KR20030029570A - current-source inverter by parallel operation control - Google Patents

Abstract

PURPOSE: A current source inverter is provided to significantly reduce losses caused due to switching operation in the inverter part, while achieving improved efficiency by reducing an energy loss in the interface circuit. CONSTITUTION: A current source inverter comprises a circuit constituted by two buck-boost converters(100) connected in parallel with each other, an inverter(200) connected to output terminals of the buck-boost converters, and a low pass filter(300) connected to an output terminals of the inverter. The current source inverter is driven by the buck-boost converters in which a switch of one converter is turned on/off when the switch of the other converter is turned on/off.

Description

Current-source inverter by parallel switching control

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a linked inverter device in a solar power generation system and to a current source inverter by parallel switching for realizing low distortion and high efficiency by constructing a converter in parallel.

Recently, with the depletion of natural resources and the problems of environment and stability for thermal power and nuclear power generation, researches on solar and wind, which are representative environmentally friendly green energy, are being actively conducted. In particular, photovoltaic power generation is attracting considerable attention from the point of view of infinite energy and clean energy, and is widely used not only for vehicles, toys, residential power generation and street lamps, but also for unmanned lighthouses, clock towers, and communication equipment that are far from grid lines.

Such photovoltaic power generation system is generally composed of solar cell module, storage battery, and inverter part, depending on the application field, load type, and location conditions of the system. In particular, solar power is affected by insolation, temperature, and surrounding environment. A storage battery is essential for storing the electrical energy generated by the battery module once.

However, the battery is expensive and bulky, and there is a problem about the environmentally friendly, spillage, or explosion of the battery liquid. As a solution, the solar energy is directly transferred to the power source without a storage device. In such a system, the inverter circuit, which is an interface circuit for connecting the DC output of the solar cell with the AC system, plays an important part.

Among the interface circuits, the current source inverter has no merit even if the output voltage of the inverter is lower than the grid voltage when the grid is connected, and there is an advantage that there is no inrush current against a short circuit during a load short circuit and an inverter accident. When constructing a photovoltaic power generation system using such a current source inverter, a controller using a processor is generally used. In order to reduce the inductance and the ripple of the output current, a high switching frequency must be controlled. There is a problem that the case is limited by the sampling frequency according to the performance of the processor.

As illustrated in FIG. 1A, a structure of a PWM inverter used as an interface circuit for linking a solar cell and a system is shown. The circuit comprises five switches, an inductor and an LC filter at the output stage. In switching operation, only the Q _A switch performs the chopping operation, and the remaining switches determine the output direction only according to the polarity of the system, so that the loss due to the switching is considerably reduced compared to the method in which the entire full-bridge switch performs the chopping operation. You can.

Figure 1 (b) shows the main operation waveform of the current source PWM inverter. The waveforms of the applied signal Q of each switch, the current I _L flowing through the inductor, the voltage V _{OUT of the} output terminal, and the current I _OUT are shown.

2 illustrates an enlarged waveform of the inductor current I _L of FIG. 1B. To briefly explain the operation of each mode, only the case where the output is positive is considered and all circuit elements are assumed to be ideal.

Mode 1: In the (t ₀ -t ₁ ) period, switch Q _A is turned on and diode D _A is reverse biased to turn off. Therefore, the inductor current starts to increase linearly with the slope of Equation 1.

During the period T _on when the switch is turned _on , the inductor current has the value of Equation 2.

Where T _on is t ₁ -t ₀ . Therefore, the energy stored in the inductor during this T _on can be expressed as Equation (3).

Mode 2: In the (t ₁ -t ₂ ) section, when switch Q _A is turned off at t ₁ , the voltage polarity across the inductor is reversed, diode D _A is biased and I _Lpeak current flows to the output side. The stored energy is transferred to the load.

Mode 3: In the (t ₂ -t ₃ ) section, switch Q _A remains turned off and there is no energy transfer from the input to the output.

The inverter circuit according to the related art has a problem in that the efficiency of the entire circuit is reduced by increasing the ripple of the output voltage because the switching frequency at which the current flowing through the input inductor operates discontinuously and the converter current is large.

