Ferroresonance destroys transformers

The author discusses 60 Hz FR (ferroresonance), presents the necessary conditions for it to occur, suggests the analytical use of the incremental input describing function for the prediction of FR in circuits, and makes recommendations to minimize the exposure of the power system to FR. It is predicted that the frequency of FR occurrence in power systems will most likely increase in the future. Previous methods of using transformer connections to counter FR may be inadequate to eliminate damaging FR overvoltage conditions. While it may be more cost effective to design a power circuit free of FR, certain corrective measures can be applied to existing systems. These include: using three-phase switching devices, providing secondary damping resistances, using special switching sequences, temporarily grounding the neutral of wye-delta banks during switching operations, using resistance-grounded wye-delta banks, and using grounded wye-grounded wye transformers