KR101556386B1 - A solar inverter of improving efficiency, using dual maximum power point tracking - Google Patents

Abstract

The present invention relates to a solar inverter for improving the conversion efficiency by using a dual maximum power point tracking (MPPT) for outputting the reference voltage to track the maximum power point (MPP) in a solar power generation system. The solar inverter of the present invention comprises an MPPT control unit for receiving an input of the current voltage and current from a solar cell array and outputting the reference voltage for tracking the MPP. The MPPT control unit receives the current voltage and current; calculates the amount of increase in the voltage and the amount of increase in the current; calculates the amount of increase in power; when the amount of increase in the current is positive, increases and decreases the reference voltage according to whether the amount of increase in power is positive or negative and to the result of comparison of the amount of increase in the current compared with the amount of increase in the voltage and the negative value of the current compared with the current voltage; and, when the amount of increase in the current is negative, increases and decreases the reference voltage according to whether the amount of increase in the power is negative or positive. The solar inverter of the present invention can more accurately track the MPP according to the slope where the solar radiation is irradiated by using not only the last amount of increase in the current and the voltage but also at least three amounts of increase and considering also the amount of increase in the current.

Description

[0001] The present invention relates to a solar inverter for improving efficiency of conversion using a dual MPPT,

The present invention relates to a dual MPPT follow-up technology for improving the efficiency of an inverter of a photovoltaic power generation system. The dual MPPT, which follows the maximum power point by using variable voltage increment and power increment according to current increment, And more particularly, to an improved solar inverter.
Here, the meaning of the dual MPPT means to follow the maximum power point, using both of the two factors of the variable voltage increment and the power increment according to the current increment. That is, the present invention relates to a solar inverter that improves the conversion efficiency by using a dual MPPT that follows the maximum power point by using both of the voltage increment-based current increment and the power increment.

Global warming and energy policies have recently become international agendas and developed countries are trying to reduce greenhouse gas emissions. In this situation, solar power generation is semi-permanent, it is easy to maintain using solar cell, and because it uses unlimited, no-limit solar energy source, it is becoming a future alternative energy source. After installing solar power, it will produce electricity according to solar radiation without emitting greenhouse gas. However, facilities for photovoltaic power generation have a high initial investment cost. By increasing the efficiency of solar power generation, profitability of solar power generation can be increased.

The efficiency of solar power generation is largely influenced by three factors. Efficiency of the PV (Photovoltaic) panel, efficiency of the inverter, and efficiency of the maximum power point tracking (MPPT) algorithm. Following this, it is easier and less costly to follow the maximum power point (MPP) with the new control algorithm than to increase the other efficiency.

The PV array requires the MPPT algorithm because it has a non-linear voltage-current characteristic with a unique point at which the generated power is at its maximum. The maximum power point (MPP) depends on the temperature of the panel and the solar radiation conditions. Both conditions change day by day and depend on the season. In addition, solar radiation can change rapidly because it changes with the state of the atmosphere, such as clouds. It is very important to accurately track the MPP under all possible conditions to always get the maximum power point.

Therefore, technologies that follow the maximum power point (MPP) are being proposed. Techniques for following the maximum power point (MPP) include a Perturbation and Observation (P & O) control method, an incremental conductance (InCond) control method, and a Hill Climbing control method.

The P & O control method is a method of periodically increasing or decreasing the output voltage of the solar cell array, comparing the previous output power with the present output power to find the maximum power operating point, and the incremental conductance control method is to determine the conductance of the solar cell output And the incremental conductance is compared to find the maximum power operating point. The hill climbing control is a method of following the maximum power point by adding or subtracting the duty ratio.

As one example of the P & O control method, techniques for following the maximum power point through an instantaneous change amount of the output power (P) of the solar cell array according to the change of the output voltage (V) of the solar cell array are proposed [Patent Document 1] . At this time, the voltage increment is added to or subtracted from the output voltage of the solar cell array module, and the output power of the added / subtracted output voltage is controlled to be the maximum. Although the P & O control technique is not complicated, the control technique can increase or decrease the voltage increment value with respect to a specific voltage value, and thus vibration occurs in the power value near the maximum power point, resulting in power loss.

Further, according to the P & O control method according to the related art, the characteristic is changed according to the magnitude? V of the reference voltage fluctuation width. When? V is large, the MPP follow-up speed is increased but the output voltage fluctuation width is widened. On the contrary, when? V is small, the MPP follow-up speed is reduced, but the advantage is that the fluctuation range of the output voltage is narrowed.

In order to solve such a problem, a technique of using a variable value of voltage increment has been proposed [Patent Document 2]. However, in the Patent Document 2, only the variable voltage increment is varied only by the value for the previous voltage increment or the output power increment. That is, in Patent Document 2, no consideration is given to an element for current.

