JP2580108B2 - Power converter - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a power conversion device, and more particularly to a power conversion device including a frequency conversion circuit that directly converts a high frequency to a low frequency.

[Conventional technology]

In a power converter with an inverter that forms an alternating current from a direct current, an attempt is made to increase the frequency in order to reduce the size and improve the performance. A typical example of such a high-frequency power conversion device is “RLStelgerwald et al.,“ Comparison of high-frequency coupling for connecting DC power supply to distribution network (A
COMRARISON OF HIGH-FREGUENCY LINK SCHEMES FOR INT
ERFACING A DCSOURCE TO A UTILITY GRID) "IEEE-IAS
-82, pp. 759-766, there is an exchange with an AC-AC switch.

Such a converter converts a direct current into an alternating current by a self-excited high-frequency inverter, and after passing through a high-frequency transformer, rectifies and converts the direct current into a direct-current having a pulsation of the same cycle as that of the commercial system, and again converts it into a separately-excited inverter. , And is characterized by being able to reduce the size and weight of the transformer by high frequency.

[Problems to be solved by the invention]

However, when used as an independent power supply without interconnection to the system, even if the above-mentioned rectifier and a converter consisting of a separately excited inverter are connected via a high-frequency transformer, there is no power supply that performs commutation. In addition, there is a problem that the separately excited inverter cannot be operated and desired AC power cannot be obtained. Also, even if the separately-excited inverter is changed to a self-excited inverter, this cannot handle the reactive power of the load,
There was a problem that desired AC power could not be obtained. By employing the methods described in JP-A-54-127530 and JP-A-57-28577, a high-frequency AC signal can be smoothly converted to a low-frequency AC signal. And in the former, each transistor of the switching circuit is bridge-connected to four diodes,
Both positive and negative currents can flow, and even when the output voltage and the output current (load current) have different phases, each transistor can be selectively operated. However, if each transistor is arbitrarily selected, the output side of the transformer may be short-circuited.Therefore, each transistor is turned on and off during the period when the output voltage is zero.However, current flows depending on the load condition. The transistor is off when
Overvoltage may occur due to transformer and wiring inductance. Further, since there are three semiconductor elements (one transistor and two diodes) that are simultaneously turned on in the circuit connecting the transformer and the smoothing circuit, the forward voltage drop of each semiconductor element is large and the loss is large. Become.

On the other hand, the latter one has two semiconductor elements (one diode and one transistor) which are simultaneously turned on in the circuit connecting the transformer and the smoothing circuit, so that the forward voltage is higher than that of the former. Although the drop can be reduced, it is not enough to suppress the loss of the circuit. Furthermore, since the input side of the transistor as a switch is connected to a bridge-connected diode, and the polarity of the voltage on the input side of the transistor is fixed by four diodes, only one-way current can flow through each transistor. If the output voltage and the output current (load current) have different phases, each transistor cannot be selectively operated. For example, even when the positive-side transistor is to be turned on, if the load power factor is small and the polarity of the load current becomes negative, the positive-side transistor cannot be turned on. Therefore, unless the load has a power factor of 1, each transistor cannot be smoothly operated.

[Object of the invention]

An object of the present invention is to provide a power conversion device that can sufficiently suppress circuit loss and can select a switching operation arbitrarily regardless of a load state.

[Means for solving the problem]

In order to achieve the above object, the present invention provides a first AC power supply for generating a high-frequency AC signal, and a high-frequency AC signal that is connected in series with the first AC power supply through a series connection point. A second AC power supply, a first switch having one terminal connected to an output side of the output side of the first AC power supply that is different from an output side connected to the series connection point, A second switch having one terminal connected to an output side different from the output side connected to the series connection point among the output sides of the AC power supply, and the other terminal of the first switch and the second switch. The first switch comprises: a smoothing means for smoothing an AC signal from the other terminal of the second switch; and a switching control means for controlling a switching operation of the first switch and the second switch. The other terminal of the switch and the other terminal of the second switch are connected to each other and Is connected to one input side of the unit,
A series connection point of the first AC power supply and the second AC power supply is connected to the other input side of the smoothing means, and an AC signal from each of the AC power supplies is turned on and off by the switching operation of each of the switches. In a power converter that is converted into an AC signal having a frequency lower than the frequency of an AC power supply and is output from the smoothing unit, the power conversion device includes a current detection unit that detects a current flowing into the smoothing unit, and each of the switches has a current. The opening / closing control means adjusts the detection current of the current detection means, the polarity of the output voltage of both AC power supplies, and the polarity of the output voltage to be output from the smoothing means. A switching control signal for each of the switching elements, and applying the generated switching signal to each of the switching elements. Configuration.

