A method and device for on-line exit of a single station of a multi-terminal direct current transmission system

Abstract

The application discloses a single-station on-line exit method of a multi-terminal direct current transmission system. After a control protection system of a converter station receives an exit-station instruction, direct current power of a station to be exited is first reduced to a minimum value at a certain rate. Then, a fixed-frequency alternating current is injected into current flowing through a circuit breaker of the station to be exited. After detecting that a circuit breaker opening condition is met, a circuit breaker opening command is issued. After detecting that the circuit breaker is tripped, the station to be exited is locked, an alternating current switch is tripped and pole isolation is performed. The technical scheme provided by the application ensures that the circuit breaker is opened when current flowing through the circuit breaker has a periodic zero point. After the application is applied, a multi-terminal direct current transmission project can select a low-cost and simple high-voltage circuit breaker, construction cost is effectively reduced, and reliability and flexibility of system operation are improved.

Description

Technical Field

[0001] This invention relates to the field of DC power transmission, and more specifically to a method and apparatus for single-station online disconnection in a multi-terminal DC power transmission system. Background Technology

[0002] In multi-terminal DC transmission applications, if some converter stations need to be temporarily shut down due to changes in system operation, faults in a certain station, or maintenance, the simplest approach is to shut down the entire transmission system, isolate the relevant converter stations, and then restart them. The optimal approach is to remove the stations to be shut down while keeping the others operational. This optimal approach requires adding a circuit breaker capable of interrupting DC current between the converter station and the DC line.

[0003] One approach is to configure DC circuit breakers. DC circuit breakers have strong breaking current capacity and fast operating speed, and can directly and quickly isolate and disconnect the station in case of a fault. However, they are complex in construction, expensive, and require a large area. Moreover, the cost and difficulty increase exponentially with the increase of voltage level. There are currently no application examples in ultra-high voltage projects.

[0004] Another approach is to configure high-voltage circuit breakers, which are modified from conventional AC circuit breakers. High-voltage circuit breakers are simple in structure, inexpensive, and suitable for almost any voltage level, but their DC current breaking capacity is very limited; in ultra-high voltage applications, they can only break DC currents of tens of amperes. Therefore, high-voltage circuit breakers cannot be directly used for rapid isolation and removal from the station; a control and protection system is needed to control the DC current to within their breaking capacity before reliable isolation can be achieved. For projects requiring self-clearing of DC line faults, the converter valve itself must also have the ability to output zero voltage (or negative voltage); high-voltage circuit breakers cannot be relied upon to interrupt fault currents.

[0005] In ultra-high voltage (UHV) applications, considering economic efficiency, reliability, and limitations of existing technology, configuring high-voltage circuit breakers for online shutdown of converter stations is currently the preferred option. UHV projects generally have large capacities and transmit high power; to ensure the safe and stable operation of the system, the power transmission of other stations should be kept as stable as possible during converter station shutdown. For some distribution network projects or projects with limited budgets, high-voltage circuit breaker solutions are also an effective cost-reduction measure. Therefore, it is urgent to study single-station online shutdown strategies for multi-terminal DC transmission based on high-voltage circuit breakers to ensure reliable isolation of converter stations. Summary of the Invention

[0006] The purpose of this invention is to provide a single-station online disconnection method and device for a multi-terminal DC transmission system, which solves the problems of reliable and rapid disconnection of high-voltage circuit breakers and is applicable to multi-terminal DC transmission projects.

[0007] To achieve the above objectives, the solution of the present invention is:

[0008] In a first aspect, the present invention proposes a method for single-station online shutdown in a multi-terminal DC transmission system, wherein the multi-terminal DC transmission system comprises at least three converter stations, one of which is a DC voltage control station and the others are DC power control stations, and the converter stations are connected via DC lines; at least one circuit breaker is configured between the converter station to be shut down and the DC system composed of the other converter stations; the single-station shutdown method includes:

[0009] After receiving the shutdown command, the converter station control and protection system will reduce the DC power to be shut down to the minimum value.

