Design method of partial energy storage modular multi-level flexible multi-state switch based on super capacitor

Abstract

The invention discloses a partial energy storage modular multi-level flexible multi-state switch design method based on a super capacitor, and belongs to the technical field of flexible power distribution of a power system. Each bridge arm comprises a first inductor, a plurality of common sub-modules NSM and a plurality of energy storage sub-modules ESM, wherein the common sub-modules NSM and the energy storage sub-modules ESM are connected in series. Through cooperative switching of the NSM and the ESM, AC / DC power transmission and control are realized. And according to the voltage and frequency of the power distribution network and the state of charge (SOC) of the super capacitor, controlling the charging and discharging behaviors of the ESM sub-module so as to cope with the power fluctuation of the system. On the basis of realizing an original AC / DC power transmission function, cross-time distribution of energy is realized through the distributed local energy storage units, short-time power fluctuation can be coped, and the problem of peak load shifting can be solved, so that the flexibility of system power regulation and control is greatly improved, and meanwhile, the stability of the system is enhanced.

Description

Technical Field

[0001] This invention relates to the field of flexible power distribution technology in power systems, specifically to a design method for a modular multilevel flexible multi-state switch based on supercapacitor partial energy storage. Background Technology

[0002] After a large number of renewable energy sources are connected to the grid, their inherent irregularity and unpredictability put enormous pressure on the safe operation of the power grid, which can easily lead to fluctuations in grid frequency and voltage, affecting system stability.

[0003] At the same time, the distribution of renewable energy bases and load centers shows a significant imbalance.

[0004] Flexible multi-state switch (FMSS) based on modular multilevel converters (MMCs) offers advantages such as high output voltage waveform quality, low switching frequency, and ease of expansion, making it well-suited for various operating scenarios in distribution networks. This FMSS utilizes power electronic conversion technology to achieve continuous and smooth regulation of active and reactive power across different distribution networks, dynamically balancing load distribution and preventing overload in heavily loaded areas and energy waste in lightly loaded areas. Simultaneously, it also incorporates multiple functions such as voltage support, power flow control, and islanded operation, seamlessly switching between different modes. With the large-scale integration of distributed energy resources and new loads, power fluctuations in distribution networks are intensifying. The MMC-based FMSS, with its fast response speed, can quickly suppress power fluctuations and ensure power quality.

[0005] However, FMSS based on MMC still has some drawbacks. In terms of control and protection systems, traditional FMSS relies on sophisticated and precise control and protection systems to coordinate the operation of distribution network equipment. If communication fails, it can affect system stability and reliability. Furthermore, since traditional MMC topologies mostly use pure power conversion topologies and lack energy storage systems, their regulation capabilities are limited when dealing with short-term power fluctuations caused by distributed energy access. They struggle to quickly and accurately achieve energy distribution across time, resulting in unsatisfactory peak shaving and valley filling effects. When the power generation of distributed energy suddenly changes, it cannot effectively and promptly stabilize the distribution network power, easily leading to problems such as voltage fluctuations and frequency deviations.

[0006] Therefore, developing a novel flexible interconnected switch topology with energy storage capabilities is imperative. This involves connecting energy storage units to the bridge arm sub-modules via bidirectional DC / DC converters. The energy storage component utilizes supercapacitors: due to their high power density and millisecond-level charge / discharge speeds, supercapacitors can rapidly respond to grid power fluctuations; their long cycle life and minimal impact from repeated charging and discharging allow for frequent participation in short-term power compensation in distribution networks (such as cross-regional load balancing); and their instantaneous power throughput avoids the regulation lag caused by electrochemical delays in batteries. Furthermore, supercapacitors possess stable chemical characteristics, eliminating safety hazards such as electrolyte leakage, overheating, and explosions, resulting in superior safety and adaptability to more complex operating conditions. More importantly, the state of charge (SOC) of a supercapacitor can be directly estimated through voltage. In contrast, battery SOC is significantly affected by temperature and aging, making estimation complex and posing safety risks due to overcharging and over-discharging. Therefore, supercapacitors offer greater adaptability. This topology enables energy distribution across time, rapidly compensating for power imbalances, effectively addressing fluctuations caused by distributed energy access, and improving power regulation flexibility and system stability. Meanwhile, compared to a fully modular energy storage topology, this structure only connects energy storage to some of the bridge arms, reducing the amount of energy storage equipment used and lowering hardware costs. Summary of the Invention

