Direct-current voltage transient stability control method for grid-connected photovoltaic inverter

Abstract

The application provides a DC voltage transient stability control method of a network-constructed photovoltaic inverter, comprising the following steps: calculating surplus energy of a DC bus capacitor according to a real-time detection value and a set value of the DC voltage, dividing the surplus energy of the capacitor by a time of expected regulation of the capacitor voltage to obtain compensation power of a grid-connected converter, and updating an instruction value of active power of the grid-connected converter by using the compensation power, so as to realize power control of the grid-connected converter and control of the capacitor voltage; during a transient fault, a deviation between an actual value and an instruction value of the DC voltage is controlled by a non-linear gain regulator and superimposed on a phase angle of a modulation voltage of the grid-connected converter, so that the DC side voltage of the grid-connected converter is always controlled within a set boundary range, and transient stability control of the DC voltage is realized. When the inverter works in a maximum power tracking mode or a limited power control mode, the application can realize stable control of the DC side voltage.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of wind power generation technology and power electronic converter control technology in power systems, in particular to a direct-current voltage transient stability control method for grid-forming photovoltaic inverters. BACKGROUND

[0002] In recent years, new energy power sources represented by wind power and photovoltaic power have developed rapidly, and the installed capacity has increased year by year. However, the current wind and photovoltaic new energy is represented by current source characteristics, which does not reflect the inertia of the power grid. The power system is facing a serious shortage of equivalent inertia, and the safety and stability of the power system are facing serious threats.

[0003] Grid-forming photovoltaic inverters have automatic synchronization with the grid and realize real-time control of active power by using a virtual synchronous control method that simulates the rotor motion equation of a synchronous generator. They also have automatic voltage regulation and real-time control of reactive power by using a virtual excitation control method that simulates the excitation regulator of a synchronous generator. They exhibit an active frequency regulation-reactive voltage regulation external characteristic similar to that of a synchronous generator at the output port, and have the ability to autonomously support the grid frequency and voltage. However, due to the small DC side capacitor of the photovoltaic inverter, when a ground fault or short circuit fault occurs in the grid, DC overvoltage or undervoltage may occur, triggering DC voltage fault protection and causing the photovoltaic inverter to shut down, resulting in a loss of active support for the grid. This cannot meet the current national standard requirements for the fault ride-through capability of photovoltaic inverters. To address this problem, the industry currently uses methods such as increasing the capacitor or adding an electrochemical energy storage device to the DC bus, which will significantly increase the cost and size of the photovoltaic inverter. Other solutions focus on optimizing the grid-forming control algorithm of the photovoltaic inverter, such as switching to a grid-following control strategy during a fault, which causes the photovoltaic inverter to lose its voltage source characteristics and still has oscillation instability problems in a weak grid. In addition, the control method of locking the frequency and amplitude of the internal potential of the converter during a fault retains the voltage source characteristics of the grid-forming photovoltaic inverter, but when the phase jumps during grid fault occurrence and recovery, the resulting active power change is likely to cause DC voltage transient instability in the grid-connected converter.

[0004] Patent application document CN104135033A discloses a grid-connected inverter voltage type control method, comprising the following steps: step one: introducing a closed-loop feedback control method into droop control, increasing a power ring, obtaining a translation amount of a droop curve U0 and a translation amount of an inverter rated output frequency f0 after Laplace transformation, step two: superimposing the translation amount of the droop curve U0 and the translation amount of the inverter rated output frequency f0 to a grid-connected droop equation of a new photovoltaic grid-connected inverter voltage type, and obtaining an equation after simplification, and replacing the power ring in the equation obtained after simplification with a DC bus voltage ring to complete the control of the photovoltaic grid-connected inverter on the DC bus voltage. However, the patent cannot completely solve the existing technical problems, and cannot meet the needs of the present application. SUMMARY

[0005] In view of the defects in the prior art, the purpose of the present application is to provide a DC voltage transient stability control method for grid-connected photovoltaic inverters.

[0006] The DC voltage transient stability control method for grid-connected photovoltaic inverters according to the present application comprises: 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 of the expected adjustment of the capacitor voltage to obtain the compensation power of the grid-connected converter, and updating the instruction value of the active power of the grid-connected converter using the compensation power, thereby realizing the control of 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 instruction value of the DC voltage are adjusted by a nonlinear gain regulator to obtain a compensation angle, the compensation angle is superimposed on the angle of the internal potential 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, thereby realizing the transient stability control of the DC voltage.

