Direct current high impedance fault protection method and system for a two-terminal hybrid direct current system

Abstract

The application discloses a DC high-resistance fault protection method and system for a double-ended hybrid DC system, and belongs to the field of power system relay protection. The method comprises the following steps: when an AC fault occurs at the local side, the double-ended hybrid DC system is switched to an AC fault ride-through mode to remove the AC fault; when a DC fault occurs at the local side, the following operations are performed: according to the fault direction and the fault type of the DC fault, the control mode of the double-ended hybrid DC system is switched; the absolute value of the average value of the DC line voltage in a preset time window under the current control mode is calculated, when the absolute value of the average value is lower than a preset threshold, the DC fault is an intra-zone fault, otherwise, the DC fault is an inter-zone fault; and the DC fault is removed by using corresponding intra-zone fault removal measures or inter-zone fault removal measures. The scheme does not depend on a communication system, has low requirements on devices, has strong tolerance to transition resistance, and is high in protection reliability.

Description

Technical Field

[0001] This invention belongs to the field of power system relay protection, and more specifically, relates to a method and system for protecting against high-resistance DC faults in a dual-terminal hybrid DC system. Background Technology

[0002] High-capacity, long-distance power transmission channels are crucial for solving the problem of reverse distribution between energy production areas and power load centers. High-voltage direct current (HVDC) transmission has advantages over high-voltage alternating current (HVAC) transmission. In HVDC technology, the rectifier side uses a line-commutated converter (LCC), and the inverter side uses a modular multilevel converter (MMC). This hybrid HVDC technology, with its advantages of eliminating commutation failure issues and high transmission flexibility, has been applied in scenarios such as offshore renewable energy transmission, island power supply, and long-distance power transmission.

[0003] Hybrid DC transmission technology still faces the following technical challenges in engineering applications. Under DC fault conditions, the fault current amplitude is high and rises rapidly. The MMC side of the hybrid DC transmission system requires current-limiting reactors or current-limiting control to protect power electronic devices. However, current-limiting reactors increase system construction costs; while current-limiting control can achieve current limiting by controlling the activation of bridge arm submodules, it weakens the system fault characteristics, thus affecting the protection principle based on transient quantity information.

[0004] Existing DC transmission line protection typically employs traveling wave protection as the primary protection and longitudinal differential protection as backup. Traveling wave protection can quickly and accurately identify metallic faults or faults with low transition resistance in DC lines, but it lacks sensitivity for high-resistance faults at the end of the DC line. Dual-ended quantitative protection and longitudinal differential protection offer absolute selectivity for faults, but the reliability of dual-ended quantitative protection depends on the path, and longitudinal differential protection suffers from transient distributed capacitance current issues, resulting in poor fast-acting performance. Therefore, how to achieve reliable single-ended quantitative protection for high-resistance faults in DC transmission lines is a pressing issue that needs to be addressed in DC line protection. Summary of the Invention

[0005] To address the shortcomings and improvement needs of existing technologies, this invention provides a method and system for protecting DC high-resistance faults in dual-ended hybrid DC systems. Its purpose is to address the problems of low reliability and reliance on communication systems in existing dual-ended quantity protection, traveling wave protection, and longitudinal differential protection during dual-ended hybrid DC transmission faults.

[0006] To achieve the above objectives, according to one aspect of the present invention, a method for protecting against high-impedance DC faults in a dual-ended hybrid DC system is provided, comprising: S1, when a fault occurs on the local side, determining whether the fault is an AC fault or a DC fault, wherein the local side is the rectifier side or the inverter side of the dual-ended hybrid DC system; S2, if the fault is an AC fault, controlling the dual-ended hybrid DC system to switch to AC fault ride-through mode to clear the AC fault; if the fault is a DC fault, executing S3-S5; S3, switching the control mode of the dual-ended hybrid DC system according to the fault direction and fault type of the DC fault; S4, calculating the absolute value of the average value of the DC line voltage within a preset time window under the current control mode; if the absolute value of the average value is lower than a preset threshold, the DC fault is an in-zone fault; otherwise, the DC fault is an out-of-zone fault; S5, adopting corresponding in-zone fault clearing measures or out-of-zone fault clearing measures to clear the DC fault.

