Hybrid direct current transmission system for new energy transmission and control method thereof

Abstract

The application provides a hybrid direct current transmission system for new energy transmission and a control method thereof, and has the characteristics that the transmission system comprises a first converter and a second converter, wherein: the first converter comprises a bypass switch, a thyristor converter valve and a first modular multilevel converter valve, the direct current side of the thyristor converter valve and the direct current side of the first modular multilevel converter valve are connected in series, and the bypass switch is connected in parallel with the thyristor converter valve; the alternating current side of the thyristor converter valve is connected in series with a first transformer, and the alternating current side of the first modular multilevel converter valve is connected in series with a second transformer; the second converter comprises a second modular multilevel converter valve, the second modular multilevel converter valve comprises a first full-bridge sub-module or a first full-bridge sub-module and a first half-bridge sub-module; the alternating current side of the second converter is connected in series with a third transformer; and the direct current side of the first converter and the direct current side of the second converter are connected in series.

Description

Technical Field

[0001] This application relates to the field of DC power transmission technology in power systems, and more specifically, to a hybrid DC power transmission system for new energy transmission and its control method. Background Technology

[0002] As a green energy source, new energy power generation can address the global warming problem caused by increased carbon emissions, and is increasingly valued and favored by governments worldwide. Wind power, as a type of new energy power generation, includes offshore wind power and onshore wind power. Compared to onshore wind power, offshore wind power has advantages such as not occupying land resources, more stable wind resources, and higher utilization hours, and has become one of the key renewable energy sources being developed in China and Europe.

[0003] Currently, my country's near-shore wind power resources have been largely developed. To acquire more offshore wind energy, offshore wind farms are gradually expanding towards deeper waters. However, as transmission distances increase, AC power transmission gradually loses its cost-effectiveness due to increased power loss, difficulty in reactive power compensation, and overall cost. Therefore, DC power transmission is the preferred option for long-distance applications.

[0004] Currently, the most common approach is to use flexible DC transmission for offshore wind power. When flexible DC transmission converter stations are used in offshore wind farms, they are typically built on offshore platforms. The entire offshore platform generally includes space for the main equipment and working areas for operators. In existing offshore flexible DC transmission topologies, a modular multilevel converter valve (MMC) topology is commonly used. However, offshore platforms are expensive: the MMC MPC converter valve is a key component in flexible DC transmission, and its cost and weight are relatively high. Therefore, reducing the weight and volume of the converter valve, reducing the weight and volume of the offshore platform, and reducing the cost of the offshore converter valve and platform are urgent problems to be solved to promote grid parity for offshore wind power.

[0005] Siemens proposed a solution for offshore wind power DC transmission using uncontrolled diode rectifier units at offshore stations and modular multilevel converters at onshore stations to reduce the cost of offshore wind power transmission systems. This solution offers advantages such as small size and low cost. However, using only diodes for offshore wind power transmission presents several problems: 1) the wind farm needs to effectively control the voltage and frequency of the offshore AC grid; 2) energy storage, diesel generators, and AC submarine cables are required to provide energy for the wind farm's startup process; and 3) reactive power compensation and harmonic issues need to be addressed.

[0006] Therefore, it is necessary to propose a hybrid DC power transmission system and its control method for new energy transmission, which can effectively solve the problems of high cost of offshore flexible DC power transmission platforms, black start of wind turbines, and reactive power and harmonic compensation, and promote the development of large-capacity long-distance offshore wind power transmission technology.

[0007] The information disclosed in the background section is only intended to enhance the understanding of the background of this application, and therefore may include information that does not constitute prior art known to those skilled in the art. Summary of the Invention

[0008] This application aims to provide a hybrid DC power transmission system and its control method for new energy transmission, which reduces the weight, volume and cost of offshore platforms and reliably realizes the access of new energy sources or the power grid.

[0009] According to a first aspect of this application, a hybrid DC power transmission system for new energy transmission is proposed, the transmission system comprising a first converter and a second converter, wherein: The first converter includes a bypass switch, a thyristor converter valve, and a first modular multilevel converter valve. The DC side of the thyristor converter valve and the DC side of the first modular multilevel converter valve are connected in series, and the bypass switch is connected in parallel with the thyristor converter valve. The AC side of the thyristor converter valve is connected in series with a first transformer, and the AC side of the first modular multilevel converter valve is connected in series with a second transformer. The second converter includes a second modular multilevel converter valve, which includes a first full-bridge submodule, or the first full-bridge submodule and a first half-bridge module; the AC side of the second converter is connected in series with a third transformer; The DC side of the first converter and the DC side of the second converter are connected in series.

[0010] According to some embodiments, the thyristor converter valve includes N series-connected 6-pulse bridge converters, where N is an integer greater than or equal to 1.

[0011] According to some embodiments, the N series-connected 6-pulse bridge converters are respectively connected in parallel with the bypass switch.

[0012] According to some embodiments, the thyristor converter valve employs a thyristor semi-controlled device or an IGCT device with turn-off thyristor characteristics.

[0013] According to some embodiments, the bypass switch is a circuit breaker, a transfer switch, or a power electronic switch.

[0014] According to some embodiments, the first modular multilevel converter valve includes a second half-bridge sub-module, or a second half-bridge module and a second full-bridge module.

[0015] According to some embodiments, the AC side of the N 6-pulse bridge converters is connected in series with the corresponding N transformers; The DC side of each of the N 6-pulse bridge converters and the DC side of the first modular multilevel converter valve are connected in series to form a DC branch. The DC branch is connected in series with the DC side of the second converter.

[0016] According to some embodiments, the DC side of the first converter and the DC side of the second converter are connected in series, including by a DC line, a DC cable, or a direct back-to-back connection.

[0017] According to some embodiments, the second full-bridge submodule of the first modular multilevel converter valve and the first full-bridge submodule of the second modular multilevel converter valve include a turn-off full-bridge power semiconductor and an energy storage capacitor, and output positive level, negative level and zero level; The first half-bridge module and the second half-bridge module include a fully controllable power semiconductor that can be turned off and an energy storage capacitor, and output positive and zero levels.

[0018] According to some embodiments, the first converter further includes an AC filter to provide reactive power compensation for the thyristor converter valve.

