Abstract:
The purpose of the present invention is to enable a power router to be more suitably managed or controlled when constructing a power network system in which power cells are asynchronously interconnected. A power router has a first master leg, a second master leg, a first stand-alone leg, and a second stand-alone leg. Based on the power transmitted and received by the first stand-alone leg and the second stand-alone leg, a control unit controls the power transmitted and received by the first master leg and the power transmitted and received by the second master leg.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a power router and a control method thereof, a computer readable medium, and a power network system. 
       BACKGROUND ART 
       [0002]    For building a power supply system, it is a significant challenge to expand a power transmission network more stably and, moreover, configure a system capable of introducing a large amount of natural energy. 
         [0003]    As a novel power network, a power network system called a digital grid (registered trademark) has been proposed (PTL 1 and PTL 2). 
         [0004]    The digital grid (registered trademark) is a power network system in which a power network is divided into small-scale cells and the cells are asynchronously connected to one another. Each power cell which is small has a scale including one house, building, or commercial facility. Each power cell which is large has a scale including a prefecture, a city, a town, and a village. Each power cell includes a load and, in some cases, a power-generating facility and a power storing facility. An example of the power-generating facility is a power-generating facility using natural energy such as solar power generation, wind power generation, and geothermal power generation. 
         [0005]    In order to allow free generation of power in each of the power cells and smooth interchange of power among the power cells, the power cells are asynchronously connected to one another. That is, even when a plurality of power cells are connected to one another, the voltage, phase, frequency of power used in each power cell are not synchronized with those of another power cell. 
         [0006]      FIG. 20  is a diagram illustrating an example of a power network system  810 . In  FIG. 20 , a utility grid  811  transmits base power from a large-scale power plant  812 . A plurality of power cells  821  to  824  are arranged. Each of the power cells  821  to  824  has loads such as a house  831  and a building  832 , power generating facilities (for example, a solar power panel  833  and a wind power generator  834 ), and a power storing facility (for example, a storage battery  835 ). 
         [0007]    In addition, in the present specification, the power generating facilities and the power storing facilities will be also collectively called a “distributed power supply”. 
         [0008]    Moreover, the power cells  821  to  824  have power routers  841  to  844 , respectively, as connection ports to be connected to the other power cells or the utility grid  811 . Each of the power routers  841  to  844  has a plurality of legs (The reference numerals of the legs are omitted in  FIG. 20  because of limited space. Blank circles attached to the power routers  841  to  844  are to be understood as connection terminals of the legs.). 
         [0009]    The legs have a connection terminal and a power conversion unit, and an address is assigned to each of the legs. Power conversion by a leg includes conversion from an alternating current to a direct current or from a direct current to an alternating current, and a change in the voltage, frequency, and phase of power. 
         [0010]    All the power routers  841  to  844  are connected to a management server  850  via a communication network  860  and are controlled integrally by the management server  850 . For example, the management server  850  instructs the power routers  841  to  844  to transmit or receive power by the legs. By the operation, power interchange is performed among the power cells via the power routers  841  to  844 . 
         [0011]    By realizing power interchange among the power cells, for example, one power generating facility (for example, the solar power panel  833  or the wind power generator  834 ) and one power storing facility (for example, the storage battery  835 ) can be commonly used by a plurality of power cells. When surplus power is interchanged among the power cells, while largely reducing the cost of the facilities, the power demand and supply balance can be stably maintained. 
       CITATION LIST 
     Patent Literature 
       [0012]    PTL 1: Japanese Patent Publication No. 4783453 
         [0013]    PTL 2: Japanese Patent Application Laid-open Publication No. 2011-182641 
       SUMMARY OF INVENTION 
     Technical Problem 
       [0014]    Asynchronous connection of a plurality of cells through power routers, if realized, would offer great advantages, and strong expectations are placed on an early practical implementation of power routers. 
         [0015]    However, putting power routers into practical use involves particular problems which are not associated with previous power transmission/distribution facilities. While a main power transmission/distribution facility at the present time is based on a power system in which voltage, phase, and frequency are completely synchronized, it is necessary to address new problems relating to power routers that interconnect power systems with different voltages, phases, and frequencies. 
         [0016]    When designated power is transmitted and received among power routers, a reception-side power router may not receive power which corresponds to a target value for the power transmission received by a transmission-side power router. For example, depending on transmission loss of a transmission line, conversion efficiency, voltage and phase differences and the like, a value smaller (or larger) than the target value may be received in the reception-side power router. 
         [0017]    An object of the present invention is to more appropriately manage power routers in building a power network system in which power cells are asynchronously connected to one another. 
       Solution to Problem 
       [0018]    A power router according to the one aspect of the present invention includes: 
         [0019]    a plurality of master legs; 
         [0020]    one or more legs other than the master legs; and 
         [0021]    a control unit that controls power transmitted/received by each of the plurality of master legs based on power transmitted/received by the one or more legs other than the master legs. 
         [0022]    A power network system according to the one aspect of the present invention includes: 
         [0023]    a power router; and 
         [0024]    a management server that controls power transmission and reception of the power router, 
         [0025]    wherein the power router includes: 
         [0026]    a plurality of master legs; 
         [0027]    one or more legs other than the master legs; and 
         [0028]    a control unit that controls power transmitted/received by each of the plurality of master legs based on power transmitted/received by the one or more legs other than the master legs, in response to an instruction from the management server. 
         [0029]    A control method of a power router according to the one aspect of the present invention includes: 
         [0030]    referring to power transmitted/received by one or more legs other than a master leg; and 
         [0031]    controlling power transmitted/received by each of a plurality of master legs based on the power transmitted/received by the one or more legs other than the master leg. 
         [0032]    A non-transitory computer readable medium storing a control program of a power router according to the one aspect of the present invention, the program causing a computer to perform: 
         [0033]    a process of referring to power transmitted/received by one or more legs other than a master leg; and 
         [0034]    a process of controlling power transmitted/received by each of a plurality of master legs based on the power transmitted/received by the one or more legs other than the master leg. 
       Advantageous Effects of Invention 
       [0035]    According to the present invention, it is possible to more appropriately manage or control power routers in building a power network system in which power cells are asynchronously connected to one another. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0036]      FIG. 1  is a block diagram illustrating a schematic configuration of a power network system  1000  according to an exemplary embodiment 1. 
           [0037]      FIG. 2  is a block diagram of a power router  101 , which illustrates an example of an internal structure of a leg. 
           [0038]      FIG. 3  is a block diagram of a power router  101 , which illustrates an internal structure of a leg in more detail. 
           [0039]      FIG. 4  is a block diagram illustrating a configuration example of a power router  170  having an AC through leg  60 . 
           [0040]      FIG. 5  is a block diagram schematically illustrating a relation between a configuration of a control unit  19  and a leg. 
           [0041]      FIG. 6  is a diagram illustrating an example that a power router is connected to a utility grid, a load, and various distributed supplies. 
           [0042]      FIG. 7A  is a diagram illustrating an example of a possible combination in a connection of power routers. 
           [0043]      FIG. 7B  is a diagram illustrating an example of a possible combination in a connection of power routers. 
           [0044]      FIG. 8A  is a diagram illustrating an example of a prohibited combination in a connection of power routers. 
           [0045]      FIG. 8B  is a diagram illustrating an example of a prohibited combination in a connection of power routers. 
           [0046]      FIG. 8C  is a diagram illustrating an example of a prohibited combination in a connection of power routers. 
           [0047]      FIG. 8D  is a diagram illustrating an example of a prohibited combination in a connection of power routers. 
           [0048]      FIG. 9A  is a diagram illustrating an example of a possible combination in a connection of power routers in consideration of an AC through leg. 
           [0049]      FIG. 9B  is a diagram illustrating an example of a possible combination in a connection of power routers in consideration of an AC through leg. 
           [0050]      FIG. 9C  is a diagram illustrating an example of a possible combination in a connection of power routers in consideration of an AC through leg. 
           [0051]      FIG. 9D  is a diagram illustrating an example of a possible combination in a connection of power routers in consideration of an AC through leg. 
           [0052]      FIG. 10  is a diagram illustrating an example that a distance between a first power router  100  and a utility grid  1035  is long. 
           [0053]      FIG. 11  is a table of combination patterns in a connection of power routers. 
           [0054]      FIG. 12  illustrates an example of an interconnection of four power routers. 
           [0055]      FIG. 13  is a block diagram illustrating a schematic configuration of a power network system  1000 , which illustrates a configuration of a management server  1010 . 
           [0056]      FIG. 14  is a block diagram schematically illustrating a configuration of a power router  600  according to an exemplary embodiment 1. 
           [0057]      FIG. 15  is a diagram illustrating a power router  600  when power received in a first stand-alone leg  63  is 2 kW (W 1 =2 kW) and power received in a second stand-alone leg  64  is 1 kW (W 2 =1 kW). 
           [0058]      FIG. 16  is a diagram illustrating a power router  600  when power received in a first stand-alone leg  63  is 1 kW (W 1 =1 kW) and power received in a second stand-alone leg  64  is 1 kW (W 2 =1 kW). 
           [0059]      FIG. 17  is a diagram illustrating a power router  600  when power received in a first stand-alone leg  63  is 2 kW (W 1 =−2 kW) and power received in a second stand-alone leg  64  is 1 kW (W 2 =−1 kW). 
           [0060]      FIG. 18  is a diagram illustrating a power router  600  when power received in a first stand-alone leg  63  is 1 kW (W 1 =1 kW) and power received in a second stand-alone leg  64  is 1 kW (W 2 =−1 kW). 
           [0061]      FIG. 19  is a block diagram schematically illustrating a configuration of a power router  700  according to an exemplary embodiment 2. 
           [0062]      FIG. 20  is a diagram illustrating an example of a power network system  810 . 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0063]    Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the drawings. In the drawings, the same reference numerals are given to the same elements and repetitive description, if not necessary, will be omitted. 
       Exemplary Embodiment 1 
       [0064]    A power network system  1000  according to an exemplary embodiment 1 will be described.  FIG. 1  is a block diagram illustrating a schematic configuration of the power network system  1000  according to the exemplary embodiment 1. The power network system  1000  has a management server  1010  and a plurality of power routers. In the present exemplary embodiment, an example in which the power network system  1000  having the management server  1010 , power routers  101  and  102 , and a transmission line  1200  will be described. The power routers  101  and  102  are specific examples of the above-described power routers  841  to  844  (see  FIG. 23 ). Hereinafter, the management server is also called a management means. 
