Torque fluctuation and critical clearing time in wind power generation by induction machines

Squirrel cage induction machines represent a common solution widely adopted in wind generation. The induction machine is simple, reliable, cheap and requires very little maintenance. Some problems are, however, connected with the use of induction machines in wind power generation systems. In particular two effects have to be taken into account: the torque fluctuations at the generator shaft and the critical clearing time (CCT). Both problems represent a serious limitation for weak networks in which the first effect can cause flicker effect while the CCT is very short. In the paper a full modelling of a wind energy plant constituted by n machines is presented. Using the set-up model the behaviour of the plant for a different number of connected machines is analysed. In particular, the system stability connected with voltage sags and network faults is studied.