Patent Publication Number: US-2022231614-A1

Title: Power conversion device

Description:
TECHNICAL FIELD 
     The present disclosure relates to a power conversion device converting AC power into DC power and vice versa. 
     BACKGROUND ART 
     The Modular Multilevel Converter (MMC) is known as a self-excited power conversion device used for a DC power transmission system. The Modular Multilevel Converter includes, for each phase of AC, an upper arm connected to a high-potential-side DC terminal and a lower arm connected to a low-potential-side DC terminal. Each arm is made up of a plurality of cascaded submodules. 
     For example, Japanese Patent Laying-Open No. 2015-130746 (PTL 1) discloses a power conversion device including a power conversion circuit capable of converting AC to DC or DC to AC. The power conversion circuit includes an arm in which a plurality of unit converters are connected in series to each other. The power conversion device further includes a first control device that perform central control for each of the unit converters, a plurality of second control devices that are daisy-chain connected to the first control device, and a third control device connected to the second control device to control each of the unit converters. 
     CITATION LIST 
     Patent Literature 
     
         
         PTL 1: Japanese Patent Laying-Open No. 2015-130746 
       
    
     SUMMARY OF INVENTION 
     Technical Problem 
     According to PTL 1, a central control device is connected to a plurality of intermediate control devices. The central control device transmits a communication frame including an object arm number, and a cell control unit of the intermediate control device sets the carrier phase of the cell control unit to a predetermined value when the object arm number  404  is identical to the ID number of the arm to which the cell control unit belongs. Therefore, in order to provide commands to all the arms, it is necessary to transmit the communication frame the same number of times as the number of arms. 
     An object of the present disclosure according to an aspect is to provide a power conversion device capable of transmitting a command to each of the submodules included in each arm in as short a time as possible. 
     Solution to Problem 
     In accordance to an embodiment, a power conversion device performing power conversion between a DC circuit and an AC circuit is provided. The power conversion device includes power conversion circuitry including a leg circuit for each phase of the AC circuit. The leg circuit includes a first arm and a second arm. The first arm and the second arm each include a plurality of submodules connected in series to each other. The power conversion device includes a host device to control each submodule included in the power conversion circuitry, and a plurality of repeating devices to relay communication between the host device and each submodule included in the power conversion circuitry. The host device includes: a command information generator to generate command information including an arm command for each arm of the arms included in the power conversion circuitry; and a communication controller provided for each leg circuit or each arm of the arms included in the power conversion circuitry. Each communication controller of a plurality of the communication controllers receives the command information transmitted from the command information generator, extracts, from the command information, an arm command associated with the communication controller, and transmits a communication frame including the extracted arm command to at least one of the repeating devices that is connected to each submodule included in a leg circuit or an arm associated with the communication controller. 
     Advantageous Effects of Invention 
     According to the present disclosure, a command can be transmitted to each of submodules included in each arm in as short a time as possible. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic configuration diagram of a power conversion device. 
         FIG. 2  is a circuit diagram showing an example of submodules forming each leg circuit in  FIG. 1 . 
         FIG. 3  is a block diagram showing a schematic configuration of a command generation device. 
         FIG. 4  illustrates a method for transmitting command information according to Embodiment 1. 
         FIG. 5  illustrates an example of a method for generating arm voltage commands. 
         FIG. 6  illustrates an example network configuration according to Embodiment 1. 
         FIG. 7  illustrates another example network configuration according to Embodiment 1. 
         FIG. 8  illustrates an example of processing of a communication frame by repeating devices according to Embodiment 1. 
         FIG. 9  illustrates processing of a communication frame by repeating devices according to Embodiment 2. 
         FIG. 10  illustrates a data aggregation process according to Embodiment 2. 
         FIG. 11  shows a network configuration according to Embodiment 3. 
         FIG. 12  illustrates an example of processing of a communication frame by submodules  7  according to Embodiment 3. 
         FIG. 13  illustrates another example of processing of a communication frame by submodules  7  according to Embodiment 3. 
         FIG. 14  illustrates a method for transmitting command information according to Embodiment 4. 
         FIG. 15  shows a network configuration according to Embodiment 5. 
         FIG. 16  illustrates a method for transmitting command information according to Embodiment 5. 
         FIG. 17  illustrates a data aggregation process according to another embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Embodiments of the present disclosure are described hereinafter with reference to the drawings. In the following description, the same components are denoted by the same reference characters. They are named identically and function identically as well. Therefore, a detailed description thereof is not herein repeated. 
     Embodiment 1 
     &lt;Configuration of Power Conversion Device&gt; 
       FIG. 1  is a schematic configuration diagram of a power conversion device. Referring to  FIG. 1 , power conversion device  1  is configured in the form of a modular multilevel converter including a plurality of submodules (corresponding to “SM” in  FIG. 1 )  7  connected in series to each other. “Submodule” is also called “converter cell” or “unit converter.” Power conversion device  1  performs power conversion between a DC circuit  14  and an AC circuit  12 . Specifically, power conversion device  1  includes power conversion circuitry  2  and a command generation device  3 . 
     Power conversion circuitry  2  includes a plurality of leg circuits  4   u ,  4   v ,  4   w  (hereinafter also referred to collectively as “leg circuit  4 ”) connected in parallel with each other between a positive DC terminal (i.e., high-potential-side DC terminal) Np and a negative DC terminal (i.e., low-potential-side DC terminal) Nn. 
     Leg circuit  4  is provided for each of a plurality of phases of AC. Leg circuit  4  is connected between AC circuit  12  and DC circuit  14  for performing power conversion between the AC circuit and the DC circuit. AC circuit  12  shown in  FIG. 1  is a three-phase AC system, and three leg circuits  4   u ,  4   v ,  4   w  are arranged for U phase, V phase, W phase, respectively. 
     AC input terminals Nu, Nv, Nw arranged respectively in leg circuits  4   u ,  4   v ,  4   w  are each connected through an interconnection transformer  13  to AC circuit  12 . AC circuit  12  is an AC power system including an AC power source, for example.  FIG. 1  does not show connection between AC input terminals Nv, Nw and interconnection transformer  13  for the sake of simplifying the drawing. 
     High-potential-side DC terminal Np and low-potential-side DC terminal Nn that are connected commonly to leg circuits  4  are connected to DC circuit  14 . DC circuit  14  is a DC terminal for a DC power system including a DC transmission network or the like, or a DC terminal for another power conversion device, for example. 
     The leg circuits may be connected to AC circuit  12  through an interconnection reactor, instead of interconnection transformer  13  in  FIG. 1 . Further, instead of AC input terminals Nu, Nv, Nw, primary windings may be arranged in respective leg circuits  4   u ,  4   v ,  4   w , and AC connection from leg circuits  4   u ,  4   v ,  4   w  to interconnection transformer  13  or the interconnection reactor may be implemented through secondary windings magnetically coupled with the respective primary windings. In this case, the primary windings may be reactors  8 A,  8 B as described below. Specifically, electrical connection (namely DC or AC connection) from leg circuit  4  to AC circuit  12  may be implemented through connecting parts such as AC input terminals Nu, Nv, Nw or the aforementioned primary windings arranged in respective leg circuits  4   u ,  4   v ,  4   w.    
     Leg circuit  4   u  includes an upper arm  5  from high-potential-side DC terminal Np to AC input terminal Nu, and a lower arm  6  from low-potential-side DC terminal Nn to AC input terminal Nu. The connection point, i.e., AC terminal Nu, between upper arm  5  and lower arm  6  is connected to interconnection transformer  13 . High-potential-side DC terminal Np and low-potential-side DC terminal Nn are connected to DC circuit  14 . Leg circuits  4   v ,  4   w  have a similar configuration to the above-described one, and therefore, leg circuit  4   u  is explained below as a representative of the leg circuits. 
     Upper arm  5  includes a plurality of cascaded submodules  7  and reactor  8 A. A plurality of submodules  7  and reactor  8 A are connected in series to each other. Lower arm  6  includes a plurality of cascaded submodules  7  and reactor  8 B. A plurality of submodules  7  and reactor  8 B are connected in series to each other. 
     The position in which reactor  8 A is inserted may be any position in upper arm  5  of leg circuit  4   u , and the position in which reactor  8 B is inserted may be any position in lower arm  6  of leg circuit  4   u . More than one reactor  8 A and more than one reactor  8 B may be arranged. Respective inductance values of the reactors may be different from each other. Alternatively, only reactor  8 A of upper arm  5 , or only reactor  8 B of lower arm  6  may be arranged. 
