JP2000350466A - System interconnection inverter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enable output of power, where DC components are removed from the output current of an inverter. SOLUTION: A switching signal generator detects the current values of output currents at prescribed sampling times, and adds them up, and sets the correction of the ROM data being the objective value, based on this addition results, and also corrects the ROM data further from the difference between the objective current value by the previous ROM data and the output current value, and outputs switching signals (steps 100-118), based on the corrected ROM data. Hereby, the output current can be brought close to the object current of the ROM data, and DC components can be removed from the output currents.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a system interconnection inverter for supplying DC power generated by a photovoltaic power generator or the like to a system power supply.

[0002]

2. Description of the Related Art In recent years, a system interconnection system such as a system interconnection generator for connecting (interconnecting) a photovoltaic power generator to a commercial power system has been widely used. In such a system interconnection system, DC power generated by a solar cell is converted into AC power by an inverter device and regenerated to the system.

On the other hand, in a system interconnection system, for example, a target value of an output current for one cycle is set, and data serving as the target value is stored as a ROM table.
By setting an on-time (on-duty) for driving the switching element of the inverter device based on the data in the ROM table, a current corresponding to a target value can be output.

[0004] A transformer is provided in a system power supply or a load connected to the system power supply. In such a transformer, particularly in a transformer provided in a system power supply line, an abnormal current may be generated due to a DC component mixed with AC power.

However, if DC power is converted into AC power corresponding to the voltage of the system power supply, a DC component may be output if the voltage waveform of the system power supply is not a regular waveform, and the system interconnection system may be output. Is required to minimize the DC component (for example, the DC current is 1% or less of the rated output current).

[0006]

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned circumstances, and proposes an inverter device capable of outputting power with a DC component removed irrespective of the voltage waveform of a system power supply. With the goal.

[0007]

In order to achieve the above object, the present invention turns on / off a switching element based on a target value of an on-duty time set at predetermined time intervals within one cycle. A grid-connected inverter that converts DC power into AC power based on a power supply frequency of the system and outputs the AC power to the system, wherein the detecting means detects at least a current value included in the output AC power, and the detecting means And correcting means for correcting the target value so as to reduce the DC component included in the AC power based on the detection result.

According to the present invention, the on-duty of the switching element is set based on the target value at a predetermined time interval which is set and stored in advance, and the switching element is turned on / off, thereby obtaining the target value. Outputs AC power with a defined current waveform.

At this time, the correcting means corrects the target value based on at least the current value of the output power detected by the detecting means so that the DC component in the output power is reduced.
As a result, the DC component is reliably removed, and output power substantially free of the DC component is obtained.

According to the present invention, the detecting means includes an integrating means for integrating the instantaneous value detected at a predetermined sampling period, and the correcting means performs a calculation based on the integrated value of at least one cycle integrated by the integrating means. Thus, the target value can be corrected.

According to this, the integrated value of the output current in one cycle of the power including no DC component is “0”. Therefore, by shifting the target value for one cycle so that the integrated value of the current approaches “0”, the waveform of the output current can be made closer to the waveform of the target value, and the DC component included in the output power can be reduced. it can.

Also, in the present invention, the detecting means determines a maximum value and a minimum value of an instantaneous value detected within at least one cycle, and the correcting means corrects the target value based on the maximum value and the minimum value. May be used.

[0013] According to this, the normal AC power has a current waveform close to a substantially sine wave. For this reason, in the AC current that does not include the DC component, the maximum value on the positive side and the maximum value on the negative side are substantially equal.

From this, the DC component included in the output current can be reduced by shifting the target value based on the difference between the maximum values so that the positive and negative maximum values become substantially equal.

Further, in the present invention, the detecting means includes measuring means for measuring a first time of a positive half cycle and a second time of a negative half cycle of the current value waveform,
The correcting means may correct the target value based on the time difference measured by the measuring means.

According to this, in the AC current having a regular waveform not including the DC current, the first time when the instantaneous value becomes a positive value and the second time when the instantaneous value becomes a negative value are substantially the same. . From this, the first time and the second time of the output current are measured, and the DC value included in the output current can be reduced by shifting the target value so as to eliminate the difference.

According to the present invention, the comparing means compares the current value detected at the predetermined time interval with the detection result of the detecting means and the current value based on the target value or the corrected target value. More preferably, the correction means sets the next on-duty time based on the comparison result of the comparison means.

