A fault identification method suitable for a hybrid multi-feed DC power transmission system

Abstract

The application provides a fault identification method suitable for a hybrid multi-feed DC power transmission system, comprising: configuring a hybrid multi-feed DC power transmission system protection; collecting the voltage and current of each protection area in the hybrid multi-feed DC power transmission system protection, comparing the collected voltage and current with the protection setting value; based on the comparison result of the collected voltage and current with the protection setting value, performing fault identification according to the logic flow of the hybrid multi-feed DC power transmission system protection signal; judging the specific fault type according to the fault identification result, outputting a protection action signal, and implementing the protection action. On the basis of collecting and processing the fault voltage and current data in the LCC-HVDC and VSC-HVDC system, the protection device discriminates the fault according to the protection criterion and outputs the protection signal, the logic of the output protection signal combination is judged to identify the fault type, the problem of multiple protection configuration misoperation in the hybrid multi-feed DC power transmission system is overcome, and the reliability of the system protection is improved.

Description

Technical Field

[0001] This invention relates to the field of DC transmission systems, specifically a fault identification method applicable to hybrid multi-infeed DC transmission systems. Background Technology

[0002] Flexible DC transmission technology, based on voltage source converters with insulated-gate bipolar transistors (IGBTs) at its core, can independently control the active and reactive power transmitted by the system. High-voltage DC transmission technology based on grid-commutated converters is widely used in long-distance, high-capacity power transmission scenarios. With the widespread application of DC transmission technology in power grids, conventional DC transmission (LCC-HVDC) and flexible DC transmission (VSC-HVDC) are fed into the same AC bus, or the electrical distance between them is relatively short, forming a hybrid multi-infeed DC transmission system. In a hybrid multi-infeed DC transmission system, the interconnection of the LCC-HVDC rectifier station and the VSC-HVDC inverter station exhibits different operating characteristics compared to a single-infeed DC transmission system.

[0003] When a DC fault occurs in a flexible DC transmission system, the short-circuit current rises rapidly. Therefore, fault identification must be completed within a short time, which places extremely high demands on the fault identification of flexible DC grids. High-voltage AC transmission is a feasible technical route for large-scale, long-distance energy transmission; however, its large transmission capacity and long transmission lines mean that faults pose a high risk of system instability or even outages. In a high-voltage operating environment, once a fault occurs, the transmission system is likely to be subjected to overvoltage and overcurrent transmission impacts, damaging the entire transmission system and causing extremely serious harm and impact on the power system and people's lives. Therefore, rapid and accurate fault identification is of great significance for the protection actions and stable operation of hybrid multi-infeed DC transmission systems.

[0004] In hybrid multi-infeed DC transmission systems, coordination among the control system, AC protection, and DC protection minimizes the impact of faults on the system. Protection should first utilize the control capabilities of the DC system to suppress fault development. If the fault cannot be suppressed by the system's control system, AC and DC protection must quickly identify and locate the fault, and then use protection actions to clear it optimally. Since hybrid multi-infeed DC transmission systems involve two types of DC transmission, their protection configuration will also be related to the protection of these two types of DC transmission. In principle, AC protection covers the AC system, and DC protection covers the DC system. AC protection should not trigger DC protection operation signals, and vice versa. However, when a fault occurs in the system, it may not only affect the local area but also cause fluctuations in voltage, current, and other physical quantities in another transmission system, potentially leading to false triggering of protection configurations. This could result in unnecessary system outages and affect the stability of system operation.

[0005] Existing research has limited coverage of fault identification in hybrid multi-infeed DC transmission systems. Most studies on DC transmission system fault identification focus on a single protection configuration, while research on identifying multiple faults in systems with multiple protection configurations is scarce. Therefore, it is necessary to conduct research in this area of ​​hybrid multi-infeed DC transmission systems. Summary of the Invention

[0006] The purpose of this invention is to provide a fault identification method suitable for hybrid multi-infeed DC transmission systems, so as to identify multiple faults when there are multiple protection configurations in the system.

