A photovoltaic VSG voltage support method based on virtual impedance and active reserve cooperation

Abstract

The application discloses a photovoltaic VSG voltage support method based on virtual impedance and active reserve cooperation and relates to the technical field of photovoltaic power generation and power system control technology. In view of the problem that the existing grid-connected photovoltaic inverter cannot simultaneously consider voltage support, overcurrent suppression and system stability under the composite fault of synchronous voltage and light drop of a power grid, the application introduces a virtual impedance in a VSG reactive ring to enhance reactive voltage support when the power grid fails, and simultaneously starts photovoltaic active reserve control to suppress grid-connected overcurrent. The virtual impedance value and the active reserve capacity are constrained by double limiting boundaries, and dynamic cooperative adjustment is achieved to realize fault ride-through. The application does not need additional energy storage devices, can effectively improve the voltage support capability of the inverter, can reliably suppress overcurrent and can guarantee stable operation of the system.

Description

Technical Field

[0001] This invention relates to the field of photovoltaic power generation and power system control technology, and in particular to a photovoltaic VSG voltage support method based on virtual impedance and active power reserve coordination. Background Technology

[0002] Against the backdrop of a global energy structure transition towards cleaner and lower-carbon energy, the penetration rate of renewable energy sources, represented by photovoltaics, in power systems continues to increase. Among them, grid-connected photovoltaic inverter systems have become a hot research and application area due to their cost advantages. Such systems typically consist of photovoltaic arrays, boost converter circuits, grid-connected inverters, and the power grid. Grid-connected inverters often employ Virtual Synchronous Generator (VSG) control technology to simulate the external characteristics of synchronous generators, thereby improving the system's inertia and damping support capabilities.

[0003] In actual engineering operation, the system often faces severe operating conditions caused by extreme weather such as thunderstorms. A typical manifestation is a deep voltage drop at the point of common coupling (PCC) due to grid faults, accompanied by significant fluctuations in the output power of the photovoltaic array caused by a sudden drop in solar irradiance. In such composite fault scenarios where grid voltage and solar irradiance drop simultaneously, the system must meet three core operational requirements: first, provide sufficient voltage support for the PCC to ensure regional voltage stability under grid fault conditions; second, strictly limit the grid-connected current amplitude to prevent damage to power electronic devices due to overcurrent; and third, maintain the stable operation of the photovoltaic array and inverter to prevent grid disconnection during fault periods and recovery phases, ultimately achieving reliable fault ride-through.

[0004] Current control technologies for photovoltaic VSG fault ride-through still cannot resolve the core contradiction between voltage support and overcurrent suppression, making it difficult to simultaneously meet the aforementioned triple operational requirements under combined fault conditions. Existing mainstream technical solutions mainly include three categories: virtual impedance method, adaptive voltage compensation coefficient method, and switching parallel grid system. Among them, the virtual impedance method is divided into fixed-value and adaptive types. The fixed-value virtual impedance method cannot adapt to different degrees of voltage dip scenarios; if the impedance value is too small, it cannot effectively limit the short-circuit current, while if the value is too large, it will deteriorate the system's operational stability. The adaptive virtual impedance method still cannot guarantee the system's voltage support capability while achieving accurate and effective suppression of short-circuit current. The adaptive voltage compensation coefficient method is limited by the adjustable range of the voltage compensation coefficient, resulting in severely insufficient voltage support capability under deep voltage dip scenarios. Although the switching parallel grid system can operate without relying on grid parameter measurements under complex fault conditions, it suffers from insufficient phase compensation accuracy and is prone to oscillations during mode switching, making it difficult to guarantee the smoothness and stability of system operation.

[0005] In summary, existing technologies generally suffer from the inherent contradiction that "strong voltage support can easily lead to current overruns, while strong current limiting measures can easily weaken voltage support capabilities." In the case of a combined fault scenario where grid voltage and solar irradiance drop simultaneously, problems such as fault ride-through failure and inverter disconnection are very likely to occur, seriously threatening the safe and stable operation of high-proportion photovoltaic grid-connected systems. Therefore, it is urgent to develop a coordinated control technology that can take into account voltage support, overcurrent suppression, and system stability. Summary of the Invention

[0006] To address this, this invention provides a photovoltaic VSG voltage support method based on virtual impedance and active power reserve coordination. This method solves the technical problems of existing technologies in the context of compound fault scenarios where grid voltage drops and irradiance decreases occur simultaneously. These problems include the inability to simultaneously balance voltage support, overcurrent suppression, and system stability. Strong voltage support can easily lead to current over-limit threats to equipment safety, while strong current limiting measures can weaken voltage support capabilities, ultimately resulting in fault ride-through failure and inverter disconnection from the grid.

