Basic Impulse Level (BIL)

     How is the reaction of materials to the impulse voltageinsulation? An examination showed that the ability to withstandincreased when the voltage that flows take place in periods of very rapid. To illustrate, we will conduct testing on the transformerinsolasi, using 50 Hz sinusoidal voltage between the windings tothe tank. Along with increasing voltage, it will reach a point where the voltage will drop. We assume That point with 46 kV (RMS) or 65 kV peak.
     If now we use dc impulses between the windings to the ground, we find that it takes twice the peak voltage (or 130 kV) before isolation of the breakdown (broken). The same thinghappened on suspension insulators, bushings, etc., unless the ratio between the impulse voltage and ac voltage peak close to1.5.
     For standardization, and to facilitate comparisons betweenthe same tool in restraining impulses, standards organizationshave determined the shape and peak value of the impulse wave.

 Voltage Standard curve Impluls The equipment used todetermine the BIL

Picture above shows the standard impulse wave. The wavereached its peak after 1.2 μs and decreased to half of its peak at 50 μs. Peak voltage has a series of values ​​ranging from 30 kV to 2400 kV (see Table)

Voltage Peak To test BIL 1.2 x 50 μs

Peak voltage is used to determine the quality of basicimpulse insulation (BIL) of a device. So, some devices(transformers, insulators, capacitors, resistors, bushings, etc.)that can withstand 1.2 x 50 μs waveform of 900 kV, is considered to have the quality of basic impulse insulation (BIL) of 900 kV.