The present invention has been made to solve the above problems, and an object thereof is to provide a current source inverter by parallel switching for reducing the switching frequency and increasing the efficiency by connecting Buck-Boost converters in parallel.

1-Conventional current source inverter circuit diagram and main operating waveform.

2-Inductor current waveform according to Q _A signal of a Buck-Boost converter in a conventional current source inverter.

3-a circuit diagram of a current source inverter by parallel switching according to the present invention.

4-Main operating waveform of the current source inverter by parallel switching according to the present invention.

5-Comparison of operating waveforms of the conventional inverter and the inverter according to the present invention.

(a) Inductor current of conventional converter

(b) Converter output current of conventional converter

(c) inductor current of the converter according to the invention

(d) the output current of the converter according to the invention

6-(a) switching signal and grid voltage and (b) output voltage and current of converter according to the present invention.

7-(a) the output voltage and current of the converter when the parallel switching is not performed and (b) the parallel switching according to the present invention.

8-Grid voltage and output current according to the present invention.

9-Efficiency comparison according to the output when the parallel switching is not performed and the parallel switching according to the present invention.

<Description of the symbols for the main parts of the drawings>

100: Buck-Boost Converter 200: Inverter

300: low-pass filter

The present invention for achieving the object as described above is connected to each other in parallel is characterized in that driven by a parallel Buck-Boost converter that is switched on / off of the other converter when the switch of one converter is turned off / on The current source inverter by parallel switching is a technical subject matter.

In addition, the parallel Buck-Boost converter is more preferably characterized in that the switching frequency is doubled by setting the phase difference between the converter to 180 degrees.

Accordingly, the size of the current of the converter is greatly reduced by reducing the switching frequency of the inverter, and the output voltage ripple is also greatly reduced, thereby providing an excellent efficiency characteristic.

Hereinafter, the present invention will be described in detail with reference to the accompanying drawings.

1 is a circuit diagram of a conventional current source inverter and main operation waveforms, FIG. 2 is an inductor current waveform according to a Q _A signal of a Buck-Boost converter in a conventional current source inverter, and FIG. 3 is a diagram of a current source inverter using parallel switching according to the present invention. 4 is a main operation waveform of the current source inverter by parallel switching according to the present invention, and FIG. 5 is a comparison of the operating waveforms of the conventional inverter and the inverter according to the present invention. (A) Inductor current of the conventional converter, (b) The converter output current of the conventional converter, (c) the inductor current of the converter according to the present invention, (d) the output current of the converter according to the present invention, Figure 6 shows (a) the switching signal and the grid voltage and (b) according to the present invention. The output voltage and current of the converter, Figure 7 is the output voltage and current of the converter when (a) not parallel switching and (b) parallel switching according to the present invention, Figure 8 is according to the present invention Grid voltage And output current, and Figure 9 is a comparison of the output efficiency in the case where the parallel switching of the present invention and if it is not the parallel switching.

As illustrated in FIG. 3, the current-type PWM inverter circuit is briefly described, and two current-type Buck-Boost converters 100 are configured in parallel, and one inverter 200 and an output terminal of the inverter are disposed at the output terminal of the converter. It consists of a low pass filter (300).

In configuring the current Buck-Boost converter 100, the use of the Buck-Boost converter 100 is required for stable operation even when the power supply voltage is higher than or lower than the input voltage of the converter which is the output voltage of the solar cell module. do.

As illustrated in FIG. 3, the parallel Buck-Boost converter 100 uses a switch Q _A , a diode D _A , an inductor L _A , a Buck-Boost converter and a switch Q _B , and a diode ( D _B ) and a Buck-Boost converter using an inductor (L _B ).

The switches Q ₁ to Q ₄ of the inverter 200 determine only the polarity of the output voltage in synchronization with the power supply voltage. That is, only the Q _A switch performs the chopping operation and the other switches are used to determine the direction of the output only according to the polarity of the system. Thus, the loss due to switching can be considerably reduced compared to the entire full-bridge switch being chopped.

