High-speed unitary protection method for low-frequency power transmission line

Abstract

The invention provides a high-speed unitary protection method for a low-frequency power transmission line. The method comprises the following steps: acquiring voltage signal data of a home terminal on a low-frequency power transmission line, and calculating a voltage second-order difference value and an average voltage increment value of a denoised signal; obtaining a line mode voltage through Karen Boolean transformation of three-phase current and voltage data, calculating to obtain a line mode voltage forward wave and a line mode voltage backward wave, and extracting a low-frequency component and a high-frequency component in the line mode voltage forward wave and the line mode voltage backward wave of a required frequency band through successive variable mode decomposition; respectively calculating an average slope ratio of low-frequency components and an energy ratio of high-frequency components in a front traveling wave and a back traveling wave of the line mode voltage in the protection time window, and when the average slope ratio and the energy ratio of the high-frequency components are both greater than a set threshold value, judging that an internal fault occurs; when the average slope ratio and the high-frequency component energy ratio are both smaller than a set threshold value, an external fault is judged. According to the method, the weak noise immunity of high-frequency quantity protection is improved, and the quickness of flexible low-frequency power transmission line protection is improved.

Description

Technical Field

[0001] The present invention relates to the technical field of relay protection of electric power systems, and in particular to a high-speed single-ended quantity protection method for low-frequency transmission lines. Background Art

[0002] Vigorously developing new energy sources such as wind and solar power in COSCO SHIPPING is an important measure to promote the construction of low-carbon power grids in my country and enhance the development of new power systems. Compared with high-voltage AC transmission and DC transmission, flexible low-frequency transmission of the same capacity has more economic advantages in the COSCO SHIPPING area. However, the construction and operation of offshore flexible low-frequency transmission systems are still in their infancy. Faced with a new transmission method, how to design and improve the system's relay protection capabilities to ensure its safe operation has become an urgent issue.

[0003] There are two main unique features of the relay protection of offshore flexible low-frequency transmission lines. First, its two-end source is a power electronic power source with a very different control method. The Modular Multilevel Matrix Converter (M3C) side adopts grid-building control, while the direct-drive wind farm side adopts grid-following control. Limited by the control strategy, the fault characteristics presented by the two-end controlled source are obviously different from those of the synchronous machine power supply. In addition, the interlaced coupling of the control response of the two-end source causes the line fault characteristics to be complex and changeable. Secondly, the transmission frequency of the flexible low-frequency transmission line is 20Hz, which is different from the common transmission frequency of 50Hz. Directly continuing to use the existing AC power grid protection method faces the problems of slow zero-crossing breaking and long fault isolation time after the fault, which may lead to the risk of low voltage crossing failure on the wind farm side and the risk of large-scale grid disconnection.

[0004] The power frequency protection method of flexible low-frequency transmission lines has a natural disadvantage in terms of speed, and cannot eliminate the coupling of the two-end power supply control response on the fault line, making it difficult to meet the needs of fast and reliable protection of flexible low-frequency transmission lines. High-frequency protection is easily affected by noise interference. Therefore, it is urgent to study high-speed protection methods suitable for flexible low-frequency transmission lines. Summary of the invention

[0005] The embodiment of the present invention provides a high-speed single-ended protection method for a low-frequency power transmission line, so as to effectively perform fault protection on the low-frequency power transmission line.

[0006] In order to achieve the above object, the present invention adopts the following technical scheme.

[0007] A high-speed single-ended protection method for a low-frequency transmission line, comprising:

[0008] Collect the voltage signal data at the local end of the low-frequency transmission line, and use the adaptive wavelet threshold denoising method to denoise the collected voltage signal data;

[0009] Calculate the voltage second-order difference value and the average voltage increment value of the signal after denoising;

[0010] Based on the second-order voltage difference value and the average voltage increment value, the line mode voltage is obtained by transforming the three-phase current and voltage data through Karen Boolean transformation, the line mode voltage forward wave and reverse wave are calculated, and the low-frequency component and high-frequency component of the line mode voltage forward wave and reverse wave in the required frequency range are extracted by successive modal decomposition;

[0011] The average slope ratio of the low-frequency component and the energy ratio of the high-frequency component in the forward and reverse waves of the line-mode voltage within the protection time window are calculated respectively. When the average slope ratio and the energy ratio of the high-frequency component are both greater than the set threshold, it is judged as an in-zone fault; when the average slope ratio and the energy ratio of the high-frequency component are both less than the set threshold, it is judged as an out-of-zone fault.

[0012] Preferably, the method further comprises: when the energy ratio of the high-frequency component is greater than a set threshold, it is judged as an in-zone fault, otherwise it is judged as an out-of-zone fault.