         [0065]    The power network system  1000  and a power network system to be described in the following exemplary embodiment have a configuration for correcting power transmission loss between power routers by power control. In general, when power is transmitted via a transmission line, transmission loss occurs due to the length of a transmission path and the difference of a path. Therefore, even when a power transmission side transmits predetermined power, power received in a power reception side is lower than the output power of the power transmission side. Thus, the power network system  1000  and the power network system to be described in the following exemplary embodiment have a function of controlling the output power of the power transmission side such that power received in the power reception side reaches an appropriate value. 
         [0066]    The power router  101  has, roughly, a DC bus  15 , a communication bus  16 , a first leg  11 , a second leg  12 , a third leg  13 , a fourth leg  14 , and a control unit  19 . In the drawing, because of limited space, the first leg to the fourth leg are indicated by a leg  1  to a leg  4 , respectively. The first leg  11 , the second leg  12 , the third leg  13 , and the fourth leg  14  are connected to the outside via terminals  115 ,  125 ,  135 , and  145 , respectively. 
         [0067]    To the DC bus  15 , the first leg  11  to the fourth leg  14  are connected in parallel. The DC bus  15  is for passing DC power. The control unit  19  controls the operation states (such as an operation of transmitting power to the outside, an operation of receiving power from the outside, or the like) of the first leg  11  to the fourth leg  14  via the communication bus  16 , thereby maintaining the bus voltage V 15  of the DC bus  15  to a predetermined value. That is, the power router  101  is connected to the outside via the first leg  11  to the fourth leg  14 , but converts all of power to be outputted/inputted to/from the outside to a direct current and transmits the current to the DC bus  15 . Conversion to the direct current thus allows asynchronous connections among power cells that are different in frequency, voltage, and phase. 
         [0068]    The configuration of the power router  101  will be described in detail.  FIG. 2  is a block diagram of the power router  101 , which illustrates an example of an internal structure of the leg. The first leg  11  to the fourth leg  14  have a similar configuration, but, for the simplification of the drawing,  FIG. 2  illustrates the internal structures of the first leg  11  and the second leg  12  and does not illustrate the internal structures of the third leg  13  and the fourth leg  14 . 
         [0069]    The first leg  11  to the fourth leg  14  are connected in parallel to the DC bus  15 . As described above, the first leg  11  to the fourth leg  14  have a similar configuration. In the present exemplary embodiment, an example in which the power router  101  having four legs will be described; however, this is for illustrative purposes only. A power router may have any number of legs more than one. In the present exemplary embodiment, the first leg  11  to the fourth leg  14  have a similar configuration, but two or more legs included in a power router may have a similar configuration or have different configurations. Hereinafter, the leg is called a power conversion leg. 
         [0070]    As illustrated in  FIG. 2 , the first leg  11  has a power conversion unit  111 , a current sensor  112 , a switch  113 , and a voltage sensor  114 . The first leg  11  is connected to a transmission line  1200  via a connection terminal  115 . The power conversion unit  111  converts AC power to DC power or converts DC power to AC power. Since DC power flows in the DC bus  15 , the power conversion unit  111  converts the DC power of the DC bus  15  to AC power of predetermined frequency and voltage and passes the AC power to the outside from the connection terminal  115 . Alternatively, the power conversion unit  111  converts AC power flowing in from the connection terminal  115  to DC power and passes the DC power to the DC bus  15 . 
         [0071]    The configuration of the leg will be described in detail.  FIG. 3  is a block diagram of the power router  101 , which illustrates the internal structure of the leg in more detail. The first leg  11  to the fourth leg  14  have a similar configuration, but, for the simplification of the drawing,  FIG. 3  illustrates the internal structure of the first leg  11  and does not illustrate the internal structure of the second leg  12 , nor include the third leg  13 , the fourth leg  14 , or a communication bus  16 . 
         [0072]    The power converter  111  has the configuration of an inverter circuit. Concretely, as illustrated in  FIG. 3 , the power converter  111  has transistors Q 1  to Q 6  and diodes D 1  to D 6 . One end of each of the transistors Q 1  to Q 3  is connected to a high-potential-side power supply line. The other ends of the transistors Q 1  to Q 3  are connected to one ends of the transistors Q 4  to Q 6 , respectively. The other ends of the transistors Q 4  to Q 6  are connected to a low-potential-side power supply line. To the high-potential-side terminals of the transistors Q 1  to Q 6 , the cathodes of the diodes D 1  to D 6  are connected, respectively. To the low-potential-side terminals of the transistors Q 1  to Q 6 , the anodes of the diodes D 1  to D 6  are connected. 
         [0073]    From the node between the transistors Q 1  and Q 4 , the node between the transistors Q 2  and Q 5 , and the node between the transistors Q 3  and Q 6 , for example, by properly controlling the on/off timings of the transistors Q 1  to Q 6 , phases of three-phase alternating current are outputted. 
         [0074]    As described above, the power conversion unit  111  has a configuration in which six antiparallel circuits configured by transistors and diodes are three-phase-bridge connected. A wire is led from the node between the transistors Q 1  and Q 4 , a wire is led from the node between the transistors Q 2  and Q 5 , and a wire is led from the node between the transistors Q 3  and Q 6 , and these wires connecting the nodes and connection terminals are called branch lines BL. Since three-phase AC is used, one leg has three branch lines BL. 
         [0075]    Since three-phase AC is used, a three-phase inverter circuit is employed here. In some cases, a single-phase inverter circuit may be used. As the transistors Q 1  to Q 6 , various self-commutated power conversion elements such as MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistors) or IGBT (Insulated Gate Bipolar Transistor) can be used. 
         [0076]    The switch  113  is disposed between the power conversion unit  111  and the connection terminal  115 . By switching of the switch  113 , the branch lines BL are switched. By the operation, the connection between the outside and the DC bus  101  are interrupted or established. The detection values of the current sensor  112  and the voltage sensor  114  are outputted to the control unit  19  via the communication bus  16 . 
         [0077]    In the above description, it is assumed that the power conversion unit uses an inverter circuit and the other side of the connection of a leg uses an alternating current. However, there is also a case where the other side of the connection of a leg uses a direct current like a storage battery (for example, the third leg  13  in  FIG. 1  is connected to a storage battery  1032 ). The power conversion in this case is DC-DC conversion. 
         [0078]    Accordingly, an inverter circuit and a converter circuit may be provided in parallel to the power conversion unit and the inverter circuit and the converter circuit may be properly used according to AC or DC being used by the other side of the connection. Alternatively, the power conversion unit may be provided with a leg dedicated for DC-DC conversion as a DC-DC conversion unit. 
         [0079]    From the viewpoint of size and cost, a power router having a leg dedicated for AC-DC conversion and a leg dedicated for DC-DC conversion is more advantageous than a power router in which an inverter circuit and a converter circuit are provided in parallel in each of all of the legs. 
         [0080]    The second leg  12  has a power conversion unit  121 , a current sensor  122 , a switch  123 , and a voltage sensor  124 . The second leg  12  is connected, for example, to a load  1031  via a connection terminal  125 . The power conversion unit  121 , the current sensor  122 , the switch  123 , and the voltage sensor  124  of the second leg  12  correspond to the power conversion unit  111 , the current sensor  112 , the switch  113 , and the voltage sensor  114  of the first leg  11 , respectively. The connection terminal  125  connected to the second leg  12  corresponds to the connection terminal  115  connected to the first leg  11 . The power conversion unit  121  has a configuration in which an antiparallel circuit  121 P configured with a thyristor  121 T and a feedback diode  121 D is three-phase-bridge connected. The thyristor  121 T, the feedback diode  121 D, and the antiparallel circuit  121 P correspond to a thyristor  111 T, a feedback diode  111 D, and an antiparallel circuit  111 P, respectively. 
         [0081]    The third leg  13  has a power conversion unit  131 , a current sensor  132 , a switch  133 , and a voltage sensor  134 . The third leg  13  is connected, for example, to the storage battery  1032  via a connection terminal  135 . The power conversion unit  131 , the current sensor  132 , the switch  133 , and the voltage sensor  134  of the third leg  13  correspond to the power conversion unit  111 , the current sensor  112 , the switch  113 , and the voltage sensor  114  of the first leg  11 , respectively. The connection terminal  135  connected to the third leg  13  corresponds to the connection terminal  115  connected to the first leg  11 . The power conversion unit  131  has a configuration in which an antiparallel circuit  131 P configured with a thyristor  131 T and a feedback diode  131 D is three-phase-bridge connected. The thyristor  131 T, the feedback diode  131 D, and the antiparallel circuit  131 P correspond to the thyristor  111 T, the feedback diode  111 D, and the antiparallel circuit  111 P, respectively. For the simplification of the drawing,  FIG. 2  does not illustrate the internal structure of the third leg  13 . 
         [0082]    The fourth leg  14  has a power conversion unit  141 , a current sensor  142 , a switch  143 , and a voltage sensor  144 . The fourth leg  14  is connected, for example, to a utility grid  1035  via a connection terminal  145 . The power conversion unit  141 , the current sensor  142 , the switch  143 , and the voltage sensor  144  of the fourth leg  14  correspond to the power conversion unit  111 , the current sensor  112 , the switch  113 , and the voltage sensor  114  of the first leg  11 , respectively. The connection terminal  145  connected to the fourth leg  14  corresponds to the connection terminal  115  connected to the first leg  11 . The power conversion unit  141  has a configuration in which an antiparallel circuit  141 P configured with a thyristor  141 T and a feedback diode  141 D is three-phase-bridge connected. The thyristor  141 T, the feedback diode  141 D, and the antiparallel circuit  141 P correspond to the thyristor  111 T, the feedback diode  111 D, and the antiparallel circuit  111 P, respectively. For the simplification of the drawing,  FIG. 2  does not illustrate the internal structure of the fourth leg  14 . 
         [0083]    The control unit  19  receives a control instruction  51  from the external management server  1010  via a communication network  1100 . The control instruction  51  includes information for instructing an operation of each leg of the power router  101 . Furthermore, the control unit  19  can output information  52  indicating an operation situation of the power router  101  to the management server  1010  via the communication network  1100 . In addition, the operation instruction to each leg includes, for example, designation of power transmission/power reception, designation of an operation mode, designation of power to be transmitted or received, or the like. More specifically, the control unit  19  monitors a bus voltage V 15  of the DC bus  15  via a voltage sensor  17  and controls the direction of power, the frequency of AC power, or the like. That is, the control unit  19  controls switching of the transistors Q 1  to Q 6  and switching of the switches  113 ,  123 ,  133 , and  143  via the communication bus  16 . 
         [0084]    In the above description, the legs are described as having a power conversion unit; however, it is also possible to provide a leg with no power conversion unit. Hereinafter, a leg with no power conversion unit is provisionally called an AC (Alternating Current) through leg  60 .  FIG. 4  is a block diagram illustrating a configuration example of a power router  170  having the AC through leg  60 . The power router  170  is described as having a configuration in which the AC through leg  60  is added to the power router  101 . For simplification of the drawing,  FIG. 4  does not illustrate the third leg  13 . 
