Method for improving stability of symmetric phase-locked grid-connected inverter based on BPF-CF collaborative architecture

Abstract

The invention relates to the technical field of power electronic control, in particular to a method for improving the stability of an inverter through a band-pass filter (BPF) and a first-order complex filter (CF). In order to solve the problem that an inverter adopting a symmetric phase-locked loop is poor in stability at a low frequency band, the method starts from output admittance, the admittance of a power grid voltage feed-forward loop and the admittance of the phase-locked loop are corrected, and the stability of the inverter is improved. According to the method, the BPF is cascaded in the voltage feed-forward path, and the CF is combined as the prefilter of the phase-locked loop to form a new symmetric phase-locked loop structure, so that the grid-connected inverter has sufficient phase margin in a weak power grid, and the stable operation capability of the inverter is improved. Experiments show that in a weak power grid environment, compared with a power grid voltage feed-forward admittance correction method, the method has higher stability under the condition of the same initial value, can be widely applied to a grid-connected inverter system in the weak power grid, and improves the stable operation capacity of an inverter in the weak power grid environment and the adaptive capacity of the inverter to power grid voltage fluctuation.

Description

Technical Field

[0001] The present invention relates to the field of power electronic control technology, and in particular to a method for improving the stability of a symmetrical phase-locked loop grid-connected inverter based on a BPF-CF collaborative architecture. Background Art

[0002] With the widespread development of renewable energy and the rapid growth of distributed generation, power systems are characterized by "double highs": a high proportion of installed renewable energy generation capacity and a high proportion of power electronics technology. In regions rich in renewable energy, the synchronization characteristics of the power grid undergo significant changes, gradually forming weak grids with weak synchronization characteristics. In such weak grid environments, the strong coupling between grid-connected converters and the grid triggers a series of instabilities, such as broadband oscillations, which seriously impact the stable operation of the power system.

[0003] As the core synchronization unit of grid-connected inverters, the phase-locked loop (PLL) generates frequency coupling during its interaction with the grid, a key factor contributing to system instability. While a symmetrical phase-locked loop (SPLL) can completely eliminate this frequency coupling effect, its transfer function differs from that of a traditional PLL, and the impact of its introduction on inverter stability remains unclear. Furthermore, in weak grid environments, low-frequency instability is closely related to the negative resistance effect introduced by the PLL, in addition to frequency coupling. Currently, little research has been conducted on the adaptability of the SPLL structure in such complex environments, and quantitative analysis of its control performance and PLL output characteristics is lacking. This makes it difficult to accurately determine the specific impact of the SPLL on inverter stability under varying grid strengths and load conditions, making it difficult to precisely optimize and control the system based on specific operating conditions in practical applications. Furthermore, due to the differences in the SPLL transfer function from traditional synchronous rotating coordinate system PLLs, traditional strategies cannot be directly applied.

[0004] To address these issues, scholars at home and abroad have conducted in-depth research from various perspectives, but existing technologies still have many shortcomings. Regarding modeling and stability analysis that considers phase-locked loop frequency coupling, various impedance modeling methods have been proposed, including the dq coordinate impedance model, the phasor impedance model, and the phase-domain impedance model. These models can be categorized as MIMO and SISO impedance models, but each method has its own shortcomings. The MIMO impedance model can accurately represent frequency coupling effects, but stability analysis is complex, requiring the use of the generalized Nyquist criterion and requiring high computational effort. While the SISO impedance model can simplify stability analysis, the MIMO impedance matrix inevitably undergoes high-order truncation to simplify the MIMO model to a SISO model, resulting in inaccurate stability analysis. In research on the mechanisms of low-frequency harmonic instability and strategies for improving stability, existing strategies for improving low-frequency stability often ignore the impact of frequency coupling on the inverter output impedance. When analyzing the impact of the negative resistance effect of the phase-locked loop on system stability based on impedance analysis, methods such as reducing the phase-locked loop bandwidth have been proposed, but this sacrifices the system's dynamic response speed and does not significantly improve stability. Existing improvement strategies mainly include improving the current control loop and the phase-locked loop, but these methods also have various problems. For example, modifying the grid voltage feedforward loop will reduce the system's ability to suppress grid voltage disturbances and reduce the quality of grid-connected current. When improving the phase-locked loop parameters or structure, it may increase the coupling between the PLL and the inverter, or make the control implementation and design complicated, which is not conducive to engineering application. Summary of the Invention

[0005] To address the above issues, this paper proposes a method to improve the stability of grid-connected inverters by adding BPF and CF. A BPF is cascaded in the voltage feedforward path and combined with CF as the prefilter of the phase-locked loop to form a new symmetrical phase-locked loop structure. By correcting the amplitude and phase of the output admittance, the inverter's stable operation capability in a weak grid environment is improved.

