JPH0764659A - Inverter control system for driving solar battery - Google Patents

Abstract

PURPOSE:To attain the appropriate control of an inverter even if the voltage fluctuation is caused at the maximum efficiency point by performing the maximum power tracking control or the constant voltage control through the comparison carried out among the maximum efficiency voltage of a solar battery, the system down voltage and the threshold voltage. CONSTITUTION:The output of a solar battery 1 is converted into the AC output by an inverter 3, and an induction motor 4 which revolves at a speed proportional to the frequency of the AC output is driven. Then a device 5 to be driven is actually driven. In such a system, the threshold voltage Vsh is set at a level lower than the voltage Vmax set at the maximum efficiency point at a normal temperature and higher than the system down voltage Voff. In a normal temperature range, Voff<Vsh<Vmax is kept. Therefore the output frequency of an inverter 3 is controlled in a accordance with a fact whether the output voltage of the battery 1 is higher or lower than the voltage Vsh. Thus the maximum power tracking control is carried out. Meanwhile the constant voltage control is carried out when Vmax<Vsh is satisfied by the rise of temperature.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an inverter control system in a drive system in which a driven device is driven by a solar cell without interposing a storage battery.

[0002]

2. Description of the Related Art When a pump or the like is driven by a solar cell, the DC output of the solar cell is converted into an alternating current by inverter control.
Although the phase induction motor is drive-controlled, the AC output is usually a constant frequency output according to the ratings of the pump and motor. Pump load is generally determined by the number of revolutions,
When the motor is rotating at a constant frequency, the pump rotation speed becomes almost constant, and the power consumption becomes almost constant. On the other hand, the output of a solar cell fluctuates greatly depending on the sunshine, so it is necessary to install the solar cell with a margin in order to obtain the specified power consumption even during low sunshine, which is disadvantageous in terms of energy use efficiency. If the amount of sunlight is large, the generated energy will be wasted. Therefore, a method has been proposed in which the maximum output of the solar cell is constantly monitored, and the motor speed is controlled according to the maximum output to effectively use the energy.

By the way, normally, in order to monitor the electric power, a voltmeter and an ammeter are provided to measure the voltage. However, although a normal inverter can monitor the voltage, it does not have an ammeter. It is necessary to install a separate ammeter. As described above, there is a problem in that the power measuring mechanism for monitoring the power and the control mechanism associated therewith are complicated.

Therefore, the present applicant has already proposed an inverter control system in a solar cell drive which can measure only a voltage without measuring a current and can control a solar cell near a maximum efficiency point with a simple structure. It is (patent application flat 3-6
6860). The content of this already proposed will be outlined.
FIG. 2 is a diagram showing the entire structure of the proposed structure, and FIG. 3 is a diagram showing the output characteristics of the solar cell. In the figure, 1 is a solar cell, 2 is a CP
U, 3 are inverters, 4 are 3-phase induction motors, 5 are pumps, 6 is ROM, and 7 is RAM.

The solar cell exhibits output characteristics as shown in FIG. 3, and changes according to the amount of sunshine such as V1 (higher sunshine amount) and V2 (decrease of sunshine amount), and accordingly the maximum output point (maximum The efficiency point) also changes like M1 and M2. Now, the characteristic V
When the pump is driven by the inverter control in the configuration as shown in FIG. 2 under the sunshine condition as shown in FIG. 1, the output voltage (operating point) of the solar cell 1 is as shown in FIG. Voc. In this state, when the CPU 2 issues a frequency command to the inverter by the control program of the ROM 6 and the inverter 3 causes the rotational speed of the pump 5 to increase stepwise by a predetermined width via the induction motor 4, the operating point is on the V1 curve. Is gradually moved upward, and the input power from the solar cell to the inverter 3 increases to reach the vicinity of the maximum efficiency point. In this state, the output from the solar cell is effectively consumed, and it is stable if the sunshine is constant. At this maximum efficiency point, if an attempt is made to further increase the rotational speed by a predetermined width, no more electric power can be extracted from the solar cell, so the operating point moves rapidly toward the Isc point, and the system goes down due to the voltage drop. Resulting in.

Therefore, the voltage is lower than the voltage Vmax at the maximum efficiency point and higher than the voltage Voff at which the system goes down.
A certain threshold voltage Vsh is set, the rotation speed is gradually increased while always observing the operating voltage Vop, and Vop is Vs.
CPU should be set to lower the rotation speed immediately when h is lowered.
2 controls the inverter.

That is, when the frequency is increased by a predetermined width Δf while the operating point Vop is in the vicinity of Vmax while the rotational speed is gradually increased, Vop suddenly falls below Vsh, so immediately decrease the frequency by Δf. Vop will return to near Vmax. Since this state is stable, the frequency is raised at a long time interval as compared with the case where the frequency is lowered. Originally, the reason for increasing the frequency even though it is in a stable state is that the sunshine conditions are constantly changing, and that point is not the maximum efficiency point forever. As described above, when Vop falls below Vsh, the frequency is immediately lowered, and when Vop is larger than Vsh, the frequency is raised at a long time interval, whereby the pump can be driven near the maximum efficiency point.

