Patent Publication Number: US-9837930-B2

Title: Power supply system and power conversion device

Description:
This application is based upon and claims priority to Chinese Patent Application No. 201510210639.7, filed Apr. 28, 2015, the entire contents of which are incorporated herein by reference. 
     TECHNICAL FIELD 
     The present disclosure relates to the technical field of power supply management, and more particularly, to a power conversion device and a power supply system including the power conversion device. 
     BACKGROUND 
     Nowaday, power generation using renewable energy sources, for example, photovoltaic power generation, wind energy power generation, fuel cell power generation, has been developed from power supplies having small power at early time to grid-connected power generation as public power at present, and are developing towards quantity production and large-scale utilization, and correspondingly the application range thereof has extended to various power utilization fields. 
     With the proliferation of power generation using renewable energy sources, grid-connected converters are increasingly upgraded. Taking a photovoltaic power generating system as an example, grid-connected converters have developed from transformers having low-frequency isolation originally to transformers having high-frequency isolation, and finally to omission of transformers. 
     An equivalent photovoltaic-panel-to-ground capacitance is relatively large, and thus, in consideration of leakage current in a system loop, if an input terminal is a photovoltaic input, then circuit topologies or control modes having low leakage current shall be selected. Accordingly, various converter circuit topologies such as H4, H5 and H6 (four-switch, five-switch and six-switch) or the like are derived. However, all of these converter circuit topologies have only considered a case where electronic outputs at an output port under a grid-connected operation and electronic outputs at an output port under a standalone operation are consistent, but none of them applies to a case where the requirements of the grid-connected operation and the standalone operation are inconsistent. 
     Taking a converter, which has a H6 topology and low leakage current in the photovoltaic grid-connected field, as an example, as shown in  FIG. 1 , during the grid-connected operation, the converter provides a single-phase two-wire output at an output port, and during the standalone operation, the converter can only provide the single-phase-two-wire output as well. Apparently, the converter does not apply to a scenario where the standalone operation requires a single-phase-three-wire output. In related arts, other converter circuit topologies also have the above problems. 
     Therefore, there exists a need to provide a power conversion device aiming at the above problems. 
     SUMMARY 
     The present disclosure provides a power conversion device and a power supply system including the power conversion device so as to apply to a scenario where the requirements of the grid-connected operation and the standalone operation are inconsistent and consequently to overcome one or more problems caused by limitations and defects in related arts. 
     Other characters and advantages of the present disclosure will become apparent through the detailed descriptions hereinafter, or be known partially through the practice of the present disclosure. 
     According to a first aspect of the present disclosure, there is provided a power conversion device for converting electric energy outputted by a power supply module, which is coupled with first and second bus capacitors in series, the power conversion device comprising: 
     an electric energy conversion module configured to convert the electric energy outputted by the power supply module into a single-phase two-wire output or a single-phase three-wire output, wherein the electric energy conversion module includes: 
     a voltage-balanced half-bridge circuit having a bridge arm midpoint coupled with each first terminal of the first and second bus capacitors; 
     a bridge conversion circuit having a first input terminal coupled with a second terminal of the first bus capacitor, a second input terminal coupled with a second terminal of the second bus capacitor, and first and second output terminals providing the single-phase two-wire output; and 
     a neutral line having a first terminal coupled with the bridge arm midpoint, and a second terminal providing the single-phase three-wire output together with the first and second output terminals of the bridge conversion circuit; and 
     a switching module coupled with the electric energy conversion module, and configured to determine the electric energy conversion module to provide the single-phase two-wire output or the single-phase three-wire output. 
