Field drying of transformer insulation

Author: Lawrence Kirchner


The reason why it is important to maintain a low level of moisture in transformer cellulose insulation has been discussed in previous articles of this and other publications. Primarily, insulation ages as a function of percent moisture and temperature. The mechanical and electrical properties of the insulation decline as the paper ages. Think of the insulation system as two chains supporting a heavy load as illustrated below.

A transformer’s life is determined by the health of the insulation system. If one tiny piece of the insulation in a critical location fails, electrically or mechanically, the transformer is potentially doomed.


The estimated life of a transformer is calculated to be approx. 20.5 years (per IEEE C.57.91) minimum when operated as follows:

  • 100% of rated load continuously

  • 110°C insulation temperature (65 rise + 15 hottest spot + 30 ambient)

  • Moisture content in insulation regulated to approximately 0.5% over life.

We all know that continuous operating conditions are rarely this consistent or severe. In fact, unless the unit is connected to a generator in a desert environment, the conditions are likely to be much less severe. This explains why transformers are often reported to still be in operation after 60 years or more of continuous operation.


As insulation ages, the length of the paper insulation glucose molecule chain breaks down. This aging effect can be measured by performing adegree of polymerization (DP) test. New insulation has a DP value of 1200. Insulation that has reached its end of life has a value of 200. Aged insulation becomes brittle and dark in color. The effects of aging cannot be reversed, but the aging process can be slowed. The three major components that are responsible for insulation aging are moisture, oxygen, and heat. It is the responsibility of the transformer owner to regulate these three components over the life of the transformer in order to get as much life out of the transformer as possible. The temperature can be controlled by maintaining the cooling system and regulating the load. Oxygen can be controlled by maintaining the oil preservation system. Assuming that the transformer insulation moisture level was 0.5% maximum as suggested by most manufacturers when it was installed, how can the insulation level increase over time?

  • Water is a byproduct of insulation aging.

  • Leaking gaskets.

  • Absorbed during transformer open time.

Best industry practice is to regasket the unit approximately every 12-15 years. This is an ideal time to remove the insulation moisture in excess of 0.5% to effectivelyresetand slow the aging process. It is common and acceptable to allow an insulation moisture content of 0.65% for older transformers.


It should now be clear why it is critical to maintain a low moisture level in your transformer insulation. Let us now discuss the theory around how this is achieved in a field environment.


There is a property of all materials known as hygroscopicity. This refers to the materials ability or willingness to hold moisture. Inside the transformer, the paper naturally tends to attract water more so than any of the other materials including the surrounding insulating media (often mineral oil). Generally (depending on temperature), 99% or more of the moisture is saturated in the paper insulation. The balance is located in the insulating liquid.


There is a technique being used today where the insulating fluid from the transformer is circulated through a desiccant media and/or a vacuum chamber continuously while the unit is energized and loaded. The device removes the small amount of moisture contained in the insulating fluid. The very dry fluid is immediately returned to the unit. The moisture in the unit will again find its equilibrium point (depending on temperature) and some of the moisture in the paper insulation will transfer back into the liquid insulation. Over time, the device will eventually remove excess moisture. The advantages of this technique are short outage time and reduced labor. The disadvantage is that it may take months or longer to achieve the desired moisture level.


The most common practice in the field for effective moisture removal is to boil the water directly out of the paper (solid) insulation. This is basically performed by raising the temperature and lowering the pressure inside the unit to convert the moisture from a liquid state to a gaseous state. The moisture then exits the transformer through the vacuum pump(s). A physical measurement of the moisture removed can be achieved by the addition of a cold trap in the vacuum stream.

There are several ways to raise the temperature of the solid insulating material, including using electrical current (short circuit method), hot oil bath, and hot air circulation. The most common equipment used in the field today utilizes the transformer oil to heat the solid insulation. The oil is circulated between the processing equipment and the transformer until the active part has reached the highest achievable temperature (typically 55-65 °C). During the circulation period the oil is heated and passed through several filters to remove particulate matter. The oil also passes through a vacuum chamber where gas and moisture is removed.


While maintaining vacuum on the transformer tank, the oil is then removed, exposing the solid insulation surface(s) to deep vacuum. During this period that can be from 12-48 hours, the combination of temperature and vacuum causes the moisture in the insulation to boil. The water vapor is then extracted through the vacuum pump system.


The length of the vacuum period necessary to sufficiently dry the insulation can be determined during by using a Moisture Equilibrium Chart (sometimes referred to as Piper Chart) similar to the one attached taken from IEEE C57.93. The data required to use the chart properly requires an accurate measurement of current vacuum and winding temperature. A very accurate measurement of the average winding temperature can be taken by measuring the winding resistance. A simple calculation based on the known winding resistance (at 20°C) provided on the unit test report can be made to determine the current average winding temperature.

A considerable amount of energy is required to convert a liquid to a vapor. In this case the energy is the heat stored in the insulation. As moisture is removed, the insulation temperature will decline. Once the insulation temperature drops below 30°C, water removal is difficult even under extremely deep vacuum. The insulation may require reheating if further moisture removal is desired.

Once the desired insulation moisture level is achieved, the unit can be refilled. The hot oil fill flow rate should be regulated to maintain a tank vacuum of one torr or less. In the case of a conservator type tank design, bleed all high points to assure that no outside air is permitted inside the tank. In the case of a nitrogen blanketed design, break the vacuum with tested dry nitrogen.


In order to assure completely re-impregnation the paper insulation with insulating fluid, the total volume of the transformer insulating fluid should then be circulated through the processing equipment at least one pass at maximum attainable temperature. For voltages 138kv and above, it is recommended to wait a minimum of 12 hours before applying test level voltages, and 24 hours before energizing with system voltage.