Voltage sag control method for cascaded power electronic transformer based on improved virtual synchronous machine

Abstract

This invention discloses a voltage sag control method for cascaded power electronic transformers based on an improved virtual synchronous machine, belonging to the field of power electronics technology. The method includes: establishing a power flow model between the high-voltage AC port of the cascaded power electronic transformer and the distribution network based on virtual synchronous machine control; constructing the exchange power function between the high-voltage AC port and the grid after introducing virtual synchronous machine control based on the power flow model; and obtaining an improved virtual synchronous machine control strategy for the input stage of the cascaded power electronic transformer under symmetrical and asymmetrical voltage sags based on the exchange power function between the high-voltage AC port and the grid. Using a virtual synchronous machine control strategy at the input stage of the cascaded power electronic transformer can solve problems such as high-voltage AC side current imbalance, power fluctuation, and low-voltage DC bus voltage fluctuation under voltage sags.

Description

Technical Field

[0001] This invention belongs to the field of power electronics technology, and specifically relates to a voltage sag control method for cascaded power electronic transformers based on an improved virtual synchronous machine. Background Technology

[0002] Power electronic transformers (PETs), as a new type of intelligent electrical equipment integrating power electronic converters and high-frequency transformers, not only possess the traditional transformer functions of voltage transformation, electrical isolation, and energy transfer, but also have AC / DC ports. They can achieve numerous functions such as power flow control, voltage sag isolation, AC / DC conversion, direct access to DC source loads, reactive power compensation, and harmonic mitigation, becoming one of the core hub devices for improving the digitalization and intelligence level of power grid equipment in new power systems. Among various topologies, cascaded H-Bridge Power Electronic Transformers (CHB-PETs) have higher power density at the same voltage level and device level, showing broad application prospects. Power electronic transformers used in distribution networks typically consist of three parts: a high-voltage input stage, an intermediate isolation stage, and a low-voltage output stage. With the increasing number of random source loads in distribution networks, the advantages of grid connection for distributed wind power, photovoltaics, and other renewable energy sources outputting in DC form are gradually becoming prominent, and PETs with both AC and DC ports demonstrate good compatibility and scalability. However, when PET (Power Electronic Transformer) is used in weak grid applications, if its AC port uses traditional grid-following control, the lack of inertia and damping will cause problems such as high-voltage AC side current imbalance, power fluctuations, and low-voltage DC bus voltage fluctuations under voltage sags. In this scenario, a control strategy with active support is needed. While virtual synchronous generator (VSG) control has similar functionality, it has limitations in addressing the high-voltage AC side current imbalance, power fluctuations, and low-voltage DC bus voltage fluctuations that occur under voltage sags due to the lack of inertia and damping in PET. Therefore, there is an urgent need to research a VSG-based PET control strategy under voltage sags. Thus, a cascaded power electronic transformer voltage sag control method based on an improved virtual synchronous generator is urgently needed to overcome the shortcomings of existing technologies. Summary of the Invention

[0003] The purpose of this invention is to propose a voltage sag control method for cascaded power electronic transformers based on an improved virtual synchronous machine, which overcomes the limitations of existing control methods and enables stable operation of such transformers under voltage sags.

[0004] This invention provides a voltage sag control method for cascaded power electronic transformers based on an improved virtual synchronous machine, comprising the following steps:

[0005] A power flow model between the high-voltage AC port of a cascaded power electronic transformer and the distribution network based on virtual synchronous machine control is established.

[0006] Based on the power flow model, the power exchange function between the high-voltage AC port and the power grid of the cascaded power electronic transformer after the introduction of virtual synchronous machine control is constructed.

[0007] Based on the aforementioned exchange power function, an improved virtual synchronous machine control strategy for the input stage of the cascaded power electronic transformer is obtained under symmetrical and asymmetrical voltage sags.

[0008] The establishment of a power flow model between the high-voltage AC port of a cascaded power electronic transformer based on virtual synchronous machine control and the distribution network includes:

[0009] Using the topology and control equations of a cascaded power electronic transformer, a mathematical model of the input stage of the cascaded power electronic transformer is established. From the perspective of the mathematical model, the feasibility of the input stage of the cascaded power electronic transformer controlled by a virtual synchronous machine and the synchronous generator being equivalent is analyzed, and an equivalent circuit for grid connection of the high-voltage AC port of the cascaded power electronic transformer with virtual synchronous machine control is established.

[0010] Based on the grid-connected equivalent circuit of the high-voltage AC port of the cascaded power electronic transformer with virtual synchronous machine control, a power flow model between the high-voltage AC port of the cascaded power electronic transformer and the distribution network is established.

[0011] The method utilizes the topology and control equations of a cascaded power electronic transformer to establish a mathematical model of its input stage. From the perspective of the mathematical model, the feasibility of using a virtual synchronous machine-controlled input stage of the cascaded power electronic transformer as an equivalent to a synchronous generator is analyzed. An equivalent circuit for grid connection of the high-voltage AC port of the cascaded power electronic transformer with virtual synchronous machine control is established, including:

[0012] Based on the topology of the cascaded power electronic transformer, the high-voltage AC side input current is taken as the state variable, and the filtering parameters of the high-voltage AC port are considered to construct a mathematical model of the input stage of the cascaded power electronic transformer.

[0013] Based on the external characteristics of the simulated synchronous machine controlled by the virtual synchronous machine introduced into the input stage of the cascaded power electronic transformer, a mathematical model of the input stage of the cascaded power electronic transformer with virtual synchronous machine control is established.

[0014] Based on the equivalent relationship between the mathematical model of the input stage of the cascaded power electronic transformer and the mathematical model of the input stage of the cascaded power electronic transformer with virtual synchronous machine control, an equivalent circuit for grid connection of the high-voltage AC port of the cascaded power electronic transformer with virtual synchronous machine control is established.

[0015] The cascaded power electronic transformer includes: an input stage, an isolation stage, a low-voltage DC load, and an output stage. The input stage adopts a cascaded H-bridge structure, connecting the high-voltage AC power supply and the DC section.

[0016] The establishment of a power flow model between the high-voltage AC port of the cascaded power electronic transformer and the distribution network based on the grid-connected equivalent circuit of the cascaded power electronic transformer with virtual synchronous machine control includes:

[0017] Based on the grid-connected equivalent circuit of the high-voltage AC port of the cascaded power electronic transformer with virtual synchronous machine control, and taking into account the filtering parameters and power flow characteristics, a power flow model between the high-voltage AC port of the cascaded power electronic transformer and the distribution network is established.