[0007] To address the aforementioned technical problems, this invention provides a photovoltaic VSG voltage support method based on virtual impedance and active power reserve coordination, applied to a grid-connected photovoltaic inverter system. The system includes a photovoltaic array, a boost converter circuit, a three-phase inverter controlled by a virtual synchronous generator, an LC filter circuit, and a power grid, connected in sequence. The method comprises the following steps: S1: Real-time acquisition of three-phase voltage and three-phase current at the grid connection point, as well as output power and incident light intensity parameters of the photovoltaic array; S2: Based on the collected grid connection point voltage, fault determination is performed. When the grid connection point voltage is lower than the preset grid connection point voltage threshold, a grid voltage fault is determined to have occurred, and fault ride-through collaborative control is initiated. S3: Introduce a virtual impedance in the reactive power loop of the virtual synchronous generator. The virtual impedance simulates an additional voltage drop, increases the voltage deviation of the reactive power loop, and improves the reactive power output capability of the inverter to support the grid connection point voltage. S4: Synchronously start photovoltaic active power reserve control. By adjusting the operating parameters of the Boost circuit, the photovoltaic array is made to deviate from the maximum power point, reducing the photovoltaic output active power and reserving active power capacity to suppress the grid connection current at the grid connection point. S5: Determine the dual limiting boundaries of coordinated control: Based on the condition that the grid connection point voltage and the grid voltage drop at the same magnitude, determine the first limiting boundary for the virtual impedance value; based on the preset grid connection current threshold and the power constraint of the photovoltaic array when the light intensity drops, determine the second limiting boundary for the active power reserve capacity. S6: Within the constraints of the dual-limit boundary, the virtual impedance value and active power reserve capacity are dynamically adjusted based on the real-time collected grid connection point voltage and grid connection current, so as to achieve stable support of the grid connection point voltage without exceeding the grid connection current limit. S7: When the grid connection point voltage is detected to have returned to the normal range and remained stable, smoothly exit the fault ride-through collaborative control and restore the normal operation of the virtual synchronous generator and photovoltaic maximum power point tracking control.

[0008] Preferably, in step S3, after introducing the virtual impedance, the voltage deviation increment of the reactive power loop is... The calculation formula is: ; in, This is the actual value of the grid connection point voltage. , The actual active and reactive power at the grid connection point. The system angular frequency, The virtual resistance value is the virtual impedance. The virtual inductance value is the virtual impedance. , These are the power components corresponding to the virtual resistance and virtual inductance.

[0009] Preferably, in step S4, the photovoltaic active power reserve control is implemented by reducing the duty cycle of the Boost circuit, causing the operating point of the photovoltaic array to move along its PU characteristic curve in the direction of increasing voltage, deviating from the maximum power point, thereby reducing the output active power of the photovoltaic array and forming active power reserve capacity.

[0010] Preferably, in step S5, the first limiting boundary is used to constrain the value of the virtual impedance, and its expression is: ; in, The virtual resistance value is the virtual impedance. This is the ratio of virtual inductance to virtual resistance. The degree of voltage drop in the power grid. , The actual active and reactive power at the grid connection point. The grid connection current amplitude at the grid connection point. This is the actual value of the grid connection point voltage. The voltage rating at the grid connection point is [value]; the value of the virtual impedance must not exceed the first limit boundary.

[0011] Preferably, in step S5, the second limiting boundary is used to constrain the active power reserve capacity, and its expression is: ; in, This is the active power reference value. This is the actual value of the grid connection point voltage. The preset grid-connected current threshold, , The actual active and reactive power at the grid connection point. This represents the maximum power point power of the photovoltaic array after the light intensity drops.

[0012] Preferably, in step S5, the determination of the active power reserve capacity must simultaneously satisfy the following three constraints: Environmental adaptability constraints: The active power reserve capacity must be greater than the natural reduction in photovoltaic power at the maximum power point caused by the drop in light intensity during the fault period; Power angle stability constraint: The active power reference value of the inverter after active power reserve adjustment must be less than the maximum transmission power of the virtual synchronous generator power angle characteristic curve under fault conditions. Overcurrent suppression constraint: The active power reference value of the inverter after active power reserve adjustment satisfies the following formula: ; in, This is the active power reference value. This is the actual value of the grid connection point voltage. The preset grid-connected current threshold, This represents the actual reactive power at the grid connection point.

[0013] Preferably, in step S6, the method for dynamically adjusting the value of the virtual impedance and the active power reserve capacity based on the real-time collected grid connection point voltage and grid connection current is as follows: When the real-time collected grid-connected current amplitude is close to the preset threshold, the active power reserve capacity is increased within the adjustable range of photovoltaic power to suppress current over-limit; when the grid connection point voltage support effect does not meet the target and there is a margin in the grid connection current, the value of the virtual impedance is increased within the first limit boundary to improve the reactive power support capability.

[0014] Preferably, the preset grid connection point voltage threshold is 0.9 pu of the rated voltage, the preset grid connection current threshold is 1.5 times the rated current, and the normal range of the grid connection point voltage is 0.9 pu to 1.1 pu.

