A control method for LCL type grid-connected inverter based on improved time delay suppression and passivity-based optimization

Abstract

This invention provides an improved parallel frequency-segment compensation and passive optimization control method for grid-connected inverters. While weighted average current control of inverters is widely used due to its loop order reduction characteristics, it fails to adequately consider the impact of control delay, leading to system instability when the resonant frequency approaches 1 / 6 of the sampling frequency. This method introduces a frequency-segment filter into the feedforward channel to construct a parallel frequency-segment compensation strategy, compensating for the phase delay caused by the reverse resonant peak frequency shift. Then, based on the stability design requirements of passive theory, corresponding closed-loop system parameters are designed to ensure that the inverter's equivalent output impedance meets the passive stability requirements, thereby improving system stability under weak grid conditions. Through passive theory analysis and impedance reshaping analysis, the proposed control method not only effectively compensates for phase delay and suppresses resonant instability but also enhances the inverter's robustness against grid inductive disturbances. Simulation and experimental results show that, compared with traditional weighted average current control, this strategy significantly improves system stability when the resonant frequency is close to 1 / 6 of the sampling frequency and exhibits stronger robustness against inductive disturbances.

Description

Technical Field

[0002] This invention belongs to the field of grid-connected inverter control technology, specifically relating to a control method for LCL-type grid-connected inverters suitable for weak grid conditions. In particular, it relates to a novel parallel frequency band compensation and passive optimization strategy that comprehensively considers digital control delay, resonant frequency offset and grid impedance disturbance under a weighted average current control framework, which can be used for high-performance grid-connected interfaces of renewable energy power generation systems. Background Technology

[0004] As a key conversion device between renewable energy generation units and the public power grid, the LCL-type grid-connected inverter's control performance directly affects the grid-connected power quality and system stability. Weighted Average Current (WAC) control is widely used because it can reduce the LCL filter to a first-order system, simplifying controller design. However, existing technologies still have the following prominent problems in practical engineering applications:

[0005] I. Current Status and Shortcomings of Weighted Average Current Control Research

[0006] While some progress has been made in the research of control strategies for LCL-type grid-connected inverters, significant limitations remain. Traditional weighted average current control achieves loop order reduction through linear combination of inverter-side and grid-side currents, yielding good steady-state performance under ideal conditions. However, this control strategy does not adequately consider the impact of digital control delay. When the resonant frequency of the LCL filter approaches 1 / 6 of the sampling frequency, the phase lag caused by the control delay leads to a reverse resonant peak in the system's open-loop transfer function, a sharp decrease in phase margin, and ultimately, system instability.

[0007] The proposed improvements include introducing a high-pass filter into the current loop, using a notch filter to compensate for phase lag, adding a negative first-order low-pass filter to the capacitor current feedback, or adding a lead compensation stage to the grid voltage feedforward channel. However, these methods generally have the following shortcomings: First, they do not systematically analyze the robustness of the grid inductive reactance under large-scale changes; second, they require real-time detection of the resonant frequency, increasing implementation complexity; third, they do not provide clear parameter design criteria based on passive theory; and fourth, the negative damping term in some methods continuously accumulates noise, affecting control performance.

[0008] II. The core problem to be solved by this invention

[0009] This invention aims to systematically solve four key technical challenges in the weighted average current control of current LCL-type grid-connected inverters:

[0010] 1. Resonance instability caused by control delay: Digital control delay causes the system to have a reverse resonance peak when the resonant frequency is close to 1 / 6 of the sampling frequency. The phase margin is insufficient, and the system is prone to instability.

[0011] 2. Problem of poor adaptability to inductive reactance disturbances under weak power grids: The inductive reactance of the power grid has time-varying and wide-range fluctuation characteristics. When the inductive reactance increases, the resonance peak shifts to the left, the phase margin is further reduced, and the robustness is insufficient.

[0012] 3. The problem of not being able to simultaneously achieve complex phase compensation and loop order reduction characteristics: Existing compensation methods often destroy the order reduction characteristics of weighted average current control, resulting in a decrease in system dynamic performance or a complex controller structure.

[0013] 4. The problem of lacking systematic passive design criteria: Most methods rely on trial and error or empirical parameters, and do not provide constraints on the equivalent output impedance from the perspective of passive theory, making it difficult to strictly guarantee stability.

