Dielectric behavior and stabilization of insulating oils used in EHV current transformers

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