[0015] Preferably, in step S7, the method for smoothly exiting the collaborative control is as follows: First, gradually reduce the value of the virtual impedance to zero according to the preset slope, and then gradually adjust the duty cycle of the Boost circuit to smoothly restore the operating point of the photovoltaic array to the maximum power point, thus completing the restart of the maximum power point tracking control and avoiding voltage and current oscillations during mode switching.

[0016] As can be seen from the above technical solutions, this invention application has the following beneficial effects: (1) This invention breaks through the dilemma of existing technology that “strong voltage support easily leads to current over-limit and strong current limiting measures will weaken the support capacity” by coordinating the virtual impedance of the reactive power loop with the photovoltaic active power reserve; without the need for additional energy storage equipment, under the extreme combined fault of simultaneous drop in grid voltage and light intensity, the grid connection point voltage that drops to 0.35pu can be stably supported to about 0.85pu, while the grid connection current is strictly limited to within 1.15 times the rated value, which is far below the safety threshold of power electronic devices, and at the same time meets the core operating requirements of voltage support and overcurrent protection.

[0017] (2) This invention avoids voltage over-adjustment and system instability by using the first limiting boundary of virtual impedance. It ensures the stability of DC bus voltage and system power angle under the fluctuation of light by using the second limiting boundary of active power reserve and the three constraints of environmental adaptability, power angle stability and overcurrent suppression. At the same time, it adopts a step-by-step smooth exit mechanism to avoid voltage and current oscillation caused by mode switching. It solves the problems of photovoltaic grid disconnection during faults, inverter grid disconnection during fault recovery and mode switching oscillation that are easy to occur in the prior art. It realizes stable operation throughout the entire process of fault occurrence, continuity and recovery.

[0018] (3) The present invention achieves the core functions by software optimization of the VSG reactive power control loop and the photovoltaic MPPT control logic. It does not require modification of the inverter main circuit hardware, addition of energy storage equipment, or increase of hardware cost. It is directly compatible with the existing mainstream grid-type photovoltaic VSG control architecture. The core parameters are designed with clear quantitative boundary formulas. It can adapt to different voltage drop and light fluctuation conditions. It does not require complex mode switching. The engineering implementation is simple and can be adapted to various power level grid-type photovoltaic inverter application scenarios. Attached Figure Description

[0019] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the embodiments will be briefly described below. Referring to the drawings will make the features and advantages of the present invention clearer. The drawings are illustrative and should not be construed as limiting the present invention in any way. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort. Wherein: Figure 1 This is a flowchart of a photovoltaic VSG voltage support method based on virtual impedance and active power reserve coordination provided by the present invention; Figure 2 This is a schematic diagram of the main circuit topology of the grid-type photovoltaic inverter system in this invention; Figure 3 This is a block diagram of the voltage-type VSG control circuit in this invention; Figure 4 This is a schematic diagram of the improved reactive power loop structure in this invention; Figure 5 This is a block diagram of the improved photovoltaic MPPT (maximum power point tracking) control principle with active power reserve regulation function in this invention; Figure 6 This is a schematic diagram of the grid connection point voltage in this invention; Figure 7 This is a schematic diagram of the grid connection point current in this invention; Figure 8 This is a schematic diagram of the DC bus voltage in this invention; Figure 9 This is a schematic diagram of the active power output of the inverter in this invention; Figure 10 This is a schematic diagram of the reactive power output of the inverter in this invention. Detailed Implementation

[0020] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0021] This invention discloses a photovoltaic VSG voltage support method based on virtual impedance and active power reserve coordination, applied to grid-connected photovoltaic inverter systems. The core inventive concept is as follows: In a composite fault scenario where grid voltage and solar intensity drop simultaneously, the voltage support capability is enhanced by introducing an adjustable virtual impedance into the VSG reactive power loop, while simultaneously suppressing fault overcurrent with the photovoltaic active power reserve strategy. Furthermore, the synergistic optimization of the two is achieved through dual-limit boundary constraints. Ultimately, without relying on additional energy storage devices, the method simultaneously meets the triple requirements of voltage support, overcurrent suppression, and stable system operation, enabling reliable fault ride-through for grid-connected photovoltaic inverters.

[0022] The main circuit topology of the grid-type photovoltaic inverter system used in this invention is as follows: Figure 2 As shown, the system, from left to right, includes a photovoltaic array, a boost converter circuit, a three-phase inverter controlled by a virtual synchronous generator (VSG), an LC filter circuit, and the power grid. The connection relationships and electrical parameter definitions of each component are as follows: The output of the photovoltaic array is connected to the input of a Boost converter circuit to convert solar energy into DC power. , These represent the output voltage and output current of the photovoltaic array, respectively; the output of the Boost circuit is connected to the DC side of the three-phase inverter, used to boost the DC voltage output by the photovoltaic array to the DC bus voltage required for inverter operation. This is the DC bus voltage; the AC side of the three-phase inverter is connected to the power grid via an LC filter circuit. The LC filter circuit is used to filter out high-frequency harmonics at the inverter output. , , These are the filter inductance, filter capacitor, and equivalent series resistance of the filter capacitor, respectively; the equivalent circuit on the mains side includes equivalent resistance and equivalent inductance, where... , These are the equivalent resistance and equivalent inductance on the grid side, respectively. This indicates the grid voltage. The common connection point between the inverter and the grid is the grid connection point (PCC).

