JP7615893B2 - Power conversion device and power supply device - Google Patents

Description

This disclosure relates to a power conversion device and a power supply device.

Conventionally, power conversion devices (power conditioners) connected to solar panels typically output 200 V over a single-phase, two-wire system when connected to the grid, and 100 V over a single-phase, two-wire system from a dedicated independent output outlet during independent operation.
Meanwhile, in recent years, highly functional hybrid power conversion devices have appeared that can be connected not only to solar panels but also to storage batteries, making it possible to output single-phase three-wire 200/100V both when connected to the grid and when operating independently.

A power conversion device capable of providing a single-phase three-wire independent output has been proposed, for example, in Patent Document 1. The power conversion device in Patent Document 1 includes a DC/DC converter connected to a DC power source, a converter that adjusts the midpoint potential of the DC bus, and an inverter that performs DC/AC conversion. The converter that controls the midpoint potential of the DC bus controls the potential of the interconnection point in a series body of a pair of capacitors connected between two wires of the DC bus so that it becomes the middle of the potential between the two wires of the DC bus. This interconnection point is directly connected to the neutral wire of the single-phase three-wire.

JP 2014-87160 A (FIG. 1)

In a power conversion device such as that disclosed in Patent Document 1, a DC/DC converter connected to a DC power source outputs a DC voltage of, for example, 350V, which is higher than the peak voltage of 200V AC, over two wires. This DC/DC converter is always necessary when the voltage of the DC power source is lower than the peak voltage of 200V AC. The converter that controls the midpoint potential of the DC bus operates to maintain 175V, which is the midpoint potential of 350V.

In this case, all DC power is first boosted to 350V before it is supplied, so the reactor in the DC/DC converter is correspondingly large. Also, since the voltage is first boosted to 350V and then stepped down to a midpoint potential of 175V, there is a large power loss due to switching. Furthermore, the switching elements in the DC/DC converter must be able to withstand a voltage of 350V or more. The higher the withstand voltage, the greater the power loss due to switching.

The purpose of this disclosure is to achieve reduced power loss and compact size in a power conversion device that provides single-phase three-wire output during standalone operation.

This disclosure includes the following inventions, however, the inventions are defined by the claims.

Disclosed is a power conversion device provided between a DC power source and a single-phase three-wire AC circuit,
an inverter for providing an AC output to the AC power path;
a DC bus on a DC side of the inverter, the DC bus having, in order of potential, a first wire, an intermediate potential wire, and a second wire, the first wire and the second wire being DC input wires to the inverter, and the intermediate potential wire being connected to a neutral wire of the AC circuit;
a first capacitor provided between the first line and the intermediate potential line;
a second capacitor provided between the intermediate potential line and the second line;
a first DC/DC converter provided between the DC power source and the DC bus, generating a first DC voltage based on an input voltage from the DC power source and outputting the first DC voltage between the first line and the second line;
a second DC/DC converter provided between the DC power source and the DC bus, converting an input voltage from the DC power source into a second DC voltage and outputting the second DC voltage between one of the first line and the second line and the intermediate potential line;
a control unit that controls the inverter, the first DC/DC converter, and the second DC/DC converter;
The power conversion device is provided with:

The power supply device includes a power conversion device as described above and a DC power supply.

According to the present disclosure, it is possible to reduce power loss and achieve compactness in a power conversion device that provides single-phase three-wire output during stand-alone operation.

FIG. 1 is a connection diagram showing the basic concept of a power conversion device and a power supply device according to the present disclosure. FIG. 2 is a circuit diagram of the power conversion device and the power supply device according to the first embodiment. FIG. 3 is a circuit diagram of a power conversion device and a power supply device according to the second embodiment. FIG. 4 is a circuit diagram of a power conversion device and a power supply device according to the third embodiment. FIG. 5 is a circuit diagram of a power conversion device and a power supply device according to the fourth embodiment. FIG. 6 is a circuit diagram of a power conversion device and a power supply device according to the fifth embodiment. For reference, FIG. 7 is a graph showing the voltage and current when a single-phase two-wire independent output is output directly between the U line and the W line of a single-phase three-wire system using one DC/DC converter and an inverter, and power is supplied to a load that is extremely unbalanced in the U phase (line U-line O) and the W phase (line W-line O). FIG. 8 is a graph showing voltage and current when the power conversion device of the first embodiment (FIG. 2) is used to output an independent output between the U line and the W line of a single-phase three-wire system and supply power to a load that is extremely unbalanced in the U phase (U line-O line) and the W phase (W line-O line). FIG. 9 is a graph showing voltage and current when a single-phase three-wire AC output is provided to a balanced load in a stand-alone operation using the power conversion device of the first embodiment. FIG. 10 is a graph showing voltages and currents when a single-phase three-wire AC output is provided to a balanced load by independent operation based on control of a voltage target value different from that in FIG. 9 using the power conversion device of the first embodiment. FIG. 11 is a graph showing voltage and current when a single-phase three-wire AC output is provided to a balanced load in a stand-alone operation using the power conversion device of the fifth embodiment (FIG. 6). FIG. 12 is a circuit diagram of a power conversion device and a power supply device which can be considered a modification of the second embodiment (FIG. 3).

[Description of the embodiments of the present disclosure]
The gist of the present disclosure includes at least the following.

(1) This is a power conversion device provided between a DC power source and a single-phase three-wire AC circuit, comprising: an inverter that provides an AC output to the AC circuit; a DC bus on the DC side of the inverter, which has, in order of potential, a first wire, an intermediate potential wire, and a second wire, the first wire and the second wire being DC input lines to the inverter, and the intermediate potential wire being connected to the neutral wire of the AC circuit; a first capacitor provided between the first wire and the intermediate potential wire; a second capacitor provided between the intermediate potential wire and the second wire; and a power conversion device that converts the DC power source and the DC The power conversion device includes a first DC/DC converter provided between the DC bus and the inverter, which generates a first DC voltage based on the input voltage from the DC power source and outputs it between the first line and the second line, a second DC/DC converter provided between the DC power source and the DC bus, which converts the input voltage from the DC power source into a second DC voltage and outputs it between one of the first line and the second line and the intermediate potential line, and a control unit that controls the inverter, the first DC/DC converter, and the second DC/DC converter.

