Photovoltaic MPPT control system based on sliding mode variable structure-global comparison composite algorithm

Abstract

The present invention discloses a photovoltaic MPPT control system based on a sliding mode variable structure-global comparison composite algorithm in the field of photovoltaic MPPT control. The control system comprises a temperature and light variable module, a photovoltaic array, a DC-DC converter, a voltage and current sensor, a temperature and light sensor, an MPPT algorithm module, a PWM module, a DSP digital signal processor, a power supply circuit, a drive circuit, and a computer display. The temperature and light variable module is used to simulate shadows caused by tree branches and poles on the photovoltaic array. The MPPT algorithm module uses a main algorithm, a sliding mode variable structure control algorithm, supplemented by a global comparison algorithm, to periodically search for the maximum power point of the photovoltaic array under shadows. The present invention can avoid the problem of traditional algorithms tracking local extreme values, prevent the photovoltaic array from being burned out due to high heat caused by shading, improve tracking speed, accuracy, and safety, and weaken oscillations near the global maximum power point.

Description

Technical Field

[0001] The present invention relates to the technical field of photovoltaic MPPT control, and in particular to a photovoltaic MPPT control system based on a sliding mode variable structure-global comparison composite algorithm. Background Art

[0002] In recent years, the consumption of traditional fossil fuels has accelerated, and energy issues have become increasingly prominent. Solar energy, as a representative of clean energy, has gradually attracted attention. The photovoltaic industry has continued to accelerate, and the number of photovoltaic power stations has also increased. These power stations can convert sunlight into electricity through a series of photovoltaic panels, effectively alleviating energy problems and environmental pollution.

[0003] Solar photovoltaic technology is increasingly being used, but low conversion efficiency is a major constraint hindering its development and application. Consequently, the study of maximum power point tracking (MPPT) algorithms has long been a hot topic in photovoltaic power generation system research. Based on the MPPT algorithm, photovoltaic power generation systems can be blocked from receiving sunlight evenly, significantly reducing their efficiency and even causing localized heating (hot spots), which can accelerate the aging of power generation modules and cause irreversible damage.

[0004] Traditional MPPT algorithms have a simple structure, low cost, and are easy to implement. However, they inevitably present challenges, such as low control accuracy, slow tracking speed, and fluctuations after power stabilization. In particular, when photovoltaic cells are exposed to the open air, temperature and light fluctuations have a greater impact on system power generation efficiency. Shadows significantly reduce the effectiveness of the MPPT algorithm. Partially shadowing a cell causes multiple power peaks in the PU curve, potentially leading the MPPT algorithm to misjudge and track the incorrect maximum power point. Furthermore, the shaded area can generate heat that can damage the photovoltaic cells.

[0005] In the prior art, there is a photovoltaic MPPT control method under partial shadow, with patent application number: CN201510228788.6; application date: 2015-05-06; authorization number: CN106200752B; authorization date: 2017-11-03; the photovoltaic MPPT control method under partial shadow adopts a power scanning method and a maximum power tracking method based on sliding mode control, and uses a method of limiting power by voltage threshold and current threshold to scan and obtain the global maximum power. Theoretical analysis is feasible, but in engineering application, the voltage and current thresholds may change with changes in the environment. The effect of using fixed values as thresholds may not be ideal. Even if they are adjustable, the testing process will be complicated. In addition, the control rate is designed by using a super-helical algorithm, which requires a large amount of engineering calculations and is highly complex. In addition, the technical solution does not provide a proof of the stability of the sliding mode control algorithm. If it is applied in practice, it may cause the system to be unstable and unusable. Summary of the Invention

[0006] In response to the shortcomings of the above-mentioned MPPT algorithm in actual photovoltaic power generation, the present invention provides a photovoltaic MPPT control system based on a sliding mode variable structure-global comparison composite algorithm, which effectively solves the problems of erroneous tracking when tracking the maximum power point; poor speed and stability; low efficiency and low accuracy. It can find the correct maximum power point under shadow conditions, improve the output efficiency of the system, and prevent the system from being damaged by local hot spots.

