System, method, rotating machine and computer program product for enhancing electric power produced by renewable facilities

• US 20030011348A1
• Filed: 07/10/2001
• Published: 01/16/2003
• Est. Priority Date: 07/10/2001
• Status: Active Grant

• Current Assignee: ABB AB (ABB Limited)
• Original Assignee: ABB AB (ABB Limited)
• Inventors: Lof, Per-anders Kristian, Ingolf Gertmar, Lars Gustaf
• Granted Patent: US 6,670,721 B2
• Time in Patent Office: Days
• US Class Current: 322/37
• CPC Class Codes: F03D 9/007 the wind motor being combined with means for converting solar radiation into useful energy
• CPC Class Codes: F03D 9/255 connected to electrical distribution networks; Arrangements therefor
• CPC Class Codes: F03D 9/257 the wind motor being part of a wind farm
• CPC Class Codes: F05B 2270/20 to optimise the performance of a machine
• CPC Class Codes: H02J 2300/10 The dispersed energy generation being of fossil origin, e.g. diesel generators
• CPC Class Codes: H02J 2300/20 The dispersed energy generation being of renewable origin
• 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 2300/30 The power source being a fuel cell
• CPC Class Codes: H02J 2300/40 wherein a plurality of decentralised, dispersed or local energy generation technologies are operated simultaneously
• CPC Class Codes: H02J 3/381 Dispersed generators
• CPC Class Codes: H02J 3/48 Controlling the sharing of the in-phase component
• CPC Class Codes: H02J 3/50 Controlling the sharing of the out-of-phase component
• CPC Class Codes: H02S 10/12 Hybrid wind-PV energy systems
• CPC Class Codes: Y02E 10/56 Power conversion systems, e.g. maximum power point trackers
• CPC Class Codes: Y02E 10/72 Wind turbines with rotation axis in wind direction
• CPC Class Codes: Y02E 10/76 Power conversion electric or electronic aspects

Abstract

An electric power system provides a coordinated and controlled intercommunication and operation of power engineering equipment and converters, e.g., rotating AC machines, power electronic converters and transformers as well as power grids in order to enhance electric power produced by renewable facilities. As viewed from the power grid and its stakeholders'"'"' perspective, enhanced renewable facilities are more stiff than conventional renewable facilities, and produce power that is as commercially valuable and fungible as electric power produced by traditional plants such as fossil fuel power plants, hydroelectric plants, nuclear plants and the like. xMs and SMs, or more generally yMs, fulfill the demands of stiffness and reduced variability, which have conventionally limited the commercial usefulness of a large scale use of renewables delivering power to a power grid.

Claims

1. An intelligent coactive converter comprising:

an input line configured to receive electric power produced by a renewable facility, said electric power being inadequate to be applied directly to a standard frequency AC power grid due to at least one of insufficient voltage stiffness, and excessive power variability;

an output line configured to deliver enhanced electric power to said standard frequency AC power grid at, at least, one point-of-connection in a grid area-of-connection, said enhanced electric power being enhanced with regard to at least one of voltage stiffness and power variability with respect to a delivery to said standard frequency AC power grid of said electric power from said renewable facility;

a constant-frequency output, rotating AC machine, yM, electrically coupled to said output line

a controller configured to control an operational state of said yM, and corresponding predetermined amounts of active and reactive power provided by, or consumed by, said yM to the standard frequency AC power grid so as to prime the electric power from the renewable facility and enable a delivery of the enhanced electric power to said at least one point-of-connection.

2. The intelligent coactive converter of claim 1, wherein:

said input line is coupled to a collection and transmission grid that collects power generated from the renewable facility and another renewable facility.

3. The intelligent coactive converter of claim 2, wherein:

said another renewable facility being at least one of a wind turbine facility, a solar cell facility, a fuel cell facility, and a gas turbine facility.

4. The intelligent coactive converter of claim 1, wherein:

said yM is at least one of an xM and a synchronous machine.

5. The intelligent coactive converter of claim 1, wherein said controller comprises:

a communication mechanism configured to coordinate power production levels between the renewable facility and at least one of a virtual energy storage facility and the constant-frequency output, rotating AC machine.

6. The intelligent coactive converter of claim 1, wherein:

said input line is configured to connect directly to said standard frequency AC power grid in said area-of-connection; and

an output of said yM being coupled to said output only.