In such a power conversion device, the first DC/DC converter and the second DC/DC converter each output a predetermined DC voltage, thereby maintaining the potential difference between the three wires of the DC bus (first wire, intermediate potential wire, second wire) at a desired value, and providing a stable single-phase three-wire output even during independent operation. Since the second DC/DC converter outputs a lower voltage than the first DC/DC converter, it is possible to reduce power loss and achieve a more compact design due to the lower voltage. In this way, it is possible to reduce power loss and achieve a more compact design in a power conversion device that provides a single-phase three-wire output during independent operation.

(2) In the power conversion device of (1), for example, during independent operation, the control unit performs constant voltage control on the first DC/DC converter so that the first DC voltage is output between the first line and the second line, and performs constant voltage control on the second DC/DC converter so that the second DC voltage is output between the intermediate potential line and the second line or the first line.
In this case, the voltage target value of the first DC/DC converter is a first DC voltage between the first and second lines, and the voltage target value of the second DC/DC converter is a second DC voltage between the midpoint line and the second line.

(3) In the power conversion device of (1), for example, during independent operation, the control unit performs constant voltage control on the first DC/DC converter so that a difference voltage between the first DC voltage and the second DC voltage is output between the first line and the intermediate potential line, and performs constant voltage control on the second DC/DC converter so that the second DC voltage is output between the intermediate potential line and the second line.
In this case, the voltage target value of the first DC/DC converter is a difference voltage between a first DC voltage between the first line and the second line and a second DC voltage between the intermediate potential line and the second line, and the voltage target value of the second DC/DC converter is the second DC voltage.

(4) In the power conversion device of (1) to (3), the first DC/DC converter may be configured to boost a voltage of the DC power supply to the first DC voltage, and the second DC/DC converter may be configured to boost a voltage of the DC power supply to the second DC voltage.
When the first DC voltage is _V1 , the second DC voltage is _V2 , and the power supply voltage of the DC power supply is _VS , if the relationship is _VS < _V2 < _V1 , both the first DC/DC converter and the second DC/DC converter can boost the power supply voltage _VS to obtain the first DC voltage _V1 and the second DC voltage _V2 .

(5) In the power conversion device of (1) to (3), the first DC/DC converter may be configured to boost the voltage of the DC power supply to obtain the first DC voltage, and the second DC/DC converter may be configured to lower the voltage of the DC power supply to obtain the second DC voltage.
If the first DC voltage is _V1 , the second DC voltage is _V2 , and the power supply voltage of the DC power supply is _VS , then if the relationship _V2 < _VS < _V1 holds, the first DC/DC converter can step up the power supply voltage _VS to the first DC voltage _V1 , and the second DC/DC converter can step down the power supply voltage _VS to the second DC voltage _V2 .

(6) In the power conversion device of (1) to (3), the second DC/DC converter may be configured to boost a voltage of the DC power supply to obtain the second DC voltage, and the first DC/DC converter may be configured to boost the second DC voltage to obtain the first DC voltage.
If the first DC voltage is _V1 , the second DC voltage is _V2 , and the power supply voltage of the DC power supply is _VS , then if the relationship is _VS < _V2 < _V1 , the second DC/DC converter can boost the power supply voltage _VS to the second DC voltage _V2 , and the first DC/DC converter can further boost the second DC voltage to the first DC voltage _V1 .

(7) In the power conversion device of (1) to (3), the DC power supply may include a first DC power supply and a second DC power supply, and when a voltage of the first DC power supply is higher than a voltage of the second DC power supply, the first DC/DC converter may be connected to the first DC power supply and the second DC/DC converter may be connected to the second DC power supply, the first DC/DC converter may step up the voltage of the first DC power supply to the first DC voltage, and the second DC/DC converter may step up the voltage of the second DC power supply to the second DC voltage.
When the first DC voltage is _V1 , the second DC voltage is _V2 , the voltage of the first DC power supply is _VSA , and the voltage of the second DC power supply is _VSB , the first DC/DC converter and the second DC/DC converter can respectively boost the voltages _VSA and _VSB to obtain the first DC voltage _V1 and the second DC voltage _V2 .

(8) In the power conversion device of (7), when the first DC voltage is V ₁ , the second DC voltage is V ₂ , the power supply voltage of the first DC power supply is V _SA , and the power supply voltage of the second DC power supply is V _SB , for example,
V _SB <V _SA <V ₂ <V ₁ , or V _SB <V ₂ <V _SA <V ₁ ,
It is preferable that the voltage relationship be as follows:
In this case, the step-up ratios of the first DC/DC converter and the second DC/DC converter can be suppressed, and power loss can be reduced.

(9) In the power conversion device according to any one of (1) to (8), the second DC voltage has a tolerance of (50±10)% of the first DC voltage.
The second DC voltage is preferably 50% of the first DC voltage, but taking into account the tolerance range of single-phase three-wire AC voltage, it is considered that (50±10)% will suffice.

(10) A power supply device includes the power conversion device according to any one of (1) to (9) above and the DC power supply.
Such a power supply device can stably provide a single-phase three-wire output in a stand-alone operation, and can also suppress power loss and achieve compactness.

[Details of the embodiment of the present disclosure]
Configuration of power conversion device and power supply device
Hereinafter, specific examples of a power conversion device and a power supply device including the power conversion device according to the present disclosure will be described with reference to the drawings.
1 is a connection diagram showing the basic concept of the power conversion device and power supply device of the present disclosure. In the figure, a power conversion device 1 is provided between a DC power source 2 and a single-phase three-wire AC circuit 3. The power conversion device 1, together with the DC power source 2, constitutes a power supply device 100. The power conversion device 1 includes a first DC/DC converter 11, a second DC/DC converter 12, a high-potential side capacitor 13, a low-potential side capacitor 14, and an inverter 15, which are connected as shown in the figure. The DC power source 2 may be, for example, a storage battery, a renewable energy source such as a solar power generation panel, or a fuel cell.

The DC bus 16 is composed of three lines with different potentials. For example, the upper first line 16a is 350V, the intermediate potential line 16c is 175V, and the lower second line 16b is 0V. Note that the voltage values are merely examples for the purpose of explanation, and are not limited to these values (same below). The second DC/DC converter 12 outputs the input voltage from the DC power source 2 at 175V, and the low-potential side capacitor 14 smoothes the line voltage between the intermediate potential line 16c and the second line 16b. The first DC/DC converter 11 outputs the input voltage from the DC power source 2 at 350V, and the high-potential side capacitor 13 and the low-potential side capacitor 14 smooth the line voltage between the first line 16a and the second line 16b. When viewed between the first line 16a and the intermediate potential line 16c of the DC bus 16, and between the intermediate potential line 16c and the second line 16b, both the first DC/DC converter 11 and the second DC/DC converter 12 are outputting 175V.

