Network following type-network construction type control switching method and system of grid-connected inverter

Abstract

The invention belongs to the technical field of power electronics, provides a network following type-network construction type control switching method and system for a grid-connected inverter, analyzes typical network following type and network construction type control methods, and reveals instability mechanisms of the two control methods. The operation characteristics of the grid-following type and grid-constructing type grid-connected inverters are analyzed, reasons for phase mutation, current impact and power oscillation during direct switching of grid-following type and grid-constructing type control are obtained, and it is revealed that it is guaranteed that phase and current reference values before and after switching are consistent to be the key for achieving smooth switching of the two types of control; by fully combining the advantages of network following type and network construction type control, the invention provides a network following type-network construction type control smooth switching method based on frequency switching and a linear step transition function, so that a grid-connected inverter can stably operate under the wide-range change of the impedance of a power grid.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of power electronics, and particularly relates to a grid-following type-grid-constructing type control switching method and system of a grid-connected inverter. BACKGROUND

[0002] The statements in this section merely provide background information related to the application and do not necessarily constitute prior art.

[0003] With the increasingly exhausted fossil energy in the global range, the application of clean new energy represented by photovoltaic and wind power is more and more extensive. High penetration is an inevitable trend of further development of new energy power generation. However, high penetration new energy has characteristics such as dispersion, intermittence, randomness and output fluctuation, which will cause the equivalent grid impedance at the grid-connected point to fluctuate greatly, making the grid characteristics increasingly complex and variable, and bringing severe challenges to the stable and efficient operation of the new energy grid-connected power generation system.

[0004] In the face of increasingly complex and variable grid states, grid-connected inverters under existing control are difficult to adapt. At present, grid-connected inverters mainly adopt two control modes of grid-following type and grid-constructing type. The traditional grid-following type control relies on a phase-locked loop to obtain the grid phase to realize synchronization with the grid, and its characteristics can be equivalent to a controlled current source. In a strong grid, the grid-following type grid-connected inverter has fast current regulation speed and high maximum power point tracking capability, but it is easy to oscillate and unstable in a weak grid, and has insufficient stability margin. At the same time, with the access of a large number of power electronic devices, the system inertia decreases, and the grid-constructing type control with active support capability is widely used. In a weak grid or even an extremely weak grid, the grid-constructing type grid-connected inverter has good stability, but it is easy to oscillate in power and has slow power regulation speed and is difficult to realize maximum power point tracking in a strong grid. From the above analysis, it can be seen that under complex grid states, a single type of grid-connected inverter is difficult to meet the requirements of system stability and efficient operation. Therefore, it is an urgent problem to be solved for new energy grids to ensure the stable operation of grid-connected inverters under wide range of grid impedance changes.

[0005] Under different grid strengths, the dynamic and steady-state characteristics of grid-following type and grid-constructing type grid-connected inverters have certain complementarity. Therefore, how to realize smooth switching between the two controls to fully combine the advantages of the two controls and improve the stability of grid-connected inverters under wide range of grid impedance changes is of great practical significance. SUMMARY

[0006] To solve the above problems, the application provides a grid-connected inverter grid-following type-grid-forming type control switching method and system, analyzes typical grid-following type and grid-forming type control methods, and discloses instability mechanisms of the two control methods.

[0007] According to some embodiments, the first aspect of the application provides a grid-connected inverter grid-following type-grid-forming type control switching method, which adopts the following technical scheme: A grid-connected inverter grid-following type-grid-forming type control switching method comprises the following steps: When the power grid is strong, the grid-connected inverter adopts voltage source type constant power control to realize grid-following type control, and a phase-locked loop is used to track the grid angle frequency, and a current loop control is used in the inner loop; The phase-locked loop causes the equivalent output impedance of the grid-following type control grid-connected inverter to have a capacitive negative resistance region, when the grid strength decreases to weak grid, the grid-connected inverter and the inductive grid impedance interact, causing oscillation instability; The grid-connected inverter is switched to grid-forming type control based on a virtual oscillator, a voltage-current double-loop control is used in the inner loop, the angle frequency is switched from the grid-following angle frequency to the grid-forming angle frequency, the input of the grid-forming voltage outer loop is switched from the current reference value deviation to the voltage deviation, and the current reference value is switched from the grid-following reference value to the grid-forming reference value, thereby completing the switching of the grid-connected inverter from the grid-following type control to the grid-forming type control; When the grid strength increases to strong grid again, the damping of the grid-forming voltage source grid-connected power loop decreases, and the grid-connected inverter is prone to power oscillation; The grid-connected inverter is switched to grid-following type control, the angle frequency is switched from the grid-forming angle frequency to the grid-following angle frequency, and the current reference value is switched from the grid-forming reference value to the grid-following reference value, thereby completing the switching of the grid-connected inverter from the grid-forming type control to the grid-following type control.