[0007] To achieve the purpose of the above invention, the technical solution provided by the present invention is as follows: a photovoltaic MPPT control system based on a sliding mode variable structure-global comparison composite algorithm includes a temperature and light variable module, a photovoltaic array, a DC-DC converter, a voltage and current sensor, a temperature and light sensor, an MPPT algorithm module, a PWM module, a DSP digital signal processor, a power supply circuit, a drive circuit and a computer display; the temperature and light variable module is used to simulate the shadow occlusion of the photovoltaic array caused by tree branches and poles; the MPPT algorithm module adopts the main algorithm - the sliding mode variable structure control algorithm and is supplemented by a global comparison algorithm to periodically search for the maximum power point of the photovoltaic array under shadow occlusion.

[0008] As a further improvement of the present invention, the photovoltaic array includes photovoltaic cell unit 1, photovoltaic cell unit 2 and photovoltaic cell unit 3 connected in series, and each of the photovoltaic cell unit 1, photovoltaic cell unit 2 and photovoltaic cell unit 3 is connected in parallel with a bypass diode.

[0009] As a further improvement of the present invention, the DC-DC converter adopts a Boost circuit, which includes a capacitor C1 and an inductor L. One end of the capacitor C1 and the inductor L is connected to the forward port of the photovoltaic array, the other end of the inductor L is connected to the drain of the switching tube MOS and the anode of the diode, the cathode of the diode is connected to one end of the capacitor C2 and the load resistor R, and the capacitor C1, capacitor C2, the source of the switching tube and the load resistor R are connected to the negative pole of the photovoltaic array.

[0010] As a further improvement of the present invention, the design method of the sliding mode variable structure control algorithm includes the following steps:

[0011] S01: Based on the photovoltaic cell equivalent circuit and the system main circuit - Boost circuit, establish the photovoltaic cell mathematical model and calculate its partial derivatives;

[0012] Mathematical model of photovoltaic cells:

[0013]

[0014]

[0015] Find its partial derivative:

[0016]

[0017] Where, I pv Represents the output current of the photovoltaic cell, V pv Represents the photovoltaic cell output voltage, I ph Represents the photocurrent of the photovoltaic cell, I D0 represents the reverse saturation current, q represents the electron charge, A represents the diode factor, K represents the Boltzmann constant, T represents the absolute temperature, and I L Inductor current, V O Indicates the output voltage at the load end, I O Indicates the output current at the load end;

[0018] S02: Based on the two states of the MOS tube when it is turned on and off, an equivalent mathematical model of the Boost circuit can be established;

[0019] (MOS tube is turned on), (MOS tube turned off)(5)

[0020] Define u=1, 0 to represent the on and off of the MOS tube respectively, and we can get the following formula:

[0021]

[0022] Rewriting this into the general state-space equation form:

[0023]

[0024] in

[0025] S03: Construct sliding surface (sliding mode switching function) S and control rate u;

[0026] Because at the maximum power point of photovoltaic, there is

[0027]

[0028] The sliding surface can be obtained

[0029] Set the control rate u by the equivalent control u eq and variable structure control sw It consists of two parts, namely:

[0030] u=u eq +u sw (11)

[0031] In order to ensure that the system can quickly reach the sliding surface while resisting interference, u sw Using the exponential approach rate, that is:

[0032] u sw =-ε*sign(s)-ks(ε>0,k>0) (12)

[0033] In the present invention, ε=0.03, k=0.005; derive u eq The process is as follows:

[0034]

[0035] Since on the sliding surface, We can get:

[0036]

[0037] In summary

[0038] S04: Using Lyapunov function to determine the stability of the sliding mode controller; To make the system stable, we need to prove The stability judgment process is as follows:

[0039]

[0040] Due to the presence of capacitor C1 in the system, I PV ≈I L ; Then we get the following formula:

[0041]

[0042] From the above formulas (3) and (4), we can get

[0043] When S>0, since ε>0, k>0, so

[0044] but at this time

[0045] When S<0, since ε>0, k>0, so

[0046] but at this time

[0047] It can be determined that the main controller, the sliding mode variable structure controller, is stable.