[0008] The DC voltage transient stability control method for grid-connected photovoltaic inverters is realized by a DC voltage transient stability control system for grid-connected photovoltaic inverters, which comprises: 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 comprises a photovoltaic panel front end, a DC capacitor cell, a grid-connected converter, and an AC filter circuit, the output positive and negative terminals of the photovoltaic panel front end are connected to the positive and negative terminals of the DC capacitor cell and the grid-connected converter respectively, 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.

[0010] The direct-current capacitor steady-state power compensation module is used to realize the stable control of the direct-current capacitor voltage while the grid-connected inverter of the grid-constructing type outputs the generated power in a steady state.

[0011] The direct-current voltage transient-state power compensation module is used to realize the control of the voltage of the direct-current capacitor within a safety boundary during the transient-state fault of the grid-connected inverter of the grid-constructing type.

[0012] 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 of the grid-constructing type.

[0013] The grid-connected converter alternating-current voltage amplitude control module is used to realize the control of the amplitude of the modulation voltage of the output bridge arm of the grid-connected inverter of the grid-constructing type.

[0014] Preferably, the direct-current capacitor steady-state power compensation module is used to realize the stable control of the direct-current capacitor voltage while the grid-connected inverter of the grid-constructing type outputs the generated power in a steady state, and is used to calculate the active power compensation instruction value that needs to be compensated by the grid-connected converter while realizing the stable control of the direct-current capacitor voltage when realizing the grid-connected generated power, and the compensation power instruction value of the grid-connected converter is obtained by dividing the surplus power of the direct-current voltage by the adjustment time setting value of the direct-current voltage, and the calculation formula is as follows:

[0015]

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

[0017] Preferably, the direct-current voltage transient-state power compensation module is used to realize the control of the voltage of the direct-current capacitor within a safety boundary during the transient-state fault of the grid-connected inverter of the grid-constructing type, and is used to calculate the phase angle compensation amount of the modulation voltage of the grid-connected converter after the direct-current voltage exceeds or is lower than the setting value, and the phase angle compensation amount is calculated in real time by the deviation of the actual value and the instruction value of the direct-current voltage through a non-linear amplification element, and the calculation formula is as follows:

[0018] Δθ dc is the compensation angle value reflecting the deviation of the direct-current voltage of the grid-connected converter; G N (s)·K A (U dc -U dcref );

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

[0020] Preferably, the expression of the nonlinear amplification element is:

[0021]

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

[0023] Preferably, the grid-connected converter synchronization angle control module is used to realize the control of the phase of the output bridge arm modulation voltage of the grid-connected inverter, and the rotating angle frequency of the grid-connected converter modulation voltage is generated by proportionally controlling the deviation of the active power and adding the grid frequency value, the rotating angle of the grid-connected converter electrical signal is obtained by integrating the rotating angle frequency and adding the compensation angle reflecting the change of the DC voltage, and the calculation formula is:

[0024]

[0025] wherein ω gsc is the rotating angle frequency of the grid-connected converter voltage signal; ω n is the rated angle 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 rotating angle of the grid-connected converter control signal; s represents the Laplace operator.

[0026] Preferably, 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 the expression is:

[0027]

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

[0029] Preferably, the grid-connected converter AC voltage amplitude control module is used to realize the control of the grid-connected inverter output bridge arm modulation voltage phase, the difference between the grid voltage set value and the actual value is passed through a proportional controller and added to the grid voltage set value to obtain the expected amplitude of the grid-connected converter modulation voltage, and the expression of the expected amplitude of the grid-connected converter modulation voltage is:

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

[0031] wherein U t is the amplitude of the grid-connected converter modulation wave; U sref is the grid voltage set value; U sm is the grid phase voltage amplitude; K V is the modulation difference coefficient of the AC voltage; and 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 application has the following beneficial effects:

[0034] (1) The present application can simplify the control loop of the grid-connected photovoltaic inverter, and when the inverter works in the maximum power tracking mode or the limited power control mode, the stable control of the DC side voltage can be realized at the same time;

[0035] (2) The present application can ensure that the DC side voltage of the grid-connected converter always works in the set boundary range when the grid voltage fault occurs, and especially when the high voltage fault occurs, the minimum value of the DC side voltage is ensured to make the grid-connected converter not have the problem of over-modulation, and the stable operation of the grid-connected converter in the transient state is ensured;