     Reactors  8 A,  8 B are arranged for preventing a sharp increase of fault current generated in the event of a fault in AC circuit  12  or DC circuit  14 , for example. Excessively large inductance values of reactors  8 A,  8 B, however, result in a problem that the efficiency of the power converter is decreased. In the event of a fault, it is therefore preferable to stop (i.e., turn off) all switching devices in each submodule  7  as quickly as possible. 
     Power conversion device  1  includes, as detection devices for measuring the amount of electricity (current, voltage, for example) to be used for control, an AC voltage detection device  10 , an AC current detection device  16 , DC voltage detection devices  11 A,  11 B, and arm current detection devices  9 A,  9 B disposed in each leg circuit  4 . 
     Signals detected by these detection devices are input to command generation device  3 . Based on these detected signals, command generation device  3  outputs operation commands  15   pu ,  15   nu ,  15   pv ,  15   nv ,  15   pw ,  15   nw  for controlling the operating states of respective submodules  7 . Command generation device  3  also receives information  17  from each submodule  7 . Information  17  is information on the inside of submodule  7  and includes a voltage value of a capacitor  24  in submodule  7  and state information indicating a state of submodule  7 , for example. 
     In the present embodiment, operation commands  15   pu ,  15   nu ,  15   pv ,  15   nv ,  15   pw ,  15   nw  are generated for the U phase upper arm, the U phase lower arm, the V phase upper arm, the V phase lower arm, the W phase upper arm, and the W phase lower arm, respectively. In the following, operation commands  15   pu ,  15   nu ,  15   pv ,  15   nv ,  15   pw ,  15   nw  may be referred to collectively or non-specifically as operation command  15 . 
     For the sake of simplifying the drawing,  FIG. 1  shows collectively some of signal lines for signals that are input from respective detection devices to command generation device  3  and signal lines for signals that are input or output between command generation device  3  and respective submodules  7 . Actually, however, the signal line is disposed individually for each detection device and each submodule  7 . In the present embodiment, these signals are transmitted through optical fibers for the sake of noise immunity. 
     AC voltage detection device  10  detects U phase AC voltage value Vacu, V phase AC voltage value Vacv, and W phase AC voltage value Vacw of AC circuit  12 . AC current detection device  16  detects U phase AC current value Iacu, V phase AC current value lacy, and W phase AC current value Iacw of AC circuit  12 . DC voltage detection device  11 A detects DC voltage value Vdcp of high-potential-side DC terminal Np connected to DC circuit  14 . DC voltage detection device  11 B detects DC voltage value Vdcn of low-potential-side DC terminal Nn connected to DC circuit  14 . 
     Arm current detection devices  9 A and  9 B disposed in U phase leg circuit  4   u  detect upper arm current Ipu flowing in upper arm  5  and lower arm current Inu flowing in lower arm  6 , respectively. Likewise, arm current detection devices  9 A and  9 B disposed in V phase leg circuit  4   v  detect upper arm current Ipv and lower arm current Inv, respectively. Arm current detection devices  9 A and  9 B disposed in W phase leg circuit  4   w  detect upper arm current Ipw and lower arm current Inw, respectively. 
     &lt;Example Configuration of Submodule&gt; 
       FIG. 2  is a circuit diagram showing an example of submodules forming each leg circuit in  FIG. 1 . Referring to  FIG. 2 , submodule  7  includes a half-bridge-type conversion circuit  25 , a capacitor  24  serving as an energy storage device, a gate controller  21 , a voltage detector  27 , and a transmission and reception device  28 . Gate controller  21 , voltage detector  27 , and transmission and reception device  28  may be implemented by a dedicated circuit, or implemented by an FPGA (Field Programmable Gate Array), or the like. 
     Conversion circuit  25  includes switching devices  22 A,  22 B connected in series to each other, and diodes  23 A,  23 B. Diodes  23 A,  23 B are connected in anti-parallel (i.e., in parallel in the reverse-bias direction) with switching devices  22 A,  22 B, respectively. Capacitor  24  is connected in parallel with the series-connected circuit made up of switching devices  22 A,  22 B for holding a DC voltage. A connection node of switching devices  22 A,  22 B is connected to a high-potential-side input/output terminal  26 P. A connection node of switching device  22 B and capacitor  24  is connected to a low-potential-side input/output terminal  26 N. 
     Gate controller  21  operates in accordance with operation command  15  received from command generation device  3  in  FIG. 1 . During a normal operation (i.e., zero voltage or positive voltage is output between input/output terminals  26 P and  26 N), gate controller  21  performs control to cause one of switching devices  22 A,  22 B to be in the ON state and the other to be in the OFF state. While switching device  22 A is in the ON state and switching device  22 B is in the OFF state, a voltage across capacitor  24  is applied between input/output terminals  26 P and  26 N. While switching device  22 A is in the OFF state and switching device  22 B is in the ON state, the voltage between input/output terminals  26 P and  26 N is 0 V. 
     Thus, submodule  7  causes switching devices  22 A,  22 B to become the ON state alternately to thereby output zero voltage or a positive voltage depending on the voltage of capacitor  24 . 
     Voltage detector  27  detects the voltage between opposite terminals  24 P and  24 N of capacitor  24 . Transmission and reception device  28  transmits, to gate controller  21 , operation command  15  received from command generation device  3  in  FIG. 1 , and transmits, to command generation device  3 , information  17  including the voltage of capacitor  24  (hereinafter also referred to simply as “capacitor voltage”) detected by voltage detector  27 . 
     As each of switching devices  22 A,  22 B, a self-arc-extinguishing-type switching device is used, of which ON operation and OFF operation can both be controlled. For example, IGBT (Insulated Gate Bipolar Transistor) or GCT (Gate Commutated Turn-off thyristor), for example, is used as switching device  22 A,  22 B. 
     The above-described configuration of submodule  7  is given as an example, and submodule  7  of any of other configurations may be applied to the present embodiment. For example, a full-bridge-type conversion circuit or a three-quarter-bridge-type conversion circuit may be used to form submodule  7 . 
     &lt;Configuration of Command Generation Device&gt; 
       FIG. 3  is a block diagram showing a schematic configuration of the command generation device. Referring to  FIG. 3 , command generation device  3  includes a control device  101 , a protection device  102 , and a repeating device group  320  made up of a plurality of repeating devices. Control device  101  and protection device  102  are each a host device of each repeating device included in repeating device group  320 . Each repeating device relays communication between the host device and each submodule  7  included in power conversion circuitry  2 . While  FIG. 3  exemplarily shows only leg circuit  4   u  for U phase in power conversion circuitry  2  of  FIG. 1 , other leg circuits  4   v ,  4   w  are similar to leg circuit  4   u.    
     Control device  101  is a device that controls operation of each submodule  7 . Control device  101  receives input of AC voltage values Vacu, Vacv, Vacw (hereinafter also referred to collectively as “AC voltage value Vac”), AC current values Iacu, lacy, Iacw (hereinafter also referred to collectively as “AC current value Iac”), DC voltage values Vdcp, Vdcn, upper arm currents Ipu, Ipv, Ipw (hereinafter also referred to collectively as “upper arm current Ip”), lower arm currents Inu, Inv, Inw (hereinafter also referred to collectively as “lower arm current In”), and capacitor voltage Vcap that are detected by respective detection devices in  FIG. 1 . Typically, capacitor voltage Vcap is an average of respective voltage values of capacitors  24  detected in respective submodules  7  of each arm circuit. 
     Based on each of the received detected values, control device  101  generates, for each period T 1  (86.8 μs, for example), a control command for controlling operation of each submodule  7  during a normal operation control period, and outputs the generated control command to repeating device group  320 . 
     The control command includes a voltage command and a current command, for example. The voltage command is, for example, an output voltage command value for upper arm  5  and an output voltage command value for lower arm  6 , in each of leg circuits  4   u ,  4   v ,  4   w . The current command is, for example, an output current command value for upper arm  5  and an output current command value for lower arm  6 , in each of leg circuits  4   u ,  4   v ,  4   w.    
     Typically, control device  101  includes, as hardware components, an auxiliary transformer, an AD (Analog to Digital) converter, and an operation unit, for example. The operation unit includes a CPU (Central Processing Unit), a RAM (Random Access Memory), and a ROM (Read Only Memory). The AD converter includes an analogue filter, a sample hold circuit, and a multiplexer, for example. Control device  101  may be configured, for example, in the form of a digital protection control device. 
