Power Conversion Device

Abstract

The purpose of the present invention is to inhibit an increase in the size and cost of a power conversion device including three or more single-phase inverters.　Provided is a power conversion device (1) comprising three or more single-phase inverters (2) that each convert DC power into AC power and a control unit (4) that controls the single-phase inverters, wherein: when the absolute value of an output voltage of each of the single-phase inverters is defined as a voltage absolute value and the minimum value of the voltage absolute value is defined, the voltage absolute value of at least one single-phase inverter is set to a value obtained by multiplying the minimum voltage value by a power of 2 or a power of 3, and the voltage absolute value of at least another one single-phase inverter is set to a value obtained by multiplying the minimum voltage value including a decimal fraction by a real number.

Description

[Technical Field]

[0001] The present application relates to a power conversion device. [Background technology]

[0002] As one type of power conversion device, a gradation control type power conversion device is known that can output a smooth AC waveform to a load without requiring a large-capacity output filter. The gradation control type power conversion device is configured by connecting multiple single-phase inverters in series. In a conventional gradation control type power conversion device, the absolute value of the output voltage of each of the multiple single-phase inverters is set to approximately 2. K A power conversion device with a multiplication factor K (K=0, 1, 2, ...) is disclosed. This power conversion device performs gradation control on the sum of the voltages output by multiple single-phase inverters and outputs the sum to a load (see, for example, Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2004-7941 Summary of the Invention [Problem to be solved by the invention]

[0004] In a conventional power conversion device, when a sine wave of a high voltage close to the rated value is output, a waveform can be formed by combining a large number of output voltage levels of the single-phase inverter, and therefore a smooth AC waveform close to a sine wave can be formed. However, when a sine wave of a low voltage is output in a conventional power conversion device, the number of output voltage levels of the single-phase inverter that must be combined to form the waveform is reduced, which makes it difficult to form a smooth AC waveform.

[0005] The present application has been made to solve the above-mentioned problems, and aims to provide a power conversion device having a plurality of single-phase inverters that can form a smooth AC waveform even when outputting a low-voltage AC waveform. [Means for solving the problem]

[0006] The power conversion device of the present application includes: three or more single-phase inverters each converting DC power into AC power; 3 or more and a control unit for controlling the single-phase inverters, wherein the three or more single-phase inverters are connected in series, and where the absolute values ​​of the output voltages of the respective single-phase inverters are defined as absolute voltage values ​​and the minimum of the absolute voltage values ​​is defined as a minimum voltage value, the absolute voltage value of at least one single-phase inverter is set to a value that is a power of two or a power of three multiple of the minimum voltage value, and the absolute voltage value of at least one other single-phase inverter is set to a value that is a real number multiple of the minimum voltage value, including a decimal point. [Effects of the Invention]

[0007] In the power conversion device of the present application, the absolute voltage value of at least one single-phase inverter among three or more single-phase inverters is set to a value that is a power of two or a power of three times the minimum voltage value, and the absolute voltage value of at least one other single-phase inverter is set to a value that is a real number multiple including a decimal point of the minimum voltage value, so that a smooth AC waveform can be formed even when outputting a low-voltage AC waveform. [Brief explanation of the drawings]

[0008] [Figure 1] 1 is a configuration diagram of a power conversion device according to a first embodiment. [Figure 2] 1 is a configuration diagram of a single-phase inverter of a power conversion device according to a first embodiment. [Figure 3] 1 is a configuration diagram of a power conversion device according to a first embodiment. [Figure 4] 3 is an explanatory diagram showing the total output voltage in the power conversion device according to the first embodiment. FIG. [Figure 5]FIG. 3 is a configuration diagram of a power conversion device of a comparative example according to the first embodiment. [Figure 6] FIG. 4 is an explanatory diagram showing the total output voltage in a power conversion device of a comparative example according to the first embodiment. [Figure 7] 3 is an explanatory diagram showing the total output voltage in the power conversion device according to the first embodiment. FIG. [Figure 8] 1 is a configuration diagram of a power conversion device according to a first embodiment. [Figure 9] 3 is an explanatory diagram showing the total output voltage in the power conversion device according to the first embodiment. FIG. [Figure 10] FIG. 10 is a configuration diagram of a power conversion device according to a second embodiment. [Figure 11] FIG. 10 is an explanatory diagram showing the total output voltage in the power conversion device according to the second embodiment. [Figure 12] FIG. 10 is a configuration diagram of a power conversion device according to a third embodiment. [Figure 13] FIG. 10 is a configuration diagram of a power conversion device according to a third embodiment. [Figure 14] FIG. 10 is an explanatory diagram showing the total output voltage in the power conversion device according to the third embodiment. [Figure 15] 2 is a diagram illustrating a hardware configuration for implementing a control unit of a power conversion device according to first to third embodiments. FIG. DETAILED DESCRIPTION OF THE INVENTION

[0009] Hereinafter, a power conversion device according to an embodiment of the present invention will be described in detail with reference to the drawings. In the drawings, the same reference numerals indicate the same or corresponding parts.

