Method for suppressing low-frequency oscillation of VSG composite damping based on equivalent circuit mapping

Abstract

This invention discloses a method for suppressing low-frequency oscillations of VSG composite damping based on equivalent circuit mapping, belonging to the field of power electronics technology. A sampling module acquires the electrical signal of the VSG and performs coordinate transformation. A power calculation module calculates active and reactive power based on the acquired electrical signal. An equivalent circuit-power control mapping module maps the VSG active power control loop to an equivalent circuit model. A composite damping control module introduces a composite damping network into the equivalent circuit model. An equivalent circuit-power control inverse mapping module inversely maps the equivalent circuit model after introducing composite damping back to an active power control loop. A voltage control module performs inverse coordinate transformation, generates a PWM signal, and drives the inverter bridge. Using this method, the problem of low-frequency power oscillations of the VSG under multiple disturbance conditions such as power command step and grid frequency disturbances is effectively solved, improving the transient damping characteristics and dynamic operating stability of the system.

Description

Technical Field

[0001] This invention relates to the field of power electronics technology, and in particular to a method for suppressing low-frequency oscillations based on VSG composite damping using equivalent circuit mapping. Background Technology

[0002] Renewable energy sources, represented by solar and wind power, are being developed and utilized on a large scale due to their abundant resources and environmental friendliness, and are gradually becoming an important pillar of the future energy system. Wind power, photovoltaic power, and other renewable energy sources are being integrated into the power system on an unprecedented scale. Against the backdrop of the continuous increase in the penetration rate of new energy sources, the sources of stable support for the power system and the ways in which they are achieved are undergoing fundamental changes. In power systems dominated by new energy generation such as photovoltaic and wind power, grid-connected inverters composed of power semiconductor devices serve as the bridge between new energy sources and the power grid.

[0003] Traditional power systems are primarily based on synchronous generators. The mechanical rotational inertia and inherent electromagnetic damping characteristics of their rotors provide crucial inertial response and damping support when the system is subjected to load disturbances or fault impacts. This is an important physical basis for maintaining system frequency stability and suppressing low-frequency power oscillations. However, with the increasing penetration of renewable energy in power systems, renewable energy generation units are typically connected to the grid via power electronic converters. These devices lack mechanical rotating parts and cannot naturally provide rotational inertia in the sense of synchronous generators. The widely used grid-following (GFL) control strategy, which uses a phase-locked loop (PLL) to track the grid phase, achieves power output control by adjusting active and reactive power commands. Its operation is essentially a controlled current source mode dependent on the grid, requiring a stable voltage and frequency reference from the external grid. Although this control method performs well in steady-state operation and power dispatch, it is difficult to actively provide voltage and frequency support under weak grid conditions or with a high proportion of renewable energy integration, and it cannot effectively participate in the system's inertial response.

[0004] With the continuous decline in the proportion of synchronous generators, the grid connection of high-proportion power electronic interface-type new energy sources has led to a significant reduction in the equivalent inertia and damping level of the power system. Under disturbances, the system is prone to problems such as increased rate of change of frequency (RoCoF), aggravated frequency deviation, and decreased dynamic stability margin. In severe cases, it may even lead to frequency instability or large-scale power grid disconnection accidents. Therefore, against the backdrop of the continuously weakening support capacity of synchronous machines, it is urgent to explore new grid connection and control technologies that can reconstruct the system stability mechanism to support the safe operation of new power systems.

[0005] Based on the above requirements, grid-forming (GFM) converter control technology has emerged. This type of control strategy no longer relies on the external power grid as a synchronization reference, but instead empowers the converter to autonomously establish voltage amplitude and frequency references, enabling it to operate in voltage source mode and actively provide inertial and damping support to the system. Among various grid-forming control methods, the Virtual Synchronous Generator (VSG) control strategy introduces the oscillation equations of a synchronous generator into the control layer, constructing virtual rotational inertia and virtual damping elements, allowing the inverter to exhibit synchronous generator-like dynamic behavior.

