Off-grid photovoltaic grid-connected control method and system based on virtual synchronous machine

Abstract

The invention discloses an off-grid photovoltaic grid-connected control method and system based on a virtual synchronous machine, and relates to the technical field of electric power. Establishing a load shedding standby control strategy of the photovoltaic unit; designing a photovoltaic pre-synchronization grid-connected control module based on double phase-locked loops; designing controllers of the virtual synchronous machine, namely an active-frequency controller and a reactive-voltage controller, building an overall control structure of the virtual synchronous machine, and combining and building a control structure for merging off-grid photovoltaic into the micro-grid; and solving the off-grid photovoltaic self-optimization-approaching grid-connected control. According to the invention, active-frequency droop control is designed to complete the working performance of a speed regulator, and a reactive-voltage controller is designed by simulating the reactive-voltage droop control characteristic of a synchronous generator, so that the photovoltaic system can autonomously sense and track the frequency change of a power grid when the frequency change occurs in the system, and the control efficiency is improved. And the operation stability after photovoltaic power generation is connected to a power grid is improved.

Description

Technical Field

[0001] This invention relates to the field of power technology, and in particular to an off-grid photovoltaic grid-connected control method and system based on a virtual synchronous machine. Background Technology

[0002] Because renewable energy grid connection can easily impact the power grid, and it cannot autonomously sense and track grid frequency changes during operation, its operational stability is poor. With the rapid development of photovoltaic (PV) power generation technology and the continuous increase in the scale of grid-connected PV, numerous problems arise during the pre-synchronization grid connection of off-grid PV systems. First, frequency and phase synchronization are crucial; the system must ensure that the generated electricity matches the grid frequency and waveform to prevent power system instability. Second, the system needs sufficient reserve capacity and adaptability so that after grid connection, the PV system can quickly sense and automatically control and track frequency fluctuations during actual grid operation, ensuring its operational stability. Therefore, off-grid PV systems achieve dynamic consistency with the grid state through adaptive regulation, successfully completing pre-synchronization grid connection. Furthermore, sensing and tracking grid frequency fluctuations requires comprehensive consideration of these factors. Appropriate technologies and management methods are needed to achieve zero-deviation tracking of the system frequency by the PV system, ensuring the stability and reliability of system operation.

[0003] Meanwhile, when the system frequency fluctuates, if the grid-connected photovoltaic system cannot track the frequency changes in time, it will lead to instability and poor frequency stability. Currently, to enhance the stability of photovoltaic power generation connected to the grid, there are roughly four control strategies for microgrids composed of photovoltaic systems: PQ (constant power) control, VF (constant voltage and constant frequency) control, VSG control, and Droop control strategies. Among them, PQ and VF control modes are suitable for grid-connected and islanded operation modes, respectively. VSG can provide rotational inertia and damping to the grid by simulating a virtual synchronous generator, improving the grid's acceptance of non-synchronous generator grid connection and facilitating the friendly access of distributed energy. However, the traditional VSG control method is relatively cumbersome. The essence of Droop control is to imitate the frequency and voltage regulation characteristics of synchronous generators to regulate the energy output of power electronic inverters. Although it imitates the frequency and voltage regulation functions of synchronous generators, it still lacks the inertia and damping characteristics of synchronous generators. Therefore, it is necessary to use a virtual synchronous machine, with the addition of two parameters, inertia and damping, to enable the inverter output to have the same working performance as a synchronous generator, thereby improving the operational stability of photovoltaic power generation after it is connected to the grid. Summary of the Invention

[0004] The purpose of this invention is to overcome the shortcomings of existing technologies and provide an off-grid photovoltaic (PV) grid-connected control method and system based on a virtual synchronous machine. This invention addresses the problem of pre-synchronization grid connection for off-grid PV and how to enable grid-connected PV to automatically control and track system frequency changes. This invention establishes a PV pre-synchronization grid connection model to achieve rapid grid synchronization of the PV system when required by the grid. Furthermore, to achieve inverter output performance similar to that of a synchronous generator, this invention designs active power-frequency droop control to achieve governor performance, and designs a reactive power-voltage droop controller by mimicking the reactive power-voltage droop control characteristics of a synchronous generator. This enables the PV system to autonomously sense and track grid frequency changes when system frequency changes occur, improving the operational stability of PV power generation after grid connection.

[0005] This invention is achieved through the following technical solution:

[0006] An off-grid photovoltaic grid-connected control method based on a virtual synchronous machine includes the following steps:

[0007] S1. Establish a load shedding and backup control strategy for photovoltaic units to reserve active power.

[0008] S2. Design a photovoltaic pre-synchronization grid-connected control module based on a double phase-locked loop, so that the terminal voltage of the off-grid photovoltaic-virtual synchronous machine system can autonomously converge to the target value through closed-loop regulation, thereby achieving amplitude self-optimization synchronization. Through closed-loop control, drive the phase angle of the off-grid photovoltaic-virtual synchronous machine system to gradually approach the grid phase, thereby completing phase self-optimization adjustment.

[0009] S3. Based on the active power reserved in step S1 and the self-optimization adjustment result in step S2, design the controller of the virtual synchronous machine, including an active power-frequency controller and a reactive power-voltage controller. Based on the active power-frequency controller and the reactive power-voltage controller, construct the control structure for off-grid photovoltaic integration into the microgrid.

[0010] S4. Solve the control structure of the off-grid photovoltaic system connected to the microgrid to realize the autonomous tracking of the grid frequency control of the off-grid photovoltaic-virtual synchronous machine system.

[0011] The load shedding and reserve control strategy for photovoltaic units described in step S1, which allows photovoltaic units to reserve active power, is as follows: After the photovoltaic unit finds its maximum power point, the output voltage of the photovoltaic unit is controlled to be lower than the voltage at which it operates at the maximum power point, so that the photovoltaic unit reserves active power to have the ability to participate in system frequency regulation, and the actual operating voltage of the photovoltaic unit is higher than the voltage at the maximum power point, ensuring that the photovoltaic unit operates stably in a preset stable region.

[0012] When a PV unit operates in Max Power Point (MPPT) mode, it cannot participate in grid frequency regulation because there is no available backup power. After finding the maximum power point of the PV array, the PV output voltage is lowered than the voltage at which it operates at the maximum power point. This control method is called load shedding reserve control, which allows the PV unit to reserve a certain amount of active power to participate in system frequency regulation.

[0013] Before finding the maximum power point of the photovoltaic array, constant power control is used, and the target power P is set through the power loop. deloading Power P less than the maximum power point MPPT This ensures the voltage operates to the left of the PV curve, resulting in a control voltage for the photovoltaic unit's output voltage lower than the voltage at the maximum power point. After finding the maximum power point of the photovoltaic array, constant voltage control is employed, at which point the voltage V at the maximum power point is known. MPPT Set the operating voltage V for control. deloading Greater than V MPPT That's all.