[0010] Embodiment 1 FIG. 1 is a configuration diagram of a power conversion device according to a first embodiment. In the power conversion device 1 of this embodiment, three or more single-phase inverters 2 are connected in series. In the power conversion device 1 shown in FIG. 1, INV1, INV2, INV3, ..., INV n-1 , INV nn single-phase inverters 2 are connected in series, where n is a natural number. Each single-phase inverter 2 is connected to a DC power supply 3. For example, INV n The output voltage of the DC power supply 3 connected to the single-phase inverter 2 is Vd n Each single-phase inverter 2 converts DC power supplied from a DC power source 3 into AC power that is gradation-controlled. A control unit 4 is connected to each single-phase inverter 2. The control unit 4 controls each single-phase inverter 2 and outputs the sum of the output voltages of the single-phase inverters 2 as an overall output voltage to a load 10. The power conversion device 1 of this embodiment has an inverter group 5 configured by connecting two or more single-phase inverters in series. In the power conversion device 1 shown in FIG. 1, INV n-1 Single-phase inverter and INV n These single-phase inverters constitute an inverter group 5. Details of the inverter group 5 will be described later.

[0011] FIG. 2 is a configuration diagram of a single-phase inverter of a power conversion device according to this embodiment. Each single-phase inverter 2 has a full-bridge circuit composed of four switching elements 11, 12, 21, and 22 and a capacitor 23. Diodes are connected in anti-parallel to each of the four switching elements 11, 12, 21, and 22. The switching elements 11, 12, 21, and 22 are, for example, insulated gate bipolar transistors (IGBTs) or metal-oxide-semiconductor field-effect transistors (MOSFETs). When MOSFETs are used for the four switching elements 11, 12, 21, and 22, the body diodes of the MOSFETs may be substituted for the diodes connected in anti-parallel. Input terminals 6a and 6b are connected to both ends of the capacitor 23, respectively. A DC power supply is connected between the input terminals 6a and 6b. An output terminal 7a is connected between the switching elements 11 and 12 that constitute one bridge circuit, and an output terminal 7b is connected between the switching elements 21 and 22 that constitute the other bridge circuit. Another single-phase inverter 2 or a load 10 is connected between the output terminals 7a and 7b.

[0012] 2 has four switching elements, but is not limited to this. For example, each switching element may be configured with multiple switching elements connected in series to achieve a higher voltage resistance, or multiple switching elements may be configured with multiple switching elements connected in parallel to achieve a higher current.

[0013] Each single-phase inverter 2 can output a positive voltage, a negative voltage, or a zero voltage between output terminals 7a and 7b. The direction of the voltage between output terminals 7a and 7b is defined as a positive voltage when the potential of output terminal 7b is higher than the potential of output terminal 7a. For example, assume that the positive terminal of a DC power supply is connected to input terminal 6b and the negative terminal is connected to input terminal 6a. The single-phase inverter 2 outputs a positive voltage between output terminals 7a and 7b by turning on switching elements 11 and 22 and turning off switching elements 12 and 21. The single-phase inverter 2 also outputs a negative voltage between output terminals 7a and 7b by turning off switching elements 11 and 22 and turning on switching elements 12 and 21. The single-phase inverter 2 also outputs a zero voltage between output terminals 7a and 7b by turning on switching elements 11 and 21 or by turning on switching elements 12 and 22.

[0014] Note that the output voltage of the single-phase inverter is assumed to be the same as the voltage of the DC power supply by assuming that the resistance components of the switching elements, wiring, etc. present between the input terminals 6a, 6b and the output terminals 7a, 7b of the single-phase inverter 2 are negligible. Therefore, from here on, the output voltage of the single-phase inverter will be described as the voltage of the DC power supply. Furthermore, when describing the ratio of the output voltage of the single-phase inverter 2, it is assumed that it is the ratio of the absolute value of the output voltage of the single-phase inverter 2.

[0015] In the power conversion device of this embodiment, consider the case where the inverter group 5 is regarded as one single-phase inverter, that is, the total voltage of the DC power supplies connected to the inverter group 5 is regarded as the voltage of the DC power supplies connected to one single-phase inverter. In this case, when the ratio of the minimum voltage value is set to 1, the voltages of the DC power supplies connected to each single-phase inverter are set at a ratio that is a power of 2 or a power of 3 times the minimum voltage value. Note that in this embodiment, a power conversion device in which the voltages of the DC power supplies connected to each single-phase inverter are set at a ratio that is a power of 3 times will be described.

