Design Insulation Level Factor for New Biodegradable Liquids

The Design Insulation Level (DIL) factor is an important parameter in the transformer industry, facilitating the design process of transformer insulation systems. This factor is based on the observation that the dielectric strength of paperliquid systems decreases with the duration of electrical stress. Consequently, all insulation levels, including Lightning Impulse (LI) and Switching Impulse (SI), can be recalculated to the AC voltage level. Historically, the values of the DIL Factor used in the industry were established for mineral oils and have not been re-evaluated for alternative insulating liquids. This paper presents the findings of a study conducted on a laboratory model designed to imitate the actual conditions found in transformer insulation systems. The study aims to evaluate the applicability of the currently utilized Design Insulation Level (DIL) Factors, originally established for mineral oils, to alternative insulating liquids. This verification is essential to ensure that these alternative liquids can meet the stringent safety and performance standards required for transformer insulation systems. The experimental setup includes a U-shaped electrode system, which simulates the electrical stresses at the winding corners in the radial oil ducts between transformer windings. The laboratory model was used to assess the impact of different liquid types on the DIL Factor. This was done by measuring the initiation voltages of partial discharges under both LI and AC voltage conditions. The results provide insights into the suitability of alternative liquids for transformer insulation, potentially leading to more efficient and reliable transformer designs.