Testing of Dielectric Transformer Oil
Q. Explain how following test is carried for transformers. Testing of dielectric transformer Oil
Dielectric strength test : The dielectric strength of transformer oil is carried out with testing kit which consist of a standard oil test which is approximately 100 X 60 mm in cross-sectional area and 85 mm high. In this cell two polished spheres 12.7 mm or 13 mm in diameter are horizontally fitted as shown in the figure. The gap spacing between the spheres is 2.5 + 0.02 mm. This gap can be adjusted attachment with the spheres. The test circuit consists of 230 V, 50 Hz supply to be connected to variac, the output is given to L.V. winding of step up transformer and the H. V. voltage from the secondary applied to the gap between spheres.
A voltmeter is connected on L. V. side, which is calibrated in terms of H.V. voltage direct really and of C.B. is also provide so that trapping of supply will take place at break down voltage.First the oil to be washed with fresh good transformer oil. Then test cell is filled with oil to be tested such that the spheres are below the oil level. Care is taken not to form the bubbles in the oil. The bubbles are removed with help of of an iron rod. The test cell is transparent.
The surrounding should be free from dust, dirt and moisture and the temperature in the man 15° C 10 35° C, then increase the voltage slowly with the help of transformer from zero value breakdown between the spheres takes place. At break down of insulating oil, the circuit breaked trips. The breakdown voltage is noted. The test is repeated for 6 times, after every 5 minutes of each test. The average of all breakdown voltage is calculated.