Due to the significant distributed nature of ultra-high voltage transmission lines， the measuring impedance is not linear but hyperbola function of the fault distance. The conventional earthing impedance protection relay can not be used directly since its high sensitivity to the interim resistance. To tackle the problem， zero sequence reactance relay（ZSRR） is widely adopted in high-voltage and extra-high-voltage transmission lines. The paper analyses the feasibility of using ZSRR in ultra-high voltage systems and key influential factors. Results indicates that the protective zone and sensitivity are largely reduced by the estimation error in the fault current phase angle based on the zero sequence current at the relay installed location， and the heavy load currents. New algorithm is needed for the application of ZSRR in UHV lines.