[0014] Therefore, this invention aims to propose an integrated control method that can simultaneously achieve delay phase compensation, loop order reduction and maintenance, improved robustness of inductive disturbances, and passive parameter design, so as to ensure stable grid connection and high-quality current output of the inverter under weak grid conditions. Summary of the Invention

[0016] The purpose of this application is to provide a control method for an LCL-type grid-connected inverter based on improved time delay suppression and passive performance optimization, including:

[0017] S1: Establish a weighted average current control mathematical model for LCL grid-connected inverters, analyze the impact of control delay on system stability, and determine the frequency range in which the reverse resonance peak occurs;

[0018] S2: Introduce a parallel frequency-division compensator in the voltage feedforward channel to form an improved parallel frequency-division compensation strategy. The high-frequency, low-frequency and mid-frequency bands are compensated by the high-pass, low-pass and band-pass branches respectively to compensate for the phase delay caused by the frequency shift of the reverse resonant peak.

[0019] S3: Design the corner frequency, gain coefficient, and delay coefficient of the high-frequency phase compensator, low-frequency amplitude compensator, and medium-frequency amplitude compensator so that the equivalent output impedance of the inverter meets the passive stability requirements in the Nyquist band.

[0020] S4: Based on the stability criterion of passive theory, analyze the open-loop frequency characteristics, closed-loop pole distribution and output impedance phase characteristics of the compensated system, and verify the stability of the system under different grid inductive reactance conditions.

[0021] S5: Through simulation and semi-physical experimental platforms, the grid-connected current waveforms, dynamic responses and robustness of traditional weighted average current control and the proposed improved strategy under different inductive reactance values ​​are compared.

[0022] A control method for an LCL-type grid-connected inverter based on improved time delay suppression and passive performance optimization, characterized in that step S1 includes:

[0023] A mathematical model for weighted average current control is established. The reference current i... ref The open-loop transfer function to the weighted average current i is: (1)

[0024] In the low-frequency range, higher-order terms and damping terms are ignored, and the current weighting coefficient and voltage feedforward gain satisfy the following conditions:

[0025] (2)

[0026] The open-loop transfer function simplifies to a form with first-order properties:

[0027] (3)

[0028] Among them G r (s) represents a quasi-proportional resonant controller. At this point, the system has sufficient phase margin and remains stable. Introducing a digital control delay is equivalent to a lag element of 1.5 times the sampling period, with a transfer function of G. d (s)=e −s·1.5Ts Through Taylor approximation, we obtain:

[0029] (4)

[0030] Delay function G d Multiplying (s) by the forward path, the inverter's open-loop transfer function T(s) is expressed as:

[0031] (5)

[0032] Analysis shows that after introducing a control delay, the transfer function no longer possesses first-order characteristics, a reverse resonance peak appears at the resonant frequency, the phase margin decreases to a critical value, and the system becomes unstable. Simultaneously, when the grid inductive reactance L... g When the resonant amplitude increases from 1.1 mH to 7.7 mH, the resonant amplitude increases and the resonant peak shifts to the left, while the phase margin decreases further.

[0033] A control method for an LCL-type grid-connected inverter based on improved time delay suppression and passive performance optimization, characterized in that step S2 includes:

[0034] A parallel frequency-segmented compensator is introduced into the voltage feedforward channel, and its transfer function G(s) is defined as:

[0035] (6)

[0036] Where a1, a2, and a3 are the gains of the high-pass, low-pass, and band-pass filters, respectively; f high f low f mid The system is divided into high-pass, low-pass, and band-pass cutoff frequencies; λ1, λ2, and λ3 are the delay coefficients of the multi-band components in the grid-connected voltage feedforward channel. Through simulation experiments, the delay coefficients of the system's multi-band components were tested and selected multiple times. The parallel frequency divider filter, through intelligent weight allocation in the frequency domain, can maintain low-frequency stability while suppressing high-frequency resonance, thereby extending the system's stable operating range. The stability problem of digital control delay is solved through control algorithm optimization.

