Synchronous control method for two-degree-of-freedom direct-current capacitor with damping characteristic

Abstract

The invention provides a synchronous control method for a two-degree-of-freedom direct-current capacitor with a damping characteristic, and belongs to the field of power systems. The method comprises the following steps: defining a direct-current capacitor synchronous control strategy with a damping characteristic; and based on the defined DC capacitor synchronous control strategy, designing a two-degree-of-freedom control framework, realizing decoupling of frequency adjustment and voltage control, and completing synchronous control of the two-degree-of-freedom DC capacitor. According to the invention, decoupling of frequency regulation and DC voltage control is realized.

Description

Technical Field

[0001] This invention belongs to the field of power systems, and particularly relates to a two-degree-of-freedom DC capacitor synchronous control method with damping characteristics. Background Technology

[0002] With the rapid development of power electronic equipment, a large number of distributed power sources are connected to the power grid through grid-connected inverters. However, due to the lack of inherent inertia and damping characteristics of synchronous generators, the large-scale connection of these inverters leads to a decrease in power system frequency stability, limiting the further absorption of new energy sources. To address this issue, the Virtual Synchronous Generator (VSG) control strategy has emerged. This strategy simulates the operating characteristics of a synchronous generator, introducing virtual inertia and virtual damping to enhance the system's frequency stability. However, existing VSG control is mainly applicable to battery systems with an ideal DC voltage source, or new energy equipment with front-end DC voltage control.

[0003] To reduce system costs, some scholars have proposed eliminating the front-end DC voltage control and replacing it with direct DC voltage regulation by the back-end inverter. To this end, two main control schemes have been developed: one is to cascade DC voltage control with VSG control, generating the active power reference value for VSG control through DC voltage control; the other is to adopt a DC capacitor synchronous control strategy, using DC capacitors to simulate the inertia of a synchronous generator, directly generating frequency and phase angle from the DC voltage.

[0004] For the first type of cascaded control strategy, due to the introduction of a DC voltage control loop, its overall dynamic characteristics no longer possess the inertia and damping characteristics of a traditional second-order system. Furthermore, some literature points out that this control method introduces negative damping, leading to a narrowing of the system's stability region and posing an instability risk in long-line scenarios—a problem that urgently needs to be avoided in grid-connected control. For the second type of DC voltage synchronization control strategy, although it can simulate the rotor inertia of a synchronous generator using a DC capacitor and possess a certain frequency support capability, it fails to mimic the damping characteristics of a synchronous generator, making the system highly susceptible to instability under disturbances. Existing technologies involve small-signal stability enhancement strategies based on the phase compensation principle, but this method carries the risk of inducing synchronization frequency oscillations. Existing technologies also involve introducing virtual resistors in the DC voltage synchronization outer loop to enhance damping, which is essentially equivalent to active power feedforward control and is also prone to inducing synchronization frequency oscillations. Summary of the Invention

[0005] To address the aforementioned shortcomings in the prior art, this invention provides a two-degree-of-freedom DC capacitor synchronous control method with damping characteristics. This method solves the problems of existing DC voltage synchronous control strategies, which, although capable of simulating the inertia of a synchronous generator, lack effective damping, leading to low-frequency oscillations and poor stability under disturbances. Furthermore, increasing the virtual inertia to enhance frequency support capability results in a decrease in DC voltage regulation performance, creating a performance conflict between frequency support and voltage control that is difficult to coordinate and optimize.

[0006] To achieve the above objectives, the technical solution adopted by this invention is: a two-degree-of-freedom DC capacitor synchronous control method with damping characteristics, comprising the following steps: S1. Define a DC capacitor synchronous control strategy with damping characteristics; S2. Based on the defined DC capacitor synchronous control strategy, a two-degree-of-freedom control framework is designed to decouple frequency regulation and voltage control, thereby completing the synchronous control of the two-degree-of-freedom DC capacitor.