[0036] (3) The present application innovates the control strategy of the grid-connected photovoltaic inverter, adds the surplus power which reacts to the DC bus voltage amplitude to the active power control, and introduces the nonlinear element to directly control the power angle change of the grid-connected converter through the deviation of the DC voltage in the transient state, so as to realize the stable control of the DC voltage in the stable boundary in the transient state and ensure the transient stability of the grid-connected converter. Compared with the traditional control method, the control algorithm of the converter is greatly simplified, and the control performance requirement of the digital controller is reduced. BRIEF DESCRIPTION OF DRAWINGS

[0037] Other characteristics, objects and advantages of the present application will become more apparent from the following detailed description of non-limiting embodiments, made with reference to the accompanying drawings.

[0038] Figure 1 DC voltage transient stability control method schematic diagram for grid-connected photovoltaic inverter;

[0039] Figure 2 DC capacitor steady-state power compensation module control block diagram for grid-connected photovoltaic inverter;

[0040] Figure 3 DC capacitor transient power compensation module control block diagram for grid-connected photovoltaic inverter;

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

[0042] Figure 5 Phase angle control block diagram of grid-connected converter for grid-connected photovoltaic inverter;

[0043] Figure 6 Amplitude control block diagram of grid-connected converter for grid-connected photovoltaic inverter;

[0044] Figure 7a And Figure 7b DC voltage transient control effect diagram before grid-connected photovoltaic inverter adopts the method of the present application;

[0045] Figure 8a And Figure 8b DC voltage transient control effect diagram after grid-connected photovoltaic inverter adopts the method of the present application. DETAILED DESCRIPTION

[0046] The present application will be described in detail below with specific embodiments. The following examples will help those skilled in the art to further understand the present application, but do not limit the present application in any form. It should be noted that for those skilled in the art, without departing from the concept of the present application, a number of changes and improvements can be made. These are within the scope of the present application.

[0047] Embodiments

[0048] According to the DC voltage transient stability control system of the grid-connected photovoltaic inverter provided by the present application, as shown in Figure 1 The grid-connected photovoltaic inverter grid-connected power generation system 100, the DC capacitor steady-state power compensation module 101, the DC voltage transient power compensation module 102, the grid-connected converter synchronous angle control module 103, and the grid-connected converter AC voltage amplitude control module 104.

[0049] More specifically, the photovoltaic inverter grid-connected power generation system 100 comprises a photovoltaic panel front end, a direct current capacitor bank, a grid-connected converter and an alternating current filter circuit, the output positive and negative terminals of the photovoltaic panel front end are connected with the positive and negative terminals of the direct current capacitor bank and the grid-connected converter respectively; the three-phase alternating current output terminals of the grid-connected converter are connected with the three-phase alternating current input terminals of the alternating current filter circuit; the three-phase alternating current output terminals of the alternating current filter circuit are connected with the power grid; wherein the grid-connected converter can be a two-level converter or a three-level converter.

[0050] The direct current capacitor steady-state power compensation module 101 is used to realize the stable control of the direct current capacitor voltage while the grid-connected inverter of the network configuration type outputs the steady-state power generation.

[0051] The direct current voltage transient-state power compensation module 102 is used to realize the control of the voltage of the direct current capacitor within the safety boundary during the transient-state fault of the grid-connected inverter of the network configuration type.

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

[0053] The grid-connected converter alternating current voltage amplitude control module 104 is used to realize the control of the amplitude of the modulation voltage of the output bridge arm of the grid-connected inverter of the network configuration type.

[0054] More specifically, the direct current capacitor steady-state power compensation module 101 adopts: as shown in the formula (1), the surplus power of the direct current voltage is calculated in real time, and the compensation power instruction value of the grid-connected converter is obtained by dividing the expected adjustment time of the direct current voltage, and the calculation formula is: Figure 2 As shown in the formula (2), the deviation of the actual value of the direct current voltage from the instruction value is calculated in real time through a nonlinear amplification element, and the phase angle compensation amount is calculated, and the calculation formula is:

[0055]

[0056] Wherein, ΔP dc is the compensation power instruction value of the grid-connected converter considering the direct current voltage control; C dc is the direct current capacitor value; T dc is the direct current voltage adjustment time setting value; U dcref is the expected direct current voltage setting value; U dc is the actual value of the direct current voltage.

