transformer aging a matter of concern

8/14/2019 Transformer Aging a Matter of Concern

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Aging Transformer: 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 canmaximize 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 electricaldevices in general, and transformers in particular. For the duration of a transformer's life, the combinationof elevated operating temperature and high ambient temperatures will have a slow degrading effect on itsinsulation. 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 thecellulose 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 agedtransformer gaskets and seals will allow moisture, present in the atmosphere, to penetrate through to theinsulation when the pressure gradient changes. This invading moisture speeds up the transformers agingprocess. Additionally, water vapor is a by-product of the degradation of cellulose insulation. Aginginsulation, itself, contributes moisture to the problem, since dielectric strength diminishes with everyincrease in moisture level.

Moisture and oxygen levels are both temperature dependent, increasing as the temperature rises. Highlevels of moisture and oxygen can lead to the formation of bubbles, which, when trapped withinthe 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. Insulationpower factor increases with increases in moisture content. In order to function reliably, atransformer 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 ofinsulation paper and oil leads to the production of moisture and furan, which can both cause furtheraccelerated aging. Overheating of the insulation system, partial discharge and arcing can all lead to therelease 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,



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temperature and water content. This data can help in detecting the type of fault, its intensity and, to someextent, 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 byincreasing temperatures within the windings. The mechanical failure of aging insulating paper can lead toelectrical breakdown. This in turn may adversely impact the insulation's performance, which can lead totransformer failure. Consequently, the condition of the insulation should be monitored regularly, as ameasure of the state of the transformer as a whole. Insulating paper can be tested directly by measuringthe degree of its polymerization.

Proper Maintenance and Fault Diagnosis

Transformer aging may also accelerate if the transformer does not undergo proper maintenance and faultdiagnosis. Proper fault diagnosis plays a vital role in enhancing the life of a transformer. The percentageof transformer failures caused by dielectric problems may be as high as 75%. Dielectric problems can becaught 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 tothermal stress that changes the mechanical and electrical properties of the material. If transformer faultsare detected at an early stage, it can greatly reduce unplanned outages and the costs that accompanythem. As insulating oils deteriorate, the chance that this deterioration will negatively impact thetransformer increases. Additionally, oil discharges may seriously damage other insulating materials. It istherefore 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. Thedecomposition of insulating oils and cellulose materials leads to the production of fault gases. Thesecombustible gases are produced when insulating oils and cellulose materials are subjected to excesselectrical 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 thegas relay. Therefore, it is important to analyze the oil for dissolved gases. This data allows one todetermine the condition of the transformer, and catch faults early. If a fault is detected, a variety ofanalysis methods can be used to predict the type of fault. Multiple dissolved gas analysis tests should betaken over time so that the rate of increase of fault gases can be monitored, and with that the progress ofthe 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 whichaccompany that, may in turn cut costs. This is possible only with the help of good diagnostic methods andrealistic interpretation of data. The usefulness of an old transformer may be improved if properoperational criteria are implemented and its insulation system is effectively maintained.

transformer aging a matter of concern

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