A multi-source and multi-terminal AC / DC integrated system for land and sea

Abstract

The present invention relates to the technical field of AC / DC transmission systems, and discloses a multi-source multi-terminal AC / DC integrated system for land and sea, which consists of a land-based multi-source energy transmission system, an offshore wind power AC collection and DC transmission system, an inland receiving-end system including an inland receiving-end power grid, an inland receiving-end collection bus, an inland receiving-end converter platform and an inland receiving-end DC overhead line, and an integrated switchyard. The land-based multi-source energy transmission system includes a multi-source energy cluster, a land-based sending-end power grid, a land-based sending-end collection bus, a land-based sending-end converter platform and a land-based sending-end 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. Based on the lightweight transmission of offshore wind power and the coordinated control of the integration of offshore wind power and land-based multi-source energy, the present invention significantly reduces the transmission cost of offshore wind power, improves the utilization rate of the transmission channel, suppresses the output fluctuation of offshore wind power, and enhances the grid's ability to absorb offshore wind power.

Description

Technical Field

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

[0002] The existing large-scale applied offshore wind power grid connection technologies mainly include the power frequency AC transmission scheme suitable for the offshore wind power transmission and the power frequency aggregation - DC transmission scheme suitable for the long-distance offshore wind power grid connection. However, both transmission schemes will lead to an increase in the technical challenges and overall construction costs of the corresponding offshore wind power grid connection system due to the increase in the engineering transmission capacity; at the same time, in terms of the 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 not only cannot achieve the interconnection and mutual assistance of large-scale offshore wind power, but also faces the problem of tight resources of the onshore coastal channels, that is, the traditional onshore AC main 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 and 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 the aggregation of offshore wind power and onshore multi-source energy, significantly reduces the transmission cost of offshore wind power, improves the utilization rate of the transmission channel and 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 and multi-terminal AC / DC integrated system for land and sea, and the system includes an onshore multi-source energy transmission system, an offshore wind power AC aggregation and DC transmission system, an inland receiving system and an integrated switching station;

[0006] The onshore multi-source energy transmission system includes a multi-source energy cluster, an onshore sending-end power grid, an onshore sending-end aggregation bus, an onshore sending-end converter 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 converter platform through the onshore sending-end aggregation bus; the onshore sending-end converter platform is connected to the integrated switching station through the onshore sending-end DC overhead line;

[0007] 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 switchyard through the DC submarine cable;

[0008] The 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.

[0009] Furthermore, the 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;

[0010] The DC collection bus is respectively connected to the onshore sending DC overhead line, the DC submarine cable, and the inland receiving 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 send the collected electric energy to the inland receiving system;

[0011] The collection ground wire is used to integrate the inland receiving metal return line of the inland receiving system, the onshore sending 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.

[0012] Furthermore, the DC collection bus includes a first loop of DC collection bus and a second loop of DC collection bus; the first loop of DC collection bus 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; the second loop of DC collection bus is used to transmit the electric energy of the onshore multi-source energy transmission system; the first loop of DC collection bus and the second loop of DC collection bus are connected through a connection switch; the first loop of DC collection bus and the second loop of DC collection bus are respectively connected to the inland receiving system through a loop of inland receiving DC overhead line.

[0013] Furthermore, the onshore sending converter platform includes an onshore sending converter and an onshore sending converter station coupling transformer; the onshore sending converter is connected to the onshore sending collection bus through the onshore sending converter station coupling transformer;

[0014] The onshore sending converter includes at least one onshore sending sub-converter, and the onshore sending sub-converter is a converter with the ability of bidirectional current flow.

[0015] Furthermore, 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 coordinating 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.

[0016] Furthermore, the first control strategy includes:

[0017] Performing outer-loop control according to 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;

[0018] 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 the 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 the q-axis current reference value of the AC current of the onshore sending-end converter platform;

[0019] Generating the reference phase angle of the onshore sending-end converter platform according to the AC side voltage of the onshore sending-end converter platform, and performing 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, b-phase voltage and c-phase voltage of the AC voltage of the onshore sending-end converter platform;

[0020] Performing circulating current control according to 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;

[0021] 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;

[0022] Generating the modulation signal of the onshore sending-end converter platform according to 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;

[0023] Generate trigger pulses for each sub-module switching device in the upper and lower bridge arms of the onshore sending-end converter platform according to the modulation signal of the onshore sending-end converter platform.

[0024] 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:

[0025] Generate 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;

[0026] Perform 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;

[0027] Generate 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.

[0028] Further, generating the a-phase voltage, b-phase voltage, and c-phase voltage of the circulating voltage of the onshore sending-end converter platform includes:

[0029] Generate 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;

[0030] 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 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;

[0031] 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 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;

[0032] Generate the a-phase voltage, b-phase voltage, and c-phase voltage of the circulating 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.

[0033] Further, generating the modulation signal of the onshore sending-end converter platform includes:

[0034] Generating the number of sub-module units to be turned on for the upper bridge arm and the number of sub-module units to be turned on for the lower bridge arm of the onshore sending-end converter platform according to the AC voltage reference value of the onshore sending-end converter platform;

[0035] Performing sub-module capacitor voltage balancing based on the number of sub-module units to be turned on for the upper bridge arm and the number of sub-module units to be turned on for 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, to generate the modulation signal of the onshore sending-end converter platform.

[0036] 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;

[0037] The offshore converter platform includes an offshore converter station and an offshore converter station connecting transformer;

[0038] 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.

[0039] Further, the offshore converter station includes an uncontrolled rectifier and a modular multilevel converter;

[0040] 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.

[0041] 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 polarities of the uncontrolled rectifier and the modular multilevel converter are symmetrical.

[0042] Further, the offshore wind power AC collection DC sending system starts based on a first startup timing sequence; the first startup timing sequence includes:

[0043] Closing 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;

[0044] After the pre-charging is completed, closing the bypass switch to start the wind turbine generators and establish the AC voltage of the AC collection busbar;

[0045] When the AC voltage of the AC collection bus meets the rectification conditions 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 series-connected uncontrolled rectifier and the modular multilevel converter.

