An inverter and a harmonic suppression method under different grid strengths

Abstract

The application discloses an inverter and a harmonic suppression method under different grid strengths, and relates to the technical field of inverters. The inverter comprises a power conversion circuit and a controller. The power conversion circuit is used for converting input direct current into alternating current output. The controller is used for changing the leading amount of phase angle leading compensation in a repetitive control link of the inverter when the total harmonic distortion (THD) of a grid is greater than a preset threshold value, until the THD is less than or equal to the preset threshold value. The repetitive control link is used for performing repetitive control and phase angle leading compensation on a current instruction value of the inverter. In the case that the grid strength is unknown, the leading amount is adjusted to obtain a leading amount suitable for the current grid strength, so that the THD of the grid is effectively reduced, and the operation stability of a system where the inverter is located is improved.

Description

Technical Field

[0001] This application relates to the field of power electronics technology, specifically to an inverter and a harmonic suppression method under different grid strengths. Background Technology

[0002] Currently, with the increasing integration of photovoltaic (PV) power generation and energy storage systems into the grid, their impact on the grid is becoming increasingly apparent. Especially when PV or energy storage systems handle a large load, the grid is not ideal; the more electrical equipment there is, the weaker the corresponding grid strength. Grid strength is generally defined by the Short Circuit Ratio (SCR), which is the system's short-circuit capacity divided by the equipment capacity. SCR is a crucial indicator of grid strength at the point of connection for new energy systems; the lower the value, the weaker the grid and the less stable the grid connection. As the number of new energy sources connected to the grid continues to increase, the grid's SCR is continuously decreasing.

[0003] Traditional harmonic suppression schemes can effectively suppress harmonics under strong grid conditions. However, under weak grid conditions, due to the higher impedance of the grid, the impedance phase angle between the existing inverter and the grid changes significantly. The parameters of harmonic suppression schemes effective under strong grid conditions are less effective under weak grid conditions, and may even increase harmonic content, potentially leading to system instability and tripping risks. Summary of the Invention

[0004] In view of this, this application provides an inverter and a harmonic suppression method under different grid strengths, which can adaptively suppress harmonics under different grid strengths.

[0005] This application provides an inverter, including: a power conversion circuit and a controller;

[0006] A power conversion circuit is used to convert the input direct current into alternating current output.

[0007] The controller is used to change the advance amount of phase angle advance compensation in the repetitive control loop of the inverter when the total harmonic distortion (THD) of the power grid is greater than a preset threshold, until the THD is less than or equal to the preset threshold. The repetitive control loop is used to repeatedly control the current command value of the inverter and perform phase angle advance compensation.

[0008] Preferably, the controller is specifically used to gradually adjust the advance amount of phase angle advance compensation in the repetitive control loop of the inverter with a preset step size until the THD is less than or equal to a preset threshold.

[0009] Preferably, the controller is specifically used to gradually adjust the lead amount N times with a preset step size, obtain the THD of the power grid after each adjustment of the lead amount to form a set of THDs, and perform phase angle lead compensation in the repeated control loop for the lead amount corresponding to the smallest THD in the set of THDs; N is an integer greater than or equal to 2.

[0010] Preferably, the controller is specifically used to gradually adjust the lead amount N times with a preset step size to obtain the THD after each adjustment of the lead amount to form a set of THDs. If each THD in the set of THDs is greater than the THD corresponding to the closing of the repetitive control loop, then the repetitive control loop is closed. If there is a THD in the set of THDs that is less than the THD corresponding to the closing of the repetitive control loop, then phase angle lead compensation is performed according to the smallest THD in the set of THDs.

[0011] Preferably, the controller is specifically used to close the repetitive control loop before adjusting the lead amount N times with a preset step size, so as to obtain the THD corresponding to closing the repetitive control loop.

[0012] Preferably, the controller is also used to gradually adjust the lead amount N times with a preset step size when the preset period is reached when the THD is less than a preset threshold, to obtain the THD of the power grid after each adjustment of the lead amount to form a set of THDs, and to perform phase angle lead compensation on the lead amount corresponding to the smallest THD in the set of THDs.

