Why we used an air circuit breaker in sub-station?

Protection devices are selected according to following parameters and many more.

1. Cuurent they need to carry.
2. Fault level.
3. ease of maintainence.
4. fault clearing time 

when you talk about substations,it does mean that its mostly if not always about very high currents. so you need Air Circuit breaker whose min ratings starts from 630 A , some starts from 400 A, for LV offcourse.

1. Air is the best medium for ark quencing in low voltage 

2. ACB can be electronically sensed and electrically controlled

3. Sensitivity is higher

A device to protect the circuit at overcurrent conditions which may reach to 10k Amp