[0014] Step S2 is as follows: Using the grid voltage amplitude as a dynamic reference signal, the off-grid photovoltaic-virtual synchronous machine terminal voltage is guided to autonomously converge to the target value through closed-loop regulation, thereby achieving self-optimization synchronization of amplitude; at the same time, in order to achieve phase consistency, the q-axis component of the grid voltage is used as a reference, and the phase angle of the off-grid photovoltaic-virtual synchronous machine is driven to gradually approach the grid phase through closed-loop control, thereby completing the self-optimization adjustment of the phase dimension.

[0015] The dq transformation angle within the dual phase-locked loop is derived from the instantaneous phase estimation of the photovoltaic-virtual synchronous machine terminal voltage.

[0016] The design of the active power-frequency controller specifically involves: based on the primary frequency regulation characteristics of a synchronous generator, introducing active power-frequency droop control, constructing a virtual speed governor control strategy, and building a virtual rotor equation by combining the synchronous generator rotor motion equation. Through the coupling of the virtual speed governor control strategy and the virtual rotor equation, the off-grid photovoltaic-virtual synchronous generator system can achieve autonomous sensing and dynamic adjustment of grid frequency fluctuations. The specific details are as follows:

[0017] The droop control between active power and frequency is incorporated to optimize the performance of the speed governor, thereby varying its output power. The formula for droop control is:

[0018] (1)

[0019] In the formula: P a P b f a f b These represent the active power and frequency corresponding to any two points a and b, respectively, and K.p This is the active power droop coefficient;

[0020] Since the relationship between frequency and angular velocity is: Therefore, the mathematical expression for the virtual speed governor control strategy algorithm is designed as follows:

[0021] (2)

[0022] In the formula: P m The active power input to the virtual prime mover; P ref This is a reference value for active power. The angular velocity at rated operating output; This refers to the actual angular velocity output during operation.

[0023] The rotor equations of the VSG are constructed using the rotor motion equations of the synchronous generator:

[0024] (3)

[0025] In the formula: P e is the electromagnetic power output by the virtual synchronizer; D is the virtual damping coefficient; J is its virtual moment of inertia;

[0026] Combining equations (2) and (3), the active power-frequency control formula for the virtual synchronous machine is:

[0027] (4)

[0028] From equation (4), we know that the angular velocity difference passes through the active droop coefficient K. P The power deviation value is obtained, and the power deviation value is compared with the given active reference power value P. ref The summation yields the active power P of the virtual prime mover input. m Then the active power P input by the virtual prime mover m The electromagnetic power P output by the virtual synchronous machine obtained through grid connection e The power difference is obtained by comparison, and then the power difference is divided by the rated angular frequency to obtain the torque difference. The torque difference is then subtracted from the damping torque to obtain the unbalanced torque, which causes the rotor module to accelerate or decelerate, thereby changing the magnitude of the actual operating output angular velocity ω. Finally, an integral stage is used to change the phase angle of the virtual synchronous machine.

[0029] The aforementioned reactive power-voltage controller design specifically involves: constructing a virtual exciter control loop based on the reactive power-voltage droop characteristics; and achieving coordinated and stable control of the generator terminal voltage and reactive power in the off-grid photovoltaic-virtual synchronous machine system through closed-loop feedback of voltage deviation and reactive power deviation. The details are as follows:

[0030] Constructing the mathematical expression for reactive power-voltage control:

[0031] (5)

[0032] In the formula: u a u b Q a Q b These represent the voltage and reactive power corresponding to any two points a and b, respectively; K q This is the reactive power droop coefficient;

[0033] Based on the reactive power-voltage droop control characteristics, the virtual exciter control loop is derived, and its mathematical formula is:

[0034] (6)

[0035] In the formula: E is the virtual electromotive force; U is the actual calculated voltage; U N This is the rated voltage value; Q ref This is the reactive power reference value; Q e 1 / k is the actual reactive power; 1 / k is the reactive power integral coefficient; s is a Laplace transform complex frequency domain variable commonly used in control systems.

[0036] Formula (5) is a voltage difference control law based on the reactive power difference, used to describe the correspondence between voltage and reactive power between any two points (a, b) in the system, i.e., the static characteristics of reactive power-voltage. Formula (6) is based on this, combined with dynamic control requirements, to construct a mathematical model of the virtual exciter control loop. It introduces feedback mechanisms (such as reactive power deviation and voltage deviation) and integral regulation, thereby realizing dynamic regulation and control of the virtual excitation of the synchronous machine. That is, Formula (6) is a dynamic regulation model formed by the control system design based on Formula (5).

[0037] By using closed-loop feedback of voltage and reactive power deviations, coordinated and stable control of the terminal voltage and reactive power of the off-grid photovoltaic-virtual synchronous machine system is achieved.

[0038] The control structure for integrating off-grid photovoltaic into a microgrid based on an active power-frequency controller and a reactive power-voltage controller is as follows: the grid-connected voltage and current of the off-grid photovoltaic-virtual synchronous machine system are collected, active power and reactive power are output after power calculation, and the active power and reactive power are respectively transmitted to the active power-frequency controller and the reactive power-voltage controller to obtain the voltage phase and amplitude. Then, the reference voltage is obtained through voltage synthesis, and finally, the pulse signal is generated by PWM pulse width modulation to control the main circuit, so as to realize the closed-loop tracking voltage control of the entire loop.

[0039] The off-grid photovoltaic-virtual synchronous machine system realizes the interaction of solar power generation energy on the DC side through a DC / DC converter, and the AC side is connected to the load and the grid through a filter. When the grid frequency fluctuates, the photovoltaic unit realizes autonomous sensing and tracking of the grid frequency through the controller of the constructed virtual synchronous machine, ensuring zero deviation between the operating frequency and the system frequency.

[0040] Step S4 involves solving the control structure for the off-grid photovoltaic system integrated into the microgrid to achieve autonomous tracking of the grid frequency control by the off-grid photovoltaic-virtual synchronous machine system, as detailed below:

[0041] The control structure for off-grid photovoltaic (PV) integration into the microgrid is initialized with parameters. Then, the DC voltage, three-phase inverter side voltage, three-phase grid voltage, and three-phase inductor current of the control structure are sampled. The active and reactive power reference quantities of the PV are set, and the pre-synchronization link for PV grid connection is started. After the closing conditions are met, grid connection control is performed to realize the autonomous tracking of grid frequency control by the off-grid PV-virtual synchronous machine system.

[0042] An off-grid photovoltaic grid-connected control device based on a virtual synchronous machine includes:

[0043] Load shedding and reserve control strategy construction module: Establish a load shedding and reserve control strategy for photovoltaic units to reserve active power.

[0044] Photovoltaic pre-synchronization grid-connected control module: A photovoltaic pre-synchronization grid-connected control module based on a double phase-locked loop is designed to enable the terminal voltage of the off-grid photovoltaic-virtual synchronous machine system to autonomously converge to the target value through closed-loop regulation, thereby achieving amplitude self-optimization synchronization. The phase angle of the off-grid photovoltaic-virtual synchronous machine system is driven to gradually approach the grid phase through closed-loop control, thus completing phase self-optimization adjustment.

