Configuration method for voltage flexible matching cascade type commutation system

Abstract

The invention discloses a configuration method for a voltage flexible matching cascade type current conversion system, which comprises the following steps of: firstly, constructing a voltage flexible matching cascade type current conversion topological structure, then establishing an initial distribution relation between the selection of a cascade converter voltage grade and transmission power, and determining a rated voltage division ratio of each station according to a receiving end net rack planning requirement; next, determining a constraint relation between a full-bridge and half-bridge hybrid MMC direct-current voltage operation range and a full-bridge sub-module proportion, calculating a modulation boundary of a unit in a voltage regulation process, and finally establishing a corresponding relation between a power distribution flexible adjustment range and a full-bridge module configuration proportion in a cascade converter operation process; and flexible mutual assistance of active power among different converter stations is realized by cooperatively adjusting the direct-current voltage proportion of the two stations. According to the method, differential matching of receiving end power is achieved in the planning stage, cross-grid flexible power scheduling is achieved in the operation stage, and the scheduling activity and engineering applicability of the cascade direct current power transmission system are remarkably improved.

Description

Technical Field

[0001] This invention belongs to the field of power system technology, specifically relating to a configuration method for a cascaded converter system with flexible voltage matching. Background Technology

[0002] Due to the significant spatial mismatch between the distribution of new energy resources and load centers, flexible DC transmission technology has become the main technical means for large-capacity, long-distance power transmission. Against this backdrop, in order to improve the receiving-end grid's capacity to absorb large-scale new energy currents and reduce the security risks of a single access point, cascaded modular multilevel converter (MMC) topologies have gradually gained attention. By connecting two or more converters in series on the DC side, the massive amount of power from the sending end can be injected into different regional AC systems at the receiving end, achieving distributed load absorption.

[0003] The literature [Wang Zhiwei, Zheng Jianhua, Wan Zhendong. Application Research of LCC-VSC Hybrid Cascaded DC Technology in Jiangsu Power Grid. Global Energy Internet, 2025, 8(1): 3-12] adopts a structure of high-end LCC (grid-commutated converter) and low-end multiple VSC (voltage source converter) in series to improve the safety and stability level and reactive power support capability of the receiving-end power grid. The literature [Zhou Baorong, Li Xuanping, Li Shoutao. High and Low Voltage Valve Group Voltage Equalization Control Strategy of LCC-MMC UHV Hybrid DC Transmission System. Southern Power Grid Technology, 2022, 16(8): 79-85] addresses the voltage distribution problem in the cascaded system by designing a voltage equalization controller to adjust the voltage deviation between the cascaded valve groups to achieve a balanced distribution of DC side voltage. However, in the research and engineering practice of these existing cascaded DC transmission systems, the following key problems still need to be solved: First, there are significant differences in the absorption capacity of the power grids in the receiving areas. After large-scale renewable energy is transmitted to the receiving end, the AC system in a single area often cannot fully absorb the generated power due to insufficient grid strength and limited load levels. Although the cascaded topology provides two access points, the traditional high- and low-voltage valve group design usually follows the principle of "symmetry," that is, the two cascaded MMC converter stations are designed to have the same DC voltage level. Since the DC current is equal everywhere in the cascaded structure, this uniformity of voltage level forces the output power of the two receiving-end converter stations to be fixed (usually allocated in a 1:1 ratio), making it impossible to design differentiated ratios based on the actual absorption capacity of different receiving-end power grids during the planning stage.

[0004] Secondly, the load demand of AC systems is dynamic. During system operation, the power load in AC Zone 1 and AC Zone 2 fluctuates periodically, or the randomness of renewable energy output causes real-time changes in the receiving-end absorption pressure. Existing cascaded topologies lack flexible power dispatching methods during operation. Due to the coupling of DC current at the receiving end or total voltage control, changing the power distribution between the two cascaded stations requires adjusting their voltage distribution ratio. However, traditional half-bridge sub-module converters are prone to entering the "overmodulation" region when the DC voltage is lowered than the peak AC voltage, leading to a surge in system harmonics or even uncontrolled collapse.

