KR20100104665A - Power converter and power conversion system using high frequency transformer - Google Patents

Abstract

PURPOSE: A power converter and a power conversion system are provided to reduce a size and manufacturing costs by maintaining the current of an AC power source as a sine wave without an inductor and a DC capacitor. CONSTITUTION: A power converter includes three unit cells(110). Each unit cell comprises an input terminal converter(130), an output terminal converter(150), and a transformer(140). An input terminal converter is connected to an AC power source. An output terminal converter is connected to an additional AC power source or AC motor. A transformer includes a single phase input unit and a three phase output unit. The single phase input unit is connected to the input terminal converter. A three phase output unit is connected to the output terminal converter.

Description

Power converter and power conversion system using high frequency transformer

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a power converter and a power conversion system, and more particularly, to a power converter and a power conversion system in which electric power is transferred from an AC to a DC, a DC to an AC, or an AC having a different voltage frequency. will be.

As a distribution distribution voltage for supplying power above MVA, so-called high voltage (Medium Voltage), which is usually from 1500V to 22000V, is commercially available worldwide.

The most commonly used high voltage AC power conversion system today is the Cascaded H Bridge. Cascaded H-bridge is a three-phase rectifier diode circuit that obtains a DC voltage and converts it into a single-phase alternating current of variable voltage and variable frequency as a unit cell, and connects the single-phase output of the unit cell in series. .

In such a connection structure, a transformer of AC commercial power frequency (50 Hz or 60 Hz) must be connected to the input, and this transformer has many secondary windings because it must be able to connect several cells. In particular, because the total power is converted to AC commercial power frequency, the size and weight of the transformer becomes very large. Such large-capacity transformers generate many problems, such as power loss and audible frequency noise.

In addition, in order to suppress the DC terminal voltage pulsation caused by the single-phase AC output, a large DC capacitor is required. These DC stage capacitors increase the volume and size of each unit cell, significantly reduce the reliability of the system and require continuous maintenance.

If a pulse width modulaton boost rectifier is used for bidirectional power conversion, a three-phase inductor is required to connect the transformer secondary-side three-phase AC voltage source, which is also the size of the power conversion system. To increase prices and reduce price competitiveness.

In order to solve the shortcomings of the above circuit, a cascaded H bridge using a high frequency transformer rather than a commercial power frequency has been proposed. This method uses high frequency transformer to isolate input and output and connect each unit cell in series.

However, in the circuit constituting each unit cell, in order to maintain the current of the AC input commercial power as a sine wave, the input stage rectifier must be a single-phase pulse width modulated step-up rectifier, resulting in switching loss and control complexity. In addition, a large amount of inductance is required between the commercial power supply and the rectifier to make the supply current sinusoidal. These inductances cause problems not only in size but also in their own loss. In addition, a large capacity electrolytic capacitor is required in order to suppress the DC stage voltage pulsation caused by the single-phase output in the input and output stage power converters. These electrolytic capacitors, installed in both DC stages, reduce the reliability of the system and lead to ongoing maintenance problems, as mentioned earlier.

In addition, since each power switch of a unit cell switches at a high frequency, power conversion efficiency is reduced due to switching loss.

Therefore, the circuit using the high frequency transformer solves the problems of volume, weight, audible noise, and complicated windings caused by the 50 Hz or 60 Hz commercial frequency transformer, but there are still problems due to the inductance connected to the AC power source and the capacitors of both DC stages of the unit cell. Done. In particular, even when bidirectional power conversion is not required, the price-competitive power is reduced because a pulse width modulated step-up rectifier must be used for the input rectifier.

A commercial frequency transformer is used for electrical isolation even when a DC power source is electrically insulated from a commercial AC power source, or when the DC power source is insulated and connected to a commercial AC power source. Type rectifier is used. These circuits also cause many of the problems caused by the commercial frequency transformers and input inductances mentioned earlier.

