Patent Publication Number: US-9906166-B2

Title: Method and device for controlling operation of inverter

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     The present application claims the priority to Chinese Patent Application No. 201610591490.6, titled “METHOD AND DEVICE FOR CONTROLLING OPERATION OF INVERTER”, filed on Jul. 25, 2016 with the State Intellectual Property Office of the PRC, which is incorporated herein by reference in its entirety. 
     FIELD 
     The present disclosure relates to a field of inverter control, and more particularly to a method and a device for controlling an operation of an inverter. 
     BACKGROUND 
     With development of a photovoltaic industry, a system voltage of a photovoltaic assembly increases from 1000V to 1500V. In order to fit a photovoltaic assembly with a system voltage being 1500V, an inverter connected to the photovoltaic assembly generally adopts a five level topology structure. 
     When the inverter with the five level topology structure normally operates, the inverter operates in a five level operation mode, that is, the inverter alternately outputs five different levels in different switching state combinations of switching transistors. As shown in  FIG. 1 , a floating capacitor voltage U fc  is U dc /4 when the inverter with the five level topology structure normally operates. For an output voltage U AN  of the inverter, the output voltage has five levels respectively being +U dc /2 (S 1  and S 7  are switched on), +U dc /4 (S 1  and S 8  are switched on or S 4 , S 5  and S 7  are switched on), 0 (S 2 , S 3  and S 7  are switched on or S 4 , S 5  and S 8  are switched on), −U dc /4 (S 6  and S 7  are switched on or S 2 , S 3  and S 8  are switched on) and −U dc /2 (S 6  and S 8  are switched on). S 1 , S 2 , S 3 , S 4 , S 5 , S 6 , S 7  and S 8  are all switching transistors, U dc  is an operation voltage setting threshold when the inverter with the five level topology structure normally operates, U dc  is less than 1500V and U dc  generally ranges from 800V to 1300V. 
     However, when the inverter starts operation, a direct current side voltage of the inverter is an open-circuit voltage which is 1500V, in order to avoid voltage stresses of the switching transistors S 1  and S 6  exceeding a withstanding range, the floating capacitor voltage U fc  is pre-charged to a preset value (such as 450V). In this case, the inverter alternately outputs seven different levels in different switching state combinations of switching transistors. For example, the floating capacitor voltage U fc  is pre-charged to 450V that is 3U dc′ /10, the output voltage U AN  of the inverter in  FIG. 1  may have seven levels respectively being +U dc′ /2 (S 1  and S 7  are switched on), +U dc′ /5 (S 1  and S 8  are switched on), +3U dc′ /10 (S 4 , S 5  and S 7  are switched on), 0 (S 2 , S 3  and S 7  are switched on or S 4 , S 5  and S 8  are switched on), −2U dc′ /10 (S 6  and S 7  are switched on), −3U dc′ /10 (S 2 , S 3  and S 8  are switched on) and −U dc′ /2 (S 6  and S 8  are switched on), U dc′  is 1500V. In this case, if the inverter is still controlled to operate in the five level operation mode, an output current of the inverter may be severely distorted, thereby affecting an operation stability of a whole photovoltaic system. 
     SUMMARY 
     In order to solve the above technical problem, a method and a device for controlling an operation of an inverter is provided according to embodiments of the disclosure to achieve an objection of preventing an output current of an inverter from being distorted caused by the inverter still being controlled according to a five level control strategy in a case that a floating capacitor voltage of the inverter being not in a preset range, thereby improving an operation stability of a whole photovoltaic system. Technical solutions are described as follows. 
     A method for controlling an operation of an inverter includes: 
     determining whether a direct current side voltage of the inverter is greater than an operation voltage setting threshold; and 
     in a case that the direct current side voltage of the inverter is not greater than the operation voltage setting threshold, controlling the inverter to operate according to a five level control strategy; and 
     in a case that the direct current side voltage of the inverter is greater than the operation voltage setting threshold:
         adjusting the direct current side voltage by using a maximum power tracking algorithm;   adjusting linearly a floating capacitor voltage of the inverter based on the adjusted direct current side voltage;   determining whether the adjusted floating capacitor voltage is in a preset range, where the preset range ranges from a quarter of the operation voltage setting threshold minus a preset threshold to a quarter of the operation voltage setting threshold plus the preset threshold; and   controlling the inverter to operate according to a five level control strategy in a case that the adjusted floating capacitor voltage is in the preset range; and   controlling the inverter to operate according to a seven level control strategy in a case that the adjusted floating capacitor voltage is not in the preset range.       

