A current-limiting and fault recovery method for a photovoltaic inverter based on adaptive variable angle

Abstract

The application provides a current-limiting and fault recovery method for a photovoltaic inverter based on adaptive angle change, belongs to the technical field of photovoltaic power generation, and is aimed at a grid voltage symmetrical drop fault, introduces an operating angle in a current-limiting angle, enhances the transient performance of a photovoltaic inverter grid-connected system, and avoids the saturation phenomenon after fault recovery caused by the continuous action of an integrator during fault current limiting by introducing an anti-saturation method of a voltage controller, simultaneously, introduces a current-limiting recovery compensation method, shortens the current-limiting mode exit time, and thus can accelerate the recovery of the system to a stable state.

Description

Technical Field

[0001] The present invention relates to the field of photovoltaic power generation, and in particular to a photovoltaic inverter current limiting and fault recovery method based on adaptive variable angle. Background Art

[0002] With the growing global demand for renewable energy, photovoltaic power generation, as a clean, renewable energy source, has gained widespread adoption. As the bridge between solar panels and the power grid, the performance of photovoltaic inverters directly impacts the efficiency and stability of the entire photovoltaic system. Traditional photovoltaic inverters primarily convert direct current (DC) to alternating current (AC), while grid-connected inverters further mimic the voltage and frequency characteristics of the power grid, enabling them to better collaborate with the grid and improve its stability and reliability.

[0003] With the continuous innovation and progress of power electronics technology and control algorithms, grid-connected photovoltaic inverters can adapt to different grid conditions and operating requirements.

[0004] Unlike synchronous generators that have strong overcurrent capabilities, power electronic converters based on semiconductor devices can usually only withstand 1.2 to 2 times the overcurrent.

[0005] Regarding the overcurrent problem during a voltage symmetric sag fault, the following studies have been conducted based on existing published references:

[0006] Reference 1 “Fan B, Wang X. Fault Recovery Analysis of Grid-Forming Inverters With Priority-Based Current Limiters[J]. IEEE Transactions on Power Systems, 2022, 38(6): 5102-5112.” (“Fault Recovery Analysis of Grid-Forming Inverters with Priority-Based Current Limiters”, IEEE Transactions on Power Systems, Vol. 38, No. 6, 2022, pp. 5102-5112) studied and analyzed the transient synchronous stability of grid-forming inverters using priority angle current limiters, and summarized the fault recovery characteristics of grid-forming inverters after adopting priority current limiters.

[0007] Reference 2 "Zhuang K, Xin H, Hu P, et al. Current Saturation Analysis and Anti-Windup Control Design of Grid-Forming Voltage Source Converter[J]. IEEE Transactions on Energy Conversion, 2022, 37(4): 2790-2802." ("Overcurrent Analysis and Anti-Windup Control of Grid-Forming Voltage Source Converter", IEEE Transactions on Energy Conversion, Vol. 37, No. 4, 2022, pp. 2790-2802) analyzes the system conditions for virtual synchronous control to recover from the current limiting state to the normal operating state after the current limiter is added. The study shows that even after the grid fault ends, it is still difficult to restore to normal operation, and there is a situation where virtual synchronous control cannot provide power to the grid according to the power command during the current limiting period.

[0008] Reference 3 “Saffar KG, Driss S, Ajaei F B. Impacts of Current Limiting onthe Transient Stability of the Virtual Synchronous Generator[J]. IEEE Transactions on Power Electronics, 2023, 38(2): 1509-1521.” compares the angle priority current limiter, d-axis priority current limiter, and q-axis priority current limiter, and studies the dynamic performance of the three current limiting strategies during the grid fault and after the fault recovery. Among them, the q-axis priority current limiter has a larger transient stability margin.

[0009] Reference 4 “Rokrok E, Qoria T, Bruyere A, et al. Transient Stability Assessment and Enhancement of Grid-Forming Converters Embedding Current Reference Saturation as Current Limiting Strategy[J]. IEEE Transactions on Power Systems, 2022, 37(2): 1519-1531.” (“Transient Stability Assessment and Enhancement of Grid-Forming Converters Embedding Current Reference Saturation as Current Limiting Strategy Based on Saturation Current Reference Limiter”, IEEE Transactions on Power Systems, Vol. 37, No. 2, 2022, pp. 1519-1531) investigates the transient stability of grid-forming control in current limiting mode under voltage sag conditions and derives the critical clearing angle (CCA) and critical clearing time (Critical Clearing Time, CCA) when considering different reference current angles. Finally, a fixed-angle current limiter based on system parameters and initial power reference values ​​was proposed, which enabled the system to withstand faults for a longer period of time. However, the optimal angle still needed to be adjusted in real time under different grid voltage drop fault conditions, and there were still problems such as being unable to exit the current limiting state and selecting the optimal angle.

