KR101761606B1 - Method for tracking maximum power point in phtovoltaic power generating system - Google Patents

Abstract

A method of tracking a maximum power point of a photovoltaic power generation system including a solar cell array according to the present invention includes calculating a first duty value (PO _duty ) using a P & O method, calculating a second duty value (PO _duty ) using an incremental conductance method INC _duty ), and summing the first duty value and the second duty value to determine a final duty value (PWMD (k)). The step of calculating the first duty value using the P & O method includes comparing the current power P (k) and the maximum power P _thmax of the solar cell array, ) Is greater than the maximum power (P _thmax ), determining whether the voltage of the solar cell array is in a voltage rising mode or a voltage falling mode, decreasing the first duty value if the voltage rising mode, And increasing the first duty value.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to a photovoltaic power generation system,

The present invention relates to a method of tracking a maximum power point of a photovoltaic power generation system.

Generally, a photovoltaic system forms an array by serially or parallelly connecting one or more solar cell modules to convert solar energy into electrical energy. Such a photovoltaic power generation system includes an inverter or the like as a power conversion device for converting DC power generated from a solar cell module into AC power suitable for a load or a system.

In the stand-alone solar inverter system, the operating point of the output power is determined by the capacity of the load. However, in the grid-connected solar inverter system, the system load can be viewed as infinitely variable load. Therefore, It requires a maximum power point tracking (MPPT) method that can maximize the generated power.

Conventional maximum power point tracking method (MPPT) mainly uses P & O (Perturbation & Obserbation) method and IncCond (Incremental conductance) method. The P & O method is simple in implementation, but has a problem of vibrating at the maximum power point, and has a problem of staying outside the maximum power point when there is a ripple in the solar cell voltage or current. In order to overcome this problem, a method of estimating the maximum power point of a grid-connected photovoltaic inverter is disclosed in Japanese Patent Application No. 10-1403556, entitled " VSolar " and " A current power calculating step of calculating a current power (PSolar) of the solar cell by measuring the current power (PSolar); A ripple power calculating step of calculating a ripple power (PRipple) for determining a minimum power (Pmin) using the calculated current power (PSolar); A direction determining step of determining a direction which is an operating direction of maximum power point tracking (MPPT) by comparing the current power (PSolar) with a predetermined maximum power (PMax); And the DC link reference voltage (VDCLink_Ref) is constantly decreased or increased, or the MPPT_Duty is constantly decreased or increased in accordance with the above-described direction determined as '0' or '1' And a point estimating step.

However, this method operates under the assumption that all solar inverters have a constant current ripple value. Therefore, even in a system having a small ripple value, it is vibrated to a constant value at the maximum power point. Therefore, this leads to power loss, which has the problem of reducing efficiency.

Japanese Patent Application Laid-Open No. 10-1403556 (entitled "Maximum Power Point Estimation Method of Grid-Connect Photovoltaic Inverter)

An embodiment of the present invention is to provide a maximum power point tracking method of a photovoltaic generation system that can increase efficiency by simultaneously using two maximum power point tracking systems.

It should be understood, however, that the technical scope of the present invention is not limited to the above-described technical problems, and other technical problems may exist.

As a technical means for achieving the above technical object, a maximum power point tracking method of a photovoltaic power generation system including a solar cell array according to an aspect of the present invention calculates a first duty value (PO _duty ) using a P & , Calculating a second duty value (INC _duty ) using an incremental conductance scheme, and summing the first duty value and the second duty value to determine a final duty value (PWMD (k)) .

The calculating of the first duty value using the P & O method may include comparing the current power P (k) and the maximum power P _thmax of the solar cell array, ) Is greater than the maximum power (P _thmax ), determining whether the voltage of the solar cell array is in a voltage rising mode or a voltage falling mode, decreasing the first duty value in the voltage rising mode, The step of increasing the first duty value may include the step of increasing the first duty value.

According to any one of the above-described objects of the present invention, the advantages of the two schemes can be maximized by using the Perturbation & Obserbation (P & O) method and the IncCond (Incremental conductance) method simultaneously.

At the initial stage of the photovoltaic power generation system, the duty value of the P & O method takes a large value, and the maximum power point can be followed while reducing the ripple by the P & O method at the ripple generating portion.

