The mechanism and characteristics of overvoltage of half-wavelength transmission line（HWTL） are different from those of the conventional lines.Overvoltage limitation is one of the key issues for the engineering application of half-wavelength transmission technology.We analyzed the generating condition and mechanism of overvoltage of HWTL

and established a simulation model of UHV half-wavelength AC transmission system.Furthermore

we studied the amplitude

duration

and waveform characteristics of overvoltage of HWTL under different conditions.According to the overvoltage characteristics under different conditions

the corresponding countermeasures were put forward.The results show that the severest overvoltage is generated when ground fault occurs on HWTL.The fault overvoltage can be limited within allowable range by metal oxide arresters（MOAs） along the line and fast active grounding measure at the middle of the line.The energy of MOA during fault can meet the requirements.