Types of Commercial Wind Turbines.


We already saw the different wind turbines we can find nowadays, from the HAWT to VAWT. However, the most commercial at this moment are the High Tip Speed Ratio HAWT. In this post, we are going to describe the different models of wind turbines and what kind of technology they use to convert the mechanical energy to electric energy.

If we pay attention at the above table, we can see there are two wind turbines models according to their Speed Control and Power Control. Thereby, we can find four models: Type A, Type B, Type C, and Type D (see the below diagram). There are three variants for the Type A (A1,A2 and A3). Although it would be possible to manufacture the same variations for the B, C and D Types, it would be not worthy (from the commercial point of view) because these models lack the capability for a fast reduction of power (see the grey-colored boxes in the first diagram). Let´s start describing the possibilities to control the power the aerodynamic forces on the turbine rotor.

POWER CONTROL

Wind turbines must have a power control system in the turbine rotor to avoid the damages of high winds. We have three methods:

  • Stall Control: Blades are bolted onto the hub at a fixed angle. The simplest, most robust and cheapest way is installing a stall control.
  • Pitch Control: Blades can be turned out or into the wind as the power output becomes too high or too low, respectively. Advantages: Good power control, assisted start-up and emergency stop. Disadvantages: Extra complexity and higher power fluctuations at high wind speeds.
  • Active Stall Control: The stall of the blade is actively controlled by pitching the blades. Advantages: Good compensation against variations in air density, easy to carry out emergency stops and to start-up the wind turbine.

SPEED CONTROL

Fixed Speed Wind Turbines (Type A):

If they work at a fixed speed ratio, they will have an induction generator (SCIG – Squirrel Cage Induction Generator) connected to the grid via a transformer, a soft-starter for a smoother grid connection and a capacitor bank for reducing reactive power compensation. The wind fluctuations are converted into mechanical fluctuations and consequently into electrical power fluctuations.  In order to increase their performance, some of them have two winding sets: one is used at low wind speeds, and the other at medium-high wind speed. Advantages: Simple, robust, reliable, well-proven, electrical components cheap. Disadvantages: Uncontrollable reactive power consumption, mechanical stress and limited power quality control.

  • A0: It has been very popular because of its relatively low price, simplicity and robustness. However, they cannot carry out assisted start-ups.
  • A1: It is quite easy to regulate the power control, the emergency stopping and the controlled start-up. However, at high wind speeds, even small variations in wind speed result in large variations in output power.
  • A2: Nowadays, they are quite popular because we the active stall control allow it to have interesting properties. The flexible coupling of the blades to the hub also facilitates emergency stopping and start-ups. The big disadvantage is the high price.

Variable Speed Wind Turbines (Types B, C and D):

They became the most numerous since time ago. They are designed to achieve maximum aerodynamic efficiency over a wide range of wind speeds by keeping constant the Tip Speed Ratio (λ) by changing the Rotational Speed (w).

The variable-speed wind turbines require a power electronic system to adjust the generator frequency and voltage to the grid. They are equipped with an induction (asynchronous) or synchronous generator connected to the grid through a power converter. The electrical system is more complicated, but they have clear advantages than the fixed speed wind turbines (see below table).

  • B1: It uses a Wound Rotor Induction Generator (WRIG) with a variable generator rotor resistance (OptiSlip®). Thus, the total rotor resistance is controllable. The generator is directly connected to the grid, the capacitor bank compensates the reactive power and the soft-starter facilities a smoother grid connection. The range of the dynamic speed control depends on the size of the variable rotor resistance (typically, the speed range is 0-10% above synchronous speed).
  • C1: The Doubly Fed Induction Generator (DFIG) corresponds to a WRIG more a Partial Scale Frequency Converted on the rotor circuit. It has a wider range of dynamic speed control compared with the OptiSlip® (typically, the speed range is -40 to +30% of the synchronous speed).
  • D1: The generator is connected to the grid through a full-scale frequency converter. The generator can be excited electrically (Wound Rotor Synchronous Generator (WRSG) or WRIG) or by a permanent magnet (Permanent Magnet Synchronous Generator (PMSG)). Some of them do not have gearbox and a direct driven multi-pole generator with a large diameter.

In the coming posts, we will detail the necessary components in the power electronics for wind turbines for a deeper understanding of the wind turbines generators.

Source: Wind Power in Power Systems by Thomas Ackermann.

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