Self-adaptive network following-network construction hybrid control method and system for grid-connected converter

Abstract

The invention relates to the technical field of grid-connected converters, and particularly discloses a grid-connected converter self-adaptive grid-following and grid-constructing hybrid control method and system, and the method comprises the steps: calculating d-axis and q-axis grid-following modulation signals and a grid-following phase angle in a grid-following control part; in a network construction control part, network construction modulation signals and network construction phase angles of a d axis and a q axis are calculated; performing angle weighting on the network following phase angle and the network construction phase angle to generate a hybrid control phase angle; weighting the d-axis and q-axis network-following modulation signals and the d-axis and q-axis network-constructing modulation signals to obtain a hybrid control modulation signal; performing coordinate transformation and PWM (Pulse Width Modulation) on the phase angle of the hybrid control and the modulation signal of the hybrid control to obtain a switching control signal of the grid-connected converter of the hybrid control; the weight of the modulation signal can be dynamically adjusted along with the change of the real-time power grid impedance, so that the converter preferentially tracks a power instruction to realize the maximum output in a strong power grid working condition, and actively enhances the voltage / frequency supporting capability in a weak power grid working condition.

Description

Technical Field

[0001] The present invention relates to the technical field of grid-connected converters, and particularly to a grid-connected converter adaptive grid-following and grid-forming hybrid control method and system. Background Art

[0002] The statements in this part only provide background technical information related to the present invention and do not necessarily constitute prior art.

[0003] Grid-connected converters play a key role in new energy power generation and are widely used in systems such as micro energy, distributed generation, energy storage, AC-DC microgrids, etc. Due to the intermittency, randomness, and output volatility of new energy power generation, the grid strength often fluctuates significantly. Traditional grid-following control or grid-forming control has poor stability in weak grids and strong grids respectively and is difficult to adapt to the working conditions of variable grid strength; traditional grid-following / grid-forming switching control faces problems such as capacity allocation calculation in multi-inverter systems and has significant engineering difficulties in complex systems. Therefore, studying the grid-connected converter control scheme under large fluctuations of grid impedance under high penetration conditions is of great significance for the reliable operation of the new energy power generation grid-connected system.

[0004] Traditional hybrid control strategies synchronize the phase angles generated by droop control (classical grid-forming control GFM), fuse PQ control (classical grid-following control GFL) and droop control, and use fixed weighting coefficients. Although the system stability can be maintained, it is difficult to achieve the optimal economic benefit; at the same time, under the premise of synchronization based on droop control, the traditional method is restricted by stability constraints and it is difficult to improve the regulation freedom and control performance by changing the weighting coefficients. This method weights the modulation signals output by PQ control and droop control, improving the stability of the converter under a wide range of short circuit ratios (SCR).

[0005] However, the existing hybrid control methods adopt a fixed weighting coefficient hybrid strategy and cannot adjust the control weight in real time when the grid strength changes dynamically, resulting in too high a proportion of grid-forming control in strong grid conditions, which limits the power output capacity, and too large a weight of grid-following control in weak grid conditions, which weakens the support strength for the grid. This rigid control mode is restricted by the constraint of droop control synchronization, resulting in limited regulation freedom and it is difficult to balance system stability and economy under wide range fluctuations of grid strength, easily triggering a series of problems such as a decline in power generation efficiency and limited new energy consumption. Summary of the Invention

[0006] To solve the above problems, the present invention proposes a grid-connected converter adaptive grid-following and grid-forming hybrid control method and system, which dynamically adjusts the grid-following / grid-forming weight coefficients by real-time sensing of the grid impedance changes, enabling the converter to preferentially track the power command to achieve the maximum output under strong grid conditions, actively enhance the voltage or frequency support ability under weak grid conditions, construct a flexible control architecture matching the grid strength fluctuations, realize the coordinated optimization of the stability and energy conversion efficiency of the new energy grid-connected system, and provide an effective and reliable converter control solution for high-penetration new energy power generation.

