Virtual impedance remodeling method applied to weak power grid for improving stability of inverter

Abstract

The invention relates to a virtual impedance remodeling method for improving the stability of an inverter under a weak power grid, and belongs to the technical field of new energy grid connection, and the method comprises the following steps: S1, constructing a Norton equivalent circuit model, considering a phase-locked loop, of a single-phase grid-connected inverter; s2, obtaining the output impedance of the grid-connected inverter considering the phase-locked loop according to the Norton equivalent circuit model considering the phase-locked loop of the single-phase grid-connected inverter; s3, introducing series virtual impedance into the Norton equivalent circuit model of the single-phase grid-connected inverter considering the phase-locked loop, and constructing an equivalent Norton circuit of the virtual series impedance grid-connected inverter; s4, constructing an output impedance expression of the grid-connected inverter after virtual impedance remodeling; and S5, determining virtual impedance by considering a stability requirement, a harmonic suppression capability requirement and a power grid current steady-state error. According to the invention, the stability of the grid-connected inverter under the weak power grid is improved, and the tracking accuracy of the output to the given value is ensured.

Description

Technical Field

[0001] The present invention belongs to the technical field of new energy grid connection, and relates to a virtual impedance reshaping method for improving the stability of an inverter under a weak power grid. Background Art

[0002] With the continuous innovation of renewable energy power generation technology, power electronic equipment with power semiconductor devices as switching units are widely used in the power generation, transmission and consumption sides of the power system, resulting in the gradual improvement of the degree of power electronics of "source-grid-load", which further promotes the rapid development of renewable energy power generation towards high penetration. The new characteristics of the power system also cause the power grid to gradually show the characteristics of a weak grid, that is, the grid impedance cannot be ignored. In a distributed power generation system, when the short circuit ratio (SCR) SCR ≤ 10, the power grid can be defined as a weak grid. For new energy grid-connected converters, the short circuit ratio SCR corresponding to the grid connection point can be determined by the ratio of the short circuit capacity of the grid at the grid connection point to the rated capacity of the new energy grid-connected converter.

[0003] Power electronic converters undertake the important task of power transmission and conversion control in renewable energy power systems. Their reliable operation is the basis for ensuring the stable operation of new power systems with renewable energy as the main body. However, the diversity of control targets, complexity of control structure and differences in control time scale of power electronic converters themselves lead to interactive effects between converters and power grids, inducing interactive instability in the frequency range of several Hz to several thousand Hz, threatening the safe and reliable operation of renewable energy power systems. At the same time, with the continuous increase of renewable energy installed capacity, the power grid is bound to become weaker and weaker. The stability problem caused by the interaction between grid-connected converters and power grids caused by low short-circuit ratio is more prominent. In addition, the complexity of the control system of the grid-connected converter itself and the interactive coupling between different control links also make it difficult to analyze the mechanism of instability of the grid-connected converter system, which seriously restricts the safe and stable operation of renewable energy power systems.

[0004] In terms of improving the stability of grid-connected inverters under weak power grids, considering the influence of phase-locked loops, some scholars have pointed out that reducing the bandwidth of the phase-locked loop can increase the frequency range of the positive damping of the output impedance of the grid-connected inverter, thereby improving the stability of the grid-connected system. However, reducing the bandwidth of the phase-locked loop will deteriorate the dynamic performance of the grid-connected inverter system. In addition, the method of improving the stability of the grid-connected system by reducing the bandwidth of the phase-locked loop is impractical because it requires a trade-off between the requirements of the entire system for the output performance of the phase-locked loop. Summary of the invention

[0005] In view of this, the purpose of the present invention is to provide a virtual impedance reshaping strategy that can improve the stability of a grid-connected inverter under a weak power grid, so as to optimize the output impedance characteristics of the inverter and ensure the stable operation of the grid-connected inverter under a weak power grid.

