Low frequency power transmission line distance protection method and system based on additional impedance angle solving

Abstract

The application discloses a low-frequency power transmission line distance protection method and system based on additional impedance angle solving, and belongs to the technical field of power system relay protection, which detects voltage signals and current signals at the protection installation position of a wind power plant side, obtains negative sequence voltage, zero sequence current, measured voltage and measured current; solves the phase angle of the low-frequency side negative sequence current according to the wind power plant side negative sequence current suppression strategy, the low-frequency side negative sequence voltage suppression strategy and the line negative sequence impedance angle, and solves the additional impedance angle in combination with the fault type; solves the line impedance value from the protection installation position to the fault point according to the measured impedance, the additional impedance angle, the measured impedance angle and the distance protection setting impedance angle, and further determines the fault position and judges the in-zone fault or the out-of-zone fault. The application can weaken the influence of the converter control strategy and the transition resistance on the measured impedance, and improves the distance measurement precision and the action reliability of the low-frequency power transmission line distance protection.

Description

Technical Field

[0001] This invention belongs to the field of power system relay protection technology, specifically relating to a method and system for distance protection of low-frequency transmission lines based on the solution of additional impedance angle. Background Technology

[0002] Currently, in offshore wind power grid connection technologies, conventional high-voltage AC transmission is typically used in nearshore areas, while offshore scenarios mainly rely on flexible direct current transmission (VSC-HVDC). New low-frequency transmission systems use frequencies lower than the power frequency for power transmission, such as 16.7 Hz. Compared to conventional high-voltage AC transmission, low-frequency transmission systems (LFTS) can extend the economical transmission distance and increase transmission capacity; compared to flexible direct current transmission, LFTS also has multi-terminal flexible networking capabilities, which can be used to construct flexible offshore wind power cluster transmission systems.

[0003] A typical low-frequency transmission system consists of key equipment at both ends. On the offshore side, a modular multilevel matrix converter (M3C) serves as the core frequency converter, while on the wind farm side, a low-frequency offshore wind farm is configured with permanent magnet synchronous generators (PMSGs). Because both ends of the low-frequency transmission line are connected via power electronic converters, the line fault characteristics are easily affected by the control strategies of the converters at both ends, causing the low-frequency transmission system to exhibit nonlinear and time-varying characteristics.

[0004] Low-frequency transmission systems differ from conventional synchronous machine systems in both topology and power supply characteristics, making it difficult to directly apply fault analysis methods used for traditional synchronous machine systems. In particular, low-frequency transmission systems exhibit weak power supply characteristics at both ends, and the converters are typically constrained by control strategies and current-limiting capabilities during faults, resulting in limited fault current amplitudes. These amplitudes are only slightly higher than the steady-state current and significantly smaller than the short-circuit current in traditional synchronous machine systems. Therefore, relying solely on changes in fault current amplitude is insufficient to accurately characterize the fault state of low-frequency transmission lines.

[0005] When offshore wind power is transmitted via flexible low-frequency transmission lines and an asymmetrical fault occurs on the line, the control strategies employed by the wind farm side and the low-frequency side will affect the amplitude and phase of the fault current. For example, if the wind farm side employs a negative-sequence current suppression strategy and the low-frequency side employs a negative-sequence voltage suppression strategy, the phase relationships of the negative-sequence voltage, negative-sequence current, and fault current will change under different fault conditions. Therefore, the fault electrical quantities of the low-frequency transmission line depend not only on the fault location and line impedance but also on the converter control strategy and the fault transition resistance.

[0006] Traditional distance protection typically calculates the measured impedance based on the voltage and current measured at the protection installation point and compares this measured impedance with the distance protection setting impedance to determine whether a fault is within the protection range. However, when a fault occurs on a low-frequency transmission line via a transition resistance... R f During an asymmetrical fault, the measured impedance of the wind farm-side impedance relay... Z M Line impedance from the protection installation point to the fault point Z MK and additional impedance Z add Together, that is Z M for Z MK and Z add The phasor sums are as follows:

[0007] Among them, line impedance Z MK It is only related to the location of the fault, while the additional impedance Z add It is affected by the transition resistance, fault current distribution and the control strategies at both ends.

[0008] In measuring impedance Z M Line impedance Z MK and additional impedance Z add In the vector relationship, φ set To set the impedance angle for distance protection, φ L The line impedance angle is typically used in distance protection. φ L Set to the same as the set impedance angle φ set equal; φ M To measure the impedance angle, αFor additional impedance angles, all angles are between -180° and 180°. Please refer to [link / reference]. Figure 1 Due to the additional impedance angle α As the fault type, fault location, transition resistance, and converter control state change, the additional impedance trajectory exhibits dynamic nonlinear characteristics, making the measured impedance... Z M The actual line impedance from the protection installation point to the fault point deviates from the actual line impedance. Z MK .

[0009] The aforementioned impedance measurement deviation directly alters the impedance relay's judgment of fault location. When the measured impedance deviates outside the protection range, a fault within the protection range may be classified as an external fault, resulting in distance protection malfunction or under-range operation. Conversely, when the measured impedance deviates within the protection range, an external fault may be classified as an internal fault, resulting in distance protection malfunction or overshoot. As the transition resistance increases, the influence of the additional impedance on the measured impedance further intensifies. Traditional distance protection methods struggle to accurately reflect the actual fault distance of low-frequency transmission lines, failing to guarantee the stable operation of low-frequency transmission systems.

[0010] Due to the line impedance from the protection installation point to the fault point Z MK The additional impedance angle is only related to the fault location and is independent of factors such as converter control strategy and transition resistance. Therefore, if the additional impedance angle can be solved based on the control strategies at both ends of the low-frequency transmission system, and the line impedance value can be determined using the geometric relationship between the measured impedance, line impedance, and additional impedance, the influence of the additional impedance on the distance protection measurement results can be reduced. Current technology lacks a distance protection method that can combine the negative sequence control characteristics of the low-frequency transmission system to solve for the additional impedance angle and accurately determine the fault line impedance and fault location based on the additional impedance angle. Summary of the Invention

[0011] The technical problem to be solved by the present invention is to address the shortcomings of the prior art by providing a method and system for distance protection of low-frequency transmission lines based on the solution of the additional impedance angle. This method is used to solve the problem that when an asymmetrical fault occurs in a low-frequency transmission line through a transition resistor, the control strategy of the converters at both ends of the line changes the amplitude and phase of the fault current, causing the additional impedance to change dynamically and nonlinearly. This results in the measured impedance deviating from the actual line impedance from the protection installation point to the fault point, which in turn causes the traditional distance protection to be inaccurate in fault distance measurement and to maloperate or fail to operate.