[Action]

According to the above-described means, since only one switching element that is turned on is present in the circuit connecting each AC power supply and the smoothing means, it is possible to suppress the loss due to the forward voltage drop of the switching element. it can. Furthermore, since each switch is composed of two switching elements having different current flowing directions, the detection current of the current detection means, the polarity of the output voltage of both AC power supplies, and the output voltage of the output voltage to be output from the smoothing means. The switching operation of each switching element can be arbitrarily selected according to the polarity.
For this reason, even when the phases of the output voltage and the output current (load current) are different depending on the state of the load, each switching element can be operated in commutation, so that the controllability of each switching element can be improved and the switching operation can be performed. Sometimes, it is possible to prevent each AC power supply from being short-circuited or to generate an overvoltage, thereby contributing to an improvement in the reliability of the device.

〔Example〕

Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a circuit diagram showing an embodiment of a power converter according to the present invention.

In the figure, 1A, 1B are AC power supplies, 2, 3 and 4 are switches, 5 is a filter capacitor for improving waveforms, 6 is a filter reactor, and the switches 2, 3 and 4 constitute a frequency conversion circuit 13. doing.

The operation of the embodiment configured as described above will be described with reference to FIG.

First, the operation in the case of the embodiment circuit without the switch 4 will be described.

From the AC power supplies 1A and 1B, as shown in FIG.
Voltages V ₁ and V ₂ pulse-width-modulated by the modulated wave S ₁ indicated by the dotted line so as to have a sine wave shape value are output, and these voltages V ₁ and V ₂ are output.
V ₂ is applied to the switch 2. As the output of the polarity of the modulating wave S ₂ shown in FIG. 2 (b) is obtained, on the switch 2 alternately as the second view (d), it is off control. By controlling the switches 2 and 3 on and off in this manner, the first
At point A in FIG. 2, voltage waveforms V ₃ and V ₄ shown in FIG. 2 (c) are obtained, and are passed through a waveform improving filter composed of the reactor 6 and the capacitor 5 to obtain a dotted line in FIG. 2 (c). the output voltage V ₀ which sinusoidal frequency-converted from a high frequency to a low frequency as shown is obtained.

Next, as shown in FIG. 1, the operation of a circuit having the switch 4 will be described.

High-frequency voltages V ₅ and V ₆ as shown in FIG. 2 (e) are output from the AC power supplies 1 A and 1 B, and applied to the switches 2 and 3. Here, using the was formed by using a modulated wave S ₁ shown in FIG. 2 (a), the switch control signal S ₃ such as shown in FIG. 2 (f), switches 2, 3 and 4 is turned on and off. For example, switch 2 and 4 at time t ₁ ~ time t ₂ is turned off, switch 3 is turned on. Then, the point A is applied a positive voltage of the power supply 1B via the switch 3, the voltage V ₃ shown in FIG. 2 (c) is output. At time t ₂ ~ time t _3, switch 3 off, to turn on the switch 4. Thus, A point is disconnected from the power supply 1B, the same potential of point N (Fig. 2 (c) reference voltages V _3). At time t ₃ ~ time t _4, performs switch 4 OFF, to turn ON the switch 2. Thus, A
Power 1A positive voltage via the switch 2 is applied to the (Figure 2 (c) reference voltages V _3). By controlling the switches 2, 3, and 4 as shown in FIG. 2 (f), the waveforms of the voltages V ₃ and V ₄ shown in FIG. 2 (c) are obtained at the point A. is, the output voltage V ₀ which has been converted to a low frequency sine wave system indicated by the dotted line in FIG. 2 (c) is obtained at the output terminal.

FIG. 3 shows an example of the waveform of the voltage V ₀ of the filter capacitor 5 and the current i ₀ flowing through the reactor 6 when the circuit operation shown in FIG. 2 is performed.

FIG. 3 is a waveform example when a conversion operation from a high-frequency power supply to a 1/20 frequency is performed.
As a result, it is understood that an output converted from high frequency to low frequency can be obtained.

