Flexible HVDC transmission line single-ended protection method based on waveform matching

Abstract

A kind of flexible HVDC transmission line single-ended protection method based on waveform matching, its steps include: establishing traveling wave expression general formula, and double distinguishing fault type, then generating matching waveform based on distinguishing fault type. Multiple fault type criteria are constructed based on matching waveform;Collect the traveling wave of line mode voltage under fault state, and determine the real-time fault type by fault type criterion. The system informs the protection device to take corresponding protection action based on real-time fault type. The present application constructs accurate matching waveform based on double distinguishing, and constructs auxiliary criterion using the waveform characteristics of matching waveform, so that the protection can accurately identify different fault types, the protection has no dead zone, and the protection has strong reliability;And all criteria of the protection method are judged based on the characteristic quantity of waveform, the method is easy to implement, and has good engineering application value.

Description

Technical Field

[0001] The present invention belongs to the technical field of power system relays, and in particular relates to a single-ended protection method for a flexible direct current transmission line based on waveform matching. Background Art

[0002] In a flexible HVDC transmission system, the safety and reliability of transmission lines are crucial. However, transmission lines are often the part of the system most prone to failure. Once a failure occurs, it can cause a rapid increase in fault current, posing a serious threat to system safety. To prevent this, effective protection measures are needed to quickly clear faults and ensure the system's high speed.

[0003] Currently, the single-ended protection in operation mainly identifies faults based on voltage mutations. This protection method has the advantages of simple principles and easy implementation. However, its sensitivity is insufficient in the event of long-distance high-resistance faults. For example, the "Single-ended Protection Method for Parallel Multi-terminal HVDC Transmission Systems" disclosed in Chinese patent CN117277234B mentions that when analyzing out-of-area faults, traditional protection methods generally only consider the case where the fault traveling wave only passes through the converter station port once. However, when there are a large number of converter stations in a multi-terminal HVDC transmission system, the out-of-area fault traveling wave may pass through the converter station port multiple times. The change in the traveling wave is different from when it only passes through the converter station port once, which will cause the relevant fault area identification method to misjudge.

[0004] In order to improve the ability of line protection to tolerate transition resistance, many technicians have introduced time-frequency processing methods into the protection structure. This type of protection based on frequency domain characteristics mainly utilizes the attenuation characteristics of high-frequency components of the transmission line boundary elements, and extracts the fault characteristics of specific frequency bands through different time-frequency processing methods, thereby realizing fault identification. This method has improved the ability of protection to tolerate transition resistance to a certain extent, but it also has defects such as overly complex algorithms and weak anti-noise interference capabilities. For example, the protection scheme proposed in the "Flexible DC Power Grid Longitudinal Protection Method Based on Frequency Domain Energy Matrix Similarity" disclosed in Chinese patent application CN117937368A, although it extracts the frequency domain characteristics of the fault current traveling wave through S transform, thereby improving the reliability and anti-interference capability of relay protection; however, this method is relatively complex and has high requirements for equipment.

[0005] In addition, many researchers have proposed improved single-ended measurement protection methods based on waveform analysis. However, these methods are significantly affected by subsequent traveling waves and cannot protect the entire length of the line. For example, Chinese patent CN114629089B, "A Single-ended Measurement Waveform Similarity Protection Method for Flexible DC Transmission Lines," states that existing protection schemes for flexible DC transmission lines need to improve their detection accuracy for different fault types, fault resistances, and fault distances, and also require high sampling rates and strict data synchronization.

[0006] Therefore, there is a need to improve this type of traveling wave protection method to enhance its operating performance, protect the entire length of the line, and ensure the safe and reliable operation of the flexible DC transmission system. Summary of the Invention

[0007] The present invention provides a single-ended protection method for a flexible DC transmission line based on waveform matching to solve the above problems.

