Long-distance offshore energy island direct current transmission system and parallel and off-grid operation control method

Abstract

The application provides a long-distance offshore energy island direct-current power transmission system and a parallel and off-grid operation control method. After offshore wind power and offshore photovoltaic power are collected on an offshore energy island, power fluctuation is first smoothed by a hydrogen production and energy storage system, and then the electric energy is transmitted to a land power grid through a flexible direct-current transmission mode. When a sending-out system fails, off-grid operation of the offshore energy island system can be realized through the coordinated control of the wind power, photovoltaic power, hydrogen production, energy storage and flexible direct-current sending-out system. The offshore new energy power generation system can be fully improved in availability, the new energy output fluctuation can be smoothed, a solution is provided for large-scale new energy sending-out in the open sea, and the application has remarkable economic benefits and good application prospect.

Description

Technical Field

[0001] This invention belongs to the field of new energy power generation technology, specifically relating to a long-distance offshore energy island DC power transmission system and a method for controlling its operation in parallel and off-grid. Background Technology

[0002] Wind and solar power generation are gradually moving from onshore to offshore, and towards the deep sea. Offshore wind and solar power have significant advantages such as being close to load centers and not occupying land resources, making them the future development direction of new energy power generation.

[0003] When offshore wind and solar power are located far from shore, issues arise such as transmission capacity limitations imposed by submarine cables and high construction costs for transmission projects. Furthermore, large-scale centralized transmission of renewable energy also leads to significant power fluctuations, which are detrimental to the stability of the onshore AC power grid. Offshore energy islands aggregate offshore wind and solar power locally and utilize some of this renewable energy through methods like hydrogen production. Existing transmission schemes for offshore energy islands consider connecting to the onshore grid via AC submarine cables. However, when the energy island is far from shore, these cables suffer from excessive reactive power, limiting active power transmission capacity. Moreover, when the transmission cable fails or is scheduled to shut down, existing grid-connected operation schemes for offshore energy islands cannot maintain voltage stability, necessitating a complete system shutdown and reducing the availability of the offshore renewable energy generation system. Therefore, research is needed on DC transmission schemes for long-distance offshore energy islands and control strategies for both grid-connected and off-grid operation modes. Summary of the Invention

[0004] The purpose of this invention is to solve the problems of difficulties in large-scale offshore renewable energy transmission and large power fluctuations. It provides a design scheme and on-grid / off-grid operation control method for a long-distance offshore energy island DC transmission system. After offshore wind power and offshore photovoltaic power are gathered on the offshore energy island, the power fluctuations are first smoothed by hydrogen production and energy storage systems, and then the power is transmitted to the onshore power grid through flexible DC transmission. When the transmission system fails, the offshore energy island system can be operated off-grid by means of coordinated control of wind power, photovoltaic power, hydrogen production, energy storage and flexible DC transmission systems, so as to fully improve the availability of the offshore renewable energy power generation system.

[0005] To achieve the above-mentioned objectives, 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-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 busbar, 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-consumption 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 turbines are connected to the AC collection submarine cable via turbine-side converters and grid-side converters, and then connected to the AC bus of the energy island via the AC collection submarine cable. The photovoltaic modules are connected to the AC collection submarine cable via photovoltaic DC / DC converters and photovoltaic grid-connected converters, and then connected to the AC bus of the energy island via the AC collection submarine cable. The hydrogen production converters are connected to the AC bus of the energy island and the hydrogen production electrolyzer on both sides, respectively, rectifying the voltage of the AC bus of the energy island into DC to power the hydrogen production electrolyzer, which is connected to the hydrogen storage system. The energy storage grid-connected converters are connected to the AC bus of the energy island and the energy storage battery on both sides, respectively. The energy island's self-use power system is connected to the AC bus of the energy island. The AC side of the energy island MMC converter station is connected to the AC bus of the energy island via a flexible DC transmission system circuit breaker, and the DC side is connected to the DC transmission submarine cable. The DC transmission submarine cable is connected to the DC side of the onshore MMC converter station on the onshore side. 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 objectives, according to a second aspect of the present invention, the present invention adopts the following technical solution:

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

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

[0011] The MMC converter station on the energy island, under the grid-connected operation mode, adopts a constant AC grid voltage control strategy to control the AC bus voltage of the energy island; under the off-grid operation mode, it is out of operation.

