Differential protection method and system for low-frequency power transmission system under negative-sequence current injection

Abstract

The invention provides a differential protection method and system for a low-frequency power transmission system under negative-sequence current injection, and the method comprises the steps: obtaining active power fluctuation caused by the low-frequency power transmission system during the inhibition of an asymmetric fault, and obtaining a negative-sequence current injection reference value; obtaining a fan side current and a low-frequency side current during a fault, and calculating an additional current and an additional current coefficient under a negative sequence current injection reference value; and calculating a sensitivity coefficient according to the additional current and the additional current coefficient, and performing fault judgment according to the sensitivity coefficient. Detailed design and improvement are carried out on a differential protection method when the low-frequency power transmission system has an asymmetric fault, active power fluctuation caused when the low-frequency power transmission system suppresses the asymmetric fault is obtained, and a reference value of negative-sequence current injection is obtained accordingly; according to the technical scheme, the negative-sequence current component under the asymmetric fault can be effectively controlled, and then power circulation between the sub-converters is adjusted.

Description

Technical Field

[0001] The present invention belongs to the field of low-frequency power transmission line protection, and in particular relates to a differential protection method and system for a low-frequency power transmission system under negative sequence current injection. Background Art

[0002] The uneven distribution of energy has caused imbalances in power generation and consumption in various regions, as well as problems in the transportation of primary energy such as coal; therefore, long-distance power transmission is one of the basic characteristics of my country's power grid technology. As offshore wind farms are gradually developing from small-scale offshore to large-scale medium and far seas, low-frequency transmission systems are gradually entering the public eye. However, as the system transmission frequency decreases and the cycle becomes longer, the judgment time of traditional phasor-type AC protection is usually a whole or half a cycle. If traditional AC protection is used directly in the low-frequency transmission system, the protection judgment time will increase. However, power electronic devices are limited by their current carrying capacity, and it is necessary to quickly judge and remove the fault when the system is protected to ensure the safety of the components. In addition, since the converter M3C in the low-frequency transmission system has a certain control effect, when the system fails, the fault current will be affected by the specific control strategy of M3C, resulting in the possibility of reduced sensitivity or even refusal of differential protection. When an asymmetric fault occurs in the low-frequency transmission line, the traditional protection judgment method caused by the traditional control strategy is unreliable. Summary of the invention

[0003] The object of the present invention is to provide a differential protection method and system for a low-frequency power transmission system under negative sequence current injection, so as to solve the above-mentioned problems.

[0004] To achieve the above object, the present invention adopts the following technical solutions: In a first aspect, the present invention provides a differential protection method for a low-frequency power transmission system under negative sequence current injection, comprising: Obtain the active power fluctuation caused by the low-frequency transmission system suppressing asymmetric faults and obtain the negative sequence current injection reference value; Obtain the fan side current and low-frequency side current during the fault, and calculate the additional current and additional current coefficient under the negative sequence current injection reference value; The sensitivity coefficient is calculated based on the additional current and the additional current coefficient, and the fault judgment is made based on the sensitivity coefficient.

[0005] Furthermore, the method of obtaining the active power fluctuation caused by suppressing asymmetric faults in the low-frequency power transmission system to obtain a negative sequence current injection reference value includes: When a low-frequency transmission line fails, additional control is added for the negative sequence current component under asymmetric faults, and the negative sequence current reference value is selected to adjust the power flow between sub-converters to reduce the power fluctuation between the sub-converters on the flexible direct current side under faults. , Control is 0 to achieve balanced control of the DC component of the three-phase active power on the fault side, and the reference value of the negative sequence current dq component is obtained as: (1) Where: U d- , U d+ , I d- , I d+ They are the positive and negative sequence d-axis components of the low-frequency side voltage and the positive and negative sequence d-axis components of the current respectively.

