Offshore wind power direct current transmission system and control method and control device thereof

Abstract

The application relates to an offshore wind power DC power transmission system and a control method and device thereof. The offshore wind power DC power transmission system, a second modular multilevel converter of an onshore converter station, can operate in a voltage reduction mode when an offshore wind farm is in a starting stage, so that the voltage level of the second modular multilevel converter matches the rated voltage level of a first modular multilevel converter, and the power supply is completed in cooperation with the first modular multilevel converter. Since the second modular multilevel converter can operate in the voltage reduction mode, the rated voltage level of the first modular multilevel converter can be smaller than the rated voltage level of the offshore DC power transmission system, so that the number of sub-modules of the first modular multilevel converter can be reduced, the size and weight of the offshore converter station are reduced, and the construction cost of the offshore converter station is reduced.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of power electronics, in particular to a sea wind power DC transmission system and a control method and control device thereof. BACKGROUND

[0002] With the acceleration of global decarbonization and emission reduction, vigorously promoting renewable clean energy has become an important means to solve the contradiction between energy supply and demand, and the sea wind power industry has a very broad development prospect. At present, the sea wind power transmission technology mainly includes high-voltage alternating current transmission and high-voltage direct current transmission. The high-voltage direct current transmission technology is not affected by the charging current and charging power of the sea cable, the transmission distance is not limited, and the investment of the sea cable is small, so it is the preferred solution for large-scale long-distance sea wind power transmission.

[0003] However, the volume and weight of the current sea converter station are large, resulting in high construction cost. SUMMARY

[0004] Therefore, it is necessary to provide a sea wind power DC transmission system with low construction cost and a control method and control device thereof aiming at the above technical problems.

[0005] In a first aspect, a sea wind power DC transmission system is provided, comprising: a DC sea cable; a sea converter station comprising a first modular multilevel converter, the rated voltage level of the first modular multilevel converter being less than the rated voltage level of the sea DC transmission system, the AC side of the first modular multilevel converter being connected with a sea wind farm, and the DC side of the first modular multilevel converter being connected with a first end of the DC sea cable when the sea wind farm is in a starting stage; and a land converter station comprising a second modular multilevel converter, the rated voltage level of the second modular multilevel converter matching the rated voltage level of the sea DC transmission system, the DC side of the second modular multilevel converter being connected with a second end of the DC sea cable, and the voltage level of the second modular multilevel converter matching the rated voltage level of the first modular multilevel converter when the sea wind farm is in the starting stage.

[0006] In one of the embodiments, the sea converter station further comprises a rectifier unit, the AC side of the rectifier unit being connected with the sea wind farm, and the DC side of the rectifier unit being connected with the first end of the DC sea cable, for transmitting the energy generated by the sea wind farm to the land converter station when the sea wind farm is in a starting completion stage.

[0007] In one of the embodiments, the rectifier unit comprises a diode rectifier, an AC side of the diode rectifier is connected with the offshore wind farm, a DC side of the diode rectifier is connected with the first end of the DC submarine cable, and the diode rectifier is configured to transmit the energy generated by the offshore wind farm to the onshore converter station when the offshore wind farm is in the start-up completion stage.

[0008] In one of the embodiments, the second modular multilevel converter is a hybrid modular multilevel converter composed of half-bridge sub-modules and full-bridge sub-modules, and the number of the full-bridge sub-modules of the second modular multilevel converter is determined according to the rated voltage level of the first modular multilevel converter.

[0009] In one of the embodiments, the number of the full-bridge sub-modules of the second modular multilevel converter is determined according to the ratio of the rated voltage level of the first modular multilevel converter to the rated voltage level of the offshore DC power transmission system and the total number of the sub-modules of the second modular multilevel converter.

[0010] In one of the embodiments, the ratio of the rated voltage level of the first modular multilevel converter to the rated voltage level of the offshore DC power transmission system is 20% to 30%.

[0011] In one of the embodiments, the offshore converter station further comprises a switch unit arranged between the first end of the DC submarine cable and the DC side of the first modular multilevel converter, and the switch unit is configured to connect the first end of the DC submarine cable and the DC side of the first modular multilevel converter when the offshore wind farm is in the start-up stage.

[0012] In one of the embodiments, the first modular multilevel converter is a modular multilevel converter composed of half-bridge sub-modules.

[0013] In a second aspect, a control method of an offshore wind power DC power transmission system is provided, which comprises: when an offshore wind farm is in a start-up stage, controlling the voltage level of a second modular multilevel converter to match the rated voltage level of a first modular multilevel converter; and controlling the DC side of the first modular multilevel converter to be connected with the first end of a DC submarine cable, wherein an AC side of the first modular multilevel converter is connected with the offshore wind farm, a second end of the DC submarine cable is connected with a DC side of the second modular multilevel converter, the rated voltage level of the first modular multilevel converter is less than the rated voltage level of the offshore DC power transmission system, and the rated voltage level of the second modular multilevel converter matches the rated voltage level of the offshore DC power transmission system.

[0014] In one of the embodiments, the control method of the offshore wind power DC transmission system further comprises: when the offshore wind farm is in a start completion stage, controlling the DC side of the first modular multilevel converter to be disconnected from the first end of the DC submarine cable, and the energy generated by the offshore wind farm is transmitted to the second modular multilevel converter through a rectifier unit; and controlling the second modular multilevel converter to operate according to the rated voltage level of the second modular multilevel converter; wherein the AC side of the rectifier unit is connected to the offshore wind farm, and the DC side of the rectifier unit is connected to the first end of the DC submarine cable.

