Patent Publication Number: US-2013241298-A1

Title: Voltage converter, voltage converting method, power adjuster, power adjusting method, photovoltaic system, and management device

Description:
TECHNICAL FIELD 
     The present invention relates to a voltage converter, a voltage converting method, a power adjuster, a power adjusting method, a photovoltaic system, and a management device, and particularly relates to a voltage converter, a voltage converting method, a power adjuster, a power adjusting method, a photovoltaic system, and a management device which are made capable of obtaining a higher output. 
     BACKGROUND ART 
     In recent years, from a viewpoint of the global environment such as reduction in emission of carbon dioxide, there has been promoted spreading of a photovoltaic system that generates power by means of a solar battery. 
     With reference to  FIG. 16 , a configuration of the photovoltaic system will be described. 
     On the lower left of  FIG. 16 , a solar battery cell  100  as a minimum unit of the configuration of a solar battery is shown, and the solar battery cell  100  generates power by a photoelectric effect due to reception of irradiation with sunlight. 
     Further, a plurality of solar battery cells  100  are connected in series, to constitute a solar battery cluster  102 . In the example shown in  FIG. 16 , the solar battery cluster  102  includes six solar battery cells  100   1  to  100   6 , and the solar battery cells  100   1  and  100   6  at both ends thereof are connected via a bypass diode  101 . 
     A plurality of solar battery clusters  102  are connected in series, to constitute a solar battery module (panel)  104 . In the example shown in  FIG. 16 , the solar battery module  104  includes three solar battery clusters  102   1  to  102   3 , and bypass diodes  101   1  to  101   3  provided in the respective solar battery clusters  102   1  to  102   3  are accommodated in a terminal box  103 . 
     Further, a plurality of solar battery modules  104  are connected in series, to constitute a solar battery string  105 . In the example shown in  FIG. 16 , the solar battery string  105  includes three solar battery modules  104   1  to  104   3 . 
     Further, a plurality of solar battery strings  105  are connected in parallel, to constitute a solar battery array  106 . In the example shown in  FIG. 16 , the solar battery array  106  includes four solar battery strings  105   1  to  105   4 . The solar battery strings  105   1  to  105   4  are connected in a connection box  107 , and thereafter connected to a power conditioner  108 . 
     The power conditioner  108  converts direct-current power outputted from the solar battery array  106  to alternating-current power and supplies it to a load  109  or returns it to a commercial power system  110  provided by a power company. Further, the power conditioner  108  has a function of performing such control as to obtain a maximum output from the solar battery array  106  based on maximum power point tracking (MPPT) control. 
     In the photovoltaic system as thus configured, it has been desired to more efficiently convert energy from sunlight to power, and a variety of techniques have been developed. For example, there has been disclosed a technique in which a DC/DC converter is provided in each solar battery array, and based on detection results of a voltage and a current of power that is outputted from the solar battery array, maximum power point tracking control is performed by the DC/DC converter while the output from the solar battery array is held in a direct-current state (refer to Patent Document 1). 
     There has further been developed a technique in which the maximum power point tracking control is performed in units of the solar battery string or the solar battery module. 
     PRIOR ART DOCUMENT 
     Patent Document 
     Patent Document 1: Japanese Unexamined Patent Publication No. 2000-112545 
     SUMMARY OF THE INVENTION 
     Problems to be Solved by the Invention 
     Incidentally, for example in such a photovoltaic system as to provide the DC/DC converter with respect to each solar battery module and perform the maximum power point tracking control, a voltage and a current of power that is outputted from each solar battery module fluctuate. In the foregoing photovoltaic system, by simply combining conventional power conditioners, the power conditioners are also controlled so as to obtain maximum power, thus causing oscillation (wobbling) of the voltage and the current of the power in the photovoltaic system as a whole, and an amount of power generation is not necessarily reliably improved. Accordingly, in the photovoltaic system provided with the DC/DC converter with respect to each solar battery module, it is necessary to perform optimum control so as to stabilize an output characteristic of the DC/DC converter and obtain a higher output. 
     The present invention was made in light of such circumstances, and serves to obtain a higher output. 
     Means for Solving the Problem 
     A voltage converter of the present invention includes: a conversion processing unit configured to execute a process to convert a voltage of power generated from sunlight; and a switching unit configured to switch an operation of the conversion processing unit to either conversion-rate fixing or maximum power point tracking control at least based on an operation status of the conversion processing unit. 
     A voltage converting method of the present invention includes the steps of: executing a conversion process for executing a process to convert a voltage of power generated from sunlight; and switching an operation of the conversion process to either conversion-rate fixing or maximum power point tracking control at least based on an operation status of the conversion process. 
     In such a configuration, the operation of the conversion process is switched to either conversion-rate fixing or maximum power point tracking control based on the operation status of the conversion process for executing a process to convert a voltage of power generated from sunlight, and hence it is possible to generate power with a higher output. 
     Moreover, the voltage converter of the present invention can further include: an operation status acquiring unit configured to communicate with another voltage converter to acquire an operation status of conversion processing unit included in the another voltage converter; and a comparison unit configured to compare an amount of output power which is the operation status acquired by the operation status acquiring unit with an amount of output power which is the operation status of the conversion processing unit to decide whether or not to fix a conversion rate of the voltage in the conversion processing unit, and notifying the switching unit of the determination. 
     In such a configuration, communication is made with another voltage converter, to acquire an operation status of the conversion processing unit included in the voltage converter; and the amount of output power as the acquired operation status is compared with the amount of output power as the operation status of the conversion processing unit, to determine whether or not to fix a conversion rate of the voltage in the voltage converter, whereby it is possible to fix a conversion rate of the conversion processing means whose output is maximum, so as to obtain a higher output. 
     A power adjuster of the present invention includes: a conversion unit configured to convert direct-current power, outputted from each of a plurality of solar battery modules under such control as to acquire a maximum power, to alternating-current power; and a voltage control unit configured to perform control such that the direct-current power that is inputted into the conversion unit has a predetermined given voltage. 
     A power adjusting method of the present invention includes the steps of: converting direct-current power, outputted from each of a plurality of solar battery modules under such control as to acquire a maximum power, to alternating-current power; and performing control such that the direct-current power has a predetermined given voltage. 
     In such a configuration, at the time of converting direct-current power, outputted from each of the plurality of solar battery modules under such control as to acquire the maximum power, to alternating-current power, control is performed such that the inputted direct-current power has a predetermined given voltage, thereby stabilizing power generation, and hence it is possible to perform power generation with a high output. 
     Moreover, the power adjuster of the present invention can further include: a direct-current voltage converting unit configured to convert a voltage of generated power for each of the plurality of solar battery modules; and a voltage deciding unit configured to decide the predetermined given voltage based on a conversion loss in the direct-current voltage converting unit and a conversion loss in the conversion unit. 
     In such a configuration, the predetermined given voltage is decided based on the conversion loss at the time of converting the voltage in the plurality of solar battery modules and based on the conversion loss at the time of converting direct-current power to alternating-current power, and hence it is possible to further increase the amount of power generation. 
     Moreover, a photovoltaic system of the present invention includes a voltage converter for converting a voltage of power generated from sunlight being provided with respect to each solar battery module, and the voltage converter has a conversion processing unit configured to execute a process to convert the voltage, and a switching unit configured to switch an operation of the conversion processing unit to either conversion-rate fixing or maximum power point tracking control at least based on an operation status of the conversion processing unit. 
     In such a configuration, the voltage converter for converting the voltage of power generated from sunlight is provided with respect to each solar battery module, and an operation of the conversion process is switched to either conversion-rate fixing or maximum power point tracking control based on an operation status of the conversion process for executing a process to convert a voltage, whereby it is possible to generate power with a higher output. 
