METHOD AND APPARATUS FOR CONVERTING WIND GENERATED ELECTRICITY TO CONSTANT FREQUENCY ELECTRICITY FOR A UTILITY GRID

Field of the Invention

The present invention relates in general to a method and apparatus of converting wind generated electricity, to constant frequency electricity. It more particularly relates to such a method and apparatus useful in providing constant frequency electricity to a utility grid.

Background Art

There is no admission that the background art disclosed in this section legally constitutes prior art.

Various different techniques have been employed in connection with the use of wind turbine generators for converting wind generated electricity, to constant frequency electricity for supplying it to a utility grid. Such techniques include the use of rectifier/inverter bridges, with a direct current (dc) link, coupled to the rotor of a doubly fed induction generator. Power factor variable correction or adjustment may be desirable for some applications. For example, reference may be made to an article by T.A. Lipo, entitled, "Variable Speed Generator Technology Options for Wind Turbine Generators," Department of Electrical and Computer Engineering, University of Wisconsin (1984). Furthermore, reference may be made to an article by T.S. Jayadev, entitled, "Novel Electric Generation Schemes for Wind Power Plants," College of Engineering and Applied Science, University of Wisconsin, Milwaukee, presented at the Second Workshop on Wind Energy Conservation Systems, Washington, D. C. (1975); and a book by Werner Leonhard, entitled "Control of Electrical Drivers," Springer-Verlag Publishing Group. Additional reference may be made to United States Patents 4,695,736 and 6,137,187.

Brief Description of the Drawings

The features of this invention and the manner of attaining them will become apparent and the invention itself will be best understood by reference to the following description of certain embodiments of the invention taken in conjunction with the accompanying drawings, wherein:

FIG. 1 is a symbolic block diagram of a wind generated electricity converting system, which is constructed in accordance with an embodiment of the present 5 invention.

Detailed Description of Certain Embodiments of the Invention

A method and apparatus are disclosed for converting wind generated electricity to constant frequency electricity for supplying it to a utility grid. The disclosed system and method uses a wind turbine generator having a doubly fed

IO induction generator. Above the synchronous speed of the generator, the electricity from the rotor is converted to direct current (dc) electricity and the dc electricity is converted back to alternating current (ac) electricity at a fixed unity power factor. Below synchronous speed, electricity flows to the rotor from the utility grid also at a fixed unity power factor. The current of the ac electricity is adjusted to be in phase

15 with the utility grid voltage, wherein the ac electricity is maintained substantially at unity power factor.

According to the disclosed embodiment of the invention, electricity is also generated from the stator of the doubly fed induction generator.

According to certain embodiments of the invention, the adjusting of the .0 current of the ac electricity to be in phase with the utility grid voltage, includes calculating a desired current magnitude and multiplying it by a normalized sensed voltage value of the utility grid voltage to provide a desired current value for current regulation of the ac electricity in phase with the utility grid voltage without any substantial phase shift. According to an embodiment of the invention, the ^>5 calculating may include subtracting the value of the dc voltage from a desired value of the dc voltage to provide an error voltage value for dc voltage regulation.

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348150.1 According to an embodiment of the invention, the calculating may also include using a closed loop controller such as a proportional and integral controller for regulating the dc voltage value to produce the desired current magnitude.

In accordance with the disclosed embodiments of the invention, the 5 determining of the normalized voltage value may be achieved by sensing the line to neutral voltage of the utility grid, and dividing the value of the sensed voltage by the peak magnitude of the sensed voltage to provide the normalized voltage value. According to another aspect of an embodiment of the invention, the actual current magnitude is regulated in response to the desired value of the ac current.

IO Referring now to FIG. 1 , there is shown a wind generated electricity converting system 10, which is constructed in accordance with an embodiment of the invention, and which is used to supply constant frequency electricity to a utility grid. The system 10 includes a wind turbine generator 12 having a doubly fed induction generator (DFIG) 14, which supplies ac electricity via its stator at a desired

15 power factor via three-phase power leads or lines 16 to the utility grid. The desired power factor of the ac electricity from the stator is achieved by rotor current control.

A generator converter 18 is connected to the rotor of the generator 14 for controlling currents of the rotor of the DFIG 14 to cause the electricity from the stator to be controlled to a desired power factor, and the generator to provide a desired

!0 torque level. The power factor of the stator may be adjusted by the control 29, which may also convert the ac electricity. The rotor electricity is supplied to a dc bus 21 via converter 18 where the dc electricity is stored in a capacitor or bank of capacitors such as a capacitor 23 connected across dc bus leads 22 and 24. Above synchronous speed of the generator, a utility converter 25 converts the dc electricity

:5 from the dc bus 21 to constant frequency ac electricity and supplies it to the utility grid via a line filter 27. Below synchronous speed, the utility converter 25 actively rectifies ac electricity from the grid and regulates it for supplying dc electricity to the generator converter 18, which in turn supplies ac current to the rotor of the generator.