[0007] To address the aforementioned problems, the purpose of this invention is to provide a modular, multi-level, flexible, multi-state switch based on supercapacitor-based partial energy storage. This switch, while maintaining the original AC / DC power transmission function, achieves cross-time energy distribution through distributed local energy storage units. It can cope with short-term power fluctuations, solve the "peak shaving and valley filling" problem, thereby greatly improving the flexibility of system power regulation and enhancing system stability.

[0008] To solve the above-mentioned technical problems, the present invention provides the following technical solution: a design method for a modular multi-level flexible multi-state switch based on partial energy storage of supercapacitors, comprising, A modular multilevel converter structure including bridge arms is provided, each of the bridge arms including a first inductor and multiple general sub-modules NSM and multiple energy storage sub-modules ESM connected in series; Through the coordinated switching of the NSM and ESM, AC and DC power transmission and control are achieved; The charging and discharging behavior of the ESM submodule is controlled based on the voltage and frequency of the power distribution network and the state of charge (SOC) of the supercapacitor to cope with system power fluctuations.

[0009] As a preferred embodiment of the modular multi-level flexible multi-state switch design method based on supercapacitor partial energy storage described in this invention, the ordinary submodule NSM is a half-bridge submodule, including a DC capacitor and two series-connected switching units, wherein the DC capacitor is connected in parallel with the two series-connected switching units.

[0010] As a preferred embodiment of the modular multi-level flexible multi-state switch design method based on supercapacitor partial energy storage described in this invention, the switching unit includes an insulated gate bipolar transistor (IGBT) and a diode VD connected in anti-parallel to the IGBT.

[0011] As a preferred embodiment of the modular multi-level flexible multi-state switch design method based on supercapacitors described in this invention, the energy storage submodule ESM includes a half-bridge submodule and an energy storage unit; the energy storage unit includes a supercapacitor and a bidirectional DC / DC converter; the bidirectional DC / DC converter is composed of two series-connected switching units, and the supercapacitor is connected to the midpoint of the two series-connected switching units through a second inductor.

[0012] As a preferred embodiment of the modular multi-level flexible multi-state switch design method based on supercapacitor partial energy storage described in this invention, the bidirectional DC / DC converter includes three working modes: Buck mode, Boost mode, and cut-off mode. The Buck mode includes controlling the switch VT4 to turn off, and periodically controlling the on and off of VT3 to realize the flow of energy from the DC capacitor of the half-bridge submodule to the supercapacitor, thereby charging the supercapacitor. The Boost mode includes controlling the switch VT3 to turn off, and periodically controlling the on and off of VT4 to realize the flow of energy from the supercapacitor to the DC capacitor, so that the supercapacitor discharges. The cut-off mode includes turning off both control switches VT3 and VT4, while the energy storage unit is not connected to the system, and the ESM submodule is equivalent to the NSM submodule.

[0013] As a preferred embodiment of the modular multi-level flexible multi-state switch design method based on supercapacitor partial energy storage described in this invention, the system operation mode includes interconnected operation mode, load switching operation mode, and islanded autonomous operation mode, depending on the grid-connected or off-grid status of the flexible multi-state switch. In the interconnected operation mode, a master-slave control strategy is adopted, with one converter MMC-PESS2 using constant DC voltage control and the other converter MMC-PESS1 using constant power control; the ESM submodule determines charging and discharging based on the grid voltage deviation and its own SOC: when the voltage exceeds the rated value by 3% or the SOC is lower than 50%, it enters Buck mode for charging; when the voltage is lower than the rated value by 3% or the SOC is higher than 80%, it enters Boost mode for discharging.