[0015] In a third aspect, a control device of an offshore DC transmission system is provided, comprising: a control module configured to control the voltage level of the second modular multilevel converter to match the rated voltage level of the first modular multilevel current when the offshore wind farm is in a start stage; and a connection module configured to control the DC side of the first modular multilevel converter to be connected to the first end of the DC submarine cable; wherein the AC side of the first modular multilevel converter is connected to the offshore wind farm, the second end of the DC cable is connected to the DC side of the second modular multilevel converter, the rated voltage level of the first modular multilevel converter is less than the rated voltage level of the offshore DC transmission system, and the rated voltage level of the second modular multilevel converter matches the rated voltage level of the offshore DC transmission system.

[0016] In a fourth aspect, a computer device is provided, comprising a memory and a processor, the memory stores a computer program, and the processor implements the control method of the offshore wind power DC transmission system according to any one of the second aspect when executing the computer program.

[0017] In a fifth aspect, a computer readable storage medium is provided, which stores a computer program, and the computer program is executed by a processor to implement the control method of the offshore wind power DC transmission system according to any one of the second aspect.

[0018] The offshore wind power DC transmission system can reduce the voltage level of the second modular multilevel converter to match the rated voltage level of the first modular multilevel converter when the offshore wind farm is in a start stage, and the first modular multilevel converter and the second modular multilevel converter can cooperate to complete power supply. Since the second modular multilevel converter can operate at a reduced voltage level, the rated voltage level of the first modular multilevel converter can be less than the rated voltage level of the offshore DC transmission system, so that the number of sub-modules of the first modular multilevel converter can be reduced, the size and weight of the offshore converter station can be reduced, and the construction cost of the offshore converter station can be reduced. BRIEF DESCRIPTION OF DRAWINGS

[0019] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the accompanying drawings needed to be used in the embodiments or the prior art description will be briefly introduced as follows. Obviously, the accompanying drawings in the following description only only some embodiments of the present application, and for those skilled in the art, other drawings can be obtained without creative labor on the basis of these drawings.

[0020] Figure 1 The schematic diagram of the offshore wind power DC power transmission system in the first embodiment is shown in FIG. 1.

[0021] Figure 2 The structural schematic diagram of the first modular multilevel converter in an embodiment is shown in FIG. 2.

[0022] Figure 3 The structural schematic diagram of the half-bridge sub-module in an embodiment is shown in FIG. 3.

[0023] Figure 4 The schematic diagram of the offshore wind power DC power transmission system in the second embodiment is shown in FIG. 4.

[0024] Figure 5 The structural schematic diagram of the second modular multilevel converter in an embodiment is shown in FIG. 5.

[0025] Figure 6 The structural schematic diagram of the full-bridge sub-module in an embodiment is shown in FIG. 6.

[0026] Figure 7 The schematic diagram of the offshore wind power DC power transmission system in the third embodiment is shown in FIG. 7.

[0027] Figure 8 The flowchart of the control method of the offshore wind power DC power transmission system in an embodiment is shown in FIG. 8.

[0028] Figure 9 The flowchart of the control method of the offshore wind power DC power transmission system in another embodiment is shown in FIG. 9.

[0029] Figure 10 The structural block diagram of the control device of the offshore DC power transmission system in an embodiment is shown in FIG. 10. DETAILED DESCRIPTION

[0030] In order to facilitate the understanding of the present application, the present application will be described more fully below with reference to the accompanying drawings. The embodiments of the present application are shown in the accompanying drawings. However, the present application can be realized in many different forms and is not limited to the embodiments described herein. On the contrary, the purpose of providing these embodiments is to make the disclosure of the present application more thorough and comprehensive.

[0031] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. The terminology used in the description herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the application.

[0032] It is to be understood that the terms “first”, “second”, and etc. can be used herein to describe various elements, but these elements are not limited by these terms. These terms are only used to distinguish a first element from another element.

[0033] It is noted that when an element is referred to as being “connected” to another element, it can be directly connected to the other element, or connected to the other element through an intervening element. Also, “connected” in the following embodiments, if there is a transmission of electrical signals or data between the connected objects, should be understood as “electrically connected”, “communicatively connected” and etc.

[0034] As used herein, the singular forms “a”, “an” and “the” include plural referents unless the context clearly dictates otherwise. It is also to be understood that the term “comprising” or “including” or “having” and the like, denotes the presence of stated features, integers, steps, operations, components, parts, and / or the like, but does not preclude the presence or addition of one or more other features, integers, steps, operations, components, parts, and / or the like.

[0035] The mainstream topology of the offshore converter of the offshore wind power DC transmission system is currently a modular multilevel converter (MMC), the modular multilevel converter can supply power to a passive system, has no characteristic harmonic and no reactive power compensation problem, can provide flexible reactive power support, and is very suitable for offshore wind power sending out scenarios in the technical level. However, since the offshore modular multilevel converter is composed of a large number of half-bridge sub-modules in series, the volume and weight of the converter valve hall are very large, which directly affects the size and weight of the offshore platform, resulting in high construction cost and construction difficulty. In order to reduce the volume and weight of the offshore converter, an offshore wind power sending out scheme based on a diode rectifier is proposed, which can greatly reduce the cost of the offshore converter. However, since the diode rectifier cannot supply power in reverse during the starting stage of the wind farm, the offshore wind farm needs to have the ability of self-starting and establishing an AC voltage self-synchronization operation, and a large number of modifications need to be made to the current wind turbine, which is difficult. In order to solve the problem of wind farm black start and voltage synchronization, a hybrid DC sending out scheme using an auxiliary converter and a diode rectifier in parallel is proposed. In order to reduce the component cost of the auxiliary converter, the auxiliary converter is composed of an MMC, a high-voltage sub-module string, and a resonance branch, and the topology structure and control strategy are relatively complex. At the same time, a large number of series and parallel capacitors and inductors need to be configured in the resonance circuit, which to some extent weakens the economic advantage of the diode rectifier.