     According to a second aspect of the present disclosure, there is provided a power supply system comprising a first power supply module and a second power supply module which are coupled with a load through at least two wires; 
     wherein the first power supply module comprises a power conversion device according to the first aspect, a first port and a second port, the first port is electrically coupled to the second power supply module, and the load is selectively coupled with the first port or the second port through a selector switch; 
     wherein the power conversion device is electrically coupled to the first port via an output switch and a grid-connected switch, and the power conversion device is electrically coupled to the second port via the output switch; and 
     wherein the power supply system employs at least one of the first power supply module and the second power supply module to provide power supply for the load according to the output switch, the grid-connected switch and the selector switch. 
     In the power conversion device and the power supply system according to exemplary embodiments of the present disclosure, the electric energy conversion module capable of converting the electric energy outputted by the power supply module into a first type output (i.e., a single-phase two-wire output) or a second type output (i.e., a single-phase three-wire output) and the switching module coupled with the electric energy conversion module are provided. Thus, during the grid-connected operation, the first type output is provided; and during the standalone operation, the second type output is provided. Consequently, the scenario where the requirements of the grid-connected operation and the standalone operation are different can be satisfied. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above-mentioned and other characteristics and advantages of the present disclosure will become more apparent from to detailed description of exemplary embodiments with reference to drawings. 
         FIG. 1  is a circuit diagram schematically showing a power conversion device having an H6 topology in related arts. 
         FIG. 2  is a diagram schematically showing a structure of a power conversion device according to an exemplary embodiment of the present disclosure. 
         FIG. 3  is a diagram schematically showing another structure of a power conversion device according to an exemplary embodiment of the present disclosure. 
         FIG. 4  is a circuit diagram when the power conversion device shown in  FIG. 3  is in a grid-connected operation. 
         FIG. 5  is a schematic diagram of a first inverter when the power conversion device shown in  FIG. 3  is in a standalone operation. 
         FIG. 6  is a simplified circuit diagram of the first inverter shown in  FIG. 5 . 
         FIG. 7  is a schematic diagram of the first inverter after further simplification with neglecting of the half-bridge circuit. 
         FIG. 8  is a schematic diagram of the second inverter when the power conversion device shown in  FIG. 3  is in a standalone operation. 
         FIG. 9  is a simplified circuit diagram of the second inverter shown in  FIG. 8 . 
         FIG. 10  is a schematic diagram showing a simplified circuit obtained by series connection of output voltages of the first inverter and the second inverter when the power conversion device is in the standalone operation. 
         FIG. 11  is a simplified circuit diagram of  FIG. 3 . 
         FIG. 12  is a control block diagram of the balanced half-bridge circuit. 
         FIG. 13  is a control block with inner current control loop of the balanced half-bridge circuit. 
         FIG. 14  is a control block with the feedforward voltage of the second capacitor. 
         FIG. 15  is a circuit diagram of another power conversion device according to exemplary embodiments of the present disclosure. 
         FIG. 16  is a circuit diagram of another power conversion device according to exemplary embodiments of the present disclosure. 
         FIG. 17  is a circuit diagram of a power supply system according to exemplary embodiments of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Now, exemplary implementations will be described more comprehensively with reference to the drawings. However, the exemplary implementations may be carried out in various manners, and shall not be interpreted as being limited to the implementations set forth herein; instead, providing these implementations will make the present disclosure more comprehensive and complete and will fully convey the conception of the exemplary implementations to one of ordinary skill in this art. Throughout the drawings, similar reference characters indicate the same or similar structures, and their detailed description will be omitted. 
