A method and system for improving the stability of grid-connected inverters under extremely weak power grid conditions

Abstract

This invention discloses a method and system for improving the stability of grid-connected inverters under extremely weak power grid conditions. The method includes: connecting compensators in series in the control loops of the DC voltage loop, current loop, and phase-locked loop of the grid-connected inverter; considering the interaction between the control loops, designing the compensator parameters in the order of designing the DC voltage loop and phase-locked loop first, followed by the current loop; and determining the compensator parameters by establishing an open-loop transfer function, selecting the gain margin, and verifying the phase margin, under the condition that the integral parameters of the compensator are fixed. This invention can enhance the adaptability of grid-connected inverters under extremely weak power grid conditions and greatly improve the stable operation capability of inverters under extremely low short-circuit ratios.

Description

Technical Field

[0001] This invention relates to the field of grid-connected inverter stability improvement technology, specifically a method and system for improving the stability of grid-connected inverters under extremely weak power grid conditions. Background Technology

[0002] The decentralized distribution of large-scale renewable energy power generation systems requires numerous transformers and long transmission lines to transport electricity from remote areas, resulting in a weak power grid with a low short-circuit ratio. In recent years, frequent accidents caused by grid weakness in renewable energy power generation systems both domestically and internationally have placed higher demands on the connection of renewable energy power generation equipment to extremely weak power grids. In this extremely weak grid environment, the oscillation modes of grid-connected inverters at different frequency bands will generate strong coupling through the grid connection point voltage, which will reduce the stability of the control system. When the inverter operates at full load or when the system experiences disturbances, the inverter faces the risk of oscillation instability. To ensure that grid-connected inverters can still generate electricity stably under low short-circuit ratio conditions, improving the stability of the inverter control loop at different frequency bands, thereby enhancing its adaptability under extremely weak power grid conditions, is a key issue that urgently needs to be addressed in renewable energy power generation.

[0003] Existing stability improvement methods mostly target oscillation problems in specific control loops or frequency bands, making it difficult to extend to other frequency bands. Therefore, different control strategies are usually required for different control loops, resulting in complex control structures and significant implementation difficulties. Moreover, as the grid short-circuit ratio further decreases, it is often difficult to simultaneously meet the stability requirements of low-frequency and mid-to-high-frequency bands, and the system still faces the risk of oscillation or even instability.

[0004] Patent application CN109301874A discloses a control method for grid-connected converters based on voltage disturbance compensation under weak power grid conditions. This method first establishes an impedance model of the three-phase grid-connected converter, including current loop, phase-locked loop, and DC voltage loop, in a synchronous rotating coordinate system. Based on the transmission relationships of variables in the impedance model, it analyzes the disturbance path of the grid connection point voltage on the outputs of the phase-locked loop and DC voltage loop, deriving expressions for the influence of disturbance components on the controller output. On this basis, disturbance compensation terms for the outputs of the phase-locked loop and DC voltage loop are added to the d-axis and q-axis current loop controllers to achieve grid voltage disturbance compensation. However, this method adds the disturbance compensation terms of the phase-locked loop and DC voltage loop to the current loop output for compensation, which is a feedforward compensation method. It does not change the characteristics of each control loop itself and can only improve stability in the low-frequency range, failing to address the instability problem in the mid-to-high frequency range dominated by the current loop.

[0005] Patent application CN113555904A discloses a stability criterion for the control loop of a grid-connected converter under weak grid conditions. The paper obtains the output current Ig and output voltage Ug of the grid-connected converter through sampling, and provides expressions for the grid impedance link G1(s), the phase-locked loop (PLL) closed-loop transfer function G2(s), and the current control loop closed-loop transfer function G3(s) without considering the PLL's influence. Essentially, this patent provides a stability criterion under weak grid conditions to further guide controller parameter design, but it does not address improvements to the control structure. Furthermore, even with optimal control parameters, each control loop still faces the risk of oscillation under weak grid conditions. Summary of the Invention

[0006] The technical problem to be solved by this invention is to provide a method for connecting a compensator in series in the control loop, thereby improving the stable operation capability of grid-connected inverters in extremely weak power grid environments.

