Dual-State Inductance Approach
As an improved analytical solution of ferroresonance, Dual-State Inductance approach presents two different operating modes for the transformer core in i) unsaturated linear part of the saturation curve and ii) the saturation region. As such, the transformer core saturation curve is simplified to a piecewise-linear characteristic with two parts which are represented by two asymptotes, as depicted in Figure 18. The slopes of these asymptotes are the linear inductances Lunsat and Lsat for unsaturated and saturated regions, respectively. In Figure 18, λk and i k are the coordinates of the intersection of the two asymptotes, which represent the knee point of the saturation curve. When the voltage is
applied to the transformer, the inductance of the core is switched between these two inductances, depending on the core flux magnitude.