[0006] Although VSG control, by introducing virtual inertia and virtual damping, enables the converter to possess inertial response capabilities similar to a synchronous generator, thereby improving frequency support performance under weak grid conditions to some extent, this advantage often comes at the cost of sacrificing system dynamic performance. Due to the introduction of oscillation equations at the control level, the active power regulation process of VSG changes from the originally rapid power electronic dynamics to a slow dynamic process dominated by virtual inertia, resulting in a reduced system dynamic response speed and greater sensitivity to control parameter configuration. Under single-unit grid-connected conditions, VSG simulates synchronous generator dynamics by introducing virtual inertia and damping, but it may also exhibit some of the drawbacks of synchronous machines. Especially when the virtual inertia value is too large, the system's equivalent inertia increases, the dynamic response slows down, and low-frequency power oscillations occur, which are more pronounced under weak grid conditions. Summary of the Invention

[0007] The purpose of this invention is to provide a method for suppressing low-frequency oscillations based on VSG composite damping using equivalent circuit mapping, thereby solving the aforementioned technical problems.

[0008] To achieve the above objectives, the present invention provides a method for suppressing low-frequency oscillations of VSG composite damping based on equivalent circuit mapping, including a sampling module, a power calculation module, an equivalent circuit-power control mapping module, a composite damping control module, an equivalent circuit-power control inverse mapping module, and a voltage control module; The sampling module acquires the electrical signal of the VSG and completes the coordinate transformation. The power calculation module calculates the active and reactive power based on the acquired electrical signal. The equivalent circuit-power control mapping module maps the VSG active power control loop to an equivalent circuit model. The composite damping control module introduces a composite damping network into the equivalent circuit model. The equivalent circuit-power control inverse mapping module inverse maps the equivalent circuit model after introducing composite damping to an active power control loop. The voltage control module realizes the inverse coordinate transformation, generates a PWM signal, and drives the inverter bridge.

[0009] Preferably, the sampling module is used to acquire the three-phase voltage and three-phase current signals at the grid connection point of the three-phase inverter in real time, and converts the three-phase voltage and current signals into two-phase voltage and current signals through Clark transformation, as shown below: ; ; In the formula, V A , V B , V C The three-phase voltage signals are in the abc coordinate system. V α 、V β The two-phase voltage signals are in the αβ coordinate system. I A , I B , I C The three-phase current signal is in the abc coordinate system. I α 、I β The two-phase current signals are in the αβ coordinate system; The voltage and current signals in the αβ coordinate system are then transformed into voltage and current signals in the dq coordinate system using the Park transform, as shown below: ; ; in, ω Angular frequency, V d , V q The two-phase voltage signals are in the dq coordinate system. I d , I q These are two-phase current signals in the dq coordinate system.

[0010] Preferably, the power calculation module performs power calculations on the voltage and current signals in the dq coordinate system obtained from the sampling module to obtain active power and reactive power, as shown below: ; in, P Active power Q This refers to reactive power.

[0011] Preferably, the equivalent circuit-power control mapping module maps the active power control loop to an equivalent circuit model based on the mapping relationship between the active power control loop parameters and the equivalent circuit parameters. The mapping relationship between the active power control loop parameters and the equivalent circuit parameters is as follows: ; in, J For virtual inertia, D p The damping coefficient is... K s Δ is the power transfer coefficient. ω For angular velocity deviation, ω 0 is the rated angular velocity. C This is the capacitance value. R This is the resistance value. L This is the inductance value. I For current, V It represents voltage.

[0012] Preferably, before constructing the equivalent circuit model, the equivalent circuit-power control mapping module first establishes a small-signal model of the VSG active power loop, and then equates the change in the reference active power command to a current source. I s Disturbances, treating grid frequency fluctuations as equivalent to voltage sources. U s Disturbance.

