State control of DFIG in converter-fixed reference frame for wind-energy plants

Doubly-fed induction generators (DFIG) still play an important role in wind-energy applications. One of the major challenges concerning their operation is to meet not only steady-state, but also dynamic requirements. State-of-the-art DFIGs are often controlled applying dq-control structures to guarantee steady-state power-quality requirements. The need of an exactly known transformation angle causes problems, especially under grid faults. Flux-based approaches offer more potential under grid faults, but no current control which is essential to fulfil certain requirements under grid conditions. A new control approach based on inverter-fixed coordinates is presented, allowing fast reaction to dynamics and satisfying also the steady-state power-quality requirements. The advantages of both, current control and inverter-fixed reference frame, are combined in this control concept. Even at grid faults, the new approach provides highly dynamic behaviour which is under analysis in this paper.