[0013] Preferably, the collecting of voltage signal data at the local end of the low-frequency transmission line and denoising the collected voltage signal data using an adaptive wavelet threshold denoising method include:

[0014] The current and voltage signal data at the local end of the flexible low-frequency transmission line are collected according to the set sampling rate, and the collected current and voltage signal data are denoised using the adaptive wavelet threshold denoising method;

[0015] The adaptive function is:

[0016] SURE(λ)=‖ww λ ‖ 2 +2σ 2 ·df(w λ )-nσ 2 (1)

[0017] Among them, SURE(·) is an adaptive method, w is the original wavelet coefficient vector, and w λ is the wavelet coefficient vector after applying the threshold λ, ||·|| represents the Euclidean norm, σ 2 is the noise variance, df(w λ ) is the degree of freedom, defined as w λ The number of non-zero coefficients in , n is the length of the wavelet coefficient vector;

[0018] The collected current and voltage signals are transformed by wavelet to obtain the wavelet coefficients w before processing. Then, the denoising threshold λ is determined according to the waveform adaptive function by formula (1). The wavelet coefficients smaller than the denoising threshold λ are set to zero, and the wavelet coefficients larger than the denoising threshold λ are shrunk to determine the retained wavelet coefficients w.λ , and then the denoised current and voltage signals are obtained through inverse wavelet transform.

[0019] Preferably, the calculation of the voltage second-order difference value and the average voltage increment value of the signal after denoising includes:

[0020] Calculate the voltage second-order difference value and the average voltage increment value of the denoised signal.

[0021]

[0022]

[0023] Among them, K1 and K2 are the second-order voltage difference value and the average voltage increment value respectively, u i-1 、u i 、u i+1 、u i+2 are the previous, current, next and next two voltage sampling values ​​after denoising, respectively, u i (k) 、u i (k-1) It represents the voltage sampling value of the current cycle and the previous cycle after denoising, i represents the sampling point number, △T represents the sampling time interval, and N represents the total number of sampling points calculated.

[0024] Preferably, the method further comprises:

[0025] Determine whether the voltage second-order differential value K1 and the average voltage increment value K2 after denoising are both greater than the corresponding thresholds. If so, determine that a fault has occurred and record the fault time. At the fault time, select the smaller value of the two and execute subsequent processing. Otherwise, if only the threshold corresponding to K1 or K2 is greater than the set threshold, or if neither K1 nor K2 is greater than the set threshold, return to continue collecting the voltage signal data at this end of the low-frequency transmission line.

[0026] Preferably, the three-phase current and voltage data are transformed by Karen Boolean transformation based on the second-order voltage difference value and the average voltage increment value to obtain the line mode voltage, the line mode voltage forward wave and reverse wave are calculated, and the low-frequency component and high-frequency component of the line mode voltage forward wave and reverse wave in the required frequency range are extracted by successive modal decomposition, including:

[0027] The line-mode voltage and line-mode current are obtained by transforming the three-phase current and voltage data through the Karlenbool transformation. The Karlenbool transformation is:

[0028]

[0029] Where x represents voltage or current, a 、xb 、x c is the three-phase voltage or current, x α 、x β , x0 are α, β, 0 mode voltages or currents respectively, α mode x is called line mode voltage U or line mode current I;

[0030] According to the line mode voltage U and line mode current I, the line mode voltage forward wave V is obtained. i and line mode voltage reverse wave V r ;

[0031]

[0032] Among them, Z0 is the line wave impedance;

[0033] The successive variational mode decomposition (SVMD) algorithm is used to decompose the line mode traveling wave obtained in the previous step, and the low-frequency and high-frequency components in the line mode voltage forward and reverse traveling waves in the required frequency band are extracted. The objective function of the SVMD algorithm is:

[0034]

[0035] Among them, s(t) is the input traveling wave signal, corresponding to the line mode voltage forward and reverse traveling waves; u k (t) is the kth voltage modal component, K is the total number of modal components; ω k is the center frequency of the kth modal component; H represents the Hilbert transform; λ is the penalty factor used to balance the energy distribution and signal reconstruction error between modal components, and t is the sampling time;

[0036] The specific calculation formula for extracting the low-frequency component and high-frequency component in the forward and reverse waves of the line-mode voltage in the required frequency band is:

[0037]

[0038] in, is the frequency domain representation of the input voltage traveling wave signal u(t), α is the weight factor used to balance the importance of different optimization criteria, ω k is the center frequency of the kth IMF, is the frequency domain representation of the k-th IMF at the n-th iteration,

[0039] is the frequency domain representation of the Lagrange multiplier at the nth iteration, ω i is the center frequency of the extracted IMF.

[0040] Through SVMD decomposition, modal components of different frequencies will be obtained According to the modal component center frequency ω k, the modal components whose center frequencies are close to the desired low-frequency band are taken as low-frequency components, and the modal components whose center frequencies are close to the desired high-frequency band are taken as high-frequency components.

