VSG self-adaption control method under unbalanced power grid voltage

Abstract

The invention discloses a VSG self-adaption control method under unbalanced power grid voltage. The method comprises the steps that inertia and damping are introduced into a VSG-based inverter control model; determining the relationship between the instantaneous change rate of the electrical angular frequency, the variable quantity of the electrical angular frequency, the inertia and the damping by combining an active power-frequency control equation; determining a steady-state inertia coefficient and a damping coefficient according to the engineering optimization parameters and the variation range of the oscillation angular frequency, and determining the variation range of the inertia coefficient and the variation range of the damping coefficient; determining a self-adaptive control method of an inertia coefficient and a damping coefficient according to the electrical angular frequency variation and the electrical angular frequency instantaneous change rate of different stages; in combination with balanced current control, active power constant control and reactive power constant control under unbalanced network voltage, three-phase balance of output current is realized, and fluctuation of active power or reactive power is suppressed. According to the invention, the influence of unbalanced power grid voltage is reduced, and the transient stability of VSG control is improved through adaptive parameter adjustment.

Description

Technical Field

[0001] The present application relates to the field of electronic control technology, and in particular to a VSG adaptive control method under unbalanced grid voltage. Background Art

[0002] Driven by rapid transformation and the pursuit of net-zero emissions, renewable energy generation continues to grow. This transformation involves the deployment of the grid, and distributed energy resources that utilize inverter interfaces have gained considerable attention and promotion. By emulating the components of a synchronous generator, the Virtual Synchronous Generator (VSG) allows the inverter to display the inertia and damping characteristics inherent to the VSG, thereby positioning the VSG as an effective control technology for inverter-based distributed energy resources.

[0003] In the related technologies, most VSG controls still use fixed inertia and damping like traditional synchronous generator (SG) controls, which makes it difficult to flexibly adjust parameters according to actual operating scenarios, greatly limiting the flexibility of VSG control. When subject to external disturbances, grid voltage imbalance can lead to severe oscillations in active power and frequency. If the relevant parameters are not adjusted adaptively in time, the output power and frequency transient process of the VSG will be long, the dynamic performance will be poor, and oscillation instability will easily occur. In severe cases, the VSG may be damaged due to large and violent oscillations. Summary of the invention

[0004] The present application aims to solve at least one of the technical problems existing in the prior art. To this end, the embodiment of the present application provides a VSG adaptive control method under unbalanced grid voltage, which reduces active power fluctuations, effectively alleviates the impact of grid voltage imbalance, and the parameters can be adaptively adjusted to improve the transient stability of VSG control.

[0005] In a first aspect, an embodiment of the present application provides a VSG adaptive control method under an unbalanced grid voltage, comprising: establishing a VSG-based inverter control model, and introducing inertia and damping into the VSG-based inverter control model;

[0006] By using the VSG-based inverter control model and combining the active-frequency control equation, the relationship between the instantaneous rate of change of the electrical angular frequency, the amount of change of the electrical angular frequency, and the inertia and damping is determined;

[0007] In combination with the active power-frequency control equation, the steady-state inertia coefficient and damping coefficient are determined according to the engineering optimization parameters of the second-order transfer function based on the VSG and the variation range of the oscillation angular frequency under the unbalanced grid voltage, and the variation range of the inertia coefficient and the variation range of the damping coefficient are determined;

[0008] Determine the adaptive control method of the inertia coefficient and the damping coefficient under the unbalanced grid voltage according to the change amount of the electrical angular frequency at different stages and the instantaneous change rate of the electrical angular frequency;

[0009] By combining balanced current control, active power constant control and reactive power constant control under unbalanced grid voltage, the three-phase output current is balanced and active power or reactive power fluctuations are suppressed.

[0010] The technical solution of the first aspect of the present application has at least one of the following advantages or beneficial effects: establishing an inverter control model based on VSG, and introducing inertia and damping into the inverter control model based on VSG, and then determining the relationship between the instantaneous rate of change of the electrical angular frequency, the amount of change of the electrical angular frequency, and the inertia and damping based on the inverter control model of the VSG with the inertia and damping introduced, and combining the active-frequency control equation, and then combining the active-frequency control equation, and determining the steady-state inertia coefficient and damping coefficient according to the engineering optimization parameters of the second-order transfer function based on VSG and the range of change of the oscillation angular frequency under the unbalanced grid voltage, and determining the range of change of the inertia coefficient and the range of change of the damping coefficient; determining the adaptive formula and adaptive control method of the inertia coefficient and damping coefficient under the unbalanced grid voltage according to the amount of change of the electrical angular frequency and the instantaneous rate of change of the electrical angular frequency at different stages, and through the adaptive formula of inertia and damping, under the unbalanced grid voltage, the values ​​of inertia and damping can be adaptively adjusted according to the changes in the amount of change of the electrical angular frequency and its instantaneous rate of change at different stages. By determining the change in electrical angular frequency, oscillation caused by frequent changes in inertia and damping is prevented, and the impact of unbalanced grid voltage is reduced, thereby ensuring steady-state operation. In addition, based on the adaptive control of inertia and damping, combined with balanced current control, active power constant control and reactive power constant control under unbalanced grid voltage, the three-phase balance of output current is achieved, and active power or reactive power fluctuations are suppressed, thereby improving the transient stability of VSG control.

[0011] According to some embodiments of the present application, the active power-frequency control equation is:

[0012]

[0013] Among them, P * is the reference of VSG active power, P is the output active power, J o is the steady-state inertia coefficient, D o is the steady-state damping coefficient, ω ref is the reference angular frequency, ω is the output angular frequency, ω g is the grid angular frequency, δ is the output power angle, ΔJ is the change of inertia coefficient, and ΔD is the change of damping coefficient;

[0014] The VSG-based inverter control model is combined with the active-frequency control equation to determine the relationship between the instantaneous rate of change of the electrical angular frequency, the amount of change of the electrical angular frequency, and the inertia and damping, which is expressed by the following equation:

[0015]

[0016] in, is the instantaneous rate of change of electrical angular frequency, Δω is the change of electrical angular frequency, P * is the reference value of VSG active power, P is the output active power, ΔP is the output active power change, J o is the steady-state inertia coefficient, D o is the steady-state damping coefficient, ω ref is the reference angular frequency, ω is the output angular frequency, ω g is the grid angular frequency, and δ is the output power angle.

[0017] According to some embodiments of the present application, a VSG-based inverter control model is controlled to change the excitation current to control the excitation electromotive force to provide a change in reactive power required by the load;

[0018] The excitation current is adjusted in combination with the reactive-voltage control equation so that the voltage of the VSG is kept close to the rated value when the reactive power of the load changes.

