Direct-current voltage transient stability control method of grid-forming type photovoltaic inverter

Abstract

The invention provides a DC voltage transient stability control method for a grid-forming photovoltaic inverter, and the method comprises the steps: calculating the surplus energy of a DC bus capacitor according to a real-time detection value and a set value of DC voltage, dividing the surplus energy of the capacitor by the expected adjustment time of the capacitor voltage, and obtaining the compensation power of a grid-connected converter, updating the instruction value of the active power of the grid-connected converter by using the compensation power, and controlling the capacitor voltage while controlling the power of the grid-connected converter; and during a transient fault period, the deviation between an actual value and an instruction value of the direct-current voltage is controlled and output by a nonlinear gain regulator and is superposed on a phase angle of a modulation voltage of the grid-connected converter, and the direct-current side voltage of the grid-connected converter is always controlled within a set boundary range, so that transient stability control of the direct-current voltage is realized. When the inverter works in the maximum power tracking mode or the limited power control mode, stable control over the direct current side voltage can be achieved at the same time.

Description

Technical Field

[0001] The present invention relates to the technical fields of wind power generation technology and power electronic converter control technology in a power system. Specifically, it relates to a DC voltage transient stability control method for a grid-forming photovoltaic inverter. Background Art

[0002] In recent years, new energy power sources represented by wind power and photovoltaic power have developed rapidly, and their installed capacities have been increasing year by year. However, the current new wind and photovoltaic energy shows a grid-following characteristic with a current source nature, and does not show inertia to the power grid. The power system faces the problem of severely insufficient equivalent inertia, and the safety and stability of the power system are seriously threatened.

[0003] The grid-forming photovoltaic can achieve automatic synchronization with the power grid and real-time control of active power by adopting a virtual synchronous control method that simulates the rotor motion equation of a synchronous generator for its grid-connected inverter, and can achieve automatic voltage regulation and real-time control of reactive power by adopting a virtual excitation control method that simulates the excitation regulator of a synchronous generator. It shows an external characteristic similar to the active frequency modulation - reactive voltage regulation of a synchronous generator at the output port and has the ability to independently support the power grid frequency and voltage. However, due to the small DC-side capacitor of the photovoltaic inverter, when a grounding or short-circuit fault occurs in the power grid, DC overvoltage or undervoltage phenomena are likely to occur, which in turn triggers the DC voltage fault protection and shuts down, resulting in the loss of the active support ability of the photovoltaic inverter to the power grid and not meeting the requirements of the current national standard for the fault uninterrupted operation ability of the photovoltaic inverter. To address this problem, the current industrial approach is to increase the capacitor or add an electrochemical energy storage device to the DC bus, which will lead to a significant increase in the cost and volume of the photovoltaic inverter; other solutions focus on optimizing the grid-forming control algorithm of the photovoltaic inverter. For example, switching to a grid-following control strategy during a fault will cause the loss of the voltage source characteristic of the grid-forming of the photovoltaic inverter, and there is still an oscillation instability problem under an extremely weak power grid; in addition, a control method of locking the frequency and amplitude of the internal potential of the converter during a fault retains the voltage source characteristic of the grid-forming photovoltaic, but when there is a phase jump during the occurrence and recovery of a power grid fault, the resulting change in active power is likely to cause transient instability of the DC voltage of the grid-connected converter.

[0004] The patent application document CN104135033A discloses a grid-connected inverter voltage control method, which includes the following steps: Step 1: Introduce a closed-loop feedback control method into the droop control, add a power loop, and after Laplace transform, obtain the translation amount △U0 of the droop curve U0 and the translation amount △f0 of the rated output frequency f0 of the inverter. Step 2: Respectively superimpose the translation amount △U0 of the droop curve U0 and the translation amount △f0 of the rated output frequency f0 of the inverter onto the grid-connected droop equation of the new type of photovoltaic grid-connected inverter voltage type, and after simplification, obtain an equation, and replace the power loop in the obtained equation after simplification with a DC bus voltage loop to complete the control of the DC bus voltage by the photovoltaic grid-connected inverter. However, this patent cannot completely solve the existing technical problems and cannot meet the requirements of the present invention. Summary of the Invention

[0005] Aiming at the defects in the prior art, the purpose of the present invention is to provide a DC voltage transient stability control method for a grid-forming photovoltaic inverter.

