Dielectric Behavior and Stabilization of Insulation Oils in EHV Current Transformers

The causes of a number of explosions of EHV current transformers in service on the Hydro-Qu6bec 765 kV network have been investigated. Thermal runaway related to unexpectedly high dielectric losses in one of the types of oils used was diagnosed. Electrical instability of the oil was related to dissolved-meta content rather than to conventional oxidation of the oil. A new way of stabilizing the oil is proposed, consisting in the addition of a metal deactivator in oil.