How to Remove Moisture from Three-Phase Oil-Immersed Transformers?

As core equipment for power transmission and distribution in power systems, the safe and stable operation of three-phase oil-immersed transformers directly determines the reliability of power grid power supply.Transformer oil, as the insulation and cooling medium of the equipment, is crucial to its performance.

 

Moisture contamination is the primary hidden danger leading to the deterioration of transformer oil performance-even trace amounts of moisture (in ppm) can significantly reduce the dielectric strength of the oil, accelerate the aging of cellulose (paper) insulation materials, trigger partial discharge, arc discharge and other faults, and ultimately cause insulation breakdown, winding short circuit, and even early scrapping of the equipment, resulting in major economic losses and power supply interruptions.

 

Therefore, accurately identifying moisture contamination and adopting scientific methods to remove it are key links in the daily maintenance and fault disposal of three-phase oil-immersed transformers.

 

Combined with industry practices, this article elaborates on the hazards, detection methods and efficient removal technologies of moisture in three-phase oil-immersed transformer oil.

 

 

Three-phase oil-immersed transformers feature high capacity, high operating load and complex insulation structure. The hazards of moisture in the oil are more prominent than those in ordinary transformers, affecting the entire life cycle of the equipment, which is mainly reflected in the following 4 aspects:

 

Sharp drop in dielectric strength

Three-phase transformers operate at high voltages. Moisture in the oil will damage the insulation performance of the oil. Even with only 30-50 ppm of moisture, the breakdown voltage of the insulating oil can drop from above 60 kV to below 30 kV, greatly increasing the risk of internal arc discharge and easily causing phase-to-phase short circuits.

 

Accelerated insulation aging

The cellulose (paper) insulation inside the transformer is in direct contact with the transformer oil. Moisture acts as a catalyst to accelerate the hydrolysis and oxidation of the paper, reducing its mechanical strength. When the moisture content of the paper exceeds 2.0%, it will become brittle and eventually lose its insulation function, leading to exposed winding faults.

 

Prominent internal fault hidden dangers

Water pockets formed by moisture in the oil will cause corona activity and gas generation. Local heating will also produce steam bubbles, leading to dielectric collapse; at the same time, moisture will promote the generation of acidic substances, cause corrosion of metal parts and oil sludge deposition, and further exacerbate equipment wear.

 

Increased thermal runaway risk

Three-phase transformers have large load fluctuations. Moisture will remain in the insulation materials, reduce heat dissipation efficiency, accelerate the deterioration of thermal performance of the insulation, and may cause thermal runaway during long-term operation, leading to abnormal temperature rise of the transformer and triggering tripping protection.

 

Moisture Impact by the Numbers

 

Moisture Content in Oil (ppm)	Dielectric Strength Loss	Transformer Risk Level
<10 ppm	Minimal	Safe (in-service oil)
20–30 ppm	20–30% reduction	Begin degradation of cellulose
40–50 ppm	Up to 50% reduction	High PD risk, flashover possible
>60 ppm	Critical	Severe insulation failure likely

Breakdown voltage of mineral oil typically drops from >60 kV to <30 kV as water increases from 10 to 50 ppm.

 

Case Study – Moisture-Induced Failure

 

Based on industry cases, a 20 MVA, 132/33 kV three-phase oil-immersed transformer tripped during heavy load in the rainy season due to a failure of the breather, resulting in moisture content in the oil exceeding 65 ppm. Eventually, the paper insulation layer carbonized and the winding short-circuited, leading to early scrapping of the equipment with maintenance costs exceeding 80,000 US dollars. This shows the hidden and destructive nature of moisture contamination.

 

 

Moisture in three-phase oil-immersed transformer oil has the characteristics of slow penetration and indistinct perceptibility. It is necessary to adopt a combination of regular detection and real-time monitoring to achieve early detection and disposal. Common detection methods are divided into laboratory precision tests and on-site rapid tests. The core methods are as follows:

 

Method	Description and Accuracy	Use Case
Karl Fischer Titration	Gold-standard chemical test for precise water ppm	Lab-based, highly accurate (±1 ppm)
Dielectric Breakdown Test (IEC 60156)	Tests oil's voltage withstand capacity	Indicates functional impact of moisture
Visual Inspection	Detects turbidity, cloudiness, or free water drops	Quick field check
Moisture Sensor (On-line)	Real-time digital moisture-in-oil monitoring	Installed in critical assets
Infrared Thermal Imaging	Detects cool spots indicating condensation or water pockets	In-service inspection
Dissolved Gas Analysis (DGA)	Indirect signs: CO₂, CO, H₂ rise from water-induced degradation	Cross-check or early failure detection

 

 

Moisture in three-phase oil-immersed transformer oil is divided into three types: dissolved water, emulsified water and free water. According to the moisture content, contamination degree and equipment operating status, select targeted removal methods.

