A high-low voltage fault ride-through control method for a grid-connected photovoltaic grid-connected inverter

Abstract

The application belongs to the technical field of new energy grid-connected control, and specifically discloses a high-low voltage fault ride-through control method for a grid-connected photovoltaic inverter, which comprises the following steps: firstly, collecting three-phase voltage, grid-connected current and DC bus voltage at a grid-connected point; obtaining the grid-connected voltage phase angle and the phase-locked angle frequency through a synchronous damping shaping phase-locked loop; and judging whether the photovoltaic grid-connected inverter is in a normal operation mode, a low-voltage ride-through mode, a high-voltage ride-through mode or an end locking mode according to the voltage amplitude at the grid-connected point. During a low-voltage fault, a forward reactive current support instruction is generated according to the voltage drop depth; during a high-voltage fault, a reverse reactive current absorption instruction is generated according to the voltage rise amplitude; and at the same time, the active current instruction is dynamically limited according to the maximum allowable current of the inverter, so that the active current and the reactive current are coordinately distributed. The application can improve the high-low voltage fault ride-through capability and operation stability of the photovoltaic grid-connected inverter under the condition of a wide short-circuit ratio power grid.

Description

Technical Field

[0001] This invention relates to the field of new energy grid-connected control technology, and in particular to a high and low voltage fault ride-through control method for grid-connected photovoltaic inverters. Background Technology

[0002] With the large-scale integration of photovoltaic power generation systems into the power grid, photovoltaic grid-connected inverters have become an important interface device for new energy grid connection. Grid-connected photovoltaic inverters typically obtain the voltage phase at the grid connection point through a phase-locked loop and realize current control, DC bus voltage control, active power regulation, and reactive power regulation in a synchronous rotating coordinate system.

[0003] The short-circuit ratio (SCR) is an important indicator characterizing the strength of grid connection points, and it can be expressed as: ,in, For the short-circuit capacity of the grid connection point, This refers to the inverter's rated capacity. A wide short-circuit ratio grid refers to a grid environment where the inverter can operate at a relatively high... A strong power grid may also operate at a lower level. In a weak grid, the voltage phase at the grid connection point is primarily determined by the grid, and traditional phase-locked loops (PLLs) can typically track the grid voltage phase quickly. However, in a weak grid, the voltage at the grid connection point is affected not only by the grid voltage but also by the grid impedance, inverter output current, control loops, and fault disturbances. In this case, strong coupling can easily form between the PLL, current control loop, and grid impedance, leading to phase oscillations, frequency abrupt changes, or even loss of lock-in.

[0004] When the power grid experiences a low-voltage drop or a high-voltage rise, grid-connected photovoltaic inverters need to quickly adjust their active and reactive currents to meet the requirements for high and low voltage fault ride-through. Traditional methods typically generate reactive current commands based on voltage amplitude, but under wide short-circuit ratio conditions, the following problems still exist: traditional phase-locked loops are prone to phase angle oscillations in weak grids and during fault recovery; the lack of unified state management during fault entry and exit processes makes it easy to trigger repeatedly near the threshold; a rapid increase in reactive current will consume the inverter's current capacity, and if the active current is not synchronously limited, it can easily trigger overcurrent protection or cause abnormal DC bus voltage.

[0005] Therefore, to solve the above problems, a high and low voltage fault ride-through control method for grid-connected photovoltaic inverters that can take into account both the rapid response of strong power grids and the synchronous stability of weak power grids is needed. Summary of the Invention

[0006] The purpose of this invention is to provide a high and low voltage fault ride-through control method for grid-connected photovoltaic inverters, which solves the problems of phase-locked oscillation, repeated mode switching, uncoordinated active current distribution, uncoordinated reactive current distribution, and fault ride-through failure in traditional grid-connected photovoltaic inverters during wide short-circuit ratio grids, high and low voltage faults, and fault recovery processes.

