TRANSFORMERLESS PHOTOVOLTAIC GRID-CONNECTING INVERTING DEVICE AND CONTROL METHOD THEREOF

• US 20100246223A1
• Filed: 06/15/2010
• Published: 09/30/2010
• Est. Priority Date: 05/15/2008
• Status: Active Grant

• Current Assignee: Wuxi Renovo Power Systems Co., Ltd.
• Original Assignee: Kun Xuan
• Inventors: XUAN, Kun
• Granted Patent: US 8,125,807 B2
• Time in Patent Office: Days
• US Class Current: 363/95
• CPC Class Codes: H02J 2300/26 involving maximum power point tracking control for photovoltaic sources maximum power point systems in particular G05F1/67
• CPC Class Codes: H02J 3/381 Dispersed generators
• CPC Class Codes: H02M 1/007 Plural converter units in cascade push-pull DC/DC converters with pre-regulator H02M3/3374; DC-AC converters following a DC-DC stage including a high frequency transformer H02M7/4807; DC-AC converters following a DC-DC conversion stage generating periodically varying voltages H02M7/4826
• CPC Class Codes: H02M 7/487 Neutral point clamped inverters
• CPC Class Codes: Y02E 10/56 Power conversion systems, e.g. maximum power point trackers
• CPC Class Codes: Y10S 323/906 Solar cell systems

Abstract

The present invention relates to a transformerless photovoltaic grid-connecting inverting device and an inverting control method thereof. The inverting device comprising a boosting unit, an inverting unit, a grid-connecting unit and a control unit. The boosting unit is connected to a solar cell, for boosting an output voltage of the solar cell and then outputting a direct voltage. The inverting unit is used to convert the direct voltage output by the boosting unit into an alternating voltage. The grid-connecting unit is connected between the inverting unit and an alternating power grid, and closes or breaks up the electrical connection between the inverting unit and the alternating power grid based on a control signal. The control unit is connected to the grid-connecting unit and the inverting unit, and determines a drive signal output to the inverting unit and a grid-connecting signal output to the grid-connecting unit based on the direct voltage and the alternating voltage and phase of the power grid, wherein the phase of the drive signal is synchronous with the phase of the alternating current in the power grid. Therefore, the present invention achieves an inverting device which has a low cost and increased efficiency and safety.

Claims

1. A transformerless photovoltaic grid-connecting inverting device comprising:

a boosting unit connected to a solar cell, for boosting an output voltage of the solar cell and then outputting a direct voltage;

an inverting unit connected to the boosting unit, for converting the direct voltage output by the boosting unit into an alternating voltage;

a grid-connecting unit connected between the inverting unit and an alternating power grid, which closes or breaks up the electrical connection between the inverting unit and the alternating power grid based on a control signal; and

a control unit connected to the grid-connecting unit and the inverting unit, which samples the direct voltage of the boosting unit and the alternating voltage and phase of the alternating power grid, and determines a drive signal output to the inverting unit and a grid-connecting signal output to the grid-connecting unit based on the direct voltage and the alternating voltage and phase of the power grid, wherein the phase of the drive signal is synchronous with the phase of the alternating current in the power grid.

2. The transformerless photovoltaic grid-connecting inverting device according to claim 1, wherein the boosting unit comprises:

an inductor, one end of which is connected to one output end of the solar cell;

a freewheeling diode, the anode end of which is connected to the other end of the inductor;

a power tube, one end of which is connected to the other end of the inductor and the other end of which is connected to the other output end of the solar cell, wherein a control end of the power tube is controlled by a switching signal having an adjustable duty ratio so as to be turned on or off, and the cathode end of the freewheeling diode outputs said direct voltage.

3. The transformerless photovoltaic grid-connecting inverting device according to claim 1, wherein the control unit comprises:

a set of sampling circuits, which sample the direct voltage of the boosting unit and the alternating voltage and phase of the alternating power grid;

a control chip, which determines the drive signal output to the inverting unit and the grid-connecting signal output to the grid-connecting unit based on the direct voltage and the alternating voltage and phase of the power grid; and

a set of driving circuits, which amplify the power of the drive signal and the grid-connecting signal and output the drive signal and the grid-connecting signal to the inverting unit and the grid-connecting unit respectively.