Further, according to the present invention, as the inverter reaches the maximum power point, the IncCond duty value occupies a large value, and a constant voltage can be maintained without vibration at the maximum power point, and the efficiency can be increased.

Finally, by using the two methods at the same time, it is possible to quickly follow the maximum power point even in an environment in which the irradiation amount rapidly varies.

1 is a block diagram of a photovoltaic power generation system according to an embodiment of the present invention.
2 is a circuit diagram of the DC / DC converter of FIG.
3 is a configuration diagram of the control unit of FIG.
4 is a diagram illustrating an operation of a controller according to an embodiment of the present invention.
5 is a graph showing a maximum power point (MPP) according to IncCond scheme of FIG.
6 is a graph showing simulation results of the maximum power point tracking method according to the present invention.
7 is a graph showing simulation results of the maximum power point estimation method using only the P & O method.
FIG. 8 is a graph showing a simulation result of the maximum power point tracking method according to the present invention when the irradiation amount rapidly changes.
9 is a graph showing the simulation result of the maximum power point estimation method using only the P & O method when the irradiation amount rapidly changes.
10 is a block diagram of a photovoltaic energy storage system.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings, which will be readily apparent to those skilled in the art. The present invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. In order to clearly explain the present invention in the drawings, parts not related to the description are omitted.

Whenever a component is referred to as "including" an element throughout the specification, it is to be understood that the element may include other elements, not the exclusion of any other element, unless the context clearly dictates otherwise.

FIG. 1 is a block diagram of a photovoltaic power generation system according to an embodiment of the present invention, FIG. 2 is a circuit diagram of the DC / DC converter of FIG. 1, and FIG. 3 is a block diagram of the control unit of FIG.

1, the solar power generation system includes a solar cell array 10, a DC / DC converter 20, a DC / AC inverter 30, and a control unit 40. [

The solar cell array 10 outputs a DC signal corresponding to sunlight.

The DC / DC converter 20 variably outputs the DC signal output from the solar cell array 10 in accordance with a PWM (Pulse Width Modulation) duty, and the PWM duty varies according to a control signal of the control unit 40 .

The DC / AC inverter 30 converts the DC signal output from the DC / DC converter 20 into AC.

The control unit 40 outputs the PWM by summing the duty value (POduty) calculated in the proposed P & O method and the duty value (INC duty) calculated in the Incremental Conductance MPPT method to thereby output the PWM Is controlled to be the maximum output. The operation of the control unit 40 will be described in more detail below.

FIG. 4 is a diagram illustrating an operation of a controller according to an embodiment of the present invention, and FIG. 5 is a graph illustrating a maximum power point MPP according to the IncCond method of FIG.

As shown in FIG. 4, the control unit 40 of the present invention calculates two PWM duties according to the two schemes by simultaneously performing the proposed P & O scheme and the IncCond scheme, adds the two PWM duties, Duty is output.

First, the proposed P & O method will be described.

The control unit 40 detects the voltage V _PV and the current I _PV from the solar cell array 10 and calculates the solar battery current power P (k) as shown in Equation (1).

Then, the control unit 40 compares the current power P (k) with the maximum power P _thmax .

If the current power P (k) is greater than the maximum power P _thmax , the controller 40 proceeds to increase the power, so that the controller 40 stores the current power as the maximum power.

The control unit 40 determines whether the solar cell array 10 operates in a voltage rising mode in which the voltage rises or in a voltage falling mode in which the voltage falls. The control unit 40 outputs a control signal POduty = PWMD (k-1) -POstep for determining that the current power is to the left of the maximum power point and decreasing the duty value. Then, the current power can continue to rise toward the maximum power point.

The control unit 40 outputs a control signal POduty = PWMD (k-1) + POstep that determines that the current power is on the right side of the maximum power point and increases the duty value. Then, the current power can be continuously lowered toward the maximum power point.

If the current power is smaller than the maximum power, the control unit 40 compares the current power with the minimum power P _thmin . The minimum power and threshold power P _th for determining the determination of the minimum power value are determined by the following equations (2) and (3).

Where I _Pvmax = maximum solar cell current, I _PV = current current, and β = constant, where P (k) = current power, P _thmin = minimum power, P _th = threshold power,

The initial value without the current value is determined by β and the value of P _th is determined. As the power generation amount increases, the value increases proportionally, and the value of maximum α + β becomes the threshold power (P _th ) value. This is to increase the threshold power proportionally as the power generation amount increases, thereby minimizing the voltage fluctuation at the maximum power point according to the power generation amount.