[0007] To solve the above-mentioned technical problems, the present invention provides the following technical solution: A method for improving the stability of grid-connected inverters under extremely weak power grid conditions includes: Compensators are connected in series in the control loops of the DC voltage loop, current loop, and phase-locked loop of the grid-connected inverter, respectively. Considering the interaction between the control loops, the compensator parameters are designed in the order of designing the DC voltage loop and phase-locked loop first, and then the current loop. Under the condition that the integral parameters of the compensator are fixed, the compensator parameters are determined by establishing the open-loop transfer function, selecting the gain margin, and verifying the phase margin.

[0008] Technical Effects: This invention simultaneously incorporates compensators in the DC voltage loop, phase-locked loop (PLL), and current loop, which improves stability in both low-frequency and mid-to-high-frequency ranges compared to incorporating a compensator in only a single control loop. However, it requires consideration of several factors: each control loop corresponds to oscillation modes on different time scales, and significant coupling exists between control loops under weak grid conditions. Incorporating a compensator alters the open-loop gain of each control loop and the distribution of system oscillation modes. Furthermore, while larger compensator parameters generally result in better compensation, excessively large parameters can increase steady-state error and overshoot. Therefore, optimizing the selection of compensator parameters after incorporating compensators in the three control loops is a key challenge this invention needs to overcome. Existing methods first assess the grid strength; then, considering that the current loop has a relatively small impact on the PLL and DC voltage loop, while the PLL and DC voltage loop have a significant impact on the stability of the current loop, this invention designs the compensator parameters for the PLL and DC voltage loop first, and then design the compensator parameters for the current loop. Meanwhile, in order to further reduce the difficulty of parameter design, the parameters of one compensator are given first, and then the parameters of the other compensator are designed based on the stability margin.

[0009] Solution: First, assess the strength of the power grid using existing methods. Then, considering that the current loop has a relatively small impact on the phase-locked loop (PLL) and DC voltage loop, while the PLL and DC voltage loop have a significant impact on the stability of the current loop, this invention designs the PLL and DC voltage loop compensator parameters first, and then the current loop compensator parameters. Furthermore, to further reduce the difficulty of parameter design, the parameters of one compensator are given first, and then the parameters of the other compensator are designed based on the stability margin (corresponding to steps S10~S40 in the embodiment). Furthermore, the compensators connected in series in the control loops of the DC voltage loop, current loop, and phase-locked loop are as follows: ; ; ; In the formula, , , These represent compensators connected in series in the control loops of the DC voltage loop, current loop, and phase-locked loop, respectively. For the Laplace operator, , as well as These are the corresponding compensator parameters.

[0010] Furthermore, the compensator corresponding to the DC voltage loop is connected in series with the output terminal of the DC voltage loop PI controller; the compensator corresponding to the current loop is connected in series with the output terminal of the current loop PI controller; and the compensator corresponding to the phase-locked loop is connected in series with the output terminal of the phase-locked loop PI controller.

[0011] Furthermore, the DC voltage loop uses the DC side voltage sampling signal as input, the phase-locked loop uses the grid side signal after coordinate transformation as input, and the current loop uses the grid current sampling signal transformed to the dq coordinate system as input.

[0012] Furthermore, when the compensator parameters At this time, compensators in the DC voltage loop and phase-locked loop are used to improve stability in the low-frequency range, while compensators in the current loop are used to improve stability in the mid-to-high frequency range; when At that time, the compensator does not function; among them, When satisfied p x > z x At this time, the above-mentioned compensator behaves as a leading compensator; by reasonably designing the compensator parameters... z x , p x The amplitude margin can be increased near the system crossover frequency, while the phase margin can be improved by increasing the phase curve within the corresponding oscillation frequency band.