[0023] This invention enables grid-connected operation of inverters based on voltage-source VSG control technology. The core control loop principle block diagram is shown below. Figure 3 As shown, the control loop mainly includes a power calculation module, an active-frequency (Pf) control loop, a reactive-voltage (QU) control loop, a virtual impedance module, a voltage-current dual closed-loop control module, and a pulse width modulation (PWM) module. The working principle and control logic of each module are as follows: Power calculation module: Real-time sampling of three-phase voltage at the grid connection point and three-phase current The instantaneous active power output of the inverter is obtained through instantaneous power calculation. and instantaneous reactive power .

[0024] Active-frequency (Pf) control loop: Used to simulate the rotor motion equations of a synchronous generator, providing virtual inertia and damping support for the system. The active power reference value... With actual output active power The difference is input to the control loop corresponding to the rotor motion equation, and its transfer function is shown in equation (1). The final output system phase angle is... : (1) in, For virtual rotational inertia, The damping coefficient is... For the Laplace operator, This is the system's rated angular frequency.

[0025] Reactive-Voltage (QU) Control Loop: Used to realize the reactive power regulation and voltage support functions of the inverter. The traditional VSG reactive power loop adopts proportional droop control, and its control expression is shown in equation (2): (2) in, This is the rated internal potential amplitude. This is the reactive power droop factor. This is a reference value for reactive power. This represents the amplitude of the internal potential output by the VSG.

[0026] In grid voltage During the drop, the PCC voltage Consequently, the inverter output reactive power decreases. Increase, by lowering the internal potential amplitude. It provides basic voltage support, but this support capability is limited by control parameters and is severely insufficient in scenarios with deep voltage drops in the power grid.

[0027] Virtual impedance module, voltage and current dual closed loop and PWM module: the phase angle of the above active loop output The amplitude of the internal potential output of the reactive power loop The virtual internal potential that together constitutes VSG Virtual internal potential Subtract virtual impedance The voltage drop across the circuit is used to obtain the reference voltage for the voltage loop. The voltage and current dual closed-loop control uses this voltage as a reference, with the inner loop quickly tracking the current command and the outer loop stabilizing the output voltage. Finally, a PWM signal is generated to drive the inverter's switching transistors, thus realizing the closed-loop control of the inverter.

[0028] Furthermore, to address the insufficient support capacity of traditional VSG reactive power loops under deep voltage dips in the power grid, this invention makes a core improvement to the reactive power-voltage control loop by introducing an adjustable virtual impedance. The core principle is to simulate an additional voltage dip using this virtual impedance, allowing the VSG controller to detect a more severe voltage dip fault than the actual operating condition. This, in turn, stimulates a stronger reactive power output capability, achieving proactive enhancement of the grid connection point voltage. The improved reactive power loop structure is as follows: Figure 4 As shown.

[0029] After introducing virtual impedance, the voltage comparison deviation term of the reactive power loop increases with the voltage deviation increment. The formula for calculating this increment is shown in equation (3): (3) in, This is the actual value of the grid connection point voltage. , The actual active and reactive power at the grid connection point. The system angular frequency, The virtual resistance value is the virtual impedance. The virtual inductance value is the virtual impedance. , These are the power components corresponding to the virtual resistance and virtual inductance.

[0030] The improved reactive power loop converts the sensed PCC voltage into the actual value minus the voltage deviation increment. This means that the PCC voltage is considered to be lower than the actual value. Based on the reactive power droop characteristics of the VSG, this equivalent voltage drop will generate an additional reactive power increment, significantly improving the reactive power output capability and voltage support effect of the VSG. In actual operation, this can be addressed by adjusting the virtual impedance. The value of is mainly used to adjust the virtual inductance. The size of the support capacity can be flexibly adjusted according to the voltage support requirements of the actual working conditions.

[0031] Furthermore, to prevent the virtual impedance value from being too large and causing the PCC voltage to be over-adjusted (exceeding 1.1 pu), and at the same time, the simulated scenario set must be close to reality and should not deviate from the actual background, an upper limit constraint must be set for the value of the virtual impedance, that is, the first limiting boundary described in this invention.

