With the increasing concentration of water in a transformer, the quality of insulation deteriorates which may result in failure during overloading conditions. Therefore, it is very important to have an effective technique to measure the concentration of water in oil.
Researchers at The University of Queensland have tested and compared methods, such as online water-activity probes together with cellulose adsorption isotherms and dielectric response measurements, for measurement of water concentration in oil and paper insulation. It would appear that the water concentration in insulation paper, measured using water activity probes, does not change significantly as the paper ages while water adsorption isotherms for paper insulation change as the paper ages. However, cellulose adsorption isotherms method may give inaccurate results if the assumptions taken under this method are invalid.
Using Karl Fischer titration, the water concentration in oil (WCO) is measured and then the cellulose adsorption isotherm is applied to estimate the water concentration in the paper (WCP). WCP is calculated using water in oil solubility coefficients for a transformer of a similar age. The WCP values are dependent on oil sampling temperature, which should also be taken into account.
In addition, the water-probe measurements are averaged to calculate WCP because the times required for water to diffuse through insulation are much longer than the typical daily temperature cycles within a transformer. Thus, very little water migrates in and out of the oil during temperature cycling.
Estimation of WCP by dielectric response measurements gives the water inside and on the surface of paper and pressboard barriers between two windings.
The results obtained by these methods conclude that the use of online water-activity probes is an effective method of measuring WCP in the insulation paper of aged power transformers, yielding data closely matching dielectric response data. It appears to avoid nearly all the problems associated with the use of adsorption isotherms.
Paper:
Determining water in transformer paper insulation: analyzing aging transformers