[0006] In order to achieve the above object, the present invention provides the following technical solutions:

[0007] A virtual impedance reshaping method for improving inverter stability under weak power grid includes the following steps:

[0008] S1: Construct the Norton equivalent circuit model of the single-phase grid-connected inverter considering the phase-locked loop;

[0009] S2: according to the Norton equivalent circuit model of the single-phase grid-connected inverter considering the phase-locked loop, the output impedance of the grid-connected inverter considering the phase-locked loop is obtained;

[0010] S3: introducing a series virtual impedance into the Norton equivalent circuit model of the single-phase grid-connected inverter considering a phase-locked loop, and constructing an equivalent Norton circuit of a virtual series impedance grid-connected inverter;

[0011] S4: Construct the output impedance expression of the grid-connected inverter after virtual impedance reshaping;

[0012] S5: Determine the virtual impedance by considering the stability requirements, harmonic suppression capability requirements and grid current steady-state error.

[0013] Further, the step S1 of constructing a Norton equivalent circuit model of a single-phase grid-connected inverter taking into account a phase-locked loop specifically includes the following steps:

[0014] S11: Based on the structure of single-phase LCL type grid-connected inverter under weak power grid, a control circuit of single-phase grid-connected inverter considering phase-locked loop is constructed;

[0015] S12: Simplify the control circuit of the single-phase grid-connected inverter considering the phase-locked loop to obtain an equivalent intermediate transfer function G 1 (s) and G 2 The relationship between (s) is as follows:

[0016]

[0017]

[0018] Where L 1 is the inductance measured for the inverter, C is the filter capacitor, L 2 is the grid-side filter inductance, K d is the capacitor current feedback coefficient, K pwm is the inverter bridge gain;

[0019] Then the system loop gain T a The expression is:

[0020] T a (s) = G c (s)G1 (s)G 2 (s)K 1

[0021] Among them G c (s) is the current controller, K 1 is the grid-connected circuit feedback coefficient;

[0022] Finally, the grid connection point voltage u is obtained pcc The relationship between the current and the grid current is:

[0023]

[0024] Among them I ref is the grid-connected current reference value, G pll (s) is the transfer function of the phase-locked loop, u pcc is the grid connection point voltage;

[0025] S13: Based on the grid connection point voltage u pcc The relationship between the phase-locked loop and the grid-connected current is calculated and the Norton equivalent circuit model of the single-phase grid-connected inverter is constructed considering the phase-locked loop.

[0026] Further, in step S2, according to the Norton equivalent circuit model of the single-phase grid-connected inverter considering the phase-locked loop, the output impedance of the grid-connected inverter considering the phase-locked loop is obtained as follows:

[0027]

[0028] Among them, Z out (s) is the output impedance of the inverter considering the phase-locked loop, Z o1 (s) and Z pll (s) is the current loop equivalent output impedance and the phase-locked loop equivalent impedance.

[0029] Further, the equivalent Norton circuit of the virtual series impedance grid-connected inverter is constructed in step S3, specifically including: the current loop equivalent impedance Z o1 (s) and the equivalent impedance Z of the phase-locked loop pll (s) in parallel, grid impedance Z g (s), virtual impedance Z v1 (s) and Z v2 (s) are connected in series, and the grid voltage u pcc (s) and grid voltage u g (s) are applied across each impedance.

[0030] Further, in step S4, the output impedance expression of the grid-connected inverter after virtual impedance reshaping is as follows:

[0031]

[0032] Where k is the error coefficient, which is used to ensure that the low and medium frequency amplitudes remain unchanged after the virtual impedance is reshaped.

[0033] Further, the stability requirement in step S5 is:

[0034] Virtual impedance Z v1 (s) and Z v2 (s) While improving the phase margin of the grid-connected inverter under weak power grid, the amplitude-frequency corresponding to the cutoff frequency fc at the grid impedance under the worst condition satisfies:

[0035] |Z out (j2πf c )|≤|Z' out (j2πf c )|

[0036] where |Z out (j2πf c )| is the amplitude of the inverter output impedance intersecting with the impedance without considering the phase-locked loop, |Z out '(j2πf c )| is the amplitude of the inverter output impedance of the phase-locked loop at the intersection with the impedance;

[0037] And the phase margin satisfies:

[0038] ∠Z' out (j2πf c )≥-(90-β)

[0039] Where β is the desired phase margin, f c is the frequency where the grid impedance intersects the inverter output impedance.

