Relay protection method applied to alternating-current line of photovoltaic power generation flexible direct-current sending-out system

Abstract

The invention provides a relay protection method applied to an alternating-current line of a photovoltaic power generation flexible direct-current sending-out system, relates to the technical field of relay protection of a power system, and aims to solve the problems of limitation of wave head feature loss, high sampling rate, large communication traffic and the like of an existing traveling wave protection method. The relay protection method comprises the following steps: acquiring voltages and currents of two detection points, and determining a current value of a front traveling wave and a current value of a reverse traveling wave of the detection points based on the voltages, the currents and wave impedance of the detection points; determining the current flowing time tau, determining the traveling wave difference of the detection points based on the current values of the reverse traveling waves of the two detection points, the set frequency and the flowing time tau, and finally determining whether the AC line has a fault based on the sum of the traveling wave difference of the detection points in the detection time interval. According to the relay protection method, the traveling wave calculation result of the whole process is used for fault judgment, the protection reliability is high, and the precision is higher.

Description

Technical Field

[0001] The present invention relates to the technical field of relay protection of power systems, and in particular to a relay protection method for an AC line applied to a photovoltaic power generation flexible DC transmission system. Background Art

[0002] In the photovoltaic power generation flexible DC transmission system, the two sides of the AC line are modular multilevel converters (MMC) and photovoltaic power generation grid-connected inverters, both of which are power electronic devices. In order to improve the continuous power supply capacity after a fault and the operating capacity under unbalanced conditions, the flexible DC converter and photovoltaic power generation grid-connected inverter will adopt complex control strategies. Typical examples include low-voltage current limiting control of flexible DC converters and negative sequence current suppression control of photovoltaic power generation. One of the fundamental goals of various types of control strategies is to limit the fault current to prevent the converter valve in the converter from overcurrent, which leads to the problem that the fault characteristics of the AC transmission line are not obvious when a fault occurs.

[0003] Specifically, the AC transmission line of the photovoltaic power generation flexible DC transmission system is a double-end weak-feed line. The meaning of double-end weak-feed is that no significant fault current is provided on both sides after the fault. This feature comes from the strict current limiting measures of power electronic devices. In order to protect the converter valve, the photovoltaic power generation grid-connected inverter and the flexible DC converter adopt control strategies to limit their output current after the fault. When the voltage drop is detected, the specific control strategies adopted by the converter include low-voltage current limiting, negative-sequence voltage suppression, negative-sequence current suppression, and double-frequency power suppression. These control strategies make the two sides of the double-end weak-feed line appear as controlled voltage sources and controlled current sources. The amplitude of the controlled power source is determined by the current limit target, which is generally 1.2 to 2 times the rated current, and the phase is determined by the voltage drop level. Therefore, the fault current has two significant characteristics: the amplitude is limited and there is a phase difference between the currents on both sides, which makes the action criteria of the existing relay protection scheme unclear, or even offset, resulting in insufficient sensitivity.

[0004] The existing renewable energy transmission system through flexible DC transmission generally adopts traditional current differential protection for the AC line protection at the sending end, but this solution has the problem of insufficient sensitivity, especially when there is transition resistance in the fault, the current differential protection has a greater risk of refusal to operate. At the same time, the distance protection widely used as the backup protection of AC lines has the problem of measuring impedance offset, which makes the fault current on both sides of the AC transmission line have a forced phase difference.

[0005] Traveling wave protection is based on the distributed parameter model of the line. The protection scheme based on the traveling wave principle has the characteristics of fast, high sensitivity, strong resistance to transition resistance, and can be unaffected by the complex control characteristics of the back side. There are protection technologies constructed using the traveling wave head and differential traveling wave principles. Single-ended and double-ended protection technologies based on the traveling wave principle have been applied to flexible DC line protection, AC line protection and other scenarios. However, the existing traveling wave protection generally has the limitations of the risk of wave head feature loss, high sampling rate, and large communication volume. Summary of the invention

[0006] The present invention aims to solve the problems of the existing traveling wave protection method, such as the loss of wave head characteristics, high sampling rate, large communication volume and the like.

[0007] A first aspect of the present invention provides a relay protection method for an AC line of a photovoltaic power generation flexible DC transmission system.

[0008] A second aspect of the present invention provides a relay protection device for an AC line of a photovoltaic power generation flexible DC transmission system.

[0009] A third aspect of the present invention provides an electronic device.

[0010] A fourth aspect of the present invention provides a readable storage medium.

[0011] The present invention provides a relay protection method for an AC line applied to a photovoltaic power generation flexible DC transmission system. The photovoltaic power generation flexible DC transmission system includes a sending-end converter and a photovoltaic power generation grid-connected inverter. The AC line includes a first endpoint and a second endpoint. The first endpoint is connected to the sending-end converter, and the second endpoint is connected to the photovoltaic power generation grid-connected inverter. The relay protection method for the AC line applied to the photovoltaic power generation flexible DC transmission system includes: obtaining a first detection signal from the first endpoint, the first detection signal includes a first voltage and a first current; obtaining a second detection signal from the second endpoint, the second detection signal includes a second voltage and a second current; obtaining the wave impedance and wave velocity of the AC line at a set frequency; determining the current value of the forward wave and the current value of the reverse wave at the first endpoint based on the first voltage, the first current and the wave impedance of the AC line; and determining the current value of the reverse wave based on the second voltage, the second current and the wave impedance of the AC line. According to the wave impedance of the AC line, the current value of the forward traveling wave and the reverse traveling wave at the second endpoint are determined; according to the distance between the first endpoint and the second endpoint and the wave velocity, the flow time τ of the current from the first endpoint to the second endpoint is determined; based on the current value of the reverse traveling wave at the first endpoint, the set frequency, the flow time τ and the current value of the reverse traveling wave at the second endpoint, the traveling wave differential quantity of the first endpoint is determined; based on the current value of the forward traveling wave of the second endpoint, the set frequency, the flow time τ and the current value of the forward traveling wave of the first endpoint, the traveling wave differential quantity of the second endpoint is determined; the sum of the traveling wave differential quantities of the first endpoint within the detection time interval is determined, wherein the detection time interval is greater than the flow time τ; the sum of the traveling wave differential quantities of the second endpoint within the detection time interval is determined; based on the sum of the traveling wave differential quantities of the first endpoint, the sum of the traveling wave differential quantities of the second endpoint and the first differential quantity threshold, it is determined whether the AC line is faulty.

[0012] The step of determining whether an AC line fault occurs based on the total traveling wave differential moment at the first endpoint, the total traveling wave differential moment at the second endpoint and the first differential moment threshold specifically includes: determining that an AC line fault occurs when the total traveling wave differential moment at the first endpoint is greater than or equal to the first differential moment threshold; or determining that an AC line fault occurs when the total traveling wave differential moment at the second endpoint is greater than or equal to the first differential moment threshold.

[0013] The relay protection method for the AC line applied to the photovoltaic power generation flexible DC transmission system also includes: when it is determined that a fault occurs in the AC line, extracting the fault signals of the first endpoint and the second endpoint within the detection time interval before the current moment: analyzing the fault signal by means of wavelet transform, and determining the ratio of the wavelet coefficients occupied by the high-frequency signal to the wavelet coefficients occupied by the low-frequency signal in the fault signal; when the ratio is less than or equal to a first preset value, determining the fault type to be an internal fault of the AC line; when the ratio is greater than the first preset value, determining the fault type to be an external fault of the AC line.

[0014] In the step of determining the traveling wave differential amount of the first endpoint based on the current value of the reverse traveling wave at the first endpoint, the set frequency, the flow time τ, and the current value of the reverse traveling wave at the second endpoint, the current value of the reverse traveling wave at the first endpoint, the set frequency, the flow time τ, the current value of the reverse traveling wave at the second endpoint, and the traveling wave differential amount of the first endpoint satisfy the following relationship:

[0015]

[0016] in, is the traveling wave differential momentum of the first endpoint, is the current value of the reverse wave at the second terminal, e is a constant, j is an imaginary unit, and j satisfies 2 =-1, ω is the angular frequency, equal to 2πf, f is the set frequency, τ is the flow time, is the current value of the reverse traveling wave at the first endpoint at the current moment;

[0017] In the step of determining the traveling wave differential momentum of the second endpoint based on the current value of the preceding traveling wave of the second endpoint, the set frequency, the flow time τ and the current value of the preceding traveling wave of the first endpoint, the current value of the preceding traveling wave of the second endpoint, the set frequency, the flow time τ, the current value of the preceding traveling wave of the first endpoint and the traveling wave differential momentum of the second endpoint satisfy the following relationship:

[0018]

[0019] in, is the traveling wave differential momentum at the second endpoint, is the current value of the forward wave at the first endpoint, e is a constant, j is an imaginary unit, and j satisfies 2 =-1, ω is the angular frequency, equal to 2πf, f is the set frequency, τ is the flow time, It is the current value of the forward wave at the second endpoint at the current moment.

[0020] In the step of determining the current value of the forward wave and the current value of the reverse wave of the first endpoint based on the first voltage, the first current and the wave impedance of the AC line at the first endpoint, the first voltage, the first current, the wave impedance and the current value of the forward wave of the first endpoint satisfy the following relationship:

[0021]

[0022] Where i1(x, t) is the current value of the forward wave of the first endpoint at time t, u(x, t) is the first voltage of the first endpoint at time t, i(x, t) is the first current of the first endpoint at time t, and Z C is the wave impedance;

[0023] The first voltage, the first current, the wave impedance of the first endpoint and the current value of the reverse wave at the first endpoint satisfy the following relationship:

[0024]

[0025] Where i2(x, t) is the current value of the reverse wave at the first endpoint at time t, u(x, t) is the first voltage at the first endpoint at time t, i(x, t) is the first current at the first endpoint at time t, and Z C is the wave impedance.