[0018] The power exchange function between the high-voltage AC port and the power grid of the cascaded power electronic transformer after the introduction of virtual synchronous machine control, based on the power flow model, includes:

[0019] Based on the power flow model of the cascaded power electronic transformer, the power exchange function between the high-voltage AC port and the power grid after the introduction of virtual synchronous machine control is constructed.

[0020] Based on the power exchange function between the high-voltage AC port and the power grid, the influence of filter parameters, inertia and damping parameters, and coupling parameters of active and reactive power on the control effect of the cascaded power electronic transformer is analyzed.

[0021] Based on the power flow model of the cascaded power electronic transformer, the power exchange function between the high-voltage AC port and the power grid after the introduction of virtual synchronous machine control is constructed as follows:

[0022] (4)

[0023] In the formula, E0∠δ0 is the port voltage of the virtual synchronous machine control port, and U f ∠0° is the grid connection point voltage, ω is the grid angular frequency, P is the active power output of the AC port, Q is the reactive power output of the AC port, Z is the equivalent circuit impedance, and α is the equivalent impedance angle.

[0024] The analysis of the impact of filter parameters, inertia and damping parameters, and coupling parameters of active and reactive power on the control effect of the cascaded power electronic transformer, based on the power exchange function between the high-voltage AC port and the power grid, includes:

[0025] The power exchange function between the high-voltage AC port and the power grid of the cascaded power electronic transformer after the introduction of virtual synchronous machine control is sorted out to obtain the influence of filtering parameters on the control effect of the cascaded power electronic transformer.

[0026] By expanding the multivariate function using the Taylor formula and removing second-order and higher-order terms, the small-signal model of the filter parameters and the transmission power between the high-voltage AC port and the distribution network is obtained as follows:

[0027] (5)

[0028] The cascaded power electronic transformer with virtual synchronous machine control is used to adjust the virtual inertia and damping parameters through an active-frequency loop to enhance the mechanical characteristics of the power electronic transformer during power regulation. The electromagnetic power of the virtual synchronous machine is calculated by the instantaneous power of the input stage of the power electronic transformer, and combined with the characteristic equation of the virtual synchronous machine, the following is obtained:

[0029] (6)

[0030] In the formula: P e Electromagnetic power; Q e Ω represents instantaneous reactive power; M represents the mechanical angular velocity of the rotor; Ω represents instantaneous reactive power. T and M e These are mechanical torque and electromagnetic torque, respectively; ω e E is the electrical angle; E is the VSG port voltage. Q is the initial voltage; ref For reference reactive power; k q is the gain coefficient of the integrator; p is the number of pole pairs; J is the inertial parameter;

[0031] When the number of rotor pole pairs p of the synchronous generator is 1, Ω = ω e And taking into account the actual electrical angle ω e At the rated electrical angle ω N The damping characteristics under varying conditions are:

[0032] (7)

[0033] In the formula, decreasing the inertia parameter J shortens the settling time and increases the response speed; increasing the damping parameter D results in faster power decay and smaller oscillation amplitude. The rated active power output of the AC port;

[0034] Considering the damping characteristics when the actual electrical angle varies near the rated electrical angle, the influence of inertia and damping parameters on the control effect of cascaded power electronic transformers is obtained.

[0035] Based on the equivalent circuit of the high-voltage AC port grid connection of the cascaded power electronic transformer with virtual synchronous machine control and the power exchange function between the high-voltage AC port and the grid after the introduction of virtual synchronous machine control, the influence of the coupling parameters of active and reactive power on the control effect of the cascaded power electronic transformer is obtained, and the power coupling model of the power electronic transformer is obtained:

[0036] (8)

[0037] In the formula, the off-diagonal parameters have a significant impact on the degree of power coupling. Under conditions of high impedance-to-inductance ratio or large power angle δ, the power coupling effect will be significantly enhanced. The coupling parameter between the power angle and active power is denoted as . The voltage-active power coupling parameter is... The coupling parameter between the power angle and reactive power is denoted as . This is the coupling parameter between voltage and reactive power.

[0038] The improved virtual synchronous machine control strategy for the input stage of the cascaded power electronic transformer under symmetrical and asymmetrical voltage sags, based on the exchange power function, includes:

[0039] The control effect of the cascaded power electronic transformer on the power exchange function between the high-voltage AC port and the power grid is analyzed by combining key parameters. By dynamically adjusting the active power reference value, the active power imbalance is eliminated to maintain a constant power angle during faults, thereby shortening the transient response process. An improved virtual synchronous machine control strategy for the input stage of the cascaded power electronic transformer under symmetrical voltage sag is obtained. The adjustment of the active power reference value is as follows:

[0040] (10)

[0041] In the formula: P ref U is the reference value for steady-state operating power. N The rated voltage at the grid connection point is δ0, and the power angle at the steady-state operating point is δ0. This is the stable initial voltage for the virtual synchronous machine control port. Here, E is the effective value of the phase voltage after the voltage dip, and Z is the voltage at the virtual synchronizer control port after the voltage dip.

[0042] Using the aforementioned dynamically adjusted active power reference value, and considering the negative sequence component that appears under asymmetrical voltage sag, an improved control strategy for the virtual synchronous machine of the input stage of the cascaded power electronic transformer under asymmetrical voltage sag is obtained. The current control loop considering the negative sequence component is as follows:

[0043] (15)

[0044] In the formula: , , , For the power grid setpoint, , , , For current control variables, , , , This is the current reference value. , , , For integral and proportional parameters, , , , This is the output voltage of the power electronic transformer. For the Laplace operator;

[0045] Based on the improved cascaded power electronic transformer input stage virtual synchronous machine control strategy under symmetrical and asymmetrical voltage sags, the analysis results under the improved virtual synchronous machine power electronic transformer voltage sag control strategy are obtained.

[0046] Another object of the present invention is to provide a computer device including a memory and a processor, wherein the memory stores a computer program, and when the processor runs the computer program stored in the memory, the processor executes the voltage sag control method for cascaded power electronic transformers based on an improved virtual synchronous machine according to the present invention.

[0047] Another object of the present invention is to provide a computer-readable storage medium having a computer program stored thereon, wherein when the computer program is executed by a processor, the processor performs the voltage sag control method for cascaded power electronic transformers based on an improved virtual synchronous machine according to the present invention.

[0048] The beneficial effects of this invention are as follows:

[0049] This invention employs a virtual synchronous machine control strategy at the input stage of a cascaded power electronic transformer, which can solve problems such as current imbalance on the high-voltage AC side, power fluctuation, and low-voltage DC bus voltage fluctuation under voltage sag.

