Insulator pf /impulse withstand for procelain & composite

[majan]

We used to have procelain string insulators in system where in pf and impulse withstand levels are defined per disc (like we use 5 disc in 33KV and 3 in 11KV)Now since silcone ruber insualtors is under proposal I would like to know How the pf & impulse values should change in specification. WIll it be a direct multiple of 5 to per disc value for 33KV ? I think NOT. but then how to define new value OR is it simply the system values based on insulation coordination?

pls throw light and Thanks

 

[cuky2000]

In general, polymer insulators perform as good as or better than the equivalent disc type units.
This may be a challenger for the new suppliers from the economic point of view since 11 kV and 34.5 kV distribution OH systems usually are satisfactory installed with 2 and 3 disk insulators instead 3 to 4 unit as state in your post.
I suspect that your system may be located in a heavy polluted environment exposed to unusual high overvoltage conditions or just overdesigned.

I suggest perform an insulation coordination assessment of the present system taking in consideration the dielectric failure history and discuss with your new insulators supplier witch product will provide a cost effective solution providing the same or better dielectric withstand voltage.
If the insulation coordination study finds that the system is overdesigned, check if there is any advantage of reducing the number of insulator or specify new insulator with lower dielectric characteristics.
_{NOTE: For instance, 3 standard insulator are good for 200 kVcrest BIL and 95 kVrms power frequency @ 60 Hz (wet or dry).}

 

[majan]

Thanks cuky for your valuable input