A current differential fast protection method and system for a flexible low-frequency power transmission line

Abstract

The application relates to the technical field of electric power automation protection, and discloses a current differential rapid protection method and system for a flexible low-frequency power transmission line. The method obtains the current of a left-end preset sampling point of a lossless power transmission line, and the current is operated to obtain a more accurate actual transformation current of the left-end preset sampling point. Meanwhile, the actual transformation current of the opposite end is obtained. The actual transformation current of the left-end preset sampling point and the actual transformation current of the right-end preset sampling point are added and processed, and the size relationship between the current addition result and a preset differential current threshold value is compared, so that it is determined that a line section between the left-end preset sampling point and the right-end preset sampling point of the power transmission line has a fault, the line section with the fault is isolated and protected, the purpose of rapidly removing the fault is achieved, and the accuracy of identifying the line fault is improved.

Description

Technical Field

[0001] This invention relates to the field of power automation protection technology, and in particular to a method and system for fast differential current protection of flexible low-frequency transmission lines. Background Technology

[0002] With the development of power electronics technology, flexible low-frequency power transmission technology, which allows for flexible frequency selection, can improve the grid's transmission capacity and flexible regulation capabilities, making it a new and efficient AC power transmission technology. In recent years, offshore low-frequency power transmission technology has received increasing attention from academia and industry. For submarine cables, the fundamental advantage of using low-frequency transmission schemes is that it can reduce the parallel capacitance effect of the cables, extend the distance of offshore AC power transmission, save investment costs for equipment such as offshore converter stations and DC circuit breakers, and facilitate multi-terminal networking of offshore wind farms, making it a highly competitive new offshore wind power grid connection technology. Currently, the frequency selected for low-frequency transmission is mostly 20Hz, with a data window length of 50ms per cycle.

[0003] Due to the current control of the converter, the short-circuit current rises briefly and then drops significantly. At the initial moment of different faults, the rise is slow due to the influence of low-frequency periodic components, and the amplitude of the short-circuit current is small or even lower than the rated current. Conventional current differential protection requires a complete data window (i.e., 50ms) and has a slow action speed. Flexible low-frequency transmission systems contain a large number of power electronic conversion devices with weak short-circuit tolerance, and there is an urgent need for rapid fault isolation when a fault occurs.

[0004] Compared to conventional overhead lines, submarine cable transmission lines have larger capacitance parameters, resulting in significant fluctuation propagation when faults occur both inside and outside the transmission line area. Conventional current sampling point differential protection is inadequate for lines with such significant fluctuation propagation, leading to lower accuracy in fault identification and difficulty in quickly clearing faults. Summary of the Invention

[0005] This invention provides a method and system for rapid current differential protection of flexible low-frequency transmission lines, which solves the technical problems of low accuracy in identifying line faults and difficulty in achieving rapid fault clearing.

[0006] In view of this, the first aspect of the present invention provides a method for fast differential current protection of flexible low-frequency transmission lines, comprising the following steps:

[0007] Collect the three-phase voltage and three-phase current at a preset sampling point on the left end of the lossless power transmission line;

[0008] The three-phase voltage and three-phase current at the preset sampling point on the left are transformed using the Kelenberg transformation matrix to obtain three voltage modulus components and three current modulus components.

[0009] The actual lossless voltage and actual lossless current at the left-end preset sampling point of the lossless transmission line are calculated using the voltage modulus component, the three current modulus components, and the line resistance at the left-end preset sampling point.

[0010] Based on the wave equation of the lossless line, the actual lossless current at the preset sampling point at the right end of the lossless transmission line is calculated using the actual lossless voltage and actual lossless current at the preset sampling point at the left end of the lossless transmission line.

[0011] The actual lossless current at the left-end preset sampling point and the actual lossless current at the right-end preset sampling point are transformed using the Kelenberg inverse transform matrix to obtain the corresponding actual transformed current at the left-end preset sampling point and the actual transformed current at the right-end preset sampling point.

