The Impact of Construction Materials of Power Transformers on Aging of Different Natural Ester Liquids

Significant increase in application of alternative liquids caused by stricter environmental and safety requirements is challenging, due to lack of information in all aspects compared to mineral liquid. Although the use of alternative liquids began around 50 years ago, there are still a plenty of unresolved questions and uncertainties regarding their behavior during exploitation of power transformers and aging of insulation system. In the current study an aging test was defined for comparison of two natural ester liquids from different suppliers but having the same base (soya), where physicochemical properties were measured during aging with different solid materials. The influence of individual construction materials aluminum, copper and core combined with two types of insulating paper on the gas production and insulating liquid and paper properties during heating (thermal stress) were investigated. The dimensions of the solid samples and the ratio of solid to liquid volume were adjusted to reflect the specific transformer design. Before aging tests, insulating liquid and cellulose samples were treated and prepared in the laboratory according to factory guidance of new transformer manufacturer. Aging was performed in the stainless steel aging vessels at 180 °C up to 21 days or 28 days depending on the type of insulating paper used. Obtained results imply that type of insulating paper has the strongest impact on the aging rate of tested insulating liquids. On the other hand, it was observed that two natural esters of the same base have different gas production pattern, which was most likely consequence of the type of antioxidant present in the liquid.