Sea-land multi-source multi-terminal alternating current and direct current integrated system

Abstract

The invention relates to the technical field of alternating-current and direct-current sending-out systems. The sea-land multi-source multi-terminal alternating current and direct current integrated system is composed of a land multi-source energy sending-out system, an offshore wind power alternating current collection and direct current sending-out system, an inland receiving end power grid, an inland receiving end collection bus, an inland receiving end commutation platform, an inland receiving end system of an inland receiving end direct current overhead line and an integrated switching station. The land multi-source energy sending-out system comprises a multi-source energy cluster, a land sending end power grid, a land sending end collection bus, a land sending end current conversion platform and a land sending end direct current overhead line. The offshore wind power alternating current collecting and direct current sending-out system comprises an offshore wind power plant, an alternating current collecting network, an offshore current conversion platform and a direct current submarine cable. According to the method, on the basis of coordination control of offshore wind power lightweight output combined with offshore wind power and onshore multi-source energy collection, the offshore wind power transmission cost is remarkably reduced, the utilization rate of a power transmission channel is increased, offshore wind power output fluctuation is stabilized, and the absorption capacity of a power grid to offshore wind power is improved.

Description

Technical Field

[0001] The present invention relates to the technical field of AC-DC transmission systems, and particularly to a multi-source multi-terminal AC-DC integrated system for land and sea. Background Art

[0002] The existing grid connection technologies for large-scale application of offshore wind power mainly include the power-frequency AC transmission solution suitable for offshore wind power transmission and the power-frequency collection-DC transmission solution suitable for long-distance offshore wind power grid connection. However, both transmission solutions will increase the technical challenges and overall construction costs of the corresponding offshore wind power grid connection system due to the increase in project transmission capacity; at the same time, in terms of land-sea coordinated networking technology, whether it is a power-frequency, low-frequency or DC transmission system, after the submarine cable lands, it depends on the AC overhead line to transmit electric energy to the load center. However, the load center is usually far from the landing point, and the transmission from the coastal landing point to the AC overhead line is difficult to effectively play the advantage of land-sea coordinated overall power transmission. It can not only achieve the interconnection and mutual assistance of large-scale offshore wind power, but also face the problem of tight resources of the onshore coastal channels, that is, the traditional onshore AC backbone grid cannot well meet the access and consumption requirements of large-scale off-site wind power.

[0003] In addition, wind power generation has strong randomness, volatility and intermittency. Without considering the solution to the power support problem between the offshore wind farm and the onshore power grid in the existing offshore wind power grid connection technology, it cannot solve the restriction of insufficient consumption capacity of the onshore power grid. Directly connecting a single offshore wind power energy to the grid will inevitably pose a severe test to the safe and stable operation of the power grid. Summary of the Invention

[0004] In order to solve the above technical problems, the present invention provides a multi-source multi-terminal AC-DC integrated system for land and sea, which is designed based on the lightweight transmission technology of offshore wind power, combines the coordinated control mechanism of offshore wind power and onshore multi-source energy collection, significantly reduces the offshore wind power transmission cost, improves the utilization rate of the transmission channel, suppresses the output fluctuation of offshore wind power, and enhances the consumption capacity of the power grid for offshore wind power.

[0005] The present invention provides a multi-source multi-terminal AC-DC integrated system for land and sea, which includes an onshore multi-source energy transmission system, an offshore wind power AC collection and DC transmission system, an inland receiving system and an integrated switching station; The onshore multi-source energy transmission system includes a multi-source energy cluster, an onshore sending-end power grid, an onshore sending-end collecting bus, an onshore sending-end conversion platform and an onshore sending-end DC overhead line; the multi-source energy cluster and the onshore sending-end power grid are connected to the onshore sending-end conversion platform through the onshore sending-end collecting bus; the onshore sending-end conversion platform is connected to the integrated switching station through the onshore sending-end DC overhead line; The above-mentioned offshore wind power AC collection and DC transmission system includes an offshore wind farm, an AC collection network, an offshore converter platform, and a DC submarine cable; the offshore wind farm is connected to the offshore converter platform through the AC collection network; the offshore converter platform is connected to the integrated switchyard through the DC submarine cable; The above-mentioned inland receiving system includes an inland receiving power grid, an inland receiving collection bus, an inland receiving converter platform, and an inland receiving DC overhead line; the inland receiving power grid is connected to the inland receiving converter platform through the inland receiving collection bus; the inland receiving converter platform is connected to the integrated switchyard through the inland receiving DC overhead line.

[0006] Furthermore, the above-mentioned onshore multi-source energy transmission system, the offshore wind power AC collection and DC transmission system, the integrated switchyard, and the inland receiving system adopt a true bipolar connection form; the integrated switchyard includes a DC collection bus and a collection ground wire; The DC collection bus is respectively connected to the onshore sending-end DC overhead line, the DC submarine cable, and the inland receiving DC overhead line, and is used for collecting the electric energy of the onshore multi-source energy transmission system and the offshore wind power AC collection and DC transmission system, and sending the collected electric energy to the inland receiving system; The collection ground wire is used for integrating the inland receiving metal return line of the inland receiving system, the onshore sending-end metal return line of the onshore multi-source energy transmission system, and the offshore-end metal return line of the offshore wind power AC collection and DC transmission system.

[0007] Furthermore, the DC collection bus includes a first DC collection bus and a second DC collection bus; the first DC collection bus is used for collecting the electric energy of the onshore multi-source energy transmission system and the offshore wind power AC collection and DC transmission system; the second DC collection bus is used for transmitting the electric energy of the onshore multi-source energy transmission system; the first DC collection bus and the second DC collection bus are connected through a connection switch; the first DC collection bus and the second DC collection bus are respectively connected to the inland receiving system through an inland receiving DC overhead line.

[0008] Furthermore, the onshore sending-end converter platform includes an onshore sending-end converter and an onshore sending-end converter station coupling transformer; the onshore sending-end converter is connected to the onshore sending-end collection bus through the onshore sending-end converter station coupling transformer; The onshore sending-end converter includes at least one onshore sending-end sub-converter, and the onshore sending-end sub-converter is a converter with the ability of bidirectional current flow.

[0009] Further, the onshore sending-end converter platform performs constant DC voltage control based on a first control strategy; the first control strategy is an outer-loop DC voltage control strategy for coordinately controlling the total output active power of the onshore multi-source energy sending system and the offshore wind power AC collection and DC sending system and the DC voltage of the offshore wind power AC collection and DC sending system.

[0010] Further, the first control strategy includes: Performing outer-loop control based on the total output active power of the onshore multi-source energy sending system and the offshore wind power AC collection and DC sending system and the total output active power reference value to generate the d-axis DC voltage reference value of the DC voltage of the onshore sending-end converter platform; Performing DC voltage control based on the d-axis DC voltage reference value and the d-axis DC voltage value of the DC voltage of the onshore sending-end converter platform to generate the d-axis current reference value of the AC current of the onshore sending-end converter platform, and performing reactive power control based on the reactive power value and the reactive power reference value of the onshore sending-end converter platform to generate the q-axis current reference value of the AC current of the onshore sending-end converter platform; Generating the reference phase angle of the onshore sending-end converter platform based on the AC-side voltage of the onshore sending-end converter platform, and performing current control based on the reference phase angle of the onshore sending-end converter platform, the d-axis current value, the d-axis current reference value, the q-axis current value, and the q-axis current reference value of the AC current of the onshore sending-end converter platform, and the d-axis voltage value and the q-axis voltage value of the AC voltage of the onshore sending-end converter platform to generate the a-phase voltage, b-phase voltage, and c-phase voltage of the AC voltage of the onshore sending-end converter platform; Performing circulating current control based on the upper-bridge-arm current and lower-bridge-arm current of the j-phase of the onshore sending-end converter platform and the d-axis second-harmonic AC current reference value and q-axis second-harmonic AC current reference value of the second-harmonic AC current of the onshore sending-end converter platform to generate the a-phase voltage, b-phase voltage, and c-phase voltage of the circulating current voltage of the onshore sending-end converter platform; Performing algebraic sum operation on the a-phase voltage, b-phase voltage, and c-phase voltage of the AC voltage of the onshore sending-end converter platform and the a-phase voltage, b-phase voltage, and c-phase voltage of the circulating current voltage of the onshore sending-end converter platform to generate the AC voltage reference value of the onshore sending-end converter platform; Generating the modulation signal of the onshore sending-end converter platform based on the AC voltage reference value of the onshore sending-end converter platform and the capacitor voltages of each sub-module and the current values of each bridge arm of the onshore sending-end converter platform; Generating the trigger pulses of the switching devices of each sub-module in the upper and lower bridge arms of the onshore sending-end converter platform based on the modulation signal of the onshore sending-end converter platform.

[0011] Further, generating the a-phase voltage, b-phase voltage, and c-phase voltage of the AC voltage of the onshore sending-end converter platform includes: Generating the d-axis modulation ratio and q-axis modulation ratio of the AC modulation ratio of the onshore sending-end converter platform according to the d-axis current value, d-axis current reference value, q-axis current value, q-axis current reference value of the AC current of the onshore sending-end converter platform, and the d-axis voltage value and q-axis voltage value of the AC voltage of the onshore sending-end converter platform; Performing an inverse Park transformation according to the reference phase angle of the onshore sending-end converter platform and the d-axis modulation ratio and q-axis modulation ratio of the AC modulation ratio of the onshore sending-end converter platform to generate the a-phase modulation ratio, b-phase modulation ratio, and c-phase modulation ratio of the AC modulation ratio of the onshore sending-end converter platform; Generating the a-phase voltage, b-phase voltage, and c-phase voltage of the AC voltage of the onshore sending-end converter platform according to the a-phase modulation ratio, b-phase modulation ratio, and c-phase modulation ratio of the AC modulation ratio of the onshore sending-end converter platform.

[0012] Further, generating the a-phase voltage, b-phase voltage, and c-phase voltage of the circulating current voltage of the onshore sending-end converter platform includes: Generating the j-phase circulating current of the onshore sending-end converter platform according to the j-phase upper bridge arm current and j-phase lower bridge arm current of the onshore sending-end converter platform; Generating the d-axis circulating current voltage reference value and q-axis circulating current voltage reference value of the circulating current voltage of the onshore sending-end converter platform according to the j-phase circulating current of the onshore sending-end converter platform and the d-axis second-harmonic AC current reference value and q-axis second-harmonic AC current reference value of the second-harmonic AC current of the onshore sending-end converter platform; Performing an inverse Park transformation according to the d-axis circulating current voltage reference value and q-axis circulating current voltage reference value of the circulating current voltage of the onshore sending-end converter platform to generate the a-phase second-harmonic AC modulation ratio, b-phase second-harmonic AC modulation ratio, and c-phase second-harmonic AC modulation ratio of the second-harmonic AC modulation ratio of the onshore sending-end converter platform; Generating the a-phase voltage, b-phase voltage, and c-phase voltage of the circulating current voltage of the onshore sending-end converter platform according to the a-phase second-harmonic AC modulation ratio, b-phase second-harmonic AC modulation ratio, and c-phase second-harmonic AC modulation ratio of the second-harmonic AC modulation ratio of the onshore sending-end converter platform.

