A control method and system for direct current transmission

Abstract

The application discloses a kind of control method and system of direct current transmission, applied to direct current transmission control field, including obtaining the topological structure data of sea-land direct current transmission topology;According to topological structure data, obtain transmission equipment data and connection relationship, connection relationship is the low-voltage interconnection information between the controllable converter valve DC side of different converter stations;Based on the first commutation characteristic of uncontrolled rectifier valve, the first switching timing is obtained, and the second switching timing is obtained based on the second commutation characteristic of controllable converter valve;Determine the start control strategy based on the first switching timing, the second switching timing and the low-voltage interconnection information;Execute start control signal.The control method of direct current transmission provided by the application solves the coordination problem of voltage source under different start conditions, realizes reliable start under two scenarios of full-station non-start and partial-station non-start, and improves the operation flexibility and start success rate of multi-terminal direct current transmission system.

Description

Technical Field

[0001] This invention relates to the field of DC power transmission control technology, and in particular to a DC power transmission control method and system. Background Technology

[0002] With the continuous expansion of offshore wind power development, especially deep-sea wind power resources, how to efficiently and economically transmit offshore wind power clusters to the onshore power grid has become an important research direction in the field of DC transmission technology. Multi-terminal DC transmission technology can interconnect multiple offshore wind farms with onshore converter stations, and has advantages such as flexible networking and high grid connection reliability, making it an important technical path for transmitting deep-sea wind power clusters.

[0003] A multi-terminal DC transmission scheme with uncontrolled rectifier valves has been proposed in the existing technology. This scheme involves offshore converter stations based on an uncontrolled rectifier valve architecture, while onshore converter stations utilize grid-commutated converters to achieve direct interconnection between the converter stations via high-voltage DC lines. However, uncontrolled rectifier valves cannot autonomously establish AC voltage and lack the ability to regulate the voltage and frequency of the AC system. This makes them unsuitable for complex operating conditions such as strong wind power fluctuations and weak grid access in multi-terminal networking scenarios. Summary of the Invention

[0004] This invention provides a control method and system for DC power transmission to solve the technical problems of autonomous black start, multi-station coordinated start-stop and low-power steady-state operation in multi-terminal heterogeneous DC topologies, so as to achieve reliable system start-up and grid connection, stable operation under off-peak conditions and take into account the economic benefits of the project.

[0005] To address the aforementioned technical problems, embodiments of the present invention provide a control method for DC power transmission, applied to a sea-land DC power transmission topology constructed from an offshore converter station containing uncontrolled rectifier valves and controllable converter valves, and an onshore converter station. The method includes: Obtain the topology data of the aforementioned land-sea DC transmission topology; Based on the topology data, the power transmission equipment data and connection relationships are obtained. The connection relationships are the low-voltage interconnection information between the DC sides of each controllable converter valve of different converter stations in the land-sea DC transmission topology. Based on the first commutation characteristics of the uncontrolled rectifier valve in the power transmission equipment data, a first switching sequence corresponding to the uncontrolled rectifier valve is obtained; based on the second commutation characteristics of the controllable converter valve in the power transmission equipment data, a second switching sequence corresponding to the controllable converter valve is obtained. Based on the first switching timing, the second switching timing, and the low-voltage interconnection information, the start-up control strategy of the land-sea DC transmission topology is determined; Execute the startup control signal generated by the startup control strategy.

[0006] As one preferred embodiment, after executing the startup control signal generated by the startup control strategy, the method further includes: Detect the real-time operating data of the offshore converter station; When the real-time operating data is detected to meet the preset operating standards, the onshore-offshore DC transmission topology is controlled to perform near-zero power operation control.

[0007] As one preferred embodiment, the step of controlling the onshore-offshore DC transmission topology to perform near-zero power operation control when the real-time operating data is detected to meet preset operating standards includes: The real-time transmission power of the offshore converter station is obtained based on the real-time operating data. The real-time transmission power is compared with a preset power threshold. When the real-time transmission power is lower than the preset power threshold, an uncontrolled rectifier valve control signal is generated. Based on the control signal of the uncontrolled rectifier valve, bypass or disconnect the uncontrolled rectifier valve; Maintain the connected state of the controllable converter valve.

[0008] As one preferred embodiment, when all the aforementioned offshore converter stations are in an inactive state, the startup control strategy includes: Reduce the DC line voltage of the onshore converter station to control the conduction of the controllable converter valve in each of the offshore converter stations; When the offshore converter station meets the first power transmission condition, the uncontrolled rectifier valve in each of the offshore converter stations is controlled to be turned on; Increase the DC line voltage of the onshore converter station to the rated value and adjust the power distribution ratio between the uncontrolled rectifier valve and the controllable converter valve to complete the start-up control.

[0009] As one preferred embodiment, when some of the offshore converter stations are in operation and others are not in operation, the startup control strategy includes: Using the controllable converter valve in the offshore converter station that is in operation as a voltage source, the low-voltage DC line between the offshore converter station that is in operation and the offshore converter station that is not in operation is controlled to be connected. By controlling the controllable converter valve in the offshore converter station that is in operation to open the controllable converter valve in the offshore converter station that is not in operation; When the offshore converter station in the non-started state meets the second power transmission condition, the uncontrolled rectifier valve in the offshore converter station is controlled to be turned on; The low-voltage DC line is disconnected to complete the start-up control.

