Remote offshore energy island direct current power transmission system and grid-connected and off-grid operation control method

Abstract

The invention provides a long-distance offshore energy island direct-current power transmission system and a grid-connected and off-grid operation control method, after offshore wind power and offshore photovoltaic power are gathered on an offshore energy island, power fluctuation is stabilized by using a hydrogen production and energy storage system, and then electric energy is transmitted to an onshore power grid in a flexible direct-current power transmission mode; when the sending-out system breaks down, off-grid operation of the offshore energy island system can be achieved through cooperative control of wind power, photovoltaic power, hydrogen production, energy storage and the flexible and straight sending-out system; the available rate of an offshore new energy power generation system can be fully improved, new energy output fluctuation is stabilized, a solution is provided for large-scale new energy output in open sea, and remarkable economic benefits and good application prospects are achieved.

Description

Technical Field

[0001] The present invention belongs to the technical field of renewable energy power generation, and in particular relates to a long-distance offshore energy island direct current transmission system and an on-grid and off-grid operation control method. Background Art

[0002] Wind power and photovoltaic power generation are gradually moving from land to sea and heading towards the deep sea. Offshore wind power and offshore photovoltaic power generation have significant advantages such as being close to the load center and not occupying land resources. They are the future development direction of new energy power generation.

[0003] When offshore wind power and offshore photovoltaic power are far from the shore, there are problems such as transmission capacity being limited by submarine cables and high construction costs of transmission projects. In addition, large-scale centralized transmission of new energy also has the problem of large power fluctuations, which is not conducive to the stability of the onshore AC power grid. The offshore energy island first gathers offshore wind power and photovoltaic power on the island, and completes the consumption of part of the new energy on the island through hydrogen production and other methods. The existing offshore energy island transmission scheme considers connecting to the onshore power grid through AC transmission submarine cables. When the energy island is far from the shore, the AC transmission submarine cable has the problem of excessive reactive power, resulting in limited active power transmission capacity. In addition, when the transmission submarine cable fails or is scheduled to be shut down, the existing offshore energy island grid-connected operation scheme cannot maintain voltage stability, and the entire system needs to be shut down, reducing the availability of the offshore new energy power generation system. Therefore, it is necessary to conduct research on the DC transmission scheme of long-distance offshore energy islands and the control strategy of the system under grid-connected and off-grid operation modes. Summary of the invention

[0004] The purpose of the present invention is to solve the problems of difficulty in transmitting large-scale offshore renewable energy and large fluctuations in electric energy, and to provide a design scheme for a long-distance offshore energy island DC transmission system and a method for controlling grid-connected and off-grid operation. After offshore wind power and offshore photovoltaic power are gathered on the offshore energy island, the power fluctuations are first smoothed by using hydrogen production and energy storage systems, and then the electric energy is transmitted to the onshore power grid through flexible DC transmission. When a failure occurs in the transmission system, the coordinated control of wind power, photovoltaic power, hydrogen production, energy storage and flexible DC transmission systems can be used to realize off-grid operation of the offshore energy island system, thereby fully improving the availability of the offshore renewable energy power generation system.

[0005] To achieve the above-mentioned object of the invention, according to the first aspect of the present invention, the present invention adopts the following technical solution:

[0006] A long-distance offshore energy island DC power transmission system, characterized in that it comprises: an offshore wind turbine, a wind turbine machine-side converter, a wind turbine grid-side converter, an offshore photovoltaic module, a photovoltaic DC / DC converter, a photovoltaic grid-connected converter, an AC collection submarine cable, an energy island AC bus, a flexible DC transmission system circuit breaker, a hydrogen production converter, a hydrogen production electrolyzer, a hydrogen storage system, an energy storage grid-connected converter, an energy storage battery, an energy island self-use power system, an energy island MMC converter station, a DC transmission submarine cable, an onshore MMC converter station, an onshore connection transformer, and an onshore AC power grid;