[0012] The onshore MMC converter station, in the grid-connected operation mode on the energy island, adopts a constant DC bus voltage and reactive power control strategy, with the control objective being to maintain the stable voltage of the DC transmission submarine cable; in the off-grid operation mode on the energy island, it operates in STATCOM mode to provide necessary reactive power support for the onshore AC power grid;

[0013] The wind turbine's converter on the machine side adopts a maximum power point tracking (MPPT) control strategy under grid-connected operation mode on the energy island. By detecting wind speed in real time and tracking the maximum power generation curve, the wind turbine can track the maximum power point at different wind speeds. Under off-grid operation mode on the energy island, a constant 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 constant DC bus voltage and reactive power control strategy in both grid-connected and off-grid operation modes on the energy island, with the control objective being to maintain the stability of the wind turbine DC bus voltage.

[0015] The photovoltaic DC / DC converter, in the grid-connected operation mode of the energy island, adopts a maximum power point tracking control strategy. By real-time detection of the photovoltaic module's power generation voltage and tracking the maximum power generation curve, the photovoltaic module can track the maximum power point under different solar radiation and temperature environments. In the off-grid operation mode of the energy island, a constant 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 constant DC bus voltage and reactive power control strategy in both grid-connected and off-grid operation modes on the energy island, with the control objective being to maintain the stability of the photovoltaic DC bus voltage.

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

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

[0019] Furthermore, under the grid-connected operation mode of the energy island, the instructions involving the overall scheduling of the long-distance offshore energy island DC transmission system include active power instructions for hydrogen production converters, active 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 in the offshore energy island transmission system.

[0020] In the off-grid operation mode of the energy island, the instructions involving the overall scheduling of the long-distance offshore energy island DC transmission system include the active power instructions of the hydrogen production converter, the active power instructions of the wind turbine generator-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 in the offshore energy island transmission system.

[0021] By adopting the technical solution of this invention, offshore wind power and offshore photovoltaic power are aggregated on an offshore energy island. First, a hydrogen production and energy storage system is used to smooth out power fluctuations, and then the electricity is transmitted to the onshore power grid through flexible DC transmission. When the transmission system fails, the offshore energy island system can also be operated off-grid through the coordinated control of the wind power, photovoltaic, hydrogen production, energy storage and flexible DC transmission systems. This can significantly improve the availability of offshore new energy power generation systems, smooth out new energy output fluctuations, and provide a solution for large-scale transmission of new energy from the open sea, with significant economic benefits and good application prospects. Attached Figure Description

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

[0023] Figure 2 This is a typical topology diagram of the wind turbine machine-side converter in this invention.

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

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

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

[0027] Figure 6 This is a schematic diagram of a specific example system of the control method for onshore MMC converter stations in this invention.

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

[0029] Figure 8 This is a schematic diagram of a specific example system for the virtual synchronous machine control method used in the off-grid operation mode of the energy storage grid-connected converter in the energy island of the present invention. Detailed Implementation

[0030] 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.

[0031] In an embodiment of the present invention, a long-distance offshore energy island DC power transmission system is as follows: Figure 1 As shown, it includes an offshore wind turbine 1, a wind turbine-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 busbar 8, a flexible DC 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-consumption 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] Offshore wind turbine 1 is connected to AC collection submarine cable 7 via turbine-side converter 2 and grid-side converter 3, and then connected to the AC busbar 8 of the energy island via AC collection submarine cable 7; offshore photovoltaic module 4 is connected to AC collection submarine cable 7 via photovoltaic DC / DC converter 5 and photovoltaic grid-connected converter 6, and then connected to the AC busbar 8 of the energy island via AC collection submarine cable 7; hydrogen production converter 10 is connected to the AC busbar 8 of the energy island and the hydrogen production electrolyzer 11 on both sides respectively, rectifying the AC busbar voltage of the energy island into DC to power the hydrogen production electrolyzer, and the hydrogen production electrolyzer 11 is connected to the energy storage... The hydrogen system 12 is connected; the energy storage grid-connected converter 13 is connected to the energy island AC bus 8 and the energy storage battery 14 on both sides respectively; the energy island self-consumption 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 onshore side. The AC side of the onshore MMC converter station 18 is transformed by the onshore connection transformer 19 and then connected to the onshore AC power grid 20.

