Sequence impedance modeling method for net-forming droop control inverter

Abstract

The invention discloses a sequence impedance modeling method for a network-forming droop control inverter, which belongs to the technical field of power electronic inverter system modeling and comprises the following steps of: S1, obtaining dq component expressions of output voltage and output current of the inverter; s2, obtaining a frequency domain expression of output voltage and output current of the dq coordinate system droop control inverter after small signal disturbance is considered; s3, obtaining a frequency domain expression of three-phase modulation wave output internal potential of the droop control inverter; and S4, obtaining small-signal positive and negative sequence impedance models of the net-forming droop control inverter. According to the method, the complete and comprehensive sequence impedance model of the grid-forming type droop control inverter with clear physical significance can be accurately established, a basis for impedance matching optimization is provided to reduce harmonic oscillation, small-disturbance stability analysis can be carried out, application of the grid-forming type inverter in a micro-grid can be promoted, and the method is suitable for popularization and application of the grid-forming type droop control inverter in the micro-grid. And an environment-friendly and economic-friendly grid-connected system is established.

Description

Technical Field

[0001] The invention belongs to the technical field of power electronic inverter system modeling, and in particular relates to a sequence impedance modeling method for a grid-type droop control inverter. Background Art

[0002] With the large-scale access of new energy power generation equipment to the power grid, grid-connected inverters, as the energy transmission interface between new energy and the power grid, have been widely used in various fields. In recent years, with the increasingly serious energy crisis and environmental pollution problems, new energy power generation technology has attracted much attention. However, in the application of droop control inverter technology, the power loop or control strategy is often ignored, the established inverter model is not complete enough, and there are some defects in analyzing system stability. Therefore, establishing a complete grid-connected droop control impedance model is of vital significance for analyzing system stability.

[0003] At present, the modeling methods of grid-connected inverter systems are mainly divided into two categories: one is the model based on the system state space problem in the time domain. Although it can better describe the dynamic behavior of the system, it has a strong need to know the main circuit structure of the system and poor scalability; the other is the impedance model based on the transfer function in the frequency domain. The impedance analysis method has the advantages of clear physical concepts and convenient measurement and verification. In recent years, it has been widely used in the method of analyzing the stability of grid-connected converters under weak power grids. The existing stability analysis methods are mainly divided into two types: dq impedance modeling and sequence impedance modeling of harmonic linearization. The dq axis impedance modeling method is a method for power system stability analysis and control design. This method is based on the dq coordinate system. The three-phase power system is converted to the dq coordinate system for analysis, and then the traditional steady-state DC operating point small signal linearization is used to derive the impedance modeling in the dq domain. However, this method has certain limitations. The three-phase system is connected to the unbalanced power grid system and there is coupling that cannot be ignored. The model has no actual physical meaning. The harmonic linearization method establishes a new energy grid-connected inverter model, uses Fourier transform to convert periodic time-varying voltage and current signals from the time domain to the frequency domain, and simplifies the originally complex time domain operations by injecting positive and negative sequence disturbance components and combining the frequency domain convolution formula. The positive and negative sequence impedances of the grid-connected inverter system are thus obtained. Therefore, compared with dq impedance modeling, the model established by the harmonic linearization modeling method is more comprehensive and has a wider range of applications.

[0004] In practical applications, the influence of line impedance cannot be ignored. The interconnected system formed between the grid-type droop control inverter and the power grid is prone to harmonic oscillation problems when the impedances of the two do not match, which poses a challenge to small-disturbance stability analysis. Therefore, a sequence impedance modeling method for grid-type droop control inverters is needed that can accurately model, perform small-disturbance stability analysis, and optimize impedance matching. Summary of the invention

[0005] The purpose of the present invention is to provide a sequence impedance modeling method for a grid-type droop control inverter, which can accurately establish a complete and comprehensive sequence impedance model of a grid-type droop control inverter with clear physical meaning, provide a basis for impedance matching optimization to reduce harmonic oscillations, and perform small disturbance stability analysis, which is helpful to promote the application of grid-type inverters in microgrids and establish an environmentally and economically friendly grid-connected system.

