Virtual synchronous machine grid-connected system, virtual impedance control method and device thereof and medium

Abstract

The invention provides a virtual synchronous machine grid-connected system, a virtual impedance control method and device thereof and a medium. The control method comprises the following steps: injecting sine waves into the grid-connected system; determining the line impedance of the grid-connected system according to the response information of the sine wave; determining an actual short-circuit ratio of the grid-connected system according to the line impedance; under the condition that the actual short-circuit ratio is smaller than the critical short-circuit ratio, first virtual impedance and second virtual impedance are determined according to rated information of the grid-connected system, a short-circuit ratio threshold value and line inductance; determining the operation state of the grid-connected system according to the active power of the grid-connected system; in the first operation state, the first virtual impedance is connected to the grid-connected system; and accessing the second virtual impedance to the grid-connected system in the second operation state. Thus, through real-time measurement of the line impedance and judgment of the operation state of the system, the self-adaptive virtual impedance is introduced, and the virtual impedance is configured through the short-circuit ratio threshold value, so that stable operation of the virtual synchronous machine under different short-circuit ratios is realized.

Description

Technical Field

[0001] The present invention relates to the technical field of renewable energy power generation, and in particular to a virtual synchronous machine grid-connected system and a virtual impedance control method, device and medium thereof. Background Art

[0002] As a renewable energy utilization method based on inverters, microgrid can be regarded as a small power system with the advantages of high efficiency, stability and low loss. The electric energy generated by renewable energy is converted into AC power through grid-connected inverters and connected to the grid. As the capacity of renewable energy connected to the grid increases, the grid-connected inverter must not only ensure its own stable operation, but also provide power support for the grid to maintain stable operation of the grid. Traditional grid-connected inverters do not have inertia, primary frequency regulation characteristics and primary voltage regulation characteristics, which leads to a decrease in the rotational inertia of the power system. The grid control technology can improve the voltage and frequency support capabilities of the converter and enhance the stability of the power system. At present, scholars at home and abroad have proposed a variety of grid control strategies, such as matching control and virtual oscillator controller. Among them, droop control based on the simulation of the operation mechanism of synchronous generators and VSG (Virtual Synchronous Generator) control are widely used. By simulating the operation characteristics of synchronous generators, the rotational inertia of the power system is supported.

[0003] VSG control technology has good performance under weak network conditions, but cannot operate stably under strong network conditions. Some scholars have proposed using virtual impedance to solve the related problems of grid-connected converters. However, the current research on virtual impedance is mainly applied to isolated island microgrids, without considering the power oscillation problem when connected to the grid under strong network conditions, and cannot solve the VSG operation instability caused by changes in short-circuit ratio due to changes in line impedance, output power, etc. Summary of the invention

[0004] The present invention aims to solve at least one of the technical problems existing in the prior art or related art.

[0005] To this end, a first aspect of the present invention is to propose a virtual impedance control method for a virtual synchronous machine grid-connected system.

[0006] A second aspect of the present invention is to provide a virtual impedance control device for a virtual synchronous machine grid-connected system.

[0007] The third aspect of the present invention is to provide a virtual synchronous machine grid-connected system.

[0008] A fourth aspect of the present invention is to provide a readable storage medium.

[0009] In view of this, according to a first aspect of the present invention, a virtual impedance control method for a virtual synchronous machine grid-connected system is proposed, the method comprising: injecting a sine wave into the grid-connected system; determining the line impedance of the grid-connected system according to response information of the sine wave; determining the actual short-circuit ratio of the grid-connected system according to the line impedance; when the actual short-circuit ratio is less than the critical short-circuit ratio, determining a first virtual impedance and a second virtual impedance according to rated information of the grid-connected system, a short-circuit ratio threshold and a line inductance; determining the operating state of the grid-connected system according to the active power of the grid-connected system; when the grid-connected system is in the first operating state, connecting the first virtual impedance to the grid-connected system; when the grid-connected system is in the second operating state, connecting the second virtual impedance to the grid-connected system.

[0010] Specifically, in the virtual impedance control method of the virtual synchronous machine grid-connected system provided by the present invention, a sine wave is injected into the line of the grid-connected system, and then the line impedance of the grid-connected system is determined according to the response information of the sine wave, and the real-time measurement value of the line impedance is obtained. Further, according to the measured line impedance, the actual short-circuit ratio of the grid-connected system in the current operating state is determined. Further, the actual short-circuit ratio and the critical short-circuit ratio of the grid-connected system are compared. When the actual short-circuit ratio is greater than the critical short-circuit ratio, the short-circuit ratio meets the requirements. At this time, it is determined that the grid-connected system does not need to access the virtual impedance. When the actual short-circuit ratio is less than the critical short-circuit ratio, the short-circuit ratio does not meet the requirements. At this time, it is determined that the grid-connected system needs to access the virtual impedance to maintain stability, and the first virtual impedance and the second virtual impedance are determined according to the rated information of the grid-connected system, the short-circuit ratio threshold and the line inductance. Among them, the first virtual impedance is the critical virtual impedance, and the second virtual impedance is the adaptive virtual impedance. Further, the operating state of the grid-connected system is determined to be the first operating state or the second operating state according to the active power of the grid-connected system. Among them, in the first operating state, the active power of the grid-connected system does not meet the dynamic requirements, and in the second operating state, the active power of the grid-connected system meets the dynamic requirements. Further, when the grid-connected system is in the first operating state, the first virtual impedance is connected to the grid-connected system. When the grid-connected system is in the second operating state, the second virtual impedance is connected to the grid-connected system so that the system operates in a critical short-circuit ratio state. In this way, through real-time measurement of line impedance and judgment of the system operating state, an adaptive virtual impedance is introduced, and the virtual impedance is configured through the short-circuit ratio threshold, so as to achieve stable operation of the virtual synchronous machine under different short-circuit ratios, and effectively improve the operation capability of the virtual synchronous machine under different short-circuit ratios.

[0011] The virtual impedance control method of the virtual synchronous machine grid-connected system according to the present invention may also have the following additional technical features:

[0012] In some technical solutions, optionally, the virtual synchronous machine grid-connected system includes an inverter, and injecting a sine wave into the grid-connected system includes: injecting a sine wave into a pulse width modulation wave of the inverter; wherein the sine wave frequency is a non-integer multiple of the fundamental wave frequency of the inverter, and the ratio of the sine wave amplitude to the fundamental wave amplitude is less than or equal to a first ratio.

[0013] In the technical solution, the virtual synchronous machine grid-connected system includes an inverter, and before pulse width modulation is performed on the modulation wave of the inverter, a sub-sine wave of a specific frequency is injected into the pulse width modulation wave of the inverter. The sine wave frequency is a non-integer multiple of the fundamental wave frequency of the inverter, and the ratio of the sine wave amplitude to the fundamental wave amplitude is less than or equal to a first ratio.

