Patent Publication Number: US-2019173289-A1

Title: Power control apparatus and power control method

Description:
TECHNICAL FIELD 
     The present disclosure relates a power control apparatus and a power control method. 
     BACKGROUND ART 
     An uninterruptible power source apparatus has been known that includes a storage battery, and can hereby keep on supplying power from the storage battery to an apparatus connected thereto for a predetermined time without causing power interruptions even when power from an input power source is cut off. Technology has been proposed in which such a power source apparatus is extended to units of customers (which will also be referred to as nodes) to supply surplus power to other customers when an abnormality occurs in supplying power due to power interruption, in the case where a storage battery has little remaining power, or the like (see Patent Literature 1, Patent Literature 2, and the like). 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: JP 2015-056976A 
     Patent Literature 2: WO 2015/072304 
     DISCLOSURE OF INVENTION 
     Technical Problem 
     Each node includes a converter (DC-DC converter or AC-DC converter) that converts the voltage between a power line and a storage battery. The conversion efficiency of the converter varies in accordance with the input and output voltage ratio. Meanwhile, the voltage of the storage battery varies in accordance with the capacity. Therefore, if the voltage of the power line is fixed at a predetermined voltage value, it is not possible to use the converter at the optimum conversion efficiency when transferring power through the power line. 
     Accordingly, the present disclosure proposes a novel and improved power control apparatus and power control method capable of using a converter at the optimum conversion efficiency when transferring power between nodes through a power line. 
     Solution to Problem 
     According to the present disclosure, there is provided a power control apparatus including: an acquisition section configured to acquire information from a node on a power reception side which receives power through a power line, the information pertaining to a characteristic of a conversion device that converts voltage between the power line and a storage battery on the power reception side; and a setting section configured to use the information acquired by the acquisition section and a characteristic of a conversion device that converts voltage between the power line and a storage battery on a power transmission side to set voltage of the power line. 
     In addition, according to the present disclosure, there is provided a power control apparatus including: an acquisition section configured to acquire information from a node on a power transmission side which transmits power through a power line, the information pertaining to a characteristic of a conversion device that converts voltage between the power line and a storage battery on the power transmission side; and a selection section configured to use the information acquired by the acquisition section and a characteristic of a conversion device that converts voltage between the power line and a storage battery on a power transmission side to select a power transmission source. 
     In addition, according to the present disclosure, there is provided a power control method including: acquiring information from a node on a power reception side which receives power through a power line, the information pertaining to a characteristic of a conversion device that converts voltage between the power line and a storage battery on the power reception side; and using the acquired information and a characteristic of a conversion device that converts voltage between the power line and a storage battery on a power transmission side to set voltage of the power line. 
     In addition, according to the present disclosure, there is provided a power control method including: acquiring information from a node on a power transmission side which transmits power through a power line, the information pertaining to a characteristic of a conversion device that converts voltage between the power line and a storage battery on the power transmission side; and using the acquired information and a characteristic of a conversion device that converts voltage between the power line and a storage battery on a power transmission side to select a power transmission source. 
     Advantageous Effects of Invention 
     According to the present disclosure as described above, it is possible to provide a novel and improved power control apparatus and power control method capable of using a converter at the optimum conversion efficiency when transferring power between nodes through a power line. 
     Note that the effects described above are not necessarily limitative. With or in the place of the above effects, there may be achieved any one of the effects described in this specification or other effects that may be grasped from this specification. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is an explanatory diagram illustrating a configuration example of a power supply system  1  according to an embodiment of the present disclosure. 
         FIG. 2  is an explanatory diagram describing a configuration example of a node  10 . 
         FIG. 3  is an explanatory diagram illustrating an example of an efficiency curve of a DCDC converter  120 . 
         FIG. 4  is an explanatory diagram illustrating an example of an efficiency curve of the DCDC converter  120  with respect to voltage of a bus line  30 . 
         FIG. 5  is an explanatory diagram illustrating efficiency curves of nodes  10   a  and  10   b  illustrated in  FIG. 1 , and an average of the two efficiency curves. 
         FIG. 6  is an explanatory diagram illustrating efficiency curves of the nodes  10   a ,  10   b ,  10   c , and  10   d  illustrated in  FIG. 1 , and an average of the four efficiency curves. 
         FIG. 7  is an explanatory diagram illustrating an efficiency curve. 
