Why is Moisture The Enemy?

Transformers are one of the most important capital assets that a Utility can own. Maintaining a healthy transformer is a core practice of any maintenance program. There are many variables that affect the life expectancy of the transformer. The most important of all being the moisture content in the solid and the liquid insulation. Moisture in the transformer is the enemy and its presence is directly related to the dielectric strength of the oil and paper.

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 Excessive moisture has a direct impact on the age reduction of the transformer. Existing technical papers and authorities around the world have linked the life of the transformer directly to the life of the solid insulation. Any attempt to protect the solid insulation will increase life and slow the aging process.
At DryTrans, we believe in a healthy, reliable and well-maintained network. The goal is to optimize the performance of transformers, extend life, protect the environment, and create a safe and efficient workplace.

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Understanding Moisture

Moisture in oil is measured in parts per million (ppm) using the weight of moisture divided by the weight of oil. Water can be present in oil in a dissolved form, as tiny droplets mixed with the oil (emulsion), or in a free state at the bottom of the tank holding the oil. Demulsification occurs when the tiny droplets unite to form larger drops which sink to the bottom and form free water. When the moisture in oil exceeds the saturation value, there will also be free water precipitated from the oil in suspension or drops. In general, the moisture content in the oil lowers the insulating system dielectric strength and allows flashover that can damage a transformer. Moisture can also accelerate paper-insulation aging rates that will reduce the expected useful life of the equipment for mineral oil, a generally accepted maximum moisture content is 30 ppm.

Moisture Testing

This test method covers the measurement of water present in insulating liquids by coulometric Karl Fischer titration.
 This test method is used commonly for test specimens below 100 % relative saturation of water in oil. The coulometric test method is known for its high degree of sensitivity (typically 10 μg H2O). This test method requires the use of equipment specifically designed for coulometric titration.
 Types of Tests conducted:

• Online Oil Filtration system (ONLINE)
• Karl Fisher Titration ASTM D1533 Method (OFFLINE)
• Scheduled maintenance Oil change (OFFLINE)

Effects of Moisture

The effect of moisture on insulation aging is well explained in various published papers and documented along with the damaging effect on insulation strength and partial discharge initiation level.
 
It has also been shown that at high temperatures, the residual moisture in winding insulation can trigger the release of free gas bubbles, thus creating an immediate threat to the dielectric integrity of the insulation structure. 
 
Deterioration in transformer cellulose decreases both its electrical and mechanical strength. In general, the greater the water content the more the mechanical strength reduces. Mineral oil has a very limited capacity to absorb moisture.
(Table Source: www.electricaltesttech.com)
The rate of paper ageing is directly related to the water content. Various studies have shown that the lifetime of the paper reduces by as much as a factor of ten for each extra 1% of water content in the cellulose. As the cellulose ages, it releases water, thus accelerating the ageing process. 

Most of the water produced during aging remains in the windings, reducing the insulation resistance of the transformer. It also reduces the transformer’s ability to withstand the mechanical and electrical stresses that occur in operation.

Over time this will result in outages and/or the necessary down rating of the transformer and ultimately could result in complete failure.

“Moisture will decrease dielectric strength, accelerate cellulose aging and cause the emission of gaseous bubbles at high temperatures”.

Where does Moisture come from?

  • Moisture accumulates in the transformer over the years of its operation.
  • There are typically three sources of moisture for a transformer

            1) New transformers are meant to carry some moisture in the paper insulation.

            2) From the atmosphere via the conservator and through the breather in free-breathing transformers.

            3) Moisture is created due to the breakdown of the solid insulation during normal loading and operating conditions.

Damages caused by Moisture

Moisture in oil reduces the insulating ability (BDV) of the oil

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During high load and high ambient temperatures, oil draws out the moisture from the paper that may decrease the BDV of the oil and cause dielectric breakdown.
  • With sudden high loads, water can evolve out of the conductor surfaces and the vapour bubbles can rise to the contacts and result in a failure.
  • During sudden cooler or low-load periods, the relative saturation of oil will increase greatly reducing the dielectric strength of the oil.

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Moisture in paper causes the following destructive effects

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Moisture and oxygen cause paper insulation to decay much faster and to form acids, metal soaps, sludge, and more moisture
  • Sludge settles on windings, causing reduced efficiency on transformer cooling. Acids increase the rate of decay, which forms more acid, sludge, and moisture at a faster rate.
  • Expansion of the paper insulation, altering the mechanical pressure of the transformer clamping system.
  • Decrease in Dielectric Breakdown strength of the oil.

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DryTrans is a brand of moisture management systems under the AVACO banner. It is a result of two decades of industry experience. AVACO Solutions is based out of SHARJAH, U.A.E. DryTrans is designed and manufactured in the United Arab Emirates.

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