Three-phase asymmetric power fluctuation suppression method and system based on virtual synchronous machine

Abstract

The invention provides a three-phase asymmetric power fluctuation suppression method and system based on a virtual synchronous machine, and the method comprises the steps: S1, constructing a virtual phase current regulation and control model, and introducing a corresponding virtual phase current gain into the virtual phase current regulation and control model for a voltage drop phase when detecting a voltage drop, performing preliminary suppression on power fluctuation through the virtual phase current gain; and S2, controlling the current reference value by adopting a virtual phase voltage regulation and control technology so as to enable the virtual synchronous machine to output a three-phase balance current, and then cooperatively inhibiting power fluctuation with a virtual phase current regulation and control technology.

Description

Technical Field

[0001] The present invention relates to the field of microgrid operation control technology, and in particular, to a method and system for suppressing three-phase asymmetric power fluctuations based on a virtual synchronous machine, and more particularly, to a method for suppressing power fluctuations by controlling a virtual synchronous machine inverter under voltage asymmetry in a three-phase grid. Background Art

[0002] The rapid development of renewable energy has led to an increasing penetration of distributed power generation (DGs) based on power electronics in power systems. However, due to the lack of inertia damping in traditional power electronics, this has threatened the stability of power systems. The emergence of virtual synchronous generator control technology effectively eliminates the low inertia and weak damping issues associated with grid-connected DGs, enhancing their ability to support grid frequency and voltage. However, most current research on VSG technology focuses solely on scenarios with symmetrical three-phase grid voltages. In actual operation, distribution network voltages are susceptible to three-phase voltage asymmetry due to factors such as load imbalance, short-circuit faults, and non-full-phase operation. In these cases, VSGs, like traditional inverters, can exhibit problems such as grid current distortion and fluctuations in active and reactive power, compromising safe and stable system operation.

[0003] Existing analysis methods for inverter output power fluctuations caused by three-phase grid voltage asymmetry include the positive-sequence separation method, which calculates positive- and negative-sequence current reference values ​​based on a given power and then employs current decoupling control; and the harmonic current compensation method, which suppresses power fluctuations by adjusting the harmonic components of the inverter output current. While these methods significantly suppress power fluctuations, they cannot be directly applied to grid-connected inverters based on virtual synchronous machine control. This is because most of these methods derive current reference values ​​from a designed outer loop. In virtual synchronous machine control, the outer loop is replaced by the virtual synchronous machine, and the reference current must be provided by the virtual synchronous machine. Consequently, subsequent research has resulted in improved positive- and negative-sequence separation or harmonic current compensation methods suitable for virtual synchronous machine technology. However, these methods also introduce new challenges: applying these improved methods to virtual synchronous machine technology significantly increases the complexity of the control process, and calculating and extracting the positive- and negative-sequence current components or harmonic components requires the use of additional modules.

[0004] In summary, although the existing methods can suppress the power fluctuations caused by asymmetric voltage in three-phase power grids, these methods not only have a large amount of calculation, but also require additional modules such as notch filters or harmonic extraction in the control loop, making the control link more complicated. To this end, the present invention is based on virtual synchronous machine technology and considers the use of a virtual phase voltage (VPVR) and virtual phase current (VPCR) collaborative control strategy. The control method proposed in the present invention can enable the inverter to output a controllable three-phase asymmetric current without changing the grid connection point voltage. It does not require positive and negative sequence separation, has a small amount of calculation, and a simple control structure. In the scenario of asymmetric grid voltage, it can significantly suppress the fluctuation of the inverter output power.