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348150.1 In accordance with the disclosed embodiment of the invention, as hereinafter described in greater detail, the current of the ac electricity from the utility converter 25 is adjusted to be in phase with the utility grid voltage. Thus, the ac electricity from the utility converter 25 supplied to the utility grid is maintained substantially at

5 all times at unity power factor. Thus, in accordance with the disclosed embodiments of the present invention, ac electricity is supplied to the utility grid via both the stator and the rotor of the doubly fed induction generator 14 at or above synchronous speeds of the generator. Below synchronous speeds, ac current is supplied to the rotor, and power is generated by the stator and supplied to the grid. The DFIG 14

10 under the control of the generator converter 18 adjusts the reactive power so that the ac electricity supplied by the stator is supplied at a selectively adjustable power factor. The ac electricity provided by the rotor of the generator 14 has a non- adjustable fixed unity power factor as a result of current regulation via the utility converter 25 in accordance with the disclosed embodiment of the present invention.

15 According to an embodiment of the invention, such as one having a DFIG with 6 poles and a synchronous speed of 1 ,200 rpm when the station is connected to a 60Hz grid, approximately two-thirds of the power may be supplied by the stator of the generator 14 at an adjustable power factor to the utility grid, while approximately one-third of the power supplied by the generator 14 from its rotor is

.0 efficiently and economically provided to the utility grid at a constant fixed unity power factor. The ratio of the division of the power between the rotor and the stator may be determined by the winding ratio. Such a technique is reliable and effective, and is a simplified approach as compared to power factor correction such as rotation of the template wave forms and other such techniques employed in the prior art.

.5 Considering now the generator converter 18 in greater detail, the generator converter 18 is a switched inverter and provides current flow to the rotor below synchronous speed until the generator 14 reaches a synchronous speed. Above synchronous speed, the generator 14 generates three-phase ac electricity from its rotor and supplies it through the generator converter 18, which then serves as an

!0 active rectifier to supply dc electricity to the dc bus 21. A control circuit 29 is employed to control the electricity through the generator converter 18 to or from the

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348150.1 rotor of the DFIG 14 during the operation thereof. A tachometer 31 monitors the shaft speed and position of the rotor of the DFIG 14, and for the control circuit 29.

A calculation of desired rotor current is then performed by the control circuit 29 utilizing conventional generator calculations and the mechanical angle sensed by 5 the tachometer 31 , together with the torque requirement and the flux requirement based upon a desired power factor. The resulting calculation is the desired rotor current supplied to the rotor of the DFIG 14 to cause it to generate the electricity from its stator at the desired power factor. The torque requirement is the amount of torque at a given wind speed (may be peak energy capture). A computer (not 10 shown) may be used to determine the desired torque by retrieving the information from a look up table stored in the computer memory (not shown). The flux requirement may be determined based upon the desired power factor at any given time as required by the utility grid.

Considering now the utility converter 25 in greater detail, the utility converter 15 25 comprises a switched inverter generally indicated at 33. The switched inverter 33 comprises three pairs of insulated gate bi-polar transistors (IGBTs), such as the A+ IGBT 35 having a parallel connected diode 37 both connected to the plus side of the dc bus 21 , and a A- IGBT 39 and its diode 42 connected to the negative side of the dc bus 21 for regulating the A phase of the power. Thus, positive or negative .0 regulation can be provided in each one of the A, B and C phases under the control of the current regulator 51.

A set of three multipliers 44, 46, and 48 together with circuits 55 and 56, help control the respective A, B, and C phases of the utility converter 25 by providing reference currents I REF a, I REF b, and I REF c via a current regulator 51 to the

>5 utility converter 25. A dc voltage regulator 53 provides a current magnitude signal lmag to each one of the three multiplier circuits 44, 46, and 48, where the signal lmag is indicative of the desired current magnitude to be processed by the utility converter 25 for the utility grid. The value lmag is multiplied by normalized voltage values VAN-NORM, VBN-NORM and VCN-NORM as calculated from the voltage iO sensed from the utility grid. In this regard, a divider circuit 55 divides the line to

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348150.1 neutral voltage VAN, VBN, and VCN sensed via a line-to-neutral voltage sense circuit 57, by the peak voltage from the grid. In this regard, the sense circuit 57 senses voltages VA, VB, and VC from the utility grid and supplies the sensed voltages VAN, VBN, and VCN to the division circuit 55. The circuit 55 generates the normalized signals VAN-NORM, VBN-NORM and VCN-NORM which are, in turn, each multiplied by the desired current signal lmag to produce reference signals I REF a, I REF b, and I REF c. These reference signals are then supplied to the current regulator 57 which controls the three pairs of the IGBTs of the utility converter 25.