[0014] As a preferred embodiment of the modular multi-level flexible multi-state switch design method based on supercapacitor partial energy storage described in this invention, in the load switching operation mode, the control strategy of the fault-side converter MMC-PESS2 is switched to droop control, and the ESM submodule immediately switches to Boost mode to discharge at a preset constant power to quickly replenish DC side energy; the normal-side converter MMC-PESS1 switches to the main converter and adopts constant DC voltage control, and its ESM submodule can enter Buck mode to charge and store energy according to system requirements. In the isolated autonomous operation mode, both converters, MMC-PESS1 and MMC-PESS2, are disconnected from the main grid and droop control is adopted. At the moment of failure, all ESM submodules are forced to switch to Boost mode to release energy. After entering stable operation, the ESM submodules start constant power discharge control. If the supercapacitor SOC is lower than 30%, it will automatically switch to the power level that only supplies critical loads.

[0015] As a preferred embodiment of the modular multi-level flexible multi-state switch design method based on supercapacitor partial energy storage described in this invention, the charging and discharging control of the ESM submodule includes the management of the supercapacitor's state of charge (SOC), where SOC is defined as the ratio of the supercapacitor's current remaining capacity to its rated total capacity; the system maintains the SOC within a preset reasonable range by adjusting the number of ESMs and their operating modes.

[0016] The present invention provides a computer device, including a memory and a processor. The memory stores a computer program, and the processor executes the computer program to implement the steps of the aforementioned modular multilevel flexible multi-state switch design method based on supercapacitor partial energy storage.

[0017] The present invention provides a computer-readable storage medium having a computer program stored thereon, wherein the computer program, when executed by a processor, implements the steps of the aforementioned modular multilevel flexible multi-state switch design method based on supercapacitor partial energy storage.

[0018] The beneficial effects of this invention are as follows: 1. It proposes a flexible multi-state switch for a modular multilevel converter based on supercapacitor local energy storage applied to AC distribution networks, enabling the converter to perform both power conversion and energy storage functions, and actively absorb / release surplus / deficient energy in the system. Simultaneously, due to the high power density and millisecond-level response speed of supercapacitors, it can quickly smooth power fluctuations within ±2MW, effectively suppressing voltage and frequency fluctuations caused by the grid connection of new energy sources, and enhancing the distribution network's ability to absorb intermittent power sources.

[0019] 2. Traditional MMC power flow is limited by AC / DC side coupling, and can only achieve basic energy conversion. This invention can achieve AC / DC side power decoupling through energy storage units, flexibly allocate active / reactive power, adapt to multiple scenarios of distributed power supply operation, support bidirectional power flow, adapt to complex operating conditions of distribution networks, and serve as a flexible power source to support the power grid in scenarios such as microgrids and islanded operation.

[0020] 3. To address the coordination issues caused by the fragmented control in traditional "FMSS + independent energy storage" systems, this invention utilizes "integrated converter-energy storage control" to adjust the charging and discharging power of the supercapacitor in real time, dynamically matching it with the feeder power command, thereby achieving precise state of charge (SOC) management and avoiding overcharging and over-discharging.

[0021] 4. Traditional MMCs are prone to instability due to power imbalance during distribution network faults and lack emergency power supply capabilities. MMC-PESS energy storage units can quickly compensate for power deficits during faults, isolate fault points, extend fault handling time, prevent fault propagation, and reduce the risk of power outages in the distribution network.

[0022] 5. Traditional MMCs requiring energy storage functionality necessitate an additional, independent energy storage system. Existing simple combinations of MMCs and energy storage suffer from poor compatibility, complex control, and equipment redundancy. MMC-PESS employs a partial energy storage design, integrating energy storage only in certain sub-modules, balancing energy storage capacity and system cost. Its modular design solves the economic challenges of full-module energy storage, reducing the amount of energy storage equipment required (hardware costs are reduced by approximately 25%). Furthermore, the supercapacitors have a cycle life of tens of thousands of cycles, more than 10 times that of batteries, significantly reducing long-term maintenance costs. Attached Figure Description

[0023] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0024] Figure 1 This is a circuit diagram of a modular multilevel flexible multi-state switch for partial energy storage based on a supercapacitor.

[0025] Figure 2 This is a sub-module circuit diagram of a modular multilevel flexible multi-state switch design method based on supercapacitor partial energy storage.

[0026] Figure 3 Power flow diagram in normal mode for a modular multilevel flexible multi-state switch design method for partial energy storage based on supercapacitors.

[0027] Figure 4Power flow diagram in load switching operation mode for a modular multilevel flexible multi-state switch design method based on supercapacitor partial energy storage.