[0048] As a further improvement of the present invention, the design method steps of the global comparison algorithm are as follows:

[0049] Step 1: When the sliding mode variable structure control algorithm starts running, the global comparison time threshold is set to 0.5s. That is, the global comparison algorithm is started every 0.5s with this time threshold as a cycle; during the cycle search process, the power difference threshold is set to 15W, as a basis for urgently starting the global comparison algorithm when the power suddenly changes due to shadow occlusion; the initial duty cycle is defined as 0.1 or 0.9;

[0050] Step 2: Discretize the voltage and current data, set the location where the power to be collected at each moment is stored as the real-time output power array Ps = [], and set the location where the system duty cycle to be collected at each moment is stored as the real-time duty cycle array Ds = [];

[0051] Step 3: Data is collected using the IV characteristic curve from open circuit to short circuit as a control process, and the real-time power and the corresponding real-time duty cycle are listed in an array as data points. The sampling cycle is set to 5 data points per cycle to ensure the efficiency of the algorithm search; the duty cycle step is set to 0.005. At this time, if the initial duty cycle is 0.1, the duty cycle is continuously added in steps of +0.005 during the power sampling process, and the duty cycle is sampled at the same time; if the initial duty cycle is 0.9, the duty cycle is continuously reduced in steps of -0.005 during the power sampling process, and the duty cycle is sampled at the same time;

[0052] Step 4: Output the maximum power of the PV array IV characteristic from open circuit to short circuit obtained by the algorithm in the form of a maximum value code instruction, and find the corresponding duty cycle;

[0053] Step 5: Compare the maximum power obtained by the global comparison algorithm with the maximum power obtained by the sliding mode variable structure algorithm. If they are equal, maintain them. If they are not equal, replace the current power with the larger value. Then, perform global maximum power tracking based on the sliding mode surface and control rate. When the 0.5s period expires, re-enter step 1 and cyclically search.

[0054] As a further improvement of the present invention, the voltage and current sensor samples the voltage and current signals output by the photovoltaic array under shadow and sends them to the MPPT algorithm module. The MPPT algorithm module is connected to the PWM module, and the PWM module outputs PWM waves to the MOSFET in the Boost circuit.

[0055] As a further improvement of the present invention, the power supply circuit and the driving circuit are both connected to a DSP digital signal processor, which has built-in A / D and D / A converters and several chips for performing read and write operations and is connected to a computer display.

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

[0057] 1. This invention utilizes a sliding mode variable structure-global comparison composite algorithm, combining software and hardware to enable the control system to rapidly track the maximum power point, minimize overshoot, and achieve high photoelectric conversion efficiency. Under simulated shadow conditions, the system can rapidly respond to changes in the external environment and accurately identify the true maximum power point even when photovoltaic panels experience multiple power peaks due to uneven light distribution.

[0058] 2. Compared with the traditional MPPT control algorithm, which cannot track the global power peak under shading, and the single sliding mode control algorithm, which can only track the global power peak under partial shading conditions, this composite algorithm can automatically search for the global power peak every 0.5 seconds. If the shaded area changes suddenly within 0.5 seconds, a global comparison process can be added to ensure that it can always stabilize at the global power peak under any circumstances. By comparing the power and duty cycle at each moment, it effectively solves the problem that the single sliding mode control cannot adaptively adjust the exponential approach rate parameters, and is more efficient in actual use.

[0059] 3. The system can effectively solve the hot spot problem that may occur in shaded environments and improve the safety of photovoltaic array operations. BRIEF DESCRIPTION OF THE DRAWINGS

[0060] Figure 1 Schematic diagram of the structure of a preferred embodiment of the present invention.

[0061] Figure 2 This is a simulation model diagram of a preferred embodiment of the present invention.

[0062] Figure 3 This is a working principle diagram of the Boost circuit when the MOS tube is turned on and off in the present invention.

[0063] Figure 4 This is a comparison chart of the global power peak effect of the sliding mode-global comparison composite algorithm and the single sliding mode control algorithm in tracking the PU curve under standard illumination (shading condition A) of the photovoltaic array of the present invention.

[0064] Figure 5 This is a comparison diagram of the global power peak effect of the sliding mode-global comparison composite algorithm and the single sliding mode control algorithm in tracking the PU curve under shading condition B of the photovoltaic array of the present invention.

[0065] Figure 6 This is a comparison diagram of the global power peak effect of the sliding mode-global comparison composite algorithm and the single sliding mode control algorithm in tracking the PU curve under shading condition C of the photovoltaic array of the present invention.

[0066] Figure 7This is a comparison diagram of the global power peak effect of the sliding mode-global comparison composite algorithm and the single sliding mode control algorithm in tracking the PU curve under shading condition D of the photovoltaic array of the present invention. DETAILED DESCRIPTION

[0067] The following will be combined with the accompanying drawings to clearly and completely describe the technical solutions of the present invention. Obviously, the embodiments described are only some of the embodiments of the present invention, not all of them. The drawings are only schematic diagrams, not actual images, and should not be understood as limiting this patent. In order to better illustrate the embodiments of the present invention, some parts of the drawings may be omitted, enlarged, or reduced. It is understandable to those skilled in the art that some well-known structures and their descriptions may be omitted in the drawings.