[0019] The reactive power-voltage control equation is:

[0020]

[0021] Among them, V o is the output terminal voltage of VSG, U g is the voltage amplitude of the three-phase power grid, K p is the ratio of reactive voltage regulation, K i is the integral coefficient of reactive voltage regulation, Q * is the reference value of the active power of VSG, and Q is the actual output reactive power of VSG.

[0022] According to some embodiments of the present application, an output electromotive force of an inverter control model based on the VSG is determined according to an output terminal voltage and an electromotive force phase angle of the VSG;

[0023] The active power-frequency control equation is determined according to the output electromotive force, grid voltage, line impedance and grid impedance angle.

[0024] According to some embodiments of the present application, determining the active power-frequency control equation according to the output electromotive force, grid voltage, line impedance and grid impedance angle of the VSG-based inverter control model includes:

[0025] The grid impedance angle is determined according to the grid angular frequency, the line inductance and the line resistance, and the grid impedance angle is expressed by the following formula:

[0026]

[0027] Among them, α is the grid impedance angle, ω g is the grid angular frequency, L g is the line inductance, R g is the line resistance;

[0028] The line impedance is determined according to the grid angular frequency, the line inductance and the line resistance, and the line impedance is expressed by the following formula:

[0029]

[0030] Among them, Z line is the line impedance, ω g is the grid angular frequency, L g is the line inductance, R g is the line resistance;

[0031] The active power is determined according to the three-phase grid voltage amplitude, the output terminal voltage of the VSG, the grid impedance angle, the output power angle and the line impedance. The active power is expressed by the following formula:

[0032]

[0033] Among them, P is active power, U is g is the voltage amplitude of the three-phase grid, V o is the output terminal voltage of VSG, α is the grid impedance angle, δ is the output power angle, Z line is the line impedance;

[0034] When the line impedance is inductive, that is, X g =ω g L g ,X g >>R g ,α≈90°, the first simplified equation of active power is obtained as:

[0035]

[0036] Among them, P is active power, U is g is the voltage amplitude of the three-phase grid, V o is the output terminal voltage of VSG, X g is the grid side admittance, δ is the output power angle;

[0037] Assuming sinδ≈δ, the second simplified equation of active power, the first derivative equation of active power, and the second derivative equation of active power are obtained:

[0038]

[0039] Among them, P is active power, U is g is the voltage amplitude of the three-phase grid, V o is the output terminal voltage of VSG, δ is the output power angle, is the first-order derivative of active power, is the second-order derivative of active power, is the first-order derivative of the output power angle, is the second-order derivative of the output power angle, Δω is the change in electrical angular frequency, is the first-order derivative of the change in electrical angular frequency, ω g is the grid angular frequency, X g is the grid side admittance;

[0040] According to the first simplified equation of active power, the second simplified equation of active power, the first-order derivative equation of active power and the second-order derivative equation of active power, an active power-frequency control equation is obtained, and the active power-frequency control equation is expressed by the following equation:

[0041]

[0042] Among them, P * is the reference value of VSG active power, P is the output active power, D g is the damping factor, X g is the grid side admittance, U g is the voltage amplitude of the three-phase grid, V o is the output terminal voltage of VSG, is the first-order derivative of active power, is the second-order derivative of active power, D o is the steady-state damping coefficient, ΔD is the change in damping coefficient, ω ref is the reference angular frequency, J g is the inertia factor, J0 is the steady-state inertia coefficient, and ΔJ is the change in the inertia coefficient.

[0043] According to some embodiments of the present application, P * is the reference of VSG active power, P is the output active power, J g is the inertia factor, D g is the damping factor, U g is the voltage amplitude of the three-phase grid, V o is the output terminal voltage of VSG, X gis the grid-side admittance, ξ is the engineering optimization parameter of the second-order transfer function of VSG, ω n is the oscillation angular frequency under unbalanced grid voltage;

[0044] The inertia factor and damping factor are tuned using the optimal damping ratio method for linear second-order systems;

[0045] The engineering optimization parameter of the second-order transfer function of the VSG is determined to be 0.707, and the variation range of the oscillation frequency under unbalanced grid voltage is determined to be 0.628 rad / s to 15.7 rad / s;

[0046] The steady-state inertia coefficient and the steady-state damping coefficient are determined according to the engineering optimization parameters of the second-order transfer function of the VSG and the variation range of the oscillation frequency under the unbalanced grid voltage, and the variation range of the inertia coefficient and the variation range of the damping coefficient are determined.

[0047] According to some embodiments of the present application, the control method for determining the adaptive control of the inertia coefficient and the damping coefficient under the unbalanced grid voltage according to the electrical angular frequency change amount at different stages and the instantaneous change rate of the electrical angular frequency includes:

[0048] When the product of the electrical angular frequency change amount and the instantaneous rate of change of the electrical angular frequency is greater than zero, the damping coefficient is reduced and the inertia coefficient is increased;

[0049] When the product of the electrical angular frequency change amount and the electrical angular frequency instantaneous change rate is less than or equal to zero, the damping coefficient is reduced, and the inertia coefficient is reduced or increased.

[0050] According to some embodiments of the present application, the control method for determining the adaptive control of the inertia coefficient and the damping coefficient under the unbalanced grid voltage according to the electrical angular frequency change amount at different stages and the instantaneous change rate of the electrical angular frequency is expressed by the following equation:

[0051]

[0052] Among them, J o is the steady-state inertia coefficient, K J is the damping adjustment coefficient under unbalanced grid voltage, K D is the inertia adjustment coefficient under unbalanced grid voltage, ω * is the electrical angular velocity threshold, is the instantaneous rate of change of electrical angular frequency, Δω is the change of electrical angular frequency, D o is the steady-state damping coefficient.

[0053] According to some embodiments of the present application, the method combines balanced current control, active power constant control and reactive power constant control under unbalanced grid voltage to balance the three-phase output current and suppress active power or reactive power fluctuations, including:

[0054] Independently control the positive sequence current component and negative sequence current component of the output current under unbalanced grid voltage to achieve balanced current control;

[0055] Control the positive sequence command value and negative sequence command value of the compensation current under the unbalanced grid voltage, and track the current command after compensation to achieve constant active power control;

[0056] The reactive power fluctuation component under the unbalanced grid voltage is controlled to be zero, the positive sequence command value and the negative sequence command value of the compensation current are compensated in the balanced current control, and the command value of the compensation current is tracked to achieve constant reactive power control.