[0012] The actual transformed current of the preset sampling point on the left end and the actual transformed current of the preset sampling point on the right end are summed to obtain a current sum result. It is then determined whether the current sum result is greater than a preset differential current threshold. If the current sum result is greater than the preset differential current threshold, it is determined that there is a fault in the line section between the preset sampling point on the left end and the preset sampling point on the right end of the transmission line, and the faulty line section is isolated and protected.

[0013] Preferably, the step of collecting the three-phase voltage and three-phase current at a preset sampling point at the left end of the lossless transmission line includes the following:

[0014] The total resistance of the transmission line is obtained as follows:

[0015] R = R0 × Len

[0016] In the formula, R represents the total resistance, R0 represents the resistance per unit length, and Len represents the line length;

[0017] Left-end resistors and right-end resistors are set at the left-end and right-end preset sampling points of the lossless power transmission line, respectively, where the left-end resistor and the right-end resistor are both R / 2.

[0018] Preferably, the step of transforming the three-phase voltage and three-phase current at the preset sampling point on the left using the Kelvin transform matrix to obtain three voltage modulus components and three current modulus components specifically includes:

[0019] Using the Kelvin transform matrix, the three-phase voltage and three-phase current at the preset sampling point on the left are transformed to obtain three voltage magnitude components and three current magnitude components.

[0020]

[0021]

[0022] In the formula, This represents the voltage magnitude component of the nth phase at the preset sampling point on the left. This represents the nth phase voltage at the preset sampling point on the left. This represents the current-mode component of the nth phase at the preset sampling point on the left. Let represent the nth phase current at the preset sampling point on the left, l represent the preset sampling point on the left, n = a, b, c, where a represents phase a, b represents phase b, c represents phase c, and S represents the Kelvin transformation matrix.

[0023]

[0024] Preferably, the steps of calculating the actual lossless voltage and actual lossless current at the left-end preset sampling point of the lossless transmission line using the voltage modulus component, the three current modulus components, and the line resistance of the preset sampling point at the left end specifically include:

[0025] Using the voltage modulus component, the three current modulus components, and the line resistance of the preset sampling point at the left end, the actual lossless voltage and actual lossless current of the lossless transmission line at the preset sampling point at the left end are calculated using the following formula:

[0026]

[0027]

[0028] In the formula, This represents the actual lossless voltage at the preset sampling point on the left. This represents the actual lossless current at the preset sampling point on the left.

[0029] Preferably, the step of calculating the actual lossless current at the pre-set sampling point at the right end of the lossless transmission line based on the wave equation of the lossless transmission line through the actual lossless voltage and actual lossless current at the pre-set sampling point at the left end of the lossless transmission line specifically includes:

[0030] Based on the wave equation of the lossless transmission line, the actual lossless current at the preset sampling point at the right end of the lossless transmission line is calculated using the actual lossless voltage and actual lossless current at the preset sampling point at the left end of the lossless transmission line. The wave equation of the lossless transmission line is as follows:

[0031]

[0032] In the formula, The current represents the actual lossless current at the preset sampling point on the right, and τ represents the line time constant. v represents wave velocity. t represents the current time, and Z represents the line impedance. Where L0 represents the inductance per unit length and C0 represents the capacitance per unit length.

[0033] Preferably, the actual lossless current at the left-end preset sampling point and the actual lossless current at the right-end preset sampling point are transformed using the Kelenberg inverse transform matrix, respectively, to obtain the corresponding actual transformed current at the left-end preset sampling point and the actual transformed current at the right-end preset sampling point.

