Marine energy island full-direct-current power transmission system and control method

Abstract

The invention discloses an offshore energy island full-direct-current power transmission system and a control method, after electric energy output by a wind driven generator is inverted into direct current, the direct current and direct-current electric energy output by a photovoltaic power generation unit are directly collected, and the direct current, hydrogen production equipment and energy storage equipment are jointly connected into an energy island direct-current bus; and the signals are transmitted to land in a high-voltage direct-current mode. According to the technical scheme, a large number of alternating current-direct current conversion links in a traditional alternating current scheme are not needed, the engineering construction cost can be remarkably reduced, the stability problem caused by the fact that a large number of power electronic devices are connected into an alternating current power grid can be avoided, 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 an offshore energy island full direct current power transmission system and a control method. Background Art

[0002] With the vigorous development of new energy, 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 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 collects 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, the offshore energy island transmission system contains a large number of power electronic equipment, all of which have DC interfaces. The existing scheme uses AC busbar collection, which requires additional DC / AC converter equipment on the DC interface, increasing the system cost. Therefore, it is urgent to propose an offshore energy island full DC transmission system and control method using DC busbar collection. Summary of the invention

[0004] The purpose of the present invention is to meet the actual needs of offshore wind power, photovoltaics, hydrogen production and long-distance power transmission systems, and to provide an offshore energy island full DC transmission system and control method. After the electric energy output by the wind turbine is inverted into DC, it is directly collected with the DC electric energy output by the photovoltaic power generation unit, and connected to the energy island DC bus together with the hydrogen production equipment and energy storage equipment. After being boosted by the energy island DC booster station, it is transmitted to land in a high-voltage DC manner. There is no need for a large number of AC-DC conversion links in traditional AC solutions, which can significantly reduce engineering construction costs.

[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] An offshore energy island full DC power transmission system, characterized in that it includes: an offshore wind turbine, a wind turbine AC / DC converter, an offshore photovoltaic module, a photovoltaic DC / DC converter, a DC collection submarine cable, an energy island DC bus, a hydrogen production DC / DC converter, a hydrogen production electrolyzer, a hydrogen storage system, an energy storage DC / DC converter, an energy storage battery, an energy island self-use power system, an energy island DC / DC booster station, a DC transmission submarine cable, an onshore DC / AC converter station, an onshore connection transformer, and an onshore AC power grid;

[0007] The offshore wind turbine generator is connected to the wind turbine AC / DC converter, and the generated electricity is inverted into DC by the wind turbine AC / DC converter and then connected to the DC bus of the energy island via a DC collection submarine cable; the photovoltaic module is connected to the photovoltaic DC / DC converter, and the generated electricity is boosted by the photovoltaic DC / DC converter and then connected to the DC bus of the energy island via a DC collection submarine cable; the two sides of the hydrogen production DC / DC converter are respectively connected to the DC bus of the energy island and the hydrogen production electrolyzer, and the voltage of the DC bus of the energy island is transformed to the same voltage as the hydrogen production electrolyzer. The hydrogen production electrolyzer is connected to the hydrogen storage system with matching voltage levels; the two sides of the energy storage DC / DC converter are connected to the energy island DC bus and the energy storage battery respectively; the energy island's self-use power system is connected to the energy island DC bus; the low-voltage side of the energy island DC / DC booster station is connected to the energy island DC bus, and the high-voltage side is connected to the DC transmission submarine cable. The DC transmission submarine cable is connected to the DC side of the onshore DC / AC converter station on the land side. The AC side of the onshore DC / AC converter station is transformed by the onshore connection transformer and connected to the onshore AC power grid;

[0008] The hydrogen production DC / DC converter adopts a DC-AC-DC topology, first inverts the DC voltage into a high-frequency AC voltage, boosts it through a high-frequency AC transformer, and then rectifies it into a DC voltage; wherein the low-voltage side inverter device adopts a parallel structure of multiple submodules, and each low-voltage side inverter device submodule adopts a single-phase controllable full-bridge converter; the high-voltage side rectifier device adopts a series structure of multiple submodules, and each high-voltage side rectifier device submodule adopts a single-phase uncontrolled full-bridge converter;

[0009] The energy island DC / DC boost station adopts an isolated MMC topology structure, and the AC sides of the high-voltage side MMC converter and the low-voltage side MMC converter are connected through an AC medium-frequency transformer, and the AC medium-frequency transformer realizes high transformation ratio boosting;

[0010] The onshore DC / AC converter station adopts an MMC topology structure.