[0045] The controller construction module of the virtual synchronous machine: Based on the reserved active power and the self-optimization adjustment results, the controller of the virtual synchronous machine is designed, including an active power-frequency controller and a reactive power-voltage controller. Based on the active power-frequency controller and the reactive power-voltage controller, the control structure for off-grid photovoltaic integration into the microgrid is constructed.

[0046] Solution module: Solve the control structure of the off-grid photovoltaic system connected to the microgrid to realize the autonomous tracking of the grid frequency control of the off-grid photovoltaic-virtual synchronous machine system.

[0047] A computer device includes a memory and a processor, the memory storing a computer program, and the processor executing the computer program to implement the steps of the off-grid photovoltaic grid-connected control method based on a virtual synchronous machine.

[0048] A computer-readable storage medium having a computer program stored thereon, which, when executed by a processor, implements the steps of the off-grid photovoltaic grid-connected control method based on a virtual synchronous machine.

[0049] The advantages of this invention are: It autonomously senses grid frequency fluctuations and rapidly adjusts based on active power-voltage and reactive power-frequency controllers, quickly sensing and tracking grid frequency changes, thus improving its operational stability. Simultaneously, it enables load shedding and standby operation of the photovoltaic array, and autonomously tracks and adapts the system frequency based on photovoltaic-virtual synchronous machine control. When the grid requires it, the off-grid photovoltaic system can quickly track the amplitude and phase of the grid voltage and maintain synchronous operation with the grid, achieving plug-and-play functionality and rapid grid connection. This strategy also maintains a certain amount of active power reserve, which is used when system frequency fluctuations occur. It enables the photovoltaic-virtual synchronous machine system to autonomously sense grid frequency fluctuations and spontaneously track system frequency changes when the grid frequency changes. Even when stable, it can still maintain good tracking of the active power reference value, achieving rapid and stable operation at frequency abrupt changes, effectively improving the dynamic characteristics of the photovoltaic system's frequency. Attached Figure Description

[0050] Figure 1 A schematic diagram of load shedding control for photovoltaic units;

[0051] Figure 2 This is a schematic diagram of photovoltaic pre-synchronization.

[0052] Figure 3 This is a schematic diagram of the droop control principle.

[0053] Figure 4 VSG active-frequency control block diagram;

[0054] Figure 5 This is a schematic diagram of the reactive power-voltage droop control principle.

[0055] Figure 6 The control block diagram for the reactive power-voltage relationship of the VSG;

[0056] Figure 7 This is the overall control block diagram of VSG;

[0057] Figure 8 A schematic diagram of the control structure for photovoltaic grid integration into a microgrid;

[0058] Figure 9 The overall control logic diagram for VSG;

[0059] Figure 10 This is a waveform diagram for pre-synchronization grid connection;

[0060] Figure 11The waveform diagram of PV-VSG tracking control at 1 second;

[0061] Figure 12 The waveform diagram of PV-VSG tracking control at 1 second. Detailed Implementation

[0062] This invention proposes an off-grid photovoltaic (PV) grid-connected self-optimizing control method based on virtual synchronous machine (VSM) technology. This method autonomously senses grid frequency fluctuations and rapidly adjusts based on active power-voltage and reactive power-frequency controllers. It quickly senses and tracks grid frequency changes, improving operational stability. Simultaneously, it enables load shedding and standby operation of the PV array, and autonomously tracks and adapts to the system frequency based on PV-VSM control. When the grid requires it, the off-grid PV system can quickly track the magnitude and phase of the grid voltage and maintain synchronous operation with the grid, achieving plug-and-play functionality and rapid grid connection. This strategy also maintains a certain amount of active power reserve for use when system frequency fluctuates. It enables the PV-VSM system to autonomously sense grid frequency fluctuations and spontaneously track system frequency changes when the grid frequency changes. Even when stable, it maintains excellent tracking of the active power reference value, achieving rapid and stable operation at frequency abrupt changes and effectively improving the dynamic characteristics of the PV system's frequency.

[0063] This invention proposes an off-grid photovoltaic grid-connected self-optimizing control method based on virtual synchronous generator (VSG) technology. It addresses the problems of voltage / frequency asynchrony, large grid impact, and difficulty in sensing dynamic changes in the traditional photovoltaic system grid connection process. By using the VSG controller to simulate the inertia and damping characteristics of a synchronous generator, the method achieves self-optimizing alignment of the photovoltaic system's frequency, voltage, and phase.

[0064] The method of this invention constructs a pre-synchronous grid connection and autonomous tracking model for photovoltaic systems. By introducing active power-frequency and reactive power-voltage droop control mechanisms, it endows off-grid photovoltaic systems with the ability to autonomously sense and respond to the grid operation status.

[0065] In terms of control strategy, this invention designs a dual closed-loop control framework. The PV-VSG system autonomously tracks grid-side frequency changes through a frequency sensing module and virtual rotor motion equations. Based on the difference between the reference power and the actual grid operation, it dynamically adjusts the inverter output to achieve self-optimizing synchronization of frequency and phase angle. Simultaneously, a voltage control loop based on voltage amplitude and reactive power deviation is introduced, enabling the system to automatically adjust reactive power output according to grid voltage fluctuations, thereby enhancing system voltage stability.

[0066] To meet the real-time and rapid response requirements of off-grid photovoltaic systems during grid connection, this embodiment employs an equivalent constraint transformation method. This transforms the synchronization criteria, which originally relied on centralized network information, into frequency / voltage / phase error indicators that are locally perceptible to the inverter, forming a "self-triggering mechanism" for grid connection. Once the PV system meets the requirements that the voltage amplitude difference, electrical angle difference, and frequency difference at the generator terminals are less than set thresholds, it automatically achieves grid connection, eliminating reliance on traditional centralized control structures.

[0067] This invention also proposes a dynamic excitation-response iterative mechanism that balances stability and flexibility. Within each scheduling cycle, the system collects frequency, voltage, and active and reactive power outputs, updates the internal state of the virtual synchronous controller, and forms the time-series evolution trajectory of the photovoltaic system output. While ensuring system voltage amplitude and frequency synchronization, a penalty function minimization mechanism constrains the system fluctuation range, achieving optimal evolution of the grid connection process.

[0068] Compared with traditional control methods, the VSG self-optimizing control strategy of this invention has significant real-time performance and local decision-making capabilities. It enables the photovoltaic system to gradually evolve from an off-grid state to a synchronous state consistent with the grid under the three-dimensional indicators of frequency, voltage and phase, and continuously track and respond to changes in grid operation after grid connection.

[0069] Simulation results show that this method can achieve frequency synchronization within two cycles and rapid voltage phase locking when the grid frequency undergoes a step change, effectively reducing the instantaneous current surge during grid connection. The system operates more stably, is highly adaptable, and possesses good engineering feasibility and distributed scalability.