[0008] The present invention adopts the following technical solutions:

[0009] A single-ended protection method for a flexible DC transmission line based on waveform matching, the main steps of which include:

[0010] Establish the general expression of the traveling wave of the initial line-mode voltage under different fault types;

[0011] Performing dual differentiation of fault types based on the traveling wave expression formula to obtain differentiated fault types;

[0012] The double distinction is provided with a primary distinction and a secondary distinction;

[0013] The primary differentiation is to classify the fault type into an intra-zone fault and an extra-zone fault according to the time difference between the initial traveling wave and the subsequent traveling wave in the subsequent traveling wave time window; the intra-zone fault is further divided into an intra-zone proximal fault and an intra-zone non-proximal fault;

[0014] The secondary differentiation classifies the types of the faults in the zone into at least a proximal zone fault, a proximal opposite zone fault, and a non-proximal zone fault based on the difference in waveform characteristics of the subsequent traveling waves;

[0015] Based on the fault type differentiation, a nonlinear optimization algorithm is used to fit the traveling wave of the line mode voltage under different fault types to generate a matching waveform;

[0016] Constructing multiple fault type criteria based on the waveform characteristics of the matching waveform;

[0017] The sensor collects the traveling wave of the line mode voltage under the fault state, and determines the actual fault type according to the fault type judgment criterion within the adaptive time window;

[0018] The system notifies the protection device to perform corresponding protection actions based on the actual fault type.

[0019] Optionally, the system in which the different fault types occur is a four-terminal flexible direct current transmission system;

[0020] The process of establishing the traveling wave expression formula includes:

[0021] The initial line mode voltage under different fault types is calculated using a composite mode domain equivalent circuit diagram to obtain a frequency domain expression;

[0022] Then, performing an inverse Laplace transform on the frequency domain expression to obtain a time domain expression;

[0023] The time domain expression obtains the general formula of the traveling wave of the initial traveling wave under different fault conditions through a classification method.

[0024] Optionally, the primary differentiation includes:

[0025] The subsequent traveling waves are divided into first-category subsequent traveling waves and second-category subsequent traveling waves according to the number of refractions and reflections at the wave impedance discontinuity point experienced by the subsequent traveling waves, and the amplitudes of the subsequent traveling waves are calculated.

[0026] Optionally, the primary differentiation further includes:

[0027] According to the classification of the subsequent traveling waves and the time difference between the initial traveling wave and the first subsequent traveling wave, the fault type is divided into an in-zone near-end fault, an in-zone non-near-end fault and an out-of-zone fault.

[0028] Optionally, the secondary differentiation includes:

[0029] The proximal area fault is divided according to the traveling wave expression formula of the initial line mode voltage traveling wave and the classification and amplitude polarity of the first subsequent traveling wave.

[0030] Optionally, the secondary differentiation further includes:

[0031] The near-opposite-end area fault is divided according to the voltage drop rate and extreme value time of the initial traveling wave.

[0032] Optionally, the secondary differentiation further includes:

[0033] The non-proximal area fault is obtained after the proximal area fault and the proximal opposite area fault are screened out from the intra-area faults.

[0034] Optionally, the nonlinear optimization algorithm is an SQP algorithm;

[0035] The basis function for traveling wave fitting of the line mode voltage under the different fault types is the traveling wave expression formula, and the traveling wave expression formula is combined into equation groups.

[0036] Optionally, the fault type criterion includes an in-zone fault criterion and an out-of-zone fault criterion;

[0037] The intra-area fault criterion at least includes a window length matching error criterion of the adaptive time window;

[0038] The window length of the adaptive time window is the product of the line fault propagation time reference value and a constant coefficient, and the step of calculating the window length matching error criterion of the adaptive time window includes:

[0039] Calculating the sum of errors between the traveling waveform of the line-mode voltage under the fault state within the adaptive time window and the matching waveform, and comparing the sum of errors with a preset error setting value to obtain a comparison result;

[0040] When the comparison result is the intra-zone fault, the actual fault type is the intra-zone fault;

[0041] When the comparison result is the out-of-zone fault, the determination process of the out-of-zone fault criterion is entered.

