Hybrid offshore wind power sending-out system based on MMC and diode uncontrolled rectification

Abstract

The invention discloses a hybrid offshore wind power sending-out system based on SiC-MOSFET type MMC and diode uncontrolled rectification, an offshore converter station is formed by connecting a DRU and an MMC in parallel, an offshore AC bus can be configured with an SVG and a capacitor, the DRU undertakes main active power transmission, the offshore MMC provides voltage support and harmonic filtering, and the MMC is connected with the DRU and the MMC in parallel. The fan provides limited reactive power compensation through an active-reactive power curve on the premise of full active power operation, and the land MMC maintains the DC voltage stable. The offshore MMC in the system adopts SiC-MOSFET straight string sub-modules and an active clamping voltage sharing technology, the number of the sub-modules is reduced, the system efficiency is improved, flexible compensation of DRU reactive power consumption is achieved through three types of reactive power compensation schemes, and economical efficiency and dynamic performance are both considered. Therefore, the system provided by the invention is suitable for high-capacity wind power transmission in deep and far sea, and has a good engineering application prospect.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of new energy grid connection and high-voltage direct current transmission, and particularly relates to a hybrid offshore wind power sending-out system based on a SiC-MOSFET type MMC and a diode uncontrolled rectification. BACKGROUND

[0002] With energy transformation, offshore wind power, as an important form of large-scale clean energy, is developing rapidly. Compared with onshore wind power, offshore wind power resources are more abundant and the wind speed is stable, with higher utilization efficiency and development potential. In recent years, offshore wind farms are gradually developing towards large capacity, long distance and deep sea, however, the distance between the wind farm and the onshore power grid is significantly increased, and how to efficiently, reliably and economically realize power sending-out becomes a key problem to be solved.

[0003] The traditional alternating current power transmission method has obvious capacitive effect in long-distance submarine cables, resulting in excessive charging current and limited reactive power transmission, which is difficult to support stable grid connection of deep-sea large-capacity wind power. The HVDC (high-voltage direct current transmission) technology has become the mainstream technical route for deep-sea wind power sending-out due to its large transmission capacity, low loss and realization of long-distance efficient power transmission. Among existing direct current sending-out schemes, the flexible direct current transmission scheme based on MMC (modular multilevel converter) is the most widely used (such as document [Luoyongjie, Huangpeng, Duanxiuchao, et al. Equivalent impedance modeling method of flexible HVDC transmission system considering the coupling of offshore wind power and MMC impedance [J]. Proceedings of the Chinese Society of Electrical Engineering, 2024, 44(07): 2655-2670]) and has the advantages of independent adjustment of active and reactive power, black start and weak grid support capability; but this scheme needs to build large-capacity MMC equipment at the offshore converter station side, resulting in large volume, high weight, high investment and operation and maintenance cost, which brings great challenges to offshore construction and maintenance.

[0004] In order to reduce the cost and land occupation, in recent years, a hybrid architecture based on DRU (diode uncontrolled rectification unit) and MMC (such as document [Ganhuiden, XiaoHuaqing, Huangying. DRU-MMC hybrid converter control strategy and capacity selection for offshore wind power [J]. Power System Automation, 2024, 48(21): 28-37]) has been proposed, which uses DRU to undertake main active power transmission and simplifies the structure of offshore converter station, and then small-capacity MMC provides voltage support and reactive power compensation for the AC bus; however, since DRU itself does not have reactive power regulation capability, and the reactive power support of small-capacity MMC is limited, the system needs additional reactive power compensation devices to maintain the stability of the bus voltage.

[0005] On the other hand, the MMC based on the conventional silicon-based IGBT device has problems of large number of sub-modules, large volume, high loss, etc., which is not conducive to the compactness and high efficiency of the offshore platform. With the development of silicon carbide (SiC) device technology, the use of SiC-MOSFET (metal oxide semiconductor field effect transistor) direct string to build sub-modules can significantly improve the withstand voltage capability, reduce the number of sub-modules, and reduce system loss and complexity. At the same time, under the premise of maintaining full active operation, the grid-following type wind turbine can provide limited reactive power support through the active-reactive curve, thereby to some extent, sharing the reactive power consumed by the DRU and enhancing the operation stability of the overall system.