[0012] The present invention adopts the following technical solution: The low-frequency transmission line distance protection method based on the additional impedance angle solution includes the following steps: S1. Detect the voltage and current signals at the wind farm-side protection installation point when an asymmetrical fault occurs in a low-frequency transmission line, and obtain the fault type, negative sequence impedance angle of the line, distance protection setting impedance angle, positive sequence impedance modulus value corresponding to the entire length of the transmission line, and distance protection line protection range. S2. Based on the voltage signal and the current signal, obtain the negative sequence voltage, zero sequence current, measured voltage and measured current at the protection installation location; S3. Based on the negative sequence current suppression strategy adopted on the wind farm side, the negative sequence voltage suppression strategy adopted on the low frequency side, the negative sequence voltage and the negative sequence impedance angle of the line, solve for the negative sequence current phase angle on the low frequency side. S4. Based on the fault type, the phase angle of the low-frequency side negative sequence current, the phase angle of the zero sequence current, and the phase angle of the measured current, calculate the additional impedance angle. S5. Solve for the measured impedance based on the measured voltage and the measured current, and solve for the line impedance value from the protection installation point to the fault point based on the measured impedance, the additional impedance angle, the measured impedance angle and the distance protection setting impedance angle; S6. Solve the fault location based on the line impedance value and the positive sequence impedance modulus value corresponding to the total length of the sending line, and compare the fault location with the protection range of the distance protection line to determine whether the asymmetrical fault is an in-zone fault or an out-of-zone fault.

[0013] Further, in step S3, the negative sequence current provided by the wind farm side is determined to be zero according to the negative sequence current suppression strategy, the negative sequence voltage provided by the low frequency side is determined to be zero according to the negative sequence voltage suppression strategy, and the negative sequence voltage at the protection installation point is determined to be consistent with the negative sequence voltage at the fault point, and the negative sequence current at the protection installation point is the same as the negative sequence current on the low frequency side.

[0014] Furthermore, the phase angle of the negative sequence voltage is subtracted from the negative sequence impedance angle of the line to obtain the phase angle of the low-frequency side negative sequence current.

[0015] Furthermore, in step S4: When the fault type is a single-phase ground fault, the additional impedance angle is obtained by adding 180° to the phase angle of the low-frequency negative sequence current and then subtracting the phase angle of the measured current. When the fault type is a phase-to-phase fault, the additional impedance angle is obtained by subtracting 90° from the phase angle of the low-frequency side negative sequence current and then subtracting the phase angle of the measured current. When the fault type is a phase-to-phase grounding fault, the additional impedance angle is obtained by subtracting the measured current phase angle from the zero-sequence current phase angle. The values ​​of the low-frequency negative sequence current phase angle, the zero sequence current phase angle, the measured current phase angle, and the additional impedance angle are all between -180° and 180°.

[0016] Furthermore, in step S2: When the fault type is a single-phase ground fault, the measured voltage is U a The measured current is I a +3 K ; When the fault type is a phase-to-phase fault, the measured voltage is U b - U c The measured current is I b - I c ; When the fault type is a phase-to-phase ground fault, the measured voltage is U b + U c The measured current is I b + I c +6 K .

[0017] Further, in step S5, the ratio of the measured voltage to the measured current is used as the measured impedance, the ratio of the amplitude of the measured voltage to the amplitude of the measured current is used as the magnitude of the measured impedance, and the difference between the phase angle of the measured voltage and the phase angle of the measured current is used as the measured impedance angle.

[0018] Furthermore, based on the geometric relationship between the measured impedance, the line impedance, and the additional impedance, and the relationship between the measured impedance angle, the distance protection setting impedance angle, and the additional impedance angle, the line impedance value from the protection installation point to the fault point is solved using the sine theorem.

[0019] Further, in step S6, the ratio of the line impedance value to the positive sequence impedance magnitude corresponding to the entire length of the transmitting line is used as the fault location.

[0020] Furthermore, the protection range of the distance protection line is 80% of the total length of the low-frequency transmission line; when the fault location is less than 80%, it is determined to be an in-zone fault; when the fault location is greater than or equal to 80%, it is determined to be an out-of-zone fault.

[0021] Secondly, embodiments of the present invention provide a low-frequency transmission line distance protection system based on the solution of the additional impedance angle, comprising: The signal acquisition module is used to detect the voltage and current signals at the wind farm-side protection installation point when an asymmetrical fault occurs in a low-frequency transmission line, and to acquire the fault type, negative sequence impedance angle of the line, distance protection setting impedance angle, positive sequence impedance modulus value corresponding to the entire length of the transmission line, and distance protection line protection range. The fault loop quantity calculation module is used to obtain the negative sequence voltage, zero sequence current, measured voltage and measured current at the protection installation location based on the voltage signal and the current signal; The additional impedance angle calculation module is used to calculate the negative sequence current phase angle on the low-frequency side based on the negative sequence current suppression strategy adopted on the wind farm side, the negative sequence voltage suppression strategy adopted on the low-frequency side, the negative sequence voltage and the negative sequence impedance angle of the line, and to calculate the additional impedance angle based on the fault type, the negative sequence current phase angle on the low-frequency side, the zero sequence current phase angle and the measured current phase angle. The line impedance calculation module is used to solve for the measured impedance based on the measured voltage and the measured current, and to solve for the line impedance value from the protection installation point to the fault point based on the measured impedance, the additional impedance angle, the measured impedance angle and the distance protection setting impedance angle. The protection determination module is used to solve the fault location based on the line impedance value and the positive sequence impedance modulus value corresponding to the total length of the sending line, and compare the fault location with the protection range of the distance protection line to determine whether the asymmetrical fault is an in-zone fault or an out-of-zone fault.

[0022] Thirdly, a computer device includes a memory, a processor, and a computer program stored in the memory and executable on the processor, wherein the processor, when executing the computer program, implements the steps of the aforementioned low-frequency transmission line distance protection method based on the solution of the additional impedance angle.

[0023] Fourthly, embodiments of the present invention provide a computer-readable storage medium including a computer program, which, when executed by a processor, implements the steps of the above-described low-frequency transmission line distance protection method based on the solution of the additional impedance angle.

[0024] Fifthly, a chip includes a memory, a processor, and a computer program stored in the memory and executable on the processor, wherein the processor, when executing the computer program, implements the steps of the aforementioned low-frequency transmission line distance protection method based on the solution of the additional impedance angle.

[0025] In a sixth aspect, embodiments of the present invention provide an electronic device including a computer program, which, when executed by the electronic device, implements the steps of the above-described low-frequency transmission line distance protection method based on the solution of the additional impedance angle.

