Research On Fault Analysis And Fault Current-limiting Scheme For VSC-based Multi-terminal DC Grid

With the shortage of traditional energy sources and the intensification of environmental degradation,the development and utilization of renewable energy have received full attention and rapid development.However,the absorption capacity of the traditional power system is limited,and most of the renewable energy is not fully utilized.Voltage Source Converter based High Voltage Direct Current(VSC-HVDC)becomes the mainstream research direction,because it has independent power control and no commutation failure.Thereinto,the HVDC system based on voltage source converter(VSC)is used widely in engineering applications.Compared to the point-to-point direct current transmission system,the multi-terminal direct current(MTDC)systems with multiple-power supply and multi-drop power reception have more flexibility and application prospects.At present,there are at least 24 HVDC projects in operation around the world,most of which are point-to-point two-terminal structures.Multi-terminal flexible DC systems and DC power grids are in the initial stages of development.However,the development of the HVDC system is constrained by the lack of the operational experience,the lack of appropriate fault analysis techniques,and the immaturity of appropriate fault isolation devices.This paper studies the fault analysis and current limiting technology of VSC flexible HVDC transmission system.The work is as follows:Firstly,based on the travelling wave analysis and Laplace transformation,this paper proposes a new expression for the approximation of current contributed by dc capacitor in VSC-HVDC networks during the pole-to-pole fault.A transformation matrix is used to eliminate the electromagnetic couple between transmission lines.To improve the accuracy of the theoretical analysis,the distributed,frequency-dependent DC line parameters and the skin effect are all considered.Compared with the existing fault calculation standard,the proposed approximation is suitable for small low-voltage(LV)and MV auxiliary networks with lumped frequency-independent series impedances,and large high-voltage(HV)networks with distributed frequency-dependent parameters as well.Secondly,to restraint the fault current,this paper proposes a novel fault current-limiting scheme for symmetrical bipolar HVDC system based on Modular Multi-level Converter(MMC),which is made up by a controllable thyristor and a current-limiting resistance.The two components are in parallel with each other and in series with the MMC converter and the neutral line.Under normal operation,the thyristor is breakover,the current flows through the thyristor,the resistance in the current limiting module does not work.When a fault occurs,the controllable thyristor shuts off immediately,and the current-limiting resistance is put into the circuit.Thus,the damping characteristic of the sub-module capacitors discharge is changed,and the fault current is effectively limited.Furthermore,due to the over-current is avoided,the IGBTs in MMC converters will not be blocked.Compared with the conventional current-limiting inductor,the novel scheme reduces the number of power outages,improves the reliability of power supply,and facilitates system recovery after the fault.What's more,this scheme is more economical,which is beneficial to large-scale popularization and application in the project.In addition,the current-limiting process of the current limiting scheme and the selection method of the current-limiting resistance are deduced.Finally,a ±200 kV VSC-HVDC system model is built by PSCAD,and lots of simulations with different fault initial conditions have been performed.Simulation results verify that the proposed approximation calculation of fault current is valid and feasible.A symmetric bipolar two-terminal MMC-HVDC system model and a symmetric bipolar four-terminal MMC-HVDC system model,which are based on the Zhangbei four-terminal HVDC grid,are constructed,and simulation results show the feasibility of the proposed fault current-limiting scheme.What's more,by means of simulations in different current limiting-schemes,fault resistances,and fault types,the reliability of the scheme are proved.