Method for determining sub-module capacitance of converter station in flexible direct-current power transmission system

Abstract

The invention relates to a method for determining sub-module capacitance of a converter station in a flexible direct-current power transmission system. The flexible direct-current power transmission system comprises a set converter station and a butt joint converter station. The method comprises the following steps: determining design capacitance of a sub-module of the established converter station according to parameters of the established converter station; determining an energy balance parameter of the flexible DC power transmission system and a capacitance voltage value range of the docking converter station sub-module; and designing the design capacitance of the docking converter station sub-module according to the design capacitance of the established converter station sub-module, the energy balance parameter and the capacitance voltage value range of the docking converter station sub-module. According to the method provided by the invention, through the differential design of the capacitors of the submodules of the offshore station and the onshore station, the minimum design capacitance of the submodules of the established converter station can be realized under the condition of ensuring the normal operation of the flexible direct-current power transmission system; therefore, the occupied area of the established converter station can be reduced, and the engineering cost of the flexible direct-current power transmission system can be reduced.

Description

Technical Field

[0001] The present application relates to the technical field of flexible DC power transmission, and particularly to a method for determining the capacitance of a sub-module of a converter station in a flexible DC power transmission system. Background Art

[0002] At present, flexible DC power transmission systems have been widely applied in various occasions in the power system and will be extended to more and more fields. It is widely regarded as one of the technical means for reliable access and effective utilization of new energy, and has become one of the important technologies for building a new power system. Flexible DC technology is applicable to offshore wind power with a longer transmission distance, and can achieve fault isolation between onshore and offshore areas, without reactive power compensation, provide support for the onshore power grid through active and reactive power decoupling control, and has network scalability. Large-scale development of far-sea wind power is the current development trend. The onshore converter station is not restricted by the floor area, while the offshore converter platform has a small space and low load. The offshore transportation and operation conditions are complex, requiring the converter valve to have the characteristics of compactness and light weight.

[0003] However, in the existing solutions, domestic offshore wind power flexible DC projects adopt the mature converter valve technology of onshore projects, without carrying out targeted research on the light weight and seaworthiness of the converter valve, resulting in problems such as low power density. Among them, the DC capacitance of the converter valve accounts for more than 50% of the volume and weight of the sub-module, and the low capacitance value design is the key to light weight.

[0004] Most of the existing literature on the strategy research for reducing the capacitance value of the sub-module in the flexible DC power transmission system is limited to the method of reducing the capacitance of the sub-module in the converter station, and there is little literature on the differential design of the sub-module capacitance between the offshore station and the onshore station. For example, an existing technology proposes a method of voltage equalization control by combining the half-bridge sub-module and the full-bridge sub-module and the complementary capacitance voltage between phases to reduce the voltage fluctuation of the full-bridge sub-module capacitance, thereby greatly reducing the sub-module capacitance value. Another existing technology proposes a method of reducing the capacitance by switching the arm reactance. Another existing technology proposes a method of reducing the capacitance by injecting a second harmonic and also injecting a third harmonic common-mode voltage into the modulation wave. Summary of the Invention

[0005] Different from the converter station capacitance reduction methods proposed by the above-mentioned existing technologies respectively, the present application achieves the purpose of reducing the converter station capacitance reduction through the differential design of the sub-module capacitance between the offshore station and the onshore station.

[0006] According to the first aspect of the present application, there is provided a method for determining the capacitance of a sub-module of a converter station in a flexible DC power transmission system, the flexible DC power transmission system including a predetermined converter station and a docking converter station, including:

[0007] Determine the designed capacitance of the sub-module of the predetermined converter station according to the parameters of the predetermined converter station;

[0008] Determine the energy balance parameters of the flexible DC power transmission system and the range of capacitor voltage values of the docking converter station sub-module; and

[0009] Design the capacitance of the docking converter station sub-module according to the designed capacitance of the established converter station sub-module, the energy balance parameters, and the range of capacitor voltage values of the docking converter station sub-module.

