Stability improvement control method for direct-current power transmission system of offshore energy island

Abstract

The invention discloses a stability improvement control method for a DC power transmission system of an offshore energy island, and the method employs the cooperative control of an energy island DC / DC booster station, an onshore DC / AC converter station, and a hydrogen production DC / DC converter, and can inhibit the fluctuation amplitude of the DC bus voltage of the energy island along with the change of wind power and photovoltaic output. When necessary, the change of new energy output can be responded in real time by adjusting the active power of the hydrogen production system, the influence of offshore new energy output fluctuation on the stability of the offshore energy island direct-current power transmission system is reduced, and the method has a good application prospect.

Description

Technical Field

[0001] The present invention belongs to the technical field of renewable energy power generation, and in particular relates to a method for improving the stability of a direct current transmission system on an offshore energy island. 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. The offshore energy island first collects offshore wind power and photovoltaic power on the island, completes the consumption of part of the new energy on the island through hydrogen production and other methods, and then sends it to the onshore AC power grid in a centralized manner, which can effectively solve the problems of deep-sea new energy power generation. Since the new energy power generation system, hydrogen production system, and offshore transmission system all have DC interfaces, the DC transmission system based on the DC collection bus on the offshore energy island can save a large number of AC-DC conversion links in the traditional transmission system.

[0004] However, existing research on methods to improve the stability of DC transmission systems on offshore energy islands is still very limited. Renewable energy generation has the problem of large power fluctuations, which is not conducive to the stability of DC transmission systems on offshore energy islands. In addition, DC transmission systems on offshore energy islands contain a large number of power electronic equipment, and the system structure and the interaction mechanism of each module are very complex. The stable operation of DC transmission systems on offshore energy islands cannot be guaranteed only by the existing DC transmission system control strategies and hydrogen production system control strategies. Summary of the invention

[0005] The purpose of the present invention is to reduce the impact of offshore renewable energy output fluctuations on the stability of an offshore energy island DC transmission system, and to provide a control method for improving the stability of an offshore energy island DC transmission system. Through the coordinated control of the energy island DC / DC booster station, the onshore DC / AC converter station and the hydrogen production DC / DC converter, the fluctuation amplitude of the energy island DC bus voltage with changes in wind power and photovoltaic output can be suppressed. When necessary, the active power of the hydrogen production system can be adjusted to respond to changes in renewable energy output in real time, thereby improving the stability of the offshore energy island DC transmission system.

[0006] In order to achieve the above-mentioned object of the invention, the present invention adopts the following technical scheme:

[0007] A method for improving the stability of an offshore energy island DC transmission system, characterized in that the offshore energy island DC transmission system comprises: 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;

[0008] 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 DC bus voltage of the energy island is transformed to a voltage level matching the hydrogen production electrolyzer, and the hydrogen production electrolyzer is connected to the hydrogen storage system; the two sides of the energy storage DC / DC converter are respectively connected to the DC bus of the energy island and the energy storage battery The energy island's self-use power system is connected to the energy island's DC busbar; the energy island's DC / DC boost station's low-voltage side is connected to the energy island's DC busbar, 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, and 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 energy island's 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 is used to boost the voltage; the onshore DC / AC converter station adopts an MMC topology structure;

[0009] The energy island DC / DC boost station adopts a virtual DC motor control strategy, the control target is to keep the DC voltage on the low-voltage side of the boost station stable and provide inertia support for the DC bus of the energy island. Its low-voltage side MMC converter adopts a DC bus voltage and reactive power control outer loop and a low-voltage side current control inner loop, responsible for controlling the low-voltage side DC bus voltage; its high-voltage side MMC converter adopts an AC voltage control outer loop and a high-voltage side current control inner loop, responsible for controlling the medium-frequency AC voltage to remain stable;

[0010] The control target of the onshore DC / AC converter station is to maintain the DC bus voltage of the converter station stable, using the DC bus voltage and reactive power control outer loop and the current control inner loop;

[0011] For the hydrogen production DC / DC converter, when the DC bus voltage of the energy island is between a first DC bus voltage threshold and a second DC bus voltage threshold, the hydrogen production active power reference value is directly given as the hydrogen production active power rated value; when the DC bus voltage of the energy island is greater than the second DC bus voltage threshold or less than the first DC bus voltage threshold, the hydrogen production active power reference value is calculated according to the hydrogen production active power rated value, the DC bus voltage rated value and the energy island DC bus voltage; then, according to the active power-DC voltage characteristic curve of the hydrogen production electrolyzer, the hydrogen production DC voltage reference value is determined; finally, the hydrogen production DC voltage is controlled by the hydrogen production DC voltage control outer loop and the hydrogen production DC current control inner loop.

