Surge Arrestor

I m not sure, but as you are saying the switch yard voltage is 220 KV. This is the line voltage and not phase voltage. We are connecting the surge arrestor in between phase and the ground.

Now, in your case, the phase to ground voltage is 220 KV/1.73 which is around 128 KV. So, you will require a surge arrestor which capacity should be more than 128 KV.

The function of surge arrestor is to arrest the high voltage surges on the line (this surges may be because of grid fluctuations or lightening) and to protect the connected equipments from high voltage. So, surge arrestor provide low resistance path to high voltage and allow current to pass through it, actuating the protection relay to trip the feeder.

The current 10 KA indicates the amount of current it can carry safely.