Grid-forming inverter and wideband impedance emulation method thereof

Abstract

The application discloses a grid-connected inverter and a wide-frequency impedance simulation method thereof. The wide-frequency impedance simulation method comprises the following steps: obtaining an output voltage and an output current of the grid-connected inverter; obtaining a reference current in a preset coordinate system through a voltage control link based on the output voltage, a virtual electromotive force and an output phase angle; obtaining a modulation voltage deviation in the preset coordinate system through a current control link based on the output current, the reference current and the output phase angle; and obtaining a modulation voltage signal based on the modulation voltage deviation, the output voltage and the output phase angle, wherein the modulation voltage signal is used to generate a modulation signal to drive the grid-connected inverter. The application helps to better match the unit impedance and the grid impedance, effectively suppresses the wide-frequency oscillation after grid connection and improves the small-signal stability of the system.

Description

Technical Field

[0001] This application relates to the field of photovoltaic and energy storage technology, and in particular to a grid-type inverter and its wideband impedance simulation method. Background Technology

[0002] New energy sources such as wind and solar power are typically connected to the power grid via inverters. However, traditional grid-connected control methods lack mechanical rotational inertia compared to hydropower and thermal power, and are prone to small-signal stability issues in weak grid environments. As the proportion of new energy sources connected to the grid continues to increase, the overall inertia level of the power system decreases, leading to increased system frequency fluctuations and greater pressure on stability.

[0003] To address these issues, numerous studies have indicated that grid-connected inverters can effectively mitigate the weak inertia challenge brought about by renewable energy grid integration. However, grid-connected inverters inherently possess complex output impedance characteristics, and if they do not match the grid impedance, small-signal instability may still occur. Currently, this problem has not been fully resolved and remains an important research direction in renewable energy grid integration technology. Summary of the Invention

[0004] This application provides a grid-type inverter and its wideband impedance simulation method to avoid the mismatch between the output impedance characteristics of the grid-type inverter and the grid impedance, thereby causing small-signal instability.

[0005] This application provides a broadband impedance simulation method for a grid-type inverter, the broadband impedance simulation method comprising:

[0006] Obtain the output voltage and output current of the grid-connected inverter;

[0007] Based on the output voltage, virtual electromotive force and output phase angle, a reference current in a preset coordinate system is obtained through a voltage control circuit.

[0008] Based on the output current, the reference current, and the output phase angle, the modulation voltage deviation in the preset coordinate system is obtained through the current control circuit.

[0009] Based on the modulation voltage deviation, the output voltage, and the output phase angle, a modulation voltage signal is obtained, which is used to generate a modulation signal to drive the grid-type inverter.

[0010] In another aspect, this application provides a grid-type inverter, the grid-type inverter including a controller configured to perform the steps of the wideband impedance simulation method described above.

[0011] The grid-connected inverter and its wideband impedance simulation method provided in this application, through the design of the current loop, make the transfer function from the output current to the output voltage of the system accurately equivalent to the virtual impedance, thereby achieving precise control of the wideband impedance characteristics of the new energy unit, which helps to better match the unit impedance with the grid impedance, effectively suppress the wideband oscillation after grid connection, and improve the small-signal stability of the system. Attached Figure Description

[0012] Figure 1 This is a schematic diagram of an integrated photovoltaic and energy storage unit provided in an embodiment of this application;

[0013] Figure 2 A schematic diagram of the broadband impedance simulation method for a grid-type inverter provided in this application embodiment;

[0014] Figure 3 This is a schematic diagram illustrating the wideband impedance simulation principle of a grid-type inverter provided in an embodiment of this application.

[0015] Figure 4 This is a schematic diagram illustrating the generation of the output phase angle provided in an embodiment of this application.

[0016] The realization of the purpose, functional features and advantages of this application will be further explained in conjunction with the embodiments and with reference to the accompanying drawings. Detailed Implementation

[0017] To make the technical problems, technical solutions, and beneficial effects to be solved by this application clearer and more understandable, the following detailed description is provided in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of this application and are not intended to limit the scope of this application.