[0005] In summary, existing cascaded flexible DC transmission technologies face dual bottlenecks when addressing the demand for multi-point absorption of renewable energy: "power ratio lock-in during the planning phase" and "limited power mutual assistance during operation." Therefore, researching and designing a cascaded topology and control method that enables flexible power allocation during the design phase and dynamic adjustment of DC voltage division relationships based on AC grid demand during operation, thereby achieving optimal power distribution and dynamic mutual assistance, is of significant practical importance for improving the absorption level of large-scale renewable energy and the operational flexibility of DC transmission networks. Summary of the Invention

[0006] In view of the above, the present invention provides a configuration method for a cascaded converter system with flexible voltage ratio, which realizes flexible mutual assistance of active power between different converter stations by coordinating the adjustment of the DC voltage ratio of the two stations, effectively solving the problem of limited power distribution in the multi-point consumption of large-scale new energy.

[0007] A method for configuring a cascaded converter system with flexible voltage matching includes the following steps: (1) Construct a voltage-flexible cascaded converter topology, which consists of a sending-end power supply and a receiving-end cascaded station. The receiving-end cascaded station includes two converters, A1 and A2. The DC sides of A1 and A2 are connected in series, with one end connected to the sending-end power supply and the other end grounded. The AC sides of A1 and A2 are connected to different receiving-end AC grids, AC1 and AC2, respectively. (2) Establish the initial allocation relationship between the voltage level selection and transmission power of the receiving-end cascaded stations, and determine the rated DC voltage ratio of converters A1 and A2 based on the active power planning requirements of AC1 and AC2 of the receiving-end AC grid. (3) Determine the modulation boundary of the converter during the voltage regulation process based on the constraint relationship between the DC voltage operating range of the converter and the proportion of the full-bridge submodule; (4) Establish the correspondence between the flexible adjustment range of power distribution and the configuration ratio of the full bridge sub-modules during the operation of the receiving end cascade station, and then coordinate the adjustment of the DC voltage ratio of converters A1 and A2 to realize the flexible mutual assistance of active power between the two converters.

[0008] Furthermore, the converters A1 and A2 adopt MMC, and the sub-modules cascaded in each of their bridge arms adopt a mixed ratio design of full-bridge sub-modules and half-bridge sub-modules.

[0009] Furthermore, due to the physical series connection of converters A1 and A2, their DC currents are strictly equal during steady-state operation. I dc = I dcA1 = I dcA2 DC voltage of the cascaded station U dc = U dcA1 + U dcA2 ,in I dcA1 and I dcA2 The DC currents of converters A1 and A2 are respectively denoted as and are collectively referred to as . I dc , U dcA1 and U dcA2 These are the DC voltages of converters A1 and A2, respectively.

[0010] Furthermore, the initial allocation relationship in step (2) satisfies the following equation:

[0011] in: P dcA1 and P dcA2 These represent the active power transmitted by converters A1 and A2, respectively.

[0012] Furthermore, in step (2), if the planned active power ratio of AC1 and AC2 of the receiving-end AC grid is N:1, then by configuring the number of sub-modules in converters A1 and A2 and adjusting the transformer ratio, the rated DC voltage ratio of A1 and A2 is N:1, where N is a positive real number.

[0013] Furthermore, in step (3), for either converter A1 or A2, its modulation boundary must satisfy the following relationship:

[0014] in: M ac The AC modulation ratio of the converter. M dc This refers to the DC voltage modulation ratio of the converter. K FBThis represents the percentage of full-bridge submodules within a single bridge arm of the converter.

[0015] Furthermore, in step (4), the DC voltage of the cascaded station at the receiving end... U dc While maintaining a constant DC voltage command value, dynamic mutual adjustment of active power between the two converters is achieved by increasing the DC voltage command value of one converter and decreasing the DC voltage command value of the other converter; for step-down converters, the proportion of full-bridge submodules within a single bridge arm... K FB It increases monotonically as the active power ratio of the converter decreases.

