Variable speed wind turbine with a doubly-fed induction generator and rotor and grid inverters that use scalar controls
The present invention relates to an improved wind turbine, of the type which employs doubly fed induction generators (DFIG), and a wind park including the same, which permits the use of lighter weight turbines, with the ability to have greater energy capture, more precise control of asymmetrical phases and enhanced maintenance and support of the grid during fault conditions.
1. A variable speed wind turbine adapted to operate in a wind farm, said turbine being adapted to supply electrical power to an associated grid at a substantially fixed grid frequency during a low voltage condition, the turbine comprising a stator and a doubly-fed generator having a rotor adapted to turn at a rotor speed, the rotor and stator each comprising polyphase windings; the variable speed turbine comprising:
a rotor inverter;
a grid inverter;
a rotor inverter controller for using scalar controls to control the rotor inverter to provide stator output power at a frequency substantially equal to a grid frequency and at unity power factor;
a link for maintaining a substantially constant DC interface between the rotor inverter and the grid inverter;
a grid inverter controller for using scalar controls to independently control individual grid inverter currents and provide output power at a frequency substantially equal to the grid frequency and at unity power factor;
blade control means for maintaining a generator rotor speed at substantially the same speed before and during a low voltage condition; and
protection means for the link and rotor for limiting damage to turbine components during a fault.
2. A variable speed wind turbine according to claim 1 , wherein the turbine is operated in a wind farm that produces substantially all reactive power needs of the grid.
3. A variable speed wind turbine according to claim 1 , wherein the scalar control utilizes a Resonance Proportional-Integral Controller.
4. A variable speed wind turbine according to claim 1 , wherein blade control means comprises pitch control means.
5. A method for operating a variable speed wind turbine in a wind farm, said turbine being adapted to supply electrical power to an associated grid at a substantially fixed grid frequency during a low voltage condition, the turbine comprising a stator and a doubly-fed generator having a rotor adapted to turn at a rotor speed, the rotor and stator each comprising polyphase windings; the method comprising the steps of:
applying scalar controls to control a rotor inverter so as to provide stator output power at a frequency substantially equal to a grid frequency and at unity power factor;
maintaining a substantially constant DC interface between the rotor inverter and a grid inverter;
applying scalar controls to independently control individual grid inverter currents and provide output power at a frequency substantially equal to the grid frequency and at unity power factor; and
applying blade control means for maintaining a generator rotor speed at substantially the same speed before and during a low voltage condition.