Control system for doubly fed induction generator

Abstract

A controller (28) for a doubly fed induction generator (12,20) adjusts control signals to a rotor side converter (24) and line side converter (22) to adjust rotor current when a voltage transient on a utility grid (10) occurs, so that the doubly fed induction generator can ride through the transient. The controller can also turn off the transistors of the rotor side converter (24) to reduce rotor current and/or activate a crowbar (42) to reduce the voltage of the dc link (26) connecting the converters (22, 24) when significant voltage transients occur on the grid (10). This permits continued operation of the dfig system without disconnecting from the grid.

Description

CROSS-REFERENCE TO RELATED APPLICATION

This application

With reference to box 65 and box 66 of FIG. 3, the sensed DC voltage (link voltage between the line side and rotor side converters) is filtered on the analog side prior to A/D conversion. FIG. 9 and Table 9 show the algorithms for both DC voltage regulation (box 65) and determination of the line current references at 66 (ILINEA−REF, etc.). Greater detail is given in FIG. 9 and Table 9.

TABLE 9

sys.debus_ref = 1050 V; DCBUS_kp = 4.0(A/V); DCBUS_ki =
1200(A/V/SEC); update rate = 4800 Hz;
CAP_DCBUS = 8 * 8200/3 = 21867 (uF); LINE_MAX_CURRENT =
sqrt(2) * 566.0 (A);
K_DA = 2 * BURDEN_RESISTOR/5000(V/A); BURDEN_RESISTOR
of Line-side inverter = 30.1 ohms

Starting at the left of FIG. 9, the nominal voltage of the DC bus between the rotor side and line side converters is compared at 110 with the actual sensed bus voltage. The result is processed by a digital proportional and integral control loop 111 to determine the ILINE_CMD signal. This provides the required magnitude of current from each phase to maintain the desired bus voltage. These values are scaled at 112 for digital to analog conversion. The scaled values are multiplied by the normalized voltages obtained as described above with reference to step 5 of Table 1. The results are the ILINE−REF values represented at 113 in FIG. 3.

Concerning converter current regulators, the line converter is modulated with a 3.06 kHz carrier (LINE−TRI in FIG. 10) triangle wave that is used to set the duty cycle. The gating logic is determined by the transfer function shown in FIG. 10. I_LINDA_REF is determined as noted above with reference to FIG. 9, and I_LINEA is the actual sensed value. The comparison is made at 120, filtered at 121, and applied to comparator 122 to obtain the gating signal. For the rotor converter, FIG. 11, modulation is with a 2.04 kHz carrier (ROTO−TRI to 130 in FIG. 11) triangle wave that is used to set the duty cycle. I_ROTOA_REF is determined as given above with reference to FIG. 7, and I_ROTOA is the actual sensed value. FIGS. 10 and 11 show the transfer functions for one phase (A), but the same functions are used for each of the other two phases.

FIG. 12 illustrates the algorithm for control of the rotor side converter switching devices during a significant transient, and crowbar actuation for a possibly greater transient. In general, gating at the rotor side converter will be stopped when the link voltage, the V_DC (also called SYS.DCBUS_V), rises above a predetermined limit, such as about 10% above nominal. If the DC link voltage reaches an even higher limit, such as about 20% above nominal, the crowbar is activated. Values for FIG. 12 are given in Table 12.

TABLE 12

V_CROWBAR_ON = 1250 V;
V_CROWBAR_OFF = 1055 V;
V_ROTOR_OFF = 1150 V;
V_ROTOR_BACK = 1055 V;
NORMAL_V_DCBUS = 1050 V

The operation of comparator 140 in FIG. 12 is to apply a “high” signal to turn off the rotor converter transistors when the sensed bus voltage (SYS.DCBUS V) is above a predetermined limit (1150 volts in the representative embodiment); and by comparator 141 to restart normal operation if system correction is sufficient to bring the DC bus voltage back to a predetermined lower level (V_ROTOR_BACK=1055V) in the representative embodiment. Similarly, comparator 142 controls activation of the crowbar if the DC bus voltage increases above a reference value (V_CROWBAR_ON=1250V) in the representative embodiment; and by comparator 143 to turn the crowbar off if the system corrects to a sufficiently low voltage (V_CROWBAR_OFF=1055V) in the representative embodiment.

The different rotor and crowbar on and off voltages provide a desired amount of hysterisis. In addition, as represented in FIG. 13, the system logic can provide for predetermined delays before activating corrective measures. Table 13 applies to FIG. 13.

