The dielectric test is carried out to check whether the dielectric strength of the oil is still within telerable limit i.e more than 30-kV after six (6) times voltage injection across 2.5mm gap (as per IEC standard).
The procedures to carry out the tests are as follows :
1. make sure the transformer oil temperature is relatively cool (about room temperature);
2. identify the location of sampling valve;
3. open the outer protective valve cap;
4. loosen the screw (valve) slowly until droplet of oil appears to flow out;
5. wet the inside beaker with oil by letting the oil flow into the beaker (rotating the beaker);
6. after the flow of oil into the beaker is constant and the beaker is about 20% full; throw away the initial oil;
7. slowly start the oil sampling by letting small jet of oil to flow into the beaker through the side wall;
8. make sure that the flow is smooth and no bubble is collected;
9. collect enough volume of oil into the beaker (at least 50% full)
10. once the sampling oil is adequate, wipe the outside beaker with clean rug/cloth
11. insert the beaker into the test compartment of the Oil Test Set
12. switch on the oil test set
13. select the desired standart of oil test
14. press the start button
The oil test will be carried out automatically by the set. The test is carried out by :
a. stirring the oil inside the beaker for 5 minutes;
b. slowly increase the voltage across the gap until the flash-over occurs
c. the voltage at which flash-over occured is the dielectric break-down voltage
d. after the flash-over, voltage will be cut-off
e. the fan will stir up the oil again for 2 mintes;
f. after 2 minutes has been reached, the voltage will be increased again until flash-over ocuur again;
g. this will be the second break-down voltage;
h. the 2-minute stirring and voltage injection test is repeated for another four (4) times;
i. after a total of 6 tests have been carried out, the average value is taken.
If the average break-down or flash-over voltage value is > 30-kV, the Oil has PASSED the test and likewise if the average break-down voltage is less than 30-kV, the oil has FAILED the test.
If the oil test fail, the oil can still be saved by carrying out Oil Filtration process. This service could be engaged from Electrical Service Contractor who has an Oil Filtration Machine.