[0042] Optionally, the out-of-zone fault criterion includes a voltage loss identification criterion, an extreme value time criterion, and a waveform graph criterion;

[0043] The calculation process of the voltage loss identification criterion includes:

[0044] Calculating the voltage loss value of the line mode voltage under the fault state and the voltage loss value of the matching waveform at different time points within the adaptive time window, and taking the minimum value and comparing it with the preset voltage loss setting value to obtain a first fault type;

[0045] The calculation process of the extreme value time criterion includes:

[0046] Calculating the first minimum time of the line mode voltage and the first minimum time of the matching waveform under the fault state, and comparing the minimum value with a preset extreme time setting value to obtain a second fault type;

[0047] The calculation process of the waveform graph criterion includes:

[0048] Calculating the second-order derivative at the maximum value of the matching waveform, and determining the concavity and convexity of the graph by comparing it with a value of 0 to obtain a third fault type;

[0049] When the first, second and third fault types are all the out-of-zone faults, the actual fault type is the out-of-zone fault;

[0050] When the first, second, and third fault types are not all the out-of-zone faults, the actual fault type is the in-zone fault.

[0051] Compared with the prior art, the present invention adopts the above technical solution and has the following technical effects:

[0052] The present invention constructs an accurate matching waveform based on double differentiation, and uses the waveform characteristics of the matching waveform to construct auxiliary criteria, so that the protection can accurately identify different fault types, there is no dead zone in the protection, and the protection reliability is strong. In addition, all criteria of the protection method are based on the characteristic quantities of the waveform. The method is easy to implement and has good engineering application value. BRIEF DESCRIPTION OF THE DRAWINGS

[0053] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0054] Figure 1 It is a flow chart of the method of the present invention;

[0055] Figure 2 Schematic diagram of a four-terminal flexible direct current transmission system in Example 1 of the present invention;

[0056] Figure 3 This is a schematic diagram of waveform simulation matching different fault locations in Example 1 of the present invention. DETAILED DESCRIPTION

[0057] To make the purpose, technical solutions, and advantages of the embodiments of the present invention more clear, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts shall fall within the scope of protection of the present invention.

[0058] It should be noted that similar reference numerals and letters denote similar items in the following drawings, and therefore, once an item is defined in one drawing, it does not need to be further defined or explained in subsequent drawings.

[0059] It should also be noted that the methods used in the present invention are all conventional methods unless otherwise specified; the raw materials and devices used are all conventional commercially available products unless otherwise specified.

[0060] The present invention provides a single-ended protection method for a flexible DC transmission line based on waveform matching, wherein: Figure 1 As shown, the main steps include:

[0061] 1. Establish a general expression of the initial line mode voltage traveling wave under different fault types; that is, analyze the initial line mode voltage traveling wave expression under different fault types and induce the general expression of the initial traveling wave under different fault conditions. Specifically, the occurrence system of different fault types in this embodiment is taken as an example of a four-terminal flexible HVDC transmission system.

[0062] The establishment process of the general expression of the traveling wave includes:

[0063] The initial line mode voltage under different fault types is calculated using a composite mode equivalent circuit diagram to obtain a frequency domain expression;

[0064] The time domain expression is obtained by inverse Laplace transform of the frequency domain expression;

[0065] The general expression of the initial traveling wave under different fault conditions is obtained by classification method from the time domain expression, and the formula is:

[0066]

[0067] In the formula, t is time; c is the ordinal of the fault type, taking values of 1, 2, 3, and 4; f1 represents a non-near-end fault in the region; f3 represents a near-opposite-end fault in the region; f4 represents an out-of-region fault; f2 represents a near-end fault in the region; m and z are constants; a and b are parameters related to the characteristics of the transmission line and the fault condition, and the values will be affected by various factors, such as the resistance, inductance, and capacitance of the line, as well as the type, location, and transition resistance of the fault, and thus the specific parameters are obtained by modeling and calculating the transmission system.

[0068] Thus, when the fault type of the transmission line corresponds to f1, f3, and f4 faults, it can be uniformly represented by the general formula f(t)=me -at +ze -bt , and when f2 fault occurs, there is an additional constant term in the general formula.

