Chemist’s Perspective nderstanding Water U in Transformer Systems The Relationship Between Relative Saturation and Parts per Million (ppm) ater content in transformer oil in parts per million (ppm) is a familiar concept to most in our industry, and limits of 30 to 35 W ppm are generally referenced. However, these simple concen- by Lance Lewand tration limits have limited value in diagnosing the condition of trans- Doble Engineering former systems and, thus, the concept of relative saturation (RS) of water in transformer oil has been re-introduced over the past 15 years. The concept of relative saturation of water in transformer oil is not a new one and was originally championed by Frank Doble as early as as the water content in the paper the mid 1940s.Thus, this article discusses and details the relationship doubles so does the aging rate of the paper. The deterioration of the between RS and ppm. paper insulation results from the It is well known that moisture continues to be a major cause of prob- weakening of the hydrogen bonds lems in transformers and a limitation to their operation. Particularly prob- of the molecular chains of the pa- lematic is excessive moisture in transformer systems, as it affects both per fibers. For these reasons it is solid and liquid insulation with the water in each being interrelated. Water important to have a means of as- affects the dielectric breakdown strength of the insulation, the tempera- sessing the moisture content of ture at which water vapor bubbles are formed, and the aging rate of the transformer systems and to main- insulating materials. In the extreme case, transformers can fail because tain transformers in a reasonably of excessive water in the insulation. The dielectric breakdown strength dry state. of the paper insulation decreases substantially when its water content In order to fully understand rises above two to three percent by weight. Similarly, the dielectric break- water and its dynamics in trans- down voltage of the oil is also affected by the relative saturation (RS) of former systems, a short explana- water in oil. The maximum loading that is possible while retaining reli- tion of the different types of water able operation (i.e., preventing the formation of water vapor bubbles) is encountered and the concepts of a function of the insulation water content. For example, dry transform- solubility and relative saturation ers (<0.5 percent water in paper) are much less susceptible to water bubble are provided. evolution. In this case, emergency loading at hot-spot temperatures be- low 180°C may be possible with little risk of bubble formation. In con- Types of Water in Oil trast, a wetter transformer, with 2.0 percent moisture in the paper, runs Water can exist in several differ- the risk of water bubble formation with hot-spot temperatures as low as ent states within the transformer. ° 139 C under the same conditions. A more long-term problem is that ex- There are three basic types of wa- cessive moisture accelerates the aging of the paper insulation, with the ter found associated with trans- aging rate being directly proportional to the water content. For example, former oil: Spring 2002 1 •Dissolved water is hydrogen bonded to the hy- drocarbon molecules of which oil is composed. Table 1 — Water in Oil Solubility as a Function of Temperature •Emulsified water is supersaturated in solution Oil Temperature Water Content in Oil, ppm but has not yet totally separated from the oil. It 0°C22 usually gives oil a milky appearance. 10°C36 •Free water is also supersaturated in solution but 20°C55 in a high enough concentration to form water 30°C83 droplets and separate from the oil. 40°C 121 In most cases, when one is analyzing or discussing 50°C 173 the amount of water in oil, dissolved water is being 60°C 242 referred to as emulsified, and free water is visually 70°C 331 apparent. 80°C 446 90°C 592 What is Water in Oil (ppm), Solubility of 100°C 772 Water in Oil, and RS of Water in Oil? The detection of water in oil performed in the labo- ratory is most often performed by an analytical tech- The solubility for mineral oil can be calculated using nique called Karl Fischer titration described in ASTM Equation 1: Test Method D 1533 or IEC Method 60814. Both meth- ods are very comparable and involve a coulometric (Equation 1) Log So = -1567/K + 7.0895 titration technique involving the reduction of an io- dine-containing reagent. The methods are used to Where: So is the solubility of water in mineral oil ° determine the amount of water in an oil sample on a K is the temperature in Kelvin ( C + 273) weight-to-weight (mg/kg) basis or what is commonly known as ppm (parts per million). Relative Saturation (RS) is the actual amount of The concepts of solubility and relative saturation water measured in the oil in relation to the solubility can sometimes be difficult to understand, but it is an level at that temperature. Relative saturation, ex- important concept when trying to assess the dryness pressed in units of percent, is the concentration of or wetness of a transformer system. Solubility is de- water (Wc) in the oil relative to the solubility (So) or fined as the total amount of water than can be dis- concentration of water the oil can hold at the mea- solved in the oil at a specific temperature. The solu- surement temperature, as shown in Equation 2. bility of water is not constant in oil but changes due to temperature. As the temperature increases, the (Equation 2) RS = Wc /So (100%) amount of water that can be dissolved in oil also in- creases. The increase is not linear but exponential in Where: Wc is in ppm wt./wt. function. For example, at 10°C only 36 ppm of water So is in ppm wt./wt. can be dissolved in the oil, whereas when the tem- perature increases to 90°C, the amount of water that For example, a sample of oil was taken for deter- can be dissolved in the oil increases tremendously to mination of the water content. The temperature of the ° almost 600 ppm. The table shown lists the calculated oil at the time of sampling was 62 C. The laboratory solubility limits for oil at various temperatures. These performed the analysis and determined the water con- levels are the greatest amount of water that can be tent to be 11 ppm. From Equation 1, it is calculated ° dissolved at the temperatures listed. If the concentra- that the solubility level at 62 C is 259 ppm. As dis- tion of water in oil is greater than that shown for that cussed previously, relative saturation is the actual specific temperature then, in all likelihood, the oil is measured value compared to the solubility value. In supersaturated with water, and free or emulsified this case it is 11 ppm divided by 259 ppm resulting in water could exist. a relative saturation of 4.25 percent. Effects of Relative Saturation on Dielectric Strength To properly maintain and operate transformers, an understanding of the effects of moisture on the dielectric breakdown strength of the electrical in- sulating liquids is necessary. Increasing moisture content reduces the dielectric breakdown voltage of insulating liquids. The correlation between the 2 NETA WORLD water content in new, filtered, mineral oils at room The dielectric breakdown voltage would be ex- temperature and the dielectric breakdown voltage pected to be about half of what it was when the rela- using ASTM method D 1816 (0.04 inch gap) is given tive saturation was very low. If the temperature is in Figure 1 (water content, ppm). Of course, the cooled to 0°C, the results of a dielectric breakdown dielectric breakdown voltage is also a function of voltage should be quite low because the solubility of the number and type of particles and their conduc- water in oil at this temperature is about 22 ppm (Table tivity, not just the water content. 1). As the water content in the oil is higher than this, Taking the same dielectric breakdown voltage data the water forms an emulsion and begins to condense. and converting it to RS (Figure 1, %RS graph) pro- During all this time the concentration of water in oil vides a much straighter curve except at the extremes. has not changed. This relationship is shown in It is evident that there is a better correlation between Figure 2. RS and dielectric breakdown voltage than with mois- ture concentration and dielectric breakdown voltage. The water Decreasing Relative High RS concentration was 48 % Saturation, 44 constant at 30 ppm. 40 The temperature was 36 changed to change the 32 Medium RS 28 relative saturation. 24 20 16 Low RS 12 8 4 Dielectric Break k Voltage, down 04812162024283236404448525660 Increasing Dielectric Strength Water Content, ppm, wt./wt. 48 44 Figure 2 — Relationship between Dielectric Strength and RS 40 36 32 Transformers are more complicated systems than 28 24 this simple example. However, the same basic prin- 20 ciples apply for the dielectric breakdown strength of 16 12 the liquid dielectric. That is, it remains a function of 8 the relative saturation of water in the oil. During the 4 Dielectric Break k Voltage, down cool-down cycle of a thermal transient in a transformer 0102030405060708090100 RS, %@22°C some of the moisture returns to the paper and some of the moisture remains in the oil. The relative satura- Figure 1— Dielectric Strength Versus Water Content and tion of water remaining in the oil will influence its Relative Saturation (RS) dielectric breakdown voltage.