7. The intelligent coactive converter of claim 6, wherein:

the point-of-connection at which the output line connects to the standard frequency AC power grid being separate from, but electrically close to, another point-of-connection within said area-of-connection at which said input line connects to said standard frequency AC power grid.

8. The intelligent coactive converter of claim 1, wherein:

the yM is configured to supply and consume power via said output line so as to improve voltage stiffness of the standard frequency AC power grid in the area-of-connection and thus aid in priming the electric power produced by the renewable facility.

9. The intelligent coactive converter of claim 1, wherein:

the yM is configured to supply and consume power via said output line so as to reduce variability in active power fed to the standard frequency AC power grid as compared to a variability in the electric power produced only by the renewable facility.

10. The intelligent coactive converter of claim 5, further comprising:

a prime mover configured to drive the yM.

11. The intelligent coactive converter of claim 10, wherein:

the controller is configured to control an operation of the prime mover and coordinate communications with the virtual energy storage facility so as to ensure a predetermined amount of power is delivered to the standard frequency AC power grid on behalf of said renewable facility.

12. The intelligent coactive converter of claim 10, wherein:

the controller is configured to coordinate a sale of the electric power from the renewable facility on a power exchange.

13. The intelligent coactive converter of claim 6, further comprising:

a power transformer coupled to the output line.

14. The intelligent coactive converter of claim 13, wherein:

the power transformer being at least one of

a On-Line Tap-Changer transformer,

two two-winding 3-Phase (2*3Φ

) transformers,

a three-winding 3-phase (2*3Φ

) transformer,

a Phase Shifting Transformer (PST),

a Thyristor Controlled Phase-Angle Regulating (TCPAR) Transformer, and

a cable-based transformer.

15. The intelligent coactive converter of claim 1, wherein:

the yM is a standard frequency AC synchronous machine.

16. The intelligent coactive converter of claim 15, further comprising:

a prime mover configured to drive the standard frequency AC synchronous machine, wherein

the standard frequency AC synchronous machine has a rotor that is attached to a shaft of the prime mover and having a moment of inertia, said moment of inertia being a source of energy available to aid in priming the electrical power from the renewable facility.

17. The intelligent coactive converter of claim 15, wherein:

said input line couples directly to said standard frequency AC power grid in said area-of-connection;

the standard frequency AC synchronous machine is positioned an electrically short distance from where the input line connects to the standard frequency AC power grid so as to improve a voltage stiffness of said standard frequency AC power grid and reduce a reactive power variability in the electric power produced by the renewable facility as it is applied to the standard frequency AC power grid.

18. The intelligent coactive converter of claim 1, wherein:

the yM is a standard frequency AC constant-frequency machine with a variable speed operation capability, including

a multi-phase stator winding coupled to the standard frequency AC power grid,

a rotor winding disposed in a rotating magnetic core attached to a shaft,

a power semiconductor converter configured as a power converter connected between a multi-phase voltage supply and the rotor winding, and

a processor configured to control an operation of the power semiconductor converter.

19. The intelligent coactive converter of claim 18, further comprising:

a prime mover having the shaft coupled to the rotor winding configured to drive the shaft of standard frequency AC constant-frequency machine with a variable speed operation capability; and

the standard frequency AC constant-frequency machine with a variable speed operation capability is configured to exhibit moment of inertia associated with a magnetic core thereof being attached to a shaft of the prime mover.

20. The intelligent coactive converter of claim 18, wherein:

the standard frequency AC constant-frequency machine with variable speed operation capability being configured to

provide negligible variability in an amount of active power fed to the standard frequency AC power grid, and

eliminate the active power variability in the electric power produced by the renewable facility.

21. The intelligent coactive converter of claim 18, wherein:

the standard frequency AC constant-frequency machine with variable speed operation capability being configured to

provide voltage stiffness at a short electrical distance from points-of-connection feeding the electric power from the renewable facilities to the standard frequency AC power grid, and

eliminate reactive power variability in the electric power produced by the renewable facility.

22. The intelligent coactive converter of claim 15, wherein:

the yM is configured to provide short circuit power to the standard frequency AC power grid.

23. The intelligent coactive converter of claim 15, wherein

the yM is embodied as a power flow controller having

a rotating AC machine connected as a shunt machine, and

a rotating AC machine connected as a series machine with a common shaft.