[0007] In some embodiments, the following technical solutions are adopted: A grid-connected converter adaptive grid-following and grid-forming hybrid control method, comprising: In the grid-following control part, calculating the d-axis and q-axis grid-following modulation signals and the grid-following phase angle; In the grid-forming control part, calculating the d-axis and q-axis grid-forming modulation signals and the grid-forming phase angle; Weighting the grid-following phase angle and the grid-forming phase angle by angle to generate the phase angle of the hybrid control; weighting the d-axis and q-axis grid-following modulation signals and the d-axis and q-axis grid-forming modulation signals to obtain the modulation signal of the hybrid control; the phase angle of the hybrid control and the modulation signal of the hybrid control are subjected to coordinate transformation and PWM modulation to obtain the switching control signal of the grid-connected converter for the hybrid control. Wherein, calculating the short-circuit ratio based on the grid impedance, combining the frequency-domain model of the converter output impedance, and respectively solving the maximum critical stability threshold corresponding to different short-circuit ratios under the Nyquist stability criterion based on the frequency-domain characteristics, and the maximum critical stability threshold is used as the weight coefficient of the grid-following modulation signal at the corresponding short-circuit ratio.

[0008] As a further solution, weighting the grid-following phase angle and the grid-forming phase angle by angle to generate the phase angle of the hybrid control, specifically: Converting the grid-following phase angle and the grid-forming phase angle into unit vectors respectively, performing a linear combination on the obtained unit vectors to obtain a composite vector, and based on the projections of the composite vector on the x and y axes and , obtaining the hybrid phase angle : .

[0009] As a further solution, the composite vector is specifically: ; Wherein, , are the unit vectors respectively converted from the grid-following phase angle and the grid-forming phase angle, is the weight coefficient of the grid-following modulation signal, is the weight coefficient of the grid-forming modulation signal.

[0010] As a further solution, calculate the short-circuit ratio based on the grid impedance, specifically: ; Among them, is the rated capacity of the converter system connected to the grid, is the short-circuit capacity of the grid, is the rated effective value of the grid voltage, is the modulus value of the grid impedance.

[0011] As a further solution, under the Nyquist stability criterion based on frequency-domain characteristics, solve the maximum critical stability threshold of the grid-connected modulation signal weight coefficient corresponding to different short-circuit ratios, specifically: Adopt the Nyquist stability criterion analysis method based on frequency-domain characteristics, and sequentially draw the zero-pole distribution diagram of the closed-loop transfer function when the short-circuit ratio is , analyze the pole distribution of the closed-loop transfer function corresponding to the grid-connected control right coefficient increasing from 0 to 1, so as to determine the corresponding maximum critical stability threshold ; is the frequency-domain model of the converter output impedance, is the grid impedance.

[0012] As a further solution, the maximum critical stability threshold is used as the weight coefficient of the grid-connected modulation signal corresponding to the short-circuit ratio, specifically: If , then ; Among them, is the short-circuit ratio, is the weight coefficient of the grid-connected modulation signal, is the maximum critical stability threshold corresponding to the short-circuit ratio of i.

[0013] As a further solution, in the grid-connected control part, calculate the grid-connected modulation signals and grid-connected phase angles of the d-axis and q-axis, specifically: Based on the active power reference value, reactive power reference value and output voltage amplitude, calculate the grid-connected output current reference values of the d-axis and q-axis; Subtract the grid-connected output current reference values from the instantaneous current measurement values of the d-axis and q-axis respectively, and then through PI regulation, obtain the grid-connected modulation signals; The three-phase output voltage is transformed from the abc-dq coordinate system to obtain the grid-connected instantaneous output voltage values of the d-axis and q-axis, and the grid-connected instantaneous output voltage value of the q-axis passes through a PI controller and an integrator to obtain the grid-connected phase angle.

[0014] As a further solution, in the grid-forming control part, calculate the grid-forming modulation signals and grid-forming phase angles of the d-axis and q-axis, specifically as follows: Calculate the grid-forming output voltage reference value and grid-forming angular frequency of the d-axis through droop control; calculate the grid-forming output current reference values of the d-axis and q-axis based on the grid-forming output voltage reference value of the d-axis. The grid-forming output current reference values of the d-axis and q-axis are respectively subtracted from the instantaneous output current values of the converters on the d-axis and q-axis, and the differences pass through PI regulators to obtain the grid-forming modulation signals of the d-axis and q-axis; Integrate the grid-forming angular frequency to obtain the grid-forming phase angle.