[0010] Another embodiment of the present invention provides a control system for DC power transmission, applied to a sea-land DC power transmission topology constructed by an offshore converter station including an uncontrolled rectifier valve and a controllable converter valve and an onshore converter station, the system comprising: The topology acquisition module is used to acquire the topology data of the land-sea DC transmission topology; The data analysis module is used to obtain power transmission equipment data and connection relationships based on the topology data. The connection relationships are the low-voltage interconnection information between the DC sides of each controllable converter valve of different converter stations in the land-sea DC transmission topology. The timing determination module is used to obtain a first switching timing corresponding to the uncontrolled rectifier valve based on the first commutation characteristics of the uncontrolled rectifier valve in the power transmission equipment data, and to obtain a second switching timing corresponding to the controllable converter valve based on the second commutation characteristics of the controllable converter valve in the power transmission equipment data. The strategy determination module is used to determine the start-up control strategy of the land-sea DC transmission topology based on the first switching timing, the second switching timing, and the low-voltage interconnection information. The signal execution module is used to execute the startup control signal generated by the startup control strategy.

[0011] As one preferred embodiment, after executing the startup control signal generated by the startup control strategy, the system further includes: The data detection module is used to detect the real-time operating data of the offshore converter station; The near-zero power control module is used to control the onshore-offshore DC transmission topology to perform near-zero power operation control when the real-time operating data is detected to meet the preset operating standards.

[0012] As one preferred embodiment, the near-zero power control module includes: The real-time power acquisition unit is used to obtain the real-time transmission power of the offshore converter station based on the real-time operating data. A control signal generation unit is used to compare the real-time transmission power with a preset power threshold, and when the real-time transmission power is lower than the preset power threshold, generate an uncontrolled rectifier valve control signal. An uncontrolled rectifier valve control unit is used to bypass or disconnect the uncontrolled rectifier valve according to the uncontrolled rectifier valve control signal; A controllable converter valve control unit is used to maintain the connected state of the controllable converter valve.

[0013] As one preferred embodiment, when all the aforementioned offshore converter stations are in an inactive state, the strategy determination module includes: A voltage regulation unit is used to reduce the DC line voltage of the onshore converter station in order to control the conduction of the controllable converter valve in each of the offshore converter stations. A valve group control unit is used to control the uncontrolled rectifier valves in each of the offshore converter stations to be turned on when the offshore converter station meets the first power transmission condition. The power distribution unit is used to increase the DC line voltage of the onshore converter station to the rated value and adjust the power distribution ratio between the uncontrolled rectifier valve and the controllable converter valve to complete the start-up control.

[0014] As one preferred embodiment, when some of the offshore converter stations are in operation and others are not in operation, the strategy determination module includes: A low-voltage line control unit is used to control the conduction of the low-voltage DC line between the offshore converter station in operation and the offshore converter station not in operation, using the controllable converter valve in the offshore converter station in operation as a voltage source. A controllable converter valve activation unit is used to control the activation of a controllable converter valve in a non-activated state in the offshore converter station by means of the controllable converter valve in the offshore converter station that is in operation. When the offshore converter station in the non-started state meets the second power transmission condition, the uncontrolled rectifier valve conduction unit controls the uncontrolled rectifier valve in the offshore converter station to conduct. The low-voltage line restoration unit is used to control the disconnection of the low-voltage DC line and complete the start-up control.

[0015] Compared with the prior art, the beneficial effects of the embodiments of the present invention are at least one of the following: (1) This invention collects structural data of the land-sea DC transmission topology, clarifies the equipment parameters of each converter station and the low-voltage interconnection relationship of the controllable converter valves on the DC side, and then generates corresponding switching sequences based on the inherent electrical characteristics of the uncontrolled rectifier valves and controllable converter valves. Combined with the low-voltage interconnection information, it determines the start-up control strategy for the appropriate operating conditions and executes the control commands. This invention forms a complete closed loop from topology data parsing, equipment characteristic matching, precise timing planning to strategy implementation. It is specifically adapted to land-sea DC topologies containing heterogeneous valve groups and low-voltage interconnection channels, making up for the shortcomings of traditional control methods that do not distinguish the operating characteristics of the two types of converter valves and do not combine the start-up and shutdown logic with the low-voltage interconnection between stations.

[0016] (2) This invention divides black starts in actual operating conditions into two categories: no start at all stations and no start at some stations, and configures corresponding control processes for each. On-site, all offshore converter stations can be started synchronously together, and low-voltage interconnection lines between stations can be used to supply power and build up voltage from the operating stations to the stations to be started, thus achieving auxiliary start-up. At the same time, for the low-peak operating conditions with low wind power output, a near-zero power operation mode is specially set up. When the output is low, the uncontrolled rectifier valve is shut down, and only the controllable converter valve is retained to maintain voltage and low power transmission, which is adapted to the current situation of large fluctuations in offshore wind power output and reduces the frequent start-up and shutdown of the units. Under the premise of ensuring reliable start-up and shutdown and stable low-power operation, the advantages of low cost and low loss of the uncontrolled rectifier valve are maximized, while taking into account the actual needs of flexible and easy-to-use on-site control. Attached Figure Description

[0017] Figure 1 This is a flowchart illustrating the control method for DC power transmission in one embodiment of the present invention; Figure 2 This is a schematic diagram of a DC power transmission control system in one embodiment of the present invention.

[0018] Figure label: Among them, 11 is the topology acquisition module, 12 is the data analysis module, 13 is the timing determination module, 14 is the strategy determination module, and 15 is the signal execution module. Detailed Implementation

[0019] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. The purpose of providing these embodiments is to make the disclosure of the present invention more thorough and comprehensive. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0020] In the description of this application, the terms "first," "second," "third," etc., are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. Therefore, a feature defined with "first," "second," "third," etc., may explicitly or implicitly include one or more of that feature. In the description of this application, unless otherwise stated, "a plurality of" means two or more.