[0019] According to a second invention of this application, a method for controlling a hybrid DC power transmission system for new energy transmission as described in any one of the first aspects is proposed, the method comprising: Close the bypass switch; Unlock the second converter and establish DC voltage; In response to the establishment of the DC voltage, the first modular multilevel converter valve is unlocked, and the AC voltage is established; The transformer connected in series with the thyristor converter valve is energized, and the thyristor converter valve is unlocked under negative pressure; In response to the unlocking of the thyristor converter valve, the bypass switch is disconnected; The thyristor converter valve raises the DC voltage, and the second converter reduces the amount of negative voltage in the full-bridge submodule to maintain the DC voltage stability.

[0020] According to some embodiments, it also includes: Inspect the primary and secondary equipment in the power transmission system; The primary equipment includes the first converter, the second converter, and the transformer; The secondary equipment includes control equipment for unlocking the thyristor converter valve, the first modular multilevel converter valve, and the second modular multilevel converter valve.

[0021] According to some embodiments, unlocking the second converter and establishing a DC voltage includes: Close the switch of the transformer connected in series with the second converter to charge the first modular multilevel converter valve and the second modular multilevel converter valve; In response to the completion of charging of the first modular multilevel converter valve and the second modular multilevel converter valve, the second modular multilevel converter valve is unlocked in DC voltage control mode; The second modular multilevel converter valve is controlled to establish a DC voltage that matches the first modular multilevel valve.

[0022] According to some embodiments, the step of unlocking the first modular multilevel converter valve in response to the establishment of DC voltage and establishing AC voltage includes: Close the switch of the transformer connected in series with the first modular multilevel converter valve; The first modular multilevel converter valve is unlocked in islanded control mode.

[0023] According to some embodiments, it also includes: Determine whether the thyristor converter valve is not energized and unlocked; If the thyristor converter valve is not unlocked, the transformer connected in series with the thyristor converter valve will be energized again to unlock the thyristor converter valve under negative pressure.

[0024] According to a third aspect of this application, a matching method is proposed for a hybrid DC transmission system for new energy transmission as described in any one of the first aspects, used to determine the ratio of the number of the first full-bridge sub-modules of the second modular multilevel converter valve to the total number of sub-modules, the matching method comprising: Determine the reactive power compensation value provided by the offshore wind farm; The reactive power required by the thyristor converter valve during operation is determined based on the reactive power compensation value. Determine the relationship between the rated active power and reactive power of the first modular multilevel converter valve. Based on the relationship between the rated active power and reactive power of the first modular multilevel converter valve, the DC voltage of the first modular multilevel converter valve is determined. Based on the DC voltage, determine the ratio of the number of the first full-bridge sub-modules of the second modular multilevel converter valve to the total number of sub-modules.

[0025] According to some embodiments, determining the reactive power compensation value includes: Based on the power factor and transmitted active power of the offshore wind farm, determine the reactive power compensation value Q1 provided by the offshore wind farm: Q1=K1P1 in, , P1 is the power factor of the wind farm and P2 is the active power transmitted by the wind farm.

[0026] According to some embodiments, determining the reactive power consumed by the thyristor converter valve during operation based on the reactive power compensation value includes: Q 11 =K 11 P 11 Among them, Q 11 The reactive power consumed by the thyristor converter valve during operation is: , U d0 For ideal open-circuit voltage, U d P11 represents the effective value of the no-load voltage on the valve side of the converter transformer, and P11 represents the active power delivered by the thyristor converter valve.

[0027] According to some embodiments, the method further includes determining the relationship between the maximum reactive power that the first modular multilevel converter valve can provide and the reactive power consumed by the thyristor converter valve: Q 12_max = (Q) 11 -Q1) Among them, Q 12_max This refers to the maximum reactive power that the modular multilevel converter valve in the first converter can provide.

[0028] According to some embodiments, including: exist When =1, it is determined that the reactive power exchanged by the first modular multilevel converter valve meets the reactive power requirement of the thyristor-converter valve. exist If the reactive power exchanged by the first modular multilevel converter valve is insufficient to meet the reactive power requirements of the thyristor converter valve, the power transmission system needs to be configured with an AC filter to provide reactive power to the thyristor converter valve.

[0029] According to some embodiments, determining the relationship between the rated active power and reactive power of the first modular multilevel converter valve includes: n=P 12 / Q 12_max Among them, P 12 The active power transmitted to the modular multilevel converter valve in the first converter.

[0030] According to some embodiments, determining the DC voltage of the first modular multilevel converter valve based on the relationship between the rated active power and reactive power of the first modular multilevel converter valve includes:

[0031] Among them, U dc12 U is the DC voltage of the modular multilevel converter valve in the first converter. dc This is the system DC voltage.

[0032] According to some embodiments, determining the ratio of the number of the first full-bridge submodules of the second modular multilevel converter valve to the total number of submodules based on the DC voltage includes: U dc2min =(1-2γ)U dc U dc2min =U dc12

[0033] Among them, U dc2min γ is the minimum DC-side voltage of the modular multilevel converter valve in the second converter, γ is the ratio of the number of full-bridge submodules to the total number of submodules, and ceiling{} represents the floor function.

[0034] This application provides a hybrid DC power transmission system for new energy transmission and its control method. The AC voltage is established by stepping down the voltage of the second converter and cooperating with the modular multilevel converter valve of the first converter. At the same time, taking advantage of the cost and space-saving advantages of the thyristor converter valve of the first converter, it is connected in series with the modular multilevel converter valve of the first converter to achieve the purpose of starting and power transmission.

[0035] This application provides a hybrid DC power transmission system and its control method for new energy transmission, which can reduce the cost and footprint of converter valves at offshore stations, thereby reducing the weight, volume and cost of offshore platforms. By adopting a modular multilevel converter with a full-bridge sub-module in the second converter, the cost and losses of onshore converter stations are reduced, ultimately contributing to the grid parity transmission of large-scale new energy.

[0036] This application provides a hybrid DC transmission system for new energy transmission and its control method: AC grid support is provided through a modular multilevel converter valve on the rectifier side, solving the black start problem of wind power, realizing power transmission, and performing reactive power and harmonic compensation in steady state; step-down operation is achieved by configuring a hybrid topology of full-bridge and half-bridge submodules in the inverter-side converter station, achieving DC voltage matching with the rectifier-side converter valve during startup; the configuration ratio of full-bridge and half-bridge submodules in the inverter-side converter station is determined based on the modular multilevel converter valve on the rectifier side, realizing the application of the hybrid DC topology engineering.

[0037] It should be understood that the above general description and the following detailed description are merely exemplary and do not limit this application. Attached Figure Description

[0038] The above and other objects, features, and advantages of this application will become more apparent from the detailed description of exemplary embodiments with reference to the accompanying drawings. The drawings described below are merely some embodiments of this application and are not intended to limit the scope of this application.