[0012] Furthermore, the energy island DC / DC boost station control system for implementing the control method includes: a virtual DC motor control module, a low-voltage side DC bus voltage and reactive power control module, a low-voltage side current control module, a low-voltage side Park reverse transformation module, a low-voltage side internal circulation control module, a low-voltage side bridge arm voltage calculation module, a high-voltage side voltage control module, a high-voltage side current control module, a high-voltage side Park reverse transformation module, a high-voltage side internal circulation control module, and a high-voltage side bridge arm voltage calculation module;

[0013] In the virtual DC motor control module, the DC bus voltage reference value U is calculated according to formula (1): dc1ref :

[0014]

[0015] Among them, U dc1ref (k+1) is the DC bus voltage reference value of the next sampling period, U dc1ref (k) is the DC bus voltage reference value of this sampling period, U dc1n is the rated DC bus voltage, P g1ref is the active power reference value, P g1 (k) is the active power of this sampling period, J is the virtual moment of inertia, D p is the active damping coefficient.

[0016] Further, the control system of the hydrogen production DC / DC converter includes: a hydrogen production active power reference value calculation module, a hydrogen production DC voltage reference value calculation module, a hydrogen production DC voltage control module, a hydrogen production DC current control module, and a hydrogen production modulation module;

[0017] Among them, in the hydrogen production active power reference value calculation module, the hydrogen production active power reference value P is calculated according to formula (2): href :

[0018]

[0019] Where: Phref is the reference value of hydrogen production active power, P hrefn is the rated value of hydrogen production active power, U dcn1 is the rated DC bus voltage, U dc is the DC bus voltage of the energy island, U dcl is the first threshold of DC bus voltage, U dch is the second threshold of DC bus voltage, F PI1 (s) is the transfer function of the DC bus voltage droop PI controller, k p1 is the proportionality coefficient, k i1 is the integration coefficient.

[0020] By adopting the technical solution of the present invention and utilizing the coordinated control of the energy island DC / DC booster station, the onshore DC / AC converter station and the hydrogen production DC / DC converter, it is possible to suppress the fluctuation amplitude of the DC bus voltage of the energy island as the wind power and photovoltaic output change. When necessary, the active power of the hydrogen production system can be adjusted to respond to the change of the new energy output in real time, thereby reducing the impact of the fluctuation of the offshore new energy output on the stability of the offshore energy island DC transmission system. The present invention has good application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 This is a typical topological diagram of the offshore energy island DC transmission system of the present invention.

[0022] Figure 2 This is a typical topology diagram of the energy island DC / DC boost station in the present invention.

[0023] Figure 3 This is a typical topological diagram of the onshore DC / AC converter station in the present invention.

[0024] Figure 4 This is a typical topology diagram of the hydrogen production DC / DC converter in the present invention.

[0025] Figure 5 This is a specific example system schematic diagram of the energy island DC / DC boost station control method in the present invention.

[0026] Figure 6 The figure is a specific example system schematic diagram of the onshore DC / AC converter station control method of the present invention.

[0027] Figure 7 The schematic diagram is a specific example system of the hydrogen production DC / DC converter control method of the present invention. DETAILED DESCRIPTION

[0028] 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.

[0029] In an embodiment of the present invention, the offshore energy island DC transmission system is as follows: Figure 1 As 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.

[0030] 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.

[0031] In the embodiment of the present invention, the energy island DC / DC boost station 13 adopts Figure 2 The isolated MMC topology 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 is used to boost the voltage. In this embodiment, the rated frequency of the AC medium-frequency transformer is 400Hz. The energy island DC / DC boost station 13 adopts a virtual DC motor control strategy, and the control target is to keep the DC voltage on the low-voltage side of the boost station stable and provide inertia support for the energy island DC bus. Its low-voltage side MMC converter adopts a DC bus voltage and reactive power control outer loop and a current control inner loop, which is responsible for controlling the low-voltage side DC bus voltage; its high-voltage side MMC converter adopts an AC voltage control outer loop and a current control inner loop, which is responsible for controlling the medium-frequency AC voltage to remain stable.

[0032] like Figure 5As shown, in an embodiment of the present invention, the control system of the energy island DC / DC boost station 13 includes: a virtual DC motor control module 101, a low-voltage side DC bus voltage and reactive power control module 102, a low-voltage side current control module 103, a low-voltage side Park inverse transformation module 104, a low-voltage side internal circulation control module 105, a low-voltage side bridge arm voltage calculation module 106, a high-voltage side voltage control module 107, a high-voltage side current control module 108, a high-voltage side Park inverse transformation module 109, a high-voltage side internal circulation control module 110, and a high-voltage side bridge arm voltage calculation module 111.