[0006] To achieve the above purpose, the technical solution adopted by the present invention is:

[0007] A sequence impedance modeling method for a grid-type droop control inverter comprises the following steps:

[0008] Step S1: adding positive and negative sequence small signal voltage disturbances at the common coupling point where the grid-type droop control inverter is connected to the grid, and obtaining the expressions of the three-phase output voltage and output current of the grid-type droop control inverter in the time domain; converting the time domain expressions to the frequency domain through the Fourier transform formula, and then transforming to the dq coordinate system to obtain the dq component expressions of the output voltage and output current of the inverter;

[0009] Step S2: According to the expression of the expression of the active and reactive droop control output power, the frequency domain expression of the droop control inverter output power is obtained in combination with the output voltage and current calculated in step S1; the disturbance phase angle is calculated using the droop control equation to obtain the frequency domain expression of the output voltage and output current of the droop control inverter in the dq coordinate system after considering the small signal disturbance;

[0010] Step S3: According to the relationship between reactive power droop control and voltage amplitude, combined with the phase angle in step S2, the frequency domain expression of the dq axis reference voltage of the droop control inverter voltage and current dual closed loop is obtained; based on the expression of the virtual impedance voltage and current dual closed loop control strategy, the output voltage and current of the inverter in step S2 are substituted to obtain the inverter output dq axis modulation wave small signal after the voltage and current dual closed loop; according to the Park inverse transformation and considering the disturbance phase angle, the frequency domain expression of the internal potential of the three-phase modulation wave output of the droop control inverter is obtained;

[0011] Step S4: Kirchhoff's voltage law is used for the main circuit model to obtain the relationship between the output internal potential, output voltage and output current, and finally the small signal positive and negative sequence impedance model Z of the grid-type droop control inverter is obtained. pp (s) and Z nn (s).

[0012] A further improvement of the technical solution of the present invention is that in step S1, after the positive and negative sequence small signal voltage disturbances are injected at the grid connection point, the time domain expressions of the output terminal voltage and output current of the a phase of the droop control inverter are respectively:

[0013]

[0014] Where, V1 and I1 are the fundamental voltage amplitude and current amplitude respectively; V p and I p are the positive sequence disturbance voltage amplitude and current amplitude respectively; V n and I n are the negative sequence disturbance voltage amplitude and the response current amplitude respectively; the initial phase of the fundamental voltage is set to zero, is the initial phase of fundamental current; and are the initial phases of the positive-sequence disturbance voltage and the negative-sequence disturbance voltage respectively; and are the initial phases of the positive sequence disturbance current response and the negative sequence disturbance current response, respectively, where the subscript "p" represents the positive sequence and "n" represents the negative sequence. Define ω p =2π(f p +f1),ω n =2π(f p -f1);

[0015]

[0016] in,

[0017] The abc to dq coordinate transformation formula T(θ1) is:

[0018]

[0019] Where, θ1 is the angle between the output d-axis and the a-axis, θ1=2πf1, f1 is the fundamental frequency;

[0020] The output voltage and current calculation formula of the grid-type droop control inverter in the dq coordinate system is as follows:

[0021]

[0022] A further improvement of the technical solution of the present invention is that: in step S2, the inverter output voltage and current are substituted, and the frequency domain convolution theorem is used to obtain the expressions of the droop control active power and reactive power in the frequency domain, and the obtained active power is substituted into the droop equation to obtain the disturbance phase angle θ; the active power and frequency loop can obtain the expression of the droop control output phase angle θ as shown below:

[0023]

[0024] The expression of θ in the frequency domain can be obtained:

