High voltage direct current link transmission system for variable speed wind turbine
A variable speed wind turbine having a doubly fed induction generator (DFIG), includes an exciter machine mechanically coupled to the DFIG and a power converter placed between a rotor of the DFIG and the exciter machine. Thus, the power converter is not directly connected to the grid avoiding the introduction of undesired harmonic distortion and achieving a better power quality fed into the utility grid. Moreover, the variable speed wind turbine includes a power control and a pitch regulation.
1. A variable speed wind turbine comprising:
a rotor including at least one blade;
a drive train coupled to the rotor, the drive train including at least a doubly fed induction generator (DFIG), said DFIG having at least a stator connectable to a high voltage direct current (HVDC) system;
at least an exciter machine coupled to the drive train; and
at least a power conversion device isolated from the HVDC system and electrically coupled to a rotor of the doubly fed induction generator and to the exciter machine to transfer electrical power between the rotor and the exciter machine.
2. The variable speed wind turbine according to claim 1 , wherein the HVDC system comprises at least one rectifier connectable to a HVDC bus, wherein the stator is connectable to the HVDC bus via the at least one rectifier.
3. The variable speed wind turbine according to claim 2 , further comprising a contactor for opening or closing a connection between the at least one rectifier and the HVDC bus.
4. The variable speed wind turbine according to claim 3 , further comprising at least two rectifiers, wherein the at least two rectifiers are connected to the HVDC bus in a series or a parallel arrangement.
5. The variable speed wind turbine according to claim 3 , wherein a HVDC bus voltage is controlled by a power conversion controller which controls the at least one power conversion device.
6. The variable speed wind turbine according to claim 3 , wherein the DFIG is controlled based on at least one of an alternating current (AC) output power of the stator, an AC output current of the stator, an HVDC bus output current and an HVDC bus output power.
7. The variable speed wind turbine according to claim 2 , wherein the at least one rectifier comprises a semiconductor device including at least one of a diode and a thyristor.
8. The variable speed wind turbine according to claim 1 , further comprising:
a connection system which controls the DFIG for connection to the HVDC system; and
a disconnection system which controls the DFIG for disconnection from the HVDC system.
9. The variable speed wind turbine according to claim 8 , wherein the stator is connected to the HVDC system;
wherein the DFIG is controlled in voltage regulation mode and the voltage is ramped up until current flows through the stator or a rectifier of the HVDC system;
wherein an operating mode of the DFIG is controlled according to at least one of a DFIG output current control, DFIG output power control, HVDC system output current control and HVDC systems output power control, after the current flows through the stator or the rectifier.
10. The variable speed wind turbine according to claim 8 , wherein the DFIG is controlled in voltage regulation mode such that an output voltage of the DFIG reaches a target voltage based on an HVDC line voltage;
wherein a contactor between the stator and the HVDC system is closed after the output voltage of the DFIG reaches the target voltage.
11. The variable speed wind turbine according to claim 1 , wherein the HVDC system comprises:
at least one power transformer connectable to the stator of the DFIG; at least one rectifier which connecting the at least one power transformer to a contactor;
an HVDC bus connectable to the at least one rectifier via the contactor.
12. The variable speed wind turbine according to claim 11 , further comprising at least two rectifiers, wherein the at least two rectifiers are connected to the bus in a series or a parallel arrangement.
13. The variable speed wind turbine according to claim 11 , wherein the at least one rectifier comprises a semiconductor device including at least one of a diode and a thyristor.
14. The variable speed wind turbine according to claim 11 , wherein a HVDC bus voltage is controlled by a power conversion controller which controls the at least one power conversion device.
15. The variable speed wind turbine according to claim 11 , wherein the DFIG is controlled based on at least one of an alternating current (AC) output power of the stator, an AC output current of the stator, an HVDC bus output current and an HVDC bus output power.
16. The variable speed wind turbine according to claim 1 , wherein the stator is directly coupled to the HVDC system using at least one of a transformer and a rectifier,
wherein the variable speed wind turbine is configured such power output from the stator is at a substantially fixed frequency under varying wind conditions.
17. The variable speed wind turbine according to claim 16 , wherein the power output from the stator is at a substantially fixed voltage.