[0057] More specifically, the direct current voltage transient-state power compensation module 102 adopts: as shown in the formula (3), the deviation of the actual value of the direct current voltage from the instruction value is calculated in real time through a nonlinear amplification element, and the phase angle compensation amount is calculated, and the calculation formula is: Figure 3

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

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

[0060] As Figure 4 shown, the expression of the nonlinear amplification element is:

[0061]

[0062] wherein, e is the input of the nonlinear amplification element; Δ H is the upper threshold value of the input error of the nonlinear amplification element; Δ L is the lower threshold value of the input error of the nonlinear amplification element; k H is the input gain coefficient of the nonlinear amplification element when the error is greater than the upper threshold value; k L is the input gain coefficient of the nonlinear amplification element when the error is less than the lower threshold value; k Z is the input gain coefficient of the nonlinear amplification element when the error is between the upper threshold value and the lower threshold value.

[0063] More specifically, the grid-connected converter synchronization angle control module 103 adopts: as Figure 5 shown, the rotating angle frequency of the grid-connected converter modulation voltage is generated by proportional control on the deviation of the active power and the addition of the grid frequency value, the rotating angle of the grid-connected converter electrical signal is obtained by integrating the rotating angle frequency and adding the compensation angle reflecting the change of the DC voltage, and the calculation formula is:

[0064]

[0065] wherein, ω gsc is the rotating angle frequency of the grid-connected converter voltage signal; ω n is the rated angle 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 rotating angle of the grid-connected converter control signal.

[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 the expression is:

[0067]

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

[0069] More specifically, the grid-connected converter AC voltage amplitude control module 104 adopts: as shown in the formula (1), the grid voltage set value and the actual value of the difference through the proportional controller and plus grid voltage set value to get the grid-connected converter modulation voltage expected amplitude, the expression of grid-connected converter modulation voltage expected amplitude is: Figure 6

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

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

[0072] Figure 7a And Figure 7b It is the DC voltage transient control effect diagram of grid-connected photovoltaic inverter before adopting the method of the present application, from the figure, it can be seen that when the grid transient fault is recovered, the grid-connected power appears serious transient overcharge, which leads to the rapid drop of DC bus capacitor voltage, and further leads to the photovoltaic inverter triggering DC under-voltage protection and shutdown;

[0073] Figure 8a And Figure 8b It is the DC voltage transient control effect diagram of grid-connected photovoltaic inverter after adopting the method of the present application, from the figure, it can be seen that, by adopting the method of the present application, since the DC capacitor voltage is controlled rapidly, when the grid transient fault is recovered, serious transient power overshoot does not appear, the DC bus capacitor voltage is also controlled within the expected range, and the photovoltaic inverter can be safely and reliably operated during the transient fault recovery.

[0074] The present application provides a kind of control method of DC voltage transient stability control system of grid-connected photovoltaic inverter, comprising:

[0075] According to the real-time detection value and set value of DC voltage, the surplus energy of DC bus capacitor is calculated, the surplus energy of capacitor is divided by the time of capacitor voltage expected regulation to obtain the compensation power of grid-connected converter, and the compensation power is used to update the instruction value of active power of grid-connected converter, to realize the control of capacitor voltage while realizing the power control of grid-connected converter; ​

[0076] When the power grid has a transient fault, the actual value of the direct current voltage is compensated by a non-linear gain regulator to obtain a compensation angle, the compensation angle is superimposed on the angle of the internal potential modulation voltage of the grid-connected converter, the direct current side voltage of the grid-connected converter is always controlled within the set boundary range, and transient stability control of the direct current voltage is realized.

[0077] Those skilled in the art know that, in addition to implementing the system, device and each module thereof provided by the present application in a pure computer readable program code manner, the same program can also be implemented in the form of logic gates, switches, application specific integrated circuits, programmable logic controllers and embedded microcontrollers by logically programming the method steps. Therefore, the system, device and each module thereof provided by the present application can be considered as a hardware component, and the modules included therein for implementing various programs can also be considered as structures in the hardware component; the modules for implementing various functions can also be considered as both software programs for implementing methods and structures in the hardware component.

[0078] The specific embodiments of the present application are described above. It needs to be understood that the present application is not limited to the specific embodiments described above, and various changes or modifications can be made by those skilled in the art within the scope of the claims, which does not affect the essential content of the present application. The embodiments of the present application and the features in the embodiments can be combined with each other in any manner without conflict.