[0007] The offshore wind turbine is connected to the AC collection submarine cable through the wind turbine side converter and the wind turbine grid side converter, and is connected to the energy island AC bus through the AC collection submarine cable; the photovoltaic module is connected to the AC collection submarine cable through the photovoltaic DC / DC converter and the photovoltaic grid-connected converter, and is connected to the energy island AC bus through the AC collection submarine cable; the two sides of the hydrogen production converter are respectively connected to the energy island AC bus and the hydrogen production electrolyzer, and the energy island AC bus voltage is rectified into DC to supply power to the hydrogen production electrolyzer, and the hydrogen production electrolyzer is connected to the hydrogen storage system; the two sides of the energy storage grid-connected converter are respectively connected to the energy island AC bus and the energy storage battery; the energy island self-use power system is connected to the energy island AC bus; the AC side of the energy island MMC converter station is connected to the energy island AC bus through the flexible DC transmission system circuit breaker, and the DC side is connected to the DC transmission submarine cable, and the DC transmission submarine cable is connected to the DC side of the onshore MMC converter station on the land side, and the AC side of the onshore MMC converter station is transformed by the onshore connection transformer and then connected to the onshore AC power grid.

[0008] To achieve the above-mentioned object of the invention, according to the second aspect of the present invention, the present invention adopts the following technical solution:

[0009] A method for controlling the on-grid and off-grid operation of a long-distance offshore energy island DC transmission system, characterized in that:

[0010] The energy storage grid-connected converter adopts a fixed power control strategy in the energy island grid-connected operation mode, and its active power and reactive power instructions are given by the energy island centralized control system; in the energy island off-grid operation mode, a virtual synchronous machine control strategy is adopted to control the energy island AC bus voltage;

[0011] The energy island MMC converter station adopts a fixed AC grid voltage control strategy to control the AC bus voltage of the energy island when the energy island is connected to the grid; and exits operation when the energy island is off-grid.

[0012] The onshore MMC converter station, in the energy island grid-connected operation mode, adopts a fixed DC bus voltage and reactive power control strategy, and the control target is to maintain the DC transmission submarine cable voltage stable; in the energy island off-grid operation mode, it works in STATCOM mode to provide necessary reactive power support for the onshore AC power grid;

[0013] The wind turbine side converter adopts a maximum power tracking control strategy in the energy island grid-connected operation mode, detects the wind speed in real time, and tracks the maximum power generation curve, so that the wind turbine can track the maximum power point operation under different wind speeds; in the energy island off-grid operation mode, a fixed power control strategy is adopted, and its active power command is given by the energy island centralized control system;

[0014] The wind turbine grid-side converter adopts a fixed DC bus voltage and reactive power control strategy in both the energy island grid-connected and off-grid operation modes, and the control target is to maintain the DC bus voltage of the wind turbine stable;

[0015] The photovoltaic DC / DC converter adopts a maximum power tracking control strategy in the energy island grid-connected operation mode, which detects the power generation voltage of the photovoltaic module in real time and tracks the maximum power generation curve, so that the photovoltaic module can track the maximum power point operation under different sunshine and temperature environments; in the energy island off-grid operation mode, a fixed power control strategy is adopted, and its active power command is given by the energy island centralized control system;

[0016] The photovoltaic grid-connected converter adopts a fixed DC bus voltage and reactive power control strategy in both the energy island grid-connected and off-grid operation modes, and the control target is to maintain the stability of the photovoltaic DC bus voltage;

[0017] The hydrogen production converter adopts a fixed DC bus voltage and reactive power control strategy in both energy island grid-connected and off-grid operation modes. The control target is to adjust the hydrogen production power by controlling the DC bus voltage of the hydrogen production electrolyzer. The DC bus voltage reference value is given according to the following method: the DC bus voltage reference value is determined based on the active power instruction of the hydrogen production converter given by the energy island centralized control system and the active power-DC voltage characteristic curve of the hydrogen production electrolyzer.

[0018] Furthermore, in the energy island grid-connected operation mode and the energy island off-grid operation mode, instructions related to the overall dispatch of the long-distance offshore energy island DC transmission system are given by the energy island centralized control system.

[0019] Furthermore, in the energy island grid-connected operation mode, the instructions involving the overall dispatch of the long-distance offshore energy island DC transmission system include active power instructions for hydrogen production converters in the offshore energy island transmission system, active power and reactive power instructions for energy storage converters, reactive power instructions for wind turbine grid-side converters, and reactive power instructions for photovoltaic grid-connected converters.