[0025] Δθ[f]=-M(s)(Vp I1 * +I p V1 * +V n I1+I n V1),f=±f p

[0026] When the disturbance phase angle Δθ corresponding to the small signal voltage disturbance component is considered, the output voltage v of the droop control in the synchronous rotating coordinate system is obtained by Park transformation. od and v oq , as shown below:

[0027]

[0028] Finally, the frequency domain expressions of the output voltage and output current of the dq coordinate system droop control inverter after considering small signal disturbance are as follows:

[0029]

[0030] A further improvement of the technical solution of the present invention is that in step S3, according to the relationship expression between the reactive power controller and the amplitude of the droop control:

[0031] E=E m +n(Q ref -Q0)

[0032] The frequency domain expression of the dq-axis reference voltage of the voltage-current double closed loop of the droop control inverter is as follows:

[0033]

[0034] V qref [f] = 0

[0035] The expression of the dual closed-loop control strategy based on virtual impedance, after the voltage and current dual closed-loop, obtains the inverter output dq axis modulation wave small signal, as shown in the following formula:

[0036]

[0037] In the formula, c d and c q are the d-axis and q-axis components of the inverter three-phase modulation wave in the synchronous rotating coordinate system; v od and v oq are the d-axis and q-axis components of the droop control inverter output voltage in the synchronous rotating coordinate system; v dref and v qref are the reference voltages; i od and i oqare the d-axis and q-axis components of the droop control inverter output current in the synchronous rotating coordinate system; L f is the filter inductance of the inverter; C f is the filter capacitor of the inverter; L v is the virtual inductance; K c is the filter capacitor decoupling coefficient, K c =ω l C f , K L is the filter inductor decoupling coefficient, K L =ω l L f , K v is the virtual inductance decoupling coefficient, K v =ω l L v , where ω l is the fundamental angular frequency; H v (s) is the PI controller of the voltage loop, H v (s) = K pv +K iv / s;H v (s) is the PI controller of the current loop, H v (s) = K iv +K ii / s;

[0038] The Prak inverse transform takes small signal disturbance into consideration and obtains the following expression in the frequency domain of the output voltage of the droop control inverter:

[0039]

[0040] According to the Park inverse transform, the expression of the modulation wave of the droop control inverter in the stationary coordinate system is as follows:

[0041]

[0042] A further improvement of the technical solution of the present invention is that in step S4, Kirchhoff's law is used for the main circuit model of the three-phase grid-connected inverter to obtain the following relationship expression between the output internal potential, output voltage and output current of the grid-connected inverter:

[0043]

[0044] Finally, the small signal positive and negative sequence impedance model Z of the grid-type droop control inverter is obtained. pp (s) and Z nn (s), as follows:

[0045]

[0046]

[0047] Where, M(s) = 3mG f (s) / s,Y c =sC f / (1+R cf C f s), Z f =L f C f s 2 / (1+C f R cf s)+1,G f (s) is the power loop filter function, L f , R L are the filter inductance and parasitic resistance of the inverter respectively; C f and R cf Filter capacitor and its parasitic resistance; Z L is the line impedance; m and n are the active power droop coefficient and reactive power droop coefficient respectively; “*” indicates the conjugate operation.

[0048] Due to the adoption of the above technical solution, the technical progress achieved by the present invention is:

[0049] The sequence impedance modeling method of the grid-type droop control inverter of the present invention can accurately establish a complete and comprehensive sequence impedance model of the grid-type droop control inverter with clear physical meaning, so as to better analyze the stability of the grid-type droop control connected to the microgrid. It is also beneficial to the small disturbance stability analysis of the grid-type droop control inverter in the grid-connected scenario, which is helpful for the application of the grid-type inverter in the microgrid and the establishment of an environmentally and economically friendly grid-connected system. BRIEF DESCRIPTION OF THE DRAWINGS

[0050] Figure 1 It is the main circuit structure and control block diagram of the grid-connected three-phase grid-connected inverter in the present invention;

[0051] Figure 2 It is a block diagram of the grid-type droop control in the present invention;