[0036] Therefore, the embodiment of the present application provides an offshore wind power DC transmission system, a second modular multilevel converter of an onshore converter station, which can operate in a step-down mode when an offshore wind farm is in a starting stage, so that the voltage level of the second modular multilevel converter matches the rated voltage level of a first modular multilevel converter, and the second modular multilevel converter cooperates with the first modular multilevel converter to complete power supply. Since the second modular multilevel converter can operate in a step-down mode, the rated voltage level of the first modular multilevel converter can be less than the rated voltage level of the offshore DC transmission system, so that the number of sub-modules of the first modular multilevel converter can be reduced, and the volume and weight of the offshore converter station can be reduced. At the same time, the high-voltage sub-module string and the resonance branch do not need to be configured in the offshore converter station, which further reduces the volume and weight of the offshore converter station, greatly reduces the platform size of the offshore converter station, and reduces the construction cost of the offshore converter station. At the same time, since the converter technology is mature, it is beneficial to engineering implementation.

[0037] At the same time, since the modular multilevel converter has the ability of flexible conversion between rectification and inversion, the first modular multilevel converter is configured in the offshore converter station, and when the offshore wind farm is in the starting stage, the first modular multilevel converter can supply power to the offshore wind farm and realize black start.

[0038] Please refer to Figure 1 which shows an offshore wind power DC transmission system provided by the first embodiment of the present application, as Figure 1As shown, the offshore wind power DC transmission system can include a DC submarine cable 120, an offshore converter station 140, and an onshore converter station 160. The offshore converter station 140 includes a first modular multilevel converter 142, and the onshore converter station 160 includes a second modular multilevel converter 162. It can be understood that the modular multilevel converter (the first modular multilevel converter 142 and the second modular multilevel converter 162) is cascaded by a plurality of sub-modules, and the modular multilevel converter can operate in an inverter working mode or a rectifier working mode. In the case that the modular multilevel converter operates in the inverter working mode, the DC side of the modular multilevel converter inputs DC power, and the AC side of the modular multilevel converter outputs corresponding AC power. In the case that the modular multilevel converter operates in the rectifier working mode, the AC side of the modular multilevel converter inputs AC power, and the DC side of the modular multilevel converter outputs corresponding DC power.

[0039] Specifically, the AC side of the first modular multilevel converter 142 is connected with the offshore wind farm 180, the DC side of the first modular multilevel converter 142 is connected with the first end of the DC submarine cable 120 when the offshore wind farm 180 is in a starting stage, and the DC side of the second modular multilevel converter 162 is connected with the second end of the DC submarine cable 120. The rated voltage level of the first modular multilevel converter 142 is less than the rated voltage level of the offshore wind power DC transmission system, and the rated voltage level of the second modular multilevel converter 162 matches the rated voltage level of the offshore wind power DC transmission system. It should be noted that the offshore wind farm 180 refers to a device that generates power by using offshore wind energy, and the power generated by the device is AC power. The rated voltage level corresponds to the transmission capacity, and the rated voltage level can be but is not limited to ±320 kV, ±400 kV, and ±500 kV. Optionally, the rated voltage level of the offshore DC transmission system can be the rated voltage level of the normal power generation and transmission after the starting of the offshore wind farm 180 is completed. Optionally, the offshore wind farm 180 can include offshore wind turbine generators, which are wind turbine generators applied to offshore wind power.

[0040] The second modular multilevel converter 162 is configured to match the voltage level of the second modular multilevel converter 162 with the rated voltage level of the first modular multilevel converter 142 when the offshore wind farm 180 is in the starting stage. It should be noted that the matching of the voltage level of the second modular multilevel converter 162 with the rated voltage level of the first modular multilevel converter 142 means that the voltage applied to the first modular multilevel converter 142 by the second modular multilevel converter 162 will not cause overvoltage of the first modular multilevel converter 142, thereby causing damage to the first modular multilevel converter 142.

[0041] It should be noted that when the offshore wind farm 180 is in the starting stage, the voltage level of the second modular multilevel converter 162 is less than the rated voltage level of the second modular multilevel converter 162, that is, the second modular multilevel converter 162 operates in a step-down mode to avoid overvoltage of the first modular multilevel converter 142. Since the number of submodules of the first modular multilevel converter 142 is related to the rated voltage of the first modular multilevel converter 142, since the second modular multilevel converter 162 can operate in a step-down mode when the offshore wind farm 180 is in the starting stage, the rated voltage of the first modular multilevel converter 142 can be reduced, thereby reducing the number of submodules of the first modular multilevel converter 142, and reducing the size and weight of the first modular multilevel converter 142. In one embodiment, as shown in Figure 2 The first modular multilevel converter 142 is a modular multilevel converter composed of half-bridge submodules HBSM, as shown in Figure 2 The first modular multilevel converter 142 is a three-phase modular multilevel converter formed by a plurality of half-bridge submodules HBSM connected in series. Please refer to Figure 3 which shows a half-bridge submodule provided by the embodiments of the present application, as shown in Figure 3 The half-bridge submodule includes IGBT Q1, IGBT Q2, diode D1, diode D2 and capacitor C1, wherein IGBT Q1 and IGBT Q2 are connected in series to form a first branch, capacitor C1 is connected in parallel with the first branch, (the emitter of IGBT Q1 is connected with the collector of IGBT Q2, one end of capacitor C1 is connected with the collector of IGBT Q1, and the other end of capacitor C1 is connected with the emitter of IGBT Q2), the positive electrode of diode D1 is connected with the emitter of IGBT Q1, the negative electrode of diode D1 is connected with the collector of IGBT Q1, the positive electrode of diode D2 is connected with the emitter of IGBT Q2, and the negative electrode of diode D2 is connected with the collector of IGBT Q2. In this embodiment, the first modular multilevel converter 142 is composed of half-bridge submodules HBSM, and the number of devices constituting the half-bridge submodule HBSM is small, that is, the weight and size of the first modular multilevel converter 142 are small, thereby further reducing the weight and size of the offshore converter.