[0016] Furthermore, the correspondence between the flexible power allocation adjustment range and the full-bridge submodule configuration ratio in step (4) is as follows: Under rated conditions, the active power ratio of converters A1 and A2 is N:1. K FBA1 = K FBA2 =0%; In a mutually supportive operation scenario, the active power ratio of converters A1 and A2 is adjusted to M:1. If M < N, then... , K FBA2 =0%; if M>N, then K FBA1 =0% ; When the active power ratio of converter A2 is 100%, then K FBA1 =50%, K FBA2 =0%; When the active power ratio of converter A1 is 100%, then K FBA1 =0% K FBA2 =50%; in: K FBA1 This refers to the percentage of full-bridge submodules within a single bridge arm of converter A1. K FBA2 This represents the percentage of full-bridge submodules within a single bridge arm of converter A2.

[0017] A computer device includes a memory and a processor, wherein the memory stores a computer program and the processor executes the computer program to implement the above-described voltage-flexible cascaded converter system configuration method.

[0018] A computer-readable storage medium storing a computer program, which, when executed by a processor, implements the above-described voltage-flexible cascaded converter system configuration method.

[0019] Based on the above technical solution, the present invention has the following beneficial technical effects: 1. It achieves demand-driven matching during the planning stage. Because this invention establishes a correspondence between voltage matching ratio and AC power grid planning requirements, designers can flexibly configure the rated voltage ratio of converters A1 and A2 according to the differences in the absorption capacity of different areas of the receiving end power grid, breaking the limitations of the traditional cascaded topology of "equal voltage sharing and power lock-in".

[0020] 2. Significantly expands the power balance range during operation. Because this invention introduces a hybrid design of full-bridge and half-bridge submodules in the MMC converter, utilizing the physical characteristic that the full-bridge submodule can output negative voltage, it expands the internal potential support boundary when the DC voltage is reduced.

[0021] 3. Improved system flexibility and security. This invention quantifies the correspondence between the proportion of full-bridge submodules and the power regulation range, enabling the receiving-end cascaded station to adjust the voltage division ratio in real time according to the dynamic fluctuations of the load in both areas. This significantly enhances the dynamic mutual assistance capability between multiple regional power grids at the receiving end while improving the level of renewable energy consumption. Attached Figure Description

[0022] Figure 1 This is a schematic diagram of a cascaded converter topology with flexible voltage matching in an embodiment of the present invention.

[0023] Figure 2 This is a block diagram of voltage coordinated control for high- and low-voltage cascaded converter stations in an embodiment of the present invention.

[0024] Figure 3 This is a DC voltage simulation waveform diagram of the cascaded commutator topology in an embodiment of the present invention. Figure 3 Udc1 and Udc2 correspond to the DC voltages of converters A1 and A2, respectively. The horizontal axis represents time (in seconds), and the vertical axis represents voltage (in kV).

[0025] Figure 4 This is a simulation waveform of the active power of the cascaded converter topology in an embodiment of the present invention. Figure 4 P1 and P2 correspond to the active power of converters A1 and A2, with the horizontal axis representing time (in seconds) and the vertical axis representing power (in MW). Detailed Implementation

[0026] To describe the present invention in more detail, the technical solution of the present invention will be described in detail below with reference to the accompanying drawings and specific embodiments.

[0027] This embodiment provides a configuration method for a cascaded converter system with flexible voltage matching, including the following steps: (1) Construction of cascaded topology and physical characteristics of system topology.

[0028] like Figure 1 As shown, the voltage-flexible cascaded converter system constructed in this embodiment is mainly applied to multi-receiving-end DC transmission scenarios. The system consists of a sending-end power supply, a DC transmission line, and a receiving-end cascade station. The receiving-end cascade station includes two converters: a first-stage converter (hereinafter referred to as station A1) and a second-stage converter (hereinafter referred to as station A2).

[0029] In terms of physical connection, stations A1 and A2 are connected to the positive and negative DC lines on the DC side via series connection. Due to the physical series connection characteristic, the DC current flowing through the valve groups of stations A1 and A2... I dc They are strictly equal during steady-state operation, that is:

[0030] This characteristic forms the physical premise for power distribution in the cascaded system. The AC sides of stations A1 and A2 are connected to independent AC receiving-end power grids AC1 and AC2 respectively via connecting transformers. Each converter consists of six arms, and each arm contains several series-connected submodules (SMs) and arm reactors. Crucially, to achieve the dynamic mutual assistance function described in this invention, the submodules of each cascaded converter employ a mixed ratio design of full-bridge submodules (FBSMs) and half-bridge submodules (HBSMs). Through the negative voltage output capability of the full-bridge submodules, the converter can still offset the back electromotive force induced on the AC side when outputting a lower DC voltage, thereby maintaining the system's modulation stability.