[0007] To achieve the above objectives, the present invention provides a high and low voltage fault ride-through control method for a grid-connected photovoltaic inverter, comprising the following steps: S1. Collect the three-phase voltage, grid-connected current, DC bus voltage, and inverter operating status of the grid-connected photovoltaic inverter. S2. Input the three-phase voltage at the grid connection point into the synchronous damping shaping phase-locked loop to obtain the grid connection voltage phase angle and the phase-locked loop frequency; S3. Perform coordinate transformation on the grid connection point voltage according to the grid connection voltage phase angle to obtain the voltage components in the synchronous rotating coordinate system, and calculate the per-unit value of the grid connection point voltage amplitude. S4. Filter the per-unit value of the voltage amplitude at the grid connection point to obtain the detection voltage used to determine the fault status and the command voltage used to generate the reactive current command. S5. Based on the detected voltage, low voltage ride-through threshold, high voltage ride-through threshold, low voltage ride-through delay, and high voltage ride-through delay, determine whether the photovoltaic grid-connected inverter is in normal operation mode, low voltage ride-through mode, or high voltage ride-through mode. S6. When the photovoltaic grid-connected inverter is in low voltage ride-through mode, a positive reactive current support command is generated based on the deviation between the command voltage and the low voltage ride-through threshold. S7. When the photovoltaic grid-connected inverter is in high voltage ride-through mode, a reverse reactive current absorption command is generated based on the deviation between the command voltage and the high voltage ride-through threshold. S8. Calculate the upper limit of active current based on the reactive current command and the maximum allowable current of the inverter, and limit the active current command output by the DC voltage outer loop. S9. Generate inverter modulation signals based on grid-connected voltage phase angle, active current command, and reactive current command to enable grid-connected photovoltaic inverters to ride through high and low voltage faults under wide short-circuit ratio grid conditions.

[0008] Preferably, in S2, the synchronous damping shaping phase-locked loop includes a phase error extraction module, a synchronous damping shaping module, a frequency generation module, and a phase integration module; The phase error extraction module is used to generate a phase-locked error signal based on the quadrature components of the grid connection point voltage; the synchronous damping shaping module is used to adjust the dynamic relationship between the phase error signal, the frequency deviation signal, and the frequency change rate; the frequency generation module is used to generate the phase-locked angle frequency; and the phase integration module is used to generate the grid connection voltage phase angle.

[0009] Preferably, the synchronous damping shaping module reduces the dynamic coupling relationship between the phase-locked loop, the current control loop, and the grid impedance under weak grid conditions by adjusting the equivalent damping and equivalent bandwidth of the phase-locked loop.

[0010] Preferably, in S3, the per-unit value of the grid connection point voltage amplitude The expression is calculated based on the voltage components in the synchronous rotating coordinate system: ; in, and These are the d-axis components of the grid connection point voltage in the synchronous rotating coordinate system and... Axial components, This is the voltage reference value.

[0011] Preferably, in S4, the per-unit value of the grid connection point voltage amplitude The voltage passes through a fault detection filter channel and a reactive power command filter channel, respectively; the fault detection filter channel outputs the detected voltage. Used for high and low voltage fault condition judgment; reactive power command filter channel outputs command voltage. It is used to generate reactive current commands during low-voltage ride-through or high-voltage ride-through. The detection voltage is obtained by using discrete first-order filtering. and command voltage : ; in, This is the current sampling sequence number. This is the previous sampling number. These are the fault detection filter coefficients. The reactive power command filtering coefficient is 0 < 0. ≤1、0< ≤1.

[0012] Preferably, in S5, when the detection voltage... Below or equal to the low voltage crossover threshold And continue to exceed the low penetration delay At this time, the photovoltaic grid-connected inverter enters the low voltage ride-through mode; When the detection voltage Higher than or equal to the high voltage ride-through threshold And continue to exceed the high penetration delay At this time, the photovoltaic grid-connected inverter enters the high-voltage ride-through mode; When the detection voltage satisfy At this time, the photovoltaic grid-connected inverter enters or maintains normal operation mode and clears the low-throughput timer and high-throughput timer.

[0013] Preferably, in S6, the reactive current command in low voltage ride-through mode It is positively correlated with the voltage drop depth, and its expression is: ; in, For low voltage ride-through reactive power support factor, This is the reactive current bias.

[0014] Preferably, in S7, the reactive current command in high voltage ride-through mode It is negatively correlated with the voltage rise rate, and its expression is: ; in, This is the high-voltage ride-through reactive power absorption coefficient.

[0015] Preferably, in S8, the upper limit of active current is calculated based on the inverter's maximum allowable current and reactive current command, and its expression is: ; in, This is the upper limit of active current. This is the maximum allowable current of the inverter. This represents the function that takes the maximum value. Final active current command satisfy: ; in, This is the original active current command output from the outer loop of the DC voltage. This represents the function that takes the minimum value.

