A protection method for new energy lines with high ratio based on the polarity difference of the first four voltage traveling waves

Abstract

The present invention provides a protection method for a new energy line with a high proportion of polarity differences of the first four voltage traveling waves. The method first derives the expression of the refraction-reflection coefficient of the wave impedance discontinuity point and obtains its value range. Then, according to the refraction-reflection process of the traveling wave when faults occur at different positions, the expression of the line mode voltage traveling wave at the protection installation is derived. The polarity characteristics of the first four voltage traveling waves when faults occur at different positions can be obtained by using the value range of the refraction-reflection coefficient, and it is concluded that the polarities of the first four voltage traveling waves are the same when there is a fault outside the zone, and the polarities of the first four voltage traveling waves are different when there is a fault inside the zone, and fault identification is performed based on this. The method uses the polarity characteristics of the traveling wave for fault identification. Compared with time characteristics and amplitude characteristics, polarity characteristics are easier to identify and have higher reliability, small calculation amount, and wide application range. At the same time, this scheme is a single-ended quantity protection, which has the advantages of no need for communication and fast action speed, and can reliably identify faults.

Description

Technical Field

[0001] The present invention relates to the field of AC line fault identification, and in particular to a high-proportion new energy line protection method based on the polarity difference of the first four voltage traveling waves. Background Art

[0002] Renewable energy represented by wind power and photovoltaic power is connected to the grid through power electronic interfaces, which promotes the widespread application of power electronic equipment on the power supply side. With the access of large-scale new energy and the re-electrification process on the load side, a large number of power sources, loads, energy storage and other equipment with different characteristics are connected to the existing power system through power electronics as interfaces. The power system is developing in the direction of "high proportion of new energy" and "high proportion of power electronic equipment", which has become one of the important characteristics of the new generation of power systems.

[0003] Compared with the traditional power system with synchronous generators as the main power source, the fault characteristics of the high-proportion renewable energy power system are more complex and diverse, and the evolution mechanism of the whole fault process is still unclear. The system short-circuit current level is reduced, the power supply response is affected by the control effect, and the relay protection characteristics change. The insufficient fault tolerance of the renewable energy unit itself can easily lead to chain disconnection from the grid, which challenges the sensitivity, rapidity, selectivity and reliability of conventional protection, seriously affecting the accurate identification and rapid isolation of faults.

[0004] Prior art, such as Chinese patent, application number: CN2012100904473, publication number: CN102623972A, discloses a differential protection method for the high-frequency component processing capacity of a single-ended transient signal of a transmission line. When the line is short-circuited, the fault generates a large amount of high-frequency signals that propagate to both ends. Due to the distributed capacitance of the busbar to the ground, the high-frequency signal will be significantly reduced when passing through the busbar. The high-frequency component value detected on the near-fault side of the busbar is always greater than the value detected on the far-fault side. The present invention utilizes the above principle to collect fault signals on both sides of the busbar inside and outside the line protection installation, and uses a complete reconstruction filter group to losslessly divide the fault transient signal, and then calculates a certain processing capacity difference of each sub-band component on both sides of the busbar inside and outside, and judges whether it is an in-area fault or an out-of-area fault by the difference according to the majority voting mechanism. The signal used to construct the criterion is a plurality of sub-band high-frequency signals covering the entire high-frequency band, and the frequency of each sub-band can be adaptively adjusted as the sampling frequency changes. Application number: CN2014101422941, publication number: CN103913676A, discloses a single-end fault location method for a transmission line based on a variable traveling wave identification time window. The method constructs an improved distributed parameter method through online estimation of the impedance of the opposite end system, effectively improves the irregularity of the error distribution of the conventional method, and improves the feasibility of constructing a combined location algorithm based on the power frequency quantity location error; calculates a variable fault distance window according to different fault conditions and error laws, locks the corresponding time area of ​​the fault traveling wave reflection wave, and thus defines a variable time window for traveling wave reflection wave identification; based on the variable time window, comprehensively considers factors such as the busbar wiring mode of the measuring end and the opposite end, the influence of the adjacent busbar reflection wave, and uses the traveling wave method to construct a reflection wave identification method and a location method within the variable time window.

