JP3217212B2 - Inverter - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention converts DC power generated by a solar cell or the like into AC power, and connects it to a single-phase three-wire distribution line of an existing commercial system and connects it to the distribution line. The present invention relates to an inverter that supplies alternating-current power to a specified load and that supplies excess power to a commercial power system in a reverse flow.

[0002]

2. Description of the Related Art Conventionally, as an inverter, as shown in FIG. 7, an input terminal is connected to a DC voltage output terminal of a solar cell 31, and a switching element for converting a DC voltage input to the input terminal into an AC voltage. Q ₁₁ to Q an inverter bridge 32 which is composed of _14, the inverter bridge 3
Reactor L ₁₁ having one end connected to one of the output terminals of L 2
And the other end of the reactor L ₁₁ inverter bridge 32
It is provided with a low-pass filter 33 comprising a capacitor C ₁₁ Metropolitan provided between the other output terminal of the.
Then, connect one of the primary-side input terminal of the power frequency transformer 34 through a connector relay 35 to the other end of the reactor L ₁₁ of the low-pass filter 33, an inverter bridge 32
The other output terminal of the commercial frequency transformer 34 is connected to the other input terminal of the commercial frequency transformer 34. Incidentally, between the input terminals of the inverter bridge 32 connects the input capacitor C _10. Then, the secondary output terminal of the commercial frequency transformer 34 is connected to the commercial power supply 3 connected in series with the commercial system 40.
6, 37 are connected to both ends via voltage lines u, v of a single-phase three-wire distribution line. The neutral point between the commercial power supplies 36 and 37 is connected to the ground GND.
The intermediate tap of the commercial frequency transformer 34 is connected to the neutral point 7 through a neutral line n.

In the inverter IV3 having the above configuration, the DC power supplied from the solar cell 31 is supplied to the commercial power source 36,
The power is converted into AC (Pulse Width Modulation) controlled AC power by the inverter bridge 32 so as to synchronize with the frequency and phase of the power 37, and the interconnection relay 35 and the commercial frequency transformer 34
(Not shown) connected to single-phase three-wire distribution line via
To supply power.

[0004] As described above, the inverter IV3 converts the DC power from the solar cell 31 into AC power by the inverter bridge 32, and has a two-wire primary-side input and a single-phase three-wire output secondary-side commercial transformer. The power supply is connected to a single-phase three-wire distribution line, which is the existing commercial system 40, to supply power to the load and also to perform reverse power flow to the commercial system 40. However, in the case of the inverter IV3, 3 kVA
Since the commercial frequency transformer 34 to 5 kVA is required, the inverter has a very large external shape and is very heavy and difficult to handle.

Therefore, as another inverter, there is an inverter IV4 which is reduced in size and weight by using a high frequency transformer as shown in FIG. The inverter IV4 includes a solar cell 41, a DC voltage output terminal of the solar cell 41 is connected to the input terminal, a high-frequency inverter bridge 42 composed of a switching element Q ₂₁ to Q _24, the output of the high frequency inverter bridge 42 A high-frequency transformer 43 having a primary-side input terminal connected to the terminal;
The diode D ₁₁ to D ₁₄ diode bridge 44 composed of a full-wave rectifying an AC voltage on a DC voltage from the secondary side output terminal, the reactor while the one end of which is connected to the output terminal of the diode bridge 44 L _12, one input terminal to the other end of the reactor L ₁₂ is connected, the other input terminal connected to the other output terminal of the diode bridge 44, commercial frequency inverter having switching elements S ₁₁ to S ₁₄ a bridge 46, which is connected between the other output terminal of the power-frequency inverter bridge reactor L ₁₃ having one end connected to one output terminal 46 and the other end commercial frequency inverter bridge 46 of the reactor L ₁₃ and a low-pass filter 47 comprising a capacitor C ₁₃ Prefecture. A commercial power supply 48 is connected to the output terminal of the low-pass filter 47 via voltage lines u and v of a single-phase two-wire distribution line of the commercial system 50. Incidentally, between the input terminals of the inverter bridge 42 connects the input capacitor C _12. In addition, between the commercial frequency inverter bridge 46 and the low-pass filter 47,
A circuit interconnection relay 49 is provided.

In the inverter IV4 having the above-described configuration, the DC power from the solar cell 41 is supplied to the high-frequency inverter bridge 42 via the input capacitor 4, and is converted into a PWM-controlled high-frequency AC voltage. Thereafter, the high-frequency AC voltage from the high-frequency inverter bridge 42 is transformed by the high-frequency transformer 43 and once rectified by the diode bridge 44 to form a high-frequency pulsating flow. Furthermore, high-frequency pulsating current rectified by the diode bridge 44, the high-frequency component is cut by the inductor L _12, is formed into a full-wave rectified waveform of the commercial frequency. Then, by the return control by the commercial frequency inverter bridge 46,
A sine wave voltage of a commercial frequency can be obtained. After the sine wave voltage from the output terminal of the diode bridge 44 is waveform-shaped by the low-pass filter 47 via the interconnection relay 49, the sine wave voltage is interconnected with the commercial system 50 by a single-phase two-wire distribution line.

Thus, in the inverter IV4,
As a method of insulating the DC side of the solar cell 41 and the AC side of the commercial system, the high-frequency transformer 43 is used instead of the commercial frequency transformer 34 of the inverter IV3, so that the size and weight can be significantly reduced. A volume ratio of about 1/4 (capacity 0.067 m ³ → capacity 0.017 m ³ ) and a weight ratio of about 1/4 (weight 67 kg → weight 17 kg) are realized.

[0008]

In the inverter IV4 shown in FIG. 8, the commercial frequency transformer 34 shown in FIG.
Instead of using the high-frequency transformer 43, the inverter could be reduced in size and weight.
At the time of interconnection with the commercial system, the inverter IV4 has only two output lines and cannot be connected to the neutral line of the single-phase three-wire distribution line. There is a problem that can not be. That is, in order to connect to a single-phase three-wire type distribution line, it is basically required that the inverters be connected by the same single-phase three-wire system.

Accordingly, an object of the present invention is to realize a compact and lightweight inverter by using a high-frequency transformer, and to use a single-phase three-wire distribution system, which is a power distribution system in which most recent newly built houses receive power. An object of the present invention is to provide an inverter that can easily connect electric wires to the same electric system.

