It is known that the occurrence and existence of ferroresonant oscillations at the subharmonic frequency (SHF) in power transmission lines (PTLs) and in power supply systems is extremely undesirable, since they cause ferroresonant overvoltages at various frequencies. At the same time, there is a wide class of nonlinear electrical circuits in which the excitation of autoparametric oscillations (APO) at the SGC frequency forms the basis of frequency-converting devices that serve as secondary power sources. It is shown that three-phase nonlinear systems are, to one degree or another, equivalent circuits for power lines, the main elements of which are: longitudinal compensation capacitors, transverse compensation reactors, and transformers with a nonlinear characteristic. To study the pattern of excitation and maintenance of SGC at frequency in three-phase electroferromagnetic circuits (EFMC), theoretical and experimental studies were carried out on an equivalent model of a three-phase circuit with nonlinear inductance. To analyze the steady-state mode of the SGC at frequency , the method of frequency-energy relations was applied. An algebraic equation is obtained that characterizes the steady-state mode of the SGC at frequency ω/3 in a three-phase nonlinear circuit. The frequency-converting properties of nonlinear inductance are also shown. The obtained results of the theoretical study were confirmed by experimental studies.