[0041] Preferably, the method of respectively calculating the average slope ratio of the low-frequency component and the energy ratio of the high-frequency component in the forward and reverse waves of the line mode voltage within the protection time window, when the average slope ratio and the energy ratio of the high-frequency component are both greater than the set threshold, it is judged as an in-zone fault; when the average slope ratio and the energy ratio of the high-frequency component are both less than the set threshold, it is judged as an out-of-zone fault, including:

[0042] In the first protection strategy, the average slope ratio of the low-frequency component and the energy ratio of the high-frequency component in the forward and reverse waves of the line-mode voltage within the protection time window are calculated respectively;

[0043] The calculation method of the average slope ratio k and the energy value E is:

[0044]

[0045] Among them, u s represents the high-frequency component of the traveling wave obtained by successive modal decomposition, △T represents the sampling time interval, N is the number of points in the calculation time window, n is the discrete point index of the high-frequency signal, and u BL and u FL Represent the low-frequency components of the forward and reverse waves, respectively. t is the fault moment determined in the previous text, and i is the discrete point index of the low-frequency component;

[0046] When the average slope ratio and the high-frequency component energy ratio are both greater than the set threshold, it is judged as an internal fault; when the average slope ratio and the high-frequency component energy ratio are both less than the set threshold, it is judged as an external fault, and the others enter the second stage protection strategy;

[0047] In the protection strategy of Section II, when the high-frequency component energy ratio is greater than the set threshold, it is judged as an internal fault, otherwise it is judged as an external fault.

[0048] It can be seen from the technical solution provided by the above-mentioned embodiment of the present invention that the embodiment of the present invention completes fault discrimination through the average slope of the low-frequency component of the forward wave and the reverse wave and the energy ratio of the high-frequency component, effectively improving the weak noise resistance of the high-frequency quantity protection. In addition, the use of the high and low-frequency components of the traveling wave effectively improves the protection speed. The high-speed single-ended quantity protection scheme can reliably operate within 0.8ms after the fault, improving the speed of flexible low-frequency transmission line protection.

[0049] Additional aspects and advantages of the present invention will be given in part in the following description, which will become obvious from the following description, or may be learned through practice of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0050] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings required for use in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other accompanying drawings can be obtained based on these accompanying drawings without paying creative work.

[0051] Figure 1 A processing flow chart of a high-speed single-ended protection method for a low-frequency transmission line provided by an embodiment of the present invention;

[0052] Figure 2 This is a schematic diagram of the offshore flexible low-frequency transmission line described in an embodiment of the present invention.

[0053] Figure 3 This is a diagram of high and low frequency components of the forward and reverse traveling waves when there is an intra-zone fault according to an embodiment of the present invention.

[0054] Figure 4 This is a diagram of the high and low frequency components of the forward and reverse traveling waves during an out-of-zone fault according to an embodiment of the present invention. DETAILED DESCRIPTION

[0055] The embodiments of the present invention are described in detail below, examples of which are shown in the accompanying drawings, wherein the same or similar reference numerals throughout represent the same or similar elements or elements having the same or similar functions. The embodiments described below with reference to the accompanying drawings are exemplary and are only used to explain the present invention, and cannot be interpreted as limiting the present invention.

[0056] It will be understood by those skilled in the art that, unless expressly stated, the singular forms "one", "said", and "the" used herein may also include plural forms. It should be further understood that the term "comprising" used in the specification of the present invention refers to the presence of the features, integers, steps, operations, elements and / or components, but does not exclude the presence or addition of one or more other features, integers, steps, operations, elements, components and / or groups thereof. It should be understood that when we refer to an element as being "connected" or "coupled" to another element, it may be directly connected or coupled to the other element, or there may be intermediate elements. In addition, the "connection" or "coupling" used herein may include wireless connection or coupling. The term "and / or" used herein includes any unit and all combinations of one or more associated listed items.

[0057] It will be understood by those skilled in the art that, unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as those generally understood by those skilled in the art in the art to which the present invention belongs. It should also be understood that terms such as those defined in common dictionaries should be understood to have meanings consistent with the meanings in the context of the prior art, and will not be interpreted with idealized or overly formal meanings unless defined as herein.

[0058] To facilitate understanding of the embodiments of the present invention, several specific embodiments will be further explained below with reference to the accompanying drawings, and each embodiment does not constitute a limitation on the embodiments of the present invention.

[0059] The processing flow of a high-speed single-ended protection method for a low-frequency transmission line provided by an embodiment of the present invention is as follows: Figure 1 As shown, the processing steps include the following:

[0060] Step S10: collecting current and voltage signal data at the local end of the flexible low-frequency transmission line, with a sampling rate of 50 kHz.

[0061] Step S20: denoising the collected current and voltage signal data using an adaptive wavelet threshold denoising method.

[0062] The adaptive function is:

[0063] SURE(λ)=‖ww λ ‖ 2 +2σ 2 ·df(w λ )-nσ 2 (1)

[0064] Among them, SURE(·) is an adaptive method, w is the original wavelet coefficient vector, and w λ is the wavelet coefficient vector after applying the threshold λ, ||·|| represents the Euclidean norm (i.e., the length of the vector), σ 2 is the noise variance, df(w λ ) is the degree of freedom, defined as w λ is the number of non-zero coefficients in , and n is the length of the wavelet coefficient vector.