[0026] The relay protection of the present invention first obtains the first voltage and the first current of the first terminal, obtains the second voltage and the second current of the second terminal, and obtains the wave impedance and the wave velocity of the AC line at a set frequency; wherein the set frequency is set arbitrarily, and the wave impedance and the wave velocity of the AC line are related to the frequency. Therefore, after determining the set frequency, the wave impedance and the wave velocity of the AC line at the set frequency can be determined, and then the current value of the forward wave and the current value of the reverse wave at the first terminal can be determined based on the first voltage, the first current and the wave impedance of the AC line at the first terminal; and then the current value of the forward wave and the current value of the reverse wave at the second terminal are determined based on the second voltage, the second current and the wave impedance of the AC line at the second terminal. ; According to the distance and wave velocity between the first endpoint and the second endpoint, the flow time τ of the current flowing from the first endpoint to the second endpoint can be determined; based on the current value of the reverse wave of the first endpoint, the set frequency, the flow time τ and the current value of the reverse wave of the second endpoint, the traveling wave differential of the first endpoint is determined; based on the current value of the front wave of the second endpoint, the set frequency, the flow time τ and the current value of the front wave of the first endpoint, the traveling wave differential of the second endpoint is determined; the sum of the traveling wave differential of the first endpoint and the sum of the traveling wave differential of the second endpoint within the detection time interval is calculated, and finally based on the sum of the traveling wave differential of the first endpoint, the sum of the traveling wave differential of the second endpoint and the first differential threshold, it is determined whether the AC line has a fault. The first differential threshold can be set according to different circuits, and it should be obtained by simulation analysis based on specific application scenarios. The relay protection method of the present invention mainly protects the AC line of the photovoltaic power generation flexible DC transmission system, detects the two endpoints of the AC line, and uses the traveling wave calculation results of the whole process to distinguish faults, with high protection reliability and higher accuracy. In addition, the integral value of the traveling wave differential momentum within the detection time is used as the judgment criterion, which has higher accuracy and will not cause misjudgment due to current mutations caused by certain special circumstances.

[0027] In addition, in the case of an in-zone fault, the high-frequency glitches in the traveling wave differential current are relatively small. This is because the fault point is close to the protection device and the high-frequency signal is quickly absorbed or attenuated. In the case of an out-of-zone fault, the fault point is far away from the protection device, and the high-frequency signal needs to travel a longer distance to reach the protection device, which allows the high-frequency signal to be retained and become an important feature of the traveling wave differential current. The present invention analyzes the fault signals of the first endpoint and the second endpoint by a wavelet transform method, and can analyze the proportion of high-frequency signals to low-frequency signals, thereby identifying the type of fault.

[0028] In some technical solutions, optionally, the relay protection method for the AC line of the photovoltaic power generation flexible DC transmission system also includes: using Karen Bell transformation to decouple the first detection signal and the second detection signal to obtain the α line mode voltage, α line mode current, β line mode voltage, β line mode current, zero mode voltage, and zero mode current of the first endpoint and the second endpoint; obtaining the positive sequence inductance, positive sequence capacitance, zero sequence inductance, zero sequence capacitance and set frequency of the AC line; based on the positive sequence inductance, positive sequence capacitance and set frequency, determining the α line mode wave impedance, β line mode wave impedance, α line mode wave velocity and β line mode wave velocity mode wave velocity; determine the zero mode wave impedance and the zero mode wave velocity based on the zero-sequence inductive reactance, the zero-sequence capacitive reactance and the set frequency; determine whether the AC line is faulty based on the α line mode wave impedance, the α line mode wave velocity, the β line mode wave impedance, the β line mode wave velocity, the α line mode voltage at the first endpoint, the α line mode current at the first endpoint, the α line mode voltage at the second endpoint, and the α line mode current at the second endpoint; or determine whether the AC line is faulty based on the zero mode wave impedance, the zero mode wave velocity, the zero mode voltage at the first endpoint, the zero mode current at the first endpoint, the zero mode voltage at the second endpoint, and the zero mode current at the second endpoint.

[0029] In this technical solution, the detection point signal is decoupled into three dimensions: α line mode, β line mode, and zero mode. Since the positive-sequence inductive reactance, positive-sequence capacitive reactance, zero-sequence inductive reactance, and zero-sequence capacitive reactance can be obtained through manufacturers or experiments, and the set frequency is set, the impedance and wave velocity of the α line mode, β line mode, and zero mode can be determined according to the positive-sequence inductive reactance, positive-sequence capacitive reactance, zero-sequence inductive reactance, zero-sequence capacitive reactance, and the set frequency. In this way, the current value of the forward wave and the current value of the reverse wave of the corresponding detection point can be calculated through the corresponding impedance and wave velocity, so that each detection point can calculate the sum of the three traveling wave differential moments, and then compare it with the corresponding differential moment threshold. It should be noted here that the differential moment threshold of the detection points corresponding to the α line mode and the β line mode is the same, which is different from the differential moment threshold of the detection point corresponding to the zero mode. These thresholds can be set according to actual conditions. The present invention adopts Karenbell transformation to decouple the first detection signal and the second detection signal, and then respectively determines the total traveling wave differential momentum of the detection points of the α line mode, the β line mode and the zero mode, which can avoid the influence of mutual coupling of the three-phase transmission lines and improve the accuracy.

[0030] In some technical schemes, optionally, the relay protection method for the AC line of the photovoltaic power generation flexible DC transmission system also includes: after a fault occurs in the AC line, determining whether the AC line fails within the next detection time interval; if the AC line is still faulty within the next detection time interval, cutting off the AC line.

[0031] In this technical solution, after an AC line fault occurs, the power supply of the AC line is not cut off immediately. Instead, it is first determined whether the AC line faults within the next detection time interval. If the AC line is still faulty within the next detection time interval, the AC line is cut off at this time. If the AC line is normal within the next detection time interval, it means that the fault may be caused by current fluctuations. At this time, there is no need to cut off the power supply of the AC line, thereby improving the accuracy.

[0032] In some technical solutions, optionally, the detection time interval is 2 ms to 4 ms.

[0033] In this technical solution, the detection time interval is 2ms to 4ms, that is, one detection cycle is 2ms to 4ms. During the detection cycle, the traveling wave differential momentum of the first endpoint and the second endpoint is respectively summed, so as to determine whether there is a fault. The detection time interval should not be too large. If the interval is too large, the calculation amount will increase. If the interval is too small, the measurement accuracy will be affected. For example, the detection time interval is 3ms.

[0034] In some technical solutions, optionally, the frequency of the high-frequency signal is greater than or equal to 5kHz and less than or equal to 10kHz, and the frequency of the low-frequency signal is greater than or equal to 1.25kHz and less than or equal to 2.5kHz. For example, the frequency of the high-frequency signal is equal to 10kHz, and the frequency of the low-frequency signal is equal to 2.5kHz. The high-frequency signal and the low-frequency signal are not limited to this, and they are mainly obtained by simulation analysis through different AC lines through scenarios, and change with the change of the AC line.