Korean Registered Patent No. 10-1065862 (Announced on September 20, 2011) Korean Registered Patent No. 10-1223611 (published on Jan. 17, 2013)

SUMMARY OF THE INVENTION An object of the present invention is to solve the above-mentioned problems, and it is an object of the present invention to solve the above problems, and to provide a PVPT- The present invention provides a solar inverter that improves the conversion efficiency by using a dual MPPT that follows a point.

According to an aspect of the present invention, there is provided a solar inverter having improved conversion efficiency using dual MPPT, comprising: an MPPT controller for receiving a current voltage and a current from a solar cell array and outputting a reference voltage for tracking a maximum power point, Wherein the MPPT control unit receives the current voltage and current, calculates a voltage increment and a current increment with respect to a previous voltage and a current, and calculates a power increment; The reference voltage is increased or decreased according to a result of comparing the current increment with respect to the current increment to a negative value of the current with respect to the current increment when the current increment is positive, ; And when the current increment is negative, the reference voltage is increased or decreased depending on whether the power increment is negative or positive.

Further, the present invention provides a solar inverter in which conversion efficiency is improved by using dual MPPT, wherein the MPPT control unit determines whether the current increment is positive and the power increment is positive and the voltage increment Decrement the reference voltage if the current increment is less than a negative value of the current current relative to the current voltage and otherwise increase the reference voltage and if the voltage increment is positive then the power increment is positive and the voltage increment vs. current Increases the reference voltage if the increment is greater than the negative value of the current current with respect to the current voltage, and otherwise decreases the reference voltage.

Further, in the solar inverter in which the conversion efficiency is improved using the dual MPPT, the MPPT control unit controls the MPPT controller such that if the current increment is positive, the voltage increment and the current increment are 0, The reference voltage is increased or decreased according to whether the current increment is positive or negative. If the current increment is positive, the reference voltage is increased. If the current increment is negative, the reference voltage is decreased.

According to another aspect of the present invention, there is provided a solar inverter having improved conversion efficiency using dual MPPT, wherein the MPPT control unit increases the reference voltage when the current increment is positive and increases the reference voltage if the current increment is positive, And if the voltage is negative, the reference voltage is decreased.

According to another aspect of the present invention, there is provided a solar inverter in which conversion efficiency is improved by using dual MPPT, wherein the MPPT control unit outputs the reference voltage and updates the value of the previous voltage and current to the value of the current voltage and current .

In addition, the present invention is characterized in that, in a solar inverter in which conversion efficiency is improved by using dual MPPT, when the reference voltage is increased or decreased, the increase / decrease magnitude of the reference voltage is determined according to the magnitude of the current increment.

In addition, the present invention provides a solar inverter having improved conversion efficiency using dual MPPT, wherein when the current increment ΔI oscillates near 0 in the most recent four samples, the increase / decrease magnitude ΔV _ref of the reference voltage is set to 0.01 decide to V, if the △ I is less than 0.001A, △ V _ref is decided to 0.05V, if the △ I is less than 0.005A, △ V _ref is decided to 0.1V, the △ I is less than 0.01A If, △ V _ref is decided to 0.5V, the △ I is less than 0.015A, △ V _ref is decided to 1V, that when other, △ V _ref is characterized in that it establishes a 2V.

As described above, according to the solar inverter in which the conversion efficiency is improved by using the dual MPPT according to the present invention, by following the maximum power point (MPP) by adopting the variable voltage increment, And it is possible to obtain the maximum power point (MPP) more quickly.

