Photovoltaic grid-connected system fault ride-through method based on VSG current limiting control

Abstract

The invention relates to the technical field of electric power, and discloses a photovoltaic grid-connected system fault ride-through method based on VSG current limiting control. When it is detected that the PCC voltage is lower than a preset voltage threshold value, the photovoltaic inverter closes power outer loop control, an active current reference value and a reactive current reference value are reconstructed according to the voltage drop depth, it is guaranteed that grid-connected operation of the photovoltaic side is kept in the fault period, and voltage support is provided; a second-order rotor motion equation and an excitation control equation are established in an energy storage converter controller to simulate a synchronous generator, when PCC voltage drop or a power grid fault is detected, dynamically-adjusted virtual impedance is injected into the output end of a VSG according to the voltage drop depth so as to increase equivalent impedance and actively limit the peak value of short-circuit current, and the voltage drop or the power grid fault is detected. And switching the active power reference value into an active power reference value in a fault state and reducing or inhibiting active output, and switching the reactive power reference value into a reactive power reference value in the fault state so as to enhance the reactive power support capability of the energy storage system and maintain the voltage stability of the grid-connected point.

Description

Technical Field

[0001] This application relates to the field of power technology, and in particular to a fault ride-through method for photovoltaic grid-connected systems based on VSG current limiting control. Background Technology

[0002] With the transformation of the global energy structure, large-scale renewable energy sources are gradually being integrated into the power system, profoundly changing the operating characteristics of the power grid. Wind power, photovoltaic, and other new energy power generation units generally use power electronic converters to connect to the grid. Their inertial support capabilities are insufficient, and their regulation characteristics differ significantly from traditional synchronous generators. This presents new challenges for the power system in responding to fault disturbances, affecting the stability and security of the power grid. Traditional synchronous generators possess inherent inertia and short-circuit support capabilities during grid faults, but new energy power generation units based on power electronic interfaces lack these characteristics and are prone to shutting down during faults, thus affecting system stability. To address these issues, Virtual Synchronous Generator (VSG) control technology has emerged. By introducing the dynamic equations of a synchronous generator into the control layer of the power electronic converter, virtual inertia and damping characteristics are achieved, thereby improving the grid adaptability and dynamic stability of new energy power generation units. However, when a grid fault occurs, the current may rise rapidly, exceeding the safety limits of the converter and electrical equipment, leading to protection actions or equipment damage. With the increasing penetration of new energy sources, the operating environment of the power grid is becoming more and more complex. In particular, under fault conditions such as voltage drop and short circuit, the application of VSG control still has many limitations.

[0003] First, due to the limited overcurrent capacity of power electronic converters, a sharp rise in current may occur when a fault occurs. If this is not limited in time, it will not only cause excessive electrical stress on the devices, triggering overcurrent protection, but may even damage critical components, thereby threatening the continuous grid-connected operation of new energy power sources.

[0004] Secondly, existing fault ride-through control strategies are still not perfect in VSG applications. Most strategies adopt fixed reactive power priority control, that is, forcibly outputting a large amount of reactive power to support the grid voltage when the voltage drops. However, this approach often ignores the dynamic distribution of active power, which may cause the grid-connected unit to exceed its capacity limit, thereby triggering protection actions or even shutting down the system. This rigid control method is not only detrimental to the safe operation of the power source itself, but may also reduce the overall voltage recovery capability of the grid.

[0005] Furthermore, the virtual inertia and damping parameters in VSG control are typically fixed during the design phase, lacking dynamic adaptive capability. Fixed parameters can lead to slow system response or increased oscillations at different fault depths, durations, and recovery stages, reducing the transient stability and fault ride-through capability of the power grid. For example, during a voltage sag, insufficient damping can cause voltage and current oscillations; while during the recovery phase, excessively high inertia settings can delay power recovery, affecting power quality.

[0006] Furthermore, most existing current-limiting control strategies rely on simple current detection and limiting methods. While this approach can prevent overcurrent to some extent, it often comes at the cost of sacrificing grid support capacity. When the current is forcibly reduced, reactive power support may be insufficient, and renewable energy sources cannot effectively help the grid maintain voltage stability, thus contradicting the fault ride-through requirements of the power system. As the installed capacity of renewable energy continues to increase, this contradiction is becoming increasingly prominent, urgently requiring the search for more scientific and rational control strategies.