[0032] This boundary is designed based on the most severe voltage drop condition, which is: when the grid voltage drops, the VSG has no voltage support capability at all; at this time, the virtual voltage drop introduced by the virtual impedance... It should not exceed ,in This represents the actual voltage at the grid connection point when no virtual impedance is deployed. This is the actual value of the grid voltage. Combining equation (3) and the line characteristics dominated by the inductive impedance of the grid, the first limiting boundary for the virtual impedance value is derived, and its expression is shown in equation (4): (4) in, The virtual resistance value is the virtual impedance. This is the ratio of virtual inductance to virtual resistance. The degree of voltage drop in the power grid. , The actual active and reactive power at the grid connection point. The grid connection current amplitude at the grid connection point. This is the actual value of the grid connection point voltage. This refers to the rated voltage at the grid connection point. In actual operation, the value of the virtual impedance must not exceed the first limit boundary and must be selected and dynamically adjusted within this upper limit according to the actual voltage support requirements.

[0033] Furthermore, the improvement in reactive power support capability is accompanied by an increase in the reactive power output of the inverter, which in turn leads to an increase in the grid-connected current amplitude, posing a risk of overcurrent damage to power electronic devices. To solve this problem, this invention, while enhancing reactive voltage support, simultaneously introduces an active power reserve control strategy on the photovoltaic side. By reducing the active power output of the photovoltaic system, capacity space is made available for the increased reactive current, effectively suppressing fault overcurrent. The improved MPPT control principle with active power reserve regulation function is as follows: Figure 5 As shown.

[0034] During normal operation, the photovoltaic array operates at its maximum power point (MPP) through maximum power point tracking (MPPT) control, maximizing the efficiency of photovoltaic power utilization. When a grid voltage fault is detected and fault ride-through coordinated control is initiated, photovoltaic active power reserve control is simultaneously activated. Specifically, this is achieved by reducing the duty cycle of the switching transistors in the Boost converter circuit. This causes the operating point of the photovoltaic array to shift from the maximum power point (MPP) along its power purifier (PU) characteristic curve towards the direction of increasing voltage, thereby reducing the output active power of the photovoltaic array. This forms a reserve of active power.

[0035] By leveraging the control characteristics of the VSG active power loop, reducing the active power input to the DC side of the inverter is equivalent to reducing the active power reference value of the VSG. According to the total current constraint, active current is suppressed by reducing active power, thereby achieving current amplitude suppression. The correspondence between grid-connected current amplitude and active and reactive power can be approximately expressed as equation (5): (5) Based on this characteristic, active power output can be appropriately reduced through active power reserve. It can provide necessary reactive power support during fault maintenance. At the same time, the total grid-connected current To ensure safety, the grid-connected current safety threshold in this invention is 1.5 times the rated current.

[0036] Furthermore, to ensure the effectiveness of the active power reserve strategy and the stability of system operation, it is necessary to set a constraint boundary for the active power reserve capacity, namely the second constraint boundary described in this invention. This boundary is determined based on a preset grid-connected current threshold and the power constraint of the photovoltaic array when the light intensity drops. Its core expression is shown in equation (6): (6) in, This is the active power reference value. This is the actual value of the grid connection point voltage. The preset grid-connected current threshold, , The actual active and reactive power at the grid connection point. This represents the maximum power point power of the photovoltaic array after the light intensity drops.

[0037] Furthermore, the determination of active power reserve capacity must simultaneously meet the following three constraints to ensure stable system operation under complex fault scenarios: (1) Environmental adaptability constraints: The active power reserve capacity must be greater than the natural reduction in photovoltaic power at the maximum power point caused by the drop in irradiance during the fault period, i.e.: ; in, For active power reserve capacity, The light intensity before the fault occurred. The light intensity dropped during the malfunction. , These represent the maximum power point power of the photovoltaic array under the corresponding light intensity; this constraint ensures that the photovoltaic array can still accurately track the power before the light intensity recovers, thus avoiding grid disconnection on the photovoltaic side.

[0038] (2) Power angle stability constraint: Based on the power angle characteristic curve of VSG, in order to ensure that the system has a stable equilibrium point during the fault, the active power reference value of the inverter is adjusted after the fault. It must be less than the maximum transmission power of the VSG power angle characteristic curve under fault conditions. .

[0039] (3) Overcurrent suppression constraint: to meet the grid-connected current amplitude Based on the safety requirements, combined with equation (5) and the reactive power required for voltage support, the maximum allowable active power output can be calculated, i.e., the active power reference value must meet the following: ; in, This is the active power reference value. This is the actual value of the grid connection point voltage. The preset grid-connected current threshold, This represents the actual reactive power at the grid connection point.

[0040] In actual operation, the active power reserve capacity must simultaneously meet the above three constraints. The maximum value among the lower limits determined by the three constraints is taken as the minimum value of the active power reserve capacity to ensure the effectiveness of the second limit boundary.

[0041] Furthermore, combined Figure 1 The control flowchart shown illustrates the photovoltaic VSG voltage support method based on virtual impedance and active power reserve coordination. The complete implementation steps are as follows: S1: Real-time acquisition of three-phase voltage and three-phase current at the grid connection point, as well as output power and incident light intensity parameters of the photovoltaic array.