[0020] In the off-grid operation mode of the energy island, the instructions involving the overall dispatch of the long-distance offshore energy island DC transmission system include the active power instructions of the hydrogen production converter in the offshore energy island transmission system, the active power instructions of the wind turbine side converter, the reactive power instructions of the wind turbine grid side converter, the active power instructions of the photovoltaic DC / DC converter, and the reactive power instructions of the photovoltaic grid-connected converter.

[0021] By adopting the technical solution of the present invention, after offshore wind power and offshore photovoltaic power are gathered on the offshore energy island, the power fluctuation is firstly smoothed by using the hydrogen production and energy storage system, and then the electric energy is transmitted to the onshore power grid through the flexible direct current transmission method; when the transmission system fails, the off-grid operation of the offshore energy island system can be realized through the coordinated control of wind power, photovoltaic power, hydrogen production, energy storage and flexible direct current transmission system; it can fully improve the availability of offshore new energy power generation system, smooth the fluctuation of new energy output, and provide a solution for large-scale new energy transmission in the distant sea, which has significant economic benefits and good application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 This is a typical topology diagram of the long-distance offshore energy island DC transmission system of the present invention.

[0023] Figure 2 This is a typical topological diagram of the wind turbine side converter in the present invention.

[0024] Figure 3 This is a typical topological diagram of the photovoltaic DC / DC converter in the present invention.

[0025] Figure 4 It is a typical topological diagram of the wind turbine grid-side converter, photovoltaic grid-connected converter, energy storage grid-connected converter, and hydrogen production converter in the present invention.

[0026] Figure 5 This is a typical topological diagram of the energy island MMC converter station and the onshore MMC converter station in the present invention.

[0027] Figure 6 It is a specific example system schematic diagram of the onshore MMC converter station control method of the present invention.

[0028] Figure 7 It is a specific example system schematic diagram of the energy island MMC converter station control method in the present invention.

[0029] Figure 8 It is a specific example system schematic diagram of the virtual synchronous machine control method adopted by the energy storage grid-connected converter in the energy island off-grid operation mode of the present invention. DETAILED DESCRIPTION

[0030] In order to describe the present invention more specifically, the technical solution of the present invention is described in detail below with reference to the accompanying drawings and specific embodiments.

[0031] In an embodiment of the present invention, a long-distance offshore energy island DC transmission system is as follows: Figure 1 As shown, it includes an offshore wind turbine 1, a wind turbine machine-side converter 2, a wind turbine grid-side converter 3, an offshore photovoltaic module 4, a photovoltaic DC / DC converter 5, a photovoltaic grid-connected converter 6, an AC collection submarine cable 7, an energy island AC bus 8, a flexible direct current transmission system circuit breaker 9, a hydrogen production converter 10, a hydrogen production electrolyzer 11, a hydrogen storage system 12, an energy storage grid-connected converter 13, an energy storage battery 14, an energy island self-use power system 15, an energy island MMC converter station 16, a DC transmission submarine cable 17, an onshore MMC converter station 18, an onshore connection transformer 19, and an onshore AC power grid 20.

[0032] The offshore wind turbine 1 is connected to the AC collection submarine cable 7 through the wind turbine machine-side converter 2 and the wind turbine grid-side converter 3, and is connected to the energy island AC bus 8 through the AC collection submarine cable 7; the offshore photovoltaic module 4 is connected to the AC collection submarine cable 7 through the photovoltaic DC / DC converter 5 and the photovoltaic grid-connected converter 6, and is connected to the energy island AC bus 8 through the AC collection submarine cable 7; the two sides of the hydrogen production converter 10 are respectively connected to the energy island AC bus 8 and the hydrogen production electrolyzer 11, the energy island AC bus voltage is rectified into DC, and the hydrogen production electrolyzer 11 is powered. The hydrogen system 12 is connected; the two sides of the energy storage grid-connected converter 13 are respectively connected to the energy island AC bus 8 and the energy storage battery 14; the energy island self-use power system 15 is connected to the energy island AC bus 8; the AC side of the energy island MMC converter station 16 is connected to the energy island AC bus 8 through the flexible DC transmission system circuit breaker 9, and the DC side is connected to the DC transmission submarine cable 17. The DC transmission submarine cable 17 is connected to the DC side of the onshore MMC converter station 18 on the land side. The AC side of the onshore MMC converter station 18 is transformed by the onshore connecting transformer 19 and connected to the onshore AC power grid 20.