The inverter 15 creates an AC voltage waveform from the 350V voltage input from the first line 16a and the second line 16b, and outputs it between the two lines, the U line and the W line, which are single-phase three-wire voltage lines. The intermediate potential line 16c of the DC bus 16 passes directly through the inverter 15 and is directly connected to the neutral line O line. When connected to the grid, the O line is grounded and is at 0V. Therefore, the intermediate potential line 16c of the DC bus 16 is also at 0V. In this case, the potential of the first line is 175V, and the potential of the second line is -175V. During independent operation, the O line is grounded inside the power conversion device 1 or inside a distribution board, switch, etc. on the AC circuit 3.

Hereinafter, a more specific embodiment of the power conversion device 1 will be described. Note that the disclosures of the respective embodiments may be partially combined.
The power conversion device 1 can operate in a grid-connected manner or in an independent manner, but the following disclosure will mainly describe the operation during independent operation. The power conversion device 1 is also bidirectional, and can output from the DC power source 2 to the AC circuit 3, or charge the DC power source 2 from the AC circuit 3 (or DC bus 16). However, the following disclosure will mainly describe the output from the DC power source 2 to the AC circuit 3.

(First embodiment: two boost sets)
Fig. 2 is a circuit diagram of the power conversion device 1 and the power supply device 100 according to the first embodiment. In Fig. 2, a first DC/DC converter 11 is connected to both ends of a DC power source 2. A second DC/DC converter 12 is also connected to both ends of the DC power source 2. A voltage V _S across both ends of the DC power source 2 is lower than 175V. For example, the voltage V _S is 100V.

The first DC/DC converter 11 includes a DC reactor 110, a high-side switching element 111, and a low-side switching element 112, which are connected as shown. The second DC/DC converter 12 includes a DC reactor 120, a high-side switching element 121, and a low-side switching element 122, which are connected as shown.

The DC bus 16 is composed of three wires with different potentials. The upper first wire 16a is 350V, the intermediate potential wire 16c is 175V, and the lower second wire 16b is 0V. A high potential side capacitor 13 is connected between the first wire 16a and the intermediate potential wire 16c. A low potential side capacitor 14 is connected between the intermediate potential wire 16c and the second wire 16b. A voltage sensor 17 is connected between the first wire 16a and the second wire 16b. A voltage sensor 18 is connected between the intermediate potential wire 16c and the second wire 16b, i.e., across the low potential side capacitor 14.

The inverter 15 is configured by connecting four switching elements 151, 152, 153, and 154 that form a full bridge circuit, an AC reactor 155 provided in the electrical path leading to the U line, an AC reactor 156 provided in the electrical path leading to the W line, a capacitor 157 provided between the U line and the O line, and a capacitor 158 provided between the O line and the W line, as shown in the figure. The intermediate potential line 16c of the DC bus 16 passes through the inverter 15 and is directly connected to the neutral line O line.

The switching elements 111, 112, 121, 122, 151 to 154 are, for example, MOSFETs (Metal-Oxide-Semiconductor Field Effect Transistors), and are controlled by the control unit 21. However, IGBTs may be used instead of the MOSFETs. The same applies to the switching elements in the second and subsequent embodiments. Detection signals from the voltage sensors 17 and 18 are sent to the control unit 21.
The control unit 21 includes, for example, a computer, and the computer executes software (computer programs) to realize necessary control functions. The software is stored in a storage device (not shown) of the control unit 21.

In FIG. 2, the second DC/DC converter 12 boosts the input voltage from the DC power source 2 and outputs it at 175V, and the low-potential side capacitor 14 smoothes the line voltage between the intermediate potential line 16c and the second line 16b. The first DC/DC converter 11 boosts the input voltage from the DC power source 2 and outputs it at 350V, and the high-potential side capacitor 13 and the low-potential side capacitor 14 smooth the line voltage between the first line 16a and the second line 16b. When viewed between the three lines of the DC bus 16, both the first DC/DC converter 11 and the second DC/DC converter 12 output 175V between the intermediate potential line 16c.

During independent operation, the control unit 21 performs constant voltage control on the second DC/DC converter 12 so that the actual output voltage of the second DC/DC converter 12 detected by the voltage sensor 18 coincides with the target voltage value. There are two ways of controlling the first DC/DC converter 11.
In the first control method, the control unit 21 performs constant voltage control on the first DC/DC converter 11 so that the voltage detected by the voltage sensor 17 coincides with the target voltage value (350 V).
In the second control method, the control unit 21 performs constant voltage control on the first DC/DC converter 11 so that the difference voltage between the voltage detected by the voltage sensor 17 and the voltage detected by the voltage sensor 18 matches the target voltage value (175 V).

Here, the second DC/DC converter 12 only needs to boost from 100V to 175V, so the DC reactor 120 can be made smaller than when boosting from 100V to 350V, and this allows the power conversion device 1 to be made more compact. In addition, since the maximum voltage that can be applied to the switching elements 121 and 122 is 175V, the withstand voltage of the switching elements 121 and 122 can be set to 175V or higher but lower than 350V. This reduces power loss due to switching and also reduces the cost of parts.

In addition to the arrangement shown in FIG. 2, the voltage sensors may be arranged in a manner such that a voltage sensor detects the voltage between the first line 16a and the second line 16b, and a voltage sensor detects the voltage between the first line 16a and the intermediate potential line 16c. Alternatively, a voltage sensor detects the voltage between the first line 16a and the intermediate potential line 16c, and a voltage sensor detects the voltage between the intermediate potential line 16c and the second line 16b may be arranged. In short, by detecting the voltage between two lines of different combinations among the three lines, it is possible to obtain the voltage between any of the lines. This is similar in other embodiments.

When providing a voltage sensor that detects the voltage between the first line 16a and the intermediate potential line 16c, and a voltage sensor that detects the voltage between the intermediate potential line 16c and the second line 16b, the reference potentials of the respective voltage sensors are not common, so it is necessary to provide them independently of each other. This is somewhat complicated and troublesome from the standpoint of ease of wiring. In contrast, providing voltage sensors 17 and 18 as shown in Figure 2 has the advantage that the reference potential (second line 16b) is common, making wiring easier.

