It is crucial to analyse transformer oil. Along with the knowledge of the insulation fluid’s current condition, analysis can also reveal other issues like partial discharges and premature cellulose insulation ageing, among others. In this blog, we’ll take a close look at the different areas of analysis and why they are so critical.
Moisture content
Transformer oils perform multiple roles. However, it is argued that electrical insulation is of the highest importance. Should the water content in the transformer oil exceed a specific amount, the performance of the insulation will be degraded, and a breakthrough occurs.
When moisture enters the oil due to rapid temperature changes
Moisture can enter the oil because of rapid changes in temperature, which will degrade the oil as well as the cellulose, which will age even faster.
Acid number
When it comes to oxidation, transformer oils are not dissimilar to other types of mineral oil. Oxygen in the air and temperature are both factors that accelerate oxidation. Another adverse factor is when metal particles are present. This acts as a catalyst for carboxyl acid formation, increasing the transformer oil’s acidity. Over time, sludge and sediment will form, and block the flow of the transformer oil, which impedes heat dissipation. Additionally, high acidity causes cellulose insulation to deteriorate. As a result, when the acid number is higher, the lower the oil’s dielectric strength and the higher that its moisture content will be.
Dielectric strength
A transformer oil’s dielectric strength is the highest voltage that the oil can take without a breakthrough resulting. Since electrical insulation is a key function of transformer oil, reduction of its dielectric strength can be a critical issue.
Furan compounds
These compounds are analysed to reveal the how much insulation has aged. This specific test shows the extent of paper polymerisation. Samples can, in theory, measure these levels, however in practice this can be a complicated process.
Furan compounds form when cellulose’s polymer structure decays. As a result, their content can show the extent of insulation aging.
Reclaiming transformer oil
When transformer oil is analysed and found to be no longer fit for purpose, it can be refreshed with new dielectric fluid. However, oil products are not inexpensive and draining oil from a transformer will not remove all oil decay products and contaminants from the transformer. As a result, they will mix with the fresh fluid, causing it to age far faster. To avoid this issue, oils should be reconditioned and reclaimed instead of constantly purchasing new transformer oil.
