JP2000069671A - System interconnection inverter apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve power factor and output current distortion by reducing an output current distortion value, even during low output period. SOLUTION: In a system interconnection inverter apparatus 10 for supplying a DC current generated by a solar battery 12 to a power system 20 through conversion to the AC current by a pulse width modulation(PWM) inverter 16, the output current of the PWM inverter 16 is detected by an AC current detecting circuit, a voltage of the power system 20 is detected by an AC voltage detecting circuit 52 and an output of a solar battery 12 is detected by a DC voltage detecting circuit 36 and a DC current detecting circuit 48. Each detected signal is converted into a digital signal and is then input to a one-chip microcomputer 30. Thereby, the width of a control pulse is calculated by deviation between the current command value and feedback current value Ia and a gain coefficient set, depending on the actual output current by the internal process of the microcomputer 30 in order to output a pulse signal for driving the PWM inverter.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a grid-connected inverter, and more particularly, to a grid-connected inverter that converts a DC input current generated in a solar cell into an AC output current and supplies the AC output current to an existing power system.

[0002]

2. Description of the Related Art In recent years, DC new energy sources represented by solar cells have been installed in ordinary homes, factories or specific areas, and connected to a commercial power system of an electric power company to generate surplus power from the new energy sources. A power system that flows backward to an existing power system has been realized. In this power system, it is important to convert a DC current generated by a solar cell into an AC current in phase with a system voltage by an inverter and output power satisfying a standard of power quality to a commercial system.

The inverter outputs an alternating current by, for example, switching of a power IC, and a general switching method includes PWM control. This PWM
The control is a method of giving a target signal waveform and controlling the feedback signal of the actual output current to approach the target signal waveform. In photovoltaic power generation, the amount of generated power increases or decreases according to the amount of solar radiation. Therefore, when the amount of alternating current output is changed according to a change in input, feedback control is performed to change the target signal waveform (increase or decrease the amplitude). That is, if the target signal waveform (command signal) is larger than the feedback signal, the output amount is increased, and if the command signal is smaller than the feedback signal, the output amount is decreased. By always performing this control, it is possible to reduce the input deviation and bring the AC output current, which is a feedback signal, closer to the target waveform. In addition, harmonics can be reduced by doing so.

[0004]

By the way, in the case of the digital processing system (for example, Japanese Patent Laid-Open No. 7-194134).
(See the case of using a high-speed digital signal processor (DSP) disclosed in Japanese Unexamined Patent Application Publication), a time delay and an AD conversion error cause the output waveform to be distorted. This is particularly noticeable when the inverter is controlled by the microcomputer.

[0005] Therefore, a main object of the present invention is to provide a grid-connected inverter device capable of reducing output current distortion even in digital control.

[0006]

SUMMARY OF THE INVENTION The present invention relates to a system interconnection inverter device for converting a DC input current generated from a DC power supply into an AC output current by a PWM inverter and supplying the AC output current to a power system. A system interconnection inverter device comprising: a current detection means for detecting; and a gain coefficient calculation means for calculating a gain coefficient according to a magnitude of a feedback current based on an output current.

[0007]

[Function] PWM for converting DC input current to AC output current
The gain coefficient of the feedback control system is adjusted according to the output current value of the inverter, and the PWM inverter composed of the semiconductor switching element is driven by the ON / OFF pulse signal output based on the adjustment result.

[0008]

According to the present invention, since the distortion value of the output current can be reduced even when the input power is low and the output is low, the output current distortion is improved irrespective of the magnitude of the input power. The above and other objects of the present invention,
Features and advantages will become more apparent from the following detailed description of embodiments, which proceeds with reference to the accompanying drawings.

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A grid-connected inverter device 10 according to an embodiment of the present invention shown in FIG.
2. The output current of the solar cell 12 is
A PWM inverter 16 for smoothing the input and converting it into an AC output current, an output filter circuit 18 for reducing the waveform distortion and noise of the AC power output from the PWM inverter 16, and a PWM inverter 16 via the output filter circuit 18. And the final output load (power system) 20 connected to the power supply.

The PWM inverter 16 is configured by connecting four semiconductor switching elements 22, 24, 26 and 28 in a bridge, and receives the semiconductor switching elements 22, 2 in response to a digital control signal sent from a control device 30.
4, 26 and 28 are ON / OFF controlled to convert DC power output from solar cell 12 into AC power in phase with the system voltage. The output filter circuit 18
Is composed of reactors 32 and 32 and a smoothing capacitor 34.

The control device 30 is constituted by, for example, a one-chip microcomputer. As shown in FIG.
An A / D converter 36 for converting detection signals on the input side (DC side) and output side (AC side) of the M inverter 16 into digital signals, an input / output unit 38, and a ROM and RAM for storing or changing necessary data. The memory unit 40 includes a memory unit 40, a comparison unit 42 for comparing a calculation result with data stored in the memory unit 40, and a CPU 44 for performing necessary calculation processing.