[0068] like Figure 1 As shown, a photovoltaic MPPT control system based on a sliding mode variable structure-global comparison composite algorithm includes: a temperature and light variable module, a photovoltaic array, a DC-DC converter, a voltage and current sensor, a temperature and light sensor, an MPPT algorithm module, a PWM module, a DSP digital signal processor, a power supply circuit, a drive circuit and a computer display; the photovoltaic array includes a photovoltaic cell unit 1, a photovoltaic cell unit 2 and a photovoltaic cell unit 3 connected in series.

[0069] In the embodiment, since the PU characteristic curve of photovoltaic cells in actual use may show multiple power peaks due to shading, the accuracy of the algorithm tracking the maximum power point should be emphasized when testing the algorithm. The temperature and light variable module is used to simulate the partial shading of the photovoltaic array caused by branches and poles in actual use. Four simulated light conditions are used here, namely:

[0070] Shading condition A (cell 1: 1000W / m 2 , battery unit 2: 1000W / m 2 , battery unit 3: 1000W / m 2 ).

[0071] Shade condition B (cell 1: 1000W / m 2 , battery unit 2: 1000W / m 2 , battery unit 3: 400W / m 2 ).

[0072] Shading condition C (battery unit 1: 1000W / m 2 , battery unit 2: 200W / m 2 , battery unit 3: 200W / m 2 ).

[0073] Shading condition D (cell 1: 1000W / m 2, battery unit 2: 500W / m 2 , battery unit 3: 100W / m 2 ).

[0074] Under varying levels of sunlight, each photovoltaic cell outputs a different voltage. The four shading conditions, A, B, C, and D, represent single, double, and triple power peaks that can occur in actual photovoltaic systems.

[0075] A bypass diode is connected in parallel to each of the three photovoltaic cell units connected in series in the embodiment. When the system is shaded, some of the diodes will be turned on. Since the current in the series circuit is consistent, different short-circuit current ranges can be divided according to different light intensities. The photogenerated currents of the three photovoltaic cell units are also different accordingly. Therefore, there may be three situations for the output power: 3 photovoltaic cell units output at the same time, 2 photovoltaic cell units output at the same time, and 1 photovoltaic cell unit outputs alone. This results in a multi-power peak PU curve under shade.

[0076] The DC-DC converter adopts a Boost circuit, which includes a capacitor C1 and an inductor L. One end of the capacitor C1 and the inductor L is connected to the forward port of the photovoltaic array, the other end of the inductor L is connected to the drain of the switching tube MOS and the anode of the diode, the cathode of the diode is connected to one end of the capacitor C2 and the load resistor R, and the capacitor C1, capacitor C2, the source of the switching tube and the load resistor R are connected to the negative pole of the photovoltaic array; when this system is working, the voltage and current signals output by the photovoltaic array under shadow are sampled and input into the voltage and current sensors, together with the temperature and light variables, into the data acquisition module; the MPPT algorithm module adopts a sliding mode-global comparison composite algorithm design in code form and is connected to the PWM module. The MPPT module processes the electrical signal and converts the control signal into a duty cycle adjustment. The PWM module outputs a PWM wave to the gate of the switching tube, which can achieve stable power tracking while finding the accurate maximum power point.

[0077] In this embodiment, the hardware circuit includes, in addition to voltage and current sensors and a DC-DC converter, a power supply circuit, a drive circuit, and a DSP digital signal processor; the power supply circuit supplies power to the DSP, the drive circuit is connected to the DSP to convert the collected electrical signals into a suitable range, and the DSP has built-in A / D and D / A converters and several chips that perform read and write operations and is connected to a computer monitor to display the output waveform processed by the sliding mode-global comparison composite algorithm on the computer monitor.

[0078] like Figure 2 、 Figure 3As shown in the figure, a simulation model diagram of a photovoltaic MPPT control system based on a sliding mode variable structure-global comparison composite algorithm under shadow occlusion is constructed. According to the mathematical expression of the photovoltaic cell and the working process of the Boost circuit when the MOS tube is turned on and off, the main algorithm of the present invention, the sliding mode variable structure control algorithm, can be designed. The algorithm includes the following steps:

[0079] S01: According to the photovoltaic cell equivalent circuit and the system main circuit - Boost circuit, set the following parameters, establish the photovoltaic cell mathematical model and calculate its partial derivatives.