[0015] In some other embodiments, the following technical solution is adopted: A grid-connected converter adaptive grid-following and grid-forming hybrid control system, comprising: A grid-following control module, used to calculate the grid-following modulation signals and grid-following phase angles of the d-axis and q-axis in the grid-following control part; A grid-forming control module, used to calculate the grid-forming modulation signals and grid-forming phase angles of the d-axis and q-axis in the grid-forming control part; A hybrid control module, used to generate the phase angle of hybrid control by weighting the grid-following phase angle and the grid-forming phase angle; weight the grid-following modulation signals of the d-axis and q-axis and the grid-forming modulation signals of the d-axis and q-axis to obtain the modulation signals of hybrid control; the phase angle of hybrid control and the modulation signals of hybrid control pass through coordinate transformation and PWM modulation to obtain the switching control signals of the grid-connected converter for hybrid control; Among them, calculate the short-circuit ratio based on the grid impedance, combine the frequency-domain model of the converter output impedance, and solve the maximum critical stability threshold corresponding to different short-circuit ratios under the Nyquist stability criterion based on frequency-domain characteristics. The maximum critical stability threshold is used as the weighting coefficient of the grid-following modulation signal corresponding to the short-circuit ratio.

[0016] In some other embodiments, the following technical solution is adopted: A terminal device, which includes a processor and a memory. The processor is used to implement instructions; the memory is used to store multiple instructions, and the instructions are suitable for being loaded and executed by the processor to perform the above-mentioned grid-connected converter adaptive grid-following and grid-forming hybrid control method.

[0017] Compared with the prior art, the beneficial effects of the present invention are: (1) The present invention determines the weights of the grid-following modulation signal and the grid-forming modulation signal according to the value of the short-circuit ratio, which is calculated based on the real-time grid impedance. Therefore, the weights of the modulation signals can be dynamically adjusted with the change of the real-time grid impedance, enabling the converter to preferentially track the power command to achieve the maximum output under strong grid conditions, and actively enhancing the voltage / frequency support ability under weak grid conditions, constructing a flexible control architecture that matches the fluctuations of the grid strength, realizing the coordinated optimization of the stability and energy conversion efficiency of the new energy grid-connected system, and providing an effective and reliable converter control solution for high-penetration new energy power generation.

[0018] (2) The traditional method uses fixed weight coefficients, resulting in insufficient control freedom. Even when using variable hybrid weight coefficients, the improvement of the control effect is relatively limited. The present invention performs angle weighting on the grid-following phase angle and the grid-forming phase angle, breaking through the above limitations of the traditional method. It not only retains the strong voltage / frequency support of the grid-forming mode in a weak grid but also exhibits good power output characteristics under a strong grid, thus realizing the coordinated optimization of stability and power output under wide-range fluctuations of the grid strength.

[0019] Other features and advantages of the present invention will be partially given in the following description, partially become obvious from the following description, or be understood through the practice of this aspect. Brief Description of the Drawings

[0020] Figure 1 It is a schematic diagram of the structure of the converter grid-connected system; Figure 2 It is a schematic diagram of the adaptive grid-following - grid-forming hybrid control architecture of the grid-connected converter in the embodiment of the present invention; Figure 3 It is a schematic diagram of the hybrid angle weighting in the embodiment of the present invention. Detailed Description of the Embodiment

[0021] It should be noted that the following detailed description is illustrative and aims to provide further explanation for the present application. Unless otherwise specified, all technical and scientific terms used in the present invention have the same meaning as commonly understood by those of ordinary skill in the technical field to which the present application belongs.

[0022] It should be noted that the terms used herein are only for describing the specific embodiments and are not intended to limit the exemplary embodiments according to the present application. As used herein, unless the context clearly indicates otherwise, the singular form is also intended to include the plural form. In addition, it should be understood that when the terms "comprising" and / or "including" are used in this specification, they indicate the presence of features, steps, operations, devices, components, and / or combinations thereof.

[0023] Embodiment 1 The structure of the converter grid-connected system is as Figure 1As shown, it includes a DC source, a three-phase bridge PWM inverter, a filter inductor and a filter capacitor to form an LC filter, a grid-side inductor and an AC grid.