[0040] Furthermore, the harmonic suppression capability requirement in step S5 is: the minimum amplitude of the output impedance at each harmonic frequency satisfies:

[0041]

[0042] where f n is the harmonic frequency, V g and I g The root mean square of the grid voltage and grid current are respectively V n and I n The distortion ratio of the grid voltage and grid current.

[0043] Furthermore, the steady-state error of the grid current introduced by the virtual series impedance is:

[0044]

[0045] where e i is the steady-state error, ig ' is the grid-connected current after connecting the virtual impedance in series, i g is the output grid-connected current without series virtual impedance, ω 0 is the fundamental frequency of the grid voltage.

[0046] The beneficial effects of the present invention are as follows: in order to solve the problem of instability of the grid-connected inverter caused by the phase-locked loop in a weak power grid, the present invention does not improve the stability by reducing the bandwidth of the phase-locked loop, thereby ensuring the dynamic performance of the grid-connected inverter system, and through the series virtual impedance method of the present invention, the amplitude of the amplitude-frequency characteristic curve of the grid-connected inverter in the medium frequency band can be further improved without reducing the amplitude of the low frequency band, and the intersection frequency of the output impedance of the inverter and the grid impedance is improved, thereby improving the stability of the grid-connected inverter in the weak power grid and effectively ensuring the accuracy of the output tracking of the given value.

[0047] Other advantages, objectives and features of the present invention will be described in the following description to some extent, and to some extent, will be obvious to those skilled in the art based on the following examination and study, or can be taught from the practice of the present invention. The objectives and other advantages of the present invention can be realized and obtained through the following description. BRIEF DESCRIPTION OF THE DRAWINGS

[0048] In order to make the purpose, technical solutions and advantages of the present invention more clear, the present invention will be described in detail below in conjunction with the accompanying drawings, wherein:

[0049] Figure 1 A structural diagram of a single-phase inverter implemented in the present invention;

[0050] Figure 2 The control block diagram of the grid-connected inverter considering the phase-locked loop;

[0051] Figure 3 A simplified block diagram of the control block diagram of a single-phase grid-connected inverter considering a phase-locked loop;

[0052] Figure 4 To consider the Norton circuit equivalent model of the phase-locked loop;

[0053] Figure 5 Bode diagram of the output impedance of the grid-connected inverter with different grid impedances;

[0054] Figure 6 It is the equivalent Norton circuit of virtual series impedance grid-connected inverter;

[0055] Figure 7 is an equivalent control block diagram of a virtual series impedance inverter; (a) is a diagram showing the implementation of series virtual impedance, and (b) is an equivalent transformation block diagram of (a);

[0056] Figure 8 Output Bode diagrams of grid-connected inverters with and without virtual impedance under different grid impedances;

[0057] Fig. 9 The grid-connected current waveform of the grid-connected inverter when the grid impedance is Lg=6mH, 10mH without using virtual series impedance;

[0058] Fig.10 It is the grid-connected current waveform of the grid-connected inverter under virtual series impedance when the grid impedance is Lg=10H, 15mH. DETAILED DESCRIPTION

[0059] The following describes the embodiments of the present invention by specific examples, and those skilled in the art can easily understand other advantages and effects of the present invention from the contents disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and the details in this specification can also be modified or changed in various ways based on different viewpoints and applications without departing from the spirit of the present invention. It should be noted that the illustrations provided in the following embodiments only illustrate the basic concept of the present invention in a schematic manner, and the following embodiments and features in the embodiments can be combined with each other without conflict.

