A method for improving stability of a multi-feed system of a grid-connected inverter based on high bandwidth

Abstract

This invention belongs to the field of grid-connected inverter technology, specifically relating to a method for improving the stability of multi-infeed systems of grid-connected inverters based on high bandwidth. The method of this invention utilizes the proportional gain of the PI controller in the current loop of a single grid-connected inverter (GCI). k p and integral coefficient k i and the proportional gain of the PI controller in the voltage loop k vp and integral coefficient k vi Using the variable as a variable, Bode plots of the closed-loop transfer function of the current loop and the closed-loop transfer function of the voltage loop of a single GCI are drawn. The frequency corresponding to -3dB in the Bode plot is selected, and this frequency is required to meet the high bandwidth requirement. High bandwidth is used as the model parameter of the GCI multi-infeed system to obtain the first model. The stability of the GCI multi-infeed system is improved based on the first model, so that the method of the present invention can effectively improve the stability of the GCI multi-infeed system.

Description

Technical Field

[0001] This invention belongs to the field of grid-connected inverter technology, specifically relating to a method for improving the stability of multi-feedback grid-connected inverter systems based on high bandwidth. Background Technology

[0002] New energy power generation mainly uses grid-connected inverters as interfaces to connect to the power grid. When a large number of grid-connected inverters are connected to the power grid, the interaction mechanism of the multi-feed system of grid-connected inverters becomes increasingly complex, involving key factors such as the inverter's own control parameters, grid structure, and grid impedance. This can easily induce harmonic oscillations in the system, threatening the safe and stable operation of the system.

[0003] Currently, the development of power electronic devices is increasingly trending towards higher frequencies. After ordinary Si devices are completely replaced by new wide-bandgap devices, the coexistence of low-frequency and high-frequency power electronic devices will be inevitable. Therefore, exploring how to effectively utilize the advantages of high frequencies is of great significance. However, existing technologies rarely utilize high-bandwidth techniques to improve the stability of multi-feed systems.

[0004] Therefore, a new solution is urgently needed to improve the stability of multi-feed systems of grid-connected inverters based on high bandwidth. Summary of the Invention

[0005] To address the aforementioned technical problems, this invention provides a method for improving the stability of a multi-infeed grid-connected inverter system based on high bandwidth. This method effectively utilizes the advantages of high frequency and also effectively improves the stability of the GCI multi-infeed system.

[0006] This invention provides a method for improving the stability of a multi-infeed grid-connected inverter system based on high bandwidth, comprising the following steps: S1: Obtain the forward channel transfer function based on the LCL filter parameters of a single grid-connected inverter; S2: The transformation matrix is ​​obtained based on the phase angle difference between the power grid dq coordinate system and the phase-locked loop controller dq coordinate system; S3: Obtain the open-loop transfer function and closed-loop transfer function of the current loop based on the PI controller transfer function and forward channel transfer function in the current loop of the grid-connected inverter. S4: Obtain the open-loop and closed-loop transfer functions of the DC voltage loop based on the PI controller transfer function in the voltage loop and the closed-loop transfer function in the current loop. S5: Based on the DC side voltage, the inverter bridge output current and the modulation signal in the inverter circuit of the grid-connected inverter, obtain the relationship matrix between DC voltage, inverter bridge output current and modulation signal; S6: The equivalent output admittance model of the grid-connected inverter is obtained based on the relationship matrix between DC voltage and inverter bridge output current, modulation signal, and current loop closed-loop transfer function. S7: Based on the output variable of the node current injected by the grid-connected inverter and the input variable of the node voltage, the node admittance matrix model and impedance matrix model of the grid structure of the multi-feed system of the grid-connected inverter are obtained. Then, considering the coupling relationship between the nodes connected to the grid-connected inverter, the interactive coupling matrix is ​​obtained. S8: The output current model of the multi-feed system of the grid-connected inverter is obtained based on the equivalent current source output of the grid-connected inverter, the circuit relationship of the equivalent Norton equivalent model of the grid-connected inverter, and the interactive coupling matrix. S9: Using high bandwidth as the output current model parameter of the grid-connected inverter multi-infeed system, the first model is obtained; when the first model is not connected to the multi-infeed system, the Nyquist diagram of the multi-infeed system is plotted. When the Nyquist diagram does not enclose the point (-1, j0), the maximum grid connection impedance Z is reached. g1 When the first model is connected to the multi-infeed system, the Nyquist diagram of the multi-infeed system is plotted. The maximum grid impedance Z reached when the Nyquist diagram does not enclose the point (-1, j0) is calculated. g2 When Z g2 >Z g1 The stability of the GCI multi-feed system is improved.