[0013] Preferably, the controller is also used to change the gain coefficient in the repetitive control loop of the inverter when the THD is greater than a preset threshold, until the THD is less than or equal to the preset threshold.

[0014] Preferably, the preset step size is 1.

[0015] This application also provides a method for harmonic suppression of inverters under different grid strengths, including:

[0016] When the total harmonic distortion (THD) of the power grid is greater than a preset threshold, the advance amount of phase angle advance compensation in the repetitive control loop of the inverter is changed until the THD is less than or equal to the preset threshold. The repetitive control loop is used to repeatedly control the current command value of the inverter and perform phase angle advance compensation.

[0017] Preferably, the lead amount of phase angle lead compensation in the repetitive control loop of the inverter is changed until the THD is less than or equal to a preset threshold, specifically including:

[0018] The phase angle advance compensation in the repetitive control loop of the inverter is gradually adjusted with a preset step size until the THD is less than or equal to the preset threshold.

[0019] Preferably, the phase angle advance compensation in the repetitive control loop of the inverter is gradually adjusted with a preset step size, specifically including:

[0020] The lead amount is gradually adjusted N times with a preset step size to obtain the THD of the power grid after each adjustment, forming a set of THDs. The lead amount corresponding to the smallest THD in the set of THDs is used for phase angle lead compensation in the repeated control loop; N is an integer greater than or equal to 2.

[0021] Preferably, the phase angle advance compensation in the repetitive control loop of the inverter is gradually adjusted with a preset step size, specifically including:

[0022] The lead amount is gradually adjusted N times with a preset step size to obtain the THD after each adjustment of the lead amount, forming a set of THDs. If each THD in the set of THDs is greater than the THD corresponding to the closing of the repetitive control loop, then the repetitive control loop is closed. If there is a THD in the set of THDs that is less than the THD corresponding to the closing of the repetitive control loop, then phase angle lead compensation is performed according to the smallest THD in the set of THDs.

[0023] Preferably, before gradually adjusting the advance amount N times with a preset step size, the method further includes:

[0024] Close the repeat control loop and obtain the THD corresponding to the time when the repeat control loop is closed.

[0025] Preferably, it further includes: when the THD is less than a preset threshold, when the preset period is reached, the lead amount is gradually adjusted N times with a preset step size, and the THD of the power grid after each adjustment of the lead amount is formed into a set of THDs, and the lead amount corresponding to the smallest THD in the set of THDs is compensated for the phase angle lead.

[0026] Preferably, the method further includes: when the THD is greater than a preset threshold, changing the gain coefficient in the repetitive control loop of the inverter until the THD is less than or equal to the preset threshold.

[0027] Therefore, this application has the following beneficial effects:

[0028] The inverter provided in this application indicates that when the total harmonic distortion (THD) of the power grid exceeds a preset threshold, the current lead value is insufficient to meet harmonic requirements, necessitating adjustment of the lead value to accommodate the current grid strength. However, instead of obtaining the grid strength, the inverter directly adjusts the lead value to obtain the corresponding THD. If the THD meets the requirements, the adjusted lead value is used as the final lead value to control the inverter's output current. This inverter, even with unknown grid strength, effectively reduces the grid's THD and improves the operational stability of the system containing the inverter by adjusting the lead value to achieve a lead value suitable for the current grid strength. Attached Figure Description

[0029] Figure 1 A block diagram illustrating the harmonic suppression principle of an inverter provided in this application embodiment;

[0030] Figure 2 A schematic diagram of an inverter provided in an embodiment of this application;

[0031] Figure 3 A flowchart illustrating a harmonic suppression method for an inverter under different grid strengths, provided as an embodiment of this application;

[0032] Figure 4 A flowchart illustrating another harmonic suppression method for inverters under different grid strengths, provided as an embodiment of this application. Detailed Implementation

[0033] To enable those skilled in the art to better understand the technical solutions provided in the embodiments of this application, the application scenarios are introduced below.

[0034] The technical solution provided in this application is applied to an inverter. The input terminal of the inverter is used to connect to a DC power source. This application does not specifically limit the type of DC power source; for example, it can be a photovoltaic string or an energy storage battery. The output terminal of the inverter is used to connect to the AC power grid, i.e., this inverter is a grid-connected inverter.