[0042] In one embodiment, as shown in Figure 1 The AC device of the second modular multilevel converter 162 is connected with the AC power grid 190, and when the offshore wind farm 180 is in the starting stage, the second modular multilevel converter 162 and the first modular multilevel converter 142 cooperate to supply power to the AC power grid 190, and at the same time, the first modular multilevel converter 142 supplies power to the offshore wind farm 180 in reverse, realizing black start.

[0043] The above embodiment provides a kind of offshore wind power DC transmission system, according to the description of the above embodiment, it is known that in the starting stage of offshore wind farm, by the cooperation of the second modular multilevel converter and the first modular multilevel converter, offshore wind farm and AC power grid are powered, then the following embodiment will provide a kind of offshore wind power DC transmission system, to explain how to realize the energy generated by offshore wind farm is transported in the completion stage of offshore wind farm startup.

[0044] Please refer to Figure 4 It shows a kind of offshore wind power DC transmission system provided by the second embodiment of the application, as shown in Figure 4 The offshore wind power DC transmission system shown in the embodiment is increased relative to the offshore wind power DC transmission system shown in the above embodiment Rectifying unit 420, that is, offshore converter station 140 can also include rectifying unit 420. Wherein, the AC side of rectifying unit 420 is connected with offshore wind farm 180, the DC side of rectifying unit 420 is connected with the first end of DC submarine cable 120, and rectifying unit 420 is used to transport the energy generated by offshore wind farm 180 to onshore converter station 160 when offshore wind farm 180 is in the completion stage of startup. Rectifying unit 420 is used to rectify the AC power generated by offshore wind farm 180 into corresponding DC power, and the rectified DC power is provided to onshore converter station 160 through DC submarine cable 120, and the second modular multilevel converter 162 of onshore converter station 160 is in the inverting mode, the DC power input from its DC side is inverted into AC power output, realizing the energy generated by offshore wind farm 180 is transported. Optionally, rectifying unit 420 is a three-phase rectifying unit.

[0045] It should be noted that during the starting process of the offshore wind farm 180, the power output by the offshore wind farm 180 gradually increases. In an embodiment, during the starting process of the offshore wind farm 180, the energy generated by the offshore wind farm 180 is transmitted to the onshore converter station 160 by the first modular multilevel converter 142 and the rectifier unit 420. When the offshore wind farm 180 is in the starting completion stage, or after the starting of the offshore wind farm 180 is completed, the energy output by the offshore wind farm 180 is completely transmitted to the onshore converter station 160 by the rectifier unit 420 through the DC submarine cable 120. According to the above description, in order to realize the reduction of the size and weight of the offshore converter station 140, the rated voltage level of the first modular multilevel converter 142 is smaller than the rated voltage level of the offshore wind DC power transmission system, so that when the offshore wind farm 180 is in the starting completion stage, the DC side of the first modular multilevel converter 142 is disconnected from the first end of the DC submarine cable 120, so as to avoid that the voltage applied to the first modular multilevel converter 142 is too large, which causes damage to the first modular multilevel converter 142. In an embodiment, when the offshore wind farm 180 is in the starting completion stage, the AC side of the first modular multilevel converter 142 is connected to the offshore wind farm 180, and the operating mode of the first modular multilevel converter 142 is the STATCOM (Static Synchronous Compensator) operating mode. In this embodiment, when the offshore wind farm 180 is in the starting completion stage, the first modular multilevel converter 142 operates in the STATCOM operating mode, so as to realize the functions of filtering harmonics, providing reactive power support, and providing synchronous voltage.

[0046] In an embodiment, the rectifier unit 420 comprises a diode rectifier. The AC side of the diode rectifier is connected to the offshore wind farm 180, and the DC side of the diode rectifier is connected to the first end of the DC submarine cable 120, for transmitting the energy generated by the offshore wind farm 180 to the onshore converter station 160 when the offshore wind farm 180 is in the starting completion stage. The diode rectifier can convert AC power into DC power. In this embodiment, the energy generated by the offshore wind farm 180 is transmitted to the onshore converter station 160 by the diode rectifier when the offshore wind farm 180 is in the starting completion stage, so that the size and cost of the offshore converter station 140 are significantly reduced. Optionally, the rectifier unit 420 is a diode rectifier, the AC side of the rectifier unit 420 is the AC side of the diode rectifier, and the DC side of the rectifier unit 420 is the DC side of the diode rectifier.

[0047] Please continue to refer to Figure 4In an embodiment, the offshore wind power DC transmission system can further include one or more of a first coupling transformer 440, a second coupling transformer 460, and a third coupling transformer 480. The first end of the first coupling transformer 440 is configured to be connected to the offshore wind farm 180, and the second end of the first coupling transformer 440 is connected to the AC side of the rectifier unit 420. The first end of the second coupling transformer 460 is configured to be connected to the offshore wind farm 180, and the second coupling transformer 460 is connected to the AC side of the first modular multilevel converter 142. The first end of the third coupling transformer 480 is connected to the AC side of the second modular multilevel converter 162, and the second end of the third coupling transformer 480 is configured to be connected to the AC power grid 190. It can be understood that the type of the first coupling transformer 440, the second coupling transformer 460, and the third coupling transformer 480 is not limited in the embodiments of the present application, and can be selected according to the energy and voltage exchange requirements.

[0048] According to the above embodiment description, in order to achieve the voltage matching of the second modular multilevel converter and the first modular multilevel converter when the offshore wind farm is in the starting stage, the second modular multilevel converter should be able to operate in a step-down mode when the offshore wind farm is in the starting stage, so as to avoid overvoltage of the first modular multilevel converter.