If the current power is greater than the minimum power, the control unit 40 changes the current power to the maximum power and determines the minimum power as the value obtained by subtracting the threshold power from the current power.

On the contrary, if the current power is smaller than the minimum power, the control unit 40 outputs a control signal for changing the traveling direction because the power is proceeding in the direction of decrease.

That is, when the solar cell array 10 is in the voltage rising mode, the control unit 40 outputs the control signal POduty = PWMD (k-1) + POstep in which the duty value is increased and the operation of the solar cell array 10 To be switched in the direction of decreasing the voltage.

Conversely, when the solar cell array 10 is in the voltage drop mode, the control unit 40 outputs a control signal (POduty = PWMD (k-1) -Postep) for decreasing the duty value so that the operation of the solar cell array 10 So that the voltage is switched in the increasing direction.

As described above, in the present invention, the control unit 40 increases or decreases the past PWM duty value PWMD (k-1) by the fioostep (POstep). The duty variation value of the controller 40 is determined according to the following equation (4).

Where k = proportional constant, MAXduty = maximum PWM duty, Vdc _ref = DC link reference voltage, and PO _step = basic duty value.

Next, the IncCond method operating simultaneously with the proposed P & O method will be described.

As shown in FIG. 5, the IncCond scheme is based on the idea that dP / dV is 0 at the maximum power point MPP. When the dP / dV > 0, the control unit 40 is located at the left of the maximum power point. It is judged that it is located on the right side.

Here, the expression for dP / dV is expanded as shown in Equation (5).

Therefore, dP / dV> 0 can be expressed as dI / dV> -I / V.

The control unit 40 detects the voltage and current values and compares the past current and voltage with the current and voltage to calculate the changes dI and dV. When the voltage fluctuation occurs, the control unit 40 determines whether the current fluctuates or not, and stores the current voltage and current as past values. When the change of the current is increased, the controller 40 determines that the power generation amount has increased due to the increase of the irradiation amount, and decreases the duty ratio as shown in Equation (6). As the duty ratio decreases, the solar cell voltage increases. When the change in the current is decreased, the control unit 40 increases the duty ratio. As the duty ratio increases, the solar cell voltage decreases.

If the voltage fluctuation occurs, the controller 40 determines that the current increases at the same rate if dI / dV = -I / V, and stores the voltage and current as past values without increasing the duty ratio. However, if dI / dV > -I / V, the controller 40 determines that it is located to the left of the maximum power point, and decreases the duty ratio. Conversely, if dI / dV <-I / V, the control unit 40 determines that it is located on the right side of the maximum power point and increases the duty ratio so that the solar cell voltage decreases. The value of INCduty according to the IncCond method according to the operation of the controller 40 is expressed by Equation (6).

Where N = proportional constant, INC _step = basic duty value.

The control unit 40 determines the final PWM value as shown in Equation (7) by combining the duty value (POduty) according to the proposed P & O method and the duty value (INCduty) according to the IncCond method.

When the photovoltaic system is initially started, the value of the PODuty of the P & O method takes a larger value for PWMD (k). Therefore, in the present invention, by using the P & O method, the portion where ripple is generated in a state where the solar cell voltage is searching for the maximum power point can follow the maximum power point while reducing the ripple.

Also, as the inverter reaches the maximum power point, the POduty value decreases proportionally and has the same ratio as the INCduty value. Therefore, the IncCond method plays a large role near the maximum power point. That is, according to the present invention, by using the IncCond method at the maximum power point oscillating at the maximum power (P _thmax ) and the minimum power (P _thmin ), it is possible to maintain a constant voltage without vibration at the maximum power point, .

In order to confirm the effect of the present invention, a PSIM simulation will be described in detail.

It shows the results when using only the proposed P & O method and using P & O method and IncCond method at the same time. The same specifications as in Table 1 were used for the solar cell. The maximum duty value (MAXduty) of the PWM boost value used in the program is set to 2000.

Table 1 shows the simulation PV module specifications.

FIG. 6 is a graph showing simulation results of the maximum power point tracking method according to the present invention, and Table 2 is a table showing the results of the maximum power tracking method of the solar power generation system of the present invention.