Another object of the present invention is to provide an inverter capable of continuing interconnection operation without stopping the inverter even when load imbalance occurs in the single-phase three-wire distribution line. It is in.

[0011]

In order to achieve the above object, an inverter according to claim 1 converts DC power supplied from a DC power supply into AC power, and converts the DC power into two voltage lines and a neutral line of a commercial system. A first power conversion unit that converts the DC power from the DC power supply into AC power in an interconnection type inverter that supplies power to a single-phase three-wire distribution line having:
The AC voltage from the first power conversion unit is transformed, and the transformed AC voltage is output from a secondary output terminal, and is provided at a substantially middle point of a winding of the secondary output terminal. A transformer in which an intermediate tap is connected to a neutral wire of the single-phase three-wire type distribution line, and a second power converter that converts AC power from a secondary output terminal of the transformer into DC power, A third power converter for converting DC power from the second power converter to AC power, and having output terminals respectively connected to both voltage lines of the single-phase three-wire distribution line; A current detector that is provided between a secondary-side intermediate tap and a neutral wire of the single-phase three-wire distribution line and detects a transient current; and a current detector that detects a transient current based on a direction of the transient current detected by the current detector. In order to prevent the transient current from flowing, the third power converter is used to convert DC power into AC power. It is characterized in that the switching elements provided on the parts and a first control unit for on-off control.

Further, the inverter of the present invention converts DC power supplied from a DC power supply into AC power, and converts the DC power into a single-phase three-wire type distribution line having two voltage lines and a neutral line of a commercial system. In an interconnection type inverter that supplies power, a first power conversion unit that converts the DC power from the DC power supply into AC power, and transforms an AC voltage from the first power conversion unit, The transformed AC voltage is output from the secondary output terminal, and an intermediate tap provided at a substantially middle point of the winding of the secondary output terminal is connected to the neutral wire of the single-phase three-wire distribution line. Transformer, a second power converter for converting AC power from a secondary output terminal of the transformer to DC power, and a high-frequency component superimposed on the DC voltage from the second power converter. And a low-pass filter that removes A third power conversion unit that converts a flow power into an AC power, and an output terminal is connected to both voltage lines of the single-phase three-wire distribution line, an intermediate tap on a secondary side of the transformer, and the single-phase three-wire distribution line. A current detector provided between the neutral wire of the phase three-wire type distribution line and detecting the transient current; and preventing the transient current from flowing based on a direction of the transient current detected by the current detector. And a first control unit that controls on / off of a switching element provided in the third power conversion unit in order to convert DC power into AC power.

[0013] The inverter according to claim 3 is the inverter according to claim 2.
In the inverter, the low-pass filter is connected between a reactor having one end connected to both DC voltage output terminals of the second power conversion unit, and another end of the reactor and an intermediate tap of the transformer, respectively. And a capacitor.

Further, the inverter according to claim 4 is the inverter according to claim 1.
In any one of the above-mentioned inverters, a voltage detector for detecting a line voltage between a neutral line and both voltage lines of the single-phase three-wire distribution line; an intermediate tap of the transformer; A switch provided between the phase three-wire distribution line and the neutral wire,
The switch is opened when the transient current detected by the current detector is equal to or more than a predetermined value, while the switch is opened when the voltage difference between the line voltages detected by the voltage detector is less than a predetermined value. And a second control unit that controls so as to be closed.

[0015] The inverter according to claim 5 converts DC power supplied from a DC power supply into AC power to form a single-phase three-wire type distribution line having two voltage lines and a neutral line of a commercial system. In an interconnection type inverter that supplies power, a first power conversion unit that converts the DC power from the DC power supply into AC power, and transforms an AC voltage from the first power conversion unit, The transformed AC voltage is output from the secondary output terminal, and an intermediate tap provided at a substantially middle point of the winding of the secondary output terminal is connected to the neutral wire of the single-phase three-wire distribution line. And a second power conversion unit that converts AC power from a secondary output terminal of the transformer into DC power, and converts DC power from the second power conversion unit into AC power. Output terminals are connected to both voltage lines of the single-phase three-wire distribution line, respectively. A power converter, a voltage detector for detecting a line voltage between a neutral line and both voltage lines of the single-phase three-wire distribution line, an intermediate tap of the transformer, and the single-phase three-wire system A switch provided between the neutral line of the distribution line and the switch when the voltage difference between the line voltages detected by the voltage detector is equal to or greater than a predetermined value. A second control unit that controls the switch to close when the value is less than a predetermined value.

[0016]

According to the inverter of the first aspect, the first inverter is provided.
The power converter converts the DC power supplied from the DC power supply into AC power. Then, the transformer transforms the AC voltage from the first power converter, and outputs the transformed AC voltage from the secondary output terminal. Next, the second power converter converts AC power from the secondary output terminal of the transformer into DC power. The third power converter converts the DC power from the second power converter into AC power, and converts the single-phase three-wire distribution line connected to the output terminal of the third power converter. A line voltage is output between both voltage lines. And, since the intermediate tap provided at the approximate middle point of the secondary winding of the transformer is connected to the neutral wire of the single-phase three-wire distribution line,
The third power converter outputs a line voltage between each voltage line and the neutral line. That is, substantially the same line voltage is generated between each voltage line and the neutral line of the single-phase three-wire distribution line, and a voltage approximately twice the line voltage is generated between the two voltage lines. It is.

Therefore, the inverter according to the present invention can be reduced in size and weight by using a high-frequency transformer instead of a commercial two-input, three-output transformer.
With three output lines having three line voltages, interconnection operation with a single-phase three-wire distribution line of a commercial system can be performed.

A transient current flowing in the neutral line is detected by a current detector inserted between the neutral line of the single-phase three-wire type distribution line and the intermediate tap of the transformer. Then, based on the direction of the transient current detected by the current detector, the first control unit controls the transient current not to flow,
On / off of the switching element of the third power converter is controlled. For example, if there is a shift in the on / off time due to the variation in the characteristics of the switching element, the on / off time of the switching element is adjusted, and the shift is corrected, so that the transition to the neutral line of the single-phase three-wire distribution line is performed. Current can be prevented from flowing. Therefore, this inverter
A stable AC voltage with low distortion can be output.