[0035] A second aspect of the present invention provides a relay protection device for an AC line of a photovoltaic power generation flexible DC transmission system. The photovoltaic power generation flexible DC transmission system includes a sending-end converter and a photovoltaic power generation grid-connected inverter. The AC line includes a first endpoint and a second endpoint. The first endpoint is connected to the sending-end converter, and the second endpoint is connected to the photovoltaic power generation grid-connected inverter. The relay protection device for the AC line of the photovoltaic power generation flexible DC transmission system includes: an acquisition unit, which is used to acquire a first detection signal of the first endpoint, the first detection signal includes a first voltage and a first current, and acquire a second detection signal of the second endpoint, the second detection signal includes a second voltage and a second current; and acquire the AC line at a set frequency. a wave impedance and a wave velocity at a certain rate; a determination unit for determining a current value of a forward wave and a current value of a reverse wave at a first endpoint based on a first voltage, a first current and a wave impedance of an AC line at the first endpoint; determining a current value of a forward wave and a current value of a reverse wave at a second endpoint based on a second voltage, a second current and a wave impedance of an AC line at a second endpoint; determining a flow time τ for a current to flow from the first endpoint to the second endpoint according to a distance and a wave velocity between the first endpoint and the second endpoint; determining a traveling wave differential amount at the first endpoint based on a current value of the reverse wave at the first endpoint, a set frequency, a flow time τ and a current value of the reverse wave at the second endpoint; determining a traveling wave differential amount at the first endpoint based on a current value of the forward wave at the second endpoint, a set frequency, a flow time τ and a current value of the reverse wave at the second endpoint; determining a traveling wave differential amount at the first endpoint based on a current value of the forward wave at the second endpoint, a set frequency, a flow time τ and a current value of the reverse wave at the second endpoint The traveling wave differential momentum of the second endpoint is determined based on the frequency, flow time τ and the current value of the previous traveling wave of the first endpoint; the sum of the traveling wave differential momentum of the first endpoint within the detection time interval is determined, wherein the detection time interval is greater than the flow time τ; the sum of the traveling wave differential momentum of the second endpoint within the detection time interval is determined; based on the sum of the traveling wave differential momentum of the first endpoint, the sum of the traveling wave differential momentum of the second endpoint and the first differential momentum threshold, it is determined whether the AC line has a fault; in the step of determining whether the AC line has a fault based on the sum of the traveling wave differential momentum of the first endpoint, the sum of the traveling wave differential momentum of the second endpoint and the first differential momentum threshold, the determining unit is specifically used to determine whether the AC line has a fault when the sum of the traveling wave differential momentum of the first endpoint is greater than or equal to the sum of the traveling wave differential momentum of the second endpoint. When the first differential quantity threshold is reached, it is determined that the AC line has a fault; or when the sum of the traveling wave differential quantities at the second endpoint is greater than or equal to the first differential quantity threshold, it is determined that the AC line has a fault; the determination unit is also used to, when determining that the AC line has a fault, extract the fault signals of the first endpoint and the second endpoint within the detection time interval before the current moment: analyze the fault signal by a wavelet transform method, and determine the ratio of the wavelet coefficients occupied by the high-frequency signal to the wavelet coefficients occupied by the low-frequency signal in the fault signal; when the ratio is less than or equal to a first preset value, determine that the fault type is an internal fault of the AC line; when the ratio is greater than the first preset value, determine that the fault type is an external fault of the AC line;Wherein, in the step of determining the traveling wave differential amount of the first endpoint based on the current value of the reverse traveling wave at the first endpoint, the set frequency, the flow time τ, and the current value of the reverse traveling wave at the second endpoint, the current value of the reverse traveling wave at the first endpoint, the set frequency, the flow time τ, the current value of the reverse traveling wave at the second endpoint, and the traveling wave differential amount of the first endpoint satisfy the following relationship:; in, is the traveling wave differential momentum of the first endpoint, is the current value of the reverse wave at the second terminal, e is a constant, j is an imaginary unit, and j satisfies 2 =-1, ω is the angular frequency, equal to 2πf, f is the set frequency, τ is the flow time, is the current value of the reverse traveling wave of the first endpoint at the current moment; in the step of determining the traveling wave differential amount of the second endpoint based on the current value of the forward traveling wave of the second endpoint, the set frequency, the flow time τ and the current value of the forward traveling wave of the first endpoint, the current value of the forward traveling wave of the second endpoint, the set frequency, the flow time τ, the current value of the forward traveling wave of the first endpoint and the traveling wave differential amount of the second endpoint satisfy the following relationship: in, is the traveling wave differential momentum at the second endpoint, is the current value of the forward wave at the first endpoint, e is a constant, j is an imaginary unit, and j satisfies 2 =-1, ω is the angular frequency, equal to 2πf, f is the set frequency, τ is the flow time, is the current value of the forward wave of the second endpoint at the current moment; in the step of determining the current value of the forward wave of the first endpoint and the current value of the reverse wave based on the first voltage, the first current and the wave impedance of the AC line of the first endpoint, the first voltage, the first current, the wave impedance of the first endpoint and the current value of the forward wave of the first endpoint satisfy the following relationship: Where i1(x, t) is the current value of the forward wave of the first endpoint at time t, u(x, t) is the first voltage of the first endpoint at time t, i(x, t) is the first current of the first endpoint at time t, and Z C is the wave impedance; the first voltage of the first endpoint, the first current, the wave impedance and the current value of the reverse wave at the first endpoint satisfy the following relationship: Where i2(x, t) is the current value of the reverse wave at the first endpoint at time t, u(x, t) is the first voltage at the first endpoint at time t, i(x, t) is the first current at the first endpoint at time t, and Z C is the wave impedance.

[0036] The third aspect of the present invention provides an electronic device, including a memory and a processor, wherein a computer program or instruction is stored in the memory, and when the processor executes the computer program or instruction, a relay protection method for an AC line applied to a photovoltaic power generation flexible DC transmission system as provided in any technical solution of the first aspect of the present application is implemented.

[0037] The fourth aspect of the present invention provides a readable storage medium, on which a program or instruction is stored. When a processor executes the program or instruction, a relay protection method for an AC line applied to a photovoltaic power generation flexible DC transmission system as provided in any technical solution of the first aspect of the present application is implemented.

[0038] Additional aspects and advantages of the present invention will become apparent in the following description or may be learned through practice of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] The above and / or additional aspects and advantages of the present invention will become apparent and easily understood from the description of the embodiments in conjunction with the following drawings, in which:

[0040] Figure 1 One of the flow charts of the relay protection method for AC line of photovoltaic power generation flexible DC transmission system of the present invention is shown;

[0041] Figure 2 A schematic structural diagram of a relay protection device for an AC line of a photovoltaic power generation flexible DC transmission system according to the present invention is shown;

[0042] Figure 3 A circuit diagram of an AC circuit of the present invention is shown;

[0043] Figure 4 A current conduction diagram of the relay protection method of the present invention applied to the AC line of the photovoltaic power generation flexible DC transmission system is shown;

[0044] Figure 5 The second flowchart of the relay protection method for AC line applied to the photovoltaic power generation flexible DC transmission system of the present invention is shown;

[0045] Figure 6 A block diagram of a relay protection device for an AC line of a photovoltaic power generation flexible DC transmission system according to the present invention is shown.

[0046] in, Figure 2 , Figure 3 and Figure 6 The corresponding relationship between the reference numerals and the component names is as follows:

[0047] 1 AC line, 102 first endpoint, 104 second endpoint, 12 capacitor, 14 inductor, 16 resistor, 18 conductance, 2 relay protection device for AC line of photovoltaic power generation flexible DC transmission system, 26 determination unit, 28 acquisition unit, 32 first protection device, 34 second protection device, 4 photovoltaic power generation flexible DC transmission system, 42 sending end converter, 44 photovoltaic power generation grid-connected inverter. DETAILED DESCRIPTION

[0048] In order to more clearly understand the above-mentioned purpose, features and advantages of the present invention, the present invention is further described in detail below in conjunction with the accompanying drawings and specific embodiments. It should be noted that the embodiments of the present application and the features in the embodiments can be combined with each other without conflict.

[0049] In the following description, many specific details are set forth to facilitate a full understanding of the present invention. However, the present invention may also be implemented in other ways different from those described herein. Therefore, the protection scope of the present invention is not limited to the specific embodiments disclosed below.

[0050] First, the application scenarios of the protection method of the present invention are introduced. Figure 3 As shown, the photovoltaic power generation flexible DC transmission system 4 includes a sending-end converter 42 and a photovoltaic power generation grid-connected inverter 44. The sending-end converter 42 is used to convert the DC power of the sending-end commutation power source into AC power. The photovoltaic power generation grid-connected inverter 44 is used to convert the DC power of the photovoltaic power source into AC power, thereby forming an AC line 1. The AC line 1 includes a first terminal 102 and a second terminal 104. The first terminal 102 is connected to the sending-end converter 42, and the second terminal 104 is connected to the photovoltaic power generation grid-connected inverter 44, that is, Figure 3 Any point on the M line can be regarded as the first endpoint 102, and any point on the N line can be regarded as the second endpoint 104. The relay protection method of the present invention detects the first endpoint 102 and the second endpoint 104 to determine whether the AC line 1 has a fault, and protects the first protection device 32 and the second protection device 34 on the AC line 1.

[0051] Specifically, Figure 1 As shown, the relay protection method for AC lines applied to the photovoltaic power generation flexible DC transmission system includes:

[0052] S102: Acquire a first detection signal of a first endpoint, the first detection signal includes a first voltage and a first current, and acquire a second detection signal of a second endpoint, the second detection signal includes a second voltage and a second current;

[0053] S104: Obtaining the wave impedance and wave velocity of the AC line at a set frequency;

[0054] S106: Determine the current value of the forward wave and the current value of the reverse wave at the first endpoint based on the first voltage, the first current and the wave impedance of the AC line at the first endpoint, and determine the current value of the forward wave and the current value of the reverse wave at the second endpoint based on the second voltage, the second current and the wave impedance of the AC line at the second endpoint;

[0055] S108: determining a flow time τ for the current to flow from the first endpoint to the second endpoint according to the distance between the first endpoint and the second endpoint and the wave velocity;

[0056] S110: Determine the traveling wave differential amount of the first endpoint based on the current value of the reverse traveling wave of the first endpoint, the set frequency, the flow time τ and the current value of the reverse traveling wave of the second endpoint, and determine the traveling wave differential amount of the second endpoint based on the current value of the forward traveling wave of the second endpoint, the set frequency, the flow time τ and the current value of the forward traveling wave of the first endpoint;

[0057] S112: Determine the sum of the traveling wave differential moments of the first endpoint within the detection time interval, wherein the detection time interval is greater than the flow time τ; determine the sum of the traveling wave differential moments of the second endpoint within the detection time interval; and determine whether a fault occurs in the AC line based on the sum of the traveling wave differential moments of the first endpoint, the sum of the traveling wave differential moments of the second endpoint, and the first differential moment threshold.

