Measures the voltage at which the oil fails, which indicates the amount of contaminants (usually moisture) in the oil. Dielectric breakdown voltage is measured by applying a voltage between two electrodes underneath the oil. New oil must have a minimum dielectric strength of 35 kV according to ASTM test methods. Dielectric strength of transformer oil also termed as the breakdown voltage (BDV) of transformer oil. BDV is that value of applied voltage where sparking occurs between two electrodes which as completely immersed in the oil with the gap separated by a specific value which is proportional to the applied voltage. If the value of BDV is quite low this explains the presence of moisture content and conducting substances in the oil. Breakdown voltage is a minimum voltage applied to an insulation, resulting in its breaking down and becoming electrically conductive. At the same time dielectric strength is a maximum electric field that the insulation can withstand in ideal conditions without breaking down
Dielectric strength of transformer oil experiment
Into a container known as a test cell with internal dimensions of 55 mm x 90 mm x 100 mm the oil is poured in it. The electrodes are polished balls with a diameter of 12.7 to 13 mm, usually of brass, which is arranged horizontally in such a way that their axis is no less than 40 mm above the cell bottom. For testing, the separation between the balls should be 4 + 0.02mm. An appropriate scale is used to adjust the gap. While preparing the oil sample, the test cell should be thoroughly cleaned avoiding moisture and suspended particles. The figure below shows an experimental setup to detect the dielectric strength of a given oil sample. Although the voltmeter is calibrated on the high voltage side it is connected on the the primary side of the high voltage transformer.
The separation between the spheres is set around 4mm with the help of a gauge and the spheres are completely dipped in oil as mentioned earlier. The voltage increases linearly and gradually until a spark or flash occurs sometime accompanied by the tripping of the MCB. Note down this value. This voltage is known as fast applied voltage. The breakdown of the gap occurred mainly due to the field effect. The thermal effect is minimal as the application time is short.
Then set the voltage to zero and restart at 40% of the rapidly applied voltage and wait for one minute. See if the break is broken. If the increase in voltage by 2.1 / 2% of the rapidly applied voltage each time and wait one minute until the MCB flashes or trips is visible. Note down this value again.
Start again at 0V and increase the voltage to the value found in the previous step and wait a minute. The breakage is expected to take place. Several attempts at this point will give us the breakdown dielectric strength. Acceptable value is 30 kV for 4 mm applied for one minute. In fact, 65 kV transformer oils are available these days at 4 mm per minute.
If it is less than 30 kV, the oil must be sent for repair. It should be noted that if the electrodes are vertically immersed in oil, the measured dielectric strength may be lower than that obtained by placing the electrodes in a horizontal position, which is a normal configuration. This is due to the fact that when the oil decomposes, the lighter carbon particles rise upward and if the electrodes are in a vertical position, they close the gap, and the breakdown will occur at a relatively lower value.