[0021] In the description of this application, it should be noted that, unless otherwise expressly specified and limited, the terms "installation," "connection," and "linking" should be interpreted broadly. For example, they can refer to fixed connections, detachable connections, or integral connections; they can refer to mechanical connections or electrical connections; they can refer to direct connections or indirect connections through an intermediate medium; and they can refer to the internal communication between two components. The terms "vertical," "horizontal," "left," "right," "upper," "lower," and similar expressions used herein are for illustrative purposes only and do not indicate or imply that the device or component referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as limiting the invention. The term "and / or" as used herein includes any and all combinations of one or more of the associated listed items. Those skilled in the art can understand the specific meaning of the above terms in this application based on the specific circumstances.

[0022] In the description of this application, it should be noted that, unless otherwise defined, all technical and scientific terms used in this invention have the same meaning as commonly understood by one of ordinary skill in the art. The terminology used in this specification is for the purpose of describing specific embodiments only and is not intended to limit the invention. Those skilled in the art can understand the specific meaning of the above terms in this application based on the specific circumstances.

[0023] One embodiment of the present invention provides a control method for direct current transmission. For details, please refer to [link to relevant documentation]. Figure 1 , Figure 1 The diagram shown is a flowchart illustrating a control method for DC power transmission in one embodiment of the present invention, which includes steps S1 to S5: S1: Obtain the topology data of the land-sea DC transmission topology; In this invention, the actual structure of the onshore-offshore DC transmission topology includes offshore converter stations and onshore converter stations. Each offshore converter station is equipped with two types of valve groups: uncontrolled rectifier valves and controlled converter valves. These two types of valve groups can be connected in series or in parallel to form heterogeneous valve groups. On the DC side of the controlled converter valves of different offshore converter stations, low-voltage submarine cables and series switching switches form low-voltage interconnection channels between stations. The onshore converter station adopts a hybrid full-bridge and half-bridge converter valve, which can output negative levels, flexibly adjust the voltage of the entire DC line network, and take into account start-up voltage reduction, steady-state voltage regulation, and fault voltage support.

[0024] The overall network connection method is that the high-voltage main transmission circuits of all offshore converter stations are connected to the onshore overhead transmission lines through high-voltage submarine cables, and finally connected to the onshore converter stations in a unified manner, forming a multi-terminal offshore-onshore DC transmission architecture with high and low voltage lines and heterogeneous valve groups working together.

[0025] Uncontrolled rectifier valves are integrated with diode devices, possessing high-voltage, high-capacity current-carrying capacity, low construction cost, and low operating power loss. However, they lack the ability to independently establish a DC support voltage and can only passively rectify power delivery by relying on external voltage. Controllable converter valves are integrated with IGBT controllable power electronic devices, configured for low-voltage, low-capacity operation. They can independently establish a DC support voltage and primarily undertake auxiliary power supply during system startup and low-voltage steady-state voltage regulation. Full-bridge and half-bridge hybrid converter valves can rely on the negative level output of their internal full-bridge submodules to achieve a wide range of flexible increases and decreases in the DC line voltage across the entire network, meeting three operational requirements: system startup voltage reduction operation, rated condition voltage regulation operation, and fault condition voltage support operation.

[0026] In this embodiment, the topology data refers to all fixed hardware parameters and wiring logic information, including the rated operating voltage and rated transmission capacity of the uncontrolled rectifier valve and the controlled converter valve, the device integration type, the series or parallel combination of valve groups, the low-voltage submarine cable laying locations, the low-voltage submarine cable matching switch numbers and control permissions, the upper and lower limits of the voltage regulation of the full-bridge and half-bridge hybrid converter valves, and the level output characteristics of the full-bridge submodules. Low-voltage interconnection refers to the low-voltage submarine cable connection loop between the DC sides of the controlled converter valves of different offshore converter stations; this loop is not connected to the high-voltage main power transmission channel.

[0027] Since the control strategy in this embodiment is designed for the unique heterogeneous valve group and low-voltage interconnection channel topology (i.e., the onshore-offshore DC topology) of this invention, the switching sequence of the uncontrolled rectifier valve and the controllable converter valve, the on / off logic of the low-voltage interconnection channel between offshore stations, and the voltage regulation range of the onshore full- and half-bridge hybrid converter valve all need to be strictly matched with the field hardware parameters. If the control system does not collect complete topology data in advance, it cannot distinguish the electrical characteristics of the two types of converter valves, nor can it adapt to the two types of black start conditions: the entire station is not started and some stations are not started. Subsequent valve group switching, voltage regulation, and low-voltage line on / off commands will deviate, and the system will be unable to complete autonomous black start based on its own architecture, nor can it guarantee stable operation at near-zero power during periods of low wind power output.

[0028] Specifically, staff can rely on existing local monitoring and control devices, industrial control acquisition systems, and on-site primary wiring ledgers at the converter stations to read and solidify the rated voltage, rated capacity, and series-parallel configuration of uncontrolled rectifier valves and controlled converter valves in each offshore converter station. Simultaneously, they can verify the proportion of controlled converter valves (not less than 18%, preferably 25%). Based on the actual on-site wiring, they can input the access points of the low-voltage submarine cables on the DC side of the controlled converter valves at each station, clarify the background control permissions of the corresponding interconnecting switches, and simultaneously compile and collect relevant parameters of the voltage regulation range and submodule level output characteristics of the onshore full- and half-bridge hybrid converter valves. Finally, all equipment parameters, wiring relationships, and hardware configuration information are uniformly integrated and archived to solidify a complete and usable topology dataset, providing solid data support for the subsequent accurate determination of valve group switching sequence and matching of two types of black start conditions.

[0029] S2: Based on the topology data, obtain the power transmission equipment data and connection relationships. The connection relationships are the low-voltage interconnection information between the DC sides of each controllable converter valve of different converter stations in the land-sea DC transmission topology. Since the topology data obtained in step S1 includes all kinds of equipment parameters, wiring information and other content, it cannot be directly used to determine the subsequent switching sequence. It is necessary to decompose the overall topology data and separate the independent power transmission equipment data and connection relationships in order to clarify the electrical operating boundaries of the two types of converter valves.

