Photovoltaic flexible power point tracking control method based on linear jump

Abstract

The invention provides a photovoltaic flexible power point tracking control method based on linear jump, and the method can employ a direct calculation mode and an indirect estimation mode to calculate the linear slope at the left side of the maximum power point of a photovoltaic power voltage characteristic curve under different conditions, and the linear slope serves as a jump current, thereby determining a reference voltage value. And when the reference power is smaller than the photovoltaic available power, a linear jump strategy is adopted, and the photovoltaic output point directly jumps to the vicinity of the reference power according to the calculated reference voltage. On the contrary, an initial jumping step is added in the maximum power point tracking method, so that the photovoltaic output point jumps to the position near the maximum power point, then the classical maximum power point tracking method is adopted to keep stable tracking of the maximum power point, and therefore rapid and stable tracking is achieved; according to the method, when the environment changes and the reference power changes, the working point of the system can be rapidly adjusted through simple logic, accurate tracking of the reference power is achieved, and therefore the reliability of the photovoltaic power station is improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of flexible power point tracking of photovoltaic power generation, and more specifically, relates to a tracking method for nonlinearly tracking a constant power point to improve the reliability of a photovoltaic power station. Background Art

[0002] The output power of the photovoltaic system is affected by many factors, such as solar radiation intensity, ambient temperature, aging of photovoltaic panels, dust accumulation on photovoltaic panels due to dust in the atmosphere, etc. These factors make the output power of the photovoltaic system change over time, resulting in the system not always working at the optimal power output point. The maximum power point tracking method adjusts the working point of the photovoltaic system so that the system always works at the maximum power output point, thereby improving the overall efficiency. However, due to the intermittent nature of solar energy, the use of maximum power point tracking control will bring some challenges to the power grid, such as reverse power flow and low inertia. According to the grid specification, a flexible power point tracking method is required to reduce the active power of the photovoltaic system in the case of fault ride-through or frequency ride-through, so as to provide sufficient room for auxiliary services. The flexible power point tracking method achieves the required constant power by temporarily deviating from the maximum power point when environmental conditions change. Once the fault is resolved, the flexible power point tracking method will resume tracking the maximum power point.

[0003] Although previous flexible power point tracking methods can achieve flexible power point tracking, they often have certain shortcomings. Classic linear search flexible power point tracking methods, such as the perturbation observation method, are relatively simple to implement. However, the fixed step size causes the method to converge slowly when the environment changes, and there is often a certain steady-state oscillation. Classic nonlinear search flexible power point tracking methods, such as the binary search method, iteratively reduce the search space to half of the previous one, thereby improving the convergence speed and eliminating the steady-state power oscillation problem. However, this method requires a separate method to set the search boundary to cope with changes in illumination, which increases the complexity of the system. Another example is a flexible power point tracking method based on Newton's method, which has a quadratic convergence characteristic and can achieve a higher convergence speed. However, this method often exhibits instability when the environment changes, resulting in significant power loss.

[0004] Based on the shortcomings of the classical method, such as slow tracking, steady-state disturbance, inability to accurately and quickly respond to changes in the external environment or reference power, and certain complexity, the present invention proposes a photovoltaic flexible power point tracking control method based on linear jump. The method is simple and can quickly and stably track the flexible power point or maximum power point to achieve the purpose of increasing power generation. Summary of the invention

[0005] A photovoltaic flexible power point tracking control method based on linear jump can calculate the linear slope on the left side of the maximum power point of the photovoltaic power-voltage characteristic curve by direct calculation and indirect estimation in different situations, so as to determine the reference voltage value corresponding to the reference power; when the reference power is less than the photovoltaic available power, a linear jump strategy is adopted, and the photovoltaic output point directly jumps to the reference power according to the calculated reference voltage; when the maximum output power of the system cannot meet the reference power, a hybrid power point tracking strategy is adopted, that is, an initial jump step is added to the maximum power method, so that the photovoltaic output point first jumps to the vicinity of the maximum power point, and then the maximum power point tracking method is used to keep tracking the maximum power point; this method greatly improves the speed of the system dynamic response, and can achieve effective tracking in complex scenarios such as continuous changes in the reference power or light, and the steps in the use process are:

[0006] Step (1): Measure the voltage and current output by the photovoltaic system, and calculate the photovoltaic output power based on the voltage and current; determine the reference power and power error , power error is the current photovoltaic output power With reference power The light intensity is measured by the light sensor, and the voltage value corresponding to the maximum power point is estimated according to the light intensity. ;

[0007] Step (2): Based on the power error and power threshold The relationship between the power threshold and the power threshold is 2% of the base power. ;