SUMMARY OF THE INVENTION The present invention has been made to solve the problems described above, and the power converter and the power conversion system according to the present invention output pulses of the single-phase inverter switches of the single-phase inverter switch of the input stage converter of each unit cell and simultaneously output the output of each single-phase inverter. A power converter for which a large inductance and a large DC stage capacitor are unnecessary, by connecting a unit to an input of a high frequency transformer and connecting an output of the high frequency transformer in series to use an input of an output stage converter to maintain a sinusoidal current of the AC power stage. Its purpose is to provide a power conversion system.

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a power converter and a power conversion system, and more particularly, to a power converter and a power conversion system in which electric power is transferred from AC to DC, DC to AC, or AC with different voltage frequencies. It is about.

A power converter in which power is transferred from AC to AC with a different voltage frequency includes three unit cells, each unit cell comprising an input stage converter connected to an AC power source, an output stage converter connected to an AC motor, and a single phase connected to the input stage converter. A transformer including an input of an output and a three-phase output connected to the output stage converter, wherein the three-phase output of the transformer of each unit cell is connected in series with each other such as three-phase outputs of transformers of other unit cells. do.

Meanwhile, a power conversion system in which power is transferred from an alternating current to an alternating current having a different voltage frequency includes a plurality of power converters, the input terminal of the first power converter is connected to an AC power source, and the output terminal of the first power converter is separate. And a plurality of power converters connected in series with each other, wherein each power converter includes three unit cells, wherein each unit cell includes an input stage converter, an output stage converter, and the input stage. A transformer comprising a single phase input connected to a converter and a three phase output connected to the output stage converter, wherein the three phase outputs of the transformer of each unit cell are the same phases as the three phase outputs of the transformers of other unit cells. Are connected in series with each other.

In addition, a power converter that transmits power from AC to DC, from DC to AC, includes three input stage converters connected to an AC power source, one output stage converter and a single phase input unit connected to the input stage converter, and a single phase connected to the output stage converter. A transformer comprising an output, wherein the output of the transformer is connected in series with the outputs of the other transformers.

A power conversion system for transmitting power from AC to DC, DC to AC, including a plurality of power converters, the input terminal of the first power converter is connected to an AC power source, the input terminals of the plurality of power converters are connected in series with each other Each power converter includes a transformer including three input stage converters, one output stage converter and a single phase input unit connected to the input stage converter, and a single phase output unit connected to the output stage converter, wherein the output unit of the transformer It is connected in series with the outputs of the other transformers.

The power converter and power conversion system according to the present invention has the following effects.

Simultaneously pulse-modulate the single-phase inverter switch of the input stage converter of each unit cell and connect the output of each single-phase inverter to the input of the high-frequency transformer, and connect the output of the high-frequency transformer in series to use as an input of the output stage converter. Since the sine wave is maintained, no large inductance and a large DC stage capacitor are required, thereby reducing the size of the power conversion system and increasing price competitiveness.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. However, the present invention is not limited by the following examples.

1 shows a circuit diagram of a power converter according to a first embodiment of the present invention. As shown in FIG. 1, the power converter according to the first embodiment is largely composed of three unit cells 110. Each unit cell is again connected to the input terminal converter 130 connected to the AC power source 120, a separate AC power source (not shown), or an output terminal converter 150 connected to the AC motor 160, and a single phase connected to the input terminal converter 130. The transformer 140 includes an input unit 141 and a three-phase output unit 142 connected to the output stage converter 150.

The input stage converter 130 and the output stage converter 150 are composed of rectifiers 132 and 152 and inverters 133 and 153, respectively. The output terminals of the rectifiers 132 and 152 are connected to the input terminals of the inverters 133 and 153.

In the circuit of the rectifier 132 of the input stage converter 130, when the power is transferred from the power source to the load, that is, in one direction, a single phase diode rectifying circuit may be used as shown, and in the case of bidirectional power transfer, the diode An active rectifying circuit (not shown) capable of connecting the active switching elements in parallel and in parallel to transmit power in both directions can be used. The active switching device has the same switching frequency as the commercial power supply frequency. Switching losses are therefore negligible and do not require a separate inductance to create sinusoidal current at the input.