[0042] Specifically, the system collects the three-phase voltage at the grid connection point in real time. Three-phase current Simultaneously, it collects operating parameters such as the output power of the photovoltaic array, incident light intensity, and DC bus voltage, providing a data foundation for fault diagnosis and coordinated control.

[0043] S2: Based on the collected grid connection point voltage, fault determination is performed. When the grid connection point voltage is lower than the preset grid connection point voltage threshold, a grid voltage fault is determined to have occurred, and fault ride-through collaborative control is initiated.

[0044] Specifically, fault determination is based on real-time collected grid connection point voltage. The preset grid connection point voltage fault threshold is 0.9 pu of the rated voltage. When the voltage drops below 0.9 pu, a grid voltage fault is detected, and fault ride-through coordinated control is immediately initiated. If the grid connection point voltage is within the normal range of 0.9 pu to 1.1 pu, the system maintains normal VSG operation and photovoltaic MPPT control.

[0045] S3: Introduce a virtual impedance in the reactive power loop of the virtual synchronous generator. The virtual impedance simulates an additional voltage drop, increases the voltage deviation of the reactive power loop, and improves the reactive power output capability of the inverter to support the grid connection point voltage.

[0046] Specifically, after the fault ride-through coordinated control is initiated, a virtual impedance is immediately added to the reactive power loop of the VSG. The virtual impedance simulates an additional voltage drop, increases the voltage deviation of the reactive power loop, enhances the reactive power output capability of the inverter, and achieves active support of the grid connection point voltage. The initial value of the virtual impedance is preset within the first limit boundary based on the real-time detected grid voltage drop depth.

[0047] S4: Synchronously start photovoltaic active power reserve control. By adjusting the operating parameters of the Boost circuit, the photovoltaic array is made to operate away from the maximum power point, reducing the photovoltaic output active power and reserving active power capacity to suppress the grid connection current at the grid connection point.

[0048] Specifically, while the virtual impedance is applied, the photovoltaic active power reserve control is started simultaneously. Based on the real-time monitored light intensity and the three constraints of the second limiting boundary, the required minimum active power reserve capacity is calculated. By adjusting the duty cycle of the switching transistor in the Boost circuit, the operating point of the photovoltaic array is moved away from the maximum power point, thereby realizing the active power reduction, forming active power reserve, and suppressing fault overcurrent.

[0049] S5: Based on the principle that the deeper the voltage drop at the grid connection point, the more reactive power compensation the VSG reactive-voltage loop provides, the dual limiting boundaries of the coordinated control are determined: the first limiting boundary for the virtual impedance value is determined based on the condition that the voltage drop at the grid connection point and the grid voltage are of equal magnitude; the second limiting boundary for the active power reserve capacity is determined based on the preset grid connection current threshold and the power constraint of the photovoltaic array when the illumination drops.

[0050] Specifically, within the constraints of the dual-limit boundary, the value of the virtual impedance and the active power reserve capacity are dynamically coordinated and adjusted based on the real-time collected grid connection point voltage and grid connection current. The specific adjustment logic is as follows: When the real-time collected grid-connected current amplitude approaches the preset 1.5 times rated current threshold, the active power reserve capacity is increased within the adjustable range of photovoltaic power, further reducing active power output and suppressing current overruns. When the voltage support effect at the grid connection point does not meet the target, and the grid connection current amplitude has sufficient margin from the threshold, the value of the virtual impedance should be appropriately increased within the first limiting boundary to further improve the reactive power support capability and optimize the voltage support effect.

[0051] S6: Within the constraints of the dual-limit boundary, the virtual impedance value and active power reserve capacity are dynamically adjusted based on the real-time collected grid connection point voltage and grid connection current, so as to achieve stable support of the grid connection point voltage without exceeding the grid connection current limit.

[0052] Specifically, the system continuously monitors the operating status of the power grid and the grid connection point. When it detects that the grid connection point voltage has recovered to the normal range of 0.9pu~1.1pu and has maintained stable operation for a preset time, it determines that the power grid fault has been cleared and enters the fault ride-through control exit process.

[0053] S7: When the grid connection point voltage is detected to have returned to the normal range and remained stable, smoothly exit the fault ride-through collaborative control and restore the normal operation of the virtual synchronous generator and photovoltaic maximum power point tracking control.

[0054] Specifically, to avoid voltage and current oscillations during mode switching and ensure a smooth system transition, a step-by-step smooth exit method is adopted to exit the fault ride-through collaborative control: first, the virtual impedance is gradually reduced to zero according to a preset slope, and the improved control of the reactive power loop is turned off; then, the duty cycle of the Boost circuit is gradually adjusted to smoothly restore the operating point of the photovoltaic array to the maximum power point, complete the restart of MPPT control, and finally restore the system to the normal VSG operating state.

