Offshore wind power direct current transmission system, auxiliary control method, medium and product

Abstract

The invention relates to the technical field of offshore wind power, and discloses an offshore wind power direct current power transmission system, an auxiliary control method, a medium and a product. In an offshore converter station, an auxiliary starting device is connected with a valve side connecting line of an offshore MMC converter valve in parallel through a second switch, and a third switch is arranged between a connecting point and a first transformer; therefore, when the second switch is switched on and the third switch is switched off, soft start boosting is only carried out on the valve side connecting line and the offshore MMC area, and low-voltage protection of other areas is prevented from being triggered. And meanwhile, intermediate connection links are reduced, and the difficulty in transforming the existing control and protection system is reduced. Furthermore, through parallel connection, the reactive capacity configuration of the auxiliary starting device can be smaller, and the economical efficiency is improved.

Description

Technical Field

[0001] The present invention relates to the technical field of offshore wind power, and in particular to an offshore wind power direct current transmission system and an auxiliary control method, medium, and product. Background Art

[0002] Because deep-sea wind farms are remote from land, flexible direct current (HVDC) transmission technology, with its low line losses and large transmission capacity, effectively solves the challenges of long-distance transmission, becoming a leading method for deep-sea wind power transmission. However, this technology faces challenges related to the weight, size, and cost of offshore platforms. Diode hybrid valve technology combines low-cost, high-efficiency uncontrollable diodes with a small number of turn-off devices, reducing converter station construction costs and providing a solution for low-cost, highly reliable, and low-cost deep-sea wind power transmission. However, offshore converter valves based on diode hybrid valve technology face the challenge of black starts due to the unidirectional conductivity of the diode valves.

[0003] Currently, diode-based offshore flexible DC transmission schemes fall into two categories. One involves a pure diode valve scheme at the offshore station. In this scheme, the system's black start requires offshore wind turbines to have grid-connected capabilities, but current wind turbines generally operate in a grid-connected manner. The other involves a hybrid valve scheme based on diode valves and auxiliary modular multilevel converter valves (MMCs). In this scheme, during normal system operation, a small-capacity MMC provides the AC voltage for the offshore AC system. When a hybrid valve employs a diode valve in series with a small-capacity MMC, the DC pole voltage of the modular multilevel converter valve can be effectively reduced, reducing the number of submodules. However, this poses the challenge of the small-capacity MMC's ability to establish the offshore AC voltage. A typical solution involves configuring an auxiliary starting device in parallel with the offshore AC busbar, which establishes the AC busbar voltage to provide temporary power to the small-capacity MMC.

[0004] However, due to factors such as low utilization and economic efficiency, auxiliary starting devices are typically configured with a smaller capacity. When establishing offshore AC voltage to power small-capacity MMCs, they are less able to withstand the surges of reactive charging currents in system transformers and cables. Therefore, the auxiliary starting device should be soft-started and boosted. However, this soft-start boost process can easily trigger the system's undervoltage protection, leading to startup failure and requiring significant modifications to existing control and protection strategies. Summary of the Invention

[0005] In view of this, the present invention provides an offshore wind power direct current transmission system and auxiliary control method, medium, and product to solve the problem that the soft start boost process of the auxiliary starting device is prone to triggering the low-voltage protection of the protection system, resulting in startup failure and requiring major modifications to the existing control and protection strategy.

[0006] In a first aspect, the present invention provides an offshore wind power direct current transmission system, the system comprising: an onshore power grid, an onshore converter station, a direct current submarine cable, an offshore converter station, a first switch, and an offshore wind farm;

[0007] The onshore power grid is connected to the onshore converter station, the onshore converter station is connected to the offshore converter station through a DC submarine cable, and the offshore converter station is connected to the offshore wind farm; the offshore converter station includes an offshore MMC converter valve, an auxiliary starting device, a second switch, a third switch, a first transformer and an offshore AC busbar, and the offshore AC busbar is connected to the offshore wind farm through the first switch; the auxiliary starting device is connected in parallel with the valve-side connecting line of the offshore MMC converter valve through the second switch, and the valve-side connecting line of the offshore MMC converter valve is connected to the first transformer through the third switch; the auxiliary starting device is used to establish an AC voltage on the valve-side connecting line through a soft start boost method when the second switch is closed and the third switch is open, so as to power the offshore MMC converter valve.

[0008] In the offshore wind power DC transmission system provided by the present invention, an auxiliary starting device in the offshore converter station is connected in parallel to the valve-side connection line of the offshore MMC converter valve via a second switch. A third switch is provided between the connection point and the first transformer. When the second switch is closed and the third switch is open, soft-start boosting is performed only on the valve-side connection line and the offshore MMC area, avoiding triggering low-voltage protection in other areas. This also reduces intermediate connection links, simplifying the modification of existing control and protection systems. Furthermore, the parallel connection allows for a smaller reactive capacity configuration for the auxiliary starting device, improving economic efficiency.

[0009] In an optional embodiment, the auxiliary starting device includes: a second transformer, a converter, a fourth switch and a power supply, the second transformer is connected to the converter, and the converter is connected to the power supply through the fourth switch.

[0010] The offshore wind power direct current transmission system provided by the present invention can realize voltage smoothing regulation in the soft start and voltage boost process through the converter, thereby avoiding hard start impact.

[0011] In an optional embodiment, the offshore converter station further includes a third transformer, a fourth transformer, a fifth switch, a sixth switch and a seventh switch; the third transformer is connected to the offshore AC bus through the fifth switch, the fourth transformer is connected to the offshore AC bus through the sixth switch, and the first transformer is connected to the offshore AC bus through the seventh switch.

[0012] The offshore wind power DC transmission system provided by the present invention achieves flexible connection between different transformers and the offshore AC busbar through the fifth to seventh switches. Furthermore, during system startup, the closing sequence of each switch can be controlled to achieve step-by-step power supply control for the third transformer, the fourth transformer, and the first transformer. This avoids the impact of large reactive charging currents on the system caused by multiple transformers receiving power simultaneously, thereby improving the stability of the startup process. Furthermore, the flexible switch configuration enables each transformer to be independently activated or deactivated according to operational requirements. When some equipment fails, the corresponding switch can be disconnected for isolated maintenance without affecting the normal operation of other equipment, thereby improving the system's reliability and operational flexibility.