[0058] The relay protection of the AC line applied to the photovoltaic power generation flexible DC transmission system of the present invention first obtains the first voltage and the first current of the first terminal, obtains the second voltage and the second current of the second terminal, and obtains the wave impedance and the wave velocity of the AC line at a set frequency; wherein the set frequency is selected by comprehensively considering the fundamental frequency of the electrical quantity and the frequency-changing characteristics of the line, and must satisfy that the frequency-changing characteristics are relatively gentle near the selected frequency and are relatively computationally resource-saving, such as 50Hz, and the wave impedance and the wave velocity of the AC line are related to the frequency. Therefore, after determining the set frequency, the wave impedance and the wave velocity of the AC line at the set frequency can be determined, and then the current value of the forward wave and the current value of the reverse wave of the first terminal can be determined based on the first voltage, the first current and the wave impedance of the AC line; then based on the second voltage, the second current of the second terminal and the wave impedance of the AC line, determine the current value of the forward and reverse waves of the second endpoint; according to the distance and wave velocity between the first endpoint and the second endpoint, determine the flow time τ of the current from the first endpoint to the second endpoint; based on the current value of the reverse wave of the first endpoint, the set frequency, the flow time τ and the current value of the reverse wave of the second endpoint, determine the traveling wave differential of the first endpoint; based on the current value of the forward wave of the second endpoint, the set frequency, the flow time τ and the current value of the forward wave of the first endpoint, determine the traveling wave differential of the second endpoint; determine the detection time interval, and calculate the sum of the traveling wave differential of the first endpoint and the sum of the traveling wave differential of the second endpoint within the detection time interval, and finally determine whether the AC line is faulty based on the sum of the traveling wave differential of the first endpoint, the sum of the traveling wave differential of the second endpoint and the first differential threshold. The first differential threshold can be set according to different circuits, and it should be obtained by simulation analysis based on specific application scenarios. The relay protection method for AC lines of photovoltaic power generation flexible DC transmission system of the present invention mainly protects the AC lines of photovoltaic power generation flexible DC transmission system, detects the two endpoints of the AC lines, does not rely on the wave head information with extremely short existence time and complex changes with the fault conditions, but uses the traveling wave calculation results of the whole process to distinguish faults, and has high protection reliability and higher accuracy. In addition, the integral value of the traveling wave differential momentum within the detection time is used as the judgment criterion, which has higher accuracy and will not cause misjudgment due to the sudden change of current caused by some special circumstances.

[0059] Among them, the directions of the forward wave and the reverse wave are consistent with the current reference direction at their respective ends, such as Figure 3 As shown, for example, for the M terminal, the forward wave is the current flowing from M to N, and the reverse wave is the current flowing from N to M; but for the N terminal, the forward wave is the current flowing from N to M, and the reverse wave is the current flowing from M to N.

[0060] In some technical schemes, optionally, the step of determining whether an AC line fault occurs based on the sum of the traveling wave differential moments at the first endpoint, the sum of the traveling wave differential moments at the second endpoint and the first differential moment threshold specifically includes: determining that an AC line fault occurs when the sum of the traveling wave differential moments at the first endpoint is greater than or equal to the first differential moment threshold; or determining that an AC line fault occurs when the sum of the traveling wave differential moments at the second endpoint is greater than or equal to the first differential moment threshold.

[0061] In this technical solution, when the sum of the traveling wave differential moments at the first endpoint is greater than or equal to the first differential moment threshold, or when the sum of the traveling wave differential moments at the second endpoint is greater than or equal to the first differential moment threshold, it is determined that a fault has occurred in the AC line. The present invention does not rely on the wave head, but uses the traveling wave calculation results of the entire process to perform fault identification, which is more accurate.

[0062] In some technical schemes, optionally, the relay protection method for the AC line of the photovoltaic power generation flexible DC transmission system also includes: when it is determined that a fault occurs in the AC line, extracting the fault signals of the first endpoint and the second endpoint within the detection time interval before the current moment: analyzing the fault signal by means of wavelet transform, and determining the ratio of the wavelet coefficients occupied by the high-frequency signal to the wavelet coefficients occupied by the low-frequency signal in the fault signal; when the ratio is less than or equal to a first preset value, determining that the fault type is an internal fault of the AC line; when the ratio is greater than the first preset value, determining that the fault type is an external fault of the AC line.

[0063] In this technical solution, in the case of an in-zone fault, the high-frequency glitches in the traveling wave differential current are relatively small. This is because the fault point is close to the protection device and the high-frequency signal is quickly absorbed or attenuated. In the case of an out-of-zone fault, the fault point is far away from the protection device, and the high-frequency signal needs to travel a longer distance to reach the protection device, which allows the high-frequency signal to be retained and become an important feature of the traveling wave differential current. The present invention analyzes the fault signals at the first endpoint and the second endpoint by a wavelet transform method, and can analyze the proportion of high-frequency signals to low-frequency signals, thereby identifying the type of fault. Among them, the first preset value is obtained based on simulation analysis, for example, 5%, 10% or 20% can be selected.

[0064] In some technical solutions, optionally, the detection time interval is 2 ms to 4 ms.

[0065] In this technical solution, the detection time interval is 2ms to 4ms, that is, one detection cycle is 2ms to 4ms. During the detection cycle, the traveling wave differential momentum of the first endpoint and the second endpoint is respectively summed, so as to determine whether there is a fault. The detection time interval should not be too large. If the interval is too large, the calculation amount will increase. If the interval is too small, the measurement accuracy will be affected. For example, the detection time interval is 3ms.

[0066] In some technical solutions, optionally, the frequency of the high-frequency signal is greater than or equal to 5kHz and less than or equal to 10kHz, and the frequency of the low-frequency signal is greater than or equal to 1.25kHz and less than or equal to 2.5kHz. For example, the frequency of the high-frequency signal is equal to 10kHz, and the frequency of the low-frequency signal is equal to 2.5kHz. The high-frequency signal and the low-frequency signal are not limited to this, and they are mainly obtained by simulation analysis through different AC lines through scenarios, and change with the change of the AC line.

[0067] In some technical solutions, optionally, the relay protection method for the AC line of the photovoltaic power generation flexible DC transmission system also includes: using Karen Bell transformation to decouple the first detection signal and the second detection signal to obtain the α line mode voltage, α line mode current, β line mode voltage, β line mode current, zero mode voltage, and zero mode current of the first endpoint and the second endpoint; obtaining the positive sequence inductance, positive sequence capacitance, zero sequence inductance, zero sequence capacitance and set frequency of the AC line; based on the positive sequence inductance, positive sequence capacitance and set frequency, determining the α line mode wave impedance, β line mode wave impedance, α line mode wave velocity and β line mode wave velocity mode wave velocity; determine the zero mode wave impedance and the zero mode wave velocity based on the zero-sequence inductive reactance, the zero-sequence capacitive reactance and the set frequency; determine whether the AC line is faulty based on the α line mode wave impedance, the α line mode wave velocity, the β line mode wave impedance, the β line mode wave velocity, the α line mode voltage at the first endpoint, the α line mode current at the first endpoint, the α line mode voltage at the second endpoint, and the α line mode current at the second endpoint; or determine whether the AC line is faulty based on the zero mode wave impedance, the zero mode wave velocity, the zero mode voltage at the first endpoint, the zero mode current at the first endpoint, the zero mode voltage at the second endpoint, and the zero mode current at the second endpoint.

[0068] In this technical solution, the detection point signal is decoupled into three dimensions: α line mode, β line mode, and zero mode. Since the positive-sequence inductive reactance, positive-sequence capacitive reactance, zero-sequence inductive reactance, and zero-sequence capacitive reactance can be obtained through manufacturers or experiments, and the set frequency is set, the impedance and wave velocity of the α line mode, β line mode, and zero mode can be determined according to the positive-sequence inductive reactance, positive-sequence capacitive reactance, zero-sequence inductive reactance, zero-sequence capacitive reactance, and the set frequency. In this way, the current value of the forward wave and the current value of the reverse wave of the corresponding detection point can be calculated through the corresponding impedance and wave velocity, so that each detection point can calculate the sum of the three traveling wave differential moments, and then compare it with the corresponding differential moment threshold. It should be noted here that the differential moment threshold of the detection points corresponding to the α line mode and the β line mode is the same, which is different from the differential moment threshold of the detection point corresponding to the zero mode. These thresholds can be set according to actual conditions. The present invention adopts Karenbell transformation to decouple the first detection signal and the second detection signal, and then respectively determines the total traveling wave differential momentum of the detection points of the α line mode, the β line mode and the zero mode, which can avoid the influence of mutual coupling of the three-phase transmission lines and improve the accuracy.

[0069] In some technical schemes, optionally, the relay protection method for the AC line of the photovoltaic power generation flexible DC transmission system also includes: after a fault occurs in the AC line, determining whether the AC line fails within the next detection time interval; if the AC line is still faulty within the next detection time interval, cutting off the AC line.

[0070] In this technical solution, after an AC line fault occurs, the power supply of the AC line is not cut off immediately. Instead, it is first determined whether the AC line faults within the next detection time interval. If the AC line is still faulty within the next detection time interval, the AC line is cut off at this time. If the AC line is normal within the next detection time interval, it means that the fault may be caused by current fluctuations. At this time, there is no need to cut off the power supply of the AC line, thereby improving the accuracy.

[0071] In some technical solutions, optionally, in the step of determining the traveling wave differential amount of the first endpoint based on the current value of the reverse traveling wave at the first endpoint, the set frequency, the flow time τ, and the current value of the reverse traveling wave at the second endpoint, the current value of the reverse traveling wave at the first endpoint, the set frequency, the flow time τ, the current value of the reverse traveling wave at the second endpoint, and the traveling wave differential amount of the first endpoint satisfy the following relationship:

[0072]

[0073] in, is the traveling wave differential momentum of the first endpoint, is the current value of the reverse wave at the second terminal, e is a constant, j is an imaginary unit, and j satisfies 2 =-1, ω is the angular frequency, equal to 2πf, f is the set frequency, τ is the flow time, is the current value of the reverse traveling wave at the first endpoint at the current moment;

[0074] In the step of determining the traveling wave differential momentum of the second endpoint based on the current value of the preceding traveling wave of the second endpoint, the set frequency, the flow time τ and the current value of the preceding traveling wave of the first endpoint, the current value of the preceding traveling wave of the second endpoint, the set frequency, the flow time τ, the current value of the preceding traveling wave of the first endpoint and the traveling wave differential momentum of the second endpoint satisfy the following relationship:

[0075]

[0076] in, is the traveling wave differential momentum at the second endpoint, is the current value of the forward wave at the first endpoint, e is a constant, j is an imaginary unit, and j satisfies 2 =-1, ω is the angular frequency, equal to 2πf, f is the set frequency, τ is the flow time, It is the current value of the forward wave at the second endpoint at the current moment.

