JP2000324847A - Current type inverter for photovoltaic power generation - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a small and high performance current type inverter for photovoltaic power generation in which reactor can be reduced by absorbing ripple of input current to a current smoothing reactor through a smoothing capacitor provided in parallel with a solar cell array thereby stabilizing the voltage. SOLUTION: The current type inverter 10 for photovoltaic power generation comprises a bridge type switching circuit 36 comprising four semiconductor switching elements 28-34 connected, respectively, in series with diodes 20-26 receiving DC power from a solar cell array section 12 through a current smoothing reactor, and a current type inverter section 14 having a PWM control circuit 38 outputting a switching control signal to each semiconductor switching element wherein DC power is converted into AC power through the inverter section and linked with a commercial power system. A smoothing capacitor 40 is connected in parallel with the solar cell array section 12 on the input side of a reactor 18.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a current-source inverter device for photovoltaic power generation, and more particularly to a current-source inverter device for photovoltaic power generation, for example, converting DC power of a solar cell into AC power and interconnecting with a commercial power system. .

[0002]

2. Description of the Related Art In recent years, attention has been paid to clean energy free from environmental pollution, especially to solar power generation using a solar cell, due to increasing awareness of global environmental protection, and practical use thereof has been promoted. In this photovoltaic power generation, the generated power fluctuates greatly in accordance with the amount of solar radiation, so in order to ensure a stable supply of power and effective use of surplus generated power, the solar power consisting of solar cells and inverters installed in buildings and ordinary households The power generation system is used in connection with a commercial power system. That is, power is normally supplied to a load, for example, an inverter air conditioner, by parallel operation of the solar power generation system and the commercial power system. Then, part or all of the electric power required for the house is covered by the solar power generation, and when the power generated by the solar cell is excessive, a reverse power flow is supplied to the commercial power system.

Generally, for converting DC power into single-phase AC power, for example, as shown in FIG. 2, four semiconductor devices, which are switching elements having a self-turn-off function,
For example, a high-speed semiconductor switching element (IGBT) Q1
To Q4 and a single-phase bridge voltage-source inverter main circuit including four feedback diodes D1 to D4 connected in anti-parallel to these switching elements.
The input side capacitor Cin is used in parallel to keep the input voltage to the inverter main circuit constant.

In this voltage-source inverter main circuit, a set of semiconductor switching elements Q1 and Q4 and a set of semiconductor switching elements Q2 and Q3 are divided, and each set is alternately switched (open / closed), so that the semiconductor switching elements Q1 and Q4 are switched. Connection point of Q2 and semiconductor switching element Q
3, a staircase-like voltage is obtained between the connection point of Q4.
Then, by applying appropriate pulse width modulation (PWM) to the switching control signal, the output voltage waveform can be made closer to a sine waveform. In other words, the voltage-source inverter main circuit that converts DC power and outputs single-phase AC power
It comprises a switching circuit and a PWM control circuit for controlling the operation of this circuit.

In principle, the input DC voltage of this voltage source inverter main circuit only needs to be a peak value (about 1.4 times the effective value) of the output AC voltage, but in practice, the semiconductor switching element Q1 The voltage is set to about twice the output AC voltage (effective value) in consideration of the voltage drop of Q4, the voltage drop of the filter circuit, the temperature characteristics of the DC power supply (solar cell), and the like.

Meanwhile, a current-source inverter main circuit system which is superior in characteristics and conversion efficiency as compared with the above-described voltage-type inverter main circuit system has been studied.

[0007]

That is, a conventional solar power generation system 1 comprising a solar cell and a current source inverter, as shown in FIG. 3, has a solar cell array section 2 and diodes 3a, 3b, 3c and Four semiconductor switching elements 4a, 4b, 4c each having 3d connected in series.
An input reactor 7 is provided in series with a current-source inverter main circuit 6 including a bridge-type switching circuit 5 composed of the inverters 4 and 4d. The reactor 7 is a device for making the input current to the current source inverter main circuit 6 constant. Further, in this inverter main circuit, even when the system voltage is higher than the input voltage, the input reactor generates a necessary voltage, so that the system interconnection is possible.

[0008] In the inverter for photovoltaic power generation, the problem of power ripple always occurs. In FIG.
Since both the AC current output and the AC voltage are sine waves, the output power has a form proportional to the square of sin (AC power output in the figure). On the other hand, since the input power of the inverter is DC, the input power is constant (DC power input in the figure). Therefore, during one cycle of the output current,
There are two cycles of “input power <output power” and “input power> output power”.

In FIG. 4, if the DC power input is DC3
Assuming that the conversion efficiency is 100 kW, the peak (maximum) power reaches 6 kW as described above, assuming that the conversion efficiency is 100% in order to output AC power of 3 kW. The power ripple indicates that a device for storing and discharging power is required somewhere in the inverter device. In the case of a current source inverter, the device that stores and releases power is the input reactor. Let the inductance of this input reactor be L
[H] and the flowing current I _L [A], the energy J _L [J stored in the reactor; Joule = W · s
ec] is represented by Equation 1.