FIG. 4 is a circuit diagram showing another embodiment of the present invention. In the figure, 10 is a DC power supply, 11 is an inverter configured using switching elements such as transistors and diodes, 12 is a transformer, and 13 is a gate turn-off thyristor.
A frequency conversion circuit constituted by using 21, 22, 23 and 24, 14 is a current detector, and 15 is a control circuit. The same members as those in FIG. 1 are denoted by the same reference numerals.

FIG. 5 is a block diagram of the control circuit 15 of FIG. 4, and FIG. 6 is an explanatory diagram of the operation of FIG. The oscillator 101 outputs a high-frequency signal as shown in FIG.
Applied as clock 104. The counter 102 divides the clock frequency to generate an inverter driving signal 201 (201, ▲) shown in FIG.
▼ drives the transistor of one arm of the inverter 11), and as shown in FIG. 7 (a), as a signal for providing the width of the inverter output voltage, an address signal corresponding to one cycle of output (the sixth signal). (C)), the data set signal of the counter 104 shown in FIG.
A signal 203 indicating the polarity of the inverter output waveform in FIG.
The signal 204 shown in FIG. 6 (h) indicating the polarity of the output is output. The ROM 103 outputs data of the address given from the counter 102 to the counter 104. In addition, the data by the address of the AVR system whose description is omitted and the address of the above time are output. Counter 104 is counter 1
By the set signal (FIG. 6 (d)) applied from 02,
The data output from the ROM 103 is set, and the oscillator 101
6 (e) after counting by the clock applied from
(A signal delayed by a time corresponding to the width of the inverter output voltage), and the latch 105 is set.
The signals 202 and ▼ are delayed from the signals 201 and ▼ by the above-described time, and the signal 202 has a waveform shown in FIG. 6F (the transistor of the other arm is driven by 202 and ▼).

The current detected by the current detector 14 shown in FIG.
106 to form "1" and "0" signals corresponding to the positive and negative polarities. Based on the output signal of the waveform shaping circuit 106 and the signals 203 and 204, the gate turn-off thyristors 21 to 24 are operated by the switch selection circuit 107 configured to operate as shown in FIGS. Is formed.

 The circuit operation will be described with reference to FIG.

The control circuit 15, an inverter 11 so that the voltage S ₁ of the sinusoidal wave shape shown by the dotted line in FIG. 7 (a) is operated in a pulse width control, an output voltage V _1, V ₂ of the shape shown by the solid line I do.
The output voltage of the inverter 11 is applied to the transformer 12, and a voltage having substantially the same shape is generated in the secondary winding of the transformer 12. The control circuit 15 controls the frequency conversion circuit 13 to output a signal S ₂ shown in FIG. 7 (b) indicating the polarity of the output voltage waveform of the power converter of the same kind as the waveform shown by the dotted line in FIG. An example of the control described below according to the signal indicating the polarity of the voltage waveform and the signal indicating the polarity of the current detected by the current detector 14 will be described. For example, at time t ₁ ~ time t _2, the polarity of the output voltage of the power converter is positive as shown in Figure No. 7 (b), the polarity of the inverter output voltage waveform is shown in Figure No. 7 (a) the voltage V ₂ Since the polarity of the current i ₀ is positive as shown in FIG.
Turn on the gate turn-off thyristor 23. By turning on the gate turn-off thyristor 23, the voltage of the secondary winding (b) of the transformer 12 is applied to the point A as shown in FIG. 7 (c). t ₂ is the inverter output voltage at the time 0 and the voltage at the point A also becomes zero, the time
Between t ₂ and time t ₃ , current i _{0 of} reactor 6 is as shown in FIG.
And flows through the secondary winding (b) and the gate turn-off thyristor 23 as shown in FIG. Since the polarity of the inverter output voltage wave system at t ₃ time is positive, subsequent to t ₅ is turned on the gate turn-off thyristor 21 turns off the gate turn-off thyristors 23. In this case, the secondary winding (A) of the transformer → the gate turn-off thyristor 21 → the gate switch can be performed without performing the shut-off operation by the gate signal of the gate turn-off thyristor 23.
Since the commutation operation is performed via the turn-off thyristor 23 → secondary winding (b), the necessity of the interruption operation is irrelevant.
Voltage V ₁ of the secondary winding (A) is applied to the point A via a gate turn-off thyristors 21. t ₄ is the inverter output voltage at the point 0, but the voltage 0 of the point A, current seventh view of a reactor 6 (d) to 2 as shown winding (a) and gate turn-off thyristors 21 Keep flowing through. Since the polarity of the inverter output voltage waveform is negative again at t ₅ when the gate turn-off thyristor
Turn on 23 and turn off gate turn-off thyristor 21. At this time, the operation is similar to the operation of t ₃ time points mentioned above. Hereinafter, t up to ₁₅ point operates in a similar manner. t
_{At 15} points, the polarity of the output voltage of the power converter becomes negative and the polarity of the current becomes negative. For this reason, the gate from the secondary winding (b)
The current flowing to the point A via the turn-off thyristor 23 may flow from the point A in a direction to turn on the gate turn-off thyristor 24. When the gate turn-off thyristor 24 is turned on, the voltage at the point A is 0, but the current of the reactor 6 becomes as shown in FIG.
It continues to flow through the gate turn-off thyristor 24 and the secondary winding (b). _At time t16, the polarity of the inverter output voltage waveform becomes positive. Since the polarity of the output voltage of the power converter is negative, the gate turn-off thyristor 24 is kept on so that the voltage applied to the point A becomes negative. A
The applied voltage at the point has a waveform shown in FIG. 7 (c). _At time t17, the inverter output voltage becomes 0, but the current of the reactor 6 becomes gate-off and off as shown in FIG. 7 (d).
It continues to flow through the thyristor 24 and the secondary winding (b). _At time t18, the polarity of the inverter output voltage waveform becomes negative, so the gate turn-off thyristor 22 is turned on,
Turn off gate turn-off thyristor 24. Operation of this point is the same as the operation of t ₃ time points mentioned above. 2
The voltage of the secondary winding (a) is a gate turn-off thyristor
It is applied to point A via 22. Thereafter, by performing the same operation, the voltage waveform shown in FIG. 7C is applied to the point A, and the low-frequency AC voltage having the waveform improved by the filter of the reactor 6 and the capacitor 5 is applied to the output. Obtainable.

