Synchronous Generators

From Wind wiki

The synchronous generator is much more expensive and mechanically more complicated than an induction generator of a similar size. However, it has one clear advantage compared with the induction generator, namely, that it does not need a reactive magnetizing current.

The magnetic field in the synchronous generator can be created by using permanent magnets or with a conventional field winding. If the synchronous generator has a suitable number of poles, (a multipole WRSG or a multi-pole PMSG) it can be used for direct drive applications without any gearbox.

As a synchronous machine, it is probably most suited for full power control as 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 caused by an inherently gusting wind energy and for the transients coming from the net side, and (2) to control the magnetization and to avoid problems by remaining synchronous with the gird frequency. Applying such a generator allows a variabl-speed operation of wind turbines.

Two classical types of synchronous generators have been used in the wind turbine industry: (1) the wound rotor synchronous generator (WRSG) and (2) the permanent magnet synchronous generator (PMSG).

Wound rotor synchronous generator

The stator winding of WRSGs are connected directly to the grid and hence the rotational speed is strictly fixed by the frequency of the supply grid. The rotor winding is excited with direct current using slip rings and brushes or with a brushless exciter with a rotating rectifier. Unlike the induction generator, the synchronous generator does not need any further reactive power compensation system. The rotor winding, through which direct current flows, generate the exciter field, which rotates with synchronous speed. 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.

Some manufacturers use a low speed WRSG and has the advantage that it does not require a gearbox. Although, a price has to be paid in that the generator is heavier and also a full-scale power converter that has to handle the full power of the system.

Permanent Magnet synchronous generator

In the permanent magnet machine, the efficiency is higher than in the induction machine, as the excitation is provided without any energy supply. However, the materials used for producing permanent magnets are expensive, and they are difficult to work with during manufacturing. Additionally, the use of PM excitation requires the use of a full-scale power converter in order to adjust the voltage and frequency of generation to the voltage and the frequency of transmission, respectively. This is an added expense. However, the benefit is that power can be generated at any speed so as to fit the current conditions. The stator of PMSGs is wound, and the rotor is provided with a permanent magnet pole system and may have salient poles or may be cylindrical. Salient poles are more common in slow-speed machines and may be the most useful version for an application for wind generators. Typical low-speed synchronous machines are of the salient-pole type and the type with many poles.

There are different topologies of PM machines presented in the literature. The most common types are the radial flux machine and the transversal flux machine.