[0049] Please refer to Figure 5 which actually shows a structural diagram of a second modular multilevel converter provided in the embodiments of the present application. As shown in Figure 5 , the second modular multilevel converter is a hybrid modular multilevel converter, which is composed of half-bridge sub-modules HBSM and full-bridge sub-modules FBSM in series. Optionally, the second modular multilevel converter is a three-phase modular multilevel converter, which can be determined according to the actual application scenario. Please refer to Figure 6 which shows a full-bridge sub-module provided in the embodiments of the present application, as shown in Figure 6As shown, the full-bridge sub-module comprises IGBT Q3, IGBT Q4, IGBT Q5, IGBT Q6, diode D3, diode D4, diode D5, diode D6 and capacitor C2, wherein IGBT Q3 and IGBT Q4 are connected in series to form a second branch, IGBT Q5 and IGBT Q6 are connected in series to form a third branch, capacitor C2 is connected in parallel with the second branch and the third branch respectively, (the emitter of IGBT Q3 is connected with the collector of IGBT Q4, the emitter of IGBT Q5 is connected with the collector of IGBT Q6, the collector of IGBT Q3 is connected with the collector of IGBT Q5, the emitter of IGBT Q4 is connected with the emitter of IGBT Q6, one end of capacitor C2 is connected with the collector of IGBT Q3, the other end of capacitor C2 is connected with the emitter of IGBT Q4), the positive electrode of diode D3 is connected with the emitter of IGBT Q3, the negative electrode of diode D3 is connected with the collector of IGBT Q3, the positive electrode of diode D4 is connected with the emitter of IGBT Q4, the negative electrode of diode D4 is connected with the collector of IGBT Q4, the positive electrode of diode D5 is connected with the emitter of IGBT Q5, the negative electrode of diode D5 is connected with the collector of IGBT Q5, the positive electrode of diode D6 is connected with the emitter of IGBT Q6, the negative electrode of diode D6 is connected with the collector of IGBT Q6. The structure of the half-bridge sub-module is described in detail above, and will not be described here.

[0050] It should be noted that the full-bridge sub-module FBSM has four working states, which are positive input state, negative input state, bypass state and blocking state. Since the full-bridge sub-module FBSM can work in the negative input state relative to the half-bridge sub-module HBSM, the second modular multilevel converter 162 composed of the half-bridge sub-module HBSM and the full-bridge sub-module FBSM has good step-down (reducing voltage level) operation effect. In an embodiment, the number of the full-bridge sub-module FBSM of the second modular multilevel converter 162 is determined by the rated voltage level of the first modular multilevel converter 142. Since the rated voltage level of the first modular multilevel converter 142 is less than the rated voltage level of the second modular multilevel converter 162, the second modular multilevel converter 162 should operate in step-down mode when the offshore wind farm 180 starts up, that is, when the second modular multilevel converter 162 and the first modular multilevel converter 142 match power supply. The step-down degree of the second modular multilevel converter 162 is related to the rated voltage level of the first modular multilevel converter 142, and the range of the voltage level that the second modular multilevel converter 162 can reduce is related to the number of the full-bridge sub-module FBSM. Therefore, in order to realize voltage matching between the second modular multilevel converter 162 and the first modular multilevel converter 142 during the start-up stage of the offshore wind farm, the number of the full-bridge sub-module FBSM of the second modular multilevel converter 162 is determined by the rated voltage level of the first modular multilevel converter 142.

[0051] In an embodiment, the number of the full-bridge sub-module FBSM of the second modular multilevel converter 162 is determined by the ratio of the rated voltage level of the first modular multilevel converter 142 to the rated voltage level of the offshore DC power transmission system and the total number of sub-modules of the second modular multilevel converter 162. In an embodiment, the ratio of the full-bridge sub-module FBSM of the second modular multilevel converter 162 to the total number of sub-modules of the second modular multilevel converter 162 is:

[0052]

[0053] wherein N is the ratio of the number of the full-bridge sub-module FBSM of the second modular multilevel converter 162 to the total number of sub-modules of the second modular multilevel converter 162 (N is the ratio of the full-bridge sub-module FBSM of the second modular multilevel converter 162), and U is the ratio of the rated voltage level of the first modular multilevel converter 142 to the rated voltage level of the offshore DC power transmission system.

[0054] In one embodiment, the ratio of the rated voltage level of the first modular multilevel converter 142 to the rated voltage level of the offshore DC power transmission system is 20%-45%. In one embodiment, the ratio of the rated voltage level of the first modular multilevel converter 142 to the rated voltage level of the offshore DC power transmission system is 20%-30%. The ratio of the rated voltage level of the first modular multilevel converter 142 to the rated voltage level of the offshore DC power transmission system in the present embodiment is 20%-30% can meet the reactive power support and filtering requirements of the offshore DC power transmission system. Alternatively, the ratio of the rated voltage level of the first modular multilevel converter 142 to the rated voltage level of the offshore DC power transmission system is 20%, 25%, 30%, 35%, 40% or 45%. The ratio of the rated voltage level of the first modular multilevel converter 142 to the rated voltage level of the offshore DC power transmission system is 30%, the required number of half-bridge sub-modules HBSM is reduced by 70%, which can reduce the construction cost and construction difficulty of the offshore converter and improve the investment returns. In one embodiment, the ratio of the full-bridge sub-modules FBSM of the second modular multilevel converter 162 to the total number of sub-modules of the second modular multilevel converter 162 is 35%-40%. Alternatively, the ratio of the full-bridge sub-modules FBSM of the second modular multilevel converter 162 to the total number of sub-modules of the second modular multilevel converter 162 is 35%, 38% or 40%.

