Application of electrical variable transmission in wind power generation system

In the widely applied non-direct driven wind power generation system, a gearbox is connected between the wind turbine and the high speed doubly-fed induction generator (DFIG) so that the system could be smaller and supply power with constant voltage and constant frequency as the wind turbine speed varies. However, the multi-level mechanical gearbox is rather vulnerable and poses many difficulties to maintain. Electrical variable transmission (EVT), an electrical continuous variable gearbox, is a very competitive alternative for vulnerable constant speed-ratio mechanical gearbox. In this paper, the application of EVT in wind power generation system with its unique power conversion characteristics is presented. EVT machine structures and the main principles of the EVT-based wind power generation system are introduced. Unique power conversion characteristics of the EVT-based system are investigated and compared among the conventional DEIG system and direct-driven permanent magnet generator (PMG) system. A controller without maximum power point tracking (MPPT) control to the outer rotor is implemented in Matlab/Simulink and speed sensorless control of the inner rotor is tested. The operational principle and the operation modes are evaluated by computer simulation and system experiments.