         FIG. 8  is an explanatory diagram illustrating that power is transferred in a case where nodes are hierarchically disposed. 
         FIG. 9  is an explanatory diagram illustrating a case where power is transferred over clusters. 
         FIG. 10  is an explanatory diagram illustrating an efficiency curve. 
         FIG. 11  is a sequence diagram describing an operation example of a node of the power supply system  1  according to the embodiment. 
     
    
    
     MODE(S) FOR CARRYING OUT THE INVENTION 
     Hereinafter, (a) preferred embodiment(s) of the present disclosure will be described in detail with reference to the appended drawings. Note that, in this specification and the appended drawings, structural elements that have substantially the same function and structure are denoted with the same reference numerals, and repeated explanation of these structural elements is omitted. 
     Note that description will be provided in the following order.
     1. Embodiment of the Present Disclosure   1.1. Overview   1.2. Configuration Example and Operation Example   2. Conclusion   

     &lt;1. Embodiment of the Present Disclosure&gt; 
     [1.1. Overview] 
     Before an embodiment of the present disclosure is described in detail, the overview of the embodiment of the present disclosure will be described. 
     As described above, the technology is disclosed for a power supply system in which, between nodes each including a power generation apparatus such as a solar power generation apparatus that uses natural energy and renewable energy to generate power and a battery that stores the power generated by the power generation apparatus, the power stored in the batteries is interchanged (see Patent Literature 1 and the like). 
     Technology is also disclosed for a system in which power is autonomously interchanged between the respective nodes in such a power supply system (see Patent Literature 2 and the like). Autonomously interchanging power between nodes individually optimizes the respective batteries. 
     Each node includes a converter (DC-DC converter or AC-DC converter) that converts the voltage between a power line and a storage battery. The conversion efficiency of the converter varies in accordance with the input and output voltage ratio. Meanwhile, the voltage of the storage battery varies in accordance with the capacity. Therefore, if the voltage of the power line is fixed at a predetermined voltage value, it is not possible to use the converter at the optimum conversion efficiency when transferring power through the power line. 
     Accordingly, in view of what has been described above, the present disclosers have assiduously studied technology capable of using a converter at the optimum conversion efficiency when transferring power through a power line. As a result, the present disclosers have devised technology capable of using a converter at the optimum conversion efficiency as described below by setting the voltage of a power line with the conversion efficiency of the converter taken into consideration when transferring power. 
     The above describes the overview of an embodiment of the present disclosure. Next, the embodiment of the present disclosure will be described in detail. 
     [1.2. Configuration Example and Operation Example] 
     First, a configuration example of the power supply system according to an embodiment of the present disclosure will be described.  FIG. 1  is an explanatory diagram illustrating a configuration example of a power supply system  1  according to an embodiment of the present disclosure. The following uses  FIG. 1  to describe a configuration example of the power supply system  1  according to an embodiment of the present disclosure. 
     The power supply system  1  illustrated in  FIG. 1  has nodes  10   a  to  10   d  (which will be referred to simply as nodes  10  in some cases) connected through a communication line  20  and a bus line  30 . The nodes  10   a  to  10   d  are power consumption units. Each node is one power generation and power consumption unit including, for example, a home, a company, a school, a hospital, a city office, and the like. The configurations of the nodes  10   a  to  10   d  will be described below. However, each of the nodes  10   a  to  10   d  includes a storage battery that stores power, and a converter that converts the voltage between the storage battery and the bus line. The following describes that the bus line  30  is an example of a power line and allows direct current to flow, but the bus line  30  may also allow alternating current to flow. That is, the converter provided to each node is either a DC-DC converter or an AC-DC converter. 
     In the case where a certain node (which will be described as the node  10   a  below) needs power, the power supply system  1  illustrated in  FIG. 1  transmits a power request from that node  10   a  to another node through the communication line  20 . The other node that receives the power request returns a supply response to the node  10   a  through the communication line  20  if the other node can accept the power request. This supply response can include, for example, information of a suppliable power amount, time slot, price, point, or the like. 
     The node  10   a  that receives the supply response from another node selects a node from which the node  10   a  is supplied with power on the basis of the content of the supply response. Then, the node  10   a  transmits, through the communication line  20 , a selection response to the selected node. Here, it is assumed that the node  10   a  selects a node  10   b  as a node from which the node  10   a  is supplied with power. 