[0055] Reference is made to Figure 7 which shows a kind of offshore DC wind power DC power transmission system provided by the third embodiment of the present application, as Figure 7 As shown, the present embodiment relative to the offshore DC wind power DC power transmission system provided by the above-mentioned embodiment, switch unit 720 is added. Wherein, switch unit 720 is arranged between the first end of DC submarine cable 120 and the DC side of first modular multilevel converter 142, for when offshore wind farm 180 is in the starting stage, the first end of DC submarine cable 120 and the DC side of first modular multilevel converter 142 are connected.

[0056] In one embodiment, switch unit 720 can also be used to disconnect the first end of DC submarine cable 120 and the DC side of first modular multilevel converter 142 when offshore wind farm 180 is in the starting completion stage.

[0057] The embodiment sets a switch unit 720 between the first end of the DC submarine cable 120 and the DC side of the first modular multilevel converter 142. When the offshore wind farm 180 is in the starting stage, the switch unit 720 connects the DC side of the first modular multilevel converter 142 and the first end of the DC submarine cable 120, realizes the cooperation of the first modular multilevel converter 142 and the second modular multilevel converter 162 for power supply, the first modular multilevel converter 142 works in the inverter mode to supply power for the offshore wind farm 180, and realizes the black start. When the offshore wind farm 180 is in the starting completion stage, the DC side of the first modular multilevel converter 142 and the first end of the DC submarine cable 120 are disconnected, the overvoltage of the first modular multilevel converter 142 is avoided, the rectifier unit 420 and the second modular multilevel converter 162 cooperate to realize the power supply, and the first modular multilevel converter 142 realizes the reactive power support of the offshore DC power transmission system.

[0058] Please continue to refer to Figure 4 , the DC side of the first modular multilevel converter 142 includes a positive end and a negative end, the first end of the DC submarine cable 120 includes a first positive end and a first negative end, the switch unit includes a positive pole knife 722, a negative pole knife 724 and a controller, the positive pole knife 722 is arranged between the positive end of the DC side of the first modular multilevel converter 142 and the first positive end of the DC submarine cable 120, and the negative pole knife 724 is arranged between the negative end of the DC side of the first modular multilevel converter 142 and the first negative end of the DC submarine cable 120. The controller controls the positive pole knife 722 and the negative pole knife 724 to be in the closed position when the offshore wind farm 180 is in the starting stage, so that the positive end of the DC side of the first modular multilevel converter 142 is connected with the first positive end of the DC submarine cable 120, and the negative end of the DC side of the first modular multilevel converter 142 is connected with the first negative end of the DC submarine cable 120. The controller controls the positive pole knife 722 and the negative pole knife 724 to be in the open position when the offshore wind farm 180 is in the starting completion stage, so that the positive end of the DC side of the first modular multilevel converter 142 is disconnected with the first positive end of the DC submarine cable 120, and the negative end of the DC side of the first modular multilevel converter 142 is disconnected with the first negative end of the DC submarine cable 120. Optionally, the DC side of the rectifier unit 420 includes a positive end and a negative end, the positive end of the DC side of the rectifier unit 420 is connected with the first positive end of the DC submarine cable 120, and the negative end of the DC side of the rectifier unit 420 is connected with the first negative end of the DC submarine cable 120.

[0059] Optionally, the DC side of the second modular multilevel converter 162 includes a positive terminal and a negative terminal, the second end of the DC submarine cable 120 includes a second positive terminal and a second negative terminal, wherein the first positive terminal of the DC submarine cable 120 is connected to the second positive terminal of the DC submarine cable 120, the first negative terminal of the DC submarine cable 120 is connected to the second negative terminal of the DC submarine cable 120, the positive terminal of the DC side of the second modular multilevel converter 162 is connected to the second positive terminal of the DC submarine cable 120, and the negative terminal of the DC side of the second modular multilevel converter 162 is connected to the second negative terminal of the DC submarine cable 120.

[0060] Reference is made to Figure 8 which shows a control method of a sea wind power DC transmission system provided by an embodiment of the present application, as Figure 8 shown, the control method can include steps S802 to S804.

[0061] S802, in the starting stage of the sea wind farm, the voltage level of the second modular multilevel is controlled to match the rated voltage level of the first modular multilevel converter.

[0062] S804, the DC side of the first modular multilevel converter is controlled to be connected to the first end of the DC submarine cable.

[0063] Wherein, the AC side of the first modular multilevel converter is connected to the sea wind farm, the second end of the DC submarine cable is connected to the DC side of the second modular multilevel converter, the rated voltage level of the first modular multilevel converter is less than the rated voltage level of the sea DC transmission system, and the rated voltage level of the second modular multilevel converter matches the rated voltage level of the sea DC transmission system. The first end and the second end of the DC submarine cable are electrically connected.

[0064] It should be noted that, since the rated voltage level of the first modular multilevel converter is less than the voltage level of the sea wind power DC transmission system, the voltage level of the second modular multilevel should be controlled to match the rated voltage level of the first modular multilevel converter, that is, the second modular multilevel is controlled to operate in step-down mode, so as to avoid overvoltage of the first modular multilevel converter.

[0065] In one embodiment, the DC side of the first modular multilevel converter is controlled to be connected to the first end of the DC submarine cable, including controlling the switch unit to be closed, so as to connect the DC side of the first modular multilevel converter to the first end of the DC submarine cable. Wherein, the switch unit is arranged between the first end of the DC submarine cable and the DC side of the first modular multilevel converter.

[0066] In one embodiment, the control of the DC side of the first modular multilevel converter to be connected with the first end of the DC submarine cable comprises the control of the positive pole isolation knife and the negative pole isolation knife to be in the closed position. The positive pole isolation knife is arranged between the positive pole end of the DC side of the first modular multilevel converter and the first positive pole end of the DC submarine cable, and the negative pole isolation knife is arranged between the negative pole end of the DC side of the first modular multilevel converter and the first negative pole end of the DC submarine cable.

[0067] Please refer to Figure 9 which shows a control method of an offshore wind power DC power transmission system provided by an embodiment of the present application, as Figure 9 shown, the control method can further comprise steps S902 to S904.

