A multi-terminal flexible direct current system control and protection method and device for realizing plug-and-play of new energy stations

Abstract

The present invention discloses a multi-terminal flexible direct current system control and protection method and device for realizing plug-and-play of new energy stations. The control and protection method of the present invention includes a new energy access form, a voltage coordination control strategy and a system rapid recovery strategy after a DC fault; the new energy access form: for the newly connected new energy station, the system DC voltage is controlled by the original flexible direct current converter station of the multi-terminal flexible direct current system, and the new energy branch including the new energy station only needs to control the power; the voltage coordination control strategy: when the new energy station is connected to the multi-terminal flexible direct current system, when the power of the fixed direct current voltage station reaches the full amount, it is necessary to increase the DC voltage coordination control function to avoid the DC voltage exceeding the safety limit; the system rapid recovery strategy after a DC fault: includes a new energy branch fault recovery strategy, a terminal line fault recovery strategy and a tie line fault recovery strategy. After the present invention is connected to new energy, the DC pole control device program and the pole protection device program of the original converter station are hardly affected.

Description

Technical Field

[0001] The present invention belongs to the technical field of flexible direct current transmission systems, and in particular to a control and protection method and device for a multi-terminal flexible direct current system that realizes plug-and-play of a new energy station. Background Art

[0002] Flexible DC transmission technology is a new generation of DC transmission technology with voltage source converter as the core. Compared with traditional DC transmission technology, flexible DC transmission technology does not have the problem of commutation failure, can increase the system dynamic reactive power reserve, improve power quality, solve the impact of nonlinear load and impact load on the system, has stronger controllability and flexibility, and has broad application prospects in large-scale clean energy grid connection, island power supply, synchronous / asynchronous interconnection of AC power grid, construction of DC power grid, etc. It is an important means to solve many challenges of modern power grid.

[0003] More and more new energy projects are in urgent need of grid connection. Since most new energy projects are located in remote areas and the AC power grid in the area is relatively weak, if AC transmission is adopted, stable transmission of new energy power cannot be achieved. In this context, the use of flexible direct current transmission technology for new energy can effectively avoid the defects of AC transmission. For new energy stations around the existing multi-terminal flexible direct current projects, the existing flexible direct current project equipment resources can be used to directly connect to the new energy stations, which not only improves the utilization rate of flexible direct current equipment, ensures the reliable, stable and efficient transmission of new energy, but also reduces the investment in new energy grid connection.

[0004] In response to DC faults, the current newly built multi-terminal flexible DC projects achieve rapid fault isolation and rapid restart of the sound system by configuring DC circuit breakers or DC resonant switches at the DC ports. However, the access of new energy stations will affect the DC fault isolation and sound system restart strategy of the multi-terminal flexible DC system. The strategy needs to be redesigned and the control and protection procedures modified, which brings great inconvenience to the access of new energy stations. Therefore, it is necessary to reasonably design the access form and control and protection strategy of new energy stations to minimize the mutual influence between new energy stations and multi-terminal flexible DC systems. Summary of the invention

[0005] The technical problem to be solved by the present invention is to overcome the defects of the above-mentioned prior art, and provide a multi-terminal flexible DC system control and protection method and device that realizes plug-and-play of new energy stations, reduces the communication information between stations, and each converter station can realize plug-and-play. After connecting to new energy, the DC pole control device program (PCP program) and pole protection device program (PPR program) of the original converter station are not affected as much as possible, reducing the additional debugging workload brought about by program upgrades, and facilitating the connection of new energy stations to the existing multi-terminal flexible DC system.

[0006] To this end, the present invention adopts the following technical solution.

[0007] In a first aspect, the present invention provides a multi-terminal flexible direct current system control and protection method for realizing plug-and-play of a new energy station, characterized in that it includes a new energy access form, a voltage coordination control strategy, and a system rapid recovery strategy after a direct current fault;

[0008] The new energy access form described above: For the newly connected new energy station, the system DC voltage is controlled by the original flexible DC converter station of the multi-terminal flexible DC system, and the new energy branch including the new energy station only needs to control the power;

[0009] The voltage coordination control strategy described above: When new energy is connected to the multi-terminal flexible DC system and the fixed DC voltage station power reaches full capacity, it is necessary to add a DC voltage coordination control function to prevent the DC voltage from exceeding the safety limit;

[0010] The system rapid recovery strategy after a DC fault includes a new energy branch fault recovery strategy, a terminal line fault recovery strategy and a tie line fault recovery strategy. The new energy branch fault recovery strategy is used to recover new energy branch faults, the terminal line fault recovery strategy is used to recover terminal line faults, and the tie line fault recovery strategy is used to recover tie line faults.