     When the node  10   b  receives the selection response transmitted from the node  10   a , the node  10   b  acquires the control right of the bus line  30  and sets a predetermined value as the voltage of the bus line  30 . Here, the node  10   b  sets the predetermined value as the voltage of the bus line  30  as described below on the basis of the characteristics of the converter of the node  10   b  that is a power transmission side of power and the characteristics of the converter of the node  10   a  that is a power reception side. 
     The node  10   b  sets the voltage of the bus line  30  on the basis of the characteristics of the converter of the node  10   b  that is a power transmission side of power and the characteristics of the converter of the node  10   a  that is a power reception side, thereby making it possible to use the converter of each node at the optimum conversion efficiency. A method for setting the voltage of the bus line  30  will be described in detail. 
     The above uses  FIG. 1  to describe a configuration example of the power supply system  1  according to an embodiment of the present disclosure. Next, a configuration example of the node  10  will be described. 
       FIG. 2  is an explanatory diagram describing a configuration example of the node  10 . The following uses  FIG. 2  to describe a configuration example of the node  10  according to an embodiment of the present disclosure. 
     As illustrated in  FIG. 2 , the node  10  according to an embodiment of the present disclosure includes a communication section  110 , a DCDC converter  120 , a storage battery  130 , an optimum efficiency curve calculation section  140 , a DC bus voltage detection section  150 , an efficiency curve calculation section  160 , a storage battery voltage detection section  170 , and a DCDC control section  180 . 
     The communication section  110  executes communication processing with another node through the communication line  20 . The communication section  110  allows various kinds of information to be communicated with another node. For example, the communication section  110  transmits a power transmission request to another node through the communication line  20 . This transmission of a power transmission request may be broadcast transmission with no destination designated, or multicast transmission with a plurality of nodes designated. In addition, for example, the communication section  110  receives a power transmission request transmitted from another node through the communication line  20 . If it is possible to transmit power, the communication section  110  returns a supply response to that node. In addition, for example, the communication section  110  receives a supply response transmitted from another node through the communication line  20 . In the case where the communication section  110  accepts power reception from that node, the communication section  110  returns a selection response to that node. 
     When the communication section  110  transmits a power transmission request, the communication section  110  transmits an efficiency curve of the own node described below along with the power transmission request. In addition, when the communication section  110  receives a supply response, the communication section  110  receives an efficiency curve of a node that transmits the supply response along with the supply response. 
     The DCDC converter  120  is provided between the bus line  30  and the storage battery  130 , and converts the direct current voltage between the bus line  30  and the storage battery  130 . In addition, the DCDC converter  120  sets the voltage of the bus line  30 . The DCDC converter  120  sets the voltage of the bus line  30  in the case where the own node has acquired the control right of the bus line  30 . The DCDC converter  120  sets the voltage of the bus line  30  as a voltage value set by the DCDC control section  180  described below. 
     The storage battery  130  is, for example, a lithium ion secondary battery, a sodium-sulfur battery, or other secondary batteries. The storage battery  130  stores power generated by a power generation apparatus that is not illustrated, but uses sunlight, solar heat, wind power, or the like to generate power. 
     The optimum efficiency curve calculation section  140  calculates the optimum efficiency curve from an efficiency curve of the storage battery  130  of the own node and an efficiency curve of the storage battery of another node. In addition, when the bus line  30  transfers power between other nodes, the optimum efficiency curve calculation section  140  calculates, in the case where the own node participates to transfer power, the optimum efficiency curve from an efficiency curve of the storage battery  130  of the own node and an efficiency curve of the storage battery of another node. A method for the optimum efficiency curve calculation section  140  to calculate an efficiency curve will be described in detail below. 
     The DC bus voltage detection section  150  detects the voltage of the bus line  30 . By detecting the voltage of the bus line  30 , the DC bus voltage detection section  150  knows whether or not the bus line  30  transfers power between other nodes. The DC bus voltage detection section  150  sends information of the voltage of the bus line  30  to the optimum efficiency curve calculation section  140 . 
     The efficiency curve calculation section  160  calculates an efficiency curve with respect to the voltage of the bus line  30  on the basis of the voltage of the storage battery  130  detected by the storage battery voltage detection section  170 . Information of the efficiency curve of the own node calculated by the efficiency curve calculation section  160  is used by the optimum efficiency curve calculation section  140  to calculate an efficiency curve. 