[0013] Furthermore, determining the compensator parameters includes the following steps: S31, Establish an open-loop transfer function for the control loop after the series compensator; S32, Select the gain margin based on the open-loop transfer function of the control loop; S33, Given the integral parameters of the compensator The compensator parameters are determined by the amplitude of the open-loop transfer function when the phase crosses 180°, which is a fixed value. ;in,

[0014] S34, verify the phase margin of the open-loop transfer function. If the phase margin does not meet the stability margin requirement, reset the gain margin and re-execute S32. Furthermore, the design methods for the compensator parameters corresponding to the DC voltage loop, phase-locked loop, and current loop are all executed according to steps S31 to S34.

[0015] Furthermore, the stability margin requirements are: a gain margin greater than 6dB and a phase margin of 30° to 60°.

[0016] Furthermore, when designing the parameters of the current loop compensator, the amplitude margin is set to 8dB.

[0017] The present invention also provides a system for improving the stability of grid-connected inverters under extremely weak power grids as described above, characterized in that it includes: The compensation module is used to connect compensators in series in the control loops of the DC voltage loop, current loop and phase-locked loop of the grid-connected inverter, respectively. The parameter design module is used to consider the interactive effects between the control loops. The compensator parameters are designed in the order of designing the DC voltage loop and phase-locked loop first, and then the current loop. Under the condition that the integral parameters of the compensator are fixed, the compensator parameters are determined by establishing the open-loop transfer function, selecting the gain margin and verifying the phase margin.

[0018] Compared with the prior art, the beneficial effects of the present invention are: This invention addresses the problem of low-frequency oscillations dominated by the DC voltage loop and phase-locked loop, and medium-to-high-frequency oscillations dominated by the current loop, that easily occur in grid-connected inverters under extremely weak grid conditions with low short-circuit ratios. By inserting compensators in series in the control loops, the stability of each control loop is enhanced, reducing the oscillation risk of the inverter in different frequency bands and effectively solving the instability problem caused by insufficient grid strength. This invention can enhance the adaptability of grid-connected inverters under extremely weak grid conditions and greatly improve the stable operation capability of inverters under extremely low short-circuit ratios.

[0019] Meanwhile, under extremely weak power grid conditions, unreasonable compensator parameter design can lead to larger steady-state errors in the system, which is detrimental to the system operation. This invention takes into account that under weak power grid conditions, the current loop has a smaller impact on the phase-locked loop and the DC voltage loop, while the phase-locked loop and the DC voltage loop have a larger impact on the current loop. Therefore, the compensator parameters of the DC voltage loop and the phase-locked loop are designed first, and then the compensator parameters of the current loop are designed, which greatly reduces the difficulty of parameter design. Attached Figure Description

[0020] Figure 1 This is a flowchart illustrating a method for improving the stability of a grid-connected inverter under extremely weak power grid conditions, according to an embodiment of the present invention.

[0021] Figure 2 This is a logic block diagram of the stability improvement method according to an embodiment of the present invention.

[0022] Figure 3 This is a comparison diagram of the open-loop transfer function at different frequency bands before and after the series compensator is inserted in an embodiment of the present invention.

[0023] Figure 4 This is a schematic diagram of the simulation waveform of the inverter when the system short-circuit ratio changes in an embodiment of the present invention.

[0024] Figure 5 This is a schematic diagram of the simulation waveform of the inverter when the phase-locked loop parameters change in an embodiment of the present invention.

[0025] Figure 6 The experimental waveforms of the inverter in this embodiment of the invention before and after the introduction of the compensator under different operating conditions are shown. Detailed Implementation

[0026] To facilitate understanding of the technical solution of the present invention by those skilled in the art, the technical solution of the present invention will now be further described in conjunction with the accompanying drawings.

[0027] The terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Therefore, a feature defined as "first" or "second" may explicitly or implicitly include one or more of that feature. In the description of this application, "multiple" means two or more, unless otherwise explicitly specified.

