Compared to the EHV, the power-frequency overvoltage of the UHV transmission lines is usually more serious, and seriously affecting the power transmission reliability. This chapter first discusses the mechanisms, characteristics, classification, and main influence factors of power-frequency overvoltage of the UHVAC transmission line, then focuses on the discussion of two restrictive measures for power-frequency overvoltage, namely, high-voltage shunt reactor and controllable high-voltage shunt reactor, and finally carries out detailed discussion on the determination of the upper and lower limits of the compensation degree of high-voltage shunt reactor.