     The expressions “couple” and “connect” used herein may refer to that two or more elements are in direct contact with each other physically or electrically, or in indirect contact with each other physically or electrically. The expressions “first”, “second”, . . . , and the like used herein are only intended to distinguish elements or operations described using the same technical terms, rather than particularly define order or sequence. The expressions “include”, “comprise”, “have”, and the like, used herein are all open terms, which means including but not limited to. Direction terms used herein, for example, “up”, “down”, “left”, “right”, “front” or “back”, and the like, are only directions relative to drawings. Therefore, the direction terms used herein are intended to illustrate rather than limit the present application. 
     In addition, the features, structures or characteristics described herein may be combined in one or more embodiments in any suitable manner. In the following description, many specific details are provided to facilitate sufficient understanding of the embodiments of the present disclosure. However, one of ordinary skill in this art will appreciate that the technical solutions in the present disclosure may be practiced without one or more of the specific details, or other structures, components, steps, or methods, and so on may be employed. In other conditions, well-known structures, components or operations are not shown or described in detail to avoid confusion of respective aspects of the present disclosure. 
       FIG. 2  is a diagram schematically showing a structure of a power conversion device according to an exemplary embodiment of the present disclosure. The power conversion device is configured to convert the electric energy output by a power supply module. The power conversion device includes an electric energy conversion module and a switching module. 
     In the exemplary embodiment, explanations will be made by taking an example where the power supply module is a Direct Current (DC) power generation device using a renewable energy source, such as a photovoltaic power generation device or a fuel cell power generation device, which will not be specifically defined in the exemplary embodiment. The power supply module is coupled in series with a bus capacitor module  10 . In the exemplary embodiment, the bus capacitor module  10  may include a first bus capacitor C 1  and a second bus capacitor C 2  that are coupled in series with each other. The electric energy conversion module is configured to convert the electric energy output by the power supply module into a first type output or a second type output. Corresponding to the DC power generation device using a renewable energy source, the electric energy conversion module is a Direct Current to Alternating Current (DC/AC) converter. In the exemplary embodiment, explanations will be made by taking an example where the first type output is a single-phase two-wire output and the second type output is a single-phase three-wire output. However, in other exemplary embodiments of the present disclosure, the first type output and the second type output may be other types. The switching module is coupled with the electric energy conversion module, and is configured to control the electric energy conversion module to provide the first type output or the second type output. 
     Again referring to  FIG. 2 , the electric energy conversion module in the exemplary embodiment may include a half-bridge circuit  20 , a bridge conversion circuit  30  and a neutral line. In a specific exemplary embodiment, the electric energy conversion module may further include a bridge output filter circuit  40 , a relay protection device  50 , related switching components and a control unit, and the like. 
     Specifically, the middle point of the bridge arm of the half-bridge circuit  20  is coupled to a first terminal of the first bus capacitor C 1  and a first terminal of the second bus capacitor C 2 , i.e., is coupled to a series-connection node of the first bus capacitor C 1  and the second bus capacitor C 2 . The half-bridge circuit  20  is configured to balance voltages across the first bus capacitor C 1  and the second bus capacitor C 2 . In the exemplary embodiment, the half-bridge circuit  20  includes a first switch QB 1  and a second switch QB 2 , each of which may be a MOSFET (Metal-Oxide-Semiconductor Field Effect Transistor), an IGBT (Insulated Gate Bipolar Transistor) or a BJT (Bipolar Junction Transistor). Materials of the switching elements may include Si, SiC, GaN or other wide bandgap semiconductor materials or the like, which will not be specifically defined in the exemplary embodiment. 