[0007] Furthermore, S1 specifically refers to: Define the transfer function for DC voltage control as follows: ; Based on transfer function The disturbance from the DC voltage reference value was obtained respectively. DC voltage disturbance Closed-loop transfer function and network-side frequency disturbances Output active power disturbance Closed-loop transfer function ; against At that time, transfer function For a proportional-differential element, where, This indicates that traditional VSG control is affected by the active power reference value disturbance. Output active power disturbance The closed-loop transfer function, This indicates frequency disturbance on the traditional VSG control network side. Output active power disturbance The closed-loop transfer function; Based on the proportional-derivative relationship, the proportional coefficient is set. and differential coefficients Among them, the proportionality coefficient Corresponding to virtual inertia, the differential coefficients Provides the missing virtual damping in the strategy to complete the definition of the DC capacitor synchronous control strategy.

[0008] Furthermore, the transfer function The expression is as follows: ; in, Represents the Laplace operator. Indicates DC capacitor. Indicates virtual damping. Indicates the reactance of the grid-side line. Represents virtual inertia. This represents the system's reference angular frequency.

[0009] Furthermore, the closed-loop transfer function The expression is as follows: ; The closed-loop transfer function The expression is as follows: ; in, Indicates the system's reference angular frequency. Indicates DC capacitor. Indicates the reactance of the grid-side line. This represents the Laplace operator.

[0010] Furthermore, the proportionality coefficient and differential coefficients The expressions are as follows: ; ; in, Indicates DC capacitor. Represents virtual inertia. Indicates virtual damping. Indicates the reactance of the grid-side line. This represents the system's reference angular frequency.

[0011] Furthermore, S2 specifically refers to: exist Based on this, a two-degree-of-freedom control framework is introduced, wherein the two-degree-of-freedom control framework includes a cutoff frequency of A low-pass filter and a differential feedforward loop, a two-degree-of-freedom control framework are used to separate the frequency control from the voltage control channels; Based on a two-degree-of-freedom control framework, the disturbance from the DC voltage reference value is obtained separately. DC voltage disturbance Closed-loop transfer function and network-side frequency disturbances Output active power disturbance Closed-loop transfer function This achieves decoupling of frequency regulation and voltage control, enabling synchronous control of a two-degree-of-freedom DC capacitor.

[0012] Furthermore, the expression for the feedforward term of the two-degree-of-freedom control is as follows: ; in, Indicates the cutoff frequency as The low-pass filter, Indicates disturbance To output The transfer function, Indicates the output. Represents the Laplace operator. Indicates the reactance of the grid-side line. Indicates the system's reference angular frequency. Indicates DC capacitor. This represents the coefficients of the differential feedforward loop.

[0013] Furthermore, the expression for the coefficients of the differential feedforward loop is as follows: ; in, Represents the coefficients of the differential feedforward loop. Indicates DC capacitor. Indicates the reactance of the grid-side line. This represents the system's reference angular frequency.

[0014] Furthermore, the closed-loop transfer function The expression is as follows: ; in, This represents the Laplace operator.

[0015] Furthermore, the closed-loop transfer function The expression is as follows: ; in, Represents the Laplace operator. Indicates the system's reference angular frequency. Indicates the reactance of the grid-side line. Represents virtual inertia. This represents virtual damping.

[0016] The beneficial effects of this invention are: Compared with existing technologies, the dual-degree-of-freedom DC capacitor synchronous control strategy with damping characteristics proposed in this invention, combined with damping introduction and feedforward compensation, achieves decoupling of frequency regulation and DC voltage control, and has the following main advantages: (1) Achieving precise control of inertia and damping simulation: Traditional DC capacitor synchronous control strategies cannot simulate virtual damping, which makes the system prone to oscillation. This invention introduces a differential element, enabling the system to not only simulate inertia but also flexibly set virtual damping, achieving second-order dynamic characteristics similar to a synchronous generator and improving the system's frequency dynamic performance.

