Variable speed wind turbine

for a fixed blade pitch angle is obtained by operating the wind turbine at the optimal tip-speed ratio as shown in the following graph.

Grey curve: Wind speed is assumed constant so that

This means that the axial and tangential forces that act on the blade vary with rotor speed.

The force in the axial direction is given by the following formula: As discussed earlier, a wind turbine would ideally operate at its maximum efficiency for below rated power.

This is for two reasons: ratings on the drivetrain equipment, such as the generator; and second to reduce the loads on the blades.

Below rated power, the wind turbine will ideally operate in such a way that

On a Torque-rotor speed diagram, this looks as follows: where the black line represents the initial section of the operating strategy for a variable speed stall-regulated wind turbine.

Ideally, we would want to stay on the maximum efficiency curve until rated power is hit.

A stall-regulated variable speed wind turbine has no pitching mechanism.

The rotor speed can either be increased or decreased by an appropriately designed controller.

In reference to the figure illustrated in the blade forces section, it is evident that the angle between the apparent wind speed and the plane of rotation is dependent upon the rotor speed.

The lift and drag co-efficients for an airfoil are related to the angle of attack.

The lift and drag forces influence the power production of a wind turbine.

So it can be established that if the angle of attack needs to be increased to limit the power production of the wind turbine, the rotor speed must be reduced.

Pitch regulation thus allows the wind turbine to actively change the angle of attack of the air on the blades.

This is preferred over a stall-regulated wind turbine as it enables far greater control of the power output.

However, due to constraints such as noise levels, this is not possible for the full range of sub-rated wind speeds.

The previous torque rotor-speed diagrams are all plots when the pitch angle,

A three dimensional plot can be produced which includes variations in pitch angle.

Ultimately, in the 2D plot, above rated wind speed, the turbine will operate at the point marked 'x' on the diagram below.

An advantage of a gearbox is that generators are typically designed to have the rotor rotating at a high speed within the stator.

[5] For variable speed wind turbines, one of two types of generators can be used: a DFIG (doubly fed induction generator) or an FRC (fully rated converter).

Unlike the DFIG, the FRC can employ a squirrel cage rotor in the generator; an example of this is the Siemens SWT 3.6-107, which is termed the industry workhorse.

[8] A disadvantage of a permanent magnet generator is the cost of materials that need to be included.

[9] Consider a variable speed wind turbine with a permanent magnet synchronous generator.

For this, power converters are employed, which results in the de-coupling of the wind turbine from the transmission system.

As more wind turbines are included in a national power system, the inertia is decreased.

This means that the frequency of the transmission system is more strongly affected by the loss of a single generating unit.

As already mentioned, the voltage generated by a variable speed wind turbine is non-grid compliant.

In order to supply the transmission network with power from these turbines, the signal must be passed through a power converter, which ensures that the frequency of the voltage of the electricity being generated by the wind turbine is the frequency of the transmission system when it is transferred onto the transmission system.