[0033] In the embodiment of the present invention, the energy storage grid-connected converter 13 adopts Figure 4 The three-phase six-bridge arm topology shown. In the energy island grid-connected operation mode, the energy storage grid-connected converter 13 adopts a fixed power control strategy, and its active power and reactive power instructions are given by the energy island centralized control system. In the energy island off-grid operation mode, the energy storage grid-connected converter 13 adopts a virtual synchronous machine control strategy, which is responsible for controlling the energy island AC bus voltage, such as Figure 8 As shown, the control system for implementing the virtual synchronous machine control strategy includes: a virtual synchronous machine mechanical module 301 , a virtual synchronous machine excitation module 302 , a voltage outer loop controller 303 , a current inner loop controller 304 , a Park inverse transformation module 305 , and a modulation module 306 .

[0034] In the virtual synchronous machine mechanical module 301, the reference phase θ is calculated according to the following method: b :

[0035] θ b (k+1)=∫ω b (k+1)dt

[0036]

[0037] Among them, θ b (k+1) is the reference phase of the next sampling period, ω b (k+1) is the angular frequency of the next sampling period, ω b (k) is the angular frequency of this sampling period, ω n is the rated angular frequency, P bref is the active power reference value, P b (k) is the active power of this sampling period, J is the virtual rotor moment of inertia, D p is the active damping coefficient.

[0038] In the virtual synchronous machine excitation module 302, the d-axis voltage reference value u is calculated according to the following method: bdref :

[0039]

[0040] Among them, u bdref (k+1) is the d-axis voltage reference value of the next sampling period, |U b (k)| is the voltage amplitude of this sampling period, U ref is the voltage amplitude reference value, Q bref is the reactive power reference value, Q b (k) is the reactive power of this sampling period, K is the virtual excitation coefficient, D q is the reactive damping coefficient.

[0041] In the embodiment of the present invention, the onshore MMC converter station 18 adopts a symmetrical monopole MMC topology structure, and its topology diagram is as follows: Figure 5 As shown, it is divided into upper and lower three-phase with a total of six bridge arms. Each bridge arm contains multiple submodules connected in series. The submodules adopt a half-bridge topology. Each submodule contains two switching devices and matching anti-parallel diodes, and also contains a capacitor. In the energy island grid-connected operation mode, the onshore MMC converter station 18 adopts a fixed DC bus voltage and reactive power control strategy, and the control target is to maintain the DC transmission cable voltage stable; in the energy island off-grid operation mode, the onshore MMC converter station 18 works in STATCOM mode to provide necessary reactive power support for the onshore AC power grid.

[0042] like Figure 6As shown, in the embodiment of the present invention, the control system of the onshore MMC converter station 18 includes: an onshore converter station DC bus voltage and reactive power control module 101, an onshore converter station current control module 102, an onshore converter station Park inverse transformation module 103, an onshore converter station internal circulation control module 104, and an onshore converter station bridge arm voltage calculation module 105.

[0043] The implementation of the DC bus voltage and reactive power control module 101 of the onshore converter station is as follows:

[0044]

[0045] Among them: F PI1 (s) is the transfer function of the PI controller, k p1 is the proportionality coefficient, k i1 is the integral coefficient, i gdref2 ,i gqref2 Corresponding to the current vector I gdqref2 The d-axis and q-axis components, U dc2ref is the DC bus voltage reference value, U dc2 is the DC bus voltage, Q g2ref is the reactive power reference value, Q g2 is the reactive power.

[0046] The implementation of the onshore converter station current control module 102 is as follows:

[0047]

[0048] Among them: F PI2 (s) is the transfer function of the PI controller, k p2 is the proportionality coefficient, k i2 is the integration coefficient, u difd2 ,u difq2 The corresponding voltage vector is U difdq2 The d-axis and q-axis components, u gd ,u gq The corresponding voltage vector is U gdq The d-axis and q-axis components, i gd2 ,i gq2 Corresponding to the current vector I gdq2 The d-axis and q-axis components, ω g is the grid voltage angular frequency, L g For the filter inductor.

