Doubly-fed-machines in wind-turbine systems: is this application limiting the lifetime of IGBT-frequency-converters?

Recently wind turbines are built at power levels above 1.5 MW. Higher power levels are anticipated for offshore applications. To limit mechanical stresses and power surges in these high-power systems speed-control is necessary. The doubly-fed induction generator (DFIG) system, used like a synchronous generator, is a viable alternative to adjust speed over a wide range while keeping the cost of the power converter minimal. A four-quadrant IGBT AC-to-AC converter is used to feed power bi-directionally to the rotor. However, with respect to reliability the power converter turns out to be the weakest link of the whole system. Contradictory to traction applications very few measurements of the real stresses in wind turbines have been done up to now. This paper describes how the DFlG-system causes very low output frequencies at full load-current. The corresponding IGBT-Chip temperature is simulated using Flomerics "Flotherm" FEM-program and verified at the test-bench. Long-term testings with three different converters have been performed and the results are presented.