Transformer Liquid Dielectric Withstand Performance

Liquid-Filled Transformers – A Crucial Part of Safe and Efficient Power Supply Norway has around 3,000 large transformers, each filled with 5-80 tonnes of electrical insulating liquid. These are essential for delivering electricity from producers to consumers. Transformer liquids must withstand high voltage to avoid electrical short circuits, which can lead to explosions, fires, and damage. Mineral oil is most used. It is reliable and well-tested but comes from non-renewable sources. The advantage of new liquids, such as esters and isoparaffins, is that they are more sustainable and fire-resistant, and may offer better cooling. However, mineral oils have been shown to withstand short-term lightning surges better than the new liquids. The most important property of an insulating liquid is its ability to withstand high electrical stress, which is not well-tested in the new liquids. The LiPe project will improve understanding of what electrical stress the liquids can withstand and improve standardised test methods to characterise the functional properties of insulating liquids. Molecular properties of the liquids, such as the energy required to release an electron, affect how an electrical short circuit occurs. This can be determined through quantum chemical calculations and can be used to model electrical conductivity at high electric fields. New measurements show a clear difference between the high-field conductivity of mineral oils and the new liquids. This has not been considered previously but may indicate why they behave differently and why mineral oils withstand short-term surges better. By calculating molecular properties, measuring and modelling conductivity, and conducting breakdown experiments, LiPe will increase the understanding of the underlying mechanisms so that different types of insulating liquids can be compared. This will make it easier to design safer transformers, and users will be more confident in choosing new, more sustainable liquids.

Considerable investments and reinvestments are needed in the power grid the coming years, all of which must be environmentally sound and highly reliable. Liquid-insulated transformers are keystones of the electrical power system. Ensuring safe transformer operation is essential for electrification, access, and security of supply of energy. There is now a large influx of new insulating liquids offered to the transformer market that have advantageous properties relating to biodegradability, reduced fire risk, and cooling properties compared to the conventional mineral oils. However, there are challenges in assessing the most crucial property of a dielectric liquid – its function as an electrical insulant, to resist an electrical breakdown and destruction of the transformer. The lack of proper test methods and standards for docu¬men¬ting their electric withstand voltage leaves industry with problems in adjusting their designs and end users with uncertainty in using new liquids. LiPe (pron. leap) will enhance the safety and reliability of power transformers by developing physical models and test methods for evaluating the dielectric performance of insulating liquids. The properties of commercial and neat liquids and additives will be investigated through atomistic modelling, high-field conductivity measurements and modelling. The effect of x-ray irradiation on breakdown inception will test the importance of free electrons to form avalanches. An experimental set-up will be developed as a tool to test a liquids dielectric performance in a short gap in combination with multivariate analysis. Large scale breakdown experiments will be performed to support results from modelling and small scale, on a transformer-relevant scale. The measurements, modelling, and experimental results will facilitate basic understanding and new and better test standards that can mitigate risks and uncertainties to promote the safe use of insulating liquids in new and old power transformers.