[0010] According to some embodiments, the method further includes:

[0011] Determine the first capacitor voltage fluctuation rate corresponding to the designed capacitance of the docking converter station sub-module;

[0012] While reducing the designed capacitance of the docking converter station sub-module, inject the harmonic signal of the docking converter station into the docking converter station to obtain the second capacitor voltage fluctuation rate of the docking converter station;

[0013] When the second capacitor voltage fluctuation rate is equal to the first capacitor voltage fluctuation rate, obtain the optimized designed capacitance of the docking converter station sub-module.

[0014] According to some embodiments, the method further includes:

[0015] Determine the optimal harmonic signal injected into the established converter station according to the designed capacitance of the established converter station sub-module, including:

[0016] Determine the predetermined capacitance of the established converter station sub-module;

[0017] Determine the predetermined capacitor voltage fluctuation rate of the established converter station according to the predetermined capacitance;

[0018] Determine the designed capacitor voltage fluctuation rate of the established converter station according to the designed capacitance of the established converter station sub-module;

[0019] Determine the harmonic signal corresponding to the case where the designed capacitor voltage fluctuation rate is equal to the predetermined capacitor voltage fluctuation rate as the optimal harmonic signal.

[0020] According to some embodiments, the method further includes:

[0021] During the operation of the flexible DC power transmission system, inject the optimal harmonic signal into the control reference wave of the established converter station.

[0022] According to some embodiments, the energy balance parameters of the flexible DC power transmission system include:

[0023] Fault buffer constraint energy;

[0024] Grid-forming support buffer energy;

[0025] The buffer energy for energy consumption and energy storage; and

[0026] The energy of the connection cable between the established converter station and the docking converter station.

[0027] According to some embodiments, the method of designing the designed capacitance of the sub-module of the docking converter station according to the designed capacitance of the sub-module of the established converter station, the energy balance, and the range of the capacitance voltage value of the sub-module of the docking converter station includes:

[0028]

[0029] Where C landc1 is the designed capacitance of the docking converter station; γ is a margin coefficient, where γ ≥ 1; E frd is the fault buffer constraint energy; E gfm is the grid-forming support buffer energy; E oe is the buffer energy for energy consumption and energy storage; E sea is the buffer energy of the established converter station; E cable is the energy of the connection cable; N 2 is the number of sub-modules of the docking converter station, U landmax is the maximum value of the capacitance voltage of the sub-module of the docking converter station, U landavg is the average value of the capacitance voltage of the sub-module of the docking converter station, U landmin is the minimum value of the capacitance voltage of the sub-module of the docking converter station.

[0030] According to some embodiments, the numbers of the established converter station and the docking converter station are each one or more.

[0031] According to some embodiments, the harmonic signals include: high-order harmonic current signals and triple-frequency harmonic voltage signals.

[0032] According to the second aspect of the present application, there is provided an electronic device, including:

[0033] A memory; and

[0034] A processor, a computer program is stored on the memory, and when the processor executes the computer program on the memory, the method described in the first aspect is implemented.

[0035] According to the third aspect of the present application, there is provided a computer-readable storage medium, the computer-readable storage medium stores a computer program, and when the computer program is executed by a processor, the method described in the first aspect is implemented.

[0036] According to the method for determining the sub-module capacitance of a converter station in the flexible DC power transmission system provided by this application, through the differential design of the sub-module capacitances of the offshore station and the onshore station, it is possible to minimize the designed capacitance of the established converter station sub-module while ensuring the normal operation of the flexible DC power transmission system, thereby reducing the floor area of the established converter station and lowering the engineering cost of the flexible DC power transmission system. Description of the Drawings

[0037] In order to more clearly illustrate the technical solutions in the embodiments of this application, the following will briefly introduce the drawings required for the description of the embodiments. Obviously, the drawings in the following description are only some embodiments of this application. For those of ordinary skill in the art, other drawings can also be obtained based on these drawings without exceeding the scope claimed by this application.

[0038] Figure 1 It is a schematic diagram of an offshore wind power flexible DC power transmission system according to an embodiment of this application.

[0039] Figure 2 It is a flowchart of the method for determining the sub-module capacitance of a converter station in the flexible DC power transmission system according to an embodiment of this application.

