Power conversion apparatus

Abstract

A power conversion apparatus (100) comprises a power converter (6), a control device (50), and a plurality of relay devices (30). The control device (50) transmits a communication frame (61) to the plurality of relay devices (30) via a ring-shaped network (510). Each of the plurality of relay devices (30): receives the communication frame (61) including first information related to the number of relay devices (30) that the communication frame (61) has passed until reaching the own relay device (30), second information indicating the maximum value among the numbers indicated by the first information, and a control command for controlling a plurality of converter cells (1); adjusts the transmission timing of the control command from the relay device (30) on the basis of the first information and the second information such that control commands are simultaneously transmitted from the plurality of relay devices (30); and transmits the control command to one or more of the converter cells (1) connected to the relay device (30) in accordance with the adjusted transmission timing.

Description

Power Conversion Device

[0001] The present disclosure relates to a power conversion device.

[0002] In recent years, modular multilevel converters (MMCs) have become known as high-voltage, large-capacity power conversion devices applied to high-voltage systems such as electric power grids. An MMC consists of arms in which multiple unit converters called converter cells are cascaded. Each converter cell has multiple semiconductor switches and capacitors.

[0003] In an MMC type power conversion device, control data is transmitted from a control device to each converter cell, and a communication method between the control device and each converter cell is required to be highly reliable and to suppress communication delays.

[0004] For example, Japanese Patent Laid-Open Publication No. 5-145569 (Patent Document 1) discloses a communication method in a system in which multiple line accommodating devices are connected by dual transmission lines. In this communication method, the line accommodating device sends out transmission frames with the same control number to both the working and backup dual transmission lines. The receiving line accommodating device uses the first of the received transmission frames to minimize transmission delays even on wide-area transmission lines.

[0005] Japanese Patent Application Publication No. 5-145569

[0006] In an MMC-type power conversion device, the control device and each converter cell must share the latest control information, so the control device and each converter cell are connected by optical cables. Typically, a daisy-chain connection method is used to reduce the number of optical cables, but this connection method results in differences in the time it takes for control data to reach each converter cell, which can degrade the control performance of the converter cells. The communication method described in Patent Document 1 considers reducing signal transmission delays and delays associated with switching transmission paths, but does not teach or suggest any solutions to the above problems.

[0007] An object in one aspect of the present disclosure is to provide a power conversion device that can prevent degradation of control performance of converter cells by synchronizing the timing of transmitting control data to each converter cell.

[0008] A power conversion device according to an embodiment includes a power converter including a plurality of converter cells connected in series with each other, a control device that controls the power converter, and a plurality of relay devices that relay communication between the control device and the plurality of converter cells. Each of the plurality of relay devices is connected to one or more of the plurality of converter cells. The control device transmits a communication frame to the plurality of relay devices via a ring-shaped network. Each of the plurality of relay devices receives the communication frame, which includes first information regarding the number of relay devices through which the communication frame transmitted from the control device to the network passes before reaching the relay device itself, second information indicating a maximum number indicated by the first information, and a control command for controlling the plurality of converter cells. The relay device adjusts transmission timing of the control command in the relay device itself based on the first information and the second information so that the control commands are transmitted simultaneously from the plurality of relay devices, and transmits the control command to one or more converter cells connected to the relay device itself according to the adjusted transmission timing.

[0009] A power conversion device according to another embodiment includes a power converter including a plurality of converter cells connected in series with each other, and a control device that controls the power converter. The control device transmits a communication frame to the plurality of converter cells via a ring-shaped network. Each of the plurality of converter cells receives the communication frame, which includes first information regarding the number of converter cells through which the communication frame transmitted from the control device to the network passes before reaching the corresponding converter cell, second information indicating the maximum number indicated by the first information, and a control command for controlling the corresponding converter cell. Based on the control command, the control device generates a gate signal to be transmitted to a conversion circuit of the corresponding converter cell, adjusts the transmission timing of the gate signal generated in the corresponding converter cell based on the first information and the second information so that the gate signals generated in each of the plurality of converter cells are transmitted simultaneously, and transmits the gate signal to the corresponding converter cell according to the adjusted transmission timing.

[0010] According to the power conversion device of the present disclosure, by synchronizing the transmission timing of control data to each converter cell, it is possible to prevent a decrease in the control performance of the converter cell.

[0011] 10 is a diagram illustrating an example of the configuration of a power conversion device. FIG. 11 is a circuit diagram illustrating an example of a converter cell. FIG. 12 is a block diagram illustrating a schematic configuration of a command generating device. FIG. 13 is a block diagram illustrating an example of the hardware configuration of a control device. FIG. 14 is a diagram illustrating a communication connection configuration according to a first embodiment. FIG. 15 is a diagram illustrating a configuration example of a communication frame according to the first embodiment. FIG. 16 is a diagram for explaining a communication system according to the first embodiment. FIG. 17 is a diagram illustrating a communication connection configuration according to a second embodiment. FIG. 18 is a diagram for explaining an adjustment parameter update system according to the second embodiment. FIG. 19 is a diagram for explaining a part of the communication system according to the second embodiment. FIG. 19 is a diagram for explaining another part of the communication system according to the second embodiment. FIG. 10 is a diagram illustrating a communication connection configuration when an abnormality has occurred in one of the networks in FIG. 8. FIG. 11 is a diagram for explaining a communication system in the event of a network abnormality. FIG. 12 is a diagram illustrating a communication connection configuration according to a third embodiment. FIG. 13 is a diagram illustrating a schematic configuration example of a communication frame according to the third embodiment. FIG. 14 is a diagram for explaining an adjustment parameter update system according to the third embodiment. FIG. 15 is a diagram illustrating a communication connection configuration according to a fourth embodiment. FIG. 16 is a diagram for explaining a communication system according to the fourth embodiment.

[0012] Hereinafter, the present embodiment will be described with reference to the drawings. In the following description, the same components are denoted by the same reference numerals. The names and functions of these components are also the same. Therefore, detailed description thereof will not be repeated.

[0013] [Configuration underlying each embodiment] <Overall configuration> Fig. 1 is a diagram showing an example configuration of a power conversion device. Referring to Fig. 1, a power conversion device 100 is connected between an AC circuit 2 and a DC circuit 4. The DC circuit 4 is, for example, a DC power system including a DC transmission network or a DC terminal of another power conversion device. In the latter case, two power converters are connected to form a back-to-back (BTB) system for connecting AC power systems with different rated frequencies, etc. The DC circuit 4 may be configured to include a power storage device connected to the DC terminal of the power converter 6. The power storage device may include, for example, an electric double layer capacitor or a storage battery such as a lithium-ion battery.

[0014] The power conversion device 100 includes a self-excited power converter 6 that performs power conversion between a DC circuit 4 and an AC circuit 2, and a command generating device 5. Typically, the power converter 6 is configured by a modular multilevel converter including a plurality of converter cells (corresponding to the "cells" in FIG. 1) 1 connected in series with each other. A "converter cell" is also called a "sub module" or "unit converter."

[0015] 1 , the power converter 6 includes a plurality of arms for each phase of the AC circuit 2. Specifically, the power converter 6 includes a plurality of leg circuits 8u, 8v, and 8w (hereinafter, collectively referred to as "leg circuits 8") connected in parallel between a positive DC terminal (i.e., a high-potential side DC terminal) Np and a negative DC terminal (i.e., a low-potential side DC terminal) Nn. The leg circuits 8 are connected between the AC circuit 2 and the DC circuit 4 and perform power conversion between the two circuits.

[0016] AC terminals Nu, Nv, and Nw provided in leg circuits 8u, 8v, and 8w, respectively corresponding to the U phase, V phase, and W phase of the AC circuit 2, are connected to the AC circuit 2 via a transformer 3. The AC circuit 2 is, for example, a three-phase AC power system including an AC power source. For ease of illustration, FIG. 1 does not show the connection between the AC terminals Nv and Nw and the transformer 3. DC terminals (i.e., a positive DC terminal Np and a negative DC terminal Nn) provided in common to each leg circuit 8 are connected to the DC circuit 4.

[0017] Instead of using the transformer 3 in Fig. 1 , the leg circuits 8u, 8v, 8w may be configured to be connected to the AC circuit 2 via an interconnection reactor. Furthermore, instead of the AC terminals Nu, Nv, Nw, primary windings may be provided in the leg circuits 8u, 8v, 8w, respectively, and the leg circuits 8u, 8v, 8w may be AC-connected to the transformer 3 or the interconnection reactor via secondary windings magnetically coupled to the primary windings. In this case, the primary windings may be reactors 7a, 7b described below. That is, the leg circuit 8 is electrically (i.e., DC- or AC-connected) to the AC circuit 2 via connection parts provided in each of the leg circuits 8u, 8v, 8w, such as the AC terminals Nu, Nv, Nw or the above-described primary windings.

[0018] The leg circuit 8u includes a positive arm 13pu extending from the positive DC terminal Np to the AC terminal Nu, and a negative arm 13nu extending from the negative DC terminal Nn to the AC terminal Nu. A connection point between the positive arm 13pu and the negative arm 13nu is connected to the transformer 3 as the AC terminal Nu. The positive DC terminal Np and the negative DC terminal Nn are connected to the DC circuit 4. The leg circuit 8v includes a positive arm 13pv and a negative arm 13nv, and the leg circuit 8w includes a positive arm 13pw and a negative arm 13nw.