In addition, according to the solar inverter having improved conversion efficiency using the dual MPPT according to the present invention, by using not only the last increment of the current and the voltage but also at least three increments, An effect of more accurate tracking of the power point (MPP) is obtained.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram of a configuration of a solar power generation system for practicing the present invention; FIG.
2 is a circuit diagram of an equivalent circuit of a solar cell according to an embodiment of the present invention.
FIG. 3 is a characteristic curve of a solar cell according to an embodiment of the present invention. FIG. 3 (a) is a VI characteristic, and FIG.
FIG. 4 is a graph showing a change in MPP according to the irradiation amount in the VI curve according to an embodiment of the present invention, wherein a red line is 1000 W / m ² , a blue line is 700 W / m ² , ² , and the graph at a temperature of 25 ° C.
FIG. 5 is a graph showing a change in MPP according to the irradiation dose in a VP curve according to an embodiment of the present invention, wherein a red line is 1000 W / m ² , a blue line is 700 W / m ² , ² , and the graph at a temperature of 25 ° C.
FIG. 6 is a graph of a change in MPP according to temperature in a VI curve according to an embodiment of the present invention. FIG. 6 is a graph for a temperature of 25 ° C. for the red line, 50 ° C. for the blue line, and 75 ° C. for the green line, Graph of solar radiation at 1000 W / m ² .
FIG. 7 is a graph showing changes in MPP according to temperature in a VP curve according to an embodiment of the present invention. In FIG. 7, a red line indicates a temperature of 25 ° C., a blue line indicates a temperature of 50 ° C., a green line indicates a temperature of 75 ° C., Graph at 1000 W / m ² .
8 is a circuit diagram of a configuration for MPPT control of a solar power generation system according to an embodiment of the present invention.
9 is a circuit diagram of an MPPT control apparatus of a photovoltaic power generation system according to an embodiment of the present invention.
10 is a table showing the operation of the solar inverter in which the conversion efficiency is improved by using the dual MPPT according to the first embodiment of the present invention.
11 is a graph showing a solar cell VP characteristic curve and an operating point in a solar inverter having improved conversion efficiency using dual MPPT according to the first embodiment of the present invention.
12 is a flowchart illustrating a solar inverter in which conversion efficiency is improved using dual MPPT according to the first embodiment of the present invention.
13 is a graph showing voltage, current and power tracking results according to the variation of the irradiation dose according to the first embodiment of the present invention (S: irradiation amount, Vpv: PV voltage, Ipv: PV current, Ppv: PV power, Maximum power).
FIG. 14 is a flowchart illustrating a solar inverter having improved conversion efficiency using a dual MPPT according to a second embodiment of the present invention; FIG.
FIG. 15 is a graph showing voltage, current and power tracking results according to the second embodiment of the present invention (S: irradiation amount, Vpv: PV voltage, Ipv: PV current, Ppv: PV power, Maximum power).
16 is a table showing the MPPT efficiencies for each section in the graph of FIG.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described in detail with reference to the drawings.

In the description of the present invention, the same parts are denoted by the same reference numerals, and repetitive description thereof will be omitted.

First, the entire configuration of a solar power generation system for carrying out the present invention will be described with reference to FIG.

1, the entire configuration of a solar power generation system for practicing the present invention includes a solar cell array 10, an inverter 20, a control device 30, and a power system 40 .

Here, the solar cell array 10 is for generating electricity by condensing sunlight incident from the outside, and silicon and a composite material are usually used mainly. Specifically, the solar cell array 10 uses a p-type semiconductor and an n-type semiconductor in combination, and utilizes a photoelectric effect to generate electricity by receiving sunlight. Most of the solar cell arrays 10 are formed of large-area P-N junction diodes, and the electromotive force generated at the opposite ends of the P-N junction diode is connected to an external circuit. The minimum unit of the solar cell array 10 is referred to as a cell. Actually, the solar cell is rarely used as it is. This is because the voltage required from one cell is very small, about 0.5V, compared to several tens of V or several hundreds of V, which is necessary for practical use. Therefore, a plurality of unit solar cells can be connected in series or parallel . In addition, when the solar cell array 10 is used outdoors, it is subjected to various harsh environments. Therefore, a plurality of cells are used as a package in order to protect a plurality of cells connected with a necessary unit capacity in a harsh environment.

The inverter 20 integrates the DC power received in the parallel group in the solar cell array 10 into one DC power and converts it into AC power for use as commercial power. The AC power output from the inverter 20 is supplied to the load of the consumer through the power system 40.

The control device 30 is an apparatus that is installed in the inverter 20 and controls the inverter 20. The control device 30 controls the power system 40 through an algorithm that tracks the maximum power point MPP of the solar cell or the solar array 10, So that the maximum generated power is transmitted.

Next, the configuration of the solar cell of the solar cell array 10 will be described with reference to Fig.

The constituent block of the solar cell array 10 is a solar cell, and as described above, it is basically a p-n junction semiconductor that directly converts light energy into electricity. 2 is an equivalent circuit of a solar cell.

The current-voltage characteristic of the solar cell is expressed by the following equation (1).

[Equation 1]

Where I and V are each a solar cell output current and voltage, I ₀ is the diode is the saturation current, q is a charge amount, A is the diode material factor, k is the Boltzmann's constant, T is the absolute temperature, R _S and R _SH The series resistance and shunt resistance of the solar cell. Here, the value of R _SH is so large that the end of equation (1) can be ignored to simplify the analysis. The solar cell array 10 is composed of many solar cells connected in parallel with the series, so that the output current and voltage of the solar cell array 10 have sufficient values for the system connection conditions. In consideration of the above-mentioned simplification, the output current-voltage characteristic of the solar cell array 10 is expressed by Equation 2, where n _p and n _s are the serial and parallel connection numbers of the solar cell.