Next, the operation when the current phase of the reactor 6 is different from the voltage phase will be described with reference to FIG. 7 (e). t
_When the current polarity of the reactor 6 becomes negative at the time point ₁₀ , from this time point, the A is output via the gate turn-off thyristor 23.
The current flowing through the point side, it is necessary to flow from the A point side to the secondary winding (B) via a gate turn-off thyristor 24 to turn on the gate turn-off thyristors 24 at t ₁₀ time. When the gate turn-off thyristor 24 is turned on, the current of the reactor 6 continues to flow through the gate turn-off thyristor 24 and the secondary winding (b) as shown in FIG. 7 (d). Since the polarity of the inverter output voltage waveform at t ₁₁ when a positive and a is the voltage of the secondary winding turns on the gate turn-off thyristors 22 (b) must be applied to the point A. However, at this time, since the polarity of the output voltage of the power converter does not match the polarity of the current of the reactor 6, the commutation operation as in the operation shown in FIG. 7D is not performed. . That is, for the secondary winding,
Is for supplying a polarity opposite to the polarity of the current in the voltage generated, even if on a gate signal to the gate turn-off thyristors 22 at t ₁₁ time, the secondary winding to a gate turn-off thyristors 22 (Lee ) And (b) are applied in opposite polarities, and the gate turn-off thyristor 22 does not turn on. Therefore, after turning on the gate turn-off thyristors 22 in the period 0 of the output voltage of the inverter 11 of t ₁₀ ~t _11, performs a cut-off operation by the gate signal of the gate turn-off thyristors 24. t ₁₁ gate turn-off thyristor 22 is turned on by the time, gate turn-off thyristor 24 is turned off, the current of the reactor 6, gate turn-off thyristor 22 → 2
It flows through the next winding (a). t ₁₁ becomes the inverter output voltage waveform is positive at the time, the point A waveform shown in Figure No. 7 (c) is obtained. t ₁₂ the output voltage of the inverter 11 at time zero
However, the current of the reactor 6 continues to flow through the gate turn-off thyristor 22. After turning on the gate turn-off thyristor 24 during the output voltage 0 of the inverter 11 of the period as well as t ₁₂ ~t ₁₃ of t ₁₀ ~t _11,
The gate turn-off thyristor 22 is turned off by a gate signal. t ₁₃ time cause the inverter output voltage waveform from the point A voltage shown in Figure No. 7 (c) is applied. From t ₁₅ time, since the polarity of the current i ₀ which flows to the polarity and the reactor 6 of the output voltage of the power converter matches, the same operation as FIG. 7 (d). By performing such ON / OFF control in accordance with the polarity of the output voltage and the polarity of the circuit current, a desired waveform as shown in FIG. 7C can be formed because it is not affected by the load condition.

