Patent Publication Number: US-2018041164-A1

Title: Solar power generation system inspection method and inspection apparatus

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation application of International Application No. PCT/JP2016/053183, filed on Feb. 3, 2016, which claims priority based on the Article 8 of Patent Cooperation Treaty from prior Japanese Patent Application No. 2015-116961, filed on Jun. 9, 2015, the entire contents of which are incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The disclosure relates to a solar power generation system inspection method and inspection apparatus for inspecting a plurality of solar cell strings included in a solar power generation system. 
     RELATED ART 
     A solar power generation system includes a plurality of solar cell strings, each solar cell string being composed of a plurality of solar cell modules that are connected in series. DC power generated by each solar cell string is converted to AC power by a power conditioner and supplied to a commercial power supply system. 
     In order to ensure safe and stable power supply, in a solar power generation system configured as described above, the solar cell strings are inspected. For example, Patent Document 1 discloses a ground fault detection apparatus that inspects a solar power generation system including a plurality of solar cell strings for the presence or absence of a ground fault in the solar cell strings. The ground fault detection apparatus is configured to perform inspection for the presence or absence of a ground fault in the solar cell strings by disengaging a solar cell string to be inspected from the solar power generation system, or in other words, electrically disconnecting the solar cell string from the power conditioner by a switch unit provided between the solar cell string and the power conditioner. 
     RELATED ART DOCUMENTS 
     Patent Documents 
     Patent Document 1: JP 2012-119382 A (published on Jun. 21, 2012) 
     SUMMARY OF THE INVENTION 
     Problem to be Solved by the Invention 
     In the ground fault detection apparatus disclosed in Patent Document 1, a large electric current flows through the switch unit that is provided between the solar cell string and the power conditioner while the solar cell string is in a normal power generating state and the power conditioner is operating. Accordingly, as the switch unit, it is necessary to use a switch that can withstand large electric currents. 
     However, a large-electric current interruption apparatus is expensive, which leads to the problem of an increase in the cost of the solar power generation system. This problem is not unique to ground fault inspection apparatuses, and is common to other inspection apparatuses configured to perform inspection by disconnecting a solar cell string in a normal power generating state from the power conditioner. 
     Accordingly, one or more embodiments may provide a solar power generation system inspection method and inspection apparatus with a low-cost configuration in which a low-cost switch can be used as a switch for disconnecting a solar cell string from the power conditioner. 
     Means for Solving the Problems 
     In order to solve the problems described above, a solar power generation system inspection apparatus according to one or more embodiments is a solar power generation system inspection apparatus that inspects a plurality of solar cell strings that are connected to a power conversion apparatus, the solar power generation system inspection apparatus including: voltage measurement units that measure output voltages of the solar cell strings; an inspection unit that inspects the solar cell strings; a first switching unit that is provided for each of the solar cell strings and switches a connection of the solar cell string between the power conversion apparatus side and the inspection unit side; a determination unit that determines that a solar cell string is in a switchable state if at least a condition that the output voltage of the solar cell string is less than or equal to a first threshold value is satisfied; and a control unit that performs control so as to switch the first switching units to the inspection unit side if it is determined that the solar cell string is in the switchable state. 
     Effects of the Invention 
     According to the configuration of one or more embodiments, a miniaturized and inexpensive switch with small power resistance (for example, a relay) can be used as the first switching unit, and thus the inspection apparatus can have a miniaturized and low-cost configuration. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a circuit diagram illustrating a configuration of a solar power generation system that includes an inspection apparatus according to one or more embodiments. 
         FIG. 2  is a flowchart illustrating operations performed by a ground fault detection apparatus, such as in  FIG. 1 . 
         FIG. 3  is a graph illustrating variations in voltage and the amount of power generated by a solar cell string included in an ordinary solar power generation system and measurement timing. 
         FIG. 4  is a circuit diagram illustrating a solar power generation system, such as in  FIG. 1 , showing a state in which an inter-electrode voltage Va-Vb is obtained. 
         FIG. 5  is a circuit diagram illustrating a solar power generation system, such as in  FIG. 1 , showing a state in which a first voltage is obtained. 
         FIG. 6  is a circuit diagram illustrating a solar power generation system, such as in  FIG. 1 , showing a state in which a second voltage is obtained. 
         FIG. 7  is a circuit diagram illustrating a solar power generation system, such as in  FIG. 1 , showing a state in which a solar cell string to be inspected has been switched. 