[0050] A. Based on the power flow model of the cascaded power electronic transformer, construct the power exchange function between the high-voltage AC port and the power grid after the introduction of virtual synchronous machine control (4); establish a quantitative mapping relationship between the virtual synchronous machine control parameters and the dynamic power behavior of the cascaded power electronic transformer, so that the dynamic power behavior of the power electronic transformer has an analytical mathematical basis.

[0051] B. Based on the power exchange function between the high-voltage AC port and the power grid, the influence of filter parameters, inertia and damping parameters, and coupling parameters of active power and reactive power on the control effect of cascaded power electronic transformers is analyzed (5)(6)(7)(8). The principle of the key parameters on the virtual inertia, damping characteristics, current and power and DC bus voltage regulation capability of the system is clarified, thus providing a theoretical basis for parameter optimization and dynamic regulation.

[0052] C. Based on the aforementioned power exchange function, obtain the improved virtual synchronous machine control strategy (10) (15) for the input stage of the cascaded power electronic transformer under symmetrical and asymmetrical voltage sags, so as to achieve coordinated regulation of power and current.

[0053] Therefore, this invention constructs a closed-loop control system that combines power exchange function, parameter influence analysis, and control strategy generation. This system enables cascaded power electronic transformers to achieve virtual inertial support, enhanced damping, and stable current, power, and DC bus voltage under voltage sag conditions. This effectively solves the problems of current imbalance, power fluctuation, and DC bus voltage fluctuation caused by the lack of physical inertia and damping characteristics of the PET during voltage sag. Attached Figure Description

[0054] Figure 1 This is a flowchart illustrating a cascaded power electronic transformer voltage sag control method based on an improved virtual synchronous machine, according to the present invention.

[0055] Figure 2 This is a typical power distribution network structure diagram based on cascaded power electronic transformers proposed in an embodiment of the present invention;

[0056] Figure 3 This is a detailed flowchart illustrating the voltage sag control method for cascaded power electronic transformers based on an improved virtual synchronous machine, as proposed in an embodiment of the present invention.

[0057] Figure 4 This is a schematic diagram of the topology of each stage of the cascaded power electronic transformer proposed in an embodiment of the present invention;

[0058] Figure 5 This is a schematic diagram of the cascaded input stage topology proposed in an embodiment of the present invention;

[0059] Figure 6 The equivalent circuit diagram of the input stage and the synchronizing machine of the cascaded power electronic transformer with virtual synchronizing machine control proposed in the embodiments of the present invention is shown below.

[0060] Figure 7 This is an equivalent model diagram of the high-voltage AC port grid connection of a cascaded power electronic transformer with virtual synchronous machine control proposed in an embodiment of the present invention;

[0061] Figure 8 This is a diagram of the improved virtual synchronous machine control strategy under symmetrical voltage sag proposed in an embodiment of the present invention;

[0062] Figure 9 This is a diagram of the improved virtual synchronous machine control strategy under asymmetric voltage sag proposed in an embodiment of the present invention.

[0063] Figure 10 This is a schematic diagram of the high-voltage side current, power, and low-voltage DC bus voltage waveforms under symmetrical voltage sag proposed in an embodiment of the present invention, wherein (a) is the high-voltage AC side current, (b) is the low-voltage DC bus voltage, (c) is the instantaneous active power on the high-voltage AC side, and (d) is the instantaneous reactive power on the high-voltage AC side.

[0064] Figure 11 This is a schematic diagram of the high-voltage side current, power, and low-voltage DC bus voltage waveforms under the asymmetric voltage sag proposed in an embodiment of the present invention, wherein (a) is the high-voltage AC side current, (b) is the low-voltage DC bus voltage, (c) is the instantaneous active power on the high-voltage AC side, and (d) is the instantaneous reactive power on the high-voltage AC side. Detailed Implementation

[0065] This invention provides a voltage sag control method for cascaded power electronic transformers based on an improved virtual synchronous machine. The invention will be further described in detail below with reference to the accompanying drawings.

[0066] like Figure 1 The embodiment of the present invention disclosed herein provides a method for controlling voltage sag of a cascaded power electronic transformer based on an improved virtual synchronous machine, comprising the following steps:

[0067] A power flow model between the high-voltage AC port of a cascaded power electronic transformer and the distribution network based on virtual synchronous machine control is established.

[0068] Based on the power flow model, the power exchange function between the high-voltage AC port and the power grid of the cascaded power electronic transformer after the introduction of virtual synchronous machine control is constructed.

[0069] Based on the aforementioned exchange power function, an improved virtual synchronous machine control strategy for the input stage of the cascaded power electronic transformer is obtained under symmetrical and asymmetrical voltage sags.

[0070] In this embodiment, for the application of power electronic transformers in weak grids, if the AC port uses the traditional grid-connected control, the lack of inertia and damping will cause problems such as high-voltage AC side current imbalance, power fluctuation and low-voltage DC bus voltage fluctuation under voltage sag. The present invention can maintain the stability of high-voltage side current and power and low-voltage DC bus voltage of cascaded power electronic transformers under symmetrical and asymmetrical voltage sag.

[0071] The following provides a detailed explanation of each step.

[0072] From such Figure 2 Based on the typical distribution network structure diagram of cascaded power electronic transformers shown, this embodiment of the invention provides a voltage sag control method for cascaded power electronic transformers based on an improved virtual synchronous machine. The specific working process is as follows: Figure 3 As shown.

[0073] Step S1: Establish a power flow model between the high-voltage AC port of the cascaded power electronic transformer and the distribution network based on virtual synchronous machine control;

[0074] In this embodiment, a mathematical model of the input stage of a cascaded power electronic transformer is established using its topology and control equations. The feasibility of equivalence between the input stage of the cascaded power electronic transformer controlled by a virtual synchronous machine and a synchronous generator is analyzed from the perspective of the mathematical model. An equivalent circuit for grid connection of the high-voltage AC port of the cascaded power electronic transformer with virtual synchronous machine control is then established. Based on this equivalent circuit, a power flow model between the high-voltage AC port of the cascaded power electronic transformer and the distribution network is established.

[0075] Step S2: Based on the power flow model, construct the power exchange function between the high-voltage AC port and the power grid of the cascaded power electronic transformer after the introduction of virtual synchronous machine control;

[0076] In this embodiment, based on the power flow model of the cascaded power electronic transformer, the power exchange function between the high-voltage AC port and the power grid after the introduction of virtual synchronous machine control is constructed; based on the power exchange function between the high-voltage AC port and the power grid, the influence of filtering parameters, inertia and damping parameters, and coupling parameters of active power and reactive power on the control effect of the cascaded power electronic transformer is analyzed.