[0080]

[0081] Among them, the ratio of the output voltage of the photovoltaic array and the photovoltaic cell is 3:1, and the ratio of the output current is 1:1.

[0082] Mathematical model of photovoltaic cells:

[0083]

[0084]

[0085] Find its partial derivative:

[0086]

[0087] S02: Based on the two states of the MOS tube when it is turned on and off, an equivalent mathematical model of the Boost circuit can be established.

[0088] (MOS tube is turned on), (MOS tube turned off)(5)

[0089] Define u=1, 0 to represent the on and off of the MOS tube respectively, and we can get the following formula:

[0090]

[0091] Rewriting this into the general state-space equation form:

[0092]

[0093] in

[0094] S03: Construct the sliding surface (sliding mode switching function) S and the control rate u.

[0095] Because at the maximum power point of photovoltaic, there is

[0096]

[0097] The sliding surface can be obtained

[0098] Set the control rate u by the equivalent control u eq and variable structure control sw It consists of two parts, namely:

[0099] u=u eq +u sw (11)

[0100] In order to ensure that the system can quickly reach the sliding surface while resisting interference, u sw Using the exponential approach rate, that is:

[0101] u sw =-ε*sign(s)-ks(ε>0,k>0) (12)

[0102] In the present invention, ε=0.03, k=0.005. Derivation u eq The process is as follows:

[0103]

[0104] Since on the sliding surface, We can get:

[0105]

[0106] In summary

[0107] S04: Use Lyapunov function to determine the stability of sliding mode controller. To make the system stable, we need to prove The stability judgment process is as follows:

[0108]

[0109] Due to the presence of capacitor C1 in the system, I PV ≈I L . Then we get the following formula:

[0110]

[0111] From the above formulas (3) and (4), we can get

[0112] When S>0, since ε>0, k>0, so

[0113] but at this time

[0114] When S<0, since ε>0, k>0, so

[0115] but at this time

[0116] It can be determined that the main controller - sliding mode variable structure controller is stable.

[0117] In the examples, the power tracking performance of a photovoltaic power generation system under four different shade conditions was tested using a single sliding mode controller. It was found that the controller could only track the global power peak under partial shade conditions. In some shade conditions, the tracked power fluctuated significantly or tracked only local power peaks. This made it impossible for the single sliding mode controller to guarantee system stability and maximize power generation efficiency. By continuously changing the exponential approach rate parameter in the sliding mode controller, the system could be re-tracked to the global maximum power under certain shade conditions. This meant that achieving global tracking with a single sliding mode controller required manually changing certain system parameter values in real time, which was clearly undesirable in practical applications.

[0118] Therefore, in order to make the exponential approach rate parameters in the sliding mode control algorithm applicable to all shadow occlusion environments and realize the global self-optimization function, a global comparison algorithm is added to the operation of the sliding mode control algorithm to periodically search for the actual power peak under shadow occlusion. The specific design steps are as follows:

[0119] Step 1: At the start of the sliding mode variable structure control algorithm, set the global comparison time threshold to 0.5s. This threshold defines a cycle, and the global comparison algorithm is initiated every 0.5s. During the cycle search process, set the power difference threshold to 15W to trigger the global comparison algorithm in the event of a sudden change in power due to shadowing. Define the initial duty cycle to be 0.1 or 0.9.

[0120] Step 2: Discretize the voltage and current data, set the location where the power to be collected at each moment is stored as the real-time output power array Ps = [], and set the location where the system duty cycle to be collected at each moment is stored as the real-time duty cycle array Ds = [].

[0121] Step 3: Collect data using the IV characteristic curve from open-circuit to short-circuit as a control process. The real-time power and the corresponding real-time duty cycle are listed in an array as data points. Sampling is performed in cycles of five data points to ensure efficient algorithm search. The duty cycle step size is set to 0.005. If the initial duty cycle is 0.1, the duty cycle is continuously added in steps of +0.005 while sampling the power, while the duty cycle is sampled. If the initial duty cycle is 0.9, the duty cycle is continuously reduced in steps of -0.005 while sampling the power, while the duty cycle is sampled.

[0122] Step 4: Output the maximum power of the PV array IV characteristic obtained by the algorithm in the global process from open circuit to short circuit in the form of code instructions for finding the maximum value, and find the corresponding duty cycle.

