Testing of Biodegradable Fluids in High Voltage Current Transformers

A comparative study was carried out on two new biodegradable hydrocarbon-based fluids

(HBB1 and HBB2), a synthetic ester (SE), and conventional mineral oil (MO) as reference.

Physical, electrical, and chemical properties of the fluids were characterized, followed by compatibility tests with elastomers and materials commonly used in instrument transformers.

The impact of the different fluids on insulation performance was assessed through tests on mock-ups based on capacitive stacks and Kraft paper insulation, including partial discharge inception voltage (PDIV) measurements, where it was found that capacitive stacks filled with synthetic oil (SO) exhibit significantly higher PDIV than those filled with the new biodegradable fluids. In addition, full-scale prototypes of 145 kV and 420 kV CTs were manufactured and subjected to dielectric, thermal, and ageing tests.

The results show that the new biodegradable hydrocarbon-based fluids exhibit dielectric and thermal behaviour close to that of MO, allowing their use in high-voltage CTs without major changes to insulation design or manufacturing processes, apart from adaptations to the compensation system due to its higher thermal expansion coefficient. In contrast, the SE shows limitations related to high viscosity and increasing dielectric dissipation factor during ageing, which may compromise the thermal stability of CTs.

From a sustainability perspective, biodegradability tests performed according to OECD 301 revealed significant variability depending on the test method, highlighting limitations in current assessment procedures. A life cycle assessment (LCA), limited to the insulating fluids, indicates that the biodegradable hydrocarbon-based fluid presents the lowest overall environmental impact, while synthetic ester shows the highest, mainly due to its production stage.

Overall, the study demonstrates that biodegradable hydrocarbon-based fluids can represent a technically viable and environmentally favourable alternative to MO in high-voltage CTs, while further research is required before SE can be reliably applied at higher voltage levels.