     The bridge conversion circuit  30  includes first and second input terminals as well as first and second output terminals. The first and second input terminals of the bridge conversion circuit  30  are coupled with a second terminal of the first bus capacitor C 1  and a second terminal of the second bus capacitor C 2 , respectively. The first and second output terminals of the bridge conversion circuit  30  are configured to provide the first type output (such as single-phase two-wire output). The bridge conversion circuit  30  herein may be a half-bridge inverter circuit or a full-bridge inverter circuit, and the like. Taking the full-bridge inverter circuit as an example, it may be a four-switch full-bridge inverter circuit (hereinafter referred to as H4 topology), a five-switch full-bridge inverter circuit (hereinafter referred to as H5 topology), a six-switch full-bridge inverter circuit (hereinafter referred to as H6 topology), or a seven-switch full-bridge inverter circuit (hereinafter referred to as H7 topology), and the like. In the exemplary embodiment, the switches in the H4 topology, the H5 topology, the H6 topology, the H7 topology or other topologies may be a MOSFET, an IGBT, or a BJT, and the like, and the materials of the switches may include Si, SiC, GaN or other wide bandgap semiconductor materials and the like, which will not be specifically defined in the exemplary embodiment. 
     A first terminal of the neutral line is coupled with the series-connection node of the first bus capacitor C 1  and the second bus capacitor C 2 , and a second terminal of the neutral line, together with the first and second output terminals of the bridge conversion circuit  30 , provides the second type output (such as a single-phase three-wire output). In the exemplary embodiment, the switching module may include a controlled switch S N  arranged between the first terminal and the second terminal of the neutral line. In the exemplary embodiment, the controlled switch S N  may be a bidirectional switch such as a MOSFET, an IGBT, a BJT, a thyristor, or a relay, and the like, which will not be specifically defined in the exemplary embodiment. 
     The operation principle and more details of the above power conversion device will be described below in detail with reference to a H6 topology. 
     As shown in  FIG. 3 , the bridge conversion circuit  30  in  FIG. 2  includes a first switching element Q 1  through a seventh switching element Q 7 . The first switching element Q 1  includes a first terminal, a second terminal and a control terminal, and the first terminal of the first switching element Q 1  is coupled with the second terminal of the first bus capacitor C 1 . The second switching element Q 2  includes a first terminal, a second terminal and a control terminal, and the first terminal of the second switching element Q 2  is coupled with the first terminal of the first switching element Q 1 . The third switching element Q 3  includes a first terminal, a second terminal and a control terminal, and the second terminal of the third switching element Q 3  is coupled with the second terminal of the second bus capacitor C 2 . The fourth switching element Q 4  includes a first terminal, a second terminal and a control terminal, and the second terminal of the fourth switching element Q 4  is coupled with the second terminal of the third switching element Q 3 . The fifth switching element Q 5  includes a first terminal, a second terminal and a control terminal, and the first terminal of the fifth switching element Q 5  is coupled with the second terminal of the first switching element Q 1  and a first terminal of the sixth switching element Q 6 . The sixth switching element Q 6  includes a first terminal, a second terminal and a control terminal, and the first terminal of the sixth switching element Q 6  is coupled with the second terminal of the first switching element Q 1  and the first terminal of the fifth switching element Q 5 , and the second terminal of the sixth switching element Q 6  is coupled with the first terminal of the third switching element Q 3 . Moreover, in consideration of demands of topology transformation, a seventh switching element Q 7  may be added as well to change the bridge conversion circuit in  FIG. 3  from the H6 topology to a H7 topology. The seventh switching element Q 7  may include a first terminal, a second terminal and a control terminal, and the first terminal of the seventh switching element Q 7  is coupled with the second terminal of the fifth switching element Q 5 , and the second terminal of the seventh switching element Q 7  is coupled with the second terminal of the second switching element Q 2  and the first terminal of the fourth switching element Q 4 . The seventh switching element Q 7  together with the fifth switching element Q 5  realizes the effect of electrically isolating two bridge arms. In an exemplary embodiment of the present disclosure, the seventh switching element Q 7  may include a switch capable of realizing electrical isolation function such as a MOSFET, an IGBT, a BJT, or a relay, and the like, which will not be specifically defined in the exemplary embodiment. 