[0007] The present invention adopts the following technical solution:

[0008] A fault identification method applicable to hybrid multi-infeed DC transmission systems includes the following steps:

[0009] S1: Configure protection for hybrid multi-infeed DC transmission systems;

[0010] S2: Collect the voltage and current of each protection zone in the hybrid multi-feed DC transmission system protection configured in step S1, and compare the collected voltage and current with the protection setting value.

[0011] S3: Based on the comparison results of voltage, current and protection setting values ​​collected in step S2, fault identification is performed according to the protection signal logic flow of the hybrid multi-feed DC transmission system.

[0012] S4: Determine the specific fault type based on the fault identification result in step S3, output the protection action signal, and implement the protection action.

[0013] Furthermore, step S1 specifically includes:

[0014] S1.1: Interconnect the rectifier station of LCC-HVDC and the inverter station of VSC-HVDC, and configure near-zone AC protection and DC line protection for LCC-HVDC system and VSC-HVDC system;

[0015] S1.2: Configure DC line traveling wave protection and AC bus current differential protection in the LCC-HVDC system, and configure DC line traveling wave protection, DC undervoltage overcurrent protection, AC connection line differential protection and AC connection line overcurrent protection in the VSC-HVDC system.

[0016] Furthermore, step S2 specifically includes:

[0017] S2.1: Use electrical measuring devices to collect the voltage and current of the AC bus and DC lines of the LCC-HVDC system, as well as the voltage and current of the AC connection lines and DC transmission lines of the VSC-HVDC system;

[0018] S2.2: The six protection devices set in the system receive the voltage and current transmitted from the electrical measuring devices and complete the corresponding calculations and comparisons of the set values.

[0019] Furthermore, step S2.2 specifically includes:

[0020] S2.21: Both the LCC-HVDC system and the VSC-HVDC system are equipped with traveling wave protection on their DC lines. The DC voltage and DC current on the DC lines of the LCC-HVDC system and the VSC-HVDC system are collected. The traveling wave protection criterion for the LCC-HVDC system is as follows (1), and the traveling wave protection criterion for the VSC-HVDC system is as follows (2):

[0021]

[0022]

[0023] In the formula, For the DC voltage drop rate of the LCC, ΔU L The change in DC voltage of the LCC, ΔI L dU is the change in DC current of the LCC. set , ΔU set ΔI set For LCC traveling wave protection setting value; For the DC voltage drop rate of VSC, ΔU dc The change in DC voltage of VSC, ΔI dc U represents the change in VSC DC line current. set1 U set2 I set This is the setting value for the VSC traveling wave protection criterion;

[0024] S2.22: The values ​​of the three-phase currents within the LCC-HVDC AC bus current differential protection zone are transmitted to the AC bus differential protection device, which calculates and compares the setting values ​​of the three-phase currents based on the following protection criteria:

[0025] |I diff |>max(I cdqd ,KI res (18)

[0026] In the formula, I diff For differential current, I cdqd I is the differential current starting setpoint. cdqd I is the proportionality coefficient.res This is the braking current;

[0027] S2.23: The VSC-HVDC DC undervoltage and overcurrent protection collects data on DC positive voltage, DC negative voltage, DC positive current, and DC negative current. The protection criteria are as follows.

[0028] |U dP -U dN |≤U d_set ,I dP ≥I d_set orI dN ≥I d_set (19)

[0029] In the formula, U dP U is the positive DC voltage. dN The negative DC voltage, U d_set I is the DC undervoltage and overcurrent protection voltage setting. dP For DC positive current, I dN For DC negative current, I d_set This is the current setting for DC undervoltage overcurrent protection;

[0030] S2.24: The VSC-HVDC AC connection line current differential protection differentially calculates the three-phase current at the beginning and end of the protected area one by one. The protection criteria are as follows.

