Grid-connected inverter dual-mode smooth switching control method suitable for power grid distortion condition

Abstract

The invention relates to the technical field of electric energy quality control, and discloses a grid-connected inverter dual-mode smooth switching control method suitable for a power grid distortion condition, and the method comprises the steps: operating a grid-connected inverter in a current source control mode, and collecting a network access current and a grid-connected point voltage; determining a system matrix and an output matrix of the grid-connected inverter; discretizing the system matrix and the output matrix, substituting the discretized matrix into a Kalman filter, and obtaining a fundamental wave voltage observation value and a fundamental wave current observation value through the Kalman filter; synthesizing the fundamental voltage observation value and the grid-connected point voltage into a harmonic voltage observation value; setting a system short-circuit ratio boundary value lambda of the grid-connected inverter, and obtaining a system short-circuit ratio SCR; and when it is judged that the current source control mode is switched, the grid-connected inverter is switched to operate in a voltage source control mode. According to the invention, the influence of the power grid fault on the grid-connected inverter can be reduced, and the reliability and stability of the whole power system are improved.

Description

Technical Field

[0001] The present invention relates to the technical field of power quality control, and in particular to a dual-mode smooth switching control method of a grid-connected inverter suitable for use under grid distortion conditions. Background Art

[0002] As the core equipment for connecting renewable energy to the power grid, grid-connected inverters play a vital role. At present, grid-connected inverters are mainly divided into two categories: grid-following grid-connected inverters and grid-building grid-connected inverters. Grid-following grid-connected inverters operate in current source mode, which observes the grid voltage phase through a phase-locked loop and controls the current amplitude and phase accordingly to achieve grid-connected output power regulation. Due to its excellent dynamic stability, current source mode controlled grid-connected inverters have been widely used in strong power grid environments; however, under weak power grid conditions, their output current characteristics will decrease.

[0003] The grid-connected inverter operates in the voltage source mode, which uses a droop control strategy to regulate the voltage amplitude and phase to achieve the purpose of regulating the output grid-connected power. The grid-connected inverter can improve the voltage and frequency support capabilities of the converter, thereby enhancing the stability of the power system. In view of the differences in the characteristics of the current source control mode and the voltage source control mode under different grid strengths, the appropriate grid-connected inverter working mode can be selected according to the actual situation of the grid strength.

[0004] At present, there are related studies on the control method of smooth switching between the current source and voltage source dual modes of grid-connected inverters. However, the existing method based on the synchronous reference frame phase-locked loop (SRF-PLL) is restricted in its seamless switching capability when the grid is distorted due to the limited control bandwidth of the phase-locked loop. Environmental factors have a significant impact on the stability of the SRF-PLL. Under ideal grid voltage conditions, the SRF-PLL can provide fast and accurate phase-locking results; however, when the grid voltage changes suddenly or the harmonic content is high, the acquisition of its phase information will no longer be accurate. Although the SRF-PLL can have a lower cutoff frequency by adjusting the PI parameters to achieve phase locking of the three-phase voltage, the lower cutoff frequency will result in a slower phase locking speed. In order to achieve accurate phase locking under grid distortion conditions, the PLL structure can be changed to improve its phase locking performance under distorted grid conditions; however, the increased complexity of the PLL structure will bring a greater computational burden.

[0005] Therefore, it is necessary to design a dual-mode smooth switching control method for grid-connected inverters under grid distortion conditions to solve the problems existing in the current technology. Summary of the invention

[0006] In view of this, the present invention proposes a dual-mode smooth switching control method for a grid-connected inverter under grid distortion conditions, aiming to solve the problem of poor reliability and stability of the power system due to the impact of grid faults on the grid-connected inverter in the current technology.