[0070] The self-optimization control method for photovoltaic and electric vehicles in this embodiment specifically includes the following steps:

[0071] Step S1: Establish a load shedding and backup control strategy for photovoltaic units.

[0072] The self-optimizing control concept emphasizes that each submodule within the system, when faced with external disturbances or changes in the operating environment, can autonomously adjust its operating strategy based on local perception and an incentive-feedback mechanism, gradually driving the entire system towards its optimal operating state without centralized coordination. This concept not only reflects the adaptability and flexibility of distributed energy systems during operation but also reveals the fundamental path for the intelligent evolution of power systems. Compared to the traditional "passive response, centralized optimization" control framework, self-optimizing control achieves an inherent unity between local objectives and system-level performance through the simultaneous implementation of real-time dynamic adjustment and local optimization. In VSG-based photovoltaic grid-connected systems, traditional control methods often struggle to respond quickly and effectively due to the high uncertainty and time-varying nature of grid frequency fluctuations. However, by introducing the self-optimizing mechanism, the photovoltaic inverter autonomously senses changes in grid frequency and voltage by simulating the inertia and damping characteristics of a synchronous generator, and actively adjusts the terminal output through droop control and virtual rotor motion equations, achieving continuous correction and optimization starting from local state errors. Especially when the system frequency rises or falls, the PV-VSG system can achieve dynamic frequency tracking and compensation by adjusting active power output and terminal voltage, thus quickly converging to a stable operating state after disturbances. This control strategy not only ensures zero frequency tracking deviation and stable operation of the photovoltaic system under small disturbances, but also reflects the self-organizing and self-coordinating characteristics of the system during operation, highlighting its significant improvement in system stability and response capability in the context of high proportion of distributed energy integration. Therefore, VSG-based photovoltaic grid-connected self-optimizing control is not only a technological evolution of control strategies, but also an important manifestation of the intelligent operation mechanism of the power system, especially suitable for complex power system environments with multi-source coordination, frequent disturbances, and high requirements for response timeliness in the future.

[0073] When a PV unit operates in the maximum power point tracking (MPPT) mode of wind power, it cannot participate in grid frequency regulation because there is no available reserve power. After finding the maximum power point of the PV array, the PV output voltage is lowered than the voltage at which the maximum power point is operated. This control method is called load shedding reserve control, which enables the PV unit to reserve a certain amount of active power to participate in system frequency regulation. Figure 1 This is a schematic diagram of the load shedding and backup control of a photovoltaic unit. The stable operating region of the photovoltaic unit is region 2, which means that the actual operating voltage of the photovoltaic array should be higher than the voltage at the maximum power point.

[0074] Before finding the maximum power point of the photovoltaic array, constant power control is used, and the target power P is set through the power loop. deloading Power P less than the maximum power point MPPTThis ensures the voltage operates to the left of the PV curve, resulting in a control voltage for the photovoltaic unit's output voltage lower than the voltage at the maximum power point. After finding the maximum power point of the photovoltaic array, constant voltage control is employed, at which point the voltage V at the maximum power point is known. MPPT Set the operating voltage V for control. deloading Greater than V MPPT That's all.

[0075] Step S2: Design a photovoltaic pre-synchronization grid-connected control module based on a dual phase-locked loop;

[0076] The photovoltaic pre-synchronization grid connection process is essentially a self-optimizing process involving coordination of both voltage amplitude and phase. To achieve natural alignment between the PV-VSG system and the grid operating state before grid connection, the grid voltage amplitude needs to be used as a dynamic reference signal to guide the PV-VSG terminal voltage to autonomously converge to the target value through closed-loop regulation, thus achieving self-optimizing synchronization of amplitude. Simultaneously, to achieve phase consistency, the system uses the q-axis component of the grid voltage as a reference and drives the PV-VSG system phase angle to gradually approach the grid phase through closed-loop control, completing the self-optimizing adjustment of the phase dimension.

[0077] To enhance the system's adaptability to operational errors, this invention constructs a photovoltaic pre-synchronization grid-connected control module based on a dual-phase-locked loop (DPLL), such as... Figure 2 As shown, the dq transformation angle within the phase-locked loop is... The instantaneous phase estimation derived from the terminal voltage enables the system to automatically correct the phase deviation introduced by the LC filter according to the actual operating state, achieving dynamic adaptation and fine alignment to grid connection conditions. Compared with traditional pre-synchronization methods, this self-optimizing control structure can achieve spontaneous synchronization of voltage amplitude and phase before the grid connection criteria are met, effectively reducing the instantaneous inrush current during grid connection and improving the system's dynamic response performance and grid connection stability.

[0078] Photovoltaic pre-synchronization grid connection includes amplitude and phase synchronization. To ensure the PV-VSG terminal voltage amplitude tracks the grid voltage amplitude, the grid voltage amplitude needs to be used as a reference for closed-loop control of the PV-VSG terminal voltage amplitude. To achieve phase synchronization between the PV-VSG and the grid voltage, closed-loop control of the grid voltage's q-axis component is required. Therefore, a photovoltaic pre-synchronization grid connection control module based on a dual phase-locked loop is designed, such as... Figure 2 As shown. It is important to note that the angle used in the dq transformation of the phase-locked loop (PLL) is... The phase of the generator terminal voltage is obtained through a phase-locked loop. Compared with the traditional pre-synchronization method, it achieves adaptive compensation for the phase deviation generated by the LC filter, and the generator terminal voltage amplitude can be completely synchronized with the power grid, theoretically reducing the current surge generated at the moment of grid connection.

[0079] First, the three-phase grid voltage Transform to a synchronous rotating coordinate system using the Park transformation:

[0080] (1)

[0081] in, Here is the Park transformation matrix. The grid phase angle estimated for a phase-locked loop (PLL).

[0082] To achieve phase synchronization between the PV-VSG terminal voltage and the grid voltage, q-axis voltage closed-loop control is employed. The reference value of the q-axis voltage is set to 0, and the angular frequency deviation of the virtual synchronizer is adjusted via a PI controller. :

[0083] (2)

[0084] (3)

[0085] in, For q-axis voltage error; , These are the proportional and integral gains, respectively. This represents the phase angle synchronization adjustment amount. Through the above steps, the phase deviation caused by the LC filter can be compensated, ensuring that the output voltage phase of the PV-VSG is consistent with the grid voltage phase, thereby achieving phase synchronization.

[0086] To synchronize the PV-VSG terminal voltage amplitude with the grid voltage amplitude, d-axis voltage closed-loop control is employed. The d-axis voltage reference value is set to the grid rated voltage amplitude.

[0087] (4)

[0088] (5)

[0089] in, This is the d-axis voltage reference value, corresponding to the grid's rated voltage; , These are the PI controller parameters for the d-axis voltage loop; This is the amplitude adjustment value, used to adjust the amplitude of the inverter's output voltage. By adjusting... The voltage amplitude at the PV-VSG generator terminal can accurately track the voltage amplitude of the grid, achieving amplitude synchronization.

