The term variable speed indicates that these wind turbines are structured to withstand and perform accurately at different wind speeds. Variable-speed wind turbines maintain optimal aerodynamic performance by allowing the generator/rotor speed to vary proportionally with wind speed. This means that the turbine can adjust its speed between the cut-in and rated speed while keeping a constant TSR (Tip Speed Ratio).
This ability to adapt to changing wind conditions helps maximize the efficiency of the turbine’s operation. In order to maintain a constant generator/rotor speed above the rated speed, it is crucial to actively control the torque.
Different types of generators are used in variable-speed wind turbines due to various factors. One of them is the doubly-fed induction generator (DFIG). The wound rotor induction generator is another type that allows for variable speed while maintaining a fixed output power.
Features of Variable-Speed Wind Turbines
Some key features of variable-speed turbines are:
- Implementing variable speed systems can significantly enhance the energy-absorbing potential during partial load operations.
- This system can either use a synchronous generator or an induction generator.
- It can function without gears, resulting in reduced costs.
- With a variable-speed wind turbine, the speed of the generator or rotor can vary in proportion to wind speed.
Also See: 2 Types of Vertical Axis Wind Turbine
Differences between Variable Speed and Fixed Speed Wind Turbines
In contrast to a variable speed turbine, a fixed turbine maintains a consistent rotational speed of the generator or rotor, irrespective of the wind velocity. Tip speed ratio changes with wind speed and rotor aerodynamics are optimal only at a specific wind speed. Generator torque is determined only by the induction generator.
- Fixed speed wind turbines only reach peak efficiency at a specific wind speed, while variable speed wind turbines achieve maximum efficiency across a wider range of wind speeds.
- Variable speed wind turbines have control over reactive power, whereas fixed speed ones do not have any control.
- Harmonics are generated by electronic converters in variable speed turbines, but none is generated in fixed speed turbines.
- Fixed-speed turbines have voltage level control, thereby eliminating voltage dips. In fixed ones in direct mode, high inrush currents are produced, causing a voltage drop.
- Variable turbines have improve voltage stability, but fixed speed turbines face voltage variations and flicker due to variations in the wind.
- Limited transient levels in variable turbines whereas fixed turbines face transients during start and stop due to shunt capacitors.
- Variable speed turbines have improved frequency variations. Due to sudden wind drop or rise in autonomous grid, fixed turbines experience variations.
Recommended: What is Vertical-Axis Wind Turbine (VAWT)?