[0040] Figure 3 It is a flowchart of the method for determining the sub-module capacitance of a converter station in the flexible DC power transmission system according to another embodiment of this application.

[0041] Figure 4 It is a flowchart of the method for determining the sub-module capacitance of a converter station in the flexible DC power transmission system according to yet another embodiment of this application.

[0042] Figure 5 It is a flowchart of determining the optimal harmonic signal injected into the established converter station according to the designed capacitance of the established converter station sub-module according to an embodiment of this application.

[0043] Figure 6 It is a system control block diagram of the flexible DC power transmission system adopting high-order harmonic current injection and triple-frequency harmonic voltage injection according to an embodiment of this application.

[0044] Figure 7 It is a structural diagram of an electronic device provided by this application.

[0045] Reference Numerals: 1 Offshore Wind Turbine Generator, 2 Incoming Line Switch, 3 Offshore Station Connection Transformer, 4 Offshore Station Converter and DC Field Equipment, 5 DC Cable, 6 Onshore Station Converter and DC Field Equipment, 7 Onshore Station Connection Transformer, 8 Primary Side Switch, 9 AC Power Grid or Power Supply. Detailed Embodiments

[0046] Next, the technical solutions in the embodiments of the present application will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present application. Obviously, the described embodiments are part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative efforts belong to the scope of protection of the present application.

[0047] According to some embodiments, the flexible DC power transmission system includes a predetermined converter station and a docking converter station. The predetermined converter station and the docking converter station are connected by a DC cable. The number of the predetermined converter station and the docking converter station can be set to one or more respectively according to actual needs. According to one embodiment, the flexible DC power transmission system is an offshore wind power flexible DC power transmission system. The predetermined converter station can be an offshore converter station, and the docking converter station can be an onshore converter station; or, the predetermined converter station can be an onshore converter station, and the docking converter station can be an offshore converter station; or, the predetermined converter station and the docking converter station can both be onshore converter stations, or both be offshore converter stations.

[0048] Figure 1 is a schematic diagram of an offshore wind power flexible DC power transmission system according to an embodiment of the present application. As Figure 1 shown, the offshore wind turbine generator 1 is connected to the primary side of the offshore station connection transformer 3 through the incoming line switch 2. The secondary side of the offshore station connection transformer 3 is connected to the offshore station converter and the offshore station DC field equipment 4. The offshore station converter and the offshore station DC field equipment 4 are connected to the onshore station converter and the DC field equipment 6 through the DC cable 5. The onshore station converter and the DC field equipment 6 are connected to the AC power grid or power supply 9 through the onshore station connection transformer 7 via the primary side switch 8 of the onshore station.

[0049] According to one aspect of the present application, a method for determining the capacitor of a converter station sub-module in a flexible DC power transmission system is provided. As Figure 2 shown, the method includes the following steps.

[0050] Step S201, determine the designed capacitor of the sub-module of the predetermined converter station according to the parameters of the predetermined converter station.

[0051] Step S202, determine the energy balance parameter of the flexible DC power transmission system and the range of the capacitor voltage value of the sub-module of the docking converter station.

[0052] Step S203, design the designed capacitor of the sub-module of the docking converter station according to the designed capacitor of the sub-module of the predetermined converter station, the energy balance, and the range of the capacitor voltage value of the sub-module of the docking converter station.

[0053] The differential design of the present invention is to determine the design capacitance of the sub-module of the docking converter station on the premise of determining the design capacitance of the sub-module of the established converter station. In this way, while ensuring the normal operation of the flexible DC transmission system, the design capacitance of the sub-module of the established converter station can meet the parameter requirements of the established converter station, thereby reducing the floor area of the established converter station and lowering the engineering cost of the flexible DC transmission system.

[0054] According to some embodiments, the parameters of the established converter station may include the design size of the established converter station. After determining the design size of the established converter station, the number of sub-modules of the established converter station can be determined according to the design size of the established converter station, and then the design capacitance of the sub-module of the established converter station can be determined.