     
    
    
     EMBODIMENTS OF THE INVENTION 
     Configuration of Solar Power Generation System 
     Embodiments will be described with reference to the drawings.  FIG. 1  is a circuit diagram showing a configuration of a solar power generation system that includes an inspection apparatus according to one or more embodiments. 
     As shown in  FIG. 1 , a solar power generation system  1  includes a plurality of solar cell strings  11 , a ground fault detection apparatus (inspection apparatus)  12 , and a power conditioning system (hereinafter referred to as PCS)  13  that serves as a power conversion apparatus. Here,  FIG. 1  shows a state (power output state) in which the power generated by the solar cell strings  11  is supplied to the PCS  13  and the ground fault detection apparatus  12  is not running. 
     Solar Cell String  11   
     Each solar cell string  11  is composed of a plurality of, for example, ten to twenty solar cell modules  21  that are connected in series. Each solar cell module  21  includes a plurality of solar cells (not shown) that are connected in series, and is formed as a panel. Ground fault resistors  22  are resistors that are each provided between a power delivery path of a solar cell string  11  and the ground. Each solar cell string  11  is connected to the PCS  13  via a pair of electric power delivery paths  23   a  and  23   b . A pair of electric power delivery paths  23   a  and  23   b  is provided for each solar cell string  11 . 
     Ground Fault Detection Apparatus  12   
     The ground fault detection apparatus  12  obtains the value of insulation resistance between a solar cell string  11  and the grounding point, and determines that a ground fault has occurred if the obtained insulation resistance value is smaller than a reference resistance value. 
     Accordingly, the ground fault detection apparatus  12  includes electric power delivery path switching switches (first switching units)  31 , PV switching switches (second switching units)  32 , a ground fault current detection circuit (inspection unit)  33 , PV current detection units (current measurement units)  34 , PV voltage detection units (voltage measurement units)  35 , a control unit (determination unit)  36 , first and second inspection power delivery paths (power delivery paths)  37   a  and  37   b , a grounding power delivery path  38 , and a voltage detection unit  39 . 
     An electric power delivery path switching switch  31  is provided to the electric power delivery paths  23   a  and  23   b  that are provided corresponding to each solar cell string  11 , and switches the connection of the solar cell string  11  between the PCS  13  side and the ground fault current detection circuit  33  side. Specifically, the connection of the electric power delivery paths  23   a  and  23   b  that are power output lines from the solar cell string  11  is switched between the PCS  13  side and the first and second inspection power delivery paths  37   a  and  37   b  side. 
     A pair of first and second inspection power delivery paths  37   a  and  37   b  are provided corresponding to each solar cell string  11 , and connect the electric power delivery path switching switch  31  of the solar cell string  11  and the ground fault current detection circuit  33 . 
     A PV switching switch  32  is provided to each pair of first and second inspection power delivery paths  37   a  and  37   b  that are provided corresponding to each solar cell string  11 , and opens and closes the first and second inspection power delivery paths  37   a  and  37   b.    
     The ground fault current detection circuit  33  detects a ground fault current that is generated if a ground fault has occurred in a solar cell string  11 . To this end, the ground fault current detection circuit  33  includes a first inspection switching switch  41 , a second inspection switching switch  42 , a third inspection power delivery path  43 , a detection resistor R 1 , and voltage dividing resistors R 2  to R 5  that are protection resistors. 
     The first inspection switching switch  41  is connected to one end portion of the third inspection power delivery path  43 , and switches the connection of the one end portion of the third inspection power delivery path  43  between the first inspection power delivery path  37   a  that is connected to the first inspection switching switch  41  and the grounding power delivery path  38  that is grounded. The second inspection switching switch  42  is connected to the other end portion of the third inspection power delivery path  43 , and switches the connection of the other end portion of the third inspection power delivery path  43  between the second inspection power delivery path  37   b  that is connected to the first inspection switching switch  41  and the grounding power delivery path  38  that is grounded. 
     In the third inspection power delivery path  43 , the detection resistor R 1  and the voltage dividing resistors R 2  to R 5  are provided in series. The voltage dividing resistors R 2  and R 3  are provided between the detection resistor R 1  and the first inspection switching switch  41 , and the voltage dividing resistors R 4  and R 5  are provided between the detection resistor R 1  and the second inspection switching switch  42 . Voltages at both ends of the detection resistor R 1  are input to the control unit  36  via the voltage detection unit  39 . In the example shown in  FIG. 1 , four voltage dividing resistors are provided, but there may be two voltage dividing resistors. However, the risk of an accident caused by a short circuit fault can be reduced by using many voltage dividing resistors. 
