Generators in Wind Turbines.

Basically, any type of three-phase generator could work in a wind turbine but they must fulfill some requirements. We have just classified them according to their generator type and the coming prototypes that nowadays we are studying. This post, as the last one, is a summary from the book Wind Power in Power Systems (Chapter 4) by Thomas Ackermann. However, for any further information, this link can be interesting.

Asynchronous (Induction) Generator.

It is the most numerous in wind turbines. Advantages: Robustness, mechanical simplicity, low price. Disadvantages: It has to receive an exciting current from another source and consumes reactive power (so, reactive power is supplied by the grid or by a power electronic system).

Squirrel Cage Induction Generator (SCIG).

This generator is used for constant-speed wind turbines (Type A) and in full variable-speed wind turbines (Type D). The generator is directly coupled to the grid and it uses a gearbox to connect it to the wind turbine. SCIG is equipped with a soft-starter mechanism and with an installation for reactive power compensation. Fluctuations in wind power are transmitted directly to the grid and it must be considered for weak grids.

Would Rotor Induction Generator (WRIG).

The electrical characteristics of the rotor can be controlled from the outside. They are more expensive and less robust than SCIG generators. Configurations:

  • OptiSlip Induction Generator (OSIG). It used for Type B concept. They are WRIGs with a variable external rotor resistance attached to the rotor windings. The generator has a variable slip (narrow range) and chooses the optimum slip, resulting in smaller fluctuation in the drive train torque and in the power output. Advantages: Simple circuit topology, no need for slip rings, an improved operating speed range, reduced mechanical loads and less power fluctuations caused by wind gusts. Disadvantages: It requires a reactive power. The speed range is limited to 0-10%. Poor control of active and reactive power.
  • Double-Fed Induction Generator (DFIG). The term Double-Fed refers to the fact the voltage on the stator is applied from the grid, and the voltage on the rotor is induced by the power converter [Bidirectional back-to-back IGBT (Insulated Gate Bipolar Transistor)]. This configuration allows the generator a variable-speed operation over a large, but restricted, range. The power converter consists of two converters, the rotor-side converter which controls the active and reactive power by controlling the rotor current components, and the grid-side converter which controls the DC-link voltage and ensures a converter operation at unity power factor. Advantages: It controls and generates reactive power that can be delivered to the stator by the grid-side converter. Disadvantages: It usually operates at unity power factor and in not involved the reactive power exchange between the turbine and the grid.

Synchronous Generator:

It is much expensive and mechanically more complicated than an induction (asynchronous) generator of a similar size. However, and it is its big advantage, they do not need a reactive magnetizing current because it can be created by using permanent magnets or with a conventional field winding. It is connected to the grid through a power electronic converter. The converter has two primary goals: 1) To act as an energy buffer for the power fluctuations. 2) To control the magnetization and to avoid problems by remaining synchronous with the grid frequency. We have two types of synchronous generators.

Wound Rotor Generator (WRSG).

It does not need any further reactive power compensation system. The speed of the synchronous generator is determined by the frequency of the rotating field and by the number of pole pairs of the rotor.

Permanent Magnet Generator (PMSG).

It is very interesting because it has self-excitation, which allows an operation at a high power factor and a high efficiency. However, the magnetic materials are sensible to temperature and they must be controlled by a cooling system. There are three topologies of PM machines; the radial, the axial, and the transversal flux machine.

Other types of Potential Interest

High –Voltage Generator (HVG).

The idea is increasing the voltage of the generator to reduce the current and thereby to reduce the heat losses. If the voltage of the machine matches the grid voltage, a grid connection is possible without a transformer.

Switch Reluctance Generator (SRG).

The SRG is a synchronous generator with a double salient construction, with salient poles on both the stator and the rotor. Like the PMSG, it has the possibility to eliminate the gearbox. However, the SRG is considered inferior to the PMSG machine because of its lower power density, lower efficiency and lower power factor than asynchronous generators.

Transverse Flux Generator (TFG).

It is fairly new, but seems to be interesting. The operation is equal to a synchronous machine, and it will function similar to any other PM (Permanent Magnet) machines. It is suitable for gearless applications but has a relatively large leakage inductance.

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