[0013] Further, generating the modulation signal of the onshore sending-end converter platform includes: Generating the number of upper bridge arm input sub-modules and the number of lower bridge arm input sub-modules of the onshore sending-end converter platform according to the AC voltage reference value of the onshore sending-end converter platform; Perform sub-module capacitor voltage balancing based on the number of sub-modules in the upper bridge arm and the number of sub-modules in the lower bridge arm of the onshore sending-end converter platform, as well as the capacitor voltages of each sub-module and the current values of each bridge arm of the onshore sending-end converter platform, and generate the modulation signal of the onshore sending-end converter platform.

[0014] Further, the offshore wind farm includes at least one offshore AC wind farm; the offshore AC wind farm includes one AC wind turbine generator, or a series of multiple AC wind turbine generators; The offshore converter platform includes an offshore converter station and an offshore converter station connecting transformer; The AC collection network includes AC collection submarine cables and an AC collection busbar; the offshore wind farm is connected to the AC collection busbar through the AC collection submarine cables, and the AC collection busbar is connected to the offshore converter station through the offshore converter station connecting transformer.

[0015] Further, the offshore converter station includes an uncontrolled rectifier and a modular multilevel converter; The onshore multi-source energy sending system and the offshore wind power AC collection DC sending system jointly charge the modular multilevel converter, and the modular multilevel converter charges the wind turbine generators in the offshore wind farm.

[0016] Further, when the uncontrolled rectifier is connected in series with the modular multilevel converter, the offshore converter station further includes a bypass switch for the uncontrolled rectifier; the positive and negative poles of the uncontrolled rectifier and the modular multilevel converter are symmetric.

[0017] Further, the offshore wind power AC collection DC sending system starts based on a first startup timing sequence; the first startup timing sequence includes: Close the bypass switch to enable the onshore multi-source energy sending system and the inland receiving-end system to jointly pre-charge the modular multilevel converter and the wind turbine generators in the offshore wind farm; After the pre-charging is completed, close the bypass switch, start the wind turbine generators, and establish the AC voltage of the AC collection busbar; When the AC voltage of the AC collection busbar meets the rectification conditions of the uncontrolled rectifier, establish the rated DC voltage of the offshore wind power AC collection DC sending system, and complete the conversion from AC to DC through the series-connected uncontrolled rectifier and modular multilevel converter.

[0018] Further, when the uncontrolled rectifier is connected in parallel with the modular multilevel converter, the offshore wind power AC collection DC sending system starts based on a second startup timing sequence; the second startup timing sequence includes: The modular multilevel converter and the wind turbines in the offshore wind farm are pre-charged jointly by the onshore multi-source energy transmission system and the inland receiving system; After the pre-charging is completed, start the wind turbines to establish the AC voltage of the AC collection bus; When the AC voltage of the AC collection bus meets the rectification conditions of the uncontrolled rectifier, establish the rated DC voltage of the offshore wind power AC collection DC transmission system, and complete the AC-to-DC conversion through the parallel-connected uncontrolled rectifier and the modular multilevel converter.

[0019] Further, when the offshore wind power AC collection DC transmission system adopts a true bipolar connection form, the offshore wind power AC collection DC transmission system includes a first offshore end metallic return line for paralleling the uncontrolled rectifier and a second offshore end metallic return line for paralleling the modular multilevel converter.

[0020] Further, the offshore conversion platform performs grid-forming control based on a second control strategy; the second control strategy includes: Perform AC voltage control according to the d-axis voltage value, d-axis voltage reference value, q-axis voltage value, and q-axis voltage reference value of the AC voltage of the offshore conversion platform to generate the d-axis current reference value and q-axis current reference value of the AC current of the offshore conversion platform; Generate the reference phase angle of the offshore conversion platform according to the AC side frequency of the offshore conversion platform, and perform current control according to the reference phase angle of the offshore conversion platform, the d-axis current value, d-axis current reference value, q-axis current value, and q-axis current reference value of the AC current of the offshore conversion platform, and the d-axis voltage value and q-axis voltage value of the AC voltage of the offshore conversion platform to generate the a-phase voltage, b-phase voltage, and c-phase voltage of the AC voltage of the offshore conversion platform; Perform circulating current control according to the j-phase upper bridge arm current and j-phase lower bridge arm current of the offshore conversion platform, and the d-axis second harmonic AC current reference value and q-axis second harmonic AC current reference value of the second harmonic AC current of the offshore conversion platform to generate the a-phase voltage, b-phase voltage, and c-phase voltage of the circulating current voltage of the offshore conversion platform; Perform algebraic sum operation on the a-phase voltage, b-phase voltage, and c-phase voltage of the AC voltage of the offshore conversion platform and the a-phase voltage, b-phase voltage, and c-phase voltage of the circulating current voltage of the offshore conversion platform to generate the AC voltage reference value of the offshore conversion platform; Generate the modulation signal of the offshore conversion platform according to the AC voltage reference value of the offshore conversion platform, and the capacitor voltages of each sub-module and the current values of each bridge arm of the offshore conversion platform; Generate trigger pulses for the switching devices of each sub-module in the upper and lower bridge arms of the offshore HVDC converter platform according to the modulation signal of the offshore HVDC converter platform.

[0021] Further, generating the phase-a voltage, phase-b voltage, and phase-c voltage of the AC voltage of the offshore HVDC converter platform includes: Generate the d-axis modulation ratio and q-axis modulation ratio of the AC modulation ratio of the offshore HVDC converter platform according to the d-axis current value, d-axis current reference value, q-axis current value, q-axis current reference value of the AC current of the offshore HVDC converter platform, and the d-axis voltage value and q-axis voltage value of the AC voltage of the offshore HVDC converter platform; Perform inverse Park transformation according to the reference phase angle of the offshore HVDC converter platform and the d-axis modulation ratio and q-axis modulation ratio of the AC modulation ratio of the offshore HVDC converter platform to generate the a-phase modulation ratio, b-phase modulation ratio, and c-phase modulation ratio of the AC modulation ratio of the offshore HVDC converter platform; Generate the phase-a voltage, phase-b voltage, and phase-c voltage of the AC voltage of the offshore HVDC converter platform according to the a-phase modulation ratio, b-phase modulation ratio, and c-phase modulation ratio of the AC modulation ratio of the offshore HVDC converter platform.

[0022] Further, the onshore receiving HVDC converter platform includes an onshore receiving HVDC converter and an onshore receiving HVDC converter station connecting transformer; the onshore receiving HVDC converter is connected to the onshore receiving collecting bus through the onshore receiving HVDC converter station connecting transformer; The onshore receiving HVDC converter includes at least one onshore receiving sub-HVDC converter, and the onshore receiving sub-HVDC converter is a converter with the ability of bidirectional current flow.

[0023] Further, the onshore receiving HVDC converter platform performs constant power control based on a third control strategy; the third control strategy includes: Perform active power control according to the active power value and active power reference value of the onshore receiving HVDC converter platform to generate the d-axis current reference value of the AC current of the onshore receiving HVDC converter platform, and perform reactive power control according to the reactive power value and reactive power reference value of the onshore receiving HVDC converter platform to generate the q-axis current reference value of the AC current of the onshore receiving HVDC converter platform; Generate the reference phase angle of the onshore receiving HVDC converter platform according to the AC side voltage of the onshore receiving HVDC converter platform, and perform current control according to the reference phase angle of the onshore receiving HVDC converter platform, the d-axis current value, d-axis current reference value, q-axis current value, q-axis current reference value of the AC current of the onshore receiving HVDC converter platform, and the d-axis voltage value and q-axis voltage value of the AC voltage of the onshore receiving HVDC converter platform to generate the phase-a voltage, phase-b voltage, and phase-c voltage of the AC voltage of the onshore receiving HVDC converter platform; Circulation control is performed based on the j-phase upper arm current and the j-phase lower arm current of the inland receiving end converter platform, as well as the d-axis second harmonic AC current reference value and the q-axis second harmonic AC current reference value of the second harmonic AC current of the inland receiving end converter platform, to generate the a-phase voltage, b-phase voltage, and c-phase voltage of the circulation voltage of the inland receiving end converter platform; An algebraic sum operation is performed on the a-phase voltage, b-phase voltage, and c-phase voltage of the AC voltage of the inland receiving end converter platform and the a-phase voltage, b-phase voltage, and c-phase voltage of the circulation voltage of the inland receiving end converter platform to generate the AC voltage reference value of the inland receiving end converter platform; Based on the AC voltage reference value of the inland receiving end converter platform, as well as the capacitor voltages of each sub-module and the current values of each arm of the inland receiving end converter platform, a modulation signal of the inland receiving end converter platform is generated; Based on the modulation signal of the inland receiving end converter platform, trigger pulses of each sub-module switching device in the upper and lower arms of the inland receiving end converter platform are generated.

[0024] Further, the generation of the a-phase voltage, b-phase voltage, and c-phase voltage of the AC voltage of the inland receiving end converter platform includes: Based on the d-axis current value, d-axis current reference value, q-axis current value, and q-axis current reference value of the AC current of the inland receiving end converter platform, as well as the d-axis voltage value and q-axis voltage value of the AC voltage of the inland receiving end converter platform, the d-axis modulation ratio and q-axis modulation ratio of the AC modulation ratio of the inland receiving end converter platform are generated; Based on the reference phase angle of the inland receiving end converter platform and the d-axis modulation ratio and q-axis modulation ratio of the AC modulation ratio of the inland receiving end converter platform, an inverse Park transformation is performed to generate the a-phase modulation ratio, b-phase modulation ratio, and c-phase modulation ratio of the AC modulation ratio of the inland receiving end converter platform; Based on the a-phase modulation ratio, b-phase modulation ratio, and c-phase modulation ratio of the AC modulation ratio of the inland receiving end converter platform, the a-phase voltage, b-phase voltage, and c-phase voltage of the AC voltage of the inland receiving end converter platform are generated.