[0037] After the parallel frequency band compensator is connected to the voltage feedforward channel, the system control block diagram is as follows: Figure 6 As shown. The improved open-loop transfer function of the system is expressed as: (7)

[0038] A control method for an LCL-type grid-connected inverter based on improved time delay suppression and passive performance optimization, characterized in that step S3 includes:

[0039] High-frequency phase compensator design:

[0040] Choose f high The frequency is 2500Hz. As the grid voltage increases, Zo(s) exhibits negative impedance characteristics near the resonant frequency, which is detrimental to suppressing high-frequency resonant components. When α1 ≤ 0.5, Zo(s) exhibits passive characteristics within the range of [-90°, 90°]. α1 is chosen to be 0.4. Simulation results show that as the value of λ1 increases, the phase lag of the compensator gradually increases. When λ1 = 2.5, the phase of the compensator at the resonant frequency approaches -180°, satisfying the requirements of closed-loop negative feedback for high-frequency components, thereby enhancing the system's ability to suppress high-frequency disturbances.

[0041] Low-frequency amplitude compensator design:

[0042] To improve the Zo(s) modulus, reduce the harmonic content in the low-frequency region, and enhance the suppression of low-frequency noise, λ2 is chosen as the minimum delay of 1.5 beats. Because there are many low-order harmonics in the power grid, f... low The value should be less than f r To avoid interaction with the resonant frequency, it is ultimately set to f. low =50Hz, set a2 to 0.4.

[0043] Intermediate frequency amplitude compensator design:

[0044] Design bandpass frequency f hmid = 800Hz, f lmid = 200Hz. Simulation tests were conducted to analyze the value of λ3. The results show that the value of λ3 affects the bandpass characteristics of the compensator in the mid-frequency range. To further optimize the phase margin of the system near the resonant frequency and without affecting the compensation effect in the high and low frequency bands, λ3 was set to a 1.0 beat delay to ensure compensation accuracy. Theoretical analysis and final simulation experiments show that when α3 ≤ 0.3, the compensator can effectively smooth the output impedance phase characteristic curve without introducing phase distortion; ultimately, α3 = 0.2.

[0045] Step S3 further includes using a quasi-proportional resonant controller as a current regulator, whose transfer function is:

[0046] (8)

[0047] In the formula, K r K is the resonance coefficient; p ω is the proportional gain coefficient. To ensure the controller still provides gain even when the fundamental frequency fluctuates, ω is set to... i =πrad / s. ω i The resonant cutoff frequency, ω o This is the fundamental angular frequency. The scaling factor K... p The following conditions must be met:

[0048] (9)

[0049] To minimize the impact of the proportional resonant controller at the cutoff frequency ω c The phase lag at this point is usually set to its resonant angular frequency as ω. c / 10, at this time the resonance coefficient K r It can be calculated using the following formula:

[0050] (10)

[0051] Once the cutoff frequency is determined, a suitable K can be obtained from the inverter parameters in Table 1. p With K r Values ​​can be obtained by substituting them into the calculation. p = 0.031623, K r =8.0.

[0052] A control method for an LCL-type grid-connected inverter based on improved time delay suppression and passive performance optimization, characterized in that step S4 includes:

[0053] Within the Nyquist band, the phase angle corresponding to the real part of the inverter's equivalent output impedance lies within the range of [-90°, 90°]. By designing the parameters of the parallel frequency band compensator, the compensated equivalent output impedance Z is made possible. eq The phase is constrained within the range of [-90°, 90°], thus satisfying the passivity stability criterion.

[0054] The expression for the equivalent output impedance is:

[0055] (11)

[0056] Where T(s) is the open-loop transfer function of the improved system; G r (s) is a quasi-proportional resonant controller, and its expression is shown in equation (6); G d (s) is the control delay transfer function; K p L1 represents the modulation gain; L2 and C represent the inverter-side inductance, grid-side inductance, and filter capacitor, respectively.

[0057] By designing the parameters of the parallel frequency band compensator, the phase of the compensated equivalent output impedance is constrained within the range of [-90°, 90°], thereby satisfying the passive stability criterion, such as... Figure 8 As shown.

[0058] The system open-loop transfer function frequency response is as follows Figure 9 As shown. After adopting the parallel frequency band compensation strategy, the open-loop transfer function still possesses first-order characteristics. When the grid inductive reactance L g When the phase varies in the range of 1.1mH to 7.7mH, the system maintains a sufficient positive phase stability margin.