[0055] According to some embodiments, the parameters of the established converter station may further include the withstand voltage level of the established converter station. When the value or range of the equipment withstand voltage level of the established converter station is determined, after the design capacitance of the sub-module of the established converter station is applied to the established converter station, the established converter station meets the withstand voltage level requirements during fault ride-through and transient overvoltages such as faults. According to one embodiment, according to the equipment withstand voltage level of the established converter station, the DC voltage of the established converter station ≤ the withstand voltage level limit value of the established converter station, where the withstand voltage level limit value of the established converter station ≥ 1.1 pu, and pu is the per-unit value unit, and the per-unit value is the relative value of the actual value to the rated value.

[0056] According to some embodiments, the design capacitance of the sub-module of the established converter station can be determined according to the size of the established converter station and the withstand voltage level requirements of the established converter station, so that the determined design capacitance of the sub-module of the established converter station can not only make the size of the established converter station reach a predetermined size, but also ensure that the withstand voltage level of the established converter station meets the requirements.

[0057] It should be noted that the above only describes the process of how to determine the design capacitance of the sub-module of the established converter station when the parameters of the established converter station include the size and withstand voltage level requirements of the established converter station. In fact, the parameters of the established converter station may also include other factors. In the process of determining the design capacitance of the sub-module of the established converter station, those skilled in the art can consider one or more factors in the parameters of the established converter station according to actual needs, and these all fall within the scope covered by this application.

[0058] According to some embodiments, after determining the design capacitance of the sub-module of the established converter station, the energy balance parameters affecting the system energy balance in the flexible DC transmission system are determined.

[0059] According to some embodiments, the energy balance parameters of the flexible DC transmission system include fault buffer constraint energy, network-forming support buffer energy, buffer energy of energy consumption and energy storage, and the energy of the connection cable between the established converter station and the docking converter station.

[0060] According to some embodiments, in the flexible DC power transmission system project, the key factor restricting the interconnected converter station is the consumption of surplus power after power transmission is damaged when a fault occurs in the interconnected converter station. To solve the surplus power problem, DC energy consumption is set in the flexible DC power transmission system project. Therefore, in the flexible DC power transmission system project, factors such as energy consumption buffer energy and energy storage buffer energy, fault buffer constraint energy during faults and fault ride-through, energy of the connection cable between the established converter station and the interconnected converter station, and network-forming support buffer energy required for network-forming support need to be considered.

[0061] According to some embodiments, the flexible DC power transmission system can be configured with energy storage devices and energy consumption devices, and the energy buffers of energy consumption and energy storage in the flexible DC power transmission system are determined according to the capacity of the energy storage and the capacity of the energy consumption. According to one embodiment, the capacity S of the configured energy storage E ≥0 MW, and the capacity S of the configured energy consumption C ≥0 MW.

[0062] According to some embodiments, the range of the capacitor voltage value of the sub-module of the interconnected converter station is determined according to the DC voltage level of the flexible DC power transmission system. The capacitor voltage fluctuation rate is determined according to the voltage value. According to one embodiment, the capacitor voltage fluctuation rate of the sub-module of the interconnected converter station is less than the capacitor voltage fluctuation rate threshold of the sub-module of the interconnected converter station, where the capacitor voltage fluctuation rate threshold of the sub-module of the interconnected converter station ≥8%.

[0063] According to some embodiments, the method for designing the designed capacitance of the sub-module of the interconnected converter station according to the designed capacitance of the sub-module of the established converter station, the energy balance, and the range of the capacitor voltage value of the sub-module of the interconnected converter station includes:

[0064]

[0065] where C landc1 is the designed capacitance of the interconnected converter station; γ is the margin coefficient, where γ≥1; E frd is the fault buffer constraint energy; E gfm is the network-forming support buffer energy; E oe is the buffer energy of energy consumption and energy storage; E sea is the buffer energy of the established converter station; E cable is the energy of the connection cable; N 2 is the number of sub-modules of the interconnected converter station, U landmax is the maximum capacitor voltage of the sub-module of the interconnected converter station, U landavg is the average capacitor voltage of the sub-module of the interconnected converter station, U landmin is the minimum capacitor voltage of the sub-module of the interconnected converter station. The designed capacitance of the interconnected converter station can be obtained by calculating with the above calculation formula.