[0006] According to the DC voltage transient stability control method for a grid-forming photovoltaic inverter provided by the present invention, it includes: calculating the surplus energy of the DC bus capacitor according to the real-time detection value and the set value of the DC voltage, dividing the surplus energy of the capacitor by the time when the capacitor voltage is expected to be adjusted to obtain the compensation power of the grid-connected converter, and using the compensation power to update the command value of the active power of the grid-connected converter, so as to control the capacitor voltage while realizing the power control of the grid-connected converter;

[0007] When a transient fault occurs in the power grid, the actual value and the command value of the DC voltage are passed through a regulator with a non-linear gain to obtain a compensation angle, and the compensation angle is superimposed on the angle of the internal potential modulation voltage of the grid-connected converter, so as to always control the DC side voltage of the grid-connected converter within the set boundary range and realize the transient stability control of the DC voltage.

[0008] The DC voltage transient stability control method for the grid-forming photovoltaic inverter is realized through a DC voltage transient stability control system for the grid-forming photovoltaic inverter. The DC voltage transient stability control system for the grid-forming photovoltaic inverter includes: a photovoltaic inverter grid-connected power generation system, a DC capacitor steady-state power compensation module, a DC voltage transient power compensation module, a grid-connected converter synchronous angle control module, and a grid-connected converter AC voltage amplitude control module;

[0009] The photovoltaic inverter grid-connected power generation system includes a front end of a photovoltaic panel, a DC capacitor bank, a grid-connected converter, and an AC filter circuit. The positive and negative output terminals of the front end of the photovoltaic panel are respectively connected to the positive and negative terminals of the DC capacitor bank and the grid-connected converter; the three-phase AC output terminals of the grid-connected converter are connected to the three-phase AC input terminals of the AC filter circuit; the three-phase AC output terminals of the AC filter circuit are connected to the power grid;

[0010] The DC capacitor steady-state power compensation module is used to achieve stable control of the DC capacitor voltage while the grid-forming grid-connected inverter outputs the generated power in the steady state;

[0011] The DC voltage transient power compensation module is used to control the voltage of the DC capacitor within the safe boundary during transient faults of the grid-forming grid-connected inverter;

[0012] The grid-connected converter synchronization angle control module is used to control the phase of the modulation voltage of the output bridge arm of the grid-forming grid-connected inverter;

[0013] The grid-connected converter AC voltage amplitude control module is used to control the amplitude of the modulation voltage of the output bridge arm of the grid-forming grid-connected inverter.

[0014] Preferably, the DC capacitor steady-state power compensation module is used to achieve stable control of the DC capacitor voltage while the grid-forming grid-connected inverter outputs the generated power in the steady state, and is used to calculate the active power compensation command value that needs to be compensated for the grid-connected converter to achieve stable control of the DC capacitor voltage while realizing the grid-connected generated power. By calculating the surplus power of the DC voltage in real time and dividing it by the expected adjustment time of the DC voltage, the compensation power command value of the grid-connected converter is obtained. Its calculation formula is:

[0015]

[0016] where, ΔP dc is the compensation power command value of the grid-connected converter considering DC voltage control; C dc is the DC capacitor value; T dc is the DC voltage adjustment time setting value; U dcref is the expected DC voltage setting value; U dc is the actual value of the DC voltage.

[0017] Preferably, the DC voltage transient power compensation module is used to control the voltage of the DC capacitor within the safe boundary during transient faults of the grid-forming grid-connected inverter, and is used to calculate the phase angle compensation amount compensated to the modulation voltage of the grid-connected converter after the DC voltage exceeds or is lower than the set value. By passing the deviation between the actual value and the command value of the DC voltage through a non-linear amplification element, the phase angle compensation amount is calculated in real time. Its calculation formula is:

[0018] Δθ dc =G N (s)·K A (U dc -U dcref );

[0019] where, Δθ dc is the compensation angle value reflecting the DC voltage deviation of the grid-connected converter; GN (s) is a non - linear amplification element; K A is the gain of the angle compensation control loop.

