Investigation on Aging of Paper and Liquid insulation during Overload Test of Natural Ester Immersed Transformer

We report a manufacturing‑scale ageing test on a natural‑ester‑filled transformer comprising (i) an initial heat‑run to establish baseline thermal performance, (ii) a two‑week accelerated ageing campaign at ~150% loading to simulate overload‑driven thermal stress, and (iii) a post‑campaign heat‑run to quantify any shift in thermal characteristics. Concurrently, we 1 sampled and analysed the liquid and paper before/after testing, measured insulation resistance, and interpreted ageing using coupled thermal and chemistry models. The protocol leverages accepted thermal‑ageing concepts from IEEE/IEC loading guides (loss‑of‑life via

Arrhenius‑based hottest‑spot acceleration factors), while integrating chemical markers whose partitioning and stability are known to challenge life estimation if used in isolation. By combining heat‑run metrics, overload thermals, and pre/post analytical chemistry of both liquid and paper, the study aims to bridge model predictions with in‑situ diagnostics under ester environments.

The results indicate that overload exposure produces measurable but characterizable changes in thermal performance and dielectric condition, and that the joint thermal–chemical modelling framework improves the precision of insulation‑lifetime estimation relative to single‑marker methods. Specifically, the observed evolution of liquid properties (e.g., acidity/moisture) and paper condition aligns with literature reporting hydrolysis‑driven moisture management in natural esters and slower DP decline versus mineral oil, supporting the premise that ester chemistry mitigates water‑catalysed cellulose ageing. The integrated analysis is consistent with multi‑year CIGRE studies showing better cellulose preservation and tractable dielectric‑response trends in natural‑ester systems, reinforcing confidence in using combined thermal profiles and chemical markers to forecast remaining life. Moreover, the findings are situated within CIGRE’s broader evidence base on alternative liquids and operational performance, underscoring that natural‑ester‑filled transformers can be assessed with refined lifetime models that account for fluid‑specific moisture kinetics and oxidation pathways. Taken together, the study demonstrates the feasibility of detecting insulation lifetime more precisely in natural‑ester transformers by fusing heat‑run thermals, overload ageing, and coupled thermal–chemical modelling, offering a practical path to more reliable end‑of‑life predictions.