[0033] In this embodiment of the invention, the energy storage grid-connected converter 13 adopts the following... Figure 4 The diagram shows a three-phase, six-arm bridge topology. In grid-connected operation on the energy island, the energy storage grid-connected converter 13 employs a constant power control strategy, with its active and reactive power commands provided by the energy island's centralized control system. In off-grid operation on the energy island, the energy storage grid-connected converter 13 employs a virtual synchronous machine control strategy, responsible for controlling the AC bus voltage of the energy island, 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 synchronizer 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] Where, θ b (k+1) is the reference phase for 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 P is the rated angular frequency. bref P is the active power reference value. b (k) represents the active power in this sampling period, J represents the virtual rotor moment of inertia, and D... p This 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 for the next sampling period, |U b (k)| represents the voltage amplitude during this sampling period, U ref Q is the reference value for voltage amplitude. bref Q is the reactive power reference value. b (k) represents the reactive power in this sampling period, K is the virtual excitation coefficient, and D q This is the reactive damping coefficient.

[0041] In this embodiment of the invention, the onshore MMC converter station 18 adopts a symmetrical single-pole MMC topology, as shown in the topology diagram below. Figure 5 As shown, the system consists of six bridge arms, divided into upper and lower three phases. 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, as well as a capacitor. In grid-connected operation on the energy island, the onshore MMC converter station 18 employs a constant DC bus voltage and reactive power control strategy, with the control objective being to maintain stable DC transmission cable voltage. In off-grid operation on the energy island, the onshore MMC converter station 18 operates in STATCOM mode, providing necessary reactive power support to the onshore AC grid.

[0042] like Figure 6As shown, in this embodiment of the 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 circulating current control module 104, and an onshore converter station bridge arm voltage calculation module 105.

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

[0044]

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

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

[0047]

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

[0049] The implementation method 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 ω is the gain coefficient. c In this embodiment of the invention, the resonant frequency of the second-order generalized integrator is chosen to be ±100Hz, and the cutoff frequency is chosen to be 12Hz. coma2 ,u comb2 and u comc2 Corresponding to voltage vector 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.

[0052] In this embodiment of the invention, the energy island MMC converter station 16 adopts a symmetrical unipolar MMC topology, the topology diagram of which is shown below. Figure 5 As shown, the system consists of six bridge arms, divided into upper and lower three phases. 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, as well as a capacitor. In the grid-connected operation mode of the energy island, the energy island MMC converter station 16 adopts a constant AC grid voltage control strategy to control the AC bus voltage of the energy island. In the off-grid operation mode of the energy island, the energy island MMC converter station 16 is taken out of operation.

[0053] like Figure 7 As shown, in this embodiment of the 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 conversion module 203, an energy island converter station internal circulating current control module 204, and an energy island converter station bridge arm voltage calculation module 205.

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

[0055]

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

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

[0058] In this embodiment of the invention, the wind turbine side converter 2 adopts the following... Figure 2 The diagram shows a three-phase six-arm bridge topology. The control strategy for the wind turbine's machine-side converter 2 employs a dual-loop control system consisting of a power outer loop and a current inner loop. The output of the power outer loop, after passing through a limiting circuit, serves as the current reference value for the current inner loop. In grid-connected operation on the energy island, the wind turbine's machine-side converter 2 uses a maximum power point tracking (MPPT) control strategy. By real-time wind speed detection and tracking of the maximum power generation curve, the wind turbine can track its maximum power point at different wind speeds. In off-grid operation on the energy island, a constant power control strategy is adopted, with the active power command provided by the energy island's centralized control system.

[0059] In this embodiment of the invention, the wind turbine grid-side converter 3 adopts the following... Figure 4 The diagram shows a three-phase six-bridge topology. The control strategy for the wind turbine grid-side converter 3 employs a dual-loop control system, consisting of an outer loop for DC bus voltage and reactive power control, and an inner loop for current control. The output of the outer loop for DC bus voltage and reactive power control is used as the current reference value for the inner loop after passing through a limiting circuit. In both grid-connected and off-grid operation modes on the energy island, the wind turbine grid-side converter 3 adopts a constant DC bus voltage and reactive power control strategy, with the control objective being to maintain a stable DC bus voltage for the wind turbine.