     The voltage dividing resistors R 2  to R 5  reduce first and second voltages V 1  and V 2  generated at both ends of the detection resistor R 1  so as to lower the voltage input to the control unit  36 . This enables the control unit  36  to be configured using a microcomputer, and the ground fault detection apparatus  12  to be miniaturized. Also, the voltage dividing resistors R 2  to R 5  are present in a circuit for obtaining the inter-electrode voltage Va-Vb, a circuit for obtaining the first voltage V 1 , and a circuit for obtaining the second voltage V 2 . 
     A PV current detection unit  34  is provided to, for example, the electric power delivery path  23   a  that is provided corresponding to each solar cell string  11 , and detects the amount of electric current flowing through the electric power delivery path  23   a , or in other words, the amount of electric current that flows from the solar cell string  11  to the PCS  13 , and outputs the result of detection to the control unit  36 . 
     A PV voltage detection unit  35  detects the inter-electrode voltage Va-Vb (the voltage between P terminal and N terminal) of the corresponding solar cell string  11 , and outputs the result of detection to the control unit  36 . 
     The voltage detection unit  39  detects the voltages at both ends of the detection resistor R 1  provided in the ground fault current detection circuit  33 , and outputs the result of detection to the control unit  36 . 
     The control unit  36  monitors the generated electric current of each solar cell string  11  detected by the PV current detection unit  34  and the generated voltage (inter-electrode voltage Va-Vb) of the solar cell string  11  detected by the PV voltage detection unit  35 , and determines whether the solar cell string  11  is in a switchable state. Also, the control unit  36  controls switching operations of the electric power delivery path switching switches  31 , the PV switching switches  32 , the first inspection switching switch  41 , and the second inspection switching switch  42 , and identifies a ground fault resistance and the location of the ground fault if a ground fault has occurred in the solar cell string  11 . 
     The control unit  36  includes a storage unit  40  that stores various types of information such as the result of a detection as to whether a ground fault has occurred in a solar cell string  11  and the location of the ground fault. 
     In one or more embodiments, a ground fault detection apparatus is used as an example of the inspection unit, but it is also possible to use a failure detection apparatus that detects a failure other than a ground fault in a solar cell without any problem. An example of a failure other than a ground fault can be the interruption of a conductive path. 
     Overall Operations of Ground Fault Detection Apparatus 
     In the configuration described above, operations performed by the ground fault detection apparatus  12  will be described below.  FIG. 2  is a flowchart illustrating operations performed by the ground fault detection apparatus  12 .  FIG. 3  is a graph that shows variations in the open circuit voltage and the amount of power generated by a solar cell string  11  on the day when the solar power generation system  1  was inspected by the ground fault detection apparatus  12 . 
     The ground fault detection apparatus  12  performs inspection for the presence or absence of a ground fault in the solar cell strings  11  when the solar cell strings  11  are in a switchable state. As used herein, “switchable state” refers to, specifically, as shown in  FIG. 3 , a state in which the amount of power generated by (the output current of) a solar cell string  11  is sufficiently small (for example, a state in which the output current of a solar cell string  11  is about the same as the standby current of the PCS  13  or a state in which the output current of a solar cell string  11  is lower than the standby current of the PCS  13 ), the state corresponding to an area indicated by P. Alternatively, “switchable state” refers to a state in which the amount of generated power (output current) is sufficiently small (for example, a state in which the output current of a solar cell string  11  is about the same as the standby current of the PCS  13  or a state in which the output current of a solar cell string  11  is lower than the standby current of the PCS  13 ) while the output voltage (open circuit voltage) of the solar cell string  11  is falling, the state corresponding to an area indicated by Q. The area P occurs in the early morning, and the area Q occurs in the early evening. Range A indicates the running time of the PCS  13 . 
     The switchable state may further include a condition that the output voltage (open circuit voltage) of the solar cell string  11  is greater than zero. It is thereby possible to perform inspection using the electric current generated by the solar cell string  11 . 
     Accordingly, the switchable state described above satisfies at least a condition that the output voltage of a solar cell string  11  is less than or equal to a first threshold value (for example, less than or equal to an upper limit value of a defined output voltage when the solar cell string  11  is in a normal power generating state). The switchable state described above may further include a condition that the output voltage of the solar cell string  11  is greater than zero. The first threshold value may be set to, for example, ½ of the average value of the output voltage of the solar cell string  11  while the PCS  13  is running. Also, the switchable state described above may be a state in which, for example, the value of the output voltage of the solar cell string  11  is greater than zero and less than or equal to a threshold value set to a value that is less than the average value of output voltage during the running time of the PCS  13 . 