[0025] The present invention provides a land-sea multi-source multi-terminal AC-DC integrated system. Compared with the prior art, the present invention has the following beneficial effects: 1) By combining the uncontrolled rectification technology with the flexible DC hybrid conversion technology to design the light-type transmission technology for offshore wind power, it can fully adapt to the complex environment of large-scale deep-sea and far-sea wind power access, greatly reduce the construction cost of the practical engineering platform for offshore wind power, and provide reliable technical support for promoting the large-scale development of far-sea wind farms; 2) By connecting offshore wind power, onshore multi-source energy and inland receiving systems through an integrated switching station, it can cross the already saturated AC grid, directly send electric energy to the load center, realize the mutual supply of DC electric energy among multiple regions, and at the same time effectively solve the problem of tight resources in the onshore coastal transmission channels, improving the utilization rate of transmission channels; 3) By sending out the electric energy of the onshore multi-source energy sending system and the AC collection and DC sending system of offshore wind power after collecting them based on the integrated switching station, combined with the reliable coordinated control design of offshore wind power and multi-source energy, it can effectively suppress the output fluctuation of offshore wind power, improve the consumption capacity of large-scale offshore wind power, and provide reliable guarantee for the safe and stable operation of the power grid system. Description of the Drawings

[0026] Figure 1 is a schematic diagram of the design process architecture of the onshore-offshore multi-source multi-terminal AC-DC integrated system in the embodiment of the present invention; Figure 2 is a schematic diagram of the structure of the onshore-offshore multi-source multi-terminal AC-DC integrated system in the embodiment of the present invention; Figure 3 is a schematic diagram of the structure of the onshore-offshore multi-source multi-terminal AC-DC integrated system including the topology of the integrated switching station in the embodiment of the present invention; Figure 4 is a schematic diagram of the constant DC voltage control logic executed by the onshore sending converter platform in the embodiment of the present invention; Figure 5 is a schematic diagram of the execution logic of the current control link in the embodiment of the present invention; Figure 6 is a schematic diagram of the execution logic of the circulating current control link in the embodiment of the present invention; Figure 7 is a schematic diagram of the execution logic of the modulation strategy control in the embodiment of the present invention; Figure 8 is a schematic diagram of the structure of the onshore-offshore multi-source multi-terminal AC-DC integrated system in the parallel connection form of the uncontrolled rectifier and the modular multilevel converter in the offshore converter platform in the embodiment of the present invention; Figure 9 is a schematic diagram of the constant power control logic executed by the inland receiving converter platform in the embodiment of the present invention; Figure 10 is a schematic diagram of the structure of the onshore-offshore multi-source multi-terminal AC-DC integrated system including another topology of the integrated switching station in the embodiment of the present invention. Detailed Embodiment

[0027] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention. Apparently, the described embodiments are some, but not all, of the embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0028] An AC / DC integrated system with multiple sources and multiple terminals on land and sea provided by an embodiment of the present invention can be designed based on Figure 1 the process framework shown and in combination with the following steps: First, determine the basic topological structure; according to the power grid planning, clarify the core requirements such as the geographical locations and capacities of the sending and receiving ends, scientifically and reasonably select the system voltage level in combination with the transmission capacity and distance, and select a suitable wiring form after comprehensively considering the necessary conditions for the safe operation of the system and the specific conditions of the geographical location, thereby determining the basic topological structure of the system; for example, the sending-end requirements include the transmission of 5 GW from the deep sea and the transmission of 3 GW from a multi-source energy cluster, the receiving-end requirements include two 4 GW load centers and the mutual assistance of UHVDC partitions, the voltage level is ±500 kV or ±800 kV, and the system wiring form is a true bipolar connection; Second, configure key equipment; considering that the system needs to have the ability to ride through AC faults, an energy-consuming device is configured on the AC side of the sending end of the system in combination with the actual project; considering that the system needs to have the ability to ride through DC faults and to improve the system stability at the same time, DC circuit breakers are equipped on both sides of the DC transmission line of the system, and a hybrid MMC is generally used for the flexible DC converter; for example, a DR-MMC converter (Diode Rectifier-Modular Multilevel Converter) or an LCC-MMC converter (Line Commutated Converter-Modular Multilevel Converter) is used for the sending-end converter; an LCC-MMC converter is used for the receiving-end converter; Third, optimize the economic cost; on the premise of ensuring the system stability, the economic cost of the system can be optimized from two aspects; one is to select a sending and receiving-end converter with a lower construction cost; the other is to select a multi-port DC circuit breaker, or by changing the wiring form of the DC switch station, the utilization rate of the DC circuit breaker can be improved, thereby achieving the effect of optimizing the economic cost; Finally, design the control strategy; specifically design the coordinated control strategy, AC / DC fault ride-through strategy, and frequency and voltage active support strategy for the AC / DC integrated system with multiple sources and multiple terminals on land and sea to cope with multiple converters and multiple working conditions, so as to ensure the safe and stable operation of the system.

[0029] Such as Figure 2As shown in the figure, an embodiment of the present invention provides a land-sea multi-source and multi-terminal AC-DC integrated system, including a land-based multi-source energy transmission system 1, an offshore wind power AC collection and DC transmission system 2, an inland receiving system 3, and an integrated switching station 4.

[0030] Among them, the land-based multi-source energy transmission system 1 includes a multi-source energy cluster 11, a land-based sending-end power grid 12, a land-based sending-end collection bus 13, a land-based sending-end converter platform 14, and a land-based sending-end DC overhead line 15; the multi-source energy cluster 11 and the land-based sending-end power grid 12 are connected to the land-based sending-end converter platform 14 through the land-based sending-end collection bus 13; the land-based sending-end converter platform 14 is connected to the integrated switching station 4 through the land-based sending-end DC overhead line 15.

[0031] The offshore wind power AC collection and DC transmission system 2 includes an offshore wind farm 21, an AC collection network, an offshore converter platform 22, and a DC submarine cable 23; the offshore wind farm is connected to the offshore converter platform 22 through the AC collection network; the offshore converter platform 22 is connected to the integrated switching station 4 through the DC submarine cable 23.

[0032] The inland receiving system 3 includes an inland receiving-end power grid 31, an inland receiving-end collection bus 32, an inland receiving-end converter platform 33, and an inland receiving-end DC overhead line 34; the inland receiving-end power grid 31 is connected to the inland receiving-end converter platform 33 through the inland receiving-end collection bus 32; the inland receiving-end converter platform 33 is connected to the integrated switching station 4 through the inland receiving-end DC overhead line 34.

[0033] To ensure the reliability, flexibility, and transmission efficiency of the operation of the land-sea multi-source and multi-terminal AC-DC integrated system, in this embodiment, it is preferably set that the land-based multi-source energy transmission system 1, the offshore wind power AC collection and DC transmission system 2, the inland receiving system 3, and the integrated switching station 4 adopt a true bipolar connection form. As Figure 3 shown, the integrated switching station 4 includes a DC collection bus 41 and a collection ground wire 42. Among them, the DC collection bus 41 includes a positive DC collection bus 411 and a negative DC collection bus 412, which are connected to the land-based sending-end DC overhead line 15, the DC submarine cable 23, and the inland receiving-end DC overhead line 34, and are used to collect the electric energy of the land-based multi-source energy transmission system 1 and the offshore wind power AC collection and DC transmission system 2, and send the collected electric energy to the inland receiving system 3; the collection ground wire 42 is used to integrate the inland receiving-end metal return line 35 of the inland receiving system 3, the land-based sending-end metal return line 16 of the land-based multi-source energy transmission system 1, and the offshore-end metal return line 26 of the offshore wind power AC collection and DC transmission system 2; the metal return line is used to ground the neutral point of the system.

[0034] This embodiment connects offshore wind power, onshore multi-source energy and multi-regional DC systems of load centers through integrated switch stations, which can not only realize DC power mutual assistance among multiple regions, but also directly transmit the power of diversified energy clusters to the load center by crossing the saturated AC grid. Compared with the traditional offshore wind power grid-connected solution that relies on AC overhead lines to realize onshore power transmission, it effectively solves the problem of tight resources in onshore coastal channels, improves the utilization rate of transmission channels, and provides new solutions for offshore wind power access methods; at the same time, it can also use onshore multi-source energy to construct a deep-sea wind power output curve smoothing technology. Compared with the single-energy offshore wind power grid-connected project, it can effectively deal with the strong randomness and volatility of wind power generation.

[0035] The following describes in detail the components of the onshore and offshore multi-source multi-terminal AC / DC integrated system of this embodiment in the order of onshore multi-source energy transmission system 1, offshore wind power AC / DC transmission system 2, inland receiving system 3, and integrated switch station 4: 1) Onshore multi-source energy delivery system In this embodiment, the onshore multi-source energy delivery system 1 is used to collect and deliver the electric energy generated by the multi-source energy cluster 11, including the multi-source energy cluster 11, the onshore sending-end power grid 12, the onshore sending-end collecting bus 13, the onshore sending-end converter platform 14 and the onshore sending-end DC overhead line 15; wherein, the multi-source energy cluster 11 includes various energy forms such as wind power generation, photovoltaic power generation, energy storage and pumped storage; the onshore sending-end power grid 12 can be understood as an onshore AC power grid, which may include power plants, transmission lines, substations, distribution lines, user terminal loads and the like; the multi-source energy cluster 11 and the onshore sending-end power grid 12 are connected to the onshore sending-end converter platform 14 through the onshore sending-end collecting bus 13, and the onshore sending-end converter platform 14 is used to convert the received electric energy from DC to AC or AC to DC.

[0036] The onshore sending-end DC overhead line 15 is used to connect the onshore multi-source energy transmission system 1 and the integrated switch station 4, and collects the DC power of the onshore multi-source energy transmission system 1 into the integrated switch station 4 and the power collected by the offshore wind power AC and DC transmission system 2, and then sends it to the inland receiving system 3.

[0037] The onshore sending-end converter platform 14 includes an onshore sending-end converter and an onshore sending-end converter station connecting transformer, and the onshore sending-end converter includes at least one onshore sending-end sub-converter; the onshore sending-end converter is connected to the onshore sending-end collecting bus 13 through the onshore sending-end converter station connecting transformer. Considering that the multi-source energy cluster 11 has a two-way regulation function and can adjust its output according to the power of the system, in this embodiment, the onshore sending-end sub-converter is preferably set as a converter with the ability of bi-directional current flow, such as a voltage-source converter, or a line-commutated converter, etc. If there are multiple onshore sending-end sub-converters, the multiple onshore sending-end sub-converters are connected in series or in parallel.

[0038] Considering the situation that the onshore sending-end DC overhead line 15, the inland receiving-end DC overhead line 34 and the DC submarine cable 23 in the integrated system are controlled by the onshore sending-end converter platform 14 to maintain stability, in order to give full play to the flexible regulation ability of the multi-source energy cluster 11 as much as possible, effectively suppress the output fluctuation of the wind farm, and improve the grid's acceptance ability for it, in this embodiment, the onshore sending-end converter platform 14 is preferably set to adopt an improved outer-loop DC voltage control strategy that introduces the coordinated control of the total output active power of the onshore multi-source energy sending system 1 and the offshore wind power AC collecting DC sending system 2 and the DC voltage of the offshore wind power AC collecting DC sending system 2 on the basis of the conventional DC voltage control, and implements constant DC voltage control.

