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

Abstract

A waveform matching-based flexible direct-current transmission line single-ended protection method comprises the steps of establishing a traveling wave expression general formula, performing dual distinguishing on fault types, and generating a matching waveform based on the distinguished fault types. Constructing a plurality of fault type criteria based on the matching waveform; and collecting the traveling wave of the line-mode voltage in a fault state, and judging the type of a real-time fault through a fault type criterion. And the system notifies the protection device to perform corresponding protection action based on the live fault type. According to the method, the accurate matching waveform is constructed based on double distinguishing, and the auxiliary criterion is constructed by using the waveform characteristics of the matching waveform, so that the protection can accurately identify different fault types, no dead zone exists in the protection, and the protection reliability is high; in addition, all criteria of the protection method are judged based on the characteristic quantity of the waveform, and the method is easy to implement and has good engineering application value.

Description

Technical Field

[0001] The 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 DC transmission system, the safety and reliability of the transmission line are crucial. However, the transmission line is often the part with the highest probability of system failure. Once a failure occurs, it may cause the fault current to rise rapidly, posing a serious threat to system safety. In order to avoid this, effective protection measures need to be adopted to quickly clear the fault to ensure the system's speed requirements.

[0003] At present, the single-terminal protection in operation mainly identifies faults based on voltage mutation. This protection method has the advantages of simple principle and easy implementation. However, in the case of long-distance high-resistance faults, its sensitivity is insufficient. For example, in the "Single-terminal protection method suitable for parallel multi-terminal high-voltage direct current transmission system" disclosed in Chinese patent CN117277234B, it is mentioned that when traditional protection methods analyze out-of-region faults, they generally only consider the situation where the fault wave only passes through the converter station port once. However, when there are many converter stations in the multi-terminal high-voltage direct current transmission system, the out-of-region fault wave may pass through the converter station port multiple times. The change of the wave is different from that 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 withstand 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 boundary elements of the transmission line, and extracts the fault characteristics of specific frequency bands through different time-frequency processing methods, thereby realizing fault identification. This method improves the ability of protection to withstand 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 the frequency domain characteristics of the fault current traveling wave are extracted through S transformation, the reliability and anti-interference ability of the relay protection are improved; however, this method is relatively complex and has high requirements on equipment.

[0005] In addition, many technicians have proposed improved single-ended quantity protection methods based on waveform analysis. However, such protection methods are greatly affected by subsequent traveling waves and cannot protect the entire length of the line. For example, the "Single-ended quantity waveform similarity protection method applicable to flexible DC transmission lines" disclosed in Chinese patent CN114629089B mentions that the detection accuracy of existing flexible DC transmission line protection schemes under different fault types, fault resistances and fault distances needs to be improved, and has high requirements for high sampling rates and strict data synchronization.

[0006] Therefore, there is a need to improve this type of traveling wave protection method to improve 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] Based on the traveling wave expression formula, the fault type is double-differentiated to obtain the differentiated fault type;

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

[0013] The primary differentiation divides the fault type into an in-zone fault and an out-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 in-zone fault is further divided into an in-zone proximal fault and an in-zone non-proximal fault;

[0014] The secondary differentiation divides the types of the faults in the zone into at least proximal zone faults, proximal opposite zone faults and non-proximal zone faults according to the difference of the 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, the frequency domain expression is subjected to an inverse Laplace transform to obtain a time domain expression;

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

[0024] Optionally, the primary differentiation includes:

[0025] According to the number of refractions and reflections of the subsequent traveling waves at the wave impedance discontinuity point, the subsequent traveling waves are divided into first-type subsequent traveling waves and second-type 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 types are divided into in-zone near-end faults, in-zone non-near-end faults and out-of-zone faults.

[0028] Optionally, the secondary differentiation includes:

[0029] The proximal area fault is divided according to the general 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 the 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 by 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 an 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 wave waveform of the line mode voltage under the fault state in 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 in-zone fault, the actual fault type is the in-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 diagram 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 the first fault type;

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

[0046] 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;

[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 judging the concavity and convexity of the graph by comparing with the 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 distinction, 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, the protection reliability is strong, and all the criteria of the protection method are based on the characteristic quantity 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 drawings required for use in the embodiments will be briefly introduced below. 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 creative labor.

