JP2012113639A - Maximum power point follow-up method for solar battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve use efficiency of a solar battery with respect to a voltage converter system which takes maximum electric power out in a device using a maximum power point follow-up method for the solar battery by efficiently performing MPPT control processing almost at a maximum power point while maintaining precision of the MPPT control by noticing that large movement of the maximum power point is less.SOLUTION: The present invention relates to maximum power point follow-up control that measures and calculates output power of the solar battery by a voltage converter and adjusts a maximum operating voltage Vm for maximizing the output power of the solar battery. The maximum power point follow-up method includes: means of operating so as to shorten measurement and calculation periods when a voltage V generated by the voltage converter is much different from the voltage Vm with respect to a period in which the output power of the solar battery is measured with the voltage V; and means of operating so as to extend the measurement and calculation periods when the voltage V is close to the Vm.

Description

The present invention relates to a voltage converter system that extracts maximum power in a device that uses maximum power point tracking control of a solar cell.

A solar cell has a VI output characteristic, and a current that can be taken out is determined by a voltage of a connected load. Therefore, it is particularly important to set the output voltage of the solar cell at a voltage point where the maximum power can be extracted (hereinafter referred to as the optimum operating point).

Control that follows the optimum operating point is called maximum power point follow-up control (hereinafter referred to as MPPT control), and is used in a voltage converter for solar cells.

There are several methods for MPPT control. Among them, a hill-climbing method with good characteristic accuracy and a simple sequence is often used.

FIG. 1 is a diagram for explaining a conventional hill-climbing method. FIG. 1 shows a characteristic curve showing the relationship between the output voltage Vo and the output power Po of the solar cell.

The output power Po of the solar cell varies with the operating voltage Vo, and its characteristic curve is as shown in FIG. The apex of the characteristic curve is the maximum power point (Pmax point) at which the output power Po is maximum, and the operating voltage Vo at that time is the optimum operating voltage (optimum operating point) Vmax. Further, the characteristic curve changes according to the solar radiation amount and the temperature variation, and therefore the optimum operating voltage Vmax also changes accordingly.

In order to make maximum use of the output power Po of the solar cell, the solar cell may be operated at the maximum power point. However, the maximum power point always fluctuates depending on the amount of solar radiation and temperature. Follow-up MPPT control is required. In the MPPT control, the output power Po of the solar cell is measured at a time interval ΔT, and the operating voltage Vo is controlled so that the output power Po increases.

In the conventional MPPT control, when the operating voltage Vo is Vbl, the output powers of Val and Vcl adjacent to each other by the change width ΔV are compared, and the power moves in the higher direction.

Similarly, the same control is performed up to the optimum operating point, and in the example of FIG. 1, the voltage converter adjusts the voltage so that the output voltage becomes high.

Similarly, when the operating voltage Vo is Vbh, the output powers of Vah and Vch adjacent to each other by the change width ΔV are compared, and the power moves in the higher direction. Similar control is performed up to the optimum operating point, and in the example of FIG. 1, the voltage converter adjusts the voltage so that the output voltage is lowered.

As described above, the hill-climbing method is realized by measuring the current value and the voltage value at the time interval ΔT and calculating the power value. Actually, a mechanism that does not perform measurement and calculation other than during MPPT control is provided to save current consumption.

However, in the conventional MPPT control, the operating voltage Vo always changes periodically, and the output power Po always changes accordingly. In particular, in the vicinity of the maximum power point, the output power Po always decreases due to the change of the operating voltage Vo, and thus the power loss is caused by that amount.

Further, since the magnitude of the change width ΔV is always constant regardless of the magnitude of the operating voltage Vo, the change width ΔV is large even in the vicinity of the maximum power point. Therefore, the loss of electric power in the vicinity of the maximum power point becomes prominent, and this contributes to lowering the utilization efficiency of the solar cell.

If the change width ΔV of the operating voltage Vo is reduced, it takes a long time from the start of the inverter until the maximum power point is reached, and the utilization efficiency of the solar cell during that time decreases. In addition, since the operating voltage Vo is constantly changing and this causes disturbance in the control stability of the inverter, in order to maintain the stability, the change speed of the operating voltage Vo, that is, the response speed of the MPPT control is not much. I can't make it bigger.

In view of the above contents, in Patent Document 1, the change width ΔV is made variable to improve the utilization efficiency of the solar cell.

Patent 2731117

However, even in the technique shown in Patent Document 1, the current value and the voltage value are periodically measured to calculate the power value, so that the current consumption increases each time, and the use efficiency of the solar cell decreases each time the number is increased. .

