Abstract: When the traditional full-cycle Fourier algorithm is directly used to process the power system fault signal including the decaying DC component， some errors will be produced. Thus， an improved Fourier algorithm which could eliminate the errors was proposed. This algorithm was based on the principle that integration of the input signal including the decaying DC component and summation of the sampled data were not zero in a full-cycle. This algorithm did not need addition of the sampled data and the approximation calculation in its derivation process， and could calculate the errors caused by the decaying DC component and its parameters. The errors caused by the decaying DC component could be eliminated from the results of the input fault signal. The simulation results showed that the parameters of fundamental frequency component and every harmonics can be accurately obtained， and the proposed algorithm can be used in power system on-line harmonic analysis.

Key words: decaying DC component, full-cycle Fourier algorithm, harmonic component, sampled data, Lagrange interpolation