[0123] Step 5: Compare the maximum power obtained by the global comparison algorithm with the maximum power obtained by the sliding mode variable structure algorithm. If they are equal, maintain them. If they are not, replace the current power with the larger value. Then, track the global maximum power based on the sliding mode surface and control rate. When the 0.5s period expires, reenter Step 1 and repeat the search.

[0124] The above-designed composite algorithm is applied to the embodiment of the system, and the results are as follows: Figure 4-Figure 7 .

[0125] like Figure 4 As shown in the figure, the photovoltaic cell PU curve under shade condition A has only one maximum power point, approximately 120W. Both the sliding mode-global comparison combined algorithm and the single sliding mode control algorithm can track the global power peak. The single sliding mode control algorithm is much faster, outperforming the combined algorithm. Because the combined algorithm requires periodic comparison, it takes 0.08 seconds.

[0126] like Figure 5 As shown in the figure, the photovoltaic cell PU curve under shading condition B has two maximum power points, and the global power peak is approximately 76 W. In this case, both the sliding mode-global comparison composite algorithm and the single sliding mode control algorithm can track the global power peak. The single sliding mode control algorithm is still faster than the composite algorithm, taking 0.07 seconds.

[0127] like Figure 6 As shown, the photovoltaic cell PU curve under shading condition C has two maximum power points, with the global peak power being approximately 32W. At this point, the sliding mode control algorithm alone experienced dramatic power fluctuations during tracking, with the upper and lower power limits differing by approximately 15W, making it impossible to maintain stable output power for the power generation system. However, the sliding mode-global comparison algorithm tracked the global peak power and maintained it stably in 0.06s.

[0128] like Figure 7 As shown, the photovoltaic cell PU curve for shade condition D has three maximum power points, and the global peak power is approximately 44W. In this case, the single sliding mode control algorithm can only track local power peaks, resulting in significant energy loss. However, the sliding mode-global comparison composite algorithm nearly tracks the global peak power, approximately 42W. Although heavy shading causes a 2W power loss during the tracking process, it still achieves global maximum power point tracking and maintains stability, achieving high energy utilization in 0.09s.

[0129] This shows that the sliding mode-global comparison composite algorithm has extremely fast response speed, extremely small overshoot, high output accuracy and efficiency, good stability, and both system security, and has good market application space.

[0130] The present invention is not limited to the above-mentioned embodiments. On the basis of the technical solutions disclosed in the present invention, those skilled in the art can make some substitutions and modifications to some of the technical features therein according to the disclosed technical content without creative labor, and these substitutions and modifications are all within the protection scope of the present invention.

Claims

1. Photovoltaic MPPT control system based on sliding mode variable structure-global comparison composite algorithm, characterized by: It includes temperature and light variable module, photovoltaic array, DC-DC converter, voltage and current sensor, temperature and light sensor, MPPT algorithm module, PWM module, DSP digital signal processor, power supply circuit, drive circuit and computer display; The temperature and light variable module is used to simulate the shadowing of the photovoltaic array caused by tree branches and poles; the MPPT algorithm module uses the main algorithm - sliding mode variable structure control algorithm and supplemented by a global comparison algorithm to periodically search for the maximum power point of the photovoltaic array under shadowing; The design method of the sliding mode variable structure control algorithm comprises the following steps: S01: Based on the photovoltaic cell equivalent circuit and the system main circuit - Boost circuit, establish the photovoltaic cell mathematical model and calculate its partial derivatives; Mathematical model of photovoltaic cells: Find its partial derivative: Where, I pv Represents the output current of the photovoltaic cell, V pv Represents the photovoltaic cell output voltage, I ph Represents the photocurrent of the photovoltaic cell, I D0 represents the reverse saturation current, q represents the electron charge, A represents the diode factor, K represents the Boltzmann constant, T represents the absolute temperature, and I L Inductor current, V O Indicates the output voltage at the load end, I O Indicates the output current at the load end; S02: Based on the two states of the MOS tube when it is turned on and off, an equivalent mathematical model of the Boost circuit can be established; Define u=1, 0 to represent the on and off of the MOS tube respectively, and we can get the following formula: Rewriting this into the general state-space equation form: in S03: Construct sliding surface (sliding mode switching function) S and control rate u; Because at the maximum power point of photovoltaic, there is The sliding surface can be obtained Set the control rate u by the equivalent control u eq and variable structure control sw It consists of two parts, namely: in=in eq +in sw (11) In order to ensure that the system can quickly reach the sliding surface while resisting interference, u sw Using the exponential approach rate, that is: u sw =-ε*sign(s)-ks(ε>0,k>0) (12) Take ε = 0.03, k = 0.005; derive u eq The process is as follows: Since on the sliding surface, We can get: In summary S04: Using Lyapunov function to determine the stability of sliding mode controller; To make the system stable, we need to prove The stability judgment process is as follows: Due to the presence of capacitor C1 in the system, I PV ≈I L ; Then we get the following formula: From the above formulas (3) and (4), we can get When S>0, since ε>0, k>0, so but at this time When S<0, since ε>0, k>0, so but at this time It can be determined that the main controller - sliding mode variable structure controller is stable.

