Virtual synchronous generator off-grid and grid-connected smooth switching control method based on adaptive parameters

Abstract

The invention discloses an off-grid and grid-connected smooth switching control method for a virtual synchronous generator based on adaptive parameters, and the method achieves the efficient switching and smooth transition of an off-grid and grid-connected mode through the dynamic adjustment of virtual impedance, rotational inertia and a damping factor based on a virtual synchronous generator control architecture of the adaptive parameters. The control architecture is divided into an inner loop current and voltage control module, an outer loop power-frequency control module and a self-adaptive parameter adjustment module, and comprises virtual impedance design, dynamic rotational inertia adjustment, self-adaptive damping factor switching and parameter optimization and modal analysis. In this way, the concept of virtual impedance is introduced into VSG control to improve the decoupling effect of active power and reactive power; a parameter alternating VSG is provided, singular value decomposition analysis and modal analysis are considered, and better performance is provided in the aspect of suppressing oscillation caused by disturbance by improving control parameters; and stable resynchronization between the converter and the power grid after grid connection and grid disconnection is realized.

Description

Technical Field

[0001] This application relates to the technical field of new energy grid connection control, and particularly to a smooth switching control method for virtual synchronous generators with adaptive parameters during grid connection and disconnection. Background Art

[0002] With the rapid development of renewable energy technologies, such as photovoltaic and wind power, grid-connected systems have become the core solution for the efficient utilization of clean energy. However, the intermittency, dynamic response characteristics of renewable energy, and the transient impact during the switching between grid-connected and off-grid modes of inverters pose severe challenges to grid stability. The specific problems are as follows:

[0003] Fluctuation of energy output: Environmental factors cause the energy output power to fluctuate frequently, and the inverter needs to quickly adjust to maintain power balance;

[0004] Dynamic charge and discharge characteristics: The energy buffer unit needs to flexibly switch the charge and discharge strategies in grid-connected / off-grid modes to support the system voltage and frequency;

[0005] Transient oscillation during mode switching: When switching from off-grid to grid-connected, the phase and frequency mismatch between the inverter and the grid are likely to cause high-frequency oscillations, resulting in equipment damage or misoperation of protection.

[0006] In the prior art, virtual synchronous control strategies can improve the transient process of inverters and provide inertial support by simulating the inertia and damping characteristics of traditional synchronous devices. However, such strategies usually adopt fixed parameter designs and are difficult to adapt to the dynamic requirements of the system, specifically manifested in:

[0007] Fixed parameters cannot effectively suppress the transient oscillations caused by energy fluctuations and mode switching;

[0008] Lack of optimization for the resynchronization process between the inverter and the grid, resulting in too long synchronization time and unstable transition process;

[0009] In complex working conditions, the power decoupling ability is insufficient, affecting the dynamic response performance of the system.

[0010] In addition, the plug-and-play ability of grid-connected inverters is the key to realizing large-scale renewable energy access, but the existing control methods do not fully solve the problem of smooth resynchronization during the switching between grid-connected and off-grid modes. Summary of the Invention

[0011] Embodiments of this application provide a smooth switching control method for virtual synchronous generators with adaptive parameters, which realizes suppressing the high-frequency oscillations caused by resynchronization to the grid and can provide better dynamic performance in both operating modes and the conversion process between these modes.

[0012] To achieve the above object, the technical solution of the embodiment of the present invention is as follows:

[0013] In a first aspect, the embodiment of the present invention provides a control method for smooth switching between grid-connected and off-grid of a virtual synchronous generator based on adaptive parameters, including: constructing a control architecture including an inner-loop voltage and current control, an outer-loop power-frequency control, and an adaptive parameter adjustment module; in the inner-loop voltage and current control, a PI controller is used to track the reference current and generate a modulation voltage command for controlling the injection of active and reactive power; in the outer-loop power-frequency control, the swing equation is used to simulate the inertial characteristics of the synchronous generator, and the mode switching is realized by adaptively adjusting the virtual impedance, moment of inertia, and damping factor; the damping factor is dynamically adjusted to a preset high value in the grid-connected mode, switched to a preset low value in the off-grid mode, and the grid frequency is obtained in real time through a phase-locked loop; based on parameter optimization and modal analysis, the control parameters are optimized by combining singular value decomposition to suppress high-frequency oscillations during the grid-connected and off-grid mode switching process.

