A DC-DC collection system for offshore wind power with medium- and high-frequency AC transmission.

Abstract

This invention discloses a full DC aggregation system for offshore wind power with medium- and high-frequency AC transmission, relating to the field of DC power transmission technology. It includes: an offshore wind turbine module for acquiring medium-voltage DC power; a multi-port DC transformer module for converting the medium-voltage DC power into high-voltage DC power; a DC submarine cable module for aggregating the high-voltage DC power and transmitting it to an onshore converter station module; an onshore converter station module for acquiring onshore high-voltage AC power based on the aggregated offshore high-voltage DC power; and an onshore AC module for receiving the onshore high-voltage AC power, thus achieving full DC aggregation of offshore wind power. This invention, through improvements to the topology and control methods of the medium-voltage-to-high-voltage DC transformer, achieves efficient and economical power transmission, improves the economic viability and engineering application value of offshore wind power aggregation systems, and meets the market demand for affordable and low-cost power generation.

Description

Technical Field

[0001] This invention relates to the field of DC power transmission technology, and in particular to a full DC collection system for offshore wind power that combines medium and high frequency AC power. Background Technology

[0002] With the continuous development of new power systems, offshore wind power is showing a trend towards deeper, larger-scale, and more market-oriented development. This increasing focus on deeper offshore wind power has led to a continuous expansion of the transmission capacity of offshore wind power systems. Simultaneously, wind turbine equipment is trending towards larger sizes and higher power, with single-unit capacities gradually increasing to over 20MW. This simplifies the medium-voltage collection process for offshore wind power, avoiding the use of low-voltage to medium-voltage DC transformers. As transmission capacity increases, the all-DC scheme employing medium- and high-frequency conversion technology for DC collection and transmission offers advantages in efficiency, power density, and cost for offshore wind power transmission systems. However, the medium-voltage to high-voltage DC transformer (DCT), as the core equipment in the all-DC scheme, currently faces challenges such as difficult insulation design, high construction costs, and difficulties in manufacturing AC isolation transformers. This results in large and heavy offshore platforms with high investment costs, making it difficult to meet the future market-oriented requirements for low-cost and affordable offshore wind power.

[0003] Furthermore, in existing conventional deep-sea wind power all-DC aggregation schemes, the high-power DC-DC converter equipment used for medium-voltage DC to high-voltage DC conversion faces a triangular contradiction: difficulty in balancing construction cost, compactness, and technological advancement. Modular series-parallel DCTs, in particular, suffer from typical drawbacks in high-voltage applications, such as difficulties in insulation design and numerous isolation transformers, thus limiting their application to medium voltage. Although various improved topologies and controls based on Modular Multi-level Converters (MMCs) for isolating high-voltage DCTs have been proposed to reduce costs and improve efficiency, the constructed DCTs contain a large number of additional control components and distributed capacitors, resulting in disadvantages in cost, size, and weight. Simultaneously, the use of non-sinusoidal modulation increases the manufacturing difficulty of the AC isolation transformer, further reducing the engineering practicality of this type of high-voltage DCT.

[0004] Therefore, there is an urgent need for a full DC collection system for offshore wind power that combines medium and high frequency AC signals to address the shortcomings of existing technologies. Summary of the Invention

[0005] This invention aims to propose a full DC collection system for offshore wind power with medium- and high-frequency AC transmission, in order to solve the problems of insulation design difficulties, high construction costs, large size and weight, and insufficient control stability of medium- and high-voltage DC transformers in existing deep-sea wind power full DC collection systems, and to achieve efficient, economical, and compact transmission of offshore wind power.

[0006] To achieve the above objectives, the present invention provides an offshore wind power all-DC collection system with medium and high frequency AC transmission, comprising: an offshore wind turbine module, a multi-port DC transformer module, a DC submarine cable module, an onshore converter station module, and an onshore AC module;

[0007] The offshore wind turbine module is used to obtain medium-voltage DC power at sea;

[0008] The multi-port DC transformer module is used to convert the medium-voltage DC power at sea into high-voltage DC power at sea.

[0009] The DC submarine cable module is used to collect the offshore high-voltage DC power, obtain the collected offshore high-voltage DC power, and transmit it to the onshore converter station module.