[0005] Application number: CN201911229871x, publication number: CN110736900A, discloses a method for single-phase grounding line selection of distribution lines based on directional traveling waves. When a traveling wave disturbance occurs in the line, the polarities of zero-mode current traveling waves and zero-mode voltage traveling waves at the head ends of all trunk lines and branch lines are collected, and the directions of the initial traveling waves of all trunk lines are determined; when the initial traveling wave of a trunk line is a reverse traveling wave, the line is determined to be a faulty line, and the other lines are normal lines; after the faulty trunk line is determined, the directions of the initial traveling waves of all branch lines connected to the trunk line are continued to be determined; when the initial traveling waves of all branch lines are forward traveling waves, it is determined that the fault occurs in the trunk line; when the initial traveling wave of a branch line is a reverse traveling wave, the branch line is determined to be a faulty line. Application number: CN202111298309x, publication number: CN114019413A, discloses a single-pole grounding fault distance measurement method based on short-time active locking of submodules, which judges whether a single-pole grounding fault occurs according to the DC voltage ratio of the positive and negative lines. When a fault occurs, the fault line is quickly determined according to the sequence of timing changes detected by the converter stations at each end; during the non-locking period, several submodules in a certain phase that are in the put-in state and the submodule input current direction is negative or submodules in the cut-off state and the submodule input current direction is positive are selected for short-time locking, and a DC voltage mutation is actively generated at the converter station exit for fault distance measurement, and the binary recursive singular value decomposition is used to accurately detect the traveling wave emission and arrival time of the mutation, and the fault distance is accurately solved by the time difference between them to achieve accurate distance measurement. Application number: CN2018112366430, publication number: CN109301800A, discloses a distribution line adaptive protection method and system based on traveling waves, belonging to the field of line protection technology. At the measurement point where the fault line is far away from the busbar, the initial current traveling wave of the fault phase of a single-phase grounding fault is large, and the non-fault phase is zero; at the measurement point close to the busbar, the initial traveling wave amplitude of the fault phase of a single-phase grounding fault is twice that of the non-fault phase, and the polarity is opposite. At the measurement point where the fault line is far away from the busbar, the initial current traveling wave of the fault phase of a two-phase grounding fault is large, and the non-fault phase is zero; at the measurement point close to the busbar, the initial current traveling waves of the fault phase and the non-fault phase of the two-phase grounding fault have no obvious characteristics. Application number: CN201710397833x, publication number: CN107315130A, discloses a transmission line fault distance measurement method using current traveling waves and voltage traveling waves at both ends of the line, belonging to the field of power system fault distance measurement technology. When a transmission line fails, the current traveling wave and voltage traveling wave of the line are obtained from the measuring points at both ends of the line respectively; the reflected wave head of the current traveling wave and voltage traveling wave obtained at both ends of the line are identified by continuous wavelet transform, and four groups of fault distance data are obtained according to the initial traveling wave head and the transmitted wave head. The fault distance is obtained by comprehensive adjustment of the four groups of data using the global geometric average optimal method, thereby realizing the fault distance measurement of the transmission line.Application number: CN2014106192611, publication number: CN104655981A, discloses a traveling wave polarity comparison directional protection method using cross-sequence differential transformation, belonging to the field of power system relay protection. When a transmission line fails, the protection measurement device extracts the fault voltage and current signals; after the extracted traveling wave signals are cross-sequence differentially transformed, the cross-sequence differential transformation of the fault voltage and current can be obtained; the discriminant of the traveling wave polarity comparison directional protection can finally distinguish the positive direction fault from the reverse direction fault; the cross-sequence differential transformation algorithm is used to detect the fault traveling wave, although the interference signal is amplified, compared with the amplification effect of the fault information, the interference information is suppressed to a certain extent. Application number: CN2018108107486, publication number: CN108923398A, discloses a DC distribution network protection method based on voltage characteristic traveling wave similarity measurement. The present invention performs high-frequency sampling of the transient voltage and current at both ends of the line after a DC fault occurs, and converts it into a line-mode voltage forward wave on one side of the protection area and a line-mode voltage reverse wave on the opposite side; based on the reflection of the traveling wave caused by impedance mismatch at the fault point and the polarity difference between the forward and reverse traveling waves at both ends and the faults inside and outside the area, a globally optimal neighborhood diffusion compatible dynamic matching similarity measurement algorithm is used to reflect the similarity of the two waveforms, thereby reliably identifying the faults inside and outside the area. Application number: CN2019105671611, publication number: CN110609204A, discloses a single-phase grounding fault location method for distribution network based on morphological wavelet analysis and denoising. The present invention first analyzes the structure of the distribution network to set a reasonable installation point for the traveling wave device, and uses the existing user-side transformer as the installation point to minimize the cost investment. In order to filter out the pulse impact noise and Gaussian white noise in the transient power sampling signal, the morphological wavelet filtering method is used to filter the collected traveling wave signal to maximize the quality of the traveling wave waveform. The wavelet analysis method is then used to extract the traveling wave head information and read the corresponding wave head arrival time. Finally, the combined D-type traveling wave positioning method is used to reasonably select the measurement combination and calculate the branch most likely to fail and the distance between the fault point and the measurement point. Application number: CN2021100099854, publication number: CN112803478A, discloses a microgrid active injection single-ended protection method based on phase polarity characteristics. First, the three-phase voltage and current signals are collected in real time at the protection device on the m side of the protected line; then, the harmonic voltage and harmonic current are obtained by fast Fourier transform, and then the harmonic voltage on the n side of the protected line is obtained; then, the phase difference between the harmonic voltage and the harmonic current on the m side, and the phase difference between the harmonic voltage on the m side and the harmonic current on the n side of the protected line are used to accurately judge the internal and external faults through the phase difference product, which solves the problems in the prior art of microgrid protection that the threshold setting is complex, the communication is dependent on the large amount of communication data, the internal and external faults cannot be quickly and accurately distinguished, and the protection time may exceed the microgrid fault tolerance time limit.Application number: CN201611062107 4, publication number: CN106505536A, discloses a UHV DC line protection method based on the current mutation characteristics under the distributed parameter model. The protection processes the voltage and current signals measured by the transformers at both ends of the DC line through an intelligent electronic device (I ED), and then identifies the faults inside and outside the area according to the polarity of the current mutation. The protection adopts a distributed parameter model to compensate for the distributed capacitance current, eliminating the influence of the distributed capacitance current and data asynchrony. The protection criterion selects the component with a frequency band of [0, 100] Hz in the current information, avoiding the influence of characteristic harmonics on the DC side, electromagnetic coupling between bipolar DC lines, and lightning interference. The polarity of the current mutation at both ends of the line is a Boolean quantity, and the communication volume of the longitudinal channel is small. Application number: CN2014100870071, publication number: CN104914351A, discloses a fault location method for power system automation, which is based on genetic algorithm and signal spectrum analysis, and uses the branch line refraction wave related to the fault line in the power grid to locate the fault. The method includes: (1) power grid structure analysis and data pairing; (2) constructing an objective function and generating variable constraints; (3) signal spectrum analysis of the branch line refraction wave to determine the frequency component of the signal and the range of the frequency correction coefficient; (4) using a genetic algorithm to solve the optimal solution of the objective function and obtain the optimal wave speed; (5) obtaining the fault point location according to the optimal wave speed and outputting the result. The method fully considers the influence of the frequency and path of each branch line. Application number: CN2014103398928, publication number: CN104133156A, discloses a single-ended traveling wave fault distance measurement method for hybrid lines based on fault distance interval, which uses single-ended power frequency to preliminarily determine the fault distance interval, focuses on the research scope of traveling waves, and then specifically analyzes the laws of traveling waves to achieve accurate fault location. This method has high ranging accuracy, and there is no ranging dead zone for faults in the neighborhood of the connection point, which can correctly identify the connection point fault or determine the fault section. Application number: CN202111298309x, publication number: CN114019413A, discloses a single-pole grounding fault distance measurement method based on short-time active locking of submodules, which judges whether a single-pole grounding fault occurs according to the DC voltage ratio of the positive and negative lines. When a fault occurs, the fault line is quickly determined according to the sequence of timing changes detected by the converter stations at each end; during the non-locking period, several submodules in a certain phase that are in the put-in state and the submodule input current direction is negative or submodules in the cut-off state and the submodule input current direction is positive are selected for short-time locking, and a DC voltage mutation is actively generated at the converter station exit for fault distance measurement, and the binary recursive singular value decomposition is used to accurately detect the traveling wave emission and arrival time of the mutation, and the fault distance is accurately solved by the time difference between them to achieve accurate distance measurement.

