Alternative liquid dielectrics in power transformer insulation: a review

Transformer liquid dielectrics evolved where mineral oil has been the dominant choice until emergence of synthetic esters and natural esters. Natural ester-based oils have been under extensive investigations to enhance their properties for replacing petroleum-based mineral oil, which is non-biodegradable and has poor dielectric properties. This paper focuses on exposition of natural ester oil application in mixed transformer liquid dielectrics. Physical, chemical, electrical, and ageing characteristics of these dielectrics and the dissolved gas analysis (DGA) were reviewed. Physical properties include viscosity, pour point, flash and fire point which are vital indicators of heat insulation and fire risk. Chemical properties considered are water content, acid number, DGA, corrosive sulphur, and sludge content to limit and detect degradation and corrosion due to oil ageing. Electrical properties including breakdown voltage were considered for consistent insulation during overload and fault conditions. These properties of evolving alternative dielectrics were reviewed based on ASTM International standards and International Electro technical Commission standards for acceptable transformer liquid dielectrics. This review paper was compiled to avail modern methodologies for both the industry and scholars, also providing the significance of using mixed dielectrics for power transformers as they are concluded to show superiority over non-mixed dielectrics.

Viscous Liquid Breakdown Voltage Measurements

The national standards on measuring the breakdown voltage of liquid dielectrics are considered, in particular, the measurement procedure and the measuring cell main parameters. The breakdown voltage of PDMS-1000, PDMS-12500 and PDMS-30000 liquids produced by various manufacturers was measured, and the main problems faced in carrying out measurements were identified. It is shown, using the example of polydimethylsiloxane liquid dielectrics, that after breakdown of viscous liquids, a channel is produced in them, which consists of gas bubbles held by the surface tension force, due to which the channel may in many cases persist for a long time. Theoretical calculations confirmed by an experiment were performed, based on which the velocity of air bubbles in polydimethylsiloxane liquids can be estimated. A conclusion has been drawn about the necessity to increase the time intervals between individual measurements, as well as the time interval before the start of measurements after pouring the test liquid into the measurement cell. It is shown that visual control of the interelectrode region and a special method of mixing the liquid are also necessary. It can be stated based on the accomplished study that the existing standards for breakdown voltage measurements in viscous liquids need to be refined.