[0011] The present invention reasonably designs the access form, coordinated control strategy and rapid recovery strategy after failure of the new energy station, so as to minimize the mutual influence between the new energy station and the multi-terminal flexible direct current system.

[0012] Furthermore, in the described new energy access form, in order to ensure that the new energy station does not go offline due to a flexible direct current submarine cable failure and that the failure of the new energy station does not affect the operation of the multi-terminal flexible direct current system, it is necessary to configure a DC energy consumption device in the new energy branch and install a DC circuit breaker at the point where the new energy branch accesses the DC bus; at the converter station where the DC bus to which the new energy branch is accessed is located, the linear bus protection program and the DC station control program related to the converter station need to be updated, and the other system programs do not need to be adjusted.

[0013] Furthermore, in the voltage coordination control strategy, when the fixed DC voltage station reaches full capacity, other converter stations assist in controlling the DC voltage by reducing power to maintain the stability of the multi-terminal flexible DC system.

[0014] Furthermore, the new energy branch fault recovery strategy is specifically as follows: protection is activated after the new energy branch fault occurs, the new energy station locks and trips the new energy branch DC circuit breaker, and the faulty branch is cut off before the flexible DC converter station protection is activated, ensuring that the fault of the new energy station and its lines does not affect the stable operation of the multi-terminal flexible DC system.

[0015] Furthermore, the terminal line fault recovery strategy is specifically as follows:

[0016] After the terminal line fault occurs, the protection of each flexible DC converter station is activated, all converter stations are locked, and the new energy station is not locked and the DC circuit breaker of the new energy branch is opened. At this time, in order to prevent the DC voltage of the new energy station from rising, the energy consumption device needs to be put into use;

[0017] It is determined that a terminal line between a converter station and its adjacent converter station is faulty, the converter station is shut down, and the DC switches on both sides of the faulty line are opened to isolate the faulty terminal line;

[0018] Finally, one converter station among the remaining converter stations is selected to be unlocked and operated again to control the DC voltage, and then other converter stations without faults are unlocked and the new energy station closes the DC circuit breaker to restore power.

[0019] Furthermore, the tie line fault recovery strategy is specifically as follows:

[0020] After the interconnection line fault occurs, the protection of each flexible DC converter station is activated, all converter stations are locked, and the new energy station needs to achieve DC fault ride-through without locking, open the DC circuit breaker of the new energy branch and put the energy consumption device into use;

[0021] Then the faulty interconnection line is determined, and the two converter stations connected to the faulty interconnection line open the DC switches of the corresponding lines;

[0022] Finally, the entire multi-terminal flexible DC system is divided into two DC systems: "one converter station connected to the faulty interconnection line + the remaining normal converter stations + the new energy station" and "another converter station connected to the faulty interconnection line + the remaining normal converter stations". In each DC system, one converter station is unlocked first to control the DC voltage, and then the remaining converter stations are unlocked respectively; the new energy station closes the DC circuit breaker. If the output power of the new energy branch is greater than the upper limit of the sum of the capacities of all converter stations in the DC system where it is located, the upper limit of the new energy output power is limited to the upper limit of the sum of the capacities of all converter stations in the DC system where it is located through the stabilization device.

[0023] In a second aspect, the present invention provides a multi-terminal flexible direct current system control and protection device for realizing plug-and-play of a new energy station, which comprises a new energy access unit, a voltage coordination control unit and a system rapid recovery unit after a direct current fault;

[0024] The new energy access unit is used to access the new energy station. The system DC voltage is controlled by the original flexible DC converter station of the multi-terminal flexible DC system. The new energy branch including the new energy station only needs to control the power. A DC energy consumption device is configured in the new energy branch, and a DC circuit breaker is installed at the place where the new energy branch accesses the DC bus.

[0025] The voltage coordination control unit described above: when new energy is connected to the multi-terminal flexible DC system and the fixed DC voltage station power reaches full capacity, a DC voltage coordination control function needs to be added to prevent the DC voltage from exceeding the safety limit;

[0026] The system rapid recovery unit after a DC fault comprises a new energy branch fault recovery subunit, a terminal line fault recovery subunit and a tie line fault recovery subunit.