[0010] Inject a fixed frequency alternating current into the current flowing through the circuit breaker of the station to be decommissioned;

[0011] Check whether the circuit breaker's permissible tripping conditions are met;

[0012] When the conditions for circuit breaker tripping are met, a circuit breaker tripping command is issued.

[0013] Upon detection of circuit breaker tripping, the station will execute interlocking, AC switch tripping, and DC side isolation commands.

[0014] Preferably, the amplitude of the fixed-frequency alternating current is greater than that of the DC component, causing the current flowing through the circuit breaker to have periodic zero-crossing points.

[0015] Preferably, the fixed-frequency AC current can be obtained by superimposing a fixed-frequency AC voltage command onto the DC voltage reference value of the station to be exited.

[0016] Preferably, the fixed-frequency AC current can be obtained by superimposing a fixed-frequency AC voltage command onto the DC voltage reference value of the DC voltage control station.

[0017] Preferably, if the station to be decommissioned is a DC voltage control station, it is switched to a DC power control station before reducing the DC power, and one of the original DC power control stations is switched to a new DC voltage control station.

[0018] Preferably, when the number of zero-crossing points of the current flowing through the circuit breaker exceeds a preset value within a first preset time, or when its DC current component is lower than the breaking allowable value for more than a second preset time, it is determined that the circuit breaker's allowable opening condition is met.

[0019] Preferably, if the circuit breaker's permissible opening condition is not met, the amplitude of the injected AC current is increased until the circuit breaker's permissible opening condition is met.

[0020] Preferably, if the exit command received by the converter station control and protection system is manually issued, the DC power of the station to be exited will be reduced to the minimum value that can maintain operation at a certain rate; if the exit command is issued by protection and the station to be exited can still maintain operation, the DC power of the station to be exited will be reduced to the minimum value at the fastest rate.

[0021] Preferably, if the decommissioning instruction received by the converter station control and protection system is a protection-issued instruction and the station to be decommissioned cannot maintain operation, then the converter to be decommissioned will be directly locked out.

[0022] Secondly, this invention proposes a single-station online decommissioning device for a multi-terminal DC transmission system. The multi-terminal DC transmission system includes at least three converter stations, one of which is a DC voltage control station and the others are DC power control stations. All converter stations are connected via DC lines. At least one circuit breaker is configured between the converter station to be decommissioned and the DC system formed by the other converter stations. The single-station online decommissioning device includes:

[0023] Power regulation unit: used to reduce the DC power to be removed from the station to the minimum value after receiving the station exit command;

[0024] Circuit breaker current injection unit: used to inject a fixed frequency AC current into the current flowing through the circuit breaker to be removed from the station;

[0025] Circuit breaker tripping unit: Used to detect the current flowing through the circuit breaker to be decommissioned from the station, and to issue a circuit breaker tripping command when the circuit breaker tripping conditions are met.

[0026] Interlocking and isolation unit: Used to detect the status of the circuit breaker, and after confirming that the circuit breaker has tripped, execute interlocking, tripping AC switch and DC side isolation commands in the station to be exited.

[0027] Preferably, the amplitude of the AC current injected into the circuit breaker current injection unit is greater than that of the DC component, causing the current flowing through the circuit breaker to exhibit periodic zero-crossing points.

[0028] Preferably, the AC current in the circuit breaker current injection unit can be obtained by superimposing a fixed frequency AC voltage command on the DC voltage reference value of the station to be exited.

[0029] Preferably, the AC current in the circuit breaker current injection unit can be obtained by superimposing a fixed frequency AC voltage command on the DC voltage reference value of the DC voltage control station.

[0030] Preferably, if the station to be decommissioned is a DC voltage control station, the power regulation unit first switches it to a DC power control station before reducing the DC power, and then switches one of the original DC power control stations to a new DC voltage control station.

[0031] Preferably, the circuit breaker tripping condition in the circuit breaker tripping unit is that the number of zero-crossing points of the current flowing through the circuit breaker exceeds a preset value within a first preset time, or its DC current component is lower than the tripping allowable value for more than a second preset time.

[0032] Preferably, if the circuit breaker tripping condition in the circuit breaker tripping unit is not met, the circuit breaker current injection unit continues to increase the amplitude of the injected AC current until the circuit breaker tripping condition is met.

