ABSTRACT
To date, type 3 system, known as the doubly fed induction generator (DFIG), is the most common type of wind turbines. Although more expensive than types 1 and 2, it offers excellent operational and control features that make their integration with power grids easy and effective. Type 3 generator is a slip-ring machine similar to that of type 2. The generator is fed from the stator as well as the rotor (thus, doubly fed), as shown in the descriptive representation in Figure 10.1. The convention for the power flow of the DFIG is depicted in the figure. The mechanical power from the turbine enters the rotor and is converted into electrical (developed power). The rotor injection circuit also inserts power into the rotor. The stator power is the developed power plus the rotor power minus the losses. When the rotor power is negative, the flow is reversed: power is extracted from the rotor and delivered to the grid.
