Reduced transformer insulation levels

FURTHER BIL reductions will result as more is learned of switching surges and as lightning arrester protective levels are improved. For system voltages of 230 kv and above, it appears likely that the ultimate reduction in BIL will be in the order of 50% of the fully insulated value. This reduction is equivalent to “4-steps-down” although this nomenclature is not very precise and is used only in the absence of a better one. With 50% BIL reduction, the transformer low-frequency test is in the range of 1.5 to 1.7 times maximum line-to-ground voltage, which appears to be a reasonable minimum ratio.