Online switching method and system for converter station of multi-terminal direct current power transmission system

Abstract

The invention belongs to the technical field of power electronics, and particularly relates to an online switching method and system for a converter station of a multi-terminal direct-current power transmission system. The online switching mode of the method comprises the following steps of: 1) confirming that a first converter station and a second converter station are respectively and correspondingly in constant voltage control and constant current control, and controlling the first converter station and the second converter station to be unlocked for operation; the first converter station and the second converter station are respectively an inverter station and a rectifier station; 2) confirming that the next inverter station is in constant current control, delaying for a certain time length, issuing an instruction for switching on a direct-current polar line switch of the inverter station, and controlling the inverter station to unlock and operate under the condition that the time length of the switch in a switching-on state is smaller than a first time length threshold value; and (3) confirming that the next rectification station is in constant current control, delaying for a certain time length, issuing an instruction for switching on the direct current polar line switch of the rectification station, controlling the rectification station to unlock and operate when confirming that the time length of the switch in the switching-on state is less than a second time length threshold value, and repeating the steps (2)-(3) if the inverter station and the rectification station to be put into use also exist.

Description

Technical Field

[0001] This invention belongs to the field of power electronics technology, specifically relating to an online commissioning and decommissioning method and system for converter stations in multi-terminal DC transmission systems. Background Technology

[0002] Multi-terminal conventional DC transmission systems (such as four-terminal conventional DC transmission systems) contain multiple converter stations, which operate in parallel. During the commissioning and decommissioning of converter stations, large impulse voltages are usually generated, and the converter valves of the converter stations are prone to bias voltage during the commissioning and decommissioning process, which may lead to the risk of valve arrester failure.

[0003] Chinese invention patent application CN106549408A discloses a method for online commissioning of converters in a multi-terminal high-voltage direct current (HVDC) transmission system. This method achieves stable online commissioning of converters by using a constant DC voltage control mode for one converter on the inverter side and a constant current control mode for the other converters on the inverter side, without interfering with the stable operation of other converters. Furthermore, this method enables precise control of the converter's power transmission, facilitating power control by dispatching and other departments. Summary of the Invention

[0004] The purpose of this invention is to provide an online commissioning and decommissioning method and system for multi-terminal DC transmission system converter stations, which solves the problems in the prior art where the inrush voltage is large and the converter valve is prone to bias voltage during the commissioning and decommissioning process of multi-terminal conventional DC transmission systems.

[0005] To achieve the above objectives, the present invention provides an online commissioning and decommissioning method for converter stations in a multi-terminal DC transmission system, wherein the online commissioning method includes: 1) Confirm that the first and second converter stations are under constant voltage control and constant current control respectively, and then unlock and operate the first and second converter stations; the first and second converter stations are the inverter station and the rectifier station respectively; 2) Confirm that the next inverter station to be put into operation is under constant current control, issue a command to close the DC pole line switch of the inverter station after a certain delay, and control the inverter station to unlock and run if the time for which the switch of the inverter station is in the closed position is less than the first time threshold. 3) Confirm that the next rectifier station to be put into operation is under constant current control, issue a command to close the DC pole line switch of the rectifier station after a certain delay, and control the rectifier station to unlock and run after confirming that the switch of the rectifier station has been in the closed position for less than the second time threshold. If there are still inverter stations and rectifier stations to be put into operation, repeat steps 2)-3).

[0006] Furthermore, online exit methods include: a) After confirming that the first and second converter stations and other inverter stations and rectifier stations put into operation in the DC transmission system are all unlocked and running, send a blocking signal to one inverter station and one rectifier station of the other inverter stations and rectifier stations respectively; b) After confirming that the corresponding inverter station and rectifier station are in a locked state, issue instructions to open the DC pole line switches of the inverter station and rectifier station respectively after a certain delay; c) If it is confirmed that the DC pole line switches of the inverter station and rectifier station are in the open state, and there are still inverter stations and rectifier stations to be decommissioned besides the first and second converter stations, then after a certain delay, issue instructions to open the DC pole line switches of the next inverter station and rectifier station in the other inverter stations and rectifier stations respectively; repeat steps b)-c) until there are no inverter stations and rectifier stations to be decommissioned besides the first and second converter stations. d) After confirming that the DC pole line switches of other inverter stations and rectifier stations are in the open state, after a certain delay, issue the blocking instructions for the first and second converter stations respectively to complete the online exit.

[0007] Furthermore, confirming that the converter station is in constant voltage control mode includes: if the converter station is not in constant voltage control mode, then the control mode of the converter station will be switched. The methods to confirm that the converter station is under constant current control include: if the converter station is not under constant current control, the control mode of the converter station will be switched.

[0008] Furthermore, the methods for confirming that the duration for which the switch in the inverter station is in the closed position is less than a first duration threshold include: Determine whether the duration of the DC pole line switch of the inverter station being in the closed position is less than a first time threshold. If yes, confirm that the duration of the switch being in the closed position of the inverter station is less than the first time threshold. If no, control the DC pole line switch of the inverter station to open and delay for a certain period of time, then re-determine and perform corresponding processing until it is confirmed that the duration of the switch being in the closed position of the inverter station is less than the first time threshold. The methods for confirming that the duration for which the switch of the rectifier station is in the closed position is less than the second duration threshold include: Determine whether the duration for which the DC pole line switch of the rectifier station is in the closed position is less than a second time threshold. If yes, confirm that the duration for which the switch of the rectifier station is in the closed position is less than the second time threshold. If no, control the DC pole line switch of the rectifier station to open and delay for a certain period of time, then re-determine and perform corresponding processing until it is confirmed that the duration for which the switch of the rectifier station is in the closed position is less than the second time threshold.