[0066] According to some embodiments, the flexible DC power transmission system is an offshore wind power flexible DC power transmission system, the established converter station is an offshore converter station, and the docking converter station is an onshore converter station. Determine E according to the low voltage ride-through characteristics of the wind farm and the detection and communication delay in the offshore wind power flexible DC power transmission system frd Fault buffer constraint; determine E according to the fast frequency regulation characteristics of the wind farm and the sea-land frequency linkage delay in the offshore wind power flexible DC power transmission system gfm Grid-forming support energy buffer; determine E according to the capacity of the energy-consuming device and the capacity of the energy storage device in the offshore wind power flexible DC power transmission system oe Energy buffer for energy consumption and energy storage; determine E according to the designed capacitance of the sub-modules of the offshore converter station sea Buffer energy of the offshore converter station; determine E according to the capacitance of the connection cable between the offshore converter station and the onshore converter station cable Energy of the connection cable; the γ margin coefficient can multiply the C calculated by the above formula landc1 Amplify the designed capacitance value of the docking converter station.

[0067] According to some embodiments, the designed capacitance of the docking converter station calculated by the above calculation formula can be further optimized. Figure 3 It is a flowchart of a method for determining the capacitance of the sub-modules of the converter station in the flexible DC power transmission system according to another embodiment of the present application. Compared with Figure 2 Figure 3 Steps S301 to S303 of Figure 2 are the same as steps S201 to S203 of Figure 3 , the difference is that

[0068] Step S304, determine the first capacitance voltage fluctuation rate corresponding to the designed capacitance of the sub-modules of the docking converter station;

[0069] Step S305, while reducing the designed capacitance of the sub-modules of the docking converter station, inject harmonic signals of the docking converter station into the docking converter station to obtain the second capacitance voltage fluctuation rate of the docking converter station;

[0070] Step S306, when the second capacitance voltage fluctuation rate is equal to the first capacitance voltage fluctuation rate, obtain the optimized designed capacitance of the sub-modules of the docking converter station.

[0071] According to some embodiments, when other factors affecting the value of the capacitance voltage fluctuation rate remain unchanged, the corresponding relationship between the capacitance voltage fluctuation rate and the capacitance is determined, and the capacitance voltage fluctuation rate can be determined according to the capacitance. According to one embodiment, the smaller the capacitance value, the greater the capacitance voltage fluctuation rate.

[0072] ​According to some embodiments, the first capacitance voltage fluctuation rate of the docking converter station corresponding to the designed capacitance of the docking converter station is determined. Appropriately reduce the value of the designed capacitance of the sub-module of the docking converter station, and at the same time inject harmonic signals (for example, high-order harmonic currents and triple-frequency harmonic voltages) into the docking converter station. According to the reduction amount of the value of the designed capacitance of the sub-module of the docking converter station and the injected harmonic signals, the second capacitance voltage fluctuation rate of the docking converter station is obtained. When the second capacitance voltage fluctuation rate is equal to the first capacitance voltage fluctuation rate, stop reducing the value of the designed capacitance of the sub-module of the docking converter station and stop injecting harmonic signals into the docking converter station. In this way, the optimization of the designed capacitance of the sub-module of the docking converter station is completed. The designed capacitance of the sub-module of the optimized docking converter station is smaller than that of the sub-module of the docking converter station before optimization, so that the project cost can be saved.

[0073] Figure 4 is a flowchart of a method for determining the capacitance of a converter station sub-module in a flexible DC power transmission system according to another embodiment of the present application. Compared with Figure 2 compared with Figure 4 Steps S401 to S403 of Figure 2 are the same as steps S201 to S203 of Figure 4 The difference is that Figure 4 the method of

[0074] Figure 5 also includes step S404, which determines the optimal harmonic signal injected into the established converter station according to the designed capacitance of the established converter station sub-module.

[0074] Figure 5 is a flowchart of a method for determining the optimal harmonic signal injected into an established converter station according to the designed capacitance of an established converter station sub-module according to an embodiment of the present application. As Figure 5 shown, the method includes the following steps.

