Contents I. Introduction
The oil-paper insulation system is the heart of an electric transformer, and its proper functioning is critical to ensure safe operation [1], [2]. Mineral oil has been used in transformers for over a century due to its excellent electrical properties and low cost [1]; however, the mineral oil-paper insulation oil system has poor thermal stability and a low explosion point [3]. Mineral oil is, moreover, a nonrenewable energy source, and its leakage can cause significant environmental harm [4]. Over the past few decades, technological innovations have emerged to address pressing energy needs, including green development and sustainability [1]. Many researchers worldwide have conducted extensive research to validate the properties of natural esters [5], [6], [7]. Natural ester is environmentally friendly and renewable, making it a preferred choice for the next generation of insulating fluids [5]; however, like any other insulating liquid, natural ester has limitations. Its dielectric loss, acid value, and viscosity are relatively high and cannot satisfy the relevant standards of mineral oil for transformers [8]. As a result, natural esters cannot be directly used to replace mineral oil in existing transformer applications. A new type of natural ester transformer must be designed based on the specific parameters of the natural ester. This development significantly increases the operating costs of power grids worldwide.