[0049] The implementation of the internal circulation control module 104 of the onshore converter station is as follows:

[0050]

[0051] Among them, FSOGI (s) is the transfer function of the second-order generalized integrator, k g is the gain coefficient, ω c is the cut-off frequency. In the embodiment of the present invention, the resonant frequency of the second-order generalized integrator is selected to be ±100 Hz, and the cut-off frequency is selected to be 12 Hz; u coma2 ,u comb2 and u comc2 The corresponding voltage vector is U comabc2 The a-axis, b-axis and c-axis components, i ca2 ,i cb2 and i cc2 Corresponding to the current vector I cabc2 The a-axis, b-axis, and c-axis components of

[0052] In the embodiment of the present invention, the energy island MMC converter station 16 adopts a symmetrical monopole MMC topology structure, and its topology diagram is as follows: Figure 5 As shown, it is divided into upper and lower three-phases with a total of six bridge arms, each bridge arm contains multiple sub-modules connected in series, the sub-modules adopt a half-bridge topology, and each sub-module contains two switching devices and matching anti-parallel diodes, and also contains a capacitor. In the energy island grid-connected operation mode, the energy island MMC converter station 16 adopts a fixed AC grid voltage control strategy and is responsible for controlling the energy island AC bus voltage; in the energy island off-grid operation mode, the energy island MMC converter station 16 is out of operation.

[0053] like Figure 7 As shown, in an embodiment of the present invention, the control system of the energy island MMC converter station 16 includes: an energy island converter station voltage control module 201, an energy island converter station current control module 202, an energy island converter station Park inverse transformation module 203, an energy island converter station internal circulation control module 204, and an energy island converter station bridge arm voltage calculation module 205.

[0054] The implementation of the energy island converter station voltage control module 201 is as follows:

[0055]

[0056] Among them: F PI3 (s) is the transfer function of the PI controller, k p3 is the proportionality coefficient, k i3 is the integral coefficient, i gdref3 ,i gqref3 Corresponding to the current vector I gdqref3 The d-axis and q-axis components, u gdref3 ,u gqref3 Corresponding to the voltage reference vector U gdqref3 The d-axis and q-axis components, u gd3 ,u gq3Corresponding to the voltage reference vector U gdq3 The d-axis and q-axis components of .

[0057] The specific implementation of the energy island converter station current control module 202 and the energy island converter station internal circulation control module 204 is consistent with the specific implementation of the onshore converter station current control module 102 and the onshore converter station internal circulation control module 104.

[0058] In the embodiment of the present invention, the fan side converter 2 adopts Figure 2 The three-phase six-bridge arm topology shown in the figure; the control strategy of the wind turbine side converter 2 adopts a dual-loop control of the power outer loop and the current inner loop. The output of the power outer loop is used as the current reference value of the current inner loop after passing through the limiting link. In the energy island grid-connected operation mode, the wind turbine side converter 2 adopts the maximum power tracking control strategy, which detects the wind speed in real time and tracks the maximum power generation curve, so that the wind turbine can track the maximum power point operation at different wind speeds; in the energy island off-grid operation mode, a fixed power control strategy is adopted, and its active power command is given by the energy island centralized control system.

[0059] In the embodiment of the present invention, the wind turbine grid-side converter 3 adopts Figure 4 The three-phase six-bridge arm topology structure shown in the figure; the control strategy of the wind turbine grid-side converter 3 adopts a dual-loop control of the DC bus voltage and reactive power control outer loop and the current inner loop. The output of the DC bus voltage and reactive power outer loop is used as the current reference value of the current inner loop after passing through the limiting link. In the energy island grid-connected and off-grid operation modes, the wind turbine grid-side converter 3 adopts a fixed DC bus voltage and reactive power control strategy, and the control target is to maintain the stability of the wind turbine DC bus voltage.

[0060] In the embodiment of the present invention, the photovoltaic DC / DC converter 5 adopts Figure 3 The Boost circuit topology shown in the figure; the control strategy of the photovoltaic DC / DC converter 5 adopts a dual-loop control of the DC bus voltage outer loop and the current inner loop. The output of the DC bus voltage outer loop is used as the current reference value of the current inner loop after passing through the limiting link. In the energy island grid-connected operation mode, the photovoltaic DC / DC converter 5 adopts a maximum power tracking control strategy, which detects the power generation voltage of the photovoltaic module in real time and tracks the maximum power generation curve, so that the photovoltaic module can track the maximum power point operation under different sunshine and temperature environments; in the energy island off-grid operation mode, a fixed power control strategy is adopted, and its active power command is given by the energy island centralized control system.

