JP2015027170A - Dc/ac conversion device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a DC/AC conversion device that has improved power conversion efficiency by outputting nine levels of three-phase line voltage with a smaller number of conducting switching elements.SOLUTION: The DC/AC conversion device includes: a multilevel DC/DC converter 101 for converting a DC voltage of a DC voltage source VDC to a plurality of levels of DC voltage; a multilevel DC/AC converter 102 comprising circuits of three phases each comprising a series circuit of switching elements S1, S2 and switching elements S3, S4 connected in series between a neutral terminal NP and an interconnection point of S1, S2; and a P side bypass circuit 103P connected between a positive terminal P1 and AC output terminals U, V, W of the converter 102. Four levels of voltage consisting of voltages of terminals N2, NP, P2, and VDC are available at the output terminal per phase to produce nine levels of voltage as line voltages. In each of current paths shown at (1)-(4), the number of conducting switching elements is two or less.

Description

  The present invention relates to a DC / AC converter capable of boosting an input DC voltage and outputting a multilevel voltage.

  FIG. 8 shows an example of a conventional DC / AC converter using a step-up chopper and a two-level inverter. The DC / AC converter of FIG. 8 boosts the voltage of the capacitor C11 using a step-up chopper for the DC voltage source VDC, and converts the positive or negative potential of the capacitor C11 to the three-phase output terminals U, This is a circuit that outputs to V and W and is linked to the AC power source 105 via the LCL filter 104.

  In FIG. 8, one end of a boosting reactor L is connected to the positive terminal P1 of the DC voltage source VDC. A series circuit of switching elements S37 and S38 for a boost chopper and a capacitor C11 are connected in parallel, and the other end of the boosting reactor L is connected to a common connection point of the switching elements S37 and S38.

  A common connection point between the switching element S37 and the capacitor C11 is connected to the positive terminal P2, and a common connection point between the switching element S38 and the capacitor C11 is connected to the negative terminals N1 and N2.

  Between the terminals P2 and N2, the DC side of a two-level inverter 200 in which switching elements S31 to S36 are connected in a three-phase bridge is connected, and the three-phase AC side is connected to three-phase output terminals U, V, and W.

  The three-phase output terminals U, V, and W are connected to an AC power source 105 via an LCL filter 104 including three-phase reactors Lf1 and Lf2 and a capacitor Cf.

  FIG. 8 shows an example in which the DC / AC converter is linked to an AC power supply, but it can also be applied to a case where a load is connected instead of the AC power supply.

  The DC / AC converter of FIG. 8 is used for a PV inverter or the like, and there are problems in improving power conversion efficiency, downsizing the boost reactor L, and downsizing the reactor Lf1 of the LCL filter 104.

  In the DC / AC converter of FIG. 8, in order to improve power conversion efficiency, as shown in Non-Patent Documents 1 and 2, an example using a T-type 3 level converter is known.

  FIG. 9 shows a configuration for one phase of the T-type 3 level converter (FIG. 9A) and the ANPC5 level converter (FIG. 9B). In the case of a three-phase circuit, FIG. The circuit in the broken lines (arms 200U, 200V, 200W for each phase) is replaced with the circuit in FIG. 9A or 9B.

  In FIG. 9A, switching elements S1 and S2 are connected in series between the positive terminal P2 and the negative terminal N2, and the neutral point terminal NP and the common connection point of the switching elements S1 and S2 are connected. Between them, switching elements S3, S4 are connected in series in the reverse direction, and a common connection point of switching elements S1, S2, S4 is connected to the AC output side.

  In FIG. 9B, switching elements S41 to S44 are sequentially connected in series between the positive terminal P2 and the negative terminal N2, and the common connection point of the switching elements S42 and S43 is connected to the neutral point terminal NP. Switching elements S45 to S48 are sequentially connected in series between the common connection point of the switching elements S41 and S42 and the common connection of the switching elements S43 and S44, and switching is performed between the common connection point of the switching elements S45 and S46. A capacitor C12 is connected between the elements S47 and S48 and the common connection point of the switching elements S46 and S47 is connected to the AC output side.

“The high efficiency transformer-less PV inverter topologies derived from NPC topology”, EPE2009 “The performance of the multilevel converter topologies for PV inverter”, CIPS 2012

  When the T-type three-level converter of FIG. 9A is used in the circuit of FIG. 8, assuming that the voltage between the terminals NP and P2 is E and the voltage between the terminals N2 and NP is E, E, 0, −E 3 levels of voltage can be output on the basis of NP. Considering the line voltage using the circuit of FIG. 9A for three phases, it is possible to output line voltages of 5 levels of 2E, E, 0, −E, and −2E.

