CN112148058A - Method for estimating multimodal MPPT (maximum power point tracking) of photovoltaic array - Google Patents

Abstract

A method of estimating a photovoltaic array multimodal Maximum Power Point Tracking (MPPT), comprising the steps of: (1) according to the most possible current steps, each step selects at least 5 points, the output voltage is divided equally from the open-circuit voltage to 0 according to at least 5 times of the maximum step number, and each voltage node is a sampling measurement point; (2) and scanning the output voltage of the photovoltaic array from the open-circuit voltage to 0 in a decreasing mode, and calculating according to the scanned measuring point information: the number of steps of the U-I characteristic curve, the stationary current and the length of each step, wherein the number of the steps is gradually increased from the right to the left of the U-I characteristic curve; (3) calculating an estimated voltage based on the stationary current of each step and the length of the step, and calculating a maximum power point power estimated value of each step; (4) and comparing the power estimation values of the maximum power points to obtain an interval where the final global peak value is located, and performing unimodal maximum power point tracking control in the interval to obtain the maximum power point.

Description

Method for estimating multimodal MPPT (maximum power point tracking) of photovoltaic array

Technical Field

The invention relates to the field of photovoltaics, in particular to a method for estimating multimodal Maximum Power Point Tracking (MPPT) of a photovoltaic array.

Background

Because the maximum output power of the photovoltaic array changes along with the change of illumination intensity and temperature due to the nonlinearity of the output characteristic of the photovoltaic cell panel, the maximum power point tracking control in photovoltaic power generation is very important. The traditional photovoltaic cell maximum power point tracking control mainly aims at a unimodal situation, and as the scale of a photovoltaic array increases, an output characteristic curve of the actual photovoltaic array can present a plurality of local maximum values (multiple peaks) when illumination is unbalanced, and at the moment, the traditional unimodal maximum power tracking algorithm is adopted to easily converge on the local peak values rather than the global peak values, so that the output power of the photovoltaic cell is wasted. With the increasingly wide application of photovoltaic power generation, the capacity of a photovoltaic power station is continuously increased, and the situation of multiple peaks is increased, so that the research on the maximum power point tracking method under the condition of multiple peaks is more and more paid attention by domestic and foreign scholars. The existing multimodal maximum power point tracking method is complex in algorithm and needs extra hardware cost.

By analyzing the output characteristics of the photovoltaic cell, the method for estimating the multimodal MPPT is provided, and the method has low complexity and computation and high tracking speed.

Disclosure of Invention

A complete theoretical proof is provided by deducing through a mathematical formula based on a U-I characteristic curve of a photovoltaic array, the method for power approximate estimation is improved according to the characteristics of a solar panel, the estimated value of the maximum power of each interval is directly obtained, and the error between the power estimated value and the actual maximum power value is reduced. The method also does not need to configure a current sensor for each string of battery plates, thereby saving the hardware cost.

In view of the above, the present invention provides a method for estimating a multi-peak MPPT of a photovoltaic array.

A method of estimating a photovoltaic array multimodal Maximum Power Point Tracking (MPPT), comprising the steps of:

(1) according to the most possible current steps, each step selects at least 5 points, the output voltage is divided equally from the open-circuit voltage to 0 according to at least 5 times of the maximum step number, and each voltage node is a sampling measurement point;

(2) and scanning the output voltage of the photovoltaic array from the open-circuit voltage to 0 in a decreasing mode, and calculating according to the scanned measuring point information: the number of steps of the U-I characteristic curve, the stationary current and the length of each step, wherein the number of the steps is gradually increased from the right to the left of the U-I characteristic curve;

(3): calculating an estimated voltage based on the stationary current of each step and the length of the step, and calculating a maximum power point power estimated value of each step;

(4) and comparing the power estimation values of the maximum power points to obtain an interval where the final global peak value is located, and performing unimodal maximum power point tracking control in the interval to obtain the maximum power point.

Preferably, the calculating the maximum power point power estimate for each step is equal to the product of the stationary current and the estimated voltage.