FIG. 6 is a graph showing changes in the solar cell current, the solar cell voltage, the solar cell power, the mode number in the INCduty, the POduty, the IncCond method, the Voltage_UP value and the boost PWM value PWMD output from the control unit 40, Respectively.

Here, the control unit 40 changes the PWMD in the direction of increasing the voltage value of the solar cell when the current power P (k) is larger than the minimum power P _thmin and the value of Voltage_UP is 1. If the current power P (k) is greater than the minimum power P _thmin and the value of the voltage_UP is 0, the control unit 40 changes the PWMD in a direction to decrease the voltage value of the solar cell.

On the contrary, when the current power P (k) is smaller than the minimum power ( _Pthmin ) and the value of Voltage_UP is 1, the control unit 40 changes the direction of the voltage of the solar cell by decreasing the direction, The control unit 40 changes the direction of the voltage of the solar cell to the increasing direction when the current power P (k) is smaller than the minimum power P _thmin and the value of Voltage_UP is 0 Therefore, the value of Voltage_UP is changed to 1 and the PWMD value is decreased.

7 is a graph showing the simulation result of the maximum power point estimation method using only the P & O method, and Table 3 is a table showing the results of the maximum power point estimation method using only the P &

In FIG. 7, the solar cell current, the solar cell voltage, the solar cell power, the variation of the voltage_UP value, the POduty, and the boost PWM value output from the control unit 40 are displayed from the top according to time.

Here, the control unit 40 changes the PWMD in the direction of increasing the voltage value of the solar cell when the current power P (k) is larger than the minimum power P _thmin and the value of Voltage_UP is 1. If the current power P (k) is greater than the minimum power P _thmin and the value of the voltage_UP is 0, the control unit 40 changes the PWMD in a direction to decrease the voltage value of the solar cell.

On the contrary, when the current power P (k) is smaller than the minimum power ( _Pthmin ) and the value of Voltage_UP is 1, the control unit 40 changes the direction of the voltage of the solar cell by decreasing the direction, The control unit 40 changes the direction of the voltage of the solar cell to the increasing direction when the current power P (k) is smaller than the minimum power P _thmin and the value of Voltage_UP is 0 Therefore, the value of Voltage_UP is changed to 1 and the PWMD value is decreased.

When comparing the results of the two simulations under the same conditions, the maximum power point estimation method according to the present invention finds the maximum power point at 2 seconds as shown in Table 2, and the voltage of 1.3 V fluctuates from 246.1 V to 247.4 V. On the other hand, the maximum power point tracking method using only P & O method has a voltage fluctuation width of 10.2V from 244.5V to 254.7V reaching the maximum power point in 3 seconds as shown in Table 3.

Therefore, the maximum power point tracking method according to the present invention has higher efficiency than the maximum power point estimation method using only the proposed P & O method, and it can be confirmed that the voltage fluctuation is small at the maximum power point.

Hereinafter, the simulation results of the maximum power point tracking method of the present invention and the maximum power point estimation method using only the P & O method when the insolation dose is rapidly changed will be described. And the radiation was set to be halved at 3 seconds.

FIG. 8 is a graph showing simulation results of the maximum power point tracking method according to the present invention when the solar radiation rapidly changes, and Table 4 is a graph showing the results of the maximum power tracking method of the solar power generation system of the present invention when the solar radiation rapidly changes. to be.

FIG. 8 is a graph showing changes in the solar cell current, the solar cell voltage, the solar cell power, the mode number in the INCduty, the POduty, the IncCond method, the Voltage_UP value and the boost PWM value PWMD output from the control unit 40, Respectively.

9 is a graph showing the simulation results of the maximum power point estimation method using only the P & O method when the irradiation amount rapidly changes, and Table 5 is a table showing the results of the maximum power point estimation method using only the P &

FIG. 9 shows changes in solar cell current, solar cell voltage, solar cell power, Voltage_UP value, POduty, and boost PWM value output from the control unit 40 from the top according to time.

Comparing the results of the two simulations under the same conditions, it can be confirmed that the maximum power point tracking method according to the present invention follows the maximum power point within a short period of time even at the sudden change of the irradiation dose, and the efficiency (%) is high at the same time. This is because in the P & O method, only the POuduty fluctuates constantly to vary the PWM duty, whereas the present invention uses both POuduty and INCduty.

