Inverter sequence admittance simplified modeling method considering frequency coupling effect between alternating current and direct current and alternating current side frequency coupling effect

Abstract

The invention discloses an inverter sequence admittance simplified modeling method considering an alternating current and direct current frequency coupling effect and an alternating current side frequency coupling effect, and belongs to the technical field of distributed new energy grid-connected power generation. Obtaining a time domain expression and a frequency domain expression of AC-DC side voltage and current based on the AC-DC frequency coupling effect and the AC side frequency coupling effect; establishing a direct current voltage outer loop control model according to the direct current side dynamic control strategy; under the condition of considering small disturbance of a phase-locked loop, an alternating current inner loop control model is established, and an inverter sequence admittance model of multiple frequency coupling effects is obtained. According to the method, the sequence admittance model considering multiple frequency coupling effects can be accurately established, the derivation process is simplified, the calculation complexity is reduced, and the influence of inverter access on the stability of an alternating current power grid is comprehensively analyzed.

Description

Technical Field

[0001] The present invention belongs to the technical field of distributed renewable energy grid-connected power generation, and in particular relates to a simplified inverter sequence admittance modeling method for frequency coupling effects between AC and DC and frequency coupling effects on the AC side. Background Art

[0002] In order to solve the fossil energy crisis and promote energy transformation, distributed power generation technology has developed rapidly. In the process of renewable energy access and re-electrification on the load side, a large number of photovoltaic power sources, loads, energy storage and other equipment with different characteristics are connected to the grid through inverters. The access of these equipment has changed the grid structure from the traditional source-grid-load unidirectional flow to a bidirectional flow. The wide-band characteristics of its control system make it easier to inject negative damping in different frequency bands into the system, thus causing a series of stability problems. Therefore, it is necessary to accurately model the impedance characteristics of the inverter system for subsequent stability analysis.

[0003] In general, most of the current impedance modeling of inverters regards the DC-side voltage-stabilizing capacitor as infinite, assuming that there is no small disturbance on the DC side, and only considers the frequency coupling characteristics of the AC-side port. However, with the continuous development of power electronics technology, the number of grid-connected micro-source inverters such as photovoltaics and energy storage has increased, resulting in an increase in the dynamic complexity of the DC side of the grid-connected system. At the same time, the multi-port characteristics of the inverter itself make it impossible to ignore the frequency coupling effect between the AC and DC sides. Existing literature has taken into account the frequency coupling effect between the AC and DC sides by performing an order-up process on the traditional second-order admittance matrix, but this model cannot be applied to the stability analysis of multi-inverter grid-connected systems.

[0004] Existing studies rarely consider the dynamic control characteristics of DC-side micro-sources and the accurate propagation path of DC-side disturbances. And with the refinement of modeling, when considering the inverter frequency coupling characteristics and complex control strategies, the modeling process becomes more and more complicated. Specifically, the parameters need to undergo multiple coordinate transformations during modeling and the expressions of each parameter are lengthy, which is not conducive to operational promotion. Therefore, there is a lack of a simple method to establish an accurate inverter model when considering multiple frequency coupling effects. Summary of the invention

[0005] The present invention aims to provide a simplified modeling method for inverter sequence admittance taking into account frequency coupling effects between AC and DC and frequency coupling effects on the AC side.

[0006] To achieve the above object, the technical solution of the present invention is: A simplified modeling method for inverter sequence admittance considering frequency coupling effect between AC and DC and frequency coupling effect on AC side includes the following steps: Step S1, determining the main circuit filter type and control structure of the three-phase inverter grid-connected system; Step S2, based on the frequency coupling effect between AC and DC and the frequency coupling effect on the AC side, obtaining the time domain expression and frequency domain expression of the voltage and current on the AC and DC sides; Step S3, establishing a DC voltage outer loop control model according to a DC side dynamic control strategy; Step S4, establishing an AC current inner loop control model while considering a small disturbance of the phase-locked loop, and obtaining frequency domain expressions of the grid-connected voltage and grid-connected current; Step S5, according to the convolution characteristics of each component, using a simplified modeling method based on the superposition theorem, to obtain an inverter order admittance model with multiple frequency coupling effects; Step S6, deriving a positive and negative sequence admittance model of a single inverter system after equivalent decoupling based on the inverter sequence admittance model of the multiple frequency coupling effects; Step S7, deriving the positive and negative sequence admittance models of multiple inverters based on the positive and negative sequence admittance models of the single inverter system after equivalent decoupling, and establishing a stability criterion for the multi-inverter grid-connected system.