Preferably, an inductor or capacitor having a cutoff frequency higher than the commercial power frequency and having a cutoff frequency lower than the inverter switching frequency of the input stage converter in order to suppress the inductance component inherent in the AC power stage and to suppress the high frequency current flowing from the load stage. The high frequency filter 131 may be configured. However, the size of the high frequency filter 131 is very small compared to the input inductor for the pulse width modulation step-up rectifier of the conventional power conversion system using a high frequency transformer.

In addition, the rectifier 132 and the inverter 133 of the input stage converter 130 for use of a snubber circuit for suppressing a voltage increase applied to the device due to the stray inductance of each circuit element connected to the input terminal. An inductor and / or a capacitor may be connected to the DC terminal 134 therebetween. However, in case of connecting the inductor, it is preferable to connect only the capacitor because the complexity of the control may be caused. In the case of the capacitor, the size of the capacitor is very small compared to the DC stage capacitor for absorbing the pulsating power shaken at twice the commercial frequency by the conventional single-phase power conversion, and can be easily implemented as a film capacitor instead of an electrolytic capacitor.

The outputs of the three phases of the transformer of each unit cell 110 are connected in series with one another, such as the outputs of the three phases of transformers of other unit cells. The current of the AC power stage is pulse width modulated on the output side single phase inverter switch of the input stage converter of each unit cell 110, and the output of each single phase inverter is connected to the primary input portion 141 of the high frequency transformer 140 and the secondary side. By connecting the three-phase output unit 142 in series as described above to use as an input of the output stage converter 150, the rectifier 132 of the input stage converter 130 without switching the rectifier 132 of the input stage converter 130 The current of the AC power supply 120 connected thereto may be maintained as a sine wave current without a low order harmonic current. In addition, the switching current generated by the inverter 133 of the input stage converter 130 is filtered by the capacitor of the DC terminal 134 of the input stage converter 130 and is blocked by the high frequency filter 131 to be alternating with the AC power supply 120. The sinusoidal current flows from the high frequency switching current.

When the input unit of the rectifier 152 of the output stage converter 150 is connected to the secondary output unit 142 of the high frequency transformer as described above, the rectifier 152 of the output stage converter 150 is modulated through the pulse width modulation of the input stage inverter 133. The average voltage during one switching cycle of the input can be made constant and its magnitude can be controlled. The rectifier 152 of the output stage converter 150 switches according to the voltage and current polarity of the rectifier input power supply without any modulation, and thus the switching frequency of the rectifier is the same as the output AC frequency of the input stage converter 130.

An inductor and / or a capacitor may be connected to the DC terminal 154 of the rectifier 152 of the output stage converter 150 to suppress the DC terminal voltage pulsation. The inductor or capacitor may minimize the size of the rectifier input because the frequency of the rectifier input is much larger than that of a commercial power supply. In the case of the rectifier 152 of the output stage converter 150, a single-phase diode rectifier circuit may be used when power is transferred from a power source to a load only. Electrolytic capacitors are required (not shown). In order to avoid this, in the case of unidirectional power transmission, an active rectifier having an active switching device connected in parallel with a diode in an output stage rectifier may be used. In this case, even though the DC stage capacitor of the output stage converter is small, the single-phase pulsating power of the load stage is transmitted to the secondary side of the high frequency transformer as it is, and the instantaneous sum of the pulsating powers collected on the secondary side is 0 when the load is three-phase symmetrical. Therefore, it is not transmitted to the primary side of the transformer but is canceled at the secondary side. Therefore, not only the voltage of the DC terminal 154 of the rectifier 152 of the output stage converter 150 but also the DC terminal 134 voltage of the rectifier 132 of the input stage converter 130 may be free from the pulsating power of the output load.

