Abstract:
A transformerless power conversion circuit is interconnected with an electric grid and converts an input DC power provided by a power generator into an AC power and feeding the AC power into the grid. The power conversion circuit includes a buck-boost converter converting the input DC power into two sets of DC power levels; and at least one half-bridge inverter converting the two sets of DC power levels into the AC power for feeding into the grid. The isolating transformer can be eliminated and the common ground problem for DC side and AC side is also solved. The power conversion circuit has significant improvement for device size, manufacture cost and conversion efficiency.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a power converter circuit of DC to AC conversion, especially to a conversion circuit, which does not use transformer and transfer the electrical power of distributed energy to electric utility.  
         [0003]     2. Description of Prior Art  
         [0004]     The request for electrical power is increased as the industry keeps developing. However, the shortage of natural resource and environmental concern for new power plant will make the electrical power more insufficient. Therefore, the renewable energy generation becomes important issue in industrialized or advanced country.  
         [0005]      FIG. 1  shows a prior art single-stage three-phase three-wire power conversion circuit. As shown in this figure, capacitors C 1  is parallel to DC power source for filtering noise and stabilizing voltage. In this conversion circuit, if the DC power source has common ground with the ground of public network. When the switch Q 3  is turned on and the switch Q 4  is turned off, the current flows along the path indicated by phantom line. The DC current will flow into the grid, this phenomenon is prohibited in many interconnection rules. Therefore, an output transformer is connected between the power conversion circuit and grid for isolation.  FIG. 2  shows power conversion circuit with a transformer arranged on the output side of the power converter therefor providing isolation to AC ground. However, the transformer will increase the size, the cost of the power conversion circuit and reduce conversion efficiency.  
         [0006]      FIG. 3  shows a prior art two stage three phase three wire power conversion circuit. In this circuit, both DC side and AC side are grounded, the DC current might flow into the grid through the common grounded points. This phenomenon is prohibited in many interconnection rules. Therefore, an transformer is connected between the power conversion circuit and the grid, as shown in  FIG. 4 .  
         [0007]      FIG. 5  and  FIG. 7  which are single stage and two stage power conversion circuits for single phase system, have both-side grounded with potential DC current flowing problem as mentioned above for  FIG. 1  and  FIG. 3 . Therefore, an transformer is connected between the power conversion circuit and the grid, as shown in  FIG. 6  and  FIG. 8 . However, the transformer will increase the size, weight, cost of the conversion circuit and reduce the conversion efficiency.  
       SUMMARY OF THE INVENTION  
       [0008]     The present invention is intended to provide a transformerless power conversion circuit, wherein transformer is not needed and the DC current problem can be solved. Therefore, the power conversion circuit has significant improvement for device size, manufacture cost and conversion efficiency.  
         [0009]     Accordingly, the present invention provides a transformerless power conversion circuit interconnection with the grid and converting an input DC power provided by a distributed generator into an AC power and feeding the AC power into the grid. The power conversion circuit includes a buck-boost converter converting the input DC power into two sets of DC power levels; and at least one half-bridge inverter converting the two sets of DC power levels into the AC power for feeding into the grid. 
     
    
     BRIEF DESCRIPTION OF DRAWING  
       [0010]     The features of the invention believed to be novel are set forth with particularity in the appended claims. The invention itself however may be best understood by reference to the following detailed description of the invention, which describes certain exemplary embodiments of the invention, taken in conjunction with the accompanying drawings in which:  
         [0011]      FIG. 1  shows a prior art single-stage three-phase three-wire power conversion circuit.  
         [0012]      FIG. 2  shows prior art single-stage three-phase three-wire power conversion circuit with a transformer arranged on the output side thereof.  
         [0013]      FIG. 3  shows a prior art two-stage three-phase three-wire power conversion circuit.  
         [0014]      FIG. 4  shows prior art two-stage three-phase three-wire power conversion circuit with a transformer arranged on the output side thereof.  
         [0015]      FIG. 5  shows a prior art single-stage two-phase two-wire power conversion circuit.  
         [0016]      FIG. 6  shows prior art single-stage two-phase two-wire power conversion circuit with a transformer arranged on the output side thereof.  
         [0017]      FIG. 7  shows a prior art two-stage single-phase two-wire power conversion circuit.  
         [0018]      FIG. 8  shows prior art two-stage single-phase two-wire power conversion circuit with a transformer arranged on the output side thereof.  
         [0019]      FIG. 9  shows the two-stage three-phase type power conversion circuit according to a preferred embodiment of the present invention.  
         [0020]      FIG. 10  shows the two-stage single-phase power conversion circuit according to another preferred embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0021]     The present invention is intended to provide a transformerless conversion circuit, which feeds the electrical power of distributed power generator to the interconnected grid.  
         [0022]      FIG. 9  shows the two-stage three-phase power conversion circuit according to a preferred embodiment of the present invention. The two-stage three-phase power conversion circuit  10  comprises a buck-boost converter  20  and two half-bridge inverters  30 . The buck-boost converter  20  converts a DC voltage source to two DC levels and comprises a first active switch Q 5 , a coupled inductor (first inductor L 1  and second inductor L 2 ) sharing the same core, two passive switches (first passive switch D 1  and second passive switch D 2 ), and two capacitors (first capacitor C 1  and second capacitor C 2 ). The half-bridge inverter  30  converts the two DC levels into alternative voltage for feeding power to grid and comprises four active switches Q 1 -Q 4 .  
         [0023]     The active switch Q 5  is electrically connected between the positive end of DC voltage source V 1  and the second inductor L 2 . Another end of the second inductor L 2  is electrically connected to the negative end of the voltage source V 1  and also electrically connected to the first ground Node  1 . One end of the first inductor L 1  is electrically connected to the first ground Node  1  and another end of the first inductor L 1  is electrically connected to the positive end of the first passive switch D 1 .  