[0010] The onshore converter station module is used to acquire onshore high-voltage alternating current based on the collected offshore high-voltage direct current power.

[0011] The onshore AC module is used to receive the high-voltage AC power from the shore, realizing full DC collection of offshore wind power.

[0012] Optionally, the offshore wind turbine module includes a plurality of offshore wind turbines, each of which is connected to the multi-port DC transformer module;

[0013] Each of the offshore wind turbines uses maximum power point tracking control to obtain medium-voltage DC power at sea.

[0014] Optionally, the multi-port DC transformer module adopts a multi-port medium-voltage-high-voltage DC transformer, which is composed of several medium-voltage units, a medium-voltage high-frequency AC bus, an AC transformer, and a high-voltage unit.

[0015] Each of the offshore wind turbines is connected to the medium-voltage unit via a medium-voltage DC port, and several medium-voltage units are connected in parallel to the medium-voltage high-frequency AC busbar. The medium-voltage high-frequency AC busbar is connected to the high-voltage unit via the AC transformer.

[0016] Optionally, several of the medium-voltage units control the voltage of the marine medium-voltage DC power supply to a constant value, and the high-voltage unit controls the AC voltage amplitude and frequency of the medium-voltage high-frequency AC collecting bus to a constant value.

[0017] Optionally, the medium-voltage unit consists of a thyristor converter and a first modular multilevel converter;

[0018] The thyristor converter adopts a fixed trigger angle control mode, and the thyristor converter and the first modular multilevel converter are connected in DC parallel-AC parallel connection.

[0019] Optionally, the high-voltage unit consists of a diode rectifier, a second modular multilevel converter, and an AC transformer;

[0020] The diode rectifier and the second modular multilevel converter are connected in series on the DC side, and the AC side of the second modular multilevel converter is connected in parallel with the diode rectifier through the AC transformer.

[0021] Optionally, both the first modular multilevel converter and the second modular multilevel converter are full-bridge modular multilevel converters, wherein the full-bridge modular multilevel converter is a modular multilevel converter composed of full-bridge sub-modules;

[0022] The first modular multilevel converter adopts a constant DC voltage mode, while the second modular multilevel converter adopts a voltage amplitude / frequency control mode.

[0023] Optionally, the control logic for the constant DC voltage mode includes:

[0024] The deviation between the medium-voltage DC voltage reference value and the medium-voltage DC voltage measurement value is generated by proportional-integral modulation to generate the first inner loop DC port current reference value.

[0025] The deviation between the FBMMC capacitor voltage reference value and the FBMMC capacitor voltage measurement value is modulated by proportional-integral modulation to generate the first inner loop AC d-axis current reference value.

[0026] The deviation between the reactive power reference value and the reactive power measurement value on the medium-voltage side of the medium-frequency transformer is modulated by proportional-integral modulation to generate the first inner-loop AC q-axis current reference value.

[0027] The first inner loop DC port current reference value, the first inner loop AC d-axis current reference value, and the first inner loop AC q-axis current reference value are used to generate trigger pulse signals for inner loop control.

[0028] Optionally, the control logic for the voltage amplitude / frequency control mode includes:

[0029] The deviation between the FBMMC capacitor voltage reference value and the FBMMC capacitor voltage measurement value is modulated by proportional-integral modulation to generate the second inner loop DC port current reference value.

[0030] The deviation between the reference value of AC voltage frequency on the high-voltage side of the intermediate frequency transformer and the measured value of AC voltage frequency on the high-voltage side of the intermediate frequency transformer is modulated by proportional-integral modulation to generate the reference value of AC d-axis current in the second inner loop.

[0031] The deviation between the reference value of the AC voltage amplitude on the high-voltage side of the intermediate frequency transformer and the measured value of the AC voltage amplitude on the high-voltage side of the intermediate frequency transformer is modulated by proportional-integral modulation to generate the reference value of the second inner loop AC q-axis current.

[0032] The second inner loop DC port current reference value, the second inner loop AC d-axis current reference value, and the second inner loop AC q-axis current reference value are used to generate trigger pulse signals for inner loop control.

[0033] Optionally, the onshore converter station module controls the voltage of the offshore high-voltage direct current power to be a constant value.

