There are three main sources of excessive moisture in a transformer

Residual Moisture

Even after the transformer core's drying, there might still be some residual moisture in some parts. These are usually in the more significant parts like wood, plastic, and resin-impregnated parts. These would need much longer drying times than cellulose paper and pressboard. The laminated pressboard is glued together with strips of pressboard. This will dry parallel to the layer direction; the glue prevents perpendicular drying. 
When the transformer starts operating, the moisture will migrate from these parts with the high moisture content to part of the core with lower moisture content. It will also move into the oil eventually. This is a lengthy process, and this will happen in a few weeks. This process will continue until the entire system

has reached an equilibrium point. This equilibrium point will depend on the unit's operating temperature as well as the total moisture content of the insulating system, the oil, and the core. 
  • Please note that the core can only be dried out in the oven for a limited time, as the paper will start to crack and decompose if all the moisture is taken out of the structure. 

Moisture ingress

There are three mechanisms for moisture ingress from the atmosphere. 

Direct exposure

This will occur when the transformer insulation is exposed to air - during the installation and repair processes. 

Molecular flow (Knudsen flow)

This type of flow will occur when there is a difference in water vapor pressure in the atmosphere and the transformer gas space or oil. This term describes the diffusion of gases through a tiny opening, like a gasket that does not seal completely. [1,2]

Viscous flow

This will happen when there is a difference in total pressure when the atmospheric pressure is higher than the tank's pressure. This will happen through the conservator system of free-breathing units. 

The main points to remember

The molecular flow will have a negligible influence on the moisture of the system.

The primary mechanism through which water will penetrate the system is through low seals by the flow of wet air into the unit because of higher air pressure outside the tank. Typical leaks that will allow this phenomenon would be at the top gasket seals at the explosion vent and forced oil-cooled units between the main tank and the coolers. Although no oil leaks are visible, there might be an ingress of atmospheric air. In water-cooled systems, water might penetrate the cooler into the oil, even if it is at a higher pressure. 

Large amounts of rainwater can be sucked into the transformer in a concise time (several hours) when a rapid drop in pressure on the inside of the tank occurs ( this can be induced by rain) in combination with insufficient sealing. This becomes a real problem when transformers are stored with some oil without the conservator preservation system. 

The contamination rate with moisture is significant for free-breathing transformers with conservator tanks, although it is limited. 

Research has shown that transformers with free-breathing systems have a moisture contamination rate of up to 0.2% per year. Most of this water is retained in wet zones and not evenly distributed throughout the complete system. It was found that only about 0.5 to 0.6 % of water is extracted from the cellulose insulation during the drying process, even if the water content can be as high as 2.5% in thin cellulose structures. 

The water contamination rate for units with membrane-sealed conservator systems is only about 0.03-0.06% water in the cellulose material.

Units with broken seals or insufficient sealing have over 50kg of free water from both open-breathing and membrane-sealed conservator systems. The damaged heat exchanger can be the source of substantial amounts of water moving into the oil in a water-cooled transformer. 

Opening a transformer and exposing its core will cause more considerable water ingress into the cellulose system than a unit with an imperfect preservation system will display after many years.

This table is sourced from [1]

Moisture Contamination during exposure of the active parts to the atmosphere

The transformers' outer insulation layers quickly absorb moisture, but the diffusion of these moisture molecules into the inner layers will take a long time.

The air's relative humidity is critical because it will determine the moisture absorbed by the insulating material.

95% of the absorbed moisture will be confined to the insulation with depths of 0.3 to 0.35 mm. Moisture is concentrated in the outer layers of the insulation components.


  1. V. Sokolov, B. Vanin, Evaluation of Power Transformer Insulation Through Measurement of Dielectric Characteristics, Proceedings of the sixty-third Annual International Conference of Doble Clients, 1996, sec 8-7
  2. V Sokolov, B Vanin, In-Service Assessment of Water Content in Power Transformers. Proceedings of the sixty - second Annual International Conference of Doble Clients, 1995, sec 8-6
  3. V. Sokolov, Methods to improve Effectiveness of Diagnostics of Insulation Condition in Large Power Transformers. Ph.D. Thesis, The Technical University of Kiev, 1982 (in Russian)

Influences on the load ability and lifetime of distribution transformers due to harmonic current content and ambient temperature - part 1


Recently, the effect of harmonic current content and ambient temperature has been linked to many of the failures seen in distribution transformers. The reason is the increase in transformer losses due to increased harmonics, specifically current harmonics. An increase in the harmonic content of the load current will create extra losses in windings, leading to an increase in hot spot temperatures and in the stress on the insulation of the transformer. These above-mentioned factors will decrease the useful life of the transformer due to the decrease of the useful life of the insulation and the transformer's loading capacity. 


Distribution transformers are typically created to provide power to devices that require a consistent and steady electrical current. However, there are instances where the voltage and current can become distorted due to non-linear loads, resulting in an increase in harmonics and other irregularities. In the past, non-linear loads made up only about 15% of the total power consumption, but in the year 2000, this number increased to 50%. As a result, the presence of non-linear loads leads to a higher harmonic content in the network. This increased non-linear load has many disadvantages such as an increase in the loss and reduction of the efficiency of power system equipment. Therefore, the impact of harmonics on the loss of useful life and reduction of efficiency in power transformers needs to be investigated. The reduction of life in transformers can be due to hot spot temperatures in the transformer winding, and this increased temperature can be a direct effect due to harmonic presence. The hot spot temperature is in direct correlation to the winding temperature and top oil temperature which is an important parameter to ensure efficient transformer monitoring. One of the most limiting factors in transformer loading is the hot spot temperatures. To evaluate loading ability awareness, the hot spot temperature is required. Thermal stresses are one of the most important factors influencing insulation deterioration, therefore hot spot temperature data are crucial to ensure better evaluation of the loading ability of a transformer, the used lifetime as well as the determination of the possible remaining lifetime of the transformer. 

At an IEEE Transformers Committee meeting in March 1980, it was recommended that a standard is provided for guidance in estimating the loading capacity of the transformer in a network with distorted currents. After this, IEEE C 57.110 entitled "Recommended procedure for determination of the transformer capacity under non-sinusoidal load currents" was published. This standard determines the procedure to decrease the level of the rated current for risen harmonics. 

There are three main sources of excessive moisture in a transformer

Residual Moisture Even after the transformer core's drying, there might still be some residual moisture in some parts. These are usually...