A composite synchronous control method and system for a grid-type converter system

Abstract

This invention discloses a composite synchronous control method and system for grid-connected converter systems, belonging to the field of new energy grid-connected converter control technology. The method integrates virtual synchronous generator control, power synchronization control, and matching control to form inertial frequency components, synchronous frequency components, and DC-side energy coordination frequency components, respectively. These three components are then fed forward to the same frequency superposition node to construct a composite synchronous frequency control law. This composite synchronous control law is used to generate the converter output angular frequency and obtain the output phase angle, thereby generating a voltage modulation signal to enable the converter to operate in a grid-connected manner. This invention improves the inertial support capability, oscillation damping capability, and dynamic convergence speed of grid-connected systems without significantly increasing the system order, reduces the active power oscillation amplitude, shortens the settling time, and expands the stable operating range of the system under different short-circuit ratio conditions. It is applicable to grid-connected energy storage, photovoltaic, wind power, and flexible DC converter systems.

Description

Technical Field

[0001] This invention belongs to the field of control technology for grid-connected converters in new energy, specifically relating to a composite synchronous control strategy and system for improving the dynamic performance of grid-connected converter systems, and particularly to a composite frequency control method for grid-connected converters that integrates virtual synchronous generator control, power synchronization control, and matching control.

[0002] This invention can be applied to grid-connected energy storage converters, photovoltaic grid-connected inverters, wind power converters, DC microgrid interface converters, flexible DC transmission converters, and new power systems containing a high proportion of power electronic equipment, to improve the system's inertial support capability, power oscillation suppression capability, weak grid adaptability, and transient dynamic performance. Background Technology

[0003] With the increasing penetration of new energy equipment such as photovoltaic power generation, wind power generation, energy storage systems, and flexible DC transmission into power systems, the traditional power system based on synchronous generators is transforming into a new type of power system based on power electronic converters. Traditional synchronous generators, due to their rotating mechanical inertia and damping characteristics, can naturally release or absorb kinetic energy when the system experiences power disturbances, thus supporting the grid frequency and voltage. However, new energy power generation systems are typically connected to the grid through power electronic converters, which lack the inherent mechanical inertia characteristics of synchronous generators. This leads to a decrease in the system's equivalent inertia, an increase in the rate of frequency change, and a weakening of damping capacity. Furthermore, they are prone to inducing low-frequency oscillations and transient instability under weak grid conditions or varying grid strength.

[0004] Traditional grid-connected converters for new energy sources mostly employ grid-following control. Grid-following control typically relies on phase-locked loops (PLLs) to obtain the grid phase and frequency, and uses the grid voltage as an external reference for current injection control. This control method is suitable for strong grid conditions, but in weak grids, islanded systems, or scenarios with a high proportion of new energy, the interaction between the PLL and the grid impedance can easily cause synchronization instability and power oscillations, making it difficult to actively establish voltage and frequency. To address these issues, grid-based control technology has gradually become an important research direction in the field of new energy grid-connected control. Grid-based control can act as a controlled voltage source, actively forming the output voltage amplitude and frequency, achieving grid-connected or islanded operation without relying on an external strong grid. Therefore, it can provide voltage support, frequency support, and inertia support for new power systems.

[0005] Existing grid-based control strategies mainly include droop control, virtual synchronous generator (VSG) control, power synchronization control, and matching control. VSG control simulates the rotor motion equations of a synchronous generator, enabling the converter to possess inertia and damping characteristics similar to a synchronous machine. Its typical control idea is to adjust the output frequency based on the active power deviation, allowing the converter to simulate the acceleration or deceleration process of a synchronous generator rotor when power disturbances occur. VSG control has good inertia support capabilities; however, because it typically introduces an equivalent inertia element, the system's dynamic response speed may decrease, and it is prone to power oscillations and insufficient damping when control parameters or grid strength change.

[0006] Power synchronization control (PSC) is a grid-based control method that directly adjusts phase or frequency using active power deviations. Compared to virtual synchronous generator (SSG) control, PSC has a relatively simple structure, faster synchronization response, and better power synchronization capabilities in weak grid environments. However, PSC itself lacks a complete synchronous generator inertia simulation mechanism, and its active inertial support capability is relatively limited. When the system experiences rapid power disturbances, PSC alone may struggle to simultaneously meet the requirements of fast response and frequency support.

[0007] Matching control is a control method based on matching the dynamics of the DC side with the frequency dynamics of the AC side. This method typically uses changes in the DC bus voltage to reflect the imbalance between the system's input and output power, and maps the DC voltage deviation to a frequency regulation, thus correlating the frequency dynamics of the converter with the energy dynamics of the DC capacitor. Matching control enhances the energy coordination capability between the DC and AC sides, and is particularly suitable for grid-type systems with DC energy storage or DC bus dynamic characteristics. However, when using matching control alone, the system's damping characteristics and parameter robustness are still limited, and it is difficult to independently achieve optimal transient performance under complex disturbance conditions.

[0008] Existing technologies typically improve upon the aforementioned single control strategies. For example, they introduce additional damping in virtual synchronous generator control, power decoupling control in power synchronization control, and DC voltage feedforward or loop reshaping methods in matching control. These methods can improve system dynamic performance to some extent, but several shortcomings remain. First, existing control strategies often rely on a single control mechanism, making it difficult to simultaneously leverage the inertial support advantage of virtual synchronous generator control, the rapid synchronization advantage of power synchronization control, and the DC-side energy coordination advantage of matching control. Second, some feedforward or reshaping control methods depend on initial system parameters and operating conditions; parameter acquisition errors may reduce control effectiveness. Third, introducing higher-order filters or additional state variables into the original control structure may increase the system order, thereby introducing new oscillation modes and increasing the complexity of stability analysis. Finally, existing methods may sacrifice steady-state performance while improving transient performance, making it difficult to simultaneously achieve fast convergence, low oscillation, and zero steady-state error. Summary of the Invention

[0009] The technical solution of this invention aims to solve the problem that a single control strategy in existing grid-type systems cannot simultaneously achieve inertial support, rapid synchronization, DC-side energy coordination, and oscillation suppression. This invention proposes a composite synchronization control strategy and system for improving the dynamic performance of grid-type systems. By integrating virtual synchronous generator control, power synchronization control, and matching control in a heterogeneous control mode, the three control mechanisms jointly participate in the formation of the converter frequency and phase angle, thereby improving the system's dynamic performance and stability margin.

[0010] The present invention solves the above-mentioned technical problems through the following technical solutions.

[0011] This invention provides a composite synchronous control method for a grid-type converter system, comprising: establishing an AC-side circuit model and a DC-side capacitor dynamic model of the converter based on the topology of the grid-type system; establishing a traditional frequency control equation based on virtual synchronous generator control, feeding forward the deviation value of active power superimposed on the DC-side bus voltage to the active power deviation value through a PI controller, thereby obtaining an inertial frequency component driven by the active power deviation; establishing a synchronous frequency component directly driven by the active power deviation based on power synchronization control; establishing an energy coordination frequency component driven by the DC bus voltage deviation based on matching control; feeding forward the inertial frequency component, the synchronous frequency component, and the DC-side energy coordination frequency component to the same frequency superposition node to form a composite synchronous frequency control law; generating the converter output angular frequency according to the composite synchronous frequency control law, and obtaining the output phase angle through integration; generating a voltage modulation signal based on the output phase angle and voltage amplitude reference value, enabling the converter to operate in a grid-type manner.

[0012] This invention proposes a novel control strategy for networked systems, which enables the organic integration of virtual synchronous generator control, power synchronization control, and matching control without significantly increasing the system order and engineering implementation complexity. This simultaneously improves inertial support capability, oscillation suppression capability, dynamic response speed, and stability margin.

[0013] Furthermore, the AC side circuit model is established based on Kirchhoff's voltage law, and the converter output voltage, grid voltage, and filter impedance satisfy the following relationship:

[0014] in, Indicates the AC side filter inductor. Indicates the AC side filter resistor. This indicates the converter output current. This indicates the converter output voltage. This indicates the grid voltage.

[0015] Furthermore, in synchronous rotation In the coordinate system, the AC side circuit model is represented as:

[0016]

[0017] in, and These represent the output current. Axial components and Axial components, and These represent the converter output voltages respectively. Axial components and Axial components, and These represent the grid voltages respectively. Axial components and Axial components, This indicates the output angular frequency of the converter.

[0018] Furthermore, the active power and reactive power output of the grid-type system are expressed as follows:

[0019]

[0020] in, This indicates the active power output of the converter. This indicates the reactive power output of the converter.

[0021] The active and reactive power outputs from the power calculation unit are processed by a low-pass filter (LPF) and used as the P and Q feedback signals in the actual control loop. The LPF attenuates high-frequency power ripples caused by switching actions, sampling noise, and electrical transients, ensuring that the frequency generation stage primarily responds to effective power changes related to grid synchronization and energy exchange, thus preventing high-frequency ripples from directly affecting the frequency generation channels of the VSG and PSC. Therefore, in subsequent small-signal analysis, ΔP and ΔQ correspond to the power disturbance values ​​after power measurement and low-pass filtering.

[0022] Furthermore, the dynamic model of the DC-side capacitor is established based on the active power balance relationship on both sides of the DC bus, and its expression is:

[0023] in, This represents the DC bus capacitance. Indicates the DC bus voltage. Indicates the DC-side input power. This indicates the AC side output power. In practical implementation, It can be taken as the active power output of the converter. .

[0024] From an energy mechanism perspective, the DC bus voltage can directly characterize the energy storage state on the DC side. The energy stored in the DC bus capacitor can be expressed as:

[0025] In the formula, This indicates the energy storage of the DC bus capacitor.

[0026] When the DC input power is less than the AC output power, the DC bus capacitor releases energy. The DC bus capacitor absorbs energy when the DC input power exceeds the AC output power. Rise. Therefore The change essentially reflects the instantaneous imbalance between the DC-side input energy and the AC-side output energy. This physical relationship forms the basis for the matching control MC to introduce the DC voltage state into the frequency formation stage.

[0027] Furthermore, the traditional frequency control equation for virtual synchronous generator control is:

[0028] in, Represents the virtual inertia coefficient. Indicates the virtual damping coefficient. This represents the angular frequency generated by the virtual synchronous generator control. Indicates the rated angular frequency. This represents the reference value for active power. This represents the actual output active power. This control equation is used to simulate the rotor motion characteristics of a synchronous generator, enabling the grid-type system to possess inertial support capabilities.

[0029] Furthermore, by performing a Laplace transform on the control equations of the virtual synchronous generator, the transfer function between the frequency deviation and the active power deviation of the traditional control is obtained as follows:

[0030] in, Represents the Laplace operator. This represents a small frequency signal generated by the control of a virtual synchronous generator. This represents a small-signal quantity indicating the active power reference value. This indicates a small signal quantity output by active power.

[0031] Furthermore, under the composite synchronous control method of this invention, a DC voltage deviation signal is first constructed based on the reference value and the actual value of the DC bus voltage:

[0032] in This is a small signal quantity for DC bus voltage. This is a small signal for the DC voltage reference value. The DC voltage deviation signal is input into the Vdc-FFC stage and combined with the active power reference value and the actual active power at the active power summation node to obtain the active power drive signal shared by VSG and PSC:

[0033] In the formula, This indicates a small DC voltage deviation signal. This represents the common active power drive signal after DC voltage feedforward correction. and These represent the proportional and integral coefficients of Vdc-FFC, respectively. This processing allows the DC-side energy state to participate in the reconstruction of the active power error signal before the frequency component is formed, thereby establishing a dynamic correlation between AC-side power regulation and DC-side energy changes.

[0034] After the common active power drive signal enters the VSG dynamic link, it forms an inertial frequency component:

[0035] in, Determines the strength of the virtual inertial effect. Provides damping for frequency shift. This is the rated angular frequency. Because this channel contains... The corresponding inertial dynamics mean that the power mismatch is not instantaneously and proportionally mapped to the output frequency. Instead, it forms a relatively smooth frequency response through inertia and damping, thereby obtaining inertial support and oscillation suppression capabilities similar to those of a synchronous generator rotor.