[0006] However, none of the above existing technologies identify faults based on polarity characteristics, and their fault characteristics are not obvious, not easy to identify, and the amount of calculation is small; at the same time, the above technical solutions are not single-ended protection and cannot reliably identify faults. Therefore, exploring new protection principles applicable to scenarios with a high proportion of new energy access is of great significance to ensuring the safe and reliable operation of my country's high-proportion new energy power system. Traveling wave protection can ignore the impact of control strategies and has a fast action speed. It is one of the effective ways to solve the protection problems of new power systems. Summary of the invention

[0007] The purpose of the present invention is to provide a high-proportion new energy line protection method and its application based on the polarity difference of the first four voltage traveling waves. The method first derives the expression of the refraction-reflection coefficient of the wave impedance discontinuity point and obtains its value range, and then derives the expression of the line mode voltage traveling wave at the protection installation according to the refraction-reflection process of the traveling wave when faults occur at different positions. The polarity characteristics of the first four voltage traveling waves when faults occur at different positions can be obtained using the value range of the refraction-reflection coefficient, and it is concluded that the polarities of the first four voltage traveling waves are the same when faults occur outside the zone, and the polarities of the first four voltage traveling waves are different when faults occur within the zone, and fault identification is performed based on this.

[0008] A new energy line protection method based on estimating the arrival time of a current traveling wave comprises the following steps:

[0009] Step (1) derive the expression of reflection coefficient at the fault point and each busbar;

[0010] Step (2) considers the analysis of the traveling wave refraction-reflection process of cross transmission;

[0011] Step (3) deriving the relationship between the expression of the traveling wave of the wire mode voltage and the fault position;

[0012] Step (4) analyzing the polarities of the first four voltage traveling waves to determine whether it is an internal fault;

[0013] Preferably, when a line fault occurs in step (1), it is equivalent to superimposing a fault voltage source at the fault point, and the voltage source has the same amplitude and opposite polarity as the voltage before the fault. The traveling wave emitted from the voltage source is transmitted to both ends of the line, and will be refracted and reflected when encountering a point where the wave impedance is discontinuous, so refracted and reflected will occur at the busbar and the fault point. Therefore, it is necessary to analyze the expression and value range of the refracted and reflected coefficient at each busbar and the fault point.

[0014] Preferably, in step (2), phase mode transformation is used to decouple the three-phase coupled lines. Since the line mode traveling wave has a small dispersion degree and a higher stability, this paper uses the line mode traveling wave as the analysis object. However, this balance relationship is destroyed at the fault point, and the zero mode will also transmit the line mode. Failure to consider the influence of this cross transmission will lead to malfunction of the protection. Therefore, it is necessary to consider the influence of cross transmission on the traveling wave.

[0015] At the same time, when faults occur at different locations, the transmission paths of the traveling waves are quite different, resulting in large differences in the arrival timing of the traveling waves measured at the protection installation. Therefore, the transmission paths of the traveling waves when faults occur at different locations are analyzed in detail.

[0016] Preferably, in step (3), the relationship between the expression of the line mode voltage traveling wave measured at the protection installation and the fault position is derived, and the polarity characteristics of the line mode voltage traveling wave can be effectively obtained based on the range of values ​​of the refraction and reflection coefficient at each wave impedance discontinuity point obtained in step (1).

[0017] Preferably, in step (4), the wavelet transform modulus maximum is used to describe the polarity characteristics of the traveling wave. The wavelet transform modulus maximum is defined as: Let W s f(x) is the wavelet transform of the function f(x), and for all x in a certain neighborhood of x0 at scale s, we have:

[0018] |W s f(x)|≤|W s f(x0)|

[0019] Where: x0 is the modulus maximum point of wavelet transform; W sf (x0) is the modulus maximum of the wavelet transform.