[0011] 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:

[0012] A control method for a full DC power transmission system of an offshore energy island, characterized by:

[0013] The AC / DC converter of the wind turbine adopts a maximum power tracking control mode, 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 under different wind speeds; the control strategy of the AC / DC converter of the wind turbine adopts a dual-loop control structure of a power outer loop and a current inner loop, the active power reference value is given according to the maximum power point tracking curve, and the output of the power outer loop is used as the current reference value of the current inner loop after passing through a limiting link;

[0014] The photovoltaic DC / DC converter adopts a maximum power tracking control 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; the control strategy of the photovoltaic DC / DC converter adopts a dual-loop control structure of a DC bus voltage outer loop and a current inner loop. The DC bus voltage reference value is given according to the maximum power point tracking curve, and the output of the DC bus voltage outer loop is used as the current reference value of the current inner loop after passing through a limiting link;

[0015] The hydrogen production DC / DC converter adopts a DC bus voltage control outer loop and a hydrogen production current control inner loop. 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 DC / DC 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;

[0016] The energy storage DC / DC converter adopts a constant DC current control strategy and calculates a DC current reference value according to an active power instruction of the energy storage DC / DC converter given by the energy island centralized control system;

[0017] The control target of the energy island DC / DC boost station is to maintain the stability of the DC voltage on the low-voltage side of the boost station. The low-voltage side MMC converter adopts the DC bus voltage and reactive power control outer loop and the low-voltage side current control inner loop to control the low-voltage side DC bus voltage; the high-voltage side MMC converter adopts the high-voltage side AC voltage control outer loop and the high-voltage side current control inner loop to control the medium-frequency AC voltage to maintain stability;

[0018] The control target of the onshore DC / AC converter station is to keep the DC bus voltage of the converter station stable, and the DC bus voltage and reactive power are controlled in an outer loop and in a current control inner loop.

[0019] Furthermore, instructions related to the overall dispatch of the offshore energy island full DC transmission system are all given by the energy island centralized control system.

[0020] Furthermore, the overall system scheduling instructions include hydrogen production DC / DC converter power instructions, energy storage DC / DC converter active power instructions, wind turbine AC / DC converter start / stop instructions, and photovoltaic DC / DC converter start / stop instructions.

[0021] The beneficial effects of the present invention are as follows: by adopting the technical solution of the present invention, the electric energy output by the wind turbine is inverted into direct current, and then directly collected with the direct current electric energy output by the photovoltaic power generation unit, and connected to the DC bus of the energy island together with the hydrogen production equipment and the energy storage equipment. After being boosted by the DC booster station of the energy island, it is transmitted to the land in a high-voltage direct current manner. There is no need for a large number of AC-DC conversion links in traditional AC solutions, which can significantly reduce the cost of engineering construction and avoid the stability problems caused by the access of a large number of power electronic equipment to the AC power grid. It has significant economic benefits and good application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

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

[0023] Figure 2 This is a typical topology diagram of the AC / DC converter for a wind turbine in the present invention.

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

[0025] Figure 4 This is a typical topology diagram of the hydrogen production DC / DC converter in the present invention.

[0026] Figure 5 This is a typical topology diagram of the energy island DC / DC boost station in the present invention.

[0027] Figure 6 This is a typical topological diagram of the onshore DC / AC converter station in the present invention.

[0028] Figure 7 This is a specific example system schematic diagram of the energy island DC / DC boost station control method in the present invention.

