How to determine if a current transformer is damp?

I. Characteristics of dampness
1. Mild dampness
The insulation resistance of the end screen decreases (<1000 MΩ), and tanδ (tangent of the dielectric loss factor) increases.
The water content in the oil increases slightly (needs to be tested by oil chromatography).
2. Severe dampness
Water may be released from the bottom, and the oil withstand voltage drops significantly.
The tanδ of the end screen significantly exceeds the standard (e.g., >0.5%), and the insulation resistance is extremely low (possibly <10 MΩ).
If moisture penetrates the main capacitor screen, the tanδ of the main screen will also increase.
3. Severe dampness
The tanδ of the main screen continues to increase (due to moisture entering the capacitor core).
The insulation resistance of the end screen is extremely low, and the tanδ exceeds the standard for a long time, indicating a high water content in the oil. II. Testing Methods
1. Insulation Resistance Test
Use a 2500V megohmmeter to measure the insulation resistance of the terminal screen to ground. The standard value should be ≥1000MΩ.
If the insulation resistance of the primary winding to ground is less than 100MΩ, be wary of moisture.
2. Dielectric Loss Test (tanδ)
A terminal screen tanδ > 0.5% (oil-immersed) or > 2% (dry-type) indicates moisture.
An abnormally increased main screen tanδ (e.g., > 0.3%) may be due to core moisture.
3. Oil Quality Analysis
Excessive water content in the oil (e.g., > 20ppm) or decreased withstand voltage (< 30kV).
4. Visual Inspection
Water seepage from the bottom, abnormal oil level, or seal failure (oil-immersed).
Cracks in the porcelain bushing or discharge marks (SF6 or resin-cast). III. Auxiliary Identification Methods
Partial Discharge Detection: Moisture may cause internal discharge, and abnormalities can be detected using ultrasound or high-frequency current methods.
Infrared Thermal Imaging: Moisture-affected areas may experience temperature abnormalities due to localized heating.
IV. Treatment Recommendations
Minor Moisture: Dry using a vacuum hot oil circulation method.
Severe Moisture: Replace the insulating oil or return the entire unit to the factory for repair.