Method and apparatus for power inverter synchronization

• US 20050135031A1
• Filed: 12/19/2003
• Published: 06/23/2005
• Est. Priority Date: 12/19/2003
• Status: Active Grant

• Current Assignee: Square D Company (Schneider Electric SE)
• Original Assignee: Square D Company (Schneider Electric SE)
• Inventors: Colby, Roy Stephen, Kocher, Mark John, Carson, Gerald Benjamin
• Granted Patent: US 7,183,667 B2
• Time in Patent Office: Days
• US Class Current: 361/78
• CPC Class Codes: H02J 3/42 with automatic parallel connection when synchronisation is achieved
• CPC Class Codes: H02M 1/0012 Control circuits using digital or numerical techniques in DC/DC converters H02M3/157, H02M3/33515; in DC-AC converters H02M7/53873
• CPC Class Codes: H02M 7/53832 in a push-pull arrangement
• CPC Class Codes: H02M 7/5395 by pulse-width modulation

Abstract

A control circuit synchronizes an ac power inverter to the mains voltage of an electrical grid by matching the fundamental phasor components of the inverter'"'"'s output voltage to the fundamental phasor components of the mains voltage. Once such matching meets an acceptable voltage error threshold, the control circuit initiates contactor closure, verifies contactor closure, and then initiates a changeover from voltage-mode control used in synchronization operations to a current-mode control of the inverter'"'"'s output. The control circuit provides corresponding disconnection control when disconnection from the grid is desired, wherein the regulated power of the inverter is ramped down in controlled fashion until it reaches a lower threshold whereupon contactor opening is initiated. Once contactor opening is verified, regulation control reverts to stand-alone voltage mode control or to shut down, as needed or desired.

Claims

1. A method of controlling interconnection of a power inverter with an electrical system, the method comprising:

controlling an inverter voltage to match an electrical system voltage;

detecting synchronization of the inverter voltage with the electrical system voltage based on calculating a voltage error between x-y phasor components of the inverter and electrical system voltages; and

commanding contactor closure to interconnect the power inverter with the electrical system responsive to detecting synchronization.

2. The method of claim 1, further comprising verifying contactor closure by monitoring one or more feedback signals.

3. The method of claim 2, further comprising changing the inverter from a voltage regulation control mode to a power regulation control mode responsive to verifying contactor closure.

4. The method of claim 2, wherein verifying contactor closure by monitoring one or more feedback signals comprises monitoring a contactor armature signal.

5. The method of claim 2, wherein verifying contactor closure by monitoring one or more feedback signals comprises measuring harmonic current flow between the inverter and the electrical system.

6. The method of claim 5, wherein measuring harmonic current flow between the inverter and the electrical system comprises generating a measurement value based on filtering a measured current signal, and comparing the measurement value to a closure detection threshold.

7. The method of claim 1, further comprising disconnecting from the electrical system by:

ramping down inverter power; and

commanding contactor opening to disconnect the inverter from the electrical system responsive to determining that the inverter power is below a defined threshold.

8. The method of claim 1, further comprising transitioning from a synchronization mode of operation comprising regulating the inverter voltage to substantially match the electrical system voltage to a grid-connected mode of operation comprising regulating an output power of the power inverter according to desired output power values.

9. The method of claim 8, further comprising transitioning from the grid-connected mode of operation to a stand-alone mode of operation by commanding contactor opening, verifying contactor opening, and changing to the stand-alone mode of operation by regulating the inverter voltage to substantially match an internal voltage reference of the power inverter.

10. The method of claim 1, wherein controlling the inverter voltage to match the electrical system voltage comprises referencing an x-y coordinate frame to the electrical system voltage by locking to a phase and frequency of the electrical system voltage, calculating the x-y phasor components of the inverter and electrical system voltages based on the x-y coordinate frame, and adjusting the inverter voltage to reduce a difference between the x-y phasor components.

11. The method of claim 10, wherein calculating the x-y phasor components of the inverter and electrical system voltages comprises calculating x-y phasor components for single-phase inverter and electrical system voltages.

12. The method of claim 11, wherein calculating the x-y phasor components for the single-phase inverter and electrical system voltages comprises integrating in-phase (x) and quadrature (y) samples of the inverter and electrical system voltages over half-cycles of a fundamental waveform period.

13. The method of claim 1, wherein controlling the inverter voltage to match the electrical system voltage comprises adjusting the inverter voltage to reduce a difference between the x-y phasor components of the inverter and electrical system voltages.