[0007] Furthermore, the forward channel transfer function includes a first transfer function and a second transfer function, the expressions of which are as follows: ; ; Among them, I m Z represents a 2×2 identity matrix; cf (s), Y L1 (s) and Y L2 (s) represent the capacitance transfer function impedance matrix of the LCL filter, the inductance transfer function admittance matrix of the inverter output port, and the inductance transfer function admittance matrix of the grid connection point, respectively. For the first transfer function, This is the second transfer function; ; ; ; ; in, For the inverter-side filter inductor, For the mains-side filter inductor, C f For the filter capacitor, R d denoted as the damping resistor in the LCL filter, s is the Laplace operator, and ω1 represents the power grid angular frequency.

[0008] Furthermore, the transformation matrix includes a first transformation matrix and a second transformation matrix, the expressions of which are as follows: ; ; in, This is the first transformation matrix. Second transformation matrix, U is the transfer function of the PI controller in the phase-locked loop; md and U mq These are the steady-state values ​​of the inverter's output port voltage modulation signal on the d-axis and q-axis, respectively; I 2d and I 2q These are the steady-state values ​​of the inverter's output current components along the d-axis and q-axis, respectively; U m0 This represents the steady-state component of the inverter grid connection point voltage on the d-axis.

[0009] Furthermore, step S3 specifically includes: The open-loop transfer function G of the current loop is derived by calculating the transfer function of the PI controller in the current loop of the grid-connected inverter and the forward channel transfer function. ac (s) and closed-loop transfer function G acclose (s): ; ; ; ; Among them, K PWM For pulse width modulation gain; G de (s) represents the delay generated during the calculation and modulation process; G i (s) represents the transfer function matrix of the PI controller in the current loop, T s Indicates the sampling period; k p k i These are the proportional and integral coefficients of the PI controller in the current loop, respectively.

[0010] Furthermore, the open-loop transfer function G of the DC voltage loop DC (s) and closed-loop transfer function G DCclose (s) are respectively: ; ; ; ; Among them, Gdc (s) is the transfer function of the PI controller in the voltage loop, G D Denotes the feedback coefficient matrix, k vp k vi These are the proportional and integral coefficients of the PI controller in the voltage loop, I. dc This is the DC side current.

[0011] Furthermore, the modulation signal relationship matrix includes a first matrix J1(s) and a second matrix J2(s): ; ; ; Among them, I power C represents the steady-state value of the output current of the new energy source. dc For the DC side capacitor, U dc I is the steady-state value of the DC-side voltage. 1d I 1q Δ is the steady-state value of the inverter bridge output current, and ∆ is the disturbance signal.

[0012] Furthermore, the equivalent output admittance model is as follows: ; Where Y(s) is the equivalent output admittance model, Z cf Y L1 and Y L2 These represent the capacitance transfer function impedance matrix of the LCL filter, the inductance transfer function admittance matrix of the inverter output port, and the inductance transfer function admittance matrix at the grid connection point, respectively.

[0013] Furthermore, the nodal admittance matrix model and impedance matrix model are as follows: ; ; Among them, Y miu (s) is the nodal admittance matrix model, Z miu (s) is the impedance matrix model, Y kk For inverter self-admittance, Y 1k Let k be the inverter mutual admittance, where k ranges from 1 to n, and n represents the nth inverter in the grid-connected inverter multi-feed system.

[0014] Furthermore, the interaction coupling matrix is: ; in, Z is the interaction coupling matrix. kk Z represents the self-impedance between GCI access nodes. 1kThis refers to the mutual impedance between the grid-connected inverter access nodes.

[0015] Furthermore, the output current model of the grid-connected inverter multi-feed system is as follows: ; ; ; ; ; Among them, Y eq (s) represents the diagonal matrix of the equivalent admittance of a single inverter in a GCI multi-infeed system, I o (s) is a matrix formed by the output currents of each grid-connected inverter in a multi-feed system, I s (s) represents the equivalent current source output matrix of each grid-connected inverter in the multi-feed system, I ok The output current of the kth inverter, I sk Y is the equivalent current source of the k-th inverter, where k ranges from 1 to n. kk Let be the equivalent output admittance model of the k-th grid-connected inverter, where k ranges from 1 to n.