[0039] Figure 1 A schematic diagram of a hybrid DC power transmission system for transmitting new energy sources is shown in an exemplary embodiment. Figure 2 Another embodiment of an exemplary hybrid DC power transmission system for transmitting new energy sources is shown in the schematic diagram. Figure 3 Another embodiment of an exemplary hybrid DC power transmission system for transmitting new energy sources is shown in the schematic diagram. Figure 4a A schematic diagram of a modular multilevel converter valve is shown as an exemplary embodiment; Figure 4b A schematic diagram of a full-bridge submodule of an exemplary embodiment is shown; Figure 4c A schematic diagram of a half-bridge submodule of an exemplary embodiment is shown; Figure 5 A flowchart illustrating a control method for a hybrid DC power transmission system for new energy transmission, as shown in an exemplary embodiment; Figure 6 A flowchart illustrating an exemplary embodiment of a method for matching the number of full-bridge submodules and half-bridge submodules in a second converter is provided. Detailed Implementation

[0040] Exemplary embodiments will now be described more fully with reference to the accompanying drawings. However, these exemplary embodiments can be implemented in many forms and should not be construed as limited to the embodiments set forth herein; rather, they are provided so that this application will be thorough and complete, and will fully convey the concept of the exemplary embodiments to those skilled in the art. The same reference numerals in the drawings denote the same or similar parts, and therefore repeated descriptions of them will be omitted.

[0041] The described features, structures, or characteristics can be combined in any suitable manner in one or more embodiments. Numerous specific details are provided in the following description to give a full understanding of embodiments of this disclosure. However, those skilled in the art will recognize that the technical solutions of this disclosure can be practiced without one or more of these specific details, or other methods, components, materials, devices, etc. In these cases, well-known structures, methods, devices, implementations, materials, or operations will not be shown or described in detail.

[0042] The flowcharts shown in the accompanying drawings are merely illustrative and do not necessarily include all content and operations / steps, nor do they necessarily have to be performed in the described order. For example, some operations / steps can be broken down, while others can be combined or partially combined; therefore, the actual execution order may change depending on the specific circumstances.

[0043] The terms "first," "second," etc., in the specification, claims, and accompanying drawings of this application are used to distinguish different objects, not to describe a specific order. Furthermore, the terms "comprising" and "having," and any variations thereof, are intended to cover non-exclusive inclusion. For example, a process, method, system, product, or apparatus that includes a series of steps or units is not limited to the listed steps or units, but may optionally include steps or units not listed, or may optionally include other steps or units inherent to these processes, methods, products, or apparatuses.

[0044] Those skilled in the art will understand that the accompanying drawings are merely schematic diagrams of exemplary embodiments, and the modules or processes in the drawings are not necessarily essential for implementing this application, and therefore cannot be used to limit the scope of protection of this application.

[0045] In existing offshore flexible DC transmission system schemes, a modular multilevel converter valve system is typically used. However, the use of fully controlled devices in the modular multilevel submodules and the configuration of capacitors in individual submodules can lead to problems such as high cost, large size, and heavy weight of the converter valves at offshore stations.

[0046] To reduce the cost, weight, and size of offshore converter stations, patents CN113922410 and CN113178886 propose a parallel scheme using diodes and modular multilevel converter valves. However, this parallel scheme requires the modular multilevel converter valves to be connected in parallel on the DC side. Furthermore, the receiving end does not use a converter valve with a full-bridge submodule, which fails to reduce the DC voltage. This results in a large number of parallel submodules, and the uncontrollable characteristics of the diodes during operation can cause problems such as the inability to effectively overcome faults at the sending end.

[0047] Therefore, in response to the existing problems of flexible DC transmission for offshore wind power, this application proposes a hybrid DC transmission system for new energy transmission and its control method.

[0048] Figure 1 A schematic diagram of a hybrid DC power transmission system for transmitting new energy sources is shown as an exemplary embodiment.

[0049] like Figure 1 As shown, the hybrid DC power transmission system for new energy transmission includes a first converter 10 and a second converter 20. The first converter 10 includes a bypass switch 105, a thyristor converter valve 101 and a modular multilevel converter valve 103, with the thyristor converter valve 101 connected in parallel with the bypass switch 105.

[0050] According to some embodiments, the thyristor converter valve 101 of the first converter 10 includes N series-connected 6-pulse bridge converters, where N is an integer greater than or equal to 1. The value of N mainly depends on the voltage level and capacity: usually, two 6-pulse bridge thyristor converter valves are connected in series to form a 12-pulse converter valve. If the voltage level is higher, two or even four 12-pulse bridge converters can be connected in series.

[0051] According to some embodiments, each thyristor converter valve 101 is connected in parallel with a bypass switch 105 on its DC side; the bypass switch 105 can ensure that the first converter 10 can continue to conduct when the thyristor converter valve 101 is turned off or stopped.

[0052] According to some embodiments, the thyristor switching valve 101 of the first converter 10 adopts a semi-controlled device such as a thyristor or an IGCT device with turn-off thyristor characteristics.

[0053] According to some embodiments, the bypass switch 105 may be a circuit breaker, a transfer switch, a power electronic switch, or other switching equipment.

[0054] According to some embodiments, during the startup phase, the new energy side, i.e. the first converter 10 side, has no power supply. A stable AC voltage is provided to the new energy side by means of the modular multilevel converter valve 103 of the first converter 10. The sub-modules of the modular multilevel converter valve 103 can be half-bridge sub-modules or a mixture of half-bridge modules and full-bridge sub-modules.

[0055] According to some embodiments, the AC side of the thyristor converter valve 101 of the first converter 10 is connected in series with a transformer 31 to a new energy source or AC system; the modular multilevel converter valve 103 is connected in series with a transformer 32 to a new energy source or AC system.

[0056] According to some embodiments, the DC side of the thyristor converter valve 101 and the DC side of the modular multilevel converter valve 103 of the first converter 10 are connected in series and then connected to the second converter 20 through the DC line 15.

[0057] According to some embodiments, DC line 15 is a DC overhead line, DC cable, or back-to-back straight busbar.

[0058] According to some embodiments, the second converter 20 includes a full-bridge submodule, or a modular multilevel converter valve 201 composed of a mixture of full-bridge and half-bridge submodules. The use of a hybrid topology of full-bridge and half-bridge submodules can reduce losses and costs.

