A grid-forming grid-connected inverter

Abstract

The invention discloses a grid-connected inverter comprising: an LCL inverter, and a first abc / dq coordinate transformation module, a second abc / dq coordinate transformation module and a PWM control module connected to the LCL inverter; a power calculation module connected to the first abc / dq coordinate transformation module; a low-pass filter connected to the power calculation module; a droop controller connected to the low-pass filter; a PI controller respectively connected to the droop controller, the first abc / dq coordinate transformation module and the second abc / dq coordinate transformation module; a dq / abc coordinate transformation module respectively connected to the PI controller and the PWM control module; and a voltage controller respectively connected to the second abc / dq coordinate transformation module and the dq / abc coordinate transformation module. The grid-connected inverter designed by the invention has stronger adaptability than a grid-following grid-connected inverter and does not affect system stability.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of new energy power systems, and particularly relates to a grid-constructing grid-connected inverter. BACKGROUND

[0002] With the proposal and gradual implementation of the "double carbon" goal, the stability problem of grid-connected inverters as an important interface element of new energy power generation units accessing the power grid is highlighted. At present, most of the power grids use grid-following grid-connected inverters, however, there is coupling between the grid-following grid-connected inverter and the phase-locked loop, the power grid in the weak power grid, which seriously affects the system stability, therefore, there is an urgent need for a grid-constructing grid-connected inverter and a construction method thereof, by designing a grid-constructing grid-connected inverter, and establishing a state space model of the grid-constructing grid-connected inverter, replacing the grid-following grid-connected inverter with the grid-constructing grid-connected inverter in the weak power grid, and reducing the influence on the system stability. SUMMARY

[0003] In order to solve the existing technical problems, the present application provides a grid-constructing grid-connected inverter, comprising:

[0004] an LCL type inverter, and a first abc / dq coordinate transformation module, a second abc / dq coordinate transformation module and a PWM control module electrically connected with the LCL type inverter;

[0005] a power calculation module electrically connected with the first abc / dq coordinate transformation module;

[0006] a low-pass filter electrically connected with the power calculation module;

[0007] a droop controller electrically connected with the low-pass filter;

[0008] a PI controller electrically connected with the droop controller, the first abc / dq coordinate transformation module and the second abc / dq coordinate transformation module respectively;

[0009] a dq / abc coordinate transformation module electrically connected with the PI controller and the PWM control module respectively;

[0010] a voltage controller electrically connected with the second abc / dq coordinate transformation module and the dq / abc coordinate transformation module respectively.

[0011] Preferably, the first abc / dq coordinate transformation module and the second abc / dq coordinate transformation module are electrically connected.

[0012] Preferably, the input end of the PI controller is electrically connected with the output end of the droop controller;

[0013] The output ends of the PI controllers are electrically connected with the first abc / dq coordinate transformation module, the second abc / dq coordinate transformation module and the dq / abc coordinate transformation module respectively.

[0014] Preferably, the space model for characterizing the meshed grid inverter of the meshed grid type comprises:

[0015] a power control loop model for automatic power distribution in the case of no communication link;

[0016] a voltage control loop model using PI control to output current command according to input voltage command;

[0017] a time delay loop model for evaluating the influence of the delay loop on system stability and revealing the interaction between states;

[0018] an LCL filter loop model for writing the state equation of the filter and the inductance coupled with the inverter by the filter structure.

[0019] Preferably, the power control loop model is constructed by a power calculation module, a low-pass filter and a droop controller, wherein the voltage and current values pass through the dq axis of the first abc / dq coordinate transformation module, and the voltage and current values of the d-axis component and the q-axis component are output into the power calculation module to obtain instantaneous active power and reactive power, and the average power is obtained through the low-pass filter; the droop controller outputs frequency control command and voltage control command based on the dq axis to the PI controller according to the average power; and the PI controller transmits the command to the first and second abc / dq coordinate transformation modules.

[0020] Preferably, the voltage controller is used to receive the voltage control command based on the abc axis transmitted by the second abc / dq coordinate transformation module.

[0021] Preferably, the time delay loop model is used to equivalent the time delay exponential term to a transfer function by using Pade approximation for eigenvalue analysis in the state space model, wherein the transfer function is expressed as:

[0022]

[0023] wherein, l and k are the order in the Pade approximation.