         [0085]    The AC through leg  60  has a current sensor  162 , a switch  163 , and a voltage sensor  164 . The AC through leg  60  is connected, for example, to another power cell via a connection terminal  165 . A branch line BL of the AC through leg  60  is connected to a branch line BL of another leg having a power conversion unit via the switch  163 . That is, the connection terminal  165 , to which the AC through leg  60  is connected, is connected to a connection terminal to which another leg having a power conversion unit is connected. For example,  FIG. 4  illustrates the case where the connection terminal  165 , to which the AC through leg  60  is connected, is connected to the connection terminal  145  to which the fourth leg  14  is connected. Only the switch  163  is provided between the connection terminal  165  of the AC through leg  60  and the connection terminal  145  to which the fourth leg  14  is connected, so that the AC through leg  60  has no power conversion unit. Therefore, power is conducted without being subjected to any conversion between the connection terminal  165  to which the AC through leg  60  is connected and the connection terminal  145  to which the fourth leg  14  is connected. Therefore, a leg having no power conversion unit is called an AC through leg. 
         [0086]      FIG. 5  is a block diagram schematically illustrating a relation between a configuration of the control unit  19  and the leg.  FIG. 5  illustrates the case where the control unit  19  controls the first leg  11 . The control unit  19  has a storage unit  191 , an operation mode management unit  192 , a power conversion instruction unit  193 , a DA/AD conversion unit  194 , and a sensor value reading unit  195 . 
         [0087]    The storage unit  191  holds the control instruction  51  from the management server  1010  as a control instruction database  196  (a first database indicated by #1DB in the drawing). In addition to the control instruction database  196 , the storage unit  191  holds a leg identification information database  197  (a second database indicated by #2DB in the drawing) for identifying each of the first leg  11  to the fourth leg  14 . The storage unit  191  can be realized, for example, by various storage units such as a flash memory. The leg identification information database  197  is information, such as an IP address, URL, and URI, assigned in order to specify each of the first leg  11  to the fourth leg  14 . Furthermore, on the basis of information INF from the operation mode management unit  192 , the storage unit  191  holds the information  52  indicating the operation situation of the power router  101  and outputs the information  52  indicating the operation situation of the power router  101  to the outside as necessary. 
         [0088]    The operation mode management unit  192  is configured, for example, by CPU. The operation mode management unit  192  reads operation mode designation information MODE which is included in the control instruction database  196  and designates an operation mode (which will be described later) of a leg (the first leg  11 ) to be stopped. Furthermore, the operation mode management unit  192  reads information (for example, an IP address) corresponding to the leg (the first leg  11 ) to be stopped by referring to the leg identification information database  197  of the storage unit  191 . By the operation, the operation mode management unit  192  can output a start instruction for the leg (the first leg  11 ) to be stopped. The operation mode management unit  192  outputs a waveform instruction signal SD 1  which is a digital signal. Furthermore, the operation mode management unit outputs a switching control signal SIG 1  to a switch (for example, the switch  113 ) of the leg to be stopped. 
         [0089]    The waveform instruction signal SD 1  is subjected to digital-to-analog conversion in the DA/AD conversion unit  194  and is outputted to the power conversion instruction unit  193  as a waveform instruction signal SA 1  which is an analog signal. The power conversion instruction unit  193  outputs a control signal SCON to a power conversion unit (for example, the power conversion unit  111 ) in response to the waveform instruction signal SA 1 . 
         [0090]    The sensor value reading unit  195  reads a value of the bus voltage V 15  detected by the voltage sensor  17 , a detection value Ir of the current sensor  112  of the leg (the first leg  11 ) to be stopped, and a detection value Vr of the voltage sensor  114 . The sensor value reading unit  195  outputs a reading result as a reading signal SA 2  which is an analog signal. The reading signal SA 2  is subjected to analog-to-digital conversion in the DA/AD conversion unit  194  and is outputted to the operation mode management unit  192  as a reading signal SD 2  which is a digital signal. On the basis of the reading signal SD 2  which is a digital signal, the operation mode management unit  192  outputs the information INF indicating the operation situation of a leg to the storage unit  191 . 
         [0091]    Next, an operation mode of the legs of the power router  101  will be described. In the present exemplary embodiment, the operation mode designation of each leg is included in the control instruction  51 . 
         [0092]    Firstly, the operation mode will be described. It has been already described that the first leg  11  to the fourth leg  14  have the power conversion units  111 ,  121 ,  131 , and  141 , respectively, and the switching operation of the transistors in the power conversion units is controlled by the control unit  19 . 
         [0093]    The power router  101  is a node in the power network system  1000  and has an important role of connecting the utility grid  1035 , the load  1031 , a distributed power supply, a power cell, or the like. The connection terminals  115 ,  125 ,  135 , and  145  of the first leg  11  to the fourth leg  14  are connected to the utility grid  1035 , the load  1031 , the distributed power supply, and power routers of other power cells, respectively. The inventors have noticed that since the roles of the first leg  11  to the fourth leg  14  vary according to the other side of connections, if the first leg  11  to the fourth leg  14  do not perform proper operations according to the roles, the power routers do not work. By the inventors, the structures of legs are the same but the way of operating the legs is changed according to the other side of the connection. 
         [0094]    The way of operating the legs is called an “operation mode”. 
         [0095]    By the inventors, three kinds are prepared as operation modes of a leg, and the mode is switched according to the other side of the connection. 
         [0096]    As the operation modes of the leg, there are a master mode, a stand-alone mode, and a designated power transmission/reception mode. 
         [0097]    Hereinafter, the modes will be described in order. 
         [0098]    (Master Mode) 
         [0099]    The master mode (Mastar) is an operation mode in the case where a leg is connected to a stable power supply source such as a grid, and an operation mode for maintaining the voltage of the DC bus  15 . In the master mode, a leg is connected to a stable AC power supply source and an AC bus voltage is maintained, or a leg is connected to a stable DC power supply source and a DC bus voltage is maintained.  FIG. 1  illustrates an example that the connection terminal  145  of the fourth leg  14  is connected to the utility grid  1035 . In the case of  FIG. 1 , the fourth leg  14  is controlled so as to operate in the master mode and plays the role of maintaining the bus voltage V 15  of the DC bus  15 . Since the other first leg  11  to third leg  13 , are connected to the DC bus  15 , power may flow in from the first leg  11  to the third leg  13  to the DC bus  15 , or power may flow out to the first leg  11  to the third leg  13 . In the case where the bus voltage V 15  of the DC bus  15  drops from a rated voltage due to the outflow of power from the DC bus  15 , the fourth leg  14  in the master mode makes up for the amount of power which has become insufficient due to the outflow, with power from the other side of the connection (in this case, the utility grid  1035 ). In the case where the bus voltage V 15  of the DC bus  15  rises from the rated voltage due to inflow of power to the DC bus  15 , the amount of power which has become excessive due to the inflow is passed on to the other side of the connection (in this case, the utility grid  1035 ). By such an operation, the fourth leg  14  in the master mode maintains the bus voltage V 15  of the DC bus  15 . 
         [0100]    Consequently, in one power router, at least one leg has to be operated in the master mode. Otherwise, the bus voltage V 15  of the DC bus  15  is not maintained constant. Although two or more legs may be operated in the master mode in one power router, it is advantageous that the number of legs in the master mode in one power router is one. 
         [0101]    A leg in the master mode may be connected to, other than a utility grid, for example, a distributed power supply (including a storage battery) having a self-commutated inverter. A distributed power supply having an externally commutated inverter and a leg in the master mode cannot be connected to each other. 
         [0102]    In the following description, the leg operated in the master mode may also be called a “master leg”. 
         [0103]    Operation control of the master leg will be described. 
         [0104]    The start-up of the master leg is as follows. 
         [0105]    First, the switch  143  is set to an open (broken) state. In this state, the connection terminal  145  is connected to the other side of a connection. In this case, the other side of the connection is the utility grid  1035 . The voltage sensor  144  measures the voltage of a grid as a connection destination, and obtains the amplitude, frequency, and phase of the voltage of the grid by using PLL (Phase-Locked-Loop) or the like. After that, the output of the power conversion unit  141  is adjusted such that the voltage of the obtained amplitude, frequency, and phase is outputted from the power conversion unit  141 . That is, the on/off pattern of the transistors Q 1  to Q 6  is decided. When the output becomes stable, the switch  143  is turned on, the power conversion unit  141  and the utility grid  1035  are connected to each other. At this time point, the output of the power conversion unit  141  and the voltage of the utility grid  1035  are synchronized with each other, so that no current flows. 
         [0106]    Operation control at the time of operating the master leg will be described. 
         [0107]    The voltage sensor  17  measures the bus voltage V 15  of the DC bus  15 . When the bus voltage V 15  of the DC bus  15  exceeds a predetermined rated bus voltage, the power conversion unit  141  is controlled so that power transmission is performed from the master leg (the fourth leg  14 ) toward the grid (at least one of the amplitude and phase of the voltage outputted from the power conversion unit  141  is adjusted, so that power transmission is performed from the DC bus  15  toward the utility grid  1035  via the master leg (the fourth leg  14 )). The rated voltage of the DC bus  15  is predetermined by a setting. 
         [0108]    On the other hand, when the bus voltage V 15  of the DC bus  15  is lower than the predetermined rated bus voltage, the power conversion unit  141  is controlled so that the master leg (the fourth leg  14 ) can receive power from the utility grid  1035  (at least one of the amplitude and phase of the voltage outputted from the power conversion unit  141  is adjusted, so that power transmission is performed from the utility grid  1035  to the DC bus  15  via the master leg (the fourth leg  14 ). It will be understood that by performing such operation of the master leg, the bus voltage V 15  of the DC bus  15  can maintain the predetermined rated voltage. 
         [0109]    (Stand-Alone Mode) 
         [0110]    The stand-alone mode (Stand Alone) is an operation mode of generating a voltage having amplitude and frequency designated by the management server  1010  and transmitting/receiving power to/from the other side of a connection. 
         [0111]    For example, it is an operation mode of supplying power to a power consumption side such as the load  1031  or an operation mode of receiving power transmitted from the other side of the connection. The stand-alone mode is an operation mode of creating designated voltage and frequency and supplying the created voltage and frequency to the other side of the connection. 
         [0112]      FIG. 1  illustrates an example that the connection terminal  125  of the second leg  12  is connected to the load  1031 . The second leg  12  is controlled so as to be operated in the stand-alone mode and supplies power to the load  1031 . 
         [0113]    When a leg is connected to another power router, there is a case where the leg is operated in the stand-alone mode as a mode of transmitting an amount of power required by the other power router. 
         [0114]    When a leg is connected to another power router, there is a case where the leg is operated in the stand-alone mode as a mode of receiving power transmitted from the other power router. 
         [0115]    Although not illustrated in the drawing, also in the case where the second leg is connected to a power generating facility instead of the load  1031 , the second leg can be operated in the stand-alone mode. In this case, the power generating facility is provided with an externally commutated inverter. 
         [0116]    An operation mode in the case of interconnecting power routers will be described later. 
         [0117]    A leg operated in the stand-alone mode will be called a “stand-alone leg”. In one power router, there may be a plurality of stand-alone legs. 