[0020] When the polarities of the four current voltage waves are all the same, it is determined to be an out-of-zone fault; when the polarities of the four current voltage waves are not completely the same, it is determined to be an in-zone fault.

[0021] The present invention also discloses a protection system for a new energy AC transmission line, which is characterized in that: the protection system includes the above-mentioned new energy line protection method based on the polarity difference of the first four voltage traveling waves and its application.

[0022] The present invention also discloses a new energy station, including a power supply protection system, which is characterized in that: the power supply protection system is a protection system for the above-mentioned new energy AC transmission line.

[0023] Beneficial Effects

[0024] The present invention identifies faults according to polarity characteristics, and the fault characteristics are obvious, easy to identify, small in calculation amount, and wide in application. At the same time, the present solution is single-ended quantity protection, has the advantages of no need for communication, fast action speed, etc., and can reliably identify faults. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] 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.

[0026] Figure 1 It is the Peterson equivalent circuit diagram of the voltage traveling wave line mode fault component at bus M;

[0027] Figure 2 It is the Peterson equivalent circuit diagram of the voltage traveling wave line mode fault component at bus Q;

[0028] Figure 3 It is the Peterson equivalent circuit diagram of the voltage traveling wave line mode fault component at the fault point;

[0029] Figure 4 This is the diagram of the refraction and reflection process of the line mode traveling wave when a fault occurs in the first half of the area;

[0030] Figure 5 This is the diagram of the refraction and reflection process of the line mode traveling wave when a fault occurs in the first half of the area;

[0031] Figure 6 This is the diagram of the refraction and reflection process of the line mode traveling wave when a fault occurs in the second half of the area;

[0032] Figure 7 This is the diagram of the refraction and reflection process of the line mode traveling wave when a fault occurs in the second half of the area;

[0033] Figure 8 This is the diagram of the refraction and reflection process of the line mode traveling wave when a fault occurs in the first half of the area outside the zone;

[0034] Fig. 9 This is the diagram of the refraction and reflection process of the line mode traveling wave when a fault occurs in the first half of the area outside the zone;

[0035] Fig.10 This is the simulation result diagram when a single-phase grounding fault occurs in the area, where Fig.10 (a) is the line mode voltage traveling wave detected at the protection installation; (b) is the wavelet transform result of the voltage traveling wave;

[0036] Fig.11 Figure 2 is a simulation result diagram when a single-phase grounding fault occurs outside the zone, where (a) is the line-mode voltage traveling wave detected at the protection installation; (b) is the wavelet transform result of the voltage traveling wave.

[0037] Fig.12 This is a schematic diagram of a high-proportion new energy line protection circuit based on the polarity difference of the first four voltage traveling waves of the present invention. DETAILED DESCRIPTION

[0038] The specific structure and method implementation of the present invention are described in detail below in conjunction with the accompanying drawings. The following detailed exemplary embodiments are disclosed only for the purpose of describing the exemplary embodiments. However, the present invention is not limited to the disclosed specific exemplary embodiments, but can cover all modifications, equivalents and alternatives falling within the scope of the present disclosure.

[0039] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the present invention is further described in detail below with reference to the accompanying drawings and specific embodiments.

[0040] The high-proportion new energy line protection method based on the polarity difference of the first four voltage traveling waves includes the following steps:

[0041] Step 1: Derive the expression of reflection coefficient at the fault point and each busbar

[0042] When a line fails, it is equivalent to superimposing a fault voltage source at the fault point. The voltage source has the same amplitude as the voltage before the fault, but opposite polarity. The traveling wave emitted from the voltage source is transmitted to both ends of the line. When it encounters a point where the wave impedance is discontinuous, it will be refracted and reflected. Therefore, refracted and reflected will occur at the busbar and the fault point. Figure 1 From the Peterson equivalent circuit of the traveling wave at the busbar M shown in the figure, it can be seen that the refraction coefficient and reflection coefficient at the busbar M determine the value range:

[0043]

[0044]

[0045] In the formula,

[0046] Among them, α M represents the refraction coefficient of the voltage traveling wave at the busbar M; β M It represents the reflection coefficient of the voltage traveling wave at the busbar M, Z c1 Indicates the line mode wave impedance of the line, Z cM It represents the equivalent wave impedance of the busbar capacitance to ground; Z2 represents the equivalent wave impedance of the back side of the busbar; n represents that the busbar has n outgoing lines in addition to the connected lines.

[0047] For the new energy AC transmission line, when multiple lines or loads are connected to the bus, the reflection coefficient of the voltage traveling wave at the bus is negative, and the range of the reflection coefficient is indicated in the formula. Due to the symmetry of the transmission network structure, the reflection coefficient at the N bus is also shown in the above formula.

[0048] However, for bus Q, the back side is connected to the transformer and the system, and the Peterson equivalent circuit of the line mode component is as follows: Figure 2 As shown, the equivalent wave impedance Z2 of the back system is much larger than the wave impedance Z of the line. C1 Therefore, the reflection coefficient at the busbar Q is greater than 0. The larger the back-side equivalent impedance Z2 is, the closer the reflection coefficient at the busbar Q is to 1, and the line-mode voltage traveling wave is approximately totally reflected. Its value range has been marked in the following formula.