Further, according to the inverter of the second aspect, the first power converter converts the DC power supplied from the DC power supply into AC power. Then, the transformer transforms the AC voltage from the first power converter, and outputs the transformed AC voltage from the secondary output terminal.
Next, the second power converter converts the AC power from the secondary output terminal of the transformer into DC power, and then superimposes the DC power on the DC voltage output from the second power converter by a low-pass filter. To remove high frequency components. The third power converter converts the DC power from the low-pass filter into AC power, and applies a voltage between the two voltage lines of the single-phase three-wire distribution line connected to the output terminal of the third power converter. To output the line voltage. Since the intermediate tap provided substantially at the midpoint of the secondary winding of the transformer is connected to the neutral wire of the single-phase three-wire distribution line, the third power converter is connected to each voltage line. And outputs a line voltage between the neutral line. That is, substantially the same line voltage is generated between each voltage line and the neutral line of the single-phase three-wire distribution line, and a voltage approximately twice the line voltage is generated between the two voltage lines. It is.

Therefore, the inverter of the present invention can be reduced in size and weight by using a high-frequency transformer instead of a commercial transformer having two input wires and three output wires.
With three output lines having three line voltages, interconnection operation with a single-phase three-wire distribution line of a commercial system can be performed.

A transient current flowing through the neutral line is detected by a current detector inserted between the neutral line of the single-phase three-wire type distribution line and the intermediate tap of the transformer. Then, based on the direction of the transient current detected by the current detector, the first control unit controls the transient current not to flow,
On / off of the switching element of the third power converter is controlled. For example, if there is a shift in the on / off time due to the variation in the characteristics of the switching element, the on / off time of the switching element is adjusted, and the shift is corrected, so that the transition to the neutral line of the single-phase three-wire distribution line is performed. Current can be prevented from flowing. Therefore, this inverter
A stable AC voltage with low distortion can be output.

According to the inverter of the third aspect, in the inverter of the second aspect, a reactor having one end connected to each of the DC voltage output terminals of the second power converter and the other end of the reactor. A low-pass filter composed of a capacitor connected between the transformer and an intermediate tap of the transformer is superimposed on each DC voltage between both DC voltage output terminals of the second power converter and the intermediate tap of the transformer. The removed high frequency components are removed.

Accordingly, the low-pass filter having the above-described simple configuration can remove the high-frequency component of the DC voltage from the second power converter and shape the waveform.

According to the inverter of the fourth aspect, in the inverter according to any one of the first to third aspects, the voltage detector is connected to the neutral line of the single-phase three-wire type distribution line and the neutral line. A line voltage between the power supply and the voltage line is detected. The second control unit controls a switch provided between an intermediate tap of the transformer and a neutral line of a single-phase three-wire distribution line to detect a transient current detected by the current detector. It opens when the voltage is equal to or more than a predetermined value, and closes when the voltage difference between the line voltages detected by the voltage detector is less than a predetermined value. Therefore,
When the load connected between the neutral line and the two voltage lines of the single-phase three-wire distribution line is in an equilibrium state, the interconnection operation is performed with the single-phase three-wire distribution line of the commercial system, and Is in an unbalanced state, that is, when the transient current detected by the current detector becomes a predetermined value or more, open the switch,
Only the neutral wire of the single-phase three-wire distribution line is cut off, and the interconnection operation is continued with the single-phase two-wire line connected to only the voltage line of the distribution line. When the voltage difference between the line voltages detected by the voltage detector is less than a predetermined value, it is determined that the load has returned to an equilibrium state, the switch is closed, and the single-phase three-wire distribution line is closed. Connect the neutral wire to the intermediate tap of the transformer, and continue the interconnection operation on the single-phase three-wire distribution line.

Therefore, usually, a single-phase three-wire distribution line and three output lines are connected to each other, so that a connection suitable for a commercial system can be performed. On the other hand, if the load becomes unbalanced, disconnect only the neutral wire of the single-phase three-wire distribution line to prevent damage to switching elements due to unbalanced load and stop system operation. In addition, since the interconnection operation can be continued with a single-phase two-wire system, power can be efficiently supplied from a solar cell to a commercial system.

Further, according to the inverter of the fifth aspect, the first power converter converts DC power supplied from the DC power supply into AC power. Then, the transformer transforms the AC voltage from the first power converter, and outputs the transformed AC voltage from the secondary output terminal.
Next, the second power converter converts AC power from the secondary output terminal of the transformer into DC power. The third power converter converts the DC power from the second power converter into AC power, and converts the single-phase three-wire distribution line connected to the output terminal of the third power converter. A line voltage is output between both voltage lines. Since the intermediate tap provided substantially at the midpoint of the secondary winding of the transformer is connected to the neutral wire of the single-phase three-wire distribution line, the third power converter is connected to each voltage line. And outputs a line voltage between the neutral line. That is, substantially the same line voltage is generated between each voltage line and the neutral line of the single-phase three-wire distribution line, and a voltage approximately twice the line voltage is generated between the two voltage lines. It is.

Therefore, the inverter of the present invention can be reduced in size and weight by using a high-frequency transformer instead of a commercial transformer having two input wires and three output wires, and
With three output lines having three line voltages, interconnection operation with a single-phase three-wire distribution line of a commercial system can be performed.

Further, the line voltage between the neutral line and the two voltage lines of the single-phase three-wire type distribution line is detected. Then, the second control unit sets a switch provided between an intermediate tap of the transformer and a neutral line of a single-phase three-wire distribution line to each line voltage detected by the voltage detector. Open when the voltage difference is equal to or greater than a predetermined value, and close when the voltage difference is less than the predetermined value.
Therefore, when the load connected between the neutral line and the voltage line of the single-phase three-wire distribution line is in an equilibrium state, while the interconnection operation is performed with the single-phase three-wire distribution line of the commercial system, If the load becomes unbalanced, that is, if the voltage difference between the respective line voltages detected by the voltage detectors is equal to or larger than a predetermined value, the switch is opened to open the single-phase three-wire distribution line. Only the sex wires are disconnected, and the interconnection operation is continued with the single-phase two-wires connected with only the voltage wires of the distribution lines. Then, when the voltage difference between the line voltages detected by the voltage detector is less than a predetermined value,
When it is determined that the load has returned to the equilibrium state, the switch is closed, the neutral wire of the single-phase three-wire distribution line is connected to the intermediate tap of the transformer, and the single-phase three-wire distribution line is connected. To continue.

Therefore, usually, a single-phase three-wire distribution line and three output lines are connected to each other so as to be connected to a commercial system. On the other hand, if the load becomes unbalanced, disconnect only the neutral wire of the single-phase three-wire distribution line to prevent damage to switching elements due to unbalanced load and stop system operation. In addition, since the interconnection operation can be continued with a single-phase two-wire system, power can be efficiently supplied from a solar cell to a commercial system.