[0035] The harmonic components in the inverter's output current directly affect the quality of the power grid. Therefore, the inverter can automatically adjust the output current according to different grid strengths to adapt to different grid strengths.

[0036] For example, when the grid's SCR (Saturation Rate) is greater than 15, it is considered a strong grid. When the grid's SCR is less than 3, it is considered a weak grid. In factory power consumption scenarios, factories include many motors. As electrical equipment, the more electrical equipment there is, the weaker the grid strength, and the lower the corresponding SCR of the photovoltaic power generation system.

[0037] The following is combined with Figure 1 First, let's introduce the internal harmonic control principle of the inverter.

[0038] See Figure 1 The figure is a block diagram of the harmonic suppression principle of an inverter provided in an embodiment of this application.

[0039] This application does not specifically limit the specific type of inverter in the embodiments. For example, it will be described using an LCL-type inverter as an example.

[0040] The embodiments of this application do not specifically limit the specific type of harmonic control; for example, it can be R-type harmonic control or integral harmonic control.

[0041] Figure 1 In this diagram, y represents the inverter's output current, r represents the current command value, and e1 represents the current output current of r and y after passing through H. iL The error between the feedback quantities after feedback control. G RPQ(z) is a repetitive controller, Q(z) is the internal model coefficient, and C(z) is the compensator, where C(z) = K r z d F(z), d is the lead value, used for phase angle lead compensation. K r The gain coefficient is F(z). F(z) is the compensation controller, and G is the repetitive controller. RP The output of (z) is u, and u and y pass through H. iL The error between the feedback quantities after feedback control is e2.

[0042] Figure 1 China G co (z) is a PI closed-loop controller, G P As the controlled object, G PI For the PI controller stage, G d This is a delayed process. V c This is the grid voltage.

[0043] It should be understood that Figure 1 The current control in the system includes a repetitive controller and a current closed-loop controller connected in series, which can be understood as an embedded series control.

[0044] Traditionally, a fixed d value is given for strong grid conditions, and another fixed d value is given for weak grid conditions. However, this cannot effectively adapt to varying grid strength, leading to a worsening of the total harmonic distortion (THD) of the grid.

[0045] Therefore, in order to adapt to the changing power grid strength, the embodiments of this application can adaptively match the lead amount d of the corresponding phase angle lead compensation, thereby reducing the harmonics of the power grid, i.e., meeting the THD requirements.

[0046] To make the above-mentioned objectives, features and advantages of this application more apparent and understandable, the embodiments of this application will be further described in detail below with reference to the accompanying drawings and specific implementation methods.

[0047] See Figure 2 The figure is a schematic diagram of an inverter provided in an embodiment of this application.

[0048] The inverter 1000 provided in this application embodiment includes: a power conversion circuit 100 and a controller 200; the output terminal of the inverter 1000 is connected to the AC power grid G.

[0049] The power conversion circuit 100 is used to convert the input DC power into AC power output. This application does not specifically limit the topology of the power conversion circuit 100; any inverter circuit can be used, including a bidirectional inverter circuit (forward inversion and reverse rectification). The power conversion circuit 100 can be two-level or multi-level.

[0050] The controller 200 is used to change the advance amount d of the phase angle advance compensation in the repetitive control loop of the inverter when the total harmonic distortion (THD) of the power grid is greater than a preset threshold, until the THD is less than or equal to the preset threshold. The repetitive control loop is used to repeatedly control the current command value of the inverter and perform phase angle advance compensation.

[0051] It should be understood that THD can be obtained using Discrete Fourier Transform (DFT) or Fast Fourier Transform (FFT), for example, by detecting the voltage or current of the power grid and calculating THD using FFT.

[0052] When the strength of the power grid changes, the lead amount of the phase angle lead compensation in the repetitive control loop will not meet the requirements. For example, when the SCR is 1.5, a d of 10 may be more suitable; when the SCR is 3, a d of 7 may be more suitable. If the inverter output current is controlled with a fixed d under weak grid conditions, it may result in large grid harmonics, i.e., the THD will not meet the requirements.