[0014] In some possible implementation manners, the inner-loop voltage and current control includes:

[0015] Establish a mathematical model of the LC filter, and describe the electrical characteristics of the AC side of the converter through the following formula;

[0016]

[0017] where L f is the inductor of the filter, C f is the capacitor of the filter, ω is the angular frequency provided by the external control loop, i m,d represents the output d-axis current of the converter before the LC filter, i m,q is the output q-axis current of the converter before the LC filter, R f is the line impedance, t is time, u d is the d-axis command voltage for modulating the converter, u q is the q-axis command voltage for modulating the converter, ν d is the d-axis AC output voltage for modulating the converter, v q is the q-axis AC output voltage for modulating the converter;

[0018] Adopt a double closed-loop PI control strategy, and the current loop generates a voltage command through the following formula;

[0019]

[0020] where K pi is the proportional controller gain of the current loop, K ii is the integral controller gain of the current loop, s is a variable of the negative frequency in the Laplace transform, is the reference d-axis current of the converter before the LC filter, is the reference q-axis current of the converter before the LC filter;

[0021] The voltage loop tracks the reference voltage through the following formula;

[0022]

[0023] where K pν is the proportional controller gain of the voltage loop, K iν is the integral controller gain of the voltage loop, is the reference d-axis voltage of the converter after passing through the LC filter, is the reference q-axis voltage of the converter after passing through the LC filter.

[0024] In some possible implementations, the outer-loop power-frequency control includes:

[0025] Implementing droop control of active power and frequency based on the swing equation; the swing equation is expressed as:

[0026]

[0027] where is the input active power of the converter, is the output active power of the converter, D is the damping factor, H is the inertia constant, ω m is the mechanical angular frequency, ω pll is the AC voltage angular frequency measured by the phase-locked loop, is the change rate of the mechanical angular frequency, is the change rate of the phase angle;

[0028] The droop control of active power and frequency is implemented through the following formula:

[0029] P = P * + K pf (ω * - ω pll );

[0030] where K pf is the reciprocal of the active power droop coefficient, P * is the active power reference, ω * is the reference angular frequency, ω pll is the grid frequency obtained by the phase-locked loop;

[0031] Combining a PI controller to implement frequency secondary regulation; the PI controller is expressed as:

[0032]

[0033] where Kif is the integral link coefficient;

[0034] Generate the voltage amplitude reference value through reactive power droop control, and the reactive power droop control is implemented by the following formula:

[0035]

[0036] where m Q is the reactive power droop coefficient, which acts on the difference between Q * and Q f , Q * is the reference reactive power, and Q f is the filtered reactive power of the frequency converter.

[0037] In some possible implementation manners, the adaptive parameter adjustment module includes:

[0038] Adjust the damping factor to the preset high value to suppress oscillation during grid connection, and adjust the damping factor to the preset low value to improve the dynamic response during islanding;

[0039] Obtain the grid phase and frequency in real time through a phase-locked loop to achieve phase synchronization with the grid; The calculation formula of ω pll is expressed as:

[0040]

[0041] ω pll = ω N + δω pll ;

[0042]

[0043] where, v pll,q is the q-axis decomposition of the grid voltage, δω pll is the frequency difference output by PI regulation, ω N is the rated frequency, ω pll is the phase-locked loop output frequency, θ pll is the phase-locked loop output phase, K ppll is the proportional controller gain of the phase-locked loop, and K ipll is the integral controller gain of the phase-locked loop.

[0044] In some possible implementation manners, the parameter optimization and modal analysis include: analyzing the system robustness based on singular value decomposition; determining the optimal combination of moment of inertia and damping factor through modal analysis.

[0045] In a second aspect, an embodiment of the present invention provides a virtual synchronous generator grid-connected and islanded smooth switching control system based on adaptive parameters, including: Main circuit module: including an LC filter and a grid-connected circuit breaker; Control module: integrating an inner-loop voltage and current controller, an outer-loop power-frequency controller, and an adaptive parameter adjustment module; Synchronization module: using an improved phase-locked loop to achieve fast phase locking; Communication module: collecting grid parameters in real time and outputting control instructions.

[0046] In some possible implementation manners, the adaptive parameter adjustment module includes: Damping factor switching unit: used to automatically switch the damping factor value according to the operating mode; Moment of inertia adjustment unit: used to dynamically adjust the inertia constant based on the generator output power.

[0047] In some possible implementation manners, the adaptive parameter adjustment module includes a PI controller for dynamically adjusting the damping factor in the outer-loop power-frequency control loop.

