There are three main forms of water in oil: free water, dissolved water, and emulsified water.
1. Free water (or sedimented water).
Moisture frees in the oil in the form of water droplets, or settles on the bottom of equipment or containers. Although it does not generally affect the breakdown voltage of the oil, it is not allowed because it indicates that the oil has dissolved water in the oil. The water settled at the bottom of equipment and containers can be directly discharged or disposed of, which is not harmful.
2. Dissolve water.
Water is dissolved in the oil as very fine particles, usually from the air into the oil, which indicates that the oil has been contaminated, and a small amount of water is generated during oil oxidation. Dissolved water can drastically reduce the breakdown voltage of the oil. When a high temperature is generated between the coil and the iron core of the transformer, the dissolved water will turn into a vapor state, and when the water touches the cold oil, it will form the dissolved water. If you want to completely remove the dissolved water, you can use a high-vacuum atomization method to remove it at a certain temperature, which is often referred to as "vacuum" filtering the oil.
3. Emulsified water.
Refers to the mixture of oil and ultra-fine water droplets (atomized water droplets). The oil is in the form of an emulsion. Whether it is heated, filtered, or clarified, such water droplets cannot be separated from the oil. Only under certain conditions, adding a suitable demulsifier can separate the oil and the water. Emulsified water is the most harmful to oil. The problems caused by emulsified water are similar to those caused by free water, namely rust, cavitation, micro-corrosion and oil degradation. The degraded oil can only separate a small amount of water, because the oxides (acids, aldehydes, ketones) are very characteristic and can encapsulate water droplets, preventing the fusion of water droplets and the separation caused by gravity.