[0016] The beneficial effects of this invention are: the method of this invention is based on the proportional coefficient k of the PI controller in the current loop of a single grid-connected inverter (GCI). p and integral coefficient k i and the proportional gain k of the PI controller in the voltage loop vp and integral coefficient k vi Using the variable as a variable, Bode plots of the closed-loop transfer function of the current loop and the closed-loop transfer function of the voltage loop of a single GCI are drawn. The frequency corresponding to -3dB in the Bode plot is selected, and this frequency is required to meet the high bandwidth requirement. High bandwidth is used as the model parameter of the GCI multi-infeed system to obtain the first model. Based on the first model, the stability of the GCI multi-infeed system is improved, so that the method of the present invention can effectively improve the stability of the GCI multi-infeed system. Attached Figure Description

[0017] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0018] Figure 1This is a schematic diagram of the control structure of a three-phase LCL grid-connected converter system in the dq domain according to a preferred embodiment of the present invention; Figure 2 This is a small-signal control block diagram of the transfer function of a single grid-connected inverter system according to a preferred embodiment of the present invention; Figure 3 This is a schematic diagram of a GCI multi-feed system network according to a preferred embodiment of the present invention; Figure 4 The preferred embodiment of the present invention is the transfer function model G of unit A. acclose (s) and G DCclose (s) conforms to the Bode plot of the frequency corresponding to -3dB, which belongs to the high bandwidth; Figure 5 The transfer function model G of units B, C, and D in the preferred embodiment of the present invention is shown. acclose (s) and G DCclose (s) conforms to the Bode plot of the frequency corresponding to -3dB, which is within the normal bandwidth; Figure 6 In a preferred embodiment of the present invention, three GCI units with normal bandwidth operate at the maximum grid impedance Z during stable operation. g1 A schematic diagram of the Nyquist multi-feed system; Figure 7 In a preferred embodiment of the present invention, three GCI units with normal bandwidth operate at the maximum grid impedance Z during stable operation. g1 A schematic diagram of the output current of a multi-feed system. Figure 8 In a preferred embodiment of the present invention, three GCI units with normal bandwidth operate beyond the maximum grid impedance Z for stable operation. g1 A schematic diagram of the Nyquist multi-feed system; Figure 9 In a preferred embodiment of the present invention, three GCI units with normal bandwidth operate beyond the maximum grid impedance Z for stable operation. g1 A schematic diagram of the output current of a multi-feed system. Figure 10 This is a preferred embodiment of the present invention, consisting of two normal bandwidth GCI units and one high bandwidth unit operating at the maximum grid impedance Z during stable operation. g2 A schematic diagram of the Nyquist multi-feed system; Figure 11 This is a preferred embodiment of the present invention, consisting of two normal bandwidth GCI units and one high bandwidth unit operating at the maximum grid impedance Z during stable operation. g2 A schematic diagram of the output current of a multi-feed system. Figure 12 This is a preferred embodiment of the present invention, consisting of two normal bandwidth GCI units and one high bandwidth unit operating beyond the maximum grid impedance Z for stable operation. g2A schematic diagram of the Nyquist multi-feed system; Figure 13 This is a preferred embodiment of the present invention, consisting of two normal bandwidth GCI units and one high bandwidth unit operating beyond the maximum grid impedance Z for stable operation. g2 A schematic diagram of the output current of a multi-feed system. Detailed Implementation

[0019] To facilitate understanding by those skilled in the art, the present invention will be further described below with reference to embodiments and accompanying drawings. The content mentioned in the embodiments is not intended to limit the present invention.

[0020] This invention provides a method for improving the stability of a multi-infeed grid-connected inverter system based on high bandwidth, comprising the following steps: Step S1: Obtain the forward channel transfer function based on the LCL filter parameters of a single grid-connected inverter (GCI); It should be further explained that, in one embodiment of the present invention, the control structure of the three-phase LCL grid-connected converter system in the dq domain is as follows: Figure 1 As shown.