[0030]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, an inverter according to the present invention will be described in detail with reference to embodiments.

(First Embodiment) FIG. 1 is a diagram showing a main part of an inverter according to a first embodiment of the present invention, wherein 1 is a solar cell, 2 is a DC voltage output terminal of the solar cell 1 and an input terminal is provided. Connected, switching element Q
A high-frequency inverter bridge as a first power converter composed of 1 to Q4 has a primary input terminal connected to the output terminal of the high-frequency inverter bridge 2, and an intermediate tap substantially at the center of the secondary winding. high frequency transformer having four input terminals to the secondary side output terminal of the high frequency transformer 3 is connected, a diode D _1, D _2, D _3, the second diode as a power conversion unit constituted by D ₄ bridge, 5a capacitor C connected between the intermediate tap of the other end and the high-frequency transformer 3 in one reactor L ₁ whose one end is connected to its reactor L ₁ DC output terminals of the diode bridge 4
A low-pass filter consisting of ₁ Tokyo, 5b are reactor L ₂ between the other end and the high-frequency transformer 3 of the reactor L ₂ of one end to the other DC output terminal of the diode bridge 4 is connected
Of the low-pass filter consisting of a capacitor connected C ₂ Metropolitan between the intermediate tap, 6 the low-pass filter 5a,
5b the input terminal to a DC voltage output terminal is connected to a commercial frequency inverter bridge as a third power converting part having switching elements S ₁ to S _4. Reference numeral 7 denotes a three-circuit interconnection relay in which the output terminal of the commercial frequency inverter bridge 6 and the intermediate tap of the high-frequency transformer 3 are connected to each input terminal.
One is connected to one end via a connector relay 7 between the center tap of the other end and the high-frequency transformer 3 of the reactor L ₃ and its reactor L ₃ which is connected via a connector relay 7 of the output terminals a low pass filter comprising a capacitor C ₃ Prefecture was,
8b is communicated between the commercial frequency inverter other intermediate taps of the other end and the high-frequency transformer 3 and the reactor L ₄ reactor L _4, one end of which is connected via a connector relay 7 of the output terminals of the bridge 6 a low-pass filter consisting of a capacitor connected C ₄ Metropolitan through system relay 7. The capacitors C ₁ and C _{2 of the} low-pass filters 5a and 5b
A current detector 9 is provided between a connection point therebetween and the interconnection relay 7. Further, upon receiving a signal indicating an overcurrent from the current detector 9, a gate signal is output to the gate terminals of the switching elements S _{1 to} S ₄ of the commercial frequency inverter bridge 6, and an open / close signal is transmitted to the interconnection relay 7. A control circuit 10 as a first control unit for outputting is provided. Incidentally, by connecting an input capacitor C ₅ between the input terminals of the high frequency inverter bridge 2, by the input capacitor C _5, the high-frequency inverter bridge 2 with respect to fluctuation of the DC voltage output of the solar cell 1 due to changes in insolation Suppress sudden changes in DC voltage input.

[0032] Then, connect the one end of the commercial power source 11 via the voltage line u of the single-phase three-wire distribution line of the grid 30 to the other end of the reactor L ₃ of the low-pass filter 8a.
One end of a commercial power supply 12 is connected to the other end of the commercial power supply 11. On the other hand, it connects the other end of the commercial power supply 12 via the voltage line v of the single-phase three-wire distribution line of the grid 30 to the other end of the reactor L ₄ of the low-pass filter 8b. The neutral point between the single-phase three-wire distribution line and the commercial power supplies 11 and 12 is connected to the ground GND, and the low-pass filter 8 is connected to the ground.
a, by connecting the neutral point of the commercial power source 11 through the neutral line n of single-phase three-wire distribution line to a connection point between the capacitor C _3, C ₄ of 8b, connecting the neutral line n It is connected to the intermediate tap of the high frequency transformer 3 via the system relay 7 and the current detector 9.

In the inverter IV1 having the above-described structure, the control circuit 10 outputs a PWM-modulated gate signal using a high-frequency carrier (20 kHz), and outputs the gate signal to the switching element S _{1 of the} high-frequency inverter bridge 2.
In response to the gate terminals of the to S _4, the high-frequency inverter bridge 2 converts the DC power supplied from the solar cell 1 into AC power. That is, the control circuit 10 generates a sine wave signal having the same phase and the same frequency (50/60 Hz) as the voltage waveform of the commercial system 30 by a sine wave generation circuit (not shown) based on the sine wave and the high frequency carrier. PW
The M-modulated gate signal is output to control the inverter output current. And the high frequency transformer 3 is
The AC voltage from the output terminal of the high-frequency inverter bridge 2 is transformed at a transformation ratio corresponding to a predetermined winding ratio. The high-frequency transformer 3 allows the DC side of the solar cell 1 and the commercial system 30
Is electrically insulated from the AC side. By providing an intermediate tap on the secondary winding of the high-frequency transformer 3, PWM modulation is performed between the intermediate tap and each output terminal of the high-frequency transformer 3 and between both output terminals of the high-frequency transformer 3. The obtained three high-frequency AC voltages are obtained.

The PWM-modulated high-frequency AC voltage from the high-frequency transformer 3 is rectified by a diode bridge 4 at the next stage, and a high-frequency ripple voltage is superimposed thereon as shown in FIGS. 2 (a) to 2 (c). The first line voltage V ₁ , the second line voltage V _2, and the third line voltage V ₃ are DC voltages. Furthermore, the reactor L ₁ and capacitor C ₁ and the reactor L ₂ and the low-pass filter 5a consists of a capacitor C _2, 5b
As a result, as shown in FIGS. 3A to 3C, the first line voltage V
_11, second line voltages V ₁₂ and third line voltage V ₁₃ from the DC voltage waveforms are smoothing the high frequency ripple component, a DC voltage waveform of the sine wave of a commercial frequency has been full-wave rectification. At this time, the peak voltage of the first line voltage V ₁₁ and the second line voltage V ₁₂ is 141 VDC, and the third line voltage V ₁₃ is 2
82 VDC.