     Also, the switchable state described above may include a condition that the output current of the solar cell string  11  is less than or equal to a second threshold value that indicates the solar cell string  11  is in a normal state so as to exclude a state in which the output current of the solar cell string is increasing abnormally due to some kind of failure from the switchable state. Also, the switchable state described above may satisfy a condition that the value of output current of the solar cell string  11  is less than or equal to a threshold value that is set to a value that is less than the average value of output current during the running time of the PCS  13 . The second threshold value may be set to, for example, ½ of the average value of the output current of the solar cell string  11  while the PCS  13  is running. 
     As shown in  FIG. 1 , when the solar power generation system  1  is in a normal operating state, the electric power delivery path switching switches  31  respectively corresponding to the solar cell strings  11  are switched to the PCS  13  side, and thus the PV switching switches  32  are off (S 11 ). 
     The control unit  36  included in the ground fault detection apparatus  12  monitors the output voltage and output current of a solar cell string  11  based on the results of detection by the PV current detection unit  34  and the PV voltage detection unit  35  (S 12 ), and determines whether the solar cell string  11  is in a switchable state (S 13 ). 
     As a result of the determination in S 13 , if it is determined that the solar cell string  11  is in a switchable state, as shown in  FIG. 4 , the control unit  36  switches all of the electric power delivery path switching switches  31  from the PCS  13  side to the ground fault current detection circuit  33  side (S 14 ). In response to the operation of switching the electric power delivery path switching switches  31 , the connections of all of the solar cell strings  11  to the PCS  13  are cut off. 
     The control unit  36  performs the above-described control on the electric power delivery path switching switches  31  when, for example, at least one solar cell string  11  is in a switchable state. That is, the solar cell strings  11  may be brought into a switchable state at slightly different timings due to slight differences in the installation angle of the solar cell modules, installation position, and the like. It should be noted that the output current of the solar cell string  11  rises after the output voltage rises. 
     Next, the control unit  36  turns on a PV switching switch  32  that corresponds to the solar cell string  11  to be inspected (S 15 , see  FIG. 4 ). In response thereto, the solar cell string  11  that corresponds to the PV switching switch  32  that has been turned on is connected to the ground fault current detection circuit  33  via the first and second inspection power delivery paths  37   a  and  37   b , and the solar cell string  11  is inspected for the presence or absence of a ground fault. 
     In a state in which the solar cell string  11  is connected to the ground fault current detection circuit  33 , the output current of the solar cell string  11  is limited by the ground fault current detection circuit  33 , and thus the operation of turning on or off the PV switching switch  32  can be easily performed. 
     Note that in S 15 , the solar cell string  11  to be inspected is one of the solar cell strings  11  that were selected in a predetermined order. The predetermined order may be, for example, the order in which the solar cell strings  11  are to be brought into a switchable state. 
     Next, inspection for the presence or absence of a ground fault is performed on the solar cell string  11  to be inspected, and if it is determined that a ground fault has occurred, the location of the ground fault is detected (S 16 ). 
     A configuration is also possible in which the control unit  36  determines that the solar cell string  11  is in an inspection ready state if a condition that the output voltage of the solar cell string  11  is greater than zero is satisfied, and turns on the PV switching switch  32  that corresponds to the solar cell string  11  to be inspected, and the ground fault current detection circuit  33  inspects the solar cell string  11  to be inspected for the presence or absence of a ground fault. 
     As a result of the inspection performed in S 16 , if it is determined that a ground fault has occurred in the inspected solar cell string  11  (S 17 ), the control unit  36  informs a management apparatus (not shown) of the solar power generation system  1  of the fact that a ground fault has occurred in the inspected solar cell string  11  and the location of the ground fault (S 18 ). The information including the presence or absence of a ground fault and the location of the ground fault is stored in the storage unit  40 . 
     Also, the control unit  36  turns off the PV switching switch  32  that corresponds to the solar cell string  11  that has been inspected (S 19 ), and the processing advances to the operation in S 21 . 
