The 100 Hz protection function in an HVDC system is a backup protection for the ac system. The misoperation of the 100 Hz protection has severe effect on the safe and stable operation of the power system. Based on the proposed sequence switching-function model of the HVDC converter, and combined with the short circuit calculation program for the ac system, a practical method of calculation and Identification the operation area in the ac system for the 100 Hz protection is present in this paper. The asymmetric switching characteristic of the converter, involving the switching instant deviation and unbalanced commutation overlaps, and effects of the ac background harmonic to the dc side harmonic are considered in this method. The method is verified to be accurate and have great engineering practicability by simulation using the PSCAD/EMTDC software and the 500 kV ac power grid in the year 2008.