A Maximum Power Point Tracking Control Method for Photovoltaic Modules under Dust Accumulation Conditions

Abstract

The present invention discloses a method for maximum power point tracking control of a photovoltaic module under the condition of dust accumulation, which solves the problems of slow tracking speed and reduced power generation efficiency of the photovoltaic module when the existing perturbation observation method is used to track the maximum power point of the photovoltaic module under the condition of dust accumulation. On the basis of the perturbation observation method, the present invention takes into account the influence of dust accumulation on the surface of the photovoltaic module in the perturbation observation method, converts the influence of dust accumulation into a perturbation factor, and adds it to the step size of the perturbation observation method to form a new step size, thereby improving the adaptability of the perturbation observation method to dust accumulation. The present invention can not only improve the speed of tracking the maximum power point of the photovoltaic module by the perturbation observation method under the condition of dust accumulation, but also has a simple control strategy, strong adaptability and low cost, and can be widely used in the tracking of the maximum power point of the photovoltaic module, having wide application value.

Description

Technical Field

[0001] The present invention relates to the output power control of photovoltaic modules, and specifically to a maximum power point tracking control method for photovoltaic modules under the condition of dust accumulation. Background Art

[0002] In modern society, the demand for energy is increasing. The shortage of traditional fossil energy and the resulting environmental problems have forced people to turn their attention to the new energy field. Solar energy has been widely concerned due to its clean and renewable characteristics, and photovoltaic power generation is the main form of solar energy utilization. The main factors affecting the efficiency of photovoltaic power generation are light intensity and working temperature. The accumulation of dust on the photovoltaic panel will produce a shading effect and a temperature effect. The shading effect will reduce the light intensity absorbed by the photovoltaic module, resulting in a decrease in the efficiency of photovoltaic power generation and an increase in the fluctuation of photovoltaic output power. Dust suspended in the atmosphere will form dust accumulation when attached to the photovoltaic panel. The influence of dust accumulation on photovoltaic modules can be attributed to three aspects: one is the shading effect. The dust accumulation reduces the sunlight absorbed by the photovoltaic module through adsorption and reflection, resulting in a decrease in the light transmittance of the photovoltaic module and a reduction in the output power of the photovoltaic module; the second is the temperature effect. The dust on the photovoltaic panel also affects the heat transfer form of the photovoltaic panel. The coverage of dust accumulation ultimately reduces the temperature of the photovoltaic module, and the greater the dust accumulation density, the lower the temperature of the photovoltaic module. The temperature difference between different dust accumulation density groups decreases with the weakening of solar radiation intensity; the third is the corrosion effect. When the dust accumulation adheres to the photovoltaic panel for a long time, the photovoltaic panel will be corroded, resulting in a pitted surface on the photovoltaic panel, which will increase the diffuse reflection of sunlight on the photovoltaic surface, reduce the light transmittance of the photovoltaic module, and reduce the amount of solar radiation absorbed by the photovoltaic module. Under the interaction of the above multiple effects, dust accumulation seriously affects the output power of photovoltaic modules, resulting in a decrease in the maximum output power point of photovoltaic modules. When photovoltaic modules operate in a high-pollution environment, it is necessary to consider the influence of dust accumulation on the power generation efficiency of photovoltaic modules and the effect of maximum power point tracking. Figure 1 The P-U output characteristic curve of the photovoltaic module under the condition of dust accumulation, where R in the figure is the dust accumulation density, P is the output power of the photovoltaic module, and U is the output voltage of the photovoltaic module.

[0003] Generally, a large number of photovoltaic cell units are packaged into a whole by series and parallel connection using a certain process to form a power generation unit that can provide a certain capacity of DC electric energy, which is called a photovoltaic module.

[0004] Photovoltaic cells have complex non-linear output characteristics. The output voltage and current of a photovoltaic array are greatly affected by sunlight intensity and temperature. When natural conditions such as light intensity and temperature change, the output characteristics and output power of the photovoltaic array also change, and the operating point of the system changes accordingly. If the system operating point is not adjusted in time, it will inevitably lead to a reduction in system efficiency. Even under the same light intensity and temperature, due to different loads, the output power of the array is also different. If it is directly connected to the load, it cannot ensure that the array operates at the maximum power point, resulting in power loss. Therefore, for a photovoltaic power generation system, the optimal operating state of the photovoltaic cells should be found to maximize the conversion of light energy into electrical energy. According to circuit theory, when the output impedance of the photovoltaic cell is equal to the load impedance, the photovoltaic cell can output the maximum power. It can be seen that the MPPT (Maximum Power Point Tracking, abbreviated as MPPT) process of the photovoltaic cell is actually a process of matching the output impedance of the photovoltaic cell with the load impedance. In practical applications, the output impedance of the photovoltaic cell is affected by environmental factors, and it is necessary to use a control method to achieve real-time adjustment of the load impedance and make it track the output impedance of the photovoltaic cell to achieve MPPT control of the photovoltaic cell. The tracking control of the maximum power point of the photovoltaic cell is one of the main technologies involved in the photovoltaic power generation system.