     The storage battery voltage detection section  170  detects the voltage of the storage battery  130  which varies in accordance with the capacity. The storage battery voltage detection section  170  sends information of the voltage of the storage battery  130  to the efficiency curve calculation section  160 . 
     On the basis of the efficiency curve calculated by the optimum efficiency curve calculation section  140 , the DCDC control section  180  controls the DCDC converter  120  such that the voltage of the bus line  30  becomes the voltage that can be used by the DCDC converter  120  the most efficiently. 
       FIG. 3  is an explanatory diagram illustrating an example of an efficiency curve of the DCDC converter  120 . The DCDC converter  120  capable of setting input voltage and output voltage shows conversion efficiency η that varies in accordance with an input and output voltage ratio N as illustrated in  FIG. 3 . Then, the DCDC converter  120  like that has a characteristic in which the conversion efficiency is the highest in the case where the input and output voltage ratio N has a certain value. 
       FIG. 4  is an explanatory diagram illustrating an example of an efficiency curve of the DCDC converter  120  with respect to the voltage of the bus line  30 . An efficiency curve of the DCDC converter  120  with respect to the voltage of the bus line  30  can be calculated by multiplying the efficiency curve illustrated in  FIG. 3  by voltage Vbat of the storage battery  130  at that time. That is, the efficiency curve calculation section  160  multiplies the efficiency curve illustrated in  FIG. 3  by a voltage value detected by the storage battery voltage detection section  170 , thereby calculating the efficiency curve as illustrated in  FIG. 4 . That is, the efficiency curve calculation section  160  calculates an efficiency curve of the DCDC converter  120  with respect to the voltage of the bus line  30  in accordance with V bus =N×V bat . 
     The efficiency curve calculated in this way can be different for each node. That is, the voltage of the bus line  30  at which the conversion efficiency of the DCDC converter  120  is the most favorable can be different for each node. Thus, the optimum efficiency curve calculation section  140  uses efficiency curves of a plurality of nodes including the own node to calculate the optimum efficiency curve. The optimum efficiency curve calculation section  140  calculates, for example, the average of a plurality of efficiency curves. Then, the DCDC control section  180  sets the voltage Vbus at which the average value reaches the maximum value as the voltage of the bus line  30 , thereby making it possible to set the voltage at which the efficiency is favorable for not only the power transmission side, but also the power reception side. 
     Specific examples for calculating the optimum efficiency curve and setting the voltage value of the bus line  30  will be described. An example of the case will be demonstrated where power is supplied from the node  10   b  to the node  10   a  in the power supply system  1  illustrated in  FIG. 1 . 
       FIG. 5  is an explanatory diagram illustrating efficiency curves of two nodes, for example, the nodes  10   a  and  10   b  illustrated in  FIG. 1 , and the average of the two efficiency curves. 
     The optimum efficiency curve calculation section  140  of the node  10   b  calculates an average η 72 (V bus ) of an efficiency curve η 1 (V bus ) of the DCDC converter  120  of the node  10   a  acquired when a power transmission request is received from the node  10   a , and an efficiency curve η 2 (V bus ) of the DCDC converter  120  of the own node on the basis of the following formula 1. 
     
       
         
           
             
               
                 
                   
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     The DCDC control section  180  sets, as the voltage of the bus line  30 , voltage Vtarget at which the conversion efficiency is the highest in the average η 12 (V bus ) of efficiency curves calculated in this way by the optimum efficiency curve calculation section  140 . The DCDC control section  180  sets the voltage Vtarget as the voltage of the bus line  30 , thereby allowing the node  10   b  to interchange power to the node  10   a  at the voltage at which the efficiency is the most favorable for both the own node and the node  10   a  to which power is transmitted. 
     Other specific examples for calculating the optimum efficiency curve and setting the voltage value of the bus line  30  will be described. Examples of the cases will be demonstrated where power is supplied from the node  10   b  to the node  10   a , and power is supplied from the node  10   c  to the node  10   d  in the power supply system  1  illustrated in  FIG. 1 . 
       FIG. 6  is an explanatory diagram illustrating efficiency curves of four nodes, for example, the nodes  10   a ,  10   b ,  10   c , and  10   d  illustrated in  FIG. 1 , and the average of the four efficiency curves. 
