AbstractNormally the incoming signal data for digital distance‐protection relays are distorted sinusoidal oscillations. To get a correct response, considering upper harmonics, it is necessary to fulfill a Fast Fourier Transformation (FFT) with signal analysis. To receive a fast reply time of protection relays, the use of differential‐equation solutions is demanded. A combination of spectrum analysis and differential solution is shown, which joins the advantages of considering upper harmonics and short‐grading time, using digital signal processors.