Decay products in the liquid insulation of power transformers

There is a general agreement that in service conditions the quality of insulating fluids gradually deteriorates under the impact of electrical, thermal and chemical stresses. An important side effect of oil gassing related to the invisible colloidal suspensions is also investigated. Two ASTM methods are used to monitor the deterioration of liquid insulation step by step. The results obtained using a laboratory-grade spectrophotometer and a ratio-turbidimeter indicate that the absorbance increases by a significant and easily observable margin with ageing rate. A comparison is made between the performances of a naphtenic commercial-based mineral oil and a synthetic and natural ester fluid. Under the same aging condition, the obtained results indicate that esters have a better capability to dissolve sludge. Currently, the gassing of oil is solely related to hidden incipient electrical failures. Consequently, these are detected and diagnosed by periodic dissolved gas analysis. The results performed in laboratory conditions reported in this contribution on oil stability under electrical stress indicate that, undetected oil-born decay products contribute to the gassing of oil.