Natural esters as a part of HV transformer insulation system

Natural esters are candidate element of transformer insulation system. Recently most common insulation in power transformers is composed of mineral oil and transformer paper. As there are efforts to preserve sustainable development, crude oil products should be eliminated and replaced by natural products, which are easily biodegradable. In past, it was shown, that natural esters may reach qualities of mineral oils in some parameters (from electrical parameters it is typically electric strength), while they remain worse in others (typically dissipation factor). Although there are some difficulties, by proper design they should be eliminated. The paper concentrates on specific aspects of designing insulation elements by preserving the same lifetime as in case of mineral oils. Not only endurance curves should be considered, but also other aspects as partial discharge resistance, oxidation and viscosity rise, regeneration and the impact on solid components. Valuable information can be obtained from FEM field calculations, which are recommended. Voltage distribution and especially electrical field intensity may point out high stressed areas where potential problems may occur during material operation as well as temperature rise can be calculated. During material aging in service high stressed areas deteriorates by continuous irreversible changes which lead to outages and downtimes or to human health threat in the worst case.