This paper presents a novel method for on-line identification of the detuning component in AC filter in high voltage transmission system. The identification principle of detuning component in each part of a double-tuned filter which is divided into three parts is given

that is

the detuned element in upper part is identified by the fundamental current change rate

the element detuning is judged by the fundamental current ratio of the two branches in middle or lower segment of the filter element

and the concrete detuned element in middle section is identified by the change rates of the fundamental current in the two branches of this section

while that in lower section is identified by the change in the phase difference of the two branches in lower section. The practical application of the detuned element online identification method is presented

and the correctness and effectiveness of this method are verified by the simulation results on the basis of a HP12/24 filter of a practical HVDC transmission project. This paper provides a practical method to realize the on-line identification of the detuning element of passive AC filter.