Photovoltaic cell maximum power tracking method based on impedance matching

Abstract

The invention relates to the field of distributed photovoltaic power generation, and aims to provide a photovoltaic cell maximum power tracking method based on impedance matching. Comprising the steps of collecting open-circuit voltage and short-circuit current output by a photovoltaic module; inquiring the ratio k by utilizing the short-circuit current and a constant I0 based on a curve that the impedance ratio k changes along with the short-circuit current; then determining impedance corresponding to the maximum power point according to the ratio of the open-circuit voltage to the short-circuit current, performing closed-loop tracking control on the impedance of the input port of the power electronic converter by using the value of the impedance, and when the tracking reaches a steady state, enabling the voltage and current of the output port of the photovoltaic module to correspond to the voltage and current of the maximum power point; therefore, the maximum power tracking of the photovoltaic module is realized. According to the invention, multiple iterative computations are not needed, the tracking dynamics is improved, computing resources can be greatly saved, and the system cost is reduced; the calculation basis is obtained by strict mathematical derivation, the accuracy is improved, the maximum power tracking can be theoretically realized without errors, and the method has universality.

Description

Technical Field

[0001] The present invention relates to the field of distributed photovoltaic power generation, and more specifically, to a photovoltaic cell maximum power tracking method based on impedance matching. Background Art

[0002] Photovoltaic power generation is a key way for humanity to effectively utilize solar energy. Solar energy is inexhaustible, renewable, clean, and environmentally friendly. Reducing costs and increasing efficiency is a perennial theme in the photovoltaic industry. From P-type to N-type, and from single-junction to tandem photovoltaic cell technologies, breakthroughs in photoelectric conversion efficiency have been achieved. The theoretical maximum conversion efficiency of solar cells has also been raised from 29% to 43%. The achievement of photovoltaic parity has enabled photovoltaics to evolve from a "supplementary source" to a "stable source" and will become the "mainstay" of future electricity.

[0003] The photoelectric effect equation shows that the voltage-current relationship at the photovoltaic cell port is highly nonlinear. This characteristic results in a single-peak power-voltage curve with the maximum power point as its extreme value. Therefore, the DC power generated by photovoltaic cells must pass through a power electronic converter before it can be connected to the grid or supplied to a load. This converter performs maximum power point tracking (MPPT) to maximize power generation efficiency and converts it into DC or AC power compatible with the grid and load.

[0004] The essence of MPPT is to adjust the impedance of the power electronic converter to match the impedance of the photovoltaic port, so that it can operate at the maximum power point. MPPT methods in traditional technologies, such as the perturbation observation method and the conductance increment method, cannot take into account both the dynamic response speed and steady-state accuracy of tracking. The maximum power point closed-loop tracking method based on impedance matching (also known as the impedance closed-loop tracking method) can overcome the contradiction between tracking dynamic response and steady-state accuracy, becoming an ideal MPPT method. However, the generation of the impedance reference value corresponding to the maximum power point requires multiple iterative calculations, which will greatly affect the dynamic response of the system, while occupying a large amount of computing resources and increasing system costs.

[0005] Therefore, the present invention proposes a new solution to solve the contradiction between dynamic response and steady-state accuracy in the existing photovoltaic maximum power point tracking method. Summary of the Invention

[0006] The technical problem to be solved by the present invention is to overcome the deficiencies in the prior art and provide a photovoltaic cell maximum power tracking method based on impedance matching.

[0007] To solve the technical problem, the solution of the present invention is:

[0008] A photovoltaic cell maximum power point tracking method based on impedance matching is provided, comprising:

[0009] Connect the photovoltaic module, power electronic converter and power load, and collect the open circuit voltage V output by the photovoltaic module oc and short-circuit current I sc ;

[0010] Based on the impedance ratio k following the short-circuit current I sc Change curve, using short-circuit current I sc And constant I0 query ratio k; then according to the ratio of open circuit voltage and short circuit current V oc / I sc Determine the impedance U corresponding to the maximum power point m / I m , recorded as the impedance reference value Z ref ;

[0011] Using the impedance reference value Z ref The impedance of the input port of the power electronic converter is tracked in a closed loop. When the tracking reaches a steady state, the voltage and current at the output port of the photovoltaic module correspond to the voltage and current at the maximum power point, thereby achieving maximum power tracking of the photovoltaic module.

