Abstract:
The noise generated from a power converter is suppressed by increasing the noise frequency to a level not lower than the maximum frequency of the human audible range. To obtain the frequency of an output current harmonic component as a noise source which has exceeded the maximum frequency of the human audible range, it is adequate to determine that the frequency of a driving carrier wave for the individual converter cells in the power converter, in which the phases of the carrier wave for the converter cells are mutually shifted by a given value between the converter cells, meets the following equation. 
     
       
      
       f 
       carrier 
       ≧f 
       audibility 
       
         — 
       
       max 
       ÷N

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    The present invention is related to PCT application No. PCT/JP2010/052833 filed on Feb. 24, 2010, claiming the invention priority based on Japanese Patent Application No. 2009-080856, filed on Mar. 30, 2009. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The present invention relates to a power converter which has converter cells configured by cascade connection, and more particularly to a power converter which suppresses a problem resulting from output current harmonics. 
         [0003]    A conventional power converter has a multiple transformer to increase the output voltage to a high level and it is linked with an AC system. Since the multiple transformers are large in weight and volume, the whole apparatus has a large weight and volume as a result. 
       SUMMARY OF THE INVENTION 
       [0004]    Since the conventional power converter is hard to output a high voltage and uses the multiple transformer to link with the AC system, the whole apparatus results in increasing its weight and volume. Especially, since the power converter is mounted on a power pole when it is used for power distribution, the increase in weight and volume of the apparatus is a big issue to be solved. To solve the problem, research and development have been conducted in a power converter which can be linked with a distribution system without using a transformer by connecting elements including IGBT (Insulated Gate Bipolar Transistor) in series to increase the output voltage as disclosed in “Series Connection of Snubberless IGBTs for 6.6 kV Transformerless Converter”, J. Narushima, IEEE/PCC Nagoya 2007. But, the above power converter produces an output current having a large harmonic component because the output voltage has a pulse state. When the current harmonic is large, a magnetomotive force is variable in the reactor or the like of the distribution system and the reactor is vibrated to generate noise. Since this reactor is directly connected to the distribution system through an electric cable, the same vibration is propagated to a distribution line, a power pole and others, which also become noise sources. In a case where the reactor is used for electric power distribution, it is installed in a residential area, so that noise generation is a particularly serious problem, and the above-described noise reduction was a big technical object. 
         [0005]    To achieve the above object, the present invention provides a power converter provided with a plurality of arms which are comprised of a plurality of converter cells connected in cascade, comprising means for mutually shifting phases of carrier waves of the converter cells by a prescribed value between the converter cells in the arms; and means for operating frequencies of the carrier waves of the converter cells in a state larger than a value which is obtained by dividing a maximum frequency of a human audible frequency band by the number of the converter cells in the arms. 
         [0006]    The power converter of the present invention also comprises interconnected reactors; buffer reactors; arms configured of a plurality of converter cells connected in cascade; a control device for controlling the converter cells; and a signal line for transmitting a control signal from the control device to the converter cells, wherein the arms are arranged on positive and negative sides of each of three phases. 
         [0007]    The power converter of the present invention also has the converter cell which is provided with a capacitor for storing electric power; fuses for cutting off the electric current in case of overcurrent; a cell configured with a plurality of switching elements connected in a half bridge state; a converter cell control circuit for outputting a control signal for the switching elements; a gate driver for applying a voltage to the switching elements according to the control signal outputted from the converter cell control circuit; a gate power supply for supplying necessary electric power to the gate driver; and a self-feeding supply for supplying necessary electric power to the gate power supply and the converter cell control circuit. 
         [0008]    The power converter of the present invention also comprises a synchronous signal generator for generating a synchronous signal at every given time; a carrier wave generator for generating a carrier wave for the converter cell after having as an input the synchronous signal which is outputted from the synchronous signal generator; and an individual-phase target voltage generator for generating a target voltage of each of three phases which are mutually shifted by ⅓ cycle. 
         [0009]    The power converter of the present invention also has the converter cell control circuit which is provided with a comparator which outputs to the gate driver an ON/OFF signal for the switching elements according to a magnitude relation between the target voltage and the carrier wave for the converter cell. 