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

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

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

[0057] In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, 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 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 creative work are 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, further definition and explanation thereof is not required 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 flexible DC transmission lines based on waveform matching, wherein: Figure 1 As shown, the main steps include:

[0061] 1. Establish a general formula for the traveling wave expression of the initial line mode voltage traveling wave under different fault types; that is, analyze the expressions of the initial line mode voltage traveling wave under different fault types, and summarize the general formula for the traveling wave expression of the initial traveling wave under different fault conditions. Specifically, in this embodiment, the occurrence system of different fault types is described by taking a four-terminal flexible direct current transmission system as an example.

[0062] The process of establishing the general expression of traveling waves includes:

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

[0064] Then perform an inverse Laplace transform on the frequency domain expression to obtain the time domain expression;

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

[0066]

[0067] Wherein, t is time; c is the ordinal number of the fault type, which takes values ​​of 1, 2, 3, and 4; f1 represents a non-proximal fault within the area; f3 represents a proximal fault within the area; f4 represents an external fault; and f2 represents a proximal fault within the area; m and z are both constants; a and b are parameters related to the characteristics of the transmission line and the fault condition, and their values ​​will be affected by many factors, such as the resistance, inductance, capacitance and other parameters of the line, as well as the type, location and transition resistance of the fault. Therefore, the specific parameters are obtained by modeling and calculating the transmission system.

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

[0069] 2. Based on the general expression of traveling wave, the fault types are double-differentiated to obtain the differentiated fault types.

[0070] Among them, double distinction has primary distinction and secondary distinction;

[0071] 2.1 One-time distinction;

[0072] The first distinction uses the subsequent traveling wave time window t n1 (t n1 <2l / v1, l is the line length, v1 represents the line mode wave velocity of the transmission line), the degree to which the initial traveling wave is affected by the subsequent traveling wave is used to distinguish between internal and external faults. Specifically, the fault type is divided into internal fault and external fault according to the time difference between the initial traveling wave and the subsequent traveling wave in the time window of the subsequent traveling wave.

[0073] Among them, the first differentiation includes the steps:

[0074] According to the number of refraction times and reflection times that the subsequent traveling wave experiences at the wave impedance discontinuity point, the subsequent traveling wave is divided into the first type of subsequent traveling wave and the second type of subsequent traveling wave, and the amplitude of the subsequent traveling wave is calculated. The amplitude formula is:

[0075]

[0076] In the formula, u -21 represents the line-mode voltage traveling wave reflected by the line-mode component at the fault point, u -20 represents the line-mode voltage traveling wave transmitted to the near end by the zero-mode component at the fault point. This zero-mode component only undergoes refraction and reflection at one impedance discontinuity point. The corresponding subsequent traveling wave is denoted as the first type of subsequent traveling wave; u -31 represents the line-mode component that is reflected by the far end and then refracted by the fault point, u -30 represents the line-mode component that is reflected by the far end and then transmitted to the near end by the zero-mode component at the fault point. This zero-mode component undergoes refraction and reflection at two impedance discontinuity points. The corresponding subsequent traveling wave is denoted 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 impedances of the transmission line respectively; R f is the fault transition resistance.

[0077] According to the classification of the subsequent traveling wave and the time difference between the initial traveling wave and the first subsequent traveling wave, the fault type is divided into in-zone fault and out-of-zone fault. Specifically, according to different fault positions, the fault is divided into in-zone near-end fault, in-zone non-near-end fault, and out-of-zone fault, and the arrival times of the subsequent traveling waves at different positions are analyzed respectively. The formula can be obtained as:

[0078]

[0079] In the formula, 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 first-type subsequent traveling wave, t 21 represents the time difference between the initial traveling wave and the first second-type subsequent traveling wave; t1 is the arrival time of the first subsequent traveling wave in the in-zone near-region fault, t2 is the arrival time of the first subsequent traveling wave in the in-zone far-region fault, and t3 is the arrival time of the first subsequent traveling wave in the in-zone near-region fault.

[0080] According to the above analysis of the subsequent traveling waves at different fault positions, it can be obtained that there are differences in the time of the first subsequent traveling wave felt at the protection for the in-zone non-near-end fault, in-zone near-end fault, and out-of-zone fault, and t1 < t3, t2 < t3 always hold. Therefore, there must be a time window length tn1 <2l / v1, so that the non-proximal faults in the zone are greatly affected by the subsequent traveling waves, while the faults outside the zone and the proximal faults in the zone are not affected by the subsequent traveling waves, so the faults inside and outside the zone can be distinguished.