[0006] Therefore, it is urgent to propose a hybrid sending architecture combining SiC-MOSFET type MMC and DRU, and design a flexible reactive power compensation scheme to realize efficient, stable and economical deep-sea wind power sending. SUMMARY

[0007] In view of the above, the present application provides a hybrid offshore wind power sending system based on SiC-MOSFET type MMC and diode uncontrolled rectification, which has both economy and dynamic performance, and is suitable for deep-sea large-capacity wind power sending scenarios.

[0008] A hybrid offshore wind power sending system based on SiC-MOSFET type MMC and diode uncontrolled rectification, comprising an offshore wind farm, an offshore converter station and a land converter station; The offshore converter station adopts a hybrid converter, which is composed of DRU and small-capacity MMC (rated capacity less than 400 MW) based on SiC-MOSFET in parallel, and the AC side of the DRU and the MMC is connected to the offshore AC bus through a converter transformer, and the DC side of the DRU and the MMC is connected in parallel and connected to the land converter station through a long-distance DC submarine cable; The offshore wind farm is composed of a plurality of grid-following type wind turbines, and the AC output ports of these wind turbines are connected to the offshore AC bus through AC submarine cables and step-up transformers; The land converter station adopts a large-capacity MMC (rated capacity above 1 GW), and the AC side of the MMC is connected to the land AC power grid through a converter transformer.

[0009] Further, the small-capacity MMC adopts an improved AC voltage control strategy for providing partial reactive power support and AC filtering function for the offshore AC bus; and the large-capacity MMC adopts a DC voltage control strategy for controlling the steady-state DC voltage of the system to be at the rated value.

[0010] Furthermore, the DRU in the offshore converter station is used for steady-state transmission of active power, and its reactive power compensation has the following three alternative schemes: Scheme 1 is that the wind turbine provides part of the reactive power, and at the same time, a reactive power compensation capacitor is configured at the offshore AC bus to assist the small-capacity MMC in joint compensation; Scheme 2 is that a large-capacity (design capacity greater than 200Mvar) SVG (Static Var Generator) is configured at the offshore AC bus to assist the small-capacity MMC in joint compensation; Scheme 3 is that the wind turbine provides part of the reactive power, and at the same time, a small-capacity (design capacity less than 150Mvar) SVG and a reactive power compensation capacitor are configured at the offshore AC bus to assist the small-capacity MMC in joint compensation.

[0011] Furthermore, the switching devices of each submodule in the small-capacity MMC adopt a SiC-MOSFET direct-series structure. By connecting multiple SiC-MOSFETs in series, the withstand voltage capability of a single submodule can be improved. Each SiC-MOSFET is connected in parallel with an active clamping voltage equalization module, which achieves voltage equalization control of the series devices through voltage sampling, sorting, and dynamic adjustment. By connecting more than 6 SiC-MOSFETs in series, the withstand voltage capability of the switching devices in each submodule reaches 7.2kV~10kV, and the current carrying capacity is 400A.

[0012] Furthermore, the improved constant AC voltage control strategy adds an active power controller to the outer loop to generate fluctuating AC voltage amplitude command values, while the inner loop adopts constant AC voltage amplitude and frequency control, and adds active AC filter control to filter out characteristic harmonic currents on the AC side of the DRU.

[0013] Furthermore, the wind turbine provides a portion of reactive power, that is, under the premise of capturing the maximum active power output in the offshore wind farm, and without exceeding the upper limit of the equipment current and capacity, it provides a certain reactive power reference to the wind turbine according to the preset active-reactive curve.

[0014] As the active power output of wind turbines increases, the active power transmitted by the DRU also increases, leading to a greater consumption of reactive power. Since the reactive power compensation capability of small-capacity MMCs is limited, the reactive power output of wind turbines should increase linearly with their active power output. Therefore, the established active-reactive power curve should satisfy the following conditions: The reactive power output of the wind turbine is 30% of the active power. When the active power output is 0, the reactive power is also 0. As the active power output increases to the rated value of 1000MW, the reactive power output also increases linearly to 300MVar. As the active power output increases to 1200MW, the reactive power output increases linearly to the upper limit of 360Mvar. After that, as the active power output increases, the reactive power output always remains at the upper limit of 360MVar.