[0007] Furthermore, when this side is the rectifier side of the dual-ended hybrid DC system, S3 includes: if the fault direction is positive, controlling the dual-ended hybrid DC system to be in constant current control mode, and maintaining the current command value before the fault in constant current control mode; if the fault direction is negative, shifting the firing angle of the rectifier-side converter to make the rectifier-side converter work in inverter mode.

[0008] Furthermore, when this side is the inverter side of the dual-terminal hybrid DC system, step S3 includes: if the fault direction is positive and the fault type is a positive fault, switching the dual-terminal hybrid DC system to a first fault current limiting mode, where the current limiting control current command value in the first fault current limiting mode is a negative pre-fault current value; if the fault direction is positive and the fault type is a negative fault, switching the dual-terminal hybrid DC system to a second fault current limiting mode, where the current limiting control current command value in the second fault current limiting mode is the pre-fault current value; if the fault direction is reverse, switching the dual-terminal hybrid DC system to a third fault current limiting mode, where the current limiting control current command value in the third fault current limiting mode is 0.

[0009] Furthermore, when this side is the rectifier side of the dual-ended hybrid DC system, the AC fault ride-through mode is used to: reduce the firing angle of the rectifier-side converter according to the AC voltage on the rectifier side, until it is reduced to the minimum firing angle.

[0010] Furthermore, in the AC fault ride-through mode, the firing angle of the rectifier-side converter and the rectifier-side AC voltage satisfy the following relationship:

[0011]

[0012] Among them, U acrU is the AC voltage on the rectifier side, α is the firing angle of the rectifier-side converter, and U is the AC voltage on the rectifier side. dcr N1 is the DC voltage on the rectifier side, and N1 is the number of six-pulse converters per pole in the converter station. r I is the commutation reactance per phase on the rectifier side. dc This refers to the direct current in the power transmission line.

[0013] Furthermore, when this side is the inverter side of the dual-ended hybrid DC system, the AC fault ride-through mode is used to: adjust the DC voltage reference value on the inverter side according to the degree of voltage drop on the AC side of the dual-ended hybrid DC system, so as to control the dual-ended hybrid DC system.

[0014] Furthermore, in the AC fault ride-through mode, the relationship between the inverter-side DC voltage reference value and the AC-side voltage sag satisfies:

[0015] U dcref =k*U dcrefe

[0016] Among them, U dcref The DC voltage reference value on the inverter side after adjustment; k is the multiple by which the AC voltage drops to normal operating conditions during a fault, used to characterize the degree of AC voltage drop; U dcrefe This is the reference value for the DC voltage on the inverter side during normal operation.

[0017] Furthermore, the preset threshold is:

[0018] ε3=K3(I b R l +I c R max )

[0019] Where ε3 is the preset threshold, K3 is the reliability coefficient of the fault area discrimination element threshold setting, and I b I is the current limiting setting value for the line current. c R is the DC current error value when both ends of the current command value are rated values. l R is the DC resistance value of the transmission line. max To set the maximum withstand transition resistance value.

[0020] According to another aspect of the present invention, a DC high-resistivity fault protection system for a dual-ended hybrid DC system is provided, comprising: a fault discrimination module, configured to determine whether the fault is an AC fault or a DC fault when a fault occurs on the local side, wherein the local side is the rectifier side or the inverter side of the dual-ended hybrid DC system; an AC fault clearing module, configured to control the dual-ended hybrid DC system to switch to AC fault ride-through mode to clear the AC fault if the fault is an AC fault, and to execute a switching module, a fault location identification module, and a DC fault clearing module if the fault is a DC fault; a switching module, configured to switch the control mode of the dual-ended hybrid DC system according to the fault direction and fault type of the DC fault; a fault location identification module, configured to calculate the absolute value of the average value of the DC line voltage within a preset time window under the current control mode, wherein when the absolute value of the average value is lower than a preset threshold, the DC fault is an in-zone fault, otherwise, the DC fault is an out-of-zone fault; and a DC fault clearing module, configured to clear the DC fault by adopting corresponding in-zone fault clearing measures or out-of-zone fault clearing measures.

[0021] According to another aspect of the present invention, a computer-readable storage medium is provided having a computer program stored thereon, characterized in that, when the program is executed by a processor, it implements the DC high-impedance fault protection method for a dual-ended hybrid DC system as described above.