When power is transferred from the output stage converter 150 to the input side AC power source, that is, in the case of regenerative operation, switching of the active rectifier by the output side leakage inductance of the high frequency transformer 140 to the active rectifier input of the output stage converter 150. A snubber circuit 155 may be added to suppress overvoltage applied to the device. Such snubber circuit may be implemented in various forms, and in order to improve energy efficiency, the output of the snubber circuit may be connected to the DC terminal of the output side converter (not shown) to regenerate switching losses due to leakage inductance to the DC terminal. .

2 is a circuit diagram of a power conversion system according to a first embodiment of the present invention. First, as shown in FIG. 2, the power conversion system according to the first embodiment includes a plurality of power converters. When the single power converter described in FIG. 1 deals with a commercial high voltage, when the voltage rating is low, the same power converter may be connected in series to handle the high voltage. The input terminal of the first power converter 200 is connected to the AC power supply 120, the output terminal of the first power converter 200 is connected to a separate AC power source (not shown) or AC motor 160, a plurality of Power converters are connected in series with each other. The series connection of these power converters will handle high voltage AC loads from 1500V to 22000V.

3 shows a circuit diagram of a power converter according to a second embodiment of the present invention. The power converter according to the second embodiment of the present invention is a power converter that converts AC power into DC power, and vice versa, three input stage converters 330 as shown in FIG. It is composed of three transformers 340 including one output stage converter 350 and a single phase input unit 341 connected to the input stage converter 330 and a single phase output unit 342 connected to the output stage converter 350. .

The input stage converter 330 includes a rectifier 332 and an inverter 333. The output terminal of the rectifier 332 is connected to the input terminal of the inverter 333. The output stage converter 350 is configured only with the rectifier 352, so as to obtain a DC power source, and does not require an inverter.

The power converter according to the second embodiment of the present invention may also need a high frequency filter 331 for suppressing the inductance component inherent in the AC power stage and suppressing the high frequency current from the load stage. In addition, the inductor and / or the DC terminal 334 between the rectifier 332 and the inverter 333 of the input stage converter 330 to suppress the voltage rise applied to the device due to the stray inductance of each circuit element connected to the input terminal. Or a capacitor may be connected.

The output 342 of the transformer 340 is connected in series with the output of the other transformer. The current of the AC power stage is pulse width modulated on the output side single phase inverter switch of each input stage converter, and the output of each single phase inverter is connected to the primary side input portion 341 of the high frequency transformer 340 and the output portion 342 on the secondary side. Are connected in series with each other as described above and used as an input of the output stage converter 350, thereby switching the AC power supply 120 connected to the rectifier 332 of the input stage converter 330 without switching the rectifier 332 of the input stage converter 330. The current can be maintained as a sinusoidal current without a low order harmonic current. In addition, the switching current generated by the inverter 333 of the input stage converter 330 is filtered by the capacitor of the DC terminal 334 of the input stage converter 330 and is cut off by the high frequency filter 331 to exchange AC power 120. The sinusoidal current flows from the high frequency switching current.

The circuits of the rectifiers 332 and 352 of the input stage converter 330 and the output stage converter 350 transfer bidirectional power by connecting an active switching element to the diode in parallel and parallel as shown in the case of bidirectional power transfer. This possible active rectifying circuit can be used. However, in the case of unidirectional power conversion in which power is only transferred from alternating current to direct current, the rectifiers 332 and 352 of the input stage converter 330 and the output stage converter 350 may simply be replaced by a single phase full-wave diode rectifier circuit (not shown). .

An inductor and / or a capacitor may be connected to the DC terminal 354 of the rectifier 352 of the output stage converter 350 to suppress the DC terminal voltage pulsation. However, if the DC power is connected to the load instead of the AC power, in this case, since there is no pulsating power, unlike the single-phase AC output shown in FIG. 1 even though a single phase full-wave diode rectifier circuit is used as the rectifier of the output stage converter. The capacitor of the DC stage 354 of the output stage converter 350 can be kept very small.