[0021] exist Figure 1 middle, I power U represents the steady-state value of the output current of the new energy source. dc C is the steady-state value of the DC voltage. dc L1 is the DC-side capacitor, L2 is the inverter-side filter inductor, and C is the grid-side filter inductor. f For the filter capacitor, R d For damping resistance, U pcc Z represents the grid connection point voltage. g U represents the mains impedance. g This represents the grid voltage, PLL is the phase-locked loop, PCC is the grid connection point, and U dcref (s) is the DC voltage reference value, G v (s) is the transfer function of the PI controller in the DC voltage loop, i1 is the inverter bridge output current, i2 is the actual grid-connected output current of a single GCI, and I 2dref I 2qref These are the reference values ​​for the components of the grid-connected output current in the d-axis and q-axis of the dq domain, respectively. 2d i 2q G represents the d-axis and q-axis components of the inverter's output current, respectively. i (s) is the transfer function of the PI controller in the current loop, U md and U mqdenoted as , where is the steady-state value of the inverter's output port voltage modulation signal on the d-axis and q-axis, PWM stands for pulse width modulation, abc / dq and dq / abc are the dq transformation and inverse dq transformation, respectively, and θ is the output phase angle of the phase-locked loop (PLL).

[0022] The forward channel transfer function includes a first transfer function and a second transfer function; The first transfer function The transmission relationship between the inverter-side filter inductor and filter capacitor is illustrated, including: ; The second transfer function The transmission relationship between the inverter-side filter inductor, filter capacitor, and grid-side filter inductor is illustrated, including: ; Among them, I m Z represents a 2×2 identity matrix; cf (s), Y L1 (s) and Y L2 (s) represent the capacitance transfer function impedance matrix of the LCL filter, the inductance transfer function admittance matrix of the inverter output port, and the inductance transfer function admittance matrix of the grid connection point, respectively.

[0023] In one embodiment of the present invention, Z cf (s), Y L1 (s) and Y L2 (s) is represented as: ; ; ; Where L1 is the inverter-side filter inductor, L2 is the grid-side filter inductor, and C... f For the filter capacitor, R d The damping resistor in the LCL filter is represented by s; s is the Laplace operator; ω1 represents the mains frequency angular frequency, including: ; Where f1 is set to 50Hz.

[0024] Step S2: Based on the influence of the phase-locked loop (PLL), obtain the phase angle difference between the grid dq coordinate system and the PLL controller dq coordinate system to obtain the transformation matrix; the transformation matrix includes the first transformation matrix and the second transformation matrix; The first transformation matrix G trans1 (s) is used for the conversion between grid-connected output current and phase-locked loop controller, including: ; The second transformation matrix G trans2 (s) is used for the conversion between grid-connected voltage and phase-locked loop controller, including: ; in, U is the transfer function of the PI controller in the phase-locked loop; md and U mq These are the steady-state values ​​of the inverter's output port voltage modulation signal on the d-axis and q-axis, respectively; I 2d and I 2q These are the steady-state values ​​of the inverter's output current components along the d-axis and q-axis, respectively; U m0 This represents the steady-state component of the inverter grid connection point voltage on the d-axis.

[0025] Step S3: Obtain the open-loop transfer function and closed-loop transfer function of the current loop based on the PI controller transfer function in the current loop of the grid-connected inverter and the forward channel transfer function obtained in step S1. Specifically, the open-loop transfer function G of the current loop is derived by calculating the PI controller transfer function in the current loop of the GCI and the forward path transfer function. ac (s) and closed-loop transfer function G acclose (s), including: ; ; Among them, K PWM For pulse width modulation gain; G de (s) represents the delay generated during the calculation and modulation process; G i (s) represents the transfer function matrix of the PI controller in the current loop, including: ; ; Among them, T s Indicates the sampling period; k p k i These are the proportional and integral coefficients of the PI controller in the current loop, respectively.

[0026] Step S4: Obtain the open-loop transfer function and closed-loop transfer function of the DC voltage loop based on the PI controller transfer function in the voltage loop and the closed-loop transfer function of the current loop obtained in step S3. Specifically, the open-loop transfer function G of the DC voltage loop is obtained based on the transfer function of the PI controller in the voltage loop and the closed-loop transfer function of the current loop. DC (s) and closed-loop transfer function G DCclose (s), including: ; ; Among them, G dc (s) is the transfer function of the PI controller in the voltage loop; G D G represents the feedback coefficient matrix; acclose (s) is the closed-loop transfer function of the current loop, including: ; ; Where, k vp k vi These are the proportional and integral coefficients of the PI controller in the voltage loop, respectively; I dc This is the DC side current.

