TESTING OF TRANSFORMER OIL

The oil is poured in a container known as test-cell which has internal dimensions of 55 mm × 90 mm
× 100 mm high. The electrodes are polished spheres of 12.7 to 13 mm diameter, preferably of brass,
arranged horizontally with their axis not less than 40 mm above the bottom of the cell. For the test, the distance between the spheres shall be 4 + 0.02 mm. A suitable gauge is used to adjust the gap. While
preparing the oil sample, the test-cell should be thoroughly cleaned and the moisture and suspended
particles should be avoided. Fig. 1.13 shows an experimental set-up for finding out the dielectric strength
of the given sample of oil. The voltmeter is connected on to the primary side of the high voltage
transformer but calibrated on the high voltage side.





The gap between the spheres is adjusted to 4 mm with the help of a gauge and the spheres are
immersed in oil to a depth as mentioned earlier. The voltage is increased gradually and continuously till
a flash over of the gap is seen or the MCB operates. Note down this voltage. This voltage is known as
rapidly-applied voltage. The breakdown of the gap has taken place mainly due to field effect. The
thermal effect is minimal as the time of application is short.
Next bring the voltage back to zero and start with 40% of the rapidly applied voltage and wait
for one minute. See if the gap has broken. If not, increase the voltage everytime by 2.1/2% of the
rapidly applied voltage and wait for one minute till the flash over is seen or the MCB trips. Note down
this voltage.
Start again with zero voltage and increase the voltage to a value just obtained in the previous
step and wait for a minute. It is expected that the breakdown will take place. A few trials around this
point will give us the breakdown value of the dielectric strength. The acceptable value is 30 kV for 4
mm applied for one minute. In fact these days transformer oils with 65 kV for 4 mm 1 minute value are
available. If it is less than 30 kV, the oil should be sent for reconditioning. It is to be noted that if the
electrodes are immersed vertically in the oil, the dielectric strength measured may turn out to be lower
than what we obtained by placing the electrodes in horizontal position which is the normal configuration. It is due to the fact that when oil decomposes carbon particles being lighter rise up and if the
electrodes are in vertical configuration, these will bridge the gap and the breakdown will take place at
a relatively lower value.