[0006] The present invention proposes a dual-loop admittance correction method for a grid voltage feedforward loop cascaded with a BPF and a phase-locked loop embedded in a CF, the technical solution of which includes the following steps:

[0007] a) Model the grid-connected inverter considering the phase-locked loop and grid voltage feedforward to obtain the stability criterion;

[0008] b) Conduct stability tests on a grid-connected inverter with a symmetrical phase-locked loop and a grid voltage feedforward loop in the medium and low frequency bands to determine the inverter's unstable frequency band and instability factors;

[0009] c) A BPF is introduced to correct the admittance of the grid voltage feedforward loop, and a CF is embedded in the SPLL loop to optimize the parameters of the BPF and CF.

[0010] d) According to the dual-loop collaborative admittance correction structure, the corrected inverter output admittance is determined and the Bode diagrams of the output admittance of the inverter using dual-loop correction under weak grid SCR=5 and very weak grid SCR=2 are obtained.

[0011] As a further improvement of the present invention, in step a, the output admittance modeling of the grid-connected inverter is performed to obtain a stability criterion based on admittance, and the specific steps are as follows:

[0012] Step a1: Based on the grid-connected inverter control process considering the phase-locked loop and grid voltage feedforward, the expression of the grid-connected current is obtained:

[0013]

[0014] Where: i L2 (s) is the expression of the grid-connected current in the complex frequency domain when the phase-locked loop and grid voltage feedforward are considered, T(s) = G i (s)G x1 (s)G x2 (s) is the loop gain, H PLL (s) is the common grid point voltage to current reference i ref The transfer function, I m is the grid-connected current reference value, G f (s)=1 / K pwm is the transfer function of the grid voltage proportional feedforward link, G i (s) is the transfer function of the grid-connected current regulator, V pcc (s) is the complex frequency domain expression of the voltage at the common grid connection point PCC, G x2 (s) is the transfer function related to the inverter structure and parameters.

[0015] After equivalent transformation, the equivalent output admittance expression is obtained:

[0016]

[0017] in:

[0018] Step a2: Establish the Norton equivalent circuit of the grid-connected inverter and obtain the grid-connected current i based on the equivalent circuit. L2 :

[0019]

[0020] In step a3, the system stability is determined based on the phase margin, a stability criterion of the output admittance. The system phase margin PM is expressed as:

[0021] PM=180°-[∠Y out (j2πf x)-(-90°)]=90°-∠Y out (j2πf x )

[0022] When the phase margin PM is greater than 0, the inverter is stable and the phase criterion of the inverter output admittance is obtained:

[0023] ∠Y out (j2πf x )<90°

[0024] As a further improvement of the present invention, a stability experiment of a grid-connected inverter including a symmetrical phase-locked loop and a grid voltage feedforward loop is performed in step b, and the specific steps are as follows:

[0025] Step b1, based on the inverter output admittance Y considering the influence of the symmetrical phase-locked loop and the feedforward loop out (S) and current loop admittance Y cur (S) Bode diagram, obtain the stability criterion based on admittance to determine the unstable area;

[0026] Step b2: Based on the two loop admittances Y spll and Y f And the combined admittance Y com The Bode diagram is used to obtain the key frequency range that causes the inverter instability and the influence of each instability factor on the inverter stability.

[0027] As a further improvement of the present invention, in step c, a bandpass filter with phase lag compensation and amplitude attenuation functions is used, which is cascaded in the grid voltage feedforward channel. At the same time, a first-order complex filter is used as a prefilter of the phase-locked loop to form a new symmetrical phase-locked loop structure, and the parameters of BPF and CF are optimized.

[0028] The specific steps are as follows:

[0029] The transfer function of a bandpass filter is:

[0030]

[0031] Where k is the proportional gain coefficient, ω n is the transition frequency

[0032] Step c1, determine the parameter ω n :

[0033] The bandpass filter is used to correct the inverter output admittance in the grid voltage feedforward channel. According to the filter transfer signal and characteristic analysis, ω n Set as the base frequency, the filter can keep the signal amplitude unchanged at the base frequency without introducing phase lag, theoretically ensuring accurate processing of the input signal and maintaining stable operation of the system.n =100pi.

