What is BIL and how does it apply to transformers?

BIL is an abbreviation for Basic Insulation Level. Insulation levels in electrical equipment are characterized by the withstand voltages used during the impulse test. Impulse test is a dielectric test which consists of the application of a high frequency steep wave front voltage between windings and between windings and ground. The BIL of a transformer is a method used to specify the magnitude of the voltage surge that a transformer can tolerate without any damage to the windings and live parts of the transformer. When lightning impulse over voltage appears in the system, it is discharged through surge protecting device before the transformer gets damaged. BIL rating specifies the minimum voltage that transformer can withstand under this condition.
BIL graph  
The method of testing of the transformer for BIL has been defined and set by IEEE and ANSI standards. The wave shape has been also defined which is commonly known as 1.2/50 μs voltage wave. The impulse wave shape shows the magnitude of the voltage in KV (Kilo volts), Rise time (tf, time that takes the voltage rise from zero to its peak value in μs (Micro seconds)),and duration of the surge (T) sometime referred as Tail time (time that takes the voltage drop to 50% of its peak value in μs (Micro seconds).
This test is done with the initial transformer design to validate the integrity of the insulation and its high frequency surge withstand capability. It is considered one of the design tests for any transformer and needs not to be repeated with every transformer manufactured. However, a quality control impulse test (QC impulse test or production impulse test) is offered as an optional test whenever required. Design impulse test consists of a reduced voltage, 2 chopped wave, and a full voltage impulse applied to the transformer. Voltage and current wave shapes are captured during the above tests for comparison. Any deviation from the reduced wave to full voltage wave shape should be studied. In general, they should be very close to each other. Any new bump in the full wave can be considered as a failure point. Based on the location of the bump, an educated guess can be made as where the failure has occurred. After subjecting the transformer to above voltage surge tests, transformer should pass hi pot test at 60Hz. and double induced voltage test 400 Hz.
During quality control, impulse-only full voltage surge is applied to all of the bushings or the terminals of the transformer before hi pot and double induced test is performed.