[0027] The beneficial effects of the present invention are as follows: the present invention can reduce the communication information between stations, each converter station can achieve plug-and-play, and after connecting to new energy, it almost does not affect the DC pole control device program (PCP program) and pole protection device program (PPR program) of the original converter station, reducing the additional debugging workload caused by program upgrades, and facilitating the connection of new energy stations to the existing multi-terminal flexible DC system. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the prior art descriptions are briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention, and for ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0029] Figure 1 This is a flow chart of a multi-terminal flexible direct current system control and protection method for realizing plug-and-play of a new energy station according to the present invention;

[0030] Figure 2 This is a schematic diagram of the new energy access form adopted by the present invention;

[0031] Figure 3 This is a schematic diagram of a DC fault in a new energy branch of the present invention;

[0032] Figure 4 This is a schematic diagram of a terminal line fault of the present invention;

[0033] Figure 5 This is a schematic diagram of a connection line fault in the present invention. DETAILED DESCRIPTION

[0034] In order to make the purpose, technical solution and advantages of the embodiments of the present invention clearer, the technical solution in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0035] Example 1

[0036] This embodiment provides a multi-terminal flexible direct current system control and protection method for realizing plug-and-play of a new energy station, such as Figure 1 As shown, it includes new energy access forms, voltage coordination control strategies and system rapid recovery strategies after DC faults.

[0037] The described new energy access form: for the newly connected new energy station, the system DC voltage is controlled by the original flexible DC converter station of the multi-terminal flexible DC system, and the new energy branch including the new energy station only needs to control the power.

[0038] In order to ensure that the new energy station does not go offline due to a flexible direct current submarine cable fault and that the fault of the new energy station does not affect the operation of the multi-terminal flexible direct current system, it is necessary to configure a DC energy consumption device on the new energy branch and install a DC circuit breaker at the point where the new energy branch is connected to the DC bus; the converter station where the DC bus to which the new energy branch is connected is located, the linear bus protection program and DC station control program related to the converter station need to be updated, and the other system programs do not need to be adjusted.

[0039] by Figure 2 For example, the original multi-terminal flexible direct current system includes five converter stations, namely the first converter station (referred to as S1), the second converter station (referred to as S2), the third converter station (referred to as S3), the fourth converter station (referred to as S4) and the fifth converter station (referred to as S5). The new energy station (referred to as S6) is connected to the DC bus of the fourth converter station through the DC circuit breaker DCCB. Due to the addition of branches to the DC bus where the fourth converter station is located, the logic changes such as bus differential protection, the DC bus protection program and DC station control program related to the fourth converter station need to be updated at this time, and the other system programs do not need to be adjusted, which is convenient for plug-and-play.

[0040] The voltage coordination control strategy described above: When new energy is connected to the multi-terminal flexible DC system, when the fixed DC voltage station reaches full power, the DC voltage coordination control function needs to be added to prevent the DC voltage from exceeding the safety limit. When the fixed DC voltage station reaches full power, other converter stations assist in controlling the DC voltage by reducing power to maintain the stability of the multi-terminal flexible DC system.

[0041] The DC voltage of the original multi-terminal flexible DC system is controlled by one operating station, and the other converter stations are all operated in the fixed active power mode. The fixed DC voltage station usually gives priority to the converter station with larger capacity. When the new energy station is connected to the multi-terminal flexible DC system, it may cause the fixed DC voltage station to reach full power. At this time, the fixed DC voltage station no longer has the ability to control the DC voltage of the system. Therefore, it is necessary to add a DC voltage coordination control function to prevent the DC voltage from exceeding the safety limit.

[0042] by Figure 2For example, the initial operating conditions are: the first converter station sets the DC voltage, the power of the second converter station, the third converter station, the fourth converter station, and the fifth converter station are all 100MW. At this time, the first converter station transmits 400MW of power to the AC system, reaching the full capacity. At this time, the 250MW new energy branch is connected to the DC bus of the fourth converter station. Since the power transmission of the first converter station has reached the full capacity, the second, third, fourth, and fifth converter stations reduce the power to assist in controlling the DC voltage and maintain system stability.

[0043] The system rapid recovery strategy after a DC fault includes a new energy branch fault recovery strategy, a terminal line fault recovery strategy and a tie line fault recovery strategy. The new energy branch fault recovery strategy is used to recover new energy branch faults, the terminal line fault recovery strategy is used to recover terminal line faults, and the tie line fault recovery strategy is used to recover tie line faults.

[0044] After the access of new energy, the system recovery strategy after the DC fault of the entire flexible DC system is divided into three situations for analysis according to the different fault locations.