[0013] Preferably, the composite damping network introduced by the composite damping control module consists of fission capacitor branches and RL The damping branch is constructed by first splitting the original equivalent capacitance of the equivalent circuit model into two split capacitances. C 1. C 2, and an additional damping resistor is connected in parallel across one of the split capacitors. R 0, forming a fission capacitor branch, and then the equivalent inductance in the equivalent circuit model. L A filter inductor is introduced in series at both ends. L f and damping resistor R f of RL Damping branch.

[0014] Preferably, the composite damping network adopts a hierarchical optimization parameter quantitative design method, first tuning the parameters of the fission capacitor branch. C 1. C 2. R 0, then adjust RL Parameters of damped branch L f , R fAmong them, the fission capacitor branch parameter design adopts frequency compensation, the zero point is placed before the cutoff frequency, the pole is set at 3 to 5 times the zero point, and the cutoff frequency is set to 1 / 3 of the traditional VSG control, with a phase margin of 60°.

[0015] Preferably, after introducing a composite damping network into the equivalent circuit model through the composite damping control module, the expression for its output current is as follows: ; ; in, I o For output current, U s A voltage source mapped to grid frequency disturbances. I s The current source is mapped to the power command step. C 1. C 2 is a split capacitor. L f for RL The filter inductor on the branch, R f for RL Damping resistor on the branch, R 0 is C 2. Damping resistors connected in parallel at both ends.

[0016] Preferably, the equivalent circuit-power control inverse mapping module inverse maps the equivalent circuit model after introducing composite damping into an active power control loop to obtain the voltage amplitude and voltage phase, as shown below: .

[0017] Preferably, the voltage control module, based on voltage amplitude and voltage phase, converts the voltage signal in the dq coordinate system into a voltage signal in the abc coordinate system through inverse Park transform and inverse Clark transform, and finally generates a PWM signal, which is then output to the three-phase inverter bridge, as shown below: ; ; in, V oα , V oβ The two-phase output voltage signals are in the αβ coordinate system. V od , V oq The two-phase output voltage signals are in the dq coordinate system. V oA , VoB , V oC The signal is the three-phase output voltage signal in the abc coordinate system.

[0018] Therefore, the present invention employs the above-mentioned VSG composite damping low-frequency oscillation suppression method based on equivalent circuit mapping, which has the following beneficial effects: 1. It can effectively solve the problem of low-frequency oscillation in VSG under multiple disturbance conditions such as reference power change and grid frequency fluctuation, and significantly improve transient damping characteristics.

[0019] 2. The fission capacitor branch increases damping in transient state through additional damping, and does not affect steady-state output power and frequency deviation in steady state.

[0020] 3. RL The damping branch improves the zero-pole distribution of the voltage source disturbance channel, enhances damping, reduces resonance peaks and increases stability margin, and can effectively suppress oscillations caused by grid frequency disturbances.

[0021] 4. By implementing hierarchical optimization, the parameters can be quantitatively designed, reducing the difficulty of tuning. The structure is simple and the engineering feasibility is strong.

[0022] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. Attached Figure Description

[0023] Figure 1 This is a structural diagram of the VSG control system of the present invention; Figure 2 This is the small-signal model of the active power of the VSG in this invention; Figure 3 This is the VSG mapping equivalent circuit model of the present invention; Figure 4 The equivalent circuit model of VSG after the introduction of fission capacitor in this invention; Figure 5 The equivalent circuit model for introducing a composite damping network in this invention; Figure 6 This is a control block diagram of the VSG after the introduction of a composite damping network in this invention; Figure 7 A comparison of the transient response of output power under different strategies; Figure 8 The simulation results show the comparison between the composite damping control strategy and the traditional VSG under reference power step and grid frequency disturbance. Detailed Implementation

[0024] To make the objectives, technical solutions, and advantages disclosed in the embodiments of the present invention clearer, the embodiments of the present invention will be further described in detail below with reference to the accompanying drawings and examples. It should be understood that the specific embodiments described herein are only used to explain the embodiments of the present invention and are not intended to limit the embodiments of the present invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of this application. Examples of the embodiments are shown in the accompanying drawings, wherein the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout.

