JPH05265582A - Inverter control system in solar battery driving - Google Patents

Abstract

PURPOSE:To control a solar battery at a point in the neighborhood of the maxi mum efficiency point with a simple constitution by setting a threshold voltage between the maximum efficiency voltage of the solar battery and a voltage at which a system goes down, and controlling the output frequency of an invert er based on whether the output voltage of the solar battery exceeds or goes less than the threshold voltage. CONSTITUTION:When a frequency command is outputted from a CPU 2 to the inverter 3 and the rotating speed of a pump 5 is increased by the inverter 3 via an induction motor 4, an input voltage from the solar battery to the inverter 3 is increased, and it arrives at a point in the neighborhood of the maximum efficiency point. When the rotating speed is tried to be increased further at the maximum efficiency point, the system goes down to prevent a power over that point from being taken out. In such a case, the threshold voltage Vsh shown as Voff<Vsh<Vmax assuming the maximum efficiency voltage of the solar battery as Vmax, the voltage at which the system goes down as Voff is set, and the output frequency of the inverter 3 is controlled based on whether the output voltage Vop of the solar battery exceeds or goes less than the threshold voltage Vsh.

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 In the case of driving a pump with a solar cell, for example, a method is used in which the pump capacity is determined from the required water amount and the head pressure difference, the power source required for this is calculated, and the solar cell corresponding to that is determined. At this time, the pump driving motor generally uses a three-phase induction motor in terms of availability, maintenance, and cost. Therefore, it is necessary to convert the DC output of the solar cell into an AC. An inverter is used for this quadrature conversion, but normally the AC output is a constant frequency output according to the ratings of the pump and motor.

The pump load is generally determined by the number of revolutions, but when the motor is rotating at a constant frequency, the number of revolutions of the pump is also almost constant and the power consumption is also almost constant.
On the other hand, the output of solar cells fluctuates greatly due to sunshine, so
In order to obtain the specified power consumption even in low sunlight, it is necessary to install solar cells with a margin, which is disadvantageous in terms of energy use efficiency, and when the amount of sunlight is large, the energy generated is wasted. It will also end up. For this reason, 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 this maximum output to effectively use energy.

[0004]

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

The present invention is intended to solve the above-mentioned problems, and an inverter in a solar cell drive which can measure a voltage without measuring a current and can control a solar cell near a maximum efficiency point with a simple structure. The purpose is to provide a control method.

[0006]

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 and the voltage at which the system goes down is Voff, a threshold voltage Vsh that satisfies Voff <Vsh <Vmax is set, and the solar cell output voltage V
It is characterized in that the output frequency of the inverter is controlled depending on whether op is larger or smaller than the threshold voltage Vsh.

[0007]

The present invention sets a threshold voltage that is less than the maximum efficiency voltage and greater than the system down voltage. The output voltage of the solar cell is the maximum when the inverter is OFF, and the voltage decreases as the frequency is increased stepwise by a predetermined width, and the voltage reaches the maximum efficiency point. If the frequency is further increased by a predetermined width in this state, the maximum output of the solar cell is exceeded, so the voltage sharply decreases and falls below the threshold voltage. Therefore, if the frequency of the predetermined width is decreased, the operating point can be brought to the previous stage, that is, near the maximum efficiency point. Then, the frequency is increased at a longer time interval than when the frequency is decreased. After that, by controlling the frequency like this, it is possible to perform the operation near the maximum efficiency point with a simple configuration.

[0008]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a diagram showing an overall configuration when the present invention is applied with a pump drive, FIG. 2 is a diagram showing an output characteristic of a solar cell, FIG. 3 is a diagram for explaining an inverter control flow in the present invention, FIG. 4 is a diagram showing an example of actual measurement of day illuminance and pump rotation speed by the inverter control method of the present invention. In the figure, 1 is a solar cell, 2 is a CPU, 3 is an inverter, 4 is a three-phase induction motor, 5 is a pump, 6 is ROM, and 7 is RAM.
Is.