[0008] Step (3): If the power error satisfies or , that is, the current output power is in the stable area of ​​the jump method or above the stable area of ​​the jump method, indicating that the reference power is less than the available photovoltaic power, and the linear jump method needs to be executed to directly calculate the reference voltage Thus, the reference power can be tracked; the jump method stable region is defined as satisfying The calculation of jump current in the linear jump method is divided into direct calculation and indirect estimation. If the voltage at the photovoltaic output point is The slope of the power voltage characteristic curve is positive and approximately constant. This approximately constant linear slope is defined as the jump current. At this time, the jump current is calculated directly, and the current at the current output point is used as the jump current. , reference voltage Equal to the reference power Divided by the jump current ; If the voltage at the photovoltaic output point is The slope of the power-voltage characteristic curve is negative. In this case, the indirect estimation method of the jump current is used to estimate the linear current on the left side of the maximum power point using light, and this current is used as the jump current. , and then calculate the reference voltage To achieve fast tracking;

[0009] Step (4): If the power error does not satisfy or , that is, the current output power is below the stable area of ​​the jump method. At this time, the reference power is either greater than or less than the photovoltaic available power. In this case, the hybrid power tracking method needs to be implemented;

[0010] Step (5): When the hybrid power tracking method is executed, the linear jump method is first executed, and the reference voltage calculated by the linear jump method is To determine whether the program should track the flexible power point or the maximum power point; if , it means that the reference power is less than the available power of the photovoltaic system, so it can directly , thereby achieving a linear jump to near the reference power to achieve fast power tracking; if , it means that the reference power is greater than the maximum power of the photovoltaic system, which means that to keep tracking the maximum power point, this method first lets the photovoltaic output point jump to Then the maximum power point is tracked by the perturbation and observation method, thus achieving fast power tracking switching;

[0011] Step (6): Repeat steps (1) to (5) to keep tracking the maximum power point or the flexible power point.

[0012] Compared with the prior art, the present invention has the following advantages and effects:

[0013] (1) It can realize power tracking in a single step, significantly improving the tracking speed of the system. When tracking a constant power point, steady-state oscillation will not occur, thus improving the tracking accuracy.

[0014] (2) By utilizing the relationship between the photovoltaic output short-circuit current and the light characteristics, the problem of excessive jumps or failure to track the reference power when the photovoltaic output point is located to the right of the maximum power point can be effectively solved, thereby improving the tracking speed and tracking accuracy.

[0015] (3) The proposed hybrid power tracking method combines the jump method and the maximum power point tracking method. It can not only quickly determine the relationship between the system maximum power and the reference power, but also has a simple structure and can smoothly switch between the constant power tracking mode and the maximum power point tracking mode.

[0016] (4) This method effectively shortens the maximum power point tracking time by adding an initial jump step to the maximum power point tracking method.

[0017] (5) The test results on the hardware platform and the comparison with other methods show that this method has the lowest cumulative error. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 It is a system framework diagram of the method of the present invention.

[0019] Figure 2 It is a schematic diagram of the jump current calculation range criterion of the method of the present invention.

[0020] Figure 3 It is a schematic diagram of jump iteration of the method of the present invention.

[0021] Figure 4 It is an overall flow chart of the method of the present invention. DETAILED DESCRIPTION

[0022] The present invention proposes a photovoltaic flexible power point tracking control method based on linear jump, which is described in detail as follows in conjunction with the accompanying drawings:

[0023] Figure 1 This is the system framework diagram of the method of the present invention, which adopts a control model of a two-stage photovoltaic power generation system. The model adopts a two-stage single-phase photovoltaic grid-connected system; the system is composed of key modules such as photovoltaic components, DC-DC converters, photovoltaic voltage control and flexible power point tracking methods. The flexible power point tracking method receives the reference power value This value is often determined by the external platform on the grid side according to the grid specifications, such as grid frequency support or low voltage ride-through requirements. The flexible power point tracking method also receives the voltage value corresponding to the maximum power point. , based on the light intensity measured by the light sensor And the photovoltaic output power calculated ; Flexible power point tracking method outputs reference voltage And sent to the voltage controller module to generate a pulse width modulation signal for the next step of power tracking.

[0024] Based on the linearity of the power-voltage characteristic curve on the left side of the maximum power point, if the reference power value to be tracked is known ,and The value is less than the maximum available power of the photovoltaic panel , the linear slope of the power-voltage characteristic curve can be used to directly calculate the power corresponding to the reference power The reference voltage value :

[0025] (1)

[0026] Where K is the slope value of the linear region of the power-voltage characteristic curve. This method defines this linear slope as the jump current .

[0027] Figure 2 It is a schematic diagram of the jump current calculation range criterion of the method of the present invention. The determination of is divided into direct calculation method and indirect estimation method, and the applicable areas correspond to Figure 2 Specifically, region ① refers to the voltage at The photovoltaic power-voltage characteristic curve corresponding to the maximum power point is the area on the left side, while area ② refers to the voltage at The corresponding photovoltaic power voltage characteristic curve is the area to the right of the maximum power point. is the open circuit voltage.

