Longitudinal Protection of LCC-VSC Bipolar HVDC Transmission Line Based on Model Identification and Current Polarity

For hybrid bipolar DC transmission systems, combined with model identification and current polarity, a new principle of longitudinal protection of transmission lines is proposed. This paper analyzes the fault network and finds: when the front end of the inverter side fails, the fault model is equivalent to the C model, and when the back side of the inverter side fails, the fault model is equivalent to the RL model; when the front end of the rectification side fails, the rectification side fault current has a positive polarity, and when the rectification side back side fails, the rectification side fault current has a negative polarity. In conclusion, DC line area failure, namely the inverter side front and rectifier side front fault at the same time, the fault model matching C model, there was a positive polarity fault current at the same time; Failure occurs outside the DC line area, namely the inverter side front and rectifier side front no fault at the same time, inverter side failure model does not match the C model, or rectifier side fault current with a negative polarity. This new principle compensates for primary protection and can be used as an improvement to backup protection.