[0019] Hereinafter, when referring to the positive arms 13pu, 13pv, and 13pw collectively or when referring to any one of them, they will be referred to as "positive arm 13p." When referring to the negative arms 13nu, 13nv, and 13nw collectively or when referring to any one of them, they will be referred to as "negative arm 13n." When referring to the positive arms 13pu, 13pv, and 13pw and the negative arms 13nu, 13nv, and 13nw collectively or when referring to any one of them, they will be referred to as "arms 13."

[0020] Since leg circuits 8v and 8w have the same configuration as leg circuit 8u, the following description will be centered on leg circuit 8u. In leg circuit 8u, positive arm 13pu includes a plurality of converter cells 1 cascaded together and a reactor 7a. The plurality of converter cells 1 and reactor 7a are connected in series. Negative arm 13nu includes a plurality of converter cells 1 cascaded together and a reactor 7b. The plurality of converter cells 1 and reactor 7b are connected in series.

[0021] The reactor 7a may be inserted at any position in the positive arm 13pu, and the reactor 7b may be inserted at any position in the negative arm 13nu. There may be a plurality of reactors 7a and a plurality of reactors 7b. The inductance values ​​of the reactors may be different from each other. Furthermore, only the reactor 7a in the positive arm 13pu or only the reactor 7b in the negative arm 13nu may be provided.

[0022] The power conversion device 100 further includes an AC voltage detector 10, an AC current detector 15, DC voltage detectors 11a and 11b, and arm current detectors 9a and 9b provided in each leg circuit 8. These detectors measure electrical quantities (i.e., current and voltage) used to control the power conversion device 100. Signals detected by these detectors are input to the command generating device 5.

[0023] The AC voltage detector 10 detects the U-phase AC voltage Vacu, the V-phase AC voltage Vacv, and the W-phase AC voltage Vacw of the AC circuit 2. The AC current detector 15 detects the U-phase AC current measured value Isysu, the V-phase AC current measured value Isysv, and the W-phase AC current measured value Isysw of the AC circuit 2. The DC voltage detector 11a detects the DC voltage Vdcp at the positive DC terminal Np connected to the DC circuit 4. The DC voltage detector 11b detects the DC voltage Vdcn at the negative DC terminal Nn connected to the DC circuit 4.

[0024] The arm current detectors 9a and 9b provided in the U-phase leg circuit 8u detect a positive arm current Ipu flowing in the positive arm 13pu and a negative arm current Inu flowing in the negative arm 13nu, respectively. The arm current detectors 9a and 9b provided in the V-phase leg circuit 8v detect a positive arm current Ipv and a negative arm current Inv, respectively. The arm current detectors 9a and 9b provided in the W-phase leg circuit 8w detect a positive arm current Ipw and a negative arm current Inw, respectively.

[0025] As shown in FIG. 1 , AC terminal Nu, which is the connection point between positive arm 13pu and negative arm 13nu of leg circuit 8u, is connected to transformer 3. Therefore, AC current Iacu flowing from AC terminal Nu to transformer 3 has a current value obtained by subtracting negative arm current Inu from positive arm current Ipu. The same is true for AC currents Iacv and Iacw. Therefore, "Iacu = Ipu - Inu", "Iacv = Ipv - Inv", and "Iacw = Ipw - Inw" are established.

[0026] The positive DC terminals of the leg circuits 8u, 8v, 8w of each phase are commonly connected as a positive DC terminal Np, and the negative DC terminals are commonly connected as a negative DC terminal Nn. From this configuration, the DC current Idc flowing from the positive terminal of the DC circuit 4 and returning to the DC circuit 4 via the negative terminal is defined as "Idc = (Ipu + Ipv + Ipw + Inu + Inv + Inw) / 2".

[0027] <Configuration Example of Converter Cell> Fig. 2 is a circuit diagram showing an example of a converter cell. The converter cell 1 shown in Fig. 2 includes a half-bridge type conversion circuit 21, a storage element 24, a voltage detector 25, a cell control unit 27, and a bypass switch 28.

[0028] The half-bridge conversion circuit 21 includes switching elements 22A and 22B and diodes 23A and 23B connected in series. The diodes 23A and 23B are connected in anti-parallel (i.e., parallel and reverse biased) with the switching elements 22A and 22B, respectively. The diodes 23A and 23B are provided to protect the switching elements 22A and 22B when a reverse voltage is applied to them. Hereinafter, the switching elements 22A and 22B and the diodes 23A and 23B will be referred to collectively or individually as the switching element 22 and the diode 23, respectively.

[0029] The storage element 24 is connected in parallel with the series-connected circuit of the switching elements 22A and 22B and holds a DC voltage. A DC capacitor is typically used as the storage element 24. The connection node between the switching elements 22A and 22B is connected to a high-potential side input / output terminal 26P. The connection node between the switching element 22B and the storage element 24 is connected to a low-potential side input / output terminal 26N.

[0030] Typically, the input / output terminal 26P is connected to the input / output terminal 26N of the adjacent converter cell 1 on the positive side. The input / output terminal 26N is connected to the input / output terminal 26P of the adjacent converter cell 1 on the negative side.

[0031] Each of the switching elements 22A, 22B is a self-extinguishing switching element that can be controlled to turn on and off, and is, for example, an insulated gate bipolar transistor (IGBT) or a gate commutated turn-off thyristor (GCT).

[0032] The conversion circuit of the converter cell 1 is not limited to the above-described half-bridge type conversion circuit 21. For example, the converter cell 1 may be configured using a full-bridge type conversion circuit or a three-quarter-bridge type conversion circuit.

[0033] The bypass switch 28 is connected between the input / output terminals 26P and 26N. For example, a mechanical switch is used as the bypass switch 28. When the bypass switch 28 is turned on, the input / output terminal 26P on the high potential side and the input / output terminal 26N on the low potential side are short-circuited.

[0034] When any element of the converter cell 1 fails, the bypass switch 28 is used to short-circuit the converter cell 1. As a result, even if any converter cell 1 among the multiple converter cells 1 fails, the operation of the power conversion device 100 can be continued by using the other converter cells 1. The decision as to whether to turn on the bypass switch 28 may be made by a control device included in the command generating device 5, or may be made by the cell control unit 27 of each converter cell 1.

[0035] The voltage detector 25 detects the voltage across the storage element 24 (i.e., the capacitor voltage Vc). The detected value of the voltage detector 25 is input to the cell control unit 27.

[0036] The cell control unit 27 receives a control command from the command generating device 5. Based on the received control command, the cell control unit 27 generates a gate signal for controlling the opening and closing of the switching elements 22A and 22B that constitute the conversion circuit 21. The cell control unit 27 also transmits to the command generating device 5 the capacitor voltage Vc detected by the voltage detector 25 and information on the opening and closing of the bypass switch 28.

[0037] Specifically, the cell control unit 27 performs phase-shift PWM (Pulse Width Modulation) control to turn one of the switching elements 22A and 22B on and the other off based on a voltage command value as a control command received. For example, when the switching element 22A is on and the switching element 22B is off, the voltage across the storage element 24 is applied between the input / output terminals 26P and 26N. Conversely, when the switching element 22A is off and the switching element 22B is on, 0 V is applied between the input / output terminals 26P and 26N. Therefore, the converter cell 1 can output zero voltage and a positive voltage that depends on the voltage of the storage element 24 by alternately turning on the switching elements 22A and 22B.

[0038] The cell control unit 27 may be configured by a dedicated circuit such as an ASIC (Application Specific Integrated Circuit), or may be configured using an FPGA (Field Programmable Gate Array), or may be configured based on a computer including a CPU (Central Processing Unit) and memory, or may be configured by a combination of two or more of the above.

[0039] The converter cell 1 is provided with a power supply circuit (not shown) that generates a drive voltage for the cell control unit 27 based on the voltage of the storage element 24. Therefore, when the voltage of the storage element 24 is low, the cell control unit 27 cannot operate.

[0040] <Configuration of Command Generating Device> Fig. 3 is a block diagram showing a schematic configuration of the command generating device. Referring to Fig. 3, the command generating device 5 includes a control device 50 that controls the power converter 6, and a relay device group 300 made up of a plurality of relay devices. The control device 50 corresponds to a higher-level device of each relay device included in the relay device group 300. Each relay device relays communication between the control device 50 and a plurality of converter cells 1 included in the power converter 6. Although Fig. 3 representatively shows only the leg circuit 8u for the U phase of the power converter 6 in Fig. 1, the same applies to the other leg circuits 8v and 8w.

[0041] The control device 50 controls each converter cell 1. The control device 50 receives inputs of AC voltages Vacu, Vacv, and Vacw (hereinafter collectively referred to as "AC voltages Vac"), AC current measurement values ​​Isysu, Isysv, and Isysw (hereinafter collectively referred to as "AC current measurement values ​​Isys"), DC voltages Vdcp and Vdcn, positive arm currents Ipu, Ipv, and Ipw (hereinafter collectively referred to as "positive arm currents Ip"), negative arm currents Inu, Inv, and Inw (hereinafter collectively referred to as "negative arm currents In"), and a capacitor voltage Vcap. Typically, the capacitor voltage Vcap is obtained by averaging the voltage values ​​of the storage elements 24 detected in each converter cell 1 for each arm circuit.

[0042] Based on the received detection values, the control device 50 generates control commands for controlling each converter cell 1 during the normal operation control period at predetermined intervals, and transmits the generated control commands to the relay device group 300.

[0043] The control commands include voltage commands, current commands, etc. The voltage commands are, for example, output voltage command values ​​for the positive-side arm 13p and negative-side arm 13n in each of the leg circuits 8u, 8v, and 8w. The current commands are, for example, output current command values ​​for the positive-side arm 13p and negative-side arm 13n in each of the leg circuits 8u, 8v, and 8w.

