Study On Fault Characteristic And Adaptability Of AC Relay Protection And Fault Location For AC-DC Cross Short Fault

HVDC(High Voltage Direct Current)system has a huge advantage in long-distance,large-capacity power transmission,and it has been widely used in China.Due to the limitation of the design and construction of transmission corridor,short-circuit faults crossing among the AC lines and the DC lines will inevitably occur.Besides,severe weather in recent years raises the probability of the faults.After the fault happens,there is direct electrical connection and interaction between the AC line and the DC line,which causes the fault characteristics distinguish from the traditional AC line fault.It will also have an effect on the relay protection and fault location methods of AC line systems.This paper mainly conducts study on the AC-DC cross short fault.Firstly,based on the±500kV Tian-Guang HVDC transmission project,an AC-DC interconnection power system with AC-DC cross lines is constructed.Then the conducting state of inverter is analyzed with different system states and failure conditions,and the AC short circuit is obtained.Furthermore,taking DC system protection action and unipolar lockout into account,further research on the fault characteristic of AC-DC cross short is carried out.Afterwards,according to the fault characteristics analysis,the adaptability of AC line protection principle on the AC-DC cross short,including distance protection,longitudinal differential protection and zero-sequence current protection,is investigated.Simulation using PSCAD/EMTDC to analyze the protection operation characteristic is also conducted to prove the accuracy of theoretical analysis.Finally,this paper analyzes the adaptability of the fault location algorithm of AC line on the AC-DC cross short fault.After the vast simulation and verification,it shows that the fault location algorithm based on frequency-domain method is unfit for AC-DC cross short fault,while time-domain method is applicative and accurate.