     As shown in  FIG. 4 , during normal grid-connected operation, the above controlled switch S N  is turned off and neither of the switching elements (the first switch QB 1  and the second switch QB 2 ) on the half-bridge circuit  20  works, and the seventh switching element Q 7  is normally on. The inverter output of the bridge conversion circuit is filtered by the bridge output filter circuit  40 , and then is coupled with a grid through the relay protection device  50 . For example, the bridge output filter circuit  40  includes an inductor L 1 , a capacitor C ac1 , an inductor L 2  and a capacitor C ac2 . The inductor L 1  and the capacitor C ac1  form an LC filter, and the inductor L 2  and the capacitor C ac2  form a LC filter. 
     When the grid has a failure and the power generator using a renewable energy source operates under a standalone mode, the fifth switching element Q 5  and the seventh switching element Q 7  are turned off, and the controlled switch S N  is turned on, and thus the bridge conversion circuit is transformed into two series half-bridge inverter topologies, which are hereinafter referred to as a first inverter and a second inverter. 
     In  FIG. 5 , the first inverter is composed of the first bus capacitor C 1 , the second bus capacitor C 2 , the second switching element Q 2 , the fourth switching element Q 4 , the first output inductor L 1  and the first output capacitor C ac1 . The topology of the first inverter after simplification is as shown in  FIG. 6 . It is supposed that the half-bridge circuit  20  can ideally control the voltage balance between the first bus capacitor C 1  and the second bus capacitor C 2 , and then the topology after further simplification is as shown in  FIG. 7 . The modulation mode of the first inverter may be a conventional half-bridge control mode, in which the output voltage closed loop control would produce the controlled duty cycle to the second switching element Q 2 , and the fourth switching element Q 4  and the second switching element Q 2  are complementary to each other. 
     In  FIG. 8 , the second inverter is composed of the first bus capacitor C 1 , the second bus capacitor C 2 , the first switching element Q 1 , the third switching element Q 3 , the sixth switching element Q 6 , the second output inductor L 2  and the second output capacitor C ac2 . The topology after simplification in the same manner as above is shown in  FIG. 9 . The modulation mode of the second inverter may be a conventional half-bridge control mode, in which the output voltage closed loop control would produce the controlled duty cycle to the first switching element Q 1 , the third switching element Q 3  and the first switching element Q 1  are complementary to each other, and the sixth switching element Q 6  are normally on. 
     When the two inverter output voltages of the first inverter and the second inverter work at the same time, an equivalent circuit diagram is as shown in  FIG. 10 . Modulating waveforms of an AN phase output voltage and a BN phase output voltage ensure a 180 degree phase difference, and a voltage obtained by series connection of the two phases of inverter voltages has an amplitude twice of the amplitude of each of the two phases of voltages and has a frequency consistent with that of each of the two phases of voltages. 
     Moreover, considering that the voltages across the first bus capacitor C 1  and the second bus capacitor C 2  will become unbalanced when two loads are unbalanced during standalone operation mode of the inverter, the voltage balance of the first bus capacitor C 1  and the second bus capacitor C 2  is controlled via the voltage-balanced half-bridge circuit  20 . The topology of the half-bridge circuit  20  after simplification is as shown in  FIG. 11 . With neglecting of the current perturbation of the middle point of the half-bridge circuit  20  (the inverter outputs the difference of the output current from the inverter AN and the inverter BN), a mathematical model of the half-bridge circuit  20  is established, a loop equation when switching on the first switch QB 1  and a loop equation when turning off the first switch QB 1  are respectively written, and a relationship between the voltage difference Δu C (s) of the first bus capacitor C 1  and the second bus capacitor C 2 , and the drive duty ratio d up (s) of the first switch QB 1  is derived as shown in Formula (1), where Cs represents the capacitance of the first bus capacitor C 1  (second bus capacitor C 2 ), Ls represents the inductance of the inductor in  FIG. 11 , r represents the equivalent resistance of the inductor, u bus  represents a voltage value provided by the power supply module, and u C     2   (s) represents the voltage across the second bus capacitor C 2 . 
     