[0007] The present invention proposes a dual-mode smooth switching control method for a grid-connected inverter under grid distortion conditions, comprising the following steps:

[0008] S100: operates the grid-connected inverter in the current source control mode and collects the grid current and grid connection point voltage;

[0009] S200: Determine a system matrix and an output matrix of the grid-connected inverter;

[0010] S300: discretizing the system matrix and the output matrix and inputting them into a Kalman filter, and obtaining a fundamental voltage observation value and a fundamental current observation value through the Kalman filter;

[0011] S400: synthesizing the fundamental voltage observation value and the grid connection point voltage into a harmonic voltage observation value;

[0012] S500: using the fundamental voltage observation value as a feedback control quantity of the PI regulator in the voltage source control mode and an input quantity of the phase-locked loop in the current source control mode, and recording the input quantity of the phase-locked loop as a first input quantity; using the fundamental voltage observation value and the fundamental current observation value as input quantities of a power calculation module in the voltage source control mode, and recording the input quantity of the power calculation module as a second input quantity;

[0013] S600: Setting a system short circuit ratio boundary value λ of the grid-connected inverter, and obtaining a system short circuit ratio SCR;

[0014] S700: Compare the system short circuit ratio SCR with the system short circuit ratio boundary value λ, and determine whether to switch the current source control mode according to the comparison result;

[0015] S800: When it is determined that the current source control mode is to be switched, the grid-connected inverter is switched to operate in the voltage source control mode.

[0016] Furthermore, the grid-connected inverter is an LCL type grid-connected inverter.

[0017] Furthermore, the LCL type grid-connected inverter includes an LCL filter, and the topological structure of the LCL filter includes:

[0018] The bridge arm side inductor is located at the output end of the grid-connected inverter and is connected to the output bridge arm of the grid-connected inverter;

[0019] A grid-side inductor, one end of which is connected to the bridge arm-side inductor, and the other end of which is connected to the grid as a filter output;

[0020] A filter capacitor is connected in parallel with the bridge arm side inductor.

[0021] Furthermore, when determining the system matrix and output matrix of the grid-connected inverter, it includes:

[0022] The system matrix:

[0023]

[0024] The output matrix:

[0025] C = (1 0 1 0);

[0026] Where A represents the system matrix; ω g represents the fundamental angular frequency of the power grid; C represents the output matrix.

[0027] Furthermore, when the system matrix and the output matrix are discretized and then input into the Kalman filter, it includes:

[0028] Discretize the system matrix as follows:

[0029]

[0030] Among them, A d represents the state transfer matrix associated with a specific linear system; A represents the system matrix; Indicates the sampling time.

[0031] Furthermore, when the fundamental voltage observation value and the grid connection point voltage are synthesized into a harmonic voltage observation value, it includes:

[0032]

[0033] in, Indicates the observed value of harmonic voltage; Indicates the grid connection point voltage; Indicates the fundamental voltage observation value.

[0034] Further, comparing the system short circuit ratio SCR with the system short circuit ratio boundary value λ, and judging whether to switch the current source control mode according to the comparison result, comprises:

[0035] When the system short circuit ratio SCR is greater than the system short circuit ratio boundary value λ, it is determined that the grid-connected inverter maintains operating in the current source control mode;

[0036] When the system short circuit ratio SCR is less than or equal to the system short circuit ratio boundary value λ, it is determined that the grid-connected inverter is switched to the voltage source control mode for operation.

[0037] Furthermore, the system short circuit ratio SCR is obtained by the following formula:

[0038]

[0039] Among them, SCR represents the system short circuit ratio; P dN Indicates the rated DC output power, L g Represents the grid impedance.

[0040] Furthermore, the system short circuit ratio boundary value λ is 3.

[0041] Compared with the prior art, the beneficial effects of the present invention are as follows: the present invention operates the grid-connected inverter in the current source control mode, and collects the grid current and the grid connection point voltage, which can effectively monitor the grid state and respond to the distortion of the grid in a timely manner; the use of the Kalman filter to process these signals can improve the measurement accuracy and the stability of the system; by setting the system short-circuit ratio boundary value λ, a clear switching threshold can be provided for the grid-connected inverter to ensure that when the grid conditions change, the inverter can smoothly switch from the current source control mode to the voltage source control mode, thereby ensuring the stable operation of the grid and the power quality.

