Soft start method and system of transformer-free multi-terminal flexible interconnection device

Abstract

The invention discloses a soft start method and system for a transformer-free multi-terminal flexible interconnection device, and relates to the technical field of AC power grid power flow control, and the method comprises the steps: inputting a current-limiting resistor, carrying out the uncontrolled rectification charging of a parallel cascading multistage inverter, cutting off the current-limiting resistor, and carrying out the uncontrolled rectification charging of the parallel cascading multistage inverter. Starting the control of the parallel cascade multilevel inverter, and carrying out uncontrolled rectification charging on the power flow control unit and the energy control unit; and performing controlled charging on the series part of the series converter, starting line power flow control, and operating the multi-terminal flexible interconnection device. According to the soft start method of the transformer-free type multi-terminal flexible interconnection device, the current-limiting resistor is used for carrying out uncontrolled rectification charging on the parallel cascade multi-stage inverter, the stability and reliability of the system are improved, the overall operation efficiency and stability of a power grid are improved through starting line power flow control, and the soft start method is suitable for large-scale popularization and application of the transformer-free type multi-terminal flexible interconnection device. The method has better effects in the aspects of stability, control precision and operation efficiency.

Description

Technical Field

[0001] The invention relates to the technical field of AC power grid flow control, and in particular to a soft starting method for a transformerless multi-terminal flexible interconnection device. Background Art

[0002] With the rapid development of economy and technology, the growth of electricity demand and the widespread application of new energy have made the structure of modern power systems increasingly complex. Traditional power grids rely on electromagnetic and mechanical technologies, such as adjusting generator output, transformer tap switches and line disconnection, to control power flow. However, these means can no longer meet the needs of new power grids, especially in the face of growing distributed energy and complex load patterns. As an emerging power flow control technology, soft normally open point (SNOP) can fully utilize the capacity of existing AC power transmission lines until their thermal limits by controlling power flow. In recent years, researchers have proposed a transformerless multi-terminal flexible interconnection device (multiport transformer-less SNOP, MTSNOP), which consists of a series converter and a parallel cascade multilevel inverter (cascade multilevel inverter, CMI). Its architecture has the freedom to independently control active and reactive power, making it a more economical and promising transmission solution.

[0003] However, there are still many deficiencies in the existing technology, which limit the widespread application of flexible interconnection devices. First, traditional SNOP devices are difficult to achieve low-cost and large-scale applications due to their large size, high installation, operation and maintenance costs. Secondly, during the operation of MTSNOP, how to effectively avoid the current shock and voltage fluctuation generated by the system at startup is a key technical problem. The existing startup method lacks a sophisticated control strategy and cannot balance the initial charging speed with the stability of current and voltage, which may cause serious voltage and current shocks at startup and even cause system oscillations. In addition, traditional series converters and parallel cascaded multi-level converters have weak isolation and recovery capabilities in the event of a fault, and cannot quickly restore normal operation, thereby affecting the overall stability of the power grid. Summary of the invention

[0004] In view of the above-mentioned problems, the present invention is proposed.

[0005] Therefore, the technical problem solved by the present invention is: the existing soft start method of the flexible interconnection device has weak fault recovery ability, large voltage fluctuation, current shock during startup, and how to achieve the optimization problem of smooth startup and rapid recovery of the multi-terminal flexible interconnection device.

[0006] In order to solve the above technical problems, the present invention provides the following technical solutions: a soft starting method for a transformerless multi-terminal flexible interconnection device, comprising inputting a current limiting resistor, performing uncontrolled rectification charging on the parallel cascade multi-level inverter, removing the current limiting resistor, performing uncontrolled rectification charging on the parallel cascade multi-level inverter; starting the parallel cascade multi-level inverter control, performing uncontrolled rectification charging on the flow control unit and the energy control unit; performing controlled charging on the series part of the series converter, starting the line flow control, and operating the multi-terminal flexible interconnection device.

[0007] As a preferred solution of the soft-start method of the transformerless multi-terminal flexible interconnection device described in the present invention, wherein: the input current limiting resistor includes inputting the current limiting resistor to limit the initial current, the parallel cascade multi-level inverter performs uncontrolled rectification charging, the current gradually increases, and exponentially decays over time, the current increases according to the undamped exponential decay law, combined with the inductance effect in the circuit, the damped oscillation term is introduced, so that the current is superimposed with an oscillation component on the basis of exponential decay, and the dynamic change process of the current is refined, which is expressed as:

[0008]

[0009] Among them, V l is the grid line voltage amplitude, R l is the current limiting resistor, C C is the capacitor of the cascaded multi-level inverter, L C is the inductance of the parallel cascade multi-level inverter, α is the time constant, β is the angular frequency, R o is the equivalent resistance after removing the current limiting resistor.

[0010] As a preferred solution of the soft-start method of the transformerless multi-terminal flexible interconnection device described in the present invention, wherein: the removal of the current limiting resistor includes reducing the circuit impedance, accelerating the charging process, and the parallel cascade multi-level inverter continues to perform uncontrolled rectification charging. The equivalent resistance after the current limiting resistor is removed is reduced, and the decay rate is slowed down. The current change is divided into an undamped exponential decay process, reflecting the trend of the current rising after the current limiting resistor is removed, and a superimposed damped oscillation term, reflecting the influence of the actual inductance effect on the current waveform, which is expressed as:

[0011]

[0012] Among them, γ is the new time constant and δ is the new angular frequency.

[0013] As a preferred solution of the soft start method of the transformerless multi-terminal flexible interconnection device described in the present invention, wherein: the starting parallel cascade multi-level inverter control includes a control voltage model of the parallel cascade multi-level inverter, which reflects the voltage rise during the controlled charging process, combines the basic characteristics of capacitor charging, and performs stable charging by increasing the voltage. The voltage control model introduces an oscillation term to superimpose sinusoidal oscillations during the exponential decay process. In the initial stage of charging, the voltage rises according to an exponential law, reflecting the response capability to instantaneous load changes. The control voltage model is expressed as:

[0014]

[0015] Among them, V s is the preset voltage value, t and τ2 are different time constants, and ω is the angular frequency.

[0016] As a preferred solution of the soft-start method of the transformerless multi-terminal flexible interconnection device of the present invention, wherein: the uncontrolled rectification charging of the power flow control unit and the energy control unit includes an uncontrolled rectification charging current model. When the power flow control unit and the energy control unit are uncontrolled rectified and charged, the characteristic of the current gradually increasing over time is displayed by the current model, and the characteristic of the capacitor in the initial charging stage is reflected by the exponential decay. The uncontrolled rectification charging current model is expressed as:

[0017]

[0018] Among them, R P is the current limiting resistor of the power flow control unit, C P is the capacitance of the power flow control unit, and θ is the angular frequency of the power flow control unit.

[0019] As a preferred solution of the soft-start method of the transformerless multi-terminal flexible interconnection device of the present invention, the controlled charging of the series part of the series converter includes a voltage model for controlled charging of the series part of the series converter, the voltage model increases the voltage to a preset value through exponential decay, and introduces a regulation response term, which reflects the different regulation stages of the voltage in the rising process through a combination of double exponential forms, and is expressed as:

[0020]

[0021] Among them, V s2 is the preset voltage value of the series converter, κ is the charging speed adjustment coefficient, and λ is the charging process adjustment coefficient.

[0022] As a preferred solution of the soft starting method of the transformerless multi-terminal flexible interconnection device described in the present invention, the starting line flow control includes a total power model, which reflects the basic power transmission situation of each line, combines voltage, current and phase angle, and reflects the basic power level of the system before any adjustment. The total power model introduces an adjustment response term, and through exponential decay adjustment, reflects the power change of the system in different time periods, describes the initial power level, adjusts the power response over time, combines power fluctuations, and responds to dynamic changes, which is expressed as:

[0023]

[0024] Where N is the total number of lines, ρ is the power flow control smoothing coefficient, μ is the power flow control adjustment coefficient, and t is the time variable. θ i are the voltage, current and phase angle of the i-th line respectively.

[0025] Another object of the present invention is to provide a soft start system for a transformerless multi-terminal flexible interconnection device, which can control the charging of the series part of the series converter through a flow control module and start line flow control, thereby solving the current problem of voltage overshoot or undershoot.

[0026] As a preferred solution of the soft start system of the transformerless multi-terminal flexible interconnection device described in the present invention, it includes: a startup cut-off module, a CMI control module, and a flow control module; the startup cut-off module starts the current limiting resistor to perform uncontrolled rectification and charging on the parallel cascade multi-level inverter, cuts off the current limiting resistor, and performs uncontrolled rectification and charging on the parallel cascade multi-level inverter; the CMI control module is used to start the control of the parallel cascade multi-level inverter, and performs uncontrolled rectification and charging on the flow control unit and the energy control unit; the flow control module is used to perform controlled charging on the series part of the series converter, start the line flow control, and operate the multi-terminal flexible interconnection device.

[0027] A computer device includes a memory and a processor, wherein the memory stores a computer program, and is characterized in that the processor executes the computer program to implement a step of a soft start method for a transformerless multi-terminal flexible interconnection device.

[0028] A computer-readable storage medium stores a computer program, wherein when the computer program is executed by a processor, the steps of a soft-start method for a transformer-free multi-terminal flexible interconnection device are implemented.

[0029] Beneficial effects of the present invention: The soft-start method of the transformerless multi-terminal flexible interconnection device provided by the present invention improves the stability and reliability of the system by inputting a current limiting resistor and performing uncontrolled rectification and charging on the parallel-connected cascaded multi-level inverters, improves the starting efficiency by cutting off the current limiting resistor, improves the control accuracy of the system by starting the control of the parallel-connected cascaded multi-level inverters, improves the overall charging efficiency and stability of the system by performing uncontrolled rectification and charging on the PFCM and PBCM in the series converter, and improves the overall operating efficiency and stability of the power grid by starting line flow control. The present invention achieves better results in terms of stability, control accuracy and operating efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following briefly introduces the drawings required for describing the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative labor. Among them:

[0031] Figure 1 An overall flow chart of a soft starting method for a transformerless multi-terminal flexible interconnection device provided in the first embodiment of the present invention.

[0032] Figure 2 This is an overall module diagram of a soft starting system of a transformerless multi-terminal flexible interconnection device provided in the third embodiment of the present invention. DETAILED DESCRIPTION

[0033] In order to make the above-mentioned purposes, features and advantages of the present invention more obvious and easy to understand, the specific implementation methods of the present invention are described in detail below in conjunction with the drawings of the specification. Obviously, the described embodiments are part of the embodiments of the present invention, but not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary persons in the art without creative work should fall within the scope of protection of the present invention.

[0034] Example 1

[0035] Reference Figure 1 , is an embodiment of the present invention, and provides a soft starting method for a transformer-free multi-terminal flexible interconnection device, comprising:

[0036] S1: put in the current limiting resistor to carry out uncontrolled rectification and charging for the parallel-connected cascade multi-level inverters, cut off the current limiting resistor to carry out uncontrolled rectification and charging for the parallel-connected cascade multi-level inverters.

[0037] Furthermore, investing in a current limiting resistor includes investing in a current limiting resistor to limit an initial current.

[0038] It should be noted that the parallel cascade multi-level inverter performs uncontrolled rectification charging, the current gradually increases, and exponentially decays over time, and the current increases according to the undamped exponential decay law. Combined with the inductance effect in the circuit, the damped oscillation term is introduced to superimpose the oscillation component on the current on the basis of exponential decay, and the dynamic change process of the current is refined, which can be expressed as:

[0039]

[0040] Among them, V l is the grid line voltage amplitude, R l is the current limiting resistor, C C is the capacitor of the cascaded multi-level inverter, L C is the inductance of the parallel cascade multi-level inverter, α is the time constant, β is the angular frequency, R o is the equivalent resistance after removing the current limiting resistor.

[0041] Furthermore, removing the current limiting resistor includes reducing the circuit impedance.

[0042] It should be noted that the charging process is accelerated, and the parallel-connected multi-level inverter continues to perform uncontrolled rectification charging. The equivalent resistance after the current limiting resistor is removed is reduced, and the decay rate is slowed down. The current change is divided into an undamped exponential decay process, which reflects the rising trend of the current after the current limiting resistor is removed, and the superimposed damped oscillation term, which reflects the influence of the actual inductance effect on the current waveform, which is expressed as:

[0043]

[0044] Among them, γ is the new time constant and δ is the new angular frequency.

[0045] It should also be noted that since the series-parallel converters are relatively independent in starting, considering that the parallel converter has the same voltage level as the system and can be used as a reactive compensator to provide voltage support for the system, the parallel CMI is started first. The starting method of the parallel CMI here adopts the self-excitation starting method, that is, the power grid provides active power to the sub-unit capacitors in the CMI. At this stage, some sub-units of the series converter are bypassed, so only the parallel CMI part is connected to the power grid, and the sub-unit capacitors between the two phases of the CMI are charged uncontrolled through the grid line voltage. It should be noted that in order to prevent the current from being too large at the moment of starting, a pre-starting resistor needs to be put into use in the first step of starting to limit the current. Under the current limiting condition, after the capacitor voltage of the CMI is charged to a stable value, the pre-charging resistor is cut off.

[0046] S2: Start the parallel cascade multi-level inverter control to perform uncontrolled rectification and charging on the power flow control unit and the energy control unit.

[0047] Furthermore, starting the parallel-connected cascaded multilevel inverter control includes a control voltage model of the parallel-connected cascaded multilevel inverter.

[0048] It should be noted that the voltage rise during the controlled charging process is reflected in combination with the basic characteristics of capacitor charging. By increasing the voltage, stable charging is performed. The voltage control model introduces an oscillation term to superimpose sinusoidal oscillations during the exponential decay process. In the early stage of charging, the voltage rises exponentially, reflecting the ability to respond to instantaneous load changes. The control voltage model is expressed as:

[0049]

[0050] Among them, V s is the preset voltage value, t and τ2 are different time constants, and ω is the angular frequency.

[0051] Furthermore, the uncontrolled rectification charging of the power flow control unit and the energy control unit includes an uncontrolled rectification charging current model.

[0052] It should be noted that when the power flow control unit and the energy control unit are charged with uncontrolled rectification, the characteristic of the current gradually increasing with time is displayed through the current model, and the characteristic of the capacitor in the initial charging stage is reflected through the exponential decay. The uncontrolled rectification charging current model is expressed as:

[0053]

[0054] Among them, R P is the current limiting resistor of the power flow control unit, C P is the capacitance of the power flow control unit, and θ is the angular frequency of the power flow control unit.

[0055] It should also be noted that after the sub-unit capacitor of the parallel CMI is charged to a certain value through uncontrolled rectification, the overall control of the parallel CMI is put into operation, and the sub-unit capacitor voltage is controlled to be charged to a preset value. At this point, the parallel converter part completes the entire startup process and pre-charges the series converter. After the parallel converter is started, the bypass of the series converter is released, and the PFCM and PBCM are charged by uncontrolled rectification. After they are charged to a stable value, the overall capacitor voltage stability control of the series converter is started, and the sub-unit capacitor voltage of the PFCM and PBCM is charged to a preset value. In order to ensure the smooth startup of the series converter and avoid instantaneous overcharging due to excessive charging speed, a ramp signal can be used when setting the preset value.

[0056] S3: Controlled charging of the series part of the series converter is performed, line flow control is started, and the multi-terminal flexible interconnection device is operated.

[0057] Furthermore, the controlled charging of the series portion of the series converter includes a voltage model for the controlled charging of the series portion of the series converter.

[0058] It should be noted that the voltage model uses exponential decay to make the voltage rise to the preset value, and introduces a regulation response term. Through the combination of double exponential forms, it reflects the different regulation stages of the voltage during the rising process, which is expressed as:

[0059]

[0060] Among them, V s2 is the preset voltage value of the series converter, κ is the charging speed adjustment coefficient, and λ is the charging process adjustment coefficient.

[0061] Furthermore, the line power flow control is started including the total power model.

[0062] It should be noted that the total power model reflects the basic power transmission of each line. It combines voltage, current and phase angle to reflect the basic power level of the system before any adjustment. The total power model introduces an adjustment response term and reflects the power changes of the system in different time periods through exponential decay adjustment. It describes the initial power level, adjusts the power response over time, combines power fluctuations, and responds to dynamic changes. It is expressed as:

[0063]

[0064] Where N is the total number of lines, ρ is the power flow control smoothing coefficient, μ is the power flow control adjustment coefficient, and t is the time variable. θ i are the voltage, current and phase angle of the i-th line respectively.

[0065] It should also be noted that after the capacitor voltage of the PFCM reaches a stable value, the line power flow control is started to achieve flexible control of the power flow of all lines and complete the soft start process of the MTSNOP device.

[0066] Example 2

[0067] An embodiment of the present invention provides a soft starting method for a transformerless multi-terminal flexible interconnection device. In order to verify the beneficial effects of the present invention, scientific demonstration is carried out through economic benefit calculation and simulation experiments.

[0068] The experimental preparation includes configuring an experimental system, which includes a multi-terminal flexible interconnection device, a series converter and a parallel cascaded multilevel inverter (CMI). The initial capacitor voltage required for the experiment is set to 0V. The experimental device is equipped with a current limiting resistor. The power flow control module includes a power flow control unit (PFCM) and a power control unit (PBCM). The system verifies the startup performance of the device by measuring the current, voltage and power data at different stages.

[0069] In the first stage of the experiment, the current limiting resistor was put into operation and the parallel cascade multilevel inverter (CMI) was charged with uncontrolled rectification. The current value after the current limiting resistor was put into operation was recorded to ensure that the current gradually increased within a safe range to prevent the initial current shock. In the second stage, the current limiting resistor was removed to allow the CMI to continue charging with uncontrolled rectification. The current value after the current limiting resistor was removed was recorded. In this stage, the charging speed was increased to stabilize the capacitor voltage at the calculated value. In the third stage, the control of the parallel cascade multilevel inverter (CMI) was started. Through controlled charging, the capacitor voltage reached the specified value and the CMI stable voltage was recorded at this time. This step ensured the smooth rise of voltage and the stable operation of the system. In the fourth stage, the power flow control unit (PFCM) and energy control unit (PBCM) in the series converter are uncontrolled rectified and charged, and the charging current of PFCM and PBCM is recorded to ensure that these units can operate normally in subsequent steps. In the fifth stage, the stable control of the overall capacitor voltage of the series converter is started, and the series part is controlled to charge until the capacitor voltage stabilizes at the specified value. In the sixth stage, the line power flow control is started to achieve normal operation of the system and the final power flow control power is recorded. In this stage, the precise control of the line power flow is ensured to ensure that the system can allocate power according to actual needs and give full play to the flexible interconnection advantages of MTSNOP.

[0070] As shown in Table 1, in terms of the current value after the current limiting resistor is put into operation, the current value of each test is maintained at about 5A, which means that by putting in the current limiting resistor, the peak value of the initial current is effectively limited, the impact of large current on system components is avoided, and the startup stability of the system is improved. In terms of the current value after the current limiting resistor is removed, it is reflected that the system can speed up the charging speed after the current limiting resistor is removed, so that the capacitor voltage can quickly stabilize at the calculated value, which not only improves the charging efficiency, but also avoids the energy waste and time delay caused by long-term charging. The stable voltage of CMI is maintained between 398V and 405V in each test, indicating that by starting the control of the cascaded multilevel inverter (CMI), the capacitor voltage can be accurately controlled near the set value to ensure the smooth operation of the system. The charging current of PFCM and PBCM is also within the expected range, showing the stability and reliability of the power flow control module. The final power flow control power data shows that after the system is in normal operation, the power value fluctuates between 49kW and 52kW, which shows that by starting the line power flow control, the system can allocate power according to actual needs and realize the efficient operation of the multi-terminal flexible interconnection device.

[0071] Table 1 Experimental data table

[0072]

[0073]

[0074] Example 3

[0075] Reference Figure 2 , which is an embodiment of the present invention, provides a soft start system for a transformerless multi-terminal flexible interconnection device, including: a startup and shutdown module, a CMI control module, and a power flow control module.

[0076] The input and removal module inputs the current limiting resistor, performs uncontrolled rectification and charging on the parallel cascade multi-level inverter, removes the current limiting resistor, and performs uncontrolled rectification and charging on the parallel cascade multi-level inverter; the CMI control module is used to start the control of the parallel cascade multi-level inverter, and performs uncontrolled rectification and charging on the flow control unit and the energy control unit; the flow control module is used to perform controlled charging on the series part of the series converter, start the line flow control, and operate the multi-terminal flexible interconnection device.

[0077] If the function is implemented in the form of a software functional unit and sold or used as an independent product, it can be stored in a computer-readable storage medium. Based on this understanding, the technical solution of the present invention, or the part that contributes to the prior art, or the part of the technical solution, can be embodied in the form of a software product. The computer software product is stored in a storage medium, including several instructions for a computer device (which can be a personal computer, server, or network device, etc.) to perform all or part of the steps of the methods of each embodiment of the present invention. The aforementioned storage medium includes: U disk, mobile hard disk, read-only memory (ROM, Read-Only Memory), random access memory (RAM, Random Access Memory), disk or optical disk, etc. Various media that can store program codes.

[0078] The logic and / or steps represented in the flowchart or otherwise described herein, for example, can be considered as an ordered list of executable instructions for implementing logical functions, and can be embodied in any computer-readable medium for use by an instruction execution system, device or apparatus (such as a computer-based system, a system including a processor, or other system that can fetch instructions from an instruction execution system, device or apparatus and execute instructions), or in conjunction with such instruction execution systems, devices or apparatuses. For the purposes of this specification, "computer-readable medium" can be any device that can contain, store, communicate, propagate or transmit a program for use by an instruction execution system, device or apparatus, or in conjunction with such instruction execution systems, devices or apparatuses.

[0079] More specific examples of computer-readable media (a non-exhaustive list) include the following: an electrical connection with one or more wires (electronic device), a portable computer disk case (magnetic device), a random access memory (RAM), a read-only memory (ROM), an erasable and programmable read-only memory (EPROM or flash memory), an optical fiber device, and a portable compact disk read-only memory (CDROM). In addition, the computer-readable medium may even be a paper or other suitable medium on which the program is printed, since the program may be obtained electronically, for example, by optically scanning the paper or other medium, followed by editing, deciphering, or processing in another suitable manner as necessary, and then stored in a computer memory.

[0080] It should be understood that the various parts of the present invention can be implemented by hardware, software, firmware or a combination thereof. In the above-mentioned embodiments, multiple steps or methods can be implemented by software or firmware stored in a memory and executed by a suitable instruction execution system. For example, if implemented by hardware, as in another embodiment, it can be implemented by any one of the following technologies known in the art or their combination: a discrete logic circuit having a logic gate circuit for implementing a logic function for a data signal, a dedicated integrated circuit having a suitable combination of logic gate circuits, a programmable gate array (PGA), a field programmable gate array (FPGA), etc. It should be noted that the above embodiments are only used to illustrate the technical solution of the present invention and are not limited. Although the present invention is described in detail with reference to the preferred embodiments, it should be understood by those skilled in the art that the technical solution of the present invention can be modified or replaced by equivalents without departing from the spirit and scope of the technical solution of the present invention, which should be included in the scope of the claims of the present invention.

[0081] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solutions of the present invention may be modified or replaced by equivalents without departing from the spirit and scope of the technical solutions of the present invention, which should all be included in the scope of the claims of the present invention.

Claims

1. A soft starting method for a transformerless multi-terminal flexible interconnection device, characterized in that: include: The current limiting resistor is put into operation to carry out uncontrolled rectification and charging for the parallel-connected cascade multi-level inverters; the current limiting resistor is removed to carry out uncontrolled rectification and charging for the parallel-connected cascade multi-level inverters; Start the parallel cascade multi-level inverter control to perform uncontrolled rectification and charging on the power flow control unit and the energy control unit; Carry out controlled charging of the series part of the series converter, start line flow control, and operate the multi-terminal flexible interconnection device.

2. The soft starting method of the transformerless multi-terminal flexible interconnection device according to claim 1, characterized in that: The input current limiting resistor includes inputting the current limiting resistor to limit the initial current, and the parallel cascade multi-level inverter performs uncontrolled rectification charging. The current gradually increases and decays exponentially with time. The current increases according to the undamped exponential decay law. Combined with the inductance effect in the circuit, a damped oscillation term is introduced to superimpose an oscillation component on the current on the basis of exponential decay, and the dynamic change process of the current is refined, which is expressed as: Among them, V l is the grid line voltage amplitude, R l is the current limiting resistor, C C is the capacitor of the cascaded multi-level inverter, L C is the inductance of the parallel cascade multi-level inverter, α is the time constant, β is the angular frequency, R o is the equivalent resistance after removing the current limiting resistor.

3. The soft starting method of the transformerless multi-terminal flexible interconnection device according to claim 2, characterized in that: The removal of the current limiting resistor includes reducing the circuit impedance, accelerating the charging process, and the parallel cascade multi-level inverter continues to perform uncontrolled rectification charging. The equivalent resistance after the current limiting resistor is removed is reduced, and the decay rate is slowed down. The current change is divided into an undamped exponential decay process, which reflects the trend of the current rising after the current limiting resistor is removed, and a superimposed damped oscillation term, which reflects the influence of the actual inductance effect on the current waveform, which is expressed as: Among them, γ is the new time constant and δ is the new angular frequency.

4. The soft starting method of the transformerless multi-terminal flexible interconnection device according to claim 3, characterized in that: The control of the start-up parallel cascade multilevel inverter includes a control voltage model of the parallel cascade multilevel inverter, which reflects the voltage rise during the controlled charging process. In combination with the basic characteristics of capacitor charging, the voltage is increased to perform stable charging. The voltage control model introduces an oscillation term to superimpose a sinusoidal oscillation in the exponential decay process. In the initial stage of charging, the voltage rises according to an exponential law, reflecting the response capability to instantaneous load changes. The control voltage model is expressed as: Among them, V s is the preset voltage value, t and τ2 are different time constants, and ω is the angular frequency.

5. The soft starting method of the transformerless multi-terminal flexible interconnection device according to claim 4, characterized in that: The uncontrolled rectification charging of the flow control unit and the energy control unit includes an uncontrolled rectification charging current model. When the flow control unit and the energy control unit are uncontrolled rectified and charged, the characteristic of the current gradually increasing over time is displayed by the current model, and the characteristic of the capacitor in the initial charging stage is reflected by the exponential decay. The uncontrolled rectification charging current model is expressed as: Among them, R P is the current limiting resistor of the power flow control unit, C P is the capacitance of the power flow control unit, and θ is the angular frequency of the power flow control unit.

6. The soft starting method of the transformerless multi-terminal flexible interconnection device according to claim 5, characterized in that: The controlled charging of the series part of the series converter includes a voltage model for controlled charging of the series part of the series converter. The voltage model causes the voltage to rise to a preset value through exponential decay, and introduces a regulation response term. Through a combination of double exponential forms, different regulation stages of the voltage in the rising process are reflected, which is expressed as: Among them, V s2 is the preset voltage value of the series converter, κ is the charging speed adjustment coefficient, and λ is the charging process adjustment coefficient.

7. The soft starting method of the transformerless multi-terminal flexible interconnection device according to claim 6, characterized in that: The startup line flow control includes a total power model, which reflects the basic power transmission of each line. It combines voltage, current and phase angle to reflect the basic power level of the system before any adjustment. The total power model introduces an adjustment response term and reflects the power change of the system in different time periods through exponential decay adjustment, describes the initial power level, adjusts the power response over time, combines power fluctuations, and responds to dynamic changes, which is expressed as: Where N is the total number of lines, ρ is the power flow control smoothing coefficient, μ is the power flow control adjustment coefficient, and t is the time variable. θ i are the voltage, current and phase angle of the i-th line respectively.

8. A system using the soft starting method of the transformerless multi-terminal flexible interconnection device according to any one of claims 1 to 7, characterized in that: Including input removal module, CMI control module, and flow control module; The input and removal module inputs a current limiting resistor to perform uncontrolled rectification and charging on the parallel-connected cascade multi-level inverters, removes the current limiting resistor, and performs uncontrolled rectification and charging on the parallel-connected cascade multi-level inverters; The CMI control module is used to start the parallel cascade multi-level inverter control and perform uncontrolled rectification and charging on the power flow control unit and the energy control unit; The power flow control module is used to controllly charge the series connection part of the series converter, start the line power flow control, and operate the multi-terminal flexible interconnection device.

9. A computer device comprising a memory and a processor, wherein the memory stores a computer program, wherein: When the processor executes the computer program, the steps of the soft starting method of the transformerless multi-terminal flexible interconnection device according to any one of claims 1 to 7 are implemented.

10. A computer-readable storage medium having a computer program stored thereon, characterized in that: When the computer program is executed by a processor, the steps of the soft starting method of the transformerless multi-terminal flexible interconnection device according to any one of claims 1 to 7 are implemented.

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