Preferably, the method further comprises: in the scanning process, the existing step stationary current and step length are analyzed according to the information of the measured points, the maximum power point power of the known steps is estimated, and the maximum power point power of the rest steps is predicted.

Preferably, when the maximum power estimation value of the known step is greater than the maximum power estimation values of all other known steps and also greater than the maximum power estimation values of the predicted remaining steps, the interval where the final global peak is located is obtained, and the scanning is ended.

Preferably, the maximum power point power estimate P for each step_{es_x}The calculation method is as follows:

wherein n is the total number of steps, F_FFor photovoltaic cell fill factor, U_{oc_x}Open circuit voltage for the xth ladder; i is_{sc_x}Short circuit current for the xth step; u shape_{k_x}The output voltage at the x-th ladder for the k-th module.

Preferably, the first and second liquid crystal materials are,

U_{m_ref}，I_{m_ref}，U_{oc_ref}，I_{sc_ref}respectively, the maximum power point voltage, the maximum power point current, the open-circuit voltage and the short-circuit current of the photovoltaic cell under the reference condition.

Drawings

In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings required in the description of the embodiments will be briefly introduced below, and it is obvious that the drawings described below are only some embodiments of the present invention, and it is obvious for those skilled in the art that other drawings can be obtained according to the drawings without creative efforts.

FIG. 1a is a U-I characteristic curve of each string of cell modules of a photovoltaic array

FIG. 1b is a U-I characteristic curve of a photovoltaic array after parallel connection

FIG. 2 is a flow chart of a method of simplifying the estimated multimodal MPPT of a photovoltaic array

FIG. 3 is a diagram illustrating a photovoltaic characteristic curve linearization

FIG. 4 is a schematic diagram of an equivalent area estimation for a single photovoltaic module

FIG. 5 is an estimated voltage analysis of series photovoltaic modules

Detailed Description

Under the condition of local shadow, when the illumination of different modules in the photovoltaic array is different, the voltage and the current output by each module are different. FIG. 1a is a U-I characteristic curve of a photovoltaic array formed by connecting 3 strings of battery plates in parallel and connecting 5 battery modules in series. The 1 st string of photovoltaic modules is illuminated by 5 different kinds of light, the 2 nd string is illuminated by 3 kinds of light, and the 3 rd string is illuminated by 2 kinds of light. After the superposition of 3 strings, 5 power peaks are shown, as shown in fig. 1 b.

As can be seen from fig. 1a and 1b, the U-I characteristic curve of the photovoltaic array with the local shadows has a plurality of steps, and the corresponding power-voltage curve has a plurality of peaks. Due to the complicated and variable shading, the height and length of each current step in the actual U-I characteristic curve are changed, which results in the change of the size and position of a plurality of power peaks. In addition, the stronger the illumination on the module, the higher the current step height is; the more modules in series under the same illumination, the longer the corresponding step length.

Flow chart of multimodal MPPT algorithm based on peak power simplified estimation, as shown in FIG. 2

Step 1: according to the most possible current steps, 5-6 points are selected for each step, the output voltage is divided equally from the open-circuit voltage to 0 according to 5-6 times of the maximum step number, and each voltage node is a sampling measurement point.

Step 2: and sequentially reducing the output voltage of the photovoltaic array from the open-circuit voltage, and continuously scanning until the output voltage is almost 0. According to the scanned measuring point information, 3 kinds of data are analyzed: the number of steps (n) of the U-I characteristic, the height of each step (i.e., the plateau current), and the length of each step (i.e., the number of photovoltaic modules under the same illumination). The number of the prescribed steps increases from the right to the left of the U-I characteristic curve.

Photovoltaic cell characteristics

The photovoltaic cell possesses some characteristics, which are used in the formula derivation of the power simplification estimation, and the basic characteristics of some photovoltaic cells are explained and formula derivation in advance.

