Dielectric and Thermal Design Guidelines for Ester Oil Transformers

Abstract

The mineral or synthetic oil is mostly being used in conjunction with paper as the dielectric medium in most of the high voltage equipment. However, impact on environment, lack of petroleum oil reserves and disposal problems with used oils, have prompted researchers to direct their focus on to biodegradable and renewable insulating materials. The new insulating liquid materials development is guided by multiple factors such as electrical and thermal performance aspects, environmental requirements, safety and economic considerations. As cautious approach is necessary for any step change in technology, transformer manufacturers have to face new specifications related to these new requirements. This work reviews the current status of vegetable oils use as transformer oil, including their production, processing, and characterization. Since transformer's reliability depends on strength of insulation involved in it, hence characterisation of insulations based on standards are important for designing of power transformers. This project utilises the concept of testing with standard impulse wave shape as per IEC 60243 for defining dielectric/insulation guidelines for 132 kV ester oil transformers along with investigation of electrical performances and electrostatic stress analysis including influence of axial and radial fields on insulation. Additionally, thermal performance evaluation of ester liquids according to IS-2026 and frequency response analysis as per IEC 60076-18 is done. The results indicate that impulse strength of ester liquids for breakdown in a quasi-uniform field is 10-15% lower as compared to mineral oil. Because of higher permittivity than mineral oil, lower electric stress in insulating oils in case of esters and slight deviations from mineral oil response are observed in frequency response analysis . Additionally because of hydrophilic nature of ester molecules, though having higher viscosity than mineral oil hence higher hotspot temperature , ester oils do not require additional increase in cooling capacity of transformers or to diminish transformer power rating. Ester oils enable life extension of transformers. In result increases the total reliability of power system