[0036] Furthermore, the PSC branch and the VSG branch use the same common active power drive signal. However, instead of going through a virtual inertia stage, the synchronization frequency component is directly formed through the power synchronization coefficient:

[0037] In the formula, This represents the power synchronization control coefficient. Compared to the VSG branch, which includes inertial dynamics, the PSC branch constitutes a fast and direct channel from power mismatch to frequency change. Its main function is to rapidly change the internal angular frequency and phase advance speed when there are sudden power changes or changes in grid conditions, thereby improving the transient synchronization speed of the converter. At the same time, the VSG branch continues to provide inertia and damping effects, thus forming a power-frequency regulation mechanism that combines fast and slow dynamics.

[0038] Furthermore, matched control introduces a matched control coefficient. A feedforward reshaping channel for DC bus voltage deviation to the frequency loop is established, and its transfer law expression is as follows:

[0039] in, To match the small frequency signal components generated by the control, this control mechanism extracts DC voltage fluctuations and dynamically maps the energy of the upstream DC capacitor to the synchronous frequency of the converter in real time, achieving adaptive matching and coordination between the DC-side capacitor voltage and the AC-side frequency.

[0040] Furthermore, the MC branch does not undergo active power error processing, but directly utilizes the DC voltage deviation signal. Characterizes the DC-side energy deviation and forms the energy coordination frequency component through matching control coefficients:

[0041] In the formula, For matching control coefficients. It participates in angular frequency generation as a negative correction component at the composite frequency forming node. When it is lower than the reference value, When positive, the negative frequency correction generated by MC reduces the rate of increase of the internal phase angle, thereby prompting the AC-side active power output to adjust in a direction favorable to DC bus energy recovery; when When the value is higher than the reference value, its effect is reversed, thus promoting the release of excess energy from the DC side to the AC side. Therefore, MC directly maps the charging and discharging state of the DC bus capacitor to the frequency and phase formation process on the AC side, achieving coordination between DC energy and the AC synchronization process.

[0042] Furthermore, the composite synchronous frequency control law is formed by obtaining three frequency components—the virtual synchronous generator control component, the power synchronization control component, and the matching control component—from the aforementioned different dynamic branches, and then superimposing them at the same frequency forming node according to the direction shown in the control block diagram to the same frequency addition point. Its small-signal relationship is:

[0043] Its unified small-signal motion oscillation equation is expressed as:

[0044] In the formula, This represents the angular frequency offset of the entire system output. and These are the proportional and integral coefficients of the DC voltage feedforward compensation link Vdc-FFC, respectively.

[0045] Furthermore, the internal angular frequency of the converter is obtained by superimposing the composite frequency offset with the rated angular frequency:

[0046] The internal phase angle of the converter is obtained by integrating the internal angular frequency, and its small-signal frequency domain relationship is as follows:

[0047] Therefore, the three frequency components in this invention have a clear division of labor in their mechanisms of action: It mainly reflects the inertial and damped response under active power mismatch. It primarily provides a fast synchronization response after power mismatch. It primarily reflects the DC bus energy deviation and provides energy coordination correction; these three factors work together to affect the internal angular frequency, and then, through integration, change the phase angle θ, ultimately influencing the converter output voltage vector E·e. jθ And the exchange of active power with the power grid.

[0048] Furthermore, this invention combines the composite synchronous frequency control swing equation, the AC side circuit small-signal equation, and the DC side power balance equation, which includes the DC bus voltage outer loop and capacitor charging and discharging dynamics, into a closed-loop system. By eliminating intermediate electrical state variables, the global open-loop transfer function of the entire system under the composite synchronous control strategy is derived. for:

[0049] In the formula, and These are the virtual inertia and damping coefficient of the reactive power control element, respectively. The intermediate decoupling transfer function matrix components of the AC link are mapped from phase and voltage amplitude to low-power signals; Let be the coupling transfer function between DC voltage fluctuations and active power, and its specific expression is:

[0050] in, This represents the initial steady-state value of the DC voltage.

[0051] From the perspective of closed-loop dynamic mechanism, the three branches VSG, PSC, and MC affect the system frequency characteristics in different ways. The VSG branch contains... and The dynamic components constitute a circuit that primarily alters the inertia, damping, and dominant pole characteristics in the low-frequency range; the PSC branch passes through... A direct power-frequency path is established to improve the response speed of power disturbances to phase adjustment without introducing new integral states, and to reshape the open-loop gain and phase characteristics; the MC branch then... For energy state feedback, through Feeding DC-side energy storage fluctuations back to the frequency formation node allows the DC capacitor dynamics to actively participate in AC-side power and phase regulation, rather than merely acting as a passive disturbance. These three mechanisms, coupled through the same angular frequency and phase state, simultaneously improve the response speed under power disturbances, low-frequency oscillation damping, and DC voltage recovery process, providing a physical explanation for subsequent stability analysis based on the global open-loop transfer function.

[0052] The present invention also provides a composite synchronous control stability evaluation system for improving the dynamic performance of a network-type system by implementing the above method, comprising: AC side small-signal modeling module: used to establish an AC small-signal model of the active and reactive power of the inverter output voltage and current in relation to the grid-side impedance, based on the circuit topology equation and the definition of first-order power filtering; DC capacitor dynamic coupling module: used to derive and establish a power balance decoupling and transfer model of the influence of DC bus voltage on the active power loop based on the relationship between DC side voltage control and capacitor charging and discharging balance; Composite Synchronous Control Motion Equation Construction Module: Used to introduce power synchronization control (PSC) feedforward and matching control (MC) voltage feedforward into the traditional VSG rotor equations to construct a hybrid frequency control model that integrates three heterogeneous control modes of network construction; The whole system closed-loop transfer function integration module is used to solve the mathematical equations corresponding to the above modules in a closed loop, and to derive the overall open-loop and closed-loop small-signal transfer function model of the system taking into account the order of all states. Stability Quantitative Assessment Module: This module is used to draw Nyquist or Bode plots based on the global open-loop transfer function, and to obtain the system's characteristic roots and control parameters based on the closed-loop characteristic equation. and The root locus changes; by extracting the real part, imaginary part, natural frequency, damping ratio, phase margin, and gain margin of the dominant oscillation mode, the oscillation attenuation capability, parameter adaptability, and dynamic stability boundary of the converter system under varying grid strength and control parameters are quantitatively evaluated.

[0053] The beneficial effects of this invention are as follows: 1. Significantly Enhanced Inertial Support and Damping Effects: The composite frequency control strategy proposed in this invention perfectly integrates the advantages of heterogeneous modes such as PSC, MC, and VSG. Without changing or increasing the system order, it provides stronger inertial support and damping effects than traditional single VSG, effectively suppressing low-frequency oscillations of the system with a faster convergence rate. Closed-loop eigenvalue analysis shows that, under the comparative operating conditions of this embodiment, the dominant closed-loop conjugate pole without the proposed control is... 2.2937±j38.163 s - ¹, after adopting the proposed control, move to 18.120±j39.199 s - ¹ The dominant mode damping ratio increased from 0.0600 to 0.4196, and the estimated 2% settling time decreased from 1.744 s to 0.221 s, indicating that the proposed control can significantly enhance the attenuation capability of key oscillation modes.

[0054] 2. Significantly improves closed-loop stability margin and robustness: After adopting the proposed composite synchronous control, the open-loop trajectory relative to the critical point ( The minimum distance (j0) is significantly increased. Correspondingly, the system phase margin increases from 6.859° to 42.134°, the gain margin increases from 5.766 dB to 12.188 dB, the peak sensitivity decreases from 8.605 to 1.436, and the allowable delay margin increases from 3.167 ms to 15.736 ms. This demonstrates that the present invention not only improves the oscillation damping under nominal operating conditions but also enhances the system's tolerance to parameter perturbations, modeling errors, and control delays.

[0055] 3. Achieve zero steady-state error control: This strategy is easy to implement in hardware and tune parameters, and achieves zero steady-state error control through adaptive optimization of the power synchronization coefficient. Matching control coefficient This approach significantly reduces power and voltage oscillations in the transient response without sacrificing the system's static steady-state response, achieving zero steady-state error control without any steady-state deviation. Under different combinations of power synchronization and matching control coefficients, the positions and damping ratios of the key conjugate eigenvalues ​​maintain analyzable and tuneable variation patterns. Therefore, the Nyquist stability margin, the real part of the dominant eigenvalue, and the damping ratio can be used as joint criteria for coordinating the tuning of control parameters, enabling the power synchronization channel and the matching control channel to achieve better damping performance while satisfying closed-loop stability constraints.

[0056] 4. Significantly Expanded Adaptability to Strong and Weak Power Grids: Traditional VSG control is prone to instability in strong power grid (large SCR) scenarios. This invention reshapes the system closed-loop trajectory through a composite synchronization mechanism, enabling the converter system to consistently achieve good phase margin and ample stable operating range within a wide range of SCR variations from 1 to 50, thus solving the instability risk of grid-connected converters during the switching between strong and weak power grids.

[0057] 5. High modeling accuracy and robustness: The small-signal closed-loop evaluation model proposed in this invention does not increase the control order of the entire system. Therefore, while fully considering the dynamic coupling of DC voltage and capacitance, it does not increase the analysis and calculation burden of the entire system. Theoretical analysis and verification on the hardware-in-the-loop (HIL) platform show that the time-domain step response and frequency-domain characteristics are highly consistent, providing an extremely accurate theoretical basis for controller design. Attached Figure Description

[0058] Figure 1 This is a block diagram illustrating the topology and composite synchronous control principle of the grid-type converter system in Embodiment 1 of the present invention. Figure 2 This is a block diagram of the closed-loop control system of the composite synchronization control strategy in Embodiment 2 of the present invention; Figures 3(a) to 3(c) are respectively from embodiments of the present invention. System frequency under disturbance DC side voltage and active power Before and after simulation images; Figures 4(a) to 4(c) are respectively from embodiments of the present invention. System frequency under disturbance DC side voltage and active power Before and after simulation images; Figures 5(a) to 5(c) are respectively from embodiments of the present invention. Control coefficients under disturbance The simulation response diagram shows the changes; Figures 6(a) to 6(c) are respectively from embodiments of the present invention. Control coefficients under disturbance The simulation response diagram shows the changes; Figures 7(a) to 7(c) show the matching control coefficients in the embodiments of the present invention. A comparison of the root loci of the grid-type system under traditional virtual synchronous generator control and the composite synchronous control of this invention during the change, wherein Figure 7(a) corresponds to Figure 7(b) corresponds to Figure 7(c) corresponds to ; Figures 8(a) and 8(b) are respectively from embodiments of the present invention. Impact and The system's active power step response diagram under the influence of; Figure 9 The diagram shows a comparison of the active power step response when the proposed control scheme is not used, when MC+VSG control is used, when PSC+VSG control is used, and when the proposed control scheme is used in the embodiments of the present invention. Figure 10 The figure shows the Nyquist stability analysis results of the system in this embodiment of the invention. Figure 10 The left figure is a global comparison of the Nyquist trajectory of the system without the proposed control and with the proposed composite synchronous control; Figure 10 The right figure shows the critical point ( A magnified view of the Nyquist trajectory near (j1,j0) is used to compare the minimum distance from the Nyquist trajectory to the critical point under the two control methods.

[0059] Figure 11 This is a phase margin analysis diagram of the system employing the proposed composite control strategy in this embodiment of the invention when the short-circuit ratio (SCR) changes. Figure 12 The diagram shows the experimental apparatus and HIL testing platform structure in the embodiments of the present invention. Figures 13(a) and 13(b) are comparative experimental results of the grid impedance under disturbance conditions in the embodiments of the present invention when the proposed control scheme is not used and when the proposed control scheme is used. Figures 14(a) and 14(b) are comparative experimental results of active power under disturbance conditions in the embodiments of the present invention when the proposed control scheme is not used and when the proposed control scheme is used. Figures 15(a) to 15(d) respectively show the control coefficients in the embodiments of the present invention. Experimental results of the adaptive test of composite frequency control when the values ​​are 0.002, 0.01, 0.1 and 0.2 respectively are shown in the figure. Figures 16(a) to 16(d) respectively show the control coefficients in the embodiments of the present invention. The experimental results of the composite frequency control adaptability test are shown in the figure when the values ​​are 0.0001, 0.0003, 0.0005 and 0.001 respectively.

