Transformer Oil Moisture Testing and Its Impact

Transformer oil moisture testing is a common practice used to assess the condition of insulation systems in oil-filled power transformers. Water is the most dangerous impurity in transformer oil, because even very small amounts significantly reduce the breakdown voltage of oil. In operating transformers, moisture may be present as free water, emulsified water, or dissolved water, and each form affects transformer operation in a different way.

When moisture is present in a dissolved state, it does not have a strong immediate impact on dielectric strength or dielectric loss tangent. However, it intensively accelerates oxidation processes in transformer oil, reduces oil stability, and contributes to the aging of both dielectric liquid and solid insulation. For this reason, transformer oil moisture testing is essential for understanding the long-term degradation mechanisms inside the transformer tank.

Sources of Moisture in Oil-Filled Transformers

The main source of moisture ingress into a transformer is atmospheric humidity. Moisture enters the transformer together with air through improper moisture protection systems. Another path is air ingress through seals under the influence of internal pressure gradients caused by load and temperature changes.

In addition to atmospheric sources, moisture is also generated inside the actual transformer. This internal water is released from solid insulation and oil as a result of aging processes. In a fully loaded transformer, cellulose insulation ages over a period of 20-30 years and releases approximately 0.5-0.75% of water by weight of insulation within this period of time.

Operational Problems Caused by Increased Moisture Content

Excessive moisture in the transformer windings insulation significantly shortens the service life of cellulose insulation by cutting this period almost in half in the worst-case scenarios. Increasing the moisture content also lowers the maximum allowable operating temperature of windings insulation. As a result, the maximum allowable load of the power transformer is reduced.

Elevated moisture levels inside the transformer tank complicate maintenance and repair procedures, thus increasing their cost. In such cases, maintenance procedures should involve time-consuming and labor-intensive insulation drying processes. For new oil-filled power transformers, the moisture content of paper insulation after manufacturing and installation normally does not exceed 1%.

During the transformer operation, moisture content of solid insulation in tranformer windings always increases and may reach up to 3% after 25 years, while the recommended limit is not more than 2%.

Forms of Water in Transformer Tank

Water can exist in several forms inside an oil-filled transformer, each with a different level of operational risk. Free or unbound water may enter the transformer tank as droplets, vapor, or aging byproducts. Since water is heavier than oil, it settles on the tank bottom. If free water does not come into direct contact with the insulation, it is not dangerous in general.

Moisture dissolved in oil constitutes a stationary form of water presence and always exists to some extent. This form is considered safe for transformer operation. Another stationary form is moisture absorbed by solid insulation, which is also safe up to a certain limit.

The most dangerous form is dispersed, fine-droplet moisture in oil. This non-stationary form develops under specific operational and thermal conditions and poses major hazards at low temperatures. Another dangerous non-stationary form is the presence of vapor bubbles inside the windings of a live transformer, which occur when the boiling point of water is exceeded in the overheated, moisture-saturated insulation.

At low temperatures, free and dispersed moisture may also exist in the form of ice or frost inside the transformer tank.

Transformer Oil Moisture Testing and Allowable Water Content

Transformer oil moisture testing should take into account the type of insulating oil used. Mineral oils are hydrophobic and have a relatively low water solubility, typically in the range of 30–60 ppm. Silicone oils have higher saturation limits of approximately 150–300 ppm, while ester-based oils can absorb a much greater amount of moisture, with saturation levels reaching 300–2,600 ppm. The oil used in transformers with film or nitrogen protection, sealed bushings, and in sealed instrument transformers shall contain not more than 10 ppm of water. For power transformers without film protection and with non-sealed bushings, the moisture content shall not exceed 25 ppm. The oil with water content of up to 10 ppm is considered to be dehydrated, although the supplied oil may contain up to 35 ppm, which requires dehydration by thermal vacuum or zeolite-based processes.

Methods for Determining the Moisture Content of Oil

The methods used for measuring the moisture content of transformer oil can be classified into three groups. Chemical extraction methods include the Karl Fischer coulometric titration and the calcium hydride method. The Karl Fischer method determines moisture content by measuring the electric charge required for electrolysis of water in the chemical reagent.

Physical extraction methods are based on equilibrium extraction followed by measurement with the use of gas chromatography or photoacoustic emission techniques. Capacitance sensing methods perform indirect measurements of relative oil saturation by detecting the conductivity changes in a sensitive film.

Laboratory methods are accurate, but time-consuming, complex, and require strict safety measures. Therefore, compact and fast-response portable instruments are needed for practical implementation in transformer oil moisture testing.

FLD 1T Express Moisture Tester by Fluidex

The FLD 1T tester by Fluidex is designed to determine the trace moisture content of insulating oils. In addition to moisture concentration in ppm, it measures oil temperature. Measurement accuracy and stability are achieved by means of a capacitance sensor that is insensitive to oil contamination.

The FLD 1T measures active water and oil temperature and calculates the moisture content of mineral transformer oil. For non-mineral oils, specific oil parameters shall be uploaded into the device.

The FLD 1T is compact and portable, with approximate dimensions of 7.5 in (length), 10.0 in (width), and 14.4 in (height). The tester is easy to operate and does not require highly skilled personnel. After pressing the start button, the moisture and temperature values are displayed, with stable results obtained within 10 minutes.

The device is suitable for operation near transformers in all weather conditions. The sensor operates reliably at ambient temperatures of −40°F to +140°F. Reliability is ensured by vandal-resistant metal buttons, metal protection of the sensing element, a regulated power supply, and integrated electrical protection.