In order to analyze the operating characteristics of power-frequency variation protections of the sending-end AC system theoretically, the mathematical model of DC system at the rectifier side which is represented by a power-frequency variation impedance in fault superimposed network is deduced in the case that fault occurs in the sending-end AC system. Based on the mathematical model and the response characteristics of HVDC control system, the characteristics of the equivalent power-frequency variation impedance of DC system are analyzed under different fault conditions. Then the impact of DC system on power-frequency variation protections is analyzed based on the fault characteristics of DC system. According to the analysis, when fault occurs in the sending-end AC system, the equivalent power-frequency variation impedance of DC system is capacitive under most fault conditions, which reduces the zone of distance protection based on power-frequency variation and results in misjudgment of directional protection based on power-frequency variation. In the end, the result is verified by simulations on PSCAD/EMTDC. This work is supported by National Natural Science Foundation of China (No. 51577072).