[0081] 2.2 Secondary differentiation;

[0082] Secondary differentiation is to classify the types of faults in the area into at least near-end area faults, near-opposite-end area faults and non-near-end area faults based on the differences in the waveform characteristics of the subsequent traveling waves. Specifically, this step mainly analyzes the differences in the waveform characteristics of the traveling waves for the near-end area and near-opposite-end area faults, thereby distinguishing the specific fault types.

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

[0084] Among them, T s is the sampling period of the protection device of the power transmission system. The secondary differentiation 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 near-end area fault is divided, and the possible protection dead zone in the near-end area of ​​the area in the previous step is supplemented. From formula (1), it can be seen that the general expression of the traveling wave of the initial fault traveling wave of the near-end fault is different from that of the other fault conditions, which can be used for fault identification; for the other near-end area faults, the first subsequent traveling wave in the fault waveform is the first type of subsequent traveling wave, and from formula (2), it can be seen that its amplitude polarity is positive, which is opposite to the polarity of the initial fault traveling wave, and the fault waveform presents a 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] The near-end fault is divided according to the voltage drop rate and extreme value time of the initial traveling wave. A supplementary analysis is performed for the possible existence of a protection dead zone in the near-end area in the previous step. There are differences in the initial traveling wave voltage drop rate and extreme value time between the near-end fault in the area and the fault outside the area, so the expression is constructed as follows:

[0087]

[0088] Where Δf3 and Δf4 are the voltage drops of the near-end fault in the zone and the fault outside the zone under the same scale, respectively.

[0089] Further, the extreme value time expression is constructed, the formula is:

[0090]

[0091] In the formula, p is the general expression of near-end fault and out-of-zone fault; τ1 represents the time constant term that is independent of transition resistance, τ2 represents the time constant term that changes with transition resistance, K1 represents the common factor of the amplitude of the two exponential terms, and K2 represents the weight coefficient. For the function extreme time, K1 has no effect on the extreme time; the larger K2 is and the smaller τ2 is, the smaller the corresponding extreme time is; in addition, the smaller the τ value is, the faster the corresponding term decays; the larger K2 is, the greater the influence of the corresponding term is; τ c1 Represents a 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 means that the fault type is an intra-zone near-end fault; when K2 is 1, it means 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-opposite area fault waveform, gradually approaches the port fault, and the voltage drop rate is slow; when the transition resistance is large, the extreme value time of the near-opposite area fault waveform gradually decreases as the fault distance increases. The out-of-area fault has a large 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 selected as the SQP algorithm. The basis function and the objective function are selected as follows:

[0098] The basis function for the traveling wave fitting of the line mode voltage under different fault types is the general expression of the traveling wave, and the general expression of the traveling wave is combined by equation groups, and the basis function can be obtained as follows:

[0099]

[0100] Objective function:

[0101]

[0102] In the formula, n is an ordinal number; X n For 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 Fault judgment criteria within the zone;

[0105] The fault judgment criteria within the zone at least include the window length matching error judgment criteria of the 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, it is combined with the matching waveform characteristics to increase the protection range, thereby realizing full-line fast-acting protection.

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

[0107] Calculate the error sum between the traveling waveform and the matching waveform of the line mode voltage under the fault state in the adaptive time window, and compare the error sum with the preset error setting value to obtain the comparison result. The formula for adaptive window length selection and matching error is:

[0108]

[0109] Where, t n1 represents the adaptive time window length, K represents a constant less than 1 (set to 0.9), represents the adaptive matching error and, ε i Indicates the error between the reference waveform and the actual waveform, i indicates 5 or more consecutive ε i Take the corresponding i value when the sign is the same. Suppose the error setting value of the criterion is S n.set ; S n.set Adjustment is carried out according to the out-of-zone metallic fault under the condition of signal-to-noise ratio of 30dB.

[0110] When S n >S n.set When the comparison result is an internal fault, the actual fault type is an internal fault;

[0111] When S n ≤S n.set When the comparison result is an out-of-zone fault, the next step of the out-of-zone fault judgment process is entered.

[0112] 4.2 Out-of-zone fault judgment criteria;

[0113] According to the in-zone fault judgment criteria, there is a protection dead zone for the near-end regional fault and the near-opposite regional fault. The distinction between it and the out-of-zone fault depends specifically on the out-of-zone fault judgment criteria. The method is to construct a protection judgment criterion by combining the waveform characteristics of the matching waveform, thereby increasing the protection range. Its expression and setting principles include voltage loss identification criterion, extreme time criterion and waveform graph criterion.

