Lightning Impulse Withstand of Natural Ester Liquid

Abstract

Due to the low biodegradability of mineral oil, intense research is conducted to define alternative liquids with comparable dielectric properties. Natural ester liquids are an alternative in focus; they are used increasingly as insulating liquid in distribution and power transformers. The main advantages of natural ester liquids compared to mineral oil are their good biodegradability and mainly high flash and fire points providing better fire safety. The dielectric strength of natural ester liquids is comparable to conventional mineral oil for homogeneous field arrangements. However, many studies showed a reduced dielectric strength for highly inhomogeneous field arrangements. This study investigates at which degree of inhomogeneity differences in breakdown voltage between the two insulating liquids occur. Investigations use lightning impulses with different electrode arrangements representing different field inhomogeneity factors and different gap distances. To ensure comparisons with existing transformer geometries, investigations are application-oriented using a transformer conductor model, which is compared to other studies. Results show significant differences in breakdown voltage from an inhomogeneity factor of 0.1 (highly inhomogeneous field) depending on the gap distance. Larger electrode gaps provide a larger inhomogeneity at which differences in breakdown voltages occur.

Alternative Dielectric Fluids for Transformer Insulation System: Progress, Challenges, and Future Prospects

“…It was also observed that streamer patterns for negative polarity are thicker than for positive polarity in natural ester and mineral oil. In [90], inhomogeneity differences in breakdown voltage between natural ester and mineral oils with different electrode arrangements representing different fields in homogeneity factors and different gap distances are reported. Various electrode and conductor arrangements reported in the literature on oil breakdown voltage studies were also documented.…”

Various Aging Processes in a Paper-Natural Ester Insulation System in the Presence of Copper and Moisture

“…inadequate legislation (standards-defined operational limits) that make the wider industrial use of natural esters more demanding, along with durability problems in terms of lower lightning impulse (LI) resistance [8], [9], oxidative stability, etc. Esters and natural esters also have many benefits, e.g., a low degree of cellulose polymerization (in the system) compared to mineral fluids [10], a high flash point and fire point [11] and a relatively high ''moisture tolerance'' of its dielectric properties [12], higher permittivity is prone a better electric field distribution in the paper-oil system [42] and esters have better thermal stress performance [43].…”

Engineering Dielectric Liquid Applications

“…Experimental investigations dedicated towards the performance of natural esters have been presented in [6,7]. Haegele et al [6] reported the degree of inhomogeneity differences in breakdown voltage between natural ester and mineral oils while using lightning impulses at different electrode arrangements representing different fields in homogeneity factors and different gap distances. Authors also envisaged different electrode and conductor arrangements reported in the literature for oil breakdown voltage studies.…”