[0065] First, the collected current and voltage signals are transformed by wavelet to obtain the wavelet coefficients w before processing. Then, the denoising threshold λ is determined according to the waveform adaptive function by formula (1). The wavelet coefficients less than the denoising threshold λ are set to zero, and the wavelet coefficients greater than the threshold are shrunk. Then the retained wavelet coefficients w are determined. λ , and then the denoised current and voltage signals are obtained through inverse wavelet transform. Since wavelet transform is not an innovative content, it is not mentioned here.

[0066] The actual complete process is: 1) Assuming the collected current and voltage signals are x(t), perform wavelet decomposition on x(t) to obtain the approximate coefficient a j and detail factor d j (where j represents the decomposition scale).

[0067]

[0068] Among them, J is the number of decomposition layers, k represents the translation parameters of the wavelet basis function and the scale function, and d j [k] is the detail coefficient of the jth layer, ψ j,k (t) is the wavelet basis function, where db4 wavelet is selected, a J [k] is the approximation coefficient of the last layer (layer J), φ k (t) is the scaling function.

[0069] 2) Determine the denoising threshold λ according to formula (1).

[0070] 3) Set the detail coefficients smaller than the denoising threshold λ to zero, and shrink the detail coefficients larger than the denoising threshold λ.

[0071] 4) Recombining according to the processed detail coefficients and approximate coefficients to obtain the denoised signal

[0072]

[0073] in, is the detail coefficient after threshold processing.

[0074] Step S30: Calculate the voltage second-order difference value and the average voltage increment value of the signal after the denoising process.

[0075]

[0076] Among them, K1 and K2 are the second-order voltage difference value and the average voltage increment value respectively, u i-1 、u i 、u i+1 、u i+2 are the previous, current, next and next two voltage sampling values ​​after denoising, respectively, u i (k) 、u i (k-1) It represents the voltage sampling value of the current cycle and the previous cycle after denoising, i represents the sampling point number, △T represents the sampling time interval, and N represents the total number of sampling points calculated.

[0077] Determine whether the voltage second-order differential value K1 and the average voltage increment value K2 after denoising are both greater than the corresponding thresholds. If so, determine that a fault has occurred, record the fault time, select the smaller value of the two at the fault time, and execute step S40; otherwise, if only the threshold corresponding to K1 or K2 is greater than the set threshold, or if neither K1 nor K2 is greater than the set threshold, return to continue executing step S10.

[0078] The threshold value corresponding to the voltage second-order differential value K1 is set to 10 times that in normal operation. Since the voltage second-order differential value in normal operation is basically within a fixed interval, the threshold value corresponding to K1 is 10 times the maximum value of the second-order differential value in one cycle of normal operation. That is, if the maximum value of the second-order differential value in the previous cycle is 1, and the second-order differential value of a certain point in the current cycle obtained by (2) is greater than 10, the protection is activated and this moment is considered to be the fault moment.

[0079] The average voltage increment value K2, after the fault, the voltage will drop suddenly, and the voltage is a periodic function. Under normal operating conditions, the values ​​of the sampling points at the same position in the current cycle and the previous cycle are almost the same. Therefore, when the values ​​of the sampling points at the same position in the current cycle and the previous cycle differ greatly, it can be considered that a fault has occurred, and the voltage amplitude can be taken as 0.2 times the threshold. For example, there are 80 points in a cycle, the voltage amplitude is 1, the values ​​of the 17th, 18th, and 19th points in the previous cycle are 0.9, 0.92, and 0.94, and the values ​​of the 17th, 18th, and 19th points in the current cycle are 0.4, 0.35, and 0.2, then corresponding to formula (3), N is 3, and it is obvious that the average value is greater than the threshold of 0.2.

[0080] Step S40, obtaining the line mode voltage by Karlenberg transformation of the three-phase current and voltage data, and calculating the line mode voltage forward wave and reverse wave;

[0081] The SVMD (Sequential Variational Mode Decomposition) algorithm is used to decompose the line-mode traveling wave obtained in the previous step, and the line-mode voltage is obtained by Karen Boolean transformation of the three-phase current and voltage data. The Karen Boolean transformation is:

[0082]

[0083] Where x represents voltage or current, a 、x b 、x c is the three-phase voltage or current, x α 、x β , x0 are α, β, 0 mode voltages or currents respectively, α mode x is called line mode voltage U or line mode current I;

[0084] Then, the line mode voltage forward wave V is obtained based on the line mode voltage U and the line mode current I. i and the anti-travelling wave V r .

[0085]

[0086] Among them, Z0 is the line wave impedance.

