A design method for voltage control loop of grid-connected converter controlled by virtual synchronous generator.

Abstract

This application relates to the field of new energy grid-connected power generation technology, and in particular to a design method for a voltage control loop of a grid-connected converter controlled by a virtual synchronous generator. In order to solve the problems and shortcomings of the existing technology in which the voltage control loop cannot take into account dynamics, stability and inaccurate estimation of the stable range, this application introduces stability constraints, active power dynamic performance constraints and reactive power dynamic performance constraints, reduces the interference of the voltage control loop on the power control loop, realizes the decoupling of the voltage control loop and the power control loop, and obtains the voltage control loop parameters through an algorithm without repeated trial and error.

Description

Technical Field

[0001] This application relates to the field of new energy grid-connected power generation technology, and in particular to a design method for a voltage control loop of a grid-connected converter controlled by a virtual synchronous generator. Background Technology

[0002] The power system is undergoing a transformation from fossil fuel-based synchronous generator power plants to renewable energy-based grid-connected converter power stations. Grid-connected technology, widely used in grid-connected converters, uses phase-locked loops (PLLs) to synchronize with the grid connection voltage, injecting a set power into the grid and exhibiting current source characteristics. However, grid-connected converters lack sufficient frequency and voltage support for the public grid, and the PLLs exhibit negative damping characteristics at low frequencies, making them prone to harmonic oscillations. Conversely, Virtual Synchronous Generators (VSGs), a type of grid-connected technology, synchronize with the grid through a power loop, simulating the characteristics of a synchronous generator to automatically adjust output voltage and frequency, allowing the converter to exhibit voltage source characteristics. Therefore, VSGs are considered a promising technology that will play a crucial role in future power systems and have attracted widespread attention.

[0003] Generally, VSG employs a cascaded controller architecture of "power outer loop + voltage and current control loops," with parameter design assuming complete decoupling of each loop to achieve independent design. For example, the voltage and current control loops are considered much faster than the power loop, so when designing the power loop parameters, the voltage and current control loops can be treated as components with a gain of 1. To achieve faster voltage and current control loops, the current control loop design typically uses the modulus optimal method or the zero-pole cancellation method. The voltage control loop design usually employs the symmetric optimal method to maximize the phase margin at the crossover frequency.

[0004] However, this application finds that, under the influence of switching frequency and system delay, the above parameter design process, especially the design of the voltage control loop, often fails to simultaneously meet the requirements of stability and VSG dynamic performance. Furthermore, repeated trial and error are required to obtain suitable parameters, and the designed voltage control loop may even lead to system instability, threatening the safe operation of the power system. Based on this, this application designs a voltage control loop design method for grid-connected converters controlled by a virtual synchronous generator to address the problems and shortcomings of existing technologies that cannot simultaneously consider dynamic performance, stability, and inaccurate estimation of the stable range in voltage control loops. Summary of the Invention

[0005] The purpose of this invention is to address the problems and shortcomings of existing technologies in which voltage control loops cannot simultaneously consider dynamics, stability, and inaccurate estimation of the stable range, and to provide a design method for a grid-connected converter voltage control loop controlled by a virtual synchronous generator.

[0006] To achieve the above objectives, this application provides the following technical solution: A method for designing a voltage control loop for a grid-connected converter controlled by a virtual synchronous generator includes the following steps: Step S1: Measure the three-phase common coupling point voltage of the grid-connected current transformer and the current on the grid-connected converter side, and obtain the voltage and current components through dq transformation; Step S2: Calculate the active power output of the grid-connected converter based on the voltage and current components. P and reactive power Q ; Step S3: The virtual synchronous generator obtains the physical parameters, active parameters, reactive parameters and current loop coefficient parameters of the grid-connected converter, and constructs the full-order state-space model of the grid-connected converter. In the voltage control loop of the full-order state-space model, grid current feedforward control is not introduced. Step S4: Based on the full-order state-space model constructed in Step S3, obtain the proportional coefficient of the voltage control loop through stability constraints, active power dynamic performance constraints, and reactive power dynamic performance constraints. k pu and integral coefficient k iu The parameter range is determined, and the adaptability of the virtual synchronous generator under different short-circuit ratios is considered. Step S5, in the proportional coefficient k pu Parameter range and integral coefficients k iu Select the proportional coefficient from the parameter range. k pu and proportionality coefficient k pu The data is then input into the grid-connected converter for grid-connected operation, and experimental verification is conducted simultaneously.

[0007] The technical effects of the above technical solution are as follows: This invention innovatively proposes a design method for the voltage control loop of a grid-connected inverter controlled by a virtual synchronous generator. It fully considers the influence of grid parameters, reactive power loop, and active power loop, and the designed parameters can meet the stability and dynamic performance requirements of the VSG. Furthermore, the proposed method has higher accuracy in stability judgment and parameter design than existing technologies, solving the problem of unreasonable parameter design caused by the simplified models used in existing technologies.

[0008] As a further improvement to the design method of grid-connected converter voltage control loop controlled by virtual synchronous generator of this application, in step S1, the dq transformation is to exchange coordinates between the system dq coordinate system aligned with the grid voltage and the controller dq coordinate system of the virtual synchronous generator control loop.

[0009] As a further improvement to the design method of voltage control loop of grid-connected converter controlled by virtual synchronous generator of this application, in step S2, the voltage component and the current component are both components in the system dq coordinate system.