[0029] Figure 8 The figure is a specific example system schematic diagram of the onshore DC / AC converter station control method 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 the embodiment of the present invention, the offshore energy island full DC transmission system is as follows Figure 1As shown, it includes an offshore wind turbine 1, a wind turbine AC / DC converter 2, an offshore photovoltaic module 3, a photovoltaic DC / DC converter 4, a DC collection submarine cable 5, an energy island DC bus 6, a hydrogen production DC / DC converter 7, a hydrogen production electrolyzer 8, a hydrogen storage system 9, an energy storage DC / DC converter 10, an energy storage battery 11, an energy island self-use power system 12, an energy island DC / DC booster station 13, a DC transmission submarine cable 14, an onshore DC / AC converter station 15, an onshore connection transformer 16, and an onshore AC power grid 17.

[0032] In the embodiment of the present invention, the offshore wind turbine 1 is connected to the wind turbine AC / DC converter 2, and the generated electricity is inverted into direct current by the wind turbine AC / DC converter 2 and then connected to the energy island DC bus 6 via the DC collection submarine cable 5; the offshore photovoltaic module 3 is connected to the photovoltaic DC / DC converter 4, and the generated electricity is boosted by the photovoltaic DC / DC converter 4 and then connected to the energy island DC bus 6 via the DC collection submarine cable 5; the two sides of the hydrogen production DC / DC converter 7 are respectively connected to the energy island DC bus 6 and the hydrogen production electrolyzer 8, and the energy island DC bus voltage is transformed to match the hydrogen production electrolyzer The voltage level of the hydrogen production electrolyzer 8 is connected to the hydrogen storage system 9; the two sides of the energy storage DC / DC converter 10 are respectively connected to the energy island DC bus 6 and the energy storage battery 11; the energy island self-use power system 12 is connected to the energy island DC bus 6; the low-voltage side of the energy island DC / DC booster station 13 is connected to the energy island DC bus 6, and the high-voltage side is connected to the DC transmission submarine cable 14, and the DC transmission submarine cable 14 is connected to the DC side of the onshore DC / AC converter station 15 on the land side, and the AC side of the onshore DC / AC converter station 15 is transformed by the onshore connecting transformer 16 and connected to the onshore AC power grid 17.

[0033] In the embodiment of the present invention, the fan AC / DC converter 2 adopts Figure 2 The three-phase six-bridge arm topology structure shown in the figure; the wind turbine AC / DC converter 2 adopts the maximum power tracking control mode, 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 under different wind speeds; the control strategy of the wind turbine AC / DC converter 2 adopts a dual-loop control structure of a power outer loop and a current inner loop. The active power reference value is given according to the maximum power point tracking curve, and 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.

[0034] In the embodiment of the present invention, the photovoltaic DC / DC converter 4 adopts Figure 3The Boost circuit topology shown in the figure; the photovoltaic DC / DC converter 4 adopts the maximum power tracking control 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; the control strategy of the photovoltaic DC / DC converter 4 adopts a dual-loop control structure of a DC bus voltage outer loop and a current inner loop. The DC bus voltage reference value is given according to the maximum power point tracking curve, and 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.

[0035] In the embodiment of the present invention, the hydrogen production DC / DC converter 7 adopts Figure 4 The DC-AC-DC topology shown in the figure first inverts the DC voltage into a high-frequency AC voltage, which is then rectified into a DC voltage of a higher voltage level after being stepped up by a high-frequency AC transformer; wherein the low-voltage side inverter device adopts a parallel structure of multiple submodules, and each low-voltage side inverter device submodule adopts a single-phase controllable full-bridge converter, that is, the switch device adopts a fully controlled type and an anti-parallel diode is set; the high-voltage side rectifier device adopts a series structure of multiple submodules, and each high-voltage side rectifier device submodule adopts a single-phase uncontrolled full-bridge converter, that is, the switch device adopts a diode. The hydrogen production DC / DC converter 7 adopts a DC bus voltage control outer loop and a hydrogen production current control inner loop. 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 DC / DC 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.

[0036] In the embodiment of the present invention, the energy storage DC / DC converter 10 adopts Figure 3 The Boost circuit topology shown; the energy storage DC / DC converter 10 adopts a constant DC current control strategy, and calculates the DC current reference value according to the energy storage DC / DC converter active power instruction given by the energy island centralized control system.