Therefore, paying attention to the fact that the maximum power point changes depending on the amount of solar radiation, but does not change significantly as shown in FIG. 2, the present invention maintains the MPPT control accuracy and MPPT control processing near the maximum power point. It aims at improving the utilization efficiency of a solar cell by making it efficient.

In order to solve the above problems, the present invention takes the following means. That is, it is maximum power point tracking control that measures and calculates the output power of the solar cell with a voltage converter and adjusts it to the maximum operating voltage Vm that maximizes the output power of the solar cell, and the voltage V generated by the voltage converter With respect to the cycle of measuring the output power of the solar cell, the means for operating to shorten the measurement and calculation cycle when the voltage V is away from the Vm, and the voltage V is in the vicinity of the Vm In some cases, means for operating to lengthen the measurement and calculation cycle.

In the present invention, when the voltage converter compares the measured and calculated output power value with the previously measured and calculated output power value, the voltage converter continuously increases or decreases the voltage value more than the set number of times. The measurement and calculation cycle is operated so as to be shortened stepwise, otherwise, the measurement and calculation cycle is set to be increased stepwise, and the measurement and calculation cycle exceeds the set maximum cycle. It is characterized by having means for operating so as not to lengthen and means for operating so as not to shorten the measurement and calculation period beyond the set minimum period.

Furthermore, the present invention is characterized by comprising means for intermittent operation in which the voltage converter repeats operation and stop in accordance with a period for measuring the output power.

By the above means, the present invention can obtain the following effects. In other words, by changing the measurement and calculation cycle, it is possible to reach the optimal operating point quickly, and when approaching the optimal operating point, the measurement and calculation cycle also becomes long, so that the current consumption can be suppressed, and the solar cell Efficiency is improved.

It is the graph which showed the relationship between the output voltage and output power of a common solar cell. It is the graph which showed the relationship between the output voltage and output power which showed the change by the solar radiation amount of a general solar cell. It is a block diagram of the voltage converter connected to a solar cell. It is the type | formula which showed the relationship between the PWM waveform and the voltage output from a chopper circuit. It is a flowchart of MPPT control. It is the flowchart which showed the example which changes the frequency | count of control.

Hereinafter, the present embodiment will be described with reference to the drawings. FIG. 3 is a block diagram showing the configuration of the voltage converter 1 connected to the solar cell according to the present invention.

The voltage converter 1 mainly includes a microcomputer 4, a PWM controller 5, a chopper circuit 6, a shunt resistor 7, and a current detection amplifier 8.

The voltage converter 1 can adjust the load voltage by performing PWM (Pulse Width Modulation) control. FIG. 4 illustrates PWM control.

PWM control is performed by changing the duty ratio of the switching frequency F. The load voltage Vr can be controlled by changing the duty well.

In general, in a chopper circuit, Equation 1 shown in FIG. 4 is established for a step-down chopper, and Equation 2 shown in FIG. 4 is established for a step-up chopper. Therefore, an arbitrary load voltage Vr can be extracted from the output voltage Vo by changing the PWM duty ratio.

The microcomputer 4 measures the output current Io and the output voltage Vo output from the solar battery 2 using the A / D conversion function. Further, the microcomputer 4 measures the load current Ir and the load voltage Vr input to the load 3 using an A / D conversion function. The shunt resistor 7 has a very low resistance value for current measurement, and the voltage drop due to the shunt resistor can be ignored. Therefore, it can be considered that the voltage output from the chopper circuit 6 is comparable to the load voltage Vr.

The current detection amplifier 8 detects the voltage applied to both ends of the shunt resistor 7 and outputs it to the microcomputer 4 as a voltage value. The microcomputer 4 can measure the voltage value using the A / D conversion function and calculate the load current Ir using the resistance value of the shunt resistor. The output current Io can also be calculated in the same procedure.

Using the measurement result, the PWM controller 5 is instructed to adjust the PWM output as will be described later. The microcomputer 4 is in a sleep state except during operation, and suppresses current consumption.

The PWM controller 5 receives an instruction from the microcomputer and outputs an appropriate PWM waveform. The PWM controller 5 is always in operation because it is necessary to change the voltage.

The chopper circuit 6 converts the output voltage Vo from the solar cell 2 into the load voltage Vr by the PWM signal. The chopper circuit 6 mainly includes a step-down type and a step-up type. The step-down type works to lower the voltage, and the step-up type works to raise the voltage.

The load 3 is connected to a battery such as a storage battery or a device that operates with a solar battery.

FIG. 5 is a flowchart of MPPT control by adjusting the PWM waveform as described above. The MPPT control will be described using a flowchart.