[0006] Furthermore, the obtaining of the wind turbine side current and the low-frequency side current during a fault, and the calculation of the additional current under the negative sequence current injection reference value, includes: The additional current expression is as follows (8) Further, calculate the additional current coefficient: Introduction fsign To change the adaptive additional current direction; when an internal fault occurs, the expected fsign is equal to 1, and Imax In phase, the working current is increased and the suppression current is reduced; on the contrary, when an external fault occurs, fsign Expected to be equal to -1; for Imax The reverse phase reduces the working current and increases the suppression current; The following relationship is obtained: (9) Will f sign Design as follows: (10) Internal fault f sign Tends to 1, when external fault occurs f sign tends to 1; when the adaptive additional current expressed by equations (8) and (10) is used, for I n The inversion of | I op | Much smaller than K|I res | ; The adaptive additional current differential protection is reliable and not affected by CT saturation.

[0007] Furthermore, the adaptive additional current differential protection is adopted, and the criterion for the fault in the zone is: (2) (3) In the formula I m 、I n 、I min 、I max 、 They are respectively the current on the wind turbine side, the current on the low-frequency side, the minimum current at the protection installations on both sides of the transmission line, the maximum current at the protection installations on both sides of the transmission line, and the additional current.

[0008] Further, calculating the sensitivity coefficient according to the additional current and the additional current coefficient, and performing fault judgment according to the sensitivity coefficient, includes: The sensitivity coefficient can be defined as follows: (11) judge K Size, identification area, internal and external faults; when K ≥0.6, the fault is identified as an internal fault; K When <0.6, the fault is identified as an external fault.

[0009] In a second aspect, the present invention provides a differential protection system for a low-frequency power transmission system under negative sequence current injection, comprising: A reference value acquisition module is used to obtain the active power fluctuation caused by the low-frequency transmission system suppressing asymmetric faults, and obtain a negative sequence current injection reference value; A calculation module, used to obtain the fan side current and the low-frequency side current during a fault, and calculate the additional current and the additional current coefficient under the negative sequence current injection reference value; The judgment module is used to calculate the sensitivity coefficient according to the additional current and the additional current coefficient, and make a fault judgment according to the sensitivity coefficient.

[0010] Furthermore, the method of obtaining the active power fluctuation caused by suppressing asymmetric faults in the low-frequency power transmission system to obtain a negative sequence current injection reference value includes: When a low-frequency transmission line fails, additional control is added for the negative sequence current component under asymmetric faults, and the negative sequence current reference value is selected to adjust the power flow between sub-converters to reduce the power fluctuation between the sub-converters on the flexible direct current side under faults. , Control is 0 to achieve balanced control of the DC component of the three-phase active power on the fault side, and the reference value of the negative sequence current dq component is obtained as: (1) Where: U d- , U d+ , I d- , I d+ They are the positive and negative sequence d-axis components of the low-frequency side voltage and the positive and negative sequence d-axis components of the current respectively.

[0011] In a third aspect, the present invention provides a computer device comprising a memory, a processor, and a computer program stored in the memory and executable on the processor, wherein the processor implements the steps of the differential protection method for a low-frequency power transmission system under negative sequence current injection when executing the computer program.

[0012] In a fourth aspect, the present invention provides a computer-readable storage medium, wherein the computer-readable storage medium stores a computer program, and when the computer program is executed by a processor, the steps of the differential protection method for a low-frequency transmission system under negative sequence current injection are implemented.

[0013] Compared with the prior art, the present invention has the following technical effects: The present invention has made a detailed design and improvement on the differential protection method of the low-frequency power transmission system when an asymmetric fault occurs. By obtaining the active power fluctuation caused by the low-frequency power transmission system when suppressing an asymmetric fault, and obtaining the reference value of the negative sequence current injection based on it, the technical solution can effectively control the negative sequence current component under an asymmetric fault, and then adjust the power flow between the sub-converters, and control the power fluctuation of the flexible direct current side sub-converter under the fault to 0. This helps to achieve balanced control of the three-phase active power DC component on the fault side, and reduce system instability and power loss caused by asymmetric faults.

[0014] By calculating the reference value of the negative sequence current dq component and applying it to the control strategy of the system, the negative sequence current can be effectively suppressed.