[0028] Figure 5 Power flow diagram in islanded autonomous operation mode for a modular multilevel flexible multi-state switch design method based on supercapacitor partial energy storage.

[0029] Figure 6 This is a block diagram of the constant power charge and discharge control of the supercapacitor in the ESM submodule under Buck mode.

[0030] Figure 7 This is a block diagram of the constant power charge and discharge control of the supercapacitor in the ESM submodule under Boost mode. Detailed Implementation

[0031] To make the present invention more apparent and understandable, the specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort should fall within the protection scope of the present invention.

[0032] Example 1, referring to Figures 1-7 This is one embodiment of the present invention, which provides a design method for a modular multilevel flexible multi-state switch based on supercapacitor partial energy storage, comprising: The flexible multi-state switching system structure of the modular multilevel converter based on supercapacitors and partially connected to energy storage in the AC distribution network of this invention is as follows: Figure 1 As shown, distribution networks 1 and 2 are connected to the ends of two AC distribution network feeders through transformers T1 and T2, respectively. The ends of the AC distribution network feeders are connected back-to-back through two MMC-PESS converters.

[0033] In this embodiment, the ESM submodule integrates an energy storage unit and has three typical operating modes: Buck mode, Boost mode, and cutoff mode. In Buck mode, the control switch VT4 is turned off. During one switching cycle, when VT3 is on, the supercapacitor is charging, and energy flows from capacitor C to capacitor S. When VT3 is off, the CSC freewheels through the diode connected in parallel with VT4. Periodically turning VT3 on and off can charge the supercapacitor. In Boost mode, VT3 is turned off. During one switching cycle, when VT4 is on, the supercapacitor is discharging, and energy flows from CSC to capacitor C. When VT4 is off, the CSC freewheels through the diode connected in parallel with VT3. Periodically turning VT4 on and off can discharge the supercapacitor. In cutoff mode, the energy storage unit does not participate in operation; both switches VT3 and VT4 are off, and the energy storage unit is not connected to the system. In this mode, the ESM can be considered as an NSM.

[0034] In this embodiment, the ordinary submodule NSM also has three typical operating modes: active, deactivated, and latched. In the active state, VT1 is on and VT2 is off, charging and discharging the capacitor according to the current direction. In the deactivated state, VT1 is off and VT2 is on, and the submodule output voltage is always 0. The latched state is a typical abnormal operating state, usually occurring during MMC-PESS startup or when a DC fault occurs; in this state, both VT1 and VT2 are off.

[0035] Based on the different grid-connected / off-grid states of the flexible multi-state switching port of the modular multilevel converter based on supercapacitor-based partial access energy storage, this paper divides the operation mode into three categories: interconnected operation mode, load switching operation mode, and autonomous operation mode.