[0031]

[0032] In the formula, For the three-phase current on the converter transformer valve side, For the three-phase current of the starting circuit, I set For braking current, K set I is the proportionality coefficient. res To protect the startup setting;

[0033] S2.25: VSC-HVDC AC overcurrent protection collects the current at both ends of the AC connection line. The protection criteria are as follows:

[0034]

[0035] in and I represents the current across the two ends of the AC connection line. set This is the current setting value.

[0036] Furthermore, step S3 specifically includes:

[0037] S3.1: Determine whether the fault is in the LCC-HVDC system or the VSC-HVDC system based on formula (7):

[0038]

[0039] If equation (7) is satisfied, it indicates that the fault occurred in the LCC-HVDC system and S3.3 continues to be executed; otherwise, the fault occurred in the VSC-HVDC system and S3.6 is executed.

[0040] S3.2: Determine whether the fault occurs on the LCC-HVDC AC line according to formula (8):

[0041] |I diff |>max(I cdqd ,KI res ) (twenty three)

[0042] If equation (8) is satisfied, the fault occurs on the LCC-HVDC AC line, and a protection action signal is output to automatically reclose the two sets of circuit breakers installed on the LCC-HVDC AC bus to clear the fault; otherwise, continue to execute S3.3.

[0043] S3.3: Determine the traveling wave protection signal according to equation (9).

[0044]

[0045] If equation (9) is satisfied, then continue executing S3.4; otherwise, there is no need to output a protection action signal, and the monitoring system continues.

[0046] S3.4: Collect the current at both ends of the VSC-HVDC AC connection line, and determine the VSC-HVDC AC overcurrent protection signal according to equation (10):

[0047]

[0048] If equation (10) is satisfied, there is no need to output a protection action signal and the monitoring system continues; otherwise, if a fault occurs on the LCC-HVDC DC line, an protection action signal is output to block the trigger pulse of the LCC-HVDC commutator.

[0049] S3.5: Preliminary judgment indicates the fault occurs in the VSC-HVDC system. The following steps are used to determine whether a protection signal is output:

[0050]

[0051] If equation (11) is satisfied, then execute S3:7; otherwise, continue executing S3:6.

[0052] S3.6: Determine whether the current in the AC connection line of the VSC-HVDC system satisfies equation (12).

[0053]

[0054] If equation (12) is satisfied, there is no need to output a protection action signal and continue to monitor the system; otherwise, it indicates that the fault is a single-pole grounding fault of the VSC-HVDC DC line. Since the VSC-HVDC can rely on its own control capability to restore the system to a stable state in a short time when a single-pole grounding fault occurs in the DC transmission line of the VSC-HVDC system, there is no need for protection action.

[0055] S3.7: Determine whether the fault occurred in the VSC-HVDC AC line, based on equation (13).

[0056]

[0057] If equation (13) is satisfied, it indicates that a VSC-HVDC AC line fault has occurred in the system. The protection action signal is output, and the trigger pulse of the VSC-HVDC system voltage source converter is blocked to avoid the impact of the fault on the system; otherwise, continue to execute S3.8.

[0058] S3.8: Determine whether the VSC-HVDC DC undervoltage overcurrent protection issues a protection signal based on equation (14):

[0059] |U dP -U dN |≤U d_set ,I dP ≥I d_set orI dN ≥I d_set (29)

[0060] If equation (14) is satisfied, then continue executing S3.9; otherwise, there is no need to output a protection action signal, and the monitoring system continues.

[0061] S3.9: Determine whether the fault is a bipolar short-circuit fault in the VSC-HVDC DC line according to formula (15):

[0062]

[0063] If equation (15) is satisfied, it indicates that the fault is a bipolar short-circuit fault of the VSC-HVDC DC line. The protection action signal is output, and the trigger pulse of the VSC-HVDC system voltage source converter is blocked to avoid the impact of the fault on the system; otherwise, there is no need to output the protection action signal, and the system continues to be monitored.

