The transformer oil or insulating oil is used in transformers as well as some electrical equipment to:

  • insulate the live parts,
  • maintain insulating properties of the material such as paper, winding, etc. used in the machine,
  • protect the machine parts from rust and corrosion,
  • increase the heat dissipation of the machine by convection.

This oil should be clean, free from moisture and other harmful contaminants and possess the following characteristics:
 
Dielectric strength (one minute) – 40 KV (RMS) with a 4 mm gap.
Viscosity at 27oC in centistokes-max. – 27
Pour point-max.  – minus 10oC.
Flash point-min. – 145oC.
Specific gravity – 0.85 to 0.88
Acidity at 27oC max. 0.05 mg KOH/g
Sludge value (using n-heptane)% by weight-max. – 1.2
 

Dielectric Strength Test of Transformer Oil

 
Oil stored in drums does not retain its original dielectric strength and should be tested and treated before use in electrical equipment. The following tests can be adopted before using the insulating oil.
 
The oil test in which spark occurs and the testing set trips is known as dielectric strength test of transformer oil. This test is performed in the oil testing set. Suitable oil testing sets, stationary as well as portable, are available for testing the oil under the specified conditions. A sample of the oil is drawn from the transformer and tested in the oil testing set. A lower than 30 KV dielectric strength for a 4 mm gap of electrodes would indicate the presence of moisture in the oil.
 
The recommended size of the glass test cell is 55 x 90 x 100 mm. The diameter of spheres or test electrode has been specified as 12.7 mm and air-gap between the spheres as 4 mm. the quantity of test sample should be drawn from the bottom of a drum, transformer, or other apparatus. The oil in the test cell shall be at least 40 mm above the electrodes and it should be allowed to stand for 20 minutes before performing the test to let air bubble escape.
 

Dielectric Strength Test of Transformer Oil – Procedure

 
The actual testing is done as follows:
 
The gap of the electrodes is first checked with a gauge. The cup is then filled with sample oil to be tested. The cup top is covered with a clean glass plate. Supply is switched on and the voltage is raised gradually by the Variac.

dielectric strength test of transformer

A stage comes when the final breakdown oil takes place and the circuit breaker is tripped which disconnects the testing set from the mains supply. This value is noted down which must be 30 KV (RMS) for 4mm gap and 20 KV (RMS) for 2.5 mm gap. The test is performed with two or three consecutive samples of oil turn by turn.
 
In the dielectric strength test of transformer oil, the test voltage should be raised to the specified value (or the breakdown voltage if it is less) at a uniform rate in 10-15 seconds and maintain for a one minute. Preliminary arcing should be ignored. The breakdown voltage is that voltage at which sustained arcing takes place. Test voltage should not be raised beyond 55 KV as flashover through the air may take place.
 
The need for taking samples in a manner to avoid contamination of the oil with dirt and moisture cannot be overstated. Only glass bottles with tin or aluminum foil caps which have been rinsed previously with oil under test should be used. Cotton waste or any other fibrous material for cleaning should be avoided. Dirt should be wiped from taps and covers before taking samples.
 
As we know, the water traces, sludge, dust particles, etc. are heavier than the oil so they are get collected at the bottom. Therefore, the dielectric strength of the oil at the bottom may be lower. Therefore, the oil should be taken from the bottom of drums with the help of a glass tube or ‘oil thief oil sampler’ sufficiently long to reach the bottom. To take the sample, the glass tube is lowered in the drum, and it’s open end is closed firmly with the thumb while withdrawing the tube.
 
The sample containers should be kept tightly covered throughout the period before the test. Proper record of breakdown strength of oil will be a good guide to the state of oil as oil with moisture and impurities has low breakdown voltage.
 

Moisture Test of Transformer Oil

 
The presence of moisture in a transformer is very harmful. It decreases the dielectric strength of oil and other insulating material such as winding insulation, paper, etc. significantly. It may be remembered that the presence of only 0.06% of the water will reduce the dielectric strength of oil to about one half..
 
Insulating paper is very hygroscopic material. It absorbs moisture content from the oil thereby decreasing the overall dielectric strength, insulation resistance and the life of the machine. As water solubility of oil increases at higher temperatures, therefore, the dielectric strength of oil may be lower at higher temperatures. So it is a good idea to take out oil samples from the machine at higher ambient temperatures.
 
Presence of free moisture in oil can be easily detected by pouring sufficient quantity of sample in test tube about 125 mm long and 12.5 mm in diameter, to fill it to a quarter of its depth and heating it rapidly in a silent flame until the oil begins to boil.  Any audible cracking during heating should be noted. The oil shall be considered to have passed the test if no cracking occurs with two out of three quantities tested.
 
Another simple way of checking the presence of free moisture in transformer oil is with the help of metal rod about 12.5 mm in diameter. The rod is heated to dull redness and the sample of oil is stirred with the heated rod. Sharp cracking sound distinct from sizzling would indicate the presence of moisture.
 
Periodic tests for acidity, sludge and flash point are very essential in the case of large-sized transformers. As even highly deteriorated oil will pass the dielectric strength test of transformer oil if it is dry. Careful recording of the oil of test samples and their comparison will help in anticipation of the impending trouble and adoption of the remedial measures.
 

Neutralization Test of Transformer Oil

 
Acids are formed in a transformer during operation. Their formation is particularly rapid at operating temperatures above 75oC. Acids are corrosive and damage the windings, core, tank, etc. They are also responsible for sludge formation.
 
To see that the oil is free from organic and inorganic acids, the neutralization test of transformer oil is performed. It is expressed in terms of the number of milligrams of potassium hydroxide (KOH) required to neutralize the total free acids in one gram of oil. This test is carried out in the laboratory or with the help of tests kits in the field. The recommended limits for acidity are as follows:

  • Acidity below 0.5 mg. KOH/gram is permissible if the oil is satisfactory otherwise.
  • Keep the oil under observation if acidity is between 0.5 and 1.0 mg. KOH/gram.
  • Discard the oil if acidity is above 1.0 mg. KOH/gram.

The acidity of oil should be checked every two years for transformers up to 1000 KVA capacity, and annually for transformers above this size. The actual period may be varied depending upon the severity of the service and observation of results.
 

Filtration of Transformer Oil

 
 If the breakdown strength of transformer oil is lower than the specified value the oil should be treated in a filtration plant. A plant which provides for heating filtration and drying under vacuum is to be preferred as filtering alone cannot remove moisture and dissolved air. Cold treatment of oil through the plant helps in removal of sludge.
 
In a filtration plant, first of all, oil enters the heating chamber where it is heated by immersion heaters. It is then passed through the filters for removal of suspended impurities and finally goes to the vacuum chamber where moisture is removed over a series of trays. The entire oil is circulated through the plant several times till satisfactory results are achieved.
 
The oil should be drawn from near the bottom and delivered back into the transformer tank at the top. The processes should be continued until the oil can withstand 30 KV for seven consecutive tests performed 4 hours apart.
 
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