8/14/2019 PCT - Is It Time to Change That Transformer

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Is it time t change that transform r?

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The USA is the largest power consumer in the world. Unfortunately a large portion of the aging USApower distribution grid was installed prior to 1990. That being said, a large installed base of transformersare nearing the end of their useful life expectancy and change-out or replacement should be part ofevery owners strategy for system reliability.

Modern liquid filled transformers are a critical link in the energy delivery chain between power producers

and end users. These transformers have no moving parts and they convert power with efficiencies thatexceed 99%, thereby leading to their useful life being measured in decades not years. Considerablecosts, however, can be associated with transformer failures, especially if such failures happen withoutwarning and no action for a planned outage can be taken.

How do you know if your transformer is in eminent danger and how did it get there? Lets answer thesecond question first.

Modern liquid filled transformers utilize a combination of oil impregnated, thermally upgraded cellulose forconductor insulation, insulation between layers, insulation between coils, and insulation between currentcarrying parts and ground within the magnetic circuit. When cellulose is dry, free from gas, and immersedin oil, its the toughest physical insulation system available. It is, however, the weakest link in thetransformer insulation system. This is not a new discovery. The Electric Journal of April 1920 states that

the arch enemies of solid insulation are moisture and heat.

If asked, a chemist would name moisture as the biggest threat. If you ask an electrical engineers thesame question, they would respond that heat is the single largest threat. Both answers would be correct.Moisture in combination with heat will destroy an insulation system. Limiting moisture and excessiveheating are the keys to getting the longest service life from your transformer.

Moisture in the solid insulation can come from three sources: 1) residual moisture from inadequate dryingduring manufacture; 2) as a by product of cellulose decomposition; 3) and recombining with latentmoisture in the oil.

Heat, on the other hand, comes largely from loading the transformer beyond its designed rating. Othercontributors can include debris within the transformer blocking oil cooling ducts, blocked cooling radiator

openings which restrict flow or oil leaks which lower oil to a level below the radiator openings thuseffectively stopping the normal convective cooling process.

Preventative steps that can be taken to extend the life of your transformer should include:

Regular scheduled maintenance that includes a visual check for oil leaks

Recording temperature readings and noting the maximum temperature indicated by the draghand on the top oil thermometer.

Since 90% of cellulose deterioration is thermal in origin, periodic inventory of connected loadsshould be conducted to ensure recent expansions have not added loads that exceed thetransformers designed capability. Evidence indicates that overloading beyond a hottest spottemperature of 140 C causes formation of gas bubbles which in turn lower dielectric withstand

and will precipitate a flash over and premature failure. If you rely on additional cooling fans or oil pumps to extend the transformer rating, be sure they

are operating as needed to limit insulation temperatures.

Limiting moisture starts during the design and manufacturing of the transformer.

Manufacturers should design to eliminate standing water around gaskets, preventing the egressof moisture via gasket absorption.

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Oil leaks should be aggressively prevented, as such leaks not only allow oil to escape andcontaminate the environment, they also provide a path for harmful moisture to enter thetransformer and migrate into the insulation. The same is true for ruptured diaphragm typepressure relief devices, or less frequently, the mis-opertation of self-resealing type pressure reliefdevices.

Manufacturers should utilize a combination of heat and vacuum to reduce the residual moisture

from the insulation during the assembly process. Insulation that has been exposed to ambientconditions during manufacture, and subsequently not dried, can have as much as 10% by weightmoisture content. Insulating paper with 1% moisture content ages 10 times faster than one withonly 0.1% moisture content by weight. Consensus opinion says that the paper insulation of a newtransformer leaving the factory should contain between 0.3% and 1% moisture content by weight.Therefore, it is necessary to use dry heat in the rage of 100C in concert with vacuum levels ofbetween 1 to 3 torr to achieve less than 1% moisture in the paper before impregnation withinsulating oil.

How do you know if your transformer is a candidate for replacement? If your records show that you havebeen systematically overloading your transformers, you should replace them with ones of greatercapacity. Continuing down that path will only result in a thermally caused failure, and quitepossibly at the most inconvenient time.

If you have not included oil sampling in your routine maintenance program, you should begin by taking anoil sample and having it analyzed for moisture content. This, combined with an insulation resistance testor an insulation power factor test, will give you an indication of insulation dryness. Insulation powerfactors of older transformers should not exceed 4%, since at that threshold it is highly likely wet insulationis the cause.

Secondly you should take an oil sample for dissolved gas analysis (DGA). When oil immersed cellulose issubjected to heat, the cellulose deteriorates, resulting in formation of water, acids, carbon dioxide andcarbon monoxide. Your sample might give indications of several gases which could indicate otherproblems not to be overlooked, but the presence of CO and or CO will indicate your transformersinsulation has been overheated and possibly compromised.

Extending the useful life of a transformer is the single most important strategy for enhancing the reliabilityof a companys power distribution infrastructure. Power transformers are expected to be long-livedequipment. Twenty to thirty years is a fairly common transformer age. If a unit has been subjected tosevere duty, including numerous faults, periods of overloading, and leaks that allow moisture egress, thelifespan could be considerably shorter. It goes without saying that replacing a transformer can be anexpensive undertaking, which is made only more expensive if the transformer should fail unexpectedlyand the replacement must be made during an unscheduled and costly outage. Taking the simplemeasures suggested can ensure the continued operation of your transformer for years to come.