[0016] Preferably, after the low-voltage ride-through mode or the high-voltage ride-through mode ends, the system enters the end-lock mode; during the duration of the end-lock mode... Within this period, it is prohibited to re-trigger the low voltage ride-through mode or the high voltage ride-through mode to avoid frequent mode switching caused by fluctuations in the grid connection point voltage near the threshold. In normal operating mode, the DC voltage outer loop is based on the DC bus voltage reference value. With actual DC bus voltage The deviation between them generates the original active current command. ; In low voltage ride-through mode or high voltage ride-through mode, the raw active current command is based on the inverter's maximum allowable current. and reactive current command Implement dynamic amplitude limiting.

[0017] Therefore, the present invention employs the above-mentioned high and low voltage fault ride-through control method for grid-connected photovoltaic inverters, and the beneficial effects are as follows: (1) By introducing a synchronous damping shaping phase-locked loop, the present invention dampens and shapes the dynamic response between the phase error signal and the phase-locked frequency output, which can suppress phase-locked oscillations during weak grid, high and low voltage faults and fault recovery stages, and improve the phase synchronization stability of grid-connected photovoltaic inverters.

[0018] (2) The present invention adopts a dual-channel filtering structure of detection voltage and command voltage, which serves the fault status judgment and reactive current command generation respectively, and can take into account both the fault identification speed and the smoothness of current command.

[0019] (3) The present invention achieves unified state management of low voltage fault, high voltage fault and fault recovery process through low penetration entry delay, high penetration entry delay and end lockout mode, avoiding false triggering and repeated switching near the threshold.

[0020] (4) The present invention achieves current capacity coordination by prioritizing reactive power and limiting active power, thereby reducing the risk of overcurrent and improving the success rate of fault ride-through.

[0021] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. Attached Figure Description

[0022] Figure 1 This is an overall flowchart of an embodiment of the high and low voltage fault ride-through control method for a grid-connected photovoltaic inverter according to the present invention; Figure 2 This is a schematic diagram of the topology and control structure of a grid-connected photovoltaic inverter connected to a grid with a wide short-circuit ratio, according to an embodiment of the high and low voltage fault ride-through control method for a grid-connected photovoltaic inverter of the present invention. Figure 3 This is a flowchart illustrating the high and low voltage fault ride-through control method of a grid-connected photovoltaic inverter according to an embodiment of the present invention. Figure 4 This is a control structure diagram of a synchronous damping shaping phase-locked loop in an embodiment of a high and low voltage fault ride-through control method for a grid-connected photovoltaic inverter according to the present invention. Figure 5 This is a control structure diagram of a generalized second-order integrator in an embodiment of a high and low voltage fault ride-through control method for a grid-connected photovoltaic inverter according to the present invention. Figure 6The following is a simulation result of high and low voltage fault ride-through under strong grid conditions when the short-circuit ratio is 10, according to an embodiment of the high and low voltage fault ride-through control method for a grid-connected photovoltaic inverter of the present invention. Among them, (a) is the grid voltage amplitude diagram, (b) is the grid voltage waveform diagram, (c) is the grid current waveform diagram, (d) is the inverter output power waveform diagram, (e) is the inverter DC voltage waveform diagram, and (f) is the inverter current command signal diagram. Figure 7 The above are simulation results of high and low voltage fault ride-through control method for grid-connected photovoltaic inverters under weak grid conditions with a short-circuit ratio of 2.5, according to an embodiment of the present invention. (a) is the grid voltage amplitude diagram, (b) is the grid voltage waveform diagram, (c) is the grid current waveform diagram, (d) is the inverter output power waveform diagram, (e) is the inverter DC voltage waveform diagram, and (f) is the inverter current command signal diagram. Detailed Implementation

[0023] The technical solution of the present invention will be further described below with reference to the accompanying drawings and embodiments.