[0044] The relay device group 300 receives a control command from the control device 50. The relay device group 300 transmits command information including the control command to each converter cell 1. Each converter cell 1 operates in accordance with the command information. In the present embodiment, each relay device included in the relay device group 300 is connected to the converter cell 1 via a star-shaped network.

[0045] Typically, each relay device is configured using circuits such as a field programmable gate array (FPGA) and an application specific integrated circuit (ASIC).

[0046] <Example of Hardware Configuration of Control Device> Fig. 4 is a block diagram showing an example of the hardware configuration of the control device. Fig. 4 shows an example of a control device 50 configured by a computer. Referring to Fig. 4, the control device 50 includes one or more input converters 70, one or more S / H (sample and hold) circuits 71, a multiplexer (MUX) 72, an A / D converter 73, one or more CPUs (Central Processing Units) 74, a RAM (Random Access Memory) 75, a ROM (Read Only Memory) 76, one or more input / output interfaces 77, an auxiliary storage device 78, and a bus 79 that interconnects the above components.

[0047] The input converter 70 includes an auxiliary transformer for each input channel, which converts the detection signal from each electrical quantity detector in FIG. 1 into a signal with a voltage level suitable for subsequent signal processing.

[0048] A sample-and-hold circuit 71 is provided for each input converter 70. The sample-and-hold circuit 71 samples and holds a signal representing an electrical quantity received from the corresponding input converter 70 at a specified sampling frequency.

[0049] The multiplexer 72 sequentially selects the signals held in the plurality of sample-and-hold circuits 71. The A / D converter 73 converts the signal selected by the multiplexer 72 into a digital value. Note that by providing a plurality of A / D converters 73, A / D conversion may be performed in parallel on detection signals of a plurality of input channels.

[0050] The CPU 74 controls the entire control device 50 and executes arithmetic processing in accordance with a program. The RAM 75 as a volatile memory and the ROM 76 as a nonvolatile memory are used as the main memory of the CPU 74. The ROM 76 stores programs, setting values ​​for signal processing, and the like. The auxiliary storage device 78 is a nonvolatile memory with a larger capacity than the ROM 76, and stores programs, data on detected values ​​of electricity, and the like.

[0051] The input / output interface 77 is an interface circuit for communication between the CPU 74 and an external device. In the example of FIG. 3, the CPU 74 and the like are connected to the relay device group 300 via one or more input / output interfaces 77.

[0052] At least a part of the control device 50 may be configured using circuits such as FPGA and ASIC, or at least a part of the control device 50 may be configured using analog circuits.

[0053] First Embodiment <Communication Connection Configuration and Communication Method> Fig. 5 is a diagram showing a communication connection configuration according to a first embodiment. Specifically, Fig. 5 shows an example in which a relay device group (corresponding to the "HUB group" in the figure) 300 is configured by four relay devices (corresponding to the "HUB" in the figure) 30, but the number of relay devices 30 is not limited to this example and may be two or three, or may be five or more. In the following description, a "relay device" is also referred to as a "HUB." In the first embodiment, the four relay devices 30 are also referred to as HUBs #1 to #4, respectively.

[0054] 5, the distance between the control device 50 and the relay device group 300 is indicated as "R1." Typically, the distance R1 is the distance between the control device 50 and the most upstream HUB #1 of the relay device group 300. More specifically, the distance R1 is the length of the optical fiber cable connecting the communication port 51 of the control device 50 and the upper port 31 of HUB #1. The distance R1 is, for example, 200 m.

[0055] 5, the control device 50 includes a communication port 51 included in the input / output interface 77. Each of the four HUBs #1 to #4 includes a communication port (hereinafter also simply referred to as an "upper port") 31 on the upstream side (e.g., on the control device 50 side) and a plurality of communication ports (hereinafter also simply referred to as "lower ports") 35 on the downstream side (e.g., on the converter cell 1 side).

[0056] The control device 50 and the four HUBs #1 to #4 are connected via a ring-shaped daisy-chain network 510 (communication paths indicated by solid lines in the figure). Specifically, the communication port 51 of the control device 50 is connected to the upper port 31 of HUB #1. The upper port 31 of HUB #1 is further connected to the upper port 31 of HUB #2. The upper port 31 of HUB #2 is further connected to the upper port 31 of HUB #3. The upper port 31 of HUB #3 is further connected to the upper port 31 of HUB #4. The upper port 31 of HUB #4 is further connected to the communication port 51 of the control device 50. The control device 50 transmits a communication frame 61 to the four HUBs #1 to #4 via the ring-shaped network 510. Therefore, the communication frame 61 is transmitted via the ring-shaped network 510 in the order of the control device 50, HUB #1, HUB #2, HUB #3, HUB #4, and control device 50.

[0057] 6 is a diagram schematically illustrating an example of a configuration of a communication frame according to the first embodiment. Referring to FIG. 6, a communication frame 61 mainly includes a flag, a communication command, a sequence number (corresponding to "SQE number" in the diagram), the numbers Nmax and Nown as adjustment parameters, a control command for controlling the converter cells 1, and an FCS (Frame Check Sequence) which is error detection information. The sequence number is a serial number assigned to the communication frame 61. As will be described in detail later, the numbers Nmax and Nown are parameters for adjusting the transmission timing of the communication frame for the converter cells transmitted from each relay device 30.

[0058] 5 again, each of HUBs #1 to #4 is connected to one or more converter cells 1 via a star-shaped network. Specifically, each lower port 35 of HUB #1 is connected to a corresponding converter cell 1. HUB #1 extracts a control command contained in a communication frame 61 and transmits a communication frame for the cell containing the control command to the converter cell 1 via the lower port 35. The same applies to HUBs #2 to #4. Note that the number of converter cells 1 corresponding to each relay device 30 may be different from the number of converter cells 1 corresponding to the other relay devices 30.

[0059] In this way, in the example of FIG. 5, a network between the control device 50, each relay device 30, and each converter cell 1 is constructed by combining a ring-shaped network topology and a star-shaped network topology.

[0060] 7 is a diagram for explaining a communication method according to the first embodiment. First, with reference to FIG. 7, a communication flow of a communication frame 61 returning from the control device 50 via each of HUBs #1 to #4 to the control device 50 will be described.

[0061] At time tf, the control device 50 starts transmitting a communication frame 61 to the network 510 via the communication port 51. At time t1, the HUB #1 starts receiving the communication frame 61 via the upstream port 31.

[0062] The difference in time between time t1 and time tf is a communication delay time caused by the distance R1 between the communication port 51 and the upper port 31 of HUB #1, and is also referred to as "delay time Dh1." Specifically, delay time Dh1 is the communication delay time from the timing when the control device 50 starts transmitting the communication frame 61 using the communication port 51 (i.e., time tf) to the timing when HUB #1 starts receiving the communication frame 61 using the upper port 31 (i.e., time t1). The longer the distance R1, the greater the delay time Dh1.

[0063] HUB #1 starts transmitting a communication frame 61 to the next HUB #2 at time t2, which is a time Dp after time t1. Here, time Dp is the delay time from when HUB #1 starts receiving the communication frame 61 from the upstream device (in this case, the control device 50) until when it starts transmitting the communication frame 61 to the downstream device (in this case, HUB #2).

[0064] During the period indicated by time Dp, HUB #1 updates the number Now included in the communication frame 61 received from upstream and transmits the communication frame 61 including the updated number Now to HUB #2. Specifically, the number Now is information indicating the number of relay devices 30 through which the communication frame 61 transmitted from the control device 50 to the network 510 passed before reaching its own relay device. HUB #1 is the most upstream relay device 30, and the number of relay devices 30 through which the communication frame 61 passed before reaching HUB #1 is 0. Therefore, the number Now included in the communication frame 61 received by HUB #1 is "0". HUB #1 counts up the "0" indicated by the number Now to "1" (i.e., +1) and transmits the communication frame 61 including the number Now indicating "1" to the next HUB #2.

[0065] At time t2, HUB #2 begins receiving a communication frame 61 from HUB #1 via the upper port 31. Here, each HUB is located in the same or adjacent control panel, and the communication distance between each HUB is short. Therefore, the communication delay time between each HUB can be considered to be substantially zero. HUB #2 counts up the number Now included in the communication frame 61 received from HUB #1 from "1" to "2." At time t3, a time Dp after time t2, HUB #2 transmits a communication frame 61 including the counted-up number Now to HUB #3.

[0066] At time t3, HUB #3 starts receiving communication frame 61 from HUB #2. HUB #3 counts up the number Now included in communication frame 61 received from HUB #2 from "2" to "3." At time t4, a time Dp after time t3, HUB #3 transmits communication frame 61 including the counted-up number Now to HUB #4.

[0067] At time t4, HUB #4 starts receiving communication frame 61 from HUB #3. HUB #4 counts up the number Now included in communication frame 61 received from HUB #3 from "3" to "4". At time t5, which is a time Dp after time t4, HUB #4 transmits communication frame 61 including the counted-up number Now to the control device 50. At time t6, which is a delay time Dh1 after time t5, the control device 50 starts receiving the communication frame 61.

[0068] Next, we will explain the transmission timing of the communication frame for cells (hereinafter also referred to as "cell frame") transmitted to the converter cell 1. In order to synchronize the transmission timing of the cell frame from each of HUBs #1 to #4, the control device 50 needs to set the number Nmax included in the communication frame 61.