[0077] In some technical solutions, optionally, in the step of determining the current value of the forward wave and the current value of the reverse wave at the first endpoint based on the first voltage, the first current and the wave impedance of the AC line at the first endpoint, the first voltage, the first current, the wave impedance and the current value of the forward wave at the first endpoint satisfy the following relationship:

[0078]

[0079] Where i1(x, t) is the current value of the forward wave of the first endpoint at time t, u(x, t) is the first voltage of the first endpoint at time t, i(x, t) is the first current of the first endpoint at time t, and Z C is the wave impedance;

[0080] The first voltage, the first current, the wave impedance of the first endpoint and the current value of the reverse wave at the first endpoint satisfy the following relationship:

[0081]

[0082] Where i2(x, t) is the current value of the reverse wave at the first endpoint at time t, u(x, t) is the first voltage at the first endpoint at time t, i(x, t) is the first current at the first endpoint at time t, and Z C is the wave impedance.

[0083] like Figure 3 and Figure 6As shown, the second aspect of the present invention provides a relay protection device 2 for an AC line of a photovoltaic power generation flexible DC transmission system, the photovoltaic power generation flexible DC transmission system 4 includes a sending-end converter 42 and a photovoltaic power generation grid-connected inverter 44, the AC line 1 includes a first endpoint 102 and a second endpoint 104, the first endpoint 102 is connected to the sending-end converter 42, and the second endpoint 104 is connected to the photovoltaic power generation grid-connected inverter 44; the relay protection device 2 for an AC line of a photovoltaic power generation flexible DC transmission system includes: an acquisition unit 28, for acquiring a first detection signal of the first endpoint 102, the first detection signal includes a first voltage and a first current, and acquiring a second detection signal of the second endpoint 104, the second detection signal includes a second voltage and a second current; obtaining the wave impedance and wave velocity of the AC line at the set frequency; the determination unit 26 is used to determine the current value of the forward wave and the current value of the reverse wave of the first terminal 102 based on the first voltage of the first terminal 102, the first current and the wave impedance of the AC line 1; determine the current value of the forward wave and the current value of the reverse wave of the second terminal 104 based on the second voltage of the second terminal 104, the second current and the wave impedance of the AC line 1; determine the flow time τ of the current flowing from the first terminal 102 to the second terminal 104 according to the distance between the first terminal 102 and the second terminal 104 and the wave velocity; determine the current value of the reverse wave of the first terminal 102, the set frequency, the flow time τ and the current value of the reverse wave of the second terminal 104, The traveling wave differential momentum of point 102; based on the current value of the previous traveling wave of the second endpoint 104, the set frequency, the flow time τ and the current value of the previous traveling wave of the first endpoint 102, determine the traveling wave differential momentum of the second endpoint 104; determine the sum of the traveling wave differential momentum of the first endpoint 102 within the detection time interval, wherein the detection time interval is greater than the flow time τ; determine the sum of the traveling wave differential momentum of the second endpoint 104 within the detection time interval; based on the sum of the traveling wave differential momentum of the first endpoint 102, the sum of the traveling wave differential momentum of the second endpoint 104 and the first differential momentum threshold, determine whether the AC line 1 has a fault; based on the sum of the traveling wave differential momentum of the first endpoint 102, the sum of the traveling wave differential momentum of the second endpoint 104 and the first differential momentum threshold, determine whether the AC line In the step of fault occurrence, the determination unit 26 is specifically used to determine that the AC line has a fault when the sum of the traveling wave differential moments at the first endpoint 102 is greater than or equal to the first differential moment threshold; or to determine that the AC line has a fault when the sum of the traveling wave differential moments at the second endpoint 104 is greater than or equal to the first differential moment threshold; the determination unit 26 is also used to extract the fault signals of the first endpoint 102 and the second endpoint 104 within the detection time interval before the current moment when determining that the AC line has a fault: analyze the fault signal by a wavelet transform method, and determine the ratio of the wavelet coefficients occupied by the high-frequency signal to the wavelet coefficients occupied by the low-frequency signal in the fault signal; when the ratio is less than or equal to the first preset value, determine that the fault type is an internal fault of the AC line;When the ratio is greater than the first preset value, the fault type is determined to be an external fault of the AC line; wherein, in the step of determining the traveling wave differential amount of the first endpoint 102 based on the current value of the reverse traveling wave of the first endpoint 102, the set frequency, the flow time τ, and the current value of the reverse traveling wave of the second endpoint 104, the current value of the reverse traveling wave of the first endpoint 102, the set frequency, the flow time τ, the current value of the reverse traveling wave of the second endpoint 104, and the traveling wave differential amount of the first endpoint 102 satisfy the following relationship:; in, is the traveling wave differential momentum of the first endpoint 102, is the current value of the reverse wave at the second terminal 104, e is a constant, j is an imaginary unit, and j satisfies 2 =-1, ω is the angular frequency, equal to 2πf, f is the set frequency, τ is the flow time, is the current value of the reverse traveling wave of the first terminal 102 at the current moment; in the step of determining the traveling wave differential amount of the second terminal 104 based on the current value of the previous traveling wave of the second terminal 104, the set frequency, the flow time τ and the current value of the previous traveling wave of the first terminal 102, the current value of the previous traveling wave of the second terminal 104, the set frequency, the flow time τ, the current value of the previous traveling wave of the first terminal 102 and the traveling wave differential amount of the second terminal 104 satisfy the following relationship: in, is the traveling wave differential momentum of the second endpoint 104, is the current value of the forward wave of the first terminal 102, e is a constant, j is an imaginary unit, and j satisfies 2 =-1, ω is the angular frequency, equal to 2πf, f is the set frequency, τ is the flow time, is the current value of the forward wave of the second terminal 104 at the current moment; in the step of determining the current value of the forward wave and the current value of the reverse wave of the first terminal 102 based on the first voltage, the first current and the wave impedance of the AC line of the first terminal 102, the first voltage, the first current, the wave impedance of the first terminal 102 and the current value of the forward wave of the first terminal 102 satisfy the following relationship: Wherein, i1(x, t) is the current value of the forward wave of the first terminal 102 at time t, u(x, t) is the first voltage of the first terminal 102 at time t, i(x, t) is the first current of the first terminal 102 at time t, and Z C is the wave impedance; the first voltage of the first terminal 102, the first current, the wave impedance and the current value of the reverse wave of the first terminal 102 satisfy the following relationship: Wherein, i2(x, t) is the current value of the reverse wave of the first terminal 102 at time t, u(x, t) is the first voltage of the first terminal 102 at time t, i(x, t) is the first current of the first terminal 102 at time t, and Z C is the wave impedance.

[0084] The third aspect of the present invention provides an electronic device, including a memory and a processor, wherein a computer program or instruction is stored in the memory, and when the processor executes the computer program or instruction, a relay protection method for an AC line applied to a photovoltaic power generation flexible DC transmission system as provided in any technical solution of the first aspect of the present application is implemented.

[0085] The fourth aspect of the present invention provides a readable storage medium, on which a program or instruction is stored. When a processor executes the program or instruction, a relay protection method for an AC line applied to a photovoltaic power generation flexible DC transmission system as provided in any technical solution of the first aspect of the present application is implemented.

[0086] Another embodiment of the present invention provides a relay protection method for an AC line of a photovoltaic power generation flexible DC transmission system.

[0087] First of all, it is important to understand that in the photovoltaic power generation flexible DC transmission system, the two sides of the AC line are modular multilevel converters (MMC, Modular Multilevel Converter) and photovoltaic power generation grid-connected inverters, both of which are power electronic devices. In order to improve the continuous power supply capacity after a fault and the operating capacity under unbalanced conditions, the flexible DC converter and photovoltaic power generation will adopt complex control strategies. Typical examples include low-voltage current limiting control of flexible DC converters and negative sequence current suppression control of photovoltaic power generation. One of the fundamental goals of various types of control strategies is to limit the fault current to prevent the converter valve in the converter from overcurrent, which leads to the problem that the fault characteristics of the AC transmission line are not obvious when a fault occurs.

[0088] Specifically, the photovoltaic power generation flexible DC transmission AC line is a double-end weak feed line. The meaning of double-end weak feed is that no significant fault current is provided on both sides after the fault. This feature comes from the strict current limiting measures of power electronic devices. In order to protect the converter valve, the photovoltaic power generation grid-connected inverter and the flexible DC converter adopt control strategies to limit their output current after the fault. When the voltage drop is detected, the specific control strategies adopted by the converter include low-voltage current limiting, negative sequence voltage suppression, negative sequence current suppression, and double frequency power suppression. These control strategies make the two sides of the double-end weak feed line appear as controlled voltage sources and controlled current sources. The amplitude of the controlled power source is determined by the current limit target, which is generally 1.2 to 2 times the rated current, and the phase is determined by the voltage drop level. Therefore, the fault current has two significant characteristics, the amplitude is limited and there is a phase difference between the currents on both sides. This makes the action criteria of the existing relay protection scheme unclear, or even offset, resulting in insufficient sensitivity.