         [0010]    The power converter of the present invention also has the converter cell which is provided with a capacitor for storing electric power; fuses for cutting off the electric current in case of overcurrent; a cell configured with a first leg and a second leg, which are configured with a plurality of switching elements connected in series, connected in a full bridge state; a converter cell control circuit for outputting a control signal for the switching elements; a gate driver for applying a voltage to the switching elements according to the control signal outputted from the converter cell control circuit; a gate power supply for supplying necessary electric power to the gate driver; and a self-feeding supply for supplying necessary electric power to the gate power supply and the converter cell control circuit. 
         [0011]    The power converter of the present invention also comprises a synchronous signal generator for generating a synchronous signal at every given time; a carrier wave generator for generating a carrier wave for the converter cell after having as an input the synchronous signal which is outputted from the synchronous signal generator; a first individual-phase target voltage generator for generating a target voltage of each of three phases which are mutually shifted by ⅓ cycle; and a second individual-phase target voltage generator for outputting a target voltage with the phase shifted by ½ cycle with respect to the target voltage which is outputted from the first individual-phase target voltage generator. 
         [0012]    The power converter of the present invention also has the converter cell control circuit which is provided with a first comparator for generating an ON/OFF signal for the switching elements configuring the first leg according to the magnitude relation between the target voltage outputted from the first individual-phase target voltage generator and the carrier wave for the converter cell outputted from the carrier wave generator; and a second comparator for generating an ON/OFF signal for the switching elements configuring the second leg according to the magnitude relation between the target voltage outputted from the second individual-phase target voltage generator and the carrier wave for the converter cell outputted from the carrier wave generator. 
         [0013]    The power converter of the present invention also comprises a synchronous signal generator for generating a synchronous signal at every given time; and an individual-phase target voltage generator for generating a target voltage of each of three phases which are mutually shifted by ⅓ cycle. 
         [0014]    The power converter of the present invention also has the converter cell control circuit which is provided with a carrier wave generator for outputting a carrier wave for the converter cell after having as an input the synchronous signal which is outputted from the synchronous signal generator; and a comparator for generating an ON/OFF signal for the switching elements according to the magnitude relation between the carrier wave for the converter cell outputted from the carrier wave generator and the target voltage outputted from the individual-phase target voltage generator. 
         [0015]    The power converter of the present invention also has the control device which is provided with a synchronous signal generator for generating a synchronous signal at every given time; a first individual-phase target voltage generator for generating a target voltage of each of three phases which are mutually shifted by ⅓ cycle; and a second individual-phase target voltage generator for outputting a target voltage with the phase shifted by ½ cycle with respect to the target voltage which is outputted from the first individual-phase target voltage generator. 
         [0016]    The power converter of the present invention also has the converter cell control circuit which is provided with a carrier wave generator for outputting a carrier wave for the converter cell after having as an input the synchronous signal which is outputted from the synchronous signal generator; a first comparator for generating an ON/OFF signal for the switching elements configuring the first cell according to the magnitude relation between the target voltage outputted from the first individual-phase target voltage generator and the carrier wave for the converter cell outputted from the carrier wave generator; and a second comparator for generating an ON/OFF signal for the switching elements configuring the second cell according to the magnitude relation between the target voltage outputted from the second individual-phase target voltage generator and the carrier wave for the converter cell outputted from the carrier wave generator. 
         [0017]    The power converter of the present invention also has the control device which is provided with an individual-phase target voltage generator for generating a target voltage of each of three phases which are mutually shifted by ⅓ cycle; a synchronous signal generator for generating a synchronous signal at every given time; a carrier wave generator for generating a carrier wave for the converter cell after having as an input the synchronous signal which is outputted from the synchronous signal generator; and a comparator which outputs to the converter cell control circuit an ON/OFF signal for the switching element according to a magnitude relation between the target voltage and the carrier wave for the converter cell. 