[0039] As Figure 4 shown, the first control strategy includes links such as outer-loop control, DC voltage and reactive power control, current control, circulating current control, modulation strategy control and trigger logic control, specifically including: Perform outer-loop control according to the total output active power of the onshore multi-source energy sending system 1 and the offshore wind power AC collecting DC sending system 2 and the total output active power reference value to generate the d-axis DC voltage reference value of the DC voltage of the onshore sending-end converter platform 14; that is, based on the total output active power of the onshore multi-source energy sending system 1 and the offshore wind power AC collecting DC sending system 2 and the total output active power reference value to perform PI control to generate the d-axis DC voltage reference value of the DC voltage of the onshore sending-end converter platform 14.

[0040] Perform DC voltage control according to the d-axis DC voltage reference value and the d-axis DC voltage value of the DC voltage of the onshore sending-end converter platform 14 to generate the d-axis current reference value of the AC current of the onshore sending-end converter platform 14, and perform reactive power control according to the reactive power value and the reactive power reference value of the onshore sending-end converter platform 14 to generate the q-axis current reference value of the AC current of the onshore sending-end converter platform 14; that is, based on the d-axis DC voltage reference value of the DC voltage of the onshore sending-end converter platform 14 and the d-axis DC voltage value perform PI control to generate the d-axis current reference value of the AC current of the onshore sending-end converter platform 14 ; meanwhile, based on the reactive power value of the onshore sending-end converter platform 14 and the reactive power reference value perform PI control to generate the q-axis current reference value of the AC current of the onshore sending-end converter platform 14 .

[0041] Generate the reference phase angle of the onshore sending-end converter platform 14 according to the AC-side voltage of the onshore sending-end converter platform 14, and perform current control based on the reference phase angle of the onshore sending-end converter platform 14, the d-axis current value of the AC current of the onshore sending-end converter platform 14, the d-axis current reference value, the q-axis current value and the q-axis current reference value, and the d-axis voltage value and the q-axis voltage value of the AC voltage of the onshore sending-end converter platform 14 to generate the a-phase voltage, b-phase voltage and c-phase voltage of the AC voltage of the onshore sending-end converter platform 14; that is, generate the reference phase angle of the onshore sending-end converter platform 14 based on the phase-locked loop (PLL) processing of the AC-side voltage of the onshore sending-end converter platform 14 act on the converter of the current control link, and combine the d-axis current reference value of the AC current of the onshore sending-end converter platform 14 , the d-axis current value , the q-axis current value and the q-axis current reference value to perform current tracking control to generate the a-phase voltage of the AC voltage of the onshore sending-end converter platform 14 , b-phase voltage

[0042] and c-phase voltage Figure 5 and other three-phase AC voltages. Specifically, as shown in , generating the a-phase voltage, b-phase voltage and c-phase voltage of the AC voltage of the onshore sending-end converter platform 14 includes: Generate the d-axis modulation ratio and q-axis modulation ratio of the AC modulation ratio of the onshore sending-end converter platform 14 according to the d-axis current value, d-axis current reference value, q-axis current value and q-axis current reference value of the AC current of the onshore sending-end converter platform 14, and the d-axis voltage value and q-axis voltage value of the AC voltage of the onshore sending-end converter platform 14; that is, based on the d-axis current reference value of the AC current of the onshore sending-end converter platform 14 , the d-axis current value , the q-axis current value , system rated angular frequency and the arm inductance value Generate the d-axis modulation ratio of the AC modulation ratio of the onshore sending-end converter platform 14 and the q-axis modulation ratio .

[0043] Perform an inverse Park transformation based on the reference phase angle of the onshore sending-end converter platform 14 and the d-axis modulation ratio and q-axis modulation ratio of the AC modulation ratio of the onshore sending-end converter platform 14 to generate the a-phase modulation ratio, b-phase modulation ratio, and c-phase modulation ratio of the AC modulation ratio of the onshore sending-end converter platform 14; that is, based on the reference phase angle of the onshore sending-end converter platform 14 and the d-axis modulation ratio of the AC modulation ratio of the onshore sending-end converter platform 14 and the q-axis modulation ratio perform an inverse Park transformation ( ) to generate the a-phase modulation ratio of the AC modulation ratio of the onshore sending-end converter platform 14 , b-phase modulation ratio and c-phase modulation ratio .

[0044] Generate the a-phase voltage, b-phase voltage, and c-phase voltage of the AC voltage of the onshore sending-end converter platform 14 according to the a-phase modulation ratio, b-phase modulation ratio, and c-phase modulation ratio of the AC modulation ratio of the onshore sending-end converter platform 14; that is, based on the a-phase modulation ratio of the onshore sending-end converter platform 14 , b-phase modulation ratio and c-phase modulation ratio , generate three-phase AC voltage.

[0045] It should be noted that the control process of the above current control can be implemented with reference to the current control process of existing converters and will not be elaborated here.

[0046] Perform circulating current control according to the j-phase upper arm current and j-phase lower arm current of the onshore sending-end converter platform 14, as well as the d-axis second-harmonic AC current reference value and q-axis second-harmonic AC current reference value of the onshore sending-end converter platform 14 to generate the a-phase voltage, b-phase voltage, and c-phase voltage of the circulating current voltage of the onshore sending-end converter platform 14.

[0047] Specifically, as Figure 6 shown, generating the a-phase voltage, b-phase voltage, and c-phase voltage of the circulating current voltage of the onshore sending-end converter platform 14 includes: Generate the j-phase circulating current of the onshore sending-end converter platform 14 according to the j-phase upper arm current and j-phase lower arm current of the onshore sending-end converter platform 14; that is, based on the j-phase upper arm current of the MMC converter in the onshore sending-end converter platform 14 and the j-phase lower arm current Generate the j-phase circulating current of the onshore sending-end converter platform 14 。

[0048] Generate the d-axis circulating voltage reference value and q-axis circulating voltage reference value of the circulating voltage of the onshore sending-end converter platform 14 according to the d-axis second-harmonic AC current reference value and q-axis second-harmonic AC current reference value of the j-phase circulating current of the onshore sending-end converter platform 14 and the second-harmonic AC current of the onshore sending-end converter platform 14; that is, based on the j-phase circulating current of the onshore sending-end converter platform 14 Perform Park transformation ( ), and then obtain the corresponding d-axis j-phase circulating current component and q-axis j-phase circulating current component. Under the condition that the d-axis second-harmonic AC current reference value of the second-harmonic AC current of the onshore sending-end converter platform 14 is set to 0 and the q-axis second-harmonic AC current reference value is set to 0, combined with the system rated angular frequency and the arm inductance value generate the d-axis circulating voltage reference value and q-axis circulating voltage reference value of the circulating voltage of the onshore sending-end converter platform 14.

[0049] Perform inverse Park transformation according to the d-axis circulating voltage reference value and q-axis circulating voltage reference value of the circulating voltage of the onshore sending-end converter platform 14 to generate the a-phase second-harmonic AC modulation ratio, b-phase second-harmonic AC modulation ratio, and c-phase second-harmonic AC modulation ratio of the second-harmonic AC modulation ratio of the onshore sending-end converter platform 14; that is, based on the d-axis circulating voltage reference value and q-axis circulating voltage reference value of the circulating voltage of the onshore sending-end converter platform 14, perform inverse Park transformation to generate the a-phase second-harmonic AC modulation ratio , b-phase second-harmonic AC modulation ratio and c-phase second-harmonic AC modulation ratio of the second-harmonic AC modulation ratio of the onshore sending-end converter platform 14.

[0050] Generate the a-phase voltage, b-phase voltage, and c-phase voltage of the circulating voltage of the onshore sending-end converter platform 14 according to the a-phase second-harmonic AC modulation ratio, b-phase second-harmonic AC modulation ratio, and c-phase second-harmonic AC modulation ratio of the second-harmonic AC modulation ratio of the onshore sending-end converter platform 14; that is, based on the a-phase second-harmonic AC modulation ratio , b-phase second-harmonic AC modulation ratio and c-phase second-harmonic AC modulation ratio of the second-harmonic AC modulation ratio of the onshore sending-end converter platform 14, generate the a-phase voltage , b-phase voltage and c-phase voltage , for suppressing the double-frequency circulating current inside the MMC.

[0051] Perform an algebraic sum operation on the a-phase voltage, b-phase voltage, and c-phase voltage of the AC voltage of the onshore sending-end converter platform 14, and the a-phase voltage, b-phase voltage, and c-phase voltage of the circulating current voltage of the onshore sending-end converter platform 14 to generate the AC voltage reference value of the onshore sending-end converter platform 14; that is, based on the a-phase voltage of the AC voltage of the onshore sending-end converter platform 14 , b-phase voltage and c-phase voltage , and the a-phase voltage of the circulating current voltage of the onshore sending-end converter platform 14 , b-phase voltage and c-phase voltage Perform an algebraic sum operation on the phase voltages of each phase to obtain the AC voltage reference value of the onshore sending-end converter platform 14 .

[0052] It should be noted that the control process of the above circulating current control can be implemented with reference to the circulating current control process of existing converters, and will not be elaborated here.

[0053] Generate the modulation signal of the onshore sending-end converter platform 14 according to the AC voltage reference value of the onshore sending-end converter platform 14, as well as the capacitor voltages of each sub-module and the current values of each bridge arm of the onshore sending-end converter platform 14. Specifically, as Figure 7 shown, generating the modulation signal of the onshore sending-end converter platform 14 includes: Generate the number of sub-modules to be put into the upper bridge arm and the number of sub-modules to be put into the lower bridge arm of the onshore sending-end converter platform 14 according to the AC voltage reference value of the onshore sending-end converter platform 14. That is, the AC voltage reference value After rounding by the ROUND function, the number of sub-modules put into the upper and lower bridge arms is obtained and .

[0054] Perform sub-module capacitor voltage equalization according to the number of sub-modules put into the upper bridge arm and the number of sub-modules put into the lower bridge arm of the onshore sending-end converter platform 14, as well as the capacitor voltages of each sub-module and the current values of each bridge arm of the onshore sending-end converter platform 14, to generate the modulation signal of the onshore sending-end converter platform 14. That is, based on the obtained number of sub-modules put into the upper and lower bridge arms and , comprehensively considering the capacitor voltages of each module and the current values of each bridge arm, adopt the capacitor voltage sorting algorithm to monitor and sort the capacitor voltages of the sub-modules and the current values of each bridge arm in real time to generate the modulation signal. According to the magnitude and direction of the modulation signal, select the sub-module with the highest or lowest capacitor voltage for input or removal. For example: when it is necessary to increase the output voltage, select the sub-module with a lower capacitor voltage for input; when it is necessary to reduce the output voltage, select the sub-module with a higher capacitor voltage for removal.

[0055] Generate trigger pulses for the switching devices of each sub-module in the upper and lower bridge arms of the onshore sending-end converter platform 14 according to the modulation signal of the onshore sending-end converter platform 14.

[0056] Compared with the traditional constant DC voltage control strategy, the improved constant DC voltage control strategy adopted by the onshore sending-end converter platform 14 in this embodiment has faster dynamic response characteristics. Without power over-limitation, it can more effectively track the fluctuations of the offshore wind power and adjust in time, effectively smoothing the wind power output, thereby improving the adaptability of the system to the changes in offshore wind power.