     The case will be considered where power is further interchanged from the node  10   c  to the node  10   d  in the case where power is interchanged from the node  10   b  to the node  10   a . The optimum efficiency curve calculation section  140  of the node  10   b  calculates an average η 1 . . . 4 (V bus ) of the efficiency curve η 1 (V bus ) of the DCDC converter  120  of the node  10   a , the efficiency curve η 2 (V bus ) of the DCDC converter  120  of the own node, an efficiency curve η 3 (V bus ) of the DCDC converter  120  of the node  10   c , and an efficiency curve η 4 (V bus ) of the DCDC converter  120  of the node  10   d  on the basis of the following formula 2. 
     
       
         
           
             
               
                 
                   
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     The DCDC control section  180  sets, as the voltage of the bus line  30 , voltage Vtarget at which the conversion efficiency is the highest in the average η 1 . . . 4 (V bus ) of efficiency curves calculated in this way by the optimum efficiency curve calculation section  140 . The DCDC control section  180  sets the voltage Vtarget as the voltage of the bus line  30 , thereby allowing the node  10   b  to set the voltage at which the efficiency is the most favorable for all the nodes that interchange power. 
     In the case where supply responses are transmitted from a plurality of nodes, a node to which power is interchanged may select a node having the efficiency curve in which the conversion efficiency is the most favorable as a source from which power is interchanged. 
     The case will be considered where the node  10   b  transmits a power supply and the nodes  10   a  and  10   c  returns supply responses to the node  10   b  in the power supply system  1  illustrated in  FIG. 1 . The nodes  10   a  and  10   c  each return an efficiency curve of the own node to the node  10   b  along with the supply response. 
     The optimum efficiency curve calculation section  140  of the node  10   b  calculates the average of an efficiency curve of the own node and an efficiency curve of each of the nodes  10   a  and  10   c .  FIG. 7  is an explanatory diagram illustrating efficiency curves of the nodes  10   a ,  10   b , and  10   c , an average η 12 (V bus ) of efficiency curves of the nodes  10   a  and  10   b , and an average η 23 (V bus ) of efficiency curves of the nodes  10   b  and  10   c.    
     If the averages η 12 (V bus ) and η 23 (V bus ) illustrated in  FIG. 7  are compared, it is η 23 (V bus ) that has higher maximum conversion efficiency. Thus, if the node  10   b  selects the node  10   c  as a source from which power is interchanged, the node  10   b  can receive power at higher efficiency. In this case, for example, the optimum efficiency curve calculation section  140  may select a node having the efficiency curve in which the conversion efficiency is the most favorable as a source from which power is interchanged. 
     The examples shown so far have described the case where all the nodes are disposed in the same layer, but the respective nodes may also be hierarchically disposed. 
       FIG. 8  is an explanatory diagram illustrating that power is transferred in the case where nodes are hierarchically disposed. In the example illustrated in  FIG. 8 , nodes  1  to  3  and nodes  5  to  7  are disposed in a lower layer, a node  4  is disposed in a higher layer of the nodes  1  to  3 , a node  8  is disposed in a higher layer of the nodes  5  to  7 , and the nodes  4 ,  8 , and  9  are disposed in the same layer. The nodes  1  to  4  are connected to a bus line  30   a , the nodes  5  to  8  are connected to a bus line  30   b , and the nodes  4 ,  8 , and  9  are connected to a bus line  30   c . Note that  FIG. 8  omits a communication line to which each node is connected. 
     The case will be considered where, for example, power is supplied to the node  6  from the node  2  through the nodes  4  and  8  in the example illustrated in  FIG. 8 . In this case, the node  2  decides voltage vbus 1  of the bus line  30   a  from the efficiency curve η 2 (V bus ) of the DCDC converter  120  of the own node. In addition, the node  4  uses the efficiency curve η 4 (V bus ) of the DCDC converter  120  of the own node and an efficiency curve η 8 (V bus ) of the DCDC converter  120  of the node  8  to decide voltage vbus 3 . For example, the node  4  sets, as the voltage vbus 3  of the bus line  30   c , the voltage at which the efficiency has the maximum value in an average η 48 (V bus ) of η 4 (V bus ) and η 8 (V bus ). In addition, the node  6  decides voltage vbus 2  of the bus line  30   b  from the efficiency curve η 6 (V bus ) of the DCDC converter  120  of the own node. 
     By deciding the voltage of the bus lines in this way, the nodes  2 ,  4 , and  6  can cause all the nodes through which power is transferred to operate at the most favorable efficiency. 