[0020] Preferably, the expression of the non - linear amplification element is:

[0021]

[0022] where e is the input of the non - linear amplification element; Δ H is the upper threshold of the input error of the non - linear amplification element; Δ L is the lower threshold of the input error of the non - linear amplification element; k H is the input gain coefficient of the non - linear amplification element when the error is greater than the upper threshold; k L is the input gain coefficient of the non - linear amplification element when the error is less than the lower threshold; k Z is the input gain coefficient of the non - linear amplification element when the error is between the upper threshold and the lower threshold.

[0023] Preferably, the grid - connected converter synchronous angle control module is used to control the phase of the modulation voltage of the output bridge arm of the grid - forming grid - connected inverter. By performing proportional control on the deviation of the active power and adding the grid frequency value, the rotational angular frequency of the modulation voltage of the grid - connected converter is generated. Integrating the rotational angular frequency and adding the compensation angle reflecting the change of the DC voltage, the rotational angle of the electrical signal of the grid - connected converter is obtained, and its calculation formula is:

[0024]

[0025] where ω gsc is the rotational angular frequency of the voltage regulation voltage signal of the grid - connected converter; ω n is the rated angular frequency of the grid voltage; P ref is the set value of the active power of the grid - connected converter; P fdbk is the feedback value of the active power of the grid - connected converter; K P is the gain of the power control loop; θ gsc is the rotational angle of the control signal of the grid - connected converter; s represents the Laplace operator.

[0026] Preferably, the feedback value P of the active power of the grid - connected converter fdbk is obtained by low - pass filtering the actual value of the active power of the grid - connected converter, and its expression is:

[0027]

[0028] where P g is the instantaneous value of the active power of the grid - connected converter; T p is the filtering time constant of the active power filter.

[0029] Preferably, the grid-connected converter AC voltage amplitude control module is used to control the phase of the modulation voltage of the output bridge arm of the grid-forming grid-connected inverter. By taking the difference between the grid voltage set value and the actual value through a proportional controller and adding the grid voltage set value, the expected amplitude of the modulation voltage of the grid-connected converter is obtained. The expression for the expected amplitude of the modulation voltage of the grid-connected converter is:

[0030] U t =(U sref -U sm )·K V +U gn ;

[0031] Wherein, U t is the amplitude of the modulation wave of the grid-connected converter; U sref is the grid voltage set value; U sm is the amplitude of the grid phase voltage; K V is the droop coefficient of the AC voltage; U gn is the rated value of the grid voltage.

[0032] Preferably, the grid-connected converter is a two-level converter or a three-level converter.

[0033] Compared with the prior art, the present invention has the following beneficial effects:

[0034] (1) The use of the present invention can simplify the control loop of the grid-forming photovoltaic inverter. When the inverter operates in the maximum power tracking mode or the power limit control mode, it can simultaneously achieve stable control of the DC side voltage;

[0035] (2) The use of the present invention can ensure that the DC side voltage of the grid-connected converter always operates within the set boundary range when a low voltage fault or a high voltage fault occurs in the power grid. Especially during a high voltage fault, by ensuring the minimum value of the DC side voltage, the grid-connected converter does not have an overmodulation problem, ensuring the transient stable operation of the grid-connected converter during the fault;

[0036] (3) Through the innovation of the control strategy of the grid-forming photovoltaic inverter, the present invention adds surplus power reflecting the amplitude of the DC bus voltage to the active power control, and at the same time introduces a non-linear element during transient faults to directly control the power angle change of the grid-connected converter through the deviation of the DC voltage, thereby realizing the transient stable control of the DC voltage within the stable boundary during transient faults to ensure the transient stability of the grid-connected converter. Compared with the traditional control method that requires complex switching operations, the control algorithm of the converter is greatly simplified, and the control performance requirements for the digital controller are reduced. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] By reading the following detailed description of the non-limiting embodiments with reference to the accompanying drawings, other features, objects, and advantages of the present invention will become more apparent:

