NGC experience on ferroresonance in power transformers and voltage transformers on HV transmission systems

Ferroresonance or nonlinear resonance can occur on circuits with power transformers connected to overhead lines and also where wound type electromagnetic voltage transformers are connected to isolated sections of busbars. Energy is coupled via the inter-circuit capacitance of the parallel lines or open circuit breaker grading capacitance to sustain the resonance. In the case of ferroresonance the relationship between voltage and current is dependent not only on frequency but also on a number of other factors such as excitation voltage magnitude and initial conditions such as the transformer magnetic flux and point on wave of the initiating switching incident. It is characterised by a sudden jump of voltage or current from one stable state to another. Ferroresonance can occur at 50 Hz fundamental frequency or sub-harmonics frequencies at 33 1/3 Hz, 16 2/3 Hz and 10 Hz. The author discusses the nature of the phenomenon in power transformers and wound voltage transformers, and then discusses the measures to predict, prevent and/or quench ferroresonance