[0087] And extract the low-frequency and high-frequency components in the forward and reverse waves of the line-mode voltage in the required frequency band. The objective function of the successive modal decomposition is:

[0088]

[0089] Among them, s(t) is the input traveling wave signal; u k (t) is the kth voltage modal component, K is the total number of modal components; ω k is the center frequency of the kth modal component; H represents the Hilbert transform; λ is the penalty factor used to balance the energy distribution and signal reconstruction error between modal components, and t is the sampling time.

[0090] Formula (4) and formula (5) are used to decompose the forward and reverse waves of the line-mode voltage. The forward and reverse waves of the line-mode voltage are time domain signals, containing information in different frequency domains. Formula (4) is used to represent the solution target of the successive modal decomposition. The goal of this step is to obtain the high-frequency and low-frequency signals of the forward and reverse waves of the line-mode voltage, and the successive modal decomposition is to gradually extract the signals of different frequency bands in the signal. Here, the signal corresponds to the forward and reverse waves of the line-mode voltage. Therefore, formula (4) and formula (5) respectively represent the extraction of the kth voltage modal component and the center frequency corresponding to the modal component. In the formula, s(t) is the input traveling wave signal, corresponding to the forward and reverse waves of the line-mode voltage, and the decomposition process is the same. If the signal s(t)=sin(10wt)+sin(20wt)+sin(100wt), then after successive modal decomposition, k=4 sub-modes are obtained, corresponding to signals and residuals of 10w, 20w, and 100w respectively. Assuming that a low-frequency signal of 10w and a high-frequency signal of 100w are required, they correspond to sub-mode 1 and sub-mode 3 respectively.

[0091] Step S50, extracting low-frequency components and high-frequency components in the forward and reverse waves of the line mode voltage in the required frequency range by using successive variable mode decomposition;

[0092] Successive modal decomposition is an existing method, but it is also used in line protection. The specific calculation formula for extracting sub-modes by successive modal decomposition can be expressed as:

[0093]

[0094] in, is the frequency domain representation of the input voltage traveling wave signal u(t), α is the weight factor used to balance the importance of different optimization criteria, ω k is the center frequency of the kth IMF, is the frequency domain representation of the k-th IMF at the n-th iteration, is the frequency domain representation of the Lagrange multiplier at the nth iteration, ω i is the center frequency of the extracted IMF.

[0095] Through SVMD decomposition, modal components of different frequencies will be obtained According to the modal component center frequency ω k , the modal component with a center frequency close to the required low frequency band (such as less than 100Hz) is used as the low frequency component, and the modal component with a center frequency close to the required high frequency band (such as greater than 1000Hz) is used as the high frequency component. If the signal frequency difference is obvious, the first modal component can be directly used as the low frequency component, and the last modal component after removing the residual can be used as the high frequency component.

[0096] Step S60, setting a two-stage protection strategy. In the first stage protection, respectively calculate the average slope ratio of the low-frequency component and the energy ratio of the high-frequency component in the forward and reverse waves of the line mode voltage within the protection time window.

[0097] The calculation method of the average slope ratio k and the energy value E is:

[0098]

[0099] Among them, u s represents the high-frequency component of the traveling wave obtained by successive modal decomposition, △T represents the sampling time interval, N is the number of points in the calculation time window, n is the discrete point index of the high-frequency signal, and u BL and u FL Represent the low-frequency components of the forward wave and the reverse wave respectively. t is the fault moment determined in the previous text, and i is the discrete point index of the low-frequency component.

[0100] When the average slope ratio and the high-frequency component energy ratio are both greater than the set threshold, it is judged as an internal fault; when the average slope ratio and the high-frequency component energy ratio are both less than the set threshold, it is judged as an external fault, and the others enter the second stage protection judgment;

[0101] In the second stage protection, the energy ratio of the high-frequency components in the forward and reverse waves of the line mode voltage within the protection time window is calculated. When the high-frequency component energy ratio is greater than the set threshold, it is judged as an internal fault, otherwise it is judged as an external fault.

[0102] First, enter the protection of Section I, and calculate the average slope ratio of the low-frequency component and the energy ratio of the high-frequency component in the forward and reverse waves of the line mode voltage within the protection time window. When the average slope ratio is greater than the set threshold 1 and the high-frequency component energy ratio is greater than the threshold p, it is judged as an internal fault; when the average slope ratio is less than the set threshold 1 and the high-frequency component energy ratio is less than the threshold p, it is judged as an external fault, and the others enter Section II protection judgment. The protection judgment criteria for Section I are:

[0103]

[0104] Among them, P and R represent the criteria for inside and outside the area respectively, and E BH and E FH They represent the energy of the high-frequency forward wave and the reverse wave respectively, and p is the high-frequency threshold, which is set to 0.8.

[0105] If there is no output result in stage I, the protection stage II is entered. The energy ratio of the high-frequency component in the forward and reverse waves of the line-mode voltage is greater than the threshold value p, and the high-frequency component energy E H Must be greater than the set threshold E set , here it is set to 0.001 (when the energy value is less than the threshold, the direct output ratio is 0), then it is judged as an internal fault, otherwise it is judged as an external fault. The protection criterion of Section II is:

[0106]

[0107] If it is an out-of-zone fault, the protection will be restored. If it is an in-zone fault, the protection will send a signal to isolate the faulty area.