[0064] This invention, based on the acquisition and processing of fault voltage and current data in LCC-HVDC and VSC-HVDC systems, allows six protection devices to identify faults and output protection signals according to protection criteria. The combination of output protection signals is logically judged to identify the fault type, overcoming the problem of false triggering of multiple protection configurations in hybrid multi-feed DC transmission systems and improving the reliability of system protection. Attached Figure Description

[0065] Figure 1 This is a schematic diagram of the topology of the hybrid multi-infeed DC transmission system of the present invention;

[0066] Figure 2 This is a logic block diagram for fault identification in the LCC-HVDC system of the hybrid multi-infeed DC transmission system of the present invention;

[0067] Figure 3 This is a logic block diagram of the VSC-HVDC system fault identification in the hybrid multi-infeed DC transmission system of the present invention. Detailed Implementation

[0068] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, 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.

[0069] This invention proposes a fault identification method applicable to hybrid multi-infeed DC transmission systems. After collecting and processing AC / DC line voltage and current data in the system, six protection devices identify faults based on protection criteria and output their respective protection signals. Different fault types output different combinations of protection signals. Logical judgment is performed on each combination of protection signals to identify the fault type. The logical judgment and protection action flow for fault identification in hybrid multi-infeed DC transmission systems are as follows: Figure 2 and Figure 3 ,in Figure 2 The fault identification logic judgment process for LCC-HVDC is as follows: Figure 3 The fault identification logic judgment process for VSC-HVDC.

[0070] The method includes the following steps:

[0071] S1: Configure protection for hybrid multi-infeed DC transmission systems;

[0072] The specific implementation steps are as follows:

[0073] S1.1: Interconnect the rectifier station of LCC-HVDC and the inverter station of VSC-HVDC, and configure near-zone AC protection and DC line protection for LCC-HVDC system and VSC-HVDC system;

[0074] S1.2: Configure DC line traveling wave protection and AC bus current differential protection in the LCC-HVDC system, and configure DC line traveling wave protection, DC undervoltage overcurrent protection, AC connection line differential protection and AC connection line overcurrent protection in the VSC-HVDC system.

[0075] S2: Collect the voltage and current of each protection zone in the hybrid multi-feed DC transmission system protection configured in step S1, and compare the collected voltage and current with the protection setting value.

[0076] The specific implementation steps are as follows:

[0077] S2.1: Use electrical measuring devices to collect the voltage and current of the AC bus and DC lines of the LCC-HVDC system, as well as the voltage and current of the AC connection lines and DC transmission lines of the VSC-HVDC system;

[0078] S2.2: The six protection devices in the system receive voltage and current data from the electrical measuring devices and perform corresponding calculations and comparisons of set values. The specific calculations and set value comparisons performed by the six protection devices are as follows:

[0079] S2.21: Both the LCC-HVDC system and the VSC-HVDC system are equipped with traveling wave protection on their DC lines. The DC voltage and DC current on the DC lines of the LCC-HVDC system and the VSC-HVDC system are collected. The traveling wave protection criterion for the LCC-HVDC system is as follows (1), and the traveling wave protection criterion for the VSC-HVDC system is as follows (2):

[0080]

[0081]

[0082] In the formula, For the DC voltage drop rate of the LCC, ΔU L The change in DC voltage of the LCC, ΔI L dU is the change in DC current of the LCC. set , ΔU set ΔI set For LCC traveling wave protection setting value; For the DC voltage drop rate of VSC, ΔU dc The change in DC voltage of VSC, ΔI dc U represents the change in VSC DC line current. set1 Uset2 I set This is the setting value for the VSC traveling wave protection criterion;

[0083] S2.22: The values ​​of the three-phase currents within the LCC-HVDC AC bus current differential protection zone are transmitted to the AC bus differential protection device, which calculates and compares the setting values ​​of the three-phase currents based on the following protection criteria:

[0084] |I diff |>max(I cdqd ,KI res (33)

[0085] In the formula, I diff For differential current, I cdqd I is the differential current starting setpoint. cdqd I is the proportionality coefficient. res This is the braking current;

[0086] S2.23: The VSC-HVDC DC undervoltage and overcurrent protection collects data on DC positive voltage, DC negative voltage, DC positive current, and DC negative current. The protection criteria are as follows.

