Research Of Directional Elements Of Midium-voltage AC System Connected With VSC-based Interconnection Devices

Open-loop operation mode is usually adopted in conventional urban medium-voltage distribution networks in China for the limitation of short-circuit capacity and electromagnetic looped network.With the expansion of the distribution network,the defects of open-loop operation mode are becoming more prominent in terms of reliability.At the same time,the demand of the reliability of power supply is enhancing constantly,so it is necessary to improve the structure of the network.The VSC-based DC technology has broad application prospects in the interconnection of AC networks,integration of large-scale renewable sources and so on.It is currently an international research hotspot.The application of back-to-back VSC-based interconnection devices based on modular multilevel converters can realize loop operation of medium-voltage distribution network.This makes the distribution network changes from a single-terminal power supply network to a multi-terminal network,and brings exclusive fault characteristics to AC system.To ensure the reliability of protection,it is necessary to install directional elements.The output characteristics of a back-to-back VSC-based interconnection device are different from conventional AC sources.Therefore,the traditional power directional component is not applicable to the AC networks connected with back-to-back VSC-based interconnection devices.Since fault characteristics are related to the control schemes of interconnectors,a fault ride-through strategy capable of eliminating negative-sequence currents under unbalanced conditions is proposed first.Then,the phase relationships among sequence currents are derived using a symmetrical component method.Theoretical analysis shows the output currents of interconnectors are controllable after faults,which may cause malfunctions of traditional power directional components.Furthermore,two novel directional criteria based on the phase relationships among sequence currents are proposed for midium-voltage AC system connected with VSC-based interconnection devices.The validity of theoretical analysis and the protection principle is verified by PSCAD simulations.