[0090] Finally, the two closed-loop control outputs are applied to the phase and amplitude control loops of the virtual synchronizer, respectively:

[0091] (6)

[0092] (7)

[0093] in, This refers to the output angular frequency after synchronization. This refers to the amplitude of the synchronized output voltage. , These are the rated frequency and rated voltage, respectively.

[0094] Step S3: Design the controllers for the virtual synchronous machine: active power-frequency controller and reactive power-voltage controller, and build the overall control structure of the virtual synchronous machine. Combine this with Step S2 to construct the control structure for off-grid photovoltaic integration into the microgrid.

[0095] In the design of the VSG controller, a self-optimizing control concept is introduced, enabling the PV-VSG system to autonomously sense grid frequency disturbances and continuously coordinate system output with grid status through a dynamic feedback adjustment mechanism. When grid frequency fluctuations occur, the PV-VSG system can proactively approach the required frequency and voltage targets by adaptively adjusting its own operating state based on local sensing information, achieving real-time tracking and stable support of the distributed photovoltaic units for the grid status. To this end, this scheme designs two types of key controllers: an active power-frequency controller and a reactive power-voltage controller, which drive the system to achieve continuous self-optimizing adjustment processes in the active power-frequency and reactive power-voltage dimensions, respectively. This design not only ensures the stable operation of the PV-VSG system under disturbance conditions but also demonstrates the proactive coordination capability and self-organizing evolution characteristics of distributed power sources in relation to the power system's operating status.

[0096] (1) Active-frequency controller

[0097] Ensuring system frequency stability is crucial among the core requirements for maintaining the normal operation of a power system. Fluctuations in system active power directly affect the system frequency. To maintain stable system active power, a dynamic balance must be maintained between the energy generated and consumed by the system. Traditional power systems typically use synchronous generators as the primary generating equipment, and synchronous generators use a unit regulating power K. G The primary frequency regulation capability can be estimated using the droop coefficient R. This regulation mechanism ensures the stable operation of the power system, and its mathematical expression is:

[0098] (8)

[0099] The control method for a typical synchronous generator is based on processing the difference between the real-time measured frequency and the system's rated frequency. After obtaining this difference value, a control algorithm generates an adjustable output value for the prime mover parameters. By adjusting the torque of the prime mover, its speed can be controlled. There are two key considerations when designing the control of a virtual synchronous generator (VSG). First, the control variable output by the virtual speed governor can be selected as the active power value, where there is a droop relationship between active power and frequency. Second, the capability of the grid-connected inverter, which can control the magnitude of the output power, needs to be considered.

[0100] When designing the control for the virtual active-frequency loop, the speed governor's performance is achieved by adding active-frequency droop control, thereby allowing the output power to change. Figure 3 This is a schematic diagram of the droop control principle. The formula for droop control is:

[0101] (9)

[0102] In the formula: K p This is the active power droop coefficient.

[0103] Since the relationship between frequency and angular velocity is: Therefore, the mathematical expression for the virtual speed governor control strategy algorithm is designed as follows:

[0104] (10)

[0105] In the formula: P m The active power input to the virtual prime mover; P ref This is a reference value for active power. The angular velocity at rated operating output; This refers to the actual angular velocity output during operation.

[0106] The frequency stability of a power system is closely related to the balance of energy supply and demand. As a mechanical and electrical device, a synchronous generator maintains power balance by releasing or absorbing inertial energy in the rotor when the load changes, ensuring torque balance to maintain frequency stability. In contrast, an inverter, as a grid-connected device, lacks a mechanical shaft and cannot maintain frequency stability by controlling torque. To reduce frequency fluctuations, a virtual rotor equation is established using the rotor motion formula of a synchronous generator. The mechanical power input of a synchronous generator has a certain inertial delay in response time, which may lead to a slower system response speed, thus affecting the overall system stability. In contrast, VSG control supplies energy through the DC side of the photovoltaic power generation system, and its response speed is faster than that of a synchronous generator, making VSG control more advantageous than traditional synchronous generators. Therefore, in order to make VSG more flexible and efficient in frequency sensing and tracking control, the rotor equation of VSG can be constructed using the rotor motion equation of a synchronous generator, and rewritten as (11):

[0107] (11)

[0108] In the formula: P e denoted as VSG, where D is the virtual damping coefficient and J is its virtual moment of inertia.

[0109] Combining equations (10) and (11), the active-frequency control formula for VSG can be derived as follows:

[0110] (12)

[0111] The control block diagram of VSG active power-frequency obtained from equation (12) is as follows: Figure 4 As shown.

[0112] Figure 3 The angular velocity difference in the middle is adjusted by the coefficient K. P The power deviation value is obtained, and this value is added to the active power reference power given by the system to obtain the input active power value P. m Then P m The active power feedback P obtained from grid connection e The power difference is obtained by comparison, then divided by the rated angular frequency to obtain the torque difference, and then the damping torque is subtracted to obtain the unbalanced torque, thereby causing the rotor module to accelerate or decelerate, thus changing the magnitude of the actual operating output angular velocity ω. Finally, an integral stage is passed to change the phase angle of the VSG.

[0113] (2) Reactive power-voltage controller

[0114] Traditional synchronous generators regulate the reactive power and voltage of the system through the excitation system. When the system voltage changes, a voltage change control quantity is generated. This change control quantity is used to adjust the excitation system, which in turn affects the electromotive force, so that the reactive power of the system reaches a balanced state, thereby keeping the system voltage stable.

[0115] This invention achieves reactive-voltage control by mimicking the reactive-voltage droop characteristics of a synchronous generator. The control principle diagram is shown below. Figure 4 As shown, the control chart is as follows Figure 5 As shown, the mathematical expression is:

[0116] (13)

[0117] In the formula: u a u b Q a Q b They are Figure 5 The voltage and reactive power corresponding to points a and b in the diagram; K q This is the reactive power droop coefficient.

[0118] Based on the concept of reactive power-voltage droop control, a design scheme for a virtual exciter control loop can be derived, the mathematical formula of which is:

[0119] (14)

[0120] In the formula: E is the virtual electromotive force; U is the actual calculated voltage; U N This is the rated voltage value; Q ref This is the reactive power reference value; Q e 1 / k is the actual reactive power; 1 / k is the reactive power integral coefficient.

[0121] Formula (13) is a voltage difference control law based on the reactive power difference, used to describe the correspondence between voltage and reactive power between any two points (a, b) in the system, i.e., the static characteristics of reactive power-voltage. Formula (14) is based on this, combined with dynamic control requirements, to construct a mathematical model of the virtual exciter control loop. It introduces feedback mechanisms (such as reactive power deviation and voltage deviation) and integral regulation, thereby realizing dynamic regulation and control of the virtual excitation of the synchronous machine. That is, Formula (14) is a dynamic regulation model formed by the control system design based on Formula (13).

[0122] Equation (14) can be used to derive the control block of the reactive voltage loop of the VSG. Figure 6 As shown.