[0048] One or more technical solutions provided in the embodiments of the present invention have at least the following technical effects or advantages:

[0049] In the embodiments of the present invention, based on the virtual synchronous generator (VSG) control architecture with adaptive parameters, by dynamically adjusting the virtual impedance, moment of inertia, and damping factor, efficient switching and smooth transition between grid-connected and islanded modes are achieved. The control architecture is divided into an inner-loop current and voltage control, an outer-loop power-frequency control, and an adaptive parameter adjustment module, including virtual impedance design, dynamic moment of inertia adjustment, adaptive damping factor switching, and parameter optimization and modal analysis. In this way, the concept of virtual impedance is introduced in VSG control to improve the decoupling effect of active and reactive power; a parameter-alternating VSG is proposed, considering singular value decomposition analysis and modal analysis, and by improving the control parameters, better performance is provided in suppressing oscillations caused by disturbances, and the grid connection synchronization time is shortened to within 0.5 seconds; smooth resynchronization between the converter and the grid after grid connection and islanding is achieved. BRIEF DESCRIPTION OF THE DRAWINGS

[0050] To more clearly illustrate the embodiments of the present invention, the drawings required for use in the embodiments of the present invention will be briefly introduced below. Obviously, the drawings in the following description are some embodiments of the present invention. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts.

[0051] Figure 1 It is a schematic flowchart of an embodiment of a virtual synchronous generator grid-connected and islanded smooth switching control method provided for the implementation of the present invention;

[0052] Figure 2Schematic diagram of the technical research model for the smooth switching control method of virtual synchronous generator for grid connection and disconnection in the embodiments of the present invention;

[0053] Figure 3 Inner loop current and voltage control block diagram in the embodiments of the present invention;

[0054] Figure 4 Overall schematic diagram of the smooth switching control method of virtual synchronous generator for grid connection and disconnection based on adaptive parameters in the embodiments of the present invention;

[0055] Figure 5 Phase-locked loop schematic diagram in the embodiments of the present invention;

[0056] Figure 6 Adaptive damping factor conversion schematic diagram in the embodiments of the present invention;

[0057] Figure 7 Grid connection frequency waveform schematic diagram using the adaptive variable damping factor method in the embodiments of the present invention;

[0058] Figure 8 Grid connection frequency waveform schematic diagram using the fixed damping factor method in the prior art;

[0059] Figure 9 Grid connection voltage waveform schematic diagram in the embodiments of the present invention;

[0060] Figure 10 Grid connection power waveform schematic diagram in the embodiments of the present invention;

[0061] Figure 11 Structural schematic diagram of a smooth switching control system for virtual synchronous generator for grid connection and disconnection based on adaptive parameters in the embodiments of the present invention. Detailed implementation manners

[0062] Next, the technical solutions in the embodiments of the present application will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present application. Obviously, the described embodiments are part of the embodiments of the present application, rather than all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the scope of protection of the present application.

[0063] In the relevant descriptions of this embodiment, terms such as "including", "containing", "having", etc. are all open terms, generally preferably understood as including but not limited to; the term "at least one" is generally preferably understood as one or more, where "a plurality" means two or more; the term "at least one of the following" or its similar expressions refer to any combination of these items, including any combination of single item or plural items. For example, "at least one of a, b or c", or "at least one of a, b and c" can all represent: a, b, c, a - b (i.e., a and b), a - c, b - c, or a - b - c, where a, b, c can be single or multiple respectively; the symbol "A / B" is used to describe the selection relationship of associated objects, generally representing an "or" relationship before and after.

[0064] In the following descriptions of this embodiment, the terms used in the embodiments of the present application are only for the purpose of describing specific embodiments, and are not intended to limit the present application. The singular forms "a" and "the" used in the embodiments of the present application and the appended claims are also intended to include the plural forms, unless the context clearly indicates otherwise.

[0065] Those skilled in the art should understand that in the following descriptions of the embodiments of the present application, the sequence numbers do not mean the sequence of execution. Some or all steps can be executed in parallel or successively, and the execution sequence of each process should be determined by its function and internal logic, and should not constitute any limitation to the implementation process of the embodiments of the present application.

[0066] Those skilled in the art should understand that the numerical ranges in the embodiments of the present application should be understood as specifically disclosing each intermediate value between the upper and lower limits of the range. The intermediate values within any stated value or stated range, as well as each smaller range between any other stated value or intermediate value within the range, are also included in the present invention. The upper and lower limits of these smaller ranges can be independently included or excluded from the range.

[0067] Unless otherwise specified, the technical / scientific terms used herein have the same meaning as commonly understood by those of ordinary skill in the art to which this application belongs. Although this application only describes preferred methods and materials, any methods and materials similar or equivalent to those described herein can also be used in the implementation or testing of this application. All documents mentioned in this specification are incorporated by reference to disclose and describe the methods and / or materials related to the documents. In case of conflict with any incorporated document, the content of this specification shall prevail.

[0068] In order to illustrate the technical solutions of the present invention, specific embodiments are used for illustration below.

