Condition assessment of the electrical insulation in a high voltage current transformer using an oil/sand mix

Moisture migration within the oil/sand mix is significantly affected by changes in internal insulation temperature. Such changes can affect adsorption occurring at the oil/sand interfaces, resulting in variations in oil moisture levels. We developed an algorithm to calculate expected moisture levels within the oil/paper insulation, based on surface oil sample temperatures. Using this algorithm, traditional methods of insulation moisture content evaluation may be applied. We have also shown that, while the rate of migration of hydrogen and acetylene from the oil/paper insulation through an oil/sand mix to a sampling port installed on the other side of the transformer is slower than in conventional oil-only current transformers, the difference is not large enough to render routine condition monitoring data misleading. However, care must be taken in the case of samples collected soon after a suspected fault condition.