     Next, the control unit  36  determines whether inspection has been performed on all of the solar cell strings  11  (S 21 ). If it is determined that inspection has been performed on all of the solar cell strings  11 , the control unit  36  performs the operation in S 22 . If, on the other hand, it is determined that inspection has not been performed on all of the solar cell strings  11 , the processing returns to S 15 , and the operations in S 15  and subsequent steps are repeated. That is, as shown in  FIG. 7 , the control unit  36  turns on the PV switching switch  32  that corresponds to the next solar cell string  11  to be inspected. After that, the solar cell string  11  is inspected for the presence or absence of a ground fault in the same manner. 
     After that, when the control unit  36  confirms, in S 21 , that inspection has been performed on all of the solar cell strings  11 , the control unit  36  switches the electric power delivery path switching switches  31  that correspond to all of the solar cell strings  11  that have been inspected to the PCS  13  side (S 22 ), and ends the processing. Through this, all of the solar cell strings  11  that have undergone ground fault inspection are connected to the PCS  13 . 
     When an electric power delivery path switching switch  31  is switched from the ground fault current detection circuit  33  side to the PCS  13  side, the electric current is limited by the ground fault current detection circuit  33 , and thus the switching operation can be easily performed without generating an arc. 
     Also, in the operations described above, each solar cell string  11  that has undergone ground fault inspection is connected to the PCS  13  irrespective of the result of inspection (the presence or absence of a ground fault) (S 21 ) so as to supply power from the solar cell string  11  that has undergone ground fault inspection to the PCS  13 . With this configuration, even if there is a minor failure (that needs to be mended but is not urgent) in the inspected solar cell string  11 , the power generation function of the solar cell string  11  can be effectively used. However, instead of the operations described above, a configuration is also possible in which only solar cell strings  11  that were determined as having no anomaly (no ground fault) as a result of ground fault inspection are connected to the PCS  13 . 
     Operations for Measuring Ground Fault Resistance 
       FIG. 4  is a circuit diagram of the solar power generation system  1  shown in  FIG. 1  in a state in which the inter-electrode voltage Va-Vb is obtained.  FIG. 5  is a circuit diagram of the solar power generation system  1  shown in  FIG. 1  in a state in which the first voltage V 1  is obtained.  FIG. 6  is a circuit diagram of the solar power generation system  1  shown in  FIG. 1  in a state in which the second voltage V 2  is obtained. 
     In the case of obtaining the inter-electrode voltage Va-Vb, as shown in  FIG. 4 , the control unit  36  switches the first inspection switching switch  41  such that one end portion of the third inspection power delivery path  43  is connected to the first inspection power delivery path  37   a , and switches the second inspection switching switch  42  such that the other end portion of the third inspection power delivery path  43  is connected to the second inspection power delivery path  37   b.    
     In this state, the positive and negative electrodes of the solar cell string  11  are connected to each other via the detection resistor R 1  and the voltage dividing resistors R 2  to R 5 . In response thereto, at both ends of the detection resistor R 1 , a voltage is generated that corresponds to the resistance value of the detection resistor R 1  when the voltage across the positive and negative electrodes of the solar cell string  11  is divided by the detection resistor R 1  and the voltage dividing resistors R 2  to R 5 . The voltage is input to the control unit  36  via the voltage detection unit  39 , and the control unit  36  obtains the inter-electrode voltage Va-Vb. 
     Next, in the case of obtaining the first voltage V 1 , as shown in  FIG. 5 , the control unit  36  switches the first inspection switching switch  41  such that one end portion of the third inspection power delivery path  43  is connected to the first inspection power delivery path  37   a , and switches the second inspection switching switch  42  such that the other end portion of the third inspection power delivery path  43  is connected to the grounding power delivery path  38 . 
     In this state, the positive electrode (P terminal) of the solar cell string  11  is grounded via the detection resistor R 1  and the voltage dividing resistors R 2  to R 5 . In response thereto, at both ends of the detection resistor R 1 , a first voltage V 1  is generated that corresponds to the resistance value of the detection resistor R 1  when the voltage across the positive electrode of the solar cell string  11  and the ground potential is divided by the detection resistor R 1  and the voltage dividing resistors R 2  to R 5 . The first voltage V 1  is input to the control unit  36  via the voltage detection unit  39 , and the control unit  36  obtains the first voltage V 1 . 
     Next, in the case of obtaining the second voltage V 2 , as shown in  FIG. 6 , the control unit  36  switches the first inspection switching switch  41  such that one end portion of the third inspection power delivery path  43  is connected to the grounding power delivery path  38 , and switches the second inspection switching switch  42  such that the other end portion of the third inspection power delivery path  43  is connected to the second inspection power delivery path  37   b.    