[0015] Furthermore, for any alternative solution, the total reactive power that needs to be compensated for by the remaining equipment, excluding the small-capacity MMC, is... Q total It is calculated using the following formula: in: K Q This is the reactive power compensation margin coefficient. Q dru The total reactive power absorbed by the DRU Q WFSiCMMC Reactive power compensated for by small-capacity MMC.

[0016] Furthermore, in Scheme 1, the reactive power to be compensated by the reactive power compensation capacitor is determined according to the following expression: in: Q wf The total reactive power compensated for by offshore wind farms. Q c This refers to the reactive power that the reactive power compensation capacitor needs to compensate.

[0017] Furthermore, in Scheme 2, the reactive power that the SVG needs to compensate is determined according to the following expression: in: Q svg This represents the reactive power that the SVG needs to compensate for.

[0018] Furthermore, in the above scheme 3, the reactive power to be compensated by the reactive power compensation capacitor and the SVG is determined according to the following expression. Since the SVG has high engineering cost and large loss volume, the reactive power compensation of the SVG should be as small as possible under the premise of meeting the dynamic performance requirements of the system. After the reactive power compensation of the SVG is determined, the remaining reactive power is all compensated by the reactive power compensation capacitor. in: Q wf The total reactive power compensated for by offshore wind farms. Q c This refers to the reactive power that the reactive power compensation capacitor needs to compensate. Q svg This represents the reactive power that the SVG needs to compensate for.

[0019] This invention's system utilizes a DRU to handle all steady-state active power transmission, employs a SiC-MOSFET-based offshore MMC for voltage support, reactive power compensation, and AC filtering, and a land-based MMC to maintain DC voltage stability. Furthermore, this invention combines a "active power priority, reactive power limitation" compensation method for grid-connected wind turbines under a set active-reactive power curve with the configuration of SVG and capacitors, forming three selectable reactive power compensation schemes to achieve a balance between cost, performance, and engineering adaptability. Therefore, this invention has the following beneficial technical effects: 1. This invention improves the withstand voltage capability and reduces the number of MMC submodules by using a SiC-MOSFET direct-connect submodule structure, thereby reducing the system control complexity and the size and weight of the offshore platform.

[0020] 2. This invention utilizes the high switching speed and low loss characteristics of SiC devices to improve system efficiency.

[0021] 3. While maintaining full active power operation of the grid-connected wind turbine, this invention enables the wind turbine to provide partial reactive power support through the active-reactive power curve, thereby enhancing the voltage stability of the offshore AC system.

[0022] 4. This invention allows for the selection of three types of reactive power compensation schemes based on actual engineering needs, achieving flexible compensation for DRU reactive power consumption while balancing dynamic performance and economy. Attached Figure Description

[0023] Figure 1 This is a topology diagram of the hybrid offshore wind power transmission system of the present invention.

[0024] Figure 2 This is a schematic diagram of a submodule switching device based on a SiC-MOSFET power device direct-series structure.

[0025] Figure 3 This is a schematic diagram of the active-reactive curve under the reactive power compensation strategy for grid-type wind turbines.

[0026] Figure 4 This is a schematic diagram of the simulation results of the hybrid offshore wind power transmission system under rated operating conditions in this embodiment, where (a) is the instantaneous value of the three-phase AC voltage at the offshore AC bus. u g,abc (b) is the effective value of the offshore AC bus voltage. U g (c) represents the active power of the offshore wind farm (WF), DRU, and WFSiCMMC. P all (d) represents the effective voltage value at the offshore sending end and the busbar of the grid-type wind turbine. U pccwt (e) represents the instantaneous DC-side voltage values ​​of DRU, WFSiCMMC, and GSMMC.u dcall (f) represents the instantaneous DC-side current values ​​of the DRU, WFSiCMMC, and GSMMC. i dcall . Detailed Implementation

[0027] To describe the present invention in more detail, the technical solution of the present invention will be described in detail below with reference to the accompanying drawings and specific embodiments.

[0028] like Figure 1 As shown, the present invention, based on a hybrid offshore wind power transmission system using SiC-MOSFET type MMC and diode uncontrolled rectification, includes an offshore wind farm, an offshore converter station, and an onshore converter station, wherein: The offshore converter station adopts a hybrid converter, which consists of a DRU and a small-capacity MMC based on SiC-MOSFET (WFSiCMMC) connected in parallel. The AC side of the MMC based on SiC-MOSFET is connected to the offshore AC bus via a converter transformer, and the DC side is connected to the onshore converter station via a long-distance DC submarine cable.