     Preferably, after controlling the inverter to operate according to the five level control strategy, the method further includes: 
     determining whether the direct current side voltage is greater than a first preset voltage, where the first preset voltage is greater than the operation voltage setting threshold; and 
     executing a step of adjusting the direct current side voltage by using the maximum power tracking algorithm in a case that the direct current side voltage is greater than the first preset voltage; and 
     controlling the inverter to operate according to the five level control strategy in a case that the direct current side voltage is not greater than the first preset voltage. 
     Preferably, the adjusting linearly the floating capacitor voltage of the inverter based on the adjusted direct current side voltage includes: 
     substituting the adjusted direct current side voltage into a preset linear relation formula to obtain a to-be-used floating capacitor voltage; and 
     adjusting the floating capacitor voltage to the to-be-used floating capacitor voltage. 
     Preferably, the preset linear relation formula is 
     
       
         
           
             
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     where U dc1  is a direct current side open-circuit voltage of the inverter, U fc1  is a floating capacitor voltage corresponding to the direct current side open-circuit voltage of the inverter, U dc2  is the operation voltage setting threshold, U fc2  a floating capacitor voltage corresponding to the operation voltage setting threshold, X is the direct current side voltage of the inverter and Y is the floating capacitor voltage of the inverter. 
     A device for controlling an operation of an inverter includes: 
     a first determining unit, configured to determine whether a direct current side voltage of the inverter is greater than an operation voltage setting threshold, where a first controlling unit is executed in a case that the direct current side voltage of the inverter is not greater than the operation voltage setting threshold, and a first adjusting unit is executed in a case that the direct current side voltage of the inverter is greater than the operation voltage setting threshold; 
     the first controlling unit, configured to control the inverter to operate according to a five level control strategy; 
     the first adjusting unit, configured to adjust the direct current side voltage by using a maximum power tracking algorithm; 
     a second adjusting unit, configured to adjust linearly a floating capacitor voltage of the inverter based on the adjusted direct current side voltage; 
     a second determining unit, configured to determine whether the adjusted floating capacitor voltage is in a preset range, where the preset range ranges from a quarter of the operation voltage setting threshold minus a preset threshold to a quarter of the operation voltage setting threshold plus the preset threshold, and where the first controlling unit is executed in a case that the adjusted floating capacitor voltage is in the preset range, and a second controlling unit is executed in a case that the adjusted floating capacitor voltage is not in the preset range; and 
     the second controlling unit, configured to control the inverter to operate according to a seven level control strategy. 
     Preferably, the device further includes a third determining unit, configured to determine whether the direct current side voltage is greater than a first preset voltage, where the first preset voltage is greater than the operation voltage setting threshold, and where the first adjusting unit is executed in a case that the direct current side voltage is greater than the first preset voltage, and the first controlling unit is executed in a case that the direct current side voltage is not greater than the first preset voltage. 
     Preferably, the second adjusting unit includes: 
     a calculating subunit, configured to substitute the adjusted direct current side voltage into a preset linear relation formula to obtain a to-be-used floating capacitor voltage; and 
     an adjusting subunit, configured to adjust the floating capacitor voltage to the to-be-used floating capacitor voltage. 
     Preferably, the calculating subunit is specifically configured to substitute the adjusted direct current side voltage into to 
             Y   =             U     dc   ⁢           ⁢   1       *     U     fc   ⁢           ⁢   2         -       U     fc   ⁢           ⁢   1       *     U     dc   ⁢           ⁢   2               U     dc   ⁢           ⁢   1       -     U     dc   ⁢           ⁢   2           +           U     fc   ⁢           ⁢   1       -     U     fc   ⁢           ⁢   2             U     dc   ⁢           ⁢   1       -     U     dc   ⁢           ⁢   2           ⁢   X             
to obtain a to-be-used floating capacitor voltage, where U dc1  is a direct current side open-circuit voltage of the inverter, U fc1  is a floating capacitor voltage corresponding to the direct current side open-circuit voltage of the inverter, U dc2  is the operation voltage setting threshold, U fc2  a floating capacitor voltage corresponding to the operation voltage setting threshold, X is the direct current side voltage of the inverter and Y is the floating capacitor voltage of the inverter.