[0057] According to some embodiments of the present application, independently controlling the positive sequence current component and the negative sequence current component of the output current under the unbalanced grid voltage to achieve balanced current control includes:

[0058] The negative sequence current component is set to zero, and a positive sequence current command and a negative sequence current command are sent to obtain a positive sequence voltage and a negative sequence voltage in a synchronous rotating dq coordinate system;

[0059] Converting the positive sequence voltage and the negative sequence voltage in the synchronous rotating coordinate system into a superposition of voltages in the synchronous rotating αβ coordinate system;

[0060] The switching of the power devices in the VSG-based inverter control model is controlled by sinusoidal pulse width modulation to achieve balanced current control.

[0061] Other features and advantages of the present application will be described in the following description, and partly become apparent from the description, or understood by practicing the present application. The purpose and other advantages of the present application can be realized and obtained by the structures specifically pointed out in the description, claims and drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0062] Figure 1 is a flow chart of a VSG adaptive control method under an unbalanced grid voltage provided by an embodiment of the present application;

[0063] Figure 2 It is an active power-frequency control block diagram provided in an embodiment of the present application;

[0064] Figure 3 is a flow chart of a method for providing a change in reactive power required by a load provided by an embodiment of the present application;

[0065] Figure 4 It is a reactive power-voltage control block diagram provided in an embodiment of the present application;

[0066] Figure 5 is a flow chart of a method for determining an active power-frequency control equation provided by an embodiment of the present application;

[0067] Figure 6 is a flow chart of a method for determining a second-order transfer function of an active power-frequency control equation proposed in an embodiment of the present application;

[0068] Figure 7 It is an electrical angular velocity variation fluctuation curve provided in an embodiment of the present application;

[0069] Figure 8 It is a flow chart of a method for ensuring three-phase balance of output current and suppressing fluctuations of active power or reactive power provided by an embodiment of the present application;

[0070] Fig. 9 is a flow chart of a method for realizing balanced current control provided by an embodiment of the present application;

[0071] Fig.10 A control block diagram provided by an embodiment of the present application by combining balanced current control, active power constant control and reactive power constant control under unbalanced grid voltage;

[0072] Fig.11 It is a structural schematic diagram of a controller provided in an embodiment of the present application. DETAILED DESCRIPTION

[0073] In order to make the purpose, technical solutions and advantages of the present application clearer, the present application is further described in detail below in conjunction with the accompanying drawings and examples. It should be understood that the specific embodiments described herein are only used to explain the present application and are not intended to limit the present application. In addition, the characteristics, operations or features described in the specification can be combined in any appropriate manner to form various implementation methods. At the same time, the steps or actions in the method description can also be replaced or adjusted in order in a manner that is obvious to those skilled in the art. Therefore, the various sequences in the specification and the accompanying drawings are only for the purpose of clearly describing a certain embodiment and are not meant to be a necessary sequence, unless otherwise specified that a certain sequence must be followed.

[0074] In the description of this application, "several" means one or more, "more" means more than two, "greater than", "less than", "exceed", etc. are understood to exclude the number itself, and "above", "below", "within", etc. are understood to include the number itself. If there is a description of "first" or "second", it is only used for the purpose of distinguishing technical features, and cannot be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features or implicitly indicating the order of the indicated technical features.

[0075] The serial numbers of the components in this document, such as "first", "second", etc., are only used to distinguish the objects described and do not have any order or technical meaning. The "connection" and "coupling" mentioned in this application, unless otherwise specified, include direct and indirect connections (couplings).

[0076] The VSG adaptive control method under unbalanced grid voltage provided in the embodiment of the present application is further explained below in conjunction with the accompanying drawings.

[0077] Reference Figure 1 , Figure 1 is a flow chart of a VSG adaptive control method under an unbalanced grid voltage provided by an embodiment of the present application. The VSG adaptive control method under an unbalanced grid voltage includes but is not limited to steps S100 to S500. Specifically,

[0078] Step S100: establishing a VSG-based inverter control model, and introducing inertia and damping into the VSG-based inverter control model;

[0079] Step S200: Determine the relationship between the instantaneous rate of change of the electrical angular frequency, the amount of change of the electrical angular frequency, and the inertia and damping through the VSG-based inverter control model and in combination with the active power-frequency control equation;

[0080] Step S300: combining the active power-frequency control equation, and determining the steady-state inertia coefficient and damping coefficient according to the engineering optimization parameters of the second-order transfer function based on the VSG and the variation range of the oscillation angular frequency under the unbalanced grid voltage, and determining the variation range of the inertia coefficient and the variation range of the damping coefficient;

[0081] Step S400: determining a control method for adaptively controlling the inertia coefficient and the damping coefficient under the unbalanced grid voltage according to the electrical angular frequency variation and the electrical angular frequency instantaneous variation rate at different stages;

[0082] Step S500: By combining balanced current control, active power constant control and reactive power constant control under unbalanced grid voltage, the three-phase output current is balanced and active power or reactive power fluctuation is suppressed.

[0083] The VSG adaptive control method under unbalanced grid voltage includes: first, establishing an inverter control model based on VSG, and introducing inertia and damping into the inverter control model based on VSG, then based on the inverter control model of VSG introduced with inertia and damping, and combining the active-frequency control equation, determining the instantaneous rate of change of electrical angular frequency, the amount of change of electrical angular frequency, and the relationship between inertia and damping, then, combining the active-frequency control equation, and according to the engineering optimization parameters of the second-order transfer function based on VSG and the range of change of the oscillation angular frequency under the unbalanced grid voltage, determining the steady-state inertia coefficient and damping coefficient, and determining the range of change of the inertia coefficient and the range of change of the damping coefficient; according to the amount of change of electrical angular frequency and the instantaneous rate of change of electrical angular frequency at different stages, determining the adaptive formula and adaptive control method of the inertia coefficient and damping coefficient under the unbalanced grid voltage, through the adaptive formula of inertia and damping, under the unbalanced grid voltage, the values ​​of inertia and damping can be adaptively adjusted according to the changes of the amount of change of electrical angular frequency and its instantaneous rate of change at different stages. By determining the change in electrical angular frequency, oscillation caused by frequent changes in inertia and damping is prevented, and the impact of unbalanced grid voltage is reduced, thereby ensuring steady-state operation. In addition, based on the adaptive control of inertia and damping, combined with balanced current control, active power constant control and reactive power constant control under unbalanced grid voltage, the three-phase balance of output current is achieved, and active power or reactive power fluctuations are suppressed, thereby improving the transient stability of VSG control.