[0042] The dual-mode smooth switching control method for a grid-connected inverter under grid distortion conditions provided by the present invention can effectively cope with grid distortion conditions and ensure the stable operation of the grid-connected inverter under various working conditions; by real-time monitoring of the system short-circuit ratio SCR and timely switching of the control mode, the method can reduce the impact of grid faults on the grid-connected inverter and improve the reliability and stability of the entire power system. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Various other advantages and benefits will become apparent to those of ordinary skill in the art by reading the detailed description of the preferred embodiments below. The accompanying drawings are only for the purpose of illustrating the preferred embodiments and are not to be considered as limiting the present invention. Moreover, the same reference symbols are used throughout the accompanying drawings to represent the same components. In the accompanying drawings:

[0044] Figure 1 A flow chart of a dual-mode smooth switching control method for a grid-connected inverter under grid distortion conditions provided in an embodiment of the present invention.

[0045] Figure 2 is a topological structure diagram of a grid-connected inverter provided by an embodiment of the present invention;

[0046] Figure 3It is a general control block diagram of seamless switching of a grid-connected inverter applicable to grid distortion conditions provided by an embodiment of the present invention;

[0047] Figure 4 is a flow chart of a Kalman filter algorithm provided by an embodiment of the present invention;

[0048] Figure 5 It is a system closed-loop pole position diagram when the filter parameter drifts when the dual-mode smooth switching control method of the grid-connected inverter suitable for grid distortion conditions of the present invention is adopted;

[0049] Figure 6 The grid-connected current and grid-connected point voltage waveforms when the grid-connected inverter switches from the current source control mode to the voltage source control mode when the dual-mode smooth switching control method of the grid-connected inverter under grid distortion conditions of the present invention is adopted;

[0050] Figure 7 The invention discloses a grid-connected current and grid-connected point voltage waveform when the grid-connected inverter switches from a voltage source control mode to a current source control mode when the dual-mode smooth switching control method of the grid-connected inverter under grid distortion conditions is adopted. DETAILED DESCRIPTION

[0051] Exemplary embodiments of the present disclosure will be described in more detail below with reference to the accompanying drawings. Although exemplary embodiments of the present disclosure are shown in the accompanying drawings, it should be understood that the present disclosure can be implemented in various forms and should not be limited by the embodiments described herein. On the contrary, these embodiments are provided in order to enable a more thorough understanding of the present disclosure and to fully convey the scope of the present disclosure to those skilled in the art. It should be noted that, in the absence of conflict, the embodiments of the present invention and the features described in the embodiments can be combined with each other. The present invention will be described in detail below with reference to the accompanying drawings and in combination with the embodiments.

[0052] See also Figure 1 As shown, in some embodiments of the present application, this embodiment provides a dual-mode smooth switching control method for a grid-connected inverter under grid distortion conditions, comprising the following steps:

[0053] S100: operates the grid-connected inverter in the current source control mode and collects the grid current and grid connection point voltage;

[0054] S200: Determine a system matrix and an output matrix of the grid-connected inverter;

[0055] S300: discretizing the system matrix and the output matrix and inputting them into a Kalman filter, and obtaining a fundamental voltage observation value and a fundamental current observation value through the Kalman filter;

[0056] S400: synthesizing the fundamental voltage observation value and the grid connection point voltage into a harmonic voltage observation value;

[0057] S500: using the fundamental voltage observation value as a feedback control quantity of the PI regulator in the voltage source control mode and an input quantity of the phase-locked loop in the current source control mode, and recording the input quantity of the phase-locked loop as a first input quantity; using the fundamental voltage observation value and the fundamental current observation value as input quantities of a power calculation module in the voltage source control mode, and recording the input quantity of the power calculation module as a second input quantity;

[0058] S600: Setting a system short circuit ratio boundary value λ of the grid-connected inverter, and obtaining a system short circuit ratio SCR;

[0059] S700: Compare the system short circuit ratio SCR with the system short circuit ratio boundary value λ, and determine whether to switch the current source control mode according to the comparison result;

[0060] S800: When it is determined that the current source control mode is to be switched, the grid-connected inverter is switched to operate in the voltage source control mode.