[0026] Compared with the prior art, the present invention has at least the following beneficial effects: A distance protection method for low-frequency transmission lines based on additional impedance angle calculation is proposed. This method acquires negative-sequence voltage, zero-sequence current, measured voltage, and measured current by detecting voltage and current signals at the protection installation point on the wind farm side. The phase angle of the low-frequency side negative-sequence current is calculated using both wind farm-side and low-frequency-side negative-sequence voltage suppression strategies. Furthermore, the additional impedance angle is calculated based on the fault type. This transforms the influence of control strategies at both ends of the low-frequency transmission system on the fault current phase into a calculable additional impedance angle, providing a basis for eliminating the influence of additional impedance on the measured impedance. Based on this, the line impedance value from the protection installation point to the fault point is calculated using the measured impedance, additional impedance angle, measured impedance angle, and distance protection setting impedance angle. The fault location is then determined based on the line impedance value, enabling distance protection criteria to differentiate between in-zone and out-of-zone faults based on the line impedance directly related to the fault location. This reduces the measured impedance offset caused by transition resistance and control strategies, improving the accuracy of fault location and protection operation on low-frequency transmission lines.

[0027] Furthermore, by utilizing negative sequence current suppression strategies on the wind farm side and negative sequence voltage suppression strategies on the low-frequency side, the negative sequence network during a fault is simplified, establishing the correspondence between the negative sequence voltage at the protection installation point, the negative sequence voltage at the fault point, and the negative sequence current on the low-frequency side. This reduces the unknown electrical quantities in the negative sequence network, providing a theoretical basis for solving the phase angle of the negative sequence current on the low-frequency side using the measurable negative sequence voltage at the protection installation point.

[0028] Furthermore, the phase angle of the negative-sequence voltage is obtained by subtracting the negative-sequence impedance angle of the line. The purpose of this technique is to convert the low-frequency negative-sequence current phase angle, which is difficult to obtain directly, into a calculation result based on the negative-sequence voltage and negative-sequence impedance parameters at the protection installation location, thus providing phase parameters for solving the additional impedance angle under different fault types.

[0029] Furthermore, for single-phase grounding faults, phase-to-phase faults, and phase-to-phase grounding faults, the additional impedance angle is calculated using the corresponding fault current phase relationships. This adapts to the sequence network composition and fault current phase differences corresponding to different fault types, avoiding errors in calculating the additional impedance angle caused by using the same calculation relationship. At the same time, limiting each phase angle to between -180° and 180° helps ensure the consistency of angle conversion and subsequent impedance geometry calculations.

[0030] Furthermore, the corresponding measurement voltages and currents are determined based on single-phase grounding faults, phase-to-phase faults, and phase-to-phase grounding faults. This ensures that the selected voltage and current quantities correspond to the specific fault circuit, accurately characterizing the electrical characteristics of the protection installation location under different fault types, and providing matching input quantities for calculating the measurement impedance and additional impedance angle.

[0031] Furthermore, the measured impedance is obtained by measuring the ratio of voltage to current, and the magnitude and impedance angle are determined based on the amplitude ratio and phase angle difference, respectively. The voltage and current signals collected at the protection installation point are converted into impedance domain parameters, providing basic data for solving the line impedance value using the geometric relationship between impedance phasors.

[0032] Furthermore, based on the geometric relationship between the measured impedance, line impedance, and additional impedance, and combined with the measured impedance angle, distance protection setting impedance angle, and additional impedance angle, the line impedance value is solved using the sine theorem. This technique eliminates the need to directly calculate the magnitude of the additional impedance; it allows the determination of the line impedance value related to the fault location from the measured impedance affected by the additional impedance. This reduces the measured impedance offset caused by transition resistance and lowers the possibility of traditional distance protection failing to operate or maloperating.

[0033] Furthermore, the line impedance value from the protection installation point to the fault point is compared with the positive sequence impedance modulus value corresponding to the entire length of the transmitting line, and the ratio of the two represents the fault location. The line impedance value is converted into the relative position of the fault point in the entire length of the line, so that the line impedance calculation results can be directly used to determine the distance protection range.

[0034] Furthermore, 80% of the total length of the low-frequency transmission line is designated as the protection range for distance protection, and the location of the fault is determined to be either an in-range or out-of-range fault. This establishes clear distance protection setting criteria, ensuring reliable operation of the protection for faults within the protection range and reliable non-operation for faults outside the protection range, thereby reducing the occurrence of distance protection exceeding or falling short of the range.

[0035] It is understood that the beneficial effects of the second to sixth aspects mentioned above can be found in the relevant descriptions in the first aspect mentioned above, and will not be repeated here.

[0036] In summary, this invention utilizes the negative sequence control strategy at both ends of a low-frequency transmission system to solve for the additional impedance angle, and combines the measured impedance phasor relationship to determine the line impedance and fault location at the fault point. This can reduce the influence of transition resistance and converter control on the measured impedance, adapt to various asymmetrical faults and different fault locations, and improve the distance protection ranging accuracy, operational reliability, and adaptability to high-impedance faults.

[0037] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. Attached Figure Description

[0038] Figure 1 This is a vector diagram showing the impedance relationship. Figure 2 This is a negative sequence current sequence network diagram; Figure 3 This is a flowchart of the method of the present invention; Figure 4This is a model diagram of a low-frequency power transmission system; Figure 5 The diagram shows the operation of different fault locations under different fault types, where (a) represents a single-phase ground fault at different fault locations, (b) represents a phase-to-phase fault at different fault locations, (c) represents a phase-to-phase ground fault at different fault locations, and (d) represents a three-phase fault at different fault locations. Figure 6 Measurements were taken at different fault locations under different transition resistances, where (a) was a transition resistance of 0.01Ω, (b) was a transition resistance of 50Ω, (c) was a transition resistance of 100Ω, (d) was a transition resistance of 200Ω, and (e) was a transition resistance of 300Ω. Figure 7 A schematic diagram of a computer device provided in an embodiment of the present invention; Figure 8 This is a block diagram of a chip provided according to an embodiment of the present invention.

[0039] Among them, 60. Computer equipment; 61. Processor; 62. Memory; 63. Computer program; 600. Electronic device; 610. Processing unit; 620. Storage unit; 6201. Random access memory unit; 6202. Cache memory unit; 6203. Read-only memory unit; 6204. Program / utility; 6205. Program module; 630. Bus; 640. Display unit; 650. Input / output interface; 660. Network adapter; 700. External device. Detailed Implementation

[0040] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0041] In the description of this invention, it should be understood that the terms "comprising" and "including" indicate the presence of the described features, integrals, steps, operations, elements and / or components, but do not exclude the presence or addition of one or more other features, integrals, steps, operations, elements, components and / or collections thereof.