[0009] Furthermore, methods to confirm that the DC pole line switch of the inverter station or rectifier station is in the open state include: Determine whether the DC pole line switch of the inverter station or rectifier station is in the open state. If yes, confirm that the DC pole line switch of the inverter station or rectifier station is in the open state. If no, after a certain delay, issue a corresponding instruction to open the DC pole line switch of the inverter station or rectifier station, and then re-determine and process accordingly until it is confirmed that the DC pole line switch of the inverter station or rectifier station is in the open state. Methods to confirm that the inverter or rectifier station is in a locked state include: Determine whether the inverter station or rectifier station is in a locked state. If so, confirm that the inverter station or rectifier station is in a locked state. If not, after a certain delay, issue a corresponding lockout command for the inverter station or rectifier station, and then re-determine and process accordingly until it is confirmed that the inverter station or rectifier station is in a locked state. The delay time in confirming that the DC pole line switch of the inverter station or rectifier station is in the open state is less than the delay time in confirming that the inverter station or rectifier station is in the locked state.

[0010] The above-described technical solution of the present invention provides a novel online commissioning and decommissioning method for converter stations in multi-terminal DC transmission systems, the beneficial effects of which include: Analysis revealed that if the converter station's DC pole line switch remains in the closed position and is not promptly unlocked, it can cause voltage bias on the valves, potentially leading to damage to the valve's surge arrester. Therefore, by confirming a time limit for unlocking the converter station after the DC pole line switch is closed, the system ensures that there is virtually no voltage bias during unlocking, thus mitigating the risk caused by voltage bias. This online commissioning / discharging method is applicable to both LCC thyristor converter valves and reverse-resistance IGCT converter valves. It can be extended to four-terminal and higher DC transmission systems.

[0011] This invention also provides an online commissioning / decommissioning system for converter stations in a multi-terminal DC transmission system, including a processor containing executable program instructions. These instructions are executed to implement the following online commissioning / decommissioning method for multi-terminal DC transmission system converter stations, wherein the online commissioning method specifically includes: 1) Confirm that the first and second converter stations are under constant voltage control and constant current control respectively, and then unlock and operate the first and second converter stations; the first and second converter stations are the inverter station and the rectifier station respectively; 2) Confirm that the next inverter station to be put into operation is under constant current control, issue a command to close the DC pole line switch of the inverter station after a certain delay, and control the inverter station to unlock and run if the time for which the switch of the inverter station is in the closed position is less than the first time threshold. 3) Confirm that the next rectifier station to be put into operation is under constant current control, issue a command to close the DC pole line switch of the rectifier station after a certain delay, and control the rectifier station to unlock and run after confirming that the switch of the rectifier station has been in the closed position for less than the second time threshold. If there are still inverter stations and rectifier stations to be put into operation, repeat steps 2)-3).

[0012] Furthermore, the online exit methods in this method specifically include: a) After confirming that the first and second converter stations and other inverter stations and rectifier stations put into operation in the DC transmission system are all unlocked and running, send a blocking signal to one inverter station and one rectifier station of the other inverter stations and rectifier stations respectively; b) After confirming that the corresponding inverter station and rectifier station are in a locked state, issue instructions to open the DC pole line switches of the inverter station and rectifier station respectively after a certain delay; c) If it is confirmed that the DC pole line switches of the inverter station and rectifier station are in the open state, and there are still inverter stations and rectifier stations to be decommissioned besides the first and second converter stations, then after a certain delay, issue instructions to open the DC pole line switches of the next inverter station and rectifier station in the other inverter stations and rectifier stations respectively; repeat steps b)-c) until there are no inverter stations and rectifier stations to be decommissioned besides the first and second converter stations. d) After confirming that the DC pole line switches of other inverter stations and rectifier stations are in the open state, after a certain delay, issue the blocking instructions for the first and second converter stations respectively to complete the online exit.

[0013] Furthermore, confirming that the converter station is in constant voltage control mode includes: if the converter station is not in constant voltage control mode, then the control mode of the converter station will be switched. The methods to confirm that the converter station is under constant current control include: if the converter station is not under constant current control, the control mode of the converter station will be switched.

[0014] Furthermore, the methods for confirming that the duration for which the switch in the inverter station is in the closed position is less than a first duration threshold include: Determine whether the duration of the DC pole line switch of the inverter station being in the closed position is less than a first time threshold. If yes, confirm that the duration of the switch being in the closed position of the inverter station is less than the first time threshold. If no, control the DC pole line switch of the inverter station to open and delay for a certain period of time, then re-determine and perform corresponding processing until it is confirmed that the duration of the switch being in the closed position of the inverter station is less than the first time threshold. The methods for confirming that the duration for which the switch of the rectifier station is in the closed position is less than the second duration threshold include: Determine whether the duration for which the DC pole line switch of the rectifier station is in the closed position is less than a second time threshold. If yes, confirm that the duration for which the switch of the rectifier station is in the closed position is less than the second time threshold. If no, control the DC pole line switch of the rectifier station to open and delay for a certain period of time, then re-determine and perform corresponding processing until it is confirmed that the duration for which the switch of the rectifier station is in the closed position is less than the second time threshold.