[0014] In some technical solutions, optionally, determining the line impedance of the grid-connected system according to the response information of the sine wave includes: obtaining the grid-connected point voltage value and the grid-connected point current value of the grid-connected system; based on a complex filter, performing coordinate transformation on the grid-connected point voltage value and the grid-connected point current value respectively through a bilinear transformation discretization algorithm to obtain a response voltage value and a response current value in an α-β coordinate system; determining the line impedance of the grid-connected system according to the response voltage value and the response current value according to the following formula; Z g =R g +j·ω r ·L g ; Among them, Z g Represents the line impedance, R g Indicates the line resistance of the grid-connected system, L g Indicates the line inductance of the grid-connected system, U pccsα Represents the voltage component of the response voltage value on the α-axis, I pccsα Indicates the current component of the response current value on the α axis, U pccsβ Represents the voltage component of the response voltage value on the β axis, I pccsβ Represents the current component of the response current value on the β axis, ω s Represents the specific angular frequency of the grid-connected system, ω r represents the fundamental angular frequency, and j represents the imaginary factor.

[0015] In this technical solution, after a sine wave is injected into the grid-connected system, the grid-connected point voltage value and grid-connected point current value of the grid-connected system are measured. Furthermore, by using a specific frequency sub-sine wave calculation method based on a complex filter, the voltage and current responses of the specific frequency sub-sine wave are extracted by means of bilinear transformation discretization, and the grid-connected point voltage value and grid-connected point current value are respectively transformed to obtain the response voltage value and response current value in the α-β coordinate system. Furthermore, according to the following formula, the line impedance of the grid-connected system is calculated based on the response voltage value and the response current value: Z g =R g +j·ωr ·L g ,

[0016] Among them, Z g Represents the line impedance, R g Indicates the line resistance of the grid-connected system, L g Indicates the line inductance of the grid-connected system, U pccsα Represents the voltage component of the response voltage value on the α-axis, I pccsα Indicates the current component of the response current value on the α axis, U pccsβ Represents the voltage component of the response voltage value on the β axis, I pccsβ Represents the current component of the response current value on the β axis, ω s Represents the specific angular frequency of the grid-connected system, ω r represents the fundamental angular frequency, j represents the imaginary factor, which means that the impedance generated by the inductor lags behind the phase of the resistor.

[0017] In some technical solutions, optionally, determining the actual short-circuit ratio of the grid-connected system according to the line impedance includes: determining the actual short-circuit ratio of the grid-connected system according to the line impedance, the line voltage of the grid-connected system and the active power output of the grid-connected point according to the following formula; Among them, K g Indicates the actual short-circuit ratio, Z g Indicates line impedance, U e Indicates the line voltage of the grid-connected system, P e Indicates the active power output by the grid-connected point.

[0018] In this technical solution, when calculating the actual short-circuit ratio of the grid-connected system, the actual short-circuit ratio of the grid-connected system is calculated according to the following formula based on the line impedance, the line voltage of the grid-connected system and the active power output of the grid-connected point: Among them, K g Indicates the actual short-circuit ratio, indicating the stability and anti-disturbance capability of the grid-connected system. g It represents the line impedance, that is, the impedance of the entire circuit, which is usually in complex form, taking into account resistance and inductance. e Indicates the line voltage of the grid-connected system, usually the voltage at the grid-connected point. e Indicates the active power output by the grid-connected point.

[0019] In some technical solutions, optionally, determining the first virtual impedance and the second virtual impedance according to the rated information of the grid-connected system, the short-circuit ratio threshold and the line inductance includes: introducing a low-pass filter, and determining the first virtual impedance according to the rated line voltage, rated angular frequency, rated active power and the short-circuit ratio threshold of the grid-connected system according to the following formula; determining the second virtual impedance according to the difference between the first virtual impedance and the line inductance of the grid-connected system; L v =L SCR -L g Among them, L SCR represents the first virtual impedance, U r Indicates the rated line voltage, ω r Indicates the rated angular frequency, P r Indicates rated active power, K SCR Indicates the short circuit ratio threshold, L v Represents the second virtual impedance, L g Indicates line inductance.

[0020] In this technical solution, in the process of calculating the first virtual impedance and the second virtual impedance, the virtual impedance is embodied in the form of virtual inductance. Specifically, a low-pass filter is introduced, and the first virtual impedance is determined according to the rated line voltage, rated angular frequency, rated active power and short-circuit ratio threshold of the grid-connected system, and the second virtual impedance is determined according to the difference between the first virtual impedance and the line inductance of the grid-connected system: L v =L SCR -L g .

[0021] Among them, L SCR It represents the first virtual impedance, i.e., the critical virtual impedance, which is associated with the electrical parameters of the system and is used to help simulate the inductance performance of the system during a short circuit fault. r Indicates the rated line voltage, which indicates the nominal voltage of the system. r Indicates the rated angular frequency, which is usually the frequency of the power grid, in radians per second. r Indicates rated active power, which indicates the nominal output power when the system is running. SCR Represents the short-circuit ratio threshold, which is used to measure the short-circuit capacity of the system, and reflects the system's immunity to disturbances under fault conditions. v represents the second virtual impedance, i.e., the adaptive virtual impedance, which is used to control the dynamic response of the system. g Indicates line inductance.

[0022] In some technical schemes, optionally, the operating state of the grid-connected system is determined based on the active power of the grid-connected system, including: when the fluctuation value of the active power is greater than the fluctuation threshold, determining that the grid-connected system is in the first operating state; when the fluctuation value of the active power is less than or equal to the fluctuation threshold, determining that the grid-connected system is in the second operating state.

[0023] In this technical solution, when determining the operating state of the grid-connected system, if the fluctuation value of the active power is greater than the fluctuation threshold, it means that the active power of the grid-connected system does not meet the dynamic requirements and the grid-connected system is in a transient process. At this time, the grid-connected system is determined to be in the first operating state. If the fluctuation value of the active power is less than or equal to the fluctuation threshold, it means that the active power of the grid-connected system meets the dynamic requirements. At this time, the grid-connected system is determined to be in the second operating state.

[0024] In some technical solutions, optionally, after the sine wave is injected into the grid-connected system, the virtual impedance control method further includes: introducing a notch filter, and the transfer function of the notch filter is: Among them, G Trap (s) is the transfer function of the notch filter, ω Trap is the specific angular frequency filtered out by the notch filter, and s is the Laplace operator.

[0025] In this technical solution, after the sine wave is injected into the grid-connected system, since the proportion of the sine wave is too small compared to the fundamental wave, a notch filter is introduced to suppress the harmonic interference in the system, ensure that the system runs more stably within a specific frequency range, reduce the impact of frequency fluctuations in the system on voltage and current, and ensure the dynamic performance indicators of the system.