[0030] The data on transmission equipment refers to the core operating parameters of uncontrolled rectifier valves, controlled converter valves, and onshore full-and-half-bridge hybrid converter valves. This includes the rated voltage, rated capacity, and device conduction characteristics of various valve groups, as well as the series and parallel connection methods of uncontrolled rectifier valves and controlled converter valves. It also covers the voltage regulation range parameters of the onshore full-and-half-bridge hybrid converter valves. The connection relationship refers to the interconnection information between the DC sides of the controlled converter valves of each offshore converter station in the onshore-offshore DC transmission topology. This interconnection is achieved through low-voltage submarine cables and matching series switching switches, and is only used for cross-station voltage support during the system startup phase, not for high-power energy transmission in the high-voltage main circuit. It should be noted that in this embodiment, the voltage and capacity of the controlled converter valves account for no less than 18% of the overall heterogeneous valve group. In practical engineering applications, a configuration ratio of 25% is preferred, and this ratio parameter needs to be confirmed simultaneously during the data analysis process in this step.

[0031] Specifically, the topology data obtained in step S1 is categorized and decomposed. First, the parameters of three core equipment categories—uncontrolled rectifier valves, controlled converter valves, and full- and half-bridge converter valves—are extracted to establish an equipment dataset that can directly characterize the electrical capabilities of the valve group. Based on this, the access nodes, contact station ranges, and switching control permissions of the DC-side low-voltage submarine cables of each offshore converter valve are checked against the actual wiring layout on site. The scattered wiring information is then integrated into a unique, clear, and controllable low-voltage interconnection correspondence.

[0032] After disassembly, two types of key information were obtained. The first was the clear electrical parameter boundaries of the two types of converter valves, which are used to generate differentiated switching sequences based on the inherent characteristics of the devices. The second was a clearly defined low-voltage auxiliary start-up path, which is used to reliably achieve cross-station voltage support when the entire station is not started or when only a portion is being restarted.

[0033] S3: Based on the first commutation characteristics of the uncontrolled rectifier valve in the power transmission equipment data, the first switching sequence corresponding to the uncontrolled rectifier valve is obtained; based on the second commutation characteristics of the controllable converter valve in the power transmission equipment data, the second switching sequence corresponding to the controllable converter valve is obtained. In step S2, the data of the power transmission equipment and the low-voltage interconnection relationship have been obtained. In this step, it is necessary to combine the characteristics of the two types of converter valves to formulate differentiated switching sequences. The uncontrolled rectifier valve focuses on "passive transmission", while the controllable converter valve focuses on "active pressure building". The timing of the two must be strictly distinguished to avoid action conflicts and ensure stable voltage support and power transmission during the startup phase, laying the foundation for subsequent operating condition matching.

[0034] The first commutation characteristic refers to the uncontrolled rectifier valve's lack of autonomous shut-off capability; it can only passively conduct, requiring external voltage triggering for commutation. It lacks additional control logic, conducts only under specific voltage conditions, and cannot actively adjust its conduction timing. The second commutation characteristic refers to the controllable converter valve's ability to actively control its conduction and shutdown, adjusting conduction duration and voltage output according to control commands, and quickly responding to the pressure build-up requirements during startup. The switching sequence refers to the preset start-up, operation, and stop times based on the converter valve's electrical characteristics.

[0035] Specifically, considering the lack of autonomous shutdown capability of uncontrolled rectifier valves, a "delayed switching" logic is set based on their rated voltage and capacity parameters. This means that the uncontrolled rectifier valve is only activated after the controlled converter valve has established a stable voltage, preventing startup failure due to insufficient voltage build-up, thus forming the first switching sequence (post-switching). For controlled converter valves with autonomous voltage build-up capability, a "pre-switching" logic is set, prioritizing their activation during startup to establish the base voltage, forming the second switching sequence (pre-switching).

[0036] S4: Based on the first switching sequence, the second switching sequence, and low-voltage interconnection information, determine the start-up control strategy for the land-sea DC transmission topology; The switching timing and low-voltage interconnection information obtained in step S3 need to be transformed into actionable startup operations to enable the system to start autonomously. However, there are two core startup scenarios (completely not running / partially running), and the voltage regulation and valve group switching logic are completely different in these scenarios, making a unified strategy insufficient. Therefore, a targeted startup control strategy needs to be developed by combining the determined first and second switching timings with the low-voltage interconnection information.

[0037] In multi-terminal systems, the startup timing of different offshore converter stations often differs. If a converter station fails to start synchronously due to an under-construction wind farm or maintenance, its controllable valves will not receive initial voltage from the onshore converter station upon subsequent startup. This is because the onshore converter station has already established a high-voltage DC connection with the operational offshore converter station, and the DC line voltage has risen to its rated value, making voltage reduction impossible. In this case, low-voltage interconnection lines are needed to provide voltage support from the controllable valves of the operational station to the controllable valves of the station awaiting startup. Low-voltage interconnection information indicates which converter stations have such connections and serves as the basis for inter-station coordinated startup.

[0038] Among them, the startup control strategy refers to the operational plan that can be directly implemented based on the switching sequence and low-voltage interconnection information for different startup scenarios (fully not running / partially running). Its core purpose is to guide the startup process of offshore converter stations and ensure that the startup process is orderly and stable.

[0039] In this embodiment, this step is implemented through a strategy generation module, which is built into the central controller of the DC transmission control system and executed by software code. The core of the module is a rule-based decision-making program. The inputs include the first switching sequence, the second switching sequence, low-voltage interconnection information, and the real-time status of each offshore converter station. The output is a structured startup control strategy.