The inverter 15 creates an AC voltage waveform from the 350V voltage input from the first line 16a and the second line 16b of the DC bus 16, and outputs it between the two lines U and W, which are single-phase three-wire voltage lines. In reality, a current sensor and a voltage sensor are also provided on the AC side of the inverter 16, but since they are not the main part of this disclosure, their description will be omitted here.

When connected to the grid, the O line is grounded on the grid side and has a potential of 0V. Therefore, the intermediate potential line 16c of the DC bus 16 is also 0V. In this case, the potential of the first line is 175V and the potential of the second line is -175V. During independent operation, the O line is grounded inside the power conversion device 1 or inside a distribution board on the AC circuit 3, so they have the same potential.

When the system is connected, the voltage (potential) of the single-phase three-wire of the AC circuit 3 is controlled by the commercial power system and is maintained at a predetermined value, so the power conversion device 1 performs current control. In this case, the inverter 15 performs constant voltage control of the DC bus 16. The first DC/DC converter 11 performs constant current control. The second DC/DC converter 12 does not need to operate and is therefore stopped.

(Second embodiment: voltage step-up and voltage step-down)
Fig. 3 is a circuit diagram of the power conversion device 1 and the power supply device 100 according to the second embodiment. In Fig. 3, the first DC/DC converter 11 is connected to both ends of the DC power supply 2. The second DC/DC converter 22 is also connected to both ends of the DC power supply 2. In this embodiment, when the first DC voltage is _V1 , the second DC voltage is _V2 , and the power supply voltage of the DC power supply is _VS , the relationship is _V2 < _VS < _V1 . That is, the voltage _VS [V] across both ends of the DC power supply 2 has a relationship of 175 < _VS < 350. For example, the voltage _VS is assumed to be 200V.

The first DC/DC converter 11 includes a DC reactor 110, a high-side switching element 111, and a low-side switching element 112, which are connected as shown. The second DC/DC converter 22 includes a DC reactor 220, a high-side switching element 221, and a low-side switching element 222, which are connected as shown.

The DC bus 16, high-potential side capacitor 13, low-potential side capacitor 14, voltage sensors 17 and 18, and inverter 15 are the same as those in the first embodiment, so their description is omitted.

In FIG. 3, the second DC/DC converter 22 steps down the input voltage from the DC power source 2 and outputs it at 175 V, and the low-potential side capacitor 14 smoothes the line voltage between the intermediate potential line 16c and the second line 16b. The first DC/DC converter 11 steps up the input voltage from the DC power source 2 and outputs it at 350 V, and the high-potential side capacitor 13 and the low-potential side capacitor 14 smooth the line voltage between the first line 16a and the second line 16b. When viewed between the three lines of the DC bus 16, both the first DC/DC converter 11 and the second DC/DC converter 22 output 175 V between the intermediate potential line 16c.

During independent operation, the control unit 21 performs constant voltage control on the second DC/DC converter 22 so that the actual output voltage of the second DC/DC converter 22 detected by the voltage sensor 18 coincides with the target voltage value. There are two ways of controlling the first DC/DC converter 11.
In the first control method, the control unit 21 performs constant voltage control on the first DC/DC converter 11 so that the voltage detected by the voltage sensor 17 coincides with the target voltage value (350 V).
In the second control method, the control unit 21 performs constant voltage control on the first DC/DC converter 11 so that the difference voltage between the voltage detected by the voltage sensor 17 and the voltage detected by the voltage sensor 18 matches the target voltage value (175 V).

Here, the second DC/DC converter 22 only requires a small step-down from 200V to 175V, so the DC reactor 220 can be made smaller, and this allows the power conversion device 1 to be made more compact. In addition, since the maximum voltage that can be applied to the switching elements 221 and 222 is 175V, the withstand voltage of the switching elements 221 and 222 only needs to be 175V or higher, which is lower than the withstand voltage of 350V. This reduces power loss due to switching and also reduces the cost of parts.

(Third embodiment: two-stage boost)
Fig. 4 is a circuit diagram of the power conversion device 1 and the power supply device 100 according to the third embodiment. In Fig. 4, the second DC/DC converter 32 is connected to both ends of the DC power source 2. The input side of the first DC/DC converter 31 is connected to the intermediate potential line 16c which is the output line of the second DC/DC converter 32. The output side of the first DC/DC converter 31 is connected to the first line 16a. The input/output common line of the first DC/DC converter 31 is the second line 16b. The voltage of the DC power source 2 is, for example, 50V.

The first DC/DC converter 31 includes a DC reactor 310, a high-side switching element 311, and a low-side switching element 312, which are connected as shown. The second DC/DC converter 32 includes a DC reactor 320, a high-side switching element 321, and a low-side switching element 322, which are connected as shown.

The DC bus 16, high-potential side capacitor 13, low-potential side capacitor 14, voltage sensors 17 and 18, and inverter 15 are the same as those in the first embodiment, so their description is omitted.

4, the second DC/DC converter 32 boosts the input voltage from the DC power source 2 and outputs it at 175 V, and the line voltage between the intermediate potential line 16c and the second line 16b is smoothed by the low-potential side capacitor 14. The first DC/DC converter 31 boosts the input voltage from the intermediate potential line 16c and outputs it at 350 V when the second line 16b is used as a reference, and boosts and outputs it at 175 V when the intermediate potential line 16c is used as a reference. The line voltage between the first line 16a and the second line 16b is smoothed by the high-potential side capacitor 13 and the low-potential side capacitor 14. When viewed between the three lines of the DC bus 16, both the first DC/DC converter 31 and the second DC/DC converter 32 output 175 V between the intermediate potential line 16c.

The second DC/DC converter 32 boosts 50 V to 175 V. The first DC/DC converter 31 boosts the voltage of 175 V input from the second DC/DC converter 32 to 350 V. In this case, the second DC/DC converter 32 boosts the voltage to 125 V, and the first DC/DC converter 31 boosts the voltage to 175 V.