According to the present invention, the difference between the current value based on the corrected target value and the current value detected by the detecting means is obtained, and the next target value is corrected based on this difference, so that the output current is corrected. By making the waveform close to the waveform of the target value, it is possible to obtain output power substantially free of a DC component.

As described above, according to the present invention, the DC component is gradually removed from the output current by repeatedly correcting the target value so as to reduce the DC component included in the output current.
It is possible to output AC power containing almost no DC component.

[0020]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of a system interconnection inverter according to the present invention will be described below with reference to the drawings.

FIG. 1 shows a schematic configuration of a photovoltaic power generator 10 to which the present invention is applied. Solar power generator 10
Includes a solar cell 12 constituted by a solar cell array connecting a plurality of solar cell modules each formed by connecting solar cells, and a system interconnection inverter (hereinafter referred to as “inverter 14”). Have.
FIG. 1 shows a solar cell 12 connected to a solar cell array formed by a plurality of modules.

The photovoltaic power generator 10 converts the DC power generated by the individual photovoltaic cells of the photovoltaic cells 12 to receive sunlight into a commercial power system (hereinafter referred to as a "system power supply 16") by an inverter 14. Converts to AC power of the corresponding frequency.

The photovoltaic power generator 10 is connected to a system power supply 16 via a distribution board (not shown), for example, and regenerates generated power to the system power supply 16 via the distribution board. At this time, the photovoltaic power generation device 10
It is connected to the line of the system power supply 16 via the protection relay 54 and a circuit breaker (not shown), and can be disconnected from the system power supply 16 by the parallel-off conductor 30 or the protection relay 54. This prevents abnormalities occurring in the system power supply 16 from spreading to the inverter 14. In addition,
The photovoltaic power generator 10 applied to the present embodiment has 200
V AC power is supplied to a single-phase three-wire 100V / 200V system power supply 16 line.

The inverter 14 of the solar power generation device 10
An inverter circuit 20 and a microcomputer (hereinafter, referred to as “microcomputer 22”) that controls the inverter circuit 20 and the like are provided. The DC power generated by the solar cell 12 is supplied to the inverter circuit 20 via a diode 24 for preventing backflow, a circuit breaker (NFB) 25 for protection, and a noise filter 26.

The DC power input to the inverter circuit 20 is converted into AC power having substantially the same frequency as the system power supply 16 by the inverter circuit 20 and output. The AC power output from the inverter circuit 20 is output to the line of the system power supply 16 via the filter circuit 28 and the noise filter 29. That is, the solar power generation device 10 uses a transformerless system in which the inverter 14 is connected to the system power supply 16 without using a transformer.

On the other hand, the inverter circuit 20 switches the DC power based on the PWM theory and
And outputs a pseudo sine wave having substantially the same frequency as. Microcomputer 2
Reference numeral 2 includes an inverter circuit 20, a generation voltage detection unit 32 including an isolation amplifier that detects a voltage of the power generated by the solar cell 12, and a current transformer (CT) that detects a DC current of the power generated by the solar cell. Generated current detector 3
4. An output current detector 38 for detecting an AC current (output current) output from the inverter circuit 20, a transformer (PT)
A system voltage detection unit 40 that detects a system voltage, a voltage waveform, and a frequency of the system power supply 16 is connected thereto. Note that the output current detection unit 38 includes a current transformer (CT) and a voltage dividing resistor, and the divided voltage (current value) is directly output from the microcomputer 22.
A / D (analog / digital) input port. The microcomputer 22 converts the voltage (current value) applied to the A / D input port into a predetermined cycle (for example, 1/220
Scan in c) and capture as instantaneous values. A at this time
The digital side of the / D input port may have 10 bits.

The inverter circuit 20 includes an IGBT
Switching elements 42Xa, 42Xb, 42Ya, 4
2Yb (hereinafter collectively referred to as “switching element 4
2) are provided in a bridge connection, and diodes (flywheel diodes) 44 are provided corresponding to the respective switching elements 42.

The microcomputer 22 generates a switching signal for driving the switching element 42 of the inverter circuit 20 based on the generated power detected by the generated voltage detector 32 and the generated current detector 34 and the voltage detected by the voltage waveform detector 40. Is controlled. Each of the switching elements 42 of the inverter circuit 20 is turned on / off in response to a switching signal output from the microcomputer 22.