[0057] 2) Offshore wind power AC collection and DC transmission system In this embodiment, the offshore wind power AC collection and DC transmission system 2 is mainly used for collecting and transmitting offshore wind power, including an offshore wind farm 21, an AC collection network, an offshore converter platform 22, and a DC submarine cable 23; among them, the offshore wind farm 21 includes at least one offshore AC wind farm, and the offshore AC wind farm includes an AC wind turbine 212, or a series of multiple AC wind turbines 211. Since the system adopts a true bipolar connection form, the offshore wind power AC collection and DC transmission system 2 also includes an offshore end metal return line 26 for connecting the system neutral point of the offshore wind power AC collection and DC transmission system 2 to the collection ground wire 42 in the integrated switchyard to achieve system grounding.

[0058] The offshore converter platform 22 includes an offshore converter station 221 and an offshore converter station coupling transformer 222; after the offshore wind power AC collection and DC transmission system 2 collects the electric energy generated by the offshore wind farm 21 through the AC collection network, it is converted from AC to DC by the offshore converter platform 22, and finally the converted DC electric energy is sent into the integrated switchyard 4 by the DC submarine cable 23.

[0059] The AC collection network includes multiple AC collection submarine cables 24 and a set of AC collection busbars 25; the AC collection submarine cables 24 can be the AC collection submarine cables inside the offshore wind farm, or the AC collection submarine cables between multiple offshore wind farms 21 and the AC collection busbars 25. The AC collection submarine cables 24 inside the offshore wind farm 21 can be the AC collection submarine cables between multiple AC wind turbines 212 that make up a series of AC wind turbines 211, or the AC collection submarine cables between multiple series of AC wind turbines 211. The offshore wind farm 21 is connected to the AC collection busbars 25 through the AC collection submarine cables 24, and the AC collection busbars 25 are connected to the offshore converter station 221 through the offshore converter station coupling transformer 222.

[0060] Specifically, the offshore converter station 221 includes an uncontrolled rectifier 2211 and a modular multilevel converter 2212; the onshore multi-source energy transmission system 1 and the offshore wind power AC collection and DC transmission system 1 can jointly charge the modular multilevel converter 2212, and the modular multilevel converter 2212 can charge the wind turbines 212 in the offshore wind farm.

[0061] In practical applications, based on the requirements of transmission capacity and construction cost, the uncontrolled rectifier 2211 and the modular multilevel converter 2212 can be set in two connection forms: series and parallel. As Figure 2 - Figure 3 shown, when the uncontrolled rectifier 2211 is in series with the modular multilevel converter 2212, the offshore converter station 221 further includes a bypass switch 2213 of the uncontrolled rectifier 2211, and since the system adopts a true bipolar wiring form, the positive and negative poles of the uncontrolled rectifier 2211 and the modular multilevel converter 2212 are symmetrically arranged. At the same time, in order to ensure that the offshore wind power AC collection and DC transmission system 2 proposed in this embodiment can become an offshore wind power grid connection system with engineering practicability, this embodiment preferably designs a startup timing applicable to the series form of the uncontrolled rectifier 2211 and the modular multilevel converter 2212 while fully considering the startup problem of the uncontrolled rectifier. Specifically, the offshore wind power AC collection and DC transmission system 2 starts based on the following first startup timing: Close the bypass switch 2213 to enable the onshore multi-source energy transmission system 1 and the inland receiving system 3 to jointly pre-charge the modular multilevel converter 2212 and the wind turbines 212 in the offshore wind farm 21.

[0062] After the pre-charging is completed, close the bypass switch 2213 to start the wind turbines 212 and establish the AC voltage of the AC collection bus 25; among them, the judgment condition for the end of pre-charging can be determined based on the actual application scenario and is not specifically limited here.

[0063] When the AC voltage of the AC collection bus 25 meets the rectification condition of the uncontrolled rectifier 2211, establish the rated DC voltage of the offshore wind power AC collection and DC transmission system 2, and complete the AC-to-DC conversion through the series form of the uncontrolled rectifier 2211 and the modular multilevel converter 2212.

[0064] In this embodiment, it is possible to select that the uncontrolled rectifier 2211 bears a larger DC voltage and the modular multilevel converter 2212 bears a smaller DC voltage to be applicable to a low-cost construction scenario, but its transmission capacity may be limited by the valve group switch current of the modular multilevel converter 2212.

[0065] As Figure 8As shown, when the uncontrolled rectifier 2211 is connected in parallel with the modular multilevel converter 2212, considering the start-up problem of the uncontrolled rectifier, the offshore wind power AC collection and DC transmission system starts based on the following second start-up timing: The modular multilevel converter 2212 and the wind turbines 212 in the offshore wind farm 21 are pre-charged jointly by the onshore multi-source energy transmission system 1 and the inland receiving system 3.

[0066] After the pre-charging is completed, the wind turbines 212 are started to establish the AC voltage of the AC collection bus 25; the judgment condition for the end of pre-charging can be determined based on the actual application scenario and is not specifically limited here.

[0067] When the AC voltage of the AC collection bus 25 meets the rectification condition of the uncontrolled rectifier 2211, the rated DC voltage of the offshore wind power AC collection and DC transmission system 2 is established, and the AC-to-DC conversion is completed through the uncontrolled rectifier 2211 and the modular multilevel converter 2212 in parallel.

[0068] In this embodiment, when the offshore wind power AC collection and DC transmission system 2 adopts a true bipolar connection form, as Figure 8 shown, the offshore wind power AC collection and DC transmission system 2 includes a first offshore end metallic return line 261 for paralleling the uncontrolled rectifier 2211 and a second offshore end metallic return line 262 for paralleling the modular multilevel converter 2212.

[0069] In this embodiment, the output capacity of the uncontrolled rectifier 2211 can be selected without being limited by the valve group switching current of the modular multilevel converter 2212, which is applicable to ultra-large-capacity offshore wind power transmission systems. However, the corresponding construction cost will be slightly higher than the series connection scenario of the uncontrolled rectifier 2211 and the modular multilevel converter 2212. In actual applications, the wiring method of the uncontrolled rectifier 2211 and the modular multilevel converter 2212 can be selected according to actual project requirements.

[0070] To better maintain the stability of the amplitude and frequency of the AC voltage of the AC collection bus 25 of the offshore wind power and ensure the safe and stable collection of large-scale offshore wind power, this embodiment preferably sets the offshore conversion platform 22 to perform grid-forming control based on the following second control strategy: Perform AC voltage control according to the d-axis voltage value, d-axis voltage reference value, q-axis voltage value, and q-axis voltage reference value of the AC voltage of the offshore conversion platform 22 to generate the d-axis current reference value and q-axis current reference value of the AC current of the offshore conversion platform 22.

[0071] Generate the reference phase angle of the offshore HVDC converter platform 22 according to the AC side frequency of the offshore HVDC converter platform 22, and perform current control based on the reference phase angle of the offshore HVDC converter platform 22, the d-axis current value of the AC current of the offshore HVDC converter platform 22, the d-axis current reference value, the q-axis current value and the q-axis current reference value, and the d-axis voltage value and the q-axis voltage value of the AC voltage of the offshore HVDC converter platform 22, to generate the a-phase voltage, b-phase voltage and c-phase voltage of the AC voltage of the offshore HVDC converter platform 22; specifically, generating the a-phase voltage, b-phase voltage and c-phase voltage of the AC voltage of the offshore HVDC converter platform 22 includes: Generate the d-axis modulation ratio and q-axis modulation ratio of the AC modulation ratio of the offshore HVDC converter platform 22 according to the d-axis current value of the AC current of the offshore HVDC converter platform 22, the d-axis current reference value, the q-axis current value and the q-axis current reference value, and the d-axis voltage value and the q-axis voltage value of the AC voltage of the offshore HVDC converter platform 22.

[0072] Generate the reference phase angle of the offshore HVDC converter platform 22 according to the AC side frequency of the offshore HVDC converter platform 22, and perform inverse Park transformation according to the reference phase angle of the offshore HVDC converter platform 22, the d-axis modulation ratio and the q-axis modulation ratio of the AC modulation ratio of the offshore HVDC converter platform 22, to generate the a-phase modulation ratio, b-phase modulation ratio and c-phase modulation ratio of the AC modulation ratio of the offshore HVDC converter platform 22.

[0073] Generate the a-phase voltage, b-phase voltage and c-phase voltage of the AC voltage of the offshore HVDC converter platform 22 according to the a-phase modulation ratio, b-phase modulation ratio and c-phase modulation ratio of the AC modulation ratio of the offshore HVDC converter platform 22.

[0074] Perform circulating current control according to the upper bridge arm current and lower bridge arm current of the j-phase of the offshore HVDC converter platform 22, and the d-axis second harmonic AC current reference value and q-axis second harmonic AC current reference value of the second harmonic AC current of the offshore HVDC converter platform 22, to generate the a-phase voltage, b-phase voltage and c-phase voltage of the circulating current voltage of the offshore HVDC converter platform 22.

[0075] Perform algebraic sum operation on the a-phase voltage, b-phase voltage and c-phase voltage of the AC voltage of the offshore HVDC converter platform 22 and the a-phase voltage, b-phase voltage and c-phase voltage of the circulating current voltage of the offshore HVDC converter platform 22, to generate the AC voltage reference value of the offshore HVDC converter platform 22.

[0076] Generate the modulation signal of the offshore HVDC converter platform 22 according to the AC voltage reference value of the offshore HVDC converter platform 22, and the capacitor voltages of each sub-module and the current values of each bridge arm of the offshore HVDC converter platform 22.

[0077] Generate the trigger pulses of the switching devices of each sub-module in the upper and lower bridge arms of the offshore HVDC converter platform 22 according to the modulation signal of the offshore HVDC converter platform 22.

[0078] In this embodiment, the offshore HVDC grid-forming platform 22 adopts grid-forming control, including links such as AC voltage control, current control, circulating current control, modulation strategy control, and trigger logic control. Among them, the current control, circulating current control, modulation strategy control, and trigger logic control links are the same as the Figure 4 - Figure 7 control logic shown, which will not be elaborated here. Among them, AC voltage control generates the d-axis current reference value and the q-axis current reference value through the d-axis voltage value and its reference value, and the q-axis voltage value and its reference value in the dq0 coordinate system of the AC voltage. At the same time, the reference phase angle of the converter acting on the current control link is generated by the AC side frequency of the offshore HVDC grid-forming platform 22. The AC voltage on the AC side in this embodiment is the AC voltage fed into the AC collection bus 25, and the AC side frequency is the frequency of the AC collection bus 25. It should be noted that the specific control steps of the above grid-forming control can refer to the conventional grid-forming control steps, which will not be elaborated here.