[0046] Further, when the uncontrolled rectifier is connected in parallel with the modular multilevel converter, the offshore wind power AC collection and DC transmission system starts based on the second startup timing; the second startup timing includes:

[0047] The onshore multi-source energy transmission system and the inland receiving system jointly pre-charge the modular multilevel converter and the wind turbines in the offshore wind farm;

[0048] After the pre-charging is completed, the wind turbines are started to establish the AC voltage of the AC collection bus;

[0049] When the AC voltage of the AC collection bus meets the rectification conditions 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 parallel-connected uncontrolled rectifier and the modular multilevel converter.

[0050] Further, 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 metallic return line for paralleling the uncontrolled rectifier and a second offshore end metallic return line for paralleling the modular multilevel converter.

[0051] Further, the offshore conversion platform performs network-forming control based on the second control strategy; the second control strategy includes:

[0052] 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;

[0053] 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;

[0054] Circulation control is performed based on the j-phase upper bridge arm current and the j-phase lower bridge arm current of the offshore HVDC 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 offshore HVDC converter platform, to generate the a-phase voltage, b-phase voltage, and c-phase voltage of the circulation voltage of the offshore HVDC converter platform;

[0055] An algebraic sum operation is performed on the a-phase voltage, b-phase voltage, and c-phase voltage of the AC voltage of the offshore HVDC converter platform and the a-phase voltage, b-phase voltage, and c-phase voltage of the circulation voltage of the offshore HVDC converter platform to generate the AC voltage reference value of the offshore HVDC converter platform;

[0056] Based on the AC voltage reference value of the offshore HVDC converter platform, as well as the capacitor voltages of each sub-module and the current values of each bridge arm of the offshore HVDC converter platform, a modulation signal of the offshore HVDC converter platform is generated;

[0057] Based on the modulation signal of the offshore HVDC converter platform, trigger pulses of each sub-module switching device in the upper and lower bridge arms of the offshore HVDC converter platform are generated.

[0058] Further, the generation of the a-phase voltage, b-phase voltage, and c-phase voltage of the AC voltage of the offshore HVDC converter platform includes:

[0059] 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 offshore HVDC converter platform, as well as the d-axis voltage value and q-axis voltage value of the AC voltage of the offshore HVDC converter platform, the d-axis modulation ratio and q-axis modulation ratio of the AC modulation ratio of the offshore HVDC converter platform are generated;

[0060] Based on 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, 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 offshore HVDC converter platform;

[0061] Based on 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, the a-phase voltage, b-phase voltage, and c-phase voltage of the AC voltage of the offshore HVDC converter platform are generated.

[0062] Further, the onshore receiving-end HVDC converter platform includes an onshore receiving-end HVDC converter and an onshore receiving-end HVDC converter station connecting transformer; the onshore receiving-end HVDC converter is connected to the onshore receiving-end collecting bus through the onshore receiving-end HVDC converter station connecting transformer;

[0063] The onshore receiving-end HVDC converter includes at least one onshore receiving-end sub-HVDC converter, and the onshore receiving-end sub-HVDC converter is a converter with the ability of bidirectional current flow.

[0064] Further, the in - land receiving - end converter platform performs constant - power control based on a third control strategy; the third control strategy includes:

[0065] Perform active - power control according to the active - power value and the active - power reference value of the in - land receiving - end converter platform, generate the d - axis current reference value of the AC current of the in - land receiving - end converter platform, and perform reactive - power control according to the reactive - power value and the reactive - power reference value of the in - land receiving - end converter platform, generate the q - axis current reference value of the AC current of the in - land receiving - end converter platform;

[0066] Generate the reference phase angle of the in - land receiving - end converter platform according to the AC - side voltage of the in - land receiving - end converter platform, and perform current control according to the reference phase angle of the in - land receiving - end converter platform, the d - axis current value, the d - axis current reference value, the q - axis current value, the q - axis current reference value of the AC current of the in - land receiving - end converter platform, and the d - axis voltage value and the q - axis voltage value of the AC voltage of the in - land receiving - end converter platform, generate the a - phase voltage, b - phase voltage, and c - phase voltage of the AC voltage of the in - land receiving - end converter platform;

[0067] Perform circulating - current control according to the upper - arm current and lower - arm current of the j - phase of the in - land receiving - 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 in - land receiving - end converter platform, generate the a - phase voltage, b - phase voltage, and c - phase voltage of the circulating - current voltage of the in - land receiving - end converter platform;

[0068] Perform algebraic - sum operation on the a - phase voltage, b - phase voltage, and c - phase voltage of the AC voltage of the in - land receiving - end converter platform and the a - phase voltage, b - phase voltage, and c - phase voltage of the circulating - current voltage of the in - land receiving - end converter platform, generate the AC - voltage reference value of the in - land receiving - end converter platform;

[0069] Generate the modulation signal of the in - land receiving - end converter platform according to the AC - voltage reference value of the in - land receiving - end converter platform, and the capacitor voltages of each sub - module and the current values of each bridge arm of the in - land receiving - end converter platform;

[0070] Generate the trigger pulses of the switching devices of each sub - module in the upper and lower bridge arms of the in - land receiving - end converter platform according to the modulation signal of the in - land receiving - end converter platform.

[0071] Further, the generating the a - phase voltage, b - phase voltage, and c - phase voltage of the AC voltage of the in - land receiving - end converter platform includes:

[0072] Generate the d-axis modulation ratio and q-axis modulation ratio of the AC modulation ratio of the inland receiving 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 inland receiving end converter platform, and the d-axis voltage value and q-axis voltage value of the AC voltage of the inland receiving end converter platform;

[0073] Perform 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;

[0074] Generate the a-phase voltage, b-phase voltage, and c-phase voltage of the AC voltage 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.