[0028] Please see Figure 1 , Figure 2 As shown, this invention provides a method for improving the stability of a grid-connected inverter under extremely weak power grid conditions, comprising: S10, compensators are connected in series in the control loops of the DC voltage loop, current loop and phase-locked loop of the grid-connected inverter.

[0029] In one embodiment of the present invention, the stability improvement method involves connecting compensators in series in the inverter control loop to improve the stability of the inverter under extremely weak power grid conditions, combined with... Figure 2 This method can be implemented. H DVC ( s ), H CC ( s )and H PLL ( s ) respectively correspond to Figure 1 The transfer functions of the PI controller for the DC voltage loop, current loop, and phase-locked loop in (a), (b), and (c) are as follows: H DVC (s)= k vp + k vi / s, H CC (s)= k ip + k ii / s and H PLL (s)= k pp + k pi / s; where, H DVC ( s ), H CC( s ), H PLL ( s These represent the transfer functions of the PI controller for the DC voltage loop, current loop, and phase-locked loop, respectively. k xp , k xi , x =v,i,p are the proportional and integral parameters of each PI controller, respectively. U dc , U dcref These are the actual and reference values ​​for the DC voltage loop, respectively. i dref / i qref , i d / i q These are the reference and actual values ​​of the disturbance for the d-axis and q-axis currents, respectively. These are the amplitude and phase angle of the grid-connected voltage signal at time t. These are the angular frequency disturbance and the fundamental angular frequency of the power grid, respectively. For the Laplace operator.

[0030] In this embodiment, the compensators connected in series in the control loops of the DC voltage loop, current loop, and phase-locked loop are respectively: ; ; ; In the formula, , , These represent compensators connected in series in the control loops of the DC voltage loop, current loop, and phase-locked loop, respectively. For the Laplace operator, z v , p v , z i , p i as well as z p , p p These are the corresponding compensator parameters.

[0031] In this embodiment, the compensator corresponding to the DC voltage loop is connected in series with the output of the DC voltage loop PI controller to improve the system's low-frequency stability. The compensator corresponding to the current loop is connected in series with the output of the current loop PI controller to improve the system's high-frequency stability. The compensator corresponding to the phase-locked loop (PLL) is connected in series with the output of the PLL PI controller to improve low-frequency stability.

[0032] The DC voltage loop uses the DC-side voltage sampling signal as input, the phase-locked loop uses the grid-side signal after coordinate transformation as input, and the current loop uses the grid-connected current sampling signal transformed to the dq coordinate system as input. When compensator parameters p x > z x At this time, compensators in the DC voltage loop and phase-locked loop are used to improve stability in the low-frequency range, while compensators in the current loop are used to improve stability in the mid-to-high frequency range; when p x = z x At that time, the compensator does not work.

[0033] S20. Considering the interaction between each control loop, the compensator parameters are designed in the order of designing the DC voltage loop and phase-locked loop first, and then the current loop.

[0034] S30, under the condition that the integral parameters of the compensator are fixed, the compensator parameters are determined by establishing the open-loop transfer function, selecting the gain margin, and verifying the phase margin.

[0035] In one embodiment of the present invention, determining the compensator parameters includes the following steps: S31, establish the open-loop transfer function for the control loop after the series compensator.

[0036] S32, select the gain margin based on the open-loop transfer function of the control loop.

[0037] S33, Given the integral parameters of the compensator z x The amplitude is a fixed value, and the compensator parameters are determined by the magnitude when the phase of the open-loop transfer function crosses 180°.

[0038] In this embodiment, considering that the operating point of the grid-connected inverter typically changes slowly, the integral parameters of the compensator are... z x Providing a suitable fixed value can not only meet the basic steady-state error requirements, but also greatly reduce the difficulty of parameter design; in this embodiment, all values ​​are given. z x It is 10.