[0028] When the output point is in the left area of ​​the maximum power point, the jump current I T It can be calculated by the ratio of the current power to the current voltage on the left side of the maximum power point:

[0029] (2)

[0030] in is the current output power of the photovoltaic system, The current output voltage of the photovoltaic system.

[0031] Figure 3 This is a schematic diagram of the jump iteration of the method of the present invention. This tracking process can meet the photovoltaic system When the system is initially located at , no matter how far the initial point is from the target power point, it can jump to the vicinity of the reference power in one step, so this process is called a linear jump process. Figure 3 Point A in the figure, tracking the reference power requires ultimately keeping tracking the reference voltage . This method approximates the current at point A to be equal to the linear slope, and calculates the reference voltage through formula (1), that is, jumping to point A1. The power at point A1 is relatively close to the reference power but there is still a certain deviation. This is because the left part of the maximum power point is not strictly linear, and its slope, that is, the current, will slowly decrease as the voltage increases. If the reference voltage is calculated based on the current current at point A1 for the next jump, that is, jumping to point A2. The power at point A2 is closer to the reference power than point A1. By performing jump iterations in this way, the output power of the system will be infinitely close to the reference power and thus achieve stable tracking. Throughout the process, the current at the output point is larger than the current corresponding to the reference power point on the left, so the iterative process is located at Left side. The initial point of the system is Figure 3 The jump process at point B is similar to the above process, but since the current at the output point in the whole process is smaller than the current corresponding to the reference power point on the left, the iteration process is located at Right side.

[0032] However, the photovoltaic output point may be located to the right of the maximum power point. For example, if the light intensity changes significantly, the slope of the power-voltage characteristic curve will be negative, which is obviously not equal to the current value. Moreover, the current value will decrease faster and change more significantly with the increase of voltage. If the jump current is obtained by direct calculation, the jump situation will be related to the relative size of the working point and the reference power. Figure 2 For example, the approximately constant linear slope on the left side of the maximum power point corresponds to the short-circuit current of the actual photovoltaic output. Figure 2 The current at point C is quite different from the short-circuit current. If the current at point C is used as the jump current to calculate the reference voltage, the power at the jump point will often deviate greatly from the reference power. It may take multiple jumps to track the reference power or even be impossible to track the reference power. Figure 2 In , at this time, the current at point C is greater than the current at the reference power point fpp-r on the right. If the current at point C is still used as the jump current, the reference voltage calculated according to formula (1) will be less than the voltage at point C, that is, , but it is often still on the right side of the maximum power point. After multiple jump iterations, the photovoltaic output point will eventually jump to the left side of the maximum power point and converge to the left reference power point. If the reference power is Figure 2 In At this time, the current at point fpp-r is greater than the current at point C. If the current at point C continues to be used as the jump current, the reference voltage calculated according to formula (1) will be greater than the voltage at point C. If the jump iteration continues, the photovoltaic output point will eventually be greater than the open-circuit voltage and will be unable to track the reference power.

[0033] Therefore, when the output point is located in the area to the right of the maximum power point, the direct calculation method is not applicable. In order to ensure the rapidity and accuracy of system tracking, the present invention adopts a method of indirectly estimating the linear slope. The short-circuit current value is indirectly estimated by using the law of short-circuit current, irradiation intensity and temperature in the photovoltaic system:

[0034] (3)

[0035] In the formula is the light intensity at the current moment, is the short-circuit current at the current moment, is the standard light intensity, is the standard short-circuit current value corresponding to the standard light intensity, is the difference between the current temperature and the standard temperature, is the temperature coefficient. This indirect estimation method can indirectly estimate the jump current when the system operating point is to the right of the maximum power point based on the known change in light intensity under the condition that the temperature remains unchanged and the light intensity changes:

[0036] (4)

[0037] In summary, the calculation method of jump current is summarized as:

[0038] (5)

[0039] in The method of determining is:

[0040] (6)

[0041] Among them is is the temperature coefficient, is the illumination coefficient, is the difference between the standard radiation intensity and the current radiation intensity, and the voltage value corresponding to the maximum power point under standard conditions It can be obtained from the manufacturer's data sheet.