14. A control circuit to control interconnection of a power inverter with an electrical system, the control circuit comprising:

a regulator circuit to control an inverter voltage to match an electrical system voltage by measuring x-y phasor components of the inverter voltage and the electrical system voltage;

a synchronization detection circuit to detect synchronization of the inverter voltage with the electrical system voltage by calculating a voltage error based on the x-y phasor components; and

a contactor control circuit to command contactor closure to interconnect the power inverter with the electrical system responsive to detecting synchronization.

15. The control circuit of claim 14, further comprising a contactor monitoring circuit to verify contactor closure by monitoring one or more feedback signals.

16. The control circuit of claim 15, wherein the regulator circuit is configured to change from a voltage-control mode to a current-control mode responsive to verifying contactor closure.

17. The control circuit of claim 15, wherein the contactor monitoring circuit verifies contactor closure by measuring harmonic current flow between the inverter and the electrical system.

18. The control circuit of claim 15, wherein the contactor monitoring circuit verifies contactor closure by monitoring a contactor armature signal.

19. The control circuit of claim 14, wherein the contactor control circuit is configured to disconnect from the electrical system by:

ramping down inverter power; and

commanding contactor opening to disconnect the inverter from the electrical system responsive to determining that the inverter power is below a defined threshold.

20. The control circuit of claim 14, wherein the regulator circuit is configured to control the inverter voltage to match the electrical system voltage by referencing an x-y coordinate frame to the electrical system voltage based on locking to a phase and frequency of the electrical system voltage, measure the x-y phasor components of the inverter and electrical system voltages based on the x-y coordinate frame, and adjust the inverter voltage to reduce a difference between the x-y phasor components.

21. The control circuit of claim 20, wherein the regulator circuit is configured as a single-phase regulator circuit that measures x-y phasor components for single-phase inverter and electrical system voltages.

22. The control circuit of claim 21, wherein the regulator circuit is configured to measure the x-y phasor components for single-phase inverter and electrical system voltages by integrating in-phase (x) and quadrature (y) samples of inverter and electrical system voltages over half-cycles of a fundamental waveform period.

23. The control circuit of claim 14, wherein the regulator circuit is configured to adjust the inverter voltage to reduce a difference between the x-y phasor components of the inverter and electrical system voltages.

24. A power inverter including a control circuit to control interconnection of a power inverter with an electrical system, the control circuit comprising:

a regulator circuit to control an inverter voltage to match an electrical system voltage by measuring x-y phasor components of the inverter voltage and the electrical system voltage;

a synchronization detection circuit to detect synchronization of the inverter voltage with the electrical system voltage by calculating a voltage error based on the x-y phasor components; and

a contactor control circuit to command contactor closure to interconnect the power inverter with the electrical system responsive to detecting synchronization.

25. The power inverter of claim 24, wherein the contactor control circuit is configured to disconnect from the electrical system by:

ramping down inverter power; and

commanding contactor opening to disconnect the inverter from the electrical system responsive to determining that the inverter power is below a defined threshold.

26. The power inverter of claim 24, wherein the regulator circuit is configured to control the inverter voltage to match the electrical system voltage by referencing an x-y coordinate frame to the electrical system voltage based on locking to a phase and frequency of the electrical system voltage, measure the x-y phasor components of the inverter and electrical system voltages based on the x-y coordinate frame, and adjust the inverter voltage to reduce a difference between the x-y phasor components.

27. The power inverter of claim 26, wherein the regulator circuit is configured as a single-phase regulator circuit that measures x-y phasor components for single-phase inverter and electrical system voltages.

28. A computer readable medium storing a computer program to control interconnection of a power inverter with an electrical system, the computer program comprising:

program instructions to control an inverter voltage to match an electrical system voltage;

program instructions to detect synchronization of the inverter voltage with the electrical system voltage based on calculating a voltage error between x-y phasor components of the inverter and electrical system voltages; and

program instructions to command contactor closure to interconnect the power inverter with the electrical system responsive to detecting synchronization.

29. The computer readable medium storing the computer program of claim 28, further comprising program instructions to verify contactor closure by monitoring one or more feedback signals.

30. The computer readable medium storing the computer program of claim 29, further comprising program instructions to change the inverter from a voltage-control mode to a current-control mode responsive to verifying contactor closure.

31. The computer readable medium storing the computer program of claim 29, wherein the program instructions to verify contactor closure by monitoring one or more feedback signals comprise program instructions to measure harmonic current flow between the inverter and the electrical system.

32. The computer readable medium storing the computer program of claim 29, wherein the program instructions to verify contactor closure by monitoring one or more feedback signals comprise program instructions to monitor a contactor armature signal.

33. The computer readable medium storing the computer program of claim 28, further comprising program instructions to disconnect from the electrical system by ramping down inverter power, and commanding contactor opening to disconnect the inverter from the electrical system responsive to determining that the inverter power is below a defined threshold.

