A method for locating one or multiple partial discharge sources in a power transformer or a Gas-Insulated Substation, whereby the partial discharges emit electromagnetic and acoustic waves, the method comprising recording a signal corresponding to the waves from the partial discharge sources by means of at least one sensor; time-reversing the recorded waves, thereafter back propagating the recorded signals into the power transformer or Gas-Insulated Substation by means of numerical simulations; and localizing the focal spots of the partial discharge sources.