[0038] Figure 1 Schematic diagram of the DC voltage transient stability control method for a grid-forming PV inverter;

[0039] Figure 2 Control block diagram of the DC capacitor steady-state power compensation module for a grid-forming PV inverter;

[0040] Figure 3 Control block diagram of the DC capacitor transient power compensation module for a grid-forming PV inverter;

[0041] Figure 4 Input-output characteristic curve of a non-linear amplification element;

[0042] Figure 5 Control block diagram of the phase angle of the grid-connected converter of a grid-forming PV inverter;

[0043] Figure 6 Control block diagram of the amplitude of the grid-connected converter of a grid-forming PV inverter;

[0044] Figure 7a and Figure 7b Effect diagram of the DC voltage transient control of a grid-forming PV inverter before adopting the method of the present invention;

[0045] Figure 8a and Figure 8b Effect diagram of the DC voltage transient control of a grid-forming PV inverter after adopting the method of the present invention. Detailed implementation manner

[0046] The present invention will be described in detail below with reference to specific embodiments. The following embodiments will help those skilled in the art to further understand the present invention, but do not limit the present invention in any form. It should be noted that those of ordinary skill in the art can make several changes and improvements without departing from the concept of the present invention. These all belong to the protection scope of the present invention.

[0047] Embodiment

[0048] According to a DC voltage transient stability control system for a grid-forming PV inverter provided by the present invention, as Figure 1 shown, it includes: a PV inverter grid-connected power generation system 100, a DC capacitor steady-state power compensation module 101, a DC voltage transient power compensation module 102, a grid-connected converter synchronous angle control module 103, and a grid-connected converter AC voltage amplitude control module 104.

[0049] More specifically, the photovoltaic inverter grid-connected power generation system 100 includes a front end of a photovoltaic panel, a DC capacitor bank, a grid-connected converter, and an AC filter circuit. The positive and negative output terminals of the front end of the photovoltaic panel are respectively connected to the positive and negative terminals of the DC capacitor bank and the grid-connected converter; the three-phase AC output terminals of the grid-connected converter are connected to the three-phase AC input terminals of the AC filter circuit; the three-phase AC output terminals of the AC filter circuit are connected to the power grid; among them, the grid-connected converter can be a two-level converter or a three-level converter.

[0050] The DC capacitor steady-state power compensation module 101 is used to realize the stable control of the DC capacitor voltage while the grid-forming grid-connected inverter outputs the generated power in the steady state.

[0051] The DC voltage transient power compensation module 102 is used to control the voltage of the DC capacitor within the safe boundary during the transient fault of the grid-forming grid-connected inverter.

[0052] The grid-connected converter synchronization angle control module 103 is used to control the phase of the modulation voltage of the output bridge arm of the grid-forming grid-connected inverter.

[0053] The grid-connected converter AC voltage amplitude control module 104 is used to control the amplitude of the modulation voltage of the output bridge arm of the grid-forming grid-connected inverter.

[0054] More specifically, the DC capacitor steady-state power compensation module 101 adopts: as Figure 2 shown, by calculating the surplus power of the DC voltage in real time and dividing it by the expected adjustment time of the DC voltage to obtain the compensation power command value of the grid-connected converter. Its calculation formula is:

[0055]

[0056] where, ΔP dc is the compensation power command value of the grid-connected converter considering the DC voltage control; C dc is the value of the DC capacitor; T dc is the set value of the DC voltage adjustment time; U dcref is the set value of the expected DC voltage; U dc is the actual value of the DC voltage.

[0057] More specifically, the DC voltage transient power compensation module 102 adopts: as Figure 3 shown, by passing the deviation between the actual value and the command value of the DC voltage through a non-linear amplification element to calculate the phase angle compensation amount in real time. Its calculation formula is:

[0058] Δθ dc =G N (s)·K A (U dc -U dcref );

[0059] Among them, Δθ dc is the compensation angle value for reflecting the DC voltage deviation of the grid-connected converter; G N (s) is a non-linear amplification element; K A is the gain of the angle compensation control loop.