[0013] In an optional embodiment, the offshore converter station further includes: a first diode converter valve and a second diode converter valve; the DC sides of the offshore MMC converter valve, the first diode converter valve, and the second diode converter valve are connected in series, and the AC sides of the offshore MMC converter valve, the first diode converter valve, and the second diode converter valve are connected in parallel; the first diode converter valve is connected to the third transformer, and the second diode converter valve is connected to the fourth transformer.

[0014] The offshore wind power DC transmission system provided by the present invention features an offshore MMC converter valve connected in series with the DC side of a first diode converter valve and a second diode converter valve. This allows the uncontrollable unidirectional conduction characteristics of the first and second diode converter valves to share the DC pole voltage, significantly reducing the number of submodules in the offshore MMC converter valves and lowering the hardware cost and construction difficulty of the converter station. Furthermore, the AC-side parallel connection structure allows an auxiliary starting device to independently power the offshore MMC converter valves, effectively resolving the black start problem caused by the unidirectional conduction of power from the diode valves. Furthermore, this series connection is compatible with the low-loss characteristics of flexible DC transmission, ensuring long-distance transmission of deep-sea wind power while achieving a balance between cost and reliability through hybrid valve technology.

[0015] In an optional embodiment, the onshore converter station includes an onshore MMC converter valve, wherein the onshore MMC converter valve is connected to the first diode converter valve and the second diode converter valve respectively through a DC submarine cable.

[0016] The offshore wind power DC transmission system provided by this invention utilizes an onshore MMC converter valve connected to first and second offshore diode converter valves via a DC submarine cable. This system's startup time is shortened by leveraging the large capacity of the onshore MMC converter valve and the offshore converter station's auxiliary startup process. Furthermore, the onshore MMC converter valve supports bidirectional power transmission, efficiently transmitting offshore wind power to the onshore power grid via the DC submarine cable while also providing voltage support for the offshore converter station during the startup phase, addressing the demand for deep-sea wind power transmission.

[0017] In a second aspect, the present invention provides an auxiliary startup control method for an offshore wind power DC transmission system, which is applied to the offshore wind power DC transmission system according to the first aspect or any corresponding embodiment thereof; the method comprises:

[0018] The auxiliary starting device is controlled to establish an AC voltage on the valve side connecting line through a soft start boost method to power the offshore MMC converter valve; when it is detected that the DC side voltage of the offshore MMC converter valve rises to a first preset value, the onshore MMC converter valve is controlled to establish a DC submarine cable voltage; when it is detected that the DC submarine cable voltage is the rated value, the offshore MMC converter valve is controlled to output the AC voltage on the valve side connecting line to the offshore AC bus, and the first diode converter valve and the second diode converter valve are controlled to output AC voltage to the offshore AC bus respectively; when it is detected that the AC measured voltage of the offshore AC bus is greater than the second preset value, the wind turbines in the offshore wind farm are controlled to start.

[0019] The auxiliary startup control method for the offshore wind power DC transmission system provided by the present invention performs voltage boosting through a soft-start boosting method, thereby avoiding the generation of impact current on the valve-side connecting line, the offshore MMC converter valve and related equipment, and reducing the reactive charging current peak of the transformer and cable. At the same time, the auxiliary startup device only acts on the local area of ​​the valve-side connecting line, is compatible with the existing control and protection system, and is not easy to trigger the low-voltage protection of the entire system. Furthermore, with the offshore MMC DC voltage as the triggering condition, the startup sequence coordination of the onshore and offshore converter stations is achieved. At the same time, the onshore MMC converter valve is controlled to establish the DC submarine cable voltage, thereby avoiding power imbalance when the offshore MMC is not ready due to premature voltage buildup, and also avoiding delaying the startup process due to late voltage buildup, thereby ensuring that the voltage establishment of the DC transmission link is accurately matched with the charging process of the offshore converter station. Furthermore, after the DC submarine cable voltage stabilizes, the offshore MMC and diode commutator valves synchronously supply power to the offshore AC busbar. This leverages the controllability of the offshore MMC to maintain AC voltage stability while also sharing the power transmission burden with the low-loss characteristics of the diode commutator valves. This also gradually expands the power supply area to the offshore AC busbar, avoiding the impact of simultaneous power supply to the entire system. Finally, using the AC busbar voltage threshold as the wind turbine startup condition ensures stable grid voltage when the wind turbine is connected, preventing wind turbine startup failures or grid connection shocks caused by low voltage.

[0020] In an optional implementation, the method further includes: acquiring an active power loss value; and controlling the power output of the auxiliary starting device based on the active power loss value.

[0021] The auxiliary starting control method for an offshore wind power direct current transmission system provided by the present invention can control the auxiliary starting device to accurately output corresponding power to compensate for the active power consumption of equipment such as transformers, cables and MMC submodules by obtaining the active power loss value, thereby avoiding the system power shortage caused by blind power output in traditional solutions, preventing DC voltage drop or starting failure, and ensuring a smooth transition of power relay from "auxiliary starting device → offshore MMC → wind turbine" in the subsequent starting process.

[0022] In an optional embodiment, the method further includes: when the output power of the wind turbine in the offshore wind farm meets the preset power, controlling the auxiliary starting device to perform reactive power compensation according to the demand of the offshore converter station.

[0023] The assisted startup control method for an offshore wind power DC transmission system provided by this invention controls the auxiliary startup device to switch to reactive power compensation mode when the wind turbines in the offshore wind farm reach a preset power output. This prevents idle equipment, improves equipment utilization, and reduces system costs. Furthermore, the reactive power output is dynamically adjusted according to the needs of the offshore station, maintaining AC voltage stability and supporting full turbine power operation. Therefore, the implementation of this invention solves the problem of low utilization of auxiliary startup devices in the prior art.

[0024] In a third aspect, the present invention provides a computer-readable storage medium having computer instructions stored thereon, the computer instructions being used to enable a computer to execute the auxiliary startup control method for an offshore wind power direct current transmission system according to the second aspect or any corresponding embodiment thereof.

