Filter circuit, and power conversion device

Abstract

To provide a filter circuit capable of suppressing output of harmonic current due to switching of a power conversion circuit, and suppressing fundamental wave current that flows as an internal current.SOLUTION: A filter circuit 3 comprises: a first terminal T1 and a second terminal T2; a first element L1 being an inductor with one end directly connected to the first terminal T1 and the other end connected to the second terminal T2; a second element C1 being a capacitor with one end directly connected to the other end of the first element L1; and a third element L2 being an inductor connected in parallel with the second element C1.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a filter circuit and a power conversion device including the filter circuit.

Background Art

[0002] In a power conversion device that converts DC power and AC power, a low-pass LC filter that suppresses the output of harmonic currents due to the switching of a power conversion circuit is arranged on the AC side of the power conversion circuit. Patent Document 1 discloses a grid-connected inverter device including an inverter circuit and an LC filter. In the grid-connected inverter device, the LC filter removes switching noise and harmonic currents of the inverter circuit and supplies them to a commercial power grid.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] A low-pass LC filter includes a reactor and a capacitor. In the capacitor, not only harmonic currents output from the power conversion circuit but also a part of the fundamental wave current flows as an internal current. Therefore, the output fundamental wave current decreases by the amount that flows as the internal current in the capacitor. Since the output of the power conversion device is controlled to be constant, the fundamental wave current output from the power conversion circuit increases by the amount that flows uselessly as the internal current. As a result, an excessive current flows through the switching element of the power conversion circuit, increasing the loss and reducing the efficiency of the power conversion device. In addition, it is necessary to increase the rated current of components such as the switching element.

[0005] The present invention has been conceived under the above circumstances, and an object thereof is to provide a filter circuit capable of suppressing the output of harmonic currents due to switching of a power conversion circuit and suppressing the fundamental wave current flowing as an internal current.

Means for Solving the Problems

[0006] The filter circuit provided by the first aspect of the present invention includes a first terminal and a second terminal, a first element which is an inductor having one end directly connected to the first terminal and the other end connected to the second terminal, a second element which is a capacitor having one end directly connected to the other end of the first element, and a third element which is an inductor connected in parallel with the second element.

[0007] In a preferred embodiment of the present invention, when the frequency of the fundamental wave is f and the capacitance of the second element is C, the inductance Lx of the third element is Lx = 1 / (4×π 2 ×f 2 ×C) That is.

[0008] In a preferred embodiment of the present invention, the frequency f is greater than 50 Hz and less than 60 Hz.

[0009] In a preferred embodiment of the present invention, a transformer having a primary winding which is the third element and a secondary winding magnetically coupled to the primary winding is further provided.

[0010] The power conversion device provided by the second aspect of the present invention includes a power conversion circuit that converts DC power and AC power, and a filter circuit provided by the first aspect of the present invention connected to the AC side of the power conversion circuit.

Effects of the Invention

[0011] According to the present invention, the first element and the second element constitute a low-pass LC filter. Therefore, when the filter circuit according to the present invention is connected to the AC side of the power conversion circuit, it can suppress the output of harmonic currents due to the switching of the power conversion circuit. Further, according to the present invention, a third element is connected in parallel to the second element of the LC filter. The fundamental wave current flowing through the second element leads the voltage by 90°. On the other hand, the fundamental wave current flowing through the third element lags the voltage by 90°. Since the fundamental wave current flowing through the second element is canceled out by the fundamental wave current flowing through the third element, the filter circuit according to the present invention can suppress the fundamental wave current flowing as the internal current as compared with the conventional LC filter without the third element.

Brief Description of the Drawings

[0012]

Figure 1

Figure 2

Embodiments for Carrying Out the Invention

[0013] Hereinafter, preferred embodiments of the present invention will be specifically described with reference to the accompanying drawings.