[0024] Preferably, the LCL filter loop model is used to generate the state equation expressed as:

[0025]

[0026] wherein, B1 = [-I 1q0 ,I 1d0 ,-Voq0 ,V od0 ,-I oq0 ,I od0 ] T ,

[0027] B2=[-A 12 ,O 2×4 ] T ,B4=[O 2×4 ,-A 31 ] T ,

[0028]

[0029]

[0030] Preferably, the space model is used to analyze the stability of the grid-forming type grid-connected inverter under a strong power grid.

[0031] Preferably, the analysis process of the stability of the grid-forming type grid-connected inverter comprises:

[0032] a first stability analysis by changing the droop control gain;

[0033] a second stability analysis by changing the voltage loop control gain;

[0034] obtaining the stability of the grid-forming type grid-connected inverter according to the result of the first stability analysis and / or the result of the second stability analysis.

[0035] The present application discloses the following technical effects:

[0036] The state space model of the grid-forming type grid-connected inverter established by the present application can effectively analyze the stability problem of the system under the condition of a strong power grid and a change of the controller parameters. And the grid-forming type grid-connected inverter shows stronger adaptability than the grid-following type grid-connected inverter under a weak power grid, and does not affect the system stability. BRIEF DESCRIPTION OF DRAWINGS

[0037] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the drawings needed to be used in the embodiments will be briefly introduced below. Obviously, the drawings in the following description only constitute some embodiments of the present application, and for those skilled in the art, other drawings can also be obtained without creative labor based on these drawings.

[0038] Figure 1 a grid-forming type grid-connected inverter topology diagram according to the present application;

[0039] Figure 2 a power control loop structure diagram based on droop control according to the present application;

[0040] Figure 3 A schematic diagram of a voltage control loop structure according to the present application;

[0041] Figure 4 A schematic diagram of the influence of droop control gain on system stability according to the present application;

[0042] Figure 5 A schematic diagram of the influence of voltage loop control gain on system stability according to the present application. DETAILED DESCRIPTION

[0043] To make the purposes, technical solutions, and advantages of the embodiments of the present application clearer, the technical solutions in the embodiments of the present application will be described below in connection with the accompanying drawings of the embodiments of the present application. Obviously, the described embodiments are only some of the embodiments of the present application but not all the embodiments. The components of the embodiments of the present application described and shown in the accompanying drawings can be arranged and designed in various different configurations. Therefore, the following detailed description of the embodiments of the present application provided in the accompanying drawings is not intended to limit the scope of the claimed present application, but only represents selected embodiments of the present application. Based on the embodiments of the present application, all other embodiments obtained by those skilled in the art without creative work fall within the scope of the present application.

[0044] As Figures 1-5 shown, the present application provides a grid-connected inverter of network construction type, comprising:

[0045] an LCL type inverter, and a first abc / dq coordinate transformation module, a second abc / dq coordinate transformation module, and a PWM control module electrically connected with the LCL type inverter;

[0046] a power calculation module electrically connected with the first abc / dq coordinate transformation module;

[0047] a low-pass filter electrically connected with the power calculation module;

[0048] a droop controller electrically connected with the low-pass filter;

[0049] a PI controller electrically connected with the droop controller, the first abc / dq coordinate transformation module, and the second abc / dq coordinate transformation module, respectively;

[0050] a dq / abc coordinate transformation module electrically connected with the PI controller and the PWM control module, respectively;

[0051] a voltage controller electrically connected with the second abc / dq coordinate transformation module and the dq / abc coordinate transformation module, respectively.

[0052] Further preferably, the first abc / dq coordinate transformation module and the second abc / dq coordinate transformation module mentioned in the present application are electrically connected.

[0053] Further preferably, the input end of the PI controller mentioned in the present application is electrically connected with the output end of the droop controller.

[0054] The output end of the PI controller is electrically connected with the first abc / dq coordinate transformation module, the second abc / dq coordinate transformation module and the dq / abc coordinate transformation module respectively.

[0055] Further preferably, the present application further constructs a space model for characterizing the networked grid-connected inverter, comprising:

[0056] A power control loop model for automatic power distribution in the case of no communication link;

[0057] A voltage control loop model using PI control to output current command according to the input voltage command;

[0058] A time delay loop model for evaluating the influence of delay link on system stability and revealing the interaction between states;

[0059] An LCL filter loop model for writing the state equation of the filter and its coupling inductance with the inverter by the filter structure.