[0007] Therefore, existing VSG current limiting control and fault ride-through technologies have certain shortcomings in terms of rapid response capability, power distribution flexibility, parameter adaptive adjustment, and grid support capability. These deficiencies restrict the continuous grid connection capability and grid support level of new energy power sources under the background of high-proportion grid access. To address the above issues, there is an urgent need to propose a new VSG current limiting control fault ride-through strategy that balances equipment safety and grid stability. Summary of the Invention

[0008] The purpose of this application is to provide a fault ride-through method for photovoltaic grid-connected systems based on VSG current limiting control. By integrating photovoltaic voltage support control scheme, virtual impedance technology and dynamic power regulation method, the method coordinates voltage support and current limiting, so that photovoltaic power generation can ensure its own safety and effectively support grid operation under fault conditions, thereby providing a solid guarantee for the stability and sustainable development of new power systems.

[0009] In a first aspect of this application, a fault ride-through method for a photovoltaic grid-connected system based on VSG current limiting control is provided, comprising:

[0010] (a) The photovoltaic side fault handling method includes: when the grid connection point PCC voltage is detected to be lower than the preset voltage threshold, the photovoltaic inverter shuts down the power outer loop control and reconstructs the active current reference value and reactive current reference value according to the voltage drop depth, wherein:

[0011] The active current reference value of the photovoltaic system is reduced or reduced to zero, so that the active power output of the photovoltaic inverter is reduced.

[0012] The reactive current reference value of the photovoltaic system is increased based on the voltage drop depth, so that the photovoltaic inverter enters the reactive power priority control mode to inject reactive power into the grid.

[0013] The output current of the photovoltaic system is limited to ensure that it does not exceed the rated current, thereby ensuring that the photovoltaic side remains connected to the grid and provides voltage support during faults.

[0014] (b) Second-order rotor motion equations and excitation control equations are established in the energy storage converter controller to simulate the synchronous generator. Fault handling methods on the energy storage side include:

[0015] When a voltage drop in the PCC or a grid fault is detected, a dynamically adjusted virtual impedance is injected into the output of the VSG according to the depth of the voltage drop, so as to increase the equivalent impedance and actively limit the peak value of the short-circuit current.

[0016] Switch the active power reference value of the energy storage system to the active power reference value under fault conditions, and reduce or suppress active power output.

[0017] The reactive power reference value of the energy storage system is switched to the reactive power reference value under fault conditions to enhance the reactive power support capability of the energy storage system, thereby maintaining the voltage stability at the grid connection point.

[0018] In a preferred embodiment, the active current reference value and the reactive current reference value are reconstructed as follows:

[0019]

[0020] Among them, i dref0 and i qref0 These represent the reference values ​​for active and reactive currents before the fault, respectively, α represents the voltage sag depth, and i N This is the rated current of the photovoltaic system.

[0021] In a preferred embodiment, the control of the VSG includes: active power-frequency control and reactive power-voltage control, wherein the active power-frequency control and reactive power-frequency control are respectively represented as follows:

[0022]

[0023] Among them, P ref P and ω represent the reference active power and actual active power of the VSG, respectively. N ω and J are the rated virtual rotor angular frequency and the actual angular frequency, respectively; J is the virtual moment of inertia; D is the damping coefficient; δ is the power angle; Q ref Q and U represent the reference reactive power and actual reactive power of the VSG, respectively. N with U *K represents the rated voltage amplitude and reference voltage of the VSG, respectively. q This is the reactive power-voltage droop control coefficient.

[0024] In a preferred embodiment, the second-order rotor motion equation and excitation control equation are respectively expressed as:

[0025]

[0026] P m =P ref +K u (ω N -ω)

[0027] Among them, P m With P e K represents mechanical power and electromagnetic power, respectively. u It represents the active power-frequency droop factor.

[0028] In a preferred embodiment, the virtual impedance includes virtual resistance and virtual reactance, and are respectively expressed as:

[0029]

[0030] Among them, R v L represents the virtual resistance. v Indicates virtual reactance, i d and i q These are the d-axis and q-axis components of the PCC current, ΔU d and ΔU q These represent the longitudinal and transverse components of the voltage drop caused by the virtual impedance between the VSG and the grid connection point, respectively.