The output of the DC power supply 12 is supplied to a DC voltage detection circuit (pulse transformer or isolation amplifier) 4.
6 and a DC current detection circuit (current transformer) 48. The output current of the PWM inverter 16 is detected by an AC current detection circuit (current transformer) 50, and the AC voltage of the power system is detected by an AC voltage detection circuit (transformer) 52.

The detection signals relating to the voltage and current on the DC side and the AC side detected by each detection circuit are applied to a one-chip microcomputer 30.
Is converted into a digital signal by the A / D converter 36 and sent to the input / output unit 38. The CPU 44 performs necessary arithmetic processing based on the digital signal sent from the input / output unit 38 and the data stored in the memory unit 40, compares the arithmetic results with the comparing unit 42, and outputs the final processing result to the input / output unit. 3
8 to the PWM inverter 16 as an ON / OFF pulse signal. Then, the PWM inverter 16 is driven by the ON / OFF pulse signal, and the power system 20 is turned on.
Is controlled in a sinusoidal waveform. That is, the feedback signal is taken into the A / D converter 36, the PWM control pulse width is calculated by the internal calculation processing of the microcomputer 30, and the ON / OFF of the pulse signal for driving the PWM inverter 16 is output.

FIGS. 3 and 4 show output power characteristics (PV characteristics) and output current characteristics (IV characteristics) of the solar cell 12.
It is shown. Generally, the generated power obtained from the solar cell 12 changes depending on the amount of solar radiation as described above. Therefore, the control device 3 composed of a one-chip microcomputer
At 0, the DC voltage is controlled by feedback control to a voltage at which the maximum power can be obtained. Controlling the DC voltage is substantially equivalent to controlling the DC current, as is apparent from the IV characteristics shown in FIG.

In outputting the maximum power thus obtained to the power system, the parameter to be changed is the amplitude value of the output current. Incidentally, the system voltage of the power system is usually within 101 ± 6V or 202 ± 20V, and the output power is controlled by increasing or decreasing the output current amplitude value with respect to that voltage. Further, a gain control method based on an output current value is effective for correcting distortion of an output waveform. In this case, as shown in FIG. 10, assuming that the output coefficient of the inverter and the deviation are fixed and the gain coefficient K is fixed, the deviation ΔZ between the control target waveform and the actual output waveform at low output and high output is Because of the difference, the responsiveness of the feedback control system changes. Generally, the gain coefficient K is often fixed to a value determined at the time of rated output. Therefore, it is necessary to increase the gain coefficient K at the time of low output.

The relationship between the actual output current value Ij (A) of the PWM inverter 16 and the gain coefficient K (dB) in the present invention is as shown in FIG.
At the time of x, the gain coefficient K is the minimum K1 and the slope is F. Therefore, the gain coefficient K at the time of the actual output current value Ij can be obtained by the proportional expression shown in Expression 2.

[0017]

K = F (Imax-Ij) + K1 Therefore, when the actual output current value Ij is small, the gain coefficient K becomes large, and when the actual output current value Ij is large, the gain coefficient K becomes small. As a result, when the output is low, the gain coefficient is higher than when the output is high.

As another method for changing the gain coefficient K, (a) a table value according to the output current, (b) a value divided at the time of low output / high output, and (c) fuzzy control are used. Values are also possible. Next, a system interconnection inverter device 10 by feedback control using a one-chip microcomputer 30 according to one embodiment of the present invention.
Will be described with reference to the flowcharts of FIGS.

In FIG. 6, the DC voltage DCV (m) and the DC current DCI (m) are detected by the DC voltage detection circuit 46 and the DC current detection circuit 48 in step S1, and the A / D converter 36 of the one-chip microcomputer 30 is used. To convert to a digital signal. In step S3, the input / output unit 3 of the microcomputer 30
The CPU 44 calculates the current DC power DCP (m) based on the digital signal input to the step S8, and then proceeds to step S5.
In the above, the previous DC power DC stored in the memory unit 40
Read P (m-1).

In step S7, the current power DCP (m)
And the previous power DCP (m-1) are compared by the comparing unit 42,
If the result is "YES", 1 is added to the previous output current command amplitude Ik (m-1) stored in the memory unit 40 as the current command amplitude Ik (m) in step S9,
If "NO", 1 is subtracted from the previous output current command amplitude Ik (m-1) in step S11. In step S13, the current power DCP (m) is changed to the previous power DCP.
Instead of (m-1), the current command amplitude Ik (m) is output in step S15.