[0075] Step S501, determine the predetermined capacitance of the established converter station sub-module;

[0076] Step S502, determine the predetermined capacitance voltage fluctuation rate of the established converter station according to the predetermined capacitance;

[0077] Step S503, determine the designed capacitance voltage fluctuation rate of the established converter station according to the designed capacitance of the established converter station sub-module;

[0078] Step S504, determine the harmonic signal corresponding to the case where the designed capacitance voltage fluctuation rate is equal to the predetermined capacitance voltage fluctuation rate as the optimal harmonic signal.

[0079] According to some embodiments, in a flexible DC power transmission system, first determine the DC voltage level of the flexible DC power transmission system. According to the DC voltage level of the flexible DC power transmission system, respectively determine the number of established converter station sub-modules and the number of docking converter station sub-modules, and then respectively determine the predetermined capacitance of the established converter station sub-modules and the predetermined capacitance of the docking converter station sub-modules according to the number of established converter station sub-modules and the number of docking converter station sub-modules.

[0080] According to some embodiments, when the flexible DC power transmission system can operate normally, according to the predetermined capacitance of the established converter station sub-modules, determine the predetermined capacitance voltage volatility of the corresponding established converter station sub-modules; according to the predetermined capacitance of the docking converter station sub-modules, determine the predetermined capacitance voltage volatility of the corresponding docking converter station sub-modules.

[0081] According to some embodiments, in some cases (for example, when the converter station is built at sea), the available floor area of the established converter station or the docking converter station in the flexible DC power transmission system is small and the load capacity is low, while the number of converter station sub-modules determined according to the DC voltage level of the flexible DC power transmission system is large and the predetermined capacitance of the sub-modules is large, which is difficult to meet the actual requirements of the flexible DC power transmission system project. In the flexible DC power transmission system project, it is necessary to determine the number of sub-modules of the converter station and the designed capacitance of the sub-modules according to the actual requirements of the project. According to one embodiment, the available floor area of the established converter station is small and the load capacity is low.

[0082] According to some embodiments, the designed capacitance of the established converter station sub-modules meets the actual requirements of the flexible DC power transmission system project, but the designed capacitance voltage volatility of the established converter station sub-modules corresponding to the designed capacitance of the established converter station sub-modules does not meet the constraint conditions of the capacitance voltage volatility of the established converter station sub-modules in the flexible DC power transmission system, and the flexible DC power transmission system cannot operate normally. According to one embodiment, the capacitance voltage volatility of the established converter station sub-modules is less than the capacitance voltage volatility threshold of the established converter station sub-modules, where the capacitance voltage volatility threshold of the established converter station sub-modules ≥ 8%.

[0083] According to some embodiments, since the designed capacitance voltage volatility of a given converter station sub-module corresponding to the designed capacitance of the given converter station sub-module does not meet the constraint conditions of the capacitance voltage volatility of the given converter station sub-module in the flexible DC transmission system, the flexible DC transmission system cannot operate properly. To ensure the normal operation of the flexible DC transmission system, when the capacitance of the given converter station sub-module is the designed capacitance, a harmonic signal is injected into the reference signal of the controller of the given converter station to achieve the capacitance voltage volatility of the given converter station sub-module corresponding to the designed capacitance of the given converter station sub-module, meeting the constraint conditions of the capacitance voltage volatility of the given converter station sub-module in the flexible DC transmission system. In this way, the DC voltage utilization rate can be improved, thereby realizing the derating design of the given converter station sub-module and the light-weighting of the converter valve.

[0084] According to some embodiments, the harmonic signal injected into the given converter station includes a high-order harmonic current signal and a high-multiple-frequency harmonic voltage signal. According to one embodiment, the high-multiple-frequency harmonic voltage signal injected into the given converter station is a triple-frequency harmonic voltage signal.