[0084] Reference Figure 2 As shown, Figure 2 It is an active power-frequency control block diagram provided in an embodiment of the present application. In some embodiments of the present application, the active power-frequency control equation is:

[0085]

[0086] Among them, P * is the reference of VSG active power, P is the output active power, J o is the steady-state inertia coefficient, D o is the steady-state damping coefficient, ω ref is the reference angular frequency, ω is the output angular frequency, ω g is the grid angular frequency, δ is the output power angle, ΔJ is the change of inertia coefficient, and ΔD is the change of damping coefficient;

[0087] Through the VSG-based inverter control model and combined with the active-frequency control equation, the relationship between the instantaneous rate of change of the electrical angular frequency, the change amount of the electrical angular frequency, inertia, and damping is determined and expressed by the following equation:

[0088]

[0089] in, is the instantaneous rate of change of electrical angular frequency, Δω is the change of electrical angular frequency, P * is the reference value of VSG active power, P is the output active power, ΔP is the output active power change, J o is the steady-state inertia coefficient, D o is the steady-state damping coefficient, ω ref is the reference angular frequency, ω is the output angular frequency, ω g is the grid angular frequency, and δ is the output power angle.

[0090] From the above equation, we can see that when When a certain value is reached, D o The increase of will lead to a decrease in the change of electrical angular frequency Δω, thereby reducing the amplitude of frequency fluctuation. o ω ref When Δω reaches a certain value, J o The increase in will lead to a decrease in the instantaneous rate of change of the electrical angular frequency, thereby slowing down the frequency fluctuation.

[0091] VSG controls the excitation electromotive force by changing the excitation current to cope with the change of reactive power required by the load, and the change of reactive power will affect the terminal voltage. In order to keep the VSG terminal voltage close to the rated value when the load changes, the excitation current must be continuously adjusted. If a reactive voltage controller is introduced, the excitation electromotive force can be reduced as the output reactive power of the VSG increases.

[0092] Therefore, in the embodiment of the present application, a reactive voltage controller is introduced, referring to Figure 3 As shown, Figure 3 is a flow chart of a method for providing a change in reactive power required by a load provided by an embodiment of the present application, including but not limited to steps S600 to S610. Specifically,

[0093] Step S600: Control the VSG-based inverter control model to change the excitation current to control the excitation electromotive force to provide a change in reactive power required by the load;

[0094] Step S610: Adjust the excitation current in combination with the reactive power-voltage control equation so as to keep the voltage of the VSG close to the rated value when the reactive power of the load changes.

[0095] Reference Figure 4 As shown, Figure 4 is a reactive power-voltage control block diagram provided in an embodiment of the present application. In some embodiments of the present application, the reactive power-voltage control equation is:

[0096]

[0097] Among them, Vo is the output terminal voltage of VSG, U g is the voltage amplitude of the three-phase power grid, K p is the ratio of reactive voltage regulation, K i is the integral coefficient of reactive voltage regulation, Q * is the reference value of the active power of VSG, and Q is the actual output reactive power of VSG.

[0098] By introducing a reactive voltage controller and controlling the VSG-based inverter control model to change the excitation current to control the excitation electromotive force to provide the change in reactive power required by the load, the increase in the output reactive power of the VSG is controlled to reduce the excitation electromotive force, ensuring that the VSG terminal voltage is kept close to the rated value when the load changes.

[0099] Reference Figure 5 As shown, Figure 5 is a flow chart of a method for determining an active power-frequency control equation provided by an embodiment of the present application, including steps S620 to S630. Specifically,

[0100] Step S620: determining the output electromotive force of the inverter control model based on the VSG according to the output terminal voltage and the electromotive force phase angle of the VSG;

[0101] Step S630: determining the active power-frequency control equation according to the output electromotive force, the grid voltage, the line impedance and the grid impedance angle.

[0102] In some embodiments of the present application, the output electromotive force of the inverter control model based on the VSG is determined according to the output terminal voltage and the electromotive force phase angle of the VSG. The output electromotive force of the inverter control model based on the VSG is: Assume the grid voltage is The line impedance is Z line =R g +jX g , the grid impedance angle is α, so the VSG active power-frequency control equation can be calculated based on the output electromotive force, grid voltage, line impedance and grid impedance angle:

[0103] First, the grid impedance angle is determined according to the grid angular frequency, line inductance and line resistance. The grid impedance angle is expressed by the following formula:

[0104]

[0105] Among them, α is the grid impedance angle, ω g is the grid angular frequency, L g is the line inductance, R g is the line resistance;

[0106] The line impedance is determined according to the grid angular frequency, line inductance and line resistance. The line impedance is expressed by the following formula:

[0107]

[0108] Among them, Z line is the line impedance, ω g is the grid angular frequency, L g is the line inductance, R g is the line resistance;

[0109] The active power is determined based on the three-phase grid voltage amplitude, the output terminal voltage of the VSG, the grid impedance angle, the output power angle and the line impedance. The active power is expressed by the following formula:

[0110]

[0111] Among them, P is active power, U is g is the voltage amplitude of the three-phase grid, V o is the output terminal voltage of VSG, α is the grid impedance angle, δ is the output power angle, Z line is the line impedance;

[0112] Assume that the line impedance is inductive, that is, X g =ω g L g ,X g >>R g ,α≈90°, the first simplified equation of active power is obtained as:

[0113]

[0114] Among them, P is active power, U is g is the voltage amplitude of the three-phase grid, V o is the output terminal voltage of VSG, X g is the grid side admittance, δ is the output power angle;

[0115] Assuming sinδ≈δ, the second simplified equation of active power, the first derivative equation of active power, and the second derivative equation of active power are obtained:

[0116]

[0117] Among them, P is active power, U is g is the voltage amplitude of the three-phase grid, V o is the output terminal voltage of VSG, δ is the output power angle, is the first-order derivative of active power, is the second-order derivative of active power, is the first derivative of the output power angle, is the second-order derivative of the output power angle, Δω is the change in electrical angular frequency, is the first-order derivative of the change in electrical angular frequency, ω g is the grid angular frequency, X g is the grid side admittance;

[0118] Based on the above derivation, according to the first simplified equation of active power, the second simplified equation of active power, the first-order derivative equation of active power and the second-order derivative equation of active power, the active-frequency control equation is obtained, and the active-frequency control equation is expressed by the following equation:

[0119]

[0120] Among them, P * is the reference value of VSG active power, P is the output active power, D g is the damping factor, X g is the grid side admittance, U g is the voltage amplitude of the three-phase grid, V o is the output terminal voltage of VSG, is the first-order derivative of active power, is the second-order derivative of active power, D o is the steady-state damping coefficient, ΔD is the change in damping coefficient, ω ref is the reference angular frequency, J g is the inertia factor, J0 is the steady-state inertia coefficient, and ΔJ is the change in the inertia coefficient.