Therefore, the switching frequency of the inverter 200 is fixed at 60 [Hz] equal to the frequency of the power supply voltage. The filter (L _F , C _F ) (300) formed at the output terminal of the inverter 200 should use a small capacity to improve the waveform of the output current, if this capacity is large due to the phase change of the current in the power supply If you look at the unit power factor does not occur.

4 shows an operation waveform of the Buck-Boost converter 100 by the proposed parallel switcher. Here, T _S is a switching cycle of each converter, and FIGS. 4A and 4B show current waveforms flowing through the inductor of each converter, and FIG. 4C shows the output current of the Buck-Boost converter 100. have.

As shown in FIG. 4, the Buck-Boost converter output current I _DC is represented by two waveforms in a switching cycle during the parallel operation of the Buck-Boost converter 100. If the phase difference between two Buck-Boost converters is set to 180 °, the same result as when switching frequency is doubled. In the present invention, the DSP (TMS320F241) is used to generate the PWM signal of the current Buck-Boost converter, and the PWM mode is used as an asymmetric mode. This is not exactly the same as doubling the switching frequency but keeps the same shape.

In addition, the condenser C _{DC at the} output terminal of the Buck-Boost converter 100 prevents the output current of the Buck-Boost converter 100 from being simultaneously used as the current of the inverter 200, thereby switching devices of the inverter 200. It reduces the current capacity of If the capacitance of the capacitor becomes too large, the output voltage V _{DC of} the Buck-Boost converter 100 is smoothed and the inverter 200 does not generate a formal current.

Figure 5 shows the inductor and output current waveforms in the conventional converter and the inductor and output current waveforms in the proposed converter under the same switching frequency. As shown, in two paralleled converter circuits with 180 ° phase difference, the inductor current has the same effect as performing the chopping operation, which almost doubles the frequency, and thus the converter output current is also nearly twice the frequency. Will have

When calculating a value of an appropriate inductor to have a desired output in the set of Buck-Boost converter 100, the switching frequency of the Buck-Boost converter 100 is f _SW and the power frequency is f _S. The number N of switching of the Buck-Boost converter 100 during the half cycle is defined as in Equation 4.

When the switching period is determined by the number of switching, the output current varies depending on the time that the switch is turned on in the switching period. In the present invention, assuming that the Buck-Boost converter 100 operates in a discontinuous mode, the switching time of each converter for supplying energy to the power side at a unit power factor uses a switching function as shown in Equation (5).

However, θ is nπ / N (n = 1,2,3, ..., N) and D is a modulation index, and the range for operating in the discontinuous mode is shown in Equation 6 below.

Energy accumulated in the inductor L in each switching section by the switching function of Equation 5 is expressed as Equation 7 below.

Where n = 1,2,3, ..., N. If the losses are ignored, the energy stored in the inductor is transferred to the output side. If the sin term is taken as the power supply voltage, it is the same as the power at the unit power factor. When the equation (7) is converted into a power equation, it is expressed by the following equation (8).

Therefore, an inductor for generating a desired output in a set of Buck-Boost converter 100 is given by Equation 9 below.

The operation of the whole interface circuit is as follows.

In two Buck-Boost converter 100 circuits connected in parallel, diode D _A is reverse biased and turned off when switch Q _A is on. Thus, the inductor current i _LA starts to increase linearly. When Q _A is turned off after a certain time, the voltage polarity across the inductor L _A is reversed and the inductor L _A is operated as a source, and the energy stored in the inductor L _A is transferred to the inverter 200 through the diode D _A to the load. .

In addition, as soon as the switch Q _A is turned off, the switch Q _B is turned on and the diode D _B is reverse biased to be turned off. Therefore, the inductor current i _LB starts to increase linearly. If Q _B is turned off after a certain time, the voltage polarity across the inductor L _B is reversed and the inductor L _B operates as a source to transfer the energy stored in the inductor L _B through the diode D _B toward the inverter.

By the parallel driving as described above, the waveform of the output current I _DC of the Buck-Boost converter 100 appears in two within the switching period. If the phase difference of each paralleled converter is set to 180 °, then the switching frequency is doubled. DSP (TMS320F241) is used as a controller to generate PWM signal of current type converter. By using PWM mode as asymmetric mode, it turns on another converter as soon as it is switched off. It produces almost the same result as one case.

