"Residual" method involves putting a relay element in the neutral (common) leg of three individual phase CTs in a wye connection. Any residual current flowing in the neutral of the wye represents ground (and/or) neutral current.
Zero sequence is also known as flux summation and involves putting a window-type CT around all three phase conductors. Normally, the flux of the three phase conductors should sum to zero so there will be no current in the CT secondary. If there is a ground fault, phase currents are not balanced, flux is not zero and there is secondary current in the CT proportional to the primary ground current.