&Quot; (2) "

Among the current-voltage characteristics, the open circuit voltage V _OC and the short-circuit current I _SC are important. The power generated at both points is zero. V _OC can be expressed as Equation (3) by deriving from Equation (1) when the current flowing through the solar cell is 0, that is, I = 0, while ignoring the shunt resistor R _SH . The short-circuit current I _SC is the current when V = 0 and is almost equal to the photocurrent I _L.

&Quot; (3) "

&Quot; (4) "

The maximum power develops at the current-voltage characteristic point where the product of V and I is the maximum. This is the MPP and is unique as in FIGS. 3A and 3B. Fig. 3A shows the voltage-current curve, and Fig. 3B shows the voltage-power curve. MPP is the highest point in Fig. 3B.

Two important factors to be considered in tracking MPP are solar radiation and temperature. These two factors have a great influence on the characteristics of the solar cell as shown in Equations (1) and (2). As a result, the MPP is constantly changing during the day, and why MPP is constantly being tracked and ensuring that the maximum active power is obtained from the panel.

The effects of solar radiation on voltage-current (VI) and voltage-power (VP) characteristics are shown in FIGS. 4 and 5 to better illustrate the effect of solar radiation on VI and VP curves. The photocurrent changes in proportion to the solar radiation. As the solar radiation increases, the short circuit current (I _SC ) increases accordingly. In contrast, the effect on the open circuit voltage (V _OC ) is less affected by the change in current depending on the irradiation dose.

In FIGS. 4 and 5, it can be seen that the change of the current with respect to the solar radiation amount is larger than the voltage. In fact, the voltage change due to irradiation is often ignored. Since the effect on both current and voltage has a positive value, the power also increases definitely when the solar radiation amount increases.

The temperature mainly affects the voltage. The open-circuit voltage changes linearly with temperature as shown in the following equation.

&Quot; (5) "

According to Equation (5), the effect of temperature on V _OC is such that the voltage decreases when the temperature rises because K _V has a negative value. However, since the rise in current with temperature is very small, it does not compensate for the voltage drop due to temperature rise. For that reason, power is reduced.

The solar cell array 10, that is, the PV panel manufacturer, presents a temperature coefficient, which is a parameter for how the open-circuit voltage, the short-circuit current and the maximum power change with temperature. The effect of temperature on the current is negligible since it is very small. It can be seen from FIGS. 6 and 7 how the voltage-current and voltage-power characteristics change with temperature.

A configuration for controlling the MPPT of the photovoltaic power generation system according to an embodiment of the present invention will be described with reference to FIGS. 8 and 9. FIG.

As shown in FIG. 8, the photovoltaic device includes a solar cell array, a DC-link capacitor, and a controlled current source that replaces a power conversion device.

As shown in FIG. 9, the MPPT controller (MPPT P & O) generates the reference voltage (V _ref ) by the MPPT algorithm. That is, the MPPT control unit (MPPT P & O) receives the PV voltage V _pv and the PV current I _pv and outputs the reference voltage V _ref . The difference (?) From the actual PV direct-current voltage (V _pv ) is calculated for the reference voltage (V _ref ), and the calculated difference is supplied to the proportional integral gain (PI). The difference between the output and the actual PV direct current (I _pv ) becomes the reference control value of the current source and is input to S1.

That is, in Fig. 9,? Outputs a value obtained by subtracting the value of the input signal of the + sign from the value of the input signal of the + sign among the two input signals as a summer. The output of the summer SIGMA in Fig. 9 is input to a limiter, and a limiter specifies an upper limit set value and a lower limit set value. If the input value is higher than the upper limit setting value, the upper limit setting value is output. If the input value is lower than the lower limit setting value, the lower limit setting value is output. The range of the triangular wave inputted to the comparator connected later is limited.

The output of S1 in Fig. 9 is input to S1 in Fig. 8, S1 becomes a PWM input, and a switching circuit (for example, the gate of the IGBT) is driven to perform a boost operation. This leads to the MPP.

In particular, the reference voltage generated by the MPPT controller (MPPT P & O) is converted into the current reference as shown in FIG. In other words, the error between the reference voltage and the actual DC voltage (the output voltage of the solar cell array) is supplied by its value, the DC link capacitance and the proportional gain according to the sampling period. The output of this gain is subtracted from the current of the solar cell module and the result is the reference current for the controlled current source.

