The monograph is devoted to the study of the problem of all-mode adjustment of relay protection devices of electric power systems. To solve it, the authors proposed an approach involving the use of detailed mathematical models of relay protection, taking into account the key software and hardware features of their implementations, as well as processes in measuring current and voltage transformers. Presented are all-mode mathematical models of differential protections of transformers, remote protections of power system equipment, as well as differential-phase protections of power transmission lines.