# Tip Speed Ratio - Cp - λ Curves - The Case For Variable Speed Wind Turbines

The Case For Variable Speed Wind Turbines

Originally, wind turbines were fixed speed. This has the benefit that the rotor speed in the generator is constant, thus the frequency of the AC Voltage is fixed. This allows the wind turbine to be directly connected to a transmission system. However, from the figure above, we can see that the power co-efficient is a function of the tip-speed ratio. By extension, the efficiency of the wind turbine is a function of the tip-speed ratio.

Ideally, one would like to have a turbine operating at the maximum value of at all wind speeds. This means that as the wind speed changes, the rotor speed must change to such that . A wind turbine with a variable rotor speed is called a variable speed wind turbine. Whilst this does mean that the wind turbine operates at or close to for a range of wind speeds, the frequency of the AC voltage generator will not be constant. This can be seen in the following equation:

$N = frac{120f}{P}$

where is the frequency of the AC voltage generated in the stator windings, is the rotor speed, is the number of poles in the generator inside the nacelle. That is, direct connection to a transmission system for a variable speed is not permissible. What is required is a power converter which converts the signal generated by the turbine generator into DC and then converts that signal to an AC signal with the grid/transmission system frequency.

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