     A photovoltaic system of the present invention includes: a plurality of solar battery modules; a direct-current voltage converting unit which is provided with respect to each of the plurality of solar battery modules and converts a voltage of generated power; a conversion unit configured to convert direct-current power, outputted from each of the plurality of solar battery modules under such control as to acquire a maximum power, to alternating-current power; and a voltage control unit configured to perform control such that the direct-current power that is inputted into the conversion unit has a predetermined given voltage. 
     In such a configuration, at the time of converting direct-current power, outputted from each of the solar battery modules under such control as to acquire the maximum power, to alternating-current power, control is performed such that the inputted direct-current power has a predetermined given voltage, thereby stabilizing power generation, and hence it is possible to perform power generation with a high output. 
     Moreover, a management device of the present invention includes: an operation status acquiring unit configured to communicate with a plurality of voltage converters each having a conversion processing unit configured to execute a process to convert a voltage of power generated from sunlight and a switching unit configured to switch an operation of the conversion processing unit to either conversion-rate fixing or maximum power point tracking control at least based on an operation status of the conversion processing unit, to acquire operation statuses of the conversion processing unit included in all of the voltage converters; and a deciding unit configured to compare amounts of output power which are the operation statuses acquired by the operation status acquiring unit, to decide the voltage converter to be fixed with a voltage conversion rate in the conversion processing unit. 
     In such a configuration, communication is made with the plurality of voltage converters to acquire operation statuses of the conversion processing units included in all the voltage converters, and amounts of output power as the acquired operation statuses are compared, to decide the voltage converter to be fixed with the voltage conversion rate in the conversion processing means, whereby it is possible to obtain a higher output. 
     A photovoltaic system of the present invention includes: a plurality of solar battery modules; a direct-current voltage converting unit which is provided with respect to each of the plurality of solar battery modules and converts a voltage of generated power; a conversion unit configured to convert direct-current power, outputted from each of the plurality of solar battery modules under such control as to acquire a maximum power, to alternating-current power; and a voltage control unit configured to perform control such that the direct-current power that is inputted into the conversion unit has a predetermined given voltage. 
     In such a configuration, at the time of converting direct-current power, outputted from each of the solar battery modules under such control as to acquire the maximum power, to alternating-current power, control is performed such that the inputted direct-current power has a predetermined given voltage, thereby stabilizing power generation, and hence it is possible to perform power generation with a high output. 
     Effect of the Invention 
     According to the voltage converter, the voltage converting method, the power adjuster, the power adjusting method, the photovoltaic system, and the management device of the present invention, it is possible to obtain a higher output. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram showing an example of a configuration of one embodiment of a photovoltaic system, to which the present invention has been applied. 
         FIG. 2  is a block diagram showing an example of a configuration of an output converter. 
         FIG. 3  is a block diagram showing an example of a configuration of a control part. 
         FIG. 4  is a flowchart illustrating a process for switching an operation of a DC/DC converting part. 
         FIG. 5  is a block diagram showing another constitutional example of the control part. 
         FIG. 6  is a flowchart illustrating a process in the control part. 
         FIG. 7  is a block diagram showing an example of a configuration of a power conditioner. 
         FIG. 8  is a flowchart illustrating a process for holding a voltage, accepted by a power conditioner, constant. 
         FIG. 9  is a block diagram showing an example of a configuration of another embodiment of the photovoltaic system, to which the present invention has been applied. 
         FIG. 10  is a block diagram showing an example of a configuration of a management unit. 
         FIG. 11  is a flowchart illustrating a process for designating a change in operation to the output converter. 
         FIG. 12  is a block diagram showing another constitutional example of the management unit. 
         FIG. 13  is a diagram showing a table where a duty value is associated with conversion efficiency. 
         FIG. 14  is a diagram showing a table where a voltage value of an input voltage is associated with conversion efficiency. 
         FIG. 15  is a flowchart illustrating a process for setting a reference voltage value in a power conditioner. 
         FIG. 16  is a view illustrating a configuration of a photovoltaic system. 
     
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     Hereinafter, a specific embodiment, to which the present invention has applied, will be described in detail with reference to the drawings. 
       FIG. 1  is a block diagram showing an example of a configuration of one embodiment of a photovoltaic system, to which the present invention has applied. It is to be noted that in the present specification, the system is one that represents the whole apparatus including a plurality of devices. 
     In  FIG. 1 , a photovoltaic system  11  is configured by connection of a power conditioner  12  with a solar battery string  13 , and power generated in the solar battery string  13  and converted to an alternating-current one in the power conditioner  12  is supplied to a load and a power system (see  FIG. 16 ). It is to be noted that, although the photovoltaic system  11  of  FIG. 1  is configured such that one solar battery string  13  is connected to the power conditioner  12 , a plurality of solar battery strings are connected in parallel to the power conditioner  12  as described with reference to  FIG. 16 , and illustration of those are omitted. 
     The power conditioner  12  is a power adjuster that adjusts power outputted from the solar battery string  13  such that the power can be supplied to a load, not shown, and outputs the power. 
     In the solar battery string  13 , eight output converters  21 - 1  to  21 - 8  are connected in series, and solar battery modules  22 - 1  to  22 - 8  are respectively connected to the output converters  21 - 1  to  21 - 8 . Further, the output converter  21 - 1  and the output converter  21 - 8  connected in series and located at both ends are connected to the power conditioner  12 . It is to be noted that the output converters  21 - 1  to  21 - 8  are each configured in a similar manner, and will hereinafter be referred to as an output converter  21  as appropriate when there is no need for distinguishing among the output converters  21 - 1  to  21 - 8 , and this also applies to internal configurations of the output converters  21 - 1  to  21 - 8  as described later with reference to  FIG. 2 . Further, the solar battery modules  22 - 1  to  22 - 8  will similarly be referred to as a solar battery module  22 . 
     In such a manner, the photovoltaic system  11  is configured such that the output converter  21  is provided with respect to each solar battery module  22 . Direct-current power, outputted from the solar battery module  22  in accordance with irradiation with sunlight, is converted to direct-current power with a predetermined voltage by the output converter  21  as the voltage converter, and is then supplied to the power conditioner  12 . 
     The configuration of the output converter  21  will be described with reference to  FIG. 2 .  FIG. 2  shows part of the solar battery string  13  (output converters  21 - 1  to  21 - 3 , solar battery modules  22 - 1  to  22 - 3 ). 
     As shown in  FIG. 2 , the output converter  21  includes a DC/DC converting part  31 , a voltage detecting part  32 , a current detecting part  33 , a power line communication part  34 , and a control part  35 . 
     The solar battery module  22  is connected to an input-side terminal of the DC/DC converting part  31  (conversion processing means, direct-current voltage converting means), direct-current power generated by the solar battery module  22  is supplied to the DC/DC converting part  31 , and the DC/DC converting part  31  converts a voltage of the power outputted from the solar battery module  22  to a voltage with a conversion rate in accordance with control of the control part  35 . Further, a power line directly or indirectly connected to the power conditioner  12  is connected to an output-side terminal of the DC/DC converting part  31 , and the DC/DC converting part  31  outputs power of the voltage after conversion to the power line. 
     The voltage detecting part  32  detects a voltage of the power supplied from the solar battery module  22  to the DC/DC converting part  31 , namely a voltage between two wires connecting the solar battery module  22  and the DC/DC converting part  31 , and supplies a signal indicating the voltage value to the control part  35 . 
     The current detecting part  33  detects a current of the power supplied from the solar battery module  22  to the DC/DC converting part  31 , and a signal indicating the current value is supplied to the control part  35 . 
     For example, the current detecting part  33  can measure a voltage at each end of a resistor (not shown) arranged on a wire connecting the solar battery module  22  and the DC/DC converting part  31 , thereby to obtain a current. 