[0024] Unless otherwise defined, the technical or scientific terms used in this invention shall have the ordinary meaning understood by one of ordinary skill in the art to which this invention pertains. The terms "first," "second," and similar terms used in this invention do not indicate any order, quantity, or importance, but are merely used to distinguish different components. Terms such as "comprising" or "including" mean that the element or object preceding the word encompasses the elements or objects listed following the word and their equivalents, without excluding other elements or objects. Terms such as "connected" or "linked" are not limited to physical or mechanical connections, but can include electrical connections, whether direct or indirect. Terms such as "upper," "lower," "left," and "right" are used only to indicate relative positional relationships; when the absolute position of the described object changes, the relative positional relationship may also change accordingly.

[0025] like Figure 1 As shown, a high and low voltage fault ride-through control method for a grid-connected photovoltaic inverter includes the following steps: S1. Collect the three-phase voltage, grid-connected current, DC bus voltage, and inverter operating status of the grid-connected photovoltaic inverter.

[0026] S2. Input the three-phase voltage at the grid connection point into the synchronous damping shaping phase-locked loop to obtain the grid connection voltage phase angle and phase-locked loop frequency.

[0027] S3. Perform coordinate transformation on the grid connection point voltage according to the grid connection voltage phase angle to obtain the voltage components in the synchronous rotating coordinate system, and calculate the per-unit value of the grid connection point voltage amplitude.

[0028] S4. Filter the per-unit value of the voltage amplitude at the grid connection point to obtain the detection voltage used to determine the fault status and the command voltage used to generate the reactive current command.

[0029] S5. Based on the detected voltage, low voltage ride-through threshold, high voltage ride-through threshold, low voltage ride-through delay, and high voltage ride-through delay, determine whether the photovoltaic grid-connected inverter is in normal operation mode, low voltage ride-through mode, or high voltage ride-through mode.

[0030] S6. When the photovoltaic grid-connected inverter is in low voltage ride-through mode, a positive reactive current support command is generated based on the deviation between the command voltage and the low voltage ride-through threshold.

[0031] S7. When the photovoltaic grid-connected inverter is in high voltage ride-through mode, a reverse reactive current absorption command is generated based on the deviation between the command voltage and the high voltage ride-through threshold.

[0032] S8. Calculate the upper limit of active current based on the reactive current command and the maximum allowable current of the inverter, and limit the active current command output by the DC voltage outer loop.

[0033] S9. Generate inverter modulation signals based on grid-connected voltage phase angle, active current command, and reactive current command to enable grid-connected photovoltaic inverters to ride through high and low voltage faults under wide short-circuit ratio grid conditions.

[0034] The control system implementing the control method of the present invention includes a voltage and current sampling module, a synchronous damping shaping phase-locked loop, a voltage amplitude calculation module, a dual-channel filtering module, a high and low voltage fault status judgment module, a reactive current command generation module, an active current limiting module, a DC voltage control module, a current control module, and a PWM modulation module.

[0035] The voltage and current sampling module is used to collect the three-phase voltage, grid current and DC bus voltage at the grid connection point, providing feedback signals for fault diagnosis and closed-loop control.

[0036] Synchronous damping shaping phase-locked loops are used to extract the positive sequence phase angle and angular frequency of grid-connected voltage and improve the dynamic stability of phase-locked loops under weak grid conditions and voltage faults.

[0037] The voltage amplitude calculation module calculates the voltage amplitude at the grid connection point based on the positive sequence voltage component, providing a basis for high and low voltage fault identification.

[0038] The dual-channel filtering module filters the fault detection voltage and the reactive power control voltage respectively, taking into account both the speed of fault identification and the smoothness of reactive power commands.

[0039] The high and low voltage fault status judgment module determines whether the system is in a normal, low voltage ride-through, or high voltage ride-through state based on the voltage amplitude and a set threshold.

[0040] The reactive current command generation module generates reactive current commands based on the fault type and voltage deviation, enabling low-voltage boost support and high-voltage buck regulation.

[0041] The active current limiting module limits the active current based on the inverter's maximum allowable current and reactive current command to prevent overcurrent during faults.

[0042] The DC voltage control module generates active current commands based on the DC bus voltage deviation to keep the DC bus voltage stable.

[0043] The current control module is used to control the grid-connected current to track active and reactive current commands and generate inverter AC side voltage commands.

[0044] The PWM modulation module is used to convert voltage commands into drive pulses for power switching devices, thereby controlling the inverter to output corresponding three-phase voltages and currents.