[0069] 2. Double distinguish the fault type based on the general expression of the traveling wave to obtain the distinguished fault type.

[0070] Among them, the double distinction is divided into primary distinction and secondary distinction;

[0071] 2.1 Primary distinction;

[0072] The primary distinction uses the degree of influence of the initial traveling wave within the subsequent traveling wave time window t n1 (t n1 <2l / v1, l is the length of the line, and v1 represents the line mode wave speed of the transmission line) on the subsequent traveling wave to distinguish the in-region and out-of-region faults, and specifically, the fault type is divided into in-region fault and out-of-region fault according to the time difference between the initial traveling wave and the subsequent traveling wave within the subsequent traveling wave time window.

[0073] Among them, a differentiation includes the following steps:

[0074] According to the number of refractions and reflections experienced by the subsequent traveling waves at the wave impedance discontinuity point, the subsequent traveling waves are divided into the first type of subsequent traveling waves and the second type of subsequent traveling waves, and the amplitude of the subsequent traveling waves is calculated. The amplitude formula is:

[0075]

[0076] Where u -21 It represents the line mode voltage traveling wave reflected by the fault point, u -20 It represents the line mode voltage traveling wave of the zero mode component transmitted from the fault point to the near end. The zero mode component is refracted and reflected only through one impedance discontinuity point. The corresponding subsequent traveling wave is recorded as the first type of subsequent traveling wave; u -31 Indicates the line mode component after being reflected by the other end and then refracted by the fault point, u -30 It represents the line mode component of the zero mode component after being reflected by the opposite end and then transmitted to the near end through the fault point. The zero mode component is folded and reflected after passing through two impedance discontinuities. The corresponding subsequent traveling wave is recorded as the second type of subsequent traveling wave; U N is the rated voltage value; Z c1 , Z c0 are the line mode and zero mode wave impedance of the transmission line respectively; R f is the fault transition resistance.

[0077] According to the classification of subsequent traveling waves and the time difference between the initial traveling wave and the first subsequent traveling wave, the fault type is divided into intra-zone fault and extra-zone fault. Specifically, according to different fault locations, the fault is divided into intra-zone near-end fault, intra-zone non-near-end fault and extra-zone fault. The arrival time of subsequent traveling waves at different locations is analyzed respectively, and the formula is obtained as follows:

[0078]

[0079] Where x represents the distance between the fault point and the protection installation location; t 11 represents the time difference between the initial traveling wave and the first subsequent traveling wave of the first kind, t 21 It represents the time difference between the initial traveling wave and the first subsequent traveling wave of the second type; t1 is the arrival time of the first subsequent traveling wave for a fault in the near area within the zone, t2 is the arrival time of the first subsequent traveling wave for a fault in the far area within the zone, and t3 is the arrival time of the first subsequent traveling wave for a fault in the near area within the zone.

[0080] According to the above analysis of subsequent traveling waves at different fault locations, it can be seen that there is a difference in the first subsequent traveling wave time felt by the protection point for non-proximal faults within the zone, proximal faults within the zone, and faults outside the zone, and t1 <t3、t2<t3恒成立,因此,一定存在时间窗长tn1 <2l / v1, so that the non-proximal fault in the area is greatly affected by the subsequent traveling wave, while the fault outside the area and the proximal fault in the area are not affected by the subsequent traveling wave, so that the faults inside and outside the area can be distinguished.

[0081] 2.2 Secondary distinction;

[0082] The secondary distinction divides the types of faults in the area according to the difference in the waveform characteristics of the subsequent traveling wave, and at least the proximal area fault, the near-end area fault and the non-proximal area fault. Specifically, the step mainly analyzes the difference in the waveform characteristics of the traveling wave for the proximal area and the near-end area fault, so as to distinguish the specific fault type.

[0083] It should be noted that the range of the proximal area fault is 0-2T s v1, the range of the near-end area fault is ≥2T s v1.