     Protection device  102  is a device that protects each submodule  7 . When at least one of the arm currents exceeds a threshold value, protection device  102  generates a protection command including a stop command for stopping operation of each submodule  7  and transmits the protection command to each repeating device  32 . In contrast, when all the arm currents are less than the threshold value, protection device  102  may not generate a protection command, or may generate a protection command including a normal command for causing each submodule  7  to operate based on the control command. Protection device  102  transmits the protection command for each period T 2  (a few microseconds, for example). 
     Repeating device group  320  receives the control command from control device  101  and receives the protection command from protection device  102 . Repeating device group  320  outputs, to each submodule  7 , operation command  15  including at least one of the control command and the protection command. Each submodule  7  operates in accordance with operation command  15 . 
     &lt;Transmission Method for Command Information&gt; 
       FIG. 4  illustrates a method for transmitting command information according to Embodiment 1. The method for transmitting the control command from control device  101  to repeating device group  320  is similar to the method for transmitting the protection command from protection device  102  to repeating device group  320 . In the following description, it is therefore supposed that control device  101  is a host device. The same applies to other embodiments as well. 
     Referring to  FIG. 4 , control device  101  includes a command information generator  151  and a plurality of communication controllers  153 A,  153 B (hereinafter also referred to collectively as “communication controller  153 ”). Respective functions of these components are implemented through execution, by a CPU of control device  101 , of a program stored in a ROM, for example. Alternatively, a part or the whole of these functions may be implemented through use of a dedicated circuit. 
     Command information generator  151  generates command information  90  including arm commands for respective arms (i.e., upper and lower arms for each of the phases) included in power conversion circuitry  2 . Because control device  101  is herein supposed to be a host device, command information  90  corresponds to the control command. Command information  90  includes a common command that is common to arms (upper and lower arms for each of the phases) included in power conversion circuitry  2 , and arm commands dedicated to upper and lower arms for each of the phases. Specifically, the arm commands include arm command U_u for upper arm  5  of the U phase, arm command U_d for lower arm  6  of the U phase, arm command V_u for upper arm  5  of the V phase, arm command V_d for lower arm  6  of the V phase, arm command W_u for upper arm  5  of the W phase, and arm command W_d for lower arm  6  of the W phase. 
     The common command includes a mode command specifying an operation mode of each submodule  7 , and the total number of submodules  7  included in power conversion circuitry  2 . The operation mode includes an activation mode for activating submodule  7 , an operation mode for causing submodule  7  to operate in a normal manner, and a pause mode for causing submodule  7  to pause. 
     Each arm command includes an arm voltage command value, an arm current command value, and an arm test command. The arm test command includes a command for starting test charging for capacitor  24  of submodule  7  in an arm, and a command for stopping the test charging. 
       FIG. 5  illustrates an example of a method for generating arm voltage commands. Referring to  FIG. 5 , command information generator  151  of control device  101  includes, as functional components for generating arm voltage commands, an AC voltage command generator  40 , a DC voltage command generator  41 , a circulating current command generator  42 , and an arm voltage command generator  44 . These functional components are configured in the form of a feedback controller such as PID controller (Proportional-Integral-Differential Controller), for example. 
     AC voltage command generator  40  generates an AC voltage command value for each phase, based on AC voltage values Vacu, Vacv, Vacw and AC current values Iacu, lacy, Iacw. DC voltage command generator  41  calculates DC current value Idc, based on upper arm currents Ipu, Ipv, Ipw and lower arm currents Inu, Inv, Inw. DC voltage command generator  41  generates a DC voltage command value based on DC voltage values Vdcp, Vdcn and DC current value Idc. 
     Circulating current command generator  42  calculates circulating currents Iccu, Iccv, Iccw flowing in respective leg circuits  4   u ,  4   v ,  4   w , based on upper arm currents Ipu, Ipv, Ipw and lower arm currents Inu, Inv, Inw of respective phases. The circulating currents circulate through a plurality of leg circuits  4 . Circulating current command generator  42  calculates a command value for the circulating current of each phase, based on circulating currents Iccu, Iccv, Iccw of respective phases and capacitor voltage Vcap which is an average determined for each arm circuit. 
     Arm voltage command generator  44  generates arm voltage commands Vprefu, Vnrefu, Vprefv, Vnrefv, Vprefw, Vnrefw for upper arm  5  and lower arm  6  for each of the phases, based on the above-described command generators. 
     Referring again to  FIG. 4 , command information generator  151  broadcasts command information  90  to communication controllers  153  for respective arms. Communication controller  153  is provided for each arm. For ease of drawing,  FIG. 4  only shows communication controller  153 A for upper arm  5  of the U phase and communication controller  153 B for lower arm  6  of the U phase. Actually, however, control device  101  additionally includes two communication controllers  153  for upper and lower arms of the V phase, and two communication controllers  153  for upper and lower arms of the W phase. Respective functions of communication controllers  153  for respective arms are similar to each other, and therefore, functions of communication controllers  153 A,  153 B for the U phase are described herein. 
     Communication controller  153 A receives command information  90  transmitted from command information generator  151 , and extracts, from command information  90 , arm command U_u associated with communication controller  153 A. Communication controller  153 A transmits a communication frame  51 A including extracted arm command U_u, to each repeating device  32  connected to associated submodules  7  in the U phase upper arm associated with communication controller  153 A. Specifically, communication controller  153 A includes a selector  31 A, a communication frame generator  52 A, and a plurality of communication ports  34 A. 
     Selector  31 A selects arm command U_u associated with select ID “Uu” that is specified for selector  31 A, and outputs the selected arm commend to communication frame generator  52 A. For example, when each communication controller  153  is formed from a circuit board, a slot number where the circuit board is mounted is specified as a select ID. Specifically, when communication controller  153 A is inserted in a slot for the U phase upper arm, select ID “Uu” is automatically specified for selector  31 A of communication controller  153 A. This makes it unnecessary to manually make settings for communication controller  153 , from which advantageous effects such as saving of the load for initial settings, and prevention of an erroneous number from being specified, for example, are expected. The function of selector  31 A may be implemented in a software manner. 
     Communication frame generator  52 A extracts the common command from command information  90 , and also extracts arm command U_u selected by selector  31 A. Communication frame generator  52 A generates communication frame  51 A including command data having the common command and arm command U_u. Communication frame generator  52 A transmits, through each communication port  34 A, communication frame  51 A to associated repeating device  32 . 
     N communication ports  34 A (N is an integer satisfying N  1 ) are provided. For the sake of convenience, N communication ports  34 A are distinguished from each other by respective numbers # 1  to #N allocated to them. Communication frame generator  52 A transmits communication frame  51 A simultaneously to communication ports  34 A# 1  to  34 A#N. Communication ports  34 A# 1  to  34 A#N each transmit communication frame  51 A to associated repeating device  32 . 
     Each communication controller  153  is connected to a plurality of repeating devices  32  through a ring network. For example, each communication port  34  of communication controller  153  is connected to four repeating devices  32  through a ring network. Four repeating devices  32  are also referred to as HUB# 1  to HUB# 4 , respectively. 
     Each repeating device  32  is connected to a predetermined number of submodules  7  through a star network. Each repeating device  32  extracts the command data included in communication frame  51 A and transmits a communication frame including the command data to each submodule  7  connected to this repeating device  32 . In the example shown in  FIG. 4 , the ring network topology and the star network topology are combined to establish a network interconnecting control device  101 , repeating devices  32 , and submodules  7 . 
     Communication controller  153 B receives command information  90  transmitted from command information generator  151 , and extracts, from command information  90 , arm command U_d associated with communication controller  153 B. Communication controller  153 B transmits a communication frame  51 B including extracted arm command U_d, to each repeating device  32  connected to associated submodules  7  in the U phase lower arm associated with communication controller  153 B. Specifically, communication controller  153 B includes a selector  31 B, a communication frame generator  52 B, and a plurality of communication ports  34 B. 
     Selector  31 B selects arm command U_d associated with select ID “Ud” that is specified for selector  31 B, and outputs the selected arm commend to communication frame generator  52 B. Communication frame generator  52 B generates communication frame  51 B including the common command and arm command U_d, and transmits, through each communication port  34 B, communication frame  51 B to associated repeating device  32 . While each communication port  34 B is connected to associated repeating devices  32  through a ring network, the connection is not shown for ease of drawing. Communication controllers  153  for the upper and lower arms of the V phase as well as communication controllers  153  for the upper and lower arms of the W phase also transmit respective communication frames to each repeating device  32 . 