[0014] FIG. 1 is a block diagram showing the overall configuration of a power conversion device A1 according to the first embodiment. The power conversion device A1 is a so-called power conditioner. The input / output terminals on the DC side of the power conversion device A1 are connected to a storage battery 5, and the input / output terminals on the AC side are connected to a power grid 9. The power conversion device A1 converts AC power supplied from the power grid 9 or a power generation facility (not shown) into DC power to charge the storage battery 5, discharges the storage battery 5 in a predetermined case, converts the DC power input from the storage battery 5 into AC power, and supplies it to the power grid 9 or a load (not shown). The storage battery 5 is a secondary battery that can perform repeated charge and discharge, and for example, is a lithium-ion battery. Note that the storage battery 5 may be other secondary batteries such as a nickel-metal hydride battery, a nickel-cadmium battery, or a lead-acid battery. Note that the storage battery 5 may be a capacitor such as an electric double layer capacitor instead of a secondary battery. The power conversion device A1 includes an inverter circuit 1, a control circuit 2, and a filter circuit 3.

[0015] The inverter circuit 1 is a bidirectional power conversion circuit that converts DC power and AC power. The input / output terminals on the DC side of the inverter circuit 1 are connected to the storage battery 5, and the input / output terminals on the AC side are connected to the filter circuit 3. The inverter circuit 1 charges and discharges the connected storage battery 5 according to a command from the control circuit 2. The inverter circuit 1 charges the connected storage battery 5 by converting the AC power input from the AC side into DC power and outputting it to the DC side. Also, the inverter circuit 1 discharges the connected storage battery 5 by converting the DC power stored in the connected storage battery 5 into AC power and outputting it to the AC side. Note that the specific internal configuration of the inverter circuit 1 is not limited. As an example of the inverter circuit 1, a single-phase full-bridge type inverter including four switching elements can be mentioned.

[0016] The control circuit 2 is realized by, for example, a microcomputer or the like, and controls the inverter circuit 1. The control circuit 2 outputs a drive signal to the inverter circuit 1 to cause the inverter circuit 1 to perform a power conversion operation and charge and discharge the storage battery 5 connected to the inverter circuit 1. The specific control method of the control circuit 2 is not limited. As an example of the control method of the control circuit 2, constant current control by PWM control and the like can be mentioned.

[0017] The filter circuit 3 is connected to the input / output terminals on the AC side of the inverter circuit 1, and is a low-pass filter that suppresses the output of harmonic currents due to the switching of the inverter circuit 1. The filter circuit 3 removes harmonic currents and supplies fundamental wave currents to the power grid 9. In the filter circuit 3, the first terminal T1 is connected to one of the input / output terminals on the AC side of the inverter circuit 1, and the third terminal T3 is connected to the other input / output terminal on the AC side of the inverter circuit 1. Also, in the filter circuit 3, the second terminal T2 is connected to one of the input / output terminals on the AC side of the power conversion device A1, and the fourth terminal T4 is connected to the other input / output terminal on the AC side of the power conversion device A1.

[0018] The filter circuit 3 includes a first reactor L1, a first capacitor C1, and a second reactor L2. The first reactor L1 is an inductor, one end of which is directly connected to the first terminal T1, and the other end of which is directly connected to the second terminal T2. The first capacitor C1 is a capacitor, one end of which is directly connected to the other end of the first reactor L1. The first reactor L1 and the first capacitor C1 constitute a low-pass LC filter.

[0019] The second reactor L2 is an inductor, one end of which is connected to one end of the first capacitor C1, and the other end is connected to the other end of the first capacitor C1. That is, the second reactor L2 is connected in parallel with the first capacitor C1. The inductance Lx of the second reactor L2 is designed such that the impedance ZL of the second reactor L2 is equal to the impedance ZC of the first capacitor C1 at the fundamental wave frequency f. The impedance ZC of the first capacitor C1 is calculated by the following formula (1) when the capacitance of the first capacitor C1 is C. The impedance ZL of the second reactor L2 is calculated by the following formula (2). From the following formula (1) and the following formula (2), by setting ZC = ZL, the inductance Lx of the second reactor L2 is calculated by the following formula (3). The fundamental wave frequency f is set to 50 Hz or 60 Hz in Japan. ZC = 1 / (2×π×f×C) ··· (1) ZL = 2×π×f×Lx ··· (2) Lx = 1 / (4×π 2 ×f 2 ×C)··· (3)

[0020] Next, the operations and effects of the filter circuit 3 and the power conversion device A1 according to this embodiment will be described.

