New energy line protection method based on voltage traveling wave head polarity and quantity difference

Abstract

The application provides a new energy line protection method based on voltage traveling wave wave head polarity and quantity difference, which derives the relationship between the number of voltage traveling waves with the same polarity before the first opposite polarity voltage traveling wave arrives and the fault position according to the voltage traveling wave reflection process. On this basis, considering that the sampling rate leads to the situation that the traveling wave is difficult to be accurately captured in the near-end or terminal fault, it can be obtained that when there are two or more than two same polarity waves before the first opposite polarity wave arrives, it is the first 50% fault in the area or the first 50% fault outside the area; when there is one same polarity wave, it is the last 50% fault in the area or the last 50% fault outside the area. Finally, the time difference between the zero mode and the line mode arriving at the protection installation is used to distinguish the faults in the area and outside the area. The method is based on the polarity and quantity difference of the voltage traveling wave to construct a single-ended protection method, solves the problem that the existing single-ended protection based on the time difference between the zero mode and the line mode has a dead zone in the terminal fault in the area, and considers the influence of the sampling rate on the protection method.

Description

Technical Field

[0001] The present invention relates to the field of fault identification of high-proportion renewable energy AC lines, and in particular to a new energy line protection method based on the polarity and quantity differences of voltage traveling wave heads. Background Art

[0002] Since the beginning of the 21st century, global warming and energy shortages have become increasingly prominent, becoming major and urgent global challenges that all humanity must address. A new round of energy transformation has also quietly emerged in the global energy sector. Accelerating the development of new energy sources, typified by wind power and photovoltaics, and promoting the green transformation of energy production are key to alleviating climate change and resource scarcity, and are the only path to achieving sustainable global development.

[0003] New energy sources represented by wind power and photovoltaic power are connected to the grid through power electronic interfaces. Compared with traditional power systems with synchronous generators as the main power source, they have significant differences in power generation principles, control methods, grid-connected equipment and external characteristics. As a result, the fault characteristics of new energy sources are different from those of traditional synchronous generators, with characteristics such as limited amplitude and controlled phase angle. This further leads to the risk of refusal to operate and false operation of traditional protection schemes based on power frequency quantities, causing comprehensive challenges to the sensitivity, speed, selectivity and reliability of conventional protection, seriously affecting the accurate identification and rapid isolation of faults.

[0004] To address these issues, existing research focuses on developing novel protection schemes for power frequency and transient quantities. For power frequency quantities, this approach primarily considers the fault characteristics of renewable energy sources, building upon existing power frequency protection schemes and proposing corresponding improvements. For transient quantities, by fully exploiting the time and frequency domain transient information contained in fault transients, a new protection principle unaffected by renewable energy control strategies is proposed. Traveling wave protection, a type of transient quantity protection, offers high-speed operation and the ability to identify faults before control systems intervene, making it an effective approach for renewable energy power systems.

