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 an estimated stator flux value corresponding to an actual generator power;
calculating a stator flux difference value between a stator flux reference value and the estimated stator flux value;
calculating a stator flux difference vector with a magnitude and direction, wherein the stator flux difference vector is a difference between a stator flux reference value and an estimated stator flux vector value; and
operating said switches of said power converter with switching times based at least on the stator flux reference value, the estimated stator flux value, the calculated stator flux difference value, the stator flux difference vector, and a DC-link voltage of the power converter to adapt at least one stator electrical quantity to obtain a desired generator power magnitude, wherein said switches are operated according to a space vector modulation scheme for controlling a switching pattern of said switches, and 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.
2. 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.
3. The method according to claim 1 , 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, τa, and the second set of the switches to an on-state during a second time interval, τb.
4. The method according to claim 3 , wherein the first and second time intervals are determined according to
τ
a
=
Δ
Ψ
s
*
(
k
)
sin
(
π
3
-
γ
)
π
3
sin
(
π
3
)
τ
b
=
Δ
Ψ
s
*
(
k
)
sin
(
γ
)
π
3
sin
(
π
3
)
.
5. 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;
calculate a stator flux difference value between the stator flux reference value and an estimated stator flux value; and
calculate a stator flux difference vector with a magnitude and direction, wherein the stator flux difference vector is a difference between a stator flux reference value and an estimated stator flux vector value; and
a switch control unit adapted to operate said switches of said power converter with switching times based at least on the determined stator flux reference value, the estimated stator flux value, the calculated stator flux difference value, the stator flux difference vector, and a DC-link voltage of the power converter to adapt at least one stator electrical quantity to obtain a desired generator power magnitude, 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, and 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.
6. The apparatus according to claim 5 , 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.
7. The apparatus according to claim 5 , 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, τa, and a second set of the switches to an on-state during a second time interval, τb.
8. The apparatus according to claim 7 , wherein the switch control unit is adapted to determine the first and second time intervals according to
τ
b
=
Δ
Ψ
s
*
(
k
)
sin
(
γ
)
π
3
sin
(
π
3
)
τ
a
=
Δ
Ψ
s
*
(
k
)
sin
(
π
3
-
γ
)
π
3
sin
(
π
3
)
.
9. The method of claim 3 , wherein the first time interval and the second time interval are determined based on the DC-link voltage of the power converter.
10. The apparatus of claim 7 , wherein the first time interval and the second time interval are determined based on the DC-link voltage of the power converter.