Nonlinear direct power control of DFIG wind turbines for VPP integration

Doubly-fed induction generators (DFIG) constitute some of the larger units met in distributed generation (DG) with significant power control capabilities. These capabilities clearly become more important in modern grids where the aggregation of the individual DG units into larger structures, known as virtual power plant (VPP) concept, is utilized. A special role of DFIGs in the VPP frame is to support the power system by offering ancillary services. To implement this task, a new modelling approach is presented for DFIGs and, through the use of grid voltage orientation, suitable controllers are proposed. In these schemes, ancillary services are implemented through the active and reactive power modulation. To this end, the total active power extracted from the DFIG is controlled by the corresponding stator active power while a droop voltage controller drives the reactive power to the appropriate levels. Simulations are carried out in order to illustrate the effectiveness of the proposed control system.