Transformer Oil BDV (Breakdown Voltage) Testing Procedure

Transformer oil breakdown voltage is BDV. The BDV, or dielectric strength, of oil is its maximum voltage withstand without breakdown. 

Transformer oil BDV testing is essential for trouble-free operation. Therefore, transformer oil BDV must be tested periodically to verify its health.

The IEC transformer oil BDV test standard requires 30 KV.   

Transformer oil insulates and cools. Insulating materials vary in dielectric strength. 

Insulating material should withstand voltage up to dielectric strength. 

The insulating material becomes permanently pierced if the voltage exceeds its dielectric strength and current flows through it.

Transformer oil absorbs moisture quickly. 

Moisture reduces oil dielectric strength. Because silica gel breathers capture external moisture, transformers employ them.

1. Core and 
2. Winding 

of power transformer submerged in the transformer oil. 

To maintain a safe transformer temperature, iron and copper loss heat must be dissipated outside. 

The transformer’s heat is transferred to oil, which transfers it to the exterior tank body and dissipates it in the air. 

The quality of transformer oil is essential for a filled transformer’s reliability.

Transformer reliability is ensured if all transformer oil characteristics are standard.

The dielectric strength of the oil, also known as the breakdown voltage (BDV), must be checked on a regular basis to ensure that the transformer functions effectively.

Transformer oil contamination can be quickly determined using the dielectric breakdown voltage test. Major contaminant is water. 

1. Conductive particles, 
2. Dirt, 
3. Debris, 
4. Insulating particles, and 
5. Transformer oil byproducts of oxidation and ageing 

can reduce mineral-insulating transformer oil dielectric strength.

Purpose of BDV Test of Transformer Oil

The breakdown voltage test (BDV) is helpful for:

• Predicting the transformer’s remaining life.
• Improving operational safety.
• Avoiding Equipment Fires
• Ensuring transformer reliability.

The transformer oil dielectric strength test is one of the principal tests used to check oil’s insulating characteristics under high voltage.

Transformer dielectric strength must above threshold.

But water, dirt, and conductive particles can weaken it. Particles and pollutants can have many causes. 

These primarily conducting particles lower oil dielectric strength. Dielectric weakness can cause HT-LT current leakage. 

In extreme circumstances, it can cause sparks and short circuits. 

Dryness and cleanliness are indicators of transformer oil health. 

This is no indication that there is no contamination.

Transformer Oil – Minimum BDV

Transformer oil should have a minimum breakdown voltage of at least 30 KV, according to IEC regulations. 

The transformer’s voltage rating has no bearing on the BDV of transformer oil. 

For 11 KV, 22 KV, or 132 KV, the minimum transformer BDV value is the same. Transformer oil BDV must be at least 30 KV.

What is BDV Tester?

The BDV tester removes the sample from the transformer tank and analyses the oil’s BDV. 

The BDV test kit can regulate voltage between 0 and 70 KV, making it essentially a high-voltage device. 

Two electrodes on the BDV tester are spaced 2.5 mm apart (units of separation of 4.00 mm are also available).

Equipment for Transformer Oil Testing

Equipment used to measure transformer dielectric strength must meet these requirements.

• Capable of supplying high voltage.
• Arrangement for oil sample storage.
• Sparking possibility.
• Voltmeter.
• The equipment should increase voltage at a rate of 2Kv/sec.
• The air gap between two electrodes in an oil cup must be in mm according to IEC 60156 standard.
• The space between electrodes should be adjustable.
• Electrode ends must be concave.

How to determine Transformer Oil – BDV Value?

The oil sample from the transformer must be at least 27°C, according to the Oil BDV Test Procedure.

Procedure for Testing BDV for Transformer Oil

Step-1: Fill a glass (or) plastic container with the 300–400 ml transformer oil sample that you removed from the transformer’s bottom valve.

Step-2: Connecting the equipment with a grounding wire. Then connect with 220VAC, 50Hz (or) 110VAC, 60Hz, as recommended by the vendor.

Step-3: First, fix the air gap between electrodes, which must be 2.5 mm per IEC 60156 standard.

Step-4: Transfer the sample of oil into the machine’s oil pot.

Step-5: Make sure that any air bubbles escape the oil pot by leaving it outside for five minutes. To ensure uniform dispersion of contaminants and to free the vessel from air bubbles, it should also be shaken.

Step-6: In the machine, insert the oil sample now.

Step-7: By using a glass window to view the oil sample, increase the KV at a rate of 2 KV/sec.

Step-8: While increasing the voltage, keep an eye on the oil sample via an inspection window continuously.

Step-9: The KV at which sparking is seen in the oil should be noted down. The breakdown voltage of oil is the voltage at which sparking is detected.

Step-10: Remove the cup’s safety lid. Shake the old to help clean a little wooden stick. Now wait ten minutes for the next reading. After 10 minutes, the same technique may be repeated.

Step-11: Take note of the breakdown KV in every circumstance as you repeat steps 8 times for the same sample.

Step-12: The BDV values that were noted in different steps should be averaged.

Step-13: Transformer oil meets quality (good) standards if its BDV value is greater than 30 KV.

Template for BDV Test Report for Transformer Oil

The high temperature will create an incorrect reading if the BDV of the transformer oil is measured immediately after oil filtering. It is recommended to wait until the transformer oil reaches a temperature of 27°C before doing the BDV test.

Oil Sample Number	Sample Test Number	BDV Value (KV)	Average BDV (KV)	Oil BDV  Value
1	1	52	(52+48+50+45+43+47+42+40)/8	46
	2	48
	3	50
	4	45
	5	43
	6	47
	7	42
	8	40

Interpretation of Results

• New Oil: Generally has a BDV value greater than 70 kV.
• In-service Oil: A BDV value greater than 30 kV indicates high insulating strength.
• Low BDV values (less than 30 kV): Can indicate contamination, deterioration, or moisture presence.

Factors affecting BDV Value of Transformer Oil

• Water is the primary pollutant in transformer oil. Moisture supplies charge carriers, reducing the dielectric withstand strength of transformer oil.
• Acids and other ageing byproducts give charge carriers via dissociation. Additionally, they’re surface-active, lowering surface tension. As a result, they promote bubble evolution in the presence of decreasing dielectric strength.
• Pressure also effects bubble evolution. As the pressure increases, so does the breakdown voltage. For pressures lower than atmospheric pressure, the BDV (Breakdown Voltage) should drop.
• Dry cellulose fibre particles reduce the breakdown voltage by promoting bubble formation.