     It is also possible to group a plurality of nodes into one cluster. In the case where a plurality of nodes are grouped into one cluster, it is also possible to cause one node to serve as a hub to transfer power over clusters. Even in this case, it is possible to set the voltage of a bus line provided to each cluster on the basis of an efficiency curve of the DCDC converter provided to each node. 
       FIG. 9  is an explanatory diagram illustrating the case where a plurality of nodes are grouped into one cluster, and power is transferred over clusters.  FIG. 9  illustrates the state in which the nodes  1  to  4  are grouped into one cluster, and the nodes  4  to  7  are grouped into one cluster. The nodes  1  to  4  are connected to the bus line  30   a , and the nodes  4  to  7  are connected to the bus line  30   b . That is, the node  4  is connected to both of the bus lines  30   a  and  30   b.    
     In the state in which the voltage V bus1  and the voltage V bus2  are respectively applied to bus lines  30   a  and  30   b , the node  4  computes efficiency η 4 (V bus ) and efficiency η 4 (V bus2 ) for the bus lines  30   a  and  30   b . Then, the node  4  makes a decision such that power is received from the bus line having more favorable efficiency.  FIG. 10  is an explanatory diagram illustrating the efficiency curve θ 4 (V bus ) of the node  4 . From the graph of the efficiency curve η 4 (V bus ) illustrated in  FIG. 10 , the efficiency at the time of the voltage V bus1  is higher than the efficiency at the time of the voltage V bus2 . Thus, the node  4  can perform such power interchange that power is received from the bus line  30  to which the voltage V bus1  is applied, or power is transmitted to the bus line  30 . 
     Next, an operation example of a node of the power supply system  1  according to an embodiment of the present disclosure will be described.  FIG. 11  is a sequence diagram describing an operation example of a node of the power supply system  1  according to an embodiment of the present disclosure. What is illustrated in  FIG. 11  is operation examples of the nodes  1  to  5  connected to the same bus line  30  and belonging to the same layer. In addition,  FIG. 11  also illustrates change in the voltage and electric current of the bus line  30 . The following uses  FIG. 11  to describe an operation example of a node of the power supply system  1  according to an embodiment of the present disclosure. 
     First, the flow of the case where the node  2  wishes to receive power from another node will be described. The node  2  transmits power requests to all the other nodes (or some nodes) through the communication line  20  (step S 101 ). These power requests include not only information such as a desired power amount, time, and price, but also information of an efficiency curve of the DCDC converter  120  of the node  2 . 
     When another node receives a power request from the node  2 , the other node determines whether to accept the power request. If it is possible to accept the power request, the other node transmits a supply response to the node  2 . In the example illustrated in  FIG. 11 , the nodes  3  and  5  each transmits a supply response to the node  2  (steps S 102  and S 103 ). When the nodes  3  and  5  each transmits a supply response to the node  2 , the nodes  3  and  5  each include not only information of a suppliable power amount, time, price and the like, but also information of an efficiency curve of the DCDC converter  120  of the own node. 
     When selecting a power supply source, the node  2  that receives the supply responses from the nodes  3  and  5  uses an efficiency curve of the DCDC converter  120  of each node and an efficiency curve of the DCDC converter  120  of the own node to select a node from which power can be efficiently received as a power supply source. In the example illustrated in  FIG. 11 , the node  2  selects the node  3  as a power supply source. 
     When the node  2  selects the node  3  as a power supply source, the node  2  transmits a selection response to the node  3  (step S 104 ). When the node  3  receives a selection response from the node  2 , the node  3  acquires the control right of the bus line  30  and sets the voltage of the bus line  30  from an efficiency curve of the DCDC converter  120  of the node  2  and an efficiency curve of the DCDC converter  120  of the own node (step S 105 ). As described above, the node  3  takes the average of efficiency curves of two nodes, and sets the voltage at which the efficiency is the highest as the voltage of the bus line  30 . When the node  3  sets the voltage of the bus line  30  at time tl, the voltage of the bus line  30  begins to gradually increase. 
     The node  3  notifies another node of the acquisition of the control right of the bus line  30 , and then begins to transmit power to the node  2  through the bus line  30  (step S 107 ). The node  2  begins to receive power from the node  3  at time t 2  (step S 108 ). When the time t 2  comes, the electric current flowing through the bus line  30  increases. 