     The power line communication part  34  is a communication part for communicating with the control part  35  of another output converter  21  via the power line for use in supplying power to the power conditioner  12 . It is to be noted that, although communication is performed by means of power-line communication in the present embodiment, communication may be performed by means other than the power-line communication. 
     Signals indicating a voltage value and a current value of power that is supplied to the DC/DC converting part  31  are supplied respectively from the voltage detecting part  32  and the current detecting part  33  to the control part  35 . Further, the control part  35  is connected with an output-side power line of the DC/DC converting part  31 , and the control part  35  measures the voltage and the current of the power that is outputted from the DC/DC converting part  31 . Based on the voltage and the current of the power that is inputted into the DC/DC converting part  31  and the voltage and the current of the power that is outputted from the DC/DC converting part  31 , the control part  35  searches a duty value with which an output of the DC/DC converting part  31  is maximum, to perform control (maximum operating point control) on the DC/DC converting part  31  so as to be operated with that duty value. That is, the control part  35  is a maximum power control means. Further, the control part  35  communicates with the control part  35  of another output converter  21  via the power line communication part  34  according to the need. 
     The output converter  21  is configured as thus described, and in each of the output converters  21 - 1  to  21 - 8  (see  FIG. 1 ), control is performed so as to obtain a maximum output. For this reason, power with a different voltage is outputted from each of the output converters  21 - 1  to  21 - 8 , and the power conditioner  12  is supplied with power with a voltage value obtained by adding those voltages. 
     In such a configuration where the maximum operating point control is performed in each of the output converters  21 - 1  to  21 - 8 , when the maximum operating point control is also performed in the power conditioner  12 , power oscillates in the photovoltaic system as a whole, which destabilizes power generation. 
     Thereat, in the photovoltaic system  11 , the control parts  35  of the output converters  21 - 1  to  21 - 8  are each communicated with one another, and in the output converter  21  whose amount of output power (amount of power generation) is maximum, a process for fixing a conversion rate of the DC/DC converting part  31  to a given rate (or setting a duty value to  100 ) is performed in each of the solar battery strings  13 . Thereby, an output characteristic of the output converter  21  with the conversion rate of the DC/DC converting part  31  held fixed becomes a reference for making output characteristics of another output converters  21  converge, thereby stabilizing the operations of the output converters  21 - 1  to  21 - 8  and the power conditioner  12 . 
       FIG. 3  is a block diagram showing an example of a configuration of the control part  35  where mutual communication is performed to allow switching of operation of the DC/DC converting part  31 . 
     In  FIG. 3 , the control part  35  is configured to include a reception part  41 , a transmission part  42 , a timer  43 , a detection part  44 , a declaration determining part  45 , a comparison part  46 , and a command part  47 . 
     The reception part  41  receives a signal transmitted from the control part  35  of another output converter  21  via the power line communication part  34  ( FIG. 2 ). For example, when the control part  35  of another output converter  21  transmits a signal declaring an amount of output power of the output converter  21 , the reception part  41  receives the signal, supplies it to the comparison part  46 , and notifies the declaration determining part  45  of the declaration. That is, the reception part  41  is operation status acquiring means for acquiring an amount of output power which is transmitted from the control part  35  of another output converter  21  and represents one of operation statuses of the DC/DC converting part  31  included in the another output converter  21 . 
     The transmission part  42  transmits a signal to the control part  35  of another output converter  21  via the power line communication part  34 . For example, when supplied with an amount of output power of the own output converter  21  from the declaration determining part  45 , the transmission part  42  transmits a signal indicating information including the amount of output power and an identification number of the own output converter  21 , to declare the amount of output power. 
     The timer  43  performs timekeeping, and when it is time to declare the amount of output power, the timer  43  notifies the declaration determining part  45  that it has been such time. For example, in the photovoltaic system  11 , the time for declaring the amount of output power among the output converters  21 - 1  to  21 - 8  has been set to every ten seconds, every one minute, or the like. 
     The detection part  44  acquires a voltage value and a current value of power that is inputted into the DC/DC converting part  31  via the voltage detecting part  32  and the current detecting part  33  of  FIG. 2 , and also acquires a voltage value and a current value of power that is outputted from the DC/DC converting part  31  via the power line connected to the output side of the DC/DC converting part  31 . Then, the detection part  44  supplies the command part  47  with the voltage value and the current value of the power that is inputted into the DC/DC converting part  31  and the voltage value and the current value of the power that is outputted from the DC/DC converting part  31 . 
     Further, when the power that is inputted into the DC/DC converting part  31  abruptly changes (abruptly rises and abruptly falls), the detection part  44  notifies the declaration determining part  45  of the abrupt change in power that is inputted into the DC/DC converting part  31 . Moreover, the detection part  44  notifies the declaration determining part  45  of the amount of output power of the DC/DC converting part  31  in accordance with a request from the declaration determining part  45 . 
     The declaration determining part  45  determines whether or not to declare the amount of output power of the output converter  21 . For example, the declaration determining part  45  determines to declare the amount of output power when notified from the timer  43  that it has been time to declare the amount of output power. 
     Further, the declaration determining part  45  also determines to declare the amount of output power when notified from the reception part  41  that the amount of output power has been declared from another output converter  21 , or when notified from the detection part  44  that the power that is inputted into the DC/DC converting part  31  has abruptly changed. 
     Then, when determining to declare the amount of output power, the declaration determining part  45  acquires the amount of output power of the DC/DC converting part  31  via the detection part  44 , supplies it to the transmission part  42 , and transmits a signal to declare the amount of output power of the DC/DC converting part  31  to the control part  35  of another output converter  21 . Further, the declaration determining part  45  supplies the acquired amount of output power of the DC/DC converting part  31  to the comparison part  46 . 
     The amount of output power of the DC/DC converting part  31  of the output converter  21 , transmitted from the control part  35  of another output converter  21 , is supplied to the comparison part  46  from the reception part  41 . The comparison part  46  is comparison means for comparing the amount of output power of the own DC/DC converting part  31  which is supplied from the declaration determining part  45  with the amount of output power of the DC/DC converting part  31  of another output converter  21  which is supplied from the reception part  41 . When the amount of output power of the own DC/DC converting part  31  is maximum as a result of the comparison, the comparison part  46  notifies the command part  47  that a conversion rate in the own DC/DC converting part  31  will be fixed, and switches the operation. It is to be noted that, when the conversion rate of the DC/DC converting part  31  has already been fixed, the comparison part  46  does not output that command, and the operation with the conversion rate of the DC/DC converting part  31  held fixed is continued. 
     On the other hand, when the amount of output power of the own DC/DC converting part  31  is not maximum as the result of the comparison, namely, when the DC/DC converting part  31  of another output converter  21  has an amount of output power larger than that of the own DC/DC converting part  31 , the comparison part  46  notifies the command part  47  that the maximum power point tracking control will be performed in the own DC/DC converting part  31 , and switches the operation. It is to be noted that, when the maximum power point tracking control has already been performed in the DC/DC converting part  31 , the comparison part  46  does not output that command, and the maximum power point tracking control is continued in the DC/DC converting part  31 . 
     The command part  47  outputs a command for designating an appropriate conversion rate for the DC/DC converting part  31  to perform an output in accordance with the maximum power point tracking control, or outputs a command for fixing the conversion rate of the DC/DC converting part  31 . That is, the command part  47  is switching means for switching the operation of the DC/DC converting part  31  to either the conversion-rate fixing or the maximum power point tracking control in accordance with the notification from the comparison part  46 . 
     For example, in the case of performing the maximum power point tracking control in the DC/DC converting part  31 , the command part  47  outputs a command for fixing the conversion rate of the DC/DC converting part  31  when it is notified from the comparison part  46  that the conversion rate will be fixed. On the other hand, in the case of the conversion rate being fixed in the DC/DC converting part  31 , when it is notified from the comparison part  46  that the maximum power point tracking control will be performed, the command part  47  starts outputting to the DC/DC converting part  31  a command for designating an appropriate conversion rate for performing an output in accordance with the maximum power point tracking control based on a voltage value and a current value detected by the detection part  44 . 