Claims

1. A method for transient stability control of DC voltage in a grid-connected photovoltaic inverter, characterized in that, include: The surplus energy of the DC bus capacitor is calculated based on the real-time detected value and the set value of the DC voltage. The surplus energy of the capacitor is divided by the expected adjustment time of the capacitor voltage to obtain the compensation power of the grid-connected converter. The command value of the active power of the grid-connected converter is updated using the compensation power, so as to realize the control of the capacitor voltage at the same time as 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 compared with the nonlinear gain regulator to obtain the compensation angle. This compensation angle is then superimposed on the angle of the internal potential modulation voltage of the grid-connected converter, so that 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 for a grid-connected photovoltaic inverter according to claim 1, characterized in that, The DC voltage transient stability control system of the grid-connected photovoltaic inverter is implemented through the DC voltage transient stability control system of the 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 includes a photovoltaic panel front end, a DC capacitor bank, a grid-connected converter, and an AC filter circuit. The positive and negative output terminals of the photovoltaic panel front end are connected to the positive and negative terminals of the DC capacitor bank and the grid-connected converter, respectively. 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. The DC capacitor steady-state power compensation module is used to enable the grid-connected inverter to achieve stable control of the DC capacitor voltage while outputting 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 transient faults in grid-connected inverters. The grid-connected converter synchronization angle control module is used to control the phase of the output bridge arm modulation voltage of the grid-connected inverter. The grid-connected converter AC voltage amplitude control module is used to control the amplitude of the output bridge arm modulation voltage of the grid-connected inverter.

3. The DC voltage transient stability control method for a grid-type photovoltaic inverter according to claim 2, characterized in that, The DC capacitor steady-state power compensation module is used to enable the grid-connected inverter to achieve stable control of the DC capacitor voltage while simultaneously outputting steady-state power generation. It calculates the active power compensation command value required for the grid-connected converter to achieve stable DC capacitor voltage control while generating grid-connected power. The compensation power command value for 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 as follows: Where, ΔP dc To compensate for the DC voltage control of the grid-connected converter; C dc This is the DC capacitance value; T dc This is the DC voltage regulation time setting value; U dcref The desired DC voltage setting; U dc This is the actual value of the DC voltage.

4. The DC voltage transient stability control method for a grid-type photovoltaic inverter according to claim 3, characterized in that, The DC voltage transient power compensation module is used to control the DC capacitor voltage within a safe boundary during transient faults in the grid-connected inverter. It calculates the phase angle compensation amount to the modulated voltage of the grid-connected converter when the DC voltage exceeds or falls below a set value. The phase angle compensation amount is calculated in real time by using a nonlinear amplification element to amplify the deviation between the actual DC voltage and the commanded value. The calculation formula is as follows: Δθ dc =G N (s)·K A (U dc -U dcref ); Where, Δθ dc This is the compensation angle value for the DC voltage deviation of the grid-connected converter; G N (s) is a nonlinear amplification element; K A This is the gain of the angle compensation control loop.

5. The DC voltage transient stability control method for a grid-type photovoltaic inverter according to claim 4, characterized in that, The expression for a nonlinear amplifier element is: Where e is the input of the nonlinear amplifier element; Δ H 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 exceeds 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 This is the input gain coefficient of the nonlinear amplifier when the error is between the upper and lower thresholds.

6. The DC voltage transient stability control method for a grid-connected photovoltaic inverter according to claim 5, characterized in that, The grid-connected converter synchronization angle control module is used to control the phase of the output arm modulation voltage of the grid-connected inverter. It generates the rotation angular frequency of the modulated voltage by proportionally controlling the active power deviation and adding the grid frequency value. The rotation angular frequency is then integrated and a compensation angle reflecting DC voltage changes is added to obtain the rotation angle of the grid-connected converter's electrical signal. The calculation formula is as follows: Where, ω gsc ω is the rotational angular frequency of the voltage regulation signal of the grid-connected converter. n P is the rated angular frequency of the mains voltage; ref P is the active power setpoint for the grid-connected converter. fdbk K is the feedback value of the active power of the grid-connected converter. P θ is the gain of the power control loop. gsc θ represents the rotation angle of the grid-connected converter control signal; s represents the Laplace operator.

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

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

9. The DC voltage transient stability control method for a grid-type 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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