[0016] In the power conversion device shown in FIG. 1, the following relationship holds between the voltages of the DC power supplies connected to the respective single-phase inverters. Vd1:Vd2:Vd3::Vd n-1 +Vd n =3 0 :3 1 :3 2 :···:3 n-2 where Vd n-1 +Vd n is the INV n-1 , INV n is the sum of the voltages of the DC power supplies connected to each of the single-phase inverters. n-1 and Vd n At least one of these is set to a ratio including a digit smaller than the ratio 1 of the minimum voltage value, such as 4.5. Hereinafter, for convenience, a ratio including a digit smaller than the ratio 1 of the minimum voltage value will be referred to as a real number including a decimal multiple of the minimum voltage value. Here, a real number including a decimal is a real number having a value other than 0 below the decimal point, such as 1.5, 2.6, or 3.65.

[0017] Hereinafter, a power conversion device configured with four single-phase inverters will be described as an example. FIG. 3 is a configuration diagram of a power conversion device including four single-phase inverters according to this embodiment. The power conversion device 1 shown in FIG. 3 includes four single-phase inverters 2, INV1, INV2, INV3, and INV4, and four DC power supplies 3 connected to the single-phase inverters 2, respectively. The voltages of the four DC power supplies 3 are Vd1, Vd2, Vd3, and Vd4, respectively. These voltages are set as follows: Vd1=V0, Vd2=3V0, Vd3=4.5V0, and Vd4=4.5V0. Here, V0 is the minimum voltage value of the DC power supplies connected to the single-phase inverters. That is, Vd1:Vd2:Vd3:Vd4=1:3:4.5:4.5. Therefore, in the power conversion device 1 shown in FIG. 3, the single-phase inverter INV3 and the single-phase inverter INV4 form an inverter group 5. In this power conversion device 1, Vd1:Vd2:Vd3+Vd4=3 0 :3 1 :3 2 (=1:3:9) holds. Vd3 and Vd4 are set to real multiples of the minimum voltage value, including decimal points.

[0018] FIG. 4 is an explanatory diagram showing the total output voltage of the power conversion device of this embodiment. FIG. 4 shows the combination of voltages output by each single-phase inverter relative to the total output voltage. In FIG. 4, the voltage ratio indicates the voltage ratio of the four DC power sources, and the total output voltage indicates the ratio of the total output voltage to the minimum voltage ratio of 1. Also, in FIG. 4, a "+" indicates that the corresponding single-phase inverter outputs a positive voltage, a "-" indicates that the corresponding single-phase inverter outputs a negative voltage, and a blank space indicates that the corresponding single-phase inverter outputs zero voltage. For example, when the total output voltage is 2.5, the single-phase inverter INV1 with a voltage ratio of 1 outputs a positive voltage, the single-phase inverter INV2 with a voltage ratio of 3 outputs a negative voltage, the single-phase inverter INV3 with a voltage ratio of 4.5 outputs a positive voltage, and the single-phase inverter INV4 with a voltage ratio of 4.5 outputs zero voltage (1-3+4.5+0=2.5).

[0019] FIG. 5 is a configuration diagram of a power conversion device of a comparative example relative to this embodiment. The power conversion device of this embodiment shown in FIG. 3 is configured with three single-phase inverters when the inverter group is considered as one single-phase inverter. Therefore, the power conversion device 1 of the comparative example shown in FIG. 5 has three single-phase inverters 2, INV1, INV2, and INV3, and three DC power supplies 3 connected to the single-phase inverters 2, respectively. If the voltages of the three DC power supplies 3 are Vd1, Vd2, and Vd3, respectively, they are set to Vd1=V0, Vd2=3V0, and Vd3=9V0. That is, in the power conversion device 1 of the comparative example, Vd1:Vd2:Vd3=3 0 :3 1 :3 2 (=1:3:9) is established. Fig. 6 is an explanatory diagram showing the total output voltage in a power conversion device of a comparative example according to this embodiment.

[0020] The total output voltage of the power conversion device of this embodiment shown in FIG. 4 is compared with the total output voltage of the power conversion device of the comparative example shown in FIG. 6. The maximum value of the total output voltage is 13, which is the same for both the power conversion device of this embodiment and the power conversion device of the comparative example. However, when comparing the gradation levels, the power conversion device of this embodiment can output the total output voltage at 22 gradation levels between 0 and 13, while the power conversion device of the comparative example can only output the total output voltage at 13 gradation levels between 0 and 13. In other words, the power conversion device of the comparative example can only output the total output voltage at gradation levels that are integer multiples of the minimum voltage value. In contrast, the power conversion device of this embodiment can output the total output voltage at 0.5 times the minimum voltage value if the total output voltage is in the range of 0 to 9.

