signal as the current input signal for the CEEMDAN algorithm; S202. Add Gaussian white noise with preset weights to the current input signal to generate a set of parallel time series to be decomposed; S203. Perform EMD decomposition on each time series to be decomposed to obtain the first IMF component of each time series to be decomposed; calculate the mean of the first IMF components of all time series to be decomposed, and use it as the IMF component output of the current iteration, thereby calculating the residual component of the current iteration. S204. Determine whether the residual term components satisfy the preset convergence condition; If satisfied, the output will be the residual term component that ultimately characterizes the linear change trend of the coupling voltage; If the condition is not met, the residual component is used as the new current input signal, and the process returns to step S202 for the next iteration of mode decomposition.

4. The adaptive reclosing method for flexible DC transmission lines based on coupled voltage analysis according to claim 3, characterized in that, In step S203, the IMF component output in the current iteration is represented as: ； In the formula, For the first The IMF components output in the next iteration; The number of time series to be decomposed in parallel; This is the first IMF component obtained by EMD decomposition and extraction; For the first The residual components obtained after the next iteration; for The weighting coefficients of the Gaussian white noise added in the next iteration; For the first Gaussian white noise; The first one obtained by EMF decomposition One IMF component.

5. The adaptive reclosing method for flexible DC transmission lines based on coupled voltage analysis according to claim 4, characterized in that, In step S203, the residual components of the current iteration are represented as follows: ； In the formula, For the first The residual components obtained from the next iteration of decomposition.

6. The adaptive reclosing method for flexible DC transmission lines based on coupled voltage analysis according to claim 1, characterized in that, In step S3, the fitting function is expressed as: ； In the formula, This is a linear function obtained by fitting the absolute values ​​of the residual components; The slope of a linear function; It is the constant term of a linear function; It is a time series; The included angle characteristic quantity is then expressed as: ； In the formula, It is an angular characteristic quantity.

7. The adaptive reclosing method for flexible DC transmission lines based on coupled voltage analysis according to claim 1, characterized in that, In step S4, the preset included angle condition is: the included angle feature quantity is continuously calculated using a preset sliding time window. When the included angle feature quantity calculated within several consecutive time windows is greater than the preset included angle threshold, it is determined that the condition is met.

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