[0025] In a fourth aspect, the present invention provides a computer program product comprising computer instructions, the computer instructions being used to enable a computer to execute the auxiliary startup control method for an offshore wind power direct current transmission system according to the second aspect or any corresponding embodiment thereof. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] In order to more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the specific embodiments or the description of the prior art. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0027] Figure 1 is a structural block diagram of an offshore wind power direct current transmission system according to an embodiment of the present invention;

[0028] Figure 2 is a flow chart of an auxiliary startup control method for an offshore wind power direct current transmission system according to an embodiment of the present invention;

[0029] Figure 3 2. This is a diagram of an auxiliary startup system for an offshore wind power direct current transmission system according to an embodiment of the present invention;

[0030] Figure 4 1 is a flow chart of an auxiliary startup control method for an offshore wind power direct current transmission system according to an embodiment of the present invention;

[0031] Figure 5 Schematic diagram of the hardware structure of a computer device according to an embodiment of the present invention. DETAILED DESCRIPTION

[0032] To make the purpose, technical solutions, and advantages of the embodiments of the present invention more clear, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without making creative efforts shall fall within the scope of protection of the present invention.

[0033] Considering factors such as low auxiliary starting device utilization and economic considerations, the capacity of auxiliary starting devices is typically small. When establishing offshore AC voltage to power small-capacity MMCs, the auxiliary starting device has limited tolerance to reactive charging current surges in the system transformers and cables. Therefore, a soft-start boost is recommended. However, the boost process can easily trigger the system's undervoltage protection, leading to startup failure and requiring significant modifications to existing control and protection strategies. Furthermore, after a successful DC system black start, the auxiliary starting device is placed in cold standby, resulting in low device utilization.

[0034] In this embodiment, an offshore wind power direct current transmission system is provided. Figure 1 As shown, it includes: an onshore power grid 1, an onshore converter station 2, a DC submarine cable 3, an offshore converter station 4, a first switch 5 and an offshore wind farm 6.

[0035] The onshore power grid 1 is connected to the onshore converter station 2 , the onshore converter station 2 is connected to the offshore converter station 4 via a DC submarine cable 3 , and the offshore converter station 4 is connected to the offshore wind farm 6 .

[0036] Furthermore, the offshore converter station 4 includes an offshore MMC converter valve 7 , an auxiliary starting device 8 , a second switch 9 , a third switch 10 , a first transformer 11 and an offshore AC busbar 12 .

[0037] Among them, the offshore AC busbar 12 is connected to the offshore wind farm 6 through the first switch 5; the auxiliary starting device 8 is connected in parallel with the valve-side connection line of the offshore MMC converter valve 7 through the second switch 9, and the valve-side connection line of the offshore MMC converter valve is connected to the first transformer 11 through the third switch 10.

[0038] Specifically, by connecting the onshore power grid 1 with the onshore converter station 2 , the onshore power grid 1 can be used to provide energy for the onshore converter station 2 .

[0039] Furthermore, the onshore converter station can convert the AC power of the onshore power grid 1 into DC power suitable for long-distance transmission. Therefore, by connecting the onshore power grid 1 with the onshore converter station 2, it can constitute the energy starting point and conversion basis of the entire transmission system.

[0040] Furthermore, the onshore power grid 1 is connected to the offshore converter station 4 via the onshore converter station 2 and the DC submarine cable 3, forming a long-distance power transmission channel.

[0041] Among them, the DC submarine cable 3, as a channel for long-distance power transmission, has the characteristics of low line loss and large transmission capacity. It can effectively solve the problem of long-distance power transmission of deep-sea wind power and realize the power transmission between onshore converter stations and offshore converter stations.

[0042] Furthermore, the offshore converter station 4 can convert and transmit the electric energy generated by the offshore wind farm 6. Therefore, by connecting the offshore converter station 4 with the offshore wind farm 6, the electric energy generated by the offshore wind farm 6 can be connected to the entire power transmission system, thereby realizing the grid-connected transmission of offshore wind power.

[0043] Furthermore, the first switch 5 can control the connection between the offshore AC busbar 12 and the offshore wind farm 6. Specifically, during the initial startup phase, the first switch 5 is opened to prevent unstable voltage from impacting the wind turbine. Once the voltage of the offshore AC busbar 12 stabilizes and meets requirements, the first switch 5 is closed, allowing the wind turbine to connect to the system and output power.

[0044] Furthermore, the auxiliary starting device 8 is connected in parallel to the valve-side connection line of the offshore MMC converter valve 7 via the second switch 9, thereby reducing the intermediate connection links between the auxiliary starting device 8 and the offshore MMC converter valve 7 and lowering the difficulty of modifying the existing control and protection system.

[0045] Furthermore, by providing a third switch 10 between the valve-side connection line and the first transformer 11, the third switch 10 can be used to control the activation and deactivation of the auxiliary starting device 8. Specifically, during startup, the third switch 10 is closed, and the auxiliary starting device 8 then uses a soft-start boost to power the offshore MMC converter valve 7. After startup is complete, the third switch 10 can be opened as needed, placing the auxiliary starting device 8 in a standby state.

[0046] Furthermore, the third switch 10 can also control the connection between the valve-side connecting line and the first transformer 11. Specifically, when the auxiliary starting device 8 is operating, opening the third switch 10 ensures that energy is centrally supplied to the offshore MMC converter valve 7, preventing energy from being diverted to the transformer. Once the offshore MMC converter valve 7 establishes a DC voltage, closing the third switch 10 connects the valve-side connecting line to the first transformer 11, thereby forming a complete AC circuit and supporting the subsequent establishment of the offshore AC bus voltage.

[0047] Furthermore, when the second switch 9 is closed and the third switch 10 is open, the auxiliary starting device 8 enters operation and gradually builds up the AC voltage on the valve-side connecting line using a soft-start voltage boosting method. This AC voltage can then provide temporary power to the offshore MMC converter valve 7, enabling it to begin charging.

[0048] Soft-start boosting refers to the process during the startup of an offshore wind turbine DC transmission system, whereby the auxiliary starting device slowly and steadily increases the output voltage until it reaches the rated value. Its core purpose is to control the rate and amplitude of voltage rise to prevent sudden voltage changes from inducing large reactive charging current shocks to system equipment (such as transformers and cables), thereby reducing the risk of triggering the low-voltage protection system and improving the startup success rate.