[0084] Wherein, T s is the sampling period of the protection device of the power transmission system. The secondary distinction step includes:

[0085] According to the general expression of the initial line mode voltage traveling wave and the classification and amplitude polarity of the first subsequent traveling wave, the proximal area fault is divided, and the possible protection dead zone in the proximal area in the foregoing step is analyzed. According to formula (1), the general expression of the initial fault traveling wave of the proximal fault and the remaining faults has a difference, which can be used for fault identification; for the remaining proximal area faults, the first subsequent traveling wave in the fault waveform is the first type of subsequent traveling wave, and according to formula (2), the amplitude polarity is positive, which is opposite to the polarity of the initial fault traveling wave, and the fault waveform presents an up convex feature, while for the far area fault (including the opposite end) and the fault outside the area, the waveform presents a concave feature, which can be used for fault identification.

[0086] According to the voltage drop rate and extreme value time of the initial traveling wave, the near-end fault is divided. The possible protection dead zone in the near-end area in the foregoing step is analyzed. The proximal near-end fault and the fault outside the area have differences in the initial traveling wave voltage drop rate and extreme value time, and the expression is constructed as follows:

[0087]

[0088] In the formula, Δf3 and Δf4 are respectively the voltage drop amount of the proximal near-end fault and the fault outside the area under the same scale.

[0089] Further, the extreme value time expression is constructed, and the algorithm is as follows:

[0090]

[0091] Where p is the general expression for near-end faults and out-of-zone faults; τ1 represents a time constant term that is independent of the transition resistance, τ2 represents a time constant term that varies with the transition resistance, K1 represents the common factor of the amplitudes of the two exponential terms, and K2 represents the weight coefficient. Regarding the function extreme value time, K1 has no effect on the extreme value time; the larger K2 is and the smaller τ2 is, the shorter the corresponding extreme value time is; in addition, the smaller the τ value is, the faster the corresponding term decays; the larger K2 is, the greater the impact of the corresponding term is; τ c1 Represents the time constant term that is independent of capacitance; τ c2 Represents the time constant term that varies with capacitance.

[0092] Therefore, when K2 is 4.85, it indicates that the fault type is an intra-zone near-end fault; when K2 is 1, it indicates that the fault type is an extra-zone fault.

[0093] As the transition resistance decreases and the fault distance increases, the influence of subsequent traveling waves gradually weakens in the near-to-end fault waveform, gradually approaching a port fault, and the voltage drop rate is slow. When the transition resistance is large, the extreme value time of the near-to-end fault waveform gradually decreases as the fault distance increases. However, out-of-zone faults have a larger voltage drop rate and extreme value time, which can be used for fault identification.

[0094] 2.3 Non-near-end fault;

[0095] After filtering out the proximal area faults and the proximal opposite area faults from the intra-area faults, the non-proximal area faults are obtained.

[0096] 3. Based on the distinction of fault types, a nonlinear optimization algorithm is used to fit the traveling wave of the line mode voltage under different fault types to generate a matching waveform.

[0097] The nonlinear optimization algorithm of this embodiment is the SQP algorithm. The basis function and the objective function are selected as follows:

[0098] The basis function for traveling wave fitting of line mode voltage under different fault types is the traveling wave expression formula, and the traveling wave expression formula is combined by equation groups to obtain the basis function:

[0099]

[0100] Objective function:

[0101]

[0102] Where n is an ordinal number; X n To obtain the matching result, the optimization goal is to make the matching waveform as close as possible to the initial fault traveling wave waveform.

[0103] 4. Construct multiple fault type criteria based on the waveform characteristics of the matching waveform; the fault type criteria include in-zone fault criteria and out-of-zone fault criteria.

[0104] 4.1 In-zone fault criterion;

[0105] The in-zone fault criterion at least comprises a window length matching error criterion of an adaptive time window; the window length matching error of the adaptive time window is used to improve the sensitivity of the protection device, and on this basis, the matching wave form feature is used to increase the protection range, so as to realize the full-line fast protection.

