Variable speed wind turbine with reduced power fluctuation and a static VAR mode of operation

A variable speed wind turbine that generates electrical power that varies dependent upon a fluctuating wind speed, said wind turbine providing substantially constant electrical power to a grid supplying multiphase electrical power to consumers, said grid having backup generators for supplying consumer needs not met by wind turbine power, said wind turbine comprising:

• a turbine rotor including at least one blade mounted to a rotatable shaft;
• a multiphase generator having a stator and a rotor coupled to said rotatable shaft for rotation therewith, said generator providing electrical power with a varying frequency;
• a power converter for converting said varying frequency electrical power into constant frequency electrical power, said power converter including a DC voltage link;
• and
• an electrical energy source coupled to the DC voltage link, said electrical energy source capable of supplying sufficient energy to smooth output power fluctuations so that the backup generators are not substantially slowed by the increased loads during wind speed fluctuations.

A variable speed wind turbine that generates electrical power that varies dependent upon a fluctuating wind speed, comprising:

• a turbine rotor including at least one blade mounted to a rotatable shaft;
• a multiphase induction generator having a stator and a rotor coupled to said rotatable shaft for rotation therewith;
• a power converter coupled to the generator, including,
• a DC voltage link,
• a rectifier for converting the electrical power generated by the generator into a constant voltage supplied to the DC voltage link,
• a bridge inverter for converting the constant voltage on the DC link into a constant frequency AC output, said bridge inverter having active switches operable for controlling output currents;
• an electrical energy source coupled to the DC voltage link, said electrical energy source capable of supplying sufficient energy to substantially reduce output power fluctuations during wind speed fluctuations;
• means for sensing output currents from the bridge inverter;
• and
• inverter controller means coupled to the bridge inverter for controlling the output currents, including means for establishing desired output currents, means for periodically determining a distortion index indicative of errors between desired and actual output currents, and means for controlling the active switches to produce output currents that minimize the distortion index.

A variable speed wind turbine that generates electrical power that varies dependent upon a fluctuating wind speed, comprising:

• a turbine rotor including at least one blade mounted to a rotatable shaft;
• a multiphase induction generator having a rotor coupled to said rotatable shaft for rotation therewith, said generator providing electrical power with a varying frequency;
• a power converter for converting the variable frequency electricity into constant frequency output electricity, the power converter including a DC voltage link and an inverter for supplying output electricity, said inverter having active switches for controlling instantaneous currents flowing therethrough;
• means for supplying a reactive power control signal indicative of the reactive power to be supplied by the inverter;
• and
• inverter controller means coupled to the inverter and responsive to said reactive power control signal for controlling the current flowing through the active switches to supply reactive power in a static mode when the wind turbine is not generating real power.

A variable speed wind turbine that generates electrical power that varies dependent upon a fluctuating wind speed, comprising:

• a turbine rotor including at least one blade mounted to a rotatable shaft;
• a multiphase induction generator having a rotor coupled to said rotable shaft for rotation therewith, said generator providing electrical power with a variable frequency;
• a power converter for converting the variable frequency electricity into constant frequency electricity, including a rectifier, an inverter, and a DC voltage link coupled between the rectifier and the inverter, wherein the rectifier includes a pair of active switches for each phase of the generator coupled between the DC voltage link and a stator power tap and operable for switching stator current therebetween, and wherein the inverter includes a pair of active switches for each phase of output power coupled between the DC voltage link and an output tap and operable for switching inverter current therebetween;
• electrical energy storage means coupled to the DC voltage link, said electrical energy storage means capable of storing sufficient energy to substantially smooth output power fluctuations;
• means for defining a reactive power control signal indicative of a predetermined reactive power;
• and
• an inverter controller coupled to the inverter and responsive to the reactive power control signal for controlling the inverter switches to supply output electricity with the predetermined reactive power, wherein the inverter controller includes
• means for creating a sinusoidal reference waveform synchronized with the output electricity,
• means for periodically calculating an imaginary waveform by shifting the reference waveform by 90.degree.;
• means for periodically calculating a real waveform from the reference waveform,
• means for adding the imaginary waveform to the real waveform to produce a current control waveform,
• means for controlling the active switches of the inverter to produce output currents that correspond to currents specified by the current control waveform.

A method for converting electricity generated by a variable speed wind turbine into fixed frequency output electricity having a selected reactive power, wherein the wind turbine includes a generator and means for supplying generated electricity to a power converter that includes an inverter supplying the output electricity, the method comprising the steps of:

• forming a reference waveform defined by the voltage of the output electricity;
• rotating the reference waveform by 90.degree. to yield an imaginary waveform;
• deriving a real waveform from the reference waveform;
• adding the real waveform to the imaginary waveform to yield a current control waveform;
• and
• controlling the inverter to produce an output current corresponding to the currents specified by the current control waveform.