During independent operation, the control unit 21 performs constant voltage control on the second DC/DC converter 32 so that the actual output voltage of the second DC/DC converter 32 detected by the voltage sensor 18 coincides with the target voltage value. There are two ways of controlling the first DC/DC converter 31 .
In the first control method, the control unit 21 performs constant voltage control on the first DC/DC converter 31 so that the voltage detected by the voltage sensor 17 coincides with the target voltage value (350 V).
In the second control method, the control unit 21 performs constant voltage control on the first DC/DC converter 31 so that the difference voltage between the voltage detected by the voltage sensor 17 and the voltage detected by the voltage sensor 18 matches the target voltage value (175 V).

By performing the above-described two-stage boost, the boost ratio of each stage is reduced compared to boosting the voltage of the DC power source 2 to 350V in one go. This allows the DC reactors 310, 320 to be miniaturized, thereby enabling the power conversion device 1 to be made more compact. In addition, since the maximum voltage that can be applied to the switching elements 321, 322 is 175V, the withstand voltage of the switching elements 321, 322 only needs to be 175V or higher, which is lower than the withstand voltage of 350V. This reduces power loss due to switching and also reduces the cost of parts.

(Fourth embodiment: two DC power supplies)
Fig. 5 is a circuit diagram of the power conversion device 1 and the power supply device 100 according to the fourth embodiment. In Fig. 5, the DC power supply 2 is composed of a DC power supply 2A and a DC power supply 2B. The first DC/DC converter 11 is connected to both ends of the DC power supply 2A. The second DC/DC converter 12 is connected to both ends of the DC power supply 2B. The voltage V _SB [V] across the DC power supply 2B has a relationship of V _SB < 175. For example, the voltage V _SB is 100V. The voltage V _SA [V] across the DC power supply 2A has a relationship of V _SA < 350. For example, the voltage V _SA is 200V.

The first DC/DC converter 11 includes a DC reactor 110, a high-side switching element 111, and a low-side switching element 112, which are connected as shown. The second DC/DC converter 12 includes a DC reactor 120, a high-side switching element 121, and a low-side switching element 122, which are connected as shown.

The DC bus 16, high-potential side capacitor 13, low-potential side capacitor 14, voltage sensors 17 and 18, and inverter 15 are the same as those in the first embodiment, so their description is omitted.

In FIG. 5, the second DC/DC converter 12 boosts the input voltage from the DC power source 2B and outputs it at 175V, and the low-potential side capacitor 14 smoothes the line voltage between the intermediate potential line 16c and the second line 16b. The first DC/DC converter 11 boosts the input voltage from the DC power source 2A and outputs it at 350V, and the high-potential side capacitor 13 and the low-potential side capacitor 14 smooth the line voltage between the first line 16a and the second line 16b. When viewed between the three lines of the DC bus 16, both the first DC/DC converter 11 and the second DC/DC converter 12 output 175V between the intermediate potential line 16c.

During independent operation, the control unit 21 performs constant voltage control on the second DC/DC converter 12 so that the actual output voltage of the second DC/DC converter 12 detected by the voltage sensor 18 coincides with the target voltage value. There are two ways of controlling the first DC/DC converter 11.
In the first control method, the control unit 21 performs constant voltage control on the first DC/DC converter 11 so that the voltage detected by the voltage sensor 17 coincides with the target voltage value (350 V).
In the second control method, the control unit 21 performs constant voltage control on the first DC/DC converter 11 so that the difference voltage between the voltage detected by the voltage sensor 17 and the voltage detected by the voltage sensor 18 matches the target voltage value (175 V).

Here, if DC power sources 2A, 2B with voltages as close as possible to the target voltage value are used, the step-up ratio of both the first DC/DC converter 11 and the second DC/DC converter 12 can be reduced. If the step-up ratio can be reduced, the DC reactors 110, 120 can be made smaller, and the power conversion device 1 can be made more compact. In addition, since the maximum voltage that can be applied to the switching elements 121, 122 is 175V, the withstand voltage of the switching elements 121, 122 only needs to be 175V or higher, and a withstand voltage lower than 350V will suffice. This reduces power loss due to switching and also reduces the cost of parts.

In generalized terms, if a first DC voltage which is a target voltage value of the line voltage between the first line 16a and the second line 16b is _V1 , a second DC voltage which is a target voltage value of the line voltage between the intermediate potential line 16c and the second line 16b is _V2 , the power supply voltage of the first DC power supply 2A is _VSA , and the power supply voltage of the second DC power supply 2B is _VSB , then:
V _SB <V _SA <V ₂ <V ₁ , or
V _SB <V ₂ <V _SA <V ₁ ,
5 is suitable when the relationship between the first DC/DC converter 11 and the second DC/DC converter 12 and the DC reactors 110 and 120 is satisfied. In this case, the step-up ratios of the first DC/DC converter 11 and the second DC/DC converter 12 can be suppressed, power loss can be reduced, and the DC reactors 110 and 120 can be made smaller.

(Fifth embodiment: combination with other DC power supplies)
Fig. 6 is a circuit diagram of the power conversion device 1 and the power supply device 100 according to the fifth embodiment. In Fig. 6, a first DC/DC converter 11 is connected to both ends of a DC power source 2 which is a storage battery. A second DC/DC converter 12 is also connected to both ends of the DC power source 2. The voltage V _S [V] across the DC power source 2 satisfies the relationship V _S < 175. For example, the voltage V _S is assumed to be 100V.

The first DC/DC converter 11 includes a DC reactor 110, a high-side switching element 111, and a low-side switching element 112, which are connected as shown. The second DC/DC converter 12 includes a DC reactor 120, a high-side switching element 121, and a low-side switching element 122, which are connected as shown.

The solar panel 19 is connected to the third DC/DC converter 23 via a diode 20 for preventing reverse current. The third DC/DC converter 23 includes a DC reactor 230, a high-side switching element 231, a low-side switching element 232, and a smoothing capacitor 233, which are connected as shown in the figure. The output of the third DC/DC converter 23 is connected between the first line 16a and the second line 16b of the DC bus 16. The other circuit configurations are the same as those in FIG. 2, so the description will be omitted.

In FIG. 6, the second DC/DC converter 12 boosts the input voltage from the DC power source 2 and outputs it at 175V, and the low-potential side capacitor 14 smoothes the line voltage between the intermediate potential line 16c and the second line 16b. The first DC/DC converter 11 boosts the input voltage from the DC power source 2 and outputs it at 350V, and the high-potential side capacitor 13 and the low-potential side capacitor 14 smooth the line voltage between the first line 16a and the second line 16b. Looking at the three lines of the DC bus 16, both the first DC/DC converter 11 and the second DC/DC converter 12 output 175V between the intermediate potential line 16c. The third DC/DC converter 23 boosts the generated voltage output from the solar power generation panel 20 to 350V and supplies it between the first line 16a and the second line 16b of the DC bus 16.

