What are the oil sample testing methods for oil-immersed transformers?
Time：2025-01-9 Auther：ZTelec-www.ztelectransformer.com
Oil sample testing for oil-immersed transformers is essential to ensure the normal operation of the transformer and extend its service life. By regularly testing the oil’s appearance, moisture content, dielectric strength and other parameters, oil deterioration or contamination can be detected in time. And then we can take corresponding maintenance measures to ensure the insulation performance and operating efficiency of the transformer. In addition, regular testing can also prevent equipment failures caused by oil quality problems and ensure the stable operation of the power system.
Oil sample testing method for oil-immersed transformers
Appearance inspection
Pour the oil sample into a clean, transparent glass container and observe it under sufficient natural light or white light.
Normal transformer oil should be clear and transparent, free of impurities and suspended matter, and light yellow or light brown in color. If the oil sample becomes darker in color, the transformer oil may be aged or contaminated; If the oil becomes cloudy or has sediment, it may contain moisture or mechanical impurities, or a chemical reaction may have occurred in the oil.
Moisture detection
The commonly used method is the Karl-Fischer method. The principle is to use iodine and sulfur dioxide to react quantitatively with water in the presence of pyridine and methanol. The amount of iodine consumed is measured by the instrument to calculate the moisture content in the oil sample. Generally speaking, the moisture content in transformer oil should be controlled at a low level, for example the moisture content in transformer oil of 35kV and below does not exceed 35mg/L, and the moisture content in transformer oil of 110kV and above does not exceed 20mg/L.
Dielectric strength test of oil
This test is used to measure the insulation capacity of transformer oil. By applying a gradually increasing voltage, the breakdown voltage in the oil is measured to determine whether the oil has good insulation properties. Low dielectric strength may indicate that the oil contains too much water or impurities.
Density measurement
Use a densitometer to measure the density of the oil sample.
Viscosity measurement
Use a viscometer to measure the viscosity of the oil sample.
Flash point test
It is the lowest temperature at which heated oil begins to emit enough vapour to combine with air to generate a flammable moisture. The purpose of this test is to determine the oil’s volatility.
Pour point test
Use a pour point meter to measure the pour point of the oil sample. This test is highly essential, especially if the transformer is situated in an extremely cold environment. This test is highly essential, especially if the transformer is situated in an extremely cold environment.
Dissolved Gas Analysis (DGA)
By detecting the type and concentration of gas dissolved in transformer oil, it is possible to determine whether there are any faults such as overheating, discharge or insulation breakdown inside the transformer. DGA can detect potential faults of the transformer at an early stage and perform preventive maintenance.
Neutralization Number or Acid Number
This test, is performed to find out how much acid is present in the oil. In new transformer oil, there is typically no acid content, however this could change due to oxidation and contamination. The test involves calculating how much potassium hydroxide (KOH, expressed in mg) is needed to neutralise the acid in one gram of oil.