[0007] Furthermore, in step S1, the outer loop of the inverter grid-connected system adopts DC voltage control, and the inner loop of the inverter grid-connected system adopts grid current and capacitor current control; the inverter is connected to the grid point through an LCL filter, and then connected to the grid through a grid impedance.

[0008] Furthermore, in step S2, a Phase grid voltage , grid-connected current and capacitor current The time domain expression of is: in, V g1 , V p , V po Respectively represent the amplitudes of the fundamental frequency component, the disturbance component and the coupling component of the grid-connected voltage; φ p , φ po Respectively represent the phases of the disturbance component and the coupling component of the grid-connected voltage; I g1 , I p , I po Respectively represent the amplitudes of the fundamental frequency component, the disturbance component and the coupling component of the grid-connected current; φi1 , φ ip , φ ipo Respectively represent the phases of the fundamental frequency component, the disturbance component and the coupling component of the grid-connected current; I c1 , I cp , I cpo represent the amplitudes of the fundamental frequency component, disturbance component and coupling component of the capacitor current respectively, φ ic1 , φ icp , φ icpo Respectively represent the phases of the fundamental frequency component, the disturbance component and the coupling component of the capacitive current; f p and f po Respectively represent the frequencies of the disturbance component and the coupling component of the grid connection point, which satisfy f po = f p -2 f 1, f 1 is the fundamental frequency; The grid voltage u ga , the grid-connected current i ga and the capacitor current i ca After Fourier transform, the corresponding frequency domain expression is: , and : in, ; , ; ; The DC side includes a DC steady-state component and a frequency f p - f 1 coupling component, the DC voltage on the DC side and DC current The time domain expression of is: in, V dc , V dcpo are respectively the fundamental frequency component and the disturbance component of the capacitor voltage on the DC side; φ dcpo is the phase of the disturbance component of the capacitor voltage on the DC side; I pv , I pvdcpo are respectively the amplitudes of the fundamental frequency component and the coupling component of the input current on the DC side; φ dcipo is the phase of the input coupling component of the DC side; is the frequency of the disturbance component on the DC side, satisfying f dcpo = f p - f 1; The DC voltage and the DC current After Fourier transform, the corresponding frequency domain expression is obtained , for: in, .

[0009] Furthermore, in step S3, the DC side dynamic control strategy is the MPPT dynamic control of the micro-source, and there is a bus voltage disturbance component on the DC side. ; The input power of the micro source is fixed, then there is an input current disturbance component on the DC side ; The bus voltage disturbance component and the input current disturbance component The following relationship exists: in, K It is the ratio of the output voltage and output current corresponding to the operating power point at a specific time section; According to the grid voltage and the grid-connected current The AC side output voltage of the inverter can be obtained and output current for: in, L f =L 1+ L 2, L 1, L 2 are the inverter side inductance and the grid side inductance of the LCL filter respectively; According to the law of conservation of energy, the bus voltage disturbance component on the DC side is obtained: for: in, in, s p1 =± ( j 2 πf p -j 2 πf 1 ), s p =±j 2 πf p , s p2 =± ( j 2 πf p -j 4 πf 1 ), s 1 =± 2 πf 1 ; The bridge arm voltage of the inverter U ina In frequency ±f 1 The steady-state value is V il , its conjugate value is V * il ; The bridge arm current of the inverter I ina In frequency ±f 1 The steady-state value is I il , its conjugate value is I * il .