That is, S1 in Fig. 9 is a PWM drive signal output for driving an IGBT that matches I _ref and I _pv , and is denoted by S1 in Fig. In a general MMPT controller, only the voltage control is performed by using the difference between the MPPT output and the PV voltage as the reference voltage. Preferably, in the present invention, current control is performed to control the current source with the difference between the reference voltage and the PV current as the reference current.

A solar inverter having improved conversion efficiency using dual MPPT according to the first embodiment of the present invention will be described with reference to FIGS. 10 to 13. FIG.

The basic principle of P & O MPPT is to _measure the solar cell output power (P _pv [n]) after controlling the voltage of the solar cell output voltage (V _pv ) The maximum power point is followed by raising or lowering the voltage V _pv ^{+ in} the direction in which the output is increased in comparison with the output power P _pv [n-1]. A brief summary of this method is shown in the table of FIG.

11 shows the PV output characteristics of the solar cell array and the movement of the operating point by P & O MPPT. In Fig. 11, when the initial operating point is A, the solar cell array voltage is V _A, and the solar cell output power at this time is P _A. Assuming that the next reference voltage is V _B , the output power at this time is P _B , and the operating point is located at point B. Here, since P _B > P _A and V _B <V _A , ΔP _pv > 0 and _{ΔV pv} <0 become Case 3 in the table of FIG. That is, the next reference voltage becomes V _C smaller than V _B by ΔV. When the operating point moves to C, P _C > P _B and V _C < V _{B are} satisfied, which is Case 3 in the table of FIG. The next reference voltage is V _D. And P _D <P _C and V _D <V _C when the operating point is D. Therefore, Case 4 in the table of FIG. The next reference voltage is V _{C, which} is larger than V _D by ΔV.

When the operating point is again C, P _C > P _D and V _C > V _D, it corresponds to Case 1 in the table of FIG. 10, and the next reference voltage becomes V _{B which} is larger by ΔV than V _C. Since P _B <P _C and V _B > V _C in the case of the operating point B, it corresponds to case 2 in the table of FIG. 10, and the next reference voltage becomes V _C smaller by ΔV than V _B.

Therefore, the operating point appears repeatedly: A → B → C → D → C → B → C. The method of determining the next reference voltage according to the polarities of _Vpv and _Ppv is P & O method. As a result, when there is no variation in solar radiation in FIG. 11, the operating point is changed between points B and D, Power Point).

Next, the basic idea of the incremental conductance (IncCond) maximum power point tracking method is that since the MPP is the maximum point of the power curve as shown in FIG. 5, the differential coefficient of power to voltage in MPP becomes zero. Here, the output characteristic of pxv is analyzed.

Also, as shown in FIG. 5, it can be seen that the power at the left side of the MPP increases with the voltage, that is, dp / dv> 0. On the right side of the MPP, it decreases with the voltage. That is, dp / dv <

This can be written as a simple equation:

(6) (in MPP)

(7) (at the left slope of the MPP)

(Equation 8) (at the right slope of the MPP)

The relationship can be written as a term using the current and voltage of the panel (solar cell)

&Quot; (9) &quot;

(10) &lt; RTI ID = 0.0 &gt; (MPP)

(11) (at the left slope of the MPP)

(12) (at the right slope of the MPP)

Therefore, the PV panel (solar cell array) terminal voltage can be adjusted in relation to the MPP voltage by measuring the increment and instantaneous panel conductivities (di / dv, I / V, respectively) and using the above equations.

Next, the P & O maximum power point tracking method and the incremental conductance maximum power point tracking method are considered together.

First, the case of maximum power point (MPP) is as follows.

(? I /? V = -I _pv / V _pv ) AND (? P = 0)

That is, the current increment relative to the voltage increment is equal to the instantaneous panel conductivity, and there is no power increment regardless of the voltage increment.

The case of the left slope of the maximum power point (MPP) is as follows.

(? I /? V> -I _pv / V _pv ) AND

({DELTA V > 0 and DELTA P > 0) OR (DELTA V &

The case of the right slope of the maximum power point (MPP) is as follows.

(? I /? V <-I _pv / V _pv ) AND

{(DELTA V > 0 and DELTA P < 0) OR (DELTA V &

The maximum power point tracking method according to the above operation polarity is shown in FIG. 12 as a flowchart.

A tracking method in a solar inverter in which conversion efficiency is improved using dual MPPT according to the first embodiment of the present invention will be described in more detail with reference to FIG.