[0106] The window length of the adaptive time window is the product of the line fault propagation time reference value and a constant coefficient, and the window length matching error criterion of the adaptive time window comprises the following steps:

[0107] The error sum of the traveling wave wave form of the line mode voltage under the in-zone fault state and the matching wave form is calculated, and the error sum is compared with a preset error setting value to obtain a comparison result. The formula for selecting the adaptive window length and the matching error is as follows:

[0108]

[0109] In the formula, t n1 represents the adaptive time window length, K represents a constant less than 1 (set to 0.9), ε i represents the error between the reference wave form and the actual wave form, i represents the i value corresponding to the same sign when the error is continuous for 5 times or more, and S i represents the error setting value of the criterion. n.set n.set The criterion is set according to the out-of-zone metallic fault under the condition of a signal-to-noise ratio of 30 dB.

[0110] When S n >S n.set , the comparison result is in-zone fault, and thus the real-time fault type is in-zone fault.

[0111] When S n ≤S n.set , the comparison result is out-of-zone fault, and then the next step of the out-of-zone fault criterion is entered.

[0112] 4.2 Out-of-zone fault criterion;

[0113] The in-zone fault criterion has a protection dead zone for the near-end region fault and the near-opposite-end region fault, and the distinction from the out-of-zone fault specifically relies on the out-of-zone fault criterion. The method is to construct a protection criterion in combination with the wave form feature of the matching wave form, so as to increase the protection range. The expression and setting principle thereof comprise a voltage loss identification criterion, a maximum value time criterion, and a wave form graph criterion.

[0114] 4.2.1 Voltage loss identification criterion;

[0115] ​The calculation process of the voltage loss identification criterion includes:

[0116] Calculate the voltage loss value of the line mode voltage under the fault state at different time points within the adaptive time window and the voltage loss value of the matching waveform, and compare the minimum value with the preset voltage loss setting value to obtain the first fault type. Specifically, the voltage loss identification criterion formula is:

[0117]

[0118] Where p i 、p j The waveform of the line mode voltage under fault conditions and the voltage loss value of the matching waveform within a given time window are respectively represented. The voltage loss when the fault is set outside the zone through high resistance grounding is used for fault setting. In order to enhance the ability of the protection to withstand high resistance faults, the transition resistance is taken as 500Ω and the voltage loss setting value is P n.set .

[0119] Therefore, when P n >P n.set When the first fault type is an out-of-zone fault; when P n <P n.set The first fault type is an intra-zone fault.

[0120] 4.2.2 Extreme value time criterion;

[0121] The calculation process of the extreme time criterion includes:

[0122] Calculate the first minimum time of the line mode voltage under the fault state and the first minimum time of the matching waveform, and compare the minimum value with the preset extreme time setting value to obtain the second fault type. The judgment formula is:

[0123]

[0124] Where, t i is the first minimum point of the matching waveform; the minimum time of the line mode voltage waveform under the fault state and the matching waveform are obtained simultaneously, and the smaller value between the two is taken for judgment setting, and the fault distance l is taken n =1-2T s In order to enhance the protection ability to withstand transition resistance, the transition resistance is set to 500Ω and the extreme value time setting value is t set .

[0125] Therefore, when t i >t set The second fault type is an out-of-zone fault; when t i <t set The second fault type is an intra-zone fault.

[0126] 4.2.3 Waveform graph criteria;

[0127] The calculation process of the waveform graph criterion includes:

[0128] Calculate the second-order derivative at the maximum value of the matching waveform and determine the concavity of the graph by comparing it with the value of 0 to obtain the third fault type. Specifically, the formula is:

[0129]

[0130] Where m is the number of time sampling points corresponding to the first maximum value; f″(t n ) is the matching waveform at time point t n The second derivative at t n2 The time window length selected for the graphical criterion.

[0131] When the first, second and third fault types are all out-of-zone faults, the actual fault type is out-of-zone fault;

[0132] When the first, second and third fault types are not all out-of-zone faults, the actual fault type is an in-zone fault.