[0025] It should be noted that the terms "comprising" and "having," and any variations thereof, are intended to cover non-exclusive inclusion, such as a process, method, system, product, or server that includes a series of steps or units, not necessarily limited to those steps or units explicitly listed, but may include other steps or units not explicitly listed or inherent to such process, method, product, or device.

[0026] The embodiments of the present invention will now be described in detail with reference to the accompanying drawings.

[0027] like Figures 1-8 As shown, this invention provides a method for suppressing low-frequency oscillations of VSG based on equivalent circuit mapping, applicable to VSG grid-connected systems. It effectively solves the problem of low-frequency power oscillations in this system under multiple disturbance conditions such as reference power changes and grid frequency fluctuations. The method includes a sampling module, a power calculation module, an equivalent circuit-power control mapping module, a composite damping control module, an equivalent circuit-power control inverse mapping module, and a voltage control module. The sampling module collects the electrical signals of the VSG and performs coordinate transformation. The power calculation module calculates active and reactive power based on the collected electrical signals. The equivalent circuit-power control mapping module maps the VSG active power control loop to an equivalent circuit model. The composite damping control module introduces a composite damping network into the equivalent circuit model to improve the system's damping characteristics and dynamic stability. The equivalent circuit-power control inverse mapping module inverse maps the equivalent circuit model after introducing composite damping to an active power control loop. The voltage control module performs inverse coordinate transformation, generates a PWM signal, and drives the inverter bridge, ultimately suppressing the low-frequency power oscillations of the VSG.

[0028] like Figure 1 As shown, the converter adopts a three-phase inverter bridge structure to convert the DC side voltage... V dc The conversion results in a three-phase AC voltage output. The three-phase inverter bridge can be either a two-level or a three-level three-phase inverter bridge, which generates a three-phase pulse-width modulated voltage under the control of a PWM modulation signal. Figure 1 middle, L fFor filtering inductors, R f The associated resistance of the filter inductor, C f This is a filter capacitor; the output terminal is connected to the line impedance. R line and L line Connected to the point of common coupling (PCC) to achieve electrical interface with the power grid, the active power control loop of the VSG is established based on the synchronous generator rotation equation, and its parameters include... J For virtual inertia, D p The damping coefficient is... P ref This is a reference value for active power. P The output active power, ω Angular velocity, ω 0 represents the rated angular velocity. This invention is based on the VSG principle. By designing and improving the active power control loop, and adjusting the converter's output angular velocity and phase according to the synchronous generator rotation equation, the dynamic characteristics and frequency response characteristics of the converter's active power are controlled.

[0029] The sampling module is used to acquire the three-phase voltage and current signals at the grid connection point of the three-phase inverter in real time, and converts the three-phase voltage and current signals into two-phase voltage and current signals through Clark transformation, as shown below: ; ; In the formula, V A , V B , V C The three-phase voltage signals are in the abc coordinate system. V α 、V β The two-phase voltage signals are in the αβ coordinate system. I A , I B , I C The three-phase current signal is in the abc coordinate system. I α 、I β The two-phase current signals are in the αβ coordinate system; The voltage and current signals in the αβ coordinate system are then transformed into voltage and current signals in the dq coordinate system using the Park transform, as shown below: ; ; in, ω Angular frequency, V d , V q The two-phase voltage signals are in the dq coordinate system. I d , I q These are two-phase current signals in the dq coordinate system.

[0030] The power calculation module performs power calculations on the voltage and current signals in the dq coordinate system obtained from the sampling module, yielding active power and reactive power, as shown below: ; in, P Active power Q This refers to reactive power.

[0031] like Figure 2 , Figure 3 As shown, the equivalent circuit-power control mapping module maps the active power control loop to an equivalent circuit model based on the mapping relationship between the active power control loop parameters and the equivalent circuit parameters. The mapping relationship between the active power control loop parameters and the equivalent circuit parameters is shown below: ; in, J For virtual inertia, D p The damping coefficient is... K s Δ is the power transfer coefficient. ω For angular velocity deviation, ω 0 is the rated angular velocity. C This is the capacitance value. R This is the resistance value. L This is the inductance value. I For current, V It represents voltage.