1) Fill factor F_FThe calculation method is as follows, and is determined only by the parameters of the battery. The fill factor describes how well the photovoltaic module performs, generally the larger the better. In the formula: u shape_{m_ref}，I_{m_ref}，U_{oc_ref}，I_{sc_ref}Respectively, the maximum power point voltage, the maximum power point current, the open-circuit voltage and the short-circuit current of the photovoltaic cell under the reference condition.

2) When the photovoltaic cell is illuminated for a certain time, the open-circuit voltage of the photovoltaic cell linearly decreases along with the increase of the temperature of the cell, namely:

U_{oc_sref}(T)＝U_{oc_ref}+β×(T-T_ref) (2)

in the formula: t is the temperature of the photovoltaic cell; t is_refIs the reference temperature (typically 25 ℃); u shape_{oc_ref}Is the open circuit voltage of the photovoltaic cell under reference illumination; beta is the temperature coefficient of the open circuit voltage, and is a negative number.

For ease of reference hereinafter, U will be referred to_{oc_sref}(T) Is marked as F₁(T)。

3) When the photovoltaic cell is illuminated for a certain time, the short-circuit current of the photovoltaic cell linearly increases along with the increase of the temperature of the cell, namely:

I_{sc_sref}(T)＝I_{sc_ref}+α×(T-T_ref) (3)

in the formula: i is_{sc_sref}Is the short-circuit current of the photovoltaic cell under reference illumination; α is a temperature coefficient of the short-circuit current.

Also, will I_{sc_sref}(T) is denoted by F₂(T)。

4) At a certain temperature, the short-circuit current of the photovoltaic cell is proportional to the illumination intensity, i.e.

I_sc/I_{sc_sref}＝S/S_ref (4)

In the formula: s is the actual illumination intensity; s_refIs a reference illumination intensity.

5) At a certain temperature, the open-circuit voltage of the photovoltaic cell panel is reduced in a logarithmic manner along with the reduction of the illumination intensity.

At constant temperature, when open, has I equal to 0, i.e.

Because of I_phIs in direct proportion to the illumination intensity and the short-circuit current, and then U is solved according to the formula (5)_oc/U_{oc_ref}The following can be obtained:

in the formula: k is a radical of₁，k₂Are coefficients.

Due to the existence of I under the reference illumination intensity_sc＝I_{sc_ref}，U_oc＝U_{oc_ref}So that k is₂Can be recorded as 1/ln (k)₁+1), formula (6) can be further simplified to

Equation (7) can be viewed as a property of the panel where the unique unknown k₁Can be obtained from actual measurements. Therefore, the open-circuit voltage of the battery can be calculated when the actual short-circuit current of the battery panel under a certain environment is known, namely

Equation (8) can improve the accuracy of the power estimation algorithm.

6) The portion of the photovoltaic cell where the current in the back section of the U-I characteristic curve rapidly decreases with increasing voltage can be linearized.

Fig. 3 is a graph illustrating the linearization of the photovoltaic characteristic curve. And solving the corresponding voltage value according to the current in the curve, wherein a complete battery panel model needs to be known, and the calculation is very complex. In order to simplify the calculation amount, the fast-falling part after the maximum power point can be linearized, and the ratio of the actual current to the short-circuit current is used as an independent variable to obtain the ratio of the corresponding voltage to the open-circuit voltage. From 2 points known on the curve: (U)_oc/U_oc，I_sc/I_sc) And (1, 0) is (U) on the abscissa_oc0) per unit value (U) of point_oc/U_oc，0/I_sc) I.e., (1, 0), the expression after linearization can be obtained as

The voltage level corresponding to the current rapid drop stage can be obtained by equation (9). For ease of reference hereinafter, equation (9) is written as

The parameters related to the photovoltaic cell characteristics corresponding to the formulas (1) to (6) can be obtained by calculation or experimental measurement of photovoltaic cell parameters provided by manufacturers before the photovoltaic inverter works, and are preset without real-time operation in the working process.