[0049] In the offshore wind power DC transmission system provided in this embodiment, the auxiliary starting device in the offshore converter station is connected in parallel to the valve-side connection line of the offshore MMC converter valve via a second switch. A third switch is provided between the connection point and the first transformer. When the second switch is closed and the third switch is open, soft-start boosting is performed only on the valve-side connection line and the offshore MMC area, avoiding triggering low-voltage protection in other areas. This also reduces intermediate connection links and eases the modification of existing control and protection systems. Furthermore, the parallel connection allows for a smaller reactive capacity configuration for the auxiliary starting device, improving economic efficiency.

[0050] In some optional embodiments, the auxiliary starting device 8 includes a second transformer 13 , a converter 14 , a fourth switch 15 and a power supply 16 .

[0051] The second transformer 13 is connected to the converter 14 , and the converter 14 is connected to the power source 16 via the fourth switch 15 .

[0052] Specifically, the second transformer 13 can convert the voltage of the input power supply to meet the input requirements of the converter, thereby playing the role of voltage matching and electrical isolation, ensuring the safe and stable operation of the converter 14 and the entire auxiliary starting device 8.

[0053] Furthermore, the converter 14 can realize the conversion of AC and DC power. Specifically, during the startup process, the converter 14 can convert the power of the power source 16 into the required AC voltage and realize the soft start boost function by controlling the amplitude and frequency of the output voltage.

[0054] Furthermore, the fourth switch 15 can control the connection between the power supply 16 and the converter 14. Specifically, when the auxiliary starting device 8 needs to operate, the fourth switch 15 is closed to enable the power supply 16 to supply power to the converter 14. When the startup is complete or the device needs to be shut down, the fourth switch 15 is opened to cut off the power input, thereby achieving control over the operating state of the auxiliary starting device 8.

[0055] Furthermore, the power supply 16 can supply power to the auxiliary starting device 8 .

[0056] Furthermore, the entire process of the auxiliary starting device 8 is as follows: the power supply 16 supplies power to the converter 14 via the fourth switch 15. The converter 14 then converts the electrical energy into an AC voltage, which is then boosted by the second transformer 13 and output through the second switch 9 to the valve-side connection line of the offshore MMC converter valve 7, thereby establishing the corresponding AC voltage on the valve-side connection line.

[0057] The offshore wind power direct current transmission system provided in this embodiment can achieve smooth voltage regulation during the soft start and voltage boost process through the converter, thereby avoiding hard start impact.

[0058] In some optional embodiments, the offshore converter station 4 further includes: a third transformer 17 , a fourth transformer 18 , a fifth switch 19 , a sixth switch 20 , and a seventh switch 21 .

[0059] The third transformer 17 and the fourth transformer 18 are three-phase transformers for realizing transmission and voltage conversion of three-phase electric energy.

[0060] The third transformer 17 is connected to the offshore AC bus 12 via the fifth switch 19 , that is, the fifth switch 19 can control the on / off connection between the third transformer 17 and the offshore AC bus 12 .

[0061] Specifically, during system startup or operation, the fifth switch 19 can be closed as needed, and by closing it, the third transformer 17 can be connected to the offshore AC bus 12, thereby providing additional AC power support for the system or sharing power transmission tasks, thereby increasing the flexibility of system operation.

[0062] Furthermore, the fourth transformer 18 is connected to the offshore AC busbar 12 via a sixth switch 20. Specifically, the sixth switch 20 can control the on / off connection between the fourth transformer 18 and the offshore AC busbar 12. Similar in function to the fifth switch 19, the fourth transformer 18 can be connected as required by the system, thereby improving system reliability and power transmission capacity.

[0063] Furthermore, the first transformer 11 is connected to the offshore AC busbar 12 via the seventh switch 21 , that is, the seventh switch 21 can control the on / off switching of the first transformer 11 and the offshore AC busbar 12 .

[0064] Specifically, during the auxiliary startup phase, by disconnecting the seventh switch 21, the energy of the auxiliary startup device 8 can be prevented from being diverted to the first transformer 11. After the offshore MMC converter valve 7 establishes a DC voltage, by closing the seventh switch 21, the first transformer 11 can be connected to the offshore AC bus 12, forming a complete AC circuit from the valve-side connecting line to the offshore AC bus.

[0065] Therefore, based on the above connections, during the startup process, the closing and opening of each switch can be controlled according to the needs of different stages. For example, when the auxiliary starting device 8 is operating, some switches can be opened to ensure centralized energy supply. When the offshore MMC converter valve 7 is operating normally, the corresponding switches are closed to connect each transformer to the system, achieving voltage stability and power transmission on the offshore AC bus 12.

[0066] The offshore wind power DC transmission system provided in this embodiment achieves flexible connection between different transformers and the offshore AC busbar through the fifth to seventh switches. Furthermore, during system startup, the closing sequence of each switch can be controlled to achieve step-by-step power supply control for the third transformer, the fourth transformer, and the first transformer. This avoids the impact of large reactive charging currents on the system caused by multiple transformers receiving power simultaneously, thereby improving the stability of the startup process. Furthermore, the flexible switch configuration enables each transformer to be independently activated or deactivated according to operational requirements. When some equipment fails, the corresponding switch can be disconnected for isolated maintenance without affecting the normal operation of other equipment, thereby improving the reliability and operational flexibility of the system.

[0067] In some optional embodiments, the offshore converter station 4 further includes: a first diode converter valve 22 and a second diode converter valve 23 .

[0068] Among them, the DC sides of the offshore MMC converter valve 7, the first diode converter valve 22 and the second diode converter valve 23 are connected in series; the AC sides of the offshore MMC converter valve 7, the first diode converter valve 22 and the second diode converter valve 23 are connected in parallel; the first diode converter valve 22 is connected to the third transformer 17, and the second diode converter valve 23 is connected to the fourth transformer 18.

[0069] Specifically, by utilizing the unidirectional conductivity of the diode converter valve and connecting it in series with the DC side of the offshore MMC converter valve 7, the DC pole voltage borne by the offshore MMC converter valve 7 can be reduced.