       FIG. 4  is a flowchart illustrating a process of the control part  35  for switching an operation of the DC/DC converting part  31 . 
     For example, when an amount of power generation, not smaller than an amount of power required for conversion by the DC/DC converting part  31 , is outputted from the solar battery module  22  in accordance with irradiation with sunlight, the process is started, and in Step S 11 , the declaration determining part  45  determines whether or not it has been time to declare the amount of output power of the DC/DC converting part  31  in accordance with the timer  43 . For example, the timer  43  counts the time from previous declaration, and when the time to declare the amount of output power elapses, the timer  43  notifies the declaration determining part  45  of such. 
     When it is determined in Step S 11  that it has not been time to declare the amount of output power of the DC/DC converting part  31 , the process goes to Step S 12 , and the declaration determining part  45  determines whether or not power that is inputted into the DC/DC converting part  31  has abruptly changed. For example, when notified from the detection part  44  that the power that is inputted into the DC/DC converting part  31  has abruptly changed, the declaration determining part  45  determines that the power that is inputted into the DC/DC converting part  31  has abruptly changed. 
     When it is determined in Step S 12  that the power that is inputted into the DC/DC converting part  31  has not abruptly changed, the process goes to Step S 13 , and the declaration determining part  45  determines whether or not an amount of output power has been declared from another output converter  21 . For example, in a case where the power that is inputted into the DC/DC converting part  31  has abruptly changed in another output converter  21 , the amount of output power is declared from that output converter  21 , and when the reception part  41  notifies the declaration determining part  45  of the declaration, the declaration determining part  45  determines that the amount of output power has been declared from another output converter  21 . 
     In Step S 13 , when the declaration determining part  45  determines that the amount of output power has not been declared from another output converter  21 , the process returns to Step S 11 , and thereafter, the same process is repeated. On the other hand, the process goes to Step S 14  when it is determined in Step S 11  that it has been time to declare the amount of output power of the DC/DC converting part  31 , or when it is determined in Step S 12  that the power that is inputted into the DC/DC converting part  31  has abruptly changed, or when it is determined in Step S 13  that the amount of output power has been declared from another output converter  21 . 
     In Step S 14 , the declaration determining part  45  acquires the voltage value and the current value of the power that is outputted from the DC/DC converting part  31  via the detection part  44 , to obtain the amount of output power of the DC/DC converting part  31 , and notifies the comparison part  46  of the amount of output power. Further, the declaration determining part  45  supplies the obtained amount of output power to the transmission part  42 , and the transmission part  42  transmits a signal indicating information that includes the amount of output power and an identification number of the own output converter  21  to all of the other output converters  21  constituting the solar battery string  13 , to notify them of the amount of output power. 
     In Step S 15 , the reception part  41  acquires information declaring the amount of output power, which is transmitted from the control part  35  of another output converter  21 , and supplies the amount of output power of the DC/DC converting part  31  of another output converter  21  to the comparison part  46 , and the process goes to Step S 16 . 
     In Step S 16 , the comparison part  46  compares the amount of output power of the DC/DC converting part  31  of another output converter  21  which was supplied in Step S 15  with the amount of output power of the own DC/DC converting part  31  which was obtained by the declaration determining part  45  in Step S 14 . 
     After the process of Step S 16 , the process goes to Step S 17 , and as a result of the comparison in Step S 16 , the comparison part  46  determines whether or not the amount of output power of the own DC/DC converting part  31  is maximum. 
     When it is determined in Step S 17  that the amount of output power of the own DC/DC converting part  31  is maximum, the process goes to Step S 18 , and the comparison part  46  notifies the command part  47  that the conversion rate in the DC/DC converting part  31  will be fixed. Accordingly, the command part  47  outputs to the DC/DC converting part  31  a command for fixing the conversion rate. 
     On the other hand, when it is determined in Step S 17  that the amount of output power of the own DC/DC converting part  31  is not maximum, the process goes to Step S 19 , and the comparison part  46  notifies the command part  47  that the maximum power point tracking control will be performed. Accordingly, the command part  47  outputs a command for designating an appropriate conversion rate for performing an output in accordance with the maximum power point tracking control. 
     After the processes of Steps S 18  and S 19 , the process returns to Step S 11 , and thereafter, the same process is repeated. It is to be noted that, for example, when an amount of irradiation of the solar battery module  22  with sunlight decreases and becomes not higher than an amount of power convertible by the DC/DC converting part  31 , the process is completed. 
     As thus described, since the conversion rate is fixed in the output converter  21  where the amount of output power of the DC/DC converting part  31  is maximum, an output characteristic of that output converter  21  becomes a convergence reference, thus stabilizing as a whole a voltage and a current of outputted power in the output converters  21 - 1  to  21 - 8 . This allows an increase in amount of power generation in the photovoltaic system  11  as a whole. That is, the photovoltaic system  11  can generate power with a higher output. 
     It is to be noted that, although the conversion rate is fixed only in the output converter  21  where the amount of output power of the DC/DC converting part  31  is maximum in the flowchart of  FIG. 4 , the conversion rate may be fixed not only in the one output converter  21  but in a plurality of output converters  21  having similar output characteristics. For example, there may be fixed conversion rates of a high-level group of output converters  21  each of whose DC/DC converting parts  31  have a large amount of output power, namely a predetermined number (e.g., three) of output converters  21  whose DC/DC converting parts  31  have largest amounts of output power, or there may be fixed conversion rates of the output converters  21  each of whose DC/DC converting parts  31  have an amount of output power within several percent from the maximum value. By fixing the conversion rate in a plurality of output converters  21  with similar output characteristics as thus described, it is possible to further stabilize the output converters  21 - 1  to  21 - 8  as a whole, and also reduce the time taken until the stabilization. 
     For example,  FIG. 5  is a block diagram showing another constitutional example of the control part where mutual communication is performed to allow switching of operation of the DC/DC converting part  31 . 
     In  FIG. 5 , a control part  35 ′ is configured to include the reception part  41 , the transmission part  42 , the detection part  44 , the command part  47  and an operation determining part  48 . The control part  35 ′ is common with the control part  35  of  FIG. 3  in including the reception part  41 , the transmission part  42 , the detection part  44 , and the command part  47 , and specific descriptions thereof will be omitted. 
     The operation determining part  48  constantly calculates a duty value of the DC/DC converting part  31  based on a voltage value and a current value of power that is outputted from the DC/DC converting part  31  which are supplied from the detection part  44 , and a maximum amount of output power of the solar battery module  22  which was set at the time of designing, and the operation determining part  48  performs a process to fix the conversion rate of the DC/DC converting part  31  when the duty value is not smaller than a reference value (e.g., 90) for a period of time not shorter than a given reference time. At this time, the operation determining part  48  notifies another output converter  21  via the transmission part  42  that the conversion rate will be fixed, and when the reception part  41  receives a response indicating reception of that notification, the operation determining part  48  notifies the command part  47  that the conversion rate in the DC/DC converting part  31  will be fixed. That is, the operation determining part  48  is determination means for determining whether or not to fix the conversion rate of the DC/DC converting part  31  based on the duty value of the DC/DC converting part  31  and the given reference time, and the transmission part  42  is notification means for notifying another output converter  21  of such. Further, the period when the duty value of the DC/DC converting part  31  is not smaller than the reference value is one of operation statuses of the DC/DC converting part  31  in the output converter  21 . 