[0015] Furthermore, methods to confirm that the DC pole line switch of the inverter station or rectifier station is in the open state include: Determine whether the DC pole line switch of the inverter station or rectifier station is in the open state. If yes, confirm that the DC pole line switch of the inverter station or rectifier station is in the open state. If no, after a certain delay, issue a corresponding instruction to open the DC pole line switch of the inverter station or rectifier station, and then re-determine and process accordingly until it is confirmed that the DC pole line switch of the inverter station or rectifier station is in the open state. Methods to confirm that the inverter or rectifier station is in a locked state include: Determine whether the inverter station or rectifier station is in a locked state. If so, confirm that the inverter station or rectifier station is in a locked state. If not, after a certain delay, issue a corresponding lockout command for the inverter station or rectifier station, and then re-determine and process accordingly until it is confirmed that the inverter station or rectifier station is in a locked state. The delay time in confirming that the DC pole line switch of the inverter station or rectifier station is in the open state is less than the delay time in confirming that the inverter station or rectifier station is in the locked state.

[0016] The technical solution of the multi-terminal DC transmission system converter station online commissioning and decommissioning system described above in this invention can achieve the same beneficial effects as the above-mentioned multi-terminal DC transmission system converter station online commissioning and decommissioning method. Attached Figure Description

[0017] Figure 1 This is a flowchart illustrating the online commissioning method of the multi-terminal DC transmission system converter station in the embodiment of the present invention. Figure 2a This is a graph showing the change in voltage and current waveforms of the online connection valve of the third converter station when the closing state time of the DC pole line switch of the third converter station is less than 20ms, as confirmed in the implementation of the online connection and deactivation method of the multi-terminal DC transmission system converter station of the present invention. Figure 2b This is a graph showing the change in voltage and current waveforms of the online connection valve of the third converter station when the closing state time of the DC pole line switch of the third converter station is greater than 20ms in the implementation of the online connection and deactivation method of the multi-terminal DC transmission system converter station of the present invention. Figure 3This is a schematic diagram of the principle of a four-terminal DC transmission system in the embodiment of the online commissioning and decommissioning method for converter stations in a multi-terminal DC transmission system of the present invention; Figure 4 This is a flowchart illustrating the online deactivation method of the online commissioning and deactivation method for multi-terminal DC transmission system converter stations in the embodiment of the present invention. Detailed Implementation

[0018] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments.

[0019] Implementation Methods for Online Commissioning and Decommissioning of Converter Stations in Multi-Terminal DC Transmission Systems This embodiment provides a technical solution for the online commissioning and decommissioning method of converter stations in multi-terminal DC transmission systems. By ensuring that the converter station is unlocked in time when the DC pole line switch is just in the closed position and before a large voltage bias is generated, the risk caused by voltage bias is avoided.

[0020] The online input methods of this method include: 1) Confirm that the first and second converter stations are under constant voltage control and constant current control respectively, and then unlock and operate the first and second converter stations; the first and second converter stations are the inverter station and the rectifier station respectively; 2) Confirm that the next inverter station to be put into operation is under constant current control, issue a command to close the DC pole line switch of the inverter station after a certain delay, and control the inverter station to unlock and run after confirming that the DC pole line switch of the inverter station has been in the closed position for less than the first time threshold. 3) Confirm that the next rectifier station to be put into operation is under constant current control, issue a command to close the DC pole line switch of the rectifier station after a certain delay, and control the rectifier station to unlock and run after confirming that the DC pole line switch of the rectifier station has been in the closed position for less than the second time threshold. If there are still inverter stations and rectifier stations to be put into operation, repeat steps 2)-3).

[0021] Therefore, analysis revealed that if the converter station's DC pole line switch remains closed and not unlocked promptly, it can cause voltage bias on the valves, potentially damaging the valve arresters. Thus, by confirming a time limit for unlocking the converter station after the DC pole line switch closes, the risk of voltage bias during unlocking is minimized, thereby mitigating the risk caused by voltage bias. This online commissioning / decommissioning method is applicable to both LCC thyristor converter valves and reverse-resistance IGCT converter valves. It can be extended to four-terminal and higher-level DC transmission systems. The strategy of prioritizing the master station and grouping multiple slave stations ensures more efficient converter station commissioning while considering control architecture, communication reliability, and startup efficiency and safety.

[0022] In this embodiment, the methods for confirming that the duration of the DC pole line switch of the inverter station being in the closed state is less than a first duration threshold include: Determine whether the duration of the DC pole line switch of the inverter station being in the closed position is less than a first duration threshold; if yes, confirm that the duration of the DC pole line switch of the inverter station being in the closed position is less than the first duration threshold; if no, control the DC pole line switch of the inverter station to open and delay for a certain period of time, then re-determine and perform corresponding processing until it is confirmed that the duration of the DC pole line switch of the inverter station being in the closed position is less than the first duration threshold. Methods to confirm that the duration for which the DC pole line switch of the rectifier station is in the closed position is less than the second duration threshold include: Determine whether the duration of the DC pole line switch of the rectifier station being in the closed position is less than the second time threshold. If yes, confirm that the duration of the DC pole line switch of the rectifier station being in the closed position is less than the second time threshold. If no, control the DC pole line switch of the rectifier station to open and delay for a certain period of time, then re-determine and perform corresponding processing until it is confirmed that the duration of the DC pole line switch of the rectifier station being in the closed position is less than the second time threshold.

[0023] In one embodiment, the first duration threshold and the second duration threshold are the same value, for example, both are 20ms.

[0024] In addition, the methods to confirm that the converter station is in constant voltage control include: if the converter station is not in constant voltage control mode, the control mode of the converter station will be switched. The methods to confirm that the converter station is under constant current control include: if the converter station is not under constant current control, the control mode of the converter station will be switched. This proactive switching method, which occurs when conditions are not met, can improve the efficiency and success rate of converter station commissioning.