As mentioned in FIG. 1, when the power is transferred from the output stage converter DC stage to the AC power, an overvoltage applied to the switching element of the active rectifier by the leakage inductance of the output side of the high frequency transformer 150 to the active rectifier input of the output stage converter 350. A snubber circuit can be added to suppress the noise (not shown).

4 to 5 are diagrams for explaining an embodiment of a system using a power converter according to a second embodiment of the present invention. 4 is a circuit diagram of a power conversion system having several independent direct current terminals according to a second embodiment of the present invention. 5 is a circuit diagram of a power conversion system having one DC terminal according to a second embodiment of the present invention.

As described above, when a single power converter handles commercial high voltage, when the voltage rating is low, the high voltage may be handled by connecting a plurality of identical power converters in series. An input terminal of the first power converter 400 is connected to the AC power supply 120 and is connected in series with an input terminal of another power converter. As shown in FIG. 4, the output stage of the power conversion system may have a plurality of independent DC stages, or as illustrated in FIG. 5, the DC stages may be connected in series to have a high voltage DC stage.

6 to 8 are views for explaining an embodiment of a transformer that can be used in the power conversion system according to the first embodiment of the present invention. FIG. 6 is a diagram of a three-phase, three-leg, 12-wound transformer according to a first embodiment of the present invention. FIG. 7 is a diagram of a single-phase 4N winding transformer according to the first embodiment of the present invention. 8 is a diagram of a three-phase three-leg 12N winding transformer according to the first embodiment of the present invention.

In the power conversion system of FIG. 2, three cells of each power converter are connected through three single-phase four-winding transformers. Therefore, if a series connection of cells of the N power converters is required to directly apply a high voltage, 3N single-phase four-wire transformers are required.

However, when three four-wound transformers for each power converter are manufactured in three legs as in a conventional three-phase transformer as shown in FIG. 6, the number of transformers can be reduced by arranging each phase for each leg. In this case, N three-phase, three-leg, 12-wound transformers are required if N-series cells are connected in series.

In addition, when N single phase transformers of N power converters are connected as shown in FIG. 7, the entire power conversion circuit may be configured by three single phase transformers having 4N windings. In addition, when the three single-phase 4N winding transformer having 4N windings to be expanded to one three-phase transformer, it can be configured as one three-phase three-leg 12N winding transformer as shown in FIG. 8 having 4N windings per leg. In this case, the number of transformers is one like the power conversion system using a commercial frequency transformer. When the switching frequency of the rectifier inverter is 10KHz or more, the size is less than 1/100 of the commercial frequency transformer, and the audible frequency noise is also large. Will be reduced.

9 to 11 are views for explaining an embodiment of a transformer that can be used in the power conversion system according to a second embodiment of the present invention. 9 is a view of a three-phase three-leg six-winding transformer according to a second embodiment of the present invention. 10 is a diagram of a single-phase 2N winding transformer according to a second embodiment of the present invention. 11 is a diagram of a three-phase three-leg 6N winding transformer according to the second embodiment of the present invention.

As described above, in the power conversion system according to the second embodiment, when three two-winding transformers for each power converter are manufactured in three legs as shown in FIG. 9, the number of transformers may be reduced by arranging each phase for each leg.

Therefore, when N single-phase transformers of N power converters are connected as shown in FIG. 10, three single-phase 2N winding transformers having 2N windings may constitute an entire power conversion circuit. In addition, when the three single-phase transformer having 2N windings to be produced as one three-phase transformer by expanding this can be configured as one three-phase three-leg 6N winding transformer as shown in Figure 11 having 2N windings per leg.

As described above, optimal embodiments have been disclosed in the drawings and the specification. Although specific terms have been used herein, they are used only for the purpose of describing the present invention and are not intended to limit the scope of the invention as defined in the claims or the claims. Therefore, those skilled in the art will understand that various modifications and equivalent other embodiments are possible therefrom. Therefore, the true technical protection scope of the present invention will be defined by the technical spirit of the appended claims.