[0042] It should also be understood that the terminology used in this specification is for the purpose of describing particular embodiments only and is not intended to limit the invention. As used in this specification and the appended claims, the singular forms “a,” “an,” and “the” are intended to include the plural forms unless the context clearly indicates otherwise.

[0043] It should also be further understood that the term "and / or" as used in this specification and the appended claims refers to any combination and all possible combinations of one or more of the associated listed items, and includes such combinations. For example, A and / or B can represent three cases: A alone, A and B simultaneously, and B alone. Additionally, the character " / " in this invention generally indicates that the preceding and following objects have an "or" relationship.

[0044] It should be understood that although terms such as first, second, third, etc., may be used in the embodiments of the present invention to describe the preset range, these preset ranges should not be limited to these terms. These terms are only used to distinguish the preset ranges from one another. For example, without departing from the scope of the embodiments of the present invention, the first preset range may also be referred to as the second preset range, and similarly, the second preset range may also be referred to as the first preset range.

[0045] Depending on the context, the word "if" as used here can be interpreted as "when," "when," "in response to determination," or "in response to detection." Similarly, depending on the context, the phrase "if determination" or "if detection (of the stated condition or event)" can be interpreted as "when determination," "in response to determination," "when detection (of the stated condition or event)," or "in response to detection (of the stated condition or event)."

[0046] The accompanying drawings illustrate various structural schematic diagrams according to embodiments disclosed in this invention. These drawings are not to scale, and some details have been enlarged for clarity, and some details may have been omitted. The shapes of the various regions and layers shown in the drawings, as well as their relative sizes and positional relationships, are merely exemplary and may deviate from reality due to manufacturing tolerances or technical limitations. Furthermore, those skilled in the art can design regions / layers with different shapes, sizes, and relative positions as needed.

[0047] To address the issue of converter control strategies at both ends of low-frequency transmission lines affecting the amplitude and phase of fault current, leading to dynamic changes in additional impedance and deviations of measured impedance from actual line impedance, this invention provides a distance protection method for low-frequency transmission lines based on the calculation of additional impedance angle. Utilizing negative-sequence current suppression strategies on the wind farm side and negative-sequence voltage suppression strategies on the low-frequency side, an equivalent analysis of the fault negative-sequence network is performed. The phase angle of the low-frequency side negative-sequence current is calculated based on the negative-sequence voltage at the protection installation point and the negative-sequence impedance angle of the line. Then, for different asymmetrical fault types, the measured voltage, measured current, and additional impedance angle are determined respectively. Subsequently, combining the phasor geometric relationship between the measured impedance, line impedance, and additional impedance, the line impedance value from the protection installation point to the fault point is solved using the sine theorem, and the fault location and whether the fault is within or outside the protection zone are determined accordingly. This improvement can reduce the influence of transition resistance and converter control strategies on the measured impedance, improve the distance measurement accuracy under different fault types, fault locations, and high-resistance grounding conditions, and reduce the risks of false tripping, failure to tripping, and protection range deviation in traditional distance protection.

[0048] Please see Figure 3 This invention discloses a method for distance protection of low-frequency transmission lines based on the solution of additional impedance angle, comprising the following steps: S1. First, detect the current and voltage signals, and then, according to the control strategies on both sides of the line, obtain the voltage on the wind farm side and the current value on the M3C side. S2. Taking into account the additional impedance angle of the negative-sequence cooperative control strategy, the formula for calculating the additional impedance angle is as follows:

[0049] Among them, measuring current M Since the wind farm side current can be measured at the protection installation point, it is necessary to determine the phase angle of the fault current in order to determine the angle of the additional impedance.

[0050] Negative sequence current sequence network diagram during a fault, such as Figure 2 As shown.

[0051] Because a negative sequence current suppression strategy is adopted on the wind farm side and a negative sequence voltage suppression strategy is adopted on the low frequency side, the negative sequence current provided by the wind farm is basically zero. （ MA2 (≈0), the negative sequence voltage provided by the low-frequency side is basically zero. （U NA2 ≈0). Therefore, it can be assumed that the negative sequence voltage measured at the protection installation point is basically the same as the negative sequence voltage at the fault point, and the negative sequence current is the same as the negative sequence current on the low frequency side.

[0052]

[0053] according to Figure 2 From the above equation, we can see that the negative sequence current on the low-frequency side can be calculated as follows:

[0054] Taking the phase angles on both sides of the above equation, we get:

[0055] Since the impedance angle of the negative sequence impedance of the wind farm side line is approximately equal to that of the negative sequence impedance on the low-frequency side, i.e., according to the above formula, we get... Therefore, the phase angle of the negative sequence current on the low-frequency side can be simplified to:

[0056] in, arg ( Z L2 () indicates the negative sequence impedance angle of the line.

[0057] S3. From the phase of the negative sequence current, the phase of the fault current is further obtained, and then the additional impedance angle is calculated. The values ​​of the additional impedance angle corresponding to different fault types are shown in Table 1. Table 1

[0058] S4. The specific expressions for the measured current and measured voltage under different fault types are shown in Table 2. Table 2

[0059] The analysis results in the table above show that the voltage and current amplitudes and phases measured at the wind farm-side protection installation point can be used to further determine the line impedance value corresponding to the fault circuit.

[0060] The mathematical expression for line impedance is shown below:

[0061] S5. By comparing the line impedance value with the positive sequence impedance corresponding to the entire length of the transmission line, the relative position of the fault point with respect to the entire length of the line can be determined. The fault location is indicated as follows:

[0062] in, α wz Indicates the location of the fault; | Z L | is the positive sequence impedance magnitude corresponding to the entire length of the transmitting line.

[0063] S6. Since the protection range of the distance protection line is 80% of the total length, a fault location less than 80% is considered an in-zone fault, and vice versa, a fault greater than 80% is considered an out-of-zone fault. The distance protection setting criteria are:

[0064] In another embodiment of the present invention, a low-frequency transmission line distance protection system based on additional impedance angle calculation is provided. This system can be used to implement the above-mentioned low-frequency transmission line distance protection method based on additional impedance angle calculation. Specifically, the low-frequency transmission line distance protection system based on additional impedance angle calculation includes a signal acquisition module, a fault loop quantity calculation module, an additional impedance angle calculation module, a line impedance calculation module, and a protection judgment module.