[0039] S34: Verify the phase margin of the open-loop transfer function. If the phase margin does not meet the stability margin requirement, reset the gain margin and re-execute S32. In this embodiment, the gain margin of the given open-loop transfer function is 6dB, and the calculated compensator parameters for the DC voltage loop are substituted into this value. p v The value is 92, which meets the requirements.

[0040] Since phase-locked loops (PLLs) and DC voltage loops dominate the low-frequency oscillation mode of inverters, introducing compensators into both PLLs and DC voltage loops can improve low-frequency stability under weak grid conditions; therefore, the parameters of PLL compensators... z p , p p The parameters can be consistent with those of a DC voltage loop compensator, i.e. z p =10, p p =92; In this embodiment, based on the quantitative indicators of inverter stability in engineering, the stability margin requirements are: amplitude margin greater than 6dB and phase margin of 30° to 60°, which are used to evaluate the rationality of the compensator parameters.

[0041] In this embodiment, the design methods for the compensator parameters corresponding to the DC voltage loop, phase-locked loop, and current loop are all executed according to steps S31 to S34. In the design of the compensator parameters in the current loop, the integral parameters are given... z i =10. Considering that the current loop mainly affects the stability at mid-to-high frequencies, the gain margin of the open-loop transfer function can be selected as 8dB. Substituting the gain margin, the compensator parameters are calculated. p i Draw the Bode plot of the open-loop transfer function and observe whether the phase margin meets the requirements. If it does, the design is complete; if not, repeat step S32.

[0042] Assuming that the designed parameters have met the requirements through the above methods, the parameters of the control loop compensator are shown in Table 1 below.

[0043] Table 1. Compensator parameter design results obtained by the method of the present invention.

[0044] To explain the rationality of the compensator parameters obtained above, Figure 3 This embodiment shows a comparison of the open-loop transfer functions of the inverter before and after the addition of the series compensator at different frequency bands. Figure 3 (a) shows the frequency characteristics in the low-frequency band. Before compensation, the amplitude margin is negative and the phase lag exceeds 180° at the crossover frequency. After compensation, both the amplitude margin and phase margin meet the design requirements. Figure 3 (b) shows the frequency characteristics of the mid-to-high frequency band, and the results are similar to those of the low frequency band, so they will not be described in detail here.

[0045] Furthermore, after obtaining the compensator parameters, the accuracy of the present invention needs to be further verified through experiments or simulations.

[0046] like Figure 4 As shown, a simulation model was built in Simulink to verify the stability of the inverter under extremely weak power grid conditions. It should be noted that the DC voltage loop, phase-locked loop, and current loop PI parameters need to be adjusted appropriately to better adapt to the extremely weak power grid environment. In this embodiment, the effectiveness of the method proposed in this invention is tested on the premise that the stability of the grid-connected inverter cannot be met under the extremely weak power grid after optimizing the controller parameters using existing traditional methods.

[0047] It should be noted that in the verification methods of the following embodiments, the change in the short-circuit ratio is achieved by the voltage drop of the power grid.

[0048] like Figure 4 As shown in Figure (a), when the system short-circuit ratio changes from 1.5 to 1.3 and no compensator is introduced in the control loop, the system dynamic response waveform oscillates and diverges.

[0049] And in Figure 4 After the compensator is introduced in (b), the inverter's state variables quickly recover to stability after the disturbance, which shows that the inverter's stability is improved under a low short-circuit ratio of 1.3. Furthermore, Figure 5 Figure (a) shows the necessity of inserting a compensator in series in the current loop. Figure 4 As shown in (a), the proportional parameters of the phase-locked loop (PLL) are as follows when no compensator is inserted in series in the current loop. k pp After the value was changed from 0.2 to 0.8, the inverter exhibited mid-to-high frequency oscillations related to the current loop. This means that the increase in the phase-locked loop parameters led to a stronger interaction between the phase-locked loop and the current loop, thereby affecting the stability of the current loop.