[0052] Figure 3 It is a schematic diagram of a voltage and current double closed-loop structure based on virtual impedance in the present invention;

[0053] Figure 4 It is the equivalent circuit diagram of the main circuit in the present invention;

[0054] Figure 5 These are the theoretical curves and simulation results of the positive and negative sequence impedances of the grid-type droop control in the present invention. DETAILED DESCRIPTION

[0055] The present invention is further described in detail below in conjunction with embodiments:

[0056] The present invention provides a sequence impedance modeling method for a grid-type droop control inverter, and the main circuit topology diagram used is as follows: Figure 1 As shown, including a DC power supply, a three-phase inverter, an LC filter and a line reactance, the method includes the following steps:

[0057] Step S1: adding positive and negative sequence small signal voltage disturbances at the common coupling point where the grid-type droop control inverter is connected to the grid, and obtaining the expressions of the three-phase output voltage and output current of the grid-type droop control inverter in the time domain; converting the time domain expressions to the frequency domain through the Fourier transform formula, and then transforming to the dq coordinate system to obtain the dq component expressions of the output voltage and output current of the inverter;

[0058] After the positive and negative sequence small signal voltage disturbances are injected at the grid connection point, the time domain expressions of the output voltage and output current of phase a of the droop control inverter are:

[0059]

[0060] Where, V1 and I1 are the fundamental voltage amplitude and current amplitude respectively; V p and I p are the positive sequence disturbance voltage amplitude and current amplitude respectively; V n and I n are the negative sequence disturbance voltage amplitude and the response current amplitude respectively; the initial phase of the fundamental voltage is set to zero, is the initial phase of fundamental current; and are the initial phases of the positive-sequence disturbance voltage and the negative-sequence disturbance voltage respectively; and are the initial phases of the positive sequence disturbance current response and the negative sequence disturbance current response, respectively, where the subscript "p" represents the positive sequence and "n" represents the negative sequence. Define ω p =2π(f p +f1),ω n =2π(f p -f1);

[0061]

[0062] in,

[0063] The abc to dq coordinate transformation formula T(θ1) is:

[0064]

[0065] Where, θ1 is the angle between the output d-axis and the a-axis, θ1=2πf1, f1 is the fundamental frequency;

[0066] The output voltage and current calculation formula of the grid-type droop control inverter in the dq coordinate system is as follows:

[0067]

[0068] Step S2: According to the expression of the expression of the active and reactive droop control output power, the frequency domain expression of the droop control inverter output power is obtained in combination with the output voltage and current calculated in step S1; the disturbance phase angle is calculated using the droop control equation to obtain the frequency domain expression of the output voltage and output current of the droop control inverter in the dq coordinate system after considering the small signal disturbance;

[0069] like Figure 2 As shown, substitute the inverter output voltage and current, use the frequency domain convolution theorem to get the expressions of the droop control active power and reactive power in the frequency domain, and use the obtained active power to substitute into the droop equation to get the disturbance phase angle θ; the active power and frequency loop can get the expression of the droop control output phase angle θ as shown below:

[0070]

[0071] The expression of θ in the frequency domain can be obtained:

[0072] Δθ[f]=-M(s)(V p I1 * +I p V1 * +V n I1+I n V1),f=±f p

[0073] When the disturbance phase angle Δθ corresponding to the small signal voltage disturbance component is considered, the output voltage v of the droop control in the synchronous rotating coordinate system is obtained by Park transformation. od and v oq , as shown below:

[0074]

[0075] Finally, the frequency domain expressions of the output voltage and output current of the dq coordinate system droop control inverter after considering small signal disturbance are as follows:

[0076]

[0077]