Current estimation algorithm

In the scanning process, known voltage and current data need to be continuously processed to obtain information such as height (steady current) of the existing step, length of the step and the like, and a processing algorithm of the information is obtained according to a U-I characteristic curve. It can be seen from the U-I characteristic that for each current step, the end of the step is very stable during the step rising (i.e. the voltage goes from large to small), and the current changes greatly when entering a higher step. The magnitude of the current at the measurement point at the end of each step can be considered as the plateau current for that step, and in fact, from a model of the photovoltaic array, this plateau current is the short circuit current for the photovoltaic module at that stage.

Taking the derivative of the U-I curve, it is clear that this end point must be in a local trough of the derivative curve. For practical digital controllers, the number of measurement points is limited, the difference between the current measurement values is the digitalization of the derivative thereof, and the local minimum value of the current difference value is the approximate position of the terminal turning point of the current step, so the processing algorithm of the current curve can be represented by the following formula:

in the formula: n is the current ladder number; u (k), I (k) are the voltage and the current of the kth measuring point respectively; i is_sThe current is stable; u shape_sIs the turning point voltage.

If Δ I (k) is simultaneously smaller than the adjacent current difference, I (k) is the stationary current I of the nth current step_s(k) The approximate turning point voltage U of the step_s(k) Is U (k).

If the length of the step is expressed by the number of photovoltaic modules in the step, the length of the current step can only be an integer, the ratio of the voltage difference between adjacent approximate turning points to the open circuit voltage of the photovoltaic modules is approximately equal to the length of the current step, and the length of the nth current step is expressed by L (n).

And step 3: and calculating the estimated voltage according to a certain mathematical model according to the analyzed stable current and the step length of each step. And multiplying the steady current by the estimated voltage to obtain the maximum power point power estimated value of each step.

And 4, step 4: and comparing the estimated values to obtain an interval where the final global peak value is located, and performing unimodal maximum power point tracking control in the interval, wherein a unimodal maximum power point tracking method (such as a variable step size disturbance observation method) is the prior art and is not described any more.

In the scanning process, the magnitude of the steady current and the length of the existing steps are analyzed according to the information of the measured points, the magnitude of the maximum power point power of the known steps is continuously estimated, and the maximum power point power of the rest steps is predicted. The former is estimated according to the principle of minimizing, and the latter is estimated according to the principle of maximizing. If the maximum power estimation value of a certain step in the known steps is larger than the maximum power estimation values of all other known steps and is also larger than the predicted maximum power estimation values of the rest steps, the interval where the maximum power point is located is considered to be found, the power estimation can be ended in advance, and the scanning can be ended.

Voltage estimation algorithm

Voltage estimation algorithm of photovoltaic module for a single photovoltaic module, there are only 1 current step and 1 power peak, so that the estimated power is equal to the actual maximum power, i.e. the area of 2 rectangles (horizontal line filled rectangle and vertical line filled rectangle) in FIG. 4 is equal, the estimated voltage U is_esAnd estimating the current I_scThe following must be satisfied:

I_sc×U_es＝I_m×U_m (12)

magnitude of the plateau of the current step (estimated current I)_sc) I.e. the magnitude of the short-circuit current I_sc. From equations (1) and (12), the calculation formula of the estimated voltage can be obtained:

the magnitude of the estimated voltage can be found by taking the product of the known fill factor and the open circuit voltage. However, the open circuit voltage varies with temperature and light, and according to the photovoltaic cell characteristics described above, the open circuit voltage of the photovoltaic cell module can be obtained by the following method when the temperature and the short circuit current are known:

1) according to the current temperature, the open-circuit voltage U under the reference illumination intensity at the temperature is obtained_{oc_sref}And short-circuit current I_{sc_sref}；

2) According to the actual short-circuit current I_scCalculating the open-circuit voltage U under the illumination intensity_oc，

Voltage estimation algorithm for series photovoltaic modules

After a plurality of photovoltaic modules are connected in series, under the condition of local shadow, the current is in a step shape, and the current output by the module with larger short-circuit current is divided into a plurality of steps. From the above, the estimated current can be obtained by analyzing the measurement data, as shown in fig. 5.