[0065] Among them, the signal acquisition module is used to detect the voltage and current signals at the wind farm-side protection installation point when an asymmetrical fault occurs in the low-frequency transmission line, and to acquire the fault type, negative sequence impedance angle of the line, distance protection setting impedance angle, positive sequence impedance modulus value corresponding to the entire length of the transmission line, and distance protection line protection range. The fault loop quantity calculation module is used to obtain the negative sequence voltage, zero sequence current, measured voltage and measured current at the protection installation location based on the voltage signal and the current signal; The additional impedance angle calculation module is used to calculate the negative sequence current phase angle on the low-frequency side based on the negative sequence current suppression strategy adopted on the wind farm side, the negative sequence voltage suppression strategy adopted on the low-frequency side, the negative sequence voltage and the negative sequence impedance angle of the line, and to calculate the additional impedance angle based on the fault type, the negative sequence current phase angle on the low-frequency side, the zero sequence current phase angle and the measured current phase angle. The line impedance calculation module is used to solve for the measured impedance based on the measured voltage and the measured current, and to solve for the line impedance value from the protection installation point to the fault point based on the measured impedance, the additional impedance angle, the measured impedance angle and the distance protection setting impedance angle. The protection determination module is used to solve the fault location based on the line impedance value and the positive sequence impedance modulus value corresponding to the total length of the sending line, and compare the fault location with the protection range of the distance protection line to determine whether the asymmetrical fault is an in-zone fault or an out-of-zone fault.

[0066] This invention provides a terminal device comprising a processor and a memory. The memory stores a computer program, which includes program instructions. The processor executes the program instructions stored in the computer storage medium. The processor may be a Central Processing Unit (CPU), or other general-purpose processors, graphics processing units (GPUs), tensor processing units (TPUs), digital signal processors (DSPs), application-specific integrated circuits (ASICs), field-programmable gate arrays (FPGAs), or other programmable logic devices, discrete gate or transistor logic devices, discrete hardware components, etc. It is the computing and control core of the terminal, suitable for implementing one or more instructions, specifically suitable for loading and executing one or more instructions to achieve a corresponding method flow or corresponding function. The processor described in this embodiment can be used for the operation of a low-frequency transmission line distance protection method based on the solution of additional impedance angle, including: When an asymmetrical fault occurs on a low-frequency transmission line, the voltage and current signals at the protection installation point on the wind farm side are detected, and the fault type, negative sequence impedance angle of the line, distance protection setting impedance angle, positive sequence impedance magnitude corresponding to the entire length of the transmitting line, and distance protection line protection range are obtained; based on the voltage and current signals, the negative sequence voltage, zero sequence current, measured voltage, and measured current at the protection installation point are obtained; based on the negative sequence current suppression strategy adopted on the wind farm side, the negative sequence voltage suppression strategy adopted on the low-frequency side, the negative sequence voltage, and the negative sequence impedance angle of the line, the phase angle of the negative sequence current on the low-frequency side is calculated; based on the... The additional impedance angle is calculated based on the fault type, the negative sequence current phase angle on the low-frequency side, the zero sequence current phase angle, and the measured current phase angle. The measured impedance is calculated based on the measured voltage and the measured current. The line impedance value from the protection installation point to the fault point is calculated based on the measured impedance, the additional impedance angle, the measured impedance angle, and the distance protection setting impedance angle. The fault location is calculated based on the line impedance value and the positive sequence impedance magnitude corresponding to the total length of the sending line. The fault location is then compared with the distance protection line protection range to determine whether the asymmetrical fault is an in-zone fault or an out-of-zone fault.

[0067] Please see Figure 7The terminal device is a computer device. In this embodiment, the computer device 60 includes a processor 61, a memory 62, and a computer program 63 stored in the memory 62 and executable on the processor 61. When executed by the processor 61, the computer program 63 implements the low-frequency transmission line distance protection method based on the additional impedance angle solution in this embodiment. To avoid repetition, these details are not elaborated here. Alternatively, when executed by the processor 61, the computer program 63 implements the functions of each model / unit in the low-frequency transmission line distance protection system based on the additional impedance angle solution in this embodiment. To avoid repetition, these details are not elaborated here.

[0068] Computer device 60 can be a desktop computer, laptop, handheld computer, cloud server, or other computing device. Computer device 60 may include, but is not limited to, a processor 61 and a memory 62. Those skilled in the art will understand that... Figure 7 This is merely an example of computer device 60 and does not constitute a limitation on computer device 60. It may include more or fewer components than shown, or combine certain components, or different components. For example, computer device may also include input / output devices, network access devices, buses, etc.

[0069] The processor 61 may be a Central Processing Unit (CPU), or other general-purpose processors, graphics processing units (GPUs), tensor processing units (TPUs), digital signal processors (DSPs), application-specific integrated circuits (ASICs), field-programmable gate arrays (FPGAs), or other programmable logic devices, discrete gate or transistor logic devices, discrete hardware components, etc. A general-purpose processor may be a microprocessor or any conventional processor.

[0070] The memory 62 can be an internal storage unit of the computer device 60, such as a hard disk or memory of the computer device 60. The memory 62 can also be an external storage device of the computer device 60, such as a plug-in hard disk, smart media card (SMC), secure digital (SD) card, flash card, etc. equipped on the computer device 60.

[0071] Furthermore, the memory 62 may include both internal storage units of the computer device 60 and external storage devices. The memory 62 is used to store computer programs and other programs and data required by the computer device. The memory 62 can also be used to temporarily store data that has been output or will be output.

[0072] Please see Figure 8 The terminal device is an electronic device 600, which is manifested in the form of a general-purpose computing device. The components of the electronic device may include, but are not limited to: at least one processing unit 610, at least one storage unit 620, a bus 630 connecting different platform components (including storage unit 620 and processing unit 610), a display unit 640, etc.

[0073] The storage unit stores program code, which can be executed by the processing unit 610 to perform the steps described in the method section of this specification according to various exemplary embodiments of the present invention. For example, the processing unit 610 can perform actions such as... Figure 3 The steps are shown in the figure.

[0074] Storage unit 620 may include readable media in the form of volatile storage units, such as random access memory (RAM) 6201 and / or cache memory 6202, and may further include read-only memory (ROM) 6203.

[0075] Storage unit 620 may also include a program / utility 6204 having a set (at least one) program module 6205, such program module 6205 including but not limited to: operating system, one or more application programs, other program modules and program data, each or some combination of these examples may include an implementation of a network environment.

[0076] Bus 630 can represent one or more of several types of bus structures, including a memory cell bus or memory cell controller, a peripheral bus, a graphics acceleration port, a processing unit, or a local bus using any of the multiple bus structures.