[0032] Before constructing the equivalent circuit model, the equivalent circuit-power control mapping module first establishes a small-signal model of the VSG active power loop, and then equates the change in the reference active power command to a current source. I s Disturbances, treating grid frequency fluctuations as equivalent to voltage sources. U s Disturbance analysis enables unified modeling and analysis of the dynamic characteristics of the system under different disturbance conditions. It also allows for intuitive analysis of the power oscillation mechanism and the impact of control parameters on the system in the circuit domain.

[0033] The composite damping network introduced by the composite damping control module consists of fission capacitor branches and RL The damping branch configuration is constrained by external factors such as maximum output power, frequency deviation, and frequency change rate RoCoF, and only the equivalent capacitance is adjusted due to the virtual inertia and droop coefficient being subject to external constraints. C With equivalent resistance R Since it is difficult to effectively improve power oscillations caused by disturbances, this method improves the transient damping of the system by introducing a composite damping network, such as... Figure 4 As shown, the method of introduction is as follows: first, the original equivalent capacitance of the equivalent circuit model is split into two split capacitances. C 1. C 2, and in one of the split capacitors C 2. Additional damping resistor connected in parallel at both ends R 0, forming a fission capacitor branch. In the equivalent circuit model after introducing the fission capacitor, the fission capacitor branch is affected by the additional damping resistor during the transient process. R 0. Improves system transient damping and transient performance, while in steady state the capacitor is equivalent to an open circuit, making R 0 does not affect the system's steady-state output power and frequency deviation. Furthermore, to ensure the improved structure does not affect frequency dynamic performance, the initial frequency change rate is kept consistent before and after the introduction of the fission capacitor. Based on this, [the following is done / the following is also done / the following is ... C 1. C 2. Apply constraints.

[0034] Based on the fission capacitor branch, although introducing only the fission capacitor can improve the zero-pole distribution of the current source disturbance channel, it may introduce new zeros in the voltage source disturbance channel, leading to increased overshoot and limiting the improvement of the overall dynamic performance of the system. Therefore, the equivalent inductance in the equivalent circuit model is further improved. L A filter inductor is introduced in series at both ends. L f and damping resistor R f of RL Damping branch, RL The damping branch can improve the zero-pole distribution of the voltage source to the output path, enhance system damping, and suppress power oscillations under multiple disturbance conditions; in terms of control implementation, RL The damping branch is equivalent to adding a typical lead-lag compensation element to the feedback branch, which improves the system's phase margin and further suppresses oscillations by providing phase lead in the mid-frequency band.

[0035] like Figure 5 As shown, the composite damping network employs a hierarchical optimization parameter quantification design method, because the composite damping network contains... C 1. C 2. R 0、 L f , Rf Five variable parameters are used to achieve optimal system damping and stability while reducing the difficulty of parameter tuning. The parameters of the fission capacitor branch are tuned first. C 1. C 2. R 0, ensuring the suppression of oscillations caused by changes in reference power and giving the system basic dynamic performance, then re-tuning. RL Parameters of damped branch L f , R f To achieve optimal overall system damping and stability, the fission capacitor branch parameters are designed using a frequency compensation method, placing the zero point before the cutoff frequency and setting the poles 3 to 5 times the zero point, while reducing the cutoff frequency to 1 / 3 of that of traditional VSG control, with a phase margin of 60°. After introducing a composite damping network into the equivalent circuit model through the composite damping control module, the expression for its output current is as follows: ; ; in, I o For the output current, and I o In the calculation formula G 1. G The specific calculation methods for the relevant parameters are as described in the formula above. U s A voltage source mapped to grid frequency disturbances. I s The current source is mapped to the power command step. C 1. C 2 is a split capacitor. L f for RL The filter inductor on the branch, R f for RL Damping resistor on the branch, R 0 is C 2. Damping resistors connected in parallel at both ends.