[0010] In summary, when grid-connected photovoltaic inverters are operating under grid voltage drops, current limiting control is required. However, existing research and technologies still have the following problems:

[0011] 1. The photovoltaic inverter based on the priority q-axis current limiter has the largest limit cut-off angle, followed by the priority angle current limiter. The priority d-axis current limiter has the smallest limit cut-off angle. An excessively large limit cut-off angle may make it difficult for the grid-connected system to exit the current limiting mode during the fault recovery phase.

[0012] 2. A fixed-angle current limiter can set the current limiting angle to the optimal solution, but an excessively large current limiting angle will make it difficult to exit the current limiting mode, and the existence of an optimal angle makes it difficult to adapt to different fault durations and different fault conditions.

[0013] In order to ensure the safe operation of power electronic devices in photovoltaic converters, a current limiting and fault recovery method based on adaptive variable angle is proposed. This method provides a current limiting strategy when the grid voltage drops symmetrically, and a compensation strategy for exiting the current limiting mode after fault recovery. Summary of the Invention

[0014] In order to solve the problems of current limiting during a grid voltage drop fault and exiting the current limiting mode after fault recovery, the present invention proposes a photovoltaic inverter current limiting and fault recovery method based on adaptive variable angle, in order to improve the current limiting and fault recovery capabilities of the photovoltaic inverter, thereby improving the transient stability during the fault period and the fault recovery process.

[0015] In order to achieve the above-mentioned object, the present invention adopts the following technical solutions:

[0016] The present invention provides a photovoltaic inverter current limiting and fault recovery method based on adaptive variable angle, which comprises:

[0017] Sample the voltage u of phase a, phase b, and phase c at the grid connection point of the photovoltaic inverter a 、u b 、u c And the output voltage u of phase a, phase b, and phase c of the photovoltaic inverter ta 、u tb 、u tc After transformation, the d-axis component u of the grid connection point voltage is obtained. d , q-axis component u d and the d-axis component u of the PV inverter output voltage td , q-axis component u tq ;

[0018] Sample the a-phase, b-phase, and c-phase currents i injected by the photovoltaic inverter into the grid a 、i b 、i c And the output current i of phase a, phase b, and phase c of the photovoltaic inverter ta 、i tb 、i tc After transformation, the d-axis component i of the grid current is obtained. d , q-axis component i q and the d-axis component i of the current to the photovoltaic inverter td , q-axis component i tq ;

[0019] Based on the d-axis component u of the grid connection point voltage d and the q-axis component u of the grid-connected point voltage q , and the d-axis component of the grid current i d and the q-axis component of the grid current i q , get the output active power P and output reactive power Q of the grid-connected point of the photovoltaic inverter;

[0020] Based on the active power P of the grid connection point and the output reactive power Q, the active power P after the low-pass filter is obtained. f and the reactive power Q after low-pass filteringf ;

[0021] For active power P f Perform virtual synchronous control to obtain the angular frequency ω and the reactive power Q f Perform droop control to obtain the voltage reference value u ref ;

[0022] The d-axis component u of the grid voltage is adjusted by the voltage loop PI regulator d and the q-axis component u q Perform voltage control to obtain the d-axis component i of the current command d,ref and the q-axis component i q,ref ;

[0023] The d-axis component i of the grid current is adjusted by the current loop PI regulator d , q-axis component i q Control is performed to obtain the d-axis component u of the inverter output voltage td and the q-axis component u tq , and after PWM modulation, generate the a-phase, b-phase, and c-phase switching signals S of the power devices in the photovoltaic inverter a 、S b 、S c , thereby controlling the on and off of the power devices of the photovoltaic inverter;

[0024] Determine whether a three-phase voltage symmetrical drop fault occurs at the grid connection point of the photovoltaic inverter. If so, set the flag bit i=1; otherwise, set the flag bit i=0;

[0025] When a three-phase voltage symmetrical drop fault occurs, determine whether the grid-connected point of the photovoltaic inverter enters the current limiting mode. If so, set the current limiting mode flag j=1, otherwise, set j=0;

[0026] When entering the current limiting mode, the integral control link of the voltage loop PI regulator is frozen, and the output active power P is corrected to obtain the corrected output active power , used for active power P f Perform virtual synchronous control to obtain the corrected angular frequency ;

[0027] when When , it indicates that the symmetrical drop fault of the grid connection point of the PV inverter has been restored.