[0040] First, the status flags of each offshore converter station are read. These status flags are reported in real time by the control system of each converter station via fiber optic communication, and the central controller receives a status update every 10 milliseconds. The status flags are divided into three categories: "Not in operation" indicates that all converter valves of the station are in the locked state and the AC side circuit breaker is open; "Standby" indicates that the controllable converter valve has completed pre-charging and is ready to unlock; "Operating" indicates that at least one converter valve is in the unlocked state and is transmitting power.

[0041] The module iterates through and statistically analyzes the status of all offshore converter stations. If all stations are in "not running," the module determines it's entering a full-station startup scenario and invokes a pre-defined full-station startup process template. If at least one station is in "running" and at least one station is in "not running," the module determines it's entering a partial-station startup scenario and invokes a pre-defined partial-station startup process template. If there are neither "running" nor "not running" stations (e.g., all are in "standby"), the module determines the system is in an intermediate state and waits for a status change before making a decision. After invoking the corresponding template, the module needs to replace the placeholders in the template with the actual parameters.

[0042] The core logic of the full-station start-up template is: first reduce the voltage, then activate the controllable valve, and finally activate the uncontrollable valve. The voltage reduction target value and voltage increase rate in the template are calculated based on the system rated voltage.

[0043] Preferably, in one embodiment of the present invention, when all offshore converter stations are in an inactive state, the activation control strategy includes: Reduce the DC line voltage of the onshore converter station to control the conduction of the controllable converter valves in each offshore converter station; When the offshore converter station meets the first power transmission condition, the uncontrolled rectifier valves in each offshore converter station are turned on. Increase the DC line voltage of the onshore converter station to the rated value and adjust the power distribution ratio between the uncontrolled rectifier valve and the controlled converter valve to complete the startup control.

[0044] Specifically, firstly, the control system reduces the DC line voltage of the onshore converter station. The reason for reducing the voltage is that unlocking the controllable converter valve requires a suitable DC side voltage; excessively high voltage may damage internal valve components, while excessively low voltage will prevent the submodule capacitor from charging. In this embodiment, the voltage is reduced to 5% to 10% of the rated value. This range satisfies the unlocking requirements of the controllable valve while ensuring a smooth subsequent voltage boosting process.

[0045] After the voltage drops, the control system issues a turn-on command to the controllable converter valves in each offshore converter station. Once the controllable converter valve detects that the DC-side voltage has reached the unlocking threshold, it closes the charging resistor to charge the submodule capacitor. After the capacitor voltage stabilizes, it bypasses the charging resistor, triggering the fully controlled device to output AC voltage. As the AC voltage gradually increases, the AC bus voltage of the offshore wind farm is established, and the wind turbines begin grid-connected power generation.

[0046] When the offshore converter station meets the first power transmission condition, the control system issues a command to turn on the uncontrolled rectifier valve. The first power transmission condition means that the voltage and frequency of the offshore AC system have reached their rated values, the wind turbines have been connected to the grid, and their output power has stabilized. At this time, the AC side has sufficient commutation voltage, and the uncontrolled rectifier valve can turn on naturally. The rated value is the rated voltage of the DC line determined during system design, such as ±320kV, ±400kV, or ±525kV, with the specific value depending on the scale of the project.

[0047] After the uncontrolled rectifier valve is turned on, the control system gradually increases the DC line voltage of the onshore converter station to the rated value, while simultaneously adjusting the power distribution ratio between the uncontrolled rectifier valve and the controlled converter valve. The boost rate is typically 1% to 2% of the rated voltage per second. Power distribution is achieved by adjusting the output voltage of the controlled converter valve: as the output voltage of the controlled valve decreases, its power load decreases, and the power load of the uncontrolled valve increases; conversely, the output voltage increases. In the final steady state, the uncontrolled valve carries most of the transmission power, and the controlled valve carries a small portion of the power.

[0048] The core logic of the partial station startup template is as follows: First, the low-voltage line is connected, and the controllable valve at the operating end provides voltage to the controllable valve at the end to be started. After the controllable valve at the starting end is unlocked, the uncontrolled valve at that end is then engaged. The switch control commands in the template directly reference the switch identifiers recorded in the low-voltage interconnection information to ensure that the commands are sent to the correct execution unit.

[0049] Preferably, in one embodiment of the present invention, when some offshore converter stations are in operation and others are not in operation, the activation control strategy includes: Using the controllable converter valve in the offshore converter station that is in operation as a voltage source, the low-voltage DC line between the offshore converter station that is in operation and the offshore converter station that is not in operation is controlled to conduct. By controlling the controllable converter valve in the offshore converter station that is in operation to open the controllable converter valve in the offshore converter station that is not in operation. When the offshore converter station in the non-started state meets the second power transmission condition, the uncontrolled rectifier valve in the offshore converter station is turned on. Disconnect the low-voltage DC line to complete the startup control.

[0050] When some offshore converter stations are in operation and others are not, the control system uses the controllable converter valves in the operating stations as a voltage source to connect the low-voltage DC lines between those stations and the non-operating stations. At this time, the DC voltage of the onshore converter stations has already risen to the rated value and cannot be lowered further to start other stations, while the controllable valves of the operating stations have a stable DC voltage on their DC sides, which can be used as a low-voltage power supply.

[0051] After the low-voltage line is connected, the DC voltage of the controllable valves in the operating station is transferred to the DC side of the controllable valves in the non-operating station. The control system controls the controllable valves in the non-operating station to open, and its unlocking process is the same as the whole station startup scenario: after detecting the DC side voltage, it charges, unlocks, outputs AC voltage, establishes the AC bus voltage of the station, and completes the grid connection of the wind turbine.

[0052] Once the voltage and frequency of the offshore AC system at the unstarted station reach their rated values ​​and the wind turbines are connected to the grid, the control system activates the uncontrolled rectifier valve at that station. After the uncontrolled valve is activated, the control system disconnects the low-voltage DC line. At this point, the station to be started is fully connected to the system. Disconnecting the low-voltage line prevents the controllable valves of the two converter stations from operating in parallel on the DC side for extended periods, thus preventing circulating currents and power oscillations.