[0061] In this embodiment, it is possible to ensure that the voltage phases before and after the switching control are the same without changing the phase-locked loop structure and without increasing the calculation burden.

[0062] In this embodiment, under ideal grid conditions, the voltage phase and current phase of the grid-connected inverter before and after mode switching are consistent, and the process of switching the working mode is smooth and seamless.

[0063] In this embodiment, under distorted grid conditions, the voltage phase and current phase of the grid-connected inverter before and after mode switching are consistent, and the process of switching the working mode is smooth and seamless.

[0064] It can be seen that the embodiment of the present invention provides a dual-mode smooth switching control method for a grid-connected inverter under grid distortion conditions, which is applicable to a three-phase DC / AC grid-connected inverter system and can smoothly switch the control mode of the grid-connected inverter under grid distortion conditions.

[0065] It is understandable that this implementation uses a power generation system including a 3.5kW three-phase grid-connected inverter with a capacity of 3.5kW as an example to illustrate how to achieve dual-mode smooth switching of the grid-connected inverter under grid distortion conditions. Figure 2 , where L1, L2 and C1 represent the LCL filter parameters, L g Represents the grid impedance. dc is the DC side voltage of the grid-connected inverter, u PCC is the grid voltage, u gis the grid voltage, which contains 3rd, 5th, 7th, 9th, and 11th background harmonics, with harmonic contents accounting for 12%, 12%, 12%, 10%, and 10% of the fundamental wave, respectively. The switching frequency of the grid-connected inverter is f s The power generation system adopts SVPWM modulation, and the parameters of the power generation system are shown in Table 1.

[0066] Table 1

[0067] System Parameters

[0068]

[0069] It can be seen that operating the grid-connected inverter in the current source control mode and collecting the grid current and grid connection point voltage can effectively monitor the grid status and respond to grid distortion in a timely manner; using the Kalman filter to process these signals can improve the measurement accuracy and system stability; by setting the system short-circuit ratio boundary value λ, a clear switching threshold can be provided for the grid-connected inverter to ensure that when the grid conditions change, the inverter can smoothly switch from the current source control mode to the voltage source control mode, thereby ensuring the stable operation of the grid and the power quality.

[0070] It can be understood that the dual-mode smooth switching control method for the grid-connected inverter under grid distortion conditions provided in this embodiment can effectively cope with the distortion of the grid and ensure the stable operation of the grid-connected inverter under various working conditions; by real-time monitoring of the system short-circuit ratio SCR and timely switching of the control mode, this method can reduce the impact of grid faults on the grid-connected inverter and improve the reliability and stability of the entire power system.

[0071] Specifically, the grid-connected inverter is an LCL-type grid-connected inverter.

[0072] Specifically, the LCL type grid-connected inverter includes an LCL filter, and the topological structure of the LCL filter includes:

[0073] The bridge arm side inductor is located at the output end of the grid-connected inverter and is connected to the output bridge arm of the grid-connected inverter;

[0074] A grid-side inductor, one end of which is connected to the bridge arm-side inductor, and the other end of which is connected to the grid as a filter output;

[0075] A filter capacitor is connected in parallel with the bridge arm side inductor.

[0076] It can be understood that the scope of application of the dual-mode smooth switching control method for grid-connected inverters under grid distortion conditions provided in the embodiment is not limited to LCL-type grid-connected inverters, but is also applicable to L-type, LLCL-type or other types. However, this embodiment takes the LCL-type grid-connected inverter as an example for explanation.

[0077] It can be seen that the topology of the LCL filter is an effective filtering solution that can reduce the high-order harmonics in the inverter output current, thereby improving the quality of the grid-connected current. The LCL filter forms a second-order low-pass filter by adding a filter capacitor on the basis of the L-type filter, which makes the LCL filter more effective than the L-type filter in suppressing high-frequency harmonics. The grid-side inductor of the LCL filter is directly connected to the grid, which helps to reduce the impact of the inverter on the grid and improve the stability and reliability of the system.

