Surge arrester at the neutral point of HV transformer

[essamalbanna]

I'm searching for a logical explanation or practical purpose for installing a lightning arrester at both neutral points of the transformer 220/66 KV, YNynd11, Although both neutral points are connected directly to earth through a disconnector!?.
What's the function of that disconnector?

 

[FPelec]

The surge arrester connected between the neutral and ground is used to ensure that, in the event the ground disconnect switch is open, no overvoltages exceeding the design specifications appear on the transformer's star point. This is typically employed when dealing with transformers with non-uniform insulation while still intending to operate the transformer also with an isolated neutral. The surge arrester is, of course, unnecessary if the star point is directly grounded, i.e. without any disconnector.