JP3789653B2 - Grid-connected inverter device - Google Patents

Description

[0001]
[Industrial application fields]
The present invention relates to a grid-connected inverter device, and more particularly to a grid-connected inverter device that converts, for example, a DC input current generated in a solar cell into an AC output current and supplies the AC power to an existing power system.
[0002]
[Prior art]
In recent years, new direct-current energy sources such as solar cells have been installed in ordinary homes, factories, or in specific areas, and connected to the commercial power grid of electric power companies, and surplus power from the new energy source has been transferred to the existing power grid. A reverse power flow power system has been realized. In this electric power system, it is important to convert the direct current generated by the solar cell into an alternating current whose phase matches that of the system voltage by an inverter and to output electric power satisfying the power quality standard to a commercial system.
[0003]
The inverter performs an alternating current output by switching of a power IC, for example, and there is PWM control as a general switching method. This PWM control is a method of giving a target signal waveform and controlling so that the feedback signal of the actual output current approaches the target signal waveform.
In solar power generation, the generated power increases or decreases depending on the amount of solar radiation. Therefore, when the AC current output amount is changed according to the input change, feedback control is performed to change the target signal waveform (increase or decrease the amplitude). That is, when the target signal waveform (command signal) is larger than the feedback signal, the output amount is increased. Conversely, when the command signal is smaller than the feedback signal, the output amount is decreased. By always performing this control, the input deviation can be reduced and the AC output current as a feedback signal can be brought close to the target waveform. This also reduces harmonics.
[0004]
[Problems to be solved by the invention]
By the way, in the case of a digital processing method (see, for example, the case of using a high-speed digital signal processor (DSP) disclosed in Japanese Patent Laid-Open No. 7-194134), time delay or AD conversion error distorts the output waveform. Cause it. This is particularly noticeable when the inverter is controlled by a microcomputer.
[0005]
Therefore, a main object of the present invention is to provide a grid-connected inverter device that can reduce output current distortion even in digital control.
[0006]
[Means for Solving the Problems]
In a grid-connected inverter device that converts a DC input current generated from a DC power source into an AC output current by a PWM inverter and supplies the AC grid to a power system, current detection means for detecting the output current of the inverter, and feedback based on the output current Gain coefficient calculation means for calculating a gain coefficient of a feedback control system according to the magnitude of current, and drive means for driving a PWM inverter by a pulse signal output based on the gain coefficient , the gain coefficient calculation The means is a grid-connected inverter device that sets a large gain coefficient when the feedback current is small .
[0007]
[Action]
The gain coefficient of the feedback control system is adjusted according to the output current value of the PWM inverter that converts the DC input current to the AC output current, and the semiconductor switching element is configured by the ON / OFF pulse signal that is output based on this adjustment result The PWM inverter to be driven is driven.
[0008]
【The invention's effect】
According to the present invention, the distortion value of the output current can be reduced even at a low output with a low input power, so that the output current distortion is improved regardless of the magnitude of the input power.
The above object, other objects, features and advantages of the present invention will become more apparent from the following detailed description of embodiments with reference to the drawings.
[0009]
【Example】
A grid-connected inverter device 10 according to an embodiment of the present invention shown in FIG. 1 is a solar cell 12 constituting a DC power source, and an output current of the solar cell 12 is smoothed and input by a DC capacitor 14 and converted into an AC output current. PWM inverter 16 that outputs, output filter circuit 18 that reduces waveform distortion or noise of AC power output from PWM inverter 16, and final output load connected to PWM inverter 16 via output filter circuit 18 ( Power system) 20.
[0010]
The PWM inverter 16 is configured by bridge-connecting four semiconductor switching elements 22, 24, 26 and 28, and the semiconductor switching elements 22, 24, 26 and 28 are turned on by a digital control signal sent from the control device 30. DC power output from the solar cell 12 under the control of / OFF is converted into AC power whose phase matches the system voltage. The output filter circuit 18 includes reactors 32 and 32 and a smoothing capacitor 34.
[0011]
The control device 30 is configured by, for example, a one-chip microcomputer, and converts the detection signals on the input side (DC side) and output side (AC side) of the PWM inverter 16 into digital signals as shown in FIG. / D conversion unit 36, input / output unit 38, memory unit 40 including ROM and RAM for storing or changing necessary data, comparison unit 42 for comparing operation results with data stored in memory unit 40, and the like A CPU 44 and the like for performing various arithmetic processes are provided inside.
[0012]
The output of the DC power supply 12 is detected by a DC voltage detection circuit (pulse transformer or isolation amplifier) 46 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.
[0013]
Detection signals relating to the voltage and current on the DC side and AC side detected by each detection circuit are converted into digital signals by the A / D conversion unit 36 of the one-chip microcomputer 30 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 calculation result with the comparison unit 42, and compares the final processing result with the input / output unit. 38 outputs to the PWM inverter 16 as an ON / OFF pulse signal. The PWM inverter 16 is driven by this ON / OFF pulse signal, and the output current supplied to the power system 20 is controlled in a sine wave form. That is, the feedback signal is taken into the A / D converter 36, the pulse width for PWM control 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.
[0014]
3 and 4 show the output power characteristic (P-V characteristic) and the output current characteristic (IV characteristic) of the solar cell 12. In general, the generated power obtained from the solar cell 12 varies depending on the amount of solar radiation as described above. Therefore, in the control device 30 composed of a one-chip microcomputer, the DC voltage is controlled to a voltage that can obtain the maximum power by feedback control. Controlling the DC voltage is substantially equivalent to controlling the DC current, as is apparent from the IV characteristics shown in FIG.
[0015]