24. The intelligent coactive converter of claim 23, wherein:

at least one of the shunt machine and the series machine includes,

a multi-phase stator winding coupled to the standard frequency AC power grid,

a rotor winding disposed in a rotating magnetic core attached to a drive shaft,

a power semiconductor converter configured as a power converter connected between a multi-phase voltage supply and the rotor winding, and

a processor configured to control an operation of the power semiconductor converter.

25. The intelligent coactive converter of claim 24, wherein:

the controller is configured to determine a timing of when to fire pulses employed in the power semiconductor converter so that

the shunt machine supplies at least one of reactive power with standard frequency AC to the AC power grid, and

active power with standard frequency AC to the AC power grid.

26. The intelligent coactive converter of claim 18, wherein:

the controller is configured to determine a timing of when to fire pulses employed in the power semiconductor converter so that

the shunt machine supplies at least one of reactive power with standard frequency AC to the AC power grid, and

active power with standard frequency AC to the AC power grid.

27. The intelligent coactive converter of claim 24, wherein:

the controller is configured to determine a timing of when to fire pulses employed in the power semiconductor converter so that

the series machine supplies at least one of reactive power with standard frequency AC to the AC power grid, and

active power with standard frequency AC to the AC power grid.

28. The intelligent coactive converter of claim 18, wherein:

the controller is configured to determine a timing of when to fire pulses employed in the power semiconductor converter so that

the series machine supplies at least one of reactive power with standard frequency AC to the AC power grid, and

active power with standard frequency AC to the AC power grid.

29. The intelligent coactive converter of claim 15, wherein:

the standard frequency AC synchronous machine includes

a multi-phase stator winding coupled to the standard frequency AC power grid,

a rotor winding disposed in a rotating magnetic core attached to a drive shaft,

a power semiconductor converter configured as a power converter connected between a multi-phase voltage supply and the rotor winding, and

a processor configured to determine a timing of when to fire pulses employed in the power semiconductor converter.

30. The intelligent coactive converter of claim 1, wherein:

the electric power produced by the renewable facility is collected and transmitted via a standard frequency AC power grid,

the standard frequency AC power grid is at least one of

a meshed power grid,

a radial power grid, and

a mixed meshed and radial power grid.

31. The intelligent coactive converter of claim 1, wherein:

the electric power produced by the renewable facility is collected and transmitted via a low frequency AC power grid,

the low frequency AC power grid is at least one of

a meshed power grid,

a radial power grid, and

a mixed meshed and radial power grid.

32. The intelligent coactive converter of claim 1, wherein:

the electric power produced by the renewable facility is collected and transmitted via a DC power grid,

the DC power grid is at least one of

a meshed power grid,

a radial power grid, and

a mixed meshed and radial power grid.

33. The intelligent coactive converter of claim 32, further comprising:

a DC-AC power semiconductor converter configured to convert the electric power produced by the renewable facility from DC to AC.

34. The intelligent coactive converter of claim 1, wherein:

said output line includes a power transformer configured to receive the electric power from the renewable facility and the yM and provide a composite output power to the standard frequency AC power grid at said point-of-connection.

35. The intelligent coactive converter of claim 34, further comprising:

a prime mover configured to drive the yM, wherein an electrical output of said prime mover being applied as an additional electrical power to said power transformer.

36. The intelligent coactive converter of claim 35, wherein:

one set of windings in a stator of the rotating AC machine stator and a winding in the power transformer being configured to be stressed by a mix of AC and DC voltage.

37. The intelligent coactive converter in claim 35, wherein:

the yM includes a stator winding that includes a cable having an insulation system configured for high voltage use and a mix of AC and DC voltage; and

a winding of the power transformer being made of a same insulation system as said cable in said stator winding.

38. The intelligent coactive converter of claim 1, wherein:

the yM includes a prime mover; and

the controller is configured to the control, an air-gap flux {overscore (Φ

)}<sub>δ

—

yM</sub> and the stator current {overscore (I)}<sub>S—

yM</sub> in the rotating AC machine to provide at least one of

reactive power, with standard frequency AC to the standard frequency AC power grid, according to an equation

Q<sub>eld—

yM</sub>=Im{3jω

·

{overscore (Φ

)}<sub>{overscore (δ

)}—

yM</sub>·

{overscore (I)}*<sub>S—

yM</sub>}

and

active power, from the prime mover, with standard frequency AC to the standard frequency AC power grid according to an equation

P<sub>eld—

yM</sub>=Re{3jω

·

{overscore (Φ

)}<sub>{overscore (δ

)}—

yM</sub>·

{overscore (I)}*<sub>S—

yM</sub>}.

