The vast majority of the world’s electricity is generated by synchronous machines directly connected to their respective power systems. This configuration works very well when the prime mover (usually an engine or a steam, gas or water turbine) provides a steady torque to the generator. Wind, however, is turbulent and this translates directly into fluctuations in drivetrain torque. Using a directly connected synchronous generator in a wind turbine would form too rigid a coupling between the mechanical and electrical systems. Wind gusts would cause large mechanical stresses in the turbine and, depending on the nature of the electrical grid, large fluctuations in the power fed into the electrical system.
The main advantages of variable speed wind turbines in terms of dynamics are:
• The total inertia of the aerodynamic rotor, the gearbox (if there is one) and the electrical generator act as an energy buffer, smoothing out the wind turbulence. Transient torques and rapid variation in electrical power as well as stresses in the drivetrain are greatly reduced
• Lower structural loads and lighter foundations are other advantages of particular importance in offshore applications
Additional advantages are:
• The power electronics may also be capable of regulating the reactive power flow in the network.
• Noise is reduced, when operating at low wind speeds.
The drawbacks of variable speed are the extra complexity of the generator (in some schemes) and of the necessary power electronic hardware, all of which increase cost and possibly reduce reliability.
To allow for wind gusts, virtually all wind turbines have generators or generator systems that provide some degree of speed variation.