[0060] Figure 3(a) to Figure 11 Figures 13(a) to 16(d) are all experiments or simulations performed under the system environment in Table 1, with only parameter changes. and .

[0061] Table 1 System Parameters Detailed Implementation

[0062] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0063] The technical solution of the present invention will be further described below with reference to the accompanying drawings, tables, and specific embodiments: Example 1 This embodiment first constructs a composite synchronization control algorithm. For example... Figure 1 As shown, the system's modulation frequency and power angle are generated by the superposition of the traditional virtual synchronous generator (VSG) stage, the power synchronization control (PSC) feedforward stage, and the matching control (MC) stage at the addition point. The basic mathematical derivation and closed-loop small-signal modeling and reconstruction process are as follows: 1. Establish a small-signal model for a composite synchronous control strategy. (1) Decoupling of active and reactive power on the AC side Based on Kirchhoff's voltage law (KVL), the AC integrated circuit equation for inverter-side output filtering and grid-side impedance is expressed in the first-order continuous domain as formula (1): (1) In the formula, and These represent the AC side output voltages of the converter, respectively. shaft and Axial components; and These represent the reference voltages of the main power grid. shaft and Axial components; and These represent the grid-connected output current of the converter, respectively. shaft and Axial components; This is the rated reference angular frequency; For continuous-domain time variables; and These represent the equivalent total resistance and equivalent total inductance of the entire system, respectively.

[0064] Among them, the equivalent total resistance of the entire system With equivalent total inductance The following relation (2) must be satisfied:

[0065] In the formula, The internal parasitic resistance of the AC-side output filter of the converter; This is the inherent inductance value of the AC side output filter of the converter; This is the equivalent grid resistance in the grid-side impedance; This is the equivalent grid inductance in the grid-side impedance.

[0066] In the frequency domain, the differential operator Replace with Laplace operator Small-signal perturbations are applied to each electrical quantity near the rated operating point. The pure small-signal relationship matrix is ​​as follows:

[0067] In the formula, For the Laplace operator; and Do not correspond to the converter output voltage shaft and Small signal offset component of the axis; and Corresponding to grid-connected current shaft and Small signal offset component of the axis; and Corresponding to the voltage of the main power grid shaft and Small signal offset component of the axis.

[0068] By using matrix inversion, the mapping relationship between small current signals and small voltage signals can be obtained:

[0069]

[0070] Assuming the external grid reference voltage is relatively stable, the small-signal disturbance of the grid-side voltage makes... Based on the internal control coordinate system of the converter, the output voltage... shaft and The axial component is composed of its internally synthesized voltage amplitude. Harmony and angle Decide: In the formula, This indicates the transient output voltage amplitude of the converter; This indicates the internal power angle phase output by the converter control system.

[0071] For formula (3) at the initial steady-state operating point After performing total differential small-signal linearization, we can obtain formula (4):

[0072] In the formula, This is the static small-signal deviation of the power angle phase of the entire system; This is the small-signal dynamic bias of the converter output voltage amplitude. This represents the initial calculation point value for the steady-state phase angle; This is the reference value for the output voltage amplitude of the converter under steady-state operating conditions.

[0073] Next, the active power output on the AC side... and reactive power The first-order low-pass filter definition equation (5) is used for small-signal linearization:

[0074] In the formula, This corresponds to the total active power output after measurement and filtering; This corresponds to the total reactive power output after measurement and filtering. Linearizing and stripping the above formula, we obtain the small-signal power equation (6):

[0075] In the formula, This refers to the small-signal disturbance of the output active power. This refers to the small-signal disturbance of the output reactive power. and The inverter-side output voltage is respectively at shaft and Steady-state operating point components on the axis; and The grid-connected current on the inverter side is respectively shaft and Steady-state operating point components on the axis.

[0076] The solution obtained earlier Expressions and Substituting the expression into formula (6), combining like terms and regularizing, the small signal from the work angle can be separated and extracted. Small amplitude signal The decoupling matrix equation for the AC side driven by the common driver (7):

[0077] In the formula, and These represent the decoupling transmission network components between power angle change, voltage amplitude change, and small-signal fluctuation of active power, respectively. and These represent the decoupling transmission network components between power angle change, voltage amplitude change, and small-signal fluctuation of reactive power, respectively. The specific frequency domain analytical expression matrix elements are shown in formula (8):

[0078] In the formula, The steady-state reference voltage amplitude corresponding to the equivalent large power grid.

[0079] (2) DC-side power balance and capacitor dynamic coupling terms Detailed derivation Considering the active power balance relationship on both sides of the DC bus capacitor, the nonlinear time-domain equation is Equation (9):

[0080] In the formula, Indicates DC bus capacitance; The DC bus voltage detected in real time; Transient input power on the DC side of the front stage of the strain flow system; This represents the active power output from the AC side of the converter. The DC input source is simulated using equivalent source current control, and the outer-loop DC voltage controller employs proportional-integral (PI) control. Its small-signal frequency domain input power characteristics satisfy: Apply a small-signal perturbation to formula (9) at the operating point and substitute it into the equation. By rearranging and simplifying, the coupling transfer function between AC-side active power and DC bus voltage variation is obtained. See Formula 10:

[0081] In the formula, The decoupling transfer function represents the cross-penetration of DC voltage disturbances into the AC active power loop; This is the small-signal state bias of the DC bus voltage. The reference setting is provided for the DC voltage given inside the control system; This is the initial steady-state operating voltage value of the DC bus; The proportional gain of the front-end DC bus voltage controller (DVC); This is the integral gain constant of the front-end DC bus voltage controller (DVC).

[0082] Combining the AC and DC side circuits, the system's total electrical quantity coupling matrix equation is extended to formula (11):

[0083] (3) Detailed derivation of the transfer functions of each control loop and the global open loop of the closed-loop system. Power Synchronous Control (PSC) feedforward branch (feedforward coefficient) ) and the matching control (MC) voltage feedforward branch (feedforward coefficient) Feeding forward to the same frequency control core addition point, we can obtain the unified small-signal composite synchronous frequency motion swing equation (12) proposed in this invention:

[0084] In the formula, This represents the offset of the small signal at the final synthesized angular frequency of the fully closed-loop output. The virtual rotational inertia coefficient corresponding to the active control loop; The virtual rotor damping coefficient corresponding to the active power control loop; The gain adjustment coefficient for the power synchronization control feedforward loop is introduced; For the introduced matching control voltage feedback reshaping coefficient; The proportional gain of the DC feedforward DC in the active-frequency control loop; It is the integral gain constant of the DC feedforward DC in the active-frequency control loop. Provide a small reference signal for the instruction input of the active power controller; This is a small signal quantity serving as the reference value for the DC voltage controller.

[0085] Using integral linear operator relations By combining formulas (11) and (12) in a closed loop, the process of eliminating intermediate state variables through multidimensional loop elimination is as follows: Step 1: Factoring the reactive power-voltage closed-loop circuit According to the reactive power control equation, its small-signal deviation relationship satisfies In the formula, The virtual inertia coefficient of the reactive power control loop voltage; The equivalent damping coefficient of the reactive power control loop; The reactive power reference input is a small signal. Substituting it into the reactive power equation of formula (11), in conventional analysis, the external reactive power command disturbance is assumed. Then, the transmission characteristics of voltage amplitude and phase angle can be obtained by combining these factors:

[0086] Step 2: Reorganization of the forward channel of the active power loop The result obtained in step one above Substituting the related terms into the active power equation of formula (11) can eliminate the intermediate voltage amplitude variable. :

[0087] Step 3: Eliminate the DC voltage term and solve for the global open-loop transfer function. When performing open-loop stability qualitative judgment on the system's characteristic trajectory, let the external independent input reference disturbance be... Formula (12) is converted into the work angle. Driver type:

[0088] Substitute into step two The expression is used to convert the DC voltage to a matching coupling factor. :

[0089] By performing polynomial algebraic rearrangement on the above equation, the forward path and negative feedback network terms are completely separated, and the global open-loop transfer function of the converter system under the improved composite synchronous control strategy is finally derived and calculated. See Formula 13:

[0090] In the formula, The global open-loop transfer function of the entire system, taking into account the dynamics of the DC capacitor, corresponds to the composite synchronous control strategy of this invention.

[0091] The small-signal topology model structure after the entire system is closed-loop interconnected is as follows: Figure 2 As shown in the figure. This figure is used to illustrate the closed-loop connection relationship between the AC side small-signal model, DC capacitor dynamic coupling, VSG inertial frequency component, PSC synchronization frequency component, and MC energy coordination frequency component in the composite synchronization control strategy, which will form the basis for subsequent Figures 3(a) to 3(b). Figure 11 The simulation response, stability analysis, and active power step response comparison shown provide a unified closed-loop model basis.

[0092] Using the derived core open-loop transfer function formula (13), its Nyquist stability characteristic diagram is plotted, as follows: Figure 10 As shown in the diagram. The analysis clearly shows that the blue curve of the traditional single VSG control curve is extremely close to the critical instability point. The system has a low stability margin; however, after adopting the composite synchronous control strategy proposed in this invention, the red curve of the Nyquist trajectory is significantly deviated from the target curve. This identifies dangerous points and thus rigorously demonstrates at the frequency domain theoretical level that composite synchronous control can significantly improve the dynamic stability margin of grid-connected converter systems.

[0093] 2. Time-domain adaptive control of key controller parameters and comparison of multiple solutions Using the constructed model, step response analysis of time-power commands is performed to quantitatively analyze the vibration damping effect of key controller parameters on the control response: Power synchronization control coefficient Adaptive testing: As shown in Figures 6(a) to 6(c), with the matching coefficient fixed, the power synchronization control coefficient is gradually increased. The step response of active power indicates that the larger the power synchronization control coefficient, the more significantly the dynamic ripple overshoot after disturbance is reduced, and the faster the convergence speed. This behavior is consistent with the inherent characteristics of power synchronization, namely, a higher coefficient can improve synchronization performance and accelerate the decay of oscillations.

[0094] Matching control coefficient Adaptive testing: As shown in Figures 5(a) to 5(c), with the power synchronization coefficient fixed, the matching control coefficient is gradually increased. Time-domain simulation results confirm that a larger matched control coefficient... It can effectively convert DC bus voltage feedback into frequency damping, significantly reducing the amplitude fluctuation of low-frequency oscillations and accelerating their convergence, thereby simultaneously improving the transient adaptive level of the converter.

[0095] To further demonstrate the vibration damping performance of the present invention's strategy, the time-domain responses of different strategies under a step disturbance with the same input active power reference value were compared, such as... Figure 9 As shown: Traditional VSG control (not employing this scheme) exhibits prolonged and violently large underdamped low-frequency power oscillations after external disturbances. Traditional damping improvement schemes, such as VSG+MC and VSG+PSC, are discussed separately. While they can slightly alleviate the overshoot of transient fluctuations to some extent, the damping effect is not optimal. The composite synchronization control strategy of this invention compresses the power oscillation amplitude in its response waveform to a minimum, demonstrating the strongest damping effect, the fastest decay rate, and the shortest settling time. Furthermore, these three network configurations are compatible and can jointly undertake synchronization and frequency regulation tasks without generating steady-state errors.

[0096] Subsequently, to verify the robustness of this strategy to drastic environmental fluctuations, the operating region of the converter system under varying generalized grid strength short-circuit ratio (SCR) was divided by changing the grid-side impedance parameters. For example... Figure 11 As shown, when the grid strength short-circuit ratio (SCR) fluctuates within an extremely wide range of 1 to 50, the system's minimum phase margin remains positive, and the global minimum is approximately [value missing]. It can always adaptively obtain ample and good phase margin, solving the instability risk of grid-connected converters when switching between strong and weak power grids and parameter disturbances.

[0097] 3. Simulation Analysis and Verification To verify the effect of the proposed composite synchronization control strategy on improving the dynamic performance of the grid-type virtual synchronous generator system, a comparative simulation model of the traditional control strategy and the composite synchronization control strategy of this invention was established on the Matlab / Simulink simulation platform. Under the same main circuit parameters, grid parameters, and operating conditions, grid inductance disturbances, active power reference value disturbances, and control parameter changes were respectively set up to conduct in-depth analysis of the transient response of system frequency, DC side voltage, and active power.

