Quasi-Z-source photovoltaic inversion direct-current chain voltage control method based on second-order sliding mode

Abstract

The invention relates to the field of quasi-Z-source direct-current chain voltage control, in particular to a quasi-Z-source photovoltaic inversion direct-current chain voltage control method based on a second-order sliding mode. The method comprises the following steps: firstly, establishing a large signal model of the quasi-Z-source inverter according to a three-phase quasi-Z-source inverter topological structure, and establishing a state space model according to the large signal model; then, according to the state space model, defining error functions, including an inductive current error function and a capacitor voltage error function; thirdly, constructing a first-order sliding mode surface according to the error function, and introducing an integral term to obtain an improved first-order sliding mode surface; then, according to the improved first-order sliding mode surface, an improved second-order sliding mode surface is constructed; and finally, according to the improved second-order sliding mode surface, solving to obtain a final improved second-order sliding mode equivalent control law.

Description

Technical Field

[0001] The present invention relates to the field of quasi-Z source DC link voltage control, and in particular to a quasi-Z source photovoltaic inverter DC link voltage control method based on a second-order sliding mode. Background Art

[0002] Because the DC voltage generated by solar photovoltaic systems fluctuates significantly, quasi-Z-source inverters have gained widespread application in the photovoltaic energy sector due to their ability to step up or down, a wide output voltage adjustment range, minimal output waveform distortion, and tolerance to inverter bridge shorts or open circuits. However, traditional quasi-Z-source DC link voltage control methods include indirect control and PID controllers. Indirect control techniques can suffer from slow response and insufficient control accuracy, while PID controllers, due to their difficult parameter adjustment, can struggle to adapt to rapidly changing dynamic systems.

[0003] In addition, the DC link voltage sliding mode control designed later has a slow output response and a large voltage overshoot because it does not have a current integration part. Therefore, designing a second-order sliding mode control method will speed up the dynamic response of the converter, reduce the voltage overshoot, and thus improve the tracking signal performance. However, integral sliding mode control will cause the system to have oscillation problems introduced by high-frequency regulation signals.

[0004] In this context, a quasi-Z-source photovoltaic inverter DC link voltage control method based on second-order sliding mode is proposed, which can not only ensure the dynamic response speed of the converter, but also reduce the voltage overshoot and have better tracking signal performance. At the same time, the second-order sliding mode can more effectively suppress the system output oscillation. Summary of the Invention

[0005] The present invention provides a quasi-Z-source photovoltaic inverter DC link voltage control method based on second-order sliding mode, which is used to solve the above-mentioned technical problems. First, based on the three-phase quasi-Z-source inverter topology, a large-signal model of the quasi-Z-source inverter is established, and based on the large-signal model, a state-space model is established; then, based on the state-space model, an error function is defined, including an inductor current error function and a capacitor voltage error function; then, based on the error function, a first-order sliding mode surface is constructed, and an integral term is introduced to obtain an improved first-order sliding mode surface; then, based on the improved first-order sliding mode surface, an improved second-order sliding mode surface is constructed; finally, based on the improved second-order sliding mode surface, the final improved second-order sliding mode equivalent control law is solved.

[0006] A quasi-Z-source photovoltaic inverter DC link voltage control method based on a second-order sliding mode comprises the following steps:

[0007] S1. Establishing a large signal model of the quasi-Z-source inverter according to the topology of the three-phase quasi-Z-source inverter, and establishing a state space model according to the large signal model;

[0008] S2. defining an error function according to the state-space model, including an inductor current error function and a capacitor voltage error function;

[0009] S3. Constructing a first-order sliding surface based on the error function, and introducing an integral term to obtain an improved first-order sliding surface, and constructing an improved second-order sliding surface based on the improved first-order sliding surface;

[0010] S4. According to the improved second-order sliding mode surface, a final improved second-order sliding mode equivalent control law is obtained.