[0008] It should be noted that the grid strength is used to measure the strength of the interaction between the grid and the connected primary equipment, including power sources, loads or various stations; the grid strength is quantitatively evaluated by the short-circuit ratio, and the criterion is that when the short-circuit ratio is greater than 3, it corresponds to strong grid, and when the short-circuit ratio is less than 3, it corresponds to weak grid; the short-circuit ratio is characterized by the equivalent grid impedance of the grid-connected point.

[0009] As a further technical limitation, before the grid-connected inverter is switched from the grid-following control to the grid-forming control, the voltage reference value of the grid-forming virtual oscillator control dynamic equation is set to the root mean square value of the grid point voltage, and during the switching process, the linear step transition function is used to switch to the root mean square value of the grid voltage to prevent the current reference value from suddenly changing.

[0010] As a further technical limitation, before the grid-connected inverter is switched from the grid-following control to the grid-forming control, the grid voltage outer loop input is the deviation of the grid-following current reference value and the grid-forming current reference value, and the current reference value output by the voltage outer loop is the same as that of the grid-following, and during the switching process, the linear step transition function is used to switch the current reference value deviation to the voltage deviation, and the current reference value is switched from the grid-following power loop output to the grid-forming voltage loop output, to ensure smooth transition of the current reference value.

[0011] As a further technical limitation, the grid-following control and the grid-forming control of the grid-connected inverter share the same current inner loop, and a linear step transition function is introduced in the control loop before the virtual oscillator dynamic equation, the grid-forming voltage outer loop and the common current inner loop to ensure smooth switching of the current reference value.

[0012] Further, the linear step transition function is:

[0013]

[0014]

[0015]

[0016] wherein, x step,k and x step,k+1 is the starting value of the first step and the first step, k k +1) step, x start and x target represent the initial value and the target value; k is the current step index, and S is the switching signal; x k is the interpolation transition value of the first step, k is a normalized interpolation coefficient; α is the total number of steps, and Δ N is the duration of each step; t is the total transition time, T e is the used transition time. t e is the used transition time.

[0017] Furthermore, the activation of the linear step transition module is controlled by a flag signal with three operating states. flag Decision, that is, when flag When = 0, the linear step transition module remains inactive, holding the current input value at the output; when flag When = 1, the linear step transition module activates smooth switching, gradually switching the initial value to the final value within a specified number of steps and time; when flag When the value is 2, the linear step transition module performs a hard switch, directly switching the initial value to the final value without the need for a smooth operation.

[0018] According to some embodiments, the second aspect of the present invention provides a grid-connected inverter grid-to-grid-structure control switching system, which adopts the following technical solution: A grid-connected inverter grid-type to grid-configuration control switching system includes: The grid-connected control module is configured to achieve grid-connected control by using voltage source constant power control when the grid is under strong grid conditions. It tracks the grid angular frequency based on phase-locked loop and uses current loop control in the inner loop. The oscillation instability module is configured as a phase-locked loop to cause the equivalent output impedance of the grid-connected inverter with grid-connected control to have a capacitive negative resistance region. When the grid strength decreases to a weak grid, the grid-connected inverter interacts with the inductive grid impedance, causing oscillation instability. The first switching module is configured to switch the grid-connected inverter to grid-building control when the grid strength decreases to a weak grid. The angular frequency is switched from the grid-following angular frequency to the grid-building angular frequency, the input of the grid-building voltage outer loop is switched from the current reference value deviation to the voltage deviation, and the current reference value is switched from the grid-following reference value to the grid-building reference value, thus completing the switching of the grid-connected inverter from grid-following control to grid-building control. The power oscillation module is configured such that when the grid-connected inverter adopts grid-type control, the damping of the grid voltage source grid-connected power loop decreases when the grid strength increases to a strong grid again, making the grid-connected inverter prone to power oscillation. The second switching module is configured to switch the grid-connected inverter to grid-following control when the grid strength is upgraded to a strong grid. The angular frequency is switched from the grid-connected angular frequency to the grid-following angular frequency, and the current reference value is switched from the grid-connected reference value to the grid-following reference value, thus completing the switching of the grid-connected inverter from grid-connected control to grid-following control.

[0019] According to some embodiments, a third aspect of the present invention provides a computer-readable storage medium, employing the following technical solution: A computer-readable storage medium having a program stored thereon, which, when executed by a processor, implements the steps in the grid-connected inverter grid-to-grid-connection control switching method as described in the first aspect of the present invention.

[0020] According to some embodiments, a fourth aspect of the present application provides an electronic device, adopting the technical scheme as follows: An electronic device comprises a memory, a processor, and a program stored in the memory and running on the processor, and the processor implements the steps in the grid-following type-grid-forming type control switching method of the grid-connected inverter according to the first aspect of the present application.

