Material Compatibility and Thermal Analysis for Process Development Guidelines of Ester Oil Transformers

Abstract

Transformer insulation is a composite system made of insulating liquid and impregnable solid insulation. Generally, the mineral oil is used for insulating medium and cooling in transformers. Recently, there is a growing interest in ester oil transformer for fire safety, environmental friendly and continuous overloading capability in comparison to conventionally used mineral oil transformer. The high interest on ester oil transformer has led to several analyses aimed at prediction of their dielectric analysis, thermal analysis and process development. In this study, material compatibility tests are performed as per ASTM D3455- 11 to check the compatibility between the insulating oil and the materials used in the transformer manufacturing such as varnishes, insulation paper, paint, gasket, core etc. for setting up the process guidelines for ester oil transformers. Further, the insulation resistance and dissipation factor of the transformer windings immersed in ester oil are measured. It reveals that the leakage current must be minimised to compensate the more dissipation factor effect of ester oil to make it commercial for transformer applications. In order to utilize the ester oil as an alternative to mineral oil, the internal and external cooling modes in thermal analysis of transformer needs to be evaluated to get a continuous overloading capability advantageous of ester oil. In this dissertation work, thermal hydraulic network models (THNM) are effectively used to predict the oil flow distribution and the temperature distribution in 12.5/16 MVA, transformer and compared with mineral oil in steady state conditions. The oil flow rate within the winding, top oil rise, winding rise, gradient of the winding and hot spot temperature rise are calculated for both natural ester oil and synthetic ester oil with respect to mineral oil under air natural and air forced cooling modes. It is observed that the materials which are compatible with mineral oil need not to give the same behaviour with ester oil, therefore separate process guidelines must be designed. Due to more viscous nature of ester oil, rate of oil flow in transformer is slow and can be increased using zig-zag cooling method.