Power electronic converters for variable speed wind turbines

The incorporation of a variable speed function into a wind turbine has a variety of benefits, including: improved power yield during low wind speed conditions, reduced mechanical loading on drive train components, reduced audible noise, reduction in tower passing effects. The pulse width modulated (PWM) inverter has now developed to the point where it can economically and technically manage the power levels associated with commercial wind turbines using variable speed drives. The four quadrant transistor based PWM inverter using insulated gate bipolar transistor (IGBTs) is now available at power ratings up to 2 MW and has superseded the line commutated inverters previously employed for developmental variable speed wind turbines. The four quadrant-inverter system discussed in this paper is in effect two voltage source inverters in cascade. Also discussed are doubly fed induction generators and direct drive configurations