[0031] (2) Mathematical modeling of the relationship between voltage level selection and transmission power allocation.

[0032] During the system planning phase, this implementation method employs initial voltage level differentiation design based on the load demand and grid strength of the two AC power grids. According to circuit theory, the active power transmitted by each cascaded converter can be expressed as:

[0033]

[0034] Due to DC current I dc The overall voltage control strategy of the sending-end converter station or cascaded system determines the power ratio allocated to the two receiving-end stations. Therefore, the power ratio allocated to the two receiving-end stations depends entirely on the voltage ratio allocated to their DC sides.

[0035] In practical engineering design, if the planned active power transmission ratio of AC1 to AC2 is 3:1, then in a system with a total DC voltage of 800kV, by configuring the number of submodules and adjusting the transformer ratio, the rated DC voltage of station A1 is set to 600kV, and the rated DC voltage of station A2 is set to 200kV. This asymmetric cascading design of voltage levels directly locks the power distribution benchmark of different converter stations at the physical topology level, which can greatly reduce the dynamic adjustment pressure of subsequent control algorithms.

[0036] (3) Mechanism of the change in DC voltage operating range of full-bridge and half-bridge hybrid MMC with the full-bridge ratio.

[0037] This implementation introduces a hybrid full-bridge and half-bridge MMC to expand the DC voltage regulation range of the cascaded station. Conventional converters containing only half-bridge modules have their DC voltage directly limited by the AC side voltage. To prevent overmodulation, i.e., to ensure that the submodule capacitor voltage can fully support the peak value of the AC sinusoidal waveform, the DC voltage modulation ratio is... M dc Must meet M dc ≥ M ac .

[0038] However, in cascaded voltage regulation scenarios, a converter station may need to significantly reduce its voltage to decrease power output. In this case, configuring a ratio of [missing information] in the bridge arm can [missing information]. K FB The full-bridge module can utilize its ability to output negative voltage to superimpose a negative level onto the DC side. The stability boundary formula for its DC voltage operation is as follows:

[0039] in: M ac For AC modulation ratio, K FB This represents the proportion of the entire bridge module to the total number of submodules. From this formula, we can see that as the proportion of the entire bridge increases... K FB Improvement, DC voltage modulation lower limit M dc,min It will decrease linearly.

[0040] Under rated conditions, the active power ratio of converters A1 and A2 is N:1. K FBA1 = K FBA2 =0%; In a mutually supportive operation scenario, the active power ratio of converters A1 and A2 is adjusted to M:1. If M < N, then... , K FBA2 =0%; if M>N, then K FBA1 =0% ; When the active power ratio of converter A2 is 100%, then K FBA1 =50%, K FBA2 =0%; When the active power ratio of converter A1 is 100%, then K FBA1 =0% K FBA2 =50%; in: K FBA1 This refers to the percentage of full-bridge submodules within a single bridge arm of converter A1. K FBA2 This represents the percentage of full-bridge submodules within a single bridge arm of converter A2.

[0041] In this process K FB The selection should be based on the actual power regulation requirements of operation. Compared to half-bridge submodules, full-bridge submodules require twice the number of Insulated Gate Bipolar Transistors (IGBTs), resulting in higher hardware construction costs and higher switching / conduction losses during operation. Therefore, when the active power load of the converter decreases, i.e., the DC voltage depth decreases, arbitrarily selecting an excessively high voltage... K FB This will lead to unnecessary huge economic costs and continuous power loss for the system; if K FB If the voltage is too low, the converter will fall into an overmodulation runaway collapse state under extremely low voltage conditions. This invention proposes a specific... K FB The ratio is the critical minimum full-bridge ratio design value derived by substituting the "lowest DC voltage command limit" that the converter station can achieve under predetermined extreme mutual assistance conditions into the aforementioned stability boundary formula. This non-arbitrary and precise ratio not only unlocks specific deep mutual assistance functions, but also embodies the beneficial technical effect of "exchanging the widest flexible power dispatch boundary of the system for the minimum cost input and operating loss".