[0075] The present invention provides a sea-land multi-source multi-terminal AC-DC integrated system. Compared with the prior art, the present invention has the following beneficial effects:

[0076] 1) By combining the uncontrolled rectification technology with the flexible DC hybrid conversion technology to design the light-weight 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;

[0077] 2) By integrating the switch station to connect offshore wind power, onshore multi-source energy, and the inland receiving end system, it can cross the already saturated AC grid, directly send electric energy to the load center, realize the mutual supply of DC electric energy between multiple regions, and at the same time effectively solve the problem of tight resources of onshore coastal transmission channels and improve the utilization rate of transmission channels;

[0078] 3) By sending out the electric energy of the onshore multi-source energy transmission system and the offshore wind power AC collection DC transmission system after being collected based on the integrated switch 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

[0079] Figure 1 is a schematic diagram of the design process architecture of the sea-land multi-source multi-terminal AC-DC integrated system in the embodiment of the present invention;

[0080] Figure 2 is a schematic diagram of the structure of the sea-land multi-source multi-terminal AC-DC integrated system in the embodiment of the present invention;

[0081] Figure 3It is a schematic structural diagram of a sea - land multi - source multi - terminal AC - DC integrated system including an integrated switchyard topology in an embodiment of the present invention;

[0082] Figure 4 It is a schematic diagram of the constant DC voltage control logic executed by the on - shore sending - end converter platform in an embodiment of the present invention;

[0083] Figure 5 It is a schematic diagram of the execution logic of the current control link in an embodiment of the present invention;

[0084] Figure 6 It is a schematic diagram of the execution logic of the circulating current control link in an embodiment of the present invention;

[0085] Figure 7 It is a schematic diagram of the execution logic of the modulation strategy control in an embodiment of the present invention;

[0086] Figure 8 It is a schematic structural diagram of a sea - land multi - source multi - terminal AC - DC integrated system in the parallel connection form of an uncontrolled rectifier and a modular multilevel converter in the off - shore converter platform in an embodiment of the present invention;

[0087] Figure 9 It is a schematic diagram of the constant power control logic executed by the inland receiving - end converter platform in an embodiment of the present invention;

[0088] Figure 10 It is a schematic structural diagram of a sea - land multi - source multi - terminal AC - DC integrated system including another integrated switchyard topology in an embodiment of the present invention. Detailed implementation manners

[0089] 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. Obviously, 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.

[0090] The embodiments of the present invention provide a sea - land multi - source multi - terminal AC - DC integrated system that can be designed based on Figure 1 the shown process framework in combination with the following steps:

[0091] First, the basic topological structure is determined. According to the power grid planning, the core requirements such as the geographical locations and capacities of the power sending and receiving ends are clarified. Combining the transmission capacity and distance, the system voltage level is scientifically and reasonably selected. After comprehensively considering the necessary conditions for the safe operation of the system and the specific conditions of the geographical location, a suitable wiring form is chosen, thereby determining the basic topological structure of the system. For example, the sending-end requirements include the transmission of 5 GW of deep-sea and far-sea power and 3 GW of multi-source energy clusters, the receiving-end requirements include two 4 GW load centers and the mutual support of UHVDC partitions, the voltage level is ±500 kV or ±800 kV, and the system wiring form is a true bipolar connection.

[0092] Second, the key equipment is configured. 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.

[0093] Third, the economic cost is optimized. On the premise of ensuring the system stability, the system economic cost 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 switchyard, the utilization rate of the DC circuit breaker can be improved, thereby achieving the effect of optimizing the economic cost.

[0094] Finally, the control strategy is designed. A coordinated control strategy, an AC / DC fault ride-through strategy, and an active frequency and voltage support strategy for the multi-source and multi-terminal AC / DC integrated system of land and sea to cope with multiple converters and multiple working conditions are designed specifically to ensure the safe and stable operation of the system.

[0095] As Figure 2 shown, the embodiment of the present invention provides a multi-source and multi-terminal AC / DC integrated system of land and sea, including an onshore multi-source energy sending system 1, an offshore wind power AC collection and DC sending system 2, an inland receiving-end system 3, and an integrated switchyard 4.

[0096] Among them, the onshore multi-source energy transmission system 1 includes a multi-source energy cluster 11, an onshore sending-end power grid 12, an onshore sending-end collecting bus 13, an onshore sending-end converter platform 14, and an onshore sending-end DC overhead line 15; 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; the onshore sending-end converter platform 14 is connected to the integrated switchyard 4 through the onshore sending-end DC overhead line 15.

[0097] 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 switchyard 4 through the DC submarine cable 23.

[0098] The inland receiving system 3 includes an inland receiving power grid 31, an inland receiving collecting bus 32, an inland receiving converter platform 33, and an inland receiving DC overhead line 34; the inland receiving power grid 31 is connected to the inland receiving converter platform 33 through the inland receiving collecting bus 32; the inland receiving converter platform 33 is connected to the integrated switchyard 4 through the inland receiving DC overhead line 34.

[0099] To ensure the reliability, flexibility, and transmission efficiency of the sea-land multi-source and multi-terminal AC-DC integrated system operation, in this embodiment, it is preferably set that the onshore 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 switchyard 4 adopt a true bipolar wiring form. As Figure 3 shown, the integrated switchyard 4 includes a DC collecting bus 41 and a collecting ground wire 42. Among them, the DC collecting bus 41 includes a positive DC collecting bus 411 and a negative DC collecting bus 412, which are connected to the onshore sending-end DC overhead line 15, the DC submarine cable 23, and the inland receiving DC overhead line 34, and are 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 the collected electric energy to the inland receiving system 3; the collecting ground wire 42 is used to integrate the inland receiving metal return line 35 of the inland receiving system 3, the onshore sending-end metal return line 16 of the onshore 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 system neutral point.

[0100] In this embodiment, a multi-region DC system integrating a switching station to connect offshore wind power, onshore multi-source energy, and load centers can not only achieve mutual supply of DC electric energy among multiple regions, but also directly send the electric energy of diverse energy clusters to the load centers by crossing the already saturated AC grid. Compared with the traditional offshore wind power grid connection scheme that relies on AC overhead lines to transmit onshore electric energy, it effectively solves the problem of tight resources of onshore coastal channels, improves the utilization rate of transmission channels, and provides a new solution for the access method of offshore wind power. At the same time, it can also use onshore multi-source energy to construct a technology for smoothing the output curve of deep-sea offshore wind power. Compared with the single-energy offshore wind power grid connection project, it can effectively address the strong randomness and volatility problems of wind power generation.

[0101] The following will explain in detail each component of the onshore and offshore multi-source and multi-terminal AC / DC integrated system of this embodiment in the order of the onshore 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:

[0102] 1) Onshore multi-source energy transmission system

[0103] In this embodiment, the onshore multi-source energy transmission system 1 is used to collect and transmit the electric energy generated by the multi-source energy cluster 11, and includes 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. Among them, 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 the onshore AC power grid, which can include power plants, transmission lines, substations, distribution lines, user terminal loads, etc. 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 from AC to DC.