[0059] Closed-loop pole distribution analysis shows that: under traditional weighted average current control, when L g When the system poles vary within the range of 1.1 mH to 7.7 mH, they shift towards the right half of the complex plane, leading to instability. Figure 10 After adopting the improved strategy of this invention, within the same range of inductive reactance variation, the system poles are all located in the left half of the complex plane, proving that the system has sufficient stability. Figure 11 ).

[0060] A control method for an LCL-type grid-connected inverter based on improved time delay suppression and passive performance optimization, characterized in that step S5 includes:

[0061] Simulation verification: A simulation model of a single-phase LCL inverter with a rated power of 20kW was built in MATLAB / Simulink. Using the parameters shown in Table 1, the traditional weighted average current control and the improved strategy of this invention were compared.

[0062] When the grid inductive reactance Lg When the current is 0mH, both strategies can guarantee power quality; when the grid inductive reactance L g When the current is 7.7mH, the traditional strategy exhibits obvious waveform oscillations and system instability, while the improved strategy of this invention maintains a stable waveform and good current sinusoidality. Under the dynamic switching conditions between full load (20kW) and half load (10kW), the grid-connected current waveform using the improved strategy of this invention remains stable and has a good dynamic response.

[0063] Hardware-in-the-loop simulation verification:

[0064] Based on the OP5600 RT-LAB platform, using the same parameters as the simulation, the inductive reactance L of the power grid was measured. g Under the condition of 7.7mH, traditional weighted average current control causes significant oscillations in the system waveform. Figure 16 The improved strategy of this invention restores system stability and improves grid-connected current quality. Figure 17 This demonstrates that the proposed control strategy is effectively robust to inductive disturbances under weak power grid conditions.

[0065] Compared with the prior art, the beneficial effects of the present invention are:

[0066] Effective suppression of resonance instability caused by time delay: By introducing a parallel frequency-segment compensator in the feedforward channel to perform phase compensation on the reverse resonance peak interval, the problem of system instability when the resonant frequency is close to 1 / 6 of the sampling frequency is solved.

[0067] Maintaining Loop Order Reduction Characteristics: Unlike traditional compensation methods, this invention retains the loop order reduction characteristics of weighted average current control while performing phase compensation, ensuring the quality of the system's dynamic response.

[0068] Enhancing robustness to inductive reactance disturbances in weak power grids: Designing closed-loop system parameters based on passive theory to ensure that the inverter's equivalent output impedance meets passive requirements, maintaining stability even when the grid's inductive reactance changes over a wide range.

[0069] The parameter design is clear and highly practical for engineering applications: it provides the selection principles and optimal values ​​for the specific cutoff frequencies, gain coefficients, and delay coefficients of the high-frequency, low-frequency, and intermediate-frequency compensators, eliminating the need for complex iterations or online identification. Attached Figure Description

[0071] Figure 1 This is a flowchart illustrating the control steps of an LCL-type grid-connected inverter based on improved time delay suppression and passive performance optimization according to the present invention.

[0072] Figure 2 This is a topology diagram of an LCL-type grid-connected inverter under weighted average current control.

[0073] Figure 3 This is a block diagram of the feedback control of a grid-connected inverter under weighted average current control.

[0074] Figure 4 To simplify the open-loop transfer function logarithmic frequency response diagram of the control system.

[0075] Figure 5 The logarithmic frequency response of the open-loop transfer function of the system under control delay.

[0076] Figure 6 To improve the system control block diagram after delay compensation.

[0077] Figure 7 The logarithmic frequency response of the transfer function is shown for different inductive reactances.

[0078] Figure 8 This is the Bode plot of the output impedance after frequency division compensation.

[0079] Figure 9 This is the logarithmic frequency response diagram of the open-loop transfer function of the inverter grid when the inductive reactance changes.

[0080] Figure 10 This is a diagram showing the closed-loop pole distribution of a traditional WAC control system under varying grid inductive reactance.

[0081] Figure 11 To improve the closed-loop pole distribution diagram of WAC control under varying grid impedance.

[0082] Figure 12 For L g WAC simulation waveform before improvement when =0mH.

[0083] Figure 13 For L g Improved WAC simulation waveform at 0mH.

[0084] Figure 14 For L g WAC simulation waveform before improvement when =7.7mH.

[0085] Figure 15 For L g The improved WAC simulation waveform at 7.7mH.