[0021] According to this embodiment, in the filter circuit 3, the first reactor L1 and the first capacitor C1 constitute a low-pass LC filter. Therefore, the filter circuit 3 connected to the AC side of the inverter circuit 1 can suppress the output of harmonic current due to the switching of the inverter circuit 1. Also, according to this embodiment, a second reactor L2 is connected in parallel to the first capacitor C1 that constitutes the LC filter. The fundamental current flowing through the first capacitor C1 leads the voltage by 90°. On the other hand, the fundamental current flowing through the second reactor L2 lags the voltage by 90°. Since the fundamental current flowing through the first capacitor C1 is canceled out by the fundamental current flowing through the second reactor L2, the filter circuit 3 can suppress the fundamental current flowing as the internal current as compared with a conventional LC filter without the second reactor L2. As a result, the current flowing through the switching element of the inverter circuit 1 can be suppressed, so that the power conversion device A1 can suppress the loss in the inverter circuit 1 and improve the efficiency. Also, by suppressing heat generation due to the suppression of loss, the power conversion device A1 can simplify the cooling mechanism of the inverter circuit 1. Further, the power conversion device A1 can use components with a low rated current as components such as the switching element of the inverter circuit 1.

[0022] Also, according to this embodiment, the inductance Lx of the second reactor L2 is designed with the value calculated by the above equation (3). Thereby, since the fundamental current flowing through the second reactor L2 can completely cancel out the fundamental current flowing through the first capacitor C1, it is possible to prevent the fundamental current from flowing as the internal current.

[0023] In addition, in this embodiment, the case where the inductance Lx of the second reactor L2 is designed with the value calculated by the above formula (3) is described such that the fundamental current flowing through the second reactor L2 completely cancels out the fundamental current flowing through the first capacitor C1. However, the present invention is not limited to this. The second reactor L2 only needs to be able to cancel at least a part of the fundamental current flowing through the first capacitor C1. Further, in this embodiment, the case where the fundamental frequency f is set to 50 Hz or 60 Hz and the inductance Lx of the second reactor L2 is designed according to the region where the power conversion device A1 is used is described. However, the present invention is not limited to this. The power conversion device A1 may be designed with an inductance Lx by setting a value greater than 50 Hz and less than 60 Hz, for example, 55 Hz, as the fundamental frequency f so that it can be used in any region in Japan.

[0024] Also, in this embodiment, the case where the power conversion device A1 performs both the conversion from DC power to AC power and the conversion from AC power to DC power is described. However, the present invention is not limited to this. The power conversion device A1 may perform only the conversion from DC power to AC power, or may perform only the conversion from AC power to DC power. For example, when a DC power source such as a solar cell is connected instead of the storage battery 5, the power conversion device A1 only needs to perform the conversion from DC power to AC power. Also, when a DC load is connected instead of the storage battery 5, the power conversion device A1 only needs to perform the conversion from AC power to DC power.

[0025] Also, in this embodiment, the case where the AC side of the power conversion device A1 is connected to the power grid 9 is described. However, the present invention is not limited to this. The AC side of the power conversion device A1 may be connected to an AC load such as an AC motor.

[0026] FIG. 2 is a block diagram showing the overall configuration of the power conversion device A2 according to the second embodiment. In FIG. 2, the same or similar elements as those of the power conversion device A1 according to the first embodiment are denoted by the same reference numerals, and redundant descriptions are omitted. The power conversion device A2 according to the present embodiment includes a transformer, and is different from the power conversion device A1 according to the first embodiment in that the primary winding of the transformer functions as an inductor instead of the second reactor L2.