[0069] During the initial communication between the control device 50 and the relay device group 300, a communication frame 61 is transmitted according to the above flow. During the initial communication, the configuration is such that no cell frames are transmitted from each of HUBs #1 to #4. The control device 50 sets the number Now (i.e., "4") contained in the communication frame 61 received from HUB #4 as the number Nmax. The number Now contained in the communication frame 61 is minimum (i.e., "0") when transmitted from the control device 50, but is maximum (i.e., "4") when received by the control device 50. Therefore, the number Nmax is the maximum value of the number indicated by the number Now.

[0070] The control device 50 transmits a communication frame 61 including the set number Nmax (here, "4") to the network 510. The HUB #1 adjusts the transmission timing of the cell frame including the control command based on the number Nmax and the number Now included in the received communication frame 61.

[0071] Specifically, HUB #1 calculates the number Nad, which is calculated as "Nmax - (Nown + 1)". Here, "Nmax = 4" and "Nown = 0", so "Nad = 3". HUB #1 sets "Dp x Nad (= 3)" as the adjustment period for the transmission timing of the cell frame. As a result, the timing at which HUB #1 starts the specified processing required to transmit the cell frame is the time tx after the end of reception of the communication frame 61 and the lapse of the adjustment period.

[0072] The prescribed processing includes, for example, a process of extracting a control command contained in the communication frame 61, a process of including the extracted control command in the cell frame, etc. The time Dx indicating the processing time of the prescribed processing is set to a fixed value and is common to each HUB. The configuration of the cell frame is the same as the configuration of the communication frame 61 shown in Figure 6, with the numbers Nmax and Nown deleted.

[0073] Therefore, at time tcf, which is a time Dx after time tx, HUB #1 transmits (outputs) a cell frame to the corresponding converter cell 1. The same explanation will be given below for the timing of transmitting cell frames in HUBs #2 to #4.

[0074] HUB #2 calculates the number Nad (=2) based on the number Nmax (=4) and the number Nown (=1) contained in the received communication frame 61. HUB #2 sets "Dp x Nad (=2)" as the adjustment period for the transmission timing of the cell frame. As a result, the timing at which the specified process starts in HUB #2 is time tx, which is the time after the end of reception of the communication frame 61 and the adjustment period has elapsed. Therefore, in HUB #2 as well, the transmission timing of the cell frame is time tcf, which is the time Dx after time tx.

[0075] HUB #3 calculates the number Nad (=1) based on the number Nmax (=4) and the number Nown (=2) contained in the received communication frame 61. HUB #3 sets "Dp x Nad (=1)" as the adjustment period for the transmission timing of the cell frame. As a result, in HUB #3 as well, the timing at which the specified process starts becomes time tx, and the timing at which the cell frame is transmitted becomes time tcf.

[0076] HUB #4 calculates the number Nad (=0) based on the number Nmax (=4) and the number Nown (=3) contained in the received communication frame 61. Therefore, the adjustment period in HUB #4 becomes "0". As a result, in HUB #4 as well, the timing at which the prescribed process starts becomes time tx, and the timing at which the cell frame is transmitted becomes time tcf.

[0077] As described above, the period from when each HUB finishes receiving the communication frame 61 to when it sends a control command to the converter cell 1 includes an adjustment period for adjusting the timing of sending the control command (i.e., a period corresponding to "Dp x Nad") and a processing period for executing the specified processing (i.e., a period corresponding to time Dx).

[0078] Each HUB #1 to #4 uses the number Nad to set an adjustment period taking into account the difference in time Dp that occurs between each HUB #1 to #4. For example, if the number Now indicated by one of the multiple HUBs received is k (where k is an integer greater than or equal to 0), and the number Now indicated by the other HUBs received is m (where m is an integer greater than k), the adjustment period for that one HUB will be longer than the adjustment periods for the other HUBs. Specifically, the difference in time between the adjustment period for one HUB and the adjustment period for the other HUBs will be (m-k) times a fixed time (for example, time Dp).

[0079] As described above, each HUB adjusts the transmission timing of the control command in its own HUB based on the number Nmax and the number Now so that control commands are transmitted simultaneously from multiple HUBs. Each HUB transmits a control command to one or more converter cells 1 connected to its own HUB according to the adjusted transmission timing. This makes it possible to match the transmission timing of the control commands transmitted from each of HUBs #1 to #4 to each converter cell 1. Therefore, since the arrival times of the control commands to each converter cell 1 are matched, it is possible to prevent variation in the control of the converter cells 1 and maintain high control performance.

[0080] Second Embodiment <Communication Connection Configuration and Communication Method> Fig. 8 is a diagram showing a communication connection configuration according to a second embodiment. Fig. 8 shows an example in which a relay device group 300A is configured by four relay devices 30A. The four relay devices 30A in the second embodiment are also referred to as HUBs #1 to #4, respectively. The distance R1 is as described in Fig. 5.

[0081] 8, control device 50A includes communication ports 51 and 52. For convenience, the control device 50A is designated by the symbol "A" to distinguish it from control device 50 according to the first embodiment. This also applies to the other embodiments described below. Each of four HUBs #1 to #4 includes upper ports 31 and 32 and a plurality of lower ports 35.

[0082] The control device 50A and the four HUBs #1 to #4 are connected via a ring-shaped daisy-chain network 510 (communication paths indicated by solid lines in the figure) and a ring-shaped daisy-chain network 520 (communication paths indicated by dotted lines in the figure). As described in Figure 5, a communication frame 61 is transmitted via the communication port 51 of the control device 50A and the upper ports 31 of each of the HUBs #1 to #4. That is, the communication frame 61 is transmitted via the network 510 in the order of the control device 50A, HUB #1, HUB #2, HUB #3, HUB #4, and control device 50A.

[0083] Furthermore, the communication port 52 of the control device 50A is connected to the upper port 32 of HUB #4. The upper port 32 of HUB #4 is further connected to the upper port 32 of HUB #3. The upper port 32 of HUB #3 is further connected to the upper port 32 of HUB #2. The upper port 32 of HUB #2 is further connected to the upper port 32 of HUB #1. The upper port 32 of HUB #1 is further connected to the communication port 52 of the control device 50A. As a result, the communication frame 61 is transmitted via the network 520 in the order of the control device 50A, HUB #4, HUB #3, HUB #2, HUB #1, and control device 50A.

[0084] As described above, in the second embodiment, the ring-shaped network for communicatively connecting the control device 50A and the relay device group 300A is duplicated into the network 510 and the network 520, in which the communication frame 61 circulates in opposite directions. The control device 50A simultaneously transmits the communication frame 61 to the network 510 and the network 520. Hereinafter, the communication frame 61 communicated via the network 510 will also be referred to as a "communication frame 61_1" for convenience, and the communication frame 61 communicated via the network 520 will also be referred to as a "communication frame 61_2" for convenience.

[0085] Here, the networks 510 and 520 have the same communication method and communication speed. Therefore, when communication frames 61 are simultaneously transmitted from the control device 50A to the networks 510 and 520, HUBs #1 and #2 receive communication frame 61_1 and then communication frame 61_2. That is, at HUBs #1 and #2, communication frame 61_1 arrives first, and communication frame 61_2 arrives second. On the other hand, HUBs #3 and #4 receive communication frame 61_1 after receiving communication frame 61_2. That is, at HUBs #3 and #4, communication frame 61_2 arrives first, and communication frame 61_1 arrives second.

[0086] Each of HUBs #1 to #4 extracts the control command contained in the communication frame 61_1 received via the network 510 and the communication frame 61_2 received via the network 520 that was received first (i.e., the communication frame that arrived first), and transmits the control command to the converter cell 1 connected to the own HUB. Specifically, each of HUBs #1 and #2 transmits the control command contained in the communication frame 61_1 to the converter cell 1, and each of HUBs #3 and #4 transmits the control command contained in the communication frame 61_2 to the converter cell 1.

[0087] 9 is a diagram for explaining a method for updating an adjustment parameter according to the second embodiment. Here, a method for updating the number Now, which is a correction parameter, will be explained. In the first embodiment, a configuration was explained in which each of HUBs #1 to #4 counts up the number Now included in a received communication frame 61 and transmits the counted-up number Now to the next device (for example, a HUB or control device 50A). In the second embodiment, only the number Now included in the first-arriving communication frame 61 is counted up, and the number Now included in the later-arriving communication frame 61 is not counted up.

[0088] 9, a method for updating the number Now included in a communication frame 61_1 will be described. The control device 50A transmits the communication frame 61_1 to HUB #1 via the communication port 51. HUB #1 counts up the number Now included in the first-arrived communication frame 61_1 from "0" to "1," and transmits the communication frame 61_1 including the counted-up number Now to HUB #2.

[0089] Each HUB determines whether a communication frame 61 is the first or last arriving communication frame based on the sequence number included in the communication frame 61. For example, communication frames 61_1 and 61_2 simultaneously transmitted from the control device 50A contain the same sequence number. Therefore, when each HUB receives a communication frame 61 (also referred to as "communication frame 61x" for convenience) having sequence number Nsqe, it determines whether it has previously received a communication frame 61 having the same sequence number as sequence number Nsqe. If each HUB has not previously received a communication frame 61 having the same sequence number as sequence number Nsqe, it determines that communication frame 61x is the first arriving communication frame. If each HUB has previously received a communication frame 61 having the same sequence number, it determines that communication frame 61x is the last arriving communication frame.

[0090] HUB#2 counts up the number Now included in the first-arrived communication frame 61_1 from "1" to "2" and transmits the communication frame 61_1 including the counted-up number Now to HUB#3.