[0033] Preferably, in the power regulation unit, if the exit command received by the converter station control and protection system is manually issued, the DC power of the station to be exited is reduced to the minimum value that can maintain operation at a certain rate; if the exit command is issued by protection and the station to be exited can still maintain operation, the DC power of the station to be exited is reduced to the minimum value at the fastest rate.

[0034] Preferably, in the power regulation unit, if the power station control and protection system receives a power outage command that is a protection-issued command and the power station to be outage cannot maintain operation, the power converter to be outage is directly locked.

[0035] With the above scheme, this invention configures a circuit breaker between the converter station and the DC line. By minimizing the DC line current and injecting a fixed-frequency AC current, the circuit breaker is opened only when the current flowing through it crosses zero. The injected AC current is very small and hardly affects the normal power transmission of other stations. The injected AC current can be achieved by superimposing an AC voltage on the DC line voltage. The conversion relationship between AC voltage and AC current is expressed by equivalent impedance. The equivalent impedance value is closely related to the parameters of the transmission line (impedance, inductive reactance, and capacitive reactance of parasitic capacitance to ground). Theoretically, the closer the frequency of the AC voltage is to the resonant frequency of the line, the smaller the equivalent impedance, that is, the smaller the AC voltage amplitude required to inject the same amplitude AC current, and thus the smaller the impact on other operating stations. Since the line parameters change with factors such as temperature and humidity, in engineering implementation, it can be simply set to a fixed frequency close to the resonant frequency. The magnitude of the AC current is changed by adjusting the amplitude of the AC voltage to ensure that the current flowing through the circuit breaker has a periodic zero-crossing point. Furthermore, if the station to be decommissioned can maintain operation during the decommissioning period, it is advisable to superimpose an AC voltage onto the DC voltage command of the decommissioning station; if the station to be decommissioned is blocked due to protection, an AC voltage should be superimposed onto the DC voltage command of the DC voltage station. By adopting this invention, multi-terminal DC transmission projects can select low-cost, simple-structured high-voltage circuit breakers, effectively reducing construction costs and improving the reliability and flexibility of system operation. Attached Figure Description

[0036] Figure 1 This is a schematic diagram of the main wiring of a three-terminal DC embodiment of the present invention;

[0037] Figure 2 This is a flowchart of a single-station exit method according to the present invention;

[0038] Figure 3 This is a flowchart of an AC current injection method according to the present invention;

[0039] Figure 4 This is a schematic diagram of the multi-terminal DC single-station exit device of the present invention. Detailed Implementation

[0040] The specific embodiments of the present invention will now be described in detail with reference to the accompanying drawings.

[0041] The single-station shutdown method provided by this invention is applicable to multi-terminal DC transmission systems, including multi-terminal conventional DC, multi-terminal flexible DC, and multi-terminal hybrid DC. A multi-terminal DC transmission system comprises at least three converter stations, one of which is a DC voltage control station and the others are DC power control stations. All converter stations are connected via DC lines. At least one circuit breaker is configured between the shut-out converter station and the DC system formed by the other converter stations. Figure 1 In a three-terminal DC implementation, the DC sides of the three converter stations are connected by positive line 11 and negative line 12. A circuit breaker 13 is configured between converter station 2 and the DC system composed of converters 1 and 3. Two circuit breakers 14 and 15 are configured between converter station 3 and the DC system composed of converters 1 and 2. No circuit breaker is configured between converter station 1 and the DC system composed of converters 2 and 3. Therefore, converter station 2 and converter station 3 meet the implementation conditions of the single-station online shutdown method proposed in this scheme.

[0042] This invention provides a first embodiment of a single-station online disconnection method for a multi-terminal DC transmission system, referring to... Figure 2 As shown, the method includes the following steps:

[0043] Step 201: After receiving the decommissioning instruction, the converter station control and protection system reduces the DC power to be decommissioned to the minimum value at a certain rate.