[0022] In summary, the above-described technical solutions of this invention can achieve the following beneficial effects: a DC high-resistance fault protection method for dual-ended hybrid DC systems is proposed. When a fault occurs at either end, the corresponding single-ended control mode is switched based on the AC / DC fault type, the positive and negative polarity and direction of the DC fault. While achieving fault crossing, the fault information on this side is converted into an electrical quantity and transmitted to the other side. Both sides can identify the fault type and fault area by detecting the single-ended quantity information. It does not rely on a communication system, has low requirements for protection devices, strong tolerance to transition resistance, and can be used as a backup for the main protection in a DC protection system, achieving reliable single-ended quantity protection. Attached Figure Description

[0023] Figure 1 A flowchart of a DC high-impedance fault protection method for a dual-ended hybrid DC system provided in an embodiment of the present invention;

[0024] Figure 2 This is a schematic diagram of a dual-end hybrid DC transmission system provided in an embodiment of the present invention;

[0025] Figure 3 The instantaneous voltage values ​​of the rectifier side port 25-30ms after a high-impedance fault at different locations within the zone, as provided in this embodiment of the invention;

[0026] Figure 4The instantaneous voltage values ​​of the inverter side port 25-30ms after a high-resistance fault at different locations within the zone, as provided in this embodiment of the invention;

[0027] Figure 5 For different transition resistances provided in the embodiments of the present invention, the fault F outside the DC region ex1 Instantaneous value of inverter-side port voltage;

[0028] Figure 6 For different transition resistances provided in the embodiments of the present invention, the fault F outside the DC region ex2 Instantaneous value of the rectifier side port voltage;

[0029] Figure 7 For different AC voltage drop levels provided in the embodiments of the present invention, AC fault F AC1 Instantaneous value of inverter-side port voltage;

[0030] Figure 8 For different AC voltage drop levels provided in the embodiments of the present invention, AC fault F AC2 Instantaneous value of the rectifier side port voltage;

[0031] Figure 9 This is a block diagram of a DC high-impedance fault protection system for a dual-ended hybrid DC system provided in an embodiment of the present invention. Detailed Implementation

[0032] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention. Furthermore, the technical features involved in the various embodiments of this invention described below can be combined with each other as long as they do not conflict with each other.

[0033] In this invention, the terms "first," "second," etc. (if present) in the invention and the accompanying drawings are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence.

[0034] Figure 1 A flowchart illustrating a DC high-impedance fault protection method for a dual-ended hybrid DC system provided in an embodiment of the present invention. (See also...) Figure 1 , combined Figures 2-8 The method for protecting against high-impedance DC faults in a dual-ended hybrid DC system in this embodiment is described in detail. The method includes operations S1-S5.

[0035] See Figure 2 The electromagnetic transient simulation model of the ±800kV hybrid DC transmission system (i.e., the dual-end hybrid DC system) built in PSCAD / EMTDC is shown. Figure 2In the middle, F AC1 F AC2 Location of AC fault on both sides; F ex1 F ex2 The fault location is outside the outlet area of ​​both valves; F in1 F in5 F indicates the location of the fault within each end zone of the line; in2 F in3 F in4 These represent the fault locations within the zone at 25%, 50%, and 75% of the distance from the LCC port on the line, respectively; M1, M2, N1, and N2 are the protection measurement point locations. During steady-state operation, the rectifier-side LCC uses constant DC current control, and the inverter-side MMC uses constant DC voltage control.

[0036] Operation S1 determines whether the fault is an AC fault or a DC fault when a fault occurs on this side, and whether this side is the rectifier side or inverter side of a dual-end hybrid DC system.

[0037] Specifically, when a fault occurs on this side, the voltage and current of the DC line on this side before and after the fault, as well as the AC voltage and current of the converter transformer system side before and after the fault, are collected. Using the AC voltage and current of the converter transformer system side, it is determined whether the fault on this side is an AC fault or a DC fault. If it is a DC fault, the direction of the fault should be further determined using the voltage and current of the DC line on this side, and the fault type should be determined using the voltages (positive and negative) of the DC line on this side. The positive current reference direction is defined as the line to which the converter valve points, and the negative current reference direction is also defined as the line to which the converter valve points.

[0038] In this embodiment, zero-mode voltage polarity selection is used to determine the type of DC fault. The discriminant is as follows:

[0039]

[0040] Where U0 is the voltage of the DC line on this side; ε1 is the threshold of the polarity selection element, which is set according to the maximum unbalanced voltage of the system when avoiding bipolar faults, for example, it is set to 0.05pu.