[0055] To verify the effectiveness of the method proposed in this invention, a model of the above-mentioned grid-type photovoltaic inverter system was built in the MATLAB / Simulink simulation platform, and an extreme combined fault scenario was set for simulation verification. The simulation parameters and results are as follows: 1. Simulation Scene Setup Simulating a complex fault caused by extreme thunderstorm weather, at t=2s, a three-phase symmetrical short-circuit fault occurred in the power grid, causing the grid voltage to drop by 65% ​​to 0.35pu; at the same time, the incident photovoltaic irradiance dropped from 1000W / m² to 700W / m², a drop of 30%; the preset grid-connected current safety threshold was 1.5 times the rated current, and the fault was cleared after t=4s, with the grid voltage and irradiance gradually recovering.

[0056] 2. Simulation Result Analysis Using the cooperative control method proposed in this invention, after implementing a virtual impedance that meets the first limiting boundary and enabling active power reserve control that meets the second limiting boundary, the simulation results are as follows: (1) Voltage characteristics at grid connection point: such as Figure 6 As shown, during the fault, the grid connection point voltage was stably supported at approximately 0.84~0.85 pu, which is significantly higher than the grid fault voltage of 0.35 pu, achieving excellent voltage support effect; (2) Grid-connected current characteristics: such as Figure 7 As shown, during the fault, the grid-connected current is effectively limited to about 1.12 to 1.15 times the rated value, which is far less than the safety threshold of 1.5 times the rated current, with no risk of overcurrent, and reliable overcurrent suppression is achieved; (3) DC bus voltage characteristics: such as Figure 8 As shown, the DC bus voltage remained stable during the fault without significant fluctuations, and the photovoltaic array and inverter did not experience grid disconnection. (4) Fault recovery characteristics: such as Figure 9 , Figure 10 As shown, after the fault is cleared, the system's active and reactive power outputs can be smoothly and quickly restored to the rated operating state, without voltage or current oscillations, and the mode switching is stable.

[0057] In contrast, traditional control strategies that do not employ the method of this invention cannot complete the voltage ride-through task under this complex fault, and the inverter disconnects from the grid at the moment the fault is cleared. Control strategies that do not meet the dual-limit boundary conditions also result in adverse outcomes such as grid disconnection on the photovoltaic side and grid disconnection at the grid connection point, fully verifying the effectiveness and superiority of the method of this invention.

[0058] This embodiment provides a computer-readable storage medium storing a computer software product. The computer software product includes several instructions to cause a computer device to execute all or part of the steps of the photovoltaic VSG voltage support method based on virtual impedance and active power reserve coordination described in any of the above embodiments of the present invention.

[0059] Those skilled in the art will understand that embodiments of this application can be provided as methods, systems, or computer program products. Therefore, this application can take the form of a completely hardware embodiment, a completely software embodiment, or an embodiment combining software and hardware aspects. Furthermore, this application 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.

[0060] This application 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... Figure 1 One or more processes and / or boxes Figure 1 A device that provides the functions specified in one or more boxes.

[0061] 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 functions specified in one or more boxes. These computer program instructions may also be loaded onto a computer or other programmable data processing apparatus to cause a series of operational steps to be performed on the computer or other programmable apparatus to produce a computer-implemented process, thereby providing instructions that execute on the computer or other programmable apparatus 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.

[0062] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A photovoltaic VSG voltage support method based on virtual impedance and active power reserve coordination, applied to a grid-type photovoltaic inverter system, the system comprising a photovoltaic array, a boost converter circuit, a three-phase inverter controlled by a virtual synchronous generator, an LC filter circuit, and a power grid connected in sequence, characterized in that, Includes the following steps: S1: Real-time acquisition of three-phase voltage and three-phase current at the grid connection point, as well as output power and incident light intensity parameters of the photovoltaic array; S2: Based on the collected grid connection point voltage, fault determination is performed. When the grid connection point voltage is lower than the preset grid connection point voltage threshold, a grid voltage fault is determined to have occurred, and fault ride-through collaborative control is initiated. S3: Introduce a virtual impedance in the reactive power loop of the virtual synchronous generator. The virtual impedance simulates an additional voltage drop, increases the voltage deviation of the reactive power loop, and improves the reactive power output capability of the inverter to support the grid connection point voltage. S4: Synchronously start photovoltaic active power reserve control. By adjusting the operating parameters of the Boost circuit, the photovoltaic array is made to deviate from the maximum power point, reducing the photovoltaic output active power and reserving active power capacity to suppress the grid connection current at the grid connection point. S5: Determine the dual limiting boundaries of coordinated control: Based on the condition that the grid connection point voltage and the grid voltage drop at the same magnitude, determine the first limiting boundary for the virtual impedance value; based on the preset grid connection current threshold and the power constraint of the photovoltaic array when the light intensity drops, determine the second limiting boundary for the active power reserve capacity. S6: Within the constraints of the dual-limit boundary, the virtual impedance value and active power reserve capacity are dynamically adjusted based on the real-time collected grid connection point voltage and grid connection current, so as to achieve stable support of the grid connection point voltage without exceeding the grid connection current limit. S7: When the grid connection point voltage is detected to have returned to the normal range and remained stable, smoothly exit the fault ride-through collaborative control and restore the normal operation of the virtual synchronous generator and photovoltaic maximum power point tracking control.