[0034] Compared with the closest existing technology, the present invention has the following advantages:

[0035] The offshore wind power all-DC collection system with shared high-frequency AC transmission proposed in this invention fully leverages the economic advantages of DR / LCC transmission of high power and the technical advantages of FBMMC's flexibility and controllability, balancing the triangular contradiction between construction cost, compactness, and technological advancement, thereby improving system efficiency and power density. This achieves efficient and economical transmission of large-capacity offshore wind power, demonstrating significant economic and engineering application value. The multi-port medium-voltage-high-voltage DC transformer topology proposed in this invention achieves compactness and lightweight design of transformers for large-capacity wind power transmission scenarios, effectively controlling the construction cost of the offshore wind power collection system, adapting to market-driven parity and low-price requirements, and significantly improving the economic efficiency of constructing the all-DC offshore wind power collection system. Furthermore, the offshore wind power all-DC collection system with shared high-frequency AC transmission and its control method proposed in this invention effectively enhance the flexibility and stability of the collection system control, ensuring the safe and stable operation of the collection system, and further promoting the engineering practice and technological development of all-DC offshore wind power collection systems. Attached Figure Description

[0036] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0037] Figure 1 This is a structural diagram of an offshore wind power all-DC collection system with medium- and high-frequency AC transmission according to an embodiment of the present invention;

[0038] Figure 2 This is a schematic diagram of the topology of the offshore medium-high frequency AC offshore wind power all-DC collection system proposed in an embodiment of the present invention;

[0039] Figure 3This is a schematic diagram of the topology of a multi-port medium-voltage DC-common high-frequency AC bus-centralized high-voltage DC high-power medium-voltage DC-high-voltage DC converter equipment for a novel offshore wind power all-DC collection system proposed in an embodiment of the present invention.

[0040] Figure 4 This is a control block diagram of the multi-port high-voltage direct current converter proposed in an embodiment of the present invention. Detailed Implementation

[0041] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions in the embodiments of this invention will be clearly and completely described below with reference to specific embodiments and corresponding drawings. Obviously, the described embodiments are only a part of the embodiments of this invention, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0042] The terminology used in the embodiments section of this invention is for the purpose of explaining specific embodiments of the invention only, and is not intended to limit the invention.

[0043] like Figure 1 As shown, this embodiment of the invention provides an offshore wind power all-DC collection system with medium and high frequency AC transmission, including: an offshore wind turbine module, a multi-port DC transformer module, a DC submarine cable module, an onshore converter station module, and an onshore AC module;

[0044] The offshore wind turbine module is used to obtain medium-voltage DC voltage at sea;

[0045] The multi-port DC transformer module is used to convert the marine medium-voltage DC voltage into a marine high-voltage DC voltage.

[0046] The DC submarine cable module is used to collect the offshore high voltage DC voltage, obtain the collected offshore high voltage DC voltage, and transmit it to the onshore converter station module.

[0047] The onshore converter station module is used to obtain onshore high-voltage AC power based on the collected offshore high-voltage DC voltage.

[0048] The onshore AC module is used to receive the high-voltage AC power from the shore, realizing full DC collection of offshore wind power.

[0049] The system of this invention specifically includes multiple offshore wind turbines, a multi-port medium-voltage to high-voltage DC transformer, a DC submarine cable, an onshore converter station, and an onshore AC module, the topology of which is as follows: Figure 2As shown in the diagram, each wind turbine is connected to a multi-port medium-voltage to high-voltage DC transformer. This transformer is connected to an onshore converter station via a DC submarine cable module. The onshore converter station is connected to the onshore AC system via an AC transmission line. Furthermore, the system is equipped with corresponding control methods.

[0050] In some optional implementations, the offshore wind turbine module includes a plurality of offshore wind turbines, each of which is connected to the multi-port DC transformer module. Specifically:

[0051] Each offshore wind turbine uses maximum power point tracking (MPPT) to obtain medium-voltage DC power at sea.

[0052] In some optional embodiments, the multi-port DC transformer module adopts a multi-port medium-voltage-high-voltage DC transformer, which is composed of several medium-voltage units, a medium-voltage high-frequency AC bus, an AC transformer, and a high-voltage unit.