[0034]

[0035]

[0036] In the formula, This represents the actual transformation current at the preset sampling point on the left. S represents the actual transformation current at the preset sampling point on the right. -1 Let Kelenberger's inverse transformation matrix be denoted as , where

[0037]

[0038] Secondly, the present invention provides a current differential fast protection system for flexible low-frequency transmission lines, comprising:

[0039] The acquisition module is used to acquire the three-phase voltage and three-phase current at a preset sampling point on the left end of the lossless power transmission line;

[0040] The transformation module is used to transform the three-phase voltage and three-phase current of the preset sampling point on the left end using the Kelvin transformation matrix to obtain three voltage modulus components and three current modulus components.

[0041] The first lossless calculation module is used to calculate the actual lossless voltage and actual lossless current at the left-end preset sampling point of the power transmission lossless line using the voltage modulus component, the three current modulus components, and the line resistance of the left-end preset sampling point.

[0042] The second lossless calculation module is used to calculate the actual lossless current at the preset sampling point at the right end of the lossless transmission line based on the actual lossless voltage and actual lossless current at the preset sampling point at the left end of the lossless transmission line according to the fluctuation equation of the lossless line.

[0043] The inverse transformation module is used to transform the actual lossless current at the left-end preset sampling point and the actual lossless current at the right-end preset sampling point using the Kelenberg inverse transformation matrix, respectively, to obtain the corresponding actual transformed current at the left-end preset sampling point and the actual transformed current at the right-end preset sampling point.

[0044] The fault protection module is used to sum the actual transformation current of the preset sampling point on the left end and the actual transformation current of the preset sampling point on the right end to obtain the current summation result, and determine whether the current summation result is greater than a preset differential current threshold. If the current summation result is greater than the preset differential current threshold, it is determined that there is a fault in the line section between the preset sampling point on the left end and the preset sampling point on the right end of the transmission line, and the faulty line section is isolated and protected.

[0045] Preferably, the system further includes:

[0046] The resistance acquisition module is used to obtain the total resistance of the transmission line.

[0047] R = R0 × Len

[0048] In the formula, R represents the total resistance, R0 represents the resistance per unit length, and Len represents the line length;

[0049] The resistance setting module is used to set the left-end resistance and the right-end resistance at the preset sampling points on the left and right ends of the lossless power transmission line, respectively, wherein the left-end resistance and the right-end resistance are both R / 2.

[0050] Preferably, the transformation module is specifically used to transform the three-phase voltage and three-phase current of the preset sampling point on the left end using the Kelvin transformation matrix, respectively, to obtain three voltage modulus components and three current modulus components.

[0051]

[0052]

[0053] In the formula, This represents the voltage magnitude component of the nth phase at the preset sampling point on the left. This represents the nth phase voltage at the preset sampling point on the left. This represents the current-mode component of the nth phase at the preset sampling point on the left. Let represent the nth phase current at the preset sampling point on the left, l represent the preset sampling point on the left, n = a, b, c, where a represents phase a, b represents phase b, c represents phase c, and S represents the Kelvin transformation matrix.

[0054]

[0055] Preferably, the first lossless calculation module is specifically used to calculate the actual lossless voltage and actual lossless current at the left-end preset sampling point of the power transmission lossless line using the voltage modulus component, the three current modulus components, and the line resistance of the preset sampling point at the left end, through the following formula:

[0056]

[0057]

[0058] In the formula, This represents the actual lossless voltage at the preset sampling point on the left. This represents the actual lossless current at the preset sampling point on the left.

[0059] As can be seen from the above technical solutions, the present invention has the following advantages:

[0060] This invention obtains the current at a preset sampling point on the left end of a lossless transmission line and processes the current to obtain a more accurate actual transformation current at the preset sampling point on the left end. Simultaneously, it obtains the actual transformation current at the opposite end. The actual transformation currents at the left and right ends of the preset sampling points are summed, and the summation result is compared with a preset differential current threshold. This allows the determination of a fault in the line section between the preset sampling points on the left and right ends of the transmission line. The faulty line section is then isolated and protected, achieving rapid fault clearance and improving the accuracy of line fault identification. Attached Figure Description

[0061] Figure 1 A flowchart of a current differential fast protection method for flexible low-frequency transmission lines provided in an embodiment of the present invention;

[0062] Figure 2 A schematic diagram of a lossless power transmission line section provided in an embodiment of the present invention;

[0063] Figure 3 This is a schematic diagram of the structure of a current differential fast protection system for a flexible low-frequency transmission line provided in an embodiment of the present invention. Detailed Implementation

[0064] To enable those skilled in the art to better understand the present invention, the technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. 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.