[0010] As a further improvement to the design method of grid-connected converter voltage control loop for virtual synchronous generator control in this application, the phase angle difference between the system dq coordinate system and the controller dq coordinate system is introduced in the dq transformation. δ , δ = θ - θ g , θ This represents the phase angle signal output by the active power control loop of the virtual synchronous generator. θ g This indicates the phase angle of the grid voltage.

[0011] As a further improvement to the design method of voltage control loop for grid-connected converter controlled by a virtual synchronous generator in this application, in step S2, the virtual synchronous generator calculates the active power output of the grid-connected converter using the following formula. P and reactive power Q : ; in, v Cd The d-axis component represents the voltage at the three-phase common coupling point. v Cq The q-axis component represents the voltage at the three-phase common coupling point. i sd The d-axis component represents the current on the grid-connected converter side. i sq The superscript s represents the q-axis component of the current on the grid-connected converter side, and indicates the component in the system dq coordinate system aligned with the grid voltage.

[0012] As a further improvement to the design method of voltage control loop for grid-connected converter controlled by virtual synchronous generator of this application, in step S3, the physical parameters of the grid-connected converter include: the filter inductance of the grid-connected converter. L f ,resistance R f Filter capacitor C f Grid inductanceL g ,resistance R g Virtual inertia J p Rated angular frequency ω n Rated voltage U Cn ; The active power parameters of a grid-connected converter include: the cutoff frequency of the active power control loop. f pc Active droop coefficient D p ; The reactive power parameters of a grid-connected converter include: the cutoff frequency of the reactive power control loop. f qc Reactive power phase margin PM q reactive power droop coefficient K q ; The current loop coefficient parameters of grid-connected converters include: grid current feedforward coefficient. k fv Current loop proportionality coefficient k pi and current loop integral coefficient k ii .

[0013] As a further improvement to the design method of voltage control loop for grid-connected converter controlled by virtual synchronous generator of this application, the full-order state-space model includes the active power control loop, reactive power control loop, voltage control loop, current control loop, system delay element, and external grid impedance of the grid-connected converter.

[0014] As a further improvement to the design method of voltage control loop of grid-connected converter for virtual synchronous generator control in this application, in step S4, the active power control loop, voltage control loop, current control loop, system delay element and external circuit model are interconnected according to the feedback relationship to form a full-order closed-loop model of grid-connected converter. The closed-loop model is linearized at the steady-state operating point of grid-connected converter to obtain a state matrix with unified state space expression.

[0015] As a further improvement to the design method of grid-connected converter voltage control loop for virtual synchronous generator control in this application, the stability constraint is: all eigenvalues ​​of the state matrix of the full-order state-space model of the control are less than zero as the constraint condition for system stability. The active power dynamic performance constraint is: by analyzing the influence of the closed-loop transfer function of the voltage control loop on the equivalent open-loop gain and phase within the operating frequency band of the active power control loop of the voltage control loop; The reactive power dynamic performance constraint is a constraint that limits the effective operating frequency band and stability margin of the reactive power control loop of the voltage control loop. Attached Figure Description

[0016] Figure 1 This is a control structure diagram of a virtual synchronous generator; Figure 2 This is a flowchart of the voltage control circuit design proposed in this application; Figure 3 Bode plot and pole-zero plot of the closed-loop transfer function of the current control loop; Figure 4 Bode plot and pole-zero diagram of the voltage control loop; Figure 5 The zero-pole diagram of the voltage control loop is given when the grid current feedforward coefficient changes. Figure 6 Bode plots of the open-loop transfer functions for the active power control loop and the reactive power control loop; Figure 7 For parameters a Bode plot of closed-loop transfer function of voltage control loop under varying conditions; Figure 8 For parameters a The constraints under change; Figure 9 When the short-circuit ratio changes according to the method of this application a Within the satisfactory range; Figure 10 The experimental verification results of the method in this application under a strong power grid are as follows; Figure 11 The experimental verification results of the method in this application under a weak power grid are as follows; Figure 12 For parameters a Bode plots of power control loop and voltage control loop under varying conditions; Figure 13 This is a comparison of the method in this application with existing methods. Detailed Implementation

[0017] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0018] To facilitate an accurate understanding of the solutions provided in the following embodiments of this application, the terms involved in this application are explained as follows before describing the technical solutions provided in this application: Virtual synchronous generators (VSRs) are a type of grid-connected technology that synchronizes with the power grid via a power loop. They simulate the characteristics of a synchronous generator, automatically adjusting output voltage and frequency to make the converter exhibit voltage source characteristics. Unlike grid-connected converters, which use phase-locked loops (PLLs) to synchronize with the grid connection point voltage and exhibit current source characteristics, VSRs offer stronger frequency and voltage support for the public power grid. They effectively avoid the negative damping characteristics and harmonic oscillations inherent in PLLs at low frequencies.

[0019] Grid-connected converters are key equipment in power systems that enable renewable energy sources (such as wind power and photovoltaics) to connect to the power grid. Their main function is to convert the electrical energy generated by new energy power generation (usually direct current or frequency-converted alternating current) into alternating current that conforms to grid standards and to achieve synchronous operation with the grid.