[0029] Each submodule switch of the WFSiCMMC uses, for example... Figure 2 The SiC-MOSFET power device direct-connected structure shown demonstrates how connecting multiple SiC-MOSFET power devices in series can improve the withstand voltage of a single submodule, thereby reducing the number of submodules used in the entire converter. This optimizes the converter's size, weight, footprint, and control and communication complexity. Furthermore, the high switching speed and low loss characteristics of SiC-MOSFETs can improve the operating efficiency of offshore wind power transmission systems. Connecting multiple SiC-MOSFET power devices in series enhances the withstand voltage of a single submodule; using six or more SiC-MOSFETs in direct connection allows the switching devices in each submodule to achieve a withstand voltage of 7.2~10kV and a current carrying capacity of 400A.

[0030] Offshore wind farms consist of multiple grid-connected wind turbines whose AC output ports are connected to an offshore AC bus via submarine AC cables and step-up transformers. The onshore converter station comprises a large-capacity gas-powered mainframe converter (GSMMC), whose AC side is connected to the onshore AC grid via a converter transformer. The onshore GSMMC submodule can adopt a hybrid full-bridge / half-bridge structure to provide DC fault current clearing and DC voltage regulation capabilities. Furthermore, its DC output should be equipped with energy dissipation devices to address the power surplus issue of the offshore wind farm in the event of an onshore AC fault.

[0031] The offshore WFSiCMMC employs an improved constant AC voltage control to provide partial reactive power support and AC filtering for the offshore AC bus; the onshore GSMMC uses a constant DC voltage control strategy to maintain the steady-state DC voltage of the DC transmission system at its rated value. The improved constant AC voltage control of the WFSiCMMC includes an outer loop active power controller to generate fluctuating AC voltage amplitude command values. The inner loop employs a constant AC voltage amplitude / frequency control strategy, including a proportional-integral (PI) voltage controller and a current controller, controlling the PCC AC voltage amplitude and frequency to the commanded values. At the same time, an active AC filter control is added to filter out the characteristic harmonic current on the AC side of the DRU.

[0032] The reactive power compensation of the offshore converter station DRU offers three alternative options: Option 1 is to provide part of the reactive power to the grid-connected wind turbines while configuring a reactive power compensation capacitor at the collection bus to assist the WFSiCMMC in joint compensation; Option 2 is to configure an SVG at the collection bus to assist the WFSiCMMC in joint compensation; Option 3 is to provide part of the reactive power to the grid-connected wind turbines while configuring a small-capacity SVG and a reactive power compensation capacitor at the collection bus to assist the WFSiCMMC in joint compensation.

[0033] Grid-connected wind turbines employ a partial reactive power compensation strategy, providing "controllable but limited" reactive power support. This allows the wind farm to capture maximum active power while maintaining the equipment's current and capacity limits, as shown in the example below. Figure 3 The active-reactive power curve shown provides a certain reactive current reference. As the active power output of the wind turbine increases, the active power transmitted by the DRU increases, and its reactive power consumption will also increase. However, the reactive power compensation capability of the small-capacity WFSiCMMC is limited. Therefore, the reactive power output of the wind turbine should increase linearly with its active power output. Combining the upper limit design curve of reactive power output for grid-connected wind turbines: grid-connected wind turbines can output reactive power of 30% of active power. When the active power output of the wind turbine is zero, the reactive power is also zero. As the active power increases to the rated value of 1000MW, the reactive power also increases linearly to 300MVar. As the active power increases to 1200MW, the reactive power output of the wind turbine increases linearly to the peak value of 360MVar. Thereafter, as the active power increases, the reactive power always remains at the upper limit of 360MVar.