[0036] like Figure 6 As shown, the equivalent circuit-power control inverse mapping module inverse maps the equivalent circuit model after introducing composite damping into an active power control loop to obtain the voltage amplitude and voltage phase, as shown below: .

[0037] This inverse mapping operation can transform the equivalent circuit model containing composite damping characteristics back into control logic adapted to the VSG, enabling the VSG to maintain good damping characteristics and stable operation under various operating conditions such as changes in reference power command and grid frequency disturbances.

[0038] The voltage control module, based on voltage amplitude and phase, transforms the voltage signal in the dq coordinate system into a voltage signal in the abc coordinate system through inverse Park and inverse Clark transformations. Finally, it generates a PWM signal and outputs the PWM signal to the three-phase inverter bridge to suppress the low-frequency oscillation of the virtual synchronous generator, as shown below: ; ; in, V oα , V oβ The two-phase output voltage signals are in the αβ coordinate system. V od , V oq The two-phase output voltage signals are in the dq coordinate system. V oA , V oB , V oC The signal is the three-phase output voltage signal in the abc coordinate system.

[0039] To verify the oscillation suppression effect of this method, an experimental verification platform was built, such as... Figure 7 , Figure 8 As shown, the operating effects of the traditional VSG control strategy and the composite damping control strategy of this invention are compared under two disturbance conditions: power command step and grid frequency fluctuation. Experimental results show that the output power and output current under the traditional VSG control strategy exhibit significant overshoot and oscillation; while the introduction of the composite damping network of this invention can effectively suppress transient oscillations, accelerate power response speed, and maintain good steady-state output even when both power command step and grid frequency fluctuation disturbances exist simultaneously, demonstrating strong robustness and stability.

[0040] A comprehensive comparison shows that using alone... RL Damping branches can enhance system damping but have a slow power response. Using fission capacitor branches alone can improve the power response but will lead to increased system overshoot. The composite damping network of this invention combines the technical advantages of both, enabling the system to achieve fast and stable power recovery and effectively solving the problem of low-frequency power oscillation under VSG multi-disturbance conditions.

[0041] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the technical solutions of the present invention, and these modifications or equivalent substitutions cannot cause the modified technical solutions to deviate from the spirit and scope of the technical solutions of the present invention.

Claims

1. A method for suppressing low-frequency oscillations based on VSG composite damping using equivalent circuit mapping, characterized in that: It includes a sampling module, a power calculation module, an equivalent circuit-power control mapping module, a composite damping control module, an equivalent circuit-power control inverse mapping module, and a voltage control module; The sampling module acquires the electrical signal of the VSG and completes the coordinate transformation. The power calculation module calculates the active and reactive power based on the acquired electrical signal. The equivalent circuit-power control mapping module maps the VSG active power control loop to an equivalent circuit model. The composite damping control module introduces a composite damping network into the equivalent circuit model. The equivalent circuit-power control inverse mapping module inverse maps the equivalent circuit model after introducing composite damping to an active power control loop. The voltage control module realizes the inverse coordinate transformation, generates a PWM signal, and drives the inverter bridge.

2. The method for suppressing low-frequency oscillations based on equivalent circuit mapping of VSG composite damping according to claim 1, characterized in that: The sampling module is used to acquire the three-phase voltage and current signals at the grid connection point of the three-phase inverter in real time, and converts the three-phase voltage and current signals into two-phase voltage and current signals through Clark transformation, as shown below: ； ； In the formula, V A , V B , V C The three-phase voltage signals are in the abc coordinate system. V α 、V β The two-phase voltage signals are in the αβ coordinate system. I A , I B , I C The three-phase current signal is in the abc coordinate system. I α 、I β The two-phase current signals are in the αβ coordinate system; The voltage and current signals in the αβ coordinate system are then transformed into voltage and current signals in the dq coordinate system using the Park transform, as shown below: ； ； in, ω Angular frequency, V d , V q The two-phase voltage signals are in the dq coordinate system. I d , I q These are two-phase current signals in the dq coordinate system.