[0027] The power conversion device A2 according to the second embodiment includes a transformer 31 instead of the second reactor L2. The transformer 31 is arranged to electrically insulate the power grid 9 and the inverter circuit 1. The transformer 31 includes a primary winding 311 and a secondary winding 312. One end of the primary winding 311 is connected to one end of the first capacitor C1, and the other end is connected to the other end of the first capacitor C1. That is, the primary winding 311 is connected in parallel with the first capacitor C1. The primary winding 311 functions as an inductor instead of the second reactor L2. The inductance Lx of the primary winding 311 is calculated by the above formula (3) in the same manner as the second reactor L2 according to the first embodiment. The secondary winding 312 is magnetically coupled to the primary winding 311, one end of which is connected to the second terminal T2 and the other end of which is connected to the fourth terminal T4.

[0028] Also in this embodiment, in the filter circuit 3, the first reactor L1 and the first capacitor C1 constitute a low-pass LC filter. Therefore, the filter circuit 3 connected to the AC side of the inverter circuit 1 can suppress the output of harmonic currents due to the switching of the inverter circuit 1. Further, according to this embodiment, the primary winding 311 of the transformer 31 is connected in parallel to the first capacitor C1 that constitutes the LC filter. Since the fundamental current flowing through the first capacitor C1 is canceled by the fundamental current flowing through the primary winding 311, the filter circuit 3 can suppress the fundamental current flowing as the internal current. Thereby, the current flowing through the switching element of the inverter circuit 1 can be suppressed, so that the power conversion device A2 can suppress the loss in the inverter circuit 1 and improve the efficiency. Further, by suppressing heat generation due to the suppression of loss, the power conversion device A2 can simplify the cooling mechanism of the inverter circuit 1. Also, the power conversion device A2 can use components with a low rated current as components such as the switching element of the inverter circuit 1.

[0029] Further, according to this embodiment, the inductance Lx of the primary winding 311 is designed with the value calculated by the above formula (3). Thereby, since the fundamental current flowing through the primary winding 311 can completely cancel the fundamental current flowing through the first capacitor C1, it is possible to prevent the fundamental current from flowing as the internal current. Furthermore, according to this embodiment, the primary winding 311 of the transformer 31 arranged to electrically insulate the power system 9 and the inverter circuit 1 functions as an inductor instead of the second reactor L2. Therefore, the power conversion device A2 does not need to separately provide the second reactor L2.

[0030] The filter circuit and the power conversion device according to the present invention are not limited to the above-described embodiments. The specific configuration of each part of the filter circuit and the power conversion device according to the present invention can be freely changed in various ways.

Explanation of Reference Numerals

[0031] A1, A2: Power conversion device, 1: Inverter circuit, 3: Filter circuit, T1: First terminal, T2: Second terminal, L1: First reactor, L2: Second reactor, C1: First capacitor, 31: Transformer, 311: Primary winding, 312: Secondary winding, 5: Battery, 9: Power system

Claims

1. a first terminal and a second terminal; a first element which is an inductor having one end directly connected to the first terminal and the other end connected to the second terminal; a second element which is a capacitor having one end directly connected to the other end of the first element; a third element which is an inductor connected in parallel with the second element; and a filter circuit.

2. When the fundamental wave frequency is f and the capacitance of the second element is C, the inductance Lx of the third element is Lx = 1 / (4 × π ２ × f ２ × C) as follows, the filter circuit according to Claim 1.

3. wherein the frequency f is greater than 50 Hz and less than 60 Hz, the filter circuit according to Claim 2.

4. further comprising a transformer having a primary winding which is the third element and a secondary winding magnetically coupled to the primary winding, the filter circuit according to Claim 2.

5. a power conversion circuit for converting DC power and AC power; the filter circuit according to any one of Claims 1 to 4, connected to the AC side of the power conversion circuit; and a power conversion device.

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