[0098] By stimulating the frequency synchronization, DC-side energy balance, and active power regulation components in a grid-type virtual synchronous generator system with external disturbances, the coupled oscillation characteristics of various state variables under different control strategies are observed. If the proposed composite synchronization control strategy can reduce the oscillation amplitudes of frequency, DC-side voltage, and active power under the same disturbance conditions and shorten their decay time, it can be shown that the strategy can enhance the equivalent damping of the system and improve the transient stability under weak grid conditions.

[0099] (1) Verification of full-state response under grid-side inductor disturbance conditions Grid inductance Changes in [the voltage / voltage] directly alter the grid-connected line impedance and the system short-circuit ratio, thus affecting the power transmission relationship between the grid-connected converter and the power grid. When When a sudden change occurs, the equivalent grid strength of the system changes, and the coupling relationships between power angle, active power, frequency, and DC-side voltage are redistributed, easily inducing low-frequency oscillations. Therefore, selecting... The disturbance behavior simulation condition can effectively test the disturbance rejection capability and full-state damping performance of the control strategy under the condition of sudden change in power grid strength.

[0100] As shown in Figures 3(a) to 3(c), in the grid inductance Under disturbance conditions, the response results of the traditional control strategy and the composite synchronization control strategy proposed in this invention are compared. Figure 3(a) shows the frequency response, Figure 3(b) shows the DC-side voltage response, and Figure 3(c) shows the active power response. As can be seen from the figures, under the traditional control strategy, the system... After the disturbance, significant damped oscillations occurred, with weak damping characteristics in frequency, active power, and DC-side voltage, and the oscillation duration was relatively long. In contrast, after adopting the composite synchronous control strategy of this invention, the oscillation amplitudes of all three electrical quantities were significantly reduced, and the waveforms after the disturbance recovered to near the steady state more quickly. This result indicates that when a sudden change in grid inductance leads to a change in grid strength, the control strategy proposed in this invention can effectively suppress coupled oscillations between frequency, DC-side voltage, and active power, and improve the transient damping performance of the system.

[0101] (2) Tracking performance verification under active power reference value command step Active power reference value The changes are primarily used to test the system's power command tracking capability. When a step change occurs, the grid-type virtual synchronous generator needs to readjust its active power output through a frequency synchronization mechanism, while the DC side needs to provide or absorb corresponding transient energy. If the control strategy lacks sufficient damping or the DC side energy coordination capability is insufficient, the active power regulation process will further cause frequency oscillations and DC side voltage fluctuations. Therefore, Disturbance conditions can reflect the dynamic tracking performance and energy balance capability of the proposed control strategy under power command variation conditions.

[0102] As shown in Figures 4(a) to 4(c), at the active power reference value Under disturbance conditions, the two control strategies are further compared and analyzed. Figure 4(a) shows the frequency response, Figure 4(b) shows the DC-side voltage response, and Figure 4(c) shows the active power response. As can be seen from the figures, under the traditional control strategy, The disturbance caused significant oscillations in frequency and active power, along with substantial transient drops and fluctuations in the DC-side voltage. This indicates that changes in the power reference value affect the DC-side energy balance through the power synchronization stage, further inducing multivariable coupled oscillations. After adopting the composite synchronization control strategy of this invention, the amplitude of frequency oscillations decreased, the active power response became smoother, and the impact and subsequent fluctuations of the DC-side voltage were weakened. These results demonstrate that the control strategy of this invention not only improves the immunity to grid inductive disturbances but also enhances the dynamic tracking capability and DC-side voltage stability when the active power reference value changes.

[0103] (3) Matching control coefficient Adaptive Impact Analysis of Changes on System Dynamic Response Matching control coefficient It is mainly used to adjust the coupling strength between the dynamic DC-side voltage and the dynamic AC-side synchronization. In the composite synchronization control structure, the DC-side voltage not only reflects the DC-side energy state of the converter, but also affects the AC-side frequency and power response through the matching control channel. Therefore, in Change under disturbance conditions This simulation can analyze the impact of changes in DC-side energy regulation capability on system frequency, active power, and DC-side voltage transient processes. It can be used to determine... Does it have the function of improving system damping and suppressing multivariable coupled oscillations?

[0104] As can be seen from Figures 5(a) to 5(c), It primarily affects the coupling and regulation capability between the dynamic DC-side voltage and the dynamic AC-side synchronization. (Smaller) It is difficult to fully utilize the regulating effect of DC-side voltage on synchronous control, and the system in It still exhibits weakly damped oscillations under disturbance; appropriately increasing It can enhance energy balance regulation capabilities, reducing the oscillation amplitude of frequency, DC-side voltage, and active power, and accelerating the decay rate. Therefore, It is an important parameter affecting the damping enhancement effect of the composite synchronous control strategy.

[0105] (4) Power synchronization control coefficient Adaptive Impact Analysis of Changes on System Synchronous Tracking Performance Power synchronization control coefficient The main influence is on the intensity of the adjustment of active power deviation to system synchronization dynamics. In a grid-type virtual synchronous generator, there is a tight coupling relationship between active power and frequency, and the gain of the power synchronization control channel directly affects the system's synchronization recovery speed and power oscillation damping after a disturbance. When the power is low, the power synchronization regulation is insufficient, and the system may experience prolonged damped oscillations; when When appropriately increased, the power deviation can participate in synchronous regulation more quickly, thereby improving the transient response of frequency and active power. Therefore, in Change under disturbance conditions This can be used to examine the impact of the power synchronization channel on the dynamic stability of the system.

[0106] As shown in Figures 6(a) to 6(c), in Under disturbance conditions, keep all other parameters unchanged and only change the power synchronization control parameters. The impact of this on the system's dynamic response is analyzed. Figure 6(a) shows the frequency response, Figure 6(b) shows the active power response, and Figure 6(c) shows the DC-side voltage response. As can be seen from the figures, Changes will directly affect the power synchronization dynamics of the system. (Small) Below this, the oscillations after system disturbance are more pronounced, and the frequency and active power decay rates are slower; with The increased power synchronization enhances the system's synchronous power regulation, reduces the frequency and active power oscillation amplitude, and improves the response convergence speed. The DC-side voltage response also improves, indicating that the enhanced power synchronization channel can indirectly weaken the disturbance effect of AC-side power oscillations on the DC-side voltage.

[0107] As can be seen from Figures 6(a) to 6(c), This mainly affects the adjustment intensity of the power synchronization control stage. Appropriately increasing... It can enhance the system's ability to The ability to synchronize and recover from disturbances improves the damping level of frequency and active power response. However... The tuning of the control signal needs to match the system's phase margin. If the value is too large, it may lead to an overly strong control channel and introduce new transient shocks. Therefore, Selection should be made under the premise of ensuring stability margin.

[0108] The simulation results from Figures 3(a) to 6(c) show that the traditional control strategy exhibits significant weakly damped oscillations under both grid inductive disturbances and active power reference value disturbances, with clear coupling between frequency, DC-side voltage, and active power. After adopting the composite synchronous control strategy of this invention, the system oscillation amplitude is reduced, the decay rate is accelerated, and the transient response is significantly improved. Further parameter variation results indicate that… The main improvement is the coordination between the DC-side voltage and the synchronous dynamics. The main benefit is enhanced power synchronization regulation capability. The combined effect of these two technologies can effectively improve the dynamic stability of grid-connected virtual synchronous generator systems under grid intensity disturbances. 4. Stability, robustness, and dominant eigenvalue analysis and control parameter tuning To further verify the impact of the composite synchronization control strategy of this invention on the system's closed-loop stability margin, robustness, and dominant oscillation mode, the matching control coefficients are further revealed from the location of the closed-loop poles. With power synchronization control coefficient The effect of oscillatory mode damping, based on the global open-loop transfer function Establish the closed-loop characteristic equation. and When explicitly used as a control parameter, the closed-loop characteristic equation can be expressed as: (14) In the formula, Represents the closed-loop characteristic function; This refers to the power synchronization control coefficient; The control coefficients are used for matching. All roots of equation (14) are the closed-loop characteristic roots of the system. For the i-th conjugate oscillation characteristic root, it can be written as: (15) in, The real part of the characteristic root determines the exponential decay or divergence rate of this oscillation mode; The imaginary part of the characteristic root determines the oscillation angular frequency of this mode. When When the value is less than 0, the corresponding mode is located in the left half of the complex plane and decays with time; when other conditions are similar, the more negative the real part of the characteristic root is, the faster the oscillation decays.

[0109] (16) (17) (18) In the formula, Let be the natural angular frequency of the i-th mode; The damping ratio; The corresponding oscillation frequency is given. The equal-damping-ratio ray in the root locus plot can be used to directly determine the damping region where the key conjugate eigenvalues ​​are located. Therefore, by observing the positional changes of the root locus relative to the imaginary axis and the equal-damping-ratio line, stability, decay rate, and oscillation damping level can be evaluated simultaneously.

[0110] Specifically, fixed power synchronization control coefficient Continuously change the matching control coefficient By solving equation (14) point by point, the key conjugate eigenvalues ​​can be obtained. The changing closed-loop root locus. Figures 7(a) to 7(c) show a comparison of the root trajectories of the key oscillation modes between the conventional VSG control and the composite synchronization control strategy of this invention.

[0111] As shown in Figure 7(a), when At that time, with As the damping ratio increases, the key conjugate oscillation characteristic roots under the composite synchronous control of this invention generally migrate to a region with a higher damping ratio, and the root locus is generally located in the left region of the key oscillation mode under traditional VSG control. As can be seen from equations (15) to (17), this change corresponds to the real part of the key mode being more negative and the damping ratio being increased, indicating that the matched control branch can transform the DC bus energy state feedback into a damping reshaping of the dominant oscillation mode, thereby improving the attenuation capability of the system's low-frequency oscillations.

[0112] As shown in Figure 7(b), when At the same time, under another set of power synchronization control parameters, the key characteristic roots still follow The damping shifts towards a stronger damping region due to changes. This result indicates that the improvement in damping of key oscillation modes by matched control does not depend on a single [factor]. The value is not determined by the power synchronization control intensity, but rather by the ability to maintain the same basic operating trend under different power synchronization control intensities, thus demonstrating the adaptability of the composite synchronization control of this invention to changes in control parameters.

[0113] As shown in Figure 7(c), when When the composite synchronization control of this invention is introduced, the key oscillation eigenvalues ​​are still in a more favorable damping region compared to the key modes of traditional VSG control, indicating that the composite control can significantly improve the damping level of the key oscillation modes. With... As the value increases further, some root loci tend to move closer to the imaginary axis within the investigated parameter range, but they remain in the left half-plane and exhibit good damping. This phenomenon indicates that... Its function is not simply "the bigger the better," but should be related to... The remaining system parameters are selected in a coordinated manner to simultaneously consider both damping improvement and stability margin.

[0114] A horizontal comparison of Figures 7(a), 7(b), and 7(c) shows that, in this embodiment, the... Within the range of values, as Increasing the value from 0.0002 to 0.0005 and 0.0012, the key oscillation modes under composite synchronization control generally show a trend of migrating to a higher damping region, indicating that appropriately increasing the strength of the power synchronization control channel is beneficial to improving system damping. Combined with different... The changes in the root locus can be seen from this. The synchronization dynamics are primarily reshaped through a direct power-frequency channel. The system mainly reshapes the energy coordination dynamics through a DC voltage-frequency channel. Both channels work together to change the coefficients and key characteristic roots of the closed-loop characteristic equation, thereby enabling the system to maintain good damping and stability over a wide range of control parameter variations.

[0115] As shown in Figures 8(a) and 8(b), respectively... and The impact on the active power step response. As shown in Figure 8(a), with... The change in the active power step response significantly alters the oscillation amplitude and decay rate, indicating that the power synchronization control parameters directly affect the damping characteristics of the active power component of the system. As shown in Figure 8(b), The changes also alter the transient process of the active power step response, indicating that the matching control channel indirectly affects the active power response by adjusting the dynamic relationship between the DC-side voltage and the AC-side power. The results in Figures 8(a) and 8(b) are consistent with the time-domain simulation patterns in Figures 5(a) to 5(c) and 6(a) to 6(c), further illustrating... and It is a key control parameter that determines the dynamic performance of the system.