[0005] Pan Zixun, Yang Xiaofeng, Zhao Rui, et al. Multi-mode coordination strategy of virtual synchronous generator under unbalanced power grid [J]. Transactions of China Electrotechnical Society, 2023, 38(16): 4274-4285. This paper discloses that the use of virtual synchronous generator (VSG) control in renewable energy grid-connected converters is beneficial to improving the grid inertia and damping level, but there are problems of output current distortion and instantaneous power pulsation when the grid voltage is unbalanced. In order to flexibly regulate the output current quality and power ripple of VSG under unbalanced power grid, this paper proposes a VSG multi-mode coordination strategy. First, the instantaneous power pulsation suppression mechanism of VSG under unbalanced power grid is studied. Based on the analysis of the grid current harmonic characteristics in the power constant mode, the current reference instruction is calculated using the VSG virtual power and the coordination coefficient is introduced to correct it; secondly, the VSG current balance-power constant multi-mode coordination is realized under the multi-resonant current inner loop control, and the influence of the coordination coefficient and the selection method of its value range are studied; finally, the effectiveness of the theoretical analysis and the proposed control strategy are verified by simulation and experimental results. This paper addresses unbalanced three-phase grid voltages by transforming the traditional VSG's dual closed-loop voltage and current control into a multi-resonant current inner loop. This inner loop, consisting primarily of a fundamental controller and a harmonic compensator, suppresses power fluctuations by regulating the harmonics in the inverter's grid-connected current. However, the control loop used in this paper is complex, requiring virtual power calculations, filtering using a doubler notch filter, and finally inputting the power into the multi-resonant current inner loop using a set coordination coefficient. This results in a high computational complexity. Summary of the Invention

[0006] In view of the defects in the prior art, the object of the present invention is to provide a method and system for suppressing three-phase asymmetric power fluctuations based on a virtual synchronous machine.

[0007] According to the present invention, a method for suppressing three-phase asymmetric power fluctuations based on a virtual synchronous machine includes:

[0008] Step S1: Construct a virtual phase current control model. When a voltage sag is detected, a corresponding virtual phase current gain is introduced into the virtual phase current control model for the voltage sag phase, and the power fluctuation is preliminarily suppressed by the virtual phase current gain.

[0009] Step S2: Use virtual phase voltage control technology to control the current reference value so that the virtual synchronous machine outputs three-phase balanced current, and then cooperate with the virtual phase current control technology to suppress power fluctuations.

[0010] Preferably, the step S1 includes:

[0011] Step S1.1: Construct a virtual phase current control model;

[0012]

[0013] Step S1.2: When a voltage drop occurs in the three-phase symmetrical grid voltage, the current three-phase voltage is expressed as:

[0014]

[0015] Among them, k a Indicates the voltage drop ratio;

[0016] Transform the coordinates to the dq coordinate system, then:

[0017]

[0018] If the inverter output current is kept constant, the d-axis current i d and q-axis current i q Constant, according to the instantaneous power theory, the active power is expressed as:

[0019]

[0020] Step S1.3: Introduce virtual phase current gain k into the virtual phase current control model through virtual phase current technology i Regulate the three-phase grid-connected current;

[0021]

[0022] Among them, k i represents the virtual phase current gain;

[0023] Due to the lack of zero-sequence current limitation in the three-phase three-wire topology, the actual three-phase grid-connected current expression is corrected to:

[0024]

[0025] At this time, the dq axis components of the current are:

[0026]

[0027] According to the instantaneous power theory, the active power is calculated as:

[0028]

[0029] Take k i =k a have to:

[0030]

[0031] In the above formula, the inverter output active power is a constant value, without pulsation components, and the power value is consistent with the symmetric power grid; for different voltage drop levels, the value of the virtual phase current gain is dynamically selected to meet k i =k a Power fluctuation suppression conditions.

[0032] Preferably, step S2 includes:

[0033] Step S2.1: Based on the synchronous generator stator electrical equation, ignoring the effect of the filter capacitor C, establish the relationship between the inverter output port voltage and current:

[0034]

[0035] Where L and R are the total inductance and total resistance between the inverter and the grid, respectively; the subscript abc represents the component in the abc coordinate system;

[0036] Step S2.2: Convert the three-phase output voltage u abc The d-axis orientation of the grid voltage is used to perform dq decomposition, and the relationship between voltage and current in the dq coordinate system is obtained:

[0037]

[0038] Among them, i dref 、i qref is the current command value in the dq coordinate system; u d 、u q The reference voltage u abc The dq axis components obtained by dq decomposition using the d-axis orientation of the grid voltage; u gd 、u gq is the dq axis component of the grid voltage; Y is the impedance matrix; X is the inductive reactance;

[0039] Step S2.3: When the voltage of phase A of the power grid drops, the voltage drop ratio is k a When k v =ka When the current reference value is constant, k v Represents the virtual phase voltage gain.