[0049]

[0050]

[0051] In the formula, Z2=Z T +Z S

[0052] Among them, α Q Represents the refraction coefficient of the voltage traveling wave at the busbar Q; β Q It represents the reflection coefficient of the voltage traveling wave at the bus Q. Z2 is the equivalent wave impedance of the measured system of bus Q, Z T is the equivalent impedance of the connected transformer, Z s is the equivalent impedance of the system, so the reflection coefficient of the line-mode voltage traveling wave at the bus Q is greater than 0.

[0053] pass Figure 3 From the Peterson equivalent circuit of the line mode traveling wave at the fault point shown in the figure, it can be seen that when there is a ground fault, the expression and value range of the refraction coefficient and reflection coefficient of the fault point are:

[0054]

[0055]

[0056] In the formula,

[0057] where α f represents the refraction coefficient of the voltage traveling wave at the fault point, β f Represents the reflection coefficient of the voltage traveling wave at the fault point, R f It represents the transition resistance at the fault point, so the reflection coefficient of the voltage traveling wave at the fault point and the busbar is less than 0.

[0058] The above analysis is all about the refraction and reflection process between line mode components. When a ground fault occurs, when the traveling wave is transmitted from other ports to the fault point, the phase mode transformation at the fault point can no longer be completely decoupled, and the zero mode and line mode components cross-transmit at the fault point. Therefore, it is necessary to analyze the impact of the cross-transmission between the zero mode and the line mode.

[0059]

[0060] where α0-1 It represents the transmission coefficient of zero-mode traveling wave to line-mode traveling wave.

[0061] From the above analysis, we can know that at each busbar (except Q port) and fault point, the reflection coefficient of the line mode voltage traveling wave is less than 0, and the refraction coefficient is greater than 0. Reflection changes polarity, while refraction does not. The line mode traveling wave generated by the transmission of the zero mode traveling wave changes polarity.

[0062] Step 2: Analysis of the traveling wave refraction-reflection process considering cross transmission

[0063] The present invention relates to the field of fault identification, and it is necessary to reliably identify faults inside and outside the zone. Since traveling wave protection can complete fault identification before the control strategy responds and has high speed, this solution uses traveling wave protection to identify faults in new energy lines. Since refraction and reflection will occur when the traveling wave reaches the wave impedance discontinuity point, this will inevitably lead to energy attenuation. After multiple refractions and reflections, the energy loss is large. Therefore, this solution considers that the energy attenuation of the traveling wave after more than three refraction and reflection processes is 0. The present invention does not consider more than three refraction and reflection processes.

[0064] In the transient analysis process of the three-phase system, the coupling between the three phases brings difficulties to the analysis of the problem. Therefore, the phase mode transformation method is usually used to decouple the coupled system and convert it into independent mode components for analysis. Line mode traveling waves have a small degree of dispersion and a high degree of stability. Therefore, this article will use line mode traveling waves as the analysis object. However, when a ground fault occurs, the modulus traveling waves are no longer independent of each other when passing through the fault point, and fault traveling waves of different modes will undergo cross-transmission. If the transmitted line mode component of the zero-mode traveling wave is not considered, the refraction and reflection process of the traveling wave is still analyzed based on the principle of independent modes in the three-phase system, which will cause malfunction of the protection device. Therefore, the present invention analyzes the refraction and reflection process of the traveling wave on the basis of considering cross-transmission.

[0065] When faults occur at different locations, the propagation paths of the traveling waves will be quite different. The amplitude, polarity, energy, time difference, etc. of the traveling waves reaching the protection installation will be different. Therefore, when analyzing the propagation path of the traveling waves, the different fault locations should be classified and described.

[0066] Step 3: Derive the relationship between the expression of the traveling wave of the wire mode voltage and the fault location

[0067] The present invention studies the new energy AC line, where L, M, N, and O represent the busbars at each end, respectively, and f represents the fault point. The line studied in the present invention is MN. When a fault occurs in the first half of the line, the second voltage traveling wave arriving at the protection installation is the reflected wave of the fault point; when a fault occurs in the second half of the line, the third traveling wave arriving at the protection installation is the reflected wave of the opposite end busbar. Therefore, when a fault occurs in the first half and the second half, the traveling waves measured at the protection installation are greatly different. The present invention discusses the first half and the second half separately.

[0068] 1) When a fault occurs in the first half of the zone, the first voltage traveling wave that reaches the protection installation is the initial fault traveling wave ΔU f ; Since the fault point is close to the protection installation and the line mode wave velocity is greater than the zero mode wave velocity, the second voltage traveling wave arriving at the protection installation at this time is the reflected wave of the line mode traveling wave through the fault point; the zero mode wave velocity is lower than the line mode wave velocity, but since the fault point is close to the protection installation, the third voltage traveling wave arriving at the protection installation at this time may be the line mode traveling wave transmitted by the zero mode traveling wave through the fault point, or it may be the reflected wave of the line mode traveling wave through the opposite end busbar, so the two cases are discussed separately:

[0069] ①The third traveling wave is a zero-mode transmitted wave

[0070] Satisfied at this time Right now Where x represents the distance from the fault point to the protection installation, and L1 represents the length of the line MN under study. At this time, the third traveling wave that reaches the protection installation is the line mode traveling wave transmitted by the zero mode traveling wave through the fault point, and the fourth traveling wave is the reflection wave of the line mode traveling wave through the opposite end busbar, as shown in Figure 4 As shown, the expressions of the first four voltage traveling waves are:

[0071]