         [0018]    The power converter of the present invention also has the control device which is provided with a first individual-phase target voltage generator for generating a target voltage of each of three phases which are mutually shifted by ⅓ cycle; a second individual-phase target voltage generator for outputting a target voltage with the phase shifted by ½ cycle with respect to the target voltage which is outputted from the first individual-phase target voltage generator; a synchronous signal generator for generating a synchronous signal at every given time; a carrier wave generator for generating a carrier wave for the converter cell after having as an input the synchronous signal which is outputted from the synchronous signal generator; a first comparator for generating an ON/OFF signal for the switching elements configuring a first leg according to the magnitude relation between the target voltage outputted from the first individual-phase target voltage generator and the carrier wave outputted from the carrier wave generator; and a second comparator for generating an ON/OFF signal for the switching elements configuring a second leg according to the magnitude relation between the target voltage outputted from the second individual-phase target voltage generator and the carrier wave outputted from the carrier wave generator. 
         [0019]    The power converter of the present invention also comprises a transformer; a buffer reactor; arms configured of a plurality of converter cells connected in cascade; a control device for controlling the converter cells; and a signal line for transmitting a control signal from the control device to the converter cells, wherein the arms are arranged on each of three phases. 
         [0020]    The power converter of the present invention also has the control signal between the control device and the converter cell control circuit which is transmitted wirelessly instead of through the signal line. 
         [0021]    According to the invention, to suppress the noise generated by the interconnected reactor and the like of the distribution system, the power converter having a plurality of converter cells, which are configured of an IGBT and the like, cascade connected for individual phases is linked with the distribution system, and the interconnected reactor is disposed between them. At this time, the noise frequency is increased to a level not lower than the maximum frequency of the human audible range to suppress the noise generated from the above-described power converter. To do so, the frequency of an output current harmonic component which is the above-described noise source needs to exceed the maximum frequency of the human audible range. To obtain the frequency of the output current harmonic component which has exceeded the maximum frequency of the human audible range, it is adequate to determine that the frequency of a driving carrier wave for each converter cell in the power converter in which the phases of the carrier waves of the converter cells are mutually shifted by a given value between the converter cells meets the following equation (1): 
         [0000]        f   carrier   ≧f   audibility     —     max   ÷N   (1)
 
         [0000]    where, f carrier  is the frequency of the converter cell carrier wave, f audibility     —     max  is the maximum frequency of the human audible range, and N is the number of the converter cells cascade connected for each phase. 
         [0022]    According to the power converter of the invention, since the frequency of the output current harmonic component which was the cause of the noise exceeds the human audible range, the sound cannot be recognized by a human being, and the noise suppression can be realized as a result. 
         [0023]    Other objects, features and advantages of the invention will become apparent from the following description of the embodiments of the invention taken in conjunction with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0024]      FIG. 1  is a configuration view of a power converter linked with a distribution system according to Embodiment 1. 
           [0025]      FIG. 2  is a schematic configuration view of a control device for controlling the power converter in Embodiment 1. 
           [0026]      FIG. 3  is a diagram showing a state of synchronous processing between converter cells performed by the control device. 
           [0027]      FIG. 4  is a configuration view of a converter cell which configures the power converter in Embodiment 1. 
           [0028]      FIG. 5  is a schematic configuration view of a converter cell control circuit which configures the converter cell in Embodiment 1. 
           [0029]      FIG. 6  is a diagram of converter cell carrier waves and target voltage waveform in individual phase arms and voltage waveforms between terminals of the converter cell in Embodiment 1. 
           [0030]      FIG. 7  is an example of a composite pulse voltage waveform of the arm. 
           [0031]      FIG. 8  is an example of output current waveforms of the power converter. 
           [0032]      FIG. 9  is a configuration view of a converter cell which configures the power converter in Embodiment 2. 
           [0033]      FIG. 10  is a schematic configuration view of a control device for controlling the power converter in Embodiment 2. 
           [0034]      FIG. 11  is a schematic configuration view of a converter cell control circuit which configures the converter cell in Embodiment 2. 
           [0035]      FIG. 12  is a diagram of converter cell carrier waves and target voltage waveform in individual phase arms and voltage waveforms of the converter cells in Embodiment 2. 