[0012] As a preferred solution of the present invention, the power electronic converter is adjusted to make the photovoltaic module work at the open circuit point, and the output voltage at this time is collected as the open circuit voltage V oc ; Adjust the power electronic converter to make the photovoltaic module work at the short-circuit point, and collect the output current at this time as the short-circuit current I sc .

[0013] As a preferred solution of the present invention, the impedance ratio k follows the short-circuit current I sc The change curve is k-ln -1 (I sc / I0+1) relationship curve; after querying the ratio k, calculate the impedance reference value Z according to the following formula ref :

[0014]

[0015] Where k is U m / I m With V oc / I sc The ratio of U m / I m is the impedance corresponding to the maximum power point, V oc / I sc It is the ratio of open circuit voltage to short circuit current; I0 ​​is a known constant, which is the reverse saturation current of the photovoltaic module in the absence of light when it leaves the factory.

[0016] As a preferred solution of the present invention, the impedance ratio k follows the short-circuit current I scThe variation curve is obtained by solving the basic current equation of the photovoltaic effect and its equivalent derivation using the Newton-Raphson method.

[0017] As a preferred solution of the present invention, the impedance ratio k follows the short-circuit current I sc The change curve is stored in the form of data pairs and called by the controller.

[0018] As a preferred solution of the present invention, when performing closed-loop tracking control, the calculated impedance reference value Z ref As a closed-loop control reference value; subtracting the reference value from the real-time impedance of the converter to obtain an error amount, and inputting the error amount into a closed-loop controller to obtain a control amount reference value; using the control amount reference value to control the switch tube of the power circuit in the power electronic converter to achieve maximum power tracking of the photovoltaic module.

[0019] The present invention also provides a system for realizing maximum power point tracking of photovoltaic cells based on impedance matching, comprising a power electronic converter, wherein the power electronic converter comprises a power circuit, a peripheral sampling circuit and a controller; wherein,

[0020] The sampling port of the peripheral sampling circuit is arranged at the output end of the photovoltaic module. The peripheral sampling circuit is electrically connected to the controller to transmit the voltage and current signals obtained by sampling. The controller is electrically connected to the switch tube in the power circuit to transmit the drive signal for adjusting the working state of the converter. The controller is electrically connected to at least one memory, and instructions are stored in the memory. The controller executes the aforementioned photovoltaic cell maximum power tracking method based on impedance matching according to the instructions.

[0021] As a preferred solution of the present invention, the photovoltaic assembly includes at least one group of photovoltaic power generation units.

[0022] As a preferred solution of the present invention, the power circuit is a DC-DC power circuit or a DC-AC power circuit.

[0023] As a preferred solution of the present invention, the controller is capable of performing floating-point operations and is any one of DSP, ARM, and FPGA.

[0024] The present invention further provides a computer-readable storage medium storing computer instructions, wherein the computer instructions are used to enable a controller to execute the aforementioned photovoltaic cell maximum power point tracking method based on impedance matching.

[0025] Compared with the prior art, the present invention has the following beneficial effects:

[0026] 1. The present invention simply samples the current open-circuit voltage and short-circuit current of the photovoltaic module to calculate the impedance reference value, which is then used for impedance closed-loop tracking control to achieve maximum power tracking. This eliminates the need for multiple iterative calculations, improves tracking dynamics, significantly saves computing resources, and reduces system costs.

[0027] 2. The calculation basis of the impedance reference value in the present invention is obtained by strict mathematical derivation, which greatly improves the accuracy compared with the existing technology and can theoretically achieve maximum power tracking without error.

[0028] 3. The method of the present invention is universal and applicable to all photovoltaic converters, including DC-DC and DC-AC power electronic converters.

[0029] 4. The present invention only needs to sample the open-circuit voltage and short-circuit current, so the operation can be realized very quickly and an impedance reference value can be generated in a short time for tracking control. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 This is a schematic structural diagram of the photovoltaic power generation system.

[0031] Figure 2 The flowchart of the photovoltaic cell maximum power point tracking method based on impedance matching is shown in FIG.

[0032] Figure 3 The impedance ratio k follows the short-circuit current I sc Change curve.

[0033] Figure 4 This is the impedance closed-loop tracking control block diagram.

[0034] Figure 1 Description of the reference numerals in FIG: photovoltaic component 101; power electronic converter 102; power load 103. DETAILED DESCRIPTION

[0035] The specific embodiments of the present invention are further described in detail below with reference to the accompanying drawings.