39. The intelligent coactive converter of claim 1, wherein:

the renewable facility includes a wind turbine, k; and

the controller is configured to control, an air-gap flux {overscore (Φ

)}<sub>δ

—

k</sub> and the stator current {overscore (I)}<sub>S—

k</sub> in a rotating AC machine in the

wind turbine, k, to provide reactive power as

Q<sub>eld—

gen—

k</sub>=Im{3jω

_k·

{overscore (Φ

)}<sub>δ

—

k</sub>·

{overscore (I)}*<sub>S—

k</sub>}

and

active power as

P<sub>eld—

gen—

k</sub>=Re{3jω

_k·

{overscore (Φ

)}<sub>δ

—

k</sub>·

{overscore (I)}*<sub>S—

k</sub>}

with variable frequency AC (ω

_k) to a generator driven by the wind turbine, k.

40. An intelligent coactive converter, coupled to a standard frequency AC power grid and configured to enhance electric power produced by a renewable facility and collected and transmitted via a DC power grid, comprising:

a DC-AC power semiconductor converter configured to convert the electric power from the DC power grid from DC to AC;

a power transformer coupled to the standard frequency AC power grid and configured to provide the enhanced electric power to the AC power grid;

a constant-frequency output, rotating AC machine, yM, including

a multi-phase stator winding being configured to couple to at least one of the power transformer and the standard frequency AC power grid,

a rotor winding located in a rotating magnetic core attached to a shaft,

a power semiconductor converter arranged as a converter connected between a multi-phase voltage supply and the rotor winding and

a controller having an external communications interface, said controller being configured to control a air-gap flux {overscore (Φ

)}<sub>δ

—

k</sub> and a stator current {overscore (I)}<sub>S—

k</sub> in the renewable facility, k, so as to provide reactive power as

P<sub>eld—

gen—

k</sub>=Im{3jω

_k·

{overscore (Φ

)}<sub>δ

—

k</sub>·

{overscore (I)}*<sub>S—

k</sub>}

and

active power as

P<sub>eld—

gen—

k</sub>=Re{3jω

_k·

{overscore (Φ

)}<sub>δ

—

k</sub>·

{overscore (I)}*<sub>S—

k</sub>}

with variable frequency AC (ω

_k) from a generator in the renewable facility, said controller being configured to

(a) set an output of the yM to a constant-frequency output, and

(b) prime power from the renewable facility when feeding power to the standard frequency AC power grid at one point-of-connection in a grid area-of-connection, wherein

the yM being configured to supply and consume reactive power, and provide voltage stiffness to the standard frequency AC power grid in the area-of-connection in order to prime the electric power produced by the renewable facility,

the yM being configured to supply and consume active power at the point-of-connection, and provide reduced variability in an active power portion of the electric power fed into the standard frequency AC power grid than a variability in the electric power produced by the renewable facility,

the yM, and the power transformer each being 2*3Φ and

windings in the yM and the power transformer being configured to be stressed by a mix of AC and DC voltage.

41. A method for priming electric power from a renewable facility, comprising steps of:

receiving electric power produced by a renewable facility, said electric power being inadequate to be applied directly to a standard frequency AC power grid due to at least one of insufficient voltage stiffness, and excessive power variability;

delivering via an output line enhanced electric power to said standard frequency AC power grid at, at least, one point-of-connection in a grid area-of-connection, said enhanced electric power being enhanced with regard to at least one of voltage stiffness and power variability with respect to a delivery to said standard frequency AC power grid of only said electric power from said renewable facility;

connecting to said output line a rotating AC machine, yM, which is configured to controllably supply at least one of active power and reactive power to the standard frequency AC power grid and draw at least one of active power and reactive power from the standard frequency AC power grid;

controlling an operational state of said rotating AC machine so as to influence respective amounts of active and reactive power provided by, or consumed by, said rotating AC machine to the standard frequency AC power grid so as to prime the electric power from the renewable facility and enable a delivery of the enhanced electric power to said point-of-connection.

42. The method of claim 41, further comprising a step of:

collecting at least one of power and information on power levels from the renewable facility and another renewable facility in a collection and transmission grid prior to performing said receiving step.

43. The method of claim 42, wherein:

said another renewable facility being at least one of a wind turbine facility, a solar cell facility, a fuel cell facility, and a gas turbine facility.