[0068] S902, in the starting completion stage of the offshore wind farm, the DC side of the first modular multilevel converter is controlled to be disconnected with the first end of the DC submarine cable, and the energy generated by the offshore wind farm is transmitted to the second modular multilevel converter through the rectifier unit.

[0069] S904, the second modular multilevel converter is controlled to operate according to the rated voltage level of the second modular multilevel converter.

[0070] The AC side of the rectifier unit is connected with the offshore wind farm, and the DC side of the rectifier unit is connected with the first end of the DC submarine cable. It should be noted that in the starting completion stage of the offshore wind farm, the DC side of the first modular multilevel converter should be controlled to be disconnected with the first end of the DC submarine cable first, and then the second modular multilevel converter is controlled to operate according to the rated voltage level thereof, so as to avoid overvoltage of the first modular multilevel converter. In one embodiment, in the starting completion stage of the offshore wind farm, the first modular multilevel converter operates in the STATCOM operating mode.

[0071] In one embodiment, the control of the DC side of the first modular multilevel converter to be disconnected with the first end of the DC submarine cable comprises the control of the switch unit to be disconnected, so as to disconnect the DC side of the first modular multilevel converter with the first end of the DC submarine cable. In one embodiment, the control of the DC side of the first modular multilevel converter to be disconnected with the first end of the DC submarine cable comprises the control of the positive pole isolation knife and the negative pole isolation knife to be in the open position.

[0072] It should be understood that, although Figure 8-9 the steps in the flowchart of the present application are shown in sequence according to the arrows, these steps are not necessarily executed in sequence according to the arrows. Unless otherwise specified herein, the execution of these steps is not strictly limited in sequence, and these steps can be executed in other sequences. Moreover, Figure 8-9At least one of the steps in the method can include multiple steps or multiple stages, which are not necessarily performed at the same time, but can be performed at different times, and the order of the steps or stages is not necessarily sequential, but can be performed alternately or alternately with at least one of the other steps or the steps or stages in the other steps.

[0073] Reference is made to Figure 10 , which shows a control device 1000 of a marine DC power transmission system provided by an embodiment of the present application, as shown in the figure, the control device 1000 of the marine DC power transmission system includes a control module 1002 and a connection module 1004. Wherein, the control module 1002 is used to control the voltage level of the second modular multilevel converter to match the rated voltage level of the first modular multilevel converter when the offshore wind farm is in the starting stage. The connection module 1004 is used to control the DC side of the first modular multilevel converter to be connected with the first end of the DC submarine cable. Figure 10

[0074] In an embodiment, the control device of the marine DC power transmission system can further include a disconnecting module and a running module. Wherein, the disconnecting module is used to control the DC side of the first modular multilevel converter to be disconnected with the first end of the DC submarine cable when the offshore wind farm is in the starting completion stage, and the energy generated by the offshore wind farm is transmitted to the second modular multilevel converter through the rectifier unit. The running module is used to control the second modular multilevel converter to operate according to the rated voltage level of the second modular multilevel converter.

[0075] For specific limitations of the control device of the marine DC power transmission system, please refer to the above limitations of the control method of the marine DC power transmission system, which will not be repeated here. Each module in the above control device of the marine DC power transmission system can be realized by software, hardware and their combination. The above modules can be embedded in or independent of the processor in the computer device in hardware form, or can be stored in the memory in the computer device in software form, so that the processor calls and executes the operations corresponding to each module. It should be noted that the division of modules in the embodiments of the present application is illustrative, and is only a logical function division. Actual implementation can have another division method.

[0076] In an embodiment, a computer device is also provided, which includes a memory and a processor, the memory stores a computer program, and the processor executes the computer program to realize the steps in each method embodiment.

[0077] In an embodiment, a computer readable storage medium is provided, which stores a computer program, and the computer program is executed by a processor to realize the steps in each method embodiment. ​

[0078] Those skilled in the art can understand that all or part of the processes in the above-mentioned embodiment methods can be completed by instructing the relevant hardware through a computer program. The computer program can be stored in a non-volatile computer readable storage medium, and when executed, can include the processes of the above-mentioned embodiment methods. Any reference to memory, storage, database or other medium used in the embodiments provided in the present application can include at least one of non-volatile and volatile memory. Non-volatile memory can include read-only memory (ROM), magnetic tape, floppy disk, flash memory or optical memory. Volatile memory can include random access memory (RAM) or external cache memory. As an illustration but not limitation, RAM can be in various forms, such as static random access memory (SRAM) or dynamic random access memory (DRAM).

[0079] In the description of the present specification, the description of the terms "some embodiments", "other embodiments", "ideal embodiments" and the like means that the specific features, structures, materials or characteristics described in conjunction with the embodiments or examples are included in at least one embodiment or example of the present application. In the present specification, the illustrative description of the above terms does not necessarily refer to the same embodiment or example.

[0080] The technical features of the above embodiments can be combined in any way. In order to make the description simple, not all possible combinations of the technical features in the above embodiments are described, but as long as the combination of the technical features does not exist, it should be considered as the scope of the present application.

[0081] The above embodiments only express several embodiments of the present application, and the description is more specific and detailed, but it should not be understood as a limitation on the scope of the patent. It should be pointed out that for ordinary skilled in the art, without departing from the concept of the present application, a number of modifications and improvements can be made, which are within the scope of the present application. Therefore, the scope of protection of the patent of the present application should be subject to the appended claims.

