Is it time to change that transformer?

The USA is the largest power consumer in the world. Unfortunately a large portion of the aging USA power distribution grid was installed prior to 1990. That being said, a large installed base of transformers are nearing the end of their useful life expectancy and change-out or replacement should be part of every owner's 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 that exceed 99%, thereby leading to their useful life being measured in decades not years. Considerable costs, however, can be associated with transformer failures, especially if such failures happen without warning 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? Let's answer the second question first.

Modern liquid filled transformers utilize a combination of oil impregnated, thermally upgraded cellulose or conductor insulation, insulation between layers, insulation between coils, and insulation between current carrying parts and ground within the magnetic circuit. When cellulose is dry, free from gas, and immersed in oil, it's the toughest physical insulation system available. It is, however, the weakest link in the transformer 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 the same 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 excessive heating 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 drying during manufacture; 2) as a by product of cellulose decomposition; 3) and recombining with latent moisture in the oil.

Heat, on the other hand, comes largely from loading the transformer beyond it's designed rating. Other contributors can include debris within the transformer blocking oil cooling ducts, blocked cooling radiator openings which restricts flow, or oil leaks which lower oil to a level below the radiator openings thus effectively 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 drag hand on the top oil thermometer.
• Since 90% of cellulose deterioration is thermal in origin, periodic inventory of connected loads should be conducted to ensure recent expansions have not added loads that exceed the transformer's designed capability. Evidence indicates that overloading beyond a hottest spot temperature of 140 ºC 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 egress of moisture via gasket absorption.
• Oil leaks should be aggressively prevented, as such leaks not only allow oil to escape and contaminate the environment, they also provide a path for harmful moisture to enter the transformer and migrate into the insulation. The same is true for ruptured diaphragm type pressure relief devices, or less frequently, the mis-opertation of self-resealing type pressure relief devices.
• 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 ambient conditions during manufacture, and subsequently not dried, can have as much as 10% by weight moisture content. Insulating paper with 1% moisture content ages 10 times faster than one with only 0.1% moisture content by weight. Consensus opinion says that the paper insulation of a new transformer 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 100ºC in concert with vacuum levels of between 1 to 3 torr to achieve less than 1% moisture in the paper before impregnation with insulating oil.

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

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

Secondly you should take an oil sample for dissolved gas analysis (DGA). When oil immersed cellulose is subjected to heat, the cellulose deteriorates, resulting in formation of water, acids, carbon dioxide and carbon monoxide. Your sample might give indications of several gases which could indicate other problems not to be overlooked, but the presence of CO² and or CO will indicate your transformer's insulation has been overheated and possibly compromised.

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