[0079] This embodiment applies the offshore wind power lightweight transmission technology that combines uncontrolled rectification technology and flexible DC hybrid conversion technology, which can fully adapt to the complex environment of large-scale deep-sea and far-sea wind power access. Compared with the pure flexible DC offshore wind power grid connection project, the construction cost of the offshore platform of this system will be greatly reduced, promoting the large-scale development of far-sea wind farms. At the same time, on the basis of fully considering the start-up problem of the uncontrolled rectifier, the start-up timing is designed specifically in combination with the wiring methods of the uncontrolled rectifier and the modular multilevel converter, making it a far-sea wind power grid connection system with engineering practicability.

[0080] 3) Inland receiving system In this embodiment, the inland receiving system 3 includes an inland receiving power grid 31, an inland receiving collection bus 32, an inland receiving HVDC grid-forming platform 33, and an inland receiving DC overhead line 34. The inland receiving power grid 31 is connected to the inland receiving HVDC grid-forming platform 33 through the inland receiving collection bus 32. Since the integrated system adopts bipolar wiring, the inland receiving system 3 also includes an inland receiving metallic return line. The inland receiving metallic return line 35 is used to connect the system neutral point of the inland receiving system 3 to the collection ground wire 42 in the integrated switchyard to realize system grounding.

[0081] As Figure 2 - Figure 3 and Figure 8 shown, the inland receiving HVDC grid-forming platform 33 includes an inland receiving converter and an inland receiving converter station coupling transformer. The inland receiving converter is connected to the inland receiving collection bus 32 through the inland receiving converter station coupling transformer, and the inland receiving converter includes at least one inland receiving sub-converter. The inland receiving sub-converter can adopt a voltage source converter, a line-commutated converter, or other converters with the ability of bidirectional current flow. If there are multiple inland receiving sub-converters, the multiple inland receiving sub-converters are connected in series or in parallel.

[0082] In this embodiment, the inland receiving-end power grid 31 is a load center. To keep the received power constant, the inland receiving-end converter platform 33 adopts a constant power control strategy to control the active power value transmitted to the inland power grid by the system to be constant. At the same time, when a system fault occurs, the reactive power on the AC side of the inland receiving-end converter platform 33 can be used to provide a certain voltage support. Specifically, as Figure 9 shown, the inland receiving-end converter platform performs constant power control based on the following third control strategy. The third control strategy includes links such as active power control, reactive power control, current control, circulating current control, modulation strategy control, and trigger logic control: Perform active power control according to the active power value and the active power reference value of the inland receiving-end converter platform 33 to generate the d-axis current reference value of the AC current of the inland receiving-end converter platform 33, and perform reactive power control according to the reactive power value and the reactive power reference value of the inland receiving-end converter platform 33 to generate the q-axis current reference value of the AC current of the inland receiving-end converter platform 33. That is, based on the active power value and the active power reference value perform PI control to generate the d-axis current reference value of the AC current of the inland receiving-end converter platform 33 ; at the same time, according to the reactive power value and the reactive power reference value of the inland receiving-end converter platform 33, perform PI control to generate the q-axis current reference value of the AC current of the inland receiving-end converter platform 33 .

[0083] Generate the reference phase angle of the inland receiving-end converter platform 33 according to the AC side voltage of the inland receiving-end converter platform 33, and perform current control according to the reference phase angle of the inland receiving-end converter platform 33, the d-axis current value, the d-axis current reference value, the q-axis current value and the q-axis current reference value of the AC current of the inland receiving-end converter platform, and the d-axis voltage value and the q-axis voltage value of the AC voltage of the inland receiving-end converter platform 33 to generate the a-phase voltage, b-phase voltage and c-phase voltage of the AC voltage of the inland receiving-end converter platform 33. That is, generate the reference phase angle of the inland receiving-end converter platform 33 through phase-locked loop (PLL) processing of the AC side voltage acting on the converter in the current control link, and combine the d-axis current reference value of the AC current of the inland receiving-end converter platform 33, the d-axis current value , the q-axis current value and the q-axis current reference value to perform current tracking control to generate the a-phase voltage of the AC voltage of the inland receiving-end converter platform 33, the b-phase voltage and the phase-c voltage and other three-phase AC voltages. The AC-side voltage of the inland receiving-end converter platform 33 is the AC-side voltage of the inland receiving-end collecting bus 32 .

[0084] Specifically, generating the phase-a voltage, phase-b voltage, and phase-c voltage of the AC voltage of the inland receiving-end converter platform includes: Generating the d-axis modulation ratio and q-axis modulation ratio of the AC modulation ratio of the inland receiving-end converter platform 33 according to the d-axis current value, d-axis current reference value, q-axis current value, and q-axis current reference value of the AC current of the inland receiving-end converter platform 33, as well as the d-axis voltage value and q-axis voltage value of the AC voltage of the inland receiving-end converter platform 33

[0085] Performing an inverse Park transformation on the d-axis modulation ratio and q-axis modulation ratio of the AC modulation ratio of the inland receiving-end converter platform 33 according to the reference phase angle of the inland receiving-end converter platform 33 to generate the phase-a modulation ratio, phase-b modulation ratio, and phase-c modulation ratio of the AC modulation ratio of the inland receiving-end converter platform 33

[0086] Generating the phase-a voltage, phase-b voltage, and phase-c voltage of the AC voltage of the inland receiving-end converter platform 33 according to the phase-a modulation ratio, phase-b modulation ratio, and phase-c modulation ratio of the AC modulation ratio of the inland receiving-end converter platform 33. It should be noted that the control flow of the above current control can refer to Figure 5 the implementation logic shown, which will not be elaborated here

[0087] Performing circulating current control according to the phase-j upper-bridge-arm current and phase-j lower-bridge-arm current of the inland receiving-end converter platform 33, as well as the d-axis second-harmonic AC current reference value and q-axis second-harmonic AC current reference value of the second-harmonic AC current of the inland receiving-end converter platform 33 to generate the phase-a voltage, phase-b voltage, and phase-c voltage of the circulating current voltage of the inland receiving-end converter platform 33; among them, the circulating current control can refer to Figure 6 the implementation logic shown, based on the phase-j upper-bridge-arm current and phase-j lower-bridge-arm current of the MMC converter in the inland receiving-end converter platform 33 assuming the d-axis second-harmonic AC current reference value is set to 0 and the q-axis second-harmonic AC current reference value is set to 0, generating the phase-a voltage 、phase-b voltage and phase-c voltage of the circulating current voltage of the inland receiving-end converter platform 33 to suppress the second-harmonic circulating current inside the MMC

[0088] The algebraic sum operation is performed on the phase-a voltage, phase-b voltage, and phase-c voltage of the AC voltage of the inland receiving converter platform 33 and the phase-a voltage, phase-b voltage, and phase-c voltage of the circulating current voltage of the inland receiving converter platform to generate the reference value of the AC voltage of the inland receiving converter platform 33; that is, based on the phase-b voltage and phase-c voltage of the AC voltage of the inland receiving converter platform 33, and the phase-a voltage phase-b voltage and phase-c voltage of the circulating current voltage of the inland receiving converter platform 33, the algebraic sum operation of the phase voltages of each phase is performed to obtain the reference value of the AC voltage of the inland receiving converter platform 33 .

[0089] According to the reference value of the AC voltage of the inland receiving converter platform 33, as well as the capacitor voltages of each sub-module and the current values of each bridge arm of the inland receiving converter platform 33, a modulation signal of the inland receiving converter platform 33 is generated; among them, the modulation signal generation logic can refer to Figure 7 for implementation, that is, based on the number of sub-modules input by the upper and lower bridge arms obtained by rounding the reference value of the AC voltage through the ROUND function, comprehensively considering the capacitor voltages of each module and the current values of each bridge arm, and using the capacitor voltage sorting algorithm, the capacitor voltages of the sub-modules and the current values of each bridge arm are monitored and sorted in real time to generate a modulation signal.

[0090] According to the modulation signal of the inland receiving converter platform 33, trigger pulses of the switching devices of each sub-module in the upper and lower bridge arms of the inland receiving converter platform 33 are generated.

[0091] In this embodiment Figure 9 The difference between the constant power control executed by the inland receiving converter platform 33 shown and Figure 4 the constant DC voltage control shown lies only in the different active power control, and the corresponding reactive power control, current control, circulating current control, modulation strategy control, trigger logic control, etc. are all the same as the implementation logic shown in Figure 4 , which will not be elaborated here; the active power control link in this constant power control is used to the active power of the MMC and its reference value to generate the reference value of the d-axis current , and the reference phase angle of the converter acting on the current control link is generated based on the AC side voltage of the inland receiving collecting bus 32.

[0092] The inland receiving converter platform 33 is connected to the integrated switch station 4 through the inland receiving DC overhead line 34, and is used to receive the collected electric energy of the onshore multi-source energy sending system and the offshore wind power AC collecting DC sending system output by the integrated switch station 4.

[0093] 4) Integrated switching station The integrated switching station 4 in this embodiment is used to collect the electric energy of the onshore multi-source energy transmission system 1 and the offshore wind power AC collection and DC transmission system 2 and send it to the inland receiving system 3, and can realize the collection and transfer of multiple types of energy.

[0094] Considering that Figure 2 The given is a relatively idealized basic topology. When applied to actual projects, it is necessary to further consider the transmission capacity of submarine cables and overhead lines for design. Taking a system with a voltage level of ±500 kV as an example, the transmission capacity of a single-circuit DC submarine cable is 2 GW, the transmission capacity of a single-circuit DC overhead line is 5 GW, a single conventional offshore wind power AC collection and DC transmission system 2 can send out 2 GW of electric energy, and the onshore multi-source energy transmission system 1 can send out 4 GW of electric energy. A total of 8 GW of electric energy needs to be transmitted, and a double-circuit DC overhead line is required for transmission. Based on this, in this embodiment, the DC collection bus 41 is preferably set to include a first DC collection bus and a second DC collection bus.

[0095] As Figure 10 shown, the first DC collection bus is used to collect the electric energy of the offshore wind power AC collection and DC transmission system; the second DC collection bus is used to transmit the electric energy of the onshore multi-source energy transmission system; the first DC collection bus and the second DC collection bus are connected by a connection switch; the first DC collection bus and the second DC collection bus are respectively connected to the inland receiving system through a single-circuit inland receiving DC overhead line. That is, one of the DC collection buses in the DC collection bus 41 is used to collect the electric energy of two offshore wind power AC collection-DC transmission systems, and the other DC collection bus transmits the electric energy of the onshore multi-source energy transmission system. The two DC collection buses are connected by a connection switch; if one of the DC collection buses is overcurrent, the connection switch can be closed, and the excess electric energy is dispersed to the other DC collection bus. Each of the two DC collection buses is sent to the inland receiving power grid through a single-circuit DC overhead line.