[0108] The method of the present invention can be applied to all low-frequency transmission lines. Figure 2 Schematic diagram of offshore flexible low-frequency transmission line.

[0109] (1) In the initial stage when a fault occurs in the line area, the high-frequency component of the traveling wave at the protection M contains both the pre-fault traveling wave and the reverse traveling wave, and the reverse traveling wave is slightly larger than the forward traveling wave; while the main component of the low-frequency component of the traveling wave is the reverse traveling wave, which is much larger than the forward traveling wave, and the ratio of the reverse traveling wave to the forward traveling wave is large.

[0110] (2) When there is an out-of-zone fault, the high-frequency and low-frequency components of the traveling wave at protection M mainly contain the traveling wave before the fault. The difference between the forward traveling wave and the reverse traveling wave is obvious, and the ratio of the reverse traveling wave to the forward traveling wave is small.

[0111] The above characteristics indicate that there are obvious differences between the high-frequency and low-frequency components of the reverse-traveling waves and the forward-traveling waves during in-zone and out-zone faults, and the high-speed single-ended protection criterion can be constructed based on this.

[0112] Example 1

[0113] In order to verify the application performance of the proposed protection method, a simulation platform was built on the PSCAD platform. Figure 2 A 220kV 20 / 50Hz offshore flexible low-frequency transmission line is selected, and M is set as the protection installation location. The output power on the wind farm side is 300MW, the low-frequency cable adopts a frequency-variable parameter model, and the length is 200km. The seawater resistivity is set to 0.5Ω·m, the sampling frequency is 50kHz, and the protection time windows of sections I and II are 0.8ms and 2.0ms.

[0114] The steps of the proposed protection process are as follows:

[0115] 1) Collect current and voltage signal data at the M end of the flexible low-frequency transmission line, with a sampling rate of 50kHz;

[0116] 2) De-noising the collected data using adaptive wavelet threshold denoising method;

[0117] 3) Calculate the voltage second-order difference value and the average voltage increment value after denoising, and determine whether the improved voltage second-order difference value and the average voltage increment value are greater than the threshold value. If they are greater than the set threshold value, it is determined that a fault has occurred and the fault time is recorded. The smaller value of the two is selected at the fault time. Otherwise, return to continue;

[0118] 4) Obtain the line-mode traveling wave from the three-phase current and voltage data after denoising through Karenbuhl transformation, and calculate the line-mode voltage forward and reverse traveling waves;

[0119] 5) The line-mode traveling wave obtained in the previous step is decomposed by the successive modal decomposition algorithm, and the low-frequency and high-frequency components in the line-mode voltage forward and reverse waves in the required frequency band are extracted. Here, the high frequency is selected to be greater than 5kHz and the low frequency is selected to be less than 1000Hz.

[0120] 6) Entering the first stage protection, respectively calculate the average slope ratio of the low-frequency component and the energy ratio of the high-frequency component in the forward and reverse waves of the line-mode voltage within the protection time window. When the average slope ratio is greater than the set threshold value 1 and the energy ratio of the high-frequency component is greater than the threshold value p=0.8, it is judged as an in-zone fault; when the average slope ratio is less than the set threshold value 1 and the energy ratio of the high-frequency component is less than the threshold value p=0.8, it is judged as an out-of-zone fault. The others enter the second stage protection judgment.

[0121] 7) If there is no output result in stage I, then stage II protection is entered. The energy ratio of the high-frequency component in the forward and reverse waves of the line-mode voltage is greater than the threshold value p, and the high-frequency component energy E H Must be greater than the set threshold E set , here it is set to 0.001 (when the energy value is less than the threshold, the output ratio is directly 0), then it is judged as an internal fault, otherwise it is judged as an external fault.

[0122] PSCAD / EMTDC is used to perform simulation tests and verify the relay protection performance for different fault types, different transition resistances, and different fault directions.

[0123] Figure 3 The high and low frequency components of the forward and reverse traveling waves during an intra-area fault provided by the embodiment of the present invention are shown in FIG. Figure 3 It can be seen from the waveform that in the 1ms time window after the fault in the area, the high-frequency component of the reverse wave is slightly larger than the high-frequency component of the forward wave, and the difference between the low-frequency components of the reverse wave and the forward wave decreases with the increase of the fault distance. The simulation results are basically consistent with the theoretical analysis. When the fault distance is greater than 95% of the line length, the protection section I may not be able to operate accurately, but at 190km of the line, the high-frequency component criterion ratio of the protection section II is 1.67, which still has a large margin, so the proposed protection can still operate accurately.

