Transformer Oil Testing, BDV.

If what you say is accurate, and the transformer is energized, then you are operating on borrowed time! It's time to de-energize the transformer and find out why the BDV (Break Down Voltage) is so low, probably by doing a DGA (Dissolved Gas Analysis). The DGA will point you in the proper direction for cleaning/replacing the oil, and/or an internal inspection. You need someone experienced in these matters on-site ASAP.

@RAMConsult.

The BDV value is not low. Its quite healthy infact by international standards ! But I would like to know the logic as to how this BDV value is considered ok. I just posted the following ...

"But let me elaborate my confusion; by International standards the oil is considered good for insulation in 132KV/11KV transformer if BDV value is greater than 55KV.

Now how will the oil be a good insulation medium for 132KV when the oil itself has a BDV value of 75KV ???

Does it has something to do with the way BDV testing is done ??? I mean electrode spacing is 2.5 mm when conducting BDV testing.... in real condition the winding spacing inside the transformer is much larger so 75KV would be good enough ???

Plz explain !"

See #1↑.

According to iec standard BDV is the break down voltage per 2.5 mm spacing between Live conductors, this 2.5 mm is not the actual spacing between electrodes of dielectric test set, infact it's a standar set to measure the break down strength of transformer oil for a minimum spacing of 2.5 mm. while In oil transformer spacing between live conductors are much larger than 2.5mm, so 77 kv BDV is acceptable for 132 and even 220kV transformer.

http://cr4.globalspec.com/thread/50425

check here

Thanks for your replies. But let me elaborate my confusion; by International standards the oil is considered good for insulation in 132KV/11KV transformer if BDV value is greater than 55KV.

Now how will the oil be a good insulation medium for 132KV when the oil itself has a BDV value of 75KV ???

Does it has something to do with the way BDV testing is done ??? I mean electrode spacing is 2.5 mm when conducting BDV testing.... in real condition the winding spacing inside the transformer is much larger so 75KV would be good enough ???

Plz explain !

Hey Nikolay !
Thanks for your answer, it really helped. I was thinking on the same track but just needed someone to confirm it. Been a great help, thanks again :) !

In 1970s we tested BDV with a gap between electrodes of 4mm at 40 kV(10kV/mm) for 33kV transformer. Have the test standard changed?.

There are two common test procedures for oil dielectric used here in the US (and many parts fo the world) per ASTM: The D-877 which uses flat-faced cylindrical electrodes spaced at 2.5mm, and the D-1816 which uses spherically-shaped electrodes spaced at either 1mm or 2mm. There are other slight differences between the two tests (rate of voltage increase, agitation of sample, etc.), but with regard to the electrodes this is the difference.

We were following the BS with spherical electrodes.

What should be Break Down Voltage value for the following Transformers?

1. 22/0.4kV, 1000kVA

2. 0.4/125 KV, 160.9 KVA

3. 132/6.3 KV, 26 MVA

Above three transformers having different voltages. Take Higher side Voltages

1. 22,000 V

2. 125, 000 V

3. 132, 000 V

According to IS 6792:1992

The oil sample is subjected to steadily increasing alternating voltage until breakdown occurs in a BDV test kit. The Breakdown voltage is the voltage reached at the time of spark appears between the electrodes. The test is carried out six time on the same cell filling and the Electric strength of the oil is the arithmetic mean of the six result obtained. The Electrodes are mounted on the horizontal axis with the test spacing of 2.5 mm. The values should be

30 KV Minimum: Transformers up to 72.5 KV

40 KV Minimum: Transformers from 72.5 KV up to 170 KV

50 KV Minimum: Transformers from 172 KV up to 420 KV

Example-1: 22/0.4kV, 1000kVA

The BDV value should be greater than 30 KV (Voltage is 22000 V)

Example-2: 0.4/125 KV, 160.9 KVA

The BDV value should be greater than 40 KV (Voltage is 125,000 V, greater than 72,500 V)

Example-3: 132/6.3 KV, 26 MVA

The BDV value should be greater than 40 KV (Voltage is 132,000 V, greater than 72,500 V)

Hope it would help