[0044] Specifically, if the exit command is issued manually, the control system will automatically reduce the DC power of the station to be exited at a certain rate to maintain operation. If the exit command is issued by protection and the station to be exited can still maintain operation, the control system will reduce the DC power of the station to be exited to the minimum rate as quickly as possible. If the exit command is issued by protection and the station to be exited cannot maintain operation, the converter to be exited can be directly locked.

[0045] Step 202: Inject a fixed frequency AC current into the current of the circuit breaker to be decommissioned.

[0046] The amplitude of the fixed-frequency alternating current is greater than that of the DC component, causing the current flowing through the circuit breaker to exhibit periodic zero-crossing points. Specifically, after the DC power drops to its minimum value, the DC current flowing through the circuit breaker is already at the minimum allowable operating value. Based on this, by controlling the injection of a certain fixed-frequency alternating current, a zero-crossing point can be generated as long as its amplitude is higher than the minimum allowable operating DC current value.

[0047] Step 203: Determine whether the circuit breaker meets the conditions for allowing tripping. If it meets the conditions, proceed to step 205; otherwise, proceed to step 204.

[0048] Specifically, in order for the circuit breaker to trip reliably and avoid damage to the circuit breaker itself due to the inability to extinguish the arc, the control system needs to detect whether the circuit breaker has met the permissible tripping conditions; all circuit breakers used for isolation must meet the tripping conditions.

[0049] In a preferred embodiment, the circuit breaker is allowed to trip when the number of zero-crossing points of the current flowing through the circuit breaker exceeds a preset value within a first preset time, or when the DC current component is lower than the tripping allowable value for more than a second preset time.

[0050] Step 204: After increasing the amplitude of the injected AC current, proceed to step 203.

[0051] Specifically, when the DC current is higher than the breaking allowable value, if no current zero-crossing point is detected or the number of current zero-crossing points is insufficient, the amplitude of the injected AC current is increased to create more current zero-crossing points.

[0052] Step 205: Issue the command to open the circuit breaker of the station to be exited.

[0053] Specifically, if a circuit breaker is detected to meet the conditions for allowing it to trip, a trip command is issued for that circuit breaker; all circuit breakers that serve an isolating function must trip.

[0054] Step 206: The circuit breaker of the station to be exited is detected to be in the open position.

[0055] Step 207: The station to be exited performs interlocking, trips the AC switch, and isolates the DC side.

[0056] Specifically, if any circuit breaker on the DC side of the station to be decommissioned is detected to be open, the station can perform blocking and tripping of the AC switch; if all circuit breakers on the DC side of the station to be decommissioned are detected to be open, the station can perform DC-side polar isolation; through DC-side polar isolation, the station to be decommissioned is reliably disconnected from the DC system. Polar isolation requires that at least one isolating switch be separated between the converter and the positive DC line, and at least one isolating switch be separated between the converter and the negative DC line.

[0057] The control and protection system injects a fixed-frequency alternating current into the current flowing through the circuit breaker, which can be achieved by superimposing an AC voltage command onto the DC line voltage reference value. Figure 3 This is an example of an injection current method.

[0058] Step 301: Determine whether the station to be exited is in an unlocked state. If the station to be exited is in an unlocked state, proceed to step 302; if it is in a locked state, proceed to step 303.

[0059] Specifically, it determines whether the station to be exited can still be unlocked and operated, and then selects the appropriate injection voltage method.

[0060] Step 302: After superimposing the DC voltage reference value of the station to be exited with the AC voltage command of a fixed frequency, proceed to step 304.

[0061] Step 303: After superimposing the DC voltage reference value of the DC voltage station with the AC voltage command of a fixed frequency, proceed to step 304.

[0062] Specifically, if the station to be exited is in an unlocked state, a fixed-frequency AC voltage command is superimposed on its DC voltage reference value to generate AC current. If the station to be exited is in a locked state, a fixed-frequency AC voltage command is superimposed on the DC voltage reference value of the control station to generate AC current. The fixed frequency can be set close to the oscillation frequency of the line, so that the amplitude of the superimposed AC voltage is relatively small.

[0063] Step 304: Determine whether the current flowing through the circuit breaker has a periodic zero-crossing point. If yes, proceed to step 306; otherwise, proceed to step 305.

[0064] Specifically, it determines whether the cumulative number of zero-crossing points of the current flowing through the circuit breaker within the first preset time exceeds a preset value.

[0065] Step 305: After increasing the amplitude of the superimposed voltage, proceed to step 304.

[0066] Specifically, if the number of current zero-crossing points is not met, increase the amplitude of the superimposed AC voltage until the number of current zero-crossing points meets the requirements.

[0067] Step 306: Maintain the injected current state until the circuit breaker opens.

[0068] Specifically, if the number of current zero-crossing points meets the requirements, the current superimposed AC voltage remains unchanged and the injected current state is maintained. The voltage superposition stops and the current injection ends when the circuit breaker tripping command is issued and any related circuit breaker is detected to have opened.

[0069] In a preferred embodiment, if the station to be decommissioned is a DC voltage control station, it is switched to a DC power control station before reducing the DC power, and one of the original DC power control stations is switched to a new DC voltage control station.

[0070] In a preferred embodiment, the circuit breaker is allowed to trip when the number of zero-crossing points of the current flowing through the circuit breaker exceeds a preset value within a first preset time, or when the DC current component is lower than the tripping allowable value for more than a second preset time.

[0071] With the above scheme, this invention configures a circuit breaker between the converter station and the DC line. By minimizing the DC line current and injecting a fixed-frequency AC current, the circuit breaker is opened only when the current flowing through it crosses zero. The injected AC current is very small and hardly affects the normal power transmission of other stations. The injected AC current can be achieved by superimposing an AC voltage on the DC line voltage. The conversion relationship between AC voltage and AC current is expressed by equivalent impedance. The equivalent impedance value is closely related to the parameters of the transmission line (impedance, inductive reactance, and capacitive reactance of parasitic capacitance to ground). Theoretically, the closer the frequency of the AC voltage is to the resonant frequency of the line, the smaller the equivalent impedance, that is, the smaller the AC voltage amplitude required to inject the same amplitude AC current, and thus the smaller the impact on other operating stations. Since the line parameters change with factors such as temperature and humidity, in engineering implementation, it can be simply set to a fixed frequency close to the resonant frequency. The magnitude of the AC current is changed by adjusting the amplitude of the AC voltage to ensure that the current flowing through the circuit breaker has a periodic zero-crossing point. Furthermore, if the station to be decommissioned can maintain operation during the decommissioning period, it is advisable to superimpose an AC voltage onto the DC voltage command of the decommissioning station; if the station to be decommissioned is blocked due to protection, an AC voltage should be superimposed onto the DC voltage command of the DC voltage station. By adopting this invention, multi-terminal DC transmission projects can select low-cost, simple-structured high-voltage circuit breakers, effectively reducing construction costs and improving the reliability and flexibility of system operation.

[0072] like Figure 4 The illustration shows an embodiment of a single-station online decommissioning device for a multi-terminal DC transmission system proposed by the present invention. The multi-terminal DC transmission system includes at least three converter stations, one of which is a DC voltage control station and the others are DC power control stations. All converter stations are connected via DC lines. At least one circuit breaker is configured between the converter station to be decommissioned and the DC line. The single-station decommissioning device includes: a power regulation unit, a circuit breaker current injection unit, a circuit breaker tripping unit, and a blocking and isolation unit. Wherein:

[0073] Power regulation unit: used to reduce the DC power to be removed from the station to the minimum value after receiving the station exit command;

[0074] Circuit breaker current injection unit: used to inject a fixed frequency AC current into the current flowing through the circuit breaker to be removed from the station;

[0075] Circuit breaker tripping unit: Used to detect the current flowing through the circuit breaker to be decommissioned from the station, and to issue a circuit breaker tripping command when the circuit breaker tripping conditions are met.

[0076] Interlocking and isolation unit: Used to detect the status of the circuit breaker, and after confirming that the circuit breaker has tripped, execute interlocking, tripping AC switch and DC side isolation commands in the station to be exited.

[0077] In a preferred embodiment, based on the above-described device embodiment, the amplitude of the AC current injected into the circuit breaker current injection unit is greater than that of the DC component, causing the current flowing through the circuit breaker to periodically cross zero.

[0078] In a preferred embodiment, based on the above-described device embodiment, the AC current in the circuit breaker current injection unit can be obtained by superimposing a fixed-frequency AC voltage command onto the DC voltage reference value of the station to be exited.

[0079] In a preferred embodiment, based on the above-described device embodiment, the AC current in the circuit breaker current injection unit can be obtained by superimposing a fixed-frequency AC voltage command onto the DC voltage reference value of the DC voltage control station.

[0080] In a preferred embodiment, based on the above device embodiment, if the station to be exited is a DC voltage control station, the power adjustment unit first switches it to a DC power control station before reducing the DC power, and then switches one of the original DC power control stations to a new DC voltage control station.

[0081] In a preferred embodiment, based on the above-described device embodiment, the circuit breaker tripping unit is allowed to trip when the number of zero-crossing points of the current flowing through the circuit breaker exceeds a preset value within a first preset time, or when the DC current component is lower than the tripping allowable value for more than a second preset time.

[0082] In a preferred embodiment, based on the above-described device embodiment, if the circuit breaker tripping condition in the circuit breaker tripping unit is not met, the circuit breaker current injection unit continues to increase the amplitude of the injected AC current until the circuit breaker tripping condition is met.

[0083] In a preferred embodiment, based on the above-described device embodiment, in the power regulation unit, if the exit command received by the converter station control and protection system is manually issued, the DC power of the station to be exited is reduced to the minimum value that can maintain operation at a certain rate; if the exit command is issued by protection and the station to be exited can still maintain operation, the DC power of the station to be exited is reduced to the minimum value at the fastest rate.

[0084] In a preferred embodiment, based on the above-described device embodiment, if the power regulation unit receives a decommissioning instruction from the converter station control and protection system that is a protection-issued instruction and the station to be decommissioned cannot maintain operation, then the converter to be decommissioned will be directly locked out.

[0085] Finally, it should be noted that the above embodiments are merely illustrative of the technical solutions of the present invention and not intended to limit it. Those skilled in the art should understand that modifications or equivalent substitutions can be made to the specific embodiments of the present invention, but such modifications or alterations are all within the scope of protection claimed in the pending patent application.

Claims

1. A method for single-station online decommissioning in a multi-terminal DC transmission system, wherein the multi-terminal DC transmission system comprises at least three converter stations, one of which is a DC voltage control station and the remaining converter stations are DC power control stations, and the converter stations are connected via DC lines; at least one circuit breaker is configured between the converter station to be decommissioned and the DC system composed of the other converter stations; characterized in that, The single-site online exit method includes: After receiving the shutdown command, the converter station control and protection system will reduce the DC power of the station to be shut down to the minimum value. A fixed-frequency alternating current is injected into the current flowing through the circuit breaker of the station to be decommissioned; the amplitude of the fixed-frequency alternating current is greater than that of the DC component, so that the current flowing through the circuit breaker exhibits periodic zero-crossing points. The circuit breaker is checked to see if the conditions for allowing it to open are met. If the number of zero-crossing points of the current flowing through the circuit breaker exceeds a preset value within a first preset time, or if its DC current component is lower than the breaking allowable value for more than a second preset time, then the conditions for allowing the circuit breaker to open are met. If the conditions for allowing the circuit breaker to open are not met, the amplitude of the injected AC current is increased until the conditions for allowing the circuit breaker to open are met. When the conditions for circuit breaker tripping are met, a circuit breaker tripping command is issued. Upon detection of circuit breaker tripping, the station will execute interlocking, AC switch tripping, and DC side isolation commands.

2. The single-station online shutdown method for a multi-terminal DC transmission system as described in claim 1, characterized in that: The fixed-frequency AC current is obtained by superimposing a fixed-frequency AC voltage command onto the DC voltage reference value of the station to be exited.

3. The single-station online shutdown method for a multi-terminal DC transmission system as described in claim 1, characterized in that: The fixed-frequency AC current is obtained by superimposing a fixed-frequency AC voltage command onto the DC voltage reference value of the DC voltage control station.

4. The single-station online shutdown method for a multi-terminal DC transmission system as described in claim 1, characterized in that: If the station to be decommissioned is a DC voltage control station, it will be switched to a DC power control station before reducing the DC power, and one of the original DC power control stations will be switched to the new DC voltage control station.

5. A single-station online shutdown method for a multi-terminal DC transmission system as described in claim 1, characterized in that: If the converter station control and protection system receives a manual exit command, the DC power to be exited will be reduced to the minimum value that can maintain operation at a certain rate. If the exit command is issued for protection purposes and the station to be exited can still maintain operation, then the DC power of the station to be exited should be reduced to the minimum value at the fastest rate.

6. The single-station online shutdown method for a multi-terminal DC transmission system as described in claim 1, characterized in that: If the converter station control and protection system receives a decommissioning command that is issued by protection and the station to be decommissioned cannot maintain operation, the converter to be decommissioned will be directly locked out.

7. A single-station online disconnection device for a multi-terminal DC transmission system, wherein the multi-terminal DC transmission system comprises at least three converter stations, one of which is a DC voltage control station and the remaining converter stations are DC power control stations, and the converter stations are connected via DC lines; at least one circuit breaker is configured between the converter station to be disconnected and the DC system composed of the other converter stations; characterized in that, The single-station online exit device includes: Power regulation unit: used to reduce the DC power to be removed from the station to the minimum value after receiving the station exit command; Circuit breaker current injection unit: used to inject a fixed frequency AC current into the current flowing through the circuit breaker to be withdrawn from the station; the amplitude of the AC current injected in the circuit breaker current injection unit is greater than the DC component, so that the current flowing through the circuit breaker will have periodic zero crossings. Circuit breaker tripping unit: Used to detect the current flowing through the circuit breaker to be decommissioned. When the circuit breaker tripping condition is met, a circuit breaker tripping command is issued. The circuit breaker tripping condition in the circuit breaker tripping unit is that the number of zero-crossing points of the current flowing through the circuit breaker exceeds a preset value within a first preset time, or its DC current component is lower than the breaking allowable value for more than a second preset time. If the circuit breaker tripping condition in the circuit breaker tripping unit is not met, the circuit breaker current injection unit continues to increase the amplitude of the injected AC current until the circuit breaker tripping condition is met. Interlocking and isolation unit: Used to detect the status of the circuit breaker, and after confirming that the circuit breaker has tripped, execute interlocking, tripping AC switch and DC side isolation commands in the station to be exited.

8. A single-station online disconnection device for a multi-terminal DC transmission system as described in claim 7, characterized in that: The AC current injected into the circuit breaker current injection unit is obtained by superimposing a fixed frequency AC voltage command onto the DC voltage reference value of the station to be exited.

9. A single-station online disconnection device for a multi-terminal DC transmission system as described in claim 7, characterized in that: The AC current injected into the circuit breaker current injection unit is obtained by superimposing a fixed frequency AC voltage command on the DC voltage reference value of the DC voltage control station.

10. A single-station online disconnection device for a multi-terminal DC transmission system as described in claim 7, characterized in that: If the station to be exited is a DC voltage control station, the power regulation unit will first switch it to a DC power control station before reducing the DC power, and then switch one of the original DC power control stations to the new DC voltage control station.

11. A single-station online disconnection device for a multi-terminal DC transmission system as described in claim 7, characterized in that: In the power regulation unit, if the shutdown command received by the converter station control and protection system is manually issued, the DC power to be shut down will be reduced to the minimum value that can maintain operation at a certain rate. If the exit command is issued for protection purposes and the station to be exited can still maintain operation, then the DC power of the station to be exited should be reduced to the minimum value at the fastest rate.

12. A single-station online disconnection device for a multi-terminal DC transmission system as described in claim 7, characterized in that: In the power regulation unit, if the power station control and protection system receives a power outage command that is a protection-issued command and the power station to be outage cannot maintain operation, the power converter to be outage will be directly locked out.

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