[0020] The voltage control loop is a key component in the virtual synchronous generator (VSG) control architecture. Together with the power outer loop and the current control loop, it forms a cascaded controller. Its main function is to regulate the converter output voltage, ensure that it stably tracks the reference voltage command, and suppress the impact of grid disturbances on the voltage.

[0021] The current control loop is the lowest level control element in the cascaded control architecture of the virtual synchronous generator (VSG). It is located after the voltage control loop and directly drives the converter switching devices. Its main function is to track the current reference command output by the voltage control loop, realize the rapid and accurate regulation of the converter side current, and suppress current harmonics and grid disturbances.

[0022] The following detailed descriptions are exemplary and intended to provide further detailed explanation of this application. Unless otherwise specified, all technical terms used in this application have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains. The terminology used in this application is for the purpose of describing particular embodiments only and is not intended to limit the exemplary embodiments according to this application.

[0023] Where there is no conflict, the embodiments and features described in this application may be combined with each other.

[0024] by Figure 1 Taking the grid-connected operation of a three-phase grid-connected converter as an example, including the DC power supply... V dc Filter inductor L f and resistance R f Filter capacitor C f Grid inductance L g and resistance R g AC power gridv gabc A three-phase full-bridge converter composed of IGBT switching transistors.

[0025] like Figure 2 As shown in the figure, the grid-connected converter voltage control loop design method for a virtual synchronous generator control provided in this embodiment includes the following steps: Step S1: Measure the three-phase common coupling point voltage of the grid-connected current transformer and the current on the grid-connected converter side, and obtain the voltage and current components through dq transformation; Step S2: Calculate the active power output of the grid-connected converter based on the voltage and current components. P and reactive power Q ; Step S3: The virtual synchronous generator acquires the physical parameters, active power parameters, reactive power parameters, and current loop coefficient parameters of the grid-connected converter, and constructs a full-order state-space model of the grid-connected converter. In this model, the voltage control loop does not incorporate grid current feedforward control. The physical parameters of the grid-connected converter include: the filter inductance of the grid-connected converter. L f ,resistance R f Filter capacitor C f Grid inductance L g ,resistance R g Virtual inertia J p Rated angular frequency ω n Rated voltage U Cn ; The active power parameters of a grid-connected converter include: the cutoff frequency of the active power control loop. f pc Active droop coefficient D p ; The reactive power parameters of a grid-connected converter include: the cutoff frequency of the reactive power control loop. f qc Reactive power phase margin PM q reactive power droop coefficient K q ; The current loop coefficient parameters of grid-connected converters include: grid current feedforward coefficient. k fv Current loop proportionality coefficient k pi and current loop integral coefficient k ii .

[0026] Step S4: Based on the full-order state-space model constructed in Step S3, obtain the proportional coefficient of the voltage control loop through stability constraints, active power dynamic performance constraints, and reactive power dynamic performance constraints. k pu and integral coefficient k iu The parameter range is determined, and the adaptability of the virtual synchronous generator under different short-circuit ratios is considered. Step S5, in the proportional coefficient k pu Parameter range and integral coefficients k iu Select the proportional coefficient from the parameter range. k pu and proportionality coefficient k pu The data is then input into the grid-connected converter for grid-connected operation, and experimental verification is conducted simultaneously.

[0027] In step S1, the dq transformation involves exchanging coordinates between the system dq coordinate system aligned with the grid voltage and the controller dq coordinate system of the virtual synchronous generator control loop. It should be noted that the control of the virtual synchronous generator in this application is implemented in the dq coordinate system. The specific reason is that, in the actual implementation, there are two dq coordinate systems: one is the controller dq coordinate system defined by the active power control loop, and the other is the system dq coordinate system aligned with the grid voltage. Variables in the controller dq coordinate system... x c With the variables in the system dq coordinate system x s The relationship is shown in equation (1), where the superscript s indicates that the variable is in the system dq coordinate system, and the superscript c indicates that the variable is in the control dq coordinate system.

[0028] δ This represents the phase angle difference between the controller's dq coordinate system and the system's control frame. δ = θ - θ g , θ This represents the phase angle signal output by the VSG active power control loop. θ g This indicates the phase angle of the grid voltage.

[0029] (1) Furthermore, the active power output of the grid-connected inverter is calculated using the following formula. P and reactive power Q : (2) in, v Cd The d-axis component represents the voltage at the three-phase common coupling point. v Cq The q-axis component represents the voltage at the three-phase common coupling point. i sd The d-axis component represents the current on the grid-connected converter side. i sq The superscript s represents the q-axis component of the current on the grid-connected converter side, and indicates the component in the system dq coordinate system aligned with the grid voltage.

[0030] Specifically, in step S3, the full-order state-space model includes the active power control loop, the voltage control loop of the grid-connected converter, the current control loop, the system delay element, and the external grid impedance.

[0031] The active power control loop is established using the following formula: (3) in, ω n Indicates the rated angular frequency. ω and ω g These represent the output angular frequency of the active power control circuit and the angular frequency of the grid voltage, respectively. J p Represents the moment of inertia. P ref and Q ref Indicates active power reference and reactive power reference. D p and K q These represent the droop coefficient of the active power control loop and the droop coefficient of the reactive power control loop, respectively. E dref This represents the output voltage of the reactive power control circuit. The symbol "·" indicates the derivative operation with respect to time.

