What are some methods for drying damp current transformers?
Feb 23, 2026| I. Vacuum Hot Oil Circulation Method (Recommended First Choice)
This method combines the advantages of vacuum dehumidification and hot oil heat transfer. It is suitable for oil-immersed capacitive current transformers, especially those with deep moisture.
Drying Principle: A vacuum is created to lower the boiling point of water, while heated insulating oil circulates, carrying away moisture from the inside.
Operating Points:
Drain and seal the oil inside the transformer;
Evacuate to a residual pressure no greater than 133 Pa and maintain for several hours;
Inject dry air or hot oil at approximately 70°C, using the vacuum negative pressure to draw it in;
Use a vacuum oil purifier for at least 8 hours of hot oil circulation for continuous dehydration.
Advantages: Thorough drying, high efficiency, minimal damage to insulation materials.
II. Vacuum Short-Circuit Drying Method (Suitable for On-Site Processing)
This is a combined method of current heating and vacuum dehumidification, particularly suitable for on-site drying of 220kV and above high-voltage current transformers.
Drying Principle: An alternating current is passed through the primary winding to generate heat (copper loss), while a vacuum is simultaneously drawn to lower the moisture evaporation temperature.
Operating Steps:
1. Connect the primary winding in series and then short-circuit it;
2. Apply an appropriate voltage to bring the winding current to its rated value, uniformly raising the temperature to 70°C;
3. Gradually increase the vacuum level, first to 53 kPa, then to 80 kPa, and finally to a high vacuum state;
4. Measure the winding resistance and insulation resistance every 2 hours to monitor the drying process.
End Criterion: Insulation resistance remains stable for 6–12 hours without decreasing, and no condensation occurs.
III. DC Drying Method (Suitable for Slightly Moisturized Transformers)
This method utilizes a direct current to generate heat for drying, is simple to operate, and suitable for small or slightly damp transformers.
Wiring Method: Connect multiple instrument transformer secondary windings in series and apply a 5-6A DC current.
Control Requirements: Use a variable resistor to adjust the current and maintain a constant value; Control the winding temperature between 70-80℃ to avoid overheating; Measure the insulation resistance every hour, stopping when it rises above 100MΩ and stabilizes.
Advantages: No disassembly required, simple equipment, and good results.
IV. Oven Drying Method (Applicable to Removable Components)
For removable secondary windings or small instrument transformers, they can be placed in an oven for heating and drying.
Operating Procedure: Place the windings on a wooden support inside the oven; Control the temperature not to exceed 80℃, generally maintaining it at 75℃; The heating and cooling processes should be carried out slowly to prevent insulation cracking.
Monitoring Requirements: Measure the insulation resistance every hour until it rises above 100MΩ and remains stable.
V. Other Auxiliary Drying Methods
Hot Air Drying: Uses external heat sources such as infrared bulbs or electric hair dryers to heat the transformer casing. Suitable for situations with slight moisture and no vacuum equipment. However, drying is uneven and inefficient.
Induction Heating: Heats the iron core by generating eddy currents through winding coils, achieving internal heating and drying. Suitable for equipment with specific structures.
Circulating Hot Air Drying: Introduces dry hot air into the transformer, combined with dehumidification. However, it suffers from long drying times and incomplete drying.
VI. Safety Precautions During Drying
Regardless of the method used, the following safety regulations must be strictly observed:
Before drying, ensure the equipment is de-energized, tested for voltage, and grounded. Live operation is strictly prohibited.
Monitor the winding temperature in real time. It is strictly prohibited to exceed the allowable value for the insulation class (usually 10°C lower than specified).
Keep away from fire sources when using hot oil or hot air. Fire extinguishers should be available on site.
Operators should wear heat-resistant gloves, goggles, and other protective equipment to prevent burns.
During the drying process, a dedicated person should monitor the process. If any abnormality is detected, the machine should be stopped immediately.