[0021] For example, let us compare the power conversion device of this embodiment and the power conversion device of the comparative example when they output a sine wave with a peak value of 3V0. In the power conversion device of this embodiment, the sine wave can be generated with a total output voltage of 0, ±0.5V0, ±1V0, ±1.5V0, ±2V0, ±2.5V0, and ±3V0, for a total of 13 gradation levels. In contrast, in the power conversion device of the comparative example, the sine wave can only be generated with a total output voltage of 0, ±1V0, ±2V0, and ±3V0, for a total of 7 gradation levels.

[0022] The power conversion device of this embodiment has three or more single-phase inverters that each convert DC power to AC power, and a control unit that sends gradation control signals to the single-phase inverters, and when the minimum value of the output voltage of each single-phase inverter is taken as the minimum voltage value, the output voltage of at least one single-phase inverter is set to a value that is a power of three times the minimum voltage value, and the output voltage of at least one other single-phase inverter is set to a value that is a real number multiple of the minimum voltage value, including a decimal point. Therefore, the power conversion device of this embodiment can generate a smooth AC waveform even when outputting a low-voltage AC waveform.

[0023] Furthermore, since the power conversion device of this embodiment has an inverter group configured by connecting two or more single-phase inverters in series, it is possible to reduce the DC voltage applied to each single-phase inverter that makes up the inverter group. Therefore, electronic components with low withstand voltages can be used as the switching elements and capacitors that make up the single-phase inverters. Generally, the lower the withstand voltage of an electronic component, the lower the power loss and the faster the response speed. By using electronic components with low withstand voltages, it is possible to achieve a power conversion device that is smaller, has lower loss, and is faster.

[0024] In the power conversion device of this embodiment, the inverter group is provided at the position of the single-phase inverter with the highest output voltage, but the position of the inverter group is not limited to this position. For example, in the power conversion device of this embodiment shown in FIG. 3, the inverter group is provided at the position of the single-phase inverter with a voltage ratio of 9 in the power conversion device of the comparative example shown in FIG. 5. However, the inverter group may be provided at the position of the single-phase inverter with a voltage ratio of 3 in the power conversion device of the comparative example. However, from the viewpoint of reducing the DC voltage applied to each single-phase inverter constituting the inverter group, it is preferable to provide the inverter group at the position of the single-phase inverter with the highest output voltage. Furthermore, although the power conversion device of this embodiment includes one inverter group, it may also include multiple inverter groups. For example, in the power conversion device of this embodiment shown in FIG. 3, the single-phase inverter with a voltage ratio of 3 may be replaced with an inverter group.

[0025] As shown in FIG. 4 , when the overall output voltage of the power conversion device of this embodiment is switched from 1 to 1.5 and from 1.5 to 2, the output of the single-phase inverter INV3 included in the inverter group 5 switches from zero output to a positive voltage output and from a positive voltage output to zero voltage output. Thus, in the power conversion device of this embodiment, the number of switching operations of the single-phase inverters included in the inverter group 5 increases, which may result in increased switching loss. When switching loss becomes a problem, for example, during heavy load operation with a large load current or load power, it is possible to reduce switching loss by not using the single-phase inverters included in the inverter group and instead using only the single-phase inverters set to an output voltage that is a power of three times the minimum voltage. In this way, it is possible to switch the operating mode depending on the load condition, taking into account the priority of output waveform smoothness and low switching loss.

[0026] In the power conversion device of this embodiment, the output voltages of the multiple single-phase inverters included in the inverter group are the same. In this case, the single-phase inverters to be switched may be interchanged within the inverter group 5. For example, in the power conversion device of this embodiment shown in FIG. 4, when a sine wave with a peak value of 13V is output, the number of switching operations of the single-phase inverter INV3 outputting 4.5V is greater than the number of switching operations of the single-phase inverter INV4 outputting the same 4.5V. In this case, the switching of the single-phase inverter INV3 when the overall output voltage is between 0V and 5V may be replaced by the switching of the single-phase inverter INV4 in the next cycle of the sine wave. Alternatively, the single-phase inverters INV3 and INV4 may be alternately switched during the same cycle of the sine wave. This operation prevents switching losses from concentrating in a single single-phase inverter.