[0042] (4) Voltage coordinated double closed-loop control equations and control structure mechanism of high and low voltage cascaded converter stations.

[0043] The proposed method for coordinating the DC voltage ratio of converters A1 and A2 refers to the real-time adjustment of the power ratio of each substation during operation in response to sudden changes in grid load in different areas of the same receiving end. In the specific implementation of dynamic mutual assistance control, this system employs a dual closed-loop control structure with mutually exclusive and complementary commands.

[0044] In the basic control principle of DC transmission systems, the fundamental prerequisite for stable system operation is constant voltage support on one side, with power control used for the rest. For the cascaded topology of this invention, the total active power injected into the system by the sending-end converter station essentially determines the common DC current flowing through the entire system. I dc Meanwhile, stations A1 and A2 at the receiving end are responsible for establishing and maintaining the system's total DC voltage reference, i.e. U dc_total = U dc1 + U dc2 Constant.

[0045] The inherent drawback of this topology is that if conventional constant power switching or power droop control strategies are used to adjust the power ratio between the two stations at the receiving end, the converter station at the receiving end must abandon its original constant voltage control and switch to a constant power loop. This will create a serious conflict with the existing constant power control at the sending end, namely, the number of constant power nodes in the system will exceed its weight, causing the entire DC system to lose its rigid voltage support point. Moreover, the forced switching of the control closed loop between constant voltage and constant power modes will also generate huge transient inrush currents on the AC and DC sides, resulting in a slow adjustment process or even system instability.

[0046] To achieve faster and more stable mutual assistance, this implementation method resolutely avoids introducing conventional power control loops at the receiving end. Both A1 and A2 stations maintain a standard constant DC voltage dual closed-loop control architecture. The innovation of this invention lies in the system-level strong coupling reverse deconstruction of the per-unit (pu) system at the reference voltage command input terminals of these two voltage outer loops. By setting the per-unit value of the cascaded total DC voltage command at the receiving end to 1.0 pu, a completely new cooperative control structure and equations are constructed: ① Cooperative voltage command generation stage.

[0047] When the system requires power balance adjustment, the control center first calculates the active power distribution ratio that station A1 needs to bear under the target operating condition, i.e., the per-unit value instruction. Based on the principle that the sum of system voltages is constant, the voltage command for station A2... The complement of the commands forcibly and rigidly locked to station A1 has the following cooperative command equations:

[0048] Note: The per-unit value generated based on the power demand of AC1 side is calibrated between 0 and 1.

[0049]

[0050] Therefore, stations A1 and A2 do not have their own independent power outer loop optimization processes. When the voltage command of station A1 increases by Δ... U At that time, the A2 station command precisely and without delay adjusted Δ within the same calculation cycle. U ,Right now .

[0051] ②Outer loop coordinated voltage PI control equation.

[0052] The aforementioned mutually exclusive linkage commands are sent in parallel to the outer loop voltage controllers of stations A1 and A2, respectively. Taking the dq rotating coordinate system as an example, the reference command control equations for the active current portion of the two converters along the d-axis are as follows: For converter A1:

[0053] For converter A2:

[0054] in: K p1 and K i1 and K p2 and K i2 These are the proportional and integral coefficients of the outer loop voltage regulators for stations A1 and A2, respectively. U dc1_pu and U dc2_pu These are the per-unit values ​​of the DC voltage actually sampled by the two converters.

[0055] ③ The decoupling control equation for the inner loop current of the converter.

[0056] Active current command output from the outer loop I d1_ref and I d2_ref and reactive current command I q_ref The circuit then enters the conventional inner-loop decoupling controller to generate the AC voltage reference value required for each phase arm of the converter; the inner-loop differential equation is as follows:

[0057]

[0058] in: U sdand U sq The d-axis and q-axis components of the grid-side AC voltage. R and L The equivalent resistance and inductance of the converter transformer and bridge arm are given. ω The angular frequency of the power grid; the obtained U cd_ref and U cq_ref After inverse coordinate transformation, it is used in conjunction with a modulator containing some full-bridge sub-modules to generate waves.