[0005] The Perturbation and Observation (P&O) method is one of the most commonly used maximum power point tracking methods for photovoltaic arrays. The basic principle is: perturb the output voltage of the photovoltaic cell, and then observe the change in its output power. Determine the next perturbation direction according to the change trend of the output power, and repeat this process until the photovoltaic cell reaches the maximum power point. Its flowchart is as Figure 2 shown. In practical applications, the output voltage of the photovoltaic module can be adjusted by adjusting the duty cycle of the MPPT controller.

[0006] The step size of the perturbation observation method determines its tracking speed and oscillation amplitude, which are determined by the slope of the output power curve of the photovoltaic module and the radian at the maximum power point. The increase in the dust accumulation density causes the slope of the output power curve of the photovoltaic module to decrease, and the curve becomes flatter near the maximum output power point, so that the original step size is not the most suitable step size under the current dust accumulation density, thus increasing the tracking time. Because the perturbation observation method fixes the step size at the beginning and uses a fixed step size to track the maximum power point, it cannot adapt to the influence of dust accumulation on the photovoltaic module.

[0007] The influence factor α of dust accumulation density on the output current of the photovoltaic module:

[0008]

[0009] Since the magnitude of the influence factor determines the degree to which the dust accumulation density affects the output current of the photovoltaic module, the greater the influence factor, the greater the impact of dust accumulation on the output current of the photovoltaic module, and the greater the degree of reduction in the power generation efficiency of the photovoltaic module. Since dust accumulation mainly affects the output current of the photovoltaic module, the influence of the dust accumulation density on the output power of the photovoltaic module can be approximately measured by the influence factor. Summary of the Invention

[0010] The present invention solves the problem that the existing perturbation observation method using a fixed tracking step size (Δs) cannot adapt to the slowdown of the tracking speed caused by the increase in the dust accumulation density, and provides a maximum power point tracking control method for photovoltaic modules under dust accumulation conditions.

[0011] The present invention is implemented by the following technical solution: A maximum power point tracking control method for photovoltaic modules under dust accumulation conditions is achieved by the following steps:

[0012] (1) Collect the dust on the surface of the reference photovoltaic module and calculate its dust accumulation density R;

[0013] (2) Since the influence of dust accumulation on the output current of the photovoltaic module is approximately linear, the influence of dust accumulation on the step size is measured by the influence factor. As the dust accumulation density increases, the influence degree of dust accumulation on the output performance of the photovoltaic module gradually weakens. The effect of the gradual weakening of the influence of dust accumulation on the output performance of the photovoltaic module is approximately represented by taking the logarithm of the dust accumulation density, and the perturbation step size s of the perturbation observation method is calculated:

[0014] s = s0 + s0·α·ln(R + 1)

[0015] where s0 is the basic step size, α is the influence factor, generally taking values in the range of 0.5 to 0.7, and R is the dust accumulation density;

[0016] (3) Collect the output voltage U(k) and output current I(k) of the photovoltaic module at time k;

[0017] (4) Calculate the output power P(k) = U(k)*I(k) of the photovoltaic module at time k, and compare it with the output power P(k - 1), output voltage U(k - 1), and output current I(k - 1) at time k - 1 (i.e., the previous sampling time), dP(k) = P(k) - P(k - 1), dU(k) = U(k) - U(k - 1), dI(k) = I(k) - I(k - 1);

[0018] (5) When dP(k) / dU(k) > 0, the operating point of the photovoltaic cell is on the left side of the maximum power point;

[0019] When U(k) > U(k - 1), add the perturbation step size s = s0 + s0·α·ln(R + 1) in the original direction; when U(k) < U(k - 1), add the perturbation step size in the opposite direction of the original

[0020] s = s0 + s0·α·ln(R + 1);

[0021] (6) When dP(k) / dU(k) < 0, the operating point of the photovoltaic cell is on the right side of the maximum power point;

[0022] When U(k) > U(k - 1), a perturbation step size s = s0 + s0·α·ln(R + 1) is added in the original opposite direction;

[0023] When U(k) < U(k - 1), a perturbation step size s = s0 + s0·α·ln(R + 1) is added in the original direction;

[0024] (7) When dP(k) / dU(k) = 0, the photovoltaic module operates at the maximum power point;

[0025] (8) U(k - 1) = U(k); I(k - 1) = I(k); P(k - 1) = P(k);

[0026] (9) Return (repeat from step 3).