[0096] In summary, an integrated AC / DC system with multiple sources and terminals on land and sea provided by the embodiments of the present invention includes a land-based multi-source energy transmission system, an offshore wind power AC collection and DC transmission system, an inland receiving system, and an integrated switching station; the land-based multi-source energy transmission system includes a multi-source energy cluster, a land-based sending grid, a land-based sending collection bus, a land-based sending converter platform, and a land-based sending DC overhead line; the multi-source energy cluster and the land-based sending grid are connected to the land-based sending converter platform through the land-based sending collection bus; the land-based sending converter platform is connected to the integrated switching station through the land-based sending DC overhead line; the offshore wind power AC collection and DC transmission system includes an offshore wind farm, an AC collection network, an offshore converter platform, and a DC submarine cable; the offshore wind farm is connected to the offshore converter platform through the AC collection network; the offshore converter platform is connected to the integrated switching station through the DC submarine cable; the inland receiving system includes an inland receiving grid, an inland receiving collection bus, an inland receiving converter platform, and an inland receiving DC overhead line; the inland receiving grid is connected to the inland receiving converter platform through the inland receiving collection bus; the inland receiving converter platform is connected to the integrated switching station through the inland receiving DC overhead line. This system can not only design a lightweight transmission technology for offshore wind power by combining uncontrolled rectification technology and flexible DC hybrid conversion technology to fully adapt to the complex environment of large-scale deep-sea and far-sea wind power access, greatly reducing the construction cost of practical engineering platforms for offshore wind power and providing reliable technical support for promoting the large-scale development of far-sea wind farms, but also connect offshore wind power, land-based multi-source energy, and the inland receiving system through the integrated switching station, cross the already saturated AC grid, directly send electric energy to the load center, realize DC energy mutual assistance between multiple regions, effectively solve the problem of tight resources of land-based coastal channels, improve the utilization rate of transmission channels, and also effectively suppress the output fluctuation of offshore wind power and improve the consumption capacity of large-scale offshore wind power by combining the electric energy of the land-based multi-source energy transmission system and the offshore wind power AC collection and DC transmission system and sending it out after being collected based on the integrated switching station, and provide reliable guarantee for the safe and stable operation of the power grid system.

[0097] Each embodiment in this specification is described in a progressive manner. For parts that are the same or similar in each embodiment, reference can be made to each other. Each embodiment focuses on the differences from other embodiments. In particular, for system embodiments, since they are basically similar to method embodiments, the description is relatively simple. For related parts, reference can be made to the partial description of the method embodiments. It should be noted that the technical features of the above embodiments can be combined arbitrarily. For the sake of brevity of description, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, it should be considered as within the scope described in this specification.

[0098] The above-described embodiments merely represent several preferred embodiments of the present invention. The description thereof is relatively specific and detailed, but it should not be construed as a limitation on the scope of the invention patent. It should be noted that for those of ordinary skill in the art, without departing from the technical principle of the present invention, several improvements and substitutions can be made, and these improvements and substitutions should also be regarded as within the protection scope of the present invention. Therefore, the protection scope of the present invention patent shall be subject to the protection scope of the claims described above.

Claims

1. A sea and land multi-source multi-terminal AC / DC integrated system, characterized in that: The system includes an onshore multi-source energy transmission system, an offshore wind power AC collection and DC transmission system, an inland receiving end system and an integrated switch station; The onshore multi-source energy delivery system comprises a multi-source energy cluster, an onshore sending-end power grid, an onshore sending-end collection busbar, an onshore sending-end commutation platform and an onshore sending-end DC overhead line; the multi-source energy cluster and the onshore sending-end power grid are connected to the onshore sending-end commutation platform via the onshore sending-end collection busbar; the onshore sending-end commutation platform is connected to the integrated switch station via the onshore sending-end DC overhead line; The offshore wind power AC collection and DC transmission system comprises an offshore wind farm, an AC collection network, an offshore converter platform and a DC submarine cable; the offshore wind farm is connected to the offshore converter platform through the AC collection network; the offshore converter platform is connected to the integrated switch station through the DC submarine cable; The inland receiving end system includes an inland receiving end power grid, an inland receiving end collecting bus, an inland receiving end converter platform and an inland receiving end DC overhead line; The inland receiving-end power grid is connected to the inland receiving-end converter platform via the inland receiving-end collecting bus; the inland receiving-end converter platform is connected to the integrated switch station via the inland receiving-end DC overhead line.

2. The sea and land multi-source multi-terminal AC / DC integrated system according to claim 1, characterized in that: The onshore multi-source energy transmission system, the offshore wind power AC collection DC transmission system, the integrated switch station and the inland receiving end system adopt a true bipolar wiring form; the integrated switch station includes a DC collection bus and a collection ground wire; The DC collection busbar is respectively connected to the onshore sending-end DC overhead line, the DC submarine cable and the inland receiving-end DC overhead line, and is used to collect the electric energy of the onshore multi-source energy transmission system and the offshore wind power AC collection and DC transmission system, and transmit the collected electric energy to the inland receiving-end system; The collecting ground wire is used to integrate the inland receiving end metal return line of the inland receiving end system, the onshore sending end metal return line of the onshore multi-source energy transmission system, and the offshore end metal return line of the offshore wind power AC collection and DC transmission system.

3. The sea and land multi-source multi-terminal AC / DC integrated system as claimed in claim 2, characterized in that: The DC busbar includes a first DC busbar and a second DC busbar; the first DC busbar is used to collect the electric energy of the onshore multi-source energy transmission system and the offshore wind power AC-DC transmission system; the second DC busbar is used to transmit the electric energy of the onshore multi-source energy transmission system; the first DC busbar and the second DC busbar are connected through a connecting switch; the first DC busbar and the second DC busbar are respectively connected to the inland receiving system through an inland receiving DC overhead line.

4. The sea and land multi-source multi-terminal AC / DC integrated system according to claim 1, characterized in that: The onshore sending-end converter platform includes an onshore sending-end converter and an onshore sending-end converter station connecting transformer; the onshore sending-end converter is connected to the onshore sending-end collecting busbar through the onshore sending-end converter station connecting transformer; The onshore sending-end converter includes at least one onshore sending-end terminal converter, and the onshore sending-end terminal converter is a converter with bidirectional current flow capability.

5. The sea and land multi-source multi-terminal AC / DC integrated system as claimed in claim 4, characterized in that: The onshore sending-end converter platform performs constant DC voltage control based on a first control strategy; the first control strategy is an outer-loop DC voltage control strategy that coordinates the total output active power of the onshore multi-source energy transmission system and the offshore wind power AC-DC transmission system with the DC voltage of the offshore wind power AC-DC transmission system.

6. The sea and land multi-source multi-terminal AC / DC integrated system according to claim 5, characterized in that: The first control strategy includes: Performing outer loop control according to the total output active power and the total output active power reference value of the onshore multi-source energy transmission system and the offshore wind power AC-to-DC transmission system to generate a d-axis DC voltage reference value of the DC voltage of the onshore sending-end converter platform; Performing DC voltage control according to the d-axis DC voltage reference value and the d-axis DC voltage value of the DC voltage of the onshore sending-end converter platform to generate a d-axis current reference value of the AC current of the onshore sending-end converter platform, and performing reactive power control according to the reactive power value and the reactive power reference value of the onshore sending-end converter platform to generate a q-axis current reference value of the AC current of the onshore sending-end converter platform; Generate a reference phase angle of the onshore sending-end converter platform according to the AC side voltage of the onshore sending-end converter platform, and perform current control according to the reference phase angle of the onshore sending-end converter platform, the d-axis current value, the d-axis current reference value, the q-axis current value and the q-axis current reference value of the AC current of the onshore sending-end converter platform, and the d-axis voltage value and the q-axis voltage value of the AC voltage of the onshore sending-end converter platform to generate the a-phase voltage, the b-phase voltage and the c-phase voltage of the AC voltage of the onshore sending-end converter platform; Performing circulating current control according to the j-phase upper bridge arm current and the j-phase lower bridge arm current of the onshore sending-end converter platform, and the d-axis double-frequency AC current reference value and the q-axis double-frequency AC current reference value of the double-frequency AC current of the onshore sending-end converter platform, to generate the a-phase voltage, the b-phase voltage and the c-phase voltage of the circulating current voltage of the onshore sending-end converter platform; Performing an algebraic sum operation on the a-phase voltage, b-phase voltage and c-phase voltage of the AC voltage of the onshore sending-end converter platform and the a-phase voltage, b-phase voltage and c-phase voltage of the circulating current voltage of the onshore sending-end converter platform to generate an AC voltage reference value of the onshore sending-end converter platform; Generate a modulation signal of the onshore sending-end converter platform according to the AC voltage reference value of the onshore sending-end converter platform, as well as the capacitor voltages of each submodule and the current values ​​of each bridge arm of the onshore sending-end converter platform; According to the modulation signal of the onshore sending-end commutation platform, trigger pulses of the switch devices of each submodule in the upper and lower bridge arms of the onshore sending-end commutation platform are generated.

7. The sea and land multi-source multi-terminal AC / DC integrated system according to claim 6, characterized in that: The generating of the a-phase voltage, the b-phase voltage and the c-phase voltage of the AC voltage of the onshore sending-end converter platform comprises: Generate a d-axis modulation ratio and a q-axis modulation ratio of the AC modulation ratio of the onshore sending-end converter platform according to the d-axis current value, the d-axis current reference value, the q-axis current value and the q-axis current reference value of the AC current of the onshore sending-end converter platform, and the d-axis voltage value and the q-axis voltage value of the AC voltage of the onshore sending-end converter platform; Performing inverse Park transformation according to the reference phase angle of the onshore sending-end converter platform and the d-axis modulation ratio and q-axis modulation ratio of the AC modulation ratio of the onshore sending-end converter platform to generate the a-phase modulation ratio, b-phase modulation ratio and c-phase modulation ratio of the AC modulation ratio of the onshore sending-end converter platform; According to the a-phase modulation ratio, b-phase modulation ratio and c-phase modulation ratio of the AC modulation ratio of the onshore sending-end converter platform, the a-phase voltage, b-phase voltage and c-phase voltage of the AC voltage of the onshore sending-end converter platform are generated.

8. The sea and land multi-source multi-terminal AC / DC integrated system according to claim 6, characterized in that: The generating of the a-phase voltage, the b-phase voltage and the c-phase voltage of the circulating current voltage of the onshore sending-end converter platform comprises: Generating a j-phase circulating current of the onshore sending-end converter platform according to a j-phase upper bridge arm current and a j-phase lower bridge arm current of the onshore sending-end converter platform; Generate a d-axis circulating current voltage reference value and a q-axis circulating current voltage reference value of the circulating voltage of the onshore sending-end converter platform according to the j-phase circulating current of the onshore sending-end converter platform and the d-axis double-frequency AC current reference value and the q-axis double-frequency AC current reference value of the double-frequency AC current of the onshore sending-end converter platform; Performing an inverse Park transformation according to the d-axis circulating current voltage reference value and the q-axis circulating current voltage reference value of the onshore sending-end converter platform to generate the a-phase double frequency AC modulation ratio, the b-phase double frequency AC modulation ratio and the c-phase double frequency AC modulation ratio of the onshore sending-end converter platform; According to the a-phase double frequency AC modulation ratio, b-phase double frequency AC modulation ratio and c-phase double frequency AC modulation ratio of the onshore sending-end converter platform, the a-phase voltage, b-phase voltage and c-phase voltage of the circulating current voltage of the onshore sending-end converter platform are generated.