[0116] like Figure 9 As shown, the active power step responses are compared under the following conditions: without the control strategy of this invention, with matched control plus VSG control, with power synchronization control plus VSG control, and with the composite synchronization control strategy of this invention. Without the control strategy of this invention, the active power response has a large overshoot, a long oscillation duration, and the system exhibits weak damping characteristics. Using matched control alone or power synchronization control alone reduces active power oscillations, but the improvement is still limited. With the composite synchronization control strategy of this invention, the active power overshoot is further reduced, the oscillation decay rate is further accelerated, and the overall dynamic response is better than that of a single auxiliary control strategy. These results indicate that there is a complementary effect between matched control and power synchronization control; their combination can simultaneously improve DC-side energy regulation and AC-side power synchronization.

[0117] Based on the aforementioned global open-loop transfer function, Nyquist stability analysis was performed, and the phase margin, gain margin, peak sensitivity, time delay margin, and dominant closed-loop eigenvalues ​​were further calculated. Figure 10 As shown in the left figure, the Nyquist trajectories corresponding to the two control methods exhibit significantly different shapes globally. Without the proposed control, the trajectory is closer to the critical point (…). (1, j0); After adopting the proposed composite synchronous control, the trajectory has a larger separation distance relative to the critical point. Figure 10 The right figure shows a magnified view of the local area near the critical point. It can be further observed that the minimum distance without the proposed control is approximately 0.116, while the minimum distance with the proposed control is approximately 0.696. This change indicates that the composite synchronization control reshapes the open-loop frequency response, moving the system away from the Nyquist critical instability boundary.

[0118] For the open-loop transfer function L(s), the closed-loop sensitivity function is defined as: (19) Nyquist trajectory to critical point ( The minimum distance between (1, j0) is defined as: (20) The peak sensitivity and minimum distance satisfy the following formula, where abs(·) represents the complex modulus: (twenty one) Therefore, a larger Nyquist minimum distance corresponds to a smaller peak sensitivity, meaning the closed-loop system has stronger robustness to model uncertainties, parameter perturbations, and external disturbances. Table 2 shows that without the proposed control, the peak sensitivity is 8.605, corresponding to a minimum distance of approximately 1 / 8.605 = 0.116; with the proposed control, the peak sensitivity decreases to 1.436, corresponding to a minimum distance of approximately 1 / 1.436 = 0.696. Figure 10 The local Nyquist results shown in the right figure are consistent.

[0119] Phase margin and gain margin reflect the margin remaining from the instability boundary when the open-loop amplitude and phase undergo additional changes, respectively. Assume the gain crossover frequency satisfies |L(jω) gc If |=1, then the phase margin can be expressed as: (twenty two) Assume the phase crossover angular frequency satisfies ∠L(jω) pc )= 180°, then the gain margin can be expressed as: (twenty three) For phase lag caused by additional pure time delay, the allowable time delay margin can be approximated by the phase margin: (twenty four) Table 2 shows the stability and robustness performance indicators of Nyquist with and without the proposed control, including phase margin, gain crossover frequency, gain margin, peak sensitivity, and time delay margin. As shown in Table 2, after adopting the proposed control, the phase margin increases from 6.859° to 42.134°, and the gain margin increases from 5.766 dB to 12.188 dB, indicating that the system has a larger tolerance range for additional phase lag and open-loop gain variations. The gain crossover frequency increases from 37.807 rad / s to 46.733 rad / s, indicating that while maintaining a significantly improved stability margin, the closed-loop regulation bandwidth is not reduced due to the increased damping. The peak sensitivity decreased from 8.605 to 1.436, a reduction of approximately 83.3%, indicating a significant decrease in sensitivity to parameter perturbations and modeling uncertainties. The delay margin increased from 3.167 ms to 15.736 ms, an increase of approximately 4.97 times, demonstrating that the proposed control has a stronger tolerance to the digital control delays introduced by sampling, calculation, and modulation updates.

[0120] Table 2. Nyquist stability and robustness indices

[0121] To further evaluate dynamic performance from the perspective of closed-loop eigenvalues, the closed-loop characteristic equation can be expressed as: (25) Let the dominant conjugate closed-loop eigenvalues ​​be: (26) Its dominant mode damping ratio is expressed as: (27) For a stable dominant second-order decaying mode, since the real part of the dominant pole σd < 0, its 2% settling time can be approximated as: (28) Table 3 shows the dominant closed-loop mode and dynamic performance indicators with and without the proposed control, including the dominant closed-loop conjugate poles, dominant mode damping ratio, and estimated 2% settling time. As shown in Table 3, the imaginary part of the dominant oscillation mode is approximately 38–39 s under both control methods. - ¹ indicates that the main comparison is still the same low-frequency oscillation mode; after adopting the proposed control, the real part of the dominant pole changes from 2.2937 s - ¹Move to 18.120 s - ¹, significantly away from the imaginary axis, indicating a marked increase in the exponential decay rate of this oscillation mode. The corresponding damping ratio increases from 0.0600 to 0.4196, approximately 7.0 times the original; the 2% settling time estimated by equation (28) is shortened from 1.744 s to 0.221 s, a reduction of approximately 87.3%. This result is consistent with... Figure 9 The variation law of the key feature roots in the root trajectory migrating to the higher damping region is consistent, which further verifies the effectiveness of composite synchronous control in oscillation suppression from the closed-loop mode level.

[0122] Table 3 Dominant Closed-Loop Mode and Dynamic Performance Indicators

[0123] Based on the root locus analysis of Figures 7(a) to 7(c), Tables 2 and 3, and... Figure 10The Nyquist results show that the proposed composite synchronous control improves stability characteristics at three levels simultaneously: First, the Nyquist trajectory moves away from the critical point, increasing the global closed-loop stability margin; second, the peak sensitivity is reduced and the time delay margin is increased, improving the system's robustness to parameter uncertainties and digital control delays; third, the dominant conjugate eigenvalues ​​move significantly to the left of the complex plane and into the higher damping region, improving the damping of the dominant oscillation mode and shortening the transient convergence time. Therefore, the PSC and MC branches do not merely change a single time-domain response index, but rather achieve a synergistic improvement in stability margin, robustness, and transient damping performance by jointly reshaping the open-loop frequency response and closed-loop eigenvalue distribution.

[0124] For any power synchronization control coefficient and matching control coefficient The candidate combinations are first determined by obtaining all eigenvalues ​​from the closed-loop characteristic equation and determining the dominant mode damping ratio; simultaneously, the phase margin, gain margin, peak sensitivity, and time delay margin are calculated from the corresponding open-loop transfer function. Only when the dominant eigenvalue is located in the left half-plane, the dominant damping ratio meets the preset requirements, and the Nyquist stability margin meets the preset threshold, is the parameter combination determined as a feasible control parameter. This avoids simply pursuing larger values. or This leads to a decrease in stability margin.

[0125] In practical implementation, the feasible region of parameters can be represented as a set of parameters that satisfy the following joint constraints: (29) In the formula, Ω represents the feasible region of the control parameters; This represents the minimum permissible dominant mode damping ratio; Indicates the minimum allowable phase margin; This represents the maximum permissible peak sensitivity. This joint constraint applies to Figures 7(a) through 7(c). , Influence on the location of eigenvalues ​​and Figure 10 The global stability margin and robustness indices represented in Table 2 can be used as the basis for implementing the coordinated tuning of control parameters in this invention.

[0126] like Figure 11 As shown, the system phase margin of the composite synchronous control strategy of this invention is analyzed under different short-circuit ratio (SCR) conditions. The results show that within the investigated SCR range, the system minimum phase margin remains positive, and the global minimum is approximately [value missing]. This indicates that even under relatively weak grid conditions, the system still possesses a certain stability margin, and no instability scenarios with zero or negative phase margins have occurred. As the SCR increases, the system phase margin generally shows an increasing trend, indicating that enhanced grid strength is beneficial to improving the system's stability margin. This result further demonstrates that the control strategy of this invention has good stability adaptability under a wide range of grid strength variations.

[0127] 5. Experimental verification To further verify the effectiveness, modeling accuracy, and adaptability to actual control environments of the method of this invention, a hardware-in-the-loop (HIL) experimental platform was built and semi-physical verification was carried out. For example... Figure 12 As shown, the experimental platform consists of a real-time HIL simulation model, a grid-type inverter controller, an I / O interface board, an oscilloscope, and a host computer responsible for parameter configuration and experimental data storage. The real-time simulator is used to run the converter main circuit and the equivalent grid model, and employs... The computational step size is used to complete real-time circuit-level simulation to ensure accurate characterization of the system's fast transient processes and oscillation characteristics. Meanwhile, the grid-type inverter controller follows... The sampling frequency is controlled by a control program, enabling the proposed control algorithm to respond to measurement signals and output control commands in real time during the HIL closed-loop experiment, thereby ensuring the reliability of the experimental verification results.

[0128] As shown in Figures 13(a) and 13(b), experimental waveforms were compared between the systems without and with the control strategy of this invention when grid impedance was disturbed. Without the control strategy of this invention, the system's active power, frequency, and DC-side voltage all exhibited significant oscillations after the grid impedance disturbance, and the oscillations lasted for a long time, indicating that the traditional control strategy was insufficiently damped under sudden changes in grid impedance. With the composite synchronous control strategy of this invention, the oscillation amplitude after the disturbance was significantly reduced, the waveform recovery speed was improved, and the system could return to a stable operating state more quickly. The experimental results are consistent with the simulation results, proving that the control strategy of this invention can effectively suppress transient oscillations caused by grid impedance disturbances.

[0129] As shown in Figures 14(a) and 14(b), the experimental responses were further compared when the active power reference value was disturbed, with and without the control strategy of this invention. Without the control strategy of this invention, the active power disturbance caused significant dynamic oscillations, accompanied by fluctuations in DC-side voltage and frequency. With the composite synchronous control strategy of this invention, the active power could be dynamically regulated more smoothly, and transient fluctuations in frequency and DC-side voltage were suppressed. These results demonstrate that the control strategy of this invention not only improves the immunity to disturbances caused by grid impedance disturbances but also enhances the dynamic tracking performance when the power reference value changes.

[0130] As shown in Figures 15(a) to 15(d), respectively in Adaptive experiments of composite frequency control were conducted with different matching control coefficients of 0.002, 0.01, 0.1, and 0.2. The experimental results show that... Changes can significantly affect the transient oscillation characteristics of a system after a disturbance. When When the voltage is small, the system's ability to coordinate with changes in DC-side voltage is insufficient, and significant oscillations still exist after disturbances; as the voltage decreases... By appropriately increasing the value, the amplitude of system waveform oscillation decreases, the recovery speed accelerates, and the DC-side voltage and power response becomes more stable. This result verifies the simulation findings regarding... The judgment of the influencing pattern, that is, appropriately increasing... It helps to enhance the matching control effect and improve the system damping performance.

[0131] As shown in Figures 16(a) to 16(d), further in Adaptive experiments on composite frequency control were conducted using different synchronization control coefficient values ​​of 0.0001, 0.0003, 0.0005, and 0.001. The results show that... Changes in this directly affect the regulation capability of the power synchronization control channel. Smaller... Below, the power and frequency oscillations of the system after disturbance are more obvious; with Within a reasonable range, increasing the value of the variable reduces the oscillation amplitude, improves the transient recovery speed, and enhances the system's synchronization stability. This experimental result is consistent with the simulation results shown in Figures 6(a) to 6(c), indicating that... It is an important parameter affecting the synchronous damping of system power.

[0132] The comprehensive experimental results show that the composite synchronous control strategy of this invention can effectively reduce the transient oscillation amplitude of the system, accelerate the oscillation decay rate, and improve the dynamic coordination performance among frequency, DC-side voltage, and active power under both grid impedance disturbances and active power reference value disturbances. and The experimental results under the given values ​​are consistent with the simulation results, further verifying the effectiveness, parameter adaptability, and engineering feasibility of the control strategy of this invention.

[0133] Example 2: Stability Evaluation System for Composite Synchronization Control Strategy This embodiment provides a stability evaluation system for a composite synchronous control strategy. This system is used to perform the composite synchronous control modeling, global open-loop transfer function derivation, and quantitative stability evaluation method described in Embodiment 1. The system can run on industrial control computers, host computers, embedded controllers, digital signal processors, microprocessors, edge computing terminals, or network-based control platforms. It can also be deployed as part of controller parameter tuning software, simulation analysis software, or online stability monitoring programs.