     In this state, the negative electrode (N terminal) of the solar cell string  11  is grounded via the detection resistor R 1  and the voltage dividing resistors R 2  to R 5 . In response thereto, at both ends of the detection resistor R 1 , a second voltage V 2  is generated that corresponds to the detection resistor R 1  when the voltage across the negative electrode of the solar cell string  11  and the ground potential is divided by the detection resistor R 1  and the voltage dividing resistors R 2  to R 5 . The second voltage V 2  is input to the control unit  36  via the voltage detection unit  39 , and the control unit  36  obtains the second voltage V 2 . Here, there is no particular limitation on the order in which the inter-electrode voltage Va-Vb, the first voltage V 1 , and the second voltage V 2  are obtained. 
     Next, the control unit  36  obtains a resistance value Rleake from the inter-electrode voltage Va-Vb and the first and second voltages V 1  and V 2  that were obtained above, as well as a total resistance value Rsum (=R 1 +R 2 +R 3 +R 4 +R 5 ) of the third inspection power delivery path  43  by using the following expression: 
         R leake= R sum×| Va−Vb|÷|V 1− V 2 |−R sum  (1).
 
     The control unit  36  compares the resistance value Rleake (insulation resistance) with a reference resistance value (threshold value), and determines that a ground fault has occurred if the resistance value Rleake is smaller than the reference resistance value. 
     Operations for Detecting Location of Ground Fault 
     The control unit  36  determines the location of where a ground fault occurred (ground fault location) based on a ratio of the absolute value of the first voltage V 1  and the absolute value of the second voltage V 2 . It is assumed here that, as an example, the solar cell string  11  includes five solar cell modules  21  that are connected in series, and a ground fault has occurred between the third solar cell module  21  and the fourth solar cell module  21  as viewed from the P terminal side of the solar cell string  11 . Reference numeral  22  indicates the ground fault resistor of the ground fault location. In this case, the ratio between the absolute value of the first voltage V 1  and the absolute value of the second voltage V 2  is as follows: 
       | V 1|:| V 2|=3:2, 
     based on which the ground fault location can be obtained. 
     Advantages of Ground Fault Detection Apparatus  12   
     As described above, the ground fault detection apparatus  12  is configured to, when a solar cell string  11  is in a switchable state, simultaneously switch all of the electric power delivery path switching switches  31  from, for example, the PCS  13  side to the ground fault current detection circuit  33  side, and thereafter, inspection is sequentially performed on each solar cell string  11  as to whether or not a ground fault has occurred. The switchable state satisfies at least a condition that the output voltage of the solar cell string  11  is less than or equal to a first threshold value (for example, less than or equal to an upper limit value of a defined output voltage when the solar cell string  11  is in a normal power generating state). Alternatively, the switchable state satisfies a condition that the value of output voltage of the solar cell string  11  is greater than zero and is less than or equal to a first threshold value that is set to a value that is less than the average value of output voltage during the running time of the PCS  13 . Furthermore, the switchable state satisfies a condition that the output current of the solar cell string  11  is less than or equal to a second threshold value that indicates the solar cell string is in a normal state. Alternatively, the switchable state satisfies a condition that the value of the output current of the solar cell string  11  is less than or equal to a second threshold value that is set to a value that is less than the average value of output current during the running time of the PCS  13 . 
     Accordingly, as the electric power delivery path switching switches  31  in the ground fault detection apparatus  12 , it is possible to use miniaturized and inexpensive relays with small power resistance, and thus the ground fault detection apparatus  12  can have a miniaturized and low-cost configuration. 
     Also, in the case of a configuration in which the ground fault current detection circuit  33  sequentially inspects a plurality of solar cell strings  11 , a situation may arise in which a long time is needed to complete inspection of all solar cell strings  11 , during which the amount of sunlight increases, and the output voltage and output current of the solar cell strings  11  increase above the switchable state. However, even if such a situation arises, the electric power delivery path switching switches  31  have already been switched from the PCS  13  side to the ground fault current detection circuit  33  side, and it is therefore possible to easily perform inspection without being affected by such changes in the solar cell strings  11 . 
     In one or more embodiments, it is determined that a solar cell string  11  is in a switchable state if at least the condition that the output voltage of the solar cell string  11  is less than or equal to the first threshold value is satisfied. Also, it is determined that the solar cell string  11  is in a switchable state if the condition that the output voltage of the solar cell string  11  is less than or equal to the second threshold value is satisfied. Also, it is determined that the solar cell string  11  is in an inspection ready state if the condition that the output voltage of the solar cell string  11  is greater than zero is satisfied. By setting the inspection ready state as described above, a state in which the output current of the solar cell string  11  is increasing abnormally due to some kind of failure can be excluded from the inspection ready state, and the safety of the ground fault detection apparatus  12  can be enhanced. 