[0091] HUB #3 determines that communication frame 61_1 arrived later than communication frame 61_2, and transmits communication frame 61_1 including the number Now contained in communication frame 61_1 to the next HUB #4, without counting up the number Now indicated by "2." Similarly, HUB #4 determines that communication frame 61_1 arrived later than communication frame 61_2, and transmits communication frame 61_1 including the number Now indicated by "2" to the control device 50A. The number Now contained in communication frame 61_2 is also updated in the same manner as above.

[0092] Therefore, after the initial communication between the control device 50A and the relay device group 300A is completed, the control device 50A sets the maximum value of the number Now (i.e., "2") contained in the communication frame 61_1 received from HUB #4 and the number Now (i.e., "2") contained in the communication frame 61_2 received from HUB #1 as the number Nmax. In this case, since the numbers Now contained in the communication frames 61_1 and 61_2 are both "2", the number Nmax is set to "2".

[0093] Fig. 10 is a diagram illustrating a part of the communication method according to the second embodiment. With reference to Fig. 10, the communication flow of a communication frame 61_1 transmitted from control device 50A via network 510 will be described.

[0094] At time tf, the control device 50A starts transmitting a communication frame 61_1 to the network 510. At time t1, HUB #1 starts receiving the communication frame 61_1. At time t2, which is a time Dp after time t1, HUB #1 starts transmitting the communication frame 61_1 to HUB #2. HUB #1 counts up the number Now included in the communication frame 61_1 from "0" to "1" and transmits the communication frame 61_1 including the number Now indicating "1" to HUB #2.

[0095] At time t2, HUB #2 starts receiving communication frame 61_1 from HUB #1. HUB #2 counts up the number Now included in communication frame 61_1 from "1" to "2." At time t3, HUB #2 transmits communication frame 61_1 including the counted-up number Now to HUB #3.

[0096] At time t3, HUB #3 starts receiving communication frame 61_1 from HUB #2. HUB #3 determines that communication frame 61_2 arrived earlier than communication frame 61_1, and maintains the number Now at 2. At time t4, HUB #3 transmits communication frame 61_1 including the number Now to HUB #4.

[0097] At time t4, HUB #4 starts receiving communication frame 61_1 from HUB #3. HUB #4 determines that communication frame 61_2 arrived earlier than communication frame 61_1, and maintains the number Now at "2". At time t5, HUB #4 transmits communication frame 61_1 including the number Now to the control device 50A. At time t6, the control device 50A receives the communication frame 61_1.

[0098] Next, the transmission timing of the cell frame transmitted to the converter cell 1 will be explained. HUB #1 calculates the number Nad based on the number Nmax and number Now contained in the first-arrived communication frame 61_1. Since "Nmax = 2" and "Nown = 0", "Nad = Nmax - (Nown + 1) = 1" is obtained. HUB #1 sets "Dp x Nad (= 1)" as the adjustment period. As a result, the timing at which the specified process starts in HUB #1 is time tx1, which is the time after the adjustment period has elapsed since the end of reception of the communication frame 61_1. HUB #1 transmits the cell frame to the converter cell 1 at time tcf1, which is the time Dx after time tx1.

[0099] HUB #2 calculates the number Nad (=0) based on the number Nmax (=2) and the number Nown (=1) included in the first-arrived communication frame 61_1. Since the adjustment period in HUB #2 is "0", the timing at which the prescribed process starts is time tx1, and the timing at which the cell frame is transmitted is time tcf1.

[0100] In HUB#3 and HUB#4, since the communication frame 61_1 arrives later, the control command included in the communication frame 61_1 is not transmitted, i.e., the cell frame based on the communication frame 61_1 is not transmitted.

[0101] Fig. 11 is a diagram illustrating another part of the communication method according to the second embodiment. With reference to Fig. 11, the communication flow of communication frame 61_2 transmitted from control device 50A via network 520 will be described.

[0102] The communication flow of the communication frame 61_2 is basically the same as the communication flow of the communication frame 61_1. At time tf, the control device 50A starts transmitting the communication frame 61_2 to the network 520. At time t1, HUB #4 starts receiving the communication frame 61_2 and counts up the number Now included in the first-arrived communication frame 61_2 from "0" to "1". At time t2, HUB #4 transmits the communication frame 61_2 including the number Now indicating "1" to HUB #3.

[0103] At time t2, HUB #3 starts receiving communication frame 61_2 from HUB #4 and counts up the number Now included in the first-arrived communication frame 61_2 from "1" to "2." At time t3, HUB #3 transmits communication frame 61_2 including the counted-up number Now to HUB #3.

[0104] At time t3, HUB #2 starts receiving communication frame 61_2 from HUB #3. HUB #2 determines that communication frame 61_1 arrived earlier than communication frame 61_2, and maintains the number Now at 2. At time t4, HUB #2 transmits communication frame 61_2 including the number Now to HUB #4.

[0105] At time t4, HUB #1 starts receiving communication frame 61_2 from HUB #2. HUB #1 determines that communication frame 61_1 arrived earlier than communication frame 61_2, and maintains the number Now at "2". At time t5, HUB #1 transmits communication frame 61_2 including the number Now to the control device 50A. At time t6, the control device 50A receives the communication frame 61_2.

[0106] Next, the transmission timing of the cell frame transmitted to the converter cell 1 will be described. HUB #4 calculates the number Nad (=1) based on the number Nmax (=2) and the number Nown (=0) contained in the first-arrived communication frame 61_2. HUB #4 sets "Dp x Nad (=1)" as the adjustment period. As a result, the timing at which the specified process starts in HUB #4 is time tx1, which is the time after the adjustment period has elapsed since the end of reception of the communication frame 61_2. HUB #4 transmits the cell frame to the converter cell 1 at time tcf1, which is the time Dx after time tx1.

[0107] HUB #3 calculates the number Nad (=0) based on the number Nmax (=2) and the number Nown (=1) contained in the first-arrived communication frame 61_2. Since the adjustment period in HUB #3 is "0", the timing at which the prescribed process starts is time tx1, and the timing at which the cell frame is transmitted is time tcf1. Since communication frame 61_2 arrives later in HUB #2 and HUB #1, the control command contained in communication frame 61_2 is not transmitted.

[0108] 10 and 11, compared with FIG. 7, the transmission timing of the cell frame according to the second embodiment (i.e., time tcf1) is earlier than the transmission timing of the cell frame according to the first embodiment (i.e., time tcf). Specifically, time tcf1 is earlier than time tcf by the time indicated by "Dp×2."

[0109] In the second embodiment, each of the HUBs #1 to #4 also uses the number Nad to set an adjustment period in consideration of the difference in time Dp that occurs between the HUBs #1 to #4. This allows the transmission timing of the control commands transmitted from the HUBs #1 to #4 to the converter cells 1 to be synchronized.

[0110] <When a network abnormality occurs> Fig. 12 is a diagram showing the communication connection configuration when an abnormality occurs in one of the networks in Fig. 8. Fig. 12 assumes that the optical cable constituting the network 520 is disconnected. As described above, the control device 50A transmits a communication frame 61_1 to the network 510 and transmits a communication frame 61_2 to the network 520. Here, if the networks 510 and 520 are healthy, the transmitted communication frames 61_1 and 61_2 return to the control device 50A.

[0111] If the control device 50A does not receive the communication frame 61 even after a certain time has elapsed since transmitting the communication frame 61, the control device 50A determines that an abnormality has occurred (for example, a disconnection) in the network that transmitted the communication frame 61. In the example of Fig. 12, the control device 50A cannot receive the communication frame 61_2, and therefore determines that an abnormality has occurred in the network 520.

[0112] 13 is a diagram for explaining a communication method when a network abnormality occurs. Referring to FIG. 13, since the network 520 is in a disconnected state, communication of the communication frame 61_2 transmitted from the control device 50A is impossible. Therefore, each of the HUBs #1 to #4 receives only the communication frame 61_1 via the network 510.

[0113] If both networks 510 and 520 were healthy, communication frame 61_2 would arrive first at each of HUBs #3 and #4. However, because network 520 is disconnected and communication frame 61_2 cannot communicate, communication frame 61_1 arrives first at each of HUBs #3 and #4.

[0114] For this reason, the communication method when an abnormality occurs in the network 520 is substantially the same as the communication method in which the communication frame 61 is communicated only over the network 510 as shown in Fig. 7. Therefore, the processing of each of the HUBs #1 to #4 shown in Fig. 13 is the same as the processing of each of the HUBs #1 to #4 shown in Fig. 7.

[0115] Typically, when an abnormality occurs in the network 520, the control device 50A transmits a communication frame 61_1 to the network 510. Each of the HUBs #1 to #4 counts up the number Now contained in the communication frame 61_1 received via the network 510, and transmits the communication frame 61_1 containing the counted-up Now to the next HUB via the network 510. Therefore, the transmission timing of the cell frame transmitted from each of the HUBs #1 to #4 is the time tcf, as in FIG.

[0116] Embodiment 3 <Communication Connection Configuration and Communication Method> Fig. 14 is a diagram showing a communication connection configuration according to embodiment 3. Referring to Fig. 14, a relay device group 300_1 is substantially the same as the relay device group 300A shown in Fig. 8.

[0117] The control device 50B includes communication ports 51 to 54. The control device 50B transmits a communication frame 62 via the communication ports 51 to 54 simultaneously.

[0118] 15 is a diagram schematically illustrating an example of a configuration of a communication frame according to the third embodiment. Referring to FIG. 15, communication frame 62 mainly includes a flag, a communication command, a sequence number, time Dhad as adjustment parameters, number Nmax and number Now, a control command for controlling converter cell 1, and FCS as error detection information. Communication frame 62 is communication frame 61 to which data of time Dhad has been added. Details of time Dhad will be described later.