[0060] It should be noted that the illustrations provided in the following embodiments are only schematic illustrations of the basic concept of the present invention, and thus the drawings only show components related to the present invention rather than being drawn according to the number, shape and size of components in actual implementation. In actual implementation, the type, quantity and proportion of each component may be changed arbitrarily, and the component layout may also be more complicated.

[0061] In the following description, numerous details are discussed to provide a more thorough explanation of the embodiments of the present invention. However, it is obvious to those skilled in the art that the embodiments of the present invention can be implemented without these specific details. In other embodiments, well-known structures and devices are shown in the form of block diagrams rather than in detail to avoid making the embodiments of the present invention difficult to understand.

[0062] The present invention provides a virtual impedance reshaping method for improving the stability of an inverter under a weak power grid. The structure diagram of the LCL type single-phase grid-connected inverter is shown in FIG. Figure 1 As shown, the output impedance output end of the single-phase full-bridge grid-connected inverter is connected to the weak grid through an LCL filter; the single-phase grid-connected inverter and the weak grid intersect at a common coupling point. dc is the DC bus voltage, L 1 is the inductance measured for the inverter, C is the filter capacitor, L 2 is the grid-side filter inductor, Z g is the grid impedance, Ug Value AC side voltage, K d is the capacitor current feedback coefficient, K 1 is the grid-connected circuit feedback coefficient, PLL is the phase-locked loop, as long as it is used for grid synchronization, G c (s) is the current controller.

[0063] The control block diagram of a single-phase grid-connected inverter considering a phase-locked loop is shown in Figure 2 As shown, the simplified block diagram of the control block diagram is as follows Figure 3 As shown, Figure 3 G in 1 (s) and G 2 The relationship of (s) is as follows

[0064]

[0065]

[0066] And the system loop gain T a The expression is:

[0067] T a (s) = G c (s)G 1 (s)G 2 (s)K 1

[0068] according to Figure 3 The grid voltage u can be obtained pcc (s) and grid-connected current i 2 The relationship between (s) is

[0069]

[0070] From the above formula, we can get the Norton equivalent circuit model of the single-phase grid-connected inverter considering the phase-locked loop, as follows: Figure 4 As shown, where Z o1 (s) is the equivalent impedance of the current loop, Z pll (s) is the equivalent impedance of the phase-locked loop, Z g (s) is the grid impedance; the current loop equivalent impedance Z o1 (s) and the equivalent impedance Z of the phase-locked loop pll (s) in parallel and then connected with the grid impedance Z g (s) in series, grid voltage u pcc (s) and grid voltage u g (s) are applied to both ends of each impedance. From its equivalent model, the output impedance of the grid-connected inverter considering the phase-locked loop can be obtained as:

[0071]

[0072] Using the established output impedance model, the output impedance Bode diagram of the grid-connected inverter with and without the phase-locked loop can be drawn under different grid impedances, such as Figure 5 As shown, the blue curve is the Bode diagram of the inverter output impedance without considering the phase-locked loop, the orange curve is the Bode diagram of the inverter output impedance considering the phase-locked loop, and the yellow, purple, and green curves represent the Bode diagrams of the grid impedance of 5mH, 8mH, and 10mH, respectively. Figure 5 The output impedance phase-frequency characteristic curve is always between -90° and 90° when the grid impedance is 5mH, 8mH and 10mH, ignoring the phase-locked loop, which means the grid-connected inverter remains stable. However, when the phase-locked loop is considered, when the grid impedance is 10mH, the phase margin at the corresponding intercept frequency is -5°, and the system loses stability.