[0010] Furthermore, in step S4, a synchronous reference coordinate system phase-locked loop is selected to obtain the phase angle required for Park coordinate transformation. , when small disturbances are not considered, the fundamental frequency f The phase angle corresponding to 1 is ,satisfy ; When the output phase angle of the phase-locked loop is slightly disturbed When the output phase angle of the phase-locked loop satisfies ; In steady state, after the phase angle component of the phase-locked loop is transformed, the frequency domain expressions of the grid-connected voltage, the grid-connected current and the capacitor current are obtained: , , for: Then the grid-connected voltage after the phase angle component transformation is dq The frequency components of the axis , The expression is: The voltage disturbance signal on the DC side will be transmitted to the AC side through the DC voltage outer loop and PWM. H v ( s ) is the transfer function of the PI control of the DC voltage outer loop, that is, H v ( s )= K pdc + K idc / s ; H i ( s ) is the transfer function of the PI control of the inner loop of the grid current, that is, H i ( s )= K p + K i / s , then the dq Frequency domain expression of the voltage modulation component of the shaft , for: in, D 0 and Q 0 are the bridge arm voltages dq The steady-state values ​​of the axial components are: .

[0011] Further, in step S5, in the frequency domain dq The voltage modulation component of the axis includes a steady-state component without small disturbances and , dq Disturbance components of shaft grid-connected current and capacitor current and , Consider the dynamics of the DC side and introduce the small signal component of the AC side and The steady-state component without small disturbances and the small signal component introduced into the AC side after considering the dynamics of the DC side contain fewer disturbance vectors; dq The disturbance components of the grid-connected current and capacitor current of the axis contain many disturbance vectors. dq The signs of the axis components are not completely symmetrical; Said dq The voltage modulation component of the axis can be expressed as: in, Get the corresponding abc The components in the coordinate system are: According to the superposition theorem, the expression of the inverter main circuit is: The second-order admittance matrix expression of the inverter is obtained as follows: in, Y 11 , Y 12 , Y 21 , Y 22 is the admittance element of the second-order admittance matrix.

[0012] Furthermore, in step S6, the equivalent positive sequence impedance of the single inverter is for: The equivalent negative sequence impedance of the single inverter for: .

[0013] Will m If two inverters with the same model and parameters are connected to the grid, then j Equivalent controlled source of an inverter and The expression satisfies the following relationship: It can be deduced that m Positive and negative sequence aggregate impedance of a parallel power generation system with 100 inverters , The expression is: Then the system admittance of a grid-following inverter looking outward is Y re Satisfies the following relationship: in, Z cline is the line impedance from each inverter to the grid connection point, Z load is the load impedance.

[0014] Furthermore, in step S7, when a plurality of the inverters form a multi-inverter grid-connected system, the stability criterion of the multi-inverter grid-connected system considering various frequency coupling effects is: the ratio of the equivalent grid impedance to the positive and negative sequence impedance of the inverter ( Z cline / m + Z load ) / Z scp and( Z cline / m + Z load ) / Z scn All satisfy the Nyquist criterion, and the multi-inverter parallel power generation system and the grid interaction system are stable.

[0015] Beneficial effect: The simplified modeling method of inverter sequence admittance considering the frequency coupling effect between AC and DC and the frequency coupling effect on the AC side proposed in the present invention can accurately establish a sequence admittance model considering multiple frequency coupling effects, greatly simplify the derivation process and reduce the calculation complexity, and can comprehensively analyze the impact of inverter access on the stability of the AC power grid, with strong applicability.

[0016] In order to make the above features and advantages of the invention more obvious and easy to understand, embodiments are given below and described in detail with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 It is the main circuit diagram of the three-phase LCL inverter grid-connected system.

[0018] Figure 2 It is the internal and external loop control block diagram of the three-phase LCL inverter.

[0019] Figure 3 It is a schematic diagram of the measurement method of the grid-connected inverter admittance.

[0020] Figure 4 It is the flow chart of the simplified modeling method of inverter sequence admittance.

[0021] Figure 5 It is a simulation verification diagram of each element of the inverter second-order admittance matrix. DETAILED DESCRIPTION

[0022] In order to make the purpose and technical solution of the embodiment of the present invention clearer, the technical solution of the embodiment of the present invention will be clearly and completely described below in conjunction with the drawings of the embodiment of the present invention. Obviously, the described embodiment is a part of the embodiment of the present invention, not all of the embodiments. Based on the described embodiment of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0023] The present invention proposes a simplified modeling method for inverter sequence admittance considering the frequency coupling effect between AC and DC and the frequency coupling effect on the AC side, comprising the following steps: Step S1, determining the main circuit filter type and control structure of the three-phase inverter grid-connected system.