[0028] The photovoltaic inverter current limiting and fault recovery method of the present invention is also characterized in that the grid-connected point output active power P and output reactive power Q of the photovoltaic inverter are obtained using formula (1);

[0029] (1)

[0030] Furthermore, the active power P after the low-pass filter is obtained using formula (2): f and the reactive power Q after low-pass filtering f ;

[0031] (2)

[0032] In formula (2), ω f is the cutoff frequency of the low-pass filter, and s is the Laplace operator.

[0033] Furthermore, the angular frequency ω and the voltage reference value u are obtained using equations (3) and (4) respectively. ref ;

[0034] (3)

[0035] In formula (3), P ref is the given active power command, ω0 is the sampled grid rated angular frequency, J is the moment of inertia, and D is the damping coefficient;

[0036] (4)

[0037] In formula (4), n is the reactive power droop coefficient, Q ref is the given reactive power instruction, and u is the grid connection point voltage.

[0038] Furthermore, according to formula (5), the d-axis component i of the current command is obtained d,ref and the q-axis component i of the current command q,ref :

[0039] (5)

[0040] In formula (5), K pu is the proportional control coefficient of the voltage loop PI regulator, K iu is the integral control coefficient of the voltage loop PI regulator;

[0041] Furthermore, the inverter output voltage d-axis component u is obtained using formula (6): td And the output voltage q-axis component u tq ;

[0042] (6)

[0043] In formula (6), K pi is the proportional control coefficient of the current loop PI regulator, K ii is the integral control coefficient of the current loop PI regulator, u ref is the voltage reference value.

[0044] Furthermore, if equation (7) holds true, it means that a three-phase voltage symmetrical drop fault occurs at the grid connection point of the photovoltaic inverter, that is, a low voltage ride-through fault occurs;

[0045] (7)

[0046] In formula (7), U n is the amplitude of the rated voltage at the grid connection point; U abc is the amplitude of the three-phase voltage at the grid connection point.

[0047] Furthermore, if equation (8) holds true, it means that the photovoltaic inverter enters the current limiting mode;

[0048] (8)

[0049] In formula (8), I max is the current limit value.

[0050] Furthermore, the corrected output active power is obtained through formula (9): :

[0051] (9)

[0052] In formula (9), represents the phase angle, φ represents the current limiter angle of the photovoltaic inverter, and φ=δ+φ0, where φ0 is the initial current limiter angle of the photovoltaic inverter.

[0053] Furthermore, the corrected angular frequency is obtained using formula (10): ;

[0054] (10)

[0055] In formula (10), k v is the current limiting recovery coefficient.

[0056] Compared with the prior art, the present invention has the following beneficial effects:

[0057] 1. The present invention proposes a photovoltaic inverter current limiting and fault recovery method based on adaptive variable angle. In fault and current limiting modes, the proposed adaptive variable angle current limiter provides overcurrent protection for photovoltaic inverter power electronic devices.

[0058] 2. The present invention provides support for exiting the current limiting mode after fault recovery by correcting the power phase angle relationship, so that the system has a stronger current limiting exit capability.

[0059] 3. The present invention has excellent effects in terms of the limit removal time and the time required for restoration of stability after a fault, especially in that it can quickly restore stability after the fault lasts for a long time.

[0060] 4. The present invention is designed based on the common topology of grid-type photovoltaic inverters, and its implementation is simple and effective. BRIEF DESCRIPTION OF THE DRAWINGS

[0061] Figure 1 This is a structural diagram of a photovoltaic inverter based on an adaptive current limiter according to the present invention.

[0062] Figure 2 This is a flow chart of the fault and current limiting detection of the present invention.

[0063] Figure 3 This is the active power-phase angle correction structure diagram of the present invention.

[0064] Figure 4 This is the active power-phase angle curve using the adaptive variable angle current limiter strategy of the present invention.

[0065] Figure 5 The different k of the present invention v The effect of size on the P-δ curve. DETAILED DESCRIPTION

[0066] The specific embodiments of the present invention will be further described in detail below with reference to the accompanying drawings.