[0031] In a preferred embodiment, after introducing the virtual impedance, the short-circuit current of the VSG is:

[0032]

[0033] Where, θ f U represents the phase angle difference between the VSG and PCC points after the voltage drop. sf This refers to the voltage amplitude at the grid connection point after the voltage drop.

[0034] The voltage drop caused by the virtual impedance between the VSG and the grid connection point is:

[0035]

[0036] In a preferred embodiment, the power injected into the grid by the energy storage converter is expressed as:

[0037]

[0038] Among them, Q f and P f S represents the reactive power reference value and active power reference value under fault conditions, respectively. N This represents the apparent power under fault conditions.

[0039] In a preferred embodiment, the preset voltage threshold is set to 0.8 pu.

[0040] In a second aspect of this application, an electronic device is provided, comprising: a memory, a processor, and a computer program stored in the memory and executable on the processor, wherein the processor executes the computer program to implement the steps of the aforementioned method.

[0041] In a third aspect of this application, a non-transitory computer-readable storage medium is provided, the non-transitory computer-readable storage medium comprising a computer program, the computer-executable instructions of which, when executed by a processor, implement the steps of the aforementioned method.

[0042] It should be understood that, within the scope of this invention, the above-described technical features of this invention and the technical features specifically described below (such as in the embodiments) can be combined with each other to form new or preferred technical solutions. Due to space limitations, they will not be described in detail here. Attached Figure Description

[0043] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the accompanying drawings used in the description of the embodiments or the prior art will be briefly introduced below. It should be understood that the accompanying drawings described below are merely some implementation examples of the present invention, and those skilled in the art can obtain other implementation examples based on these drawings without creative effort.

[0044] Figure 1 This is a flowchart illustrating a fault ride-through method for a photovoltaic grid-connected system based on VSG current limiting control, according to one embodiment.

[0045] Figure 2 A VSG voltage-current two-loop control diagram considering virtual impedance is shown.

[0046] Figure 3 A VSG control diagram with power regulation is shown.

[0047] Figure 4 A schematic diagram of a photovoltaic-energy storage system is shown when a fault occurs.

[0048] Figure 5 The system dynamics before and after the failure are shown. Detailed Implementation

[0049] In the following description, many technical details are presented to help the reader better understand this application. However, those skilled in the art will understand that the technical solutions claimed in this application can be implemented even without these technical details and various variations and modifications based on the following embodiments.

[0050] Through extensive and in-depth research, the inventors have proposed a fault ride-through control strategy based on virtual synchronous generator (VSG) current limiting control. This method organically combines photovoltaic voltage support control, virtual impedance technology, and dynamic power regulation methods to effectively address the overcurrent problem of the VSG grid-connected converter during grid voltage sag faults. In this control strategy, the photovoltaic system provides reactive power support under fault conditions, while the VSG-based energy storage system actively supports the PCC voltage and limits the amplitude of the short-circuit current, avoiding the overcurrent risk under traditional control methods. Simulation results show that the proposed control method can significantly improve the transient performance of the photovoltaic energy storage system under grid fault conditions, ensuring stable operation of the system during symmetrical grid faults, while providing sufficient voltage support capability and effectively suppressing overcurrent phenomena. Therefore, the coordinated fault ride-through control strategy proposed in this invention not only improves the stability and reliability of the photovoltaic energy storage system but also enhances its grid fault tolerance capability, possessing strong engineering application value and promising prospects for widespread application.

[0051] Compared with existing technologies, the fault ride-through control strategy for grid-connected systems based on VSG current limiting control proposed in this invention has the following significant advantages and technical effects:

[0052] First, by introducing photovoltaic voltage support control, the present invention enables the photovoltaic system to quickly provide reactive power support when the grid experiences a voltage dip fault, thereby improving the voltage stability at the grid connection point. In contrast, traditional methods often have a delayed response and limited voltage support capability in this process.

[0053] Secondly, this invention employs virtual impedance technology, which can dynamically adjust impedance characteristics during faults to achieve flexible control of grid voltage and current, thereby effectively reducing the impact of short-circuit current. In contrast, most existing technologies rely solely on fixed control parameters, failing to balance voltage support and current limiting requirements under sudden voltage drops.