         [0118]    Operation control of a stand-alone leg will be described. 
         [0119]    First, the switch  123  is opened (broken). The connection terminal  125  is connected to the load  1031 . The amplitude and frequency of power (a voltage) to be supplied to the load  1031  are instructed from the management server  1010  to the power router  101 . The control unit  19  performs control so that the power (the voltage) of the instructed amplitude and frequency is outputted from the power conversion unit  121  toward the load  1031  (that is, the control unit  19  decides the on/off pattern of the transistors Q 1  to Q 6 ). When the output becomes stable, the control unit  19  connects the power conversion unit  121  to the load  1030  by turning on the switch  123 . After that, when the power is consumed by the load  1031 , the power of the consumed amount flows from the stand-alone leg (the second leg  12 ) to the load  1301 . 
         [0120]    (Designated-Power Transmission/Reception Mode) 
         [0121]    A designated-power transmission/reception mode (Grid Connect) is an operation mode for transmitting/receiving power of an amount decided by designation. In the designated-power transmission/reception mode, designated active power is transmitted/received between connection destinations. Designated reactive power is generated. That is, there are a case of transmitting designated power to the other side of a connection and a case of receiving designated power from the other side of a connection. 
         [0122]    When a leg is connected to a leg of another power router, power of a decided amount is interchanged from one side to the other side. 
         [0123]    The third leg  13  is connected to the storage battery  1032 . 
         [0124]    In such a case, the power of the decided amount is transmitted toward the storage battery  1032 , so that the storage battery  1032  is charged. 
         [0125]    Alternatively, a distributed power supply (including a storage battery) having a self-commutated inverter and a designated-power transmission/reception leg may be connected to each other. However, a distributed power supply having an externally commutated inverter and a designated-power transmission/reception leg cannot be connected to each other. 
         [0126]    A leg operated in the designated-power transmission/reception mode will be called a “designated-power transmission/reception leg”. In one power router, a plurality of designated-power transmission/reception legs may exist. 
         [0127]    Operation control of a designated-power transmission/reception leg will be described. Since the control at the time of startup is basically the same as that of the master leg, a description thereof will not be repeated. 
         [0128]    Operation control at the time of operating a designated-power transmission/reception leg will be described. In  FIG. 1 , the first leg  11  transmits/receives power designated power to/from a first leg  21  of a power router  102  operating in the stand-alone mode via the transmission line  1200 . In the first leg  11  of the power router  101 , the voltage of a grid of the other side of a connection is measured by the voltage sensor  114  and the frequency and phase of the voltage of the other side of the connection are obtained using PLL (Phase-Locked-Loop) or the like. On the basis of an active power value and a reactive power value designated by the management server  1010  and the frequency and phase of the voltage of the other side of the connection, the power conversion unit  111  obtains a target value of a current which is inputted/outputted. The current sensor  112  measures a present current value. The power conversion unit  111  is adjusted so that a current corresponding to the difference between the target value and the present value is additionally outputted (by adjusting at least one of the amplitude and phase of the voltage outputted from the power conversion unit  111 , desired power flows between the designated-power transmission/reception leg and the other side of the connection). 
         [0129]    As described above, it will be understood that the first leg  11  to the fourth leg  14  having the same configuration can play the roles of three patterns according to the way of the operation control. 
         [0130]    The power router  101  can operate each leg in the above-described three operation modes by referring to the designation information of operation modes included in the control instruction  51 . The power router  101  can thus appropriately operate each leg in accordance with the role of each leg. 
         [0131]    Next, connection restrictions between power routers will be described. Since the operations of legs vary according to the operation modes, restriction naturally occurs between the selection of the other side of a connection and the selection of an operation mode. That is, when the other side of the connection is decided, an operation mode which can be selected is decided. Conversely, when an operation mode is decided, the other side of the connection which can be selected is decided (when the other side of the connection changes, it becomes necessary to change the operation mode of the leg accordingly). 
         [0132]    Patterns of possible connection combinations will be described below. 
         [0133]    In the following description, signage in the drawings will be simplified as illustrated in  FIG. 6 . 
         [0134]    That is, the master leg is expressed by “M”. 
         [0135]    The stand-alone leg is expressed by “5”. 
         [0136]    The designated-power transmission/reception legs are expressed by “D”. 
         [0137]    The AC through leg is expressed by “AC”. 
         [0138]    By assigning numbers like “#1” at the shoulder of a leg as necessary, the legs may be distinguished. 
         [0139]    In  FIG. 6  to  FIG. 12 , systematic reference numerals are assigned. However, the same reference numerals are not always designated to the same elements among the drawings. 
         [0140]    For example, reference numeral  200  in  FIG. 6  and reference numeral  200  in  FIG. 4A  do not refer to the same element. 
         [0141]    Connection combinations illustrated in  FIG. 6  are possible connections. A first leg  210  is connected as a master leg to the utility grid  1035  as also already described. A second leg  220  is connected as a stand-alone leg to the load  1031  as also already described. A third leg  230  and a fourth leg  240  are connected as designated-power transmission/reception legs to the storage battery  1032  as also already described. 
         [0142]    A fifth leg  250  is an AC through leg. The AC through leg  250  is connected to a designated-power transmission/reception leg of another power router  300  and is also connected to the storage battery  1032  via a connection terminal  245  of the fourth leg  240 . Since the AC through leg  250  does not have a power conversion unit, the connection relation is equivalent to that of the designated-power transmission/reception leg of another power router  300  being directly connected to the storage battery  1032 . It will be understood that such a connection is allowed. 
         [0143]    A sixth leg  260  is connected to the utility grid  1035  as a designated-power transmission/reception leg. When it is assumed that determined power is received from the utility grid  1035  via the sixth leg  260 , it will be understood that such a connection is allowed. As for the relation with the first leg  210 , which is the master leg, if the power received by the sixth leg  260  is insufficient to maintain a rated voltage of a DC bus M 201 , the master leg  210  receives necessary power from the utility grid  1035 . Conversely, when the power received by the sixth leg  260  exceeds an amount necessary to maintain the rated voltage of the DC bus M 201 , the master leg  210  passes excessive power to the utility grid  1035 . 
         [0144]    Next, the case of connecting power routers will be described. Connection of power routers means connection of a leg of one power router and a leg of another power router. In the case of connecting legs, there is a restriction in an operation mode for the combination. 
         [0145]    Both of combinations of connections illustrated in  FIGS. 7A and 7B  are examples of possible combinations. In  FIG. 7A , the master leg  110  of the first power router  100  and the stand-alone leg  210  of the second power router  200  are connected to each other. Although it will not be specifically described, the master leg  220  of the second power router  200  is connected to the utility grid  1035 , thereby maintaining the voltage of the DC bus M 201  of the second power router  200  at the rated voltage. 
         [0146]    In  FIG. 7A , when power is supplied from the first power router  100  to the load  1031 , the voltage of the DC bus M 101  drops. The master leg  110  obtains power from the other side of a connection so as to maintain the voltage of the DC bus M 101 . In other words, the master leg  110  draws power of an insufficient amount from the stand-alone leg  210  of the second power router  200 . The stand-alone leg  210  of the second power router  200  transmits power of the amount required from the other side of the connection (in this case, the master leg  110 ). In the DC bus M 201  of the second power router  200 , although the voltage drops only by the power transmission amount from the stand-alone leg  210 , it is compensated from the utility grid  1035  by the master leg  220 . In such a manner, the first power router  100  can obtain the power of a necessary amount from the second power router  200 . 
         [0147]    As described above, even when the master leg  110  of the first power router  100  and the stand-alone leg  210  of the second power router  200  are connected, since the role of the master leg  110  and that of the stand-alone leg  210  fit together, no inconvenience occurs in the operations. It is therefore understood that a master leg and a stand-alone leg may be connected as illustrated in  FIG. 7A . 
         [0148]    In  FIG. 7B , a designated-power transmission/reception leg  310  of the third power router  300  and a stand-alone leg  410  of a fourth power router  400  are connected to each other. Although it will not be specifically described, a master leg  320  of the third power router  300  and a master leg  420  of the fourth power router  400  are connected to the utility grid  1035 , so that DC buses M 301  and M 401  of the third and fourth power routers  300  and  400  maintain the rated voltage. 
         [0149]    It is assumed that, by an instruction from the management server  1010 , the designated-power transmission/reception leg  310  of the third power router  300  is instructed to receive designated power. The designated-power transmission/reception leg  310  draws the designated power from the stand-alone leg  410  of the fourth power router  400 . The stand-alone leg  410  of the fourth power router  400  transmits power of an amount required by the other side of a connection (in this case, the designated-power transmission/reception leg  310 ). In the DC bus M 401  of the fourth power router  400 , although the voltage drops only by the amount of power transmitted from the stand-alone leg  410 , it is compensated from the utility grid  1035  by the master leg  420 . 
         [0150]    As described above, even when the designated-power transmission/reception leg  310  of the third power router  300  and the stand-alone leg  410  of the fourth power router  400  are connected, since the role of the designated-power transmission/reception leg  310  and that of the stand-alone leg  410  fit together, no inconvenience occurs in the operations. It is therefore understood that a designated-power transmission/reception leg and a stand-alone leg may be connected as illustrated in  FIG. 7B . 
         [0151]    Although the case where the third power router  300  receives power from the fourth power router  400  has been described, it will be understood that there is similarly no inconvenience also in the case where power is conversely given from the third power router  300  to the fourth power router  400 . 
         [0152]    In such a manner, designated power can be given between the third and fourth power routers  300  and  400 . 
         [0153]    In the case of directly connecting legs having power converters, only two patterns illustrated in  FIGS. 7A and 7B  are allowed. In other words, only the pattern of connecting a master leg and a stand-alone leg and the pattern of connecting a designated-power transmission/reception leg and a stand-alone leg are allowed. 
         [0154]    Next, combinations of legs which cannot be connected will be described. 
         [0155]      FIGS. 8A to 8D  illustrate patterns of legs which cannot be connected to each other. 
         [0156]    As will be seen from  FIGS. 8A, 8B, and 8C , legs in the same operation mode cannot be connected to each other. 
         [0157]    For example, in the case of  FIG. 8A , master legs  510  and  610  are connected to each other. 
         [0158]    As described above in relation to the operation, first, a master leg performs the process of generating power synchronized with the voltage, frequency, and phase of the other side of the connection. 
         [0159]    In the case where the other side of the connection is also a master leg, the master legs try to synchronize with the voltage and frequency of the other side. However, since the master legs do not autonomously establish the voltage and frequency, such a synchronizing process cannot succeed. 
