Cores and secondary windings are located inside the head of the transformer. The high voltage insulation is fully obtained with SF6 gas instead of paper and oil.
The dielectric quality is no longer depending on long and complex treatments or delicate checks during the equipment's life-time. Partial discharge measurements are not necessary as the only piece of solid insulation is the outer porcelain or polymeric envelope. The probability of internal discharge is none, due to an insulation coordination with gaseous dielectric medium.
A device having a preset breakage protects the CT against possible over-pressure. The internal insulation can be continuously monitored by means of a pressure-gauge fitted with alarm and/or trip contacts in case of gas pressure decrease or increase.