Research On Low Voltage Ride Through Technology Of Doubly Fed Induction Generator

As the increasing penetration levels of wind power,the low voltage ride-through(LVRT)capability has become one of the basic grid code requirements for the large scale wind farms.Under grid faults,the LVRT strategy can not only protect the safety of wind turbine itself,but also improve the transient stability and fault recovery ability of grid.For doubly fed induction generator(DFIG),the limited converter rating makes it difficult to ride-through serious faults.In order to ride-through various faults,the rotor current of DFIG need to be suppressed.Usually,the LVRT controllers are designed to accelerate the transient decaying rate of the stator flux,then the rotor current is suppressed,indirectly.This paper establishes the transient mathematical model of doubly-fed induction generator based on the research of doubly-fed wind power generation system,and analyze the transient behaviors of DFIG under grid faults.Then the reason of overcurrent phenomenon in rotor windings is studied.Considering the strong coupling between stator and rotor of DFIG,the transient current model is deduced to directly analyze the dynamic behaviors of the stator and rotor currents.Based on the transient current model,the direct current amplitude control is proposed to suppress the stator and rotor currents,respectively.As the reactive support is required,a reactive current of rotor is directly controlled.In consideration of asymmetrical faults,the transient current model is extended to study the dynamic behaviors of negative sequence component,and the control system is designed to suppress both positive and negative sequence currents.The simulation of a 0.9-MW DFIG in PSCAD well demonstrates the enhanced capability of DFIG to ride-through symmetrical and asymmetrical faults.