Transformer Oil Test
The transformer oil should be checked during periodic maintenance as well as suitability of new oil to be decided. The sample is taken into a standard oil test cell. These tests are recommended as per I.S.
(a) Crackle Test of Transformer Oil :
It is initially a test aimed to determine the presence of water or moisture in the oil. It is a supplementary test to a di-electric strength test and not a substitute for the same. The sample of the oil to be tested is taken into a clean and dry container and a metal rod 12.5 mm in diameter heated to a dull redness be lowered down and the oil is stirred with this rod in the container. If the cracking sound is heard this indicates the presence of moisture in the oil. If no sound is heard then oil is moisture-free.
(b) Flash Point Test of Transformer Oil :
These test should be taken only for the oils which are subjected to frequent faults which increase temperature to a very high value, or due to internal faults or if the oil shows any sign of un-usual orders. In this test, the sample of oil is heated continuously and its temperature is measured. Oil vapour is formed due to such continuous heating. The vapour when mixes with air forms a mixture which in ignitable. Under the prescribed conditions a small pilot pen flame is applied and the mixture give momentary flash. This temperature if less than 130°C to 140°C the oil is good for using as insulation.
(c) Di-electric Strength Test of Transformer Oil :
C.B. = Circuit breaker
AT = Auto
transformer
HVT = High voltage step-up transformer
As shown in the Figure, the sample of transformer oil is taken into the test cell. The test cell is made up of transparent glass. Its volume is about 500 ml and is covered with a lit on the top. Two stainless steel polished electrodes in the form of spheres of diameter 12.5 mm are located in the test cell and the gap of 2.5 mm is maintained between them. They are connected to the high voltage secondary of the transformer. The primary of the high voltage transformer is supplied with a variable voltage from autotransformer. The autotransformer is connected to the ordinary 230 volts A.C. 50 Hz supply. For safety, a suitable circuit breaker is connected between the supply and autotransformer.
The location of test spherical electrodes is such that their axis is dipped in oil approximately 40 mm. The gap between them is adjusted to 2.5 mm which can be checked by filler gauge.
Procedure of Transformer Oil Test :
After setting properly the electrodes in their place, a test sample of transformer oil is poured in the test cell-using dry and clean glass rod so that while pouring, gas bubbles are not formed. The surroundings should be dust, dirt and moisture free and temperature in the range of 15 to 35°C preferably 27°C. Increase slowly the voltage with help of autotransformer 2 kV per second from initial zero value till breakdown between the spheres takes place. At the breakdown, the circuit breakes trip. The breakdown voltage is noted. The test is repeated at least 6 times. These breakdown tests are taken after every five minutes. Each time the breakdown voltage is noted. The average of the six readings is worked out. This is di-electric strength of the oil under test. The oil should show the permissible limits as per table.
|
Equipment’s rated voltage |
Breakdown voltage in test |
|
Less than 70 kV |
Above 30 kV |
|
70 to 170 kV |
Above 40 kV |
|
Above 170 kV |
Above 50 kV |
(d) Acidity Test of Transformer Oil :
This test is carried out to know the acid contents in the oil. As explained earlier, presence of acid is harmful.
The test procedure is as follows :
Take 10 grams of oil in the conical flask of 250 ml capacity. Add to it an amount of neutralized alcohol. This mixture is heated for 5 minutes for boiling it. To ensure that the acid is extracted, the boiled mixture is taken into unstoppered flask.
Then 1 ml of phenolphthalein is added and the mixture is kept for cooling upto the temperature of 45º. Then potassium hydroxide (KOH) is mixed for quick titration.
The total acidity = 56.1 x N x V/W
Where, N = normality
of KOH
V =
volume of KOH
W = mass
of sample in grams
If acidity is between 0.5 to 1 mg.KOH/ gr the oil should be reconditioned; and if I is more than 1 mg KOH/ gr the oil should be rejected. The acidity should be less than 0.5 mg KOH/ gr for a good oil.
(e) Sludge Test of Transformer Oil :
The oxidation results in the formation of insoluble material called as ‘sludge’. Its formation is due to exposure to atmosphere and also due to high temperatures.
Testing procedure
The used oil is the sample. Take it in the container and dilute it with n-heptane. This diluted solution is then filtered through paper. The filter paper is washed. Through filter paper 3-parts of xylene and 1-part of acetone is poured. If the filtrate appears brown in colour then this is the indication of presence of lot of sludge in the coil.