[0060] Further preferably, the power control loop model mentioned in the present application is constructed by a power calculation module, a low-pass filter and a droop controller, wherein the voltage and current values pass through the dq axis of the first abc / dq coordinate transformation module, and the voltage and current values of the d-axis component and the q-axis component are output into the power calculation module to obtain the instantaneous active power and the reactive power, and the average power is obtained through the low-pass filter; the droop controller outputs the frequency control command and the voltage control command based on the dq axis to the PI controller according to the average power; and the PI controller transmits the command to the first and second abc / dq coordinate transformation modules.

[0061] Further preferably, the voltage controller mentioned in the present application is used to receive the voltage control command based on the abc axis transmitted by the second abc / dq coordinate transformation module.

[0062] Further preferably, the time delay loop model mentioned in the present application is used to equivalent the time delay index term to a transfer function by using Pade approximation for eigenvalue analysis in the state space model, wherein the transfer function is expressed as:

[0063]

[0064] wherein, l and k are the orders of Pade approximation.

[0065] Further preferably, the state equation generated by the LCL filter link model mentioned in the present invention is expressed as:

[0066]

[0067] Where, B1=[-I lq0 , I ld0 , -V oq0 , V od0 , -I oq0 , I od0 ] T ,

[0068] B2=[-A 12 ,O 2×4 ] T ,B4=[O 2×4 ,-A 31 ] T ,

[0069]

[0070] Further preferably, the spatial model mentioned in the present invention is used to analyze the stability of a grid-connected inverter under a strong power grid.

[0071] Further preferably, the stability analysis process of the grid-connected inverter mentioned in the present invention includes:

[0072] The first stability analysis was performed by varying the droop control gain;

[0073] A second stability analysis was performed by changing the voltage loop control gain;

[0074] The stability of the grid-connected inverter is obtained according to the result of the first stability analysis and / or the result of the second stability analysis.

[0075] Example 1: This paper presents the details of state-space modeling for a grid-connected inverter, including the power control loop, voltage control loop, time lag, and LCL filter. The system stability is analyzed by varying the droop control gain and voltage loop control gain.

[0076] The model building includes:

[0077] Step 1: Establish a typical topology of a grid-connected inverter.

[0078] Step 2: Establish the models of each link of the grid-connected inverter.

[0079] Step 3: Stability analysis of grid-connected inverter under strong power grid.

[0080] Step 1: establish a grid-connected inverter topology. Specifically, it includes:

[0081] abc / dq coordinate transformation module, PWM control module and LCL type inverter, power calculation module, low-pass filter, droop controller and voltage controller. It is assumed that the input of the inverter is powered by a rigid DC power supply, which is represented by a capacitor.

[0082] Step 2: establish a grid-connected inverter model. Model each link in the topology structure, including:

[0083] 1) Power control loop model establishment.

[0084] 2) Voltage control link model establishment

[0085] 3) Time delay link model establishment.

[0086] 4) LCL filter link model establishment.

[0087] Step 3: stability analysis of grid-connected inverter under strong grid. By changing the system parameters, the characteristic roots are analyzed to determine the stability of the system. If the characteristic roots are located in the left half plane (i.e. the real part of the characteristic roots is negative), the system is stable; if the characteristic roots are located in the right half plane (i.e. the real part of the characteristic roots is positive), the system is unstable. Different characteristic root points can be drawn according to the change of system parameters, and the change trajectory formed by different characteristic root points is called the root trajectory of the system.

[0088] System parameter changes include:

[0089] Change the droop control gain for stability analysis.

[0090] Change the voltage loop control gain for stability analysis.

[0091] The grid-connected inverter state space model disclosed in the application can solve the problem of coupling between the grid-connected inverter and the phase-locked loop and the power grid in the weak grid, which seriously affects the stability of the system, and includes the following processes:

[0092] Step 1: Figure 1 The grid-connected inverter topology is shown in Figure 1 The grid-connected inverter topology is shown in

[0093] Step 2: establish a grid-connected inverter model.