[0087] |U dP -U dN |≤U d_set ,I dP ≥I d_set orI dN ≥I d_set (34)

[0088] In the formula, U dP U is the positive DC voltage. dN The negative voltage of DC, U d_set I is the DC undervoltage and overcurrent protection voltage setting. dP For DC positive current, I dN For DC negative current, I d_set This is the current setting for DC undervoltage overcurrent protection;

[0089] S2.24: The VSC-HVDC AC connection line current differential protection differentially calculates the three-phase current at the beginning and end of the protected area one by one. The protection criteria are as follows.

[0090]

[0091] In the formula, For the three-phase current on the converter transformer valve side, For the three-phase current of the starting circuit, I set For braking current, K set I is the proportionality coefficient. res To protect the startup setting;

[0092] S2.25: VSC-HVDC AC overcurrent protection collects the current at both ends of the AC connection line. The protection criteria are as follows:

[0093]

[0094] in and I represents the current across the two ends of the AC connection line. set This is the current setting value.

[0095] S3: Based on the comparison results of the voltage and current collected in step S2 with the protection setting values, fault identification is performed according to the protection signal logic flow of the hybrid multi-feed DC transmission system.

[0096] When LCC-HVDC and VSC-HVDC are fed into the same AC bus, and a fault occurs in the mixed multi-feed DC transmission system, in addition to the protection systems configured within this protection zone operating, external protection systems may also operate. Therefore, the fault cannot be accurately and reliably identified based solely on the protection system's operation; specific protection signals are required for fault identification. Step S3 is implemented as follows:

[0097] S3.1: Determine whether the fault is in the LCC-HVDC system or the VSC-HVDC system based on formula (7):

[0098]

[0099] If equation (7) is satisfied, it indicates that the fault occurred in the LCC-HVDC system and S3.3 continues to be executed; otherwise, the fault occurred in the VSC-HVDC system and S3.6 is executed.

[0100] S3.2: Determine whether the fault occurs on the LCC-HVDC AC line according to formula (8):

[0101] |I diff |>max(I cdqd ,KI res (38)

[0102] If equation (8) is satisfied, the fault occurs on the LCC-HVDC AC line, and a protection action signal is output to automatically reclose the two sets of circuit breakers installed on the LCC-HVDC AC bus to clear the fault; otherwise, continue to execute S3.3.

[0103] S3.3: Determine the traveling wave protection signal according to equation (9).

[0104]

[0105] If equation (9) is satisfied, then continue executing S3.4; otherwise, there is no need to output a protection action signal, and the monitoring system continues.

[0106] S3.4: Collect the current at both ends of the VSC-HVDC AC connection line, and determine the VSC-HVDC AC overcurrent protection signal according to equation (10):

[0107]

[0108] If equation (10) is satisfied, there is no need to output a protection action signal and the monitoring system continues; otherwise, if a fault occurs on the LCC-HVDC DC line, an protection action signal is output to block the trigger pulse of the LCC-HVDC commutator.

[0109] S3.5: Preliminary judgment indicates the fault occurs in the VSC-HVDC system. The following steps are used to determine whether a protection signal is output:

[0110]

[0111] If equation (11) is satisfied, then execute S3:7; otherwise, continue executing S3:6.

[0112] S3.6: Determine whether the current in the AC connection line of the VSC-HVDC system satisfies equation (12).

[0113]

[0114] If equation (12) is satisfied, there is no need to output a protection action signal and continue to monitor the system; otherwise, it indicates that the fault is a single-pole grounding fault of the VSC-HVDC DC line. Since the VSC-HVDC can rely on its own control capability to restore the system to a stable state in a short time when a single-pole grounding fault occurs in the DC transmission line of the VSC-HVDC system, there is no need for protection action.