[0050] After inserting a compensator in series in the current loop, Figure 5 In (b), it is shown that when the phase-locked loop proportional parameter k pp During switching, the inverter's mid-to-high frequency oscillations decayed rapidly, proving that the current loop compensator can improve the inverter's stability in the mid-to-high frequency range. To further verify the stability advantage of the proposed method under extremely weak power grids, it was validated on an experimental platform. Assuming that the compensator parameters have been adjusted according to the actual experimental prototype parameters, the adjustment method still follows the compensator parameter design process, and the adjusted parameters are shown in Table 2 below; Table 2 Compensator parameters under experimental platform parameters

[0051] Continuously reduce the short-circuit ratio, such as Figure 6 The results show the experimental comparison before and after the control loop was connected in series with the compensator when the short-circuit ratio decreased from 1.4 to 1.3 and from 1.3 to 1.1, respectively. Before the addition of the compensator, when the short-circuit ratio switched from 1.4 to 1.3, the inverter exhibited oscillation and divergence, indicating that as the short-circuit ratio decreased while disturbances were present, the inverter's stability declined, and it could not operate stably. After introducing the compensator, the inverter could operate stably in a weak grid environment with a short-circuit ratio of 1.3, such as... Figure 6 As shown in (a); Furthermore, when the short-circuit ratio switches from 1.3 to 1.1, the inverter can still return to stability, indicating that the compensator greatly improves the inverter's stable operation capability under extremely low short-circuit ratios. Figure 6 As shown in (b). The above verification results reflect Figure 2 The effectiveness of the stability improvement method shown is demonstrated, and the rationality of the compensator parameter design process is also verified.

[0052] The present invention also provides a system for improving the stability of grid-connected inverters under extremely weak power grid conditions as described above, comprising: The compensation module is used to connect compensators in series in the control loops of the DC voltage loop, current loop, and phase-locked loop of the grid-connected inverter.

[0053] The parameter design module is used to consider the interactive effects between the control loops. The compensator parameters are designed in the order of designing the DC voltage loop and phase-locked loop first, and then the current loop. Under the condition that the integral parameters of the compensator are fixed, the compensator parameters are determined by establishing the open-loop transfer function, selecting the gain margin and verifying the phase margin.

[0054] The compensation module includes a DC voltage loop compensation unit, a phase-locked loop compensation unit, and a current loop compensation unit. The DC voltage loop compensation unit is located at the output of the DC voltage loop PI controller and is used to input the transfer function as follows: G dv ( s )=( s + z v ) / ( s + p vA compensator is used to improve the system's low-frequency stability. The phase-locked loop (PLL) compensation unit is also located at the output of the PLL PI controller, used to input the transfer function... G dp ( s )=( s + z p ) / ( s + p p A compensator is used to improve low-frequency stability. The current loop compensation unit is located at the output of the current loop PI controller and is used to input the transfer function as follows: G di ( s )=( s + z i ) / ( s + p i The compensator is used to improve the stability of the high-frequency band in the system.

[0055] The system also includes an inverter basic control unit, which is used to acquire DC side voltage signal, grid side voltage signal and grid-connected current signal, and form control inputs for DC voltage loop, phase-locked loop and current loop respectively, so that each compensation unit works together to improve the stable operation capability of the grid-connected inverter under extremely weak grid conditions.

[0056] It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, and that the invention can be implemented in other specific forms without departing from its spirit or essential characteristics. Therefore, the embodiments should be considered illustrative and non-limiting in all respects, and the scope of the invention is defined by the appended claims rather than the foregoing description. Thus, all variations falling within the meaning and scope of equivalents of the claims are intended to be included within the present invention, and no reference numerals in the claims should be construed as limiting the scope of the claims.

[0057] The above embodiments are merely examples of implementation methods of the invention. The scope of protection of the present invention is not limited to the above embodiments. For those skilled in the art, several modifications and improvements can be made without departing from the concept of the present invention, and these all fall within the scope of protection of the present invention.