By multiplying the desired value of the current (lmag) by the normalized sensed values of the voltage from the grid, the current is caused to flow to and from the utility grid always substantially in phase with the voltage of the grid to maintain a constant unity power factor in accordance with the disclosed embodiments of the invention. Such an arrangement is highly efficient and effective.

Considering now the dc voltage regulator 53 in greater detail, the dc voltage regulator 53 regulates the voltage of the dc bus 21 by adjusting the current flowing to or from the utility grid in a manner that is in phase with the utility voltages, and therefore operating substantially at a unity power factor substantially at all times. The magnitude of the desired current to or from the utility grid is lmag. The value of lmag is calculated by a closed loop proportional and integral controller 58. The desired value of the voltage of the dc bus 21 is defined as V_Dc and the sensed value is defined as VBUS with units of volts for each. A summation circuit 61 causes the signal V_Dc to be subtracted from the signal V_Bus from the dc bus 21 representative of the voltage on the capacitor 23, to generate an error term. This error term is regulated by the proportional and integral (Pl) controller control loop such that the error is regulated toward zero, and the actual value of the V_Dc is driven to the desired value VBUS- The output of the dc voltage regulator 53 is the signal lmag which is the desired peak magnitude of current flowing through the utility converter 25.

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348150.1 The value of lmag is multiplied by the multiplication circuits 44, 46, and 48 by signals that represent the normalized line to neutral voltages for each of the three phases of the utility grid voltage. As a result, the desired current level is provided and is used for the current regulation for each phase. In this regard, the signals 5 I REF a, I REF b, and I REF c, for the respective phases A, B, and C of the utility converter 25, are in phase with the utility voltage with substantially no phase shift.

Considering now the current regulator 51 in greater detail, the current regulator 51 receives the desired utility matrix currents I REF a, I REF b and I REF c. The current regulator 51 also receives sensed inputs from the output of the utility

IO converter 25. The sensed currents are designated as Ia, Ib, and Ic, which may be measured by current transducers. For each phase, the sensed current value may be subtracted from the desired current value by a summation circuit such as the summation circuit 62 for the A phase. Error terms are then generated. For example, in phase A, Ia is subtracted from I REF a to generate an error signal. The

15 error terms such as the error term generated by the circuit 62 are compared by three comparators, such as a comparator 63 of the A phase, to a triangular shaped signal such as a signal LINE-TRI-A for the A phase. Thus, control signals A+, A-, B+, B-, C+ and C- are generated for turning on or off the utility converter IGBTs such as IGBTs 35 and 39 for the A phase. If the error is above the triangle wave, the proper

.0 IGBT, such as the IGBT 39, is turned on to cause the utility matrix current to go down which in this case, is the A- signal for the IGBT 39 of the A phase. The reverse process is used when the error is below the triangle wave to control the proper IGBT, such as the IGBT 35 for the A phase. Either a positive or a negative IGBT is turned on, but not both at the same time for any one of the three phases.

!5 For example, if A+ is on, then A- is off and if A- is on, then A+ is off. In this regard, the current regulator 51 includes an inverter for each phase such as an inverter 64 is used to invert the A+ signal from the comparator such as the comparator 63 for the A phase to provide the control signal A-. There is also a short delay between the switchings to avoid a short circuit across the dc bus 21. In this implementation, a

IO triangle wave of 2kHz is used and the current regulation calculations are made by analog hardware (not shown). A delay between changing the state of devices may

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348150.1 be set to a value such as 5 micro-seconds, for certain applications. The current regulation may be accomplished by a variety of techniques including, but not limited to, the use of digital logic (not shown). This may also include filtering of the actual and reference signals.

While particular embodiments of the present invention have been disclosed, it is to be understood that various different modifications are passing and one contemplated within the true spirit and scope of the appended claims. For example, while discrete circuits have been described for certain functions, it will become apparent to those skilled in the art that such functions may be implemented in whole or in part by software. There is no intention, therefore, of limitations to the exact abstract or disclosure herein presented.

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348150.1