Oil-Filled Transformer Service Life

In many countries around the world today most of the transformers in operation were commissioned in the end of the previous century. That is why the issue of equipment replacement or lifetime extension is becoming important now. Power generation and distribution facilities must make decisions on the future strategy of improving the reliability of operating their equipment.

The reality is that an oil-filled transformer is certainly not the strongest link in the chain that brings power from the generation facility to the users. At the same time, no moving parts in the transformer makes the potential lifetime of such equipment quite long (several tens of years.

However, operating oil-filled transformers does involve significant repairs and part costs. The amount of required investment grows in case of sudden damage without the possibility for a planned down time.

Therefore, it is logical that preventing such situations by analyzing their probability and identifying possible causes is an important priority.

The modern power transformers rely on oil-impregnated heat resistant cellulose for conductor insulation. If the conditions in the transformer are such that the insulation is dry, contains no gas and is fully immersed in oil, such insulation is the most reliable kind.

It was in the 1920s that one of the most authoritative electrotechnical magazines warned that the worst enemies of transformer insulation are water and increased temperature. Simultaneous impact of these factors can destroy the insulation system. so the limits on these two factors are certainly the right way to extend oil-filled transformer life.

Moisture can also be found in transformers as a result of:

• inefficiency and poor quality of drying the equipment by the manufacturer;
• decomposition of cellulose;
• rejoining with moisture hidden in the oil.

Temperature is increased when the equipment is overloaded. Overheating is also caused by a disruption of the normal convective cooling process due to, for instance, reduced flow of oil. This is usually caused by contamination of cooling system channels or transformer oil leaks.

Extension of oil-filled transformer lifetime can be achieved by the following measures:

• regular planned tests of equipment, including visual control for transformer oil leaks;
• accumulation of statistical data, which helps identify the dependency of temperature on transformer load.

Since in most cases the cellulose insulation decomposes due to heating, the load must be checked from time to time, to prevent it from exceeding the nominal limit.

Measures to reduce moisture content must be taken at the stage of equipment design and manufacturing. Oil leaks must be watched for and repaired during transformer operation and repaired immediately. If the oil is leaking from the tank for one reason or another, it does not only contaminate the environment, but also causes harmful moisture to come in contact with the oil.

If the content of oil in used oil is nearing its limit for some reason, special equipment must be used. The Oil Drying units by GlobeCore are designed to purify electrical insulation oils to remove moisture and particulate matter with zeolite. After processing, the moisture content in the oil does not exceed 10 ppm. Filtration fineness is 5 μm due to the use of two filters.

How to determine that the transformer must be changed?

The following facts indicate the possible need to replace the power transformer:

• systematic overloads of equipment (the transformer must be replaced with one more powerful);
• insulation strength exceeds the original value by more than 4%;
• CO and CO2 gases in DGA samples.

Considering the current new transformer prices, extension of transformer service life is a good strategy for power generation and distribution companies. The industry experts note that transformer age of 20-30 years is quite realistic, and can be achieved by taking the measures described in this article.