12, first, the current voltage V (k) and the current I (k) are measured and the cycle is started (S111). The output power P (k) at this time is calculated as a product of voltage and current (S112). Then, using the corresponding values I (k-1) and V (k-1) stored at the end of the preceding cycle, the incremental change approaches (S112) as follows.

di? I (k) - I (k-1)

dv? V (k) - V (k-1)

First, the main check is performed by comparing di / dv against -I / V and by power increment (the difference between the current power and the previous power, P = P (k) -P (k-1)). This comparison is made when -I / V = di / dv, or when both DELTA V and DELTA I are not zero. To this end, it is determined whether di / dv is equal to -I / V (S131). Then, it is checked whether? V and? I are 0 (S132, S134). In this case, if it is determined to be true, the reference voltage is not adjusted and the current state is judged as the optimum state. In MPP, that is, di / dv = -I / V, no control action is required. Therefore, the adjustment step is ignored, the reference does not change, and the driving point remains at the MPP. The algorithm updates the parameters stored at the end of the cycle as usual (S182).

Next, it is determined whether the voltage increment? V is positive or negative (S133).

If the voltage increment ΔV is negative, the reference voltage V _{ref can be} adjusted depending on whether the power increment ΔP is positive or negative and whether the voltage increment vs. current increment di / dv is greater or less than the negative of the current -I / (S141 to S143). That is, the power increment △ P is positive, the voltage increment compared to current incremental di / dv is less than negative -I / V of voltage against current reduces the reference voltage V _ref (S142), otherwise increasing the reference voltage V _ref (S143).

Further, even when the voltage increment DELTA V is positive, depending on whether or not the power increment DELTA P is positive or negative, the reference voltage DELTA V is increased or decreased depending on whether the voltage increment vs. current increment di / dv is larger or smaller than the negative- V _ref adjustment is adjusted (S151 to S153). That is, if the power increment ΔP is positive and the current increment di / dv with respect to the voltage increment is greater than the negative -I / V of the current versus voltage, the reference voltage V _ref is increased (S152), otherwise the reference voltage V _ref is decreased (S153).

It is also included in the algorithm to detect that the control action is required when the panel is operating in the MPP at the preceding period (dv = 0). In this case, the change in the state of the atmosphere is detected using (di? 0) (S132, S134). Now the control signal V _ref adjustment will depend on whether the current increment di is positive or negative (S161) as in the flow chart. That is, if the current increment di is positive, the reference voltage V _ref adjustment is adjusted by multiplying (S162). If di is negative, the V _ref adjustment is adjusted by adjusting (S163).

The adjusted reference voltage V _ref is outputted (S181), and the parameters I (k-1) and V (k-1) stored at the end of the period are updated to the current values (S182).

In FIG. 13, when the solar irradiance changes in a stepwise manner as in the case of 0 to 2 seconds, the change momentarily occurs and the solar irradiance remains unchanged, so that the MPP is tracked well. However, if the solar radiation changes along the slope and the algorithm corresponds to the continuously changing curve according to the irradiation dose as shown in Fig. 5, the change in voltage and current is not only the fluctuation of the voltage. As a result, it is not easy for the algorithm to determine whether the change in power is due to an increase in the reference voltage according to the algorithm or a change in solar radiation.

As a result of the change in the irradiation dose, it is apparent that the method according to the first embodiment of the present invention is such that the voltage is far from the MPP voltage in the solar cell. Furthermore, the method according to the first embodiment can move the DC voltage in the wrong direction. It can be seen from Fig. 13 that the power is the same.

Next, a tracking method in the solar inverter in which the conversion efficiency is improved by using the dual MPPT according to the second embodiment of the present invention will be described with reference to FIG. 14 to FIG. The above method is performed by the MPPT control unit.

Investigation of solar irradiance Accurate tracking of the MPP along the slope requires at least three as well as the last increment of current and voltage. To do this, four consecutive samples of voltage and current are considered. Whereby the last three increments of the power [Delta] P and the current [Delta] I can be calculated. If the average of the current increase is within the allowable range of the last increase in current, use a ± 20% difference of the last increase. (1.2 DELTA I, 0.8 DELTA I).

And since the last three increases in current and power have the same sign, the irradiation dose is forced to move the reference voltage in the right direction according to the varying slopes. It increases when the current increases and decreases when the current increases.

Particularly, in the first embodiment described above, some problems occur when the irradiation amount fluctuates. It determines whether to use? V /? P or? I /? P when determining the reference voltage V _ref . The problem appears in the following two cases. The first case △ △ V P and the sign of the both is a negative value of △ V / △ P is down the instruction to have a positive value and increase the V _ref without reducing the V _ref. In the second case, if ΔP and ΔI are positive values, the sign of ΔI / ΔP also has a positive value, and the reference voltage decreases instead of increasing. The solution is simple because the problem appears only in these two cases. That is, when ΔI is a positive value, the algorithm uses ΔV / ΔP, and in the opposite case, ΔI / ΔP is used.

