Testing of breakdown strength of transformer oil.

Experiment No.: – 11

Aim of the Experiment: –

Testing of breakdown strength of transformer oil.

Apparatus Required: –

Sl. No.	Name	Remarks
1	BDV Test Kit	1 Nos.
2	Transformer Oil	As per required

Circuit Diagram: –

Theory: –

Insulating oil used in electrical power transformer is commonly known as transformer oil. It is normally obtained by fractional distillation and subsequent treatment of crude petroleum. That is why this oil is also known as mineral insulating oil. Transformer oil serves mainly two purposes one it is liquid insulation in electrical power transformer and second, it is dissipates heat of the transformer i.e. acts as coolant. In addition to these, this oil serves other two purposes, it helps to preserve the core and winding as these are fully immersed inside oil and another important purpose of this oil is, it prevents direct contact of atmospheric oxygen with cellulose made paper insulation of windings, which is susceptible to oxidation.

Types of Transformer Oil: –

Generally there are two types of transformer oil used in transformer,

1. Paraffin based transformer oil.
2. Naphtha based transformer oil.

Naphtha oil is more easily oxidized than paraffin oil. But oxidation product i.e. sludge in the Naphtha oil is more soluble than paraffin oil. Thus Sludge of naphtha based oil is not precipitated in bottom of the transformer. Hence it does not obstruct convection circulation of the oil, means it does not disturb the transformer cooling system. But in the case of paraffin oil although oxidation rate is lower than that of Naphtha oil but the oxidation product or sludge is insoluble and precipitated at bottom of the tank and obstruct the transformer cooling system. Although Paraffin based oil has above mentioned disadvantage but still in our country it is generally used because of its easy availability. Another problem with paraffin based oil is its high pour point due to the wax content, but this does not effect its use due to warm climate condition of India.

Properties of transformer Insulating oil: –

Some specific parameters of insulating oil should be considered to determined the serviceability of that oil.

Parameters of transformer oil

The parameters of transformer oil are categorized as,

1. Electrical Parameters: – Dielectric Strength, Specific resistance, Dielectric dissipation factor.
2. Chemical Parameters: – Water content, acidity, sludge content.
3. Physical Parameters: – Inter facial tension, viscosity, flash point, pour point.

Electrical Parameter of transformer oil: –

Dielectric Strength of transformer oil:

Dielectric strength of transformer oil is known as breakdown voltage of transformer oil or BDV of transformer oil. Break down voltage is measured by observing at what voltage, sparking starts between wo electrodes immerged in the oil, separated by specific gap.

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

Procedures:

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. Check the gap between the two electrodes of the machine and pour the oil sample in oil pot of the machine. (the gap between the two electrodes of the testing kit is generally 4mm)
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. (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.)

Observation Table:

Conclusion:

To be written by student.

for your knowledge.

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.