Abstract: This paper analyzes the causes of frequent abnormal detuning protection alarms in AC filters at a ±500 kV converter station of China Southern Power Grid. First, it is found that the detuning alarms of AC filter branches are mainly triggered by an abnormal increase in the 12th harmonic component. Then, an electromagnetic transient simulation model of the converter station is established in PSCAD/EMTDC, and the frequency–impedance characteristics under different filter switching configurations are analyzed. The results show that, under certain specific operating conditions, a 12th-order parallel resonance point exists in the system. Meanwhile, the inrush current generated during AC filter energization contains a 12th harmonic component. Finally, the setting values of the AC filter detuning protection are discussed, and it is concluded that the existing protection settings are unreasonable.