         [0160]    Therefore, the master legs cannot be connected to each other. 
         [0161]    There is also another reason. 
         [0162]    A master leg has to draw power from the other side of the connection in order to maintain the voltage of the DC bus (or has to pass excessive power to the other side of the connection in order to maintain the voltage of the DC bus). When the master legs are connected to each other, they cannot mutually satisfy requirements of the other sides of connection (if master legs are connected to each other, neither of the power routers cannot maintain the voltages of respective DC buses. Therefore, problems such as black-out may occur in each of the power cells). As described above, since the roles of the master legs conflict (do not fit together), the master legs cannot be connected to each other. 
         [0163]      FIG. 8B  illustrates designated-power transmission/reception legs being connected to each other, but it will be understood that this is not successful, either. 
         [0164]    Like the above master legs and as described above in relation to the operation, first, a designated-power transmission/reception leg performs the process of generating power synchronized with the voltage, frequency, and phase of the other side of a connection. 
         [0165]    In the case where the other side of the connection is also a designated-power transmission/reception leg, the legs try to synchronize with the voltage and frequency of the other side. However, since the designated-power transmission/reception legs do not autonomously establish a voltage and frequency, such a synchronizing process cannot succeed. 
         [0166]    Therefore, the designated-power transmission/reception legs cannot be connected to each other. 
         [0167]    There is also another reason. 
         [0168]    Even if designated transmission power to be transmitted from one designated-power transmission/reception leg  510  and designated reception power to be received by the other designated-power transmission/reception leg  610  are matched, such the designated-power transmission/reception legs cannot be connected to each other. For example, it is assumed that the one designated-power transmission/reception leg  510  adjusts a power conversion unit to transmit the designated transmission power (for example, the designated-power transmission/reception leg  510  allows an output voltage to be higher than the output voltage of the other side of a connection by a predetermined value). On the other hand, the other designated-power transmission/reception leg  610  adjusts a power conversion unit to receive the designated reception power (for example, the other designated-power transmission/reception leg  610  allows the output voltage to be lower than that of the other side of the connection by a predetermined value). When such adjusting operations are performed simultaneously in both of the designated-power transmission/reception legs  510  and  610 , it will be understood that both of the legs become out of control. 
         [0169]      FIG. 8C  illustrates stand-alone legs being connected to each other. However, such a connection is not allowed. 
         [0170]    A stand-alone leg generates a voltage and a frequency by itself. 
         [0171]    If any of the voltages, frequencies, and phases generated by two stand-alone legs differ even slightly in a state where the stand-alone legs are connected to each other, an unintended power flows between the two stand-alone legs. 
         [0172]    Since it is difficult to keep a state in which the voltages, frequencies, and phases generated by two stand-alone legs are perfectly matched, it is not allowed to connect stand-alone legs to each other. 
         [0173]      FIG. 8D  illustrates a master leg and a designated-power transmission/reception leg being connected to each other. 
         [0174]    From the above description, it will be understood that this connection does not work, either. Even when the master leg  510  tries to transmit/receive power to/from the other side of a connection so as to maintain the voltage of a DC bus M 501 , the designated-power transmission/reception leg  610  does not transmit/receive power in response to a request of the master leg  510 . Therefore, the master leg  510  cannot maintain the voltage of the DC bus M 501 . Even when the designated-power transmission/reception leg  610  tries to transmit/receive designated power to/from the other side ( 510 ) of a connection, the master leg  510  does not transmit/receive the power in response to a request of the designated-power transmission/reception leg  610 . Therefore, the designated-power transmission/reception leg  610  cannot transmit/receive the designated power to/from the other side of the connection (in this case, the master leg  510 ). 
         [0175]    Although the cases in which legs having a power conversion unit are connected with each other have been considered, patterns illustrated in  FIGS. 9A to 9D  are also possible when an AC through leg is taken into consideration. Since an AC through leg has no power conversion unit, it is simply a bypass. Therefore, as illustrated in  FIGS. 9A and 9B , a connection of the master leg  110  of the first power router  100  to the utility grid  1035  via the AC through leg  250  of the second power router  200  is substantially the same as a direct connection of the master leg  110  to the utility grid  1035 . Similarly, as illustrated in  FIGS. 9C and 9D , a connection of the designated-power transmission/reception leg  110  of the first power router  100  to the utility grid  1035  via the AC through leg  250  of the second power router  200  is substantially the same as a direct connection of the designated-power transmission/reception leg  110  to the utility grid  1035 . 
         [0176]    Even so, when an AC through leg is provided, there are the following advantages. For example, a case is considered in which, as illustrated in  FIG. 10 , the distance from the first power router  100  to the utility grid  1035  is very long and the first power router  100  has to be connected to the utility grid  1035  via some power routers  200  and  300 . If there is no AC through leg, the first power router  100  has to be connected via one or more stand-alone legs as illustrated in  FIG. 7A . When the first power router  100  is connected via a leg having a power converter, output is subjected to conversion from AC power to DC power and conversion from DC power to AC power. In the power conversion, although only a few percent, energy loss occurs. Therefore, a plurality of times of power conversion required only for a connection to the utility grid is insufficient. Therefore, it is meaningful to provide a power router with an AC through leg having no power conversion unit. 
         [0177]      FIG. 11  illustrates the summary of the above description.  FIG. 12  is a diagram illustrating an example of the case of connecting the four power routers  100 ,  200 ,  300 , and  400  to one another. In  FIG. 12 , by reference numeral “ 71 A” is designated power transmission lines as a part of the utility grid, and by reference numeral “ 71 B” is designated power transmission lines detached from the utility grid. When a connection line connecting a power router and a load (or a distributed power supply) is called a power distribution line  72 , the power distribution lines  72  are detached from the utility grid  1035 . That is, the power distribution line  72  connecting the power router to the load (or the distributed power supply) is not connected to the utility grid  1035 . Reference numerals  1035 A to  1035 C indicate utility grids. Since all of the connection relations are described above, each connection destinations will not be described in detail. However, it will be understood that all the connection relations are allowable connection relations. 
         [0178]    Next, referring once again to  FIG. 1 , the power router  102  will be described. The power router  102  has a configuration similar to that of the power router  101 . The power router  102  has, roughly, the DC bus  15 , the communication bus  16 , a first leg  21 , a second leg  22 , a third leg  23 , a fourth leg  24 , and the control unit  19 . In the drawing, because of limited space, the first leg to the fourth leg are indicated as leg  1  to leg  4 , respectively. The first leg  21 , the second leg  22 , the third leg  23 , and the fourth leg  24  have configurations similar to those of the first leg  11 , the second leg  12 , the third leg  13 , and the fourth leg  14  of the power router  101 , respectively. The first leg  21 , the second leg  22 , the third leg  23 , and the fourth leg  24  are connected to the outside via terminals  215 ,  225 ,  235 , and  245 , respectively. Operation modes of the power router  102  are similar to those of the power router  101 , a description thereof will not be repeated. 
         [0179]    In the present exemplary embodiment, the first leg  11  of the power router  101  and the first leg  21  of the power router  102  are connected to each other via the transmission line  1200 . The second leg  22  is connected to a load  1033  via the terminal  225 . The third leg  23  is connected to a storage battery  1034  via the terminal  235 . The fourth leg  24  is connected to the utility grid  1035  via the terminal  245 . Thus, the fourth leg  24  operates as a master leg. 
         [0180]    Next, the management server  1010  will be described.  FIG. 13  is a block diagram illustrating a schematic configuration of the power network system  1000 , which illustrates a configuration of the management server  1010 . The management server  1010  can be configured, for example, as hardware such as a computer. The management server  1010  has a storage device  1012 . The storage device  1012  stores information required for the control of power routers. 
         [0181]    An operation of a power router according to the present exemplary embodiment will be described below in detail. As described above, the power router is normally provided with a plurality of legs. In the state in which a bus voltage is maintained at a predetermined value, when each of the plurality of legs performs power transmission and reception, it is necessary to balance transmission power and reception power in terms of an entire power router. In order to balance the transmission power and the reception power in terms of the entire power router, the control unit  19  has to control each leg. 
         [0182]    In the present exemplary embodiment, based on the above-described assumption, the case where a plurality of master legs exist in one power router will be described. Providing a plurality of master legs has the following technical meaning. For example, there is considered a case where a power router is requested to transmit power to a destination requiring large power such as a high power household appliance. In this case, a designated-power transmission/reception leg or a stand-alone leg is connected to the destination. Therefore, in order to satisfy a request of the destination, it is necessary to use a high-output designated-power transmission/reception leg or stand-alone leg. In this case, in order to normally perform power transmission and reception of a leg other than a master leg of the power router, the capacity (rating) of the master leg has to be large. However, an increase in the capacity of the master leg causes an increase in the size and the cost of the master leg. 
         [0183]    On the other hand, in the present exemplary embodiment, a plurality of master legs are provided. The entire capacity of the plurality of master legs can be increased thereby. However, when the plurality of master legs are provided, a special problem occurs as compared with the case where one master leg is provided. For example, when one master leg is provided, it is sufficient if the master leg performs power transmission and reception with the outside such that a bus voltage is simply maintained constant. However, when a plurality of master legs exist, if each of the master legs independently performs power transmission and reception similarly to the case where one master leg is provided, it is probable that an amount to be transmitted and received will be excessively increased. In this case, it is probable that overshoot and undershoot of a bus voltage, destabilization of the bus voltage, and extension of a time required for stabilizing the bus voltage will occur. Therefore, in the present exemplary embodiment, when a plurality of master legs perform power transmission and reception with the outside, amounts to be transmitted and received by the respective master legs are decided and set for the respective master legs, thereby preventing the occurrence of such a problem. 
         [0184]    More specifically, in the present exemplary embodiment, in the state in which a plurality of master legs exist in one power router and a bus voltage is maintained to a predetermined value, the plurality of master legs are controlled to balance transmission power and reception power in terms of the entire power router. 
         [0185]    When a target voltage value of the DC bus  15  is Vdc target , an actually measured value of the DC bus  15  is Vdc measure , and an actually measured value of an AC current flowing in the master leg is I measure , a target value I target  of the AC current to flow in the master leg can be defined by the following Equation 1 by using a coefficient s (s is a real number). 
         [0000]      Equation 1 
         [0000]        I   target   =s ( Vds   target   −Vdc   measure )· I   measure   (1)
 
