Demonstrating Moisture as a Major Contributor to Transformer Failures: The Elusive Link Revealed Through Detailed Case Studies

For each case, the paper presents the transformer details and relevant history, the nature of the failure and the subsequent failure analysis. It then goes on to assess the moisture state of the transformer and discuss the root cause of failure, highlighting the difficulties inherent in performing accurate root cause analysis, particularly identifying moisture as a contributing factor when relying only on periodic offline condition data. With the benefit of hindsight and greater understanding of the dynamics of moisture in transformers gained from continuous monitoring, the paper reassesses the role of moisture as a major contributor to these failures.

While in some cases onerous, the duty required of the transformers at the time of failure was within normal operating requirements, and a healthy, dry transformer would be expected to withstand the stresses. Each failure highlights a different major cause of failure that was precipitated by high moisture levels:

 Case study 1 illustrates the effect on the dielectric strength of the insulation system of a wet transformer when temperature cycled through a period of sustained hot summer days followed by a cool day.  Case study 2 highlights the effect of oil regeneration on the solubility of water in oil in a wet transformer without subsequent dry out.  Case studies 3 and 4 demonstrate the susceptibility of a wet transformer to the stresses imposed by through faults and switching events. By providing robust, real-world examples this paper serves not only as a historical record of significant failures but also as a contemporary reminder and argument for the importance of rigorous, proactive moisture management and the advantages provided by advanced online monitoring systems in both proactive management, and post-failure analysis. For the utility, a key outcome from these incidents was the continual development of continuous online monitoring systems and moisture mitigation strategies over the past 20 years which continues into the present.