[0037] In the embodiment of the present invention, the energy island DC / DC boost station 13 adopts Figure 5 In the structure shown, the AC side of the high-voltage side MMC converter and the low-voltage side MMC converter are connected through an AC medium-frequency transformer, and the AC medium-frequency transformer realizes high transformation ratio boosting. The control target of the energy island DC / DC boost station 13 is to keep the DC voltage on the low-voltage side of the boost station stable. Its low-voltage side MMC converter adopts the DC bus voltage and reactive power control outer loop and the low-voltage side current control inner loop, which is responsible for controlling the low-voltage side DC bus voltage; its high-voltage side MMC converter adopts the high-voltage side AC voltage control outer loop and the high-voltage side current control inner loop, which is responsible for controlling the medium-frequency AC voltage to keep stable.

[0038] like Figure 7 As shown, in an embodiment of the present invention, the control system of the energy island DC / DC boost station 13 includes: a low-voltage side DC bus voltage and reactive power control module 101, a low-voltage side current control module 102, a low-voltage side Park inverse transformation module 103, a low-voltage side internal circulation control module 104, a low-voltage side bridge arm voltage calculation module 105, a high-voltage side voltage control module 106, a high-voltage side current control module 107, a high-voltage side Park inverse transformation module 108, a high-voltage side internal circulation control module 109, and a high-voltage side bridge arm voltage calculation module 110.

[0039] The specific implementation of the control method of the energy island DC / DC boost station 13 is as follows:

[0040] The low-voltage side DC bus voltage U dc1 The first PI controller is used to control the reference value U dc1ref The output of the controller is used as the reference value of the d-axis current of the low-voltage side MMC converter after passing through the limiting link. gdref1 ; For reactive power Q g1 The second PI controller is used to control the reference value Q g1ref The output of the controller is used as the reference value of the q-axis current of the low-voltage side MMC converter after passing through the limiting link. gqref1 ;

[0041] The low-voltage side current control module 102 is used to control the d-axis and q-axis currents I gd1 and I gq1 The third PI controller is used to control it to follow the reference value I gdref1 and I gqref1 The output of the controller is used as the reference differential mode voltage U of the low-voltage side MMC converter difdq1 ;

[0042] The low-voltage side Park inverse transformation module 103 is used to measure the reference differential mode voltage U difdq1 Perform Park inverse transformation to obtain the reference differential mode voltage U in the stationary three-phase coordinate system difabc1 The angle used by the Park inverse transformation is the AC grid phase θ g1 ;

[0043] The internal circulation control module 104 on the low-voltage side is used to control the internal circulation I of the low-voltage side MMC converter. cabc1 A resonant controller is used for control, and the output of the internal circulating current controller is used as the reference common mode voltage U of the low-voltage side MMC converter. comabc1 ;

[0044] The low-voltage side bridge arm voltage calculation module 105 is used to calculate the reference differential mode voltage U difabc1 and the reference common mode voltage U comabc1 , the reference voltage U of the upper and lower bridge arms of the low-voltage side MMC converter is obtained by calculation prefabc1 with U nrefabc1 And realize the control of the low-voltage side MMC converter through the valve control module;

[0045] The high-voltage side voltage control module 106 is used to control the d-axis and q-axis voltages U gd2 and U gq2 The fourth PI controller is used to control it to follow the reference value U gdref2 and U gqref2 The output of the controller is used as the reference current I of the high-voltage side MMC converter gdqref2 ; U gdref2 Set to the rated AC voltage amplitude, U gqref2 Set to 0;

[0046] The high-voltage side current control module 107 is used to control the d-axis and q-axis currents I gd2 and I gq2 The fifth PI controller is used to control it to follow the reference value I gdref2 and I gqref2 The output of the controller is used as the reference differential mode voltage U of the high-voltage side MMC converter difdq2 ;

[0047] The high-voltage side Park inverse transformation module 108 is used to measure the reference differential mode voltage U difdq2 Perform Park inverse transformation to obtain the reference differential mode voltage U in the stationary three-phase coordinate system difabc2 The angle used by the Park inverse transformation is the AC grid phase reference value θ g2 In this embodiment, θ g2 Set to a standard sine wave with a frequency of 400Hz;