[0005] The prior art, such as Chinese patent, application number: 201010149567.7, publication number: CN101923139A, discloses an intelligent method for single-ended traveling wave fault location of power transmission line, the polarity and time difference of the first three wave heads of the traveling wave of the fault of the power transmission line are taken as sample attributes, and the BP neural network is used to realize fault location. When the length of the shortest healthy line is greater than one fourth of the full length of the fault line and the length of the second shortest healthy line is greater than one half of the full length of the fault line, the first three wave heads detected at the protection installation contain at least two traveling waves from the fault line; when the above condition is not met, the direction traveling wave is used to identify whether the traveling wave is from the fault line. The time difference between the last two wave heads and the first wave head and the polarity of the wave heads are taken as sample attributes, the fault location neural network is trained and tested, and the initial fault distance is realized. On this basis, the relationship between the fault distance and the wave speed and the transmission time is applied to correctly identify the property of the second traveling wave head, and then the accurate fault distance is obtained; application number: 201110175930.7, publication number: CN102253315A, discloses a fault location method based on single-ended location. When a short circuit occurs in the power transmission line, the power frequency electrical quantity of the local bus of the power transmission line is measured by a traveling wave fault location device of the power transmission line, the fault point range is determined by the impedance method, and the transition resistance value is estimated; then the transient voltage / current traveling wave is analyzed by wavelet transform, it is judged that the fault is an ordinary short circuit fault, then the single-ended traveling wave method combined with the line length is used to directly calculate the fault point position; for a special short circuit fault, the single-ended fault location method of the power transmission line combined with the impedance method and the traveling wave method is used to calculate the fault point position; application number: 201110398088.3, publication number: CN102520315A, discloses a single-ended fault location method of power transmission line based on traveling wave multi-scale information, different phases are selected as reference phases according to different fault types, the phase-mode transformation is performed on the traveling wave signal of the fault current of the power transmission line, and the fault current traveling wave mode signal used for fault location is obtained; the continuous wavelet transform is performed on the mode signal, the mode maximum values of the first and second traveling wave heads and the corresponding time are extracted, the fault occurring section is judged according to the wavelet transform mode maximum value, the frequency components of the first and second traveling wave heads used for location are determined, and the traveling wave mode wave speeds and the wave head arrival time of two different frequency components are obtained; finally, the fault distance of the power transmission line is comprehensively calculated in combination with the mode wave speeds and the arrival time of the two traveling waves. The method accurately determines the frequency components, the mode wave speed and the arrival time of the traveling wave head by using the multi-scale information of the traveling wave, so that the fault distance of the power transmission line can be accurately calculated.Application number: 202111391311.1, publication number: CN114113890A, discloses a multi-terminal fusion distribution network fault location method based on traveling wave modulus transmission time difference. By analyzing the line mode and zero mode components of the fault traveling wave, the arrival time of the zero mode and line mode components is calibrated by wavelet transform to obtain the time difference, and deep learning is used to fit the relationship curve between the zero mode wave velocity and the fault distance to determine the zero mode traveling wave velocity. The fault distance is obtained by the modulus transmission time difference ranging principle.

[0006] Application number: 201810545632.4, publication number: CN108767819A, discloses a traveling wave protection method for transmission lines that takes into account the traveling wave velocity characteristics. After a line fault occurs, this method collects transient current traveling wave signals at each bus node in the power grid, calculates the modulus time difference between the arrival of the first wave heads of the zero-mode traveling wave and the line-mode traveling wave at each bus node; the bus node with the smallest modulus time difference is regarded as the key bus node; the fault area is identified based on the key bus node, the average modulus transit time difference of each line in the fault area is calculated, the line with the smallest average modulus transit time difference is identified as the fault line, and line protection action is performed on the fault line. Application number: 202210596379.1, publication number: CN114937978A, discloses a new energy line protection method based on a high proportion of polarity differences of the first four voltage traveling waves. 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 locations, 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 locations can be obtained using the value range of the refraction and 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.

[0007] However, most of these existing technologies rely on the characteristics of traveling wave headers for fault location, which cannot accurately distinguish between internal and external faults. Existing single-ended traveling wave protection based on zero-mode line-mode time difference has a blind spot for end-of-zone faults. Furthermore, these existing technologies fail to consider the impact of sampling rate on traveling wave header capture for near-end and end-of-zone faults, resulting in low fault tolerance. Summary of the Invention

[0008] The purpose of the present invention is to solve the problem of dead zone in the existing single-ended protection for end faults within the zone. At the same time, considering the influence of sampling rate on the accurate capture of traveling wave fronts for near-end faults and end faults, a protection method for renewable energy transmission lines is provided. The technical solution is as follows:

[0009] A new energy line protection method based on the polarity and number differences of voltage traveling wave heads is characterized by: first, the expression and value range of the voltage traveling wave refraction and reflection coefficient at the wave impedance discontinuity point are derived; second, the relationship between the number of same-polarity voltage traveling waves before the arrival of the first reverse polarity voltage traveling wave and the fault location is derived based on the voltage traveling wave refraction and reflection process; then, the influence of the sampling rate on the relationship between the number of same-polarity voltage traveling waves before the arrival of the first reverse polarity voltage traveling wave and the fault location is considered; finally, according to the judgment criteria: when there are two or more same-polarity waves before the arrival of the first reverse polarity wave, it is the first 50% fault within the zone or the first 50% fault outside the zone; when there is one same-polarity wave, it is the last 50% fault within the zone or the last 50% fault outside the zone, and the time difference between the zero mode and the line mode arriving at the protection installation point is further used to distinguish between internal and external faults.