[0070] Furthermore, by series voltage division on the DC side, the number of submodules required for the offshore MMC converter valve 7 can be reduced, thereby reducing the construction cost of the offshore converter station while ensuring unidirectional transmission of DC side electrical energy.

[0071] Furthermore, by connecting the AC sides of the offshore MMC valve 7, first diode valve 22, and second diode valve 23 in parallel to the offshore AC bus 12, they can jointly provide voltage support for the offshore AC system. Specifically, during normal system operation, the offshore MMC valve 7 and diode valves work together to stabilize the offshore AC bus voltage while sharing power transmission tasks on the AC side, thereby improving system reliability and operational efficiency.

[0072] Furthermore, the connection between the first diode converter valve 22 and the third transformer 17 forms an electrical energy transmission path. Specifically, the third transformer 17 modifies the AC voltage output by the first diode converter valve 22 to meet the voltage requirements of the offshore AC busbar, thereby establishing an electrical connection between the first diode converter valve 22 and the offshore AC system. The converted electrical energy is then transmitted to the offshore AC busbar 12.

[0073] Furthermore, the second diode converter valve 23 is connected to the fourth transformer 18. The fourth transformer 18 then changes the AC voltage output by the second diode converter valve 23 to ensure that it matches the voltage of the offshore AC busbar. This allows the second diode converter valve 23 to effectively transmit electrical energy to the offshore AC busbar 12, supporting the operation of the offshore AC system.

[0074] In the offshore wind power DC transmission system provided by this embodiment, the offshore MMC valve is connected in series with the DC side of the first and second diode valves. This allows the uncontrollable unidirectional conduction characteristics of the first and second diode valves to share the DC pole voltage, significantly reducing the number of submodules in the offshore MMC valves and lowering the hardware cost and construction difficulty of the converter station. Furthermore, the AC-side parallel connection structure allows the auxiliary starting device to independently power the offshore MMC valves, effectively resolving the black start problem caused by the unidirectional conduction of the diode valves. Furthermore, this series connection is compatible with the low-loss characteristics of flexible DC transmission, ensuring long-distance transmission of deep-sea wind power while achieving a balance between cost and reliability through hybrid valve technology.

[0075] In some optional embodiments, the onshore converter station 2 includes: an onshore MMC converter valve 24 .

[0076] The onshore MMC converter valve 24 is connected to the first diode converter valve 22 and the second diode converter valve 23 through the DC submarine cable 3 .

[0077] Specifically, the onshore MMC converter valve 24 is the core conversion equipment between the onshore power grid 1 and the DC submarine cable 3, responsible for converting the AC power of the onshore power grid 1 into DC power and transmitting it to the offshore converter station 4 through the DC submarine cable 3.

[0078] Furthermore, during system startup, the onshore MMC converter valve 24 can establish the DC submarine cable voltage to the rated value according to the status of the offshore MMC converter valve 7, thereby providing stable voltage support for the entire DC system; during normal operation, it can also realize two-way transmission of electric energy between the onshore power grid and the offshore wind farm.

[0079] The offshore wind power DC transmission system provided in this embodiment uses an onshore MMC converter valve connected to the first and second offshore diode converter valves via a DC submarine cable. The large capacity of the onshore MMC converter valves, combined with the offshore converter station's assisted startup process, shortens the overall system startup time. Furthermore, the onshore MMC converter valves support bidirectional power transmission, efficiently transmitting offshore wind power to the onshore power grid via the DC submarine cable and providing voltage support for the offshore converter station during the startup phase, addressing the demand for deep-sea wind power transmission.

[0080] According to an embodiment of the present invention, an embodiment of an auxiliary startup control method for an offshore wind power direct current transmission system is provided. It should be noted that the steps shown in the flowchart of the accompanying drawings can be executed in a computer system such as a set of computer-executable instructions, and although a logical order is shown in the flowchart, in some cases, the steps shown or described can be executed in an order different from that shown here.

[0081] In this embodiment, an auxiliary startup control method for an offshore wind power DC transmission system is provided, which can be used for the offshore wind power DC transmission system provided in the above embodiment. Figure 2 FIG. 1 is a flow chart of an auxiliary startup control method for an offshore wind power direct current transmission system according to an embodiment of the present invention. Figure 2 As shown, the process includes the following steps:

[0082] Step S201: Control the auxiliary starting device to establish an AC voltage on the valve-side connecting line through a soft start boosting method to power the offshore MMC converter valve.

[0083] Specifically, the offshore converter station 4 is controlled to close the second switch 9 and the fourth switch 15. At the same time, the first switch 5, the third switch 10, the fifth switch 19, the sixth switch 20, and the seventh switch 21 are controlled to open, so that the auxiliary starting device 8 is connected to the circuit and isolated from other non-essential circuits.

[0084] Furthermore, the auxiliary starting device 8 is controlled to transition from hot standby to operational mode. Its internal converter 14 then slowly increases the AC voltage on the valve-side connection line of the offshore MMC converter valve 77 to its rated value (soft start) by controlling the amplitude and frequency of the output voltage. During this process, the voltage rise rate is controlled, preventing the generation of excessive reactive charging current and the triggering of system low-voltage protection.

[0085] After the AC voltage on the valve-side connection line is established, it is fed into the offshore MMC converter valve 7 via the AC side to charge its internal submodule capacitors. During the charging process, the auxiliary starting device 8 maintains a stable output voltage, ensuring that the charging current remains within a safe range and avoiding any impact on the equipment.

[0086] Step S202: When it is detected that the DC side voltage of the offshore MMC converter valve rises to a first preset value, the onshore MMC converter valve is controlled to establish the DC submarine cable voltage.

[0087] Specifically, as the neutron module capacitor of the offshore MMC converter valve 7 is continuously charged, the DC side voltage of the offshore MMC converter valve 7 gradually increases.

[0088] Furthermore, the DC side voltage of the offshore MMC converter valve 7 is monitored in real time. When the voltage rises to a first preset value, a*Udcref, it indicates that the offshore MMC converter valve 7 is fully charged and capable of withstanding the DC submarine cable voltage. Here, a∈[1,1.1) and Udcref is the rated DC voltage of the submarine cable.