[0069] With the rapid development of renewable energy technologies, such as photovoltaic and wind power generation. Taking the photovoltaic energy storage system as an example, the photovoltaic energy storage grid-connected system has become the core solution for the efficient utilization of renewable energy. However, the intermittency of photovoltaic power generation, the dynamic response characteristics of the energy storage system, and the transient impact during the grid-connected and off-grid mode switching of the inverter pose challenges to grid stability. Although the traditional virtual synchronous generator (VSG) control strategy can simulate the inertia support of the synchronous generator, it does not fully consider the following characteristics of the photovoltaic energy storage system:

[0070] Photovoltaic power volatility: The change of illumination leads to frequent fluctuations in the output power, and the inverter needs to quickly adjust to maintain power balance;

[0071] Energy storage dynamic charge and discharge: The energy storage unit needs to flexibly switch the charge and discharge strategies in the grid-connected / off-grid modes to support voltage and frequency;

[0072] Mode switching transient oscillation: When switching from off-grid to grid-connected, the phase and frequency mismatch between the inverter and the grid are likely to trigger high-frequency oscillations, resulting in equipment damage or misoperation of protection.

[0073] In the existing technologies, the VSG control with fixed parameters is difficult to adapt to the dynamic requirements of the photovoltaic energy storage system. There is an urgent need for a smooth switching control method combined with adaptive parameter adjustment. To solve the above problems, the VSG control strategy is introduced. The VSG control strategy can enable the converter to have the inertia and damping characteristics of a traditional synchronous generator by simulating the mechanical characteristics and electromagnetic characteristics of the synchronous generator, improve the transient process of the converter control, and provide inertia support for the system.

[0074] However, an important feature of the grid-connected converter is to resynchronize with the grid to ensure the plug-and-play ability, but this has not been considered in the existing works. In the future converter-based power grid, the grid-connected converter enables the large-scale access of renewable energy.

[0075] Based on this, the embodiments of the present invention provide a smooth switching control method for virtual synchronous generator grid-connected and off-grid based on adaptive parameters, which realizes the suppression of high-frequency oscillations caused by resynchronization with the grid and can provide better dynamic performance in both operating modes and the conversion process between these modes.

[0076] Figure 1 For the schematic flow chart of the embodiment of a smooth switching control method for virtual synchronous generator grid-connected and off-grid based on adaptive parameters provided by the embodiments of the present invention, see Figure 1 As shown, the above method may include:

[0077] S101, constructing a control architecture including an inner-loop voltage and current control, an outer-loop power-frequency control, and an adaptive parameter adjustment module;

[0078] S102. In the inner - loop voltage - current control, a PI controller is used to track the reference current and generate a modulation voltage command for controlling the injection of active and reactive power.

[0079] In some embodiments, the main - circuit topology of the VSG can be referred to Figure 2 as shown. An LC filter is used to filter the injected power of the converter. The filter inductance and capacitance are represented by L f and C f respectively. When the breaker ( Figure 2 the switch in it) is closed, the converter is connected to the power grid with grid impedance Rg + jLg, where j represents the imaginary unit. When the breaker is open, the converter operates in stand - alone mode. The inner - loop voltage - current control loop, as a grid - connected unit to support the power grid, its core is to regulate the amplitude and frequency of the AC voltage by controlling the injection of its active and reactive power.

[0080] Specifically, in some embodiments, the inner - loop voltage - current control includes:

[0081] Establish a mathematical model of the LC filter to describe the electrical characteristics of the AC side of the converter through the following formula;

[0082]

[0083] where L f is the inductance of the filter, C f is the capacitance of the filter. For example, in the embodiments of the present invention, the filter inductance L f = 2mH, and the filter capacitance C f = 500μF.

[0084] ω is the angular frequency provided by the external control loop, i m,d represents the output current of the converter before passing through the LC filter, i m,q is the output current of the converter after passing through the LC filter, R f is the line impedance, t is time, u d is the d - axis command voltage for modulating the converter, u q is the q - axis command voltage for modulating the converter, ν d is the d - axis AC output voltage for modulating the converter, v q is the q - axis AC output voltage for modulating the converter;

[0085] The inner - loop voltage - current control adopts a double - closed - loop PI control strategy, including voltage and current control, and can be embedded into, for example, Figure 3In the voltage source converter control loop shown. In the current control loop, the converter output current tracks its reference current through a proportional-integral (PI) controller, and the current reference voltage can be obtained by calculating the AC current control loop.