4. The transformerless photovoltaic grid-connecting inverting device according to claim 3, wherein the inverting unit comprises:

a first power tube, a second power tube, a third power tube and a fourth power tube, which are connected in turn between the two output ends of said boosting unit;

a first capacitor and a first diode, which are connected in series between one output end of the boosting unit and the connecting point of the first power tube and the second power tube;

a second capacitor and a second diode, which are connected in series between the other output end of the boosting unit and the connecting point of the third power tube and the fourth power, tube;

a filtering inductor, one end of which is connected to the connecting point of the second power tube and the third power tube, and the other end of which is connected to the grid-connecting circuit;

wherein the second power tube and the third power tube are respectively controlled so as to be turned on and off alternately at a frequency of the power grid;

when the second power tube is turned on, the first power tube is turned on and off continuously at a frequency that is 100˜

200 times higher than the frequency of the power grid;

when the second power tube is turned off, the first power tube is also turned off;

when the third power tube is turned on, the fourth power tube is turned on and off continuously at a frequency that is 100˜

200 times higher than the frequency of the power grid; and

when the third power tube is turned off, the fourth power tube is also turned off; and

wherein the first power tube, the second power tube, the third power tube and the fourth power tube are driven by the drive signal output from the control unit via the set of driving circuits.

5. The transformerless photovoltaic grid-connecting inverting device according to claim 1, wherein the control unit further monitors the maximum output power of the solar cell based on the direct voltage of the boosting unit, thereby adjusting the grid-connecting power, and wherein if the direct voltage is within the stabilized voltage range, the grid-connecting power will be increased and if the direct voltage is below the stabilized voltage range, the grid-connecting power will be decreased.

6. An inverting control method of a transformerless photovoltaic grid-connecting inverting device, the inverting device comprising:

a boosting unit connected to a solar cell, for boosting an output voltage of the solar cell and then outputting a direct voltage;

an inverting unit connected to the boosting unit, for converting the direct voltage output by the boosting unit into an alternating voltage;

a grid-connecting unit connected between the inverting unit and an alternating power grid, which closes or breaks up the electrical connection between the inverting unit and the alternating power grid based on a control signal; and

a control unit connected to the inverting unit and the grid-connecting unit, which controls the operation of the inverting unit and the grid-connecting unit;

wherein the inverting control method comprises the following steps;

a) the control unit samples the direct voltage of the boosting unit and the voltage and phase of the alternating power grid;

b) the control unit determines whether the direct voltage and the voltage and phase of the alternating power grid are normal, and if any of the parameters is abnormal, the method returns to the step a), otherwise the method continues to the step c);

c) the control unit outputs a drive signal to the inverting unit, so as to cause the inverting from a direct current to an alternating current to begin, wherein the phase of the drive signal is synchronous with the phase of the alternating current in the power grid;

d) the control unit controls the grid-connecting unit to close the electrical connection between the inverting unit and the alternating power grid, so that the alternating current produced by the inverting unit is coupled to the alternating power grid; and

e) during the grid-connecting, the control unit performs a maximum power point tracking step which monitors the maximum output power of the solar cell, thereby adjusting the grid-connecting power.

7. The inverting control method of a transformerless photovoltaic grid-connecting inverting device according to claim 6, wherein it further comprises the following steps:

f) the control unit continues to sample the direct voltage of the boosting unit and the voltage and phase of the alternating power grid; and

g) the control unit determines whether the direct voltage and the voltage and phase of the alternating power grid are normal, and if any of the parameters is abnormal, the control unit disconnects the electrical connection between the inverting unit and the alternating power grid and stops the inverting of the inverting unit, and the method returns to the step a).

8. The inverting control method of a transformerless photovoltaic grid-connecting inverting device according to claim 6, wherein the maximum power point tracking step in the step c) comprises:

determining whether the direct voltage of the boosting unit is within the stabilized voltage range, wherein if the direct voltage is within the stabilized voltage range, the grid-connecting power will be increased and if the direct voltage is below the stabilized voltage range, the grid-connecting power will be decreased.

9. The inverting control method of a transformerless photovoltaic grid-connecting inverting device according to claim 8, wherein the step of increasing and decreasing the grid-connecting power comprises increasing and decreasing the duty ratio of the drive signal.

10. The inverting control method of a transformerless photovoltaic grid-connecting inverting device according to claim 8, wherein it further comprises a step before the step of increasing the grid-connecting power:

determining whether the grid-connecting power achieves the maximum output power, and wherein if the grid-connecting power has increased to the maximum output power, the grid-connecting power will not be increased any more.