[0054] Furthermore, this invention incorporates a dynamic power regulation method, automatically switching the distribution strategy of active and reactive power when a fault occurs, enabling the energy storage system to maintain sufficient voltage support while limiting overcurrent. This not only avoids the risk of grid disconnection caused by converter overcurrent protection activation but also improves the system's continuous operation capability.

[0055] Through the above technical solution, this invention can significantly improve the transient performance of photovoltaic energy storage systems under symmetrical grid faults. Specifically, the grid connection point voltage can quickly recover to a stable level, the peak short-circuit current of the system is effectively suppressed, and the entire system maintains stable operation during the fault. Therefore, this invention significantly improves the stability, reliability, and grid fault tolerance of photovoltaic energy storage systems, and has broad prospects for engineering applications.

[0056] Explanation of some concepts:

[0057] PCC voltage (Point of Common Coupling Voltage) refers to the voltage at the common coupling point between the photovoltaic grid-connected inverter / energy storage converter and the public power grid. It is a key electrical quantity reflecting the grid's operating status and fault depth. This invention achieves control functions such as fault identification, reactive power support, current limiting, and power switching by real-time detection of PCC voltage changes.

[0058] To make the objectives, technical solutions, and advantages of this application clearer, the embodiments of this application will be described in further detail below with reference to the accompanying drawings.

[0059] The first aspect of this application relates to a fault ride-through method for a photovoltaic grid-connected system based on VSG current limiting control, the process of which is as follows: Figure 1 As shown, it includes photovoltaic-side fault handling methods and energy storage-side fault handling methods, specifically including VSG-based current limiting control strategies, photovoltaic voltage support control methods, virtual impedance dynamic adjustment technology, and power switching-based dynamic adjustment mechanisms.

[0060] The photovoltaic side fault handling method includes: when the grid connection point PCC voltage is detected to be lower than the preset voltage threshold (e.g., set to 0.8pu), the photovoltaic inverter shuts down the power outer loop control and reconstructs the active current reference value and reactive current reference value according to the voltage drop depth.

[0061] Specifically, the steps include: reducing or reducing the active current reference value of the photovoltaic system to zero, so that the photovoltaic inverter reduces active power output; increasing the reactive current reference value of the photovoltaic system according to the voltage drop depth, so that the photovoltaic inverter enters the reactive power priority control mode to inject reactive power into the grid; and limiting the output current of the photovoltaic system to ensure that it does not exceed the rated current, thereby ensuring that the photovoltaic side maintains grid-connected operation and provides voltage support during faults.

[0062] When the grid voltage drops, the photovoltaic voltage support control method automatically switches the photovoltaic side to reactive power priority mode, providing fast and sufficient reactive power support to effectively boost and stabilize the grid connection point voltage without affecting the normal power output of the system after the fault is recovered.

[0063] A second-order rotor motion equation and excitation control equation are established in the energy storage converter controller to simulate a synchronous generator. A VSG-based current limiting control strategy is adopted. By introducing a current limiting link into the VSG control framework, the peak short-circuit current can be actively limited when the grid experiences faults and voltage drops, thus preventing the converter from disconnecting from the grid due to overcurrent protection and improving the system's continuous operation capability under severe fault conditions.

[0064] When a voltage dip in the PCC or a grid fault is detected, a dynamically adjusted virtual impedance is injected into the output of the VSG based on the depth of the voltage dip to increase the equivalent impedance and actively limit the peak value of the short-circuit current. This virtual impedance dynamic adjustment technology utilizes the adaptive adjustment capability of the virtual impedance to dynamically change the equivalent impedance characteristics during a fault, thereby enhancing voltage support and reducing fault current surges, achieving coordinated control of current and voltage.

[0065] Furthermore, when a PCC voltage drop or grid fault is detected, the active power reference value of the energy storage system is switched to the active power reference value under fault conditions, and the active power output is reduced or suppressed. The reactive power reference value of the energy storage system is also switched to the reactive power reference value under fault conditions to enhance the reactive power support capability of the energy storage system, thereby maintaining the voltage stability at the grid connection point.

[0066] The dynamic adjustment mechanism based on power switching designs the switching logic between active and reactive power. During grid faults, the energy storage system actively reduces active power output and increases reactive power support, ensuring the dual objectives of PCC voltage support and current limitation.