         [0186]    An AC voltage value Vac target  to be set for the master leg so as to achieve the target voltage value of the DC bus  15  at Vdc target  is expressed by the following Equation 2, by using the target value I target  of the AC current to flow in the master leg and a coefficient t (t is a real number). 
         [0000]      Equation 2 
         [0000]        Vac   target   =t·I   target   (2)
 
         [0187]    The aforementioned coefficient s and coefficient t are determined by the characteristics of a power router and a leg such as a structure and a manufacturing error. For example, the coefficient s and the coefficient t can be determined by actually measuring current/voltage characteristics of a leg. 
         [0188]    In the master leg, the AC voltage value Vac target  to be set in the master leg is set, so that it is possible to control transmission power or reception power. 
         [0189]    In a power router according to the present exemplary embodiment, a plurality of legs serve as master legs. Therefore, the control unit  19  needs to control the AC voltage value with respect to each of the plurality of master legs. A description will be provided below for power control which is performed for each of the plurality of master legs. For the sake of simplicity, the following description will be provided for the case where the control unit  19  controls transmission and reception power of the master legs. 
         [0190]    A description will be provided below for an example in which a power router having a plurality of master legs has two master legs and two stand-alone legs other than the master legs.  FIG. 14  is a block diagram schematically illustrating a configuration of a power router  600  according to an exemplary embodiment 1. 
         [0191]    The power router  600  has a first master leg  61 , a second master leg  62 , a first stand-alone leg  63 , and a second stand-alone leg  64 . A rated value of the first master leg  61  is represented as RM 1 , a rated value of the second master leg  62  is RM 2 , a rated value of the first stand-alone leg  63  is RS 1 , and a rated value of the second stand-alone leg  64  is RS 2 . 
         [0192]    Although not illustrated, the first master leg  61  and the second master leg  62  are connected to a utility grid and a power supply such as a storage battery. Although not illustrated, the first stand-alone leg  63  and the second stand-alone leg  64  are connected to a power supply such as a storage battery, an exterior load or the like. 
         [0193]    In the power router  600 , the control unit  19  distributes power, which is to be received by or transmitted from a master leg, to the first master leg  61  and the second master leg  62  in response to the situation of power transmission and power reception of the first stand-alone leg  63  and the second stand-alone leg  64 . 
         [0194]    The transmission and reception power of the first stand-alone leg  63  and the second stand-alone leg  64  is designated, for example, by the management server  1010  in accordance with the control instruction  51 . The transmission and reception power of the first stand-alone leg  63  and the second stand-alone leg  64  designated in accordance with the control instruction  51  is stored, for example, in the storage unit  191  of the control unit  19 . The control unit  19  can thereby appropriately refer to the transmission and reception power of the first stand-alone leg  63  and the second stand-alone leg  64  stored in the storage unit  191 . 
         [0195]    An operation to be described below can be performed, for example, when the management server  1010  has newly designated the transmission and reception power of the first stand-alone leg  63  and the second stand-alone leg  64  or when the management server  1010  has changed the designation of the transmission and reception power of the first stand-alone leg  63  and the second stand-alone leg  64 . 
         [0196]    Hereinafter, power transmitted to the outside of the power router  600  from the first master leg  61 , the second master leg  62 , the first stand-alone leg  63 , or the second stand-alone leg  64  is taken as negative. Power received by the first master leg  61 , the second master leg  62 , the first stand-alone leg  63 , or the second stand-alone leg  64  from the outside of the power router  600  is taken as positive. 
         [0197]    The transmission and reception power of the first master leg  61  is represented as P 1  [kW]. When the first master leg  61  transmits power to the outside, P 1  has a negative value (P 1 &lt;0). When the first master leg  61  receives power from the outside, P 1  has a positive value (P 1 &gt;0). 
         [0198]    The transmission and reception power of the second master leg  62  is represented as P 2  [kW]. When the second master leg  62  transmits power to the outside, P 2  has a negative value (P 2 &lt;0). When the second master leg  62  receives power from the outside, P 2  has a positive value (P 2 &gt;0). 
         [0199]    The transmission and reception power of the first stand-alone leg  63  is represented as W 1  [kW]. When the first stand-alone leg  63  transmits power to the outside, W 1  has a negative value (W 1 &lt;0). When the first stand-alone leg  63  receives power from the outside, W 1  has a positive value (W 1 &gt;0). 
         [0200]    The transmission and reception power of the second stand-alone leg  64  is represented as W 2  [kW]. When the second stand-alone leg  64  transmits power to the outside, W 2  has a negative value (W 2 &lt;0). When the second stand-alone leg  64  receives power from the outside, W 2  has a positive value (W 2 &gt;0). 
         [0201]    Accordingly, W total , which is the total power transmitted/received by the first stand-alone leg  63  and the second stand-alone leg  64  is (W 1 +W 2 ) [kW]. In this case, when W total &gt;0, it is necessary to transmit power to the outside via a master leg in order to maintain the voltage of the DC bus  15  to the target voltage value Vdc target . When W total &lt;0, it is necessary to receive power from the outside via the master leg in order to maintain the voltage of the DC bus  15  to the target voltage value Vdc target . The control unit  19  distributes transmission power or reception power to the first master leg  61  and the second master leg  62 , so that power transmission and reception are performed. 
         [0202]    First, the control unit  19  calculates a coefficient u (also called a first coefficient) for defining the output of the first master leg  61  and the second master leg  62 . The coefficient u is calculated by the following Equation. 
         [0000]    
       