[0040] Preferably, the compensated grid voltage is:

[0041]

[0042] Transform it to the dq coordinate system:

[0043]

[0044] According to the above formula:

[0045]

[0046] When k v =k a When i is constant, dref 、i qref .

[0047] Preferably, step S2 includes: a constant current reference value is subjected to current inner loop PI control and SPWM modulation so that the virtual synchronous machine outputs a three-phase balanced current; then the A phase current is regulated by using virtual phase current control technology, thereby ultimately suppressing power fluctuations.

[0048] According to the present invention, a three-phase asymmetric power fluctuation suppression system based on a virtual synchronous machine is provided, comprising:

[0049] Module M1: Builds a virtual phase current control model. When a voltage sag is detected, the corresponding virtual phase current gain is introduced into the virtual phase current control model for the voltage sag phase. This virtual phase current gain is used to initially suppress power fluctuations.

[0050] Module M2: Uses virtual phase voltage control technology to control the current reference value so that the virtual synchronous machine outputs three-phase balanced current, and then cooperates with the virtual phase current control technology to suppress power fluctuations.

[0051] Preferably, the module M1 includes:

[0052] Module M1.1: Construct virtual phase current control model;

[0053]

[0054] Module M1.2: When a voltage drop occurs in the three-phase symmetrical grid voltage, the current three-phase voltage is expressed as:

[0055]

[0056] Among them, ka Indicates the voltage drop ratio;

[0057] Transform the coordinates to the dq coordinate system, then:

[0058]

[0059] If the inverter output current is kept constant, the d-axis current i d and q-axis current i q Constant, according to the instantaneous power theory, the active power is expressed as:

[0060]

[0061] Module M1.3: Introducing virtual phase current gain k into the virtual phase current control model through virtual phase current technology i Regulate the three-phase grid-connected current;

[0062]

[0063] Among them, k i represents the virtual phase current gain;

[0064] Due to the lack of zero-sequence current limitation in the three-phase three-wire topology, the actual three-phase grid-connected current expression is corrected to:

[0065]

[0066] At this time, the dq axis components of the current are:

[0067]

[0068] According to the instantaneous power theory, the active power is calculated as:

[0069]

[0070] Take k i =k a have to:

[0071]

[0072] In the above formula, the inverter output active power is a constant value, without pulsation components, and the power value is consistent with the symmetric power grid; for different voltage drop levels, the value of the virtual phase current gain is dynamically selected to meet k i =k a Power fluctuation suppression conditions.

[0073] Preferably, the module M2 includes:

[0074] Module M2.1: Based on the synchronous generator stator electrical equation, ignoring the effect of the filter capacitor C, establish the relationship between the inverter output port voltage and current:

[0075]

[0076] Where L and R are the total inductance and total resistance between the inverter and the grid, respectively; the subscript abc represents the component in the abc coordinate system;

[0077] Module M2.2: Converts the three-phase output voltage u abc The d-axis orientation of the grid voltage is used to perform dq decomposition, and the relationship between voltage and current in the dq coordinate system is obtained:

[0078]

[0079] Among them, i dref 、i qref is the current command value in the dq coordinate system; u d 、u q The reference voltage u abc The dq axis components obtained by dq decomposition using the d-axis orientation of the grid voltage; u gd 、u gq is the dq axis component of the grid voltage; Y is the impedance matrix; X is the inductive reactance;

[0080] Module M2.3: When the grid A phase voltage drops, the voltage drop ratio is k a When k v =k a When the current reference value is constant, k v Represents the virtual phase voltage gain.

[0081] Preferably, the compensated grid voltage is:

[0082]

[0083] Transform it to the dq coordinate system:

[0084]

[0085] According to the above formula:

[0086]

[0087] When k v =k a When i is constant, dref 、i qref .