[0067] This invention proposes a fault ride-through method for photovoltaic-storage grid-connected systems based on virtual synchronous generator current-limiting control. This method comprehensively utilizes photovoltaic voltage support, dynamic virtual impedance adjustment, and power switching mechanisms to achieve coordinated control of the photovoltaic and energy storage systems. During grid faults, this method can quickly suppress short-circuit currents, effectively support the grid connection point voltage, and ensure stable system operation.

[0068] To better understand the technical solution of this application, a specific example is provided below. The details listed in this example are mainly for ease of understanding and are not intended to limit the scope of protection of this application.

[0069] (1) Control Strategy

[0070] a.VSG

[0071] VSG control mainly consists of two parts: active power-frequency control and reactive power-voltage control. Its active power control and reactive power control components can be expressed as follows (1):

[0072]

[0073] Among them, P ref P and ω represent the reference active power and actual active power of the VSG, respectively; N ω and Q represent the rated virtual rotor angular frequency and the actual angular frequency, respectively; ref Q and U represent the reference reactive power and actual reactive power of the VSG, respectively; N with U * These represent the rated voltage amplitude and reference voltage of the VSG, respectively; J is the virtual moment of inertia; D is the damping coefficient; δ is the power angle; K q This is the reactive power-voltage droop control coefficient.

[0074] The primary frequency regulation capability, damping characteristics, and moment of inertia of a synchronous generator are typically modeled using rotor motion equations and governor equations. In this invention, a classic second-order synchronous machine model is adopted, and established based on rotor motion equations and governor equations, as shown in equations (2) and (3). Where P... m With P e K represents mechanical power and electromagnetic power, respectively. u It represents the active power-frequency droop factor.

[0075]

[0076] P m =P ref +K u (ω N -ω)(3)

[0077] When a voltage shift occurs in the power grid, the synchronous generator maintains a stable terminal voltage through excitation system regulation. The virtual excitation controller of the VSG is established by simulating the synchronous generator excitation system, and its control equation is expressed as the following formula (4):

[0078] U * =U N +K q (Q ref -Q)(4)

[0079] b. Photovoltaic voltage support strategy

[0080] According to the Chinese national standard "Technical Specification for Testing Grid-Connected Inverters of Photovoltaic Power Stations" (GB / T37409—2019), photovoltaic power stations need to maintain grid-connected operation for a certain period of time during grid faults to support grid stability. The national grid connection standard clearly stipulates that during grid faults, the outer loop power control loop will be disconnected, while the active current reference value i of the inner loop current loop will be maintained. dref and reactive current reference value i qref The setting is based on the depth of the voltage drop, as shown in formulas (5) and (6) below:

[0081]

[0082] Among them, i dref0 and i qref0 These represent the reference values ​​for active and reactive currents before the fault, respectively; α represents the voltage sag depth; i N This refers to the rated current of the photovoltaic system. Specifically, when α > 0.9, the active current reference value i dref and reactive current reference value i qref i dref0 and i qref0 When 0.2≤α≤0.9, the active current reference value i dref and reactive current reference value i qref They are respectively and 1.5(1-α)i N When α < 0.2, the active current reference value i dref and reactive current reference value i qref 0 and 1.1i respectively. N .

[0083] c. Current limiting control based on virtual impedance

[0084] During a power grid fault, a virtual impedance can be quickly introduced to increase the system's output impedance, thereby mitigating overcurrent. Figure 2 A block diagram of VSG voltage-current dual-loop control considering virtual impedance is given, where PI is a proportional-integral controller, PWM is a pulse width modulator, and U... pc c、i pcc Let represent the voltage and current at point PCC, respectively; the control equation corresponding to the VSG voltage-current dual-loop control is expressed as the following formula (7):

[0085]

[0086] Among them, R v L represents the virtual resistance. v Indicates virtual reactance, i d and i q These represent the d-axis and q-axis components of the PCC current, respectively. dref u qref These represent the d-axis and q-axis reference currents at point PCC, respectively. 0ref This indicates the reference value for the VSG output voltage;

[0087] After introducing the virtual impedance, the short-circuit current of the VSG is given by the following formula (8):

[0088]

[0089] Where, θ fθ represents the phase angle difference between the VSG and PCC points after the voltage drop. f =arctan(ωL) f / R f );U sf This represents the voltage amplitude at the grid connection point after a voltage drop. L f R f These are the inductance and resistance between VSG and PCC points after the grid voltage drops.