[0053] Each of the offshore wind turbines is connected to the medium-voltage unit via a medium-voltage DC port, and several medium-voltage units are connected in parallel to the medium-voltage high-frequency AC busbar. The medium-voltage high-frequency AC busbar is connected to the high-voltage unit via the AC transformer.

[0054] A multi-port medium-voltage to high-voltage DC transformer consists of multiple medium-voltage units, a medium-voltage high-frequency AC bus, an AC transformer, and a high-voltage unit. Its topology is as follows: Figure 3 As shown, each offshore wind turbine is connected to the medium-voltage side of a multi-port medium-voltage-high-voltage DC transformer. The medium-voltage unit provides a medium-voltage DC port for the offshore wind turbine to access the transformer. Each medium-voltage unit is connected in parallel to the medium-voltage high-frequency AC busbar. The medium-voltage high-frequency AC busbar is connected to the high-voltage unit through a transformer. The high-voltage unit provides a high-voltage DC port for high-voltage DC transmission.

[0055] In some alternative implementations, several medium-voltage units control the voltage of the marine medium-voltage DC power supply to a constant value, and the high-voltage unit controls the AC voltage amplitude and frequency of the medium-voltage high-frequency AC collecting bus to a constant value.

[0056] Specifically, the high-voltage unit of the multi-port medium-voltage to high-voltage DC transformer controls the AC voltage amplitude and frequency of the medium-voltage high-frequency AC collection bus to a constant value, and multiple medium-voltage units control the marine medium-voltage DC voltage of multiple DC ports to a constant value.

[0057] In some alternative implementations, the medium-voltage unit consists of a thyristor converter and a first modular multilevel converter;

[0058] The thyristor converter adopts a fixed trigger angle control mode, and the thyristor converter and the first modular multilevel converter are connected in DC parallel-AC parallel connection.

[0059] Specifically, the medium-voltage unit of a multi-port medium-voltage to high-voltage DC transformer consists of a line-commutated converter (LCC) and an MMC. The LCC and MMC are connected in parallel on the DC side and in parallel on the AC side. For example... Figure 4 As shown, the LCC section of the medium-voltage unit adopts a constant firing angle control mode.

[0060] The calculation formula for the LCC control principle is as follows:

[0061]

[0062] Where dc represents direct current, U dcL k is the DC-side voltage of the LCC section. L For the converter transformer turns ratio of the LCC section, V gd For v ga,b,c d component, V gq For v ga,b,c The q component, v ga,b,c It is the AC three-phase voltage at the point of common connection (PCC) to which the medium-voltage unit is connected, a* is the firing angle setpoint of the LCC section, and ω g For v ga,b,c angular frequency, L eL I represents the equivalent inductance of the LCC converter transformer. dc U is the DC side current. dcM U is the DC-side voltage of the MMC section. dc This refers to the DC side voltage of the medium-voltage unit.

[0063] In some alternative implementations, the high-voltage unit consists of a diode rectifier, a second modular multilevel converter, and an AC transformer;

[0064] The diode rectifier and the second modular multilevel converter are connected in series on the DC side, and the AC side of the second modular multilevel converter is connected in parallel with the diode rectifier through the AC transformer.

[0065] Specifically, the high-voltage unit of the multi-port medium-voltage to high-voltage DC transformer consists of a diode rectifier (DR), an MMC, and an AC transformer. The DR and MMC are connected in series on the DC side, and the MMC on the AC side is connected in parallel with the DR through the transformer.

[0066] In some optional implementations, both the first modular multilevel converter and the second modular multilevel converter are full-bridge modular multilevel converters, wherein the full-bridge modular multilevel converter is a modular multilevel converter composed of full-bridge sub-modules;

[0067] The first modular multilevel converter adopts a constant DC voltage mode, while the second modular multilevel converter adopts a voltage amplitude / frequency control mode.

[0068] Specifically, the MMC used in both the medium-voltage and high-voltage units of the multi-port medium-voltage-high-voltage DC transformer is a Full-Bridge Modular Multilevel Converter (FBMMC). An FBMMC is an MMC whose submodules are full-bridge submodules. For example... Figure 3 As shown, the FBMMC section of the medium-voltage unit uses a constant DC voltage U. dc The Q control mode is used to stabilize the DC voltage; the FBMMC section of the high-voltage unit adopts the voltage amplitude / frequency (V / f) control mode to maintain the AC voltage amplitude and frequency stability of the medium-voltage high-frequency AC bus.