When the maximum power obtained in this way is output to the power system, the parameter to be changed is the amplitude value of the output current. By the way, the system voltage of the power system is usually within 101 ± 6V or 202 ± 20V, and the output power is controlled by increasing / decreasing the output current amplitude value with respect to the voltage.
In addition, a gain control method using an output current value is effective for correcting distortion of the output waveform. In this case, the relationship between the output waveform of the inverter and the deviation as shown in FIG. 10 is that when the gain coefficient K is fixed, the deviation ΔZ between the control target waveform and the actual output waveform is low and high. Because of the difference, the responsiveness of the feedback control system changes. In general, 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.
[0016]
The relationship between the actual output current value Ij (A) and the gain coefficient K (dB) of the PWM inverter 16 according to the present invention is such that the gain coefficient K is the minimum when the actual output current value Ij is the maximum value Imax, as shown in FIG. Since the slope is F at K1, the gain coefficient K at the actual output current value Ij is obtained by the proportional expression shown in Equation 2.
[0017]
[Expression 2]
K = F (Imax-Ij) + K1
Therefore, when the actual output current value Ij is small, the gain coefficient K increases. Conversely, when the actual output current value Ij is large, the gain coefficient K decreases. As a result, the gain coefficient is increased at the time of low output than at the time of high output.
[0018]
Other methods for changing the gain coefficient K include (a) a table value corresponding to the output current, (b) a value divided during low output / high output, and (c) a value determined by fuzzy control. Is possible.
Next, the operation of the grid-connected inverter device 10 by feedback control using the one-chip microcomputer 30 according to one embodiment of the present invention will be described with reference to the flowcharts of FIGS.
[0019]
In FIG. 6, DC voltage DCV (m) and DC current DCI (m) are detected by DC voltage detection circuit 46 and DC current detection circuit 48 in step S1, and digital signals are detected by A / D conversion unit 36 of one-chip microcomputer 30. Convert to In step S3, the CPU 44 calculates the current DC power DCP (m) based on the digital signal input to the input / output unit 38 of the microcomputer 30. In step S5, the previous DC power DCP (m) stored in the memory unit 40 is calculated. -1) is read.
[0020]
In step S7, the current power DCP (m) and the previous power DCP (m-1) are compared by the comparison unit 42. If the result is "YES", the current command amplitude Ik (m) is stored in step S9. 1 is added to the previous output current command amplitude Ik (m−1) stored in the unit 40. If “NO”, 1 is added from the previous output current command amplitude Ik (m−1) in step S11. Is subtracted. In step S13, the current power DCP (m) is changed to the previous power DCP (m-1) and stored in the memory unit 40. In step S15, the current command amplitude Ik (m) is output.
[0021]
In FIG. 7, in step S17, the output of the current command amplitude Ik (n) from step S15 in FIG. 6 is input to the CPU 44. In step S19, the voltage obtained by digitally converting the voltage phase of the power system by the A / D converter 36. The command phase Ip (n) is input to the CPU 44.
In step S21, a current command value Is (n) as a control target is output. Further, the actual output current detected by the alternating current detection circuit 50 in step S23 is digitally converted by the A / D converter 36, and a feedback current is obtained. The value Ia (n) is obtained. In step S25, the output current detected by the alternating current detection circuit 50 is digitally converted by the A / D converter 36 to obtain the actual output current value Ij (n).
[0022]
In step S27, a gain coefficient K = F (Imax−Ij (n)) + K1 set according to the actual output current value Ij (n) is obtained by calculation in the CPU 44, and the control target is determined in step S29. A control deviation (n) = Is (n) −Ia (n) due to a deviation between the current command value Is (n) and the feedback current value Ia (n) is calculated. Finally, in step S31, the previous control deviation (n−1) is subtracted from the current control deviation (n), the difference is multiplied by the gain coefficient K obtained in step S29, and the previous PWM width (n− 1) is added to obtain the current PWM width (n) by calculation. In step S33, the PWM width (n) is output and a pulse signal based on the PWM width (n) is input to the PWM inverter 16.
[0023]
FIG. 8 and FIG. 9 show the relationship between gain and power factor and the relationship between gain and harmonics (distortion) using the output (500 W, 2 kW, 4 kW) as parameters when the grid-connected inverter device 10 according to the present invention is used. It is a graph to show.
As is clear from the graphs of FIGS. 8 and 9, when the output is low (500 W), if the gain is low, the power factor is poor, the harmonics increase, and the distortion increases. 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, in the case of high output (4 kW), if the gain is too high, the power factor is not so bad, but oscillation symptoms are mixed in and harmonics increase and distortion increases. When the gain is 8, the power factor is good, the harmonics are few, and the distortion is improved. Further, at the time of intermediate output (2 kW), it is understood that if the gain is too low, the power factor is bad and the number of harmonics increases, so that a higher gain is desirable.
[0024]
Therefore, it is effective to set the gain according to the output amount. In particular, when the output is low, the output distortion is improved as well as the power factor by setting the gain high. Since the feedback current is controlled so as to approach the command signal, the gain coefficient can also 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 grid-connected inverter device according to an embodiment of the present invention.
2 is a block diagram of a one-chip microcomputer constituting the control device shown in FIG. 1. 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 the solar cell.
FIG. 5 is a graph showing a relationship between a gain coefficient K and an actual output current value Ij in the example.
6 is a flowchart for explaining the operation of the embodiment shown in FIG. 1;
7 is a flowchart constituting part of FIG. 6. FIG.
FIG. 8 is a graph showing the relationship between gain and power factor with the output of the grid-connected inverter device in the example as a parameter.
9 is a graph showing the relationship between gain and high frequency (distortion) using output as a parameter, as in FIG.
FIG. 10 is a graph showing the relationship between the output waveform of the inverter and the deviation.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 10 ... Grid connection inverter apparatus 12 ... Solar cell 16 ... PWM inverter 20 ... Load (electric power system)
22, 24, 26, 28 ... semiconductor switching element 30 ... control device (one-chip microcomputer)
36 ... A / D converter 38 ... Input / output unit 40 ... Memory unit 42 ... Comparison unit 44 ... CPU
46 ... DC voltage detection circuit 48 ... DC current detection circuit 50 ... AC current detection circuit 52 ... AC voltage detection circuit