[0027] Step S5: Based on the DC side voltage, the inverter bridge output current in the inverter circuit of the grid-connected inverter, and the modulation signal, obtain the relationship matrix between DC voltage, inverter bridge output current, and modulation signal; Specifically, the modulation signal relationship matrix includes a first matrix J1(s) and a second matrix J2(s): ; ; ; Among them, I power C represents the steady-state value of the output current of the new energy source. dc For DC side capacitor; U dc This represents the steady-state value of the DC-side voltage; I 1d I 1q Δ represents the steady-state value of the inverter bridge output current; ∆ represents the disturbance signal.

[0028] Step S6: Based on the relationship matrix between DC voltage, inverter bridge output current, and modulation signal obtained in step S5, and the current loop closed-loop transfer function obtained in step S3, obtain the equivalent output admittance model of the grid-connected inverter. In one embodiment of the present invention, based on Figure 2 The equivalent control block diagram, based on the relationship matrix between DC voltage and inverter bridge output current, modulation signal, and current loop closed-loop transfer function, yields the equivalent output admittance model Y(s) of the grid-connected inverter considering the influence of DC-side power transmission, including: ; Step S7: Based on the node current I injected by the grid-connected inverter 1-i For output variables, node voltage V 1-i Using the input variables, we obtain the node admittance matrix model and impedance matrix model of the grid structure of the multi-feed system of the grid-connected inverter. Then, considering the coupling relationship between the nodes connected to the grid-connected inverter, we obtain the interactive coupling matrix. It should be further noted that, in one embodiment of the present invention, based on Figure 3 Schematic diagram of a multi-feed inverter system, where G k Let I represent the k-th GCI. ok Z represents the grid-connected output current of the k-th GCI. kg V represents the impedance of the k-th GCI line. ok Z represents the voltage at the grid connection point of the k-th GCI; load V represents the common load of the GCI multi-feed system. g I represents the grid connection point voltage of a multi-feed system. g U represents the total grid-connected output current of a multi-infeed system. g This indicates the grid voltage of a multi-feed system.

[0029] Injecting node current with GCI Output variable, node voltage Using these as input variables, we obtain the node admittance matrix model and impedance matrix model of the GCI multi-feed system grid structure, including: Based on the GCI injected node current as the output variable and the node voltage as the input variable, the node admittance matrix model Y of the GCI multi-feed system grid structure is obtained. miu (s) and impedance matrix model Z miu (s), including: ; ; ; Among them, Y kk Y is the inverter's self-admittance; 1k Let k be the inverter mutual admittance, where k ranges from 1 to n, and n represents the nth inverter in the GCI multi-infeed system.

[0030] In Z miu Based on (s), considering the coupling relationship between nodes accessing GCI, from Z bus (s) Select elements containing GCI access node information to construct the interaction coupling matrix Z describing the interaction between GCI access nodes. m (s), including: ; Among them, Z kk Z represents the self-impedance between GCI access nodes. 1k denoted as the mutual impedance between GCI access nodes, where k ranges from 1 to n, and n represents the nth inverter in the GCI multi-feed system.

[0031] Step S8: Obtain the output current model of the multi-feed system of the grid-connected inverter based on the equivalent current source output of the grid-connected inverter, the equivalent Norton equivalent model circuit of the grid-connected inverter, and the interactive coupling matrix obtained in step S7. Specifically, the output current model of the GCI multi-feed system is obtained based on the relationship between the output of the GCI equivalent current source and the GCI equivalent Norton equivalent model circuit, including: ; ; Among them, Y eq (s) is the diagonal matrix of the equivalent admittance of a single inverter in a GCI multi-infeed system, I o (s) is a matrix formed by the output currents of each GCI in the multi-feed system, I s (s) represents the output matrix of each GCI equivalent current source in the multi-feed system, including: ; ; ; Among them, I ok The output current of the kth inverter; I sk Y is the equivalent current source of the k-th inverter, where k ranges from 1 to n; kk Let be the equivalent output admittance model for the k-th GCI, where k ranges from 1 to n.

[0032] Step S9: Using high bandwidth as the output current model parameter of the grid-connected inverter multi-infeed system, obtain the first model; when the first model is not connected to the multi-infeed system, draw the Nyquist diagram of the multi-infeed system. When the Nyquist diagram does not enclose the point (-1, j0), the maximum grid connection impedance Z is obtained. g1 When the first model is connected to the multi-infeed system, the Nyquist plot of the multi-infeed system is plotted. The maximum grid impedance Z reached when the Nyquist plot does not enclose the point (-1, j0) is calculated. g2 When Z g2 >Z g1 The stability of the GCI multi-feed system is improved.