[0036] In this embodiment, under normal operating conditions, MMC-PESS2 is the master converter and MMC-PESS1 is the slave converter. Distribution networks 1 and 2 can exchange power between MMC-PESS1 and MMC-PESS2 to meet load demands (power flow direction is as follows). Figure 3The main converter employs constant DC voltage control, while the slave converters use constant power control based on grid commands. The main converter achieves system power balance and controls bidirectional power transmission between the AC and DC grids within its capacity. When the cross-grid power difference, voltage, frequency fluctuations, and SOC (state of charge of the supercapacitor) are all within allowable ranges, the energy storage unit only maintains basic monitoring, with no high-frequency Buck / Boost operations to reduce losses. Once a voltage rise exceeding 3% of the rated value or SOC < 50% is detected, the energy storage unit charges in Buck mode to absorb excess reactive power, suppress voltage rise, and increase SOC. Simultaneously, to obtain maximum charging power, when the arm current is positive, the ESM needs to be engaged for as long as possible; however, if the arm voltage is greater than the sum of the capacitor voltages of the corresponding NSMs within the arm, the ESM needs to be engaged even if the arm current is negative to meet the arm's output voltage requirements. When a voltage drop exceeding 3% of the rated value or SOC > 80% is detected, the energy storage unit discharges in Boost mode, rapidly injecting reactive power to support the voltage and reducing SOC, maintaining it within a reasonable range. Meanwhile, to obtain maximum discharge power, when the arm current is negative, if the sum of the voltages of the ESM modules exceeds the current arm output voltage requirement, the entire arm voltage is generated by the ESM according to the modulation command. Conversely, when the arm output voltage exceeds the sum of the capacitor voltages of the NSM modules, even if the arm current is positive, a certain amount of ESM needs to be activated to meet the arm output voltage requirement. For example, in 10kV distribution network 2 (load center), due to a sudden increase in distributed wind power output (short-term output from 0.6MVA to 0.9MVA), the voltage of distribution network 2 rises to 10.45kV. At the same time, the current SOC of the supercapacitor is 42% (<50%), and the rated capacity of the MMC-PESS system is 1MVA. Detecting the direction of the bridge arm current—At this time, the power distribution network has a power surplus, and the bridge arm current is positive (energy flows from the distribution network to the energy storage unit). When the bridge arm current is positive, the ESM needs to be deployed for as long as possible. Originally, two ESMs (each with a capacity of 0.1MVA) were deployed, but now this is increased to four ESMs, increasing the total charging power to 0.4MVA (not exceeding the rated capacity of 1MVA). Verifying the bridge arm voltage and NSM capacitor voltage—Assuming that the current bridge arm needs to output 10.4kV to absorb the surplus power, and the total capacitor voltage of the 12 NSMs in this bridge arm is 9.8kV (<10.4kV), even if the bridge arm current is positive (an ideal scenario where no additional ESM is needed), ESMs still need to be deployed to fill the voltage gap. Ultimately, through the coordinated operation of four ESMs (total voltage 0.6kV) and twelve NSMs (total voltage 9.8kV), the bridge arm output voltage reaches 10.4kV. The system operates under this scenario most of the time.At this point, the supercapacitor absorbs the surplus reactive power with a power output of 0.4 MVA, reducing the voltage of distribution network 2 from 10.45 kV to 10.25 kV (returning to the ±3% rated voltage range) within 10 minutes. The supercapacitor's SOC rises from 42% to 53% (exceeding the 50% charging trigger threshold), meeting the requirement of "maintaining SOC balance." Subsequently, the number of ESMs deployed is reduced from 4 to 2, entering a low-loss standby state, maintaining the SOC only within the 50%~55% range. The system operates under this scenario for most of the time.

[0037] In a further embodiment, when a temporary grounding or short-circuit fault occurs on a feeder of a distribution network (assumed to occur at the feeder of distribution network 2 in this paper); due to the short-circuit fault, the voltage of distribution network 2 drops sharply from 10kV to 8kV (a drop of 20% > 15% threshold). Within 20ms, the system switches from interconnection mode to load switching mode, transferring the critical load of distribution network 2 (such as hospital load, total power 0.3MVA) to distribution network 1. A stable voltage is output through the coordinated output of ESM and NSM to ensure power supply. Simultaneously, MMC-PESS2 switches to droop control, real-time monitoring and maintaining grid voltage and frequency; at this time, ESM immediately switches to Boost mode, using constant power control via the bidirectional Buck / Boost interface (such as...). Figure 6 , 7 As shown), the system discharges at a preset power to quickly replenish DC-side energy, preventing voltage collapse and ensuring the normal operation of critical loads at feeder 2. Simultaneously, it switches from converter MMC-PESS1 to the main converter, employing constant DC voltage control to maintain DC voltage stability. The rectifier side of MMC-PESS1 draws power from distribution network 1, with a portion transmitted to MMC-PESS2 via the DC side, and the other portion driving the ESM into Buck mode to charge the supercapacitor at constant power, storing energy for future needs. When the output power of distribution network 1 is less than the transfer demand, the ESM immediately switches to Boost mode, using PD-SPWM (carrier superimposed modulation technology) and a capacitor voltage sorting method to prioritize the operation of submodules with higher capacitor voltages, ensuring longer operation time and constant power discharge (superimposed with the rectifier output). This achieves capacitor voltage balance among submodules, preventing over-discharge of submodules with lower capacitor voltages, while ensuring the total power transmitted to MMC-PESS2 via the DC side meets the demand of load 2 (power flow direction as shown). Figure 4 When distribution network 2 is repaired, MMC-PESS2 is reconnected to the grid. At this time, the energy storage unit switches to Buck mode for charging, load 2 switches back to distribution network 2 for power supply, the system returns to normal, and switches to interconnected operation mode.