[0124] Figure 4 The figure shows the high and low frequency components of the forward and reverse waves when an out-of-zone fault is provided in an embodiment of the present invention. It can be seen that the forward waves of the high and low frequency components are much larger than the reverse waves within 1ms after the fault, and the low frequency forward wave changes significantly more than the reverse wave within 1ms after the fault. The simulation results are consistent with the theoretical analysis. The proposed protection identification result is an out-of-zone fault, and the protection operates correctly.

[0125] The above results are all correctly identified.

[0126] In summary, the embodiments of the present invention effectively improve the weak noise resistance of high-frequency quantity protection through adaptive wavelet threshold denoising and successive variable mode decomposition methods. In addition, the use of high and low frequency components of traveling waves effectively improves the protection speed. The high-speed single-ended quantity protection scheme can reliably operate within 0.8ms after a fault, which improves the speed of flexible low-frequency transmission line protection.

[0127] Those skilled in the art can understand that the accompanying drawings are only schematic diagrams of an embodiment, and the modules or processes in the accompanying drawings are not necessarily required to implement the present invention.

[0128] It can be known from the description of the above implementation methods that those skilled in the art can clearly understand that the present invention can be implemented by means of software plus a necessary general hardware platform. Based on such an understanding, the technical solution of the present invention is essentially or the part that contributes to the prior art can be embodied in the form of a software product, which can be stored in a storage medium such as ROM / RAM, a magnetic disk, an optical disk, etc., and includes a number of instructions for enabling a computer device (which can be a personal computer, a server, or a network device, etc.) to execute the methods described in the various embodiments of the present invention or certain parts of the embodiments.

[0129] Each embodiment in this specification is described in a progressive manner, and the same or similar parts between the embodiments can be referred to each other, and each embodiment focuses on the differences from other embodiments. In particular, for the device or system embodiment, since it is basically similar to the method embodiment, the description is relatively simple, and the relevant parts can be referred to the partial description of the method embodiment. The device and system embodiments described above are merely schematic, wherein the units described as separate components may or may not be physically separated, and the components displayed as units may or may not be physical units, that is, they may be located in one place, or they may be distributed on multiple network units. Some or all of the modules may be selected according to actual needs to achieve the purpose of the scheme of this embodiment. Ordinary technicians in this field can understand and implement it without paying creative labor.

[0130] The above is only a preferred specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily thought of by a person skilled in the art within the technical scope disclosed by the present invention should be included in the protection scope of the present invention. Therefore, the protection scope of the present invention should be based on the protection scope of the claims.

Claims

1. A high-speed single-ended protection method for a low-frequency transmission line, characterized in that: include: Collect the voltage signal data at the local end of the low-frequency transmission line, and use the adaptive wavelet threshold denoising method to denoise the collected voltage signal data; Calculate the voltage second-order difference value and the average voltage increment value of the signal after denoising; Based on the second-order voltage difference value and the average voltage increment value, the line mode voltage is obtained by transforming the three-phase current and voltage data through Karen Boolean transformation, the line mode voltage forward wave and reverse wave are calculated, and the low-frequency component and high-frequency component of the line mode voltage forward wave and reverse wave in the required frequency range are extracted by successive modal decomposition; The average slope ratio of the low-frequency component and the energy ratio of the high-frequency component in the forward and reverse waves of the line-mode voltage within the protection time window are calculated respectively. When the average slope ratio and the energy ratio of the high-frequency component are both greater than the set threshold, it is judged as an in-zone fault; when the average slope ratio and the energy ratio of the high-frequency component are both less than the set threshold, it is judged as an out-of-zone fault.

2. The method according to claim 1, characterized in that The method further includes: when the energy ratio of the high-frequency component is greater than a set threshold, it is determined to be an in-zone fault, otherwise it is determined to be an out-of-zone fault.

3. The method according to claim 1 or 2, characterized in that: The method of collecting the voltage signal data at the local end of the low-frequency transmission line and performing denoising processing on the collected voltage signal data using an adaptive wavelet threshold denoising method includes: The current and voltage signal data at the local end of the flexible low-frequency transmission line are collected according to the set sampling rate, and the collected current and voltage signal data are denoised using the adaptive wavelet threshold denoising method; The adaptive function is: SURE(λ)=‖w-w λ ‖ 2 +2σ 2 ·df(w λ )-nσ 2 (1) Among them, SURE(·) is an adaptive method, w is the original wavelet coefficient vector, and w λ is the wavelet coefficient vector after applying the threshold λ, ||·|| represents the Euclidean norm, σ 2 is the noise variance, df(w λ ) is the degree of freedom, defined as w λ The number of non-zero coefficients in , n is the length of the wavelet coefficient vector; The collected current and voltage signals are transformed by wavelet to obtain the wavelet coefficients w before processing. Then, the denoising threshold λ is determined according to the waveform adaptive function by formula (1). The wavelet coefficients smaller than the denoising threshold λ are set to zero, and the wavelet coefficients larger than the denoising threshold λ are shrunk to determine the retained wavelet coefficients w. λ , and then the denoised current and voltage signals are obtained through inverse wavelet transform.