TABLE 13

DIP_LIMIT = 0.7 * 575 * sqrt(2) = 375 (V);
RECOVER_TIME = 40/4 = 10;
RECOVER_LIMIT = 500 (V);
DIP_CONFIRM_NUMBER = 3;
operated at a rate of 4800 Hz.

Starting at the top of FIG. 13, a “SYS.SAG” flag is set at “false” during normal operation, indicating that no significant under voltage event is occurring in the utility grid. At box 151, a decision is reached as to whether or not grid voltage has sagged below the predetermined limit, such as 70% of nominal. If not, no action is taken and the logic recycles to the initial box 150. If the measured value of the magnitude of the AC grid voltage is below the DIP_LIMIT value, a down counter 152 is triggered, and at box 153 an evaluation is made as to whether or not the counter has reached zero. In the representative embodiment, counter 152 starts at 3 and counts downward to 0 (i.e., three, then two, then one, then zero), provided that the VMAG value has continued to be below the reference value. The recycling frequency is 4800 Hz, so this would correspond to a voltage dip or sag in excess of 3 divided by 4,800 or 1/1600 second.

At that point, the SYS.SAG flag is set at true (box 154) and the system evaluates whether or not the VMAG value has recovered for a predetermined number of cycles, similar to the procedure described above. In the case of recovery, the count starts at 10 and decreases for each cycle that the recovery limit has been met (boxes 155, 156, 157, 158), ultimately resulting in resetting the SYS.SAG flag to false if the recovery voltage has been exceeded for ten 4800 Hz decision cycles.

The logic for an over voltage event (grid surge) is somewhat different. With reference to FIG. 4, some moderate increase in rotor current is achieved when a high voltage is measured, but there is no “ramping” of the type described with reference to the SAG adjustment. However, the logic of FIG. 12 concerning monitoring of the DC bus voltage still applies. Thus, if the surge is sufficient to raise the DC link voltage, corrective measures are taken at the same voltages for an over voltage event as for an under voltage event, and recovery also is achieved at the same voltages.

While the preferred embodiment of the invention has been illustrated and described, it will be appreciated that various changes can be made therein without departing from the spirit and scope of the invention.

Claims

1. A method of controlling a doubly fed induction generator (dfig) system, such dfig system having a generator with a stator energized by a grid having a voltage with a nominal value, a driven rotor coupled with the stator, a grid side converter electrically connected to the grid, a rotor side converter electrically connected to the rotor, a dc link connecting the converters, a controller supplying control signals to the converters for control of the torque and reactive power from the dfig system, which method comprises:

providing rotor current command signals from the controller;

monitoring the voltage of the grid for transients from nominal; and

if a transient greater than a first predetermined transient occurs, adjusting the rotor current command signals to permit continued operation of the dfig system without disconnecting the dfig from the grid.

2. The method defined in claim 1, including, if a grid transient greater than a second predetermined transient (different from the first predetermined transient) occurs, automatically reducing the rotor current to a minimum value.

3. The method defined in claim 2, in which the rotor side converter has switching transistors, and including turning off the switching transistors to reduce rotor current to a minimum level if a transient greater than the second predetermined transient occurs.

4. The method defined in claim 2 or claim 3, including, if a grid transient greater than a third predetermined transient (different from both of the first and second predetermined transients) occurs, activating a crowbar to reduce the voltage of the dc link.

5. The method defined in claim 1, including monitoring the voltage of the grid for a voltage sag from nominal, and if a voltage sag greater than a first predetermined sag occurs, adjusting the rotor current command signals to reduce rotor torque and reactive power, whereby the dfig system rides through the transient.

6. The method defined in claim 1 or claim 5, in which the torque producing component of the rotor current is adjusted.

7. The method defined in claim 1 or claim 5, in which the flux producing component of the rotor current is adjusted.

8. The method defined in claim 1 or claim 5, in which both the torque producing and flux producing components of the rotor current are adjusted.

9. The method defined in claim 1 or claim 5, in which the rotor current is reduced progressively during the transient.

10. The method defined in claim 9 in which the rotor current is increased progressively following the transient.

11. The method defined in claim 1, including, if a grid transient greater than a second predetermined transient (different from the first predetermined transient) occurs, automatically activating a crowbar to reduce the voltage of the dc link.

12. The method defined in claim 11, including monitoring the voltage of the grid for transients greater than the second predetermined transient by monitoring the voltage of the dc link.