[0033] The specific implementation of the control method of the energy island DC / DC boost station 13 is as follows:

[0034] The virtual DC motor control module 101 is used to calculate the DC bus voltage reference value U according to formula (1): dc1ref :

[0035]

[0036] Among them, U dc1ref (k+1) is the DC bus voltage reference value of the next sampling period, U dc1ref (k) is the DC bus voltage reference value of this sampling period, U dc1n is the rated DC bus voltage, P g1ref is the active power reference value, P g1 (k) is the active power of this sampling period, J is the virtual moment of inertia, D p is the active damping coefficient.

[0037] The low-voltage side DC bus voltage U dc1 Controlled by PI controller to follow 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 Controlled by PI controller to follow 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 ;

[0038] The low-voltage side current control module 103 is used to control the d-axis and q-axis currents I gd1 and I gq1 Use PI controller 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;

[0039] The low-voltage side Park inverse conversion module 104 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 ;

[0040] The internal circulation control module 105 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 ;

[0041] The low-voltage side bridge arm voltage calculation module 106 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;

[0042] The high-voltage side voltage control module 107 is used to control the d-axis and q-axis voltages U gd2 and U gq2 Use PI controller 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;

[0043] The high-voltage side current control module 108 is used to control the d-axis and q-axis currents I gd2 and I gq2 Use PI controller 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 ;

[0044] The high-voltage side Park inverse transformation module 109 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, θg2 Set to a standard sine wave with a frequency of 400Hz;

[0045] The internal circulating current control module 110 on the high-voltage side is used to control the internal circulating current I 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 ;

[0046] The high-voltage side bridge arm voltage calculation module 111 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 U nrefabc2 The high-voltage side MMC converter is controlled through the valve control module.

[0047] 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 3 As shown, it is divided into upper and lower three phases with a total of six bridge arms, each of which contains multiple submodules connected in series. The control target 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.

[0048] like Figure 6 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.

[0049] The specific implementation of the control method of the onshore DC / AC converter station 15 is as follows:

[0050] The DC bus voltage U dc3 The first PI controller is used to control 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 The second PI controller is used to control 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 ;

[0051] The DC / AC converter station d and q axis currents I gd3 and I gq3 The third PI controller is used 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 ;

[0052] 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 difabc3 The angle used by the Park inverse transformation is the AC grid phase θ g3 ;

[0053] 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 ;

[0054] 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.

[0055] In the embodiment of the present invention, the hydrogen production DC / DC converter 7 adopts Figure 4 The boost circuit topology shown in the figure. When the DC bus voltage of the energy island is between the first DC bus voltage threshold and the second DC bus voltage threshold, the active power reference value of hydrogen production is directly given as the active power rating of hydrogen production; when the DC bus voltage of the energy island is greater than the second DC bus voltage threshold or less than the first DC bus voltage threshold, the active power reference value of hydrogen production is calculated according to the active power rating of hydrogen production, the DC bus voltage rating and the DC bus voltage of the energy island; then, the DC voltage reference value of hydrogen production is determined according to the active power-DC voltage characteristic curve of the hydrogen production electrolyzer; finally, the DC voltage of hydrogen production is controlled by the outer loop of the DC voltage control of hydrogen production and the inner loop of the DC current control of hydrogen production.

[0056] like Figure 7As shown, in an embodiment of the present invention, the control system of the hydrogen production DC / DC converter 7 includes: a hydrogen production active power reference value calculation module 301, a hydrogen production DC voltage reference value calculation module 302, a hydrogen production DC voltage control module 303, a hydrogen production DC current control module 304, and a hydrogen production modulation module 305.

[0057] In the hydrogen production active power reference value calculation module 301, the hydrogen production active power reference value P is calculated according to formula (2): href :

[0058]

[0059] Where: P href is the reference value of hydrogen production active power, P hrefn is the rated value of hydrogen production active power, U dcn1 is the rated DC bus voltage, U dc is the DC bus voltage of the energy island, U dcl is the first threshold of DC bus voltage, U dch is the second threshold of DC bus voltage, F PI1 (s) is the transfer function of the DC bus voltage droop PI controller, k p1 is the proportionality coefficient, k i1 is the integration coefficient.

[0060] In the hydrogen production DC voltage reference value calculation module 302, according to the hydrogen production active power reference value P href And the active power-DC voltage characteristic curve of the hydrogen production electrolyzer, determine the DC voltage reference value U dchref .