[0027] In the power conversion device of this embodiment, the output voltages of the multiple single-phase inverters included in the inverter group may be different. For example, in the power conversion device of this embodiment shown in FIG. 3, the Vd1:Vd2:Vd3:Vd4 ratios, which were set to 1:3:4.5:4.5, may instead be set to 1:3:4.33:4.66. FIG. 7 is an explanatory diagram showing the total output voltage in the power conversion device of this embodiment configured in this manner. As shown in FIG. 7, by setting the output voltages of the multiple single-phase inverters included in the inverter group 5 to different values, the power conversion device of this embodiment can output the total output voltage at 32 gradation levels ranging from 0 to 13. In this way, a power conversion device in which the output voltages of the multiple single-phase inverters included in the inverter group are set to different values ​​can increase the number of gradation levels compared to a power conversion device in which the output voltages of the multiple single-phase inverters included in the inverter group are set to the same value. It is desirable that the output voltages be output at equal intervals. For example, by setting the decimal point of the real multiple of the minimum voltage value (including the decimal point) to 1 / 2 as shown in Figure 3, it is possible to output voltages in 1 / 2 increments, or by setting it to 1 / 3 or 2 / 3 as shown in Figure 7, it is possible to output voltages in 1 / 3 increments.

[0028] The power conversion device according to the present embodiment described so far has an inverter group formed of two single-phase inverters. The number of single-phase inverters forming the inverter group may be three or more. FIG. 8 is a configuration diagram of the power conversion device according to the present embodiment. The power conversion device 1 according to the present embodiment shown in FIG. 8 has five single-phase inverters 2, INV1, INV2, INV3, INV4, and INV5, and five DC power supplies 3 connected to the single-phase inverters 2, respectively. If the voltages of the five DC power supplies 3 are Vd1, Vd2, Vd3, Vd4, and Vd5, respectively, the voltages are set as follows: Vd1=V0, Vd2=3V0, Vd3=2.5V0, Vd4=3V0, and Vd5=3.5V0. That is, in the power conversion device 1 shown in FIG. 8, the inverter group 5 is formed of three single-phase inverters, INV3, INV4, and INV5. In this power conversion device 1, Vd1:Vd2:Vd3+Vd4+Vd5=30 :3 1 :3 2 (=1:3:9) holds. Vd3 and Vd5 are set to real multiples of the minimum voltage value, including decimal points.

[0029] Fig. 9 is an explanatory diagram showing the total output voltage in the power conversion device of this embodiment configured as described above. As shown in Fig. 9, by increasing the number of single-phase inverters included in the inverter group 5, the power conversion device of this embodiment can output the total output voltage at 24 gradation levels ranging from 0 to 13. Note that, as shown in Fig. 4, the gradation levels in a power conversion device in which the number of single-phase inverters included in the inverter group 5 is two are 22. In this way, a power conversion device in which the number of single-phase inverters included in the inverter group is set to three can increase the number of gradation levels compared to a power conversion device in which the number of single-phase inverters included in the inverter group is set to two.

[0030] In the power conversion device of this embodiment shown in Fig. 8, the voltages of the DC power supplies connected to the three single-phase inverters included in the inverter group are set to 2.5V0, 3V0, and 3.5V0. In this way, in the power conversion device shown in Fig. 8, one of the three DC power supplies included in the inverter group is set to a voltage that is an integer multiple of the minimum voltage value. In the power conversion device of this embodiment, it is not necessary for the voltages of all DC power supplies connected to the single-phase inverters included in the inverter group to be set to values ​​that are real multiples of the minimum voltage value, including decimals.

[0031] As described above, in a power conversion device having three or more single-phase inverters each converting DC power to AC power and a control unit sending gradation control signals to the single-phase inverters, an inverter group is formed from the plurality of single-phase inverters, and the ratio of the sum of the output voltages of the single-phase inverters included in the inverter group to the output voltages of the single-phase inverters not included in the inverter group is set to a power of 3. The output voltage of at least one single-phase inverter included in the inverter group is set to a real number multiple, including a decimal point, of the minimum voltage value of the output voltage of the single-phase inverters. A power conversion device configured in this manner can generate a smooth AC waveform even when outputting a low-voltage AC waveform.

[0032] Furthermore, the power conversion device configured in this manner can reduce the DC voltage applied to the single-phase inverters included in the inverter group, allowing the use of electronic components with low withstand voltage as the switching elements and capacitors that make up the single-phase inverters, thereby achieving a smaller power conversion device, lower loss, and higher speed.

[0033] Embodiment 2 In the first embodiment, a power conversion device has been described in which the voltage of the DC power supply connected to each single-phase inverter is set to a value that is a power of 3. In the second embodiment, a power conversion device will be described in which the voltage of the DC power supply connected to each single-phase inverter is set to a value that is a power of 2.