Between the first line voltage V ₁₁ , the second line voltage V ₁₂ and the third line voltage V ₁₃ of the outputs of the low-pass filters 5 a and 5 b, V ₁₃ = V ₁₁ + V ₁₂ Have a relationship. That is, since an intermediate tap of the high frequency transformer 3 in a substantially middle point of the secondary winding, a first line voltage V ₁₁ is a second line voltage V ₁₂ becomes approximately equal between the third line The voltage V ₁₃ is the first line voltage V ₁₁ or the second line voltage V
The voltage value is about twice as large as ₁₂ . The current waveform flowing through the output terminals of the low-pass filters 5a and 5b is such that the inverter output current has the same phase as the voltage waveform of the commercial system 3, and
Since PWM control is performed using a sine wave signal of the same frequency, the waveform has the same phase as the voltage waveform as shown in FIGS. 3 (a) and 3 (b).

Further, the third line voltage V ₁₃ is inputted to the input terminal of the commercial frequency inverter bridge 6 at the next stage. Here, based on the commercial frequency, the control circuit 10 sets the gate terminals of the four IGBT switching elements S1 to S4 constituting the commercial frequency inverter bridge 6 based on the commercial frequency.
The switching elements S1, S4 and the switching elements S2, S3 are alternately turned on and off. That is, FIG. 3 (a), line voltages V ₁₁ shown in (b), the valley of the voltage value of V ₁₂ bottom in synchronization with (OV point), switching when the switching element S1, S4 are on device S2 , S3 are turned on and off alternately, such as off. As a result, the voltage waveforms and the current waveforms of the line voltages V ₁₁ and V ₁₂ are inverted into every other sine wave peak of the full-wave rectified waveform and converted into a sine wave AC waveform of a commercial frequency. Is done. Further, the sine wave AC voltage from the commercial frequency inverter bridge 6 is supplied to the low-pass filters 8a and 8a.
The harmonic components are removed by b, and as shown in FIGS. 4A to 4C, it is possible to obtain an AC voltage waveform of a commercial frequency whose waveform is shaped. That is, FIG. 4 (a) shows a first line voltage V ₂₁ across the capacitor C ₃ of the low-pass filter 8a, Fig. 4 (b) capacitor C ₃ of the low-pass filter 8b
Of shows the second line voltage V ₂₂ across, Fig. 4 (c) shows the low pass filter 8a, third line voltage V ₂₃ between the two output terminals of 8b.

Here, the third line voltage V ₂₃ has a voltage value approximately twice as large as the first line voltage V ₂₁ or the second line voltage V ₂₂ , and the DC voltage of the output of the solar cell 1 On the other hand, the transformer ratio of the high-frequency transformer 3 is designed so that the first line voltage V ₂₁ and the second line voltage V ₂₂ are 100 VAC (AC), and the third line voltage V ₂₃ is 200 VAC ( In the inverter IV1 of the first embodiment, since the rated input voltage is 200 VDC (DC), the turns ratio of the high-frequency transformer 3 is 1: 2.2 to 2.7,
The intermediate tap of the high-frequency transformer 3 is set to a substantially middle point of the secondary winding number).

The commercial frequency inverter bridge 6
IGBT switching element S1
3A and 3B, the switching elements S1, S4 and the switching elements S2, S3 are alternately synchronized with each other by synchronizing the gate terminals of S4 to the bottom of the valley (OV point) of the voltage value of each line voltage shown in FIGS. By turning on and off, every other peak of each sine wave that has been full-wave rectified is inverted and converted into a commercial frequency sine wave AC waveform as shown in FIGS. 4 (a) and 4 (b). To do this, the switching elements S1 and S4 or the switching elements S2 and S3, which are the four IGBTs of the commercial frequency inverter bridge 6, are used.
Must be turned on and off at the same time. However, strictly speaking, the on / off timing of the switching elements S1 to S4 may be deviated due to variations in the characteristics of the individual IGBTs, even if the same gate signal is applied.

For example, in FIG. 1, the control circuit 1 controls both the switching elements S1 and S4 to turn on simultaneously.
When the same gate signal is applied to the gate terminals of the switching elements S1, S4 from 0, the switching elements S1, S4
The variation in properties, when turned on quickly towards the switching element S1 by a minute time, the reactor L ₁ from the upper side of the output terminal of the high-frequency transformer 3, the switching element S1, the low-pass filter 8a, the voltage line u, the neutral line n Then, the current flows through a path extending to the intermediate tap of the high-frequency transformer 3 via the current detector 9. Then, after the switching element S4 after the minute time is turned on, the center tap of the high frequency transformer 3, a current detector 9, the neutral line n, the voltage line v, the low pass filter 8b, the switching element S4 and the reactor L _2,
A current flows through a path leading to the lower output terminal of the high-frequency transformer 3. Therefore, when the load connected to the single-phase three-wire type distribution line is in a balanced state, the short-circuit from the neutral line n of the commercial system 30 to the high-frequency Transformer 3
A transient current corresponding to the load flows in the direction of the intermediate tap, and when the switching element S4 is turned on, an equivalent current flows in the above-described reverse direction, so that the current apparently becomes zero.

As described above, when the load of the single-phase three-wire distribution line of the commercial system 30 is in a balanced state and the switching element S1 and the switching element S4 are simultaneously turned on, the current flowing through the current detector 9 is zero. However, if the timing at which each switching element is turned on is shifted, a current flows through the neutral line n for a minute time during which the timing is shifted. That is, when the switching element S1 is turned on for a minute time earlier than the switching element S4,
From the neutral line n to the intermediate tap of the high-frequency transformer 3
If the switching element S4 is turned on earlier than the switching element S1 by a very short time while the transient current flows in the (negative direction) for a very short time, then the Transient current flows for a time.

Incidentally, even when the load of the single-phase three-wire type distribution line of the commercial system 30 is in an unbalanced state, the same phenomenon as described above occurs as shown in FIG. In this case, some current has previously flowed through the neutral line n, but the timing at which the switching element is turned on is shifted, and a transient current flows through the current detector 9 at the zero-cross point every half cycle of the commercial frequency sine wave. Flows.

Similarly, this also occurs when the switching elements S1 and S4 are turned off and when the switching elements S2 and S3 are turned off.
Regardless of whether the load is balanced or unbalanced, the waveform of the current flowing through the neutral line n is such that the transient current is positive or negative at the zero-cross point every half cycle of the commercial frequency sine wave as shown in FIGS. 5 (a) and 5 (b). Will flow either way.