[0121] In some embodiments of the present application, after obtaining the active-frequency control equation according to the first simplified equation of active power, the second simplified equation of active power, the first-order derivative equation of active power and the second-order derivative equation of active power, the VSG adaptive control method under unbalanced grid voltage also includes performing Laplace transform on the active-frequency control equation to obtain the second-order transfer function of the active-frequency control equation, referring to Figure 6 , Figure 6 1 is a flow chart of a method for determining a second-order transfer function of an active power-frequency control equation proposed in an embodiment of the present application. The method for determining a second-order transfer function of an active power-frequency control equation includes but is not limited to steps S700 to S730. Specifically,

[0122] Step S700: Perform Laplace transform on the active power-frequency control equation to obtain the second-order transfer function of the active power-frequency control equation. The second-order transfer function is represented by the following equation:

[0123]

[0124] Among them, P *is the reference of VSG active power, P is the output active power, J g is the inertia factor, D g is the damping factor, U g is the voltage amplitude of the three-phase grid, V o is the output terminal voltage of VSG, X g is the grid-side admittance, ξ is the engineering optimization parameter of the second-order transfer function of VSG, ω n is the oscillation angular frequency under unbalanced grid voltage;

[0125] Step S710: using an optimal damping ratio method for a linear second-order system to tune the inertia factor and the damping factor;

[0126] Step S720: determining that the engineering optimization parameter of the second-order transfer function of the VSG is 0.707, and determining that the variation range of the oscillation frequency under the unbalanced grid voltage is 0.628 rad / s to 15.7 rad / s;

[0127] Step S730: Determine the steady-state inertia coefficient and the steady-state damping coefficient according to the engineering optimization parameters of the second-order transfer function of the VSG and the variation range of the oscillation frequency under the unbalanced grid voltage, and determine the variation range of the inertia coefficient and the variation range of the damping coefficient.

[0128] The VSG parameters have the ability to adjust and influence the time response characteristics as described in the following equation:

[0129]

[0130] Among them, D g is the damping factor, J g is the inertia factor, U g is the voltage amplitude of the three-phase grid, V o is the output terminal voltage of VSG, X g is the grid side admittance;

[0131] By performing Laplace transform on the active power-frequency control equation, the second-order transfer function of the active power-frequency control equation is obtained. Combined with the second-order transfer function of the active power-frequency control equation, and based on the engineering optimization parameters of the second-order transfer function based on VSG and the variation range of the oscillation angular frequency under the unbalanced grid voltage, the steady-state inertia coefficient and damping coefficient, as well as the variation range of the inertia coefficient and the variation range of the damping coefficient are determined.

[0132] It can be understood that the engineering optimization parameter ξ of the second-order transfer function of the VSG is defined as ξ = 0.707, indicating the critical damping condition, and the range of the oscillation angular frequency under the unbalanced grid voltage is set to (0.628, 15.7) rad / s. Then, the optimal damping ratio method of the linear second-order system is used to calculate the inertia factor J.g and the damping factor D g The steady-state inertia coefficient and the steady-state damping coefficient are determined by tuning, and the variation range of the inertia coefficient and the damping coefficient are determined by combining the engineering optimization parameters of the second-order transfer function of the VSG and the variation range of the oscillation angular frequency under the unbalanced grid voltage.

[0133] Reference Figure 7 As shown, Figure 7 is an electrical angular velocity variation fluctuation curve provided in an embodiment of the present application, Figure 7 The horizontal axis is the horizontal axis, and the vertical axis is the electrical angular velocity. When the grid voltage is unbalanced, when the product of the electrical angular frequency change and the instantaneous rate of change of the electrical angular frequency is Δω When it is greater than 0, it indicates that the electrical angular frequency acceleration stage has begun. In this case, the inertia coefficient should be appropriately increased to minimize the amplitude of the frequency offset and improve the response speed of the system. On the contrary, when the product of the electrical angular frequency change and the instantaneous rate of change of the electrical angular frequency Δω When it is less than or equal to 0, it indicates that the frequency deceleration stage has begun. At this time, the inertia coefficient should be reduced to slow down the fluctuation of the angular frequency, reduce the system overshoot, and ensure that the frequency value is quickly restored to a stable value. At the same time, by appropriately reducing the damping coefficient, the adverse effects of unbalanced voltage on the load can be minimized.

[0134] Therefore, the inertia coefficient J and the damping coefficient D are given by Δω and The specific relationship is shown in the table below.

[0135]

[0136] In the present application, a control method for adaptively determining the inertia coefficient and the damping coefficient under an unbalanced grid voltage is provided according to the amount of change of the electrical angular frequency and the instantaneous rate of change of the electrical angular frequency at different stages, including: when the product of the amount of change of the electrical angular frequency and the instantaneous rate of change of the electrical angular frequency is greater than zero, reducing the damping coefficient and increasing the inertia coefficient; when the product of the amount of change of the electrical angular frequency and the instantaneous rate of change of the electrical angular frequency is less than or equal to zero, reducing the damping coefficient and reducing and increasing the inertia coefficient.

[0137] Based on the above analysis, the adaptive adjustment formulas of the inertia coefficient J and the damping coefficient D are:

[0138]

[0139] Among them, J o is the steady-state inertia coefficient, K J is the damping adjustment coefficient under unbalanced grid voltage, K D is the inertia adjustment coefficient under unbalanced grid voltage, ω * is the electrical angular velocity threshold, is the instantaneous rate of change of electrical angular frequency, Δω is the change of electrical angular frequency, D o is the steady-state damping coefficient.

[0140] It should be noted that the damping adjustment coefficient K under the unbalanced grid voltage in the embodiment of the present application is J The value range is Inertia adjustment factor K under unbalanced grid voltage D The range is By setting ω * It is the electrical angular velocity threshold, which can prevent system oscillation caused by frequent adjustment of the inertia coefficient J and the damping coefficient D, so that the system can operate in a stable state.

[0141] In some embodiments of the present application, when the grid voltage is unbalanced, the instantaneous composite power output of the grid-connected inverter can be expressed as follows:

[0142]

[0143] Where S is the instantaneous composite power output of the grid-connected inverter, V oαβ is the output terminal voltage of VSG in the synchronously rotating αβ coordinate system, I o is the actual excitation current, the superscript “^” is conjugate; the superscript “+” is the positive sequence component; the superscript “-” is the negative sequence component; the subscript “dq” is the synchronous rotating dq coordinate system, and the subscript “αβ” is the synchronous rotating αβ coordinate system.

