Direct power and stator flux vector control of a generator for wind energy conversion system
A method for controlling a variable speed wind turbine generator is disclosed. The generator is connected to a power converter comprising switches. The generator comprises a stator and a set of terminals connected to the stator and to the switches of the power converter. The method comprises: determining a stator flux reference value corresponding to a generator power of a desired magnitude, determining an estimated stator flux value corresponding to an actual generator power, determining a difference between the determined stator flux reference value and the estimated stator flux value, and operating said switches in correspondence to the determined stator flux reference value and the estimated stator flux value to adapt at least one stator electrical quantity to obtain said desired generator power magnitude.
1. A method for controlling a variable speed wind turbine generator connected to a power converter comprising switches, said generator comprising a stator and a set of terminals connected to said stator and to said switches, said method comprising:
determining a stator flux reference value corresponding to a generator power of a desired magnitude,
determining an estimated stator flux value corresponding to an actual generator power, and
operating said switches in correspondence to the determined stator flux reference value and the estimated stator flux value to adapt at least one stator electrical quantity to obtain said desired generator power magnitude.
2. The method of claim 1 , wherein a stator flux difference value between the determined stator flux reference value and the estimated stator flux value is determined and the switches are operated in correspondence to said determined stator flux difference value.
3. The method according to claim 2 , wherein a stator flux difference vector with a magnitude and direction is determined by means of the difference between the stator flux vector reference value and the estimated stator flux vector value, and said switches are operated based on said stator flux difference vector.
4. The method according to claim 1 , wherein said switches are operated according to a pulse width modulation scheme in order to generate a synthesized voltage waveform at the stator terminals.
5. The method according to claim 1 , wherein said switches are operated according to a space vector modulation scheme for controlling a switching pattern of said switches.
6. The method according to claim 5 , wherein said switching pattern is formed by applying one or more vectors during one or more switching times, and said switching times for the switching pattern is determined from the magnitude and direction of the stator flux difference vector.
7. The method according to claim 6 , wherein the switches comprise a first and a second set of switches and first set of the switches are operated to an on-state during a first time interval, τ a , and the second set of the switches to an on-state during a second time interval, τ b .
8. The method according to claim 7 , wherein the first and second time intervals are determined according to
τ
a
=
ΔΨ
s
*
(
k
)
sin
(
π
3
-
γ
)
π
3
sin
(
π
3
)
τ
b
=
ΔΨ
s
*
(
k
)
sin
(
γ
)
π
3
sin
(
π
3
)
.
9. An apparatus for controlling a variable speed wind turbine generator connected to a power converter comprising switches, said generator comprising a stator and a set of terminals connected to said stator and to said switches, said apparatus comprising:
a power controller adapted to determine a stator flux reference value corresponding to a generator power of a desired magnitude,
a flux estimator adapted to determine an estimated stator flux value corresponding to an actual generator power, and
a switch control unit adapted to operate said switches in correspondence to the determined stator flux reference value and the estimated stator flux value to adapt at least one stator electrical quantity to obtain said desired generator power magnitude.
10. The apparatus according to claim 9 , comprising a comparator adapted to determine a stator flux difference value between the determined stator flux reference value and the estimated stator flux value, and said switch control unit is adapted to operate said switches in correspondence to said determined stator flux difference value.
11. The apparatus according to claim 10 , wherein the comparator is adapted to establish, from a difference between the stator flux reference value and the estimated stator flux value, a stator flux difference vector with a magnitude and direction and operating said switches based on said stator flux difference vector.
12. The apparatus according to claim 9 , wherein said switch control unit is adapted to operate the switches according to a pulse width modulation scheme in order to generate a synthesized voltage waveform at the stator terminals.
13. The apparatus according to claim 9 , wherein said switch control unit is adapted to operate the switches according to a space vector modulation scheme for controlling a switching pattern of said switches.
14. The apparatus according to claim 13 , wherein said switch control unit is adapted to determine the switching times for the switching pattern from the magnitude and direction of the stator flux difference vector.
15. The apparatus according to claim 14 , wherein the switch control unit is adapted to establish a switching pattern by providing control signals which operates a first set of the switches to an on-state during a first time interval, τ a , and a second set of the switches to an on-state during a second time interval, τ b .
16. The apparatus according to claim 15 , wherein the switch control unit is adapted to determine the first and second time intervals according to
τ
b
=
ΔΨ
s
*
(
k
)
sin
(
γ
)
π
3
sin
(
π
3
)
τ
a
=
ΔΨ
s
*
(
k
)
sin
(
π
3
-
γ
)
π
3
sin
(
π
3
)
.