[0025] Taking a converter station in a four-terminal DC transmission system as an example, if the next inverter station to be put into operation is the third converter station, and the next rectifier station to be put into operation is the fourth converter station, then refer to... Figure 1The specific steps for the above online investment method are as follows: 1) Confirm that the first converter station is in constant voltage control mode and the second converter station is in constant current control mode; control the first and second converter stations to unlock and operate stably according to the conventional converter station sequential control operation; 2) Confirm that the third converter station to be put into operation is in constant current control mode, and issue a closing command for the DC pole line switch of the third converter station to be put into operation after a delay of 40ms; confirm that the closing state time of the DC pole line switch of the third converter station is less than 20ms, and immediately issue a unlocking command for the third converter station. Assuming the DC pole line switch of the third converter station is in the closed position for less than 20ms, the voltage and current waveforms of the online connection valve at the third converter station are shown below. Figure 2a At this time, there is no bias in the voltage across the valve of the third converter station, and the maximum valve voltage is -242kV, achieving safe and stable system operation. If the DC pole switch of the third converter station is in the closed position for more than 20ms, the voltage across the valve of the third converter station will be biased, and the maximum valve voltage will be -396kV, posing a risk of damage to the valve arrester. For example, if the DC pole switch of the third converter station is in the closed position for no less than 20ms, the waveforms of the online valve voltage and current of the third converter station are shown in the figure. Figure 2b .

[0026] If the closed state time of the DC pole switch of the third converter station is greater than the threshold of 20ms, then the opening command of the DC pole switch of the third converter station is issued, so that the switch is in the open state and delayed for 100ms, and then the closing command of the DC pole switch of the third converter station is issued.

[0027] 3) Confirm that the fourth converter station to be put into operation is in constant current control mode, and issue a closing command for the DC pole line switch of the fourth converter station to be put into operation after a delay of 40ms; confirm that the closing state time of the DC pole line switch of the fourth converter station is less than 20ms, and immediately issue a unlocking command for the fourth converter station. Similarly, if the closed state time of the DC pole switch of the fourth converter station is greater than the threshold of 20ms, a trip command for the DC pole switch of the fourth converter station is issued, so that the switch is in the open state and delayed for 100ms, and then a closing command for the DC pole switch of the fourth converter station is issued.

[0028] In summary, the online commissioning of the four-terminal DC transmission system is complete. Considering the DC transmission unlocking control principles, the master station priority principle prioritizes unlocking the master station responsible for constant DC voltage control (usually the inverter-side master station) to establish a stable DC voltage reference for the master-slave control system. The inverter priority principle, in conventional LCC-HVDC, requires the inverter side to establish commutation voltage in advance to avoid the risk of commutation failure due to unlocking the rectifier side first; this is a common principle for both two-terminal and multi-terminal DC transmission. Therefore, the first converter station adopts constant voltage control. The second, third, and fourth converter stations adopt constant current control to ensure the output of the required operating power.

[0029] The principle of this four-terminal DC transmission system is as follows: Figure 3 As shown.

[0030] Furthermore, in this embodiment, the online deactivation method of the online commissioning and deactivation method for multi-terminal DC transmission system converter stations includes: a) After confirming that the first and second converter stations and other inverter stations and rectifier stations put into operation in the DC transmission system are all unlocked and running, send a blocking signal to one inverter station and one rectifier station of the other inverter stations and rectifier stations respectively; b) After confirming that the corresponding inverter station and rectifier station are in a locked state, issue instructions to open the DC pole line switches of the inverter station and rectifier station respectively after a certain delay; c) If it is confirmed that the DC pole line switches of the inverter station and rectifier station are in the open state, and there are still inverter stations and rectifier stations to be decommissioned besides the first and second converter stations, then after a certain delay, issue instructions to open the DC pole line switches of the next inverter station and rectifier station in the other inverter stations and rectifier stations respectively; repeat steps b)-c) until there are no inverter stations and rectifier stations to be decommissioned besides the first and second converter stations. d) After confirming that the DC pole line switches of other inverter stations and rectifier stations are in the open state, after a certain delay, issue the blocking instructions for the first and second converter stations respectively to complete the online exit.

[0031] Finally, the strategy of grouping and exiting multiple slave stations ensures that the commissioning of converter stations and the shutdown of DC transmission systems are more efficient, taking into account the control architecture, communication reliability, and startup efficiency and security.

[0032] Reference Figure 4 In one specific embodiment, taking a converter station in a four-terminal DC transmission system as an example, the other inverter stations are the third converter station, and the other rectifier stations are the fourth converter station. The specific steps of the above-mentioned online shutdown method are as follows: a) Confirm that the first, second, third, and fourth converter stations are unlocked and operating stably; issue interlock signals to the third and fourth converter stations respectively; b) After confirming that the third and fourth converter stations are in the locked state, delay for 20ms and issue the DC pole line switch opening command for the third and fourth converter stations respectively; c) Confirm that the DC pole line switches of the third and fourth converter stations are in the open state, delay for 20ms, and issue blocking instructions to the first and second converter stations respectively; since there are no inverter stations and rectifier stations to be decommissioned, the online decommissioning of the four-terminal DC transmission system is completed.

[0033] Methods to confirm that the DC pole line switch of the inverter station or rectifier station is in the open state include: Determine whether the DC pole line switch of the inverter station or rectifier station is in the open state. If yes, confirm that the DC pole line switch of the inverter station or rectifier station is in the open state. If no, after a certain delay, issue a corresponding instruction to open the DC pole line switch of the inverter station or rectifier station, and then re-determine and process accordingly until it is confirmed that the DC pole line switch of the inverter station or rectifier station is in the open state. Methods to confirm that the inverter or rectifier station is in a locked state include: Determine whether the inverter station or rectifier station is in a locked state. If so, confirm that the inverter station or rectifier station is in a locked state. If not, after a certain delay, issue a corresponding lockout command for the inverter station or rectifier station, and then re-determine and process accordingly until it is confirmed that the inverter station or rectifier station is in a locked state. The delay time in confirming that the DC pole line switch of the inverter station or rectifier station is in the open state is less than the delay time in confirming that the inverter station or rectifier station is in the locked state.