     Afterward, the flow of the case where the node  4  also wishes to receive power from another node will be described. The node  4  transmits power requests to all the other nodes (or some nodes) through the communication line  20  (step S 109 ). These power requests include not only information such as a desired power amount, time, and price, but also information of an efficiency curve of the DCDC converter  120  of the node  4 . 
     When another node receives a power request from the node  4 , the other node determines whether to accept the power request. If it is possible to accept the power request, the other node transmits a supply response to the node  4 . In the example illustrated in  FIG. 11 , the nodes  1  and  5  each transmits a supply response to the node  4  (steps S 110  and S 111 ). When the nodes  1  and  5  each transmits a supply response to the node  4 , the nodes  1  and  5  each include not only information of a suppliable power amount, time, price and the like, but also information of an efficiency curve of the DCDC converter  120  of the own node. 
     When selecting a power supply source, the node  4  that receives the supply responses from the nodes  1  and  5  uses an efficiency curve of the DCDC converter  120  of each node and an efficiency curve of the DCDC converter  120  of the own node to select a node from which power can be efficiently received as a power supply source. In the example illustrated in  FIG. 11 , the node  4  selects the node  1  as a power supply source. 
     When the node  4  selects the node  1  as a power supply source, the node  4  transmits a selection response to the node  1  (step S 112 ). In addition, the node  4  also transmits a selection response indicating that power is supplied from the node  1  to the node  3  that has acquired the control right of the bus line  30  (step S 112 ). 
     The node  3  that has acquired the control right of the bus line  30  sets the voltage of the bus line  30  again on the basis of efficiency curves of the nodes  1  to  4  (step S 113 ). As described above, the node  3  takes the average of efficiency curves of four nodes, and sets the voltage at which the efficiency is the highest as the voltage of the bus line  30 . When the node  3  sets the voltage of the bus line  30  at time t 3 , the voltage of the bus line  30  further increases. 
     The node  3  transmits information of the voltage value of the bus line  30  to the nodes  1  and  4  (step S 114 ). The node  1  begins to transmit power to the node  4  through the bus line  30  (step S 115 ). The node  4  begins to receive power from the node  1  at time t 4  (step S 116 ). When the time t 4  comes, the electric current flowing through the bus line  30  increases. 
     In this way, power is supplied from the node  3  to the node  2  and from the node  1  to the node  4  through the bus line  30 . 
     Afterward, when power supply terminates from the node  3  to the node  2 , the node  2  transmits a termination notification to the node  3  at time t 5  (step S 117 ). At the time t 5 , the amount of electric current flowing through the bus line  30  decreases. When the node  3  receives the termination notification from the node  2  at time t 6 , the node  3  causes the control right of the bus line  30  to transition to the node  1  that is transmitting power at that time (step S 118 ). 
     The node  1  that has acquired the control right of the bus line  30  sets the voltage of the bus line  30  (step S 119 ). The node  1  sets the voltage of the bus line  30  on the basis of an efficiency curve of the DCDC converter  120  of the node  4  and an efficiency curve of the DCDC converter  120  of the own node. As described above, the node  1  takes the average of efficiency curves of two nodes, and sets the voltage at which the efficiency is the highest as the voltage of the bus line  30 . When the voltage of the bus line  30  is set at time t 7  in the example of  FIG. 11 , the voltage of the bus line  30  further increases. 
     Afterward, when power supply terminates from the node  1  to the node  4 , the node  4  transmits a termination notification to the node  1  at time t 8  (step S 120 ). At the time t 8 , the amount of electric current flowing through the bus line  30  decreases. At that time, no power is transferred through the bus line  30 . Accordingly, the amount of electric current flowing through the bus line  30  is 0. 
     When the node  1  receives the termination notification from the node  4  at the time t 8 , the node  1  discards the control right of the bus line  30  at time t 9  because no other power is transferred through the bus line  30  at the time t 8  (step S 121 ). When the node  1  discards the control right of the bus line  30 , voltage applied to the bus line  30  decreases to 0. 
     By performing the above-described operation, each node of the power supply system  1  according to an embodiment of the present disclosure can set the voltage of the bus line with the conversion efficiency of the DCDC converter of the node taken into consideration when transferring power through the bus line  30 . Each node sets the voltage of the bus line with the conversion efficiency of the DCDC converter taken into consideration, thereby allowing the converter to be used at the optimum conversion efficiency. 