[0078] Step S3: According to the relationship between reactive power droop control and voltage amplitude, combined with the phase angle in step S2, the frequency domain expression of the dq axis reference voltage of the droop control inverter voltage and current dual closed loop is obtained; based on the expression of the virtual impedance voltage and current dual closed loop control strategy, the output voltage and current of the inverter in step S2 are substituted to obtain the inverter output dq axis modulation wave small signal after the voltage and current dual closed loop; according to the Park inverse transformation and considering the disturbance phase angle, the frequency domain expression of the internal potential of the three-phase modulation wave output of the droop control inverter is obtained;

[0079] According to the relationship expression between the reactive power controller and the amplitude of droop control:

[0080] E=E m +n(Q ref -Q0)

[0081] The frequency domain expression of the dq-axis reference voltage of the voltage-current double closed loop of the droop control inverter is as follows:

[0082]

[0083] V qref [f] = 0

[0084] like Figure 3 As shown, the grid-type droop control inverter control is the decoupling control of the voltage outer loop and the current inner loop. On this basis, the virtual impedance strategy is added. The expression of the dual closed-loop control strategy based on virtual impedance is obtained through the voltage and current double closed loops. The inverter output dq axis modulation wave small signal is shown in the following formula:

[0085]

[0086] In the formula, c d and c q are the d-axis and q-axis components of the inverter three-phase modulation wave in the synchronous rotating coordinate system; v od and v oq are the d-axis and q-axis components of the droop control inverter output voltage in the synchronous rotating coordinate system; v dref and v qref are the reference voltages; i od and i oq are the d-axis and q-axis components of the droop control inverter output current in the synchronous rotating coordinate system; L f is the filter inductance of the inverter; C f is the filter capacitor of the inverter; L v is the virtual inductance; K c is the filter capacitor decoupling coefficient, K c =ω l C f , KL is the filter inductor decoupling coefficient, K L =ω l L f , K v is the virtual inductance decoupling coefficient, K v =ω l L v , where ω l is the fundamental angular frequency; H v (s) is the PI controller of the voltage loop, H v (s) = K pv +K iv / s;H v (s) is the PI controller of the current loop, H v (s) = K iv +K ii / s;

[0087] The Prak inverse transform takes small signal disturbance into consideration and obtains the following expression in the frequency domain of the output voltage of the droop control inverter:

[0088]

[0089] According to the Park inverse transform, the expression of the modulation wave of the droop control inverter in the stationary coordinate system is as follows:

[0090]

[0091] Step S4: Kirchhoff's voltage law is used for the main circuit model to obtain the relationship between the output internal potential, output voltage and output current, and finally the small signal positive and negative sequence impedance model Z of the grid-type droop control inverter is obtained. pp (s) and Z nn (s);

[0092] like Figure 4 As shown, Kirchhoff's law is used for the main circuit model of the three-phase grid-connected inverter, and the relationship between the output potential, output voltage and output current of the grid-connected inverter is obtained as follows:

[0093]

[0094] Finally, the small signal positive and negative sequence impedance model Z of the grid-type droop control inverter is obtained. pp (s) and Z nn (s), as follows:

[0095]

[0096] Where, M(s) = 3mG f (s) / s,Y c =sC f / (1+R cf C f s), Z f =L f C f s 2 / (1+C f R cf s)+1,G f (s) is the power loop filter function, L f , R L are the filter inductance and parasitic resistance of the inverter respectively; C f and R cf Filter capacitor and its parasitic resistance; Z L is the line impedance; m and n are the active power droop coefficient and reactive power droop coefficient respectively; “*” indicates the conjugate operation.

[0097] In order to verify the sequence impedance modeling method of the grid-type droop control inverter proposed in the present invention, simulation verification is carried out in Simulink: Figure 5 As shown in the figure, the positive and negative sequence impedance theoretical curves and simulation results of the grid-type droop control inverter are shown. pp (s) and Z nn (s) are the small signal positive and negative sequence impedances of the grid-type droop control inverter, respectively. It can be observed from the figure that the impedance measurement results are basically consistent with the constructed impedance model, which proves the correctness of the sequence impedance modeling of the grid-type droop control inverter.

