How to remove moisture from transformer oil?

If you’ve ever worked with a transformer, you know that one of the key factors in keeping it in top condition is its oil. But there’s a hidden danger lurking in that oil moisture. Moisture in transformer oil can cause a variety of problems, including malfunctions, overheating, and reduced efficiency. So, let’s delve deeper into how to remove moisture from transformer oil and which methods are best for keeping transformer oil clean and dry.

First， why is it a problem to have moisture in the transformer oil?

First and foremost, water and electricity don’t mix. The same goes for transformer oil. Excessive water content in the oil can lead to several serious problems:

• Insulation failure: Transformer oil is an insulator. When water is present in the oil, its insulating properties degrade, leading to short circuits and even electrical fires.
• Oil degradation: Water accelerates the oil’s oxidation process. Over time, this creates acidic sludge, which can damage transformer components.
• Overheating: Water in the oil absorbs heat, causing overheating, which reduces the transformer’s efficiency and lifespan.
• Increased risk of failure: As humidity increases, the likelihood of arcing increases, making the transformer more susceptible to failure.

So， what is the moisture limit of transformer oil?

How much is it? Generally, the maximum safe level of moisture is 20 ppm (parts per million). Ideally, for optimal performance, keep the moisture content below 5 ppm.

Now, let’s talk about how to remove this moisture.

Next， how to remove moisture from transformer oil?

There are many methods, but today I want to focus on one of the most effective and common: vacuum dehydration using a transformer oil filter. However, I’ll also mention a few other methods if you’re interested.

Vacuum Dehydration

Aspect	Details
Features	In a vacuum environment, the boiling point of water is lower, allowing the water to evaporate quickly and be separated through the condensation system.
Efficiency	Highly effective in removing free water, dissolved water, and emulsified water from transformer oil.
Advantages	Rapidly removes moisture from transformer oil, restoring its insulating properties. Does not damage the oil’s chemical composition, preserving its original characteristics. Suitable for different types of transformer oils (such as mineral oils or synthetic oils).
Disadvantages	High initial investment for equipment. – Requires a longer operational period, making it more suitable for transformers with larger oil volumes or those in long-term use.
Application	Ideal for large-scale transformer oil purification and moisture removal, especially for transformer oils that have been in use for long periods and have accumulated significant moisture.

Working Principle

The core of vacuum dehydration is to exploit the lower boiling point of water in a low-pressure environment, causing it to rapidly evaporate into gas. With the help of a vacuum system, the water is separated from the oil and recovered through a condensation system. This process is accomplished through the following steps:

• Oil Heating: Transformer oil enters the oil filter system and is heated to a certain temperature by a heating device, causing the water in the oil to evaporate.
• Vacuum Extraction: The system draws air in through a vacuum pump, creating a low-pressure environment. Water evaporates more easily under low pressure, effectively removing moisture.
• Condensation and Separation: The evaporated water condenses into droplets through a condensation pipe and is collected and removed by the system.
• Purified Oil Output: The clean oil after vacuum dehydration is discharged, with the water content reduced to a minimum.

Process Flow and Steps

The steps of vacuum dehydration using a transformer oil purifier:

• Pretreatment: Initial filtration and impurity removal of the transformer oil ensures that the oil is as pure as possible before entering the dehydration system.
• Oil Heating: The transformer oil is heated to a certain temperature to cause the water in the oil to evaporate. Vacuum Extraction: A vacuum pump extracts air from the system, creating a negative-pressure environment that lowers the boiling point of water and accelerates evaporation.
• Moisture Separation: A condensation system collects and removes the evaporated water, ensuring complete removal of moisture from the oil.
• Purified Oil Output: The purified transformer oil is output, with the moisture content reduced to below the specified standard.

Best Transformer Oil Dehydrator

read more：7 Best Transformer Oil Filtration Machines in 2025: Reviews & Buying Guide

Other Methods

• Heat Dehydration： Heat is used to evaporate the water from the oil, which is then separated through a condensation system. This method is typically used for oils with low moisture content, but the treatment effect is not as good as vacuum dehydration and can easily affect the oil’s properties.
• Condensation Separation： Condensation separation primarily uses temperature differences to condense the evaporated water. It is suitable for removing free water and small amounts of dissolved water. Its advantage is that it does not require high energy consumption, but it is not effective in removing emulsified water.
• Water Filtration： This method removes water from the oil through a filter. It is suitable for the initial purification of water from oil, but the treatment efficiency is relatively low.
• Centrifugal Separation： Centrifugal separation uses high-speed centrifugal force to separate water and impurities from the oil. It is suitable for oils with high moisture content, but its efficiency is relatively low for oils of good quality.