From the Editors' desk

The first article in this issue is "Fundamental Concepts of Using Water Activity Probes to Assess Transformer Insulation Water Content" by Daniel Martin and Tapan Saha from the University of Queensland, Brisbane; Caesar Perkasa and Nick Lelekakis from Monash University, Melbourne; and Tim Gradnik from Milan Vidmar Power Research Institute, Ljubljana. It is concerned with variation in the measured water content of the paper insulation of a transformer, due to changes in the properties of the oil. This variation was analyzed by measuring the solubility of water in various oils, of different ages and levels of degradation, and combining these measurements with the existing models to calculate the dimensionless water content of the paper dedefined as (mass of water adsorbed on the surface of the paper)/(mass of the dry paper). A 3-L glass vessel incorporating a water activity probe, and a port carrying a valve through which oil samples could be extracted for Karl-Fischer titration, was used in order to study the relationship between the water vapor pressure above an oil and the mass of water absorbed by the oil. The extraction port was located so that the oil sample was drawn from a volume very close to the tip of the water activity probe. The main conclusions were that water activity measurements should be corrected for any difference in oil temperature between the water activity probe location and the insulation hot-spot location, and that oil circulation speed significantly influences water activity measurements. It follows that the tips of water activity probes should be located in regions where the oil speed is as high as possible, e.g., near cooler inlet or outlet points.