[0010]

[Equation 1] J_L= 1/2 (LI_L ^Two) [J] From Equation 1 above, the reactor stores and releases energy.
If you do _LVary accordingly.

In order to reduce the distortion of the output current in the system interconnection, it is desirable that the input current has little fluctuation.
From Equation 1, the smaller the variation of the current I _L flowing through the input reactor while performing accumulation and release of certain energy, it is necessary to sufficiently increase the value of the inductance L of the input reactor.

[0012] Similarly, as another problem peculiar to the photovoltaic power generation system, the ripple of the input current cannot be made too large. FIG. 5 shows an IV characteristic diagram of a general solar cell. The point (voltage) at which the maximum power is obtained is Vmax,
Assuming that the current is Imax and the current at the time of output short circuit is Isc, if the inverter is operating at the maximum power point,
The input ripple current increases / decreases around Imax. When the input current increases, the generated power sharply decreases as the voltage decreases. If when the input current increases to I _1, the corresponding power decreases the power generation amount to P _2. Conversely, when the power decreases to I2, the amount of power generation also decreases.

In FIG. 5 described above, the input current is I _{1 to} I ₂
If changes in width, the power generation amount will vary with the width of P ₁ to P _2. P1 is the maximum generated power. Obviously, this occurs twice in one cycle of the alternating current output. In the photovoltaic power inverter, the input current Iin
= Imax, it is desirable to always operate. However, since it always vibrates around Imax due to power ripple, it always deviates from the maximum power point of the solar cell.

Therefore, it is difficult to design an inverter with a large current ripple of the input current Iin for photovoltaic power generation. If a certain amount of change in J _L is required according to the equation 1, the inductance of the input reactor can be reduced. Value (L value)
Will have to be large to some extent.

As described above, in a current source inverter for photovoltaic power generation, it is difficult to reduce the size of the input reactor to reduce the inductance. Depending on the current,
There is also a problem that necessary power cannot be taken out. That is, there is a problem that a current equal to or higher than the short-circuit current Isc cannot be secured as an input current due to the characteristics of the solar cell, and the output voltage of the solar cell decreases due to an increase in the current value, resulting in a decrease in output power. is there.

SUMMARY OF THE INVENTION Therefore, a main object of the present invention is to provide a current-source inverter for photovoltaic power generation at a low cost, in which the input reactor is reduced in size and easy to handle.

[0017]

SUMMARY OF THE INVENTION The present invention provides a bridge-type switching circuit which receives DC power from a solar cell via a reactor and includes four semiconductor switching elements, and supplies a switching control signal to each semiconductor switching element. A current-source inverter unit having a pulse width modulation control circuit for outputting the current-source inverter unit. The current-source inverter unit converts the DC power of the solar cell into AC power and interconnects with a commercial power system. And a current-source inverter for photovoltaic power generation, wherein a smoothing capacitor is connected in parallel with the solar cell on the input side of the reactor.

[0018]

The smoothing capacitor provided in parallel with the solar cell on the input side of the switching circuit connected via the reactor absorbs the ripple of the input current to the reactor and stabilizes the voltage. As a result, the value of the reactor can be reduced as compared with the conventional current source inverter in which all of the input current ripple depends on the reactor.

[0019]

According to the present invention, it is possible to provide a small-sized and high-performance current-source inverter for photovoltaic power generation at low cost.

The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description of embodiments with reference to the accompanying drawings.

[0021]

DESCRIPTION OF THE PREFERRED EMBODIMENTS A current source inverter device 10 for photovoltaic power generation according to one embodiment of the present invention shown in FIG.
And an output filter circuit 16.

The solar cell array section 12 comprises the same number of reverse current blocking diodes as a plurality of solar cells (not shown). The required number of solar cells and reverse current blocking diodes are connected in series or in a required number according to the output capacity. Connected in parallel.

The current source inverter section 14 converts a DC output generated by the solar cell array section 12 into an AC output, and includes a current smoothing reactor 18 for smoothing high-frequency ripples and diodes 20, 22, 24,. Semiconductor switching elements 2 each of which is connected in series with
And a pulse width modulation control circuit (PWM) for outputting a switching control signal to each of the semiconductor switching elements 28 to 34 of the switching circuit 36.
(Control circuit) 38. The PWM control circuit 38 is, for example, a feedback control system that detects the output current of the switching circuit 36 by a current detector (not shown) and feeds back the current. The PWM control circuit 38 has a plurality of sets in which the pulse width is adjusted so that the output state of the switching circuit 36 becomes appropriate. Is supplied to each of the semiconductor switching elements 28 to 34 constituting the switching circuit 36 as a switching control signal.