Claims

1. A method for improving stability of a grid-connected inverter under extremely weak grid, characterized in that, include: Compensators are connected in series in the control loops of the DC voltage loop, current loop, and phase-locked loop of the grid-connected inverter, respectively. Considering the interaction between the control loops, the compensator parameters are designed in the order of designing the DC voltage loop and phase-locked loop first, and then the current loop. Under the condition that the integral parameters of the compensator are fixed, the compensator parameters are determined by establishing the open-loop transfer function, selecting the gain margin, and verifying the phase margin.

2. The method for improving stability of a grid-connected inverter under extremely weak grid according to claim 1, characterized in that, The compensators connected in series in the control loops of the DC voltage loop, current loop, and phase-locked loop are as follows: ； ； ； wherein , , are the compensators in series in the control loop of the DC voltage loop, the current loop and the phase-locked loop, respectively, is the Laplace operator, , and are the corresponding compensator parameters, respectively.

3. The method for improving the stability of grid-connected inverters under extremely weak power grid conditions according to claim 1, characterized in that, The compensator corresponding to the DC voltage loop is connected in series with the output terminal of the DC voltage loop PI controller; the compensator corresponding to the current loop is connected in series with the output terminal of the current loop PI controller; and the compensator corresponding to the phase-locked loop is connected in series with the output terminal of the phase-locked loop PI controller.

4. The method for improving the stability of grid-connected inverters under extremely weak power grid conditions according to claim 3, characterized in that, The DC voltage loop uses the DC side voltage sampling signal as input, the phase-locked loop uses the grid side signal after coordinate transformation as input, and the current loop uses the grid current sampling signal transformed to the dq coordinate system as input.

5. The method for improving the stability of grid-connected inverters under extremely weak power grid conditions according to claim 2, characterized in that, When compensator parameters At this time, compensators in the DC voltage loop and phase-locked loop are used to improve stability in the low-frequency range, while compensators in the current loop are used to improve stability in the mid-to-high frequency range; when At that time, the compensator does not function; among them, .

6. The method for improving the stability of grid-connected inverters under extremely weak power grid conditions according to claim 1, characterized in that, Determining the compensator parameters includes the following steps: S31, Establish an open-loop transfer function for the control loop after the series compensator; S32, Select the gain margin based on the open-loop transfer function of the control loop; S33, Given the integral parameters of the compensator The compensator parameters are determined by the amplitude of the open-loop transfer function when the phase crosses 180°, which is a fixed value. ;in, S34 verifies the phase margin of the open-loop transfer function. If the phase margin does not meet the stability margin requirement, the gain margin is reset and S32 is executed again.

7. The method for improving the stability of grid-connected inverters under extremely weak power grids according to claim 5, characterized in that, The compensator parameters for the DC voltage loop, phase-locked loop, and current loop are designed according to steps S31 to S34.

8. The method for improving the stability of grid-connected inverters under extremely weak power grid conditions according to claim 5, characterized in that, The stability margin requirements are: a gain margin greater than 6dB and a phase margin of 30° to 60°.

9. The method for improving the stability of grid-connected inverters under extremely weak power grid conditions according to claim 8, characterized in that, When designing the parameters of the current loop compensator, the amplitude margin is set to 8dB.

10. A system for improving the stability of a grid-connected inverter under extremely weak power grid conditions according to any one of claims 1-9, characterized in that, include: The compensation module is used to connect compensators in series in the control loops of the DC voltage loop, current loop and phase-locked loop of the grid-connected inverter, respectively. The parameter design module is used to consider the interactive effects between the control loops. The compensator parameters are designed in the order of designing the DC voltage loop and phase-locked loop first, and then the current loop. Under the condition that the integral parameters of the compensator are fixed, the compensator parameters are determined by establishing the open-loop transfer function, selecting the gain margin and verifying the phase margin.

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