Study Of Travelling Wave Based MMC-HVDC Transmission Line Protection

Non-renewable energy is gradually drying up,and the development of renewable energy such as wind energy and solar energy is imperative.Modular Multilevel Converter based High Voltage Direct Current(MMC-HVDC)transmission technologies have gained worldwide attentions,due to the capability of consuming large scale renewable energies such as variable wind and solar energies.It is of massive transmission capacity,lower iron losses,high reliability,subject to no commutation failures,and is capable of independently control active and reactive powers.MMC-HVDC transmission is becoming the star technology in the power industry.However,MMC-HVDC transmission also has its inherited vulnerability as a new technology,the rapidly rising fault currents and the difficulty to quench the arc in the HVDC transmission system,when a short-circuit fault occurs.Therefore,it is of great importance to study the characteristics of short-circuit faults and the protection of MMC-HVDC transmission lines.In this paper,key issues regarding fault analysis and protection of MMC-HVDC transmission lines are deeply studied:(1)Thoroughly studies MMC-HVDC transmission system.This includes investigating the topology of the MMC based converter,its sub-modules and the related operational principles,analyzing its operational modes and its modulation means,as well the AC system models,DC transmission line models,and the pseudo bi-polar system.(2)Investigates the pseudo bi-polar two terminal MMC-HVDC system and its fault characteristics,analyzes polar-to-ground and polar-to-polar faults in detail.It provides important information for MMC-HVDC transmission line protection and is of great significance to line protection.(3)Proposes a single-terminal-measurement based protection algorithm based on zero-mode of voltage traveling waves,which eliminates the influence of the electromagnetic coupling between the positive and negative poles in principle.The simulation model is built in PSCAD/EMTDC.Simulation results indicate that the proposed protection is capable of clear faults even with higher transit fault resistance.It operates reliable,fast,requires no communication assistance,and is of excellent selectivity.It is fast to operate,sensitive to fault detection,can serve as main protection of MMC-HVDC transmission lines.(4)Researches onto backup protection of MMC-HVDC transmission lines,and proposes a protection algorithm based on the aerial-mode current travelling wave,utilizes the directional characteristics of sudden changes of the aerial-mode current travelling waves at the rectifier and the inverter sides of the line.The proposed protection method is of excellent selectivity,requires transmits fewer data only.A PSCAD/EMTDC backup protection element is built,and a large number of fault cases are simulated and tested,results show that the proposed backup protection always acts reliably,is of high sensitivity,and excellent selectivity.It can be used as a useful auxiliary for the main protection of MMC-HVDC transmission lines.