Aging Transformers: A Matter Of Concern

Aging Transformers: A Matter Of Concern

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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.

Insulating Paper
The mechanical properties of insulating paper are greatly reduced as it ages, although electrical properties may not show significant change. Insulating paper's mechanical strength can be degraded by increasing temperatures within the windings. The mechanical failure of aging insulating paper can lead to electrical breakdown. This in turn may adversely impact the insulation's performance, which can lead to transformer failure. Consequently, the condition of the insulation should be monitored regularly, as a measure of the state of the transformer as a whole. Insulating paper can be tested directly by measuring the degree of its polymerization.

Proper Maintenance and Fault Diagnosis
Transformer aging may also accelerate if the transformer does not undergo proper maintenance and fault diagnosis. Proper fault diagnosis plays a vital role in enhancing the life of a transformer. The percentage of transformer failures caused by dielectric problems may be as high as 75%. Dielectric problems can be caught by testing for the presence of furanic compounds in the oil, which are an indication of solid dielectric deterioration.

The dielectric temperature of an oil/cellulose insulation system can impact the aging process, leading to thermal stress that changes the mechanical and electrical properties of the material. If transformer faults are detected at an early stage, it can greatly reduce unplanned outages and the costs that accompany them. As insulating oils deteriorate, the chance that this deterioration will negatively impact the transformer increases. Additionally, oil discharges may seriously damage other insulating materials. It is therefore important to regularly monitor the transformer's insulation.

Dissolved Gas Analysis
One way to deduce the type and severity of a transformer fault is through dissolved gas analysis. The decomposition of insulating oils and cellulose materials leads to the production of fault gases. These combustible gases are produced when insulating oils and cellulose materials are subjected to excess electrical or thermal stresses. As a fault slowly evolves these gases will build up, dissolved within the oil. Initial gas quantities are very small, and it takes time for free gas to migrate to, and accumulate, in the gas relay. Therefore, it is important to analyze the oil for dissolved gases. This data allows one to determine the condition of the transformer, and catch faults early. If a fault is detected, a variety of analysis methods can be used to predict the type of fault. Multiple dissolved gas analysis tests should be taken over time so that the rate of increase of fault gases can be monitored, and with that the progress of the fault.

Life Extensions
If a transformer is assessed with regularity and thoroughness, its aging process may be controlled and its life extended. An extended life for the transformer, along with the increased safety and reliability which accompany that, may in turn cut costs. This is possible only with the help of good diagnostic methods and realistic interpretation of data. The usefulness of an old transformer may be improved if proper operational criteria are implemented and its insulation system is effectively maintained.

Insulating Paper
The mechanical properties of insulating paper are greatly reduced as it ages, although electrical properties may not show significant change. Insulating paper's mechanical strength can be degraded by increasing temperatures within the windings. The mechanical failure of aging insulating paper can lead to electrical breakdown. This in turn may adversely impact the insulation's performance, which can lead to transformer failure. Consequently, the condition of the insulation should be monitored regularly, as a measure of the state of the transformer as a whole. Insulating paper can be tested directly by measuring the degree of its polymerization.

Proper Maintenance and Fault Diagnosis
Transformer aging may also accelerate if the transformer does not undergo proper maintenance and fault diagnosis. Proper fault diagnosis plays a vital role in enhancing the life of a transformer. The percentage of transformer failures caused by dielectric problems may be as high as 75%. Dielectric problems can be caught by testing for the presence of furanic compounds in the oil, which are an indication of solid dielectric deterioration.

The dielectric temperature of an oil/cellulose insulation system can impact the aging process, leading to thermal stress that changes the mechanical and electrical properties of the material. If transformer faults are detected at an early stage, it can greatly reduce unplanned outages and the costs that accompany them. As insulating oils deteriorate, the chance that this deterioration will negatively impact the transformer increases. Additionally, oil discharges may seriously damage other insulating materials. It is therefore important to regularly monitor the transformer's insulation.

Dissolved Gas Analysis
One way to deduce the type and severity of a transformer fault is through dissolved gas analysis. The decomposition of insulating oils and cellulose materials leads to the production of fault gases. These combustible gases are produced when insulating oils and cellulose materials are subjected to excess electrical or thermal stresses. As a fault slowly evolves these gases will build up, dissolved within the oil. Initial gas quantities are very small, and it takes time for free gas to migrate to, and accumulate, in the gas relay. Therefore, it is important to analyze the oil for dissolved gases. This data allows one to determine the condition of the transformer, and catch faults early. If a fault is detected, a variety of analysis methods can be used to predict the type of fault. Multiple dissolved gas analysis tests should be taken over time so that the rate of increase of fault gases can be monitored, and with that the progress of the fault.

Life Extensions
If a transformer is assessed with regularity and thoroughness, its aging process may be controlled and its life extended. An extended life for the transformer, along with the increased safety and reliability which accompany that, may in turn cut costs. This is possible only with the help of good diagnostic methods and realistic interpretation of data. The usefulness of an old transformer may be improved if proper operational criteria are implemented and its insulation system is effectively maintained.


About the Author:

To know more about Liquid filled

distribution transformers check out Pacific crest transformers website.

http://www.pacificcresttrans.com/home.html



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