As an important measuring device in converter stations

the all-fiber optic current transformer (AFOCT) is essential for ensuring the safe and stable operation of DC systems.This paper presents a case study of a circuit breaker tripping event caused by abnormal measurement data from an unbalanced AC filter AFOCT in a ±800 kV converter station .Firstly

the basic working principle of AFOCT is described in detail

followed by the derivation of its measurement principles.Secondly

an in-depth research is conducted through methods including correctness analysis of protection actions

on-site fault troubleshooting

on-site verification

and vibration impact testing of the polarizationmaintaining cable.The root cause of the abnormal measurement data is identified

as well as the fault mechanism related to cable vibration effects.Finally

specific improvement measures and engineering suggestions are proposed to address the issue of abnormal measurement data caused by vibration interference

providing theoretical support for the engineering design

on-site construction acceptance

and reliability enhancement of AFOCT in converter stations.