[0069] The FBMMC provides three inner-loop control variables for the medium-voltage unit. The specific control principle is as follows:

[0070] V' g and V g These are the three-phase AC voltages of PCC, respectively. ga,b,c The vector and magnitude are then expressed as V' g Based on, i.e., V gd =V g And V gq =0, the AC-side dynamics of FBMMC can be expressed as:

[0071]

[0072] Among them, u Md,q The valve-side voltage u of FBMMC Ma,b,c The d and q components, u Ma,b,c L is the valve-side voltage of the FBMMC. cal i' represents the equivalent inductance of the medium-voltage unit. Md,q For the grid-side current i' of FBMMC Ma,b,c The d and q components, i' Ma,b,c R is the grid-side current of the FBMMC, t is time, and R is the current. cal Representing the resistance of the medium-voltage unit, J1 is the coefficient matrix, V Md,q The grid-side voltage v of the FBMMC Ma,b,cThe d and q components. The coefficient matrix J1 is represented as:

[0073]

[0074] Since the regulation variable of the FBMMC section is the bridge arm voltage u of the FBMMC. a,b,cn and u a,b,cp The subscripts "p" and "n" represent the upper arm and lower arm, respectively, i.e., u a,b,cn U is the lower arm voltage of the FBMMC. a,b,cp Let E be the upper arm voltage of the FBMMC. To decouple the two, an intermediate variable E is defined. dcM :

[0075]

[0076] Ignoring circulating current, the DC dynamics of the FBMMC section can be expressed as:

[0077]

[0078] Among them, L arm For the bridge arm inductance of FBMMC, U dcM This is the DC voltage of the FBMMC section.

[0079] In some optional implementations, the control logic for the constant DC voltage mode includes:

[0080] The deviation between the medium-voltage DC voltage reference value and the medium-voltage DC voltage measurement value is generated by proportional-integral modulation to generate the first inner loop DC port current reference value.

[0081] The deviation between the FBMMC capacitor voltage reference value and the FBMMC capacitor voltage measurement value is modulated by proportional-integral modulation to generate the first inner loop AC d-axis current reference value.

[0082] The deviation between the reactive power reference value and the reactive power measurement value on the medium-voltage side of the medium-frequency transformer is modulated by proportional-integral modulation to generate the first inner-loop AC q-axis current reference value.

[0083] The first inner loop DC port current reference value, the first inner loop AC d-axis current reference value, and the first inner loop AC q-axis current reference value are used to generate trigger pulse signals for inner loop control.

[0084] like Figure 4 As shown, U dcIn Q control mode, the deviation between the medium-voltage DC voltage reference value and the measured medium-voltage DC voltage value is modulated by proportional-integral (PI) to generate the inner-loop DC port current reference value. The deviation between the FBMMC capacitor voltage reference value and the measured FBMMC capacitor voltage value is modulated by PI to generate the inner-loop AC d-axis current reference value. The deviation between the medium-voltage side reactive power reference value and the measured medium-voltage side reactive power value of the medium-frequency transformer is modulated by PI to generate the inner-loop AC q-axis current reference value. Among them, the inner-loop DC port current reference value, the inner-loop AC d-axis current reference value, and the inner-loop AC q-axis current reference value are used to generate the trigger pulse signal for all switching devices in the FBMMC in the "three degrees of freedom" inner-loop control.

[0085] In some optional implementations, the control logic of the voltage amplitude / frequency control mode includes:

[0086] The deviation between the FBMMC capacitor voltage reference value and the FBMMC capacitor voltage measurement value is modulated by proportional-integral modulation to generate the second inner loop DC port current reference value.

[0087] The deviation between the reference value of AC voltage frequency on the high-voltage side of the intermediate frequency transformer and the measured value of AC voltage frequency on the high-voltage side of the intermediate frequency transformer is modulated by proportional-integral modulation to generate the reference value of AC d-axis current in the second inner loop.

[0088] The deviation between the reference value of the AC voltage amplitude on the high-voltage side of the intermediate frequency transformer and the measured value of the AC voltage amplitude on the high-voltage side of the intermediate frequency transformer is modulated by proportional-integral modulation to generate the reference value of the second inner loop AC q-axis current.