[0036] like Figure 1 As shown, the photovoltaic power generation system of the present invention includes a photovoltaic module 101, a power electronic converter 102, and an electrical load 103. The photovoltaic module 101 is used to receive sunlight, generating a photovoltaic effect to generate current. The photovoltaic module 101 includes at least one set of photovoltaic power generation units. The power electronic converter 102 is used to convert the direct current (DC) generated by the photovoltaic module 101 into usable DC or AC power, while also performing photovoltaic maximum power point tracking (PVMPT). The electrical load 103 is used to receive the power output from the converter.

[0037] The power electronic converter 102 includes a power circuit, a peripheral sampling circuit, and a controller. The power circuit can be a DC-DC circuit or a DC-AC circuit, and is used to achieve power conversion and photovoltaic maximum power tracking. The sampling port of the peripheral sampling circuit is located at the output of the photovoltaic module 101, sampling and obtaining voltage and current signals, performing corresponding modulation processing, and then outputting them to the ADC port of the controller. The controller is electrically connected to the switching transistors in the power circuit to transmit drive signals used to adjust the operating state of the converter. After receiving the signals from the peripheral sampling circuit, the controller completes internal calculations and outputs PWM control signals to implement the relevant control functions of the converter. The controller is capable of floating-point operations and can be any of the general-purpose control chips such as DSP, ARM, and FPGA, supporting real-time online calculations and precise control functions. When the controller outputs a steady-state control variable (such as duty cycle), the power circuit will regulate the photovoltaic module 101 to operate to the corresponding unique state (including the voltage and current at the output of the photovoltaic module 101).

[0038] Based on the knowledge of those skilled in the art, the controller can be electrically connected to at least one memory, in which instructions and data are stored in the form of an impedance ratio k following the short-circuit current I sc The controller executes the photovoltaic cell maximum power tracking method based on impedance matching according to the instruction.

[0039] The photovoltaic cell maximum power tracking method based on impedance matching in the present invention is implemented based on the following principles:

[0040] By adjusting the power circuit to an open circuit or short circuit state, the peripheral sampling circuit samples the open circuit voltage V oc And the short-circuit current I sc After completing signal modulation and processing, the controller outputs it to the controller. After receiving the current open-circuit voltage and short-circuit current values ​​of the photovoltaic module, the controller reads the impedance ratio k stored in the memory in the form of data pairs and follows the short-circuit current I sc Change the curve and perform query operations to obtain the corresponding impedance U at the maximum power point at this time m / I m With V oc / I sc The ratio k of the maximum power point is calculated to obtain the corresponding impedance U m / I m , which is used as the impedance reference value Z for the converter closed-loop tracking ref Used for maximum power point tracking control of photovoltaic modules.

[0041] like Figure 2As shown, the workflow of the maximum power tracking method of the present invention includes three parts in sequence: open circuit voltage and short circuit current sampling, impedance reference value calculation and impedance closed-loop tracking control.

[0042] In the sampling phase, the impedance of the power electronic converter is adjusted to make the photovoltaic module work in an open circuit state, and the output voltage at this time is collected as the open circuit voltage V oc Adjust the impedance of the power electronic converter to make the photovoltaic module work in a short-circuit state, and collect the output current at this time as the short-circuit current I sc .

[0043] In the calculation phase, based on the impedance ratio k following the short-circuit current I sc Change curve, using short-circuit current I sc And constant I0 query ratio k; then according to the ratio of open circuit voltage and short circuit current V oc / I sc Determine the impedance U corresponding to the maximum power point m / I m , recorded as the impedance reference value Z ref .

[0044] Impedance ratio k follows short-circuit current I sc The change curve is k-ln -1 (I sc / I0+1) relationship curve, for example Figure 3 The middle curve can be obtained by solving the basic current equation of the photovoltaic effect and its equivalent derivation using the Newton-Raphson method. The ratio k follows the short-circuit current I sc The conversion of the constant I0 is the data basis for online query. After the ratio k is obtained, the impedance reference value Z is calculated according to the following formula ref :

[0045]

[0046] Where k is U m / I m With V oc / I sc The ratio of U m / I m is the impedance corresponding to the maximum power point, V oc / I sc It is the ratio of open circuit voltage to short circuit current; I0 ​​is a known constant, which is the reverse saturation current of the photovoltaic module in the absence of light when it leaves the factory.