[0026] The transfer function of the notch filter is: Among them, G Trap (s) is the transfer function of the notch filter, which describes the system's response to the input signal at a specific angular frequency ω Trap The following response. Trap is the specific angular frequency filtered out by the notch filter, and s is the Laplace operator.

[0027] According to a second aspect of the present invention, a virtual impedance control device for a virtual synchronous machine grid-connected system is proposed, the device comprising: an input unit for injecting a sine wave into the grid-connected system; a processing unit for determining the line impedance of the grid-connected system according to response information of the sine wave; the processing unit is also used to determine the actual short-circuit ratio of the grid-connected system according to the line impedance; the processing unit is also used to determine the first virtual impedance and the second virtual impedance according to the rated information, the short-circuit ratio threshold and the line inductance of the grid-connected system when the actual short-circuit ratio is less than the critical short-circuit ratio; the processing unit is also used to determine the operating state of the grid-connected system according to the active power of the grid-connected system; a control unit is used to connect the first virtual impedance to the grid-connected system when the grid-connected system is in the first operating state; the control unit is also used to connect the second virtual impedance to the grid-connected system when the grid-connected system is in the second operating state.

[0028] The virtual impedance control device of the virtual synchronous machine grid-connected system provided by the present invention comprises an input unit, a processing unit and a control unit. The input unit injects a sine wave into the line of the grid-connected system, and the processing unit further determines the line impedance of the grid-connected system according to the response information of the sine wave, and obtains the real-time measurement value of the line impedance. Further, the processing unit determines the actual short-circuit ratio of the grid-connected system in the current operating state according to the measured line impedance. Further, the processing unit compares the actual short-circuit ratio and the critical short-circuit ratio of the grid-connected system. When the actual short-circuit ratio is greater than the critical short-circuit ratio, the short-circuit ratio meets the requirements. At this time, the processing unit determines that the grid-connected system does not need to access the virtual impedance. When the actual short-circuit ratio is less than the critical short-circuit ratio, the short-circuit ratio does not meet the requirements. At this time, the processing unit determines that the grid-connected system needs to access the virtual impedance to maintain stability, and determines the first virtual impedance and the second virtual impedance according to the rated information of the grid-connected system, the short-circuit ratio threshold and the line inductance. Among them, the first virtual impedance is the critical virtual impedance, and the second virtual impedance is the adaptive virtual impedance. Further, the processing unit determines that the operating state of the grid-connected system is the first operating state or the second operating state according to the active power of the grid-connected system. Among them, in the first operating state, the active power of the grid-connected system does not meet the dynamic requirements, and in the second operating state, the active power of the grid-connected system meets the dynamic requirements. Further, when the grid-connected system is in the first operating state, the control unit connects the first virtual impedance to the grid-connected system. When the grid-connected system is in the second operating state, the control unit connects the second virtual impedance to the grid-connected system so that the system operates in a critical short-circuit ratio state. In this way, through real-time measurement of line impedance and judgment of the system operating state, an adaptive virtual impedance is introduced, and the virtual impedance is configured through the short-circuit ratio threshold, so as to achieve stable operation of the virtual synchronous machine under different short-circuit ratios, and effectively improve the operation capability of the virtual synchronous machine under different short-circuit ratios.

[0029] According to the third aspect of the present invention, a virtual synchronous machine grid-connected system is proposed, comprising a processor and a memory, wherein the memory stores a program or instruction that can be run on the processor, and when the program or instruction is executed by the processor, the steps of the virtual impedance control method of the virtual synchronous machine grid-connected system in any of the above technical solutions are implemented. Therefore, the virtual synchronous machine grid-connected system proposed in the third aspect of the present invention has all the beneficial effects of the virtual impedance control method of the virtual synchronous machine grid-connected system in any of the above technical solutions in the first aspect, which will not be repeated here.

[0030] According to a fourth aspect of the present invention, a readable storage medium is provided, on which a program or instruction is stored, and when the program or instruction is executed by a processor, a virtual impedance control method for a virtual synchronous machine grid-connected system as in any of the above technical solutions is implemented. Therefore, the readable storage medium provided in the fourth aspect of the present invention has all the beneficial effects of the virtual impedance control method for a virtual synchronous machine grid-connected system in any of the technical solutions in the first aspect, which will not be described in detail here.

[0031] Additional aspects and advantages of the present invention will become apparent from the following description or may be learned by practice of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] The above and / or additional aspects and advantages of the present invention will become apparent and easily understood from the description of the embodiments in conjunction with the following drawings, in which:

[0033] Figure 1 One of the flow charts of the virtual impedance control method of the virtual synchronous machine grid-connected system according to the embodiment of the present invention is shown;

[0034] Figure 2 A second flow chart of a virtual impedance control method of a virtual synchronous machine grid-connected system according to an embodiment of the present invention is shown;

[0035] Figure 3 A schematic diagram showing a virtual impedance control method of a virtual synchronous machine grid-connected system according to an embodiment of the present invention;

[0036] Figure 4 A structural block diagram showing a virtual impedance control device of a virtual synchronous machine grid-connected system according to an embodiment of the present invention;

[0037] Figure 5 A structural block diagram of a virtual synchronous machine grid-connected system according to an embodiment of the present invention is shown. DETAILED DESCRIPTION

[0038] In order to more clearly understand the above-mentioned purpose, features and advantages of the present invention, the present invention is further described in detail below in conjunction with the accompanying drawings and specific embodiments. It should be noted that the embodiments of the present invention and the features in the embodiments can be combined with each other without conflict.

[0039] In the following description, many specific details are set forth to facilitate a full understanding of the present invention. However, the present invention may also be implemented in other ways different from those described herein. Therefore, the protection scope of the present invention is not limited to the specific embodiments disclosed below.

[0040] Combine the following Figures 1 to 5, through specific embodiments and application scenarios, the virtual synchronous machine grid-connected system and its virtual impedance control method, device and medium provided in the embodiments of the present application are described in detail.

[0041] In one embodiment of the present invention, Figure 1 As shown, the virtual impedance control method of the virtual synchronous machine grid-connected system may specifically include the following steps 102 to 114:

[0042] Step 102, injecting a sine wave into the grid-connected system;

[0043] Step 104, determining the line impedance of the grid-connected system according to the response information of the sine wave;

[0044] Step 106, determining the actual short-circuit ratio of the grid-connected system according to the line impedance;

[0045] Step 108, when the actual short circuit ratio is less than the critical short circuit ratio, determining the first virtual impedance and the second virtual impedance according to the rated information of the grid-connected system, the short circuit ratio threshold and the line inductance;

[0046] Step 110, determining the operating state of the grid-connected system according to the active power of the grid-connected system;

[0047] Step 112, when the grid-connected system is in the first operating state, connecting the first virtual impedance to the grid-connected system;

[0048] Step 114: When the grid-connected system is in the second operating state, connect the second virtual impedance to the grid-connected system.