[0088] Preferably, the module M2 includes: a constant current reference value is subjected to current inner loop PI control and SPWM modulation to enable the virtual synchronous machine to output a three-phase balanced current; and then the A phase current is regulated by using virtual phase current control technology to ultimately suppress power fluctuations.

[0089] Compared with the prior art, the present invention has the following beneficial effects:

[0090] 1. This invention proposes a VPCR and VPVR coordinated control strategy, which can theoretically provide strong support for the control method of VSG inverters in asymmetric voltage scenarios;

[0091] 2. The control strategy proposed in this invention can meet the actual engineering requirements for system response speed and stability, avoiding the complex calculation process of traditional control schemes;

[0092] 3. The present invention can enable the inverter to output controllable three-phase asymmetric current without changing the grid-connected point voltage. It does not require positive and negative sequence separation, has low calculation complexity, and a simple control structure. In the scenario of asymmetric grid-connected voltage, it can significantly suppress the fluctuation of the inverter output power. BRIEF DESCRIPTION OF THE DRAWINGS

[0093] Other features, objects and advantages of the present invention will become more apparent upon reading the detailed description of non-limiting embodiments with reference to the following drawings:

[0094] Figure 1 This is the topology diagram of the meshed VSG inverter.

[0095] Figure 2 This is the principle diagram of the coordinated control of VPVR and VPCR.

[0096] Figure 3a to Figure 3b Schematic diagram of the output waveform of the grid-type VSG inverter. DETAILED DESCRIPTION

[0097] The present invention will be described in detail below with reference to specific embodiments. The following examples will help those skilled in the art to further understand the present invention, but are not intended to limit the present invention in any form. It should be noted that, for those skilled in the art, several changes and improvements can be made without departing from the scope of the present invention. These all fall within the scope of protection of the present invention.

[0098] Example 1

[0099] According to the present invention, a method for suppressing three-phase asymmetric power fluctuations based on a virtual synchronous machine is provided, which includes: first, modeling the virtual phase current control VPCR technology, introducing the corresponding virtual phase current gain for the voltage drop phase to compensate for the voltage drop phase, and realizing the preliminary suppression of power fluctuations; then, combining the characteristics of the virtual synchronous machine inverter, proposing a virtual phase voltage VPVR and virtual phase current VPCR collaborative control strategy, and finally achieving complete suppression of the inverter output power fluctuations through the virtual phase voltage gain and virtual phase current gain. Compared with the existing power fluctuation suppression methods such as current positive and negative sequence separation control and harmonic current compensation, the method for suppressing three-phase asymmetric power fluctuations based on a virtual synchronous machine provided by the present invention adopts VPVR and VPCR collaborative control to simplify the system control structure and make the three-phase current output of the inverter controllable.

[0100] In this embodiment, the virtual synchronous machine topology is as follows Figure 1 As shown in the figure, U dc is the DC side voltage; u o =(u oa ,u ob ,u oc ) T is the output voltage of the grid-connected inverter; L and C are the inductance and capacitance of the filter respectively; i o =(i oa ,i ob ,i oc ) T is the inverter output current; L g is the line inductance; u g =(u ga ,u gb ,u gc ) T is the three-phase grid voltage. ref , Q ref are the reference values ​​of active power and reactive power respectively; P e , Q e are the active power and reactive power output by the inverter respectively; θ and u are the reference phase and amplitude of the inverter output port voltage obtained after the VSG control algorithm. The VPVR and VPCR coordinated control strategy proposed in this invention is as follows: Figure 2 As shown in the figure, u v gd 、u v gq 、i v gd 、i v gq are the dq-axis components of the regulated virtual phase voltage and virtual phase current respectively.

