SURGE ARRESTER (SELECTION)

[electpower]

I have trouble with understanding the selection of surge arrester

This is regarding the surge arreter for 200kV sytem connected to the overhead line on one side and to a cable which is 50 meter long connected to the transformer on the other side, this surge arrester has to protect the transformer
The transformer has a Lightning impulse withstand of 1050kV
And the tranformer is connected to a generator
the load rejection factor k=1.4 (approximately)
Now my continous voltage of the arrester is Uc= (Phase to earth voltage of the highest system voltage)*1.05
This 1.05 takes into account the harmonics and other transients
so my Uc= (245/root 3)*1.05
Uc=149kV
Now the rated voltage of the arrester
Ur= Utov/kTov
Utov= k*(245/root 3) = 198kv
kTOV is a factor taken from the plot of (Utov/Ur) verses time
Now my kTOv factor is 1.07 ( for 10s - as suggested by IEC)
from which my rated voltage is now 185kv
However the IEC suggests that the rated voltage to be higher than the U tov
But the actual selection is 228kv rated surge arrester with Uc= Ur/1.25
The possible arresters in the 245kV ranges are with rated voltages 192, 198 and 228 kv
Can some one explain why is this selection made ?(WIth regard to the protective level )

 

[RRaghunath]

198kV should have been adequate for your system. 228kV will be required if the system is non-effectively grounded.
Having said that going in for higher Uc than required (228kV in this case) is permissible as long as the equipment protective level is ensured / BIL is not exceeded.
I have seen people specifying higher voltage SAs with the hope that the SA life will be better. But, if caution is not exercised, you could be having a situation that the SA lives long but the transformer dies.