[0089] Furthermore, when the DC side voltage of the offshore MMC converter valve 7 rises to a first preset value, the auxiliary starting device 8 is controlled to switch from the operating state to the hot standby state, and voltage control of the valve-side connecting line is stopped. Simultaneously, the offshore MMC converter valve 7 is controlled to switch from the charging state to the hot standby state.

[0090] Furthermore, the onshore MMC converter valve 24 is controlled to operate in a typical manner, gradually building the DC cable voltage to its rated value. During this process, the onshore MMC converter valve 24 ensures a steady rise in the DC cable voltage by regulating its own output, providing stable voltage support for the entire DC transmission system.

[0091] Step S203: When it is detected that the DC submarine cable voltage is at the rated value, the offshore MMC converter valve is controlled to output the AC voltage of the valve-side connecting line to the offshore AC bus, and the first diode converter valve and the second diode converter valve are controlled to output AC voltages to the offshore AC bus, respectively.

[0092] Specifically, when it is detected that the DC submarine cable voltage reaches the rated value, it indicates that the system has a stable DC power input, and the voltage establishment process of the offshore AC system can be started.

[0093] Furthermore, the offshore MMC converter valve 7 is controlled to switch from the hot standby state to the operating state, so that the AC voltage of the output valve side connecting line is transmitted to the offshore AC bus 12, and the voltage gradually rises to the rated value, providing initial voltage support for the offshore AC bus 12.

[0094] At the same time, the first diode converter valve 22 and the second diode converter valve 23 are controlled to output the AC voltage to the offshore AC bus 12 through the third transformer 17 and the fourth transformer 18 respectively.

[0095] By connecting the AC sides of the offshore MMC converter valve 7, the first diode converter valve 22, and the second diode converter valve 23 in parallel to the offshore AC bus 12, the voltage of the offshore AC bus 12 can be jointly stabilized, thereby forming a complete offshore AC power supply system.

[0096] Step S204 , when it is detected that the AC voltage of the offshore AC busbar is greater than a second preset value, the wind turbines in the offshore wind farm are controlled to start.

[0097] Specifically, the AC side voltage of the offshore AC bus 12 is monitored in real time. When the voltage is greater than a second preset value (such as 0.9 per unit), it indicates that the offshore AC bus voltage is stable and meets the wind turbine startup requirements.

[0098] Furthermore, the wind turbines in the offshore wind farm 6 are controlled to start up, and the output power of the wind turbines is gradually increased to full power output, thereby realizing grid-connected power generation and power transmission of offshore wind power.

[0099] The auxiliary startup control method for the offshore wind power DC transmission system provided in this embodiment performs voltage boosting through a soft-start boosting method, thereby avoiding the generation of impact current on the valve-side connecting line, the offshore MMC converter valve and related equipment, and reducing the reactive charging current peak of the transformer and cable. At the same time, the auxiliary startup device only acts on the local area of ​​the valve-side connecting line, is compatible with the existing control and protection system, and is not easy to trigger the low-voltage protection of the entire system. Furthermore, with the offshore MMC DC voltage as the trigger condition, the startup sequence coordination of the onshore and offshore converter stations is achieved. At the same time, the onshore MMC converter valve is controlled to establish the DC submarine cable voltage, thereby avoiding power imbalance caused by premature voltage buildup when the offshore MMC is not ready, and also avoiding delaying the startup process due to late voltage buildup, thereby ensuring that the voltage establishment of the DC transmission link is accurately matched with the charging process of the offshore converter station. Furthermore, after the DC submarine cable voltage stabilizes, the offshore MMC and diode commutator valves synchronously supply power to the offshore AC busbar. This leverages the controllability of the offshore MMC to maintain AC voltage stability while also sharing the power transmission burden with the low-loss characteristics of the diode commutator valves. This also gradually expands the power supply area to the offshore AC busbar, avoiding the impact of simultaneous power supply to the entire system. Finally, using the AC busbar voltage threshold as the wind turbine startup condition ensures stable grid voltage when the wind turbine is connected, preventing wind turbine startup failures or grid connection shocks caused by low voltage.

[0100] In some optional implementations, the method further includes: obtaining a power loss value; and controlling the power output of the auxiliary starting device based on the power loss value.

[0101] Specifically, the system active power loss can be estimated using the following relationship (1):

[0102] P=P0*(V / V ref ) 2 (1)

[0103] Where: P represents the active power loss value; P0 represents the AC rated voltage V ref The losses of each device in the AC system during operation can be obtained through simulation calculation; V represents the AC voltage; V ref Indicates the AC rated voltage.

[0104] Furthermore, according to the acquired active power loss value P, the auxiliary starting device 8 can be controlled to switch to the power operation mode and change from the hot standby state to the operation state, so that its output power is equal to the estimated active power loss value P.

[0105] Furthermore, in the process of controlling the output power of the auxiliary starting device, it is ensured that the output current does not exceed the rated current limit of the device, thereby avoiding device damage or system failure due to overload.

[0106] Furthermore, the power output by the auxiliary starting device 8 is used to compensate for the active power loss of the system, maintain the active power balance of the system, ensure that each device is not affected by power imbalance during startup and operation, and ensure stable operation of the system.

[0107] The auxiliary startup control method for an offshore wind power DC transmission system provided in this embodiment can control the auxiliary startup device to accurately output corresponding power to compensate for the active power consumption of equipment such as transformers, cables, and MMC submodules by obtaining the active power loss value. This avoids system power shortages caused by blind power output in traditional solutions, prevents DC voltage drops or startup failures, and ensures a smooth transition of power relay from "auxiliary startup device → offshore MMC → wind turbine" during subsequent startup.

[0108] In some optional implementations, the above method further includes: when the output power of the wind turbine in the offshore wind farm meets the preset power, controlling the auxiliary starting device to perform reactive power compensation according to the demand of the offshore converter station.

[0109] Specifically, the output power of the wind turbines in the offshore wind farm 6 can be monitored in real time. When the output power of the wind turbines reaches the preset power (i.e., full power output), it indicates that the system has entered the normal operation stage. At this time, the power output of the offshore wind power is stable, and there is no need for the auxiliary starting device 8 to continue to provide active power compensation.