[0134] This system is primarily used to establish a global small-signal model considering AC-side power coupling, DC-side capacitor dynamics, and a composite synchronization control law, given converter system topology parameters, DC bus parameters, controller parameters, and grid operating conditions. Based on the model, it outputs the system's phase margin, amplitude margin, dominant oscillation mode, parameter sensitivity, and stability boundaries under wide short-circuit ratio conditions. Unlike stability analysis methods based solely on the traditional VSG frequency-power closed loop, this embodiment integrates power synchronization control, matching control, and virtual synchronization control into a unified frequency regulation framework, thereby more accurately reflecting the dynamic stability characteristics of the grid-type system under grid strength disturbances and power command disturbances.

[0135] Specifically, the composite synchronization control strategy stability evaluation system includes the following functional modules.

[0136] 1. Parameter acquisition and operating condition initialization module.

[0137] This module is used to acquire or read electrical parameters, controller parameters, and operating condition parameters of the converter system. Electrical parameters include AC-side filter inductance, AC-side filter resistance, grid-side equivalent inductance, grid-side equivalent resistance, DC bus capacitance, DC bus steady-state voltage, rated angular frequency, output voltage steady-state value, grid-connected current steady-state value, active power steady-state value, and reactive power steady-state value. Controller parameters include the virtual inertia coefficient and damping coefficient in virtual synchronous generator control, and power synchronization control coefficient. Matching control coefficient DC voltage outer loop proportional and integral coefficients, reactive voltage loop parameters, and power measurement filter parameters.

[0138] This module can also generate a set of operating conditions to be evaluated based on the input short-circuit ratio (SCR), grid-side impedance parameters, or disturbance type. For example, when it is necessary to evaluate the system's adaptability to switching between strong and weak power grids, the parameter acquisition and operating condition initialization module can sequentially generate multiple equivalent grid-side impedance combinations according to a preset SCR range; when it is necessary to evaluate the sensitivity of controller parameters, this module can generate different... and The set of parameters under the given values. This module enables subsequent modeling and stability analysis to be completed on a unified data baseline.

[0139] 2. Small signal modeling module on the AC side.

[0140] This module is used to establish a small-signal coupling model between AC side voltage, current, power angle, voltage amplitude and output active and reactive power based on the AC side circuit topology and power measurement filtering relationship of the converter.

[0141] Specifically, the module first establishes the frequency domain relationship between the converter output voltage, grid voltage, filter impedance, and grid-connected current based on Kirchhoff's voltage law; then, it performs small-signal linearization processing on the output voltage, grid-connected current, power angle, and voltage amplitude near the specified steady-state operating point; and finally, it obtains the matrix mapping relationship between power angle disturbance, voltage amplitude disturbance, and power disturbance by combining the first-order filtering definition of active power and reactive power.

[0142] Through the above processing, the AC side small-signal modeling module can obtain... , , and The transfer function components for decoupling the AC link. Among them, Characterizing the effect of power angle disturbance on active power. Characterizing the impact of voltage amplitude disturbances on active power. Characterizes the effect of power angle disturbance on reactive power. This module characterizes the impact of voltage amplitude disturbances on reactive power. Therefore, it provides the AC-side power coupling basis for deriving the closed-loop transfer function of the entire system.

[0143] 3. DC capacitor dynamic coupling module.

[0144] This module is used to establish the active power balance relationship on both sides of the DC bus capacitor and to provide a unified description of the dynamics of DC bus voltage control, capacitor charging and discharging, and AC side active power.

[0145] Specifically, this module constructs a small-signal dynamic equation for the DC bus voltage based on the energy balance relationship between the DC-side input power, AC-side output active power, DC bus voltage, and DC bus capacitance. Simultaneously, the module incorporates the proportional-integral (PI) regulation characteristics of the DC voltage outer-loop controller into the model to describe the effect of DC bus voltage deviation on the upstream input power or equivalent input current. By performing frequency domain transformation and linearization on the above relationships, a decoupling transfer function considering the charging and discharging characteristics of the DC capacitor can be obtained. .

[0146] The purpose of this module is to incorporate the DC-side energy state into the converter's frequency and power dynamic analysis. This allows stability assessment to move beyond simply considering AC-side impedance and power, and instead reflect the coupling relationship between DC bus voltage fluctuations, upstream energy throughput, and AC-side active power output. This coupling relationship is crucial for constructing the matching control frequency channel and evaluating the system's damping reshaping capability.

[0147] 4. Composite synchronous control motion equation construction module.

[0148] This module is used to introduce power synchronization control feedforward and matching control feedforward, and to build a composite synchronization frequency control model based on the motion equations of a traditional virtual synchronous generator.

[0149] Specifically, traditional VSG control drives the virtual rotor motion equation through the deviation between the active power reference value and the actual active power, thereby forming the inertial frequency component. Power synchronization control uses the power synchronization coefficient... The active power deviation is directly fed forward to the frequency regulation node to improve the synchronization establishment speed after power disturbances. Matching control uses matching control coefficients. The DC bus voltage deviation is mapped to the frequency loop, which is used to feed back the DC side capacitor energy status to the AC side frequency adjustment process in real time.

[0150] This module superimposes the VSG inertial frequency component, PSC synchronization frequency component, and MC energy coordination frequency component to the same frequency addition point to generate a small-signal regulation equation for the system output angular frequency. Subsequently, by integrating the output angular frequency, the change in phase angle within the converter is obtained and further used to generate the voltage modulation signal. Thus, the system can integrate inertial support, fast synchronization, and DC-side energy coordination control without introducing additional complex high-order state variables.

[0151] 5. Integrated module for closed-loop transfer function of the whole system.

[0152] This module is used to establish the mathematical equations output by the closed-loop AC side small-signal modeling module, the DC capacitor dynamic coupling module, and the composite synchronous control motion equation construction module, eliminating intermediate electrical state variables and obtaining the global open-loop transfer function model under the composite synchronous control strategy. .

[0153] Specifically, this module will first connect the communication side. The power decoupling matrix is ​​combined with the reactive voltage closed-loop relationship to obtain the closed-loop transfer relationship between power angle, voltage amplitude, and output power; then the DC side... The dynamic coupling term is introduced into the active frequency regulation channel; finally, the dynamic coupling term is introduced into the active frequency regulation channel; and The feedforward adjustment of the output frequency is uniformly incorporated into the open-loop channel, and the global open-loop transfer function is obtained after algebraic simplification. .

[0154] It is used to characterize the overall open-loop stability of a composite synchronous control system under specified operating points and specified control parameters. It is the core model for subsequent Nyquist plots, Bode plots, dominant mode analysis, and parameter tuning judgment.

[0155] 6. Stability quantitative assessment and judgment module.

[0156] This module is used to call the analytical unit or frequency domain analysis unit, based on the global open-loop transfer function. Plot Nyquist plots, Bode plots, pole distribution plots, or root locus plots, and automatically extract system stability evaluation metrics. Evaluation metrics include phase margin, gain margin, gain crossover frequency, phase crossover frequency, dominant oscillation mode frequency, damping ratio, and closed-loop dominant pole location.

[0157] When the Nyquist curve maintains a sufficient distance from the critical point, and the phase margin and amplitude margin meet preset thresholds, this module determines that the system has stable operation capability under the current parameters and operating conditions. When the Nyquist trajectory approaches the critical point, the phase margin decreases, the dominant pole approaches the imaginary axis, or the dominant oscillation mode damping is insufficient, this module outputs a stability risk warning and records the corresponding SCR, grid-side impedance, and other parameters. , And other control parameter information.

[0158] 7. Parameter sensitivity scanning and tuning auxiliary module.

[0159] This module is used to scan and analyze key parameters of composite synchronization control within a preset range. Preferably, this module can change the parameters of the composite synchronization control within a preset range. , Virtual inertia, virtual damping, DC voltage outer loop proportional-integral parameters, and short-circuit ratio (SCR) are calculated and recalculated for each set of parameters. And the corresponding stability index.

[0160] This module allows us to understand how parameter changes affect the system's stability margin and dynamic performance. For example, increasing parameters within a reasonable range... At this time, the damping reshaping effect of the DC bus voltage deviation on the frequency loop is enhanced, and the oscillation decay rate of the system's active power, DC voltage, and frequency waveform increases; increasing within a reasonable range At this time, the direct synchronization guidance capability of active power deviation on the output frequency is enhanced, and the system synchronization establishment speed is improved. This module can further output recommended tuning ranges for controller parameters, providing a basis for engineering debugging.

[0161] 8. Evaluation result output module.

[0162] This module outputs system stability assessment results in the form of data tables, graphs, assessment reports, or control parameter files. The output may include the global open-loop transfer function. Nyquist stability plot, Bode plot, phase margin, amplitude margin, dominant oscillation frequency, damping ratio, short-circuit ratio range, parameter sensitivity curves, and recommended values ​​for controller parameters.

[0163] Through the aforementioned functional modules, the system in this embodiment can perform stability prediction of the composite synchronous control strategy before the controller is put into operation, and can also assist in parameter tuning during the controller commissioning process. This system can not only determine whether the converter system is stable under a fixed grid strength, but also quantitatively assess the dynamic stability boundary of the system under conditions of wide short-circuit ratio variation, sudden changes in grid impedance, and changes in active power command, thereby improving the reliability and verifiability of the composite synchronous control strategy in engineering applications.

[0164] Example 3: Simulation Analysis and Model Matching Verification To verify the accuracy and reliability of the composite synchronization control strategy in Example 1 and the stability evaluation system in Example 2, this example conducts multi-condition time-domain simulations under a typical new energy grid-connected system topology, and compares the simulation results with those based on the global open-loop transfer function. The obtained frequency domain stability analysis results are compared and verified.

[0165] The simulation objects in this embodiment include the DC-side power supply, DC bus capacitor, three-phase voltage source converter, AC-side filter, grid-side impedance, and an equivalent model of the large power grid. The control system is compared using both traditional VSG control and the composite synchronous control strategy of this invention. The core state variables recorded in the simulation include the system output angular frequency, DC bus voltage, AC-side active power, and, when necessary, reactive power and phase angle changes. Through the time-domain response of these state variables, the system's overshoot, oscillation amplitude, decay rate, settling time, and steady-state error after disturbances can be comprehensively evaluated.

[0166] 1. Verification of full-state response under power grid intensity disturbances.

[0167] After the converter system has reached a steady state at a specified time, the equivalent inductance on the grid side is changed abruptly. Alternatively, equivalent impedance parameters can be used to simulate disturbances caused by changes in grid strength from a weak grid to a strong grid, or vice versa. These disturbances can be used to test the disturbance rejection and transient vibration damping capabilities of the composite synchronization control strategy under conditions of sudden changes in grid impedance.

[0168] In the traditional VSG control mode, when When abrupt changes occur, the system primarily relies on the motion equations of the virtual synchronous machine for frequency and power angle regulation. Because traditional single-VSG control fails to fully utilize the rapid synchronous feedforward effect of active power deviations and fails to map DC bus voltage fluctuations to the frequency loop in a timely manner, the system exhibits a noticeable weak damping response during disturbances. Specifically, this manifests as a large offset in the output angular frequency waveform, significant overshoot in the active power waveform accompanied by low-frequency decaying oscillations, and a prolonged recovery process in the DC bus voltage due to the temporary imbalance between input and output energy. This phenomenon indicates that traditional VSG control is prone to insufficient transient damping under conditions of sudden changes in grid strength.

[0169] After adopting the composite synchronization control strategy of this invention, the power synchronization control channel can quickly apply the active power deviation to the output frequency, improving the system's synchronous response speed to power changes; the matching control channel can introduce the DC bus voltage deviation into the frequency regulation stage, allowing the energy state of the front-stage DC capacitor to participate in the dynamic regulation of AC side power. These two, together with the inertial support of the traditional VSG, constitute a composite frequency regulation mechanism. Simulation results show that after adopting composite synchronization control, the transient overshoot of the output angular frequency, DC bus voltage, and active power are significantly reduced, the oscillation decay speed is accelerated, and the system can transition to a new steady-state operating point in a shorter time. This result verifies that the composite synchronization control strategy has stronger damping reshaping capability and dynamic stability maintenance capability against grid intensity disturbances.

[0170] 2. Tracking performance verification under step change of active power reference value.