     Further, when the reception part  41  receives a notification that the conversion rate will be fixed from the control part  35  of another output converter  21  at the time of the conversion rate of the own DC/DC converting part  31  being fixed, the operation determining part  48  makes a response transmitted which indicates reception of that notification, and also notifies the command part  47  that the maximum power point tracking control will be performed in the own DC/DC converting part  31 . 
       FIG. 6  is a flowchart illustrating a process in the control part  35 ′. 
     For example, when the solar battery module  22  outputs power in amount not smaller than a predetermined minimum amount of power generation in accordance with irradiation with sunlight, the process is started, and in Step S 21 , the operation determining part  48  determines whether or not to fix the conversion rate of the own DC/DC converting part  31 . For example, the operation determining part  48  determines to fix the conversion rate of the own DC/DC converting part  31  when a period of the duty value of the DC/DC converting part  31  being not smaller than the reference value (e.g., 90) is not shorter than the given reference time. 
     In Step S 21 , the process is held until the operation determining part  48  determines to fix the conversion rate of the own DC/DC converting part  31 , and the process goes to Step S 22  when the operation determining part  48  determines to fix the conversion rate of the own DC/DC converting part  31 . 
     In Step S 22 , the operation determining part  48  notifies the transmission part  42  that the conversion rate will be fixed, and the transmission part  42  transmits a notification that the conversion rate will be fixed, including an identification number of the own output converter  21 , to all of the other output converters  21  constituting the solar battery string  13 . 
     After the process of Step S 22 , the process goes to Step S 23 , and the reception part  41  receives a response, transmitted in response to the notification transmitted in Step S 22 , and supplies it to the operation determining part  48 . Then, when the responses from all of the other output converters  21  are supplied to the operation determining part  48 , the process goes to Step S 24 . 
     In Step S 24 , the operation determining part  48  notifies the command part  47  that the conversion rate in the DC/DC converting part  31  will be fixed. Accordingly, the command part  47  outputs to the DC/DC converting part  31  a command for fixing the conversion rate, and the process goes to Step S 25 . 
     In Step S 25 , the operation determining part  48  transmits a notification that the control part  35  of another output converter  21  will fix the conversion rate, to determine whether or not the reception part  41  has received the notification, and holds the process until determining that the reception part  41  has received the notification. 
     In Step S 25 , when it is determined that the reception part  41  has received the notification, the process goes to Step S 26 , and the operation determining part  48  transmits a response to that notification via the transmission part  42 , and the process goes to Step S 27 . 
     In Step S 27 , the operation determining part  48  notifies the command part  47  that the maximum power point tracking control will be performed. Accordingly, the command part  47  starts outputting a command for designating an appropriate conversion rate for performing an output in accordance with the maximum power point tracking control, and the process returns to Step S 21 , whereafter the same process is repeated. 
     By such a process as thus described, the conversion rate is fixed in the output converter  21  where the amount of output power of the DC/DC converting part  31  is maximum, and an output characteristic of the output converter  21  is taken as a convergence reference, thereby stabilizing a voltage and a current of outputted power in the output converters  21 - 1  to  21 - 8  as a whole, and it is thus possible to increase the amount of power generation. 
     Further, after reception of the responses transmitted from all of the other output converters  21 , the process is performed so as to fix the conversion rate in the DC/DC converting part  31 , whereby it is possible to confirm that communication has been reliably performed, so as to perform a more stable process. 
     It is to be noted that the processes for fixing the conversion rates are performed substantially simultaneously in a plurality of DC/DC converting parts  31 , and when those operations coincide, fixing and fluctuation of the conversion rate are repeated. Hence, for example, even when it is notified from another output converter  21  that the conversion rate will be fixed, the conversion rate of the own DC/DC converting part  31  is held fixed within a given time (e.g., the order of several seconds) from fixing of the conversion rate of the DC/DC converting part  31 , whereby it is possible to avoid such repetition as above. 
     Further, other than determining whether or not to fix the conversion rate based on the duty value of the DC/DC converting part  31 , for example, the operation determining part  48  may perform determination based on a voltage ratio or a current ratio of the input/output of the DC/DC converting part  31  or may perform determination based on whether or not a conversion loss in the DC/DC converting part  31  is not smaller than a given value. Moreover, fixing of the conversion rate, for example, includes setting the duty value to a given value not smaller than 90. 
     Incidentally, in the photovoltaic system  11 , the power conditioner  12  can be configured so as to receive power at a voltage value (hereinafter referred to as reference voltage value as appropriate) previously set as a reference in order to avoid destabilization of power generation. Thereby, the voltage value obtained by adding voltages of power outputted from the respective output converters  21 - 1  to  21 - 8  constituting the solar battery string  13  converge to the reference voltage value set in the power conditioner  12 . 
     An example of a configuration of the power conditioner  12  will be described with reference to  FIG. 7 . 
     In  FIG. 7 , the power conditioner  12  is configured to include a DC/AC converter  51 , a voltage detecting part  52 , a current adjusting part  53  and a control part  54 . 
     In accordance with control of the control part  54 , the DC/AC converter  51  is conversion means for converting direct-current power, which is supplied from the solar battery string  13  to the power conditioner  12 , to alternating-current power and outputting the power. The voltage detecting part  52  detects a voltage of the power supplied to the power conditioner  12 , and supplies the control part  54  with a signal indicating that voltage value. In accordance with control of the control part  54 , the current adjusting part  53  adjusts a current passing through the current adjusting part  53  such that the power that is inputted into the DC/AC converter  51  becomes a reference voltage value. 
     The control part  54  is voltage control means for making the current adjusting part  53  adjust a current such that the power that is inputted into the DC/AC converter  51  maintains the reference voltage value, in accordance with a voltage value detected by the voltage detecting part  52 . For example, when the voltage value detected by the voltage detecting part  52  falls below the reference voltage value, the control part  54  controls the current adjusting part  53  to reduce the current and increase the voltage of the power that is inputted into the DC/AC converter  51  to the reference voltage value. On the other hand, when the voltage value detected by the voltage detecting part  52  exceeds the reference voltage value, the control part  54  controls the current adjusting part  53  to increase the current and decrease the voltage of the power that is inputted into the DC/AC converter  51  to the reference voltage value. 
     It is to be noted that this reference voltage value is, for example, set in the control part  54  in accordance with characteristics of the solar battery module  22  at the time of designing the photovoltaic system  11 . Further, it is set via a terminal connected to the power conditioner  12 , or the like, or set in a wired or wireless manner via a communication part, not shown, in accordance with an actual power generation status at the time of maintenance of the photovoltaic system  11 . For example, the reference voltage value is set as a rated voltage (such as 250 V) with which conversion is efficient in the power conditioner  12 , or as a voltage value proportional to the number of series of the solar battery modules  22 . For example, since eight solar battery modules  22  are connected in series in the example of  FIG. 1 , when a (nominal maximum operation) voltage value of one solar battery module  22  is 25V, 200 V is set as the reference voltage value. 
     Control by the control part  54  to hold the voltage of the power, inputted into the DC/AC converter  51 , constant is periodically performed in each predetermined period, for example. 
       FIG. 8  is a flowchart illustrating a process of the control part  54  for holding a voltage, accepted by the power conditioner  12 , constant. 
     For example, when the voltage value of the power generated by the solar battery string  13  in accordance with irradiation with sunlight becomes not smaller than a predetermined voltage value with which the power conditioner  12  can convert the power to an alternating-current one, the process is started, and in Step S 31 , the control part  54  acquires a voltage value detected by the voltage detecting part  52 . 
     After the process of Step S 31 , the process goes to Step S 32 , and the control part  54  determines whether or not the voltage value acquired from the voltage detecting part  52  in Step S 31  is less than a range of the reference voltage value (range of several percent of the reference voltage value with the reference voltage value at the center). 
