Voltage-sensed system and method for anti-islanding protection of grid-connected inverters

• US 8,423,312 B2
• Filed: 03/17/2011
• Issued: 04/16/2013
• Est. Priority Date: 09/04/2007
• Status: Active Grant

• Current Assignee: Enphase Energy Incorporated
• Original Assignee: Solarbridge Technologies Incorporated (Totalenergies Se)
• Inventors: Krein, Philip T.
• Primary Examiner(s): PATEL, RAJNIKANT B
• Application Number:
  US13/050,592
• Publication Number:
  US 20110164440A1
• Time in Patent Office: 761 Days
• Field of Search: 363 34- 41, 363/66, 363/71, 363/95, 363/98, 363/101, 363/89, 363/127, 363/55, 363/6.01, 363/57, 363/60, 363/48, 361/18, 361/93, 361/94, 361/97, 361/103, 361/106, 323/282, 323/258, 323/259, 323/266, 323/340, 320/32, 320/46, 320/48, 307/16, 307/32, 307/36, 307/38, 307/43, 307/64, 307 44- 46, 307/125, 307/127, 307/129, 702/57, 702/64, 702/75, 702/60, 702/111, 702/108, 702/109
• US Class Current: 702/111
• CPC Class Codes: H02M 1/32 Means for protecting converters other than automatic disconnection

Abstract

A method is provided for preventing islanding of a power source connected to an electric AC grid via an interface. The method senses an output voltage waveform of the interface, controls an output current waveform of the interface to track a reference current waveform having a mathematical relationship with the sensed output voltage waveform, and discontinues the output current waveform when the output voltage waveform is sensed to be outside a predetermined waveform range.

Claims

1. A method for operating a power interface connected to a power source to provide power from the power source to an electric alternating current (AC) grid, the method comprising:

generating a reference current waveform such that the reference current waveform is proportional to the product of a predetermined transfer function and a sensed output voltage waveform of the power interface, wherein (i) the predetermined transfer function includes a time delay and (ii) the product yields one of;

a stable and an unstable action;

controlling an output current waveform of the power interface to track the reference current waveform; and

discontinuing the output current waveform in response to the output voltage waveform being outside a predetermined waveform range.

2. The method of claim 1, wherein the power interface is a power inverter configured to control the output current waveform flowing from the power source into the grid based on the reference current waveform.

3. The method of claim 1, wherein the power source is a solar panel and controlling the output current waveform comprises tracking a maximum power point of the solar panel.

4. The method of claim 1, wherein discontinuing the output current waveform comprises at least one of (i) turning off the power source and (ii) turning off the power interface.

5. The method of claim 1, wherein discontinuing the output current waveform comprises disconnecting the power source from the electric AC grid.

6. The method of claim 1, wherein discontinuing the output current waveform comprises discontinuing the output current waveform in response to the output voltage waveform being outside a predetermined waveform range for a predetermined number of line cycles of operation.

7. The method of claim 1, further comprising sensing a frequency of the output voltage waveform.

8. The method of claim 1, wherein the predetermined waveform range of the output voltage waveform includes a predetermined nominal value.

19. The method of claim 1, wherein the reference current waveform is non-unity proportional to the product of the predetermined transfer function and the sensed output voltage waveform.

9. A system for providing power to an alternating current (AC) grid, the system comprising:

a power inverter electrically connectable between an energy source and the AC grid, the power inverter to supply an output voltage waveform to the AC grid;

a control unit coupled to the power inverter and configured to (i) establish a reference current waveform proportional to the product of a predetermined transfer function and the output voltage waveform and (ii) control the inverter to generate an output current waveform that tracks the reference current waveform, wherein the predetermined transfer function includes a time delay, and the product yields one of;

a stable and an unstable action; and

a discontinuing unit configured to discontinue the output current waveform in response to the output voltage waveform being outside a predetermined waveform range.

10. The system of claim 9, wherein the energy source is a solar panel and the control unit is further configured to control the output current waveform based on a maximum power point of the solar panel.

11. The system of claim 9, wherein the discontinuing unit is configured to discontinue the output current waveform by performing at least one of (i) turning off the power source and (ii) turning off the power interface.

12. The system of claim 9, wherein the discontinuing unit is configured to discontinue the output current waveform by disconnecting the power source from the electric AC grid.

13. The system of claim 9, wherein the discontinuing unit is configured to discontinue the output current waveform in response to the output voltage waveform being outside a predetermined waveform range for a predetermined number of line cycles of operation.

20. The system of claim 9, wherein the reference current waveform is non-unity proportional to the product of the predetermined transfer function and the output voltage waveform.

14. A method for controlling a power inverter connected to an energy source to provide power to an electric alternating current (AC) grid, the method comprising:

sensing an output voltage waveform of the power inverter;

generating an output current waveform of the power inverter that tracks a reference current waveform such that the reference current waveform is proportional to the product of a predetermined transfer function and the sensed output voltage waveform, wherein (i) the predetermined transfer function includes a time delay and (ii) the product yields one of;

a stable and an unstable action; and

discontinuing the output current waveform in response to the output voltage waveform being outside a predetermined waveform range.

15. The method of claim 14, wherein discontinuing the output current waveform comprises discontinuing the output current waveform in response to the output voltage waveform being outside a predetermined waveform range for a predetermined number of line cycles of operation.

16. The method of claim 14, wherein sensing an output voltage waveform comprises sensing a frequency of the output voltage waveform.

17. A system for controlling a power inverter connected to an energy source to provide power to an electric alternating current (AC) grid, the system comprising:

an input to receive an output voltage waveform of the power inverter;

a processor programmed (i) to control the power inverter to generate an output current waveform that tracks a reference current waveform such that the reference current waveform is proportional to the product of a predetermined transfer function and the output voltage waveform and (ii) to discontinue the output the output current waveform in response to the output voltage waveform being outside a predetermined waveform range, wherein (i) the predetermined transfer function includes a time delay and (ii) the product yields one of;

a stable and an unstable action.

18. The system of claim 17, wherein the reference current waveform is non-unity proportional to the product of the predetermined transfer function and the output voltage waveform.