[0098] In view of the limitations of traditional dq impedance modeling, the present invention proposes a full-band small signal modeling method for a grid-type droop control inverter based on a sequence impedance model and verifies the correctness of the constructed model, solving the problem of small signal impedance modeling of a grid-type droop control inverter. The model established by the present invention can accurately reflect the impedance characteristics of the grid-type droop control inverter in the full frequency band, not only including the LC filter parameters and line impedance of the main circuit, but also covering all aspects of the inverter control strategy including power calculation, active and reactive droop control loops, virtual impedance, voltage and current loops, providing a theoretical model basis for studying the active and reactive droop coefficients of the grid-type droop control inverter, voltage and current controller parameters, and line impedance, which is helpful to promote the application of grid-type inverters in microgrids and establish an environmentally and economically friendly grid-connected system.

[0099] It is to be understood that the present invention is described by some embodiments, and it is known to those skilled in the art that various changes or equivalent substitutions may be made to these features and embodiments without departing from the spirit and scope of the present invention. In addition, under the teachings of the present invention, these features and embodiments may be modified to adapt to specific circumstances and materials without departing from the spirit and scope of the present invention. Therefore, the present invention is not limited by the specific embodiments disclosed herein, and all embodiments falling within the scope of the claims of this application are within the scope of protection of the present invention.

Claims

1. A sequence impedance modeling method for a grid-type droop control inverter, characterized in that The following steps are involved: Step S1: adding positive and negative sequence small signal voltage disturbances at the common coupling point where the grid-type droop control inverter is connected to the grid, and obtaining expressions of the three-phase output voltage and output current of the grid-type droop control inverter in the time domain; The time domain expression is converted to the frequency domain by the Fourier transform formula, and then transformed to the dq coordinate system to obtain the dq component expression of the output voltage and output current of the inverter; Step S2: According to the expression of the expression of the active and reactive droop control output power, the frequency domain expression of the droop control inverter output power is obtained in combination with the output voltage and current calculated in step S1; the disturbance phase angle is calculated using the droop control equation to obtain the frequency domain expression of the output voltage and output current of the droop control inverter in the dq coordinate system after considering the small signal disturbance; Step S3: According to the relationship between reactive power droop control and voltage amplitude, combined with the phase angle in step S2, the frequency domain expression of the dq axis reference voltage of the droop control inverter voltage and current dual closed loop is obtained; based on the expression of the virtual impedance voltage and current dual closed loop control strategy, the output voltage and current of the inverter in step S2 are substituted to obtain the inverter output dq axis modulation wave small signal after the voltage and current dual closed loop; according to the Park inverse transformation and considering the disturbance phase angle, the frequency domain expression of the internal potential of the three-phase modulation wave output of the droop control inverter is obtained; Step S4: Kirchhoff's voltage law is used for the main circuit model to obtain the relationship between the output internal potential, output voltage and output current, and finally the small signal positive and negative sequence impedance model Z of the grid-type droop control inverter is obtained. pp (s) and Z nn (s).

2. The sequence impedance modeling method of a grid-type droop control inverter according to claim 1, characterized in that: In step S1, after the positive and negative sequence small signal voltage disturbances are injected at the grid connection point, the time domain expressions of the output voltage and output current of the phase a of the droop control inverter are: Where, V1 and I1 are the fundamental voltage amplitude and current amplitude respectively; V p and I p are respectively the positive sequence disturbance voltage amplitude and current amplitude; V n and I n are the negative sequence disturbance voltage amplitude and the response current amplitude respectively; the initial phase of the fundamental voltage is set to zero, is the initial phase of fundamental current; and are the initial phases of the positive-sequence disturbance voltage and the negative-sequence disturbance voltage respectively; and are the initial phases of the positive sequence disturbance current response and the negative sequence disturbance current response, respectively, where the subscript "p" represents the positive sequence and "n" represents the negative sequence. Define ω p =2π(f p +f1),ω n =2π(f p -f1); Where V1 = V1 / 2, The abc to dq coordinate transformation formula T(θ1) is: Where, θ1 is the angle between the output d-axis and the a-axis, θ1=2πf1, f1 is the fundamental frequency; The output voltage and current calculation formula of the grid-type droop control inverter in the dq coordinate system is as follows:

