13 Technology and Control of Wind Turbines

Wind speed 𝑣Wind at any height can be determined approximately based on a measured wind speed at a reference height of 10 m

alpha=1,6 at Neutral air above flat open coast

• 30.243 Wind Turbines (1/1/2024)

• 77.6 GW total installed power (January 2026)

  − Onshore (88 %) and offshore (12 %)

Ground-mounted PV systems (4.1 - 14.4 €cents/kWh)

and onshore wind turbines (4.3 - 9.2 €cents/kWh) are

the most cost-effective technologies in Germany

• The wind speed before the wind turbine is larger than after. Because the mass flow must be continuous ( ሶ𝑚 = 𝜌 ⋅ 𝐴 ⋅ 𝑣 = 𝑐𝑜𝑛𝑠𝑡. ), the area A3 after the wind turbine is bigger than the area A1 before.

= 59%

Real wind turbines achieve a 𝐶P,max of 45 to 52 %

▪ Position of rotor axis (horizontal, vertical)

▪ Rotor speed (slow-, fast-running)

▪ Rotor speed (constant, variable)

▪ Possibility of influencing power (stall, pitch controlled)

▪ Possibility of storm protection (switching off, fixing)

▪ Generator type (asynchronous, synchronous, permanent magnet)

▪ Network coupling (direct, indirect)

Geared Doubly-Fed Induction Generator

• generator speed must be constant

• Advantages

  − Simple construction

  − A converter is not required

  − Low investment costs and less maintenance

• Disadvantages

  − Only IG feasible

  − Stronger fluctuations in wind speed are fed to grid

  − High mechanical stresses on the wind turbine

  − Operation at optimum operating point not always possible → lower Cp

• Generator speed may vary

• A Converter is used: Inverter generates the fixed grid frequency and grid voltage

• Typically used in high power plants

• Advantages

  − The system is very flexible with regard to wind fluctuations

  ➢ SG gearless and with permanent magnets possible

  − System can always be operated at the optimum operating point

  − Less mechanical stress on wind turbine (stall and pitch control)

• Disadvantages

  − More complex and expensive construction

  − Efficiency of the converter lowers overall system efficiency

• Passive Stall

  The blades of the wind turbine are designed in such a way that, when the wind is too strong, turbulences occure and cause a stall. This causes the wind turbine to be braked in a passive way.

• Active Stall

  When wind speed is too high, the wind turbine blade is turned against the wind in order to achieve a stall.

• Pitch

  If the wind speed exceeds the maximum allowed value, the blades of the wind turbine are actively turned by servo drive systems into the wind. The rotor blades are positioned as close to the wind as necessary to avoid overloading.

• Active stall control is specifically about stalling the blades to reduce power, whereas pitch control is about finetuning the blade angles to optimize power output and protect the turbine.