[0033] It should be further noted that, in one embodiment of the present invention, the proportional coefficient k of the PI controller in the GCI current loop is used. p and integral coefficient k i and the proportional gain k of the PI controller in the voltage loop vp and integral coefficient k viUsing the variable as a variable, plot the Bode plots of the closed-loop transfer function of the current loop and the closed-loop transfer function of the voltage loop for a single GCI. Select the frequency corresponding to -3dB in the Bode plot, requiring this frequency to meet the high bandwidth requirement, as shown in Tables 2 and 3. Use high bandwidth as the model parameter for the GCI multi-infeed system to obtain the first model. Based on the first model, improve the stability of the GCI multi-infeed system, specifically including: taking a 3-GCI multi-infeed system as an example, first connect 3 GCIs with normal bandwidth to the multi-infeed system, and test the maximum grid impedance Z that the output stable grid-connected current can withstand when the system is running stably. g1 Draw the Nyquist diagram of the system at this point; then replace the three normal bandwidth GCIs in the multi-infeed system with one high bandwidth and two normal bandwidth GCIs. The maximum new grid impedance Z that the new GCI multi-infeed system can withstand when its output stable grid-connected current is running stably. g2 And draw the Nyquist diagram of the system at this time. If Z g2 >Z g1 Therefore, the stability of the GCI multi-feed system is improved; the method of the present invention can effectively improve the stability of the GCI multi-feed system.

[0034] Table 2 Current Loop Bandwidth Type Parameter Table ; Table 3 Voltage Loop Bandwidth Type Parameter Table ; To verify the effectiveness of the method of the present invention, a three-phase LCL grid-connected inverter multi-feed system with three generator units was built in MATLAB / Simulink. The main parameters are shown in Table 1.

[0035] Table 1 Main Parameters of Multi-Infeed System ; Figure 4 For the transfer function model G of unit A acclose (s) and G DCclose (s) conforms to the Bode plot of a frequency corresponding to -3dB, which belongs to a high bandwidth. From Figure 4 It can be determined that the frequency corresponding to -3dB of the Bode plot of the closed-loop transfer function of the current loop of Unit A is... The current loop bandwidth is high, and the Bode plot of the voltage loop closed-loop transfer function corresponds to a frequency of 141Hz at -3dB, which also indicates a high bandwidth voltage loop.

[0036] Figure 5 Transfer function models G for units B, C, and D respectively. acclose (s) and G DCclose(s) The -3dB frequency corresponds to a Bode plot within the normal bandwidth. Among them, the Bode plot of the closed-loop transfer function of the current loop of units B, C, and D corresponds to a frequency of 501Hz at -3dB, which corresponds to a normal bandwidth for the current loop. The Bode plot of the closed-loop transfer function of the voltage loop corresponds to a frequency of 85.5Hz at -3dB, which corresponds to a normal bandwidth for the voltage loop.

[0037] Figure 6 The three GCI units, B, C, and D, with normal bandwidth, are operating stably at the maximum grid impedance Z. g1 A schematic diagram of the Nyquist multi-infeed system at a current density of 3.3 mH. In the diagram, the curve does not enclose the point (-1, j0), indicating that the multi-infeed system is stable at this point. Figure 7 The corresponding output current diagram of the multi-feed system at this time.

[0038] Figure 8 The three GCI units, B, C, and D, with normal bandwidth, are operating at a grid impedance of 3.8mH, exceeding Z. g1 A schematic diagram of the Nyquist multi-infeed system at a current density of 3.3 mH. In the diagram, the curve encloses the point (-1, j0), indicating that the multi-infeed system is unstable at this point. Figure 9 This corresponds to a schematic diagram showing the instability of the output current in the multi-feed system at this time.

[0039] Figure 10 Unit A (high bandwidth) and two GCI units (B and C) with normal bandwidth are operating stably at the maximum grid impedance Z. g2 A schematic diagram of the Nyquist multi-infeed system at a current density of 6.1 mH. In the diagram, the curve does not enclose the point (-1, j0), indicating that the multi-infeed system is stable at this point. Figure 11 The corresponding output current diagram of the multi-feed system at this time.