[0034] The selection of small-capacity SVG and reactive power compensation capacitors should be determined based on the maximum reactive power that the grid-connected wind turbine and WFSiCMMC can provide, and the reactive power compensation of WFSiCMMC under rated operating conditions. Q WFSiCMMC Based on the comprehensive determination of WFSiCMMC and the design capacity of the offshore platform, under the rated power of 1000MW, and with a typical reactive power factor of 0.4, the reactive power absorption of the DRU is estimated.Q dru Approximately 400 Mvar, reactive power compensation margin factor K Q The value ranges from 0.9 to 1.5, therefore, the total reactive power that all devices except WFSiCMMC need to compensate for in each scheme is... Q total All can be calculated using the following formula: For reactive power compensation scheme 1: by Figure 3 The active-reactive power curve of the wind turbine shown can be used to determine the reactive power compensation of the wind turbine. Q wf The reactive power compensation capacitor can be calculated using the following formula. Q c No effect: For reactive power compensation scheme 2: The reactive power that the SVG needs to compensate can be calculated according to the following formula. Q svg : For reactive power compensation scheme 3: the reactive power compensation calculated by combining the small-capacity SVG and the reactive power compensation capacitor can be obtained according to the following formula: Because SVG projects are expensive and have large volumes of losses, reactive power compensation should be minimized while meeting the system's dynamic performance requirements. After determining the capacity of the SVG, all remaining reactive power is compensated by reactive power compensation capacitors.

[0035] Example Considering that among the three reactive power compensation alternatives, Scheme 3 involves all equipment configurations and has a relatively balanced economy and dynamic performance, this implementation method will take Scheme 3 as an example for introduction.

[0036] The hybrid offshore wind power transmission system in this embodiment includes grid-connected wind turbines, a SiC-MOSFET-based offshore small-capacity MMC (WFSiCMMC), a DRU, a high-voltage DC submarine cable, an onshore large-capacity converter (GSMMC), and an onshore AC grid. The DRU and WFSiCMMC are connected in parallel to the offshore collecting bus on the AC side and to the high-voltage DC bus on the DC side, transmitting active power to the land via the submarine cable. In the system, the DRU handles all steady-state active power transmission, while the WFSiCMMC is used to construct AC voltage, provide controlled reactive power support, and filter harmonic currents injected by the DRU. To meet the reactive power balance requirements of the system operation, an SVG and capacitor compensator are configured on the offshore AC bus PCC (Point of Common Coupling). Typical system parameters are shown in Table 1. Table 1 Then, the system architecture, steady-state operation control strategy, and reactive power compensation configuration method are designed in the DRU-WFSiCMMC hybrid offshore wind power transmission system according to the following methods: (1) The WFSiCMMC submodule adopts a direct-connect structure of SiC-MOSFET power devices. By connecting more than 6 SiC-MOSFETs in series, the voltage withstand capability of the switching devices in each submodule reaches 7.2~10kV and the current carrying capacity is 400A, thereby reducing the number of submodules required in the converter and reducing the size, weight and hardware design complexity of the offshore converter platform. To ensure voltage balance between devices during high-frequency switching, an active clamping voltage equalization module is connected in parallel to each device. Through voltage sampling, sorting and dynamic control, the voltage balance control of the series devices is realized.

[0037] (2) Under steady-state operation, all active power is rectified and output by the DRU, transmitted to land via submarine high-voltage DC cables, and then inverted by the GSMMC and connected to the onshore power grid. Due to its uncontrollable rectifier structure, the DRU continuously absorbs a large amount of reactive power under rated operating conditions, requiring coordinated reactive power support from multiple sources such as the WFSiCMMC, wind turbines, SVG, and capacitors. Under a rated power of 1000MW, based on a typical reactive power coefficient of 0.4, the reactive power absorption of the DRU is estimated to be approximately 400Mvar. Q WFSiCMMC Based on the WFSiCMMC and the design capacity of the offshore platform (design capacity 120Mvar), a reactive power compensation margin of 1.5 times is considered. K Q Wind turbine generators, SVG and capacitors compensate for reactive power Q total Must meet: in: Q wf The reactive power provided to the offshore wind farm is 300 Mvar. Due to the high cost and large volume of SVG engineering, the reactive power compensation should be minimized while meeting the system's dynamic performance requirements. Therefore, the design utilizes parallel capacitor banks to provide this power. Q C Static compensation (design capacity 120Mvar), Q SVG Provides rapid dynamic adjustment (design capacity 60Mvar).

[0038] (3) The WFSiCMMC adopts an improved constant AC voltage control strategy, and introduces active power regulation logic in the outer loop to set the dynamic voltage reference value. To ensure that all steady-state active power is transmitted by the DRU; the inner loop realizes voltage-current dual closed-loop control, controlling the PCC point voltage to stabilize near the rated value. Its current reference adopts the dq coordinate system decomposition method, where the q-axis component is used to output reactive power and the d-axis component is used to cooperate with the DRU to build a stable voltage. In addition, the current controller integrates a selective active filtering function to filter out the characteristic harmonics such as the 11th and 13th order injected by the DRU.