[0078] Specifically, determining the system matrix and output matrix of the grid-connected inverter includes:

[0079] The system matrix:

[0080]

[0081] The output matrix:

[0082] C = (1 0 1 0);

[0083] Where A represents the system matrix; ω g represents the fundamental angular frequency of the power grid; C represents the output matrix.

[0084] It can be seen that the fundamental angular frequency ω of the power grid g It is obtained by measuring the grid frequency, which is usually monitored and released in real time by the grid dispatching center. The output matrix C is related to the output characteristics of the grid-connected inverter.

[0085] Specifically, when the system matrix and the output matrix are discretized and input into the Kalman filter, it includes:

[0086] Discretize the system matrix as follows:

[0087]

[0088] Among them, A d represents the state transfer matrix associated with a specific linear system; A represents the system matrix; Indicates the sampling time.

[0089] In this embodiment, the state transfer matrix A associated with a specific linear system is d Substitute into the Kalman filter to get the fundamental voltage observation value And fundamental current observation value

[0090] See also Figure 4 As shown, the implementation steps of the Kalman filter algorithm for parameter estimation are as follows:

[0091] Step 1: Initial state of the predictor variables,

[0092] Step 2: Calculate the error covariance matrix, P-(k) = A d P(k-1)A d T +Q(k-1).

[0093] Step 3: Update Kalman gain, K(k) = P -1 (k)C T [CP - (k)C T +R(k)] -1 .

[0094] Step 4: Update the state observation,

[0095] Step 5: Update the error covariance matrix, P(k) = [IK(k)C]P - (k).

[0096] Step 6: Return to step 1.

[0097] Where X(k) is the state vector at time k, A d is the state transfer matrix associated with a specific linear system, C is the observation matrix, Q(k) is the noise inside the system, and R(k) is the measurement noise of the actual observation. Covariances Q and R can be selected based on experience. According to practical experience, the measurement noise covariance matrix R has a significant impact on the estimation accuracy and even the stability of the algorithm. In the implementation case of the present invention, Q is selected as 0.01 and R is selected as 0.2. and P - (k) is the estimate at time k, using only the information available at time k-1, K(k) is the Kalman gain, is an estimate of X(k), and P(k) is the covariance matrix of the estimation errors.

[0098] Specifically, when the fundamental voltage observation value and the grid connection point voltage are synthesized into a harmonic voltage observation value, it includes:

[0099]

[0100] in, Indicates the observed value of harmonic voltage; Indicates the grid connection point voltage; Indicates the fundamental voltage observation value.

[0101] In this embodiment, when the grid-connected inverter is operating normally, the phase information of the fundamental wave can be accurately obtained through the Kalman filter. When the working mode is switched, the fundamental wave voltage observation value is used as the input of the phase-locked loop. Since it only contains fundamental wave information, the speed and accuracy of phase locking can be guaranteed.

[0102] In this embodiment, when the fundamental voltage observation value And fundamental current observation value When used as the input of droop control, the calculated real-time active power is a constant value, and the phase-locking speed and accuracy can be guaranteed, thus ensuring the voltage phase synchronization at the switching moment.

[0103] See also Figure 3 As shown in Figure 1, the harmonic voltage observation value is used as the feedforward of pulse modulation to eliminate the influence of grid harmonics. The voltage feedforward can be expressed as:

[0104] G feedforward (s)=(L1C1s 2 +1) / G d (s)K SVPWM

[0105] Where, C1 represents the filter capacitor; L1 represents the DC side filter inductor; G d Indicates time delay; K SVPWM Represents the rectifier gain.