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a power converter and a power conversion system, and more particularly, to a power converter and a power conversion system in which electric power is transferred from an AC to a DC, a DC to an AC, or an AC having a different voltage frequency. will be. In the power converter and the power conversion system according to the present invention, the pulse width modulation of the single-phase inverter switch of the input stage converter of each unit cell, the output of each single-phase inverter is connected to the single-phase input of the high-frequency transformer and the three-phase of the high-frequency transformer By connecting the outputs in series and using them as inputs of the output stage converter, the current of the AC commercial power stage is maintained as a sine wave, which eliminates the need for a large inductance and a large DC stage capacitor, thereby reducing the size of the power conversion system and increasing the price competitiveness. .

1 is a circuit diagram of a power converter according to a first embodiment of the present invention.

2 is a circuit diagram of a power conversion system according to a first embodiment of the present invention.

3 is a circuit diagram of a power converter according to a second embodiment of the present invention.

4 is a circuit diagram of a power conversion system having a plurality of independent direct current terminals according to a second embodiment of the present invention.

5 is a circuit diagram of a power conversion system having one DC terminal according to a second embodiment of the present invention.

6 is a view of a three-phase three-leg 12 winding transformer according to the first embodiment of the present invention.

7 is a view of a single-phase 4N winding transformer according to the first embodiment of the present invention.

8 is a view of a three-phase three-leg 12N winding transformer according to the first embodiment of the present invention.

9 is a view of a three-phase three-leg six-winding transformer according to a second embodiment of the present invention.

10 is a view of a single-phase 2N winding transformer according to a second embodiment of the present invention.

11 is a view of a three-phase three-leg 6N winding transformer according to a second embodiment of the present invention.

Claims (42)