[0144] The calculation formulas for instantaneous active power and reactive power are as follows:

[0145]

[0146] Among them, P is the instantaneous active power, Q is the instantaneous reactive power, is the instantaneous average value of active power, is the instantaneous average value of reactive power; P c2 is the second cosine fluctuation amplitude of active power, Q c2 is the second cosine fluctuation amplitude of reactive power; P s2 is the amplitude of the secondary sinusoidal fluctuation of active power, Q s2 It is the amplitude of the secondary sinusoidal fluctuation of reactive power.

[0147] As can be seen from the above formula, both active power and reactive power have power components, which can easily cause active power and reactive power to fluctuate, causing oscillation and causing the system to be unable to operate in a steady state. Therefore, it is necessary to achieve three-phase current balance and minimize the fluctuation of active power and reactive power.

[0148] Reference Figure 8 As shown, Figure 81 is a flow chart of a method for ensuring three-phase balance of output current and suppressing fluctuation of active power or reactive power provided by an embodiment of the present application. The method for ensuring three-phase balance of output current and suppressing fluctuation of active power or reactive power includes but is not limited to steps S510 to S530. Specifically,

[0149] Step S510: independently controlling the positive sequence current component and the negative sequence current component of the output current under the unbalanced grid voltage to achieve balanced current control;

[0150] Step S520: controlling the positive sequence command value and the negative sequence command value of the compensation current under the unbalanced grid voltage, and tracking the compensated current command to achieve constant active power control;

[0151] Step S530: Control the size of the reactive power fluctuation component under the unbalanced grid voltage to zero, compensate for the positive sequence command value and the negative sequence command value of the current in the balanced current control, and track the command value of the compensation current to achieve constant reactive power control.

[0152] On the basis of adaptive control, combined with balanced current control, active power constant control and reactive power constant control under unbalanced grid voltage, the positive sequence current component and negative sequence current component of the output current under unbalanced grid voltage are independently controlled to achieve balanced current control, thereby achieving three-phase balance of output current; the positive sequence command value and negative sequence command value of the compensation current under unbalanced grid voltage are controlled, and the current command after compensation is tracked to achieve active power constant control, thereby suppressing active power fluctuation; the size of the reactive power fluctuation component under unbalanced grid voltage is controlled to be zero, the positive sequence command value and negative sequence command value of the compensation current are used in balanced current control, and the command value of the compensation current is tracked to achieve reactive power constant control, thereby suppressing reactive power fluctuation. By balancing the three phases of output current and suppressing active power or reactive power fluctuation, active or reactive power fluctuation is reduced, the impact of grid voltage imbalance is effectively reduced, and the parameters can be adaptively adjusted to improve the transient stability of VSG control.

[0153] Since the calculation formula of instantaneous active power and reactive power is:

[0154]

[0155] Among them, P is the instantaneous active power, Q is the instantaneous reactive power, is the instantaneous average value of active power, is the instantaneous average value of reactive power; P c2 is the second cosine fluctuation amplitude of active power, Q c2 is the second cosine fluctuation amplitude of reactive power; P s2 is the amplitude of the secondary sinusoidal fluctuation of active power, Qs2 It is the amplitude of the secondary sinusoidal fluctuation of reactive power.

[0156] Therefore, in the dq coordinate system, the calculation formulas of instantaneous active power and reactive power can be used to obtain the following formula:

[0157]

[0158] in, and It is the dq negative sequence component of the actual excitation electromotive force; and It is the dq positive sequence component of the actual excitation electromotive force; and is the dq negative sequence component of the actual excitation current; and It is the dq positive sequence component of the actual excitation current.

[0159] In order to achieve balanced current control, the positive and negative sequence components of the output current must be controlled independently when the grid voltage is unbalanced.

[0160] Reference Fig. 9 As shown, Fig. 9 is a flow chart of a method for implementing balanced current control provided by an embodiment of the present application. The method for implementing balanced current control includes but is not limited to steps S511 to S513. Specifically,

[0161] Step S511: setting the negative sequence current component to zero, and sending the positive sequence current command and the negative sequence current command to obtain the positive sequence voltage and the negative sequence voltage in the synchronous rotating dq coordinate system;

[0162] Step S512: converting the positive sequence voltage and the negative sequence voltage in the synchronous rotating coordinate system into a superposition of voltages in the synchronous rotating αβ coordinate system;

[0163] Step S513: Control the switches of the power devices of the VSG-based inverter control model through sinusoidal pulse width modulation to achieve balanced current control.

[0164] In an embodiment of the present application, a method for realizing balanced current control includes: first, setting the negative sequence current component to zero, and sending a positive sequence current command and a negative sequence current command to obtain a positive sequence voltage and a negative sequence voltage in a synchronous rotating dq coordinate system; then converting the positive sequence voltage and the negative sequence voltage in the synchronous rotating dq coordinate system into a superposition of voltages in a synchronous rotating αβ coordinate system, and then controlling the switch of a power device based on a VSG inverter control model through sinusoidal pulse width modulation to realize balanced current control; the current calculation equation of the power device is shown as follows:

[0165]

[0166] in, and It is the reference value of the dq negative sequence component of the actual excitation current.

[0167] When dealing with fixed unbalanced parameters in the power grid, balanced current control includes the positive and negative sequence command values ​​of the compensation current, because there is a fixed relationship between these values ​​and the current command value. By independently controlling the positive and negative sequence components of the output current and tracking the compensated current command, the control goal of suppressing the active power grid frequency fluctuation by 2 times can be achieved, realizing balanced current control.

[0168] Reference Fig.10 As shown, Fig.10 It is a control block diagram provided by an embodiment of the present application by combining balanced current control, active power constant control and reactive power constant control under unbalanced grid voltage. In the case of active power constant control, the amplitude of the fluctuation component related to the active power must be zero, and its calculation equation is as follows:

[0169]

[0170] in, and It is the reference value of the dq positive sequence component of the actual excitation current.

[0171] The negative sequence reference current can be calculated using the above formula:

[0172]

[0173] In some embodiments of the present application, when constant reactive power control is required, the magnitude of the reactive power fluctuation component must be zero, and the positive and negative sequence command values ​​of the compensation current are compensated in the balanced current control, and the command value of the compensation current is tracked, so as to achieve the control target of suppressing the reactive power to be twice the grid frequency fluctuation, and obtain the following relationship:

[0174]

[0175] The negative sequence reference current for reactive power constant control can be calculated by the following formula:

[0176]

[0177] Through the above analysis of balanced current control, active power constant control and reactive power constant control, we can get Figure 6The VSG voltage and current double closed-loop control block diagram is shown. According to the change of the electrical angular frequency at different stages and the instantaneous change rate of the electrical angular frequency, the adaptive control method of the inertia coefficient and the damping coefficient under the unbalanced grid voltage is determined, and the effectiveness of the adaptive control method is further improved. On the basis of the adaptive control of inertia and damping, combined with the balanced current control, active power constant control and reactive power constant control under the unbalanced grid voltage, the three-phase balance of the output current is achieved, and the fluctuation of active power or reactive power is suppressed, thereby improving the transient stability of the VSG control.

