AC filter is important equipment in the converter station of ±800 kV Yunnan—Guangdong ultra-high voltage direct current（UHVDC）transmission system

with such features as large capacity and frequentswitching

and its switching control logic affects directly safe and stable operation of the DC transmission system. In this paper

two abnormal events are analyzed: rise of AC busbar voltage due to abnormal operation of group filter with thermal stand by condition and power limitation due to undefined group filter blade. And this issue that the operated group filters has no way to remove automatically after the tripping of the line circuit breaker is discussed. In view of the control logic defect of them

corresponding improvement suggestion and proposal are propped

which provides reference for the design and operation of actual DC engineering.