[0017] (2) Decoupling of DC voltage control to improve response speed: In traditional single-degree-of-freedom control, the DC voltage response is affected by inertia, making it difficult to simultaneously meet the requirements of fast voltage tracking and frequency support. This invention uses a dual-degree-of-freedom control strategy to make the DC voltage control independent of the inertia parameter and only subject to the low-pass filter cutoff frequency adjustment, thereby achieving both fast voltage response and stable frequency support. Attached Figure Description

[0018] Figure 1 This is a control block diagram of the control strategy proposed in this invention.

[0019] Figure 2 This is an existing traditional VSG control block diagram.

[0020] Figure 3 This is a closed-loop small-signal model for traditional VSG control.

[0021] Figure 4 The control block diagram is for the existing DC voltage control and VSG cascade.

[0022] Figure 5 This is a closed-loop small-signal model of DC voltage control and VSG cascade structure.

[0023] Figure 6 The control block diagram is for the existing synchronous control strategy of DC capacitors.

[0024] Figure 7 This is a closed-loop small-signal model for DC capacitor synchronous control.

[0025] Figure 8 This is a closed-loop small-signal model for the designed DC capacitor synchronous control.

[0026] Figure 9 This is a DC capacitor synchronous control strategy structure with damping characteristics.

[0027] Figure 10 This is a schematic diagram of a feedforward design for two-degree-of-freedom control.

[0028] Figure 11 This is a closed-loop small-signal model for a two-degree-of-freedom DC capacitor synchronous control strategy with damping characteristics.

[0029] Figure 12 A comparison of the power response of the traditional VSG and the proposed control strategy is presented.

[0030] Figure 13 The effect of different inertia parameters on DC voltage response under two-degree-of-freedom control.

[0031] Figure 14 This illustrates the effect of different low-pass filter cutoff frequencies on the DC voltage response under two-degree-of-freedom control.

[0032] Figure 15 This is a flowchart of the method of the present invention. Detailed Implementation

[0033] The specific embodiments of the present invention are described below to enable those skilled in the art to understand the present invention. However, it should be understood that the present invention is not limited to the scope of the specific embodiments. For those skilled in the art, various changes are obvious as long as they are within the spirit and scope of the present invention as defined and determined by the appended claims. All inventions utilizing the concept of the present invention are protected.

[0034] Example This invention proposes a two-degree-of-freedom DC capacitor synchronous control method with damping characteristics, the control structure of which is as follows: Figure 1 As shown in the figure. In this invention, the DC voltage control employs a proportional-derivative (PD) circuit to simulate the inertia and damping characteristics of a synchronous generator. Simultaneously, to achieve decoupling of the control channels, feedforward compensation based on a DC voltage reference value is added after the DC voltage control module, realizing dual-degree-of-freedom adjustment of frequency control and voltage control.

[0035] like Figure 15 As shown, this invention provides a two-degree-of-freedom DC capacitor synchronous control method with damping characteristics, the implementation method of which is as follows: S1. Define a DC capacitor synchronous control strategy with damping characteristics, and its implementation method is as follows: Define the transfer function for DC voltage control as follows: ; Based on transfer function The disturbance from the DC voltage reference value was obtained respectively. DC voltage disturbance Closed-loop transfer function and network-side frequency disturbances Output active power disturbance Closed-loop transfer function ; against At that time, transfer function For a proportional-differential element, where, Indicates the disturbance from the active power reference value Output active power disturbance The closed-loop transfer function, Indicates network-side frequency disturbance Output active power disturbance The closed-loop transfer function; Based on the proportional-derivative relationship, the proportional coefficient is set. and differential coefficients Among them, the proportionality coefficient Corresponding to virtual inertia, the differential coefficients Provides the missing virtual damping in the strategy to complete the definition of the DC capacitor synchronous control strategy; S2. Based on the defined DC capacitor synchronous control strategy, a two-degree-of-freedom control framework is designed to decouple frequency regulation and voltage control, thereby achieving synchronous control of the two-degree-of-freedom DC capacitor. The implementation method is as follows: exist Based on this, a two-degree-of-freedom control framework is introduced, wherein the two-degree-of-freedom control framework includes a cutoff frequency of A low-pass filter and a differential feedforward loop, a two-degree-of-freedom control framework are used to separate the frequency control from the voltage control channels; Based on a two-degree-of-freedom control framework, the disturbance from the DC voltage reference value is obtained separately. DC voltage disturbance Closed-loop transfer function and network-side frequency disturbances Output active power disturbance Closed-loop transfer function This achieves decoupling of frequency regulation and voltage control, enabling synchronous control of a two-degree-of-freedom DC capacitor.

