The actual transmission distance of UHV half-wavelength transmission lines is difficult to meet the conditions of half-wavelength, or even far less than half wavelength. Therefore, tuned half-wavelength transmission lines will become the main realization form of half wavelength AC transmission. In this paper, the PSCAD/EMTDC simulation model is used to analyze three main relay protection schemes for UHV AC transmission: current differential protection, distance protection and directional elements. At the same time, the difference of signal before and after the tuning device is analyzed. The results show that the tuning device will affect the amplitude of electrical quantity and cause phase deviation, except directional protection, current differential protection and distance protection are not suitable for transmission lines with tuned half-wavelength. © 2020, Electrical Technology Press Co. Ltd. All right reserved.