Mineral insulating oils in service
There is precious little basic information openly available on how insulating materials in transformers behave over longer periods of time. However, such information is very important for ensuring that aging equipment works adequately. It is valuable both from the perspective of transformer maintenance/asset management, and as background information necessary for making informed choices on which type of insulating liquid to employ in new equipment.
Keywords: transformer oil, inhibitor, oil oxidation, in-service oil, inhibitor top up
Infrastructure of electricity grids in the hitherto industrialized world was to a large extent built up in the decades following World War II, with a peak in investments around 1960 – 1980. This means that the flotilla of power transformers is now starting to reach projected lifetime of around 40 years. Over the last couple of decades there has also been an ongoing deregulation and privatization of electricity generation and distribution. This has led to higher utilization of existing equipment to meet an ever higher power demand. Demands for planned asset management, investments and reliability of power delivery have also increased. The need to ensure that aging equipment works adequately and will do so for some time yet has generated an increased interest in aging behaviour of power equipment. Yet there is precious little basic information openly available on how insulating materials in transformers behave over longer periods of time. Such information is valuable both from the perspective of transformer maintenance/asset management, and as background information necessary for making informed choices on which type of insulating liquid to employ in new equipment.
In this study there is a total of 145 separate oil samples from separate transformers (all samples appeared around the same time for analysis), 114 of which are inhibited and 31 uninhibited (as defined by IEC 60296). The inhibited oil samples, comparable to ASTM D3487 Type II, came from transformers in the Nordic region in Europe, whereas the uninhibited samples came mainly from the Middle East. The age of transformers ranges from only a few years up to 45 years.
The samples were analysed for acidity (neutralization number), interfacial tension, dielectric loss (DDF or power factor) and peroxide content. The inhibited samples were also analysed for inhibitor content. All of these parameters except peroxide content are standard measurements, but little information exists on how they relate to each other statistically. The peroxide measurements, performed according to a method we have developed, should tell us more about oxidation behaviour and efficiency of oxidation inhibitors.
Monitoring and maintenance of insulating oils in service
Insulating oil in a power transformer accounts for only about 5 % of the initial cost of the complete system (information collected from OEMs), but it is a very vital part both for monitoring of the functioning of the transformer and for the ultimate lifetime under which the investment cost should be regained with interest. Ideally, a high quality insulating oil should not contribute to premature ageing of non-replaceable parts such as winding or solid insulation. Through DGA the oil also serves as an information carrier about what is happening inside the transformer.
However, it is vital to ensure that aging of the oil is not excessive and that it performs adequately. For this reason there are guides to follow to check the oil performance including IEC 60422  and IEEE C57.106 . IEC 60422 is a much more elaborate document with more detail than the corresponding IEEE document, which provides very few direct limiting values for physical and chemical properties.