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It's a challenge that is now common to many utilities in the US: managing aging substation transformers installed in the 1960s and 1970s and fast approaching the end of their 'life'. These transformers didn't cause a blip in the radar during the last two decades, but with every year of the 21st century, their failure rates have become increasingly difficult to predict. This means that resource allocation and repair/replacement decisions are also becoming more and more exigent.
Transformer Aging Factors
The main factors responsible for transformer aging are given underneath. Controlling these variables can maximize the life of a transformer:
- Temperature
- Oxygen
- Moisture
Other factors can include extreme operational conditions, and adverse conditions within its surroundings (such as a high temperature and humidity index), through faults and electrical surges.
Degrading Insulation
The cumulative effect of elevated temperature over time will adversely affect the useful life of electrical devices in general, and transformers in particular. For the duration of a transformer's life, the combination of elevated operating temperature and high ambient temperatures will have a slow degrading effect on its insulation. Insulation degradation can ultimately lead to catastrophic failures in the transformer.
Moisture in a transformer's insulation system can cause molecular chains to decompose, speed up the cellulose aging process and adversely affect the tensile and dielectric properties of the insulation.
One source of moisture is from humidity in the ambient air surrounding the transformer. Improper or aged transformer gaskets and seals will allow moisture, present in the atmosphere, to penetrate through to the insulation when the pressure gradient changes. This invading moisture speeds up the transformers aging process. Additionally, water vapor is a by-product of the degradation of cellulose insulation. Aging insulation, itself, contributes moisture to the problem, since dielectric strength diminishes with every increase in moisture level.
Moisture and oxygen levels are both temperature dependent, increasing as the temperature rises. High levels of moisture and oxygen can lead to the formation of bubbles, which, when trapped within the insulating materials can cause voids and localized stress, leading to flashovers and failures. Water present in the insulation can also impact the insulation's dielectric properties. Insulation power factor increases with increases in moisture content. In order to function reliably, a transformer must stay within acceptable moisture limits, which vary with load and temperature. The moisture content of an oil sample is normally measured with the Karal Fischer reaction test. This has been adopted by the industry as a standard test due to its high selectivity, sensitivity, repeatability and reliability.
The Importance of Constant Monitoring
The physical parameters and behavior of an insulation system change as it degrades. The degradation of insulation paper and oil leads to the production of moisture and furan, which can both cause further accelerated aging. Overheating of the insulation system, partial discharge and arcing can all lead to the release of gases. Moisture within the insulation chain can help lead to its degradation and failure. Temperature can have an effect on moisture content, and how it moves between the cellulose and the oil. One way to minimize damage in an aging transformer is through constant monitoring of fault gases, temperature and water content. This data can help in detecting the type of fault, its intensity and, to some extent, its location.
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