EE6623 Chap.2 Wind Turbines and Generator Control
Wind Turbines and Generator Control
Control changes across the power curve
A wind turbine faces different objectives at different speeds. Below cut-in, useful generation is unavailable. Between cut-in and rated speed, the controller tries to capture energy efficiently by coordinating rotor speed with wind speed.
Once rated output is reached, maximising capture would overload equipment, so pitch or another power-limiting action holds output near its rating. At extreme wind, protection stops the machine.
Tip-speed ratio joins the atmosphere to the shaft
Blade-tip speed is angular speed multiplied by rotor radius. Dividing it by wind speed gives the dimensionless tip-speed ratio.
A blade design has an efficient operating region, so variable-speed control changes rotor speed as the wind changes. Always convert revolutions per minute to radians per second before using angular equations, and distinguish a calculated set point from a mechanical speed limit.
Keep aerodynamic and electrical efficiencies separate
The power coefficient describes capture at the rotor.
Generator and converter efficiencies describe later electrical stages. These factors multiply in physical order. Betz efficiency is a theoretical aerodynamic ceiling, not an instruction to assign the same coefficient to every operating point.
A practical rotor remains below that ceiling because of wake rotation, drag, blade losses and off-design operation.
Generator topology decides the converter burden
A full-rated converter electrically decouples a synchronous or cage-rotor machine from grid frequency and processes the entire output.
A doubly-fed induction generator connects its stator more directly to the grid and processes slip power through a rotor converter. That can reduce converter rating while narrowing the decoupled speed range and increasing exposure to grid conditions.
Slip needs a declared convention
Synchronous speed follows grid frequency and pole count.
Slip compares rotor speed with that synchronous reference and can change sign when the machine moves above it. State the adopted sign convention before interpreting operating direction. Reactive power is another independent dimension: magnetising requirements and converter capability affect voltage support even when real-power direction is already known.
What this chapter covers
- 01
Betz efficiency and practical coefficients
- 02
Tip-speed ratio and rotor speed
- 03
Cut-in, rated and cut-out regions
- 04
Pitch and power limiting
- 05
Synchronous and induction machines
- 06
Doubly-fed slip power
- 07
Reactive-power capability
Worked example · free
Recover a rotor-speed set point
- 2Rearrange the tip-speed-ratio definition.
- 2Calculate angular speed in radians per second.
- 3Convert to revolutions per minute and interpret.
Key terms
- Tip-speed Ratio
- Blade-tip speed divided by wind speed, used to describe aerodynamic operating condition.
- Rated Power
- The output level equipment is designed to maintain once further capture must be limited.
- Cut-out Speed
- The high wind speed at which protection stops the turbine.
- Synchronous Speed
- The rotating-field speed set by electrical frequency and machine pole count.
- Slip
- The fractional difference between synchronous and rotor speeds under a stated sign convention.
Wind Turbines and Generator Control FAQ
Why does turbine output stop following the cube law?
The cube law describes available power in the undisturbed wind. Once equipment reaches rated output, control deliberately reduces aerodynamic capture to protect the generator and converter. At still higher wind speed, shutdown protection can drive output to zero even while the resource keeps rising.
What does tip-speed ratio control?
It relates blade-tip speed to incoming wind speed and helps locate the rotor operating condition associated with a strong power coefficient. Variable-speed control changes rotor speed as wind changes so the ratio can remain near its desired value below rated operation.
Why use a doubly-fed induction generator?
Its stator carries the main grid power while a rotor converter processes slip power, permitting variable-speed operation with a converter rated for a fraction of total output. The trade includes a narrower speed range and a more direct electrical connection to grid conditions.
Exam move
Draw the turbine power curve from memory and label cut-in, the variable-speed region, rated operation and cut-out. Under each region write the control objective rather than only the name: begin generation, preserve efficient tip-speed ratio, limit power, or protect equipment. Practise the tip-speed relation in both directions, finding rotor speed from wind and finding wind from rotor speed.
Convert revolutions per minute to radians per second on its own line and carry rotor radius rather than diameter. Build a second conversion chain from available wind power through power coefficient, generator efficiency and converter efficiency, labelling every stage. For induction-machine questions, calculate synchronous speed from frequency and pole count before evaluating slip.
State whether slip is synchronous minus rotor speed or another convention, then interpret its sign. Compare full-converter and doubly-fed arrangements with a small diagram showing which path carries total power and which carries slip power. Finally, add a reactive-power question to each real-power practice. Ask whether the converter has rating headroom and whether the proposed control supports voltage or energy delivery.
Review errors by region: cube-law extrapolation above rated speed, a theoretical Betz limit treated as a practical coefficient, and a correct slip magnitude with no sign meaning.
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