  By using three levels of output voltage, the voltage skip amount of the output pulse voltage can be halved compared to the conventional two-level inverter (Fig. 8) at the same DC voltage and the same switching frequency, thus reducing switching loss. And the volume of the reactor Lf of the LCL filter can be reduced.

  9A, the switching element S1 is conductively connected between the terminal P2 and the output, and the switching elements S3 and S4 are conductively connected between the terminal NP and the output, and the terminal N2 From the output to the output, the switching element S2 is conducted and connected to output the potentials of the terminals P2, NP, and N2. At this time, since one or two switches are connected to the current output path, the conduction loss can be reduced.

  On the other hand, when the circuit of FIG. 9B is used, if the voltage between the terminals NP and P2 is 2E, the voltage between the terminals N2 and NP is 2E, and the voltage of the capacitor C12 is E, 2E, E, 0 , -E, -2E can be output on the basis of NP. When the line voltage is considered using the circuit of FIG. 9B for three phases, 9-level line voltages of 4E, 3E, 2E, E, 0, -E, -2E, -3E, and -4E are output. it can.

  When the circuit of FIG. 9B is used, since the number of levels of the line voltage is 9, the effect of reducing the output harmonics and the effect of reducing the size of the LCL filter reactor Lf1 are obtained using the circuit of FIG. Than there is. However, in the ANPC5 level converter of FIG. 9B, when the potential at the terminal P2 is output, the three switching elements S41, S45, and S46 are conducted, so that conduction loss increases. For this reason, there is a problem that improvement in conversion efficiency cannot be expected.

  The present invention solves the above-described problems, and an object of the present invention is to boost the input DC voltage and enable output of a nine-level three-phase line voltage with a small number of switching element conductions. An object of the present invention is to provide a direct current / alternating current converter with improved power.

  The DC / AC converter according to claim 1 for solving the above-mentioned problem is a DC / AC converter for converting a DC power source into an AC output of a plurality of levels, the DC power source and the input side being the DC power source. A DC / DC converter connected to each of the positive and negative side terminals, converting the voltage of the DC power source into a plurality of levels of DC voltage and outputting the voltage to the positive side output terminal, neutral point side terminal and negative side output terminal; First and second switching elements connected in series between a positive output terminal and a negative output terminal of the DC / DC converter, a neutral point terminal of the DC / DC converter, and the first and second switching elements A DC / AC conversion unit connected between the common connection points of the second switching elements and having a first bidirectional switching unit that can be controlled in the reverse withstand voltage direction is provided for three phases, and is provided for three phases. Each of the first and second switches of the DC / AC converter A DC / AC converter having a common connection point of the chucking elements as a three-phase AC output terminal and a second bidirectional switching means capable of controlling in a reverse withstand voltage direction are provided for three phases. Each one end of the bidirectional switching means is commonly connected to the positive terminal or the negative terminal of the DC power supply, and each other end of the second bidirectional switching means is connected to the three-phase AC output of the DC / AC converter. And a bypass auxiliary circuit connected to each of the terminals.

  The DC / AC converter according to claim 2, wherein the DC / DC converter includes seventh to tenth switching elements sequentially connected in series between the positive output terminal and the negative output terminal. , A first capacitor connected between a common connection point of the seventh and eighth switching elements and a common connection point of the ninth and tenth switching elements, and the eighth and ninth switching elements, A boosting reactor connected between a common connection point and a positive terminal of the DC power supply; and second and third capacitors connected in series between the positive output terminal and the negative output terminal. The common connection point of the second and third capacitors is the neutral point side terminal.

  According to the above configuration, nine-level three-phase line voltage can be output with a small number of switching element conductions, and power conversion efficiency is improved.

  The DC / AC converter according to claim 3, wherein the DC / DC converter includes: 11th to 14th switching elements sequentially connected in series between the positive output terminal and the negative output terminal; A fourth capacitor connected between a common connection point of the eleventh and twelfth switching elements and a common connection point of the thirteenth and fourteenth switching elements, and connected in parallel to the fourth capacitor. A series body of the fifteenth and sixteenth switching elements, a boosting reactor connected between a common connection point of the fifteenth and sixteenth switching elements and a positive terminal of the DC power supply, and the positive output The fifth and sixth capacitors connected in series between the terminal and the negative output terminal, the common connection point of the twelfth and thirteenth switching elements, and the fifth and sixth capacitors. And a seventeenth and eighteenth switching elements connected in series with the connection point, and the common connection point of the fifth and sixth capacitors is the neutral point side terminal. Yes.

