If the moisture content in transformer oil exceeds the specified standard, it will affect the performance and service life of the transformers. It will reduce the insulation performance of the transformer oil, and increase the risk of electrical failures in the equipment if the moisture exceeds the standard; the presence of moisture in the transformer oil will accelerate the oxidation process of the oil, leading to deterioration of the oil quality and ultimately shortening the service life of the equipment. Therefore, It is vitally important to strictly control the moisture content in transformer oil, which ensures the stable operation of the equipment.
The standard for the moisture content in transformer oil depends on the voltage level. Based on the National standard GB50150 - 2006, the moisture content of transformer oil for 500kV transformers should not exceed 10mg/L, for 220kV - 330kV transformers it should not exceed 15mg/L, and for 110kV transformers and below, it should not exceed 20mg/L.
How to do the oil moisture test?
Usually, It adopts the Karl-Fischer coulometric method
Principle: When there is water present, iodine is reduced by sulfur dioxide in the presence of methanol, and hydrogen iodic acid pyridine is generated. During the electrolysis process, the iodine produced reacts with the water in the sample to form hydrogen iodic acid until all the water has reacted. The Oil moisture tester uses a test unit composed of a pair of platinum electrodes for sampling. During the test, reagents must also be added. This method is complex in principle, has been in use for 30 years, and has not changed much.
The test method is complex and time-consuming, with low accuracy, but it is much cheaper. The oil moisture tester with Karl-Fischer testing method is relatively easy to identify in appearance. It has glassware typically and widely used in China.