[0015] Based on the current on the wind turbine side and the low-frequency side during the fault, the additional current and the additional current coefficient are calculated, and the sensitivity coefficient is further calculated. This technical solution can significantly improve the sensitivity of the differential protection. This enables the protection to operate more quickly and accurately when an internal fault occurs, reducing the impact of the fault on the system. The concept of adaptive additional current is introduced. By adjusting the direction and size of the additional current, the working current is increased and the suppression current is reduced in the case of an internal fault, and the opposite is true in the case of an external fault. At the same time, by designing the sensitivity coefficient K and distinguishing the internal and external faults according to the size of K, the accuracy and reliability of the differential protection are further improved.

[0016] This technical solution not only improves the traditional control strategy and the sensitivity of the traditional differential protection, but also improves the traditional differential protection itself. By adding a negative sequence current injection control strategy, the differential current is increased and the braking current is reduced, thereby further improving the performance of the differential protection.

[0017] The adaptive additional current differential protection method is adopted to clearly define the criteria for faults in the zone. At the same time, by calculating the sensitivity coefficient K and judging the type and location of the fault according to the value of K, the optimization and improvement of the traditional differential protection is achieved.

[0018] This technical solution considers the impact of negative sequence current injection strategy on differential protection of low frequency transmission system and proposes corresponding improvement methods. This makes this technical solution have broad application prospects in the field of low frequency transmission line protection. Whether in improving system stability, reducing power loss, or improving the sensitivity and reliability of differential protection, this technical solution has shown significant advantages.

[0019] In summary, this technical solution has achieved remarkable technical results by suppressing active power fluctuations caused by asymmetric faults, improving the sensitivity of differential protection, and improving the performance of traditional differential protection. At the same time, its broad application prospects also provide strong support for its practical application in the field of low-frequency transmission line protection. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 This is a logic block diagram of the present invention.

[0021] Figure 2 This is the structural topology of the offshore low-frequency power transmission system in an embodiment of the present invention.

[0022] Figure 3 (a)- Figure 3 (i) is the value and setting value of the differential current when different types of faults occur at different locations of the low-frequency transmission line of the present invention; Figure 3 (a) Results of improved differential protection for AG, ABG, and AB faults at 10% of the line. Figure 3 (b) Results of improved differential protection for AG, ABG, and AB faults at 20% of the line. Figure 3 (c) Results of improved differential protection for AG, ABG, and AB faults at 30% of the line. Figure 3 (d) Results of improved differential protection for AG, ABG, and AB faults at line 40. Figure 3 (e) Results of improved differential protection for AG, ABG, and AB faults at 50% of the line. Figure 3 (f) Results of improved differential protection for AG, ABG, and AB faults at 60% of the line. Figure 3(g) Results of improved differential protection for AG, ABG, and AB faults at 70% of the line. Figure 3 (h) Results of improved differential protection for AG, ABG, and AB faults at 80% of the line. Figure 3 (i) Results of improved differential protection for AG, ABG, and AB faults at 90% of the line.

[0023] Figure 4 It is a flow chart of the present invention. DETAILED DESCRIPTION

[0024] The present invention is further described below in conjunction with the accompanying drawings: Example 1, please refer to Figure 4 The present invention provides a differential protection method for a low-frequency power transmission system under negative sequence current injection, comprising: Obtain the active power fluctuation caused by the low-frequency transmission system suppressing asymmetric faults and obtain the negative sequence current injection reference value; Obtain the fan side current and low-frequency side current during the fault, and calculate the additional current and additional current coefficient under the negative sequence current injection reference value; The sensitivity coefficient is calculated based on the additional current and the additional current coefficient, and the fault judgment is made based on the sensitivity coefficient.

[0025] By obtaining the active power fluctuation caused by the low-frequency transmission system suppressing asymmetric faults and calculating the reference value of negative sequence current injection based on it, the present invention realizes accurate control of negative sequence current. This accurate control helps to reduce the negative impact of asymmetric faults on the system, such as power fluctuations, voltage imbalance, etc., thereby improving the stability and reliability of the system.

[0026] When a fault occurs, the invention provides a more accurate data basis for differential protection by obtaining the fan side current and the low-frequency side current, and calculating the additional current and the additional current coefficient in combination with the negative sequence current injection reference value. These additional information helps the differential protection to more accurately determine the location and type of the fault, avoids false operation or refusal to operate due to insufficient or misleading information, and improves the accuracy and reliability of protection.