In response to the signal indicating the overcurrent from the current detector 9, the control circuit 10 determines the direction (positive direction or negative direction) of the transient current, and the transient current flows in the negative direction. When it is determined that the switching element S1 has been turned on and the switching element S3 has been turned off, the gate signal is controlled so as to delay the switching element S2 and the switching element S4 for a short time. on the other hand,
When the control circuit 10 determines that the transient current has flowed in the positive direction, the control circuit 10 delays the turning off of the switching element S2 and the turning on of the switching element S4 for a short time, and delays the turning on of the switching element S1 and the turning off of the switching element S3 for a short time. Control the gate signal so that it hastens. The control circuit 10 controls the gate signal so that the upper and lower limits of the on / off times of the switching elements S1 to S4 do not exceed predetermined upper and lower limits. Thus, the transient current flowing through the current detector 9 inserted between the neutral line n of the single-phase three-wire distribution line and the intermediate tap of the high-frequency transformer 3 (on / off timing due to characteristic variations of the switching elements S1 to S4) (Caused by the deviation).

As described above, the inverter IV1 is inserted between the solar cell 1 and the single-phase three-wire distribution line of the existing commercial system 30, and converts the DC power generated by the solar cell 1 into 60.
The power is converted into AC power of / 50 Hz, connected to the commercial system 30 and supplied to the load, and also the reverse flow of the power is performed to the commercial system 30. Therefore, the inverter IV1 is reduced in size and weight by using the high-frequency transformer 3 (conventional capacity ratio: about 1 /
4, weight ratio: about 1/6), and the neutral line n, the voltage line u and the voltage line v
Therefore, interconnection operation with the single-phase three-wire distribution line of the commercial system 30 can be performed.

Further, based on the direction of the transient current detected by the current detector 9, the control circuit 10 controls the on / off timing of the switching elements S1 to S4 of the commercial frequency inverter bridge 6 for a very short time. Further, it is possible to prevent a transient current from flowing through the neutral line n due to a shift in the on / off time due to a variation in the characteristics of the switching element.

(Second Embodiment) FIG. 6 is a block diagram showing a main part of an inverter according to a second embodiment of the present invention. In addition to the configuration of the inverter IV1 according to the first embodiment, an intermediate portion of the high-frequency transformer 3 is provided. A switch 18 is provided between the tap and the neutral line n of the single-phase three-wire distribution line of the commercial system 30 instead of the interconnection relay. A load 2 is connected between the voltage line u and the neutral line n of the single-phase three-wire distribution line of the commercial system 30.
1 and a load 22 is connected between the voltage line v and the neutral line n. As means for detecting that the loads 21 and 22 have become unbalanced, the current detector 9 includes:
It detects that an overcurrent has flown in the neutral line n of the single-phase three-wire distribution line connected to the intermediate tap of the high-frequency transformer 3. As means for detecting the restoration of the unbalanced state, the line voltage V _un between the voltage line u and the neutral line n and the voltage line v and the neutral line n of the single-phase three-wire type distribution line are used. And voltage detectors 13 for detecting line voltages _Vvn between the two. Further, based on the line voltages V _un , V _vn from the voltage detector 13, it is determined whether the load is unbalanced or not.
And a control circuit 20 as a second control unit for controlling the opening and closing of the power supply. The same components as those of the inverter IV1 are denoted by the same reference numerals, and description thereof will be omitted.

In the inverter IV2 having the above-described configuration, when the output terminal is connected to the single-phase three-wire distribution line of the commercial power system 30 to perform the interconnection operation, if the unbalanced state of the loads 21 and 22 increases, the control is performed. The circuit 20 determines whether or not the signal indicating the transient current from the current detector 9 is equal to or greater than a preset set value, and opens if the signal indicating the transient current is equal to or greater than the set value. Is output to the switch 18. Then, the switch 18 is opened, the connection between the inverter IV2 and the neutral line n is cut off, the inverter IV2 is connected only to the voltage lines u and v, and the inverter IV2 is connected to the commercial system 30 with a single-phase two-line 200V. Perform system operation.

On the other hand, when the inverter IV2 is connected with the single-phase two-line 200V of the voltage line u and the voltage line v of the commercial system 30 with the switch 18 opened as described above, the loads 21 and 22 the line voltage V _vn of the line voltage V _un and the voltage line v and the neutral line n voltage lines u and neutral n is across voltage detected by the voltage detector 13, from the voltage detector 13 of the Upon receiving signals representing line voltages V _un and V _vn , the control circuit 20
When it is determined that the voltage difference between the line voltages V _un and V _vn is equal to or _smaller than a preset value, the control signal is output to the switch 18 to indicate that the switch is closed. And the switch 1
8 is closed, and the inverter IV2 that has been connected and operated with the single-phase two-wire 200V can return to the commercial system 30 and the single-phase three-wire interconnected operation state again. The switch 18 is detected by detecting the voltage difference between the line voltage V _un between the voltage line u and the neutral line n and the line voltage V _vn between the voltage line v and the neutral line n. Closed because the above voltage difference becomes small,
This is because it can be determined that the load imbalance state has returned.

From the above, this inverter IV2
In the normal operation state, when the interconnection operation with the commercial system 30 is performed by the single-phase three-wire type distribution line, the loads 21 and 22 connected between the neutral line n of the commercial system 30 and each voltage line are inoperable. In the equilibrium state, only the neutral line n of the single-phase three-wire is connected to the inverter IV.
2, the inverter IV2 can continue the interconnection operation with a single-phase two-wire 200V. Therefore, even if the loads 21 and 22 become unbalanced, the interconnection operation is not stopped, so that efficient power supply to the commercial system 30 can be performed.

In the first and second embodiments, the solar cell 1 is used. However, the DC power supply is not limited to this, but may be a DC power supply such as a fuel cell.

In the first and second embodiments, the high-frequency transformer 3 is provided with an intermediate tap substantially at the center of the secondary winding. However, two secondary windings of the transformer are prepared. Alternatively, the start and end of each winding may be connected, and the connection point may be used as an intermediate tap.

In the first and second embodiments, the low-pass filters 5a and 5b composed of the reactors L ₁ and L ₂ and the capacitors C ₁ and C ₂ are used. However, the low-pass filter is not limited to this. Any device that removes a high-frequency component superimposed on the DC voltage from the power conversion unit may be used. That is, a low-pass filter or the like composed of only a reactor having one end connected to the output terminal of the second power converter may be used.