[0045] (1) The first type of fault is a new energy branch fault. After the new energy branch fault occurs, the protection is activated, the new energy station locks and trips the new energy branch DC circuit breaker, and the faulty branch is cut off before the flexible DC converter station protection is activated, ensuring that the fault of the new energy station and its line does not affect the stable operation of the multi-terminal flexible DC system.

[0046] by Figure 3 For example, the protection is activated after the new energy branch fails. The current transformers CT on both sides of the new energy station and the fourth converter station determine that the fault occurred in the new energy branch. The new energy station locks and trips the DC circuit breaker of the new energy branch. The first converter station, the second converter station, the third converter station, the fourth converter station, and the fifth converter station are all operating normally.

[0047] (2) The second type of fault is a terminal line fault. Figure 4 Take the line fault from the first converter station to the second converter station as an example. After the fault occurs, the five flexible direct current converter stations are protected, the converter station is locked, and the new energy station is not locked and the DC circuit breaker of the new energy branch is opened. At this time, in order to prevent the DC voltage of the new energy converter station from rising, it is necessary to put the energy-consuming device into operation; the first converter station and the second converter station determine that the line from the first converter station to the second converter station is faulty, and the first converter station and the second converter station open the DC switch of the faulty line to isolate the faulty line from the first converter station to the second converter station. Finally, the second converter station is unlocked again to control the DC voltage, and the third converter station, the fourth converter station, and the fifth converter station are unlocked separately. The new energy station closes the DC circuit breaker to restore power.

[0048] (3) The third type of fault is the interconnection line fault. Figure 5Take the fault of the interconnection line from the second converter station to the fourth converter station as an example. After the fault occurs, the protection of each flexible DC converter station is activated, and the five stations are locked. The new energy station needs to achieve DC fault crossing, so it is not locked, the DC circuit breaker of the new energy branch is opened, and the energy consumption device is put into use. Subsequently, the second converter station and the fourth converter station determine that the interconnection line from the second converter station to the fourth converter station is faulty, and the second converter station and the fourth converter station open the DC switches of the corresponding lines. Finally, the entire multi-terminal flexible DC system is decoupled into two DC systems, "S4+S5+S6" and "S1+S2+S3". S1 and S4 are unlocked first to control the DC voltage, and then S2, S3, and S5 are unlocked respectively. The new energy station closes the DC circuit breaker. Since the output power of the new energy branch is greater than the upper limit of the sum of the capacities of the two converter stations S4 and S5, the upper limit of the new energy output power is limited to the upper limit of the sum of the capacities of the two converter stations S4 and S5 through the stabilization device.

[0049] Example 2

[0050] This embodiment is a multi-terminal flexible direct current system control and protection device that realizes plug-and-play of new energy stations, which includes a new energy access unit, a voltage coordination control unit and a system rapid recovery unit after a direct current fault.

[0051] The new energy access unit is used to access the new energy station. The system DC voltage is controlled by the original flexible DC converter station of the multi-terminal flexible DC system. The new energy branch only needs to control the power. A DC energy consumption device is configured in the new energy branch, and a DC circuit breaker is installed at the place where the new energy branch accesses the DC bus.

[0052] The voltage coordination control unit described above: when new energy is connected to the multi-terminal flexible DC system and the fixed DC voltage station power reaches full capacity, a DC voltage coordination control function needs to be added to prevent the DC voltage from exceeding the safety limit;

[0053] The system rapid recovery unit after a DC fault comprises a new energy branch fault recovery subunit, a terminal line fault recovery subunit and a tie line fault recovery subunit.

[0054] The new energy branch fault recovery subunit is used for: protection action after the new energy branch fault, the new energy station locks and trips the new energy branch DC circuit breaker, and cuts off the faulty branch before the flexible DC converter station protection action, ensuring that the fault of the new energy station and its line does not affect the stable operation of the multi-terminal flexible DC system.

[0055] The terminal line fault recovery subunit is used for:

[0056] After the terminal line fault occurs, the protection of each flexible DC converter station is activated, all converter stations are locked, and the new energy station is not locked and the DC circuit breaker of the new energy branch is opened. At this time, in order to prevent the DC voltage of the new energy station from rising, the energy consumption device needs to be put into use;

[0057] It is determined that a terminal line between a converter station and its adjacent converter station is faulty, the converter station is shut down, and the DC switches on both sides of the faulty line are opened to isolate the faulty terminal line;

[0058] Finally, one converter station among the remaining converter stations is selected to be unlocked and operated again to control the DC voltage, and then other converter stations without faults are unlocked and the new energy station closes the DC circuit breaker to restore power.