[0034] Step c2, determine the parameter k:

[0035] According to the Bode diagram comparing the inverter output admittance for different k values:

[0036] When K is 0.8, the amplitude of the inverter output admittance near the fundamental frequency is improved compared with that before correction, and it will not affect the tracking effect of the grid-connected current. Take k = 0.8.

[0037] The transfer function of a first-order complex filter is:

[0038]

[0039] Among them, ω0 is the center frequency and ω1 is the corner frequency

[0040] Step c3, determine the parameter ω0:

[0041] In the inverter phase-locked loop (PLL), ω0 is the grid fundamental angular frequency, which ensures the PLL accurately tracks the grid voltage output frequency and phase signal, achieving synchronous operation between the inverter and the grid, and ensuring efficient and stable power transmission. For example, ω0 = 100pi.

[0042] Step c4, determine the parameter ω1:

[0043] Obtain the intercept frequency between the output admittance of the grid-connected inverter and the grid admittance after the grid voltage feedforward admittance is corrected by the bandpass filter:

[0044] f x =107Hz

[0045] The filter parameter ω1 is designed by a design process that considers multiple parameters:

[0046] The output admittance amplitude at the intercept frequency is positively correlated with ω1, and the phase is negatively correlated with ω1;

[0047] The output admittance at 150HZ is positively correlated with ω1;

[0048] Considering the above, we take ω1=60.

[0049] As a further improvement of the present invention, step d corrects the grid voltage feedforward by cascading a filter in the grid voltage feedforward loop and modifying the phase-locked loop structure loop. The two loops work together to comprehensively correct the amplitude and phase of the inverter output admittance.

[0050] The final inverter output admittance Y after correction out1 :

[0051]

[0052] Current loop to inverter output admittance:

[0053]

[0054] After correction, the output admittance of the symmetrical phase-locked loop to the inverter is:

[0055]

[0056] The grid voltage feedforward loop after correction has an output admittance to the inverter:

[0057]

[0058] Compared with the prior art, the technical solution of the present invention has the following advantages:

[0059] (1) BPF corrects the admittance of the grid voltage feedforward loop, raises the phase-frequency curve, and expands the stable frequency range; CF transforms the phase-locked loop structure and optimizes the inverter output admittance amplitude and phase.

[0060] (2) Adding BPF and CF to the symmetrical PLL inverter improves the inverter's stability. Under different grid strengths, such as SCR = 5 and SCR = 2, the inverter's phase margin is improved after correction, and its stable operation capability is enhanced. Compared with the method of reducing the PLL bandwidth, this control strategy allows the PLL to use a high bandwidth, improving the grid-connected current response speed while ensuring the inverter's stable operation under weak grid conditions.

[0061] (3) The parameters of the added CF are selected in multiple dimensions, taking into account the filter impulse response speed and the ability to suppress the third harmonic, reducing the output admittance amplitude, reducing the disturbance component in the grid-connected current, and improving the quality of the grid-connected current. BRIEF DESCRIPTION OF THE DRAWINGS

[0062] Figure 1 is a system block diagram of the present invention;

[0063] Figure 2 This is the Norton equivalent circuit diagram of the grid-connected inverter of the present invention;

[0064] Figure 3 It is a block diagram of a symmetrical phase-locked loop control based on a first-order complex filter structure of the present invention;

[0065] Figure 4 It is a flow chart of dual-loop admittance correction parameter design of the present invention;

[0066] Figure 5 This is a control block diagram of the dual-loop admittance correction control strategy of the present invention;

[0067] Figure 6This is a comparative Bode diagram of the output admittance of the inverter using grid voltage feedforward admittance correction and dual-loop correction under weak grid SCR=5 and very weak grid SCR=2 of the present invention;

[0068] Figure 7 This is a comparison diagram of the grid current waveforms of the grid voltage feedforward admittance correction and the dual-loop admittance correction under different grid short-circuit ratios of the present invention;

[0069] Figure 8 This is a grid-connected current waveform diagram of the inverter in an extremely weak power grid under the dual-loop admittance correction strategy of the present invention; DETAILED DESCRIPTION

[0070] The specific embodiments of the present invention will be described in further detail below with reference to the accompanying drawings. Specific implementation method 1

[0072] In this specific embodiment, a method for improving the stability of a symmetrical phase-locked loop grid-connected inverter based on a BPF-CF collaborative architecture is provided. Figure 1 As shown, the following steps are included:

[0073] Step a, modeling the grid-connected inverter considering the phase-locked loop and grid voltage feedforward to obtain the stability criterion;

[0074] Step b: conducting a stability experiment of a grid-connected inverter including a symmetrical phase-locked loop and a grid voltage feedforward loop in a medium and low frequency band to obtain an unstable frequency band and instability factor of the inverter;

[0075] Step c: introduce the BPF to correct the grid voltage feedforward loop admittance, embed the CF in the SPLL loop, and optimize the parameters of the BPF and CF;

[0076] Step d: determining the corrected inverter output admittance according to the dual-loop cooperative admittance correction structure. Specific implementation method 2

[0078] In this specific embodiment, a method for improving the stability of a symmetrical phase-locked loop grid-connected inverter based on a BPF-CF collaborative architecture is further defined on the basis of the first specific embodiment:

[0079] like Figure 2 As shown, in step a, the output admittance model of the grid-connected inverter is performed to obtain a stability criterion based on admittance. The specific steps are as follows:

[0080] Step a1: Based on the grid-connected inverter control process considering the phase-locked loop and grid voltage feedforward, the expression of the grid-connected current is obtained:

[0081]

[0082] Where: i L2 (s) is the expression of the grid-connected current in the complex frequency domain when the phase-locked loop and grid voltage feedforward are considered, T(s) = G i (s)G x1 (s)G x2 (s) is the loop gain, H PLL (s) is the common grid point voltage to current reference i ref Transfer function, Im is the grid current reference value, Gf(s)=1 / K pwm is the transfer function of the grid voltage proportional feedforward link, G i (s) is the transfer function of the grid-connected current regulator, V pcc (s) is the complex frequency domain expression of the voltage at the common grid connection point PCC, G x2 (s) is the transfer function related to the inverter structure and parameters.

[0083] After equivalent transformation, the equivalent output admittance expression is obtained:

[0084]

[0085] in:

[0086] Step a2: Establish the Norton equivalent circuit of the grid-connected inverter and obtain the grid-connected current i based on the equivalent circuit. L2 :

[0087]

[0088] Step a3: The stability of the system is determined by judging the phase margin based on the stability of the output admittance. The system phase margin PM is expressed as:

[0089] PM=180°-[∠Y out (j2πf x )-(-90°)]=90°-∠Y out (j2πf x )

[0090] When the phase margin PM is greater than 0, the inverter is stable and the phase criterion of the inverter output admittance is obtained:

[0091] ∠Y out (j2πf x )<90° Specific implementation method three

[0093] In this specific embodiment, a method for improving the stability of a symmetrical phase-locked loop grid-connected inverter based on a BPF-CF collaborative architecture is further defined on the basis of the second specific embodiment:

[0094] In step b, a stability experiment of a grid-connected inverter including a symmetrical phase-locked loop and a grid voltage feedforward loop is conducted, and the specific steps are as follows:

[0095] Step b1, based on the inverter output admittance Y considering the influence of the symmetrical phase-locked loop and the feedforward loop out (S) and current loop admittance Y cur (S) Bode diagram, obtain the stability criterion based on admittance to determine the unstable area;

[0096] Step b2: Based on the two loop admittances Y spll and Y f And the combined admittance Y com The Bode diagram is used to obtain the key frequency range that causes the inverter instability and the influence of each instability factor on the inverter stability. Specific implementation method four

[0098] In this specific embodiment, a method for improving the stability of a symmetrical phase-locked loop grid-connected inverter based on a BPF-CF collaborative architecture is further defined as follows:

[0099] like Figure 3 and Figure 4 As shown, in step c, the BPF is a bandpass filter with phase lag compensation and amplitude attenuation functions, which is cascaded in the grid voltage feedforward channel. The CF is a first-order complex filter, which serves as the prefilter of the phase-locked loop to form a new symmetrical phase-locked loop structure, and the parameters of the BPF and CF are optimized.

[0100] The specific steps are as follows:

[0101] The transfer function of a bandpass filter is:

[0102]

[0103] Where k is the proportional gain coefficient, ω n is the transition frequency

[0104] Step c1, determine the parameter ω n :

[0105] The bandpass filter is used to correct the inverter output admittance in the grid voltage feedforward channel. According to the filter transfer signal and characteristic analysis, ω n Set as the base frequency, the filter can keep the signal amplitude unchanged at the base frequency without introducing phase lag, theoretically ensuring accurate processing of the input signal and maintaining stable operation of the system. n =100pi.