         
           
             
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   3 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   u 
                   = 
                   
                      
                     
                       
                         
                           W 
                            
                           
                               
                           
                            
                           1 
                         
                         + 
                         
                           W 
                            
                           
                               
                           
                            
                           2 
                         
                       
                       
                         
                           RM 
                            
                           
                               
                           
                            
                           1 
                         
                         + 
                         
                           RM 
                            
                           
                               
                           
                            
                           2 
                         
                       
                     
                      
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
         [0203]    That is, the coefficient u is calculated by dividing the sum of the transmission and reception power of legs other than the master legs by the sum of the rating of the master legs. 
         [0204]    As expressed by Equation 4 below, the control unit  19  multiplies the rating RM 1  of the first master leg  61  by the coefficient u, thereby calculating the power instruction value P 1  of the first master leg  61 . 
         [0000]      Equation 4 
         [0000]        P 1= u·RM 1  (4)
 
         [0205]    As expressed by Equation 5 below, the control unit  19  multiplies the rating RM 2  of the second master leg  62  by the coefficient u, thereby calculating the power instruction value P 2  of the second master leg  62 . 
         [0000]      Equation 5 
         [0000]        P 2= u·RM 2  (5)
 
         [0206]    Detailed examples (cases 1 to 4) will be described below. 
       Case 1 
       [0207]    The case where power received in the first stand-alone leg  63  is 2 [kW] (W 1 =2 [kW]) and power received in the second stand-alone leg  64  is 1 [kW] (W 2 =1 [kW]) will be described.  FIG. 15  is a diagram illustrating the power router  600  when power received in the first stand-alone leg  63  is 2 kW (W 1 =2 kW) and power received in the second stand-alone leg  64  is 1 kW (W 2 =1 kW). In this case, the power router  600  receives power of 3 [kW] from the outside. Thus, the power router  600  has to be able to transmit power of 3 [kW] at maximum via the master legs. In this case, the control unit  19  calculates the coefficient u from Equation 3 above as expressed by the following Equation 6. 
         [0000]    
       
         
           
             
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   6 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   u 
                   = 
                   
                     
                        
                       
                         
                           2 
                           + 
                           1 
                         
                         
                           3 
                           + 
                           2 
                         
                       
                        
                     
                     = 
                     0.6 
                   
                 
               
               
                 
                   ( 
                   6 
                   ) 
                 
               
             
           
         
       
     
         [0208]    In this case, the coefficient u is 0.6. Thus, by Equation 4 above, the transmission power of the first master leg  61  is 1.8 [kW] (=0.6×3 [kW]). By Equation 5 above, the transmission power of the second master leg  62  is 1.2 [kW] (=0.6×2 [kW]). 
       Case 2 
       [0209]    The case where power received in the first stand-alone leg  63  is 1 [kW] (W 1 =1 [kW]) and power received in the second stand-alone leg  64  is 1 [kW] (W 2 =1 [kW]) will be described.  FIG. 16  is a diagram illustrating the power router  600  when power received in the first stand-alone leg  63  is 1 kW (W 1 =1 kW) and power received in the second stand-alone leg  64  is 1 kW (W 2 =1 kW). In this case, the power router  600  receives power of 2 [kW] from the outside. Thus, the power router  600  has to be able to transmit power of 2 [kW] at maximum via the master leg. In this case, the control unit  19  calculates the coefficient u from Equation 3 above as expressed by the following Equation 7. 
         [0000]    
       
         
           
             
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   7 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   u 
                   = 
                   
                     
                        
                       
                         
                           1 
                           + 
                           1 
                         
                         
                           3 
                           + 
                           2 
                         
                       
                        
                     
                     = 
                     0.4 
                   
                 
               
               
                 
                   ( 
                   7 
                   ) 
                 
               
             
           
         
       
     
         [0210]    In this case, the coefficient u is 0.6. Thus, by Equation 4 above, the transmission power of the first master leg  61  is 1.2 [kW] (=0.4×3 [kW]). By Equation 5 above, the transmission power of the second master leg  62  is 0.8 [kW] (=0.4×2 [kW]). 
         [0211]    When, for example, the setting of the transmission and reception power of the first stand-alone leg  63  and the second stand-alone leg  64  has been changed from the case 1 to the case 2 by an instruction of the management server  1010 , the control unit  19  can change the coefficient u from 0.6 to 0.4. 
       Case 3 
       [0212]    The case where power transmitted in the first stand-alone leg  63  is 2 [kW] (W 1 =−2 [kW]) and power transmitted in the second stand-alone leg  64  is 1 [kW] (W 2 =−1 [kW]) will be described.  FIG. 17  is a diagram illustrating the power router  600  when power transmitted in the first stand-alone leg  63  is 2 [kW] (W 1 =−2 [kW]) and power transmitted in the second stand-alone leg  64  is 1 [kW] (W 2 =−1 [kW]). In this case, the power router  600  receives power of 3 [kW] from the outside. Thus, the power router  600  has to be able to receive power of 3 [kW] at maximum via the master legs. In this case, the control unit  19  calculates the coefficient u from Equation 3 above as expressed by the following Equation 8. 
         [0000]    
       
         
           
             
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   8 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   u 
                   = 
                   
                     
                        
                       
                         
                           
                             - 
                             2 
                           
                           - 
                           1 
                         
                         
                           3 
                           + 
                           2 
                         
                       
                        
                     
                     = 
                     0.6 
                   
                 
               
               
                 
                   ( 
                   8 
                   ) 
                 
               
             
           
         
       
     
         [0213]    In this case, the coefficient u is 0.6. Thus, by Equation 4 above, the reception power of the first master leg  61  is 1.8 [kW] (=0.6×3 [kW]). By Equation 5 above, the reception power of the second master leg  62  is 1.2 [kW] (=0.6×2 [kW]). 
       Case 4 
       [0214]    The case where power received in the first stand-alone leg  63  is 1 [kW] (W 1 =1 [kW]) and power transmitted in the second stand-alone leg  64  is 1 [kW] (W 2 =−1 [kW]) will be described.  FIG. 18  is a diagram illustrating the power router  600  when power received in the first stand-alone leg  63  is 1 [kW] (W 1 =1 [kW]) and power transmitted in the second stand-alone leg  64  is 1 [kW] (W 2 =−1 [kW]). In this case, the power router  600  balances transmission power and reception power between the first stand-alone leg  63  and the second stand-alone leg  64 . Thus, the power router  600  does not need to perform power transmission and reception via the master legs. In this case, the control unit  19  calculates the coefficient u from Equation 3 above as expressed by the following Equation 9. 
         [0000]    
       
         
           
             
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   9 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   u 
                   = 
                   
                     
                        
                       
                         
                           1 
                           - 
                           1 
                         
                         
                           3 
                           + 
                           2 
                         
                       
                        
                     
                     = 
                     0 
                   
                 
               
               
                 
                   ( 
                   9 
                   ) 
                 
               
             
           
         
       
     
         [0215]    In this case, the coefficient u is 0. Thus, by Equation 4 above, the reception power of the first master leg  61  is 0 [kW] (=0× 3  [kW]). By Equation 5 above, the reception power of the second master leg  62  is 0 [kW] (=0×2 [kW]). By the operation, it can be understood that the first master leg  61  and the second master leg  62  perform no power transmission and reception. 
         [0216]    Furthermore, the case with a generalized configuration of a power router will be described. The number of master legs of the power router is represented as N (N is an integer equal to or more than 2) and the number of legs other than the master legs is M (M is an integer equal to or more than 1). In this case, Equation 3 above can be generalized as expressed by the following Equation 10. 
         [0000]    
       
         
           
             
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   10 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   u 
                   = 
                   
                      
                     
                       
                         
                           ∑ 
                           
                             i 
                             = 
                             1 
                           
                           M 
                         
                          
                         
                             
                         
                          
                         Wi 
                       
                       
                         
                           ∑ 
                           
                             j 
                             = 
                             1 
                           
                           N 
                         
                          
                         
                             
                         
                          
                         RMj 
                       
                     
                      
                   
                 
               
               
                 
                   ( 
                   10 
                   ) 
                 
               
             
           
         
       
     
         [0217]    In this case, Equation 4 and Equation 5 above can be generalized as expressed by the following Equation 11. As expressed by Equation 10 above, j is an integer satisfying the relation of 1≦j≦N. 
         [0000]      Equation 11 
         [0000]        Pj=u·RMj   (11)
 