[0032] Furthermore, the voltage control loop is established using the following formula: (4) in, x vd and x vq This represents two state variables of the voltage control loop. k pu and k iu These represent the proportional coefficient and integral coefficient of the voltage control loop, respectively. G piv (s) =k pu + k iu / s . k fv Represents the grid current feedforward coefficient. B C = ω n C f , i csdref and i c and sqref represent the reference signals of the current control loop in the dq coordinate system.

[0033] Establish the current control loop using the following formula: (5) in, x id and x iq This represents two state variables of the voltage control loop. k pi and k ii These represent the proportional coefficient and integral coefficient of the voltage control loop, respectively. G pii ( s ) = k pi + k ii / s . X f = ω n L f , v c dref and v c and qref represent the reference signals of the current control loop in the dq coordinate system.

[0034] The system delay element is established using the following formula: (6) in, v s td and v s and tq represent the output voltage of the converter in the system dq coordinate system, respectively. T del This represents the system delay, including the computational delay of one sampling period and the pulse width modulation (PWM) delay of half a sampling period. T del ≈ 1.5T s , T s Indicates the switching cycle.

[0035] The external circuit model of the system is established using the following formula: (7) in, i s gd and i s and gq represent the grid-side currents in the system's dq coordinate system, respectively. v s gd and v s and gq represent the grid voltages in the system's dq coordinate system, respectively. X g ≈ ω n L g .

[0036] Furthermore, this application interconnects the active power control loop, voltage control loop, current control loop, system delay element, and external circuit model according to feedback relationships to form a full-order closed-loop model of the grid-connected converter. The closed-loop model is linearized at the steady-state operating point of the grid-connected converter to obtain a state matrix with a unified state space expression.

[0037] The proportional gain of the voltage control loop is obtained through stability constraints, active power dynamic performance constraints, and reactive power dynamic performance constraints. k pu and integral coefficient k iu During the parameter range process, the stability constraint is: all eigenvalues ​​of the state matrix of the full-order state-space model are less than zero as the constraint condition for system stability; The active power dynamic performance constraint is: by analyzing the influence of the closed-loop transfer function of the voltage control loop on the equivalent open-loop gain and phase within the operating frequency band of the active power control loop of the voltage control loop; The reactive power dynamic performance constraint is a constraint that limits the effective operating frequency band and stability margin of the reactive power control loop of the voltage control loop.

[0038] Furthermore, considering that traditional current control loop models are typically simplified to a second-order system: (8) in, ω CL and ξ CL These represent the equivalent natural frequency and damping coefficient, respectively.

[0039] Linearizing equations (1) and (5)-(7), the current control loop can be expressed as: (9) Wherein, the state vector x i = [ Δ i s sd, Δ i s sq, Δ v s Cd, Δ v s Cq, Δ i s gd, Δ i sgq, Δ v s td, Δ v s tq, Δ x s id, Δ x s iq] T Input vector u i = [Δ i c sdref, Δ i c sqref] T Output vector y i = [Δ i c sd, Δ i c sq] T , A i , B i , C i and D i It is a coefficient matrix.

[0040] The full-order model of the current control loop can be derived as follows: (10) in, I i Represents a 10×10 identity matrix. G icdd_fo ( s ), G icdq_fo ( s ), G icqd_fo ( s )and G icqq_fo ( sLet be the closed-loop transfer function of the current control loop, defined as: (11) in G icdd_fo ( s ) = G icqq_fo ( s ), G icdq_fo ( s ) = - G icqd_fo ( s ).

[0041] Specifically, Figure 3 (ab) demonstrates different ξ CL Transfer function of the lower current control loop G icdd_sim ( s )and G icdd_fo ( s） The comparison. For example... Figure 3 As shown in (a), in the 100-1000Hz frequency band, the simplified model and the full-order model show a large error. Specifically, in the 600-700Hz frequency band, the amplitude of the full-order model is significantly attenuated, and the phase angle jumps at the amplitude attenuation point. This indicates that the full-order model has a pair of zeros close to the imaginary axis, while the simplified model has the characteristics of a second-order underdamped system.

[0042] Furthermore, Figure 3 (a)-(b) demonstrate G icdd_sim ( s )and G icdd_fo ( s ) at different damping coefficients ξ CL From the zero-pole plot below, we can see that: For both simplified and full-order models, changes in the damping coefficient primarily affect the zeros and poles in the frequency band above 100Hz.

[0043] Compared to the simplified model, the dominant poles of the full-order model are closer to the imaginary axis, and changes in the damping coefficient have almost no effect on them.

[0044] Around 660Hz, full-order model G icdd_fo ( s There exists a pair of closed-loop zeros, which is consistent with... Figure 2 The results reflected are consistent.

[0045] Specifically, traditional voltage control loop models typically employ the symmetric optimal method, and their open-loop transfer model can be simplified to: (12) in, a This represents the bandwidth tuning factor. T eqi This represents the equivalent time constant of the current control loop. T eqi = 2 ξ CL / ω CL proportional coefficient of voltage control circuit k pu With integral coefficient k iu The relationship can be represented as: (13) Linearizing equations (1) and (4)-(7), the current control loop can be expressed as: (14) Wherein, the state vector x v = [ Δ i s sd, Δ i s sq, Δ v s Cd, Δ v s Cq, Δ i s gd, Δ i sgq, Δ v s td, Δ v s tq, Δ x s id, Δ x s iq] T Input vector u v = [Δ v c Cdref, Δ v c Cqref] T Output vector y v = [Δ v c Cd, Δ v c Cq] T , A v , B v , C v andD v It is a coefficient matrix.