[0048] The internal circulating current control module 109 on the high-voltage side is used to control the internal circulating current I of the high-voltage side MMC converter. cabc2 A resonant controller is used for control, and the output of the internal circulating current controller is used as the reference common mode voltage U of the high-voltage side MMC converter. comabc2 ;

[0049] The high-voltage side bridge arm voltage calculation module 110 is used to calculate the reference differential mode voltage U difabc2 and the reference common mode voltage U comabc2 , the reference voltage U of the upper and lower bridge arms of the high-voltage side MMC converter is obtained by calculation prefabc2 with Unrefabc2 The high-voltage side MMC converter is controlled through the valve control module.

[0050] In the embodiment of the present invention, the onshore DC / AC converter station 15 adopts a symmetrical monopolar MMC topology structure, and its topology diagram is as follows: Figure 6 As shown, it is divided into upper and lower three phases 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 a matching anti-parallel diode, and also contains a capacitor. The control goal of the onshore DC / AC converter station 15 is to maintain the DC bus voltage of the converter station stable, using the DC bus voltage and reactive power control outer loop and current control inner loop.

[0051] like Figure 8 As shown, in the embodiment of the present invention, the control system of the onshore DC / AC converter station 15 includes: an onshore converter station DC bus voltage and reactive power control module 201, an onshore converter station current control module 202, an onshore converter station Park inverse transformation module 203, an onshore converter station internal circulation control module 204, and an onshore converter station bridge arm voltage calculation module 205.

[0052] The specific implementation of the control method of the onshore DC / AC converter station 15 is as follows:

[0053] The DC bus voltage U dc3 Controlled by PI controller to follow the reference value U dc3ref The output of the controller is used as the reference value of the d-axis current of the DC / AC converter station after passing through the limiting link. gdref3 ; For reactive power Q g3 Controlled by PI controller to follow the reference value Q g3ref The output of the controller is used as the reference value of the q-axis current of the DC / AC converter station after passing through the limiting link. gqref3 ;

[0054] The DC / AC converter station d and q axis currents I gd3 and I gq3 Use PI controller to control it to follow the reference value I gdref3 and I gqref3 The output of the controller is used as the reference differential voltage U of the DC / AC converter station. difdq3 ;

[0055] The DC / AC converter station reference differential mode voltage U is measured by using the onshore converter station Park inverse transformation module 203. difdq3 Perform Park inverse transformation to obtain the reference differential mode voltage U in the stationary three-phase coordinate system difabc3The angle used by the Park inverse transformation is the AC grid phase θ g3 ;

[0056] The internal circulation control module 204 of the onshore converter station is used to control the internal circulation I of the DC / AC converter station. cabc3 A resonant controller is used for control, and the output of the internal circulating current controller is used as the reference common mode voltage U of the DC / AC converter station. comabc3 ;

[0057] The onshore converter station bridge arm voltage calculation module 205 is used to calculate the DC / AC converter station reference differential mode voltage U difabc3 and the reference common mode voltage U comabc3 , the reference voltage U of the upper and lower bridge arms of the DC / AC converter station is obtained by calculation prefabc3 with U nrefabc3 The DC / AC converter station is controlled through the valve control module.