[0104] The onshore sending-end DC overhead line 15 is used to connect the onshore multi-source energy transmission system 1 and the integrated switching station 4, and to collect the DC electric energy of the onshore multi-source energy transmission system 1 and the electric energy collected by the offshore wind power AC collection and DC transmission system 2 into the integrated switching station 4 and then send it to the inland receiving system 3.

[0105] 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 sub-sending-end 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 sub-sending-end 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 sub-sending-end converters, the multiple onshore sub-sending-end converters are connected in series or in parallel.

[0106] 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 and DC sending system 2 and the DC voltage of the offshore wind power AC collecting and DC sending system 2 on the basis of the conventional DC voltage control, and perform constant DC voltage control.

[0107] 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:

[0108] 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 and DC sending system 2 and the total output active power reference value, and 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 and 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.

[0109] 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 14, perform DC voltage control to generate the d-axis current reference value of the AC current of the onshore sending-end converter platform 14, and based on the reactive power value and the reactive power reference value of the onshore sending-end converter platform 14, perform reactive power control 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 ; at the same time, 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 .

[0110] According to the AC side voltage of the onshore sending-end converter platform 14, generate the reference phase angle of the onshore sending-end converter platform 14, and according to the reference phase angle of the onshore sending-end converter platform 14, 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 14, and the d-axis voltage value and the q-axis voltage value of the AC voltage of the onshore sending-end converter platform 14, perform current control 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, based on the phase-locked loop (PLL) processing of the AC side voltage of the onshore sending-end converter platform 14 to generate the reference phase angle of the onshore sending-end converter platform 14, which acts on the converter of the current control link, and combines 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 、b-phase voltage and c-phase voltage

[0111] of the AC voltage of the onshore sending-end converter platform 14 and other three-phase AC voltages Figure 5 Specifically, as

[0112] Generate the d-axis modulation ratio and q-axis modulation ratio of the AC modulation ratio of the onshore sending-end converter platform 14 based on 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 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 , d-axis current value , q-axis current value and q-axis current reference value , the d-axis voltage value of the AC voltage of the onshore sending-end converter platform 14 and q-axis voltage value , system rated angular frequency and arm inductance value to generate the d-axis modulation ratio of the AC modulation ratio of the onshore sending-end converter platform 14 and q-axis modulation ratio .

[0113] 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 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 .

[0114] Generate the a-phase voltage, b-phase voltage, and c-phase voltage of the AC voltage of the onshore sending-end converter platform 14 based on 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 to generate three-phase AC voltage.

[0115] 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, which will not be elaborated here.

[0116] Circulation control is performed based on the j-phase upper-bridge-arm current and the j-phase lower-bridge-arm current of the onshore sending-end converter platform 14, 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 onshore sending-end converter platform 14, to generate the a-phase voltage, b-phase voltage, and c-phase voltage of the circulation voltage of the onshore sending-end converter platform 14.

[0117] Specifically, as Figure 6 shown, generating the a-phase voltage, b-phase voltage, and c-phase voltage of the circulation voltage of the onshore sending-end converter platform 14 includes:

[0118] Based on the j-phase upper-bridge-arm current and the j-phase lower-bridge-arm current of the onshore sending-end converter platform 14, generate the j-phase circulation current of the onshore sending-end converter platform 14; that is, based on the j-phase upper-bridge-arm current and the j-phase lower-bridge-arm current of the MMC converter in the onshore sending-end converter platform 14, generate the j-phase circulation current .

[0119] Based on the j-phase circulation current of the onshore sending-end converter platform 14 and 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 onshore sending-end converter platform 14, generate the d-axis circulation voltage reference value and the q-axis circulation voltage reference value of the circulation voltage of the onshore sending-end converter platform 14; that is, based on the j-phase circulation current of the onshore sending-end converter platform 14, perform Park transformation ( ) to obtain the corresponding d-axis j-phase circulation current component and q-axis j-phase circulation 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 circulation voltage reference value and the q-axis circulation voltage reference value of the circulation voltage of the onshore sending-end converter platform 14.

[0120] Based on the d-axis circulation voltage reference value and the q-axis circulation voltage reference value of the circulation 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; that is, based on the d-axis circulation voltage reference value and the q-axis circulation voltage reference value of the circulation voltage of the onshore sending-end converter platform 14, perform inverse Park transformation to generate the a-phase second-harmonic AC modulation ratio and the double-frequency AC modulation ratio of phase b and the double-frequency AC modulation ratio of phase c .

[0121] Generate the phase-a voltage, phase-b voltage, and phase-c voltage of the circulating current voltage of the onshore sending-end converter platform 14 according to the phase-a double-frequency AC modulation ratio, phase-b double-frequency AC modulation ratio, and phase-c double-frequency AC modulation ratio of the double-frequency AC modulation ratio of the onshore sending-end converter platform 14; that is, based on the phase-a double-frequency AC modulation ratio of the double-frequency AC modulation ratio of the onshore sending-end converter platform 14 and the double-frequency AC modulation ratio of phase b and the double-frequency AC modulation ratio of phase c generate the phase-a voltage of the circulating current voltage of the onshore sending-end converter platform 14 and the phase-b voltage and the phase-c voltage to suppress the double-frequency circulating current inside the MMC

[0122] Perform an algebraic sum operation on the phase-a voltage, phase-b voltage, and phase-c voltage of the AC voltage of the onshore sending-end converter platform 14 and the phase-a voltage, phase-b voltage, and phase-c 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 phase-a voltage of the AC voltage of the onshore sending-end converter platform 14 and the phase-b voltage and the phase-c voltage and the phase-a voltage of the circulating current voltage of the onshore sending-end converter platform 14 and the phase-b voltage and the phase-c voltage to obtain the AC voltage reference value of the onshore sending-end converter platform 14 through the algebraic sum operation of the voltages of each phase .