           [0036]      FIG. 13  is a schematic configuration view of a control device for controlling the power converter in Embodiment 3. 
           [0037]      FIG. 14  is a schematic configuration view of the converter cell control circuit configuring the converter cell in Embodiment 3. 
           [0038]      FIG. 15  is a schematic configuration view of a control device for controlling the power converter in Embodiment 4. 
           [0039]      FIG. 16  is a schematic configuration view of the converter cell control circuit configuring the converter cell in Embodiment 4. 
           [0040]      FIG. 17  is a schematic configuration view of a control device for controlling the power converter when the circuit configuration of the converter cell is a half bridge type in Embodiment 5. 
           [0041]      FIG. 18  is a schematic configuration view of a control device for controlling the power converter when the circuit configuration of the converter cell is a full bridge type in Embodiment 5. 
           [0042]      FIG. 19  is a configuration view of a power converter linked with a distribution system in Embodiment 6. 
           [0043]      FIG. 20  is a schematic configuration view of a control device for controlling the power converter in Embodiment 9. 
           [0044]      FIG. 21  is a schematic configuration view of the converter cell control circuit configuring the converter cell in Embodiment 9. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0045]    One embodiment of the invention is described below with reference to the drawings. The following embodiment shows one mode of the invention and the invention also includes other modes without departing from the scope of the invention. 
       Embodiment 1 
       [0046]      FIG. 1  is a configuration view of a power converter having converter cells cascade connected and in a state linked with a system  107  according to one embodiment of the invention. A power converter  101  comprises converter cells  102 , a control device  103 , a signal line  104  for transmitting control signals from the control device  103  to the individual converter cells, buffer reactors  105 , and interconnected reactors  106 . In the drawing,  102 _U,  102 _V,  102 _W,  102   —   u ,  102   —   v  and  102   —   w  have the plural converter cells  102  connected in cascade and are defined as arms. Thus, the upper arms  102 _U,  102 _V and  102 _W are called a U-phase positive side arm, a V-phase positive side arm and a W-phase positive side arm, and the lower arms  102   —   u ,  102   —   v  and  102   —   w  are called a U-phase negative side arm, a V-phase negative side arm and a W-phase negative side arm. And, the converter cells  102  and the control device  103  are connected in a row by the signal line  104  as shown in  FIG. 1 . 
         [0047]      FIG. 2  shows a schematic configuration view of the control device  103 . The control device  103  comprises an individual-phase target voltage generator  108  which outputs a target voltage of each of the U-phase, the V-phase and the W-phase, a synchronous signal generator  109  which generates an execution signal (hereinafter called a synchronous signal) for interrupt processing to be conducted by the control device  103  at every given time, and a carrier wave generator  110  which outputs a carrier wave for all converter cells within the power converter. Target voltages outputted from the individual-phase target voltage generator  108  are of the U-phase, the V-phase and the W-phase, and the phases of these three-phase target voltages are shifted by ⅓ cycle respectively. And, when it is determined that the number of the converter cells in the above-described arm is N, the number of data of the carrier wave for the converter cell outputted from the carrier wave generator  110  is 6N. 
         [0048]    The control device  103  conducts interrupt processing of the carrier waves of the converter cells at every given time ΔT in order to synchronize the k-th (k=1, . . . , N) converter cells mutually in each arm, and the synchronous signal generator  109  outputs a synchronous signal at the every given time ΔT. When the above-described synchronous signal is inputted, the carrier wave generator  110  immediately outputs a carrier wave which is forcibly corrected to a prescribed value as shown in  FIG. 3 . Here, in a case where the k-th (k=1, . . . , N) carrier wave for the upper arms is corrected to a prescribed value A 1 , the k-th (k=1, . . . , N) carrier wave for the lower arms is corrected to a prescribed value −A 1 . For the synchronization of the carrier wave by using the above-described synchronous signal, one of the converter cells in the individual arms is synchronized for each input of the synchronous signal to the carrier wave generator  110 . This procedure is conducted sequentially from the first to N-th converter cells in the individual arms. A series of work to synchronize between the above converter cells is called synchronous processing below. The above-described target voltages and carrier waves outputted from the individual-phase target voltage generator  108  and the carrier wave generator  110  are transmitted to the appropriate converter cells through the signal line  104 . 
