During the process of the rectifying by thyristor in the three phase symmetric AC power system the high order harmonic current is produced.It is harmful to the power system not only influencing on the power quality but also resulting in the maloperation of the relay protection or burning out the electrical board of micro computer protection and digital electrometer and other devices in the computer system.It is

nevertheless

possible to filter the high order harmonic voltage by using AC filter equipment reasonably designed.On the basis of description of the harm of harmonic current and the method of restraining the harmonic

this paper presents the theoretical calculation of AC harmonic filter

and also the way of reasonable selection of the AC filter for eliminating the high order harmonic current is discussed as well.