Abstract: According to an AC（alternating current） filter unbalanced protection action trip accident in ±800 kV UHV（ultra high vacuum） Ximeng Converter Station,it introduces the working principle and structure of the optical fiber CT（current transformer）,and analyzes the unbalance protection principle and action of AC filter capacitor. The cause of the accident is the protection misoperation caused by the program setting of unbalanced optical fiber CT electronic unit. The program logic of optical fiber CT electronic unit is improved and upgraded. The hidden danger of equipment is eliminated,and the operation and maintenance suggestions of optical fiber CT are put forward.