 
     An inverter includes any one of the above devices for controlling an operation of an inverter. 
     A photovoltaic power generation system includes the above inverter. 
     Compared with the conventional technology, the disclosure has the following advantages. In the disclosure, it is firstly determined whether a direct current side voltage of an inverter is greater than an operation voltage setting threshold. In a case that it is determined that the direct current side voltage is greater than the operation voltage setting threshold, the direct current side voltage of the inverter may be an open-circuit voltage which is 1500V (for example when the inverter starts operation, the direct current side voltage of the inverter is the open-circuit voltage). The inverter alternatively outputs seven different levels. Since a normal operation mode of the inverter is a five level operation mode, in this case, the direct current side voltage is adjusted by using a maximum power tracking algorithm and a floating capacitor voltage of the inverter is linearly adjusted based on the adjusted direct current side voltage, the floating capacitor voltage is adjusted to be in a preset range so to convert the inverter from a seven level operation mode to the five level operation mode. In a case that the floating capacitor voltage is adjusted to be in the preset range, the inverter is controlled to operate according to the five level control strategy. In a case that the floating capacitor voltage is not adjusted to be in the preset range, the inverter is controlled to operate according to the seven level control strategy rather than the five level control strategy to prevent an output current of the inverter from being distorted caused by the inverter still being controlled according to the five level control strategy in a case that the floating capacitor voltage of the inverter being not in the preset range, thereby improving an operation stability of a whole photovoltaic system. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In order to describe technical solutions in embodiments of the present disclosure more clearly, hereinafter drawings to be used in the description of the embodiments are introduced simply. Apparently, the drawings described below only describe some embodiments of the present disclosure. Those skilled in the art can obtain other drawings based on these drawings without any creative work. 
         FIG. 1  is a flowchart of a method for controlling an operation of an inverter according to the disclosure; 
         FIG. 2A  to  FIG. 2H  are schematic diagrams of an current flowing path in a case that an inverter operates in a seven level operation mode according to the disclosure; 
         FIG. 3  is another flowchart of a method for controlling an operation of an inverter according to the disclosure; 
         FIG. 4  is a sub-flowchart of a method for controlling an operation of an inverter according to the disclosure; 
         FIG. 5  is a schematic logic structural diagram of a device for controlling an operation of an inverter according to the disclosure; 
         FIG. 6  is another schematic logic structural diagram of a device for controlling an operation of an inverter according to the disclosure; and 
         FIG. 7  is a schematic logic structural diagram of an adjusting unit according to the disclosure. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Technical solutions of embodiments of the present disclosure will be described clearly and completely below in conjunction with the drawings of the embodiments of the disclosure. Apparently, the described embodiments are some rather than all of the embodiments of the present disclosure. Any other embodiments obtained based on the embodiments of the present disclosure by those skilled in the art without any creative work fall within the protection scope of the present disclosure. 
     Embodiment 1 
     Reference is made to  FIG. 1 , which is a flowchart of a method for controlling an operation of an inverter according to the disclosure. The method may include steps S 11  to S 16 . 
     In step S 11 , it is determined whether a direct current side voltage of the inverter is greater than an operation voltage setting threshold. Step S 12  is executed in a case that the direct current side voltage of the inverter is not greater than the operation voltage setting threshold. And step S 13  is executed in a case that the direct current side voltage of the inverter is greater than the operation voltage setting threshold. 
     It should be noted that, in a case that the direct current side voltage of the inverter is not greater than the operation voltage setting threshold, a voltage stress of each switching transistor in the inverter is in a withstanding range. The inverter operates in a five level operation mode, so step S 12  is executed. 
