reasons for flashover

Few months back we had a flash over on the surge arrestor on primary side of power transformer. There is a network of surge arrestor connected in delta for phase to phase and in star for phase to line.

can overvoltage create this problem? or there are some other reasons?

Power transformers normally experience huge surges during switching on. You have not mentioned trafo ratings and voltage levels, but I would suggest to check the switching breaker especially the bounces during closing. A particular phase with high bounce will induce surges in other two phases and can damage the surge arrestors.

It is 125MVA trafo with 33kv on its primary side where the surge arrestor are connected. Same time when the flashover occurred there was some flash over at the feeding side (sub-station) of this transformer. The problem at our side may be due to flashover at substation. Second question is if there is some flashover on surge arrestor. Is it possible to check it whether it can be used again or not? In fact we changed it with new one as we didn't want to take any risk.