[0089] The existing AC line protection for renewable energy transmission projects via flexible DC transmission generally adopts traditional current differential protection, but this solution has the problem of insufficient sensitivity, especially when there is transition resistance in the fault, the current differential protection has a greater risk of refusal to operate. At the same time, the distance protection widely used as the backup protection of AC lines has the problem of measured impedance offset. This is because the amplitude and phase of the power supply on both sides under the control strategy are controlled, resulting in a forced phase difference between the fault currents on both sides.

[0090] Traveling wave protection is based on the distributed parameter model of the line. The protection scheme based on the traveling wave principle has the characteristics of fast, high sensitivity, strong resistance to transition resistance, and can be unaffected by the complex control characteristics of the back side. There are protection technologies constructed using the traveling wave head and differential traveling wave principles. Single-ended and double-ended protection technologies based on the traveling wave principle have been applied to flexible DC line protection, AC line protection and other scenarios. However, the existing traveling wave protection generally has the limitations of the risk of wave head feature loss, high sampling rate, and large communication volume.

[0091] In addition, a relay protection scheme for long AC lines has been proposed, which utilizes the different energy frequency domain information during faults inside and outside the zone, and forms auxiliary judgment criteria based on this principle to prevent incorrect braking of relay protection under high-resistance faults within the zone.

[0092] In addition, the fault current amplitude of the double-terminal weak-feed line is limited, and the phase-controlled characteristics cause impacts such as insignificant action criteria, reversed relationship between the action quantity and the braking quantity, and failure of directional element judgment. The fundamental reason is that the corresponding protection scheme does not fully extract the electrical quantity information and does not clearly distinguish between the AC line and the back power supply of the protection equipment.

[0093] In view of this factor, the present invention provides a relay protection method for AC lines applied to photovoltaic power generation flexible DC transmission systems, that is, a high-sensitivity traveling wave differential protection method suitable for photovoltaic power generation flexible DC transmission double-end weak-feed lines, including a modeling method for double-end weak-feed lines, decoupling of three-phase systems and calculation of modulus traveling waves, calculation and setting principles of relay protection criteria using full-process information of faults, and a sensitivity verification method.

[0094] Specifically, the double-terminal weakly fed AC transmission line is first modeled using a relatively essential distributed parameter circuit model, such as Figure 2 As shown, the AC line 1 is mainly composed of a capacitor 12, an inductor 14, a resistor 16 and a conductor 18. During the assembly process, the two ends of the AC line 1 are installed between the sending-end converter 42 of the relay protection device 2 of the AC line applied to the photovoltaic power generation flexible DC transmission system and the photovoltaic power generation grid-connected inverter 44. Or, as Figure 3As shown, another protected circuit structure includes a first protection device 32 and a second protection device 34, and the first protection device 32 and the second protection device 34 are arranged between the sending-end converter 42 and the photovoltaic power generation grid-connected inverter 44. The AC line 1 of the present invention is a double-end weak feed line. Since the line of the double-end weak feed line is a transmission line with a limited length, the resistance and conductance can be ignored, so that the relationship shown in the following formula is obtained:

[0095]

[0096] Among them, each parameter and Chinese explanation are as follows (it should be noted that the following explanation is not limited to the above formula, and is applicable to the formula of the entire application. The subsequent repeated parameters will not be explained again):

[0097] x: represents the position coordinates of the detection point on the line.

[0098] t: indicates time.

[0099] L: represents inductive reactance.

[0100] C: represents capacitive reactance.

[0101] u(x,t): represents the voltage at position x and time t.

[0102] i(x,t): represents the current at position x at time t.

[0103] Represents the partial derivative of the voltage u(x,t) with respect to the position x, that is, the voltage gradient.

[0104] It represents the partial derivative of current i(x,t) with respect to time t, that is, the rate of change of current with time.

[0105] Represents the partial derivative of the current i(x,t) with respect to the position x, that is, the current gradient.

[0106] It represents the partial derivative of voltage u(x,t) with respect to time t, that is, the rate of change of voltage with time.

[0107] Then there is the relationship shown by the following wave equation:

[0108]

[0109] The general solution is:

[0110]

[0111] Among them, each parameter and Chinese explanation are as follows (it should be noted that the following explanation is not limited to the above formula, and is applicable to the formula of the entire application. The subsequent repeated parameters will not be explained again):

[0112] u1: voltage value of the forward wave;

[0113] u2: voltage value of the reverse wave;

[0114] i1: current value of the forward wave;

[0115] i2: current value of the reverse wave;

[0116] Z C is the wave impedance, v is the wave velocity, For the traveling waves u1,u2,i1,i2 on the fault-free line, when When the traveling waves are the same, the traveling wave quantities are the same due to the invariance of traveling wave transmission.

[0117] The voltage and current of the forward and reverse waves at the detection point can be obtained by the following formula:

[0118]

[0119] The present invention constructs the traveling wave differential momentum according to the current traveling waves i1, i2, and uses the traveling wave differential momentum as the protection criterion. Figure 4 For the circuit shown, the traveling wave differential momentum used for calculation on the M side is the difference between the reverse wave of the current on the M side at the current moment and the forward wave of the current on the N side before time τ, and the traveling wave differential momentum used for calculation on the N side is the difference between the reverse wave of the current on the N side at the current moment and the forward wave of the current on the M side before time τ.

[0120] The present invention performs calculation in the form of phasor. The frequency corresponding to the phasor can be selected with reference to the frequency variation of line parameters, sampling conditions, etc. The traveling wave differential momentum is calculated as follows:

[0121]

[0122] Among them, the parameters and Chinese explanations are as follows:

[0123] is the traveling wave differential momentum on the M side.

[0124] is the travelling wave differential momentum on the N side.

[0125] is the current value of the reverse traveling wave on the N side.

[0126] is the current value of the forward wave on the M side.

[0127] is the current value of the reverse traveling wave on the M side.

[0128] is the current value of the forward wave on the N side.

[0129] e is a constant.

[0130] j is an imaginary unit, satisfying j 2 =-1.

[0131] ω is the angular frequency, equal to 2πf, where f is the set frequency.

[0132] Therefore, according to this formula, when the current values ​​of the forward and reverse waves on the M and N sides are calculated, the traveling wave differential amount on the M side and the traveling wave differential amount on the N side can be calculated.

[0133] The present invention constructs the protection principle based on the amplitude of the traveling wave differential current phasor, and the judgment formula is as follows:

[0134]

[0135] Among them, the parameters and Chinese explanations are as follows:

[0136] t s is the detection start time, t w is the detection duration, is the current traveling wave differential momentum expressed in phasor form, and ε is the threshold value, which can be set according to different situations.

[0137] The setting principle is that when the most serious non-fault disturbance occurs, the protection principle will not operate reliably.

[0138] Among them, ε=KI max ;

[0139] I maX It is the maximum value of the integral within the time window of the most serious non-fault disturbance, and K is a coefficient greater than 1.

[0140] Thus, when the integral of the traveling wave differential momentum on the M side or the N side during the detection period is greater than the threshold value ε, it indicates that a fault has occurred in the AC line.

[0141] In addition, in order to improve the protection sensitivity and reliability, when the amplitude time window integral value criterion is met, the present invention can also perform internal and external fault judgment.

[0142] The principle of the internal and external fault judgment criteria is that when there is an internal fault, the traveling wave differential current is basically dominated by the power frequency and attenuated DC components, and the high-frequency burrs are relatively small. When there is an external fault, the traveling wave differential current has significant high-frequency burrs.

[0143] In the case of an in-zone fault, the high-frequency burrs in the traveling wave differential current are relatively small. This is because the fault point is close to the protection device and the high-frequency signal is quickly absorbed or attenuated. In the case of an out-of-zone fault, the fault point is far away from the protection device and the high-frequency signal needs to travel a longer distance to reach the protection device, which allows the high-frequency signal to be retained and become an important feature of the traveling wave differential current.

[0144] Dyadic wavelet transform is a signal time-frequency analysis method with great computational convenience. The present invention uses the derivative of the cubic B-spline function as the mother wavelet to transform the t s to s +t w The signal during the detection period is subjected to wavelet transform, and the wavelet transform result at each scale is modulo the maximum value. The formula for the fault inside and outside the construction area is as follows:

[0145]

[0146] Among them, the parameters and Chinese explanations are as follows:

[0147] WTMM1 is the wavelet transform modulus maximum at scale 1, WTMM 3,4 is the maximum value of the wavelet transform modulus at scale 3 and scale 4, max is the maximum value, is the ratio parameter, is the threshold, i X (t) is the current value of endpoint X at time t, t s is the starting time of the fault signal, t w For the time interval.

[0148] That is, the ratio of the maximum value of the scale 1 modulus maximum to the maximum value of the scale 3 and 4 modulus maximum. Scale 1 represents high frequency, and scales 3 and 4 represent low frequency. When the ratio parameter Less than the set value When the ratio parameter is Greater than or equal to the set value This indicates that there are more high-frequency signals and fewer low-frequency signals, and it is determined to be an out-of-zone fault.

[0149] In addition, in order to eliminate the influence of mutual coupling of three-phase transmission lines and improve the real-time performance of protection calculation, the present invention adopts Karen Bell transformation to decouple the three-phase system into α line mode, β line mode and zero mode. Karen Bell transformation is as follows:

[0150]

[0151] Among them, u α and u β is the line mode component after decoupling, u0 is the zero mode component, ua 、u b 、u c is the three-phase voltage (it should be noted that this explanation is not limited to the above formula, and is applicable to the formulas of the entire application. The subsequent repeated parameters will not be explained again).