In the second embodiment, the current detector 9 inserted between the intermediate tap of the high-frequency transformer 3 and the neutral line n is used as means for detecting the load imbalance state.
However, the present invention is not limited to this, and the voltage difference between the neutral line and each voltage line is detected by using a voltage detector to determine whether the potential difference is equal to or greater than a predetermined value. , The load imbalance state may be detected.

[0054]

As is apparent from the above description, the inverter according to the first aspect of the present invention converts DC power supplied from a DC power supply into AC power and has two voltage lines and a neutral line of a commercial system. In an interconnected inverter that supplies power to a single-phase three-wire distribution line, the DC power from the DC power supply is converted to AC power by a first power conversion unit, and the AC power is supplied from the first power conversion unit. The AC voltage is transformed by a transformer, the transformed AC voltage is outputted from a secondary output terminal, and the intermediate tap provided at a substantially middle point of the winding of the secondary output terminal of the transformer is the above-mentioned. While being connected to the neutral line of the single-phase three-wire distribution line, AC power from the secondary output terminal of the transformer is converted to DC power by the second power converter, and the third power converter is Converting the DC power from the second power converter into AC power, It is connected to the commercial system by three lines of both voltage line and neutral line of the three-wire distribution line, and the intermediate tap on the secondary side of the transformer and the neutral line of the single-phase three-wire type distribution line And a first control unit detects a transient current based on a direction of the transient current detected by the current detector so that the transient current does not flow based on a direction of the transient current detected by the current detector. To convert on / off control of a switching element provided in the third power converter in order to convert the power to AC power.

Therefore, according to the inverter of the first aspect of the present invention, the inverter can be reduced in size and weight, and can be connected to a single-phase three-wire distribution line used in most new houses. it can. Also, for example, by correcting a shift in the on / off time due to the variation in the characteristics of the switching element, it is possible to prevent a transient current from flowing through the neutral wire of the single-phase three-wire distribution line. Therefore, this inverter can be connected to a commercial system with a stable AC voltage output with little distortion.

The inverter according to the second aspect of the present invention converts DC power supplied from a DC power supply into AC power,
In an interconnection type inverter that supplies power to a single-phase three-wire distribution line having two voltage lines and a neutral line of a commercial system, the DC power from the DC power supply is converted to AC power by a first power conversion unit. And the AC voltage from the first power conversion unit is transformed by a transformer, and the transformed AC voltage is output from a secondary output terminal, and is wound around a secondary output terminal of the transformer. An intermediate tap provided substantially at the midpoint of the line connects to the neutral line of the single-phase three-wire distribution line, while the second power converter converts AC power from the secondary output terminal of the transformer. The DC power is converted into DC power, and a high-frequency component superimposed on the DC voltage from the second power converter is removed by a low-pass filter. The third power converter converts the DC power from the low-pass filter into AC power. And single-phase three-wire distribution lines, both voltage and neutral A three-wire system is connected to the commercial power system. Transient current is detected by a current detector provided between the intermediate tap on the secondary side of the transformer and the neutral wire of the single-phase three-wire distribution line. The first control unit detects the third power conversion based on the direction of the transient current detected by the current detector and converts the DC power into the AC power so that the transient current does not flow. The on / off control of the switching element provided in the section is performed.

Therefore, according to the inverter of the second aspect of the present invention, the inverter can be reduced in size and weight, and can be connected to a single-phase three-wire power distribution line used in most new houses. it can. Also, for example, by correcting a shift in the on / off time due to the variation in the characteristics of the switching element, it is possible to prevent a transient current from flowing through the neutral wire of the single-phase three-wire distribution line. Therefore, this inverter can be connected to a commercial system with a stable AC voltage output with little distortion.

According to a third aspect of the present invention, in the inverter according to the second aspect, the low-pass filter includes a reactor having one end connected to each of the DC voltage output terminals of the second power converter, and a reactor connected to the reactor. And capacitors connected between the other end of the transformer and the intermediate tap of the transformer.

Therefore, according to the inverter of the present invention, the low-pass filter is superimposed on each DC voltage between both DC voltage output terminals of the second power converter and the intermediate tap of the transformer. High-frequency components are removed. Therefore, the low-pass filter having a simple configuration allows the second
The waveform shaping can be performed by removing the high-frequency component of the DC voltage from the power converter.

According to a fourth aspect of the present invention, there is provided the inverter according to any one of the first to third aspects, wherein a line voltage between a neutral line and both voltage lines of the single-phase three-wire distribution line is provided. Is detected by the voltage detector, and the second controller detects the switch provided between the intermediate tap of the transformer and the neutral line of the single-phase three-wire distribution line by the current detector. When the detected transient current is equal to or more than a predetermined value, the current is opened, and when the voltage difference between the line voltages detected by the voltage detector is less than the predetermined value, the control is performed so as to close.

Therefore, according to the inverter of the fourth aspect of the present invention, even when the load connected to the single-phase three-wire distribution line becomes unbalanced, the switch is opened without stopping the inverter, When the load is restored to the equilibrium state while the interconnection operation with the single-phase two-wire (200 V) is continued, the switch is closed and the operation is returned to the single-phase three-wire interconnection, whereby the output of the solar cell is reduced. Can efficiently flow backward to the commercial system.

Further, the inverter according to the present invention converts DC power supplied from a DC power supply into AC power,
In an interconnection type inverter that supplies power to a single-phase three-wire distribution line having two voltage lines and a neutral line of a commercial system, the DC power from the DC power supply is converted to AC power by a first power conversion unit. And the AC voltage from the first power conversion unit is transformed by a transformer, and the transformed AC voltage is output from a secondary output terminal, and is wound around a secondary output terminal of the transformer. An intermediate tap provided substantially at the midpoint of the line connects to the neutral line of the single-phase three-wire distribution line, while the second power converter converts AC power from the secondary output terminal of the transformer. The third power converter converts the DC power from the second power converter into AC power, and converts the DC power into single-phase power.
The line is connected to the commercial system with three lines, the voltage line and the neutral line of the line distribution line, and the line voltage between the neutral line and both voltage lines of the single-phase three-line distribution line is A voltage detector detects, and a second control unit detects a switch provided between an intermediate tap of the transformer and a neutral wire of the single-phase three-wire distribution line by the voltage detector. When the voltage difference between the line voltages is equal to or greater than a predetermined value, the switch is opened, and when the voltage difference is less than the predetermined value, the switch is closed.