[0053] The inverter provided in this application continuously monitors the THD of the power grid. When the THD exceeds a preset threshold, the value of d is adjusted until the THD meets the requirements. This application does not specifically limit the adjustment range or trend of d; for example, it can be adjusted to a larger value or a smaller value, such as increasing in increments of 1. Each time d is increased, the THD is re-obtained under the control of the adjusted d, and it is determined whether the THD has decreased. If it has decreased and meets the requirements, the adjusted d can be used as the final d for control.

[0054] It should be understood that the inverter provided in this application embodiment does not continuously adjust the value of d, but adjusts d when the THD does not meet the requirements. The adjusted d can stabilize the power grid for a period of time, because the power grid strength generally does not change suddenly in a short period of time, so d will not be adjusted frequently.

[0055] In this embodiment of the application, when adjusting d, multiple d can be adjusted to obtain the THD corresponding to each d, and finally the d corresponding to the smallest THD is used to control the current of the inverter.

[0056] The inverter provided in this application, when the THD of the power grid exceeds a preset threshold, indicates that the current lead value is insufficient to meet harmonic requirements and needs to be adjusted to meet the current grid strength. However, it does not require obtaining the grid strength; instead, it directly adjusts the lead value to obtain the THD corresponding to the adjusted lead value. If the THD meets the requirements, the adjusted d value is used as the final lead value to control the inverter's output current. This inverter, even with unknown grid strength, adjusts the lead value to obtain a lead value suitable for the current grid strength, thereby effectively reducing the grid's THD and improving the operational stability of the system. This inverter is particularly suitable for weak grid conditions, enabling automatic optimization of the d value under weak grid conditions and exhibiting strong adaptability.

[0057] The controller is specifically used to gradually adjust the advance amount of phase angle advance compensation in the repetitive control loop of the inverter with a preset step size until the THD is less than or equal to the preset threshold.

[0058] In this embodiment, the size of the preset step size is not specifically limited. For example, the preset step size can be an integer or a decimal, such as an integer of 1. Each time, the advance amount is increased by 1 or decreased by 1.

[0059] To reduce the total electrical discharge (THD) of the power grid and improve its performance, the THD corresponding to each adjustment of d can be searched multiple times to find the d with the minimum THD. For example, the number of adjustments can be preset to N, where N is an integer greater than or equal to 2, such as N=5. It should be understood that the time for multiple adjustments of d is relatively short, generally within tens of seconds, which can complete the adjustment of d and find the minimum THD, quickly stabilizing the power grid and reducing THD.

[0060] The controller is specifically used to gradually adjust the lead amount N times with a preset step size to obtain the THD of the power grid after each adjustment, thus forming a set of THDs. For example, if the adjustment is made N times, there will be N THDs. The lead amount corresponding to the smallest THD in the set of THDs is used for phase angle lead compensation in the repeated control loop, that is, the d corresponding to the smallest THD among the N THDs is taken as the final lead amount.

[0061] Because the THD (Total Distance) after adjustment using the repetitive control loop may be lower than the THD after the repetitive control loop is turned off when the power grid is under different strengths, meaning that no adjustment is actually more effective, the repetitive control loop is turned off and no adjustment is made. The specific implementation method is described below.

[0062] The controller is specifically used to gradually adjust the lead amount N times with a preset step size to obtain the THD after each adjustment of the lead amount, forming a set of THDs. If each THD in the set of THDs is greater than the THD corresponding to the closing of the repetitive control loop, then the repetitive control loop is closed. If there is a THD in the set of THDs that is less than the THD corresponding to the closing of the repetitive control loop, then phase angle lead compensation is performed according to the THD corresponding to the smallest THD in the set of THDs.

[0063] Specifically, before making advance adjustment, the repetitive control loop can be turned off to obtain the current THD, and then N adjustments can be made to obtain the corresponding N THDs.

[0064] For example, the controller is specifically used to close the repetitive control loop before adjusting the lead amount N times with a preset step size, so as to obtain the THD corresponding to closing the repetitive control loop.