[0010] Preferably, the new energy line protection method based on the difference in polarity and quantity of voltage traveling wave heads comprises the following steps:

[0011] Step (1): Derive the expression of the refraction-reflection coefficient at the fault point and the busbar;

[0012] Step (2): Based on the refraction and reflection process of the voltage traveling wave, derive the relationship between the number of voltage traveling waves of the same polarity detected before the first voltage traveling wave of the opposite polarity arrives at the protection installation and the fault location;

[0013] Step (3): For near-end faults and end-end faults, considering that the traveling wave head may be difficult to capture accurately under the influence of sampling rate, analyze its influence on the relationship between the number of same-polarity voltage traveling waves before the arrival of the first reverse-polarity voltage traveling wave and the fault location;

[0014] Step (4): determining the line fault section based on the number of voltage traveling waves of the same polarity before the first voltage traveling wave of the opposite polarity arrives;

[0015] Step (5): Determine whether the fault is inside or outside the zone based on the time difference between the zero-mode line mode and the protection installation location.

[0016] The present invention also discloses a protection system for a new energy AC transmission line, including a protection module for the new energy AC transmission line, which is characterized in that the protection module for the new energy AC transmission line is provided with the above-mentioned new energy line protection method based on the polarity and quantity difference of the voltage traveling wave head.

[0017] The present invention also discloses a new energy station, including a protection system, which is characterized in that: the protection system is a protection system for the above-mentioned new energy AC transmission line.

[0018] Beneficial effects

[0019] The application is based on the single-ended protection method constructed by the polarity and quantity difference of voltage traveling wave, and solves the problem of dead zone of the existing single-ended protection based on the time difference of zero-mode and mode in the end fault in the area; meanwhile, the method considers the influence of sampling rate on the protection method in the near-end fault and end fault, and analyzes that the protection method can still correctly act in the case of the existence of traveling wave which cannot be accurately captured, and has certain fault tolerance. BRIEF DESCRIPTION OF DRAWINGS

[0020] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the following will briefly introduce the drawings needed to be used in the embodiments. Obviously, the drawings in the following description only constitute some embodiments of the present application, and for those skilled in the art, other drawings can also be obtained from these drawings without creative labor.

[0021] Figure 1 Peterstein equivalent circuit diagram of voltage traveling wave mode fault component at bus M and N;

[0022] Figure 2 Peterstein equivalent circuit diagram of voltage traveling wave mode fault component at bus P and O;

[0023] Figure 3 Peterstein equivalent circuit diagram of voltage traveling wave mode fault component at fault point;

[0024] Figure 4 Folded reflection process diagram of line mode traveling wave when fault occurs at (0, 1 / 2) in the area;

[0025] Figure 5 Folded reflection process diagram of line mode traveling wave when fault occurs at (1 / 2, 1) in the area;

[0026] Figure 6 Folded reflection process diagram of line mode traveling wave when fault occurs at (0, 1 / 2) outside the area;

[0027] Figure 7 Folded reflection process diagram of line mode traveling wave when fault occurs at (1 / 2, 1) outside the area;

[0028] Figure 8 Folded reflection process diagram of line mode traveling wave when fault occurs at (0, 1 / 3) in the area;

[0029] Figure 9 Folded reflection process diagram of line mode traveling wave when fault occurs at (2 / 3, 1) in the area;

[0030] Wherein: Figure 4-Figure 9 In the figure, the solid line reaching the protection installation R1 of the line MN represents the traveling wave with the same polarity as the initial traveling wave of the fault point; and the dashed line represents the traveling wave with the opposite polarity to the initial traveling wave of the fault point.

[0031] Figure 10 This diagram shows a typical renewable energy line protection circuit based on the polarity and number differences in voltage traveling waves. P, M, N, and O represent the busbars at each end, respectively. Each busbar is assumed to have n outgoing lines, with n ≥ 2. Lines MN, LM, and NO are 200 km, 100 km, and 60 km long, respectively. R1, R2, and R3 represent the protections configured for Lines MN, PM, and NO, respectively. This study focuses on protection R1 configured on Line MN.