     In Step S 32 , when it is determined that the voltage value acquired from the voltage detecting part  52  is less than the range of the reference voltage value, the process goes to Step S 33 . In Step S 33 , the control part  54  controls the current adjusting part  53 , and reduces the current that is inputted into the DC/AC converter  51 , and the process goes to Step S 36 . It is to be noted that, when the current value that passes through the current adjusting part  53  is set to a minimum value, such as at the time of starting the process, the process of Step S 33  is skipped and the process goes to Step S 36 . 
     On the other hand, when it is determined in Step S 32  that the voltage value acquired from the voltage detecting part  52  is not less than the range of the reference voltage value (namely, not smaller than a lower limit of the range of the reference voltage value), the process goes to Step S 34 . 
     In Step S 34 , the control part  54  determines whether or not the voltage value acquired from the voltage detecting part  52  in Step S 31  is not less than the range of the reference voltage value. 
     In Step S 34 , when it is determined that the voltage value acquired from the voltage detecting part  52  is not less than the range of the reference voltage value (namely not smaller than an upper limit of the range of the reference voltage value), the process goes to Step S 35 . In Step S 35 , the control part  54  controls the current adjusting part  53 , and increases the current that is inputted into the DC/AC converter  51 , and the process goes to Step S 36 . 
     On the other hand, when it is determined in Step S 34  that the voltage value acquired from the voltage detecting part  52  is not more than the range of the reference voltage value, the process goes to Step S 36 . That is, in this case, with the process in Step S 32  included, the voltage value acquired from the voltage detecting part  52  is within the range of the reference voltage value. 
     In Step S 36 , after the control part  54  holds the process just for a predetermined period, the process returns to Step S 31 , and thereafter, the same process is repeated. It is to be noted that, for example, when an amount of irradiation of the solar battery module  22  with sunlight decreases and becomes not higher than an amount of power convertible by the power conditioner  12 , the process is completed. 
     As thus described, the control part  54  performs control such that the voltage of the power that is inputted into the DC/AC converter  51  is held within the range of the reference voltage value, and a voltage that is accepted by the power conditioner  12  is held within a given range. 
     Herewith, voltages of power outputted from the output converters  21 - 1  to  21 - 8  which are connected in series converge with the reference voltage of the power conditioner  12  as the reference, thus stabilizing the power generation in the photovoltaic system  11  as a whole. As thus described, since power in maximum amount is stably outputted from each of the output converters  21 - 1  to  21 - 8 , it is possible to increase the amount of power generation in the photovoltaic system  11  as a whole. 
     It is to be noted that a schedule for setting a reference voltage value associated with a time and a date has previously been set in the control part  54 , and in accordance with the schedule, the control part  54  can perform control such that power that is inputted into the DC/AC converter  51  is held at the reference voltage value. That is, for example, respective appropriate reference voltage values are used in morning hours, daytime hours and the like during the day, or respective appropriate reference voltages are used in accordance with seasons, thereby allowing a further increase in the amount of power generation. 
     Incidentally, when such control is performed as to accept power at the reference voltage value in the power conditioner  12 , it is assumed that a voltage is converted at a high conversion rate in the output converter  21 . In that case, as a result of a decrease in the duty value, power conversion efficiency may deteriorate, depending on the output converter  21 . In this case, for example, in order to improve the amount of power generation in the photovoltaic system  11  as a whole, such management as to perform a high-output operation also in the output converter  21  can also be performed in the power conditioner  12 , thereby to avoid a decrease in the amount of power generation. 
       FIG. 9  is a block diagram showing an example of a configuration of another embodiment of the photovoltaic system, to which the present invention has been applied. 
     In  FIG. 9 , the photovoltaic system  11 ′ is configured to include the power conditioner  12 , the solar battery string  13 , and a management unit  14 . It should be noted that the photovoltaic system  11 ′ is configured in a similar manner to the photovoltaic system  11  of  FIG. 1  in being configured by connection of the power conditioner  12  with the solar battery string  13 , and detailed descriptions thereof will be omitted. On the other hand, the photovoltaic system  11 ′ is different from the photovoltaic system  11  of  FIG. 1  by including the management unit  14 . 
     The management unit  14  communicates with the output converters  21 - 1  to  21 - 8  of the solar battery string  13 , and in consideration of the state of each thereof, the management unit  14  designates to switch the process to either the process for performing the maximum power point tracking control or the process for fixing the conversion efficiency. 
     For example, the management unit  14  is connected with the respective control parts  35  ( FIG. 2 ) of the output converters  21 - 1  to  21 - 8  via a signal line, and the control part  35  supplies the management unit  14  with a signal showing a current amount of output power from the DC/DC converting part  31 . It is to be noted that other than that the communication between the management unit  14  and the control part  35  is performed via the signal line, for example, the communication may be performed via a power line to supply power to the power conditioner  12 , or the communication may be performed wirelessly. 
     An example of a configuration of the management unit  14  will be described with reference to  FIG. 10 . 
     In  FIG. 10 , the management unit  14  is configured to include a communication part  61 , a timer  62 , a request determining part  63 , a comparison part  64 , and a command part  65 . 
     The communication part  61  communicates with the respective control parts  35  of the output converters  21 - 1  to  21 - 8 . The communication part  61  acquires amounts of output power of the respective DC/DC converting parts  31  of the output converters  21 - 1  to  21 - 8  by communication with the control part  35 , and supplies them to the comparison part  64 . That is, the communication part  61  is operation status acquiring means for acquiring an amount of output power representing one of operation statuses of the DC/DC converting part  31  included in the output converter. 
     The timer  62  performs timekeeping, and when it is time to make a request for the amount of output power, the timer  62  notifies the request determining part  63  that it has been such time. For example, in the photovoltaic system  11 ′, the time for acquiring the amount of output power from the output converters  21 - 1  to  21 - 8  has been set to every ten seconds, every one minute or the like. 
     The request determining part  63  determines whether or not to make a request to the output converters  21 - 1  to  21 - 8  for the amount of output power, and when making a request for the amount of output power, the request determining part  63  transmits to the communication part  61  a notification of making a request for the amount of output power. For example, when notified by the timer  62  that it has been time to make a request for the amount of output power, the request determining part  63  determines to make a request of the output converters  21 - 1  to  21 - 8  for the amount of output power. Further, when the amount of output power is transmitted from any of the output converters  21 , the request determining part  63  determines to make a request to the output converters  21  other than the output converter  21  having transmitted the amount of output power for the amount of output power. 
     The amounts of output power of all of the output converters  21  which are received by the communication part  61  are supplied to the comparison part  64 , and the comparison part  64  compares those amounts of output power, and when the output converter  21  whose amount of output power is maximum is not the same output converter  21  in the previous process, the comparison part  64  notifies the command part  65  of an identification number (identification number transmitted along with the amount of output power) of the output converter  21  whose amount of output power has newly become maximum. That is, the comparison part  64  is deciding means for deciding the output converter  21  whose amount of output power has newly become maximum as one to fix the conversion rate of the voltage. 
     Next, in accordance with the notification from the comparison part  64 , the command part  65  transmits a command for changing an operation so as to fix the conversion rate to the output converter  21  whose amount of output power has newly become maximum, and also transmits a command for changing an operation so as to perform the maximum power point tracking control to the output converter  21  whose amount of output power was maximum in the previous process. 
       FIG. 11  is a flowchart illustrating a process for designating a change in operation to the output converters  21 - 1  to  21 - 8  by the management unit  14 . 
     In Step S 41 , in accordance with the notification from the timer  62 , the request determining part  63  determines whether or not it has been time to make a request for the amount of output power. 
     When it is determined in Step S 41  that it has not been time to make a request for the amount of output power, the process goes to Step S 42 , and the request determining part  63  determines whether or not the amount of output power has been transmitted from any of the output converters  21 . 