[0018] In the description of this application, it should be understood that the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance.

[0019] Definitions of relevant variables:

[0020] Per-unit value of output power command

[0021] Per-unit value of output power acquisition

[0022] per-unit value of power deviation

[0023] Damping power

[0024] Rotational speed damping coefficient

[0025] Frequency offset per unit value

[0026] Frequency offset value

[0027] Output frequency

[0028] Rated frequency of power grid

[0029] Output phase angle

[0030] dq axis reference current per unit value

[0031] Per-unit value of AC current acquisition

[0032] dq axis current per unit value

[0033] Current difference

[0034] Modulation voltage difference

[0035] dq axis modulation voltage per unit value

[0036] AC modulated voltage signal

[0037] Virtual electromotive force

[0038] Output voltage

[0039] : Per-unit value of dq axis voltage acquisition

[0040] Voltage difference

[0041] Filter impedance

[0042] Virtual impedance

[0043] DC bus capacitor

[0044] AC side voltage of grid-connected inverter

[0045] DC bus voltage

[0046] Energy storage unit

[0047] : Power grid impedance

[0048] Figure 1 This is a schematic diagram of an integrated photovoltaic and energy storage system provided in an embodiment of this application.

[0049] like Figure 1 As shown, the integrated photovoltaic and energy storage unit includes a photovoltaic array, an energy storage unit B, a first DC / DC converter, a second DC / DC converter, and a grid-type inverter T1.

[0050] The DC terminal of the grid-type inverter T1 is connected to the output terminals of the first DC / DC converter and the second DC / DC converter via positive and negative DC buses (i.e., positive DC bus and negative DC bus). The input terminal of the first DC / DC converter is connected to the photovoltaic array, and the input terminal of the second DC / DC converter is connected to the energy storage unit B. The AC terminal of the grid-type inverter T1 is connected to the power grid or the load.

[0051] The picture This is the DC bus voltage. For the AC side voltage of grid-connected inverter T1, For filter impedance, For grid impedance, For output voltage, This is the output current.

[0052] In some examples, the first DC / DC converter can operate in maximum power point tracking mode, and the second DC / DC converter can operate in constant voltage charge / discharge mode.

[0053] In some examples, a DC bus capacitor is also connected between the positive DC bus and the negative DC bus. .

[0054] In some examples, the grid-connected inverter T1 also includes a controller configured to perform the steps of the wideband impedance simulation method, as detailed below.

[0055] Based on this, such as Figure 2 As shown in the figure, this application provides a broadband impedance simulation method for a grid-connected inverter, the broadband impedance simulation method including the following steps:

[0056] S11. Obtain the output voltage and output current of the grid-type inverter;

[0057] S12. Based on the output voltage, virtual electromotive force and output phase angle, a reference current in a preset coordinate system is obtained through a voltage control circuit.

[0058] In some examples, the step of obtaining a reference current in a preset coordinate system based on the output voltage, virtual electromotive force, and output phase angle through a voltage control circuit includes:

[0059] Based on the output phase angle, the output voltage is transformed to obtain the corresponding output voltage in the preset coordinate system;

[0060] The voltage difference is obtained by subtracting the virtual electromotive force from the corresponding output voltage in the preset coordinate system.

[0061] The voltage difference is passed through a preset voltage controller to obtain the reference current.

[0062] The transfer function of the voltage controller is determined by the following formula: ;in, For reference current, This is the voltage difference. and These are the proportional coefficient and integral coefficient of the proportional-integral component, respectively.

[0063] Please combine Figure 3 To understand, based on the output phase angle , output voltage go through After coordinate transformation (i.e., Clarke transformation and Park transformation), we obtain Output voltage in coordinate system ; virtual electromotive force and Output voltage in coordinate system Perform the subtraction to obtain the voltage difference value. ,Right now ; the voltage difference After passing through the preset voltage controller (in the diagram) The proportional-integral controller shown obtains the reference current. .

