# Patent

## Title
Transformerless power conversion circuit for grid-connected power generation systems

## 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.

## Background
BACKGROUND OF THE INVENTION
1. Field of the Invention
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.
2. Description of Prior Art
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.
FIG. 1 shows a prior art single-stage three-phase three-wire power conversion circuit. As shown in this figure, capacitors C1 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 Q3 is turned on and the switch Q4 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.
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.
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
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.
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
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:
FIG. 1 shows a prior art single-stage three-phase three-wire power conversion circuit.
FIG. 2 shows prior art single-stage three-phase three-wire power conversion circuit with a transformer arranged on the output side thereof.
FIG. 3 shows a prior art two-stage three-phase three-wire power conversion circuit.
FIG. 4 shows prior art two-stage three-phase three-wire power conversion circuit with a transformer arranged on the output side thereof.
FIG. 5 shows a prior art single-stage two-phase two-wire power conversion circuit.
FIG. 6 shows prior art single-stage two-phase two-wire power conversion circuit with a transformer arranged on the output side thereof.
FIG. 7 shows a prior art two-stage single-phase two-wire power conversion circuit.
FIG. 8 shows prior art two-stage single-phase two-wire power conversion circuit with a transformer arranged on the output side thereof.
FIG. 9 shows the two-stage three-phase type power conversion circuit according to a preferred embodiment of the present invention.
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
The present invention is intended to provide a transformerless conversion circuit, which feeds the electrical power of distributed power generator to the interconnected grid.
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 Q5, a coupled inductor (first inductor L1 and second inductor L2) sharing the same core, two passive switches (first passive switch D1 and second passive switch D2), and two capacitors (first capacitor C1 and second capacitor C2). The half-bridge inverter 30 converts the two DC levels into alternative voltage for feeding power to grid and comprises four active switches Q1-Q4.
The active switch Q5 is electrically connected between the positive end of DC voltage source V1 and the second inductor L2. Another end of the second inductor L2 is electrically connected to the negative end of the voltage source V1 and also electrically connected to the first ground Node 1. One end of the first inductor L1 is electrically connected to the first ground Node 1 and another end of the first inductor L1 is electrically connected to the positive end of the first passive switch D1.
The positive end of the second passive switch D2 is electrically connected to the negative end of the second capacitor C2. The positive end of the passive switch D1 is electrically connected to another end of the first inductor L1, while the negative end of the passive switch D1 is electrically connected to the positive end of the first capacitor C1.
The positive end of the second capacitor C2 is electrically connected to the ground Node 1, the negative end of the second capacitor C2 is electrically connected to the positive end of the second passive switch D2. The positive end of the first capacitor C1 is electrically connected to the negative end of the passive switch D1. The negative end of the first capacitor C1 is electrically connected to the ground Node 1.
When the current of the DC voltage source V1 flows through the first active switch Q5, energy is stored in the second inductor L2. The energy is coupled from iron core to the first inductor L1 because the first inductor L1 and the second inductor L2 share the same iron core. The buck-boost converter 20 will produce two DC voltage levels V5, V6 on the capacitors C1 and C2. The two DC voltage levels V5, V6 are converted into alternative voltage by the two half-bridge inverters 30.
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 Q1 is turned on, the current of the switch control circuit flows from the positive end of the capacitor C1, and through the switch Q1 and the capacitor C3, and then flows back to the negative end of the capacitor C1. When the switch Q2 is turned on, the current of the switch control circuit flows from the negative end of the capacitor C2, and through the switch Q2 and the capacitor C3, and then flows back to the positive end of the capacitor C2.
Through the inductor couple circuit L1 and L2, the single DC input V1 is converted to two DC voltages V5 and V6. 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 V5 and V6 can transmit energy to alternative source side by suitable circuit path.
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 Q3, a set of inductors (first inductor L1 and second inductor L2) sharing the same core, two passive switches (first passive switch D1 and second passive switch D2), and two capacitors (first capacitor C1 and second capacitor C2). The half-bridge inverter 60 comprises two active switches Q1-Q2.
When the current of the DC voltage source V1 flows through the first active switch Q3, energy is stored in the second inductor L2. The energy is coupled from iron core to the first inductor L1 because the first inductor L1 and the second inductor L2 share the same iron core. The buck-boost converter 50 will produce two DC voltage levels V3, V4 on the capacitors C1 and C2. The two DC voltage levels V3, V4 are converted into alternative voltage by the two half-bridge inverters 60.
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 Q1 is turned on, the current of the switch control circuit flows from the positive end of the capacitor C1, and through the switch Q1 and the capacitor C3, and then flows back to the negative end of the capacitor C1. When the switch Q2 is turned on, the current of the switch control circuit flows from the negative end of the capacitor C2, and through the switch Q2 and the capacitor C3, and then flows back to the positive end of the capacitor C2.
Through the inductor couple circuit L1 and L2, the single DC input V1 is converted to two DC inputs V3 and V4. 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 V3 and V4 can transmit energy to alternative source side by suitable circuit path.
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









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.
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.

## Claims

1. A transformerless power conversion circuit for distributed generation system interconnected with an electric grid, the power conversion circuit converting 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 comprising:
a buck-boost converter converting the input DC power into two sets of DC power levels, wherein the buck-boost converter comprising:
a first active switch having an input end connected to a positive end of the input DC power;
a first inductor and a second inductor sharing a same iron core; wherein one end of the first inductor is electrically connected to a grounded end, one end of the second inductor is electrically connected to an output end of the first active switch, another end of the second inductor is electrically connected to a negative end of the input DC power and the ground;
a first passive switch with a positive end connected to another end of the first inductor;
a second passive switch with a negative end connected to the output end of the first active switch;
a first capacitor with a positive end connected to a negative end of the first passive switch and a negative end connected to the ground;
a second capacitor with a positive end connected to the ground and a negative end connected to a positive end of the second passive switch; and
at least one half-bridge inverter converting the two sets of DC power levels into the AC power for feeding into the grid.

2. The transformerless power conversion circuit as in claim 1, wherein the at least one half-bridge inverter generates three-phase electrical power and comprises four active switches.

3. The transformerless power conversion circuit as in claim 1, wherein the at least one half-bridge inverter generates single-phase electrical power and comprises two active switches.