     Thus, when communication controller  153  is provided for each arm, each of a plurality of communication controllers  153  transmits, to repeating devices  32  connected to associated submodules  7  included in the arm (e.g., U phase upper arm) which is associated with this communication controller  153 , a communication frame (communication frame  51 A, for example) including the arm command (arm command U_u, for example) for that arm. 
     In the above-described configuration, the communication frame is transmitted for each arm, and therefore, it is unnecessary for the communication frame to include information specifying the arm. Control device  101  can therefore transmit a relevant arm command simultaneously to submodules  7  in each arm, which enables shortening of the communication time required for control device  101  to transmit the command. 
     The above description regarding  FIG. 4  is given of the case where the host device is control device  101 . If the host device is protection device  102 , command information  90  corresponds to the protection command. In this case, the common command in command information  90  includes a stop command for stopping operation of submodules  7  in all the arms. Each arm command in command information  90  includes a stop command for stopping operation of each submodule in the associated arm. For example, arm command U_u generated by the command information generator of protection device  102  includes a stop command for stopping operation of each submodule  7  in the U phase upper arm. This configuration similar to that of control device  101  described above also enables shortening of the communication time required for protection device  102  to transmit the command. 
     Referring next to  FIGS. 6 and 7 , specific examples of network configuration interconnecting control device  101  and repeating devices  32  are described.  FIG. 6  illustrates an example network configuration according to Embodiment 1. Specifically,  FIG. 6  shows a network configuration where the number N of communication ports in  FIG. 4  is one. 
     Referring to  FIG. 6 , control device  101  includes a communication port  34 A for the U phase upper arm, a communication port  34 B for the U phase lower arm, a communication port  34 C for the V phase upper arm, a communication port  34 D for the V phase lower arm, a communication port  34 E for the W phase upper arm, and a communication port  34 F for the W phase lower arm. 
     Communication port  34 A is connected to four repeating devices  32  through the ring topology. Each repeating device  32  is connected to a plurality of submodules  7  through the star topology. Communication frame  51 A that is output from communication port  34 A is transmitted in the order of the numerals in the parentheses in  FIG. 6 , i.e., (1), (2), (3), (4), (5). The method for transmitting communication frames that are output from respective communication ports  34 B to  34 F is similar to the method for transmitting the communication frame that is output from communication port  34 A. 
     Specifically, control device  101  transmits communication frame  51 A to HUB# 1  through communication port  34 A. HUB# 1  processes communication frame  51 A received from control device  101  and transmits the frame to HUB# 2 . HUB# 2  processes communication frame  51 A received from HUB# 1  and transmits the frame to HUB# 3 . HUB# 3  processes communication frame  51 A received from HUB# 2  and transmits the frame to HUB# 4 . HUB# 4  processes communication frame  51 A received from HUB# 3  and transmits the frame to control device  101 . Details of how communication frame  51 A is processed are described later herein. 
       FIG. 7  illustrates another example network configuration according to Embodiment 1. Specifically,  FIG. 7  shows a network configuration where the number N of communication ports in  FIG. 4  is two. Referring to  FIG. 7 , control device  101  includes two communication ports  34 A, two communication ports  34 B, two communication ports  34 C, two communication ports  34 D, two communication ports  34 E, and two communication ports  34 F. 
     Each of two communication ports  34 A is connected to four repeating devices  32  through the ring topology. Each repeating device  32  is connected to a plurality of submodules  7  through the star topology. Communication frame  51 A that is output from each communication port  34 A is transmitted in the order of the numerals in parentheses in  FIG. 7 , i.e., (1), (2), (3), (4), (5), like the one shown in  FIG. 6 . 
     &lt;Example of Processing of Communication Frame by Repeating Devices&gt; 
       FIG. 8  illustrates an example of processing of a communication frame by repeating devices according to Embodiment 1. Communication frames transmitted through respective communication ports of control device  101  are processed in a similar manner. Therefore, an example of processing of communication frame  51 A transmitted from control device  101  through communication port  34 A is described. The numerals in parentheses (1), (2), (3), (4), (5) in  FIG. 8  correspond to those in  FIG. 6 . 
     Communication frame  51 A transmitted from control device  101  to HUB# 1  is also referred to as “communication frame  51 A_ 1 ,” communication frame  51 A transmitted from HUB# 1  to HUB# 2  is also referred to as “communication frame  51 A_ 2 ,” communication frame  51 A transmitted from HUB# 2  to HUB# 3  is also referred to as “communication frame  51 A_ 3 ,” communication frame  51 A transmitted from HUB# 3  to HUB# 4  is also referred to as “communication frame  51 A_ 4 ,” and communication frame  51 A transmitted from HUB# 4  to control device  101  is also referred to as “communication frame  51 A_ 5 .” 
     Referring to  FIG. 8 , communication frame  51 A_ 1  includes a flag region  81  (corresponding to “flg” in the drawing), a header region  82 , a payload region  83 , and an FCS (Frame Check Sequence) region  84  where error detection information is stored. In header region  82 , information such as a communication command (corresponding to “cmd” in the drawing), a sequence number (corresponding to “seq” in the drawing), and a payload length (corresponding to “pl” in the drawing), for example, is stored. In payload region  83 , command data X including the common command and arm command U_u is stored. 
     Receiving communication frame  51 A_ 1  from communication controller  153 A connected to HUB# 1 , HUB# 1  processes communication frame  51 A_ 1  to generate communication frame  51 A_ 2 . Specifically, HUB# 1  adds HUB data Y 1  to the payload region of communication frame  51 A_ 1 , and updates the payload length and FCS region  84  to generate communication frame  51 A_ 2 . HUB# 1  transmits communication frame  51 A_ 2  to HUB# 2  connected to HUB# 1 . 
     HUB# 1  generates HUB data Y 1  based on internal information (hereinafter also referred to as “SM internal information”) received from each submodule  7  connected to HUB# 1 . HUB data Y 1  includes the SM internal information received from each submodule  7  and state information about HUB# 1  (“HUB state” in the drawing). 
     The SM internal information includes the capacitor voltage of each submodule  7  (corresponding to “SM# 1  voltage to SM#N voltage” in the drawing), and the state information of each submodule  7  (“SM# 1  state to SM#N state” in the drawing). In the following description, the capacitor voltage of submodule  7  is also referred to as “SM voltage,” the state information of submodule  7  is also referred to as “state information Dsm,” and the state information of the HUB is also referred to as “state information Dh.” 
     State information Dsm includes operational information indicating an operational state of submodule  7 , failure rank information indicating the degree of failure, and failure type information indicating the type of failure. The operational information of submodule  7  includes an activation bit indicating whether the submodule is active or not, a failure bit indicating whether the submodule is failing or not, and a separation bit depending on the failure bit indicating a failure state. The separation bit is information indicating whether failing submodule  7  has been separated from the arm. The failure rank information includes information indicating whether submodule  7  can operate or not. The operational information may also include a control bit indicating whether operation of the submodule is being controlled or not, and a stop bit indicating whether the submodule is being stopped or not. 
     State information Dh of HUB# 1  includes operational information indicating an operational state of HUB# 1 , failure rank information indicating the degree of failure of HUB# 1 , and failure type information indicating the type of failure. The operational information of HUB# 1  includes an operational bit indicating whether HUB# 1  is operating in a normal manner, or failing. The failure rank information includes information indicating whether HUB# 1  can operate or not. 
     HUB# 2  adds HUB data Y 2  to communication frame  51 A_ 2 , and updates the payload length and FCS region  84  to generate communication frame  51 A_ 3 . HUB# 3  adds HUB data Y 3  to communication frame  51 A_ 3 , and updates the payload length and FCR region  84  to generate communication frame  51 A_ 4 . HUB# 4  adds HUB data Y 4  to communication frame  51 A_ 4  received from HUB# 3  connected to HUB# 4 , and updates the payload length and FCS region  84  to generate communication frame  51 A_ 5 . HUB# 4  transmits communication frame  51 A_ 5  to control device  101 . 