     Other Configuration Examples of Inspection Apparatus 
     In the embodiments described above, the solar power generation system  1  is configured to include the ground fault detection apparatus  12  as an example of the inspection apparatus. However, the inspection apparatus is not limited to an apparatus that detects a ground fault in a solar cell string  11 , and may be an apparatus that detects a line interruption in a solar cell string  11  by using a conventionally known configuration. That is, the inspection apparatus included in the solar power generation system  1  may be an inspection apparatus that disconnects a solar cell string  11  from the PCS  13  and performs inspection when the solar cell string  11  is in a switchable state, such as, for example, a ground fault inspection apparatus or a line interruption inspection apparatus for solar cell strings  11 . 
     Also, in one or more embodiments, an example was described in which the solar cell strings  11  have an equal output voltage. However, for example, a so-called multi-string PCS may be used in which a booster is provided for each solar cell string  11 . In this case, the output voltage varies from solar cell string  11  to solar cell string  11 , and thus the timing of switching solar cell strings  11  may be determined based on a reference string, which is determined by extracting and selecting from past measurement history, as the reference string, a solar cell string  11  whose voltage increases earliest in the early morning. 
     Also, the configuration of the ground fault detection apparatus  12  is not limited to that shown in the embodiments above that includes the ground fault current detection circuit  33 , and the ground fault detection apparatus  12  may detect a ground fault by using a conventionally known configuration. 
     Also, the ground fault detection apparatus  12  may include, as auxiliary means for detecting the switchable state, a solarimeter that measures the amount of sunlight on the solar cell strings  11 , a clock that detects early morning and early evening, and the like. 
     SUMMARY 
     A solar power generation system inspection apparatus according to one or more embodiments is a solar power generation system inspection apparatus that inspects a plurality of solar cell strings that are connected to a power conversion apparatus, the solar power generation system inspection apparatus including: voltage measurement units that measure output voltages of the solar cell strings; an inspection unit that inspects the solar cell strings; a first switching unit that is provided for each of the solar cell strings and switches a connection of the solar cell string between the power conversion apparatus side and the inspection unit side; a determination unit that determines that a solar cell string is in a switchable state if at least a condition that the output voltage of the solar cell string is less than or equal to a first threshold value is satisfied; and a control unit that performs control so as to switch the first switching units to the inspection unit side if it is determined that the solar cell string is in the switchable state. 
     With the configuration described above, the voltage measurement unit measures the output voltage of the solar cell string, and the determination unit determines that the solar cell string is in a switchable state if at least a condition that the output voltage of the solar cell string is less than or equal to the first threshold value (for example, in early morning or early evening) is satisfied. If the solar cell string is in the switchable state, the control unit performs control so as to switch the first switching units to the inspection unit side. Through this, the inspection unit can inspect each solar cell string. 
     As described above, when a solar cell string is in the switchable state, the first switching units provided for each solar cell string, for example, simultaneously switch the solar cell strings from the power conversion apparatus side to the inspection unit side. Accordingly, as the first switching units, miniaturized and inexpensive switches with small power resistance (for example, relays) can be used, and thus the inspection apparatus can have a miniaturized and low-cost configuration. 
     Also, in the case where the inspection unit sequentially inspects a plurality of solar cell strings that are in a switchable state, for example, early in the morning, a situation may arise in which if it takes a long time to complete inspection of all solar cell strings, the amount of sunlight increases, and the output voltage and output current of the solar cell strings increase. In this case, an electric current flows from the solar cell string that is connected to the power conversion apparatus to the power conversion apparatus, as a result of operations of the power conversion apparatus. If the first switching unit is a miniaturized switch, it is difficult for the first switching unit to switch the solar cell string from the power conversion apparatus side to the inspection unit side. 
     However, according to the configuration of one or more embodiments, even if the amount of sunlight increases, and the output voltage and output current of the solar cell strings increase, the first switching units have already switched the solar cell strings from the power conversion apparatus side to the inspection unit side. Accordingly, inspection can be performed easily without being affected by such changes of the solar cell strings as described above. That is, the output current of the solar cell string is controlled to a micro current by the inspection unit, and thus inspection can be performed easily without being affected by changes of the solar cell strings. 