[0119] 14 again, similar to the configuration in FIG. 8, the ring-shaped network for communicatively connecting the control device 50B and the relay device group 300_1 is duplicated into networks 510 and 520. The communication method of the communication frame 62 via the networks 510 and 520 is the same as the communication method of the communication frame 61 via the networks 510 and 520 described in embodiment 2. The distance R1 is as described in FIG. 5.

[0120] The relay device group 300_2 includes two relay devices 30A. The two relay devices 30A included in the relay device group 300_2 are also referred to as HUB#1* and HUB#2*, respectively. Each of HUB#1* and HUB#2* is connected to one or more converter cells 1.

[0121] 14, the distance between the control device 50B and the relay device group 300_2 is indicated as "R2." Specifically, the distance R2 is the length of the optical fiber cable connecting the communication port 53 of the control device 50B and the upper port 31 of the HUB #1*. Alternatively, the distance R2 is the length of the optical fiber cable connecting the communication port 54 of the control device 50B and the upper port 32 of the HUB #2*. The distance R2 is, for example, 50 m, which is shorter than the distance R1.

[0122] The control device 50B and the two HUBs #1* and #2* included in the relay device group 300_2 are connected via a ring-shaped daisy-chain network 530 (communication path indicated by a solid line in the figure) and a ring-shaped daisy-chain network 540 (communication path indicated by a dotted line in the figure). In one aspect, the communication frame 62 is transmitted via the network 530 in the order of the control device 50B, HUB #1*, HUB #2*, and control device 50B. In another aspect, the communication frame 62 is transmitted via the network 540 in the order of the control device 50B, HUB #2*, HUB #1*, and control device 50B.

[0123] In this way, the ring-shaped network for connecting the control device 50B and the relay device group 300_2 for communication is duplicated into the network 530 and the network 540 in which the communication frame 62 circulates in opposite directions.

[0124] The control device 50B simultaneously transmits communication frames 62 to the networks 510 to 540. For convenience, the communication frame 62 communicated via the network 510 is also referred to as a "communication frame 62_1," and the communication frame 62 communicated via the network 520 is also referred to as a "communication frame 62_2." For convenience, the communication frame 62 communicated via the network 530 is also referred to as a "communication frame 62_3," and the communication frame 62 communicated via the network 540 is also referred to as a "communication frame 62_4."

[0125] Here, communication frames 62 are simultaneously transmitted from the control device 50B to the networks 510 to 540, and the networks 510 to 540 use the same communication method and communication speed. Therefore, communication frame 62_1 arrives first at HUBs #1 and #2, and communication frame 62_2 arrives first at HUBs #3 and #4. Furthermore, communication frame 62_3 arrives first at HUB #1*, and communication frame 62_4 arrives first at HUB #2*.

[0126] As explained in the second embodiment, each of HUBs #1 to #4 transmits the control command contained in the communication frame that arrived first to the converter cell 1. Specifically, HUBs #1 and #2 transmit the control command contained in the communication frame 62_1 to the converter cell 1. HUBs #3 and #4 transmit the control command contained in the communication frame 62_2 to the converter cell 1. Similarly, HUB #1* transmits the control command contained in the communication frame 62_3 to the converter cell 1, and HUB #2* transmits the control command contained in the communication frame 62_4 to the converter cell 1.

[0127] 16 is a diagram for explaining a method for updating an adjustment parameter according to the third embodiment. Here, a method for setting the time Dhad, which is an adjustment parameter, and a method for updating the number Nown will be explained. FIG. 16 shows the communication flow of a communication frame 62_1 communicated via a network 510.

[0128] The method of updating the number Now contained in the communication frames 62_1 and 62_2 is the same as that described in Fig. 9. Specifically, only the number Now contained in the first-arriving communication frame 62 is counted up, and the number Now contained in the later-arriving communication frame 62 is not counted up. Therefore, HUB #1 counts up the number Now indicated by "0" contained in the first-arriving communication frame 62_1 to "1", and HUB #2 counts up the number Now indicated by "1" contained in the first-arriving communication frame 62_1 to "2". In HUB #3 and #4, the number Now contained in the communication frame 62_1 is maintained at "2".

[0129] The number Now contained in the communication frame 62_2 is also updated in the same manner. Specifically, HUB #3 counts up the number Now contained in the first-arrived communication frame 62_2 from "0" to "1," and HUB #4 counts up the number Now contained in the first-arrived communication frame 62_2 from "1" to "2." In HUB #1 and #2, the number Now contained in the communication frame 62_2 is maintained at "2."

[0130] The method of updating the number Now contained in the communication frames 62_3 and 62_4 can be considered similar to the above. Specifically, HUB #1* counts up the number Now contained in the first-arrived communication frame 62_3 from "0" to "1." In HUB #2*, the number Now contained in the later-arrived communication frame 62_3 remains at "1." On the other hand, HUB #2* counts up the number Now contained in the first-arrived communication frame 62_4 from "0" to "1." In HUB #1*, the number Now contained in the later-arrived communication frame 62_4 remains at "1."

[0131] After the initial communication with the relay device groups 300_1 and 300_2 is completed, the control device 50B sets the maximum value of the numbers Now included in each of the communication frames 62_1 to 62_4 as the number Nmax. In this case, the number Nmax is set to "2".

[0132] In the third embodiment, as shown in FIG. 14, distances R1 and R2 are different, and therefore delay time Dh1 corresponding to distance R1 and delay time Dh2 corresponding to distance R2 are different. Therefore, the transmission timing of the cell frame is adjusted taking into consideration time Dhad, which indicates the difference between these communication delay times. The calculation method for time Dhad will be described below. Note that, since distance R1 is greater than distance R2, delay time Dh1 is greater than delay time Dh2.

[0133] The control device 50B calculates the delay time Dh1 based on the total communication delay time Dall required for the communication frame 62_1 transmitted from the control device 50B via the network 510 to pass through each of the HUBs #1 to #4 and reach the control device 50B, and the number of each of the HUBs #1 to #4. Specifically, the control device 50B measures the total communication delay time Dall from when the communication frame 62_1 is transmitted to the network 510 until when the communication frame 62_1 is received.

[0134] In the example of FIG. 16 , the total communication delay time Dall corresponds to the time from time tf to time t6. Because there are four hubs on the route of the network 510, the relational expression "Dall = Dh1 × 2 + Dp × number of hubs (= 4)" holds. The control device 50B calculates the delay time Dh1 caused by the distance R1 based on the fixed value of time Dp, the measured total communication delay time Dall, and this relational expression. The calculation of the delay time Dh1 may be performed using the communication frame 62_2.

[0135] Similarly, the control device 50B calculates the delay time Dh2 caused by the distance R2. Specifically, the control device 50B measures the total communication delay time Dallx from when the communication frame 62_3 is transmitted to the network 530 until the communication frame 62_3 is received. Because there are two hubs on the route of the network 530, the relational expression "Dallx = Dh2 × 2 + Dp × number of hubs (= 2)" holds. The control device 50B calculates the delay time Dh2 caused by the distance R2 based on the time Dp, the measured total communication delay time Dallx, and this relational expression. The calculation of the delay time Dh2 may be performed using the communication frame 62_4.

[0136] After the initial communication with the relay device groups 300_1 and 300_2 is completed, the control device 50B calculates a time Dhad, which is a subtraction value obtained by subtracting the delay time Dh2 from the delay time Dh1. The control device 50B transmits a communication frame 62 including the time Dhad via each of the networks 510 to 540.

[0137] Next, the communication flow of communication frames 62_1 to 62_4 and the transmission timing of cell frames will be described. Here, the time Dhad is taken into consideration only when setting the adjustment period for each of HUBs #1* and #2* located at a short distance R2 from the control device 50B, and is not taken into consideration when setting the adjustment period for each of HUBs #1 to #4 located at a long distance R1 from the control device 50B. Note that the control device 50B may notify each of HUBs #1 to #4 to ignore the time Dhad when setting the adjustment period.

[0138] Specifically, the method for setting the adjustment period in each of HUBs #1 to #4 according to the third embodiment is the same as in Figures 10 and 11. Therefore, the communication flow of communication frames 62_1 and 62_2 and the transmission timing of cell frames in each of HUBs #1 to #4 are the same as in Figures 10 and 11. Therefore, each of HUBs #1 to #4 transmits a cell frame to converter cell 1 at time tcf1, which is a time Dx after time tx1.

[0139] Fig. 17 is a diagram illustrating a communication system according to the third embodiment. Specifically, Fig. 17 shows the communication flow of communication frames 62_3 and 62_4.

[0140] 17 , at time tf, the control device 50B starts transmitting a communication frame 62_3 to the network 530 and a communication frame 62_4 to the network 540. At time t1a, HUB #1* starts receiving the communication frame 62_3, and HUB #2* starts receiving the communication frame 62_4. The delay time Dh2 corresponds to the time from time tf to time t1a.

[0141] HUB #1* counts up the number Now included in communication frame 62_3 from "0" to "1". HUB #2* counts up the number Now included in communication frame 62_4 from "0" to "1". At time t2a, which is a time Dp after time t1a, HUB #1* starts transmitting communication frame 62_3 to HUB #2*, and HUB #2* starts transmitting communication frame 62_4 to HUB #1*.

[0142] At time t4a, which is a time Dp after time t2a, HUB #2* starts transmitting a communication frame 62_3 to the control device 50B, and HUB #1* starts transmitting a communication frame 62_4 to the control device 50B. HUB #1* maintains the number Now included in the later-arrived communication frame 62_4 as "1". HUB #2* maintains the number Now included in the later-arrived communication frame 62_3 as "1". At time t5a, the control device 50B receives the communication frames 62_3 and 62_4.