[0058] 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. An offshore energy island full DC power transmission system, characterized in that: include: Offshore wind turbines, wind turbine AC / DC converters, offshore photovoltaic modules, photovoltaic DC / DC converters, DC collection submarine cables, energy island DC busbars, hydrogen production DC / DC converters, hydrogen production electrolyzers, hydrogen storage systems, energy storage DC / DC converters, energy storage batteries, energy island self-use power systems, energy island DC / DC booster stations, DC transmission submarine cables, onshore DC / AC converter stations, onshore connection transformers, onshore AC power grids; The offshore wind turbine generator is connected to the wind turbine AC / DC converter, and the generated electricity is inverted into DC by the wind turbine AC / DC converter and then connected to the DC bus of the energy island via a DC collection submarine cable; the photovoltaic module is connected to the photovoltaic DC / DC converter, and the generated electricity is boosted by the photovoltaic DC / DC converter and then connected to the DC bus of the energy island via a DC collection submarine cable; the two sides of the hydrogen production DC / DC converter are respectively connected to the DC bus of the energy island and the hydrogen production electrolyzer, and the voltage of the DC bus of the energy island is transformed to the same voltage as the hydrogen production electrolyzer. The hydrogen production electrolyzer is connected to the hydrogen storage system with matching voltage levels; the two sides of the energy storage DC / DC converter are connected to the energy island DC bus and the energy storage battery respectively; the energy island's self-use power system is connected to the energy island DC bus; the low-voltage side of the energy island DC / DC booster station is connected to the energy island DC bus, and the high-voltage side is connected to the DC transmission submarine cable. The DC transmission submarine cable is connected to the DC side of the onshore DC / AC converter station on the land side. The AC side of the onshore DC / AC converter station is transformed by the onshore connection transformer and connected to the onshore AC power grid; The hydrogen production DC / DC converter adopts a DC-AC-DC topology, first inverts the DC voltage into a high-frequency AC voltage, boosts it through a high-frequency AC transformer, and then rectifies it into a DC voltage; wherein the low-voltage side inverter device adopts a parallel structure of multiple submodules, and each low-voltage side inverter device submodule adopts a single-phase controllable full-bridge converter; the high-voltage side rectifier device adopts a series structure of multiple submodules, and each high-voltage side rectifier device submodule adopts a single-phase uncontrolled full-bridge converter; The energy island DC / DC boost station adopts an isolated MMC topology structure, and the AC sides of the high-voltage side MMC converter and the low-voltage side MMC converter are connected through an AC medium-frequency transformer, and the AC medium-frequency transformer realizes high transformation ratio boosting; The onshore DC / AC converter station adopts an MMC topology structure.

2. The control method of the offshore energy island full DC power transmission system according to claim 1 is characterized by: The AC / DC converter of the wind turbine adopts a maximum power tracking control mode, 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 under different wind speeds; the control strategy of the AC / DC converter of the wind turbine adopts a dual-loop control structure of a power outer loop and a current inner loop, the active power reference value is given according to the maximum power point tracking curve, and the output of the power outer loop is used as the current reference value of the current inner loop after passing through a limiting link; The photovoltaic DC / DC converter adopts a maximum power tracking control 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; the control strategy of the photovoltaic DC / DC converter adopts a dual-loop control structure of a DC bus voltage outer loop and a current inner loop. The DC bus voltage reference value is given according to the maximum power point tracking curve, and the output of the DC bus voltage outer loop is used as the current reference value of the current inner loop after passing through a limiting link; The hydrogen production DC / DC converter adopts a DC bus voltage control outer loop and a hydrogen production current control inner loop. 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 DC / DC 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; The energy storage DC / DC converter adopts a constant DC current control strategy and calculates a DC current reference value according to an active power instruction of the energy storage DC / DC converter given by the energy island centralized control system; The control target of the energy island DC / DC boost station is to maintain the stability of the DC voltage on the low-voltage side of the boost station. The low-voltage side MMC converter adopts the DC bus voltage and reactive power control outer loop and the low-voltage side current control inner loop to control the low-voltage side DC bus voltage; the high-voltage side MMC converter adopts the high-voltage side AC voltage control outer loop and the high-voltage side current control inner loop to control the medium-frequency AC voltage to maintain stability; The control target of the onshore DC / AC converter station is to keep the DC bus voltage of the converter station stable, and the DC bus voltage and reactive power are controlled in an outer loop and in a current control inner loop.

3. The control method according to claim 2, characterized in that: The instructions for the overall dispatch of the offshore energy island's full DC transmission system are all given by the energy island's centralized control system.

4. The control method according to claim 3, characterized in that: The overall dispatching instructions of the system include power instructions for the hydrogen production DC / DC converter, active power instructions for the energy storage DC / DC converter, start / stop instructions for the wind turbine AC / DC converter, and start / stop instructions for the photovoltaic DC / DC converter.

Images