basic impluse insulation level
DESCRIPTION
SADGTRANSCRIPT
B.I.L :-
In addition it is the capacity of an equipment to withstand mechanical stress..like lightning strike without causing any damage to the equipment...
The Basic Impulse Insulation Levels refers to the ability of electric equipment such as transformers to withstand lightning surges. When lightning strikes a transmission line, a traveling waveis created. This traveling wavetravels along the line and damages the transformer winding.
Surge arresters mounted on the line can mitigate the surge, however, they cannot totally eliminate it. Voltage surges can also be created by the switching of circuit breakers and switches.
The BIL or the Basic Impulse Insulation level indicates the ability of the transformer to withstand these heavy surges.
1. Basic Impulse Insulation Level (BIL)
This is the reference insulation level expressed as an impulse crest (or peak) voltage with a standard wave not longer than a 1.2 x 50 microsecond wave.
A 1.2 x 50 microsecond wave means that the impulse takes 1.2 microseconds to reach the peak and then decays to 50% of the peak in 50 microseconds. (Click here for a figure of the BIL waveform)
Surge Arresters
Modern Surge arresters are of the gapless Zinc Oxide type. Previously, Silicon Carbide arresters were used, but their use has been superceeded by the ZnO arresters, which have a non-linear resistance characteristic. Thus, it is possible to eliminate the series gaps between the individual ZnO block making up the arrester.
Selection Procedure for Surge arresters
1. Determine the continuous arrester voltage. This is usually the system rated voltage. 2. Select a rated voltage for the arrester.
3. Determine the normal lightning discharge current. Below 36kV, 5kA rated arresters are chosen. Otherwise, a 10kA rated arrester is used. 4. Determine the required long duration discharge capability. For rated voltage < 36kV, light duty surge arrester may be specified. For rated voltage between 36kV and 245kV, heavy duty arresters may be specified. For rated voltage >245kV, long duration discharge capabilities may be specified. 5. Determine the maximum prospective fault current and protection tripping times at the location of the surge arrester and match with the surge arrester duty. 6. Select the surge arrester having porcelain creepage distance in accordance with the environmental conditions. 7. Determine the surge arrester protection level and match with standard IEC 99 recommendations.