Insulation level

The insulation levels expressed in impulse crest voltage with a standard wave is not longer than a 1.2 X 50 µs wave. System insulation as calculated by suitable methods shall be equal or greater than the BIL.


The withstand voltage is the level that the insulator will withstand for a given length of time or number of applications without disruptive discharge occurring, i.e. a failure of insulation resulting in a collapse of voltage and passage of current (sometimes termed ‘sparkover’ or‘flashover’ when the discharge is on the external surface). The BIL is usually expressed as a per unit of the peak (crest) value of the normal operating voltage to earth.
Example: for a maximum operating voltage of 362 kV.
1 p.u. = v2 X (362 / v3) = 300 kV
so that a BIL of 2.7 p.u. = 810 kV
Up to an operating voltage of 345 kV the insulation level is determined by lightning, and the standard impulse consideration is suffice. Above this value, however, the overvoltage resulting from switching is higher in magnitude and therefore they decide the insulation.

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