Design Study of Doubly-Fed Induction Generators for a 2MW Wind Turbine

A design study for a 2 MW commercial wind turbine is presented to illustrate two connection methods for a standard doubly-fed induction machine which can extend the low speed range down to 80% slip without an increase in the rating of the power electronic converter. This far exceeds the normal 30% lower limit. The low speed connection is known as induction generator mode and the machine is operated with a short circuited stator winding with all power flow being through the rotor circuit. A two loop cascaded PI control scheme has been designed and tuned for each mode. The purpose of this paper is to present simulation results which illustrate the dynamic performance of the controller for both doubly-fed induction generator connection methods for a 2 MW wind turbine. A simple analysis of the rotor voltage for the doubly-fed connection method is included as this demonstrates the dominant components that need to be considered when designing such advanced control strategies.