     HUB data Y 2  includes SM internal information received from each submodule  7  connected to HUB# 2 , and the state information of HUB# 2 . The same applies as well to HUB data Y 3 , Y 4 . Accordingly, as shown in  FIG. 8 , command data X and HUB data Y 1  to Y 4  are stored in payload region  83  of communication frame  51 A_ 5 . Thus, to the communication frame transmitted from control device  101 , an HUB data field is coupled each time the communication frame is passed through the HUB. Communication frames  51 A_ 1  to  51 A_ 5  therefore have respective formats different from each other. 
     In the above-described configuration, communication frame  51 A is transmitted from control device  101  through communication port  34 A to thereby allow command data X to be conveyed to each repeating device  32  for the U phase upper arm and also allow HUB data Y 1  to Y 4  of respective repeating devices  32  to be acquired. Likewise, communication frames for other arms are transmitted from other communication ports to thereby allow command data to be conveyed to respective repeating devices  32  for other arms and allow HUB data of respective repeating devices  32  to be acquired. 
     In other words, as control device  101  performs communication once, conveyance of the command data to all repeating devices  32  connected to control device  101  as well as acquisition of HUB data of all repeating devices  32  are performed simultaneously. Thus, according to the present embodiment, the frequency at which communication is performed can be reduced and the time required for communication can be shortened, as compared with the method, for example, specifying the ID number of repeating device  32  and the ID number of submodule  7  to make communication individually. Moreover, depending on the number of repeating devices  32 , the communication frame is extended automatically. For example, even when one repeating device  32  is added afterword, it is unnecessary to change the structure of the communication frame to be transmitted from control device  101 . 
     The above description regarding  FIG. 8  is given of the case where the host device is control device  101 . If the host device is protection device  102 , SM internal information includes state information Dsm and does not include the SM voltage. Specifically, HUB data includes state information Dsm and state information Dh. The foregoing is applied similarly in other respects. 
     Advantages 
     According to Embodiment 1, because the communication frame is not required to include information that specifies an arm, the command for all arms can be transmitted simultaneously. Therefore, the communication time required for conveying the command can be shortened. Moreover, because information that specifies an arm is unnecessary, the frame length of the communication frame can be shortened. 
     Moreover, as the host device makes communication once, conveyance of command data from the host device to repeating device  32  and acquisition of HUB data of repeating device  32  are performed simultaneously. Therefore, the frequency at which communication is performed between the host device and repeating device  32  can be reduced and the time required for communication can be shortened. Further, because the communication frame is extended automatically depending on the number of repeating devices  32 , it is unnecessary to change the structure of the communication frame, even when repeating device  32  is added afterward. 
     Embodiment 2 
     The above description regarding Embodiment 1 is given of the configuration where HUB data is added to the communication frame as the communication frame is passed through each HUB. In connection with Embodiment 2, a description is given of a configuration where aggregate data, which is generated by performing data aggregation, is added to the communication frame when the communication frame is passed through each HUB. 
       FIG. 9  illustrates processing of a communication frame by repeating devices according to Embodiment 2. For ease of description, example processing of communication frame  51 A is described. The numerals in the parentheses (1), (2), (3), (4), (5) in  FIG. 9  correspond respectively to the numerals in the parentheses in  FIG. 6 . 
     Referring to  FIG. 9 , a communication frame  51 A_a 1  transmitted from control device  101  to HUB# 1  is identical to communication frame  51 A_ 1  in  FIG. 8 . A communication frame  51 A_a 2  transmitted from HUB# 1  to HUB# 2  differs from communication frame  51 A_ 2  in  FIG. 8  in terms of the contents of payload region  83 . 
     HUB# 1  generates HUB aggregate data Z 1  and HUB data Yla, based on SM internal information received from each submodule  7  connected to HUB# 1 . HUB# 1  adds HUB aggregate data Z 1  and HUB data Yla to payload region  83  of communication frame  51 A_a 1 , and updates the payload length and FCS region  84  to generate communication frame  51 A_a 2 . 
     Specifically, payload region  83  of communication frame  51 A_a 2  includes command data X, HUB aggregate data Z 1 , and HUB data Yla. HUB aggregate data Z 1  includes an aggregate value of the SM voltage, an aggregate value of state information Dsm, and an aggregate value of state information Dh. HUB data Yla includes partial information of state information Dsm of submodules  7  connected to HUB# 1 , and state information Dh of HUB# 1 . Details of HUB aggregate data Z 1  and HUB data Y 1   a  are described later herein. 
     HUB# 2  adds HUB data Y 2   a  to communication frame  51 A_a 2 , and generates HUB aggregate data Z 2  by updating HUB aggregate data Z 1 . Further, HUB# 2  updates the payload length and FCS region  84  to generate a communication frame  51 A_ 3   a.    
       FIG. 10  illustrates a data aggregation process according to Embodiment 2. A data aggregation process performed by HUB# 2  is chiefly described herein. Referring to  FIG. 10 , HUB# 1  uses SM internal information of each submodule  7  connected to HUB# 1  to perform a data aggregation process and thereby generate HUB aggregate data Z 1 . An SM voltage aggregate value  301  in HUB aggregate data Z 1  is the sum of SM voltages received from respective submodules  7  connected to HUB# 1 . 
     In aggregate data Z 1 , the aggregate value of state information Dsm includes an activation bit aggregate value  303 , an active SM number aggregate value  305 , and a failure bit aggregate value  307 . Activation bit aggregate value  303  is the logical conjunction of activation bits received from respective submodules  7 . For example, when all submodules  7  connected to HUB# 1  are active, the logical conjunction is “1” and, when at least one submodule  7  is not active, the logical conjunction is “0.” The value of the logical conjunction can be used to determine whether or not all submodules  7  connected to HUB# 1  are active. 
     Active SM number aggregate value  305  is the sum of activation bits received from respective submodules  7 . The sum corresponds to the number of submodules  7  that have been activated, among submodules  7  connected to HUB# 1 . 
     Failure bit aggregate value  307  is the logical disjunction of failure bits received from respective submodules  7 . For example, when none of all submodules  7  connected to HUB# 1  has failed, the logical disjunction is “0” and, when at least one submodule  7  has failed, the logical disjunction is “1.” The value of the logical disjunction can be used to determine whether any submodule  7  is failing among submodules  7  connected to HUB# 1 . 
     HUB data Y 1   a  includes partial information of state information Dsm, and state information Dh. Specifically, HUB data Y 1   a  includes failure bit, separation bit, failure rank information and failure type information that are included in state information Dsm of each submodule  7 , and also includes state information Dh of HUB# 1 . Thus, because HUB data Y 1   a  does not include the SM voltage and the activation bit of submodule  7 , the size of HUB data Y 1   a  is smaller than the size of HUB data Y 1  in  FIG. 8 . 
     HUB# 2  updates HUB aggregate data Z 1  to generate HUB aggregate data Z 2 . HUB aggregate data Z 2  includes an SM voltage aggregate value  301 F generated by updating SM voltage aggregate value  301 , an activation bit aggregate value  303 F generated by updating activation bit aggregate value  303 , an active SM number aggregate value  305 F generated by updating active SM number aggregate value  305 , and a failure bit aggregate value  307 F generated by updating failure bit aggregate value  307 . 
     SM voltage aggregate value  301 F is the sum of SM voltage aggregate value  301  and respective SM voltages received by HUB# 2 . Activation bit aggregate value  303 F is the logical conjunction of activation bit aggregate value  303  and respective activation bits received by HUB# 2 . Active SM number aggregate value  305 F is the sum of active SM number aggregate value  305  and respective activation bits received by HUB# 2 . Failure bit aggregate value  307 F is the logical disjunction of failure bit aggregate value  307  and respective failure bits received by HUB# 2 . 
     HUB data Y 2   a  includes failure bit, separation bit, failure rank information and failure type information that are included in state information Dsm received by HUB# 2 , and also includes state information Dh of HUB# 2 . Because HUB data Y 2   a  includes only a part of state information Dsm, the size of HUB data Y 2   a  is smaller than the size of HUB data Y 2  in  FIG. 8 . 
     Although not shown in  FIG. 10 , the aggregate value of state information Dh in HUB aggregate data Z 1  is the operational bit of HUB# 1 . Further, the aggregate value of state information Dh in HUB aggregate data Z 2  is the logical conjunction of the operational bit of HUB# 1  and the operational bit of HUB# 2 . 
