Testing of Electrical Transformer Oil

Testing of Transformer Oil
Question
Q. Explain 1. Crackle 2. flash point 3. Acidity 4. Sludge test.
Q. State the properties of good transformer oil. List out the various tests to be performed on transformer oil.
Q. Explain how following test is carried for transformers : Testing of dielectric strength of transformer oil.
Q. Explain how following test is carried for transformers : Crackle test.
Q. With the help of neat figure explain the dielectric strength test on transformer oil.


Answer


The transformer oil should be checked periodically, and its suitability is justified.

 (a) Flash point test :

• In this test the transformer oil is heated continuously till a point is reached at which the vapour is formed on the surface of oil fires spontaneously.

• The temperature of oil at this point should not be less than 130°C to 140°C.

(b) Crackle test :

• The objective of this test is to determine the presence of water or moisture in the oil.

• In this test a sample of transformer oil under test is taken in a test tube. It is heated on a gas burner, if water or moisture is present in the oil, cracking noise is.heard.
• In other method, sample oil taken in the glass beaker of 250 ml. An iron rod of 12.5 mm size is heated to red hot and then plugged into oil under test, if a crackling noise is heard it indicates the presence of moisture in oil.

(c) 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 is noted. The test is repeated for 6 times, after every 5 minutes of each test. The average of all breakdown voltage is calculated.

• It is conducted to determine acid content in the oil.
• Take 10 gms of sample oil in the 250 ml conical flask. Then take 50 ml of alcohol in another flask and add 10 ml of phenolphthalein solution. This mixture is heated to 40-50° C and neutralized with a solution of KOH, by quick titration.
• Now this neutralized alcohol is added to the oil sample taken in the first flask. The sample is heated to boiling point for at least 5 minutes.
• To this, is then added 1 C.C. of phenolphthalein and is cooled to 40 to 50°C and titrated quickly with KOH solution.

• If the acidity is both 0.5 to 1 mg KOH/gram the oil should be reconditioned.
• And if it is more than 1 mg KOH/gram the oil should be rejected.
• The acidity should be less than 0.5 mg KOH/gram for good Transformer oil