[0047] In the closed-loop tracking control link, the impedance reference value Z refThe impedance of the power electronic converter input port is tracked in a closed loop. When the tracking reaches a steady state, the voltage and current at the output port of the photovoltaic module correspond to the voltage and current at the maximum power point, thereby achieving maximum power tracking of the photovoltaic module. Figure 4 As shown, the impedance closed-loop tracking control block diagram described in the present invention includes: an error generation link between the actual impedance of the converter and the impedance reference value, a closed-loop controller tracking link, and a transfer function link between the control quantity and the controlled quantity, which are used to achieve closed-loop tracking of the impedance reference value.

[0048] The following is a more detailed description of the working principle of the photovoltaic cell maximum power point tracking method based on impedance matching with the accompanying drawings:

[0049] First, the ratio k is determined by the short-circuit current I sc There is a strict and unique correspondence between and the constant I0 (that is, there is a functional relationship between the two), and this fact is proved below.

[0050] The relationship between the photovoltaic cell terminal current I and voltage V can be obtained from the basic equation of the photovoltaic effect (1):

[0051]

[0052] Among them, I sc represents the short-circuit current of the photovoltaic module, I0 represents the reverse saturation current of the photovoltaic cell in the absence of light, and e represents the natural logarithm; Represents the comprehensive parameter value of the photovoltaic cell, where q is the charge of the electron, n is the constant factor, kB is the Boltzmann constant, T is the PN junction temperature; V represents the output voltage of the photovoltaic cell; I represents the output current of the photovoltaic cell.

[0053] Let I = 0 in formula (1), and the open circuit voltage expression of the photovoltaic module shown in formula (2) can be obtained:

[0054]

[0055] It should be noted that the reverse saturation current I0 of the photovoltaic cell is constant when there is no light and does not change with the external environment. At the same time, when there is no sudden change in any light, the open circuit voltage V oc and short-circuit current I sc Corresponds to a unique value and is a constant at that moment.

[0056] For the convenience of analysis, we define the variables C2 and C1 as shown in the following equations (3) and (4). C2 and C1 have a monotonic function relationship:

[0057]

[0058] According to equations (3), (4) and (2), equation (1) can be rewritten as equation (5):

[0059]

[0060] The open circuit voltage V oc and short-circuit current I sc As the denominator, the port voltage and current of the photovoltaic module are normalized to obtain the per-unit value expressions (6), (7), and (8):

[0061]

[0062] Among them, I * 、V * 、P * They refer to the normalized per-unit values ​​of current, voltage, and power respectively; I, V, and P refer to the output current, voltage, and power of the PV module ports respectively.

[0063] From formula (1), it can be deduced that the photovoltaic module power-voltage (PV) curve has a unique maximum point, that is, the unique maximum point (V m ,I m ), so the partial derivative of power with respect to voltage is 0 at the maximum value. The per-unit values ​​of voltage and current at this time are (V * m ,I * m ). At the same time, due to the open circuit voltage V oc and short-circuit current I sc is a constant, so the power per unit value to voltage per unit value is also 0 at the maximum value point, that is:

[0064]

[0065] Further expansion yields:

[0066]

[0067] Obviously, from formula (10) we can know that V * m There is a functional relationship between and C2.

[0068] According to formula (6), in (V * m ,I * m ) can satisfy the formula (11):

[0069]

[0070] From formula (11), we can know that I * m With V *m There is a functional relationship between and C2, and since formula (10) shows that V * m There is a functional relationship between V and C2, so V * m with I * m The ratio of can be expressed as a certain functional relationship of C2, as shown in formula (12):

[0071]

[0072] Since C2 and the current determine the short-circuit current I sc and constant I0 have a strict unique correspondence, so V * m with I * m The ratio k of the short-circuit current I sc There is also a strict unique correspondence between and the constant I0, as shown in formula (13), and the functional relationship between the two is proved.

[0073]

[0074] However, the ratio k is determined by the short-circuit current I sc There is no explicit expression between the constant I0, so the comparison value k and the currently determined short-circuit current I sc The method for solving the curve between and constant I0 is explained.

[0075] Because I sc Represents the short-circuit current of the photovoltaic module, I0 represents the saturation current when there is no light, so when there is no light, I sc =I0, when there is light I sc >I0, so I sc >=I0. So the domain satisfies formula (14):

[0076]

[0077] Use Matlab script file to write program and set variable I sc / I0 starts from 1 and increases to infinity with a fixed step size, and solves the corresponding (V * m ,I * m ), and finally draw k=V * m / I * m with I scThe corresponding data can also be exported and stored in a memory electrically connected to the controller for online query operation by the controller.