[0036] like Figure 2 As shown, the main circuit diagram of a traditional VSG includes a converter, an LC filter, and a transmission line connected to the power grid (note: reactive power control and voltage / current inner loop control are omitted in the diagram). In this structure, the VSG output is processed by the filter and then fed into the power grid, primarily achieving frequency support through the active power control loop. The main circuit parameters are defined as follows: and These represent the filter reactance and the filter capacitor, respectively. This represents the parasitic resistance of the filter inductor. and These represent the reactance and resistance of the grid-side lines, respectively. Indicates the DC side voltage. V Indicates the filter port voltage. V g Indicates grid-side voltage. ω gThis indicates its frequency. The main control loop of VSG control is the active power control loop, which generates the filter port voltage by simulating the rotor equation of a synchronous generator. V frequency and phase angle In the control process, J Represents virtual inertia. D Indicates virtual damping. P This indicates the active power output at the filter port. This indicates that the reference was helpful. This represents the system's reference angular frequency. All control variables are expressed in per-unit values. s This represents the Laplace operator.

[0037] Neglecting line resistance, the active power transmitted by the grid-side lines It can be represented as: (1) in, δ Indicates the filter port voltage V With grid voltage V g The phase angle difference between them is expressed as follows: (2) in, Indicates time t The differential.

[0038] Linearizing equation (1) yields: (3) in, V 0 and V g0 This represents the steady-state values ​​of the filter port voltage and the grid-side voltage. δ 0 indicates phase angle difference δ steady-state value, and These represent the active power and phase angle difference, respectively, as small disturbances relative to the steady-state operating point.

[0039] Ignoring the dynamics of the phase-locked loop, combining equations (2), (3) and Figure 2 The active power control block diagram can be used to obtain the closed-loop small-signal model of the traditional VSG, such as Figure 3 As shown. By Figure 3 Yes, we can obtain: from the active power reference value disturbance and network-side frequency disturbance Output active power disturbance Closed-loop transfer function and : (4) (5) in, This indicates that traditional VSG control is affected by the active power reference value disturbance. Output active power disturbance The closed-loop transfer function, This indicates frequency disturbance on the traditional VSG control network side. Output active power disturbance The closed-loop transfer function, the closed-loop transfer function Describes the output active power in the VSG control strategy Useful for reference Its response characteristics reflect its basic control performance. Closed-loop transfer function This describes the VSG's response to power grid frequency disturbances. The active power response capability is a key indicator of its ability to simulate a synchronous generator and provide frequency support. It is worth noting that the closed-loop transfer function... In the denominator, the coefficient of the first-order term is virtual damping. D With virtual inertia J The ratio, and the constant term is the angular frequency reference value. Divide by virtual inertia J Reactance of grid-side lines The product of the two. This structure is highly similar to the power frequency response of a synchronous generator, and can serve as an important reference when evaluating the degree to which other control strategies simulate the characteristics of a synchronous generator. It also provides direction for the design of new control strategies.

[0040] The aforementioned VSG control strategy is typically applied to converter systems where the DC-side voltage is already stable. However, for new energy systems lacking upstream DC voltage control, the primary side of the converter can usually be modeled as a constant power source. With a DC capacitor The parallel structure has the same main circuit as the traditional VSG, such as... Figure 4 As shown in the diagram, the control system primarily stabilizes the DC voltage through DC voltage control and provides frequency support through active power control. To achieve DC voltage stability, a PI controller is often used to regulate the DC voltage and generate an active power reference value for VSG control. Figure 4 The control block diagram in the image illustrates the control path, where... Indicates the DC voltage reference value. and These represent the proportional and integral coefficients of the PI controller, respectively. This represents the reference value for angular frequency.