[0046] Furthermore, the full-order model of the voltage control loop can be derived as follows: (15) in, I v Represents a 12×12 identity matrix. G vcdd_fo ( s ), G vcdq_fo ( s ), G vcqd_fo ( s )and G vcqq_fo ( s Let be the closed-loop transfer function of the voltage control loop, defined as: (16) in G vcdd_fo ( s ) = G vcqq_fo ( s ), G vcdq_fo ( s ) = - G vcqd_fo ( s ).

[0047] Furthermore, Figure 4 (a) Shows the simplified model and the full-order model of the open-loop transfer function of the voltage control loop at different levels. a Bode plot comparison below, Figure 4 (b) shows the open-loop pole-zero plot of the full-order model. In the selected... a Down, G vodd_sim ( s It has the largest phase margin at the cutoff frequency and remains stable, consistent with the previous analysis. However, the full-order model... G vodd_fo ( s A pair of open-loop zeros appear in the right half-plane, which is a characteristic of a non-minimum-phase element. Furthermore, Figure 4 (c) Full-order model of voltage control closed-loop transfer function G vcdd_fo ( s This indicates that the voltage control loop is unstable. The above results show that the traditional simplified model of the voltage control loop misjudges parameters that are actually unstable as stable.

[0048] Therefore, this application finds that the grid current feedforward term is an important cause of voltage control loop instability. Figure 4 It shows the grid current feedforward coefficient k fv When varying within the range of 0-1 G vodd_fo ( s )and G vcdd_fo ( s The zero-pole plot of ) as k fv The gradual decrease, such as Figure 5 As shown in (a), the open-loop zero point in the right half-plane gradually moves to the left half-plane, and the voltage control loop changes from unstable to stable.

[0049] like Figure 5 As shown in (b). The above results indicate that the voltage control loop instability is caused by the introduction of an open-loop zero located in the right half-plane, resulting in a non-minimum phase effect that leads to instability. Therefore, this application proposes to eliminate grid current feedforward control in the voltage control loop, i.e. k fv Setting it to 0 improves stability while avoiding the use of an additional current sensor, which helps reduce the manufacturing cost of the converter.

[0050] A simplified model using an active power control loop T po_sim ( s Simplified model of reactive power control loop T qo_sim ( s Design the power loop parameters: (17) (18) According to equation (17), the following can be drawn: T po_sim ( s )and T qo_sim ( s Bode plot, such as Figure 6 As shown, the cutoff frequency of the active power control loop can be obtained from this. f pcut Reactive power control loop cutoff frequency f qc and phase margin PM q .

[0051] Specifically, step S4 also includes the following steps: Linearizing equations (1) to (7), the state-space model of VSG can be written as: (19) Wherein, the state vector is: x p = [ Δ δ , Δ ω , Δ i s sd, Δ i s sq, Δ v s Cd, Δ v s Cq, Δ i s gd, Δ i sgq, Δ v s td, Δ v s tq, Δ x s id, Δ x s iq] T , A p It is a coefficient matrix.

[0052] Through calculation A p Eigenvalues ​​and stability constraints can be expressed as: (20) Here, Re(·) represents taking the real part of the matrix, and max[·] represents returning the maximum value element of the matrix.

[0053] Furthermore, Figure 7 Given G vcdd_fo ( s ) in different a The Bode plot is shown below, and the stability condition is marked by equation (22). When a Take a smaller value, for example a When = 0.1, in the 1-100Hz frequency band, G vcdd_fo ( s ) ≈ 1, however, at this time the voltage control circuit becomes unstable, when a When the value is large, for example a = 2.1, although the voltage control loop is stable, but Figure 6 The cutoff frequency of the active power control loop shown is around 6.9 Hz. G vcdd_fo ( sThis can cause a large negative phase, potentially leading to instability in the power control loop. Furthermore, G vcdd_fo ( s The amplitude-frequency characteristics of the circuit also affect the dynamic performance of the power control loop.

[0054] In order to reduce the interference of the voltage control loop on the active power control loop, the proposed dynamic performance constraint condition of the active power control loop is shown in equation (21).

[0055] The cutoff frequency of the active power control loop should be designed to vary within the range of several Hz to tens of Hz, not exceeding 2 Hz. f n , f n This indicates the rated frequency of the mains voltage, which is 50 Hz. Figure 7 It can be seen that, G vcdd_fo ( s The amplitude-frequency response curve of the ) has a maximum value in the 1-100 Hz frequency band. f p When varying within the 1-100 Hz frequency band, the frequency corresponding to the maximum amplitude of the voltage control loop is denoted as... f pmax Equation (25) means that in f pmax Place, T po ( s The amplitude should be less than -3dB. Furthermore, in Figure 6 The cutoff frequency shown f pc Place, G vcdd_fo ( s） The resulting amplitude change should not exceed 3dB.

[0056] Through the above settings a This ensures that the voltage control circuit will not cause significant interference to the power control circuit.