[0089] The second inner loop DC port current reference value, the second inner loop AC d-axis current reference value, and the second inner loop AC q-axis current reference value are used to generate trigger pulse signals for inner loop control.

[0090] like Figure 4 As shown, in the V / f control mode, the deviation between the FBMMC capacitor voltage reference value and the measured FBMMC capacitor voltage value is modulated by PI to generate the inner loop DC port current reference value. The deviation between the AC voltage frequency reference value and the measured AC voltage frequency value on the high-voltage side of the intermediate frequency transformer is modulated by PI to generate the inner loop AC d-axis current reference value. The deviation between the AC voltage amplitude reference value and the measured AC voltage amplitude value on the high-voltage side of the intermediate frequency transformer is modulated by PI to generate the inner loop AC q-axis current reference value. Among them, the inner loop DC port current reference value, the inner loop AC d-axis current reference value, and the inner loop AC q-axis current reference value are used to generate the trigger pulse signal for all switching devices in the FBMMC in the "three degrees of freedom" inner loop control.

[0091] In some alternative implementations, the onshore converter module controls the voltage of the offshore high-voltage direct current power to a constant value.

[0092] Specifically, the offshore wind power DC collection system, which uses medium- and high-frequency AC transmission, is controlled by an onshore converter station to maintain a constant high-voltage DC voltage.

[0093] Those skilled in the art will understand that embodiments of the present invention can be provided as methods, systems, or computer program products. Therefore, the present invention can take the form of a completely hardware embodiment, a completely software embodiment, or an embodiment combining software and hardware aspects. Furthermore, the present invention can take the form of a computer program product embodied on one or more computer-usable storage media (including, but not limited to, disk storage, CD-ROM, optical storage, etc.) containing computer-usable program code.

[0094] This invention is described with reference to flowchart illustrations and / or block diagrams of methods, apparatus (systems), and computer program products according to embodiments of this application. It will be understood that each block of the flowchart illustrations and / or block diagrams, and combinations of blocks in the flowchart illustrations and / or block diagrams, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, special-purpose computer, embedded processor, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, generate instructions for implementing the flowchart illustrations and / or block diagrams. Figure 1 One or more processes and / or boxes Figure 1 A device that provides the functions specified in one or more boxes.

[0095] These computer program instructions may also be stored in a computer-readable storage medium that can direct a computer or other programmable data processing device to function in a particular manner, such that the instructions stored in the computer-readable storage medium produce an article of manufacture including instruction means, which are implemented in a process Figure 1 One or more processes and / or boxes Figure 1 The function specified in one or more boxes.

[0096] These computer program instructions may also be loaded onto a computer or other programmable data processing equipment to cause a series of operational steps to be performed on the computer or other programmable equipment to produce a computer-implemented process, thereby providing instructions that execute on the computer or other programmable equipment for implementing the process. Figure 1 One or more processes and / or boxes Figure 1 The steps of the function specified in one or more boxes.

[0097] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the specific implementation of the present invention. Any modifications or equivalent substitutions that do not depart from the spirit and scope of the present invention should be covered within the scope of protection of the claims of the present invention.

Claims

1. A fully DC collection system for offshore wind power with medium- and high-frequency AC transmission, characterized in that, include: Offshore wind turbine modules, multi-port DC transformer modules, DC submarine cable modules, onshore converter station modules, and onshore AC modules; The offshore wind turbine module is used to obtain medium-voltage DC power at sea; The multi-port DC transformer module is used to convert the medium-voltage DC power at sea into high-voltage DC power at sea. The DC submarine cable module is used to collect the offshore high-voltage DC power, obtain the collected offshore high-voltage DC power, and transmit it to the onshore converter station module. The onshore converter station module is used to acquire onshore high-voltage alternating current based on the collected offshore high-voltage direct current power. The onshore AC module is used to receive the high-voltage AC power from the shore, realizing full DC collection of offshore wind power; The offshore wind turbine module includes several offshore wind turbines, each of which is connected to the multi-port DC transformer module. The multi-port DC transformer module adopts a multi-port medium-voltage-high-voltage DC transformer, which is composed of several medium-voltage units, a medium-voltage high-frequency AC busbar, an AC transformer, and a high-voltage unit. The medium-voltage unit consists of a thyristor converter and a first modular multilevel converter; The thyristor converter adopts a fixed trigger angle control mode, and the thyristor converter and the first modular multilevel converter are connected in DC parallel-AC parallel connection.