[0034] The configuration of the power conversion device of this embodiment is similar to the configuration of the power conversion device of embodiment 1 shown in Fig. 1. However, in the power conversion device of this embodiment, the following relationship holds between the voltages of the DC power supplies connected to the respective single-phase inverters in the power conversion device shown in Fig. 1. Vd1:Vd2:Vd3::Vd n-1 +Vd n =2 0 :2 1 :2 2 :···:2 n-2 where Vd n-1 +Vd n is the INV n-1 , INV n is the sum of the voltages of the DC power supplies connected to each of the single-phase inverters. n-1 and Vd n At least one of the voltages is set to a real number multiple including a decimal point of the minimum voltage value.

[0035] Hereinafter, a power conversion device configured with four single-phase inverters will be described as an example. FIG. 10 is a configuration diagram of a power conversion device according to this embodiment, which is configured with four single-phase inverters. The power conversion device 1 shown in FIG. 10 includes four single-phase inverters 2, INV1, INV2, INV3, and INV4, and four DC power supplies 3 connected to the single-phase inverters 2, respectively. The voltages of the four DC power supplies 3 are Vd1, Vd2, Vd3, and Vd4, respectively. These voltages are set as follows: Vd1=V0, Vd2=2V0, Vd3=1.5V0, and Vd4=2.5V0. Here, V0 is the minimum voltage value of the DC power supplies connected to the single-phase inverters. That is, Vd1:Vd2:Vd3:Vd4=1:2:1.5:2.5. Therefore, in the power conversion device 1 shown in FIG. 10, INV3 and INV4 form an inverter group 5. In this power conversion device 1, Vd1:Vd2:Vd3+Vd4=2.5. 0 :2 1 :2 2 (=1:2:4) is established. Vd3 and Vd4 are set to real multiples of the minimum voltage value, including decimal points.

[0036] FIG. 11 is an explanatory diagram showing the total output voltage in the power conversion device of this embodiment. FIG. 11 shows combinations of voltages output by each single-phase inverter with respect to the total output voltage. As shown in FIG. 11, in the power conversion device of this embodiment, it is possible to adjust the gradation level at a voltage value that is half the minimum voltage value up to a total output voltage of 6. From this, even in a power conversion device in which the voltage of the DC power supply connected to each single-phase inverter is set to a value that is a power of 2, by providing an inverter group, it is possible to form a smooth AC waveform even when outputting an AC waveform of a low voltage.

[0037] As described above, in a power conversion device having three or more single-phase inverters each converting DC power to AC power and a control unit sending gradation control signals to the single-phase inverters, an inverter group is formed from the plurality of single-phase inverters, and the ratio of the sum of the output voltages of the single-phase inverters included in the inverter group to the output voltages of the single-phase inverters not included in the inverter group is set to a power of 2. The output voltage of at least one single-phase inverter included in the inverter group is set to a real number multiple, including a decimal point, of the minimum voltage value of the output voltage of the single-phase inverters. A power conversion device configured in this manner can generate a smooth AC waveform even when outputting a low-voltage AC waveform.

[0038] Furthermore, the power conversion device configured in this manner can reduce the DC voltage applied to the single-phase inverters included in the inverter group, allowing the use of electronic components with low withstand voltage as the switching elements and capacitors that make up the single-phase inverters, thereby achieving a smaller power conversion device, lower loss, and higher speed.

[0039] Embodiment 3 FIG. 12 is a configuration diagram of a power conversion device according to a third embodiment. In the power conversion device 1 of this embodiment, k+m+1 single-phase inverters 2 are connected in series. Here, k is a natural number, and m is an integer equal to or greater than 0. However, k+m is equal to or greater than 2. Each single-phase inverter 2 is connected to a DC power supply 3. For example, INVn The output voltage of the DC power supply 3 connected to the single-phase inverter 2 is Vd n Each single-phase inverter 2 converts DC power supplied from a DC power source 3 into gradation-controlled AC power. A control unit 4 is connected to each single-phase inverter 2. The control unit 4 controls each single-phase inverter 2 and outputs the sum of the output voltages of each single-phase inverter 2 to a load 10 as an overall output voltage.

[0040] In the power conversion device of this embodiment, as in the first embodiment, the output voltage of the single-phase inverter is the same as the voltage of the DC power supply. Therefore, hereinafter, the output voltage of the single-phase inverter will be described as the voltage of the DC power supply. In the power conversion device of this embodiment, the minimum value of the output voltage of the single-phase inverter is set to V0. Then, Vd1=V0.