Therefore, according to the inverter of the fifth aspect of the present invention, the inverter can be reduced in size and weight, and can be connected to a single-phase three-wire distribution line used in most new houses. it can. Also, when an imbalance occurs in the load connected to the single-phase three-wire distribution line, the switch is opened without stopping the inverter, and the single-phase two-wire
(200V), while the load returns to the equilibrium state, the switch is closed, and the operation returns to the single-phase three-wire connection operation. Reverse power can flow into the grid.

[Brief description of the drawings]

FIG. 1 is a main part configuration diagram of an inverter according to a first embodiment of the present invention.

FIGS. 2 (a) to 2 (c) are diagrams showing voltage waveforms of line voltages output from a diode bridge of the inverter.

FIGS. 3A to 3C are diagrams showing a voltage waveform and a current waveform of each line voltage of the output of the low-pass filter of the inverter.

FIGS. 4A to 4C are diagrams showing voltage waveforms of respective line voltages output from a commercial frequency inverter bridge of the inverter.

FIG. 5 is a diagram showing a waveform of a current flowing in a neutral line with turning on / off of an IGBT constituting a commercial frequency inverter bridge of the inverter.

FIG. 6 is a main part configuration diagram of an inverter according to a second embodiment of the present invention.

FIG. 7 is a configuration diagram of a main part of a conventional inverter.

FIG. 8 is a main part configuration diagram of another conventional inverter.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Solar cell, 2 ... High frequency inverter bridge, 3 ... High frequency transformer, 4 ... Diode bridge, 5a, 5b, 8a, 8
b: low-pass filter, 6: commercial frequency inverter bridge, 7, 17: interconnection relay, 9: current detector, 10, 20
... control circuit, 11, 12 ... utility power, 13 ... voltage detector, 18 ... switch, C ₁ -C ₄ ... capacitor, C ₅ ... input capacitor, D ₁ to D ₄ ... diodes, Q ₁ to Q _4, S _{1 to} S
₄ ... Switching element.

──────────────────────────────────────────────────続 き Continuation of front page (72) Inventor Hiroshi Nakata 22-22, Nagaike-cho, Abeno-ku, Osaka-shi, Osaka Inside Sharp Corporation (56) References JP-A-4-1655963 (JP, A) Shokai Sho 62- 84393 (JP, U) (58) Fields investigated (Int. Cl. ⁷ , DB name) H02M 7/48 H02M 1/14

Claims (5)

(57) [Claims] An interconnection type for converting DC power supplied from a DC power supply to AC power and supplying power to a single-phase three-wire type distribution line having two voltage lines and a neutral line of a commercial system. A first power conversion unit that converts the DC power from the DC power supply into AC power; transforms the AC voltage from the first power conversion unit, and converts the transformed AC voltage into two. A transformer that outputs from the secondary output terminal and has an intermediate tap provided at a substantially middle point of a winding of the secondary output terminal connected to a neutral wire of the single-phase three-wire distribution line; A second power conversion unit that converts AC power from a secondary output terminal of the transformer into DC power; and a DC power from the second power conversion unit that converts the DC power into AC power. A third power converter in which output terminals are respectively connected to both voltage lines of the distribution line; A current detector that is provided between a secondary-side intermediate tap and a neutral wire of the single-phase three-wire distribution line, and that detects a transient current; and in a direction of the transient current detected by the current detector. And a first control unit that controls on / off of a switching element provided in the third power conversion unit to convert DC power into AC power so that the transient current does not flow. Features inverter. 2. An interconnection type for converting DC power supplied from a DC power supply into AC power and supplying power to a single-phase three-wire distribution line having two voltage lines and a neutral line of a commercial system. A first power conversion unit that converts the DC power from the DC power supply into AC power; transforms the AC voltage from the first power conversion unit, and converts the transformed AC voltage into two. A transformer that outputs from the secondary output terminal and has an intermediate tap provided at a substantially middle point of a winding of the secondary output terminal connected to a neutral wire of the single-phase three-wire distribution line; A second power converter that converts AC power from a secondary output terminal of the transformer into DC power, and a low-pass filter that removes a high-frequency component superimposed on the DC voltage from the second power converter, DC power from the low-pass filter is converted to AC power. A third power converter in which output terminals are respectively connected to both voltage lines of the single-phase three-wire distribution line, an intermediate tap on a secondary side of the transformer, and a neutral terminal of the single-phase three-wire distribution line; A current detector that is provided between the power supply line and the line, and converts a DC power into an AC power based on a direction of the transient current detected by the current detector so that the transient current does not flow. And a first control unit for performing on / off control of a switching element provided in the third power conversion unit. 3. The inverter according to claim 2, wherein
The low-pass filter includes a reactor having one end connected to both DC voltage output terminals of the second power converter, and a capacitor connected between the other end of the reactor and an intermediate tap of the transformer. An inverter characterized by becoming. 4. The voltage detector according to claim 1, wherein a line voltage between a neutral line and both voltage lines of the single-phase three-wire distribution line is detected. A switch provided between an intermediate tap of the transformer and a neutral wire of the single-phase three-wire distribution line, and the transient current detected by the current detector is equal to or more than a predetermined value. A second control unit that controls to close the switch when the voltage difference between the line voltages detected by the voltage detector is less than a predetermined value while opening the switch. And the inverter. 5. An interconnection type for converting DC power supplied from a DC power supply to AC power and supplying power to a single-phase three-wire type distribution line having two voltage lines and a neutral line of a commercial system. A first power conversion unit that converts the DC power from the DC power supply into AC power; transforms the AC voltage from the first power conversion unit, and converts the transformed AC voltage into two. A transformer that outputs from the secondary output terminal and has an intermediate tap provided at a substantially middle point of a winding of the secondary output terminal connected to a neutral wire of the single-phase three-wire distribution line; A second power conversion unit that converts AC power from a secondary output terminal of the transformer into DC power; and a DC power from the second power conversion unit that converts the DC power into AC power. A third power converter in which output terminals are respectively connected to both voltage lines of the distribution line; A voltage detector for detecting a line voltage between a neutral line and both voltage lines of a line type distribution line, and a voltage detector between an intermediate tap of the transformer and a neutral line of the single-phase three-line type distribution line. The provided switch, the switch is opened when the voltage difference between the line voltages detected by the voltage detector is equal to or more than a predetermined value, and the switch is opened when the voltage difference is less than a predetermined value. An inverter comprising: a second control unit that performs control to close the inverter.