[0027] The present invention significantly enhances the sensitivity of differential protection to faults by calculating the sensitivity coefficient and making fault judgment based on the coefficient. The introduction of the sensitivity coefficient enables the protection to capture fault signals more sensitively, and respond in time even in the early stage of a fault or when the fault signal is weak, effectively shortening the fault removal time and reducing the damage of the fault to the system.

[0028] Through the implementation of the above technical methods, the present invention not only improves the performance of differential protection, but also improves the safety of low-frequency power transmission systems as a whole. Accurately controlling the injection of negative sequence current reduces the power fluctuation and voltage imbalance of the system, and improves the stable operation capability of the system; improving the accuracy and sensitivity of differential protection ensures that faults can be detected and removed in time, preventing the expansion and spread of faults. These technical effects work together on the system to form a multi-level, all-round safety guarantee.

[0029] Example 2, please refer to Figure 1 The present invention provides a differential protection method for a low-frequency power transmission system under negative sequence current injection, the specific steps of which are: Step 1. Obtain a negative sequence current injection reference value by suppressing active power fluctuation caused by an asymmetric fault; Step 2. According to the impact of negative sequence current injection on the traditional differential protection of low-frequency transmission lines; Step 3. Adaptive additional current differential protection is proposed, and the additional current is designed for faults within the zone; Step 4. When an out-of-zone fault occurs, design the additional coefficient based on whether the CT is saturated. f sign .

[0030] Step 5. According to K sen The size of the fault area can be used to determine whether the fault is inside or outside the area.

[0031] Furthermore, in step 1, when a low-frequency transmission line fails, additional control is added for the negative-sequence current component under an asymmetric fault, and a suitable negative-sequence current reference value is selected to adjust the power flow between the sub-converters. In order to eliminate the influence of this part of the unbalanced DC component on the capacitor voltage, the power fluctuation between the sub-converters on the flexible DC side under the fault is adjusted. , Control it to 0 to achieve balanced control of the DC component of the three-phase active power on the fault side, and the reference value of the negative sequence current dq component can be obtained as: (1) Where: U d- , U d+ , I d- , I d+ They are respectively the positive and negative sequence d-axis components of the voltage and the positive and negative sequence d-axis components of the current on the low-frequency side. The range of the negative sequence current injection reference value is .

[0032] Furthermore, in step 2, a negative sequence current suppression control strategy is adopted during the fault on the wind turbine side, and a negative sequence current injection and voltage reduction VF coordinated control strategy is adopted on the M3C low frequency side. Considering that the change of control strategy may affect the stability of the system, a short-time injection is adopted, and a negative sequence current of one cycle is injected after one cycle after the fault, and then the original control strategy is switched. The sensitivity of the differential protection during the injection of negative sequence current is significantly improved compared with the traditional sensitivity of the negative sequence current suppression strategy adopted at both ends.

[0033] (2) Furthermore, in step 3, for the design of the additional current during the fault in the zone, the specific steps are: The proposed scheme's criterion for faults within the area is: (3) (4) In the formula I m 、I n 、I min 、I max 、 They are respectively the current on the wind turbine side, the current on the low-frequency side, the minimum current at the protection installations on both sides of the transmission line, the maximum current at the protection installations on both sides of the transmission line, and the additional current.

[0034] Adaptive additional current Can be designed as: (5) According to the amplitude ratio and phase difference of the current on both sides, The amplitudes of the operating current and the suppression current can be adaptively changed. Always with I max Therefore, when an internal fault occurs, The introduction increases the amplitude of the action current, but reduces the amplitude of the braking current.

[0035] Furthermore, in step 4, the additional coefficient is designed according to whether there is CT saturation f sign , the specific design steps are as follows: When there is no CT saturation due to external fault, the currents on both sides satisfy the following relationship: (6) (7) is approximately 0. In this case, the adaptive additional current differential protection degenerates into the traditional differential protection. Therefore, in the case of external faults, the criterion of the proposed scheme is the same as that of the traditional differential protection. I op Significantly less than K|I res | , external faults can be correctly identified.