44. The method of claim 41, wherein:

said yM is at least one of an xM and a synchronous machine.

45. The method of claim 41, further comprising a step of:

coordinating power production levels between the renewable facility and a virtual energy storage facility.

46. The method of claim 41, wherein:

the controlling step includes controlling the yM to supply and consume power so as to provide voltage stiffness to the standard frequency AC power grid to compensate for the electric power provided by the renewable facility.

47. The method of claim 41, wherein:

the controlling step includes controlling the yM to supply and consume power so as to reduce variability of power provided to the standard frequency AC power grid from the renewable facility.

48. The method of claim 41, further comprising a step of:

driving the yM with a prime mover.

49. The method of claim 48, further comprising a step of:

ensuring a delivery of a predetermined amount of power from the renewable facility by controlling an operation of the prime mover and coordinating communications with the virtual energy storage facility so as to ensure the predetermined amount of power is delivered to the standard frequency AC power grid on behalf of said renewable facility.

50. The method of claim 49, further comprising a step of:

coordinating a sale of the electric power from the renewable facility on a power exchange.

51. The method of claim 48, further comprising a step of:

using a predetermined moment of inertia associated with a magnetic core of the AC synchronous machine attached to a shaft of the prime mover to help prime the electrical power from the renewable facility.

52. The method of claim 41, further comprising a step of:

providing short circuit power to the standard frequency AC power grid.

53. The method of claim 41, further comprising a step of:

determining a timing when to fire pulses employed in a semiconductor converter that converts the electric power from the renewable facility to a standard frequency AC.

54. The method of claim 41, further comprising a step of:

controlling in the rotating AC machine, an air-gap flux {overscore (Φ

)}<sub>δ

—

yM</sub> and stator current {overscore (I)}<sub>S—

yM</sub> to provide at least one of

reactive power, with standard frequency AC to the standard frequency AC power grid, according to an equation

Q<sub>eld—

yM</sub>=Im{3jω

·

{overscore (Φ

)}<sub>{overscore (δ

)}—

yM</sub>·

{overscore (I)}*<sub>S—

yM</sub>}

and

active power, from a prime mover, with standard frequency AC to the standard frequency AC power grid according to an equation

P<sub>eld—

yM</sub>=Re{3jω

·

{overscore (Φ

)}<sub>{overscore (δ

)}—

yM</sub>·

{overscore (I)}*<sub>S—

yM</sub>}

55. The method of claim 41, further comprising a step of:

controlling in the rotating AC machine, an air-gap flux {overscore (Φ

)}<sub>δ

—

k</sub> and stator current {overscore (I)}<sub>S—

k</sub> in the at least one wind turbine, k, to provide reactive power as

Q<sub>eld—

gen—

k</sub>=Im{3jω

_k·

{overscore (Φ

)}<sub>δ

—

k</sub>·

{overscore (I)}*<sub>S—

k</sub>}

and

active power as

P<sub>eld—

gen—

k</sub>=Re{3jω

_k·

{overscore (Φ

)}<sub>δ

—

k</sub>·

{overscore (I)}*<sub>S—

k</sub>}

with variable frequency AC (ω

_k) to a generator driven by the wind turbine, k.

57. The system of claim 56, further comprising:

means for collecting power from the renewable facility and another renewable facility prior to receiving said electric power from said renewable facility.

58. The system of claim 56, wherein:

said means for controllably supplying includes at least one of an xM and a synchronous machine.

59. The system of claim 56, further comprising:

means for coordinating power production levels between the renewable facility and a virtual energy storage facility.

60. The system of claim 56, wherein:

the means for controlling includes means for providing voltage stiffness to the standard frequency AC power grid to compensate for the electric power provided by the renewable facility.

61. The system of claim 56, wherein:

the means for controlling includes means for reducing variability of power provided to the standard frequency AC power grid from the renewable facility.

62. The system of claim 56, further comprising:

means for using a moment of inertia associated with a magnetic core of a AC synchronous machine to aid in priming the electrical power from the renewable facility.

63. The system of claim 56, further comprising:

means for providing short circuit power to the standard frequency AC power grid.