[0064] S13. Based on the output current, the reference current, and the output phase angle, the modulation voltage deviation in the preset coordinate system is obtained through the current control loop.

[0065] In some examples, the process of obtaining the modulation voltage deviation in the preset coordinate system based on the output current, the reference current, and the output phase angle through a current control circuit includes:

[0066] Based on the output phase angle, the output current is transformed to obtain the corresponding output current in the preset coordinate system;

[0067] The difference between the reference current and the corresponding output current in the preset coordinate system is obtained to obtain the current difference value;

[0068] The current difference is passed through a preset current controller to obtain the modulation voltage deviation.

[0069] The transfer function of the current controller is determined by the following formula: , The impedance corresponding to the filter inductor. The virtual broadband impedance to be simulated. Let be the transfer function of the proportional-integral element.

[0070] Please combine Figure 3 To understand, based on the output phase angle , output current go through After coordinate transformation, we get Output current in coordinate system ; reference current and Output current in coordinate system By performing the subtraction, the current difference value is obtained. ,Right now ; to the current difference The modulation voltage deviation is obtained after passing through a preset current controller. ,Right now .

[0071] S14. Based on the modulation voltage deviation, the output voltage, and the output phase angle, a modulation voltage signal is obtained, which is used to generate a modulation signal to drive the grid-type inverter.

[0072] In some examples, obtaining the modulated voltage signal based on the modulated voltage deviation, the output voltage, and the output phase angle includes:

[0073] Based on the output phase angle, the output voltage is transformed to obtain the corresponding output voltage in the preset coordinate system;

[0074] The modulation voltage deviation is summed with the corresponding output voltage in the preset coordinate system to obtain the modulation voltage in the preset coordinate system.

[0075] Based on the output phase angle, the modulation voltage signal is obtained by inverse coordinate transformation of the modulation voltage corresponding to the preset coordinate system.

[0076] Please combine Figure 3 To understand, based on the output phase angle , output voltage go through After coordinate transformation (i.e., Clarke transformation and Park transformation), we obtain Output voltage in coordinate system ; Modulation voltage deviation and Output voltage in coordinate system Summing yields Modulation voltage in coordinate system ,Right now Based on the output phase angle ,Will Modulation voltage in coordinate system The modulated voltage signal is obtained after inverse coordinate transformation (i.e., Park transform and Clarke transform). This modulated voltage signal A PWM drive signal is generated through the PWM modulation stage to drive the grid-type inverter T1.

[0077] In some examples, the broadband impedance simulation method further includes:

[0078] Obtain the output active power of the grid-connected inverter;

[0079] Based on the output active power, the output phase angle is obtained through the active power and frequency inertia control loop.

[0080] In some specific examples, obtaining the output phase angle based on the output active power, through an active power and frequency inertia control loop, includes:

[0081] The power deviation value is obtained by subtracting the output power command value from the output active power.

[0082] The power deviation value is subtracted from the damping power and then passed through an inertia circuit to obtain the per-unit value of the frequency offset.

[0083] The frequency offset per unit value is multiplied by the grid rated frequency to obtain the frequency offset value;

[0084] The output phase angle is obtained by summing the frequency offset value with the rated frequency of the power grid and then integrating the results.

[0085] Please combine Figure 4 To understand, the output power command value With output active power The difference is calculated to obtain the power deviation value. Power deviation value With damping power After difference, through inertia element The per-unit value of frequency offset is obtained. Damping power Frequency offset per unit value With rotational damping coefficient The product is obtained; the per-unit value of frequency offset. With the rated frequency of the power grid After multiplication, the frequency offset value is obtained. ; frequency offset value With the rated frequency of the power grid After summing, the output frequency is obtained. After the points system Obtain the output phase angle .

[0086] The preferred embodiments of this application have been described above with reference to the accompanying drawings, but this does not limit the scope of the claims. Any modifications, equivalent substitutions, and improvements made by those skilled in the art without departing from the scope and spirit of this application shall be within the scope of the claims.