[0106] Step c2, determine the parameter k:

[0107] According to the Bode diagram comparing the inverter output admittance for different k values:

[0108] When K is 0.8, the amplitude of the inverter output admittance near the fundamental frequency is improved compared with that before correction, and it will not affect the tracking effect of the grid-connected current. Take k = 0.8.

[0109] The transfer function of a first-order complex filter is:

[0110]

[0111] Among them, ω0 is the center frequency and ω1 is the corner frequency

[0112] Step c3, determine the parameter ω0:

[0113] In the inverter phase-locked loop (PLL), ω0 is the grid fundamental angular frequency, which ensures the PLL accurately tracks the grid voltage output frequency and phase signal, achieving synchronous operation between the inverter and the grid, and ensuring efficient and stable power transmission. For example, ω0 = 100pi.

[0114] Step c4, determine the parameter ω1:

[0115] Obtain the intercept frequency between the output admittance of the grid-connected inverter and the grid admittance after the grid voltage feedforward admittance is corrected by the bandpass filter:

[0116] f x =107Hz

[0117] The filter parameter ω1 is designed by a design process that considers multiple parameters:

[0118] The output admittance amplitude ω1 at the intercept frequency is positively correlated with ω1, and the phase is negatively correlated with ω1;

[0119] The output admittance at 150HZ is positively correlated with ω1;

[0120] Considering the above, we take ω1=60. Specific implementation method five

[0122] In this specific embodiment, a method for improving the stability of a symmetrical phase-locked loop grid-connected inverter based on a BPF-CF collaborative architecture is further defined on the basis of the fourth specific embodiment:

[0123] like Figure 5 and Figure 6 As shown, in step d, the grid voltage feedforward is corrected by cascading a filter in the grid voltage feedforward loop and modifying the phase-locked loop structure loop. The two loops work together to fully correct the amplitude and phase of the inverter output admittance.

[0124] The final inverter output admittance Y after correction out1 :

[0125]

[0126] Current loop to inverter output admittance:

[0127]

[0128] After correction, the output admittance of the symmetrical phase-locked loop to the inverter is:

[0129]

[0130] The grid voltage feedforward loop after correction has an output admittance to the inverter:

[0131]

[0132] The experimental results are as follows Figure 7 and Figure 8 As shown in the figure, it can be further seen that compared with the method of correcting the grid voltage feedforward admittance, this method comprehensively improves the low-frequency stability of the inverter under different grid strengths, especially under extremely weak grid conditions, where this method still has a significant effect.

[0133] It should be noted that the above-described specific embodiments are merely illustrative of the principles and effects of the present invention and are not intended to limit the present invention. Anyone skilled in the art may modify or alter the above-described specific embodiments without departing from the spirit and scope of the present invention. Therefore, all equivalent modifications or alterations made by one of ordinary skill in the art without departing from the spirit and technical concepts disclosed herein shall be encompassed by the claims of the present invention.

Claims

1. A method for improving the stability of a symmetrical phase-locked grid-connected inverter based on a BPF-CF collaborative architecture, characterized in that: The method comprises the following steps: a) Model the grid-connected inverter considering the phase-locked loop and grid voltage feedforward to obtain the stability criterion; b) Conduct stability tests on a grid-connected inverter with a symmetrical phase-locked loop and a grid voltage feedforward loop in the medium and low frequency bands to determine the inverter's unstable frequency band and instability factors; c) A BPF is introduced to correct the admittance of the grid voltage feedforward loop, and a CF is embedded in the SPLL loop to optimize the parameters of the BPF and CF. d) According to the dual-loop collaborative admittance correction structure, the corrected inverter output admittance is determined and the Bode diagrams of the output admittance of the inverter using dual-loop correction under weak grid SCR=5 and very weak grid SCR=2 are obtained.