     In a case that the direct current side voltage of the inverter is greater than the operation voltage setting threshold, the voltage stress of the switching transistor in the inverter may exceed the withstanding range of the switching transistor. In this case, in order to prevent the voltage stress of the switching transistor from exceeding the withstanding range of the switching transistor, the inverter does not operate in the five level operation mode (For example, when the inverter starts operation, the direct current side voltage of the inverter is an open-circuit voltage which is 1500V. In order to prevent voltage stresses of the switching transistor S 1  and the switching transistor S 6  exceeding the withstanding range, a floating capacitor voltage U fc  is pre-charged to a preset value (such as 450V). In this case, the inverter alternately outputs seven different levels in different switching state combinations of switching transistors). So the inverter is not controlled to operate according to a five level control strategy and step S 13  is executed. 
     The operation voltage setting threshold is generally 1200V. 
     In step S 12 , the inverter is controlled to operate according to a five level control strategy. 
     In this embodiment, in a case that the inverter is controlled to operate according to the five level control strategy, the inverter operates in the five level operation mode. 
     A specific process of controlling the inverter to operate according to the five level control strategy includes: adjusting a duty cycle of the inverter according to U fc =¼U dc1  to make the inverter alternately output five levels respectively being 0, ±¼U dc1 , ±½U dc1  in different switching state combinations of switching transistors. U dc1  is the direct current side voltage of the inverter. 
     In step  13 , the direct current side voltage is adjusted by using a maximum power tracking algorithm. 
     In this embodiment, adjusting the direct current side voltage by using the maximum power tracking algorithm includes: tracking an output power of the inverter by using the maximum power tracking algorithm until a maximum output power of the inverter is tracked, and adjusting the direct current side voltage based on the tracked output power. 
     A specific process of tracking an output power of the inverter by using the maximum power tracking algorithm until a maximum output power of the inverter is tracked and adjusting the direct current side voltage based on the tracked output power in the disclosure is the same as a process of tracking an output power of the inverter by using the maximum power tracking algorithm until a maximum output power of the inverter is tracked and adjusting the direct current side voltage based on the tracked output power in the conventional technology, which is not described here. 
     In step S 14 , a floating capacitor voltage of the inverter is linearly adjusted based on the adjusted direct current side voltage. 
     In step S 15 , it is determined whether the adjusted floating capacitor voltage is in a preset range. The preset range ranges from a quarter of the operation voltage setting threshold minus a preset threshold to a quarter of the operation voltage setting threshold plus the preset threshold. Step S 12  is executed in a case that the adjusted floating capacitor voltage is in the preset range, or step S 16  is executed in a case that the adjusted floating capacitor voltage is not in the preset range. 
     In a case that the adjusted floating capacitor voltage is not in the preset range, in order to prevent the voltage stress of the switching transistor from exceeding the withstanding range of the switching transistor, the inverter has to operate in a seven level operation mode, so step S 16  is executed. 
     In step S 16 , the inverter is controlled to operate according to a seven level control strategy. 
     In this embodiment, in a case that the inverter is controlled to operate in the seven level control strategy, the inverter operates in a seven level operation mode. 
     A specific process of controlling the inverter to operate according to the seven level control strategy includes: adjusting a duty cycle of the inverter according to a requirement of the seven level operation mode to make the inverter alternately output seven different levels in different switching state combinations of switching transistors. For example, when the inverter starts operation, the direct current side voltage of the inverter is an open-circuit voltage which is 1500V. In order to prevent a voltage stress of the switching transistor from exceeding the withstanding range, a floating capacitor voltage U fc  is pre-charged to 450V which is 3U dc2 /10. A voltage outputted by the inverter may be respectively +U dc2 /2, +U dc2 /5, +3U dc2 /10, 0, −2U dc2 /10, −3U dc2 /10 and −U dc2 /2. U dc2  is the open-circuit voltage which is 1500V. 
     A current flowing path in a case that the inverter operates in the seven level operation mode is the same as a current flowing path in a case that the inverter operates in the five level operation mode. 
