Transformer Oil BDV Test

The BDV of transformer oil is breakdown voltage of transformer oil. The BDV or dielectric strength of oil is  the maximum voltage withstand capacity of the oil without breakdown. BDV test of transformer oil oil is very important for trouble free operation of transformer. Therefore, periodical testing of the transformer oil BDV is compulsory for ensuring healthiness  of the transformer oil.

As per IEC, the minimum  BDV of transformer oil should be 30 KV as per transformer oil BDV test standard.   

The transformer oil is used in the transformer for insulation as well as for cooling purpose. The different insulating materials have different dielectric strength. The insulating material should sustain the voltage up to its dielectric strength. If the voltage is increased above the specified dielectric strength of the insulating material, the current starts flowing through it and eventually the material get permanently punctured.

The transformer oil is hygroscopic in nature i.e it tends to absorb the moisture quickly. The dielectric strength of the oil gets deteriorated with the absorption of the moisture. That is why the breather filled with silica gel (Moisture absorbent material) is used in transformer so that external moisture during breathing can be trapped in silica gel.

Core and winding of the power transformer is submerged in transformer oil. The heat generated inside the transformer due to iron loss and copper loss needs to be dissipated outside of the transformer to limit the temperature to its safe limit. The heat generated inside the transformer is transferred to oil and the oil transfers the heat to the outer tank body and, the temperature is dissipated in the atmospheric air. The quality of the transformer oil has utmost importance for reliable operation of oil filled transformer.

The reliability of the transformer can be ensured if all the parameters of the transformer oil are as per the standard.

The one very important parameter – Dielectric strength or breakdown voltage (BDV) of the oil must be periodically checked to ensure trouble-free operation of the transformer.

The dielectric breakdown voltage test is a quick and easy way of determining the contamination in the transformer oil. The water is the major contaminant however, other contaminants like conductive particles,dirt,debris,insulating particles and by product of oxidation and aging of the transformer oil can deteriorate the dielectric strength of the mineral insulating transformer oil.

Purpose of   BDV Test of  Transformer Oil

The breakdown voltage test (BDV test) is useful for;

• Predicting the remaining life of the transformer
• Enhancing operational safety
• Preventing equipment fires
• Maintaining transformer reliability

Minimum BDV of Transformer oil

As per IEC, the minimum breakdown voltage of transformer oil should be not less than 30 KV. The BDV of transformer oil does not depend on the voltage rating of the transformer. Minimum transformer BDV   value  for either 11 KV, 22 KV, 132 KV  is the same. The minimum transformer oil BDV  is 30 KV.

What is BDV Tester?

The sample is taken out from the transformer tank and the BDV of the oil is checked by BDV tester. The BDV test kit is basically a high voltage unit and the voltage can be regulated from 0–70 KV. The BDV tester has two electrodes separated by 2.5 mm distance( 4.00 mm separation units are also available).

How to Calculate BDV Value of Transformer Oil?

Test Procedure of Oil BDV Test:

” The temperature of the transformer oil sample must be 27°C.”

Steps for Transformer oil BDV Testing

1. Collect the 300-400 ml  oil sample of transformer oil from the bottom valve of the transformer in a glass or plastic vessel.
2. Pour the oil sample in oil pot of the machine.
3. Keep the oil pot in open air for 5 Minutes so that air bubble if any gets out. Further,the vessel should be disturbed to get out of the air bubbles and for homogeneous distribution of impurities.
4. Now put the oil sample in the machine.
5. Increase the KV at the rate of 2 KV/Sec and observe the oil sample through a glass window.
6. Continuously observe the oil sample through an inspection window while increasing the voltage.
7. Note down the KV at which sparking is observed in the oil. The voltage at which sparking observed is the breakdown voltage of oil.
8. Repeat the step No. 4,5,6 and  7 for the same sample for  six times and note down the breakdown KV in each case.
9. Take the average of BDV values observed in step 7 and 8.
10. If oil  BDV value is above 30 KV, transformer oil is good.

Transformer oil BDV Test Report Format:

” If the BDV of transformer oil is measured just after filtration of oil, it will give the erroneous reading because of high temperature. The transformer oil should be cooled down to 27°C temperature  before carrying out BDV test.”

Factors which affect the Transformer Oil BDV Value

The main factors affecting the breakdown voltage of insulating oil are follows.

1. The water is the major contaminant in transformer oil. Moisture delivers charge carriers and therefore decreases  the dielectric withstand strength of the oil.
2. Aging by-products such as acids also deliver charge carriers through dissociation. Additionally they are surface-active, decreasing the surface tension. Thus they support bubble evolution following into a decreased dielectric strength.
3. Pressure influences bubble evolution too. With increasing pressure the breakdown voltage increases. For pressures below the atmospheric pressure the breakdown voltage should decreases.
4. Dry particles of cellulose fibers decrease the breakdown voltage  since they support bubble generation. 

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