       
         
           
             
               
                 
                   
                     Δ 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     
                       
                         u 
                         C 
                       
                       ⁡ 
                       
                         ( 
                         s 
                         ) 
                       
                     
                   
                   = 
                   
                     
                       
                         
                           d 
                           up 
                         
                         ⁢ 
                         
                           u 
                           bus 
                         
                       
                       - 
                       
                         
                           u 
                           
                             C 
                             2 
                           
                         
                         ⁡ 
                         
                           ( 
                           s 
                           ) 
                         
                       
                     
                     
                       Cs 
                       ⁡ 
                       
                         ( 
                         
                           Ls 
                           + 
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                         ) 
                       
                     
                   
                 
               
               
                 
                   Formula 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     ( 
                     1 
                     ) 
                   
                 
               
             
           
         
       
     
     With neglecting of the voltage perturbation across the second bus capacitor C 2 , there is: 
     
       
         
           
             
               
                 
                   
                     Δ 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     
                       
                         u 
                         C 
                       
                       ⁡ 
                       
                         ( 
                         s 
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                   = 
                   
                     
                       
                         d 
                         up 
                       
                       ⁢ 
                       
                         u 
                         bus 
                       
                     
                     
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                       ⁡ 
                       
                         ( 
                         
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                   Formula 
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                     ( 
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     A PI (proportion integral) control block diagram as shown in  FIG. 12  can be obtained according to the Formula (2), and in this control block diagram, a delay elements includes a sampling delay and control delay. When the current perturbation on the middle point is larger, the voltage balance on the middle point cannot be better realized using a PI control loop. If it is desired to quickly suppress the perturbation and satisfy the voltage balance on the common node of the first bus capacitor C 1  and the second bus capacitor C 2 , a huge fluctuation of the voltage across the second bus capacitor C 2  will be caused when the current perturbation on the middle point is larger. The exemplary embodiment provides two solutions: one solution is to introduce an inner-loop control using a sampling current of an inductor of the voltage-balanced half-bridge  20 ; and the second solution is to introduce a feedforward of the voltage across the second bus capacitor C 2 . The two solutions are specifically and respectively shown in  FIGS. 13 and 14 , and both of the two solutions may have a strong suppression effect on the perturbation. 
     A voltage balance control circuit is used to suppress the unbalance between the voltage across the first bus capacitor C 1  and the second bus capacitor C 2 . Therefore, when the loads are symmetric and relatively light, the half-bridge circuit  20  may be arranged to do not work. Nevertheless, when the loads are asymmetric or relatively heavy, the half-bridge circuit  20  employs a burst control mode so that the half-bridge circuit  20  suppresses the voltage difference when the voltage difference is relatively large, while the first switch QB 1  and the second switch QB 2  are turned off so as to reduce losses when the voltage difference is relatively small. 
     A DC bus voltage control value is based on k times of the peak output voltage of the inverter, and k can be 1.05 to 1.35 usually. It is supposed that the voltages across the first bus capacitor C 1  and the second bus capacitor C 2  are balanced, the voltage values across the first bus capacitor C 1  and the second bus capacitor C 2  are respectively a half of the DC bus voltage control value, then there is: 
     
       
         
           
             
               
                 
                   
                     
                       V 
                       bus 
                     
                     = 
                     
                       
                         2 
                       
                       ⁢ 
                       k 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         V 
                         
                           rm 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           s 
                         
                       
                     
                   
                   ⁢ 
                   
                     
 
                   
                   ⁢ 
                   
                     
                       V 
                       
                         bus 
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                         up 
                       
                     
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                         V 
                         
                           bus 
                           ⁢ 
                           
                               
                           
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                           ⁢ 
                           
                               
                           
                           ⁢ 
                           dw 
                         
                       
                       = 
                       
                         
                           
                             1 
                             2 
                           
                           ⁢ 
                           
                             V 
                             bus 
                           
                         
                         = 
                         
                           
                             
                               
                                 2 
                               
                               ⁢ 
                               k 
                             
                             2 
                           
                           ⁢ 
                           
                             V 
                             
                               r 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
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                               ⁢ 
                               
                                   
                               
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     where V rms  represents the root mean square(RMS) value of line voltage of the inverter output, and k is a coefficient set in consideration of the voltage drop across a inverter bridge arm and a dynamic margin, V bus   _   up  represents the voltage across the first bus capacitor C 1 , and V bus   _   dw  represents the voltage across the second bus capacitor C 2 . 
     Considering that the perturbation of the output voltage on the voltage-balanced half-bridge  20  is a sinusoidal signal, 0.4%˜10% of the output voltage of the inverter bridge arm is reserved for the regulation of the voltage-balanced half-bridge circuit  20 . Taking the voltage across the first bus capacitor C 1  as an example, the minimum of a half bus voltage needs to meet the following condition: 
     
       
         
           
             