The control unit 21 controls the second DC/DC converter 12 so that the actual output voltage of the second DC/DC converter 12 detected by the voltage sensor 18 coincides with the target voltage value. There are two ways of controlling the first DC/DC converter 11.
In the first control method, the control unit 21 controls the first DC/DC converter 11 so that the voltage detected by the voltage sensor 17 coincides with the target voltage value (350 V).
In the second control method, the control unit 21 controls the first DC/DC converter 11 so that the difference voltage between the voltage detected by the voltage sensor 17 and the voltage detected by the voltage sensor 18 matches the target voltage value (175 V).

6, the power supplied from the third DC/DC converter 23 to the DC bus 16 can be supplied to the load connected to the inverter 15. When surplus power occurs in relation to the load connected to the inverter 15, the first DC/DC converter 11 can be operated in the reverse direction as a step-down chopper using the power output from the third DC/DC converter 23 to charge the DC power source 2, which is a storage battery.
In addition, a DC load can also be connected to the DC bus 16.

"verification"
(Reference example)
For reference, FIG. 7 is a graph showing the voltage and current when a single-phase two-wire independent output is output as is between the U-line and W-line of a single-phase three-wire by one DC/DC converter and an inverter, and power is supplied to a load that is extremely unbalanced in the U-phase (U-line-O line) and W-phase (W-line-O line). The load is a resistive load connected to the U-phase, and a parallel body of a resistor and a capacitor connected in series to a diode connected to the W-phase. The voltage of the DC power supply is 200V, which is boosted to 350V by the DC/DC converter. The intermediate potential line of the DC bus is a pair of series bodies of capacitors placed between the two lines of the DC bus, and the interconnection point is set as the intermediate potential line. That is, the potentials of the three lines of the DC bus are 350V, 175V, and 0V, from the top. The target AC output voltage is 200V (effective value) between the U-line and the W-line. The horizontal axis of the three graphs at the top, middle, and bottom is a common time axis, which corresponds to three cycles (0.06 seconds) of an alternating current (50 Hz).

In the upper graph, Vdc1 is the voltage between the first line and the mid-potential line of the DC bus, and Vdc2 is the voltage between the mid-potential line and the second line. Ideally, both should be 175 V, but both are significantly different from 175 V.
In the middle graph, Vuo is the U-phase voltage, and Vwo is the W-phase voltage. The centers of oscillation of both Vuo and Vwo deviate from 0.
In the lower graph, Iu is the current flowing through line U, and Iw is the current flowing through line W. The centers of oscillation for both Iu and Iw deviate from 0.

(Power conversion device of the first embodiment (FIG. 2))
Next, the power conversion device of the first embodiment will be examined as a representative embodiment.
FIG. 8 is a graph showing the voltage and current when the power conversion device 1 of the first embodiment (FIG. 2) is used to output an independent output between the U-line and W-line of a single-phase three-wire and supply power to a load that is extremely unbalanced in the U-phase (U-line-O line) and the W-phase (W-line-O line). As with the above-mentioned reference example, the load is a resistive load connected to the U-phase, and a diode connected in series to a parallel body of a resistor and a capacitor connected to the W-phase. The voltage of the DC power supply is 100V, which is boosted to 350V/175V by the first and second DC/DC converters 11 and 12. The target output voltage of the AC is 100V (effective value) between the U-line and O-line, and 100V (effective value) between the W-line and O-line. The horizontal axis of the three graphs in the upper, middle, and lower stages is a common time axis, and corresponds to three cycles (0.06 seconds) of AC (50 Hz).

8, Vdc1 in the upper graph is the voltage between the first line 16a and the intermediate potential line 16c of the DC bus 16, and Vdc2 is the voltage between the intermediate potential line 16c and the second line 16b. Both are controlled to approximately 175 V.
In the middle graph, Vuo is the U-phase voltage, and Vwo is the W-phase voltage. Both Vuo and Vwo are sine waves with the center of oscillation at zero.
In the lower graph, Iu is the current flowing through the U line, and Iw is the current flowing through the W line. Iu is a sine wave with the center of oscillation at 0. Because of the presence of the diode, Iw has a waveform that changes every half cycle, but it can be seen that it is a half-wave rectified sine wave with the center of oscillation at 0.

As described above, favorable results have been obtained, and similarly favorable results are obtained for other embodiments.

(How to set the voltage target value)
Next, the difference depending on the method for setting the voltage target value will be examined. As described in the above embodiments, there are the following two types of methods for setting the voltage target value.
(a) The control unit 21 performs constant voltage control on the first DC/DC converter 11 so that the voltage detected by the voltage sensor 17 coincides with the target voltage value (350 V).
(b) The control unit 21 performs constant voltage control on the first DC/DC converter 11 so that the difference voltage between the voltage detected by the voltage sensor 17 and the voltage detected by the voltage sensor 18 coincides with the target voltage value (175 V).

Figure 9 is a graph showing the voltage and current when the power conversion device 1 of the first embodiment is used to provide a single-phase three-wire AC output to a balanced load in autonomous operation by performing the control described above in (a). The horizontal axis of the top, middle, and bottom graphs is a common time axis, which corresponds to three cycles of AC (0.06 seconds). The top graph has an expanded vertical scale to exaggerate the fluctuations.

9, Vdc1 in the upper graph is the voltage between the first line 16a and the intermediate potential line 16c of the DC bus 16, and Vdc2 is the voltage between the intermediate potential line 16c and the second line 16b. Both fluctuate around 175 V. Since it is three cycles, the fluctuation is noticeable, but there is no particular problem if it is averaged over a longer period.
In the middle graph, Vuo is the U-phase voltage, and Vwo is the W-phase voltage. Both Vuo and Vwo are sine waves with the center of oscillation at zero.
In the lower graph, Iu is the current flowing through line U, and Iw is the current flowing through line W. Both Iu and Iw are sine waves with the center of oscillation at zero.