[0059] According to some embodiments, the modular multilevel converter valve 201 of the second converter 20 adopts a full-bridge submodule, which can make full use of the negative voltage output characteristic of the full-bridge submodule. During restart, the DC voltage can be reduced to a voltage that matches the DC voltage generated by the modular multilevel converter valve 103 of the first converter 10.

[0060] According to some embodiments, the second converter 20 is connected to the AC power grid via a transformer 33.

[0061] According to some embodiments, in the first converter 10, the modular multilevel converter valve 103 and in the second converter 20: the full-bridge submodule topology includes a turn-off full-bridge power semiconductor and an energy storage capacitor, which can output positive, negative and zero levels; the half-bridge submodule topology includes a turn-off fully controllable power semiconductor and an energy storage capacitor, which can output positive and zero levels.

[0062] According to an embodiment, the first converter 10 further includes an AC filter for providing reactive power compensation when the reactive power provided by the offshore wind turbine, the modular multilevel converter valve 103 and the second modular multilevel converter valve 201 cannot meet the reactive power of the thyristor converter valve 101.

[0063] This application provides a hybrid DC power transmission system for new energy transmission. By proposing a topology of a first converter 10 and a second converter 20, it enables large-scale new energy access, startup, and grid connection. At the same time, the first converter 10 can effectively reduce cost, weight, and volume, promoting grid parity for new energy access.

[0064] This application provides a hybrid DC power transmission system for new energy transmission. The AC voltage is established by stepping down the voltage through the second converter 20 and cooperating with the modular multilevel converter valve 103 of the first converter 10. At the same time, taking advantage of the cost and space-saving advantages of the thyristor converter valve 101 of the first converter 10, it is connected in series with the modular multilevel converter valve 103 of the first converter 10 to achieve the purpose of starting and power transmission.

[0065] This application provides a hybrid DC power transmission system for new energy transmission. The first converter 10 can employ a thyristor converter valve 101 connected in series with a modular multilevel converter valve 103. The thyristor converter valve 101 bears the main DC voltage, reducing cost, weight, and footprint. If the thyristor converter valve 101 fails, the operational reliability can be improved by closing the parallel bypass switch 105, ensuring the stability of new energy power supply, or relying on the continuity of power supply from land to offshore platforms.

[0066] This application provides a hybrid DC power transmission system for new energy transmission. The first converter 10 is connected in series with a small number of modular multilevel converter valves 103 via thyristor converter valves 101, which reduces the number of modular multilevel converter valves at offshore stations, saving costs and space. The second converter 20 is configured with a full-bridge submodule to achieve step-down operation, thereby enabling the second converter 20 to operate at the modular multilevel low voltage of the first converter 10, realizing the AC test unlocking voltage establishment of the first converter 10, and then using the established AC voltage to establish the thyristor converter valves 101 of the first converter 10.

[0067] Figure 2 This is yet another embodiment of an exemplary hybrid DC power transmission system for transmitting new energy sources.

[0068] like Figure 2 As shown, the hybrid DC power transmission system for new energy transmission includes a first converter 10, AC side transformers 311, 312, 32 and switches 3113, 3114, 3123, 3124, 323, 324 on both sides of the transformers, a second converter 20, transformer 33 and switches 331 and 332 on both sides of the transformers, and a DC line 15 connecting the first converter 10 and the second converter 20.

[0069] According to an example embodiment, the first converter 10 includes thyristor converter valves 1011 and 1013, bypass switches 1051 and 1053, and a modular multilevel converter valve 103.

[0070] According to the example embodiment, thyristor converter valves 1011 and 1013 are 6-pulse bridge rectifier thyristor converter valves. Thyristor converter valves 1011 and 1013 are connected in series. The DC side of thyristor converter valve 1011 is connected in parallel with bypass switch 1051, and the DC side of thyristor converter valve 1013 is connected in parallel with bypass switch 1053.

[0071] According to some embodiments, thyristor converter valves 1011 and 1013 and modular multilevel converter valve 103 can be arbitrarily connected in series, as long as the high voltage output line is connected to the high voltage end of the second converter 20 through the positive line and the low voltage output line is connected to the low voltage end of the second converter 20 through the negative line.

[0072] According to the example embodiment, the AC side of the thyristor converter valve 1011 of the first converter 10 is connected to the new energy or AC system through the transformer 311. The valve side of the transformer 311 is equipped with a switch 3113 connected to the thyristor converter valve 1011, and the valve side of the transformer 311 is equipped with a switch 3114 connected to the new energy or AC system.

[0073] According to the example embodiment, the AC side of the thyristor converter valve 1013 of the first converter 10 is connected to the new energy or AC system through the transformer 312. The valve side of the transformer 312 is equipped with a switch 3123 connected to the thyristor converter valve 1013, and the valve side of the transformer 312 is equipped with a switch 3124 connected to the new energy or AC system.

[0074] According to some embodiments, the modular multilevel converter valve 103 of the first converter 10 adopts a half-bridge sub-module, or a hybrid scheme of half-bridge module and full-bridge sub-module. The half-bridge module can be given priority to reduce cost and loss.

[0075] According to the example embodiment, the AC side of the multilevel converter valve 103 is connected to a new energy source or AC system through a transformer 32. A switch 323 is configured on the valve side of the transformer 32 and connected to the modular multilevel converter valve 103. A switch 324 is configured on the valve side of the transformer 32 and connected to the new energy source or AC system.

[0076] According to some embodiments, the first converter 10 and the second converter 20 are connected by a DC line 15, which is a DC overhead line, a DC cable, or a back-to-back straight bus.

[0077] According to some embodiments, the second converter 20 includes a full-bridge submodule, or a modular multilevel converter valve 201 composed of a mixture of full-bridge and half-bridge submodules. Using a mixed topology of full-bridge and half-bridge submodules can reduce losses and costs.

[0078] According to some embodiments, the modular multilevel converter valve 201 of the second converter 20 adopts a full-bridge submodule, which can make full use of the negative voltage output characteristic of the full-bridge submodule. During restart, the DC voltage can be reduced to a voltage that matches the DC voltage generated by the modular multilevel converter valve 103 of the first converter 10.

[0079] According to some embodiments, the thyristor switching valve 101 of the first converter 10 adopts a semi-controlled device such as a thyristor or an IGCT device with turn-off thyristor characteristics.

[0080] According to some embodiments, bypass switches 1051 and 1053 may be switching devices such as circuit breakers, transfer switches, and power electronic switches.