[0101] In this embodiment, the virtual phase current control VPCR technology includes:

[0102] When the three-phase symmetrical grid voltage drops, the voltage drop ratio of phase A is k a (0 <k a <1) as an example, the three-phase voltage can be expressed as:

[0103]

[0104] Transform the coordinates to the dq coordinate system, then:

[0105]

[0106] If the inverter output current is kept constant, the d-axis current i d and q-axis current i q Constant, according to the instantaneous power theory, the active power can be expressed as:

[0107]

[0108] At this point, it can be seen that the active power contains a double frequency fluctuation component. In order to suppress the power fluctuation under the voltage asymmetry of the three phases, the inverter needs to output a controllable asymmetric current to offset the impact of the grid voltage imbalance. VPCR technology introduces a virtual phase current gain k i The three-phase grid-connected current is regulated and the sampled A-phase current is amplified by k i times, and then perform Park transformation to obtain the dq axis virtual current and Used to track the dq axis reference current i dref and i qref At this time, the three-phase grid-connected current can be expressed as:

[0109]

[0110] Due to the lack of zero-sequence current limitation in the three-phase three-wire topology, the actual three-phase grid-connected current expression is corrected to:

[0111]

[0112] At this time, the dq axis components of the current are:

[0113]

[0114] According to the instantaneous power theory, the active power is calculated as:

[0115]

[0116] Take k i =k aWe can get:

[0117]

[0118] In the above formula, the inverter output active power is constant, without pulsation components, and the power value is consistent with the symmetric power grid. For different voltage drop levels, the value of the virtual phase current gain can be dynamically selected to meet k i =k a Power fluctuation suppression conditions.

[0119] In this embodiment, the collaborative control strategy of the virtual phase voltage VPVR and the virtual phase current VPCR includes:

[0120] In grid-following inverter control, the current reference is a fixed value designed by the outer loop parameters, so VPCR technology can be directly applied to grid-following inverter control. However, in grid-forming VSG inverters, that is, virtual synchronous machine inverters, the current reference is obtained through the virtual synchronous machine control loop and can vary during operation due to voltage asymmetry. Therefore, it is proposed to use virtual phase voltage technology to control the current reference value, and then cooperate with virtual phase current technology to suppress power fluctuations.

[0121] According to the synchronous generator stator electrical equation, ignoring the effect of the filter capacitor C, the relationship between the inverter output port voltage and current can be established:

[0122]

[0123] Where L and R are the total inductance and total resistance between the inverter and the grid respectively; the subscript "abc" represents the components in the abc coordinate system. abc By using the d-axis orientation of the grid voltage to perform dq decomposition, the relationship between voltage and current in the dq coordinate system can be obtained:

[0124]

[0125] where i dref 、i qref is the current command value in the dq coordinate system; u d 、u q The reference voltage u abc The dq axis components obtained by dq decomposition using the d-axis orientation of the grid voltage; u gd 、u gq is the dq-axis component of the grid voltage; Y is the impedance matrix; X is the inductive reactance, X = ωL.

[0126] Substitute the instantaneous average active and reactive power output of the inverter into the reactive-voltage and active-frequency control loops of the VSG to obtain a constant reference voltage amplitude u and phase angle θ, and then obtain the three-phase reference voltage u on the inverter side. abc , will u abc The grid voltage d-axis orientation is used to perform dq decomposition to obtain u d 、u q , because u abc The three-phase balanced voltage has only positive sequence components. At this time, the calculation result of the dq axis current command value is mainly determined by the dq axis component u of the grid voltage. gd 、u gq Therefore, unbalanced grid voltage will cause unbalanced inverter output current.

[0127] The voltage of phase A of the power grid still drops and the drop ratio is k a For example(0 <k a <1), virtual phase voltage regulation (VPVR) is used to compensate the grid voltage in the current calculation formula, and the virtual phase voltage gain k is taken v , so that the sampled A-phase voltage is amplified. At this time, the compensated grid voltage is:

[0128]

[0129] Transforming it to the dq coordinate system yields:

[0130]

[0131] According to the above formula, we can get:

[0132]

[0133] It is not difficult to see that the current reference instruction i dref 、i qref Affected by the asymmetric grid voltage, it contains double frequency fluctuation components, so k v =k a Then we can get a constant i dref 、i qref , k v The value of can be adjusted according to the degree of voltage drop. After the current inner loop PI control and SPWM modulation, the inverter can output three-phase balanced current. Then, the VPCR technology is used to regulate the A-phase current, ultimately achieving the purpose of suppressing power fluctuations.