[0041] For the DC capacitor of the converter, its DC voltage and active power satisfy the following relationship: (6) in, This indicates the DC-side power.

[0042] Assuming DC-side power Assuming no fluctuations occur in the short term, linearizing the above relationship yields: (7) in, This represents the amount of disturbance in the DC voltage. This represents the steady-state value of the DC voltage, typically 1.0.

[0043] Combination formula (7) and Figure 4 A closed-loop small-signal model of a cascaded structure of DC voltage control and VSG control can be constructed, such as Figure 5 As shown. By Figure 5 Yes, we can obtain the following from the DC voltage reference value disturbance: DC voltage disturbance Closed-loop transfer function and network-side frequency disturbances Output active power disturbance Closed-loop transfer function : (8) (9) It can be observed that the closed-loop transfer function and closed-loop transfer function The denominators are all fourth-order polynomials. (Based on the closed-loop transfer function) It can be seen that the control structure behaves as a fourth-order system in regulating DC voltage. The dynamic process is complex, and the response speed and regulation accuracy are affected by multiple dynamic couplings, making parameter tuning difficult and significantly reducing the system's controllability and predictability. Further comparison of the closed-loop transfer function... With closed-loop transfer function As can be seen, after introducing DC voltage control, the system's power-frequency response no longer possesses the virtual inertia and virtual damping characteristics of traditional VSG control. Closed-loop transfer function. The fourth-order characteristic means that the system contains four poles, with the newly added pair originating from the DC voltage control loop. This can easily lead to a decrease in the system's stability margin and even trigger instability. Simultaneously, the positions of the original poles will shift, significantly altering the system's frequency response characteristics. In summary, while this cascaded control structure achieves DC voltage regulation, it undermines the essential characteristics of the VSG analog synchronous generator, negatively impacting the system's frequency support and dynamic stability.

[0044] Compared to the traditional cascaded scheme of "DC voltage control + VSG control", some scholars have proposed a DC capacitor synchronous control strategy. This strategy directly utilizes DC voltage regulation to generate a frequency signal, such as... Figure 6 As shown. Among them, K This represents the proportional coefficient for DC voltage control.

[0045] based on Figure 6 The structure allows for the derivation of the closed-loop small-signal model of the control strategy, such as... Figure 7 As shown. By Figure 7 Therefore, this strategy can be derived from the DC voltage reference value disturbance. DC voltage disturbance Closed-loop transfer function and network-side frequency disturbances Output active power disturbance Closed-loop transfer function : (10) (11) By comparing closed-loop transfer functions With closed-loop transfer function As can be seen from the denominator structure, this control strategy successfully simulates the inertia characteristics of a synchronous generator, with an equivalent inertia of [value missing]. However, the lack of a first-order term in the denominator of the transfer function indicates that this control strategy fails to introduce effective damping. Therefore, when the system itself has weak damping, this control strategy is prone to causing system instability.

[0046] In this embodiment, to make the DC capacitor synchronization control more closely match the dynamic characteristics of the synchronous generator, a new control strategy needs to be designed that can not only simulate the system inertia but also introduce appropriate virtual damping. Assume the transfer function of the DC voltage control is... Its closed-loop small-signal model for DC capacitor synchronous control is as follows: Figure 8 As shown. By Figure 8 Yes, we can obtain the following from the DC voltage reference value disturbance: DC voltage disturbance Closed-loop transfer function and network-side frequency disturbances Output active power disturbance Closed-loop transfer function : (12) (13) Comparison of closed-loop transfer functions , and closed-loop transfer function , It can be seen that when the transfer function When it is (14), .