     &lt;2. Conclusion&gt; 
     According to an embodiment of the present disclosure as described above, there is provided a node that can, when power is transferred between nodes connected to a common bus line (power line), set the voltage of the bus line with the conversion efficiency of a converter provided to each node taken into consideration. 
     According to an embodiment of the present disclosure, there is provided a node that can, when power is transferred between nodes connected to a common bus line, select a power transmission source with the conversion efficiency of the converter of the own node taken into consideration. 
     Note that each node may set the voltage at which the conversion efficiency is the most favorable in a converter on a power reception side as the voltage of a bus line, or set the voltage at which the conversion efficiency is the most favorable in a converter on a power transmission side as the voltage of a bus line. 
     The preferred embodiment(s) of the present disclosure has/have been described above with reference to the accompanying drawings, whilst the present disclosure is not limited to the above examples. A person skilled in the art may find various alterations and modifications within the scope of the appended claims, and it should be understood that they will naturally come under the technical scope of the present disclosure. 
     Further, the effects described in this specification are merely illustrative or exemplified effects, and are not limitative. That is, with or in the place of the above effects, the technology according to the present disclosure may achieve other effects that are clear to those skilled in the art from the description of this specification. 
     Additionally, the present technology may also be configured as below. 
     (1) 
     A power control apparatus including: 
     an acquisition section configured to acquire information from a node on a power reception side which receives power through a power line, the information pertaining to a characteristic of a conversion device that converts voltage between the power line and a storage battery on the power reception side; and 
     a setting section configured to use the information acquired by the acquisition section and a characteristic of a conversion device that converts voltage between the power line and a storage battery on a power transmission side to set voltage of the power line. 
     (2) 
     The power control apparatus according to (1), in which 
     the setting section sets, as the voltage of the power line, voltage at which an average value of conversion efficiency of each conversion device reaches a maximum value. 
     (3) 
     The power control apparatus according to (1), in which 
     the setting section sets, as the voltage of the power line, voltage at which conversion efficiency is most favorable in a conversion device on the power reception side. 
     (4) 
     The power control apparatus according to (1), in which 
     the setting section sets, as the voltage of the power line, voltage at which conversion efficiency is most favorable in a conversion device on the power transmission side. 
     (5) 
     The power control apparatus according to any of (1) to (4), in which 
     the conversion device is a DC-DC converter. 
     (6) 
     The power control apparatus according to any of (1) to (4), in which 
     the conversion device is an AC-DC converter. 
     (7) 
     The power control apparatus according to any of (1) to (6), in which 
     the power line is a bus line. 
     (8) 
     A power control apparatus including: 
     an acquisition section configured to acquire information from a node on a power transmission side which transmits power through a power line, the information pertaining to a characteristic of a conversion device that converts voltage between the power line and a storage battery on the power transmission side; and 
     a selection section configured to use the information acquired by the acquisition section and a characteristic of a conversion device that converts voltage between the power line and a storage battery on a power transmission side to select a power transmission source. 
     (9) 
     The power control apparatus according to (8), in which 
     the acquisition section acquires the information of a node that responds to a power transmission request of power, the information pertaining to the characteristic of the conversion device. 
     (10) 
     The power control apparatus according to (8) or (9), in which 
     the selection section selects, as a power transmission source, a node in which a maximum value of an average value of conversion efficiency in each conversion device becomes highest. 
     (11) 
     The power control apparatus according to any of (8) to (10), in which 
     the conversion device is a DC-DC converter. 
     (12) 
     The power control apparatus according to any of (8) to (10), in which 
     the conversion device is an AC-DC converter. 
     (13) 
     The power control apparatus according to any of (8) to (12), in which 
     the power line is a bus line. 
     (14) 
     A power control method including: 
     acquiring information from a node on a power reception side which receives power through a power line, the information pertaining to a characteristic of a conversion device that converts voltage between the power line and a storage battery on the power reception side; and 
     using the acquired information and a characteristic of a conversion device that converts voltage between the power line and a storage battery on a power transmission side to set voltage of the power line. 
     (15) 
     A power control method including: 
     acquiring information from a node on a power transmission side which transmits power through a power line, the information pertaining to a characteristic of a conversion device that converts voltage between the power line and a storage battery on the power transmission side; and 
     using the acquired information and a characteristic of a conversion device that converts voltage between the power line and a storage battery on a power transmission side to select a power transmission source. 
     REFERENCE SIGNS LIST 
     
         
           1  power supply system 
           10  node 
           20  communication line 
           30  bus line