Although the gate turn-off thyristors 21, 22, 23, and 24 are used as the elements constituting the frequency conversion circuit 13 shown in FIG. 4, it is of course possible to use other switching elements having a cutting function. . Further, a normal thyristor having no turn-off function can be used. in this case,
For example t _3, t ₁₈ the time of the operation is the same, t ₁₀ ~t ₁₄ when the operation is as follows. Since a normal thyristor cannot perform a cut-off operation, it is necessary to perform a commutation operation during a period in which a commutation operation is possible. When the inverter 11 does not perform the operation for assisting the commutation operation of the frequency conversion circuit, it is necessary to perform the commutation operation only with the output voltages V ₁ and V ₂ shown in FIG. In the example shown in FIG.
Since the polarity of the output voltage V _1, V ₂ 0 period of the reactor 6 of the current is inverted, commutation from thyristor 24 until t ₁₁ time to the thyristor 22 can not perform. Fig. 8 (a)
As shown in the figure, when the current polarity of the reactor 6 is inverted at a certain time point t ₉ ′ of the inverter output voltage, the time Toff required for the commutation operation before the time point t ₁₀ ″ at which the inverter output voltage becomes 0 is obtained. Turn on the thyristor at t ₉
It is commutated from thyristor 24 to thyristor 22. Also, t
_The thyristor 24 is turned on at the time t ₁₁ ′, which is Toff earlier than the time ₁₂ , and the thyristor 22 is commutated to the thyristor 24. In the example shown in FIG. 7 (e), 1 wave component at t ₁₁ ~t ₁₂ is applied to the point A as a reverse polarity voltage than the inverter frequency is the output frequency of the power converter, very high At times, the deterioration of the output voltage of the power conversion device due to the voltage of one wave having the opposite polarity can be ignored, and there is no problem.

FIG. 9 shows another embodiment of the present invention, in which the configuration of the frequency conversion circuit 13 is changed. In the figure, 1A, 1B
Corresponds to the AC power supply shown in FIG. 1, and is equivalent to the voltage generated by the secondary windings (a) and (b) of the transformer 12 shown in FIG. Reference numerals 21, 22, 23, 24 and 25,
Reference numeral 26 denotes a gate turn-off thyristor corresponding to the switches 2, 3, and 4 in FIG.

The operation will be described with reference to FIG. FIG. 10 is a time diagram showing a part of one cycle of the output voltage of the power converter. As shown in FIG. 10 (a), square wave voltages are supplied from the current transformers 1A and 1B. In order to obtain a point A voltage as shown in FIG. 10 (b), when the current i ₀ flowing through the reactor 6 has the same polarity, as shown in FIG. 10 (c), the gate turn-off thyristor 21, This is achieved by controlling on / off of 23 and 25. Similarly, the voltage shown in FIG. 10 (d) is obtained by controlling the gate turn-off thyristors 22, 24, 26 to be on / off as shown in FIG. 10 (e). At the time of the polarity of the current mismatch i ₀ flowing through the polarity and the reactor 6 of the voltage at the point A, as with the embodiment of FIG. 4, to the ON operation of the gate turn-off thyristor which satisfies the voltage polarity and current polarity As a result, a desired waveform can be obtained. Further, the AC power supply 1A, late 1B of the voltage waveform (for example, t ₃ ~t ₄ between t ₂ ~t ₄₎ has been described in example using, it is also possible to use the first half of the voltage waveform. Although the frequency conversion circuit 13 is constituted by the gate / turn-off thyristors 21 to 26, it can be changed to another switching element having another cutting function.

Although the embodiment of FIGS. 1 to 10 has been described with a single-phase circuit,
Of course, it can be applied in three phases and other polyphases.