10 is a block diagram of a photovoltaic generation energy storage system including a solar cell 11, a first DC-DC converter 21, a second DC-DC converter 22, a DC-AC inverter 31, 41 and a battery tank 50.

1. The configuration and operation of the control unit 41 are similar to those of the control unit 40 of FIG. 1 except that the first and second DC-DC converters 21 and 22 are similar in configuration and operation to the DC- And its configuration and operation are similar. That is, the maximum power point tracking method of the present invention can be applied not only to a solar power generation system but also to a solar energy storage system.

The maximum power point tracking method of the control unit 40 according to an embodiment of the present invention may also be implemented in the form of a recording medium including a computer program stored in a medium executed by the computer or instructions executable by the computer. Computer readable media can be any available media that can be accessed by a computer and includes both volatile and nonvolatile media, removable and non-removable media.

In addition, the computer-readable medium may include both computer storage media and communication media. Computer storage media includes both volatile and nonvolatile, removable and non-removable media implemented in any method or technology for storage of information such as computer readable instructions, data structures, program modules or other data. Communication media typically includes any information delivery media, including computer readable instructions, data structures, program modules, or other data in a modulated data signal such as a carrier wave, or other transport mechanism.

While the methods and systems of the present invention have been described in connection with specific embodiments, some or all of those elements or operations may be implemented using a computer system having a general purpose hardware architecture.

It will be understood by those skilled in the art that the foregoing description of the present invention is for illustrative purposes only and that those of ordinary skill in the art can readily understand that various changes and modifications may be made without departing from the spirit or essential characteristics of the present invention. will be. It is therefore to be understood that the above-described embodiments are illustrative in all aspects and not restrictive. For example, each component described as a single entity may be distributed and implemented, and components described as being distributed may also be implemented in a combined form.

The scope of the present invention is defined by the appended claims rather than the detailed description and all changes or modifications derived from the meaning and scope of the claims and their equivalents are to be construed as being included within the scope of the present invention do.

10: Solar cell array
20: DC / DC converter
30: DC / AC inverter
40:

Claims (4)

1. A method of tracking a maximum power point of a solar power system comprising a solar cell array,
Calculating a first duty value (PO _duty ) using the P & O method,
Calculating a second duty value (INC _duty ) using an incremental conductance scheme, and
And summing the first duty value and the second duty value to determine a final duty value (PWMD (k)),
The step of calculating the first duty value using the P &
Comparing the current power P (k) and the maximum power P _thmax of the solar cell array,
_Determining whether the voltage of the solar cell array is in a voltage rising mode or a voltage falling mode if the current power P (k) is greater than the maximum power (P _thmax )
And decreasing the first duty value if the voltage is in the voltage increase mode and increasing the first duty value if the voltage is in the voltage decrease mode.
The method according to claim 1,
The step of calculating the first duty value using the P &
_{Comparing the} current power P (k) to a minimum power P _thmin if the current power P (k) is less than the maximum power P _thmax ,
If the current power P (k) is greater than the minimum power P _thmin , changing the current power to the maximum power,
_Determining whether the voltage of the solar cell array is in a voltage rising mode or a voltage falling mode if the current power P (k) is smaller than the minimum power ( _Pthmin )
Further comprising increasing the first duty value if the voltage is in the voltage rise mode and decreasing the first duty value if the voltage is in the voltage fall mode.

I _Pvmax = maximum solar cell current, I _PV = current current, and β = constant. P (k) = current power, P _thmin = minimum power, P _th = threshold power,
3. The method of claim 2,
Wherein the first duty value (POduty) is a value which is increased or decreased by a pseudo step (POstep) in a past PWM duty value (PWMD (k-1)).

K = proportional constant, MAXduty = maximum PWM duty, Vdc _ref = DC link reference voltage, PO _step = basic duty value
The method of claim 3,
The step of calculating the second duty value (INC _duty ) using the incremental conductance method includes:
Calculating a change dI, dV by comparing past current, voltage and current, and voltage, and
Determining whether the current fluctuates when a change in voltage occurs, decreasing the second duty value if the change in current is increased, and increasing the second duty value if the change in current is decreasing; Maximum power point tracking method of power generation system.

INC _step = basic duty value, N = proportional constant

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