Dynamic Analysis of a Gearless Wind Turbine Coupled to a DFIG

Abstract

Most modern wind turbines use power electronic converters to maintain voltage phase, frequency, and magnitude at the grid-dictated values. However, such converters have often been reported to have high failure rates and cost. Further, failure of conventional wind turbine gearboxes adds to the overall cost and downtime. One remedy to limit the size of these converters is to implement a continuously variable transmission (CVT) which has fewer moving parts, e.g. a belt/chain CVT. Further, a CVT may completely eliminate the conventional gearbox architecture used in current wind turbine drivetrains. However, several dynamical issues related to CVTs prevent their widespread use. Current dynamical understanding of the most common CVTs (i.e. a belt/chain CVT) is limited by formulations of shift speed, belt-pulley friction torques, as well as belt-pulley slip. This paper aims to redress the shift speed formulation which has been widely based on quasi-static equilibrium analyses and, surprisingly, on slip definitions that provide minimal detail on the inertial interactions between the belt and the pulleys. Consequently, the paper proposes a new definition of slip to capture such interactions and uses it to develop more accurate representations of belt-pulley friction torques. Using MATLAB/Simulink, the CVT model is incorporated into a wind turbine model with a doubly-fed induction generator (DFIG). Further, the entire turbine/rotor-CVT-generator model is coupled to the grid through the conventional grid- and rotor-side converters (i.e. GSC and RSC respectively). The results for the overall integrated powertrain are presented and discussed in detail with the CVT operated in open-loop and the DFIG in closed-loop. The intent is to study how control inputs of a CVT affect power flow through the entire drivetrain to meet the objectives of a) maximal power extraction from the wind and b) tracking the grid demands without degrading the CVT performance (with regard to slip, torque capacity, etc.). Further, the results presented herein examine the ability of a CVT to provide speed control (which traditionally is achieved via RSC), thereby, offering the potential to downsize RSC and thus the overall converter.