[0123] 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, which will not be elaborated here

[0124] 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, generate the modulation signal of the onshore sending-end converter platform 14, including:

[0125] Generate 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 according to the AC voltage reference value of the onshore sending-end converter platform 14. That is, the AC voltage reference value is rounded by the ROUND function to obtain the number of sub-modules put into the upper and lower bridge arms and 。

[0126] Perform sub-module capacitor voltage balancing based on the number of sub-modules put into operation in the upper bridge arm and 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, and generate the modulation signal of the onshore sending-end converter platform 14. That is, based on the number of sub-modules put into operation in the upper and lower bridge arms obtained and , comprehensively consider the capacitor voltages of each module and the current values of each bridge arm, adopt the capacitor voltage sorting algorithm, monitor and sort the capacitor voltages of the sub-modules and the current values of each bridge arm in real time, and 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.

[0127] Generate 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 14 according to the modulation signal of the onshore sending-end converter platform 14.

[0128] 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-limit, it can more effectively track the fluctuations of the offshore wind power and adjust in time, effectively smooth the wind power output, thereby improving the adaptability of the system to the changes in the offshore wind power.

[0129] 2) Offshore wind power AC collection and DC transmission system

[0130] In this embodiment, the offshore wind power AC collection and DC transmission system 2 is mainly used to collect and transmit the offshore wind power, including the offshore wind farm 21, the AC collection network, the offshore converter platform 22 and the 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 the 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.

[0131] 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, the offshore converter platform 22 performs the conversion from AC to DC, and finally the DC submarine cable 23 sends the converted DC electric energy into the integrated switchyard 4.

[0132] The AC collection network includes multiple AC collection submarine cables 24 and a group of AC collection busbars 25; the AC collection submarine cables 24 can be the AC collection submarine cables within an 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 within the offshore wind farm 21 can be the AC collection submarine cables between multiple AC wind turbine units 212 that form an AC wind turbine string 211, or the AC collection submarine cables between multiple AC wind turbine strings 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 connection transformer 222.

[0133] 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 DC transmission system 1 can jointly charge the modular multilevel converter 2212, and the modular multilevel converter 2212 can charge the wind turbine units 212 in the offshore wind farm.

[0134] 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:

[0135] 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 DC transmission system 2 proposed in this embodiment can become an offshore wind power grid connection system with engineering practicability for the lightweight transmission technology of offshore wind power, this embodiment preferably designs the 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 DC transmission system 2 starts based on the following first startup timing:

[0136] Close the bypass switch 2213 so that the onshore multi-source energy transmission system 1 and the inland receiving system 3 jointly pre-charge the modular multilevel converter 2212 and the wind turbine units 212 in the offshore wind farm 21.

[0137] After the pre-charging is completed, close the bypass switch 2213 and start the wind turbine units 212 to establish the AC voltage of the AC collection busbars 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.

[0138] When the AC voltage of the AC collection bus 25 meets the rectification conditions 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 series form.

[0139] In this embodiment, it can be selected 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 the low-cost construction scenario. However, its transmission capacity may be limited by the valve group switching current of the modular multilevel converter 2212.

[0140] As Figure 8 shown, when the uncontrolled rectifier 2211 is 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:

[0141] 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.

[0142] After the pre-charging is completed, the wind turbines 212 are started to establish the AC voltage of the AC collection bus 25; among them, the judgment condition for the end of the pre-charging can be determined based on the actual application scenario and is not specifically limited here.

[0143] When the AC voltage of the AC collection bus 25 meets the rectification conditions 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 form.

[0144] 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 metal return line 261 for paralleling the uncontrolled rectifier 2211 and a second offshore end metal return line 262 for paralleling the modular multilevel converter 2212.

[0145] In this embodiment, it can be selected that the output capacity of the uncontrolled rectifier 2211 is not limited by the valve group switching current of the modular multilevel converter 2212, which is applicable to the ultra-large-capacity sea breeze transmission system. 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 the actual engineering requirements.

[0146] To better maintain the stability of the AC voltage amplitude and frequency of the AC collection bus 25 of the offshore wind power and ensure the safe and stable collection of large-scale offshore wind power, in this embodiment, it is preferably set that the offshore conversion platform 22 performs grid-forming control based on the following second control strategy:

[0147] 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, and generate the d-axis current reference value and q-axis current reference value of the AC current of the offshore conversion platform 22.

[0148] Generate the reference phase angle of the offshore conversion platform 22 according to the AC side frequency of the offshore conversion platform 22, and perform current control according to the reference phase angle of the offshore conversion platform 22, 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 22, and the d-axis voltage value and q-axis voltage value of the AC voltage of the offshore conversion platform 22 to generate the a-phase voltage, b-phase voltage, and c-phase voltage of the AC voltage of the offshore conversion platform 22; specifically, generating the a-phase voltage, b-phase voltage, and c-phase voltage of the AC voltage of the offshore conversion platform 22 includes:

[0149] Generate the d-axis modulation ratio and q-axis modulation ratio of the AC modulation ratio of the offshore conversion platform 22 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 offshore conversion platform 22, and the d-axis voltage value and q-axis voltage value of the AC voltage of the offshore conversion platform 22.

[0150] Generate the reference phase angle of the offshore conversion platform 22 according to the AC side frequency of the offshore conversion platform 22, and perform inverse Park transformation according to the reference phase angle of the offshore conversion platform 22, the d-axis modulation ratio and q-axis modulation ratio of the AC modulation ratio of the offshore conversion 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 conversion platform 22.

[0151] Generate the a-phase voltage, b-phase voltage, and c-phase voltage of the AC voltage of the offshore conversion 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 conversion platform 22.

[0152] Perform circulating current control according to the upper-bridge arm current and lower-bridge arm current of the j-phase of the offshore conversion 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 conversion platform 22 to generate the a-phase voltage, b-phase voltage, and c-phase voltage of the circulating current voltage of the offshore conversion platform 22.

[0153] Perform 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 22 and the a-phase voltage, b-phase voltage, and c-phase voltage of the circulating current voltage of the offshore converter platform 22 to generate the AC voltage reference value of the offshore converter platform 22.

[0154] Generate the modulation signal of the offshore converter platform 22 based on the AC voltage reference value of the offshore converter platform 22, as well as the capacitor voltages of each sub-module and the current values of each bridge arm of the offshore converter platform 22.

[0155] Generate the trigger pulses of the switching devices of each sub-module in the upper and lower bridge arms of the offshore converter platform 22 according to the modulation signal of the offshore converter platform 22.