[0073] The virtual impedance-based reshaping method under weak power grid proposed by the present invention is as follows: Figure 6 As shown, Figure 6 Middle Z o1 (s) is the equivalent impedance of the current loop, Z pll (s) is the equivalent impedance of the phase-locked loop, Z g (s) is the grid impedance, u pcc ,u g is the grid connection point voltage and grid side voltage, Z v1 (s) and Z v2 (s) is the series virtual impedance proposed by the present invention, which is used to improve the stability of the inverter system under weak power grid; the current loop equivalent impedance Z o1 (s) and the equivalent impedance Z of the phase-locked loop pll (s) in parallel, grid impedance Z g (s), virtual impedance Z v1 (s) and Z v2 (s) are connected in series, and the grid voltage u pcc (s) and grid voltage u g (s) are applied to both ends of each impedance. When the phase-locked loop is considered, when the phase-locked bandwidth is too large, negative damping will be introduced in the low- and medium-frequency bands of the output impedance of the grid-connected inverter. In order to reduce the influence of the phase-locked loop on the output impedance of the grid-connected inverter without reducing the dynamic capability of the phase-locked loop under weak power grids, and to improve the phase-frequency characteristics of the output impedance of the grid-connected inverter, the present invention introduces a virtual string impedance method. Through the virtual impedance Z v1 (s) and Z v2 (s), thereby improving the phase-frequency characteristics of the inverter's output impedance. v1 (s) is a virtual positive resistor, which is mainly used to ensure the improvement of the output impedance of the reshaped grid-connected inverter. However, due to the introduction of the positive resistor Z v1(s), which will lead to a decrease in the amplitude of the low-frequency amplitude-frequency curve of the output impedance of the reshaped grid-connected inverter. In order to ensure the tracking capability of the fundamental current and reduce the error of the fundamental current, Z is introduced. v2 (s), and Z v2 The value of (s) is (k-1)*Z out (s),Z v2 (s) is used to increase the amplitude of the output impedance at low frequencies.

[0074] The output impedance expression of the grid-connected inverter after virtual impedance reshaping is as follows, where k is the error coefficient, which is used to ensure that the medium and low frequency amplitudes remain unchanged after virtual impedance reshaping.

[0075] Z' out (s) = Z o1 (s) / / Z pll (s)+Z v1 (s)+Z v2 (s)

[0076] =Z out (s)+Z v1 (s)+Z v2 (s)

[0077] =kZ out (s)+Z v2 (s)

[0078] In order to determine the virtual impedance Z v1 (s) and Z v2 (s), the main factors considered are as follows

[0079] 1. Stability requirements: virtual impedance Z v1 (s) and Z v2 (s) While improving the phase margin of the grid-connected inverter under weak power grid, the amplitude-frequency corresponding to the cutoff frequency fc at the grid impedance under the worst condition must satisfy

[0080] |Z out (j2πf c )|≤|Z' out (j2πf c )|

[0081] And the phase margin should meet the desired phase margin, β is the desired phase margin

[0082] ∠Z' out (j2πf c )≥-(90-β)

[0083] 2. Harmonic suppression capability requirements: In the grid-connected inverter, in order to suppress the background harmonics of the grid voltage, the output impedance Z at each harmonic frequencyout The amplitude of '(s) should not be too small. Therefore, the minimum amplitude of the output impedance at each harmonic frequency should satisfy:

[0084]

[0085] 3. Too big Z v1 (s) will make the output impedance too small at low frequency, which will increase the steady-state error of the grid current compared to when there is no virtual series impedance. Therefore, the steady-state error of the grid current introduced by the virtual series impedance is:

[0086]

[0087] Then the virtual impedance Z v1 (s)Z v2 The value of (s) is mainly determined by the above three factors.

[0088] The realization form of series virtual impedance is mainly composed of Figure 7 As shown in (a) and (b), (a) is the implementation form of series virtual impedance. Figure 7 (b) is the equivalent transformation block diagram of (a), in which the grid-connected current is fed forward, thus realizing the series virtual impedance reshaping strategy.

[0089] like Figure 8 The output impedance Bode diagram of the series virtual impedance is shown, where the blue curve is the system Bode diagram without the series virtual impedance, and the orange curve is the system Bode diagram after the series virtual impedance. Compared with the output impedance of the inverter without the series virtual impedance, the output impedance of the inverter with the series virtual impedance not only improves the amplitude of the amplitude-frequency characteristic of the grid-connected inverter in the low and medium frequency bands under the weak power grid, increases the intersection frequency of the output impedance of the inverter and the grid impedance, greatly improves the phase margin at the intersection frequency, but also further expands the applicable range of the grid impedance, so that the inverter can operate under extremely weak power grids, and also ensures that the amplitude of the grid-connected inverter in the low frequency band remains unchanged, further ensuring the tracking of the given current.