[0059] The overall control structure of this invention is as follows: Figure 2 As shown in the equations above, receiving-end stations A1 and A2 always operate in constant DC voltage control mode, completely avoiding control conflicts with the sending-end caused by receiving-end power control switching, thus ensuring a robust voltage reference for the entire DC grid. During the mutual assistance transient process, the mutual exclusion mapping formula guarantees... Therefore, for the external full-loop circuit, the total equivalent voltage step at the receiving end is zero, which makes the system cascaded common DC current... I dc There will be no transient fluctuations or shocks. The power transfer between the two stations is directly and instantaneously issued through the feedforward voltage command. The response time is only limited by the electrical following response of the voltage inner loop, thus achieving ultra-fast dynamic mutual assistance for large-capacity transmission of active power while ensuring extremely high dynamic stability.

[0060] (5) Quantitative correspondence between the power distribution adjustment range and the full bridge ratio during the cascade mutual assistance process.

[0061] The "mutual assistance" mentioned in this invention refers to the real-time adjustment of the power ratio of each level of inter-station in response to sudden changes in grid load during operation. Based on the aforementioned mathematical model, this implementation presents a set of typical power mutual assistance scenarios: The system is set to a rated state where station A1 receives 75% of the power (600kV) and station A2 receives 25% of the power (200kV). At this time, the full-bridge ratio requirement of station A1 is 0% to achieve stability.

[0062] Scenario 1: Balanced Power Distribution Mode. If the AC2 grid requires power support, the control center issues an instruction requiring both substations to share 50% of the power (i.e., each bearing 400kV DC voltage). For substation A1, its DC voltage is reduced from 600kV to 400kV. To compensate for the insufficient modulation margin caused by the voltage reduction, calculations show that the minimum full-bridge ratio required for substation A1 is 16.7%.

[0063] Scenario 2: Extreme Mutual Aid Mode. If a serious fault or load shedding occurs in the AC1 system, almost all of the power received by AC1 needs to be transferred to AC2. Assume that the power ratio of station A1 is adjusted to 10%, corresponding to a voltage drop to 80kV, while the voltage of station A2 rises to 720kV. At this time, the demand for full-bridge modules in station A1 operating at reduced voltage surges. Calculations show that station A1 must be configured with full-bridge submodules at a ratio of 43.4% to avoid overmodulation. This implementation provides a set of flexible adjustment ranges and the corresponding full-bridge configuration ratios, as shown in Table 1: Table 1

[0064] (6) Simulation verification.

[0065] The constructed model topology is as follows Figure 1 As shown, PSCAD / EMTDC (electromagnetic transient simulation software) simulation verification was performed on the two typical power mutual assistance scenarios given in step (5). To meet the power mutual assistance requirements of these two scenarios, the proportion of the full-bridge submodules of station A1 was set to 43.4%, and the proportion of the full-bridge submodules of station A2 was set to 0%, with a total active power of 1000MW connected to the sending end. The system reached steady state at rated conditions after 1.5s of simulation. At 2s, the system began to switch to scenario one, and at 3s, the system began to switch to scenario two. During this process, the simulated DC voltage waveforms of stations A1 and A2 are shown below. Figure 3 As shown, the simulated active power waveforms of stations A1 and A2 are as follows: Figure 4 As shown in the simulation results, the voltage ratio and active power ratio of the two converter stations can be successfully adjusted during operation, proving the effectiveness of the proposed method for configuring a cascaded converter system with flexible voltage matching that supports flexible power allocation and mutual assistance.

[0066] The above description of the embodiments is provided to enable those skilled in the art to understand and apply the present invention. Those skilled in the art can readily make various modifications to the above embodiments and apply the general principles described herein to other embodiments without creative effort. Therefore, the present invention is not limited to the above embodiments, and any improvements and modifications made to the present invention by those skilled in the art based on the disclosure thereof should be within the scope of protection of the present invention.