     Referring again to  FIG. 9 , HUB# 3  performs an aggregation process on HUB aggregate data Z 2  of communication frame  51 A_a 3  to update the data to HUB aggregate data Z 3 , adds HUB data Y 3   a , and updates the payload length and FCS region  84  to generate a communication frame  51 A_a 4 . Likewise, HUB# 4  performs an aggregation process on HUB aggregate data Z 3  to update the data to HUB aggregate data Z 4 , adds HUB data Y 4   a  to communication frame  51 A_a 4 , and updates the payload length and FCS region  84  to generate a communication frame  51 A_a 5 . HUB# 4  transmits communication frame  51 A_a 5  to control device  101 . 
     Advantages 
     According to Embodiment 2, data that is not required to be identified for each submodule  7  (e.g. SM voltage, activation bit, and the like) is subjected to an aggregation process and added as an aggregate value to the communication frame. Therefore, the data size of HUB data Y 1   a  to Y 4   a  in which individual data for submodules  7  is stored can be made smaller than the data size of HUB data Y 1  to Y 4  in  FIG. 8 , respectively. Specifically, the frame lengths of respective communication frames  51 A_a 2  to  51 A_a 5  can be made shorter than the frame lengths of respective communication frames  51 A_ 2  to  51 A_ 5  in  FIG. 8 , respectively. 
     Thus, the frame length can be shortened to shorten the time required per communication. Accordingly, the memory amount and the amount of operation that are necessary for control device  101  or protection device  102  can also be reduced. Moreover, a slow and low-cost communication module (e.g. communication module of 1 to 2 Gbps class) can be used to form a communication channel. Further, even when many submodules  7  are connected, the time for communication can shortened, which facilitates establishment of a large-scale HDVC system. 
     Embodiment 3 
     The above description regarding Embodiment 1 is given of the configuration where repeating device  32  is connected to each submodule  7  through a star network. In connection with Embodiment 3, a description is given of a configuration where repeating device  32  is connected to each submodule  7  through a ring network. 
       FIG. 11  shows a network configuration according to Embodiment 3. The network configuration in  FIG. 11  corresponds to a configuration formed by changing the connection topology between repeating device  32  and each submodule  7  in  FIG. 6  to the ring topology. Referring to  FIG. 11 , each of communication ports  351  to  354  of each repeating device  32  is connected to a plurality of submodules  7  through the ring topology. For the sake of convenience, numbers # 1  to # 16  are allocated to respective submodules to distinguish the submodules from each other. Specifically, 16 submodules  7  are also referred to as submodules # 1  to # 16 , respectively. 
     Communication port  351  of HUB# 1  is connected to submodules # 1  to # 4  through the ring topology, communication port  352  is connected to submodules # 5  to # 8  through the ring topology, communication port  353  is connected to submodules # 9  to # 12  through the ring topology, and communication port  354  is connected to submodules # 13  to # 16  through the ring topology. For example, a communication frame that is output from communication port  351  is transmitted in the order of the numerals in the parentheses in  FIG. 11 , i.e., (1), (2), (3), (4), (5). The same applies as well to HUB# 2  to HUB# 4 . 
       FIG. 12  illustrates an example of processing of a communication frame by submodules  7  according to Embodiment 3. Processing of a communication frame described in connection with  FIG. 12  is basically similar to the processing of the communication frame described in connection with  FIG. 8 . Moreover, communication frames transmitted through respective communication ports of repeating device  32  are processed similarly. Therefore, example processing of a communication frame transmitted from communication port  351  is described herein. The numerals in the parentheses (1), (2), (3), (4), (5) in  FIG. 12  correspond to the numerals in the parentheses in  FIG. 11 . 
     A communication frame  61 _ 1  transmitted from HUB# 1  to submodule # 1  includes a flag region  86 , a header region  87 , a payload region  88 , and an FCS region  89 . Header region  87  includes information such as a communication command, a sequence number, and a payload length, for example. Payload region  88  includes command data X having the common command and arm command U_u. 
     Submodule # 1  adds SM data K 1  to the payload region of communication frame  61 _ 1  received from HUB# 1 , and also updates the payload length and FCS region  89  to generate a communication frame  61 _ 2 . SM data K 1  includes SM internal information of submodule # 1 . The SM internal information includes the capacitor voltage and state information Dsm of submodule # 1 . 
     Likewise, submodule # 2  generates a communication frame  61 _ 3 , submodule # 3  generates a communication frame  61 _ 4 , and submodule # 4  generates a communication frame  61 _ 5 . In payload region  88  of communication frame  61 _ 5 , command data X and SM data K 1  to K 4  are stored. SM data K 2  to K 4  include the SM internal information of submodules # 2  to # 4 , respectively. 
     Thus, to the communication frame transmitted from repeating device  32 , the SM data field is coupled each time the communication frame is passed through submodule  7 . Communication frames  61 _ 1  to  61 _ 5  therefore have respective formats different from each other. 
       FIG. 13  illustrates another example of processing of a communication frame by submodules  7  according to Embodiment 3. Processing of a communication frame described in connection with  FIG. 13  is basically similar to the processing of the communication frame described in connection with  FIG. 9 . Example processing of a communication frame transmitted from communication port  351  is described herein. 
     Referring to  FIG. 13 , a communication frame  63 _ 1  transmitted from HUB# 1  to submodule # 1  is identical to communication frame  61 _ 1  in  FIG. 12 . A communication frame  63 _ 2  transmitted from submodule # 1  to submodule # 2  differs from communication frame  61 _ 2  in  FIG. 12  in terms of the contents of payload region  88 . 
     Specifically, payload region  88  of communication frame  63 _ 2  includes command data X, SM aggregate data Zs 1 , and SM data K 1   a . SM aggregate data Zs 1  includes an aggregate value of the SM voltage of submodule # 1 , and an aggregate value of state information Dsm (e.g. an activation bit aggregate value, an active SM number aggregate value, a failure bit aggregate value) of submodule # 1 . 
     In SM aggregate data Zs 1 , only the information of submodule # 1  is stored. Therefore, the aggregate value of the SM voltage of submodule # 1  is the SM voltage of submodule # 1 . The activation bit aggregate value and the active SM number aggregate value are the activation bit of submodule # 1 . The failure bit aggregate value is the failure bit of submodule # 1 . 
     SM data K 1   a  includes a failure bit, a separation bit, failure rank information and failure type information of submodule # 1 . Because SM data K 1   a  includes only partial information of state information Dsm, the data size of SM data K 1   a  is smaller than the data size of SM data K 1  in  FIG. 12 . 
     Submodule # 2  performs an aggregation process on SM aggregate data Zs 1  to update SM aggregate data Zs 1  to SM aggregate data Zs 2 , adds SM data K 2   a  to communication frame  63 _ 2 , and updates the payload length and FCS region  89  to generate a communication frame  63 _ 3 . 
     Specifically, SM data K 2   a  includes a failure bit, a separation bit, failure rank information and failure type information of submodule # 2 . SM aggregate data Zs 2  includes an aggregate value of the SM voltages of submodules # 1  to # 2 , and an aggregate value of state information Dsm (e.g. an activation bit aggregate value, an active SM number aggregate value, a failure bit aggregate value) of submodules # 1  to # 2 . 
     The activation bit aggregate value of SM aggregate data Zs 2  is the logical conjunction of the activation bit aggregate value of SM aggregate data Zs 1  and the activation bit of submodule # 2 . The active SM number aggregate value of SM aggregate data Zs 2  is the sum of the active SM number aggregate value of SM aggregate data Zs 1  and the activation bit of submodule # 2 . The failure bit aggregate value of SM aggregate data Zs 2  is the logical disjunction of the failure bit aggregate value of SM aggregate data Zs 1  and the failure bit of submodule # 2 . 
     The same applies as well to processing performed by submodules # 3 , # 4 . Consequently, command data X, SM aggregate data Zs 4 , and SM data Ka 1  to Ka 4  are stored in the payload region of a communication frame  63 _ 5 . 
     Advantages 
     According to Embodiment 3, advantages similar to those of Embodiments 1 and 2 are obtained. 
     Embodiment 4 
     The above description regarding Embodiment 1 is given of the configuration where the communication controller is provided for each arm. In connection with Embodiment 4, a description is given of a configuration where a communication controller is provided for each phase of AC circuit  12 . 