[0171] Under stable system operation, the active power reference value A step disturbance is applied to assess the converter system's ability to dynamically track changes in power command and its proactive frequency support capabilities. The step disturbance can simulate adjustments to power command from renewable energy generation, changes in energy storage power dispatch, or the need for rapid response to grid-connected power.

[0172] In traditional VSG control mode, active power can eventually track the new reference value, but the transient process is quite drastic. Since power changes first affect the frequency response through a virtual inertia link, the system's synchronous adjustment speed is constrained by this inertia link. Simultaneously, the lack of sufficient coordination between the DC-side energy throughput process and the AC-side power regulation leads to a significant drop or rise in the DC bus voltage after a power step, accompanied by weakly damped oscillating recovery. The active power waveform exhibits a large peak overshoot, a long settling time, and a prolonged duration of transient oscillations.

[0173] After adopting the composite synchronization control strategy of this invention, the active power deviation is reduced through... A fast synchronization feedforward channel is formed, enabling the system to respond more quickly to changes in power commands; DC bus voltage deviation is addressed through... An energy coordination channel is established, enabling the DC-side capacitor to dynamically participate in frequency regulation. Simulation results show that the actual active power output can quickly and smoothly track the new target power value, the frequency fluctuation amplitude is reduced, and the DC bus voltage can recover to near steady state with higher damping after transient disturbances. After the transient ends, the system can maintain zero steady-state tracking error, indicating that the composite synchronous control strategy improves dynamic response speed without sacrificing steady-state accuracy.

[0174] 3. Matching control coefficient Analysis of the effect of changes on the virtual damping characteristics of the system.

[0175] To analyze the impact of the matching control channel on the system damping performance, this embodiment keeps the virtual synchronization control parameters, power synchronization control coefficients, and DC voltage controller parameters unchanged under grid intensity disturbance conditions, and only changes the matching control coefficients. The values ​​of are determined, and the changes in the output angular frequency, DC bus voltage, and active power are observed.

[0176] when When the value is small, the DC bus voltage deviation has a weak impact on the frequency regulation stage, and the energy state of the upstream DC capacitor is difficult to participate in the system power angle and frequency regulation in a timely manner. Therefore, after a sudden change in grid impedance, the system will still experience a large active power overshoot and a long oscillation decay process, the DC bus voltage recovery speed is slow, and the frequency waveform also exhibits certain underdamped characteristics.

[0177] along with As the value gradually increases within a reasonable range, the mapping effect of the DC bus voltage deviation to the output frequency is enhanced, and the change in DC-side capacitor energy can be more fully converted into damping regulation in the frequency loop. Simulation results show that a larger value... It can reduce the transient oscillation amplitude of active power and DC bus voltage, accelerate the oscillation decay rate, and improve the smoothness of the frequency recovery process. This result shows that the matching control coefficient... It is an important parameter affecting the virtual damping level of a composite synchronous control system. Appropriately increasing... It can enhance the energy coordination capability between the DC side and the AC side, thereby improving the transient stability of the grid-type system under grid intensity disturbances.

[0178] 4. Power synchronization control coefficient Analysis of the effect of changes on the synchronous tracking performance of the system.

[0179] To analyze the impact of the power synchronization feedforward channel on the system's synchronization response capability, this embodiment keeps the matching control coefficient, DC voltage controller parameters, and traditional VSG parameters unchanged under grid intensity disturbance conditions, only changing the power synchronization control coefficient. And observe the dynamic changes in system frequency, active power and DC bus voltage.

[0180] when At lower levels, the system primarily relies on the traditional VSG equation of motion for synchronization. The direct guiding effect of active power deviation on the output frequency is weak, resulting in a slow initial response speed of the active power and frequency waveforms after disturbance, significant overshoot in the first cycle, and a longer time required for the system to transition to a new steady state.

[0181] when When increased within a reasonable range, the active power deviation can act more directly on the frequency regulation node, improving the system synchronization locking speed and shortening the active power settling time after the disturbance. Simulation results show that a larger deviation... This improves the regulation capability of the power synchronization control channel, enabling the active power output to approach the new steady-state value more quickly, while reducing the transient fluctuation amplitude of the frequency and power waveforms. This result demonstrates that the power synchronization control coefficient... It is an important parameter affecting the system's fast synchronization performance. Appropriately increasing this coefficient can help improve the robustness of grid-type converters to changes in strong and weak power grids.

[0182] 5. Matching and verification between the global small-signal model and the time-domain simulation model.

[0183] To verify the accuracy of the model established by the stability evaluation system in Example 2, this example further uses a global open-loop transfer function. The obtained frequency domain analysis results are compared with the time domain simulation results.

[0184] First, different SCRs, different and different Substituting system parameters under the given conditions Nyquist and Bode plots were plotted, and phase margin, gain margin, and dominant oscillation mode frequency were extracted. Frequency domain analysis results show that, compared with traditional VSG control, the Nyquist trajectory of the system is further away from the critical instability point after adopting the composite synchronous control strategy, and the phase margin and gain margin in the Bode plot are improved, indicating that the system has a greater stability margin.

[0185] Secondly, the damping variation predicted by the frequency domain model is compared with the time domain simulation results. The results show that when When the value increases, the frequency domain model shows that the system damping margin improves, and the corresponding time domain simulation waveform shows that the oscillation attenuation of DC voltage and active power accelerates; when As the frequency domain increases, the synchronization response capability is enhanced, and the corresponding time domain simulation waveform shows a shortened active power settling time and a reduced frequency fluctuation amplitude. The consistent trends of these two changes indicate that the established global small-signal model accurately reflects the dynamic characteristics of the composite synchronization control system.

[0186] Finally, the traditional VSG control, the combined VSG and MC control, the combined VSG and PSC control, and the composite synchronization control of this invention are compared. Simulation results show that introducing MC or PSC alone can improve the dynamic performance of the system to some extent, but the improvement effect is limited by the single control mechanism. With the simultaneous introduction of MC and PSC, the system can balance DC-side energy coordination and AC-side rapid synchronization, further reducing active power overshoot, DC bus voltage fluctuation, and transient settling time. This demonstrates that the composite synchronization control strategy of this invention can improve the dynamic performance and stability margin of a networked system without increasing the system control order.

[0187] In summary, this embodiment utilizes grid strength disturbances, active power reference value step changes, and Parameter scanning, Parameter scanning and frequency-domain-time domain model matching verification demonstrate that the composite synchronization control strategy of this invention can effectively improve the damping performance, synchronization speed, and wide grid strength adaptability of the converter system. The theoretical analysis results obtained from the stability evaluation system in Example 2 are consistent with the time-domain simulation results of this example, indicating that the global small-signal model can be used for parameter tuning and engineering stability evaluation of the composite synchronization controller.

[0188] Example 4: Hardware-in-the-Loop (HIL) Semi-Physical Experiment Verification of the Controller To further verify the feasibility, effectiveness, and robustness of the composite synchronization control strategy of this invention in a real controller operating environment, this embodiment conducts experimental verification based on a hardware-in-the-loop (HIL) semi-physical experimental platform. This experiment is used to verify the dynamic response effect of the control strategy of Embodiment 1 when running in a real controller, and to verify the accuracy of the stability evaluation system of Embodiment 2 and the simulation model of Embodiment 3.

[0189] 1. The structure of the HIL hardware-in-the-loop experimental platform.

[0190] This embodiment of the HIL semi-physical experimental platform includes a real-time simulator, an actual grid-type converter controller, physical I / O interface boards, a multi-channel digital oscilloscope, and a host computer. The real-time simulator is used to run the converter main circuit, grid impedance, DC bus capacitance, AC filter, and equivalent grid model; the actual grid-type converter controller is used to run the composite synchronous control algorithm of this invention; the physical I / O interface boards are used to realize analog and digital interaction between the controller and the real-time simulator; the multi-channel digital oscilloscope is used to acquire key experimental waveforms such as active power, DC bus voltage, frequency, current, or voltage; and the host computer is used for parameter configuration, experimental condition switching, data recording, and control parameter debugging.

[0191] Preferably, the real-time simulator is The simulation step size is used to perform real-time calculations on the main circuit model to ensure accurate capture of the high-frequency transients and low-frequency oscillations of the network system during disturbances. The actual controller executes a composite synchronous control program according to the engineering control cycle and sends modulation signals or control commands to the real-time simulator through the I / O board, while simultaneously receiving voltage, current, DC bus voltage, and power measurement signals fed back by the real-time simulator. Through this closed-loop connection method, the dynamic performance and stability of the controller can be semi-physical verified without connecting a real high-power main circuit.

[0192] 2. Power grid impedance sudden change disturbance experiment.

[0193] During the experiment, the grid-type system in the HIL platform was first operated under steady-state conditions. Then, the grid-side impedance parameters were abruptly changed at a preset time to simulate external disturbances caused by changes in grid strength or line impedance. The experiment was conducted under conditions without and with the composite synchronization control strategy of this invention, and the response waveforms of active power, DC bus voltage, and frequency were recorded.

[0194] Without the composite synchronous control strategy of this invention, the system exhibits significant underdamped oscillations after a sudden change in grid-side impedance. Specifically, this manifests as a large overshoot in the active power waveform, slow oscillation decay, a long DC bus voltage recovery time, and fluctuations in the frequency waveform. This experimental phenomenon is consistent with the response characteristics of traditional VSG control in simulations, indicating that single VSG control suffers from insufficient damping and slow dynamic recovery under grid intensity disturbances.

[0195] After adopting the composite synchronous control strategy of this invention, experimental waveforms show that the transient oscillations of the system are significantly suppressed. The peak value of active power disturbance is reduced, the waveform decay rate is accelerated, the amplitude of DC bus voltage fluctuation is reduced, and the frequency recovery process is smoother. These results indicate that the PSC feedforward channel and MC energy coordination channel can effectively play their roles in practical controllers, enabling the system to have stronger anti-disturbance capability and faster steady-state recovery capability under the condition of sudden changes in grid impedance.

[0196] 3. Active power reference value step disturbance test.

[0197] Under stable system operation, a step change was applied to the active power reference value to verify the dynamic tracking capability of the composite synchronous control strategy of this invention to power command changes. Experiments compared the active power, DC bus voltage, and frequency response with and without the control strategy of this invention.

[0198] Without the control strategy of this invention, a step in the power reference value will cause significant power overshoot and oscillation. Although the active power can eventually reach the new reference value, the transient adjustment process is long, the DC bus voltage fluctuates significantly due to energy imbalance, and the frequency response also has a large transient offset.

[0199] After adopting the composite synchronization control strategy of this invention, the active power can track the new reference value more quickly and smoothly. Experimental waveforms show that the power overshoot amplitude is reduced, the oscillation convergence speed is improved, the settling time is shortened, and the DC bus voltage disturbance is effectively suppressed. These results demonstrate that the composite synchronization control strategy of this invention can not only improve the dynamic stability under grid impedance disturbances, but also improve the dynamic tracking performance when the power command changes, and maintain good steady-state accuracy after the transient ends.

[0200] 4. Matching control coefficient HIL parameter adaptability experiment.

[0201] To verify the damping adjustment effect of the matching control channel in the actual controller, this embodiment sets up multiple different sets of damping adjustment channels on the HIL platform. The value was determined, and the dynamic response of the system was observed under the same disturbance conditions.

[0202] when When the voltage is small, the DC bus voltage deviation has a weak reshaping effect on the frequency loop, and the system still exhibits significant active power oscillations and DC voltage fluctuations after the disturbance. With... Within a reasonable range, increasing the DC-side energy state enhances its involvement in frequency regulation. Experimental waveforms show that the active power oscillation amplitude decreases, the DC bus voltage recovery speed accelerates, and the frequency response becomes more stable.

[0203] The experimental results are consistent with the simulation conclusions in Example 3, indicating that... It can effectively adjust the damping level of the composite synchronous control system. Appropriately increase... This helps to enhance the matching degree between the dynamic regulation of the DC-side capacitor and the frequency regulation of the AC side, thereby improving the system's ability to suppress transient oscillations.

[0204] 5. Power synchronization control coefficient HIL parameter adaptability experiment.

[0205] To verify the synchronization adjustment capability of the power synchronization control channel in the actual controller, this embodiment further sets up multiple different sets of... The system response was collected under the same grid impedance disturbance or power step disturbance conditions.