[0034] In one specific embodiment, refer to Figure 4 If the DC pole line switches of the third and fourth converter stations are not in the open state, a 20ms delay is made before issuing the open command for the DC pole line switches of the third and fourth converter stations. Conversely, if the third and fourth converter stations are not in the locked state when it is confirmed that they are in the locked state, a 100ms delay is made before re-issuing the locked signal for the third and fourth converter stations. The delay duration in confirming that the DC pole line switches of the inverter or rectifier station are in the open state corresponds to a mechanical switch, whose opening process mainly involves contact separation and the establishment of the arc-extinguishing medium, naturally having a time scale in the tens of milliseconds. A shorter delay (e.g., 20ms) can avoid the mechanical bounce and transient interference of switch opening while ensuring rapid isolation of the fault source in case of a fault, conforming to the protection design principle of "rapid fault clearing".

[0035] The delay time in confirming the inverter or rectifier station is in a locked state corresponds to the deep coupling between the locked state and the control and protection of the DC system. This requires waiting for the DC current to drop to zero (the current will slowly decay during the locked state), coordinating the locked sequence of the master and slave stations (the master station locked last to ensure DC voltage stability), and coordinating the timing of auxiliary equipment such as bypass switch switching and surge arrester operation. A relatively long delay (e.g., 100ms) provides sufficient time for these complex system coordinations, ensuring the smoothness and safety of the locked state process.

[0036] It is evident that this method can ensure the flexible commissioning and decommissioning of other converter stations without power interruption at the first and second converter stations. It can reduce the impact voltage during the commissioning and decommissioning process of converter stations in multi-terminal conventional DC transmission systems, and reduce the bias voltage of converter valves during commissioning and decommissioning, thus ensuring the flexible, safe and stable operation of four-terminal and above DC transmission systems.

[0037] Implementation of Online Commissioning and Decommissioning System for Converter Stations in Multi-Terminal DC Transmission Systems This embodiment provides a technical solution for the online commissioning and decommissioning of converter stations in a multi-terminal DC transmission system. The system includes a processor storing executable program instructions, which are executed to implement the following online commissioning and decommissioning method for multi-terminal DC transmission system converter stations. The online commissioning method specifically includes: 1) Confirm that the first and second converter stations are under constant voltage control and constant current control respectively, and then unlock and operate the first and second converter stations; the first and second converter stations are the inverter station and the rectifier station respectively; 2) Confirm that the next inverter station to be put into operation is under constant current control, issue a command to close the DC pole line switch of the inverter station after a certain delay, and control the inverter station to unlock and run after confirming that the DC pole line switch of the inverter station has been in the closed position for less than the first time threshold. 3) Confirm that the next rectifier station to be put into operation is under constant current control, issue a command to close the DC pole line switch of the rectifier station after a certain delay, and control the rectifier station to unlock and run after confirming that the DC pole line switch of the rectifier station has been in the closed position for less than the second time threshold. If there are still inverter stations and rectifier stations to be put into operation, repeat steps 2)-3).

[0038] Therefore, analysis revealed that if the converter station's DC pole line switch remains closed and not unlocked promptly, it can cause voltage bias on the valves, potentially leading to damage to the valve's surge arrester. Thus, by confirming a time limit for unlocking the converter station after the DC pole line switch is closed, the system ensures that there is virtually no voltage bias during unlocking, thereby mitigating the risk caused by voltage bias. This online commissioning / decommissioning system is applicable to both LCC thyristor converter valves and reverse-resistance IGCT converter valves. It can be extended to four-terminal and higher-level DC transmission systems. The strategy of prioritizing the master station and grouping multiple slave stations ensures more efficient converter station commissioning while considering control architecture, communication reliability, and startup efficiency and safety.

[0039] In this embodiment, the methods for confirming that the duration of the DC pole line switch of the inverter station being in the closed state is less than a first duration threshold include: Determine whether the duration of the DC pole line switch of the inverter station being in the closed position is less than a first duration threshold; if yes, confirm that the duration of the DC pole line switch of the inverter station being in the closed position is less than the first duration threshold; if no, control the DC pole line switch of the inverter station to open and delay for a certain period of time, then re-determine and perform corresponding processing until it is confirmed that the duration of the DC pole line switch of the inverter station being in the closed position is less than the first duration threshold. Methods to confirm that the duration for which the DC pole line switch of the rectifier station is in the closed position is less than the second duration threshold include: Determine whether the duration of the DC pole line switch of the rectifier station being in the closed position is less than the second time threshold. If yes, confirm that the duration of the DC pole line switch of the rectifier station being in the closed position is less than the second time threshold. If no, control the DC pole line switch of the rectifier station to open and delay for a certain period of time, then re-determine and perform corresponding processing until it is confirmed that the duration of the DC pole line switch of the rectifier station being in the closed position is less than the second time threshold.

[0040] In one embodiment, the first duration threshold and the second duration threshold are the same value, for example, both are 20ms.

[0041] In addition, the methods to confirm that the converter station is in constant voltage control include: if the converter station is not in constant voltage control mode, the control mode of the converter station will be switched. The methods to confirm that the converter station is under constant current control include: if the converter station is not under constant current control, the control mode of the converter station will be switched.

[0042] This proactive switching method, which allows for switching even when conditions are not met, can improve the efficiency and success rate of converter station deployment.