[0041] In this embodiment, the fault direction of a DC fault is determined by the rate of change of current, and the discriminant is as follows:

[0042]

[0043] Among them, I DC ε1 represents the current of the DC line on this side; ε2 is the threshold value of the directional element, which is set according to the system simulation results, for example, 500kA / s.

[0044] Operation S2: If the fault is an AC fault, control the dual-end hybrid DC system to switch to AC fault ride-through mode to clear the AC fault. If the fault is a DC fault, execute operations S3-S5.

[0045] According to an embodiment of the present invention, when this side is the rectifier side of a dual-ended hybrid DC system, the AC fault ride-through mode is used to: reduce the firing angle of the rectifier side converter according to the AC voltage on the rectifier side until it is reduced to the minimum firing angle.

[0046] In AC fault ride-through mode, the firing angle of the rectifier-side converter and the rectifier-side AC voltage satisfy the following relationship:

[0047]

[0048] Among them, U acr U is the AC voltage on the rectifier side, α is the firing angle of the rectifier-side converter, and U is the AC voltage on the rectifier side. dcr X is the DC voltage on the rectifier side, N1 is the number of six-pulse converters in each pole of the converter station, and X is the DC voltage on the rectifier side. r I is the commutation reactance per phase on the rectifier side. dc This refers to the direct current in the power transmission line.

[0049] According to an embodiment of the present invention, when this side is the inverter side of a dual-ended hybrid DC system, the AC fault ride-through mode is used to: adjust the DC voltage reference value on the inverter side according to the degree of voltage drop on the AC side of the dual-ended hybrid DC system, so as to control the dual-ended hybrid DC system.

[0050] In AC fault ride-through mode, the relationship between the inverter-side DC voltage reference value and the AC-side voltage sag satisfies:

[0051] U dcref =k*U dcrefe

[0052] Among them, U dcref The DC voltage reference value on the inverter side after adjustment; k is the multiple by which the AC voltage drops to normal operating conditions during a fault, used to characterize the degree of AC voltage drop; U dcrefe This is the reference value for the DC voltage on the inverter side during normal operation.

[0053] Operate S3 to switch the control mode of the dual-end hybrid DC system according to the fault direction and fault type of the DC fault.

[0054] According to an embodiment of the present invention, for a DC fault occurring on this side, when this side is the rectifier side of a dual-ended hybrid DC system, operation S3 is divided into the following two cases.

[0055] (1) The fault direction of the DC fault is positive, and the control dual-end hybrid DC system is in constant current control mode. The current command value in constant current control mode is maintained at the current command value before the fault.

[0056] (2) The fault direction of DC fault is reversed. The firing angle of the rectifier-side converter is shifted to make the rectifier-side converter work in the inverter state. It is preferable to shift the phase to 120°.

[0057] According to an embodiment of the present invention, for a DC fault occurring on this side, when this side is the inverter side of a dual-end hybrid DC system, operation S3 is divided into the following three cases.

[0058] (1) When the fault direction of the DC fault is positive and the fault type is positive, the dual-end hybrid DC system is switched to the first fault current limiting mode. The current limiting control current command value in the first fault current limiting mode is the negative fault current value.

[0059] (2) If the fault direction of the DC fault is positive and the fault type is negative, switch the dual-end hybrid DC system to the second fault current limiting mode. The current limiting control current command value in the second fault current limiting mode is the current value before the fault.

[0060] (3) The fault direction of DC fault is reversed. Switch the dual-end hybrid DC system to the third fault current limiting mode. The current limiting control current command value in the third fault current limiting mode is 0.

[0061] Operation S4 calculates the absolute value of the average DC line voltage within the preset time window under the current control mode. When the absolute value of the average value is lower than the preset threshold, the DC fault is an in-zone fault; otherwise, the DC fault is an out-of-zone fault.

[0062] The average value U of the DC line voltage within the preset time window dc_avg for:

[0063]

[0064] Among them, U dc_i Let be the DC line voltage on this side at the i-th sampling time; n is the number of sampling points within the time interval for calculating the average voltage, with the time interval being Δt ms and the sampling rate being N, so n = N * Δt.