Thus, from the inverter circuit 20,
The AC power whose voltage and phase match the system power supply 16 is output according to the zero crossing of the voltage detected by the voltage waveform detection unit 40. Switching element 4
The voltage waveform of the AC power output from the inverter circuit 20 due to the on / off of 2 is a sawtooth waveform,
Filter circuit 28 removes harmonic components from the output power of inverter circuit 20 and outputs AC power having a predetermined waveform.

A booster circuit is provided between the inverter circuit 20 and the noise filter 26, and after the power generated by the solar cell 12 is boosted to a DC voltage having a predetermined voltage corresponding to the system voltage by the booster circuit, the power is supplied to the inverter circuit 20. May be input.

As shown in FIG. 2, the microcomputer 22 is provided with a signal generator 50 for generating a switching signal for obtaining a sine wave signal based on the PWM theory. FIG. 2 shows a functional block diagram of the signal generation unit 50.

The signal generating section 50 includes a ROM 52 for storing data (an ON / OFF signal pattern) for generating a preset current waveform (hereinafter, stored data is referred to as “ROM data”). , ROM 52 to ROM
The control unit 56 includes a control unit 56 that reads data and generates a switching signal based on the ROM data, and a driving unit 58 that drives the switching element 42 based on the switching signal output from the control unit 56. In the inverter 14, the switching element 4
2 is driven by a switching signal of 10 kHz to 15 kHz.

The control unit 56 controls the phase with respect to the frequency fs of the system power supply 16 (for example, fs = 50 Hz when the power supply frequency of the system power supply 16 is 50 Hz) and the frequency of the system power supply 16.
It outputs AC power in accordance with the power supply voltage of. At this time,
The control unit 56 sets a time (on duty) for turning on the switching element 42 based on the ROM data stored in the ROM 52.

This switching signal is output as a switching signal Xa and also as a switching signal Ya having a phase shifted by 180 ° from the switching signal Xa. Also, the switching signal X
a and Ya are input to an inverter 64, and a switching signal Xa obtained by inverting the switching signals Xa and Ya.
b and Yb are output. The drive unit 58 uses the switching signals Xa, Xb, Ya, and Yb to switch the switching element 42X.
a, 42Xb, 42Ya, 42Yb are switched (ON /
Off). As a result, the inverter 14 outputs power according to the frequency and voltage of the system power supply 16.

On the other hand, the parallel contactor 30 is connected to the microcomputer 2
The microcomputer 22 connects and disconnects the photovoltaic power generator 10 and the system power supply 16 by the parallel contactor 30. For example,
The microcomputer 22 disconnects the photovoltaic power generator 10 from the system power supply 16 when the power generated by the solar cell 12 is low or does not generate power, and the operation of the inverter 14 is stopped. Immediately before starting, the photovoltaic power generator 10 and the system power supply 16 are connected.

When the microcomputer 22 detects an increase in voltage, a decrease in voltage, an increase in frequency, or a decrease in frequency of the system power supply 16, the microcomputer 22 operates the disconnecting conductor 30 or the protection relay 54 to disconnect the inverter 14 from the system power supply 16 and disconnect the system power supply 16. Connection protection is provided. A set value is set in advance for the voltage rise, the voltage drop, the frequency rise, and the frequency drop. When the voltage or frequency of the system power supply 16 reaches the set value, the system interconnection protection is performed.

The configuration and control of the photovoltaic power generator 10 can be applied to a conventionally known configuration and control method, and a detailed description thereof will be omitted in the present embodiment.

The ROM 5 of the control unit 56
2 stores, as ROM data, a target value of an on-time (on duty) of a switching signal when a preset current waveform such as a sine wave is generated, and the ROM data is based on a phase. Is read.
In the inverter 14, the switching element 42 is driven by an on-duty switching signal based on the ROM data, thereby outputting a predetermined current and power having a current waveform based on the ROM data. This RO
The M data is set based on a predetermined current waveform that does not include a DC component such as a sine wave.

On the other hand, the control unit 52 receives the system voltage from the system voltage detecting unit 40 and the output current from the output current detecting unit 38. When generating the switching signal, the control unit 56 corrects the ROM data based on the output current so as to prevent the output current from outputting power including a DC component due to the influence of the system power supply. ing.

That is, the control section 56 is provided with an output current integrating section 60 and a correction section 62. The output current integrating unit 60 reads the output current Io detected by the output current detecting unit 38 at a predetermined sampling time, integrates the integrated value Iw of the output current for one cycle, and includes a DC component in the output current. It is determined whether or not.