When the solar radiation amount is constant, the operation is performed at the maximum power point by the MPP tracking method according to the first embodiment. However, if the amount of solar radiation fluctuates, the reference voltage in the wrong direction is increased or decreased.

First, when ΔI is negative due to a decrease in the amount of solar radiation, ΔP and ΔV are both negative values when the curve is changed from a red curve to a blue curve in FIG. In this case, the sign of DELTA V / DELTA P has a positive value and gives a false instruction to increase Vref without decreasing the reference voltage Vref. In this case, Vref is decreased by using? I /? P instead of? V /? P (hereinafter, the right part of the flowchart of FIG. 14).

Secondly, when the solar radiation amount increases and ΔI has a positive value, it is changed from a blue curve to a red curve in FIG. At this time, both DELTA P and DELTA I become positive values so that the sign of DELTA I / DELTA P becomes a positive value and gives an erroneous instruction to decrease Vref without increasing Vref. In this case, Vref is increased by using the first embodiment and? V /? P instead of? I /? P (hereinafter, the left part of the flowchart of FIG. 14).

That is, when ΔI is a positive value, the same method as in the first embodiment is used, and when ΔI is negative, V _ref is increased or decreased in accordance with decrease in ΔI and ΔP. The flowchart according to the second embodiment is shown in Fig.

The maximum power point tracking method according to the second embodiment will be described in more detail with reference to FIG.

As shown in FIG. 14, first, the current voltage V (k) and the current I (k) are measured and the cycle is started (S111). The output power P _pv (k) at this time is calculated as a product of the voltage and the current, and the current increment ΔI, the voltage increment ΔV, and the power increment ΔP are calculated (S112). That is, the incremental change approaches as follows.

di? I (k) - I (k-1)

dv? V (k) - V (k-1)

(k-1) = V (k) x I (k) -V (k-1) x I (k-1).

Next, it is determined whether? I is positive or negative (S220).

When? I is positive, it is performed as in the first embodiment. That is, di / dv is compared with -I / V and power increment (difference between current power and previous power, P = P (k) -P (k-1)) is performed. This comparison is made when -I / V = di / dv, or when both DELTA V and DELTA I are not zero. To do this, it is compared whether di / dv is equal to -I / V (S231). Then, it is checked whether? V and? I are 0 (S232, S234). In this case, if it is determined to be true, the reference voltage is not adjusted and the current state is judged as the optimum state. In MPP, that is, di / dv = -I / V, no control action is required. Therefore, the adjustment step is ignored, the reference does not change, and the driving point remains at the MPP. The algorithm updates the parameters stored at the end of the cycle as usual (S282).

Next, it is determined whether the voltage increment DELTA V is positive or negative (S233). If the power increment ΔP is positive and the voltage increment vs. current increment di / dv is less than the negative of the current -I / V of the current (S242), the reference voltage V _ref is decreased (S242), otherwise the reference voltage V _ref is increased S243). If the voltage increment DELTA V is positive and the power increment DELTA P is positive and the current increment di / dv relative to the voltage increment is greater than the negative-I / V of the current versus voltage, the reference voltage _Vref is increased (S252) The reference voltage _Vref is decreased (S253).

It is also included in the algorithm to detect that the control action is required when the panel is operating in the MPP at the preceding period (dv = 0). In this case, the change in the state of the atmosphere is detected using (di? 0) (S232, S234). Now the control signal V _ref adjustment will depend on whether the current increment di is positive or negative (S261) as in the flowchart. That is, if the current increment di is a positive number, the reference voltage V _ref adjustment is adjusted by multiplying (S262). If di is negative, the V _ref adjustment is adjusted by adjusting (S263).

Next, when? I is negative, the reference voltage V _ref is adjusted depending on whether the voltage increment dP is positive or negative (S272). That is, when the voltage increment is positive, the reference voltage is increased (S273) and decreased when negative (S274). When the power increment ΔP is 0, the reference voltage is not adjusted (S271).

The adjusted reference voltage V _ref is outputted (S281), and the parameters I (k-1) and V (k-1) stored at the end of the cycle are updated to the current values (S282).

In the steady state, the vibration around the MPP can be minimized, so that the change in the reference voltage proportional to the current increase can be minimized. If? I is smaller than a certain value _{,? Vref} is reduced. In this case, we considered six options that were determined through trial and error. That is, the value is adjusted until satisfactory performance is given. The six cases are as follows.

(1) If? I oscillates near zero in the last four samples,? V _ref is 0.01V.

(2) If? I is less than 0.001A,? V _ref is 0.05V.

(3) △ if I is less than 0.005A, △ V _ref is 0.1V.

