In this paper,several principle questions of half-wavelength transmission are studied,including power frequency overvoltage in steady-state operation,power frequency overvoltage caused by faults,static power angle synchronization stability and transient power angle synchronization stability.The circuit model of the long-distance transmission system is established at first for steady-state and transient analysis.The sending-end system and the receiving-end system are both considered in the model.A test system based on an actual transmission line is given to facilitate the description of system characteristics.Based on the circuit model,the resonant transmission distance of the system is found and calculated.Theoretical analysis and numerical calculations are carried out to determine the feasibility transmission distance.It is demonstrated that the transmission distance should be in a certain range,which is larger than the resonant transmission distance,to satisfy the steady-state overvoltage and the small signal synchronization stability as well as the frequency deviation constraints.For transmission distances in the feasible range,the three-phase short circuit fault at a certain point of the transmission line will cause the most serious transient power-frequency overvoltage,and the system is very likely to lose synchronization stability.Considering the transient power-frequency overvoltage and the transient synchronization stability,the half-wavelength transmission system is technically impossible to operate.