     When the request determining part  63  determines that the amount of output power has not been transmitted in Step S 42 , the process returns to Step S 41 , and thereafter, the same process is repeated. On the other hand, when the request determining part  63  determines that it has been time to make a request for the amount of output power in Step S 41  or when the request determining part  63  determines that the amount of output power has been transmitted in Step S 42 , the process goes to Step S 43 . Herein, other than that the amount of output power is transmitted from the output converter  21  in response to a request from the management unit  14  (after-mentioned Step S 43 ), for example, the amount of output power is transmitted by determination of the output converter  21  itself when it is sensed that the power that is inputted into the DC/DC converting part  31  has abruptly changed. That is, it is determined in Step S 42  that the amount of output power has been transmitted when the output converter  21  transmits the amount of output power in accordance with the abrupt change in power. 
     In Step S 43 , the request determining part  63  transmits to the output converter  21  a notification to make a request for the amount of output power via the communication part  61 . It is to be noted that, when it is determined in Step S 41  that the it has been time to make a request for the amount of output power, a notification to make a request for the amount of output power is transmitted to all of the output converters  21 , and when it is determined in Step S 42  that the amount of output power has been transmitted, a notification to make a request for the amount of output power is transmitted to the output converters  21  other than the output converter  21  having transmitted the amount of output power. In accordance with that notification, the communication part  61  receives information indicating the amount of output power transmitted from the output converter  21  and supplies it to the comparison part  64 , and the process goes to Step S 44 . 
     In Step S 44 , the comparison part  64  compares the amounts of output power supplied in Step S 43 , and in Step S 45 , the comparison part  64  determines whether or not the operation of the output converter  21  needs changing. For example, when the output converter  21  whose amount of output power is maximum has been changed since the last process, the comparison part  64  determines that the operation of the output converter  21  needs changing. 
     When it is determined in Step S 45  that the operation of the output converter  21  needs changing, the process goes to Step S 46 , and the comparison part  64  notifies the command part  65  of an identification number of the output converter  21  whose amount of output power has newly become maximum. 
     Accordingly, the command part  65  transmits a command for changing an operation so as to fix the conversion rate to the output converter  21  identified by means of the identification number, namely the output converter  21  whose amount of output power has newly become maximum, via the communication part  61 . Further, the command part  65  transmits a command for changing an operation so as to operate the maximum power point tracking control to the output converter  21  whose amount of output power was maximum in the previous process, namely the output converter  21  having fixed the conversion rate. 
     After the process of Step S 46 , or when it is determined in Step S 45  that the operation of the output converter  21  does not need changing, the process returns to Step S 41 , and thereafter, the same process is repeated. 
     As thus described, the amount of processing as a whole can be reduced more by comparing the amount of output power of each of the DC/DC converting parts  31  of the output converters  21 - 1  to  21 - 8  and transmitting the command for switching the process in the management unit  14  than by performing the process for comparing the amount of output power in each of the output converters  21 - 1  to  21 - 8 . It is thereby possible to stably operate switching between the process for performing the maximum power point tracking control and the process for fixing the conversion efficiency, so as to increase the amount of power generation. 
     In addition, other than fixing the conversion rate only in the output converter  21  where the amount of output power of the DC/DC converting part  31  is maximum, the management unit  14  may fix conversion rates of a high-level group of output converters  21  each of whose DC/DC converting parts  31  have a large amount of output power. 
     Further, based on the conversion efficiency in accordance with the duty value as described above and the conversion efficiency in the power conditioner  12 , the management unit  14  may decide such a reference voltage value with which an output becomes high in the photovoltaic system  11 ′ as a whole, and set the value in the power conditioner  12 . 
     With reference to  FIG. 12 , another constitutional example of the management unit will be described. 
     In  FIG. 12 , a management unit  14 ′ is configured to include a communication part  71 , a storage part  72 , a loss calculating part  73 , a voltage deciding part  74 , a command part  75 , and a connection terminal  76 . 
     The communication part  71  communicates with the respective control parts  35  of the output converters  21 - 1  to  21 - 8  and the control part  54  ( FIG. 7 ) of the power conditioner  12 . The communication part  71  acquires duty values of the respective DC/DC converting parts  31  of the output converters  21 - 1  to  21 - 8  by communication with the respective control parts  35  of the output converters  21 - 1  to  21 - 8 . Further, the communication part  71  acquires a current reference voltage value of the power conditioner  12  by communication with the control part  54 . The communication part  71  then supplies the duty values and the reference voltage value to the loss calculating part  73 . 
     The storage part  72  stores a variety of tables required for management of the photovoltaic system  11 ′. For example, in the storage part  72 , there are stored a table associating the duty value of the DC/DC converting part  31  with the conversion efficiency as shown in  FIG. 13 , a table associating the voltage value of the input voltage of the power conditioner  12  with the conversion efficiency as shown in  FIG. 14 , and the like. 
     Based on the duty values of the respective DC/DC converting parts  31  of the output converters  21 - 1  to  21 - 8  which are supplied from the communication part  71 , the loss calculating part  73  refers to the table ( FIG. 13 ) stored in the storage part  72 , to calculate a conversion loss with respect to each of the DC/DC converting parts  31 . Further, based on the current reference voltage value of the power conditioner  12  which is supplied from the communication part  71 , the loss calculating part  73  refers to the table ( FIG. 14 ) stored in the storage part  72 , to calculate a conversion loss in the DC/AC converter  51  of the power conditioner  12 . As thus described, the loss calculating part  73  is conversion loss acquiring means for calculating and acquiring conversion losses in the DC/DC converting part  31  and the DC/AC converter  51 . 
     The voltage deciding part  74  is voltage deciding means for deciding a reference voltage value, with which an output becomes high in the photovoltaic system  11 ′ as a whole, based on conversion losses in the respective DC/DC converting parts  31  of the output converters  21 - 1  to  21 - 8  and conversion loss in the power conditioner  12 . For example, when the conversion loss in the power conditioner  12  is larger than a total of the conversion losses in the respective DC/DC converting parts  31  of the output converters  21 - 1  to  21 - 8 , the voltage deciding part  74  changes the reference voltage value of the power conditioner  12  to a conversion-efficient voltage value (a larger value than the current reference voltage value). Then, the voltage deciding part  74  repeats the process for changing the voltage value based on a conversion loss after the change, thereby to make the reference voltage value converge and decide an optimum reference voltage value. 
     Herein, basically, the reference voltage value is uniquely decided in the power conditioner  12 , and for example when about 300 V is the most conversion-efficient voltage value, the voltage deciding part  74  first gets the reference voltage value closer to 300 V, to reduce the conversion loss in the power conditioner  12 . The voltage deciding part  74  then searches and decides a conversion-efficient reference voltage value as a whole, while fluctuating the reference voltage value such that the conversion losses in the respective DC/DC converting parts  31  of the output converters  21 - 1  to  21 - 8  are reduced. There is ideally obtained such a reference voltage value as to make a sum of the conversion loss in the power conditioner  12  and the conversion losses in the respective DC/DC converting parts  31  of the output converters  21 - 1  to  21 - 8  minimum. 
     The command part  75  (command means) outputs a command to the control part  54  of the power conditioner  12  via the communication part  71  so as to set the voltage value decided in the voltage deciding part  74  as the reference value of the power conditioner  12 . It is to be noted that in the case of storing the reference voltage value set in the power conditioner  12  in the command part  75 , the communication part  71  may not acquire the current reference voltage value of the power conditioner  12  by communication with the control part  54 , but may acquire the reference voltage value stored in the command part  75 . 
     The connection terminal  76  is, for example, connected with a terminal for maintenance (not shown) or the like, and that terminal communicates with the management unit  14 ′ via the communication part  71 . For example, by operating that terminal, the user can make the reference voltage value of the power conditioner  12  displayed in a display part of the terminal, and update the table stored in the storage part  72 . 