2. The photovoltaic MPPT control system based on sliding mode variable structure-global comparison composite algorithm according to claim 1 is characterized in that: The photovoltaic array includes a photovoltaic cell unit 1, a photovoltaic cell unit 2, and a photovoltaic cell unit 3 connected in series, and each of the photovoltaic cell unit 1, the photovoltaic cell unit 2, and the photovoltaic cell unit 3 is connected in parallel with a bypass diode.

3. The photovoltaic MPPT control system based on sliding mode variable structure-global comparison composite algorithm according to claim 2 is characterized in that: The DC-DC converter adopts a Boost circuit, which includes a capacitor C1 and an inductor L. One end of the capacitor C1 and the inductor L is connected to the forward port of the photovoltaic array, the other end of the inductor L is connected to the drain of the switching tube MOS and the anode of the diode, the cathode of the diode is connected to one end of the capacitor C2 and the load resistor R, and the capacitor C1, capacitor C2, the source of the switching tube and the load resistor R are connected to the negative pole of the photovoltaic array.

4. The photovoltaic MPPT control system based on sliding mode variable structure-global comparison composite algorithm according to claim 1 is characterized in that: The design method steps of the global comparison algorithm are as follows: Step 1: When the sliding mode variable structure control algorithm starts running, the time threshold of the global comparison is set to 0.5s. That is, the global comparison algorithm is started every 0.5s with this time threshold as a cycle; During the cycle search process, the power difference threshold is set to 15W, which serves as the basis for urgently starting the global comparison algorithm when the power suddenly changes under the shadow shading. The initial duty cycle is defined as 0.1 or 0.

9. Step 2: Discretize the voltage and current data, set the location where the power to be collected at each moment is stored as the real-time output power array Ps = [], and set the location where the system duty cycle to be collected at each moment is stored as the real-time duty cycle array Ds = []; Step 3: Collect data using the IV characteristic curve from open circuit to short circuit as a control process. The real-time power and the corresponding real-time duty cycle are listed in an array as data points. The sampling cycle is set to 5 data points to ensure the efficiency of the algorithm search. Set the duty cycle step size to 0.

005. If the initial duty cycle is 0.1, the duty cycle will be continuously superimposed in steps of +0.005 during the power sampling process, while the duty cycle is sampled at the same time. If the initial duty cycle is 0.9, the duty cycle is continuously reduced in steps of -0.005 during the power sampling process while the duty cycle is sampled; Step 4: Output the maximum power of the PV array IV characteristic from open circuit to short circuit obtained by the algorithm in the form of a maximum value code instruction, and find the corresponding duty cycle; Step 5: Compare the maximum power obtained by the global comparison algorithm with the maximum power obtained by the sliding mode variable structure algorithm. If they are equal, maintain them. If they are not equal, replace the current power with the larger value. Then, perform global maximum power tracking based on the sliding mode surface and control rate. When the 0.5s period expires, re-enter step 1 and cyclically search.

5. The photovoltaic MPPT control system based on sliding mode variable structure-global comparison composite algorithm according to claim 3 or 4, characterized in that: The voltage and current sensor samples the voltage and current signals output by the photovoltaic array under shadow and sends them to the MPPT algorithm module. The MPPT algorithm module is connected to the PWM module, and the PWM module outputs PWM waves to the MOSFET in the Boost circuit.

6. The photovoltaic MPPT control system based on the sliding mode variable structure-global comparison composite algorithm according to any one of claims 1 to 4, characterized in that: The power supply circuit and the driving circuit are both connected to a DSP digital signal processor. The DSP digital signal processor has built-in A / D and D / A converters and several chips for performing read and write operations and is connected to a computer display.

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