[0065] In addition, the inverter provided in this application embodiment, besides using the grid's THD to trigger the adjustment of d, can also combine the control cycle to trigger the adjustment of d. For example, if the grid's THD meets the requirements and is less than or equal to a preset threshold, but the control cycle has expired, d can still be adjusted to find a better d corresponding to THD. Specifically, the controller is also used to gradually adjust the lead amount N times with a preset step size when the THD is less than the preset threshold and the preset cycle has expired, obtaining the grid's THD after each adjustment to form a set of THDs, and performing phase angle lead compensation on the lead amount corresponding to the smallest THD in the set of THDs.

[0066] The embodiments of this application do not specifically limit the preset period, such as 2 hours or 5 hours.

[0067] The embodiments in this application do not specifically limit the value of d, which varies depending on the system. For example, the value of d is different when the inverter is applied to photovoltaic grid-connected power generation and when the inverter is applied to wind power grid-connected power generation.

[0068] For example, in a weak grid, such as when the SCR is 1.5, after continuous adjustment of d, a value of around 15 is more suitable, resulting in a lower THD. In a strong grid, a value of around 3 is more suitable, resulting in a lower THD. It can be seen that the value of d varies considerably between strong and weak grids.

[0069] The inverter described in the above embodiments only illustrates adjusting d to make the THD meet the requirements. In addition, the controller is also used to change the gain coefficient K in the repetitive control loop of the inverter when the THD is greater than a preset threshold. rThe THD is adjusted until it is less than or equal to a preset threshold, i.e., by jointly adjusting d and the gain coefficient to minimize the THD of the power grid. Generally, the gain coefficient is a small value, which can be a decimal greater than 0, such as 0.1, typically less than 1.5. The specific value of the gain coefficient is not specifically limited here; the gain coefficient can be increased or decreased by adjusting it.

[0070] Based on the inverter provided in the above embodiments, this application also provides a method for harmonic suppression of the inverter under different grid strengths, which will be described in detail below with reference to the accompanying drawings.

[0071] See Figure 3 The figure is a flowchart of a harmonic suppression method for an inverter under different grid strengths, provided in an embodiment of this application.

[0072] The harmonic suppression method for inverters under different grid strengths provided in this application includes:

[0073] S301: Obtain the total harmonic distortion (THD) of the power grid;

[0074] S302: Determine whether THD is greater than the preset threshold. If so, execute S303.

[0075] S303: When the total harmonic distortion (THD) of the power grid is greater than the preset threshold, the advance amount of the phase angle advance compensation in the repetitive control loop of the inverter is changed until the THD is less than or equal to the preset threshold. The repetitive control loop is used to repeatedly control the current command value of the inverter and perform phase angle advance compensation.

[0076] In this embodiment, the size of the preset step size is not specifically limited. For example, the preset step size can be an integer or a decimal, such as an integer of 1. The advance amount is increased or decreased by 1 each time.

[0077] To reduce the THD of the power grid and improve its performance, the THD corresponding to each adjustment of d can be searched multiple times to find the d with the minimum THD. For example, the number of adjustments can be preset to N, where N is an integer greater than or equal to 2, such as N being 5.

[0078] The inverter provided in this application continuously monitors the THD of the power grid. When the THD exceeds a preset threshold, the value of d is adjusted until the THD meets the requirements. This application does not specifically limit the adjustment range or trend of d; for example, it can be adjusted to a larger value or a smaller value, such as increasing in increments of 1. Each time d is increased, the THD is re-obtained under the control of the adjusted d, and it is determined whether the THD has decreased. If it has decreased and meets the requirements, the adjusted d can be used as the final d for control.

[0079] It should be understood that the inverter provided in this application embodiment does not continuously adjust the value of d, but adjusts d when the THD does not meet the requirements. The adjusted d can stabilize the power grid for a period of time, because the power grid strength generally does not change suddenly in a short period of time, so d will not be adjusted frequently.

[0080] The method provided in this application addresses situations where the total harmonic distortion (THD) of the power grid cannot meet harmonic requirements. It necessitates adjusting the lead value to accommodate the current grid strength, obtaining the THD corresponding to the adjusted lead value. If the THD meets the requirements, the adjusted lead value (d) is used as the final lead value to control the inverter's output current. This inverter, even with unknown grid strength, effectively reduces the grid's THD and improves the operational stability of the system containing the inverter by adjusting the lead value to suit the current grid strength.

