Wind energy conversion systems

Wind turbine development has become an attractive research topic in renewable energy resources in recent years. Wind energy conversion systems (WECSs) have been intensively developed to contribute the “green energy” demand up 20% by 2030. Clearly, energy specialists and policy makers have realized the potential importance of wind energy resource in common energy crisis in the world nowadays. For technical aspects, many of opportunities being pursued through advanced control techniques are utilized to achieve higher contribution of wind energy.

This writing briefly summarizes important characteristics of WECSs based on a DFIG and PMSG, which are widely used in research topics as well as commercial markets.

Wind turbine is a device that converts mechanical energy from wind into electrical energy to be used in stand-alone grid or to be connected with the grid. A typical wind turbine consists of necessary components, illustrated in the following figure

Wind Turbine Components

There are two basic configurations in WECSs, depending on the electrical machine type.

1- Fixed-speed WECSs

This wind power system operates at a constant rotor speed regardless of the wind speed variations. The rotor speed is controlled according to the grid frequency. The electrical machine equipped with such wind turbines is squirrel-cage induction generator. Sometimes a permanent-magnetic synchronous generator can be used. Fixed-speed WECSs have advantage of being simple, robust, and reliable with a low-cost generator and easy control. However, such wind power systems also have drawbacks due to limited control when wind speed changes continuously.

Large Scale Wind Turbine

2- Variable-speed WECS

Comparing with fixed-speed wind turbines, variable-speed WECSs based on a doubly-fed induction generator (DFIG) offer a number of merits such as simple control, four-quadrant active and reactive power regulation, and low cost converter. With a DFIG-based wind system, the stator side is directly connected to the grid, whereas the rotor side is connected to a back-to-back voltage source inverter. The stator outputs power into the grid. The rotor is capable of delivering or absorbing power to/from the grid, depending on the rotor speed.

With a PMSG-based wind system, the generator output voltage and frequency are proportional to the rotor speed and the current is proportional to the torque on the shaft. The output is rectified and fed through a buck-boost regulator to an inverter which generates the required fixed amplitude and frequency AC voltage.

Domestic Wind Power System