Claims

1. A configuration method for a cascaded converter system with flexible voltage matching, characterized in that, Includes the following steps: (1) Construct a voltage-flexible cascaded converter topology, which consists of a sending-end power supply and a receiving-end cascaded station. The receiving-end cascaded station includes two converters, A1 and A2. The DC sides of A1 and A2 are connected in series, with one end connected to the sending-end power supply and the other end grounded. The AC sides of A1 and A2 are connected to different receiving-end AC grids, AC1 and AC2, respectively. (2) Establish the initial allocation relationship between the voltage level selection and transmission power of the receiving-end cascaded stations, and determine the rated DC voltage ratio of converters A1 and A2 based on the active power planning requirements of AC1 and AC2 of the receiving-end AC grid. (3) Determine the modulation boundary of the converter during the voltage regulation process based on the constraint relationship between the DC voltage operating range of the converter and the proportion of the full-bridge submodule; (4) Establish the correspondence between the flexible adjustment range of power distribution and the configuration ratio of the full bridge sub-modules during the operation of the receiving end cascade station, and then coordinate the adjustment of the DC voltage ratio of converters A1 and A2 to realize the flexible mutual assistance of active power between the two converters.

2. The configuration method of the voltage-flexible cascaded converter system according to claim 1, characterized in that: The converters A1 and A2 adopt MMC, and the sub-modules cascaded in each of their bridge arms adopt a mixed ratio design of full-bridge sub-modules and half-bridge sub-modules.

3. The configuration method of the voltage-flexible cascaded converter system according to claim 1, characterized in that: Due to the physical series connection of converters A1 and A2, their DC currents are strictly equal during steady-state operation. I dc = I dcA1 = I dcA2 DC voltage of the cascaded station U dc = U dcA1 + U dcA2 ,in I dcA1 and I dcA2 The DC currents of converters A1 and A2 are respectively denoted as and are collectively referred to as . I dc , U dcA1 and U dcA2 These are the DC voltages of converters A1 and A2, respectively.

4. The configuration method of the voltage-flexible cascaded converter system according to claim 3, characterized in that: The initial allocation relationship in step (2) satisfies the following relationship: ; in: P dcA1 and P dcA2 These represent the active power transmitted by converters A1 and A2, respectively.

5. The configuration method of the voltage-flexible cascaded converter system according to claim 1, characterized in that: In step (2), if the planned active power ratio of AC1 and AC2 of the receiving-end AC grid is N:1, then by configuring the number of sub-modules in converters A1 and A2 and adjusting the transformer ratio, the rated DC voltage ratio of A1 and A2 is N:1, where N is a positive real number.

6. The configuration method of the voltage-flexible cascaded converter system according to claim 1, characterized in that: In step (3), for either converter A1 or A2, its modulation boundary must satisfy the following relationship: ; in: M ac The AC modulation ratio of the converter. M dc This is the DC voltage modulation ratio of the converter. K FB This represents the percentage of full-bridge submodules within a single bridge arm of the converter.

7. The configuration method of the voltage-flexible cascaded converter system according to claim 1, characterized in that: In step (4), the DC voltage of the receiving-end cascade station is... U dc While maintaining a constant DC voltage command value, dynamic mutual adjustment of active power between the two converters is achieved by increasing the DC voltage command value of one converter and decreasing the DC voltage command value of the other converter; for step-down converters, the proportion of full-bridge submodules within a single bridge arm... K FB It increases monotonically as the active power ratio of the converter decreases.

8. The configuration method of the voltage-flexible cascaded converter system according to claim 5, characterized in that: The correspondence between the flexible power allocation adjustment range and the full-bridge submodule configuration ratio in step (4) is as follows: Under rated conditions, the active power ratio of converters A1 and A2 is N:

1. K FBA1 = K FBA2 =0%; In a mutually supportive operation scenario, the active power ratio of converters A1 and A2 is adjusted to M:

1. If M < N, then... , K FBA2 =0%; if M>N, then K FBA1 =0% ; When the active power ratio of converter A2 is 100%, then K FBA1 =50%, K FBA2 =0%; When the active power ratio of converter A1 is 100%, then K FBA1 =0% K FBA2 =50%; in: K FBA1 This refers to the percentage of full-bridge submodules within a single bridge arm of converter A1. K FBA2 This represents the percentage of full-bridge submodules within a single bridge arm of converter A2.

9. A computer device comprising a memory and a processor, wherein the memory stores a computer program, characterized in that: The processor is used to execute the computer program to implement the voltage-flexible cascaded converter system configuration method as described in any one of claims 1 to 8.

10. A computer-readable storage medium storing a computer program, characterized in that: When the computer program is executed by the processor, it implements the voltage-flexible cascaded converter system configuration method as described in any one of claims 1 to 8.

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