[0090] Fig. 9The output impedance Bode diagram of the grid-connected inverter of the phase-locked loop is considered when the grid impedance is 8mH, 10mH, 15mH and 16mH. The other orange curves are the output Bode diagrams of the grid-connected inverter after the virtual series impedance proposed by the present invention is adopted. The blue curve is the Bode diagram without the virtual series impedance. It can be seen from the figure that when the grid impedance is 10mH, the output impedance of the virtual series impedance intersects with the grid impedance at 403Hz, and the corresponding phase margin is 50°. At this time, the grid-connected inverter is stable. The output impedance of the grid-connected inverter without the virtual series impedance intersects with the grid impedance at 250Hz, and the corresponding phase margin is -6.4°. At this time, the grid-connected inverter is unstable. Moreover, when the grid impedance is 16mH, the phase margin of the grid-connected inverter system using the series virtual impedance proposed by the present invention is 10°, which is still stable.

[0091] A single-phase inverter grid-connected system was built based on the MATLAB / Simulink simulation platform, and the proposed impedance reshaping strategy was simulated and verified. The simulated current waveform is shown in Fig. 9 and Fig.10 As shown. Fig. 9 The grid-connected current waveform of the grid-connected inverter when the grid impedance is Lg=6mH and 10mH without using virtual series impedance. Fig.10 for Fig. 9 When the grid impedance is Lg = 10mH, 15mH, the grid-connected current waveform of the grid-connected inverter under virtual series impedance is used. Fig. 9 It shows that when the grid impedance is 6mH, the grid-connected current waveform is good and the system is stable, but when the grid impedance is 10mH, the waveform is disordered and the system becomes unstable. The inverter system using the virtual series impedance strategy proposed by the present invention is stable when the grid impedance is 10mH and 15mH.

[0092] Both theoretical analysis and simulation results confirm that the series virtual impedance reshaping strategy proposed in the present invention can effectively improve the output impedance characteristics of the inverter, enhance the robust stability of the grid-connected inverter under weak grid conditions, and propose a new solution to the problem of improving current quality under weak grid conditions.

[0093] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solution of the present invention can be modified or replaced by equivalents without departing from the purpose and scope of the technical solution, which should be included in the scope of the claims of the present invention.

Claims

1. A virtual impedance reshaping method for improving inverter stability under weak power grid, characterized in that: The following steps are involved: S1: Construct the Norton equivalent circuit model of the single-phase grid-connected inverter considering the phase-locked loop; S2: according to the Norton equivalent circuit model of the single-phase grid-connected inverter considering the phase-locked loop, the output impedance of the grid-connected inverter considering the phase-locked loop is obtained; S3: introducing a series virtual impedance into the Norton equivalent circuit model of the single-phase grid-connected inverter considering a phase-locked loop, and constructing an equivalent Norton circuit of a virtual series impedance grid-connected inverter; S4: Construct the output impedance expression of the grid-connected inverter after virtual impedance reshaping; S5: Determine the virtual impedance by considering the stability requirements, harmonic suppression capability requirements and grid current steady-state error.

