Q12Wind and Solar Energy Systems

Generator-Converter Configurations in Wind Turbines

Wind turbine generator-converter configurations are classified into four standard types based on how the generator interfaces with the grid. Type A uses a fixed-speed squirrel-cage induction generator (SCIG) directly connected to the grid with no (or minimal) power electronic converter, the simplest and lowest-cost configuration, but unable to optimize aerodynamic efficiency across varying wind speeds and requiring reactive power compensation (capacitor banks) since induction generators consume reactive power from the grid.

Type B uses a wound-rotor induction generator with variable external rotor resistance (adjusted via a power-electronic-controlled resistor circuit), providing limited variable-speed operation (typically a narrow range around synchronous speed) without requiring a full power converter, an intermediate-cost, intermediate-flexibility configuration. Type C uses a Doubly-Fed Induction Generator (DFIG), examined in detail in relation to another question in this examination, with a partial-scale back-to-back power converter connected only to the rotor circuit (rated for roughly 25-30% of total generator power), providing wide variable-speed operation (typically plus/minus 30% around synchronous speed) at considerably lower converter cost than a full-scale converter would require, making Type C the most widely deployed configuration in modern large wind turbines.

Type D uses a full-scale back-to-back power converter (rated for 100% of generator power) between the generator (either a synchronous generator, often a permanent-magnet synchronous generator, PMSG, or occasionally an induction generator) and the grid, providing complete decoupling between generator and grid frequency, allowing fully flexible variable-speed operation across the entire wind speed range and enabling gearless (direct-drive) turbine designs when paired with a multi-pole PMSG, at the cost of the highest power-electronic converter cost among the four configurations, but offering the greatest operational flexibility and grid-support capability (reactive power control, fault ride-through) of the four types.