[0060] In this embodiment of the invention, the photovoltaic DC / DC converter 5 adopts the following... Figure 3 The Boost circuit topology is shown. The control strategy of the photovoltaic DC / DC converter 5 adopts a dual-loop control with an outer loop for DC bus voltage and an inner loop for current. The output of the outer loop for DC bus voltage is used as the current reference value for the inner loop after passing through a limiting circuit. In the grid-connected operation mode of the energy island, the photovoltaic DC / DC converter 5 adopts a maximum power point tracking (MPPT) control strategy. By real-time detection of the photovoltaic module's generation voltage and tracking the maximum power generation curve, the photovoltaic module can track the maximum power point under different solar radiation and temperature environments. In the off-grid operation mode of the energy island, a constant power control strategy is adopted, and its active power command is given by the energy island centralized control system.

[0061] In this embodiment of the invention, the photovoltaic grid-connected converter 6 adopts the following... Figure 4The diagram shows a three-phase, six-arm bridge topology. The control strategy for the photovoltaic grid-connected converter 6 employs a dual-loop control system: an outer loop for DC bus voltage and reactive power control, and an inner loop for current control. The output of the outer loop for DC bus voltage and reactive power control is used as the current reference value for the inner loop after passing through a limiting circuit. In both grid-connected and off-grid operation modes on the energy island, the photovoltaic grid-connected converter 6 adopts a constant DC bus voltage and reactive power control strategy, with the control objective being to maintain a stable photovoltaic DC bus voltage.

[0062] In this embodiment of the invention, the hydrogen converter 10 employs, as shown in the example below. Figure 4 The diagram shows a three-phase six-bridge topology. The control strategy for the hydrogen converter 10 employs a dual-loop control system consisting of an outer loop for DC bus voltage and reactive power, and an inner loop for current. The output of the outer loop for DC bus voltage and reactive power is used as the current reference value for the inner loop after passing through a limiting circuit. In both grid-connected and off-grid operation modes on the energy island, the hydrogen converter 10 adopts a constant DC bus voltage and reactive power control strategy. The control objective is to regulate the hydrogen production power by controlling the DC bus voltage of the hydrogen electrolyzer. The DC bus voltage reference value is determined using the following method: based on the active power command of the hydrogen converter given by the energy island centralized control system and the active power-DC voltage characteristic curve of the hydrogen electrolyzer, the DC bus voltage reference value is determined.

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

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

[0065] The above description of the embodiments is provided to enable those skilled in the art to understand and apply the present invention. It will be apparent to those skilled in the art that various modifications can be made to the above embodiments, and the general principles described herein can be applied to other embodiments without inventive 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 long-distance offshore energy island DC power transmission system, characterized in that, include: Offshore wind turbines, wind turbine generator-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 DC transmission system circuit breakers, hydrogen production converters, hydrogen production electrolyzers, hydrogen storage systems, energy storage grid-connected converters, energy storage batteries, energy island self-consumption power systems, energy island MMC converter stations, DC transmission submarine cables, onshore MMC converter stations, onshore connection transformers, and onshore AC power grids; The offshore wind turbine is connected to the AC collection submarine cable via a turbine-side converter and a grid-side converter, and then connected to the AC bus of the energy island via the AC collection submarine cable. The photovoltaic modules are connected to the AC collection submarine cable via a photovoltaic DC / DC converter and a photovoltaic grid-connected converter, and then connected to the AC bus of the energy island via the AC collection submarine cable. The hydrogen production converter is connected to the AC bus of the energy island and the hydrogen production electrolyzer on both sides, respectively, rectifying the voltage of the AC bus of the energy island into DC to power the hydrogen production electrolyzer, which is connected to the hydrogen storage system. The energy storage grid-connected converter is connected to the AC bus of the energy island and the energy storage battery on both sides, respectively. The energy island's self-use power system is connected to the AC bus of the energy island. The AC side of the energy island MMC converter station is connected to the AC bus of the energy island via a circuit breaker of the flexible DC transmission system, and the DC side is connected to the DC transmission submarine cable. The DC transmission submarine cable is connected to the DC side of the onshore MMC converter station on the onshore side. The AC side of the onshore MMC converter station is transformed by the onshore connection transformer and then connected to the onshore AC grid. In the grid-connected operation mode of the energy island, the grid-connected energy storage converter adopts a constant power control strategy, and its active power and reactive power commands are given by the energy island centralized control system; in the off-grid operation mode of the energy island, the grid-connected energy storage converter adopts a virtual synchronous machine control strategy, which is responsible for controlling the AC bus voltage of the energy island. The control system for implementing the virtual synchronous machine control strategy includes: a virtual synchronous machine mechanical module, a virtual synchronous machine excitation module, a voltage outer loop controller, a current inner loop controller, a Park inverse transformation module, and a modulation module; In the virtual synchronizer mechanical module, the reference phase θ is calculated using the following method. b : θ b (k+1)=∫ω b (k+1)dt Where, θ b (k+1) is the reference phase for 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 P is the rated angular frequency. bref P is the active power reference value. b (k) represents the active power in this sampling period, J represents the virtual rotor moment of inertia, and D... p This is the active damping coefficient.

