By proceeding from the implementation of the digital algorithm of relays， a discussion is made of the contradiction between the action speed of rapid relays and the length of data window. In order to solve the contradiction， various algorithms are adopted for relays. Short data window algorithms are fast and long ones are accurate. The long data window full wave Fourier algorithm is selected to ensure the accuracy of the calculation and simplify the Fourier algorithm used in variation relays. The operation equation of amplitude comparison relays of rapid relays is expressed as the operating value being greater than the braking value. To meet the requirement on the variation relay to serve as the rapid relay， the braking coefficient is introduced into the braking value according to the variation features of the operating value to correct the operation characteristics in real-time so as to meet the requirement of action speed.