Grounding Fault Analysis And The Control And Protection Strategies Of Hybrid Dual-Infeed HVDC System

With its economy,stability,fast controllability and power grid asynchronous interconnection and many other advantages,Line Commutated Converter based on High Voltage Direct Current(LCC-HVDC)plays an important role in the"West-to-East Power Transmission Strategy" in China.With the progress of high-power full-control power electronic devices and the development of large-scale renewable energy,Voltage Source Converter based on High Voltage Direct Current(VSC-HVDC)has also attained rapidly development.During recent years,hybrid double-fed DC transmission system consisting LCC-HVDC and VSC-HVDC has gained more and more attention and research.The grounding fault,as one of the most common faults of power system,has always been the focus of research on fault protection strategy of HVDC system.So far,there is no systematic study on the ground fault characteristics and the protection strategy for hybrid double-fed DC system.In this paper,the mathematical model of LCC-HVDC and VSC-HVDC in hybrid double-fed DC system is established,and the corresponding control strategy is designed.The simulation results verify the effectiveness of the control strategy.On this basis,the electro-magnetic transient model of hybrid double feeding system is established.Then,the rectifier side of LCC-HVDC subsystem,the rectifier side of VSC-HVDC subsystem,and the inverter side of hybrid double-fed DC system are selected as the three main research objectives,and the main ground fault in the region is simulated and analyzed,and the fault mechanism is also studied.For the VSC-HVDC subsystem,a positive sequence and negative sequence passive controller based on VSC Euler-Lagrangian mathematical model is designed for the three-phase unbalance of commutation voltage caused by single-phase ground fault of AC system.The simulation experiment is further studied.The simulation results show that the designed control strategy has good dynamic and steady state control characteristics.On this basis,the improved active power control is designed to ensure the active power stable output of the VSC-HVDC subsystem in the fault condition.The simulation results show that the method effectively restrains the enlargement of fault current.