When the output current integrating section 60 determines that a DC component is included, the correcting section 62 removes the DC component from the output current based on the integrated value Iw so as to remove the DC component.
Shift and correct the data.

For example, the integrated value Iw for one cycle of the power supply frequency integrated by the output current integrating unit 60 is “0 (Iw =
0) ", it is determined that the output current does not include a DC component, but the integrated value Iw is a positive value (Iw> 0).
, It is determined that a positive DC component is included in the output current, and the ROM data read from the ROM 52 is shifted in a direction in which the positive current component decreases (in a direction in which the current value on the positive side decreases). ). When the integrated value Iw is a negative value (Iw <0),
It is determined that the output current contains a negative DC component, and RO
The ROM data read from M52 is corrected so as to shift in the direction in which the negative current component decreases (in the direction in which the current value on the positive side increases).

On the other hand, the control section 56 is provided with a current difference calculating section 66. The current difference calculator 66 drives the switching element 42 based on the target output current value Ia of the inverter 14, ie, the data Ds, from the ROM data or the ROM data (hereinafter referred to as data Ds) corrected based on the integrated value Iw. Then, the output current value is calculated, and the calculated target output current value Ia is compared with the actual output current value Is detected by the output current detection unit 38.

The correction section 62 of the control section 56 further corrects Ds when the next switching signal is generated based on the comparison result. That is, if there is almost no difference between the target output current value Ia and the output current value Is (Ia
−Is = 0), it is determined that the DC current is not included in the output current from the inverter 14, but the target output current value Ia
When there is a difference between the output current value Is and the output current value Is, it is determined that the output current from the inverter 14 includes a DC component, and the data Ds for generating the switching signal is corrected based on the current difference.

That is, when the target output current Ia is higher than the output current value Is, it is determined that the output current contains a positive DC component, and the data Ds is set so that the output current value at the next switching becomes smaller. Is corrected, and a switching signal is generated based on the corrected data. When the target output current value Ia is lower than the output current value Is, it is determined that the output current contains a negative DC component,
The data Ds is corrected so that the current value of the next switching signal increases the output current value of the inverter 14, and a switching signal is generated based on the corrected data.

As a result, the control unit 56 reduces the DC component included in the output power of the inverter 14 and outputs power substantially free of the DC component to the system power supply 16.

Hereinafter, the removal of the DC component from the output power of the inverter 14 will be described as the operation of the present embodiment. The ROM 52 stores, for example, 720 pieces of data as ROM data for one cycle, and the control unit 56 reads the ROM data at intervals corresponding to the cycle of the switching signal. For example, when the cycle of the switching signal is 11 kHz, 220 data is read out per cycle at a power supply frequency of 50 Hz (fs = 50 Hz) to generate a switching signal.

In the flowchart shown in FIG. 3, in the first step 100, a correction value of the ROM data is set.
FIG. 4 shows an outline of the ROM data correction value setting process.

In this flowchart, when the integrated value Iw is reset in the first step 120, the next step 1
At 22, the output current detection unit 3 is provided at a predetermined sampling interval.
In step 124, the instantaneous value (current value Is) of the output current detected is read, and in step 124, the instantaneous value is integrated with a sign. In step 126, it is checked whether the integration of one cycle of the power supply frequency fs is completed.

When the integration of the current value for one cycle of the power supply frequency fs is completed (YES in step 126), the process proceeds to step 128, where it is determined whether the integrated value Iw is "0" or not. When the value Iw is Iw = 0, an affirmative determination is made in step 128 and the process proceeds to step 130,
Set no correction.

On the other hand, when the integrated value Iw is not "0" (negative determination in step 126), step 13
The process proceeds to 2 to check whether the integrated value Iw is positive or negative. If the integrated value Iw is positive (Iw> 0, a positive determination in step 132), the process proceeds to step 134 to lower the ROM data. Set to correct in the direction.

When the integrated value Iw is negative (Iw <
0, negative determination in step 132) includes step 136
Is set so that the ROM data is corrected in the upward direction. Note that the correction amount of the ROM data is the integrated value Iw
May be set to shift based on the absolute value of
It may be set so as to shift by a preset correction amount.

In the flowchart shown in FIG. 3, the correction value set in this way is read. After this,
Steps 102 to 116 are executed based on the output interval of the switching signal, that is, based on the switching cycle, whereby the switching element 42 is driven in the switching cycle.