[0086] Exemplarily, the current loop can generate a voltage command through the following formula;

[0087]

[0088] where, K pi is the proportional controller gain of the current loop, K ii is the integral controller gain of the current loop, s is the complex frequency domain variable in the Laplace transform, is the reference d-axis current of the converter before the LC filter, is the reference q-axis current of the converter before the LC filter;

[0089] The current reference current can be obtained by calculating the AC voltage control loop. For example, the voltage loop can track the reference voltage through the following formula;

[0090]

[0091] where, K pν is the proportional controller gain of the voltage loop, K iν is the integral controller gain of the voltage loop, is the reference d-axis voltage of the converter after passing through the LC filter, is the reference q-axis voltage of the converter after passing through the LC filter.

[0092] S103. In the outer-loop power-frequency control, the swing equation is used to simulate the inertia characteristics of the synchronous generator, and the mode switching is realized by adaptively adjusting the virtual impedance, moment of inertia, and damping factor;

[0093] In some embodiments, the PI controller is also used to adjust the AC output voltage to the reference voltage provided by the reactive power (Q-V) droop control. For example, assuming that the direction of the D-axis vector in the Park transformation is consistent with the direction of the AC output voltage vector, the Q-axis component of the output voltage is zero. Therefore, the Q-V droop controller defines the reference value of the D component of the AC output voltage, and the Q component is set to zero.

[0094] In some embodiments, the outer-loop power-frequency control specifically includes:

[0095] Based on the swing equation, the droop control of active power and frequency is realized; Inertial damping is used as the most important characteristic of the synchronous generator to extract the phase angle and angular frequency, as Figure 4As shown, the phase angle and angular frequency are calculated. In an embodiment of the present invention, the swing equation can be expressed as:

[0096]

[0097]

[0098] Wherein, is the input active power of the converter, is the output active power of the converter, D is the damping factor, H is the inertia constant, ω m is the mechanical angular frequency, ω pll is the AC voltage angular frequency measured by the phase-locked loop, is the change rate of the mechanical angular frequency, is the change rate of the phase angle;

[0099] In some embodiments, distributed power sources in the power grid are equipped with an active power-frequency (P-f) droop curve to simulate the function of the main frequency control of a static synchronous generator. Also see Figure 4 As shown, this relationship between frequency and power makes the frequency adjustable and manages the order in which various distributed power sources participate in the main frequency, so that the VSG can operate in parallel with other VSGs and static synchronous generators.

[0100] In some embodiments, the droop control of active power and frequency can be achieved through the following formula:

[0101] P = P * + K pf (ω * - ω pll );

[0102] Wherein, K pf is the reciprocal of the active power droop coefficient, P * is the active power reference, ω * is the reference angular frequency, ω pll is the grid frequency obtained by the phase-locked loop;

[0103] In some embodiments, in independent operation, the secondary frequency control for restoring the frequency to its nominal value is an advantageous feature of a static synchronous generator. Therefore, the primary and secondary frequency controls are achieved through a proportional control and an integral control in total. Therefore, the frequency secondary regulation can be achieved by combining a PI controller; wherein, the PI controller can be expressed as:

[0104]

[0105] Wherein, K if is the integral link coefficient;

[0106] Generate the voltage amplitude reference value through reactive power droop control and calculate the d-axis reference voltage of the voltage control loop. The reactive power droop control is implemented by the following formula:

[0107]

[0108] where m Q is the reactive power droop coefficient, which acts on the difference between Q * and Q f . Q * is the reference reactive power, and Q f is the filtered inverter reactive power.

[0109] In the embodiment of the present invention, in the outer loop control, the swing equation is added to the primary and secondary frequency controls to jointly provide the reference mechanical phase angle and angular frequency for the inner loop control. The mechanical angular frequency is used to implement the coupling term between the d-axis and q-axis in the inner loop control.

[0110] S104. Dynamically adjust the damping factor to a preset high value in the grid-connected mode, switch to a preset low value in the off-grid mode, and obtain the grid frequency in real time through a phase-locked loop (PLL);

[0111] In some embodiments, the above step S104 specifically includes:

[0112] S1041. Adjust the damping factor to the preset high value to suppress oscillations during grid connection, and adjust the damping factor to the preset low value to improve the dynamic response during off-grid;

[0113] It can be understood that during the operation of the PV energy storage grid-connected inverter, the reasonable adjustment of the damping factor plays a crucial role in the stable and efficient operation of the system in both grid-connected and off-grid modes. When the PV energy storage grid-connected inverter is in the grid-connected mode, parameters such as the voltage and frequency of the grid will affect the operation of the inverter. At the same time, factors such as the power output fluctuation of the inverter itself, harmonic interference in the grid, and changes in line impedance may cause oscillations in the system. If such oscillations are not effectively suppressed, it may lead to large fluctuations in the current and voltage output by the inverter, affecting the power quality. The damping factor is an important parameter in the control system. It is similar to the damping force in a physical system and can consume the energy of system oscillations, thereby suppressing the amplitude and duration of oscillations. In the control strategy of the inverter, by adjusting the damping factor, the dynamic response characteristics of the system can be changed. When the damping factor is adjusted to the preset high value (for example, D big = 238), the oscillation suppression ability of the system is enhanced. A high damping factor can increase the energy loss of the system, enabling the oscillation energy to be consumed in a short time, thus enabling the system to quickly return to a stable state.