[0024] like Figure 1 The figure shows the main circuit diagram of the three-phase LCL inverter grid-connected system. In the three-phase LCL inverter grid-connected system, the inverter is connected to the grid connection point (PCC) through the LCL filter, and then connected to the AC distribution network through the grid impedance. Figure 2As shown, the inverter is controlled by an AC current inner loop and a DC voltage outer loop; the grid current and capacitor current form a double current inner loop, and the double current inner loop and the grid voltage form a full feedforward structure; the weighted average of the inverter's output current and the grid-connected current is used as the feedback value, which can effectively suppress resonance.

[0025] Step S2, based on the frequency coupling effect between AC and DC and the frequency coupling effect on the AC side, obtain the time domain expression and frequency domain expression of the voltage and current on the AC and DC sides.

[0026] Considering the frequency coupling effect of the inverter AC side, the frequency is injected at the grid connection point. f p When the disturbance component is a Phase grid voltage , grid-connected current and capacitor current The time domain expression of is: in, V g1 , V p , V po They represent the amplitudes of the fundamental frequency component, disturbance component and coupling component of the grid-connected voltage respectively; φ p , φ po They represent the phases of the disturbance component and coupling component of the grid-connected voltage respectively. The initial phase of the fundamental frequency component is usually considered to be 0. I g1 , I p , I po Respectively represent the amplitudes of the fundamental frequency component, disturbance component and coupling component of the grid-connected current; φ i1 , φ ip , φ ipo is the phase of the fundamental frequency component, disturbance component and coupling component of the grid-connected current; I c1 , I cp , I cpo They represent the amplitudes of the fundamental frequency component, disturbance component and coupling component of the capacitor current respectively, φ ic1 , φ icp, φ icpo Respectively represent the phases of the fundamental frequency component, disturbance component and coupling component of the capacitor current; f p and f po They represent the frequencies of the disturbance component and coupling component of the grid connection point, respectively, which satisfy f po = f p -2 f 1.

[0027] Furthermore, the grid voltage u ga , grid-connected current i ga and capacitor current i ca After Fourier transform, the corresponding frequency domain expression is: , and : in, ; , ; ; according to a , b , c The phase angle relationship between the three phases can be obtained at the grid connection point. b Mutually, c The frequency domain expressions of the phase grid-connected voltage, grid-connected current and capacitor current will not be repeated here one by one.

[0028] As a multi-port power device, the frequency coupling component on the AC side of the inverter will be transferred to the DC port under the action of coordinate transformation. Therefore, in addition to the DC steady-state component, the DC side also contains a frequency f p - f 1 coupling component, at this time the DC voltage on the DC side and DC current The time domain expression of is: in V dc , V dcpoThey are the fundamental frequency component and disturbance component of the DC side capacitor voltage respectively; φ dcpo is the phase of the disturbance component of the DC side capacitor voltage; f dcpo = f p - f 1 is the frequency of the disturbance component; I pv , I pvdcpo are the amplitudes of the fundamental frequency component and coupling component of the DC side input current respectively; φ dcipo is the phase of the DC side input coupling component; is the frequency of the disturbance component on the DC side, satisfying f dcpo = f p - f 1.

[0029] Furthermore, the DC voltage and DC current After Fourier transform, the corresponding frequency domain expression is obtained , for: in, .

[0030] Step S3, establishing a DC voltage outer loop control model according to the DC side dynamic control strategy.

[0031] The dynamic control strategy of the DC side is the MPPT dynamic control of the micro source. It can be considered that there is a bus voltage disturbance component on the DC side. , because the input power of the micro source is fixed, there is an input current disturbance component on the DC side , the two have the following relationship: in, K It is the ratio of the output voltage and output current corresponding to the operating power point at a specific time section.

[0032] More specifically, the DC side power and AC side power The time domain expression and frequency domain expression of are: in, L 1, L2 are the inverter side inductance and grid side inductance of the LCL filter respectively.

