Transformer Life

Power transformers are generally designed for an operating life of 30 to 35 years. However, depending on how the transformer is operated, maintained and exposed to external risk these values may be less. Over the last 20 years trends have indicated that the transformer life has been much lower than expected. Even within Eskom, from statistics since the early 1990’s it was found that the average life of the larger GSU transformers was around 15 years.

A major contribution to transformer failure is windings, leads, tap changer and bushings. These failures are usually catastrophic to the transformer and in most cases the transformer has to be either rewound or replaced.

The two major components of the transformer that have the faster degradation rate are the oil and paper insulation. These components degrade as a result of thermal, electrical and chemical stress. Paper insulation degrades very early with elevated operating temperatures. Chemical stress usually takes a long time to affect transformer health and predominantly affects the oil. Electrical stress is the most destructive and with rapid degrading mechanisms on both oil and paper insulation.

It was clear from the literature thus far that the key components to focus on when assessing transformer health are the oil and cellulose/paper insulation under chemical, thermal and electrical stress. 

The key aspect of transformer winding construction is the insulation of each conductor with paper that is impregnated with insulating oil, providing at least a minimum life of 25 years at an operating temperature of 65-95 °C [Emsley2]. The end of life of a transformer occurs when the mechanical strength of the solid insulation in the windings was lost and failures are then triggered by severe conditions like lightning strikes, switching transients and short-circuits [Emsley2, Kachler1, Wang1]. Wang et al. highlighted that transformer failures can be categorized as electrical, mechanical or thermal where the cause of failure being either internal or external [Wang1].

Generator and furnace transformers exhibit accelerated aging in comparison to grid transformers due to the high loading factors [Kachler1]. Generator transformers are often loaded to full rating, which can increase the aging rate due to higher winding temperatures causing faster rates of insulation paper de-polymerisation [Checksfield1].

Insulating materials age thermally and from chemical reactions occurring within the materials caused by pyrolysis, oxidation and hydrolysis, which are accelerated by increased levels of temperature, oxygen and moisture content [McNutt1, Wang1].

An unusual gassing state defined as ‘stray gassing’ was identified as a phenomenon where some types of insulating oils produced hydrogen and hydrocarbons when heated at low temperatures of around 100°C to 120°C. However, the gas formation subsides to plateau off and this gas level can be removed by filtering the oil [Hohlein1].

External faults easily increase the short circuit forces to dangerous proportions such that any hidden defects and aged insulation causes inter-turn shorting [Wang2]. Internal faults frequently occur on one phase of the HV winding and can be formed either by direct contact or by an electric arc [Jablonski1]. With system growth, transformer loading increases causing the operating stress to increase. This causes the conductor insulation to weaken in an aging transformer to the point where it can no longer sustain mechanical stresses of a fault. Dielectric failure of turn-to-turn insulation causes loosening of winding clamping pressure, which reduces the transformer's ability to withstand future short circuit forces [Bartley1].

References

[Bartley1]	Bartley, W. H., “Analysis of International Transformer Failures,” International Association of Engineering Insurers 36th Annual Conference, Stockholm, 2003
[Checksfield1]	Checksfield, M., Westlake, A., “Experiences with Operating and Monitoring Generator Transformers,” IEE Colloquium on IET, Transformer Life Management (Ref. No. 1998/510), pages 4/1-7, 22 October 1998
[Emsley2]	Emsley, A. M., Stevens, G. C., “Review of Chemical Indicators of Degradation of Cellulosic     Electric Paper Insulation in Oil-filled Transformers,” IEE Proceedings - Science, Measurement and Technology, Vol. 141, No. 5, pages 324-334, 1994
[Hohlein1]	Hohlein, I., “Unusual Cases of Gassing in Transformers in Service,” IEEE Electrical Insulation Magazine, Vol. 22, No. 1, pages 24-27, 2006
[Jablonski1]	Jablonski, M., Napieralska-Juszczak, E., “Internal Faults in Power Transformers,” IET Electrical Power Applications, Vol. 1, No. 1, pages 105-111, 2007
[Kachler1]	Kachler, A. J., Hohlein, I., “Ageing of Cellulose at Transformer Service Temperatures. Part1: Influence of Type of Oil and Air on the Degree of Polymerisation of Pressboard, Dissolved Gases and Furanic Compounds in oil,” IEEE Electrical Insulation Magazine, Vol. 21, No. 2, pages 15-21, 2005
[McNutt1]	McNutt, W. J., “Insulation Thermal Life Considerations for Transformer Loading Guides,” IEEE Transactions on Power Delivery, Vol. 7, No. 1, pages 392-401, January 1992
[Wang1]	Wang, H., Butler, K. L., “Modeling Transformers with Internal Incipient Faults,” IEEE Transactions on Power Delivery, Vol. 17, No. 2, pages 500-509, April 2002