[0024] Traditional hybrid methods synchronize based on the phase angles generated by droop control In the grid-following control part and the grid-forming control part, modulation signals are obtained respectively, and then the modulation signals obtained from the grid-following and grid-forming controls are weighted; however, the weight coefficients of the two are usually fixed and cannot be adjusted in real time when the grid strength changes dynamically, resulting in an excessive proportion of grid-forming control in strong grid conditions, which limits the power output capacity, and an excessive weight of grid-following control in weak grid conditions, which weakens the support strength for the grid.

[0025] Based on this, in one or more embodiments, a grid-connected converter adaptive grid-following-grid-forming hybrid control method is disclosed, combined with Figure 2 , specifically including the following process: S101: In the grid-following control part, calculate the d-axis and q-axis grid-following modulation signals and the grid-following phase angle.

[0026] Specifically, in the grid-following control part, given the active and reactive power reference values , and the output voltage amplitude , calculate according to the instantaneous power: , to obtain the d-axis and q-axis grid-following output current reference values and , and then subtract them from the instantaneous current measurement values and respectively, and after PI regulation, obtain the grid-following modulation signals and ; where, ; , respectively represent the instantaneous output voltage values of the d-axis and q-axis converters.

[0027] The phase-locked loop adopts a typical synchronous rotating coordinate structure, and the three-phase output voltage is transformed from the abc-dq coordinate to obtain the d-axis and q-axis grid-following instantaneous output voltage values and , and the q-axis component passes through a PI controller and an integrator to obtain the grid-following phase angle .

[0028] S102: In the grid-forming control part, calculate the d-axis and q-axis grid-forming modulation signals and the grid-forming phase angle.

[0029] Specifically, in the grid-forming control section, the calculation formula for droop control is as follows: ; Wherein, 、 、 、 respectively represent the grid-forming output voltage reference value on the d-axis, the reactive power droop coefficient, the actual reactive power output of the converter, and the given voltage reference value, 、 、 、 respectively represent the grid-forming angular frequency, the active power droop coefficient, the actual active power output of the converter, and the angular frequency reference value.

[0030] 、 The calculation formulas of ; 、 are respectively the grid-forming voltage deviation value and the grid-forming angular frequency deviation value, and ; Integrating gives the grid-forming phase angle .

[0031] 、0 are respectively subtracted from the instantaneous output voltage values 、 of the converter on the d-axis and q-axis. The difference passes through a PI regulator to obtain the grid-forming output current reference values and on the d-axis and q-axis; 、 are respectively subtracted from the instantaneous output current values 、 of the converter on the d-axis and q-axis. The difference passes through a PI regulator to obtain the grid-forming modulation signals and on the d-axis and q-axis.

[0032] S103: Weight the grid-following phase angle and the grid-forming phase angle to generate the phase angle for hybrid control; weight the grid-following modulation signals on the d-axis and q-axis and the grid-forming modulation signals on the d-axis and q-axis to obtain the modulation signal for hybrid control; the phase angle for hybrid control and the modulation signal for hybrid control undergo coordinate transformation and PWM modulation to obtain the switching control signal for the grid-connected converter in hybrid control.

[0033] Traditional methods use fixed weight coefficients and have insufficient control degrees of freedom. If one attempts to use variable hybrid weight coefficients within the framework of traditional methods (generating phase angle synchronization through droop control), the improvement in control effect is also limited. For example, in a strong power grid, the ideal situation is that the grid-following weight coefficient is 1 and the grid-forming weight coefficient is 0, which can improve the new energy consumption rate of the converter. However, restricted by stability constraints, the grid-following weight coefficient cannot be set to 1 under the premise of droop synchronization.

[0034] In this embodiment, an adjustable hybrid control based on hybrid phase angle synchronization is adopted, which breaks through the above limitations of traditional methods. It not only retains the strong voltage / frequency support in the grid-forming mode in a weak power grid (grid-forming weight coefficient = 1, grid-following weight coefficient = 0, equivalent to grid-forming control), but also exhibits good power output characteristics in a strong power grid (grid-following weight coefficient = 1, grid-forming weight coefficient = 0, equivalent to grid-following control), thereby achieving the coordinated optimization of stability and power output under the condition of wide-range fluctuations in grid strength.