[0106] At this time, the current inner loop and voltage outer loop transfer functions of the control system in the current source control mode and voltage source control mode will also change. The transfer functions of the new voltage outer loop control and the new current inner loop control are as follows:

[0107]

[0108]

[0109] Where, L2 represents the grid-side filter inductor; I gref Indicates the reference value of the grid current; G PI_I represents the transfer function of the current control loop; K p2 and K i2 They represent the proportional coefficient and integral coefficient of the PI controller in the current control loop respectively; u PCC_q Indicates the component of the grid connection point voltage on the q axis; K PLL_P and K PLL_I They represent the proportional coefficient and integral coefficient in the PI controller in SRF-PLL respectively.

[0110] See also Figure 5 As shown, according to the formula The closed-loop main pole positions under different parameter deviation conditions can be obtained.

[0111] Specifically, comparing the system short circuit ratio SCR with the system short circuit ratio boundary value λ, and judging whether to switch the current source control mode according to the comparison result, includes:

[0112] When the system short circuit ratio SCR is greater than the system short circuit ratio boundary value λ, it is determined that the grid-connected inverter maintains operating in the current source control mode;

[0113] When the system short circuit ratio SCR is less than or equal to the system short circuit ratio boundary value λ, it is determined that the grid-connected inverter is switched to the voltage source control mode for operation.

[0114] Specifically, the system short circuit ratio SCR is obtained by the following formula:

[0115]

[0116] Among them, SCR represents the system short circuit ratio; P dN Indicates the rated DC output power, L g Represents the grid impedance.

[0117] In this embodiment, the system short circuit ratio boundary value λ is preferably 3.

[0118] It can be seen that when the system short circuit ratio SCR is greater than 3, the inverter can maintain operation in the current source control mode. This is because in the current source control mode, the inverter can provide a stable current output and maintain good performance even when the grid voltage is distorted. However, when the system short circuit ratio SCR is less than or equal to 3, the inverter needs to switch to the voltage source control mode. In the voltage source control mode, the inverter focuses on the stable output of the voltage, which is especially important when the grid voltage quality is poor. Through this mode switching, it can be ensured that the grid-connected inverter can provide stable and reliable power output under a variety of different grid conditions.

[0119] Those skilled in the art will appreciate that the embodiments of the present application may be provided as methods, systems or computer program products. Therefore, the present application may adopt the form of a complete hardware embodiment, a complete software embodiment, or an embodiment in combination with software and hardware. Moreover, the present application may adopt the form of a computer program product implemented on one or more computer-usable storage media (including but not limited to disk storage, CD-ROM, optical storage, etc.) containing computer-usable program codes.

[0120] The present application is described with reference to flowcharts and / or block diagrams of methods, devices (systems) and computer program products according to embodiments of the present application. It should be understood that each process and / or box in the flowchart and / or block diagram, as well as the combination of processes and / or boxes in the flowchart and / or block diagram, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, a special-purpose computer, an embedded processor or other programmable data processing device to generate a machine, so that the instructions executed by the processor of the computer or other programmable data processing device generate instructions for implementing the processes in the flowchart and / or block diagram. Figure 1 A process or multiple processes and / or boxes Figure 1 A device that provides the functions specified in a block or multiple blocks.

[0121] These computer program instructions may also be stored in a computer-readable memory capable of directing a computer or other programmable data processing device to operate in a specific manner, so that the instructions stored in the computer-readable memory produce an article of manufacture comprising an instruction device, which implements the process Figure 1 A process or multiple processes and / or boxes Figure 1 A function specified in one or more boxes.

[0122] These computer program instructions can also be loaded onto a computer or other programmable data processing device so that a series of operating steps are executed on the computer or other programmable device to produce a computer-implemented process, thereby providing instructions for implementing the process. Figure 1 A process or multiple processes and / or boxes Figure 1 The steps for the functions specified in one or more boxes.

[0123] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the above embodiments, ordinary technicians in the relevant field should understand that the specific implementation methods of the present invention can still be modified or replaced by equivalents. Any modification or equivalent replacement that does not depart from the spirit and scope of the present invention should be covered within the scope of protection of the claims of the present invention.