[0059] The tie line fault recovery subunit is used for:

[0060] After the interconnection line fault occurs, the protection of each flexible DC converter station is activated, all converter stations are locked, and the new energy station needs to achieve DC fault ride-through without locking, open the DC circuit breaker of the new energy branch and put the energy consumption device into use;

[0061] Then the faulty interconnection line is determined, and the two converter stations connected to the faulty interconnection line open the DC switches of the corresponding lines;

[0062] Finally, the entire multi-terminal flexible DC system is divided into two DC systems: "one converter station connected to the faulty interconnection line + the remaining normal converter stations + the new energy station" and "another converter station connected to the faulty interconnection line + the remaining normal converter stations". In each DC system, one converter station is unlocked first to control the DC voltage, and then the remaining converter stations are unlocked respectively; the new energy station closes the DC circuit breaker. If the output power of the new energy branch is greater than the upper limit of the sum of the capacities of all converter stations in the DC system where it is located, the upper limit of the new energy output power is limited to the upper limit of the sum of the capacities of all converter stations in the DC system where it is located through the stabilization device.

[0063] For the specific limitations of a multi-terminal flexible direct current system control and protection device for realizing plug-and-play of a new energy station, please refer to the limitations of a multi-terminal flexible direct current system control and protection method for realizing plug-and-play of a new energy station in Example 1. The two have the same functions and effects and will not be repeated here.

[0064] It is obvious that those skilled in the art can easily make various modifications to the above embodiments and apply the general principles described herein to other embodiments without creative work. Therefore, the present invention is not limited to the above embodiments, and improvements and modifications made by those skilled in the art to the present invention based on the disclosure of the present invention should be within the protection scope of the present invention.

Claims

1. A multi-terminal flexible direct current system control and protection method for realizing plug-and-play of new energy stations, characterized in that: Including new energy access forms, voltage coordination control strategies and system rapid recovery strategies after DC faults; The new energy access form described above: For the newly connected new energy station, the system DC voltage is controlled by the original flexible DC converter station of the multi-terminal flexible DC system, and the new energy branch including the new energy station only needs to control the power; The voltage coordination control strategy described above: When new energy is connected to the multi-terminal flexible DC system and the fixed DC voltage station power reaches full capacity, it is necessary to add a DC voltage coordination control function to prevent the DC voltage from exceeding the safety limit; The system rapid recovery strategy after a DC fault: includes a new energy branch fault recovery strategy, a terminal line fault recovery strategy and a tie line fault recovery strategy. The new energy branch fault recovery strategy is used to recover new energy branch faults, the terminal line fault recovery strategy is used to recover terminal line faults, and the tie line fault recovery strategy is used to recover tie line faults; In the above-mentioned new energy access form, in order to ensure that the new energy station does not go offline due to the fault of the flexible direct current submarine cable and the fault of the new energy station does not affect the operation of the multi-terminal flexible direct current system, it is necessary to configure a DC energy consumption device on the new energy branch and install a DC circuit breaker at the point where the new energy branch is connected to the DC bus; the converter station where the DC bus to which the new energy branch is connected is located, the linear bus protection program and the DC station control program related to the converter station need to be updated, and the other system programs do not need to be adjusted; The tie line fault recovery strategy is specifically as follows: After the interconnection line fault occurs, the protection of each flexible DC converter station is activated, all converter stations are locked, and the new energy station needs to achieve DC fault ride-through without locking, open the DC circuit breaker of the new energy branch and put the DC energy consumption device into use; Then the faulty interconnection line is determined, and the two converter stations connected to the faulty interconnection line open the DC switches of the corresponding lines; Finally, the entire multi-terminal flexible DC system is divided into two DC systems: "one converter station connected to the faulty interconnection line + the remaining normal converter stations + the new energy station" and "another converter station connected to the faulty interconnection line + the remaining normal converter stations". In each DC system, one converter station is unlocked first to control the DC voltage, and then the remaining converter stations are unlocked respectively; the new energy station closes the DC circuit breaker. If the output power of the new energy branch is greater than the upper limit of the sum of the capacities of all converter stations in the DC system where it is located, the stabilization device is used to limit the upper limit of the output power of the new energy to the upper limit of the sum of the capacities of all converter stations in the DC system where it is located.