Figure 10 is a graph showing voltage and current when the power conversion device 1 of the first embodiment is used to provide a single-phase three-wire AC output to a balanced load in autonomous operation by performing the control described above in (b). The horizontal axis of the top, middle, and bottom graphs is a common time axis, and corresponds to three cycles (0.06 seconds) of AC (50 Hz). The top graph has an expanded vertical scale to exaggerate the fluctuations.

In Fig. 10, Vdc1 in the upper graph is the voltage between the first line 16a and the intermediate potential line 16c of the DC bus 16, and Vdc2 is the voltage between the intermediate potential line 16c and the second line 16b. Both fluctuate around 175V. As in Fig. 9, the fluctuations are somewhat noticeable because it is three cycles, but there is no particular problem if the fluctuations are averaged over a longer period. In comparison with Fig. 9, the fluctuations are more suppressed in Fig. 10.
In the middle graph, Vuo is the U-phase voltage, and Vwo is the W-phase voltage. Both Vuo and Vwo are sine waves with the center of oscillation at zero.
In the lower graph, Iu is the current flowing through line U, and Iw is the current flowing through line W. Both Iu and Iw are sine waves with the center of oscillation at zero.

(When used in conjunction with charging)
11 is a graph showing voltages and currents when a single-phase three-wire AC output is provided to a balanced load in a stand-alone operation using the power conversion device 1 of the fifth embodiment (FIG. 6). The horizontal axis of the upper, middle, and lower three graphs is a common time axis, and corresponds to three cycles (0.06 seconds) of AC (50 Hz).

10, Vdc1 in the upper graph is the voltage between the first line 16a and the intermediate potential line 16c of the DC bus 16, and Vdc2 is the voltage between the intermediate potential line 16c and the second line 16b. It can be seen that both of them fluctuate slightly around 175 V, and are maintained almost constant.
In the middle graph, Vuo is the U-phase voltage, and Vwo is the W-phase voltage. Both Vuo and Vwo are sine waves with the center of oscillation at zero.
In the lower graph, Iu is the current flowing through the U line, Iw is the current flowing through the W line, and Ibat is the charging current to the DC power source 2. Charging is indicated by a negative value. Although it swings slightly to the positive side, it is essentially a pulsating DC current. The pulsation is influenced by the AC current and has a period twice that of commercial AC.

(Sixth embodiment: modified example)
In all of the first to fifth embodiments, two DC/DC converters are used to output the voltage of the first line 16a as seen from the second line 16b and the voltage of the intermediate potential line 16c, but it is also possible to output the voltage of the second line 16b as seen from the first line 16a and the voltage of the intermediate potential line 16c.
Fig. 12 is a circuit diagram of the power conversion device 1 and the power supply device 100, which can be considered as a modification of the second embodiment (Fig. 3). In Fig. 12, the first DC/DC converter 11 is connected to both ends of the DC power source 2. The second DC/DC converter 42 is connected between the first line 16a and the positive terminal of the DC power source 2. A capacitor 43 is provided between the first line 16a and the positive terminal of the DC power source 2 to stabilize the voltage between the first line 16a and the positive terminal of the DC power source 2. The voltage V _S across the DC power source 2 is, for example, 100V.

The first DC/DC converter 11 includes a DC reactor 110, a high-side switching element 111, and a low-side switching element 112, which are connected as shown. The second DC/DC converter 42 includes a DC reactor 420, a high-side switching element 421, and a low-side switching element 422, which are connected as shown.

The DC bus 16, high-potential side capacitor 13, low-potential side capacitor 14, voltage sensors 17 and 18, and inverter 15 are the same as those in the first embodiment, so their description is omitted.

In FIG. 12, the first DC/DC converter 11 boosts the input voltage from the DC power source 2 and outputs it at 350V, and the high-potential side capacitor 13 and the low-potential side capacitor 14 smooth the line voltage between the first line 16a and the second line 16b. The second DC/DC converter 42 steps down the difference voltage (250V) between the first line 16a and the positive terminal of the DC power source 2 and outputs it at 175V, and the high-potential side capacitor 13 smooths the line voltage between the intermediate potential line 16c and the first line 16a. When viewed between the three lines of the DC bus 16, both the first DC/DC converter 11 and the second DC/DC converter 22 output 175V between the intermediate potential line 16c.

The control during the independent operation is similar to that in the second embodiment.
Since the second DC/DC converter 42 only needs to step down the voltage from 250V to 175V, the DC reactor 420 can be made smaller, which enables the power conversion device 1 to be made more compact. Furthermore, since the maximum voltage that can be applied to the switching elements 421, 422 is 250V, the withstand voltage of the switching elements 421, 422 only needs to be 250V or higher, and a withstand voltage lower than 350V will suffice. This reduces power loss due to switching, and also reduces the cost of components.

Summary of disclosure
What has been disclosed above is a power conversion device 1 provided between a DC power source 2 and a single-phase three-wire AC circuit 3. This power conversion device 1 includes an inverter 15 that provides an AC output to the AC circuit 3, a DC bus 16 on the DC side of the inverter 15 and having, in order of potential, a first line 16a, an intermediate potential line 16c, and a second line 16b, the first line 16a and the second line 16b being DC input lines to the inverter 15, and the intermediate potential line 16c being connected to the neutral line (O line) of the AC circuit 3, a high-potential side capacitor 13 provided between the first line 16a and the intermediate potential line 16c, and a high-potential side capacitor 13 provided between the first line 16a and the intermediate potential line 16c. The power supply circuit includes a low-potential side capacitor 14 provided between the first line 16c and the second line 16b, a first DC/DC converter 11 (31) provided between the DC power supply 2 and the DC bus 16, a second DC/DC converter 12 (22, 32, 42) provided between the DC power supply 2 and the DC bus 16, and a control unit 21 that controls the inverter 15 as well as the first DC/DC converter 11 (31) and the second DC/DC converter 12 (22, 32, 42).

The first DC/DC converter generates a first DC voltage (e.g., 350 V) based on the input voltage from the DC power source 2 and outputs it between the first line 16a and the second line 16b. The second DC/DC converter converts the input voltage from the DC power source 2 to a second DC voltage (e.g., 175 V) and outputs it between either the first line 16a or the second line 16b and the intermediate potential line 16c.