[0134] The present invention also provides a three-phase asymmetric power fluctuation suppression system based on a virtual synchronous machine. The three-phase asymmetric power fluctuation suppression system based on a virtual synchronous machine can be implemented by executing the process steps of the three-phase asymmetric power fluctuation suppression method based on a virtual synchronous machine, that is, those skilled in the art can understand the three-phase asymmetric power fluctuation suppression method based on a virtual synchronous machine as a preferred implementation of the three-phase asymmetric power fluctuation suppression system based on a virtual synchronous machine.

[0135] Example 2

[0136] Example 2 is a preferred example of Example 1

[0137] To mitigate the double-frequency fluctuations in grid-connected inverter output power caused by three-phase grid voltage asymmetry, existing measures include positive- and negative-sequence current separation control, harmonic current compensation, and direct power control. However, these measures require additional control elements and are complex. Therefore, it is necessary to improve inverter control strategies for asymmetric voltages to ensure the safety and stability of the power system.

[0138] According to the present invention, a three-phase asymmetric power fluctuation suppression method based on a virtual synchronous machine includes: first, modeling the virtual phase current control (VPCR) technology, introducing the corresponding virtual phase current gain for the voltage drop phase to compensate for the voltage drop, so as to achieve preliminary suppression of power fluctuations; then, combining the characteristics of the grid-type VSG inverter model, proposing a virtual phase voltage (VPVR) and virtual phase current (VPCR) collaborative control strategy, and finally achieving complete suppression of the inverter output power fluctuation through the virtual phase voltage gain and virtual phase current gain. In order to prove the advantages of the present invention, it is verified by simulation. First, according to the grid-type VSG grid-connected inverter topology, such as Figure 1 As shown in the figure, a simulation model is built, and the voltage of phase A grid is set to drop to 0.8pu, and the voltage of phase B and C grid is normal to simulate the asymmetric operation state of three-phase voltage; secondly, as Figure 2 As shown in Figure 2, the VPVR and VPCR coordinated control strategy proposed in this invention is applied to the VSG inverter control loop. Finally, the simulation model is run to obtain the output waveform of the traditional VSG inverter as shown in Figure 2. Figure 3a As shown, the VPVR+VPCR coordinated control output waveform is as follows Figure 3b In the initial stage of system operation, the inverter output active power is set to 15kW. Figure 3a 、 Figure 3bA comparison shows that when three-phase grid voltage asymmetry occurs, the VSG grid-connected inverter using a traditional control method experiences severe current waveform distortion, resulting in double-frequency fluctuations in output power with a fluctuation amplitude of ΔP ≈ 3000W. However, the VSG inverter using the control strategy proposed in this invention achieves controllable output current and significantly suppresses power fluctuations caused by asymmetric voltage, with a ΔP of ≈ 120W. Simulations validate the theoretical validity.

[0139] Comparing the proposed VPVR and VPCR coordinated control strategy with traditional VSG control strategies, the control strategy designed in this invention suppresses power oscillations caused by three-phase voltage asymmetry. Furthermore, the proposed solution enables the inverter to output controllable three-phase asymmetric current without changing the grid voltage. This approach requires minimal computation, has a simple control structure, and significantly suppresses power fluctuations. This invention provides a new power fluctuation suppression strategy for the stable operation of grid-connected VSG inverter systems under three-phase asymmetric voltages.

[0140] Those skilled in the art will appreciate that, in addition to implementing the system, device, and various modules provided by the present invention in purely computer-readable program code, it is entirely possible to implement the same program in the form of logic gates, switches, application-specific integrated circuits, programmable logic controllers, embedded microcontrollers, and the like by logically programming the method steps. Therefore, the system, device, and various modules provided by the present invention can be considered a hardware component, and the modules included therein for implementing various programs can also be considered structures within the hardware component; the modules for implementing various functions can also be considered both software programs for implementing the method and structures within the hardware component.

