1. What is BDV Test in transformer?
BDV means break-down voltage, also named as Dielectric Strength Valve. BDV Test is an very important testing procedure for the insulation oils in transformer. During the transformer operation for a period, insulation oils will become dirty/ degradation which results in loosing function of the oil in transformer as insulation and cooling, in that case, transformer will face some serious problems.
Periodically BDV Testing is the main way to monitor and check the oil’s quality.
When the testing result shows a very low value, the oil has to be replaced or purified.
2. How to proceed the Dielectric Strength Test for transformer oil?
The transformer oil under test is subjected to an AC electric field with continuously increasing voltage till the oil breaks down. The test cell shall be of glass or plastic and transparent and non-absorbent (Eg. Methyl Methacrylate Vessel) with effective volume between 300 to 500 ml.
The electrodes must be Copper, Brass, Bronze or Stainless steel and well polished having spherical shape with dip 12.5 mm to 13 mm. It shall have a spherical front as shown in the figure. The electrodes shall be mounted horizontally in the cell and the axis shall be immersed at a depth of approximately 40 mm, The electrode gap shall be 2.5 ± 0.1 mm.
Before carrying out the test the cell shall be cleaned by rinsing with the test oil. The sampling vessel containing the test oil shall be gently agitated to have homogeneous distribution of impurities and air bubbles to escape. The oil temperature at the time of testing shall be around 27°C (15°C to 35°C preferable).
An increasing AC voltage of rated frequency is applied to the electrodes, approximately at the rate of 2 kV/Sec, starting from zero upto the value producing breakdown. The test kit will have provision for automatic switching off of the supply voltage within 0.02 second.
The test shall be repeated six times on the same cell filling and the arithmetic mean of the under test six of in results is noted as the electric strength or BDV (Break Down Voltage) of the oil under test.
The test shall be conducted in a dry place free from dust and voltage applied every time after disappearance of any air bubbles. The time intervals shall be five minutes, if the disappearance of air bubble cannot be observed.
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