PCB decontamination of insulating oils and their electrical behaviour evaluation

Today, international laws forbid the use of Poly Chlorinated Biphenils (PCB) in new electrical equipments and have introduced restrictions for those already in service, filled with mineral and syntetic oil, when containing only few ppm of PCS. In Italy, a large number of chemical analyses have shown [1] that about the 25% of the distribution transformers, filled with mineral oils, contain a PCB amount over the limit imposed by the new standards. This is due by the fact that in the 70's the transformers, containing both mineral oil or PCB, were built using the same construction factory equipment, without a preliminary complete cleaning. To overcome this situation, recently different physical and chemical treatments for PCB decontamination of such used mineral oils have been proposed. This paper presents the performances of a new process [2], based on the dechlorination reaction of a PCB, as a consequence of the intimate contact between a contaminated oils and a solution of KOH in PolyEthilenGlycols (PEG), in presence of an anionic surfactant as reactor promoter. The same treatment allows to speed up the decontamination reaction below the recommended limit of 25 ppm. The present paper reports also experimental data on the electrical properties of such oils, showing the effects of the different treatment of PCB decontamination, with particular regard to ac dielectric strength, impulse breakdown, tn delta and partial discharge tests.