[0049] Specifically, in the virtual impedance control method of the virtual synchronous machine grid-connected system provided by the present invention, a sine wave is injected into the line of the grid-connected system, and then the line impedance of the grid-connected system is determined according to the response information of the sine wave, and the real-time measurement value of the line impedance is obtained. Further, according to the measured line impedance, the actual short-circuit ratio of the grid-connected system in the current operating state is determined. Further, the actual short-circuit ratio and the critical short-circuit ratio of the grid-connected system are compared. When the actual short-circuit ratio is greater than the critical short-circuit ratio, the short-circuit ratio meets the requirements. At this time, it is determined that the grid-connected system does not need to access the virtual impedance. When the actual short-circuit ratio is less than the critical short-circuit ratio, the short-circuit ratio does not meet the requirements. At this time, it is determined that the grid-connected system needs to access the virtual impedance to maintain stability, and the first virtual impedance and the second virtual impedance are determined according to the rated information of the grid-connected system, the short-circuit ratio threshold and the line inductance. Among them, the first virtual impedance is the critical virtual impedance, and the second virtual impedance is the adaptive virtual impedance. Further, the operating state of the grid-connected system is determined to be the first operating state or the second operating state according to the active power of the grid-connected system. Among them, in the first operating state, the active power of the grid-connected system does not meet the dynamic requirements, and in the second operating state, the active power of the grid-connected system meets the dynamic requirements. Further, when the grid-connected system is in the first operating state, the first virtual impedance is connected to the grid-connected system. When the grid-connected system is in the second operating state, the second virtual impedance is connected to the grid-connected system so that the system operates in a critical short-circuit ratio state. In this way, through real-time measurement of line impedance and judgment of the system operating state, an adaptive virtual impedance is introduced, and the virtual impedance is configured through the short-circuit ratio threshold, so as to achieve stable operation of the virtual synchronous machine under different short-circuit ratios, and effectively improve the operation capability of the virtual synchronous machine under different short-circuit ratios.

[0050] In some embodiments of the present invention, optionally, the virtual synchronous machine grid-connected system includes an inverter, and the above step 102 may specifically include the following step 102a:

[0051] Step 102a, injecting a sine wave into the pulse width modulation wave of the inverter;

[0052] The sine wave frequency is a non-integer multiple of the fundamental wave frequency of the inverter, and the ratio of the sine wave amplitude to the fundamental wave amplitude is less than or equal to the first ratio.

[0053] In this embodiment, the virtual synchronous machine grid-connected system includes an inverter, and before pulse width modulation is performed on the modulation wave of the inverter, a sub-sine wave of a specific frequency is injected into the pulse width modulation wave of the inverter. The sine wave frequency is a non-integer multiple of the fundamental wave frequency of the inverter, and the ratio of the sine wave amplitude to the fundamental wave amplitude is less than or equal to a first ratio.

[0054] In actual application, the first ratio may be 5% or other values. Those skilled in the art may set the specific value of the first ratio according to actual conditions, and no specific limitation is made here.

[0055] In some embodiments of the present invention, optionally, the above step 104 may specifically include the following steps 104a and 104c:

[0056] Step 104a, obtaining a grid connection point voltage value and a grid connection point current value of the grid connection system;

[0057] Step 104b, based on a complex filter, coordinate transformation is performed on the grid connection point voltage value and the grid connection point current value respectively by a bilinear transformation discretization algorithm to obtain a response voltage value and a response current value in an α-β coordinate system;

[0058] Step 104c, according to formula Z g =R g +j·ω r ·L g , and Determine the line impedance of the grid-connected system according to the response voltage value and the response current value;

[0059] Among them, Z g Represents the line impedance, R g Indicates the line resistance of the grid-connected system, L g Indicates the line inductance of the grid-connected system, U pccsα Represents the voltage component of the response voltage value on the α-axis, I pccsα Indicates the current component of the response current value on the α axis, U pccsβ Represents the voltage component of the response voltage value on the β axis, I pccsβ Represents the current component of the response current value on the β axis, ω s Represents the specific angular frequency of the grid-connected system, ω r represents the fundamental angular frequency, and j represents the imaginary factor.

[0060] In this embodiment, after a sine wave is injected into the grid-connected system, the current and voltage at the PCC (Point of Common Coupling) of the grid-connected system are measured to obtain the grid-connected point voltage value and grid-connected point current value of the grid-connected system. Furthermore, by using a specific frequency sub-sine wave calculation method based on a complex filter, the voltage and current responses of the specific frequency sub-sine wave are extracted by means of bilinear transformation discretization, and the grid-connected point voltage value and grid-connected point current value are respectively transformed to obtain the response voltage value and response current value in the α-β coordinate system. Furthermore, according to the following formula, the line impedance of the grid-connected system is calculated based on the response voltage value and the response current value:

[0061] Z g =R g +j·ω r ·L g (1)

[0062]

[0063] Among them, Z g Represents the line impedance, R g Indicates the line resistance of the grid-connected system, L g Indicates the line inductance of the grid-connected system, U pccsα Represents the voltage component of the response voltage value on the α-axis, I pccsα Indicates the current component of the response current value on the α axis, U pccsβ Represents the voltage component of the response voltage value on the β axis, I pccsβ Represents the current component of the response current value on the β axis, ω s Represents the specific angular frequency of the grid-connected system, ω r represents the fundamental angular frequency, j represents the imaginary factor, which means that the impedance generated by the inductor lags behind the phase of the resistor.

[0064] Furthermore, the transfer function of the complex filter is:

[0065]

[0066] Among them, ω c is the cutoff angular frequency of the complex filter, which represents the bandwidth of the complex filter and determines the response speed and frequency selectivity of the complex filter. s is the Laplace operator, representing the complex variable in the complex frequency domain, and j is the imaginary factor, indicating that the formula processes complex domain signals. ω k is the specific angular frequency extracted by the complex filter, G ωk (s) is the transfer function of the complex filter, which is used to extract the specific angular frequency ω k of a sine wave.

[0067] Furthermore, the above formula (1) shows that the line impedance is the total complex impedance of the line resistance and the line inductance.

[0068] Furthermore, the above formula (2) calculates the line resistance of the grid-connected system through the response voltage value and the response current value of the sub-sine wave of a specific frequency in the α-β coordinate system. Formula (2) shows that the line resistance is a linear combination of the components of the response current value and the response voltage value on the α-axis and the β-axis.