[0033] Filter capacitor C The capacitor voltage on both sides and capacitor current satisfy: According to the grid voltage and grid current The AC side output voltage of the inverter can be obtained and output current for: in, L f = L 1+ L 2, L 1, L 2 are the inverter side inductance and grid side inductance of the LCL filter respectively.

[0034] According to the law of conservation of energy, the power on the AC side and the DC side can be considered equal, and the bus voltage disturbance component on the DC side is obtained: for: in, in, s p1 =± ( j 2 πf p -j 2 πf 1 ), s p =±j 2 πf p , s p2 =± ( j 2 πf p -j 4 πf 1 ), s 1 =± 2 πf 1 ; Inverter bridge arm voltage U ina In frequency ±f1 The steady-state value is V il , its conjugate value is V * il ; Inverter bridge arm current I ina In frequency ±f 1 The steady-state value is I il , its conjugate value is I * il .

[0035] Step S4, establishing an AC current inner loop control model while taking into account a small disturbance of the phase-locked loop, and obtaining frequency domain expressions of the grid-connected voltage and grid-connected current.

[0036] refer to Figure 3 Schematic diagram of the measurement method of grid-connected inverter admittance, using a synchronous reference frame phase-locked loop (SRF-PLL) to obtain the phase angle required for Park coordinate transformation θ PLL , in order to achieve synchronization with the grid voltage. When small disturbances are not considered, the fundamental frequency f The phase angle corresponding to 1 is expressed as ,satisfy ; When considering the small disturbance of the phase angle of the phase-locked loop output When the output angle satisfies ; Therefore, the coordinate transformation module can be To split, the splitting formula is: Furthermore, the expressions of grid voltage, grid current and capacitor current after the phase angle component transformation of the steady-state phase-locked loop are: , , for: Furthermore, considering the small perturbation of the phase angle Very small, grid-connected voltage dq Axis Component u gd , u gq , u gdu , u gqu The following relationship is satisfied in the time domain and frequency domain: Furthermore, in the steady state, the small perturbation of the phase angle is zero; in ( f p - f 1) and -( f p - f 1) At the frequency, the output phase angle is slightly disturbed for dq Axis Component u gq The transfer function of a small signal through a phase-locked loop is H PLL (s) income, including: Combining the above two equations, we can get: Then the grid-connected voltage after considering the phase angle component transformation is dq The frequency components of the axis are: Similarly, the grid current and capacitor current can be obtained dq The frequency components of each axis.

[0037] The voltage disturbance signal on the DC side will be transmitted to the AC side through the DC voltage outer loop and PWM. H v ( s ) is the transfer function of the PI control of the DC voltage outer loop, that is, H v ( s )= K pdc + K idc / s ; H i ( s ) is the transfer function of the PI control of the inner loop of the grid current, that is, H i ( s )= K p + K i / s ,but dq Frequency domain expression of the voltage modulation component of the shaft , for: in, D 0 and Q 0 are the bridge arm voltages dqThe steady-state value of the axis component can be obtained by calculating the inverter main circuit at the steady-state operating point: .

[0038] Step S5, according to the convolution characteristics of each component, using a simplified modeling method based on the superposition theorem, obtain the inverter order admittance model of multiple frequency coupling effects.