[0021] According to some embodiments, a fifth aspect of the present application provides a computer program product, adopting the technical scheme as follows: A computer program product comprises software codes, and the program in the software codes implements the steps in the grid-following type-grid-forming type control switching method of the grid-connected inverter according to the first aspect of the present application.

[0022] Compared with the prior art, the present application has the following beneficial effects: The present application effectively reveals the instability mechanism of the grid-following type and grid-forming type control under wide range of grid strength, provides an effective method for realizing smooth switching of the two types of control, points out the reason for phase mutation, current impact, and power oscillation caused by direct switching of the grid-following type and grid-forming type control, reveals the key for smooth switching of the two types of control, effectively suppresses the impact current of the grid-connected inverter in the grid-following type and grid-forming type control switching, and effectively improves the stable operation ability of the grid-connected inverter under wide range of grid impedance. BRIEF DESCRIPTION OF DRAWINGS

[0023] The accompanying drawings, which form a part of this embodiment, are used to provide a further understanding of the embodiment, and the schematic embodiment and its description of the embodiment are used to explain the embodiment, and do not constitute an improper limitation on the embodiment.

[0024] Figure 1 The flowchart of the grid-following type-grid-forming type control switching method of the grid-connected inverter in the first embodiment of the present application; Figure 2 The control block diagram of the smooth switching strategy of the grid-following type and grid-forming type control in the first embodiment of the present application; FIG. 3(a) is a simulation waveform diagram of the grid point voltage of the grid-connected inverter in the first embodiment of the present application, which is switched from the grid-following type to the grid-forming type control by using the smooth switching strategy; FIG. 3(b) is a simulation waveform diagram of the grid point current of the grid-connected inverter in the first embodiment of the present application, which is switched from the grid-following type to the grid-forming type control by using the smooth switching strategy; FIG. 4(a) is the total harmonic distortion of the Fourier analysis of the grid point voltage under the grid-following type control in the first embodiment of the present application; FIG. 4(b) is the total harmonic distortion of the Fourier analysis of the grid point voltage under the grid-forming type control in the first embodiment of the present application; Fig. 5(a) is a simulation waveform diagram of the grid-connected point voltage in the embodiment one of the present application, which is switched from the grid-forming type to the grid-following type control by using the smooth switching strategy; Fig. 5(b) is a simulation waveform diagram of the grid-connected point current in the embodiment one of the present application, which is switched from the grid-forming type to the grid-following type control by using the smooth switching strategy; Figure 6 Fig. 6 is a simulation waveform diagram of the active and reactive power in the embodiment one of the present application, which is switched from the grid-forming type to the grid-following type control by using the smooth switching strategy; Fig. 7(a) is the total harmonic distortion of the Fourier analysis of the grid-connected point current under the grid-following type control in the embodiment one of the present application; Fig. 7(b) is the total harmonic distortion of the Fourier analysis of the grid-connected point current under the grid-forming type control in the embodiment one of the present application; Figure 8 Fig. 8 is a structure block diagram of the grid-following type-grid-forming type control switching system of the grid-connected inverter in the embodiment two of the present application. DETAILED DESCRIPTION

[0025] The present application will be further described below with reference to the accompanying drawings and embodiments.

[0026] It should be noted that the following detailed description is exemplary in nature and is intended to provide further description of the present application. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs.

[0027] It is also important to note that the terms used herein are not intended to limit the exemplary embodiments of the present application, unless otherwise explicitly defined. As used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms "comprises" and / or "comprising," when used in this specification, specify the presence of stated features, steps, operations, devices, components and / or combinations thereof, but do not preclude the presence or addition of one or more other features, steps, operations, devices, components and / or combinations thereof.

[0028] In the present application, the terms such as "upper", "lower", "left", "right", "front", "back", "vertical", "horizontal", "side", "bottom" and the like indicate the orientation or positional relationship based on the orientation or positional relationship shown in the drawings, which are only the relationship words determined for the convenience of describing the structural relationship of the components or elements of the present application, and are not intended to specify any component or element in the present application, and cannot be understood as a limitation of the present application.

[0029] In this invention, terms such as "fixed connection," "connected," and "linked" should be interpreted broadly, indicating a fixed connection, an integral connection, or a detachable connection; a direct connection or an indirect connection through an intermediate medium. Those skilled in the art can determine the specific meaning of these terms in this invention based on the specific circumstances, and they should not be construed as limitations on the invention.

[0030] Where there is no conflict, the embodiments and features in the embodiments of the present invention can be combined with each other.

[0031] Example 1 Embodiment 1 of the present invention introduces a grid-connected inverter grid-type to grid-structured control switching method.