[0069] Furthermore, the above formula (3) shows that the line inductance is the cross product (i.e., vector product) of the components of the response voltage value and the response current value on the α-axis and the β-axis, divided by the sum of the squares of the components of the response voltage value on the α-axis and the β-axis and the specific angular frequency ω s As a result, formula (3) also takes into account the inductive effect in the motor.

[0070] Furthermore, the above formula (4) describes the transfer function of the complex filter used for virtual synchronous machine control, which is used to extract the specific angular frequency ω k Specifically, the transfer function of formula (4) describes a second-order filter that extracts a specific angular frequency ω from the input signal. k The bandwidth of the filter is controlled by the sine wave, allowing the filter to have a good selective response near a specific corner frequency. In formula (4), s-jω k Represents the behavior of the grid-connected system input signal in the complex frequency domain, minus a specific angular frequency ω k This allows the filter to focus on extracting signal components at specific corner frequencies.

[0071] In some embodiments of the present invention, optionally, the above step 106 may specifically include the following step 106a:

[0072] Step 106a, according to the formula Determine the actual short-circuit ratio of the grid-connected system based on the line impedance, the line voltage of the grid-connected system and the active power output at the grid-connected point;

[0073] Among them, K g Indicates the actual short-circuit ratio, Z g Indicates line impedance, U e Indicates the line voltage of the grid-connected system, P e Indicates the active power output by the grid-connected point.

[0074] In this embodiment, when calculating the actual short-circuit ratio of the grid-connected system, the actual short-circuit ratio of the grid-connected system is calculated according to the following formula based on the line impedance, the line voltage of the grid-connected system and the active power output by the grid-connected point.

[0075]

[0076] Among them, K g Indicates the actual short-circuit ratio, indicating the stability and anti-disturbance capability of the grid-connected system. g It represents the line impedance, that is, the impedance of the entire circuit, which is usually in complex form, taking into account resistance and inductance. e Indicates the line voltage of the grid-connected system, usually the voltage at the PCC. e Indicates the active power output by the grid-connected point.

[0077] Furthermore, the above formula (5) shows that the actual short-circuit ratio is obtained by dividing the ratio of the square of the line voltage to the active power by the modulus of the line impedance. The actual short-circuit ratio indicates the impedance response capability of the grid-connected system to a short-circuit fault. A higher short-circuit ratio generally means that the grid-connected system has higher stability during a fault.

[0078] In some embodiments of the present invention, optionally, the step of determining the first virtual impedance and the second virtual impedance according to the rated information of the grid-connected system, the short-circuit ratio threshold and the line inductance may specifically include the following steps 116 and 118:

[0079] Step 116, introduce a low-pass filter, according to the formula Determining a first virtual impedance according to a rated line voltage, a rated angular frequency, a rated active power and a short-circuit ratio threshold of the grid-connected system;

[0080] Step 118, according to formula L v =L SCR -L g , determining a second virtual impedance according to a difference between the first virtual impedance and the line inductance of the grid-connected system;

[0081] Among them, L SCR represents the first virtual impedance, U r Indicates the rated line voltage, ω r Indicates the rated angular frequency, P r Indicates rated active power, K SCR Indicates the short circuit ratio threshold, L v Represents the second virtual impedance, L g Indicates line inductance.

[0082] In this embodiment, in the process of calculating the first virtual impedance and the second virtual impedance, the virtual impedance is embodied in the form of virtual inductance. Specifically, a low-pass filter is introduced, and the first virtual impedance is determined according to the rated line voltage, rated angular frequency, rated active power and short-circuit ratio threshold of the grid-connected system according to the following formula:

[0083]

[0084] Among them, L SCR It represents the first virtual impedance, i.e., the critical virtual impedance, which is associated with the electrical parameters of the system and is used to help simulate the inductance performance of the system during a short circuit fault. r Indicates the rated line voltage, which indicates the nominal voltage of the system. r Indicates the rated angular frequency, which is usually the frequency of the power grid, in radians per second. r Indicates rated active power, which indicates the nominal output power when the system is running. SCRRepresents the short-circuit ratio threshold, which is used to measure the short-circuit capacity of the system, and it reflects the system's immunity to disturbances under fault conditions.

[0085] Furthermore, the second virtual impedance is determined according to the difference between the first virtual impedance and the line inductance of the grid-connected system according to the following formula:

[0086] L v =L SCR -L g (7)

[0087] Among them, L v represents the second virtual impedance, i.e., the adaptive virtual impedance, which is used to control the dynamic response of the system. g Indicates line inductance.

[0088] Furthermore, the transfer function of the low-pass filter is:

[0089]

[0090] Among them, G LSCR (s) is the transfer function of the low-pass filter, which represents the response characteristics of the system to the input signal in frequency. LSCR is the cutoff frequency of the low-pass filter, which determines the highest frequency that the low-pass filter can pass. LSCR When , the low-pass filter will significantly attenuate the signal. s is the Laplace operator, which is used to describe the behavior of the system in the complex frequency domain.

[0091] Furthermore, the function of the low-pass filter is to allow low-frequency signals to pass through while suppressing or attenuating high-frequency signals. In the above formula (8), the numerator is ω LSCR , the denominator is s+ω LSCR , which means that when the frequency of the input signal is close to or below ω LSCR When the input signal frequency is much higher than ω, the signal is almost not attenuated. LSCR By introducing a low-pass filter, when virtual inductance is introduced into the system, the effect of virtual inductance can be limited to the subsynchronous frequency band, thereby avoiding high-frequency interference in differential operations.

[0092] In some embodiments of the present invention, optionally, the above step 110 may specifically include the following steps 110a and 110b:

[0093] Step 110a, when the fluctuation value of the active power is greater than the fluctuation threshold, determining that the grid-connected system is in the first operating state;

[0094] Step 110b: when the fluctuation value of the active power is less than or equal to the fluctuation threshold, determining that the grid-connected system is in the second operating state.

[0095] In this embodiment, when determining the operating state of the grid-connected system, if the fluctuation value of the active power is greater than the fluctuation threshold, it means that the active power of the grid-connected system does not meet the dynamic requirements and the grid-connected system is in a transient process. At this time, the grid-connected system is determined to be in the first operating state. If the fluctuation value of the active power is less than or equal to the fluctuation threshold, it means that the active power of the grid-connected system meets the dynamic requirements. At this time, the grid-connected system is determined to be in the second operating state.

[0096] It should be noted that after determining that the grid-connected system is in the first operating state and connecting the first virtual impedance, namely the critical virtual impedance, the operating state of the grid-connected system will continue to be evaluated until the grid-connected system is in the second operating state and the second virtual impedance, namely the adaptive virtual impedance, is connected to make the line operate stably in the critical short-circuit ratio state.