         [0049]    A configuration of the converter cell  102  is shown in  FIG. 4 . The converter cell  102  is comprised of a cell  114  comprising two IGBT elements  111 , a DC capacitor  112  and two fuses  113 , a converter cell control circuit  115 , a gate driver  116 , a self-feeding supply  117 , and a gate power supply  118 . As electric power for the converter cell control circuit  115  and the gate driver  116 , the electric power charged into the DC capacitor  112  is supplied via the self-feeding supply  117  and the gate power supply  118 . 
         [0050]      FIG. 5  shows a schematic configuration view of the converter cell control circuit. In the converter cell control circuit  115 , the target voltage and the carrier wave for the converter cell transmitted from the control device  103  through the signal line  104  are compared for magnitude by a comparator  119 , and the generated switch ON/OFF signal is outputted to the gate driver  116 . According to the ON/OFF information, the gate driver  116  applies the voltage from the gate power supply  118  to the gate terminals of the two IGBT elements  111  to drive the IGBT elements  111 . At this time, when one of the ON/OFF information of the two IGBT elements is ON, the other is OFF. And, an inversion of the ON/OFF signal for the positive side arm becomes the ON/OFF signal for the negative side arm. 
         [0051]    To suppress the noise generated due to the output current harmonic component, the phases of the carrier waves of the k-th and k+first (k=1, . . . , N−1) converter cells of each arm are shifted by a prescribed value respectively. This phase shift is performed by using the above-described synchronous processing. The frequency of the carrier wave for the converter cell is determined to meet the above-described equation. And, an output cycle ΔT of the above-described synchronous signal is determined according to the following equation (2). 
         [0000]      Δ T= 1/( n×f   carrier )  (2)
 
         [0052]      FIG. 6  shows a magnitude relation between the carrier waves and the target voltage of the converter cell and the output voltages of the converter cell at the time. The upper section of  FIG. 6  shows the carrier waves and the target voltage waveform of the converter cell. Meanwhile, the lower section shows the voltage waveforms between terminals of the individual converter cells when it is determined that the voltage between the terminals of the converter cell is E [V]. The each converter cell compares the target voltage and the converter cell carrier wave for magnitude as described above, and when the target voltage is larger, the voltage E [V] is generated in the converter cell by switching ON an IGBT element  111   a  and switching OFF an IGBT element  111   b . Conversely, when the target voltage is smaller than the carrier wave for the each converter cell, the voltage applied to the converter cell  102  becomes 0 [V] by switching OFF the IGBT element  111   a  and switching ON the IGBT element  111   b . Thus, the output voltages of the individual phases become a total of the voltages applied to the converter cells  102 . As described above, the ON/OFF timing of the converter cells in the arms is shifted when the phases of the individual converter cell carrier waves in the arms are shifted. 
         [0053]      FIG. 7  shows a composite pulse voltage waveform (arm voltage waveform) of the arm when the power converter is operated as described above. 
         [0054]    When the present invention is used, an output current waveform becomes as shown in  FIG. 8 , and the frequency f ripple  of the output current harmonic component exceeds the maximum frequency of the human audible range. Therefore, sounds generated from the interconnected reactor, distribution system, etc. due to the f ripple  cannot be recognized by human beings. As a result, it becomes possible to suppress the noise. 
       Embodiment 2 
       [0055]    In Embodiment 1 described above, the converter cells  102  are configured of a so-called half bridge type circuit as shown in  FIG. 4 , but they may be configured of a full bridge type circuit as shown in  FIG. 9 . In the drawing,  111 _A designates an IGBT element  111   a  and an IGBT element  111   b , and  111 _B designates an IGBT element  111   c  and an IGBT element  111   d . When 111_A and  111 _B are defined as legs respectively, the converter cell  102  in this embodiment is configured of a cell  114 , which comprises the legs  111 _A and  111 _B each having two IGBT elements  111 , a DC capacitor  112  and four fuses  113 , a converter cell control circuit  115 , a gate driver  116 , a self-feeding supply  117  and a gate power supply  118 . 