[0047] (14) From equation (14), we can see that the transfer function at this time is This is a proportional-derivative (PD) element. If the converter is expected to have an equivalent virtual inertia of... J Virtual damping is D The dynamic characteristics are then determined by setting proportional coefficients. and differential coefficients for: (15) Therefore, the proportionality coefficient Corresponding to virtual inertia This is consistent with the traditional DC capacitor synchronous control strategy, but the newly introduced differential coefficient... This provides the virtual damping that was missing in the original strategy. Based on this, a DC capacitor synchronous control strategy with virtual damping characteristics can be obtained, the structure of which is as follows: Figure 9 As shown.

[0048] In this embodiment, a two-degree-of-freedom control framework is designed based on S3 to achieve decoupling of frequency regulation and voltage control.

[0049] Based on the design of S3, F 4( s ) = F 1( s ), D 4( s ) = D 1( s Observing their expressions, we can see that the two closed-loop transfer functions have the same denominator, that is, the same characteristic polynomial. This means that once the virtual inertia... and virtual damping With the parameters determined, the two closed-loop transfer functions share the same poles. This constraint stems from... Figure 9 The control strategy shown belongs to a single-degree-of-freedom control system. For converters with DC voltage control, the DC voltage control closed-loop transfer function is set independently. and grid-side frequency disturbance active response function The poles are crucial. For example, to achieve fast DC voltage reference tracking, the grid-side frequency disturbance active response function is required. It has high bandwidth, which usually corresponds to a small virtual inertia. However, smaller virtual inertia It will also reduce the active response function of the grid-side frequency disturbance. Frequency support capability. In the closed-loop transfer function. F 4( s and closed-loop transfer function D 4( s Under extreme constraints such as , these performance requirements are difficult to meet simultaneously.

[0050] To address the above problems, this invention proposes a two-degree-of-freedom control system, the specific structure of which is as follows: Figure 1 As shown. The system consists of a cutoff frequency of It consists of a low-pass filter and a differential feedforward loop. The coefficients of the differential feedforward loop are... Set as (16): (16) Its design principle is as follows Figure 10 As shown, where, Indicates input to disturbance u 2. Transfer function at the feed point For disturbance To output y The transfer function, Represents the transfer function Input, y Indicates the output. Figure 10 (a) proceed Figure 10 The modification in (b) is due to the introduction of a control module that only depends on the input. Therefore, it will not change the disturbance. To output y The transfer function, but will take the input To output y The transfer function is adjusted to the desired objective function. .

[0051] Objective function Designed with a cutoff frequency of The low-pass filter, and substitute it into Figure 10 From the feedforward expression in (b), we can obtain the feedforward term as follows: (17) Therefore, the closed-loop small-signal model of the modified control can be obtained as follows: Figure 11 .according to Figure 11 After modification, the following can be obtained: disturbance from the DC voltage reference value. DC voltage disturbance Closed-loop transfer function and network-side frequency disturbances Output active power disturbance Closed-loop transfer function : (18) (19) At this point, the control system can independently set the DC voltage control closed-loop transfer function according to actual needs. The bandwidth, and the active response function of the network-side frequency disturbance. The required equivalent inertia and virtual damping enable both fast DC voltage tracking and high frequency support capabilities.

[0052] To verify whether the proposed control strategy can simulate virtual inertia like a traditional VSG. and virtual damping And whether two-degree-of-freedom control can make DC voltage control only subject to the cutoff frequency. The impact of the proposed two-degree-of-freedom DC capacitor synchronous control strategy with damping characteristics was investigated. A traditional VSG grid-connected system and a converter grid-connected system controlled by the proposed two-degree-of-freedom DC capacitor synchronous control strategy with damping characteristics were built on the MATLAB / Simulink platform. Simulation experiments were conducted to verify the control effect. The main simulation parameters of the system are shown in Table 1. Table 1 is the simulation parameter table, and the proportional coefficient of the proposed control strategy is... k p Differential coefficients k d Sum of coefficients k f It can be calculated according to equations (15) and (16).