〔The invention's effect〕

As described above, according to the present invention, since only one switching element that is turned on is present in each circuit connecting each AC power supply and the smoothing means, it is possible to sufficiently suppress the loss of the circuit. Can be. Further, since each switch is composed of two switching elements having different current flowing directions, each switching element can be commutated even when the output voltage and the output current have different phases depending on the load condition. Controllability of each switching element can be improved, and it is possible to prevent short-circuiting of each AC power supply or occurrence of overvoltage during switching operation, thereby contributing to improvement of the reliability of the device. Can be.

[Brief description of the drawings]

FIG. 1 is a block diagram showing an embodiment of the power converter according to the present invention, FIG. 2 is a time chart for explaining the operation of the embodiment of FIG. 1, and FIG. 3 is used for explaining the operation. Waveform diagram, FIG. 4 is a block diagram showing another embodiment of the power converter, FIG. 5 is a block diagram showing a control circuit of the embodiment, FIG. 6 is a time chart for explaining the operation of FIG. 7
FIGS. 8 and 9 are time charts for explaining the operation of the fourth embodiment, FIG. 9 is a block diagram showing still another embodiment of the power converter, and FIG. 10 is an embodiment of FIG. 3 is a time chart shown to explain the operation of FIG. 1A, 1B ... AC power supply, 2, 3, 4 ... switch, 5 ... capacitor, 6 ... reactor, 11 ... inverter, 12 ... transformer, 13 ... frequency conversion circuit, 14 ... current Detector, 15 ……
Control circuit, 21, 22, 23, 24, 25, 26 ... Gate turn-off thyristor.

 ──────────────────────────────────────────────────続 き Continuation of the front page (72) Inventor Junto Jifuku 3-1-1, Yachimachi, Hitachi-shi Hitachi, Ltd. Hitachi Plant (56) References JP-A-54-127530 (JP, A) JP-A-57-28577 (JP, A)

Claims (6)

(57) [Claims] 1. A first AC power supply for generating a high-frequency AC signal, a second AC power supply connected in series with the first AC power supply via a series connection point to generate a high-frequency AC signal, A first switch having one terminal connected to an output side different from an output side connected to the series connection point, among an output side of the first AC power supply, and an output side of a second AC power supply. A second switch having one terminal connected to an output side different from the output side connected to the series connection point, the other terminal of the first switch and the other of the second switch; A smoothing means for smoothing an AC signal from a terminal of the first switch, and switching control means for controlling a switching operation of the first switch and the second switch, the other terminal of the first switch And the second
The other terminals of the switch are connected to each other and connected to one input side of the smoothing means, and a series connection point of the first AC power supply and the second AC power supply is connected to the other input terminal of the smoothing means. The power converter is connected to the side, and the AC signal from each of the AC power supplies is converted into an AC signal having a lower frequency than the frequency of each of the AC power supplies by the switching operation of each of the switches, and is output from the smoothing unit. A current detecting means for detecting a current flowing into the smoothing means, wherein each of the switches is constituted by two switching elements having different current flowing directions, and wherein the switching control means detects the current detected by the current detecting means. A switching control signal for each of the switching elements is generated and generated according to the current and the polarity of the output voltage of the two AC power supplies and the polarity of the output voltage to be output from the smoothing means. A power converter characterized by applying a switching signal to each switching element. 2. The power converter according to claim 1, wherein said first AC power supply and said second AC power supply generate pulse width modulated AC signals. Power converter. 3. The power conversion device according to claim 1, wherein the switching elements of the switches are connected in anti-parallel with each other. 4. The power converter according to claim 1, wherein said switching control means includes a polarity of an output voltage to be output from said smoothing means. And when the polarities of the detection currents of the current detection means do not match, a switching control signal for switching each of the switches is generated during a period in which the output voltages of the two AC power supplies are zero. Power converter. 5. The power converter according to claim 1, wherein said switching control means includes a polarity of an output voltage to be output from said smoothing means. And a switching control signal for switching each of the switches in accordance with the output voltages of the two AC power supplies when the polarity of the detection current of the current detection means coincides with that of the current detection means. . 6. The power converter according to claim 1, wherein the switching control means includes a polarity of an output voltage to be output from the smoothing means. When it is determined that the transition to the commutation operation is to be performed from the polarity of the detection current of the current detection means and the commutation operation, a switching control signal for switching the switches according to the output voltages of the two AC power supplies is generated. A power converter, comprising: