JP2001145266A - System connection inverter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent conversion efficiency from decreasing as compared with a conventional method where the power factor of output power from an inverter part is decreased when the amount of generation of a solar battery reaches low output for continuing driving an inverter. SOLUTION: The inverter for converting the output power of a DC power supply to AC power for supplying to an AC power system is provided with an inverter part 5 consisting of a plurality of switching elements Q2-Q5, an inverter control means 32 for controlling the inverter part 5 by the multiple switching system, and a low-power control means 33 for performing the duty control of the inverter part according to the decrease in the output power when the DC power supply or the output power of the inverter part decreases.

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 system interconnection inverter that outputs to an AC power system.

[0002]

2. Description of the Related Art Conventionally, for example, Japanese Patent Application Laid-Open No. H11-41817.
No. This will be described below.

FIG. 7 shows a photovoltaic power generation system that uses a solar cell 1 as a DC power supply, converts DC power obtained from the solar cell 1 into AC power by an inverter device 2, and then flows backward to the commercial power system 7. It represents.

FIG. 8 shows a configuration of a transformerless inverter circuit unit provided in the inverter device 2. In this circuit, the positive input terminal P and the negative input terminal N are connected to the positive output terminal and the negative output terminal of the solar cell 1 shown in FIG. 7, respectively, and P-phase and N-phase voltages are applied.

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

[0006] A DC 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 subjected to PWM modulation by the switching operation of the switching elements Q2 to Q5 of the inverter unit 5. Is done. Then, after being shaped into a sine wave AC waveform through the filter unit 8, it is supplied to the commercial power system 7.

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

[0008]

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

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

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

This loss is a loss of a power source for driving the system-linked inverter body, a loss caused by switching of switching elements of the chopper unit 4 and the inverter unit 5, and a loss caused by an increase in inverter current.

Among these losses, the loss is a substantially fixed value, while the loss is a fluctuation value that increases as the output current increases.

For example, if the inverter is a 4.5 kW output inverter, the output power will be 4.5 kW if the rated conversion efficiency is 100%, but the input power will be 4.74 kW if the efficiency is 95%. The relationship between the input power and the output power of the inverter in consideration of the inverter loss (fixed loss and fluctuation loss) is shown by a solid line in FIG.

A point to be noted here is an intersection A between the solid line and the 0 kW line of the input power. This represents the fixed loss of the aforementioned loss. In other words, this indicates 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 photovoltaic power generation system, the time at the time of low output sometimes reaches tens of minutes, and is long enough to be ignored.

In order to prevent the inverter from stopping due to the low output, a conventional technique (Japanese Patent Laid-Open No. 11-4 / 1999) is used.
No. 1817), there is a method of reducing the power factor of the output power to reduce the active power, but this is not a preferable method from the viewpoint of inverter conversion efficiency.

The present invention solves such a problem.

[0018]

SUMMARY OF THE INVENTION A system-linked inverter according to 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. Inverter control means for controlling the inverter section by a multiplex switching method; and low-power control means for performing duty control of the inverter section in response to a decrease in the output power when the output power of the DC power supply or the inverter section decreases. It is a configuration provided.

Further, the low power control means determines whether or not the output power of the DC power supply or the inverter section is smaller than a predetermined threshold, and when it is determined that the output power is smaller than the predetermined threshold. Is a configuration for setting a duty ratio that is proportional to the magnitude of the output power or that changes in accordance with the magnitude of the output power.

Further, the low-power control means performs duty control using a plurality of system frequency cycles as one duty cycle, or performs phase control using one cycle or a plurality of system frequency cycles as one duty period. Configuration.

[0021]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of the present invention applied to a photovoltaic power generation system will be specifically described with reference to the drawings. Note that the configurations denoted by the same reference numerals as those of the above-described conventional example have the same functions, and thus description thereof is omitted.

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

The control means 3 comprises a chopper control means 31 for turning on and off the switching element Q1 of the chopper section 4 to adjust the boosted voltage, and an inverter control means 3 for on / off controlling the switching elements Q2 to Q5 of the inverter section 5.
2. If the voltage detected by the input voltage detecting means 9 is lower than a predetermined threshold (for example, 170 V), the duty control is performed by synchronizing the switching elements Q2 to Q5 of the inverter unit 5 with the timing detected by the zero timing detecting means 10. This is the low power control means 33 that performs the operation.

The operation of the above configuration will be described with reference to FIG.

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

Although the input voltage detecting means 9 has set the reference voltage to 100 V, a voltage suitable for the solar cell 1 is generally set. It is also possible to detect and determine the generated power instead of the voltage in some way.

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

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

As shown in FIG. 3, the duty cycle x / 16 means that 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 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 using a plurality of system cycles of two or more cycles as a basic cycle.

After driving the chopper unit 4 and the inverter unit 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 170 V or more. If the voltage is equal to or higher than 170 V in this step, the process proceeds to step S5.

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

In the section in which the inverter unit 5 is driven by the duty control, the low-power control means 33 controls the switching elements Q2 to Q5 in order to output a sinusoidal current.
Is operated normally. In a section where the inverter unit 5 is not driven by the duty control, the switching elements Q2 to Q5 maintain an off state.

In step S5, since the detected voltage is 170 V or more, 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
It is determined whether or not 16/16 has been set as a result of the increase in the duty ratio. If it is determined in this step that the setting is not 16/16, the process returns to step S4.

In the step S7, when the duty ratio is 16
/ 16, that is, to determine whether or not the continuous drive setting has been made. When the continuous drive setting has been made, the step for terminating the control by the low power control means 33 and switching to the normal operation by the inverter control means 32 described later. It is.

If it is determined in step S4 that the voltage is not 170v or more, 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 this step, the flow shifts to step S9. In step S9, the duty ratio is decreased by one. It should be noted that 1/16 in step S8.
When it is determined that the setting is made, the setting cannot be set below the setting, so that step S9 is skipped.

In step S10, the input voltage detecting means 9
Is determined to be 100 V or less. 100
If it is not less than V, the process proceeds to step S6, and waits until one duty period elapses before returning to step S4. When it is determined in step S10 that the voltage is equal to or lower than 100 V, the system-linked inverter cannot operate.
Then, the process goes to 11 to stop driving the chopper unit 4 and the inverter unit 5. Thereafter, the process shifts to step S1, and waits in the drive stop state until the input voltage detecting means 9 confirms that the voltage is 100 V or more.

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

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

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

When it is determined in step S13 that the voltage has become 170 V or less, the flow returns to step S4 to switch to duty control, the duty ratio is reduced by 1 from the 16/16 setting, and the duty control by the low power control means 33 is started. It is.

By repeatedly executing the above steps, the system-linked inverter can be driven in a wide range of input power from the solar cell 1 from low power to high power.

As described above, when the amount of power generated by the solar cell 1 is low, the duty control of the present invention is executed, and the loss of the inverter unit 5 is reduced as compared with the conventional method of reducing the power factor of the inverter unit. 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 obtained by adding the loss and the output power, as in the conventional control, but when the duty control is off, only the loss is generated. Since the conversion efficiency is obtained from the average power between the ON state and the OFF state, the loss decreases according to the duty ratio. Therefore, the conversion efficiency is improved.

In the above embodiment, the low power control means 33 performs the duty control 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, the phase of the system frequency may be controlled in a plurality of cycles, for example, two cycles as one duty period.

However, in this case, since the DC component is included in the output current, a means for detecting and correcting the DC component or controlling the output current every half cycle so that the DC component is not included in principle can be used. Ingenuity is required.

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

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 unit 5 is a low constant value. However, even during this control, it is possible to obtain an appropriate output power by changing the current amplitude.

In the above-described embodiment, the inverter 5
Although it is assumed that only the duty control is performed, only the chopper unit 4 can perform the duty control 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 according to the present invention, the conversion efficiency at the time of low-output operation is improved, and a long-time operation with low output becomes possible.

[Brief description of the drawings]

FIG. 1 is a diagram showing a schematic configuration of a system-linked inverter of the present invention.

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

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

FIG. 4 is a diagram illustrating a relationship between losses during duty control.

FIG. 5 is a schematic diagram of another example of the duty control;

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

FIG. 7 is a block diagram illustrating a configuration of a photovoltaic power generation system.

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

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

[Explanation of symbols]

 DESCRIPTION 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 part 7 Commercial system power 9 Input voltage detection means 10 Zero timing detection means

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Masaki Bankoji 2-5-2-5 Keihanhondori, Moriguchi-shi, Osaka Sanyo Electric Co., Ltd. F-term (reference) 5F051 KA03 5G003 AA06 BA01 CC07 DA04 DA15 GA01 GB06 GC05 5G066 HA30 HB06 5H007 BB07 CA01 CB05 CC12 DA03 DA04 DB01 DC03 DC05

Claims (4)

[Claims] 1. An apparatus for converting output power of a DC power supply into AC power and supplying the AC power to an AC power system, comprising: an inverter unit including a plurality of switching elements; inverter control means for controlling the inverter unit; When the output power of the DC power supply or the inverter unit decreases, the system interconnection inverter includes low power control means for performing duty control of the inverter unit in accordance with the decrease of the output power. 2. The low power control means determines whether the output power of the DC power supply or the inverter unit is smaller than a predetermined threshold, and determines that the output power is smaller than the predetermined threshold. 2. The system interconnection inverter according to claim 1, wherein a duty ratio that is proportional to the output power or changes according to the output power is set. 3. The system-coupling inverter according to claim 1, wherein said low-power control means performs duty control with a plurality of system frequency cycles as one duty cycle. 4. The system according to claim 1, wherein the low-power control unit is configured to control a system frequency of 1
3. The system-linked inverter according to claim 1, wherein the phase control is performed with one cycle or a plurality of cycles as one duty period. 4.