[0039] (4) The SVG and the reactive power compensation capacitor are coordinated and adjusted through the station control system to maintain the static and dynamic reactive power support of the system and suppress the PCC voltage fluctuation; the capacitor bank adopts group configuration and is logically switched in combination with wind conditions and operating mode. The SVG quickly compensates for reactive power demand in the form of voltage-reactive power droop.

[0040] (5) The onshore GSMMC adopts a constant DC voltage control mode. Its AC side is connected to the 220kV grid through a converter transformer with a transformation ratio of 220kV / 320kV and a capacity of 1100MVA. The DC side is equipped with energy consumption devices and monitoring and protection components to prevent the sub-module from overvoltage caused by the power surplus of the wind farm under special operating conditions. The GSMMC has voltage control capability and can stably maintain the DC voltage of the system to ensure the stable operation of submarine DC transmission.

[0041] (6) A simulation model of the DRU-WFSiCMMC hybrid offshore wind power transmission system based on the parameters in Table 1 was built. The simulation results are as follows: Figure 4 As shown, the results indicate that the hybrid offshore wind power transmission system can operate stably under rated conditions. Under steady-state conditions, all 1000MW of active power output from the wind farm is transmitted to the onshore GSMMC via the DRU. Furthermore, with the combined reactive power compensation of wind turbine + WFSiCMMC + SVG + capacitor, the offshore AC system voltage remains stable at the rated value of 66kV under rated conditions, verifying the feasibility and effectiveness of the system and steady-state operation strategy of this invention.

[0042] The system of this invention operates collaboratively based on the concept of "SiC devices reducing losses and decreasing the size of the converter platform + DRU undertaking active power steady-state transmission + WFSiCMMC, wind turbine, and reactive power compensation device providing multi-source reactive power support," achieving simplified system structure, improved efficiency, and diversified configuration of reactive power support equipment. The control strategy of this invention does not require grid connection upgrades for wind turbine units. Combined with the advantages of SiC-MOSFET offshore modular multilevel converters (small size and weight) and three types of reactive power compensation schemes for flexible compensation of DRU reactive power consumption, it can greatly reduce the manufacturing and maintenance difficulty of offshore platforms, exhibiting excellent engineering adaptability and practicality. It is particularly suitable for system retrofitting, upgrading, and operational efficiency improvement of large-scale deep-sea offshore wind power projects with a capacity of 1GW or more.

[0043] The above description of the embodiments is provided to enable those skilled in the art to understand and apply the present invention. Those skilled in the art can readily make various modifications to the above embodiments and apply the general principles described herein to other embodiments without creative effort. Therefore, the present invention is not limited to the above embodiments, and any improvements and modifications made to the present invention by those skilled in the art based on the disclosure thereof should be within the scope of protection of the present invention.

Claims

1. A hybrid offshore wind power transmission system based on SiC-MOSFET type MMC and diode uncontrolled rectification, comprising an offshore wind farm, an offshore converter station, and an onshore converter station, characterized in that: The offshore converter station adopts a hybrid converter, which consists of a DRU and a small-capacity MMC based on SiC-MOSFET connected in parallel. The AC sides of the DRU and MMC are connected to the offshore AC bus via a converter transformer, and the DC sides of the DRU and MMC are connected in parallel and connected to the onshore converter station via a long-distance DC submarine cable. The offshore wind farm consists of multiple grid-connected wind turbines, whose AC output ports are connected to the offshore AC bus via AC submarine cables and step-up transformers. The onshore converter station uses a large-capacity MMC, and its AC side is connected to the onshore AC power grid via a converter transformer.

2. The hybrid offshore wind power transmission system based on SiC-MOSFET type MMC and diode uncontrolled rectification as described in claim 1, characterized in that: The small-capacity MMC adopts an improved constant AC voltage control strategy to provide partial reactive power support and AC filtering function for the offshore AC bus; the large-capacity MMC adopts a constant DC voltage control strategy to control the steady-state DC voltage of the system to the rated value.