[0123] (3) Constructing a virtual synchronous machine overall control model

[0124] The overall simulation diagram of VSG is as follows: Figure 7 As shown, the grid-connected voltage and current are first obtained, and the active and reactive power are calculated through power calculation. Then, the active and reactive power are respectively transmitted to the active power control loop and the reactive power voltage control loop. After that, the voltage phase and amplitude are obtained, and the reference voltage is obtained through voltage synthesis. Finally, the pulse signal is obtained through PWM pulse width modulation and transmitted to the main circuit. Thus, the closed-loop tracking voltage control of the entire circuit is achieved.

[0125] As shown in Figure 7, the overall control block diagram of VSG first obtains the grid-connected voltage and current, and then calculates the active and reactive power. The active and reactive power are then fed into the active power control loop and the reactive power voltage control loop, respectively. After that, the voltage phase and amplitude are obtained, and the reference voltage is obtained through voltage synthesis. Finally, the pulse signal is obtained through PWM pulse width modulation and transmitted to the main circuit, thus achieving closed-loop tracking voltage control of the entire circuit.

[0126] To achieve voltage amplitude and phase synchronization at the grid connection point, a VSG dual-loop structure as shown in the diagram is adopted. First, three-phase quantities are obtained through voltage and current sampling. After synchronous coordinate system transformation, the following was obtained , Then, the instantaneous power relationship is as follows:

[0127] (15)

[0128] (16)

[0129] Obtaining merit or no merit , .

[0130] voltage amplitude Calculated by the RMS module, relative to the rated voltage Constituting error The reactive power-voltage loop employs an integral control law with droop:

[0131] (17)

[0132] In the formula: E is the virtual electromotive force; U is the actual calculated voltage; U N This is the rated voltage value; Q ref This is the reactive power reference value; Q e 1 / k is the actual reactive power; 1 / k is the reactive power integral coefficient; s is a Laplace transform complex frequency domain variable commonly used in control systems.

[0133] The above equation is equivalent to connecting parallel integration channels over voltage error and reactive power deviation, making the virtual electromotive force... Capable of accurate tracking in steady state At the same time, according to The reactive power distribution and voltage support are completed, and the two 1 / s and 1 / k integral blocks in the figure correspond to the implementation of the control law.

[0134] Phase synchronization is achieved by the active-frequency loop. Based on the rotor power angle dynamics of the virtual synchronous generator, let... Determined by the proportional-integral active power deviation:

[0135] (18)

[0136] (19)

[0137] (20)

[0138] in, Indicates virtual damping; This is virtual inertia; The rated angular frequency; in the figure These correspond to the damping, inertia, and angle integral components, respectively. This is a phase integrator module normalized to the rated frequency. The result is obtained from the above equation. Used for coordinate transformation and phase adjustment to ensure that the generator terminal phase remains synchronized with the power grid, and in It provides a frequency modulation characteristic when the frequency changes.

[0139] By combining the two control signals, a three-phase voltage reference is constructed:

[0140] (twenty one)

[0141] (twenty two)

[0142] The PWM generates the bridge arm duty cycle and applies it to the main circuit and filter. Considering the effects of the inner current loop and the filter network, the voltage path from the generator terminal to the grid connection point can be approximated as a first-order object. Therefore, the amplitude closed-loop transfer function is:

[0143] (twenty three)

[0144] in, , To achieve the equivalent gain, the above formulas are combined to achieve simultaneous closed-loop control of voltage amplitude and phase, ensuring that the PCC voltage responds to U under disturbances and power command changes. N Steady-state error-free tracking and fast dynamic response, and achieve The schedulable output.

[0145] The off-grid photovoltaic-virtual synchronous machine system realizes the interaction of solar power generation energy on the DC side through a DC / DC converter, and the AC side is connected to the load and the grid through a filter. When the grid frequency fluctuates, the photovoltaic unit realizes autonomous sensing and tracking of the grid frequency through the controller of the constructed virtual synchronous machine, ensuring zero deviation between the operating frequency and the system frequency.

[0146] (4) Constructing a control structure for off-grid photovoltaic systems to be integrated into the microgrid

[0147] Figure 8 This is a schematic diagram of the photovoltaic grid-connected inverter using VSG control technology. The energy exchange of solar power on the DC side of the grid-connected inverter is accomplished through a DC / DC converter. On the AC side, it connects to the load and the grid after passing through a filter. When the grid frequency fluctuates, the circuit, through the VSG control module, participates in the sensing and tracking of the system frequency, enabling the photovoltaic system to autonomously sense and track changes in the system frequency, ensuring zero deviation between its operating frequency and the system frequency.

[0148] To obtain the system voltage U oabc and current i oabc The vector parameters, after being processed by the power calculation module, output the active power P. e And reactive power Q, P e The system, via the VSG control module, performs droop control on both active power (frequency) and reactive power (voltage). Then, through rotor motion equations and stator voltage equations, the system acquires inertial damping and electromagnetic transient characteristics, mimicking the operating characteristics of a synchronous generator. The obtained voltage amplitude and phase angle command values ​​are then processed through a voltage-current dual-loop module to obtain three-phase voltage command values. Finally, PWM pulse generation is used to control the grid-connected inverter.

[0149] Step 4: Solve for the off-grid photovoltaic self-optimizing grid connection control.

[0150] The solution framework of the off-grid photovoltaic grid-connected self-optimizing control method based on virtual synchronous machine technology proposed in this invention is as follows: Figure 7 As shown. First, the control circuit is initialized with parameter settings. Then, the DC voltage, three-phase inverter side voltage, three-phase grid voltage, and three-phase inductor current are sampled. Reference values ​​for the active and reactive power of the photovoltaic system are set. Based on this information, the pre-synchronization phase of photovoltaic grid connection can begin. After the closing conditions are met, grid connection control is initiated, according to... Figure 7 The PV-VSG system achieves autonomous tracking of the grid frequency. The overall VSG control logic diagram is as follows: Figure 9 As shown.

[0151] Example:

[0152] (1) Experimental data and parameter settings

[0153] To verify the accuracy and superiority of the off-grid photovoltaic grid-connected self-optimizing control method based on virtual synchronous machine technology in this invention, firstly, a simulation analysis of off-grid photovoltaic pre-synchronization grid connection is conducted. Based on this, to verify the photovoltaic's ability to track grid frequency changes, scenarios of grid frequency increase and decrease are set up respectively. Firstly, a simulation model of the grid-connected autonomous tracking control method system based on VSG is built on MATLAB / Simulink software, and then the simulation results are analyzed.

[0154] The simulation example settings and photovoltaic array parameters are shown in Table 1: Before pre-synchronization begins, the photovoltaic VSG operates under no-load conditions; after pre-synchronization begins, the photovoltaic VSG tracks the voltage and phase of the grid (pre-synchronization begins at 0.35 seconds); after pre-synchronization ends, the photovoltaic VSG operates in grid-connected mode. The active power reference value P of the PV-VSG is set. ref =16kW, corresponding voltage reference value E ref =1650V, so that it operates within region 2 of Figure (1) to ensure that it has a certain adjustable capacity. The grid frequency was set to rise from 50Hz to 50.2Hz and fall from 50.2Hz to 50Hz at 1S and 2S respectively to test its frequency tracking response characteristics.