[0085] According to some embodiments, when the harmonic signal injected into the given converter station satisfies the condition that the designed capacitance voltage volatility of the given converter station sub-module after injection is equal to the predetermined capacitance voltage volatility of the given converter station sub-module, the corresponding harmonic signal is determined as the optimal harmonic signal. When the DC voltage level of the flexible DC transmission system is determined, injecting the optimal harmonic signal into the given converter station can achieve the design capacitance of the given converter station sub-module to be minimized while meeting the constraint conditions of the capacitance voltage volatility of the given converter station sub-module in the flexible DC transmission system.

[0086] Figure 6 is a system control block diagram of high-order harmonic current injection and triple-frequency harmonic voltage injection in a flexible DC transmission system according to an embodiment of the present application. As Figure 6 shown, a harmonic signal is injected into the given converter station, where U 3j_ref represents a triple-frequency voltage modulation signal, U cirj_ref represents a high-multiple-frequency circulating current modulation voltage signal, e j_ref is the differential-mode voltage reference value, and U dc is the DC-side voltage.

[0087] According to some embodiments, the optimal harmonic signal includes an optimal high-order harmonic current signal and an optimal triple-frequency harmonic voltage signal. Injecting the optimal harmonic signal (optimal high-order harmonic current signal and optimal triple-frequency harmonic voltage signal) into the reference signal of the controller of the given converter station to obtain a new reference signal of the given converter station, and obtaining a modulation wave through a modulation algorithm, finally realizing the coupled injection of the optimal high-order harmonic current signal and the optimal triple-frequency harmonic voltage signal.

[0088] According to another aspect of the present application, there is provided an electronic device. Referring to Figure 7 , the electronic device includes a processor and a memory. The memory stores computer instructions, and when the computer instructions are executed by the processor, the processor executes the computer instructions to implement the method and refinement scheme as Figures 2 to 5 shown.

[0089] It should be understood that the above device embodiments are illustrative only, and the devices disclosed in the present invention can also be implemented in other ways. For example, the division of the above-mentioned units / modules is only a logical function division, and there may be other division methods in actual implementation. For example, multiple units, modules or components can be combined, or can be integrated into another system, or some features can be ignored or not executed.

[0090] In addition, without special instructions, in each embodiment of the present invention, each functional unit / module can be integrated in one unit / module, or each unit / module can exist physically alone, or two or more units / modules can be integrated together. The above integrated unit / module can be implemented in the form of hardware or in the form of a software program module.

[0091] When the above integrated unit / module is implemented in the form of hardware, the hardware can be a digital circuit, an analog circuit, etc. The physical implementation of the hardware structure includes but is not limited to transistors, memristors, etc. Without special instructions, the processor or chip can be any suitable hardware processor, such as CPU, GPU, FPGA, DSP, and ASIC, etc. Without special instructions, the on-chip cache, off-chip memory, and memory can be any suitable magnetic storage medium or magneto-optical storage medium, such as resistive random access memory RRAM (Resistive Random Access Memory), dynamic random access memory DRAM (Dynamic Random Access Memory), static random access memory SRAM (Static Random-Access Memory), enhanced dynamic random access memory EDRAM (Enhanced Dynamic Random Access Memory), high-bandwidth memory HBM (High-Bandwidth Memory), hybrid memory cube HMC (Hybrid Memory Cube), etc.

[0092] When the integrated unit / module is implemented in the form of a software program module and sold or used as an independent product, it can be stored in a computer-readable memory. Based on this understanding, the technical solution of the present invention, in essence, or the part that contributes to the prior art, or all or part of the technical solution, can be embodied in the form of a software product. The computer software product is stored in a memory and includes several instructions for causing a computer electronic device (which can be a personal computer, a server, or a network electronic device, etc.) to execute all or part of the steps of the methods described in various embodiments of this disclosure. The aforementioned memory includes various media that can store program codes, such as USB flash drives, read-only memories (ROM, Read-Only Memory), random access memories (RAM, Random Access Memory), external hard drives, magnetic disks, or optical discs.

[0093] The embodiments of this application also provide a non-transitory computer storage medium storing a computer program, which, when executed by multiple processors, causes the processors to execute the method and refinement solutions as Figures 2 to 5 shown.