[0045] Example 1: I. System Sampling and Synchronous Damping Shaping Phase-Locked Loop: like Figure 2 As shown, in S1, the grid-connected photovoltaic inverter collects the three-phase voltage through the grid connection point voltage sensor. v a , v b , v c Grid-connected current is collected through a current sensor. i a , i b , i c DC bus voltage is acquired through a DC voltage sensor. v dc .

[0046] like Figure 4 As shown, the synchronous damping shaping phase-locked loop in S2 of the present invention includes a phase error extraction module, a synchronous damping shaping module, a frequency generation module, and a phase integration module. The synchronous damping shaping phase-locked loop is used to dampen and shape the dynamic response between the phase error signal and the phase-locked frequency output in order to suppress phase-locked oscillations caused by high and low voltage faults under weak short-circuit ratio conditions.

[0047] The phase error extraction module is used to generate a phase-locked error signal based on the quadrature components of the grid connection point voltage; the synchronous damping shaping module is used to adjust the dynamic relationship between the phase error signal, the frequency deviation signal, and the frequency change rate; the frequency generation module is used to generate the phase-locked angle frequency; and the phase integration module is used to generate the grid connection voltage phase angle.

[0048] The synchronous damping shaping module reduces the dynamic coupling between the phase-locked loop, current control loop, and grid impedance under weak grid conditions by adjusting the equivalent damping and equivalent bandwidth of the phase-locked loop, thereby improving the phase synchronization stability during low-voltage faults, high-voltage faults, and fault recovery phases.

[0049] S3. Perform coordinate transformation on the grid connection point voltage based on the grid connection voltage phase angle to obtain the voltage components in the synchronous rotating coordinate system, and calculate the per-unit value of the grid connection point voltage amplitude. The specific process is as follows: First, transform the three-phase voltage at the grid connection point to a two-phase stationary coordinate system: ; in, v α and v β These are the grid connection point voltages in a two-phase stationary coordinate system. α Axial components and β Axis components. Then, v α and v β Enter them respectively as follows Figure 5 The second-order generalized integrator shown yields in-phase and quadrature components. For any input voltage component... v x The output relationship of the second-order generalized integrator is as follows: ; in, v x ( s () represents the input voltage component v x The Laplace expression, v x ′( s () represents the in-phase component of the output of the second-order generalized integrator. qv x ′( s () represents the orthogonal components output by the second-order generalized integrator. s For the Laplace operator, The damping coefficient of the second-order generalized integrator. ω g This is the grid angular frequency. (After obtaining...) v α ′、 qv α ′、 v β 'and qv β Afterwards, the positive-sequence voltage component and the negative-sequence voltage component are obtained through the positive-sequence separation module. The positive-sequence voltage component is: .

[0050] The negative sequence voltage component is: ; in, v α + and v β + These are the positive sequence components of the grid connection point voltage in a two-phase stationary coordinate system. α Axial components and β Axial components, v α - and v β - These are the negative sequence components of the grid connection point voltage in a two-phase stationary coordinate system. α Axial components and β Axial components.

[0051] Subsequently, the positive sequence voltage component is transformed into a synchronous rotating coordinate system: ; Among them, v d + and v q + These are the positive sequence voltages in a synchronous rotating coordinate system. d Axial components and q Axial components, θ This is the phase-locked angle. In positive order... q Shaft voltage serves as a phase-locked loop error signal: ; in, e pll ( k ) is the first k Phase-locked error signal at each sampling time, v q + ( k ) is the first k The q-axis component of the positive sequence voltage at each sampling time in the synchronous rotating coordinate system.

[0052] The damping-shaping frequency controller can be expressed as: ; in, ω ( k ) is the first k The phase-locked angle frequency at each sampling time, K ω For frequency integral gain, Ts The controller sampling period, T k This is the adjustment coefficient for the phase-locked loop error signal. T p This is the frequency damping feedback coefficient. The phase angle generation module generates the phase-locked angle based on the phase-locked angle frequency: ; in, ω ( k ) is the first k The phase-locked angle frequency at each sampling time. The damped shaping frequency controller described above... This term is used to form frequency damping feedback. When the phase-locked loop (PLL) angular frequency deviates from the rated angular frequency, this term produces a reverse adjustment effect, causing the PLL angular frequency to smoothly recover to the rated angular frequency. When the power grid is in a weak short-circuit ratio, high or low voltage fault, or fault recovery phase, this damping feedback can suppress sudden changes in the PLL angular frequency and phase oscillations.