[0086] Figure 16 For L g Waveform diagram of traditional WAC hardware-in-the-loop simulation at 7.7mH.

[0087] Figure 17 For L g Improved WAC hardware-in-the-loop waveform at 7.7mH.

[0088] Figure 18Improve the WAC dynamic waveform diagram during full-load / half-load switching. Detailed implementation method:

[0089] The present invention will be further described in detail below with reference to the embodiments and accompanying drawings, but the embodiments of the present invention are not limited thereto.

[0090] Example 1: Simulation Verification of Single-Phase LCL Grid-Connected Inverter

[0091] This embodiment uses the inverter parameters shown in Table 1. The grid voltage is 220V, the bus voltage is 360V, the switching frequency is 10kHz, the sampling frequency is 20kHz, the inverter-side inductance is 1.5mH, the grid-side inductance is 0.5mH, the filter capacitor is 6μF, and the carrier amplitude is 1V.

[0092] Step 1: Establish a mathematical model

[0093] The open-loop transfer function of the weighted average current control is established according to equations (1) to (5), and the influence of control delay is analyzed. It is determined that the system has the risk of instability when the resonant frequency is close to 1 / 6 of the sampling frequency.

[0094] Step 2: Design a parallel frequency band compensator

[0095] Design the compensator transfer function according to equation (6). High-frequency compensator parameters:

[0096] f high =2500Hz, a1=0.4, λ1=2.5; Low-frequency compensator parameters: flow=50Hz, a2=0.4, λ2=1.5 beats; Medium-frequency compensator parameters: f lmid =200Hz, f hmid =800Hz, a3=0.2, λ3=1.0 beats.

[0097] Step 3: Design the current regulator

[0098] A quasi-proportional resonant controller is used, and the parameters are calculated according to equations (8) to (10). Let K... p =0.031623, K r =8.0, ω i =πrad / s.

[0099] Step 4: Passive stability analysis

[0100] The equivalent output impedance Zo(s) is calculated according to equation (12), and its Bode plot is plotted. It is verified that the phase angle of Zo(s) in the Nyquist band is in the interval [-90°, 90°], which satisfies the passive stability criterion.

[0101] Step 5: Simulation Comparison

[0102] Build a simulation model in MATLAB / Simulink. Set the grid inductive reactance L. g The values ​​are 0mH and 7.7mH, respectively. The simulation duration is 0.2s, and a variable step size ode45 solver is used.

[0103] When L g When the current is 0mH, both the traditional WAC strategy and the improved strategy of this invention can output grid-connected current with good sinusoidal properties, and the THD is less than 3%.

[0104] When L g When the current is 7.7mH, the grid-connected current of the traditional WAC strategy oscillates significantly, the THD exceeds 15%, and the system becomes unstable; the improved strategy of this invention maintains a sinusoidal grid-connected current, the THD is below 3.5%, and the waveform is stable.

[0105] Dynamic response test: When the reference current is switched from full load (20kW) to half load (10kW) at 0.1s, the grid-connected current under the improved strategy of this invention recovers to stability within 0.01s without overshoot.

[0106] Example 2: Hardware-in-the-loop simulation verification

[0107] Based on the OP5600 RT-LAB real-time simulation platform, the control algorithm was downloaded to the FPGA, and hardware-in-the-loop testing was performed using the parameters in Table 1. The grid-connected current waveform was acquired using an oscilloscope.

[0108] When L g At a current density of 7.7mH, the traditional WAC strategy causes severe oscillations in the grid-connected current waveform and significant frequency distortion. After switching to the improved strategy of this invention, the grid-connected current waveform immediately recovers to a sine wave, and the current quality meets the requirements of the IEEE 1547 standard. The experimental results are consistent with the simulation, verifying the effectiveness of the proposed control strategy under real digital control delay and hardware constraints.

[0109] Example 3: Inductive robustness test

[0110] Set L g The inductive reactance was gradually increased from 1.1 mH to 7.7 mH in 1.1 mH increments. At each inductive reactance value, the phase margin of the system's open-loop transfer function and the grid-connected current THD were measured.

[0111] Traditional WAC strategy in L g When the inductive reactance is >4.4mH, the phase margin drops below 0°, and the THD exceeds 10%. The improved strategy of this invention maintains a phase margin of over 30° under all inductive reactance values, and the THD is always below 4%. This shows that the present invention has excellent robustness to inductive reactance disturbances.