[0065] The preset threshold setting takes into account the maximum transition resistance of the fault zone within the area and leaves sufficient margin. When both ends of the fault zone are under current limiting control, the DC voltage is generated by the voltage drop across the line resistor and the voltage drop caused by the difference in DC current flowing through the transition resistor. Therefore, according to an embodiment of the present invention, the preset threshold is:

[0066] ε3=K3(I b R l +Ic R max )

[0067] Where ε3 is the preset threshold, K3 is the reliability coefficient of the fault area discrimination element threshold setting, and I b I is the current limiting setting value for the line current. c R is the DC current error value when both ends of the current command value are rated values. l R is the DC resistance value of the transmission line. max To set the maximum withstand transition resistance value. In the above formula, the first part of the voltage drop depends on the line parameters, and the second part of the voltage drop depends on the current difference when the two-terminal current command values ​​are the same and the resistance value when the maximum transition resistance is considered.

[0068] Operate S5 to implement the corresponding fault clearing measures within or outside the zone to clear the DC fault.

[0069] Specifically, if the DC fault is an intra-zone fault, intra-zone fault clearance measures are used to clear the DC fault on this side; if the DC fault is an extra-zone fault, extra-zone fault clearance measures are used to clear the DC fault on this side.

[0070] by Figure 2 Taking the dual-ended hybrid DC system model shown in Figure 1 and the system parameters shown in Table 1 as examples, this embodiment verifies the DC high-impedance fault protection method for the dual-ended hybrid DC system. The simulation sampling rate is set to 10kHz, and the data window length Δt for calculating the average protection voltage is 5ms, i.e., 50 sampling points.

[0071] Table 1

[0072]

[0073]

[0074] When determining the type of DC fault, the threshold of the polarity selection element is set to avoid the maximum unbalanced voltage of the system when a bipolar fault occurs. In this embodiment, it is set to 0.05 pu. The threshold of the directional element is set to avoid the current change rate when the maximum unbalanced current fluctuation occurs under normal steady-state conditions, multiplied by the reliability coefficient. In this embodiment, the threshold is set to 500 kA / s.

[0075] When determining the preset threshold, the DC resistance value of the transmission line can be calculated from the system parameters shown in Table 1. When the current command values ​​at both ends are the same, the current difference between the two ends is affected by the asymmetry of the components on both sides of the system and the adjustment accuracy of the controller. At the same time, there is also a small amount of line capacitive current. In this embodiment, a current error value of 3% of the rated current at both ends is considered, the maximum withstand resistance value is set to 500Ω, and the reliability coefficient is taken as 1.3. Substituting the parameters, the voltage threshold of the fault discrimination element can be calculated to be 55.4kV.

[0076] 1) Faults within the DC zone

[0077] by Figure 2 The system shown has a positive line midpoint fault (F) in3 Taking a fault occurring at 1.105s, with a 300Ω transition resistance (a permanent fault) as an example... Figure 3 , Figure 4 The figures show the instantaneous voltage waveforms of the rectifier and inverter ports 25-30ms after a high-resistance fault occurs at different locations within the zone. In the figures, Threshold represents the threshold value (i.e., the preset threshold). From... Figure 3 and Figure 4 As can be seen, after the fault, the port voltage drops below the threshold value under the action of the dual-end control system, and the protection operates reliably.

[0078] 2) Fault outside DC zone

[0079] by Figure 2 The system shown has a circuit valve outlet fault (F) ex1 F ex2 Taking a permanent fault as an example (the fault occurred at 1.105s), Figure 5 , Figure 6 F represents the DC region fault under different transition resistances. ex1 F ex2 The waveform of the instantaneous voltage at the port is shown. In the figure, Threshold is the threshold value. From Figure 5 and Figure 6 As can be seen from the data, after a fault, the port voltage rises above the threshold value under the action of the dual-end control system, and the protection reliably does not operate.

[0080] 3) AC fault

[0081] by Figure 2 The system shown has an AC line fault (F) AC1 F AC2 Taking a permanent fault as an example (the fault occurred at 1.105s), Figure 7 , Figure 8 AC fault F under different AC voltage drop levels AC1 F AC2 The waveform of the instantaneous voltage at the port is shown. In the figure, Threshold is the threshold value. From Figure 7 and Figure 8 As can be seen from the data, after a fault, the port voltage rises above the threshold value under the action of the dual-end control system, and the protection reliably does not operate.

