Influence of ageing by-products on the stability of some transformer fluids

There is no doubt that the service reliability of power transformer largely depends upon the condition of the fluid. The gradual deterioration (under the impact of electrical, thermal and environmental stresses) of purity due to its ageing has an important impact upon the solid insulation. In this contribution, the dielectric and physicochemical properties of aged commercially available ester fluids are studied under electrical/thermal stress. Many comparisons are made to the quality test results of mineral oil, as this is something transformer industry is very familiar with. A reactive free radical reagent, 2,2-diphenyl-1-picrylhydrazyl (DPPH), is added to new oil and aged oil to determine free radical concentration. It is shown that with the production of oxidative aging by-products, the gassing tendency under thermal and electrical stresses along with the physicochemical properties of the fluids are affected. The results indicate that the gassing tendency of natural esters is much lower than their counterparts. The theoretical premises that, oil born decay products contribute to the oil gassing is experimentally confirmed under laboratory conditions