This paper presents a new concept in ultra-high speed distance relay for power line protection. The idea is based on the use of a sampling window shorter than usual to speed commutation. The reduction in protection time is obtained through a variable window that is expanded sample by sample until a satisfactory precision is reached in the calculation of fault quantities. The right window length is obtained automatically via software and time-by-time changes depending on the characteristics of the fault. The algorithm is based on the equation associated to the R-L fault circuit, whose solution is obtained using the phase modified Fourier transform. To validate the proposed procedure, a large number of simulations are performed with reference to an existing 150-kV subtransmission line. In comparison with other solutions, the relay proposed here is very cost-effective and makes it possible to achieve ultra-high speed fault localization not only on high-voltage (HV) networks but also on medium-voltage (MV) distribution networks.