For this purpose, in step 102, the ROM data before correction when the previous switching signal is generated is read, and the target current value Ia is calculated from the uncorrected ROM data (step 104). In step 106, the output current value Is output from the inverter 14 is read from the output current detection unit 38 by the switching signal generated based on the previous ROM data. In step 108, the target current value Ia and the output current value Is Is calculated.

On the other hand, in step 110, the ROM data of the next switching signal is read from the ROM 52, and in step 112, the ROM data is corrected based on the integrated value Iw to generate data Ds.

In the next step 114, the data Ds
Is corrected based on the difference between the target current value Ia calculated in step 104 and the output current value Is.

In the next step 116, a switching signal is generated based on the data thus corrected, and the switching element 42 is driven. In step 118, it is confirmed whether or not the processing for one cycle of the power supply frequency fs has been completed, and the driving of the switching element for one cycle has been completed.
After shifting to 0, a correction value for the next cycle is set.

That is, in addition to the correction set for each cycle of the power supply frequency fs, the ROM data is corrected every time a switching signal is generated, and the output of the switching signal is suppressed while suppressing the change in on-duty every time the switching signal is generated. The DC component of the current is quickly removed.

By outputting the switching signal in this manner, for example, as shown in FIG. 5A, the output current includes a negative DC component and the output current is entirely negative as shown by the solid line. Side, the ROM data which is the target value of the output current indicated by the two-dot chain line is
The correction is made so as to shift to the positive side as shown by the broken line.

As shown in FIG. 5B, when the output current shifts to the positive side due to the inclusion of a positive DC component, the RO is shifted so that the output current shifts to the negative side.
Correct the M data.

As a result, the output current is brought closer to the target value of the ROM data containing no DC component. That is, the DC component of the output current is reduced.

When the switching signal is generated for each switching cycle, the target current value Ia based on the ROM data at the time of generating the previous switching signal is compared with the actual output current value Is. To correct the next switching signal. For example, if the output current value Is is smaller than the previous target current value Ia, it is determined that the output current includes a negative DC component. At the next switching, the negative DC component is suppressed, and the output current value Ia
s is made to approach the target current value Ia. If the output current value Is is larger than the previous target current value Ia, it is determined that the output current includes a positive DC component. At the next switching, the positive DC component is suppressed, and the output current value Is is reduced. To approach the target current value Ia.

As a result, the output current can be approximated to the current value based on the ROM data that does not include the DC component, and the DC component can be further removed and the waveform of the output current can be further shaped.

As described above, in the inverter 14, the output current detecting section 38 accurately detects the current value output from the inverter 14, thereby appropriately determining whether or not the output current includes a DC component. By generating a switching signal based on this determination result and the detected output current value, it is possible to obtain appropriate output power that does not include a DC component.

In this embodiment, the inverter 14
Was determined based on the integrated value of the output current to determine whether or not the output power contained a DC component. Based on the determination result, the ROM data was corrected so that the DC component was removed from the output power. The determination of the presence or absence and the correction of the ROM data are not limited to this.

When the current waveform of the output power does not have a large deformation and is close to a waveform in which the positive half cycle and the negative half cycle are similar to each other, such as a predetermined sine wave, the maximum value on the positive side of the output current is obtained. It is determined whether or not a DC component is included based on the value and the maximum value on the negative side. If it can be determined that a DC component is included, the ROM data may be corrected based on the difference between the maximum values. good.

That is, as shown in FIG.
A maximum value detecting unit 70 may be provided instead of the flow integrating unit 60.
No. As shown in FIGS. 7A and 7B, the maximum
The value detection unit 70 calculates the maximum value I of the positive half cycle._{+ MAX}When,
Maximum value I of the negative half cycle _-MAXAnd this difference is approximately
Shift the target current value of ROM data so that it becomes the same
Output current by generating a switching signal.
Is closer to the target current, and the DC component is
Remove.

As shown in FIG. 8, a time measuring section 72 may be provided instead of the output power integrating section 60. FIG.
As shown in FIG. 9A and FIG.
2, the time T ₁ of the positive half cycle, the time T ₂ of the negative half cycle measuring time T ₁ on the basis of the measurement result
When correcting the ROM data as time T ₂ are equal, by generating a switching signal, close the output current to the target current, for removing a DC component from the output current.

