Learnings

Outcomes

WS1 showed that FT-IR was a suitable and sensitive technique for ester in mineral oil.  The detection limit of the measurement procedure used is between 100 mg/kg and 200 mg/kg.  The same procedure was applied to an in-service aged mineral oil, and similar results were obtained, with no apparent interference from ageing products.

WS2 results showed that the chosen marker can be detected at 100 μg/l in unused mineral oil.  The same marker was tested at concentrations of 1 g/l to 10 g/l in unused mineral oil and found to have detectable effects on some performance tests (tan δ and interfacial tension) but without compromising its ability to meet required specifications i.e. IEC 60296.  The detection limit was unaffected when the marker was added to lightly and heavily aged mineral oils.

Recommendations for further work

The results suggest that further evaluation using either technique could be used in a demonstration project where a simulated credible leak might be set up to test whether switch oil markers could be detected in a separate compartment

Lessons Learnt

In this project, FTIR Spectroscopy has been used to detect low concentrations of MIDEL 7131 in mineral oil. A procedure for sample preparation and FTIR measurement has been developed, which shows good repeatability.

The results showed that there is a linear correlation between absorbance of a characteristic peak at a wavenumber of 1748 cm-1 and MIDEL 7131 concentration in a mixture with mineral oil.

As well as serving the purpose intended for this project.  This method may also be used for determining residual mineral oil in ester after a transformer retrofill.

The addition of the chemical tracer to mineral oil was found to affect functional properties, but, up to 10 g/kg, not significantly and not always negatively, the most affected property was interfacial tension, which would limit the amount that could be added.  The addition of 5 g/kg was enough to give the mineral oil a slight odour, a characteristic that could be potentially problematic, but could also be an opportunity for rapid identification – there are electronic olfactory sensory systems (e-Nose) that are portable and sensitive.

Dissemination 

A paper entitled “Determination of Remaining Mineral Oil in Synthetic-Ester Retrofilled Transformers Based on Fourier-Transform Infrared Spectroscopy” has been accepted for presentation at the IEEE 14th International Conference on the Properties and Applications of Dielectric Materials (ICPADM), in Phuket, Thailand in August 2024.  An article for the IEEE Electrical Insulation Magazine is planned on the same topic.

A summary of both workstreams has been proposed for the EuroDoble conference in Manchester in October 2024.