Fault Characteristics Analysis And Protection Principle Research Of Flexible HVDC Grid

The flexible HVDC grid is a new type of power transmission network based on voltage source converters,DC circuit breakers,and DC buses.It has advanced advantages such as flexible power flow control,multi-terminal source and load interaction,easy accessing to large-scale clean energy,and high power supply reliability.Therefore,the flexible HVDC grid has become an important development direction of HVDC transmission technology.Relay protection is an important basis for ensuring the safety operation of power grid.The structure,operation control and fault characteristics of the flexible HVDC grid are different from traditional AC power grids and HVDC power transmission systems,which brings about many new problems and challenges for analysis of fault characteristics and relay protection research.Under the support of National Science and Technology Major Project and other projects,according to the technical characteristics of the flexible HVDC grid and the application requirements of relay protection,the systematic theoretical research and simulation analysis are conducted focusing on the modeling method,control strategy,short-circuit current characteristics analysis and relay protection principle of the flexible HVDC grid.This study has important theoretical and practical significance for ensuring the safe and reliable operation of the flexible HVDC grid.The mathematics and simulation modeling of flexible HVDC grid is an important foundation for the study of control strategy,fault characteristic analysis and protection principle.The modular multilevel converter(MMC)is a difficult point in flexible HVDC grid modeling.Starting from the energy conservation of MMC submodule,the mathematical model of submodule is established by averaging the dynamic characteristics of N submodules in the same arm,which can greatly reduce the mathematical model dimension.What's more,according to the topology structure of the converter,the arm mathematical models are integrated to form a DC side mathematical model,which realizes the mathematical expression of the DC side dynamic characteristics.The simulation results verify the correctness of the established mathematical model,which lays a theoretical foundation for the following control strategies research and fault characteristics analysis of flexible HVDC grid.Aiming at the prominent problems of complicated topology structure of MMC,large number of electrical nodes and accurate simulation time-consuming,the MMC equivalent simulation model based on Thevenin's equivalent circuit is established.Compared with the transient characteristics of the detailed MMC simulation model,the complete simulation results show that the equivalent MMC simulation model can maintain high transient simulation accuracy while significantly improving simulation speed.Therefore,the equivalent MMC simulation model can better meet the transient simulation requirement of the flexible HVDC grid.The control strategy and control parameters of flexible HVDC grid have a direct and important influence on the grid operation and fault characteristics.Based on the mathematical model of the converter control system,transfer functions of the AC side current,the DC side circulating current,the DC voltage and the grid side power are deduced.In accordance with the requirements of rapidity,a PI parameter setting method for the current inner loop and circulating current suppressing was presented.According to the requirements of followability and immunity,a control parameter tuning method for the power outer loop and the outer loop of DC voltage was proposed,which provides method guidance for the reasonable setting of converter control parameters.Aiming at the problems of complex structure,number of controllers,and excessive control parameters in AC fault control strategies based on bi-sequence PI controllers,a new AC fault control strategy based on multi-frequency quasi-proportional resonator controller is proposed.Under the premise of ensuring the control effect to AC side fault,the proposed AC fault control strategy can effectively simplify control structure,reduce the number of controllers and the control parameters to adjust,which is convenient for engineering applications.The fault characteristics of the flexible HVDC grid are complex,and its theoretical analysis is difficult.At present,simulation and qualitative analysis are the main analysis method,which is difficult to meet the requirements of relay protection principle research and setting calculation.Based on the background of AC grid faults and DC system faults,this thesis carries out the theoretical research work on the short circuit current calculation method of the flexible HVDC grid.For AC grid faults,based on the established mathematical model of the converter,a mathematical expression of the short circuit current generated by the converter was deduced taking into account the effects of converter fault control strategy and control system delay.Moreover,a short circuit current equivalent model for AC grid with flexible HVDC system is established.As for DC system faults,according to the analysis method of the short-circuit current for two terminal flexible HVDC system,the RLC equivalent circuit for the bipolar short circuit of the flexible HVDC grid with multi-terminal sources is established,and the solving equation of the bipolar short-circuit fault current is put forward.For above fault characteristic research,the simulation results verify the correctness of the proposed short-circuit current calculation method.After the fault of the flexible HVDC grid transmission line,the short-circuit current has the characteristics of large amplitude and fast rising speed.In addition,in order to improve the reliability of power supply,fault DC lines need to be correctly detected and effectively isolated.As a result,there are high requirements on the quickness and selectivity of DC line protection.Based on the structural characteristic of the transmission line in the flexible HVDC grid,the initial traveling wave characteristic of the DC line for different fault types is analyzed by utilizing the Peterson principle,including bipolar short circuit and unipolar ground short circuit.Based on above traveling wave characteristic analysis of the DC line,a traveling wave protection criterion for DC line based on voltage reverse traveling wave is proposed,drawing on the application experience of traveling wave protection in conventional HVDC transmission lines.The results of digital simulation show that the proposed protection criteria has the characteristic of high operation speed and good selectivity,which can better meet the application requirements of transmission line protection in the flexible HVDC grid.At the end of this thesis,the whole research work is summarized and the future study is looked forward.