[0057] (twenty one) For the reactive power control loop, by Figure 7 From the established stable range of the voltage control loop, it can be seen that the specific effect of the voltage control loop on the reactive power control loop is as follows: f pmax nearby, G vcdd_fo ( s The positive amplitude provided will change the low-frequency characteristics of the reactive power control loop; in | G vcdd_fo ( s In the frequency band where )| < 1,G vcdd_fo ( s ) will make Figure 6 middle T qo_sim ( s The amplitude frequency and curve shifted downwards; G vcdd_fo ( s The voltage control loop always provides a negative phase angle. Therefore, the voltage control loop will inevitably change the cutoff frequency of the reactive power control loop. Thus, only by ensuring that the cutoff frequency of the reactive power control loop is within [1, ... f qc And satisfy | T qo ( s Within the range of 1, the stability margin does not decay. The proposed constraints are as follows: (twenty two) Furthermore, step S4 also includes the following steps: Figure 2 The flowchart illustrates the voltage control loop parameter optimization method proposed in this application, making... a from a min Start with step size a step Get the value to a max The voltage control circuit is calculated using equation (13). k pu and k iu The value of is recorded once the constraints shown in equations (20)-(22) are satisfied. a Otherwise, continue executing the process until... a = a max .

[0058] Figure 8 The constraints are given as follows a Regarding the changes, please note that the vertical axis of the curve does not represent the actual physical quantity; a value greater than 0 only indicates that the corresponding constraint conditions are met. In addition to the constraints expressed in equations (20)-(22), Figure 8 It also demonstrates the stability of the voltage control loop; a curve greater than 0 indicates that max[Re( λ p If )] < 0, the voltage control loop is stable. When a When the voltage is less than 0.67, the voltage control loop becomes unstable, thus causing the power control loop to become unstable. Subsequently, a When varying within [0.67, 0.96], all constraints are satisfied.a When the range changes from [0.96, 1.19], although the power control loop and reactive power control loop are constrained, the dynamic performance constraint of the active power control loop is not satisfied. Finally, when a If the value is greater than 1.19, although the voltage control loop is stable, it does not meet the stability and dynamic performance constraints of the active power control loop. Therefore, the parameters of the voltage control loop must satisfy the range [0.67, 0.96].

[0059] according to Figure 2 Process acquisition a To determine the satisfactory range, the system parameters must first be obtained. Generally, the filter parameters are easily obtained from the manufacturer or through offline measurement. However, the equivalent impedance of the power grid changes with the number of grid-connected converters and transmission lines, thus altering the stability and dynamic performance of the VSG. Therefore, it is necessary to determine... Figure 8 Received a Whether the satisfactory range is applicable to different grid impedances.

[0060] Under different short-circuit ratios (SCR) a Satisfaction range such as Figure 9 As shown. By Figure 8 (SCR=7) and Figure 9 It can be seen that as the SCR gradually decreases, a The satisfactory range is constantly increasing, mainly because it meets the stability constraints of the power control loop and the dynamic performance constraints of the active power loop. a The range of values ​​has been expanded. This also indicates that VSG has strong stability under weak power grid conditions.

[0061] according to Figure 8 and Figure 9 , a Ultimately, it was designed to be 0.85 to maximize the VSG's adaptability to both strong and weak power grids.

[0062] Step S5 further includes the following steps: Two scenarios were tested, one under a strong power grid and the other under a weak power grid. Scenario 1 was a stability test. P ref The power output is 1.5 kW (1.0 pu), varying at different times. a The possible values ​​are as follows: 1) a = p 1. All constraints are satisfied, namely the voltage control loop coefficients in Table 1.

[0063] 2) a = p2. The dynamic performance constraint of the active power control loop is not satisfied in equation (21).

[0064] 3) a = p 3. The stability constraint of equation (20) is not satisfied. p 1. p 2 and p 3. The values ​​for strong and weak power grids are shown in Table 1.

[0065] Table 1 Parameter values ​​under different power grids

[0066] Scenario 2 is a dynamic performance test, under different conditions. a Change P ref The possible values ​​are as follows: 1) a = p 1: P ref From 0.5 pu to 1.0 pu 2) a = p 2: P ref From 0.5 pu to 1.0 pu In scenario 1, the flag signal is changed. a , a = p At time 1, flag = 1 V; a = p At time 1, flag = 2 V, and after 2 seconds it switches to... a = p 3. At this point, flag = 3 V.

[0067] The test results for the above two scenarios under strong and weak power grids are as follows: Figure 10 and 11 As shown. In scenario 1, as... Figure 10 (a) and Figure 11 As shown in (a), due to p 1 and p Both conditions satisfy the stability constraints, therefore the system can operate stably before and after parameter switching. When a Depend on p 2 Transformed to p When the current exceeds the threshold of 1.5 pu, the system oscillates and becomes unstable, failing to meet the stability constraint. When the current exceeds the threshold of 1.5 pu, the converter is locked out to prevent damage.

[0068] In scenario 2, a = pAt time 1, all constraints are satisfied, such as Figure 10 As shown in (b) and 11(b), after the change in active power command, the changes in output power and current are smooth with almost no oscillation. In contrast, a = p At time 2, the dynamic performance constraints of the active power control loop are not met. After the active power command changes, such as Figure 10 As shown in (c) and 11(c), the output power and current exhibited significant oscillations.