[0090] The voltage drop caused by the virtual impedance between VSG and PCC can be expressed by the following formula (9):

[0091]

[0092] The voltage drop in equation (9) is decomposed into a longitudinal component ΔU. d With the transverse component ΔU q Transforming it to the dq coordinate system, we can obtain the following formula (10):

[0093]

[0094] In the dq coordinate system, the required virtual impedance value can be expressed as the following formula (11):

[0095]

[0096] c. Current limiting control based on virtual impedance

[0097] According to the grid connection requirements specified in the State Grid standards, when the per-unit value of the grid voltage is U... fg At that time, the power injected into the grid by the converter can be expressed by the following formulas (12) and (13):

[0098]

[0099] Among them, Q f and P f S represents the reference values ​​of reactive power and active power after the fault, respectively. N U represents the apparent power after a fault. fg This is the per-unit value of the grid voltage. Specifically, when α > 0.9, the reference value of reactive power Q after the fault is... f When 0.2 ≤ α ≤ 0.9, the reactive power reference value Q after the fault is 0. f For 1.55S N (0.9-U fg When α < 0.2, the reference value of reactive power Q after the fault is... f 1.05S N .

[0100] Figure 3 The diagram shown is a control block diagram of a virtual synchronous generator (VSG) with power regulation. During normal grid operation, switch S... P and S Q When in the disconnected state, the active power reference is P. ref The feedback quantity in the reactive power droop control loop corresponds to the actual output power Q. When a grid fault occurs and causes a voltage drop, switch S... P and S Q When activated by closure, the active power reference switches to P. f The feedback quantity in the reactive power droop control loop is switched to Q. f .

[0101] (2) Control strategy verification

[0102] To verify the effectiveness of the proposed fault ride-through strategy under power grid fault conditions, electromagnetic transient simulations were conducted using the MATLAB / Simulink platform. At the fault point on the AC power grid side (e.g., Figure 4 As shown in the figure, a three-phase ground fault is applied. When the fault occurs, the proposed fault ride-through scheme is adopted.

[0103] At the initial moment, the system operates under rated conditions (P ref =1pu,Q ref =0). At t=3s, a three-phase symmetrical fault occurs in the power grid, causing a voltage dip, and the fault is cleared at t=5s. Under the proposed strategy, the dynamic response characteristics of the system before and after the fault are as follows: Figure 5 As shown.

[0104] like Figure 5 As shown, at the moment of the fault, the PCC voltage dropped to 0.78 pu and recovered to 1 pu after the fault was cleared. During the fault, the active power output of the energy storage system dropped to 0, while the reactive power output rose to 0.96 pu, indicating that the energy storage system provided voltage support at the PCC and effectively reduced the fault current. Figure 5 The output current values ​​of the energy storage system are displayed, with the steady-state short-circuit current reaching 1.2 pu and the maximum transient peak current reaching 1.4 pu. These results demonstrate that the system not only provides effective voltage support but also successfully suppresses overcurrent in the VSG during operation.

[0105] The second aspect of this application provides a fault ride-through system for a photovoltaic grid-connected system based on VSG current limiting control, including a photovoltaic-side fault handling module and an energy storage-side fault handling module. When the PCC voltage at the grid connection point is detected to be lower than a preset voltage threshold, the photovoltaic-side fault handling module instructs the photovoltaic inverter to shut down the power outer loop control and reconstructs the active current reference value and reactive current reference value according to the voltage drop depth. Specifically: the active current reference value of the photovoltaic system is reduced or reduced to zero, causing the photovoltaic inverter to reduce active power output; the reactive current reference value of the photovoltaic system is increased according to the voltage drop depth, causing the photovoltaic inverter to enter a reactive power priority control mode to inject reactive power into the grid; and the output current of the photovoltaic system is limited to ensure that it does not exceed the rated current, thereby ensuring that the photovoltaic side maintains grid-connected operation and provides voltage support during the fault period. A second-order rotor motion equation and excitation control equation are established in the energy storage converter controller to simulate a synchronous generator. When a PCC voltage drop or grid fault is detected, the energy storage side fault handling module injects a dynamically adjusted virtual impedance into the output terminal of the VSG according to the voltage drop depth to increase the equivalent impedance and actively limit the peak value of the short-circuit current. The active power reference value of the energy storage system is switched to the active power reference value under the fault state, and the active power output is reduced or suppressed. The reactive power reference value of the energy storage system is switched to the reactive power reference value under the fault state to enhance the reactive power support capability of the energy storage system, thereby maintaining the voltage stability at the grid connection point.