     An example is taken to illustrate the current flowing path in a case that the inverter operates in the seven level operation mode. For example, when the inverter starts operation, the direct current side voltage of the inverter is an open-circuit voltage which is 1500V. In order to prevent voltage stresses of the switching transistor S 1  and the switching transistor S 6  from exceeding the withstanding range, a floating capacitor voltage U fc  is pre-charged to 450V which is 3U dc2 /10. Reference is made to  FIG. 2A  to  FIG. 2H . As shown in  FIG. 2A , the switching transistors S 1  and S 7  are switched on, the output voltage of the inverter is U AN =U dc2 /2. As shown in  FIG. 2B , the switching transistors S 4 , S 5  and S 7  are switched on, the output voltage of the inverter is U AN =U fc , i.e., (+3U dc2 /10). As shown in  FIG. 2C , the switching transistors S 1  and S 8  are switched on, the output voltage of the inverter is U AN =U dc2 /2−U fc . As shown in  FIG. 2D , the switching transistors S 2 , S 3  and S 7  are switched on, the output voltage of the inverter is U AN =0. As shown in  FIG. 2E , the switching transistors S 4 , S 5  and S 8  are switched on, the output voltage of the inverter is U AN =0. As shown in  FIG. 2F , the switching transistors S 6  and S 7  are switched on, the output voltage of the inverter is U AN =−U dc2 /2+U fc . As shown in  FIG. 2G  the switching transistors S 2 , S 3  and S 8  are switched on, the output voltage of the inverter is U AN =−U fc . As shown in  FIG. 2H , the switching transistors S 6  and S 8  are switched on, the output voltage of the inverter is U AN =−U dc2 /2. 
     In the disclosure, it is firstly determined whether a direct current side voltage of an inverter is greater than an operation voltage setting threshold. In a case that it is determined that the direct current side voltage is greater than the operation voltage setting threshold, the direct current side voltage of the inverter may be an open-circuit voltage which is 1500V (for example when the inverter starts operation, the direct current side voltage of the inverter is the open-circuit voltage). The inverter alternatively outputs seven different levels. Since a normal operation mode of the inverter is a five level operation mode, in this case, the direct current side voltage is adjusted by using a maximum power tracking algorithm and a floating capacitor voltage of the inverter is linearly adjusted based on the adjusted direct current side voltage, the floating capacitor voltage is adjusted to be in a preset range so to convert the inverter from a seven level operation mode to the five level operation mode. In a case that the floating capacitor voltage is adjusted to be in the preset range, the inverter is controlled to operate according to the five level control strategy. In a case that the floating capacitor voltage is not adjusted to be in the preset range, the inverter is controlled to operate according to the seven level control strategy rather than the five level control strategy to prevent an output current of the inverter from being distorted caused by the inverter still being controlled according to the five level control strategy in a case that the floating capacitor voltage of the inverter being not in the preset range, thereby improving an operation stability of a whole photovoltaic system. 
     Embodiment 2 
     In this embodiment, another method for controlling an operation of an inverter is provided based on the method for controlling an operation of an inverter shown in  FIG. 1 . Reference is made to  FIG. 3 , the method may include steps S 31  to S 37 . 
     In step S 31 , it is determined whether a direct current side voltage of the inverter is greater than an operation voltage setting threshold. Step S 32  is executed in a case that the direct current side voltage of the inverter is not greater than the operation voltage setting threshold. And step S 33  is executed in a case that the direct current side voltage of the inverter is greater than the operation voltage setting threshold. 
     In step S 32 , the inverter is controlled to operate according to a five level control strategy. 
     In step  33 , the direct current side voltage is adjusted by using a maximum power tracking algorithm. 
     In step S 34 , a floating capacitor voltage of the inverter is linearly adjusted based on the adjusted direct current side voltage. 
     In step S 35 , it is determined whether the adjusted floating capacitor voltage is in a preset range. The preset range ranges from a quarter of the operation voltage setting threshold minus a preset threshold to a quarter of the operation voltage setting threshold plus the preset threshold. Step S 32  is executed in a case that the adjusted floating capacitor voltage is in the preset range and step S 36  is executed in a case that the adjusted floating capacitor voltage is not in the preset range. 
     In step S 36 , the inverter is controlled to operate according to a seven level control strategy. 
     A specific process of step S 31  to step S 36  is the same as a process of step S 11  to step S 16  of the method for controlling an operation of an inverter shown in  FIG. 1 , which is not described here. 
     In step S 37 , it is determined whether the direct current side voltage is greater than a first preset voltage. The first preset voltage is greater than the operation voltage setting threshold. Step S 33  is executed in a case that the direct current side voltage is greater than the first preset voltage, and step S 32  is executed in a case that the direct current side voltage is not greater than the first preset voltage. 