     Next,  FIG. 15  is a flowchart illustrating a process for the management unit  14 ′ setting a reference voltage value in the power conditioner  12 . 
     For example, when conversion of power is started in the power conditioner  12 , the process is started, and in Step S 51 , the communication part  71  communicates with the respective control parts  35  of the output converters  21 - 1  to  21 - 8 . Then, the communication part  71  acquires duty values of the respective DC/DC converting parts  31  of the output converters  21 - 1  to  21 - 8 , and supplies those duty values to the loss calculating part  73 . The loss calculating part  73  calculates conversion losses in the respective DC/DC converting parts  31  of the output converters  21 - 1  to  21 - 8  with reference to the table ( FIG. 13 ) stored in the storage part  72 , to notify them to the voltage deciding part  74 . 
     In Step S 52 , the communication part  71  communicates with the control part  54  of the power conditioner  12 , acquires the current reference voltage value of the power conditioner  12 , and supplies that reference voltage value to the loss calculating part  73 . The loss calculating part  73  calculates a conversion loss in the DC/AC converter  51  of the power conditioner  12  based on the current reference voltage value with reference to the table ( FIG. 14 ) stored in the storage part  72 , to notify it to the deciding part  74 . 
     In Step S 53 , the voltage deciding part  74  determines whether or not the conversion loss in the DC/AC converter  51  of the power conditioner  12  which was calculated in Step S 52  is larger than the conversion losses in the respective DC/DC converting parts  31  of the output converters  21 - 1  to  21 - 8  which were calculated in Step S 51 . 
     When it is determined in Step S 53  that the conversion loss in the DC/AC converter  51  of the power conditioner  12  is larger, the process goes to Step S 54 , and the voltage deciding part  74  decides a reference voltage value, with which an output becomes high in the photovoltaic system  11 ′ as a whole. That is, in this case, since the conversion loss in the DC/AC converter  51  of the power conditioner  12  is larger, such a voltage value is decided as to increase the reference voltage value of the power conditioner  12  so that the conversion efficiency in the DC/AC converter  51  improves. 
     On the other hand, when it is determined in Step S 53  that the conversion loss in the DC/AC converter  51  of the power conditioner  12  is not larger (the conversion loss in the DC/AC converter  51  of the power conditioner  12  is not larger than the conversion losses in the respective DC/DC converting parts  31  of the output converters  21 - 1  to  21 - 8 ), the process goes to Step S 55 . 
     In Step S 55 , the voltage deciding part  74  decides a reference voltage value with which an output becomes high in the photovoltaic system  11 ′ as a whole. That is, in this case, since the conversion loss in the DC/AC converter  51  of the power conditioner  12  is not larger, such a voltage value is decided as to decrease the reference voltage value of the power conditioner  12  so that the conversion efficiency in the respective DC/DC converting parts  31  of the output converters  21 - 1  to  21 - 8  improves. 
     After the process of Step S 54  or S 55 , the process goes to Step S 56 , and the voltage deciding part  74  notifies the command part  75  of the reference voltage value decided in Step S 54  or S 55 . Then, the command part  75  transmits a command for updating of reference voltage value of the power conditioner  12 , to the control part  54  of the power conditioner  12  via the communication part  71 , and the process returns to Step S 51 , whereafter, the same process is repeated. 
     As thus described, the management unit  14 ′ sets the reference voltage value of the power conditioner  12  in consideration of the duty value of the DC/DC converting part  31  of the output converter  21 , thereby allowing conversion of power with optimum conversion efficiency in a well-balanced manner in the photovoltaic system  11 ′ as a whole. It is thus possible to operate the photovoltaic system  11 ′ with a higher output. 
     It should be noted that, although the management unit  14 ′ has an independent configuration in the present embodiment, the control part  54  of the power conditioner  12  may include a function of the management unit  14 ′. That is, the power conditioner  12  may include the communication part  71 , the storage part  72 , the loss calculating part  73 , the voltage deciding part  74 , the command part  75 , and the connection terminal  76  of the management unit  14 ′. Further, the control part  35  of any of the output converters  21 - 1  to  21 - 8  may include the function of the management unit  14 ′. 
     In addition, although the example was described where the conversion losses in the output converters  21 - 1  to  21 - 8  are calculated in the management unit  14 ′ in the photovoltaic system  11 ′, for example, individual conversion losses may be calculated in the respective control parts  35  of the output converters  21 - 1  to  21 - 8 , and the conversion losses may be notified to the management unit  14 ′. Further, similarly, the conversion loss may be calculated in the control part  54  of the power conditioner  12 , and the conversion loss may be notified to the management unit  14 ′. 
     Moreover, other than that the respective conversion losses in the output converters  21 - 1  to  21 - 8  are calculated with reference to the table based on the duty values, for example, the control part  35  may calculate a voltage value and a current value of power that is inputted into the DC/DC converting part  31  and a voltage value and a current value of power that is outputted from the DC/DC converting part  31  and may calculate output power/input power, to calculate the conversion loss. Similarly, as for the conversion loss in the power conditioner  12 , input power and output power are detected by a sensor built in the power conditioner  12 , to calculate the conversion loss. 
     Moreover, the management unit  14 ′ can communicate with an external server via a network by the communication part  71  (communication means), to acquire time information, insolation information, temperature information and the like, for example, and store the reference voltage value of the power conditioner  12  associated with the information into the storage part  72  (storage means). Then, the management unit  14 ′ can make reference to the storage part  72  based on the insolation information or the temperature information, to decide a more optimum reference voltage value of the power conditioner  12  through use of the reference voltage value associated with the past information close to the current insolation or temperature. Moreover, the use of that information can improve the decision accuracy and decision speed for the reference voltage. 
     Furthermore, the process for the management unit  14 ′ setting the reference voltage value in the power conditioner  12  is repeated in each predetermined given period, such as every one minute, every ten minutes or every one hour. Alternatively, when the management unit  14 ′ can acquire insolation information, temperature information or the like, by executing the process with a change in insolation or a temperature taken as a trigger, it is possible to calculate a conversion loss suitable for the insolation or the temperature at that time, and perform such control as to make the amount of power generation more optimum. For example, when the weather changes from sunny to cloudy, specifically when an insolation intensity changes by not less than 500 w/m 2  from one at the time of calculating the conversion loss before, the management unit  14 ′ can execute the process. Moreover, when the temperature changes by not less than 10 degrees from one at the time of calculating the conversion loss before, the management unit  14 ′ can execute the process. 
     It should be noted that each of the processes described with reference to the foregoing flowchart does not need performing in time sequence in the described order of the flowcharts, and the processes include a process that is parallelly or individually executed (e.g., parallel process or process by an object). 
     Further, each control part is configured to include a CPU (Central Processing Unit), a ROM (Read Only Memory), a RAM (Random Access Memory), a flash memory, (e.g., EEPROM (Electronically Erasable and Programmable Read Only Memory)), and the like, and a program stored in the ROM or the flash memory is loaded to the RAM and executed, thereby to control each part of the photovoltaic system  11 . It should be noted that as for a program to be executed by the CPU, other than the programs previously stored in the ROM and the flash memory, a program can be downloaded to the flash memory and updated as appropriate. Further, a program may be one to be transferred to a remote computer and executed. 
     It is to be noted that the embodiment of the present invention is not restricted to the foregoing embodiments, and a variety of changes are possible in the range not deviating from the gist of the present invention. 
     DESCRIPTION OF SYMBOLS 
       11  photovoltaic system 
       12  power conditioner 
       13  solar battery string 
       14  management unit 
       21  output converter 
       22  solar battery module 
       31  DC/DC converting part 
       32  voltage detecting part 
       33  current detecting part 
       34  power line communication part 
       35  control part