Eliminating VT uncertainties in modeling ferroresonance phenomena caused by single phase-to-ground faults in isolated neutral network

•Ferroresonance modes after single phase-to-ground faults in isolated neutral network are studied.•The way to topological model of a voltage transformer (VT) is explained.•Parameters of the VT model are found under uncertainties in its design and characteristics.•Possibilities to simplify the VT model by neglecting core losses are considered. It is commonly recognized that correct transformer model (particularly topological one) is a key element for any ferroresonance simulations. This paper represents a line of reasoning by which a reliable model of a voltage transformer (VT) can be constructed given substantial uncertainties in its parameters and characteristics. Starting with catalog data for electric steels and typical leakage inductances of 6-kV VTs, a model of an isolated neutral network with three inductive VTs is initially fitted to reproduce in detail its ferroresonant behavior for a chosen network capacitance. Then, validity of the network model is verified by accurate predictions of ferroresonance processes for all other capacitance values available and thus different ferroresonance modes recorded during full-scale factory tests. Possibilities to simplify the VT model by neglecting dynamic, and then entire core losses and hysteresis are also shown.