[0053] II. Grid Connection Point Voltage Detection and Dual-Channel Filtering: Based on the phase angle output by the phase-locked loop, the grid connection point voltage is transformed into a synchronous rotating coordinate system to obtain... d Axis voltage components v d and q Axis voltage components v q The per-unit value of the grid connection point voltage amplitude is expressed as: ; in, V pu ( k ) is the first k The per-unit value of the grid connection point voltage amplitude at each sampling time. V n This is the voltage reference value.

[0054] To balance fault identification speed and current command smoothness, two filtering channels are provided in S4 of this invention. The fault detection filtering channel outputs a detection voltage. V det It is used to determine low voltage ride-through, high voltage ride-through, or normal operating conditions; the reactive power command filter channel outputs the command voltage. V iq This is used to generate reactive current commands during low-level or high-level ride-through. The detection voltage and command voltage are expressed as follows: ; in, a det These are the fault detection filter coefficients. a iq These are the reactive power command filtering coefficients.

[0055] III. High and Low Voltage Fault Status Judgment: In S5, based on the detected voltage V det Low voltage ride-through threshold V LV High voltage pass-through threshold V HV Low-profile entry delay T LV and high penetration into time delay T HV Determine the running status. Specifically, as follows: Figure 3 As shown, the system first acquires input quantities such as the grid connection point voltage, the active current command and reactive current command output from the DC voltage outer loop, as well as voltage and current commands from various channels. Then, it proceeds to the high and low voltage fault status judgment stage. ; in, V det ( k ) is the first k The detected voltage at each sampling time. V LV Low voltage ride-through threshold, V HV For high voltage ride-through threshold, t LV ( k (This is for the low-penetration entry timer.) t HV ( k (This is for the high-penetration entry timer.) T LV For low-level entry delay, T HV For high-speed entry delay, L end ( k () is the end of the lock.

[0056] That is, when the detection voltage V det Below or equal to the low voltage crossover threshold V LV And continue to exceed the low penetration delay T LV At that time, the photovoltaic grid-connected inverter enters low voltage ride-through mode; when the detected voltage... V det Higher than or equal to the high voltage ride-through threshold V HV And continue to exceed the high penetration delay T HV At that time, the photovoltaic grid-connected inverter enters the high-voltage ride-through mode; when the detected voltage... Vdet satisfy V LV ≤ V det ≤ V HV When in use, enter or maintain normal operating mode and clear the low-penetration timer and high-penetration timer.

[0057] IV. Reactive current command generation: In S6, under low voltage ride-through mode, a positive reactive current support command is generated based on the voltage sag depth: ; in, i q * indicates reactive current command. K LV For low voltage ride-through reactive power support factor, V LV Low voltage ride-through threshold, V iq The reactive power command filtering channel outputs the command voltage. i q0 This is the reactive current bias.

[0058] In S7, during high-voltage ride-through mode, a reverse reactive current absorption command is generated based on the voltage rise magnitude: ; in, This is a reactive current command. K HV The high-voltage ride-through reactive power absorption coefficient. V iq The reactive power command filtering channel outputs the command voltage. V HV For high voltage ride-through threshold, i q0 This is the reactive current bias.

[0059] V. Coordination of active and reactive current capacities: In both low-voltage ride-through and high-voltage ride-through modes, reactive current consumes inverter current capacity. To avoid overcurrent, step S8 of this invention calculates the upper limit of active current based on the inverter's maximum allowable current and the reactive current command: ; in, i d,lim This is the upper limit of active current. I max This is the maximum allowable current of the inverter. This represents the function that takes the maximum value. i q* indicates reactive current command.

[0060] Final active current command i d *satisfy: ; in, i d0 This is the original active current command output from the outer loop of the DC voltage. This represents the function that takes the minimum value.

[0061] In S9, the inverter modulation signal is generated based on the grid-connected voltage phase angle, active current command, and reactive current command, so as to realize the high and low voltage fault ride-through of the grid-connected photovoltaic inverter under the grid conditions with a wide short-circuit ratio.

[0062] VI. Maintain control after the crossing ends: After the low-voltage ride-through mode or high-voltage ride-through mode ends, the system enters the ride-through end hold state. The duration of the ride-through end hold state is... T lock Within this timeframe, the low-voltage ride-through mode or high-voltage ride-through mode is prohibited from being retried to avoid frequent mode switching when the grid connection point voltage fluctuates near the threshold, and related timers and state variables are cleared.