[0097] In actual application, the specific value of the fluctuation threshold can be set by those skilled in the art according to actual conditions, and no specific limitation is made here.

[0098] In some embodiments of the present invention, optionally, after the above step 102, the above method may further include the following step 120:

[0099] Step 120, introducing a notch filter;

[0100] The transfer function of the notch filter is: G Trap (s) is the transfer function of the notch filter, ω Trap is the specific angular frequency filtered out by the notch filter, and s is the Laplace operator.

[0101] In this embodiment, after a sine wave is injected into the grid-connected system, since the proportion of the sine wave is too small compared to the fundamental wave, a notch filter is introduced to suppress harmonic interference in the system, ensuring that the system operates more stably within a specific frequency range, reducing the impact of frequency fluctuations in the system on voltage and current, and ensuring the dynamic performance indicators of the system.

[0102] The transfer function of the notch filter is:

[0103]

[0104] Among them, G Trap (s) is the transfer function of the notch filter, which describes the system's response to the input signal at a specific angular frequency ω Trap The following response. Trap is the specific angular frequency filtered out by the notch filter, and s is the Laplace operator.

[0105] Furthermore, the notch filter is a filter with strong selectivity, which is used to suppress or eliminate the signal of a specific angular frequency, while retaining the signals of other frequencies, so as to suppress the harmonic interference of the specific angular frequency, thereby ensuring the dynamic performance and stability of the system. In the above formula (9), the numerator is The denominator is This means that the energy of the input signal will be suppressed to a minimum around a specific corner frequency.

[0106] In summary, if Figure 2 As shown, the virtual impedance control method of the virtual synchronous machine grid-connected system provided by the present invention may specifically include the following steps 202 to 216:

[0107] Step 202, injecting a sub-sine wave of a specific frequency;

[0108] Step 204, calculating the line impedance according to the response information of the sine wave;

[0109] Step 206, calculating the actual short circuit ratio according to the line impedance;

[0110] Step 208, judging: the short circuit ratio meets the requirement, if so, the process ends, if not, executing step 210;

[0111] Step 210, calculating the critical virtual impedance and the adaptive virtual impedance;

[0112] Step 212, determine: the system meets the dynamic requirements, if so, execute step 214, if not, execute step 216;

[0113] Step 214, connecting the adaptive virtual impedance;

[0114] Step 216: connect the critical virtual impedance.

[0115] Specifically, the virtual impedance control method of the virtual synchronous machine grid-connected system provided by the present invention is as follows: Figure 3 As shown, first, the inductor current i output on the inverter side is Labc , inductor voltage u nabc And the grid side voltage u gabc , grid side current i gabc Sampling is performed, and then the inductor voltage u in the abc coordinate system is transformed through coordinate transformation. nabc , grid side voltage u gabc , inductor current i Labc and the grid-side current i gabc Transformed into the inductor current i in the dq coordinate system respectively Ldq , inductor voltage u ndq , grid side voltage u gdq And the grid-side current i gdqFurthermore, by sampling the grid-side voltage u gdq And the grid-side current i gdq Calculate the instantaneous power and input the calculation result into the VSG control, then calculate the frequency and amplitude reference values ​​through the active loop and reactive loop to obtain the output voltage reference value u of the inverter ref Furthermore, based on the sampled grid-side voltage u gdq and the grid-side current i gdq According to the discretization method of bilinear transformation, the response voltage and current values ​​of the sub-sine wave of a specific frequency are extracted, and the line impedance R is calculated. v , L v , and calculate the actual short-circuit ratio SCR of the system under the current operating conditions according to the line impedance. Further, based on the adaptive impedance control method, according to the actual short-circuit ratio SCR and the grid-side current i gdq , calculate the voltage value u of the virtual impedance vdq , and then based on the voltage value u of the virtual impedance vdq , inductor current i Ldq And the inductor voltage u ndq , and perform voltage and current PI (Proportion-Integration) dual closed-loop control. Furthermore, a sub-sine wave of a specific frequency is injected into the modulation wave to generate a PWM (Pulse Width Modulation) signal to act on the grid-side converter.

[0116] In this way, the virtual impedance control method of the virtual synchronous machine grid-connected system provided by the present invention calculates the real-time short-circuit ratio of the system through real-time measurement of the line impedance, introduces adaptive virtual impedance based on the judgment of the system operation status, and configures the virtual impedance through the short-circuit ratio threshold, thereby realizing the stable operation of VSG under different short-circuit ratios.

[0117] In one embodiment of the present invention, a virtual impedance control device for a virtual synchronous machine grid-connected system is also provided. Figure 4 As shown, Figure 4 The structure block diagram of the virtual impedance control device 300 of the virtual synchronous machine grid-connected system according to the embodiment of the present invention is shown. The virtual impedance control device 300 of the virtual synchronous machine grid-connected system may specifically include the following input unit 302, processing unit 304 and control unit 306:

[0118] An input unit 302, for injecting a sine wave into the grid-connected system;

[0119] The processing unit 304 is used to determine the line impedance of the grid-connected system according to the response information of the sine wave;

[0120] The processing unit 304 is further used to determine the actual short-circuit ratio of the grid-connected system according to the line impedance;

[0121] The processing unit 304 is further configured to determine the first virtual impedance and the second virtual impedance according to the rated information of the grid-connected system, the short circuit ratio threshold and the line inductance when the actual short circuit ratio is less than the critical short circuit ratio;

[0122] The processing unit 304 is further used to determine the operation state of the grid-connected system according to the active power of the grid-connected system;

[0123] A control unit 306, configured to connect the first virtual impedance to the grid-connected system when the grid-connected system is in the first operating state;

[0124] The control unit 306 is further configured to connect the second virtual impedance to the grid-connected system when the grid-connected system is in the second operating state.