[0011] Furthermore, according to the topology of the three-phase quasi-Z-source inverter, a large signal model of the quasi-Z-source inverter is established. The large signal model is:

[0012] The large signal model of the through state is:

[0013]

[0014] Among them, i L1 is the current flowing through the first inductor, for i L1 The first derivative of L2 is the current flowing through the second inductor, for i L2 The first derivative of u c1 is the voltage across the first capacitor, for u c1 The first derivative of u c2 is the voltage across the second capacitor, for u c2 The first-order derivative of , L represents the inductance value, and L1=L2=L, L1 is the first inductor in the quasi-Z source inverter, L2 is the second inductor in the quasi-Z source inverter, C represents the capacitance value, and C1=C2=C, C1 is the first capacitor in the quasi-Z source inverter, and C2 is the second capacitor in the quasi-Z source inverter;

[0015] The large signal model in the non-through state is:

[0016]

[0017] Among them, i L1 is the current flowing through the inductor L1, for i L1 The first derivative of L2 is the current flowing through the inductor L2, for iL2 The first derivative of u c1 is the voltage across capacitor C1, for u c1 The first derivative of u c2 is the voltage across capacitor C2, for u c2 The first-order derivative of , L represents the inductance value, and L1=L2=L, C represents the capacitance value, and C1=C2=C, V0 is the DC input voltage, i dc is the DC link current.

[0018] Furthermore, a state space model is established based on the large signal model;

[0019]

[0020] in, A and B are represented as:

[0021]

[0022] Among them, u is the system input, expressed as:

[0023]

[0024] Among them, i L1 is the current flowing through the inductor L1, for i L1 The first derivative of L2 is the current flowing through the inductor L2, for i L2 The first derivative of u c1 is the voltage across capacitor C1, for u c1 The first derivative of u c2 is the voltage across capacitor C2, for u c2 The first derivative of , L represents the inductance value, and L1 = L2 = L, C represents the capacitance value, and C1 = C2 = C, i dc is the DC link current, and V0 is the DC input voltage.

[0025] Furthermore, according to the state-space model, an error function is defined, including an inductor current error function and a capacitor voltage error function; the error function is:

[0026] Inductor current error function:

[0027]

[0028] Among them, e1 is the inductor current error, i L1is the current flowing through the inductor L1, is the current i L1 Reference current:

[0029] Capacitor voltage error function:

[0030]

[0031] Among them, e2 is the capacitor voltage error, u C1 is the current flowing through the inductor C1, is the voltage u C1 The reference voltage;

[0032] and Expressed as:

[0033]

[0034] Among them, K i is the integral coefficient, K p is the proportional coefficient, D is the pass-through duty cycle, and V0 is the DC input voltage.

[0035] Furthermore, a first-order sliding mode surface is constructed according to the error function, and the first-order sliding mode surface is:

[0036] s=λ1e1+λ2e2;

[0037] Wherein, s is the first-order sliding mode surface, λ1 and λ2 are parameters to be designed, e1 is the inductor current error, and e2 is the capacitor voltage error function.

[0038] Furthermore, an integral term is introduced to obtain an improved first-order sliding mode surface, which is:

[0039] s1=λ1e1+λ2e2+λ3∫(e1+e2)dt;

[0040] Wherein, s1 is the improved first-order sliding mode surface, λ1, λ2 and λ3 are parameters to be designed, e1 is the inductor current error, and e2 is the capacitor voltage error function.

[0041] Furthermore, based on the improved first-order sliding surface, an improved second-order sliding surface is constructed, and the improved second-order sliding surface is:

[0042]

[0043] Wherein, s1 is the improved first-order sliding surface, is the first-order derivative of s1, and α is the parameter to be designed.

[0044] Furthermore, according to the improved second-order sliding mode surface, the final improved second-order sliding mode control law is solved, and the final improved second-order sliding mode equivalent control law is:

[0045]

[0046] Among them, u eq is the equivalent control law, is the current flowing through the inductor L1, for i L1 The first derivative of L2 is the current flowing through the inductor L2, for i L2 The first derivative of u c1 is the voltage across capacitor C1, for u c1 The first derivative of u c2 is the voltage across capacitor C2, for u c2 The first derivative of , L represents the inductance value, and L1 = L2 = L, C represents the capacitance value, and C1 = C2 = C, i dc is the DC link current, s1 is the improved first-order sliding surface, e1 is the inductor current error, e2 is the capacitor voltage error function, λ1, λ2, λ3 and α are the parameters to be designed, K i is the integral coefficient, K p is the proportional coefficient.

[0047] Compared with the prior art, the present invention has the following beneficial effects:

[0048] 1. Establishing a large signal model can provide a more intuitive understanding of the dynamic response relationship between input and output of a nonlinear system, thereby improving the accuracy and speed of the control strategy. The large signal model can also be used to predict and optimize the performance of the inverter. It can also be used to analyze the dynamic behavior of the system under large signal disturbances, thereby improving the reliability of the system.

