Why Do Transformers Need Drying? Understanding Moisture in Insulation and Common Drying Methods

Drying is one of the most important processes in transformer manufacturing, although its significance is sometimes underestimated.

In oil-immersed transformers, insulating paper, pressboard, and other cellulose-based materials are hygroscopic, meaning they can absorb moisture from the surrounding air. From material storage and winding production to active-part assembly, exposure to ambient air may introduce moisture into the insulation system.

If this moisture is not properly controlled before oil filling, it can affect the dielectric properties of the insulation and accelerate its aging.

Transformer drying, therefore, is not simply a process of heating the transformer. Its primary purpose is to remove moisture from the solid insulation to an acceptable level and establish suitable insulation conditions for subsequent vacuum oil filling, factory testing, and long-term operation.

Why Is Moisture a Problem for Transformers?

A transformer’s insulation system must withstand electrical, thermal, and mechanical stresses throughout its service life. Excessive moisture can significantly affect the electrical and physical properties of insulating materials.

• Reduced insulation resistance;
• Increased dielectric losses;
• Lower dielectric strength;
• Increased risk of partial discharge and insulation failure;
• Accelerated aging of cellulose insulation;
• Reduced long-term transformer reliability.

Moisture does not remain only on the surface of the insulation. During manufacturing, storage, or maintenance, it can gradually penetrate insulating paper and pressboard. In large transformers with thick insulation structures, removing this deeply absorbed moisture can take considerable time.

For this reason, modern transformer drying processes generally rely on a combination of heat, vacuum, and time rather than simply applying a high temperature.

Where Does Moisture in a Transformer Come From?

Moisture can enter the insulation system at several stages of production. Cellulose-based materials can absorb moisture during storage and processing, while winding production and active-part assembly may expose the insulation to ambient air for extended periods.

For transformers already in service, moisture may be introduced during maintenance, winding replacement, tank opening, or prolonged exposure of the active part. Abnormal sealing conditions during transportation or storage can also increase the risk of moisture ingress.

Therefore, the need for transformer drying should not be determined solely by voltage rating. It should be evaluated based on insulation condition, exposure time, ambient humidity, maintenance history, and insulation test results.

How Does Transformer Drying Actually Work?

From a physical perspective, transformer drying is essentially the process of moving moisture from inside the insulation to its surface and then removing it from the surrounding environment.

Increasing the temperature within an appropriate range helps accelerate moisture diffusion through cellulose insulation. Reducing the surrounding pressure creates more favorable conditions for evaporation and helps moisture continue migrating outward.

This is why modern transformer drying is based on the coordinated control of temperature, vacuum, and time. The effectiveness of the drying process depends on the balance between all three.

Common Transformer Drying Methods

Hot-Air Drying

Heated dry air is circulated around the transformer active part, gradually increasing the temperature of the insulation. Moisture migrates toward the surface and is carried away by the circulating air. The method uses relatively simple equipment and can be suitable for certain smaller transformers and maintenance applications, but its efficiency is limited for thick insulation structures.

Induction Heating

Induction heating uses electromagnetic induction to generate heat, which is then transferred to the transformer active part. It can be practical under certain manufacturing or field-maintenance conditions, but temperature distribution must be carefully monitored to prevent local overheating.

Vacuum Drying

Vacuum drying applies appropriate heat to the active part and then reduces the surrounding pressure. Under reduced pressure, moisture can evaporate more readily and be removed by the vacuum system. For transformers with thicker winding insulation, this is generally more effective than simple atmospheric hot-air drying.

Vapor Phase Drying

Vapor phase drying (VPD) uses the vapor of a suitable heat-transfer medium. When the vapor reaches the relatively cooler transformer active part, it condenses and releases heat, providing efficient and relatively uniform heating while a vacuum system removes moisture released from the insulation.

• Efficient heat transfer;
• More uniform heating of the active part;
• Effective drying of thick insulation structures;
• Shorter drying cycles for large transformers;
• Improved process consistency.

Why Can’t We Simply Increase the Drying Temperature?

Higher temperatures can accelerate moisture migration, but transformers contain large quantities of cellulose-based insulation, and temperature itself is an important factor in insulation aging.

Excessive temperatures may accelerate thermal degradation of insulating paper and negatively affect its mechanical strength and long-term service life.

An effective drying process therefore does not aim for the highest possible temperature. Instead, the objective is to achieve sufficient moisture removal by combining an appropriate temperature, suitable vacuum conditions, and adequate drying time.

Why Are Large Transformers More Difficult to Dry?

The real challenge is the insulation structure. Large transformers generally contain more solid insulation and may have thicker and more complicated insulation systems.

Moisture located deep inside these materials must pass through internal diffusion, migration to the surface, evaporation, and removal from the drying system.

As a result, dry insulation at the surface does not necessarily mean that the deeper layers have reached the required moisture level. This is one reason why vacuum drying and vapor phase drying are particularly important for large and high-voltage power transformers.

When Should a Transformer Be Evaluated for Drying?

Drying is not limited to the manufacturing of new transformers. The insulation condition should also be evaluated after certain maintenance, transportation, or storage events, including:

• Replacement of windings or major insulation components;
• Prolonged exposure of the active part to ambient air;
• Abnormal sealing conditions during transportation or storage;
• Unusual insulation resistance measurements;
• Abnormally high moisture content in transformer oil;
• Major maintenance involving significant exposure of the insulation system.

An abnormal result from a single test does not automatically mean that the transformer must be dried again. The condition of the insulation should be evaluated using multiple factors, including historical data, environmental conditions, oil analysis, and electrical test results.

How Do We Know When Transformer Drying Is Complete?

A transformer cannot simply be considered dry because it has been heated for a certain number of hours or because it has reached a particular temperature. The required drying conditions and acceptance criteria vary depending on transformer design, voltage class, insulation structure, applicable standards, and manufacturer specifications.

• Active-part temperature;
• Vacuum system conditions;
• Moisture removal trends;
• Insulation resistance;
• Absorption ratio or polarization index;
• Dielectric dissipation factor;
• Moisture content in insulating oil;
• Dielectric properties of the insulating oil;
• Other parameters specified by the design and manufacturing process.

For large power transformers in particular, manufacturers may evaluate trends recorded throughout the drying cycle rather than relying on a single measurement.

Why Is Vacuum Oil Filling Important After Drying?

For an oil-immersed transformer, the insulation treatment does not end when the drying process is completed. Once the active part has been dried, unnecessary exposure to humid air should be avoided because the cellulose insulation can begin absorbing moisture again.

Vacuum oil filling helps minimize the re-entry of air and moisture while allowing transformer oil to thoroughly impregnate the solid insulation.

From a manufacturing perspective, drying, vacuum processing, and oil filling form a continuous insulation-treatment process rather than three completely independent operations.

Заключение

Moisture is one of the important factors affecting transformer insulation reliability. During manufacturing, transportation, installation, and maintenance, cellulose-based insulation may absorb moisture from the surrounding environment.

The fundamental principle of transformer drying is straightforward: control temperature, vacuum, and time so that moisture can migrate out of the solid insulation and be removed from the system.

Hot-air drying, induction heating, vacuum drying, and vapor phase drying use different methods to achieve this objective, but the goal is the same: to achieve the required insulation condition without causing unnecessary thermal damage to the insulating materials.

For transformer manufacturers, drying is therefore much more than an auxiliary production step. It is an important part of establishing a reliable insulation system and can influence factory test performance, operational reliability, and transformer service life.