Improvement Of Low Voltage Ride Through Of Asynchronized Extra High Voltage Generator Based On Fault Current Limiter

As a new type of generator,asynchronized extra high voltage generators（AEHVGs）combine asynchronized synchronous generators with extra high voltage generators and inherit the advantages of both generators.Round cross-linked polyethylene cable is employed as stator winding of AEHVGs which enables high stator voltage and high capacity.Thanks to the variable speed constant frequency operation,AEHVGs are suitable for pumped storage as well as wind energy conversion system（WECS）.When adopted in wind energy conversion system,AEHVGs are vulnerable to grid voltage dips.According to the national standard GB/T₁9963-2011: Technical Rule for Connecting Wind Farm to Power System,when grid voltage dips,the wind farm should possess fault ride through ability.As a result,it is necessary for AEHVGs based wind energy conversion system to improve its low voltage ride through ability.Low voltage ride through improvement of AEHVGs based WECS is researched in this paper.The mathematical model of AEHVGs based WECS is established.The model includes wind turbine aerodynamic model,AEHVGs and back to back converters.AEHVGs based WECS control strategy is determined based on the mathematic model.AEHVGs transient mathematical model is achieved under certain simplification and assumption.The transient performance of AEHVGs based WECS is analyzed under symmetric voltage dips and asymmetrical voltage dips（including single-phase voltage dips and phase-to-phase voltage dips）.In order to study the sensitivity of AEHVGs based WECS when suffered from grid voltage dips and the impact of voltage dips on wind turbines,the simulations of AEHVGs based WECS suffering symmetric and asymmetrical voltage dips are conducted in MATLAB/Simulink,respectively.A segmented switching bridge-type fault current limiter（SSBFCL）is proposed to improve low voltage ride through of AEHVG based WECS.The simulation of AEHVGs based WECS equipped with SSBFCL is carried out under different grid voltage dips scenarios.SSBFCL is compared with Crowbar circuit.The improvement of low voltage ride through of AEHVGs based on SSBFCL is analyzed under single-phase voltage dips,phase-to-phase voltage dips and different depth symmetric voltage dips,respectively.The results show that AEHVGs based WECS is vulnerable to voltage dips,SSBFCL is able to improve the low voltage ride through of AEVHGs based WECS and SSBFCL surpasses Crowbar circuit in low voltage ride through.