[0106] On the other hand, the present invention also provides an electronic device, which may include: a processor, a communication interface, a memory, and a communication bus, wherein the processor, the communication interface, and the memory communicate with each other through the communication bus. The processor can call logical instructions in the memory to execute a fault ride-through method for photovoltaic grid-connected systems based on VSG current limiting control.

[0107] Furthermore, the logical instructions in the aforementioned memory can be implemented as software functional units and sold or used as independent products, and can be stored in a computer-readable storage medium. Based on this understanding, the technical solution of the present invention, or the part that contributes to the prior art, or a part of the technical solution, can be embodied in the form of a software product. This computer software product is stored in a storage medium and includes several instructions to cause a computer device (which may be a personal computer, server, or network device, etc.) to execute all or part of the steps of the methods described in the various embodiments of the present invention. The aforementioned storage medium includes various media capable of storing program code, such as USB flash drives, portable hard drives, read-only memory, random access memory, magnetic disks, or optical disks.

[0108] On the other hand, the present invention also provides a computer program product, which includes a computer program stored on a non-transitory computer-readable storage medium. The computer program includes program instructions, and when the program instructions are executed by a computer, the computer is able to execute the photovoltaic grid-connected system fault ride-through method based on VSG current limiting control provided by the above methods.

[0109] In another aspect, the present invention also provides a non-transitory computer-readable storage medium having a computer program stored thereon, which, when executed by a processor, is implemented to perform the aforementioned photovoltaic grid-connected system fault ride-through methods based on VSG current limiting control.

[0110] Through the above description of the embodiments, those skilled in the art can clearly understand that each embodiment can be implemented by means of software plus necessary general-purpose hardware platforms, and of course, it can also be implemented by hardware. Based on this understanding, the above technical solutions, in essence or the part that contributes to the prior art, can be embodied in the form of a software product. This computer software product can be stored in a computer-readable storage medium, such as ROM / RAM, magnetic disk, optical disk, etc., and includes several instructions to cause a computer device (which may be a personal computer, server, or network device, etc.) to execute the methods described in the various embodiments or some parts of the embodiments.

[0111] The various technical features disclosed in the above-described invention, the various technical features disclosed in the following embodiments and examples, and the various technical features disclosed in the accompanying drawings can be freely combined to form various new technical solutions (all of which are considered to have been recorded in this specification), unless such a combination of technical features is technically infeasible. For example, in one example, feature A+B+C is disclosed, and in another example, feature A+B+D+E is disclosed. Features C and D are equivalent technical means that serve the same function, and technically only one needs to be used; it is impossible to use both simultaneously. Feature E can be technically combined with feature C. Therefore, the solution A+B+C+D should not be considered as recorded because it is technically infeasible, while the solution A+B+C+E should be considered as recorded.

[0112] All documents mentioned in this application are considered to be incorporated in their entirety into the disclosure of this application so that they can serve as a basis for modifications if necessary. Furthermore, it should be understood that after reading the foregoing disclosure of this application, those skilled in the art can make various alterations or modifications to this application, and these equivalent forms also fall within the scope of protection claimed in this application.