3. The sequence impedance modeling method of a grid-type droop control inverter according to claim 2, characterized in that: In step S2, the inverter output voltage and current are substituted, and the frequency domain convolution theorem is used to obtain the expressions of the droop control active power and reactive power in the frequency domain, and the obtained active power is substituted into the droop equation to obtain the disturbance phase angle θ; the active power and frequency loop can obtain the expression of the droop control output phase angle θ as shown below: The expression of θ in the frequency domain can be obtained: Δθ[f]=-M(s)(V p I1 * +I p V1 * +V n I1+I n V1),f=±f p When the disturbance phase angle Δθ corresponding to the small signal voltage disturbance component is considered, the output voltage v of the droop control in the synchronous rotating coordinate system is obtained by Park transformation. od and v oq , as shown below: Finally, the frequency domain expressions of the output voltage and output current of the dq coordinate system droop control inverter after considering small signal disturbance are as follows:

4. The sequence impedance modeling method of a grid-type droop control inverter according to claim 3 is characterized in that: In step S3, according to the relationship expression between the reactive power controller and the amplitude of the droop control: E=E m +n(Q ref -Q0) The frequency domain expression of the dq-axis reference voltage of the droop control inverter voltage and current double closed loop is as follows: V qref [f]=0 The expression of the dual closed-loop control strategy based on virtual impedance, after the voltage and current dual closed-loop, obtains the inverter output dq axis modulation wave small signal, as shown in the following formula: In the formula, c d and c q are the d-axis and q-axis components of the inverter three-phase modulation wave in the synchronous rotating coordinate system; v od and v oq are the d-axis and q-axis components of the droop control inverter output voltage in the synchronous rotating coordinate system; v dref and v qref are the reference voltages; i od and i oq are the d-axis and q-axis components of the droop control inverter output current in the synchronous rotating coordinate system; L f is the filter inductance of the inverter; C f is the filter capacitor of the inverter; L v is the virtual inductance; K c is the filter capacitor decoupling coefficient, K c =ω l C f , K L is the filter inductor decoupling coefficient, K L =ω l L f , K v is the virtual inductance decoupling coefficient, K v =ω l L v , where ω l is the fundamental angular frequency; H v (s) is the PI controller of the voltage loop, H v (s) = K pv +K iv / s;H v (s) is the PI controller of the current loop, H v (s) = K iv +K ii / s; The Prak inverse transform takes small signal disturbance into consideration and obtains the following expression in the frequency domain of the output voltage of the droop control inverter: According to Park inverse transformation, the expression of the modulation wave of the droop control inverter in the abc stationary coordinate system is as follows:

5. The sequence impedance modeling method of a grid-type droop control inverter according to claim 4, characterized in that: In step S4, Kirchhoff's law is applied to the main circuit model of the three-phase grid-connected inverter to obtain the relationship between the output internal potential, output voltage and output current of the grid-connected inverter as follows: Finally, the small signal positive and negative sequence impedance model Z of the grid-type droop control inverter is obtained. pp (s) and Z nn (s), as follows: Where, M(s) = 3mG f (s) / s,Y c =sC f / (1+R cf C f s), Z f =L f C f s 2 / (1+C f R cf s)+1,G f (s) is the power loop filter function, L f , R L are the filter inductance and parasitic resistance of the inverter respectively; C f and R cf Filter capacitor and its parasitic resistance; Z L is the line impedance; m and n are the active power droop coefficient and reactive power droop coefficient respectively; "*" indicates the conjugate operation.

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