A multilevel wind power conversion system with an open winding squirrel cage induction generator

The growing use of wind energy conversion systems around the world is pushing the individual power of wind turbines to higher values. Especially in offshore farms, where installation and maintenance have higher costs, high power wind turbines and their reliability become critical issues to make investments economically viable. In this context, multilevel converters, widely used in high power drive systems, have been gaining attention also for variable frequency generation systems. Among the several topologies developed for multilevel converters, there is a structure that employs 2 conventional two-level converters connected to an open winding machine, resulting in a three-level behavior. This structure will be studied in this paper, applied to wind power conversion with a squirrel cage induction generator. The structure is interesting for its high number of redundant switching states, compared to the conventional system, and for the possibility to sustain operation if one of the converters fails. The squirrel cage induction machine is a cheap and robust solution, which is very important when dealing with high power and offshore exploration. The resulting switching states and an initial modulator design will be presented along with the machine modeling, leading to a dynamic simulation of the system.