[0065] For easier understanding, please refer to Figure 1 The present invention provides a method for fast differential current protection of flexible low-frequency transmission lines, comprising the following steps:

[0066] S1. Collect the three-phase voltage and three-phase current at the preset sampling point on the left end of the lossless power transmission line.

[0067] Among them, such as Figure 2 As shown, it represents a schematic diagram of the section where the lossless transmission line is located. The preset sampling point on the left is located at the left end of the section where the lossless transmission line is located, and the preset sampling point on the right is located at the right end of the section where the lossless transmission line is located. The left and right ends are set relative to the position of the section where the lossless transmission line is located.

[0068] S2. Using the Kelvin transform matrix, the three-phase voltage and three-phase current of the preset sampling point on the left are transformed to obtain three voltage modulus components and three current modulus components.

[0069] S3. Calculate the actual lossless voltage and actual lossless current at the left-end preset sampling point of the lossless transmission line using the voltage modulus component, the three current modulus components, and the line resistance of the preset sampling point on the left.

[0070] S4. Based on the wave equation of the lossless line, calculate the actual lossless current at the preset sampling point at the right end of the lossless transmission line using the actual lossless voltage and actual lossless current at the preset sampling point at the left end of the lossless transmission line.

[0071] S5. Use the Kelenberger inverse transform matrix to transform the actual lossless current of the preset sampling point on the left and the actual lossless current of the preset sampling point on the right, respectively, to obtain the corresponding actual transformed current of the preset sampling point on the left and the actual transformed current of the preset sampling point on the right.

[0072] S6. Sum the actual transformation current of the preset sampling point on the left end and the actual transformation current of the preset sampling point on the right end to obtain the current summation result. Determine whether the current summation result is greater than the preset differential current threshold. If the current summation result is greater than the preset differential current threshold, it is determined that there is a fault in the line section between the preset sampling point on the left end and the preset sampling point on the right end of the transmission line, and the faulty line section is isolated and protected.

[0073] To improve the reliability of the judgment, a fault can be identified only if several preset sampling points meet the above conditions consecutively. For example, a fault can be confirmed only if three consecutive sampling points meet the conditions, and a trip command can be issued to the circuit breaker to isolate the fault.

[0074] This invention provides a current differential fast protection method for flexible low-frequency transmission lines. By acquiring the current at a preset sampling point on the left end of the lossless transmission line and operating the current, a more accurate actual transformed current at the preset sampling point on the left end is obtained. Simultaneously, the actual transformed current at the opposite end is acquired. The actual transformed currents at the preset sampling points on the left and right ends are summed, and the summation result is compared with a preset differential current threshold. This allows for the determination of a fault in the line section between the preset sampling points on the left and right ends of the transmission line. The faulty line section is then isolated for protection, achieving rapid fault clearing and improving the accuracy of line fault identification.

[0075] In one specific embodiment, the steps preceding step S1 include:

[0076] S10. Obtain the total resistance of the transmission line.

[0077] R = R0 × Len

[0078] In the formula, R represents the total resistance, R0 represents the resistance per unit length, and Len represents the line length;

[0079] S11. Set left-end resistors and right-end resistors at the left-end and right-end preset sampling points of the lossless power transmission line, respectively, wherein the left-end resistors and right-end resistors are both R / 2.

