Inverter control methodology for distributed generation sources connected to a utility grid

• US 7,248,946 B2
• Filed: 05/11/2004
• Issued: 07/24/2007
• Est. Priority Date: 05/11/2004
• Status: Expired due to Fees

• Current Assignee: Advanced Energy Conversion LLC (Systematic Power Solutions LLC)
• Original Assignee: Advanced Energy Conversion LLC (Systematic Power Solutions LLC)
• Inventors: Bashaw, Travis B., Carpenter, Robert T., Torrey, David A.
• Primary Examiner(s): Bahta; Kidest
• Application Number:
  US10/843,064
• Publication Number:
  US 20050254191A1
• Time in Patent Office: 1,169 Days
• Field of Search: 700/276, 700/286, 700/291, 700/21, 323/205, 323/299, 323/906, 361/42, 361/62, 363/95, 363/56, 363/43, 363/41, 363/207, 363/131, 363/71, 290/40, 290/44
• US Class Current: 700/286
• CPC Class Codes: H02J 2300/24 of photovoltaic origin
• CPC Class Codes: H02J 2300/28 The renewable source being wind energy wind motors F03D
• CPC Class Codes: H02J 3/381 Dispersed generators
• CPC Class Codes: H02J 3/388 Islanding, i.e. disconnection of local power supply from the network
• CPC Class Codes: Y02E 10/56 Power conversion systems, e.g. maximum power point trackers
• CPC Class Codes: Y02E 10/76 Power conversion electric or electronic aspects

Abstract

The present invention provides an improved control methodology for maximum power point tracking (MPPT), anti-islanding, and output current regulation for distributed generation sources connected to a utility grid. The control includes enhancements for MPPT and regulating the inverter output-current that is sourced into the utility grid. In addition, the functions associated with MPPT, anti-islanding detection and output current regulation are integrated together; the MPPT algorithm operates in conjunction with the anti-islanding detection, and the output current regulation operates in conjunction with the MPPT algorithm.

Claims

1. A method for maximum power point tracking (MPPT) in a distributed generation system coupled to an alternating current (AC) power grid, comprising:

comparing a current gain of the distributed generation system between zero crossings of the AC power grid;

if the current gain has decreased in the current gain comparing step, comparing a change in power magnitude of the distributed generation system between the zero crossings of the AC power grid with an expected power decrement and, if the change in power magnitude is greater than a factor q of the expected power decrement, incrementing the current gain to set a peak operating point of the distributed generation system, otherwise decrementing the current gain to set the peak operating point of the distributed generation system;

if the current gain has not decreased in the current gain comparing step, comparing the change in power magnitude of the distributed generation system between the zero crossings of the AC power grid with an expected power increment and, if the change in power magnitude is less than the factor q of the expected power increment, decrementing the current gain to set the peak operating point of the distributed generation system, otherwise comparing the change in power magnitude to a power incline function and incrementing or decrementing the current gain based on the comparison to set the peak operating point of the distributed generation system.

2. The method of claim 1, wherein an inverter is used to couple the distributed generation system to the AC power grid, and wherein the power incline function comprises a measure of how the efficiency of the inverter varies with output current.

3. The method of claim 1, wherein, if the change in power magnitude of the distributed generation system between the zero crossings of the AC power grid is less than the power incline function, decrementing the current gain, otherwise incrementing the current gain.

4. The method of claim 3, wherein the method is repeated after an output power update period.

5. The method of claim 4, wherein the output power update period comprises a predetermined number of zero crossings of the AC power grid.

6. The method of claim 5, wherein an inverter is used to couple the distributed generation system to the AC power grid, and wherein the predetermined number of zero crossings is dependent upon a capacitance in parallel with the inverter input.

7. The method of claim 5, wherein the predetermined number of zero crossings is between 2 and 10.

8. The method of claim 3, wherein the step of comparing the change in power magnitude to the power incline function increases stability near a maximum output power capability of the distributed generation system.

9. The method of claim 1, wherein the distributed generation system comprises any distributed generation source where it is desirable to deliver energy to the AC power grid while optimally loading the source.

10. The method of claim 1, wherein q is between 0 and 1.

11. The method of claim 10, wherein q is between 0.25 and 0.5.

12. The method of claim 1, wherein the expected power decrement and the expected power increment comprise exact expected power changes into the AC power grid that should occur for a given change in the current gain.