[0039] More specifically, refer to Figure 4 The simplified modeling method flow chart of the inverter sequence admittance is shown in the frequency domain. dq The voltage modulation component of the axis includes a steady-state component without small disturbances , ; dq Disturbance components of shaft grid-connected current and capacitor current , ; Consider the dynamics of the DC side and introduce the small signal component of the AC side , The steady-state component without small disturbances and the small signal component introduced into the AC side after considering the dynamics of the DC side contain fewer disturbance vectors, and the convolution process is simple. dq The disturbance components of the shaft grid current and capacitor current contain many disturbance vectors, and due to dq Due to the incomplete symmetry of the axis component signs, direct convolution will be very complicated. In the frequency domain, dq The disturbance components of the shaft grid current and capacitor current are essentially abc The phase current is obtained by Park transformation, and then Park transformation is performed again to return to abc coordinate system, so their calculations can be expressed directly, each part dq The voltage modulation component of the axis can be expressed as: in A and B The expression is: According to the proposed simplified modeling method, the steady-state component without small disturbances Convert to abc The frequency domain components of the coordinate system can be written directly without calculation: Frequency domain dq Disturbance components of shaft grid-connected current and capacitor current After a Park inverse transform, we can get: Considering the dynamics of the DC side, the small signal component of the AC side is introduced After a Park inverse transform, we can get: According to the superposition theorem, the expression of the inverter main circuit is: The inverter output impedance model considering the frequency coupling effect between AC and DC and the characteristics of the DC side dynamic control strategy obtains the second-order admittance matrix expression as follows: After sorting: More specifically, the specific expressions of each parameter are: Combining the above equations, the admittance element in the inverter second-order admittance matrix considering various frequency coupling effects can be derived: Y 11 , Y 12 , Y 21 , Y 22 The expression is: refer to Figure 5 The simulation verification diagram of each element of the inverter second-order admittance matrix is ​​shown. The time domain simulation model of the three-phase photovoltaic grid-connected inverter is established using PSCAD / EMTDC simulation software. The accuracy of the established inverter admittance model and the effectiveness of the proposed simplified modeling method are verified by frequency sweeping. It can be seen that the theory is consistent with the simulation results, which verifies the correctness of the model.

[0040] Step S6, deriving the positive and negative sequence admittance models of a single inverter after equivalent decoupling based on the inverter sequence admittance models of the multiple frequency coupling effects.

[0041] The inverter sequence admittance model considering multiple frequency coupling effects is equivalently decoupled, and the single inverter system is equivalent to two subsystems: positive sequence and negative sequence. The applied frequency is f p The disturbance voltage source, is the reciprocal of the sum of line impedance and load impedance, and The controlled current source introduced for the positive and negative sequence coupling of the grid-following inverter satisfies the following relationship: Then the equivalent positive sequence impedance of a single inverter is for: Similarly, the equivalent negative sequence impedance of a single inverter can be obtained: for: Step S7, deriving the positive and negative sequence admittance models of multiple inverters based on the positive and negative sequence admittance models of the single inverter after equivalent decoupling, and establishing a stability criterion for the multi-inverter grid-connected system.

[0042] Will m If two inverters with the same model and parameters are connected to the grid, then j Equivalent controlled source of an inverter and The expression satisfies the following relationship: It can be deduced that m Positive and negative sequence aggregate impedance of a parallel power generation system with 100 inverters , The expression is: Then the system admittance of a grid-following inverter looking outward is Y re Satisfies the following relationship: in, Z cline is the line impedance from each inverter to the grid connection point, Z load is the load impedance.

[0043] Then the stability criterion of multi-inverter grid connection considering various frequency coupling effects can be obtained as follows: the ratio of equivalent grid impedance to inverter positive and negative sequence impedance ( Z cline / m + Zload ) / Z scp and( Z cline / m + Z load ) / Z scn All of them satisfy the Nyquist criterion, and the multi-inverter parallel power generation system and the grid interaction system are stable.

[0044] The simplified modeling method of inverter sequence admittance considering the frequency coupling effect between AC and DC and the frequency coupling effect on the AC side proposed in the present invention can accurately establish a sequence admittance model considering multiple frequency coupling effects, greatly simplify the derivation process and reduce the calculation complexity, and can comprehensively analyze the impact of inverter access on the stability of the AC power grid, with strong applicability.

[0045] Although the present invention has been disclosed as above by way of embodiments, it is not intended to limit the present invention. Any person having ordinary knowledge in the technical field may make some changes and modifications without departing from the spirit and scope of the present invention. Therefore, the protection scope of the present invention shall be determined by the scope of the attached patent application.