[0032] like Figure 1 The grid-connected inverter control switching method shown includes: In the case of a strong power grid, the grid-connected inverter adopts voltage source constant power control to achieve grid-following control, based on phase-locked loop to track the grid angular frequency, and the inner loop adopts current loop control; The phase-locked loop causes the equivalent output impedance of the grid-connected inverter with grid-connected control to have a capacitive negative resistance region. When the grid strength decreases to a weak grid, the grid-connected inverter interacts with the inductive grid impedance, causing oscillation and instability. The grid-connected inverter switches to grid-type control based on a virtual oscillator. The inner loop adopts voltage and current dual closed-loop control. The angular frequency switches from the grid-following angular frequency to the grid-connecting angular frequency. The input of the grid-connecting voltage outer loop switches from current reference value deviation to voltage deviation. The current reference value switches from grid-following reference value to grid-connecting reference value, thus completing the switch of the grid-connected inverter from grid-following control to grid-connecting control. When the grid strength is increased to a strong grid again, the damping of the grid-connected voltage source power loop decreases, and the grid-connected inverter is prone to power oscillation. When the grid-connected inverter switches to grid-following control, the angular frequency changes from the grid-connected angular frequency to the grid-following angular frequency, and the current reference value changes from the grid-connected reference value to the grid-following reference value, thus completing the switch of the grid-connected inverter from grid-connected control to grid-following control.

[0033] In weak grid environments, grid-connected inverters utilize grid-based control. When the grid strength increases to that of a strong grid, the inverter switches to grid-following control, improving its adaptability to a wide range of grid impedance variations. However, due to the mismatch between grid-following and grid-based control states, direct switching can lead to problems such as phase abrupt changes, current surges, and power fluctuations.

[0034] This embodiment provides a method for smooth switching of grid-connected inverters based on grid type and grid configuration, including the following steps: The voltage source type constant power control technology is adopted to realize the grid-following control, the phase-locked loop is used to track the grid angle frequency, the current loop control is adopted in the inner loop, and the power loop provides the current reference value for the current loop. The voltage source type virtual oscillator control technology is adopted to realize the grid-forming control, the voltage and frequency support is provided for the system, the voltage and current double closed loop control is adopted in the inner loop, the current inner loop is shared by the grid-following control and the grid-forming control, and the voltage outer loop provides the current reference value for the current inner loop. The reasons for causing the phase mutation, current impact and power fluctuation when the grid-following control and the grid-forming control are directly switched are analyzed, and it is revealed that ensuring the matching of the two control states is the key to realizing the smooth switching. The grid angle frequency and the angle frequency output by the virtual oscillator are connected to the same angle frequency integrator through a selection switch to generate a phase angle. The linear step transition function is established, and the initial value, final value, transition time and step number of the transition function are determined. The linear step transition function is introduced before the voltage outer loop and before the current inner loop, the angle frequency is directly switched when the control strategy is switched, and the linear step transition link is enabled to ensure the smooth switching of the phase and the current reference value.

[0035] Specifically, the grid-following and grid-forming control smooth switching method mainly comprises a main circuit, a coordinate transformation and power calculation link, a power loop, a synchronization loop and a control loop. The phase-locked loop tracks the grid voltage to obtain the grid angle frequency, the power loop obtains the grid-following current reference value, the dynamic equation of the virtual oscillator generates the voltage loop reference value and the grid-forming angle frequency, and the voltage outer loop of the grid-forming control generates the current reference value. A single angle frequency integrator is used, and only the angle frequency is switched when switching to ensure the continuity of the phase. The linear step transition link is used to ensure the smooth transition of the current reference value.

[0036] The distributed photovoltaic inverter commonly uses a two-level topology, Figure 2 the main circuit of which is a two-level grid-connected inverter topology structure diagram, L f is a filter inductance, C f is a filter capacitance, R d is a damping resistance, L g is a grid inductance. The inverter comprises three-phase bridge arms, each bridge arm comprises two switching tubes, each switching tube is directly controlled by a controller in a control system through a conditioning circuit and a driving circuit, and the output side of the bridge arm is connected to the grid through a filter.

[0037] The sampling conditioning circuit collects and conditions the grid-connected point voltage v abc , the inverter side output current i Labc and the grid current valuei gabc .

[0038] Figure 2 The power calculation section introduces three-phase... v abc , i Labc and i gabc After coordinate transformation to a two-phase rotating coordinate system d shaft and q From the diagram, the output power can be written as: (Shaft component) (1) In the formula, P and Q These represent the active and reactive power outputs of the inverter, respectively. v d , v q These are the grid connection point voltages. v abc of d shaft and q Axial components, i gd , i gq These are the three-phase power grid currents. i gabc of d shaft and q Axial components.

[0039] Network-based control includes a power loop, a phase-locked loop, and a current loop. Figure 2 The upper part of the intermediate synchronization loop is a phase-locked loop (PLL), which tracks the grid voltage to obtain the grid frequency. ω PLL The power loop output current loop reference value is determined by... Figure 2 The reference value for the grid current in the medium power loop can be written as follows: (2) In the formula, i Ldref_GFL , i Lqref_GFL They are respectively controlled by the network type. d shaft and q Shaft current reference value, P ref and Q ref These are the reference values ​​for active and reactive power, respectively.

