JP3427021B2 - Grid-connected inverter - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention converts DC power obtained from a DC power source such as a solar cell into AC power,
The present invention relates to a grid-connected inverter that outputs to an AC power system.

[0002]

2. Description of the Related Art Conventionally, for example, Japanese Patent Laid-Open No. 11-41817.
It is shown in the publication. The contents will be described below.

FIG. 7 shows a solar power generation system in which the solar cell 1 is used as a direct current power source, the direct current electric power obtained from the solar cell 1 is converted into alternating current electric power by the inverter device 2, and then the reverse flow is made to the commercial power system 7. It represents.

Further, FIG. 8 shows a configuration of a transformerless inverter circuit portion provided in the inverter device 2. In the circuit, the plus side input terminal P and the minus side input terminal N are connected to the plus output terminal and the minus output terminal of the solar cell 1 shown in FIG. 7, respectively, and the voltages of the P phase and the N phase are applied.

In FIG. 8, the positive side input terminal P and the negative side input terminal N are connected to the commercial power system 7 via the chopper section 4, the inverter section 5 and the filter section 8. Further, the inverter unit 7 is configured by bridge-connecting four switching elements Q2 to Q5.

A direct current voltage (for example, 200 V) obtained from the solar cell 1 is first boosted to a predetermined voltage (for example, 400 V) by the chopper unit 4 and then PWM-modulated by the switching operation of the switching elements Q2 to Q5 of the inverter unit 5. To be done. Then, after being shaped into a sinusoidal AC waveform through the filter unit 8, it is supplied to the commercial power system 7.

In such a configuration, even when the generated power from the solar cell 1 is reduced and the inverter output is reduced,
The amplitude of output current was reduced and operation was continued. Therefore, when the output of the inverter unit 5 is low, the conversion efficiency of the inverter unit 5 is lowered.

[0008]

The input voltage from the solar cell 1 is about 100 to 300V. Since the system voltage is usually linked to an AC voltage of 200V, the chopper unit 4 boosts the voltage to a voltage (DC voltage 360 to 400V) at which current can flow backward with respect to the system voltage, and the inverter unit 5 converts the system voltage into an AC current. It is necessary to reverse flow to the grid.

Since the system voltage on the output side of the inverter section 5 is constant, the inverter section 5 outputs AC power as AC current and controls the output by changing the current value.

Further, the inverter unit 5 usually has a loss inside. Therefore, the generated power from the solar cell 1 is the output power obtained by subtracting this loss.

This loss is the loss of the power supply for driving the main body of the system-linked inverter, the loss caused by the switching of the switching elements of the chopper section 4 and the inverter section 5, and the loss caused by the increase of the current of the inverter.

Of these losses, the losses of and are almost fixed values, while the losses of are variable values that increase with an increase in the output current.

For example, assuming that the inverter has a 4.5 kw output, the output power is 4.5 kw if the rated conversion efficiency is 100%, but if the efficiency is 95%, the input power is 4.74 kw. The solid line in FIG. 9 shows the relationship between the input power and the output power of the inverter in consideration of the inverter loss (fixed loss and variable loss).

The point to be noted here is the intersection A between this solid line and the 0 kw line of the input power. This represents the fixed loss among the above-mentioned losses. In other words, it shows that the inverter does not operate when the input power below the point A and the amount of power generated by the solar cell are below the point A.

In an actual solar power generation system, the time at low output can reach several tens of minutes, which is too long to ignore.

In order to prevent the inverter from stopping due to this low output, there is a conventional technique (Japanese Patent Laid-Open No. 11-4).
No. 1817), there is a method of reducing the power factor of output power to reduce active power, but this is not a preferable method from the viewpoint of conversion efficiency of the inverter.

The present invention solves this problem.

[0018]

A grid interconnection inverter of the present invention converts an output power of a DC power supply into an AC power and supplies the AC power to an AC power system, and an inverter section including a plurality of switching elements. An inverter control unit that controls the inverter unit; and a low power control unit that performs duty control on the inverter unit according to the decrease in the output power when the output power of the DC power supply or the inverter unit decreases, The low power control means is characterized by performing duty control with a plurality of cycles of the system frequency as one duty cycle, or performing phase control with one cycle or a plurality of cycles of the system frequency as one duty period.

The low power control means determines whether or not the output power of the DC power source or the inverter unit is smaller than a predetermined threshold value, and when it is determined that the output power is smaller than the predetermined threshold value. Is a configuration for setting a duty ratio which is proportional to the magnitude of the output power or changes according to the magnitude of the output power.

[0020]

[0021]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments in which the present invention is applied to a solar power generation system will be specifically described below with reference to the drawings. Incidentally, the configurations given the same reference numerals as those of the above-mentioned conventional example have the same functions, and therefore the description thereof will be omitted.

In FIG. 1, 3 is a control means for driving and controlling each switching element of the chopper section 4 and the inverter section 5, 9 is an input voltage detecting means for detecting the power generation output of the solar cell 1, that is, the input voltage of the chopper section 4. Reference numeral 10 denotes a zero timing detecting means for detecting the timing of zero voltage at the system frequency of the commercial power system 7.

The control means 3 turns on and off the switching element Q1 of the chopper section 4 to adjust the boosted voltage, and the chopper control means 3 controls the switching elements Q2 to Q5 of the inverter section 5 to turn on and off.
2. If the voltage detected by the input voltage detecting means 9 detects a predetermined threshold value (for example, 170 V) or less, each of the switching elements Q2 to Q5 of the inverter unit 5 is duty-controlled in synchronization with the timing detected by the zero timing detecting means 10. It is the low power control means 33 that operates.

The operation of this structure will be described with reference to FIG.

First, in step S1, it is determined whether or not the voltage detected by the input voltage detecting means 9 is 100V or more. If the detected voltage is not 100 V or higher, the process stands by in step S1 until it exceeds 100 V. In this step, when the amount of power generated by the solar cell 1 is small, for example, at night, the grid-linkage inverter cannot operate, so the system is automatically stopped.

Although the input voltage detecting means 9 is set to 100V as a criterion for the determination, the voltage corresponding to the solar cell 1 is generally set. It is also possible to detect the generated power instead of the voltage by some method and make the determination.

When it is determined in step S1 that the voltage is 100 V or more, the process proceeds to step S2. In step S2, an on / off signal is first supplied from the chopper control means 31 to the switching element Q1 of the chopper section 4 to start driving.

At step S3, the control means 3 outputs the lowest output, for example, a duty ratio of 1/16 to the low power control means 33.
Then, the driving of the inverter unit 5 by the low power control means 33 is started.

Here, the duty ratio x / 16 means that, as shown in FIG. 3, 16 cycles of the system frequency are one duty period, only x cycles of the 16 cycles are turned on, and the remaining (16-x) cycles are set. It means the duty ratio to turn off.

That is, duty control is performed with one cycle of the system frequency as a basic cycle and 16 cycles as one duty period. Duty control may be performed with a plurality of cycles of two or more system frequencies as a basic cycle.

After driving the chopper section 4 and the inverter section 5, the process proceeds to step S4. In step S4,
It is determined whether the voltage detected by the input voltage detecting means 9 is 170V or higher. If the voltage is 170 V or more in this step, the process proceeds to step S5, and if it is not 170 V or more, the process proceeds to step S8.

170V which is the criterion of the above step S4
Is an index voltage for determining whether sufficient power generation can be obtained. Therefore, it is a numerical value that should be given an appropriate value depending on the solar cell 1 used. When a voltage exceeding 170V is constantly obtained by the control described below, the operation of the inverter is configured to shift from the intermittent operation state by duty control to the normal operation.

Further, in the section in which the inverter section 5 is driven by duty control, the low power control means 33 outputs the sine wave current to the switching elements Q2 to Q5.
Drive normally. The switching elements Q2 to Q5 maintain the OFF state in the section in which the inverter unit 5 is not driven by the duty control.

In step S5, since the detected voltage is 170 V or higher, the duty ratio set by the low voltage control means 33 is increased by 1 and set to 2/16. Step S6
Then, after the increase in step S5, the process waits until one duty period elapses. In step S7, step S5
As a result of the increase of the duty ratio in step 16, it is determined whether or not the setting is 16/16. If it is determined that the setting is not 16/16 in this step, the process returns to step S4.

In step S7, the duty ratio is 16
/ 16, that is, whether or not the continuous drive setting is set. When the continuous drive setting is set, the low power control means 33 ends the control, and the step for switching to the normal operation by the inverter control means 32 described later. Is.

When it is determined in step S4 that the voltage is not 170 V or higher, the process proceeds to step S8. In step S8, it is determined whether the duty ratio is currently set to 1/16.

If the setting is not 1/16 in the step, the process proceeds to step S9. In step S9, the duty ratio is decreased by 1. In addition, in step S8, 1/16
When it is determined that the setting is made, the setting cannot be made below the setting, and thus the step S9 is skipped.

In step S10, the input voltage detecting means 9
To determine if it is 100V or less. 100
If it is not V or less, the process proceeds to step S6, waits until one duty period elapses, and then returns to step S4. If it is determined in step S10 that the voltage is 100 V or less, the grid-connected inverter cannot operate.
11, the driving of the chopper unit 4 and the inverter unit 5 is stopped. After that, the process proceeds to step S1 and the input voltage detection means 9 waits in the drive stopped state until it is confirmed that the voltage is 100 V or more.

The low power control means 33 executes the duty control of the inverter section 5 by repeatedly executing the steps S4 to S10. Then, as a result of increasing the duty ratio in step S5, 16/1
When it is determined in step S7 that 6 settings have been made, the process proceeds to step S12 in order to switch from duty control to normal operation control.

In step S12, the inverter control means 32 continues normal operation.

In step S13, it is determined whether the voltage detected by the input voltage detecting means 9 is 170V or less.
If the voltage is not 170 V or less, step S12 is repeatedly executed and the inverter control means 32 continues normal operation.

When it is determined in step S13 that the voltage is 170 V or less, the process returns to step S4 to switch to duty control, the duty ratio is decreased from 16/16 setting by 1, and the low power control means 33 starts duty control. Of.

By repeating the above steps, the grid-connected inverter can be driven in a wide range of input power from the solar cell 1 from low power to high power.

From the above, when the power generation amount of the solar cell 1 is low, the loss of the inverter unit 5 is reduced as compared with the conventional method of reducing the power factor of the inverter unit by executing the duty control of the present invention. Can be reduced, and the conversion efficiency of the inverter unit 5 can be improved.

That is, as shown in FIG. 4, in the conventional control, the total power is obtained by adding the loss and the output power as shown in (a), and the conversion efficiency is (output power / total power). On the other hand, in the duty control of the present invention, when the duty control is on, the total power is the sum of the loss and the output power as in the conventional control, but when the duty control is off, there is only the loss. Since the conversion efficiency is obtained from the average electric power at the time of ON and the electric power at the time of OFF, the loss decreases according to the duty ratio. Therefore, the conversion efficiency is improved.

In the above-mentioned embodiment, the low power control means 33 controls the duty based on one cycle of the system frequency as shown in FIG. 3, but the present invention is not limited to this. For example, as shown in FIG. 5, the phase control may be executed with one cycle of the system frequency as one duty period. Further, as shown in FIG. 6, phase control may be performed with a system frequency being a plurality of cycles, for example, two cycles as one duty period.

In this case, however, since the output current contains a DC component, a means for detecting and correcting the DC component or, in principle, controlling the output current every half cycle so that the DC component is not included. Ingenuity is needed.

Although the duty control and the normal operation are switched by the detection voltage of the input voltage detecting means 9 in the above-mentioned embodiment, the output power detecting means for detecting the output power of the inverter section 5 is used as the input voltage detecting means 9. Alternatively, the duty control and the normal operation may be switched in the same manner as described above based on the detection value of the output power detecting means.

Since the control by the low power control means 33 is duty control, it is desirable that the amplitude of the output current from the inverter section 5 be a low constant value. However, it is possible to obtain appropriate output power by changing the current amplitude even during this control.

Further, in the above embodiment, the inverter unit 5
Although it is premised that only the duty control of the inverter is performed, it is possible to perform the duty control of the chopper unit 4 together with the inverter unit 5. In this case, it is necessary to design so that the output voltage of the chopper section 4 does not drop significantly by stopping the chopper section 4.

[0051]

According to the grid-connected inverter of the present invention, the conversion efficiency at the time of low output operation can be improved, and the operation at a low output for a long time becomes possible.

[Brief description of drawings]

FIG. 1 is a diagram showing a schematic configuration of a grid interconnection inverter of the present invention.

FIG. 2 is a diagram showing an operation flowchart of the grid-connected inverter of the present invention.

FIG. 3 is a schematic diagram showing an example of duty control.

FIG. 4 is a diagram showing a relationship of losses during duty control.

FIG. 5 is a schematic diagram of phase control as another example of duty control.

FIG. 6 is a schematic diagram of phase control as another example of duty control.

FIG. 7 is a block diagram showing a configuration of a solar power generation system.

FIG. 8 is a circuit diagram showing a configuration of an inverter device.

FIG. 9 is a diagram showing a relationship between input power and output power of the inverter unit.

[Explanation of symbols]

1 solar cell 2 Inverter device 3 control means 31 Chopper control means 32 Inverter control means 33 Low power control means 4 Chopper part 5 Inverter section 7 Commercial grid power 9 Input voltage detection means 10 Zero timing detection means

─────────────────────────────────────────────────── ─── Continuation of the front page (56) Reference JP-A-5-122855 (JP, A) JP-A-7-191767 (JP, A) JP-A-6-165518 (JP, A) JP-A-8- 237884 (JP, A) JP-A-7-72942 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) H02J 3/00-5/00 H02M 7/42-7/98

Claims (3)

(57) [Claims] 1. In an apparatus for converting output power of a DC power supply into AC power and supplying the AC power to an AC power system, an inverter section comprising a plurality of switching elements, inverter control means for controlling the inverter section, and When the output power of the DC power source or the inverter unit drops, the low power control unit performs duty control of the inverter unit according to the drop of the output power , and the low power control unit sets a plurality of cycles of the system frequency to 1 Du
A grid-connected inverter characterized by duty control as the duty cycle . 2. The output power of the DC power supply is converted into AC power.
The power supply to the AC power system.
Inverter unit composed of switching element and the inverter
Inverter control means for controlling the unit, the DC power source,
Is the output power of the inverter when it decreases.
Duty control of the inverter section according to
Low power control means for performing, wherein the low power control means is one cycle or a plurality of cycles of the system frequency.
Characteristic of phase control with cycle as one duty period
And a grid-connected inverter. 3. The low power control means controls the DC power supply or
Or the output power of the inverter is less than the specified threshold
If the output power is smaller than the specified threshold,
Is determined to be proportional to the magnitude of the output power,
Is the duty ratio that changes according to the magnitude of the output power.
It sets , The grid-connected inverter of Claim 1 or Claim 2 characterized by the above-mentioned.