[0035] Specifically, the hybrid angle weighting structure is as Figure 3 shown. Through angle weighting, the phase angle of hybrid control is generated ; First, the vector space projection method is used to convert , into unit vectors: ; ; , are the unit vectors obtained by converting the grid-following phase angle and the grid-forming phase angle respectively. The vectors , are linearly combined to obtain the composite vector . The projections of on the x and y axes are , respectively, and the hybrid phase angle .

[0036] is the weight coefficient of the grid-following modulation signal, is the weight coefficient of the grid-forming modulation signal. The process of determining the value of k is described in detail later.

[0037] In this embodiment, the modulation signals obtained from grid-following and grid-forming controls are weighted to obtain the modulation signal of hybrid control: ; Among them, is the weight coefficient of the grid-following modulation signal, is the weight coefficient of the grid-forming modulation signal. The modulation signal of hybrid control , and phase angle After dq / abc coordinate transformation, the input is subjected to PWM modulation to obtain the switching control signal for hybrid control.

[0038] In this embodiment, the weight coefficient of the grid-following modulation signal is determined as follows: Based on the grid impedance, calculate the short-circuit ratio. Combining with the frequency-domain model of the converter output impedance, under the Nyquist stability criterion based on frequency-domain characteristics, solve the maximum critical stability threshold corresponding to different short-circuit ratios respectively, and use the maximum critical stability threshold as the weight coefficient of the grid-following modulation signal corresponding to the short-circuit ratio.

[0039] As a specific implementation, considering that the grid impedance is usually inductive, the formula for the grid impedance is , represents the grid inductance, and the modulus of the grid impedance and the grid inductance The relationship is , is the angular frequency reference value.

[0040] Calculate the SCR by detecting the grid impedance, , where is the rated capacity of the converter system connected to the grid, is the short-circuit capacity of the grid, is the rated effective value of the grid voltage, is the modulus of the grid impedance.

[0041] Construct the frequency-domain model of the converter output impedance under grid-following and grid-forming hybrid control Specifically:

[0042] Among them, represents the complex variable, represents the imaginary unit; , represent the weight coefficients of the grid-following modulation signal and the grid-forming modulation signal respectively; is the filter inductor, is the DC source voltage, is the PWM coefficient; , are the grid-following output current reference values on the d-axis and q-axis respectively, , are the grid-forming active droop coefficient and the reactive droop coefficient respectively; is the angular frequency reference value, is the voltage reference value, is the fundamental voltage, is the phase angle of the fundamental current; represents the transfer function of the grid-following control current loop, , respectively represent the transfer functions of the grid-forming control current loop and voltage loop, represents the transfer function of the phase-locked loop.

[0043] Adopt the Nyquist stability criterion analysis method based on frequency domain characteristics, and sequentially plot the zero-pole distribution diagram of the closed-loop transfer function when, analyze the weight coefficient of the grid-following modulation signal as it increases from 0 to 1, the pole distribution of the corresponding closed-loop transfer function, so as to determine the corresponding maximum critical stability threshold .

[0044] Determine the weights of the grid-following modulation signal and the grid-forming modulation signal according to the short-circuit ratio value: ; For example: when SCR = 1, i = 1, at this time, , then .

[0045] Thus, on the premise of ensuring that the interaction process between the power electronic device and the power grid does not induce oscillation instability, the advantage of the grid-following control for flexible regulation of new energy is maintained to the greatest extent.

[0046] In this embodiment, by real-time sensing the change of the grid impedance and dynamically adjusting the grid-following and grid-forming weight coefficients, the converter gives priority to tracking the power command to achieve the maximum output under strong grid conditions, and actively enhances the voltage / frequency support ability under weak grid conditions, constructs a flexible control architecture matching the grid strength fluctuation, realizes the collaborative optimization of the stability and energy conversion efficiency of the new energy grid-connected system, and provides an effective and reliable converter control solution for high-penetration new energy power generation.