[0115] S3.7: Determine whether the fault occurred in the VSC-HVDC AC line, based on equation (13).

[0116]

[0117] If equation (13) is satisfied, it indicates that a VSC-HVDC AC line fault has occurred in the system. The protection action signal is output, and the trigger pulse of the VSC-HVDC system voltage source converter is blocked to avoid the impact of the fault on the system; otherwise, continue to execute S3.8.

[0118] S3.8: Determine whether the VSC-HVDC DC undervoltage overcurrent protection issues a protection signal based on equation (14):

[0119] |U dP -UdN |≤U d_set ,I dP ≥I d_set orI dN ≥I d_set (44)

[0120] If equation (14) is satisfied, then continue executing S3.9; otherwise, there is no need to output a protection action signal, and the monitoring system continues.

[0121] S3.9: Determine whether the fault is a bipolar short-circuit fault in the VSC-HVDC DC line according to formula (15):

[0122]

[0123] If equation (15) is satisfied, it indicates that the fault is a bipolar short-circuit fault of the VSC-HVDC DC line. The protection action signal is output, and the trigger pulse of the VSC-HVDC system voltage source converter is blocked to avoid the impact of the fault on the system; otherwise, there is no need to output the protection action signal, and the system continues to be monitored.

[0124] S4: Based on the fault identification result in step S3, determine the specific fault type, output a protection action signal, and implement the protection action. Step S4 specifically includes the following steps:

[0125] S4.1: If a protection action signal is issued in step S3.2, the fault occurs on the LCC-HVDC AC bus, including single-phase grounding, two-phase grounding, three-phase grounding, and phase-to-phase faults on the LCC-HVDC AC bus. The protection action is to automatically reclose the two sets of circuit breakers installed on the LCC-HVDC AC bus to clear the fault; otherwise, execute S4.2.

[0126] S4.2: If a protection action signal is issued in step S3.4, the fault occurs on the LCC-HVDC DC line, including single-pole grounding fault and double-pole short-circuit fault of the LCC-HVDC DC line. The trigger pulse of the LCC-HVDC commutator is blocked to avoid the impact of the fault on the system; otherwise, S4.3 is executed.

[0127] S4.3: If a protection action signal is issued in step S3.7, the fault is a VSC-HVDC AC line fault, including single-phase grounding, two-phase grounding, three-phase grounding, and phase-to-phase faults of the VSC-HVDC AC bus. The protection action is to block the trigger pulse of the VSC-HVDC system voltage source converter; otherwise, execute S4.4.

[0128] S4.4: If a protection action signal is issued in step S3.9, it indicates that the fault in the system is a bipolar short circuit fault in the VSC-HVDC DC line, and the protection action is to block the trigger pulse of the VSC-HVDC system voltage source converter; otherwise, no protection action signal needs to be output, and the system continues to be monitored.

[0129] Implement this fault identification scheme in PSCAD / EMTDC, and combine it with Figure 1 The hybrid multi-infeed DC transmission system model shown verifies this scheme. The following simulation experiments validate the method of this invention.

[0130] The method of this invention was verified in PSCAD / EMTDC software based on a simulation model of a hybrid multi-infeed DC transmission system. The LCC-HVDC has a rated power of 3000MW and a rated DC voltage of ±500kV, the VSC-HVDC has a rated power of 1250MW and a rated DC voltage of 420kV, and the length of the tie line between the LCC-HVDC system and the VSC-HVDC system is 20km.