[0072] Among them: U i (i=1,2,3,4..) represents the i-th voltage traveling wave measured at the protection installation; ΔU f It represents the initial fault wave transmitted to both ends of the line. For example, a single-phase grounding fault occurs when the voltage is greater than 0. At this time, it is equivalent to superimposing a negative voltage source at the fault point. At this time, ΔU f <0;β M represents the reflection coefficient of the line mode voltage traveling wave at the M port; β f Represents the reflection coefficient of the line mode voltage traveling wave at the fault point; β M0 represents the reflection coefficient of the zero-mode voltage traveling wave at the M port; α 0-1 It represents the transmission coefficient of zero-mode traveling wave transmitted through the fault point to line-mode traveling wave; α fIt represents the transmission coefficient of line-mode traveling wave; the polarity of the first four voltage traveling waves can be determined by the range of the refraction-reflection coefficient and indicated in the above formula.

[0073] ②The third traveling wave is the reflected wave of the opposite bus

[0074] Satisfied at this time Right now Where x represents the distance from the fault point to the protection installation, and L1 represents the length of the line MN under study. At this time, the third traveling wave that reaches the protection installation is the line mode traveling wave reflected by the opposite bus, and the fourth traveling wave is the line mode traveling wave transmitted by the zero mode traveling wave through the fault point, as shown in Figure 5 As shown, the expressions of the first four voltage traveling waves are:

[0075]

[0076] Among them: U i (i=1,2,3,4..) represents the i-th voltage traveling wave measured at the protection installation; ΔU f It represents the initial fault wave transmitted to both ends of the line. For example, a single-phase grounding fault occurs when the voltage is greater than 0. At this time, it is equivalent to superimposing a negative voltage source at the fault point. At this time, ΔU f <0;β M represents the reflection coefficient of the line mode voltage traveling wave at the M port; β f Represents the reflection coefficient of the line mode voltage traveling wave at the fault point; β M0 represents the reflection coefficient of the zero-mode voltage traveling wave at the M port; α 0-1 It represents the transmission coefficient of zero-mode traveling wave transmitted through the fault point to line-mode traveling wave; α f It represents the transmission coefficient of line-mode traveling wave; the polarity of the first four voltage traveling waves can be determined by the range of the refraction-reflection coefficient and indicated in the above formula.

[0077] 2) When a fault occurs in the second half of the zone, the first voltage wave reaching the protection installation is still the initial fault wave ΔU f ; However, since the fault point is close to the opposite bus at this time, the second voltage traveling wave reaching the protection installation is the reflected wave of the line mode traveling wave through the opposite bus; the zero mode wave speed is lower than the line mode wave speed, but since the fault point is far from the protection installation, the third voltage traveling wave reaching the protection installation at this time may be the zero mode traveling wave reflected by the opposite bus and transmitted through the fault point. It may also be the reflected wave of the line mode traveling wave through the fault point. Therefore, the two cases are discussed separately:

[0078] ①The third traveling wave is the reflected wave of the line mode traveling wave passing through the fault point

[0079] Satisfied at this time Right now Where x represents the distance from the fault point to the protection installation, and L1 represents the length of the line MN under study. At this time, the third traveling wave that reaches the protection installation is the line mode traveling wave reflected from the fault point, and the fourth traveling wave is the zero mode traveling wave reflected from the opposite end busbar and transmitted from the fault point. Figure 6 As shown, the expressions of the first four voltage traveling waves are:

[0080]

[0081] Where: U i (i=1,2,3,4..) represents the i-th voltage traveling wave measured at the protection installation; ΔU f It represents the initial fault wave transmitted to both ends of the line. For example, a single-phase grounding fault occurs when the voltage is greater than 0. At this time, it is equivalent to superimposing a negative voltage source at the fault point. At this time, ΔU f <0;β M represents the reflection coefficient of the line mode voltage traveling wave at the M port; β f Represents the reflection coefficient of the line mode voltage traveling wave at the fault point; β M0 represents the reflection coefficient of the zero-mode voltage traveling wave at the M port; α 0-1 It represents the transmission coefficient of zero-mode traveling wave transmitted through the fault point to line-mode traveling wave; α f It represents the transmission coefficient of line-mode traveling wave; the polarity of the first four voltage traveling waves can be determined by the range of the refraction-reflection coefficient and indicated in the above formula.

[0082] ②The third traveling wave is the zero-mode traveling wave transmitted through the fault point

[0083] At this time, the line length meets Right now Where x represents the distance from the fault point to the protection installation, and L1 represents the length of the line MN under study. At this time, the third traveling wave that reaches the protection installation is the line mode traveling wave that is transmitted through the fault point after the zero mode traveling wave is reflected by the opposite busbar, and the fourth traveling wave is the reflected wave of the line mode traveling wave through the fault point, as shown in Figure 7 As shown, the expressions of the first four voltage traveling waves are:

[0084]

[0085] Among them: U i (i=1,2,3,4..) represents the i-th voltage traveling wave measured at the protection installation; ΔU f It represents the initial fault wave transmitted to both ends of the line. For example, a single-phase grounding fault occurs when the voltage is greater than 0. At this time, it is equivalent to superimposing a negative voltage source at the fault point. At this time, ΔU f <0;β Mrepresents the reflection coefficient of the line mode voltage traveling wave at the M port; β f Represents the reflection coefficient of the line mode voltage traveling wave at the fault point; β M0 represents the reflection coefficient of the zero-mode voltage traveling wave at the M port; α 0-1 It represents the transmission coefficient of zero-mode traveling wave transmitted through the fault point to line-mode traveling wave; α f It represents the transmission coefficient of line-mode traveling wave; the polarity of the first four voltage traveling waves can be determined by the range of the refraction-reflection coefficient and indicated in the above formula.

