A method for detecting faulted phases of three phases of a transmission line of an AC power system, the method including: sampling electric signals including currents i(t) and voltages u(t) at one end of the transmission line at a series of time points; computing instantaneous voltage values of the electric signals at a preset point (q) on the transmission line from values of the sampled electric signals based on a time domain lumped parameter differential equation for the transmission line for the series of time points; recording the computed instantaneous voltage values of the electric signals at the preset point; determining pre-fault instantaneous voltage values at a first time point of the series of time points; determining post-fault instantaneous voltage values at a second time point of the series of time points; computing fault component instantaneous voltage values of the electrical signals based on the pre-fault and post-fault instantaneous voltage values at the preset point; detecting faulted phases or fault type by comparing the computed fault component instantaneous voltage values at the preset point; and generating fault signals (SFault) indicating the faulted phases or fault types, characterized in that: the computed instantaneous voltage values comprise phase-phase instantaneous voltage values corresponding to each phase-phase loop electric signal and phase-ground instantaneous voltage values corresponding to each phase-ground loop electric signal; and the computed fault component instantaneous voltage values comprise phase-phase fault instantaneous voltage values corresponding to each of the phase-phase loop electric signals and phase-ground fault instantaneous voltage values corresponding to each of the phase-ground loop electric signals. sampling electric signals including currents i(t) and voltages u(t) at one end of the transmission line at a series of time points; computing instantaneous voltage values of the electric signals at a preset point (q) on the transmission line from values of the sampled electric signals based on a time domain lumped parameter differential equation for the transmission line for the series of time points; recording the computed instantaneous voltage values of the electric signals at the preset point; determining pre-fault instantaneous voltage values at a first time point of the series of time points; determining post-fault instantaneous voltage values at a second time point of the series of time points; computing fault component instantaneous voltage values of the electrical signals based on the pre-fault and post-fault instantaneous voltage values at the preset point; detecting faulted phases or fault type by comparing the computed fault component instantaneous voltage values at the preset point; and generating fault signals (SFault) indicating the faulted phases or fault types, characterized in that: the computed instantaneous voltage values comprise phase-phase instantaneous voltage values corresponding to each phase-phase loop electric signal and phase-ground instantaneous voltage values corresponding to each phase-ground loop electric signal; and the computed fault component instantaneous voltage values comprise phase-phase fault instantaneous voltage values corresponding to each of the phase-phase loop electric signals and phase-ground fault instantaneous voltage values corresponding to each of the phase-ground loop electric signals. the computed instantaneous voltage values comprise phase-phase instantaneous voltage values corresponding to each phase-phase loop electric signal and phase-ground instantaneous voltage values corresponding to each phase-ground loop electric signal; and the computed fault component instantaneous voltage values comprise phase-phase fault instantaneous voltage values corresponding to each of the phase-phase loop electric signals and phase-ground fault instantaneous voltage values corresponding to each of the phase-ground loop electric signals.