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## Moisture analysis for power transformers

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In this chapter, the problem of moisture in transformer insulation is tackled. The physical processes involved in moisture dynamics are first described, then the different methodologies that can be applied to estimate the value of this variable are explained. Finally, the challenges that lie ahead regarding moisture monitoring are discussed.

Chapter Contents:

• 3.1 Introduction
• 3.2 Moisture in transformer insulation
• 3.2.1 Risks associated to the presence of high levels of moisture in transformers
• 3.2.2 Sources of moisture contamination in transformers
• 3.2.2.1 Residual moisture
• 3.2.2.2 Ingress of water from the atmosphere
• 3.2.2.3 Internal generation of water by the decomposition of cellulose and oil
• 3.3 Moisture dynamics in transformers
• 3.3.1 Adsorption and desorption of moisture in cellulosic insulation
• 3.3.2 Moisture distribution within transformer solid insulation
• 3.3.3 Solubility of water in oil
• 3.3.4 Moisture equilibrium between paper and oil
• 3.3.5 Moisture equilibrium in alternative fluids
• 3.3.6 Moisture dynamics in a transformer under operation
• 3.4 Monitoring of moisture content in oil
• 3.4.1 Periodical sampling of oil
• 3.4.2 On-line measure of oil moisture with capacitive sensors
• 3.4.3 Interpretation of the moisture content of oil
• 3.5 Estimation of the moisture content of solid insulation from moisture in oil measures
• 3.5.1 Determination of moisture content of paper using the equilibrium charts
• 3.5.2 Improved methodologies to estimate the moisture content of paper from the measures of moisture content of oil
• 3.6 Dielectric response methods for the estimation of moisture in solid insulation
• 3.6.1 Theoretical principles
• 3.6.2 Frequency dielectric spectroscopy
• 3.6.3 Recovery voltage method
• 3.6.4 Polarisation and depolarisation currents
• 3.7 Conclusions, future trends and challenges
• References

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