[0047] Embodiment 2 In one or more embodiments, a grid-connected converter adaptive grid-following-grid-forming hybrid control system is disclosed, which specifically includes: A grid-following control module, which is used to calculate the d-axis and q-axis grid-following modulation signals and the grid-following phase angle in the grid-following control part; A grid-forming control module, which is used to calculate the d-axis and q-axis grid-forming modulation signals and the grid-forming phase angle in the grid-forming control part; A hybrid control module, which is used to generate the phase angle of the hybrid control by angle weighting the grid-following phase angle and the grid-forming phase angle; weight the d-axis and q-axis grid-following modulation signals and the d-axis and q-axis grid-forming modulation signals to obtain the modulation signal of the hybrid control; the phase angle of the hybrid control and the modulation signal of the hybrid control are subjected to coordinate transformation and PWM modulation to obtain the switching control signal of the grid-connected converter for hybrid control; Among them, the short-circuit ratio is calculated based on the grid impedance. Combining with the frequency-domain model of the converter output impedance, under the Nyquist stability criterion based on frequency-domain characteristics, the maximum critical stability thresholds corresponding to different short-circuit ratios are respectively solved, and the maximum critical stability thresholds are used as the weight coefficients of the grid-following modulation signals under the corresponding short-circuit ratios.

[0048] It should be noted that the specific implementation manners of the above-mentioned modules are exactly the same as those in Embodiment 1 and will not be elaborated here.

[0049] Embodiment 3 In one or more embodiments, a terminal device is disclosed, which includes a processor and a memory. The processor is used to implement instructions; the memory is used to store multiple instructions, and the instructions are adapted to be loaded and executed by the processor to perform the grid-connected converter adaptive grid-following and grid-forming hybrid control method described in Embodiment 1.

[0050] It should be understood that in this embodiment, the processor may be a central processing unit CPU, and the processor may also be other general-purpose processors, digital signal processors DSP, application-specific integrated circuits ASIC, off-the-shelf programmable gate arrays FPGA or other programmable logic devices, discrete gate or transistor logic devices, discrete hardware components, etc. The general-purpose processor may be a microprocessor or the processor may also be any conventional processor, etc.

[0051] The memory may include a read-only memory and a random access memory, and provide instructions and data to the processor. A part of the memory may also include a non-volatile random access memory. For example, the memory may also store information about the device type.

[0052] In the implementation process, each step of the above method may be completed by the integrated logic circuit in the hardware of the processor or the instructions in the form of software.

[0053] Although the specific implementation manners of the present invention have been described above in conjunction with the accompanying drawings, it is not a limitation to the protection scope of the present invention. Those skilled in the art should understand that based on the technical solutions of the present invention, various modifications or deformations that can be made by those skilled in the art without creative efforts are still within the protection scope of the present invention.

Claims

1. A grid-connected converter adaptive grid-following-grid-building hybrid control method, characterized in that: include: In the grid-following control part, the d-axis and q-axis grid-following modulation signals and grid-following phase angles are calculated; In the network control part, the network modulation signal and network phase angle of the d-axis and q-axis are calculated; The grid-following phase angle and the grid-forming phase angle are weighted by angle to generate a hybrid controlled phase angle; the d-axis and q-axis grid-following modulation signals and the d-axis and q-axis grid-forming modulation signals are weighted to obtain a hybrid controlled modulation signal; the hybrid controlled phase angle and the hybrid controlled modulation signal are subjected to coordinate transformation and PWM modulation to obtain a hybrid controlled grid-connected converter switch control signal; Among them, the short-circuit ratio is calculated based on the grid impedance, and combined with the frequency domain model of the converter output impedance, the maximum critical stability thresholds corresponding to different short-circuit ratios are solved respectively under the Nyquist stability criterion based on frequency domain characteristics. The maximum critical stability threshold is used as the weight coefficient of the grid-following modulation signal under the corresponding short-circuit ratio.

2. The method for adaptively following the grid and building the grid of a grid-connected converter according to claim 1, characterized in that: The phase angle of the grid-following phase angle and the grid-building phase angle are weighted by angle to generate the phase angle of the mixed control, specifically: The phase angle of the grid and the phase angle of the grid are converted into unit vectors respectively, and the obtained unit vectors are linearly combined to obtain a synthetic vector. Based on the projection of the synthetic vector on the x and y axes and , and the mixing phase angle is obtained : 。 3. The method for adaptively following the grid and building the grid of a grid-connected converter according to claim 2, characterized in that: The synthetic vector is specifically: ； in, , are the unit vectors obtained by transforming the phase angle of the grid and the phase angle of the grid, is the weight coefficient of the network modulation signal, is the weight coefficient of the network modulation signal.