The solar cell exhibits the output characteristics as shown in FIG. 2, and changes according to the amount of sunshine such as V1 (high amount of sunshine) and V2 (decrease of the amount of sunshine). The efficiency point) also changes like M1 and M2. Now, the characteristic V
If the pump is driven by inverter control with the configuration shown in FIG.
2 does not operate, no current flows, so the output voltage (operating point) of the solar cell 1 is Voc in FIG. In this state, the CPU 2 issues a frequency command to the inverter by the control program of the ROM 6, and the inverter 3 gradually increases the rotation speed of the pump 5 through the induction motor 4 by a predetermined width. 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 rotation speed by a predetermined width, no more electric power can be taken out 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, in the present invention, the voltage Vm at the maximum efficiency point is
A threshold voltage Vsh that is smaller than ax and larger than the voltage Voff at which the system goes down is set, and the rotation speed is gradually increased while constantly observing the operating voltage Vop,
The inverter is controlled by the CPU 2 so that the rotation speed is immediately reduced when op falls below Vsh.

That is, if 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 will suddenly fall below Vsh, so immediately increase the frequency by Δf. Vop will return to near Vmax. Since this state is stable, the frequency is raised at long time intervals as compared with the case where the frequency is lowered. Originally, the reason why the frequency is increased even though it is in a stable state is that the sunshine condition is constantly changing, and that point is not the maximum efficiency point forever. Thus, the pump can be driven near the maximum efficiency point by lowering the frequency immediately after Vop falls below Vsh and raising the frequency at long time intervals when Vop is higher than Vsh.

FIG. 3 is a diagram for explaining a control flow in the CPU.

The CPU reads the solar cell output voltage, determines whether the voltage Vop is greater than Vsh (step), and waits for the time T0 to elapse when Vop is greater than Vsh (at long time intervals). ) Increase the frequency by Δf (step,) and Vop becomes V
If it is smaller than sh, the frequency is immediately decreased by Δf and the value is output as the frequency command value for the inverter, and thereafter this process is repeated to perform control.

FIG. 4 is a diagram showing a change in pump rotation speed with respect to a change in daily illuminance measured by the control method of the present invention. From this figure, it can be seen that it can almost follow the changes in the daily illuminance.

In the above embodiment, an example of driving a pump has been described, but the present invention is not limited to this, and can be applied to any device such as a fan that can be driven by a solar cell. is there.

[0017]

As described above, according to the present invention, when the operating voltage is lower than the threshold voltage, the frequency is immediately lowered, and when the operating voltage is higher than the threshold voltage, the frequency is slowly raised to obtain the maximum efficiency point. It is possible to drive in the vicinity, and since the normal inverter has an input voltage detection unit and a central processing unit, it can be handled only by changing the program,
The output of the solar cell can be effectively used at low cost without complicating the mechanism. Further, even if the illuminance of the sun, the solar cell, the load of the motor, etc. are changed, it is possible to always operate near the maximum efficiency point only by roughly determining an appropriate threshold value.

[Brief description of drawings]

FIG. 1 is an overall configuration diagram when a control method according to the present invention is applied to pump driving.

FIG. 2 is a diagram showing output characteristics of a solar cell.

FIG. 3 is a diagram for explaining a control flow.

FIG. 4 is a diagram showing changes in pump rotation speed with respect to changes in sunshine.

[Explanation of symbols]

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

Claims (2)

[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, where Voff is the voltage at which the system goes down, a threshold voltage Vsh that satisfies Voff <Vsh <Vmax is set, and the solar cell output voltage V
An inverter control method in solar cell drive, wherein the output frequency of the inverter is controlled depending on whether op is larger or smaller than a threshold voltage Vsh. 2. The inverter control method for driving a solar cell according to claim 1, wherein when the solar cell output voltage Vop is higher than the threshold voltage Vsh, the frequency is increased at long time intervals.