Including three unit cells, wherein each unit cell, An input stage converter connected to an AC power source; An output stage converter connected to a separate AC power or AC motor; And A transformer including a single-phase input connected to the input stage converter and an output of a three-phase connected to the output stage converter, And the outputs of the three phases of the transformer of each unit cell are connected in series with each other, such as the outputs of the three phases of transformers of other unit cells. The method of claim 1, And the transformer is a single-phase four-winding transformer. The method of claim 1, The three transformers of the power converter is replaced with one three-phase three-leg 12-winding transformer having three legs and arranged a single-phase four-winding transformer for each leg. The method of claim 1, And the input stage converter and the output stage converter each include a rectifier and an inverter, and the output stage of the rectifier is connected to an input terminal of the inverter. The method of claim 4, wherein And a high frequency filter is connected to an input of the rectifier of the input stage converter. The method of claim 4, wherein And an inductor or a capacitor is connected to a DC terminal between the rectifier and the inverter of the input stage converter and the output stage converter. The method of claim 4, wherein And the rectifier of the input stage converter and the output stage converter comprises a single phase diode rectifier circuit. The method of claim 4, wherein And the rectifier of the input stage converter and the output stage converter comprises an active rectifier circuit. The method of claim 4, wherein And a snubber circuit is connected to an input of the rectifier of the output stage converter. Including a plurality of power converter, the input terminal of the first power converter is connected to the AC power source, the output terminal of the first power converter is connected to a separate AC power source or AC motor, the plurality of power converters in series with each other Each power converter includes three unit cells, wherein each unit cell includes: Input stage converter; An output stage converter; And A transformer including a single phase input unit connected to the input stage converter and an output unit three phase connected to the output stage converter, And the outputs of the three phases of the transformer of each unit cell are connected in series with each other, such as the outputs of the three phases of transformers of other unit cells. The method of claim 10, The transformer is a power conversion system, characterized in that the single-phase four-winding transformer. The method of claim 10, Wherein the three transformers of the power converter are replaced by one three-phase three-leg twelve winding transformer having three legs and a single-phase four-winding transformer disposed for each leg. The method of claim 10, And a plurality of transformers of the power conversion system are replaced by three single-phase 4N winding transformers. The method of claim 10, And a plurality of transformers of the power conversion system are replaced with one three-phase three-leg 12N winding transformer. The method of claim 10, And the input stage converter and the output stage converter each include a rectifier and an inverter, and the output stage of the rectifier is connected to an input terminal of the inverter. The method of claim 15, And a high frequency filter is connected to an input of the rectifier of the input stage converter. The method of claim 15, And an inductor or a capacitor is connected to a DC terminal between the rectifier and the inverter of the input converter and the output converter. The method of claim 15, And the rectifier of the input stage converter and the output stage converter comprises a single phase diode rectifier circuit. The method of claim 15, And said rectifier of said input stage converter and said output stage converter comprises an active rectifying circuit. The method of claim 15, And a snubber circuit is connected to an input of the rectifier of the output stage converter. Three input stage converters connected to an AC power source; One output stage converter; And A transformer including a single phase input connected to the input stage converter and a single phase output coupled to the output stage converter,  And the output of the transformer is connected in series with the outputs of other transformers. The method of claim 21, And said transformer is a single-phase two-winding transformer. The method of claim 21, The three transformers of the power converter is replaced by one three-phase three-leg six-winding transformer having three legs and arranged a single-phase two-winding transformer for each leg. The method of claim 21, And the input stage converter includes a rectifier and an inverter, the output stage of the rectifier is connected to an input stage of the inverter, and the output stage converter includes a rectifier. The method of claim 24, And a high frequency filter is connected to an input of the rectifier of the input stage converter. The method of claim 24, And an inductor or a capacitor is connected to a DC terminal after the rectifier of the input stage converter and the output stage converter. The method of claim 24, And the rectifier of the input stage converter and the output stage converter comprises a single phase diode rectifier circuit. The method of claim 24, And the rectifier of the input stage converter and the output stage converter comprises an active rectifier circuit. The method of claim 24, And a snubber circuit is connected to an input of the rectifier of the output stage converter. Including a plurality of power converters, the input terminal of the first power converter is connected to the AC power source, the input terminals of the plurality of power converters are connected in series with each other, wherein each power converter, Three input stage converters; One output stage converter; And A transformer including a single phase input connected to the input stage converter and a single phase output coupled to the output stage converter, And the output of the transformer is connected in series with the outputs of other transformers. 31. The method of claim 30, And the output stage of the power converter has a plurality of independent DC stages. 31. The method of claim 30, The power conversion system, characterized in that the output terminal of the power converter has a DC terminal by connecting the DC terminal of the output stage converter in series. 31. The method of claim 30, The transformer is a power conversion system, characterized in that the single-phase two-winding transformer. 31. The method of claim 30, Wherein the three transformers of the power converter are replaced by one three-phase three-leg six-winding transformer having three legs and disposing a single-phase two-winding transformer for each leg. 31. The method of claim 30, And a plurality of transformers of the power conversion system are replaced with three single-phase 2N winding transformers. 31. The method of claim 30, And a plurality of transformers of the power conversion system are replaced with one three-phase three-leg 6N winding transformer. 31. The method of claim 30, And the input stage converter includes a rectifier and an inverter, the output stage of the rectifier is connected to an input stage of the inverter, and the output stage converter includes a rectifier. The method of claim 37, And a high frequency filter is connected to an input of the rectifier of the input stage converter. The method of claim 37, And an inductor or a capacitor is connected to a DC terminal after the rectifier of the input stage converter and the output stage converter. The method of claim 37, And the rectifier of the input stage converter and the output stage converter comprises a single phase diode rectifier circuit. The method of claim 37, And said rectifier of said input stage converter and said output stage converter comprises an active rectifying circuit. The method of claim 37, And a snubber circuit is connected to an input of the rectifier of the output stage converter.

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