[0043] Taking a converter station in a four-terminal DC transmission system as an example, if the next inverter station to be put into operation is the third converter station, and the next rectifier station to be put into operation is the fourth converter station, then the specific steps for the above-mentioned online commissioning method are as follows: 1) Confirm that the first converter station is in constant voltage control mode and the second converter station is in constant current control mode; control the first and second converter stations to unlock and operate stably according to the conventional converter station sequential control operation; 2) Confirm that the third converter station to be put into operation is in constant current control mode, and issue a closing command for the DC pole line switch of the third converter station to be put into operation after a delay of 40ms; confirm that the closing state time of the DC pole line switch of the third converter station is less than 20ms, and immediately issue a unlocking command for the third converter station. If the closed state of the DC pole line switch of the third converter station is confirmed to be less than 20ms, then the voltage on the valve of the third converter station is not biased, and the maximum valve voltage is -242kV, thus achieving safe and stable operation of the system. If the closed state of the DC pole line switch of the third converter station is greater than 20ms, then the voltage on the valve of the third converter station is biased, and the maximum valve voltage is -396kV, which poses a risk of damage to the valve surge arrester.

[0044] If the closed state time of the DC pole switch of the third converter station is greater than the threshold of 20ms, then the opening command of the DC pole switch of the third converter station is issued, so that the switch is in the open state and delayed for 100ms, and then the closing command of the DC pole switch of the third converter station is issued.

[0045] 3) Confirm that the fourth converter station to be put into operation is in constant current control mode, and issue a closing command for the DC pole line switch of the fourth converter station to be put into operation after a delay of 40ms; confirm that the closing state time of the DC pole line switch of the fourth converter station is less than 20ms, and immediately issue a unlocking command for the fourth converter station. Similarly, if the closed state time of the DC pole switch of the fourth converter station is greater than the threshold of 20ms, a trip command for the DC pole switch of the fourth converter station is issued, so that the switch is in the open state and delayed for 100ms, and then a closing command for the DC pole switch of the fourth converter station is issued.

[0046] In summary, the online commissioning of the four-terminal DC transmission system is complete. Considering the master station priority principle in DC transmission unlocking control, the master station responsible for constant DC voltage control (usually the inverter-side master station) is unlocked first to establish a stable DC voltage reference for the master-slave control system. The inverter priority principle, in conventional LCC-HVDC, requires the inverter side to establish commutation voltage in advance to avoid the risk of commutation failure due to unlocking the rectifier side first; this is a common principle for both two-terminal and multi-terminal DC transmission. Therefore, the first converter station adopts constant voltage control. The second, third, and fourth converter stations adopt constant current control to ensure the output of the required operating power.

[0047] Furthermore, in this embodiment, the online deactivation method of the online commissioning and deactivation method for multi-terminal DC transmission system converter stations includes: a) After confirming that the first and second converter stations and other inverter stations and rectifier stations put into operation in the DC transmission system are all unlocked and running, send a blocking signal to one inverter station and one rectifier station of the other inverter stations and rectifier stations respectively; b) After confirming that the corresponding inverter station and rectifier station are in a locked state, issue instructions to open the DC pole line switches of the inverter station and rectifier station respectively after a certain delay; c) If it is confirmed that the DC pole line switches of the inverter station and rectifier station are in the open state, and there are still inverter stations and rectifier stations to be decommissioned besides the first and second converter stations, then after a certain delay, issue instructions to open the DC pole line switches of the next inverter station and rectifier station in the other inverter stations and rectifier stations respectively; repeat steps b)-c) until there are no inverter stations and rectifier stations to be decommissioned besides the first and second converter stations. d) After confirming that the DC pole line switches of other inverter stations and rectifier stations are in the open state, after a certain delay, issue the blocking instructions for the first and second converter stations respectively to complete the online exit.

[0048] Finally, the strategy of grouping and exiting multiple slave stations ensures that the commissioning of converter stations and the shutdown of DC transmission systems are more efficient, taking into account the control architecture, communication reliability, and startup efficiency and security.

[0049] In one specific embodiment, taking a converter station in a four-terminal DC transmission system as an example, the other inverter stations are the third converter station, and the other rectifier stations are the fourth converter station. The specific steps of the above-mentioned online shutdown method are as follows: a) Confirm that the first, second, third, and fourth converter stations are unlocked and operating stably; issue interlock signals to the third and fourth converter stations respectively; b) After confirming that the third and fourth converter stations are in the locked state, delay for 20ms and issue the DC pole line switch opening command for the third and fourth converter stations respectively; c) Confirm that the DC pole line switches of the third and fourth converter stations are in the open state, delay for 20ms, and issue blocking instructions to the first and second converter stations respectively; since there are no inverter stations and rectifier stations to be decommissioned, the online decommissioning of the four-terminal DC transmission system is completed.

[0050] Methods to confirm that the DC pole line switch of the inverter station or rectifier station is in the open state include: Determine whether the DC pole line switch of the inverter station or rectifier station is in the open state. If yes, confirm that the DC pole line switch of the inverter station or rectifier station is in the open state. If no, after a certain delay, issue a corresponding instruction to open the DC pole line switch of the inverter station or rectifier station, and then re-determine and process accordingly until it is confirmed that the DC pole line switch of the inverter station or rectifier station is in the open state. Methods to confirm that the inverter or rectifier station is in a locked state include: Determine whether the inverter station or rectifier station is in a locked state. If so, confirm that the inverter station or rectifier station is in a locked state. If not, after a certain delay, issue a corresponding lockout command for the inverter station or rectifier station, and then re-determine and process accordingly until it is confirmed that the inverter station or rectifier station is in a locked state. The delay time in confirming that the DC pole line switch of the inverter station or rectifier station is in the open state is less than the delay time in confirming that the inverter station or rectifier station is in the locked state.

[0051] In one specific embodiment, if the DC pole line switches of the third and fourth converter stations are not in the open state, a 20ms delay is made before issuing the open command for the DC pole line switches of the third and fourth converter stations. Conversely, if the third and fourth converter stations are not in the locked state when it is confirmed that they are in the locked state, a 100ms delay is made before re-issuing the locked signal for the third and fourth converter stations. The delay duration in confirming that the DC pole line switches of the inverter or rectifier station are in the open state corresponds to a mechanical switch, whose opening process mainly involves contact separation and the establishment of an arc-extinguishing medium, naturally having a time scale in the tens of milliseconds. A shorter delay (e.g., 20ms) can avoid the mechanical bounce and transient interference of switch opening while ensuring rapid isolation of the fault source in case of a fault, conforming to the protection design principle of "rapid fault clearing".

[0052] The delay time in confirming the inverter or rectifier station is in a locked state corresponds to the deep coupling between the locked state and the control and protection of the DC system. This requires waiting for the DC current to drop to zero (the current will slowly decay during the locked state), coordinating the locked sequence of the master and slave stations (the master station locked last to ensure DC voltage stability), and coordinating the timing of auxiliary equipment such as bypass switch switching and surge arrester operation. A relatively long delay (e.g., 100ms) provides sufficient time for these complex system coordinations, ensuring the smoothness and safety of the locked state process.

[0053] It is evident that this system can flexibly enable or disable other converter stations without power interruption at the first and second converter stations. It can reduce the impact voltage during the online enabling or disabling process of converter stations in multi-terminal conventional DC transmission systems, and reduce the bias voltage of converter valves during enabling or disabling, thus ensuring the flexible, safe, and stable operation of four-terminal and above DC transmission systems.

[0054] It should be understood that the above-described specific embodiments of the present invention are merely illustrative or explanatory of the principles of the present invention, and do not constitute a limitation thereof.

Claims

1. A method for commissioning and decommissioning converter stations in a multi-terminal DC transmission system, characterized in that, Online payment methods include: 1) Confirm that the first and second converter stations are under constant voltage control and constant current control respectively, and then unlock and operate the first and second converter stations; the first and second converter stations are the inverter station and the rectifier station respectively; 2) Confirm that the next inverter station to be put into operation is under constant current control, issue a command to close the DC pole line switch of the inverter station after a certain delay, and control the inverter station to unlock and run if the time for which the switch of the inverter station is in the closed position is less than the first time threshold. 3) Confirm that the next rectifier station to be put into operation is under constant current control, issue a command to close the DC pole line switch of the rectifier station after a certain delay, and control the rectifier station to unlock and run after confirming that the switch of the rectifier station has been in the closed position for less than the second time threshold. If there are still inverter stations and rectifier stations to be put into operation, repeat steps 2)-3).

2. The method for commissioning and decommissioning converter stations in a multi-terminal DC transmission system according to claim 1, characterized in that, Online exit methods include: a) After confirming that the first and second converter stations and other inverter stations and rectifier stations put into operation in the DC transmission system are all unlocked and running, send a blocking signal to one inverter station and one rectifier station of the other inverter stations and rectifier stations respectively; b) After confirming that the corresponding inverter station and rectifier station are in a locked state, issue instructions to open the DC pole line switches of the inverter station and rectifier station respectively after a certain delay; c) If it is confirmed that the DC pole line switches of the inverter station and rectifier station are in the open state, and there are still inverter stations and rectifier stations to be decommissioned besides the first and second converter stations, then after a certain delay, issue instructions to open the DC pole line switches of the next inverter station and rectifier station in the other inverter stations and rectifier stations respectively; repeat steps b)-c) until there are no inverter stations and rectifier stations to be decommissioned besides the first and second converter stations. d) After confirming that the DC pole line switches of other inverter stations and rectifier stations are in the open state, after a certain delay, issue the blocking instructions for the first and second converter stations respectively to complete the online exit.

3. The method for commissioning and decommissioning a converter station in a multi-terminal DC transmission system according to claim 1 or 2, characterized in that, The methods for confirming that the converter station is in constant voltage control include: if the converter station is not in constant voltage control mode, the control mode of the converter station will be switched. The methods to confirm that the converter station is under constant current control include: if the converter station is not under constant current control, the control mode of the converter station will be switched.

4. The online commissioning and decommissioning method for converter stations in a multi-terminal DC transmission system according to claim 1 or 2, characterized in that, The methods for confirming that the switch in the inverter station has been in the closed position for less than a first duration threshold include: Determine whether the duration of the DC pole line switch of the inverter station being in the closed position is less than a first time threshold. If yes, confirm that the duration of the switch being in the closed position of the inverter station is less than the first time threshold. If no, control the DC pole line switch of the inverter station to open and delay for a certain period of time, then re-determine and perform corresponding processing until it is confirmed that the duration of the switch being in the closed position of the inverter station is less than the first time threshold. The methods for confirming that the duration for which the switch of the rectifier station is in the closed position is less than the second duration threshold include: Determine whether the duration for which the DC pole line switch of the rectifier station is in the closed position is less than a second time threshold. If yes, confirm that the duration for which the switch of the rectifier station is in the closed position is less than the second time threshold. If no, control the DC pole line switch of the rectifier station to open and delay for a certain period of time, then re-determine and perform corresponding processing until it is confirmed that the duration for which the switch of the rectifier station is in the closed position is less than the second time threshold.