2. The method for improving the stability of a symmetrical phase-locked grid-connected inverter based on a BPF-CF collaborative architecture according to claim 1, characterized in that: In step a, the output admittance model of the grid-connected inverter is performed to obtain a stability criterion based on admittance. The specific steps are as follows: Step a1: Based on the grid-connected inverter control process considering the phase-locked loop and grid voltage feedforward, the expression of the grid-connected current is obtained: Where: i L2 (s) is the expression of the grid-connected current in the complex frequency domain when the phase-locked loop and grid voltage feedforward are considered, T(s) = G i (s)G x1 (s)G x2 (s) is the loop gain, H PLL (s) is the common grid point voltage to current reference i ref Transfer function, Im is the grid current reference value, Gf(s)=1 / K pwm is the transfer function of the grid voltage proportional feedforward link, G i (s) is the transfer function of the grid-connected current regulator, V pcc (s) is the complex frequency domain expression of the voltage at the common grid connection point PCC, G x2 (s) is the transfer function related to the inverter structure and parameters. After equivalent transformation, the equivalent output admittance expression is obtained: in: Step a2: Establish the Norton equivalent circuit of the grid-connected inverter and obtain the grid-connected current i based on the equivalent circuit. L2 : In step a3, the system stability is determined based on the phase margin, a stability criterion of the output admittance. The system phase margin PM is expressed as: PM=180°-[∠Y out (j2πf x )-(-90°)]=90°-∠Y out (j2πf x ) When the phase margin PM is greater than 0, the inverter is stable and the phase criterion of the inverter output admittance is obtained: ∠Y out (j2πf x )<90°。 3. The method for improving the stability of a symmetrical phase-locked grid-connected inverter based on a BPF-CF collaborative architecture according to claim 2, characterized in that: In step b, a stability experiment of a grid-connected inverter including a symmetrical phase-locked loop and a grid voltage feedforward loop is conducted, and the specific steps are as follows: Step b1, based on the inverter output admittance Y considering the influence of the symmetrical phase-locked loop and the feedforward loop out (S) and current loop admittance Y cur (S) Bode diagram, obtain the stability criterion based on admittance to determine the unstable area; Step b2: Based on the two loop admittances Y spll and Y f And the combined admittance Y com The Bode diagram is used to obtain the key frequency range that causes the inverter instability and the influence of each instability factor on the inverter stability.

4. The method for improving the stability of a symmetrical phase-locked grid-connected inverter based on a BPF-CF collaborative architecture according to claim 3 is characterized in that: In step c, the BPF is a bandpass filter with phase lag compensation and amplitude attenuation functions, which is cascaded in the grid voltage feedforward channel. The CF is a first-order complex filter, which serves as the prefilter of the phase-locked loop to form a new symmetrical phase-locked loop structure, and the parameters of the BPF and CF are optimized. The specific steps are as follows: The transfer function of a bandpass filter is: Where k is the proportional gain coefficient, ω n is the transition frequency Step c1, determine the parameter ω n : The bandpass filter is used to correct the inverter output admittance in the grid voltage feedforward channel. According to the filter transfer signal and characteristic analysis, ω n Setting ω as the base frequency allows the filter to maintain the signal amplitude unchanged at the base frequency without introducing phase lag, ensuring accurate processing of the input signal and maintaining stable operation of the system. n =100pi. Step c2, determine the parameter k: According to the Bode diagram comparing the inverter output admittance for different k values: When K is 0.8, the amplitude of the inverter output admittance near the fundamental frequency is improved compared with that before correction, and it will not affect the tracking effect of the grid-connected current. Take k = 0.

8. The transfer function of a first-order complex filter is: Among them, ω0 is the center frequency and ω1 is the corner frequency Step c3, determine the parameter ω0: In the inverter phase-locked loop, when ω0 is the grid base frequency, the phase-locked loop can accurately track the grid voltage output frequency and phase signal, achieving synchronous operation of the inverter and the grid, and ensuring efficient and stable power transmission. Assume ω0 = 100pi. Step c4, determine the parameter ω1: Obtain the intercept frequency between the output admittance of the grid-connected inverter and the grid admittance after the grid voltage feedforward admittance is corrected by the bandpass filter: f x =107Hz The filter parameter ω1 is designed by a design process that considers multiple parameters: The output admittance amplitude at the intercept frequency is positively correlated with ω1, and the phase is negatively correlated with ω1; The output admittance at 150HZ is positively correlated with ω1; Considering the above, we take ω1=60.

5. The method for improving the stability of a symmetrical phase-locked grid-connected inverter based on a BPF-CF collaborative architecture according to claim 4 is characterized in that: In the step d, the grid voltage feedforward is corrected by cascading a filter in the grid voltage feedforward loop and modifying the phase-locked loop structure loop. The two loops work together to fully correct the amplitude and phase of the inverter output admittance. The final inverter output admittance Y after correction out1 : Current loop to inverter output admittance: After correction, the output admittance of the symmetrical phase-locked loop to the inverter is: The grid voltage feedforward loop after correction has an output admittance to the inverter: 。

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