Abstract:
Provided is a solar power conversion apparatus which includes at least one solar array receiving light and generating a DC power, a converter unit converting amplitude of the generated DC power, a multilevel inverter unit receiving the DC power from the converter unit to output AC power with multi levels and comprising a plurality of multilevel inverters, an AC filter insulating the inverter unit from a power grid, and a control unit applying a control signal to the converter unit and the multilevel inverter.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application claims priority under 35 U.S.C. 119 and 35 U.S.C. 365 to Korean Patent Application No. 10-2011-0055338, filed on Jun. 8, 2011, which is hereby incorporated by reference in its entirety. 
       BACKGROUND 
       [0002]    The present disclosure relates to a solar power conversion apparatus, and more particularly, to a solar power conversion apparatus using a neutral-point-clamped multilevel method as a hybrid method and having higher output level and lower distortion than a solar power conversion apparatus of the related art. 
         [0003]    Solar power conversion apparatuses are apparatuses for converting a direct current generated in solar cells to an alternating current and transmit the alternating current to a gird. Full bridge type hardware is mainly used in solar power conversion. However, because of bad quality of power and low efficiency, recently, studies and demands on multilevel power conversion products are increasing. 
         [0004]    Solar power conversion apparatuses of the related art have low power conversion efficiency because solar power conversion is inefficient and whole arrays are used as one input. Also, because of being operated usually in two or three output levels, solar power conversion apparatuses of the related art may have some problems such as not various power levels, low quality of power, and impossible modularization. 
         [0005]    In order to solve those problems mentioned above, a multilevel power conversion apparatus has been developed. A multilevel power conversion apparatus has better efficiency and more stable quality than a full bridge type. There are three multilevel power conversion apparatus topologies: an H-bridge, a neutral-point-clamped, and a flying capacitor. 
         [0006]    However, such a multilevel type also has a little high distortion and low power level. 
       SUMMARY 
       [0007]    Embodiments provide a solar power conversion apparatus using multi levels, and more particularly, a solar power conversion apparatus having lower distortion and higher output level than a related art solar power conversion apparatus by using a neutral-point-clamped multilevel method. 
         [0008]    Embodiments also provide a solar power conversion apparatus having high efficiency by connecting two solar cells generating electric energy to a module as a solar power converting unit in order to increase efficiency. 
         [0009]    In one embodiment, a solar power conversion apparatus includes: at least one solar array receiving light and generating a DC power; a converter unit converting amplitude of the generated DC power; a multilevel inverter unit receiving the DC power from the converter unit to output AC power with multi levels and comprising a plurality of multilevel inverters; an AC filter insulating the inverter unit from a power grid; and a control unit applying a control signal to the converter unit and the multilevel inverter. 
         [0010]    Also, the converter unit may include at least one converter corresponding to the number of the solar arrays. 
         [0011]    Also, the number of the multilevel inverters connected may correspond to the number of the converters. 
         [0012]    Also, when a plurality of the multilevel inverters are connected in series, each of the multi level inverters may output power of five levels. 
         [0013]    Also, when a plurality of the multilevel inverters connected in series, the multilevel inverters may output power of nine levels. 
         [0014]    Also, the multilevel inverters may include: a plurality of switching elements; a plurality of clamping diodes connected between the switching elements; and a plurality of bus capacitors. 
         [0015]    Also, the switching elements may be insulated gate bipolar transistors (IGBTs). 
         [0016]    Also, the multilevel inverter unit may include (m −1 ) clamping diodes where the m denotes the number of output levels. 
         [0017]    Also, the multilevel inverter unit may include (m−1)/2 DC bus capacitors limiting a ripple of DC power. 
         [0018]    Also, the multilevel inverter unit may generate five power levels according to turned-on states of switching element of the multilevel inverter unit. 
         [0019]    Also, the control unit may include: a converter gating unit for applying a gating signal to perform MPPT (maximum power point tracking) control on the converter; and an inverter gating unit for applying a gating signal to control the inverter 
         [0020]    Also, the gating signal may be a PWM signal based on a carrier or a space vector PWM signal. 
         [0021]    Also, the converter unit may perform maximum power point tracking (MPPT) control about the DC power. 
         [0022]    The details of one or more embodiments are set forth in the accompanying drawings and the description below. Other features will be apparent from the description and drawings, and from the claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]      FIG. 1  is a block diagram schematically illustrating a solar power conversion apparatus according to an embodiment. 
           [0024]      FIG. 2  is a view illustrating the solar power conversion apparatus in detail according to the embodiment. 
           [0025]      FIG. 3  is a view illustrating one operation of the solar array, the DC-DC converter, and the multilevel inverter according to the embodiment. 
           [0026]      FIG. 4  is a view illustrating states of the switching elements in the inverter according to each output level of the embodiment. 
           [0027]      FIG. 5  is a view showing a switching order of the switching elements in the inverter according to the embodiment. 
           [0028]      FIG. 6  is a view illustrating an output signal of the solar power conversion apparatus according to the embodiment. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0029]    Reference will now be made in detail to the embodiments of the present disclosure, examples of which are illustrated in the accompanying drawings. 