         [0056]      FIG. 10  shows a schematic configuration view of the control device  103  in this embodiment. In the full bridge type circuit of this embodiment, the above-described legs  111 _A and  111 _B control the switching operation according to two types of target voltages which are called as target voltages A and B, and the control device  103  outputs the two types of target voltages and the converter cell carrier wave. The target voltage A is a target voltage for controlling the switching operation of the leg  111 _A, and the target voltage B is a target voltage for controlling the switching operation of the leg  111 _B, and it is determined that the phases of the target voltages A and B are mutually shifted by ½ cycle. 
         [0057]      FIG. 11  shows a schematic configuration view of the converter cell control circuit. The carrier wave outputted from the control device  103  and the individual-phase target voltages A and B also outputted from the control device  103  are respectively compared for magnitude by the comparators  119 , and ON/OFF signals for the legs  111 _A and  111 B are outputted. 
         [0058]      FIG. 12  shows a magnitude relation between carrier waves and target voltages of the converter cells and output voltages of the converter cells at the time in this embodiment. In this embodiment, the target voltage A and the converter cell carrier waves are compared for magnitude, and when the target voltage A is larger in the leg  111 _A, the IGBT element  111   a  is switched ON, and the IGBT element  111   b  is switched OFF. Similarly, when the target voltage B is larger in the leg  111 _B, the IGBT element  111   c  is switched ON, and the IGBT element  111   d  is switched OFF. In  FIG. 12 , the lower part shows output current waveforms of the converter cells when the IGBT element  111   a  is switched ON, the IGBT element  111   b  is switched OFF, the IGBT element  111   c  is switched OFF, the IGBT element  111   d  is switches ON and the voltage between terminals of the converter cells is determined to be E [V]. When the IGBT element  111   a  is switched OFF, the IGBT element  111   b  is switched ON, the IGBT element  111   c  is switched ON and the IGBT element  111   d  is switched OFF, and the voltage between the terminals of the converter cell becomes −E [V], and when the IGBT elements  111   a  and  111   c  and those  111   b  and  111   d  have the same ON/OFF, the voltage between the terminals of the converter cell becomes 0 [V]. It is adequate to determine that the frequency of a driving carrier wave for each converter cell in the power converter of this embodiment meets the following equation (3). 
         [0000]        f   carrier   ≧f   audibility     —     max ÷2 N   (3)
 
       Embodiment 3 
       [0059]    In Embodiments 1 and 2, the outputs of the control device  103  are the target voltages of each of the U-phase, the V-phase and the W-phase and the carrier waves of the converter cells but may be individual-phase target voltages and synchronous signals. 
         [0060]      FIG. 13  shows a schematic configuration view of the control device  103  when the converter cell  102  is configured of a half bridge type circuit, and  FIG. 14  shows a schematic configuration view of the converter cell control circuit  115 . In this embodiment, the synchronous signal is outputted to the appropriate converter cell control circuit  115  at the every given time ΔT. And, in the converter cell control circuit  115 , the carrier wave outputted from the carrier wave generator  110 , which forcefully corrects a value by the synchronous signal inputted at every cycle ΔT, is compared with the target voltage outputted from the control device  103  for the magnitude by the comparator  119 , and the ON/OFF signal is outputted to the gate driver  116  according to the magnitude relation. 
       Embodiment 4 
       [0061]    In Embodiment 3, the converter cell  102  is configured of the half bridge type circuit as shown in  FIG. 4 , but it may be a full bridge type circuit as shown in  FIG. 9 . 