[0155] Table 1 Photovoltaic Array Parameters

[0156] numerical values parameter numerical values Short circuit current / A 15.68 Maximum power point voltage / V 1450 Open circuit voltage / V 1800 Light intensity (W / ²) 1000 Maximum power / W 21315 Temperature / °C 25 Maximum power point current / A 14.7

[0157] (2) Analysis of the results of photovoltaic pre-synchronous grid connection

[0158] Before grid connection begins, the PV-VSG undergoes pre-synchronization, autonomously tracking the grid voltage and frequency, maintaining a constant operating frequency of 50Hz. When the grid requires PV grid connection, a closing signal is issued, and simultaneously, the pre-synchronization error is checked to ensure that the angle difference between the PV-VSG and the grid is less than 0.02 rad, the speed difference is less than 0.1 r / s, and the voltage amplitude difference is less than 0.8V. When the grid connection conditions are met, PV-VSG synchronization and grid connection are performed. To simulate this process, PV grid connection pre-synchronization is performed 0.35 seconds in advance, and the grid sends a signal to connect at 0.35 seconds to verify whether the proposed control strategy can be used immediately when the grid requires it.

[0159] Depend on Figure 10It can be seen that the photovoltaic operating frequency was maintained at 50Hz before the grid connection signal was issued. After the grid connection signal was issued, the frequency of the PV-VSG fluctuated slightly and quickly tracked the grid frequency, stabilizing with the grid frequency after two cycles. Furthermore, after the closing signal was issued, the PV-VSG quickly tracked the phase and amplitude of the grid voltage, and completely tracked the amplitude and phase of the system voltage after 0.2s. Thus, the PV-VSG synchronous grid connection was completed. The VSG-based photovoltaic grid connection autonomous tracking control strategy proposed in this paper can quickly control the photovoltaic system to connect to the grid when needed, achieving synchronization with the grid, and ensuring the pre-set tracking voltage of 1650V and tracking frequency of 16kW.

[0160] (3) Analysis of PV-VSG frequency autonomous tracking control when grid frequency rises

[0161] When the active power output by the engine exceeds the active power consumed by the load, the grid frequency rises. In order to maintain frequency stability, the PV-VSG system needs to sense system changes and track the system frequency.

[0162] Depend on Figure 11 Analysis shows that when the grid frequency fluctuates and rises, the PV-VSG system can quickly and autonomously sense the change in grid frequency, thereby tracking the grid frequency and maintaining synchronization with it, thus ensuring its operational stability. Based on the load shedding and backup control theory of photovoltaic units analyzed above, it can be seen that within region 2 of the PV-VSG operating summary diagram (1), the output active power decreases when its voltage rises. Therefore, when the grid frequency rises, the PV-VSG voltage rises, but after fluctuations, it can still maintain the set voltage, and the output active power of the PV-VSG system decreases. After a brief fluctuation, it can stably track the active power reference value.

[0163] Therefore, the PV-VSG frequency autonomous tracking control strategy proposed in this invention can effectively sense and track changes in the grid frequency in a timely manner. Furthermore, it exhibits a fast response speed and minimal fluctuations, and once stable, the PV-VSG voltage and active power continue to track the reference values ​​well, maintaining excellent stability.

[0164] (4) Analysis of PV-VSG frequency autonomous tracking control when grid frequency drops

[0165] When the active power output by the engine is less than the active power consumed by the load, the grid frequency drops. In order to maintain frequency stability, the PV-VSG system should sense the system change and track the system frequency.

[0166] right Figure 12Analysis shows that when the grid frequency drops, the PV-VSG system can quickly sense grid frequency fluctuations, track grid frequency changes, and quickly reach stability after a short period of fluctuation, tracking the set values ​​of PV-VSG voltage and active power to ensure the stability of system operation.

[0167] This invention is not limited to the above embodiments. All technical solutions formed by equivalent substitutions fall within the protection scope claimed by this invention.

Claims

1. A method for controlling off-grid photovoltaic grid connection based on a virtual synchronous machine, characterized in that: Includes the following steps: S1. Establish a load shedding and backup control strategy for photovoltaic units to reserve active power. S2. Design a photovoltaic pre-synchronization grid-connected control module based on a double phase-locked loop, so that the terminal voltage of the off-grid photovoltaic-virtual synchronous machine system can autonomously converge to the target value through closed-loop regulation, thereby achieving amplitude self-optimization synchronization. Through closed-loop control, drive the phase angle of the off-grid photovoltaic-virtual synchronous machine system to gradually approach the grid phase, thereby completing phase self-optimization adjustment. S3. Based on the active power reserved in step S1 and the self-optimization adjustment result in step S2, design the controller of the virtual synchronous machine, including an active power-frequency controller and a reactive power-voltage controller. Based on the active power-frequency controller and the reactive power-voltage controller, construct the control structure for off-grid photovoltaic integration into the microgrid. S4. Solve the control structure of the off-grid photovoltaic system connected to the microgrid to realize the autonomous tracking of the grid frequency control of the off-grid photovoltaic-virtual synchronous machine system.

2. The off-grid photovoltaic grid-connected control method based on a virtual synchronous machine according to claim 1, characterized in that: The load shedding and reserve control strategy for photovoltaic units described in step S1, which allows photovoltaic units to reserve active power, is as follows: After the photovoltaic unit finds its maximum power point, the output voltage of the photovoltaic unit is controlled to be lower than the voltage at which it operates at the maximum power point, so that the photovoltaic unit reserves active power to have the ability to participate in system frequency regulation, and the actual operating voltage of the photovoltaic unit is higher than the voltage at the maximum power point, ensuring that the photovoltaic unit operates stably in a preset stable region.

3. The off-grid photovoltaic grid-connected control method based on a virtual synchronous machine according to claim 1, characterized in that: Step S2 is as follows: Using the grid voltage amplitude as a dynamic reference signal, the off-grid photovoltaic-virtual synchronous machine terminal voltage is guided to autonomously converge to the target value through closed-loop regulation, thereby achieving self-optimization synchronization of amplitude; at the same time, in order to achieve phase consistency, the q-axis component of the grid voltage is used as a reference, and the phase angle of the off-grid photovoltaic-virtual synchronous machine is driven to gradually approach the grid phase through closed-loop control, thereby completing the self-optimization adjustment of the phase dimension.

4. The off-grid photovoltaic grid-connected control method based on a virtual synchronous machine according to claim 3, characterized in that: The dq transformation angle within the dual phase-locked loop is derived from the instantaneous phase estimation of the photovoltaic-virtual synchronous machine terminal voltage.