[0110] Furthermore, the auxiliary starting device 8 can be controlled to switch from the power operation mode to the reactive operation mode, and to switch from the current operation state to a working state of performing reactive compensation according to the requirements of the offshore converter station 4 .

[0111] Furthermore, the auxiliary starting device 8 can be controlled to dynamically adjust reactive power output according to the real-time demand of the offshore converter station and perform reactive power compensation on the system.

[0112] Furthermore, reactive power compensation can stabilize the offshore AC bus voltage, improve the system power factor, improve the power transmission efficiency and system stability, and at the same time improve the utilization rate of the auxiliary starting device 8 so that it continues to play a role when the system is operating normally.

[0113] This embodiment provides an assisted startup control method for an offshore wind power DC transmission system. When a wind turbine in an offshore wind farm reaches a preset power output, the assisted startup device is controlled to switch to reactive power compensation mode. This prevents idle equipment, improves equipment utilization, and reduces system costs. Furthermore, the reactive power output is dynamically adjusted based on offshore station requirements, maintaining AC voltage stability and supporting full turbine power operation. Therefore, the implementation of this invention addresses the low utilization rate of assisted startup devices in the prior art.

[0114] In one example, a method and device for auxiliary startup control of an offshore wind power direct current transmission system are provided.

[0115] Specifically, the auxiliary starting device of the offshore wind power DC transmission system is as follows: Figure 3 As shown, the outlet of the auxiliary starting device is connected to the valve side connection line of the small-capacity MMC of the offshore station through the switch Q2, and a switch Q3 needs to be set between the connection point and the transformer T2.

[0116] Furthermore, if Figure 4 As shown in the figure, the starting sequence of the auxiliary starting device includes:

[0117] 1) The offshore station closes switches Q1 and Q2 and opens switches Q3 to Q7;

[0118] 2) The auxiliary starting device switches from hot standby to operation, the outlet soft start voltage is increased to the rated value, and the AC voltage of the valve side connection line is established;

[0119] 3) The small-capacity MMC is charged and its DC side voltage is raised to a*Udcref, and then switched to hot standby, and the auxiliary starting device is switched from operation to hot standby;

[0120] Where a∈[1,1.1), Udcref is the DC voltage rating of the submarine cable.

[0121] 4) The onshore station MMC establishes the DC submarine cable voltage to the rated value according to the typical operation mode;

[0122] 5) The small-capacity MMC switches from hot standby to operation, and establishes an AC voltage to the rated value for the valve-side connection line;

[0123] 6) Close switches Q3 to Q7, and the small-capacity MMC switches from hot standby to operation, soft-starting and boosting the AC side while stabilizing its own DC side voltage;

[0124] 7) The auxiliary starting device switches to power operation mode, switching from hot standby to operation, and does not control the valve side AC voltage;

[0125] 8) Estimate the system active power loss based on the above relationship (1), and make the auxiliary starting device output the power, but its output current does not exceed the limit;

[0126] 9) When the voltage on the offshore AC side is greater than 0.9 per unit, start the wind turbine and continuously increase the output power to full power output;

[0127] 10) The auxiliary starting device switches to reactive operation mode and performs reactive power compensation according to the requirements of the offshore station.

[0128] This example provides an assisted startup control method and device for an offshore wind power DC transmission system. First, the auxiliary startup device is connected in parallel to the valve-side connection line of the offshore station's small-capacity MMC. Compared to existing solutions, this reduces the number of intermediate connections between the two. Consequently, when charging the small-capacity MMC, the soft-start boost process only involves the valve-side connection line and the small-capacity MMC area. This is consistent with existing control methods, making it less likely to trigger low-voltage protection in other areas, and allowing the auxiliary startup device to have a smaller reactive capacity configuration. Second, after the small-capacity MMC's own DC voltage is established, the AC voltage of the valve-side connection line is controlled to the rated value. The auxiliary startup device's active power output is then gradually controlled based on demand, gradually energizing each area. This power supply method aligns with existing offshore station operations, requiring minimal modification to existing equipment control and protection systems. Furthermore, the black start device ensures active power balance throughout the system, unaffected by the impact of equipment connection. Finally, after system startup is complete, the black start device operates in reactive power compensation mode, improving equipment utilization.

[0129] The embodiment of the present invention also provides a computer device for executing the above Figure 2 The auxiliary startup control method of the offshore wind power direct current transmission system is shown.

[0130] See also Figure 5 , Figure 5 is a structural diagram of a computer device provided by an optional embodiment of the present invention, such as Figure 5 As shown, the computer device includes: one or more processors 10, a memory 20, and interfaces for connecting various components, including high-speed interfaces and low-speed interfaces. Various components utilize different buses to communicate with each other and can be installed on a common mainboard or installed in other ways as needed. The processor can process the instructions executed in the computer device, including instructions stored in or on the memory to display the graphical information of a GUI on an external input / output device (such as, a display device coupled to the interface). In some optional embodiments, if necessary, multiple processors and / or multiple buses can be used together with multiple memories. Equally, multiple computer devices can be connected, and each device provides part of the necessary operations (for example, as a server array, a group of blade servers, or a multi-processor system). Figure 5 A processor 10 is taken as an example.

[0131] The processor 10 may be a central processing unit, a network processor, or a combination thereof. The processor 10 may further include a hardware chip. The hardware chip may be an application-specific integrated circuit, a programmable logic device, or a combination thereof. The programmable logic device may be a complex programmable logic device, a field programmable gate array, a general purpose array logic, or any combination thereof.

[0132] The memory 20 stores instructions that can be executed by at least one processor 10, so as to enable at least one processor 10 to execute the method shown in the above embodiment.

[0133] The memory 20 may include a program storage area and a data storage area, wherein the program storage area may store an operating system and application programs required for at least one function; the data storage area may store data created based on the use of the computer device, etc. In addition, the memory 20 may include a high-speed random access memory, and may also include a non-transient memory, such as at least one disk storage device, a flash memory device, or other non-transient solid-state storage device. In some optional embodiments, the memory 20 may optionally include a memory remotely located relative to the processor 10, and these remote memories may be connected to the computer device via a network. Examples of the above-mentioned network include, but are not limited to, the Internet, an intranet, a local area network, a mobile communication network, and combinations thereof.