[0114] In practical applications, the operating state of the system can be monitored in real time. When it is detected that the inverter is in the grid-connected mode, the control system automatically adjusts the damping factor to a preset high value. For example, a mode judgment module can be added to the control algorithm of the inverter to judge the current operating mode according to the detected grid connection signal. Once it is determined to be in the grid-connected mode, the damping factor parameter is updated to the preset high value.

[0115] When the photovoltaic-storage grid-connected inverter is in the off-grid mode, it needs to independently supply power to the load. At this time, the dynamic characteristics and stability of the system mainly depend on the control ability of the inverter itself. Different from the grid-connected mode, the load changes are more frequent and intense in the off-grid mode. For example, the sudden connection or disconnection of the load will cause a sharp change in the power demand of the system. In this case, the inverter needs to have a fast dynamic response ability to be able to adjust the output power and voltage in a timely manner to meet the load demand and ensure the stability and reliability of power supply.

[0116] When the damping factor is adjusted to a preset low value (for example, D small = 38), the dynamic response speed of the system will be significantly improved. The low damping factor reduces the inertia of the system and enables it to respond faster to load changes. When the load suddenly increases, the inverter can quickly increase the output power to meet the load demand; when the load suddenly decreases, the inverter can also quickly reduce the output power to avoid problems such as overvoltage in the system. This can effectively improve the power supply quality and stability of the system in the off-grid mode.

[0117] In the embodiments of the present invention, to solve the vibration damping problem of the system in different operating modes and achieve smooth grid resynchronization, a strategy of dynamically adjusting the damping factor D using a PI controller in the external power supply frequency control loop is adopted. As Figure 6 shown, for the off-grid and grid-connected modes, different damping factor value ranges are designed for the swing equation, that is, from D small to D big . Specifically, the embodiments of the present invention abandon the traditional method of using a fixed damping factor D, but apply an adaptive damping factor in the external power supply frequency control loop. Through the PI controller, according to the real-time operating state of the system, the size of the damping factor D is dynamically adjusted in the external power supply frequency control loop.

[0118] This adaptive adjustment strategy can ensure that the system exhibits the best vibration damping characteristics in both the off-grid and grid-connected operating modes. In the off-grid mode, the system can quickly respond to load changes and reduce vibration interference; in the grid-connected mode, it can effectively suppress the oscillations generated during the interaction with the grid, thereby achieving smooth resynchronization of the system and the grid.

[0119] S1042. Obtain the grid phase and frequency in real time through a phase-locked loop to achieve phase synchronization with the grid. The phase-locked loop is only used to measure the actual grid frequency for frequency regulation. The schematic diagram of the phase-locked loop is as shown in Figure 5 shown.

[0120] Exemplarily, the calculation formula of ω pll is expressed as:

[0121]

[0122] ω pll = ω N + δω pll ;

[0123]

[0124] wherein, v pll,q is the q-axis decomposition of the grid voltage, δω pll is the frequency difference output by PI regulation, ω N is the rated frequency (e.g., 50 HZ), ω pll is the output frequency of the phase-locked loop, θ pll is the output phase of the phase-locked loop, K ppll is the proportional controller gain of the phase-locked loop, K ipll is the integral controller gain of the phase-locked loop.

[0125] S105. Based on parameter optimization and modal analysis, optimize the control parameters by combining singular value decomposition to suppress high-frequency oscillations during the off-grid and grid-connected mode switching process.

[0126] In some embodiments, the above step S105 specifically includes:

[0127] S1051. Analyze the system robustness based on singular value decomposition;

[0128] S1052. Determine the optimal combination of moment of inertia and damping factor through modal analysis.

[0129] Exemplarily, in step S1051, a mathematical model of the photovoltaic energy storage grid-connected inverter during mode switching can be constructed first to obtain a system matrix. Perform singular value decomposition on this matrix and decompose it into the product of three matrices. Evaluate the system robustness according to the magnitude and distribution of the singular values after decomposition. If the singular values change sharply during the switching, the system robustness in the corresponding direction decreases and oscillations are likely to be triggered. In step S1052, determine the optimal combination of rotational inertia and damping factor through modal analysis. Modal analysis can solve modal parameters such as the natural frequency and damping ratio of the system. According to the system design and operating conditions, determine the value range of the rotational inertia and damping factor. Change the parameter values within this range to perform multiple modal analyses, evaluate different combinations based on the system performance indicators, and find the optimal combination that can make the natural frequency of the system avoid high-frequency interference and has a high damping ratio, so as to suppress high-frequency oscillations and improve the system stability.