[0060] As Figure 4 shown, the expression of the non-linear amplification element is:

[0061]

[0062] Among them, e is the input of the non-linear amplification element; Δ H is the upper threshold of the input error of the non-linear amplification element; Δ L is the lower threshold of the input error of the non-linear amplification element; k H is the input gain coefficient of the non-linear amplification element when the error is greater than the upper threshold; k L is the input gain coefficient of the non-linear amplification element when the error is less than the lower threshold; k Z is the input gain coefficient of the non-linear amplification element when the error is between the upper threshold and the lower threshold.

[0063] More specifically, the grid-connected converter synchronization angle control module 103 adopts: As Figure 5 shown, by performing proportional control on the deviation of the active power and adding the grid frequency value to generate the rotational angular frequency of the modulation voltage of the grid-connected converter, integrating the rotational angular frequency and adding the compensation angle reflecting the DC voltage change to obtain the rotational angle of the electrical signal of the grid-connected converter, and its calculation formula is:

[0064]

[0065] Among them, ω gsc is the rotational angular frequency of the voltage regulation voltage signal of the grid-connected converter; ω n is the rated angular frequency of the grid voltage; P ref is the set value of the active power of the grid-connected converter; P fdbk is the feedback value of the active power of the grid-connected converter; K P is the gain of the power control loop; θ gsc is the rotational angle of the control signal of the grid-connected converter.

[0066] The feedback value P fdbk of the active power of the grid-connected converter is obtained by low-pass filtering the actual value of the active power of the grid-connected converter, and its expression is:

[0067]

[0068] Among them, P gis the instantaneous value of the active power of the grid-connected converter; T p is the filtering time constant of the active power filter.

[0069] More specifically, the grid-connected converter AC voltage amplitude control module 104 adopts: as Figure 6 shown, by taking the difference between the grid voltage set value and the actual value through a proportional controller and adding the grid voltage set value to obtain the expected amplitude of the grid-connected converter modulation voltage, the expression of the expected amplitude of the grid-connected converter modulation voltage is:

[0070] U t =(U sref -U sm )·K V +U gn ;

[0071] wherein, U t is the amplitude of the modulation wave of the grid-connected converter; U sref is the grid voltage set value; U sm is the amplitude of the grid phase voltage; K V is the droop coefficient of the AC voltage; U gn is the rated value of the grid voltage.

[0072] Figure 7a and Figure 7b are the DC voltage transient control effect diagrams of the grid-forming PV inverter before adopting the method of the present invention. It can be seen from the figure that when the grid transient fault is restored, the grid-connected power appears serious transient overcharge, resulting in a rapid drop in the DC bus capacitor voltage, and then causing the PV inverter to trigger DC under-voltage protection and shut down;

[0073] Figure 8a and Figure 8b are the DC voltage transient control effect diagrams of the grid-forming PV inverter after adopting the method of the present invention. It can be seen from the figure that by adopting the method of the present invention, due to the fast power compensation control of the DC capacitor voltage, when the grid transient fault is restored, there is no serious transient power overshoot, and the DC of the DC bus capacitor voltage is also controlled within the expected range, and the PV inverter can operate safely and reliably during the transient fault recovery.

[0074] The present invention provides a control method for a DC voltage transient stability control system of a grid-forming PV inverter, including:

[0075] Calculating the surplus energy of the DC bus capacitor according to the real-time detection value and the set value of the DC voltage, dividing the surplus energy of the capacitor by the time for the expected regulation of the capacitor voltage to obtain the compensation power of the grid-connected converter, and updating the command value of the active power of the grid-connected converter by using the compensation power, so as to control the capacitor voltage while realizing the power control of the grid-connected converter;

[0076] When a transient fault occurs in the power grid, the actual value of the DC voltage and the command value are used to obtain a compensation angle through a regulator with non-linear gain. This compensation angle is superimposed on the angle of the internal electromotive force modulation voltage of the grid-connected converter, and the DC-side voltage of the grid-connected converter is always controlled within the set boundary range to achieve transient stability control of the DC voltage.