[0027] In a photovoltaic power generation system under actual dust accumulation conditions, through further research on the P-U characteristic curves of photovoltaic modules under different dust accumulation densities, it can be seen that as the dust accumulation density increases, the output power of the photovoltaic module will decrease significantly, the slope of the curve decreases, and the arc of the curve near the maximum power point increases; this indicates that under different dust accumulation densities, the perturbation observation method can use different sizes of step lengths for tracking. Since the dust accumulation density will reduce the slope of the output power curve, the perturbation amount of the perturbation observation method can be increased. Since near the maximum power point, the larger arc of the output power curve also means that increasing the step length will not cause a greater oscillation in the tracking result.

[0028] Based on the perturbation observation method, the present invention combines the dust accumulation density and the step length of the perturbation observation method, making the fixed-step perturbation observation method into a variable-step perturbation observation method, improving the adaptability of the perturbation observation method, solving the problem that the tracking speed of the perturbation observation method for tracking the maximum power point of photovoltaic modules becomes slower under dust accumulation conditions, and improving the tracking speed and the power generation efficiency of photovoltaic modules. The present invention can not only improve the speed of tracking the maximum power point of photovoltaic modules by the perturbation observation method under dust accumulation conditions, but also has a simple control strategy, strong adaptability, low cost, and can be widely used in the tracking of the maximum power point of photovoltaic modules, having broad application value. Description of the Drawings

[0029] Figure 1 It is the P-U output characteristic curve of the photovoltaic module under dust accumulation conditions;

[0030] Figure 2It is the control flow chart of the existing disturbance observation method;

[0031] Figure 3 It is the flow chart of the maximum power point tracking control method for photovoltaic modules under the dust accumulation condition of the present invention;

[0032] Figure 4 It is the simulation waveform when the step size of the traditional disturbance observation method is -0.001 provided by the example of the present invention;

[0033] Figure 5 It is the simulation waveform of the disturbance observation method adopting the control strategy of the present invention provided by the example of the present invention. Specific implementation manners

[0034] In order to make the purpose, technical solutions and advantages of the present invention clearer, the present invention will be further described in detail below in conjunction with embodiments.

[0035] The present invention is implemented by adopting the following technical solutions: A maximum power point tracking control method for photovoltaic modules under the dust accumulation condition is realized by the following steps:

[0036] (1) Collect the dust on the surface of the reference photovoltaic module and calculate its dust accumulation density R;

[0037] (2) Since the influence of dust accumulation on the output current of the photovoltaic module is approximately linear, the influence of dust accumulation on the step size is measured by an influence factor. As the dust accumulation density increases, the influence degree of dust accumulation on the output performance of the photovoltaic module gradually weakens. The effect of the gradual weakening of the influence of dust accumulation on the output performance of the photovoltaic module is approximately represented by taking the logarithm of the dust accumulation density. The formula for calculating the step size s of the disturbance observation method is:

[0038] s = s0 + s0·α·ln(R + 1)

[0039] Where s0 is the basic step size, α is the influence factor, generally taking values in the range of 0.5 to 0.7, and R is the dust accumulation density;

[0040] (2) Collect the output voltage U(k) and output current I(k) of the photovoltaic module at time k;

[0041] (3) Calculate the output power P(k) = U(k) * I(k) of the photovoltaic module at time k, and compare it with the output power P(k - 1), output voltage U(k - 1), and output current I(k - 1) at time k - 1 (i.e., the previous sampling time), dP(k) = P(k) - P(k - 1), dU(k) = U(k) - U(k - 1), dI(k) = I(k) - I(k - 1);

[0042] (4) When dP(k) / dU(k) > 0, the operating point of the photovoltaic cell is on the left side of the maximum power point;

[0043] When U(k) > U(k - 1), a perturbation step size s = s0 + s0·α·ln(R + 1) is added in the original direction; when U(k) < U(k - 1), a perturbation step size

[0044] s = s0 + s0·α·ln(R + 1) is added in the opposite direction of the original one;

[0045] (5) When dP(k) / dU(k) < 0, the operating point of the photovoltaic cell is on the right side of the maximum power point;

[0046] When U(k) > U(k - 1), a perturbation step size s = s0 + s0·α·ln(R + 1) is added in the opposite direction of the original one;

[0047] When U(k) < U(k - 1), a perturbation step size s = s0 + s0·α·ln(R + 1) is added in the original direction,

[0048] (7) When dP(k) / dU(k) = 0, the photovoltaic module operates at the maximum power point;

[0049] (8) U(k - 1) = U(k); I(k - 1) = I(k); P(k - 1) = P(k);

[0050] (9) Return (repeat from step 3).