[0094] 1) Figure 2 The power control loop model is shown in FIG. 2. The power control loop is used for automatic power distribution in the case of no communication link. The power control loop based on droop control includes a power calculation module, a low-pass filter and a droop controller. Voltage and current values are dq-transformed to output voltage and current values of d-axis component and q-axis component into the power calculation module, and instantaneous active power and reactive power can be calculated by the power calculation block. The calculated instantaneous power is further obtained by the low-pass filter to obtain average power. The droop controller receives the processed average power to output frequency control instructions and voltage control instructions based on dq-axis, and finally outputs voltage control instructions based on abc coordinate axes through PI controller and coordinate transformation to act on the voltage controller. Figure 2 The state space model of the power control loop based on droop control can be realized by combining the power calculation module, the low-pass filter and the droop controller, as shown in equation (1).

[0095]

[0096] In the equation, u p = [v od ,v oq ,i od ,i oq ] T ,x p = [P,Q] T ,γ p = [ω] T ,

[0097]

[0098] C p = [-m p 0],D p = [O 1×4 ]

[0099] v odq0 and i odq0 are voltage operating point and current operating point.

[0100] 2)

[0101] The voltage control loop is shown in FIG. 3. The voltage control loop adopts PI control to output current instructions according to the input voltage instructions. Figure 3 Figure 3

[0102] The state space model of the voltage control loop is shown in equations (2)-(4):

[0103]

[0104] ​​wherein, V is the voltage command processed by the power control loop, odq V is the voltage signal of the inverter bus voltage input to the voltage control loop through coordinate transformation, V is the difference between the voltage command processed by the power control loop and the voltage signal input to the voltage control loop, I is the current command output by the voltage control loop, ω is the angular frequency of the voltage control loop, n ω is the rated frequency of the voltage control loop, C is the coupling capacitance of the voltage control loop, f K is the coupling capacitance of the voltage control loop, pv K is the coupling capacitance of the voltage control loop, iv K is the coupling capacitance of the voltage control loop.

[0105] The state space model of equations (2)-(4) can be obtained by linearization, as shown in equations (5)-(6).

[0106]

[0107] wherein,

[0108]

[0109] 3) Establish the time delay link. The voltage signal will be delayed after passing through the hardware device, which can be represented by the equation:

[0110] v = e -τ·s v * (7)

[0111] wherein, v * is the voltage signal value without passing through the hardware device, v is the voltage signal value delayed after passing through the hardware device, τ = 1.5T s is the delay time caused by digital calculation delay (Ts) and pulse width modulation delay (0.5Ts), T s is the sampling period of the inverter.

[0112] In order to evaluate the influence of the delay link on the stability of the system and reveal the interaction between the states, the Pade approximation is used to equivalent the time delay exponential term to the transfer function shown in equation (8), which is used for eigenvalue analysis in the state space model.

[0113]

[0114] wherein l and k are the order of the Pade approximation, respectively.

[0115] In order to analyze the relationship between the factors in the time delay link, equation (8) can be further converted into the state space expression as shown in equation (11):

[0116]

[0117] 4) Establish LCL filter link. From the filter structure, the state equation of the filter and the coupling inductance between the filter and the inverter can be written as shown in equations (13)-(18):

[0118]

[0119] In the formula, L f is the filter inductance, C f is the filter capacitance, R f is the filter resistance, Lc is the coupling inductance between the filter and the inverter, v bdq is the bus voltage connected to the inverter. ω is the angular frequency of the inverter output voltage. The bus voltage v bdq and the angular frequency ω are regarded as the input variables of the inverter.

[0120] After linearization of equations (13)-(18), it can be shown by equation (19):

[0121]

[0122] In the formula,

[0123] B1 = [-I q0 , I ld0 , -V oq0 , V od0 , -I oq0 , I od0 ] T ,

[0124] B2 = [-A 12 , O 2×4 ] T , B4 = [O 2×4 , -A 31 ] T ,

[0125]

[0126] Step 3: Stability analysis of grid-connected inverter of grid-forming type under strong power grid.

[0127] 1) Change the droop control gain for stability analysis. The root locus curve of the state space model obtained by changing the droop control gain is shown in Figure 4 . In the process of changing the droop control gain m from 0.5 to 3.5, the eigenvalues of the system state space model also change, gradually shifting to the right half plane. This shows that as m increases, the system tends to be unstable.

[0128] 2) Change the voltage loop control gain for stability analysis. The root locus curve of the state space model obtained by changing the voltage loop control gain is shown in Figure 5The characteristic value of the system state space model changes when the voltage loop control gain n changes from 1.1 to 0.5, and gradually shifts to the right half plane. This shows that the system gradually loses stability as n decreases.