[0134] The memory 20 may include a volatile memory, such as a random access memory; the memory may also include a non-volatile memory, such as a flash memory, a hard disk or a solid-state drive; the memory 20 may also include a combination of the above types of memory.

[0135] The computer device further includes a communication interface 30 for the computer device to communicate with other devices or a communication network.

[0136] The embodiment of the present invention also provides a computer-readable storage medium. The above-mentioned method according to the embodiment of the present invention can be implemented in hardware, firmware, or implemented as a computer code that can be recorded in a storage medium, or implemented as a computer code that is originally stored in a remote storage medium or a non-temporary machine-readable storage medium and downloaded through a network and will be stored in a local storage medium, so that the method described herein can be stored in such software processing on a storage medium using a general-purpose computer, a dedicated processor, or programmable or dedicated hardware. Among them, the storage medium can be a magnetic disk, an optical disk, a read-only storage memory, a random access memory, a flash memory, a hard disk or a solid-state drive, etc.; further, the storage medium can also include a combination of the above-mentioned types of memory. It can be understood that a computer, a processor, a microprocessor controller or programmable hardware includes a storage component that can store or receive software or computer code. When the software or computer code is accessed and executed by a computer, a processor or hardware, the method shown in the above embodiment is implemented.

[0137] A portion of the present invention may be applied as a computer program product, such as a computer program instruction, which, when executed by a computer, can call or provide the method and / or technical solution according to the present invention through the operation of the computer. Those skilled in the art should understand that the form in which the computer program instruction exists in a computer-readable medium includes, but is not limited to, a source file, an executable file, an installation package file, etc. Accordingly, the way in which the computer program instruction is executed by the computer includes, but is not limited to: the computer directly executes the instruction, or the computer compiles the instruction and then executes the corresponding compiled program, or the computer reads and executes the instruction, or the computer reads and installs the instruction and then executes the corresponding installed program. Here, the computer-readable medium may be any available computer-readable storage medium or communication medium that can be accessed by the computer.

[0138] Although the embodiments of the present invention have been described with reference to the accompanying drawings, those skilled in the art may make various modifications and variations without departing from the spirit and scope of the present invention. Such modifications and variations are all within the scope defined by the appended claims.

Claims

1. An offshore wind power direct current transmission system, characterized in that: The system includes: an onshore power grid, an onshore converter station, a DC submarine cable, an offshore converter station, a first switch and an offshore wind farm; The onshore power grid is connected to the onshore converter station, the onshore converter station is connected to the offshore converter station via the DC submarine cable, and the offshore converter station is connected to the offshore wind farm; The offshore converter station includes an offshore MMC converter valve, an auxiliary starting device, a second switch, a third switch, a first transformer and an offshore AC busbar, wherein the offshore AC busbar is connected to the offshore wind farm via the first switch; The auxiliary starting device is connected in parallel to the valve-side connection line of the offshore MMC converter valve through the second switch, and the valve-side connection line of the offshore MMC converter valve is connected to the first transformer through the third switch; The auxiliary starting device is used to establish an AC voltage on the valve-side connecting line through a soft-start boosting method when the second switch is closed and the third switch is open, so as to power the offshore MMC converter valve.

2. The system according to claim 1, wherein: The auxiliary starting device includes: a second transformer, a converter, a fourth switch and a power supply. The second transformer is connected to the converter, and the converter is connected to the power supply via the fourth switch.

3. The system according to claim 1, wherein: The offshore converter station further includes a third transformer, a fourth transformer, a fifth switch, a sixth switch and a seventh switch; The third transformer is connected to the offshore AC busbar through the fifth switch, the fourth transformer is connected to the offshore AC busbar through the sixth switch, and the first transformer is connected to the offshore AC busbar through the seventh switch.

4. The system according to claim 3, characterized in that The offshore converter station further includes: a first diode converter valve and a second diode converter valve; The DC sides of the offshore MMC converter valve, the first diode converter valve, and the second diode converter valve are connected in series, and the AC sides of the offshore MMC converter valve, the first diode converter valve, and the second diode converter valve are connected in parallel; The first diode converter valve is connected to the third transformer, and the second diode converter valve is connected to the fourth transformer.

5. The system according to claim 4, characterized in that The onshore converter station includes an onshore MMC converter valve, which is connected to the first diode converter valve and the second diode converter valve respectively through the DC submarine cable.

6. An auxiliary startup control method for an offshore wind power direct current transmission system, characterized in that: Applicable to an offshore wind power direct current transmission system according to any one of claims 1 to 5; the method comprising: The control auxiliary starting device establishes the valve side connection line AC voltage through soft start boost mode to power the offshore MMC converter valve; When it is detected that the DC side voltage of the offshore MMC converter valve rises to a first preset value, controlling the onshore MMC converter valve to establish the DC submarine cable voltage; When it is detected that the DC submarine cable voltage is at a rated value, the offshore MMC converter valve is controlled to output the AC voltage of the valve-side connecting line to the offshore AC bus, and the first diode converter valve and the second diode converter valve are controlled to output AC voltages to the offshore AC bus respectively; When it is detected that the AC voltage of the offshore AC busbar is greater than a second preset value, the wind turbines in the offshore wind farm are controlled to start.

7. The method according to claim 6, characterized in that The method further comprises: Get active power loss value; Based on the active power loss value, the power output of the starting auxiliary device is controlled.

8. The method according to claim 6, characterized in that The method further comprises: When the output power of the wind turbine in the offshore wind farm meets the preset power, the auxiliary starting device is controlled to perform reactive power compensation according to the demand of the offshore converter station.

9. A computer-readable storage medium, characterized in that The computer-readable storage medium stores computer instructions, and the computer instructions are used to enable a computer to execute the auxiliary startup control method for an offshore wind power direct current transmission system according to any one of claims 6 to 8.

10. A computer program product, characterized in that The method comprises computer instructions for causing a computer to execute the auxiliary startup control method for an offshore wind power direct current transmission system according to any one of claims 1 to 6 to 8.

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