[0053] The criteria for determining the first and second power transmission conditions are the same (both involve AC voltage and frequency reaching rated values ​​and wind turbines being connected to the grid). The difference lies in the voltage source: the voltage in the first scenario is provided by the onshore converter station after voltage reduction, while the voltage in the second scenario is provided by the controllable valve of the operating station through a low-voltage line. Due to the different voltage sources, the unlocking conditions of the controllable valve and the methods for ensuring voltage stability during startup also differ. Therefore, "first" and "second" are used to distinguish them during strategy generation to ensure that control commands are directed to the correct voltage source and line switch.

[0054] S5: Execute the startup control signal generated by the startup control strategy.

[0055] After startup control is completed, the system enters normal operation. The control system continuously monitors real-time operating data of each offshore converter station, including DC voltage, DC current, transmission power, AC voltage, and AC frequency. Monitoring is achieved through the internal control and protection systems of each converter station. Each converter station is equipped with voltage and current transformers, and the measured signals are transmitted to the central controller via optical fiber after analog-to-digital conversion. The central controller updates the data every 10 milliseconds and compares it with preset operating standards.

[0056] Preferably, in one embodiment of the present invention, after executing the startup control signal generated by the startup control strategy, the method further includes: Monitor the real-time operation data of the offshore converter station; When the real-time operating data is detected to meet the preset operating standards, the control system will execute near-zero power operation control for the onshore-offshore DC transmission topology.

[0057] Among the preset operating standards, the most critical is the power transmission threshold. This threshold is determined based on the minimum stable operating power of the uncontrolled rectifier valve. The uncontrolled rectifier valve is composed of diodes, and its conduction requires sufficient commutation voltage from the AC side. When the transmission power falls below a certain value, the AC side voltage drops, the commutation margin is insufficient, and commutation failure may occur. This threshold is typically taken as 5% to 10% of the rated transmission power, and the specific value is determined through simulation calculations based on the valve model and system parameters.

[0058] Preferably, in one embodiment of the present invention, when real-time operating data is detected to meet preset operating standards, the onshore-offshore DC transmission topology is controlled to perform near-zero power operation control, including: The real-time transmission power of the offshore converter station is obtained based on real-time operating data. The real-time transmission power is compared with a preset power threshold. When the real-time transmission power is lower than the preset power threshold, an uncontrolled rectifier valve control signal is generated. Based on the control signal of the uncontrolled rectifier valve, bypass or disconnect the uncontrolled rectifier valve; Maintain the connected state of the controllable converter valve.

[0059] Near-zero power operation refers to an operating mode where, when the output of an offshore wind farm is extremely low, the uncontrolled rectifier valve is bypassed or disconnected, and only the controlled converter valve transmits power. Because the operating characteristics of uncontrolled rectifier valves and controlled converter valves differ under low-power conditions, uncontrolled rectifier valves, composed of diodes, require sufficient commutation voltage from the AC side to operate normally. The lower the power, the lower the AC voltage, and the smaller the commutation margin. When the power falls below a certain critical value, the commutation margin is insufficient, the diodes cannot reliably turn off, commutation failure occurs, and the system trips. Controlled converter valves, using fully controlled devices, regulate power by actively controlling the amplitude and phase of the output voltage, independent of the AC system's commutation voltage. Even at extremely low power, they can maintain stable DC voltage and current.

[0060] Therefore, when the transmission power is too low, the uncontrolled rectifier valve will shut down, and the power will be transmitted only by the controllable commutator valve, which can avoid commutation failure and achieve stable operation under low power.

[0061] The start-up condition for near-zero power operation is that the transmission power falls below a preset threshold, which is set based on the minimum stable operating power of the uncontrolled rectifier valve. The minimum stable operating power of the uncontrolled rectifier valve depends on its commutation margin, which is measured by the arc-extinguishing angle. The smaller the arc-extinguishing angle, the higher the risk of commutation failure. In engineering, the safe lower limit of the arc-extinguishing angle is usually set at around 15 degrees. Through simulation calculations, the minimum transmission power corresponding to an arc-extinguishing angle of 15 degrees can be found. This power value is typically 5% to 10% of the rated transmission power, with the specific value varying depending on the valve model and system parameters. In actual engineering, the threshold setting will have a certain margin. For example, if the simulation calculation shows that the minimum stable power is 5% of the rated power, the threshold can be set to 8% to ensure early switching before the power drops to the danger zone.

[0062] Specifically, when the control system detects that the transmission power is lower than a preset threshold, it generates a control signal for the uncontrolled rectifier valve. The type of control signal depends on the configuration of the uncontrolled rectifier valve. If the valve assembly is equipped with a bypass switch, a bypass signal is generated to short-circuit the valve assembly from the transmission circuit; if no bypass switch is equipped, a disconnect signal is generated to disconnect the valve assembly via a DC circuit breaker.

[0063] Then, the controllable converter valve remains connected. After the uncontrolled valve is deactivated, the controllable valve maintains its original operating state and independently undertakes all transmission power. At this time, the operation mode of the controllable valve is the same as that of conventional flexible DC transmission, maintaining a constant DC voltage by adjusting the output voltage, and the current naturally adjusts with the power.