[0141] The above describes specific embodiments of the present invention. It should be understood that the present invention is not limited to the specific embodiments described above, and those skilled in the art may make various changes or modifications within the scope of the claims, which do not affect the essence of the present invention. The embodiments of this application and the features in the embodiments may be combined with each other in any manner unless there is a conflict.

Claims

1. A method for suppressing three-phase asymmetric power fluctuations based on a virtual synchronous machine, characterized in that: include: Step S1: Construct a virtual phase current control model. When a voltage sag is detected, a corresponding virtual phase current gain is introduced into the virtual phase current control model for the voltage sag phase, and the power fluctuation is preliminarily suppressed by the virtual phase current gain. Step S2: Use virtual phase voltage control technology to control the current reference value so that the virtual synchronous machine outputs three-phase balanced current, and then cooperate with the virtual phase current control technology to suppress power fluctuations.

2. The method for suppressing three-phase asymmetric power fluctuations based on a virtual synchronous machine according to claim 1, characterized in that: The step S1 comprises: Step S1.1: Construct a virtual phase current control model; Among them, u ga 、u gb 、u gc Represents the three-phase grid voltage, U represents the amplitude of the three-phase grid phase voltage, i a 、i b 、i c Represents the instantaneous value of the three-phase current, I represents the amplitude of the three-phase current, θ i represents the initial phase angle of the current, w represents the angular frequency; t represents the time variable; Step S1.2: When a voltage drop occurs in the three-phase symmetrical grid voltage, the current three-phase voltage is expressed as: Among them, k a Indicates the voltage drop ratio; Transform the coordinates to the dq coordinate system, then: If the inverter output current is kept constant, the d-axis current i d and q-axis current i q Constant, according to the instantaneous power theory, the active power is expressed as: Step S1.3: Introduce virtual phase current gain k into the virtual phase current control model through virtual phase current technology i Regulate the three-phase grid-connected current; Among them, k i represents the virtual phase current gain; Due to the lack of zero-sequence current limitation in the three-phase three-wire topology, the actual three-phase grid-connected current expression is corrected to: At this time, the dq axis components of the current are: According to the instantaneous power theory, the active power is calculated as: Take k i =k a have to: Among them, u d Voltage d-axis component; u q represents the q-axis component of voltage; In the above formula, the inverter output active power is a constant value, without pulsation components, and the power value is consistent with the symmetric power grid; for different voltage drop levels, the value of the virtual phase current gain is dynamically selected to meet k i =k a Power fluctuation suppression conditions.

3. The method for suppressing three-phase asymmetric power fluctuations based on a virtual synchronous machine according to claim 1, characterized in that: The step S2 comprises: Step S2.1: Based on the synchronous generator stator electrical equation, ignoring the effect of the filter capacitor C, establish the relationship between the inverter output port voltage and current: Where L and R are the total inductance and total resistance between the inverter and the grid, respectively; the subscript abc represents the component in the abc coordinate system; Step S2.2: Convert the three-phase output voltage u abc The d-axis orientation of the grid voltage is used to perform dq decomposition, and the relationship between voltage and current in the dq coordinate system is obtained: Among them, i dref 、i qref is the current command value in the dq coordinate system; u d 、u q The reference voltage u abc The dq axis components obtained by dq decomposition using the d-axis orientation of the grid voltage; u gd 、u gq is the dq axis component of the grid voltage; Y is the impedance matrix; X is the inductive reactance; Step S2.3: When the voltage of phase A of the power grid drops, the voltage drop ratio is k a When k v =k a When the current reference value is constant, k v Represents the virtual phase voltage gain.

4. The method for suppressing three-phase asymmetric power fluctuations based on a virtual synchronous machine according to claim 3, characterized in that: The grid voltage after compensation is: Transform it to the dq coordinate system: According to the above formula: When k v =k a When i is constant, dref 、i qref .

