Variable speed wind turbine drive and control system
A variable speed wind turbine utilizes a hydraulic transmission including a hydraulic pump coupled to a rotor and a hydraulic motor coupled to an induction generator to produce variable frequency power that is converted to grid frequency power by electronic controls.
1. A variable speed wind turbine transmission system comprising:
a turbine rotor driven by wind;
a fixed displacement hydraulic pump coupled to the rotor;
a fixed displacement hydraulic motor coupled to a generator;
a hydraulic circuit to transfer fluid from the pump to the motor;
wherein the generator rotates at a variable speed directly related to rotation of the turbine rotor and produces an electrical output at a variable frequency; and
a power converter to convert the variable frequency electrical output of the generator into grid frequency power.
2. The system of claim 1 , wherein the power converter comprises:
an inverter which converts AC power to DC power,
an active front end which converts the DC power into grid frequency AC power; and
an L-C-L filter which reduces power line harmonics.
3. The system of claim 2 , wherein the active front end synthesizes a sinusoidal input voltage from the inverter by Pulse-Width-Modulation (PWM).
4. The system of claim 3 , wherein the power converter is capable of controlling a load on the generator based on the speed of rotation of the rotor.
5. The system of claim 3 , wherein the power converter controls the inverter by a generator rotor speed feedback device to regulate the amount of negative slip of the induction generator.
6. The system of claim 3 , wherein the power converter controls the inverter using a sensorless vector control.
7. The system of claim 3 , wherein the power converter is capable of controlling both speed and torque of the generator shaft.
8. The system of claim 3 which utilizes the energy from the spinning rotor to maintain control during a failure of the power grid.
9. The system of claim 1 , wherein the system does not utilize a gearbox.
10. A variable speed wind turbine system comprising:
a turbine rotor driven by the wind;
a fixed displacement hydraulic pump directly driven by the rotor;
a fixed displacement hydraulic motor;
a closed loop hydraulic fluid circuit for carrying pressurized oil to the hydraulic motor from the hydraulic pump and to return hydraulic fluid to the hydraulic pump from the hydraulic motor;
an induction generator directly driven by the fixed displacement hydraulic motor, the induction generator producing AC power at variable frequency;
a power converter including an inverter which converts the AC power to DC power, an active front end which converts the DC power into grid frequency AC power, and an L-C-L filter which reduces power line harmonics.
11. The system of claim 10 , wherein the active front end synthesizes a sinusoidal input voltage from the inverter by Pulse-Width-Modulation (PWM).
12. The system of claim 11 , wherein the power converter is capable of controlling a load on the generator based on the speed of rotation of the rotor.
13. The system of claim 11 , wherein the power converter controls the inverter by a generator rotor speed feedback device to regulate the amount of negative slip of the induction generator.
14. The system of claim 11 , wherein the power converter controls the inverter using a sensorless vector control.
15. The system of claim 11 , wherein the power converter is capable of controlling both speed and torque of the generator shaft.
16. A method for producing a grid frequency power with a variable speed wind turbine, the method comprising the steps of:
providing a variable speed wind turbine comprising a turbine rotor driven by wind, a fixed displacement hydraulic pump coupled to the rotor, a fixed displacement hydraulic motor coupled to a generator, and a hydraulic circuit to transfer fluid from the pump to the motor;
subjecting the rotor of the variable speed wind turbine to a source of wind such that the generator produces an electrical output at a variable frequency related to a speed of rotation of the rotor; and
converting the variable frequency electrical output of the generator into grid frequency power.
17. The method of claim 16 , wherein the step of converting the variable frequency electrical output of the generator into a fixed grid frequency power comprises the steps of:
inverting the variable frequency electrical output to direct current power;
converting the direct current power into a fixed grid frequency AC power output.
18. The method of claim 17 , wherein the step of converting the variable frequency electrical output of the generator into a fixed grid frequency power further comprises the step of:
filtering the fixed frequency grid power using an L-C-L filter.