Claims

1. A simplified modeling method for inverter sequence admittance considering the frequency coupling effect between AC and DC and the frequency coupling effect on the AC side, characterized in that: The steps include: Step S1, determining the main circuit filter type and control structure of the three-phase inverter grid-connected system; Step S2, based on the frequency coupling effect between AC and DC and the frequency coupling effect on the AC side, obtaining the time domain expression and frequency domain expression of the voltage and current on the AC and DC sides; Step S3, establishing a DC voltage outer loop control model according to a DC side dynamic control strategy; Step S4, establishing an AC current inner loop control model while considering a small disturbance of the phase-locked loop, and obtaining frequency domain expressions of the grid-connected voltage and grid-connected current; Step S5, according to the convolution characteristics of each component, using a simplified modeling method based on the superposition theorem, to obtain an inverter order admittance model with multiple frequency coupling effects; Step S6, deriving a positive and negative sequence admittance model of a single inverter system after equivalent decoupling based on the inverter sequence admittance model of the multiple frequency coupling effects; Step S7, deriving the positive and negative sequence admittance models of multiple inverters based on the positive and negative sequence admittance models of the single inverter system after equivalent decoupling, and establishing a stability criterion for the multi-inverter grid-connected system.

2. A simplified modeling method for inverter sequence admittance considering the frequency coupling effect between AC and DC and the frequency coupling effect on the AC side as claimed in claim 1, characterized in that: In step S1, the outer loop of the inverter grid-connected system adopts DC voltage control, and the inner loop of the inverter grid-connected system adopts grid current and capacitor current control; the inverter is connected to the grid connection point through an LCL filter, and then connected to the grid through a grid impedance.

3. A simplified modeling method for inverter sequence admittance considering the frequency coupling effect between AC and DC and the frequency coupling effect on the AC side as claimed in claim 2, characterized in that: In the step S2, a Phase grid voltage , grid-connected current and capacitor current The time domain expression of is: in, V g1 , V p , V po Respectively represent the amplitudes of the fundamental frequency component, the disturbance component and the coupling component of the grid-connected voltage; φ p , φ po Respectively represent the phases of the disturbance component and the coupling component of the grid-connected voltage; I g1 , I p , I po Respectively represent the amplitudes of the fundamental frequency component, the disturbance component and the coupling component of the grid-connected current; φ i1 , φ ip , φ ipo Respectively represent the phases of the fundamental frequency component, the disturbance component and the coupling component of the grid-connected current; I c1 , I cp , I cpo represent the amplitudes of the fundamental frequency component, disturbance component and coupling component of the capacitive current respectively, φ ic1 , φ icp , φ icpo Respectively represent the phases of the fundamental frequency component, the disturbance component and the coupling component of the capacitive current; f p and f po Respectively represent the frequencies of the disturbance component and the coupling component of the grid connection point, which satisfy f po = f p -2 f 1, f 1 is the fundamental frequency; The grid voltage u ga , the grid-connected current i ga and the capacitor current i ca After Fourier transform, the corresponding frequency domain expression is: , and : in, ; , ； ； The DC side includes a DC steady-state component and a frequency f p - f 1 coupling component, the DC voltage on the DC side and DC current The time domain expression of is: in, V dc , V dcpo are respectively the fundamental frequency component and the disturbance component of the capacitor voltage on the DC side; φ dcpo is the phase of the disturbance component of the capacitor voltage on the DC side; I pv , I pvdcpo are respectively the amplitudes of the fundamental frequency component and the coupling component of the input current on the DC side; φ dcipo is the phase of the input coupling component of the DC side; is the frequency of the disturbance component on the DC side, satisfying f dcpo = f p - f 1; The DC voltage and the DC current After Fourier transform, the corresponding frequency domain expression is obtained , for: in, .

4. The simplified modeling method of inverter sequence admittance considering the frequency coupling effect between AC and DC and the frequency coupling effect on the AC side as claimed in claim 3, characterized in that: In step S3, the DC side dynamic control strategy is the MPPT dynamic control of the micro source, and there is a bus voltage disturbance component on the DC side. ; The input power of the micro source is fixed, then there is an input current disturbance component on the DC side ; The bus voltage disturbance component and the input current disturbance component The following relationship exists: in, K It is the ratio of the output voltage and output current corresponding to the operating power point at a specific time section; According to the grid voltage and the grid-connected current The AC side output voltage of the inverter can be obtained and output current for: in, L f = L 1+ L 2, L 1, L 2 are the inverter side inductance and the grid side inductance of the LCL filter respectively; According to the law of conservation of energy, the bus voltage disturbance component on the DC side is obtained: for: in, in, s p1 =± ( j 2 πf p -j 2 πf 1 ), s p =±j 2 πf p , s p2 =± ( j 2 πf p -j 4 πf 1 ), s 1 =± 2 πf 1 ; The bridge arm voltage of the inverter U ina In frequency ±f 1 The steady-state value is V il , its conjugate value is V * il ; The bridge arm current of the inverter I ina In frequency ±f 1 The steady-state value is I il , its conjugate value is I * il .