5. The method for suppressing three-phase asymmetric power fluctuations based on a virtual synchronous machine according to claim 4, characterized in that: The step S2 includes: a constant current reference value is subjected to current inner loop PI control and SPWM modulation to enable the virtual synchronous machine to output a three-phase balanced current; then the A phase current is regulated by using a virtual phase current regulation technology to ultimately suppress power fluctuations.

6. A three-phase asymmetric power fluctuation suppression system based on virtual synchronous machine, characterized in that: include: Module M1: Builds a virtual phase current control model. When a voltage sag is detected, the corresponding virtual phase current gain is introduced into the virtual phase current control model for the voltage sag phase. This virtual phase current gain is used to initially suppress power fluctuations. Module M2: Uses virtual phase voltage control technology to control the current reference value so that the virtual synchronous machine outputs three-phase balanced current, and then cooperates with the virtual phase current control technology to suppress power fluctuations.

7. The three-phase asymmetric power fluctuation suppression system based on virtual synchronous machine according to claim 6, characterized in that: The module M1 includes: Module M1.1: Construct virtual phase current control model; Among them, u ga 、u gb 、u gc Represents the three-phase grid voltage, U represents the amplitude of the three-phase grid phase voltage, i a 、i b 、i c Represents the instantaneous value of the three-phase current, I represents the amplitude of the three-phase current, θ i represents the initial phase angle of the current, w represents the angular frequency; t represents the time variable; Module M1.2: When a voltage drop occurs in the three-phase symmetrical grid voltage, the current three-phase voltage is expressed as: Among them, k a Indicates the voltage drop ratio; Transform the coordinates to the dq coordinate system, then: If the inverter output current is kept constant, the d-axis current i d and q-axis current i q Constant, according to the instantaneous power theory, the active power is expressed as: Module M1.3: Introducing virtual phase current gain k into the virtual phase current control model through virtual phase current technology i Regulate the three-phase grid-connected current; Among them, k i represents the virtual phase current gain; Due to the lack of zero-sequence current limitation in the three-phase three-wire topology, the actual three-phase grid-connected current expression is corrected to: At this time, the dq axis components of the current are: According to the instantaneous power theory, the active power is calculated as: Take k i =k a have to: Among them, u d Voltage d-axis component; u q represents the q-axis component of voltage; In the above formula, the inverter output active power is a constant value, without pulsation components, and the power value is consistent with the symmetric power grid; for different voltage drop levels, the value of the virtual phase current gain is dynamically selected to meet k i =k a Power fluctuation suppression conditions.

8. The three-phase asymmetric power fluctuation suppression system based on virtual synchronous machine according to claim 6, characterized in that: The module M2 includes: Module M2.1: Based on the synchronous generator stator electrical equation, ignoring the effect of the filter capacitor C, establish the relationship between the inverter output port voltage and current: Where L and R are the total inductance and total resistance between the inverter and the grid, respectively; the subscript abc represents the component in the abc coordinate system; Module M2.2: Converts the three-phase output voltage u abc The d-axis orientation of the grid voltage is used to perform dq decomposition, and the relationship between voltage and current in the dq coordinate system is obtained: Among them, i dref 、i qref is the current command value in the dq coordinate system; u d 、u q The reference voltage u abc The dq axis components obtained by dq decomposition using the d-axis orientation of the grid voltage; u gd 、u gq is the dq axis component of the grid voltage; Y is the impedance matrix; X is the inductive reactance; Module M2.3: When the grid A phase voltage drops, the voltage drop ratio is k a When k v =k a When the current reference value is constant, k v Represents the virtual phase voltage gain.

9. The three-phase asymmetric power fluctuation suppression system based on virtual synchronous machine according to claim 8, characterized in that: The grid voltage after compensation is: Transform it to the dq coordinate system: According to the above formula: When k v =k a When i is constant, dref 、i qref .

10. The three-phase asymmetric power fluctuation suppression system based on virtual synchronous machine according to claim 9, characterized in that: The module M2 includes: a constant current reference value is subjected to current inner loop PI control and SPWM modulation to enable the virtual synchronous machine to output three-phase balanced current; then the A phase current is regulated by using virtual phase current regulation technology to ultimately suppress power fluctuations.

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