[0077] Electronic device 600 can also communicate with one or more external devices 700 (e.g., keyboard, pointing device, Bluetooth device, etc.), and with one or more devices that enable a user to interact with electronic device 600, and / or with any device that enables electronic device 600 to communicate with one or more other computing devices (e.g., router, modem). This communication can be performed via input / output interface 650. Furthermore, electronic device 600 can also communicate with one or more networks (e.g., local area network, wide area network, and / or public network, such as the Internet) via network adapter 660. Network adapter 660 can communicate with other modules of electronic device 600 via bus 630. It should be understood that, although not shown in the figures, other hardware and / or software modules can be used in conjunction with electronic device 600, including but not limited to: microcode, device drivers, redundant processing units, external disk drive arrays, RAID systems, tape drives, and data backup storage platforms.

[0078] This invention also provides a storage medium, specifically a computer-readable storage medium, which is a memory device in a terminal device for storing programs and data. It is understood that the computer-readable storage medium here can include both built-in storage media in the terminal device and extended storage media supported by the terminal device; it can be any tangible medium containing or storing a program that can be used by or in conjunction with an instruction execution system, apparatus, or device. The computer-readable storage medium provides storage space that stores the terminal's operating system. Furthermore, the storage space also stores one or more instructions suitable for loading and execution by a processor, which can be one or more computer programs (including program code). More specific examples of the computer-readable storage medium include: an electrical connection with one or more wires, a portable disk, a hard disk, random access memory, read-only memory, erasable programmable read-only memory, optical fiber, portable compact disk read-only memory, optical storage device, magnetic storage device, or any suitable combination thereof.

[0079] Computer-readable storage media also include data signals propagated in baseband or as part of a carrier wave, carrying readable program code. Such propagated data signals can take various forms, including but not limited to electromagnetic signals, optical signals, or any suitable combination thereof. A readable storage medium can also be any readable medium other than a readable storage medium that can send, propagate, or transmit a program for use by or in connection with an instruction execution system, apparatus, or device. The program code contained on the readable storage medium can be transmitted using any suitable medium, including but not limited to wireless, wired, optical fiber, radio frequency, etc., or any suitable combination thereof.

[0080] Program code for performing the operations of this invention can be written in any combination of one or more programming languages, including object-oriented programming languages ​​such as Java and C++, and conventional procedural programming languages ​​such as C or similar languages. The program code can execute entirely on the user's computing device, partially on the user's device, as a standalone software package, partially on the user's computing device and partially on a remote computing device, or entirely on a remote computing device or server. In cases involving remote computing devices, the remote computing device can be connected to the user's computing device via any type of network, including a local area network (LAN) or a wide area network (WAN), or it can be connected to an external computing device (e.g., via the Internet using an Internet service provider).

[0081] One or more instructions stored in a computer-readable storage medium can be loaded and executed by a processor to implement the corresponding steps of the low-frequency transmission line distance protection method based on the additional impedance angle solution in the above embodiments; one or more instructions in the computer-readable storage medium are loaded and executed by the processor to perform the following steps: When an asymmetrical fault occurs on a low-frequency transmission line, the voltage and current signals at the protection installation point on the wind farm side are detected, and the fault type, negative sequence impedance angle of the line, distance protection setting impedance angle, positive sequence impedance magnitude corresponding to the entire length of the transmitting line, and distance protection line protection range are obtained; based on the voltage and current signals, the negative sequence voltage, zero sequence current, measured voltage, and measured current at the protection installation point are obtained; based on the negative sequence current suppression strategy adopted on the wind farm side, the negative sequence voltage suppression strategy adopted on the low-frequency side, the negative sequence voltage, and the negative sequence impedance angle of the line, the phase angle of the negative sequence current on the low-frequency side is calculated; based on the... The additional impedance angle is calculated based on the fault type, the negative sequence current phase angle on the low-frequency side, the zero sequence current phase angle, and the measured current phase angle. The measured impedance is calculated based on the measured voltage and the measured current. The line impedance value from the protection installation point to the fault point is calculated based on the measured impedance, the additional impedance angle, the measured impedance angle, and the distance protection setting impedance angle. The fault location is calculated based on the line impedance value and the positive sequence impedance magnitude corresponding to the total length of the sending line. The fault location is then compared with the distance protection line protection range to determine whether the asymmetrical fault is an in-zone fault or an out-of-zone fault.

[0082] The databases involved in the embodiments provided in this application may include at least one type of relational database and non-relational database. Non-relational databases may include, but are not limited to, blockchain-based distributed databases. The processors involved in the embodiments provided in this application may be general-purpose processors, central processing units, graphics processing units, digital signal processors, programmable logic devices, quantum computing-based data processing logic devices, etc., and are not limited to these.

[0083] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. The components of the embodiments of the present invention described and shown in the accompanying drawings can generally be arranged and designed in various different configurations. Therefore, the following detailed description of the embodiments of the present invention provided in the accompanying drawings is not intended to limit the scope of the claimed invention, but merely to illustrate selected embodiments of the invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

[0084] To fully verify the correctness, reliability, and practicality of the proposed low-frequency transmission line distance protection method based on additional impedance angle calculation in low-frequency transmission systems, a distribution network simulation model closely resembling actual engineering scenarios was built using the PSCAD / EMTDC electromagnetic transient simulation platform. By setting typical fault conditions, the consistency between fault identification results and theoretical criteria was compared and analyzed to comprehensively verify the effectiveness of the method. To verify the correctness of the proposed distance protection method, a simulation model of a distribution network closely resembling actual engineering scenarios was built in PSCAD. Figure 4 The model shown is a distribution network system with a high proportion of renewable energy access. The model is as follows: The low-frequency transmission line is 100km long.

[0085] 1. Operating conditions with different fault types To verify the adaptability of the proposed protection algorithm under complex fault conditions, AG, BC, BCG, and ABC faults were introduced at 20%, 50%, and 90% fault locations on a low-frequency transmission system line. Taking metallic grounding as an example, the operation at different fault locations under different fault types was verified.

[0086] Depend on Figure 5 Simulation results show that the system can accurately reflect the location of single-phase grounding, phase-to-phase short circuit, phase-to-phase grounding, and three-phase faults at different locations on the transmission line, enabling the distance protection scheme to operate accurately. This demonstrates that the protection method can operate accurately under different fault types and locations, and can still meet the requirements for fault location accuracy in engineering applications, even under complex conditions at remote locations.

[0087] 2. Different fault transition resistance conditions To verify the adaptability of the proposed protection algorithm under complex fault conditions, single-phase ground faults (AG) occurring at 20%, 50%, and 90% of the line in a low-frequency transmission system were used as examples. The operation at different fault locations was verified when the transition resistance was 0.01Ω (metallic short circuit), 50Ω, 100Ω, 200Ω, and 300Ω (high resistance ground).