[0061] In the embodiment of the present invention, the photovoltaic grid-connected converter 6 adopts Figure 4The three-phase six-bridge arm topology shown in the figure; the control strategy of the photovoltaic grid-connected converter 6 adopts a dual-loop control of the DC bus voltage and reactive power control outer loop and the current inner loop. The output of the DC bus voltage and reactive power outer loop is used as the current reference value of the current inner loop after passing through the limiting link. In the energy island grid-connected and off-grid operation modes, the photovoltaic grid-connected converter 6 adopts a fixed DC bus voltage and reactive power control strategy, and the control target is to maintain the stability of the photovoltaic DC bus voltage.

[0062] In the embodiment of the present invention, the hydrogen production converter 10 adopts Figure 4 The three-phase six-bridge arm topology shown in the figure; the control strategy of the hydrogen production converter 10 adopts a dual-loop control of the DC bus voltage and reactive power outer loop and the current inner loop. The output of the DC bus voltage and reactive power outer loop is used as the current reference value of the current inner loop after passing through the limiting link. In the energy island grid-connected and off-grid operation modes, the hydrogen production converter 10 adopts a fixed DC bus voltage and reactive power control strategy. The control target is to adjust the hydrogen production power by controlling the DC bus voltage of the hydrogen production electrolyzer. The DC bus voltage reference value is given according to the following method: According to the active power instruction of the hydrogen production converter given by the energy island centralized control system and the active power-DC voltage characteristic curve of the hydrogen production electrolyzer, the DC bus voltage reference value is determined.

[0063] In the embodiment of the present invention, in the energy island grid-connected operation mode, the active power instruction of the hydrogen production converter in the offshore energy island power transmission system, the active power and reactive power instructions of the energy storage converter, the reactive power instruction of the wind turbine grid-side converter, the reactive power instruction of the photovoltaic grid-connected converter and other instructions involving the overall scheduling of the system are given by the energy island centralized control system;

[0064] In the off-grid operation mode of the energy island, the active power instructions of the hydrogen production converter in the offshore energy island transmission system, the active power instructions of the wind turbine side converter, the reactive power instructions of the wind turbine grid side converter, the active power instructions of the photovoltaic DC / DC converter, the reactive power instructions of the photovoltaic grid-connected converter and other instructions involving the overall scheduling of the system are given by the energy island centralized control system.

[0065] The above description of the embodiments is to facilitate the understanding and application of the present invention by those skilled in the art. It is obvious that those skilled in the art can easily make various modifications to the above embodiments and apply the general principles described herein to other embodiments without creative work. Therefore, the present invention is not limited to the above embodiments, and improvements and modifications made to the present invention by those skilled in the art based on the disclosure of the present invention should be within the scope of protection of the present invention.

Claims

1. A long-distance offshore energy island DC transmission system, characterized in that: include: Offshore wind turbines, wind turbine side converters, wind turbine grid side converters, offshore photovoltaic modules, photovoltaic DC / DC converters, photovoltaic grid-connected converters, AC collection submarine cables, energy island AC busbars, flexible direct current transmission system circuit breakers, hydrogen production converters, hydrogen production electrolyzers, hydrogen storage systems, energy storage grid-connected converters, energy storage batteries, energy island self-use power systems, energy island MMC converter stations, DC transmission submarine cables, onshore MMC converter stations, onshore connection transformers, onshore AC power grids; The offshore wind turbine is connected to the AC collection submarine cable through the wind turbine side converter and the wind turbine grid side converter, and is connected to the energy island AC bus through the AC collection submarine cable; the photovoltaic module is connected to the AC collection submarine cable through the photovoltaic DC / DC converter and the photovoltaic grid-connected converter, and is connected to the energy island AC bus through the AC collection submarine cable; the two sides of the hydrogen production converter are respectively connected to the energy island AC bus and the hydrogen production electrolyzer, and the energy island AC bus voltage is rectified into DC to supply power to the hydrogen production electrolyzer, and the hydrogen production electrolyzer is connected to the hydrogen storage system; the two sides of the energy storage grid-connected converter are respectively connected to the energy island AC bus and the energy storage battery; the energy island self-use power system is connected to the energy island AC bus; the AC side of the energy island MMC converter station is connected to the energy island AC bus through the flexible DC transmission system circuit breaker, and the DC side is connected to the DC transmission submarine cable, and the DC transmission submarine cable is connected to the DC side of the onshore MMC converter station on the land side, and the AC side of the onshore MMC converter station is transformed by the onshore connection transformer and then connected to the onshore AC power grid.