[0112] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A control method for LCL-type grid-connected inverter based on improved time delay suppression, characterized in that, Includes the following steps: S1: Establish a weighted average current control mathematical model for LCL grid-connected inverters, analyze the impact of control delay on system stability, and determine the frequency range in which the reverse resonance peak occurs; S2: Introduce a parallel frequency-division compensator in the voltage feedforward channel to form an improved parallel frequency-division compensation strategy. The high-frequency, low-frequency and mid-frequency bands are compensated by the high-pass, low-pass and band-pass branches respectively to compensate for the phase delay caused by the frequency shift of the reverse resonant peak. S3: Design the corner frequency, gain coefficient, and delay coefficient of the high-frequency phase compensator, low-frequency amplitude compensator, and medium-frequency amplitude compensator so that the equivalent output impedance of the inverter meets the passive stability requirements in the Nyquist band. S4: Based on the stability criterion of passive theory, analyze the open-loop frequency characteristics, closed-loop pole distribution and output impedance phase characteristics of the compensated system, and verify the stability of the system under different grid inductive reactance conditions. S5: Through simulation and a semi-physical experimental platform, the grid-connected current waveforms, dynamic responses, and robustness of the traditional weighted average current control and the proposed improved strategy under different inductive reactance values ​​(1.1mH~7.7mH) are compared.

2. The method of claim 1, building a grid-tie inverter feedback control system characterized by, The mathematical model for weighted average current control of the grid-connected inverter feedback control system in step S1 is as follows: (1) For now, we can disregard the effect of the control delay Gd, approximate the higher-order terms and damping terms in the low-frequency range, and assume that the current weighting coefficient and voltage feedforward gain satisfy the following conditions: (2) The open-loop transfer function simplifies to a form with first-order properties: (3) Gr(s) is a quasi-proportional resonant controller. After introducing the control delay Gd(s), the open-loop transfer function no longer has first-order characteristics and a reverse resonance peak appears at the resonant frequency, causing the phase margin to decrease to the critical value and the system to become unstable.

3. The method of claim 1, wherein, In step S2, the transfer function G(s) of the parallel frequency band compensator is defined as: (6) Where a1, a2, and a3 are the gains of the high-pass, low-pass, and band-pass filters, respectively; f high f low f mid The system is divided into high-pass, low-pass, and band-pass cutoff frequencies; λ1, λ2, and λ3 are the delay coefficients of the multi-band components in the grid-connected voltage feedforward channel. Through simulation experiments, the delay coefficients of the system's multi-band components were tested and selected multiple times. The parallel frequency divider filter, through intelligent weight allocation in the frequency domain, can maintain low-frequency stability while suppressing high-frequency resonance, thereby extending the system's stable operating range. The stability problem of digital control delay is solved through control algorithm optimization.

4. The method of claim 3, wherein, In step S3, the design parameters of the high-frequency phase compensator are as follows: Choose f high The frequency is 2500Hz. As the grid voltage increases, Zo(s) exhibits negative impedance characteristics near the resonant frequency, which is detrimental to suppressing high-frequency resonant components. When α1 ≤ 0.5, Zo(s) exhibits passive characteristics within the range of [-90°, 90°]. α1 is chosen to be 0.

4. Simulation results show that as the value of λ1 increases, the phase lag of the compensator gradually increases. When λ1 = 2.5, the phase of the compensator at the resonant frequency approaches -180°, satisfying the requirements of closed-loop negative feedback for high-frequency components, thereby enhancing the system's ability to suppress high-frequency disturbances. According to the method described in claim 3, the design parameters of the low-frequency amplitude compensator in step S3 are as follows: to improve the Zo(s) modulus, reduce the harmonic content of the system in the low-frequency region, and enhance the suppression capability of low-frequency noise, λ2 is selected as the minimum delay of 1.5 beats. Because there are many low-order harmonics in the power grid, f low The value should be less than f r To avoid interaction with the resonant frequency, it is ultimately set to f. low =50Hz, set a2 to 0.

4.