2. The offshore wind power all-DC collection system with medium- and high-frequency AC transmission as described in claim 1, characterized in that, Each of the offshore wind turbines uses maximum power point tracking control to obtain medium-voltage DC power at sea.

3. The offshore wind power all-DC collection system with medium- and high-frequency AC transmission as described in claim 2, characterized in that, Each of the offshore wind turbines is connected to the medium-voltage unit via a medium-voltage DC port. Several medium-voltage units are connected in parallel to the medium-voltage high-frequency AC busbar, and the medium-voltage high-frequency AC busbar is connected to the high-voltage unit via the AC transformer.

4. The offshore wind power all-DC collection system with medium- and high-frequency AC transmission as described in claim 3, characterized in that, The medium-voltage units control the voltage of the marine medium-voltage DC power supply to a constant value, and the high-voltage unit controls the AC voltage amplitude and frequency of the medium-voltage high-frequency AC collecting bus to a constant value.

5. A marine wind power all-DC collection system with medium- and high-frequency AC transmission as described in claim 3, characterized in that, The high-voltage unit consists of a diode rectifier, a second modular multilevel converter, and an AC transformer. The diode rectifier and the second modular multilevel converter are connected in series on the DC side, and the AC side of the second modular multilevel converter is connected in parallel with the diode rectifier through the AC transformer.

6. A marine wind power all-DC collection system with medium- and high-frequency AC transmission as described in claim 5, characterized in that, Both the first modular multilevel converter and the second modular multilevel converter are full-bridge modular multilevel converters, and the full-bridge modular multilevel converter is a modular multilevel converter composed of full-bridge sub-modules; The first modular multilevel converter adopts a constant DC voltage mode, while the second modular multilevel converter adopts a voltage amplitude mode. / Frequency control mode.

7. A marine wind power all-DC collection system with medium- and high-frequency AC transmission as described in claim 6, characterized in that, The control logic for the constant DC voltage mode includes: The deviation between the medium-voltage DC voltage reference value and the medium-voltage DC voltage measurement value is generated by proportional-integral modulation to generate the first inner loop DC port current reference value. The deviation between the FBMMC capacitor voltage reference value and the FBMMC capacitor voltage measurement value is modulated by proportional-integral modulation to generate the first inner loop AC d-axis current reference value. The deviation between the reactive power reference value and the reactive power measurement value on the medium-voltage side of the medium-frequency transformer is modulated by proportional-integral modulation to generate the first inner-loop AC q-axis current reference value. The first inner loop DC port current reference value, the first inner loop AC d-axis current reference value, and the first inner loop AC q-axis current reference value are used to generate trigger pulse signals for inner loop control.

8. A marine wind power all-DC collection system with medium- and high-frequency AC transmission as described in claim 6, characterized in that, The voltage amplitude / The control logic for frequency control mode includes: The deviation between the FBMMC capacitor voltage reference value and the FBMMC capacitor voltage measurement value is modulated by proportional-integral modulation to generate the second inner loop DC port current reference value. The deviation between the reference value of AC voltage frequency on the high-voltage side of the intermediate frequency transformer and the measured value of AC voltage frequency on the high-voltage side of the intermediate frequency transformer is modulated by proportional-integral modulation to generate the reference value of AC d-axis current in the second inner loop. The deviation between the reference value of the AC voltage amplitude on the high-voltage side of the intermediate frequency transformer and the measured value of the AC voltage amplitude on the high-voltage side of the intermediate frequency transformer is modulated by proportional-integral modulation to generate the reference value of the second inner loop AC q-axis current. The second inner loop DC port current reference value, the second inner loop AC d-axis current reference value, and the second inner loop AC q-axis current reference value are used to generate trigger pulse signals for inner loop control.

9. A marine wind power all-DC collection system with medium- and high-frequency AC transmission as described in claim 1, characterized in that, The onshore converter station module controls the voltage of the offshore high-voltage direct current power to a constant value.

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