Claims

1. A fault ride-through method for a photovoltaic grid-connected system based on VSG current limiting control, characterized in that, include: (a) The photovoltaic side fault handling method includes: when the grid connection point PCC voltage is detected to be lower than the preset voltage threshold, the photovoltaic inverter shuts down the power outer loop control and reconstructs the active current reference value and reactive current reference value according to the voltage drop depth, wherein: The active current reference value of the photovoltaic system is reduced or reduced to zero, so that the active power output of the photovoltaic inverter is reduced. The reactive current reference value of the photovoltaic system is increased based on the voltage drop depth, so that the photovoltaic inverter enters the reactive power priority control mode to inject reactive power into the grid. The output current of the photovoltaic system is limited to ensure that it does not exceed the rated current, thereby ensuring that the photovoltaic side remains connected to the grid and provides voltage support during faults. (b) Second-order rotor motion equations and excitation control equations are established in the energy storage converter controller to simulate the synchronous generator. Fault handling methods on the energy storage side include: When a voltage drop in the PCC or a grid fault is detected, a dynamically adjusted virtual impedance is injected into the output of the VSG according to the depth of the voltage drop, so as to increase the equivalent impedance and actively limit the peak value of the short-circuit current. Switch the active power reference value of the energy storage system to the active power reference value under fault conditions, and reduce or suppress active power output. The reactive power reference value of the energy storage system is switched to the reactive power reference value under fault conditions to enhance the reactive power support capability of the energy storage system, thereby maintaining the voltage stability at the grid connection point.

2. The fault ride-through method for photovoltaic grid-connected systems based on VSG current limiting control as described in claim 1, characterized in that, The active current reference value and reactive current reference value are reconstructed as follows: Among them, i dref0 and i qref0 These represent the reference values ​​for active and reactive currents before the fault, respectively, α represents the voltage sag depth, and i N This is the rated current of the photovoltaic system.

3. The fault ride-through method for photovoltaic grid-connected systems based on VSG current limiting control as described in claim 1, characterized in that, The control of the VSG includes: active power-frequency control and reactive power-voltage control, wherein the active power-frequency control and reactive power-frequency control are respectively represented as follows: Among them, P ref P and ω represent the reference active power and actual active power of the VSG, respectively. N ω and J are the rated virtual rotor angular frequency and the actual angular frequency, respectively; J is the virtual moment of inertia; D is the damping coefficient; δ is the power angle; Q ref Q and U represent the reference reactive power and actual reactive power of the VSG, respectively. N with U * K represents the rated voltage amplitude and reference voltage of the VSG, respectively. q This is the reactive power-voltage droop control coefficient.

4. The fault ride-through method for photovoltaic grid-connected systems based on VSG current limiting control as described in claim 3, characterized in that, The second-order rotor motion equation and excitation control equation are respectively expressed as follows: P m =P ref +K u (oh N -oh) Among them, P m With P e K represents mechanical power and electromagnetic power, respectively. u It represents the active power-frequency droop factor.

5. The fault ride-through method for photovoltaic grid-connected systems based on VSG current limiting control as described in claim 4, characterized in that, The virtual impedance includes virtual resistance and virtual reactance, which are respectively expressed as: Among them, R v L represents the virtual resistance. v Indicates virtual reactance, i d and i q These are the d-axis and q-axis components of the PCC current, ΔU d and ΔU q These represent the longitudinal and transverse components of the voltage drop caused by the virtual impedance between the VSG and the grid connection point, respectively.

6. The fault ride-through method for photovoltaic grid-connected systems based on VSG current limiting control as described in claim 5, characterized in that, After introducing the virtual impedance, the short-circuit current of the VSG is: Where, θ f U represents the phase angle difference between the VSG and PCC points after the voltage drop. sf This refers to the voltage amplitude at the grid connection point after the voltage drop. The voltage drop caused by the virtual impedance between the VSG and the grid connection point is:

7. The fault ride-through method for photovoltaic grid-connected systems based on VSG current limiting control as described in claim 5, characterized in that, The power injected into the grid by the energy storage converter is expressed as: Among them, Q f and P f S represents the reactive power reference value and active power reference value under fault conditions, respectively. N This represents the apparent power under fault conditions.

8. The fault ride-through method for photovoltaic grid-connected systems based on VSG current limiting control as described in claim 1, characterized in that, The preset voltage threshold is set to 0.8 pu.

9. An electronic device, comprising: A memory, a processor, and a computer program stored in the memory and executable on the processor, characterized in that the processor, when executing the computer program, implements the method according to any one of claims 1 to 8.

10. A non-transitory computer-readable storage medium, characterized in that, The non-transitory computer-readable storage medium includes a computer program that, when executed by a processor, implements the steps of the method as described in any one of claims 1 to 8.

Images