[0086] In summary, when a fault occurs in the zone, the polarities of the first four voltage traveling waves arriving at the protection installation are different.

[0087] The analysis ideas of out-of-area faults are the same as those of in-area faults, and the fault conditions of the first half and the second half of the out-of-area faults are classified and discussed. However, when a fault occurs in the second half of the area, in order to ensure the selectivity of the protection, the protection of the next-level line can be activated by judging the voltage change rate, the voltage change amount, etc. Therefore, the discussion of the fault in the second half is not focused. The difficulty lies in the difficulty of accurately identifying the fault in the area and the fault at the head end of the lower-level line. Therefore, the present invention focuses on analyzing the fault at the head end of the next-level line.

[0088] Since the transmission line is long and the transmission line of the back-side connection system is short, when an out-of-area fault occurs, the influence of line MN on traveling wave twists and reflections is not considered.

[0089] 1) When a fault occurs in the first half of the zone, the first voltage traveling wave that reaches the protection installation is the initial fault traveling wave ΔU f ; Since the fault point is close to the protection installation and the line mode wave velocity is greater than the zero mode wave velocity, the second voltage traveling wave arriving at the protection installation at this time is the reflected wave of the line mode traveling wave through the fault point; the zero mode wave velocity is lower than the line mode wave velocity, but since the fault point is close to the protection installation, the third voltage traveling wave arriving at the protection installation at this time may be the line mode traveling wave transmitted by the zero mode traveling wave through the fault point, or it may be the reflected wave of the line mode traveling wave through the opposite end busbar, so the two cases are discussed separately:

[0090] ①The third traveling wave is a zero-mode transmitted wave

[0091] Satisfied at this time Right now Where x represents the distance from the fault point to the protection installation, and L1 represents the length of the line MN under study. At this time, the third traveling wave that reaches the protection installation is the line mode traveling wave transmitted by the zero mode traveling wave through the fault point, and the fourth traveling wave is the reflection wave of the line mode traveling wave through the opposite end busbar, as shown in Figure 8 As shown, the expressions of the first four voltage traveling waves are:

[0092]

[0093] Among them: U i (i=1,2,3,4..) represents the i-th voltage traveling wave measured at the protection installation; ΔU f It represents the initial fault wave transmitted to both ends of the line. For example, a single-phase grounding fault occurs when the voltage is greater than 0. At this time, it is equivalent to superimposing a negative voltage source at the fault point. At this time, ΔU f <0;β M represents the reflection coefficient of the line mode voltage traveling wave at the M port; β f Represents the reflection coefficient of the line mode voltage traveling wave at the fault point; β N0 Represents the reflection coefficient of zero-mode voltage traveling wave at the N port; α 0-1 It represents the transmission coefficient of zero-mode traveling wave transmitted through the fault point to line-mode traveling wave; α f It represents the transmission coefficient of line-mode traveling wave; the polarity of the first four voltage traveling waves can be determined by the range of the refraction-reflection coefficient and indicated in the above formula.

[0094] ②The third traveling wave is the reflected wave of the opposite bus

[0095] Satisfied at this time Right now Where x represents the distance from the fault point to the protection installation, and L1 represents the length of the line MN under study. At this time, the third traveling wave that reaches the protection installation is the line mode traveling wave reflected by the opposite bus, and the fourth traveling wave is the line mode traveling wave transmitted by the zero mode traveling wave through the fault point, as shown in Fig. 9 As shown, the expressions of the first four voltage traveling waves are:

[0096]

[0097] Among them: U i (i=1,2,3,4..) represents the i-th voltage traveling wave measured at the protection installation; ΔU f It represents the initial fault wave transmitted to both ends of the line. For example, a single-phase grounding fault occurs when the voltage is greater than 0. At this time, it is equivalent to superimposing a negative voltage source at the fault point. At this time, ΔU f <0;β M represents the reflection coefficient of the line mode voltage traveling wave at the M port; β f Represents the reflection coefficient of the line mode voltage traveling wave at the fault point; β M0 represents the reflection coefficient of the zero-mode voltage traveling wave at the M port; α 0-1 It represents the transmission coefficient of zero-mode traveling wave transmitted through the fault point to line-mode traveling wave; α f It represents the transmission coefficient of line-mode traveling wave; the polarity of the first four voltage traveling waves can be determined by the range of the refraction-reflection coefficient and indicated in the above formula.

[0098] In summary, when there is an in-zone fault, the polarities of the first four voltage traveling waves are different, but when there is an out-zone fault, the polarities of the first four voltage traveling waves are the same. The present invention uses this as a criterion for fault identification.

[0099] Step 4: Analyze the polarity of the first four voltage waves to determine whether it is an internal fault

[0100] The scheme adopted by the present invention is to use the polarity of the first four line mode voltage traveling waves to identify faults inside and outside the zone, so it is necessary to be able to accurately identify the polarity of the traveling wave. This paper adopts the wavelet transform modulus maximum (WTMM) under discrete binary wavelet transform to quantitatively describe the local steepness of the signal. The amplitude of the modulus maximum represents the mutation strength of the signal, and the polarity represents the mutation direction of the signal.