[0067] In this embodiment, the photovoltaic inverter grid-connected system consists of a photovoltaic inverter, an AC grid, a system control circuit, and a current limiting and fault exit circuit. The voltage u at the a, b, and c phase terminals of the photovoltaic inverter grid-connected point is collected. a 、u b 、u c And the output voltage u of phase a, phase b, and phase c of the photovoltaic inverter ta 、u tb 、u tc After transformation, the d-axis component u of the grid connection point voltage is obtained. d , q-axis component u d and the d-axis component u of the PV inverter output voltage td , q-axis component u tq ;

[0068] Sample the a-phase, b-phase, and c-phase currents i injected by the photovoltaic inverter into the grid a 、i b 、i c And the output current i of phase a, phase b, and phase c of the photovoltaic inverter ta 、i tb 、i tc After transformation, the d-axis component i of the grid current is obtained. d , q-axis component i q and the d-axis component i of the current to the photovoltaic invertertd , q-axis component i tq .

[0069] Based on the d-axis component u of the grid connection point voltage d and the q-axis component u of the grid-connected point voltage q , and the d-axis component of the grid current i d and the q-axis component of the grid current i q , get the output active power P and output reactive power Q of the grid-connected point of the photovoltaic inverter;

[0070] Based on the active power P of the grid connection point and the output reactive power Q, the active power P after the low-pass filter is obtained. f and the reactive power Q after low-pass filtering f ;

[0071] like Figure 1 As shown, the photovoltaic inverter grid-connected system control circuit includes power control, voltage control, current control, current limiting, and fault recovery control.

[0072] Active power P f Use virtual synchronous control to obtain angular frequency ω and reactive power Q f Use droop control to obtain the voltage reference value u ref ;

[0073] The voltage and current control both use PI regulator, which is composed of proportional control and integral control. The voltage loop PI regulator controls the d-axis component u of the grid voltage. d and the q-axis component u q Perform voltage control to obtain the d-axis component i of the current command d,ref and the q-axis component i q,ref ;

[0074] The d-axis component i of the grid current is controlled by the current loop PI regulator d , q-axis component i q Control is performed to obtain the d-axis component u of the inverter output voltage td and the q-axis component u tq , and after PWM modulation, generate the a-phase, b-phase, and c-phase switching signals S of the power devices in the photovoltaic inverter a 、S b 、S c , thereby controlling the on and off of the power devices of the photovoltaic inverter.

[0075] like Figure 2 As shown, by sampling the three-phase voltage amplitude of the photovoltaic inverter grid connection point and comparing it with the rated voltage, it is determined whether the photovoltaic inverter grid connection point has a three-phase voltage symmetrical drop fault. If so, set the flag bit i=1, otherwise, set the flag bit i=0;

[0076] When a three-phase voltage symmetrical drop fault occurs, the d-axis component i of the photovoltaic inverter current command is sampled. d,ref and the q-axis component i of the current command q,ref , compared with the current limit value to determine whether the grid-connected point of the photovoltaic inverter enters the current limiting mode. If so, set the current limiting mode flag j=1, otherwise, set j=0;

[0077] When entering the current limiting mode, the integral control of the voltage loop PI regulator is frozen and the output active power P is corrected, such as Figure 3 As shown, the corrected output active power is obtained , used for active power P f Perform virtual synchronous control to obtain the corrected angular frequency ;

[0078] when When , it indicates that the symmetrical drop fault of the grid connection point of the PV inverter has been restored.

[0079] like Figure 4 As shown in the active power-phase angle curve of the present invention, since the current limiting angle changes with the power angle, the current limiter angle of the photovoltaic inverter increases with the increase of the power angle during the fault period. After the fault is recovered, due to inertia, the power angle will continue to increase, and the normal current limiting curve will shift to the right accordingly, and the deceleration area will also increase. The normal current limiting curve will also shift to the left as the power angle decreases until the current limiting mode is exited. Therefore, the system can return to the stable state before the fault, thereby improving transient stability.

[0080] like Figure 5 The present invention is different from v The effect of size on the P-δ curve, as k v Increase, the amplitude of the normal current limiting curve increases, the deceleration area increases accordingly, and the time it takes for the power angle to return to its initial state is reduced. The specific steps of the method of the present invention are as follows:

[0081] Step 1: Sample the voltage u of phase a, phase b, and phase c at the grid connection point of the photovoltaic inverter. a 、u b 、u c And the output voltage u of phase a, phase b, and phase c of the photovoltaic inverter ta 、u tb 、u tc After transformation, the d-axis component i of the grid connection point voltage is obtained. d , q-axis component u d and the d-axis component u of the PV inverter voltage td , q-axis component u tq ;

[0082] Sample the a-phase, b-phase, and c-phase currents i injected by the photovoltaic inverter into the grid a 、i b 、i c And the output current i of phase a, phase b, and phase c of the photovoltaic inverter ta 、i tb 、i tc After transformation, the d-axis component i of the grid current is obtained. d , q-axis component i q and the d-axis component i of the current to the photovoltaic inverter td , q-axis component i tq .

