The effectiveness of alternative drying technology for the maintenance of insulation quality in air breathing transformers

The role of excessive moisture concentration in accelerated ageing and increased risk of dielectric failure of oil-paper based insulation systems has been well established in published literature. Traditionally for air breathing systems, two technologies have been applied to prevent moisture ingress and promote low humidity of the bulk oil: silica-gel columns (adsorptive drying) and refrigerant dryers. The object of this paper is to present basic studies that have been carried out to establish the efficiency and effectiveness of these two air dryer types with a view to selection of the most effective option for a given transformer. As well, comparisons of silica-gel breathers, with and without flow directive internal baffles have been undertaken; both on the laboratory bench and under substation working conditions in terms of air flow rate, air dryness and gel-bed moisture distribution. The point of exhaustion of a silica-gel bed has been determined for different flow conditions to better quantify the point of replacement for transformer preventative maintenance programs. Deductions about the application and the relative merits of use of the different types of breathers on oil immersed power transformers are made