[0032] Figures 11(a) and 11(b) are the reverse line mode voltage traveling wave and its wavelet transform modulus maximum detected at protection R1 when a single-phase grounding fault occurs on line MN at a distance of 40 km from protection R1.

[0033] Figures 12(a) and 12(b) are the reverse line mode voltage traveling wave and its wavelet transform modulus maximum detected at protection R1 when a single-phase grounding fault occurs on line MN at a distance of 170 km from protection R1.

[0034] Figures 13(a) and 13(b) show the reverse line mode and zero mode voltage traveling waves and their wavelet transform modulus maxima detected at protection R1 when a single-phase grounding fault occurs on line MN at a distance of 40 km from protection R1.

[0035] Figures 14(a) and 14(b) show the reverse line mode and zero mode voltage traveling waves and their wavelet transform modulus maxima detected at protection R1 when a single-phase grounding fault occurs on line NO at a distance of 15 km from protection R1.

[0036] Figure 15 This is a flow chart of the new energy line protection method based on the polarity and quantity differences of voltage traveling wave heads according to the present invention. DETAILED DESCRIPTION

[0037] The following describes the specific structure and method implementation of the present invention in detail in conjunction with the accompanying drawings. The following detailed exemplary embodiments are disclosed for the purpose of describing the exemplary embodiments only. However, the present invention is not limited to the specific exemplary embodiments disclosed, but can cover all modifications, equivalents and alternatives that fall within the scope of the present disclosure.

[0038] 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.

[0039] A new energy line protection method based on the polarity and number differences of voltage traveling wave heads is characterized by: first, the expression and value range of the voltage traveling wave refraction and reflection coefficient at the wave impedance discontinuity point are derived; second, the relationship between the number of same-polarity voltage traveling waves before the arrival of the first reverse polarity voltage traveling wave and the fault location is derived based on the voltage traveling wave refraction and reflection process; then, the influence of the sampling rate on the relationship between the number of same-polarity voltage traveling waves before the arrival of the first reverse polarity voltage traveling wave and the fault location is considered; finally, according to the judgment criteria: when there are two or more same-polarity waves before the arrival of the first reverse polarity wave, it is the first 50% fault within the zone or the first 50% fault outside the zone; when there is one same-polarity wave, it is the last 50% fault within the zone or the last 50% fault outside the zone, and the time difference between the zero mode and the line mode arriving at the protection installation point is further used to distinguish between internal and external faults.

[0040] Next, we further explain the new energy transmission line protection method of the present invention:

[0041] A new energy line protection method based on the difference in polarity and quantity of voltage traveling wave heads includes the following steps:

[0042] Step (1): Derive the expression of the refraction-reflection coefficient at the fault point and each busbar

[0043] When a line fault occurs, it is equivalent to superimposing a fault additional 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 fault additional voltage source is transmitted to both ends of the line, and refraction and reflection will occur at the wave impedance discontinuity point between the busbar and the fault point. Figure 1 From the Peterson equivalent circuit of the traveling wave at the busbars M and N shown in the figure, it can be seen that the expressions and value ranges of the refractive coefficient and reflection coefficient at the busbars are:

[0044]

[0045]

[0046] Where: n is the number of outgoing lines on each busbar, n≥2; α M / N , β M / N Respectively represent the refraction coefficient and reflection coefficient of the voltage traveling wave at busbars M and N; Z C1 Indicates the line mode wave impedance of the line; Z CM It represents the equivalent impedance of the busbar-to-ground capacitance; Z2 represents the equivalent impedance of the back side of the busbar.

[0047] according to Figure 2 From the Peterson equivalent circuit of the traveling wave at the busbars P and O shown in the figure, it can be seen that the expressions and value ranges of the refractive coefficient and reflection coefficient at the busbars are:

[0048]

[0049]

[0050] Where: n is the number of outgoing lines on each busbar, n≥2; α P / O , β P / O Represent the refraction coefficient and reflection coefficient of the voltage traveling wave at the busbars P and O respectively; Z 2T Indicates the equivalent impedance of the back side of the busbar.