[0038] In this further embodiment, when both feeders of distribution network 1 and distribution network 2 experience faults, the voltage of both distribution networks is <9.5kV or >10.5kV (exceeding 10kV ±5%), and the frequency is <49.8Hz or >50.2Hz (deviation from rated frequency ±0.2Hz) for 5 seconds. The system switches to islanded autonomous operation mode. At this time, both MMC-PESS1 and MMC-PESS2 are disconnected from the main grid and operate independently. Their control strategies are switched to droop control to share the load power and maintain the voltage / frequency stability of the local load. Simultaneously, at the moment of the fault, the ESM submodules in MMC-PESS1 and MMC-PESS2 are forcibly switched to Boost mode, releasing supercapacitor energy through the bidirectional Buck / Boost interface to quickly fill the DC-side energy gap (power flow direction as follows). Figure 5 After entering the stable autonomous operation phase, the ESM initiates constant power discharge control based on the rated power of each load to ensure that the power transmitted to each load is stable. However, if the supercapacitor's SOC is below 30%, constant power discharge will automatically switch to "critical load power," reducing the power supply to non-critical loads and extending the runtime of critical loads. Simultaneously, when load power fluctuates, the output frequency is maintained stable at 50Hz±0.2Hz by fine-tuning the discharge power (within ±5%) in conjunction with the connection / disconnection of bridge arm submodules, preventing load disconnection due to frequency instability. Once the indicators of distribution networks 1 and 2 return to normal, the voltage rises above 9.5kV (drop <5%) and the fault current disappears, remaining stable for 30 seconds. MMC-PESS1 and MMC-PESS2 gradually connect to the grid, with MMC-PESS1 switching to constant active power control and MMC-PESS2 switching to constant DC voltage control. The system returns to normal and switches to interconnected operation mode.

[0039] This embodiment also provides an electronic device applicable to the design method of a modular multilevel flexible multi-state switch based on partial energy storage of supercapacitors, comprising: a memory and a processor; the memory is used to store computer-executable instructions, and the processor is used to execute the computer-executable instructions to realize the design method of a modular multilevel flexible multi-state switch based on partial energy storage of supercapacitors as proposed in the above embodiment.

[0040] This embodiment also provides a storage medium storing a computer program that, when executed by a processor, implements the modular multilevel flexible multi-state switch design method based on supercapacitor partial energy storage proposed in the above embodiment.

[0041] The storage medium proposed in this embodiment and the design method for a modular multi-level flexible multi-state switch based on supercapacitor partial energy storage proposed in the above embodiments belong to the same inventive concept. Technical details not described in detail in this embodiment can be found in the above embodiments, and this embodiment has the same beneficial effects as the above embodiments.

[0042] Based on the above description of the implementation methods, those skilled in the art can clearly understand that the present invention can be implemented using software and necessary general-purpose hardware, and of course, it can also be implemented using hardware, but in many cases the former is a better implementation method. Based on this understanding, the technical solution of the present invention, or the part that contributes to the prior art, can be embodied in the form of a software product. This computer software product can be stored in a computer-readable storage medium, such as a computer floppy disk, read-only memory (ROM), random access memory (RAM), flash memory, hard disk, or optical disk, etc., including several instructions to cause a computer device (which may be a personal computer, server, or network device, etc.) to execute the methods of the various embodiments of the present invention.

[0043] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A design method for a modular multi-level flexible multi-state switch based on partial energy storage of supercapacitors, characterized in that: include, A modular multilevel converter structure including bridge arms is provided, each of the bridge arms including a first inductor and multiple general sub-modules NSM and multiple energy storage sub-modules ESM connected in series; Through the coordinated switching of the NSM and ESM, AC and DC power transmission and control are achieved; The charging and discharging behavior of the ESM submodule is controlled based on the voltage and frequency of the power distribution network and the state of charge (SOC) of the supercapacitor to cope with system power fluctuations.