[0040] Network-based control includes a virtual oscillator control loop, an outer voltage loop, and an inner current loop. Figure 2The lower half of the synchronous ring is a virtual oscillator control loop, and the dynamic equation of the virtual oscillator generates the voltage reference value and the grid forming angular frequency ω VOC The dynamic equation is: (3) (4) In the formula, ζ is a speed constant, k v , k i and are voltage and current regulation coefficients, respectively, V ref is the grid voltage reference value, V is the actual output voltage of the inverter, C is the virtual capacitance, ω ref is the angular frequency reference value.

[0041] Figure 2 The voltage outer loop of the control loop generates the grid forming current reference value, which can be specifically expressed as: (5) wherein, i Ldref_GFM , i Lqref_GFM and are the d axis and q axis current reference values of the grid forming control, G v (PI controller) s v dref , v qref and are the d axis and q axis components of the grid voltage reference value, ω g is the grid angular frequency, v gd , v gq and are the d axis and q axis components of the grid voltage.

[0042] As can be seen from the control loop in Figure 2 , the grid following type and the grid forming type control share the current inner loop, but there are differences in the generation of the current reference value and the phase between the two control methods. The key to smooth switching between the two controls is to ensure that the current reference value and the phase do not change abruptly before and after switching. First, it is necessary to ensure that the reference values of the grid following type and the grid forming type control remain consistent, and the steady-state operating point of the system remains unchanged before and after mode switching.​

[0043] Due to the network type control of the angular frequency reference value ω ref Set to the grid angular frequency ω g , the phase-locked loop tracks the grid voltage to obtain the network angular frequency ω PLL Therefore, the network angular frequency ω VOC is equal to the network angular frequency ω PLL . As shown in the synchronization loop of Figure 2 , the phase-locked loop and the angular frequency output by the virtual oscillator control loop are connected to the same angular frequency integrator through a selection switch, and the ω PLL and ω VOC Smooth switching of the phase angle can be ensured.

[0044] A linear step transition function is introduced in the control loop part of FIG. 3 to ensure smooth switching of the current reference value, and the linear step transition function is: (6) (7) (8) (9) Where x step,k and x step,k+1 is the starting value of the first k step and the first k +1) step, x start and x target represent the initial value and the target value; k is the current step index, and S is the switching signal; x k is the interpolation transition value of the first k step, α is the normalized interpolation coefficient; N is the total number of steps, and Δ t is the duration of each step; T is the total transition time, t e is the used transition time.

[0045] The linear step transition module is implemented in the voltage reference value of the virtual oscillator dynamic equation V ref before, and before the outer voltage outer loop and the current inner loop. For brevity,V ref The preceding linear step transition module is not explicitly shown in the diagram. Activation of the linear step transition module is indicated by a flag signal with three operating states. flag Decide: flag = 0: The linear step transition module remains inactive, keeping the current input value at the output.

[0046] flag = 1: The linear step transition module activates smooth switching, gradually switching the initial value to the final value within a specified number of steps and time.

[0047] flag = 2: The linear step transition module performs a hard switch, directly switching the initial value to the final value without the need for a smooth operation.

[0048] When the grid-connected inverter switches from grid-following mode to grid-connected mode, the input of the PI controller in the grid-connected voltage outer loop is initialized to the current reference deviation to ensure... i Ldqref_GFM Tracking before mode switching i Ldqref_GFL .when flag When the linear step transition module is activated (1=1), the controller input gradually switches from current reference deviation to voltage deviation.

[0049] In grid-connected control, due to the droop relationship between reactive power and grid-connected voltage, the steady-state grid-connected voltage typically differs from that in grid-following mode. During the switch from grid-following to grid-connected mode, the significant deviation between the voltage reference value and the grid-connected voltage, after PI regulation, causes a sudden change in the current reference value. Therefore, preprocessing of the voltage reference value is necessary. V ref Initialize to grid connection point voltage before switching. v abc The root mean square value. Activated during mode switching. V ref The preceding linear step transition module enables V ref from v abc The root mean square value is switched to the grid voltage. v gabc The root mean square value. This ensures that mode switching does not cause large voltage deviations, thus preventing sudden changes in the current reference value. Therefore, flag Setting 2 to 2 makes the current reference value from i Ldqref_GFL Switch directly i Ldqref_GFM After the switching is complete, the flag signal is reset to ensure input stability.

[0050] When the grid-connected inverter switches from grid-forming mode to grid-following mode, flag1=0 makes the input of the voltage loop controller the voltage deviation. The reference values in grid-following and grid-forming modes remain consistent, d The axis current reference values are the same, while there is a certain difference between the axis current reference values due to the droop relationship between reactive power and voltage in grid-forming control, q The axis current reference values are the same, while there is a certain difference between the axis current reference values due to the droop relationship between reactive power and voltage in grid-forming control, i Ldqref_GFM switches to i Ldqref_GFL After the switching is completed, flag2 is reset to maintain the stability of the controller input. The example method ensures smooth switching of the modes, and there is no sudden change in the current reference value and phase angle, thereby enhancing the stability of the grid-connected inverter.