2. The grid-connected and off-grid operation control method for the long-distance offshore energy island DC transmission system as described in claim 1, characterized in that: The energy storage grid-connected converter adopts a constant power control strategy in the grid-connected operation mode of the energy island, and its active power and reactive power commands are given by the energy island centralized control system; in the off-grid operation mode of the energy island, it adopts a virtual synchronous machine control strategy to control the AC bus voltage of the energy island. The MMC converter station on the energy island, under the grid-connected operation mode, adopts a constant AC grid voltage control strategy to control the AC bus voltage of the energy island; under the off-grid operation mode, it is out of operation. The onshore MMC converter station, in the grid-connected operation mode on the energy island, adopts a constant DC bus voltage and reactive power control strategy, with the control objective being to maintain the stable voltage of the DC transmission submarine cable; in the off-grid operation mode on the energy island, it operates in STATCOM mode to provide necessary reactive power support for the onshore AC power grid; The wind turbine's converter on the machine side adopts a maximum power point tracking (MPPT) control strategy under grid-connected operation mode on the energy island. By detecting wind speed in real time and tracking the maximum power generation curve, the wind turbine can track the maximum power point at different wind speeds. Under off-grid operation mode on the energy island, a constant 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 constant DC bus voltage and reactive power control strategy in both grid-connected and off-grid operation modes on the energy island, with the control objective being to maintain the stability of the wind turbine DC bus voltage. The photovoltaic DC / DC converter, in the grid-connected operation mode of the energy island, adopts a maximum power point tracking control strategy. By real-time detection of the photovoltaic module's power generation voltage and tracking the maximum power generation curve, the photovoltaic module can track the maximum power point under different solar radiation and temperature environments. In the off-grid operation mode of the energy island, a constant 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 constant DC bus voltage and reactive power control strategy in both grid-connected and off-grid operation modes on the energy island, with the control objective being to maintain the stability of the photovoltaic DC bus voltage. The hydrogen converter adopts a constant DC bus voltage and reactive power control strategy in both grid-connected and off-grid operation modes on the energy island. The control objective is to adjust the hydrogen production power by controlling the DC bus voltage of the hydrogen electrolyzer. The reference value of the DC bus voltage is given according to the following method: the reference value of the DC bus voltage is determined based on the active power command of the hydrogen converter given by the energy island centralized control system and the active power-DC voltage characteristic curve of the hydrogen electrolyzer.

3. The on-grid and off-grid operation control method according to claim 2, characterized in that: In both grid-connected and off-grid operation modes on the energy island, instructions for the overall dispatch of the long-distance offshore 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 3, characterized in that: Under the grid-connected operation mode of the energy island, the instructions involving the overall scheduling of the long-distance offshore energy island DC transmission system include the active power instructions of the hydrogen production converter, the active 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 offshore energy island transmission system. In the off-grid operation mode of the energy island, the instructions involving the overall scheduling of the long-distance offshore energy island DC transmission system include the active power instructions of the hydrogen production converter, the active power instructions of the wind turbine generator-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 in the offshore energy island transmission system.

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