[0061] In the hydrogen production DC voltage control module 303, a fourth PI controller is used to control the hydrogen production DC voltage U dch Control it to follow the hydrogen production DC voltage reference value U dchref The output of the fourth PI controller is used as the reference value of the hydrogen production DC current after passing through the limiting link. dchref .

[0062] In the hydrogen production DC current control module 304, five PI controllers are used to control the hydrogen production DC current I dch Control it to follow the hydrogen production DC current reference value I dchref The output of the fifth PI controller is used as the input of the hydrogen production modulation module 305 after passing through the limiting link. The hydrogen production modulation module 305 outputs a switch action signal to realize the control of the hydrogen production DC / DC converter.

[0063] 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 method for improving the stability of an offshore energy island DC transmission system, characterized in that: The offshore energy island DC power transmission system includes: 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, and 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 DC bus voltage of the energy island is transformed to a voltage level matching the hydrogen production electrolyzer, and the hydrogen production electrolyzer is connected to the hydrogen storage system; the two sides of the energy storage DC / DC converter are respectively connected to the DC bus of the energy island and the energy storage battery The energy island's self-use power system is connected to the energy island's DC busbar; the energy island's DC / DC boost station's low-voltage side is connected to the energy island's DC busbar, 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, and 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 energy island's 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 is used to boost the voltage; the onshore DC / AC converter station adopts an MMC topology structure; The energy island DC / DC boost station adopts a virtual DC motor control strategy, the control target is to keep the DC voltage on the low-voltage side of the boost station stable and provide inertia support for the DC bus of the energy island. Its low-voltage side MMC converter adopts a DC bus voltage and reactive power control outer loop and a low-voltage side current control inner loop, responsible for controlling the low-voltage side DC bus voltage; its high-voltage side MMC converter adopts an AC voltage control outer loop and a high-voltage side current control inner loop, responsible for controlling the medium-frequency AC voltage to remain stable; The control target of the onshore DC / AC converter station is to maintain the DC bus voltage of the converter station stable, using the DC bus voltage and reactive power control outer loop and the current control inner loop; For the hydrogen production DC / DC converter, when the DC bus voltage of the energy island is between a first DC bus voltage threshold and a second DC bus voltage threshold, the hydrogen production active power reference value is directly given as the hydrogen production active power rated value; when the DC bus voltage of the energy island is greater than the second DC bus voltage threshold or less than the first DC bus voltage threshold, the hydrogen production active power reference value is calculated according to the hydrogen production active power rated value, the DC bus voltage rated value and the energy island DC bus voltage; then, according to the active power-DC voltage characteristic curve of the hydrogen production electrolyzer, the hydrogen production DC voltage reference value is determined; finally, the hydrogen production DC voltage is controlled by the hydrogen production DC voltage control outer loop and the hydrogen production DC current control inner loop.

2. The offshore energy island DC transmission system stability improvement control method according to claim 1 is characterized in that: The energy island DC / DC boost station control system for implementing the control method includes: a virtual DC motor control module, a low-voltage side DC bus voltage and reactive power control module, a low-voltage side current control module, a low-voltage side Park reverse transformation module, a low-voltage side internal circulation control module, a low-voltage side bridge arm voltage calculation module, a high-voltage side voltage control module, a high-voltage side current control module, a high-voltage side Park reverse transformation module, a high-voltage side internal circulation control module, and a high-voltage side bridge arm voltage calculation module; In the virtual DC motor control module, the DC bus voltage reference value U is calculated according to formula (1): dc1ref : Among them, U dc1ref (k+1) is the DC bus voltage reference value of the next sampling period, U dc1ref (k) is the DC bus voltage reference value of this sampling period, U dc1n is the rated DC bus voltage, P g1ref is the active power reference value, P g1 (k) is the active power of this sampling period, J is the virtual moment of inertia, D p is the active damping coefficient.

3. The offshore energy island DC transmission system stability improvement control method according to claim 1 is characterized in that: The control system of the hydrogen production DC / DC converter includes: a hydrogen production active power reference value calculation module, a hydrogen production DC voltage reference value calculation module, a hydrogen production DC voltage control module, a hydrogen production DC current control module, and a hydrogen production modulation module; Among them, in the hydrogen production active power reference value calculation module, the hydrogen production active power reference value P is calculated according to formula (2): href : Where: P href is the reference value of hydrogen production active power, P hrefn is the rated value of hydrogen production active power, U dcn1 is the rated DC bus voltage, U dc is the DC bus voltage of the energy island, U dcl is the first threshold of DC bus voltage, U dch is the second threshold of DC bus voltage, F PI1 (s) is the transfer function of the DC bus voltage droop PI controller, k p1 is the proportionality coefficient, k i1 is the integration coefficient.

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