[0042] Figure 4 is the overall flow chart of the method of the present invention. When it is detected that the photovoltaic output power is in the stable region of the jump method or above the stable region of the jump method, the linear jump strategy is adopted. When the photovoltaic output power is below the stable region of the jump method, the hybrid power point tracking strategy needs to be used. The hybrid power tracking strategy first obtains the reference voltage through the linear jump strategy .when Less than the voltage corresponding to the maximum power point When , it can be judged that the reference power is less than the available photovoltaic power, and the linear jump strategy is maintained. Greater than the voltage corresponding to the maximum power point When the reference power is greater than the available photovoltaic power, the maximum power point should be tracked. Different from the classic maximum power point tracking method, when it is determined that the maximum power point needs to be tracked, the hybrid power point tracking strategy adds an initial jump step, that is, it first jumps to the vicinity of the maximum power point, and then uses the maximum power point tracking method to keep tracking the maximum power, thereby significantly improving the response speed of the system. Figure 4 middle Indicates the difference between the current power of the photovoltaic system and the power at the previous moment. Indicates the difference between the current voltage of the photovoltaic system and the voltage at the previous moment. represents the voltage step size of the disturbance applied to the system, Represents the reference voltage calculated according to the maximum power point method. The key to the photovoltaic flexible power point tracking method based on linear jump is to reasonably judge and switch to the linear jump strategy or maximum power point tracking method when the light changes, so as to effectively respond to the changes in light conditions and achieve fast and stable tracking effects, thereby improving the control effect of flexible power point tracking.

[0043] The photovoltaic flexible power point tracking control method based on linear jump was verified by comparative experiments on Yanxu's semi-physical simulation platform, using the cumulative error CE as the evaluation index:

[0044] (7)

[0045] The cumulative errors of each method in the experimental results are: linear jump method 1.772%, Newton method 6.057%, bisection method 4.693%, adaptive perturbation observation method 5.180%, general method 3.950%, and secant method 3.114%.

[0046] The above description is only a preferred embodiment of the present invention, and does not limit the patent scope of the present invention. Any equivalent structure or equivalent process transformation made by using the contents of the present invention specification and drawings, or directly or indirectly applied in other related technical fields, are also included in the patent protection scope of the present invention.

Claims

1. A photovoltaic flexible power point tracking control method based on linear jump, characterized in that: The method can calculate the linear slope on the left side of the maximum power point of the photovoltaic power-voltage characteristic curve as the jump current by direct calculation and indirect estimation in different situations, so as to determine the reference voltage value corresponding to the reference power; when the reference power is less than the photovoltaic available power, a linear jump strategy is adopted, and the photovoltaic output point directly jumps to the vicinity of the reference power according to the calculated reference voltage; when the maximum output power of the system cannot meet the reference power, the method adopts a hybrid power point tracking strategy, that is, an initial jump step is added to the maximum power point tracking method, so that the photovoltaic output point first jumps to the vicinity of the maximum power point, and then the maximum power point tracking method is used to keep tracking the maximum power point; the method greatly improves the speed of the system dynamic response, and can achieve effective tracking in complex scenarios such as continuous changes in the reference power or light. The steps in the use process are: Step (1): Measure the voltage and current output by the photovoltaic system, and calculate the photovoltaic output power based on the voltage and current; determine the reference power and power error , power error is the current photovoltaic output power With reference power The light intensity is measured by the light sensor, and the voltage value corresponding to the maximum power point is estimated according to the light intensity. ; Step (2): Based on the power error and power threshold The relationship between the power threshold and the power threshold is 2% of the base power. ; Step (3): If the power error satisfies or , that is, the current output power is in the stable area of ​​the jump method or above the stable area of ​​the jump method, indicating that the reference power is less than the available photovoltaic power, and the linear jump method needs to be executed to directly calculate the reference voltage Thus, the reference power can be tracked; the jump method stable region is defined as satisfying The calculation of jump current in the linear jump method is divided into direct calculation and indirect estimation. If the voltage at the photovoltaic output point is The slope of the power voltage characteristic curve is positive and approximately constant. This approximately constant linear slope is defined as the jump current. At this time, the jump current is calculated directly, and the current at the current output point is used as the jump current. , reference voltage Equal to the reference power Divided by the jump current ; If the voltage at the photovoltaic output point is The slope of the power-voltage characteristic curve is negative. At this time, the indirect estimation method of the jump current is used to estimate the linear current on the left side of the maximum power point using light, and this current is used as the jump current. , and then calculate the reference voltage To achieve fast tracking; Step (4): If the power error does not satisfy or , that is, the current output power is below the stable area of ​​the jump method. At this time, the reference power is either greater than or less than the photovoltaic available power. In this case, the hybrid power tracking method needs to be implemented; Step (5): When the hybrid power tracking method is executed, the linear jump method is first executed, and the reference voltage calculated by the linear jump method is To determine whether the program should track the flexible power point or the maximum power point; if , it means that the reference power is less than the available power of the photovoltaic system, so it can directly , thereby achieving a linear jump to near the reference power to achieve fast power tracking; if , it means that the reference power is greater than the maximum power of the photovoltaic system, which means that to keep tracking the maximum power point, this method first lets the photovoltaic output point jump to Then the maximum power point is tracked by the perturbation and observation method, thus achieving fast power tracking switching; Step (6): Repeat steps (1) to (5) to keep tracking the maximum power point or the flexible power point.

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