This paper proposes a new method based on estimation of an error by approximating the given curve into a straight line. The given curve is approximated with set of straight lines in which each straight line consists three points. In these three points, the middle point value is the arithmetic mean of first and last points. In a regular curve, the deviation between estimated point and actual point is small whereas in an irregular curve the error is large. In this paper, instantaneous current signal is taken as input and current phasor is estimated using Discrete Fourier Transform Technique. In general, many conventional methods failed during power swing due to the variations in current and voltage but the proposed method effectively detects the faults as well as power swing. Simulations are carried out in MATLAB/SIMULINK.