[0063] This end-of-crossover hold control is used to avoid repeated switching between low-crossover, high-crossover, and normal operation modes when the grid connection point voltage fluctuates near the threshold, thereby reducing sudden changes in current command during the fault recovery phase.

[0064] VII. Implementation Results: To verify the control effect of this invention, a grid-connected photovoltaic inverter model can be built on the Matlab / Simulink platform. The system includes a photovoltaic array, a Boost converter, a DC bus capacitor, a three-phase grid-connected inverter, an LCL filter, grid impedance, and an AC grid. The control system includes maximum power point tracking control, DC voltage control, a synchronous damping shaping phase-locked loop, high and low voltage fault ride-through control, positive sequence current vector control, negative sequence current vector control, and a PWM modulation module.

[0065] Under power grid conditions with a high short-circuit ratio, such as Figure 6 When the short-circuit ratio is shown to be 10, the system experiences both low-voltage sag and high-voltage rise. Simulation results demonstrate that this invention can quickly identify low-voltage or high-voltage faults and generate corresponding reactive current commands based on the voltage deviation, while simultaneously limiting active current to ensure that the inverter current does not exceed the maximum allowable value.

[0066] Under power grid conditions with a low short-circuit ratio, such as Figure 7When the short-circuit ratio is 2.5, traditional phase-locked loops are prone to phase-locked frequency oscillations during faults or fault recovery phases. This invention employs a synchronous damped shaping phase-locked loop, which suppresses phase-locked frequency fluctuations through positive and negative sequence separation and a damped shaping frequency controller, thereby stabilizing grid-connected current, active power, reactive power, and DC bus voltage, and improving the fault ride-through success rate.

[0067] Therefore, the high and low voltage fault ride-through control method of the grid-connected photovoltaic inverter described above can achieve stable low voltage ride-through and high voltage ride-through under grid conditions with strong and weak short circuit ratios, and has good engineering applicability and promotion value.

[0068] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the technical solutions of the present invention, and these modifications or equivalent substitutions cannot cause the modified technical solutions to deviate from the spirit and scope of the technical solutions of the present invention.

Claims

1. A high and low voltage fault ride-through control method for a grid-connected photovoltaic inverter, characterized in that, Includes the following steps: S1. Collect the three-phase voltage, grid-connected current, DC bus voltage, and inverter operating status of the grid-connected photovoltaic inverter. S2. Input the three-phase voltage at the grid connection point into the synchronous damping shaping phase-locked loop to obtain the grid connection voltage phase angle and the phase-locked loop frequency; S3. Perform coordinate transformation on the grid connection point voltage according to the grid connection voltage phase angle to obtain the voltage components in the synchronous rotating coordinate system, and calculate the per-unit value of the grid connection point voltage amplitude. S4. Filter the per-unit value of the voltage amplitude at the grid connection point to obtain the detection voltage used to determine the fault status and the command voltage used to generate the reactive current command. S5. Based on the detected voltage, low voltage ride-through threshold, high voltage ride-through threshold, low voltage ride-through delay, and high voltage ride-through delay, determine whether the photovoltaic grid-connected inverter is in normal operation mode, low voltage ride-through mode, or high voltage ride-through mode. S6. When the photovoltaic grid-connected inverter is in low voltage ride-through mode, a positive reactive current support command is generated based on the deviation between the command voltage and the low voltage ride-through threshold. S7. When the photovoltaic grid-connected inverter is in high voltage ride-through mode, a reverse reactive current absorption command is generated based on the deviation between the command voltage and the high voltage ride-through threshold. S8. Calculate the upper limit of active current based on the reactive current command and the maximum allowable current of the inverter, and limit the active current command output by the DC voltage outer loop. S9. Generate inverter modulation signals based on grid-connected voltage phase angle, active current command, and reactive current command to enable grid-connected photovoltaic inverters to ride through high and low voltage faults under wide short-circuit ratio grid conditions.

