Transformer Oil

A1) Why would it be suitable if its bdv is less than 25% of what is required (rhetorical question - NNTR)?

A2) Ask the oil and transformer manufacturers.

Yes, 50kV dielectric breakdown voltage should be acceptable for use in a 220kV transformer, assuming other characteristics of the oil are acceptable (including specific gravity, acidity, interfacial tension, moisture content, dissolved combustible gases). The BDV is usually a measure of the moisture and other contaminants in the oil, and 50kV is well above the ASTM D-1816 and D-877 test limits.

Is there a chart to read the BDV strength of oil for various voltage levels (6.6,11.33,66,132,275,400,500,750kV) of transformer.

Breakdown strength doesn't necessarily relate directly to transformer voltages. It relates to the electrode type and gap used in the BDV test apparatus. ASTM D-877 is the older test method, using flat, coin-shaped electrodes spaced 2.54mm (0.1") apart, and "passing" BDV is 30kV. The newer D-1816 uses electrodes with rounded surfaces, at a 1mm or 2mm gap. There are differences in the acceptable values of BDV for different voltage ranges (for NEW oil), and also depends on the electrode gap used. See the following link for more explanation. However, although the BDV value is part of the equation, the rest is in the "thickness" of oil (it's more than 1 or 2mm !) the transformer manufacturer has designed for when he located the energized components in the transformer tank.

Based on a D-877 test, the OP's BDV is quite acceptable. Based on D-1816, the OP's BDV is acceptable if this is in-service oil. Overall, I would probably accept this oil test value for use, unless it were brand new oil (fresh from manufacturer) and the test was the D-1816 @ 2mm. Then I would expect the BDV to be better.

http://sdmyers.com/techcorner/tech-2009-12-18-dbvt.html