13. A method of controlling a doubly fed induction generator (dfig) system, such dfig system having a generator with a stator energized by a grid having a voltage with a nominal value, a driven rotor coupled with the stator, a grid side converter electrically connected to the grid, a rotor side converter electrically connected to the rotor, a dc link connecting the converters, a controller supplying control signals to the converters for control of the torque and reactive power from the dfig system, which method comprises:

monitoring the voltage of the grid for transients from nominal; and

if a grid transient greater than a predetermined transient occurs, activating a crowbar to reduce the voltage of the dc link connecting the converters, without disconnecting the dfig system from the grid.

14. The method defined in claim 13, including monitoring the voltage of the grid for transients greater than the predetermined transient by monitoring the voltage of the dc link, and activating the crowbar if the dc link voltage increases above a predetermined voltage, without disconnecting the dfig system from the grid.

15. A method of controlling a dfig system, such dfig system having a generator with a stator energized by an ac utility grid having a voltage with a nominal value, a variable speed wind driven rotor coupled with the stator, a grid side ac-dc converter electrically connected to the grid at the ac side, a rotor side ac-dc converter electrically connected to the rotor at the ac side, a dc link connecting the dc sides of the converters, a controller supplying control signals to the converters for controlling operation of switching transistors thereof, which method comprises:

calculating rotor current command signals to control the converter switching transistors to maintain a desired rotor current;

monitoring the voltage of the utility grid for transients from nominal; and

if a grid transient greater than a first predetermined transient occurs, adjusting the rotor current command signals to reduce rotor current and thereby reduce rotor torque and reactive power to permit continued rotation of the rotor without disconnecting the dfig system from the grid, whereby the dfig system rides through the transient; and

following the transient, returning the rotor current command signals to operate as before occurrence of the grid transient.

16. The method defined in claim 15, including reducing the rotor current progressively during the transient.

17. The method defined in claim 15 including, if a grid transient greater than the first predetermined transient occurs, automatically turning off the rotor side converter to reduce rotor current to minimum.

18. The method defined in claim 17 including, if a grid transient greater than a second predetermined transient (different from the first predetermined transient) occurs, automatically activating a crowbar to reduce the voltage of the dc link.

19. The method defined in claim 18, including monitoring the voltage of the utility grid for transients greater than the second predetermined transient by monitoring the voltage of the dc link.

20. A controller for a doubly fed induction generator (dfig) system, such dfig system having a generator with a stator energized by a grid having a voltage with a nominal value, a driven rotor coupled with the stator, a grid side converter electrically connected to the grid, a rotor side converter electrically connected to the rotor, a dc link connecting the converters, said controller comprising means for supplying control signals to the converters for control of the torque and reactive power from the dfig system, said controller further comprising:

means for providing rotor current command signals from the controller;

means for monitoring the voltage of the grid for transients from nominal; and

means for adjusting the rotor current command signals to permit continued operation of the dfig system without disconnecting the dfig system from the grid if a transient greater than a first predetermined transient occurs.

21. A controller for a doubly fed induction generator (dfig) system, such dfig system having a generator with stator energized by a grid having a voltage with a nominal value, a driven rotor coupled with the stator, a grid side converter electrically connected to the grid, a rotor side converter electrically connected to the rotor, a dc link connecting the converters, said controller comprising means for supplying control signals to the converters for control of the torque and reactive power from the dfig system, said controller further comprising:

means for monitoring the voltage of the grid for transients from nominal; and

means for activating a crowbar to reduce the voltage of the dc link connecting the converters if a grid transient greater than a predetermined transient occurs, without disconnecting the dfig system from the grid.

22. A controller for a dfig system, such dfig system having a generator with a stator energized by an ac utility grid having a voltage with a nominal value, a variable speed wind driven rotor coupled with the stator, a grid side ac-dc converter electrically connected to the grid at the ac side, a rotor side ac-dc converter electrically connected to the rotor at the ac side, a dc link connecting the dc sides of the converters, said controller comprising means for supplying control signals to the converters for controlling operation of switching transistors thereof, said controller further comprising:

means for calculating rotor current command signals to control the converter switching transistors to maintain a desired rotor current;

means for monitoring the voltage of the utility grid for transients from nominal; and

means for adjusting the rotor current command signals to reduce rotor current and thereby reduce rotor torque and reactive power to permit continued rotation of the rotor without disconnecting the dfig system from the grid if a grid transient greater than a first predetermined transient occurs, whereby the dfig system rides through the transient; and

means for returning the rotor current command signals to operate as before occurrence of the grid transient following the transient.

