Check Schweitzer Engineering Labs webpage. They have a good selection of technical papers. Also, see GE Industrial Systems website under Protection Relays (Multilin).
Digital relays detect negative sequence current by measuring all three phase currents and computing the sequence currents using standard sequence current equations. If all three phase currents magnitudes and phase angles are known, all sequence currents can be computed and vice versa.
Zero sequence currents can be computed this way, but are often detected using residual current detection or flux summation methods.
