Abstract
Moisture has a range of different harmful effects on liquid-immersed transformers and reactors, especially accelerating chemical decomposition of the cellulose molecules and hence solid insulation ageing. This chapter will consider some of the possible sources of moisture in transformers and reactors, especially residual moisture which was not removed during dryout in the factory and moisture absorption during the life of the transformer or reactor. This chapter will then consider moisture equilibrium and moisture migration in the liquid and solid insulation in transformers and reactors. A number of different techniques are used to estimate moisture levels in transformer insulation, and this chapter will consider the main methods used both for periodic condition assessment and continuous online monitoring. Finally, this chapter will briefly consider techniques which can be used to dry liquid and solid insulation in transformers and reactors at the installation site.