13. The method of claim 1, wherein the expected power decrement is about 1.0 to 2.0 times the expected power increment.

14. The method of claim 1, wherein the power incline function varies between zero and no greater than the expected power increment.

15. The method of claim 1, wherein the step of comparing the change in power magnitude to a power incline function and incrementing or decrementing the current gain based on the comparison prevents power greater than that available at a maximum power point (MPP) from being drawn from the distributed generation system.

16. A system for improving regulation of an output current of an inverter coupled between a distributed generation system and an alternating current (AC) power grid, the inverter converting a direct current (DC) provided by the distributed generation system into the output current, comprising:

a proportional-integral-derivative (PID) controller coupled to a DC output voltage of the distributed generation system;

a feed forward controller coupled to the DC output voltage of the distributed generation system and to an AC output voltage of the AC power grid; and

a power normalization controller.

17. The system of claim 16, wherein the PID controller and the feed forward controller are normalized to an output voltage of the distributed generation system.

18. The system of claim 17, wherein the normalization of the PID controller and the feed forward controller to the output voltage of the distributed generation system reduces distortion in an output current of the inverter.

19. The system of claim 16, wherein the feed forward controller is configured to reduce integral control effort within the PID controller.

20. The system of claim 19, wherein reduction of the integral control effort within the PID controller reduces distortion in an output current of the inverter.

21. The system of claim 16, wherein the power normalization controller is normalized to an RMS voltage of the AC power grid.

22. The system of claim 16, wherein the inverter further includes a maximum power point tracking (MPPT) controller, and wherein the system for improving regulation of an output current improves stability of the tracking provided by the MPPT controller.

23. The system of claim 22, wherein the system for improving regulation of an output current ensures that the MPPT controller operates under consistent conditions regardless of an RMS voltage of the AC power grid.

24. A method for maximum power point tracking (MPPT) in an inverter coupled between a distributed generation system and an alternating current (AC) power grid, comprising:

enhancing a quality of an output current of the inverter through combined use of a proportional-integral-derivative (PID) controller, a feed forward controller, and a power normalization controller; and

performing the MPPT on the enhanced quality output current to improve stability of the MPPT.

25. The method of claim 24, further comprising:

normalizing the PID controller and the feed forward controller to an output voltage of the distributed generation system.

26. The method of claim 25, wherein the normalization of the PID controller and the feed forward controller to the output voltage of the distributed generation system reduces distortion in an output current of the inverter.

27. The method of claim 24, further comprising:

configuring the feed forward controller to reduce integral control effort within the PID controller.

28. The method of claim 27, wherein reduction of the integral control effort within the PID controller reduces distortion in an output current of the inverter.

29. The method of claim 24, further comprising:

normalizing the power normalization controller to an RMS voltage of the AC power grid.

30. A method for detecting an islanding condition in a system including a distributed generation system and an inverter coupled between the distributed generation system and an AC power grid, comprising:

comparing an RMS voltage of the AC power grid with an island threshold;

if the RMS voltage is greater than the island threshold, determining if a change in the RMS voltage reflects a change made to a current gain of the inverter when islanding;

if the change in the RMS voltage reflects a change made to the current gain of the inverter when islanding, then incrementing a number of island hits;

processing the number of island hits that occurred during a collection period; and

enabling or disabling the inverter based on the number of island hits that occurred during the collection period.

31. The method of claim 30, wherein the current gain of the inverter is provided by a maximum power point tracking (MPPT) system.

32. The method of claim 30, further comprising:

repeating the comparing step at a next zero crossing of the AC power grid.

33. The method of claim 30, wherein the step of processing the number of island hits that occurred during a collection period further comprises:

comparing the number of island hits that occurred during the collection period to a predetermined threshold; and

if the number of island hits that occurred during the collection period exceeds the predetermined threshold, thereby indicating an islanding condition, then disabling the inverter, otherwise enabling or continuing to enable the inverter.

34. The method of claim 30, wherein the collection period comprises a plurality of zero crossings of the AC power grid.

35. The method of claim 30, wherein the collection period comprises a window of recent zero crossings of the AC power grid.

36. The method of claim 35, wherein the window is moving.