9. The sea and land multi-source multi-terminal AC / DC integrated system according to claim 6, characterized in that: The generating of the modulation signal of the onshore sending-end commutation platform comprises: According to the AC voltage reference value of the onshore sending-end converter platform, the number of upper bridge arm submodules and the number of lower bridge arm submodules of the onshore sending-end converter platform are generated; The submodule capacitor voltage is balanced according to the number of upper bridge arm submodules and the number of lower bridge arm submodules of the onshore sending-end commutation platform, as well as the capacitor voltage of each submodule and the current value of each bridge arm of the onshore sending-end commutation platform, to generate a modulation signal of the onshore sending-end commutation platform.

10. The sea and land multi-source multi-terminal AC / DC integrated system according to claim 1, characterized in that: The offshore wind farm includes at least one offshore AC wind farm; the offshore AC wind farm includes one AC wind turbine group, or a plurality of AC wind turbine group strings; The offshore converter platform includes an offshore converter station and an offshore converter station connecting transformer; The AC collection network includes an AC collection submarine cable and an AC collection busbar; the offshore wind farm is connected to the AC collection busbar through the AC collection submarine cable, and the AC collection busbar is connected to the offshore converter station through the offshore converter station coupling transformer.

11. The sea and land multi-source multi-terminal AC / DC integrated system according to claim 10, characterized in that: The offshore converter station includes an uncontrolled rectifier and a modular multilevel converter; The onshore multi-source energy transmission system and the offshore wind power AC collection and DC transmission system jointly charge the modular multilevel converter, and the modular multilevel converter charges the wind turbines in the offshore wind farm.

12. The sea and land multi-source multi-terminal AC / DC integrated system according to claim 11, characterized in that: When the uncontrolled rectifier is connected in series with the modular multilevel converter, the offshore converter station further includes a bypass switch of the uncontrolled rectifier; and the positive and negative poles of the uncontrolled rectifier and the modular multilevel converter are symmetrical.

13. The sea and land multi-source multi-terminal AC / DC integrated system according to claim 12, characterized in that: The offshore wind power AC collection and DC transmission system is started based on a first startup sequence; the first startup sequence includes: Closing the bypass switch so that the onshore multi-source energy delivery system and the inland receiving system jointly pre-charge the modular multi-level converter and the wind turbines in the offshore wind farm; When the pre-charging is completed, the bypass switch is closed, the wind turbine generator set is started, and the AC voltage of the AC collection bus is established; When the AC voltage of the AC collection bus meets the rectification condition of the uncontrolled rectifier, the rated DC voltage of the offshore wind power AC collection and DC transmission system is established, and the AC to DC conversion is completed through the uncontrolled rectifier and the modular multilevel converter in series.

14. The sea and land multi-source multi-terminal AC / DC integrated system according to claim 11, characterized in that: When the uncontrolled rectifier is connected in parallel with the modular multilevel converter, the offshore wind power AC collection and DC transmission system is started based on a second startup sequence; the second startup sequence includes: The onshore multi-source energy delivery system and the inland receiving system jointly pre-charge the modular multi-level converter and the wind turbines in the offshore wind farm; When the pre-charging is completed, the wind turbine generator set is started to establish an AC voltage of the AC collection bus; When the AC voltage of the AC collection bus meets the rectification condition of the uncontrolled rectifier, the rated DC voltage of the offshore wind power AC collection and DC transmission system is established, and the AC to DC conversion is completed through the uncontrolled rectifier and the modular multilevel converter in parallel.

15. The sea and land multi-source multi-terminal AC / DC integrated system according to claim 14, characterized in that: When the offshore wind power AC collection and DC transmission system adopts a true bipolar connection form, the offshore wind power AC collection and DC transmission system includes a first offshore end metal loop for parallel connection of uncontrolled rectifiers and a second offshore end metal loop for parallel connection of the modular multilevel converters.

16. The sea and land multi-source multi-terminal AC / DC integrated system according to claim 11, characterized in that: The offshore converter platform performs network construction control based on the second control strategy; The second control strategy includes: Performing AC voltage control according to the d-axis voltage value, d-axis voltage reference value, q-axis voltage value and q-axis voltage reference value of the AC voltage of the offshore converter platform to generate a d-axis current reference value and a q-axis current reference value of the AC current of the offshore converter platform; Generate a reference phase angle of the offshore converter platform according to the AC side frequency of the offshore converter platform, perform current control according to the reference phase angle of the offshore converter platform, the d-axis current value of the AC current of the offshore converter platform, the d-axis current reference value, the q-axis current value and the q-axis current reference value, and the d-axis voltage value and the q-axis voltage value of the AC voltage of the offshore converter platform, and generate an a-phase voltage, a b-phase voltage and a c-phase voltage of the AC voltage of the offshore converter platform; Performing circulating current control according to the j-phase upper bridge arm current and the j-phase lower bridge arm current of the offshore converter platform, and the d-axis double frequency AC current reference value and the q-axis double frequency AC current reference value of the double frequency AC current of the offshore converter platform, to generate the a-phase voltage, the b-phase voltage and the c-phase voltage of the circulating current voltage of the offshore converter platform; Performing an algebraic sum operation on the a-phase voltage, b-phase voltage and c-phase voltage of the AC voltage of the offshore converter platform and the a-phase voltage, b-phase voltage and c-phase voltage of the circulating voltage of the offshore converter platform to generate an AC voltage reference value of the offshore converter platform; Generate a modulation signal of the offshore converter platform according to an AC voltage reference value of the offshore converter platform, as well as capacitor voltages of each submodule and current values ​​of each bridge arm of the offshore converter platform; According to the modulation signal of the offshore converter platform, trigger pulses of the switch devices of each submodule in the upper and lower bridge arms of the offshore converter platform are generated.

17. The sea and land multi-source multi-terminal AC / DC integrated system according to claim 16, characterized in that: The generating of the a-phase voltage, the b-phase voltage and the c-phase voltage of the AC voltage of the offshore converter platform comprises: Generate a d-axis modulation ratio and a q-axis modulation ratio of an AC modulation ratio of the offshore converter platform according to a d-axis current value, a d-axis current reference value, a q-axis current value and a q-axis current reference value of the AC current of the offshore converter platform, and a d-axis voltage value and a q-axis voltage value of the AC voltage of the offshore converter platform; Performing inverse Park transformation according to the reference phase angle of the offshore converter platform and the d-axis modulation ratio and the q-axis modulation ratio of the AC modulation ratio of the offshore converter platform to generate the a-phase modulation ratio, the b-phase modulation ratio and the c-phase modulation ratio of the AC modulation ratio of the offshore converter platform; According to the a-phase modulation ratio, b-phase modulation ratio and c-phase modulation ratio of the AC modulation ratio of the offshore converter platform, the a-phase voltage, b-phase voltage and c-phase voltage of the AC voltage of the offshore converter platform are generated.

18. The sea and land multi-source multi-terminal AC / DC integrated system according to claim 1, characterized in that: The inland receiving-end converter platform includes an inland receiving-end converter and an inland receiving-end converter station connecting transformer; the inland receiving-end converter is connected to the inland receiving-end collecting busbar through the inland receiving-end converter station connecting transformer; The inland receiving-end converter includes at least one inland receiving-end converter, and the inland receiving-end converter is a converter with bidirectional current flow capability.

19. The sea and land multi-source multi-terminal AC / DC integrated system according to claim 1, characterized in that: The inland receiving-end converter platform performs constant power control based on a third control strategy; the third control strategy includes: Performing active power control according to the active power value and active power reference value of the inland receiving-end converter platform to generate a d-axis current reference value of the AC current of the inland receiving-end converter platform, and performing reactive power control according to the reactive power value and reactive power reference value of the inland receiving-end converter platform to generate a q-axis current reference value of the AC current of the inland receiving-end converter platform; Generate a reference phase angle of the inland receiving-end converter platform according to the AC side voltage of the inland receiving-end converter platform, and perform current control according to the reference phase angle of the inland receiving-end converter platform, the d-axis current value, the d-axis current reference value, the q-axis current value and the q-axis current reference value of the AC current of the inland receiving-end converter platform, and the d-axis voltage value and the q-axis voltage value of the AC voltage of the inland receiving-end converter platform to generate the a-phase voltage, the b-phase voltage and the c-phase voltage of the AC voltage of the inland receiving-end converter platform; Performing circulating current control according to the j-phase upper bridge arm current and the j-phase lower bridge arm current of the inland receiving-end converter platform, and the d-axis double-frequency AC current reference value and the q-axis double-frequency AC current reference value of the double-frequency AC current of the inland receiving-end converter platform, to generate the a-phase voltage, the b-phase voltage and the c-phase voltage of the circulating current voltage of the inland receiving-end converter platform; Performing an algebraic sum operation on the a-phase voltage, b-phase voltage and c-phase voltage of the AC voltage of the inland receiving-end converter platform and the a-phase voltage, b-phase voltage and c-phase voltage of the circulating current voltage of the inland receiving-end converter platform to generate an AC voltage reference value of the inland receiving-end converter platform; Generate a modulation signal of the inland receiving-end converter platform according to the AC voltage reference value of the inland receiving-end converter platform, and the capacitor voltages of each submodule and the current values ​​of each bridge arm of the inland receiving-end converter platform; According to the modulation signal of the inland receiving-end commutation platform, trigger pulses of the switch devices of each submodule in the upper and lower bridge arms of the inland receiving-end commutation platform are generated.

20. The sea and land multi-source multi-terminal AC / DC integrated system according to claim 19, characterized in that: The generating of the a-phase voltage, the b-phase voltage and the c-phase voltage of the AC voltage of the inland receiving-end converter platform comprises: Generate a d-axis modulation ratio and a q-axis modulation ratio of the AC modulation ratio of the inland receiving-end converter platform according to the d-axis current value, the d-axis current reference value, the q-axis current value and the q-axis current reference value of the AC current of the inland receiving-end converter platform, and the d-axis voltage value and the q-axis voltage value of the AC voltage of the inland receiving-end converter platform; Performing inverse Park transformation according to the reference phase angle of the inland receiving-end converter platform and the d-axis modulation ratio and q-axis modulation ratio of the AC modulation ratio of the inland receiving-end converter platform to generate the a-phase modulation ratio, b-phase modulation ratio and c-phase modulation ratio of the AC modulation ratio of the inland receiving-end converter platform; According to the a-phase modulation ratio, b-phase modulation ratio and c-phase modulation ratio of the AC modulation ratio of the inland receiving-end converter platform, the a-phase voltage, b-phase voltage and c-phase voltage of the AC voltage of the inland receiving-end converter platform are generated.

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