[0133] 5. The traveling wave of the line mode voltage under the fault state is collected by the sensor. Based on the above content, the actual fault type is determined by the fault type judgment criterion within the adaptive time window.

[0134] Afterwards, the system notifies the protection device to perform corresponding protection actions based on the acquired actual fault type.

[0135] Example 1: Build the following on PSCAD / EMTDC Figure 2 The ±500kV true bipolar four-terminal flexible DC transmission system shown in the figure uses constant DC voltage control on the rectifier side and constant active power control on the inverter side. The DC line length is 300km and the line mode impedance Z c1 =251Ω, zero mode wave impedance Z c0 =380Ω, converter equivalent inductance L m =0.033H. Current limiting reactor inductance L b =0.1H; the protection sampling frequency is 50kHz, that is, T s =20μs, set a fault on the transmission line, where f1 is a non-near-end fault within the zone; f2 and f3 are the near-end faults (including the near-opposite end) at the protection R and I exits (at least one of the left and right sides is not an overhead line); f4 represents an out-of-zone fault.

[0136] Figure 3 The following is a schematic diagram of waveform simulation matching different fault conditions of the proposed protection. f =0Ω and R f= 200Ω and the near-end and near-opposite faults to generate matching waveforms. Figure 3 It can be seen that there is a large error between the actual value and the matching value when there is an intra-zone fault (non-port) and an intra-zone near-end fault; the actual value and the matching value are highly consistent when there is an extra-zone fault and an intra-zone near-opposite-end fault; when a metallic fault occurs at the intra-zone near-end and near-opposite-end, the actual value waveform shows a fluctuating characteristic, and the first subsequent traveling wave shows an upward convex characteristic when there is a near-end fault. When there is a near-opposite-end fault, the actual waveform extreme value time is short and the matching waveform is flat, which is consistent with the previous analysis.

[0137] Furthermore, the protection performance under different fault conditions is verified. Table 1 shows the setting values ​​as follows:

[0138] Set value <![CDATA[f″(t n )]]> P n.set ]]> <![CDATA[t set ]]> <![CDATA[S n.set =80]]> <0 0.25 5Ts

[0139] The protection action results of Table 1 are shown in Tables 2 and 3 as follows:

[0140]

[0141]

[0142] Table 2

[0143]

[0144]

[0145] Table 3

[0146] Table 2 shows the protection action for faults within the zone, and Table 3 shows the protection action for faults outside the zone. Simulation results show that the proposed protection principle can reliably identify faults across the entire line, and maintains high sensitivity even in the event of high-resistance faults.

[0147] The above are only preferred embodiments of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the technical principles of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A single-ended protection method for a flexible DC transmission line based on waveform matching, characterized in that the steps include: Establish the general expression of the traveling wave of the initial line-mode voltage under different fault types; Performing dual differentiation of fault types based on the traveling wave expression formula to obtain differentiated fault types; The double distinction is provided with a primary distinction and a secondary distinction; The primary differentiation is to classify the fault type into an intra-zone fault and an extra-zone fault according to the time difference between the initial traveling wave and the subsequent traveling wave in the subsequent traveling wave time window; the intra-zone fault is further divided into an intra-zone proximal fault and an intra-zone non-proximal fault; The secondary differentiation classifies the types of the faults in the zone into at least a proximal zone fault, a proximal opposite zone fault, and a non-proximal zone fault based on the difference in waveform characteristics of the subsequent traveling waves; Based on the fault type differentiation, a nonlinear optimization algorithm is used to fit the traveling wave of the line mode voltage under different fault types to generate a matching waveform; Constructing multiple fault type criteria based on the waveform characteristics of the matching waveform; The sensor collects the traveling wave of the line mode voltage under the fault state, and determines the actual fault type according to the fault type judgment criterion within the adaptive time window; The system notifies the protection device to perform corresponding protection actions based on the actual fault type; The nonlinear optimization algorithm is an SQP algorithm; The basis function for traveling wave fitting of the line mode voltage under the different fault types is the traveling wave expression formula, and the traveling wave expression formula is combined into an equation group; The fault type criterion includes an in-zone fault criterion and an out-of-zone fault criterion; The intra-area fault criterion at least includes a window length matching error criterion of the adaptive time window; The window length of the adaptive time window is the product of the line fault propagation time reference value and a constant coefficient, and the step of calculating the window length matching error criterion of the adaptive time window includes: Calculating the sum of errors between the traveling waveform of the line-mode voltage under the fault state within the adaptive time window and the matching waveform, and comparing the sum of errors with a preset error setting value to obtain a comparison result; When the comparison result is the intra-zone fault, the actual fault type is the intra-zone fault; When the comparison result is the out-of-zone fault, the determination process of the out-of-zone fault criterion is entered.