[0041] INV1 to INV k The following relationship holds for the voltage of the DC power supply connected to the single-phase inverter up to Vd1:Vd2:Vd3::Vd k =3 0 :3 1 :3 2 :···:3 K-1

[0042] And INV k+1 The output voltage Vd of the DC power supply 3 connected to the single-phase inverter 2 k+1 is set to the following range: Vd1+Vd2+Vd3++Vd k <Vd k+1 <Vd1+Vd2+Vd3+···+Vd k +Vd1

[0043] Furthermore, INV k+1 From INV k+m+1 The following relationship holds for the voltage of the DC power supply connected to the single-phase inverter up to Vd k+1 :Vd k+2 :Vdk+3 :···:Vd k+m+1 =3 0 :3 1 :3 2 :···:3 m Note that m is an integer equal to or greater than 0, and m may be set to 0. In that case, the above INV k+1 The single-phase inverter shown in FIG.

[0044] Hereinafter, a power conversion device configured with four single-phase inverters will be described as an example. FIG. 13 is a configuration diagram of a power conversion device configured with four single-phase inverters according to this embodiment. The power conversion device 1 shown in FIG. 13 has a configuration in which k=2 and m=1 in the power conversion device shown in FIG. 12. The power conversion device 1 shown in FIG. 13 has four single-phase inverters 2, INV1, INV2, INV3, and INV4, and four DC power supplies 3 connected to the single-phase inverters 2, respectively. If the voltages of the four DC power supplies 3 are Vd1, Vd2, Vd3, and Vd4, respectively, they are set as follows: Vd1=V0, Vd2=3V0, Vd3=4.5V0, and Vd4=13.5V0. Here, V0 is the minimum voltage value of the DC power supplies connected to the single-phase inverters. That is, Vd1:Vd2=3 0 :3 1 (=1:3). Vd3 is 4.5V, which is greater than Vd1+Vd2=4V and less than Vd1+Vd2+Vd1=5V. In other words, Vd3 is set to a real number multiple including a decimal point of the minimum voltage value. Furthermore, Vd3:Vd4=3 0 :3 1 (=1:3).

[0045] Fig. 14 is an explanatory diagram showing the total output voltage in the power conversion device of this embodiment. Fig. 14 shows the combination of voltages output by each single-phase inverter with respect to the total output voltage. As shown in Fig. 14, in the power conversion device of this embodiment, it is understood that the gradation level can be adjusted at a voltage value that is half the minimum voltage value up to a total output voltage of 18. This means that the power conversion device of this embodiment can form a smooth AC waveform even when outputting a low-voltage AC waveform.

[0046] In the power conversion device of this embodiment, Vd3 is set to a value 4.5 times the minimum voltage value. Therefore, the gradation levels of the overall output voltage are spaced at intervals of 0.5 times the minimum voltage value. The decimal point of the real multiplication value, including a decimal point, is not limited to 0.5. For example, Vd3 may be set to a value 4.3 or 4.7 times the minimum voltage value. However, as shown in FIG. 14, since the intervals of the gradation levels of the overall output voltage are constant, it is preferable to set the decimal point of the real multiplication value, including a decimal point, to 0.5.

[0047] As shown in FIG. 14 , when the overall output voltage of the power conversion device of this embodiment is switched from 4.5 to 5 and from 5 to 5.5, the output of the single-phase inverter INV4, which outputs the maximum voltage, switches from zero to a positive voltage output and then from a positive voltage output to zero. Thus, in the power conversion device of this embodiment, the number of switching operations of the single-phase inverter that outputs the maximum voltage increases, which may increase switching loss. When switching loss is a problem, for example, during heavy load operation where the load current or load power is large, it is possible to reduce switching loss by not using the single-phase inverter that outputs the maximum voltage, but instead using only a single-phase inverter set to an output voltage that is a power of three times the minimum voltage. In this way, it is possible to switch the operating mode depending on the load condition, taking into account the priority of output waveform smoothness and low switching loss.

[0048] The power conversion device of this embodiment described so far has a configuration in which the ratio of the output voltages of the single-phase inverters is a power of three. In the power conversion device of this embodiment, the ratio of the output voltages of the single-phase inverters may be a power of two. In this case, the following relationship holds in the power conversion device of this embodiment shown in FIG. 12. Note that in this case as well, the minimum value of the output voltage of the single-phase inverter is set to V0. And, Vd1=V0.