First, the current output voltage Vo and output current Io are calculated, and the values are multiplied to calculate the output power Po. The calculated value is set as a power value 1 (S2).

Next, the PWM duty is changed by a specified value. In the flowchart, the PWM duty is increased to increase the load voltage Vr output from the chopper circuit (S3). Further, the output power Po at that time is calculated in the same manner as the previous time, and is set to a power value 2 (S4).

The power value 1 calculated last time is compared with the power value 2 calculated this time (S5). If the power value 2 calculated this time is larger, the PWM duty is further increased (the power value 2 ≧ the power value 1 portion of S5). If the previously calculated power value 2 is larger, the PWM duty is reduced (the power value 2 <the power value 1 portion of S5).

When the PWM duty is reduced, the power value is calculated again (S8), and the previous value is compared (S9). When the power value 2 calculated this time is larger, the PWM duty is further decreased (the power value 2 ≧ the power value 1 portion of S10). When the previously calculated power value 2 is larger, the PWM duty is increased (the power value 2 <the power value 1 portion of S10).

As described above, the power value is calculated repeatedly, compared with the power value calculated last time, and the PWM duty is changed.

However, as the number of repetitions increases, more current is consumed for measurement and calculation. However, if the cycle of the control interval is increased, the optimal operating point is not easily reached, and the efficiency of the solar cell is deteriorated. Therefore, the current consumption is suppressed by adjusting the length of the control interval based on the number of times the PWM duty is increased or the number of times of DOWN.

FIG. 6 is a flowchart for changing the control interval.

When MPPT control is performed (S14) and the voltage is continuously increased by m times or more by PWM control (YES in S15), the control interval is made shorter than the current interval (S16). However, the control interval is within Tmin.

When the voltage is continuously decreased by n times or more by PWM control (YES in S17), the control interval is made shorter than the current interval (S18). However, the control interval is not less than Tmin.

If not, the control interval is set longer than the current interval (S19). However, the control interval is set to Tmax or less. By repeating the above, the control interval can be changed.

Since the calculation is not complicated and the flow chart is very simple, calculation of the control interval does not take time, and wasteful current consumption can be suppressed.

In addition, by setting the parameters m and n and the control intervals Tmin and Tmax in the flowchart to various values, it is possible to adjust the MPPT control to be optimal for the installation site of the solar cell.

From the above, if the sunshine is stable, the measurement and calculation cycle becomes long, so that current consumption can be suppressed. Furthermore, even if the sunshine is unstable and the optimum operating point moves, the measurement and calculation cycle is shortened, so that the optimum operating point can be reached, so that current consumption can be suppressed. Therefore, the efficiency of the solar cell is improved without requiring complicated control.

In this embodiment, a chopper circuit is used as means for controlling the voltage applied to the load. However, the present invention can be implemented by means other than the chopper circuit, and is not limited thereto.

Further, in this embodiment, the details of the duty ratio of the PWM waveform and the number of times of increase or decrease of the voltage value are not shown by specific numerical values, but it varies depending on the environment to be implemented and the configuration of the apparatus. In the present invention, these are not limited at all.

1 ... Voltage converter,
2 ... solar cells,
3 ... Load 4 ... Microcomputer 5 ... PWM controller 6 ... Chopper circuit 7 ... Shunt resistor 8 ... Current detection amplifier

Claims (3)

Maximum power point tracking control that measures and calculates the output power of the solar cell with a voltage converter and adjusts the maximum output voltage Vm to maximize the output power of the solar cell,
Regarding the period of measuring the output power of the solar cell by the voltage V generated by the voltage converter,
Means for operating to shorten the measurement and calculation period when the voltage V is away from the Vm;
Means for operating to lengthen the measurement and calculation cycle when the voltage V is in the vicinity of Vm;
A method for following the maximum power point of a solar cell, comprising: In the maximum power point tracking method according to claim 1,
When the voltage converter compares the measured and calculated output power value with the previously measured and calculated output power value and continuously increases or decreases the voltage value more than the set number of times, the measurement and calculation cycle is changed. Means to operate in a stepwise manner, otherwise, to operate in a stepwise manner to increase the measurement and calculation cycle;
Means to operate so as not to lengthen the measurement and calculation cycle beyond the set maximum cycle;
Means to operate so as not to shorten the measurement and calculation cycle beyond the set minimum cycle;
A method for following the maximum power point of a solar cell, comprising: The maximum of the solar cell according to any one of claims 1 to 2, further comprising means for intermittent operation in which the voltage converter repeats operation and stop according to a period of measuring the output power. Power point tracking method.