34. The computer readable medium storing the computer program of claim 28, wherein the program instructions to control the inverter voltage to match the electrical system voltage comprise program instructions to reference an x-y coordinate frame to the electrical system voltage by locking to a phase and frequency of the electrical system voltage, calculate the x-y phasor components of the inverter and electrical system voltages based on the x-y coordinate frame, and adjust the inverter voltage to reduce a difference between the x-y phasor components.

35. The computer readable medium storing the computer program of claim 34, wherein the program instructions to calculate the x-y phasor components of the inverter and electrical system voltages comprise program instructions to calculate x-y phasor components for single-phase inverter and electrical system voltages.

36. The computer readable medium storing the computer program of claim 35, wherein the program instructions to calculate the x-y phasor components for the single-phase inverter and electrical system voltages comprise program instructions to integrate in-phase (x) and quadrature (y) samples of the inverter and electrical system voltages over half-cycles of a fundamental waveform period.

37. The computer readable medium storing the computer program of claim 28, wherein the program instructions to control the inverter voltage to match the electrical system voltage comprise program instructions to adjust the inverter voltage to reduce a difference between the x-y phasor components of the inverter and electrical system voltages.

38. A method of connecting an initially disconnected power inverter with an electrical system, the method comprising:

locking a phase and frequency of an inverter voltage to an electrical system voltage, and adjusting the inverter voltage to approximate the electrical system voltage;

commanding contactor closure to connect the inverter to the electrical system responsive to determining that the inverter voltage substantially matches the electrical system voltage; and

verifying contactor closure by measuring harmonic current flow between the inverter and the electrical system.

39. The method of claim 38, wherein measuring harmonic current flow between the inverter and the electrical system comprises generating a measurement signal corresponding to current flow between the inverter and the electrical system.

40. The method of claim 39, wherein verifying contactor closure comprises determining whether the measurement signal is above a defined threshold.

41. The method claim 39, wherein generating the measurement signal includes filtering a current measurement signal using an n-point moving average filter.

42. The method of claim 38, further comprising regulating the inverter voltage under feedback control to approximate the electrical system voltage.

43. The method of claim 42, wherein regulating the inverter voltage under feedback control includes calculating x-y phasor components of the inverter and electrical system voltages and determining whether the inverter voltage substantially matches the electrical system voltage based on the x-y phasor components.

44. The method of claim 43, further comprising generating x-y reference signals based on the electrical system voltage for calculation of the x-y phasor components.

45. A method of controlling interconnection of a power inverter with an electrical system, the method comprising:

controlling an inverter voltage to match an electrical system voltage based on comparing x-y phasor components of the inverter voltage with x-y phasor components of the electrical system voltage;

detecting synchronization of the inverter voltage with the electrical system voltage; and

commanding contactor closure to interconnect the power inverter with the electrical system responsive to detecting synchronization.

46. The method of claim 45, further comprising changing from controlling the inverter voltage to controlling real and reactive inverter power responsive to verifying contactor closure.

47. The method of claim 45, further comprising disconnecting from the electrical system by:

ramping down inverter power; and

commanding contactor opening to disconnect the inverter from the electrical system responsive to determining that the inverter power is below a defined threshold.

48. The method of claim 45, further comprising transitioning from a synchronization mode of operation wherein the power inverter regulates its output voltage to match the electrical system voltage to a grid-connected mode of operation wherein the power inverter regulates its real and reactive output power according to desired output power values.

49. The method of claim 48, further comprising transitioning from the grid-connected mode of operation to a stand-alone mode of operation by commanding contactor opening, verifying contactor opening, and changing to the stand-alone mode of operation wherein the power inverter regulates its output voltage to match a reference voltage of the power inverter.

50. The method of claim 45, wherein controlling an inverter voltage to match an electrical system voltage based on comparing x-y phasor components of the inverter voltage with x-y phasor components of the electrical system voltage comprises:

referencing an x-y coordinate frame to the electrical system voltage by locking to a phase and frequency of the electrical system voltage;

calculating the x-y phasor components of the inverter and electrical system voltages based on the x-y coordinate frame; and

adjusting the inverter voltage to reduce a difference between the x-y phasor components.

51. The method of claim 50, wherein calculating the x-y phasor components of the inverter and electrical system voltages comprises calculating x-y phasor components for single-phase inverter and electrical system voltages.

52. The method of claim 51, wherein calculating the x-y phasor components for the single-phase inverter and electrical system voltages comprises integrating in-phase (x) and quadrature (y) samples of the inverter and electrical system voltages over half-cycles of a fundamental waveform period.