[0053] Table 1

[0054] (1) Virtual inertia and virtual damping Simulation verification of characteristics To verify whether the proposed control strategy can simulate the virtual inertia of a synchronous generator like a traditional VSG, and virtual damping Based on the characteristics of the proposed control strategy, a 0.1 Hz frequency sag disturbance was applied to the power grid at 3 s in the simulation to obtain the power response of the traditional VSG and the proposed control strategy, such as... Figure 12 As shown in the figure, P represents the active power output of the converter, measured in pu. The blue line represents the traditional VSG control strategy, and the red dashed line represents the control strategy proposed in this invention. Figure 12 As can be seen, the power response curve of the proposed control strategy is consistent with that of the traditional VSG, exhibiting typical second-order dynamic characteristics, indicating that the strategy can effectively simulate the virtual inertia of a synchronous generator. and virtual damping The behavior.

[0055] (2) Verification of two degrees of freedom of the control system To verify whether DC voltage control under the two-degree-of-freedom control strategy is independent of virtual inertia J Parameters, virtual inertia J Set the CPU to 3 PU and 6 PU respectively, and apply the perturbation after 3 seconds. The DC voltage response curve is obtained by jumping from 1.0 pu to 1.1 pu, as shown below. Figure 13 As shown in the figure, the horizontal axis t The vertical axis represents time, measured in seconds (s). V dc This represents the DC-side voltage. (From...) Figure 13 It is evident that even virtual inertia J The DC voltage response curve of the system remains consistent despite the change in parameters, indicating that the DC voltage control is no longer affected by parameters such as virtual inertia.

[0056] Furthermore, to verify that the DC voltage control is only affected by the low-pass filter cutoff frequency... The impact on the cutoff frequency The values ​​are set to 10, 20, and 30 respectively, and the perturbation is applied after 3 seconds. The DC voltage response curve is obtained by jumping from 1.0 pu to 1.1 pu, as shown below. Figure 14 As shown. By Figure 14 It can be seen that the DC voltage response varies with the cutoff frequency. It changes with the frequency of change, and the cutoff frequency The larger the frequency, the higher the system bandwidth and the faster the response, verifying that DC voltage control is mainly affected by the low-pass filter cutoff frequency. Influence.

[0057] This invention simulates the virtual inertia of a synchronous generator using a proportional-differential element. and virtual damping Characteristics, including the scaling factor Corresponding virtual inertia, differential coefficients Corresponding to virtual damping. Meanwhile, considering that while increasing inertia improves frequency support capability, it weakens the dynamic regulation performance of DC voltage, this invention introduces a two-degree-of-freedom control structure to address this contradiction. This structure ensures that the DC voltage control loop is unaffected by the virtual inertia parameter, thus balancing frequency stability and voltage response speed. Compared to traditional control strategies, the method proposed in this invention better aligns with the physical characteristics of synchronous generators, significantly optimizing the dynamic performance of DC voltage while improving system stability. This provides an efficient and reliable solution for the stable operation of high-penetration renewable energy grids.

[0058] In summary, this invention proposes a dual-degree-of-freedom DC voltage synchronization control strategy with damping characteristics. This strategy can simulate the virtual inertia and virtual damping characteristics of a synchronous generator. Based on this, a dual-degree-of-freedom control framework is constructed to separate the frequency control and voltage control channels so that they do not interfere with each other. This improves system stability while ensuring that the DC voltage has good dynamic regulation capability.

Claims

1. A two-degree-of-freedom DC capacitor synchronous control method with damping characteristics, characterized by, The method comprises the following steps: S1, defining a direct current capacitor synchronous control strategy with damping characteristics; S2, based on the defined direct current capacitor synchronous control strategy, designing a two-degree-of-freedom control framework to realize the decoupling of frequency regulation and voltage control and complete the synchronous control of the two-degree-of-freedom direct current capacitor.