[0080] In one specific embodiment, step S2 specifically includes:

[0081] Using the Kelvin transform matrix, the three-phase voltage and three-phase current at the preset sampling point on the left are transformed to obtain three voltage magnitude components and three current magnitude components.

[0082]

[0083]

[0084] In the formula, This represents the voltage magnitude component of the nth phase at the preset sampling point on the left. This represents the nth phase voltage at the preset sampling point on the left. This represents the current-mode component of the nth phase at the preset sampling point on the left. Let represent the nth phase current at the preset sampling point on the left, l represent the preset sampling point on the left, n = a, b, c, where a represents phase a, b represents phase b, c represents phase c, and S represents the Kelvin transformation matrix.

[0085]

[0086] In one specific embodiment, step S3 specifically includes:

[0087] Using the voltage modulus component, the three current modulus components, and the line resistance of the preset sampling point at the left end, the actual lossless voltage and actual lossless current of the lossless transmission line at the preset sampling point at the left end are calculated using the following formula:

[0088]

[0089]

[0090] In the formula, This represents the actual lossless voltage at the preset sampling point on the left. This represents the actual lossless current at the preset sampling point on the left.

[0091] In one specific embodiment, step S4 specifically includes:

[0092] Based on the wave equation of the lossless transmission line, the actual lossless current at the preset sampling point at the right end of the lossless transmission line is calculated using the actual lossless voltage and actual lossless current at the preset sampling point at the left end of the lossless transmission line. The wave equation of the lossless transmission line is as follows:

[0093]

[0094] In the formula, The current represents the actual lossless current at the preset sampling point on the right, and τ represents the line time constant. v represents wave velocity. t represents the current time, and Z represents the line impedance. Where L0 represents the inductance per unit length and C0 represents the capacitance per unit length.

[0095] In one specific embodiment, step S5 specifically includes:

[0096] Using the inverse Kalenberger transform matrix, the actual lossless currents at the left and right preset sampling points are transformed respectively, yielding the corresponding actual transformed currents at the left and right preset sampling points.

[0097]

[0098]

[0099] In the formula, This represents the actual transformation current at the preset sampling point on the left. S represents the actual transformation current at the preset sampling point on the right. -1 Let Kelenberger's inverse transformation matrix be denoted as , where

[0100]

[0101] The above is a detailed description of an embodiment of a current differential fast protection method for flexible low-frequency transmission lines provided by the present invention. The following is a detailed description of an embodiment of a current differential fast protection system for flexible low-frequency transmission lines provided by the present invention.

[0102] For easier understanding, please refer to Figure 3 The present invention provides a current differential fast protection system for flexible low-frequency transmission lines, comprising:

[0103] The acquisition module 100 is used to acquire the three-phase voltage and three-phase current of the preset sampling point at the left end of the lossless power transmission line;

[0104] The transformation module 200 is used to transform the three-phase voltage and three-phase current of the preset sampling point on the left end using the Kelvin transform matrix to obtain three voltage modulus components and three current modulus components.

[0105] The first lossless calculation module 300 is used to calculate the actual lossless voltage and actual lossless current at the left-end preset sampling point of the power transmission lossless line using the voltage modulus component, the three current modulus components and the line resistance of the left-end preset sampling point;

[0106] The second lossless calculation module 400 is used to calculate the actual lossless current at the preset sampling point at the right end of the lossless transmission line based on the actual lossless voltage and actual lossless current at the preset sampling point at the left end of the lossless transmission line according to the fluctuation equation of the lossless line.

[0107] The inverse transformation module 500 is used to transform the actual lossless current of the preset sampling point on the left and the actual lossless current of the preset sampling point on the right using the Kelenberg inverse transformation matrix, respectively, to obtain the corresponding actual transformed current of the preset sampling point on the left and the actual transformed current of the preset sampling point on the right.