[0083] Step 2: Use formula (1) to obtain the grid-connected point output active power P and output reactive power Q of the photovoltaic inverter;

[0084] (1)

[0085] Step 3: Use formula (2) to obtain the active power P after the low-pass filter f and the reactive power Q after low-pass filtering f ;

[0086] (2)

[0087] In formula (2), ω f is the cutoff frequency of the low-pass filter, and s is the Laplace operator.

[0088] Step 4: Use equations (3) and (4) to obtain the angular frequency ω and voltage reference value u respectively. ref ;

[0089] (3)

[0090] In formula (3), P ref is the given active power command, ω0 is the sampled grid rated angular frequency, J is the moment of inertia, and D is the damping coefficient;

[0091] (4)

[0092] In formula (4), n is the reactive power droop coefficient, Q ref is the given reactive power instruction, and u is the grid connection point voltage.

[0093] Step 5: Obtain the d-axis component i of the current command according to formula (5): d,ref and the q-axis component i of the current command q,ref :

[0094] (5)

[0095] In formula (5), K pu is the proportional control coefficient of the voltage loop PI regulator, K iu is the integral control coefficient of the voltage loop PI regulator.

[0096] Step 6: Use formula (6) to obtain the d-axis component u of the inverter output voltage td And the output voltage q-axis component u tq ;

[0097] (6)

[0098] In formula (6), K pi is the proportional control coefficient of the current loop PI regulator, K ii is the integral control coefficient of the current loop PI regulator, u ref is the voltage reference value.

[0099] Step 7: If equation (7) holds true, it means that a three-phase voltage symmetrical drop fault occurs at the grid connection point of the photovoltaic inverter, that is, a low voltage ride-through fault occurs;

[0100] (7)

[0101] In formula (7), U n is the amplitude of the rated voltage at the grid connection point; U abc is the amplitude of the three-phase voltage at the grid connection point.

[0102] Step 8: If equation (8) holds true, it means that the photovoltaic inverter enters the current limiting mode;

[0103] (8)

[0104] In formula (8), I max is the current limit value.

[0105] Step 9: Get the corrected output active power through formula (9): :

[0106] (9)

[0107] In formula (9), represents the phase angle, φ represents the current limiter angle of the photovoltaic inverter, and φ=δ+φ0, where φ0 is the initial current limiter angle of the photovoltaic inverter.

[0108] Step 10: Use formula (10) to get the corrected angular frequency ;

[0109] (10)

[0110] In formula (10), k v is the current limiting recovery coefficient.

[0111] In this embodiment, an electronic device includes a memory and a processor, wherein the memory is used to store a program that supports the processor to execute the above method, and the processor is configured to execute the program stored in the memory.

[0112] In this embodiment, a computer-readable storage medium stores a computer program, and when the computer program is executed by a processor, the steps of the above method are executed.