[0036] The external slices have CT saturation. Deviation from 0. Increased operating current I op , reducing the suppression current I res , resulting in | I op |Greater than K|I res | This external fault is misjudged as being within the protection zone, resulting in the generation of a false trip signal. An improved solution is proposed to address the safety issues caused by CT saturation. Equation (5) is modified and a unified expression for the adaptive additional current is designed, as shown in the formula: (8) Introduction fsign To change the adaptive additional current When an internal fault occurs, the expected fsign is equal to 1. Therefore, and Imax In phase, the working current is increased and the suppression current is reduced. Conversely, when an external fault occurs, fsign Expected to be equal to -1. for Imax of the reverse phase, reducing the operating current and increasing the suppression current.

[0037] It is close to 0 in the linear transition region and very large in the CT saturation region. and Are much greater than Due to the weak feed characteristics of the converter interface source, even in the case of a fault in the high impedance area, The magnitude is also greater than The amplitude is large.

[0038] Based on the previous analysis, we can get the following relationship: (9) Therefore, the f sign Design as follows: (10) Internal fault f sign Tends to 1, when external fault occurs f sign tends to 1. When the adaptive additional current expressed by equations (8) and (10) is used, for I n The inversion of | I op | Much smaller than K|I res | The adaptive additional current differential protection is reliable and is not affected by CT saturation.

[0039] Furthermore, in step 5, the sensitivity coefficient can be defined as follows: (11) judge K Size, identify area, internal and external faults. K ≥0.6, the fault is identified as an internal fault. K When <0.6, the fault is identified as an external fault.

[0040] Establish as attached Figure 2 The offshore low-frequency power transmission system shown in the figure is used as a simulation model. The system lines are set as shown in the figure. Figure 2 The topology is shown.

[0041] When AG, ABG, and AB faults occur at the end of the line, in order to suppress the active power fluctuation caused by asymmetric faults, the reference value of negative sequence current injection is obtained. Then, the traditional differential protection is improved by adding a negative sequence current injection control strategy to increase the sensitivity coefficient of the fault in the area, and the action of the differential protection is judged according to the sensitivity coefficient. The above simulation results are shown in the following table.

[0042] Table 1 Action conditions under different transition resistances

[0043] Establish as attached Figure 2 The offshore low-frequency power transmission system shown in the figure is used as a simulation model. The system lines are set as shown in the figure. Figure 2 The topology shows that AG, ABG, and AB faults occur after 300 Whether the differential protection can operate accurately when the transition resistance is different at the fault position. The above simulation results are shown in the following table.

[0044] Table 2 Action conditions under different fault locations

[0045] In yet another embodiment of the present invention, a differential protection system for a low-frequency power transmission system under negative sequence current injection is provided, which can be used to implement the above-mentioned differential protection method for a low-frequency power transmission system under negative sequence current injection. Specifically, the system includes: A reference value acquisition module is used to obtain the active power fluctuation caused by the low-frequency transmission system suppressing asymmetric faults, and obtain a negative sequence current injection reference value; A calculation module, used to obtain the fan side current and the low-frequency side current during a fault, and calculate the additional current and the additional current coefficient under the negative sequence current injection reference value; The judgment module is used to calculate the sensitivity coefficient according to the additional current and the additional current coefficient, and make a fault judgment according to the sensitivity coefficient.

[0046] The division of modules in the embodiments of the present invention is schematic and is only a logical function division. There may be other division methods in actual implementation. In addition, each functional module in each embodiment of the present invention may be integrated into one processor, or may exist physically separately, or two or more modules may be integrated into one module. The above-mentioned integrated modules may be implemented in the form of hardware or in the form of software functional modules.

[0047] In another embodiment of the present invention, a computer device is provided, the computer device comprising a processor and a memory, the memory being used to store a computer program, the computer program comprising program instructions, and the processor being used to execute the program instructions stored in the computer storage medium. The processor may be a central processing unit (CPU), or may be other general-purpose processors, digital signal processors (DSP), application-specific integrated circuits (ASIC), field-programmable gate arrays (FPGA) or other programmable logic devices, discrete gates or transistor logic devices, discrete hardware components, etc. It is the computing core and control core of the terminal, which is suitable for implementing one or more instructions, and is specifically suitable for loading and executing one or more instructions in a computer storage medium to implement a corresponding method flow or corresponding function; the processor described in the embodiment of the present invention can be used for the operation of a differential protection method for a low-frequency transmission system under negative sequence current injection.