         [0030]    It should be understood that the terms and words used in the specification and the appended claims should not be construed as limited to general and dictionary meanings, but interpreted based on the meanings and concepts corresponding to technical aspects of the present invention on the basis of the principle that the inventor is allowed to define terms appropriately for the best explanation for the invention. 
         [0031]    Therefore, embodiments described in the specification and the example illustrated in the accompanying drawings herein is just a mere example for the purpose of illustrations only, not intended to represent all the technical aspects of the embodiment, the scope of the invention, so it should be understood that various equivalents and modifications thereof could be made at the time of filing. 
         [0032]      FIG. 1  is a block diagram schematically illustrating a solar power conversion apparatus according to an embodiment. 
         [0033]    Referring to  FIG. 1 , a solar power conversion apparatus  10  includes the first and second solar arrays  11  and  12 , a converter  20 , a multilevel inverter  30 , an AC filter  40 , a transformer  50 , and a control unit  60 . 
         [0034]    The solar arrays  11  and  12  receive light and generate DC power. That is, the solar arrays  11  and  12  may be solar cell arrays in which a plurality of solar cells are arranged to receive sunlight and generate power. 
         [0035]    The converter  20  may include a DC-DC converter or a plurality of DC-DC converters respectively corresponding to the first and second solar arrays  11  and  12 . 
         [0036]    The multilevel inverter  30  may include a first multilevel inverter  31  and a second multilevel inverter  32 . The first and second multilevel inverters  31  and  32  may output five power levels, respectively. If the first and second multilevel inverters  31  and  32  are connected in series, the first and second multilevel inverters  31  and  32  may output nine power levels. That is, the output level of the first and the second multilevel inverters  31  and  32  may depend on the configuration of the multilevel inverter  30 . 
         [0037]    In addition, although the first and the second multilevel inverters  31  and  32  are connected in series to generate nine power levels normally, if an error occurs at one of the first and the second multilevel inverters  31  and  32 , the other normal multilevel inverter may be operated alone to output five power levels. 
         [0038]    The AC filter  40  insulates the inverter  30  from a power grid. 
         [0039]    The control unit  60  may apply a control signal to the converter and the multilevel inverter  30 . 
         [0040]      FIG. 2  is a view illustrating the solar power conversion apparatus in detail according to the embodiment. 
         [0041]    Referring to  FIG. 2 , first and second converters  21  and  22  of the AC-DC converter  20  may include diodes D 1 , D 2 , D 3 , and D 4 , capacitors C 1  and C 2 , inductors L 1  and L 2 , and switching elements T 1  and T 2 , respectively. 
         [0042]    The multilevel inverter  30  may include a plurality of switching elements, an insulated gate bipolar (IGBT), and a bus capacitor limiting a ripple of DC voltage. The respective inverters  31  and  32  are connected in series. The detail structure and operation of the multilevel inverter  30  will be described later. 
         [0043]    The control unit  60  may include a converter gating unit  61  and an inverter gating unit  62 . 
         [0044]    The converter gating unit  61  applies a gating signal to the converter  20  for maximum power point tracking (MPPT) control. 
         [0045]    The inverter gating unit  62  applies a gating signal for controlling the operation of the inverter  30 . The inverter gating unit  62  may use a pulse width modulation signal (PWM) signal based on a carrier or a space vector PWM signal as a gating signal for controlling the operation of the inverter  30 . 
         [0046]      FIG. 3  is a view illustrating one operation of the solar array, the DC-DC converter, and the multilevel inverter according to the embodiment. If one of the inverters  31  and  32  of the solar power conversion apparatus in  FIG. 2  cannot be operated due to an error, the apparatus may be operated on the condition illustrated in  FIG. 3 . 
         [0047]    A DC power generated in the solar array  11  is MPPT-controlled by the converter  21  in order to output a maximum output value and is transmitted to the multilevel inverter  31 . The first converter  21  and the inverter  31  may be controlled by gating signals of the control unit  60 , respectively. 
         [0048]    The inverter  31  may output total five power levels according to the gating signal of the control unit  60 . The inverter  31  may include a plurality of switching elements and two bust capacitors C 3  and C 4  limiting a ripple of DC voltage. Also as illustrated in  FIG. 3 , the switching elements are arranged in two rows each having 4 elements, and clamping diodes may be arranged between the switching elements. The switching elements may preferably be insulated gate bipolar transistors (IGBTs). 
         [0049]      FIG. 4  is a view illustrating states of the switching elements in the inverter according to each output level of the embodiment. 
         [0050]    The multilevel inverter  31  may output total five power levels such as 2E, E, 0, −E, and −2E. ‘2E’ means a voltage of power applied to the inverter  31 . When outputting the power levels such as 2E, E, 0, −E, and −2E, the switching elements may be turned on as shown in the table of  FIG. 5 . 
         [0051]      FIG. 5  is a view showing a switching order of the switching elements in the inverter according to the embodiment. 
         [0052]    As shown in  FIG. 5 , if the state of switching is 1, the switching element may be on. If the state of switching also is 0, the switching element may be off. The five power levels may be generated by turning on the switching elements in an order according to a system clock as described above. 
         [0053]    In  FIG. 3  or  FIG. 4 , the multilevel inverter  31  includes four pairs of interacting IGBT switches: (TA 11 +, TA 11 −), (TA 22 +, TA 22 −), (TB 11 +, TB 11 −), and (TB 22 +, TB 22 −). 
         [0054]    In  FIG. 4 , (a), (b), (c), and (d) are exemplary views illustrating turned-on states of the switching elements according to output levels, wherein (a) shows the output power level 2E, (b) shows the output power level E, (C) shows the output power level −E, and (d) shows the output power level −2E. 
         [0055]    Meanwhile, the number of elements required according to the number (m) of outputting multi levels is shown in Table 1 below. 
         [0000]    
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Flying 
                 H-bridge 
                   