The following describes a voltage estimation calculation method for each current step by taking the voltage estimation of the 3 rd step as an example. The estimated voltage of step 3 consists of 2 parts: one is the estimated voltage U1 when only the ladder itself is present; the other is the voltage offset U due to other higher steps₂。U₁Can be calculated by the algorithm of the previous subsection. U shape₂The voltage offset of each current step on the left is added up, and U is corresponding to step 3 in FIG. 5₂The voltage offsets caused by the 4 th and 5 th steps need to be accumulated.

The voltage offset values of the output voltage of each module for different steps can be calculated according to equation (10):

in the formula: u shape_{i_x}The output voltage at the xth step for the ith module; i is_{sc_x}Short circuit current for the xth step; u shape_{oc_i}，I_{sc_i}The open circuit voltage and the short circuit current of the ith module under the current temperature and illumination are respectively.

Considering that the output voltage and the current of the module under the same illumination are completely the same, the U-I curves are completely overlapped. Thus, a module under the same illumination can be considered as 1 module with an open circuit output voltage proportional to the number of modules, i.e.

In the formula: l is_iIs the length of the ith step (with L)_iA module of the same illumination).

Therefore, if the short-circuit current of all steps is known, the estimated voltage of the xth step is affected by all steps with step numbers larger than x, and can be calculated by the following formula:

in the formula: n is the maximum step number.

Correspondingly, the maximum power point power estimation calculation formula of each step is as follows:

the foregoing is merely an example of the present invention and is not intended to limit the invention in any manner. Those skilled in the art can make various other improvements or modifications equivalent to the above-described embodiments without departing from the scope of the present invention, and any simple modification, equivalent change or modification made to the above embodiments according to the technical essence of the present invention will still fall within the scope of the present invention.

Claims (6)

1. A method of estimating a photovoltaic array multimodal Maximum Power Point Tracking (MPPT), comprising the steps of:

(1) according to the most possible current steps, each step selects at least 5 points, the output voltage is divided equally from the open-circuit voltage to 0 according to at least 5 times of the maximum step number, and each voltage node is a sampling measurement point;

(2) and scanning the output voltage of the photovoltaic array from the open-circuit voltage to 0 in a decreasing mode, and calculating according to the scanned measuring point information: the number of steps of the U-I characteristic curve, the stationary current and the length of each step, wherein the number of the steps is gradually increased from the right to the left of the U-I characteristic curve;

(3) calculating an estimated voltage based on the stationary current of each step and the length of the step, and calculating a maximum power point power estimated value of each step;

(4) and comparing the power estimation values of the maximum power points to obtain an interval where the final global peak value is located, and performing unimodal maximum power point tracking control in the interval to obtain the maximum power point.

2. The method of claim 1, wherein: the calculating a maximum power point power estimate for each step is equal to the product of the stationary current and the estimated voltage.

3. The method of claim 1, further comprising: in the scanning process, the existing step stationary current and step length are analyzed according to the information of the measured points, the maximum power point power of the known steps is estimated, and the maximum power point power of the rest steps is predicted.

4. The method of claim 3, further comprising: and when the maximum power estimation value of the known step is larger than the maximum power estimation values of all other known steps and is also larger than the predicted maximum power estimation value of the rest steps, obtaining the interval where the final global peak value is located, and ending the scanning.

5. The method of claim 2, wherein the maximum power point power estimate P for each step_{es_x}The calculation method is as follows:

wherein n is the total number of steps, F_FFor photovoltaic cell fill factor, U_{oc_x}Open circuit voltage for the xth ladder; i is_{sc_x}Short circuit current for the xth step; u shape_{k_x}The output voltage at the x-th ladder for the k-th module.

6. The method of claim 5, wherein:

U_{m_ref}，I_{m_ref}，U_{oc_ref}，I_{sc_ref}respectively, the maximum power point voltage, the maximum power point current, the open-circuit voltage and the short-circuit current of the photovoltaic cell under the reference condition.

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