23. A doubly fed induction generator (dfig) system comprising:

a generator with a stator energized by a grid having a voltage with a nominal value;

a driven rotor coupled with the stator;

a grid side converter electrically connected to the grid;

a rotor side converter electrically connected to the rotor;

a dc link connecting the converters; and

a controller supplying control signals to the converters for control of the torque and reactive power from the dfig system;

means for providing rotor current command signals from the controller;

means for monitoring the voltage of the grid for transients from nominal; and

means for adjusting the rotor current command signals to permit continued operation of the dfig system without disconnecting the dfig system from the grid if a transient greater than a first predetermined transient occurs.

24. A doubly fed induction generator (dfig) system comprising:

a generator with a stator energized by a grid having a voltage with a nominal value;

a driven rotor coupled with the stator,

a grid side converter electrically connected to the grid;

a rotor side converter electrically connected to the rotor;

a dc link connecting the converters;

a controller supplying control signals to the converters for control of the torque and reactive power from the dfig system;

means for monitoring the voltage of the grid for transients from nominal;

a crowbar constructed and arranged to reduce the voltage of the dc link; and

means for activating a crowbar to reduce the voltage of the dc link connecting the converters if a grid transient greater than a predetermined transient occurs.

25. A dfig system comprising:

a generator with a stator energized by an ac utility grid having a voltage with a nominal value;

a variable speed wind driven rotor coupled with the stator;

a grid side ac-dc converter electrically connected to the grid at the ac side;

a rotor side ac-dc converter electrically connected to the rotor at the ac side;

a dc link connecting the dc sides of the converters;

a controller supplying control signals to the converters for controlling operation of switching transistors thereof;

means for calculating rotor current command signals to control the converter switching transistors to maintain a desired rotor current;

means for monitoring the voltage of the utility grid for transients from nominal; and

means for adjusting the rotor current command signals to reduce rotor current and thereby reduce rotor torque and reactive power to permit continued rotation of the rotor without disconnecting the dfig system from the grid if a grid transient greater than a first predetermined transient occurs, whereby the dfig system rides through the transient; and

means for returning the rotor current command signals to operate as before occurrence of the grid transient following the transient.

26. A doubly fed induction generator (dfig) system comprising

a generator with a stator energized by a grid having a voltage with a nominal value,

a driven rotor coupled with the stator,

a grid side converter electrically connected to the grid,

a rotor side converter electrically connected to the rotor,

a dc link connecting the converters,

a controller monitoring the voltage of the grid for transients from nominal and supplying control signals to said converters for control of torque and reactive power from the dfig system, said controller being programmed to provide rotor current command signals and to adjust said rotor current command signals to permit continued operation of the dfig system without disconnecting the dfig system from the grid if a transient greater than a first predetermined transient occurs.

27. A doubly fed induction generator (dfig) system comprising

a generator with stator energized by a grid having a voltage with a nominal value,

a driven rotor coupled with the stator,

a grid side converter electrically connected to the grid,

a rotor side converter electrically connected to the rotor,

a dc link connecting the converters,

a crowbar constructed and arranged to reduce the voltage of the dc link, and

a controller monitoring the voltage of the grid for transients from nominal and supplying control signals to said converters for control of the torque and reactive power from the dfig system, said controller being programmed to activate a crowbar to reduce the voltage of the dc link connecting the converters if a grid transient greater than a predetermined transient occurs, without disconnecting the dfig system from the grid.

28. A doubly fed induction generator (dfig) system comprising

a generator with a stator energized by an ac utility grid having a voltage with a nominal value,

a variable speed wind driven rotor coupled with the stator,

a grid side ac-dc converter electrically connected to the grid at the ac side,

a rotor side ac-dc converter electrically connected to the rotor at the ac side,

a dc link connecting the dc sides of the converters, and

a controller monitoring the voltage of the grid for transients from nominal and supplying control signals to said converters for controlling operation of switching transistors in the dfig system, said controller being programmed to calculate rotor current command signals to control the converter switching transistors to maintain a desired rotor current, to adjust said rotor current command signals to reduce rotor current and thereby reduce rotor torque and reactive power to permit continued rotation of the rotor without disconnecting the dfig system from the grid if a grid transient greater than a first predetermined transient occurs, whereby the dfig system rides through the transient, and to return said rotor current command signals to operate as before occurrence of the grid transient following the transient.