[0152] In the modulus network, there is no mutual coupling of the three-phase transmission lines, so the traveling wave calculation can be performed according to the method of a single transmission line. At this time, the wave impedance and wave velocity of the line mode and zero mode need to be calculated.

[0153] Taking α mode and 0 mode as examples, suppose the self-impedance and mutual impedance in the three-phase system are Z s ,Z m ,Right now:

[0154]

[0155] Among them, Z s is the self-impedance, Z m is the mutual impedance, I a ,I b ,I c is the three-phase current.

[0156] Then we have:

[0157] u α =u a -u b =i a Z s +i b Z m +i c Z m -i a Z m -i b Z s -i c Z m =(Z s -

[0158] Z m )(i a -i b );

[0159]

[0160] From the above formula, we can know that the line mode and zero mode impedance are exactly the same as the positive sequence and zero sequence networks. The line mode and zero mode inductive reactance and capacitive reactance can be obtained from the positive sequence and zero sequence impedance, and the line mode and zero mode wave impedance and wave velocity can be further calculated. The calculation formula is as follows:

[0161]

[0162] Among them, ZCα is the α line mode wave impedance, Z Cβ is the β line mode wave impedance, Z C0 is the zero mode wave impedance, v α is the α line mode wave velocity, v β is the β line mode wave velocity, v0 is the zero mode wave velocity;

[0163] In addition, X L+ ,X C+ ,X L0 ,X C0 They are respectively the positive-sequence inductive reactance, positive-sequence capacitive reactance, zero-sequence inductive reactance and zero-sequence capacitive reactance of the line, all of which are frequency-variable parameters. ω is the angular frequency, which is equal to 2πf, where f is the set frequency.

[0164] Through the above formula, the positive-sequence inductive reactance, positive-sequence capacitive reactance, zero-sequence inductive reactance and zero-sequence capacitive reactance can all be obtained through previous experiments, so that the wave impedance and wave velocity under the α line mode, β line mode and zero mode can be calculated, and then the sum of the traveling wave differential momentum under the α line mode, β line mode and zero mode can be obtained according to the same method as above, and then compared with the corresponding differential momentum thresholds under the α line mode, β line mode and zero mode. As long as the sum of the traveling wave differential momentum of one of the α line mode, β line mode and zero mode is greater than the differential momentum threshold, it is determined to be a fault.

[0165] Referring to the sensitivity verification method of the traditional protection scheme, the sensitivity verification of this scheme is as follows:

[0166] The time window integral value of the traveling wave differential current amplitude of the fault in the zone under different fault types and transition resistance after entering the fault steady state is calculated, and its ratio to the set value is calculated as the sensitivity coefficient.

[0167] Another embodiment of the present invention provides a relay protection method for an AC line applied to a photovoltaic power generation flexible DC transmission system.

[0168] Firstly, the line mode and zero mode traveling wave impedance and wave velocity are calculated according to the positive sequence and zero sequence parameters of the transmission line parameters and the selected frequency, the transmission delay of the line mode and zero mode traveling wave is calculated, and the three-phase voltage and current are transformed into phase mode to obtain the line mode and zero mode voltage and current;

[0169] Secondly, calculate the line mode and zero mode current forward waves and at the protection installations at both ends of the line, and calculate their phasors at the selected frequency respectively, and calculate the phasors of the line mode and zero mode traveling wave differential currents at the selected frequency at both ends;

[0170] Secondly, calculate the integral value of the phasor amplitude in the corresponding time window, and apply the amplitude time window integral criterion to determine whether it is a fault. Here, the time window width is selected as 3ms. When any integral value of the line mode and zero mode exceeds the set threshold, it is determined to be a fault, and the fault criterion calculation inside and outside the zone is performed.

[0171] After the amplitude time window integral criterion is established, a delay is performed to ensure that the traveling wave differential current used for calculation inside and outside the zone is post-fault data. The wavelet transform modulus maximum ratio of the time domain traveling wave differential current is calculated. When it is less than the set value, it is determined to be an inside fault, otherwise it is determined to be an outside fault. Here, it is set according to the average value of the most serious outside fault and the weakest inside fault.

[0172] Finally, the sensitivity coefficient of the protection scheme is verified through simulation models and other means.

[0173] like Figure 5 As shown, this embodiment provides another relay protection method for an AC line of a photovoltaic power generation flexible DC transmission system, comprising the following steps:

[0174] S502: protection starts;

[0175] S504: Acquire positive-sequence and zero-sequence frequency conversion parameters of the AC line;

[0176] S506: Obtain line mode, zero mode wave impedance, and wave velocity;

[0177] S508: Obtaining a double-ended line mode, a zero mode forward wave, and a reverse wave;

[0178] S510: Calculate the two-terminal line mode and zero mode traveling wave differential current i x (t);

[0179] S512: Calculation of two-terminal modulus traveling wave differential current vector

[0180] S514: Judgment The absolute value of s -t w to s Is the integral of t greater than ε? If so, execute S516; if not, execute S518:

[0181] S516: Determine whether max(WTMM1(ix(t)) / max(WTMM3,4(ix(t))) is less than Lset; if so, execute S522; if not, execute S520;

[0182] S518: Determine that it is an interference fault;

[0183] S520: Determine that the fault is outside the zone;

[0184] S522: Determine that it is an in-zone fault.

[0185] The above is only an implementation method of the present invention and is not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A relay protection method for AC lines in a photovoltaic power generation flexible DC transmission system, characterized in that: The photovoltaic power generation flexible direct current transmission system comprises a sending-end converter and a photovoltaic power generation grid-connected inverter, the AC line comprises a first endpoint and a second endpoint, the first endpoint is connected to the sending-end converter, and the second endpoint is connected to the photovoltaic power generation grid-connected inverter, and the relay protection method for the AC line of the photovoltaic power generation flexible direct current transmission system comprises: Acquire a first detection signal of the first endpoint, where the first detection signal includes a first voltage and a first current; Acquire a second detection signal of the second endpoint, where the second detection signal includes a second voltage and a second current; Obtaining the wave impedance and wave velocity of the AC line at a set frequency; Determine a current value of a forward wave and a current value of a reverse wave at the first endpoint based on the first voltage at the first endpoint, the first current, and a wave impedance of the AC line; Determine a current value of a forward wave and a current value of a reverse wave at the second endpoint based on the second voltage at the second endpoint, the second current, and the wave impedance of the AC line; determining a flow time τ for a current to flow from the first endpoint to the second endpoint based on the distance between the first endpoint and the second endpoint and the wave velocity; Determine the traveling wave differential amount of the first endpoint based on the current value of the reverse traveling wave of the first endpoint, the set frequency, the flow time τ, and the current value of the reverse traveling wave of the second endpoint; determining a traveling wave differential momentum at the second end point based on the current value of the preceding traveling wave at the second end point, the set frequency, the flow time τ, and the current value of the preceding traveling wave at the first end point; Determine the sum of the traveling wave differential momentum of the first endpoint within a detection time interval, wherein the detection time interval is greater than the flow time τ; Determining the sum of the traveling wave differential momentum of the second endpoint within the detection time interval; Determining whether a fault occurs in the AC line based on the total of the traveling wave differential quantities of the first endpoint, the total of the traveling wave differential quantities of the second endpoint, and a first differential quantity threshold; The step of determining whether the AC line fails based on the total traveling wave differential quantity of the first endpoint, the total traveling wave differential quantity of the second endpoint, and the first differential quantity threshold specifically includes: When the sum of the traveling wave differential quantities of the first endpoints is greater than or equal to the first differential quantity threshold, determining that a fault occurs in the AC line; or When the sum of the traveling wave differential quantities at the second endpoints is greater than or equal to the first differential quantity threshold, determining that a fault occurs in the AC line; The relay protection method for AC lines applied to the photovoltaic power generation flexible DC transmission system also includes: When it is determined that the AC line fails, the fault signals of the first endpoint and the second endpoint within the detection time interval before the current moment are extracted: Analyzing the fault signal by wavelet transform method, and determining the ratio of wavelet coefficients occupied by high-frequency signal to wavelet coefficients occupied by low-frequency signal in the fault signal; When the ratio is less than or equal to a first preset value, determining that the fault type is an internal fault of the AC line; When the ratio is greater than the first preset value, determining that the fault type is an external fault of the AC line; Wherein, in the step of determining the traveling wave differential amount of the first endpoint based on the current value of the reverse traveling wave of the first endpoint, the set frequency, the flow time τ, and the current value of the reverse traveling wave of the second endpoint, the current value of the reverse traveling wave of the first endpoint, the set frequency, the flow time τ, the current value of the reverse traveling wave of the second endpoint, and the traveling wave differential amount of the first endpoint satisfy the following relationship: in, is the traveling wave differential momentum of the first endpoint, is the current value of the reverse wave at the second terminal, e is a constant, j is an imaginary unit, and j satisfies 2 = -1, ω is the angular frequency and is equal to 2πf, f is the set frequency, τ is the flow time, is the current value of the reverse traveling wave at the first endpoint at the current moment; In the step of determining the traveling wave differential amount of the second endpoint based on the current value of the preceding traveling wave of the second endpoint, the set frequency, the flow time τ, and the current value of the preceding traveling wave of the first endpoint, the current value of the preceding traveling wave of the second endpoint, the set frequency, the flow time τ, the current value of the preceding traveling wave of the first endpoint, and the traveling wave differential amount of the second endpoint satisfy the following relationship: in, is the traveling wave differential momentum of the second end point, is the current value of the forward wave at the first endpoint, e is a constant, j is an imaginary unit, and j satisfies 2 =-1, ω is the angular frequency, equal to 2πf, f is the set frequency, τ is the flow time, is the current value of the forward wave of the second end point at the current moment; In the step of determining the current value of the forward wave and the current value of the reverse wave at the first endpoint based on the first voltage, the first current and the wave impedance of the AC line at the first endpoint, the first voltage, the first current, the wave impedance and the current value of the forward wave at the first endpoint satisfy the following relationship: Where i1(x, t) is the current value of the forward wave of the first endpoint at time t, u(x, t) is the first voltage of the first endpoint at time t, i(x, t) is the first current of the first endpoint at time t, and Z C is the wave impedance; The first voltage at the first endpoint, the first current, the wave impedance, and the current value of the reverse wave at the first endpoint satisfy the following relationship: Where i2(x, t) is the current value of the reverse wave at the first endpoint at time t, u(x, t) is the first voltage at the first endpoint at time t, i(x, t) is the first current at the first endpoint at time t, and Z C is the wave impedance.