(4) If? I is less than 0.01 A,? V _ref is 0.5 V.

(5) If? I is less than 0.015 A,? V _ref is 1 V.

(6) In other cases,? V _ref is 2V.

The six settings are constant set values for reducing the self-excited vibration when reaching MPPT nearer while reaching MPPT sooner.

That is, four successive samples V (k), V (k -1), V (k-2), V = V (k-3) extracts the three increment _{△ V 3 (k-2)} - of V (k-3), △ V 2 = V (k-1) -V (k-2), △ V 1 = V (k) -V (k-1) is calculated. Similarly, four successive samples I (k), I (k -1), (k-2), extracts the I (k-3), 3 of increment _{△ I 3 = I (k-} 2) - the I (k-3), △ I 2 = I (k-1) -I (k-2), △ I 1 = I (k) -I (k-1) is calculated.

The preceding ΔI means ΔI = I (k) -I (k-1). That is, the value obtained by subtracting the sampled current value from the sampled current value as the PV current increment.

Also, it is as follows that &quot;? I oscillates near zero in the last four samples &quot;. That is, three ΔI values are extracted and stored using the four most recently stored sampled current values. If these three ΔIs are continuously smaller than 0.001A, it is determined that they are close to the MPP, and ΔV _ref is changed from 0.05V to 0.01V to reduce the self-excited vibration.

The performance of MPP tracking is very good and is similar on all ramps. In all cases there is a slight delay. The dynamic efficiency is calculated as follows.

&Quot; (6) &quot;

Where P _PV is the power delivered by the solar cell (or PV panel) and P _MPP is the theoretical maximum power. The method according to the second embodiment described so far can correctly track the MPP voltage on the solar irradiance slope. As shown in the table of FIG. 16, the efficiency in the solar radiation inclined plane is 99.4% or more.

Although the present invention has been described in detail with reference to the above embodiments, it is needless to say that the present invention is not limited to the above-described embodiments, and various modifications may be made without departing from the spirit of the present invention.

10: solar cell array 20: inverter
30: control device 40: power system

Claims (7)

1. A solar inverter in which a conversion efficiency is improved by using a dual MPPT that follows a maximum power point by using both the voltage incremental current increment and the power increment,
And an MPPT control unit for receiving current voltage and current from the solar cell array and outputting a reference voltage for tracking the maximum power point,
The MPPT control unit,
Receiving current voltage and current, calculating voltage increment and current increment relative to previous voltage and current, and calculating power increment;
The reference voltage is increased or decreased according to a result of comparing the current increment with respect to the current increment to a negative value of the current with respect to the current increment when the current increment is positive, ;
And the reference voltage is increased or decreased according to whether the power increment is negative or positive when the current increment is negative.
The apparatus of claim 1, wherein the MPPT control unit, when the current increment is positive,
Decreasing the reference voltage if the power increment is positive and the current increment relative to the voltage increment is less than a negative value of the current current relative to the current voltage; if not, increasing the reference voltage;
And increasing the reference voltage if the power increment is positive and the current increment relative to the voltage increment is greater than a negative value of the current current relative to the current voltage and decreasing the reference voltage if the voltage increment is positive A solar inverter that improves conversion efficiency by using dual MPPT.
The apparatus of claim 1, wherein the MPPT control unit, when the current increment is positive,
Increasing or decreasing the reference voltage according to whether the current increment is positive or negative, if the voltage increment and the current increment are zero, increasing the reference voltage if the current increment is positive and increasing the reference voltage if the current increment is negative, And the conversion efficiency is improved by using the dual MPPT.
The method according to claim 1,
The MPPT control unit, when the current increment is negative,
Wherein the reference voltage is increased when the power increment is positive and is decreased when the power increment is negative.
The method according to claim 1,
Wherein the MPPT control unit outputs the reference voltage and updates the value of the previous voltage and the current with the value of the current voltage and current, thereby improving the conversion efficiency using the dual MPPT.
The method according to claim 1,
Wherein the increase / decrease magnitude of the reference voltage is determined according to the magnitude of the current increment when the reference voltage is increased or decreased.
The method according to claim 6,
The current increment △ I is the most recent four samples of the at least three △ I is less than 0.001A streak decide the increase and decrease in size △ V _ref of the reference voltage to 0.01V, if the △ I is less than 0.001A , △ V _ref is decided to 0.05V, the △ I is less than 0.005A, △ V _ref is decided to 0.1V, if the △ I is less than 0.01A, △ V _ref is decided to 0.5V, the △ Wherein the conversion efficiency is improved by using a dual MPPT characterized in that? V _ref is set to 1 V when I is less than 0.015 A, and? V _ref is set to 2 V in other cases.

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