[0108] The fault protection module 600 is used to sum the actual transformation current of the preset sampling point on the left end and the actual transformation current of the preset sampling point on the right end to obtain a current summation result, and determine whether the current summation result is greater than a preset differential current threshold. If the current summation result is greater than the preset differential current threshold, it is determined that there is a fault in the line section between the preset sampling point on the left end and the preset sampling point on the right end of the transmission line, and the faulty line section is isolated and protected.

[0109] In one specific embodiment, the system further includes:

[0110] The resistance acquisition module is used to obtain the total resistance of the transmission line.

[0111] R = R0 × Len

[0112] In the formula, R represents the total resistance, R0 represents the resistance per unit length, and Len represents the line length;

[0113] The resistance setting module is used to set the left-end resistance and the right-end resistance at the preset sampling points on the left and right ends of the lossless power transmission line, respectively, wherein the left-end resistance and the right-end resistance are both R / 2.

[0114] In one specific embodiment, the transformation module is specifically used to transform the three-phase voltage and three-phase current of the preset sampling point on the left end using the Kelvin transformation matrix, respectively, to obtain three voltage modulus components and three current modulus components.

[0115]

[0116]

[0117] In the formula, This represents the voltage magnitude component of the nth phase at the preset sampling point on the left. This represents the nth phase voltage at the preset sampling point on the left. This represents the current-mode component of the nth phase at the preset sampling point on the left. Let represent the nth phase current at the preset sampling point on the left, l represent the preset sampling point on the left, n = a, b, c, where a represents phase a, b represents phase b, c represents phase c, and S represents the Kelvin transformation matrix.

[0118]

[0119] In one specific embodiment, the first lossless calculation module is specifically used to calculate the actual lossless voltage and actual lossless current at the left-end preset sampling point of the power transmission lossless line using the voltage modulus component, the three current modulus components, and the line resistance of the preset sampling point at the left end, according to the following formula:

[0120]

[0121]

[0122] In the formula, This represents the actual lossless voltage at the preset sampling point on the left. This represents the actual lossless current at the preset sampling point on the left.

[0123] In one specific embodiment, the second lossless calculation module is specifically used to calculate the actual lossless current at the right-end preset sampling point of the lossless transmission line based on the actual lossless voltage and actual lossless current at the preset sampling point at the left end of the lossless transmission line, wherein the wave equation of the lossless line is:

[0124]

[0125] In the formula, The current represents the actual lossless current at the preset sampling point on the right, and τ represents the line time constant. v represents wave velocity. t represents the current time, and Z represents the line impedance. Where L0 represents the inductance per unit length and C0 represents the capacitance per unit length.

[0126] In one specific embodiment, the inverse transformation module is specifically used to transform the actual lossless current at the left-end preset sampling point and the actual lossless current at the right-end preset sampling point using the Kelvin inverse transformation matrix, respectively, to obtain the corresponding actual transformed current at the left-end preset sampling point and the actual transformed current at the right-end preset sampling point.

[0127]

[0128]

[0129] In the formula, This represents the actual transformation current at the preset sampling point on the left. S represents the actual transformation current at the preset sampling point on the right. -1 Let Kelenberger's inverse transformation matrix be denoted as , where

[0130]

[0131] Those skilled in the art will clearly understand that, for the sake of convenience and brevity, the specific working processes of the systems, devices, and units described above can be referred to the corresponding processes in the foregoing method embodiments, and will not be repeated here.

[0132] In the several embodiments provided by this invention, it should be understood that the disclosed apparatus and methods can be implemented in other ways. For example, the apparatus embodiments described above are merely illustrative; for instance, the division of 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 apparatuses or units may be electrical, mechanical, or other forms.

[0133] 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.

[0134] 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.

[0135] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for fast differential current protection of flexible low-frequency transmission lines, characterized in that, Includes the following steps: Collect the three-phase voltage and three-phase current at a preset sampling point on the left end of the lossless power transmission line; The three-phase voltage and three-phase current at the preset sampling point on the left are transformed using the Kelenberg transformation matrix to obtain three voltage modulus components and three current modulus components. The actual lossless voltage and actual lossless current at the left-end preset sampling point of the lossless transmission line are calculated using the three voltage-mode components, the three current-mode components, and the line resistance of the preset sampling point at the left end, including: Using the three voltage-mode components, the three current-mode components, and the line resistance of the preset sampling point at the left end, the actual lossless voltage and actual lossless current of the lossless transmission line at the preset sampling point at the left end are calculated using the following formula: In the formula, This represents the actual lossless voltage at the preset sampling point on the left. R represents the actual lossless current at the preset sampling point on the left, and R represents the total resistance. This represents the voltage magnitude component of the nth phase at the preset sampling point on the left. This represents the current-mode component of the nth phase at the preset sampling point on the left. Based on the wave equation of the lossless transmission line, the actual lossless current at the preset sampling point at the right end of the lossless transmission line is calculated using the actual lossless voltage and actual lossless current at the preset sampling point at the left end. This includes: Based on the wave equation of the lossless transmission line, the actual lossless current at the preset sampling point at the right end of the lossless transmission line is calculated using the actual lossless voltage and actual lossless current at the preset sampling point at the left end of the lossless transmission line. The wave equation of the lossless transmission line is as follows: In the formula, This represents the actual lossless current at the preset sampling point on the right. Represents the line time constant, where, v represents wave velocity. t represents the current time, and Z represents the line impedance. Where L0 represents the inductance per unit length and C0 represents the capacitance per unit length; The actual lossless current at the left-end preset sampling point and the actual lossless current at the right-end preset sampling point are transformed using the Kelenberg inverse transform matrix to obtain the corresponding actual transformed current at the left-end preset sampling point and the actual transformed current at the right-end preset sampling point. The actual transformed current of the preset sampling point on the left end and the actual transformed current of the preset sampling point on the right end are summed to obtain a current sum result. It is then determined whether the current sum result is greater than a preset differential current threshold. If the current sum result is greater than the preset differential current threshold, it is determined that there is a fault in the line section between the preset sampling point on the left end and the preset sampling point on the right end of the transmission line, and the faulty line section is isolated and protected.

2. The current differential fast protection method for flexible low-frequency transmission lines according to claim 1, characterized in that, Before collecting the three-phase voltage and three-phase current at a preset sampling point on the left end of a lossless transmission line, the following steps are included: The total resistance of the transmission line is obtained as follows: In the formula, R0 represents the resistance per unit length, and Len represents the line length; Left-end resistors and right-end resistors are set at the left-end and right-end preset sampling points of the lossless power transmission line, respectively, where the left-end resistor and the right-end resistor are both R / 2.

3. The current differential fast protection method for flexible low-frequency transmission lines according to claim 2, characterized in that, The specific steps for transforming the three-phase voltage and three-phase current at the preset sampling points on the left using the Kelvin transform matrix to obtain three voltage magnitude components and three current magnitude components include: Using the Kelvin transform matrix, the three-phase voltage and three-phase current at the preset sampling point on the left are transformed to obtain three voltage magnitude components and three current magnitude components. In the formula, This represents the nth phase voltage at the preset sampling point on the left. Let represent the nth phase current at the preset sampling point on the left, l represent the preset sampling point on the left, n = a, b, c, where a represents phase a, b represents phase b, c represents phase c, and S represents the Kelvin transformation matrix. 。 4. The current differential fast protection method for flexible low-frequency transmission lines according to claim 1, characterized in that, Using the inverse Kalenberger transform matrix, the actual lossless currents at the left and right preset sampling points are transformed respectively, yielding the corresponding actual transformed currents at the left and right preset sampling points. In the formula, This represents the actual transformation current at the preset sampling point on the left. This represents the actual transformation current at the preset sampling point on the right. Let Kelenberger's inverse transformation matrix be denoted as , where 。 5. A current differential fast protection system for flexible low-frequency transmission lines, characterized in that, include: The acquisition module is used to acquire the three-phase voltage and three-phase current at a preset sampling point on the left end of the lossless power transmission line; The transformation module is used to transform the three-phase voltage and three-phase current of the preset sampling point on the left end using the Kelvin transformation matrix to obtain three voltage modulus components and three current modulus components. The first lossless calculation module is used to calculate the actual lossless voltage and actual lossless current at the left-end preset sampling point of the lossless transmission line using the three voltage-mode components, the three current-mode components, and the line resistance of the left-end preset sampling point. Specifically, the first lossless calculation module is used to calculate the actual lossless voltage and actual lossless current at the left-end preset sampling point of the lossless transmission line using the three voltage-mode components, the three current-mode components, and the line resistance of the left-end preset sampling point through the following formula: In the formula, This represents the actual lossless voltage at the preset sampling point on the left. R represents the actual lossless current at the preset sampling point on the left, and R represents the total resistance. This represents the voltage magnitude component of the nth phase at the preset sampling point on the left. This represents the current-mode component of the nth phase at the preset sampling point on the left. The second lossless calculation module is used to calculate the actual lossless current at the preset sampling point at the right end of the lossless transmission line based on the actual lossless voltage and actual lossless current at the preset sampling point at the left end of the lossless transmission line according to the fluctuation equation of the lossless line. Based on the wave equation of the lossless transmission line, the actual lossless current at the preset sampling point at the right end of the lossless transmission line is calculated using the actual lossless voltage and actual lossless current at the preset sampling point at the left end. This includes: Based on the wave equation of the lossless transmission line, the actual lossless current at the preset sampling point at the right end of the lossless transmission line is calculated using the actual lossless voltage and actual lossless current at the preset sampling point at the left end of the lossless transmission line. The wave equation of the lossless transmission line is as follows: In the formula, This represents the actual lossless current at the preset sampling point on the right. Represents the line time constant, where, v represents wave velocity. t represents the current time, and Z represents the line impedance. Where L0 represents the inductance per unit length and C0 represents the capacitance per unit length; The inverse transformation module is used to transform the actual lossless current at the left-end preset sampling point and the actual lossless current at the right-end preset sampling point using the Kelenberg inverse transformation matrix, respectively, to obtain the corresponding actual transformed current at the left-end preset sampling point and the actual transformed current at the right-end preset sampling point. The fault protection module is used to sum the actual transformation current of the preset sampling point on the left end and the actual transformation current of the preset sampling point on the right end to obtain the current summation result, and determine whether the current summation result is greater than a preset differential current threshold. If the current summation result is greater than the preset differential current threshold, it is determined that there is a fault in the line section between the preset sampling point on the left end and the preset sampling point on the right end of the transmission line, and the faulty line section is isolated and protected.

6. The current differential fast protection system for flexible low-frequency transmission lines according to claim 5, characterized in that, Also includes: The resistance acquisition module is used to obtain the total resistance of the transmission line. In the formula, R0 represents the resistance per unit length, and Len represents the line length; The resistance setting module is used to set the left-end resistance and the right-end resistance at the preset sampling points on the left and right ends of the lossless power transmission line, respectively, wherein the left-end resistance and the right-end resistance are both R / 2.

7. The current differential fast protection system for flexible low-frequency transmission lines according to claim 6, characterized in that, The transformation module is specifically used to transform the three-phase voltage and three-phase current at the preset sampling point on the left using the Kelvin transform matrix, respectively, to obtain three voltage modulus components and three current modulus components. In the formula, This represents the nth phase voltage at the preset sampling point on the left. Let represent the nth phase current at the preset sampling point on the left, l represent the preset sampling point on the left, n = a, b, c, where a represents phase a, b represents phase b, c represents phase c, and S represents the Kelvin transformation matrix. 。

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