[0082] Figure 9 This is a block diagram of a DC high-impedance fault protection system for a dual-ended hybrid DC system provided in an embodiment of the present invention. (See also...) Figure 9The DC high-resistance fault protection system 900 for dual-ended hybrid DC systems includes a fault discrimination module 910, an AC fault clearing module 920, a switching module 930, a fault location identification module 940, and a DC fault clearing module 950.

[0083] The fault identification module 910, for example, performs operation S1 to determine whether the fault is an AC fault or a DC fault when a fault occurs on this side, and whether this side is the rectifier side or the inverter side of a dual-end hybrid DC system.

[0084] For example, the AC fault clearing module 920 performs operation S2, which controls the dual-end hybrid DC system to switch to AC fault ride-through mode to clear the AC fault if the fault is an AC fault, and executes the switching module 930, the fault location identification module 940 and the DC fault clearing module 950 if the fault is a DC fault.

[0085] The switching module 930, for example, performs operation S3 to switch the control mode of the dual-end hybrid DC system according to the fault direction and fault type of the DC fault.

[0086] For example, the fault location identification module 940 performs operation S4 to calculate the absolute value of the average value of the DC line voltage within a preset time window under the current control mode. When the absolute value of the average value is lower than a preset threshold, the DC fault is an in-zone fault; otherwise, the DC fault is an out-of-zone fault.

[0087] The DC fault clearing module 950, for example, performs operation S5 to clear the DC fault by adopting the corresponding intra-zone fault clearing measures or inter-zone fault clearing measures.

[0088] The DC high-resistance fault protection system 900 for dual-ended hybrid DC systems is used to perform the above... Figures 1-8 The illustrated embodiment presents a DC high-impedance fault protection method for a dual-ended hybrid DC system. For details not covered in this embodiment, please refer to the foregoing. Figures 1-8 The DC high-impedance fault protection method for a dual-ended hybrid DC system shown in the embodiment will not be described in detail here.

[0089] This invention also provides a computer-readable storage medium storing a computer program thereon. When the program is executed by a processor, it implements the power system frequency stability early warning method based on steady-state operating information as described above, which will not be elaborated further here.

[0090] Those skilled in the art will readily understand that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for protecting against high-impedance DC faults in a dual-ended hybrid DC system, characterized in that, include: S1, when a fault occurs on this side, determine whether the fault is an AC fault or a DC fault, and this side is the rectifier side or inverter side of the dual-end hybrid DC system; S2, if the fault is an AC fault, control the dual-end hybrid DC system to switch to AC fault ride-through mode to clear the AC fault; if the fault is a DC fault, execute S3-S5. S3, switch the control mode of the dual-end hybrid DC system according to the fault direction and fault type of the DC fault; S4, calculate the absolute value of the average value of the DC line voltage within a preset time window under the current control mode. When the absolute value of the average value is lower than a preset threshold, the DC fault is an intra-zone fault; otherwise, the DC fault is an extra-zone fault. S5, adopt the corresponding in-zone fault clearing measures or out-of-zone fault clearing measures to clear the DC fault; Wherein, when this side is the rectifier side of the dual-ended hybrid DC system, S3 includes: If the fault direction is positive, the dual-end hybrid DC system is controlled to be in constant current control mode, and the current command value in constant current control mode is maintained at the current command value before the fault. If the fault direction is reversed, the firing angle of the rectifier-side converter will be shifted to make the rectifier-side converter work in inverter mode. When this side is the inverter side of the dual-ended hybrid DC system, S3 includes: If the fault direction is positive and the fault type is a positive fault, the dual-ended hybrid DC system is switched to the first fault current limiting mode. The current limiting control current command value in the first fault current limiting mode is the negative pre-fault current value. If the fault direction is positive and the fault type is negative, the dual-ended hybrid DC system is switched to the second fault current limiting mode. The current limiting control current command value in the second fault current limiting mode is the current value before the fault. If the fault direction is reversed, the dual-ended hybrid DC system is switched to the third fault current limiting mode, and the current limiting control current command value in the third fault current limiting mode is 0. The preset threshold is: in, The preset threshold, The reliability coefficient is set for the threshold of the fault area discrimination component. This is the current limiting setting for the line current. This represents the DC current error value when both ends of the current command value are at their rated values. This represents the DC resistance value of the transmission line. To set the maximum withstand transition resistance value.