[0048] In another embodiment of the present invention, the present invention further provides a storage medium, specifically a computer-readable storage medium (Memory), which is a memory device in a computer device for storing programs and data. It is understandable that the computer-readable storage medium here can include both built-in storage media in the computer device and, of course, extended storage media supported by the computer device. The computer-readable storage medium provides a storage space, which stores the operating system of the terminal. In addition, one or more instructions suitable for being loaded and executed by the processor are also stored in the storage space, and these instructions can be one or more computer programs (including program codes). It should be noted that the computer-readable storage medium here can be a high-speed RAM memory or a non-volatile memory, such as at least one disk memory. The processor can load and execute one or more instructions stored in the computer-readable storage medium to implement the corresponding steps of the differential protection method for a low-frequency transmission system under negative sequence current injection in the above embodiment.

[0049] It will be appreciated by those skilled in the art that embodiments of the present invention may be provided as methods, systems, or computer program products. Therefore, the present invention may take the form of a complete hardware embodiment, a complete software embodiment, or an embodiment combining software and hardware. Furthermore, the present invention may take the form of a computer program product implemented on one or more computer-usable storage media (including but not limited to disk storage, CD-ROM, optical storage, etc.) containing computer-usable program code.

[0050] The present invention is described with reference to flowcharts and / or block diagrams of methods, devices (systems), and computer program products according to embodiments of the present invention. It should be understood that each process and / or block in the flowchart and / or block diagram, as well as the combination of processes and / or blocks in the flowchart and / or block diagram, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, a special-purpose computer, an embedded processor, or other programmable data processing device to produce a machine, so that the instructions executed by the processor of the computer or other programmable data processing device generate instructions for implementing the processes in the flowchart and / or block diagram. Figure 1 A process or multiple processes and / or boxes Figure 1 A device that provides the functions specified in a block or multiple blocks.

[0051] These computer program instructions may also be stored in a computer-readable memory capable of directing a computer or other programmable data processing device to operate in a specific manner, so that the instructions stored in the computer-readable memory produce an article of manufacture comprising an instruction device, which implements the process Figure 1 A process or multiple processes and / or boxes Figure 1 A function specified in one or more boxes.

[0052] These computer program instructions can also be loaded onto a computer or other programmable data processing device so that a series of operating steps are executed on the computer or other programmable device to produce a computer-implemented process, thereby providing instructions for implementing the process. Figure 1 A process or multiple processes and / or boxes Figure 1 The steps for the functions specified in one or more boxes.

[0053] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the above embodiments, ordinary technicians in the relevant field should understand that the specific implementation methods of the present invention can still be modified or replaced by equivalents. Any modification or equivalent replacement that does not depart from the spirit and scope of the present invention should be covered within the scope of protection of the claims of the present invention.

Claims

1. A differential protection method for a low-frequency power transmission system under negative sequence current injection, characterized in that: include: Obtain the active power fluctuation caused by the low-frequency transmission system suppressing asymmetric faults and obtain the negative sequence current injection reference value; Obtain the fan side current and low-frequency side current during the fault, and calculate the additional current and additional current coefficient under the negative sequence current injection reference value; The sensitivity coefficient is calculated based on the additional current and the additional current coefficient, and the fault judgment is made based on the sensitivity coefficient.

2. The differential protection method for a low-frequency power transmission system under negative sequence current injection according to claim 1, characterized in that: The method of obtaining the active power fluctuation caused by the low-frequency power transmission system suppressing asymmetric faults and obtaining the negative-sequence current injection reference value includes: When a low-frequency transmission line fails, additional control is added for the negative sequence current component under asymmetric faults, and the negative sequence current reference value is selected to adjust the power flow between sub-converters to reduce the power fluctuation between the sub-converters on the flexible direct current side under faults. , Control is 0 to achieve balanced control of the DC component of the three-phase active power on the fault side, and the reference value of the negative sequence current dq component is obtained as: (1) Where: U d- , U d+ , I d- , I d+ They are the positive and negative sequence d-axis components of the low-frequency side voltage and the positive and negative sequence d-axis components of the current, respectively.