       FIG. 14  illustrates a method for transmitting command information according to Embodiment 4. Referring to  FIG. 14 , a control device  101 A includes a command information generator  151 A, and a plurality of communication controllers  70 U,  70 V,  70 W (hereinafter also referred to collectively as “communication controller  70 ”). Communication controllers  70 U,  70 V, and  70 W are communication controllers corresponding to U phase, V phase, and W phase, respectively. In other words, communication controllers  70 U,  70 V, and  70 W are communication controllers provided for leg circuits  4   u ,  4   v , and  4   w , respectively. 
     Command information generator  151 A generates command information  92  for each submodule  7 . Because control device  101  is herein supposed to be a host device, command information  92  corresponds to the control command. Command information  92  includes a common command, an arm command for each arm, and a communication port number M. Communication port number M is the number of communication ports provided for each communication controller  153 U,  153 V,  153 W. 
     Command information generator  151 A broadcasts command information  92  to communication controllers  70  for respective phases. Communication controllers  70  for respective phases function similarly, and therefore, the functions of communication controller  70 U are described herein. 
     Communication controller  70 U receives command information  92  transmitted from command information generator  151 A, and extracts, from command information  92 , an arm command associated with communication controller  70 U. Specifically, because communication controller  70 U is a communication controller for leg circuit  4   u  of the U phase, communication controller  70 U extracts an arm command U_u for upper arm  5  of leg circuit  4   u , and an arm command U_d for lower arm  6  of leg circuit  4   u.    
     Communication controller  70 U transmits, to each repeating device connected to associated submodules  7  included in leg circuit  4   u , a communication frame including the extracted arm command. Specifically, communication controller  70 U transmits a communication frame  53 A including arm command U_u to each repeating device  32  connected to associated submodules  7  included in upper arm  5  of leg circuit  4   u . Communication controller  70  also transmits a communication frame  53 B including arm command U_d to each repeating device  32  connected to associated submodules  7  included in lower arm  6  of leg circuit  4   u.    
     Communication controller  70 U includes selectors  58 A,  58 B, communication frame generators  54 A,  54 B, and a plurality of communication ports  36 A. M communication ports  36 A are provided (M is an integer satisfying M  2 ). For the sake of convenience, M communication ports  36 A are distinguished from each other by respective numbers # 1  to #M allocated to them. 
     Selector  58 A selects arm command U_u, based on select ID “U” specified for selector  58 A, and information  501  indicating that a half (e.g., communication ports with port numbers # 1  to #M/2) of a plurality of communication ports  36 A are to be used as communication ports for the upper arm, and outputs the selected arm command to communication frame generator  54 A. 
     Communication frame generator  54 A generates communication frame  53 A including the common command and arm command U_u. Communication frame generator  54 A transmits communication frame  53 A to each repeating device  32  through communication ports  36 A with respective port numbers # 1  to #M/2. Specifically, communication frame generator  54 A transmits communication frame  53 A simultaneously to communication ports  36 A# 1  to  36 A#M/2. Communication ports  36 A# 1  to  36 A#M/2 transmit communication frame  53 A to repeating devices  32 . 
     Selector  58 B selects arm command U_d, based on select ID “U” specified for selector  58 B, and information  502  indicating that the remaining half (e.g., communication ports with port numbers #(M/2+1) to #M) of a plurality of communication ports  36 A are to be used as communication ports for the lower arm, and outputs the selected arm command to communication frame generator  54 B. 
     Communication frame generator  54 B generates communication frame  53 B including command data having the common command and arm command U_d. Communication frame generator  54 B transmits communication frame  53 B to each repeating device  32  through communication ports  36 A with respective port numbers #(M/2+1) to #M. 
     Thus, when communication controller  70  is provided for each leg circuit  4 , each of a plurality of communication controllers  70  transmits, to repeating device  32  connected to associated submodules  7  included in leg circuit  4  (e.g. leg circuit  4   u ) associated with this communication controller  70 , the communication frames (e.g. communication frames  53 A,  53 B) including the extracted arm commands (e.g. arm commands U_u, U_d) for upper and lower arms respectively. 
     Regarding the example shown in  FIG. 14 , the above description is given of the configuration where command information  92  including communication port number M used for each phase is provided to communication controller  70 . The configuration, however, is not limited to the above-described one. For example, communication port number M may be stored in advance as a fixed value in each communication controller  70 . For example, when communication port number M in each communication controller  70  is fixed to 10 and used, communication controller  70  transmits a communication frame including the arm command for the upper arm, through communication ports  36 A with port numbers # 1  to # 5 , and transmits a communication frame including the arm command for the lower arm, through communication ports  36 A with port numbers # 6  to # 10 . 
     Advantages 
     According to Embodiment 4, advantages similar to those of Embodiment 1 are obtained. 
     Embodiment 5 
     According to Embodiment 1, repeating devices  32  connected through a single ring network are connected to submodules  7  in the same arm. In connection with Embodiment 5, a description is given of a configuration where some repeating devices  32  connected through a single ring network are connected to submodules  7  in the upper arm of a certain phase, and the other repeating devices  32  connected through the same ring network are connected to submodules  7  in the lower arm of the same phase. 
       FIG. 15  shows a network configuration according to Embodiment 5. Referring to  FIG. 15 , a control device  101 B includes a communication port  40 U for the U phase, a communication port  40 V for the V phase, and a communication port  40 W for the W phase. Communication port  40 U is connected to HUB# 1  to HUB# 4  through the ring topology. Each of HUB# 1  and HUB# 2  is connected to associated submodules  7  for the U phase upper arm through the star topology. Each of HUB# 3  and HUB# 4  is connected to associated submodules  7  for the U phase lower arm through the star topology. The same applies as well to the V phase and the W phase. 
     A communication frame that is output from communication port  40 U is transmitted in the order of the numerals in the parentheses (1), (2), (3), (4), (5) in  FIG. 15 . A method for transmitting the communication frame is described later herein. The method for transmitting a communication frame that is output from each communication port  40 V,  40 W is similar to the method for transmitting a communication frame that is output from communication port  40 U. 
       FIG. 16  illustrates a method for transmitting command information according to Embodiment 5. Referring to  FIG. 16 , control device  101 B includes a command information generator  151 B, and a plurality of communication controllers  72 U,  72 V,  72 W (hereinafter also referred to collectively as “communication controller  72 ”). Communication controllers  72 U,  72 V, and  72 W are communication controllers corresponding to the U phase, the V phase, and the W phase, respectively. 
     Command information generator  151 B generates command information  94  for each submodule  7 . Because control device  101 B is herein supposed to be a host device, command information  94  corresponds to the control command. Command information  94  includes a common command, arm commands for respective arms, and HUB number P. HUB number P is the number of repeating devices  32  allocated to each phase. Specifically, HUB number P is the number of repeating devices  32  connected to submodules  7  in each leg circuit  4 . 
     Command information generator  151 B broadcasts command information  94  to communication controllers  72  for respective phases. Communication controllers  72  for respective phases function similarly, and therefore, the functions of communication controller  72 U are described herein. 
     Communication controller  72 U receives command information  94  transmitted from command information generator  151 B, and extracts, from command information  94 , an arm command associated with communication controller  72 U. Specifically, communication controller  72 U extracts an arm command U_u for the upper arm of leg circuit  4   u , and an arm command U_d for the lower arm of leg circuit  4   u.    
     Communication controller  72 U transmits, to each repeating device  32  connected to associated submodules  7  included in leg circuit  4   u , a communication frame  55  including arm commands U_u, U_d. Specifically, communication controller  72 U includes a selector  59 , a communication frame generator  75 , and communication port  40 U. 
     Selector  59  selects arm commands U_u, U_d, based on select ID “U” that is specified for selector  59 , and outputs the selected arm commends to communication frame generator  75 . Communication frame generator  75  extracts, from command information  94 , the common command, HUB number P, and arm commands U_u, U_d. Communication frame generator  75  generates communication frame  55  including the extracted common command, HUB number P, and arm commands U_u, U_d, as well as HUB No. J. The initial value of HUB No. J stored in communication frame  55  that is transmitted from communication port  40 U is set to “0.” Communication frame generator  75  transmits communication frame  55  to each repeating device  32  through communication port  40 U. 
     HUB# 1  receives communication frame  55 . HUB# 1  adds 1 (+1) to (namely increments) the value of HUB No. J stored in received communication frame  55  to generate the HUB No. of HUB# 1  itself, and determines whether the generated HUB No. is less than or equal to the value P/2. Because the initial value of HUB No. J is “0,” HUB# 1  generates its HUB No. “1.” Because HUB# 1  satisfies J P/2, HUB# 1  identifies itself as a repeating device  32  for the upper arm. HUB# 1  therefore extracts, from communication frame  55 , arm command U_u for the upper arm. HUB# 1  also extracts the common command from communication frame  55 . 