[0094] The embodiments of this application have been introduced in detail above. Specific examples are used herein to elaborate on the principles and implementation manners of this application. The description of the above embodiments is only used to help understand the method and its core idea of this application. At the same time, any changes or deformations made by those skilled in the art based on the idea of this application, within the specific implementation manners and application scope of this application, fall within the protection scope of this application. In summary, the content of this specification should not be construed as a limitation to this application.

Claims

1. A method for determining the capacitance of sub-modules in a flexible DC transmission system, the flexible DC transmission system including a given converter station and a connected converter station, characterized in that, it includes: Determining the designed capacitance of the sub-modules of the given converter station according to the parameters of the given converter station; Determining the energy balance parameters of the flexible DC transmission system and the range of capacitance voltage values of the sub-modules of the connected converter station; and Designing the designed capacitance of the sub-modules of the connected converter station according to the designed capacitance of the sub-modules of the given converter station, the energy balance parameters and the range of capacitance voltage values of the sub-modules of the connected converter station.

2. The method according to claim 1, characterized in that, it further includes: Determining the first capacitance voltage fluctuation rate corresponding to the designed capacitance of the sub-modules of the connected converter station; Injecting harmonic signals of the connected converter station into the connected converter station while reducing the designed capacitance of the sub-modules of the connected converter station to obtain the second capacitance voltage fluctuation rate of the connected converter station; Obtaining the optimized designed capacitance of the sub-modules of the connected converter station when the second capacitance voltage fluctuation rate is equal to the first capacitance voltage fluctuation rate.

3. The method according to claim 1, characterized in that, it further includes: Determining the optimal harmonic signal injected into the given converter station according to the designed capacitance of the sub-modules of the given converter station, including: Determining the predetermined capacitance of the sub-modules of the given converter station; Determining the predetermined capacitance voltage fluctuation rate of the given converter station according to the predetermined capacitance; Determining the designed capacitance voltage fluctuation rate of the given converter station according to the designed capacitance of the sub-modules of the given converter station; Determining the harmonic signal corresponding to the case where the designed capacitance voltage fluctuation rate is equal to the predetermined capacitance voltage fluctuation rate as the optimal harmonic signal.

4. The method according to claim 3, characterized in that, it further includes: During the operation of the flexible DC transmission system, injecting the optimal harmonic signal into the control reference wave of the given converter station.

5. The method according to claim 1, characterized in that, The energy balance parameters of the flexible DC transmission system include: Fault buffer constraint energy; Grid-forming support buffer energy; Buffer energy for energy consumption and energy storage; and The energy of the connection cable between the given converter station and the connected converter station.

6. The method according to claim 5, characterized in that, The manner of designing the designed capacitance of the sub-modules of the connected converter station according to the designed capacitance of the sub-modules of the given converter station, the energy balance and the range of capacitance voltage values of the sub-modules of the connected converter station includes: Among them, C landc1 is the designed capacitance of the docking converter station; γ is the margin coefficient, where γ ≥ 1; E frd is the fault buffer constraint energy; E gfm is the grid-forming support buffer energy; E oe is the buffer energy of the energy consumption and energy storage; E sea is the buffer energy of the established converter station; E cable is the energy of the connecting cable; N 2 is the number of sub-modules of the docking converter station, U landmax is the maximum value of the capacitor voltage of the sub-module of the docking converter station, U landavg is the average value of the capacitor voltage of the sub-module of the docking converter station, U landmin is the minimum value of the capacitor voltage of the sub-module of the docking converter station.

7. The method according to any one of claims 1 to 6, characterized in that, The number of the given converter station and the connected converter station is one or more respectively.

8. The method according to any one of claims 2 to 4, characterized in that, The harmonic signals include: high-order harmonic current signals and triple-frequency harmonic voltage signals.

9. An electronic device, characterized in that, it includes a memory and a processor, a computer program is stored on the memory, and the processor implements the method according to any one of claims 1 to 8 when executing the computer program on the memory.

10. A computer-readable storage medium, characterized in that, the computer-readable storage medium stores a computer program, and when the computer program is executed by a processor, the method according to any one of claims 1 to 8 is implemented.

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