3. The differential protection method for a low-frequency power transmission system under negative sequence current injection according to claim 1, characterized in that: The obtaining of the fan side current and the low-frequency side current during a fault, and the calculation of the additional current under the negative sequence current injection reference value, includes: The additional current expression is as follows （8）。 4. The differential protection method for a low-frequency power transmission system under negative sequence current injection according to claim 3, characterized in that: Calculate the additional current factor: Introduction fsign To change the adaptive additional current direction; when an internal fault occurs, the expected fsign is equal to 1, and Imax In phase, the working current is increased and the suppression current is reduced; on the contrary, when an external fault occurs, fsign Expected to be equal to -1; for Imax The reverse phase reduces the working current and increases the suppression current; The following relationship is obtained: （9） Will f sign Design as follows: （10） Internal fault f sign Tends to 1, when external fault occurs f sign tends to 1; when the adaptive additional current expressed by equations (8) and (10) is used, for I n The inversion of | I op | Much smaller than K|I res | ; The adaptive additional current differential protection is reliable and not affected by CT saturation.

5. The differential protection method for a low-frequency power transmission system under negative sequence current injection according to claim 3, characterized in that: Adaptive additional current differential protection is used, and the criterion for faults within the zone is: （2） （3） In the formula I m 、I n 、I min 、I max 、 They are respectively the current on the wind turbine side, the current on the low-frequency side, the minimum current at the protection installations on both sides of the transmission line, the maximum current at the protection installations on both sides of the transmission line, and the additional current.

6. The differential protection method for a low-frequency power transmission system under negative sequence current injection according to claim 1, characterized in that: The method of calculating the sensitivity coefficient according to the additional current and the additional current coefficient and making a fault judgment according to the sensitivity coefficient includes: The sensitivity coefficient can be defined as follows: （11） judge K Size, identification area, internal and external faults; when K ≥0.6, the fault is identified as an internal fault; K When <0.6, the fault is identified as an external fault.

7. A differential protection system for a low-frequency power transmission system under negative sequence current injection, characterized in that: include: A reference value acquisition module is used to obtain the active power fluctuation caused by the low-frequency transmission system suppressing asymmetric faults, and obtain a negative sequence current injection reference value; A calculation module, used to obtain the fan side current and the low-frequency side current during a fault, and calculate the additional current and the additional current coefficient under the negative sequence current injection reference value; The judgment module is used to calculate the sensitivity coefficient according to the additional current and the additional current coefficient, and make a fault judgment according to the sensitivity coefficient.

8. The differential protection system of a low-frequency power transmission system under negative sequence current injection according to claim 7, characterized in that: The method of obtaining the active power fluctuation caused by the low-frequency power transmission system suppressing asymmetric faults and obtaining the negative-sequence current injection reference value includes: When a low-frequency transmission line fails, additional control is added for the negative sequence current component under asymmetric faults, and the negative sequence current reference value is selected to adjust the power flow between sub-converters to reduce the power fluctuation between the sub-converters on the flexible direct current side under faults. , Control is 0 to achieve balanced control of the DC component of the three-phase active power on the fault side, and the reference value of the negative sequence current dq component is obtained as: (1) Where: U d- , U d+ , I d- , I d+ They are the positive and negative sequence d-axis components of the low-frequency side voltage and the positive and negative sequence d-axis components of the current, respectively.

9. A computer device comprising a memory, a processor, and a computer program stored in the memory and executable on the processor, characterized in that: When the processor executes the computer program, the steps of the differential protection method for a low-frequency power transmission system under negative sequence current injection as described in any one of claims 1 to 6 are implemented.

10. A computer-readable storage medium storing a computer program, characterized in that: When the computer program is executed by a processor, the steps of the differential protection method for a low-frequency power transmission system under negative sequence current injection as claimed in any one of claims 1 to 6 are implemented.

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