[0156] In this embodiment, the offshore converter 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 Figure 4 - Figure 7 the control logic shown, which will not be elaborated here. Among them, the 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 converter platform 22. The AC voltage on the AC side in this embodiment is the AC voltage fed into by 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.

[0157] This embodiment applies the offshore wind power lightweight transmission technology that combines uncontrolled rectification technology and flexible DC hybrid commutation 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.

[0158] 3) Inland receiving system

[0159] In this embodiment, the inland receiving system 3 includes an inland receiving power grid 31, an inland receiving collecting bus 32, an inland receiving converter platform 33, and an inland receiving DC overhead line 34. The inland receiving power grid 31 is connected to the inland receiving converter platform 33 through the inland receiving collecting bus 32. Since the integrated system adopts a bipolar connection, the inland receiving system 3 further 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 collecting ground wire 42 in the integrated switchyard to achieve system grounding.

[0160] As Figure 2 - Figure 3 and Figure 8 shown, the inland receiving converter platform 33 includes an inland receiving converter and an inland receiving converter station connecting transformer. The inland receiving converter is connected to the inland receiving collecting bus 32 through the inland receiving converter station connecting 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.

[0161] In this embodiment, the inland receiving power grid 31 is a load center. To keep the received power constant, the inland receiving converter platform 33 adopts a constant power control strategy to control the active power value transmitted to the inland power grid to be constant. At the same time, when the system fails, the reactive power on the AC side of the inland receiving converter platform 33 can be used to provide a certain voltage support. Specifically, as Figure 9 shown, the inland receiving 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:

[0162] Perform active power control according to the active power value and the active power reference value of the inland receiving converter platform 33 to generate the d-axis current reference value of the AC current of the inland receiving converter platform 33, and perform reactive power control according to the reactive power value and the reactive power reference value of the inland receiving converter platform 33 to generate the q-axis current reference value of the AC current of the inland receiving converter platform 33. That is, based on the active power value and the active power reference value of the inland receiving converter platform 33, perform PI control to generate the d-axis current reference value of the AC current of the inland receiving converter platform 33 ; at the same time, according to the reactive power value and the reactive power reference value of the inland receiving converter platform 33, perform PI control to generate the q-axis current reference value of the AC current of the inland receiving converter platform 33 。

[0163] 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 based on the reference phase angle of the inland receiving-end converter platform 33, the d-axis current value of the AC current of the inland receiving-end 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 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, based on the phase-locked loop (PLL) processing of the AC-side voltage of the inland receiving-end converter platform 33 to generate the reference phase angle of the inland receiving-end converter platform 33 act on the converter of the current control link, and combine the d-axis current reference value of the AC current of the inland receiving-end converter platform 33 、d-axis current value 、q-axis current value and 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 、b-phase voltage and c-phase 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 。

[0164] Specifically, generating the a-phase voltage, b-phase voltage and c-phase voltage of the AC voltage of the inland receiving-end converter platform includes:

[0165] Generate 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 of the AC current of the inland receiving-end converter platform 33, 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 q-axis voltage value of the AC voltage of the inland receiving-end converter platform 33.

[0166] Perform inverse Park transformation according to the reference phase angle of the inland receiving-end converter platform 33 and the d-axis modulation ratio and q-axis modulation ratio of the AC modulation ratio of the inland receiving-end converter platform 33 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 33.

[0167] Generate the a-phase voltage, b-phase voltage and c-phase voltage of the AC voltage of the inland receiving-end converter platform 33 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 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.

[0168] Circulation control is performed based on the current of the upper arm of phase j and the current of the lower arm of phase j of the inland receiving - end converter platform 33, 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 33, to generate the phase - a voltage, phase - b voltage, and phase - c voltage of the circulation voltage of the inland receiving - end converter platform 33; among them, the circulation control can refer to Figure 6 the implementation logic shown, and based on the current of the upper arm of phase j of the MMC converter in the inland receiving - end converter platform 33 and the current of the lower arm of phase j , under the condition that the d - axis second - harmonic AC current reference value of the second - harmonic AC current of the inland receiving - end converter platform 33 is set to 0 and the q - axis second - harmonic AC current reference value is set to 0, the phase - a voltage , phase - b voltage and phase - c voltage of the circulation voltage of the inland receiving - end converter platform 33 are generated to suppress the second - harmonic circulation inside the MMC.

[0169] An 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 - end converter platform 33 and the phase - a voltage, phase - b voltage, and phase - c voltage of the circulation voltage of the inland receiving - end converter platform 33 to generate the AC voltage reference value of the inland receiving - end converter platform 33; that is, based on the phase - a voltage , phase - b voltage and phase - c voltage of the AC voltage of the inland receiving - end converter platform 33, and the phase - a voltage , phase - b voltage and phase - c voltage of the circulation voltage of the inland receiving - end converter platform 33, an algebraic sum operation of the phase voltages of each phase is performed to obtain the AC voltage reference value of the inland receiving - end converter platform 33.

[0170] According to the AC voltage reference value of the inland receiving - end converter platform 33 and the capacitor voltages of each sub - module and the current values of each bridge arm of the inland receiving - end converter platform 33, a modulation signal of the inland receiving - end converter platform 33 is generated; among them, the modulation signal generation logic can refer to Figure 7 the implementation, that is, based on the number of sub - modules put into the upper and lower arms obtained by rounding the AC voltage reference value 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 the modulation signal.

[0171] According to the modulation signal of the inland receiving - end converter platform 33, trigger pulses of the switching devices of each sub - module in the upper and lower arms of the inland receiving - end converter platform 33 are generated.

[0172] In this embodiment Figure 9 The constant power control executed by the inland receiving end converter platform 33 shown and Figure 4 The difference between the constant DC voltage control shown lies only in the different active power controls. The corresponding reactive power control, current control, circulating current control, modulation strategy control, trigger logic control and other links are all the same as Figure 4 The implementation logic shown, which will not be elaborated here; the active power control link in this constant power control is used to control the active power of the MMC And its reference value Generate the d-axis current reference value And act on the reference phase angle of the converter in the current control link Generated based on the AC side voltage of the inland receiving end collecting bus 32 .

[0173] The inland receiving end converter platform 33 is connected to the integrated switch station 4 through the inland receiving end DC overhead line 34, and is used to receive the collected electric energy of the onshore multi-source energy transmission system and the offshore wind power AC collection DC transmission system output by the integrated switch station 4.