[0101] The maximum value of wavelet transform modulus is defined as: Let W s f(x) is the wavelet transform of the function f(x), and for all x in a certain neighborhood of x0 at scale s, we have:

[0102] |W s f(x)|≤|W s f(x0)|

[0103] Where: x0 is the modulus maximum point of wavelet transform; W s f(x0) is the modulus maximum of the wavelet transform.

[0104] Since the initial phase angle of the fault is 0°~90°, ΔU f is negative; when the initial phase angle of the fault is 90°~180°, ΔU f is positive; therefore, when a fault occurs at different times, the polarity of the voltage wave measured at the protection installation is different, so the fault criterion cannot be directly set to be greater than 0 or less than 0. The present invention uses the polarity difference of the first four voltage waves to identify the fault. If the polarities of the first four voltage waves are the same, it is an out-of-zone fault; if the polarities of the first four voltage waves are different, it is an out-of-zone fault.

[0105] Figure 7 The current traveling wave simulation diagram and its wavelet transform modulus maximum of a single-phase grounding fault occurring on a line MN with a length of 200 km and a distance of 40 km from the M end are given. The fault occurs at 0.5 ms. From the results of the wavelet transform modulus maximum in the figure, it can be seen that the polarities of the first four voltage traveling waves arriving at the protection installation are not exactly the same, so it can be determined as an in-zone fault.

[0106] Figure 8The current traveling wave simulation diagram and its wavelet transform modulus maximum of a single-phase grounding fault occurring on a line MN with a length of 200km and 15km away from the N end are given. The fault occurs at 0.5ms. From the results of the wavelet transform modulus maximum in the figure, it can be seen that the first four voltage traveling waves reaching the protection installation have the same polarity, so it can be determined as an out-of-zone fault.

[0107] It can be seen from the above simulation analysis that when a fault occurs inside or outside the zone, the present invention can effectively identify the fault.

[0108] The present invention provides a high-proportion new energy line protection method based on the polarity difference of the first four voltage traveling waves and its application. The method first derives the expression of the refraction and reflection coefficient of the wave impedance discontinuity point and obtains its value range. Then, according to the refraction and reflection process of the traveling wave when faults occur at different positions, the expression of the line mode voltage traveling wave at the protection installation is derived. The polarity characteristics of the first four voltage traveling waves when faults occur at different positions can be obtained by using the value range of the refraction and reflection coefficient, and it is concluded that the polarity of the first four voltage traveling waves is the same when the fault occurs outside the zone, and the polarity of the first four voltage traveling waves is different when the fault occurs inside the zone, and fault identification is performed based on this. The method uses the polarity characteristics of the traveling wave for fault identification. Compared with time characteristics and amplitude characteristics, polarity characteristics are easier to identify and have higher reliability, small calculation amount, and wide application range. At the same time, this scheme is a single-ended quantity protection, which has the advantages of no need for communication and fast action speed, and can reliably identify faults.

[0109] The present invention uses specific examples to illustrate the principles and implementation methods of the invention. The description of the above embodiments is only used to help understand the method of the present invention and its core idea. 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 work are within the scope of protection of the present invention.

Claims

1. A high-proportion new energy line protection method based on the polarity difference of the first four voltage traveling waves. The method obtains its value range by deriving the expression of the refraction-reflection coefficient of the wave impedance discontinuity point; and derives the expression of the line mode voltage traveling wave at the protection installation location according to the refraction-reflection process of the traveling wave when faults occur at different locations; The polarity characteristics of the first four voltage traveling waves at different fault locations are obtained by using the range of the refraction-reflection coefficient, and it is concluded that the polarities of the first four voltage traveling waves are the same when there is an out-of-zone fault, and the polarities of the first four voltage traveling waves are different when there is an in-zone fault. Fault identification is performed based on this, and the protection method specifically includes the following steps: Step (1): deriving the expressions of the reflection coefficients at the fault point and each busbar; this step requires analyzing the expressions of the refraction coefficient and the reflection coefficient at the busbar and the fault point, and determining their value ranges; Step (2): Analysis of the traveling wave refraction and reflection process considering cross transmission; This step uses phase mode transformation to decouple the three-phase coupled line and analyzes the refraction and reflection process of the traveling wave affected by cross transmission; Step (3): deriving the relationship between the expression of the line mode voltage traveling wave and the fault position; this step derives the relationship between the expression of the line mode voltage traveling wave measured at the protection installation and the fault position, and effectively derives the polarity characteristics of the line mode voltage traveling wave according to the value range of the refraction-reflection coefficient at each wave impedance discontinuity point obtained in step (1); Step (4): Analyze the polarity of the first four voltage traveling waves to determine whether it is an internal fault. In this step, the wavelet transform modulus maximum is used to describe the polarity characteristics of the traveling wave. The wavelet transform modulus maximum is defined as: Let W s f(x) is the wavelet transform of the function f(x), and for all x in a certain neighborhood of x0 at scale s, we have: |W s f(x)|≤|W s f(x0)| Where: x0 is the modulus maximum point of wavelet transform; W sf (x0) is the modulus maximum of the wavelet transform; When the polarities of the four current voltage waves are all the same, it is determined to be an out-of-zone fault; when the polarities of the four current voltage waves are not completely the same, it is determined to be an in-zone fault.

2. A protection system for a new energy AC transmission line, characterized by: The protection system includes the high-proportion new energy line protection method based on the polarity difference of the first four voltage traveling waves as described in claim 1 above.

3. A new energy station, including a power supply protection system, characterized by: The power supply protection system is the protection system for the new energy AC transmission line as described in claim 2.

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