[0051] pass Figure 3 From the Peterson equivalent circuit of the line mode traveling wave at the fault point, it can be seen that when there is a ground fault, the expressions and value ranges of the refractive index and reflection index of the fault point are:

[0052]

[0053]

[0054] Where: Z f =Z C1 / / R f =(Z C1 ·R f ) / (Z C1 +R f ); α f , β f They represent the refraction coefficient and reflection coefficient of the voltage traveling wave at the fault point respectively; R f Indicates the transition resistance at the fault point.

[0055] From the range of values ​​of the expressions for the refraction coefficient and reflection coefficient of the voltage traveling wave at the busbar and the fault point in step (1), it can be seen that the reflection coefficient of the traveling wave at the busbar and the fault point is less than 0 and the refraction coefficient is greater than 0.

[0056] Step (2): Based on the refraction and reflection process of the voltage traveling wave, deduce the relationship between the number of voltage traveling waves of the same polarity detected before the first voltage traveling wave of the opposite polarity arrives at the protection installation and the fault location:

[0057] The research object of this paper is the new energy AC line. There is electromagnetic coupling between the three-phase lines, which can be decoupled by using Karen Bell transformation. The transformation matrix is:

[0058]

[0059] The Karen Bell transformation formula above can be used to transform the phase components of the three-phase coupling into mode components, achieving decoupling. The mode components include the zero mode component and the line mode component, which in turn includes the α mode component and the β mode component.

[0060] In order to obtain the polarity of the traveling wave head, the wavelet transform modulus maximum can be used to extract the polarity of the traveling wave head.

[0061] 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 expressions and polarities of the first four line-mode voltage traveling waves, they will be classified and described according to different fault locations.

[0062] This paper studies new energy AC lines, where P, M, N, and O represent the busbars at each end, and f represents the fault point. The research object is line MN. Based on the polarity relationship between the second traveling wave detected at the protection installation and the initial traveling wave at the fault point, the fault section is divided into two parts: (0, 1 / 2) and (1 / 2, 1), and these are discussed separately.

[0063] (1) When a fault occurs in the (0,1 / 2) section of the zone, the second traveling wave detected at the protection installation is the reflected wave at the fault point, which has the same polarity as the initial traveling wave, such as Figure 4 shown. Figure 4 In the example, the third traveling wave detected by the protection installation is the reflection wave from the opposite busbar, which has the opposite polarity to the initial traveling wave. Therefore, when a fault occurs in the (0,1 / 2) section, the protection installation detects two waves of the same polarity before the arrival of the first wave of opposite polarity.

[0064] (2) When a fault occurs in the (1 / 2,1) section of the zone, the second traveling wave detected at the protection installation is the reflected wave from the opposite busbar, which has the opposite polarity to the initial traveling wave, such as Figure 5 Therefore, when a fault occurs in the (0,1 / 2) section, the protection installation detects that there is a wave of the same polarity before the arrival of the first wave of the opposite polarity.

[0065] (3) When a fault occurs in the (0,1 / 2) section outside the zone, the second traveling wave detected at the protection installation is the reflected wave at the fault point, which has the same polarity as the initial traveling wave, such as Figure 6 shown. Figure 6 In the example, the third traveling wave detected by the protection installation is a reflection from the busbar at the other end of the faulted line, which has the opposite polarity to the initial traveling wave. Therefore, when a fault occurs in the (0,1 / 2) section, the protection installation detects two waves of the same polarity before the arrival of the first wave of opposite polarity.

[0066] (4) When a fault occurs in the (1 / 2,1) section outside the zone, the second traveling wave detected at the protection installation is the reflected wave from the busbar at the opposite end of the lower fault line, which has the opposite polarity to the initial traveling wave, such as Figure 7 Therefore, when a fault occurs in the (0,1 / 2) section, the protection installation detects that there is a wave of the same polarity before the arrival of the first wave of the opposite polarity.

[0067] Step (3): For the near-end fault and the end fault, considering the influence of the sampling rate, there may be a situation that the wave front is difficult to capture accurately, and the influence of the number of same polarity voltage waves before the first opposite polarity voltage wave arriving on the fault location is analyzed:

[0068] For the near-end fault and the end fault, due to the influence of the sampling rate, there may be a situation that the wave front is difficult to capture accurately, and considering the relatively ideal situation, it is considered that there is a wave that fails to be captured accurately for the near-end fault and the end fault. Different fault locations result in different wave reflection processes, which further cause different wave capturing situations. Taking the near-end fault and the end fault in the region as examples, the wave capturing situations for the near-end (0, 1 / 3) segment fault and the end (2 / 3, 1) segment fault are analyzed.