Claims (4)

In a grid-connected inverter device that converts a DC input current generated from a DC power source into an AC output current by a PWM inverter and supplies it to a power system,
Current detection means for detecting an output current of the inverter;
A gain coefficient calculating means for calculating a gain coefficient of the feedback control system according to the magnitude of the feedback current based on the output current;
Driving means for driving the PWM inverter by a pulse signal output based on the gain coefficient,
The gain coefficient calculation means is a grid-connected inverter device that sets a gain coefficient large when the feedback current is small . In a grid-connected inverter device that converts a DC input current generated from a DC power source into an AC output current by a PWM inverter and supplies it to a power system,
Current detection means for detecting an output current of the inverter;
Gain coefficient calculating means for calculating a gain coefficient according to the magnitude of the feedback current based on the output current,
The gain coefficient calculation means is a grid-connected inverter device that calculates a gain coefficient K based on Equation (1).
[Expression 1]
K = F (Imax-Ij) + K1
However, Imax: Maximum output current value Ij: Actual output current value K1: Gain coefficient corresponding to the maximum output current value F: Constant In a grid-connected inverter device that converts a DC input current generated from a DC power source into an AC output current by a PWM inverter and supplies it to a power system,
Current detection means for detecting an output current of the inverter;
Gain coefficient calculating means for calculating a gain coefficient according to the magnitude of the feedback current based on the output current,
The gain coefficient calculation means is a grid-connected inverter device including a one-chip microcomputer. The grid-connected inverter device according to any one of claims 1 to 3 , wherein the DC power source includes a solar battery.

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