[0178] In addition, an embodiment of the present application also provides a controller, including a memory, a processor, and a computer program stored in the memory and executable on the processor, wherein the processor implements the aforementioned control method when executing the computer program.

[0179] Reference Fig.11 , Fig.11 It is a structural diagram of a controller 1000 provided in an embodiment of the present application, including a processor 1001, which can be implemented by a general-purpose CPU (Central Processing Unit), a microprocessor, an application-specific integrated circuit (Application Specific Integrated Circuit, ASIC), or one or more integrated circuits, and is used to execute relevant programs to implement the method provided in the embodiment of the present application; a memory 1002, which can be implemented in the form of a read-only memory 1002 (Read Only Memory, ROM), a static storage device, a dynamic storage device or a random access memory 1002 (Random Access Memory, RAM), etc. The memory 1002 can store an operating system and other application programs. When the technical solutions provided in the embodiments of this specification are implemented by software or firmware, the relevant program codes are stored in the memory 1002 and are called by the processor 1001 to execute the embodiments of this application; the input / output interface 1003 is used to implement information input and output; the communication interface 1004 is used to implement communication interaction between this device and other devices, and communication can be achieved through wired methods (such as USB, network cables, etc.) or wireless methods (such as mobile networks, WIFI, Bluetooth, etc.); a bus transmits information between various components of the device (such as the processor 1001, the memory 1002, the input / output interface 1003 and the communication interface 1004); wherein the processor 1001, the memory 1002, the input / output interface 1003 and the communication interface 1004 are connected to each other within the device through the bus.

[0180] It will be appreciated by those skilled in the art that all or some of the steps and systems in the disclosed method above may be implemented as software, firmware, hardware and appropriate combinations thereof. Some physical components or all physical components may be implemented as software executed by a processor, such as a central processing unit, a digital signal processor or a microprocessor, or may be implemented as hardware, or may be implemented as an integrated circuit, such as an application specific integrated circuit. Such software may be distributed on a computer-readable medium, which may include a computer-readable storage medium (or a non-transitory medium) and a communication medium (or a temporary medium). As known to those skilled in the art, the term computer-readable storage medium includes volatile and non-volatile, removable and non-removable media implemented in any method or technology for storing information (such as computer-readable instructions, data structures, program modules or other data). Computer-readable storage media include, but are not limited to, RAM, ROM, EEPROM, flash memory or other memory technologies, CD-ROM, digital versatile disks (DVD) or other optical disk storage, magnetic cassettes, magnetic tapes, disk storage or other magnetic storage devices, or any other medium that may be used to store desired information and may be accessed by a computer. Furthermore, it is well known to those skilled in the art that communication media typically embodies computer readable instructions, data structures, program modules, or other data in a modulated data signal such as a carrier wave or other transport mechanism, and may include any information delivery media.

[0181] The above is a specific description of the preferred implementation of the present application, but the present application is not limited to the above-mentioned implementation mode. Technical personnel familiar with the field can also make various equivalent deformations or substitutions without violating the spirit of the present application. These equivalent deformations or substitutions are all included in the scope defined by the claims of the present application.

Claims

1. A VSG adaptive control method under unbalanced grid voltage, characterized in that: include: Establishing a VSG-based inverter control model, and introducing inertia and damping into the VSG-based inverter control model; By using the VSG-based inverter control model and combining the active-frequency control equation, the relationship between the instantaneous rate of change of the electrical angular frequency, the amount of change of the electrical angular frequency, and the inertia and damping is determined; In combination with the active power-frequency control equation, the steady-state inertia coefficient and damping coefficient are determined according to the engineering optimization parameters of the second-order transfer function based on the VSG and the variation range of the oscillation angular frequency under the unbalanced grid voltage, and the variation range of the inertia coefficient and the variation range of the damping coefficient are determined; Determine the adaptive control method of the inertia coefficient and the damping coefficient under the unbalanced grid voltage according to the change amount of the electrical angular frequency at different stages and the instantaneous change rate of the electrical angular frequency; By combining balanced current control, active power constant control and reactive power constant control under unbalanced grid voltage, the three-phase output current is balanced and active power or reactive power fluctuations are suppressed.

2. The VSG adaptive control method under unbalanced grid voltage according to claim 1, characterized in that: The active power-frequency control equation is: Among them, P * is the reference of VSG active power, P is the output active power, J o is the steady-state inertia coefficient, D o is the steady-state damping coefficient, ω ref is the reference angular frequency, ω is the output angular frequency, ω g is the grid angular frequency, δ is the output power angle, ΔJ is the change of inertia coefficient, and ΔD is the change of damping coefficient; The VSG-based inverter control model is combined with the active-frequency control equation to determine the relationship between the instantaneous rate of change of the electrical angular frequency, the amount of change of the electrical angular frequency, and the inertia and damping, which is expressed by the following equation: in, is the instantaneous rate of change of electrical angular frequency, Δω is the change of electrical angular frequency, P * is the reference value of VSG active power, P is the output active power, ΔP is the output active power change, J o is the steady-state inertia coefficient, D o is the steady-state damping coefficient, ω ref is the reference angular frequency, ω is the output angular frequency, ω g is the grid angular frequency, and δ is the output power angle.

3. The VSG adaptive control method under unbalanced grid voltage according to claim 2 is characterized in that: Also includes: The VSG-based inverter control model changes the excitation current to control the excitation electromotive force to provide the change of reactive power required by the load; The excitation current is adjusted in combination with the reactive-voltage control equation so that the voltage of the VSG is kept close to the rated value when the reactive power of the load changes. The reactive power-voltage control equation is: Among them, V o is the output terminal voltage of VSG, U g is the voltage amplitude of the three-phase power grid, K p is the ratio of reactive voltage regulation, K i is the integral coefficient of reactive voltage regulation, Q * is the reference value of the active power of VSG, and Q is the actual output reactive power of VSG.

4. The VSG adaptive control method under unbalanced grid voltage according to claim 3 is characterized in that: include: Determining the output electromotive force of the inverter control model based on the VSG according to the output terminal voltage and the electromotive force phase angle of the VSG; The active power-frequency control equation is determined according to the output electromotive force, grid voltage, line impedance and grid impedance angle.