  According to the above configuration, in addition to the above effects, a ripple current of the boosting reactor can be reduced because a 5-level DC / DC converter is used for the DC / DC converter.

(1) According to the first to third aspects of the invention, nine-level three-phase line voltage can be output with a small number of conductions of switching elements, and power conversion efficiency is improved.
(2) According to the invention described in claim 3, since the 5-level DC / DC converter is used for the DC / DC converter, the ripple current of the boosting reactor can be reduced.

The block diagram which shows the basic circuit structure of this invention. The circuit configuration of the multilevel DC / DC converter in the example of embodiment of the present invention is shown, (a) is a circuit diagram used in Example 1, and (b) is a circuit diagram used in Example 2. The circuit diagram of the DC / AC converter in the example of an embodiment of the present invention. The circuit diagram of the bypass auxiliary circuit in the example of an embodiment of the present invention. 1 is a circuit diagram showing Embodiment 1 of the present invention. The circuit diagram explaining the current pathway in Example 1 of the present invention. The circuit diagram which shows Example 2 of this invention. The circuit diagram which shows an example of the conventional DC / AC converter. The circuit diagram which shows the conventional T-type level converter and ANPC5 level converter. The circuit diagram which shows the other example of the bidirectional | two-way switching means of this invention.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings, but the present invention is not limited to the following embodiments. The basic configuration of the present invention includes a DC voltage source, a multilevel DC / DC converter, a multilevel DC / AC converter, and a bypass auxiliary circuit.

  FIG. 1 shows the configuration of a DC / AC converter in this embodiment. In FIG. 1, a multi-level DC / DC converter 101 is connected to the positive terminal P1 and the negative terminal N1 of the DC voltage source VDC.

  This multi-level DC / DC converter 101 uses a neutral point clamp type converter in a configuration in which a DC voltage is divided into two, and a DC output side is a positive side terminal P2, a neutral point side terminal NP, The negative terminal N2.

  A multi-level DC / AC converter 102 is connected to each of the terminals P2, NP, and N2, and its three-phase output side is connected to an AC power source 105 through harmonic elimination and system interconnection LCL filters 104. ing.

  When the bypass auxiliary circuit for bypassing the multilevel DC / DC converter 101 and the multilevel DC / AC converter 102 is provided on the positive side, the positive side terminal P1 and the three-phase output side of the multilevel DC / AC converter 102 are provided. P-side bypass circuit 103P is connected between the N-side bypass circuit 103N and the N-side bypass circuit 103N is connected between the negative-side terminal N1 and the three-phase output side of the multi-level DC / AC converter 102 (FIG. Shows both the positive side and the negative side).

  As the multi-level DC / DC converter 101, any one of the three-level FCDC / DC converter of FIG. 2A and the five-level DC / DC converter of FIG. 2B is used.

  The three-level FCDC / DC converter of FIG. 2A includes switching elements S7 to S10 (seventh to tenth) sequentially connected in series between an output-side positive terminal P2 and an output-side negative terminal N2. Switching element), a first capacitor C1 (flying capacitor) connected between a common connection point of the switching elements S7 and S8 and a common connection point of the switching elements S9 and S10, and the switching elements S8 and S9. And a boosting reactor L connected between the common connection point of the DC voltage source VDC and the positive side terminal P1 of the DC voltage source VDC, and second and third capacitors connected in series between the output side terminals P2 and N2. C2 and C3, and a common connection point of the capacitors C2 and C3 is a neutral point side terminal (NP).

  The 5-level DC / DC converter shown in FIG. 2B includes switching elements S11 to S14 (11th to 14th) sequentially connected in series between an output-side positive terminal P2 and an output-side negative terminal N2. Switching element), a fourth capacitor C4 connected between a common connection point of the switching elements S11 and S12 and a common connection point of the switching elements S13 and S14, and a switching connected in parallel to the capacitor C4. Step-up reactor L connected between a series body of elements S15 and S16 (15th and 16th switching elements), a common connection point of switching elements S15 and S16, and positive terminal P1 of DC voltage source VDC And fifth and sixth capacitors C5 and C6 connected in series between the positive terminal P2 and the negative terminal N2, and the switching element S1 , S13 and switching elements S17, S18 (third bidirectional switching means) connected in series in the opposite direction between the common connection point of the capacitors C5, C6, and the capacitor The common connection point of C5 and C6 is a neutral point side terminal (NP).