[0040] Figure 12 Unit A (high bandwidth) and two GCI units (B and C) with normal bandwidth are operating at a grid impedance of 6.5H, exceeding Z. g2 A schematic diagram of the Nyquist multi-infeed system at a current density of 6.1 mH. In the diagram, the curve encloses the point (-1, j0), indicating that the multi-infeed system is unstable at this point. Figure 13 This corresponds to a schematic diagram showing the instability of the output current in the multi-feed system at this time.

[0041] In summary, by adjusting the proportional and integral coefficients of the GCI current and voltage loops, high bandwidth is achieved in both loops, thus improving the stability of the multi-infeed system after replacing the normal bandwidth unit with the high-bandwidth unit. The method of this invention achieves an experimental stability improvement of 84.84%.

[0042] The above embodiments are preferred implementations of the present invention. In addition, the present invention can be implemented in other ways. Any obvious substitutions without departing from the concept of the present technical solution are within the protection scope of the present invention.

[0043] To facilitate understanding by those skilled in the art of the improvements of this invention over the prior art, some of the accompanying drawings and descriptions have been simplified, and for clarity, some other elements have been omitted from this application. Those skilled in the art should realize that these omitted elements may also constitute the content of this invention.

Claims

1. A method for improving the stability of a multi-infeed grid-connected inverter system based on high bandwidth, characterized in that, Includes the following steps: S1: Obtain the forward channel transfer function based on the LCL filter parameters of a single grid-connected inverter; S2: The transformation matrix is ​​obtained based on the phase angle difference between the power grid dq coordinate system and the phase-locked loop controller dq coordinate system; S3: Obtain the open-loop transfer function and closed-loop transfer function of the current loop based on the PI controller transfer function and forward channel transfer function in the current loop of the grid-connected inverter. S4: Obtain the open-loop and closed-loop transfer functions of the DC voltage loop based on the PI controller transfer function in the voltage loop and the closed-loop transfer function in the current loop. S5: Based on the DC side voltage, the inverter bridge output current and the modulation signal in the inverter circuit of the grid-connected inverter, obtain the relationship matrix between DC voltage, inverter bridge output current and modulation signal; S6: The equivalent output admittance model of the grid-connected inverter is obtained based on the relationship matrix between DC voltage and inverter bridge output current, modulation signal, and current loop closed-loop transfer function. S7: Based on the output variable of the node current injected by the grid-connected inverter and the input variable of the node voltage, the node admittance matrix model and impedance matrix model of the grid structure of the multi-feed system of the grid-connected inverter are obtained. Then, considering the coupling relationship between the nodes connected to the grid-connected inverter, the interactive coupling matrix is ​​obtained. S8: The output current model of the multi-feed system of the grid-connected inverter is obtained based on the equivalent current source output of the grid-connected inverter, the circuit relationship of the equivalent Norton equivalent model of the grid-connected inverter, and the interactive coupling matrix. S9: Using high bandwidth as the output current model parameter of the grid-connected inverter multi-infeed system, the first model is obtained; when the first model is not connected to the multi-infeed system, the Nyquist diagram of the multi-infeed system is plotted. When the Nyquist diagram does not enclose the point (-1, j0), the maximum grid connection impedance Z is reached. g1 When the first model is connected to the multi-infeed system, the Nyquist diagram of the multi-infeed system is plotted. The maximum grid impedance Z reached when the Nyquist diagram does not enclose the point (-1, j0) is calculated. g2 When Z g2 >Z g1 The stability of the GCI multi-feed system is improved.

2. The method for improving the stability of a multi-infeed grid-connected inverter system based on high bandwidth as described in claim 1, characterized in that, The forward channel transfer function includes a first transfer function and a second transfer function, the expressions of which are as follows: ； ； Among them, I m Z represents a 2×2 identity matrix. cf (s), Y L1 (s) and Y L2 (s) represent the capacitance transfer function impedance matrix of the LCL filter, the inductance transfer function admittance matrix of the inverter output port, and the inductance transfer function admittance matrix of the grid connection point, respectively. For the first transfer function, This is the second transfer function; ； ； ； ； in, For the inverter-side filter inductor, For the mains-side filter inductor, C f For the filter capacitor, R d denoted as the damping resistor in the LCL filter, s is the Laplace operator, and ω1 represents the power grid angular frequency.