[0125] The virtual impedance control device 300 of the virtual synchronous machine grid-connected system provided by the embodiment of the present invention includes an input unit 302, a processing unit 304 and a control unit 306. The input unit 302 injects a sine wave into the line of the grid-connected system, and the processing unit 304 further determines the line impedance of the grid-connected system according to the response information of the sine wave, and obtains the real-time measurement value of the line impedance. Further, the processing unit 304 determines the actual short-circuit ratio of the grid-connected system in the current operating state according to the measured line impedance. Further, the processing unit 304 compares the actual short-circuit ratio and the critical short-circuit ratio of the grid-connected system. When the actual short-circuit ratio is greater than the critical short-circuit ratio, the short-circuit ratio meets the requirements. At this time, the processing unit 304 determines that the grid-connected system does not need to be connected to the virtual impedance. When the actual short-circuit ratio is less than the critical short-circuit ratio, the short-circuit ratio does not meet the requirements. At this time, the processing unit 304 determines that the grid-connected system needs to be connected to the virtual impedance to maintain stability, and determines the first virtual impedance and the second virtual impedance according to the rated information of the grid-connected system, the short-circuit ratio threshold and the line inductance. Among them, the first virtual impedance is a critical virtual impedance, and the second virtual impedance is an adaptive virtual impedance. Further, the processing unit 304 determines that the operating state of the grid-connected system is the first operating state or the second operating state according to the active power of the grid-connected system. Among them, in the first operating state, the active power of the grid-connected system does not meet the dynamic requirements, and in the second operating state, the active power of the grid-connected system meets the dynamic requirements. Further, when the grid-connected system is in the first operating state, the control unit 306 connects the first virtual impedance to the grid-connected system. When the grid-connected system is in the second operating state, the control unit 306 connects the second virtual impedance to the grid-connected system so that the system operates in a critical short-circuit ratio state. In this way, through real-time measurement of line impedance and judgment of system operating state, adaptive virtual impedance is introduced, and virtual impedance is configured through short-circuit ratio threshold, so as to realize stable operation of virtual synchronous machine under different short-circuit ratios, and effectively improve the operation capability of virtual synchronous machine under different short-circuit ratios.

[0126] In some embodiments of the present invention, optionally, the virtual synchronous machine grid-connected system includes an inverter, and the input unit 302 is specifically used to: inject a sine wave into the pulse width modulation wave of the inverter; wherein the sine wave frequency is a non-integer multiple of the fundamental wave frequency of the inverter, and the ratio of the sine wave amplitude to the fundamental wave amplitude is less than or equal to the first ratio.

[0127] In some embodiments of the present invention, optionally, the processing unit 304 is specifically used to: obtain the grid-connected point voltage value and the grid-connected point current value of the grid-connected system; based on the complex filter, the grid-connected point voltage value and the grid-connected point current value are respectively converted into coordinates by a bilinear transformation discretization algorithm to obtain a response voltage value and a response current value in an α-β coordinate system; according to the following formula, the line impedance of the grid-connected system is determined according to the response voltage value and the response current value; Z g=R g +j·ω r ·L g ; Among them, Z g Represents the line impedance, R g Indicates the line resistance of the grid-connected system, L g Indicates the line inductance of the grid-connected system, U pccsα Represents the voltage component of the response voltage value on the α-axis, I pccsα Indicates the current component of the response current value on the α axis, U pccsβ Represents the voltage component of the response voltage value on the β axis, I pccsβ Represents the current component of the response current value on the β axis, ω s Represents the specific angular frequency of the grid-connected system, ω r represents the fundamental angular frequency, and j represents the imaginary factor.

[0128] In some embodiments of the present invention, optionally, the processing unit 304 is specifically used to: determine the actual short-circuit ratio of the grid-connected system according to the line impedance, the line voltage of the grid-connected system and the active power output of the grid-connected point according to the following formula; Among them, K g Indicates the actual short-circuit ratio, Z g Indicates line impedance, U e Indicates the line voltage of the grid-connected system, P e Indicates the active power output by the grid-connected point.

[0129] In some embodiments of the present invention, optionally, the processing unit 304 is specifically used to: introduce a low-pass filter, and determine the first virtual impedance according to the rated line voltage, rated angular frequency, rated active power and short-circuit ratio threshold of the grid-connected system according to the following formula; determine the second virtual impedance according to the difference between the first virtual impedance and the line inductance of the grid-connected system; L v =L SCR -L g Among them, L SCR represents the first virtual impedance, U r Indicates the rated line voltage, ω r Indicates the rated angular frequency, P r Indicates rated active power, K SCR Indicates the short circuit ratio threshold, L v Represents the second virtual impedance, L g Indicates line inductance.

[0130] In some embodiments of the present invention, optionally, the processing unit 304 is specifically used to: when the fluctuation value of the active power is greater than the fluctuation threshold, determine that the grid-connected system is in the first operating state; when the fluctuation value of the active power is less than or equal to the fluctuation threshold, determine that the grid-connected system is in the second operating state.

[0131] In some embodiments of the present invention, optionally, after the sine wave is injected into the grid-connected system, the processing unit 304 is further used to: introduce a notch filter, and the transfer function of the notch filter is: Among them, G Trap (s) is the transfer function of the notch filter, ω Trap is the specific angular frequency filtered out by the notch filter, and s is the Laplace operator.

[0132] In one embodiment of the present invention, a virtual synchronous machine grid-connected system is also proposed. Figure 5 As shown, Figure 5 The structure block diagram of the virtual synchronous machine grid-connected system 400 provided by the embodiment of the present invention is shown. The virtual synchronous machine grid-connected system 400 includes:

[0133] Memory 402, where programs or instructions are stored;

[0134] The processor 404 implements the steps of the virtual impedance control method of the virtual synchronous machine grid-connected system in any of the above embodiments when the processor 404 executes the above program or instruction.

[0135] The virtual synchronous machine grid-connected system 400 provided in this embodiment includes a memory 402 and a processor 404. When the program or instruction in the memory 402 is executed by the processor 404, the steps of the virtual impedance control method of the virtual synchronous machine grid-connected system in any of the above embodiments are implemented. Therefore, the virtual synchronous machine grid-connected system 400 has all the beneficial effects of the virtual impedance control method of the virtual synchronous machine grid-connected system in any of the above embodiments, which will not be repeated here.

[0136] Specifically, the memory 402 and the processor 404 may be connected via a bus or other means. The processor 404 may include one or more processing units, and the processor 404 may be a central processing unit (CPU), a digital signal processor (DSP), an application specific integrated circuit (ASIC), a field programmable gate array (FPGA), and other chips.

[0137] In one embodiment of the present invention, a readable storage medium is also provided, on which a program or instruction is stored, and when the program or instruction is executed by a processor, the steps of the virtual impedance control method of the virtual synchronous machine grid-connected system in any of the above embodiments are implemented.

[0138] The readable storage medium provided by the embodiment of the present invention can implement the steps of the virtual impedance control method of the virtual synchronous machine grid-connected system in any of the above embodiments when the program or instruction stored therein is executed by the processor. Therefore, the readable storage medium has all the beneficial effects of the virtual impedance control method of the virtual synchronous machine grid-connected system in any of the above embodiments, which will not be described in detail here.

[0139] Specifically, the above-mentioned readable storage medium may include any medium capable of storing or transmitting information. Examples of readable storage media include electronic circuits, semiconductor memory devices, read-only memory (ROM), random access memory (RAM), compact disc read-only memory (CD-ROM), flash memory, erasable ROM (EROM), magnetic tape, floppy disk, optical disk, hard disk, optical fiber medium, radio frequency (RF) link, optical data storage device, etc. The code segment can be downloaded via a computer network such as the Internet, an intranet, etc.