2. The high and low voltage fault ride-through control method for a grid-connected photovoltaic inverter according to claim 1, characterized in that, In S2, the synchronous damping shaping phase-locked loop includes a phase error extraction module, a synchronous damping shaping module, a frequency generation module, and a phase integration module; The phase error extraction module is used to generate a phase-locked error signal based on the quadrature components of the grid connection point voltage; the synchronous damping shaping module is used to adjust the dynamic relationship between the phase error signal, the frequency deviation signal, and the frequency change rate; the frequency generation module is used to generate the phase-locked angle frequency; and the phase integration module is used to generate the grid connection voltage phase angle.

3. The high and low voltage fault ride-through control method for a grid-connected photovoltaic inverter according to claim 2, characterized in that, The synchronous damping shaping module reduces the dynamic coupling between the phase-locked loop, the current control loop, and the grid impedance under weak grid conditions by adjusting the equivalent damping and equivalent bandwidth of the phase-locked loop.

4. The high and low voltage fault ride-through control method for a grid-connected photovoltaic inverter according to claim 1, characterized in that, In S3, the per-unit value of the grid connection point voltage amplitude The expression is calculated based on the voltage components in the synchronous rotating coordinate system: ； in, and These are the d-axis components of the grid connection point voltage in the synchronous rotating coordinate system and... Axial components, This is the voltage reference value.

5. The high and low voltage fault ride-through control method for a grid-connected photovoltaic inverter according to claim 1, characterized in that, In S4, the per-unit value of the grid connection point voltage amplitude They pass through the fault detection filter channel and the reactive power command filter channel, respectively. Fault detection filter channel output detection voltage Used for high and low voltage fault status judgment; The reactive power command filter channel outputs the command voltage. It is used to generate reactive current commands during low-voltage ride-through or high-voltage ride-through. The detection voltage is obtained by using discrete first-order filtering. and command voltage : ； in, This is the current sampling sequence number. This is the previous sampling number. These are the fault detection filter coefficients. The reactive power command filtering coefficient is 0 < 0. ≤1、0< ≤1.

6. The high and low voltage fault ride-through control method for a grid-connected photovoltaic inverter according to claim 1, characterized in that, In S5, when the detection voltage Below or equal to the low voltage crossover threshold And continue to exceed the low penetration delay At this time, the photovoltaic grid-connected inverter enters the low voltage ride-through mode; When the detection voltage Higher than or equal to the high voltage ride-through threshold And continue to exceed the high penetration delay At this time, the photovoltaic grid-connected inverter enters the high-voltage ride-through mode; When the detection voltage satisfy At this time, the photovoltaic grid-connected inverter enters or maintains normal operation mode and clears the low-throughput timer and high-throughput timer.

7. The high and low voltage fault ride-through control method for a grid-connected photovoltaic inverter according to claim 1, characterized in that, In S6, reactive current command in low voltage ride-through mode It is positively correlated with the voltage drop depth, and its expression is: ； in, For low voltage ride-through reactive power support factor, This is the reactive current bias.

8. The high and low voltage fault ride-through control method for a grid-connected photovoltaic inverter according to claim 1, characterized in that, In S7, reactive current command in high voltage ride-through mode It is negatively correlated with the voltage rise rate, and its expression is: ； in, This is the high-voltage ride-through reactive power absorption coefficient.

9. The high and low voltage fault ride-through control method for a grid-connected photovoltaic inverter according to claim 1, characterized in that, In S8, the upper limit of active current is calculated based on the inverter's maximum allowable current and reactive current command, and its expression is: ； in, This is the upper limit of active current. This is the maximum allowable current of the inverter. This represents the function that takes the maximum value. Final active current command satisfy: ； in, This is the original active current command output from the outer loop of the DC voltage. This represents the function that takes the minimum value.

10. The high and low voltage fault ride-through control method for a grid-connected photovoltaic inverter according to claim 1, characterized in that, After the low-voltage ride-through mode or high-voltage ride-through mode ends, it enters the end-lock mode; during the duration of the end-lock mode... Within this period, it is prohibited to re-trigger the low voltage ride-through mode or the high voltage ride-through mode to avoid frequent mode switching caused by fluctuations in the grid connection point voltage near the threshold. In normal operating mode, the DC voltage outer loop is based on the DC bus voltage reference value. With actual DC bus voltage The deviation between them generates the original active current command. ; In low voltage ride-through mode or high voltage ride-through mode, the raw active current command is based on the inverter's maximum allowable current. and reactive current command Implement dynamic amplitude limiting.

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