[0081] According to some embodiments, in the first converter 10, the modular multilevel converter valve 103 and in the second converter 20: the full-bridge submodule topology includes a turn-off full-bridge power semiconductor and an energy storage capacitor, which can output positive, negative and zero levels; the half-bridge submodule topology includes a turn-off fully controllable power semiconductor and an energy storage capacitor, which can output positive and zero levels.

[0082] Figure 3This is yet another embodiment of an exemplary hybrid DC power transmission system for transmitting new energy sources.

[0083] like Figure 3 As shown, the hybrid DC power transmission system for new energy transmission includes a first converter 10, AC side transformers 311, 312, 32 and switches 3113, 3114, 3115, 3123, 3124, 3125, 323, 324 on both sides of the transformers, a second converter 20, transformer 33 and switches 331 and 332 on both sides of the transformers, and a DC line 15 connecting the first converter 10 and the second converter 20.

[0084] According to an example embodiment, the first converter 10 includes thyristor converter valves 1011, 1013, 1015, and 1017, bypass switches 1051, 1053, 1055, and 1057, and a modular multilevel converter valve 103.

[0085] According to the example embodiment, thyristor converter valves 1011, 1013, 1015, and 1017 are 6-pulse bridge rectifier thyristor converter valves. Thyristor converter valves 1011 and 1013 are connected in series to form a 12-pulse bridge thyristor converter valve, and thyristor converter valves 1015 and 1017 are connected in series to form a 12-pulse bridge thyristor converter valve. The DC side of thyristor converter valve 1011 is connected in parallel with bypass switch 1051, the DC side of thyristor converter valve 1013 is connected in parallel with bypass switch 1053, the DC side of thyristor converter valve 1015 is connected in parallel with bypass switch 1055, and the DC side of thyristor converter valve 1017 is connected in parallel with bypass switch 1057.

[0086] According to some embodiments, thyristor converter valves 1011, 1013, 1015, and 1017 and modular multilevel converter valve 103 can be arbitrarily connected in series, as long as the high-voltage output line is connected to the high-voltage end of the second converter 20 through the positive line and the low-voltage output line is connected to the low-voltage end of the second converter 20 through the negative line.

[0087] According to the example embodiment, the AC side of the thyristor converter valves 1011 and 1013 of the first converter 10 is connected to the new energy or AC system through the transformer 311. The valve side of the transformer 311 is equipped with a switch 3113 connected to the thyristor converter valve 1011, the valve side of the transformer 311 is equipped with a switch 3115 connected to the thyristor converter valve 1013, and the valve side of the transformer 311 is equipped with a switch 3114 connected to the new energy or AC system.

[0088] According to the example embodiment, the AC side of the thyristor converter valves 1015 and 1017 of the first converter 10 is connected to the new energy or AC system through the transformer 312. The valve side of the transformer 312 is equipped with a switch 3123 connected to the thyristor converter valve 1015, the valve side of the transformer 312 is equipped with a switch 3125 connected to the thyristor converter valve 1017, and the valve side of the transformer 312 is equipped with a switch 3124 connected to the new energy or AC system.

[0089] According to some embodiments, the modular multilevel converter valve 103 of the first converter 10 adopts a half-bridge sub-module, or a hybrid scheme of half-bridge module and full-bridge sub-module. The half-bridge module can be given priority to reduce cost and loss.

[0090] According to the example embodiment, the AC side of the multilevel converter valve 103 is connected to a new energy source or AC system through a transformer 32. A switch 323 is configured on the valve side of the transformer 32 and connected to the modular multilevel converter valve 103. A switch 324 is configured on the valve side of the transformer 32 and connected to the new energy source or AC system.

[0091] According to some embodiments, the first converter 10 and the second converter 20 are connected by a DC line 15, which is a DC overhead line, a DC cable, or a back-to-back straight bus.

[0092] According to some embodiments, the second converter 20 includes a full-bridge submodule, or a modular multilevel converter valve 201 composed of a mixture of full-bridge and half-bridge submodules. Using a mixed topology of full-bridge and half-bridge submodules can reduce losses and costs.

[0093] According to some embodiments, the modular multilevel converter valve 201 of the second converter 20 adopts a full-bridge submodule, which can make full use of the negative voltage output characteristic of the full-bridge submodule. During restart, the DC voltage can be reduced to a voltage that matches the DC voltage generated by the modular multilevel converter valve 103 of the first converter 10.

[0094] According to some embodiments, the thyristor switching valve 101 of the first converter 10 adopts a semi-controlled device such as a thyristor or an IGCT device with turn-off thyristor characteristics.

[0095] According to some embodiments, bypass switches 1051, 1053, 1055, and 1057 may be switching devices such as circuit breakers, transfer switches, and power electronic switches.

[0096] According to some embodiments, in the first converter 10, the modular multilevel converter valve 103 and in the second converter 20: the full-bridge submodule topology includes a turn-off full-bridge power semiconductor and an energy storage capacitor, which can output positive, negative and zero levels; the half-bridge submodule topology includes a turn-off fully controllable power semiconductor and an energy storage capacitor, which can output positive and zero levels.

[0097] Figure 4a A schematic diagram of a modular multilevel converter valve is shown as an exemplary embodiment.

[0098] like Figure 4a As shown, the modular multilevel converter valves in the first and second converters can be divided into three phases, each consisting of upper and lower bridge arms.

[0099] According to some embodiments, each arm of the modular multilevel converter valve in the first converter includes M sub-modules connected in series and an arm inductor. The sub-modules may be half-bridge sub-modules or a hybrid topology of half-bridge sub-modules and full-bridge sub-modules.

[0100] According to some embodiments, each arm of the modular multilevel converter valve in the second converter includes N sub-modules connected in series and an arm inductor. The sub-modules can adopt a topology of full-bridge sub-modules or a mixture of half-bridge sub-modules and full-bridge sub-modules.

[0101] like Figure 4b As shown, the full-bridge submodule topology includes one energy storage capacitor, four fully controlled power semiconductors, and four anti-parallel diodes. Each submodule can output positive, zero, and negative voltage levels.

[0102] like Figure 4c As shown, the half-bridge submodule topology includes one energy storage capacitor, two fully controlled power semiconductors, and two anti-parallel diodes. Each submodule can output both positive and zero voltage levels.

[0103] According to the example embodiment, the number M of submodules in each arm of the modular multilevel converter valve in the first converter is less than the number N of submodules in each arm of the modular multilevel converter valve in the second converter.