[0064] Another embodiment of the present invention provides a control system for DC power transmission; for details, please refer to [link to relevant documentation]. Figure 2 , Figure 2The diagram shown illustrates a control system for DC power transmission according to one embodiment of the present invention, comprising: Topology acquisition module 11 is used to acquire topology data of the sea-land DC transmission topology; Data analysis module 12 is used to obtain power transmission equipment data and connection relationships based on topology data. The connection relationships are the low-voltage interconnection information between the DC sides of each controllable converter valve of different converter stations in the land-sea DC transmission topology. The timing determination module 13 is used to obtain the first switching timing corresponding to the uncontrolled rectifier valve based on the first commutation characteristics of the uncontrolled rectifier valve in the power transmission equipment data, and to obtain the second switching timing corresponding to the controllable converter valve based on the second commutation characteristics of the controllable converter valve in the power transmission equipment data. The strategy determination module 14 is used to determine the start-up control strategy of the land-sea DC transmission topology based on the first switching sequence, the second switching sequence and low-voltage interconnection information. The signal execution module 15 is used to execute the startup control signal generated by the startup control strategy.

[0065] Preferably, in one embodiment of the present invention, after executing the startup control signal generated by the startup control strategy, the system further includes: The data detection module is used to detect the real-time operating data of the offshore converter station; The near-zero power control module is used to control the onshore-offshore DC transmission topology to perform near-zero power operation control when the real-time operating data is detected to meet the preset operating standards.

[0066] Preferably, in one embodiment of the present invention, the near-zero power control module includes: The real-time power acquisition unit is used to obtain the real-time transmission power of the offshore converter station based on real-time operating data. The control signal generation unit is used to compare the real-time transmission power with the preset power threshold. When the real-time transmission power is lower than the preset power threshold, it generates an uncontrolled rectifier valve control signal. The uncontrolled rectifier valve control unit is used to bypass or disconnect the uncontrolled rectifier valve according to the uncontrolled rectifier valve control signal; The controllable converter valve control unit is used to maintain the connected state of the controllable converter valve.

[0067] Preferably, in one embodiment of the present invention, when all offshore converter stations are in an inactive state, the strategy determination module includes: The voltage regulation unit is used to reduce the DC line voltage of the onshore converter station in order to control the conduction of the controllable converter valves in each offshore converter station. The valve group control unit is used to control the uncontrolled rectifier valves in each offshore converter station to open when the first power transmission condition is met. The power distribution unit is used to raise the DC line voltage of the onshore converter station to the rated value and adjust the power distribution ratio between the uncontrolled rectifier valve and the controlled converter valve to complete the start-up control.

[0068] Preferably, in one embodiment of the present invention, when some offshore converter stations are in operation and others are not in operation, the strategy determination module includes: The low-voltage line control unit is used to control the conduction of the low-voltage DC line between the operating offshore converter station and the non-operating offshore converter station, using the controllable converter valve in the operating offshore converter station as a voltage source. The controllable converter valve conduction unit is used to control the conduction of the controllable converter valve in the offshore converter station that is not in operation through the controllable converter valve in the offshore converter station that is in operation. When the offshore converter station, which is in an unstarted state, meets the second power transmission condition, the uncontrolled rectifier valve conduction unit controls the uncontrolled rectifier valve in the offshore converter station to conduct. The low-voltage line restoration unit is used to control the disconnection of the low-voltage DC line and complete the start-up control.

[0069] Compared with the prior art, the beneficial effects of the embodiments of the present invention are at least one of the following: (1) This invention collects structural data of the land-sea DC transmission topology, clarifies the equipment parameters of each converter station and the low-voltage interconnection relationship of the controllable converter valves on the DC side, and then generates corresponding switching sequences based on the inherent electrical characteristics of the uncontrolled rectifier valves and controllable converter valves. Combined with the low-voltage interconnection information, it determines the start-up control strategy for the appropriate operating conditions and executes the control commands. This invention forms a complete closed loop from topology data parsing, equipment characteristic matching, precise timing planning to strategy implementation. It is specifically adapted to land-sea DC topologies containing heterogeneous valve groups and low-voltage interconnection channels, making up for the shortcomings of traditional control methods that do not distinguish the operating characteristics of the two types of converter valves and do not combine the start-up and shutdown logic with the low-voltage interconnection between stations.

[0070] (2) This invention divides black starts in actual operating conditions into two categories: no start at all stations and no start at some stations, and configures corresponding control processes for each. On-site, all offshore converter stations can be started synchronously together, and low-voltage interconnection lines between stations can be used to supply power and build up voltage from the operating stations to the stations to be started, thus achieving auxiliary start-up. At the same time, for the low-peak operating conditions with low wind power output, a near-zero power operation mode is specially set up. When the output is low, the uncontrolled rectifier valve is shut down, and only the controllable converter valve is retained to maintain voltage and low power transmission, which is adapted to the current situation of large fluctuations in offshore wind power output and reduces the frequent start-up and shutdown of the units. Under the premise of ensuring reliable start-up and shutdown and stable low-power operation, the advantages of low cost and low loss of the uncontrolled rectifier valve are maximized, while taking into account the actual needs of flexible and easy-to-use on-site control.

[0071] The embodiments described above are merely illustrative of several implementations of the present invention, and while the descriptions are specific and detailed, they should not be construed as limiting the scope of the present invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these modifications and improvements all fall within the scope of protection of the present invention. Therefore, the scope of protection of this patent should be determined by the appended claims.

Claims

1. A control method for direct current transmission, characterized in that, The method, applied to a sea-to-land DC transmission topology comprised of an offshore converter station containing uncontrolled rectifier valves and controlled converter valves, and an onshore converter station, includes: Obtain the topology data of the aforementioned land-sea DC transmission topology; Based on the topology data, the power transmission equipment data and connection relationships are obtained. The connection relationships are the low-voltage interconnection information between the DC sides of each controllable converter valve of different converter stations in the land-sea DC transmission topology. Based on the first commutation characteristics of the uncontrolled rectifier valve in the power transmission equipment data, a first switching sequence corresponding to the uncontrolled rectifier valve is obtained; based on the second commutation characteristics of the controllable converter valve in the power transmission equipment data, a second switching sequence corresponding to the controllable converter valve is obtained. Based on the first switching timing, the second switching timing, and the low-voltage interconnection information, the start-up control strategy of the land-sea DC transmission topology is determined; Execute the startup control signal generated by the startup control strategy.

