The main bulk of electric power systems use a three¿phase configuration with an alternating current flow. Pursuing the optimal performance of these networks and avoiding possible technical problems, it is preferred balanced operating conditions, i.e., the currents of all phases have the same magnitude and form a direct sequence with a phase of 120º. However, if several single¿phase loads (such as high speed traction systems) are present in these systems, the operating condition turns unbalanced, provoking an asymmetric voltage supply. This undesirable working condition can be avoided by the use of two reactances connected with the single-phase load using a triangle configuration, in such a way that the total current consumption turns out to be balanced. This approach is commonly known as Steinmetz circuit or symmetrizing circuit. Due to the reactances of the symmetrizing circuit, the connection of the circuit in an electric network changes the system frequency response, appearing new resonances of several types and presenting impedance values too small or too large. Moreover, the quantity of nonlinear loads (i.e., loads with consume nonsinusoidal currents, such as arc furnaces, power electronics devices like high-speed rail systems...) is increasing nowadays. The harmonic current injection by these charges may interact with the resonances caused by the Steinmetz circuit, resulting in a large harmonic distortion in voltage. The aim of this doctoral thesis is to analyze the presence of several types of resonances in order to avoid this problem. It is used a simplified scheme of a network in which the single¿phase load and the symmetrized circuit are connected. This system also encloses nonlinear loads that generate harmonic currents. This set is analyzed from the viewpoint of nonlinear loads and of the network for parallel and series resonance frequencies location, respectively. By obtaining these resonant frequencies and knowing the harmonic injection of nonlinear loads, the trouble of voltage supply distortion can be anticipated and avoided.