Drying-Out Method of Transformer – Complete Guide ⚡💧

Moisture is one of the most dangerous enemies of a transformer. It drastically reduces insulation resistance, lowers dielectric strength of oil, accelerates ageing of paper insulation, and can lead to failures during operation. Therefore, drying-out the transformer—especially new units or those kept out of service for long—is a critical commissioning step.

This article explains the methods, procedures, drying-out duration, and purification techniques in a simple and professional manner. 😊📘

⭐ What Is the Need for Drying-Out a Transformer?

The primary challenge in drying out a transformer is not drying the oil, because oil can be filtered easily.

The real difficulty lies in removing moisture absorbed by the windings and insulation 🧵💧. These materials are hygroscopic and absorb moisture from the atmosphere.

If left untreated:

👉 Insulation resistance drops

👉 Dielectric strength reduces

👉 Partial discharge risk increases

👉 Breakdown of cellulose paper accelerates

Thus, drying-out ensures safe commissioning and long life of the transformer. ⚡🛡️

1. Methods of Drying Out a Transformer 🔧🔥

There are two main approaches:

A) Drying with the Tank Empty (No Oil) 🧰

The core and windings remain in the tank, but without oil. Heat is applied in two ways:

1. Hot Air Drying Method 💨🔥

Hot air is blown into the tank through the drain pipe or radiator outlet.

🌡️ Inlet air temperature: 85°C – 100°C

🌡️ Outlet air temperature: 65°C – 75°C

⚙️ Heating capacity: approx. 15 kW per 10 m³ tank capacity

Advantages:

✔️ Quick removal of moisture due to low-humidity air

✔️ Suitable for both small and large transformers

Drying Time:

🕒 ~4 days → 11 kV transformer

🕒 ~15 days → 220 kV transformer

2. Short-Circuit Heating Method 🔌⚡🔥

The secondary winding is short-circuited and a reduced voltage is applied to the primary.

This causes internal heating of the windings.

Limitations:

⚠️ Temperature distribution becomes uneven without oil

⚠️ Not recommended for large transformers

⚠️ Must keep top cover open for air flow

⚠️ Winding temperature must never exceed 90°C

This method is usually combined with hot air drying to reduce heater load.

B) Drying with the Transformer Filled With Oil 🛢️🔥

Heat can be introduced in three ways:

1. Short-Circuit Heating (with oil)

Used cautiously to raise oil and winding temperature.

Maintains safer heat distribution. 🌡️

2. Oil Circulation Through Purifying Plant ♻️💧

Purifiers heat the oil and continuously remove:

👉 Moisture

👉 Sludge

👉 Dissolved gases

👉 Particles

Ensures clean, high dielectric strength oil. ⚡🛢️

3. Immersion Type Heaters 🔥

Heaters are inserted directly into the tank to raise oil temperature uniformly.

Role of Vacuum in Drying-Out 🌬️🧭

Applying vacuum accelerates moisture removal drastically.

🔧 Vacuum should be at least 28 inches of Hg

🔧 Tank must withstand 15 psi external air pressure

💨 Vacuum vaporizes moisture at lower temperature

🛡️ Prevents overheating of cellulose insulation

Vacuum-based drying is one of the best industrial standards.

2. Time Required for Drying-Out Operation ⏳⚡

Drying time depends on:

👉 Initial moisture level

👉 Transformer size

👉 Purifier capacity

👉 Oil temperature

👉 Ambient humidity

Typical Duration:

🕒 1 day → small transformers

🕒 Up to 1 month → large power transformers

Important Note ⚠️

Raising temperature increases drying rate, but:

🚫 Temperatures above 90°C damage insulation

🚫 High heat decomposes cellulose → releases more water

Therefore, temperature must be strictly controlled. 🌡️🛑

When Is Drying Complete? 🎯

Drying is considered complete when:

🟢 Oil samples from top and bottom show high BDV

🟢 Insulation resistance (IR value) becomes high and stable

Let the transformer cool for 24 hours and test again.

If BDV and IR remain high → transformer is sufficiently dry. ✔️

3. Methods of Purifying and Drying-Out Transformer Oil 🧼🛢️

There are three commonly used oil purifiers:

1. Centrifugal Purifiers (De Laval type) 🌀

Use high-speed rotation to separate moisture and impurities.

2. Filter Pack Type Purifiers (Streamline type) 🧺

Oil passes through layered filter media to remove sludge and particles.

3. Activated Earth Media Filters 🌍

Absorb dissolved moisture and acidic products effectively.

Why Oil Purification Is Essential? 🔍⚙️

Transformer oil contamination occurs due to:

💧 Water (free or dissolved)

🖤 Carbon particles

🍂 Dirt and sludge

💭 Gases

🔥 Deposits from ageing or arcing

Low BDV indicates moisture or impurity presence → risk of internal flashover.

Regular purification maintains:

✔️ High dielectric strength

✔️ Better cooling performance

✔️ Longer insulation life

Final Summary 📘✨

Drying-out of a transformer is a critical commissioning process that ensures the insulation system is moisture-free and healthy.

Based on transformer size and condition, different methods like:

🔥 Hot air blowing

🔥 Short-circuit heating

🔥 Oil circulation

🔥 Immersion heating

🌬️ Vacuum drying

are used.

A properly dried transformer will have:

🟢 High BDV of oil

🟢 High and stable insulation resistance

🟢 Uniform temperature distribution

This ensures long-term reliability and safe operation. ⚡🛡️