     After the inverter is controlled to operate in the five level operation mode according to the five level control strategy, it is considered that a low temperature makes an effect on a photovoltaic assembly, which may cause an output voltage of the inverter rise from an operation voltage setting threshold (such as 1200 V) to a first preset voltage (such as 1300V) or more. When the output voltage of the inverter rises from the operation voltage setting threshold to the first preset voltage or more, regarding an influence of a voltage spike under full power, a voltage stress of the switching transistor in the inverter rises and may even exceed a voltage stress withstanding range of the switching transistor. In this case, a floating capacitor voltage of the inverter needs to be adjusted to lower the voltage stress of the switching transistor and further ensure an operation stability of a photovoltaic system. Specifically, step S 37  is executed after step S 32 . 
     The above preset threshold is influenced by a specific circuit of an inverter or a photovoltaic system, the preset threshold may be variable. The above preset threshold may be, but not limited to, 20V. 
     In the embodiment 1 and the embodiment 2, a process of adjusting linearly the floating capacitor voltage of the inverter based on the adjusted direct current side voltage may be referred to  FIG. 4  and may include steps S 41  and S 42 . 
     In step S 41 , the adjusted direct current side voltage is substituted into a preset linear relation formula to calculate a to-be-used floating capacitor voltage. 
     The preset linear relation formula is obtained based on the above operation voltage setting threshold, a floating capacitor voltage corresponding to the above operation voltage setting threshold, the direct current side voltage of the inverter (that is the open-circuit voltage) when the inverter starts operation and a floating capacitor voltage corresponding to the direct current side voltage of the inverter when the inverter starts operation. 
     In particular, the preset linear relation formula may be 
               Y   =             U     dc   ⁢           ⁢   1       *     U     fc   ⁢           ⁢   2         -       U     fc   ⁢           ⁢   1       *     U     dc   ⁢           ⁢   2               U     dc   ⁢           ⁢   1       -     U     dc   ⁢           ⁢   2           +           U     fc   ⁢           ⁢   1       -     U     fc   ⁢           ⁢   2             U     dc   ⁢           ⁢   1       -     U     dc   ⁢           ⁢   2           ⁢   X         ,         
where U dc1  is a direct current side open-circuit voltage of the inverter, U fc1  is a floating capacitor voltage corresponding to the direct current side open-circuit voltage of the inverter, U dc2  is the operation voltage setting threshold, U fc2  a floating capacitor voltage corresponding to the operation voltage setting threshold, X is the direct current side voltage of the inverter and Y is the floating capacitor voltage of the inverter.
 
     In step S 42 , the floating capacitor voltage is adjusted to the to-be-used floating capacitor voltage. 
     Embodiment 3 
     Corresponding to the above method embodiments, a device for controlling an operation of an inverter is provided according to this embodiment. Referring to  FIG. 5 , the device for controlling an operation of an inverter includes: a first determining unit  51 , a first controlling unit  52 , a first adjusting unit  53 , a second adjusting unit  54 , a second determining unit  55  and a second controlling unit  56 . 
     The first determining unit  51  is configured to determine whether a direct current side voltage of the inverter is greater than an operation voltage setting threshold. The first controlling unit  52  is executed in a case that the direct current side voltage of the inverter is not greater than the operation voltage setting threshold, and the first adjusting unit  53  is executed in a case that the direct current side voltage of the inverter is greater than the operation voltage setting threshold. 
     The first controlling unit  52  is configured to control the inverter to operate according to a five level control strategy. 
     The first adjusting unit  63  is configured to adjust the direct current side voltage by using a maximum power tracking algorithm. 
     The second adjusting unit  54  is configured to adjust linearly a floating capacitor voltage of the inverter based on the adjusted direct current side voltage. 
     The second determining unit  55  is configured to determine whether the adjusted floating capacitor voltage is in a preset range, where the preset range ranges from a quarter of the operation voltage setting threshold minus a preset threshold to a quarter of the operation voltage setting threshold plus the preset threshold. The first controlling unit  52  is executed in a case that the adjusted floating capacitor voltage is in the preset range, and the second controlling unit  56  is executed in a case that the adjusted floating capacitor voltage is not in the preset range. 
     The second controlling unit  56  is configured to control the inverter to operate according to a seven level control strategy. 