[0051] In the MATLAB / Simulink environment, an MPPT control model of the photovoltaic module Boost circuit is built. (1) Set the basic parameters of the perturbation observation method controller. The starting duty cycle Dref is 1, the basic step size s0 is -0.001, α is 0.7, and the dust accumulation density R is assumed to be 10 g / m 2 ; where, under standard conditions, the light intensity S = 1000 W / m 2 , the operating temperature T = 25 °C, and the basic parameters of the photovoltaic module U OC = 36.3 V; U m = 29 V; I SC = 7.84 A; I m = 7.35 A. The simulation waveform of the power output of the photovoltaic module when the step size is set to -0.001 using the traditional perturbation observation method is as Figure 4 shown, and the simulation waveform of the perturbation observation method under the control strategy of the present invention is as Figure 5 shown.

[0052] Therefore, it can be seen that the disturbance observation method of the present invention has great superiority over the traditional disturbance observation method under the dust accumulation condition. It can not only quickly track the maximum power point, but also has very small fluctuations around the maximum power point. Generally speaking, the present invention solves the problem that the tracking speed of the traditional disturbance observation method slows down under the dust accumulation condition, and has a simple control strategy, strong adaptability and low cost. It can be widely used in the maximum power point tracking of photovoltaic modules and has broad application value.

[0053] As mentioned above, it is not intended to impose any form of limitation on the present invention. Although the present invention has been disclosed through the above embodiments, it is not intended to limit the present invention. Any person skilled in the art can make some changes or modifications to equivalent embodiments with equivalent changes within the scope of the technical solution of the present invention by using the disclosed technical content. However, as long as the content does not depart from the technical solution of the present invention, any simple modification, equivalent change and modification made to the above embodiments based on the technical essence of the present invention still fall within the scope of the technical solution of the present invention.

Claims

1. A maximum power point tracking control method for photovoltaic modules under dust accumulation conditions, characterized by being implemented by the following steps: (1) Collect the dust accumulation on the surface of the reference photovoltaic module and calculate its dust accumulation density R; (2) Since the influence of dust accumulation on the output current of the photovoltaic module is approximately linear, the influence of dust accumulation on the step size is measured by an influence factor. As the dust accumulation density increases, the influence degree of dust accumulation on the output performance of the photovoltaic module gradually weakens. The gradually weakening effect of dust accumulation on the output performance of the photovoltaic module is approximately represented by taking the logarithm of the dust accumulation density, and calculate the perturbation step size s of the perturbation observation method: s = s0 + s0·α·ln(R + 1) where s0 is the basic step size, α is the value of the influence factor ranging from 0.5 to 0.7, and R is the dust accumulation density; (3) Collect the output voltage U(k) and output current I(k) of the photovoltaic module at time k; (4) Calculate the output power P(k) = U(k)*I(k) of the photovoltaic module at time k, and compare it with the output power P(k - 1), output voltage U(k - 1), and output current I(k - 1) at the previous sampling time k - 1. dP(k) = P(k) - P(k - 1), dU(k) = U(k) - U(k - 1), dI(k) = I(k) - I(k - 1); (5) When dP(k) / dU(k) > 0, the operating point of the photovoltaic cell is on the left side of the maximum power point; When U(k) > U(k - 1), add the perturbation step size s = s0 + s0·α·ln(R + 1) in the original direction; when U(k) < U(k - 1), add the perturbation step size s = s0 + s0·α·ln(R + 1) in the opposite direction; (6) When dP(k) / dU(k) < 0, the operating point of the photovoltaic cell is on the right side of the maximum power point; When U(k) > U(k - 1), add the perturbation step size s = s0 + s0·α·ln(R + 1) in the opposite direction; when U(k) < U(k - 1), add the perturbation step size s = s0 + s0·α·ln(R + 1) in the original direction; (7) When dP(k) / dU(k) = 0, the photovoltaic module operates at the maximum power point; (8) When dP(k) / dU(k) ≠ 0, save the current U(k), I(k), and P(k) as U(k - 1), I(k - 1), and P(k - 1) in the next cycle; and return to step 3 to repeat.

2. The maximum power point tracking control method for a photovoltaic module under the dust accumulation condition according to claim 1, wherein: Set a reference photovoltaic module and determine the dust accumulation density R on its panel; Calculate the step size s of the perturbation observation method according to the magnitude of the dust accumulation density; add the dust accumulation density to the tracking process of the perturbation observation method according to the relationship between the dust accumulation density and the perturbation observation method.

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