Analytical analysis of PT ferroresonance in the transient-state

Potential transformer (PT) ferroresonance might happen due to a capacitance when a circuit breaker is opened. In this case, the iron core repeats saturation and unsaturation. This paper describes an analytical analysis of PT ferroresonance in the transient-state. In this paper, the iron core is modelled by simplified two-segment core model to analyze ferroresonance analytically. Thus, a nonlinear ordinary differential equation (ODE) for the flux linkage is changed into a linear ODE with constant coefficients, which enables an analytical analysis. In this simplified model, each state, which is either saturated or unsaturated state, corresponds to one of the three modes, i.e. overdamping, critical damping and underdamping. The flux linkage and the voltage in each state are obtained analytically by solving the linear ODE with constant coefficients. The proposed transient analysis seems simple but effective in the more understanding of ferroresonance and thus can be used to design a ferroresonance suppression circuit of a PT.