[0049] 2. By applying second-order sliding mode control, the quasi-Z-source inverter system has a stronger ability to resist load disturbances; and by expanding the sliding variable in the sliding mode control from 1 to 2, it can effectively weaken the vibration, thereby improving the robustness and stability of the system.

[0050] 3. The sliding mode control introduces the integral term, which can eliminate the steady-state error of the system, thereby ensuring that the system output can reach the sliding mode surface quickly and accurately, thereby improving the response speed and control accuracy of the system; it can also improve the transient performance and robustness of the system, thereby ensuring the stability and accuracy of the system output. BRIEF DESCRIPTION OF THE DRAWINGS

[0051] Figure 1 It is a schematic flow diagram of the present invention;

[0052] Figure 2 It is a three-phase quasi-Z source inverter topology;

[0053] Figure 3 This is the capacitor voltage waveform change caused by load changes under the improved second-order sliding mode controller;

[0054] Figure 4 This is a diagram showing the capacitor voltage waveform changes caused by load changes under a traditional sliding mode controller. DETAILED DESCRIPTION

[0055] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0056] With the rapid development of the photovoltaic power generation industry, quasi-Z-source photovoltaic inverters are an indispensable component. However, the DC link voltage output has problems such as slow response speed and insufficient control accuracy, and there is an oscillation problem, which will affect the stable output and application efficiency of the photovoltaic power generation system. To this end, the present invention provides a quasi-Z-source photovoltaic inverter DC link voltage control method based on second-order sliding mode. By adopting a second-order sliding mode control method and introducing an integral term, this method not only effectively optimizes the stability and control accuracy of the system, but also improves the response speed and anti-interference performance of the system, and can effectively suppress oscillations. In order to illustrate the feasibility of the present invention, two specific embodiments will be described below.

[0057] Example 1

[0058] In the embodiment of the present application, a quasi-Z source photovoltaic inverter DC link voltage control method based on second-order sliding mode is adopted; the specific implementation process is as follows Figure 1 As shown, first, according to the three-phase quasi-Z-source inverter topology, a large signal model of the quasi-Z-source inverter is established, and based on the large signal model, a state space model is established; then, according to the state space model, an error function is defined, including an inductor current error function and a capacitor voltage error function; then, according to the error function, a first-order sliding mode surface is constructed, and an integral term is introduced to obtain an improved first-order sliding mode surface; then, according to the improved first-order sliding mode surface, an improved second-order sliding mode surface is constructed; finally, according to the improved second-order sliding mode surface, the final improved second-order sliding mode equivalent control law is obtained.

[0059] Furthermore, the specific implementation process of the embodiment of the present application is as follows: In the embodiment of the present application, based on the photovoltaic cell simulation system, the parameter values ​​of the photovoltaic module are: rated power of 300W, voltage at maximum power of 38.2V, current at maximum power of 7.853A, open circuit voltage of 46.5V, short circuit current of 8.21, the number of series-connected battery cells in the component is 54, and the number of parallel-connected photovoltaic modules is 1; the parameter selection of the quasi-Z source inverter is: input voltage V0 = 43.25V, output capacitor voltage V C =75.68V, output power P=180W, duty cycle is DC link voltage peak V dc =108.125V, the source inductance of Z is L=13.65×10 -4 H, Z source capacitance is C = 3.12 × 10 -4 F.

[0060] Further, according to Figure 2 The three-phase quasi-Z-source inverter topology is shown, and a large signal model of the quasi-Z-source inverter is established. The large signal model is:

[0061] The large signal model of the through state is:

[0062]

[0063] Among them, i L1 is the current flowing through the inductor L1, for i L1 The first-order derivative of iL2 is the current flowing through the inductor L2. for i L2 The first derivative of u c1 is the voltage across capacitor C1, for u c1 The first derivative of u c2 is the voltage across capacitor C2, for u c2 The first derivative of ;

[0064] The large signal model in the non-through state is:

[0065]

[0066] Among them, i L1 is the current flowing through the inductor L1, for i L1 The first derivative of L2 is the current flowing through the inductor L2, for i L2 The first derivative of u c1 is the voltage across capacitor C1, for uc1 The first derivative of u c2 is the voltage across capacitor C2, for u c2 The first derivative of dc is the DC link current.