[0077] Those skilled in the art know that in addition to implementing the systems, devices, and their respective modules provided by the present invention in the form of pure computer-readable program code, the method steps can be logically programmed to enable the systems, devices, and their respective modules provided by the present invention to be implemented in the form of logic gates, switches, application-specific integrated circuits, programmable logic controllers, and embedded microcontrollers, etc., to implement the same program. Therefore, the systems, devices, and their respective modules provided by the present invention can be considered as a kind of hardware component, and the modules included therein for implementing various programs can also be regarded as the structures within the hardware component; the modules for implementing various functions can also be regarded as either software programs for implementing the method or the structures within the hardware component.

[0078] The specific embodiments of the present invention have been described above. It should be understood that the present invention is not limited to the above specific embodiments, and those skilled in the art can make various changes or modifications within the scope of the claims, which do not affect the essence of the present invention. Without conflict, the embodiments of the present application and the features in the embodiments can be arbitrarily combined with each other.

Claims

1. A method for controlling transient stability of direct current voltage of a grid-connected photovoltaic inverter, characterized in that: include: The surplus energy of the DC bus capacitor is calculated according to the real-time detection value and the set value of the DC voltage, and the compensation power of the grid-connected converter is obtained by dividing the surplus energy of the capacitor by the expected adjustment time of the capacitor voltage. The command value of the active power of the grid-connected converter is updated by using the compensation power, so as to realize the control of the capacitor voltage while realizing the power control of the grid-connected converter; When a transient fault occurs in the power grid, the actual value of the DC voltage and the command value are used to obtain a compensation angle through a nonlinear gain regulator, and the compensation angle is superimposed on the angle of the internal potential modulation voltage of the grid-connected converter. The DC side voltage of the grid-connected converter is always controlled within the set boundary range, thereby achieving transient stability control of the DC voltage.

2. The DC voltage transient stability control method of the grid-connected photovoltaic inverter according to claim 1 is characterized in that: It is realized by a DC voltage transient stability control system of a grid-connected photovoltaic inverter, which includes: a photovoltaic inverter grid-connected power generation system, a DC capacitor steady-state power compensation module, a DC voltage transient power compensation module, a grid-connected converter synchronization angle control module, and a grid-connected converter AC voltage amplitude control module; The photovoltaic inverter grid-connected power generation system comprises a photovoltaic panel front end, a DC capacitor pool, a grid-connected converter and an AC filter circuit, wherein the output positive and negative terminals of the photovoltaic panel front end are respectively connected to the DC capacitor pool and the positive and negative terminals of the grid-connected converter; the three-phase AC output terminals of the grid-connected converter are connected to the three-phase AC input terminals of the AC filter circuit; and the three-phase AC output terminals of the AC filter circuit are connected to the power grid; The DC capacitor steady-state power compensation module is used to achieve stable control of the DC capacitor voltage while the grid-connected inverter outputs power in a steady state; The DC voltage transient power compensation module is used to control the voltage of the DC capacitor within a safe boundary during a transient fault of the grid-connected inverter; The grid-connected converter synchronization angle control module is used to realize the control of the modulation voltage phase of the output bridge arm of the grid-connected inverter; The grid-connected converter AC voltage amplitude control module is used to realize the control of the output bridge arm modulation voltage amplitude of the grid-connected inverter.

3. The DC voltage transient stability control method of the grid-connected photovoltaic inverter according to claim 2 is characterized in that: The DC capacitor steady-state power compensation module is used to realize the stable control of the DC capacitor voltage while the grid-connected inverter outputs the generated power in the steady state, and is used to calculate the active power compensation command value that needs to be compensated for the DC capacitor voltage when the grid-connected converter realizes the grid-connected generated power and the stable control of the DC capacitor voltage. The compensation power command value of the grid-connected converter is obtained by calculating the surplus power of the DC voltage in real time and dividing it by the expected adjustment time of the DC voltage. The calculation formula is: Where ΔP dc The compensation power command value for the grid-connected converter taking into account the DC voltage control; C dc is the DC capacitance value; T dc is the DC voltage regulation time setting value; U dcref is the desired DC voltage setting value; U dc is the actual value of the DC voltage.

