Transformer Moisture Monitoring | Moisture in the Insulation

Moisture in a Transformer

Where does moisture occur in a transformer?

Moisture occurs in the oil and paper insulation systems of a transformer. Moisture is a significant concern for power transformers and can lead to unexpected failure of equipment and unplanned outages. Excessive moisture in the oil of a transformer reduces the oil’s dielectric strength. This introduces the opportunity for flashover and arcing.

Power Transformer Image

What causes moisture inside a transformer?

In power transformers, moisture comes from several different sources:
  • Solid insulation inside the transformer – both paper and pressboard; as solid insulation ages, depolymerization of paper generates moisture
  • Gaskets deteriorated by aging or improper installation provides a “two-way path” for oil to egress and air and moisture to ingress from the environment
  • Broken conservator bags and non-functioning nitrogen gas blanket systems also allow ingress of air and moisture into the system

What is “bubbling” or “bubble evolution”?

Bubbling from the conductor paper insulation happens when the temperature of the conductor is high and the paper wrapped around the conductor is sufficiently wet. In this situation, water vapor bubbles are created in the voids of the paper. These bubbles tend to rise and accumulate on the surfaces of the paper. Thereby reducing the dielectric withstand between turns and sections of the winding. This can cause a turn-to-turn failure (arcing) which can result in a catastrophic failure of the transformer.

Transformer Insulation Moisture Monitoring

How to manage moisture in a transformer?

Management of moisture within a transformer is critical to aging, reliability, performance, and safety. It is important to know the amount of moisture dissolved in the oil. The dielectric strength diminishes as relative saturation increases.

Processing the oil to remove moisture and contaminants will aid in returning the oil to a higher dielectric strength for better performance. While this process removes moisture from the oil, it does not extract moisture from the insulating paper. So drying the oil will have only short term benefits. The important measurement is the % Relative Saturation of the oil.

The acceptable amount of moisture is dependent on temperature, the chemical composition of the oil, and the aging level of the oil. Moisture sensors and monitors are a continuous way to manage moisture of both insulation system components.

In practice, only the relative saturation of moisture in oil can be measured and the moisture in paper must be inferred from it, based on the water absorption characteristics of both oil and paper and time taken for the migration characteristics of water between them under variable temperature conditions.

Karl Fischer Titration Test

Most utilities employ a schedule for oil sampling and subject the insulating fluid to dissolved gas analysis and moisture content testing. The Karl Fischer Titration test measures the absolute moisture content in the oil and reports the Parts Per Million content. Moisture exists in three states: dissolved, bound, and free water. Dissolved moisture is free to move between oil and paper. Bound moisture is attached to acid, fibrous particles, dust, etc. The Karl Fischer Titration test measures all three but cannot differentiate dissolved from bound water.

Introducing the E3 Transformer Monitor

E3 Transformer Monitor

Dynamic Ratings’ E3 Transformer Monitor integrates moisture sensor data and other calculated parameters as part of the moisture management model. Measured values include moisture content in oil in terms of percentage relative saturation, bubble evolution, temperature at the moisture sensor’s location, top oil temperature of the transformer, load current measured on the winding of interest and bottom oil temperature of the transformer.


Additional Resources


Moisture Content in Transformer Insulation


The effects of moisture in the insulation system of a transformer


Understanding how moisture works inside of a transformer.