Ferroresonance in networks - Definition, description and classification

Ferroresonance is a non-linear resonance phenomenon that can affect electricity transmission and distribution networks. It refers to all oscillatory phenomena, most often harmonic, but also pseudo-periodic, or even chaotic in extreme cases, which occur in an electrical circuit made up, on the one hand, of one or more non-linear inductors (comprising saturable ferromagnetic materials) and, on the other hand, of a network comprising at least one capacitor supplied by one or more generally sinusoidal voltage sources.

The essential and characteristic property of such a phenomenon is to present at least two stable regimes for the same excitation. Classically, in electrical engineering, the electrical characteristics of components are considered to be linear, which implies that the steady state reached is unique and independent of initial conditions. Here, the presence of inductances with non-linear characteristics can lead to radically different and even surprising behavior for electrical engineers. Several different permanent regimes can occur in a given circuit, depending on initial conditions (remanent flux, switch-on times, etc.). Generally speaking, one of these is the one we usually expect, while the others are abnormal and sometimes even dangerous for electrical equipment, as they present overvoltages or overcurrents.

The phenomenon has already been observed on several occasions in networks, and it is therefore possible to classify the most frequently encountered behaviours. Periodic regimes with a period many times that of the source, and even pseudo-periodic regimes, can appear. It should be noted that the waveform generated, which illustrates the overall behavior of the network, provides information on the typology of the network under consideration and its more or less non-linear behavior.

Temporary harmonic overvoltages on networks are different in nature from ferroresonance phenomena, although they are generated by networks of the same type. The phenomena generally subside after a few seconds, the case most frequently encountered on EHV networks.

Non-linear phenomena, as encountered here in electrical engineering, are also the subject of growing interest in many fields of physics. They are also encountered in fluid mechanics, thermodynamics, mechanics, chemistry, and the best-known example is meteorology, with its sensitivity to initial conditions (butterfly effect) and the difficulty of predicting over a horizon of more than a few days.

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