2. The control method for DC power transmission as described in claim 1, characterized in that, After executing the startup control signal generated by the startup control strategy, the method further includes: Detect the real-time operating data of the offshore converter station; When the real-time operating data is detected to meet the preset operating standards, the onshore-offshore DC transmission topology is controlled to perform near-zero power operation control.

3. The control method for DC power transmission as described in claim 2, characterized in that, When the real-time operating data is detected to meet the preset operating criteria, the near-zero power operation control of the onshore-offshore DC transmission topology is controlled, including: The real-time transmission power of the offshore converter station is obtained based on the real-time operating data. The real-time transmission power is compared with a preset power threshold. When the real-time transmission power is lower than the preset power threshold, an uncontrolled rectifier valve control signal is generated. Based on the control signal of the uncontrolled rectifier valve, bypass or disconnect the uncontrolled rectifier valve; Maintain the connected state of the controllable converter valve.

4. The control method for DC power transmission as described in claim 1, characterized in that, When all of the aforementioned offshore converter stations are in a non-operational state, the startup control strategy includes: Reduce the DC line voltage of the onshore converter station to control the conduction of the controllable converter valve in each of the offshore converter stations; When the offshore converter station meets the first power transmission condition, the uncontrolled rectifier valve in each of the offshore converter stations is controlled to be turned on; Increase the DC line voltage of the onshore converter station to the rated value and adjust the power distribution ratio between the uncontrolled rectifier valve and the controllable converter valve to complete the start-up control.

5. The control method for DC power transmission as described in claim 1, characterized in that, When some of the offshore converter stations are in operation and others are not in operation, the startup control strategy includes: Using the controllable converter valve in the offshore converter station that is in operation as a voltage source, the low-voltage DC line between the offshore converter station that is in operation and the offshore converter station that is not in operation is controlled to be connected. By controlling the controllable converter valve in the offshore converter station that is in operation to open the controllable converter valve in the offshore converter station that is not in operation; When the offshore converter station in the non-started state meets the second power transmission condition, the uncontrolled rectifier valve in the offshore converter station is controlled to be turned on; The low-voltage DC line is disconnected to complete the start-up control.

6. A control system for direct current transmission, characterized in that, The system is applied to a sea-land DC transmission topology consisting of an offshore converter station containing uncontrolled rectifier valves and controlled converter valves, and an onshore converter station. The system includes: The topology acquisition module is used to acquire the topology data of the land-sea DC transmission topology; The data analysis module is used to obtain power transmission equipment data and connection relationships based on the topology data. The connection relationships are the low-voltage interconnection information between the DC sides of each controllable converter valve of different converter stations in the land-sea DC transmission topology. The timing determination module is used to obtain a first switching timing corresponding to the uncontrolled rectifier valve based on the first commutation characteristics of the uncontrolled rectifier valve in the power transmission equipment data, and to obtain a second switching timing corresponding to the controllable converter valve based on the second commutation characteristics of the controllable converter valve in the power transmission equipment data. The strategy determination module is used to determine the start-up control strategy of the land-sea DC transmission topology based on the first switching timing, the second switching timing, and the low-voltage interconnection information. The signal execution module is used to execute the startup control signal generated by the startup control strategy.

7. The DC power transmission control system as described in claim 6, characterized in that, After executing the startup control signal generated by the startup control strategy, the system further includes: The data detection module is used to detect the real-time operating data of the offshore converter station; The near-zero power control module is used to control the onshore-offshore DC transmission topology to perform near-zero power operation control when the real-time operating data is detected to meet the preset operating standards.

8. The DC power transmission control system as described in claim 7, characterized in that, The near-zero power control module includes: The real-time power acquisition unit is used to obtain the real-time transmission power of the offshore converter station based on the real-time operating data. A control signal generation unit is used to compare the real-time transmission power with a preset power threshold, and when the real-time transmission power is lower than the preset power threshold, generate an uncontrolled rectifier valve control signal. An uncontrolled rectifier valve control unit is used to bypass or disconnect the uncontrolled rectifier valve according to the uncontrolled rectifier valve control signal; A controllable converter valve control unit is used to maintain the connected state of the controllable converter valve.

9. A DC power transmission control system as described in claim 6, characterized in that, When all the aforementioned offshore converter stations are in an inactive state, the strategy determination module includes: A voltage regulation unit is used to reduce the DC line voltage of the onshore converter station in order to control the conduction of the controllable converter valve in each of the offshore converter stations. A valve group control unit is used to control the uncontrolled rectifier valves in each of the offshore converter stations to be turned on when the offshore converter station meets the first power transmission condition. The power distribution unit is used to increase the DC line voltage of the onshore converter station to the rated value and adjust the power distribution ratio between the uncontrolled rectifier valve and the controllable converter valve to complete the start-up control.

10. A DC power transmission control system as described in claim 6, characterized in that, When some of the offshore converter stations are in operation and others are not in operation, the strategy determination module includes: A low-voltage line control unit is used to control the conduction of the low-voltage DC line between the offshore converter station in operation and the offshore converter station not in operation, using the controllable converter valve in the offshore converter station in operation as a voltage source. A controllable converter valve activation unit is used to control the activation of a controllable converter valve in a non-activated state in the offshore converter station by means of the controllable converter valve in the offshore converter station that is in operation. When the offshore converter station in the non-started state meets the second power transmission condition, the uncontrolled rectifier valve conduction unit controls the uncontrolled rectifier valve in the offshore converter station to conduct. The low-voltage line restoration unit is used to control the disconnection of the low-voltage DC line and complete the start-up control.

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