5. The method of claim 3, wherein, In step S3, the design parameters of the intermediate frequency amplitude compensator are as follows: Design bandpass frequency f hmid = 800Hz, f lmid = 200Hz. Simulation tests were conducted to analyze the value of λ3. The results show that the value of λ3 affects the bandpass characteristics of the compensator in the mid-frequency range. To further optimize the phase margin of the system near the resonant frequency and without affecting the compensation effect in the high and low frequency bands, λ3 was set to a 1.0 beat delay to ensure compensation accuracy. Theoretical analysis and final simulation experiments show that when α3 ≤ 0.3, the compensator can effectively smooth the output impedance phase characteristic curve without introducing phase distortion; ultimately, α3 = 0.

2.

6. The method according to claim 1, characterized in that, In step S4, the stability requirement based on the passivity theory is: Within the Nyquist band, the phase angle corresponding to the real part of the inverter's equivalent output impedance lies within the range of [-90°, 90°]. By designing the parameters of the parallel frequency band compensator, the compensated equivalent output impedance Z is made possible. eq With the phase constrained within the range of [-90°, 90°], its equivalent output impedance is: (7) This satisfies the passive stability criterion. According to the method of claim 1, step S3 further includes using a quasi-proportional resonant controller as a current regulator, the transfer function of which is: (8) In the formula, K r K is the resonance coefficient. p ω is the proportional gain coefficient. To ensure the controller still provides gain even when the fundamental frequency fluctuates, ω is set to... i =πrad / s. ω i ω is the resonant cutoff frequency. o This is the fundamental angular frequency. The scaling factor K... p The following conditions must be met: (9)。 7. To minimize the impact of the proportional resonant controller at the cutoff frequency ω c The phase lag at this point is usually set to its resonant angular frequency as ω. c / 10, at this time the resonance coefficient K r It can be calculated using the following formula: (10) Finally, the value of K p and the value of K r are calculated.

8. The method of claim 1, wherein, In step S4, the closed-loop pole distribution analysis is as follows: Under traditional weighted average current control, when the grid inductive reactance L... g When the inductive reactance varies within the range of 1.1mH to 7.7mH, the system poles shift to the right half of the complex plane, causing the system to become unstable. After adopting the improved strategy of this invention, within the same range of inductive reactance variation, the system poles are all located in the left half of the complex plane, proving that the system has sufficient stability. The method of claim 1, wherein In step S5, a simulation verification is performed by building a 20kW rated power single-phase LCL inverter simulation model in MATLAB / Simulink. Using the parameters shown in Table 1, the grid-connected current waveforms of the traditional weighted average current control and the improved strategy of this invention are compared under the following operating conditions: When the grid inductive reactance L g When the current is 0mH, both strategies can guarantee power quality; when the grid inductive reactance L g When the current is 7.7mH, the traditional strategy exhibits obvious waveform oscillations and system instability, while the improved strategy of this invention maintains a stable waveform and good current sinusoidality. Under the dynamic switching conditions between full load (20kW) and half load (10kW), the grid-connected current waveform using the improved strategy of this invention remains stable and has a good dynamic response. The method of claim 1, wherein In step S5, the hardware-in-the-loop simulation verification is based on the OP5600 RT-LAB platform, using the same parameters as the simulation, and is performed on the grid inductive reactance L. g Under the condition of 7.7mH, traditional weighted average current control causes significant oscillations in the system waveform, while the improved strategy of this invention restores the system to stability and improves the grid-connected current quality, proving that the proposed control strategy has effective robustness against inductive disturbances under weak grid conditions. The method according to any one of claims 1 to 11, characterized in that The control delay G d (s) is equivalent to a lag element with a transfer function of: (11) It is approximated by Taylor as a rational fraction; the resonant frequency f of the LCL filter is determined by the inverter-side inductance L1, the grid-side inductance L2, and the filter capacitor C. The method according to claim 1, characterized in that, In step S4, after introducing the parallel frequency band compensator, the improved open-loop transfer function of the system is expressed as: (12) Where T(s) is the open-loop transfer function of the improved system; G r (s) is a quasi-proportional resonant controller, and its expression is shown in equation (6); G d (s) is the control delay transfer function; K p L1 represents the modulation gain; L2 and C represent the inverter-side inductance, grid-side inductance, and filter capacitor, respectively; the open-loop transfer function, after introducing a parallel frequency band compensator, retains the first-order characteristics of the system and effectively suppresses the phase lag caused by the reverse resonance peak.

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