METHOD AND SYSTEM TO CONVERT DIRECT CURRENT (DC) TO ALTERNATING CURRENT (AC) USING A PHOTOVOLTAIC INVERTER

• US 20090121549A1
• Filed: 11/14/2007
• Published: 05/14/2009
• Est. Priority Date: 11/14/2007
• Status: Active Grant

• Current Assignee: General Electric Company
• Original Assignee: General Electric Company
• Inventors: Leonard, John Andrew
• Granted Patent: US 8,018,748 B2
• Time in Patent Office: Days
• US Class Current: 307/51
• CPC Class Codes: H02J 2300/24 of photovoltaic origin
• 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: H02J 3/40 Synchronising a generator for connection to a network or to another generator
• CPC Class Codes: H02M 1/0022 the disturbance parameters being input voltage fluctuations
• 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 1/36 Means for starting or stopping converters
• CPC Class Codes: H02M 3/156 with automatic control of output voltage or current, e.g. switching regulators
• CPC Class Codes: Y02B 10/10 Photovoltaic [PV]
• CPC Class Codes: Y02E 10/56 Power conversion systems, e.g. maximum power point trackers

Abstract

A method to convert direct current (DC) power generated by a photovoltaic (PV) arrays to alternating current (AC) power using a PV inverter, the method includes: applying the DC power generated the PV array to the PV inverter; boosting the DC power from the PV array from a predetermined voltage level to a predetermine DC link voltage, wherein the PV array is regulated by the boosting to output a voltage at the predetermined voltage level; converting the boosted DC power to AC power, and disabling the boost of the power generated by the PV array after the array has a predetermined voltage level at least as great as the DC link voltage.

Claims

1. A method to increase photovoltaic (PV) inverter efficiency by disabling a DC-DC boost comprising:

applying a DC power generated by a PV array to the PV inverter;

boosting the DC power from the PV array from a predetermined voltage level to a predetermine DC link voltage, wherein the PV array is regulated by the boosting to output a voltage at the predetermined voltage level;

converting the boosted DC power to AC power, and

disabling the boost of the power generated by the PV array after the array has a predetermined voltage level at least as great as the DC link voltage.

2. The method of claim 1 wherein the AC power is compatible with AC power in a power utility grid.

3. The method of claim 1 wherein the predetermined voltage level for a PV array corresponds to a maximum power output of the PV array and the method further includes periodically determining the maximum power output of the PV array.

4. The method of claim 1 wherein the boosting is by a DC boost circuit having at least one switch transistor and the switch transistor is controlled by a duty cycle, wherein the method further comprises determining a duty cycle to operating the PV array at the predetermined voltage level.

5. The method of claim 1 wherein the conversion of the boosted DC power to AC power includes regulating the array with the disabled boost to output a maximum power voltage corresponding to a maximum power of the array.

6. The method of claim 1 wherein the array is the first PV array and power from additional PV arrays is input to the PV inverter and separately boosting the power from each additional PV array while disabling the boost by the first PV array, wherein the predetermined DC link voltage is the maximum power voltage for the first PV array.

7. A method to convert direct current (DC) power generated by a plurality of photovoltaic (PV) arrays to alternating current (AC) power using a PV inverter having a DC boost circuit for each of the PV arrays and a DC to AC inverter, the method comprising:

applying the DC power generated by each PV array to a respective input to the PV inverter;

operating each PV array at a maximum power voltage corresponding to a maximum power output of the array to produce DC power;

determining a duty cycle for each PV array which regulates the PV array to output the maximum power voltage for the array and boosts the maximum power voltage to at least a minimum DC link voltage;

applying the duty cycle to each boost circuit to boost the DC power from the array connected to the boost circuit from the maximum power voltage of the array to at least the minimum DC link voltage;

converting the boosted DC power to AC power, and

disabling the boost circuit connected to a PV array having a maximum power voltage at least as great as the minimum DC link voltage.

8. The method of claim 7 wherein the AC power is compatible with AC power in a power utility grid.

9. The method of claim 8 further periodically calculating the minimum DC link based on the AC power in the power utility grid.

10. The method of claim 7 further comprising periodically updating the maximum power voltage and determining the duty cycle for the updated maximum power voltage.

11. The method of claim 7 wherein the conversion of the boosted DC power to AC power includes regulating the one array with the disabled boost to output a maximum power voltage corresponding to a maximum power of the one array.

12. A photovoltaic inverter comprising:

a plurality of input connections each adapted to receive direct current (DC) power from a photovoltaic array;

a separate DC boost circuit electrically connected to each of the input connections, wherein the boost circuit includes at least one switch operated in accordance with a duty cycle;

a DC link circuit electrically connected to a DC output from each of the DC boost circuits, wherein the DC link circuit includes a DC to alternating current (AC) inverter and an AC power output adapted to delivery AC power from to an AC power grid, and

a controller executing processes for;

determining a voltage at which each of the photovoltaic arrays connected to an input connection produces a predetermined power;

generating the duty cycle for each boost circuit to control the at least one switch to cause the PV array produce the predetermined power and the boost circuit to output a predetermined DC link voltage, and disabling one of the boost circuits coupled to a PV array generating a voltage greater than the predetermined DC link voltage.

13. The photovoltaic inverter of claim 12 wherein the predetermined power is a maximum power for the photovoltaic array.

14. The photovoltaic inverter of claim 12 wherein the AC inverter is a bridge circuit.

15. The photovoltaic inverter of claim 12 wherein the boost circuits each includes an inductor and capacitor.

17. The method of claim 15 wherein the Vmpp is periodically updated every 15 to 60 minutes and the Vmpp is dithered at least every one second.

18. The method of claim 17 wherein the Vmpp is dithered at least every tenth of a second.

19. The method of claim 15 wherein the varying of the voltage output of the photovoltaic array is performed by varying a duty cycle of a boost circuit coupled to the array.

20. The method of claim 19 wherein the duty cycle is varied from a cycle in which switches in the boost circuit are off to a cycle in which the duty cycle

16. A method to determine, on an ongoing basis, a maximum power point for a photovoltaic array, the method comprising:

a. varying a voltage output of the photovoltaic array over an operating range of voltages for the array;

b. measuring current output from the photovoltaic array as the voltage output is varied over the operating array;

c. using the measured current and the voltage corresponding to the measured current to determine a power output of the array for each of the various voltages in the operating range;

d. selecting a maximum power output from the power outputs in the operating range and designating a corresponding voltage as a voltage at maximum power point (Vmpp) for the array;

e. periodically repeating steps (a) to (d) to update Vmpp;

f. operating the photovoltaic array at the Vmpp;

g. dithering the Vmpp by slightly shifting the voltage of the array away from the Vmpp, and

h. if the power output of the array at the shifted voltage is greater than the power output of the array at Vmpp, designating the shifted voltage as the Vmpp and repeating steps (f) to (h).