In such a power conversion device 1, the first DC/DC converter 11 and the second DC/DC converter 12 each output a predetermined DC voltage, thereby maintaining the potential difference between the three wires (first wire 16a, intermediate potential wire 16c, second wire 16b) of the DC bus 16 at a desired value, and providing a stable single-phase three-wire output even during independent operation. The second DC/DC converter 12 outputs a lower voltage than the first DC/DC converter 11, so that the lower voltage makes it possible to reduce power loss and achieve compactness. In this way, in the power conversion device 1 that provides a single-phase three-wire output during independent operation, it is possible to reduce power loss and achieve compactness.

The first DC/DC converter 11 can be controlled by feedback control to adjust the detected value of the first DC voltage to a voltage target value (e.g., 350 V), or by feedback control to adjust the difference voltage between the first DC voltage and the second DC voltage to a voltage target value (e.g., 175 V).

The circuit configuration of the first DC/DC converter 11 and the second DC/DC converter 12 can be varied in various ways depending on the voltage of the DC power supply 2. As mentioned above, two sets of boost (Fig. 2), boost and step-down (Fig. 3), two-stage boost (Fig. 4), and boost with separate DC power supplies (Fig. 5) are possible. In addition, two sets of step-down are also possible when the voltage of the DC power supplies is high. When separate DC power supplies are provided, one may be step-up and the other step-down.

Note that in the above embodiments, the DC bus 16 is described in order of increasing potential as the first line, the intermediate potential line, and the second line. However, the terms "first line" and "second line" are used for convenience, and the DC bus 16 may be described in order of decreasing potential as the first line, the intermediate potential line, and the second line.

《Addendum》
The embodiments disclosed herein should be considered to be illustrative and not restrictive in all respects. The scope of the present invention is defined by the claims, and it is intended to include all modifications within the meaning and scope of the claims.

LIST OF SYMBOLS 1 Power conversion device 2, 2A, 2B DC power supply 3 AC circuit 11 First DC/DC converter 12 Second DC/DC converter 13 High potential side capacitor 14 Constant potential side capacitor 15 Inverter 16 DC bus 16a First line 16b Second line 16c Intermediate line 17, 18 Voltage sensor 19 Photovoltaic power generation panel 20 Diode 21 Control unit 22 Second DC/DC converter 23 Third DC/DC converter 31 First DC/DC converter 32 Second DC/DC converter 42 Second DC/DC converter 43 Capacitor 100 Power supply device 110 DC reactor 111, 112 Switching element 120 DC reactor 121, 122 Switching element 151, 152, 153, 154 Switching element 155, 156 AC reactor 157, 158 Capacitor 220 DC reactor 221, 222 Switching element 230 DC reactor 231, 232 Switching element 233 Capacitor 310 DC reactor 311, 312 Switching element 320 DC reactor 321, 322 Switching element 420 DC reactor 421, 422 Switching element

Claims (7)

A power conversion device provided between a DC power source and a single-phase three-wire AC circuit,
an inverter for providing an AC output to the AC power path;
a DC bus on a DC side of the inverter, the DC bus having, in order of potential, a first wire, an intermediate potential wire, and a second wire, the first wire and the second wire being DC input wires to the inverter, and the intermediate potential wire being connected to a neutral wire of the AC circuit;
a first capacitor provided between the first line and the intermediate potential line;
a second capacitor provided between the intermediate potential line and the second line;
a first DC/DC converter provided between the DC power source and the DC bus, converting a predetermined input voltage to generate a first DC voltage and outputting the first DC voltage between the first line and the second line;
a second DC/DC converter provided between the DC power source and the DC bus, converting an input voltage from the DC power source into a second DC voltage and outputting the second DC voltage between one of the first line and the second line and the intermediate potential line;
a control unit that controls the inverter, the first DC/DC converter, and the second DC/DC converter;
Equipped with
the second DC/DC converter boosts a voltage of the DC power supply to generate the second DC voltage;
The first DC/DC converter is a power conversion device that boosts the second DC voltage to the first DC voltage . A power conversion device provided between a DC power source and a single-phase three-wire AC circuit,
an inverter for providing an AC output to the AC power path;
a DC bus on a DC side of the inverter, the DC bus having, in order of potential, a first wire, an intermediate potential wire, and a second wire, the first wire and the second wire being DC input wires to the inverter, and the intermediate potential wire being connected to a neutral wire of the AC circuit;
a first capacitor provided between the first line and the intermediate potential line;
a second capacitor provided between the intermediate potential line and the second line;
a first DC/DC converter provided between the DC power source and the DC bus, generating a first DC voltage based on an input voltage from the DC power source and outputting the first DC voltage between the first line and the second line;
a second DC/DC converter provided between the DC power source and the DC bus, converting an input voltage from the DC power source into a second DC voltage and outputting the second DC voltage between one of the first line and the second line and the intermediate potential line;
a control unit that controls the inverter, the first DC/DC converter, and the second DC/DC converter;
Equipped with
When the DC power supply includes a first storage battery and a second storage battery, and the voltage of the first storage battery is higher than the voltage of the second storage battery,
the first DC/DC converter is connected to the first storage battery, and the second DC/DC converter is connected to the second storage battery;
The first DC/DC converter boosts a voltage of the first storage battery to the first DC voltage;
The second DC/DC converter is a power conversion device that boosts a voltage of the second storage battery to the second DC voltage . The control unit during independent operation is
The first DC/DC converter is subjected to constant voltage control so that the first DC voltage is output between the first line and the second line; and
performing constant voltage control on the second DC/DC converter so that the second DC voltage is output between the intermediate potential line and the second line or the first line;
The power conversion device according to claim 1 or 2 . The control unit during independent operation is
performing constant voltage control on the first DC/DC converter so that a difference voltage between the first DC voltage and the second DC voltage is output between the first line and the intermediate potential line; and
performing constant voltage control on the second DC/DC converter so that the second DC voltage is output between the intermediate potential line and the second line;
The power conversion device according to claim 1 or 2 . When the first DC voltage is V ₁ , the second DC voltage is V ₂ , the power supply voltage of the first storage battery is V _SA , and the power supply voltage of the second storage battery is V _SB ,
V _SB <V _SA <V ₂ <V ₁ , or
V _SB <V ₂ <V _SA <V ₁ ,
The power conversion device according to claim 2, wherein the following relationship is satisfied: The power conversion device according to claim 1 , wherein the second DC voltage is (50±10)% of the first DC voltage. A power supply device comprising: the power conversion device according to claim 1 ; and the DC power supply.

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