Transformer dew point measurement and insulation moisture estimation part 1

Author: Tom Melle


It is no secret that water and electricity do not play well together. Even the smallest amounts of moisture in a power transformer can have varying degrees of problems ranging from limited output to catastrophic failure. In this article, we will explore the fundamentals of measuring the dew point for estimating moisture content of dry insulation inside of power transformers. This is beneficial during installation, after performing routine maintenance and ongoing, over the life of the transformer.


Background on dew point measurements


Dew point is defined as the temperature to which air must be cooled for water condensation to begin. At dew point, the partial water vapor pressure in air equals the water vapor saturation pressure. In this state, condensation and evaporation are in equilibrium and take place at the same rate. Since the correlation between water vapor saturation pressure and temperature is known, the dew point can be calculated from the measured temperature and the relative humidity. Pressure is also a component of dew point – any variation in total pressure changes the partial water vapor pressure of the gas being measured.


A traditional method to measure dew point is a chilled mirror instrument where the mirror is cooled down until dew forms. The dew formation causes light to be scattered on the surface, which is detected with optics. At the start of dew formation, the temperature of the mirror denoting dew point is read by a thermometer. This fundamental measuring technology is traditionally used as a calibration reference in laboratories because it gives very accurate results over a wide range of dew points. However, it cannot tolerate dusty or hydrocarbon-rich environments due to its sensitive optics and thus is less commonly used for process control purposes.


Recent technology incorporates capacitive sensors to measure the dew point of a gas sample and is the basis for most modern dew point sensors. Regardless of the technology, the dew point meter must be capable of operating at low inlet pressure at ambient conditions and with good resolution to -60°C (-76°F) dew point or lower.


Estimation of insulation moisture content


For many years, the dew point measurement technique has been widely used to estimate moisture content of dry insulation inside  power transformers. Experience strongly suggests that the technique provides an accurate and reliable method of moisture estimation in a gas space. When used with proper understanding, it can be a very useful tool.


The dew point within a closed vessel is responsive to the surface moisture on the insulation. A reliable measurement requires that a state of equilibrium be achieved between the surface moisture level and that of the surrounding gas space. When the internal atmospheric environment is changed by the introduction of dry air or dry nitrogen, a period of equilibration must occur for the dew point to adequately represent the surface moisture. Equilibrium exists when, in an otherwise static condition, the moisture content is constant for 6-12 hours.


Knowing the dew point in degrees Celsius can be used to obtain the vapor pressure. If the total pressure at the dew point instrument is different from the tank pressure, the measured vapor pressure must be corrected by multiplying the measured value by the ratio of the absolute tank pressure to the absolute pressure at the dew point instrument. The average surface moisture content of the insulation structure can be found based on the insulation temperature and the vapor pressure of the insulation environment.


An example using the following measured conditions illustrates this procedure:

  • Dew point –40°C ( –40°F)

  • Insulation temperature 20°C (68°F)

  • Tank gauge pressure 14 kPa (2 psig)

  • Pressure at dew point instrument 0 kPa (0 psig)

  • Atmospheric pressure 101.35 kPa (14.7 psi)

Per C57.93-2007 (Annex B) Figure B1 – conversion from dew point or frost point to vapor pressure - the vapor pressure is 100 μm (13.3 Pa). Using this figure and the information above, correct the vapor pressure to tank conditions:


100 * (14.7 + 2) / (14.7 + 0) = 114um (15Pa)


Per C57.93-2007 Figure B.2 - Moisture equilibrium chart - the moisture content equates to 0.75% of the dry weight of insulation at the junction of 114 μm (15.2 Pa) vapor pressure and the insulation temperature of 20 °C.


It is advisable to measure pressure, temperature and dew point several times during the equalizing period (12–24 hours) to assure that equilibrium is achieved. Equilibrium exists when, in an otherwise static condition, the moisture content remains constant for at least 12 hours.


The following four precautions need emphasis when making a dew point measurement:

  1. Equilibrium conditions must be reached.

  2. Insulation temperature must be known accurately.

  3. The measuring equipment shall be properly calibrated and be in good working order.

  4. It may be necessary to remove the dew point probe or other detector after each use to minimize the risk of contamination from oil vapor and splashed oil. Contamination may destroy its calibration.



VAISALA DEWPOINT METER USER’S GUIDE  (M010091EN-F ) 2007; Vaisala Oyj; Helsenki, Finland 

SHAW AUTOMATIC DEWPOINT METER MANUAL  (SADPMAN0704 ) 2004; Moisture Meters Ltd; Westgate, Bradford, England