Condition Monitoring Maintenance Preventive Maintenance Reliability

Drive Moisture From Your Transformers

EP Editorial Staff | July 1, 2021

Moisture dissolved in oil is relatively minor compared with what is bound in transformer cellulose insulation.

By Bob Rasor, PE, and Kyle Johannes, SDMyers LLC

If one is familiar with power transformers, the presence of elevated moisture will naturally bring caution to mind. Proven over decades, moisture in a liquid-filled transformer can cause issues that result in irreversible damage to the paper insulation. The common problem is oxidation but, in extreme cases, arcing and flashovers can occur, leading to dielectric failure.

Some experts have estimated that, when a moisture problem exists, approximately a year of reliable service is lost for every year the repair is delayed. This is supported by the long-accepted fact that doubling the moisture content in a transformer can cut its life in half. This is especially important because, once oxidation damage occurs to the insulation, it is irreversible.

In a recent survey, 9.7% of 85,210 in-service transformers tested had unacceptable moisture levels (defined as 18% or greater relative saturation). This demonstrates that, as the moisture content rises, oxidation and acid levels increase and the life of the transformer decreases, as indicated by furan analysis. The average lost life of those units deemed unacceptable was nearly 24%, compared to transformers with acceptable moisture levels that, on average, had 2% lost life.

Additionally, 2% of the transformers with unacceptable moisture levels exceeded 50% relative saturation. Close to half of this 2% had furans tested and it was found that 15% were at the end of their reliable life (30% or less life remaining).

Moisture intrusion

Moisture in a transformer is primarily absorbed by the solid insulation, but it can exist in several other forms. These can include dissolved water in the oil, free water suspended as droplets in the oil (sometimes at the bottom of the tank), or emulsified water contained in the decay products of oil oxidation.

Other means of moisture entry include:

• aged/leaking gaskets for bushings, radiator flanges, covers, pressure-relief devices, lids, and gauges

• moisture that is chemically bound to cellulose in paper insulation, released as the insulation ages

• improper drying of transformers during manufacturing can cause issues at the outset. Note that this is rare.

It is important to ensure that oil samples are taken accurately and represent the true moisture levels in a transformer. When sampling a transformer, the oil inside the pipe and valve spaces can contain disproportionately more moisture than the transformer itself, due to accumulating condensation. To account for this issue, and to eliminate the possibility of an erroneous reading, it’s important to adequately flush the valve before obtaining the sample.

Moisture locations

To maximize the reliable life of a transformer, moisture must be removed from the oil and the paper insulation. This is a more involved task because moisture in the solid insulation is not removed during standard oil-cleaning processes. The solid insulation, which consists of paper on the coils, cardboard, pressboard, and the wood structure, holds as much as 100 times more moisture than the oil.

Dehydrating the oil is only temporary. Within weeks of an oil-dehydration procedure, moisture content will become unacceptable again, as moisture trapped in the solid insulation migrates into the oil. Dehydrating the oil doesn’t remove the moisture in the cellulose unless the moisture was within the top surface layer.

Although there are several methods for dehydrating transformer oil, the goal of drying the insulation is much more difficult.

Moisture levels in oil

After an oil sample is taken, a Karl Fischer (KF) test is performed to determine the parts per million of moisture in the oil. Temperature is an important factor and will cause variations in the results. For this example, we’ll use a 1,000-gal. transformer and an oil sample at 42 C. After the KF test was performed, the result was 30 ppm of moisture in the oil. The calculation below allows us to determine the weight of water in the oil:

(30 ppm/1,000,000) x 1,000-gal. x 7.34 lb./gal. of oil = 0.22 lb. of water or 3.4 fl. oz.

Moisture levels in paper

On average, a 1,000-gal. transformer might have 1,170 lb. of solid insulation. Using 30 ppm and one of several industry algorithms that estimate the solid insulation dryness from an oil sample, the moisture by dry weight (M/DW) can be estimated at about 2%. The total moisture in the insulation would then simply be 2% of 1,170 or 23.4 lb.

At this moisture level, there is 106 times (23.4 lb./0.22 lb.) more moisture in the paper than in the oil. Therefore, moisture must be removed from the oil and the solid insulation to ensure the transformer delivers its maximum reliable life.

Moisture reduction

Depending upon the voltage class, most power transformers are delivered at 0.5% M/DW from the factory. Retaining that level is ideal. For our example transformer with 2% M/DW, the moisture that needs to be removed from the paper would be 1.5%, if the desired level is 0.5%. In the example, 1,170 lb. of solid insulation, multiplied by 0.015 equates to 17.5 lb. of water that needs to be removed. Remember, dehydrating the transformer oil alone would only remove 0.22 lb. of water.

Drying methods

There are three main methods for drying the insulation:

• An effective, but less common, technique is the factory dry-out. This involves draining the transformer, uninstalling it, and transporting it to a facility to perform several drying techniques, including bake-outs, heat and vacuum applications, and vapor-phase drying. Downtime is a significant drawback.

• The field vacuum dry-out method uses equipment that applies vacuum and heat to a drained transformer. Because this is done onsite, there is less cost in shipping and packaging. The downtime required, however, is still significant. Most important, this method requires that the transformer be rated for full vacuum, so many 34-kV class and smaller transformers are not eligible.

• The most cost-effective solution is to use an online dryer. This is a longer-term solution that involves installation of drying-column equipment onto the transformer. Oil is passed through drying media that filters out the moisture over the course of several months. This solution is performed on site and often installed with the transformer energized. Moisture levels are monitored until they reach an acceptable level. The time it takes to reach this acceptable level is related directly to the initial oil quality, the moisture levels, and the temperatures involved.

For most applications, using an online dryer allows you to maintain power during the drying process and eliminates the costs associated with downtime and transportation logistics. Moisture can prematurely end the reliable life of a transformer but mitigating this issue early and consistently can be straightforward and relatively inexpensive when compared to the alternative. EP

Bob Rasor, PE, is Technical Director for SDMyers LLC, Talmadge, OH ( and Director of Transformer Services. He has more than 40 years of industry service and is also currently active in several IEEE Working Groups including chairing a new moisture-guide development task force.

Kyle Johannes is a Product Manager at SDMyers, responsible for transforming ideas into products, services, and customer experiences to improve the reliability of electrical equipment and decrease failure modes. His focus is on filtering and safety equipment.



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