[0131] The fault point and fault time settings are as follows: For LCC-HVDC DC line single-pole grounding faults and LCC-HVDC DC line short-circuit faults, the fault point is set on the DC line. For LCC-HVDC AC bus single-phase grounding, two-phase grounding, three-phase grounding, and phase-to-phase faults, the fault point is set on the AC bus. For VSC-HVDC DC line single-pole grounding faults and VSC-HVDC DC line short-circuit faults, the fault point is set on the DC line. For VSC-HVDC AC connection line single-phase grounding, two-phase grounding, three-phase grounding, and phase-to-phase short-circuit faults, the fault point is set on the AC connection line. The fault start time is 1 second, and the duration is 0.1 seconds.

[0132] The simulation steps were performed according to S1-S4, and the simulation results are shown in Table 1.

[0133] Table 1 Simulation results of fault identification methods under different fault conditions

[0134]

[0135]

[0136] As shown in Table 1, when a fault occurs in a hybrid multi-infeed DC transmission system, in addition to the protection signals output by the protection systems configured within the protected area, protection signals are also output by the protection systems outside the protected area. According to... Figure 2 and Figure 3 The fault identification logic judgment process shown for LCC-HVDC and VSC-HVDC, namely simulation steps S3-S4, is to identify the fault type and execute the corresponding protection action.

[0137] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A fault identification method applicable to hybrid multi-infeed DC transmission systems, characterized in that, Includes the following steps: S1: Configure protection for hybrid multi-infeed DC transmission systems; S2: Collect the voltage and current of each protection zone in the hybrid multi-feed DC transmission system protection configured in step S1, and compare the collected voltage and current with the protection setting value. S3: Based on the comparison results of voltage, current and protection setting values ​​collected in step S2, fault identification is performed according to the protection signal logic flow of the hybrid multi-feed DC transmission system. S4: Determine the specific fault type based on the fault identification result in step S3, output a protection action signal, and implement the protection action; Step S2 specifically includes: S2.1: Use electrical measuring devices to collect the voltage and current of the AC bus and DC lines of the LCC-HVDC system, as well as the voltage and current of the AC connection lines and DC transmission lines of the VSC-HVDC system; S2.2: The six protection devices set in the system receive the voltage and current transmitted from the electrical measuring devices and complete the corresponding calculations and comparisons of the set values; Step S2.2 specifically includes: S2.21: Both the LCC-HVDC system and the VSC-HVDC system are equipped with traveling wave protection on their DC lines. The DC voltage and DC current on the DC lines of the LCC-HVDC system and the VSC-HVDC system are collected. The traveling wave protection criterion for the LCC-HVDC system is as follows (1), and the traveling wave protection criterion for the VSC-HVDC system is as follows (2): （1）； （2）； In the formula, For LCC DC voltage drop rate, This represents the change in DC voltage of the LCC. This represents the change in DC current of the LCC. , , For LCC traveling wave protection setting value; VSC DC voltage drop rate, This represents the change in DC voltage at VSC. This represents the change in VSC DC line current. , , This is the setting value for the VSC traveling wave protection criterion; S2.22: The values ​​of the three-phase currents within the LCC-HVDC AC bus current differential protection zone are transmitted to the AC bus differential protection device, which calculates and compares the setting values ​​of the three-phase currents based on the following protection criteria: （3）； In the formula, For differential current, The differential current starting setpoint, This is the proportionality coefficient. This is the braking current; S2.23: The VSC-HVDC DC undervoltage and overcurrent protection collects data on DC positive voltage, DC negative voltage, DC positive current, and DC negative current. The protection criteria are as follows. （4）； In the formula, It is the positive DC voltage. This is the negative DC voltage. This is the DC undervoltage and overcurrent protection voltage setting. It is the positive DC current. It is the negative DC current. This is the current setting for DC undervoltage overcurrent protection; S2.24: The VSC-HVDC AC connection line current differential protection calculates the difference between the three-phase currents at the beginning and end of the protected area, and the protection criteria are as follows. （5）； In the formula, For the three-phase current on the converter transformer valve side, For the three-phase current of the starting circuit, For braking current, This is the proportionality coefficient. To protect the startup setting; S2.25: The VSC-HVDC AC overcurrent protection collects the current at both ends of the AC connection line. The protection criteria are as follows: （6）； in and The current at both ends of the AC connection line, This is the current setting value.