[0077] Step S3: Based on the exchange power function, obtain the improved virtual synchronous machine control strategy for the input stage of the cascaded power electronic transformer under symmetrical and asymmetrical voltage sags.

[0078] In this embodiment, the control effect of the power electronic transformer is analyzed based on the power exchange function between the high-voltage AC port of the cascaded power electronic transformer and the power grid, combined with key parameters, to obtain an improved virtual synchronous machine control strategy for the input stage of the cascaded power electronic transformer under symmetrical and asymmetrical voltage sags.

[0079] The control effect of the cascaded power electronic transformer on the high-voltage AC port and the power grid is analyzed by combining key parameters. By dynamically adjusting the active power reference value, active power imbalance can be eliminated to maintain a constant power angle during faults, thus shortening the transient response process. An improved virtual synchronous machine control strategy for the input stage of the cascaded power electronic transformer under symmetrical voltage sags is obtained. Using the dynamically adjusted active power reference value, and considering the negative sequence component appearing under asymmetrical voltage sags, an improved virtual synchronous machine control strategy for the input stage of the cascaded power electronic transformer under asymmetrical voltage sags is obtained. Based on the improved virtual synchronous machine control strategies for the input stage of the cascaded power electronic transformer under symmetrical and asymmetrical voltage sags, simulation analysis results of the improved virtual synchronous machine power electronic transformer voltage sag control strategy are obtained.

[0080] In a preferred embodiment, step S1 specifically includes the following steps in the above embodiments:

[0081] Step S1-1: Using the topology and control equations of the cascaded power electronic transformer, establish a mathematical model of the input stage of the cascaded power electronic transformer; from the perspective of the mathematical model, analyze the feasibility of the virtual synchronous machine controlled input stage of the cascaded power electronic transformer and the synchronous generator being equivalent, and establish the equivalent circuit of the high-voltage AC port grid connection of the cascaded power electronic transformer with virtual synchronous machine control.

[0082] Step S1-2: Based on the grid-connected equivalent circuit of the cascaded power electronic transformer high-voltage AC port with virtual synchronous machine control, establish a power flow model between the cascaded power electronic transformer high-voltage AC port and the distribution network.

[0083] Step S1-1 includes: based on the topology of the cascaded power electronic transformer, taking the high-voltage AC side input current as a state variable, considering the filtering parameters of the high-voltage AC port, and constructing a mathematical model of the input stage of the cascaded power electronic transformer.

[0084] Based on the external characteristics of the simulated synchronous machine controlled by the virtual synchronous machine introduced into the input stage of the cascaded power electronic transformer, a mathematical model of the input stage of the cascaded power electronic transformer with virtual synchronous machine control is established.

[0085] Based on the equivalent relationship between the mathematical model of the input stage of the cascaded power electronic transformer and the mathematical model of the input stage of the cascaded power electronic transformer with virtual synchronous machine control, an equivalent circuit for grid connection of the high-voltage AC port of the cascaded power electronic transformer with virtual synchronous machine control is established.

[0086] The cascaded power electronic transformer includes: an input stage, an isolation stage, a low-voltage DC load, and an output stage. The input stage adopts a cascaded H-bridge structure, connecting the high-voltage AC power supply and the DC section.

[0087] Step S1-2 includes: establishing a power flow model between the high-voltage AC port of the cascaded power electronic transformer and the distribution network based on the grid-connected equivalent circuit of the cascaded power electronic transformer with virtual synchronous machine control, taking into account filtering parameters and power flow characteristics.

[0088] In a preferred embodiment, step S1-1 specifically includes the following steps in the above embodiments:

[0089] Step S1-1-1: Obtain the topology of the cascaded power electronic transformer. The cascaded power electronic transformer includes an input stage, an isolation stage, a low-voltage DC load, and an output stage. The input stage adopts a cascaded H-bridge structure to connect the high-voltage AC power supply and the DC section.

[0090] like Figure 4 As shown, a cascaded power electronic transformer includes an input stage, an isolation stage, and an output stage. The input stage uses an H-bridge cascaded structure, the isolation stage uses a dual active bridge structure, the output stage uses a three-phase three-wire inverter structure, and the filtering stage uses an LC filter. The cascaded input stage structure, taking phase a as an example, is shown below. Figure 5 As shown.

[0091] Step S1-1-2: Using the high-voltage AC side input current as a state variable, and considering the filtering parameters of the high-voltage AC port, construct a mathematical model of the input stage of the cascaded power electronic transformer;

[0092] In this embodiment, the mathematical model of the input stage of the cascaded power electronic transformer is the mathematical model of the AC side of the input stage in the cascaded power electronic transformer; such as Figure 5 As shown, with the high-voltage AC input current as the state variable, the mathematical model of the AC input stage in a cascaded power electronic transformer is established as follows:

[0093] (1)

[0094] In the formula, L f R f These are the filter inductance before virtual synchronous machine control is introduced and the equivalent resistance of the line from the grid common point to the cascaded power electronic transformer port, respectively.a u b u c For AC port voltage, i a i b i c For AC input current, d m U represents the duty cycle of the m-th submodule. am U bm U cm Where n is the DC port voltage and n is the number of submodules.

[0095] Step S1-1-3: Based on the fact that virtual synchronous machine control can be introduced into the input stage of the cascaded power electronic transformer to simulate the external characteristics of the synchronous machine, a mathematical model of the input stage of the cascaded power electronic transformer with virtual synchronous machine control is established.

[0096] like Figure 6 As shown, the grid-side voltage u a u b u c Equivalent to the terminal voltage of a synchronous generator, CHB output voltage e a e b e c Equivalent to the internal electromotive force of a synchronous generator, i a i b i c For grid-connected current, the DC portion of CHB is equivalent to a prime mover, providing virtual mechanical torque T. m Virtual electromagnetic torque T e It is then supplied by the instantaneous power of the AC power grid, U x i x Let voltage and current represent the three phases, respectively. The mathematical model for the input stage of a cascaded power electronic transformer with virtual synchronous machine control is as follows:

[0097] (2)

[0098] In the formula, L and R are the filter inductance introduced by the virtual synchronous machine control and the equivalent resistance of the line from the grid common point to the cascaded power electronic transformer port, respectively. a u b u c This is the grid-side voltage, equivalent to the terminal voltage of a synchronous generator, e a e b e c The output voltage of CHB is equivalent to the internal electromotive force of a synchronous generator, i a i b i c This is the grid-connected current.

[0099] Step S1-1-4: Based on the equivalent relationship between the mathematical model of the input stage of the cascaded power electronic transformer and the mathematical model of the input stage of the cascaded power electronic transformer with virtual synchronous machine control, establish the grid-connected equivalent circuit of the high-voltage AC port of the cascaded power electronic transformer with virtual synchronous machine control.