[0069] Furthermore, Figure 12 A comparison of Bode plots for the open-loop transfer function of the active power loop and the closed-loop transfer function of the voltage control loop is presented. a When = 0.85, due to the voltage loop closed-loop transfer function G vcdd_fo ( s )exist f vmax1 There is a peak value at this point, therefore compared to the simplified model of the active power control loop... T po_sim ( s） , T po ( s )exist f vmax1 The amplitude at that point increases, but is less than -3dB, and G vcdd_fo ( s It did not lead to T po ( s )and T po_sim ( s The cutoff frequency and gain margin of the signal deviate significantly. T po_sim ( s cutoff frequency f pc The phase deviation is only -0.9°. The above results all satisfy the constraint condition of equation (21), indicating that the voltage control loop will not have an adverse effect on the power control loop. Therefore, the converter has a good dynamic response after the power command changes. a When = 1.07, f vmax2 Compared to f vmax1 reduce, T po ( s )exist f vmax2 A frequency band with an amplitude greater than 0 appeared nearby, causing the cutoff frequency of the active power control loop to increase, and, T po( s )exist f vmax2 The amplitude exceeds -3dB, which does not satisfy the constraint condition of equation (21). Furthermore, f vmax2 Nearby by G vcdd_fo ( s The introduced negative phase reduces T po ( s The phase margin of the phase margin compromises the stability of the system. a When = 1.50, it is obvious T po ( s The system experienced a negative phase angle crossover in the frequency band where the amplitude was greater than 0 dB, thus causing the system to become unstable.

[0070] Under strong power grid conditions, this application was compared with existing voltage control loop design methods, and the results are as follows: Figure 13 As shown, the design basis for the parameters of the compared methods is as follows: Method I is the symmetrical optimal method. Using equation (12), the phase margin of the open-loop transfer function of the voltage control loop is set to 30°-75°, from which the following can be obtained: a Within a satisfactory range.

[0071] Method II obtains the satisfactory range of a through the stability constraint of equation (20).

[0072] Method III adopts C f << k pu , C f << k iu and C f T eqi The local simplified model ≈ 0, from which the stability criterion is derived as shown in equation (23), can be calculated by combining equation (13). a Within a satisfactory range.

[0073] (twenty three) Method I assumes that the current control loop is fast enough, thus the effect of the grid current feedforward term can be ignored. However, as... Figure 3 As shown, under finite switching frequencies, the current control loop is difficult to approximate as a second-order circuit as shown in equation (8). Therefore, the grid current feedforward term cannot be ignored, and instability caused by grid feedforward is difficult to avoid. Furthermore, even if we let k fv = 0, by Figure 8 The results show that, although the voltage control circuit is in Figure 13 It is stable in region I, but unstable in VSG.

[0074] Method II, by employing a full-order model, can guarantee performance in region II. a It will not cause system instability, but the subsequent parameter selection can only rely on trial and error to ensure that the converter has good output characteristics, thus increasing the workload.

[0075] After applying the stability criterion of equation (27), as shown in region III, a The calculated satisfactory range is too aggressive. a When the voltage is low, incorrect judgments may be made regarding the stability of voltage control.

[0076] In comparison, the beneficial effects of the method proposed in this application can be summarized as follows: 1) It is not limited to stability constraints, but simultaneously considers stability, dynamic performance constraints of active power control circuit and dynamic performance constraints of reactive power control circuit. The voltage control circuit designed in this way will not have an adverse effect on the power control circuit, and the current and power output of the converter are more stable.

[0077] 2) The estimation of the VSG stability range is more accurate, avoiding overly conservative or aggressive results, and the impact of SCR changes on parameters is taken into account, thus improving the adaptability of VSG to strong and weak power grids.

[0078] 3) The parameters of the voltage control loop are automatically obtained through an algorithm, rather than through trial and error, which significantly reduces the workload.

[0079] It is worth noting that those skilled in the art will understand that embodiments of this application can be provided as methods, systems, or computer program products. Therefore, this application can take the form of a completely hardware embodiment, a completely software embodiment, or an embodiment combining software and hardware aspects. Furthermore, this application can take the form of a computer program product embodied on one or more computer-usable storage media (including but not limited to disk storage, CD-ROM, optical storage, etc.) containing computer-usable program code.

[0080] This application is described with reference to flowchart illustrations and / or block diagrams of methods, apparatus (systems), and computer program products according to embodiments of this application. It will be understood that each block of the flowchart illustrations and / or block diagrams, and combinations of blocks in the flowchart illustrations and / or block diagrams, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, special-purpose computer, embedded processor, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, generate instructions for implementing the flowchart... Figure 1 One or more processes and / or boxes Figure 1 A device that provides the functions specified in one or more boxes.

[0081] These computer program instructions may also be stored in a computer-readable storage medium that can direct a computer or other programmable data processing device to function in a particular manner, such that the instructions stored in the computer-readable storage medium produce an article of manufacture including instruction means, which are implemented in a process Figure 1 One or more processes and / or boxes Figure 1 The function specified in one or more boxes.

[0082] These computer program instructions can also be loaded onto a computer or other programmable data processing equipment to cause a series of operational steps to be performed on the computer or other programmable equipment to produce a computer-implemented process, thereby providing instructions that execute on the computer or other programmable equipment for implementing the process. Figure 1 One or more processes and / or boxes Figure 1 The steps of the function specified in one or more boxes.