Further, on the input side of the current smoothing reactor 18, a smoothing capacitor 40 is connected in parallel with the solar cell array section 12 to smooth voltage fluctuations occurring in the solar cell array section 12. As a result, the solar cell array unit 12
Is stabilized by the smoothing capacitor 40, a current necessary for the current smoothing reactor 18 flows, and a current whose high-frequency ripple is smoothed by the reactor flows to the switching circuit 36. That is, a low-pass filter is formed by the current smoothing reactor 18 and the smoothing capacitor 40, so that flat generated power can be supplied from the solar cell array unit 12.

The capacity of the capacitor 40 must be set so that the low-pass filter formed by the current smoothing reactor 18 and the smoothing capacitor 40 can sufficiently absorb fluctuations with respect to twice the output frequency. The calculation formula used as a guide is shown in Expression 2.

[0026]

(Equation 2)

For example, when the system frequency is 50 Hz, the ripple of 100 Hz occurs twice as described above.
The cutoff frequency fc is 1/10 of 10 as 30 mH.
Assuming that the frequency is Hz, the capacitance is about 8,400 μF from Equation 2.

The output filter circuit 16 includes a capacitor 42 and reactors 44, 44 connected to the output terminal of the switching circuit 36, and has an inverted L-shape in order to obtain an output with less high frequency components. A load and a commercial power system (not shown) are interconnected to the output filter circuit 16.

Next, an outline of the operation of the current-source inverter device 10 for photovoltaic power generation having the above-described configuration will be described.

First, the electromotive force generated in the solar cell array section 12 is stabilized in voltage by a smoothing capacitor 40, smoothed in a high-frequency ripple by a current smoothing reactor 18, and input to a switching circuit 36. In this switching circuit 36, a plurality of sets of Ps whose pulse widths are adjusted by a PWM control circuit 38 so that the output state becomes appropriate.
Each of the semiconductor switching elements 28 to 3 constituting the switching circuit 36 using the WM pulse as a switching control signal
4 As a result, an AC current with less distortion is output from the switching circuit 36, and the output filter circuit 16
Then, the high-frequency component is further removed, and the high-frequency component is connected to a load or a commercial power system.

As described above, according to the present invention, the smoothing capacitor for absorbing the ripple of the input current and stabilizing the voltage is added in parallel to the solar cell array unit while the operation of the inverter main circuit remains the current type. As a result, the value of the reactor can be smaller than that of the conventional current-source inverter device shown in FIG. Incidentally, in the conventional current source inverter device, for example, 10
Although a very large reactor such as 0 mH was required, according to the present invention, it is possible to set the ripple current as large as possible as long as the ripple current at the inverter input portion can be tolerated. It is.

The fact that the reactor can be small in size means that the power loss can be reduced. That is, in order to set the inductance of the reactor to be large, it is necessary to wind a considerable winding, thereby increasing the wire length, increasing the DC resistance, and causing a large loss (copper loss) due to the flow of current. There was a tendency. However, by replacing the power ripple with a smoothing capacitor, the number of reactor windings can be reduced, resulting in a low-loss circuit.

Therefore, according to the present invention, the current source inverter for photovoltaic power generation can be reduced in size, thereby reducing the cost and the installation area of the inverter, which contributes to practical use.

[Brief description of the drawings]

FIG. 1 is a circuit diagram illustrating a schematic configuration of a current source inverter device for photovoltaic power generation according to an embodiment of the present invention.

FIG. 2 is a circuit diagram of a conventional general voltage-source inverter device that converts DC power into single-phase AC power.

FIG. 3 is a circuit diagram of a conventional current source inverter device corresponding to FIG.

FIG. 4 is a waveform diagram showing the relationship between input and output power of an inverter that converts DC to AC.

FIG. 5 is an IV characteristic diagram of a general solar cell, including a PV characteristic diagram.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 10 ... Current-source inverter apparatus for photovoltaic power generation 12 ... Solar cell array part 14 ... Current-source inverter part 16 ... Output filter circuit 18 ... Current smoothing reactor (input reactor) 20-26 ... Diode (diode for short-circuit prevention) 28- 34 semiconductor switching element 36 switching circuit 38 pulse width modulation control circuit (PWM control circuit) 40 smoothing capacitor (input capacitor)

Claims (2)

[Claims] 1. A bridge-type switching circuit that receives DC power from a solar cell via a reactor and includes four semiconductor switching elements, and a pulse width modulation control circuit that outputs a switching control signal to each of the semiconductor switching elements. A current source inverter section having
In the photovoltaic power generation inverter device, which converts the DC power into AC power by the current source inverter unit and interconnects with a commercial power system, a smoothing capacitor is arranged in parallel with the solar cell on the input side of the reactor. A current source inverter device for photovoltaic power generation, wherein the inverter is connected. 2. The current source inverter for photovoltaic power generation according to claim 1, wherein the capacity of said smoothing capacitor is set to a value that reduces the frequency of twice the AC output frequency together with said reactor.