[0174] 4) Integrated switch station

[0175] The integrated switch 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 DC transmission system 2 and send it to the inland receiving end system 3, and can realize the collection and transfer of multiple types of energy.

[0176] Considering that Figure 2 What is given is a relatively idealized basic topological structure. 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 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 by double-circuit DC overhead lines. Based on this, this embodiment preferably sets the DC collection bus 41 to include a first DC collection bus and a second DC collection bus.

[0177] Such as Figure 10As shown, the first DC collection bus is used to collect the electric energy of the offshore wind power AC collection - 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 through a connection switch; the first DC collection bus and the second DC collection bus are respectively connected to the inland receiving end system through a DC overhead line for the receiving end. 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 through a connection switch. If one of the DC collection buses is over - current, the connection switch can be closed, and the excess electric energy is dispersed to the other DC collection bus. The two DC collection buses are each sent to the inland receiving end power grid through a DC overhead line.

[0178] In summary, an integrated AC / DC system with multiple sources and terminals on land and sea provided by an embodiment 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-end power grid, a land-based sending-end collection bus, a land-based sending-end converter platform, and a land-based sending-end DC overhead line; the multi-source energy cluster and the land-based sending-end power grid are connected to the land-based sending-end converter platform through the land-based sending-end collection bus; the land-based sending-end converter platform is connected to the integrated switching station through the land-based sending-end 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-end power grid, an inland receiving-end collection 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 through the inland receiving-end collection bus; the inland receiving-end converter platform is connected to the integrated switching station through the inland receiving-end 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 reduce the construction cost of practical engineering platforms for offshore wind power, and provide 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 an integrated switching station, cross the already saturated AC grid, directly send electric energy to the load center, realize DC power mutual assistance between multiple regions, effectively solve the problem of tight resources of onshore coastal channel resources, improve the utilization rate of transmission channels, and also, by sending out the electric energy of the land-based multi-source energy transmission system and the offshore wind power AC collection and DC transmission system after being collected based on the integrated switching station, combined with the reliable coordinated control design of offshore wind power and multi-source energy, 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.

[0179] 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 the system embodiment, since it is basically similar to the method embodiment, the description is relatively simple. For related parts, reference can be made to the partial description of the method embodiment. It should be noted that the above technical features of the embodiments can be combined arbitrarily. For the sake of brevity of description, not all possible combinations of the above technical features in the embodiments are described. However, as long as there is no contradiction in the combination of these technical features, it should be considered as the scope described in this specification.

[0180] The above-described embodiments merely represent several preferred embodiments of the present invention. The description 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 principles 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 multi-source and multi-terminal AC-DC integrated system for land and sea, 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 system, and an integrated switching station; The onshore multi-source energy transmission system includes a multi-source energy cluster, an onshore sending grid, an onshore sending collection bus, an onshore sending converter platform, and an onshore sending DC overhead line; the multi-source energy cluster and the onshore sending grid are connected to the onshore sending converter platform through the onshore sending collection bus; the onshore sending converter platform is connected to the integrated switching station through the onshore 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; The onshore multi-source energy transmission system, the offshore wind power AC collection and DC transmission system, the integrated switching station, and the inland receiving system adopt a true bipolar connection form; the integrated switching station includes a DC collection bus and a collection ground wire; The DC collection bus is respectively connected to the onshore sending 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 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; 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.

2. The multi-source multi-terminal AC-DC integrated system on land and sea according to claim 1, characterized in that The onshore sending converter platform includes an onshore sending converter and an onshore sending converter station connecting transformer; the onshore sending converter is connected to the onshore sending collection bus through the onshore sending converter station connecting transformer; The onshore sending converter includes at least one onshore sending sub-converter, and the onshore sending sub-converter is a converter with the ability of bidirectional current flow.

3. The multi-source and multi-terminal AC / DC integrated system on land and sea according to claim 2, characterized in that The onshore sending-end converter platform performs constant DC voltage control based on the first control strategy; the first control strategy is an outer-loop DC voltage control strategy for coordinating 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.

4. The multi-source and multi-terminal AC-DC integrated system on land and sea according to claim 3, characterized in that, 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, 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, and the d-axis voltage value and 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.

5. The multi-source and multi-terminal AC / DC integrated system on land and sea according to claim 4, characterized in that The 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 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 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; Perform an inverse Park transformation based on the reference phase angle of the onshore sending-end converter platform, the d-axis modulation ratio and the 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; Generate 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.

6. The multi-source multi-terminal AC-DC integrated system for land and sea as claimed in claim 4, wherein The generating the a-phase voltage, b-phase voltage and c-phase voltage of the circulating voltage of the onshore sending-end converter platform includes: Generate the j-phase circulating current of the onshore sending-end converter platform according to the j-phase upper-arm current and the j-phase lower-arm current of the onshore sending-end converter platform; 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 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; Perform an 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 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; Generate the a-phase voltage, b-phase voltage and c-phase voltage of the circulating 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.

7. The multi-source and multi-terminal AC / DC integrated system for land and sea as claimed in claim 4, wherein The generating the modulation signal of the onshore sending-end converter platform includes: Generate the number of upper-arm input sub-modules and the number of lower-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 according to the number of upper-arm input sub-modules and the number of lower-arm input sub-modules of the onshore sending-end converter platform, as well as the capacitor voltages of each sub-module and the current values of each arm of the onshore sending-end converter platform to generate the modulation signal of the onshore sending-end converter platform.

8. The multi-source multi-terminal AC-DC integrated system on land and sea 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 an AC wind turbine, or a series of multiple AC wind turbines; 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 AC collection busbars; the offshore wind farm is connected to the AC collection busbars through the AC collection submarine cables, and the AC collection busbars are connected to the offshore converter station through the offshore converter station connecting transformer.

9. The multi-source multi-terminal AC / DC integrated system for land and sea as claimed in claim 8, wherein 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 turbines in the offshore wind farm.

10. The multi-source and multi-terminal AC / DC integrated system on land and sea according to claim 9, 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 for the uncontrolled rectifier; the positive and negative poles of the uncontrolled rectifier and the modular multilevel converter are symmetrical.