[0069] (1) For the (0, 1 / 3) segment fault in the region, the wave propagation process is as shown in Figure 8 The first three waves are all from the fault point and have the same polarity as the initial wave. If there is no leakage wave, there are three same polarity waves before the first opposite polarity wave arrives; if there is one leakage wave, there are three situations, respectively: the first wave and the second wave are captured accurately, and the third wave is not captured accurately; the first wave and the third wave are captured accurately, and the second wave is not captured accurately; the second wave and the third wave are captured accurately, and the first wave is not captured accurately. Therefore, two same polarity waves can be captured before the first opposite polarity wave arrives.

[0070] According to the analysis in (1) and the wave propagation process, for the near-end fault, the closer the fault point is to the protection installation, the more same polarity waves will be detected before the first opposite polarity wave arrives. In the case of no leakage wave, three or more same polarity waves can be detected; in the case of one leakage wave, two or more same polarity waves can be detected.

[0071] (2) For the (2 / 3, 1) segment fault in the region, the wave propagation process is as shown in Figure 9 The first wave can be captured accurately, and the second wave and the third wave may have a leakage wave. Considering one leakage wave, there are two situations, respectively: the first wave and the second wave are captured accurately, and the third wave is not captured accurately; the first wave and the third wave are captured accurately, and the second wave is not captured accurately. Therefore, one same polarity wave can be captured before the first opposite polarity wave arrives.

[0072] According to the analysis in (2) and the wave propagation process, for the end fault, the closer the fault point is to the protection installation, the number of same polarity waves detected before the first opposite polarity wave arrives does not change, and it is always one same polarity wave.

[0073] Similarly, when the (0,1 / 3) and (2 / 3,1) sections outside the zone are faulty and leakage waves are considered, the analysis is consistent with that inside the zone. That is, when the (0,1 / 3) section outside the zone is faulty, two or more same-polarity waves can be captured before the first reverse-polarity wave arrives; when the (2 / 3,1) section outside the zone is faulty, one same-polarity wave can be captured before the first reverse-polarity wave arrives.

[0074] In summary, from the analysis of (1)-(4) in step (2) and (1)-(2) in step (3), it can be seen that when the first 50% of faults occur within the zone or the first 50% of faults occur outside the zone, there are two or more waves of the same polarity before the first reverse polarity wave reaches the protection installation; when the last 50% of faults occur within the zone or the last 50% of faults occur outside the zone, there is one wave of the same polarity before the first reverse polarity wave reaches the protection installation.

[0075] Step (4): Determine the fault section of the line based on the number of voltage waves of the same polarity before the first voltage wave of the opposite polarity arrives.

[0076] Based on the capture and quantity differences of the same-polarity waves before the first reverse-polarity wave reaches the protection installation in step (3), it can be seen that for the first 50% of faults within the zone or the first 50% of faults outside the zone, there are two or more same-polarity waves before the first reverse-polarity wave reaches the protection installation; for the last 50% of faults within the zone or the last 50% of faults outside the zone, there is one same-polarity wave before the first reverse-polarity wave reaches the protection installation. Therefore, the fault section can be identified based on the difference in the quantity of same-polarity waves captured before the first reverse-polarity wave reaches the protection installation.

[0077] Step (5): Determine the fault inside and outside the zone based on the time difference between the zero-mode line mode and the protection installation location

[0078] According to the analysis in step (4), the fault section can be identified based on the difference in the number of same-polarity waves captured before the first reverse-polarity wave reaches the protection installation, but it cannot distinguish between internal and external faults. The existing single-ended protection scheme based on zero-mode line-mode time difference can effectively identify internal and external faults, but there are dead zones for near-end faults and end-end faults. This is mainly because the initial traveling wave may not be accurately captured in the case of near-end faults; in the case of end-end faults, due to under-range setting, it cannot be distinguished from the fault at the head end of the lower line.