         [0218]    So far, according to the present configuration, when a plurality of master legs are used in a power router, power to be transmitted and received via the master legs can be distributed to the respective master legs. A specific power designation value is set for each of the plurality of master legs so that it is possible to maintain a bus voltage to a proper value. 
         [0219]    Legs other than master legs include the above-described AC through leg. However, the AC through leg simply passes the transmission and reception power of another stand-alone leg or designated-power transmission/reception leg and performs no direct power transmission and reception with the outside. Therefore, when the transmission and reception power passing through the AC through leg is included in the sum (the numerator of the right side of Equation 10 above) of the transmission and reception power of the legs other than master legs, the transmission and reception power of a stand-alone leg or a designated-power transmission/reception leg connected to the AC through leg is doubly counted. Thus, when calculating the sum (the numerator of the right side of Equation 10 above) of the transmission and reception power of the legs other than master legs, the transmission and reception power passing through the AC through leg is to be excluded. 
       Exemplary Embodiment 2 
       [0220]    Next, a power router  700  according to an exemplary embodiment 2 will be described.  FIG. 19  is a block diagram schematically illustrating a configuration of the power router  700  according to the exemplary embodiment 2. The power router  700  has a configuration in which the first master leg  61  and the second master leg  62  of the power router  600  according to the exemplary embodiment 1 have been replaced with a first master leg  65  and a second master leg  66 , respectively. 
         [0221]    In the power router  600  according to the exemplary embodiment 1, the ratings of a plurality of legs have been multiplied by the coefficient u. On the other hand, in the power router  700  according to the present exemplary embodiment, a priority is predetermined among a plurality of legs of the power router  900 . The control unit  19  sets a larger power designation value in a master leg with a high priority. The priority is a value indicating the importance of a leg, and for example, is expressed by a numeral value. 
         [0222]    In the present exemplary embodiment, the control unit  19  multiplies the rating RM 1  of the first master leg  65  by an adjustment coefficient v 1  (also called a second coefficient) as well as the coefficient u. Thus, the power instruction value P 1  of the first master leg  65  is expressed by the following Equation 12. 
         [0000]      Equation 12 
         [0000]        P 1= v   1   ·u·RM 1  (12)
 
         [0223]    The control unit  19  multiplies the rating RM 2  of the second master leg  66  by an adjustment coefficient v 2  (also called a second coefficient) as well as the coefficient u. Thus, the power instruction value P 2  of the second master leg  66  is expressed by the following Equation 13. 
         [0000]      Equation 13 
         [0000]        P 2= v   2   ·RM 2  (13)
 
         [0224]    Here, to a master leg with a higher priority is allocated a larger value of adjustment coefficient which is multiplied into the rating of the leg. For example, when the priority of the first master leg  65  is higher than that of the second master leg  66 , v 1 &gt;v 2 . Bearing in mind, however, that power transmission and reception cannot be performed for the first master leg  65  and the second master leg  66  beyond the ratings of these legs, it is necessary to set v 1  such that 0&lt;(v 1 ×u)&lt;1 and set v 2  such that 0&lt;(v 2 ×u)&lt;1. 
         [0225]    Furthermore, the case with a generalized configuration of a power router will be described. The number of master legs of the power router is represented as N (N is an integer equal to or more than 2) and the number of legs other than the master legs is M (M is an integer equal to or more than 1). In this case, Equation 12 and Equation 13 above can be generalized as expressed by the following Equation 14. As expressed by Equation 10 above, j is an integer satisfying the relation of 1≦j≦N. 
         [0000]      Equation 14 
         [0000]        Pj=v   j   ·u·RMj   (14)
 
         [0226]    It is necessary to set v j  such that 0&lt;(v j ×u)&lt;1. 
         [0227]    So far, according to the present configuration, it is possible to adjust the power designation value of a master leg in response to the priory among each of a plurality of master legs. Thus it is possible to determine the power designation value so as to correspond to the characteristics of each of the plurality of master legs. 
         [0228]    The priority can be set as follows. For example, it is possible to place a higher priority on a power leg connected to a power supply with high stability such as a commercial power supply system. It is possible thereby to expect stable supply of power to a power router. 
         [0229]    For example, it is also possible to change a priority in accordance with time. By such an operation, a power supply source with a time-variable rates system, such as night hour rates, may be effectively utilized to reduce electricity cost. 
         [0230]    For example, it is also possible to place a higher priority on a master leg with a larger rating. By such an operation, the master leg with a larger rating is mainly used, so that it is possible to achieve stable power transmission and reception. The priority of the master leg may be set by the management server  1010  or the control unit  19 . 
         [0231]    For example, it is also possible to place a higher priority on a master leg with a shorter accumulated operation time. By such an operation, it is possible to reduce a load of the master leg with a longer accumulated operation time and to even out accumulated loads among a plurality of master legs. As a consequence, it is possible to reduce a failure rate and extend a lifetime of a power router. 
       Other Embodiments 
       [0232]    The present invention is not limited to the aforementioned exemplary embodiments and can be appropriately changed without departing from the spirit thereof. For example, although the control unit  19  is described as the configuration of hardware in the foregoing exemplary embodiments, the present invention is not limited thereto. For example, the control unit  19  can be configured by a computer which performs any processing with a CPU (Central Processing Unit) executing a computer program. Furthermore, a control device is installed in a power conversion unit of a leg, and the control device, for example, is configured as a dynamic reconfiguration logic (FPGA: Field Programmable Gate Array). A control program of the FPGA is adapted to a mode of a leg and operated. By the operation, the FPGA is rewritten in accordance with the type and operation of a leg, so that it is possible to perform control suitable to the operation mode of the leg, thereby reducing hardware capacity requirement and cost. Furthermore, the above-described program is stored by using a non-transitory computer readable medium of various types and can be supplied to a computer. Non-transitory computer readable media includes substantive recording media (tangible storage media) of various types. Examples of the non-transitory computer readable media include magnetic recording media (for example, a flexible disk, a magnetic tape, and a hard disk drive), a magnetic optical recording medium (for example, a magnetic optical disk), a CD-ROM (Read Only Memory) a CD-R, a CD-R/W, semiconductor memories (for example, a mask ROM, a PROM (Programmable ROM), an EPROM (Erasable PROM), a flash ROM, and a RAM (Random Access Memory)). A program may be supplied to a computer by any of transitory computer readable medium of various types. Examples of the transitory computer readable medium include an electric signal, a light signal, and an electromagnetic wave. With the transitory computer readable medium, a program can be supplied to a computer via a wired communication path such as an electric line or an optical fiber or a wireless communication path. 
         [0233]    In the exemplary embodiments 1 and 2, the number of master legs is 2, but this is for illustrative purposes only. The number of master legs can be 3 or more. Furthermore, in the exemplary embodiments 1 and 2, the number of legs other than the master legs is assumed 2, but this is for illustrative purposes only. The number of legs other than the master legs can be any number equal to or more than 1. Furthermore, the legs other than the master legs may be stand-alone legs or designated-power transmission/reception legs. 
         [0234]    So far, the present invention has been described with reference to the exemplary embodiments. However, the present invention is not limited thereto. Various modifications which can be understood by a person skilled in the art can be made to the configuration and details of the present invention within the scope of the invention. 
         [0235]    This application claims priority based on Japanese Patent Application No. 2014-4919 filed on Jan. 15, 2014, the contents of which are incorporated herein in its entirety by reference. 
       REFERENCE SIGNS LIST 
       [0000]    
       
         BL branch line 
         D 1  to D 6  diode 
         INF information 
         MODE operation mode designation information 
         P 1 , P 2  power instruction value 
         Q 1  to Q 6  transistor 
         RM 1 , RM 2  rating 
         SA 1 , SD 1  waveform instruction signal 
         SA 2 , SD 2  reading signal 
         SCON control signal 
         SIG 1  switching control signal 
         V 15  bus voltage 
         Vr detection value 
           11 ,  21  first leg 
           12 ,  22  second leg 
           13 ,  23  third leg 
           14 ,  24  fourth leg 
           51 , M 101 , M 201 , M 301 , M 401 , M 501 , M 601 , DC bus 
           16  communication bus 
           17  voltage sensor 
           19  control unit 
           51  control instruction 
           52  information 
           60  through leg 
           61 ,  65  first master leg 
           62 ,  66  second master leg 
           63  first stand-alone leg 
           64  second stand-alone leg 
           71 A,  71 B power transmission line 
           72  distribution line 
           100 ,  101 ,  102 ,  170 ,  200 ,  300 ,  400 ,  600 ,  700 ,  841  to  844  power router 
           821  to  824  power cell 
           111 ,  121 ,  131 ,  141  power conversion unit 
           112 ,  122 ,  132 ,  142 ,  162  current sensor 
           113 ,  223 ,  133 ,  143 ,  163  switch 
           114 ,  224 ,  134 ,  144 ,  164  voltage sensor 
           115 ,  125 ,  135 ,  145 ,  165 ,  215 ,  225 ,  235 ,  245  connection terminal 
           191  storage unit 
           192  operation mode management unit 
           193  power conversion instruction unit 
           194  DA/AD conversion unit 
           195  sensor value reading unit 
           196  control instruction database 
           197  leg identification information database 
           110 ,  210 ,  220 ,  320 ,  420 ,  560  master leg 
           210 ,  410  stand-alone leg 
           250  AC through leg 
           610  designated-power transmission/reception leg 
           810 ,  1000 ,  2000  power network system 
           811 ,  1035 ,  1035 A to  1035 C utility grid 
           812  large-scale power plant 
           831  house 
           832  building 
           833  solar power panel 
           834  wind power generator 
           835 ,  1032 ,  1034  storage battery 
           850 ,  1010 ,  1020  management server 
           860 ,  1100  communication network 
           1011  power conversion unit 
           1012  storage device 
           1031 ,  1033  load 
           1200 ,  1201  to  1203 ,  1211  to  1213  transmission line 
           1300  communication line