11. The multi-source multi-terminal AC-DC integrated system for land and sea as claimed in claim 10, characterized in that, The offshore wind power AC collection and DC transmission system starts based on the first startup timing; the first startup timing includes: Closing the bypass switch to enable the onshore multi-source energy transmission system and the inland receiving system to jointly pre-charge the modular multilevel converter and the wind turbines in the offshore wind farm; After the pre-charging is completed, closing the bypass switch to start the wind turbines and establish the AC voltage of the AC collection bus; When the AC voltage of the AC collection bus meets the rectification condition of the uncontrolled rectifier, establish the rated DC voltage of the offshore wind power AC collection and DC transmission system, and complete the AC-to-DC conversion through the series-connected uncontrolled rectifier and modular multilevel converter.

12. The multi-source multi-terminal AC-DC integrated system on land and sea according to claim 9, 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 starts based on the second startup timing; the second startup timing includes: The onshore multi-source energy transmission system and the inland receiving system jointly pre-charge the modular multilevel converter and the wind turbines in the offshore wind farm; After the pre-charging is completed, start the wind turbines and establish the AC voltage of the AC collection bus; When the AC voltage of the AC collection bus meets the rectification condition of the uncontrolled rectifier, establish the rated DC voltage of the offshore wind power AC collection and DC transmission system, and complete the AC-to-DC conversion through the parallel-connected uncontrolled rectifier and modular multilevel converter.

13. The multi-source and multi-terminal AC-DC integrated system on land and sea according to claim 12, wherein, 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 metal return line for paralleling the uncontrolled rectifier and a second offshore metal return line for paralleling the modular multilevel converter.

14. The multi-source and multi-terminal AC / DC integrated system on land and sea according to claim 9, characterized in that The offshore converter platform performs grid-forming control based on the 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 converter platform to generate the d-axis current reference value and q-axis current reference value of the AC current of the offshore converter platform; Generate the reference phase angle of the offshore converter platform according to the AC side frequency of the offshore converter platform, and perform current control according to the reference phase angle of the offshore converter 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 converter platform, and the d-axis voltage value and q-axis voltage value of the AC voltage of the offshore converter platform to generate the a-phase voltage, b-phase voltage, and c-phase voltage of the AC voltage of the offshore converter platform. Circulation control is performed based on the j-phase upper-bridge-arm current and the j-phase lower-bridge-arm current of the offshore HVDC 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 offshore HVDC converter platform, to generate the a-phase voltage, b-phase voltage, and c-phase voltage of the circulation voltage of the offshore HVDC 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 offshore HVDC converter platform and the a-phase voltage, b-phase voltage, and c-phase voltage of the circulation voltage of the offshore HVDC converter platform to generate the AC voltage reference value of the offshore HVDC converter platform; Based on the AC voltage reference value of the offshore HVDC converter platform, as well as the capacitor voltages of each sub-module and the current values of each bridge arm of the offshore HVDC converter platform, a modulation signal of the offshore HVDC converter platform is generated; Based on the modulation signal of the offshore HVDC converter platform, trigger pulses of each sub-module switching device in the upper and lower bridge arms of the offshore HVDC converter platform are generated.

15. The multi-source and multi-terminal AC / DC integrated system for land and sea as described in claim 14, characterized in that The generation of the a-phase voltage, b-phase voltage, and c-phase voltage of the AC voltage of the offshore HVDC 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 offshore HVDC converter platform, as well as the d-axis voltage value and q-axis voltage value of the AC voltage of the offshore HVDC converter platform, the d-axis modulation ratio and q-axis modulation ratio of the AC modulation ratio of the offshore HVDC converter platform are generated; Based on 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, 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 offshore HVDC 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 offshore HVDC converter platform, the a-phase voltage, b-phase voltage, and c-phase voltage of the AC voltage of the offshore HVDC converter platform are generated.

16. The multi-source multi-terminal AC-DC integrated system on land and sea according to claim 1, characterized in that The onshore receiving-end HVDC converter platform includes an onshore receiving-end HVDC converter and an onshore receiving-end converter station connecting transformer; the onshore receiving-end HVDC converter is connected to the onshore receiving-end collecting bus through the onshore receiving-end converter station connecting transformer; The onshore receiving-end HVDC converter includes at least one onshore receiving-end sub-converter, and the onshore receiving-end sub-converter is a converter with the ability of bidirectional current flow.

17. The multi-source multi-terminal AC-DC integrated system for land and sea as claimed in claim 1, wherein The onshore receiving-end HVDC converter platform performs constant power control based on a third control strategy; the third control strategy includes: Active power control is performed based on the active power value and the active power reference value of the onshore receiving-end HVDC converter platform to generate the d-axis current reference value of the AC current of the onshore receiving-end HVDC converter platform, and reactive power control is performed based on the reactive power value and the reactive power reference value of the onshore receiving-end HVDC converter platform to generate the q-axis current reference value of the AC current of the onshore receiving-end HVDC converter platform; Generate the 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 based on the reference phase angle of the inland receiving end converter platform, the d-axis current value of the AC current of the inland receiving end 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 inland receiving end converter platform, to generate the a-phase voltage, b-phase voltage and c-phase voltage of the AC voltage of the inland receiving end converter platform; Perform circulating current control according to the upper bridge arm current and lower bridge arm current of the j-phase of the inland receiving 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 inland receiving end converter platform, to generate the a-phase voltage, b-phase voltage and c-phase voltage of the circulating current voltage of the inland receiving end converter platform; Perform 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 the AC voltage reference value of the inland receiving end converter platform; Generate the 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 sub-module and the current values of each bridge arm of the inland receiving end converter platform; Generate the trigger pulses of the switching devices of each sub-module in the upper and lower bridge arms of the inland receiving end converter platform according to the modulation signal of the inland receiving end converter platform.

18. The multi-source multi-terminal AC-DC integrated system for land and sea as claimed in claim 17, wherein The generating the a-phase voltage, b-phase voltage and c-phase voltage of the AC voltage of the inland receiving end converter platform includes: Generate the d-axis modulation ratio and q-axis modulation ratio of the AC modulation ratio of the inland receiving end converter platform according to the d-axis current value of the AC current of the inland receiving end 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 inland receiving end converter platform; Perform an 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; Generate the a-phase voltage, b-phase voltage and c-phase voltage of the AC voltage 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.

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