[0079] To address the above problem, the fault section criterion in step (4) is first used to distinguish the first 50% fault and the last 50% fault. If it is the first 50% fault, it may be the first 50% fault within the zone or the first 50% fault outside the zone. Therefore, the zero-mode line-mode time difference criterion is further used to distinguish the fault within and outside the zone. If it is the last 50% fault, it may be the last 50% fault within the zone or the last 50% fault outside the zone. Therefore, the zero-mode line-mode time difference criterion is further used to distinguish the fault within and outside the zone.

[0080] exist Figure 10In a typical renewable energy AC transmission system topology, the research object is protection R1 installed on line MN, with a fault occurring at 0.5 ms. Figure 11 shows the reverse voltage traveling wave and its wavelet transform modulus maximum detected at protection R1 when a single-phase ground fault occurs 40 km from protection R1 on line MN. Figure 11(b) shows that before the first reverse polarity voltage traveling wave reaches the protection installation, four waves of the same polarity exist. Even when considering leakage waves, the condition of two or more waves of the same polarity is still met for the first 50% of faults.

[0081] Figure 12 shows the reverse voltage traveling wave and its wavelet transform modulus maximum detected at protection R1 when a single-phase ground fault occurs on line MN, 170 km from protection R1. Figure 12(b) shows that a wave of the same polarity exists before the first reverse polarity voltage traveling wave reaches the protection installation. Even when considering the presence of leakage waves, the condition of a wave of the same polarity existing in the first 50% of faults is still met.

[0082] Figure 13 shows the line-mode and zero-mode components of the reverse voltage traveling wave detected at protection R1, along with their wavelet transform modulus maxima, for a single-phase ground fault occurring 40 km from protection R1 on line MN. Figure 14 shows the line-mode and zero-mode components of the reverse voltage traveling wave detected at protection R1, along with their wavelet transform modulus maxima, for a single-phase ground fault occurring 215 km from protection R1 on line NO. Comparing Figures 13(b) and 14(b) shows that the zero-mode-line-mode time difference for intra-zone faults is smaller than for extra-zone faults, which can be used to identify intra-zone and extra-zone faults.

[0083] The present invention provides a new energy power line protection method based on the difference in polarity and number of voltage traveling wave heads. This method derives the relationship between the number of same-polarity voltage traveling waves before the arrival of the first reverse-polarity voltage traveling wave and the fault location based on the voltage traveling wave refraction and reflection process. Based on this, considering the difficulty in accurately capturing traveling waves for near-end or end-of-line faults due to the sampling rate, it can be concluded that when two or more same-polarity waves are present before the arrival of the first reverse-polarity wave, it indicates a fault in the first 50% of the zone or in the first 50% of the zone; when one same-polarity wave is present, it indicates a fault in the last 50% of the zone or in the last 50% of the zone. Finally, the time difference between the zero-mode and line-mode time differences at the protection installation is used to distinguish between faults within and outside the zone. This method constructs a single-ended protection method based on the difference in polarity and number of voltage traveling waves, solving the problem of dead zones in existing single-ended protection methods based on zero-mode and line-mode time differences for end-of-zone faults within the zone, and taking into account the impact of the sampling rate on the proposed protection method. The above shows and describes the basic principles, main features, and advantages of the present invention. Those skilled in the art will appreciate that the present invention is not limited to the foregoing embodiments. The foregoing embodiments and description merely illustrate the principles of the present invention. Various changes and modifications may be made to the present invention without departing from the spirit and scope of the present invention. Such changes and modifications are intended to fall within the scope of the present invention. The scope of protection claimed in the present invention is defined by the appended claims and their equivalents.

