Analysis of the electromagnetic transient time in DFIG during grid faults

During grid faults, grid codes required wind turbine (WT) to provide dynamic reactive support within tens of ms as well as maintain grid connection continuously, namely low voltage ride through (LVRT). But owing to the limited DC bus voltage, the rotor side convertor in doubly fed induction generator (DFIG) cannot actually output sufficient voltage to provide reactive support until the rotor-side electromotive force (EMF) decays to a certain extent. This paper tries to assess the electromagnetic transient time of DFIG and its influential factors, which is crucial to the feasibility of rapid reactive support under serve grid faults. For this issue, the transient time constant under various operating conditions was calculated, including crowbar protection and typical control strategies. Simulation models were built in Matlab and results verify the accuracy of relevant conclusions. Among typical LVRT schemes, demagnetizing control without current limitation has the fastest transient, and larger crowbar resistor would slow down its transient. Considering the ranges of typical parameters in commercial MW-level DFIGs, the transient was assessed to last 100ms at least. That means under severe grid faults, it is impossible for DFIG to provide reactive support within 20ms as some grid code required, unless some measures were taken in the stator side rather than in the rotor side.