  As the multi-level DC / AC converter 102 in FIG. 1, a circuit in which the T-type 3-level DC / AC converter (for one phase) in FIG. 3 is provided for three phases is used. In FIG. 3, the first and second switching elements S1, S2 are connected in series between the positive terminal P2 and the negative terminal N2, and the neutral point terminal NP and the switching elements S1, S2 are common. Between the connection points, the third and fourth switching elements S3, S4 are connected in series in the reverse direction (first bidirectional switching means), and the common connection point of the switching elements S1, S2, S4 is AC output.

  As the P-side bypass circuit 103P or the N-side bypass circuit 103N in FIG. 1, a circuit in which the two-level bypass circuit in FIG. 4 is provided for three phases is used. In the upper circuit of FIG. 4, one end of a series circuit (second bidirectional switching means) in which the fifth and sixth switching elements S5P and S6P are connected in series in the reverse direction is connected to the positive terminal P1 of the DC voltage source VDC. The other end is connected to the output side of the multilevel DC / AC converter 102.

  In the lower circuit of FIG. 4, one end of the series circuit (second bidirectional switching means) in which the fifth and sixth switching elements S5N and S6N are connected in series in the reverse direction is the negative terminal N1 of the DC voltage source VDC. The other end is connected to the output side of the multilevel DC / AC converter 102.

  The relationship between the combination of the multi-level DC / DC converter 101, the multi-level DC / AC converter 102, the P-side bypass circuit 103P or the N-side bypass circuit 103N and the embodiment is as shown in Table 1 below.

  In the first embodiment, as shown in Table 1, the DC / DC converter has a three-level FCDC / DC converter, the DC / AC converter has a T-type 3-level DC / AC converter, and the bypass auxiliary circuit has two levels. Using each bypass circuit, a three-phase DC / AC converter was configured as shown in FIG. In FIG. 5, the positive terminal of the DC voltage source VDC is P1, and the negative terminal is N1.

  As the multi-level DC / DC converter 101, a 3-level FC type DC / DC converter shown in FIG. 2A is used. Switching elements S7 to S10, a capacitor C1 (flying capacitor), a boosting reactor L, and a smoothing are used. Capacitors C2 and C3. This unit receives DC terminals P1 and N1, outputs DC terminals P2 and N2, and has a positive side of the series circuit of the smoothing capacitors C2 and C3 as P2, a neutral point as NP, and a negative side as N2. .

  As the multi-level DC / AC converter 102, a T-type 3-level DC / AC converter shown in FIG. 3 is used for three phases, which are switching elements S1U, S1V, S1W, S2U, S2V, S2W, S3U, S3V, It is comprised by S3W, S4U, S4V, and S4W. The unit receives DC terminals P2, NP and N2 and outputs three-phase output terminals U, V and W.

  As the P-side bypass circuit 103P, the bypass auxiliary circuit shown in the upper side of FIG. 4 is used for three phases (switching elements S5PU, S5PV, S5PW, S6PU, S6PV, S6PW), and the input side is the positive side terminal of the DC voltage source VDC. The output side is connected to the three-phase output terminals U, V, and W, respectively. Three-phase output terminals U, V, and W are connected to a three-phase AC power source 105 through an LCL filter 104.

  In FIG. 5, the multilevel DC / DC converter 101 boosts the input DC voltage source VDC to generate voltage sources in the smoothing capacitors C2 and C3.

  The switching pattern of the multilevel DC / DC converter 101 is as follows. Assuming that the voltage of the flying capacitor C1 is E and the voltages of the smoothing capacitors C2 and C3 are E, when the switching elements S7 and S8 are on, the terminal on the opposite side to P1 of the boosting reactor L (that is, between S8 and S9) A voltage of 2E appears at the connection point.

  When the switching elements S7 and S9 are on, the flying capacitor C1 is discharged under the condition that the current IL of the boosting reactor L> 0, and the terminal on the opposite side of P1 of the boosting reactor L (that is, the connection point between S8 and S9) ) Appears with a voltage of + E.

  When switching elements S8 and S10 are on, flying capacitor C1 is charged under the condition of current IL of boosting reactor L> 0, and terminal on the side opposite to P1 of boosting reactor L (that is, the connection point between S8 and S9) ) Appears with a voltage of + E.