4. The DC voltage transient stability control method of the grid-connected photovoltaic inverter according to claim 3 is characterized in that: The DC voltage transient power compensation module is used to control the voltage of the DC capacitor within the safety boundary during the transient fault period of the grid-connected inverter, and is used to calculate the phase angle compensation amount of the modulation voltage of the grid-connected converter after the DC voltage exceeds or falls below the set value. The phase angle compensation amount is calculated in real time by passing the deviation between the actual value of the DC voltage and the command value through a nonlinear amplification element. The calculation formula is: Δθ dc =G N (s)·K A (U dc -U dcref ); Among them, Δθ dc G is the compensation angle value of the DC voltage deviation of the grid-connected converter; N (s) is a nonlinear amplification element; K A is the gain of the angle compensation control loop.

5. The DC voltage transient stability control method of the grid-connected photovoltaic inverter according to claim 4 is characterized in that: The expression of the nonlinear amplification element is: Where, e is the input of the nonlinear amplifier element; Δ H is the upper threshold of the input error of the nonlinear amplifier element; Δ L k is the lower threshold of the input error of the nonlinear amplifier element; H k is the input gain coefficient of the nonlinear amplifier element when the error is greater than the upper threshold; L k is the input gain coefficient of the nonlinear amplifier element when the error is less than the lower threshold; Z is the input gain coefficient of the nonlinear amplifier element when the error is between the upper threshold and the lower threshold.

6. The DC voltage transient stability control method of the grid-connected photovoltaic inverter according to claim 5 is characterized in that: The grid-connected converter synchronization angle control module is used to realize the control of the phase of the modulation voltage of the output bridge arm of the grid-connected inverter. The rotation angle frequency of the modulation voltage of the grid-connected converter is generated by proportionally controlling the deviation of the active power and adding the grid frequency value. The rotation angle frequency is integrated and a compensation angle reflecting the change of the DC voltage is added to obtain the rotation angle of the electrical signal of the grid-connected converter. The calculation formula is: Among them, ω gsc is the rotation angular frequency of the voltage regulation signal of the grid-connected converter; ω n is the rated angular frequency of the grid voltage; P ref is the active power setting value of the grid-connected converter; P fdbk is the feedback value of the active power of the grid-connected converter; K P is the gain of the power control loop; θ gsc is the rotation angle of the grid-connected converter control signal; s represents the Laplace operator.

7. The DC voltage transient stability control method of the grid-connected photovoltaic inverter according to claim 6, characterized in that: Feedback value P of active power of grid-connected converter fdbk The actual value of the active power of the grid-connected converter is obtained after low-pass filtering, and its expression is: Among them, P g is the instantaneous value of active power of the grid-connected converter; T p is the filtering time constant of the active power filter.

8. The DC voltage transient stability control method of the grid-connected photovoltaic inverter according to claim 7, characterized in that: The grid-connected converter AC voltage amplitude control module is used to realize the control of the modulation voltage phase of the output bridge arm of the grid-connected inverter. The expected amplitude of the modulation voltage of the grid-connected converter is obtained by adding the difference between the grid voltage setting value and the actual value through a proportional controller and the grid voltage setting value. The expression of the expected amplitude of the modulation voltage of the grid-connected converter is: IN t =(U sref -IN sm )·K V +U gn ; Among them, U t is the amplitude of the modulation wave of the grid-connected converter; U sref is the grid voltage setting value; U sm is the grid phase voltage amplitude; K V is the regulation coefficient of AC voltage; U gn is the rated value of the grid voltage.

9. The DC voltage transient stability control method of a grid-connected photovoltaic inverter according to claim 2, characterized in that: The grid-connected converter is a two-level converter or a three-level converter.

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