3. A method for improving the stability of a multi-infeed grid-connected inverter system based on high bandwidth, as described in claim 1 or 2, characterized in that... The transformation matrix includes a first transformation matrix and a second transformation matrix, and the expressions for the first transformation matrix and the second transformation matrix are as follows: ； ； in, This is the first transformation matrix. Second transformation matrix, U is the transfer function of the PI controller in the phase-locked loop; md and U mq These are the steady-state values ​​of the inverter's output port voltage modulation signal on the d-axis and q-axis, respectively; I 2d and I 2q These are the steady-state values ​​of the inverter's output current components along the d-axis and q-axis, respectively; U m0 This represents the steady-state component of the inverter grid connection point voltage on the d-axis.

4. The method for improving the stability of a multi-infeed grid-connected inverter system based on high bandwidth as described in claim 3, characterized in that, Step S3 specifically involves: The open-loop transfer function G of the current loop is derived by calculating the transfer function of the PI controller in the current loop of the grid-connected inverter and the forward channel transfer function. ac (s) and closed-loop transfer function G acclose (s): ； ； ； ； Among them, K PWM For pulse width modulation gain; G de (s) represents the delay generated during the calculation and modulation process; G i (s) represents the transfer function matrix of the PI controller in the current loop, T s Indicates the sampling period; k p k i These are the proportional and integral coefficients of the PI controller in the current loop, respectively.

5. The method for improving the stability of a multi-infeed grid-connected inverter system based on high bandwidth as described in claim 4, characterized in that, The open-loop transfer function G of the DC voltage loop DC (s) and closed-loop transfer function G DCclose (s) are respectively: ； ； ； ； Among them, G dc (s) is the transfer function of the PI controller in the voltage loop, G D Denotes the feedback coefficient matrix, k vp k vi These are the proportional and integral coefficients of the PI controller in the voltage loop, I. dc This is the DC side current.

6. The method for improving the stability of a multi-infeed grid-connected inverter system based on high bandwidth as described in claim 5, characterized in that, The modulation signal relationship matrix includes a first matrix J1(s) and a second matrix J2(s): ； ； ； Among them, I power C represents the steady-state value of the output current of the new energy source. dc For the DC side capacitor, U dc I is the steady-state value of the DC-side voltage. 1d I 1q This represents the steady-state value of the inverter bridge output current. This is a disturbance signal.

7. The method for improving the stability of a multi-infeed grid-connected inverter system based on high bandwidth as described in claim 6, characterized in that, The equivalent output admittance model is as follows: ； in, For the equivalent output admittance model, Z cf (s), Y L1 (s) and Y L2 (s) represent the capacitance transfer function impedance matrix of the LCL filter, the inductance transfer function admittance matrix of the inverter output port, and the inductance transfer function admittance matrix of the grid connection point, respectively.

8. The method for improving the stability of a multi-infeed grid-connected inverter system based on high bandwidth as described in claim 7, characterized in that, The node admittance matrix model and impedance matrix model are as follows: ； ； Among them, Y miu (s) is the nodal admittance matrix model, Z miu (s) is the impedance matrix model, Y kk For inverter self-admittance, Y 1k Let k be the inverter mutual admittance, where k ranges from 1 to n, and n represents the nth inverter in the grid-connected inverter multi-feed system.

9. A method for improving the stability of a multi-infeed grid-connected inverter system based on high bandwidth as described in claim 8, characterized in that, The interaction coupling matrix is: ； in, Z is the interaction coupling matrix. kk Z represents the self-impedance between GCI access nodes. 1k This refers to the mutual impedance between the grid-connected inverter access nodes.

10. The method for improving the stability of a multi-infeed grid-connected inverter system based on high bandwidth as described in claim 9, characterized in that, The output current model of the grid-connected inverter multi-feed system is as follows: ； ； ； ； ； Among them, Y eq (s) is the diagonal matrix of the equivalent admittance of a single inverter in a GCI multi-infeed system, I o (s) is a matrix formed by the output currents of each grid-connected inverter in a multi-feed system, I s (s) represents the equivalent current source output matrix of each grid-connected inverter in the multi-feed system, I ok The output current of the kth inverter, I sk Y is the equivalent current source of the k-th inverter, where k ranges from 1 to n. kk Let be the equivalent output admittance model of the k-th grid-connected inverter, where k ranges from 1 to n.

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