Research Of Low Voltage Ride Through On Doubly Fed Induction Generator

Variable-speed constant-frequency power generation technology is often adopted as wind power generation technology in China. In variable-speed constant-frequency generator, double-fed wind generator takes up a higher and higher proportion in wind turbine generator systems owing to its high wind energy capture ability and conversion efficiency as well as low loss. As the application of wind power started late in our country, the technological level of wind power in control and protection is low. As a result, there are still many defects in protection technology for rotor side of double-fed generator. By directing at defects of existing technology, this paper studies low voltage ride through technology of double-fed wind generator by starting from increasing switching speed of Crowbar protective circuit at rotor side.Firstly, based on researches about physical model of double-fed generator, mathematical model of double-fed generator set is established under ABC static three-phase coordinate. The mathematical model of multiple inputs, multiple outputs, nonlinearity and strong coupling is transformed into mathematical model of constant coefficient differential equation under synchronous d-q coordinate system through transformation of space coordinates. This has provided convenience for follow-up studies.Secondly, by aiming at characteristics of converter in double-fed wind turbine system, converter of double-fed wind turbine system is controlled by applying stator voltage vector control strategy and double closed loop control strategy of voltage and current. Besides, grid model of double-fed wind turbine system is set up under Matlab/simulink. By the simulation study on dynamic characteristics of double-fed wind turbine system under different voltage drop degrees and fault types, it find that rotor side of double-fed wind generator will produce serious excess current when asymmetric short circuit fault occurs in the system. This would directly damage converter at rotor side and endanger safety of wind turbine generator system.Finally, Crowbar protective circuit model at rotor side is established under Matlab/Simulink environment. Moreover, this paper proposes that IGBT of Crowbar protective circuit at rotor side can be controlled by applying D-type latch. Simulation proves that this control method can start and cut off protective circuit in time, and restrain excess current at rotor side of double-fed generator. Thus low voltage ride through ability of double-fed wind turbine system will be improved.