                   
               
               
                   
                 Capacitor 
                 cascade 
                 Diode clamp 
                 Hybrid diode 
               
               
                   
                 method 
                 method 
                 method 
                 clamp method 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 Main 
                 (m − 1) × 2 
                 (m − 1) × 2 
                 (m − 1) × 2 
                 (m − 1) × 2 
               
               
                 switching 
               
               
                 element 
               
               
                 Clamping 
                 0 
                 0 
                 (m − 1) × 
                 (m − 1) 
               
               
                 diode 
                   
                   
                 (m − 2) 
               
               
                 DC bus 
                 (m − 1) 
                 (m − 1 )/2 
                 (m − 1) 
                 (m − 1)/2 
               
               
                 capacitor 
               
               
                 Balance 
                 (m − 1 ) × 
                 0 
                 0 
                 0 
               
               
                 capacitor 
                 (m − 2)/2 
               
               
                   
               
             
          
         
       
     
         [0056]    In Table 1, the hybrid diode clamp method is the method that the multilevel inverters are connected in series according to the embodiment. 
         [0057]    Also, if two multilevel inverters are connected in series as illustrated in  FIG. 2 , total nine power levels such as 4E, 3E, 2E, E, 0, −E, −2E, −3E, and −4E, may be outputted. 
         [0058]      FIG. 6  is a view illustrating an output signal of the solar power conversion apparatus according to the embodiment. 
         [0059]    As illustrated in  FIG. 6 , a signal of a required power level may be extracted from the output signal and then be transmitted to the power grid. 
         [0060]    Although embodiments have been described with reference to a number of illustrative embodiments thereof, it should be understood that numerous other modifications and embodiments can be devised by those skilled in the art that will fall within the spirit and scope of the principles of this disclosure. More particularly, various variations and modifications are possible in the component parts and/or arrangements of the subject combination arrangement within the scope of the disclosure, the drawings and the appended claims. In addition to variations and modifications in the component parts and/or arrangements, alternative uses will also be apparent to those skilled in the art.