  When the switching elements S9 and S10 are on, the voltage at the terminal opposite to P1 of the step-up reactor L (that is, the connection point between S8 and S9) is zero.

  Note that the direction of the reactor current IL is positive from the DC voltage source VDC to the multilevel DC / DC converter 101.

  The current IL flowing through the boosting reactor L is determined by the voltage difference between the DC voltage source VDC and the terminal P1 of the boosting reactor L of the multilevel DC / DC converter 101 opposite to the terminal P1 (that is, the connection point between S8 and S9). By controlling, the voltage of the smoothing capacitors C2 and C3 can be boosted with respect to VDC.

  In FIG. 5, when the negative terminal N1 is used as a reference, the positive potential of the DC voltage source VDC is + VDC, the potential of the terminal P2 of the smoothing capacitor C2 is 2E, and the neutral points of the smoothing capacitors C2 and C3. The voltage that can be output from the output terminals U, V, W of the multilevel DC / AC converter 102 is four levels of 0, E, 2E, and VDC when the potential of the terminal NP is E.

  + VDC is output from the P-side bypass circuit 103P, and + E, + 2E, and 0 are output from the multilevel DC / AC converter 102.

  Table 2 shows ON / OFF patterns of the switching elements related to the U-phase output phase voltage.

  In Table 2, the reference point is the terminal N2. Further, this is an example of switching when E> VDC (when U-phase current IU> 0), “1” represents on control, and “0” represents off control.

  In Table 2, the U-phase output voltage is 2E when the switching elements S1U and S3U are on, E when the switching elements S3U and S4U are on, and VDC when the switching elements S3U and S5PU are on. It becomes 0 when the switching elements S2U and S3U are on.

  Although the output voltage of Table 2 is four levels, when considering the line voltage in the three-phase AC voltage, as shown in Table 3, 2E, 2E-VDC, E, E-VDC, 0, -2E,- It becomes a 9-level voltage of 2E + VDC, -E, E + VDC. Table 3 shows the relationship among the U-phase output voltage, the V-phase output voltage, and the UV-phase line voltage.

  Here, in the conventional circuit using the ANPC5 level converter of FIG. 9B, the three switching elements S41, S45, and S46 are conducted from the terminal P2 to the output. On the other hand, in the first embodiment, the number of conductions of the switching element is 2 or less in any of the paths (1) to (4) in FIG. 6 representing the current path flowing through the circuit in FIG.

That is,
Path (1): conduction switching element: S5PU and S6PU
Path (2): conduction switching element: S1U
Path (3): conduction switching element: S3U and S4U
Path (4): conduction switching element: S2U
It becomes.

  Thus, according to the first embodiment, since the number of conduction of the switching element can be reduced, there is an effect of reducing conduction loss.

  In FIG. 5, for example, there are two series circuits (first bidirectional switching means) of U-phase switching elements S3U and S4U and two series circuits (second bidirectional switching means) of switching elements S5PU and S6PU, respectively. The switching elements are not limited to be connected in series in the reverse direction, and a configuration in which two switching elements are connected in antiparallel as shown in FIG.

  That is, in FIG. 10A, a switching circuit S61 having a freewheeling diode and a series circuit comprising a diode D1 connected in series in the forward direction to S61, and a switching element S62 having a freewheeling diode and in the reverse direction to S61. And the series circuit which consists of the diode D2 of the direction opposite to said D1 is connected in parallel.

  In FIG. 10B, a switching circuit S63 and a series circuit composed of a diode D3 connected in series to the S63 in the forward direction, a switching element S64 in the direction opposite to the S63, and a diode D4 in the direction opposite to the D3. A series circuit consisting of

  The anti-parallel configuration as shown in FIG. 10 may be adopted for the V-phase switching elements S3V and S4V, S5PV and S6PV, and the W-phase switching elements S3W and S4W, S5PW and S6PW in FIG. is there.

  In FIG. 5, the bypass auxiliary circuit is provided on the P side. However, the present invention is not limited to this, and the N side bypass circuit 103N provided with the switching elements S5N and S6N of FIG. , V, W may be connected.

  In the second embodiment, as shown in Table 1, the DC / DC converter is a 5-level DC / DC converter, the DC / AC converter is a T-type 3 level DC / AC converter, and the bypass auxiliary circuit is 2 levels. Using the bypass circuits, a three-phase DC / AC converter was constructed as shown in FIG.