[0088] Depend on Figure 6 It can be seen that under different fault conditions where the transition resistance increases from 0.01Ω to 300Ω, the protection principle proposed in this invention can achieve correct and reliable operation, and can accurately represent the actual fault distance in the low-frequency transmission system. Simulation results show that as the transition resistance increases, the proposed method does not exhibit any failure or maloperation of the distance protection, indicating that this invention has strong adaptability to transition resistance disturbances, and that the distance protection can reliably not operate when an external fault occurs.

[0089] To address the issue of traditional distance protection systems in low-frequency transmission systems being susceptible to failure to operate or maloperation due to transition resistance, a novel protection criterion is proposed to eliminate the influence of transition resistance. This method combines the characteristics of directional impedance relays, geometric relationships, and triangle side-angle relationships, and utilizes the sine theorem to solve the problem. Z M and Z MK This method, by fundamentally reducing the impact of transition resistance on protection performance, utilizes the relationship between the additional impedance angle obtained through control strategy and the coordinated design of the protection scheme to achieve accurate fault location determination. Results show that this method exhibits high ranging accuracy under asymmetrical faults, can withstand a maximum transition resistance of 300Ω, and is unaffected by changes in fault location, thus improving the reliability of distance protection.

[0090] In summary, this invention presents a method and system for distance protection of low-frequency transmission lines based on the solution of additional impedance angle. It utilizes negative-sequence current suppression strategies on the wind farm side and negative-sequence voltage suppression strategies on the low-frequency side of the transmission system to analyze the fault negative-sequence network. Based on the negative-sequence voltage at the protection installation point and the negative-sequence impedance angle of the line, the phase angle of the negative-sequence current on the low-frequency side is solved. For single-phase grounding, phase-to-phase, and phase-to-phase grounding faults, the additional impedance angle, measured voltage, and measured current are determined respectively. Furthermore, combining the phasor geometric relationship between the measured impedance, line impedance, and additional impedance, the line impedance value from the protection installation point to the fault point is obtained using the sine theorem. The fault location is determined by the ratio of the line impedance value to the positive-sequence impedance magnitude of the entire transmission line. This method can reduce the influence of converter control strategies and transition resistance on the measured impedance, enabling the distance protection to more accurately reflect the actual fault distance. It is applicable to different asymmetrical fault types, different fault locations, and high-resistance grounding conditions, reducing the risks of false tripping, failure to tripping, and protection range offset in traditional distance protection, and improving the ranging accuracy, operational reliability, and operational adaptability of low-frequency transmission line distance protection.

[0091] Those skilled in the art will clearly understand that, for the sake of convenience and brevity, the above-described division of functional units and modules is merely an example. In practical applications, the above functions can be assigned to different functional units and modules as needed, that is, the internal structure of the device can be divided into different functional units or modules to complete all or part of the functions described above. The functional units and modules in the embodiments can be integrated into one processing unit, or each unit can exist physically separately, or two or more units can be integrated into one unit. The integrated unit can be implemented in hardware or as a software functional unit. Furthermore, the specific names of the functional units and modules are only for easy differentiation and are not intended to limit the scope of protection of this application. The specific working process of the units and modules in the above system can be referred to the corresponding process in the foregoing method embodiments, and will not be repeated here.

[0092] In the above embodiments, the descriptions of each embodiment have different focuses. For parts that are not described in detail or recorded in a certain embodiment, please refer to the relevant descriptions of other embodiments.

[0093] Those skilled in the art will recognize that the units and algorithm steps of the various examples described in conjunction with the embodiments disclosed in this invention can be implemented in electronic hardware, or a combination of computer software and electronic hardware. Whether these functions are implemented in hardware or software depends on the specific application and design constraints of the technical solution. Those skilled in the art can use different methods to implement the described functions for each specific application, but such implementations should not be considered beyond the scope of this invention.

[0094] In the embodiments provided by this invention, it should be understood that the disclosed devices / terminals and methods can be implemented in other ways. For example, the device / terminal embodiments described above are merely illustrative. For instance, the division of modules or units is only a logical functional division, and in actual implementation, there may be other division methods. For example, multiple units or components may be combined or integrated into another system, or some features may be ignored or not executed. Furthermore, the coupling or direct coupling or communication connection shown or discussed may be through some interfaces; the indirect coupling or communication connection between devices or units may be electrical, mechanical, or other forms.

[0095] The units described as separate components may or may not be physically separate. The components shown as units may or may not be physical units; that is, they may be located in one place or distributed across multiple network units. Some or all of the units can be selected to achieve the purpose of this embodiment according to actual needs.

[0096] Furthermore, the functional units in the various embodiments of the present invention can be integrated into one processing unit, or each unit can exist physically separately, or two or more units can be integrated into one unit. The integrated unit can be implemented in hardware or as a software functional unit.

[0097] If the integrated module / unit is implemented as a software functional unit and sold or used as an independent product, it can be stored in a computer-readable storage medium. Based on this understanding, all or part of the processes in the methods of the above embodiments of the present invention can also be implemented by a computer program instructing related hardware. The computer program can be stored in a computer-readable storage medium, and when executed by a processor, it can implement the steps of the various method embodiments described above. The computer program includes computer program code, which can be in the form of source code, object code, executable files, or certain intermediate forms. The computer-readable medium can include: any entity or device capable of carrying the computer program code, a recording medium, a USB flash drive, a portable hard drive, a magnetic disk, an optical disk, a computer memory, a read-only memory (ROM), a random-access memory (RAM), an electrical carrier signal, a telecommunication signal, and a software distribution medium, etc.

[0098] This application is described with reference to flowchart illustrations and / or block diagrams of methods, apparatus, and computer program products according to embodiments of this application. It will be understood that each block of the flowchart illustrations and / or block diagrams, and combinations of blocks in the flowchart illustrations and / or block diagrams, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, special-purpose computer, embedded processor, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, generate instructions for implementing the flowchart... Figure 1 One or more processes and / or boxes Figure 1 A device that provides the functions specified in one or more boxes.

[0099] These computer program instructions may also be stored in a computer-readable storage medium that can direct a computer or other programmable data processing device to function in a particular manner, such that the instructions stored in the computer-readable storage medium produce an article of manufacture including instruction means, which are implemented in a process Figure 1 One or more processes and / or boxes Figure 1 The function specified in one or more boxes.

[0100] These computer program instructions may also be loaded onto a computer or other programmable data processing equipment to cause a series of operational steps to be performed on the computer or other programmable equipment to produce a computer-implemented process, thereby providing instructions that execute on the computer or other programmable equipment for implementing the process. Figure 1 One or more processes and / or boxes Figure 1 The steps of the function specified in one or more boxes.

