Study On Control Technology Of Low Voltage Ride-through For Grid-connected Photovoltaic Power System

With the decrease of traditional fossil energy reserves and increase of environmental pollution problems, the grid-connected PV generation industry has developed rapidly in the world. The large-scale grid-connected PV generation is the main trend of new energy development. With the large capacity of grid-connected PV systems putting into operation widely, some problems follow. The low voltage ride-through is regarded as one of the biggest challenges of the grid-connected PV equipment design and manufacturing control technology, and is directly related to the large-scale application of grid-connected PV generation.This thesis mainly focuses on control strategies of grid-connected PV inverter under grid symmetric and asymmetric fault conditions. Firstly, the mathematical model of PV inverter in three-phase ABC static coordinate system is obtained, the PARK transformation is used to calculate the mathematical model of two phase dq synchronous rotating coordinate system. Secondly, the LVRT control strategy based on voltage oriented vector is proposed, combining with the DC unloading circuits under grid symmetric fault. The strategy ensures the stable operation of grid-connected PV generation system. Finally, the LVRT control strategy based on predictive current is presented in order to research the operation characteristics of PV inverter under grid asymmetric fault. Meanwhile, a dual second order generalized integrator control method is proposed to separate positive and negative sequence components of grid voltage and current, this method can ensure grid-connected PV generation system normal operation and realize LVRT.In this thesis, the3MW grid-connected PV generation system model is set in the simulation software PSCAD/EMTDC to verify LVRT control strategy. The simulation results can prove the effectiveness of proposed control strategy. The voltage fluctuation of DC side is restrained and PV generation system can provide reactive power to support voltage recovery according to the depth of grid voltage sags during grid faults, so the system continues to operate stably.