     In this embodiment, the device for controlling the operation of the inverter shown in  FIG. 5  may further include a third determining unit  57  as shown in  FIG. 6 . The third determining unit  57  is configured to determine whether the direct current side voltage is greater than a first preset voltage, where the first preset voltage is greater than the operation voltage setting threshold. The first adjusting unit  53  is executed in a case that the direct current side voltage is greater than the first preset voltage, and the first controlling unit  52  is executed in a case that the direct current side voltage is not greater than the first preset voltage. 
     In this embodiment, the second adjusting unit  54  further includes: a calculating subunit  541  and an adjusting subunit  542 , as shown in  FIG. 7 . 
     The calculating subunit  541  is configured to substitute the adjusted direct current side voltage into a preset linear relation formula to obtain a to-be-used floating capacitor voltage. 
     In this embodiment, the preset linear relation formula may be 
               Y   =             U     dc   ⁢           ⁢   1       *     U     fc   ⁢           ⁢   2         -       U     fc   ⁢           ⁢   1       *     U     dc   ⁢           ⁢   2               U     dc   ⁢           ⁢   1       -     U     dc   ⁢           ⁢   2           +           U     fc   ⁢           ⁢   1       -     U     fc   ⁢           ⁢   2             U     dc   ⁢           ⁢   1       -     U     dc   ⁢           ⁢   2           ⁢   X         ,         
where U dc1  is a direct current side open-circuit voltage of the inverter, U fc1  is a floating capacitor voltage corresponding to the direct current side open-circuit voltage of the inverter, U dc2  is the operation voltage setting threshold, U fc2  a floating capacitor voltage corresponding to the operation voltage setting threshold, X is the direct current side voltage of the inverter and Y is the floating capacitor voltage of the inverter.
 
     The calculating subunit  541  is configured to substitute the adjusted direct current side voltage into 
             Y   =             U     dc   ⁢           ⁢   1       *     U     fc   ⁢           ⁢   2         -       U     fc   ⁢           ⁢   1       *     U     dc   ⁢           ⁢   2               U     dc   ⁢           ⁢   1       -     U     dc   ⁢           ⁢   2           +           U     fc   ⁢           ⁢   1       -     U     fc   ⁢           ⁢   2             U     dc   ⁢           ⁢   1       -     U     dc   ⁢           ⁢   2           ⁢   X             
to obtain a to-be-used floating capacitor voltage.
 
     The adjusting subunit  542  is configured to adjust the floating capacitor voltage to the to-be-used floating capacitor voltage. 
     The above preset threshold is influenced by a specific circuit of an inverter or a photovoltaic system, the preset threshold may be variable. The above preset threshold may be, but not limited to, 20V. 
     Embodiment 4 
     In this embodiment, it is further provided an inverter including the device for controlling the operation of the inverter illustrated in the embodiment 3. 
     Of course, in this embodiment, it is further provided a photovoltaic power generation system including the above inverter. 
     It should be noted that, the various embodiments in the disclosure are described progressively. Each embodiment focuses on the difference from other embodiments. The same or similar parts of the various embodiments can be referred mutually. The device provided by the embodiment is described simply since it corresponds to the method provided by the embodiment, and part of the method description can be referred to, to explain the corresponding parts of the device. 
     Finally, it should be noted that, in the disclosure, relationship terms such as “first” and “second” are merely used to distinguish one entity or operation from another entity or operation, which does not require or indicate there exist any real relationships or sequences among those entities or operations. And terms “include”, “contain” or any other variants mean to cover non-exclusive containing so to make a process, a method, a product or a device including a series of essential factors not only include these essential factors but also include those essential factors not clearly listed or further include fixed essential factors of the process, the method, the product or the device. In a situation without many limits, an essential factor limited by a sentence “include a . . . ” does not exclude another same essential factor existing in a process, a method, a product or a device including the descried essential factor. 
     Above, a method and a device for controlling an operation of an inverter provided by the disclosure are specifically illustrated. A principle and the embodiments of the disclosure are illustrated by using specific examples in the description. The above description of the embodiments is used to help understanding a method and a core idea of the disclosure. In addition, for those skilled in the arts, according to the idea of the disclosure, a change is made in the specific embodiments and an applying filed. In summary, contents of the specification should not be understood to limit the disclosure.