[0067] In this embodiment, a large-signal model of the quasi-Z-source inverter in both the direct-flow and non-direct-flow states is established based on the topology of a three-phase quasi-Z-source inverter. This model describes the voltage and current behavior of the quasi-Z-source inverter in both direct-flow and non-direct-flow states. The output voltage can be controlled by adjusting the time proportion of the direct-flow state, thereby achieving a voltage boost. Furthermore, in the direct-flow state, the impedance network stores energy, allowing for wide-range regulation of the DC link voltage, thereby enhancing system reliability.

[0068] Furthermore, a state space model is established based on the large signal model;

[0069]

[0070] in, A and B are represented as:

[0071]

[0072] Among them, u is the system input, expressed as:

[0073]

[0074] Among them, i L1 is the current flowing through the inductor L1, for i L1 The first-order derivative of iL2 is the current flowing through the inductor L2. for i L2 The first derivative of u c1 is the voltage across capacitor C1, for u c1 The first derivative of u c2 is the voltage across capacitor C2, for u c2 The first derivative of dc is the DC link current.

[0075] In the embodiments of this application, a state-space model is established based on the large-signal model, accurately describing the dynamic behavior of the system under various operating conditions. This state-space model enables analysis of the system's controllability, observability, and stability, thereby ensuring system performance. It also enables the design of more precise control laws, further ensuring system performance and reliability.

[0076] Furthermore, according to the state-space model, an error function is defined, including an inductor current error function and a capacitor voltage error function; the error function is:

[0077] Inductor current error function:

[0078]

[0079] Among them, e1 is the inductor current error, i L1 is the current flowing through the inductor L1, is the current i L1 Reference current:

[0080] Capacitor voltage error function:

[0081]

[0082] Among them, e2 is the capacitor voltage error, u C1 is the current flowing through the inductor C1, is the voltage u c1 The reference voltage;

[0083] and Expressed as:

[0084]

[0085] Among them, K i is the integral coefficient, K p is the proportional coefficient.

[0086] In this embodiment, error functions for the inductor current and capacitor voltage are defined based on the state-space model. These describe the difference between the inductor current and the desired current, and the capacitor voltage and the desired voltage. By calculating the inductor current error and capacitor voltage error in real time, system deviations can be adjusted promptly and quickly, ensuring the stability of the quasi-Z-source inverter's output voltage and improving system control accuracy.

[0087] Furthermore, a first-order sliding mode surface is constructed according to the error function, and the first-order sliding mode surface is:

[0088] s=λ1e1+λ2e2;

[0089] Wherein, s is the first-order sliding mode surface, λ1 and λ2 are parameters to be designed, e1 is the inductor current error, and e2 is the capacitor voltage error function.

[0090] In the embodiments of the present application, a first-order sliding surface is constructed based on the inductor current error and capacitor voltage error. This allows the system state to be guided onto the sliding surface and maintained there after reaching it. The system state can converge to an equilibrium point or desired trajectory within a finite time, thereby achieving rapid and stable control of the system.

[0091] Furthermore, an integral term is introduced to obtain an improved first-order sliding mode surface, which is:

[0092] s1=λ1e1+λ2e2+λ3∫(e1+e2)dt;

[0093] Wherein, s1 is the improved first-order sliding mode surface, λ1, λ2 and λ3 are parameters to be designed, e1 is the inductor current error, and e2 is the capacitor voltage error function.

[0094] In an embodiment of the present application, an integral term is introduced into the first-order sliding surface to obtain an improved first-order sliding surface. By introducing the integral term, the steady-state error of system tracking is better eliminated, thereby improving the control accuracy of the system and ensuring the output voltage of the quasi-Z-source inverter; the system state can converge to the equilibrium point or the desired trajectory within a limited time, thereby achieving rapid and stable control of the system.

[0095] Furthermore, based on the improved first-order sliding surface, an improved second-order sliding surface is constructed, and the improved second-order sliding surface is:

[0096]

[0097] Wherein, s1 is the improved first-order sliding surface, is the first-order derivative of s1, and α is the parameter to be designed.

[0098] In the embodiment of the present application, a second-order sliding surface is constructed. By introducing the second-order derivative term, the system chattering phenomenon is effectively reduced and the control accuracy is improved. The second-order sliding mode control also inherits the advantages of the first-order sliding mode in being insensitive to parameter changes and external disturbances. Moreover, the second-order sliding surface enables the system to have faster convergence speed and better stability when approaching the sliding surface, thereby effectively improving the stability of the output voltage of the quasi-Z-source inverter.