3. The hybrid offshore wind power transmission system based on SiC-MOSFET type MMC and diode uncontrolled rectification as described in claim 1, characterized in that: The DRU in the offshore converter station is used for steady-state transmission of active power, and its reactive power compensation has the following three alternative schemes: Scheme 1 is that the wind turbine provides part of the reactive power, and at the same time, a reactive power compensation capacitor is configured at the offshore AC bus to assist the small-capacity MMC in joint compensation; Scheme 2 is that a large-capacity SVG is configured at the offshore AC bus to assist the small-capacity MMC in joint compensation; Scheme 3 is that the wind turbine provides part of the reactive power, and at the same time, a small-capacity SVG and a reactive power compensation capacitor are configured at the offshore AC bus to assist the small-capacity MMC in joint compensation.

4. The hybrid offshore wind power transmission system based on SiC-MOSFET type MMC and diode uncontrolled rectification as described in claim 1, characterized in that: The switching devices of each sub-module in the small-capacity MMC adopt a SiC-MOSFET direct-series structure. By connecting multiple SiC-MOSFETs in series, the withstand voltage capability of a single sub-module can be improved. Furthermore, each SiC-MOSFET is connected in parallel with an active clamping voltage equalization module, which achieves voltage equalization control of the series devices through voltage sampling, sorting, and dynamic adjustment.

5. The hybrid offshore wind power transmission system based on SiC-MOSFET type MMC and diode uncontrolled rectification according to claim 2, characterized in that: The improved constant AC voltage control strategy involves adding an active power controller to the outer loop to generate fluctuating AC voltage amplitude command values, while the inner loop uses constant AC voltage amplitude and frequency control, and also adds active AC filter control to filter out characteristic harmonic currents on the AC side of the DRU.

6. The hybrid offshore wind power transmission system based on SiC-MOSFET type MMC and diode uncontrolled rectification according to claim 3, characterized in that: The wind turbine provides a portion of reactive power, that is, under the premise of capturing the maximum active power output in the offshore wind farm, and without exceeding the upper limit of the equipment current and capacity, it provides a certain reactive power reference to the wind turbine according to the preset active-reactive curve.

7. The hybrid offshore wind power transmission system based on SiC-MOSFET type MMC and diode uncontrolled rectification according to claim 6, characterized in that: The active-reactive power curve should meet the following conditions: The reactive power output of the wind turbine is 30% of the active power. When the active power output is 0, the reactive power is also 0. As the active power output increases to the rated value of 1000MW, the reactive power output also increases linearly to 300MVar. As the active power output increases to 1200MW, the reactive power output increases linearly to the upper limit of 360Mvar. After that, as the active power output increases, the reactive power output always remains at the upper limit of 360MVar.

8. The hybrid offshore wind power transmission system based on SiC-MOSFET type MMC and diode uncontrolled rectification according to claim 3, characterized in that: For any alternative, the total reactive power that needs to be compensated for, excluding the small-capacity MMC, is... Q total It is calculated using the following formula: in: K Q This is the reactive power compensation margin coefficient. Q dru The total reactive power absorbed by the DRU Q WFSiCMMC Reactive power compensated for by small-capacity MMC.

9. The hybrid offshore wind power transmission system based on SiC-MOSFET type MMC and diode uncontrolled rectification according to claim 8, characterized in that: In Scheme 1, the reactive power to be compensated by the reactive power compensation capacitor is determined according to the following expression: In Scheme 2, the reactive power that the SVG needs to compensate is determined according to the following expression: in: Q wf The total reactive power compensated for by offshore wind farms. Q c This refers to the reactive power that the reactive power compensation capacitor needs to compensate. Q svg This represents the reactive power that the SVG needs to compensate for.

10. The hybrid offshore wind power transmission system based on SiC-MOSFET type MMC and diode uncontrolled rectification according to claim 8, characterized in that: In Scheme 3, the reactive power to be compensated by the reactive power compensation capacitor and the SVG is determined according to the following expression. Since the SVG has high engineering cost and large loss volume, the reactive power compensation of the SVG should be as small as possible under the premise of meeting the dynamic performance requirements of the system. After the reactive power compensation of the SVG is determined, the remaining reactive power is all compensated by the reactive power compensation capacitor. in: Q wf The total reactive power compensated for by offshore wind farms. Q c This refers to the reactive power that the reactive power compensation capacitor needs to compensate. Q svg This represents the reactive power that the SVG needs to compensate for.

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