2. The fault identification method for hybrid multi-infeed DC transmission systems as described in claim 1, characterized in that: Step S1 specifically includes: S1.1: Interconnect the rectifier station of LCC-HVDC and the inverter station of VSC-HVDC, and configure near-zone AC protection and DC line protection for LCC-HVDC system and VSC-HVDC system; S1.2: Configure DC line traveling wave protection and AC bus current differential protection in the LCC-HVDC system, and configure DC line traveling wave protection, DC undervoltage overcurrent protection, AC connection line differential protection and AC connection line overcurrent protection in the VSC-HVDC system.

3. The fault identification method for hybrid multi-infeed DC transmission systems as described in claim 1, characterized in that: Step S3 specifically includes: S3.1: Determine whether the fault is in the LCC-HVDC system or the VSC-HVDC system according to formula (7): （7）； If equation (7) is satisfied, it indicates that the fault occurred in the LCC-HVDC system and S3.3 continues to be executed; otherwise, the fault occurred in the VSC-HVDC system and S3.6 is executed. S3.3: Determine whether the fault occurs on the LCC-HVDC AC line according to formula (8): （8）； If equation (8) is satisfied, the fault occurs on the LCC-HVDC AC line, and a protection action signal is output to automatically reclose the two sets of circuit breakers installed on the LCC-HVDC AC bus to clear the fault; otherwise, continue to execute S3.

3. S3.3: Determine the traveling wave protection signal according to equation (9). （9）； If equation (9) is satisfied, then continue executing S3.4; otherwise, there is no need to output a protection action signal, and the monitoring system continues. S3.4: Collect the current at both ends of the VSC-HVDC AC connection line, and determine the VSC-HVDC AC overcurrent protection signal according to equation (10): （10）； If equation (10) is satisfied, there is no need to output a protection action signal and the system continues to monitor; otherwise, if a fault occurs on the LCC-HVDC DC line, an protection action signal is output to block the trigger pulse of the LCC-HVDC commutator. S3.5: Preliminary judgment indicates the fault occurs in the VSC-HVDC system. The following steps are used to determine whether a protection signal is output: （11）； If equation (11) is satisfied, then execute S3:7; otherwise, continue executing S3:

6. S3.6: Determine whether the current of the AC connection line in the VSC-HVDC system satisfies equation (12). （12）； If equation (12) is satisfied, there is no need to output a protection action signal and continue to monitor the system; otherwise, it indicates that the fault is a single-pole grounding fault of the VSC-HVDC DC line. Since the VSC-HVDC can rely on its own control capability to restore the system to a stable state in a short time when a single-pole grounding fault occurs in the DC transmission line of the VSC-HVDC system, there is no need for protection action. S3.7: Determine whether the fault occurred in the VSC-HVDC AC line, based on formula (13). （13）； If equation (13) is satisfied, it indicates that a VSC-HVDC AC line fault has occurred in the system. The protection action signal is output, and the trigger pulse of the VSC-HVDC system voltage source converter is blocked to avoid the impact of the fault on the system; otherwise, continue to execute S3.

8. S3.8: Determine whether the VSC-HVDC DC undervoltage overcurrent protection issues a protection signal based on equation (14): （14）； If equation (14) is satisfied, then continue to execute S3.9; otherwise, there is no need to output a protection action signal, and the monitoring system continues. S3.9: Determine whether the fault is a bipolar short-circuit fault in the VSC-HVDC DC line according to formula (15): （15）； If equation (15) is satisfied, it indicates that the fault is a bipolar short circuit fault of the VSC-HVDC DC line. The protection action signal is output, and the trigger pulse of the VSC-HVDC system voltage source converter is blocked to avoid the impact of the fault on the system; otherwise, there is no need to output the protection action signal, and the system continues to be monitored.

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