2. The dual-degree-of-freedom direct-current capacitor synchronous control method with damping characteristics according to claim 1, characterized in that, The S1 is specifically: The transfer function of the direct voltage control is defined as ; Based on transfer function , the closed-loop transfer function from the DC voltage reference disturbance to the DC voltage disturbance and the closed-loop transfer function from the grid-side frequency disturbance to the output active power disturbance are obtained, respectively; For the traditional VSG control , the transfer function is a proportional-differential element, where represents the closed-loop transfer function from the active reference disturbance to the output active disturbance , and represents the closed-loop transfer function from the grid frequency disturbance to the output active disturbance . Based on the proportional-differential element, set the proportional coefficient and the differential coefficient Wherein, the proportional coefficient Corresponding to the virtual inertia, the differential coefficient Provide the missing virtual damping in the strategy, complete the definition of the direct current capacitor synchronous control strategy.

3. The dual-degree-of-freedom direct-current capacitor synchronous control method with damping characteristics according to claim 2, characterized in that, The transfer function The expression for the transfer function is as follows: ； wherein, represents the Laplace operator, represents the direct current capacitor, represents the virtual damping, represents the line-side reactance, represents the virtual inertia, represents the system reference angular frequency.

4. The dual-degree-of-freedom DC capacitor synchronous control method with damping characteristics according to claim 2, characterized in that, The closed loop transfer function The expression for the closed loop transfer function is as follows: ； The closed loop transfer function The expression for the closed loop transfer function is as follows: ； wherein, represents the system reference angular frequency, represents the direct current capacitor, represents the line side circuit reactance, represents the Laplace operator.

5. The dual-degree-of-freedom DC capacitor synchronous control method with damping characteristics according to claim 2, characterized in that, The proportional coefficient And the differential coefficient The expressions of the proportional coefficient, the integral coefficient and the differential coefficient are as follows, respectively: ； ； wherein, represents a direct current capacitor, represents a virtual inertia, represents a virtual damping, represents a line reactance of the grid side, represents a system reference angular frequency.

6. The dual-degree-of-freedom DC capacitor synchronous control method with damping characteristics according to claim 2, characterized in that, The S2 is specifically: exist Based on this, a two-degree-of-freedom control framework is introduced, wherein the two-degree-of-freedom control framework includes a cutoff frequency of A low-pass filter and a differential feedforward loop, a two-degree-of-freedom control framework are used to separate the frequency control from the voltage control channels; Based on a two-degree-of-freedom control framework, the disturbance from the DC voltage reference value is obtained separately. DC voltage disturbance Closed-loop transfer function and network-side frequency disturbances Output active power disturbance Closed-loop transfer function This achieves decoupling of frequency regulation and voltage control, enabling synchronous control of a two-degree-of-freedom DC capacitor.

7. The dual-degree-of-freedom direct-current capacitor synchronous control method with damping characteristics according to claim 6, characterized in that, The expression of the feedforward term of the two-degree-of-freedom control is as follows: ； wherein, represents a low-pass filter with a cut-off frequency of represents a perturbation to the transfer function of the output represents the output, represents the Laplace operator, represents the reactance of the grid-side line, represents the system reference angular frequency, represents the direct current capacitor, represents the coefficient of the derivative feedforward loop.​​ 8. The dual-degree-of-freedom DC capacitor synchronous control method with damping characteristics according to claim 6, characterized in that, The expression of the coefficient of the differential feedforward loop is as follows: ； wherein, represents the coefficient of the differential feedforward loop, represents the direct current capacitor, represents the reactance of the grid-side line, represents the system reference angular frequency.

9. The dual-degree-of-freedom DC capacitor synchronous control method with damping characteristics according to claim 6, characterized in that, The closed loop transfer function The expression for the closed loop transfer function is as follows: ； wherein denotes the Laplacian operator.

10. The dual-degree-of-freedom DC capacitor synchronous control method with damping characteristics according to claim 6, characterized in that, The closed loop transfer function The expression for the closed loop transfer function is as follows: ； wherein, represents a Laplacian operator, represents a system reference angular frequency, represents a line-side line reactance, represents a virtual inertia, represents a virtual damping.

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