[0143] Next, the transmission timing of the cell frame transmitted to the converter cell 1 will be described. Each HUB #1*, #2* adjusts the transmission timing of the cell frame containing the control command for its own HUB based on the number Now, the number Nmax, and the time Dhad. Specifically, HUB #1* calculates the number Nad (=1) using the number Nmax (=2) and the number Now (=0) contained in the received communication frame 62_3 and the calculation formula "Nad = Nmax - (Nown + 1)". HUB #1* sets "Dhad + Dp × Nad (=1)" as the adjustment period. From this, it can be understood that the adjustment period is set longer by the time Dhad (=Dh1 - Dh2) than the adjustment period set in each of HUBs #1 to #4 included in the relay device group 300_1.

[0144] As a result, the timing at which HUB #1 starts the specified process is time tx1, which is the adjustment period after the end of reception of communication frame 62_3. HUB #1* transmits the cell frame to converter cell 1 at time tcf1, which is the time Dx after time tx1. At HUB #2*, since communication frame 62_3 arrives later, the control command included in communication frame 62_3 is not transmitted.

[0145] HUB #2* calculates the number Nad (=1) based on the number Nmax (=2) and the number Nown (=0) included in the received communication frame 62_4. HUB #2* sets "Dhad + Dp × Nad (=1)" as the adjustment period. As a result, the timing at which HUB #2 starts the specified process is time tx1, which is the adjustment period after the end of reception of communication frame 62_4. HUB #2* transmits the cell frame to converter cell 1 at time tcf1, which is the time Dx after time tx1.

[0146] In this way, even when the control device 50B communicates with multiple relay device groups 300_1, 300_2 located in different locations, each HUB sets the adjustment period by using the number Nad and the time Dhad, taking into account the delay time difference that occurs between each HUB. This makes it possible to synchronize the transmission timing of control commands transmitted from each HUB to each converter cell 1.

[0147] <Modifications> In the third embodiment, as shown in Fig. 14, a configuration has been described in which the network used for communication between the control device 50B and the relay device group 300_1 and the network used for communication between the control device 50B and the relay device group 300_2 are duplicated, but the present invention is not limited to this configuration, and a configuration in which the networks are not duplicated as in the first embodiment may also be used. For example, a configuration in which the communication ports 52 and 54 and the networks 520 and 540 are deleted from Fig. 14 may also be used.

[0148] In this case, each of HUBs #1 to #4 included in relay device group 300_1 communicates with control device 50B via network 510. Each of HUBs #1* and #2* included in relay device group 300_2 communicates with control device 50B via network 530. As in the above, an adjustment period is set in each of HUBs #1 to #4 without using time Dhad, and an adjustment period is set using time Dhad only in each of HUBs #1* and #2*.

[0149] The method for updating the number Now in each of HUBs #1 to #4 and each of HUBs #1* and #2* is the same as the method for updating described in Fig. 9. Therefore, after the initial communication with the relay device groups 300_1 and 300_2 is completed, the control device 50B receives the number Now (= 4) included in the communication frame 62_1 and receives the number Now (= 2) included in the communication frame 62_3. Therefore, the number Nmax, which is the maximum value of the number Now (= 4) and the number Now (= 2), is set to "4".

[0150] Each of HUBs #1 to #4 adjusts the timing of transmitting control commands in its own HUB based on the number Now (= 4) and the number Nmax. Each of HUBs #1* and #2* adjusts the timing of transmitting control commands in its own relay device based on the number Now (= 2), the number Nmax, and the time Dhad.

[0151] The control device 50B calculates a delay time Dh1 based on the total communication delay time Dall required for a communication frame 62_1 transmitted from the control device 50B via the network 510 to pass through each of HUBs #1 to #4 and reach the control device 50B, and the number of HUBs #1 to #4. Similarly, the control device 50B calculates a delay time Dh2 based on the total communication delay time Dallx required for a communication frame 62_3 transmitted from the control device 50B via the network 530 to pass through each of HUBs #1* and #2* and reach the control device 50B, and the number of HUBs #1* and #2*.

[0152] As mentioned above, time Dhad is not used in each of HUBs #1 to #4, so time Dhad may not be included in the communication frames sent to each of HUBs #1 to #4, and time Dhad may be included only in the communication frames sent to each of HUBs #1* and #2*.

[0153] Fourth Embodiment In the above-described first to third embodiments, a configuration has been described in which a relay device group 300 is provided between the control device 50 and each converter cell 1. In a fourth embodiment, a configuration will be described in which the control device 50 communicates directly with each converter cell 1 without providing a relay device group 300. Therefore, in the fourth embodiment, the configuration shown in FIG. 3 is not applied.

[0154] Fig. 18 is a diagram showing a communication connection configuration according to the fourth embodiment. Specifically, Fig. 18 shows an example in which the cell group 400 is configured by four converter cells 1, but the number of converter cells 1 is not limited to this example and may be two or three, or may be five or more. The four converter cells 1 are also referred to as cells #1 to #4, respectively.

[0155] In the example of FIG. 18 , the distance between the control device 50C and the cell group 400 is indicated as "R1." Typically, the distance R1 is the distance between the control device 50C and the most upstream cell #1 of the cell group 400. More specifically, the distance R1 is the length of the optical fiber cable connecting the communication port 56 of the control device 50C and the communication port 81 of the cell #1. Each of the four cells #1 to #4 includes the communication port 81, a cell control unit 27, and a conversion circuit 21.

[0156] The control device 50C and the four cells #1 to #4 are connected via a ring-shaped daisy-chain network 560. Specifically, the communication port 56 of the control device 50C is connected to the communication port 81 of cell #1. The communication port 81 of cell #1 is further connected to the communication port 81 of cell #2. The same applies to cells #2 to #4. Therefore, the communication frame 61 is transmitted via the network 560 in the order of the control device 50C, cell #1, cell #2, cell #3, cell #4, and control device 50C.

[0157] The cell control unit 27 of cell #1 extracts the control command contained in the communication frame 61, generates a gate signal based on the control command, and outputs (transmits) the gate signal to the conversion circuit 21. The same applies to cells #2 to #4.

[0158] Fig. 19 is a diagram for explaining a communication method according to the fourth embodiment. The communication method shown in Fig. 19 is generally similar to the communication method shown in Fig. 7, and will be described briefly. The following processing in each of cells #1 to #4 is executed by the corresponding cell control unit 27. In addition, in the fourth embodiment, the number Now indicates the number of converter cells through which a communication frame 61 transmitted from the control device 50C to the network 560 passes before reaching the own converter cell.

[0159] At time tf, the control device 50C starts transmitting a communication frame 61 to the network 560 via the communication port 56. At time t1, which is a delay time Dh1 after time tf, the cell #1 starts receiving the communication frame 61.

[0160] Cell #1 starts transmitting a communication frame 61 to the next cell #2 at time t2, which is a time Dp after time t1. The time Dp according to the fourth embodiment is a delay time from when cell #1 starts receiving the communication frame 61 from the upstream device (in this case, the control device 50C) until when it starts transmitting the communication frame 61 to the downstream device (in this case, cell #2).

[0161] Cell #1 counts up the "0" indicated by the number Now contained in the communication frame 61 to "1" and transmits the communication frame 61 including the number Now indicating "1" to the next cell #2. At time t2, cell #2 starts receiving the communication frame 61 from cell #1. The cells are arranged adjacent to each other, and the communication distance between the cells is short. Therefore, the communication delay time between the cells can be considered to be substantially zero.

[0162] Cell #2 counts up the number Now from "1" to "2". At time t3, cell #2 transmits a communication frame 61 including the counted-up number Now to cell #3. Cell #3 counts up the number Now included in the received communication frame 61 from "2" to "3". At time t4, cell #3 transmits a communication frame 61 including the counted-up number Now to cell #4.

[0163] Cell #4 counts up the number Now included in the received communication frame 61 from "3" to "4." At time t5, cell #4 transmits the communication frame 61 including the counted-up number Now to the control device 50C. At time t6, the control device 50C receives the communication frame 61.

[0164] Next, we will explain the transmission timing of the gate signal transmitted to the conversion circuit 21. In order to synchronize the transmission timing of the gate signal from the cell control unit 27 of each of cells #1 to #4, the control device 50C sets the number Nmax included in the communication frame 61.

[0165] The communication frame 61 is transmitted according to the above flow during the initial communication between the control device 50C and the cell group 400. During the initial communication, no gate signal is transmitted from each cell control unit 27.

[0166] The control device 50C sets the number Now (i.e., "4") included in the communication frame 61 received from cell #4 as the number Nmax. The control device 50C transmits the communication frame 61 including the set number Nmax to the network 560.

[0167] Cell #1 adjusts the transmission timing of the gate signal based on the numbers Nmax and Now included in the received communication frame 61. Specifically, cell #1 calculates the number Nad, which is calculated as "Nmax - (Nown + 1)". In this case, "Nad = 3". Cell #1 sets "Dp x Nad (= 3)" as the adjustment period for the transmission timing of the gate signal. As a result, the timing at which cell #1 starts the specified processing required to transmit the gate signal is time tx, which is the time after the end of reception of the communication frame 61 and the adjustment period has elapsed. Cell #1 transmits (outputs) the gate signal to the conversion circuit 21 at time tg, which is a time Dy after time tx.

[0168] The above-mentioned specified processing includes, for example, a process of extracting a control command contained in the communication frame 61, a process of generating a gate signal based on the extracted control command, etc. A time Dy indicating the processing time of the specified processing is set to a fixed value and is common to each converter cell.