64. The system of claim 56, further comprising:

means for controlling in an rotating AC machine, an air-gap flux {overscore (Φ

)}<sub>δ

—

yM</sub> and a stator current {overscore (I)}<sub>S—

yM</sub> to provide at least one of

reactive power, with standard frequency AC to the standard frequency AC power grid, according to an equation

Q<sub>eld—

yM</sub>=Im{3jω

·

{overscore (Φ

)}<sub>{overscore (δ

)}—

yM</sub>·

{overscore (I)}*<sub>S—

yM</sub>}

and

active power, from a prime mover, with standard frequency AC to the standard frequency AC power grid according to an equation

P<sub>eld—

yM</sub>=Re{3jω

·

{overscore (Φ

)}<sub>{overscore (δ

)}—

yM</sub>·

{overscore (I)}*<sub>S—

yM</sub>}

65. The system of claim 56, further comprising:

means for controlling in a rotating AC machine, an air-gap flux {overscore (Φ

)}<sub>δ

—

k</sub> and a stator current {overscore (I)}<sub>S—

k</sub> in wind turbine, k, to provide reactive power as

Q<sub>eld—

gen—

k</sub>=Im{3jω

_k·

{overscore (Φ

)}<sub>δ

—

k</sub>·

{overscore (I)}*<sub>S—

k</sub>}

and

active power as

P<sub>eld—

gen—

k</sub>=Re{3jω

_k·

{overscore (Φ

)}<sub>δ

—

k</sub>·

{overscore (I)}*<sub>S—

k</sub>}

with variable frequency AC (ω

_k) to a generator driven by the wind turbine, k.

66. The system of claim 65, further comprising:

a computer program product for controlling at least one of the air-gap flux, {overscore (Φ

)}<sub>δ

—

k</sub>, to set a torque output from the renewable power production facility to a highest level of power transfer with regard to a fault condition in at least one of a collection and transmission grid and a transmission and distribution grid by controlling at least one of the air-gap flux to a level below a nominal pre-fault level and the stator current to a predetermined level below the nominal pre-fault level.

56. A system for priming electric power from a renewable facility, comprising:

receiving electric power produced by a renewable facility, said electric power being inadequate to be applied directly to a standard frequency AC power grid due to at least one of insufficient voltage stiffness, and excessive power variability with respect to a delivery to said standard frequency AC power grid of only said electric power from said renewable facility;

means for delivering via an output line enhanced electric power to said standard frequency AC power grid at, at least, one point-of-connection in a grid area-of-connection, said enhanced electric power being enhanced with regard to at least one of voltage stiffness and power variability; and

means for controllably supplying at least one of active power and reactive power to the standard frequency AC power grid and drawing at least one of active power and reactive power from the standard frequency AC power grid.

67. A computer program product for controlling an intelligent coactive converter, comprising:

a memory having computer readable instructions encoded therein, said computer readable instructions implementing process steps when executed by a processor, said process steps including,

controlling in an rotating AC machine, an air-gap flux {overscore (Φ

)}<sub>δ

—

yM</sub> and a stator current {overscore (I)}<sub>S—

yM</sub> to provide at least one of

reactive power, with standard frequency AC to the standard frequency AC power grid, according to an equation

Q<sub>eld—

yM</sub>=Im{3jω

·

{overscore (Φ

)}<sub>{overscore (δ

)}—

yM</sub>·

{overscore (I)}*<sub>S—

yM</sub>}

and

active power, from a prime mover, with standard frequency AC to the standard frequency AC power grid according to an equation

P<sub>eld—

yM</sub>=Re{3jω

·

{overscore (Φ

)}<sub>{overscore (δ

)}—

yM</sub>·

{overscore (I)}*<sub>S—

yM</sub>}

68. A computer program product for controlling an intelligent coactive converter, comprising:

a memory have computer readable instructions encoded therein, said computer readable instructions implementing process steps when executed by a processor, said process steps including,

controlling in a rotating AC machine, an air-gap flux {overscore (Φ

)}<sub>δ

—

k</sub> and a stator current {overscore (I)}<sub>S—

k</sub> wind turbine, k, to provide reactive power as

Q<sub>eld—

gen—

k</sub>=Im{3jω

_k·

{overscore (Φ

)}<sub>δ

—

k</sub>·

{overscore (I)}*<sub>S—

k</sub>}

and

active power as

P<sub>eld—

gen—

k</sub>=Re{3jω

_k·

{overscore (Φ

)}<sub>δ

—

k</sub>·

{overscore (I)}*<sub>S—

k</sub>}

with variable frequency AC (ω

_k) to a generator driven by the wind turbine, k.