[0081] In addition, the inverter provided in this application embodiment, besides using the grid's THD to trigger the adjustment of d, can also combine the control cycle to trigger the adjustment of d. For example, if the grid's THD meets the requirements and is less than or equal to a preset threshold, but the control cycle has expired, d can still be adjusted to find a better d corresponding to THD. Specifically, if THD is less than the preset threshold, but the preset cycle has expired, the lead amount is gradually adjusted N times with a preset step size. The THD of the grid after each adjustment of the lead amount is used to form a set of THDs. The lead amount corresponding to the smallest THD in the set of THDs is used for phase angle lead compensation.

[0082] The following describes a specific method for triggering adjustment d by using THD as the judgment adjustment.

[0083] See Figure 4 The figure is a flowchart of another harmonic suppression method for inverters under different grid strengths provided in the embodiments of this application.

[0084] S401: THD is greater than the preset threshold.

[0085] Before gradually adjusting the advance amount N times with a preset step size, the process also includes:

[0086] S402: Close the repeat control loop and obtain the corresponding THD after the repeat control loop is closed.

[0087] S403: The lead amount is gradually adjusted N times with a preset step size to obtain the THD of the power grid after each adjustment of the lead amount, forming a set of THDs.

[0088] The lead amount is gradually adjusted N times with a preset step size to obtain the THD of the power grid after each adjustment, forming a set of THDs. The lead amount corresponding to the smallest THD in the set of THDs is used for phase angle lead compensation in the repetitive control loop; N is an integer greater than or equal to 2.

[0089] Alternatively, the lead amount can be gradually adjusted N times with a preset step size to obtain the THD after each adjustment, forming a set of THDs. If each THD in the set is greater than the THD corresponding to the closing of the repetitive control loop, then the repetitive control loop is closed. If there is a THD in the set that is less than the THD corresponding to the closing of the repetitive control loop, then phase angle lead compensation is performed according to the smallest THD in the set.

[0090] In addition, after each adjustment of d and before the next adjustment, a preset time period can be allowed to allow the system to stabilize. The embodiments of this application do not specifically limit the length of the preset time period, but it is generally much shorter than the preset period.

[0091] S404: Determine if each THD in a set of THDs is greater than the THD corresponding to the closed repetitive control loop. If so, execute S405; otherwise, execute S406.

[0092] S405: Close the repeat control loop.

[0093] S406: Perform phase angle advance compensation according to the minimum THD in the set of THDs.

[0094] The inverter described in the above embodiments only illustrates adjusting d to make the THD meet the requirements. In addition, the controller is also used to change the gain coefficient K in the repetitive control loop of the inverter when the THD is greater than a preset threshold. r The THD is adjusted until it is less than or equal to a preset threshold, i.e., by jointly adjusting d and the gain coefficient to minimize the THD of the power grid. Generally, the gain coefficient is a small value, which can be a decimal greater than 0, such as 0.1, and is generally less than 1.5. The specific value of the gain coefficient is not specifically limited here; the gain coefficient can be increased or decreased by adjusting it.

[0095] It should be noted that the various embodiments in this specification are described in a progressive manner, with each embodiment focusing on the differences from other embodiments. Similar or identical parts between embodiments can be referred to interchangeably. For the systems or apparatus disclosed in the embodiments, since they correspond to the methods disclosed in the embodiments, the descriptions are relatively simple, and relevant parts can be referred to the method section.

[0096] The above description of the disclosed embodiments enables those skilled in the art to make or use this application. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of this application. Therefore, this application is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. An inverter, characterized in that, include: Power conversion circuits and controllers; The power conversion circuit is used to convert the input DC power into AC power output. The controller is used to change the advance amount of phase angle advance compensation in the repetitive control loop of the inverter when the total harmonic distortion (THD) of the power grid is greater than a preset threshold, until the THD is less than or equal to the preset threshold. The repetitive control loop is used to repeatedly control the current command value of the inverter and perform phase angle advance compensation.

2. The inverter according to claim 1, characterized in that, The controller is specifically used to gradually adjust the advance amount of phase angle advance compensation in the repetitive control loop of the inverter with a preset step size until the THD is less than or equal to the preset threshold.