[0051] The simulation results of the grid-connected point voltage, current and inverter output power by using the method introduced in the example are shown as follows: When the short-circuit ratio is 2.1, the grid-connected inverter switches from grid-following mode to grid-forming control by using the example method. FIG. 3(a) and FIG. 3(b) are respectively the simulation waveform diagrams of the grid-connected point voltage and current when the grid-connected inverter switches from grid-following mode to grid-forming control, and it can be seen that the switching process is smooth, and there is no overshoot in the voltage and current. FIG. 4(a) and FIG. 4(b) are respectively the total harmonic distortion diagrams of the grid-connected point voltage in grid-following mode and grid-forming control after Fourier analysis, and it can be seen that the total harmonic distortion of the grid-connected point voltage decreases after the mode switching. When the short-circuit ratio is 6.5, the grid-connected inverter switches from grid-forming control to grid-following control by using the example method. FIG. 5(a) and FIG. 5(b) are respectively the simulation waveform diagrams of the grid-connected point voltage and current when the grid-connected inverter switches from grid-forming control to grid-following control, and it can be seen that the switching process is smooth, and there is no overshoot in the voltage and current. Figure 6 FIG. 6(a) and FIG. 6(b) are respectively the simulation waveform diagrams of the inverter output power when the grid-connected inverter switches from grid-forming control to grid-following control, and it can be seen that the low-frequency oscillation of the power disappears after the mode switching. FIG. 7(a) and FIG. 7(b) are respectively the total harmonic distortion diagrams of the grid-connected point current in grid-forming control and grid-following control after Fourier analysis, and it can be seen that the total harmonic distortion of the grid-connected point current decreases after the mode switching.

[0052] The example effectively reveals the instability mechanism of grid-following control and grid-forming control under wide range of grid strength, provides an effective method for realizing smooth switching of the two controls, points out the reasons for the phase mutation, current impact and power oscillation caused by direct switching of grid-following control and grid-forming control, reveals the key to smooth switching of the two controls, effectively suppresses the impact current of the grid-connected inverter when switching between grid-following control and grid-forming control, and effectively improves the stable operation ability of the grid-connected inverter under wide range of grid impedance.

[0053] Example Two The embodiment two of the present application introduces a grid-connected inverter grid-following-grid-forming control switching system.

[0054] As shown in a grid-connected inverter grid-following-grid-forming control switching system in the prior art, the grid-connected inverter is controlled by the grid-following control mode when the power grid is strong, and the grid-connected inverter is controlled by the grid-forming control mode when the power grid is weak. Figure 8 The grid-connected inverter grid-following-grid-forming control switching system comprises: The grid-following control module is configured to realize the grid-following control of the grid-connected inverter by the voltage source constant power control when the power grid is strong, and to track the grid angle frequency based on the phase-locked loop, and to realize the inner loop control by the current loop. The oscillation instability module is configured to make the equivalent output impedance of the grid-connected inverter in the grid-following control mode have a capacitive negative resistance region by the phase-locked loop, and to cause the oscillation instability when the grid-connected inverter interacts with the inductive grid impedance when the power grid is weak. The first switching module is configured to switch the grid-connected inverter to the grid-forming control mode when the power grid is weak, to switch the angle frequency from the grid-following angle frequency to the grid-forming angle frequency, to switch the input of the grid-forming voltage outer loop from the current reference value deviation to the voltage deviation, and to switch the current reference value from the grid-following reference value to the grid-forming reference value, so as to complete the switching of the grid-connected inverter from the grid-following control mode to the grid-forming control mode. The power oscillation module is configured to reduce the damping of the grid-forming voltage source grid-connected power loop when the power grid is strong again after the grid-connected inverter adopts the grid-forming control mode, and to easily cause the power oscillation of the grid-connected inverter. The second switching module is configured to switch the grid-connected inverter to the grid-following control mode when the power grid is strong, to switch the angle frequency from the grid-forming angle frequency to the grid-following angle frequency, and to switch the current reference value from the grid-forming reference value to the grid-following reference value, so as to complete the switching of the grid-connected inverter from the grid-forming control mode to the grid-following control mode.

[0055] The detailed steps are the same as those of the grid-connected inverter grid-following-grid-forming control switching method provided in the embodiment one, and will not be described here.

[0056] Embodiment three The embodiment three of the present application provides a computer readable storage medium.

[0057] A computer readable storage medium has a program stored thereon, and the program is executed by a processor to implement the steps in the grid-connected inverter grid-following-grid-forming control switching method according to the embodiment one of the present application.

