BDV of transformer in high voltage

Not 100% sure but my guess would be that the existing internal clearances and insulation within the transformer tank (between windings and the metal parts of the tank) mean that the expected breakdown voltage the oil has to handle is less than 60kV (hence no need for a higher rating).

There are differences in your BDV for oil dielectric strength testing and the BDV oil in your transformer for your transformer rated at 220KV or 400KV.

Look at the construction of your transformer and your dielectric strength of your oil tester equipment.

In your dielectric strength equipment, your transformer oil is tested by injection of high voltage across the two test pole with the distance of 5mm or 10mm and when the high voltage flash over the two test pole, the maximum voltage will be register and that will be your BDV of the oil.

Now look at your construction of your transformer. Most of the transformer winding are insulated and the only bare copper wire or the bare parts of your transformer is your transformer HV and your LV side that connected to the termination for the outer connection.

The distance from the termination with respect to the transformer body or between both phase is very far away. It about 800mm to 1000mm. Therefore, the transformer with 60KV BDV is normal.

You can try another dielectric strength with your transformer oil with the testing pole slightly far away. You will get higher BDV for your oil. Of course, all the dielectric strength for the oil test has to follow BS standard or IEC standard.

BDV of 60 KV is at a gap of 2.5 mm, inactual gap between winding & tank body(earth) is aproximate 1000 mm, so there is no issue of flashover.

Sorry, Mr. Punit Sharma was correct. The gap for the dielectric oil test is 2.5mm and not 5mm or 10mm as I stated early.

Boy, I haven't touch that equipment for a long time an forget about the testing gap.

Just a guess, but I think that if your transformer was initially designed and tested for 220 or 440kV, then your oil is contaminated. The most likely contaminate is water. Send an oil sample out for testing or replace it with new oil.

This is an important topic. None of the postings so far have been in enough depth to be very meaningful. The test method must be quoted along with the result. 60 kV at 1 mm on D1816 is very good. 60kV at 2 mm in an actual test is not bad. D877 is less sensitive to particulate issues. The scatter of data on these tests is also very meaningful. Large particles will give a certain pattern, for example. The oil going into a unit should be at least 58 kV at 2mm by D1816. After some years of service, it will be much less. Air saturation, moisture, and particulate all take their toll on the dielectric strength of an oil. If this is your job, you should join ASTM D27, or the IEEE Dielectric or Electrical Insulation subgroups. You will learn a lot, and save your employer money in the long term.

There are points of 'near contact' in any unit. The stuff you can see (like leads, switches and bushings) is never the critical issue. Paper wrapped conductors may be only a few thousandths of an inch apart at significant voltage differential.

why BDV is 60KV in 220KV Transformers

Whoever generated the 60 kV number will also have quoted the test used (ASTM D1816??) This test will also have mentioned the test gap, if that is an option. We use D1816 at 2mm gap. If the gap had been 1 mm, you could not get it to withstand 60kV. There will always be several mm of gap between the grounded parts and the windings. The adjacent copper turns will be at least a mm apart, but only a few volts different in potential.

Like a lot of engineering topics, it may take a bit of explanation and understanding, but it all makes sense. The nominal rating of the transformer bears no relation to the material properties that it requires.

Always read the units of a measurement. Remember a 2 pound hammer can always break a 10 pound concrete block, even if it is made from 3000 pound concrete (3000 PSI??)