[0140] In the description of this specification, the terms "first" and "second" are used for descriptive purposes only and cannot be understood as indicating or implying relative importance unless otherwise clearly specified and limited; the terms "connection", "installation", "fixation" and the like should be understood in a broad sense, for example, "connection" can be a fixed connection, a detachable connection, or an integral connection; it can be directly connected or indirectly connected through an intermediate medium. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0141] In the description of this specification, the description of the terms "one embodiment", "some embodiments", "specific embodiments", etc. means that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representation of the above terms does not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described can be combined in any one or more embodiments or examples in a suitable manner.

[0142] In addition, the technical solutions between the various embodiments of the present invention can be combined with each other, but it must be based on the fact that ordinary technicians in the field can implement it. When the combination of technical solutions is contradictory or cannot be implemented, it should be deemed that such combination of technical solutions does not exist and is not within the scope of protection required by the present invention.

[0143] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A virtual impedance control method for a virtual synchronous machine grid-connected system, characterized in that: include: Inject a sine wave into the grid-connected system; Determining the line impedance of the grid-connected system according to the response information of the sine wave; Determining an actual short-circuit ratio of the grid-connected system according to the line impedance; When the actual short-circuit ratio is less than the critical short-circuit ratio, determining a first virtual impedance and a second virtual impedance according to the rated information of the grid-connected system, a short-circuit ratio threshold and a line inductance; Determining the operating state of the grid-connected system according to the active power of the grid-connected system; When the grid-connected system is in a first operating state, connecting the first virtual impedance to the grid-connected system; When the grid-connected system is in the second operating state, the second virtual impedance is connected to the grid-connected system.

2. The virtual impedance control method of the virtual synchronous machine grid-connected system according to claim 1, characterized in that: The virtual synchronous machine grid-connected system includes an inverter, and the injecting of a sine wave into the grid-connected system includes: injecting a sine wave into a pulse width modulation wave of the inverter; The sine wave frequency is a non-integer multiple of the fundamental wave frequency of the inverter, and the ratio of the sine wave amplitude to the fundamental wave amplitude is less than or equal to the first ratio.

3. The virtual impedance control method of the virtual synchronous machine grid-connected system according to claim 1, characterized in that: The determining the line impedance of the grid-connected system according to the response information of the sine wave comprises: Obtaining a grid-connected point voltage value and a grid-connected point current value of the grid-connected system; Based on a complex filter, coordinate transformation is performed on the grid connection point voltage value and the grid connection point current value respectively through a bilinear transformation discretization algorithm to obtain a response voltage value and a response current value in an α-β coordinate system; According to the following formula, the line impedance of the grid-connected system is determined based on the response voltage value and the response current value; Z g =R g +j·ω r ·L g ; Among them, Z g represents the line impedance, R g Represents the line resistance of the grid-connected system, L g represents the line inductance of the grid-connected system, U pccsα represents the voltage component of the response voltage value on the α-axis, I pccsα represents the current component of the response current value on the α axis, U pccsβ represents the voltage component of the response voltage value on the β axis, I pccsβ represents the current component of the response current value on the β axis, ω s represents the specific angular frequency of the grid-connected system, ω r represents the fundamental angular frequency, and j represents the imaginary factor.

4. The virtual impedance control method of the virtual synchronous machine grid-connected system according to claim 1, characterized in that: Determining the actual short-circuit ratio of the grid-connected system according to the line impedance includes: According to the following formula, the actual short-circuit ratio of the grid-connected system is determined based on the line impedance, the line voltage of the grid-connected system and the active power output by the grid-connected point; Among them, K g represents the actual short circuit ratio, Z g represents the line impedance, U e represents the line voltage of the grid-connected system, P e Represents the active power output by the grid-connected point.

5. The virtual impedance control method of the virtual synchronous machine grid-connected system according to claim 1, characterized in that: The determining of the first virtual impedance and the second virtual impedance according to the rated information of the grid-connected system, the short-circuit ratio threshold and the line inductance comprises: A low-pass filter is introduced to determine the first virtual impedance according to the rated line voltage, rated angular frequency, rated active power and short-circuit ratio threshold of the grid-connected system according to the following formula; Determining the second virtual impedance according to a difference between the first virtual impedance and the line inductance of the grid-connected system; L v =L SCR -L g ; Among them, L SCR represents the first virtual impedance, U r represents the rated line voltage, ω r Denotes the rated angular frequency, P r Indicates the rated active power, K SCR represents the short circuit ratio threshold, L v The second virtual impedance, L g represents the line inductance.

6. The virtual impedance control method of the virtual synchronous machine grid-connected system according to claim 1, characterized in that: Determining the operating state of the grid-connected system according to the active power of the grid-connected system includes: When the fluctuation value of the active power is greater than the fluctuation threshold, determining that the grid-connected system is in the first operating state; When the fluctuation value of the active power is less than or equal to the fluctuation threshold, it is determined that the grid-connected system is in the second operating state.

7. The virtual impedance control method of a virtual synchronous machine grid-connected system according to any one of claims 1 to 6, characterized in that: After the sine wave is injected into the grid-connected system, the virtual impedance control method further comprises: A notch filter is introduced, and the transfer function of the notch filter is: Among them, G Trap (s) is the transfer function of the notch filter, ω Trap is the specific angular frequency filtered out by the notch filter, and s is the Laplace operator.

8. A virtual impedance control device for a virtual synchronous machine grid-connected system, characterized in that: include: An input unit for injecting a sine wave into the grid-connected system; A processing unit, configured to determine the line impedance of the grid-connected system according to the response information of the sine wave; The processing unit is further used to determine the actual short-circuit ratio of the grid-connected system according to the line impedance; The processing unit is further configured to determine the first virtual impedance and the second virtual impedance according to the rated information of the grid-connected system, the short-circuit ratio threshold and the line inductance when the actual short-circuit ratio is less than the critical short-circuit ratio; The processing unit is further used to determine the operating state of the grid-connected system according to the active power of the grid-connected system; A control unit, configured to connect the first virtual impedance to the grid-connected system when the grid-connected system is in a first operating state; The control unit is further configured to connect the second virtual impedance to the grid-connected system when the grid-connected system is in a second operating state.

9. A virtual synchronous machine grid-connected system, characterized in that: It comprises a processor and a memory, wherein the memory stores a program or instruction that can be run on the processor, and when the program or instruction is executed by the processor, the steps of the virtual impedance control method of the virtual synchronous machine grid-connected system as described in any one of claims 1 to 7 are implemented.

10. A readable storage medium, characterized in that: The readable storage medium stores a program or an instruction, and when the program or the instruction is executed by the processor, the steps of the virtual impedance control method of the virtual synchronous machine grid-connected system according to any one of claims 1 to 7 are implemented.

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