Research On Line Protection And Fault Location Of Three-terminal Hybrid DC Transmission System

China’s land is vast,but the energy distribution is uneven.The central and western regions are energy reserve centers,while the eastern coastal regions are load centers.It can be seen that load centers and energy resources are inversely distributed,and the global demand for new energy is growing in recent years.The hybrid DC transmission system combines the technical advantages of traditional DC transmission and flexible DC transmission and ovecomes their technical shortcomings;thus it has solved the problems such as long-distance,large-capacity transmission and new energy dispatch and comsumption.The DC transmission line is long,and it is easy to cause the flashover from the line to the ground under lightning strikes,pollution,etc,which makes it difficult to extinguish the arc or destroys the system stability.Research on both the safe and reliable line protection,and the accurate and fast line fault location has very important practical significance for maintaining the safe operation of the system.At present,there are few studies on the protection and fault location of hybrid DC transmission lines.Considering the topology particularity of the hybrid DC transmission systems,the existing single-type DC transmission line protection and fault location methods are not suitable for hybrid DC transmission systems.Therefore,it is of great practical significance to study the protection and fault location methods applicable to hybrid DC transmission lines.In this thesis,a hybrid three terminal DC transmission true bipolar system model is built in PSCAD simulation software,and the corresponding line protection and fault location methods suitable for the hybrid DC transmission are explored.Firstly,this thesis introduces the topology of the hybrid DC transmission,and analyzes the structure,the individul and comprehensive operation control principle of the LCC converter station and the MMC converter station based on the actual model and parameters of the hybrid three terminal DC transmission system.On this basis,typical fault characteristics of each converter station and the impact of different fault conditions on the transient characteristics of the line fault are studied,which provides theoretical basis for the design of line protection and fault location methods.Secondly,this thesis proposes to apply the inductive boundary characteristics of the single-end traveling wave as the protection method for hybrid DC transmission lines,and then analyzes and verifies its effectiveness.According to the obvious boundary characteristics of the fault voltage traveling wave when it is transmitted to the inductive boundary of the line,this method analyzes the difference of the transient energy value of the voltage traveling wave when the fault occurs inside and outside the area,as the basis for determining the setting value of the line fault protection;and then it constructs the protection criterion to realize the fast and reliable line protection.The effectiveness of this method is verified through the simulation.When the fault is far away from the detection end in the area or close to the detection end outside the area,the fault region can be accurately determined under the interference of the transition resistance,fault type,fault distance and other factors.At the same time,it satisfies the requirements of the quickness and selectivity for this system.Finally,in order to avoid the complex catadioptric characteristics of traveling wave in the process of hybrid multi-terminal DC transmission line and the influence of long-distance transmission environment on line parameters,this thesis studies the fault location method of the Artifatial Neural Network(ANN)based on the line natural frequency.According to the natural frequency characteristics of traveling waves in different line faults,the layered distributed BP neural network optimized by PSO algorithm is used to perform line fault location.Through simulation experiments and the comparison with the single-end traveling wave ranging method,it is verified that the method has good adaptability under the condition of high resistance grounding,different fault types and positions,and the ranging results are accurate and reliable.