         [0062]      FIG. 15  shows a schematic configuration view of the control device  103 , and  FIG. 16  shows a schematic configuration view of the converter cell control circuit  115 . The outputs of the control device  103  are the target voltages A and B outputted from individual-phase target voltage generators  108 _A and  108 _B and the synchronous signal outputted from the synchronous signal generator  109 . The converter cell control circuit  115  has as an input the synchronous signal outputted from the control device  103 , compares the carrier wave outputted from the carrier wave generator  110  and the target voltages A and B also outputted from the control device  103  for magnitude by the comparator  119 , and outputs the ON/OFF signal to the gate driver  116  according to the magnitude relation. 
       Embodiment 5 
       [0063]    In Embodiments 1 to 4, the individual-phase target voltages and the switching carrier waves of the individual converter cells or the synchronous signal is outputted from the control device  103 , but the switching ON/OFF signals for the individual converter cells may be used instead. 
         [0064]      FIG. 17  shows a schematic configuration view of the control device  103  when the converter cell  102  is configured of the half bridge type circuit.  FIG. 18  shows a schematic configuration view of the control device  103  when the converter cell  102  is configured of the full bridge circuit. In this embodiment, the switching operations of all the converter cells  102  within the power converter  101  are computed by the control device  103 . 
       Embodiment 6 
       [0065]    The power converter of the invention can have the configuration shown in  FIG. 19  in addition to that shown in  FIG. 1 . In this embodiment, the power converter  101  is configured of individual arms  102 _U,  102 _V and  102 _W of the U-phase, the V-phase and the W-phase, a signal line  104 , buffer reactors  105  and a transformer  120  which is linked with a distribution system  107 . The converter cells  102  and the control device  103  are connected in a row by the signal line  104  as shown in  FIG. 19 . The individual-phase target voltages of the U-phase, the V-phase and the W-phase and the carrier waves of the converter cells are outputted by the control device  103  and transmitted to the individual converter cells  102  through the signal line  104 . The converter cell  102  may be configured of either the half bridge type circuit shown in  FIG. 4  or the full bridge type circuit shown in  FIG. 9 . 
       Embodiment 7 
       [0066]    In Embodiment 6, the outputs of the control device  103  are the individual-phase target voltages and the carrier waves of the converter cells but may be the individual-phase target voltages and synchronous signals. At the time, the control device  103  and the converter cell control circuit  115  are configured in the same manner as those described in Embodiment 3 or Embodiment 4. 
       Embodiment 8 
       [0067]    The outputs of the control device  103  in Embodiments 6 and 7 are the individual-phase target voltages and the phases of the switching carrier waves of the individual converter cells or the synchronous signals but may be the individual-phase target voltages and the switching ON/OFF signals for the converter cells. At the time, the control device  103  and the converter cell control circuit  115  are configured in the same manner as those described in Embodiment 5. 
       Embodiment 9 
       [0068]    In Embodiments 1 to 8, the control signal is transmitted from the control device  103  to the converter cell control circuit  115  through the signal line  104 , but it may be transmitted wirelessly. Thus, the signal line  104  in the power converter can be eliminated, and maintenance and inspection works for the apparatus can be reduced. 
         [0069]      FIG. 20  shows a schematic configuration view of the control device  103  configured as shown in Embodiment 1, and  FIG. 21  shows a schematic configuration view of the converter cell control circuit  115 . The control device  103  is newly provided with a data transmission portion  121  for transmitting data, and the converter cell control circuit  115  is newly provided with a data receiving portion  122  for receiving data. The data transmission portion  121  transmits a target voltage of each of the U-phase, the V-phase and the W-phase and carrier waves of 6N converter cells  102 . The receiving portion  122  receives the appropriate target voltages and carrier waves respectively. 
         [0070]    The control devices  103  and the converter cell control circuits  115  of Embodiments 2 to 8 are also configured in the same manner. 
         [0071]    The present invention can be used for a static synchronous compensator (STATCOM), a back-to-back system (such as a frequency converter), a high voltage direct current system (HVDC), a motor drive, etc. in addition to the power converter for electric power distribution. 
         [0072]    It should be further understood by those skilled in the art that although the foregoing description has been made on embodiments of the invention, the invention is not limited thereto and various changes and modifications may be made without departing from the spirit of the invention and the scope of the appended claims.