4. The method for adaptively following the grid and building the grid of a grid-connected converter according to claim 1, characterized in that: The short-circuit ratio is calculated based on the grid impedance, specifically: ； in, is the rated capacity of the converter system connected to the grid, is the short-circuit capacity of the power grid, is the rated effective value of the grid voltage, is the grid impedance modulus.

5. The method for adaptively following the grid and building the grid of a grid-connected converter according to claim 1, characterized in that: Under the Nyquist stability criterion based on frequency domain characteristics, the maximum critical stability threshold of the weight coefficient of the grid modulation signal corresponding to different short-circuit ratios is solved respectively, which is: The Nyquist stability criterion analysis method based on frequency domain characteristics is used to plot the short circuit ratio Closed loop transfer function Zero-pole distribution diagram of the network control weight coefficient The closed-loop transfer function pole distribution corresponding to the increase from 0 to 1 is determined The maximum critical stability threshold corresponding to ; is the frequency domain model of the converter output impedance, is the grid impedance.

6. A grid-connected converter adaptive grid-following-grid-building hybrid control method as claimed in claim 1 or 5, characterized in that: The maximum critical stability threshold is used as the weight coefficient of the network modulation signal under the corresponding short circuit ratio, specifically: like ,but ; in, is the short circuit ratio, is the weight coefficient of the network modulation signal, is the maximum critical stability threshold corresponding to the short circuit ratio i.

7. The method for adaptively following the grid and building the grid of a grid-connected converter according to claim 1, characterized in that: In the grid-following control part, the d-axis and q-axis grid-following modulation signals and grid-following phase angles are calculated, specifically: Based on the active power reference value, the reactive power reference value and the output voltage amplitude, the grid-following output current reference values ​​of the d-axis and the q-axis are calculated; The grid-following output current reference value is subtracted from the instantaneous current measurement values ​​of the d-axis and the q-axis respectively, and then subjected to PI regulation to obtain a grid-following modulation signal; Three-phase output voltage The instantaneous grid-following output voltage values ​​of the d-axis and q-axis are obtained through abc-dq coordinate transformation, and the grid-following phase angle is obtained through a PI controller and an integrator through the instantaneous grid-following output voltage value of the q-axis.

8. The method for adaptively following the grid and building the grid of a grid-connected converter according to claim 1, characterized in that: In the network control part, the network modulation signal and network phase angle of the d-axis and q-axis are calculated, specifically: The d-axis grid-forming output voltage reference value and the grid-forming angular frequency are calculated by droop control; the d-axis and q-axis grid-forming output current reference values ​​are calculated based on the d-axis grid-forming output voltage reference value, and the d-axis and q-axis grid-forming output current reference values ​​are respectively subtracted from the d-axis and q-axis converter instantaneous output current values, and the difference is passed through a PI regulator to obtain the d-axis and q-axis grid-forming modulation signals; The meshing phase angle is obtained by integrating the meshing angular frequency.

9. A grid-connected converter adaptive grid-following-grid-building hybrid control system, characterized in that: include: A grid-following control module is used to calculate the d-axis and q-axis grid-following modulation signals and grid-following phase angles in the grid-following control part; A network control module is used to calculate the network modulation signal and network phase angle of the d-axis and q-axis in the network control part; A hybrid control module is used to weight the grid-following phase angle and the grid-forming phase angle by angle to generate a hybrid controlled phase angle; weight the d-axis and q-axis grid-following modulation signals and the d-axis and q-axis grid-forming modulation signals to obtain a hybrid controlled modulation signal; the hybrid controlled phase angle and the hybrid controlled modulation signal are subjected to coordinate transformation and PWM modulation to obtain a hybrid controlled grid-connected converter switch control signal; Among them, the short-circuit ratio is calculated based on the grid impedance, and combined with the frequency domain model of the converter output impedance, the maximum critical stability thresholds corresponding to different short-circuit ratios are solved respectively under the Nyquist stability criterion based on frequency domain characteristics. The maximum critical stability threshold is used as the weight coefficient of the grid-following modulation signal under the corresponding short-circuit ratio.

10. A terminal device, comprising a processor and a memory, wherein the processor is used to implement instructions; and the memory is used to store multiple instructions, characterized in that: The instructions are suitable for being loaded by a processor and executing the grid-connected converter adaptive grid-following-grid-building hybrid control method described in any one of claims 1-8.

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