  7, the same parts as those in FIG. 5 are denoted by the same reference numerals. 7 is different from FIG. 5 in that the multi-level DC / DC converter 101 uses the 5-level DC / DC converter of FIG. 2B, and the other parts are the same as those in FIG. Has been.

  In FIG. 7, the positive potential of the DC voltage source voltage is + VDC, the negative potential is 0, the potentials of the capacitors C2 and C3 are E, the neutral point is 0, and the negative point is -E. Sometimes, the voltages that can be output from the output terminals U, V, and W of the multi-level DC / AC converter 102 are four levels of 0, E, 2E, and VDC as in the first embodiment. Therefore, the line voltage can be set to 9 levels of 2E, 2E-VDC, E, E-VDC, 0, -2E, -2E + VDC, -E, E + VDC as in the first embodiment.

  Also in the second embodiment, the current path when outputting the four-level voltage to each phase is the same path as (1) to (4) shown in FIG. The number of conduction is 2 or less.

  For this reason, similarly to the case of the first embodiment, the number of conductions of the switching element can be reduced as compared with the case of using the conventional circuit of FIG. In the second embodiment, since the five-level DC / DC converter is used, the ripple current of the boosting reactor L can be reduced.

  Instead of the switching elements S17 and S18 of FIG. 7, the circuit configurations of FIGS. 10 (a) and 10 (b) may be employed.

DESCRIPTION OF SYMBOLS 101 ... Multi-level DC / DC converter 102 ... Multi-level DC / AC converter 103P ... P side bypass circuit 103N ... N side bypass circuit 104 ... LCL filter 105 ... AC power supply C1, C4 ... Capacitor (flying capacitor)
C2, C3, C5, C6 ... Smoothing capacitors D1-D4 ... Diodes S1-S4, S5P, S5N, S6P, S6N, S7-S18, S61-S64 ... Switching elements VDC ... DC voltage source L ... Boosting reactor

Claims (3)

A DC / AC converter for converting a DC power source into a multi-level AC output,
DC power supply,
DC / input is connected to the positive and negative terminals of the DC power supply, and the DC power supply voltage is converted into a plurality of levels of DC voltage and output to the positive output terminal, neutral point terminal, and negative output terminal. A DC converter,
First and second switching elements connected in series between a positive output terminal and a negative output terminal of the DC / DC converter, a neutral point side terminal of the DC / DC converter, and the first And a DC / AC converter having a first bidirectional switching means connected between the common connection points of the second switching elements and having a first bidirectional switching means that can be controlled in the reverse withstand voltage directions. A DC / AC converter having a common connection point of each of the first and second switching elements of the DC / AC converter of the three-phase AC output terminal;
Second bidirectional switching means that can be controlled in the reverse withstand voltage directions are provided for three phases, and each one end of the second bidirectional switching means for the three phases is used as a positive terminal or a negative terminal of the DC power supply. A bypass auxiliary circuit connected in common and connected to each of the other ends of the second bidirectional switching means to a three-phase AC output terminal of the DC / AC converter;
A DC / AC conversion device characterized by comprising:   The DC / DC converter includes seventh to tenth switching elements sequentially connected in series between the positive output terminal and the negative output terminal, and a common connection point of the seventh and eighth switching elements. And a first capacitor connected between a common connection point of the ninth and tenth switching elements, and a common connection point of the eighth and ninth switching elements and a positive terminal of the DC power supply. A boosting reactor connected, and a second and a third capacitor connected in series between the positive output terminal and the negative output terminal, and a common connection point of the second and third capacitors. The DC / AC converter according to claim 1, wherein the neutral point side terminal is used.   The DC / DC converter has a common connection point between the eleventh and twelfth switching elements, and the eleventh and twelfth switching elements, which are sequentially connected in series between the positive output terminal and the negative output terminal. A fourth capacitor connected between a common connection point of the thirteenth and fourteenth switching elements, and a series body of fifteenth and sixteenth switching elements connected in parallel to the fourth capacitor, A boosting reactor connected between a common connection point of the fifteenth and sixteenth switching elements and a positive terminal of the DC power source, and connected in series between the positive output terminal and the negative output terminal The fifth and sixth capacitors are connected between the common connection point of the twelfth and thirteenth switching elements and the common connection point of the fifth and sixth capacitors, and have mutually opposite breakdown voltages. 2. The DC / DC converter according to claim 1, further comprising a third bidirectional switching unit that can be controlled in a direction, wherein a common connection point of the fifth and sixth capacitors is the neutral point side terminal. AC converter.

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