 

The core technology is vacuum dehydration, combined with other auxiliary methods, to ensure that the moisture content is reduced to a safe range (<30 ppm). The details are as follows:

 

Method	Water Form Removed	Typical Moisture Level Achievable	Use Case Scenario
Vacuum Dehydration	Dissolved + Free	≤10 ppm	Most effective for large transformers
Thermal Vacuum Drying	Water + Gases from Oil and Paper	≤5 ppm + paper drying	Offline method used during major overhauls
Hot Oil Circulation + Filtration	Free/emulsified	~30–50 ppm	Used for moderate contamination
Molecular Sieve Drying	Dissolved moisture	≤15 ppm	On-line or by-pass system for slow drying
Centrifugal Separation	Free water only	Doesn't remove dissolved water	Pre-filtration step for high water presence

 

 

For three-phase oil-immersed transformers, prevention of moisture contamination is more important than removal. Establishing a complete maintenance system can significantly reduce moisture infiltration, extend equipment life and oil service cycle. The core prevention measures are as follows:

 

Strengthen sealing protection

Regularly check the gaskets of transformer flanges, valves and cable bushings, replace aged gaskets every 5-7 years, install weatherproof sealing strips and covers to prevent rainwater and environmental moisture from infiltrating through sealing gaps; adopt oil tanks with excellent sealing performance to avoid direct contact between oil and air.

 

Maintain breather function

The silica gel breather is the key to preventing moist air from entering the transformer. Check the color of the silica gel monthly (discoloration of discolored silica gel to pink indicates saturation), and replace or regenerate it in a timely manner. For high-humidity areas, adopt a two-stage breathing system to improve dehumidification effect.

 

Install protection systems

Three-phase transformers with critical loads can be equipped with bladder protection systems or nitrogen sealing systems. Through a sealed rubber diaphragm or inert gas pressurization, the tank breathing cycle is eliminated, and the infiltration of moist air is completely blocked; for idle units, install electric heaters to prevent condensation water accumulation during cooling.

 

Standardize oil handling

When sampling or refueling, use dry tools and containers to avoid wet operations; store new oil in a sealed manner to prevent moisture absorption, detect moisture content before refueling, and use it only if qualified; avoid open oil drums during rain, and transport oil in a closed and constant temperature environment.

 

Establish a regular maintenance plan

Check the breather silica gel monthly, detect the moisture content in the oil every 6-12 months, check the gasket tightness every 6 months, inspect the nitrogen system pressure quarterly, and conduct on-site sealing inspection after heavy rain or sudden temperature drop to form a full-process maintenance closed loop.

 

Real Example

Unit: 25 MVA, 66/11 kV oil-immersed transformer

Initial issue: Moisture 62 ppm in oil, 1.9% in paper

Corrective action:

• Bladder conservator installed
• Breather replaced with 2-stage silica + oil trap
• Flange gaskets renewed

Follow-up: Moisture <15 ppm sustained for 3 years

Result: No further breakdown voltage loss; insulation life preserved
Key takeaway: Prevention pays off exponentially in extended life and reduced risk.

 

 

Moisture control of three-phase oil-immersed transformer oil shall comply with the following industry standards: IEC 60422 (In-service Oil Maintenance and Moisture Limits), IEEE C57.106 (Guide for Receipt and Maintenance of Insulating Liquids), IS 1866 (Indian Oil Maintenance Standards), ASTM D1533 (Standard Test Method for Moisture in Electrical Insulating Liquids). Among them, the moisture content of in-service oil shall be controlled below 30 ppm, and the moisture content of cellulose insulation shall be lower than 0.5%.

 

Combined with industry operation and maintenance practices, the following suggestions are put forward for three-phase oil-immersed transformers:

 

• For key transformers, adopt the method of on-line moisture monitoring + regular laboratory detection to timely capture the change trend of moisture and avoid fault hidden dangers.
• Prioritize vacuum dehydration for dehydration disposal, and match with appropriate auxiliary methods according to moisture content and equipment status to ensure dehydration effect.
• Establish an emergency disposal plan for moisture contamination. Immediately detect the moisture content in the oil after heavy rain or breather failure, and start emergency dehydration if necessary to prevent fault expansion.
• Regularly carry out training for operation and maintenance personnel to standardize the detection and dehydration process, and avoid secondary pollution caused by improper operation.

 

 

The core of removing moisture from three-phase oil-immersed transformers is "accurate detection, scientific disposal and active prevention".

 

As the most effective dehydration technology, vacuum dehydration can quickly restore oil quality, and combined with thermal vacuum drying, molecular sieve drying and other methods, it can meet the disposal needs of different contamination degrees; and perfect sealing protection and regular maintenance can reduce moisture infiltration from the source and reduce equipment fault risk.

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As core equipment of power systems, the oil quality management of three-phase oil-immersed transformers is directly related to the safe and stable operation of the power grid. Only by attaching importance to moisture contamination prevention and adopting scientific detection and dehydration technologies can we extend equipment service life, ensure power transmission and distribution safety, and provide reliable support for the efficient operation of power systems.

 

 If you are planning a transformer project, contact GNEE today to get expert technical support, customized solutions, and a competitive quotation for your 630kVA oil immersed transformer.