2. The photovoltaic VSG voltage support method based on virtual impedance and active power reserve coordination according to claim 1, characterized in that, In step S3, after introducing the virtual impedance, the voltage deviation increment of the reactive power loop... The calculation formula is: ； in, This is the actual value of the grid connection point voltage. , The actual active and reactive power at the grid connection point. The system angular frequency, The virtual resistance value is the virtual impedance. The virtual inductance value is the virtual impedance. , These are the power components corresponding to the virtual resistance and virtual inductance.

3. The photovoltaic VSG voltage support method based on virtual impedance and active power reserve coordination according to claim 1, characterized in that, In step S4, the specific implementation of photovoltaic active power reserve control is as follows: by reducing the duty cycle of the Boost circuit, the operating point of the photovoltaic array is moved along its PU characteristic curve in the direction of increasing voltage, deviating from the maximum power point, thereby reducing the output active power of the photovoltaic array and forming active power reserve.

4. The photovoltaic VSG voltage support method based on virtual impedance and active power reserve coordination according to claim 1, characterized in that, In step S5, the first limiting boundary is used to constrain the value of the virtual impedance, and its expression is: ； in, The virtual resistance value is the virtual impedance. This is the ratio of virtual inductance to virtual resistance. The degree of voltage drop in the power grid. , The actual active and reactive power at the grid connection point. The grid connection current amplitude at the grid connection point. This is the actual value of the grid connection point voltage. The voltage rating at the grid connection point is [value]; the value of the virtual impedance must not exceed the first limit boundary.

5. The photovoltaic VSG voltage support method based on virtual impedance and active power reserve coordination according to claim 1, characterized in that, In step S5, the second limiting boundary is used to constrain the active power reserve capacity, and its expression is: ； in, This is the active power reference value. This is the actual value of the grid connection point voltage. The preset grid-connected current threshold, , The actual active and reactive power at the grid connection point. This represents the maximum power point power of the photovoltaic array after the light intensity drops.

6. The photovoltaic VSG voltage support method based on virtual impedance and active power reserve coordination according to claim 1, characterized in that, In step S5, the determination of the active power reserve capacity must simultaneously satisfy the following three constraints: Environmental adaptability constraints: The active power reserve capacity must be greater than the natural reduction in photovoltaic power at the maximum power point caused by the drop in light intensity during the fault period; Power angle stability constraint: The active power reference value of the inverter after active power reserve adjustment must be less than the maximum transmission power of the virtual synchronous generator power angle characteristic curve under fault conditions. Overcurrent suppression constraint: The active power reference value of the inverter after active power reserve adjustment satisfies the following formula: ； in, This is the active power reference value. This is the actual value of the grid connection point voltage. The preset grid-connected current threshold, This represents the actual reactive power at the grid connection point.

7. The photovoltaic VSG voltage support method based on virtual impedance and active power reserve coordination according to claim 1, characterized in that, In step S6, the method for dynamically adjusting the virtual impedance and active power reserve capacity based on the real-time collected grid connection point voltage and grid connection current is as follows: When the real-time collected grid-connected current amplitude is close to the preset threshold, the active power reserve capacity is increased within the adjustable range of photovoltaic power to suppress current overruns; when the grid connection point voltage support effect does not meet the target and there is a margin in the grid-connected current, the value of the virtual impedance is increased within the first limit boundary to improve the reactive power support capability.

8. The photovoltaic VSG voltage support method based on virtual impedance and active power reserve coordination according to claim 1, characterized in that, The preset grid connection point voltage threshold is 0.9 pu of the rated voltage, the preset grid connection current threshold is 1.5 times the rated current, and the normal range of the grid connection point voltage is 0.9 pu to 1.1 pu.

9. The photovoltaic VSG voltage support method based on virtual impedance and active power reserve coordination according to claim 1, characterized in that, In step S7, the method for smoothly exiting the collaborative control is as follows: First, gradually reduce the value of the virtual impedance to zero according to the preset slope, and then gradually adjust the duty cycle of the Boost circuit to smoothly restore the operating point of the photovoltaic array to the maximum power point, thus completing the restart of the maximum power point tracking control and avoiding voltage and current oscillations during mode switching.

10. A computer storage medium, characterized in that, The computer storage medium stores a computer software product, which includes several instructions to cause a computer device to execute the photovoltaic VSG voltage support method based on virtual impedance and active power reserve coordination as described in any one of claims 1 to 9.

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