[0129] In the description of the present specification, the description referring to the terms "one embodiment", "some embodiments", "an example", "a specific example", or "some examples" and the like means that the specific feature, structure, material or characteristic described in connection with the embodiment or example is included in at least one embodiment or example of the present application. In the present specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Also, the specific feature, structure, material or characteristic described can be combined in an appropriate manner in any one or more embodiments or examples. Furthermore, the person skilled in the art can combine and combine the different embodiments or examples described in the present specification and the features of the different embodiments or examples, without contradiction.

[0130] Although the embodiments of the present application have been shown and described above, it is understood that the above-described embodiments are exemplary and are not to be construed as limiting the present application, and the person skilled in the art can make changes, modifications, replacements and variations to the above-described embodiments within the scope of the present application.

Claims

1. A grid-connected inverter, characterized in that: include: An LCL inverter, and a first abc / dq coordinate conversion module, a second abc / dq coordinate conversion module, and a PWM control module electrically connected to the LCL inverter; a power calculation module, electrically connected to the first abc / dq coordinate transformation module; a low-pass filter, electrically connected to the power calculation module; a droop controller electrically connected to the low-pass filter; a PI controller electrically connected to the droop controller, the first abc / dq coordinate transformation module, and the second abc / dq coordinate transformation module respectively; a dq / abc coordinate transformation module, electrically connected to the PI controller and the PWM control module respectively; The voltage controller is electrically connected to the second abc / dq coordinate transformation module and the dq / abc coordinate transformation module respectively.

2. The grid-connected inverter according to claim 1, characterized in that: The first abc / dq coordinate transformation module is electrically connected to the second abc / dq coordinate transformation module.

3. The grid-connected inverter according to claim 2, characterized in that: The input end of the PI controller is electrically connected to the output end of the droop controller; The output end of the PI controller is electrically connected to the first abc / dq coordinate transformation module, the second abc / dq coordinate transformation module, and the dq / abc coordinate transformation module respectively.

4. The grid-connected inverter according to claim 3, characterized in that: The spatial model used to characterize the grid-connected inverter includes: Power control loop model for automatic power allocation in the absence of a communication link; The voltage control link model adopts PI control and outputs current command according to the input voltage command; Delay link model, used to evaluate the impact of delay links on system stability and reveal the interaction between states; The LCL filter link model is used to write the state equation of the filter and its coupled inductance with the inverter based on the filter structure.

5. The grid-connected inverter according to claim 4, characterized in that: The power control loop model is constructed by the power calculation module, the low-pass filter and the droop controller, wherein the voltage and current values ​​pass through the first abc / dq coordinate transformation module, and the output voltage and current values ​​of the d-axis component and the q-axis component enter the power calculation module to obtain instantaneous active power and reactive power, and obtain the average power through the low-pass filter; the droop controller outputs a frequency control instruction based on the average power, as well as a voltage control instruction based on the dq axes, to the PI controller; the PI controller then transmits the instruction to the first and second abc / dq coordinate transformation modules.

6. The grid-connected inverter according to claim 5, characterized in that: The voltage controller is used to receive the voltage control instruction based on the abc axis transmitted by the second abc / dq coordinate transformation module.

7. The grid-connected inverter according to claim 6, characterized in that: The time-delay link model is used to equate the time-delay exponential term to a transfer function by adopting the Pade approximation, and is used to perform eigenvalue analysis in the state-space model, wherein the transfer function is expressed as: in, l and k are the orders of Pade approximation.

8. The grid-connected inverter according to claim 7, characterized in that: The state equation generated by the LCL filter link model is expressed as: where B1 = [-I lq0 , I ld0 , -V oq0 , V od0 , -I oq0 + I od0 T ,​ B2=[-A 12 ,Oh 2×4 ] T ,B4=[O 2×4 ,-A 31 ] T , 9. The grid-connected inverter according to claim 8, characterized in that: The spatial model is used to analyze the stability of the grid-connected inverter under a strong power grid.

10. The grid-connected inverter according to claim 9, characterized in that: The stability analysis process of the grid-connected inverter includes: The first stability analysis was performed by varying the droop control gain; A second stability analysis was performed by changing the voltage loop control gain; The stability of the grid-connected inverter is obtained according to the result of the first stability analysis and / or the result of the second stability analysis.

Images