[0130] Exemplarily, Figure 7 Schematic diagram of the grid-connected frequency waveform using the adaptive variable damping factor method in the embodiment of the present invention, Figure 8 Schematic diagram of the grid-connected frequency waveform using the fixed damping factor method in the prior art. Among them, Figure 7 For variable damping grid connection switching, H = 6s, D big = 238, D small = 38. Figure 8 For grid connection switching with a fixed damping factor, H = 6s, D = 38. Comparing Figure 7 and Figure 8 it can be seen that the variable damping grid connection switching shortens the grid connection synchronization time to within 0.5 seconds.

[0131] Figure 9 Schematic diagram of the grid-connected voltage waveform in the embodiment of the present invention, Figure 10 Schematic diagram of the grid-connected power waveform in the embodiment of the present invention. As Figure 9 shown, the converter voltage is consistent with the grid voltage at about 0.7 seconds, proving successful grid connection. Where V0 is the converter voltage and V g is the grid voltage. As Figure 10 shown, the output power of the converter is consistent with the grid power.

[0132] In the embodiments of the present invention, a virtual synchronous generator (VSG) control architecture based on adaptive parameters realizes efficient switching and smooth transition between grid-connected and off-grid modes by dynamically adjusting virtual impedance, moment of inertia, and damping factor. The control architecture is divided into an inner-loop current-voltage control, an outer-loop power-frequency control, and an adaptive parameter adjustment module, including virtual impedance design, dynamic moment of inertia adjustment, adaptive damping factor switching, and parameter optimization and modal analysis. In this way, the concept of virtual impedance is introduced in VSG control to improve the decoupling effect of active and reactive power; a parameter-alternating VSG is proposed, which considers singular value decomposition analysis and modal analysis, and provides better performance in suppressing oscillations caused by disturbances by improving control parameters, and the grid connection synchronization time is shortened to within 0.5 seconds; the smooth re-synchronization of the converter with the grid after grid connection and disconnection is realized.

[0133] Based on the same inventive concept, an embodiment of the present application further provides a virtual synchronous generator grid-connected and off-grid smooth switching control system based on adaptive parameters, which is used to implement the above-mentioned virtual synchronous generator grid-connected and off-grid smooth switching control method. Figure 11 It is a schematic structural diagram of a virtual synchronous generator grid-connected and off-grid smooth switching control system in an embodiment of the present invention. Refer to Figure 11 As shown, the virtual synchronous generator grid-connected and off-grid smooth switching control system 1100 based on adaptive parameters may include:

[0134] Main circuit module 1101: includes an LC filter and a grid connection breaker;

[0135] Control module 1102: integrates an inner-loop voltage-current controller, an outer-loop power-frequency controller, and an adaptive parameter adjustment module;

[0136] Synchronization module 1103: uses an improved phase-locked loop to achieve fast phase locking;

[0137] Communication module 1104: collects grid parameters in real time and outputs control instructions.

[0138] In some possible implementation manners, the adaptive parameter adjustment module includes:

[0139] Damping factor switching unit: used to automatically switch the damping factor value according to the operating mode;

[0140] Moment of inertia adjustment unit: used to dynamically adjust the inertia constant based on the generator output power.

[0141] In some possible implementation manners, the adaptive parameter adjustment module includes a PI controller, which is used to dynamically adjust the damping factor in the outer-loop power-frequency control loop.

[0142] The various embodiments in this specification are described in a progressive manner. For the same or similar parts between the various embodiments, reference can be made to each other, and the key points of each embodiment are the differences from other embodiments.

[0143] The above embodiments are only used to illustrate the technical solutions of the present application, rather than limiting the present application. Although the present application has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that they can still modify the technical solutions described in the foregoing embodiments, or perform equivalent replacements on some or all of the technical features. These modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the present application.

Claims

1. A virtual synchronous generator off-grid and on-grid smooth switching control method based on adaptive parameters, characterized in that Including: Construct a control architecture including an inner - loop voltage - current control, an outer - loop power - frequency control, and an adaptive parameter adjustment module; In the inner - loop voltage - current control, track the reference current through a PI controller and generate a modulation voltage command to control the injection of active and reactive power; In the outer - loop power - frequency control, use the swing equation to simulate the inertia characteristics of the synchronous generator, and achieve mode switching by adaptively adjusting the virtual impedance, moment of inertia, and damping factor; Dynamically adjust the damping factor to a preset high value in the grid - connected mode, switch to a preset low value in the off - grid mode, and obtain the grid frequency in real - time through a phase - locked loop; Based on parameter optimization and modal analysis, optimize the control parameters by combining singular value decomposition to suppress high - frequency oscillations during the grid - connected / off - grid mode switching process.