               V 
               
                 bus 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 _ 
                 ⁢ 
                 
                     
                 
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                 up 
               
             
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                       ⁢ 
                       
                           
                       
                       ⁢ 
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     Therefore, it is supposed that:
 
Δ V≦V   bus   _   up   −V   bus   _   up   _   min  
 
     For example, the output voltage of the inverter bridge arm is 200V, and the peak of the output voltage of the inverter bridge arm is 200V*1.414, and then the designed working range of the DC bus voltage according to the above method is 297V˜382V (200V×1.414×[1.05, 1.35]). It is supposed that the bus voltage is 330 v, then the reserved voltage difference value can be 33V (i.e., 330V-297V). When the actual voltage difference is less than 16.5V, the half-bridge circuit  20  does not work, while when the actual voltage difference is greater than or equal to 16.5V, the half-bridge circuit  20  works. 
     Although it is explained by taking the H6 topology for example in the foregoing exemplary embodiment, the bridge conversion circuit  30  may also be other circuit topologies such as the H4 topology, the H5 topology and the like in other exemplary embodiments of the present disclosure. 
     As shown in  FIG. 15 , the bridge conversion circuit  30  has a conventional H4 topology. During a grid-connected operation, the controlled switch S N  is turned off, and the control method can either be unipolar or bipolar, and thus a single-phase two-wire voltage source is usually output. During a standalone operation, the controlled switch S N  is turned on, and two independent voltage sources having half-bridge inverter output circuit topologies can be formed and are connected in series at the output side, and consequently, u AN =u BN  is similarly ensured, i.e., a single-phase three-wire voltage source can be output during the standalone operation. 
     As shown in  FIG. 16 , the bridge conversion circuit  30  is a H5 topology with low leakage current. When the H5 topology is incorporated into the power conversion device in the exemplary embodiment, both outputting a single-phase two-wire voltage source during a grid-connected operation and outputting a single-phase three-wire voltage source during a standalone operation can be realized. For example, during the grid-connected operation, the controlled switch S N  is turned off, and the fifth switching element Q 5  operates according to the modulating approach for a single-phase current source; during the standalone operation, the controlled switch S N  is turned on, and the fifth switching element Q 5  is normally off. 
     For details of the bridge output filter circuit, the relay protection device, related switching components and control units in the foregoing power conversion device may be found in relevant prior art. For example, the first output capacitor C ac1 , the second output capacitor C ac2 , the first output inductor L 1 , the second output inductor L 2 , preload switches S preload , a relay switching element S 1 , a relay switching element S 2 , a relay switching element S 3 , a resistor R, a resistor R 1 , a resistor R 2  and a bidirectional thyristor SCR and the like may be included, which will not be described in details in the exemplary embodiment. 
     In an exemplary embodiment, there is also provided a power supply system. As shown in  FIG. 17 , the power supply system includes a first power supply module and a second power supply module. The first power supply module and the second power supply module are coupled with a load through at least two wires. The first power supply module includes any one of the power conversion devices in the foregoing exemplary embodiments, and may further include a first port 0 and a second port 1. The second power supply module may be a grid. The first port 0 is electrically coupled to the second power supply module, and the load is selectively coupled with the first port 0 or the second port 1 through a selector switch. The power conversion device is electrically coupled to the first port 0 via an output switch S 10  and grid-connected switches S 11  and S 12 , and the power conversion device is electrically coupled to the second port 1 via the output switch S 10 . With the on/off corporation among the output switch S 10 , the grid-connected switches S 11  and S 12 , and the selector switch, the power supply system may use the first power supply module and/or the second power supply module to supply power for the load. For instance, when both the first power supply module and the second power supply module work normally, the selector switch is electrically coupled with the first port 0 or the second port 1, the output switch S 10  and the grid-connected switches S 11  and S 12  are all turned on, and the first power supply module and the second power supply module supply power to the load at the same time. When the second power supply module works abnormally, the selector switch is electrically coupled with the second port 1, the output switch S 10  is turned on, the grid-connected switches S 11  and S 12  are turned off, and only the first power supply module supplies power to the load. When the second power supply module works normally, the selector switch is electrically coupled with the first port 0, the output switch S 10  is turned off, the grid-connected switches S 11  and S 1  are turned off, and only the second power supply module supplies power to the load. Or, when the second power supply module works normally, the selector switch is electrically coupled with the second port 1, the output switch S 10  is turned off, the grid-connected switches are turned on, and only the second power supply module supplies power to the load. 
