Transformer oil dehydration is a highly effective maintenance procedure, but it is a rigorous physical process that can sometimes reveal or create underlying issues. Success isn't just defined by achieving a low ppm moisture level; it's confirmed by the transformer operating correctly post-service. This article outlines common troubleshooting issues that can arise after dehydration and provides a practical guide for diagnosing and resolving them to ensure a successful outcome.

Failure to Achieve Target Moisture Levels

Cause: The most common issue is a leak in the system that allows moist air to be pulled into the transformer under vacuum, sabotaging the process. This could be a faulty gasket, a valve, or a leaky bushing.

Troubleshooting: The service crew should perform a vacuum hold test before processing to check the integrity of the tank. If moisture levels plateau, a thorough inspection of all seals and gaskets is required.

High Dielectric Strength Failure Post-Processing

Cause: While moisture is the primary culprit for low dielectric strength, the dehydration process can agitate and suspend solid contaminants (sludge, cellulose fibers from degraded paper) throughout the oil. These particles can lower the dielectric strength even in dry oil.

Troubleshooting: The solution is often additional filtration (e.g., using a bypass particle filter) in conjunction with vacuum dehydration to remove both moisture and particulates. A post-processing oil sample should be sent for laboratory analysis to identify the contaminant.

Gassing or Sudden Rise in Dissolved Gas Analysis (DGA) Values

Cause: The application of heat and vacuum can strip volatile gases from the oil and solid insulation. This can lead to a temporary and misleading rise in combustible gas levels (especially methane and carbon monoxide) that is not indicative of a new fault.

Troubleshooting: It is critical to take a DGA sample immediately before and then several days after the dehydration process to establish a new baseline. Comparing these values helps distinguish process-related gassing from genuine fault gases. A follow-up DGA test 2-4 weeks later is recommended to confirm the trend is stable.

Loss of Oil or Foaming During Processing

Cause: Applying too much heat too quickly can cause the oil to foam violently, potentially leading to oil loss through the vacuum pump and, in extreme cases, inadequate processing.

Troubleshooting: This is an operator error. Reputable service providers will carefully control the heat and vacuum ramp rates according to the oil type and volume to prevent this issue.

Conclusion

Summarize that most post-dehydration issues are preventable with proper procedures, experienced operators, and thorough pre-job inspections. Communication between the asset owner and the service provider is key. Understanding that a temporary shift in DGA or the need for supplemental filtration are part of the process ensures a correct interpretation of results and confirms the long-term health benefits of the dehydration service.