Claims

1. A new energy power line protection method based on the polarity and quantity differences of voltage traveling wave heads is characterized by: Firstly, the expression and value range of the voltage wave reflection coefficient at the fault point and busbar are derived; Secondly, the relationship between the number of traveling waves of the same polarity before the arrival of the first traveling wave of the opposite polarity voltage detected at the protection installation and the fault location is derived based on the refraction and reflection process of the voltage traveling wave. The three-phase coupled line is decoupled using phase mode transformation to obtain the line mode traveling wave component. The polarity characteristics of the traveling wave are described using the wavelet transform modulus maximum. 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, it has: Where: x0 is the modulus maximum point of wavelet transform; W s f(x0) is the modulus maximum of the wavelet transform; Set up a new energy AC line, where P, M, N, and O represent the busbars at each end, f represents the fault point, and the research object is line MN. Based on the polarity relationship between the second traveling wave detected at the protection installation and the initial traveling wave at the fault point, the fault section is divided into two parts (0, 1 / 2) and (1 / 2, 1) for determination: (1) When a fault occurs in the (0,1 / 2) section of the zone, the second traveling wave detected by the protection installation is the reflected wave from the fault point, which has the same polarity as the initial traveling wave. The third traveling wave detected by the protection installation is the reflected wave from the opposite busbar, which has the opposite polarity as the initial traveling wave. Therefore, when a fault occurs in the (0,1 / 2) section, the protection installation detects two waves of the same polarity before the arrival of the first wave of opposite polarity. (2) When a fault occurs in the (1 / 2, 1) section of the zone, the second traveling wave detected by the protection installation is the reflected wave from the opposite busbar, which has the opposite polarity to the initial traveling wave. Therefore, when a fault occurs in the (0, 1 / 2) section, the protection installation detects a wave of the same polarity before the arrival of the first wave of opposite polarity. (3) When a fault occurs in the (0,1 / 2) section outside the zone, the second traveling wave detected by the protection installation is the reflected wave from the fault point, which has the same polarity as the initial traveling wave. The third traveling wave detected by the protection installation is the reflected wave from the busbar at the opposite end of the lower fault line, which has the opposite polarity to the initial traveling wave. Therefore, when a fault occurs in the (0,1 / 2) section, the protection installation detects two waves of the same polarity before the arrival of the first wave of opposite polarity. (4) When a fault occurs in the (1 / 2, 1) section outside the zone, the second traveling wave detected by the protection installation is the reflected wave from the busbar at the opposite end of the lower fault line, which has the opposite polarity to the initial traveling wave. Therefore, when a fault occurs in the (0, 1 / 2) section, the protection installation detects a wave of the same polarity before the arrival of the first wave of opposite polarity. Then, for near-end faults and end-end faults, considering that the sampling rate may make it difficult to accurately capture the traveling wave head, the influence of the sampling rate on the relationship between the number of same-polarity voltage traveling waves before the arrival of the first reverse-polarity voltage traveling wave and the fault location is analyzed. Finally, the fault section of the line is identified based on the number of same-polarity voltage waves before the arrival of the first reverse-polarity voltage wave. According to the judgment criteria: when there are two or more same-polarity waves before the arrival of the first reverse-polarity wave, it is the first 50% fault within the zone or the first 50% fault outside the zone; when there is one same-polarity wave, it is the last 50% fault within the zone or the last 50% fault outside the zone. The time difference between the zero mode and the line mode arriving at the protection installation point is further used to distinguish between internal and external faults.

2. The new energy line protection method based on the difference in polarity and number of voltage traveling wave heads according to claim 1 is characterized in that: The expressions and value ranges of the refraction coefficient and reflection coefficient of the fault point are as follows: Where: Z f =Z C1 / / R f =(Z C1 ·R f ) / (Z C1 +R f ); α f , β f They represent the refraction coefficient and reflection coefficient of the voltage traveling wave at the fault point respectively; R f Indicates the transition resistance of the fault point; Z C1 Indicates the line mode wave impedance of the line.

3. The new energy line protection method based on the difference in polarity and number of voltage traveling wave heads according to claim 1 is characterized in that: When the fault in step (4) is the first 50% fault within the zone or the first 50% fault outside the zone, the time difference of the zero mode line mode is used to distinguish the fault inside and outside the zone; when the fault in step (4) is the last 50% fault within the zone or the last 50% fault outside the zone, the time difference of the zero mode line mode is used to distinguish the fault inside and outside the zone.

4. The protection system for the new energy AC transmission line includes a protection module for the new energy AC transmission line, which is characterized by: The protection module of the new energy AC transmission line is provided with the new energy line protection method based on the difference in polarity and quantity of voltage traveling wave heads as described in any one of claims 1-3.

5. A new energy station, including a protection system, characterized by: The protection system is the protection system for the new energy AC transmission line as described in claim 4.

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