Claims

1. A photovoltaic inverter current limiting and fault recovery method based on adaptive variable angle, characterized in that: The method comprises: Sample the voltage u of phase a, phase b, and phase c at the grid connection point of the photovoltaic inverter a 、u b 、u c And the output voltage u of phase a, phase b, and phase c of the photovoltaic inverter ta 、u tb 、u tc After transformation, the d-axis component u of the grid connection point voltage is obtained. d , q-axis component u q and the d-axis component u of the PV inverter output voltage td , q-axis component u tq ; Sample the a-phase, b-phase, and c-phase currents i injected by the photovoltaic inverter into the grid a 、i b 、i c And the output current i of phase a, phase b, and phase c of the photovoltaic inverter ta 、i tb 、i tc After transformation, the d-axis component i of the grid current is obtained. d , q-axis component i q and the d-axis component i of the current to the photovoltaic inverter td , q-axis component i tq ; Based on the d-axis component u of the grid connection point voltage d and the q-axis component u of the grid-connected point voltage q , and the d-axis component of the grid current i d and the q-axis component of the grid current i q , get the output active power P and output reactive power Q of the grid-connected point of the photovoltaic inverter; Based on the active power P of the grid connection point and the output reactive power Q, the active power P after the low-pass filter is obtained. f and the reactive power Q after low-pass filtering f ; For active power P f Perform virtual synchronous control to obtain the angular frequency ω and the reactive power Q f Perform droop control to obtain the voltage reference value u ref ; The d-axis component u of the grid voltage is adjusted by the voltage loop PI regulator d and the q-axis component u q Perform voltage control to obtain the d-axis component i of the current command d,ref and the q-axis component i q,ref ; The d-axis component i of the grid current is adjusted by the current loop PI regulator d , q-axis component i q Control is performed to obtain the d-axis component u of the inverter output voltage td and the q-axis component u tq , and after PWM modulation, generate the a-phase, b-phase, and c-phase switching signals S of the power devices in the photovoltaic inverter a 、S b 、S c , thereby controlling the on and off of the power devices of the photovoltaic inverter; Determine whether a three-phase voltage symmetrical drop fault occurs at the grid connection point of the photovoltaic inverter. If so, set the flag bit i=1; otherwise, set the flag bit i=0; When a three-phase voltage symmetrical drop fault occurs, determine whether the grid-connected point of the photovoltaic inverter enters the current limiting mode. If so, set the current limiting mode flag j=1, otherwise, set j=0; When entering the current limiting mode, the integral control link of the voltage loop PI regulator is frozen, and the output active power P is corrected to obtain the corrected output active power , used for active power P f Perform virtual synchronous control to obtain the corrected angular frequency ; when When , it indicates that the symmetrical drop fault of the grid connection point of the photovoltaic inverter has been restored, where U n is the amplitude of the rated voltage at the grid connection point; U abc is the amplitude of the three-phase voltage at the grid connection point.

2. The photovoltaic inverter current limiting and fault recovery method according to claim 1, characterized in that: Using formula (1), we can get the grid-connected output active power P and reactive power Q of the photovoltaic inverter. (1)。 3. The photovoltaic inverter current limiting and fault recovery method according to claim 2, characterized in that: Using formula (2), we can get the active power P after the low-pass filter: f and the reactive power Q after low-pass filtering f ; (2) In formula (2), ω f is the cutoff frequency of the low-pass filter, and s is the Laplace operator.

4. The photovoltaic inverter current limiting and fault recovery method according to claim 3, characterized in that: Use equations (3) and (4) to obtain the angular frequency ω and voltage reference value u respectively ref ; (3) In formula (3), P ref is the given active power command, ω0 is the sampled grid rated angular frequency, J is the moment of inertia, and D is the damping coefficient; (4) In formula (4), n is the reactive power droop coefficient, Q ref is the given reactive power instruction, and u is the grid connection point voltage.

5. The photovoltaic inverter current limiting and fault recovery method according to claim 4, characterized in that: According to formula (5), the d-axis component i of the current command is obtained d,ref and the q-axis component i of the current command q,ref : (5) In formula (5), K pu is the proportional control coefficient of the voltage loop PI regulator, K iu is the integral control coefficient of the voltage loop PI regulator.

6. The photovoltaic inverter current limiting and fault recovery method according to claim 5, characterized in that: Use formula (6) to obtain the d-axis component u of the inverter output voltage td And the output voltage q-axis component u tq ; (6) In formula (6), K pi is the proportional control coefficient of the current loop PI regulator, K ii is the integral control coefficient of the current loop PI regulator.

7. The photovoltaic inverter current limiting and fault recovery method according to claim 6, characterized in that: If equation (7) holds true, it means that a three-phase voltage symmetrical drop fault occurs at the grid connection point of the photovoltaic inverter, that is, a low voltage ride-through fault occurs; (7)。 8. The photovoltaic inverter current limiting and fault recovery method according to claim 7, characterized in that: If equation (8) holds true, it means that the photovoltaic inverter enters the current limiting mode; (8) In formula (8), I max is the current limit value.

9. The photovoltaic inverter current limiting and fault recovery method according to claim 8, characterized in that: The corrected output active power is obtained by formula (9): : (9) In formula (9), represents the phase angle, φ represents the current limiter angle of the photovoltaic inverter, and φ=δ+φ0, where φ0 is the initial current limiter angle of the photovoltaic inverter.

10. The photovoltaic inverter current limiting and fault recovery method according to claim 9, characterized in that: The corrected angular frequency is obtained using formula (10): ; (10) In formula (10), k v is the current limiting recovery coefficient.

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