[0078] Finally, the impedance reference value Z corresponding to the maximum power point is calculated by formula (15): ref .

[0079]

[0080] When performing closed-loop tracking control, the impedance reference value Z ref This is used as a closed-loop control reference value. This reference value is subtracted from the converter's real-time impedance to generate an error, which is then fed into a closed-loop controller to generate a control reference value. This control reference value is then used to control the switches in the power circuit of the power electronic converter. When tracking reaches steady state, the voltage and current at the PV module's output port correspond to those at the maximum power point, thus achieving maximum power point tracking (MPPT).

[0081] The above describes the specific embodiments of the present invention. It should be understood that the present invention is not limited to the above specific embodiments, and those skilled in the art may make various variations or modifications within the scope of the claims, which do not affect the essence of the present invention.

Claims

1. A photovoltaic cell maximum power tracking method based on impedance matching, characterized in that: include: Connect the photovoltaic module, power electronic converter and power load, and collect the open circuit voltage V output by the photovoltaic module oc and short-circuit current I sc ; Based on the impedance ratio k following the short-circuit current I sc Change curve, using short-circuit current I sc And constant I0 query ratio k; then according to the ratio of open circuit voltage and short circuit current V oc / I sc Determine the impedance U corresponding to the maximum power point m / I m , recorded as the impedance reference value Z ref ; Using the impedance reference value Z ref The impedance of the input port of the power electronic converter is tracked in a closed loop. When the tracking reaches a steady state, the voltage and current at the output port of the photovoltaic module correspond to the voltage and current at the maximum power point, thereby achieving maximum power tracking of the photovoltaic module.

2. The method according to claim 1, characterized in that Adjust the power electronic converter to make the photovoltaic module work at the open circuit point, and collect the output voltage at this time as the open circuit voltage V oc ; Adjust the power electronic converter to make the photovoltaic module work at the short-circuit point, and collect the output current at this time as the short-circuit current I sc .

3. The method according to claim 1, characterized in that The impedance ratio k follows the short-circuit current I sc The change curve is k-ln -1 (I sc / I0+1) relationship curve; after querying the ratio k, calculate the impedance reference value Z according to the following formula ref : Where k is U m / I m With V oc / I sc The ratio of U m / I m is the impedance corresponding to the maximum power point, V oc / I sc It is the ratio of open circuit voltage to short circuit current; I0 ​​is a known constant, which is the reverse saturation current of the photovoltaic module in the absence of light when it leaves the factory.

4. The method according to claim 1, wherein The impedance ratio k follows the short-circuit current I sc The variation curve is obtained by solving the basic current equation of the photovoltaic effect and its equivalent derivation using the Newton-Raphson method.

5. The method according to claim 1, characterized in that The impedance ratio k follows the short-circuit current I sc The change curve is stored in the form of data pairs and called by the controller.

6. The method according to claim 1, characterized in that When performing closed-loop tracking control, the calculated impedance reference value Z ref As a closed-loop control reference value; subtracting the reference value from the real-time impedance of the converter to obtain an error amount, and inputting the error amount into a closed-loop controller to obtain a control amount reference value; using the control amount reference value to control the switch tube of the power circuit in the power electronic converter to achieve maximum power tracking of the photovoltaic module.

7. A system for implementing maximum power point tracking of photovoltaic cells based on impedance matching, comprising a power electronic converter, characterized in that: The power electronic converter includes a power circuit, a peripheral sampling circuit and a controller; wherein, The sampling port of the peripheral sampling circuit is arranged at the output end of the photovoltaic module, and the peripheral sampling circuit is electrically connected to the controller to transmit the voltage and current signals obtained by sampling; The controller is electrically connected to the switch tube in the power circuit to transmit a drive signal for adjusting the working state of the converter; The controller is electrically connected to at least one memory, and instructions are stored in the memory. The controller executes the photovoltaic cell maximum power point tracking method based on impedance matching according to any one of claims 1 to 6 according to the instructions.

8. The system according to claim 7, characterized in that The photovoltaic assembly includes at least one group of photovoltaic power generation units.

9. The system according to claim 7, wherein: The power circuit is a DC-DC power circuit or a DC-AC power circuit.

10. The system according to claim 7, wherein: The controller can perform floating-point operations and is any one of DSP, ARM, and FPGA.

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