Claims

1. Offshore wind power DC transmission system, characterized in that The offshore converter station further comprises a rectifier unit, an AC side of the rectifier unit being connected with the offshore wind farm, and a DC side of the rectifier unit being connected with the first end of the DC submarine cable, for transmitting energy generated by the offshore wind farm to the onshore converter station when the offshore wind farm is in a start-up completion stage. The rectifier unit comprises a diode rectifier, an AC side of the diode rectifier being connected with the offshore wind farm, and a DC side of the diode rectifier being connected with the first end of the DC submarine cable, for transmitting energy generated by the offshore wind farm to the onshore converter station when the offshore wind farm is in a start-up completion stage. The number of the full-bridge sub-modules of the second modular multilevel converter is determined by the ratio of the rated voltage level of the first modular multilevel converter to the rated voltage level of the offshore wind power DC transmission system and the total number of sub-modules of the second modular multilevel converter. The ratio of the rated voltage level of the first modular multilevel converter to the rated voltage level of the offshore wind power DC transmission system is 20% to 30%. The offshore converter station further comprises: ​ N=0.5- ， ​ 2. The offshore wind power DC transmission system according to claim 1, characterized in that, ​ 3. The offshore wind power DC transmission system according to claim 2, characterized in that, ​ 4. The offshore wind power DC transmission system according to claim 1, characterized in that, ​ 5. The offshore wind power DC transmission system according to claim 1, characterized in that, ​ 6. The offshore wind power DC transmission system according to claim 1, characterized in that, ​ A switch unit is arranged between the first end of the DC submarine cable and the DC side of the first modular multilevel converter, and is used to connect the first end of the DC submarine cable and the DC side of the first modular multilevel converter when the offshore wind farm is in a starting stage.

7. The offshore wind power DC transmission system according to claim 1, characterized in that, The first modular multilevel converter is a modular multilevel converter composed of half-bridge submodules.

8. A control method of an offshore wind power DC transmission system, characterized by, Comprise: When the offshore wind farm is in a starting stage, the voltage level of the second modular multilevel converter is controlled to match the rated voltage level of the first modular multilevel converter; wherein the second modular multilevel converter operates in a step-down mode when the offshore wind farm is in the starting stage; The DC side of the first modular multilevel converter is controlled to be connected with the first end of the DC submarine cable; Wherein, the AC side of the first modular multilevel converter is connected with the offshore wind farm, the second end of the DC submarine cable is connected with the DC side of the second modular multilevel converter, the rated voltage level of the first modular multilevel converter is less than the rated voltage level of the offshore wind power DC transmission system, and the rated voltage level of the second modular multilevel converter matches the rated voltage level of the offshore wind power DC transmission system, and the power supply is completed in cooperation with the first modular multilevel converter; Wherein, the second modular multilevel converter is a hybrid modular multilevel converter composed of half-bridge submodules and full-bridge submodules, the number of full-bridge submodules of the second modular multilevel converter is determined by the rated voltage level of the first modular multilevel converter; the working state of the full-bridge submodule includes positive input state, negative input state, bypass state and lock state; The proportion of the full-bridge submodule of the second modular multilevel converter to the total number of submodules of the second modular multilevel converter is: N=0.5- ， Wherein, N is the proportion of the number of full-bridge submodules of the second modular multilevel converter to the total number of submodules of the second modular multilevel converter, and U is the proportion of the rated voltage level of the first modular multilevel converter to the rated voltage level of the offshore wind power DC transmission system.

9. The control method of the offshore wind power DC transmission system according to claim 8, characterized by, Also include: When the offshore wind farm is in a starting completion stage, the DC side of the first modular multilevel converter is disconnected from the first end of the DC submarine cable, and the energy generated by the offshore wind farm is transmitted to the second modular multilevel converter through a rectifier unit; The second modular multilevel converter is controlled to operate according to the rated voltage level of the second modular multilevel converter; Wherein, the AC side of the rectifier unit is connected with the offshore wind farm, and the DC side of the rectifier unit is connected with the first end of the DC submarine cable.

10. A control device of an offshore wind power DC transmission system, characterized by, Comprise: A control module is arranged to control the voltage level of the second modular multilevel converter to match the rated voltage level of the first modular multilevel converter when the offshore wind farm is in a starting stage; wherein the second modular multilevel converter operates in a step-down mode when the offshore wind farm is in the starting stage; The connecting module is configured to control connection of a DC side of the first modular multilevel converter with a first end of a DC submarine cable; wherein an AC side of the first modular multilevel converter is connected with the offshore wind farm, a second end of the DC submarine cable is connected with a DC side of a second modular multilevel converter, a rated voltage level of the first modular multilevel converter is less than a rated voltage level of the offshore wind power DC transmission system, and a rated voltage level of the second modular multilevel converter matches the rated voltage level of the offshore wind power DC transmission system, and the first modular multilevel converter cooperates with the second modular multilevel converter to complete power supply. The second modular multilevel converter is a hybrid modular multilevel converter composed of half-bridge sub-modules and full-bridge sub-modules, and a number of the full-bridge sub-modules of the second modular multilevel converter is determined by the rated voltage level of the first modular multilevel converter; and working states of the full-bridge sub-modules include a positive input state, a negative input state, a bypass state and a blocking state. A ratio of the full-bridge sub-modules of the second modular multilevel converter to a total number of sub-modules of the second modular multilevel converter is: N=0.5- ， wherein N is the ratio of the number of the full-bridge sub-modules of the second modular multilevel converter to the total number of sub-modules of the second modular multilevel converter, and U is a ratio of the rated voltage level of the first modular multilevel converter to the rated voltage level of the offshore wind power DC transmission system. 11.A computer device, comprising a memory and a processor, wherein the memory stores a computer program, and the computer device is configured to perform the method according to any one of claims 1-10. The processor executes the computer program to implement the steps of the control method of the offshore wind power DC transmission system according to any one of claims 8 to 9.

12. A computer readable storage medium having stored thereon a computer program, characterized in that, The computer program is executed by the processor to implement the steps of the control method of the offshore wind power DC transmission system according to any one of claims 8 to 9.

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