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Non-Standard Wave-Shape break-down/flashover propability prediction

Non-Standard Wave-Shape break-down/flashover propability prediction

Non-Standard Wave-Shape break-down/flashover propability prediction

(OP)
Hi all,

In a 380 kV System, After running a study for insulation coordination with PSCAD, with a lightning impulse of 1.7/77 us, the resultant voltage on bushing was showing oscillatory response with a peak of 846 kV and 2.5/103 us overall characteristic(the oscillation isn't reaching the half-value before 103 us). System BIL is 1425 kV (estimated as per testing with 1.2/50 us impulses).
The recommendation of the IEEE is to provide a sufficient margin above the BIL ~15-25%.(i understand that this value is for 1.2/50 us waves since lightning impulses flash-over is most dependent on tail time "IEEE 1313.2-1999 cl.4.5!!)
My question is how to estimate the withstand of any bushing against this non standard wave?i.e. if we are in a stage to select the insulation level of such equipment, what shall be the minimum BIL to withstand this 2.5/103 us wave?  

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