Alternating current line protection method and system based on wavefront index coefficient

The invention discloses an alternating-current line protection method and system based on a wavefront index coefficient, and the method comprises the steps: collecting a, b and c three-phase voltages and currents of a power transmission line through a traveling wave voltage sensor, and carrying out the phase-mode transformation, and obtaining a modulus voltage um and a current im; starting protection according to the break variable of the modulus voltage [mu] m; calculating a voltage reverse traveling wave according to the modulus voltage um and the current im break variable; extracting the arrival time of the wave head of the second traveling wave in a given time window by using a wavelet modulus maximum method by using the voltage reverse traveling wave, then extracting data window data from the starting time to the arrival time of the wave head of the second traveling wave, and judging the fault direction through a fault direction criterion; and when the fault direction is a positive direction fault, fitti