BDV (break down voltage) test is a regular test conducted on insulating materials. Accordingly, transformer insulating materials viz. transformer oil, paper, pressboard, too, are subjected to this test.
It is measured by observing at what voltage, sparking starts between the two electrodes (mushroom, disc, or spherical types). These electrodes are connected to an alternating voltage source of specified frequency.
Low value of the measured BDV indicates presence of moisture and conducting substances in the oil.
For measuring the dielectric strength of transformer oil, portable High Voltage BDV Insulation Test kits are generally available at site. Oil is kept in a test vessel (as prescribed by IS 6792 or IEC 60156) in which one pair of electrodes are fixed with a gap of 2.5 mm / 4.0 mm between them. The voltage is then raised gradually from Zero, at rate of rise of 2kV/Sec. The electronic modules keep on observing the voltage at which sparking starts between the electrodes. And, this voltage is recorded and is called the BDV of transformer oil.
For the material to pass the test, observed BDV should not be below the prescribed minimum limit.