2. The single-ended protection method for flexible DC transmission lines based on waveform matching according to claim 1, characterized in that: The system in which the different fault types occur is a four-terminal flexible direct current transmission system; The process of establishing the traveling wave expression formula includes: The initial line mode voltage under different fault types is calculated using a composite mode domain equivalent circuit diagram to obtain a frequency domain expression; Then, performing an inverse Laplace transform on the frequency domain expression to obtain a time domain expression; The time domain expression obtains the general formula of the traveling wave of the initial traveling wave under different fault conditions through a classification method.

3. The single-ended protection method for flexible DC transmission lines based on waveform matching according to claim 2, characterized in that: The primary distinction includes: The subsequent traveling waves are divided into first-category subsequent traveling waves and second-category subsequent traveling waves according to the number of refractions and reflections at the wave impedance discontinuity point experienced by the subsequent traveling waves, and the amplitudes of the subsequent traveling waves are calculated.

4. The single-ended protection method for flexible DC transmission lines based on waveform matching according to claim 3 is characterized in that: The primary distinction also includes: According to the classification of the subsequent traveling waves and the time difference between the initial traveling wave and the first subsequent traveling wave, the fault type is divided into an in-zone near-end fault, an in-zone non-near-end fault and an out-of-zone fault.

5. The single-ended protection method for flexible DC transmission lines based on waveform matching according to claim 4 is characterized in that: The secondary distinction includes: The proximal area fault is divided according to the traveling wave expression formula of the initial line mode voltage traveling wave and the classification and amplitude polarity of the first subsequent traveling wave.

6. The single-ended protection method for flexible DC transmission lines based on waveform matching according to claim 5, characterized in that: The secondary distinction also includes: The near-opposite-end area fault is divided according to the voltage drop rate and extreme value time of the initial traveling wave.

7. The single-ended protection method for flexible DC transmission lines based on waveform matching according to claim 6, characterized in that: The secondary distinction also includes: The non-proximal area fault is obtained after the proximal area fault and the proximal opposite area fault are screened out from the intra-area faults.

8. The single-ended protection method for flexible DC transmission lines based on waveform matching according to claim 1, characterized in that: The out-of-zone fault criterion includes a voltage loss identification criterion, an extreme value time criterion, and a waveform diagram criterion; The calculation process of the voltage loss identification criterion includes: Calculating voltage loss values ​​of the line mode voltage under the fault state and voltage loss values ​​of the matching waveform at different time points within the adaptive time window, and comparing the minimum value with a preset voltage loss setting value to obtain a first fault type; The calculation process of the extreme value time criterion includes: Calculating the first minimum time of the line mode voltage and the first minimum time of the matching waveform under the fault state, and comparing the minimum value with a preset extreme time setting value to obtain a second fault type; The calculation process of the waveform graph criterion includes: Calculating the second-order derivative at the maximum value of the matching waveform, and determining the concavity and convexity of the graph by comparing it with a value of 0 to obtain a third fault type; When the first, second and third fault types are all the out-of-zone faults, the actual fault type is the out-of-zone fault; When the first, second, and third fault types are not all the out-of-zone faults, the actual fault type is the in-zone fault.

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