[0169] Cell #2 sets the adjustment period to "Dp x Nad (= 2)". As a result, the timing at which the specified process starts in cell #2 is time tx, and therefore the timing at which the gate signal is transmitted in cell #2 is time tg. Similarly, cell #3 sets the adjustment period to "Dp x Nad (= 1)". Cell #4 sets the adjustment period to "0". As a result, the timing at which the gate signal is transmitted in cells #3 and #4 is time tg.

[0170] As described above, the period from when each cell finishes receiving the communication frame 61 until when it transmits a gate signal to the conversion circuit 21 includes an adjustment period (i.e., a period corresponding to "Dp x Nad") and a processing period for executing the specified processing (i.e., a period corresponding to time Dy).

[0171] Each cell #1 to #4 uses the number Nad to set an adjustment period taking into account the difference in time Dp that occurs between each cell #1 to #4. For example, if the number Nown received by one of the multiple cells #1 to #4 is k (where k is an integer greater than or equal to 0), and the number Nown received by the other cells is m (where m is an integer greater than k), the adjustment period in that cell will be longer than the adjustment periods in the other cells. Specifically, the difference in time between the adjustment period in one cell and the adjustment period in the other cells is (m-k) times a fixed time (for example, time Dp).

[0172] As described above, each converter cell 1 adjusts the transmission timing of the gate signal generated in its own converter cell based on the number Nmax and the number Now so that the gate signals generated in each of the multiple converter cells 1 are transmitted simultaneously. The converter cell 1 transmits the gate signal to itself in accordance with the adjusted transmission timing. This makes it possible to match the transmission timing of the gate signal transmitted to each conversion circuit 21. Therefore, since the arrival times of the gate signals at each conversion circuit 21 are matched, it is possible to prevent variation in the control of the converter cells 1 and maintain high control performance.

[0173] Here, the above-mentioned embodiment 4 has been described based on the communication connection configuration of embodiment 1. However, the configuration of embodiment 4 may also be based on the communication connection configuration of embodiments 2 and 3. For example, as in embodiment 2, the network connecting the control device 50C and the cell group 400 may be duplicated into two networks in which communication frames circulate in opposite directions. As in embodiment 3, the control device 50C may be configured to communicate with the cell group 400 and other cell groups. In these cases, the processing performed in each HUB in embodiments 2 and 3 is typically performed by the cell control unit 27 of each converter cell 1 according to embodiment 4. This synchronizes the transmission timing of gate signals transmitted to each conversion circuit 21.

[0174] Other Embodiments. The configurations exemplified as the above-described embodiments are examples of the configurations of the present disclosure, and may be combined with other known technologies, or may be modified, such as by omitting some parts, within the scope of the gist of the present disclosure. Furthermore, the above-described embodiments may be implemented by appropriately adopting the processes and configurations described in other embodiments.

[0175] The embodiments disclosed herein should be considered to be illustrative in all respects and not restrictive. The scope of the present disclosure is defined by the claims, not by the above description, and is intended to include all modifications within the meaning and scope of the claims.

[0176] 1 Converter cell, 2 AC circuit, 3 Transformer, 4 DC circuit, 5 Command generating device, 6 Power converter, 7a, 7b Reactor, 8u, 8v, 8w Leg circuit, 9a, 9b Arm current detector, 10 AC voltage detector, 11a, 11b DC voltage detector, 13nu to 13nw Negative arm, 13pu to 13pw Positive arm, 15 AC current detector, 21 Conversion circuit, 22A, 22B Switching element, 23A, 23B Diode, 24 Storage element, 25 Voltage detector, 27 Cell control unit, 28 Bypass switch, 30, 30A Relay device, 31, 32 Upper port, 35 Lower port, 50, 50A to 50C Control device, 70 Input converter, 71 Sample and hold circuit, 72 Multiplexer, 73 A / D converter, 75 RAM, 76 ROM, 77 input / output interface, 78 auxiliary storage device, 79 bus, 100 power conversion device, 300, 300A relay device group, 400 cell group, 510 to 540, 560 network.

Claims

1. A power conversion device comprising: a power converter including a plurality of converter cells connected in series with each other; a control device that controls the power converter; and a plurality of relay devices that relay communication between the control device and the plurality of converter cells, wherein each of the plurality of relay devices is connected to one or more of the plurality of converter cells; the control device transmits a communication frame to the plurality of relay devices via a ring-shaped network; and each of the plurality of relay devices receives the communication frame including first information regarding the number of relay devices through which the communication frame transmitted from the control device to the network has passed before reaching the own relay device, second information indicating the maximum number indicated by the first information, and a control command for controlling the plurality of converter cells; adjusts a transmission timing of the control command in the own relay device based on the first information and the second information so that the control commands are transmitted from the plurality of relay devices simultaneously; and transmits the control command to one or more converter cells connected to the own relay device according to the adjusted transmission timing.

2. The power conversion device of claim 1, wherein the period from when the relay device finishes receiving the communication frame to when it transmits the control command to the converter cell includes an adjustment period for adjusting the transmission timing of the control command and a processing period for executing specified processing, and when the number indicated by the first information received by one of the plurality of relay devices is k (where k is an integer greater than 0) and the number indicated by the first information received by another of the plurality of relay devices is m (where m is an integer greater than k), the adjustment period in the one relay device is longer than the adjustment period in the other relay devices.

3. The power conversion device according to claim 2, wherein the difference time between the adjustment period in the one relay device and the adjustment period in the other relay device is (m−k) times the fixed time.

4. The power conversion device according to claim 3, wherein the fixed time is the time from when the relay device starts receiving the communication frame to when the relay device starts transmitting the communication frame.

5. A power conversion device according to any one of claims 1 to 4, wherein each of the plurality of relay devices counts up the number indicated by the first information contained in the received communication frame, and transmits the communication frame including the first information after the count-up to the next relay device.

6. A power conversion device according to any one of claims 1 to 4, wherein the network is duplicated into a first network and a second network in which the communication frames circulate in opposite directions, and the control device simultaneously transmits the communication frames to the first network and the second network.

7. A power conversion device as described in claim 6, wherein each of the plurality of relay devices transmits the control command contained in the communication frame that is received first, of the communication frame received via the first network and the communication frame received via the second network, to one or more converter cells connected to the relay device itself.

8. The power conversion device of claim 7, wherein, when the previously received communication frame is the communication frame received via the first network, each of the plurality of relay devices counts up the number indicated by the first information included in the communication frame received via the first network, and transmits the communication frame including the first information after counting up to a next relay device via the first network, and transmits the communication frame including the first information to a next relay device via the second network without counting up the number indicated by the first information included in the communication frame received via the second network.

9. A power conversion device as described in claim 6 or claim 7, wherein, when an abnormality occurs in the second network, the control device transmits the communication frame to the first network, and each of the plurality of relay devices counts up the number indicated by the first information included in the communication frame received via the first network, and transmits the communication frame including the first information after the count-up to a next relay device via the first network.

10. The power conversion device according to any one of claims 1 to 4, wherein the network includes a third network and a fourth network, the plurality of relay devices include a first relay device group including a first plurality of relay devices that communicate with the control device via the third network, and a second relay device group including a second plurality of relay devices that communicate with the control device via the fourth network, the plurality of converter cells include a first plurality of converter cells and a second plurality of converter cells, each of the first plurality of relay devices is connected to one or more converter cells of the first plurality of converter cells, and each of the second plurality of relay devices is connected to one or more converter cells of the second plurality of converter cells, and a first distance between the control device and the first relay device group is longer than a second distance between the control device and the second relay device group.

11. The power conversion device described in claim 10, wherein the control device calculates a subtraction value by subtracting a second communication delay time corresponding to the second distance from a first communication delay time corresponding to the first distance, and transmits the communication frame including the subtraction value to the second plurality of relay devices via the fourth network, and each of the second plurality of relay devices adjusts the transmission timing of the control command in its own relay device based on the first information, the second information, and further the subtraction value.

12. The power conversion device described in claim 11, wherein the first information includes a first device number indicating the number of relay devices through which the communication frame transmitted from the control device to the third network passes before reaching the relay device itself, and a second device number indicating the number of relay devices through which the communication frame transmitted from the control device to the fourth network passes before reaching the relay device itself, the second information indicates the maximum value of the first device number and the second device number, each of the first plurality of relay devices adjusts the transmission timing of the control command at its own relay device based on the first device number and the second information, and each of the second plurality of relay devices adjusts the transmission timing of the control command at its own relay device based on the second device number, the second information, and the subtraction value.

13. A power conversion device as described in claim 11 or claim 12, wherein the control device calculates the first communication delay time based on a first time required for the communication frame transmitted from the control device via the third network to reach the control device via the first plurality of relay devices and the number of the first plurality of relay devices.

14. A power conversion device comprising: a power converter including a plurality of converter cells connected in series with each other; and a control device that controls the power converter, wherein the control device transmits a communication frame to the plurality of converter cells via a ring-shaped network, and each of the plurality of converter cells receives the communication frame, which includes first information regarding the number of converter cells through which the communication frame transmitted from the control device to the network passes before reaching the own converter cell, second information indicating the maximum number indicated by the first information, and a control command for controlling the own converter cell, generates a gate signal to be transmitted to a conversion circuit of the own converter cell based on the control command, adjusts transmission timing of the gate signal generated in the own converter cell based on the first information and the second information so that the gate signals generated in each of the plurality of converter cells are transmitted simultaneously, and transmits the gate signal to the own converter cell in accordance with the adjusted transmission timing.

Images