In FIG. 7, in step S17, FIG.
The output of the current command amplitude Ik (n) from step S15 is input to the CPU 44. In step S19, the voltage command phase Ip (n) obtained by digitally converting the voltage phase of the power system by the A / D converter 36 is input to the CPU 44. I do. In step S21, a current command value Is (n) as a control target is output, and the actual output current detected by the AC current detection circuit 50 in step S23 is converted to the A / D converter 3
6, and is converted to a feedback current value Ia (n).
Get. In step S25, the output current detected by the AC current detection circuit 50 is digitally converted by the A / D converter 36 to obtain an actual output current value Ij (n).

Then, in a step S27, the CPU 44
Gain coefficient K = F (Imax−Ij) set according to the actual output current value Ij (n) by the arithmetic processing in
(N)) + K1 is obtained by calculation, and in step S29, a control deviation (n) = Is (n) -Ia () due to a deviation between the current command value Is (n), which is a control target, and the feedback current value Ia (n). n) is calculated. Finally, in step S31, the previous control deviation (n-1) is subtracted from the current control deviation (n), and the difference is calculated in step S2.
9 and the previous coefficient P
The PWM width (n) of this time is calculated by adding the WM width (n-1). Then, in step S33, the PWM width (n)
And a pulse signal based on the output is input to the PWM inverter 16.

FIGS. 8 and 9 show the output (500) when the grid interconnection inverter device 10 according to the present invention is used.
7 is a graph showing a relationship between a gain and a power factor and a relationship between a gain and a harmonic (distortion) using W, 2 kW, and 4 kW as parameters. As is clear from the graphs of FIGS. 8 and 9, in the case of low output (500 W), when the gain is low, the power factor is poor, the number of harmonics is large, and the distortion is large. When the gain is as high as 16, the power factor is improved, the harmonics are reduced, and the distortion is improved. On the other hand, at the time of high output (4 kW), if the gain is too high, the power factor is not so bad, but the oscillation symptom is mixed in, the harmonics increase, and the distortion increases. When the gain is 8, the power factor is good, the harmonics are small, and the distortion is improved. Also, at the time of intermediate output (2 kW), if the gain is too low, the power factor is poor and the number of harmonics increases, so it is understood that a higher gain of 16 is desirable.

Therefore, it is effective to set the gain according to the output amount. In particular, when the output is low, the gain is set high, so that not only the power factor but also the output distortion is improved. Since the feedback current is controlled so as to approach the command signal, the gain coefficient can be calculated from the magnitude of the command signal.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a schematic configuration of a system interconnection inverter device according to one embodiment of the present invention.

FIG. 2 is a block diagram of a one-chip microcomputer constituting the control device shown in FIG.

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

FIG. 4 is an IV characteristic diagram showing output current characteristics of a solar cell.

FIG. 5 shows a gain coefficient K and an actual output current value I in the embodiment.
It is a graph which shows the relationship of j.

FIG. 6 is a flowchart for explaining the operation of the embodiment shown in FIG. 1;

FIG. 7 is a flowchart constituting a part of FIG. 6;

FIG. 8 is a graph showing a relationship between a gain and a power factor using the output of the grid-connected inverter device in the embodiment as a parameter.

FIG. 9 is a graph showing a relationship between a gain using output as a parameter and a high frequency (distortion) as in FIG. 8;

FIG. 10 is a graph showing a relationship between an output waveform of an inverter and a deviation.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 10 ... Grid connection inverter device 12 ... Solar cell 16 ... PWM inverter 20 ... Load (power system) 22, 24, 26, 28 ... Semiconductor switching element 30 ... Control device (one-chip microcomputer) 36 ... A / D conversion part 38 ... I / O section 40 ... Memory section 42 ... Comparison section 44 ... CPU 46 ... DC voltage detection circuit 48 ... DC current detection circuit 50 ... AC current detection circuit 52 ... AC voltage detection circuit

Continued on the front page F-term (reference)

Claims (5)

[Claims] 1. A DC input current generated from a DC power supply is represented by PW
In a system interconnection inverter device for converting into an AC output current by an M inverter and supplying the AC power to a power system, a current detection means for detecting an output current of the inverter, and a gain coefficient according to a magnitude of a feedback current based on the output current Characterized in that it comprises a gain coefficient calculating means for calculating. 2. The system interconnection inverter device according to claim 1, wherein said gain coefficient calculation means sets a large gain coefficient when said feedback current is small. 3. The system interconnection inverter device according to claim 1, wherein said gain coefficient calculation means calculates a gain coefficient K based on equation (1). K = F (Imax-Ij) + K1 where Imax: maximum output current value Ij: actual output current value K1: gain coefficient corresponding to maximum output current value F: constant 4. The system interconnection inverter device according to claim 1, wherein said gain coefficient calculation means includes a one-chip microcomputer. 5. The system interconnection inverter device according to claim 1, wherein said DC power supply includes a solar cell.