5. The VSG adaptive control method under unbalanced grid voltage according to claim 4, characterized in that: The active power-frequency control equation is determined according to the output electromotive force, grid voltage, line impedance and grid impedance angle of the VSG-based inverter control model, including: The grid impedance angle is determined according to the grid angular frequency, the line inductance and the line resistance, and the grid impedance angle is expressed by the following formula: Among them, α is the grid impedance angle, ω g is the grid angular frequency, L g is the line inductance, R g is the line resistance; The line impedance is determined according to the grid angular frequency, the line inductance and the line resistance, and is expressed by the following formula: Among them, Z line is the line impedance, ω g is the grid angular frequency, L g is the line inductance, R g is the line resistance; The active power is determined according to the three-phase grid voltage amplitude, the output terminal voltage of the VSG, the grid impedance angle, the output power angle and the line impedance. The active power is expressed by the following formula: Among them, P is active power, U is g is the voltage amplitude of the three-phase grid, V o is the output terminal voltage of VSG, α is the grid impedance angle, δ is the output power angle, Z line is the line impedance; When the line impedance is inductive, that is, X g =ω g L g ,X g >>R g ,α≈90°, the first simplified equation of active power is obtained as: Among them, P is active power, U is g is the voltage amplitude of the three-phase grid, V o is the output terminal voltage of VSG, X g is the grid side admittance, δ is the output power angle; Assuming sinδ≈δ, the second simplified equation of active power, the first derivative equation of active power, and the second derivative equation of active power are obtained: Among them, P is active power, U is g is the voltage amplitude of the three-phase grid, V o is the output terminal voltage of VSG, δ is the output power angle, is the first-order derivative of active power, is the second-order derivative of active power, is the first-order derivative of the output power angle, is the second-order derivative of the output power angle, Δω is the change in electrical angular frequency, is the first-order derivative of the change in electrical angular frequency, ω g is the grid angular frequency, X g is the grid side admittance; According to the first simplified equation of active power, the second simplified equation of active power, the first-order derivative equation of active power and the second-order derivative equation of active power, an active power-frequency control equation is obtained, and the active power-frequency control equation is expressed by the following equation: Among them, P * is the reference value of VSG active power, P is the output active power, D g is the damping factor, X g is the grid side admittance, U g is the voltage amplitude of the three-phase grid, V o is the output terminal voltage of VSG, is the first-order derivative of active power, is the second-order derivative of active power, D o is the steady-state damping coefficient, ΔD is the change in damping coefficient, ω ref is the reference angular frequency, J g is the inertia factor, J0 is the steady-state inertia coefficient, and ΔJ is the change in the inertia coefficient.

6. The VSG adaptive control method under unbalanced grid voltage according to claim 5, characterized in that: Also includes: The active power-frequency control equation is Laplace transformed to obtain a second-order transfer function of the active power-frequency control equation, and the second-order transfer function is represented by the following equation: Among them, P * is the reference of VSG active power, P is the output active power, J g is the inertia factor, D g is the damping factor, U g is the voltage amplitude of the three-phase grid, V o is the output terminal voltage of VSG, X g is the grid-side admittance, ξ is the engineering optimization parameter of the second-order transfer function of VSG, ω n is the oscillation angular frequency under unbalanced grid voltage; The inertia factor and damping factor are tuned using the optimal damping ratio method for linear second-order systems; The engineering optimization parameter of the second-order transfer function of the VSG is determined to be 0.707, and the variation range of the oscillation frequency under unbalanced grid voltage is determined to be 0.628 rad / s to 15.7 rad / s; The steady-state inertia coefficient and the steady-state damping coefficient are determined according to the engineering optimization parameters of the second-order transfer function of the VSG and the variation range of the oscillation frequency under the unbalanced grid voltage, and the variation range of the inertia coefficient and the variation range of the damping coefficient are determined.

7. The VSG adaptive control method under unbalanced grid voltage according to claim 2, characterized in that: The control method for determining the adaptive inertia coefficient and damping coefficient under the unbalanced grid voltage according to the electrical angular frequency variation at different stages and the instantaneous rate of change of the electrical angular frequency includes: When the product of the electrical angular frequency change amount and the instantaneous rate of change of the electrical angular frequency is greater than zero, the damping coefficient is reduced and the inertia coefficient is increased; When the product of the electrical angular frequency change amount and the electrical angular frequency instantaneous change rate is less than or equal to zero, the damping coefficient is reduced, and the inertia coefficient is reduced or increased.

8. The VSG adaptive control method under unbalanced grid voltage according to claim 7, characterized in that: The control method for determining the adaptive control of the inertia coefficient and the damping coefficient under the unbalanced grid voltage according to the electrical angular frequency variation at different stages and the instantaneous rate of change of the electrical angular frequency is expressed by the following equation: Among them, J o is the steady-state inertia coefficient, K J is the damping adjustment coefficient under unbalanced grid voltage, K D is the inertia adjustment coefficient under unbalanced grid voltage, ω * is the electrical angular velocity threshold, is the instantaneous rate of change of electrical angular frequency, Δω is the change of electrical angular frequency, D o is the steady-state damping coefficient.

9. The VSG adaptive control method under unbalanced grid voltage according to claim 1, characterized in that: The method combines balanced current control, active power constant control and reactive power constant control under unbalanced grid voltage to balance the three-phase output current and suppress active power or reactive power fluctuations, including: Independently control the positive sequence current component and negative sequence current component of the output current under unbalanced grid voltage to achieve balanced current control; Control the positive sequence command value and negative sequence command value of the compensation current under the unbalanced grid voltage, and track the current command after compensation to achieve constant active power control; The reactive power fluctuation component under the unbalanced grid voltage is controlled to be zero, the positive sequence command value and the negative sequence command value of the compensation current are compensated in the balanced current control, and the command value of the compensation current is tracked to achieve constant reactive power control.

10. The VSG adaptive control method under unbalanced grid voltage according to claim 9, characterized in that: The independently controlling the positive sequence current component and the negative sequence current component of the output current under the unbalanced grid voltage to achieve balanced current control includes: The negative sequence current component is set to zero, and a positive sequence current command and a negative sequence current command are sent to obtain a positive sequence voltage and a negative sequence voltage in a synchronous rotating dq coordinate system; Converting the positive sequence voltage and the negative sequence voltage in the synchronous rotating coordinate system into a superposition of voltages in the synchronous rotating αβ coordinate system; The switching of the power devices in the VSG-based inverter control model is controlled by sinusoidal pulse width modulation to achieve balanced current control.

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