[0083] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of this application and not to limit them. Although this application has been described in detail with reference to the above embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the specific implementation of this application. Any modifications or equivalent substitutions that do not depart from the spirit and scope of this application should be covered within the protection scope of the claims of this application.

[0084] It should be noted that, in this document, relational terms such as "first" and "second" are used only to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus.

[0085] Although embodiments of this application have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and variations can be made to these embodiments without departing from the principles and spirit of this application, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A design method for a voltage control loop of a grid-connected converter controlled by a virtual synchronous generator, characterized in that, Includes the following steps: Step S1: Measure the three-phase common coupling point voltage of the grid-connected current transformer and the current on the grid-connected converter side, and obtain the voltage and current components through dq transformation; Step S2: Calculate the active power output of the grid-connected converter based on the voltage component and the current component. P and reactive power Q ; Step S3: The virtual synchronous generator acquires the physical parameters, active parameters, reactive parameters and current loop coefficient parameters of the grid-connected converter, and constructs a full-order state-space model of the grid-connected converter. In the voltage control loop of the full-order state-space model, grid current feedforward control is not introduced. Step S4: Based on the full-order state-space model constructed in step S3, obtain the proportional coefficient of the voltage control loop through stability constraints, active power dynamic performance constraints, and reactive power dynamic performance constraints. k pu and integral coefficient k iu The parameter range is determined, and the adaptability of the virtual synchronous generator under different short-circuit ratios is considered. Step S5, in the proportionality coefficient k pu The parameter range and the integral coefficient k iu Select the proportional coefficient from the parameter range. k pu and the proportionality coefficient k pu The data is then input into the grid-connected converter for grid-connected operation, and experimental verification is conducted simultaneously.

2. The design method for a grid-connected converter voltage control loop controlled by a virtual synchronous generator according to claim 1, characterized in that, In step S1, the dq transformation is performed by exchanging coordinates between the system dq coordinate system aligned with the grid voltage and the controller dq coordinate system of the virtual synchronous generator control loop.

3. The design method for a voltage control loop of a grid-connected converter controlled by a virtual synchronous generator according to claim 2, characterized in that, In step S2, both the voltage component and the current component are components in the system's dq coordinate system.

4. The design method for a grid-connected converter voltage control loop controlled by a virtual synchronous generator according to claim 2, characterized in that, In the dq transformation, the phase angle difference between the system dq coordinate system and the controller dq coordinate system is introduced. δ , δ = θ - θ g , θ This represents the phase angle signal output by the active power control loop of the virtual synchronous generator. θ g This indicates the phase angle of the grid voltage.

5. The design method for a grid-connected converter voltage control loop controlled by a virtual synchronous generator according to claim 2, characterized in that, The virtual synchronous generator calculates the active power output of the grid-connected converter using the following formula. P and reactive power Q : ； in, v Cd This represents the d-axis component of the voltage at the three-phase common coupling point. v Cq This represents the q-axis component of the voltage at the three-phase common coupling point. i sd This represents the d-axis component of the current on the grid-connected converter side. i sq The superscript s represents the q-axis component of the current on the grid-connected converter side, and the superscript s represents the component in the system dq coordinate system aligned with the grid voltage.

6. The design method for a grid-connected converter voltage control loop controlled by a virtual synchronous generator according to claim 2, characterized in that, In step S3, the physical parameters of the grid-connected converter include: the filter inductance of the grid-connected converter. L f ,resistance R f Filter capacitor C f Grid inductance L g ,resistance R g Virtual inertia J p Rated angular frequency ω n Rated voltage U Cn ; The active power parameters of the grid-connected converter include: the active power control loop cutoff frequency. f pc Active droop coefficient D p ; The reactive power parameters of the grid-connected converter include: the cutoff frequency of the reactive power control loop. f qc Reactive power phase margin PM q reactive power droop coefficient K q ; The current loop coefficient parameters of the grid-connected converter include: grid current feedforward coefficient. k fv Current loop proportionality coefficient k pi and current loop integral coefficient k ii .

7. The design method for a grid-connected converter voltage control loop controlled by a virtual synchronous generator according to claim 1, characterized in that, The full-order state-space model includes the active power control loop, reactive power control loop, voltage control loop, current control loop, system delay element, and external grid impedance of the grid-connected converter.

8. The design method for a voltage control loop of a grid-connected converter controlled by a virtual synchronous generator according to claim 7, characterized in that, In step S4, the active power control loop, the voltage control loop, the current control loop, the system delay element, and the external circuit model are interconnected according to the feedback relationship to form the full-order closed-loop model of the grid-connected converter. The closed-loop model is linearized at the steady-state operating point of the grid-connected converter to obtain a state matrix with a unified state space expression.

9. The design method for a voltage control loop of a grid-connected converter controlled by a virtual synchronous generator according to claim 8, characterized in that, The stability constraint is: all eigenvalues ​​of the state matrix of the full-order state-space model are less than zero as a constraint on system stability; The active power dynamic performance constraint is: by analyzing the influence of the closed-loop transfer function of the voltage control loop on the equivalent open-loop gain and phase within the operating frequency band of the active power control loop of the voltage control loop; The reactive power dynamic performance constraint is a constraint that limits the effective operating frequency band and stability margin of the reactive power control circuit of the voltage control circuit.

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