         [0024]     The positive end of the second passive switch D 2  is electrically connected to the negative end of the second capacitor C 2 . The positive end of the passive switch D 1  is electrically connected to another end of the first inductor L 1 , while the negative end of the passive switch D 1  is electrically connected to the positive end of the first capacitor C 1 .  
         [0025]     The positive end of the second capacitor C 2  is electrically connected to the ground Node  1 , the negative end of the second capacitor C 2  is electrically connected to the positive end of the second passive switch D 2 . The positive end of the first capacitor C 1  is electrically connected to the negative end of the passive switch D 1 . The negative end of the first capacitor C 1  is electrically connected to the ground Node  1 .  
         [0026]     When the current of the DC voltage source V 1  flows through the first active switch Q 5 , energy is stored in the second inductor L 2 . The energy is coupled from iron core to the first inductor L 1  because the first inductor L 1  and the second inductor L 2  share the same iron core. The buck-boost converter  20  will produce two DC voltage levels V 5 , V 6  on the capacitors C 1  and C 2 . The two DC voltage levels V 5 , V 6  are converted into alternative voltage by the two half-bridge inverters  30 .  
         [0027]     The Node  1  is grounded for DC side and the Node  2  is grounded for AC side. The switch control circuit of the half-bridge inverters  30  is shown by phantom line. When the switch Q 1  is turned on, the current of the switch control circuit flows from the positive end of the capacitor C 1 , and through the switch Q 1  and the capacitor C 3 , and then flows back to the negative end of the capacitor C 1 . When the switch Q 2  is turned on, the current of the switch control circuit flows from the negative end of the capacitor C 2 , and through the switch Q 2  and the capacitor C 3 , and then flows back to the positive end of the capacitor C 2 .  
         [0028]     Through the inductor couple circuit L 1  and L 2 , the single DC input V 1  is converted to two DC voltages V 5  and V 6 . When both the DC side and the AC side have grounded system, a common ground point Node  1  can be provided and the two DC inputs V 5  and V 6  can transmit energy to alternative source side by suitable circuit path.  
         [0029]      FIG. 10  shows the two-stage single-phase type power conversion circuit according to another preferred embodiment of the present invention. The two-stage single-phase type power conversion circuit  40  comprises a buck-boost converter  50  and a half-bridge inverter  60 . In other word, one half-bridge inverter is removed in comparison with preferred embodiment of  FIG. 9 . The buck-boost converter  50  comprises a first active switch Q 3 , a set of inductors (first inductor L 1  and second inductor L 2 ) sharing the same core, two passive switches (first passive switch D 1  and second passive switch D 2 ), and two capacitors (first capacitor C 1  and second capacitor C 2 ). The half-bridge inverter  60  comprises two active switches Q 1 -Q 2 .  
         [0030]     When the current of the DC voltage source V 1  flows through the first active switch Q 3 , energy is stored in the second inductor L 2 . The energy is coupled from iron core to the first inductor L 1  because the first inductor L 1  and the second inductor L 2  share the same iron core. The buck-boost converter  50  will produce two DC voltage levels V 3 , V 4  on the capacitors C 1  and C 2 . The two DC voltage levels V 3 , V 4  are converted into alternative voltage by the two half-bridge inverters  60 .  
         [0031]     The Node  1  is grounded for DC side and the Node  2  is grounded for AC side. The switch control circuit of the half-bridge inverters  60  is shown by phantom line. When the switch Q 1  is turned on, the current of the switch control circuit flows from the positive end of the capacitor C 1 , and through the switch Q 1  and the capacitor C 3 , and then flows back to the negative end of the capacitor C 1 . When the switch Q 2  is turned on, the current of the switch control circuit flows from the negative end of the capacitor C 2 , and through the switch Q 2  and the capacitor C 3 , and then flows back to the positive end of the capacitor C 2 .  
         [0032]     Through the inductor couple circuit L 1  and L 2 , the single DC input V 1  is converted to two DC inputs V 3  and V 4 . When both the DC side and the AC side have grounded system, a common ground point Node  1  can be provided and the two DC inputs V 3  and V 4  can transmit energy to alternative source side by suitable circuit path.  
         [0033]     To sum up, the power conversion circuit according to the present invention can solve the problem of common ground and has significant improvement for device size, manufacture cost and conversion efficiency. Moreover, the manufacture cost and power loss can be further reduced by using less active switch.  
                                                                     TABLE                           Comparison between the conventional power conversion circuit and       the power conversion circuit according to the present invention                Active switch           number                Isolating   Applicable DC/AC   Single   Three       Cases   transformer   grounded system   phase   phase                    Conventional   None   One side grounded   4   6       single stage   Output   One side/two sides   4   6       converter   transformer   grounded       Conventional   None   One side grounded   3   5       two stage   Output   One side/two sides   3   5       converter   transformer   grounded           High   One side/two sides   8   10           frequency   grounded           transformer       Present   None   One side/two sides   3   5       invention       grounded                  
 
         [0034]     As can be seen from above table, the present invention solves the one side or two sides grounded problem for conventional power conversion circuit. The isolating transformer is removed to reduce size and weight of the power conversion circuit. The power conversion circuit according to the present invention has significant improvement for device size, manufacture cost and conversion efficiency. Moreover, the manufacture cost and power loss can be further reduced by using less active switches.  
         [0035]     Although the present invention has been described with reference to the preferred embodiment thereof, it will be understood that the invention is not limited to the details thereof. Various substitutions and modifications have suggested in the foregoing description, and other will occur to those of ordinary skill in the art. Therefore, all such substitutions and modifications are intended to be embraced within the scope of the invention as defined in the appended claims.