Claims

1. A broadband impedance simulation method for a grid-connected inverter, characterized in that, The broadband impedance simulation method includes: Obtain the output voltage and output current of the grid-connected inverter; Based on the output voltage, virtual electromotive force and output phase angle, a reference current in a preset coordinate system is obtained through a voltage control circuit. Based on the output current, the reference current, and the output phase angle, the modulation voltage deviation in the preset coordinate system is obtained through the current control circuit. Based on the modulation voltage deviation, the output voltage, and the output phase angle, a modulation voltage signal is obtained, which is used to generate a modulation signal to drive the grid-type inverter.

2. The broadband impedance simulation method according to claim 1, characterized in that, The process of obtaining a reference current in a preset coordinate system based on the output voltage, virtual electromotive force, and output phase angle through a voltage control circuit includes: Based on the output phase angle, the output voltage is transformed to obtain the corresponding output voltage in the preset coordinate system; The voltage difference is obtained by subtracting the virtual electromotive force from the corresponding output voltage in the preset coordinate system. The voltage difference is passed through a preset voltage controller to obtain the reference current.

3. The broadband impedance simulation method according to claim 2, characterized in that, The transfer function of the voltage controller is determined by the following formula: ;in, For reference current, This is the voltage difference. and These are the proportional coefficient and integral coefficient of the proportional-integral component, respectively.

4. The broadband impedance simulation method according to claim 1, characterized in that, The process of obtaining the modulation voltage deviation in the preset coordinate system based on the output current, the reference current, and the output phase angle through a current control circuit includes: Based on the output phase angle, the output current is transformed to obtain the corresponding output current in the preset coordinate system; The difference between the reference current and the corresponding output current in the preset coordinate system is obtained to obtain the current difference value; The current difference is passed through a preset current controller to obtain the modulation voltage deviation.

5. The broadband impedance simulation method according to claim 4, characterized in that, The transfer function of the current controller is determined by the following formula: , The impedance corresponding to the filter inductor. The virtual broadband impedance to be simulated. Let be the transfer function of the proportional-integral element.

6. The broadband impedance simulation method according to claim 1, characterized in that, The process of obtaining the modulation voltage signal based on the modulation voltage deviation, the output voltage, and the output phase angle includes: Based on the output phase angle, the output voltage is transformed to obtain the corresponding output voltage in the preset coordinate system; The modulation voltage deviation is summed with the corresponding output voltage in the preset coordinate system to obtain the modulation voltage in the preset coordinate system. Based on the output phase angle, the modulation voltage signal is obtained by inverse coordinate transformation of the modulation voltage corresponding to the preset coordinate system.

7. The broadband impedance simulation method according to claim 1, characterized in that, The broadband impedance simulation method also includes: Obtain the output active power of the grid-connected inverter; Based on the output active power, the output phase angle is obtained through the active power and frequency inertia control loop.

8. The broadband impedance simulation method according to claim 7, characterized in that, The process of obtaining the output phase angle based on the output active power, through an active power and frequency inertia control loop, includes: The power deviation value is obtained by subtracting the output power command value from the output active power. The power deviation value is subtracted from the damping power and then passed through an inertia circuit to obtain the per-unit value of the frequency offset. The frequency offset per unit value is multiplied by the grid rated frequency to obtain the frequency offset value; The output phase angle is obtained by summing the frequency offset value with the rated frequency of the power grid and then integrating the results.

9. A grid-connected inverter, characterized in that, The grid-type inverter includes a controller configured to perform the steps of the broadband impedance simulation method according to any one of claims 1-8.

10. The grid-type inverter according to claim 9, characterized in that, The DC terminal of the grid-type inverter is connected to the output terminals of the first DC / DC converter and the second DC / DC converter via positive and negative DC buses. The input terminal of the first DC / DC converter is connected to the photovoltaic array, and the input terminal of the second DC / DC converter is connected to the energy storage unit. The AC terminal of the grid-type inverter is connected to the power grid or the load.

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