2. The method according to claim 1, wherein The inner - loop voltage - current control includes: Establish a mathematical model of the LC filter and describe the electrical characteristics of the AC side of the converter through the following formula; Among them, L f is the inductor of the filter, C f is the capacitor of the filter, ω is the angular frequency provided by the external control loop, i m,d represents the output q-axis current of the converter before passing through the LC filter, i m,q is the output d-axis current of the converter before passing through the LC filter, R f is the line impedance, t is time, u d is the d-axis command voltage for modulating the converter, u q is the q-axis command voltage for modulating the converter, ν d is the d-axis AC output voltage for modulating the converter, v q is the q-axis AC output voltage for modulating the converter; Adopt a double - closed - loop PI control strategy, and the current loop generates a voltage command through the following formula; Among them, K pi is the proportional controller gain of the current loop, K ii is the integral controller gain of the current loop, s is a variable of this negative frequency in the Laplace transform, is the reference d-axis current of the converter before the LC filter, is the reference q-axis current of the converter before the LC filter; The voltage loop tracks the reference voltage through the following formula; Among them, K pν is the proportional controller gain of the voltage loop, K iν is the integral controller gain of the voltage loop, is the reference d-axis voltage of the converter after the LC filter, is the reference q-axis voltage of the converter after passing through the LC filter.

3. The method according to claim 2, wherein The outer - loop power - frequency control includes: Implement droop control of active power and frequency based on the swing equation; the swing equation is expressed as: Among them, is the input active power of the converter, is the output active power of the converter, D is the damping factor, H is the inertia constant, ω m is the mechanical angular frequency, ω pll is the AC voltage angular frequency measured by the phase-locked loop, is the change rate of the mechanical angular frequency, is the change rate of the phase angle; The droop control of active power and frequency is achieved through the following formula: P = P * + K pf (ω * - ω pll ); Among them, K pf is the reciprocal of the active power droop coefficient, P * is the active power reference, ω * is the reference angular frequency, ω pll is the grid frequency obtained by the phase-locked loop; Combine a PI controller to achieve secondary frequency regulation; the PI controller is expressed as: Among them, K if is the integral link coefficient; Generate a reference value of voltage amplitude through reactive - power droop control, and the reactive - power droop control is achieved through the following formula: where m Q is the reactive power droop coefficient, which acts on the difference between Q * and Q f ; Q * is the reference reactive power, and Q f is the filtered reactive power of the frequency converter.

4. The method according to claim 3, wherein The adaptive parameter adjustment module includes: Adjust the damping factor to the preset high value during grid connection to suppress oscillations, and adjust the damping factor to the preset low value during off - grid operation to improve dynamic response; The phase and frequency of the power grid are obtained in real time through a phase-locked loop to achieve phase synchronization with the power grid; ω pll The calculation formula of is expressed as: ω pll = ω N + δω pll ; where, v pll,q is the q-axis decomposition of the grid voltage, δω pll is the frequency difference output by the PI regulation, ω N is the rated frequency, ω pll is the output frequency of the phase-locked loop, θ pll is the output phase of the phase-locked loop, K ppll is the proportional controller gain of the phase-locked loop, K ipll is the integral controller gain of the phase-locked loop.

5. The method according to claim 4, wherein The parameter optimization and modal analysis include: Analyze the system robustness based on singular value decomposition; Determine the optimal combination of moment of inertia and damping factor through modal analysis.

6. An adaptive-parameter-based virtual synchronous generator grid-connected and islanded seamless switching control system based on the control method according to any one of claims 1-5, characterized in that, Including: Main - circuit module: including an LC filter and a grid - connection breaker; Control module: integrating an inner - loop voltage - current controller, an outer - loop power - frequency controller, and an adaptive parameter adjustment module; Synchronization module: adopt an improved phase - locked loop to achieve fast phase locking; Communication module: collect grid parameters in real - time and output control commands.

7. The system according to claim 6, characterized in that, The adaptive parameter adjustment module includes: Damping - factor switching unit: used to automatically switch the damping - factor value according to the operating mode; Moment - of - inertia adjustment unit: used to dynamically adjust the inertia constant based on the generator output power.

8. The system according to claim 7, wherein The adaptive parameter adjustment module includes a PI controller for dynamically adjusting the damping factor in the outer - loop power - frequency control loop.

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