**4. Wind Turbine Generators**

One of limiting factors in wind turbines lies in their generator technology. There is no con‐ sensus among academics and industry on the best wind turbine generator technology. Tra‐ ditionally, there are three main types of wind turbine generators (WTGs) which can be considered for the various wind turbine systems, these being direct current (DC), alternating current (AC) synchronous and AC asynchronous generators. In principle, each can be run at fixed or variable speed. Due to the fluctuating nature of wind power, it is advantageous to operate the WTG at variable speed which reduces the physical stress on the turbine blades and drive train, and which improves system aerodynamic efficiency and torque transient behaviors.

#### *(a) DC Generator Technologies*

In conventional DC machines, the field is on the stator and the armature is on the rotor. The stator comprises a number of poles which are excited either by permanent magnets or by DC field windings. If the machine is electrically excited, it tends to follow the shunt wound DC generator concept.

**Figure 5.** System level drive train technologies [24].

An example of the DC wind generator system is illustrated in Fig. 6. It consists of a wind turbine, a DC generator, an insulated gate bipolar transistor (IGBT) inverter, a controller, a transformer and a power grid. For shunt wound DC generators, the field current (and thus magnetic field) increases with operational speed whilst the actual speed of the wind turbine is determined by the balance between the WT drive torque and the load torque. The rotor includes conductors wound on an armature which are connected to a split-slip ring com‐ mentator. Electrical power is extracted through brushes connecting the commentator which is used to rectify the generated AC power into DC output. Clearly, they require regular maintenance and are relatively costly due to the use of commutators and brushes.

In general, these DC WTGs are unusual in wind turbine applications except in low power demand situations [47; 23; 33; 54] where the load is physically close to the wind turbine, in heating applications or in battery charging.

**Figure 6.** Schematic of a DC generator system [33].

operate the WTG at variable speed which reduces the physical stress on the turbine blades and drive train, and which improves system aerodynamic efficiency and torque transient

In conventional DC machines, the field is on the stator and the armature is on the rotor. The stator comprises a number of poles which are excited either by permanent magnets or by DC field windings. If the machine is electrically excited, it tends to follow the shunt wound

behaviors.

182 Advances in Wind Power

*(a) DC Generator Technologies*

**Figure 5.** System level drive train technologies [24].

DC generator concept.