4. The method according to claim 3, characterized in that The calculation of the voltage second-order difference value and the average voltage increment value of the signal after the denoising process includes: Calculate the voltage second-order difference value and the average voltage increment value of the denoised signal. Among them, K1 and K2 are the second-order voltage difference value and the average voltage increment value respectively, u i-1 、u i 、u i+1 、u i+2 are the previous, current, next and next two voltage sampling values ​​after denoising, respectively, u i (k) 、u i (k-1) It represents the voltage sampling value of the current cycle and the previous cycle after denoising, i represents the sampling point number, △T represents the sampling time interval, and N represents the total number of sampling points calculated.

5. The method according to claim 4, characterized in that The method further comprises: Determine whether the voltage second-order differential value K1 and the average voltage increment value K2 after denoising are both greater than the corresponding thresholds. If so, determine that a fault has occurred and record the fault time. At the fault time, select the smaller value of the two and execute subsequent processing. Otherwise, if only the threshold corresponding to K1 or K2 is greater than the set threshold, or if neither K1 nor K2 is greater than the set threshold, return to continue collecting the voltage signal data at this end of the low-frequency transmission line.

6. The method according to claim 5, characterized in that The method of obtaining the line mode voltage by transforming the three-phase current and voltage data based on the second-order voltage difference value and the average voltage increment value through Karen Boolean transformation, calculating the line mode voltage forward wave and reverse wave, and extracting the low-frequency component and high-frequency component in the line mode voltage forward wave and reverse wave of the required frequency band by successive variable mode decomposition includes: The line-mode voltage and line-mode current are obtained by transforming the three-phase current and voltage data through the Karlenbool transformation. The Karlenbool transformation is: Where x represents voltage or current, a 、x b 、x c is the three-phase voltage or current, x α 、x β , x0 are α, β, 0 mode voltages or currents respectively, α mode x is called line mode voltage U or line mode current I; According to the line mode voltage U and line mode current I, the line mode voltage forward wave V is obtained. i and line mode voltage reverse wave V r ; Among them, Z0 is the line wave impedance; The successive variational mode decomposition (SVMD) algorithm is used to decompose the line mode traveling wave obtained in the previous step, and the low-frequency and high-frequency components in the line mode voltage forward and reverse traveling waves in the required frequency band are extracted. The objective function of the SVMD algorithm is: Among them, s(t) is the input traveling wave signal, corresponding to the line mode voltage forward and reverse traveling waves; u k (t) is the kth voltage modal component, K is the total number of modal components; ω k is the center frequency of the kth modal component; H represents the Hilbert transform; λ is the penalty factor used to balance the energy distribution and signal reconstruction error between modal components, and t is the sampling time; The specific calculation formula for extracting the low-frequency component and high-frequency component in the forward and reverse waves of the line-mode voltage in the required frequency band is: in, is the frequency domain representation of the input voltage traveling wave signal u(t), α is the weight factor used to balance the importance of different optimization criteria, ω k is the center frequency of the kth IMF, is the frequency domain representation of the k-th IMF at the n-th iteration, is the frequency domain representation of the Lagrange multiplier at the nth iteration, ω i is the center frequency of the extracted IMF. Through SVMD decomposition, modal components of different frequencies will be obtained According to the modal component center frequency ω k , the modal components whose center frequencies are close to the desired low-frequency band are taken as low-frequency components, and the modal components whose center frequencies are close to the desired high-frequency band are taken as high-frequency components.

7. The method according to claim 6, characterized in that The method of respectively calculating the average slope ratio of the low-frequency component and the energy ratio of the high-frequency component in the forward and reverse waves of the line mode voltage within the protection time window, when both the average slope ratio and the energy ratio of the high-frequency component are greater than the set threshold, it is judged as an in-zone fault; when both the average slope ratio and the energy ratio of the high-frequency component are less than the set threshold, it is judged as an out-of-zone fault, including: In the first protection strategy, the average slope ratio of the low-frequency component and the energy ratio of the high-frequency component in the forward and reverse waves of the line-mode voltage within the protection time window are calculated respectively; The calculation method of the average slope ratio k and the energy value E is: Among them, u s represents the high-frequency component of the traveling wave obtained by successive modal decomposition, △T represents the sampling time interval, N is the number of points in the calculation time window, n is the discrete point index of the high-frequency signal, and u BL and u FL Represent the low-frequency components of the forward and reverse waves, respectively. t is the fault moment determined in the previous text, and i is the discrete point index of the low-frequency component; When the average slope ratio and the high-frequency component energy ratio are both greater than the set threshold, it is judged as an internal fault; when the average slope ratio and the high-frequency component energy ratio are both less than the set threshold, it is judged as an external fault, and the others enter the second stage protection strategy; In the protection strategy of Section II, when the high-frequency component energy ratio is greater than the set threshold, it is judged as an internal fault, otherwise it is judged as an external fault.

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