The Impact of Ferroresonance and Low Frequency Phenomena on Power Transformers and Transmission systems

Abstract

Ferroresonance is a low frequency phenomenon that can occur when one side of a double circuit transmission line connected to a transformer is switched out. This can result in the transfer of power from the adjacent circuit through mutual coupling into the de-energised circuit. This can lead to saturation of the transformer and stressing of the disconnector during opening. More typically, ferroresonance will affect control and protection functions preventing circuit operations and possibly requiring an unplanned double circuit outage to safely isolate the distressed transformer. In the UK a number of power transformers are exposed to ferroresonance and remote switching where they are connected to mesh corner and circuit tee configurations. Ferroresonance will continue for a few minutes until it can be detected and quenched. In cases where it cannot be detected, this can be much longer until the transformer is fully isolated. There are a number of ferroresonance modes, which may affect the unit differently, little is known to what degree this is damaging the transformer. This paper briefly discusses the results of system tests carried out to investigate the effect of ferroresonance and subsequent modelling work. The validation of a detailed transient model has been carried in an attempt to understand the effect of ferroresonance and quantify the energy injected into the transformer core during various ferroresonance modes. The article concentrates on the early stages of this work and the impact of ferroresonance on network infrastructure including transformers, switchgear, protection and control.