5. A simplified modeling method for inverter sequence admittance considering the frequency coupling effect between AC and DC and the frequency coupling effect on the AC side as claimed in claim 4, characterized in that: In step S4, a synchronous reference coordinate system phase-locked loop is selected to obtain the phase angle required for Park coordinate transformation. , when small disturbances are not considered, the fundamental frequency f The phase angle corresponding to 1 is ,satisfy ; When the output phase angle of the phase-locked loop is slightly disturbed When the output phase angle of the phase-locked loop satisfies ; In steady state, after the phase angle component of the phase-locked loop is transformed, the frequency domain expressions of the grid-connected voltage, the grid-connected current and the capacitor current are obtained: , , for: Then the grid-connected voltage after the phase angle component transformation is dq The frequency components of the axis , The expression is: The voltage disturbance signal on the DC side will be transmitted to the AC side through the DC voltage outer loop and PWM. H v ( s ) is the transfer function of the PI control of the DC voltage outer loop, that is, H v ( s )= K pdc + K idc / s ; H i ( s ) is the transfer function of the PI control of the inner loop of the grid current, that is, H i ( s )= K p + K i / s , then the dq Frequency domain expression of the voltage modulation component of the shaft , for: in, D 0 and Q 0 are the bridge arm voltages dq The steady-state values ​​of the axial components are: 。 6. A simplified modeling method for inverter sequence admittance considering the frequency coupling effect between AC and DC and the frequency coupling effect on the AC side as claimed in claim 5, characterized in that: In the step S5, in the frequency domain dq The voltage modulation component of the axis includes a steady-state component without small disturbances and , dq Disturbance components of shaft grid-connected current and capacitor current and , Consider the dynamics of the DC side and introduce the small signal component of the AC side and The steady-state component without small disturbances and the small signal component introduced into the AC side after considering the dynamics of the DC side contain fewer disturbance vectors; dq The disturbance components of the grid-connected current and capacitor current of the axis contain many disturbance vectors. dq The signs of the axis components are not completely symmetrical; Said dq The voltage modulation component of the axis can be expressed as: in, Get the corresponding abc The components in the coordinate system are: According to the superposition theorem, the expression of the inverter main circuit is: The second-order admittance matrix expression of the inverter is obtained as follows: in, Y 11 , Y 12 , Y 21 , Y 22 is the admittance element of the second-order admittance matrix.

7. A simplified modeling method for inverter sequence admittance considering the frequency coupling effect between AC and DC and the frequency coupling effect on the AC side as claimed in claim 6, characterized in that: In step S6, the equivalent positive sequence impedance of the single inverter for: The equivalent negative sequence impedance of the single inverter for: 。 8. A simplified modeling method for inverter sequence admittance considering the frequency coupling effect between AC and DC and the frequency coupling effect on the AC side as claimed in claim 7, characterized in that: Will m If two inverters with the same model parameters are connected to the grid, then j Equivalent controlled source of an inverter and The expression satisfies the following relationship: It can be deduced that m Positive and negative sequence aggregate impedance of a parallel power generation system with 100 inverters , The expression is: Then the system admittance of a grid-following inverter looking outward is Y re Satisfies the following relationship: in, Z cline is the line impedance from each inverter to the grid connection point, Z load is the load impedance.

9. A simplified modeling method for inverter sequence admittance considering the frequency coupling effect between AC and DC and the frequency coupling effect on the AC side as claimed in claim 8, characterized in that: In step S7, when a plurality of the inverters form a multi-inverter grid-connected system, the stability criterion of the multi-inverter grid-connected system considering various frequency coupling effects is: the ratio of the equivalent grid impedance to the positive and negative sequence impedance of the inverter ( Z cline / m + Z load ) / Z scp and( Z cline / m + Z load ) / Z scn All satisfy the Nyquist criterion, and the multi-inverter parallel power generation system and the grid interaction system are stable.

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