Claims

1. A dual-mode smooth switching control method for a grid-connected inverter under grid distortion conditions, characterized in that: include: Operate the grid-connected inverter in current source control mode and collect grid current and grid connection point voltage; Determining a system matrix and an output matrix of the grid-connected inverter; Discretizing the system matrix and the output matrix and inputting them into a Kalman filter, and obtaining a fundamental voltage observation value and a fundamental current observation value through the Kalman filter; The fundamental voltage observation value and the grid connection point voltage are synthesized into a harmonic voltage observation value; The fundamental voltage observation value is used as a feedback control quantity of the PI regulator in the voltage source control mode and an input quantity of the phase-locked loop in the current source control mode, and the input quantity of the phase-locked loop is recorded as a first input quantity; Using the fundamental voltage observation value and the fundamental current observation value as input quantities of a power calculation module in the voltage source control mode, and recording the input quantity of the power calculation module as a second input quantity; Setting a system short circuit ratio boundary value λ of the grid-connected inverter and obtaining a system short circuit ratio SCR; Comparing the system short circuit ratio SCR with the system short circuit ratio boundary value λ, and determining whether to switch the current source control mode according to the comparison result; When it is determined that the current source control mode is to be switched, the grid-connected inverter is switched to operate in the voltage source control mode.

2. The dual-mode smooth switching control method for grid-connected inverters under grid distortion conditions according to claim 1 is characterized in that: The grid-connected inverter is an LCL type grid-connected inverter.

3. The dual-mode smooth switching control method for grid-connected inverters under grid distortion conditions according to claim 2 is characterized in that: The LCL type grid-connected inverter includes an LCL filter, and the topological structure of the LCL filter includes: The bridge arm side inductor is located at the output end of the grid-connected inverter and is connected to the output bridge arm of the grid-connected inverter; A grid-side inductor, one end of which is connected to the bridge arm-side inductor, and the other end of which is connected to the grid as a filter output; A filter capacitor is connected in parallel with the bridge arm side inductor.

4. The dual-mode smooth switching control method for grid-connected inverters under grid distortion conditions according to claim 1, characterized in that: When determining the system matrix and output matrix of the grid-connected inverter, it includes: The system matrix: The output matrix: C＝(1 0 1 0)； Where A represents the system matrix; ω g represents the fundamental angular frequency of the power grid; C represents the output matrix.

5. The dual-mode smooth switching control method for grid-connected inverters under grid distortion conditions according to claim 1, characterized in that: When the system matrix and output matrix are discretized and input into the Kalman filter, it includes: Discretize the system matrix as follows: Among them, A d represents the state transfer matrix associated with a specific linear system; A represents the system matrix; Indicates the sampling time.

6. The dual-mode smooth switching control method for grid-connected inverters under grid distortion conditions according to claim 1, characterized in that: When the fundamental voltage observation value and the grid connection point voltage are synthesized into a harmonic voltage observation value, it includes: in, Indicates the observed value of harmonic voltage; Indicates the grid connection point voltage; Indicates the fundamental voltage observation value.

7. The dual-mode smooth switching control method for grid-connected inverters under grid distortion conditions according to claim 1, characterized in that: The system short circuit ratio SCR is compared with the system short circuit ratio boundary value λ, and whether to switch the current source control mode is determined according to the comparison result, including: When the system short circuit ratio SCR is greater than the system short circuit ratio boundary value λ, it is determined that the grid-connected inverter maintains operating in the current source control mode; When the system short circuit ratio SCR is less than or equal to the system short circuit ratio boundary value λ, it is determined that the grid-connected inverter is switched to the voltage source control mode for operation.

8. The dual-mode smooth switching control method for grid-connected inverters under grid distortion conditions according to claim 7, characterized in that: The system short circuit ratio SCR is obtained by the following formula: Among them, SCR represents the system short circuit ratio; P dN Indicates the rated DC output power, L g Represents the grid impedance.

9. The dual-mode smooth switching control method for grid-connected inverters under grid distortion conditions according to claim 7, characterized in that: The system short-circuit ratio boundary value λ is 3.

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