Dielectric Strength Test on Transformer Oil
Q. With the help of neat figure explain the dielectric strength test on transformer Oil.
Dielectric strength test :
The dielectric strength of transformer oil is carried out with testing kit which consists of a standard oil test cell 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 by the 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 is applied to the gap between spheres. A voltmeter is connected on L.V. side, which is calibrated in terms of H.V. voltage directly and of C.B. is also provided so that tripping of supply will take place at break down voltage.
First the oil to be tested is taken out from the drain cock of the transformer tank in clean bottle. The test cell is first 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 the help of iron rod. The test cell is transparent. The surrounding should be free from dust, dirt and moisture and the temperature in the range of 15°C to 35°C, then increase the voltage slowly with the help of transformer from zero value till the breakdown between the spheres takes place. At break down of insulating oil, the circuit breaker trips. The breakdown voltage noted. The test is repeated for 6 times, after every 5 minutes of each test. The average of all breaks voltage is calculated.