2. The relay protection method for AC lines applied to a photovoltaic power generation flexible DC transmission system according to claim 1 is characterized in that: Also includes: Decoupling the first detection signal and the second detection signal by using Karenberg transformation to obtain the α line mode voltage, α line mode current, β line mode voltage, β line mode current, zero mode voltage, and zero mode current of the first endpoint and the second endpoint; Obtaining positive-sequence inductive reactance, positive-sequence capacitive reactance, zero-sequence inductive reactance, zero-sequence capacitive reactance and a set frequency of the AC line; Determining the α line mode wave impedance, the β line mode wave impedance, the α line mode wave velocity, and the β line mode wave velocity based on the positive sequence inductive reactance, the positive sequence capacitive reactance, and the set frequency; Determining zero-mode wave impedance and zero-mode wave velocity based on the zero-sequence inductive reactance, the zero-sequence capacitive reactance and the set frequency; Determining whether a fault occurs in the AC line based on the α line mode wave impedance, the α line mode wave velocity, the β line mode wave impedance, the β line mode wave velocity, the α line mode voltage at the first endpoint, the α line mode current at the first endpoint, the α line mode voltage at the second endpoint, and the α line mode current at the second endpoint; or Whether a fault occurs in the AC line is determined based on the zero-mode wave impedance, the zero-mode wave velocity, the zero-mode voltage at the first endpoint, the zero-mode current at the first endpoint, the zero-mode voltage at the second endpoint, and the zero-mode current at the second endpoint.

3. The relay protection method for AC lines applied to a photovoltaic power generation flexible DC transmission system according to claim 1 is characterized in that: Also includes: After the AC line fails, it is determined whether the AC line fails within a next detection time interval. If the AC line still fails within the next detection time interval, the AC line is cut off.

4. The relay protection method for AC lines applied to a photovoltaic power generation flexible DC transmission system according to claim 1 is characterized in that: The detection time interval is 2 ms to 4 ms.

5. The relay protection method for AC lines applied to a photovoltaic power generation flexible DC transmission system according to claim 4 is characterized in that: The detection time interval is 3 ms.

6. The relay protection method for AC lines applied to a photovoltaic power generation flexible DC transmission system according to claim 1 is characterized in that: The frequency of the high-frequency signal is greater than or equal to 5 kHz and less than or equal to 10 kHz, and the frequency of the low-frequency signal is greater than or equal to 1.25 kHz and less than or equal to 2.5 kHz.

7. The relay protection method for AC lines applied to a photovoltaic power generation flexible DC transmission system according to claim 6 is characterized in that: The frequency of the high-frequency signal is equal to 10 kHz, and the frequency of the low-frequency signal is equal to 2.5 kHz.

8. A relay protection device for AC lines used in a photovoltaic power generation flexible DC transmission system, characterized in that: The photovoltaic power generation flexible direct current transmission system comprises a sending-end converter and a photovoltaic power generation grid-connected inverter, the AC line comprises a first endpoint and a second endpoint, the first endpoint is connected to the sending-end converter, the second endpoint is connected to the photovoltaic power generation grid-connected inverter, and the relay protection device for the AC line of the photovoltaic power generation flexible direct current transmission system comprises: an acquisition unit, configured to acquire a first detection signal of the first endpoint, wherein the first detection signal includes a first voltage and a first current; Acquire a second detection signal of the second endpoint, where the second detection signal includes a second voltage and a second current; Obtaining the wave impedance and wave velocity of the AC line at a set frequency; a determining unit, configured to determine a current value of a forward wave and a current value of a reverse wave at the first endpoint based on the first voltage at the first endpoint, the first current, and a wave impedance of the AC line; Determine a current value of a forward wave and a current value of a reverse wave at the second endpoint based on the second voltage at the second endpoint, the second current, and the wave impedance of the AC line; determining a flow time τ for a current to flow from the first endpoint to the second endpoint according to a distance between the first endpoint and the second endpoint and a wave velocity; Determine the traveling wave differential amount of the first endpoint based on the current value of the reverse traveling wave of the first endpoint, the set frequency, the flow time τ, and the current value of the reverse traveling wave of the second endpoint; determining a traveling wave differential momentum at the second end point based on the current value of the preceding traveling wave at the second end point, the set frequency, the flow time τ, and the current value of the preceding traveling wave at the first end point; Determine the sum of the traveling wave differential momentum of the first endpoint within a detection time interval, wherein the detection time interval is greater than the flow time τ; Determining the sum of the traveling wave differential momentum of the second endpoint within the detection time interval; Determining whether a fault occurs in the AC line based on the total of the traveling wave differential quantities of the first endpoint, the total of the traveling wave differential quantities of the second endpoint, and a first differential quantity threshold; In the step of determining whether the AC line fails based on the total traveling wave differential quantity of the first endpoint, the total traveling wave differential quantity of the second endpoint, and the first differential quantity threshold, the determining unit is specifically configured to: When the sum of the traveling wave differential quantities of the first endpoints is greater than or equal to the first differential quantity threshold, determining that a fault occurs in the AC line; or When the sum of the traveling wave differential quantities at the second endpoints is greater than or equal to the first differential quantity threshold, determining that a fault occurs in the AC line; The determination unit is further configured to extract the fault signals of the first endpoint and the second endpoint within a detection time interval before a current moment when it is determined that the AC line has a fault; Analyzing the fault signal by wavelet transform method, and determining the ratio of wavelet coefficients occupied by high-frequency signal to wavelet coefficients occupied by low-frequency signal in the fault signal; When the ratio is less than or equal to a first preset value, determining that the fault type is an internal fault of the AC line; When the ratio is greater than the first preset value, determining that the fault type is an external fault of the AC line; Wherein, in the step of determining the traveling wave differential amount of the first endpoint based on the current value of the reverse traveling wave of the first endpoint, the set frequency, the flow time τ, and the current value of the reverse traveling wave of the second endpoint, the current value of the reverse traveling wave of the first endpoint, the set frequency, the flow time τ, the current value of the reverse traveling wave of the second endpoint, and the traveling wave differential amount of the first endpoint satisfy the following relationship: in, is the traveling wave differential momentum of the first endpoint, is the current value of the reverse wave at the second terminal, e is a constant, j is an imaginary unit, and j satisfies 2 =-1, ω is the angular frequency, equal to 2πf, f is the set frequency, τ is the flow time, is the current value of the reverse traveling wave at the first endpoint at the current moment; In the step of determining the traveling wave differential moment of the second end point based on the current value of the preceding traveling wave of the second end point, the set frequency, the flow time τ and the current value of the preceding traveling wave of the first end point, The current value of the traveling wave before the second endpoint, the set frequency, the flow time τ, the current value of the traveling wave before the first endpoint, and the traveling wave differential momentum of the second endpoint satisfy the following relationship: in, is the traveling wave differential momentum of the second end point, is the current value of the forward wave at the first endpoint, e is a constant, j is an imaginary unit, and j satisfies 2 =-1, ω is the angular frequency, equal to 2πf, f is the set frequency, τ is the flow time, is the current value of the forward wave of the second end point at the current moment; In the step of determining the current value of the forward wave and the current value of the reverse wave of the first endpoint based on the first voltage, the first current and the wave impedance of the AC line at the first endpoint, The first voltage at the first endpoint, the first current, the wave impedance, and the current value of the forward wave at the first endpoint satisfy the following relationship: Where i1(x, t) is the current value of the forward wave of the first endpoint at time t, u(x, t) is the first voltage of the first endpoint at time t, i(x, t) is the first current of the first endpoint at time t, and Z C is the wave impedance; The first voltage at the first endpoint, the first current, the wave impedance, and the current value of the reverse wave at the first endpoint satisfy the following relationship: Where i2(x, t) is the current value of the reverse wave at the first endpoint at time t, u(x, t) is the first voltage at the first endpoint at time t, i(x, t) is the first current at the first endpoint at time t, and Z C is the wave impedance.

9. An electronic device, characterized in that: include: A memory and a processor, wherein the memory stores a computer program or an instruction, and when the processor executes the computer program or the instruction, a relay protection method for an AC line applied to a photovoltaic power generation flexible DC transmission system is implemented as described in any one of claims 1 to 7.

10. A readable storage medium, characterized in that: The readable storage medium stores a program or an instruction, and when the processor executes the program or the instruction, the relay protection method for the AC line applied to the photovoltaic power generation flexible DC transmission system according to any one of claims 1 to 7 is implemented.

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