5. The method for commissioning and decommissioning converter stations in a multi-terminal DC transmission system according to claim 2, characterized in that, Methods to confirm that the DC pole line switch of the inverter station or rectifier station is in the open state include: Determine whether the DC pole line switch of the inverter station or rectifier station is in the open state. If yes, confirm that the DC pole line switch of the inverter station or rectifier station is in the open state. If no, after a certain delay, issue a corresponding instruction to open the DC pole line switch of the inverter station or rectifier station, and then re-determine and process accordingly until it is confirmed that the DC pole line switch of the inverter station or rectifier station is in the open state. Methods to confirm that the inverter or rectifier station is in a locked state include: Determine whether the inverter station or rectifier station is in a locked state. If so, confirm that the inverter station or rectifier station is in a locked state. If not, after a certain delay, issue a corresponding lockout command for the inverter station or rectifier station, and then re-determine and process accordingly until it is confirmed that the inverter station or rectifier station is in a locked state. The delay time in confirming that the DC pole line switch of the inverter station or rectifier station is in the open state is less than the delay time in confirming that the inverter station or rectifier station is in the locked state.

6. A multi-terminal DC transmission system converter station online commissioning / decommissioning system, comprising a processor, wherein the processor stores executable program instructions, characterized in that, The executable program instructions are executed to implement the following online commissioning and decommissioning method for converter stations in multi-terminal DC transmission systems, wherein the online commissioning method specifically includes: 1) Confirm that the first and second converter stations are under constant voltage control and constant current control respectively, and then unlock and operate the first and second converter stations; the first and second converter stations are the inverter station and the rectifier station respectively; 2) Confirm that the next inverter station to be put into operation is under constant current control, issue a command to close the DC pole line switch of the inverter station after a certain delay, and control the inverter station to unlock and run if the time for which the switch of the inverter station is in the closed position is less than the first time threshold. 3) Confirm that the next rectifier station to be put into operation is under constant current control, issue a command to close the DC pole line switch of the rectifier station after a certain delay, and control the rectifier station to unlock and run after confirming that the switch of the rectifier station has been in the closed position for less than the second time threshold. If there are still inverter stations and rectifier stations to be put into operation, repeat steps 2)-3).

7. The online commissioning and decommissioning system for converter stations in a multi-terminal DC transmission system according to claim 6, characterized in that, The online exit methods in this method specifically include: a) After confirming that the first and second converter stations and other inverter stations and rectifier stations put into operation in the DC transmission system are all unlocked and running, send a blocking signal to one inverter station and one rectifier station of the other inverter stations and rectifier stations respectively; b) After confirming that the corresponding inverter station and rectifier station are in a locked state, issue instructions to open the DC pole line switches of the inverter station and rectifier station respectively after a certain delay; c) If it is confirmed that the DC pole line switches of the inverter station and rectifier station are in the open state, and there are still inverter stations and rectifier stations to be decommissioned besides the first and second converter stations, then after a certain delay, issue instructions to open the DC pole line switches of the next inverter station and rectifier station in the other inverter stations and rectifier stations respectively; repeat steps b)-c) until there are no inverter stations and rectifier stations to be decommissioned besides the first and second converter stations. d) After confirming that the DC pole line switches of other inverter stations and rectifier stations are in the open state, after a certain delay, issue the blocking instructions for the first and second converter stations respectively to complete the online exit.

8. The online commissioning and decommissioning system for converter stations in a multi-terminal DC transmission system according to claim 6 or 7, characterized in that, The methods for confirming that the converter station is in constant voltage control include: if the converter station is not in constant voltage control mode, the control mode of the converter station will be switched. The methods to confirm that the converter station is under constant current control include: if the converter station is not under constant current control, the control mode of the converter station will be switched.

9. The online commissioning and decommissioning system for converter stations in a multi-terminal DC transmission system according to claim 6 or 7, characterized in that, The methods for confirming that the switch in the inverter station has been in the closed position for less than a first duration threshold include: Determine whether the duration of the DC pole line switch of the inverter station being in the closed position is less than a first time threshold. If yes, confirm that the duration of the switch being in the closed position of the inverter station is less than the first time threshold. If no, control the DC pole line switch of the inverter station to open and delay for a certain period of time, then re-determine and perform corresponding processing until it is confirmed that the duration of the switch being in the closed position of the inverter station is less than the first time threshold. The methods for confirming that the duration for which the switch of the rectifier station is in the closed position is less than the second duration threshold include: Determine whether the duration for which the DC pole line switch of the rectifier station is in the closed position is less than a second time threshold. If yes, confirm that the duration for which the switch of the rectifier station is in the closed position is less than the second time threshold. If no, control the DC pole line switch of the rectifier station to open and delay for a certain period of time, then re-determine and perform corresponding processing until it is confirmed that the duration for which the switch of the rectifier station is in the closed position is less than the second time threshold.

10. The online commissioning and decommissioning system for converter stations in a multi-terminal DC transmission system according to claim 7, characterized in that, Methods to confirm that the DC pole line switch of the inverter station or rectifier station is in the open state include: Determine whether the DC pole line switch of the inverter station or rectifier station is in the open state. If yes, confirm that the DC pole line switch of the inverter station or rectifier station is in the open state. If no, after a certain delay, issue a corresponding instruction to open the DC pole line switch of the inverter station or rectifier station, and then re-determine and process accordingly until it is confirmed that the DC pole line switch of the inverter station or rectifier station is in the open state. Methods to confirm that the inverter or rectifier station is in a locked state include: Determine whether the inverter station or rectifier station is in a locked state. If so, confirm that the inverter station or rectifier station is in a locked state. If not, after a certain delay, issue a corresponding lockout command for the inverter station or rectifier station, and then re-determine and process accordingly until it is confirmed that the inverter station or rectifier station is in a locked state. The delay time in confirming that the DC pole line switch of the inverter station or rectifier station is in the open state is less than the delay time in confirming that the inverter station or rectifier station is in the locked state.

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