Mitigation of Ferroresonance Induced by Single-Phase Opening of a Three-Phase Transformer Feeder

This paper presents a practical mitigation technique for the ferroresonance induced by the single-phase primary opening of a three-phase 5-legged (wound core) station service transformer. This ferroresonance problem was causing erratic behaviour of the automatic transfer switch between the main and reserve station service transformers. A nonlinear three-phase transformer model, based on the magnetic-circuit theory, was used to reproduce and acquire a deeper understanding of the ferroresonance phenomenon. The model, validated by field measurements, allowed comparison of the effectiveness of resistive and inductive loads for reducing the voltage on the open phase. In previous literature, it has been generally recommended to use a resistive load to damp ferroresonance. This paper demonstrates that an inductive load is more effective and has the advantage of being a more energy-efficient solution. The inductive-load mitigation technique is now in service in Hydro-Québec's Deschambault substation.