[0206] when When the voltage is low, the system mainly relies on the VSG inertial element to complete frequency and power angle regulation. The power waveform recovery speed after disturbance is slow, and the synchronization establishment time is long. With... Within a reasonable range, increasing the active power deviation enhances its direct guiding effect on the output frequency. Experimental waveforms show that the system oscillation decay rate increases, the active power stabilizes to the target value more quickly, and the frequency fluctuation amplitude decreases.

[0207] The experimental results are consistent with the simulation analysis. The consistent impact on system synchronization performance indicates that the power synchronization control coefficient... These are important parameters affecting the fast response capability and power synchronization capability of a composite synchronous control system. Proper tuning is crucial. This can improve the synchronization stability of the converter system under different grid strength and disturbance conditions.

[0208] 6. Verification of the consistency between HIL experimental results and simulation and theoretical models.

[0209] To further verify the accuracy of the model established in this invention, this embodiment compares the HIL experimental waveform with the time-domain simulation results in Embodiment 3 and the frequency-domain stability evaluation results in Embodiment 2. The comparison results show that under conditions of grid impedance disturbance, active power reference value step, ... Parameter changes and Under operating conditions such as parameter changes, the oscillation decay trend, steady-state recovery process, and parameter influence law shown in the HIL experiment are consistent with the simulation analysis results.

[0210] Specifically, when a composite synchronous control strategy is adopted, the active power and DC bus voltage oscillation amplitudes in the HIL experiment are significantly smaller than those under traditional control conditions, and the system settling time is significantly shortened; when When the value increases, the DC-side voltage fluctuation suppression capability is enhanced in the experiment, which is consistent with the theoretical judgment that the matched control improves the system's damping level; when When the power synchronization speed is increased, the power synchronization speed in the experiment improves, which is consistent with the frequency domain analysis results of the power synchronization control enhancing the fast response capability.

[0211] Under full operating conditions, the dynamic response trends of the HIL experimental waveforms are highly consistent with those of the theoretical and simulation models, and the dynamic relative average error can be controlled within 4%. This result demonstrates that the composite synchronous control strategy of this invention can operate stably in actual controllers, and that the global small-signal modeling and stability assessment framework can accurately reflect the dynamic characteristics of the system, providing a reliable basis for controller design, parameter tuning, and stability boundary assessment in engineering applications.

[0212] In summary, this embodiment verifies the feasibility and effectiveness of the composite synchronization control strategy of the present invention in a real controller environment through hardware-in-the-loop semi-physical experiments on the controller. Experimental results show that the strategy can significantly reduce the transient oscillation amplitude of the system under grid impedance disturbances and power command disturbances, shorten the steady-state recovery time, improve the power synchronization speed and DC-side energy coordination capability, thereby enhancing the dynamic stability and robust adaptability of the grid-type system under wide grid intensity variation scenarios.

Claims

1. A composite synchronous control method for a grid-type converter system, characterized in that, Includes the following steps: Step 1: Establish a small-signal model of the AC side circuit of the converter in the synchronous rotating dq coordinate system, and establish a DC side capacitor power coupling model that includes the dynamics of the DC bus voltage based on the DC bus capacitor charging and discharging balance relationship. Step 2: Construct a composite synchronous frequency control loop and introduce virtual synchronous generator (VSG) control to obtain the inertial frequency component driven by active power deviation. Power synchronization control (PSC) is introduced to obtain the synchronization frequency component directly induced by the active power deviation. Matching control MC is introduced to obtain the energy coordination frequency component driven by the DC bus voltage deviation. ; Step 3: Convert the inertial frequency components Synchronization frequency components and energy coordination frequency components Feedforward superposition to the frequency fusion node constructs a unified composite synchronous frequency control law, generating the internal control angular frequency of the converter. The output phase angle is obtained by integration using an integrator. Step 4: Combine the internal voltage amplitude reference value generated by the reactive power-voltage control loop with the output phase angle to form a voltage modulation signal, which drives the converter to operate in a grid-type mechanism; Step 5: Combine the small-signal model of the AC side circuit, the power coupling model of the DC side capacitor, and the composite synchronous frequency control law to eliminate intermediate transient variables of the system and construct a global open-loop small-signal transfer function model for the entire system; Step 6: Based on the global open-loop small-signal transfer function model, the stability analysis and dynamic performance evaluation of the network system are carried out through the Nyquist stability criterion, phase margin stability region analysis under different short-circuit ratios, and active power step response.

2. The composite synchronization control method according to claim 1, characterized in that, In step 1: The small-signal model of the AC side circuit of the converter is based on the small-signal disturbance of the active power output of the AC side of the converter. and reactive power small signal disturbance For the output, linearization modeling is performed using the d-axis and q-axis components of the AC side output voltage and current of the converter at the steady-state operating point, as well as the corresponding small-signal fluctuations of voltage and current, as state variables; DC-side capacitor power coupling model based on DC bus capacitor DC bus voltage steady-state value DC bus voltage small-signal disturbance and small-signal disturbances from the input power of the preceding DC source. Establish.

3. The composite synchronization control method according to claim 1 or 2, characterized in that, In step 2: The active power control loop of the virtual synchronous generator (VSG) is transformed by Laplace, and the inertial frequency component is... The relationship between the active power deviation and the virtual rotational inertia coefficient J and the virtual rotor damping coefficient D is a second-order transfer function relationship. Power Synchronization Control (PSC) uses power synchronization coefficients. Establish a direct feedforward path from active power deviation to angular frequency, and synchronize frequency component. Equal to power synchronization coefficient The product of the active power small-signal deviation; Matched control (MC) uses matched control coefficients. Establish a feedforward reshaping channel for DC bus voltage deviation to the frequency loop, and coordinate the frequency components with energy. Equal to the matching control coefficient The product of the small-signal deviation of the DC bus voltage.

4. The composite synchronization control method according to claim 1 or 2, characterized in that, In step 3, within the unified form of the composite synchronous frequency control law, the small signal fluctuation of the control angular frequency inside the converter is... From inertial frequency components Synchronization frequency components and energy coordination frequency components The three components are linearly superimposed and synthesized at the frequency fusion node to form a unified small-signal motion oscillation equation after the recombination of multiple heterogeneous loops.

5. The composite synchronization control method according to claim 1 or 2, characterized in that, Step 5, the construction of the global open-loop small-signal transfer function model for the entire system includes the following sub-steps: The AC link is decoupled by mapping the AC side voltage amplitude fluctuation and converter power angle fluctuation to active and reactive power small signals, resulting in an AC decoupling matrix determined by the AC side impedance, modulation ratio and steady-state operating point. Proportional coefficient based on the front-end DC bus voltage controller (DVC) and integral coefficient The coupling transfer function between DC bus voltage fluctuation and active power is derived. By simultaneously eliminating all intermediate state variables, we obtain the global open-loop transfer function.

6. The composite synchronization control method according to claim 1 or 2, characterized in that, Step 6, stability analysis and dynamic performance evaluation include: Quantitative evaluation mapping construction: The Nyquist stability criterion is executed to determine the system stability tone of the closed-loop control model, the phase margin stability region boundary is plotted under wide short-circuit ratio variation conditions, and an active power step response is applied in the time domain to evaluate the dynamic response speed, oscillation suppression effect and steady-state recovery time domain index. System stability determination: The Nyquist trajectory is analyzed in real time. When the Nyquist trajectory maintains a preset safe distance from the critical point, and the system phase margin and amplitude margin both meet the preset minimum stability threshold, the network configuration is determined to have stable operation capability under the current operating conditions. Dynamic instability risk warning and parameter correlation storage: When the Nyquist trajectory approaches the critical point, the phase margin is lower than the minimum stability threshold, the dominant pole approaches the imaginary axis, or the damping ratio of the dominant oscillation mode of the system is insufficient, a synchronization stability risk warning is triggered, and the current short-circuit ratio, grid-side inductance disturbance, active power disturbance, and power synchronization control coefficient are recorded in correlation. Matching control coefficient And the proportional and integral coefficients of the front-end DC bus voltage controller (DVC).

7. A composite synchronous control system for a grid-type converter system, characterized in that, include: The model building module is used to build a small-signal model of the AC side circuit of the converter in the synchronous rotating dq coordinate system, and to build a DC side capacitor power coupling model that includes the dynamics of the DC bus voltage based on the DC bus capacitor charging and discharging balance relationship. The composite synchronous frequency control module includes a virtual synchronous generator (VSG) control unit, a power synchronization control (PSC) unit, and a matching control (MC) unit; the VSG control unit is used to generate inertial frequency components driven by active power deviation. ; The PSC unit is used to establish a direct feedforward channel for active power deviation towards angular frequency, generating synchronous frequency components. The MC unit is used to establish a feedforward reshaping channel for the DC bus voltage deviation to the frequency loop, generating energy-coordinated frequency components. ; The frequency fusion and phase output module is used to convert inertial frequency components. Synchronization frequency components and energy coordination frequency components Feedforward superposition to the frequency fusion node generates the internal control angular frequency of the converter according to the composite synchronous frequency control law. The phase angle is then integrated by an integrator and output. The modulation signal generation module is used to synthesize the internal voltage amplitude reference value output by the reactive power-voltage control loop and the output phase angle into a voltage modulation signal to drive the converter operation with a grid-type control mechanism. The transfer function modeling module is used to combine the small-signal model of the AC side circuit, the DC side capacitor power coupling model, and the composite synchronous frequency control law to eliminate intermediate transient variables of the system and establish a global open-loop small-signal transfer function model of the entire system. The stability assessment module is used to perform stability analysis and dynamic performance evaluation of the system based on the global open-loop small-signal transfer function model, utilizing the Nyquist stability criterion, phase margin stable region analysis, and active power step response.

8. The composite synchronous control system according to claim 7, characterized in that, When establishing the small-signal model of the AC side circuit of the converter, the model building module uses the small-signal disturbance of active power ΔP and the small-signal disturbance of reactive power at the AC side output of the converter. For the output, linear modeling is performed using the d-axis and q-axis components of the AC side output voltage and current at the steady-state operating point, along with their corresponding small-signal voltage and current fluctuations, as state variables; the DC side capacitor power coupling model uses the DC bus capacitor... DC bus voltage steady-state value DC bus voltage small-signal disturbance and small-signal disturbances from the input power of the preceding DC source. Establish.

9. The composite synchronous control system according to claim 7, characterized in that: The VSG control unit has a virtual rotational moment of inertia coefficient J and a virtual rotor damping coefficient D. After Laplace transform, the inertial frequency component... The relationship between the active power deviation and the active power deviation satisfies a second-order transfer function relationship with parameters J and D; The PSC unit has a power synchronization coefficient. Synchronization frequency component Equal to power synchronization coefficient The product of the active power small-signal deviation; The MC unit has matching control coefficients. Energy coordination frequency components Equal to the matching control coefficient The product of the small-signal deviation of the DC bus voltage.

10. The composite synchronous control system according to claim 7, characterized in that, The transfer function modeling module includes: The AC decoupling submodule is used to decouple the AC link, mapping the AC side voltage amplitude fluctuation and converter power angle fluctuation to active and reactive power small signals, and obtaining the AC decoupling matrix determined by the AC side impedance, modulation ratio and steady-state operating point. DC-coupled submodule for use with proportional gain based on the front-end DC bus voltage controller (DVC) and integral coefficient The coupling transfer function between DC bus voltage fluctuation and active power is derived. The global integration submodule is used to simultaneously eliminate all intermediate state variables to obtain the global open-loop transfer function; The stability assessment module includes: The stability determination submodule is used to analyze the Nyquist trajectory in real time. When the Nyquist trajectory maintains a preset safe distance from the critical point and the system's phase margin and amplitude margin both meet the preset minimum stability threshold, the system is determined to have stable operation capability. The early warning storage submodule is used to trigger a synchronization stability risk warning when the Nyquist trajectory approaches the critical point, the phase margin is lower than the minimum stability threshold, the dominant pole approaches the imaginary axis, or the damping ratio of the dominant oscillation mode is insufficient. It also records the current short-circuit ratio, grid-side inductance disturbance, active power disturbance, and power synchronization control coefficient. Matching control coefficient And the proportional and integral coefficients of the front-end DC bus voltage controller (DVC).

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