     Again referring to  FIG. 17 , the power conversion device is applied into a PCS (Power Conversion System) converter part, and a BAT (DC cell) input terminal as shown in  FIG. 17  may either be directly connected to an inverter input port, or connected to an inverter input bus port through a converter. Through the wiring solution as shown in  FIG. 17 , the original power distribution solution of a user is not influenced at all; during a grid-connected operation, energy can be provided through interaction of a single-phase two-wire output and the grid; during an standalone operation, the PCS converter provides a single-phase three-wire voltage source output which is consistent with the power supply output of the grid under the original power distribution of the user. Therefore, the original power distribution structure of the user does not need to be adjusted. 
     Taking a household application as an example (a single-phase three-wire system is as shown in  FIG. 17 , and other systems are similar), according to the household power distribution connection manner as shown in  FIG. 17 , the grid after passing through an indoor meter, is connected to a capacity-limitation breaker K 1  of a power company, and then is connected to an indoor electrical load after passing through an indoor leakage protection breaker K 2 . It should be noted that the capacity-limitation breaker K 1  of the power company is not always connected indoors, and may also be connected outdoors. The two connection manners have no essential differences for user&#39;s indoor, and only need to connect three power wires (L 1 , L 2  and N) into the interior of the household, and do not involve in structural modifications of an indoor power distribution connection system. In order to improve the power supply reliability and quality for the user, to improve the economic benefits of using electric energy and to save the energy and protect the environment, another independent power source (PCS) is connected to the household via the power distribution wiring manner as shown in  FIG. 17 , and this does not need to make any structural modification on the original power distribution connection system inside the household; i.e., only the parts in the dotted box are inserted between the capacity-limitation breaker K 1  of the power company and the leakage protection breaker K 2 , while the indoor parts do not need to be altered, and only three incoming wires are connected indoors (the present embodiment illustrates a single-phase three-wire system, and other systems are similar), and it is very easy to upgrade the existing household power supply distribution system. Rather, due to the connection of another independent power source (PCS), the energy at the grid side can flow bidirectionally. Therefore, a measure meter needs to be changed into a bidirectional charging meter, such as a first measure meter Meter 1  and a second measure meter Meter 2  as shown in  FIG. 17 ; moreover, the measuring directions of the first measure meter Meter 1  and the second measure meter Meter 2  are opposite. When the grid is normal, the system is in a grid-connected operation; and when the grid encounters with a failure, the system is in a standalone operation. During the standalone operation, if the grid returns back to a normal state, the system switches into the grid-connected operation mode seamlessly. 
     In conclusion, the power conversion device provided by the exemplary embodiments of the application may works not only in the grid-connected operation but also in the standalone operation, and can provide a single-phase three-wire output during the standalone operation. For a scenario where the respective requirement of the grid-connected operation and the standalone operation are different, the exemplary embodiments of the present disclosure modify the existing low leakage current H4, H5, or H6 topologies and the like to satisfy that, during the grid-connected operation, the input power of the renewable energy source at the input terminal of the inverter is converted to the power at the output port of the inverter and thereby is connected to a single-phase line voltage; during the standalone operation, a single-phase three-wire voltage source output can be realized by adding small number of components. 
     The present disclosure has been described in the foregoing related embodiments. However, the foregoing embodiments are examples of implementing the present invention merely. It should be noted that the disclosed embodiments do not limit the scope of the present invention. Instead, alternations and modification figured out without departing from the spirit and scope of the present invention shall all pertain to the patent protection scope of the present invention.