2. The method for controlling the on-grid and off-grid operation of the long-distance offshore energy island DC transmission system according to claim 1, characterized in that: The energy storage grid-connected converter adopts a fixed power control strategy in the energy island grid-connected operation mode, and its active power and reactive power instructions are given by the energy island centralized control system; in the energy island off-grid operation mode, a virtual synchronous machine control strategy is adopted to control the energy island AC bus voltage; The energy island MMC converter station adopts a fixed AC grid voltage control strategy to control the AC bus voltage of the energy island when the energy island is connected to the grid; and exits operation when the energy island is off-grid. The onshore MMC converter station, in the energy island grid-connected operation mode, adopts a fixed DC bus voltage and reactive power control strategy, and the control target is to maintain the DC transmission submarine cable voltage stable; in the energy island off-grid operation mode, it works in STATCOM mode to provide necessary reactive power support for the onshore AC power grid; The wind turbine side converter adopts a maximum power tracking control strategy in the energy island grid-connected operation mode, detects the wind speed in real time, and tracks the maximum power generation curve, so that the wind turbine can track the maximum power point operation under different wind speeds; in the energy island off-grid operation mode, a fixed power control strategy is adopted, and its active power command is given by the energy island centralized control system; The wind turbine grid-side converter adopts a fixed DC bus voltage and reactive power control strategy in both the energy island grid-connected and off-grid operation modes, and the control target is to maintain the DC bus voltage of the wind turbine stable; The photovoltaic DC / DC converter adopts a maximum power tracking control strategy in the energy island grid-connected operation mode, which detects the power generation voltage of the photovoltaic module in real time and tracks the maximum power generation curve, so that the photovoltaic module can track the maximum power point operation under different sunshine and temperature environments; in the energy island off-grid operation mode, a fixed power control strategy is adopted, and its active power command is given by the energy island centralized control system; The photovoltaic grid-connected converter adopts a fixed DC bus voltage and reactive power control strategy in both the energy island grid-connected and off-grid operation modes, and the control target is to maintain the stability of the photovoltaic DC bus voltage; The hydrogen production converter adopts a fixed DC bus voltage and reactive power control strategy in both energy island grid-connected and off-grid operation modes. The control target is to adjust the hydrogen production power by controlling the DC bus voltage of the hydrogen production electrolyzer. The DC bus voltage reference value is given according to the following method: the DC bus voltage reference value is determined based on the active power instruction of the hydrogen production converter given by the energy island centralized control system and the active power-DC voltage characteristic curve of the hydrogen production electrolyzer.

3. The on-grid and off-grid operation control method according to claim 2, characterized in that: In the energy island grid-connected operation mode and the energy island off-grid operation mode, the instructions for the overall dispatch of the long-distance offshore energy island DC transmission system are given by the energy island centralized control system.

4. The on-grid and off-grid operation control method according to claim 2, characterized in that: Under the energy island grid-connected operation mode, the instructions involving the overall dispatch of the long-distance offshore energy island DC transmission system include the active power instructions of the hydrogen production converter in the offshore energy island transmission system, the active power and reactive power instructions of the energy storage converter, the reactive power instructions of the wind turbine grid-side converter, and the reactive power instructions of the photovoltaic grid-connected converter. In the off-grid operation mode of the energy island, the instructions involving the overall dispatch of the long-distance offshore energy island DC transmission system include the active power instructions of the hydrogen production converter in the offshore energy island transmission system, the active power instructions of the wind turbine side converter, the reactive power instructions of the wind turbine grid side converter, the active power instructions of the photovoltaic DC / DC converter, and the reactive power instructions of the photovoltaic grid-connected converter.

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