[0099] Furthermore, according to the improved second-order sliding mode surface, the final improved second-order sliding mode control law is solved, and the final improved second-order sliding mode equivalent control law is:

[0100]

[0101] Among them, u eq is the equivalent control law, i L1 is the current flowing through the inductor L1, for i L1 The first derivative of L2 is the current flowing through the inductor L2, for i L2 The first derivative of u c1 is the voltage across capacitor C1, for u c1 The first derivative of u c2 is the voltage across capacitor C2, for u c2 The first derivative of dc is the DC link current, s1 is the improved first-order sliding surface, e1 is the inductor current error, e2 is the capacitor voltage error function, λ1, λ2, λ3 and α are the parameters to be designed, K i is the integral coefficient, K p is the proportional coefficient.

[0102] In the embodiment of the present application, the final improved second-order sliding mode control law is obtained. By introducing the second-order derivative of the control quantity, the second-order sliding mode control not only retains the advantage of the first-order sliding mode control that is insensitive to parameter changes and external disturbances, but also significantly improves the robustness and dynamic performance of the system; moreover, by designing the dynamic sliding mode surface, the system state can be quickly and accurately guided to the desired trajectory, thereby improving the convergence speed of the system and ensuring the system's ability to suppress chattering, thereby improving the tracking accuracy and stability of the system.

[0103] Example 2

[0104] In the embodiment of the present application, the load applied to the system of the first embodiment is changed to observe the waveform changes of the capacitor voltage under the action of the improved second-order sliding mode controller and the traditional sliding mode controller. Figure 3 To improve the second-order sliding mode controller, at 0.5s, the capacitor voltage waveform changes after the load changes from 10.6Ω to 5.35Ω; Figure 4 Under the traditional sliding mode controller, at 0.5s, the capacitor voltage waveform changes after the load changes from 10.6Ω to 5.35Ω.

[0105] These two figures show the impact of two different controllers (improved second-order sliding mode controller and traditional sliding mode controller) on the capacitor voltage when the load changes. Figure 3 To improve the second-order sliding-mode controller, the capacitor voltage waveform changes after the load changes from 10.6Ω to 5.35Ω at 0.5s. The voltage begins to drop from 108.1V at 0.5s, reaches its lowest point at 0.498s, then gradually recovers. At 0.583s, the voltage stabilizes at around 105.1V, demonstrating good stability, minimal voltage fluctuation, and rapid recovery. Figure 4Under a traditional sliding-mode controller, at 0.5s, the capacitor voltage waveform changes after the load changes from 10.6Ω to 5.35Ω. At 0.5s, the voltage begins to drop from 106.91V and rapidly decreases, reaching its lowest point at 0.51s. At 0.5693s, the voltage stabilizes at around 92.5V, demonstrating poor stability, large voltage fluctuations, and slow recovery.

[0106] Comparative analysis shows that the improved second-order sliding mode controller exhibits better stability and faster response speed, and the voltage recovery is closer to the initial value, while the traditional sliding mode controller exhibits larger voltage fluctuations and slower recovery speed.

[0107] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principles of the present invention. These improvements and modifications should also be regarded as within the scope of protection of the present invention.

Claims

1. A quasi-Z-source photovoltaic inverter DC link voltage control method based on second-order sliding mode, characterized in that: The following steps are involved: S1. Establishing a large signal model of the quasi-Z-source inverter according to the topology of the three-phase quasi-Z-source inverter, and establishing a state space model according to the large signal model; S2. defining an error function according to the state-space model, including an inductor current error function and a capacitor voltage error function; S3. Constructing a first-order sliding surface based on the error function, and introducing an integral term to obtain an improved first-order sliding surface, and constructing an improved second-order sliding surface based on the improved first-order sliding surface; S4. According to the improved second-order sliding mode surface, a final improved second-order sliding mode equivalent control law is obtained.