[0100] Comparing equation (1) and equation (2):

[0101] (3)

[0102] In the formula, This is the output voltage of CHB. Let m be the duty cycle of the m-th submodule. This is the DC port voltage. For grid-connected current, Input current to the AC port. This is the grid-side voltage. This refers to the AC port voltage.

[0103] As can be seen from equation (3), the input stage of the cascaded power electronic transformer with virtual synchronous machine control can be equivalent to a synchronous generator, and a system can be established as follows: Figure 7 The diagram shows the equivalent circuit for grid connection of the high-voltage AC port of a cascaded power electronic transformer with virtual synchronous machine control.

[0104] In a preferred embodiment, step S1-2 specifically includes the following steps in the above embodiments:

[0105] Step S1-2-1: Based on the grid-connected equivalent circuit of the cascaded power electronic transformer high-voltage AC port with virtual synchronous machine control, and taking into account the filtering parameters and power flow characteristics, establish a power flow model between the cascaded power electronic transformer high-voltage AC port and the distribution network.

[0106] In a preferred embodiment, step S2 specifically includes the following steps in the above embodiments:

[0107] Step S2-1: Based on the power flow model of the cascaded power electronic transformer, construct the power exchange function between the high-voltage AC port and the power grid after the introduction of virtual synchronous machine control for the cascaded power electronic transformer.

[0108] Step S2-2: Based on the power exchange function between the high-voltage AC port and the power grid, analyze the influence of filter parameters, inertia and damping parameters, and coupling parameters of active power and reactive power on the control effect of the cascaded power electronic transformer.

[0109] In a preferred embodiment, step S2-1 specifically includes the following steps in the above embodiments:

[0110] Step S2-1-1: Based on the power flow model, obtain the exchange power function between the high-voltage AC port of the cascaded power electronic transformer and the power grid;

[0111] like Figure 7 As shown, considering the filtering parameters and power flow characteristics, a power flow model is established between the high-voltage AC port of the cascaded power electronic transformer and the distribution network. Based on the power flow model, the exchange power function is obtained as follows:

[0112] (4)

[0113] In the formula, E0∠δ0 is the port voltage of the virtual synchronous machine control port, and U f ∠0° is the grid connection point voltage, ω is the grid angular frequency, P is the active power output of the AC port, Q is the reactive power output of the AC port, Z is the equivalent circuit impedance, and α is the equivalent impedance angle.

[0114] In a preferred embodiment, step S2-2 specifically includes the following steps in the above embodiments:

[0115] S2-2-1. The power exchange function between the high-voltage AC port and the power grid of the cascaded power electronic transformer after the introduction of virtual synchronous machine control is sorted out. The second-order and higher-order terms are removed by expanding the multivariate function Taylor formula to obtain the small-signal model of the filter parameters and the transmission power between the high-voltage AC port and the distribution network, and the influence of the filter parameters on the control effect of the cascaded power electronic transformer is obtained.

[0116] according to Figure 7 Equation (4) is expanded using the Taylor formula for multivariate functions, and second-order and higher-order terms are removed to obtain the small-signal model of the filter parameters L, R, and the transmission power between the high-voltage AC port and the distribution network:

[0117] (5)

[0118] When the fluctuations in the filter parameters are small, the impact on the output power of the power electronic transformer is minimal. Therefore, in practical applications of power electronic transformers, it is essential to strictly control their filter parameters and minimize their deviation from the preset settings.

[0119] Step S2-2-2: Adjust the virtual inertia and damping parameters of the cascaded power electronic transformer with virtual synchronous machine control through the active-frequency loop to enhance the mechanical characteristics of the power electronic transformer during power regulation. Consider the damping characteristics when the actual electrical angle changes near the rated electrical angle, and obtain the influence of inertia and damping parameters on the control effect of the cascaded power electronic transformer.

[0120] Depend on Figure 7It can be seen that the electromagnetic power of the virtual synchronous machine is obtained by calculating the instantaneous power of the input stage of the power electronic transformer, and combined with the characteristic equation of the virtual synchronous machine, we get:

[0121] (6)

[0122] In the formula: P e Electromagnetic power; Q e Ω represents instantaneous reactive power; M represents the mechanical angular velocity of the rotor; Ω represents instantaneous reactive power. T and M e These are mechanical torque and electromagnetic torque, respectively; ω e E is the electrical angle; E is the VSG port voltage. Q is the initial voltage; ref For reference reactive power; k q is the gain coefficient of the integrator; p is the number of pole pairs; J is the inertial parameter.

[0123] When the number of rotor pole pairs p of the synchronous generator is 1, Ω = ω e And taking into account the actual electrical angle ω e At the rated electrical angle ω N The damping characteristics under varying conditions are:

[0124] (7)

[0125] In the formula, decreasing the inertia parameter J shortens the settling time and increases the response speed; increasing the damping parameter D results in faster power decay and smaller oscillation amplitude. The rated active power output of the AC port.

[0126] Step S2-2-3: Based on the equivalent circuit of the high-voltage AC port grid connection of the cascaded power electronic transformer with virtual synchronous machine control and the power exchange function between the high-voltage AC port and the grid after the introduction of virtual synchronous machine control, the power coupling model of the power electronic transformer is obtained, and the influence of the coupling parameters of active power and reactive power on the control effect of the cascaded power electronic transformer is obtained.

[0127] according to Figure 7 Combined with equation (4), the power coupling model of the virtual synchronous machine is obtained as follows:

[0128] (8)

[0129] In the formula, the off-diagonal parameters have a significant impact on the degree of power coupling. Under conditions of high impedance-to-inductance ratio or large power angle δ, the power coupling effect will be significantly enhanced. The coupling parameter between the power angle and active power is denoted as . The voltage-active power coupling parameter is... The coupling parameter between the power angle and reactive power is denoted as . This is the coupling parameter between voltage and reactive power.

[0130] In a preferred embodiment, step S3 specifically includes the following steps in the above embodiments:

[0131] Step S3-1: Analyze the control effect of the power electronic transformer by combining the exchange power function between the high voltage AC port of the cascaded power electronic transformer and the power grid with key parameters. By dynamically adjusting the active power reference value, the active power imbalance can be eliminated to maintain a constant power angle during the fault, thereby shortening the transient response process and obtaining an improved virtual synchronous machine control strategy for the input stage of the cascaded power electronic transformer under symmetrical voltage sag.