C _DC coupled to the output of the Buck-Boost converter 100 prevents the converter output current I _{DC from} becoming the input current of the inverter 200, thereby reducing the current capacity of the switching element of the inverter unit.

Since the switches Q ₁ to Q ₄ constituting the inverter 200 determine only the polarity of the output voltage in synchronization with the power supply voltage, the switching frequency of the inverter 200 is fixed to 60 [Hz], which is the frequency of the power supply voltage. In the circuit diagram of FIG. 3, the switches Q ₁ and Q ₂ remain on to generate a positive output, and the switches Q ₃ and Q ₄ remain controlled to generate a negative output. The switch of the inverter 200 determines only the direction of the output according to the polarity of the system without performing the chopping operation.

The output terminal of the inverter 200 includes a low-pass filter 300 composed of L _F and C _F , so that the current flowing through the input inductor operates discontinuously so as to supply a continuous output current to the grid. It is located between the interface circuits.

Experimental results according to the invention are shown in FIGS. 6, 7 and 8.

6 (a) shows one signal Q _A of the Buck-Boost converter 100 driven in parallel with the power supply voltage V _S and the switching control signals Q ₁ and Q ₂ of the inverter 200 for polarity determination. ). Q _{A which} is a switching signal of the Buck-Boost converter 100 among the switch elements performs a chopping operation so as to have a unit power factor, and the switch of the inverter 200 is controlled by a control signal for determining whether the output polarity is positive or negative. On and off operation will be performed.

6 (b) shows the output voltage V _DC of the Buck-Boost converter 100 and the inductor current I _LA of the Buck-Boost converter in parallel operation. The inductor current which was discontinuously operated is converted into a continuous voltage after passing through a filter and used as an applied voltage of the inverter 200.

FIG. 7 shows the output voltage V _DC and the inductor current I _LA of the Buck-Boost converter 100 when the parallel switching is not performed (a) and (b). Compared with the case where the parallel switching is not performed, the current magnitude of the Buck-Boost converter 100 is also greatly reduced, and the output voltage ripple is also greatly reduced. In addition, due to parallel switching, the switching frequency of the Buck-Boost converter 100 is 40k [Hz], but due to the parallel operation, the output waveform is operated at 80k [Hz].

8 shows the system voltage and output current when the output is 150 [W] while controlling the output voltage of the solar module, which is the maximum output voltage, to 52.3 [V]. It can be seen that the output current is supplied sine with a 180 ° phase difference from the grid voltage. Therefore, it can be seen that a high power factor can be obtained by operating the inductor current discontinuously without a separate current controller so that the peak value of the current follows the voltage.

This is because the output characteristics of the solar cell fluctuate due to the continuously changing solar radiation, temperature, load condition, etc., so the maximum output point of the solar cell is changed.

FIG. 9 compares the efficiency according to the output when the inverter 200 is not parallel-switched and in one case. As can be seen from the figure, the case of parallel operation does not appear to be higher than the efficiency.

According to the present invention, the magnitude of the current is instantaneously controlled by a Buck-Boost type parallel current converter, and the frequency of the output waveform operates at twice the converter switching frequency, thereby reducing the inductance of the output. There is no need to increase the size of or control the inverter with high switching frequency.

Therefore, the inverter is used only to determine the positive and negative directions of the AC output stage, which has the effect of significantly reducing the loss due to switching in the inverter portion.

In addition, the parallel current switching reduces the current size of the converter and reduces the ripple of the output voltage, thereby reducing the energy loss in the entire interface circuit, thereby achieving high efficiency.

Claims (2)

Parallel-connected current source inverter, characterized in that driven by a parallel Buck-Boost converter 100 is connected to each other in parallel when the switch of one converter is turned on / off. The method of claim 1, wherein the parallel Buck-Boost converter 100, A parallel source switching current source inverter, characterized in that the switching frequency is doubled by setting the phase difference between the converters at 180 °.