2. The virtual impedance reshaping method for improving inverter stability in a weak power grid according to claim 1 is characterized in that: Step S1 constructs a Norton equivalent circuit model of a single-phase grid-connected inverter taking into account a phase-locked loop, specifically comprising the following steps: S11: Based on the structure of single-phase LCL type grid-connected inverter under weak power grid, a control circuit of single-phase grid-connected inverter considering phase-locked loop is constructed; S12: Simplify the control circuit of the single-phase grid-connected inverter considering the phase-locked loop, and obtain the relationship between the equivalent intermediate transfer functions G1(s) and G2(s) as follows: Where L1 is the inverter inductor, C is the filter capacitor, L2 is the grid-side filter inductor, K d is the capacitor current feedback coefficient, K pwm is the inverter bridge gain; Then the system loop gain T a The expression is: T a (s)=G c (s)G1(s)G2(s)K1 Among them G c (s) is the current controller, K1 is the grid-connected circuit feedback coefficient; Finally, the grid connection point voltage u is obtained pcc The relationship between the current and the grid current is: Among them I ref is the grid-connected current reference value, G pll (s) is the transfer function of the phase-locked loop, u pcc is the grid connection point voltage; S13: Based on the grid connection point voltage u pcc The relationship between the phase-locked loop and the grid-connected current is calculated and the Norton equivalent circuit model of the single-phase grid-connected inverter is constructed considering the phase-locked loop.

3. The virtual impedance reshaping method for improving inverter stability in a weak power grid according to claim 1 is characterized in that: In step S2, according to the Norton equivalent circuit model of the single-phase grid-connected inverter considering the phase-locked loop, the output impedance of the grid-connected inverter considering the phase-locked loop is obtained as follows: Where Z out (s) is the output impedance of the inverter considering the phase-locked loop, Z o1 (s) and Z pll (s) is the current loop equivalent output impedance and the phase-locked loop equivalent impedance.

4. The virtual impedance reshaping method for improving inverter stability in a weak power grid according to claim 1 is characterized in that: Step S3 constructs an equivalent Norton circuit of a virtual series impedance grid-connected inverter, specifically including: current loop equivalent impedance Z o1 (s) and the equivalent impedance Z of the phase-locked loop pll (s) in parallel, grid impedance Z g (s), virtual impedance Z v1 (s) and Z v2 (s) are connected in series, and the grid voltage u pcc (s) and grid voltage u g (s) are applied across each impedance.

5. The virtual impedance reshaping method for improving inverter stability in a weak power grid according to claim 1 is characterized in that: In step S4, the output impedance expression of the grid-connected inverter after virtual impedance reshaping is as follows: Z' out (s)=Z o1 (s) / / Z pll (s)+Z v1 (s)+Z v2 (s) =Z out (s)+Z v1 (s)+Z v2 (s) =kZ out (s)+Z v2 (s) Where k is the error coefficient, which is used to ensure that the low and medium frequency amplitudes remain unchanged after the virtual impedance is reshaped.

6. The virtual impedance reshaping method for improving inverter stability in a weak power grid according to claim 1 is characterized in that: The stability requirement in step S5 is: Virtual impedance Z v1 (s) and Z v2 (s) While improving the phase margin of the grid-connected inverter under weak power grid, the amplitude-frequency corresponding to the cutoff frequency fc at the grid impedance under the worst condition satisfies: |From out (j2πf c )|≤|From' out (j2πf c )| where |Z out (j2πf c )| is the magnitude of the inverter output impedance at the impedance intersection frequency without considering the phase-locked loop, |Z out '(j2πf c )| is the amplitude of the inverter output impedance of the phase-locked loop at the frequency where it intersects with the impedance; And the phase margin satisfies: ∠Z' out (j2πf c )≥-(90-β) Where β is the desired phase margin, f c is the frequency where the grid impedance intersects the inverter output impedance.

7. The virtual impedance reshaping method for improving inverter stability in a weak power grid according to claim 1 is characterized in that: The harmonic suppression capability requirement in step S5 is: the minimum amplitude of the output impedance at each harmonic frequency satisfies: where f n is the harmonic frequency, V g and I g The root mean square of the grid voltage and grid current are respectively V n and I n The distortion ratio of the grid voltage and grid current.

8. The virtual impedance reshaping method for improving inverter stability in a weak power grid according to claim 1 is characterized in that: The steady-state error of the grid current introduced by the virtual series impedance is: where e i is the steady-state error, i g ' is the grid-connected current after connecting the virtual impedance in series, i g is the output grid-connected current without series virtual impedance, and ω0 is the fundamental frequency of the grid voltage.

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