[0058] The detailed steps are the same as those of the grid-connected inverter grid-following-grid-forming control switching method provided in the embodiment one, and will not be described here.

[0059] Embodiment four The embodiment four of the present application provides an electronic device.

[0060] An electronic device includes a memory, a processor, and a program stored in the memory and running on the processor, and the processor implements the steps in the grid-connected inverter grid-following-grid-forming control switching method according to the embodiment one of the present application when executing the program.

[0061] The detailed steps are the same as the grid-connected inverter grid-following-grid-forming control switching method provided in the embodiment one, and will not be repeated here.

[0062] Embodiment five The embodiment five of the present application provides a computer program product.

[0063] A computer program product includes software codes, and the program in the software codes implements the steps in the grid-connected inverter grid-following-grid-forming control switching method according to the embodiment one of the present application.

[0064] The detailed steps are the same as the grid-connected inverter grid-following-grid-forming control switching method provided in the embodiment one, and will not be repeated here.

[0065] Those skilled in the art should understand that the embodiments of the present application can be provided as a method, a system, or a computer program product. Therefore, the present application can take the form of an entirely hardware embodiment, an entirely software embodiment, or an embodiment combining software and hardware aspects. Moreover, the present application can take the form of a computer program product implemented on one or more computer-usable storage media (including, but not limited to, disk storage, CD-ROMs, optical storage media, etc.) containing computer-usable program code. The solutions in the embodiments of the present application can be implemented in various computer languages, such as the object-oriented programming language Java and the interpreted scripting language JavaScript.

[0066] The present application is described with reference to flowcharts and / or block diagrams of the methods, devices (systems), and computer program products according to the embodiments of the present application. It should be understood that each flow and / or block in the flowcharts and / or block diagrams, and the combination of the flows and / or blocks in the flowcharts and / or block diagrams can be implemented by computer program instructions. These computer program instructions can be provided to a general-purpose computer, a special-purpose computer, an embedded processor, or other programmable data processing devices to produce a machine, so that the instructions executed by the computer or other programmable data processing devices produce the functions described in the flowcharts and / or block diagrams. Figure 1 The device that implements the functions specified in one flow or multiple flows and / or blocks. Figure 1 The device that implements the functions specified in one flow or multiple flows and / or blocks.

[0067] These computer program instructions may also be stored in a computer-readable storage medium that can direct a computer or other programmable data processing device to function in a particular manner, such that the instructions stored in the computer-readable storage medium produce an article of manufacture including instruction means, which are implemented in a process Figure 1 One or more processes and / or boxes Figure 1 The function specified in one or more boxes.

[0068] These computer program instructions may also be loaded onto a computer or other programmable data processing equipment to cause a series of operational steps to be performed on the computer or other programmable equipment to produce a computer-implemented process, thereby providing instructions that execute on the computer or other programmable equipment for implementing the process. Figure 1 One or more processes and / or boxes Figure 1 The steps of the function specified in one or more boxes.

[0069] Although preferred embodiments of the invention have been described, those skilled in the art, upon learning the basic inventive concept, can make other changes and modifications to these embodiments. Therefore, the appended claims are intended to be interpreted as including both the preferred embodiments and all changes and modifications falling within the scope of the invention.

[0070] Obviously, those skilled in the art can make various modifications and variations to this invention without departing from its spirit and scope. Therefore, if these modifications and variations fall within the scope of the claims of this invention and their equivalents, this invention also intends to include these modifications and variations.

[0071] The above description is merely a preferred embodiment of this practice and is not intended to limit the scope of this practice. Various modifications and variations can be made to this practice by those skilled in the art. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of this practice should be included within the protection scope of this practice.

Claims

1. A grid-connected inverter grid-following-grid-forming control switching method, characterized by, Comprising: In strong grid, grid-connected inverter adopts voltage source type constant power control to realize grid-following type control, and the inner loop adopts current loop control based on phase-locked loop tracking grid angular frequency; The phase-locked loop makes the equivalent output impedance of the grid-connected inverter in the grid-following type control appear a capacitive negative resistance region, when the grid strength decreases to weak grid, the grid-connected inverter interacts with inductive grid impedance, causing oscillation instability; The grid-connected inverter is switched to grid-forming type control based on virtual oscillator, the inner loop adopts voltage-current double closed loop control, the angular frequency is switched from grid-following angular frequency to grid-forming angular frequency, the input of grid-forming voltage outer loop is switched from current reference value deviation to voltage deviation, and the current reference value is switched from grid-following reference value to grid-forming reference value, completing the switching of the grid-connected inverter from grid-following type control to grid-forming type control; When the grid strength increases to strong grid again, the damping of grid-forming voltage source grid-connected power loop decreases, and the grid-connected inverter is prone to power oscillation; The grid-connected inverter is switched to grid-following type control, the angular frequency is switched from grid-forming angular frequency to grid-following angular frequency, and the current reference value is switched from grid-forming reference value to grid-following reference value, completing the switching of the grid-connected inverter from grid-forming type control to grid-following type control.