3. The method for suppressing low-frequency oscillations based on equivalent circuit mapping of VSG composite damping according to claim 2, characterized in that: The power calculation module performs power calculations on the voltage and current signals in the dq coordinate system obtained from the sampling module, yielding active power and reactive power, as shown below: ； in, P Active power Q This refers to reactive power.

4. The method for suppressing low-frequency oscillations based on equivalent circuit mapping of VSG composite damping according to claim 3, characterized in that: The equivalent circuit-power control mapping module maps the active power control loop into an equivalent circuit model based on the mapping relationship between the active power control loop parameters and the equivalent circuit parameters. The mapping relationship between the active power control loop parameters and the equivalent circuit parameters is shown below: ； in, J For virtual inertia, D p The damping coefficient is... K s Δ is the power transfer coefficient. ω For angular velocity deviation, ω 0 is the rated angular velocity. C This is the capacitance value. R This is the resistance value. L This is the inductance value. I For current, V It represents voltage.

5. The method for suppressing low-frequency oscillations based on equivalent circuit mapping of VSG composite damping according to claim 4, characterized in that: Before constructing the equivalent circuit model, the equivalent circuit-power control mapping module first establishes a small-signal model of the VSG active power loop, and then equates the change in the reference active power command to a current source. I s Disturbances, treating grid frequency fluctuations as equivalent to voltage sources. U s Disturbance.

6. The method for suppressing low-frequency oscillations based on equivalent circuit mapping of VSG composite damping according to claim 5, characterized in that: The composite damping network introduced by the composite damping control module consists of fission capacitor branches and RL The damping branch is constructed by first splitting the original equivalent capacitance of the equivalent circuit model into two split capacitances. C 1. C 2, and an additional damping resistor is connected in parallel across one of the split capacitors. R 0, forming a fission capacitor branch, and then the equivalent inductance in the equivalent circuit model. L A filter inductor is introduced in series at both ends. L f and damping resistor R f of RL Damping branch.

7. The method for suppressing low-frequency oscillations based on equivalent circuit mapping of VSG composite damping according to claim 6, characterized in that: The composite damping network employs a hierarchical optimization parameter quantification design method, first tuning the parameters of the fission capacitor branch. C 1. C 2. R 0, then adjust RL Parameters of damped branch L f , R f Among them, the fission capacitor branch parameter design adopts frequency compensation, the zero point is placed before the cutoff frequency, the pole is set at 3 to 5 times the zero point, and the cutoff frequency is set to 1 / 3 of the traditional VSG control, with a phase margin of 60°.

8. The method for suppressing low-frequency oscillations based on equivalent circuit mapping of VSG composite damping according to claim 7, characterized in that: After introducing a composite damping network into the equivalent circuit model through the composite damping control module, the expression for its output current is as follows: ； ； in, I o For output current, U s A voltage source mapped to grid frequency disturbances. I s The current source is mapped to the power command step. C 1. C 2 is a split capacitor. L f for RL The filter inductor on the branch, R f for RL Damping resistor on the branch, R 0 is C 2. Damping resistors connected in parallel at both ends.

9. The method for suppressing low-frequency oscillations based on equivalent circuit mapping of VSG composite damping according to claim 8, characterized in that: The equivalent circuit-power control inverse mapping module inversely maps the equivalent circuit model after introducing composite damping into an active power control loop to obtain the voltage amplitude and voltage phase, as shown below: 。 10. The method for suppressing low-frequency oscillations based on equivalent circuit mapping of VSG composite damping according to claim 9, characterized in that: The voltage control module, based on voltage amplitude and phase, converts the voltage signal in the dq coordinate system into a voltage signal in the abc coordinate system through inverse Park and inverse Clark transformations. Finally, it generates a PWM signal and outputs the PWM signal to the three-phase inverter bridge, as shown below: ； ； in, V oα , V oβ The two-phase output voltage signals are in the αβ coordinate system. V od , V oq The two-phase output voltage signals are in the dq coordinate system. V oA , V oB , V oC The signal is the three-phase output voltage signal in the abc coordinate system.

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