5. The off-grid photovoltaic grid-connected control method based on a virtual synchronous machine according to claim 1, characterized in that: The design of the active power-frequency controller specifically involves: based on the primary frequency regulation characteristics of a synchronous generator, introducing active power-frequency droop control, constructing a virtual speed governor control strategy, and building a virtual rotor equation by combining the synchronous generator rotor motion equation. Through the coupling of the virtual speed governor control strategy and the virtual rotor equation, the off-grid photovoltaic-virtual synchronous generator system can achieve autonomous sensing and dynamic adjustment of grid frequency fluctuations. The specific details are as follows: The droop control between active power and frequency is incorporated to optimize the performance of the speed governor, thereby varying its output power. The formula for droop control is: （1） In the formula: P a P b f a f b These represent the active power and frequency corresponding to any two points a and b, respectively, and K. p This is the active power droop coefficient; Since the relationship between frequency and angular velocity is: Therefore, the mathematical expression for the virtual speed governor control strategy algorithm is designed as follows: （2） In the formula: P m The active power input to the virtual prime mover; P ref This is a reference value for active power. The angular velocity at rated operating output; This refers to the actual angular velocity output during operation. The rotor equations of the VSG are constructed using the rotor motion equations of the synchronous generator: （3） In the formula: P e is the electromagnetic power output by the virtual synchronizer; D is the virtual damping coefficient; J is its virtual moment of inertia; Combining equations (2) and (3), the active power-frequency control formula for the virtual synchronous machine is: （4） From equation (4), we know that the angular velocity difference passes through the active droop coefficient K. P The power deviation value is obtained, and the power deviation value is compared with the given active reference power value P. ref The summation yields the active power P of the virtual prime mover input. m Then the active power P input by the virtual prime mover m The electromagnetic power P output by the virtual synchronous machine obtained through grid connection e The power difference is obtained by comparison, and then the power difference is divided by the rated angular frequency to obtain the torque difference. The torque difference is then subtracted from the damping torque to obtain the unbalanced torque, which causes the rotor module to accelerate or decelerate, thereby changing the magnitude of the actual operating output angular velocity ω. Finally, an integral stage is used to change the phase angle of the virtual synchronous machine.

6. The off-grid photovoltaic grid-connected control method based on a virtual synchronous machine according to claim 1, characterized in that: The aforementioned reactive power-voltage controller design specifically involves: constructing a virtual exciter control loop based on the reactive power-voltage droop characteristics; and achieving coordinated and stable control of the generator terminal voltage and reactive power in the off-grid photovoltaic-virtual synchronous machine system through closed-loop feedback of voltage deviation and reactive power deviation. The details are as follows: Constructing the mathematical expression for reactive power-voltage control: （5） In the formula: u a u b Q a Q b These represent the voltage and reactive power corresponding to any two points a and b, respectively; K q This is the reactive power droop coefficient; Based on the reactive power-voltage droop control characteristics, the virtual exciter control loop is derived, and its mathematical formula is: （6） In the formula: E is the virtual electromotive force; U is the actual calculated voltage; U N This is the rated voltage value; Q ref This is the reactive power reference value; Q e is the actual reactive power; 1 / k is the reactive power integral coefficient; s is the Laplace transform complex frequency domain variable in the control system; By using closed-loop feedback of voltage and reactive power deviations, coordinated and stable control of the terminal voltage and reactive power of the off-grid photovoltaic-virtual synchronous machine system is achieved.

7. The off-grid photovoltaic grid-connected control method based on a virtual synchronous machine according to claim 1, characterized in that: The control structure for integrating off-grid photovoltaic into a microgrid based on an active power-frequency controller and a reactive power-voltage controller is as follows: the grid-connected voltage and current of the off-grid photovoltaic-virtual synchronous machine system are collected, active power and reactive power are output after power calculation, and the active power and reactive power are respectively transmitted to the active power-frequency controller and the reactive power-voltage controller to obtain the voltage phase and amplitude. Then, the reference voltage is obtained through voltage synthesis, and finally, the pulse signal is generated by PWM pulse width modulation to control the main circuit, so as to realize the closed-loop tracking voltage control of the entire loop. The off-grid photovoltaic-virtual synchronous machine system realizes the interaction of solar power generation energy on the DC side through a DC / DC converter, and the AC side is connected to the load and the grid through a filter. When the grid frequency fluctuates, the photovoltaic unit realizes autonomous sensing and tracking of the grid frequency through the controller of the constructed virtual synchronous machine, ensuring zero deviation between the operating frequency and the system frequency.

8. The off-grid photovoltaic grid-connected control method based on a virtual synchronous machine according to claim 1, characterized in that: Step S4 involves solving the control structure for the off-grid photovoltaic system integrated into the microgrid to achieve autonomous tracking of the grid frequency control by the off-grid photovoltaic-virtual synchronous machine system, as detailed below: The control structure for off-grid photovoltaic (PV) integration into the microgrid is initialized with parameters. Then, the DC voltage, three-phase inverter side voltage, three-phase grid voltage, and three-phase inductor current of the control structure are sampled. The active and reactive power reference quantities of the PV are set, and the pre-synchronization link for PV grid connection is started. After the closing conditions are met, grid connection control is performed to realize the autonomous tracking of grid frequency control by the off-grid PV-virtual synchronous machine system.

9. An off-grid photovoltaic grid-connected control system based on a virtual synchronous machine, characterized in that: Including: Load shedding and reserve control strategy construction module: Establish a load shedding and reserve control strategy for photovoltaic units to reserve active power. Photovoltaic pre-synchronization grid-connected control module: A photovoltaic pre-synchronization grid-connected control module based on a double phase-locked loop is designed to enable the terminal voltage of the off-grid photovoltaic-virtual synchronous machine system to autonomously converge to the target value through closed-loop regulation, thereby achieving amplitude self-optimization synchronization. The phase angle of the off-grid photovoltaic-virtual synchronous machine system is driven to gradually approach the grid phase through closed-loop control, thus completing phase self-optimization adjustment. The controller construction module of the virtual synchronous machine: Based on the reserved active power and the self-optimization adjustment results, the controller of the virtual synchronous machine is designed, including an active power-frequency controller and a reactive power-voltage controller. Based on the active power-frequency controller and the reactive power-voltage controller, the control structure for off-grid photovoltaic integration into the microgrid is constructed. Solution module: Solve the control structure of the off-grid photovoltaic system connected to the microgrid to realize the autonomous tracking of the grid frequency control of the off-grid photovoltaic-virtual synchronous machine system.

10. A computer device comprising a memory and a processor, wherein the memory stores a computer program, characterized in that, When the processor executes the computer program, it implements the steps of the off-grid photovoltaic grid-connected control method based on a virtual synchronous machine as described in any one of claims 1-8.

11. A computer-readable storage medium having a computer program stored thereon, characterized in that, When the computer program is executed by the processor, it implements the steps of the off-grid photovoltaic grid-connected control method based on a virtual synchronous machine as described in any one of claims 1-8.

Images