2. The design method for a modular multi-level flexible multi-state switch based on supercapacitor partial energy storage as described in claim 1, characterized in that: The general submodule NSM is a half-bridge submodule, which includes a DC capacitor and two series-connected switching units. The DC capacitor is connected in parallel with the two series-connected switching units.

3. The design method for a modular multi-level flexible multi-state switch based on supercapacitor partial energy storage as described in claim 2, characterized in that: The switching unit includes an insulated gate bipolar transistor (IGBT) and a diode VD connected in antiparallel to the IGBT.

4. The design method for a modular multi-level flexible multi-state switch based on supercapacitor partial energy storage as described in claim 3, characterized in that: The energy storage submodule ESM includes a half-bridge submodule and an energy storage unit; the energy storage unit includes a supercapacitor and a bidirectional DC / DC converter; the bidirectional DC / DC converter consists of two series-connected switching units, and the supercapacitor is connected to the midpoint of the two series-connected switching units through a second inductor.

5. The design method for a modular multi-level flexible multi-state switch based on supercapacitor partial energy storage as described in claim 4, characterized in that: The bidirectional DC / DC converter includes three operating modes: Buck mode, Boost mode, and cut-off mode. The Buck mode includes controlling the switch VT4 to turn off, and periodically controlling the on and off of VT3 to realize the flow of energy from the DC capacitor of the half-bridge submodule to the supercapacitor, thereby charging the supercapacitor. The Boost mode includes controlling the switch VT3 to turn off, and periodically controlling the on and off of VT4 to realize the flow of energy from the supercapacitor to the DC capacitor, so that the supercapacitor discharges. The cut-off mode includes turning off both control switches VT3 and VT4, while the energy storage unit is not connected to the system, and the ESM submodule is equivalent to the NSM submodule.

6. The design method for a modular multi-level flexible multi-state switch based on supercapacitor partial energy storage as described in claim 5, characterized in that: Based on the grid-connected or off-grid status of the flexible multi-state switch, the system operation modes include interconnected operation mode, load switching operation mode, and islanded autonomous operation mode; In the interconnected operation mode, a master-slave control strategy is adopted, with one converter MMC-PESS2 using constant DC voltage control and the other converter MMC-PESS1 using constant power control; the ESM submodule determines charging and discharging based on the grid voltage deviation and its own SOC: when the voltage exceeds the rated value by 3% or the SOC is lower than 50%, it enters Buck mode for charging; when the voltage is lower than the rated value by 3% or the SOC is higher than 80%, it enters Boost mode for discharging.

7. The design method for a modular multi-level flexible multi-state switch based on supercapacitor partial energy storage as described in claim 6, characterized in that: In the load switching operation mode, the control strategy of the fault-side converter MMC-PESS2 is switched to droop control, and the ESM submodule immediately switches to Boost mode to discharge at a preset constant power to quickly replenish DC side energy; the normal-side converter MMC-PESS1 switches to the main converter and adopts constant DC voltage control, and its ESM submodule can enter Buck mode to charge and store energy according to system requirements. In the isolated autonomous operation mode, both converters, MMC-PESS1 and MMC-PESS2, are disconnected from the main grid and droop control is adopted; at the moment of failure, all ESM submodules are forcibly switched to Boost mode to release energy. Once the system reaches stable operation, the ESM submodule initiates constant power discharge control. If the supercapacitor's SOC is below 30%, it automatically switches to a power level that supplies only critical loads.

8. The design method for a modular multi-level flexible multi-state switch based on supercapacitor partial energy storage as described in claim 7, characterized in that: The charging and discharging control of the ESM submodule includes the management of the supercapacitor's state of charge (SOC), which is defined as the ratio of the supercapacitor's current remaining capacity to its rated total capacity. The system maintains the SOC within a preset reasonable range by adjusting the number of ESMs and their operating modes.

9. A computer device comprising a memory and a processor, wherein the memory stores a computer program, characterized in that, When the processor executes the computer program, it implements the steps of the design method for a partially energy storage modular multilevel flexible multi-state switch based on supercapacitors as described in any one of claims 1 to 8.

10. A computer-readable storage medium having a computer program stored thereon, characterized in that, When the computer program is executed by the processor, it implements the steps of the design method for a modular multilevel flexible multi-state switch based on supercapacitor partial energy storage as described in any one of claims 1 to 8.

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