[0101] The above content is only for illustrating the technical concept of the present invention and should not be construed as limiting the scope of protection of the present invention. Any modifications made to the technical solution based on the technical concept proposed in this invention shall fall within the scope of protection of the claims of this invention.

Claims

1. A method for distance protection of low-frequency transmission lines based on the solution of additional impedance angle, characterized in that, Includes the following steps: S1. Detect the voltage and current signals at the wind farm-side protection installation point when an asymmetrical fault occurs in a low-frequency transmission line, and obtain the fault type, negative sequence impedance angle of the line, distance protection setting impedance angle, positive sequence impedance modulus value corresponding to the entire length of the transmission line, and distance protection line protection range. S2. Based on the voltage signal and the current signal, obtain the negative sequence voltage, zero sequence current, measured voltage and measured current at the protection installation location; S3. Based on the negative sequence current suppression strategy adopted on the wind farm side, the negative sequence voltage suppression strategy adopted on the low frequency side, the negative sequence voltage and the negative sequence impedance angle of the line, solve for the negative sequence current phase angle on the low frequency side. S4. Based on the fault type, the phase angle of the low-frequency side negative sequence current, the phase angle of the zero sequence current, and the phase angle of the measured current, calculate the additional impedance angle. S5. Solve for the measured impedance based on the measured voltage and the measured current, and solve for the line impedance value from the protection installation point to the fault point based on the measured impedance, the additional impedance angle, the measured impedance angle and the distance protection setting impedance angle; S6. Solve the fault location based on the line impedance value and the positive sequence impedance modulus value corresponding to the total length of the sending line, and compare the fault location with the protection range of the distance protection line to determine whether the asymmetrical fault is an in-zone fault or an out-of-zone fault.

2. The low-frequency transmission line distance protection method based on additional impedance angle calculation according to claim 1, characterized in that, In step S3, the negative sequence current provided by the wind farm side is determined to be zero according to the negative sequence current suppression strategy, the negative sequence voltage provided by the low frequency side is determined to be zero according to the negative sequence voltage suppression strategy, and the negative sequence voltage at the protection installation point is determined to be consistent with the negative sequence voltage at the fault point, and the negative sequence current at the protection installation point is the same as the negative sequence current on the low frequency side.

3. The low-frequency transmission line distance protection method based on the additional impedance angle solution according to claim 2, characterized in that, The phase angle of the negative sequence voltage is subtracted from the negative sequence impedance angle of the line to obtain the phase angle of the low-frequency side negative sequence current.

4. The low-frequency transmission line distance protection method based on additional impedance angle calculation according to claim 1, characterized in that, In step S4: When the fault type is a single-phase ground fault, the additional impedance angle is obtained by adding 180° to the phase angle of the low-frequency negative sequence current and then subtracting the phase angle of the measured current. When the fault type is a phase-to-phase fault, the additional impedance angle is obtained by subtracting 90° from the phase angle of the low-frequency side negative sequence current and then subtracting the phase angle of the measured current. When the fault type is a phase-to-phase grounding fault, the additional impedance angle is obtained by subtracting the measured current phase angle from the zero-sequence current phase angle. The values ​​of the low-frequency negative sequence current phase angle, the zero sequence current phase angle, the measured current phase angle, and the additional impedance angle are all between -180° and 180°.

5. The low-frequency transmission line distance protection method based on additional impedance angle calculation according to claim 1, characterized in that, In step S2: When the fault type is a single-phase ground fault, the measured voltage is U a The measured current is I a +3 K ; When the fault type is a phase-to-phase fault, the measured voltage is U b - U c The measured current is I b - I c ; When the fault type is a phase-to-phase ground fault, the measured voltage is U b + U c The measured current is I b + I c +6 K .

6. The low-frequency transmission line distance protection method based on additional impedance angle calculation according to claim 1, characterized in that, In step S5, the ratio of the measured voltage to the measured current is used as the measured impedance, the ratio of the amplitude of the measured voltage to the amplitude of the measured current is used as the magnitude of the measured impedance, and the difference between the phase angle of the measured voltage and the phase angle of the measured current is used as the measured impedance angle.

7. The low-frequency transmission line distance protection method based on additional impedance angle calculation according to claim 6, characterized in that, Based on the geometric relationship between the measured impedance, the line impedance, and the additional impedance, and the relationship between the measured impedance angle, the distance protection setting impedance angle, and the additional impedance angle, the line impedance value from the protection installation point to the fault point is solved using the sine theorem.

8. The low-frequency transmission line distance protection method based on additional impedance angle calculation according to claim 1, characterized in that, In step S6, the ratio of the line impedance value to the positive sequence impedance magnitude corresponding to the entire length of the transmitting line is used as the fault location.

9. The low-frequency transmission line distance protection method based on the solution of the additional impedance angle according to claim 8, characterized in that, The distance protection line covers 80% of the total length of the low-frequency transmission line; when the fault location is less than 80%, it is determined to be an in-zone fault; when the fault location is greater than or equal to 80%, it is determined to be an out-of-zone fault.

10. A low-frequency transmission line distance protection system based on additional impedance angle calculation, characterized in that, include: The signal acquisition module is used to detect the voltage and current signals at the wind farm-side protection installation point when an asymmetrical fault occurs in a low-frequency transmission line, and to acquire the fault type, negative sequence impedance angle of the line, distance protection setting impedance angle, positive sequence impedance modulus value corresponding to the entire length of the transmission line, and distance protection line protection range. The fault loop quantity calculation module is used to obtain the negative sequence voltage, zero sequence current, measured voltage and measured current at the protection installation location based on the voltage signal and the current signal; The additional impedance angle calculation module is used to calculate the negative sequence current phase angle on the low-frequency side based on the negative sequence current suppression strategy adopted on the wind farm side, the negative sequence voltage suppression strategy adopted on the low-frequency side, the negative sequence voltage and the negative sequence impedance angle of the line, and to calculate the additional impedance angle based on the fault type, the negative sequence current phase angle on the low-frequency side, the zero sequence current phase angle and the measured current phase angle. The line impedance calculation module is used to solve for the measured impedance based on the measured voltage and the measured current, and to solve for the line impedance value from the protection installation point to the fault point based on the measured impedance, the additional impedance angle, the measured impedance angle and the distance protection setting impedance angle. The protection determination module is used to solve the fault location based on the line impedance value and the positive sequence impedance modulus value corresponding to the total length of the sending line, and compare the fault location with the protection range of the distance protection line to determine whether the asymmetrical fault is an in-zone fault or an out-of-zone fault.

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