2. The method for controlling the DC link voltage of a quasi-Z-source photovoltaic inverter based on a second-order sliding mode according to claim 1, characterized in that: According to the three-phase quasi-Z-source inverter topology, a large signal model of the quasi-Z-source inverter is established. The large signal model is: The large signal model of the through state is: Among them, i L1 is the current flowing through the first inductor, for i L1 The first derivative of L2 is the current flowing through the second inductor, for i L2 The first derivative of u c1 is the voltage across the first capacitor, for u c1 The first derivative of u c2 is the voltage across the second capacitor, for u c2 The first-order derivative of , L represents the inductance value, and L1=L2=L, L1 is the first inductor in the quasi-Z source inverter, L2 is the second inductor in the quasi-Z source inverter, C represents the capacitance value, and C1=C2=C, C1 is the first capacitor in the quasi-Z source inverter, and C2 is the second capacitor in the quasi-Z source inverter; The large signal model in the non-through state is: Among them, i L1 is the current flowing through the inductor L1, for i L1 The first derivative of L2 is the current flowing through the inductor L2, for i L2 The first derivative of u c1 is the voltage across capacitor C1, for u c1 The first derivative of u c2 is the voltage across capacitor C2, for u c2 The first-order derivative of , L represents the inductance value, and L1=L2=L, C represents the capacitance value, and C1=C2=C, V0 is the DC input voltage, i dc is the DC link current.

3. The method for controlling the DC link voltage of a quasi-Z-source photovoltaic inverter based on a second-order sliding mode according to claim 1, characterized in that: According to the large signal model, a state space model is established: in, A and B are represented as: Among them, u is the system input, expressed as: Among them, i L1 is the current flowing through the inductor L1, for i L1 The first derivative of L2 is the current flowing through the inductor L2, for i L2 The first derivative of u c1 is the voltage across capacitor C1, for u c1 The first derivative of u c2 is the voltage across capacitor C2, for u c2 The first derivative of , L represents the inductance value, and L1 = L2 = L, C represents the capacitance value, and C1 = C2 = C, i dc is the DC link current, and V0 is the DC input voltage.

4. The method for controlling the DC link voltage of a quasi-Z-source photovoltaic inverter based on a second-order sliding mode according to claim 1, wherein: According to the state space model, an error function is defined, including an inductor current error function and a capacitor voltage error function; the error function is: Inductor current error function: Among them, e1 is the inductor current error, i L1 is the current flowing through the inductor L1, is the current i L1 Reference current: Capacitor voltage error function: Among them, e2 is the capacitor voltage error, u C1 is the current flowing through the inductor C1, is the voltage u C1 The reference voltage; and Expressed as: Among them, K i is the integral coefficient, K p is the proportional coefficient, D is the pass-through duty cycle, and V0 is the DC input voltage.

5. The method for controlling DC link voltage of a quasi-Z-source photovoltaic inverter based on a second-order sliding mode according to claim 1, characterized in that: According to the error function, a first-order sliding surface is constructed, and the first-order sliding surface is: s=λ1e1+λ2e2; Wherein, s is the first-order sliding mode surface, λ1 and λ2 are parameters to be designed, e1 is the inductor current error, and e2 is the capacitor voltage error function.

6. The method for controlling DC link voltage of a quasi-Z-source photovoltaic inverter based on a second-order sliding mode according to claim 1, characterized in that: By introducing the integral term, an improved first-order sliding mode surface is obtained, which is: s1=λ1e1+λ2e2+λ3∫(e1+e2)dt; Wherein, s1 is the improved first-order sliding surface, λ1, λ2 and λ3 are parameters to be designed, e1 is the inductor current error, and e2 is the capacitor voltage error function.

7. The method for controlling DC link voltage of a quasi-Z-source photovoltaic inverter based on a second-order sliding mode according to claim 1, characterized in that: According to the improved first-order sliding surface, an improved second-order sliding surface is constructed. The improved second-order sliding surface is: Wherein, s1 is the improved first-order sliding surface, is the first-order derivative of s1, and α is the parameter to be designed.

8. The method for controlling DC link voltage of a quasi-Z-source photovoltaic inverter based on a second-order sliding mode according to claim 1, characterized in that: According to the improved second-order sliding mode surface, the final improved second-order sliding mode control law is obtained. The final improved second-order sliding mode equivalent control law is: Among them, u eq is the equivalent control law, i L1 is the current flowing through the inductor L1, for i L1 The first derivative of L2 is the current flowing through the inductor L2, for i L2 The first derivative of u c1 is the voltage across capacitor C1, for u c1 The first derivative of u c2 is the voltage across capacitor C2, for u c2 The first derivative of , L represents the inductance value, and L1 = L2 = L, C represents the capacitance value, and C1 = C2 = C, i dc is the DC link current, s1 is the improved first-order sliding surface, e1 is the inductor current error, e2 is the capacitor voltage error function, λ1, λ2, λ3 and α are the parameters to be designed, K i is the integral coefficient, K p is the proportional coefficient.

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