[0049] INV1 to INV k The following relationship holds for the voltage of the DC power supply connected to the single-phase inverter up to Vd1:Vd2:Vd3::Vd k =2 0 :2 1 :2 2 :···:2 k―1

[0050] And INV k+1 The output voltage Vd of the DC power supply 3 connected to the single-phase inverter 2 k+1 is set to the following range: Vd1+Vd2+Vd3++Vd k <Vd k+1 <Vd1+Vd2+Vd3+···+Vd k +Vd1

[0051] Furthermore, INV k+1 From INV k+m+1 The following relationship holds for the voltage of the DC power supply connected to the single-phase inverter up to Vd k+1 :Vd k+2 :Vd k+3 :···:Vd k+m+1 =2 0 :2 1 :2 2 :···:2 m

[0052] The power conversion device configured in this manner can also generate a smooth AC waveform even when outputting a low voltage AC waveform.

[0053] The control unit 4 is configured with a processor 100 and a storage device 101, as shown in FIG. 15, which is an example of hardware. The storage device includes a volatile storage device such as a random access memory and a non-volatile auxiliary storage device such as a flash memory, although not shown. Alternatively, a hard disk auxiliary storage device may be used instead of the flash memory. The processor 100 executes a program input from the storage device 101. In this case, the program is input to the processor 100 from the auxiliary storage device via the volatile storage device. The processor 100 may output data such as calculation results to the volatile storage device of the storage device 101, or may store the data in the auxiliary storage device via the volatile storage device. The control unit 4 may also be a digital controller such as an FPGA (Field Programmable Gate Array) or an MCU (Micro Controller Unit), or may be configured as a mixture of an analog circuit and a digital controller.

[0054] Although the present application describes various exemplary embodiments, the various features, aspects, and functions described in one or more embodiments are not limited to application to a particular embodiment, but may be applied to the embodiments alone or in various combinations. Therefore, countless variations not illustrated are conceivable within the scope of the technology disclosed in this application, including, for example, cases where at least one component is modified, added, or omitted, and cases where at least one component is extracted and combined with a component of another embodiment. [Explanation of symbols]

[0055] 1 power conversion device, 2 single-phase inverter, 3 DC power supply, 4 control unit, 5 inverter group, 6a, 6b input terminals, 7a, 7b output terminals, 10 load, 11, 12, 21, 22 switching elements, 23 capacitor, 100 processor, 101 storage device

Claims

1. A power conversion device having three or more single-phase inverters that respectively convert DC power into AC power, and a control unit that controls the three or more single-phase inverters, a power conversion device in which three or more of the single-phase inverters are connected in series, and when the absolute value of the output voltage of each of the single-phase inverters is defined as a voltage absolute value and the minimum of the voltage absolute values ​​is defined as a minimum voltage value, the voltage absolute value of at least one of the single-phase inverters is set to a value that is a power of two or a power of three multiple of the minimum voltage value, and the voltage absolute value of at least one other of the single-phase inverters is set to a value that is a real number multiple including a decimal point of the minimum voltage value.

2. 2. The power conversion device according to claim 1, further comprising one or more inverter groups each formed by connecting two or more of the single-phase inverters in series, wherein the voltage absolute values ​​of the single-phase inverters not included in the inverter group are set to a value that is a power of two or a power of three multiple of the minimum voltage value, and the voltage absolute value of at least one of the single-phase inverters included in the inverter group is set to a value that is a real number multiple including a decimal point of the minimum voltage value.

3. 3. The power conversion device according to claim 2, wherein the voltage absolute value of at least one of the single-phase inverters included in the inverter group is a maximum value of the voltage absolute values ​​of the respective single-phase inverters.

4. 4. The power conversion device according to claim 2, wherein a sum of the voltage absolute values ​​of the single-phase inverters constituting the inverter group is greater than the voltage absolute value of the single-phase inverter not included in the inverter group.

5. 4. The power conversion device according to claim 1, wherein a value after the decimal point of the real number including a decimal multiple of the minimum voltage value is 1 / 3, 1 / 2, or 2 / 3.

6. where k is a natural number and m is an integer equal to or greater than 0, k+m is equal to or greater than 2, and the power conversion device includes k+m+1 single-phase inverters; 2. The power conversion device according to claim 1, wherein the absolute voltage values ​​of the k single-phase inverters are set to a value that is a power of two or a power of three multiple of the minimum voltage value, the absolute voltage value of one of the remaining single-phase inverters is set to a value that is greater than a sum of the absolute voltage values ​​of the k single-phase inverters and smaller than a sum of the minimum voltage value and the sum of the absolute voltage values ​​of the k single-phase inverters, and the absolute voltage values ​​of the other m single-phase inverters are set to a value that is a power of two or a power of three multiple of the absolute voltage value of one of the remaining single-phase inverters.

7. A power conversion device as described in Claim 6, characterized in that the voltage absolute value of another of the single-phase inverters is set to a value obtained by adding 1 / 2 of the minimum voltage value to the sum of the voltage absolute values ​​of the k single-phase inverters.

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