[0104] According to the example embodiment, by controlling the number of negative voltage levels in the full-bridge submodule of the modular multilevel converter, the magnitude of the DC voltage can be controlled while maintaining a constant AC voltage. This allows the modular multilevel converter valves in the first and second converters to operate in parallel during startup. After the thyristor converter valve in the first converter operates, the negative voltage levels in the full-bridge submodule of the modular multilevel converter in the second converter are gradually switched to positive voltage, thereby achieving high DC voltage operation.

[0105] Figure 5 A flowchart illustrating a control method for a hybrid DC power transmission system for new energy transmission, as shown in an exemplary embodiment, is provided.

[0106] S610 refers to the primary and secondary equipment in a detection system.

[0107] According to the example embodiment, primary and secondary equipment in a hybrid DC transmission system for new energy transmission are tested. The primary equipment includes a first converter, a second converter, a transformer, etc., in the hybrid DC transmission system for new energy transmission. The secondary equipment includes control equipment used in system control, such as control equipment for unlocking the thyristor converter valve and the modular multilevel converter valve of the first converter, and control equipment for unlocking the modular multilevel converter valve of the second converter. When all equipment in the system is ready, proceed to S620.

[0108] S620, closes the bypass switch of the thyristor converter valve in the first converter.

[0109] According to the example embodiment, the bypass switch of the thyristor converter valve in the first converter is closed, so that the modular multilevel converter valve in the first converter is connected to the second converter; S630, close the switches on both sides of the transformer on the second converter side.

[0110] According to the example embodiment, closing the switches on both sides of the transformer on the second converter side energizes the modular multilevel converter valve of the first converter and the modular multilevel converter valve of the second converter.

[0111] S640, the second converter control unlocks and runs, establishing voltage.

[0112] According to the example embodiment, the transformer on the second converter side charges the modular multilevel converter valve of the first converter and the modular multilevel converter valve of the second converter. After a period of delay, the second converter operates in DC voltage control mode, and the sub-module of the modular multilevel converter valve of the second converter is unlocked and runs to establish a voltage matching the modular multilevel valve of the first converter.

[0113] According to some embodiments, the delay ranges from 5s to 5000s.

[0114] S650, first converter control unlock, establish AC voltage.

[0115] According to the example embodiment, after the DC side of the modular multilevel converter valve of the first converter is established with voltage, the modular multilevel converter valve of the first converter closes the switches on both sides of the transformer connected in series to connect to the new energy or AC system, and the modular multilevel converter valve of the first converter is unlocked by islanded control to establish AC voltage.

[0116] S660, closes the switches on both sides of the transformer connected in series with the thyristor converter valve of the first converter.

[0117] According to the example embodiment, Figure 2To illustrate, first close the switches 3124 and 3123 on both sides of the transformer 312, which are connected in series with the thyristor converter valve 1013, and the transformer becomes energized.

[0118] S670, negative pressure unlocking operation of thyristor converter valve.

[0119] According to the example embodiment, Figure 2 For example, the thyristor converter valve 1013 is first unlocked with negative pressure, and the current gradually transfers from the bypass switch 1053 to the thyristor converter valve 1023.

[0120] S680, disconnect the bypass switch.

[0121] According to the example embodiment, Figure 2 For example, after the current is transferred to the thyristor converter valve 1023, the bypass switch 1053 is disconnected.

[0122] S690, the second converter reduces the amount of negative voltage in the full-bridge submodule.

[0123] According to the example embodiment, Figure 2 For example, in the first converter, the thyristor converter valve 1023 gradually increases the DC voltage by controlling the conduction angle, while the second converter simultaneously reduces the number of negative voltages in the full-bridge submodules and gradually increases the voltage to maintain a stable DC current.

[0124] S691, determine if there is a thyristor converter valve that is not energized for unlocking.

[0125] According to the example embodiment, when the thyristor switching valve 1023 reaches its rated voltage, the system determines whether any thyristor switching valves are not energized and unlocked. If any thyristor switching valves are not energized and unlocked, the process proceeds to S660. Figure 2 For example, if the thyristor converter valve 1011 is not energized and unlocked, then proceed to S660; if all thyristor converter valves are energized and unlocked, then proceed to S600.

[0126] S600, a new energy source, achieves energy output through a hybrid topology.

[0127] If all thyristor converter valves are working properly, the DC voltage is maintained at the rated voltage, and the system eventually operates at the rated DC voltage. The new energy source achieves energy output through the hybrid topology.

[0128] According to an example embodiment, this application provides a hybrid DC power transmission system and its control method for new energy transmission: AC grid support is provided through modular multilevel converter valves on the rectifier side to solve the problem of black start of wind power, realize power transmission, and perform reactive power and harmonic compensation in steady state; step-down operation is achieved by configuring a hybrid topology of full-bridge and half-bridge sub-modules in the inverter-side converter station, so as to achieve DC voltage matching with the rectifier-side converter valve during startup, and realize the application of hybrid DC topology engineering.

[0129] According to an example embodiment, this application provides a hybrid DC transmission system and its control method for new energy transmission: the hybrid DC transmission topology achieves step-down operation by configuring a hybrid submodule of full-bridge and half-bridge in the second converter, thereby matching it with the modular multilevel converter valve of the first converter; then, new energy or grid access is achieved through the modular multilevel converter valve of the first converter, gradually establishing a system that unlocks the thyristor converter valve to transmit new energy. By reducing the number of modular multilevel converters in the first converter, the system of thyristor converter valves connected in series with modular multilevel converter valves reduces the cost and footprint of converter valves at offshore stations, thereby reducing the weight, volume, and cost of offshore platforms. By using modular multilevel converter valves with full-bridge submodules in the second converter, the cost and losses of onshore converter stations are reduced, ultimately achieving grid parity transmission of large-scale new energy. In the event of a thyristor converter valve failure, power transmission can still continue through step-down operation of the modular multilevel converter.

[0130] Figure 6 A flowchart illustrating an exemplary embodiment of a method for matching the number of full-bridge submodules and half-bridge submodules in a second converter is provided.

[0131] S1, determine the reactive power compensation value.

[0132] According to the example embodiment, based on the power factor and transmitted active power of the offshore wind farm, the reactive power compensation value Q1 provided by the offshore wind farm is determined: Q1=K1P1 in, , P1 is the power factor of the wind farm and P2 is the active power transmitted by the wind farm.

[0133] S2, determine reactive power.

[0134] According to the example embodiment, the reactive power Q consumed by the thyristor converter valve in the first converter during operation is determined. 11 : Q 11 =K 11 P 11 in, , U d0 For ideal open-circuit voltage, Ud P represents the effective value of the open-circuit voltage on the valve side of the converter transformer. 11 It supplies active power to the thyristor converter valve.

[0135] According to some embodiments, K 11 It is approximately 40% to 60%.

[0136] According to the example embodiment, the reactive power Q consumed by the thyristor converter valve in the first converter is... 11 The relationship between the maximum reactive power provided by the modular multilevel converter valve in the first converter and the reactive power consumed by the thyristor converter valve is determined as follows: Q 12_max = α (Q) 11 -Q1) Among them, Q 12_max This refers to the maximum reactive power that the modular multilevel converter valve in the first converter can provide.

[0137] According to some embodiments, When =1, it indicates that the reactive power exchanged by the modular multilevel converter valve in the first converter meets the reactive power requirements of the thyristor-converter converter valve. When the value is less than 1, it indicates that the modular multilevel converter valve in the first converter is insufficient to provide reactive power to the thyristor converter valve, and an AC filter needs to be configured to provide additional reactive power to the thyristor converter valve.

[0138] According to some embodiments, The capacity rating is determined by the modular multilevel converter valves in the first converter.

[0139] S3, determine the relationship between the rated active power and reactive power transmitted by the modular multilevel converter valve in the first converter.

[0140] According to the example embodiment, the active power P transmitted by the modular multilevel converter valve in the first converter is... 12 With reactive power Q 12 Due to limitations imposed by the operating mode, there are constraint boundaries. Under rated operating conditions, based on specific engineering parameters, the relationship between the rated active power and reactive power transmitted by the modular multilevel converter valve in the first converter is determined as follows: n=P 12 / Q 12_max Among them, P 12 The active power transmitted to the modular multilevel converter valve in the first converter.

[0141] S4, determine the DC voltage of the modular multilevel converter valve in the first converter.

[0142] According to the example embodiment, based on the distribution of active power transmitted by the wind farm in the first converter, the DC voltage of the modular multilevel converter valve in the first converter is determined:

[0143] Among them, U dc12 U is the DC voltage of the modular multilevel converter valve in the first converter. dc This is the system DC voltage.

[0144] S5, determine the ratio of the number of full-bridge sub-modules of the second converter modular multilevel converter valve to the total number of sub-modules.

[0145] According to the embodiment, during the startup phase, the modular multilevel converter valve in the second converter controls all full-bridge submodules to output a negative level, thereby obtaining the lowest DC-side voltage of the modular multilevel converter valve in the second converter: U dc2min =(1-2γ)U dc Among them, U dc2min γ is the minimum DC-side voltage of the modular multilevel converter valve in the second converter, and γ is the ratio of the number of full-bridge sub-modules to the total number of sub-modules.

[0146] According to the example embodiment, based on the matching relationship between the minimum DC voltage of the modular multilevel converter valve in the second converter and the DC voltage of the modular multilevel converter valve in the first converter during startup, the following is true: U dc2min =U dc12 , According to the formula and U dc2min =(1-2γ)U dc Determine the ratio of the number of full-bridge submodules to the total number of half-bridge submodules for the second converter modular multilevel converter valve:

[0147] Here, ceiling{} represents the floor function.

[0148] This application provides a hybrid DC transmission system for new energy transmission and its control method: AC grid support is provided through a modular multilevel converter valve on the rectifier side, solving the black start problem of wind power, realizing power transmission, and performing reactive power and harmonic compensation in steady state; step-down operation is achieved by configuring a hybrid topology of full-bridge and half-bridge submodules in the inverter-side converter station, achieving DC voltage matching with the rectifier-side converter valve during startup; the configuration ratio of full-bridge and half-bridge submodules in the inverter-side converter station is determined based on the modular multilevel converter valve on the rectifier side, realizing the application of the hybrid DC topology engineering.

[0149] It should be clearly understood that this application describes how specific examples are formed and used, but this application is not limited to any details of these examples. Rather, based on the teachings of the disclosure of this application, these principles can be applied to many other embodiments.

[0150] Furthermore, it should be noted that the above figures are merely illustrative representations of the processes included in the method according to exemplary embodiments of this application, and are not intended to be limiting. It is readily understood that the processes shown in the above figures do not indicate or limit the temporal order of these processes. Additionally, it is readily understood that these processes may be executed synchronously or asynchronously, for example, in multiple modules.

[0151] Exemplary embodiments of this application have been specifically shown and described above. It should be understood that this application is not limited to the detailed structures, arrangements, or implementation methods described herein; rather, this application is intended to cover various modifications and equivalent arrangements contained within the spirit and scope of the appended claims.

Claims

1. A hybrid DC power transmission system for transmitting new energy sources, characterized in that, The power transmission system includes a first converter and a second converter, wherein: The first converter includes a bypass switch, a thyristor converter valve, and a first modular multilevel converter valve. The DC side of the thyristor converter valve and the DC side of the first modular multilevel converter valve are connected in series, and the bypass switch is connected in parallel with the thyristor converter valve. The AC side of the thyristor converter valve is connected in series with a first transformer, and the AC side of the first modular multilevel converter valve is connected in series with a second transformer. The second converter includes a second modular multilevel converter valve, which includes a first full-bridge submodule, or the first full-bridge submodule and a first half-bridge module; the AC side of the second converter is connected in series with a third transformer; The DC side of the first converter and the DC side of the second converter are connected in series; The power transmission system is configured as follows: Close the bypass switch; Unlock the second converter to establish a DC voltage; In response to the establishment of the DC voltage, the first modular multilevel converter valve is unlocked to establish the AC voltage; The transformer connected in series with the thyristor converter valve is energized, and the thyristor converter valve is unlocked under negative pressure. In response to the unlocking of the thyristor converter valve, the bypass switch is disconnected; The thyristor converter valve raises the DC voltage, and the number of negative voltages in the full-bridge submodules in the second converter is reduced to maintain DC voltage stability.

2. The power transmission system as described in claim 1, characterized in that, The thyristor converter valve comprises N 6-pulse bridge converters connected in series, where N is an integer greater than or equal to 1.

3. The power transmission system as described in claim 2, characterized in that, The N series-connected 6-pulse bridge converters are each connected in parallel with the bypass switch.

4. The power transmission system as described in claim 1, characterized in that, The thyristor converter valve uses a thyristor semi-controlled device or an IGCT device with turn-off thyristor characteristics.

5. The power transmission system as described in claim 1, characterized in that, The bypass switch is a circuit breaker, a transfer switch, or a power electronic switch.

6. The power transmission system as described in claim 1, characterized in that, The first modular multilevel converter valve includes a second half-bridge sub-module, or a second half-bridge module and a second full-bridge module.

7. The power transmission system as described in claim 2, characterized in that, The AC side of each of the N 6-pulse bridge converters is connected in series with the corresponding N transformers. The DC side of each of the N 6-pulse bridge converters and the DC side of the first modular multilevel converter valve are connected in series to form a DC branch. The DC branch is connected in series with the DC side of the second converter.

8. The power transmission system as described in claim 1, characterized in that, The DC side of the first converter and the DC side of the second converter are connected in series, including by DC lines, DC cables or direct back-to-back connection.

9. The power transmission system as described in claim 6, characterized in that, The second full-bridge submodule of the first modular multilevel converter valve and the first full-bridge submodule of the second modular multilevel converter valve include a turn-off full-bridge power semiconductor and an energy storage capacitor, and output positive level, negative level and zero level; The first half-bridge module and the second half-bridge module include a fully controllable power semiconductor that can be turned off and an energy storage capacitor, and output positive and zero levels.

10. The power transmission system as described in claim 1, characterized in that, The first converter also includes an AC filter to provide reactive power compensation for the thyristor converter valve.

11. A method for controlling a hybrid DC transmission system for new energy transmission as described in any one of claims 1-10, characterized in that, The method includes: Close the bypass switch; Unlock the second converter and establish DC voltage; In response to the establishment of the DC voltage, the first modular multilevel converter valve is unlocked, and the AC voltage is established; The transformer connected in series with the thyristor converter valve is energized, and the thyristor converter valve is unlocked under negative pressure; In response to the unlocking of the thyristor converter valve, the bypass switch is disconnected; The thyristor converter valve raises the DC voltage, and the second converter reduces the amount of negative voltage in the full-bridge submodule to maintain the DC voltage stability.

12. The method as described in claim 11, characterized in that, Also includes: Inspect the primary and secondary equipment in the power transmission system; The primary equipment includes the first converter, the second converter, and the transformer; The secondary equipment includes control equipment for unlocking the thyristor converter valve, the first modular multilevel converter valve, and the second modular multilevel converter valve.

13. The method as described in claim 11, characterized in that, The process of unlocking the second converter and establishing a DC voltage includes: Close the switch of the transformer connected in series with the second converter to charge the first modular multilevel converter valve and the second modular multilevel converter valve; In response to the completion of charging of the first modular multilevel converter valve and the second modular multilevel converter valve, the second modular multilevel converter valve is unlocked in DC voltage control mode; The second modular multilevel converter valve is controlled to establish a DC voltage that matches the first modular multilevel valve.

14. The method as described in claim 11, characterized in that, The process of unlocking the first modular multilevel converter valve in response to the establishment of DC voltage and establishing AC voltage includes: Close the switch of the transformer connected in series with the first modular multilevel converter valve; The first modular multilevel converter valve is unlocked in islanded control mode.

15. The method as described in claim 11, characterized in that, Also includes: Determine whether the thyristor converter valve is not energized and unlocked; If the thyristor converter valve is not unlocked, the transformer connected in series with the thyristor converter valve will be energized again to unlock the thyristor converter valve under negative pressure.

16. A method for proportioning a hybrid DC transmission system for new energy transmission as described in any one of claims 1-10, used to determine the proportion of the number of the first full-bridge sub-modules of the second modular multilevel converter valve to the total number of sub-modules, characterized in that, The proportioning method includes: Determine the reactive power compensation value provided by the offshore wind farm; The reactive power required by the thyristor converter valve during operation is determined based on the reactive power compensation value. Determine the relationship between the rated active power and reactive power of the first modular multilevel converter valve. Based on the relationship between the rated active power and reactive power of the first modular multilevel converter valve, the DC voltage of the first modular multilevel converter valve is determined. Based on the DC voltage, determine the ratio of the number of the first full-bridge sub-modules of the second modular multilevel converter valve to the total number of sub-modules.

17. The proportioning method as described in claim 16, characterized in that, The determination of the reactive power compensation value includes: Based on the power factor and transmitted active power of the offshore wind farm, determine the reactive power compensation value Q1 provided by the offshore wind farm: Q1=K1P1 in, , P1 is the power factor of the wind farm and P2 is the active power transmitted by the wind farm.

18. The proportioning method as described in claim 17, characterized in that, The determination of the reactive power consumed by the thyristor converter valve during operation based on the reactive power compensation value includes: Q 11 =K 11 P 11 Among them, Q 11 This refers to the reactive power consumed by the thyristor converter valve during operation. , U d0 For ideal open-circuit voltage, U d P is the effective value of the no-load voltage on the valve side of the converter transformer. 11 It supplies active power to the thyristor converter valve.

19. The proportioning method as described in claim 18, characterized in that, It also includes determining the relationship between the maximum reactive power that the first modular multilevel converter valve can provide and the reactive power consumed by the thyristor converter valve: Q 12_max = α (Q 11 -Q1) Among them, Q 12_max This refers to the maximum reactive power that the modular multilevel converter valve in the first converter can provide.

20. The proportioning method as described in claim 19, characterized in that, include: exist α When =1, it is determined that the reactive power exchanged by the first modular multilevel converter valve meets the reactive power requirement of the thyristor-converter valve. exist α If the reactive power exchanged by the first modular multilevel converter valve is insufficient to meet the reactive power requirements of the thyristor converter valve, the power transmission system needs to be configured with an AC filter to provide reactive power to the thyristor converter valve.

21. The proportioning method as described in claim 19, characterized in that, The determination of the relationship between the rated active power and reactive power of the first modular multilevel converter valve includes: n=P 12 / Q 12_max Among them, P 12 The active power transmitted to the modular multilevel converter valve in the first converter.

22. The proportioning method as described in claim 21, characterized in that, The determination of the DC voltage of the first modular multilevel converter valve based on the relationship between the rated active power and reactive power of the first modular multilevel converter valve includes: Among them, U dc12 U is the DC voltage of the modular multilevel converter valve in the first converter. dc This is the system DC voltage.

23. The proportioning method as described in claim 22, characterized in that, The determination of the ratio of the number of the first full-bridge sub-modules to the total number of sub-modules of the second modular multilevel converter valve based on the DC voltage includes: And dc2min =(1-2γ)U dc IN dc2min =U dc12 Among them, U dc2min γ is the minimum DC-side voltage of the modular multilevel converter valve in the second converter, γ is the ratio of the number of full-bridge submodules to the total number of submodules, and ceiling{} represents the floor function.

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