[0008]

By the way, in the proposed method, the voltage Vmax at the maximum efficiency point changes depending on the temperature, and at a high temperature, Vmax and the voltage Voff that causes system down are Voff≈Vmax. In some cases, it becomes difficult to set the threshold voltage Vsh, and even if it is set, the control becomes extremely unstable. The present invention is intended to solve the above problems, and an object of the present invention is to provide an inverter control system in a solar cell drive capable of performing appropriate control even if the voltage Vmax at the maximum efficiency point fluctuates.

[0009]

According to the present invention, a solar cell output is converted into an AC output by an inverter, and an induction motor rotating at a speed proportional to the frequency of the AC output is driven to drive a driven device. In the driving method described above, when the maximum efficiency voltage of the solar cell is Vmax, the voltage at which the system goes down is Voff, and the threshold voltage is Vsh, maximum power tracking control is performed when Voff <Vsh <Vmax, and Vmax < It is characterized in that constant voltage control is performed when Vsh.

[0010]

The present invention is smaller than the voltage Vmax at the maximum efficiency point at normal temperature, for example, 25 ° C.
A threshold voltage Vsh that is higher than the voltage Voff at which the system goes down is set. Since Voff <Vsh <Vmax is maintained in the state of the normal temperature range, the output frequency of the inverter is controlled according to whether the solar cell output voltage is higher or lower than the threshold voltage Vsh to perform maximum power tracking control. To do. On the other hand, the temperature rises to a high temperature such as 70 ° C., and Voff <Vmax <Vs
When it becomes h, the operating point Vop is controlled so as to become Vsh. That is, when Vop> Vsh, the output frequency of the inverter is raised, and when Vop <Vsh, the output frequency of the inverter is lowered. By such control, it is possible to perform operation near the maximum efficiency point with a simple structure at normal temperature, and to perform stable operation even if Vmax decreases due to high temperature.

[0011]

EXAMPLES Examples of the present invention will be described below. The system configuration of the present invention is the same as that shown in FIG.
Since the control method by U is different, only the control method will be described below. FIG. 1 is a diagram showing a flow for explaining a control method of the present invention. Smaller than the voltage Vmax at the maximum efficiency point at a normal temperature of about 25 ° C,
Set a certain threshold voltage Vsh which is higher than the voltage Voff at which the system goes down.

In the state where Voff <Vsh <Vmax is maintained, the operating voltage Vop is constantly observed and the output frequency of the inverter is controlled in the same manner as in the case of FIG. 3 to control the maximum power tracking.

On the other hand, if the same control is performed when the temperature becomes high (for example, about 70 ° C.) and Vmax <Vsh, the system goes down. So V
When max <Vsh is detected, the control is immediately switched to a constant frequency within the capacity range of the solar cell, and constant voltage control is performed. By such control, the operation similar to that described with reference to FIG. 3 can be performed at the normal temperature, and the appropriate operation by the constant voltage can be performed at the high temperature state.

The above control will be described with reference to FIG. 1. The CPU reads the solar cell output voltage, compares the set threshold voltages Vsh and Vmax, and Vsh <Vma
When it is x, it is judged whether Vop is larger than Vsh (step ~), and when Vop is larger than Vsh, wait for T0 time to elapse (in a long time interval) and increase the frequency by Δf (step,). , V
When op is smaller than Vsh, the frequency is immediately decreased by Δf (), and the value is output as the frequency command value for the inverter. Thereafter, this process is repeated to perform control. On the other hand, when Vmax <Vsh, Vo
Constant voltage control is performed so that p becomes Vsh. For this reason,
When Vop> Vsh, the frequency is increased by Δf (step), and when Vop is smaller than Vsh, the frequency is decreased by Δf (step). As described above, the difference in control between when Vmax is smaller than Vsh and when Vmax is larger is only the difference in the interval, and basically Vm
Control when Vmax is Vsh when ax is smaller than Vsh
It is also possible to substitute by the control at the time of larger. In this case, the time response of the constant voltage control becomes slightly longer, but almost constant voltage operation is possible.

[0015]

As described above, according to the present invention, the setting range of the threshold voltage is widened to facilitate the setting, and it is possible to operate near the maximum efficiency point with a simple structure at normal temperature. Further, even if the temperature becomes high and Vmax decreases, it becomes possible to operate at a predetermined voltage.

[Brief description of drawings]

FIG. 1 is a diagram for explaining a control flow of the present invention.

FIG. 2 is an overall configuration diagram when a control system is applied to pump driving.

FIG. 3 is a diagram showing output characteristics of a solar cell.

[Explanation of symbols]

1 ... Solar cell, 2 ... CPU, 3 ... inverter, 4 ... 3-phase induction motor, 5 ... pump.

Claims (1)

[Claims] 1. A drive system in which an output of a solar cell is converted into an AC output by an inverter, and an induction motor rotating at a speed proportional to the frequency of the AC output is driven to drive a driven device. Maximum efficiency voltage of Vm
ax, the voltage at which the system goes down is Voff, and the threshold voltage is Vsh. When Voff <Vsh <Vmax, maximum power tracking control is performed, and when Vmax <Vsh, constant voltage control is performed. Inverter control method for solar cell drive.