[0132] When a voltage sag occurs on the input side, its sag tolerance is related to three factors: modulation ratio constraint, DC-side capacitor withstand voltage limit, and switching device current withstand level. Among these, the switching device current limit is the primary factor affecting the input side sag tolerance. As the input current increases, the transmitted power P and the effective value of the phase voltage U after the sag also change. sag The relationship is:

[0133] (9)

[0134] In the formula: α is the current margin of the switching device, I max I is the effective value of the device's maximum input current. n This represents the effective value of the current passing through the device under rated operating conditions. This is the minimum effective value of the grid phase voltage when the input current is at its maximum after a temporary sag.

[0135] Based on the voltage sag depth, the active power reference value is adjusted in real time to provide a stable current reference value for the current closed-loop control, thereby achieving stable control of the output current at the high-voltage AC port. Adjust according to the following rules:

[0136] (10)

[0137] In the formula: P ref U is the reference value for steady-state operating power. N The rated voltage at the grid connection point is δ0, and the power angle at the steady-state operating point is δ0. This is the stable initial voltage for the virtual synchronous machine control port. is the effective value of the phase voltage after the sag, E is the voltage of the virtual synchronous machine control port after the sag, and Z is the equivalent circuit impedance.

[0138] like Figure 8As shown, the improved virtual synchronous machine control strategy block diagram can be obtained according to equation (10). The strategy is designed for the working condition of symmetrical voltage sag in the power grid. The core idea is to dynamically adjust the power reference value. The active power reference value calculated in real time is used instead of the fixed active power reference value in the traditional scheme. At the same time, in order to improve the dynamic response speed of the system, the instantaneous power of the input stage of the power electronic transformer is used as the actual feedback quantity in the feedback branch to shorten the feedback delay and enhance the real-time performance of the control.

[0139] Step S3-2: Using the dynamically adjusted active power reference value, and considering the negative sequence component that appears under asymmetrical voltage sag, obtain the improved cascaded power electronic transformer input stage virtual synchronous machine control strategy under asymmetrical voltage sag.

[0140] The output voltage of the virtual synchronous machine is affected by grid asymmetry, resulting in negative-sequence and zero-sequence components. This leads to current imbalance and power fluctuations in the virtual synchronous machine's output current. For a three-phase three-wire system, the sum of the three-phase zero-sequence voltages is zero; therefore, the influence of the zero-sequence component is ignored in the derivation. The positive-sequence and negative-sequence voltages are:

[0141] (11)

[0142] In the formula: U sag(1) U sag(2) Let be the positive and negative sequence voltages under voltage sag, respectively, and a = 1∠120° be the rotation factor.

[0143] Transform the terminal voltage and output current from the abc coordinate system to the two-phase stationary αβ coordinate system. Based on the expression for instantaneous power, the instantaneous complex power of the output under unbalanced voltage sag is... for:

[0144] (12)

[0145] In the formula, The terminal voltage is in the αβ coordinate system. The terminal voltage is in the αβ coordinate system. Let be the positive sequence terminal voltage in the dq coordinate system. Let be the negative sequence terminal voltage in the dq coordinate system. Let be the positive sequence terminal current in the dq coordinate system. Let be the negative sequence terminal current in the dq coordinate system.

[0146] The real and imaginary parts of instantaneous complex power correspond to instantaneous active power, respectively. and reactive power for:

[0147] (13)

[0148] In the formula: p0 and q0 are the average power of the cascaded power electronic transformer, p sin2 p cos2 q sin2 q cos2 These are the second harmonic power components of the cascaded power electronic transformer.

[0149] Furthermore, the relationship between the dq components and each power component in both positive and negative sequences can be derived as follows:

[0150] (14)

[0151] In the formula, the voltage component , , , Given the grid value, four current components , , , Determine the six power components p0, q0, p sin2 p cos2 q sin2 q cos2 Therefore, controlling the current component can improve the control effect on the power component under unbalanced voltage sags, thereby enhancing the stability of current, power, and low-voltage DC bus voltage at the high-voltage AC port. The current closed-loop control loop needs to consider the positive and negative sequence components in the two-phase stationary coordinate system, specifically expressed as:

[0152] (15)

[0153] In the formula: , , , For the power grid setpoint, , , , For current control variables, , , , This is the current reference value. , , , For integral and proportional parameters, , , , This is the output voltage of the power electronic transformer. For the Laplace operator.

[0154] like Figure 9As shown, the block diagram of the improved virtual synchronous machine control strategy under asymmetric voltage sag is obtained according to equation (15). When an asymmetric voltage sag occurs in the power grid, the independent control of each power component is achieved by adding a control loop for the negative sequence component, which can enhance the stability of the current and power on the high voltage AC side and the bus voltage on the low voltage DC side.

[0155] Step S3-3: Based on the improved cascaded power electronic transformer input stage virtual synchronous machine control strategy under symmetrical and asymmetrical voltage sags, obtain the analysis results under the improved virtual synchronous machine cascaded power electronic transformer voltage sag control strategy.

[0156] Based on the AC / DC distribution network simulation model built using cascaded power electronic transformers, the number of cascaded H-bridges is 12, the filter inductance is 11mH, the equivalent resistance is 0.1Ω, the high-frequency transformer turns ratio is 2.962:1, and the power supply voltage is 10kV. Under normal operating conditions, the active power reference value of the cascaded power electronic transformer is 1.2MW. If the current margin α of the switching devices is taken as 1.2, then... 0.67U N (U N The effective value of the phase voltage under rated operating conditions on the high-voltage side is given. The active power reference value is adjusted in real time according to the voltage sag depth by formula (10). The low-voltage DC bus load is 1MW. Simulation analysis is performed based on the above parameters and operating conditions.

[0157] The waveforms of high-voltage side current, power, and low-voltage DC bus voltage under symmetrical voltage sag proposed in this embodiment of the invention are as follows: Figure 10 As shown, (a) is the current on the high-voltage AC side, (b) is the voltage on the low-voltage DC bus, (c) is the instantaneous active power on the high-voltage AC side, and (d) is the instantaneous reactive power on the high-voltage AC side.

[0158] like Figure 10 Simulation results show that during the symmetrical voltage sag, the high-voltage side current of the cascaded power electronic transformer controlled by the improved virtual synchronous machine remains stable, while the low-voltage DC bus voltage quickly recovers to near the rated voltage of 750V after a brief drop. By reducing the instantaneous active and reactive power output of the cascaded power electronic transformer, the current during the sag is kept stable. The active power is reduced from 0.9MW to -0.8MW, and the reactive power is reduced from 2.0Mvar to 1Mvar. The power remains stable during the sag without significant power fluctuations.

[0159] The waveforms of high-voltage side current, power, and low-voltage DC bus voltage under asymmetric voltage sag proposed in this embodiment of the invention are as follows: Figure 11As shown, (a) is the high-voltage AC side current, (b) is the low-voltage DC bus voltage, (c) is the instantaneous active power on the high-voltage AC side, and (d) is the instantaneous reactive power on the high-voltage AC side.

[0160] like Figure 11 Simulation results show that during the duration of the asymmetrical voltage sag, the high-voltage side current of the cascaded power electronic transformer controlled by the improved virtual synchronous machine remains stable, while the low-voltage DC bus voltage quickly recovers to near the rated voltage of 750V after a brief drop. By reducing the instantaneous active and reactive power output of the cascaded power electronic transformer, the current during the sag is kept stable. The active power is reduced from 0.9MW to -0.6MW, and the reactive power is reduced from 2.0Mvar to 1.5Mvar. The power remains stable during the sag without significant power fluctuations.

[0161] Another embodiment of the present invention provides a computer device including a memory and a processor, wherein the memory stores a computer program, and when the processor runs the computer program stored in the memory, the processor executes the voltage sag control method for cascaded power electronic transformers based on an improved virtual synchronous machine according to the present invention.

[0162] Another embodiment of the present invention provides a computer-readable storage medium having a computer program stored thereon, wherein when the computer program is executed by a processor, the processor performs the voltage sag control method for cascaded power electronic transformers based on an improved virtual synchronous machine according to the present invention.

[0163] Those skilled in the art will understand that embodiments of the present invention can be provided as methods, systems, or computer program products. Therefore, the present invention can take the form of a completely hardware embodiment, a completely software embodiment, or an embodiment combining software and hardware aspects. Furthermore, the present invention can take the form of a computer program product embodied on one or more computer-usable storage media (including, but not limited to, disk storage, CD-ROM, optical storage, etc.) containing computer-usable program code.

[0164] This invention is described with reference to flowchart illustrations and / or block diagrams of methods, apparatus (systems), and computer program products according to embodiments of this application. It will be understood that each block of the flowchart illustrations and / or block diagrams, and combinations of blocks in the flowchart illustrations and / or block diagrams, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, special-purpose computer, embedded processor, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, generate instructions for implementing the flowchart illustrations and / or block diagrams. Figure 1 One or more processes and / or boxes Figure 1 A device that provides the functions specified in one or more boxes.

[0165] These computer program instructions may also be stored in a computer-readable storage medium that can direct a computer or other programmable data processing device to function in a particular manner, such that the instructions stored in the computer-readable storage medium produce an article of manufacture including instruction means, which are implemented in a process Figure 1 One or more processes and / or boxes Figure 1 The function specified in one or more boxes.

[0166] These computer program instructions may also be loaded onto a computer or other programmable data processing equipment to cause a series of operational steps to be performed on the computer or other programmable equipment to produce a computer-implemented process, thereby providing instructions that execute on the computer or other programmable equipment for implementing the process. Figure 1 One or more processes and / or boxes Figure 1 The steps of the function specified in one or more boxes.

[0167] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the specific implementation of the present invention. Any modifications or equivalent substitutions that do not depart from the spirit and scope of the present invention should be covered within the protection scope of the claims of the present invention.

Claims

1. A method for controlling voltage sag of cascaded power electronic transformers based on an improved virtual synchronous machine, characterized in that, Includes the following steps: A power flow model between the high-voltage AC port of a cascaded power electronic transformer and the distribution network based on virtual synchronous machine control is established. Based on the power flow model, the power exchange function between the high-voltage AC port and the power grid of the cascaded power electronic transformer after the introduction of virtual synchronous machine control is constructed. Based on the aforementioned exchange power function, an improved virtual synchronous machine control strategy for the input stage of the cascaded power electronic transformer is obtained under symmetrical and asymmetrical voltage sags.

2. The voltage sag control method for cascaded power electronic transformers based on an improved virtual synchronous machine according to claim 1, characterized in that, The establishment of a power flow model between the high-voltage AC port of a cascaded power electronic transformer based on virtual synchronous machine control and the distribution network includes: Using the topology and control equations of a cascaded power electronic transformer, a mathematical model of the input stage of the cascaded power electronic transformer is established. From the perspective of the mathematical model, the feasibility of the input stage of the cascaded power electronic transformer controlled by a virtual synchronous machine and the synchronous generator being equivalent is analyzed, and an equivalent circuit for grid connection of the high-voltage AC port of the cascaded power electronic transformer with virtual synchronous machine control is established. Based on the grid-connected equivalent circuit of the high-voltage AC port of the cascaded power electronic transformer with virtual synchronous machine control, a power flow model between the high-voltage AC port of the cascaded power electronic transformer and the distribution network is established.

3. The voltage sag control method for cascaded power electronic transformers based on an improved virtual synchronous machine according to claim 2, characterized in that, The method utilizes the topology and control equations of a cascaded power electronic transformer to establish a mathematical model of its input stage. From the perspective of the mathematical model, the feasibility of using a virtual synchronous machine-controlled input stage of the cascaded power electronic transformer as an equivalent to a synchronous generator is analyzed. An equivalent circuit for grid connection of the high-voltage AC port of the cascaded power electronic transformer with virtual synchronous machine control is established, including: Based on the topology of the cascaded power electronic transformer, the high-voltage AC side input current is taken as the state variable, and the filtering parameters of the high-voltage AC port are considered to construct a mathematical model of the input stage of the cascaded power electronic transformer. Based on the external characteristics of the simulated synchronous machine controlled by the virtual synchronous machine introduced into the input stage of the cascaded power electronic transformer, a mathematical model of the input stage of the cascaded power electronic transformer with virtual synchronous machine control is established. Based on the equivalent relationship between the mathematical model of the input stage of the cascaded power electronic transformer and the mathematical model of the input stage of the cascaded power electronic transformer with virtual synchronous machine control, an equivalent circuit for grid connection of the high-voltage AC port of the cascaded power electronic transformer with virtual synchronous machine control is established. The cascaded power electronic transformer includes: an input stage, an isolation stage, a low-voltage DC load, and an output stage. The input stage adopts a cascaded H-bridge structure, connecting the high-voltage AC power supply and the DC section.

4. The voltage sag control method for cascaded power electronic transformers based on an improved virtual synchronous machine according to claim 2, characterized in that, The establishment of a power flow model between the high-voltage AC port of the cascaded power electronic transformer and the distribution network based on the grid-connected equivalent circuit of the cascaded power electronic transformer with virtual synchronous machine control includes: Based on the grid-connected equivalent circuit of the high-voltage AC port of the cascaded power electronic transformer with virtual synchronous machine control, and taking into account the filtering parameters and power flow characteristics, a power flow model between the high-voltage AC port of the cascaded power electronic transformer and the distribution network is established.