[0114] 4.2.1 Voltage loss identification criteria;

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

[0116] Calculate the voltage loss value of the line mode voltage under the fault state at different time points in 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] In the formula, p i 、p j The waveform of the line mode voltage under the fault state and the voltage loss value of the matching waveform in a given time window are respectively represented. The voltage loss when the outside of the zone is grounded through high resistance 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 value 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 criterion 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 and the matching waveform under the fault state is obtained simultaneously, and the smaller value between the two is taken for judgment and setting, and the fault distance l is taken n =1-2T s v1 is used to set the criterion for high-resistance fault. To enhance the ability of protection 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 judge 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, and the actual fault type is determined by the fault type judgment criterion within the adaptive time window based on the above content.

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

[0135] Example 1: Build the following on PSCAD / EMTDC Figure 2 The ±500kV true bipolar four-terminal flexible DC transmission system is shown. The rectifier side adopts constant DC voltage control, and the inverter side adopts constant active power control; the DC line length is 300km, and the line mode wave impedance of the transmission line is Z c1 =251Ω, zero mode wave impedance Z c0 =380Ω, inverter 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, a fault is set on the transmission line, where f1 is a non-proximal fault within the zone; f2 and f3 are proximal faults (including the proximal 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 schematic diagram of waveform simulation matching different fault conditions of the proposed protection is shown in Figure 2. f =0Ω and R f= 200Ω and near-end and near-opposite-end fault matching waveform generation, 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; when there is an out-of-zone fault and an intra-zone near-opposite-end fault, the actual value and the matching value are highly coincident; when a metallic fault occurs at the intra-zone near-end and near-opposite-end, the actual value waveform shows a fluctuating characteristic, and when there is a near-end fault, the first subsequent traveling wave shows an upward convex characteristic, and 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] Setting value <![CDATA[f″(t n )]]> <![CDATA[P n.set ]]> <![CDATA[t set ]]> <![CDATA[S n.set =80]]> <0 0.25 5Ts

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

[0140]

[0141]

[0142] Table 2

[0143]

[0144]

[0145] Table 3

[0146] Among them, Table 2 shows the protection action for faults within the zone, and Table 3 shows the protection action for faults outside the zone. The simulation results show that the proposed protection principle can reliably identify faults within the entire line range, and still maintains a high sensitivity when a high-resistance fault occurs.

[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 flexible DC transmission lines 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; Based on the traveling wave expression formula, the fault type is double-differentiated to obtain the differentiated fault type; The double distinction is provided with a primary distinction and a secondary distinction; The primary differentiation divides the fault type into an in-zone fault and an out-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 in-zone fault is further divided into an in-zone proximal fault and an in-zone non-proximal fault; The secondary differentiation divides the types of the faults in the zone into at least proximal zone faults, proximal opposite zone faults and non-proximal zone faults according to the difference of the 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.

2. The single-ended protection method for flexible DC transmission line based on waveform matching according to claim 1 is 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, the frequency domain expression is subjected to an inverse Laplace transform to obtain a time domain expression; The time domain expression obtains the general traveling wave expression formula of the initial traveling wave under different fault conditions through classification method.

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

4. The single-ended protection method for flexible DC transmission line 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 types are divided into in-zone near-end faults, in-zone non-near-end faults and out-of-zone faults.

5. The single-ended protection method for flexible DC transmission line 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 general 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 line based on waveform matching according to claim 5 is 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 line based on waveform matching according to claim 6 is 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 line based on waveform matching according to any one of claims 1 to 7, characterized in that: The nonlinear optimization algorithm is an SQP algorithm; The basis function for the 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 by equation groups.

9. The single-ended protection method for flexible DC transmission line based on waveform matching according to claim 8, characterized in that: 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 an 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 wave waveform of the line mode voltage under the fault state in 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 in-zone fault, the actual fault type is the in-zone fault; When the comparison result is the out-of-zone fault, the determination process of the out-of-zone fault criterion is entered.

10. The single-ended protection method for flexible DC transmission line based on waveform matching according to claim 9, 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 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 the first fault type; The calculation process of the extreme value time criterion includes: 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 calculation process of the waveform graph criterion includes: Calculating the second-order derivative at the maximum value of the matching waveform, and judging the concavity of the graph by comparing with the 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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