     HUB# 1  generates a communication frame  57 A including the common command and arm command U_u, and transmits this communication frame  57 A to each submodule  7  connected to HUB# 1 . HUB# 1  transmits, to HUB# 2 , a communication frame  55  generated by updating HUB No. J to its HUB No. (namely updating to HUB No. J=1). 
     Each of HUB# 2  to HUB#P/2 compares, with the value P/2, its HUB No. J determined by incrementing the value of received HUB No. J, to identify itself as repeating device  32  for the upper arm. Each of HUB# 2  to HUB#P/2 therefore performs a process similarly to HUB# 1 , and transmits communication frame  57 A to each submodule  7  connected to itself. 
     Subsequently, HUB#(P/2+1) receives communication frame  55  from HUB#P/2. HUB#(P/2+1) adds 1 (+1) to the value of received HUB No. J to generate its HUB No. In this case, the generated HUB No. is J=(P/2+1). HUB#(P/2+1) determines whether generated HUB No. J is less than or equal to the value P/2. Because HUB#(P/2+1) satisfies J&gt;P/2, HUB#(P/2+1) identifies itself as repeating device  32  for the lower arm. HUB#(P/2+1) therefore extracts, from communication frame  55 , arm command U_d for the lower arm. HUB#(P/2+1) also extracts the common command from communication frame  55 . 
     HUB#(P/2+1) generates a communication frame  57 B including the common command and arm command U_d, and transmits this communication frame  57 B to each submodule  7  connected to HUB#(P/2+1) itself. HUB#(P/2+1) transmits, to HUB#(P/2+2), communication frame  55  generated by updating HUB No. J to its HUB No. (namely updating to HUB No. J=(P/2+1)). 
     Each of HUB#(P/2+2) to HUB#P compares, with the value P/2, its HUB No. J generated by incrementing the value of received HUB number, to thereby identify itself as repeating device  32  for the lower arm. Each of HUB#(P/2+1) to HUB#P therefore performs a process similarly to HUB#(P/2+1), and transmits communication frame  57 B to each submodule  7  connected to itself. 
     Advantages 
     According to Embodiment 5, advantages similar to those of Embodiment 1 are obtained. 
     OTHER EMBODIMENTS 
     (1) Regarding the above example in  FIG. 9  according to Embodiment 2, the above description is given of the configuration where each of HUB data Y 1   a  to Y 4   a  includes information about all submodules  7  connected to a respective one of HUB# 1  to HUB# 4 . The configuration, however, is not limited to the above-described one. For example, if it is unnecessary to transmit information regarding all submodules  7  simultaneously to control device  101 , the amount of data for submodules  7  transmitted per communication may be limited. Specifically, each of HUB data Y 1   a  to Y 4   a  may include only the information regarding specified submodule  7  to thereby shorten the communication frame length. 
       FIG. 17  illustrates a data aggregation process according to another embodiment. A description is given herein of a data aggregation process performed by HUB# 2 . Referring to  FIG. 17 , a communication frame  51 C_a 2  received by HUB# 2  includes the contents of communication frame  51 A_a 2  in the example in  FIG. 9  to which an SM No. specification value  309  is added. 
     SM No. specification value  309  is information specifying the number of submodule  7  connected to HUB# 1  to HUB# 4  each. For example, when SM No. specification value  309  is “2,” HUB# 1  to HUB# 4  each cause HUB data to include information regarding the second submodule  7  among submodules  7  connected to the HUB itself. 
     In the example in  FIG. 17 , HUB# 2  selects, by a selector  371 , a failure bit of the second submodule  7  among submodules  7 , and stores the failure bit in HUB data Y 2   a . HUB# 2  selects, by a selector  372 , a separation bit of the second submodule  7  among submodules  7 , and stores the separation bit in HUB data Y 2   a . Regarding the failure rank information and the failure type information as well, only the information regarding the second submodule  7  is stored in HUB data Y 2   a.    
     HUB data Y 2   a  therefore includes failure bit, separation bit, failure rank information and failure type information that are included in state information Dsm of the second submodule  7 , and also includes state information Dh of HUB# 2 . Thus, in the example in  FIG. 17 , HUB data Y 2   a  does not include the information regarding submodules  7  other than the second submodule, and therefore, the data size is smaller than that of HUB data Y 2   a  in  FIG. 9 or 10 . Regarding HUB data Y 1   a  generated by HUB# 1  as well, its data size is smaller than that of HUB data Y 1   a  in  FIG. 9 or 10 . Regarding HUB aggregate data Z 2 , its data size is similar to the one in  FIG. 9  or  FIG. 10 . HUB# 2  transmits, to HUB# 3 , a communication frame  51 C a 3  including such HUB aggregate data Z 2  and HUB data Y 1   a , Y 2   a.    
     Control device  101  transmits the communication frame in which SM No. specification value  309  is set to “2” to each of HUB# 1  to HUB# 4  and, when control device  101  next transmits the communication frame, it sets SM No. specification value  309  to “3” and transmits the communication frame including this SM No. specification value  309 . Thus, each time control device  101  transmits the communication frame, it transmits the communication frame including a different SM No. specification value  309 . For example, when R submodules are connected to each of HUB# 1  to HUB# 4 , control device  101  can transmit the communication frame R times to thereby acquire information about all submodules  7  connected to each of HUB# 1  to HUB# 4 . More than a single number may be set for SM No. specification value  309 . For example, “1” and “2” may be set for SM No. specification value  309  in the present communication frame, and “3” and “4” may be set for SM No. specification value  309  in the next communication frame. 
     (2) Regarding the above embodiments, the above description is given of the configuration where repeating devices are provided between a host device and submodules, supposing that the system is a large-scale HDVC system including many submodules  7 . If the embodiments are applied to a small-scale system like STATCOM or the like, the host device may be connected directly to submodules without using repeating devices. 
     (3) Regarding the above embodiments, the above description is given of the configuration where a plurality of repeating devices are connected to each communication port. The configuration, however, is not limited to the above-described one, and may be a configuration where a single repeating device is connected to each communication port. In this case, each communication port is connected to the repeating device by the star topology. 
     (4) Each configuration presented above as an embodiment by way of example is an example of the configuration of the present disclosure, and may be combined with another known technique, or modified by being omitted partially to the extent that does not go beyond the scope of the present disclosure. Moreover, the above-described embodiments may be implemented by appropriately employing a process(es) and/or a configuration(s) described in connection with other embodiments. 
     It should be construed that the embodiments disclosed herein are given by way of illustration in all respects, not by way of limitation. It is intended that the scope of the present disclosure is defined by claims, not by the description above, and encompasses all modifications and variations equivalent in meaning and scope to the claims. 
     REFERENCE SIGNS LIST 
     
         
         
           
               1  power conversion device;  2  power conversion circuitry;  3  command generation device;  4   u ,  4   v ,  4   w  leg circuit;  5  upper arm;  6  lower arm;  7  submodule;  8 A,  8 B reactor;  9 A,  9 B arm current detection device;  10  AC voltage detection device;  11 A,  11 B DC voltage detection device;  12  AC circuit;  13  interconnection transformer;  14  DC circuit;  15  operation command;  16  AC current detection device;  21  gate controller;  22 A,  22 B switching device;  23 A,  23 B diode;  24  capacitor;  25  conversion circuit;  26 N,  26 P input/output terminal;  27  voltage detector;  28  transmission and reception device;  31 A,  31 B,  58 A,  58 B,  59  selector;  32  repeating device;  34 A- 34 F,  36 A,  40 U- 40 W,  351 - 354  communication port;  40  AC voltage command generator;  41  DC voltage command generator;  42  circulating current command generator;  44  arm voltage command generator;  52 A,  52 B,  54 A,  54 B,  75  communication frame generator;  70 U- 70 W,  72 U- 72 W,  153 ,  153 A,  153 B,  153 U- 153 W communication controller;  81 ,  86  flag region;  82 ,  87  header region;  83 ,  88  payload region;  84 ,  89  FCS region;  101 ,  101 A,  101 B control device;  102  protection device;  151 A,  151 B command information generator;  320  repeating device group.