5. The voltage sag control method for cascaded power electronic transformers based on an improved virtual synchronous machine according to claim 1, characterized in that, The power exchange function between the high-voltage AC port and the power grid of the cascaded power electronic transformer after the introduction of virtual synchronous machine control, based on the power flow model, includes: Based on the power flow model of the cascaded power electronic transformer, the power exchange function between the high-voltage AC port and the power grid after the introduction of virtual synchronous machine control is constructed. Based on the power exchange function between the high-voltage AC port and the power grid, the influence of filter parameters, inertia and damping parameters, and coupling parameters of active and reactive power on the control effect of the cascaded power electronic transformer is analyzed.

6. The voltage sag control method for cascaded power electronic transformers based on an improved virtual synchronous machine according to claim 5, characterized in that, Based on the power flow model of the cascaded power electronic transformer, the power exchange function between the high-voltage AC port and the power grid after the introduction of virtual synchronous machine control is constructed as follows: （4） In the formula, E0∠δ0 is the port voltage of the virtual synchronous machine control port, and U f ∠0° is the grid connection point voltage, ω is the grid angular frequency, P is the active power output of the AC port, Q is the reactive power output of the AC port, Z is the equivalent circuit impedance, and α is the equivalent impedance angle.

7. The voltage sag control method for cascaded power electronic transformers based on an improved virtual synchronous machine according to claim 5, characterized in that, The analysis of the impact of filter parameters, inertia and damping parameters, and coupling parameters of active and reactive power on the control effect of the cascaded power electronic transformer, based on the power exchange function between the high-voltage AC port and the power grid, includes: The power exchange function between the high-voltage AC port and the power grid of the cascaded power electronic transformer after the introduction of virtual synchronous machine control is sorted out to obtain the influence of filtering parameters on the control effect of the cascaded power electronic transformer. By expanding the multivariate function using the Taylor formula and removing second-order and higher-order terms, the small-signal model of the filter parameters and the transmission power between the high-voltage AC port and the distribution network is obtained as follows: （5） The cascaded power electronic transformer with virtual synchronous machine control is used to adjust the virtual inertia and damping parameters through an active-frequency loop to enhance the mechanical characteristics of the power electronic transformer during power regulation. The electromagnetic power of the virtual synchronous machine is calculated by the instantaneous power of the input stage of the power electronic transformer, and combined with the characteristic equation of the virtual synchronous machine, the following is obtained: （6） In the formula: P e Electromagnetic power; Q e Ω represents instantaneous reactive power; M represents the mechanical angular velocity of the rotor; Ω represents instantaneous reactive power. T and M e These are mechanical torque and electromagnetic torque, respectively; ω e E is the electrical angle; E is the VSG port voltage. Q is the initial voltage; ref For reference reactive power; k q is the gain coefficient of the integrator; p is the number of pole pairs; J is the inertial parameter; When the number of rotor pole pairs p of the synchronous generator is 1, Ω = ω e And taking into account the actual electrical angle ω e At the rated electrical angle ω N The damping characteristics under varying conditions are: （7） In the formula, decreasing the inertia parameter J shortens the settling time and increases the response speed; increasing the damping parameter D results in faster power decay and smaller oscillation amplitude. The rated active power output of the AC port; Considering the damping characteristics when the actual electrical angle varies near the rated electrical angle, the influence of inertia and damping parameters on the control effect of cascaded power electronic transformers is obtained. Based on the equivalent circuit of the high-voltage AC port grid connection of the cascaded power electronic transformer with virtual synchronous machine control and the power exchange function between the high-voltage AC port and the grid after the introduction of virtual synchronous machine control, the influence of the coupling parameters of active and reactive power on the control effect of the cascaded power electronic transformer is obtained, and the power coupling model of the power electronic transformer is obtained: （8） In the formula, the off-diagonal parameters have a significant impact on the degree of power coupling. Under conditions of high impedance-to-inductance ratio or large power angle δ, the power coupling effect will be significantly enhanced. The coupling parameter between the power angle and active power is denoted as . The voltage-active power coupling parameter is... The coupling parameter between the power angle and reactive power is denoted as . This is the coupling parameter between voltage and reactive power.

8. The voltage sag control method for cascaded power electronic transformers based on an improved virtual synchronous machine according to claim 1, characterized in that, The improved virtual synchronous machine control strategy for the input stage of the cascaded power electronic transformer under symmetrical and asymmetrical voltage sags, based on the exchange power function, includes: The control effect of the cascaded power electronic transformer on the power exchange function between the high-voltage AC port and the power grid is analyzed by combining key parameters. By dynamically adjusting the active power reference value, the active power imbalance is eliminated to maintain a constant power angle during faults, thereby shortening the transient response process. An improved virtual synchronous machine control strategy for the input stage of the cascaded power electronic transformer under symmetrical voltage sag is obtained. The adjustment of the active power reference value is as follows: （10） In the formula: P ref U is the reference value for steady-state operating power. N The rated voltage at the grid connection point is δ0, and the power angle at the steady-state operating point is δ0. This is the stable initial voltage for the virtual synchronous machine control port. Here, E is the effective value of the phase voltage after the voltage dip, and Z is the voltage at the virtual synchronizer control port after the voltage dip. Using the aforementioned dynamically adjusted active power reference value, and considering the negative sequence component that appears under asymmetrical voltage sag, an improved control strategy for the virtual synchronous machine of the input stage of the cascaded power electronic transformer under asymmetrical voltage sag is obtained. The current control loop considering the negative sequence component is as follows: （15） In the formula: , , , For the power grid setpoint, , , , For current control variables, , , , This is the current reference value. , , , For integral and proportional parameters, , , , This is the output voltage of the power electronic transformer. For the Laplace operator; Based on the improved cascaded power electronic transformer input stage virtual synchronous machine control strategy under symmetrical and asymmetrical voltage sags, the analysis results under the improved virtual synchronous machine power electronic transformer voltage sag control strategy are obtained.

9. A computer device, characterized in that, It includes a memory and a processor, wherein the memory stores a computer program, and when the processor runs the computer program stored in the memory, the processor executes the voltage sag control method for cascaded power electronic transformers based on an improved virtual synchronous machine according to any one of claims 1 to 8.

10. A computer-readable storage medium having a computer program stored thereon, characterized in that, When the computer program is executed by the processor, the processor performs the voltage sag control method for cascaded power electronic transformers based on an improved virtual synchronous machine, as described in any one of claims 1 to 8.

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