2. A grid-tie inverter grid-connection type-configuration type control switching method as claimed in claim 1, characterized in that, Before the grid-connected inverter is switched from grid-following type control to grid-forming type control, the voltage reference value of the dynamic equation of the grid-forming virtual oscillator control is set as the root mean square value of the grid point voltage, and in the switching process, the linear step transition function is used to switch to the root mean square value of the grid voltage, so as to prevent the sudden change of the current reference value.

3. A grid-tie inverter grid-connection type-configuration type control switching method as claimed in claim 1, characterized in that, Before the grid-connected inverter is switched from grid-following type control to grid-forming type control, the input of the grid-forming voltage outer loop is the deviation of the grid-following current reference value and the grid-forming current reference value, the current reference value output by the voltage outer loop is the same as that of the grid-following, and in the switching process, the linear step transition function is used to switch the current reference value deviation to the voltage deviation, and the current reference value is switched from the output of the grid-following power loop to the output of the grid-forming voltage loop, so as to ensure the smooth transition of the current reference value.

4. A grid-tie inverter grid-tie configuration grid-tie control switching method as claimed in claim 1, characterized in that, The grid-following type control and the grid-forming type control of the grid-connected inverter share the same current inner loop, and the linear step transition function arranged in front of the dynamic equation of the virtual oscillator, the grid-forming voltage outer loop and the common current inner loop is introduced in the control loop to ensure the smooth switching of the current reference value.

5. A grid-tie inverter grid-connection type-configuration type control switching method as claimed in claim 4, characterized in that, The linear step transition function is: wherein x step,k and x step,k+1 is the initial value of the first step, k is the initial value of the first step, k is the initial value of the first step, x start and x target represent the initial value and the target value; k is the current step index, S is the switching signal; x k is the interpolation transition value of the first step, k is the interpolation transition value of the first step, α is the normalized interpolation coefficient; N is the total number of steps, Δ t is the duration of each step; T is the total transition time, t e is the used transition time.

6. A grid-tie inverter grid-tie configuration grid-tie control switching method as claimed in claim 5, characterized in that, Activation of the linear step transition module is controlled by a flag signal having three operating states flag The decision is that when flag = 0, the linear step transition module remains inactive, keeping the current input value at the output; when flag = 1, the linear step transition module activates smooth switching, gradually switching the initial value to the final value within a specified number of steps and time; when flag = 2, the linear step transition module performs hard switching, directly switching the initial value to the final value without smooth operation.

7. A grid-connected inverter grid-type to grid-configuration type control switching system, characterized in that, Comprising: The grid-following type control module is configured to, in strong grid, the grid-connected inverter adopts voltage source type constant power control to realize grid-following type control, and the inner loop adopts current loop control based on phase-locked loop tracking grid angular frequency; The oscillation instability module is configured to the phase-locked loop makes the equivalent output impedance of the grid-connected inverter in the grid-following type control appear a capacitive negative resistance region, when the grid strength decreases to weak grid, the grid-connected inverter interacts with inductive grid impedance, causing oscillation instability; The first switching module is configured to when the grid strength decreases to weak grid, the grid-connected inverter is switched to grid-forming type control, the angular frequency is switched from grid-following angular frequency to grid-forming angular frequency, the input of grid-forming voltage outer loop is switched from current reference value deviation to voltage deviation, and the current reference value is switched from grid-following reference value to grid-forming reference value, completing the switching of the grid-connected inverter from grid-following type control to grid-forming type control; The power oscillation module is configured to reduce the damping of the grid-connected voltage source grid-connected power loop when the grid strength is improved to the strong grid again after the grid-connected inverter adopts the grid-forming control, and the grid-connected inverter is prone to power oscillation; The second switching module is configured to switch the grid-connected inverter to the grid-following type control when the grid strength is improved to the strong grid, switch the angular frequency from the grid-forming angular frequency to the grid-following angular frequency, and switch the current reference value from the grid-forming reference value to the grid-following reference value, so as to complete the switching of the grid-connected inverter from the grid-forming type control to the grid-following type control.

8. A computer-readable storage medium having stored thereon a computer program, characterized in that, The program is executed by the processor to implement the steps of the grid-following type-grid-forming type control switching method of the grid-connected inverter according to any one of claims 1-6.

9. An electronic device comprising a memory, a processor, and a computer program stored on the memory and running on the processor, characterized in that, The processor executes the program to implement the steps of the grid-following type-grid-forming type control switching method of the grid-connected inverter according to any one of claims 1-6.

10. A computer program product comprising software code, characterized in that, The program in the software code executes the steps of the grid-following type-grid-forming type control switching method of the grid-connected inverter according to any one of claims 1-6.

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