2. The multi-terminal flexible DC system control and protection method according to claim 1, characterized in that: In the voltage coordination control strategy, when the fixed DC voltage station reaches full capacity, other converter stations assist in controlling the DC voltage by reducing power to maintain the stability of the multi-terminal flexible DC system.

3. The multi-terminal flexible DC system control and protection method according to claim 1, characterized in that: The new energy branch fault recovery strategy described is specifically as follows: protection is activated after the new energy branch fault, the new energy station locks and trips the new energy branch DC circuit breaker, and the faulty branch is cut off before the flexible DC converter station protection is activated, ensuring that the fault of the new energy station and its lines does not affect the stable operation of the multi-terminal flexible DC system.

4. The multi-terminal flexible DC system control and protection method according to claim 1, characterized in that: The terminal line fault recovery strategy is specifically as follows: After the terminal line fault occurs, the protection of each flexible DC converter station is activated, all converter stations are locked, and the new energy station is not locked and the DC circuit breaker of the new energy branch is opened. At this time, in order to prevent the DC voltage of the new energy station from rising, the energy consumption device needs to be put into use; It is determined that a terminal line between a converter station and its adjacent converter station is faulty, the converter station is shut down, and the DC switches on both sides of the faulty terminal line are opened to isolate the faulty terminal line; Finally, one converter station among the remaining converter stations is selected to be unlocked and operated again to control the DC voltage, and then other converter stations without faults are unlocked and the new energy station closes the DC circuit breaker to restore power.

5. A multi-terminal flexible direct current system control and protection device for realizing plug-and-play of new energy stations, characterized in that: It includes a new energy access unit, a voltage coordination control unit and a system rapid recovery unit after a DC fault; The new energy access unit is used to access the new energy station. The system DC voltage is controlled by the original flexible DC converter station of the multi-terminal flexible DC system. The new energy branch only needs to control the power. A DC energy consumption device is configured in the new energy branch, and a DC circuit breaker is installed at the place where the new energy branch accesses the DC bus. The voltage coordination control unit described above: when new energy is connected to the multi-terminal flexible DC system and the fixed DC voltage station power reaches full capacity, a DC voltage coordination control function needs to be added to prevent the DC voltage from exceeding the safety limit; The system rapid recovery unit after DC fault: includes a new energy branch fault recovery subunit, a terminal line fault recovery subunit and a tie line fault recovery subunit; The tie line fault recovery subunit is used for: After the interconnection line fault occurs, the protection of each flexible DC converter station is activated, all converter stations are locked, and the new energy station needs to achieve DC fault ride-through without locking, open the DC circuit breaker of the new energy branch and put the energy consumption device into use; Then the faulty interconnection line is determined, and the two converter stations connected to the faulty interconnection line open the DC switches of the corresponding lines; Finally, the entire multi-terminal flexible DC system is divided into two DC systems: "one converter station connected to the faulty interconnection line + the remaining normal converter stations + the new energy station" and "another converter station connected to the faulty interconnection line + the remaining normal converter stations". In each DC system, one converter station is unlocked first to control the DC voltage, and then the remaining converter stations are unlocked respectively; the new energy station closes the DC circuit breaker. If the output power of the new energy branch is greater than the upper limit of the sum of the capacities of all converter stations in the DC system where it is located, the stabilization device is used to limit the upper limit of the output power of the new energy to the upper limit of the sum of the capacities of all converter stations in the DC system where it is located.

6. The multi-terminal flexible DC system control and protection device according to claim 5, characterized in that: The new energy branch fault recovery subunit is used for: protection action after the new energy branch fault, the new energy station locks and trips the new energy branch DC circuit breaker, and cuts off the faulty branch before the flexible DC converter station protection action, ensuring that the fault of the new energy station and its line does not affect the stable operation of the multi-terminal flexible DC system.

7. The multi-terminal flexible DC system control and protection device according to claim 5, characterized in that: The terminal line fault recovery subunit is used for: After the terminal line fault occurs, the protection of each flexible DC converter station is activated, all converter stations are locked, and the new energy station is not locked and the DC circuit breaker of the new energy branch is opened. At this time, in order to prevent the DC voltage of the new energy station from rising, the energy consumption device needs to be put into use; It is determined that a terminal line between a converter station and its adjacent converter station is faulty, the converter station is shut down, and the DC switches on both sides of the faulty terminal line are opened to isolate the faulty terminal line; Finally, one converter station among the remaining converter stations is selected to be unlocked and operated again to control the DC voltage, and then other converter stations without faults are unlocked and the new energy station closes the DC circuit breaker to restore power.

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