Grid side converter control of dfig and mitigation of voltage sag

Abstract

Now days wind power energy is playing a major role in power industry. With the increase in application of wind power variety of new topologies are coming into picture. Among the different form of variable speed fixed frequency topologies DFIG is most popular form due to its efficiency and ability to allow wide range of speed variation at reduced converter size. Doubly Fed Induction Generator (DFIG) is basically a wound rotor induction generator which is used to fed power from both stator and rotor circuit. Stator feeds power directly to grid which is unidirectional. Rotor circuit is connected to a bidirectional ac/dc/ac converter having a common dc link bus. In rotor, power flow is bidirectional i.e. depending on the mode of operation the power flow is either from rotor side to grid or it may be from grid side to rotor. The rotor side converter also fed reactive power to DFIG so as it can run at a near unity power factor. The function of the grid voltage converter is to maintain the DC link voltage constant, which ultimately fed a constant amplitude ac voltage to rotor side for maintaining the flux constant. But when grid voltage variation occurs, the dc link voltage also varies, ultimately rotor input voltage varies. This causes abnormal input of reactive power to rotor circuit. So to maintain the reactive power demand of machine, it draw reactive power from grid, which may lead to a condition of voltage fluctuation at PCC. One solution to this problem may be compensation of grid voltage variation before grid converter circuit. This compensation of voltage sag is done by a custom power device, known as Dynamic Voltage Restorer (DVR). The current Topic discusses about the application of DVR to DFIG to compensate voltage sag of grid so that the voltage of DC link will remain constant.