3. The inverter according to claim 2, characterized in that, The controller is specifically used to gradually adjust the lead amount N times with a preset step size, obtain the THD of the power grid after each adjustment of the lead amount to form a set of THDs, and perform phase angle lead compensation in the repetitive control loop on the lead amount corresponding to the smallest THD in the set of THDs; where N is an integer greater than or equal to 2.

4. The inverter according to claim 3, characterized in that, The controller is specifically configured to gradually adjust the lead amount N times with a preset step size to obtain the THD after each adjustment of the lead amount, forming a set of THDs. If each THD in the set of THDs is greater than the THD corresponding to the closing of the repetitive control loop, then the repetitive control loop is closed. If there is a THD in the set of THDs that is less than the THD corresponding to the closing of the repetitive control loop, then phase angle lead compensation is performed according to the smallest THD in the set of THDs.

5. The inverter according to claim 4, characterized in that, Specifically, the controller is used to close the repetitive control loop before adjusting the lead amount N times with a preset step size, so as to obtain the THD corresponding to closing the repetitive control loop.

6. The inverter according to any one of claims 1-5, characterized in that, The controller is further configured to adjust the lead amount N times with a preset step size when the THD is less than a preset threshold and the preset period is reached, to obtain a set of THDs of the power grid after each adjustment of the lead amount, and to perform phase angle lead compensation on the lead amount corresponding to the smallest THD in the set of THDs.

7. The inverter according to any one of claims 1-5, characterized in that, The controller is further configured to change the gain coefficient in the repetitive control loop of the inverter when the THD is greater than a preset threshold, until the THD is less than or equal to the preset threshold.

8. The inverter according to any one of claims 2-5, characterized in that, The preset step size is 1.

9. A method for harmonic suppression of an inverter under different grid strengths, characterized in that, include: When the total harmonic distortion (THD) of the power grid is greater than a preset threshold, the advance amount of the phase angle advance compensation in the repetitive control loop of the inverter is changed until the THD is less than or equal to the preset threshold. The repetitive control loop is used to repeatedly control the current command value of the inverter and perform phase angle advance compensation.

10. The method according to claim 9, characterized in that, The step of changing the phase angle advance compensation amount in the repetitive control loop of the inverter until the THD is less than or equal to the preset threshold specifically includes: The phase angle advance compensation in the repetitive control loop of the inverter is gradually adjusted by a preset step size until the THD is less than or equal to the preset threshold.

11. The method according to claim 10, characterized in that, The step-wise adjustment of the phase angle advance compensation in the repetitive control loop of the inverter with a preset step size specifically includes: The lead amount is gradually adjusted N times with a preset step size to obtain the THD of the power grid after each adjustment, forming a set of THDs. The lead amount corresponding to the smallest THD in the set of THDs is used for phase angle lead compensation in the repetitive control loop; N is an integer greater than or equal to 2.

12. The method according to claim 10, characterized in that, The step-wise adjustment of the phase angle advance compensation in the repetitive control loop of the inverter with a preset step size specifically includes: The lead amount is gradually adjusted N times with a preset step size to obtain the THD after each adjustment, forming a set of THDs. If each THD in the set is greater than the THD corresponding to the closing of the repetitive control loop, then the repetitive control loop is closed. If there is a THD in the set that is less than the THD corresponding to the closing of the repetitive control loop, then phase angle lead compensation is performed according to the smallest THD in the set.

13. The method according to claim 11 or 12, characterized in that, Before gradually adjusting the advance amount N times with a preset step size, the process also includes: Close the repeat control loop and obtain the THD corresponding to when the repeat control loop is closed.

14. The method according to any one of claims 9-12, characterized in that, Also includes: When the THD is less than a preset threshold, the lead amount is gradually adjusted N times with a preset step size when the preset period is reached. The THD of the power grid after each adjustment of the lead amount is formed into a set of THDs. The lead amount corresponding to the smallest THD in the set of THDs is compensated for with phase angle lead compensation.

15. The method according to any one of claims 9-12, characterized in that, Also includes: When the THD is greater than a preset threshold, the gain coefficient in the repetitive control loop of the inverter is changed until the THD is less than or equal to the preset threshold.

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