2. The DC high-resistivity fault protection method for a dual-ended hybrid DC system as described in claim 1, characterized in that, When this side is the rectifier side of the dual-ended hybrid DC system, the AC fault ride-through mode is used to: reduce the firing angle of the rectifier-side converter according to the AC voltage on the rectifier side, until it is reduced to the minimum firing angle.

3. The DC high-resistance fault protection method for a dual-ended hybrid DC system as described in claim 2, characterized in that, In the AC fault ride-through mode, the firing angle of the rectifier-side converter and the rectifier-side AC voltage satisfy the following relationship: in, This is the AC voltage on the rectifier side. The firing angle of the rectifier-side converter. This is the DC voltage on the rectifier side. This refers to the number of six pulse converters in each pole of the converter station. For the commutation reactance of each phase on the rectifier side, This refers to the direct current of the power transmission line.

4. The DC high-resistance fault protection method for a dual-ended hybrid DC system as described in claim 1, characterized in that, When this side is the inverter side of the dual-ended hybrid DC system, the AC fault ride-through mode is used to: adjust the DC voltage reference value on the inverter side according to the degree of voltage drop on the AC side of the dual-ended hybrid DC system, so as to control the dual-ended hybrid DC system.

5. The DC high-resistance fault protection method for a dual-ended hybrid DC system as described in claim 4, characterized in that, In the AC fault ride-through mode, the relationship between the inverter-side DC voltage reference value and the AC-side voltage sag satisfies: in, This is the adjusted DC voltage reference value for the inverter side; It represents the multiple by which the AC side voltage drops to normal operating conditions during a fault, and is used to characterize the degree of AC side voltage drop. This is the reference value for the DC voltage on the inverter side during normal operation.

6. A DC high-impedance fault protection system for a dual-ended hybrid DC system, characterized in that, include: The fault detection module is used to determine whether the fault is an AC fault or a DC fault when a fault occurs on this side, where this side is the rectifier side or the inverter side of the dual-ended hybrid DC system. An AC fault clearing module is used to control the dual-end hybrid DC system to switch to AC fault ride-through mode to clear the AC fault if the fault is an AC fault, and to execute a switching module, a fault location identification module, and a DC fault clearing module if the fault is a DC fault. The switching module is used to switch the control mode of the dual-ended hybrid DC system according to the fault direction and fault type of the DC fault. When this side is the rectifier side of the dual-ended hybrid DC system, the switching method is as follows: if the fault direction is positive, the dual-ended hybrid DC system is controlled to be in constant current control mode, and the current command value in constant current control mode maintains the current command value before the fault; if the fault direction is negative, the firing angle of the rectifier-side converter is phase-shifted to make the rectifier-side converter operate in inverter mode; when this side is the inverter side of the dual-ended hybrid DC system, the switching method is as follows: if the fault direction is negative... If the fault direction is positive and the fault type is a positive fault, the dual-ended hybrid DC system is switched to a first fault current limiting mode, where the current limiting control current command value is negative and equal to the pre-fault current value. If the fault direction is positive and the fault type is a negative fault, the dual-ended hybrid DC system is switched to a second fault current limiting mode, where the current limiting control current command value is equal to the pre-fault current value. If the fault direction is reverse, the dual-ended hybrid DC system is switched to a third fault current limiting mode, where the current limiting control current command value is 0. The fault location identification module is used to calculate the absolute value of the average DC line voltage within a preset time window under the current control mode. When the absolute value of the average value is lower than a preset threshold, the DC fault is an in-zone fault; otherwise, the DC fault is an out-of-zone fault. The preset threshold is: In the formula, The preset threshold, The reliability coefficient is set for the threshold of the fault area discrimination component. This is the current limiting setting for the line current. This represents the DC current error value when both ends of the current command value are at their rated values. This represents the DC resistance value of the transmission line. To set the maximum withstand transition resistance value; The DC fault clearing module is used to clear the DC fault by adopting corresponding in-zone fault clearing measures or out-of-zone fault clearing measures.

7. A computer-readable storage medium having a computer program stored thereon, characterized in that, When the program is executed by the processor, it implements the DC high-resistance fault protection method for a dual-end hybrid DC system as described in any one of claims 1-5.

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