     The solar power generation system inspection apparatus described above may be configured such that the determination unit further determines that the solar cell string is in an inspection ready state if a condition that the output voltage of the solar cell string is greater than zero is satisfied. 
     With the configuration described above, the solar cell string is switched from the power conversion apparatus side to the inspection unit side while the solar cell string is in a power generation state or a substantially power generation state, and thus the inspection of the solar cell string can be performed by the inspection unit. 
     In the inspection apparatus, the operation of switching the connections of the solar cell strings from the power conversion apparatus side to the inspection unit side (switchable state) by the first switching units is limited based on the upper limit voltage of a predetermined voltage range, and the operation of carrying out inspection performed by the inspection unit (inspection ready state) is limited based on the lower limit voltage of the predetermined voltage range. 
     The solar power generation system inspection apparatus described above may further include current measurement units that measure output currents of the solar cell strings, and the determination unit may be configured to further determine that the solar cell string is in the switchable state if a condition that the output current of the solar cell string is less than or equal to a second threshold value is satisfied. 
     With the configuration described above, the determination unit further determines that the solar cell string is in the switchable state if the output current of the solar cell string is less than or equal to the second threshold value (for example, less than or equal to the upper limit value of an output current that indicates that the solar cell string is in a normal state). 
     If the solar cell string is in a normal state, in conformity with the PV panel characteristics, the solar cell string is in a low voltage/low current state, but if there is an anomaly (for example, string short circuit), a large current may flow despite the fact that the voltage is low. With the configuration described above, a state in which the output current of the solar cell string is increasing abnormally due to some kind of failure can be excluded from the switchable state, and the safety of the detection apparatus can be enhanced. 
     The solar power generation system inspection apparatus described above may be configured such that the inspection unit sequentially inspects the solar cell strings, and the control unit controls the first switching units so as to connect the solar cell strings that have been inspected by the inspection unit to the power conversion apparatus. 
     With the configuration described above, because an inspected solar cell string is connected to the power conversion apparatus, even if there is a minor failure (that needs to be mended but is not urgent) in the inspected solar cell string, the power generation function of the solar cell string can be effectively used. 
     The solar power generation system inspection apparatus described above may further include second switching units that each open and close a power delivery path between the first switching unit and the inspection unit, and the control unit may be configured to control the second switching units so as to sequentially connect each of the solar cell strings to be inspected to the inspection unit and at the same time disconnect each of the solar cell strings that have been inspected from the inspection unit. 
     With the configuration described above, each solar cell string to be inspected is sequentially connected to the inspection unit, and at the same time, each solar cell string that has been inspected is disconnected from the inspection unit. Accordingly, the inspection unit can appropriately inspect each solar cell string in sequence. 
     A solar power generation system inspection method according to one or more embodiments is a solar power generation system inspection method for inspecting a plurality of solar cell strings that are connected to a power conversion apparatus, the method including the steps of; inspecting the solar cell strings; determining that a solar cell string is in a switchable state if at least a condition that an output voltage of the solar cell string is less than or equal to a first threshold value is satisfied; and if it is determined in the determining step that the solar cell string is in the switchable state, switching connections of the solar cell strings from the power conversion apparatus side to the inspection unit side. 
     With the configuration described above, it is possible to produce the same advantages effects as those of the solar power generation system inspection apparatus described above. 
     The present invention is not limited to the embodiments described above, and various modifications can be made within the scope recited in the appended claims. Embodiments obtained by combining technical means disclosed in different embodiments as appropriate also fall within the technical scope of the present invention. 
     INDUSTRIAL APPLICABILITY 
     One or more aspects are applicable to an apparatus that performs inspection for a ground fault, a line interruption or the like on a plurality of solar cell strings included in a solar power generation system. 
     INDEX TO THE REFERENCE NUMERALS 
     
         
           1  Solar power generation system 
           11  Solar cell string 
           12  Ground fault detection apparatus (inspection apparatus) 
           13  Power conditioning system (power conversion apparatus) 
           21  Solar cell module 
           22  Ground fault resistor 
           23   a ,  23   b  Electric power delivery path 
           31  Electric power delivery path switching switch (first switching unit) 
           32  PV switching switch (second switching unit) 
           33  Ground fault current detection circuit (inspection unit) 
           34  PV current detection unit (current measurement unit) 
           35  PV voltage detection unit (voltage measurement unit) 
           36  Control unit (determination unit) 
           37   a ,  37   b  First and second inspection power delivery path (power delivery path)