Dielectric Strength of Transformer Oil

The Dielectric Strength of Transformer Oil is also known as the Breakdown Voltage (BDV) of Transformer Oil. BDV is measured by observing at what voltage, sparking strands between two electrodes immersed in the transformer oil, separated by a specific gap, which can be tested by a portable transformer oil bdv tester. A low value of BDV indicates presence of moisture content and conducting substances in the transformer oil.  

Insulating oil is the main insulation part of oil-filled electrical equipment such as power transformers, circuit breakers, etc. The insulation strength of transformer oil is one of the main insulation performance indicators that evaluates its ability to adapt to the dielectric strength of the electric field voltage without causing damage to electrical equipment. It is to ensure the safe operation of oil-filled electrical equipment. 

If impurities fall into the oil or get damp due to absorbing moisture in the air, or deteriorate due to aging of the oil, the dielectric strength of the oil will be reduced, which will affect the good insulation of the equipment, or even break down the equipment, causing an accident. Therefore, testing the oil in operation on a regular basis can facilitate the discovery of problems and timely treatment. This has certain significance for preventing accidents and ensuring safety.

Electrodes gap in transformer oil test kit for measuring BDV of transformer oil, portable BDV measuring kit is generally available at site. In this kit, oil is kepin a #pot in which one pair of electrodes arefixed with a gap of 2.5 mm (in some kit it 4mm) between them. Now slowly rising voltage is applied between the electrodes. The rate of rising voltage is controlled at 2 KV/s and observe the voltageat which sparking starts between the electrodes.That means at which voltaqe dielectric strength of transformer oil between the electrodes has been broken down.

Generally, there are three types of electrodes of a withstand voltage tester, namely, semisphere, mushroom and flat. 

Experiments have proved that the same oil sample uses three different shapes of electrodes under the same conditions, and the measurement results are different. Generally, the measured dielectric strength value of the mushroom electrode is the highest, followed by the semisphere shape, and the flat plate shape is the lowest. When the breakdown voltage of the oil is above 30kV, if the test value of the flat electrode is the standard, the mushroom electrode is roughly 4-6kV higher, and the semisphere cap shape is roughly 1-2kV higher. For example, when the breakdown voltage of oil is below 30kV, the above difference tends to be shortened or is basically the same. The use of Semisphere electrodes or  Mushrrom shaped electrodes is more popular in current electric fields.

In order to ensure safety, both new oil and purified oil should be tested for dielectric strength before use. Oils in operation should also be tested regularly for their dielectric strength. Only those that meet the requirements of national standards can be used. This measurement is taken 3 to 6 times in the sam sample of oil, and we take the average value of these readings. Dry and clean oil gives better BDV value results than the contaminated transformer oil with moisture content and other conducting impurities. Minimum breakdown voltage of transformer oil or dielectric strength of transformer oil at which this oil can safely be used in transformer is considered as above 30 KV. Portable transformer oil tester is required at site to perform the regular testing before filling new transformer oil into power transformers, or judge the effect of transformer oil filtration.

If the test BDV value is below the required value, vacuum dehydration & filtration plant is needed to remove moisture, gases and solid particles, mechanical impurities, as well as acids and other oxides. Thereby improving the dielectric strength of transformer oil. When the oil bdv test result is ok, and only need to filter it before filling into power transformers, the portable oil transfer unit can be used to realize this function.

PureTech is a manufacturer specialized in oil filtration, oil regeneration, oil testing, and waste oil recycling, if you have any problems in oil treatment, please contact us for support.