In the present embodiment described above, the description has been made using the solar cell power generation device 10 including the inverter 14 for converting the DC power generated by the solar cell 12 into AC power corresponding to the system power supply 16. In addition, the present invention can be applied to various types of grid-connected power generators that convert DC power into AC power.

[0070]

As described above, according to the present invention, the current value of the output current is accurately detected, and the target value of the on-duty of the switching element is corrected based on the detection result. An excellent effect of removing a DC component and outputting an appropriate power according to the system power supply can be obtained.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a schematic configuration of a photovoltaic power generator applied to the present embodiment.

FIG. 2 is a functional block diagram illustrating a schematic configuration of a switching signal generator.

FIG. 3 is a flowchart showing an outline of generation of a switching signal.

FIG. 4 is a flowchart showing the integration of an output current value and the determination of the presence or absence of a DC component based on the integration result.

FIGS. 5A and 5B are diagrams schematically illustrating current waveforms based on an output current, a target value, and a target value corrected based on an integrated value of the output current, and FIG. An example including a negative DC component is shown, and (B) shows an example in which a positive DC component is included in the output current.

FIG. 6 is a functional block diagram illustrating a schematic configuration of a switching signal generation unit showing an example of correcting a target value based on a maximum value of an output current.

FIGS. 7A and 7B are diagrams schematically showing current waveforms based on an output current, a target value, and a target value corrected based on the maximum value of the output current, and FIG. An example including a negative DC component is shown, and (B) shows an example in which a positive DC component is included in the output current.

FIG. 8 is a functional block diagram showing a schematic configuration of a switching signal generation unit showing an example of correcting a target value based on a difference between a positive half cycle time and a negative half cycle time of an output current. .

FIGS. 9A and 9B are output currents, target values, and current waveforms based on a target value corrected based on a difference between a positive half cycle time and a negative half cycle time of the output current. FIGS. 3A and 3B are schematic diagrams schematically showing an example in which (A) shows an example in which a negative DC component is included in an output current and (B) shows an example in which a positive DC component is included in an output current.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 10 Photovoltaic power generator 12 Solar cell 14 Inverter (system interconnection inverter) 16 System power supply 20 Inverter circuit 22 Microcomputer 38 Output current detection part 42 Switching element 50 Switching signal generation part 52 ROM 56 Control part 60 Output current integration part 62 Correction part 66 Current difference calculation unit 70 Maximum value detection unit 72 Time measurement unit

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Isao Morita 2-5-5 Keihanhondori, Moriguchi-shi, Osaka Sanyo Electric Co., Ltd. (72) Hisashi Tokizaki 2-chome Keihanhondori, Moriguchi-shi, Osaka No. 5-5 Sanyo Electric Co., Ltd. F term (reference) 5G003 AA06 BA04 CB05 CC02 GA01 GB06 GC05 5G066 EA00 GC02 HA04 HA06 HB06 KC02 5H007 AA07 BB07 CA01 CB05 DB02 DB12 DB13 DC02 DC05 FA14 FA19 GA09

Claims (5)

[Claims] 1. A switching element is turned on / off based on a target value of an on-duty time set at predetermined time intervals within one cycle to convert DC power into AC power based on a system power frequency. A grid interconnection inverter that converts and outputs the DC power to a system, comprising: a detection unit configured to detect at least a current value included in the output AC power; and a DC component included in the AC power based on a detection result of the detection unit. Correction means for correcting the target value so as to decrease it. 2. An accumulating means for accumulating instantaneous values detected in a predetermined sampling cycle by said detecting means, wherein said correcting means calculates said target value based on an accumulated value of at least one cycle accumulated by said integrating means. The system interconnection inverter according to claim 1, wherein the correction is performed. 3. The method according to claim 1, wherein the detecting means determines a maximum value and a minimum value of the instantaneous value detected within at least one cycle, and the correcting means corrects the target value based on the maximum value and the minimum value. The system interconnection inverter according to claim 1, wherein: 4. The detecting means includes measuring means for measuring a first time of a positive half cycle and a second time of a negative half cycle of the waveform of the current value, and the measuring means measures the time. 2. The system interconnection inverter according to claim 1, wherein the correction unit corrects the target value based on the obtained time difference. 3. 5. A method according to claim 1, further comprising comparing means for comparing a current value detected at said predetermined time interval with a detection result of said detecting means and a current value based on said target value or said corrected target value. 5. The system interconnection inverter according to claim 1, wherein the means sets a next on-duty time based on a comparison result of the comparison means. 6.