Research On Outlet Line Protection Scheme For Flexible Substation

With the development of renewable energy and load,more and more distributed generation and DC load are connected to the distribution network.Flexible DC distribution grids have become one of the important development directions of the distribution power systems and DC technology due to several advantages over AC grids.In flexible DC distribution grids,the flexible substations are used to achieve the mutual conversion of electric energy as energy transfer node,grid control node and load control node.However,the DC protection is one of the challenges to be addressed associated with the VSC-based DC distribution grids mainly because the characteristics of the DC and AC fault currents are quite different.The difficulty lies in the rapid and selective identification of failure.For dc power distribution system based on converters with dc fault isolation capability,this thesis analyzes the fault characteristics of three types of typical faults of DC line.On this basis,a flexible substation AC and DC outlet protection scheme,AC and DC protection setting principle and DC protection setting calculation method are proposed.This will provide an effective theoretical basis and technical support for the planning,construction and operation of flexible substations and will promote the large-scale development and efficient use of renewable energy as well as the friendly acceptance of diversified AC and DC loadsIn this thesis,to solve the fast and selective fault identification problem for DC line bipolar short circuit fault,the power electronic converters are locally linearized into a constant impedance at high frequency band.When a fault occurs in DC systems,the voltage at the point of failure will mutate,resulting in attenuation of high frequency signals.The characteristic of transient high frequency impedances is analyzed when faults occur in different position.On the basis of this,this thesis proposes a directional pilot protection based on the comparison of the amplitude of transient high frequency impedance,which can quickly and reliably remove the faulty line and then lay the foundation for recovery of the flexible DC distribution network.Finally,the four-terminal open-loop flexible DC distribution network model is built by using PSCAD/EMTDC simulation platform to verify the feasibility and effectiveness of the proposed protection.