PCB decontamination of insulating oils and their electrical behaviour evaluation

Today, international laws forbid the use of Poly Chlorinated Biphenyls (PCB) in new electrical equipment and have introduced restrictions for those already in service, filled with mineral and synthetic oil, when containing only few ppm of PCB. In Italy, a large number of chemical analyses have shown that about 25% of the distribution transformers, filled with mineral oils, contain a PCB amount over the limit imposed by the new standards. This is due to 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, different physical and chemical treatments for PCB decontamination of such mineral oils have been proposed. This paper presents the performances of a new process, based on the dechlorination reaction of a PCB, as a consequence of the intimate contact between contaminated oils and a solution of KOH in PolyEthylenGlycols (PEG), in presence of an anionic surfactant as reactor promoter. The same treatment allows one to speed up the decontamination reaction below the recommended limit of 25 ppm. The paper reports 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 δ and partial discharge tests